Composite electrolytes composed of Cs-substituted phosphotungstic acid and sulfonated poly(ether–ether ketone) for fuel cell systems

Composite electrolytes composed of cesium hydrogen sulfate containing phosphotungstic acids (CsHSO4–H3PW12O40) and sulfonated poly(ether–ether ketone) (SPEEK) were prepared by casting the corresponding precursor for application in fuel cells. Partially Cs-substituted phosphotungstic acids (CsxH3−xPW12O40) were formed in the CsHSO4–H3PW12O40 system by mechanochemical treatment. SPEEK was prepared from PEEK by sulfonation using concentrated sulfuric acid. Flexible composite electrolytes were obtained and their electrochemical properties were markedly improved with the addition of CsxH3−xPW12O40, into the SPEEK matrix. A maximum power density of 213 mW cm−2 was obtained from the single cell test for 50H3PW12O40–50CsHSO4 in SPEEK (1/5 by weight) composite electrolyte at 80 °C and at 80 RH%. Electrochemical properties and transmission electron microscopy (TEM) results suggest that three-dimensional cluster particles were formed and homogeneously distributed in the SPEEK matrix. The mechanochemically synthesized CsxH3−xPW12O40 incorporated into the SPEEK matrix increased the number of protonate sites in the electrolyte. The composite electrolytes were successfully formed with CsxH3−xPW12O40, which consist of hydrogen bonding between surface of inorganic solid acids and not only –HSO4− dissociated from CsHSO4 but also –SO3H groups in the SPEEK.