High thermal stability 1D borophosphate proton conducting polyelectrolytes

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, Na 5 [BOB(PO 4 ) 3 )]) and ammonium borophosphate (NH 4 BOB, (NH 4 ) 3 H 2 [BOB(PO 4 ) 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 NH 4 BOB and NaBOB to be on the order of 2 µS cm −1 and 0.1 µS cm −1 at 200 °C, respectively. Although NH 4 BOB 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 H 2 ) 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. Graphical abstract