FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Gas Diffusion Layers

FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Gas Diffusion Layers

Gas Diffusion Layers D Gervasio, Arizona State University, Tempe, AZ, USA & 2009 Elsevier B.V. All rights reserved. Introduction This article is mean...

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Gas Diffusion Layers D Gervasio, Arizona State University, Tempe, AZ, USA & 2009 Elsevier B.V. All rights reserved.

Introduction This article is meant to be an introduction to gas diffusion layers (GDLs) in fuel cells for the reader with a general scientific background. Recommended references to recent reviews from the scientific literature are present at the end of the article to direct the reader to the more detailed treatments of the topic. Figure 1 shows a schematic diagram for a protonexchange membrane fuel cell (PEMFC) that operates on hydrogen as fuel and oxygen from air as oxidant. The polymer electrolyte membrane (PEM) cell consists primarily of three components. These components are the anode, where the hydrogen is split into protons (Hþ) and electrons (e); a PEM that transports the protons through the cell from the anode to the cathode; and the cathode, where the protons, electrons, and oxygen (from air) combine to make the only chemical product of the fuel cell, water. Special emphasis is given to the gas supply structures of each electrode, which is divided into three parts: the active layer, the GDL, and the bipolar plate (BPP). All those parts are electron conductors. The active layer is a porous layer about 10–50 mm thick where electrochemical reactions occur. The active layer is typically made of nanoparticulate platinum catalyst supported on a submicron carbon particle and held together by a Teflons binder. The active layer is coated on the GDL. Reactants and product are brought in and out of the active layer through the GDL. The GDL is about 0.1–1 mm thick and is typically made of a mesoporous graphite paper or cloth material (pores i

e−

e−

V −

+

H+

Anode bipolar plate

Cathode bipolar plate

PEM

H2

O2 Active layer

GDL

Figure 1 Schematic diagram of structures features in a proton-exchange membrane fuel cell (PEMFC) including the gas diffusion layer (GDL).

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around 10  10 mm in cross-sectional area). A GDL is adjacent to a BPP. The BPP brings chemicals to and from the GDL and separates the anode and cathode gas from each other. The BPP is typically a block of solid nonporous material, like graphite or metal. Large fluidic channels about a square millimeter in cross-sectional area and tens of centimeters long are cut or stamped into the BPP. The BPP is the ultimate inlet of reactants and outlet of product for each cell in a set of cells in a fuel cell stack. In fuel cell electrodes, mass transport of reactants to and products from the reaction zone must not be rate limiting relative to the desired chemical reaction rates. If the mass transport is rate limiting, concentration gradients occur, which leads to losses in cell voltage and efficiency. Fuel cells use a porous GDL between the gas flow field of the BPP and the active layer of the electrode to ensure mass transport is not rate limiting. Often, a socalled microporous layer is added to improve the electric contact and to reduce mass transport limitations between the electrode and GDL. The BPP has several functions. It provides the housing for the macroscopic gas flow field and is used to building up voltage by connecting single fuel cells in a series sense. A BPP needs to be a good electrical conductor and a corrosion resistant, and provide structural rigidity. BPPs are impermeable to fuel cell gases (hydrogen and oxygen) and separate the gases in adjacent fuel cells while at the same time electrically connecting anode of one cell to the cathode of the adjacent cell. As seen in Figure 1, a reactant gas enters into a fuel cell through a BPP. The flow field of a BPP is like a long rectangular pipe with a cross-sectional area on the order of a square millimeter and a length on the order of 10 cm to 1 m. One wall of the cross section of the rectangular pipe is formed by the GDL and the other three walls are formed by the solid nonporous BPP. The reason for using this fluidic structure in the BPP is to ensure that as little pressure drop as possible (a few pounds per square inch to no more than 15 psig, which is 103 241 Pa over atmospheric pressure) is developed as gas flows through this channel at the required flow rates (typically liters of gas per minute). Gas flow from the BPP supplies the GDL. Typically, the GDL is made of a porous and electrically conducting carbon paper or cloth. The GDL is about 2–3 orders of magnitude thicker than the active layer, which is about 10 mm thick. The permeability of gas in the GDL is a function of its structure (pore size) and water wetting properties (composition effects, e.g., presence and amount of polytetrafluoroethylene (PTFE) on

