Source: https://electrochemical.asmedigitalcollection.asme.org/article.aspx?articleid=2665812
Timestamp: 2019-04-23 22:44:30+00:00

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Manuscript received June 21, 2016; final manuscript received August 21, 2017; published online February 6, 2018. Assoc. Editor: Jan Van herle.
We present a macrohomogeneous two-phase model of a proton exchange membrane fuel cell (PEMFC). The model takes into account the mechanical compression of the gas diffusion layer (GDL), the two-phase flow of water, the transport of the gas species, and the electrochemical reaction of the reactant gases. The model was used to simulate the behavior of a PEMFC with a patterned GDL. The results of the reduced model, which considers only the mechanical compression and the two-phase flow, are compared to the experimental ex-situ imbibition data obtained by neutron radiography imaging. The results are in good agreement. Additionally, by using all model features, a simulation of an operating fuel cell has been performed to study the intricate couplings in an operating fuel cell and to examine the patterned GDL effects. The model confirms that the patterned GDL design liberates the predefined domains from liquid water and thus locally increases the oxygen diffusivity.
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Geometry of the fuel cell model with hydrophilic/hydrophobic pattern of the cathode side GDL. A bipolar plate is positioned on the top of the GDL and the CL is at the bottom. Corresponding boundary conditions to the channel and the rib regions are defined at the top. Channels/ribs are perpendicular to the GDL pattern. At the bottom, a boundary CL interface is assumed.
Capillary pressure-saturation curves of the hydrophobic and the hydrophilic regions. The two curves are constructed from the experimental results given in Ref. . The experimental data set have been divided into hydrophilic and hydrophobic regions. For each region, the average value of the saturation was determined at every value of the applied capillary pressure (black and gray dots on the curves). The final form of each curve is determined by a piecewise cubic interpolation of the obtained averaged values.
Finite element mesh used in the ex-situ capillary pressure simulation. The mesh consists of 1740 cuboid finite elements. 3 × 17,995 DOFs are used to represent the displacement field u and 2 × 17,995 DOFs are used to represent the pressures pliq and pgas.
The finite element mesh used in the simulation of the operating cell. Computation time was reduced by considering the pattern repetitions in both X and Y directions, thus reducing the simulated domain to LX/4 × LY/4. The mesh consists of 12,276 cuboid finite elements. 3 × 106,533 DOFs are used to represent the displacement field u, 2 × 106,533 DOFs are used to represent the pressures pliq and pgas, 2 × 28,800 DOFs are used to represent the concentrations cO2 and cv, and 1440 DOFs are used to represent the planar distribution of iloc.
Experimentally obtained distribution of the total water thickness in the patterned GDL at a capillary pressure of 3 kPa. Neutron imaging also captured the water in the injection channel spanning from left to right. The water accumulates in the hydrophilic domains. The width of the water strip is slightly wider than the width of the modified material.
Simulated distribution of the total water thickness in the patterned GDL at a capillary pressure of 3 kPa. The geometry consists of three full-sized hydrophilic regions and two full-sized hydrophobic regions confined by two half-sized hydrophobic regions. The water is uniformly distributed in the Y direction. In X direction, a pattern emerges with more water accumulation in the hydrophilic and less water accumulation in hydrophobic regions.
Comparison of experimental (dotted) and simulated (full line) water thicknesses as a function of position for different applied capillary pressures. The succession of hydrophobic and hydrophilic regions is clearly recognizable along the X-axis in both the simulated and the experimentally obtained results.
Simulated distribution of the effective porosity of compressed GDL. The values are lower under the ribs due to the higher level of material compression.
Simulated distribution of the capillary pressure of the patterned GDL at a fuel cell voltage of 0.6 V. The highest values of the capillary pressure are under the ribs in the hydrophilic region (3.41 mbar), slightly lower values are observed under the ribs in the hydrophobic region (3.36 mbar), and the lowest are in the channel region (3.20 mbar) where the boundary conditions are set.
Simulated distribution of the liquid water saturation of the patterned GDL at a fuel cell voltage of 0.6 V. The hydrophilic/hydrophobic pattern is most pronounced in this figure. The highest values (0.236) are on the catalyst side under the ribs in the hydrophilic region. The lowest values (0.100) are observed on the bipolar plate side under the channels in the hydrophobic region.
Simulated distribution of the phase change rates of the patterned GDL at a fuel cell voltage of 0.6 V. Positive values indicate the condensation process. Low humidity of the inlet gases causes the lowest rates (evaporation) under the channel regions. Different levels of saturation cause the rate difference between the hydrophobic and the hydrophilic regions under the channels. The condensation takes place under the ribs and in the vicinity of the CL boundary, which is related to higher water vapor concentration caused by the chemically produced water and the water dragged from the anode side.
