Enhanced performance of diamond electrodes with heavily N-doped surface nanolayers grown by CVD for high reduction current density

• Fundamental electrochemical behavior of diamond electrodes with heavily N-doped surface nanolayers. • Polarization curve measurements for hydrogen evolution reaction in the dark and upon visible-light irradiation. • Comparison between N-doped diamond layer with and without detonation-synthesized nanodiamonds (DNDs). • Electron transfer at the electrode surface was limited with DNDs. • Effective conversion of visible-light energy to the photovoltaic effect was observed without DNDs. The diamond electrode with a heavily N-doped surface nanolayer has the potential to revolutionize not only the production of CO from CO 2 reduction, but also the production of various chemicals using reduction reactions. However, the current density attributed to the reduction reaction remains insufficient for industrial requirements. In this study, we investigated the fundamental electrochemical behavior of diamond electrodes with heavily N-doped surface nanolayers through polarization curve measurements for hydrogen evolution reaction in the dark and upon visible-light irradiation. Two types of layers, a chemical vapor deposition (CVD)-processed diamond layer containing detonation-synthesized nanodiamonds (DNDs) and a heavily N-doped CVD-processed diamond (NDD) layer without DNDs, were used as heavily N-doped surface nanolayers and compared mainly in terms of the electron transfer coefficient and visible-light responsiveness. Electron transfer at the electrode surface for the DND-based layer was limited, whereas it was unrestricted for the NDD layer; however, the current density was suppressed in the larger negative-current-density region. Moreover, the NDD layer enabled a more effective conversion of visible-light energy to the photovoltaic effect than the DND-based layer. These results suggest that the electrode surface is better capped by less-defective diamond crystals, whereas the N-doped layer has sufficient conductivity to enable a larger reduction current density. Such a diamond electrode could revolutionize the research and development in the field of diamond green chemistry.