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Gas Diffusion Layers

the carbon paper or cloth). The pores in the GDL are on the order of microns in diameter. Gas moves through the GDL mainly by Knudsen diffusion. Knudsen diffusion occurs when the mean free path of the gas molecule is relatively long compared to the pore size, so the molecules collide frequently with the pore wall. Liters of gas per minute can pass a GDL when only a very small source pressure (a few psi, which is equal to about 10 000 Pa) is applied. Accordingly, pressure drops through a GDL are negligible when only gas flows are involved, because PEM fuel cells typically generate only 100 mW to no more than one or two watts per square centimeter of geometric active area, and 10 mL per minute of hydrogen corresponds to a watt of electricity. The GDL may not only have to transport gases but also have to pass liquid water when a fuel is operated under 100 1C. Two-phase mass transport is complicated, and recently attempts have been made to model it using computation fluid dynamics, which is beyond the scope of the present discussion. Some appreciation for this situation is given qualitatively below. Why bother placing a porous GDL between the macroscopic flow field of the BPP and the active layer of the electrode? The primary function of the GDL is to promote efficient uniform mass transport between the gas flow field in the BPP and the whole active layer. Recall that the active layer is where the electrochemical reactions take place. The good uniform mass transport between the BPP flow field and the active layer ensures that the fuel cell electrodes’ reaction can proceed at its maximum rate and is not limited by mass transport. So, the reaction is limited only by electrode activation overpotential and the fuel cell can generate the maximum power possible. This summarizes the behavior of the GDL for high temperature (>100 1C) fuel cells, such as the phosphoric acid, high-temperature PEM, and solid oxide fuel cell. For fuel cells operating below 100 1C, such as room temperature PEM and mixed liquid–gas-fed fuel cells, like direct methanol fuel cells, the GDL also has a major impact on water management. Simultaneous liquid (e.g., water and/or methanol) and vapor (e.g., hydrogen or oxygen) flow is a two-phase mass transport that is still challenging to model and understand. The GDL controls the rate at which water vapor diffuses into or out of the electrode. Water diffusion needs to be balanced in any fuel cell, but achieving that balance is particularly challenging in a PEMFC operating below 100 1C. Too little water diffusion through the GDL can result in water condensing leading to ‘flooding’ in the electrode. Flooding is the formation of a liquid water layer over the catalyst in the active layer. Flooding impedes gas supply to the catalyst. Gas supply rates become slower than reaction rates and this results in a cell voltage loss called mass transport polarization.

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Whereas too little water diffusion through the GDL causes flooding, too much diffusion of water through the GDL can lead to drying out of the active layer – and in severe cases the polymer electrolyte membrane itself. When the membrane needs water to conduct protons then this drying can cause excessive cell resistance called ohmic polarization. Water diffusion is usually controlled by carefully varying the PTFE content in the active layer as well as the GDL. More PTFE makes the electrode layer hydrophobic and lowers liquid water transport from or to the electrode and less PTFE makes the electrode layer hydrophilic and draws water into or out of the electrode. Active layer flooding occurs in an electrode that is hydrophilic because it has low PTFE content and too little water diffusion. Membrane layer drying occurs in hydrophobic high-PTFE-content electrodes that have too much water diffusion. A fine balance of hydrophobic and hydrophilic forces must be achieved to get the maximum power out of a PEM fuel cell. Clearly, water management is not simple in a fuel cell operating below 100 1C. A fuel cell using mixed liquid and gas feeds might not be possible without using well-designed GDLs.

Properties of Gas Diffusion Layers The properties of GDLs have been recently summarized by J. P. Feser and coworkers, in particular, the most important property, namely, the permeability of gases and liquids for woven, nonwoven, and carbon fiber-based GDLs at various levels of compression. Permeation In simplest terms, permeation can be quantified by a resistance to flow, R, through the GDL; R can be defined as R ¼ DP/J, in which J is a flow of a fluid and DP is the measured pressure drop to sustain that flow through a specified area of a GDL. Compressibility The simplest model of a fluid flowing through a GDL is the viscous flow of that fluid in a pipe which has a length the thickness of the GDL and area equal to is the pore area. The pressure drop, DP, is directly proportional to the viscosity of the flowing fluid and the length of a pore and inversely proportional to the cross-sectional area of the pore. According to this simple model the resistance to flow through a GDL should be proportional to the compressive pressure applied to it, since the greater the pressure the smaller the cross-sectional area of the pore. In fact, J. P. Feser and coworkers have shown that the permeability decreases as compressive pressure increases, as expected.

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Fuel Cells – Proton-Exchange Membrane Fuel Cells | Gas Diffusion Layers

Conductivity The electronic conductivity is proportional to the porosity (the apparent density) of the GDL. Accordingly, for a given material (woven and nonwoven carbon cloth, carbon paper, etc.), the electronic conductivity increases as the compressive pressure increases, because the apparent density and electrical conductivity are approaching that of carbon itself. For a given compression, the conductivity of carbon paper is usually greater than that of carbon cloth; however, the permeability of carbon cloth is usually greater than that of carbon paper since the pore density of carbon cloth is greater than that of carbon paper. Mechanical Stability Mechanical stability usually concerns massive fissures in the GDL that can lead to loss of active area of the electrode. Carbon cloth is soft and pliable and does not suffer cracking but GDL cracking can occur when using carbon papers. Recently, cracking has been suppressed by using partially ordered graphitized nano-carbon black (PUREBLACKs Carbon), a new class of nano-carbons jointly developed by Superior Graphite Co. and Columbian Chemicals Co. as well as mats of oriented conducting carbon nanotubes.

hydrophobicity, diameter, and length of the pipes. The hydrophobicity of a GDL is characterized by contact angle measurement. The pore diameter and thickness can be considered directly and isotropically dependent on compressive pressure.

