| Zinc air fuel cell(ZAFC) was considered highly promising for off-grid power generation and electric vehicles because zinc possesses a unique set of characteristics,including easy storage and transportation, low cost, no emission and safe handling. In order to meet the commercialization requirement, the power density and lifetime of ZAFC should be significantly improved. Therefore, it is necessary to carry out overall research on factors affecting the performance and degradation mechanism of ZAFC, it is essential for designing and using zinc air fuel cell scientificly.A ZAFC scheme, which using inexpensive manganese dioxide(MnO2) as the catalyst of air cathode and zinc pellets as fuel, was proposed. The anode chamber with an open structure could bring benefits to cells in a stack with a uniform supply of the zinc fuel and electrolyte. The circulating flowing electrolyte with low pressure can prevent the hydrophobic hole in air cathode from flooding by electrolyte. After optimizing the anode chamber structure to maximaze the reaction area and electron transfer path between the air cathode and bipolar, the peak power density of fuel cell stack with an air cathode area of 215 cm2 can be as high as 435 m W/cm2. Furthermore, the influence of the temperature and flux of electrolyte and oxygen partial pressure were investigated, and finnaly power density of ZAFC can be 544 mW/cm2 and 899 mW/cm2 when air or oxygen was feed, respectively. Tests results shown that the time need for voltages of the fuel cell stack to reach steady state while current step-up or current step-down are both in milliseconds, and did not exist voltage overshoot phenomenon.The degradation phenomena and mechanism were studied. An equivalent circuit model was set up to analysis the impedance characteristic of ZAFC. The results of electrochemical impedance spectroscopy showed that the charge transfer resistance and mass transfer resistance of air cathode were increased when the ZAFC had operated for a long time. The investigation results of air permeability and the microstructure and morphology of air cathode by scanning electron microscopes(SEM)indicated that the degradation mechanism of ZAFC was the clogging of micropores and loss of catalyst particles of air cathode. During the shutdown tests of ZAFC, anagglomeration of zinc pellets and expanding of anode were found when shutdowm with a dry model. The air cathode would be corroded by electrolyte when shutdowm with a soaking model, which would lead to hydrophobicity of air cathode decresing and pore structure changeing. A phenomenon of water diffusion through the air cathode was found when the catalytic layer and gas diffusion layer were soaked with electrolyte and water respectively, which could be used to penetrate the clogged pores.A ZAFC system with an electronic control unit was developed. The tests of starting up and operating repeatedly prove that the technology road-map for ZAFC system is applicable. Moreover, regenerating of fresh zinc particles directly from strong alkaline solutions by electrowinning was developed successfully.The Innovative research results improved the power density of ZAFC enormously, and revealed the performance influence factors and degradation mechanisms. Furthermore, technologies to penetrate clogged pore of air cathode and regenerate zinc particls were achieved. Therefore, it can provide theoretical guidance for designing and using ZAFC, and layed a foundation for its application. |
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