Asymmetric Diaphragms and Their Influence on Gas Crossover in Alkaline Electrolysis

Many novel lab-scale diaphragms, as well as commercial state-of-the-art diaphragms for alkaline water electrolysis are prepared by non-solvent based phase inversion processes. On the lab-scale, the fabrication process is often asymmetric. The polymer dope is coated onto a glass plate similar to conventional film forming techniques, after which the glass substrate carrying the film is immersed into a non-solvent bath to rapidly precipitate the constituting polymer. As the film is initially only exposed to the non-solvent on the top surface, the resulting pore structure at the top surface of the film differs from that at the glass plate side. This contribution explores a number of phase-inversion prepared diaphragms towards their pore structure, and in particular investigate how the asymmetric nature influences the electrolysis behavior at single cell level. Reference diaphragms based on polysulfone (PSU) are used as reference materials, and the investigation is extended to include diaphragms based on more novel polymers systems. Significant differences are observed in the relative crossover of hydrogen to oxygen (HTO) as well as the oxygen to hydrogen (OTH), depending on the orientation of the diaphragm. Specifically, towards which electrode the skin layer is oriented matters. More than an order of magnitude in apparent crossover flux can be observed in some cases at otherwise identical conditions. Introducing an electrode-diaphragm gap can alleviate severe supersaturation and reduce crossover flux, whereas over compressed cells exhibit more severe degrees of crossover.