Understanding the Low-Overpotential Production of CH4 from CO2 on Mo2C Catalysts

While Cu is the only electrocatalyst that converts CO2 into meaningful quantities of CH4 fuel, it requires significant overpotentials (onset potential of ∼−0.80 V vs RHE), decreasing energy conversion efficiencies. We report that Mo2C is capable of catalyzing CO2 into CH4 at low potentials (onset potential of ∼−0.55 V vs RHE), where Cu electrocatalysts do not convert CO2. This low-overpotential catalyst was first identified as a candidate by electronic structure calculations, which indicated the free energetics of CO hydrogenation to be more favorable than that on conventional transition metals such as Cu. Despite the low onset potential for CH4, the CH4 has a steep Tafel slope (∼−280 mV/dec), resulting in most of the current passing through the Mo2C electrocatalysts being utilized for the competitive hydrogen evolution reaction. We conducted a detailed theoretical analysis on the basis of density functional theory calculations, microkinetic analysis, and simulated Pourbaix diagrams to suggest the reasons for these characteristics. These analyses suggest that the potential-limiting step in CH4 evolution is the clearing of OH from the surface, while the rate-limiting step is the nonelectrochemical C–O bond scission, resulting in a high OH coverage and a high Tafel slope. Our calculations suggest that this high coverage weakens H binding, causing enhancement of the H2 evolution reaction in comparison to that under CO2-free conditions. This analysis shows that the detailed interaction of theory and experiment can be used to design and analyze operational electrocatalysts for CO2 reduction and other complicated electrocatalytic reactions.