Formation and precipitation of insulating discharge products, such as Li2S, in the lithium-sulfur (Li-S) battery cathode leads to deleterious performance decay. Physicochemical interactions underlying the cathode microstructure evolution due to precipitation are hitherto poorly understood. In this work, a mesoscale understanding of the microstructure – precipitate interplay owing to surface passivation and pore blockage is presented. Mesoporous, carbon-based cathode microstructures are examined for disparate precipitate morphology and growth. Pore-scale manifestation of the microstructural and transport limitations on the electrochemical performance is elucidated. Surface passivation and pore blockage effects are examined for complexations due to sulfur loading, electrolyte volume, pore size and precipitate morphology. This study provides critical insights into the underlying mesoscale physics and microstructural stochasticity on the Li-S battery performance.
Lithium–sulfur chemistry, despite being a promising candidate for energy storage due to its higher theoretical capacity, is faced with several critical challenges. Practical operation of Li–S batteries demonstrates lower capacity, poor rate capability, and insufficient cycle life, which can be related to the underlying physicochemical interactions at the electrodes. Typical carbon-based porous cathodes undergo coupled electrochemical, chemical, and microstructural evolution during operation. In this work, the mesoscale interaction resulting from the underlying chemical/electrochemical complexations and microstructural evolution is studied in order to elucidate the transient impedance behavior in Li–S battery mesoporous carbon cathodes. The discharge product (e.g., Li2S) precipitation is shown to affect impedance evolution with correlational dependence on the porous cathode microstructure attributes. This mesoscale impedance analytics can be a valuable virtual probing tool for Li–S battery electrochemical performance.
Capacity fade in lithium-ion batteries largely originates from the undesired electrolyte decomposition, which results in the formation of solid electrolyte interphase (SEI) and the anode surface passivation. In this work, a mesoscale interfacial modeling approach is developed to investigate the formation and growth of the SEI film on typical graphite based anode over several cycles. It is found that lithium diffusion kinetics in the SEI film significantly affects the SEI growth rate. Lower lithium diffusion barrier leads to higher growth rate. The present model demonstrates that the SEI thickness is a linear function of the square root of the charging time over long-time cycling. Growth of multi-component SEI film is also elucidated. It is found that the heterogeneity of the SEI film may lead to instability in lithium ion concentration distribution.
High-capacity anode materials for lithium-ion batteries, such as silicon, are prone to large volume change during lithiation/delithiation which may cause particle cracking and disintegration, thereby resulting in severe capacity fade and reduction in cycle life. In this work, a stochastic analysis is presented in order to understand the mechano-electrochemical interaction in silicon active particles along with a surface film during cycling. Amorphous silicon particles exhibiting single-phase lithiation incur lower amount of cracking as compared to crystalline silicon particles exhibiting two-phase lithiation for the same degree of volumetric expansion. Rupture of the brittle surface film is observed for both amorphous and crystalline silicon particles and is attributed to the large volumetric expansion of the silicon active particle with lithiation. The mechanical property of the surface film plays an important role in determining the amount of degradation in the particle/film assembly. A strategy to ameliorate particle cracking in silicon active particles is proposed.
Galvanostatic intermittent titration technique (GITT) – a popular method for characterizing kinetic and transport properties of battery electrodes – is predicated on the proper evaluation of electrode active area. LiNi0.5044Co0.1986Mn0.2970O2 (NCM523) material exhibits a complex morphology in which sub-micron primary particles aggregate to form secondary particle agglomerates. This work proposes a new active area formulation for primary/secondary particle agglomerate materials to better mimic the morphology of NCM523 electrodes. This formulation is then coupled with macro-homogeneous models to simulate GITT and half-cell performance of NCM523 electrodes. Subsequently, the model results are compared against the experimental results to refine the area formulation. A single parameter, the surface roughness factor, is proposed to mimic the change in interfacial area, diffusivity and exchange current density simultaneously and detailed modeling results are presented to provide valuable insights into the efficacy of the formulation.
The shuttle effect and poor conductivity of the discharge products are among the primary impediments and scientific challenges for lithium–sulfur batteries. The lithium–sulfur battery is a complex energy storage system, which involves multistep electrochemical reactions, insoluble polysulfide precipitation in the cathode, soluble polysulfide transport, and self-discharge caused by chemical reactions between polysulfides and Li metal anode. These phenomena happen at different length and time-scales and are difficult to be entirely gauged by experimental techniques. In this paper, we reviewed the multiscale modeling studies on lithium–sulfur batteries: (1) the atomistic simulations were employed to seek alternative materials for mitigating the shuttle effect; (2) the growth kinetics of Li2S film and corresponding surface passivation were investigated by the interfacial model based on findings from atomistic simulations; (3) the nature of Li2S2, which is the only solid intermediate product, was revealed by the density functional theory simulation; and (4) macroscale models were developed to analyze the effect of reaction kinetics, sulfur loading, and transport properties on the cell performance. The challenge for the multiscale modeling approach is translating the microscopic information from atomistic simulations and interfacial model into the meso-/macroscale model for accurately predicting the cell performance.
Poor electronic conductivity of bulk lithium sulfide (Li2S) is a critical challenge for the debilitating performance of the lithium–sulfur battery. This study focuses on investigating the thermodynamic and kinetic properties of native defects in Li2S based on a first-principles approach. It is found that the hole polaron p+ can form in Li2S by removing a 3p electron from an S2– anion. The p+ diffusion barrier is only 90 meV, which is much lower than the Li vacancy (VLi-) diffusion barrier. Hence p+ has the potential to serve as a charge carrier in the discharge product. Once the vacancy–polaron complex (VLi- – 2p+) forms, the charge transport will be hindered due to the relatively higher diffusion barrier of the complex. Heteroatom dopants, which can decrease the p+ formation energy and increase VLi- formation energy, are expected to be introduced to the discharge product to improve the electronic conductivity.
The cathode surface passivation caused by Li2S precipitation adversely affects the performance of lithium–sulfur (Li–S) batteries. Li2S precipitation is a complicated mesoscale process involving adsorption, desorption and diffusion kinetics, which are affected profoundly by the reactant concentration and operating temperature. In this work, a mesoscale interfacial model is presented to study the growth of Li2S film on carbon cathode surface. Li2S film growth experiences nucleation, isolated Li2S island growth and island coalescence. The slow adsorption rate at small S2- concentration inhibits the formation of nucleation seeds and the lateral growth of Li2S islands, which deters surface passivation. An appropriate operating temperature, especially in the medium-to-high temperature range, can also defer surface passivation. Fewer Li2S nucleation seeds form in such an operating temperature range, thereby facilitating heterogeneous growth and potentially inhibiting the lateral growth of the Li2S film, which may ultimately result in reduced surface passivation. The high specific surface area of the cathode microstructure is expected to mitigate the surface passivation.
Besides lithium sulfide (Li2S), lithium persulfide (Li2S2) is another solid discharge product in lithium–sulfur (Li–S) batteries. Revealing the charge transport mechanism in the discharge products is important for developing an effective strategy to improve the performance of Li–S batteries. Li2S2 cannot transport free electrons due to its wide bandgap between the valence band maximum (VBM) and conduction band minimum (CBM). However, electron polarons (p-) and hole polarons (p+) can appear in solid Li2S2 due to the unique molecular orbital structure of the S22- anion. The thermodynamic and kinetic properties of native defects are investigated. It is found that negatively charged Li vacancies (VLi-) and p+ are the main native defects with a low formation energy of 0.77 eV. The predominant charge carrier is p+ because p+ has a high mobility. The electronic conductivity related to p+ diffusion is dependent on temperature, and high temperatures are preferred to increase the conductivity.
Processing induced nanoparticle agglomeration and binder distribution affect the electrode microstructure formation and corresponding electrochemical performance in lithium-ion batteries. In the present study, stochastic dynamics computations based on a morphologically detailed mesoscale model are performed to illustrate the microstructural variability of electrode films affected by the evaporation condition (drying temperature) and the binder length (molecular weight). Micropores are observed at the surface of the electrode film when dried at a lower temperature. The pore formation depth tends to increase as the binder length increases. The solvent chemical potential also affects the surface topography of the electrode film. The solvent with higher volatility (more negative chemical potential) tends to produce more micropores. A lower drying temperature is beneficial for improving the electronic conductivity of the porous electrode film due to the better distribution of the conductive additive nanoparticles on and around the active particles, thereby facilitating the electron transport network formation. Agglomeration between active material nanoparticles can also be mitigated at a lower drying temperature. Additionally, better adhesion of the porous electrode film can be achieved due to preferential localization of the binder on the substrate at relatively low-temperature evaporation.
Electrode processing based on the state-of-the-art materials represents a scientific opportunity toward a cost-effective measure for improving the lithium-ion battery performance. In this regard, perhaps the most important is the drying step in a typical non-aqueous based slurry processing which can profoundly impact the electrode microstructure and hence performance. Solvent evaporation during drying plays a critical role in the redistribution of the particulate phases consisting of active particle, conductive additive and binder. In this work, we attempt to provide a mechanistic understanding of the role of solvent evaporation on the electrode characteristics and performance via a combined experimental and theoretical analysis. This study elucidates that a non-uniform distribution of the constituent phases, especially the relatively mobile conductive additive and binder, can develop which depends on the solvent evaporation, particle diffusion and sedimentation attributes. Experimental results and theoretical analysis reveal the impact of evaporation rate on the conductive additive and binder distribution in the electrode microstructure and resulting electrochemical performance. Our analysis has shown that a slower two-stage drying, as opposed to a high-rate single-stage drying, allows for a favorable distribution of binder and conductive additive, thus reducing internal cell resistance and improving electrochemical performance.
Safety and performance of lithium-ion batteries over a wide temperature window are of paramount importance, especially for electric vehicles. The safety concerns are predicated on the thermal behavior as the occurrence of local temperature excursions may lead to thermal runaway. In this work, the role of electrode microstructure and implications on the cell thermal behavior are examined. A microstructure-aware electrochemical-thermal coupled model has been proposed, which delineates the electrode-level thermal complexations due to the structure-transport-electrochemistry interactions. Detailed analysis of the spatio-temporal variation of the heat generation rates (ohmic, reaction and reversible contributions) for different electrode microstructural configurations is presented to explain the dominant factors causing temperature rise. The tradeoff between the cell performance and safety is discussed from an electrode-level, bottom-up view point. This study aims to provide valuable insights into potentially tuning electrode-level structural features as an internal safety switch toward limiting the Li-ion cell temperature rise during operation.
A three-electrode cell can be a useful tool for measuring electrode-level and cell-level electrochemical characteristics, such as the impedance response and potential variations in lithium-ion cells. In this paper, a reliable three-electrode coin cell setup is introduced, which improves the stability and accuracy of electrochemical measurements by modifying the electrode alignment and employing Li4Ti5O12 as a reference electrode. An important highlight is the ability to obtain impedance evolution characteristics at different depth of discharge (DOD) for an individual electrode and the full cell based on both the frequency response analysis and the carrier function Laplace transform characteristics. The reliability of the proposed modified three-electrode coin cell setup has been validated by analyzing the impedance response of symmetric and full cells, and the voltage profiles of the full cell along with the positive/negative electrode contributions. The importance of the resistance contributions from the negative and positive electrodes to the full cell impedance evolution at different DOD is highlighted.
