Study On Mechanical And Electrical Performance Of Epoxy Packaged μDMFC

Micro direct methanol fuel cells (μDMFCs) have broad application prospects in portable electronics, MEMS and so on, due to their advantages of room temperature operation, few pollution and high energy density. In fact the techniques of epoxy resin packaged μDMFC developed on the basis of microelectronics and MEMS technology, which can further improve the cell energy density. However, the inner stress changes during the assembly process and owing to creep characteristic of epoxy inner stress keeps up receding in long term, which will decrease the cell performance and life. Consequently, several μDMFCs with different endplate structure were designed and fabricated in this dissertation. Meanwhile, AC impedance spectra and polarization curves are tested intermittently in long term to investigate the effect of structure on cell performance and degradation.Firstly, the process of epoxy resin packaging μDMFC was designed. Taking liquid epoxy leakage, curing and demoulding into consideration, the process is divided into two steps: pre-assembly and curing. In first step the pre-assembled μDMFC was positioned in the mould and then epoxy resin was poured into the mould. In second step, the fuel cell within mould was cured by epoxy on platform.Secondly, four endplates with different structure were designed to assemble the μDMFC and the assembly processes were simulated. It indicated that the residual mean normal stress in GDL-MEA interface is lower than the initial assembly pressure due to resilience. When the initial assembly pressure is2.0MPa, the residual mean normal stress will approximate to optimal pressure which is experimentally proved to be1.5MPa in our paper, so the value of2.0MPa is used as initial assembly pressure. The simulation results also revealed that the normal stress and its distribution were influenced by the endplate structure. The residual mean normal stress in cell with endplate of9point-like convex array was almost equal to1.5MPa, but the normal stress standard deviation in that was0.65MPa, meaning uneven stress distribution. Comprehensively taking into consideration residual normal stress, stress distribution and contact resistance, endplates of16and25point-like convex array are better.Finally, AC impedance characteristic of epoxy packaged μDMFC and cell performance degradation were studied. The initial AC impedance spectra were fitted by equivalent circuit to investigate the inner electrochemical information. The results showed that the Rs, Rmt and Rct were associated with the normal stress and its distribution which were influenced by endplate structure. The inner resistance in μDMFC with endplate of9point-like convex array increased rapidly from1.02Ω (the first day) to4.1Ω (the100th day). In the end, the AC impedance spectra varying with time demonstrated that the main reasons of cell performance degradation were epoxy creep and transport resistance increasing.