Efficient electrolyzer for CO₂ splitting in neutral water using earth-abundant materials

Low-cost, efficient CO₂-to-CO+O₂ electrochemical splitting is a key step for liquid-fuel production for renewable energy storage and use of CO₂ as a feedstock for chemicals. Heterogeneous catalysts for cathodic CO₂-to-CO associated with an O₂-evolving anodic reaction in high-energy-efficiency cells are not yet available. An iron porphyrin immobilized into a conductive Nafion/carbon powder layer is a stable cathode producing CO in pH neutral water with 90% faradaic efficiency. It is coupled with a water oxidation phosphate cobalt oxide anode in a home-made electrolyzer by means of a Nafion membrane. Current densities of approximately 1 mA/cm² over 30-h electrolysis are achieved at a 2.5-V cell voltage, splitting CO₂ and H₂O into CO and O₂ with a 50% energy efficiency. Remarkably, CO₂ reduction outweighs the concurrent water reduction. The setup does not prevent high-efficiency proton transport through the Nafion membrane separator: The ohmic drop loss is only 0.1 V and the pH remains stable. These results demonstrate the possibility to set up an efficient, low-voltage, electrochemical cell that converts CO₂ into CO and O₂ by associating a cathodic-supported molecular catalyst based on an abundant transition metal with a cheap, easy-to-prepare anodic catalyst oxidizing water into O₂.