The water management in the membrane electrode assembly (MEA) is one of the most critical factors for both performance and durability. Operating conditions and material properties of gas diffusion layer (GDL) should be optimized to provide proper water management in the MEA. Some experimental investigations such as synchrotron x-ray computed tomography (CT) and neutron imaging have been studied to visualize two-phase thermodynamic fluid in the MEA. However, model-based engineering capability is significantly efficient to optimize the two-phase water management and hydration control. Conventional fuel cell models count the water saturation in the GDL with Darcy’s law and define water saturate as uniform inside the GDL, although it is varied with locations in the real system. The simulations with such a homogeneous model sometimes underestimate the effect of mass transport overpotential in the GDL [1]. A two-phase calculation technique that incorporates detailed micro-scale structure of the GDL was developed with Lattice Boltzmann Method (LBM) and validated with experimental data of water breakthrough pressures [2]. This direct GDL model with LBM was replaced with the homogeneous GDL model in the computational fluid dynamics (CFD) based fuel cell performance model [3, 4]. Simulations of fuel cell performance with this integrated model show better matching with the performance data for wet and dry conditions respectively, although simulations from the conventional homogeneous model cannot match the performance of dry and wet conditions (Fig. 1). This integrated model can properly estimate the effect of water saturation in the GDL. A model-based engineering approach is explored with this integrated model to optimize water management in the MEA. Simulations of fuel cell performance are pursued with various thermal conductivities of micro porous layer (MPL) in the GDL. The GDL with higher thermal conductivity MPL shows higher water evolution inside the GDL and resulted in lower oxygen concentration at the border between GDL and cathode catalyst layer. Figure 2 shows the temperature and water saturation profiles in the GDLs with various thermal conductivities. Temperature of higher thermal conductivity MPL is lowered and it causes higher evolution of water. Fuel cell performance of this higher thermal conductivity MPL in the GDL shows the highest among three cases although the lowest oxygen concentration at the cathode catalyst layer. The model prediction is extended to the hydration of the membrane of each thermal conductivity cases. The results clearly show that higher thermal conductivity GDL makes higher membrane hydration and higher proton conductivity which overcomes mass transport loss due to lowered oxygen concentration at the cathode catalyst layer. This work can be extended to water management with various operating conditions to show capability of the model-based engineering.

References:

1. Hayashi et al., SAE Technical Paper 2017-01-1188 (2017) doi:10.4271/2017-01-1188.
2. Satjaritanun, J.W. Weidner, S. Hirano, Z. Lu, Y. Khunatorn, S. Ogawa, S.E. Litster, A.D. Shum, I.V. Zenyuk, S. Shimpalee, Journal of the Electrochemical Society, 164 (11) (2017) E3359-E3371.
3. Shimpalee et al., Abstract# 1418, 232nd Electrochemical Society Meeting, October 1-5, 2017, National Harbor, Maryland.
4. Hirano et al., Abstract# 1419, 232nd Electrochemical Society Meeting, October 1-5, 2017, National Harbor, Maryland.