Synthesis of Ag-Sn Intermetallic Compounds via Mechanical Alloying as Selective Electrocatalysts for CO₂ Reduction Reaction

The electrochemical CO₂ reduction reactions (CO₂RR) have attracted attention as a method to convert CO₂ into value-added products using electricity derived from renewable energy sources[1]. In CO₂RR, various products such as carbon monoxide (CO) and formate (HCOO⁻) can be generated. Therefore, for CO₂RR catalysts, the higher selectivity for the desired products and suppression of the competing hydrogen evolution reaction (HER) is important.<br/><br/>Previous research has developed highly active and selective CO₂RR catalysts using various materials, such as polymers, organometallic complexes, and metal nanoparticles[2-4]. Among them, metal alloys catalysts are attracting attention as promising CO₂RR catalysts, as the catalytic activity and product selectivity can be tuned by choice of metal elements and their composition[5-7]. Intermetallic compounds, which have unique crystal structures and composition of metal elements compared to conventional metallic alloys, are expected to exhibit different reactivity and selectivity from conventional solid-solution alloy catalysts due to their unique atomic structures and electronic states. In this study, we synthesized intermetallic compounds composed of silver (Ag) and tin (Sn), which show high CO₂RR selectivity for producing CO and HCOO⁻, and investigated whether the CO₂RR activity and selectivity could be enhanced. Specifically, we employed mechanical alloying as a synthetic method since it has been gaining attention in recent years to synthesize various kinds of materials[8].<br/><br/>Intermetallic compounds of Ag and Sn (Ag-Sn) were synthesized by mechanical alloying using a planetary ball mill using Ag powder (particle diameter 50-80 nm) and Sn powder (particle diameter 60-80 nm) at the desired ratio (Ag: Sn = 3:1 ~ 7:1) as the starting materials. The CO₂RR activity was evaluated by constant current measurements using gas diffusion electrodes as the working electrode and 1 M KHCO₃ as the electrolyte.<br/><br/>The powder x-ray diffraction (XRD) pattern showed that the sample at Ag: Sn = 7:1 can be attributed to the solid solution alloy of Ag and Sn, whereas the XRD pattern for Ag: Sn = 4:1 could be attributed to the ζ phase, which is an intermetallic compound phase. In both samples, no peaks of the unreacted precursor were observed, suggesting that the single phase of the Ag-Sn solid solution and the intermetallic compound phase was successfully synthesized. For the Ag: Sn = 4:1 sample, the concentration of Ag and Sn in the depth direction was analyzed by X-ray photoelectron spectroscopy (XPS) measurement. The results showed that the ratio of Sn on the surface was higher than the original Ag: Sn ratio (4:1). This result suggests that Sn atoms of the Ag: Sn = 4:1 sample are segregated on the surface. In contrast, the sample showed an intermetallic compound phase (ζ phase) as the bulk crystal structure. For CO₂RR activity at a constant current density (53.0 mA/cm²), the synthesized Ag-Sn catalysts mainly produced HCOO⁻. The faradaic efficiency of HCOO⁻ generation for Ag: Sn = 4:1 sample was 84 %. This value was larger than that of the Ag: Sn = 7:1 sample (78%). For comparison, we evaluated the CO₂RR activity of the pure Ag and Sn powders, which were used as precursors. They predominantly produced CO and HCOO⁻ with a respective faradaic efficiency of 89% and 62%. These findings imply that the formation of the ζ-phase, an intermetallic compound phase, may contribute to the generation of HCOO⁻.<br/><br/>[1] K. P. Kuhl <i>et al</i>., <i>J. Am. Chem. Soc</i>., <b>36</b>, 14107-14113 (2014).<br/>[2] Y. Ma <i>et al., Nat. Commun.</i>, <b>13</b>, 1400 (2022).<br/>[3] R. Angamuthu <i>et al</i>., <i>Science</i>, <b>327</b>, 313-315(2010).<br/>[4] R. Reske <i>et al</i>., <i>J. Am. Chem. Soc</i>., <b>136</b>, 6978-6986 (2014).<br/>[5] T. Gunji <i>et al., Chem. Mater.</i>, 32, 6855-6863 (2020).<br/>[6] J. Zeng <i>et al., ACS Appl. Mater. Interfaces</i>, <b>11</b>, 33074-33081 (2019).<br/>[7] K. Iwase <i>et al., Chem. Commun.</i>, <b>58</b>, 4865-4868 (2022).<br/>[8] J. D. Bellis <i>et al</i>., <i>Chem. Mater</i>., <b>33</b>, 2037-2045 (2021).