2, AECOM, Pittsburgh, Pennsylvania, United States
The production of renewable fuels and valuable chemicals from CO2 reduction reaction by highly efficient, selective, Earth-abundant, robust electrocatalytic materials is of vital interest not only to attenuate the climate change but also to find alternative ways to maintain carbon resources. Here we report the hierarchical CuO inverse opal material which is composed of uniform 2D-3D porous network with voids about 200 nm in diameter for promoting electrochemical CO2 reduction (EC-CO2RR) activity. The 3D highly ordered, interconnected mesopores in a hexagonal close packed structure arrange in unique 2D sheet-like architecture. The cycle voltammetry reveals the reduction of monoclinic CuO to Cu2O and metallic Cu in the presence of CO2. The current densities at different constant potentials are very stable, reaching -10 and -27 mA cm-2 at -0.8 and -1.1 V vs. RHE. Such a unique mesostructure exhibits a Faradaic efficiency (FE) for CO production of 63% at -0.8 V vs. RHE with mass-normalized production rate of 28.2 mmol g-1h-1. It is surprising that hierarchical CuO porous structure significantly suppresses the hydrogen evolution reaction, producing H2 with 11-13% of FE only at -1.0 V and -1.1 V vs. RHE. While methane is always found together with CO, ethylene is detected in a higher overpotential region (-0.9 to -1.1 V vs. RHE). The formate is also detected by ion chromatography. The results demonstrate that such a 2D-3D hierarchical mesostructure is beneficial to simultaneous production of CO and other C1-C2 products as well as inhibition of undesirable H2 evolution. The in situ Raman, XRD and X-ray absorption spectroscopy will be further employed to gain deep understanding of the interaction between hierarchical CuO inverse opal and CO2 during the EC-CO2RR as well as the chemical and structural changes of CuO under cycling.