Investigation of polyphenylene oxide-based membranes for anion exchange water electrolyser

Abstract

The energy crisis and resources shortage of conventional fossil fuels such as coal, natural gas and oil are becoming severe with the rapid development of society and industrialisation. Renewables hold the key to the increasing energy demand and environmental issues. Green hydrogen (H2) is considered one of the most promising energy carriers for the future due to its high energy density and CO2-free emission. Anion exchange membrane water electrolyser (AEMWE) is a promising technology for producing hydrogen. AEMWE could offer significant cost reduction by enabling earth abundant catalyst materials while providing pure hydrogen due to good H2 and O2 separation. Anion exchange membrane (AEM) is a significant component in AEMWE. AEM acts as an electrolyte to conduct the negative ions, e.g., OH- and the separator between the anode and cathode compartments where the O2 and H2 are produced. However, the performance of AEMs still needs to be improved to meet the requirement of the long-term operation of water electrolysis. This project aims to prepare a stable AEM and investigate its degradation mechanism under pH 7-14, relevant to AEMWE fed with deionised water to 1M KOH supporting electrolyte. Cross-linked quaternised poly(2,6-dimethyl-1,4-phenylene oxide) (QPPO)- based membranes were prepared via Friedel-Crafts reactions using SnCl4 catalyst and environmentally-friendly chloromethylating reagents. New equations to calculate the degree of chloromethylation and cross-linking degree were proposed. The ionic conductivity can reach 133 mS cm−1 at 80 °C. Ex situ stability testing after 500 h in 1 M KOH showed membranes retained up to 94 % of their original Ion Exchange Capacity (IEC). QPPO was employed as both membranes and ionomers in electrolyser tests and compared with previously prepared polystyrene-b-poly(ethylene-co butylene)-b-polystyrene (SEBS) and low-density polyethylene (LDPE)-based membbrane. QPPO membranes exhibited area-specific resistance of 104 mΩ cm−2 and electrolyser current density of 814 mA cm−2 at 2.0 V when 0.1 M NaOH supporting electrolyte feed at 40 °C. The oxidative stability of QPPO and LDPE-based membranes was studied. Compared with LDPE-based membrane, QPPO-based membrane shows better oxidative stability. The degradation mechanism of PPO-based membrane under DI water conditions was studied. The residual degradation solution and extracted sample after the degradation test were characterised by NMR. The possible degradation mechanism is that oxygen or OH radicals attack the methyl group on the rearranged ylide, forming aldehyde or carboxyl attached to the CH2 group. To increase their mechanical strength, reduce thier water swelling and improve thier dimension stability, QPPO membranes were reinforced using the pore filling technique inside porous fluoropolymer of tetrafluoroethylene (PTFE) to prepare the PPO/PTFE based composite membranes. Reinforced membranes significantly increased tensile strength, 31 MPa from 14 MPa for unreinforced membranes. This increased membrane lifetime in working electrolyser to >200 h compared with otherwise identical electrolyser assembled with PPO-based membrane (50 h). The water uptake of the composite membrane is 77.5 %, lower than that of the PPO-based membrane (430 %). However, the PPO/PTFE conductivity was 6.25 mS cm-1 lower than 30 mS cm-1 of PPO-based membrane at 20 °C. This is caused by lower water uptake of PPO/PTFE composite membranes and lower volume fraction of QPPO in the composite membrane. At 40 % RH, the net change mass of composite membrane is 1.59 %, much lower than that of PPO-based membrane (10.98 %) at 40 °C.