Interfacial engineering for redirecting CO2 electroreduction selectivity on boron-doped diamond: from formic acid to carbon monoxide

Abstract

The electrochemical carbon dioxide reduction reaction (eCO2RR) on boron-doped diamond (BDD) electrodes predominantly yields formic acid (HCOOH) as the primary product. Redirecting the reaction pathway toward highly valuable carbon monoxide (CO) remains a significant challenge due to the chemical inertness of the BDD surface. Herein, we present a comprehensive interfacial engineering strategy to tune the product selectivity from HCOOH to CO on BDD electrodes. First, functionalisation of the BDD surface with nitrogen-containing molecules such as aniline and pyrazole effectively captured CO2, successfully directing the reaction pathway toward CO. Second, utilising an aqueous KClO4 electrolyte dynamically facilitated CO production by minimising specific anion adsorption. Through systematic optimisation of the eCO2RR conditions in 0.1 M KClO4, we achieved a maximum Faradaic efficiency for CO production (FECO) of 72% at an applied potential of −1.9 V (vs. Ag/AgCl), an electrolyte flow rate of 500 mL min−1, and an operating temperature of 5 °C. Electrochemical analyses revealed that increasing the flow rate enhanced the CO2 supply by significantly reducing the diffusion layer thickness. Moreover, decreasing the temperature not only increased CO2 solubility but also selectively suppressed the competing hydrogen evolution reaction (HER). These findings provide fundamental insights into the reaction mechanisms on inert sp3-carbon surfaces and demonstrate the vast potential of interfacial engineering for developing highly selective and stable CO2 conversion systems.