Abstract:

Phosphoric acid (PA) doped polybenzimidazoles (PBI) has become a preferred material for high-temperature proton exchange membrane fuel cells (HT-PEMFCs) due to its excellent thermal and chemical stability, as well as its high glass transition temperature. However, the weak dissociation and diffusion rate of phosphoric acid molecules at low temperatures resulted in poor proton conductivity of the membrane and difficulties in cold start-up of the fuel cell. Therefore, developing high-temperature proton exchange membranes that can operate efficiently over a wide range of temperatures and humidities is currently a challenge. In particular, extending the low-temperature operating window and achieving cold start-up capability are of great significance for the practical applications of fuel cells, such as in vehicles. In this study, a series of PA doped gel state polybenzimidazole proton exchange membranes with flexible alkyl sulfonic acid side chains were designed and synthesized through a polyphosphoric acid sol-gel process and a lactone ring-opening reaction. The effects of introducing sulfonic acid and varying the side-chain length on PA doping level, proton conductivity, and stability under different temperatures and humidities were investigated. The results showed that the prepared gel state membranes had a porous structure with self-assembled layer stacking, which facilitated the absorption of a large amount of PA and provided fast pathways for proton transmission. Among them, PA/PS-PBI exhibited proton conductivity performance superior to other reported works over a wide temperature range. Specifically, at room temperature, its proton conductivity increased from 0.0286S/cm of the original membrane to 0.0694S/cm, which was further increased to 0.1619S/cm and 0.3578S/cm at 80℃ and 200℃, respectively. In addition, the proton conductivity values of these membranes below 80℃ under 0% relative humidity (RH) were comparable to those of Nafion membrane under 100% RH, providing a new solution for breaking the classic definition of proton exchange membranes and achieving for applications in wide temperature range (25—240℃) fuel cell operations.

Key words: fuel cells, gel, polybenzimidazole, membrane, proton conductivity

摘要：

磷酸（PA）掺杂聚苯并咪唑（PBI）以其优异的热化学稳定性和高玻璃化转变温度成为高温质子交换膜燃料电池（HT-PEMFCs）的首选材料。然而，由于低温下磷酸较弱的解离度和传递速率，导致膜的质子传导性能不佳，电池冷启动困难。因此，研发可在宽温湿度范围内高效运行的高温质子交换膜成为当前挑战。特别是拓宽其低温运行窗口、实现冷启动对这类质子交换膜燃料电池在新能源汽车领域的实际应用具有重要意义。本文通过多聚磷酸溶胶凝胶工艺与内酯开环反应设计并合成了一系列磷酸掺杂的具有柔性烷基磺酸侧链的凝胶型聚苯并咪唑质子交换膜。重点探究了烷基磺酸的引入以及侧链长度对磷酸掺杂水平、不同温湿度下的质子传导率及稳定性的影响规律。研究结果表明，所制备的质子交换膜具有凝胶型自组装片层堆叠的多孔结构，有利于吸收大量磷酸并提供质子快速传输通道。其中，PA/PS-PBI展现出了在宽温域范围内均优于目前所报道的其他工作的质子传导性能。特别是常温下，其质子传导率从原膜的0.0286S/cm提升至0.0694S/cm。80℃下，其质子传导率从原膜的0.1117S/cm提升至0.1619S/cm。200℃下，其质子传导率从原膜的0.2609S/cm提升至0.3578S/cm。此外，该膜在80℃和0%相对湿度（RH）条件下仍可具有与Nafion膜在100%RH时相当的质子传导率，为打破质子交换膜经典定义、实现宽温域（25~240℃）运行提供新的方案。

关键词: 燃料电池, 凝胶, 聚苯并咪唑, 膜, 质子传导率