Investigating the Effect of Catalyst Configuration on the Performance of Silver-Based Membrane Electrode Assemblies for Electrochemical CO2 Reduction
Authors: Weseler Lydia , Turek Thomas 
Published: December 2024

As one of the most dominant greenhouse gases, CO2 plays a critical role in climate change. Electrochemical CO2 reduction (eCO2R) is a promising approach to mitigate this impact, as it is capable of converting the excess gas into valuable chemicals, especially concerning electrical energy storage from renewable sources at peak times. However, due to high overpotentials, the technology is still in a rather early stage of development and not yet feasible for economic implementation on industrial scale. One of these limitations results from the poor solubility of CO2 in water or commonly used aqueous electrolytes, leading to insufficient transport of the reactant gas to the cathode in classical H-cell setups. Therefore, gas diffusion electrodes (GDEs) are typically employed to reduce the according mass transport resistances [1]. Furthermore, ohmic resistances, among others caused by the electrolyte gaps in the system, contribute to overpotentials. To decrease these constraints, conducting CO2 reduction at membrane electrode assemblies (MEAs) is an interesting opportunity that has been gaining more and more attention in research throughout the past years. Exemplarily, fig. 1 shows that the elimination of only the catholyte compartment and thus implementation of an exchange MEA can already decrease the cell potential by more than 30 % at 2 kA m-2 in comparison to GDE [2] setup.
Depending on the cathodic catalyst, there are various reaction routes of eCO2R, one of which aims to synthesize the target product carbon monoxide. This is possible e. g. at silver catalysts, providing high CO selectivity and hydrogen as the only byproduct [3]. The combination of these products, syngas, is an important feedstock for many chemical processes such as Fischer-Tropsch synthesis. Although the majority of studies regarding silver-based MEAs for eCO2R uses pure silver as a catalyst, the configuration of the material is varying: While we usually disperse a mix of powder and flakes to obtain our catalyst suspension, nanoparticles are more frequently used in literature [3-5]. Moreover, porous silver membranes [6, 7] or silver deposited from solutions such as AgNO3 [8] and various other structures have already been investigated. Anyways, when comparing the results yielded with the different configurations of silver (cf. fig. 1), it becomes clear that there is no obvious trend in which material generates the best performances. A closer look at the conditions of the corresponding experiments reveals that this is quite plausible, as there is, on the one hand, no uniform MEA manufacturing procedure: Not only are there deviations in the employed material form, but also the gas diffusion layer, ionomer and membrane as well as ink fabrication and processing. On the other hand, the electrochemical measurements are alternating between galvanostatic and potentiostatic methods with no constant active electrode area, let alone type of test cell or setup.
Therefore, this study aims to investigate the influence of differing catalyst configuration used for MEAs while maintaining other parameters concerning manufacturing and characterization procedure. The base of all MEAs is a carbon-based gas diffusion layer of the same supplier and type. An anion exchange ionomer is chosen to enhance the mass transport of the reactants to the catalyst. Only the form of the catalyst, e. g. powder, nanoparticles or fixed structures, is varied. The catalyst and ionomer are attached to the gas diffusion layer which is then sandwiched with the membrane corresponding to the ionomer. Conducting galvanostatic step experiments in exchange MEA setup with a fixed type of anode, the performance of the resulting MEAs is evaluated. This way, important effects on parameters such as Faradaic efficiency for the target product and cell potential can be unraveled.
Literature:
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[8] W. H. Lee et al., J. Mater. Chem. 9 (29), 2021.
[9] Y. Hori et al., Electrochim. Acta 48 (18), 2003.

Figure 1