Electrocatalysis at the electrode-electrolyte interface: a combined DFT and classical force-field approach

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Electrochemistry offers a route to manufacturing fuels and chemicals from renewable or sustainable energy sources such as solar, wind, or hydroelectric power. The efficiency, rates, and product selectivity of such electrochemical processes can be further improved by incorporating catalysts into the electrochemical technology. Although effective, electrocatalytic manufacturing processes are highly complex, making it difficult to predict the best combinations of catalytic materials and operating conditions needed for optimal performance. The project will develop theoretical and mathematical methods to understand the energetics of electrocatalytic reactions in liquid solvents, thereby facilitating the search for improved catalysts and manufacturing processes. The project also involves training of students at various educational levels and outreach to underrepresented groups. Computational methods to model electrocatalytic processes at the electrode/electrolyte interface are essential to guide rational design. Much progress has been made in theoretical descriptions of surface-bound reactant and product states, particularly using density functional theory (DFT). More recently, advances in describing electrocatalytic transition states have enabled estimates of reaction barriers. However, in these advances the description of the surrounding electrolyte is comparatively primitive, despite the evident importance of solvation effects on reaction rates. The project combines state-of-the-art DFT methods to compute the transition state, with classical molecular dynamics (MD) simulation methods to compute solvation free energies. The enabling concept is to treat the DFT transition state described at the metal/vacuum interface as a molecule, to be inserted into the electrolyte in the double layer. MD simulation methods have been developed for solvation free energies of molecules in solution, by computing the thermodynamic work required to slowly "turn on" the interactions with surrounding fluid. These methods will be applied to the transition state itself, as well as to the surface-bound reactants and products. Relatively polar reactants, transition states, and products should more strongly interact with solvent and electrolyte ions, and their free energies lowered accordingly, with corresponding effects on reaction barriers. These important solvation effects on barriers and reaction rates depend crucially on proper averaging over local arrangements of water molecules and ions near the surface. Atomistic MD simulations are uniquely suited to describe and average over these local arrangements. The first reactions to be studied with the new methods will include electrocatalytic hydrogen oxidation and propanol reduction, reactions of technical relevance for fuel cells and biomass electroreduction. The developed approach will be validated against available experimental data, including solution phase propanol dehydration barriers, interfacial water structure, double-layer capacitance, hydrogen oxidation barriers on single-crystal Pt electrodes, and propanol electroreduction barriers. Both graduate and undergraduate students will develop and apply the proposed methods, and receive broad education in electrochemistry, electronic structure, and molecular modeling. Both investigators will continue their strong record of mentoring undergraduates in research, which together has resulted in 48 advised undergraduates who have co-authored 22 peer reviewed publications over the past 5 years.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.