Electrochemical exfoliation of poly(triazine imide)-coated graphene oxide for the in-situ synthesis of composite proton exchange membranes with enriched phosphoric acid channels

Abstract

Phosphoric acid-doped polybenzimidazole (PA-PBI) membranes suffer from phosphoric acid (PA) loss, insufficient proton conduction sites, and severe plasticization, limiting their application in fuel cells. This work synthesized poly(triazine imide) (PTI) and poly(heptazine imide) (PHI) via ionothermal polycondensation, followed by the preparation of PTI-coated exfoliated graphene oxide (PTI@EGO) and PHI-coated exfoliated graphene oxide (PHI@EGO) nanosheets using electrochemical exfoliation. Structural characterization and theoretical calculations revealed strong interfacial interactions between PTI/PHI and exfoliated graphene oxide (EGO), effectively preventing the aggregation of EGO nanosheets. Furthermore, PTI@EGO provides more PA binding sites compared to PHI@EGO. Consequently, PTI@EGO was incorporated into poly(2,5-benzimidazole) (ABPBI) via in-situ polymerization to prepare PTI@EGO/ABPBI composite proton exchange membranes. The composite membranes exhibit exceptional mechanical strength, enhanced PA retention, and superior proton conductivity. Specifically, the ABPBI composite membrane containing 1 wt% PTI@EGO (1PTI@EGO/ABPBI) with a phosphoric acid doping level of 3.37 exhibited a tensile strength of 30 MPa, which is 1.5 times that of the pristine ABPBI membrane. The proton conductivity of the 1PTI@EGO/ABPBI membrane exceeded 0.01 S cm⁻¹ across a wide-temperature range (40–180 °C). At 180 °C/0 % relative humidity (RH), the conductivity reached 66 mS cm⁻¹, 2.44 times higher than that of pure ABPBI. The single-cell assembled with the 1PTI@EGO/ABPBI membrane delivered a peak power density of 0.522 W cm⁻² at 160 °C, nearly 2.51 times that of the pure ABPBI membrane. These results indicate that PTI@EGO constructs PA-enriched, efficient proton conduction pathways within the ABPBI membrane.