Influence of crosslinking in phosphoric acid-doped poly(phenylene oxide) membranes on their proton exchange membrane properties

•Proton conductivity and PA retention are highly dependent on membrane crosslinking.•Interactions between quaternary ammonium and phosphoric acid enhances PA retention.•Highest conductivity of membrane with lowest crosslinking and shortest crosslinker.•Highest PA retention with the membrane having the highest degree of crosslinking.•Crosslinking optimization crucial for high-performance PA-doped membranes.