Two-dimensional materials are competitive candidates as cathode materials in lithium–sulfur batteries for immobilizing soluble polysulfides and mitigating the shuttle effect. In this study, a mesoscale modeling approach, which combines first-principles simulation and kinetic Monte Carlo simulation, is employed to evaluate titanium silicide (Ti2Si and TiSi2) monolayers as potential host materials in lithium–sulfur batteries. It is found that the Ti2Si monolayer has much stronger affinities to Li2Sx (x = 1, 2, 4) molecules than the TiSi2 monolayer. Also, Ti2Si can facilitate the dissociation of long-chain Li2S4 to LiS2. On the other hand, TiSi2 can only provide a weak chemical interaction for trapping soluble Li2S4. Therefore, the Ti2Si monolayer can be considered as the next-generation cathode material for lithium–sulfur batteries. However, the strong interaction between Ti2Si and Li2S also causes fast surface passivation. How to control the Li2S precipitation on Ti2Si should be answered by further studies.
Thermal implications related to heat generation and potential temperature excursions during operation in lithium-ion batteries are of critical importance for electric vehicle safety, performance and life. Concurrently, appropriate thermal management strategies for lithium-ion batteries are crucial to maintain cell temperatures within a desired range. Different battery thermal management strategies have been proposed, each with various advantages and disadvantages depending on the applications. This work proposes the use of nanofluids, colloidal suspensions of nanoparticles in a base fluid, as a heat transfer fluid for active thermal management. To analyze the efficacy of nanofluids for thermal management in lithium-ion batteries, different nanofluids and their effect on the temperature distribution within typical battery modules are investigated for two different flow configurations. In particular, the study is focused on battery performance, heat dissipation capability under high discharge rates and ambient temperatures, and design considerations relevant to electric vehicle applications. This study underscores the potential of this innovative thermal management technique toward effective thermal safety without performance penalty of lithium-ion batteries for vehicle electrification.
Aqueous processing of thick electrodes for Li-ion cells promises to increase energy density due to increased volume fraction of active materials, and to reduce cost due to the elimination of the toxic solvents. This work reports the processing and characterization of aqueous processed electrodes with high areal loading and associated full pouch cell performance. Cracking of the electrode coatings becomes a critical issue for aqueous processing of the positive electrode as areal loading increases above 20–25 mg/cm2 (~4 mAh/cm2). Crack initiation and propagation, which was observed during drying via optical microscopy, is related to the build-up of capillary pressure during the drying process. The surface tension of water was reduced by the addition of isopropyl alcohol (IPA), which led to improved wettability and decreased capillary pressure during drying. The critical thickness (areal loading) without cracking increased gradually with increasing IPA content. The electrochemical performance was evaluated in pouch cells. Electrodes processed with water/IPA (80/20 wt%) mixture exhibited good structural integrity with good rate performance and cycling performance.
Forging a stronger connection between mesoscale geometry, performance, and processing techniques can realize practical approaches for controlling battery performance using mesoscale geometry. To this end, 3D X-ray imaging, microstructural characterization, and computational modeling have been applied to analyze the intercalation behavior of Li(Ni1/3Mn1/3Co1/3)O2(NMC) cathodes. Samples extracted from pristine cathodes were imaged using X-ray nanotomography. Active material particle geometry was characterized and compared for samples from four cathodes treated with distinct preparation steps. Significant size reduction was observed in calendered and ball milled samples, and distinct differences were observed in particle morphology. Tomographic data for a representative particle was applied in a numerical transport model to assess the effect of particle geometry on intercalation. This assessment proved critical in determining an appropriate estimate of particle size for defining dimensionless parameters that permit rapid estimation of intercalation time. Defining an effective particle radius based on a sphere of equivalent surface area to volume ratio was found to provide the most accurate estimate of intercalation time. Informed by this analysis, dimensionless parameters were used to assess intercalation behavior of the cathode materials. This assessment revealed a substantial change in rate capability connected to particle size reductions achieved in calendering and ball milling.
The electrochemical performance of anodes made of transition metal oxides (TMOs) in lithium-ion batteries (LIBs) often suffers from their chemical and mechanical instability. In this research, a novel electrode with a hierarchical current collector for TMO active materials is successfully fabricated. It consists of porous nickel as current collector on a copper substrate. The copper has vertically aligned microchannels. Anatase titanium dioxide (TiO2) nanoparticles of ∼100 nm are directly synthesized and cast on the porous Ni using a one-step process. Characterization indicates that this electrode exhibits excellent performance in terms of capacity, reliable rate, and long cyclic stability. The maximum insertion coefficient for the reaction product of LixTiO2 is ∼0.85, a desirable value as an anode of LIBs. Cross-sectional SEM and EDS analysis confirmed the uniform and stable distribution of nanosized TiO2 nanoparticles inside the Ni microchannels during cycling. This is due to the synergistic effect of nano-TiO2 and the hierarchical Cu/Ni current collector. The advantages of the Cu/Ni/TiO2 anode include enhanced activity of electrochemical reactions, shortened lithium ion diffusion pathways, ultrahigh specific surface area, effective accommodation of volume changes of TiO2 nanoparticles, and optimized routes for electrons transport.
The performance, safety, and reliability of Li-ion batteries are determined by a complex set of multiphysics, multiscale phenomena that must be holistically studied and optimized. This paper provides a summary of the state of the art in a variety of research fields related to Li-ion battery materials, processes, and systems. The material presented here is based on a series of discussions at a recently concluded bilateral workshop in which researchers and students from India and the U.S. participated. It is expected that this summary will help understand the complex nature of Li-ion batteries and help highlight the critical directions for future research.
Fundamental understanding of the underlying diffusion-mechanics interplay in the intercalation electrode materials is critical toward improved life and performance of lithium-ion batteries for electric vehicles. Especially, diffusion induced microcrack formation in brittle, intercalation active materials, with emphasis on the grain/grain-boundary (GB) level implications, has been fundamentally investigated based on a stochastic modeling approach. Quasistatic damage evolution has been analyzed under lithium concentration gradient induced stress. Scaling of total amount of microcrack formation shows a power law variation with respect to the system size. Difference between the global and local roughness exponent indicates the existence of anomalous scaling. The deterioration of stiffness with respect to microcrack density displays two distinct regions of damage propagation; namely, diffused damage evolution and stress concentration driven localized crack propagation. Polycrystalline material microstructures with different grain sizes have been considered to study the diffusion-induced fracture in grain and GB regions. Intergranular crack paths are observed within microstructures containing softer GB region, whereas, transgranular crack paths have been observed in microstructures with relatively strong GB region. Increased tortuosity of the spanning crack has been attributed as the reason behind attaining increased fracture strength in polycrystalline materials with smaller grain sizes.
Tin (Sn) anode active particles were electrochemically lithiated during simultaneous imaging in a scanning electron microscope. Relationships among the reaction mechanism, active particle local strain rate, particle size, and microcrack formation are elucidated to demonstrate the importance of strain relaxation in the mechano-electrochemical interaction in Sn-based electrodes under electrochemical cycling. At low rates of operation, due to significant creep relaxation, large Sn active particles, of size 1 μm, exhibit no significant surface crack formation. Microcrack formation within Sn active particles occurs due to two different mechanisms: (i) large concentration gradient induced stress at the two-phase interface, and (ii) high volume expansion induced stress at the surface of the active particles. From the present study, it can be concluded that majority of the microcracks evolve at or near the particle surface due to high volume expansion induced tension. Concentration gradient induced damage prevails near the center of the active particle, though significantly smaller in magnitude. Comparison with experimental results indicates that at operating conditions of C/2, even 500 nm sized Sn active particles remain free from surface crack formation, which emphasizes the importance of creep relaxation. A phase map has been developed to demonstrate the preferred mechano-electrochemical window of operation of Sn-based electrodes.
Transition metal oxides are usually used as catalysts in the air cathode of lithium–air (Li–air) batteries. This study elucidates the mechanistic origin of the oxygen reduction reaction catalyzed by δ-MnO2 monolayers and maps the conditions for Li2O2 growth using a combination of first-principles calculations and mesoscale modeling. The MnO2 monolayer, in the absence of an applied potential, preferentially reacts with a Li atom instead of an O2 molecule to initiate the formation of LiO2. The oxygen reduction products (LiO2, Li2O2, and Li2O molecules) strongly interact with the MnO2 monolayer via the stabilization of Li–O chemical bonds with lattice oxygen atoms. As compared to the disproportionation reaction, direct lithiation reactions are the primary contributors to the stabilization of Li2O2 on the MnO2 monolayer. The energy profiles of (Li2O2)2 and (Li2O)2 nucleation on δ-MnO2 monolayer during the discharge process demonstrate that Li2O2 is the predominant discharge product and that further reduction to Li2O is inhibited by the high overpotential of 1.21 V. Interface structures have been examined to study the interaction between the Li2O2 and MnO2 layers. This study demonstrates that a Li2O2 film can be homogeneously deposited onto δ-MnO2 and that the Li2O2/MnO2 interface acts as an electrical conductor. A mesoscale model, developed based on findings from the first-principles calculations, further shows that Li2O2 is the primary product of electrochemical reactions when the applied potential is smaller than 2.4 V.
The formation of solid electrolyte interphase and diffusion induced microcrack in the lithium-ion battery electrodes are predominant degradation mechanisms, which cause capacity fade and cell impedance rise. Physics-based degradation models reveal new insights and allow fundamental understanding of the transport–chemistry–mechanics interactions. In addition, simulation-based diagnostics (e.g. electrochemical impedance spectroscopy, acoustic emission characteristics) can enable virtual probing and interrogation of electrode degradation behavior. This short perspective highlights the recent progress in physics-based degradation modeling and virtual diagnostics in lithium-ion battery electrodes.
Formation of bacterial colonies as biofilm on the surface/interface of various objects has the potential to impact not only human health and disease but also energy and environmental considerations. Biofilms can be regarded as soft materials, and comprehension of their shear response to external forces is a key element to the fundamental understanding. A mesoscale model has been presented in this article based on digitization of a biofilm microstructure. Its response under externally applied shear load is analyzed. Strain stiffening type behavior is readily observed under high strain loads due to the unfolding of chains within soft polymeric substrate. Sustained shear loading of the biofilm network results in strain localization along the diagonal direction. Rupture of the soft polymeric matrix can potentially reduce the intercellular interaction between the bacterial cells. Evolution of stiffness within the biofilm network under shear reveals two regimes: a) initial increase in stiffness due to strain stiffening of polymer matrix, and b) eventual reduction in stiffness because of tear in polymeric substrate.
The lithium-ion battery electrode represents a complex porous composite, consisting of multiple phases including active material, conductive additive and polymeric binder. This study proposes a mesoscale model to probe the effects of the cathode composition, e.g. the ratio of active material, conductive additive and binder content, on the electrochemical properties and performance. The results reveal a complex non-monotonic behavior in the effective electrical conductivity as the amount of conductive additive is increased. Insufficient electronic conductivity of the electrode limits the cell operation to lower currents. Once sufficient electron conduction (i.e., percolation) is achieved, the rate performance can be a strong function of ion-blockage effect and pore phase transport resistance. Even for the same porosity, different arrangements of the solid phases may lead to notable difference in the cell performance, which highlights the need for accurate microstructural characterization and composite electrode preparation strategies.
Lithium-ion batteries are the most commonly used portable energy storage technology due to their relatively high specific energy and power, but face thermal issues that raise safety concerns, particularly in automotive and aerospace applications. In these environments, there is zero tolerance for catastrophic failures such as fire or cell rupture, making thermal management a strict requirement to mitigate thermal runaway potential. The optimum configurations for such thermal management systems are dependent on both the thermo-electrochemical properties of the batteries and operating conditions/engineering constraints. The aim of this study is to determine the effect of various combined active (liquid heat exchanger) and passive (phase change material) thermal management techniques on cell temperatures and thermal balancing. The cell configuration and volume/weight constraints have important roles in optimizing the thermal management technique, particularly when utilizing both active and passive systems together. A computational modeling study including conjugate heat transfer and fluid dynamics coupled with thermo-electrochemical dynamics is performed to investigate design trade-offs in lithium-ion battery thermal management strategies. It was found that phase change material properties and cell spacing have a significant effect on the maximum and gradient of temperature in a module cooled by combined active and passive thermal management systems.
