Membrane electrode assembly functions

The function of the electrolyte membrane is to facilitate transport of protons from anode to cathode and to serve as an effective barrier to reactant crossover. The electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and pathways for reactant supply to the catalyst and removal of products from the catalyst [96], The GDL is a carbon paper of 0.2 0.5 mm thickness that provides rigidity and support to the membrane electrode assembly (MEA). It incorporates hydrophobic material that facilitates the product water drainage and prevents... 

The multi-faceted functionality of a GDL includes reactant distribution, liquid water transport, electron transport, heat conduction and mechanical support to the membrane-electrode-assembly. 

The membrane electrode assembly (MEA), which consists of three components (two gas diffusion electrodes with a proton exchange membrane in between), is the most important component of the PEMFC. The MEA exerts the largest influence on the performance of a fuel cell, and the properties of each of its parts in turn play significant roles in that performance. Although all the components in the MEA are important, the gas diffusion electrode attracts more attention because of its complexity and functions. In AC impedance spectra, the proton exchange membrane usually exhibits resistance characteristics the features of these spectra reflect the properties of the gas diffusion electrode. In order to better understand the behaviour of a gas diffusion electrode, we introduce the thin-film/flooded agglomerate model, which has been successfully applied by many researchers to... 

Clearly, a fundamental understanding of the key strac-ture/property relationships, particularly membrane morphology and conductivity as a function of polymer electrolyte architecture and water content - both in the bulk hydrated membrane and at the various interfaces within the membrane electrode assembly (MEA), can provide guidance in the synthesis of novel materials or MEA manufacturing techniques that lead to the improvement in the efficiency and/or operating range of PEMFCs. 

In experiments, we used an ion-exchange membrane made of MF-4SK (Nafion-type) perfluorinated polymer with functional sulfo-groups for preparation of membrane-electrode assemblies (MEAs). The membrane thickness was 130 Xm, the exchange capacity was 0.86 mg-equiv/g, the catalyst was 20 wt % Pt on a hydrophobic carbon carrier, the catalyst loading at the anode and cathode was 0.35 mg/cm. ... 

The development of membranes for fuel cells is a highly complex task. The primary functionalities, (i) transport of protons and (ii) separation of reactants and electrons, have to be provided and sustained for the required operating time. Optimization of the composition and structure of the material to maximize conductivity and mechanical robustness involves careful balancing of synthesis and process parameters. The ultimate membrane qualification test is the fuel cell experiment. It is evident that the membrane is not a stand-alone component, but is combined with the electrodes in the membrane electrode assembly (MEA). Interfacial properties, influence on anode and cathode electrocatalysis, and water management are the key aspects to be considered and optimized in this ensemble. 

The three components of the fuel cell, anode, cathode, and electrolyte form a membrane-electrolyte assembly, as, by analogy with polymer electrolyte fuel cells, one may regard the thin layer of solid electrolyte as a membrane. Any one of the three membrane-electrode assembly components can be selected as the entire fuel cell s support and made relatively thick (up to 2 mm) in order to provide mechanical stability. The other two components are then applied to this support in a different way as thin layers (tenths of a millimeter). Accordingly, one has anode-supported, electrolyte-supported, and cathode-supported fuel cells. Sometimes though an independent metal or ceramic substrate is used to which, then, the three functional layers are applied. 

Bipolar plates connecting adjacent cells can be considered as combination the functions, current collector, gas distribution (flow field), gas separation and coolant layer into one subunit. Integration of electrolyte membrane, catalytic layers and gas diffusion layers into a Membrane Electrode Assembly (MEA) results into a second major subunit for fuel cell stack integration. Combination of a catalytic layer and a gas diffusion layer to a gas diffusion electrode is yet another possibility of integrating functional layers to subunits. 

Figure 9.8 (a) Schematic representation of oxidized to H. (b) Structure of the the structure and reactivity of the bio-inspired membrane-electrode assembly used for the H2-evolving nickel catalyst grafted on a carbon electrocatalytic characterization of the nanotube [51]. Electrons are exchanged Ni-functionalized CNTs under conditions... 

