Investigation For Air-breathing Proton Exchange Membrane Fuel Cell

Air-breathing proton exchange membrane fuel cells(ab-PEMFCs) are widely recognized as the mostly promising portable power supplier due to its advantages such as high energy density, pollution-free, no complicated system needed and so on. It has the potential of becoming an ideal power source for portable electronic devices such as laptops, Pads, smartphones etc. However, its commercialization is obstructed by the low output power, and the oxygen reducing reaction. Enhancing the cathode water management ability and improving the charge transfer could be the solution to these issues. In this paper, we investigated the effects of gas diffusion layer, cathode catalyst layer, flow field of cathode plate on the performance of the cells.Firstly, the effect of hydrophobicity and thickness of cathode gas diffusion layer on the performance of the cells was investigated. The results show that the optimal PTFE content is 25 wt.% and the optimal thickness of microporous layer is 0.14 mm. Higher or inferior hydrophobicity would cause flooding. Higher hydrophobicity would block the outflow of water; inferior hydrophobicity would not be able to remove the liquid in the catalyst layer. The optimized thickness of microporous layer was beneficial to the relief of flooding due to the high specific surface area and Kelvin effect of microporous layer.Secondly, we investigated the influence of carbon materials in microporous layer. The results show that the cell with the gas diffusion layer which use graphite as microporous layer shows the best stability among other materials used, the voltage deceased by 7% after 7h constant current discharge. The study shows the macropores of microporous layer are attributed to enhance the mass transfer property. Meanwhile, the discharge stability of cell was related to the hydrophobicity of carbon materials. The improved graphitization degree of carbon materials was beneficial to maintaining good thermal management which consequently enhance the stability of discharge.Thirdly, we studied the effect of the dual catalyst layer structure on cell performance. The inner catalyst layer which bonds the proton exchange membrane was combined with Pt/C and Nafion ionomer which was hydrophilic. The outer catalyst layer was composed of Pt/C, Nafion ionomer and PTFE which was partly hydrophobic. The test results show that the dual catalyst layer at 0.7V the current density was 172 m A cm-2, which is higher than 152 m A/cm-2 of single catalyst layer. The research shows that cells with dual catalyst layer containing 5wt.% PTFE of total binder can obtain higher performance than cells with single catalyst layer containing 10 wt.% PTFE of total binder, with almost no decrease in water management ability. This is due to the dual catalyst layer decreased the content of PTFE without hydrophobic reduction, obtained higher proton transfer rate, at the meantime, the PTFE in out layer would retain water in low humidity and remove excess water in high current density.Fourthly, to enhance the mass transfer rate of cathode catalyst layer, it was crucial to improve the porous structure and hydrophobicity. We fabricated the Pt/CNT by sol-gel method and mixed it with JM Pt/C in different proportions. The result shows the Pt/CNT30-Pt/C35 owns better performance than that of a single catalyst. The research shows the high graphitization of carbon nanotube possesses better electro conductivity than Vulcan XC-72 R, which greatly improves the conductivity of catalyst layer. Meanwhile, reflexed nanotube gives the catalyst layer a more porous structure and improves gas permeability. Furthermore, the carbon nanotube improves the hydrophobicity of cathode catalyst layer, which enhance the water management ability and eventually the performance of the cell.Finally, four kinds of cathode flow field plate have been designed and tested to study the performance change on air-cooling proton exchange membrane fuel cells. The result shows that by reducing the cross-sectional area of flow channel outlet, the current density was increased by 40 m A/cm-2. Observation on the cell with the dotted grid ribs flow field plate indicates the voltage changed little after 8h constant current discharge. The research shows that the decrease in the cross-sectional area of flow field outlet would reduce the pressure drop. The gas turbulence at the connection of each runner in flow field could enhance the cathode water thermal management ability which was beneficial to improving cell performance and stability.