Two-phase Transport Characteristics In Cathode Porous Layers Of Proton Exchange Membrane Fuel Cells

Since the conventional fossil energy has low efficiency and can result in severeenvironment destruction, every country has paid much attention to develop the cleanand efficient renewable energy. The fuel cells, which can generate electricity directlyfrom the chemical reaction, have the advantages of high efficiency, no pollution andfuel diversity, and hence are considered as the most promising energy suppliers in thefields of vehicles, power stations, and spacecraft. Among the various fuel cells, theproton exchange membrane fuel cell has been attracting more and more attentionsbecause of its low operating temperature, ease of start-up, and compactness.A typical proton exchange membrane fuel cell consists of bipolar plate, gasdiffusion layer (GDL), catalyst layer (CL), and proton exchange membrane. Its workingprinciple is as follows. At the anode, hydrogen is supplied, which diffuses through thegas diffusion layer to the catalyst layer, where it dissociates into protons and electrons.The protons are conducted through the proton exchange membrane to the cathodecatalyst layer, while the electrons are forces to transfer through the external circuit. Atthe cathode, oxygen is supplied to the catalyst layer, where it reacts with protons andelectrons from the anode to produce water and heat. Since the oxygen reaction rate israther lower, the cell performance losses occur mainly at the cathode. Therefore, theunderstanding the transport and reaction process in the cathode porous layers can helpthe optimization of the fuel cell. Up to now, many researchers have performed thenumerical studies to investigate the coupled transport and reaction processes in thecathode porous layers. However, most of current studies are based on the macroscopicmethod, which therefore cannot elucidate the effects of the porous layer microstructures.Because of the shortcomings of the macroscopic models, the pore network model isemployed in this work. The main studies and conclusions are as follows.①Pore network model is used to investigate the oxygen transport process in theGDL. It is revealed that the oxygen effective diffusivity becomes higher with decreasingthe heterogeneity and the anisotropy, as well as increasing the pore connectivity.According to the simulation results and the experimental data reported elsewhere, a newcorrelation considering GDL structure impacts is proposed to determine the oxygeneffective diffusivity.②Pore network model is used to explore the liquid water and oxygen transport in the GDL of mixed wettability. An optimum fraction of hydrophilic pore in the GDLexists leading to the largest limiting current density. As compared to the GDL ofuniformly distributed hydrophilic pores, the non-uniform case results in better fuel cellperformance.③The pore network model is used to investigate the liquid distribution in thebi-layer GDL. Inserting the micro porous layer (MPL) between the GDL and the CL canreduce the liquid saturation in the GDL, but the amount of reduction depends on theinlet boundary condition. The liquid saturation in the GDL is reduced with thicker MPL,greater in-plane connectivity, lower through-plane connectivity, and the reduced inletflooding coverage.④Pore network model is used to determine the transport properties of the bilayerGDL. With increasing the MPL thickness, the liquid permeability increases, but theoxygen effective diffusivity reduces. With the increase of the width of MPL crack, theliquid permeability increases; the oxygen effective diffusivity also increases when theliquid saturation is high, but at lower liquid saturation, it changes little. At the momentof breakthrough, a liquid saturation jump is observed at the interface of MPL and GDLin the plain bilayer GDL; and with the increase of the MPL thickness, both the liquidpermeability and the oxygen effective diffusivity decrease. In the defective bilayer GDL,however, a liquid saturation drop at the interface of micro porous layer and gas diffusionlayer is revealed; and as the MPL crack width increases, the liquid permeabilitybecomes greater, opposite to the behavior of the oxygen effective diffusivity.⑤The pore network model is used to simulate the transfer and reaction processesin the CL. With decreasing the CL thickness and the Nafion film volume, the cellvoltage increases at the high current density, but reduces at the low current density. Thecell voltage is also increased by adding the proton conductivity of Nafion layer. Ascompared to the CL of the uniformly distributed Nafion, the CL of non-uniform Nafiondistribution shows better performance at the high current density.