Particle size plays an important role in the electrochemical performance of cathodes for lithium-ion (Li-ion) batteries. High energy planetary ball milling of LiNi1/3Mn1/3Co1/3O2 (NMC) cathode materials was investigated as a route to reduce the particle size and improve the electrochemical performance. The effect of ball milling times, milling speeds, and composition on the structure and properties of NMC cathodes was determined. X-ray diffraction analysis showed that ball milling decreased primary particle (crystallite) size by up to 29%, and the crystallite size was correlated with the milling time and milling speed. Using relatively mild milling conditions that provided an intermediate crystallite size, cathodes with higher capacities, improved rate capabilities, and improved capacity retention were obtained within 14 μm-thick electrode configurations. High milling speeds and long milling times not only resulted in smaller crystallite sizes but also lowered electrochemical performance. Beyond reduction in crystallite size, ball milling was found to increase the interfacial charge transfer resistance, lower the electrical conductivity, and produce aggregates that influenced performance. Computations support that electrolyte diffusivity within the cathode and film thickness play a significant role in the electrode performance. This study shows that cathodes with improved performance are obtained through use of mild ball milling conditions and appropriately designed electrodes that optimize the multiple transport phenomena involved in electrochemical charge storage materials.
Understanding interfacial phenomena such as ion and electron transport at dynamic interfaces is crucial for revolutionizing the development of materials and devices for energy-related applications. Moreover, advances in this field would enhance the progress of related electrochemical interfacial problems in biology, medicine, electronics, and photonics, among others. Although significant progress is taking place through in situ experimentation, modeling has emerged as the ideal complement to investigate details at the electronic and atomistic levels, which are more difficult or impossible to be captured with current experimental techniques. Among the most important interfacial phenomena, side reactions occurring at the surface of the negative electrodes of Li-ion batteries, due to the electrochemical instability of the electrolyte, result in the formation of a solid-electrolyte interphase layer (SEI). In this work, we briefly review the main mechanisms associated with SEI reduction reactions of aprotic organic solvents studied by quantum mechanical methods. We then report the results of a Kinetic Monte Carlo method to understand the initial stages of SEI growth.
In lithium-sulfur (Li-S) batteries, during discharge, solid sulfur (S8(s)) gets dissolved and undergoes successive reduction and finally precipitates as lithium sulfide (Li2S) in a typical carbon-based, porous cathode. Deposition of Li2S leads to 80% volume expansion compared to solid S8(s). During the dissolution-precipitation process, the total volume change of the electrolyte in the pore space can be attributed to two factors: (a) precipitation/dissolution of the solid sulfur phase; and (b) the cathode microstructure shrinks or swells to accommodate the changes in the pore volume resulting from the electrolyte induced hydrostatic pressure. Current lithium-sulfur performance models neglect this contribution. In this work, a computational methodology has been developed to quantify the impact of precipitation induced volume change, pore morphology and confinement attributes in a Li-S cathode. Impact of volume expansion on cell voltage has also been analyzed using a performance model. It is found that the poromechanical interaction significantly affects the second voltage plateau. Cathode microstructures with relatively smaller pores tend to experience less volume expansion, for the same operating conditions. It has been found that non-uniform precipitation may lead to significant pore confinement, which has the potential to cause microcrack formation in the pore walls of a typical carbon-based cathode microstructure.
High-capacity anode materials (such as, silicon) are of critical importance for lithium-ion batteries aimed at achieving longer drive range for electric vehicles. Large lithium retention in these alloying materials is, however, accompanied by high volume expansion, which results in severe mechanical degradation and capacity decay. The inherently coupled mechano-electrochemical stochastics is elucidated in this work. A stochastic computational methodology has been developed to capture the large deformation and mechanical degradation in high-capacity anode materials. Lithiation and delithiation in such active particles follow a two-phase diffusive interface formation and propagation. Mechano-electrochemical interactions lead to different tensile forces acting on the active particle that may lead to microcrack formation. In this study, we have demonstrated that: (a) concentration gradient induced stress at the two-phase interface does not lead to severe mechanical degradation; and (b) large volume expansion induced tensile force at the particle surface actually gives rise to multiple spanning crack formation and further propagation during delithiation. Anode materials with higher partial molar volume of the lithiated phase can lead to enhanced mechanical degradation. Functionally graded materials, with reduced elastic modulus near the surface, hold potential for significant reduction in crack formation.
Mechanical degradation, owing to intercalation induced stress and microcrack formation, is a key contributor to the electrode performance decay in lithium-ion batteries (LIBs). The stress generation and formation of microcracks are caused by the solid state diffusion of lithium in the active particles. In this work, scaling relations are constructed for diffusion induced damage in intercalation electrodes based on an extensive set of numerical experiments with particle-level description of microcrack formation under disparate operating and cycling conditions, such as temperature, particle size, C-rate, and drive cycle. The microcrack formation and evolution in active particles is simulated based on a stochastic methodology. A reduced order scaling law is constructed based on an extensive set of data from the numerical experiments. The scaling relations include combinatorial constructs of concentration gradient, cumulative strain energy, and microcrack formation. The reduced order relations are further employed to study the influence of mechanical degradation on cell performance and validated against the high order model for the case of damage evolution during variable current vehicle drive cycle profiles.
The internal shuttle effect caused by polysulfides dissolution and migration negatively impacts the lithium-sulfur battery performance. In this work, a mesoscale simulation strategy, which involves atomistic calculation and coarse-grained molecular modeling, is employed to evaluate silicene as a potential cathode host material to immobilize polysulfides. Adsorption energies of insoluble polysulfides (Li2Sx with x = 1, 2) and soluble polysulfide Li2S4 on pristine and doped silicene sheets are calculated. Results show that the adsorption is thermodynamically favorable and N-doped silicene is helpful in trapping intermediate discharge products, Li2S2 and Li2S4. The dissociation and reduction of long-chain polysulfides to short-chain polysulfides are observed. Electronic structure analysis shows that Li2Sx molecules interact with silicene via strong chemical bonds. The atomistic structure evolution of Li2S layer formation on silicene is also investigated in this study. It is found that Li2S (110) layer forms first and then it is converted to Li2S (111) layer by introducing more Li2S molecules to the substrate. Li2S (111)/silicene interfacial structure is thermodynamically stable and the interaction is dominated by Li-Si bonds. A coarse-grained model is developed to study and compare the growth of Li2S on silicene and graphene. It is found that the Li2S induced surface coverage is faster on silicene than on graphene, which indicates that a silicene-based cathode host will experience more acute surface passivation, which will adversely affect cathode performance.
The precipitation of lithium sulfide (Li2S) on the Li metal anode surface adversely impacts the performance of lithium–sulfur (Li2S) batteries. In this study, a first-principles approach including density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations is employed to theoretically elucidate the Li2S/Li metal surface interactions and the nucleation and growth of a Li2S film on the anode surface due to long-chain polysulfide decomposition during battery operation. DFT analyses of the energetic properties and electronic structures demonstrate that a single molecule adsorption on Li surface releases energy forming chemical bonds between the S atoms and Li atoms from the anode surface. Reaction pathways of the Li2S film formation on Li metal surfaces are investigated based on DFT calculations. It is found that a distorted Li2S (111) plane forms on a Li(110) surface and a perfect Li2S (111) plane forms on a Li(111) surface. The total energy of the system decreases along the reaction pathway; hence Li2S film formation on the Li anode surface is thermodynamically favorable. The calculated difference charge density of the Li2S film/Li surface suggests that the precipitated film would interact with the Li anode via strong chemical bonds. AIMD simulations reveal the role of the anode surface structure and the origin of the Li2S formation via decomposition of Li2S8 polysulfide species formed at the cathode side and dissolved in the electrolyte medium in which they travel to the anode side during battery cycling.
In this work, a novel heterofunctional, bimodally-porous carbon morphology, termed the carbon compartment (CC), is utilized as a sulfur host within a lithium-sulfur battery cathode. A multi-scale model explores the physics and chemistry of the lithium-sulfur battery cathode. The CCs are synthesized through a rapid, low cost process to improve electrode-electrolyte interfacial contact and accommodate volumetric expansion associated with sulfide formation. The CCs demonstrate controllable sulfur loading and ca. 700 mAh g-1 (at 47%-wt S) reversible capacity with high coulombic efficiency due to their unique structures. Density functional theory and ab initio molecular dynamics characterize the interface between the C/S composite and electrolyte during the sulfur reduction mechanism. Stochastic realizations of 3D electrode microstructures are reconstructed based on representative SEM micrographs to study the influence of solid sulfur loading and lithium sulfide precipitation on microstructural and electrochemical properties. A macroscale electrochemical performance model is developed to analyze the performance of lithium-sulfur batteries. The combined multi-scale simulation studies explain key fundamentals of sulfur reduction and its relation to the polysulfide shuttle mechanism: how the process is affected due to the presence of carbon substrate, thermodynamics of lithium sulfide formation and deposition on carbon, and microstructural effects on the overall cell performance.
Research into new and improved materials to be utilized in lithium-ion batteries (LIB) necessitates an experimental counterpart to any computational analysis. Testing of lithium-ion batteries in an academic setting has taken on several forms, but at the most basic level lies the coin cell construction. In traditional LIB electrode preparation, a multi-phase slurry composed of active material, binder, and conductive additive is cast out onto a substrate. An electrode disc can then be punched from the dried sheet and used in the construction of a coin cell for electrochemical evaluation. Utilization of the potential of the active material in a battery is critically dependent on the microstructure of the electrode, as an appropriate distribution of the primary components are crucial to ensuring optimal electrical conductivity, porosity, and tortuosity, such that electrochemical and transport interaction is optimized. Processing steps ranging from the combination of dry powder, wet mixing, and drying can all critically affect multi-phase interactions that influence the microstructure formation. Electrochemical probing necessitates the construction of electrodes and coin cells with the utmost care and precision. This paper aims at providing a step-by-step guide of non-aqueous electrode processing and coin cell construction for lithium-ion batteries within an academic setting and with emphasis on deciphering the influence of drying and calendaring.
Solid electrolyte interphase (SEI) formation, due to the electrochemical reaction between the salt and solvent in the electrolyte, is a key contributor to the electrode performance decay in lithium-ion batteries. The active particle morphology and electrode microstructure affect the side reaction rate and hence the SEI induced interfacial transport and impedance behavior. The change resistance due to the variation of SEI thickness can be inferred from electrochemical impedance spectroscopy. In this study, we proposed a microstructure-aware impedance model to predict the effect of electrode microstructure on impedance response. Our model successfully captures the influence of active particle morphology on the SEI formation and corresponding impedance characteristics. Different electrode realizations with microstructural and compositional variations have been considered. The critical influence of active material morphology, mean particle size, binder and electrolyte volume fractions on the SEI formation and impedance behavior reveals the underlying interdependences of the interfacial and transport resistance modes.
In lithium–sulfur batteries, the growth of insulating discharge product Li2S film affects the cathode microstructure and the related electron as well as lithium ion transport properties. In this study, chemical reactions of insoluble lithium polysulfides Li2Sx (x = 1, 2) on crystal Li2S substrate are investigated by a first-principles approach. First-principles atomistic thermodynamics predicts that the stoichiometric (111) and (110) surfaces are stable around the operating cell voltage. Li2Sx adsorption is an exothermic reaction with the (110) surface being more active to react with the polysulfides than the stoichiometric (111) surface. There is no obvious charge transfer between the adsorbed molecule and the crystal Li2S substrate. Analysis of the charge density difference suggests that the adsorbate interacts with the substrate via a strong covalent bond. The growth mechanism of thermodynamically stable surfaces is investigated in the present study. It is found that direct Li2S deposition is energetically favored over Li2S2 deposition and reduction process.
