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 report the eCO2RR performance of two-dimensional transition metal carbide class of materials. Our results indicate a maximum methane (CH4) current density of −421.63 mA/cm2 and a CH4 faradic efficiency of 82.7% ± 2% for di-tungsten carbide (W2C) nanoflakes in a hybrid electrolyte of 3 M potassium hydroxide and 2 M choline-chloride. Powered by a triple junction photovoltaic cell, we demonstrate a flow electrolyzer that uses humidified CO2 to produce CH4 in a 700-h 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 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.
Highlights
An overarching challenge of the electrochemical carbon dioxide reduction reaction is finding an earth-abundant, highly active catalyst that selectively produces hydrocarbons at relatively low overpotentials
To improve the CO2RR performance in competing with hydrogen evolution reaction (HER), we have employed a mixture of 3 M potassium hydroxide (KOH) and 2 M choline chloride (CC) solution (KOH:CC 3 M:2 M) as the electrolyte in this study[47]
The linear sweep voltammetry (LSV) experiments and a realtime product stream analysis show that CO2RR on the W2C surface starts at a potential of −122.7 mV vs reversible hydrogen electrode (RHE) by producing carbon monoxide (CO) and H2 and reach maximum CO2RR current density of −548.9 mA/cm[2] at −1.05 V vs RHE (Supplementary Figs. 2–4 and Fig. 1a)
Summary
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. The electrocatalytic performance of TMC NFs with similar crystallite sizes (25.4 ± 5 nm) were studied in a three-electrode cell and compared with Au and Cu nanoparticles (NPs), conventional catalysts for this reaction,[46] under identical experimental conditions (Supplementary section 2).
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