ChE research unravels mysteries of two superstar industry catalysts; discovery could advance renewable energy sources
Discovery will potentially lead to more accurate design of new catalysts for renewable energy sources
An international team of scientists, including Davidson School of Chemical Engineering Professor Jeffrey Greeley and Research Scientist Dr. Zhenhua Zeng, has unraveled mysteries of two superstar industry catalysts, which could lead to more accurate design of new catalysts to advance greener technologies by combining solar and wind energy.
The Purdue researchers, in collaboration with scientists and engineers in Germany, France, and China, studied the atomic-scale structure and mechanism of complex water splitting electrocatalysts, which are used for hydrogen fuel production. The team was able to identify, for the first time, the molecular-scale features of two of the most popular such catalysts.
Their study, In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution, is published in the recent issue of Nature Communications.
“Although these catalysts have been studied for many years, our team has a unique combination of experimental and computational expertise that allowed us to unravel the mysteries surrounding their atomic-scale structures. This information can accelerate the design of still better electrocatalysts” said Professor Greeley (left).
When a high enough voltage is applied to a water molecule, the molecule is split into the separate molecules – oxygen molecules and hydrogen molecules - in a process called water electrolysis. The role of a catalyst is to make the water electrocatalysis happen at as low a voltage as possible.
“Water electrolysis is essential in the production of fuels for hydrogen fuel cells, which generate power using a highly-efficient electrochemical reaction instead of combustion,” explained Dr. Zhenhua Zeng (right). Since hydrogen fuels are a clean energy source with zero emissions, hydrogen fuel cells are considered to be greener technology and can be used in many practical applications, including fuel cell cars, portable power sources, and in homes.
Separating oxygen from the water molecule, called the oxygen evolution reaction (OER), is orders of magnitude more difficult than producing hydrogen. Thus, OER accounts for the majority of energy losses in the water electrolysis process.
To overcome this difficulty, and ultimately design new catalysts with significantly improved performance, the researchers sought to identify the atomic-scale structure, including the active phase and the reaction center, of the two most popular and complex electrocatalysts used in alkaline water electrolysis, and to understand how the reaction happens.
Dr. Fabio Dionigi and Professor Peter Strasser at Technical University Berlin measured the crystal structure of catalysts under operation conditions through advanced synchrotron experiments. They discovered that the lattice parameters of the active phase are actually different from both the as-prepared phase and those phases which were assumed in previous studies.
To understand the measured structure, the researchers performed a large number of expensive molecular dynamics simulations and rigorous calculations. “Our modeling not only self-consistently identified the active phase with lattice parameters perfectly matching the measurements, but it also provided atomic-scale details of the crystal structure that are inaccessible in experiments,” said Dr. Zeng.
“The transformation to the catalytically active phase is reversible upon letting the catalysts returning to their resting state at open circuit potential,” said Professor Strasser. “Therefore, it is important to study these catalysts in operando conditions.” Professor Strasser noted these are challenging due to the aqueous environment in which the catalysts work.
“After optimizing the setup conditions, our operando X-ray scattering experiments performed at the synchrotron were able to provide the lattice parameters of the active phase of the catalysts during oxygen evolution,” said Dr. Dionigi.
In the past, due to the complexity of those real-world catalysts, simplified structures were used in modeling studies, rather than the exact structure which exists under reaction conditions. “In the current work, we performed rigorous calculations to identify the exact structure of the active phase,” explained Dr. Zeng. “We then identified the reaction center and catalytic mechanism self-consistently, i.e. without approximation. We have been developing many previously unavailable tools and highly-accurate methodologies in the last eight years, and we were well prepared to attack such a highly challenging system.”
The research was recently selected as the Editor’s Highlights in Nature Communications: https://www.nature.com/collections/dmmhtcypsc/content/adam-weingarten
“This is fundamental research because our team’s goal is to understand, at the atomic scale, the exact site where the reaction occurs and how it happens,” explained Dr. Zeng. He compares the current research to opening a “black box.
“By opening the ‘black box’ now, we and the community can perform well-targeted research to significantly improve the efficiency of water electrolysis by reducing the difficulty of oxygen generation in comparison with hydrogen generation,” he said. This reduction would lower the cost of hydrogen fuel production, a key composition of oncoming hydrogen economy and greener energy.
Figure 1 (above): Unraveling the mystery of NiFe layered double hydroxide for oxygen evolution. Measurement using advanced synchrotron techniques indicated a change of the lattice parameters under operation conditions (middle figure). Large-scale computational modeling not only self-consistently identified the active phase with lattice parameters perfectly matching the measurement, but also provided atomic-scale details of the crystal structure that are inaccessible in experiment. The identification of the exact structure of the active phase make it possible to understand, at the atomic scale, the exact site where the reaction occurs and how it happens (far right). (Purdue University/Zhenhua Zeng).
Read the full Nature Communications publication at: https://www.nature.com/articles/s41467-020-16237-1
Learn more about the Davidson School of Chemical Engineering at Purdue University: https://engineering.purdue.edu/ChE
All illustrations and graphics files can be accessed in a Google folder at this link: https://drive.google.com/drive/folders/1nENQb-FZJHEUINuFF5py3VcrtoD8IndW?usp=sharing
Source: Dr. Zhenhua Zeng, zeng46@purdue.edu, (765)-494-1291
Dr. Jeffrey Greeley, jgreeley@purdue.edu, (765) 494-1282
Writer: Jennifer Merzdorf, jmerzdo@purdue.edu, (765) 496-8779