Gold-Like Activity, Copper-Like Selectivity of Heteroatomic Transition Metal Carbides (M2C) for Electrocatalytic Carbon Dioxide Reduction Reaction

Abstract

An overarching challenge of the electrochemical carbon dioxide reduction reaction (eCO2RR) is finding an earth-abundant, highly active catalyst that selectively produces hydrocarbons at relatively low overpotentials. Here, we have studied the two-dimensional transition metal carbide (TMC) class of materials and found that di-tungsten carbide (W2C) nanoflakes exhibit maximum methane (CH4) current density of -421.63 mA/cm2 and a CH4 faradic efficiency of 82.7%±2% in a hybrid electrolyte of 3 M potassium hydroxide (KOH) and 2 M choline-chloride (CC). Powered by a triple junction photovoltaic cell, we have demonstrated a flow electrolyzer that uses humidified CO2 to produce CH4 in a 700-hours process under one sun illumination with a CO2RR energy efficiency of about 62.3% and a solar-to-fuel efficiency of 20.7%. Density functional theory (DFT) calculations reveal that dissociation of water, chemisorption of CO2 and cleavage of the C-O bond – the most energy consuming elementary steps in other catalysts such as copper – become nearly spontaneous at the W2C surface. This results in instantaneous formation of adsorbed CO – an important reaction intermediate – and an unlimited source of protons near the tungsten surface sites that are the main reasons for the observed superior activity, selectivity, and small potential.