Study On The Current Density Distribution Of Proton Exchange Membrane Fuel Cells

At present, the main problem of scaling up PEMFC to commercial dimensionsis the attenuation of performance. To investigate into current distribution in protonexchange membrane fuel cell (PEMFC), as well as to analysis the influence of theoperating conditions and key components on the current distribution is thefoundation of solving the attenuation of performance during the magnification ofsingle cell. Therefore, the issue is crucial for the development, application andindustrialization of PEMFCs.A modified subcell approach has been developed to measure the currentdistribution in PEMFCs. The effects of operation condition such as temperature,pressure and flow rate on the current distribution of PEMFC were discussed. A newflow pattern of PEMFC with metal mesh flow field was put forward from theexperimental results. In the metal mesh flow field, the flow mainly tracked a shortcut along the diagonal direction from the inlet to the outlet. The experimental resultswith parallel channels flow field demonstrated that the current density distributionover the whole electrode was heterogeneous and was consistent with the velocitydistribution resulted from the mathematic model. For PEMFC with parallel channelsflow field, subcells in inlet channels presented higher performances than those inmiddle channels, but showed lower performances than those in outlet channels. Gasflow rate was one of prime factors to influence the current distribution of PEMFCcathode. With the decrease of reactant flow rate, the flooding process began at theflow field outlet region of a cell, and then it propagated to the inlet area.For the first time, the measurement of current density distribution in PEMFCwas taken as a method for investigating the influence of the flow field structure, gasdiffusion layer and catalyst layer structures on the electrode performance. The fuelcell performance with metal mesh flow field was much better than that with parallelchannels flow field. Furthermore, it was noted that no severe flooding occurredthroughout the entire metal mesh flow field. It was beneficial to alleviate theflooding condition at high current density to use Acetylene black instead of VulcanXC-72. At low current density (ohmic polarization), as a consequence of resistancedecrease, the performance increased with the decrease of PTFE loading. While underflooding condition, it was essential to improve the PTFE loading properly in gasdiffusion layer to obtain a higher limiting current density. It was demonstrated thatthe conventional hydrophobic electrode showed better performance under floodingcondition compared to the hydrophilic electrode. The thin film hydrophilic electrodeperformed better in absence of liquid water, but it was more readily to be flooded. Anovel composite catalyst layer,which including both the advantages of hydrophilicand hydrophobic electrodes was designed, and an improved performance wasobtained in the concentration polarization region.