Monitoring the Activation of a Aucu Aerogel Catalyst for Electrochemical CO2 Reduction via in Situ XAS
Authors: Winzely Maximilian , Clark Adam Hugh , Chauhan Piyush , Leidinger Paul Maurice , Fikry Meriem , de Wild Tym , Schmidt Thomas J. 
Published: December 2024

To drive the further development of electrochemical CO2 reduction technologies, there is an urgent need for highly active catalysts that minimize unwanted side reactions and that also possess a large specific surface area. While nanostructured catalysts typically fulfill the latter requirement, they often use porous carbon supports that improve the nanoparticles’ dispersion but can shift the product selectivity towards undesirable H2 formation.[1] This challenge could be solved by using unsupported aerogels consisting of interconnected nanodomain networks of highly porous materials such as nano-wires or -particles, which provide a large surface area while minimizing unwanted side reactions.[2] So far, precious metals such as gold (Au) and silver (Ag) have shown remarkable activity and selectivity as CO2-reduction catalysts for CO production.[3] However, due to the high cost of such noble metals, improving their mass-specific activity is essential. One strategy to attain this, especially for Au, is to reduce the adsorption energy of the catalyst’s surface towards CO by changing the electronic structure of the d-band through alloying with other metals. In this context, ordered AuCu structures have proven to be promising candidates for improving the CO2-to-CO activity and selectivity.[4]

With the motivation to combine both of the above approaches (i.e., Au-alloying with Cu and the absence of a C-support), in this study we present an AuCu aerogel with an average domain size of ≈ 7 nm that exhibits an exceptionally high faradaic efficiency of 87 % for CO at -0.6 V versus the reversible hydrogen electrode (RHE). This corresponds to a ≈ 2-fold higher Au-mass-specific partial current density for CO when compared to an equivalent, monometallic Au aerogel. Notably, this enhanced activity and selectivity are achieved by performing a potential cycling procedure prior to the CO2 reduction potential hold that involves cyclic voltammetry (CV) between 0.1 and 1.7 V vs. RHE at a scan rate of 50 mV/s until a stable voltammogram is achieved. To investigate the changes in the electronic and structural properties which have happened to the catalyst during this potential cycling, we performed an in situ grazing incidence X-ray absorption spectroscopy (XAS) measurements in the course of these CVs. Chiefly, these spectroelectrochemcial tests were carried out in the same cell used for the assessment of the CO2 reduction performance, ensuring for the first time that the mass-transport conditions encountered by the catalyst during these XAS measurements are identical to those in the CO2-reduction activity and selectivity tests. Figure 1 illustrates the changes observed throughout the potential cycling procedure in the Cu K- and Au L3-absorption edges, whereby the acquired spectra were submitted to a multivariate curve resolution (MCR) analysis. For the Cu K spectra, a total of three components were identified to describe the whole dataset, and it becomes evident that as the number of cycles increases, the copper oxide phase (component 1 in Figs. 1b and 1c) diminishes while a metallic phase (component 2, identified as an AuCu alloy through EXAFS fitting) becomes more prominent. In the case of the Au L3 data, only two components were discerned to describe the dataset, and both of them were identified as distinct AuCu alloy phases through EXAFS fitting. With an increasing number of cycles, component 2 (featuring a higher oxide content than component 1) becomes dominant at positive potentials, suggesting an increasing Au content on the aerogel’s surface as the potential cycling procedure progresses.
In summary, in this contribution we present an AuCu aerogel catalyst exhibiting a high activity and selectivity for CO production that is achieved by cyclic voltammetry treatment prior to holding CO2 reduction potential. The results derived from the in situ XAS measurements on this material indicate that this process effectively removes the copper oxide domains initially present in the catalyst, forming a novel AuCu alloy phase while simultaneously enriching the aerogel’s surface with Au.

References

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Cai, B. and A. Eychmuller, Promoting Electrocatalysis upon Aerogels. Adv Mater, 2019. 31(31): p. e1804881.

Hori, Y.M., A.; Kikuchi, K.; Suzuki, S, Electrochemical Reduction of Carbon Dioxides to Carbon Monoxide at a Gold Electrode in Aqueous Potassium Hydrogen Carbonate. J. Chem. Soc., Chem. Commun., 1987. 10: p. 728-729.

Liu, K., et al., Electronic Effects Determine the Selectivity of Planar Au-Cu Bimetallic Thin Films for Electrochemical CO(2) Reduction. ACS Appl Mater Interfaces, 2019. 11(18): p. 16546-16555.

Figure 1