Highly active oxygen evolution integrated with efficient CO₂ to CO electroreduction

Electrochemical reduction of CO₂ to useful chemicals has been actively pursued for closing the carbon cycle and preventing further deterioration of the environment/climate. Since CO₂ reduction reaction (CO₂RR) at a cathode is always paired with the oxygen evolution reaction (OER) at an anode, the overall efficiency of electrical energy to chemical fuel conversion must consider the large energy barrier and sluggish kinetics of OER, especially in widely used electrolytes, such as the pH-neutral CO₂-saturated 0.5 M KHCO₃. OER in such electrolytes mostly relies on noble metal (Ir- and Ru-based) electrocatalysts in the anode. Here, we discover that by anodizing a metallic Ni–Fe composite foam under a harsh condition (in a low-concentration 0.1 M KHCO₃ solution at 85 °C under a high-current ∼250 mA/cm²), OER on the NiFe foam is accompanied by anodic etching, and the surface layer evolves into a nickel–iron hydroxide carbonate (NiFe-HC) material composed of porous, poorly crystalline flakes of flower-like NiFe layer-double hydroxide (LDH) intercalated with carbonate anions. The resulting NiFe-HC electrode in CO₂-saturated 0.5 M KHCO₃ exhibited OER activity superior to IrO₂, with an overpotential of 450 and 590 mV to reach 10 and 250 mA/cm², respectively, and high stability for >120 h without decay. We paired NiFe-HC with a CO₂RR catalyst of cobalt phthalocyanine/carbon nanotube (CoPc/CNT) in a CO₂ electrolyzer, achieving selective cathodic conversion of CO₂ to CO with >97% Faradaic efficiency and simultaneous anodic water oxidation to O₂. The device showed a low cell voltage of 2.13 V and high electricity-to-chemical fuel efficiency of 59% at a current density of 10 mA/cm².