Electrolyte Membranes Based on Molten KH₅(PO₄)₂ for Intermediate Temperature Fuel Cells

Abstract

Electrolyte membranes loaded with KH 5 (PO 4 ) 2 are prepared by immersing SiO 2 /polybenzimidazole (PBI) matrices into molten KH 5 (PO 4 ) 2 . The morphologies, chemical structures, thermal properties, mechanical properties and proton conductivities of the electrolyte membranes are examined. The utilization of SiO 2 /PBI matrices ensures that the KH 5 (PO 4 ) 2 -loaded electrolyte membranes possess excellent mechanical properties. With the highest SiO 2 doping rate, the K/(3SiO 2 /7PBI) electrolyte membrane has favorable mechanical properties with a tensile strength of 55.83 MPa and an elongation of~5 5%, which satisfy the requirements for membrane electrode assemblies in intermediate temperature fuel cells. In addition, this electrolyte membrane possesses a superior conductivity of 0.139 S cm -1 at 180°C under an H 2 O/N 2 atmosphere and shows desirable conductivity stability at elevated temperatures. A fuel cell equipped with the K/(3SiO 2 /7PBI) electrolyte membrane exhibits an open-circuit voltage of 1.01 V and its peak power density reaches 36 mWcm -2 .

Enhanced MEA Performance for an Intermediate-Temperature Fuel Cell with a KH5(PO4)2-Doped Polybenzimidazole Membrane

“…The weight loss continues up to 600 °C, and then the second sharp degradation occurs due to the decomposition of PBI membrane. In Figure 4 b, both KH 5 (PO 4 ) 2 powder and KH 5 (PO 4 ) 2 -doped PBI membrane have an endothermic peak at 130 °C, attributed to the melting of KH 5 (PO 4 ) 2 -doped PBI membrane [ 14 ].…”

“…As shown in Figure 6 , the proton conductivity increases with temperature in any gas conditions. From 120 °C to 140 °C, a sharp increment occurs, which is due to the melting point of KH 5 (PO 4 ) 2 at ~130 °C [ 14 ]. Under dry gas conditions, the maximum conductivity appears when the temperature rises up to 180 °C, but as temperature continues to increase, the conductivity decays, due to dehydration of phosphate salt [ 10 , 27 ].…”

“…Pure KH 5 (PO 4 ) 2 was synthesized by K 2 CO 3 and H 3 PO 4 according to the method described elsewhere [ 14 ]. To obtain KH 5 (PO 4 ) 2 -doped PBI membrane, pure PBI membrane was immersed in molten KH 5 (PO 4 ) 2 .…”

“…Although such electrolytes composed with MH 5 (PO 4 ) 2 retain high conductivity, the area specific resistance of electrolytes was still high because the electrolyte’s thickness staying in the millimeter range. In our previous work, aiming to reduce the thickness of the electrolyte with promising mechanical strength, we reported the preparation of composite membranes based on the molten proton conductor KH 5 (PO 4 ) 2 doped within a polybenzimidazole (PBI) polymer support matrix, respectively [ 14 ]. The PBI membranes doped with molten conductor showed high proton conductivity and good mechanical properties, though this was not reflected in a good fuel cell performance.…”

“…The PBI membranes doped with molten conductor showed high proton conductivity and good mechanical properties, though this was not reflected in a good fuel cell performance. The employed membrane electrode assembly (MEA) prepared with electrolytes based on KH 5 (PO 4 ) 2 exhibited a poor peak power density of 40 mW cm −2 under H 2 /O 2 [ 14 ]. Such low fuel cell performance has been a major impediment to the application of electrolyte membranes for molten proton conductors.…”