An operationally broadened alkaline water electrolyser enabled by highly stable poly(oxindole biphenylene) ion-solvating membranes

Ion-solvating membranes (ISMs) are an alternative to proton-exchange and anion-exchange membranes for use in water electrolysers. ISMs do not have fixed ionic groups in their structure but instead gain their ionic conductivity through the uptake of liquid electrolyte. Although in principle they could offer improved stability over anion-exchange membranes due to the absence of easily degradable anion-exchange groups, stability gains have been modest. Here we report poly(oxindole biphenylene)-based ISMs with highly stable oxindole/KOH complex ion pairs for use in water electrolysers. These ISMs exhibit promising alkaline stability at 80 °C with a negligible conductivity decay over more than 15,000 h and, thus, allow durable alkaline electrolysis over 2,500 h, even at elevated temperatures and high operating voltages of 2.3 V. Moreover, they show ultralow gas permeation and, thus, low transient response times (<1 s). They allow the use of non-precious-metal catalysts (Ni and Ni/Fe) and can be operated over a broad temperature range (−35 to 120 °C).