Gold Deposited Gas Diffusion Electrodes by RF Sputtering and Their CO2 Reduction Activity to CO
Authors: Yamada Takuya , Iwase Kazuyuki , Todoroki Naoto , Honma Itaru 
Published: December 2024

The electrochemical CO2 reduction reaction (CO2RR) is attracting attention as a promising method to convert CO2 into value-added products such as CO, which is used as a feedstock in the Fischer-Tropsch process. Previous research demonstrated that precious metal catalysts, such as Ag or Au, proceed CO2 reduction to CO generation with high efficiency[1]. Despite their high Faradaic efficiency (current efficiency) and current density in using porous gas diffusion electrode (GDE) systems, a higher utilization efficiency of those catalysts is needed due to their high cost of those catalysts. Based on the above background, in the present study, we employed the sputtering process as a direct formation method of catalysts’ nanoparticles on the catalyst layer on GDEs. We used gold as a model catalyst, fabricated Au-supported GDEs (Au-GDEs), and evaluated their CO2RR activity.

Au-GDEs were fabricated using an RF sputtering process, and Au nanoparticles were directly deposited onto commercially available GDEs. The thicknesses of the catalysts’ layer were controlled by adjusting the sputtering time. The Au-modified GDEs with the conventional spray-coating method were also prepared using Au nanoparticles synthesized by a conventional solution process using citric acid as the reduction agent(Au_Spray) [2] for comparison. The CO2RR activity was evaluated by galvanostatic measurements using 1 M KHCO3 as electrolytes.

Fig. 1(a) shows a cross-section scanning electron microscopy (SEM) image of an Au-GDE with a typical catalyst layer thickness of 100 nm (Au_100). The catalyst layer composed of Au (the white part of the figure) was uniformly formed on the surface of GDE. The X-ray absorption structure (XAS) analyses revealed that the valence state of Au was Au(0), and that the catalyst layer on the GDEs was composed of Au nanoparticles. Next, the CO2RR activity of the Au-GDE was evaluated. Carbon monoxide (CO) was detected as the main CO2RR product, and formate (HCOO−) and hydrogen (H2) as a side reaction product were also detected. The effect of film thickness of Au-GDE on CO2RR selectivity was evaluated at a current density of 50 mA cm-2. Au-GDE with a film thickness of 10 nm (Au_10) and Au_100 produced CO with the Faradaic efficiencies (FEs) of 72.7 % and 79.8 %, respectively. The FEs of hydrogen evolution reaction (HER) of Au_10 and Au_100 were 26.1% and 10.3%, respectively. The CO efficiencies for Au_10 and Au_100 were significantly higher than those of Au_Spray, which had a similar Au loading to that of Au_100. The higher CO selectivity of the Au-GDEs prepared by sputtering might be attributed to the higher coverage of Au on GDEs, which was confirmed by SEM-EDX mapping.

The potential dependence of partial current density for CO (j
CO) normalized by Au mass loading was evaluated as an indicator of Au utilization efficiency for CO2RR (Fig. 1(b)). The electrode with a catalyst layer thickness of 10 nm showed the 1882 A g-1 (potential of -0.85 V vs. RHE). As far as we know, this is the highest value among the Au-based electrodes using GDEs [3]. The value is about one order of magnitude higher than that of Au_Spray. This high CO production activity can be attributed to the formation of a thin Au catalyst layer composed of Au nanoparticles on the electrode surface by sputtering. [4] The results of the electrode structure analysis and details of the CO2RR activity will be discussed in the presentation.

[1]A. Fen wick et. al., ACS Energy Lett., 2022, 7, 871−879 [2] X. Ji et. al., J. Am. Chem. Soc., 2007, 129, 13939–13948 [3] M. Sassenburg et. al., ACS Appl. Energy Mater., 2022, 5, 5983–5994. [4] T. Yamada
et. al., in preparation.

Figure 1