Fuel cell membranes high performance

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Since PEM, like living organisms, need water to function and the amount and state of water are critical for an efficient operation, secondary requirements on this type of fuel cell membranes emerge. These include the necessity of sufficient humidification and the ability to retain water under operation conditions. Associated problems under fuel cell operation include the electroosmotic transport of water to the cathode side accompanied by dehydration at the anode side [45]. In the cathode the accumulation of water at high current densities, typically greater than 1 A cm-2, causes performances losses due to blocking of catalytically active sites and restriction of oxygen transport. 

Fuel cell membranes are close to optimization with PFSA materials. These materials give excellent performance when fully humidified and have adequate oxidative stability. These materials limit the temperature of operation of the FC to <100°C and have unacceptably high methanol crossover for direct methanol... 

While Nafion , a perfluorinated polymer developed by DuPont, is the most commonly used proton conductive polymer electrolyte membrane it is an insufficient solution in a number of areas. It has high cationic transport (approximately 9.56 5/cm) [8] but also has high levels of methanol fuel crossover, slow anode kinetics and very high cost [12]. Fuel cell membrane performance can be estimated from the ratio of proton conductivity (a) to methanol permeability (P). The higher the value of a/P, the better the membrane performance would be [13]. Chitosan has been shown to have a much lower methanol permeability than Nafion [14], and as such, a great deal of attention focused on developing chitosan membranes with high levels of ionic conduction and low methanol permeability as delineated in Table 3.1. 

At high current densities, the ohmic resistance of the membrane still affects the properties of the fuel cell. For improved performance, thinner membranes of Nafion 112, 50 pm thick, were introduced in the place of the standard Nafion 115 membranes (100 pm). Even thinner membranes have been developed (Nafion 111,25 pm), but fuel cells with such membranes occasionally failed because of small membrane defects allowing the gases to mix. 

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. 

Kubo, N., and Shinohara, K. (2010) Effects of heat and water transport on the performance of polymer electrolyte membrane fuel cell under high current density operation. Electrochim. Acta, 56, 352-360. 

A number of technical and cost issues face polymer electrolyte fuel cells at the present stage of development (35, 38, 39, 40, 41). These concern the cell membrane, cathode performance, and cell heating limits. The membranes used in present cells are expensive, and available only in limited ranges of thickness and specific ionic conductivity. Lower-cost membranes that exhibit low resistivity are needed. This is particularly important for transportation applications characterized by high current density operation. Less expensive membranes promote lower-cost PEFCs, and thinner membranes with lower resistivities could contribute to power density improvement (41). It is estimated that the present cost of membranes could fall (by a factor of 5) if market demand increased significantly (to millions of square meters per year) (33). 

These sulfonated polysulfone membranes 100 °C for the cell equipped with sulfonated have been investigated in DMFCs operating at polysulfone membrane. The fuel cell perfor-high temperatures (100-120 °C). Figure 2.11 mance at 100 °C compared to 90 °C shows a shows the DMFC performances at 90 and lower open circuit voltage (OCV) (0.77... 

Recent fuel cell membrane research and development (R D) efforts are summarized with a focus on (1) membranes for high-temperature, low-hmnidity PEMFC operation, (2) low-cost alternatives to PFSA membranes, and (3) direct methanol fuel cell membranes. A listing of fuel cell membrane performance data is given in Tables 29.4-29.6 for each membrane subcategory. The review material is by no means exhaustive, but it is representative of the kinds of fuel cell membranes previously/cmrently under investigation. For different viewpoints on the evolutionary development of various fuel cell membranes, the reader is directed to other review articles in the open hterature (Rikukawa and Sanui, 2000 Li et al., 2003 Haile, 2003 Jannasch, 2003 Savadogo, 2004 Hickner et al., 2004 Hogarth et al., 2005 Smitha et al., 2005). 

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