质子交换膜燃料电池催化剂层中多相反应输运机理的孔尺度研究

One of the key components in proton exchange membrane fuel cells is cathode catalyst layer, which is a complex porous medium consisting of carbon, Platinum, electrolyte and pores. In the cathode catalyst layer, multiple physicochemical processes simultaneously occur including multiphase flow, heat transfer, mass transport, conduction of proton and electron, as well as electrochemical reaction. Understanding the coupled multiple processes in cathode catalyst layers and thus enhancing Pt utilization is of great importance for improving cell performance and reducing cell cost.This proposal aims at revealing mechanisms of the reactive transport processes in cathode catalyst layers at pore-scale. First, the complex nanoscale porous structures of catalyst layer are observed experimentally and reconstructed numerically. Then, a pore-scale model considering multiphase flow with phase change, heat and mass transfer, and electrochemical reactions inside the catalyst layer will be developed. The emphasis will be put on three basic problems, the mechanisms for which are currently not well understood. The first one is the effects of discrete distributions of Pt particles on the reactive transport processes at nanoscale; the second one is the mechanisms of transport across phase interface and the corresponding local transport resistance which becomes higher for lower Pt loading; the third one is the generation, transfer and distributions of liquid water and its effects on the mass transport and chemical reaction. Based on the above work, a porous agglomerate model will be developed to fully account for the porous structures and the multiple constituents of and the coupled reactive transport processes in the catalyst layer. With the help of the porous agglomerate model, effects of porous structures, multiple constituents, and phase interface on the reactive transport processes will be explored in detail. The ultimate goal of the proposal is to enhance the reactive transport processes in cathode catalyst layer and reducing Pt loading. The work of the proposal is helpful for the development of proton exchange membrane fuel cell vehicles.

阴极催化剂层是质子交换膜燃料电池的核心部件，是包含碳、催化剂Pt、电解液、孔隙等多组分微纳尺度多孔介质，其中发生着耦合的多相流、传热传质、质子/电子传导和电化学反应过程。深入理解催化剂层中多场耦合传递过程，是提高Pt利用率与电池性能、降低电池成本的关键。本项目表征并重构催化剂层实际三维多孔结构，发展考虑相变两相流、传热传质和电化学反应过程的孔尺度方法，研究催化剂层多场耦合反应输运过程规律。重点研究催化剂层中机理尚不完全明晰的三个问题：催化剂离散分布对反应输运过程的影响机制；跨成分界面反应输运过程和低Pt传质阻力机理；液态水迁移分布规律及其对反应输运过程的影响。在此基础上，构建基于物理而非经验的催化剂层模型，系统查明多孔结构、成分、界面特性、运行参数对传质阻力和电流密度的作用机制。最终，优化催化剂层结构，强化反应输运过程，降低Pt用量，为推动我国燃料电池新能源汽车行业的发展提供理论支撑。

由多种组分构成的微纳多孔阴极催化层是决定质子交换膜燃料电池性能及寿命的关键部件，其中发生着多个相互耦合的物理化学过程。实时动态的相关实验研究面临着极大挑战，本项目基于实验观测结果构建了与真实催化层高度一致的催化层结构，发展了描述催化层中多相反应输运过程的物理化学模型及相应的孔尺度数值方法；重点探讨了减铂方式、相对湿度、液态水含量等对催化层反应输运过程的影响规律。.对比减铂方式影响的研究中，重点探讨了低铂碳载铂、掺加裸碳、降低厚度三种减铂方式对催化层中反应速率的影响。研究发现，采用低铂碳载铂时，总反应速率最高，局部传质阻力Rlocal最小；简化模型解析发现，低铂碳载铂显著降低了局部铂颗粒密度，减少相邻铂颗粒之间对氧气的“竞争”，从而获得更高的总反应速率；提出了在催化层表面打孔降低局部孔隙率过低引起的传输阻力升高的问题，通过打孔，催化层中的总反应速率最高提升了15.7%。.考察相对湿度（RH）影响的研究中，发展了包含液态水分布的孔尺度模型和考虑超薄水膜的亚格子模型；重点研究了高比表面积碳载体（HSC）中发生毛细凝结对催化层性能的影响。研究发现，当大量铂颗粒处于HSC内孔中时，离聚物无法覆盖进入内孔，在RH较低时，铂颗粒利用率很低，Rlocal较大；在RH较高时，内孔中液态水能够活化其中的铂颗粒，提高铂颗粒利用率，降低Rlocal。.考察液态水含量S影响的研究中，重点关注了S与润湿性共同作用下Rlocal的变化。研究发现，当催化层为亲水表面时，液态水优先覆盖于固体表面，Rlocal升高；如果部分微孔中的水能够传导质子，少量液态水也能大大减低Rlocal。当催化层为疏水表面时，液态水优先形成水滴占据大孔，S≤0.6时，水对Rlocal的影响较小；如果部分微孔的水能够传导质子，S≥0.4时，液态水对催化层性能的促进作用才变得显著。因此，不同润湿性催化层当采用不同的结构优化策略。