Phase-dependent catalytic activity of in situ-oxidized molybdenum carbide for formic acid electro-oxidation
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Fuel cells enable clean and efficient energy generation in portable power sources to be used in automotive, military, and space applications. The platinum group metals (PGM) are some of the most active fuel cell electrocatalysts, but their high price and scarcity hinder large-scale fuel cell commercialization. Transition metal carbides (TMCs) have long been regarded as cost-effective PGM substitutes offering improved physical properties and high catalytic activity towards some reactions. Recently, nanosized TMCs have been synthesized that exhibit a complex size- and phase-dependent stability and catalytic properties. In this work, we employ a combination of density functional theory, ab initio thermodynamics, ab initio molecular dynamics, and electronic structure analysis of MoxCy catalysts to reveal phase-, potential-, and pH-dependent surface structure and composition, nature of the active site, and its catalytic activity for formic acid electro-oxidation. We develop Pourbaix diagrams for representative MoxCy surfaces and reveal a substantial degree of surface oxidation (either monolayer-limited or penetrating to the bulk) in an electrochemical environment. Mechanistic studies unfold a phase-dependent inhibiting or promoting effect of an oxygen overlayer that modifies the free energies of co-adsorbed reaction intermediates. Finally, machine learning tools shed light on the structure of TMC nanoparticles and help bridge the materials gap.