“…Several different types of membranes are currently being investigated, including poly(perfluorosulfonic acids) [3,4], sulfonated poly(arylenes) [5][6][7][8][9][10], and acid-doped polymer complexes [11][12][13][14][15][16]. A cross-linked linear poly(ethylenimine) hydrochloride/phosphoric acid system, using 1,1,3,3-tetramethoxypropane as the cross-linker, has been presented as a potential membrane alternative that has good conductivity, mechanical integrity and thermal stability up to 150ºC [17,18].…”