A one-dimensional computational framework is developed that can solve for the evolution of voltage and current in a lithium-ion battery electrode under different operating conditions. A reduced order model is specifically constructed to predict the growth of mechanical degradation within the active particles of the carbon anode as a function of particle size and C-rate. Using an effective diffusivity relation, the impact of microcracks on the diffusivity of the active particles has been captured. Reduction in capacity due to formation of microcracks within the negative electrode under different operating conditions (constant current discharge and constant current constant voltage charge) has been investigated. At the beginning of constant current discharge, mechanical damage to electrode particles predominantly occurs near the separator. As the reaction front shifts, mechanical damage spreads across the thickness of the negative electrode and becomes relatively uniform under multiple discharge/charge cycles. Mechanical degradation under different drive cycle conditions has been explored. It is observed that electrodes with larger particle sizes are prone to capacity fade due to microcrack formation. Under drive cycle conditions, small particles close to the separator and large particles close to the current collector can help in reducing the capacity fade due to mechanical degradation.
While the popularity of lithium-ion batteries (LIBs) has increased significantly in recent years, safety concerns due to the high thermal instability of LIBs limit their use in applications with zero tolerance for a catastrophic failure. Industries such as aerospace and automotive must be very stringent in their selection and design of lithium-ion cells and modules to meet safety requirements. A safety issue of particular interest is a scenario called thermal runaway in which one or more exothermic side-reactions occur, leading to elevated temperature ranges that in turn lead to an uncontrollable and excessive release of heat. This work aims to characterize the effect of these reactions by utilizing a thermal abuse model that predicts single-cell behavior when subjected to an elevated-temperature. The experimental test of the thermal safety behavior includes a constant-power heating element to trigger a thermal runaway event. This study takes an existing thermal abuse model and modifies it to emulate the conditions during a constant-power heating test. The result is found to be in agreement with the experimental data for different cell configurations. The influence of convection condition, cell physical configuration, and electrolyte combustion on the cell thermal behavior is also investigated.
While the energy and power density of lithium-ion batteries (LIBs) are steadily improving, thermal safety continues to remain a critical challenge. Under abuse conditions, exothermic reactions may lead to the release of heat that can trigger subsequent unsafe reactions. The situation worsens in a module configuration, as the released heat from an abused cell can activate a chain of reactions in the neighboring cells, causing catastrophic thermal runaway. This work focuses on experimental elucidation and analysis of different LIB module configurations to characterize the thermal behavior and determine safe practices. The abuse test consists of a heat-to-vent setting where a single cell in a module is triggered into thermal runaway via a heating element. The cell-to-cell thermal runaway propagation behavior has been characterized. Results have shown that increasing the inter-cell spacing in a module containing cylindrical cells significantly decreases the probability of thermal runaway propagation. Additionally, it was determined that appropriate tab configuration combined with cell form factors exhibit a major influence on thermal runaway propagation. Different thermal insulation materials have been analyzed to determine their ability to ameliorate and/or potentially mitigate propagation effects.
The active particle morphology and microstructure affect the impedance behavior of intercalation electrodes due to the underlying charge transport, active material/electrolyte interfacial surface area, and solid-phase diffusion in lithium-ion batteries (LIB). In order to capture the impact of the electrode microstructural variability on the impedance response, an integrated electrochemical impedance predictive analysis is presented. In the analysis, stochastically reconstructed 3-D microstructures of representative LIB electrodes are considered with variations in the active material morphology and particle size distribution. With the properties evaluated from the virtual 3-D microstructures, the corresponding impedance response is predicted. The concept of electrochemical Sauter mean diameter (ESMD) has been introduced to investigate the effect of active particle morphology, such as particle agglomeration. This integrated analysis is envisioned to offer a virtual impedance response probing framework to elucidate the influence of electrode microstructural variability and underlying electrochemical and transport interactions.
Li intercalation and diffusion in pristine and modified SnS2 interlayer are studied by a first-principles approach. The results predict that the octahedral interstitial site is energetically favored for Li intercalation. The minimum energy path of Li diffusion in SnS2 interlayer is investigated by climbing image nudged elastic band method. It is found that Li atom diffuses from one energetically favored octahedral interstitial site to the neighbor one via tetrahedral interstitial site. The expansion of interlayer spacing is beneficial for decreasing the diffusion barrier. Ce dopant negatively impacts the Li diffusivity although it can optimize the interlayer spacing. Geometric structures of LixSnS2 (0 < x ≤ 3) are investigated to understand the lithiation-induced volume expansion and atomic structure change. The lithiation process can be divided into two stages. When Li content (x in LixSnS2) is less than 1, the volume expansion is not dramatic and only S atoms capture electrons from Li atoms. When Li content is larger than 1, Sn4+ cations are significantly reduced, S-Sn-S trilayer gradually decomposes, and LixS2 (1 ≤ x ≤ 3) layer forms between two Sn monolayers. The mechanism of volume expansion is elucidated in this study.
The performance of lithium-ion batteries is limited by suboptimal energy density and power capability. A feasible approach is designing 3D electrode architectures where lithium ion transport in the electrolyte and active material can be optimized for improving the energy/power density. In this study, the influence of active material morphology and 3D electrode configurations is investigated with particular emphasis on solid-state transport and resulting implications on the performance. A morphology-detailed computational modeling is presented which simulates lithium transport in disparate 3D electrode configurations. The resulting lithium concentration in the 3D electrode constructs during discharging, relaxation, and charging process reveal a local sate of charge map. This is correlated with the electrode performance. This study demonstrates the role of active particle morphology and 3D architecture on the electrode relaxation behavior, which determines the resulting concentration gradient and performance.
Fast charging is critical for efficient electrical vehicle operation. In this study, the fast charging performance limitations are elucidated with emphasis on the influence of temperature extremes. The role of charging protocols and electrode design parameters on charging time reduction are analyzed. Conventional charging protocol at high C-rate (e.g. at 3C) demonstrates that the performance limitation in rapid charging primarily originates from lithium ion transport in the electrolyte (electrolyte resistance) at moderate and high operating temperatures. However, interfacial kinetics resistance becomes the limiting mechanism for the 1C charging rate at low temperatures. To overcome the low temperature limitation, self-heating has been found to boost the cell performance and coulombic efficiency. An effective charging protocol has been suggested by maximizing lithium concentration at the anode/separator interface. Based on the advantageous combination of charging protocol and temperature, we propose a pulse-charging protocol with adiabatic operation, which can reduce charging time and increase performance. Furthermore, appropriate electrode design, such as reducing the electrode thickness and increasing the porosity, results in improved charging performance.
Nanostructured tin (Sn) is a promising high-capacity electrode for improved performance in lithium-ion batteries for electric vehicles. In this work, Sn nanoisland growth for nanostructured electrodes assisted by the pulse laser irradiation has been investigated based on a mesoscale modeling formalism. The influence of pertinent processing conditions, such as pulse duration, heating/cooling rates, and atom flux, on the Sn nanostructure formation is specifically considered. The interaction between the adsorbed atom and the substrate, represented by the adatom diffusion barrier, is carefully studied. It is found that the diffusion barrier predominantly affects the distribution of Sn atoms. For both α-Sn and β-Sn, the averaged coordination number is larger than 3 when the diffusion barrier equals to 0.15 eV. The averaged coordination number decreases as the diffusion barrier increases. The substrate temperature, which is determined by heating/cooling rates and pulse duration, can also affect the formation of Sn nanoislands. For α-Sn, when applied low heating/cooling rates, nanoislands cannot form if the diffusion barrier is larger than 0.35 eV.
Cost reduction is a key issue for commercialization of fuel cell electric vehicles (FCEV). High current density operation is a solution pathway. In order to realize high current density operation, it is necessary to reduce mass transport resistance in the gas diffusion media commonly consisted of gas diffusion layer (GDL) and micro porous layer (MPL). However, fundamental understanding of the underlying mass transport phenomena in the porous components is not only critical but also not fully understood yet due to the inherent microstructural complexity. In this study, a comprehensive analysis of electron and oxygen transport in the GDL and MPL is conducted experimentally and numerically with three-dimensional (3D) microstructural data to reveal the structure–transport relationship. The results reveal that the mass transport in the GDL is strongly dependent on the local microstructural variations, such as local pore/solid volume fractions and connectivity. However, especially in the case of the electrical conductivity of MPL, the contact resistance between carbon particles is the dominant factor. This suggests that reducing the contact resistance between carbon particles and/or the number of contact points along the transport pathway can improve the electrical conductivity of MPL.
A new composite containing silver nanoparticles and graphite is developed in order to improve electrochemical energy storage. The nanocomposite uses silver (Ag) nanoparticles as a catalyst to enhance the electrochemical performance. Results indicate that the graphite composite decorated with Ag shows up to a six-fold improvement in specific capacitance. Electron/charge transfer is enhanced through a shift from double-layer to pseudocapacitive behavior, mediated by Li+ intercalation. Decoration with Ag nanoparticles allows for improvements in electrochemical impedance response, ease of electronic/ionic charging, and overall energy storage capability. This research provides a promising alternative solution for the next generation of safe and cost-effective lithium-ion devices.
The present work discusses the implications of wall wettability and inclination of the surface on droplet dynamics. In this work, mesoscopic illustration of droplet dynamics in a duct with different inclination angles, based on the two-phase lattice Boltzmann model is reported. Temporal evolution of wetted length, wetted area and maximum height of the droplet for surfaces with different inclination angles and wettabilities is furnished in detail in order to elucidate the droplet displacement dynamics. It has been observed that the effect of inclination of the surface on droplet dynamics is more pronounced on a hydrophobic surface as compared to a hydrophilic surface. The time evolution of height and contact line motion of the droplet shows that higher angle of inclination of substrate affects the dynamics strongly irrespective of wettability.
The present work presents the capillarity–wettability interaction of the trapped non-wetting phase in a model pore, characterized by a circular tube with varying constriction. The displacement of a three-dimensional immiscible binary large object (blob) subjects to oscillatory acoustic excitation in a tube of varying radius is examined using the lattice Boltzmann method. The impact of the physicochemical parameters, including radius of the tube, amplitude of the force, wettability, viscosity and frequency on blob dynamics is discussed. The response of the blob with pinned and sliding contact line exhibits underdamped and overdamped characteristics, respectively. It is observed that the mean displacement and frequency response increase with amplitude of the force. The resonance behavior of the blob with different surface wettability and capillary number is discussed. It has been observed that the displacement behavior is not affected to a large extent at high capillary number. However, wettability plays a crucial role in mobilization of trapped blob at low capillary number. The displacement of the blob is greater in case of the surfaces with uniformly wettability as opposed to mixed wettability.
Mixing sequence during electrode processing affects the internal microstructure and resultant performance of a lithium-ion battery. In order to fundamentally understand the microstructure evolution during electrode processing, a mesoscale model is presented, which investigates the influence of mixing sequence for different evaporation conditions. Our results demonstrate that a stepwise mixing sequence can produce larger conductive interfacial area ratios than that via a one-step mixing sequence. Small-sized cubical nanoparticles are beneficial for achieving a high conductive interfacial area ratio when a stepwise mixing sequence is employed. Two variants of multistep mixing have been investigated with constant temperature and linearly increasing temperature conditions. It is found that the temperature condition does not significantly affect the conductive interfacial area ratio. The homogeneity of binder distribution in the electrode is also studied, which plays an important role along with the solvent evaporation condition. This study suggests that an appropriate combination of mixing sequence and active particle size and morphology plays a critical role in the formation of electrode microstructures with improved performance.
