Halloysite ionogels enabling poly(2,5-benzimidazole)-based proton-exchange membranes for wide-temperature-range applications

Abstract

Simultaneous excellent proton conductivity and high mechanical properties from ambient temperatures to 200 °C are the most pressing challenges to the long-term applications of polybenzimidazole-based proton-exchange membranes for methanol steam reformer-proton-exchange membrane fuel cell units. Their performance is subject to the loss and plasticizing effects of free phosphoric acid (PA) during their long-term operation. Herein, novel proton carriers, termed halloysite ionogels (IL@HNTs), were prepared by filling the ionic liquid (IL) into the inorganic framework of halloysite nanotubes (HNTs) with the assistance of supercritical CO2 to replace free PA in polybenzimidazole membranes. IL@HNTs-embedded poly(2,5-benzimidazole) (ABPBI) composite membranes (ABPBI/IL@HNTs) were obtained by in situ synthesis and then doped with low levels of PA. Experimental characterization results showed that the ILs were confined within the lumen of the HNTs. Benefiting from the introduction of IL@HNTs, the composite membranes showed excellent proton conductivity (>10 mS/cm) from ambient temperature to 180 °C and a greatly enhanced mechanical strength (>75 MPa), water uptake, and PA absorbability. The ABPBI/5IL@HNTs composite membrane achieved peak power outputs of 219 and 380 mW/cm2 under anhydrous conditions at 80 and 160 °C, respectively, which were respectively 1.9 and 2.1 times greater than those of PA-doped ABPBI membrane. Satisfactory single-cell performance was obtained at a low PA doping level and without free PA. The results suggest that this approach of introducing novel ionogels to construct wide-temperature proton-exchange membranes can overcome the limitations of traditional low-temperature and high-temperature membranes, thus broadening the application temperature range of existing PEMFCs.