Investigating the Origin of Enhanced C2+ Selectivity in Oxide-/Hydroxide-derived Copper Electrodes during CO2 Electroreduction

Abstract

Oxide-/hydroxide-derived copper electrodes exhibit excellent selectivity toward C2+ products during electrocatalytic CO2 reduction reaction (CO2RR). However, the origin of such enhanced selectivity remains controversial. Here, we prepared two Cu-based electrodes with mixed oxidation states, namely HQ-Cu (containing Cu, Cu2O, CuO) and AN-Cu (containing Cu, Cu(OH)2). We extracted ultra-thin specimen from the electrodes using a focused ion beam to investigate the distribu-tion and evolution of various Cu species by electron microscopy and electron energy loss spectroscopy. We found that at the steady stage of CO2RR, the electrodes have all been reduced to Cu0, regardless of the initial states, suggesting that the high C2+ selectivities are not associated with specific oxidation states of Cu. We verified this conclusion by control experi-ments, in which HQ-Cu and AN-Cu were pretreated to fully reduce oxides/hydroxides to Cu0, and the pretreated elec-trodes showed even higher C2+ selectivity, compared with their un-pretreated counterparts. We observed that the ox-ide/hydroxide crystals in HQ-Cu and AN-Cu were fragmented into nano-sized irregular Cu grains under the applied nega-tive potentials. Such a fragmentation process, which is the consequence of an oxidation-reduction cycle and does not oc-cur in electropolished Cu, not only built an intricate network of grain boundaries, but also exposed a variety of high-index facets. These two features greatly facilitated the C-C coupling, thus accounting for the enhanced C2+ selectivity. Our work demonstrates that the use of advanced characterization techniques enables investigating the structural and chemical states of electrodes in unprecedented detail, to gain new insights into a widely studied system.