Active particles inside the lithium-ion battery electrode experience diffusion induced stress and volume change during intercalation. High-rate or low-temperature operation can cause large concentration gradients resulting in higher probability of microcrack formation and propagation in the active particles and ultimately performance decay. Acoustic emission is a non-destructive technique for the detection of mechanical degradation. In this work, a computational methodology has been developed, based on a dynamic lattice spring model (DLSM), to study acoustic emission characteristics resulting from intercalation induced microcrack formation in electrodes. This method allows relating the acoustic response with the mechanical damage experienced during lithiation/delithiation in the active particles. Energy released due to brittle fracture is rendered as the major source of acoustic emission response. Predictions from this analysis suggest that during cycling maximum amount of acoustic activity is observed in the first couple of cycles. A phase map has been developed to demonstrate the influence of elastic modulus and damping coefficient on the damage evolution and identify a potential window of reduced mechanical degradation.
Mechanical degradation, owing to intercalation induced stress and fracture, is a key contributor to the electrode performance decay in lithium-ion batteries. Solid state diffusion of lithium ions in the active particles causes concentration gradients, which results in stress generation and formation of microcracks or propagation of preexisting cracks. Formation and propagation of microcracks in turn affects the solid state transport of lithium ions and the interfacial charge transfer resistance. In this work, a systematic investigation of the influence of mechanical degradation on the resistance to diffusion and charge transport is provided. In this regard, a modeling approach combining fracture formation and electrochemical impedance is presented, which predicts the mechanical damage induced impedance response and resistance evolution in the electrode. The impact of particle size, charge/discharge rate and operating temperature on the electrode impedance response is illustrated.
The processing induced active particle assembly determines the internal microstructure and resultant performance of the electrode in a lithium-ion battery. A morphology-detailed mesoscale model has been developed to gain fundamental understanding of the influence of active particle morphology, size, volume fraction, solvent evaporation, and multi-phase (active particle, conductive additive, binder and solvent) interaction. Our results demonstrate that smaller isometric active particles tend to form favorable aggregation with conductive additive particles. Two regimes, namely spontaneous aggregation and evaporation induced aggregation, have been identified. Low solvent evaporation rate promotes spontaneous aggregation resulting in an enhanced interfacial area than that in evaporation-induced aggregation. The influence of active material morphology and volume fraction on conducting pathway formation has been conjectured.
In a lithium-ion cell, heat generation and temperature evolution during operation pose a significant bearing on the mechanical degradation and cell performance. The thermal implications on the electrode mechano-electrochemical behavior have been elucidated. Crack formation due to diffusion-induced stress in the active particles has been analyzed. Temperature dependence of the mechano-physicochemical parameters has been taken into account. Total amount of diffusion-induced damage has been estimated for different current density, ambient temperature and particle size. For subzero temperatures, adiabatic operation can boost the cell performance significantly. Increased mechanical degradation has been observed for high C-rate and larger particle sizes. Decreasing ambient temperature results in aggravated crack formation resulting in severe capacity loss. However, at subzero temperatures and under high C-rate conditions, significant concentration gradient exists near the active particle peripheral region resulting in reduced damage penetration. The cell performance analysis reveals that the impact of mechanical degradation on the capacity loss is most prominent at subzero temperatures. The effect of cycling shows accelerated damage in the first few cycles followed by a plateau in the damage evolution. Existence of a critical particle size for maximum damage has been suggested which depends significantly on the cell temperature.
The field of Mechano-Electro-Chemical (MEC) Coupling examines the multi-directional couplings that exist between a material's physico-electro-chemical state and its properties, processing, and structure. While increasing electro-chemical performance (e.g. reaction rates in fuel cell or battery applications) has largely been the focus in energy related materials, its coupling with mechanical properties, as well as mechano-chemical and mechano-electrical coupling, are becoming of increasing interest, as demonstrated by Barai and Mukherjee,1 Kim and Qi,2 Malavé et al.,3 Zenyuk et al.,4 Zuev et al.,5 and the many other articles in this Focus Issue. Furthermore, as described by the definition above and conceptually highlighted in Fig. 1, MEC coupling also influences material structure and processing. Examples of this include the stress-induced ferroelastic domain switching discussed in Kimura et al.,6 and the chemically induced changes in cation diffusion coefficients, and hence sintering kinetics, discussed in Ni et al.7
Non-equilibrium phase transformation and effect of interfacial Li flux on miscibility gap in two-phase transformation of LiFePO4 have been explored in this study. Our previously developed “Mushy-Zone” (MZ) model, accounting for sluggish Li diffusion across the two-phase interface, has been employed to study the non-equilibrium phase transformation in LiFePO4 materials of Li-ion batteries. Phase transformation rate, variation of two-phase miscibility gap, and interfacial Li composition profiles have been studied for different particle shapes at varying discharge rates. Sluggish Li flux across the two-phase interface, which is believed to be the origin of the kinetically-induced non-equilibrium phase transformation, has been calculated to explain the obtained results. It is found that small particle sizes (radii of 20 nm) and slow discharge rates tend to create homogenous phase transformation (i.e., shrunk or no miscibility gap). Two-phase transformation is remarkably delayed for spherical particles at low discharge rates, leading to a lower capacity compared to that of plated-shaped particles. However, at higher discharge rates spherical particles show better capacity.
Several bacterial species possess the ability to attach to surfaces and colonize them in the form of thin films called biofilms. Biofilms that grow in porous media are relevant to several industrial and environmental processes such as wastewater treatment and CO2 sequestration. We used Pseudomonas fluorescens, a Gram-negative aerobic bacterium, to investigate biofilm formation in a microfluidic device that mimics porous media. The microfluidic device consists of an array of micro-posts, which were fabricated using soft-lithography. Subsequently, biofilm formation in these devices with flow was investigated and we demonstrate the formation of filamentous biofilms known as streamers in our device. The detailed protocols for fabrication and assembly of microfluidic device are provided here along with the bacterial culture protocols. Detailed procedures for experimentation with the microfluidic device are also presented along with representative results.
We report capillarity–wettability interaction of the trapped nonwetting phase. The displacement of immiscible nonwetting phase subject to oscillatory acoustic excitation in a sinusoidal channel is analyzed with the lattice Boltzmann method. The effect of the surface wettability, frequency and geometry of the channel on two-phase behavior is discussed. The effect of capillarity induced resonance on mobilization of trapped nonwetting phase blob is explored for a range of capillary number for uniform and mixed-wet surfaces. It is observed that transport of a nonwetting phase can be achieved by exciting with the frequencies near to the resonant frequency which is vital in mobilization of the trapped nonwetting phase in the application like recovery of oil and two-phase flows.
The importance of fundamental understanding of droplet dynamics and the concomitant implications of wall wettability are critical in the emergent science and technology areas including digital microfluidics and clean energy conversion. In this work, mesoscopic illustration, based on the two-phase lattice Boltzmann model, of droplet dynamics in a microchannel is presented in order to unveil the role of superhydrophobicity and mixed wettability. The impact of critical physicochemical determinants, including capillary number and droplet size, is explored in the context of droplet–wettability interactions. Temporal evolution of wetted length and wetted area for a combination of wettability scenarios is furnished in detail in order to elucidate the droplet displacement dynamics. Capillary number plays an important role with disparate droplet behavioral patterns stemming from superhydrophobic and mixed-wet wall characteristics.
Fracture due to diffusion induced stress of electrode active particles has been identified as one of the critical factors for capacity fade and impedance rise in lithium-ion batteries. The inherent stochastics underlying the crack formation and propagation in brittle intercalation materials is critical toward fundamental understanding of the degradation phenomena limiting battery life and performance. A stochastic methodology has been developed to characterize the diffusion-induced damage inside lithium-ion battery electrode active particles. Presence of a “critical” initial crack length at which maximum stress occurs has been identified. A damage parameter has been introduced to characterize the impact of fracture on solid state lithium diffusion. The importance of active particle size, concentration dependent elastic moduli and cycling on the fracture stochastics and concomitant electrochemical performance decay has been elucidated along with the concept of a fracture phase-map.
The majority of bacteria in nature live in biofilms, where they are encased by extracellular polymeric substances (EPS) and adhere to various surfaces and interfaces. Investigating the process of biofilm formation is critical for advancing our understanding of microbes in their most common mode of living. Despite progress in characterizing the effect of various environmental factors on biofilm formation, work remains to be done in the realm of exploring the inter-relationship between hydrodynamics, microbial adhesion and biofilm growth. We investigate the impact of secondary flow structures, which are created due to semi-confined features in a microfluidic device, on biofilm formation of Shewanella oneidensis MR-1. Secondary flows are important in many natural and artificial systems, but few studies have investigated their role in biofilm formation. To direct secondary flows in the creeping flow regime, where the Reynolds number is low, we flow microbe-laden culture through microscale confinement features. We demonstrate that these confinement features can result in pronounced changes in biofilm dynamics as a function of the fluid flow rate.
The high-rate, high-capacity potential of LiFePO4-based lithium-ion battery cathodes has motivated numerous experimental and theoretical studies aiming to realize such performance through nano-sizing, tailoring of particle shape through synthesis conditions, and doping. Here, a granular mechanics study of microstructures formed by dense jammed packings of experimentally and theoretically inspired LiFePO4 particle shapes is presented. A strong dependence of the resultant packing structures on particle shapes is observed, in which columnar structures aligned with the  direction inhibit diffusion along  in anisotropic LiFePO4. Transport limitations are induced by  columnar order and lead to catastrophic performance degradation in anisotropic LiFePO4 electrodes. Further, judicious mixing of nanoplatelets with additive nanoparticles can frustrate columnar ordering and thereby enhance the rate capability of LiFePO4 electrodes by nearly an order of magnitude.
In this work, we investigate the cold-start operation of polymer electrolyte fuel cells (PEFCs) through high-resolution neutron radiography, experimental testing, theoretical evaluation, and comparison with model prediction. Ice formation location, voltage evolution, and loss of the electro-catalyst surface area (ECSA) are examined. A dimensionless parameter h¯ , characterizing the spatial variation of the reaction rate across the cathode catalyst layer, is discussed at subfreezing temperature using newly determined membrane ionic conductivity. The h¯ evaluation identifies the operating range that the reaction rate can be treated uniform across the catalyst layer, in which the model is valid.
Using a microfabricated porous media mimic platform, we investigated how fluid flow influences the formation of filamentous structures, known as streamers, between porous media structures. We demonstrate how hydrodynamics govern the formation, morphology and the distribution of these biofilm streamers in the device. Our work establishes that, under favorable hydrodynamic conditions, streamers can often act as precursors to mature microbial structures found in complex geometries, such as those involved in porous media.
Recent years have witnessed an explosion of research and development efforts in the area of polymer electrolyte fuel cells (PEFC), perceived as the next generation clean energy source for automotive, portable and stationary applications. Despite significant progress, a pivotal performance/durability limitation in PEFCs centers on two-phase transport and mass transport loss originating from suboptimal liquid water transport and flooding phenomena. Liquid water blocks the porous pathways in the gas diffusion layer (GDL) and the catalyst layer (CL), thus hindering oxygen transport from the flow field to the electrochemically actives sites in the catalyst layer. Different approaches have been examined to model the underlying transport mechanisms in the PEFC with different levels of complexities. Due to the macroscopic nature, these two-phase models fail to resolve the underlying structural influence on the transport and performance. Mesoscopic modeling at the pore-scale offers great promise in elucidating the underlying structure-transport-performance interlinks in the PEFC porous components. In this article, a systematic review of the recent progress and prospects of pore-scale modeling in the context of two-phase transport in the PEFC is presented. Specifically, the efficacy of lattice Boltzmann (LB), pore morphology (PM) and pore network (PN) models coupled with realistic delineation of microstructures in fostering enhanced insight into the underlying liquid water transport in the PEFC GDL and CL is highlighted.
