Cu Vacancy Induced Product Switching from Formate to CO for CO2 Reduction on Copper Sulfide

Cu is commonly modified with sulfur to obtain high selectivity for formate since S can promote the formation of the key *OCHO intermediate along the formate pathway. In the present work, we demonstrate that Cu-vacancies on copper sulfide can surprisingly switch the formate pathway to the CO pathway, and the concentration of Cu vacancies can deterministically regulate the CO faradaic efficiency and partial current density. The JCO of SNC@Cu1.96S (Cu1.96S coated sulfur, nitrogen-co-doped carbon) can reach 37.2 mA cm–2 in an H cell, which is the highest among the Cu-based catalysts and comparable to other top CO production catalysts. According to DFT calculations, the Cu vacancies formed in copper sulfide change the electronic structures of the S sites in such a way that the H* takes a large Gibbs free energy, which in turn suppresses the formation of formate. However, the resulting fewer surface Cu cations and more surface S anions weakens the adsorbate–metal interaction, synergizing the adsorption structural transition of the surface intermediates from *OCHO (two O–Cu bonds) to *COOH (one C–Cu bond) in favor of CO production.