Membrane electrode assembly structure

SD is routinely used to deposit thin films and has proven benefits from economies of scale in the metallization of plastics. The technique has already been used to create enhanced and unique MEAs for H2 -air proton exchange membrane fuel cell (PEMFC) systems. In this project, JPL is pursuing the use of SD to create DMFC membrane electrode assembly structures with highly electro-active catalyst layers that will reduce the amount and cost of the Pt-alloy catalyst at the fuel cell anode. 

Sol-gel techniques have been successfidly applied to form fuel cell components with enhanced microstructures for high-temperature fuel cells. The apphcations were recently extended to synthesis of hybrid electrolyte for PEMFC. Although die results look promising, the sol-gel processing needs further development to deposit micro-structured materials in a selective area such as the triple-phase boundary of a fuel cell. That is, in the case of PEMFC, the sol-gel techniques need to be expanded to form membrane-electrode-assembly with improved microstructures in addition to the synthesis of hybrid membranes to get higher fuel cell performance. 

Recent studies (in the 1990s) focused on the optimization of structures and compositions of electrodes, membranes, and membrane electrode assemblies (MEA) and of operating conditions. The major accomplishments in these areas are summarized as follows ... 

Figure 2.1 shows a schematic structure of the fuel cell membrane electrode assembly (MEA), including both anode and cathode sides. Each side includes a catalyst layer and a gas diffusion layer. Between the two sides is a proton exchange membrane (PEM) conducting protons from the anode to the cathode. 

Schematic structure of a fuel ceU membrane electrode assembly (MEA), including both anode and cathode catalyst layers. (Based on Lister. S. and McLean, G. Journal of Power Sources 2004 130 61-76. With permission from Elsevier.)...

Bose, A. B., Shaik, R., and Mawdsley, J. Optimization of the performance of polymer electrolyte fuel cell membrane electrode assemblies Roles of curing parameters on the catalyst and ionomer structures and morphology. Journal of Power Sources 2008 182 61-65. 

Fig. 1.6 Illustration of a planar-stack, solid-oxide fuel cell (SOFC), where an membrane-electrode assembly (MEA) is sandwiched between an interconnect structure that forms fuel and air channels. There is homogeneous chemical reaction within the flow channels, as well as heterogeneous cehmistry at the channel walls. There are also electrochemical reactions at the electrode interfaces of the channels. A counter-flow situation is illustrated here, but co-flow and cross-flow configurations are also common. Channel cross section dimensions are typically on the order of a millimeter.

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

The main advantage of the GDE technique is that the electrode structure is similar to the fuel cell membrane electrode assembly. Therefore, the obtained results may be closer to those tested in a real fuel cell. However, the GDE technique is still rarely used in fuel cell studies due to the complicated design of the electrochemical cell, as well as the instability and poor-repeatability of the results. Furthermore, prior to the electrochemical measurements, the GDE needs to... 

In a H2/02 (air) fuel cell, in general, the spectra, i.e., the Nyquist plots obtained by EIS measurements, very often have three features, which are denoted as high-frequency, medium-frequency, and low-frequency. The high-frequency region of an impedance spectrum is associated with the internal ohmic resistance and the contact capacitance in the granular electrode structure of the membrane electrode assemblies, whereas the medium- and low-frequency regions represent the charge-... 

The MCFC membrane electrode assembly (MEA) comprises three layers a porous lithiated NiO cathode structure and a porous Ni/NiCr alloy anode structure, sandwiching an electrolyte matrix (see detail below). To a first approximation, the porous, p-type semiconductor, nickel oxide cathode structure is compatible with the air oxidant, and a good enough electrical conductor. The nickel anode structure, coated with a granular proprietary reform reaction catalyst, is compatible with natural gas fuel and reforming steam, and is an excellent electrical conductor. As usual, the oxygen is the actual cathode and the fuel the anode. Hence the phrase porous electrode structure . 

Figure 12.1 is a schematic view of a typical PEM fuel cell. A membrane electrode assembly (MEA) usually refers to a five-layer structure that includes an anode gas diffusion layer (GDL), an anode electrode layer, a membrane electrolyte, a cathode electrode layer, and a cathode GDL. Most recently, several MEA manufacturers started to include a set of membrane subgaskets as a part of their MEA packages. This is often referred to as a seven-layer MEA. In addition to acting as a gas and... 

Molecular-Level Modeling of the Structure and Proton Transport within the Membrane Electrode Assembly of Hydrogen Proton Exchange Membrane Fuel Cells... 

To build an efficient, high-quality microscale fuel cell, microfabrication techniques need to be combined with appropriate materials such as Nation based membrane electrode assemblies (MEAs). These techniques must be able to produce three-dimensional structures, allow reactant and product flow into and out of the device, process appropriate materials, and should be of low cost. Fortimately, traditional thin film techniques can be modified for microscale fuel cell fabrication, while maintaining their advantages of surface preparation, sensor integration, and finishing or packaging. In addition, other techniques are also available and are discussed in the following sections. 

Membrane electrode assemblies (MEAs) are typically five-layer structures, as shown in Figure 10.1. The membrane is located in the center of the assembly and is sandwiched by two catalyst layers. The membrane thickness can be from 25 to 50 pm and, as mentioned in Chapter 10, made of perfluorosulfonic acid (Figure 11.3). The catalyst-coated membranes are platinum on a carbon matrix that is approximately 0.4 mg of platinum per square centimeter the catalyst layer can be as thick as 25 pm [12], The carbon/graphite gas diffusion layers are around 300 pm. Opportunities exist for chemists to improve the design of the gas diffusion layer (GDF) as well as the membrane materials. The gas diffusion layer s ability to control its hydrophobic and hydrophilic characteristics is controlled by chemically treating the material. Typically, these GDFs are made by paper processing techniques [12],... 

Cells usually have a bipolar configuration. The electrocatalysts are bonded to each side of the membrane (15), and the resulting SPE is a structurally stable membrane-electrode assembly as shown in Figure 1. A multi-layer package of expanded metal screens which presses up against the electrode on one side serves as the current collector and fluid distributor. 

The Jet Propulsion Laboratory (JPL) has researched the stated objectives by investigating sputter-deposition (SD) of designed anode and cathode nanostructures of Pt-alloys, and electronic structures and microstructures of sputter-deposited catalyst layers. JPL has used the information derived from these investigations to develop novel catalysts and membrane electrode assemblies (MEAs) that... 

J. Xie, D. L. Wood III, K. L. More, P. Atanassov, and R. L. Borup, Micro-structural Changes of Membrane Electrode Assemblies During PEFC Durability Testing at High Humidity Conditions, /. Electrochem. Soc., 12, AlOl 1 (2005). 

There are multiple transport and reaction steps in a fuel cell. Many of these reaction and transport processes are discussed in other chapters. PEM fuel cell designs have been heuristically derived to achieve high power output. Many proprietary methods of membrane-electrode assemblies have been developed, as well as complex structures of the flow fields, to provide the... 

This introductory chapter provides a brief outline and history of the PEFC technology, and important requirements and aspects in the development of polymer membranes for fuel cells. To obtain materials that meet specific requirements, the relationship of composition/structure and the properties has to be estabUshed. For the preparation of the membrane electrode assembly, it is important to understand the interfacial properties between membrane and electrodes. 

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. 

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