High proton conductivity of ammonium polyphosphate/PTFE composites in water

Ammonium polyphosphate (APP)-based composite electrolytes are promising proton conductors for fuel cell applications, yet their practical implementation is hindered by stability and conductivity challenges. In this study, we developed a series of APP/polytetrafluoroethylene (PTFE) composite electrolytes, where hydrophobic PTFE serves as a binder to enhance mechanical stability while mitigating APP hydrolysis. Remarkably, the APP95 %/PTFE5 % composite exhibited a notable proton conductivity of 0.023 S⋅cm −1 at 90 °C in water, surpassing the threshold for efficient fuel cell operation (>0.01 S⋅cm −1 ). Structural and spectroscopic analyses revealed that water-induced hydrolysis of APP generates ammonium pyrophosphate ((NH 4 ) 4 P 2 O 7 ), facilitating proton transport through hydrogen bond reorganization. The incorporation of PTFE not only improved electrolyte durability but also reduced excessive hydrolysis. Leveraging this composite, we fabricated a direct ammonia fuel cell (DAFC), achieving a peak power density of 12 mW⋅cm −2 at 80 °C with an open-circuit voltage of 0.67 V. This work demonstrates the high ionic conductivity of APP/PTFE composite in water with reasonable durability due to the presence of strongly hydrophobic PTFE.