Soft chemical synthesis and the role of potassium pentahydrogen bis(phosphate) in a proton conducting composite electrolyte based on potassium dihydrogen phosphate

• Proton conducting phosphates have been prepared by using a wet impregnation method.
• Crystalline KH5(PO4)2 is introduced to proton conducting electrolyte based on KH2PO4.
• Molten KH5(PO4)2 plays a decisive role in increasing the proton conductivity.
• SiO2 addition ensures long-term viability under relatively low humidification.

Proton conducting composites based on potassium dihydrogen phosphate (KH2PO4) have been successfully prepared and characterized as a potential electrolyte for intermediate temperature fuel cells. X-ray diffraction, thermal analysis, and conductivity measurements reveal that the molten potassium pentahydrogen bis(phosphate) (KH5(PO4)2) phase enhances the proton conductivity of KH2PO4–KH5(PO4)2 composites by more than four orders of magnitude. The addition of 10 wt.% SiO2 is sufficient to alleviate the dehydration of KH2PO4–KH5(PO4)2, and long-term stable conductivity is exhibited under a relatively low humidification level (pH2O = 0.03 atm).