The nonequilibrium phase transformation and particle shape effects in LiFePO4 materials of Li-ion batteries are explored in this work. A continuum model employing the “mushy-zone” (MZ) approach, accounting for sluggish Li diffusion across the two-phase boundary, has been developed to study the kinetically-induced nonequilibrium phenomenon in Li-ion batteries. A theoretical analysis is presented to show that the nonequilibrium miscibility gap expands and shifts to higher Li composition at high discharge rates, due to insufficient compositional readjustments at the two-phase boundary. Furthermore, critical effects of particle shape on nonequilibrium phase transformation and discharge capacity have been discovered by the model.
A response of the aggregation dynamics of enteroaggregative Escherichia coli under low magnitude steady and oscillating electric fields is presented. The presence of uniform electric fields hampered microbial adhesion and biofilm formation on a transverse glass surface, but instead promoted the formation of flocs. Extremely heterogenous distribution of live and dead cells was observed among the flocs. Moreover, floc formation was largely observed to be independent of the frequency of alternating electric fields.
In this paper, the electrochemical kinetics, oxygen transport and solid water formation within the cathode electrode of polymer electrolyte fuel cells (PEFCs) during cold start is investigated. We specifically evaluate the key parameters that govern the self-startup of PEFCs by considering a wide range of the relevant factors. These parameters include characteristic time scales of cell warm-up, ionomer hydration in the catalyst layer, ice build-up and melting, as well as the ratios of the time constants. Supporting experimental observation using neutron imaging and isothermal cold start experiment is discussed. Gas purge is found to facilitate the PEFC cold start but the improvement may be relatively small compared with other methods such as selecting suitable materials and modifying the cell design. We define a three-step electrode process for cold start and conduct a one-dimensional analysis, which enables the evaluation of the impact of ice volume fraction and temperature variations on the cell cold start performance. The ionic conductivity data of Nafion® 117 membrane at subfreezing temperature, evaluated from experiment, is utilized to analyze the temperature dependence of the ohmic polarization during cold start.
We present our recent progress on mesoscopic modeling of multiphysicochemical transport phenomena in porous media based on the lattice Boltzmann method. Simulation examples include injection of CO2-saturated brine into a limestone rock, two-phase behavior and flooding phenomena in polymer electrolyte fuel cells, and electroosmosis in homogeneously charged porous media. It is shown that the lattice Boltzmann method can account for multiple, coupled physicochemical processes in these systems and can shed some light on the underlying physics occurring at the fundamental scale. Therefore, it can be a potential powerful numerical tool to analyze multiphysicochemical processes in various energy, earth, and environmental systems.
A key performance limitation in polymer electrolyte fuel cells (PEFC), manifested in terms of mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. Liquid water covers the electrochemically active sites in the catalyst layer (CL) rendering reduced catalytic activity and blocks the available pore space in the porous CL and fibrous gas diffusion layer (GDL) resulting in hindered oxygen transport to the active reaction sites. The cathode CL and the GDL play a major role in the mass transport loss and hence in the water management of a PEFC. In this work the development of a mesoscopic modeling formalism coupled with realistic microstructural delineation is presented to study the influence of the pore structure and surface wettability on liquid water transport and interfacial dynamics in the PEFC catalyst layer and gas diffusion layer. The two-phase regime transition phenomenon in the capillary dominated transport in the CL and the influence of the mixed wetting characteristics on the flooding dynamics in the GDL are highlighted.
This paper presents a first approximation to the theoretical analysis of fractal-like electrodes for lithium-ion batteries, and demonstrates that fractals constitute an optimal electrode configuration for electrochemical energy storage systems. The model considers a generalized description of three-dimensional, non-deterministic branching structures composed of cylindrical elements for the electrode design. Each element is attached to a branch in an iterative process. At every branching step, the “parent” branch divides into N “child” branches (N being a random variable with a defined probability distribution). At the same time, the dimensions of the radii and lengths of the branches are also determined by a stochastic process. With this model, the charge characteristics of several fractal electrodes corresponding to different geometric configurations are simulated, and the optimal parameters are obtained.
A pore-scale description of species and charge transport through a bilayer cathode catalyst layer (CL) of a polymer electrolyte fuel cell using a direct numerical simulation (DNS) model is presented. Two realizations of the bilayer catalyst layer structure are generated using a stochastic reconstruction technique with varied electrolyte and void phase volume fractions. The DNS calculations predict that a higher electrolyte phase volume fraction near the membrane–CL interface provides an extended active reaction zone and exhibits enhanced performance. A higher void phase fraction near the gas diffusion layer aids in better oxygen transport. The effects of cell operating conditions in terms of low inlet relative humidity and elevated cell operating temperature on the bilayer CL performance are also investigated. Low humidity and elevated temperature operations exhibit overall poorer performance compared to the high humidity and the low-temperature operations, respectively.
A key performance limitation in polymer electrolyte fuel cells (PEFC), called the mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. The cathode gas diffusion layer (GDL) is a primary contributor to mass transport loss owing to the blockage of available pore space by liquid water thereby rendering hindered oxygen transport to the active reaction sites in the electrode. The GDL, typically a fibrous non-woven carbon paper or a woven carbon cloth, thus plays an important role in the water management of a PEFC. This Feature Article provides a systematic description of the development of pore-scale models coupled with realistic microstructural delineation as well as micron-resolution imaging techniques to study the profound influence of the underlying structure and surface wettability on liquid water transport and interfacial dynamics in the fuel cell GDL. A pore-network model and a two-phase lattice Boltzmann model coupled with stochastic generation of GDL microstructures are elaborated. Concurrently, optical diagnostics of water dynamics at GDL interfaces and X-ray micro-tomographic imaging of liquid water distribution inside the GDL of an operating fuel cell are discussed.
The cathode catalyst layer (CL), due to sluggish oxygen reduction reaction and several transport losses therein, plays an important role in the overall performance of polymer electrolyte fuel cells (PEFCs). The relative volume fractions of the constituent phases, i.e. the electronic, electrolyte and void phases, of the cathode CL need to be selected appropriately in order to achieve an optimal balance between oxygen diffusion and proton conduction. In this work, the influence of electrolyte and void phase fractions of the cathode CL on the cell performance is investigated based on a pore-level description of species and charge transport through a random CL microstructure via the direct numerical simulation (DNS) model. Additionally, the effects of inlet relative humidity and net water transport from the anode on the cathode performance have been studied which indicate the interdependence between the CL composition and the cell operating conditions. The results indicate that the low humidity operation benefits the performance by enhancing the oxygen transport especially under high current densities. Finally, the DNS model predicts the volume fractions of 0.4 and 0.26 for the void and electrolyte phases, respectively, as the optimal composition of the catalyst layer for the best performance.
A nonisothermal, two-phase model was developed to investigate simultaneous heat and mass transfer in the cathode gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). The model was applied in two-dimensions with the in-plane (i.e., channel-to-land) and through-plane (i.e., catalyst layer-to-channel) directions to investigate the effects of anisotropy of GDL. For the first time, the anisotropy in the GDL properties was taken into account and found to be an important factor controlling the temperature distribution in the GDL. The maximum temperature difference in the GDL was found to be a strong function of GDL anisotropy. A temperature difference of up to 5°C at a cell voltage of 0.4 V was predicted for an isotropic GDL while it reduced to 3°C for an anisotropic GDL. Significant effect of temperature distribution on liquid water transport and distribution was also observed. In addition, the latent heat effects due to condensation/evaporation of water on the temperature and water distributions were analyzed and found to strongly affect the two-phase transport.
A full morphology (FM) model has been developed for studying the two-phase characteristics of the gas diffusion medium in a polymer electrolyte fuel cell (PEFC). The three-dimensional (3D) fibrous microstructure for the nonwoven gas diffusion layer (GDL) microstructure has been reconstructed using a stochastic technique for Toray090 and SGL10BA carbon papers. The FM model directly solves for the capillary pressure-saturation relations on the detailed morphology of the reconstructed GDL from drainage simulations. The estimated capillary pressure-saturation curves can be used as valuable inputs to macroscopic two-phase models. Additionally, 3D visualization of the water distribution in the gas diffusion medium suggests that only a small number of pores are occupied by liquid water at breakthrough. Based on a reduced compression model, the two-phase behavior of the GDL under mechanical load is also investigated and the capillary pressure-saturation relations are evaluated for different compression levels.
A direct numerical simulation (DNS) model of species and charge transport in the cathode catalyst layer of a polymer electrolyte fuel cell has been developed. The 3D porous microstructure of the catalyst layer has been reconstructed based on a stochastic technique using the low-order statistical information (porosity, two-point correlation function) as obtained from 2D transmission electron microscopy (TEM) micrographs of a real catalyst layer. In this microscopically complex structure, the DNS model solves point-wise accurate conservation equations, thereby obtaining a pore-scale description of concentration and potential fields. DNS predictions are further compared with the one-dimensional macrohomogeneous results to establish appropriate correlations for effective transport properties as input into macroscopic computational fuel cell models. Finally, the utility of the stochastic reconstruction technique coupled with the DNS model is demonstrated through addressing the influence of microstructural inhomogeneity on the fuel cell performance.
A direct numerical simulation (DNS) model is developed to achieve pore-level description of polymer electrolyte fuel cell (PEFC) electrodes. The DNS method solves point-wise accurate conservation equations directly on an electrode microstructure comprising of various phases and hence utilizes the intrinsic transport properties of each phase. Idealized two- and three-dimensional regular microstructures are constructed to represent the porous cathode catalyst layer. Various voltage losses identified from the simulation results are compared with experimental observations. This pore-scale model is further applied to study the morphological effects, such as pore size, layer thickness and porosity, on the performance of the cathode catalyst layer.
The direct numerical simulation (DNS) method, developed for modeling the cathode catalyst layer (CL) of a polymer electrolyte fuel cell (PEFC) in Part I, is further extended wherein the catalyst layer is described as a random three-dimensional porous structure. A random CL microstructure is obtained using a computer-generated random number with specified porosity and pore size as the input structural parameters. Some statistical features of the CL and their dependence on the porosity are identified and demonstrated. The charge and species conservation equations are solved directly on this microscopically complex structure. The results from the DNS calculation are compared with the one-dimensional macrohomogeneous predictions and the Bruggeman factor for transport property correction is evaluated, which can be used as direct input into the macroscopic fuel cell models.
A numerical reservoir simulation model for the study of enhanced oil recovery (EOR) from a porous formation has been presented. The resistance to oil movement arises from viscous forces in the fluid phase as well as surface tension. Viscous forces can be lowered by hot water injection into the formation or by raising the formation temperature. These methods have been numerically analyzed in the present study. The role of the operating parameters such as the injection pressure and temperature on oil recovery has been reported. Displacement of oil by water is clearly brought out by the saturation and the temperature profiles.The numerical solution of the EOR problem experiences growth of errors during long time integration, particularly on large regions. Possible reasons are scatter in the constitutive relationship data, inexact outflow boundary condition and round-off errors in the calculation of the matrix inverse. The nature of these errors has been addressed in the present work. To solve the computationally intensive field-scale problems, two domain decomposition algorithms namely, Schwarz's and Uzawa's algorithms have been evaluated.Results show that oil recovery can be improved when the formation temperature is higher, or the injection temperature and pressure are raised. Adverse results can however be obtained when the injection temperature exceeds a critical value. Optimum conditions prevail when the speed of the oil–water interface is matched with that of the thermal front. As a computational tool, the domain decomposition algorithms are conditionally seen to improve the numerical performance of the oil recovery codes.
