Operando Measurements of Copper Catalyst Morphology in Gas Diffusion Electrodes during Electrochemical CO2 Reduction for the Formation of C2+ Compounds
Authors: Takamatsu Daiko , Fukatani Naoto , Yoneyama Akio , Hirano Tatsumi , Takahashi Hiromasa , Yabuuchi Shin , Watanabe Koichi 
Published: December 2024

Carbon capture and conversion technologies have been widely studied as promising approaches to solving the global warming problem. The conversion of CO2 to carbon-based chemicals and fuels by electrochemical CO2 reduction (CO2RR) using renewable electricity is one of the attractive strategies to achieve carbon neutrality. Among the various products of CO2RR, multi-carbon products (C2+) such as ethylene and ethanol are essential chemicals with high energy density and value-added. Various metals, alloys, and metal oxides have been widely studied as electrocatalysts for CO2RR, but copper (Cu) and its derivatives are the only electrocatalysts with selectivity for the formation of C2+ compounds [1]. Current electrocatalysts for CO2RR have limitations concerning current density enhancement, faradaic efficiency (FE) of the desired product, and catalyst durability. The use of gas diffusion electrodes (GDEs), which allow CO2RR to occur at the solid catalyst/liquid electrolyte/gaseous CO2 interface, can effectively accelerate CO2RR and solve the problem of limited mass transport due to the inherently low diffusivity and solubility of CO2 in water. Recently, high C2+ product selectivity was achieved at current densities above 100 mA/cm2 using GDEs with Cu-based catalysts [2,3]. However, the influence of the catalytic morphology of GDEs on reactivity and product selectivity is not fully understood yet because of the complex CO2RR mechanism and catalyst structure in GDEs. In this study, in situ and ex situ synchrotron radiation spectroscopy and imaging techniques were used to investigate the dynamic behavior of the catalyst morphology and electrolyte permeability changes inside Cu-GDEs during CO2RR.

Cu(x)-GDEs were prepared by magnetron sputtering of Cu on a commercial carbon-based gas diffusion layer (GDL) with a microporous layer (MPL). Cu thicknesses of x = 70, 300, and 1000 nm were used. CO2RR experiments were performed in a homemade three-compartment flow cell using a neutral electrolyte (e.g. KCl aq, KHCO3 aq) with Cu-GDE as the working electrode, Pt mesh as the counter electrode, and Ag/AgCl as the reference electrode. The FEs of each product were obtained by the constant current CO2 electrolysis (chronopotentiometry, CP). The gas products were quantified using gas chromatography with a thermal conductivity detector. As for the liquid products, alcohols were evaluated using gas chromatography with a flame ionization detector, and organic acids were detected by high-performance liquid chromatography with a conductivity detector, respectively. The in situ and ex situ evaluation of Cu-GDE before, during, and after CO2RR was performed by scanning X-ray fluorescence microscopy (SXFM), X-ray computed tomography (CT), and X-ray absorption spectroscopy (XAS). Synchrotron radiation experiments were carried out at SPring-8 BL16XU/B2, PF BL14B, and saga-LS BL07, respectively.

Figure (a) shows the FEs of CO2RR products as a function of total current density (Jtotal) with the Cu(300)-GDE. FEs for C2+ products (FEC2+), including C2H4, C2H5OH, CH3COOH, C3H7OH, reached up to 82% at Jtotal = 400 mA/cm2. When the total current density increased above 1000 mA/cm2, FEC2+ decreased and FEH2 increased. This suggests that the state of the triple-phase boundary changes with current density during electrolysis. To elucidate the following questions: i) what morphology of Cu catalyst leads to high C2+ selectivity, and ii) why FEH2 increases at high current densities, the internal structure of the Cu-GDE after electrolysis was observed using X-ray CT. Figures (b), and (c) show ex situ CT slice images of Cu(70)-GDE before and after CO2 electrolysis at CP 400 mA/cm2. In the CT images contrasting the absorption coefficients corresponding to the electron density, the denser Cu and K are observed brighter than carbon (MPL/GDL). Before CO2 electrolysis, Cu was deposited on the MPL surface with a uniform thickness (Fig. (b)), but after CP 400 mA/cm2, Cu on the MPL surface decreased and bright spots were dispersed within the MPL region (Fig. (c)). Elemental discrimination imaging using spectral CT and SXFM confirmed that Cu and K were dispersed within the MPL region after CO2 electrolysis, while only K and no Cu were observed within the MPL region after electrolysis with Ar gas supply. These results suggest that the morphology of the Cu catalyst within the GDEs changes from its initial state during CO2RR. In the presentation, we also report on the operando SXFM and XAS during CO2 and Ar electrolysis and discuss the effect of changes in the morphology and oxidation state of Cu catalyst on C2+ selectivity in Cu-GDE during CO2 electrolysis.

[1] S. Nitopi, et al., Chem. Rev., 119, 7610 (2019).

[2] G. Zhang, et al., Nat. Commun., 12, 5745 (2021).

[3] A. Inoue, et al., EES Catal., 1, 9 (2023).

Figure 1