Mechanistic Insights into the Selective Electroreduction of Carbon Dioxide to Ethylene on Cu2O-Derived Copper Catalysts

In this work, we made a comprehensive investigation to unravel the underlying causes for the selectivity of CO2 electroreduction toward ethylene on Cu2O-derived Cu catalysts. Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, chronopotentiometry, online gas chromatography, nuclear magnetic resonance spectroscopy, and numerical simulations of local pH were used toward this end. Ten Cu2O-derived Cu films of different thicknesses and morphologies were prepared and extensively characterized. Aqueous 0.1 M KHCO3 was used as the electrolyte. We report here, for the first time, a remarkably strong correlation between the statistically relevant crystallite sizes of the Cu2O-derived Cu particles and selective CO2 electroreduction to C2H4. Specifically, as the crystallite size of the particles decreased from 41 to 18 nm, the Faradaic efficiency (FE) of C2H4 formation increased from 10 to 43%. Using cyclic voltammetry, samples with smaller particle crystallite sizes were found to possess more diverse adsorption sites for CO (a known reaction intermediate), which we interpret to be important for the C–C coupling of C1 adsorbates to C2 intermediates. The effect of local pH on the yield of C2H4 for the different Cu2O-derived Cu catalysts was less significant when compared to the effects of crystallite sizes and mass transport limitations. We also show here that remarkable amounts of C2 and C3 products could be achieved using these Cu2O-derived Cu catalysts. Driven at a fixed total current density of −31.2 mA cm–2, the catalysts could reduce CO2 to C2H4, ethanol, and n-propanol with optimized FEs of 42.6% (jC2H4 = −13.3 mA cm–2), 11.8% (jC2H5OH = −3.7 mA cm–2), and 5.4% (jC3H7OH = −1.7 mA cm–2), respectively.