Search

Porous gas diffusion electrodes GDEs are widely used in electrochemical applications where a gaseous reactant is converted to a target product. Important applications for silver based GDEs are the chlor alkali and the CO2 electrolysis processes in which silver catalyzes the oxygen or carbon dioxide reduction reaction. The wetting of the porous GDEs is of utmost importance for the achieved performance of the electrode a completely dry electrode will result in low current densities due to the reduced active surface area while on the other hand, a completely flooded electrode will deteriorate the access of the gaseous reactant. Therefore, we investigated silver based GDEs for the oxygen reduction reaction with different amounts of the hydrophobic agent polytetrafluoroethylene PTFE and analyzed the potential induced wetting behavior electrowetting . The electrolyte breakthrough was recorded by a digital microscope and subsequently evaluated via imaging analysis of the observed breached electrolyte droplets. In order to characterize the wetting state during transition to the steady state, we applied electrochemical impedance spectroscopy measurements and retrieved the double layer capacitance. Our results indicate that a higher overvoltage facilitates the breakthrough of electrolytes through the gas diffusion electrode. Surprisingly, a faster breakthrough of electrolyte was observed for electrodes with higher PTFE content. Porometry measurements revealed that the GDE with low PTFE content has a monomodal pore size distribution, whereas electrodes with higher PTFE amount exhibit a bimodal pore size distribution. In GDEs with monomodal pore size distribution the time in which the double layer capacitance is leveling off correlates with the breakthrough time of the electrolyte. In summary, we emphasize that the wetting of GDEs is a complex interplay of the applied potential, electrode composition, and resulting porous structure which requires further advanced measurements and analysis considering the parameters affecting the wetting behavior as a whole

Metal-based gas diffusion electrodes (GDE) are utilized to achieve high current densities during the reaction in energy conversion systems, such as chlor-alkali electrolysis or the electrochemical reduction of carbon dioxide. By intense contact between the liquid and the gas phase with the catalytically active surfaces within the metallic electrodes, industrially required current densities can be accomplished. Their almost bulk-like silver structures with the 3D porous design and the strong absorption of the extended material make radiographic measurements extremely demanding. To receive in-depth information on electrolyte intrusion into the electrode's porous structure and the distribution during electrochemical measurements, lab-based and synchrotron radiography allows for monitoring this process operando 1, 2 . In this work, we describe the development of a cell design that can be modularly adapted and successfully used to monitor both the ORR and the CO2RR as exemplary and currently very relevant examples of gas-liquid reactions. The applied half-cell set-up designed for ORR allowed for operando synchrotron radiation measurements at electrochemically stable operation at viable industrial conditions and current densities to follow the electrolyte distribution within Ag-based GDE during chlor-alkali-electrolysis. By adding one additional electrolyte compartment, extending the X-ray window, and implementing an anion exchange membrane, the in-house built half-cell set-up designed for operando synchrotron radiation measurements of ORR, was converted into a full-cell set-up for imaging CO2RR of strongly absorbing metal-based GDEs. With the reported cell design, we were able to observe the electrolyte distribution within GDEs during cell operation at technically relevant current densities for CO2RR up to 300 mAcm-2 (Figure 1). By high-resolution synchrotron radiation operando measurements, we demonstrated crystallization of electrolyte caused by the hydrophobic character of the GDE. At high overpotentials, hydrogen evolution reaction caused blockade of the Ag-based GDE, resulting in declining current densities, which could be imaged and monitored during realistic potentiostatic operation. M. C. Paulisch, M. Gebhard, D. Franzen, A. Hilger, M. Osenberg, S. Marathe, C. Rau, B. Ellendorff, T. Turek, C. Roth and I. Manke, ACS Appl. Energy Mater., 4(8), 7497–7503 (2021). M. Gebhard, M. Paulisch, A. Hilger, D. Franzen, B. Ellendorff, T. Turek, I. Manke and C. Roth, Materials (Basel, Switzerland), 12(8) (2019). Figure 1