Membrane electrode assembly stability

Ramani, V. Kunz, H. R. Fenton, J. M., Stabilized composite membranes and membrane electrode assemblies for elevated temperature/low relative humidity PEFC operation. Journal of Power Sources 2005, 152, 182-188. 

The heart of a fuel cell is the membrane electrode assembly (MEA). In the simplest form, the electrode component of the MEA would consist of a thin film containing a highly dispersed nanoparticle platinum catalyst. This catalyst layer is in good contact with the ionomeric membrane, which serves as the reactant gas separator and electrolyte in this cell. The membrane is about 25-100 p,m thick. The MEA then consists of an ionomeric membrane with thin catalyst layers bonded on each side. Porous and electrically conducting carbon paper/cloth current collectors act as gas distributors (Figure 27.1). Since ohmic losses occur within the ionomeric membrane, it is important to maximize the proton conductivity of the membrane, without sacrificing the mechanical and chemical stability. 

Long-term Stability of Membrane/Electrode Assemblies in PEFCs... 

During the course of membrane electrode assembly (MEA), manufacture and PEMFC operation, the membranes are exposed to the impacts of temperature, humidity, and pressure. Consequently, it is important that the membranes possess a good mechanical stability, and in particular a high mechanical strength and minimal swelling. 

In the PEFC, the membrane, together with the electrodes, forms the basic electrochemical unit, the membrane electrode assembly (MEA). The first and foremost function of the electrolyte membrane is the transport of protons from anode to cathode. On one hand, the electrodes host the electrochemical reactions within the catalyst layer and provide electronic conductivity, and, on the other hand, they provide pathways for reactant supply to the catalyst and removal of products from the catalyst. The components of the MEA need to be chemically stable for several thousands of hours in the fuel ceU under the prevailing operating and transient conditions. PEFC electrodes are wet-proofed fibrous carbon sheet materials of a few 100 ttm thickness. The functionality of the proton exchange membrane (PEM) extends to requirements of mechanical stability to also ensure effective separation of anode and... 

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. 

With respect to fuel-cell technology itself, the small portable units use commercially available membrane electrode assemblies (MEA) and gas diffusion layers (GDL). As the operating temperature of small fuel-cell stacks usually lies below 50 °C, the requirements with respect to material stability of MEA and GDL, but also of sealing gaskets and bipolar plates are comparable lower than for other applications. For example, it is well known that metallic bipolar plates show significantly lower corrosion below 50 °C than at typical operation temperature of 80 °C [6,7], so that a sufficient lifetime for portable applications can be achieved with stainless steel. 

Alcohol crossover and cell resistance are the relevant properties determining the DAFC performance, which are closely related to the membrane used in the preparation of the membrane-electrode assembly (MEA). Mechanical properties, as well as the chemical and thermal stability, of the membrane could also be important when durability is considered. 

Hamon C, Purdy G, Kim YS, Pivovar B, Zelenay P (2006) Novel process for improved longterm stability of DMFC membrane-electrode assemblies. Proc ECS P2004—21 352-356... 

A membrane electrode assembly (MEA), where the fuel cell anode and cathode halfreactions occur, is the heart and the most delicate part of a fuel cell system. The performance, stability, and durability of a fuel cell largely depend on the quality of the MEAs. Therefore, MEA qualification should be critical in developing durable, high performanee fuel eell systems. 

The performanee and durability of a membrane electrode assembly (MEA) is affected signifieantly by the eathode eleetrode eomposition and structure, due to the poor kineties of oxygen reduetion and reaetant transport limitations. Utilization and stability of platinum or its alloys in the PEMFC play important roles in fuel cell efficiency, durability, and the drive for eost reduction through reduced Pt loadings. Cathode catalyst layer degradation is a critical issue for fuel cell durability to meet the requirement of > 5000 hours for automotive applications and > 40,000 for stationary applications. 

In general, PFSA membranes are characterized by excellent performance, electrochemical stability, suitable mechanical properties, and allow rapid startup. However, it appears necessary to ameliorate the PFSA membranes and ionomers to improve the operating efficiency of membrane-electrode assemblies of PEM electrolysers at practical current densities useful to reduce capital costs. PESA membranes used in electrochemical devices are essentially based on Nafion however, several alternative PSFA membranes with shorter pendant side chain have been developed by Dow, 3 M, Gore, Asahi Glass, Solvay Specialty Polymers, etc.. 

In addition to these two main reactions, there are some other processes having an impact on the electrode behavior. The surface mobility of adsorbed species in the catalytic layer is relevant to analyze the performance and degradation of the fuel cell membrane-electrode assembly. This study is not an easy task and, therefore, these phenomena have been scarcely explored. The diffusion of hydrogen and oxygen adsorbed species is particularly relevant for the performance and stability of PEMFC electrodes. 

High thermal stability for membrane electrode assembly (MEA) processing... 

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