In these two Ni-functionalized CNT materials, the Ni-molecular catalyst is located at the crossroads of the three interpenetrated networks allowing percolation of protons (the Nafion membrane), hydrogen (the pores in the gas diffusion layer), and electrons (the carbon fibers of the gas diffusion layer relayed by the conducting CNTs). In a way and even if it is not as well defined as in the protein, the catalyst environment in this membrane-electrode assembly reproduces that found in the active sites of hydrogenases buried into the polypeptidic framework but connected to the surfece of the protein via a gas diffusion channel, a network of hydrogen-bonded amino acids for proton transport and the array of electrontransferring iron-sulfur clusters. 

Usually, the starting point of model derivation is either a physical description along the channel or across the membrane electrode assembly (MEA). For HT-PEFCs, the interaction of product water and electrolyte deserves special attention. Water is produced on the cathode side of the fuel cell and will either be released to the gas phase or become adsorbed in the electrolyte. As can be derived from electrochemical impedance spectroscopy (EIS) measurements [14], water production and removal are not equally fast Water uptake of the membrane is very fast because the water production takes place inside the electrolyte, whereas the transport of water vapor to the gas channels is difiusion limited. It takes several minutes before a stationary state is reached for a single cell. The electrolyte, which consists of phosphoric add, water, and the membrane polymer, changes composition as a function of temperature and water content [15-18]. As a consequence, the proton conductivity changes as a function of current density [14, 19, 20). 

Figure 3.4 The system of coordinates in the membrane-electrode assembly. Each functional layer is equipped with its own rr-axis.

In a single-cell configuration, there are no bipolar plates. The two plates on each side of the membrane electrode assembly may be considered as two halves of a bipolar plate. The fully functioning bipolar plates are essential for multicell configurations (as shown in the Figure 16), by electrically connecting the anode of one cell to the cathode of the adjacent cell. 

The functions of porous electrodes in fuel cells are 1) to provide a surface site for gas ionization or de-ionization reactions, 2) to provide a pathway for gases and ions to reach the catalyst surface, 3) to conduct water away from the interface once these are formed, and 4) to allow current flow. A membrane electrode assembly (MEA) forms the core of a fuel cell and the key electrochemical reactions take place in the MEA. MEA performance is severely affected by electrode composition, structure, and geometry, and especially by cathode structure and composition, due to poor oxygen reduction kinetics and transport liniitations of the reactants in the cathode catalyst layer. 

The single characteristic that encompasses phenomena at all scales and in all fuel cell components is the fuel cell polarization curve. The polarization curve of a membrane-electrode assembly (MEA), a single cell, or a fuel cell stack furnishes the link between microscopic structure and physicochemical properties of distinct cell components on the one hand and macroscopic cell engineering on the other. It thus condenses an exuberant number of parameters, which lies in the 50s to 100s, into a single response function. Analysis of parametric dependencies in the polariz-tion curve could be extremely powerful at the same time, it could as well be highly misleading if applied blindly. ... 

To produce a structure capable of performing the above-mentioned electrochemical task, an anode and cathode, each containing catalyst particles, are adhered to opposite sides of the PEM to form a layered composite structure. This composite structure is responsible for the electrochemical conversions and directed flow of fuel, byproducts, ions, electrolytes and electrons requisite for the electrochemical functioning of a fuel cell. This layered composite structure is also referred to as a membrane electrode assembly (MEA). 

FIGURE 5.24 Membrane conductivity as a function of current density at 120 °C and 1.0 atm backpressure, with different RHs. Nafion -112-based membrane electrode assembly with an active area of 4.4 cm. The thickness of the Nafion - 12 membrane was taken to be 50 pm [45]. (For color version of this figure, the reader is referred to the online version of this book.)... 

FIGURE 9.1 Oxygen partial pressure as a function of current density at 70 °C and 100% RH with different backpressures. Gore -membrane-based membrane electrode assembly (MEA) area 46 cm. Gas diffusion layer (GDL) 25-DC. Stoichiometries of H2 and air 1.2 and 2.5, respectively. Single serpentine flow channel with both a width and depth of 1.0 mm [1]. 

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