质子交换膜燃料电池中微纳输运过程的数值研究进展

Numerical investigation on the nano/microscale transport processes in proton exchange membrane fuel cells:A review

质子交换膜燃料电池是氢能利用的典型装置.在燃料电池的多尺度空间内发生着复杂的相变多相流、传热传质、电子质子传导、电化学反应等物理化学过程.上述过程对电池的性能、寿命及成本影响显著.近年来,随着先进实验手段、数值方法和计算资源的不断发展,研究者基于微纳米尺度研究燃料电池中发生的复杂多场耦合输运过程,不断发现新的微纳输运过程特征及耦合机制.本文回顾了近年来针对燃料电池关键组件(包括催化层、气体扩散层和气体通道)中发生的多场耦合输运过程的微纳尺度数值仿真工作.针对催化层,主要介绍了孔尺度数值仿真在预测有效传输系数、揭示传质阻力机理、查明微纳结构对反应输运过程影响方面的进展.针对扩散层,重点介绍了孔尺度仿真在研究扩散层气液两相流动及查明结构和润湿特性对液态水运动和分布影响的工作,还讨论了气体扩散层薄层多孔介质输运特性及典型代表单元是否成立.针对气体通道,着重介绍了通道中液态水运动及其对传质反应的影响.此外,还讨论了各组件跨尺度界面行为特性.最后,对采用微纳尺度数值方法研究燃料电池内多场耦合输运过程进行了总结和展望.

The proton exchange membrane fuel cell(PEMFC)is a typical device for utilizing hydrogen energy.Inside the multiscale space of PEMFCs,complicated processes simultaneously take place,including phase change multiphase flow,heat and mass transfer,proton and electron conduction as well as electrochemical reaction.These processes are closely coupled,and significantly affect the cell performance,durability and cost.Recently,with the development of advanced experiment techniques,numerical methods and computer technologies,more efforts have been devoted to exploring the nano/microscale reactive transport processes inside the PEMFCs,and new nano/microscale transport phenomena and coupled mechanisms have been revealed.The present work reviews the recent nano/microscale numerical studies of multiphase reactive transport processes in porous electrodes and gas channels of PEMFCs.For the catalyst layers,pore-scale numerical studies of structure-process-performance relationship are introduced,including predicting the macroscopic transport properties,revealing the mechanisms of transport resistance and investigating the effects of nanoscale structures on the reactive transport processes.Particularly,recent work about the extra local transport resistance under low Pt loading inside catalyst layers is discussed in detail.The pore-scale study is helpful for understanding the local transport processes across the pore-ionomer interface,inside the ionomer film and around the reactive sites.Pore-scale study of multiphase flow inside catalyst layers is rare and future pore-scale work is highly required for understanding the behaviors of water generation,condensation and migration.For the gas diffusion layers,pore-scale studies of air-water two phase flow and effects of structures and surface wettability on the liquid water distribution are reviewed.Effects of the liquid water distributions on the transport properties(permeability,diffusivity,and thermal conductivity)are also discussed.Note that the gas diffusion layer is a thin porous medium,whose thickness is comparable to its typical pore size,leading to a possible failure of the concept of representative element volume(REV).Pore-scale numerical modeling of different transport processes inside the gas diffusion layer is required to identify the existence of the REV.Further pore-scale studies taking into account coupled multiphase flow and heat transfer process are required to understand the phase change multiphase flow inside the gas diffusion layer.For the gas channels,emphasis is placed on the liquid water dynamic behaviors under different structural and operating parameters.Future work should pay more attention to comprehensively evaluate the effects of dynamic liquid water behaviors and distributions on the mass transport and electrochemical reactions.The multiscale transport processes across the interface between different components are also discussed in this review.Finally,conclusions and perspectives of nano/microscale numerical studies of multiphase reactive transport processes inside PEMFCs are provided.It is suggested that porous structure reconstruction schemes,models for the coupled multiphase flow reactive transport processes as well as more accurate and efficient numerical methods should be further developed.Besides,advanced experiment techniques should be developed to provide more details of the distribution of important variables at the nano/microscale,thus to validate and improve the current nano/microscale numerical modeling.In addition,upscaling schemes are required to incorporate the nano/microscale numerical results into the cell-scale models,thus to fully understand the effects of nano/microscale processes on the cell performance.