Electrode material

The electrodes are adjacent to the diaphragm and are made of corrosion-resistant metal. While solid electrodes of pure nickel were often used in the past, this is expensive and does not allow reaching maximum efficiency. 

The oxygen-producing anode is either made of pure nickel, of a nickel coating on a steel core or of a nickel-iron alloy coated on a simple steel or nickel plate. Note that stainless steel has a lower overpotential than nickel (0.28V vs 0.61V) for the production of oxygen but a worst electric conductivity (1.45 MS/m vs 14.3 MS/m), so that choosing one or the other material implies a trade-off. Research about stainless steel electrodes or nickel-iron alloys with high iron content is on-going to offer a cheap alternative to nickel. 

The hydrogen-producing cathode generally has a steel core with some catalytic coating . Depending on manufacturers and application type, the coating ranges from plain unactivated nickel to activated nickel alloys (NiMo, NiSn, NiS) or even platinum group metals. In contrast to the oxygen reaction, platinum group metals have very low overpotentials for the hydrogen reaction (0.09V for Pa and Pt or even 0.01V for platinized Pt), bettering both nickel (0.32V) and stainless steel (0.42V). Yet, a more cost-effective approach is to use some activated coatings where nickel is electroplated with some Mo, Sn or S (10-20%mass in the resulting alloy) to reduce the overpotential within the range of 0.11-0.27V. Nickel-sulphur is notably a very good catalyser, but the nickel-sulphur layer partly dissolves each time the electrolyser is stopped under the transient the increase of the electric potential. The hydrogen conversion efficiency therefore drops with time. 

While conventional (gapped) electrolysers tend to use solid electrodes, zero-gap electrolysers use thin porous or perforated plates. For example, a thin 0.3mm pure nickel plate with 50% perforation can be used for the anode, while a similarly thin 0.3mm perforated nickel plate coated with NiS can be used for the cathode .