Search

One critical issue to phosphoric acid (PA) doped high-temperature proton exchange membranes (HT-PEMs) is to balance the proton conductivity and mechanical properties for overall application performance in fuel cells. Addressing the issue, we prepare durable HT-PEMs having the dual crosslinking structure by employing poly(vinylimidazole-divinylbenzene-styrene) (poly(VIm-DVB-St)) copolymer as a crosslinker and using the poly (aromatic ether ketone) (PAEK) polymer containing four methyl groups as the host membrane matrix. The imidazole groups of poly(VIm-DVB-St) react with benzyl bromide groups of brominated PAEK for both the primary cross-linking network and high PA doping. The divinylbenzene crosslinked poly (styrene-co-vinylimidazole) network generates the secondary cross-linking structure. The formed reticular polymer chain structure brings on low swelling and high mechanical strength of the HT-PEMs. The fuel cell based on the acid doped PAEK41-85%VIm/233.0 PA shows a H2-air fuel cell peak power density of 306 mW cm−2 at 200 °C without back pressure, and a low degradation rate of 3.9 × 10−5 V h−1 during a period of 600 h under a constant current density of 200 mA cm−2 at 160 °C.

The sulfuric acid (SA) doped dense polybenzimidazole (PBI) membrane exhibits low vanadium crossover, and is of great interest as the separator in vanadium redox flow battery (VRFB) applications. However, further reducing the area resistance is needed to increase the voltage efficiency. Towards this target, a series of quaternary ammonium groups grafted PBI membranes are designed and synthesized by using 2,3-epoxypropyltrimethylammonium chloride (EPTMA-Cl) as the grafting agent. The membranes were prepared through a gentle and facile heating curing method without the usage of catalyst, and physicochemical properties and single cell performance of SA doped PBI-EPTMA membranes are systematically investigated. The results indicate that grafted EPTMA groups increase the hydrophilicity and free volume of membranes, resulting in enhanced SA doping content and decreased area resistance. Ultimately, VRFB assembled with the PBI-21%EPTMA membrane displays higher energy efficiencies (87.6%–75.3%) at current densities of 50–150 mA cm−2, comparing with Nafion115 (81.7–61.2%). The stable performance over 450 charge-discharge cycles of VRFB demonstrates the good stability of the PBI-EPTMA membrane.

Phosphoric acid doped polybenzimidazole membranes have been explored as proton exchange membranes for high temperature polymer electrolyte membrane fuel cells. Long-term durability of the membrane is of critical concern and has been evaluated by accelerated degradation tests under Fenton conditions. In this study effects of phosphoric acid and ferrous ions were investigated by measurements of the weight loss, intrinsic viscosity and size exclusion chromatography (SEC) of the polymer membranes. Ferrous ions resulted in, as expected, catalytic formation of peroxide radicals and hence the accelerated polymer degradation in terms of weight loss and molecular weight decrease. The presence of phosphoric acid as an inevitable dopant of the membranes, on the other hand, significantly impeded the membrane degradation by means of metal ion complexing, decreased pH, and acid–base interactions with the amino groups of the polymer. Fuel cell durability tests with contaminations of ferrous ions did show considerable performance degradation, however, primarily due to the catalyst deterioration rather than the membrane degradation.

A novel acid–base polymer membrane is prepared by doping of imidazolium polysulfone with phosphoric acid for high temperature proton exchange membrane fuel cells. Polysulfone is first chloromethylated, followed by functionalization of the chloromethylated polysulfone with alkyl imidazoles i.e. methyl (MePSU), ethyl (EtPSU) and butyl (BuPSU) imidazoliums, as revealed by 1H NMR spectra. The imidazolium polysulfone membranes are then doped with phosphoric acid and used as a proton exchange membrane electrolyte in fuel cells. An acid doping level of about 10–11 mol H3PO4 per mole of the imidazolium group is achieved in 85 wt% H3PO4 at room temperature. The membranes exhibit a proton conductivity of 0.015–0.022 S cm−1 at 130–150 °C under 15 mol% water vapor in air, and a tensile strength of 5–6 MPa at 130 °C under ambient humidity. Fuel cell tests show an open circuit voltage as high as 0.96 V and a peak power density of 175–204 mW cm−2 at 150 °C with unhumidified hydrogen and air under ambient pressure.