Conjugate Analysis of Heat-Species-Charge Transport for Evaluating Effects of the Temperature Gradient on Cell Performance
Management of water in the PEMFC is very important to improve the cell performance. However, the water transport mechanism in the PEMFC is complicated and it is difficult to understand by only experimental methods because of the thinness of the components of the PEMFC. Thus, numerical analyses are very useful methods.
The two-dimensional analytical model considering distributions of temperature, spices and reaction in components of the PEMFC is developed to estimate the unsteady performance of the PEMFC. The absolute value of the temperature of the cell impacts on the cell performance. However, the gradient of temperature has also great impacts on the cell performance. Moreover, temperature, chemical reaction and transport phenomena have interaction, for example the phase change and impeding the transport of the reactant gases by liquid water. So, it is important to consider not only temperature but also water transport. MPL was reported to moisturize CL and PEM and at the same time MPL enhances liquid water drainage to Channel. However, the detailed mechanism how MPL works on the cell performance is yet unclear. Finally, in this study, the effects of the gradient of temperature are investigated, and the models with MPL and without MPL are compared to validate the work of MPL.
2. Analytical methods
The unsteady cell performance has been studied with two-dimensional analytical model considered heat, species and charge transport in components of the PEMFC, such as GDL, MPL, CL and PEM. The impact of temperature gradients has been investigated by comparison of non-isothermal and isothermal condition at 72 deg. C.
3. Results and discussions
In Fig. 1, the distribution of temperature was shown. From this, the temperature of the area nearby CL and PEM was the highest and the value was 72 deg. C. And the temperature around separators was lowest and the value was 70 deg. C. The difference of temperature is only 2 deg. C, however, the current density had more than 5 times difference between non-isothermal and isothermal condition as shown in Fig. 2. The distributions of the liquid water saturation of each condition were shown in Fig. 3 and Fig. 4, respectively. Under isothermal condition, the value of liquid water saturation in CL was about approximately s=1, on the other hand, there is little liquid water in CL under non-isothermal condition. Then, current density drastically decreased from 40 s under isothermal condition with the liquid water accumulation in the CL.