Electrochemical CO₂ Reduction over Nanoparticles derived from an Oxidized Cu–Ni Intermetallic Alloy

Research and development on the electrocatalytic CO₂reduction reaction (CO₂RR) to valuable hydrocarbons and alcohols has attracted great interest because of the potential to produce sustainable fuels. To date, copper (Cu) and Cu-based compounds are the most studied metal electrocatalysts that have demonstrated the ability to convert CO₂ and CO into large amounts of C2 and C3 molecules [1]. Furthermore, studies of Cu particle catalysts have reported that their particle size, crystalline phase, and morphology are highly correlated with C2 selectivity [2]. As one approach to control C2 selectivity on Cu catalysts, the use of Cu-based bimetallic alloys has been reported [3], especially the combination of Cu with CO-producing metal species such as Ag, Au, Zn, and Pd. On the other hand, combinations of Cu with hydrogen-producing elements such as Fe, Ni, and Pt have been reported to improve the activity and selectivity of hydrogen production compared to pure Cu electrodes, making it difficult to improve C2 and C3 activity with these combinations.<br/>In this study, we demonstrate that Ni species have a positive effect on the formation of C2 compounds in the CO₂RR of Cu-based catalysts. To avoid size-dependent selectivity of copper catalysts, Cu-Ni interatomic alloy NPs (0-83 at% Ni as Ni/(Cu + Ni)) of various compositions with diameters around 10 nm were synthesized by one-pot and two-step synthesis using copper(II) acetylacetonate and nickel(II) chloride [4]. Since these Cu-Ni NPs were easily spontaneously oxidized in air at room temperature, these partially oxidized NPs were loaded onto carbon paper supports and used as electrocatalyst.<br/>The electrochemical CO₂ reduction reaction was evaluated using a three-electrode system in a closed batch single-compartment reactor filled with 0.05 M KHCO₃ aqueous solution bubbled with CO₂. Electrolysis was performed at -1.2 V (vs. RHE) using a catalytic electrode as the working electrode, a Pt wire as the counter electrode, and an Ag/AgCl electrode as the reference electrode. The CuNi-NP was driven as a catalyst for hydrogen production in the early stages of the electrolysis, but activated into an excellent CO₂ reduction catalyst in the subsequent electrolysis. The dependence of the C2 selectivity on the Ni content was investigated using various activated Cu–Ni NPs electrodes. Activated Cu–Ni (3–32 at%-Ni) alloy nanoparticles enhance selectivity for ethylene and ethanol formation over oxide-derived Cu nanoparticles by electrochemical CO₂RR in aqueous solution. The Cu–Ni (19 at%) NPs exhibited 35% efficiency for C2 and the suppression of H₂ formation down to 9%. These results demonstrate that the addition of Ni to the Cu NPs facilitates C–C bond formation. The activated catalyst consists of metallic Cu, Cu–O, Ni, and Ni–O species, as confirmed by X-ray absorption spectroscopy measurements [5].<br/>The present research suggests that further investigation is worthwhile to broaden the possibility of an unprecedented oxide-derived Cu–Ni and other CO₂RR catalysts.<br/><br/><b>References</b><br/>[1] Y. Hori, et al., <i>Chem. Lett.,</i> (1985) 1695-1698. [2] A. Loiudice, et al., <i>Angew. Chem. Int. Ed.,</i> 55 (2016) 5789-5792. [3] S. Nitopi, et al., <i>Chem. Rev.,</i> 119 (2019) 7610-7672. [4] R. Watanabe, T. Ishizaki, <i>J. Mater. Chem. C.,</i> 2 (2014) 3542-3548. [5] T. M. Suzuki, T. Morikawa, et al., <i>Chem. Commun.,</i> 56 (2020) 15008-15011.