Electrochemical reduction of carbon dioxide in an aqueous solution using phosphorus-doped polycrystalline diamond electrodes

Phosphorus-doped (P-doped) polycrystalline diamond thin films were deposited on a conductive Si substrate by microwave plasma-enhanced chemical vapor deposition (MWCVD), using a liquid source containing phosphorus atoms. The thin films were then used as electrodes for the electrochemical reduction of carbon dioxide (CO₂) dissolved in aqueous solution. The P-doped diamond electrode exhibited approximately 1 V higher overpotential for the hydrogen evolution reaction than conventional boron-doped (B-doped) diamond electrodes in an aqueous electrolyte. A cathodic current attributed to the CO₂ reduction reaction was observed at the P-doped diamond electrode in 0.2 M Na₂SO₄ bubbled with CO₂. An analysis of the soluble products showed that the main reduction product was formic acid. We have experimentally demonstrated that P-doped diamond thin film is ideal as an electrode material for the electrochemical reduction of CO₂ in aqueous solution, as it does not suffer from the interference of hydrogen evolution by water electrolysis.