Silver Nanocluster Assemblies As Electrocatalyst for Electrochemical Carbon Dioxide Reduction Reaction
Authors: Das Subhabrata , Takahashi Shuntaro , Das Saikat , Negishi Yuichi 
Published: December 2024

Recent rise in the consumption of fossil fuels have raised the levels of carbon dioxide (CO2) in the atmosphere causing global warming1. Electrochemical reduction of carbon dioxide (CO2RR) to value-added chemicals at low temperature and ambient pressure using green renewable energy sources has emerged as a propitious technology to realize carbon neutrality and mitigate global warming2.
Supramolecular assembly of noble metal nanoclusters (NMNC) is one of the alluring areas of modern material science. Nanocluster assemblies formed by integrating NMNC using a crosslinking ligand, are reported to be applied in various applications such as nanodevices and as luminescent sensor materials3. Silver nanocluster assemblies (Ag-CAM) are composed of regularly crosslinked silver nanoclusters4 and were evaluated as cathodic electrocatalysts for CO2RR. In this study, we evaluated the reaction selectivity and structural stability of an Ag-CAM ([Ag12(StBu)6(CF3COO)6(2EB)3]n; Ag12-2EB) crosslinked by linker with triple bonds5 when used as a cathode electrocatalyst (Figure 1).
Synthesized Ag-CAMs were supported onto the surface of commercially available gas diffusion electrode (GDE) by spray coating method (Figure 2). The catalyst-loaded electrodes were attached to a self-fabricated GDE flow cell, and electrocatalytic properties were evaluated. Ag/AgCl was used as the reference electrode, and Pt mesh was taken as the counter electrode for the electrochemical reactions. Single crystal-XRD and powder-XRD were measured to determine the exact crystal structure, while SEM-EDS was used to characterize the electrode surface to determine the structural stability. We used gas chromatography (GC) and 1H-nuclear magnetic resonance (NMR) spectroscopy to calculate the reaction selectivity and quantify the product concentration. The CO2RR products in outlet gas streams from the gas chamber and electrolytes from the catholyte room were analyzed using GC and 1H-NMR spectroscopy.
For silver nanocluster assembly cross-linked with 4,4`-bipyridine ([Ag12(StBu)8(CF3COO)4(BPY)4]n; Ag12-BPY) most of the constituent elements of the aggregate (S, N, F, etc.) were eluted out after performing the electrochemical (EC) reactions up to a voltage of 3.2V. The catalyst structure underwent significant transformations and was confirmed by SEM-EDS analysis with samples before and after EC analysis. However, in EC experiments using Ag12-2EB as the cathode electrocatalyst, the structural decompositions were suppressed and the electrocatalyst could withstand long-term use. Figure 3 shows the Faraday selectivity (%) of CO2RR products against the applied potential for Ag12-2EB/GDE in 0.1M KHCO3. Carbon monoxide (CO) was the main product obtained from electrochemical CO2RR of Ag12-2EB and hydrogen evolution reaction (HER) could be suppressed when the applied potential was shifted to the negative side. Additionally, regarding the reaction selectivity, it was found that the selectivity was approximately the same regardless of the structure of the crosslinking agent. Knowledge obtained from this study will help to bridge the gap for utilizing Ag-CAMs as novel cathode catalyst materials for electrochemical CO2RR experiments.
References
(1) Liu, Y. Y., Huang, J. R., Zhu, H. L., Liao, P. Q., & Chen, X. M. (2023). Simultaneous Capture of CO2 Boosting Its Electroreduction in the Micropores of a Metal–organic Framework. Angewandte Chemie, 135(52), e202311265.
(2) Kawawaki, T., Okada, T., Hirayama, D., & Negishi, Y. (2024). Atomically precise metal nanoclusters as catalysts for electrocatalytic CO 2 reduction. Green Chemistry, 26(1), 122-163.
(3) Nag, A., & Pradeep, T. (2022). Assembling atomically precise noble metal nanoclusters using supramolecular interactions. ACS Nanoscience Au, 2(3), 160-178.
(4) Huang, R. W., Wei, Y. S., Dong, X. Y., Wu, X. H., Du, C. X., Zang, S. Q., & Mak, T. C. (2017). Hypersensitive dual-function luminescence switching of a silver-chalcogenolate cluster-based metal–organic framework. Nature chemistry, 9(7), 689-697.
(5) Das, S., Sekine, T., Mabuchi, H., Hossain, S., Das, S., Aoki, S., Takahashi, S & Negishi, Y. (2023). Silver cluster-assembled materials for label-free DNA detection. Chemical Communications, 59(27), 4000-4003.

Figure 1