Direct ethanol-based fuel cells (DEFC) are a viable source for renewable energy production, with numerous environmentally friendly ethanol sources, such as biomass and CO₂ reduction,^1,2 available. Despite these advantages, DEFC’s require high overpotential and suffer from low selectivity to the fully oxidized product CO₂ (12 e^- per ethanol) on pure metal catalysts.^3,4 To systematically understand these phenomena, and thereby facilitate design of improved DEFC’s, studies have aimed to probe the fundamentals of the reaction chemistry on single crystal surfaces of noble metal catalysts such as Pt. However, the mechanistic details of the reactions are not fully known, and even basic information such as the nature of the rate-limiting step is not understood.

In this work, we consider a detailed analysis to understand the reaction mechanism of ethanol (CH₃CH₂OH) electrooxidation on the Pt(100) single crystal surface in the lower overpotential region (0.2-0.3 V vs. SHE). To elucidate the reaction mechanism, we make use of periodic Density Functional Theory calculations combined with theoretical electrochemistry analyses. First, we determine the optimal coverage of H* on the surface in the potential region of interest. Then, using the equilibrium H* coverage as a basis,⁵ we elucidate the reaction mechanism by incorporating various effects such as reactant coverages and solvation,⁶ as well as chemical and electrochemical reaction barriers, determined using techniques such as explicit proton transfer from water bilayers, potential of mean force simulations using ab-initio molecular dynamics, and the Climbing Image Nudged Elastic Band (CINEB) algorithm.⁷ Finally, we discuss the detailed reaction mechanism in terms of selectivity-determining reaction steps and coverages of reaction intermediates determined from a microkinetic analysis.

References:

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(2) Li, F.; Li, Y. C.; Wang, Z.; Li, J.; Nam, D.-H.; Lum, Y.; Luo, M.; Wang, X.; Ozden, A.; Hung, S.-F.; Chen, B.; Wang, Y.; Wicks, J.; Xu, Y.; Li, Y.; Gabardo, C. M.; Dinh, C. T.; Wang, Y.; Zhuang, T.-T.; Sinton, D.; Sargent, E. H. Nature Catalysis 2019, 2, 648.

(3) Marinkovic, N. S.; Li, M.; Adzic, R. R. Top Curr Chem (Z) 2019, 377 (3), 315.

(4) Shao, M. H.; Adzic, R. R. Electrochim Acta 2005, 50 (12), 2415.

(5) Deshpande, S.; Maxson, T.; Greeley, J. npj Computational Materials 2020, 6, 79.

(6) Deshpande, S.; Greeley, J. ACS Catal. 2020, 9320.

(7) Chun, H.-J.; Apaja, V.; Clayborne, A.; Honkala, K.; Greeley, J. ACS Catal. 2017, 3869.