(Invited) Preparation of Aucu Alloy Nanoparticles Via Ionic Liquid/Metal Sputtering for Electrocatalytic CO2 Reduction
Authors: Torimoto Tsukasa , Ito Ryuichi , Akiyoshi Kazutaka , Kameyama Tatsuya , Yui Yuhki , Ibe Masaya 
Published: December 2024

Development of highly active electrocatalysts for carbon dioxide reduction has been intensively investigated for the efficient utilization of CO2 and the lowering of electric energy consumption. Among various strategies, alloy nanoparticles (NPs) have attracted significant interest due to their tunable catalytic properties, which can be adjusted by altering their chemical composition and particle size. Recently, we reported novel preparation methods of metal nanoparticles through the sputter deposition of metals onto room-temperature ionic liquids (RTILs), a technique we termed “RTIL/metal sputtering technique”. This method enabled the formation of metal and alloy NPs, such as Au, Ag, Pd, Pt, AuAg, AuCu, AuPt and AuRh,(1-6) with sizes in the nanometer ranges, in the absence of additional stabilizing agents. Here we report the preparation of AuCu alloy NPs via RTIL/metal sputtering and evaluate their electrocatalytic activity for CO2 reduction as a function of the alloy composition.

The simultaneous sputtering of Au and Cu onto RTILs produced AuCu alloy NPs with the size ranging from 3 to 4 nm. Subsequent loading of these sputter-deposited AuCu NPs in RTIL on carbon black powders (CB) not only increased the AuCu NP size to 3-7 nm but also facilitated alloying between Au and Cu. The chemical composition of AuCu NPs was controlled by changing the area fraction of Au plates in Au-Cu binary targets used for sputtering. The XRD analysis revealed that thus-obtained AuCu NPs had the fcc structure, in which each peak shifted to a lower angle with an increase in the Au fraction.

The prepared AuCu/CB powers were immobilized on glassy carbon electrodes and used for the electrocatalyst for CO2 reduction in a 0.5 M KHCO3 aqueous solution. Cyclic voltammetry revealed that cathodic currents were observed at potentials more negative than -1.2 V, with the current intensity being higher under a CO2 atmosphere compared to an N2 atmosphere. This confirms the CO2 reduction activity of AuCu nanoparticles. Constant potential electrolysis was carried out at -2 V vs. Ag/AgCl in a CO2 atmosphere. As CO2 reduction products, CO and HCOOH were formed. The selectivity of these products was remarkably influenced by the composition of AuCu alloy NPs: The HCOOH selectivity increased with a higher Cu ratio, while the CO selectivity showed a volcano-type relationship, reaching an optimum at the Au fraction in NPs of ca. 0.8. H2 was also produced as a byproduct, with its selectivity being minimized at the Au fraction of 0.6-0.9. In conclusion we demonstrated the importance of alloy composition in optimizing the electrocatalytic performance of AuCu NPs for CO2 reduction.

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