Ether-free polymer based bipolar electrolyte membranes without an interlayer catalyst for water electrolysis with durability at a high current density

In this study, bipolar membranes (BPMs) were fabricated employing the sulfonated poly(fluorine biphenyl indole) based proton exchange membrane (PEM) and the poly(arylene piperidinium) based anion exchange membrane (AEM) for the production of green hydrogen by water electrolysis. As those polymers comprise similar aromatic backbone structures without ether groups, excellent chemical durability of the constituent membranes was expected for the long-term energy efficient operation in a wide pH range. Three BPMs with different ionic conductivity pairs between PEM and AEM were designed to explore its effect on water electrolysis performance. At the fixed anionic conductivity of AEM of 0.0370–0.0923 S cm⁻¹ at 30–70 °C, the proton conductivity of PEMs was tuned as 0.0379–0.1196 S cm⁻¹, 0.0909–0.1598 S cm⁻¹, and 0.1104–0.1739 S cm⁻¹ at 30–70 °C. BPMs were steadily operated at low voltages below 1.5 V in the wide current density range of 0–1500 mA cm⁻². Among them, the water electrolytic cell assembled with BPM 80/20 showed the best water electrolysis performance, as the cell voltage of BPM 80/20, BPM 70/30, and BPM 60/40 was 0.75 V, 0.98 V, and 1.08 V, respectively, under the current density of 700 mA cm⁻² at 70 °C. It was much lower than that of AEM, 2.24 V and PEM 80/20, 1.81 V even in the absence of interlayer catalyst. Also, the synthesized BPM 80/20 was not only thermally and mechanically stable up to 298 °C and 19.15 MPa (tensile strength), but also chemically stable in the concentrated acidic and basic environment, as its weight loss was less than 5.4 % and 4.62 % in 4 M H₂SO₄ and 4 M KOH solutions for 4 weeks, respectively.