Proton conductivity properties of tin(IV) N,Nbis( phosphonomethyl)glycine phosphonates and their pyrophosphate-based derivatives

Abstract

Metal phosphonates (MPs), a subclass of coordination polymers, are distinguished by their acidic functional groups, such as –P–OH, –SO₃H, and –COOH. Combined with their chemical and thermal stability, commercial availability, and synthetic versatility, these features make MPs particularly appealing for fuel cell applications. They have shown remarkable potential not only as proton-conductive materials for proton exchange membrane fuel cells (PEMFCs), but also as precursors to metal diphosphate-based proton conductors suitable for intermediate-temperature fuel cells (ITFCs), between 100 and 300 ºC. In this work, we present the synthesis and characterization of a novel family of amorphous tin(IV) phosphonates, Sn[(COOH-CH2)N((CH2-PO(OH)2)2](OH)0.8 (H₂O)3 (SnBPMGLY); doped with variable amounts (0.1 - 0.3) of Al3+ or Mg2+ ions. Both, undoped and doped materials, were subjected to pyrolytic treatment at 750 °C in air, resulting predominantly in tin(IV) pyrophosphates with slight compositional variations. All materials were thoroughly characterized with respect to their proton conduction properties. The as-synthesized undoped material, SnBPMGLY, exhibited notable conductivity, reaching up to 5.05 × 10⁻⁴ S·cm-1, at 90 °C and 95% relative humidity (RH). After pyrolysis, the resulting tin pyrophosphates showed higher proton conductivities, with values up to 1.3 × 10⁻² S·cm-1 in the case of Sn0.8Al0.2BPMGLY Δ750 ºC under the same conditions. The low activation energy values (E < 0.35 eV) determined from the Arrhenius a plots suggest a water-mediated Grotthuss proton transfer mechanism. Current efforts are focused on evaluating the performance of these pyrolyzed materials at intermediate temperature (125–300 °C) under high vapor pressure conditions.