A Multiscale And Multiphysics Study On Coupled Mechanical Stress And Transport Properties Of PEMFC

Proton Exchange Membrane Fuel Cell(PEMFC)has received widespread attention and tremendous research has been carried out,mainly because of its high energy conversion efficiency and power density,low operating temperature and no pollutant emissions,etc.The PEMFC has been considered as the most promising energy device that can achieve carbon neutrality.Mechanical stress is critical to enhance the performance and durability of the PEMFC during the assembly process of the PEMFC stack.Mechanical stress can effectively reduce the thermoelectric contact loss between each component and prevent the leakage of reaction gases.However,it can also cause mechanical compression deformation of the porous medium components in the cell.The gas diffusion layer(GDL)is prone to deformation due to its low Young’s modulus.Mechanical stress directly affects the transport properties in GDL and then affect the PEMFC performance.To study the effects of mechanical stress on transport properties,the present study uses numerical simulation and experimental verification methods,as well as multi-scale tools to investigate the transport phenomena and performance of PEMFC under compression force.This thesis consists of four themes,i.e.,(1)two-dimensional single-phase macroscopic model(2)twodimensional two-phase flow macroscopic model(3)GDL microstructure reconstruction and transport properties analysis(4)multi-scale two-phase flow model to study the internal transport properties of PEMFC.In the first part of this thesis,a macroscopic,single-phase,steady-state model that couples solid mechanics,electrochemical reactions,and heat and mass transfer is established.This model investigates the effects of a few design parameters,such as the tilt angle of the metallic bipolar plate(BPP),the width of flow channel on the stress distribution in GDL and BPP,and the contact resistance between the BPP and GDL.Then the effect of mechanical stress on the heat and mass transfer and i-V curve of PEMFC is studied.It shows that stress reduces the gas transfer capacity and improves the heat transfer capacity in the GDL,particularly under the ribs region.The impact of mechanical compression on heat and mass transfer becomes more pronounced when the current density increases to a high level.It is found that the cell performance for under 20% GDL strain mechanical stress is optimal.In the second part of this work,a macroscopic,two-phase,steady-state model that couples solid mechanics,electrochemical reactions,heat and mass transfer,and gasliquid phase-change is developed.Oxygen,temperature,and liquid water saturation distributions over a range of current density are obtained at the baseline compression level of 20% GDL compression strain.Five compression strain ratios ranging from 5%,10%,15%,20% and 25% are then investigated to study the effect on stress distribution,transport properties and cell performance.The simulation results show that significant GDL deformation and channel intrusion are caused due to the existence of ribs and channels in BPP.The maximum liquid water saturation occurs at the interface between the GDL and the ribs.Liquid water saturation is found to increase with increasing current density.The performance of PEMFC firstly increases and then decreases as the mechanical compression strain ratio increases from 5% to 25%.A compression strain ratio of 20% is found to yield the optimal performance,balancing between high contact resistance caused at low compression and high transport resistance at high compression.In the third part of this work,two microscopic reconstruction methods,stochastic numerical and X-ray computed tomography(XCT)are used to reconstruct two commonly commercial GDLs: Toray GDL and Freudenberg GDL,respectively.Then,the porosity and pore size distribution of the two different GDL materials are compared and analyzed.A pore scale model(PSM)is employed to compute the effective transport properties in the in-plane direction and the through-plane direction,including the gas diffusivity,electrical conductivity and thermal conductivity.The correlations of mechanical compression strain ratio and effective transport properties of both types of GDL in the in-plane direction and the through-plane direction are obtained.Furthermore,a lattice Boltzmann method(LBM)is used to determine the relationship between the effective liquid water permeability and the mechanical compression strain ratio as well as the saturation and capillary pressure in the in-plane direction and the through-plane direction.The results show significant anisotropic transport properties of the Toray GDL due to its disc-shape binder at the intersection of the fibers.On the other hand,the Freudenberg GDL shows mostly isotropic transport properties due to its uniformly distributed carbon fiber without binder.This part of research not only provides a reliable framework for studying the GDL microstructure,but also provides useful guidelines in the parameters for the simulation and experiment of PEMFC.In the last part of this work,the relationships between mechanical stress and transport properties obtained by PSM and LBM are combined with the macroscale twophase flow model to build a comprehensive multi-scale,multi-physics two-phase model.The distribution of saturation in the PEMFC with Toray GDL and with the Freudenberg GDL are compared and studied.The model results show that the saturation with the Toray GDL is mainly condensed in the catalyst layer area,while the saturation with Freudenberg GDL mainly occurs in the GDL area under the ribs.The results of two-phase flow model are verified by the experimental data of neutron technique to ensure the reliability and accuracy of the model.Compared with the existing simple macro-scale or micro-scale analysis,the present multi-scale,multiphysics model can provide accurate and reliable predictions.It can play a critical and beneficial guiding role for future PEMFC engineering applications.