Flow field channels

In conclusion, at an intermediate optimum thickness, a diffusion layer allows for (1) gas diffusion toward the CL, (2) liquid transport from the CL toward the flow field channels, (3) good contact with both the bipolar plate... 

Kowal et al. [235] used this method to compare the liquid water distribution in the fuel cell with CFP and CC as cathode DLs at different operating conditions and with a parallel flow field channel design for both anode and cathode plates. It was observed that the CFP DL experienced more flooding at lower current densities than the CC, and it retained more water near the landing widths than in or under the channels (60 vs. 40%, respectively). In addition to showing better performance and water removal, the CC resulted in more uniform water coverage on the landing widths and in the channels of the FF. 

An optimum relationship between the DL and the flow field channels is a key factor in the overall improvement of fhe fuel cell s performance at both high and low current densities. Currently, flow field designs are typically serpentine, interdigitated, or parallel [207,264]. The FF plate performs several functions If is a current collector, provides mechanical support for the electrodes, provides access channels for the reactants to their respective electrode surfaces and for the removal of producf water, and it prevents mixing of oxidant, fuel, and coolant fluids. 

Figure 19. Predicted carbon loss distribution along anode flow-field channel over a complete H2/air-front start—stop cycle using the pseudo-capacitance model in comparison with one-dimensional, normalized mass activity from Fig. 17. The pseudo-capacitance value used in the model is obtained from AC-impedance measurements as described in references (42, 43).

The influence of the physical heterogeneity on the flow regime non-imiform flow field, channel/macropore flow... 

Water-management optimization within gas diffusion electrodes and flow field channels... 

The appearance of flow visualization methods [61, 62, 63, 64] has made possible the study of two-phase flows in flow field channels. These methods should be perfected considering the potential measurement artifacts introduced by the transparent element (change in thermal and current distribution, and flow field channel surface properties). Mathematical representations of the pressure drop in presence of two-phase flow will be needed to modify existing stack reactant flow distribution models [65]. 

In applications, the reduced system is embedded in a 1 + ID computational scheme for the overall fuel cell. This includes a model of the membrane s water content and temperature, the anode GDL, and the variation of the oxygen and water vapor contents in the flow field channels in the along-the-channel direction, providing the channel conditions and fluxes which were taken as prescribed in the analysis. To present numerical results from the reduced system, we simulate this coupling by providing along-the-channel data for the oxygen and water vapor concentrations, temperature, current density, and catalyst layer production of heat and total water from a previous 1 + ID computation reported in [3]. These values vary in the y direction but are constant in time and do not couple back to the reduced simulations. 

Sufficient rigidity to bridge the flow field channels without sagging. However, it should be flexible and compressible to compensate for mechanical tolerances and to maintain electric and thermal contact. 

Reactant supply and water removal is optimized by so called interdigitated flow fields where gas flow is forced across the ribs separating the flow field channels through the macroporous part of the gas diffusion electrodes at the expense of high pressure drop between reactant inlet and outlet. 

This chapter of "PEM Fuel Cell Modeling" looks at how engineers can model PEM fuel cells to get optimal results for any application. A review of recent literature on PEM fuel cell modeling was presented. A full three-dimensional, non-isothermal CFD model of a PEM fuel cell with straight flow field channels has been developed in this chapter. The model was developed to improve fundamental understanding of transport phenomena in PEM fuel cells and to investigate the impact of various operation parameters on performance. This comprehensive model accounts for the major transport phenomena in a PEM fuel cell convective and diffusive heat and mass transfer, electrode kinetics, transport and phase change... 

In the present work, a CFD model of PEM fuel cell with straight flow field channels is developed. The model developed in this chapter is different from those in the literature in that it is fully three-dimensional as opposed to two-dimensional (e g. [51]). It is accounts for liquid water transport through the gas diffusion layer as well as transport across the membrane, rather than restricted to transport of liquid water through GDL only (e.g. [60]). Furthermore, it is non-isothermal, rather than assumed constant cell temperature (e.g. [61], [64], and [65]). Also, the present model incorporates the effect of hygro and thermal stresses into actual three-dimension fuel cell model, rather than assumed simplified temperature and humidity profile, with no internal heat generation (e.g. [68] and [69]). 

As mentioned previously, the catalyst layer is applied to either a GDM or to a decal (applying to the PEM directly is also explored). The GDM allows the reactant to transport from the flow-field channels on the plates to the catalyst layer, and allows the product to transport from the catalyst layer to the flow-field channels on the plates so it must be porous, typically with porosity as high up to 80% and with pore size in lO s pm. The GDM also provides mechanical support for the catalyst layer or the catalyst-coated membrane (CCM). The GDM transports heat and electrons between the plates and the catalyst layers as well, so it must conduct electrons and heat. 

Another popular GDM is carbon cloth, made of carbon threads woven together. It is quite soft and thus could intrude into the flow-field channels on the plates due to deformation. It appears that carbon cloth type GDM started to fade out since around 2000. 

Please note that diffusion of O2 to the anode through the PEM does not form an O2/H2 boundary. This is different from situations that O2 from fhe environmenf diffuses into the anode from the flow field channels. 

The air pressure drop using 80% RH air was significantly higher (about 1 time more) than that using dry air. With 80% RH at °C, the total air volume entering the stack will be about 15% more than that of dry air, and this accounts for a smaller portion of the total air pressure drop. The larger portion is believed to be related to the partial blockage of the flow field channels by liquid water. Since the stack was intended to operate at a current density of less than 0.5 A cm , the air supply device had to be able to overcome around a 10 kPa pressure drop. 

To reduce the necessary gas flow and minimize the need for purging, the use of capillary forces may be helpful. On the one hand, the flow-field channel design could be arranged to make use of capillary forces to drive water droplets out of the channel (Figure 5.6). This could be realized by a T-shaped channel geometry with a tapered ceiling of the bottom channel, which leads into a smaU top channel [45]. 

DMFCs are-low temperature cells with typical operating temperatures around 70 °C, fueled with an aqueous methanol solution at the anode side while air is fed to the cathode. The electrochemical reactions form carbon dioxide at the anode and liquid water at the cathode side. CO2 formed at the catalytic layer migrates through the GDL and forms bubbles in the anodic flow fleld channels which are carried away by the methanol stream. At the cathode side, the water is transported through the GDL to form droplets in the cathodic flow field channels which must be removed by the air stream. As neutrons and X-rays offer undistorted insights... 

CO2 to exit into the flow field channels. It was found that gas bubbles ejected into the flow field show an eruptive cyclic behavior when a threshold current density of 20 mA cm is exceeded. 

A perspective view of the interior of the visualized fuel-ceU section is depicted in Figure 18.23. The fiber structure of the GDL is clearly visible, and also hquid water agglomerations in the flow field channels at both the anode and cathode sides. At the back side of the channel, water agglomerates to form a film. Most droplets gather at the bottom of the chaimel, reflecting the gravitational influence. 

The reconstructed 3D volume can be sliced to obtain cross-sections of the test cell for any desired position and perspective. In Figure 18.24, cross-sections of the flow field channel structure and the GDL were generated to analyze the through-plane water distribution in the chaimel and inside the GDL at positions close to the interfaces to the flow field and microporous layer (MPL), respectively, and also for the central region of the GDL. Most water present in the GDL resides close to the flow field interface underneath the land area from where it migrates into the flow... 

Figure 18.23 Visualization of the tomographed fuel cell part. The cathode is shown on the front side and the anode on the back side of the 3D image. Water films and separated droplets in the flow field channels could be visualized. The carbon fiber structure of the applied GDL is visible. Reproduced from [32] by permission of Elsevier.

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