Ken S. Chen

Computational fluid dynamics modeling of proton exchange membrane fuel cells

A transient, multi-dimensional model has been developed to simulate proton exchange membrane (PEM) fuel cells. The model accounts simultaneously for electrochemical kinetics, current distribution, hydrodynamics and multi-component transport. A single set of conservation equations valid for flow channels, gas-diffusion electrodes, catalyst layers and the membrane region are developed and numerically solved using a finite-volume-based computational fluid dynamics (CFD) technique. The numerical model is validated against published experimental data with good agreement. Subsequently, the model is applied to explore hydrogen dilution effects in the anode feed. The predicted polarization cubes under hydrogen dilution conditions are found to be in qualitative agreement with recent experiments reported in the literature. The detailed two-dimensional electrochemical and flow/transport simulations further reveal that in the presence of hydrogen dilution in the fuel stream, hydrogen is depleted at the reaction surface resulting in substantial kinetic polarization and hence a lower current density that is limited by hydrogen transport from the fuel stream to the reaction site.

In Situ High-Resolution Neutron Radiography of Cross-Sectional Liquid Water Profiles in Proton Exchange Membrane Fuel Cells

High-resolution neutron radiography was used to image an operating proton exchange membrane fuel cell in situ. The cross-sectional liquid water profile of the cell was quantified as a function of cell temperature, current density, and anode and cathode gas feed flow rates. Detailed information was obtained on the cross-sectional water content in the membrane electrode assembly and the gas flow channels. At low current densities, liquid water tended to remain on the cathode side of the cell. Significant liquid water in the anode gas flow channel was observed when the heat and water production of the cell were moderate, where both water diffusion from the cathode and thermal gradients play a significant role in determining the water balance of the cell. Within the membrane electrode assembly itself, the cathode side was moderately more hydrated than the anode side of the assembly from 0.1 to 1.25 A cm -2 . The total liquid water content of the membrane electrode assembly was fairly stable between current densities of 0.25 and 1.25 A cm -2 , even though the water in the gas flow channels changed drastically over this current density range. At 60°C, the water content in the center of the gas diffusion layer was depleted compared to the membrane or gas flow channel interfaces. This phenomenon was not observed at 80°C where evaporative water removal is prevalent.

Two-Phase Transport in Polymer Electrolyte Fuel Cells with Bilayer Cathode Gas Diffusion Media

2framework is developed to analyze the two-phase transport in polymer electrolyte fuel cells with bilayer cathode gas diffusion media GDM, consisting of a coarse gas diffusion layer GDL with an average pore size around 10-30 m and a microporous layer MPL with an average pore size ranging from 0.1 to 1 m. Effects of the relevant properties of the MPL on liquid water transport are examined, including average pore size, wettability, thickness, and porosity. It is quantitatively shown that the MPL increases the rate of water back-flow across the membrane toward the anode by increasing the hydraulic pressure differential across the membrane, consequently reducing the net amount of water to be removed from the cathode. Furthermore, it is seen that different microporous and wetting characteristics of the MPL cause a discontinuity in the liquid saturation profile at the MPL-GDL interface, which in turn reduces the amount of liquid water in the catalyst layer-MPL interface. Our analyses show that the back-flow of liquid water increases with increasing hydrophobicity and thickness, and decreasing pore size and porosity of the MPL.

Real-Time Imaging of Liquid Water in an Operating Proton Exchange Membrane Fuel Cell

Neutron imaging experiments were carried out to measure the water content of an operating proton exchange membrane fuel cell (PEMFC) under varying conditions of current density and temperature. It was found that the water content of the PEMFC is strongly coupled to the current density and temperature of the cell. These measurements indicate that changes in water content lag changes in current density by at least 100 s, both when the current density was increased and decreased. Less liquid water was measured in the cells when operating at 80°C than at 40°C. At 60°C cell temperature, a peak in water content was observed around 650 mA/cm 2 and the water content was found to decrease with increasing current density. This is explained in the context of cell heating by performing a simple thermal analysis of an operating PEMFC so as to yield quantitative information on the waste heat and its effects on the liquid water contained in the cell.

Anisotropic Heat and Water Transport in a PEFC Cathode Gas Diffusion Layer

A nonisothermal, two-phase model was developed to investigate simultaneous heat and mass transfer in the cathode gas diffusion layer GDL of a polymer electrolyte fuel cell PEFC. The model was applied in two-dimensions with the in-plane i.e., channel-to-land and through-plane i.e., catalyst layer-to-channel directions to investigate the effects of anisotropy of GDL. For the first time, the anisotropy in the GDL properties was taken into account and found to be an important factor controlling the temperature distribution in the GDL. The maximum temperature difference in the GDL was found to be a strong function of GDL anisotropy. A temperature difference of up to 5°C at a cell voltage of 0.4 V was predicted for an isotropic GDL while it reduced to 3°C for an anisotropic GDL. Significant effect of temperature distribution on liquid water transport and distribution was also observed. In addition, the latent heat effects due to condensation/evaporation of water on the temperature and water distributions were analyzed and found to strongly affect the two-phase transport.

