Influence of binder content in silver-based gas diffusion electrodes on pore system and electrochemical performance

The influences of the polytetrafluoroethylene (PTFE) content in silver-based gas diffusion electrodes on the resulting physical properties and the electrochemical performance during oxygen reduction in concentrated sodium hydroxide electrolyte were investigated through half-cell measurements. A systematic variation of the pore system was achieved by application of different silver/PTFE ratios during the production of the gas diffusion electrodes (GDE). In all electrodes, a silver skeleton structure with relatively constant properties was formed, while the PTFE fills up part of the open pore space. The resulting structures were characterized with a variety of methods for the physical properties supported by focused ion beam milling and scanning electron microscope (FIB/SEM) tomography. It could be shown that variations in the obtained pore system strongly influence the electrochemical performance of the electrodes. Determination of the Tafel slopes revealed that this is not due to changes in the electrocatalytic activity but rather caused by variations in the electrolyte uptake. While too small amounts of PTFE (1 wt%) lead to decreased performance through electrolyte flooding, higher PTFE contents above about 5 wt% also deteriorate the electrode performance because the extent of the three-phase boundary diminishes. The decisive role of the electrolyte intrusion was confirmed by measurements at higher electrolyte pressure. While the best electrochemical performance was achieved with an electrode containing 98 wt% silver, a slightly higher PTFE content is advisable to prevent breakthrough of the electrolyte.