Ugur Pasaogullari

Liquid Water Transport in Gas Diffusion Layer of Polymer Electrolyte Fuel Cells

High-current-density performance of polymer electrolyte fuel cells ~PEFCs! is known to be limited by transport of reactants and products. In addition, at high current densities, excessive amount of water is generated and condenses, filling the pores of electrodes with liquid water, and hence limiting the reactant transport to active catalyst. This phenomenon known as ‘‘flooding’’ is an important limiting factor of PEFC performance. In this work, the governing physics of water transport in both hydrophilic and hydrophobic diffusion media is described along with one-dimensional analytical solutions of related transport processes. It is found that liquid water transport across the gas diffusion layer ~GDL! is controlled by capillary forces resulting from the gradient in phase saturation. A one-dimensional analytical solution of liquid water transport across the GDL is derived, and liquid saturation in excess of 10% is predicted for a local current density of 1.4 A/cm 2

Two-Phase Modeling and Flooding Prediction of Polymer Electrolyte Fuel Cells

A newly developed theory of liquid water transport in hydrophobic gas diffusion layers is applied to simulate flooding in polymer electrolyte fuel cells ~PEFCs! and its effects on performance. The numerical model accounts for simultaneous two-phase flow and transport of species and electrochemical kinetics, utilizing the well-established multiphase mixture formulation to efficiently model the two-phase transport processes. The two-phase model is developed in a single domain, yielding a single set of governing equations valid in all components of a PEFC. The model is used to explore the two-phase flow physics in the cathode gas diffusion layer. Multidimensional simulations reveal that flooding of the porous cathode reduces the rate of oxygen transport to the cathode catalyst layer and causes a substantial increase in cathode polarization. Furthermore, the humidification level and flow rate of reactant streams are key parameters controlling PEFC performance and two-phase flow and transport characteristics. It is also found that minimization of performance limitations such as membrane dry-out and electrode flooding depends not only on material characteristics but also on the optimization of these operating parameters.

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.

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.

Computational Thermal-Fluid Analysis of a Microtubular Solid Oxide Fuel Cell

A computational fluid dynamics model is developed to study the steady-state behavior of a microtubular solid oxide fuel cell (SOFC). The model incorporates mass, momentum, species, and heat balances along with ionic and electronic charge transfers. The anode-supported SOFC studied in this work consists of a ceria-based electrolyte which is known as an electronic conductor in reducing atmospheres, letting electrons leak through the electrolyte. Related internal leakage currents are calculated implicitly in the model to incorporate the performance losses. Moreover, to have a more realistic approach while cutting down the computational effort, in this study a fuel cell test furnace is also modeled separately to evaluate the distribution of the oxygen concentration and temperature field inside the furnace. Results from the furnace model are used as boundary conditions for the fuel cell model. Fuel cell model results are compared with the experimental data which shows good agreement.

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.

Consideration of the Role of Micro-Porous Layer on Liquid Water Distribution in Polymer Electrolyte Fuel Cells

Evidence of a region of gradual property change between the micro-porous layer and the macroporous layer of bilayer diffusion media is presented, and a mathematical model describing the effects of this gradual interfacial region is developed. The model results in a continuous liquid water saturation distribution across the diffusion media compared to the sharp discontinuity in the liquid phase saturation predicted by the earlier sudden interface models. High-resolution neutron radiography is used to measure the water content profile across the diffusion media, and the results are compared with the model predictions. The effect of the geometric blur, and uncertainty in-neutron radiography, is accounted for by applying the effects of geometric blur to model results. When the blur is considered, the neutron radiography results are found to be very similar to model predictions.

Modeling and Diagnostics of Polymer Electrolyte Fuel Cells

Durability of PEM Fuel Cell Membranes.- Modeling of Membrane-Electrode-Assembly Degradation in Proton-Exchange-Membrane Fuel Cells - Local H2 Starvation and Start-Stop Induced Carbon-Support Corrosion.- Cold Start of Polymer Electrolyte Fuel Cells.- Species, Temperature, and Current Distribution Mapping in Polymer Electrolyte Membrane Fuel Cells.- High-Resolution Neutron Radiography Analysis of Proton Exchange Membrane Fuel Cells.- Magnetic Resonance Imaging and Tunable Diode Laser Absorption Spectroscopy for In-Situ Water Diagnostics in Polymer Electrolyte Membrane Fuel Cells.- Characterization of the Capillary Properties of Gas Diffusion Media.- Mesoscopic Modeling of Two-Phase Transport in Polymer Electrolyte Fuel Cells.- Atomistic Modeling in Study of Polymer Electrolyte Fuel Cells - A Review.

Influence of Formic Acid Impurity on Proton Exchange Membrane Fuel Cell Performance

The effect of trace amounts of formic acid (HCOOH) in hydrogen fuel on proton exchange membrane fuel cell (PEMFC) performance is reported. Long-term stability tests (100 h), periodic cyclic voltammetry scans, and electrochemical impedance spectroscopy analyses are used to evaluate and characterize the effects of this impurity on fuel cell performance. The results show that trace amounts of HCOOH cause degradation in fuel cell performance and significantly contaminate the electrodes. Furthermore, full recovery from the contamination could not be achieved by applying pure hydrogen to the anode while operating the fuel cell. However, this degradation may also be caused by the coarsening or dissolution of Pt, in addition to any permanent effects of HCOOH contamination. Mechanisms of contamination of the electrodes and performance degradation of the PEMFC are also postulated.

Water Transport Across a Polymer Electrolyte Membrane under Thermal Gradients

A fundamental experimental and numerical study of the water transport across a perfluorosulfonic acid membrane under a temperature gradient is presented. The water transport phenomenon was experimentally investigated through water flux measurement and neutron radiography. The experimental observations found that water is transported in the direction from the high temperature side to the low temperature side, when both sides of the membrane are sufficiently humidified, and suggest that the transport mechanism is concentration gradient driven. The neutron radiography measurements detected the presence of water content gradient across the membrane and higher water content is seen at a larger thermal gradient. A numerical model was developed to investigate the experimental results. Water transport predictions agreed qualitatively but more accurate material and transport property characterizations are needed for further improvement.