Membrane-electrode assembly water transport

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... 

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... 

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. 

Schematic illustration of the membrane electrode assembly (MEA) of a PEM fuel cell (top) and details which have been subjected to modeling and simulation work described in the following chapters. Atomic level simulations have been performed for water and proton transport within the hydrophilic domaine of hydrated ionomers and for the electrochemical processes taking place at the electrocatalysts surfaces. The latter include the introduction of polarizable solvents and electrostatic potential variations. Mesoscale modeling is aiming at a better description and understanding of the development of ionomer microstructures.

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. 

Supply the water required for the methanol oxidation in the anode by transporting water from the cathode, so that the fuel cell can be operated with pure methanol and with minimum flooding at the cathode. This can be achieved by innovation in the design of the membrane electrode assembly. 

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). 

Water transport capability (high water flux) from the cathode to the anode These properties have to be assured under a wide range of temperature and humidity (—30-120°C, nominal 0-100% relative humidity (RH)) considering the fabrication of membrane electrode assemblies (MEAs)... 

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. 

A modem FC used in transportation and other applications is shown in Fig. 2. Its key elements are the electrodes, the catalyst, and the proton exchange membrane (PEM) the cell is fueled by hydrogen or methanol at the anode and oxygen or air at the cathode. The membrane electrode assembly (MEA) that is the heart of ECs includes the proton exchange membrane, a polymer modified to include ions, typically sulfonic groups an ionomer In the presence of water, ionomers self-assem-ble into microphase separated domains that allow the movement of in one direction only, from the anode to the cathode. The membrane performance was first demonstrated by Nafion, the ionomer made by DuPont, which consists of a perflu-orinated backbone and pendant chains terminated by sulfonic groups, -SOs . Nafion was the major component in the PEMEC developed by General Electric for... 

Ye, X., and Wang, C. Y. 2007. Measurement of water transport properties through membrane-electrode assemblies. ZSecfroc ggjj 5oc., 154, B676-B682. 

Fig. 6 Accelerating effect of temperature on FRR into protonicaUy transported water (water electrochemicaUy transported from the anode to the cathode side) at 1.08 A cm" for anode water feed PEM electrolysis cells with Nafion membrane electrode assemblies (MEAs) and 670-kPa balanced pressure (assumes projected end of useful life is approximately 10% loss of total fluorine) (LaConti et al. 2005)...

R. Zaffou, J. S. Yi, H. R. Kunz and J. M. Fenton, Temperature-Driven Water Transport through Membrane Electrode Assembly of Proton Exchange Membrane Fuel Cells, Electrochem. Solid-State Lett., Vol. 9, No. 9, pp. A418-A422, 2006. 

For fundamental research where the thermal parameters are not precisely known and to define unknown thermal transport parameters, we would like to directly measure the temperature profile within the electrolyte. To approach this problem, a thermocouple can be embedded directly in the diffusion media of a PEFC [32, 33]. However, the contact resistance between the diffusion media and the thermocouple becomes another unknown parameter. To circumvent these difficulties, Burford et al. invented a method to embed an array of microthermocouples directly between two 25-pm-thick Nafion electrolyte sheets of a membrane electrode assembly [34, 35]. Local temperature variation in PEFCs was determined to reach > 10°C at high current density for a thick diffusion media (>400 pm for woven cloth media). This proved that an isothermal assumption is typically not justified over a full range of performance and indicates phase change plays a role in water transport in PEFCs. An even smaller MEMs-based thermosensor array has been developed using vapor deposition [36] and has been embedded within a PEFC electrolyte, providing precise locational control of the sensor position. 

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