高温聚合物电解质膜燃料电池膜电极中磷酸分布及调控策略研究进展

Advancement in Distribution and Control Strategy of Phosphoric Acid in Membrane Electrode Assembly of High-Temperature Polymer Electrolyte Membrane Fuel Cells

高温聚合物电解质膜燃料电池(HT-PEMFC)由于其较高的运行温度(140–200℃)而具有较快的电极反应动力学和良好的抗CO等杂质气体毒化能力以及简化水热管理等优势,是PEMFC的重要发展方向之一。HT-PEMFC的核心部件为基于磷酸掺杂聚合物电解质膜(HT-PEM)组装的膜电极(MEA)。在高温膜电极(HT-MEA)中,一方面聚合物电解质膜和催化层中的离子传导极大地依赖于磷酸的含量;而另一方面磷酸分子填充在高分子链周围会引起聚合物膜力学性能的下降,迁移进催化层中的磷酸容易导致阴阳极催化层的“酸淹”以及在铂催化剂表面吸附而降低催化剂活性。因此,研究磷酸在高温聚合物电解质膜电极中的分布状态和迁移过程,对构建高性能和高稳定性的HT-PEMFC至关重要。基于此,本文对近年来HT-MEA中磷酸的分布、动态迁移过程的研究现状进行了梳理分析,对HT-MEA(包括高温聚合物电解质膜和催化层)中磷酸分布和迁移的调节与优化策略研究进展进行了较全面的综述,并对其未来发展趋势进行了评述和展望。

High-temperature polymer electrolyte membrane fuel cells(HT-PEMFCs)have the unique advantages of fast electrode reaction kinetics,high CO tolerance,and simple water and thermal management at their operating temperature(140–200℃),which are considered as one of the important research directions of PEMFCs.Membrane electrode assemblies(MEAs),as the core component of HT-PEMFCs,are usually fabricated by sandwiching phosphoric acid(PA)-doped polymer membrane(HT-PEM)between two electrodes.Technically,high PA content is required in HT-PEMs to ensure fast proton conduction,since PA acts as a proton transport carrier,while a high content of PA decreases the interaction among polymer molecules,thus enhancing the movement of the polymer molecules and leading to a decrease in the mechanical strength of the polymer membranes.In addition,PA is driven into catalyst layers owing to capillary force caused by micropore structures,crack connectivity,and accessibility.The PA content in the electrodes is also affected by the hydrophilic/hydrophobic characteristics of the catalyst layers and the surface tension of the acid when it is in close contact with the catalyst layers.Furthermore,PA plays an important role in the construction of electrochemical triple-phase boundaries to promote electrochemical reactions in the catalyst layers.Simultaneously,as a liquid or"free molecule",the migration of PA may be accelerated by the current and the water produced,owing to the formation of charged phosphates or hydronium ions.This process encourages the redistribution of PA within the catalyst layers,and results in acid flooding of the catalytic layers and adsorption on the surface of the platinum catalyst,leading to increased mass transfer resistance for the gas reaction and reduced catalyst activity.Moreover,the increase in supplied absolute flow rate and the temperature elevation in the HT-PEMFC process could accelerate the evaporation of PA from the electrolyte membrane,resulting in a decrease in the stability of HT-PEMFC and corrosion of the metal end plate.Therefore,it is crucial to regulate the distribution and migration of PA in MEAs for the construction of HT-PEMFCs with high performance and stability.Hence,this paper reviews the research status of PA distribution in HT-PEM electrodes in recent years,and summarizes the corresponding regulations and optimization strategies as well as its future development trend.