High-Yield and Selective Photoelectrocatalytic Reduction of CO₂ to Formate by Metallic Copper Decorated Co₃O₄ Nanotube Arrays

Abstract

Carbon dioxide (CO₂) reduction to useful chemicals is of great significance to global climate and energy supply. In this study, CO₂ has been photoelectrocatalytically reduced to formate at metallic Cu nanoparticles (Cu NPs) decorated Co₃O₄ nanotube arrays (NTs) with high yield and high selectivity of nearly 100%. Noticeably, up to 6.75 mmol·L^–1·cm^–2 of formate was produced in an 8 h photoelectrochemical process, representing one of the highest yields among those in the literature. The results of scanning electron microscopy, transmission electron microscopy and photoelectrochemical characterization demonstrated that the enhanced production of formate was attributable to the self-supported Co₃O₄ NTs/Co structure and the interface band structure of Co₃O₄ NTs and metallic Cu NPs. Furthermore, a possible two-electron reduction mechanism on the selective PEC CO₂ reduction to formate at the Cu–Co₃O₄ NTs was explored. The first electron reduction intermediate, CO_2 ads^•–, was adsorbed on Cu in the form of Cu–O. With the carbon atom suspended in solution, CO_2 ads^•– is readily protonated to form the HCOO^– radical. And HCOO^– as a product rapidly desorbs from the copper surface with a second electron transfer to the adsorbed species