燃料电池超低铂阴极的结构设计及动力学和传质机理研究

It is necessary for the cathode of PEMFCs to use ultra-low platinum loading (≦0.1 gPt/kW) in order to commercialize the fuel cell electric vehicles. However, the principles of electrode kinetics of oxygen reduction reaction (ORR) and mass transport for conventional cathodes with low platinum loading do not apply to the cathodes with ultra-low platinum loading. Moreover, the issue of local mass transport surrounding Pt particles in the catalyst layer has a more impact on the fuel cell performance for ultra-low platinum cathodes than for low platinum cathodes. In this project, we will first prepare PEMFC cathodes with ultra-low platinum loading using different types of Pt-based catalysts, and then investigate the effects of catalyst type, specific activity, Pt loading as well as the cathode structure on the cell performance. Furthermore, the dynamic coverage of the Pt-oxide on the cathode with ultra-low platinum loading will be measured at low current densities and its influence on the ORR kinetics will be elucidated. The IR- and mass transport-corrected ORR kinetics on the cathode with ultra-low platinum loading at high current densities is investigated by changing the pore size, and the influence of both molecular diffusion and Knudsen diffusion on the ORR kinetics is discussed. The influence of water film thickness as well as the dimension and morphology of the Nafion ionomer on the Pt surface on the local oxygen mass transport is studied. The mechanism of effective local oxygen mass transport on the cathode with ultra-low platinum loading is clarified. In a summary, this project will not only provide a solid scientific guideline for developing cathodes with ultra-low platinum loading via finding novel high-performance catalysts or designing new cathode structures, but also contribute to improving the theories of both electrocatalysis and porous electrodes, thus accelerating the commercialization of PEMFCs using cathodes with ultra-low platinum loading.

阴极超低铂化(≦0.1gPt/kW)是车用燃料电池走向商业化应用的必由之路。然而，传统的低Pt阴极动力学和传质规律并不适合于超低Pt情况。同时，超低Pt阴极氧跨催化层Pt粒子界面的局域传输对电池性能影响更加突出。本项目将首先研究不同Pt催化剂的超低Pt阴极，探索催化剂类型、比活性、Pt载量和阴极结构等对性能的影响，阐明超低Pt阴极在低电流下含氧物种的动态变化对氧还原动力学的影响规律。通过研究不同尺度通道的超低Pt阴极在大电流下欧姆和传质校正的动力学，探索通道中氧分子扩散和克努森扩散对动力学行为的影响规律。通过覆盖于Pt表面水膜厚度与Nafion尺寸和形貌及其对氧分子有效局域传输影响的研究，阐明超低Pt阴极氧的有效局域传质机制。本研究不仅可指导新型超低Pt催化剂和阴极的结构设计，为超低Pt阴极开发奠定科学基础，还可以丰富电催化理论以及超低Pt多孔电极理论，推进超低Pt燃料电池的商业化进程。

本项目针对质子交换膜燃料电池超低Pt化带来的大电流密度区性能急剧恶化、高电位催化剂衰减加剧等问题，对低电流密度区氧还原动力学和大电流密度区传质规律进行了深入的研究，取得了一系列创新性成果。首先，创新设计“催化层-类催化层”双层阴极结构，首次通过实验方式量化了氧气分子跨越Pt基纳米粒子表面超薄水膜和Nafion树脂膜的局域传输阻力，揭示了其对超低Pt燃料电池性能的主导作用；阐明了测试条件对超低Pt阴极局域传质的影响规律，提出吸附控制的“吸附-扩散”模型。其次，通过调节阴极孔道的尺寸、离聚物分布状态和催化剂载体形貌等方式，探明了通道里的氧气分子扩散动力学行为与纳米孔道尺寸的关系，对经典的有效扩散系数公式进行了修正，提出有效孔隙率概念，指导超低Pt电极结构和性能的不断优化。第三，揭示了低电流密度下Pt基催化剂表面含氧物种的动态变化规律，指导设计合成了一系列具有特殊结构的新型高性能Pt基催化剂（如具有成分渐变内核及单原子层外壳的核-壳催化剂、新型三明治结构Pt-Ni二十面体催化剂、双壳层截角八面体Pt-Ni合金催化剂等），进一步地，阐明了新型低Pt、超低Pt催化剂的合成机理及其载体、尺寸、形貌、组分等与氧还原活性的构~效关系。第四，系统地研究了Pt基合金催化剂的衰减机制，提出采取核壳模型来描述Pt基合金催化剂结构，构建模型预测电池运行中电化学活性面积、催化层中的粒径分布以及催化剂结构的演变，进一步地，揭示了外部电势加载模式对耐久性的影响机制。最后，基于上述理论，项目提出了发展高性能、长寿命超低Pt电极的高比表面积催化剂设计合成策略、纳米尺度离子树脂形貌调控策略以及有序纳米结构催化层构建策略等。相关成果为超低Pt阴极的开发奠定科学基础并提供技术支撑，极大地推进超低Pt质子交换燃料电池的商业化应用。