Recent years have witnessed an explosion of interest and research endeavor in lithium-ion batteries to enable vehicle electrification. In particular, a critical imperative is to accelerate innovation for improved performance, life and safety of lithium-ion batteries for electric drive vehicles. Lithium ion batteries are complex, dynamical systems which include a multitude of coupled physicochemical processes encompassing electronic/ionic/diffusive transport in solid/electrolyte phases, electrochemical and phase change reactions and diffusion induced stress generation in multi-scale porous electrode microstructures. While innovations in nanomaterials and nanostructures have spurred the recent advancements, fundamental understanding of the electrode processing – microstructure – performance interplay is of paramount importance. In this presentation, mesoscale physicochemical interactions in lithium-ion battery electrodes will be elucidated.
Cost reduction is the most important issue for commercialization of Fuel Cell Electric Vehicle (FCEV). High current density operation is one of the solutions for it. In order to realize high current density operation, it is necessary to reduce both of electron and oxygen transport resistance in the porous materials such as gas diffusion layer (GDL) and microporous layer (MPL). However, the impacts of MPL microstructure on their properties are not fully understood yet compared with GDL because of the necessity of higher spatial resolution. In previous study, the transport analysis on the micro-structure which were visualized by Nano X-ray CT and FIB-SEM were conducted for it. However, it was not enough to understand both of the electron and oxygen transport phenomena and find the dominant factors, because there is no study which focused on the comparison of the numerical and experimental results on both of the electron and oxygen transport. In this study, the comprehensive analysis on both of electron and oxygen transport phenomena in GDL and MPL was conducted with experimental and numerical study based on the three-dimensional (3D) micro structure data. As a result, it was found that pore structure, such as a local porosity and/or tortuosity significantly affected the oxygen transport phenomena. On the other hands, especially in the case of electron transport phenomena in MPL, our results suggested that the dominant factor is not the solid structure such as local solid fraction and/or tortuosity but the contact resistance between carbon particles. This fact revealed that it is effective way to reduce the contact resistance between carbon particles and/or the number of contact points in unit length of a transport path in order to improve electrical transport of MPL.
The displacement of a three-dimensional immiscible blob subject to oscillatory acoustic excitation in a channel is studied with the Lattice Boltzmann method. The effects of amplitude of the force, viscosity and frequency on blob dynamics are investigated. The trend for variation of mean displacement of blob and frequency response is in agreement to that of the previous two-dimensional studies reported in literature. The response of the blob with pinned contact line shows underdamped behavior. It is also found that increasing the amplitude of the force increases the mean displacement and frequency response.
Lithium-ion batteries (LiB) are widely used in the electronics industry (such as, cell phones and laptop computers) because of their very high energy density, which reduced the size and weight of the battery significantly. LiB also serves as a renewable energy source for the transportation industry (see Ref. [1,2]). Graphite and LiCoO2 are most frequently used as anode and cathode material inside LiB (see Ref. [2,3]). During the charging and discharging process, intercalation and de-intercalation of Li occur inside the LiB electrodes. Non-uniform distributions of Li induce stress inside the electrodes, also known as diffusion induced stress (DIS). Very high charge or discharge rate can lead to generation of significant amount of tensile or compressive stress inside the electrodes, which can cause damage initiation and accumulation (see Ref. ). Propagation of these micro-cracks can cause fracture in the electrode material, which impacts the solid electrolyte interface (SEI) (see Ref. [2,3,5]). Concurrent to the reduction of cyclable Li, resistance between the electrode and electrolyte also increases, which affects the performance and durability of the electrode and has a detrimental consequence on the LiB life (see Ref. ).
Shewanella oneidensis is a metal reducing bacterium, which is of interest for bioremediation and clean energy applications. S. oneidensis biofilms play a critical role in several situations such as in microbial energy harvesting devices. Here, we use a microfluidic device to quantify the effects of hydrodynamics on the biofilm morphology of S. oneidensis. For different rates of fluid flow through a complex microfluidic device, we studied the spatiotemporal dynamics of biofilms, and we quantified several morphological features such as spatial distribution, cluster formation and surface coverage. We found that hydrodynamics resulted in significant differences in biofilm dynamics. The baffles in the device created regions of low and high flow in the same device. At higher flow rates, a non-uniform biofilm develops, due to unequal advection in different regions of the microchannel. However, at lower flow rates, a more uniform biofilm evolved. This depicts competition between adhesion events, growth and fluid advection. Atomic force microscopy (AFM) revealed that higher production of extra-cellular polymeric substances (EPS) occurred at higher flow velocities.
The nonequilibrium phase transformation and particle shape effects in LiFePO4 materials of Li-ion batteries are explored in this work. A continuum model employing the “mushy-zone” (MZ) approach, accounting for sluggish Li diffusion across the two-phase boundary, has been developed to study the kinetically-induced nonequilibrium phenomenon in Li-ion batteries. A theoretical analysis is presented to show that the nonequilibrium miscibility gap expands and shifts to higher Li composition at high discharge rates, due to insufficient compositional readjustments at the two-phase boundary. Furthermore, critical effects of particle shape on nonequilibrium phase transformation and discharge capacity have been discovered by the model.
Despite tremendous progress in recent years, a pivotal performance limitation in PEM fuel cells manifests in terms of mass transport loss owing to liquid water transport and resulting flooding. A key contributor to the mass transport loss is the cathode gas diffusion layer (GDL) due to the blockage of available pore space by liquid water thus rendering hindered oxygen transport to the active reaction sites in the electrode. The GDL, typically a non-woven carbon paper or woven carbon cloth, thus plays an important role in the overall water management in PEM fuel cells. The underlying poremorphology and the pore wetting characteristics have significant influence on the flooding dynamics in the GDL. Another important factor is the role of cell compression on the GDL microstructural change. In this work, we investigate the influence of GDL microstructure change under compression on the transport behavior. We will present an improved compression model based on the micro fini te element approach. The compression of reconstructed GDL microstructures along with effective property estimation and transport characterization are elucidated.
The gas diffusion layer (GDL) plays a key role in the overall performance/durability of a polymer electrolyte fuel cell (PEFC). Of profound importance, especially in the context of water management and flooding phenomena, is the influence of the underlying pore morphology and wetting characteristics of the GDL microstructure. In this article, we present the digital volumetric imaging (DVI) technique in order to generate the 3-D carbon paper GDL microstructure. The internal pore structure and the local microstructural variations in terms of fiber alignment and fiber/binder distributions are investigated using the several 3-D thin sections of the sample obtained from DVI.
Sufficient water content within a polymer electrolyte membrane (PEM) is necessary for adequate ionic conductivity. Membrane hydration is therefore a fundamental requirement for fuel cell operation. The hydration state of the membrane affects the water transport within, as both the diffusion coefficient and electro-osmotic drag depend on the water content. Membrane's water uptake is conventionally measured ex situ by weighing free-swelling samples equilibrated at controlled water activity. In the present study, water profiles in Nafion® membranes were measured using high-resolution neutron imaging. The state-of-the-art, 13 μm resolution neutron detector is capable of resolving water distributions across N1120, N1110 and N117 membranes. It provides a means to measure the water uptake and transport properties of fuel cell membranes in situ.
In recent years, the polymer electrolyte fuel cell (PEFC) has emerged as a promising clean energy conversion device for various applications. One key research direction requiring significant breakthrough in order to alleviate performance limitations in PEFCs involves enhanced understanding of the coupled multi-physics transport phenomena and interfacial processes catering over multiple length scales in the constituent porous components. Multi-physics, multi-scale modeling is envisioned to hold the key toward enhanced understanding of the underlying structure-transport-performance interactions. In this article, a brief overview of several major aspects pertaining to the multi-physicochemical modeling of electrochemical reaction kinetics, species transport, two-phase heat and water transport, and phase change in the PEFC is presented.
In the present scenario of a global initiative toward a sustainable energy future, the polymer electrolyte fuel cell (PEFC) has emerged as one of the most promising alternative energy conversion devices for various applications. Despite tremendous progress in recent years, a pivotal performance limitation in the PEFC comes from liquid water transport and the resulting flooding phenomena. Liquid water blocks the open pore space in the electrode and the fibrous diffusion layer leading to hindered oxygen transport. The electrode is also the only component in the entire PEFC sandwich which produces waste heat from the electrochemical reaction. The cathode electrode, being the host to several competing transport mechanisms, plays a crucial role in the overall PEFC performance limitation. In this work, an electrode model is presented in order to elucidate the coupled heat and water transport mechanisms. Two scenarios are specifically considered: (1) conventional, Nafion® impregnated, three-phase electrode with the hydrated polymeric membrane phase as the conveyer of protons where local electro-neutrality prevails; and (2) ultra-thin, two-phase, nano-structured electrode without the presence of ionomeric phase where charge accumulation due to electro-statics in the vicinity of the membrane-CL interface becomes important. The electrode model includes a physical description of heat and water balance along with electrochemical performance analysis in order to study the influence of electro-statics/electro-migration and phase change on the PEFC electrode performance.
In this paper, we investigate the electrochemical reaction kinetics, species transport, and solid water dynamics in a polymer electrolyte fuel cell (PEFC) during cold start. A simplified analysis is developed to enable the evaluation of the impact of ice volume fraction on cell performance during cold-start. Supporting neutron imaging data are also provided to reveal the real-time water evolution. Temperature-dependent voltage changes due to the reaction kinetics and ohmic loss are also analyzed based on the ionic conductivity of the membrane at subfreezing temperature. The analysis is valuable for the fundamental study of PEFC cold-start.
The gas diffusion layer (GDL) plays a critical role in the overall performance of a polymer electrolyte fuel cell (PEFC), especially in the mass transport control regime due to suboptimal liquid water transport. Liquid water blocks the porous pathways in the catalyst layer and gas diffusion layer thereby causing hindered oxygen transport from the channel to the active reaction sites. This phenomenon is known as “flooding” and is perceived as the primary mechanism leading to the limiting current behavior in the cell performance. The pore morphology and wetting characteristics of the cathode GDL are of paramount importance in the effective PEFC water management. Typical beginning-of-life GDLs exhibit hydrophobic characteristics, which facilities liquid water transport and hence reduces flooding. Experimental data, however, suggest that the GDL loses hydrophobicity over prolonged PEFC operation and becomes prone to enhanced flooding. In this work, we present a pore-scale modeling framework to study the structure-wettability-durability interplay in the context of flooding behavior in the PEFC GDL.
In this work, we present a neutron radiography and analysis, as well as modeling study on cold-start operation of polymer electrolyte membrane (PEM) fuel cells. Fuel cells with Gore™ or LANL MEAs and SGL or E-Tek ELAT GDLs are tested in varying subfreezing temperatures (−40 to 0°C) to determine the time scale of cold-start failure, amount of solid water formation, solid water formation location, and. A higher PTFE-loading in the MPL is found to decrease loss in electrocatalytic surface area in our case. Theoretical analysis is also conducted and model predictions are compared with the experimental data in terms of the cell voltage evolution.
A critical performance limitation in the polymer electrolyte fuel cell (PEFC) is attributed to the mass transport loss originating from suboptimal liquid water transport and flooding phenomena. Liquid water can block the porous pathways in the fibrous gas diffusion layer (GDL) and the catalyst layer (CL), thus hindering oxygen transport from the flow field to the electrochemically actives sites in the catalyst layer. The cathode GDL is the component primarily responsible for facilitating gas and liquid transport, therefore plays a major role in determining the water management of a PEFC and hence the mass transport loss. The underlying pore morphology and wetting characteristics have significant influence on the flooding dynamics in the GDL. In this paper, the study of the two-phase behavior and the durability implications due to the wetting characteristics in the carbon paper GDL are presented using a pore-scale modeling framework.
