Monitoring the Dynamics of the Galvanic Replacement Reaction to Develop Free-Standing Electrocatalysts
Authors: Ben Abderrahmane Amira , Tingry Sophie , Cornu David , Holade Yaovi 
Published: August 2024

Hydrogen (H2) is well-known for its potential to become the future fuel and bring a more sustainable energy system [1]. The selective electrooxidation of organic compounds from biomass appears to be a possible substitute for the kinetically slow oxygen evolution reaction (OER) at the anode, which increases the electrical energy input for a conventional water electrolysis in alkaline media [2]. However, this approach is currently impeded by limited fundamental knowledge of the different active sites of electrocatalysts. The latter can be used to control selectivity at low potential and high current density to co-generate value-added products at the anode instead of CO2 by complete oxidation. Nanostructured gold-based multi-metal electrocatalysts can meet such objectives. Particularly in the electro-valorization of cellulosic biomass at electrode potentials unfavorable to C-C bond breaking and high overall cell voltage.

To develop such electrocatalysts, we have initiated a multivariate methodology to study the dynamics of galvanic replacement between silver and gold. This procedure allows us to control nanoporosity and nanoalloying in the resulting gas diffusion electrode (GDE): GDE-Ag1-xAux. GDE-Ag represents the electrode upon which Ag particles were electrochemically grown prior to reaction with Au(III). Thermodynamically, silver with E°(Ag+/Ag) = 0.8 V vs SHE (standard hydrogen electrode) can be replaced by gold with E°(AuCl4
–/Au) = 1.0 V vs SHE according to: 3Ag + xKAuCl4(aq) → 3xAgCl + xKCl + Ag3-3xAux.[2,3] From a kinetic point of view, the challenge is to capture in real time the formation of metal cages of Ag-Au alloys after a '3by1' atomic exchange that introduces nanoporosity and electronic strain as silver and gold have a similar atomic radius. To interrogate the driving force behind this redox process, we have also established an electrochemical methodology to extract quantitative data, allowing detailed capture of the mixed electrochemical and mass transport (diffusion) kinetics. As the precipitation product AgCl leads to pore fouling, we have developed an efficient methodology to trigger its instant dissolution as soon as it is formed, thus promoting the production of a high-quality alloy. As shown in Figure 1, upon injection of a KAuCl4 solution into a degassed solution in contact with the GDE-Ag electrode, the open circuit potential (OCP) begins to change. This evolution is due to the modification of the electrochemical interface as silver atoms are progressively replaced by gold atoms. Thereby, leading to a regulated potential defined either by the Ag(I)/Ag redox couple, or by the Au(III)/Au pair. Interestingly, we found that the OCP variation logically depends on initial conditions such as Ag loading on the GDE, KAuCl4 concentration, Ag particle size, etc. This fundamental knowledge has led to the synthesis of efficient electrocatalysts for the oxidation of cellulose subunits in alkaline media with an onset electrode potential much lower than OER (E < 1 V vs RHE (reversible hydrogen electrode)). Indeed, the formation of an alloy phase, the creation of nanoporosity within it and the free-standing nature amplify, through synergistic and electronic effects, the electrocatalytic efficiency.

Acknowledgments

This work was funded by French National Research Agency (ANR-22-CE43-0004).

References

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[2] R. Boukil, N. Tuleushova, D. Cot, B. Rebiere, V. Bonniol, J. Cambedouzou, S. Tingry, D. Cornu and Y. Holade, J. Mater. Chem. A 2020, 8 (18), 8848–8856, https://doi.org/10.1039/D0TA01063D.

[3] D. K. Pattadar, R. A. Masitas, C. D. Stachurski, D. E. Cliffel, and F. P. Zamborini, J. Am. Chem. Soc. 2020, 142, 19268−19277, https://dx.doi.org/10.1021/jacs.0c09426.

Figure 1