Dispersion engineering of ethylenediamine tetramethylene phosphonic acid in composite Membranes: Toward enhanced proton conductivity and phosphoric acid retention for high-temperature proton exchange membranes

Phosphoric acid (PA) and ethylenediamine tetramethylene phosphonic acid (EDTMPA) have emerged as a promising dual-proton-conducting system for high-temperature proton exchange membrane fuel cells (HT-PEMFCs). However, the correlation between the morphological distribution of solid-state EDTMPA within composite membranes and the resultant proton conductivity/PA retention properties remains unexplored in EDTMPA-PA dual-conductor PEMs. This study addresses this gap by strategically modulating the polymer matrix composition to optimize EDTMPA dispersion. By incorporating polymers with enhanced compatibility for EDTMPA into the membrane matrix, a uniform nanoscale distribution of EDTMPA particles is achieved. The experimental results showed that the decrease in the distribution size of EDTMPA in the composite membrane resulted in an increase in proton conductivity and enhanced PA retention ability under the same PA content. This particle-size-dependent enhancement is attributed to the increase in interfacial contact area between EDTMPA and PA, which enhances their intermolecular interactions with PA. These findings establish a microstructure-property relationship that advances the rational design of dual-proton-conductor PEMs, offering a scalable strategy to reconcile high performance with operational stability in next-generation HT-PEMFCs.