Chlorine in NiO promotes electroreduction of CO₂ to formate

We report the exceptional finding that NiO, a known electroactive catalyst for the reduction of CO₂ to CO, can be tuned to become an active electrocatalyst for CO₂ reduction to formate when chlorine is synthetically incorporated into NiO. The CO₂ reduction reaction (CO₂RR) is carried out on chlorine-containing NiO octahedral particles made by a solid-state synthesis method yielding a Faradaic efficiency (FE) of 70 % for formate production at -0.8 V vs. RHE with a partial current density of 14.7 mA/cm². XPS confirms the presence of Ni³⁺ and Ni²⁺ species, indicating the existence of uncoordinated Ni. The Ni³⁺/Ni²⁺ ratio increases with the Cl concentrations on NiO. Cl concentrations are also confirmed with STEM-EDX. DFT modeling provides insights into the thermodynamic stability and CO₂RR mechanism over the Cl-containing NiO surface. It is suggested that Cl can occupy the defective sites created by oxygen vacancies on the NiO model with Cl (O-alpha+Cl). The surface Pourbaix diagrams constructed from DFT indicate the preferred surface terminations favorable at the operating conditions for the CO₂RR, which closely agrees with the experimental findings. The O-alpha+Cl has been found to promote CO₂RR to formate.