Proton bipolar plates

A PEFC consists of two electrodes in contact with an electrolyte membrane (Fig. 14.7). The membrane is designed as an electronic insulator material separating the reactants (H2 and 02/air) and allowing only the transport of protons towards the electrodes. The electrodes are constituted of a porous gas diffusion layer (GDL) and a catalyst (usually platinum supported on high surface area carbon) containing active layer. This assembly is sandwiched between two electrically conducting bipolar plates within which gas distribution channels are integrated [96]. 

Figure 4.1 shows a schematic of a typical polymer electrolyte membrane fuel cell (PEMFC). A typical membrane electrode assembly (MEA) consists of a proton exchange membrane that is in contact with a cathode catalyst layer (CL) on one side and an anode CL on the other side they are sandwiched together between two diffusion layers (DLs). These layers are usually treated (coated) with a hydrophobic agent such as polytetrafluoroethylene (PTFE) in order to improve the water removal within the DL and the fuel cell. It is also common to have a catalyst-backing layer or microporous layer (MPL) between the CL and DL. Usually, bipolar plates with flow field (FF) channels are located on each side of the MFA in order to transport reactants to the... 

Due to their high electrical and thermal conductivity, materials made out of metal have been considered for fuel cells, especially for components such as current collectors, flow field bipolar plates, and diffusion layers. Only a very small amount of work has been presented on the use of metal materials as diffusion layers in PEM and DLFCs because most of the research has been focused on using metal plates as bipolar plates [24] and current collectors. The diffusion layers have to be thin and porous and have high thermal and electrical conductivity. They also have to be strong enough to be able to support the catalyst layers and the membrane. In addition, the fibers of these metal materials cannot puncture the thin proton electrolyte membrane. Thus, any possible metal materials to be considered for use as DLs must have an advantage over other conventional materials. 

T. Matsuura, M. Kato, and M. Hori. Study on metallic bipolar plate for proton exchange membrane fuel cell. Journal of Power Sources 161 (2006) 74-78. 

Dobrovolskii, Y. A., A. E. Ukshe, A. V. Levchenko, et al. 2007. Materials for bipolar plates for proton-conducting membrane fuel cells. Russian Journal of General Chemistry 4 752-765. 

In PEMFCs working at low temperatures (20-90 °C), several problems need to be solved before the technological development of fuel cell stacks for different applications. This concerns the properties of the components of the elementary cell, that is, the proton exchange membrane, the electrode (anode and cathode) catalysts, the membrane-electrode assemblies and the bipolar plates [19, 20]. This also concerns the overall system vdth its control and management equipment (circulation of reactants and water, heat exhaust, membrane humidification, etc.). 

Besmann, T.M. et al., Carbon/carbon composite bipolar plate for proton exchange membrane fuel cells, J. Electrochem. Soc., 147, 4083, 2000. 

As documented in and expressed by these various contributions, the topic Polymers for Fuel Cells is a vast one and concerns numerous synthetic and physico-chemical aspects, derived from the particular application as a solid polymer electrolyte. In this collection of contributions, we have emphasized work which has already led to tests of these polymers in the real fuel cell environment. There exist other synthetic routes for proton-conducting membrane preparation, which are not discussed in this edition. Furthermore, certain polymers are utilized as fuel-cell structure materials, e.g., as gaskets or additives (binder, surface coating) to bipolar plate materials. These aspects are not covered here. 

Between any two bipolar plates providing reactant access, a multilayer MEA is set up that consists of a positive electrode and a negative electrode pressed into the two sides of a proton-conducting membrane. 

This section is devoted to a brief description of the main comptments of DAFC as an introduction to the most exhaustive analysis in Chaps. 2, 3,4, and 5 for electrocatalysts for methanol, ethanol, and higher alcohols, in Chap. 6 for proton exchange and alkaline membranes, and Chap. 7 for carbonous materials used as catalysts support, gas diffusion layers and bipolar plates. 

Kim M, Yu HN, Lim JW, Lee DG (2012) Bipolar plates made of plain weave carbon/epoxy composite for proton exchange membrane fuel cell. Int J Hydrogen Energy 37 4300 308... 

T. M. Besmann et al., Carbon/Carbon Composite Bipolar Plate for Proton Exchange Membrane Fuel Cells, Journal of The Electrochemical Society, 147(11), 4083-4086 (2000). 

Two types of ohmic losses occur in fuel cells. These are potential losses due to electron transport through electrodes, bipolar plates, and collector plates and potential loss due to proton transport through the membrane. The magnitudes of these potential losses depend on the materials used in the construction of the fuel cells and its operating conditions [27]. Membrane conductivity increases with membrane water content. Reduction in the thickness of the membrane between anode and cathode may be thought of as an expedient way to eliminate ohmic overpotential. However, thin membrane may cause the problem of crossover or intermixing of anodic and cathodic reactants [27]. 

Combinations of Dirichlet and Neumann boundary conditions are used to solve the electronic and protonic potential equations. Dirichlet boundary conditions are applied at the land area (interface between the bipolar plates and the gas diffusion layers). Neumann boundary conditions are applied at the interface between the gas charmels and the gas diffusion layers to give zero potential flux into the gas charmels. Similarly, the protonic potential field requires a set of potential boimdary condition and zero flux boundary condition at the anode catalyst layer interface and cathode catalyst layer interface respectively. 

PEMFGs use a proton-conducting polymer membrane as electrolyte. The membrane is squeezed between two porous electrodes [catalyst layers (CLs)]. The electrodes consist of a network of carbon-supported catalyst for the electron transport (soHd matrix), partly filled with ionomer for the proton transport. This network, together with the reactants, forms a three-phase boundary where the reaction takes place. The unit of anode catalyst layer (ACL), membrane, and cathode catalyst layer (CCL) is called the membrane-electrode assembly (MEA). The MEA is sandwiched between porous, electrically conductive GDLs, typically made of carbon doth or carbon paper. The GDL provides a good lateral delivery of the reactants to the CL and removal of products towards the channel of the flow plates, which form the outer layers of a single cell. Single cells are connected in series to form a fuel-cell stack. The anode flow plate with structured channels is on one side and the cathode flow plate with structured channels is on the other side. This so-called bipolar plate... 

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