A facile synthesis of Cu catalysts with multiple high-index facets for the suppression of competing H 2 evolution during electrocatalytic CO 2 reduction

The electrochemical CO reduction reaction (CO RR) over the high-index facets of Cu nanoparticles (NPs) is favourable towards the formation of multi-carbon products, such as hydrocarbons and oxygenates. However, the facile synthesis of Cu NPs with multiple high-index facets remains a great challenge in the research community. Herein, we have prepared numerous Cu catalysts with flat surfaces by electropolishing polycrystalline Cu foils before and after annealing at different temperatures ranging from 200 °C to 1000 °C under an argon atmosphere. The individual electrode crystal orientations were investigated via X-ray diffraction (XRD) and electron backscattering diffraction (EBSD) techniques. As confirmed by the EBSD technique, the formation of high-index facets increases with an increase in the annealing temperature and reaches a high quantity of high-index facets enclosed mainly by (211) and (431) facets with about 94% of those on the electrode annealed at 1000 °C. As a possible application, we have used the different electrodes for CO RR at -1.0 V vs. RHE with special emphasis on the formation of H gas and C1 products. Thus, the electrodes prepared at higher temperatures enable the suppression of competing H evolution due to the increased amount of high-index facets. Moreover, the formation rates of C1 products were inhibited as well at the electrodes with increased number of high-index facets. The drops in the formation rates of both H and C1 products indicate that they are consumed in the chemical reaction to commence the formation of multi-carbon products. However, further study is still required with superior attention on CO RR towards the C product formation at a range of applied potentials.