Understanding the Highly Selective Electroreduction of Carbon Dioxide into Fuels on Hierarchical Oxide-Derived Inverse Opals

Abstract

The production of value-added chemicals and fuels from CO2 and water by highly efficient, selective, robust electrocatalytic materials is of vital interest to address the energy challenges. In this work, we report a hierarchical CuO-derived inverse opal (CuO-HIO) electrocatalyst which demonstrated impressive CO selectivity and negligible H2 evolution at high current densities (achieved ~35 mA cm-2 at -1.2 V vs. RHE). Such a three-dimensional interconnected porous network with voids about 200 nm surprisingly exhibited a Faradaic efficiency (FE) of 72.5% for CO production at -0.6 V vs. RHE and was very stable during 24-hour CO2 electrolysis. The in situ Cu K-edge XANES and Fourier transformed EXAFS spectra dynamically revealed the direct transformation of Cu2+ to Cu0 species within few minutes at -0.6 V vs. RHE and CuO was entirely reduced to metallic copper after 60 min. Such an observation was well consistent with the in situ time-resolved XRD patterns which illustrated the gradual emergence of (111), (200), (220), (311), and (222) diffractions of cubic copper under the working condition. CO2-to-CO outperformance of CuO-HIO catalyst over typical oxidized copper catalysts could be ascribed to (i) the mass transport limitation and local pH effect induced by unique hierarchical morphology, (ii) highly roughened surface, and (iii) in situ reduction of Cu2+ to stabilized metallic Cu active sites during the CO2 electrolysis. Our findings may open an opportunity for rational design of promising, high-performance, less expensive electrocatalysts for renewable fuel production from fossil fuel-generated CO2 emission.