Vladimir Gurau

An Analytical Solution of a Half‐Cell Model for PEM Fuel Cells

A mathematical model for polymer electrolyte membrane (PEM) fuel cells is developed and rigorous analytical solutions of the model are obtained. The modeling domain consists of the cathode gas channel, gas diffuser, catalyst layer, and the membrane. To account for the composite structure of the gas diffuser and for its gradient in liquid water content, the gas diffuser is modeled as a series of parallel layers with different porosity and tortuosity. Starting from the oxygen transport equations and Ohms law for proton migration, expressions for the oxygen mass fraction distribution in the gas channel, gas diffuser, and catalyst layer, and current density and membrane phase potential in the catalyst layer and membrane are derived. The solutions are presented in terms of the physical and thermodynamic parameters of the fuel cell. The polarization curve is expressed parametrically as a function of the surface overpotential. Expressions for cathode internal and overall effectiveness factors, active fraction of the catalyst layer, catalyst layer resistance, limiting current density, and the slope of the polarization curve are also presented. Due to the advantage of the closed-form solutions this model can he easily used as a diagnostic tool for a PEM fuel cell operating on H 2 and air.

Two-Phase Transport in PEM Fuel Cell Cathodes

To date, multiphase computational fluid dynamics models for proton exchange membrane (PEM) fuel cells failed to provide even a qualitative depiction of the fuel cell water management. This was primarily due to the inability to capture two-phase phenomena in the cathode catalyst layer and the water saturation equilibrium at the interface between the fuel cell components. A model without the cathode catalyst layer cannot capture dominant mechanisms of water transfer and cannot explain correctly the fuel cell performance. We propose a multifluid, multiphase model consisting of separate transport equations for each phase. The model accounts for gas- and liquid-phase momentam and species transport in the cathode channel, gas diffusion layer (GDL), and catalyst layer and for the current density, ionomer-phase potential, and water content in the catalyst coated membrane. The model considers water produced at cathode by (I) electrochemical reaction, (II) change of phase, and (III) parallel, competing mechanisms of water transfer between the ionomer distributed in the catalyst layer and the catalyst layer pores. Liquid water is transported in the GDL and the catalyst layer due to liquid pressure gradient and in the channel due to gravity and two-phase drag. We have developed a transport equation for the water content. The source/sink terms of the transport equation represent the parallel, competing mechanisms of water transfer between the ionomer phase and the catalyst layer pores. They are (I) sorption/desorption at nonequilibrium and (II) electro-osmotic drag by the secondary current. Another distinguishing feature of this model is the capability to capture water saturation equilibrium at channel-GDL and GDL-catalyst layer interfaces. The computational results are used to study the dynamics of water transport within and between the fuel cell components and the impact of the GDL and catalyst layer properties on the amount of water retained in the fuel cell components during operation. A new dominant mechanism of water transfer between the ionomer distributed in the catalyst layer and the catalyst layer pores is identified. The amount of water retained in GDL is determined by GDL permeability and its pore size at the interface with the channel. The amount of water retained in the cathode catalyst layer is determined by the saturation equilibrium at the interface with the GDL. Models based on the two-phase mixture model are not applicable to PEM fuel cell electrodes.

A Critical Overview of Computational Fluid Dynamics Multiphase Models for Proton Exchange Membrane Fuel Cells

This paper presents an overview of the mathematical issues and the current situation in the computational fluid dynamics (CFD) modeling of multiphase transport in hydrogen-operated proton exchange membrane fuel cells (PEMFCs) at a macroscopic scale. The paper overviews multiphase models which are based on the finite volume approach and which cover water transport from anode to cathode throughout the membrane electrode assembly (MEA). We review conceptual models of water transport in the diffusion media focusing on the formulation of the balance equations and of the constitutive relations and discuss weaknesses and inconsistencies of current approaches based on experimental and theoretical evidence. A major incentive of this review is to stress the impact on water management of the widely ignored phenomena at the subgrid-scale distributed interfaces and at the macroscopic-scale interfaces between the fuel cell components. We discuss how misinterpretation of the physical meaning of various terms in the macr...

A Look at the Multiphase Mixture Model for PEM Fuel Cell Simulations

Two-phase flows in polymer electrolyte membrane fuel cells (PEMFCs) are complex dynamic processes involving phase transitions and phase production due to concurrent processes. There have been attempts to simulate these phenomena using the multiphase mixture (M 2 ) model. The conjecture is that M 2 is mathematically equivalent to classical two-fluid models without invoking any additional approximations. We show that the M 2 model has a narrow applicability, limited to flows without phase transitions or phase production due to other processes. For more complex situations, including those encountered in PEMFCs, the M 2 model ceases to correctly reflect the conservation principles and may lead to predictions of unrealistic velocity and scalar fields.

Technique for characterization of the wettability properties of gas diffusion media for proton exchange membrane fuel cells.

