High thermal stability 1D borophosphate proton conducting polyelectrolytes

Abstract

The ionic conductivity, thermal stability, and general viability for use as electrolytes in fuel cells are examined for two borophosphate polyelectrolyte compounds (e.g., sodium borophosphate (NaBOB, Na5[BOB(PO4)3)]) and ammonium borophosphate (NH4BOB, (NH4)3H2[BOB(PO4)3])). Bulk synthesis methods are presented for laboratory scale reactions (> 5 g) using low-temperature ionic liquid fluxes. Electrochemical impedance spectroscopy (EIS) was used to determine temperature-dependent ionic conductivities of the NH4BOB and NaBOB to be on the order of 2 µS cm−1 and 0.1 µS cm−1 at 200 °C, respectively. Although NH4BOB displays higher ionic conductivity compared to NaBOB at equivalent temperatures, thermal gravimetric analysis (TGA) shows much higher thermal stability for NaBOB, exhibiting no mass loss below 600 °C. The thermal stability of NaBOB was also assessed under reducing (~ 1 atm H2) conditions, finding no reductive thermal degradation below 400 °C. Ab initio molecular dynamics (AIMD) simulations show free proton (H+) movement is related to gyrational mobility of the polyanionic borophosphate chains.