Silver-Based Supportless Membrane Electrode Assemblies for Electrochemical CO₂ Reduction

CO 2 is one of the major contributors to the emission of greenhouse gases boosting climate change. Meanwhile, renewable energy production is fluctuating due to weather conditions, demanding for appropriate storage, e. g. by Power-to-X technology. The first step of these processes, the production of hydrogen via water electrolysis, is typically linked to a second step to obtain hydrocarbons [1]. Meanwhile, electrochemical CO 2 reduction (eCO 2 R) is capable of generating chemical feedstocks by converting excess CO 2 , simultaneously using renewable energy sources during peak times. A promising pathway of eCO 2 R focuses on CO, as it can be produced with high selectivity at silver catalysts, concurrently generating hydrogen as the only byproduct [2]. To partly overcome the mass transport limitations resulting from the very limited solubility of CO 2 in aqueous electrolytes, gas diffusion electrodes (GDEs) are typically used for eCO 2 R in a three-chamber setup with anolyte, catholyte and separate gas compartment [3]. However, there are still considerable overpotentials in this setup, among others caused by ohmic losses such as the electrolyte resistance. Employing membrane electrode assemblies (MEAs), either both electrodes or one of them can be combined with the membrane to form a full- or half-MEA, respectively, resulting in a significant decrease in cell voltage. Although there have already been studies on the fabrication of MEAs employing silver catalysts for eCO 2 R [4, 5], they are still very limited in options and mostly based on carbon gas diffusion layers (GDLs). The manufacturing approach applied in this work is based on a catalyst ink recipe for sintered silver GDEs originally developed for chlor-alkali electrolysis by Moussallem et al. [6]. Instead of using Nickel mesh as a substrate, the suspension is spray-coated on a stainless steel plate to enable the required treatment at temperatures above 300 °C. Afterwards, the catalyst layer is hot-pressed on the prepared anion exchange membrane at more moderate temperatures, forming a supportless cathodic half-MEA. Resulting from variations in the manufacturing procedure, different MEAs are electrochemically characterized, examining Faradaic efficiencies as well as cell voltages, also in comparison to measurements performed in three-chamber setup. Addressing challenges in product efficiency and membrane degradation, it is shown that this type of MEA is capable of eCO 2 R to CO, already reducing the cell potential at elevated current densities by nearly 50 %, see fig. 1. [1] Tom Kober et al. Report: perspectives of power-to-X technologies in Switzerland: supplementary report to the white paper . en. Technical report. 2019. doi: 10.3929/ETHZ-B-000525806. [2] Yoshio Hori et al. “Electrocatalytic process of CO selectivity in electrochemical reduction of CO 2 at metal electrodes in aqueous media”. Electrochimica Acta , 39 (11-12), (1994), 1833–1839. [3] Thomas Burdyny and Wilson A. Smith. “CO 2 reduction on gas-diffusion electrodes and why catalytic performance must be assessed at commercially-relevant conditions”. Energy & Environmental Science , 12 (5), (2019), 1442–1453. doi: 10.1039/C8EE03134G. [4] Zengcai Liu et al. “CO 2 electrolysis to CO and O 2 at high selectivity, stability and efficiency using Sustainion membranes”. Journal of The Electrochemical Society , 165 (15), (2018), J3371–J3377. doi: 10.1149/2.0501815jes. [5] Jonghyeok Lee et al. “Electrochemical CO 2 reduction using alkaline membrane electrode assembly on various metal electrodes”. Journal of CO 2 Utilization , 31 (2019), 244–250. doi: 10.1016/j.jcou.2019.03.022. [6] Imad Moussallem et al. “Development of high-performance silver-based gas-diffusion electrodes for chlor-alkali electrolysis with oxygen depolarized cathodes”. Chemical Engineering and Processing: Process Intensification , 52 (2012), 125–131. doi: 10.1016/j.cep.2011.11.003. Figure 1