Fatigue Fracture Properties And Mechanism Of Proton Exchange Membrane

As a core component of fuel cells,the proton exchange membranes(PEMs)functions to transfer protons and isolate reactive gases.Therefore,the structural integrity of the PEMs is critical to the durability of fuel cells.For outstanding problems that limit the durability of PEM machinery,the effects of multiple factors on PEM fracture and fatigue properties were investigated,providing reference and guidance for the development and optimization of PEM.According to the influence of mechanical reinforcement layer(external factors),the fracture properties of two kind of PEMs were studied by J-R curve method and EWF method quantitatively.The results shown that the crack propagation resistance of the Nafion XL in the MD direction was the largest and that of Nafion 212 was smallest.While in water environment,the crack propagation resistance of the Nafion 212 was significantly reduced,while the Nafion XL was almost unaffected.The presence of the mechanical reinforcement layer reduced the sensitivity of the membrane to the humidity environment,thereby improving the fracture performance and durability of the mechanically enhanced PEMs in the humidity cycle.Aiming at the effect of cation interaction(internal factors)on the fracture properties of PEMs,the fracture properties and fracture morphology of different monovalent alkali metal cation membrane materials in air and water were compared.It was found that both cation contamination and water environment can change the crack propagation mode from toughness to brittle fracture.This phenomenon can be explained by effective crosslink density theory.Although the inter-ionic force is enhanced after ion exchange,the amount of ionic cross-linking is also reduced,thereby reducing the effective cross-linking density and the crack propagation resistance.In water environment,the shielding effect of water reduces the overall effective crosslink density by reducing the amount and strength of ionic crosslinks.The effects of different temperature and humidity environments on the fracture properties of Nafion 212 were investigated.Based on GTN model,the fracture process of PEMs under different temperature and humidity conditions was simulated by means of FEM.The results shown that the experimental and simulation data fit well at room temperature in air and water,which indicates that the model could better describe the fracture and failure law of PEMs under different temperature and humidity environments.The effect of the reinforcement layer on the fatigue crack growth performance of the PEM was investigated.It was found that the fatigue crack growth(FCG)rate of Nafion 212 was affected by factors such as stress ratio and increases with the increasing of crack length.However,the FCG rate of the Nafion XL was almost constant and closely related to the initial crack length and material orientation.The Nafion XL exhibited crack propagation in the outer ionomer layer and the interfacial delamination between the ionomer layer and the reinforcement layer during fatigue crack propagation through microstructural observing.The balance between two factors is the main cause of the difference.In order to simulate the true stress state of PEMs during service,the biaxial fatigue crack propagation behavior of Nafion 212 was studied by in-plane biaxial cyclic loadings.The FCG rates of flat cracks under different loading conditions were compared,and the effect of transverse stress on FCG rate was clarified too.From the perspective of experimental and theoretical analysis,the FCG rate and propagation paths of slant cracks under uniaxial and biaxial loading conditions were studied.The crack propagation path of the bifurcation and turning were similar to the crack shapes of the membrane under the real fuel cells.This phenomenon clarified the mechanism and mechanical driving force of PEM crack formation under operating conditions.