3-D simulation of heat and water transport processes in PEFCs during evaporative cooling and humidification

Abstract

Evaporative cooling (EC) is a promising concept to increase the power density and reduce the complexity of polymer electrolyte fuel cells (PEFCs) by using gas diffusion layers (GDLs) modified with hydrophilic lines (HPIL). While this concept has been demonstrated in recent experiments by use of a thermal test cell, a detailed understanding of the simultaneous cooling and humidification is missing. To close this gap, we have developed a non-isothermal, two-phase continuum model of an EC cell. This 3-D model consists of a membrane electrode assembly (MEA) with an anode GDL with of one HPIL. The MEA is sandwiched by flow channel plates that consist of one gas flow channel (GFC) and liquid water channel on the anode side and two GFCs on the cathode side. In the base case model, water evaporates mostly within a thin film at the anode GFC/HPIL interface and – to a smaller degree – at the interfaces of the HPIL with the hydrophobic anode GDL. The largest part of the generated water vapor leaves the cell through the anode GFC and only about a tenth reaches the cathode side. The membrane humidification varies on the anode side being the highest below the HPIL and ribs and the lowest below the anode GFC, and decreases towards the cathode side. The temperature drop due to latent heat of evaporation is the largest along the interface between the HPIL and anode GFC. As our model essentially simulates the first contact surface between the HPIL and anode GDL in gas flow direction, it represents an extreme case in comparison to the experiments in that higher local evaporation rates and heat fluxes and lower membrane humidification levels are simulated. The role of evaporation kinetics on the results is analyzed by varying the evaporation rate constant over several orders of magnitude. While a plateau is reached for the water vapor flux via the anode GDL and towards the cathode side at a sufficiently large evaporation rate constant, a much larger rate constant would be necessary to reach a transport limited regime in the anode GFC. This is shown to be challenging to resolve numerically due to increasingly sharper gradients within a thinner film along the borders of the HPIL. Future studies could couple this continuum model with a pore-scale simulation in combination with ex-situ experiments to improve the simulation of the two-phase flow in the modified GDL, which includes the upscaling of the Hertz-Knudsen-Schrage equation to the continuum level