Synthesis And Properties Of High Temperature Proton Exchange Membranes

Proton exchange membranes are critical materials utilized for proton exchange membrane fuel cell. The commercial available proton exchange membrane is Nafion produced by Dupont with the fluorocarbon chain as main chain and sulfonic acid groups in side chain. Proton exchange property of Nafion membrane is mainly depended upon the water molecules and when temperature over 80℃, its proton conductivity was reduced dramatically. Therefore, the performance of Nafion membrane could not meet the requirement when it works at the high temperature due to poisoning of CO. Many researchers consider about the modification of water management and the purity of hydrogen to enhance the proton conductivity. It is very important to develop a kind of membrane which can work at the high temperature. Currently, one of the most potential membrane is polybenzimidazole (PBI)/phosphate composite membrane to replace Nafion. However, PBI/phosphate membrane has its own shortcomings. In this system proton transport by phosphate molecules, so the contain number of phosphate molecules influence the conductivity of the membranes. If phosphate content is lower, the proton conductivity is not enough. But if the content is much more, the PBI membrane may swell too serious to keep the mechanical property. To solve this problem, we must increase the proton conductivity in the case of membrane swelling less. Two methods are developed to solve the problem. The first one is doping some proton conductor in the PBI membrane. These conductors such as zirconium phosphate (ZrP), phosphotungstic acid (PWA) and silicotungstic acid (SiWA) were used. The second one is using the crosslink to reduce swelling in the case of increasing the phosphate content. In this paper following experiments were conducted:1. Synthesis of PBI from 3,3',4,4'-amino-diphenyl ether,5-sulfonated isophthalate sodium and isophthalic acid by direct polycondensation and then different sulfonated PBI were produced. The membranes were doping with phosphate and conductivity was tested using electrochemical workstation. Fenton test was also done. We choose the basic membrane to doping with inorganic proton conductor based on the excellent oxidative property. The results show that SPBI-10 is the candidate because its acceptable oxidative property.2. Preparation of the inorganic proton conductor zirconium phosphate (a-ZrP). In the previous studies, small size and large surface is benefit to proton transfer. The smallest size of a-ZrP was chosen throughout controlling the experimental conditions. The results show that reacting at 90℃in 5M phosphate for 48 hours it would get the smallest size. 3. Doping the conductor into PBI membranes to get the inorganic-organic hybrid membrane. The conductivity changed with the conductor content was tested. The results show that, to a certain extent, the conductivity rises up in the conditions that other properties keep no change.4. Synthesis of PBI-OH from 3,3',4,4'-amino-diphenyl ether, 5-sulfonated isophthalate sodium,5-hydroxyl isophthalic acid and isophthalic acid by direct polycondensation in polyphosphoric acid (PPA). The conductivity, phosphate doping level and other physical properties were tested. The results show that SiO2 inorganic network structure was formed, which was useful to improve the conductivity.