Determination of Water Evaporation Rates in Gas Diffusion Layers of Fuel Cells

Abstract

The efficiency of polymer electrolyte fuel cells is closely linked to effective heat and water management. Evaporation of water in the gas diffusion layers (GDL) is an important process influencing water management and can even be used for evaporative cooling. In this study, evaporation rates in Toray-GDL materials are obtained for different temperatures, gas flow rates, carrier gases and water saturation levels. Evaporation rates are correlated with water distribution profiles in the GDL, obtained using X-ray tomographic microscopy imaging. The influence of temperature on the evaporation rates is predicted by considering the effect of temperature on vapor diffusion alone. However, changing the carrier gas type points to a deviation from diffusive-driven vapor transport, which could be due to entrainment of gas inside the GDL, among other factors. © The Author ( Polymer electrolyte fuel cells (PEFC) efficiently convert the chemical energy stored in hydrogen to electrical energy, with water and heat as the byproducts. PEFCs are therefore considered a key technology for future hydrogen-based energy scenarios, especially in the mobility and energy storage sectors, as the usage of renewable electricity gains momentum worldwide. The central component of PEFCs is the membrane-electrode-assembly (MEA), which consists of a membrane at the center that is coated with catalyst layers (CL) on both sides, which are contacted by gas diffusion layers (GDL). Water management in the MEA is a critical topic that is closely related to power density and durability of PEFCs.1 Water management (i.e. having maximum water content in the membrane and a minimum saturation in the porous layers) is a complex issue governed by the fluid and energy flows in the MEA. Most of this complexity arises from the fluxes of heat and water in the GDLs, which are difficult to localize and quantify.Gas diffusion layers are porous structures with porosities in the range of 70-85% and average pore sizes around 10-40 μm.2 The GDL materials have to provide maximum heat and electron conductivity in the solid structure while simultaneously ensuring high permeability of the reactant gases, product water and water vapor in the void. At low operating-temperatures or high current densities, gas transport in the pore space can be significantly hindered by liquid water occupying pore space, especially at the cathode side. Consequently, oxygen transport to the cathode CL is significantly affected, which in turn reduces fuel cell performance or efficiency.

Mass Transport Limitations of Water Evaporation in Polymer Electrolyte Fuel Cell Gas Diffusion Layers

“…Thus, for any gas velocity, a GDL thickness reduction only increases the total evaporative water removal if L is small enough to avoid fully saturating the gas channel. This effect of increased evaporation rates has been observed experimentally by Lal et al [21] who, with a comparable setup, observed that a thickness reduction from TGP-H 120 to TGP-H 060 resulted in an evaporation rate increase that was lower than expected from the reduction in diffusive path length.…”

“…This GDL was chosen to keep overall evaporative fluxes moderate to avoid thermal gradients as much as possible. The onset of thermal limitations was observed for thinner GDLs at 80 • C in a similar setup [21]. A seal was placed around the GDL, preventing leakage of vapor and controlling the GDL compression.…”

“…The artificial distribution of hydrophobic and hydrophilic GDL domains enables tailormade patterns of water filled lines with high levels of evaporation rates and necessitates a good understanding of the vapor removal mechanisms, ultimately limiting the rate of evaporation. Understanding the characteristic mechanisms and limitations to evaporative water removal in the framework of PEFCs and quantifying them has been the aim of many scientific works, both experimentally [21][22][23][24][25] and numerically [26][27][28][29][30][31][32]. However, the reported evaporation values show a wide spread [31,33] and some measurements performed on differential cell scales leave open questions of how evaporative flux scales with GDL thickness and gas type are used in the evaporation rate measurements, as reported by Lal et al [21].…”

“…Understanding the characteristic mechanisms and limitations to evaporative water removal in the framework of PEFCs and quantifying them has been the aim of many scientific works, both experimentally [21][22][23][24][25] and numerically [26][27][28][29][30][31][32]. However, the reported evaporation values show a wide spread [31,33] and some measurements performed on differential cell scales leave open questions of how evaporative flux scales with GDL thickness and gas type are used in the evaporation rate measurements, as reported by Lal et al [21]. They observed that doubling the diffusive distance did not halve the diffusive flux, as was expected under differential conditions, while the evaporation increase due to changing from nitrogen to hydrogen in the gas channel fell short of the theoretical effect the increased diffusivity should have.…”

Experimental parameter uncertainty in proton exchange membrane fuel cell modeling. Part I: Scatter in material parameterization

“…These formulas explicitly include the liquid-gas interfacial area density a lg , which is unknown a priori, making even an estimation of the rates difficult. [179,180] and Toray TGP-H-060 [181] were recast into the form of Eq. 18 and shown as heart symbols.…”