Pretreatment of diamond electrodes toward CO₂ electroreduction at high current density

Boron-doped diamond (BDD) electrodes are promising candidates for long-term and scalable CO₂ electroreduction, a key technology to realize a sustainable society. BDD electrodes have been shown to maintain a high Faradaic efficiency in CO₂ electroreduction for over 1,000 h of continuous operation. Furthermore, a preliminary CO₂ electroreduction step at low current density performed prior to main electrolysis (hereafter referred to as pre-electrolysis treatment) has been proposed as an effective surface treatment unique to BDD electrodes. However, it remains unclear whether this pre-electrolysis treatment is still effective for CO₂ electroreduction at high current density. In this work, we optimized a pre-electrolysis treatment protocol for BDD electrodes to achieve an efficient CO₂ electroreduction even at high current density. X-ray photoelectron spectroscopy studies revealed that the proportion of C–O/C=O and COOH bonding states at the BDD surface were increased after the pre-electrolysis treatment. Consequently, linear sweep voltammetry studies in a CO₂-saturated electrolyte solution showed that the BDD electrode after the pre-electrolysis treatment exhibited a significant increase in current compared to the BDD electrode without the pre-electrolysis treatment. We propose that interactions between CO₂ molecules in the electrolyte solution and the oxygen-containing functional groups introduced at the BDD surface by the pre‑electrolysis treatment enhances the local CO₂ concentration, thereby promoting the CO₂ electroreduction. With the optimized pre-electrolysis treatment protocol, the Faradaic efficiency for formic acid production was enhanced from 19% to 71%, representing a 3.7-fold increase.