In this paper, a measurement technique based on the capillary penetration method is presented for use in estimating the wettability properties of gas diffusion media (GDM), a component for proton exchange membrane fuel cells (PEMFCs). The present method solves several critical issues, including the formation of an external meniscus and the evaporation of imbibed solvent, both of which greatly affect the apparent rate of solvent imbibition. Solvent evaporation is prevented by inserting a GDM sample between two thin stainless steel plates to form a tri-layer structure having non-porous evaporation covers on each side of the porous GDM sample. The presence of stainless steel plates in contact with the GDM sample was demonstrated to have a negligible impact on the evaluation of the Washburn material constant.

Robotic Arm for Automated Assembly of Proton Exchange Membrane Fuel Cell Stacks

We present an innovative, inexpensive end-effector, the robot workcell, and the fuel cell components used to demonstrate the automated assembly process of a proton exchange membrane fuel cell stack. The end-effector is capable of handling a variety of fuel cell components including membrane electrode assemblies, bipolar plates and gaskets using vacuum cups mounted on level compensators and connected to a miniature vacuum pump. The end-effector and the fuel cell components are designed with features that allow an accurate component alignment during the assembly process within a tolerance of 0.02 in. and avoiding component overlapping which represents a major cause of overboard gas leaks during the fuel cell operation. The accurate component alignment in the stack is achieved with electrically nonconductive alignment pins permanently mounted on one fuel cell endplate and positioning holes machined on the fuel cell components and on the end-effector. The alignment pins feature a conical tip which eases the engagement between them and the positioning holes. A passive compliance system consisting of two perpendicularly mounted miniature linear blocks and rails allow compensating for the robots limitations in accuracy and repeatability.

A Continuum Model for Water Transport in the Ionomer-Phase of Catalyst Coated Membranes for PEMFCs

We study the problem of water transport in the ionomer-phase of catalyst coated membranes (CCMs) for proton exchange membrane fuel cells (PEMFCs), where microscopic-scale phenomena at the distributed interfaces between structural components control the water management. Existing models for water transport in CCMs describe the transport in systems which consist exclusively of an ionomer-phase. Interfacial water fluxes across distributed interfaces representing various mechanisms of water transfer between ionomer and catalyst layer pores are not captured properly in these models. Here we develop a continuum model for water transport in CCMs using the method of volume averaging. Water is exchanged between ionomer and the catalyst layer pores by electro-osmotic discharge (EOD) through the three-phase boundary (TPB) regions and by sorption and desorption across the ionomer-pore interfaces. While the former mechanism does not affect directly the water content in the ionomer-phase, it represents an effective mechanism for water transfer during fuel cell operation and controls directly the water saturation in the catalyst pores.

In-Situ Characterization of GRAFCELL® Flexible Graphite Film as Gas Diffusion Layers for PEMFCs

Historically, gas diffusion layers (GDLs) for PEM fuel cells have been based on carbon or graphite fiber, which are costly to manufacture, offer limited design flexibility, and can lead to fiber penetration through the polymer electrolyte membrane. GrafTech has achieved a significant milestone by demonstrating, for the first time, expanded natural graphite GDLs for use in PEMFCs. Like other GRAFCELL materials, GRAFCELL GDL retains a continuous graphite phase. This unique phase continuity, combined with an extremely low contact resistance, provides superior electrical and thermal properties in comparison to industry standard fiber-based GDLs. GRAFCELL materials with different structural properties have been tested in-situ under a range of operating conditions in order to correlate their properties with the fuel cell performance. Seven structural properties were monitored using a “sensitivity testing” approach. The perforated flexible graphite films (Fig. 1) were used as macro-porous substrate for cathode or anode GDLs. To obtain systematic results, the macro-porous substrates were tested in combination with identical micro-porous layers and catalyst coated membranes. Two cases were analyzed: (i) macro-porous substrates with GrafTech integrated micro-porous layers and (ii) macro-porous substrates with experimental Gore Carbel MP30z microporous films. The other electrode used LT 1400W of BASF Fuel Cell, Inc. as gas diffusion media. After a leak test and a break-in procedure consisting of a series of polarization curves, the single cells containing the GDL samples were tested for sensitivity at different operating conditions using impedance spectroscopy. From this research, we conclude that GDLs based on perforated flexible graphite have the potential to be a viable commercial product. Experimental materials provided nearly equivalent performance on the anode and adequate cathode performance over a range of operating conditions. Future refinements to the finished GDL, such as optimization of the microporous layer, should enable GRAFCELL materials to replace fiber-based gas diffusion layer products in a number of applications. Fig. 1 Perforated GRAFCELL flexible graphite film used as macro-porous substrate for PEMFC GDLs

Spatiotemporal Behavior of Water and Two-Phase Transport in the Porous Electrodes for PEM Fuel Cells

We analyze the impact of the interfacial phenomena at the macroscopic interfaces between fuel cell components on the water management and on the two-phase transport in proton exchange membrane fuel cell (PEMFC) electrodes using multiphase multi-fluid computational fluid dynamics (CFD). We present the numerical approach used to capture multiphase phenomena at the cathode gas diffusion layer (GDL) - channel interface and explain the mechanisms that trigger the phenomenon referred to as the eruptive water ejection. Notwithstanding that they have been widely ignored, these are phenomena which ultimately control the amount of water that reside in the fuel cell components during operation.