质子交换膜燃料电池内氧气局部传质阻力的多尺度机理研究

Catalyst layers with high loading of platinum dominate the high cost of the polymer electrolyte membrane fuel cell (PEMFC), and reducing the platinum loading of the catalyst has been the main focus in recent years. When a PEMFC with low-loaded catalyst operates under the condition of high current density, the local resistance of oxygen caused by the ionomer covering on the platinum leads to a significant concentration activation loss. Fully understanding of the mechanisms of the local resistance is essential for the prediction of the transport resistance of oxygen and the performance of PEMFC accurately. Based on a down-top idea of the multiscale method, a microscale system with oxygen, thin film and platinum is firstly built and adopted to investigate the influence of the ionomer structure, water content and temperature on the transport properties of oxygen under the confinement effect with the molecular dynamics simulation, aiming to explain the mechanisms of the local resistance. Then, a pore scale numerical method is developed to study the transport processes in the complex microstructure of the catalyst layer considering the solution-diffusion of the oxygen, charges transports and chemical reactions, aiming to obtain the quantificational relationships between the local transport resistance and the structures of the catalyst layer. Finally, the limiting current measurement is conducted to validate the coupling mechanisms of the operation conditions and the oxygen transport resistance in the catalyst. These achievements are expected to provide a multiscale method for the optimization of the structures of the catalyst layer and instruct the design of PEMFC with low-loaded platinum and improved performance.

高铂载量的催化层是导致质子交换膜燃料电池成本高昂的重要因素，降低催化层铂载量是近年来的研究热点。高电流密度工况下，覆盖在铂表面的离子聚合物薄膜所产生的氧气局部传质阻力是导致低铂载量燃料电池浓差极化损失显著增加的原因。探究该局部传质阻力的来源机理是准确预测氧气传质阻力和燃料电池性能的关键基础。本项目基于“分层求解，尺度提升”的多尺度思想，首先构建氧气-薄膜-铂微观体系，采用分子动力学方法查明空间限域效应下薄膜结构、水含量和温度对氧气在该微观体系内的吸附传输特性的影响规律，揭示氧气局部传质阻力的来源机理；然后发展催化层内耦合电荷传输、氧气溶解扩散和电化学反应的孔隙尺度数值方法，归纳催化层结构参数与氧气传质阻力的定量关系；最后结合极限电流密度测试方法对实验操作条件与氧气传质阻力的耦合机制予以验证。本项目成果既可为优化催化层微观结构提供多尺度模型，又有望为降低铂载量并提高燃料电池性能提供指导依据。

离子聚合物薄膜所产生的局部传质阻力是导致电池性能降低的原因。探究其来源机理是准确预测电池性能的关键基础。本项目基于“分层求解，尺度提升”的思想，首先发展催化层内耦合电荷传输、氧气溶解传质及电化学反应的孔隙尺度数值方法，归纳催化层电极参数与氧气传质阻力的定量关系；其次，开发了一种全面考虑膜电极参数的团聚模型，全面研究燃料电池内的气-水-电-热耦合机制。主要研究结果包括：开发一种两相界面非平衡态溶解传质过程的耦合传质数值方法，应用于催化层内复杂界面的耦合传质研究中，该方法具有二阶数值精度；开发一种氢泵数值模型，研究了阳极催化层内反应气体的传质阻力，结果表明局部传质阻力由薄膜引起，氢气在薄膜内传输系数降低是局部传质阻力的主要来源，该传质阻力随着电解质含量和Pt/C比值的增加的增加；研究了氧气在阴极催化层内局部传质阻力，表明氧气在薄膜内扩散系数降低以及氧气在铂表面的吸附是额外局部传质阻力的来源；研究了催化层内液态水存在对局部传质阻力的影响，结果表明当不考虑氧气在液态水内的渗透时，液态水对局部传质阻力影响较大；研究了多种团聚结构的催化层内气体传质和电化学反应过程，结果表明当团聚结构能够全面反映电解质和碳载体不均质分布特征时，该种团聚结构能够应用于气体传质阻力的研究；最后通过引入聚集因子，开发一种全面考虑氧气传质阻力的团聚结构模型，并应用于燃料电池内多物理场耦合输运和气体在不同燃料电池组件内传输过程的研究。本项目成果对于优化燃料电池微观结构具有指导意义。本项目在执行期间共计发表国内与国际学术期刊论文10篇，其中SCI期刊论文8篇，影响因子大于6的期刊论文共计6篇，包括Applied Energy, Sustainable Energy and Fuels, Electrochimica Acta等国际权威期刊。培养在读硕士研究生3名。