Ag-Based Gas Diffusion Electrodes for the Electrochemical CO2 Reduction by Pulsed Hydrogen Bubble Templation

Abstract

Energy-conversion technologies like the electrocatalytic nitrogen reduction reaction (N2RR), carbon monoxide reduction reaction (CORR) and in particular the electrochemical carbon dioxide reduction reaction (CO2RR) can contribute decisively to overcome the challenge of reducing greenhouse gas emissions. By reducing CO2 electrochemically, the carbon-cycle can be closed and industrially desired chemicals like syngas (CO + H2), formic and acetic acid, or hydrocarbons, such as methanol and ethylene, can be produced fossil-fuel free, depending on the catalyst material applied. Ag-based catalysts are very promising in the field of CO2RR, since they show high selectivities towards the reduction of CO2 to CO and current efficiencies of up to 100 %. In aqueous-fed systems, a significant drawback is the limited CO2 solubility, resulting in long diffusion pathways of CO2 within the bulk electrolyte 1, 2. Consequently, industrial conditions cannot be reached for CO2 electrolysis in aqueous solution in H-cell configurations due to the CO2 depletion within the porous electrodes that limit lab scale testing of CO2RR to current densities of approx. 35 mAcm-². One way to achieve commercially relevant current densities (> 200 mAcm-²) is the utilization of gas diffusion electrodes (GDEs) in flow cell configuration. These enable a decrease in CO2 diffusion lengths to approx. 50 nm, enhancing the mass transport to the catalyst 3.In our contribution, we will demonstrate a facile and fast production process of Ag-based GDEs by combining pulsed electrochemical deposition of the Ag catalyst with ionomer infiltration of the electrode. Using the dynamic hydrogen bubble templation method (DHBT), we utilized the parasitic hydrogen evolution reaction (HER) to aid in a solvent free structuring of the 3D catalyst network. In this work, we focused on the deposition parameters by varying different pulse-to-pause ratios in order to increase the amount of deposited catalyst and thus, successfully reduced the overpotential during CO2RR operation (Fig. 1, left). To inhibit electrode flooding and decrease CO2 mass transport limitations during CO2RR, we infiltrated the electrode with a perfluorinated ionomer. SEM and EDS analyses showed a uniform Ag/F distribution along the cross section of the electrodes (Fig. 1, right), which resulted in the conversion of CO2 to CO at industrially viable current densities.References S. Hernandez-Aldave and E. Andreoli, Catalysts, 10(6), 713 (2020).Z. Sun, T. Ma, H. Tao, Q. Fan and B. Han, Chem, 3(4), 560–587 (2017).T. Burdyny and W. A. Smith, Energy Environ. Sci., 12(5), 1442–1453 (2019). Figure 1