Ab Initio Thermodynamic Stability of Carbide Catalysts under Electrochemical Conditions

Transition metal carbides (TMCs) are considered as promising electrocatalysts to break adsorption energy scaling relations that limit the catalytic function of the parent transition metals. Yet, their stability under electrochemical conditions is uncertain, with at least hexagonal Mo2C in an aqueous electrolyte known to form a surface oxide layer already at very negative potentials. Here, we use ab initio thermodynamics to systematically investigate the stability of all low-index facets of a series of TMCs (TM = Ti, Zr, V, Nb, Cr, and Mo) with different metal/carbon ratios. The deduced electrochemical stability window indeed challenges an intended use of TMCs as CO2 reduction catalysts. Only MoC and CrC are found to stably exhibit facets with accessible, methanol-selective C-rich active sites.