Development of high-performance silver-based gas-diffusion electrodes for chlor-alkali electrolysis with oxygen depolarized cathodes

Silver-based gas-diffusion electrodes were prepared, characterized and tested during chlor-alkali electrolysis with oxygen depolarized cathodes. Spraying of aqueous suspensions containing silver catalyst particles, a PTFE suspension, a surfactant and an organic thickening agent on nickel nets followed by drying, hot-pressing and sintering allowed for the preparation of highly active electrodes. Electrodes obtained from the most suitable catalyst SF9ED (Ferro) were systematically investigated. The best electrodes contain 97–98 wt.% silver and 2–3 wt.% PTFE, are hot-pressed at ca. 130 °C with moderate pressure and subsequently sintered at temperatures around 340 °C. For these electrodes, half cell potentials of ca. 650 mV vs. RHE corresponding to electrolysis cell voltages of ca. 2.17 V at current densities of 4 kA/m2 were obtained. Both half cell and electrolysis cell voltages were found to be reproducible within about ±30 mV during repeated measurements using electrodes from different batches. While the Ag/PTFE loading had little influence on the observed voltages in the range studied (100–300 mg/cm2), loadings of at least 200 mg/cm2 are recommended to prevent leakage of NaOH electrolyte and to allow for operation under oxygen overpressures of more than 100 mbar without breakthrough of gas to the electrolyte.