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 an 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, and established molecular modification as a proof of concept for redirecting selectivity. Second, utilising an aqueous KClO₄ electrolyte dynamically facilitated CO production by minimising specific anion adsorption, providing a considerably more facile and practical route to the same selectivity switch. Through systematic optimisation of the eCO₂RR conditions in 0.1 M KClO₄, we achieved a maximum faradaic efficiency for CO production (FE_CO) of 72% at an applied potential of -1.9 V (vs. Ag/AgCl; -1.46 V vs. RHE), 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). Temperature-dependent linear sweep voltammetry experiments further revealed that this suppression is driven primarily by a negative shift of the HER onset potential rather than by any appreciable shift of the CO₂ reduction onset. These findings provide fundamental insights into the reaction mechanisms on inert sp³-carbon surfaces and demonstrate that interfacial engineering is an effective strategy for developing selective CO₂ conversion systems on BDD, although practical implementation will require further advances in electrode fabrication, operating conditions, and energy efficiency.