Study On Scale-up Effects And Flow Distribution And Water Management Processes Of Proton Exchange Membrane Fuel Cells

Higher power output is a big trend in the development of proton exchange membrane fuel cells(PEMFC),where the core issue lies in the scaling up of the fuel cell stack.The scale-up effects would arise in the sizing up and numbering up processes and consequently influence the fuel cell performance and durability.Therefore,the study on the scale-up processes is of theoretical and practical significance to the development of PEMFCs with higher power,higher power density,and higher durability.In this thesis,the scientific and technological issues involved in the scaling up of PEMFCs are systematically studied through numerical simulation,theoretical analysis,and experimental investigation.The transport-reaction interactions behind the size-up and number-up processes are revealed.The key technologies including water management and flow distribution which greatly impact the scaling up of PEMFCs are investigated.The main contents are given below.(1)The effects of channel length,electrode structure,and flow distribution on cell performance are investigated.The inherent relationship between scale-up effects and water management is revealed.Results show that the cell performance decreases with a longer channel length(i.e.,a cell with a larger area).This size effect is closely related to the water accumulation phenomenon inside the cell,which is caused by the increased capillary diffusion distance of liquid water.Additionally,its diffusion coefficient would decrease due to the defective dent structure of the microporous layer(MPL),further aggravating the water flooding and gas blockage and degrading the cell performance.It is found that when the channel length is extended from 50 mm to 100 mm.200 mm,300 mm.400 mm.and 500 mm,the maximum power density nonlinearly decreases by 1.0%,3.9%,11.9%,18.6%,and 30.5%,respectively.With the numbering up of flow channels or single cells,the flow uniformity becomes worse.The channel or cell with less reactant supply causes the co-suppression effect on gas-liquid transport processes and dominates the power output of the fuel cell stack.It can be summarized that the weakened water removal rate is the common derivative effect of the size-up and number-up processes.The scaling up of PEMFC s relies on key technologies including enhanced water management and flow distribution,etc.(2)The stability analysis approach is tactfully applied to the PEMFCs and the multiplicity,dynamic,and stability features of the water and heat management processes are investigated.For the cold-start process,the nonlinear heat generation term and linear removal term are constructed against ice fraction.On this basis,the critical ice fraction of the transition from ice formation to the melting process is obtained,which is corresponding to the ice fraction of the unstable point.The explicit criteria for the successful cold-start are proposed.The first is the mass transfer prerequisite and the second is that the ice fraction inside the cell must be lower than the critical value.For the normal operation process,the nonlinear water generation term and linear removal term are constructed against the current density and three steady states are obtained including the shutdown point,intermediate unstable point,and right-hand stable point.When operated at the intermediate unstable state,dehydration or flooding phenomena would arise due to disturbance,consequently deteriorating the cell performance and durability.Instead,the stable state is the desired operating point where the water removal and cell performance could be enhanced under the premise of membrane hydration by optimizing the operating parameters.(3)The two-level flow distribution issues are investigated and the mechanism and technology therein are highlighted.The essences of the stack-level and cell-level flow distribution processes are the flow rate and velocity distribution,respectively.Regarding the stack-level issue,the optimized design scheme of the stack manifold is formulated.The pressure drop of headers is dominated by the outlet header because the pressure recovery is found in the inlet header.To reduce the flow resistance of headers and improve the uniformity,the sufficient size of the outlet header to make the resistance ratio of headers less than 1/4 is the prerequisite.The second is the consistency of pressure distribution in the inlet and outlet headers and the optimal ratio of cross-sections is found to be 1:2.In the cell-level issue,the flow uniformity mechanism based on the radial flow regulation is further revealed considering the inherent relationship between the flow structure and distributor geometry.On this basis,a novel combined-mesh-type transition zone design is proposed,which effectively improves flow uniformity.In this design,the generation and distribution of radial flow are proactively regulated by the central horizontal meshes and lateral vertical meshes.The efficient transport of axial kinetic energy and the uniform velocity distribution are achieved in the transition zone with an abrupt expansion geometry.The critical configuration parameters including the porosities of center and lateral meshes are determined by the undesired radial velocity components above the parallel gas channels.(4)The manipulation schemes of MPL manufacturing are formulated and the relationship between manufacturing and operation processes is discussed.The aim is to form the MPL with a homogeneous porous structure to improve water removal.Results show that a well-dispersed ink with small agglomerates yields a homogeneous MPL structure while ink with high agglomeration produces a heterogeneous porous structure with defective dents and cracks.Therefore,the core regulation principle of the MPL manufacturing processes lies in the uniform dispersion of MPL inks.Based on the ink preparation experiments,the effects of components on the ink microstructure are clarified and an optimal ink formulation is proposed:the polytetrafluoroethylene(PTFE)loading is 30%and the weight ratio between isopropanol and water is 1:2.Concerning the slot die coating process,the fundamentals of flow pattern transition(Poiseuille-Couette-bubbly flow)behind coating limits are clarified.The Couette flow is advantageous for uniform shear rate and dispersion.Furthermore,phase-field simulations systematically investigate the effect of fluid and geometrical parameters and quantitatively present the upper flow limit of the new coating window.Validated by the coating experiments,the new coating window could effectively regulate the coating shear flow,therefore promoting the ink dispersion and the uniformity of the porous structure.