Biocompatible Anodes for Electrochemical CO Reduction

Introduction: Hybrid Electro-Bio Carbon Utilization Systems (HEBCUS) aim to convert CO₂ into value-added products via electrocatalytic and biological means. CO electrolysis, an intermediate to CO₂ conversion, requires a highly alkaline electrolyte. Long-term CO electrolysis produces neutral and acidic conditions, oxidizing Ni-based anodes. Our aim is to find low-cost, robust anode materials viable in neutral pH conditions and an NH₄OH-based electrolyte to allow for higher acetate accumulation and biocompatibility.

Methods: All experiments used a Membrane Electrode Assembly (MEA) electromechanical cell configuration using a CuO-based cathode catalyst. Ni, IrO_x, and Co anodes were tested in 1 M KOH (pH 14), 0.5 M K₂SO₄ (pH 7), and 0.8 M NH₄OH + 0.2 M KOH (pH 13.5) using a variety of current densities. Gas products were characterized using gas chromatography. Liquid samples were characterized via Nuclear magnetic resonance spectroscopy. Cell voltage was noted, and Faradaic efficiencies for the resulting products were calculated. The effect of CuO cathode age was also tested.

Results: Co rivaled Ni for acetate production with low cell voltage in 1 M KOH. IrO_x had less favorable acetate production and relatively high cell voltage. In 0.5 M K₂SO₄, both Ni and Co oxidized while IrO_x remained stable. The age of CuO has a significant effect on the product distribution, with new CuO cathodes producing more acetate than older ones. Acetate production in 0.8 M NH₄OH + 0.2 M KOH using a Ni anode and a new CuO cathode is favorable.

Conclusions: IrOx is a viable biocompatible anode option. New CuO cathodes produce more acetate, less ethylene, and less 1-propanol than older ones. Replacing up to 80% of KOH with NH₄OH for CO electrochemical reduction shows promise.