Polybenzimidazole/ionic-liquid-functional silica composite membranes with improved proton conductivity for high temperature proton exchange membrane fuel cells

A series of polybenzimidazole containing hydroxyl groups (PBIOH)/ionic-liquid-functional silica (ILS) composite membranes (PBIOH-ILS) have been successfully prepared for the high temperature proton exchange membrane fuel cells (HT-PEMFCs). These ionic-liquid-functional silica (ILS) nanoparticles, which germinate from the polymer chains by an in situ sol-gel reaction, enhance the performance of polybenzimidazole-based composite membranes. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were used to characterize and confirm the Si-O-Si structure in the membranes. The PBIOH-ILS composite membranes have shown excellent thermal stability, oxidative stability and improved mechanical strength. Considering the trade-off of mechanical properties and proton conductivity, 5% ILS in weight was demonstrated to be the optimum content in the membranes. The proton conductivity was significantly enhanced because the ILS nanoparticles with high specific area in the membranes could absorb more phosphoric acid, which just like stones flooded by the sea after doping with phosphoric acid. The doping level of PBIOH-ILS 5 was as high as 7.54 after doping with phosphoric acid for 48 h and 9.65 after doping with phosphoric acid for 72 h. High doping levels lead to high proton conductivities. The PBIOH-ILS 5 had a proton conductivity of 0.106 S cm−1 at 170 °C, which is higher than some of the known PBI-based membranes measured under similar conditions.