Numerical Simulation of Local Entropy Generation of Oxygen Transport in Cathode Diffusion Media of PEFC

Abstract

To further improve the power density and efficiency of polymer electrolyte fuel cells (PEFCs), it’s necessary to review the conventional flow fields consisting of porous electrodes and gas channels and reduce the overpotentials due to the electrochemical reaction and mass transport. In the past works, the voltage loss in fuel cells had been generally classified into the activation, ohmic and concentration polarizations and evaluated comprehensively. However, the factors of the energy loss should be identified in more detail to design the better electrode/channel structure, because the interaction of the heat and mass transfer, ionic and electronic transport and electrochemical reaction occurs complicatedly in porous electrodes. Several researchers [1-2] focused on entropy that is a physical property representing the thermodynamic irreversibility and evaluated the energy losses in fuel cells and electrochemical devices from the view of entropy generation. Especially, Charoen-amornkitt et al. [2] has attempted to design the diffusion field in a reaction-diffusion system with the use of a combination of entropy generation analysis and topology optimization. The analysis of entropy generation enables to identify the dominant process causing the overpotential in PEFCs and contributes to design the optimum electrode/channel structure minimizing the energy loss. This study introduced the evaluation technique using entropy generation into the conventional two-phase flow simulation for the gas diffusion layer (GDL) of a PEFC and succeeded to numerically estimate the distribution of entropy production rate due to the oxygen diffusion in the cathode GDL. Furthermore, the author investigated the effect of land-channel configuration on the local entropy generation of oxygen transport within the GDL in detail and discussed how to design the electrode/channel structure reducing the entropy generation.Fig. 1 presents the numerical results of the two-phase flow simulation and the entropy generation analysis in the cathode GDL. The two-phase model used in this study was developed by Natarajan and Nguyen [3]. The model domain consists of the flow channel, both lands, GDL and catalyst layer in the cross section of the cathode. Figs. 1(a) and 1(b) denote the distributions of the oxygen concentration (mole fraction) and the entropy production rate due to the oxygen diffusion, respectively. In the simulation, the fuel cell was operated for 200 s at the temperature of 80 oC and the constant current density of 1.0 A/cm2. The pressure and humidification of air supplied to the cathode were set to 1 atm and 80 %RH. As shown in Fig. 1(a), the oxygen concentration is lowered under the land because of its insufficient supply. The result of entropy generation analysis in Fig. 1(b) revealed that the entropy production rate due to the oxygen diffusion becomes remarkably high under the boundary between the flow channel and land that causes its large concentration gradient. To reduce the entropy generation in this area, it’s necessary to design the electrode/channel structure alleviating the gradient of oxygen concentration.