| Proton exchange membrane fuel cell(PEMFC)is considered to be one of the most important green-energy power devices in the future due to its outstanding energy conversion efficiency,fast start-up and zero emissions.Water and heat management of fuel cells is the key issue to the commercial development of fuel cells.Recently,as the research deepens,it is found that the optimization design of the flow field could enhance the performance of PEMFC,improving its durability and thus reducing costs.The ideal flow field should have high electrical and thermal conductivity,provide uniform distribution and high concentration of reactant gases throughout the electrochemical active area and offer an efficient water removal capacity and a high membrane hydration.Presently,the flow fields used in PEMFC mainly include parallel,interdigitated,and serpentine flow fields.There are many related studies on the flow field of these rib-channel structures,which inevitable are faced with uneven distribution of gas and temperature,resulting in lower water and heat management.Additionally,plenty of liquid water accumulated under the rib which is difficult to effectively removed,limiting the power density improvement.Researchers found that the metal foam flow field and the dot matrix flow fields can effectively solve the problems mentioned above.However,there are still few studies on these two new flow fields.The heat and mass transfer behaviors inside these two new flow fields and how the operating and structure parameters affect cell performance are still unknown.In this paper,the effects of metal foam flow field and the dot matrix flow field as the cathode flow field on the cell performance are studied.The electrochemical impedance analysis and high-speed camera are used to analyze the impedance of the cell,effective surface area of the electrode and offline observation of gas-liquid flow.With taking the porosity,the polytetrafluoroethylene(PTFE)loading relative humidity of the inlet air of metal foam,single dot area and arrangement of the blocks as the object variables,the effects of different operating and structure parameters of flow fields on the cell performance are discussed.It is found that the overall pressure drop of the cell with metal foam is larger than that with the parallel design,which is much smaller than that of the serpentine flow field.Its performance is much higher than parallel flow field cell but worse than the serpentine one.The cell got better performance when the metal foam is with lager porosity,besides it showed a large ohmic impedance and a small activation impedance.Compared to the conventional flow fields,the metal foam can reduce the sensitivity of the cell to the inlet humidity.When the PTFE loading of the metal foam is low,it contributes to hydration of the membrane,thus the proton conductivity of the membrane is enhanced.When the inlet air humidity is high,the metal foam with a low PTFE loading is insufficient to remove the water generated effectively.But the ohmic impedance increases owing to the excessively high PTFE loading exceeding the hydrophobicity required.When the dot matrix flow field is used on cathode side,the pressure drop of the cell is lower and the cell performance shows better,compared with that of the conventional parallel and serpentine flow fields.When the single block of the dot matrix flow field is totally the same,the distance between the blocks excessively reduces the cell performance.For the flow field with blocks non-uniformly arranged,the dot matrix flow field cell with fewer number of rows has good performance when the column interval is the same.On the contrary,the dot matrix flow field cell with more columns shows better performance.Besides,dot matrix flow fields with larger single block area shows better drainage capacity. |
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