Study On The Structure And Performance Of Open-Cathode Air-Cooling Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cell(PEMFC)is a device that directly converts the chemical energy of hydrogen and oxygen into the electrical energy.It has many advantages such as high efficiency,clean,quick startup and ambient operating temperature,which make it a very promising power source for transportation,portable,stationary and backup applications.The open-cathode PEMFCs have the advantages of simple structure and easy to maintain,since they need neither humidifying equipment nor air compressor.However,air is the reactant for cathode as well as the cooling medium for an open-cathode PEMFC stack,thus bring significant effects on the performance and long-term stability of the fuel cell.In this paper,the performance and stability of PEMFC under open-cathode conditions was improved by optimizing the structure design of stack,the electrolyte and the electrode structure.The details are summarized as follows:We designed and built a kilo-watt scale open-cathode PEMFC stack and characterized its overall discharge performance as well as the corresponding state of single cells.The distribution of temperature and air velocity of the stack were also measured.The results showed that internal temperature and flow rate of the reactant gasarevary in a large extent in the stack.In normal operation condition,the temperature differences of different electrodes in the stack exceeded 12?,while the differences of the air flow rate of the stack reached up to 65%,thus resulting in the poor performance uniformity of different single cells,and then reducing the overall performance and stability of the stack.The parameters such as temperature,air humidity and so on were coupled in the stack,which makes it difficult to evaluate the accurate performance of the membrane electrode assembly(MEA)by a stack test.On the basis of a study on a kilo-watt stack,we developed a method to evaluate the MEA using a single cell,by which the performance of the MEA structure could be easily studied under the stimulating conditions of an open-cathode stack.Therefore,the influence of specific factors on the performance of the MEAs could be systematically examined.Firstly,we measured the performance of two MEAs with Nafion211 and Nafion212 membranes with the thicknesses of 25?m and 50?m,respectively.The results indicated that the performance of the cell with Nafion211 membrane is higher than Nafion212 membrane,and the cell with Nafion211 membrane can also withstand a harsh working condition of high temperature and low humidity.It could keep a relatively stable performance under 800 mA.cm-2 in a wide range of fluctuation of cell temperature and air flow rate,while the Nafion212 membrane is not suitable for the air-cooling working condition over 55? and higher.The key materials in the electrodes of MEA were also investigated.The effects on the performance of various factors in a MEA were evaluated and compared by preparing MEAs with different manufacture processes,different catalyst layers,different proton exchange membranes and different gas diffusion layers.The results showed that under such a high temperature and low humidity working condition,the water management,especially the water retention capacity of the MEA,is the key factor that affects the cell performance and stability.The catalyst-coated membrane(CCM)structure was more suitable for a MEA to be used in the open-cathode stack than a traditional gas diffusion electrode(GDE)structure.When discharging under 0.6V at 55?,the CCM cell showed a current density of 527 mA.cm-2 while only 334 mA.cm-2 for the GDE cell.The 60%Pt/C catalyst showed the best overall cell performance,compared to other catalysts with same Pt loading in the electrode.The discharging performance and stability was obviously improved by increasing the carbon loading of the micro-porous layer to increase the thickness of the gas diffusion layer.With a carbon loading of 2 mg.cm-2 in the MPL,the cell voltage exceeded 0.62 V while discharging under 800 mA.cm-2 at 55?.The employment of thinner Nafion/PTFE composite proton exchange membranes could further improve the discharging performance and stability of the fuel cell.