Simulated relative humidity of the patterned GDL at a fuel cell voltage of 0.6 V. The lowest values under the channels (90%) are related to the prescribed water vapor boundary condition. The highest values (above 100%) are observed in the vicinity of the CL boundary due to the electrochemical production and the transport of water vapor in the through-plane direction of the membrane.
Simulated distribution of the normalized oxygen diffusion coefficient (DO2/DO2int) of the patterned GDL at a fuel cell voltage of 0.6 V. Both the effective porosity and the liquid water saturation level influence the diffusion coefficient. The lowest value of 0.28 is observed under the ribs in the hydrophilic region, where the porosity is the lowest and the saturation is the highest. The highest value of 0.40 is found under the channels in the hydrophobic region, where the saturation is the lowest and the porosity is the highest.
Top side view of the simulated oxygen concentration of the patterned GDL at a fuel cell voltage of 0.6 V. The highest value is observed under the channels (6.72 mol/m3), where the oxygen concentration boundary condition is set. The lowest value on the top (bipolar plate) side is observed under the ribs in the hydrophilic region (2.45 mol/m3).
Bottom side view of the simulated oxygen concentration of the patterned GDL at a fuel cell voltage of 0.6 V. The highest value at the CL boundary is observed under the channels in the hydrophobic region (5.79 mol/m3) and the lowest is under the ribs in the hydrophilic part (1.72 mol/m3).
Simulated oxygen streamlines. The oxygen originates in the channel zones from which it is transported under the ribs and toward the CL boundary.
Simulated water vapor concentration of the patterned GDL at a fuel cell voltage of 0.6 V. The lowest values are observed under the channels (approximately 4.13 mol/m3) and are related to the prescribed water vapor concentration boundary condition. The highest values (approximately 4.65 mol/m3) are observed in the vicinity of the CL boundary due to the electrochemical production and the through the membrane transport of the water vapor.
Simulated distribution of the current density of the patterned GDL at a fuel cell voltage of 0.6 V. The highest values are observed in the channel regions in the hydrophobic parts where the resistance to the transport of gases is the lowest. Contrarily, the lowest values are found in the ribs section in the hydrophilic region where the gas diffusion coefficient is the lowest.
Simulated distribution of the water vapor flux of the patterned GDL at a fuel cell voltage of 0.6 V. The boundary flux at the CL/GDL interface includes the contribution of the chemical reaction in the CL as well as the contribution of the water transport through the membrane.
Relative errors of the local current density and the oxygen diffusion coefficient for different mesh sizes. The influence of mesh size has very little effect on the oxygen diffusion and the current density distribution. All values are below 1.6%.
Relative errors of the relative humidity observed on the catalyst boundary and for the liquid water saturation. The results range between 0% and −0.8%. The largest error is observed for the maximal value of liquid water saturation when using the coarsest mesh.
Relative errors of the average phase change mass transfer rate Qpc. The curves are obtained by considering the base case humidity (RHC = 90%) and a case with higher humidity of cathode side (RHC = 110%). Both curves show significant influence of mesh size on results. When both condensation and evaporation processes are present in one simulation (the base case scenario), the errors are larger.
Local current density and oxygen diffusion coefficient versus relative humidity RHC. The values are normalized to the base case (RHC = 90%) results.
Relative humidity at the catalyst boundary and liquid water saturation versus relative humidity RHC. The values are normalized to the base case (RHC = 90%) results.
Local current density and oxygen diffusion coefficient as a function of the interfacial area accommodation coefficient Γs. The values are normalized to the base case results (Γs = 0.1).
Relative humidity at the catalyst boundary and liquid water saturation as a function of the interfacial area accommodation coefficient Γs curves. The values are normalized to the base case results (Γs = 0.1).
Liquid water saturation as a function of the boundary capillary pressure pcBC. The values are normalized to the base case results (pcBC=3.2 m bar).
Local current density, oxygen diffusion coefficient, and relative humidity at the catalyst boundary versus the boundary capillary pressure pcBC. The values are normalized to the base case results (pcBC=3.2 m bar).
Dujc J, Forner-Cuenca A, Marmet P, et al. Modeling the Effects of Using Gas Diffusion Layers With Patterned Wettability for Advanced Water Management in Proton Exchange Membrane Fuel Cells. ASME. J. Electrochem. En. Conv. Stor.. 2018;15(2):021001-021001-14. doi:10.1115/1.4038626.

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