Pore-scale Modelling Of The Mass Transport Characteristics In Air-breathing Cathode Of Membraneless Microfluidic Fuel Cells

As a miniatured power source for portable mobile electronic devices,membraneless microfluidic fuel cell?MMFC?is an important branch of the fuel cell technology,holding significant scientific impact and application prospects.Conventional direct liquid fuel cells suffer from the complicated water management and membrane degradation problems.By contrast,the MMFC could naturally segregate the fuel and oxidant by the co-laminar flow in microchannel,thus removing proton exchange membrane and complex bipolar plates.The air-breathing cathode MMFC could exploit the atmospheric oxygen from the ambient environment as the oxidant in the passive manners,and thus eliminates the mass transfer limitation caused by the dissolved oxygen and improves the cell performance.Very recently,however,the implementation of advanced anode configuration and fuel transfer enhancement greatly promoted the anode performance,making the air-breathing cathode the limiting factor at high current density.Thereby,aiming at optimizing the air-breathing cathode,the mass transfer and electrochemical reaction should be investigated and the coupled interaction should be detailed.At the present stage,however,numerical simulation of air-breathing MMFC is mainly based on macroscopic continuum models which assuming the GDL and CL as homogeneous porous component with isotropic transport properties.The impact and interaction of microstructure and mass transfer and electrochemical reaction are largely unknown,and the fundamental understanding of coupled mechanisms is needed.In this thesis,in order to provide in-depth understanding of the multiphysics processes for mass transport enhancement and structural design optimization in air-breathing cathode MMFC,pore-scale modeling is performed to investigate the coupled mechanism of multi-component transport processes and electrochemical reaction using the Lattice Boltzmann method?LBM?.This thesis covers the following work:?1?Based on the stochastic reconstruction method,the 3D microstructure of porous GDL and CL is reconstructed.The structure and morphology of the porous media can be manipulated by parameter control.Then the pore-scale simulation is conducted to predict important structural and transport properties.?2?Based on the reconstructed CL,a single-phase numerical model is developed using BGK-LBM.The main flow channel is also considered in this model to investigate the effect of flowing electrolyte on dissolved oxygen transfer.The effects of porosity,overpotential,isotropic/anisotropic structure,ionomer loading?weight ratio of ionomer to total electrode weight?,electrolyte flow rate and platinum loading on the oxygen transport and electrochemical reaction kinetics are investigated and discussed in detail.?3?A LBM model is developed based on the Fortran programming for the coupled physic-electrochemical processes in the air-breathing cathode with the main flow channel.The numerical instability caused by very large diffusivity ratio?10⁴?is solved.The effects of porosity,overpotential,electrolyte flow rate,isotropic/anisotropic structure,relative humidity on oxygen/water vapor diffusion and electrochemical reaction within pores,catalytic interface and electrolyte are investigated.The LBM modelling can provide these findings:?1?The conventional Bruggeman correlation overestimates the oxygen effective diffusivity and effective proton conductivity,thus overestimating the cell performance.Fitting the LBM predicted D_effff is determined as D_eff=?^3.5D_bulk.?2?The main factors affecting the oxygen transport and reaction rate are pore size,distribution of ionomer,number of reaction sites differences caused by changes of CL morphology.The increased porosity can reduce mass transfer resistance and reaction sites number,leading to the increased oxygen concentration and current density.The oxygen mass transfer resistance and reaction sites number will change with ionomer loading??_N?,the current density is higher when?_N=0.3.Catalyst preparation requires deliberate selection of ionomer loading,both to ensure sufficient reaction sites number,and to avoid excessive oxygen transport resistance due to redundant ionomer loading.The longitudinal anisotropic CL outperforms the isotropically and transverse anisotropically structured one,therefore developing ordered electrode is the direction of the next stage of CL structure design.The electrolyte flow rate has no prominent influence on oxygen transport,but increasing the electrolyte inlet flow rate will slightly reduce the average current density.While the influence of platinum particles is further considered,the dominant limiting factors on cell performance are sluggish oxygen reduction reaction?ORR?kinetics and platinum particles quantity.Catalyst preparation couldn't simply increase or decrease the platinum loading,and should consider both the cost and platinum utilization.?3?The GDL microstructure plays a significant role in oxygen transport.The decreased porosity can improve the mass transfer resistance,leading to the decreased local oxygen and water vapor concentration.The complex porous structure would cause non-uniform distribution of the local reaction rate.Besides,the cathode with anisotropic GDL outperforms the isotropically structured one.It's also found that the high relative humidity in the air can improve the water vapor concentration within the GDL.At high current density,in particular,the water vapor may condense into liquid phase and block the pores to hinder oxygen transfer.Therefore,it's suggested that the air-breathing cathode of MMFCs should operate at a lower relative humidity environment.