Pore-Scale Modeling Of The Physicochemical Phenomena In The Electrode Of Proton Exchange Membrane Fuel Cells

For the widespread commercialization of proton exchange membrane(PEM)fuel cells,there remains two major issues to be addressed:cost and durability,which are both related to the electrode.The electrode is the heart of PEM fuel cells,including gas diffusion layer(GDL),micro porous layer(MPL),and catalyst layer(CL).Each layer has a district heterogeneous microstructure controlling the complex phenomena.Changes in the microstructure in turn affect its performance and durability.Therefore,the electrode should be further optimized to achieve a better performance under a low Pt loading,which bases on an in-depth understanding of the inner physicochemical phenomena along with its dependency on the electrode microstructure.With a focus on the microstructure-transport-performance interlink,pore-scale modeling has evolved to a powerful tool to develop the electrode.In this dissertation,a comprehensive pore-scale modeling platform is developed to obtain a deeper insight into the electrode,including stochastic reconstruction algorithms to build the electrode microstructure as computation domain and a versatile lattice Boltzmann(LB)model to resolve the reactive transport and two-phase flow phenomena.Each layer of the electrode is realistically reconstructed with every ingredient considered and gets validated by comparing with electrode samples.The numerical tool,LB model,is further developed to fix the inherent issues on high spurious velocity,low density&viscosity ratio,thermodynamic inconsistency,limited computation time-scale,etc.The model is validated by traditional benchmarks and also by comparing with experimental data.The self-developed LB model is efficiently paralleled on the graphics processing unit(GPU)workstation to conduct large-scale simulations.With the developed modeling platform,for the first time,the effect of GDL on droplet dynamic behavior is studied with realistic multicomponent multiphase flow considered.The GDL microstructure can amplify the material wettability,affect the motion direction,and impede the droplet motion.More hydrophobic GDL can effectively accelerate the transport.It is observed that the droplet could reach the sidewall due to the presence of GDL and get stuck in the channel,which is hard to avoid but deadly for the water management.For this issue,the flow channel is adjusted to be hydrophilic and the liquid water will be removed from the GDL surface to be stored in upper corner of the flow filed,which can effectively decrease the pressure drop and prevent reactant maldistribution.Then,the structural effect of GDL/MPL assembly on two-phase flow is investigated in terms of hydrophobicity,pore-size distribution,and assembly design.It is found that more hydrophobic material and smaller pore apply higher local capillary force against the water transport.For the assembly without MPL,GDL suffers flooding and the reactant transport space is significantly decreased.Crack-free MPL blocks the liquid water in the CL,which ensures the gas transport space in the GDL but could cause flooding in the CL.To balance the water content in the electrode,a systematical perforated GDL/MPL design is proposed and can automatically separate the transport of liquid water and reactant,which is an ideal water management strategy.Regarding the CL,the effects of ingredient content,structure design,and carbon support are investigated for the production optimization.Under the 0.3 mg cm-2 Pt loading,a higher Platinum/Catalyst(Pt/C)ratio yields a thinner CL structure,which can significantly decrease the reactant transport loss and improve the performance.For the same Pt/C ratio,a higher Ionomer/Catalyst(I/C)ratio yields a larger electrochemical surface area(ECSA)but also a higher transport loss,which should be delicately balanced to reach an optimum performance.To simultaneously ensure the transport and ECSA,a novel CL design with artificial mesoscale pore is proposed.The mesoscale pore served as an efficient transport channel significantly decreases the transport loss under a high I/C ratio and can boost the performance by 50%.Besides,high-surface-area carbon support can also improve the performance by decreasing transport resistance and enlarging the ECSA,which helps to achieve a lower Pt loading.Lastly,for a deeper insight into the contamination mechanism of hydrogen sulfide(H2S),for the first time,the pore-scale model is applied to investigate the poisoning phenomena in the anode CL.A novel iteration algorithm is proposed to overcome the inherent time-scale issue in the LB model,which can extend its engineering application.The proposed model is validated by comparing with experimental data and can accurately predict the effect of H2S contamination on performance with time.The H2 S affects both the reactive area and catalytic efficiency of Pt particles.When around 70%reactive area of the platinum particle is poisoned,its catalytic efficiency will drop rapidly.For the anode side,the performance is not sensitive to Pt loading under a clean condition,but a slightly higher loading could significantly prolong the life against the contamination for a larger buffer reactive area and a lower average concentration of H2S.Furthermore,the concentration of H2S in the fuel gas should be strictly restricted as it directly affects the poisoning rate and determines the electrode durability.