A key performance limitation in the polymer electrolyte fuel cell (PEFC), manifested in terms of mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. A key contributor to the mass transport loss is the cathode gas diffusion layer (GDL) due to the blockage of available pore space by liquid water thus rendering hindered oxygen transport to the active reaction sites in the electrode. The GDL, therefore, plays an important role in the overall water management in the PEFC. The underlying pore-morphology and the wetting characteristics have significant influence on the flooding dynamics in the GDL. Another important factor is the role of cell compression on the GDL microstructural change and hence the underlying two-phase behavior. In this article, we present the development of a pore-scale modeling formalism coupled with realistic microstructural delineation and reduced order compression model to study the structure-wettability influence and the effect of compression on two-phase behavior in the PEFC GDL.
Water transport in the ionomeric membrane, typically Nafion®, has profound influence on the performance of the polymer electrolyte fuel cell, in terms of internal resistance and overall water balance. In this work, high resolution neutron imaging of the Nafion® membrane is presented in order to measure water content and through-plane gradients in situ under disparate temperature and humidification conditions.
This paper investigates the electrochemical kinetics, oxygen transport, and solid water formation in polymer electrolyte fuel cell (PEFC) during cold start. Following [Y. Wang, J. Electrochem. Soc., 154 (2007) B1041-B1048.], we develop a pseudo one-dimensional analysis, which enables the evaluation of the impact of ice volume fraction and temperature variations on cell performance during cold-start. The oxygen profile, starvation ice volume fraction, and relevant overpotentials are obtained. This study is valuable for studying the characteristics of PEFC cold-start.
This study examined the hydration state of the PEM and GDLs in four PEM fuel cells, with various combinations of SGL Carbon carbon paper GDLs with different PTFE loadings, in response to current step up and step down transients. The goal was to determine the effect of these GDLs and their PTFE loadings in the substrates and MPLs on the water dynamics and performance. The fuel cells used GoreTM MEAs and were operated at 80oC with stoich tracking gas flows at 50% inlet RHs. In situ HFR measurements were used for monitoring PEM hydration and in situ neutron imaging of the fuel cells was conducted at NIST with quantitative analysis to determine the liquid water content in the GDLs and channels. High PTFE loadings in the porous, open GDL substrates allow for greater water flux and lower water holding capacity, which facilitates cathode reaction water removal at high currents. In contrast, high PTFE loadings in the less porous, smaller pore size MPLs increase the water barrier attributes and seal in PEM water. Counter flow gas feed results in more even water distribution than co-flow feed. Counter flow feed with cathode air flowing against gravity increases cell hydration in dry operating conditions.
In the present scenario of a global initiative toward a sustainable energy future, the polymer electrolyte fuel cell (PEFC) has emerged as one of the most promising alternative energy conversion devices for different applications. Despite tremendous progress in recent years, a pivotal performance/durability limitation in the PEFC arises from liquid water transport, perceived as the Holy Grail in PEFC operation. The porous catalyst layer (CL), fibrous gas diffusion layer (GDL) and flow channels play a crucial role in the overall PEFC performance due to the transport limitation in the presence of liquid water and flooding phenomena. Although significant research, both theoretical and experimental, has been performed, there is serious paucity of fundamental understanding regarding the underlying structure-transport-performance interplay in the PEFC. The inherent complex morphologies, micro-scale transport physics involving coupled multiphase, multicomponent, electrochemically reactive phenomena and interfacial interactions in the constituent components pose a formidable challenge. In this paper, the impact of capillary transport, wetting characteristics and interfacial dynamics on liquid water transport is presented based on a comprehensive mesoscopic modeling framework with the objective to gain insight into the underlying electrodics, two-phase dynamics and the intricate structure-transport-interface interactions in the PEFC.
A key performance limitation in polymer electrolyte fuel cells (PEFC), manifested in terms of mass transport loss, originates from liquid water transport and resulting flooding phenomena in the constituent components. Liquid water leads to the coverage of the electrochemically active sites in the catalyst layer (CL) rendering reduced catalytic activity and blockage of the available pore space in the porous CL and fibrous gas diffusion layer (GDL) resulting in hindered oxygen transport to the active reaction sites. The cathode CL and the GDL therefore play a major role in the mass transport loss and hence in the water management of a PEFC. In this article, we present the development of a mesoscopic modeling formalism coupled with realistic microstructural delineation to study the profound influence of the pore structure and surface wettability on liquid water transport and interfacial dynamics in the PEFC catalyst layer and gas diffusion layer.
Water transport in the ionomeric membrane, typically Nafion®, has profound influence on the performance of the polymer electrolyte fuel cell, in terms of internal resistance and overall water balance. In this work, the response of Nafion® membrane water content and the resulting water gradient to fuel cell operational variability are investigated and the results from a theoretical modeling study are furnished along with reference to neutron imaging observations.
This paper investigates interaction of the electrochemical kinetics, oxygen transport and solid water formation within polymer electrolyte fuel cell (PEFC) electrode during cold start. Followed by the analysis of Wang , we simplify the one-dimensional model of electrode processes, which allows solving the profiles of important quantities and directly relating the ice impact mechanisms to surface overpotential. The key parameters that govern these profiles are evaluated in the range of the relevant factors for a typical fuel cell. We also decouple the mechanisms of solid water impacts and compare their importance. This study is valuable for studying the characteristics of cold-start for PEFCs.
It is widely recognized that the performance degradation and the limiting current behavior in polymer electrolyte fuel cells (PEFC) are mainly attributed to the excessive build up of liquid water in the cathode side and the resulting flooding phenomena. Liquid water blocks the open pore space in the catalyst layer (CL) and the gas diffusion layer (GDL) leading to hindered oxygen transport and covers the electrochemically active sites in the CL thereby rendering reduced catalytic activity. The CL flooding therefore plays a crucial role in the overall PEFC performance limitation. In order to elucidate the primary mechanisms of liquid water removal out of the CL, the factors affecting CL flooding and to discern the role and contribution of CL flooding on the overall PEFC voltage loss, a CL flooding model has been developed. The flooding model is based on a simplified structure-wettability representation of the PEFC CL and a physical description of water and heat balance along with electrochemical performance analysis. The model shows that the evaporation mechanism, depending upon the cell operating temperature and the GDL thermal conductivity, plays a crucial role in the CL flooding behavior and the cell performance.
A comprehensive pore-scale modeling framework has been developed for the catalyst layer of a polymer electrolyte fuel cell in order to study the effects of site coverage and volume blockage owing to liquid water. The catalyst layer microstructure is reconstructed using a stochastic generation method. The liquid water distribution is obtained from a two- phase lattice Boltzmann simulation and the catalytic surface coverage leading to reduced electrochemical activity is estimated from the saturation maps. The pore blockage effect due to liquid water is evaluated using a direct numerical simulation model from the reduced oxygen diffusivity owing to the resistance offered by the blocked pores.
In the current work, we present a comprehensive modeling framework to predict the effective gas diffusivity, as a function of liquid water saturation, based on realistic 3-D microstructures of the uncompressed as well as compressed gas diffusion layer (GDL). The presented approach combines the generation of a virtual microscopic GDL and different physical modeling. We develop a reduced model in order to simulate the compression of the GDL layer since its compression has a strong impact on the material properties such as the water transport or its gas diffusion. Then, we determine the two-phase distribution of a non-wetting fluid, i.e. water, and a wetting fluid, i.e. air, within the GDL for different saturations. This is done using a full morphology (FM) model. Finally, solving the Laplace equation for the partly saturated medium we determine the relative gas diffusion, i.e. the gas diffusion depending on the saturation. In the present work, our approach is applied to a typical GDL medium, a SGL10BA carbon paper.
A non-isothermal, two-phase model is applied to a two- dimensional model of GDL to study the heat and mass transport in the in-plane direction (i.e. channel-to-land) and in the through-plane direction (i.e. catalyst layer-to-channel). For the first time, the anisotropy in the GDL thermal conductivity is taken into account, and found to be an important factor governing the temperature distribution in the GDL. The maximum temperature difference in the GDL is found to be a strong function of the GDL anisotropy. A temperature difference of up to 5ºC is predicted, and a significant effect of temperature distribution on the water transport and distribution is observed. In addition, the latent heat effects due to condensation/evaporation of water on the temperature and water distributions are also analyzed and found to strongly affect the two-phase transport.
Bacterial electrical conduction; Cellular electronic energy transfer; Microbial electrolysis cell; Microbial fuel cell.
Biofilms are aggregations of microbes that are encased by extracellular polymeric substances (EPS) and adhere to surfaces or interfaces. Biofilms exist in a very wide diversity of environments, and microfluidic devices are being increasingly utilized to study and understand their formation and properties.
Fundamentals of two-phase transport in a porous medium with emphasis on pore-scale physics are discussed. Lattice Boltzmann modeling and pore network modeling techniques are reviewed and are deployed to study two-phase flow in the engineered porous materials used in a polymer electrolyte fuel cell (PEFC). Computation of fluid distribution, capillary pressure, and relative permeability is discussed. Porous material microstructure–PEFC performance relation is showcased, highlighting the potential of deploying pore-scale modeling to the advancement of fundamental two-phase transport understanding and PEFC porous material development. Additionally, the need for further advancement of pore-scale models is highlighted.
Fuel cells, being highly energy efficient, environmentally benign, and minimally noisy, are widely considered as the 21st century energy-conversion devices for mobile, stationary, and portable power. Unlike the conventional Carnot cycle based energy conversion devices with intermediate heat and mechanical energy generation, fuel cells convert the chemical energy of a fuel directly into electricity. Among the several types of fuel cells, the polymer electrolyte fuel cell (PEFC) has emerged as the most promising power source for a wide range of applications.
Fuel cells, owing to their high energy efficiency, environmental friendliness and low noise, are widely considered as the twenty-first century energy-conversion devices for mobile, stationary and portable power. Among the different types of fuel cells, the polymer electrolyte fuel cell (PEFC) has emerged as a promising power source for a wide range of applications.
This chapter focuses on glass-like carbons, their method of micro and nanofabrication and their electrochemical and microfluidic applications. At first, the general properties of this material are exposed, followed by its advantages over other forms of carbon and over other materials. After an overview of the carbonization process of organic polymers we delve into the history of glass-like carbon. The bulk of the chapter deals with different fabrication tools and techniques to pattern polymers. It is shown that when it comes to carbon patterning, it is significantly easier and more convenient to shape an organic polymer and carbonize it than to machine carbon directly. Therefore the quality, dimensions and complexity of the final carbon part greatly depend on the polymer structure acting as a precursor. Current fabrication technologies allow for the patterning of polymers in a wide range of dimensions and with a great variety of tools. Even though several fabrication techniques could be employed such as casting, stamping or even Computer Numerical Controlled (CNC) machining, the focus of this chapter is on photolithography, given its precise control over the fabrication process and its reproducibility. Next Generation Lithography (NGL) tools are also covered as a viable way to achieve nanometer-sized carbon features. These tools include electron beam (e-beam), Focused-ion beam (FIB), Nano Imprint Lithography (NIL) and Step-and-Flash Imprint Lithography (SFIL). At last, the use of glass-like carbon in three applications, related to microfluidics and electrochemistry, is discussed: Dielectrophoresis, Electrochemical sensors, and Fuel Cells. It is exposed how in these applications glass-like carbon offers an advantage over other materials.
Fuel cells, due to their high energy efficiency, zero pollution and low noise, are widely considered as the 21st century energy-conversion devices for mobile, stationary and portable power. Among the several types of fuel cells, polymer electrolyte fuel cell (PEFC) has emerged as the most promising power source for a wide range of applications.