Preparation And Properties Of High Temperature Proton Membranes Based On Polybenzimidazoles

Direct methanol fuel cells (DMFCs) are efficient and attractive fuel cells that can be used as power sources for mobile applications and in small electronic devices. For this reason, increasingly more attention is now being focused on DMFCs in recent years. The research of high-temperature DMFC has been a hotspot of DMFC. Current commercial proton exchange membranes are perfluorinated ionic polymer, such as DuPont’s Nafion. However, it has low ion conductivity at high temperatures (>120℃) due to its low water uptake. Membranes with high proton conductivity at temperatures as high as 120-200℃are one of the main goals in current PEMFC research.The earliest research of high temperature proton exchange membrane is polybenzimidazoles (PBI) membrane. However, there are also some problems with them:Firstly, the conductivities of PBI composite membranes, especially acid doped PBI membranes are not high due to their limited acid doped quantity. Secondly, the doping acid is easy to wash away under high-temperature and humidity. In addition, the amino-terminated in PBI chains tend to degradation in oxidation condition, and these affects the life of composite membranes. Thirdly, PBI is almost insoluble in organic solvents and its membranes are cast from methanesulfonic acid (MSA) or phosphoric acid suffers from inferior mechanical properties. In this paper, the novel proton conductors, with high-performance at high temperature, were synthesized. Poly (2,5-benzimidazole) (ABPBI) and poly[2,2’-(m-phenylene)-5,5’-bibenzimidazole] (mPBI) were synthesized and their terminated amino groups were protected via reacting with carbamide. ABPBI or PBI composite membranes were prepared by direct casting or hot-pressed method.SiO2 nanopowder-riveted phosphotungstic acid (T-PWA-SiO2) was synthesized as a proton conductor to improve the conductivity of ABPBI. The research shows that PWA has good fastness in T-PWA-SiO2. The T-PWA-SiO2 samples were combined with ABPBI to prepare ABPBI/(T-PWA-SiO2) composite membranes (30 wt% and 46 wt%) by polyphosphoric acid direct-casting method. The T-PWA-SiO2 particles combined with ABPBI by hydrogen bonding between PWA and C=N in ABPBI. The membrane morphologies were investigated by scanning electron microscopy (SEM). Thermogravimetric analysis (TGA) indicates thermal stability of the composite membranes below 200℃. The PWA exhibited good fastness in ABPBI/(T-PWA-SiO2) composite membranes. The conductivity, life, and heat resistance of the composite membranes were enhanced. The conductivity of the ABPBI/(T-PWA-SiO2) (46 wt.%) composite membrane was about 0.055 S/cm at 180℃under 100% relative humidity.Cerium sulfophenyl phosphate (CeSPP), a novel inorgano-organic solid proton conductor, was firstly synthesized. The synthesized CeSPP was characterized by Fourier-transform infrared spectroscopy (FT-IR), x-ray diffraction (XRD), TGA, SEM and energy-dispersive x-ray spectroscopy (EDX). Combined FT-IR with the XRD spectra, the data indicates the CeSPP has layered structure. The TGA results reveal that CeSPP has good thermal stability. The CeSPP exhibited considerable high-proton conductivity and reached about 0.13 S/cm at 150℃. The activation energy of proton transfer was 29.64 kJ·mol-1.The CeSPP samples are combined with ABPBI and mPBI to prepare ABPBI/CeSPP and mPBI/CeSPP composite membranes, respectively. The SEM indicates that the composite membranes have dense structure and CeSPP particles are homogenous dispersed into the composite membranes. FT-IR revealed that intense hydrogen bondings have formed between-OH (in CeSPP) and C=N in PBI. TGA indicates thermal stability of the composite membranes below 200℃Addition of CeSPP was in favor of increasing their proton conductivity at high temperature, improving mechanical properties and oxidation-resistant properties. The conductivities of the ABPBI/CeSPP (38 wt.%) and PBI/CeSPP (25 wt.%) composite membrane were 0.14 S/cm and 0.11 S/cm at 180℃under 100% relative humidity, respectively. The activation energy of proton transfer were 22.004 kJ·mol-1 and 24.665 kJ-mol-1, respectively.