Mapping a route for more efficient production of sustainable fuels

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This perspective article, led by RASEI Fellow Tanja Cuk, brings together researchers at six research institutions from across the United States, to describe how advances in spectroscopy and theory can map out the elementary details of the oxygen evolution reaction, a critical reaction to enable the production of fuels from sustainable energy sources.

The oxygen evolution reaction (or OER for short), is a critical step in the creation of sustainable, decarbonized fuels, such as hydrogen. Water (H₂O) can be split into hydrogen (H₂) and oxygen (O₂) using electricity. This process pulls apart strong chemical bonds – it takes a lot of energy! If we can better understand this process, we can make it more efficient, which will enable us to create clean fuels and store renewable energy, like solar and wind power, to smooth out variations in the supply.  Specifically, the OER is the half-reaction that occurs at the anode (positive electrode) during electrolysis, in which the water molecules are oxidized to produce oxygen gas (O₂), protons (H⁺), and electrons. Though this sounds straightforward, the process involves multiple intermediates, or steps, many of which are currently poorly defined. Understanding this complex process requires a collaborative approach. Jin Suntivich (Cornell University) and Dhananjay Kumar (North Carolina A&T) bring expertise in making advanced materials and electrochemistry, Geoffroy Hautier (Dartmouth College) and Ismaila Dabo (Carnegie Mellon University) develop theoretical models, and Ethan Crumlin (Lawrence Berkeley National Laboratory), Tanja Cuk (CU �鶹ӰԺ), and Jin Suntivich use X-ray and optical spectroscopy to visualize the small molecular intermediates.     

Imagine that you have to drive from Denver, Colorado, to Greensboro, North Carolina. If someone gave you a map that only showed your starting location and destination, it would be quite difficult. You would know that you had to head east, but you wouldn’t know what roads to take, which were the fastest moving, or where any good stops were along the way. You could probably get there, but you would get lost a few times on the way, use some of the slow roads, and maybe be stuck staying in places you didn’t want to. It would be a very inefficient journey. Now compare this to using a modern navigation app, one that has details of every road along the way, the speed limits, the traffic levels, where all the gas stations are, the good restaurants and coffee shops, and good places to stop for the night. You would be far more efficient (and happy) using the navigation app.

It is the same with a chemical reaction. If you understand the elementary steps of a reaction, you can design a system that is more efficient and effective at getting to the final product. Creating this ‘map’ for the OER is a central mission of the Center for Electrochemical Dynamics and Reactions on Surfaces (CEDARS). CEDARS is a Department of Energy funded Energy Frontier Research Center (EFRC), that brings together twelve research groups at five universities and two DOE national labs across the chemical, materials, and computational sciences.  CEDARS is headed by Director Dhananjay Kumar at North Carolina A&T, with a strong program in thin materials research.  This is the first EFRC awarded to an HBCU as a lead institution in the country.  There are several challenges that need to be overcome before the OER process can be scaled up. Currently OER is expensive, energy intensive and not reliable for continuous long-term operation. OER requires large inputs of electricity, the catalysts used in the reaction are based on scarce materials that are unstable under long-term exposure to the harsh conditions present in the OER process. By better understanding the elementary steps of the OER reaction researchers can design cheaper, more efficient processes.

RASEI Fellow, and Associate Director of CEDARS Tanja Cuk explains that there have been a series of proposed oxygen-related intermediates (e.g. OH*, O*, O-O), but it has been hard to capture experimental evidence for them and the elementary steps that create them. “The article is a perspective on how to get at the intermediates and their dynamics within the buried electrode-electrolyte interface. The approach involves model crystalline materials, targeted spectroscopies to isolate the intermediates, and theoretical investigations that predict how they appear in the electrochemistry and the spectroscopy.  We also use materials that bind the intermediates at different strengths, so that they appear statically and transiently.” This fundamental and basic energy sciences approach combines expertise from across CEDARS bringing together computational theoretical modeling, materials synthesis, and spectroscopy.  The diversity of institutions involved has already provided for many student and postdoctoral exchanges that further deepen the background of the team and broaden the scope of the research.  Just last month the Center Director and his graduate student visited NREL and CU to test the samples made at NCAT.

Precise control of the materials under investigation is required for effective characterization and theoretical modeling. Dhananjay Kumar, Jin Suntivich, and collaborators within CEDARS use a process called epitaxial layer deposition, a procedure where a thin crystalline layer is grown on top of a substrate. For these investigations the epitaxial layers are the OER catalysts made from ruthenium and titanium oxides that are then tested electrochemically. Geoffroy Hautier is a materials theorist who uses computational models to calculate the structure and defects that intermediates create in the materials and their impacts on x-ray and optical spectra. Ismaila Dabo takes these configurations and creates a model of the electrical and water environment around the electrode interface, describing a more realistic environment for the OER processes. To provide a more detailed understanding, the theoretical models are tested and refined based on feedback from advanced spectroscopic observations. The spectroscopies highlight static spectra of intermediate coverages and transient intermediates during OER. Jin Suntivich brings expertise in combining in-situ electrochemistry with non-linear optical techniques; Ethan Crumlin develops in-situ and time-resolved x-ray spectroscopies; Tanja Cuk combines in-situ electrochemistry with ultrafast optical spectroscopy.  Integrating the computational advances with the experimental observations provides a powerful toolkit. Accurate interpretation of the spectral observations relies on the findings from the computational techniques. 

While the ‘map’ for the OER has not been solved, this interdisciplinary and fundamental approach to interrogating the OER process is providing invaluable insights into how different catalysts bind to the intermediates and how this impacts different reaction pathways.  By characterizing the nature of the intermediates bound to the catalyst an understanding of their equilibrium behavior during the OER process can be developed. The CEDARS team are already thinking about next steps for this powerful approach.  These include understanding the non-equilibrium dynamics of these intermediates by fully time resolved x-ray and optical probes and investigating more complex material structures.  The observations from these ‘in-process’ reactions will help define the roadmap to a more efficient and cost-effective approach to generate clean fuels from renewable energy sources. 

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