Inorganic–organic composite electrolytes consisting of polybenzimidazole and Cs-substituted heteropoly acids and their application for medium temperature fuelcells

Abstract

Inorganic–organic composite electrolytes were fabricated from partially Cs⁺-substituted heteropoly acids (Cs-HPAs) and polybenzimidazole (PBI) for application in medium temperature fuel cells. PBI was synthesized using diaminobenzidine (DAB) and isophthalic acid (IPA) in polyphosphoric acid (PPA) in the temperature range of 170 to 200 °C under nitrogen atmosphere. Heteropoly acids, such as phosphotungstic acids (H₃PW₁₂O₄₀), and silicotungstic acid (H₄SiW₁₂O₄₀), were mechanochemically treated with caesium hydrogen sulfate (CsHSO₄) to obtain Cs-HPAs with a molar ratio of 50/50. The PBI-Cs-HPAs composite electrolytes were fabricated by solvent casting, and FT-IR results showed that the PBI-Cs-HPAs composite formed successfully. High proton conductivity and good fuel cell performance of PBI composite electrolytes were comparable to those of phosphoric acid-doped pure PBI electrolytes, even though they have lower phosphoric acid doping level (PADL) than that of pure PBI. The high proton conductivities of 1.91 × 10⁻² S cm⁻¹ and 1.71 × 10⁻² S cm⁻¹ were achieved at 160 °C under anhydrous conditions for PBI-50H₃PW₁₂O₄₀·50CsHSO₄ with 87 wt.% PADL and PBI-50H₄SiW₁₂O₄₀·50CsHSO₄ with 82 wt.% PADL, respectively. These observations implied that the mechanochemically synthesized Cs-HPA composites are promising materials to achieve high electrochemical properties with a low level of PADL for PBI electrolytes.