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.
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