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By means of an initial electrochemical carbon dioxide reduction reaction (eCO 2 RR), both the reaction current and Faradaic efficiency of the eCO 2 RR on boron-doped diamond (BDD) electrodes were significantly improved. Here, this effect is referred to as the self-activation of BDD. Generally, the generation of carbon dioxide radical anions (CO 2 •– ) is the most recognized pathway leading to the formation of hydrocarbons and oxygenated products. However, the self-activation process enabled the eCO 2 RR to take place at a low potential, that is, a low energy, where CO 2 •– is hardly produced. In this work, we found that unidentate carbonate and carboxylic groups were identified as intermediates during self-activation. Increasing the amount of these intermediates via the self-activation process enhances the performance of eCO 2 RR. We further evaluated this effect in long-term experiments using a CO 2 electrolyzer for formic acid production and found that the electrical-to-chemical energy conversion efficiency reached 50.2% after the BDD self-activation process.

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The electrochemical reduction of carbon dioxide (CO 2 ) has been attracting great interest in these days. In particular, the electrochemical production of formic acid from carbon dioxide in a halogen-free electrolyte is practically important for recently developed applications such as hydrogen carriers and fuel cells. In the present work, we have demonstrated highly selective electrochemical CO 2 reduction in a halogen-free electrolyte by an "activation" process which involves an initial electrochemical CO 2 reduction reaction at a boron-doped diamond (BDD) electrode. The BDD activation increased the Faradaic efficiency of formic acid from 10 to 90%. A mechanistic study on the BDD activation is presented, which can explain the drastic change of reaction selectivity from the perspective of reducing the energy of electron transfer from the BDD electrode and the mass transport of CO 2 molecules to the electrode surface.

We investigated the electrochemical CO2 reduction reaction (CO2RR) properties of boron- and nitrogen-codoped diamond (BNDD) electrodes. The BNDD electrodes exhibited higher CO2RR performance than boron-doped diamond (BDD) electrodes, indicating that the nitrogen doping can promote CO2RR reactivity. In particular, the faradaic efficiencies of the CO2RR products, obtained at a relatively low negative potential (−2.0 V vs Ag/AgCl) in this study, were significantly higher using BNDD electrodes with high nitrogen-doping levels. At this potential, it should be difficult to generate CO2 •– as an initial intermediate of CO2RR; therefore, nitrogen doping promoted the generation of the intermediate through another reaction pathway. Linear sweep voltammetry revealed that nitrogen doping could promote the generation of adsorbed hydrogen atoms on the electrode surface by proton reduction. Thus, we concluded that nitrogen doping promoted the initial step of CO2RR through the adsorption of hydrogen atoms on the electrode surface. In addition, electrolysis using various electrolyte solutions confirmed that the influence of dopants on the BNDD electrode surface was significant for CO2RR, particularly at low overpotentials.