Compatible layers bring benefits

Electrochemical devices, such as electrolysers, often use ion-conducting membranes to separate the anode from the cathode. In these devices, typically the pH on each side of the membrane must be equal. Conversely, owing to their unique structure comprising a water dissociation catalyst sandwiched between anion and cation exchange layers, bipolar membranes (BPMs) allow a pH gradient from one side to the other. This brings potential benefits including greater activity and stability of catalysts at the electrodes. Yet, they are held back by poor water dissociation kinetics, limited mass transport and inadequate durability of the various interfaces within the BPM. Now, Liang Wu, Tongwen Xu and colleagues at The University of Science and Technology of China report a BPM that can operate stably over 1,000 h in a water electrolyser at 300–500 mA cm^–2 and support a current density of 1 A cm^–2 at 2.68 V, half that of a commercial material.

The researchers select commercially available nanoparticulate SnO₂ as the water dissociation catalyst. This is ultrasonically sprayed onto the anion exchange layer, before being covered by the cation exchange layer — also applied via ultrasonic spraying. The anion and cation exchange layers are composed of identical poly(terphenyl alkylene) polymeric skeletons, differing only in their side groups that confer the specific ionic conductivity. Using such similar polymers for each layer means that they are highly compatible with one another and have similar mechanical properties, strengthening the interfaces and allowing very thin membranes to be produced. In turn, high durability and rapid mass transport result. The research team note that their BPM may also be an enabling material for other electrochemical applications, such as CO₂ electrolysers.