Modification of ZnO gas-diffusion-electrodes for enhanced electrochemical CO2 reduction: optimization of operational conditions and mechanism investigation

Abstract

The development of new catalysts based on Earth-abundant materials, with high activity, stability, and selectivity for the electrochemical CO₂ reduction, is of great importance in upgrading greenhouse gases to valuable fuels and feedstocks. In this work, we developed an amine-functionalized ZnO catalyst with improved performance for CO₂ reduction to CO. We observed that electrodes prepared with ZnO modified with amine groups, combined with 25% carbon black as a support, exhibit the highest performance in the electrochemical conversion of CO₂ to CO. Under optimized conditions, we achieved a current density of −130 mA cm⁻², with a faradaic efficiency exceeding 80% at −1.2 V vs. RHE. The XPS and FTIR results confirmed the presence of nitrogen, related to amine groups, on the modified ZnO samples. EIS results confirmed that the presence of amine groups decreased the resistance to charge transfer. We investigated the mechanism of CO₂ reduction using in situ FTIR spectroscopy, at potentials more negative than −0.8 V vs. RHE, there was consumption of CO₂ at 2343 cm⁻¹ and formation of the CO₂⁻ intermediate at 1668 cm⁻¹. Additionally, we confirmed the adsorption of CO₂ on the catalyst surface by detecting bidentate CO₂ at 1378 cm⁻¹. Amino-functionalized ZnO catalyst was stable during 100 h in a membrane electrode assembly (MEA) cell at −50 mA cm⁻² with a faradaic efficiency higher than 70% and a full cell potential at 2.7 V. The catalyst exhibited a remarkable CO₂ reduction performance, and its modification strategy is promising to be applied to different catalysts.