Understanding Liquid Water Distribution and Removal Phenomena in an Operating PEMFC via Neutron Radiography

A proton exchange membrane fuel cell (PEMFC) was imaged using neutron radiography under pseudo steady-state operating conditions to determine the total liquid water content of the cell and the liquid water content distribution across the active cell area as a function of cell temperature, current density, and cathode air flow rate. A simple cathode-based model was formulated to rationalize the observed dry inlet regions which were most strongly influenced by temperature and current density. Between temperatures of 40 and 80°C and current densities of 0.5 and 1.5 A cm -2 , the outlet gas temperature was measured to be 1-5°C greater than the cell bulk temperature. This small temperature difference was enough to account for drying of 20-40% of the cell area, depending on the bulk cell temperature. For the cell construction used in this work, the temperature and cathode stoichiometric flow had a marginal effect on the polarization curve performance but had a large effect on the liquid water content and distribution within the cell.

Phase Change in a Polymer Electrolyte Fuel Cell

Stable high performance in a polymer electrolyte fuel cell (PEFC) requires efficient removal of product water and heat from the reaction sites. The most important coupling between water and heat transport in PEFC, through the liquid-vapor phase change, remains unexplored. This paper sheds light on physical characteristics of liquid-vapor phase change and its role in PEFC operation. A two-phase, nonisothermal numerical model is used to elucidate the phase-change effects inside the cathode gas diffusion layer (GDL) of a PEFC. Locations of condensation and evaporation are quantified. Operating conditions such as the relative humidity (RH) of inlet gases and materials properties such as the thermal conductivity of GDL are found to have major influence on phase change. Condensation under the cooler land surface is substantially reduced by decreasing the inlet RH or increasing the GDL thermal conductivity. The RH effect is more pronounced near the cell inlet, whereas the GDL thermal conductivity affects the phase-change rate more uniformly throughout the flow length.

Current Ramping: A Strategy for Rapid Start-up of PEMFCs from Subfreezing Environment

A rapid start-up of proton exchange membrane fuel cells (PEMFCs) from subzero temperatures is essential for fuel cell vehicle commercialization. We explore the limits of the start-up time and the required cell material properties and operating conditions using an experimentally validated model. A linear class of current-ramping protocols is proposed and optimized for a rapid cold start. A PEMFC with a standard cell thermal mass of 0.4 J/cm 2 K starting from -30°C is of primary interest and is extensively studied in this work. Either a small initial current density (e.g., 100 mA/cm 2 ) combined with an intermediate ramping rate or a relatively large initial current density (e.g., 200 mA/cm 2 ) in combination with a small ramping rate can lead to a successful self-start if the membrane electrode assembly (MEA) is sufficiently dry before the start-up. However, a current-ramping cold start using an insufficient initial current density (e.g., ≤50 mA/cm 2 ) shuts down with any ramping rate. A more rapid self-start can be achieved by increasing the initial current density, which is mainly limited by the initial water content in the MEA. Hence, keeping the MEA mildly hydrated before the cold start can be favorable to a rapid start-up of a PEMFC using current ramping. This strategy is particularly effective for the rapid start-up of next-generation PEMFCs with reduced thermal mass. A PEMFC with a thermal mass of 0.2 J/cm 2 K and a relatively wet MEA can be successfully started from -30°C in about 5 s by applying an initial current density of as high as 1 A/cm 2 .

Liquid Water Transport in Polymer Electrolyte Fuel Cells With Multi-Layer Diffusion Media

A two-phase, multi-component, full cell model is developed in order to analyze the two-phase transport in polymer electrolyte fuel cells with multi-layer cathode gas diffusion media, consisting of a coarse gas diffusion layer (GDL) (average pore size ~10 µm) and a micro-porous layer (MPL) (average pore size ~0.2-2 µm). The relevant structural properties of MPL, including average pore size, wettability, thickness and porosity are examined and their effects on liquid water transport are discussed. It is found that MPL promotes back-flow of liquid water across the membrane towards the anode, consequently alleviating cathode flooding. Furthermore, it is seen that unique porous and wetting characteristics of MPL causes a discontinuity in the liquid saturation at MPL-GDL interface, which in turn reduces the amount of liquid water in cathode catalyst layer-gas diffusion medium interface in some cases. Our analyses show that the back-flow of liquid water increases with the increasing thickness and decreasing pore size, hydrophobicity and bulk porosity of the MPL.

A Mathematical Model for Electroless Copper Deposition on Planar Substrates

A mathematical model for the electroless deposition of copper on a planar electrode is presented and used to make time-dependent predictions on the various quantities in the system. The model takes into account mass transport by diffusion and migration, Butler-Volmer kinetics at the electrode surface, and mixed potential theory. A finite difference approach is used to solve the equations, and the resultant model is used to predict the concentration profiles, potential response, and plating rate as a function of time and concentration of various reactive components.