One-pot synthesis of proton-conductive inorganic-organic hybrid membranes from organoalkoxysilane and phosphonic acid derivatives

Inorganic-organic hybrid membranes consisting of an aliphatic main-chain polymer and Si-O linkages were synthesized via a simple one-pot process from an alkoxysilane derivative, methyl-substituted phenylvinylphosphonic acid, and phosphonic acid acrylate. (Methyldimethoxysilylmethyl)styrene (MDMSMS) was copolymerized with 3-methylphenylvinylphosphonic acid (CH₃C₆H₄VPA, or MPVPA), and 2-hydroxyethyl methacrylate acid phosphate (HEMAP), and then hydrolyzed, yielding inorganic-organic hybrid composites. The formation of the inorganic-organic hybrid structure was confirmed by IR, ¹³C, and ²⁹Si NMR spectroscopy. The membranes exhibited good thermal stability up to 180°C. The combination of aliphatic chains of the methacrylate derivative and Si-O networks in the hybrid membranes improved the mechanical properties of the membranes, enabling the construction of a membrane electrode assembly. The tensile modulus of the MDMSMS/MPVPA/HEMAP membrane with a composition of 1:1:5 was 688MPa. The conductivity of the 1:1:5 membranes was 4.1×10^-2Scm^-1 at 130°C and 100% relative humidity (RH), while it was 4.7×10^-4Scm^-1 at 130°C and 19.3% RH. The peak power of the 1:1:5 membrane was 4.8mW/cm² at 140°C and 30% RH. The cell utilizing the hybrid membrane was operated at 120°C and 30% RH for 24h without any drop in cell voltage.