Interaction of Gas Diffusion Layer with the Adjoining Components The GDL and BPP are set and sealed to each other by simply applying a compressive mechanical pressure. The interaction of the GDL with the BPP reflects the sealing gasket between the two and effects of the magnitude and uniformity of the force used to set the BPP against the sealing gasket and the sealing gasket against the GDL. The active layer, on the other hand, is chemically coated onto the GDL. This is usually done by a spray coating process followed by a heat treatment. The interaction of the active layer with the GDL involves the adhesion of the active layer to the GDL as well as the relative thermal expansion and swelling, for example, in the presence of water. The active layer may delaminate from the GDL if the dimensions of the active layer and the GDL are very different as the temperature or humidity is varied and their adhesion is poor.

Conclusion Corrosion Graphite paper and cloth are popular as GDLs in fuel cells because graphite has good corrosion resistance at the temperature, pressure, humidity, and electrical potentials involved. However, iron, nickel, titanium, and other metal foams and cermets have been used in some cases but, as expected, these metals are more costly and much more susceptible to corrosion. Hydrophobicity The hydrophobicity of a GDL can be altered by treating a carbon cloth or paper or metal foam or frit with a Teflon coating. Graphite-based GDLs are well understood. Typically a GDL will not be wetted by liquid water when the composition of the solid components in the GDL consists of 20 wt% of Teflon and 80% graphite. Similar trends are found with metal GDLs.

Characterization of Gas Diffusion Layers As mentioned above, the GDL can be thought of as a collection of parallel pipe with diameters on the order of microns and lengths on the order of millimeters. Accordingly, the flow properties of the GDL are mechanical in nature and the change of flow responds to the

The GDL ensures efficient and uniform mass transport between the macroscopic flow field and the active layer where the electrochemical reactions take place. The performance of the GDL has tremendous impact on fuel cell performance. If the fuel cannot reach the active layer of an electrode at a sufficient rate, the cell voltage is lowered because the electrode has a potential loss called the mass transport overpotential. Conventional GDLs employ an electrically conducting material, and graphite is most popular due to its corrosion resistance under fuel cell operating conditions. The most popular GDLs use porous carbon cloth or paper to control permeability and conductivity, mixed with another material, such as PTFE, to control water wetting. In addition to providing reactant feeds to the active layer, the GDL controls the rate at which water vapor diffuses into or out of the fuel cell. This is particularly important when using a membrane–electrode assembly that uses a PEM that requires water for the ionic conductivity of the PEM. Inadequate water diffusion results in flooding of the catalyst, so that cell voltage is lowered due to mass transport polarization; too high water diffusion can contribute to drying out of the PEM, so that cell voltage is lowered due to ohmic polarization. Clearly, optimization of GDL properties is a complicated process that can strongly influence fuel cell performance.

Fuel Cells – Proton-Exchange Membrane Fuel Cells | Gas Diffusion Layers

Nomenclature Symbols and Units J P

R

flow of fluid (sccm, standard centimeters per minute) pressure (psi pounds per square inch, 1 psi = approximately 6.895 kPa) resistance to flow (psi/sccm)

Abbreviations and Acronyms BPP GDL PEM PEMFC PTFE SOFC

bipolar plate gas diffusion layer polymer electrolyte membrane proton-exchange membrane fuel cell polytetrafluoroethylene solid oxide fuel cell

See also: Electrochemical Theory: Corrosion; Electrodes: Porous Electrodes; Fuel Cells – Molten Carbonate Fuel Cells: Overview; Fuel Cells – Overview: Introduction; Modeling; Fuel Cells – Phosphoric Acid Fuel Cells: Cells and Stacks; Fuel

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Cells – Proton-Exchange Membrane Fuel Cells: Bipolar Plates; Cells; Membrane–Electrode Assemblies; Overview Performance and Operational Conditions; Stacks; Fuel Cells – Solid Oxide Fuel Cells: Overview.History: Fuel Cells; Measurement Methods: Structural Properties: Neutron and Synchrotron Imaging, In-Situ for Water Visualization.

Further Reading Arunachala MK, Anupam M, and Igor VB (2006) Gas diffusion layer using a new type of graphitized nano-carbon PUREBLACKs for proton exchange membrane fuel cells. Electrochemistry Communications 8: 887--891. Feser JP, Prasad AK, and Advani SG (2006) On the relative influence of convection in serpentine flow fields of PEM fuel cells. Journal of Power Sources 162: 1226--1231. Larminie J and Dicks A (2003) Fuel Cell Systems Explained, 2nd edn. Chichester. West Sussex: John Wiley & Sons. Trung VN (2003) A liquid water management strategy for PEM fuel cell stacks. Journal of Power Sources 114(1): 70--79. Ugur P and Wang CY (2004) Liquid water transport in gas diffusion layer of polymer electrolyte fuel cells. Journal of the Electrochemical Society 151(3): A399--A406. Vielstich W, Gasteiger HA, and Lamm A (eds.) (2003) Handbook of Fuel Cells: Fundamentals, Technology, and Applications. Chichester: John Wiley & Sons.