Degradation and lifetime

The maximum power density of 0.093 W cm shown in Fig. 3.53 is one of the highest currently demonstrated, but still nearly ten times lower than that of hydrogen PEM cells (see Fig. 3.51), which also achieves more than twice the current density. Typical single cell overall efficiencies are of the order of 40% (Muller et al, 2003). 

The crossover of methanol has caused problems in finding a suitable membrane material. On the positive electrode side, methanol combines with oxygen to form CO2. Among the alternatives to pure Nafion are Nafion filled with zirconium phosphate or grafted with styrene to inhibit methanol transport (Bauer and Willert-Porada, 2003 Sauk et al., 2004), as well as non-Nafion membrane materials such as sulfonated polyimide (Woo et al., 2003). None have achieved performance as good as the one shown in Fig. 3.53, which, however, has a substantial methanol crossover rate. 

Cell modelling techniques for flow and electrochemistry are basically the same for DMFC as for other PEM fuel cells, i.e., the techniques described in section 3.1.5 and 3.5.1-3.5.3 have been employed for DMFCs by Fuhrman and Gartner (2003). 

Fuels other than methanol have been contemplated, such as formic acid, which exhibits lower crossover fluxes than methanol and thus may be an alternative for use e.g., in small systems contemplated for portable applications (Ha et al, 2004 Zhu et al, 2004). 

The theoretical power density of a DMFC at 0.5 V is about 1600 Wh per kg of methanol fuel, but in practice, small DMFCs for portable applications have achieved much less. If small DMFCs are designed like conventional PEM cells, including a membrane-electrode assembly (MEA), two gas diffusion layers, fuel and air channels with forced flows and current collectors, they may achieve power densities of about 0.015-0.050 W cm at temperatures in the range of 23-60°C (Lu et al., 2004), consistent with the value found at 85°C in Fig. 3.53. 

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THERMO- AND PHOTOOXIDATIVE DEGRADATION AND LIFETIME OF COMPOSITE BUILDING MATERIALS... 

Degradation and lifetime investigations are intended to characterize the membrane, the catalyst, and the corrosion of the carbon support as a function of time and... 

DEGRADATION AND LIFETIME OF RUBBER VIBRATION INSULATORS FOR VEHICLES... 

Rasmussen, J. R., Miller, G. R. and Gordon, R. S. (1982) Degradation and lifetime of -alumina electrolytes. One Hundred Sixty-First Meeting of the Electrochemical Society, Montreal, Quebec, Canada, 9-14 May 1982... 

Table 4 shows some elements that can be included in perovskite materials, highlighting those that would be expected to allow a decrease in coordination number in the perovskite lattice, and hence alford catalytic behaviour, with the elements stable in the reducing conditions of an SOFC anode indicated on the bottom hne. The use of these species produce a dimensionally stable anode, that does not change significantly between fully oxidised and fully reduced. This reduces many degradation and lifetime issues. 

Gillen, K.T., M. Celina, and R. Bernstein, Validation of improved methods for predicting long-term elastomeric seal lifetimes from compression stress-relaxation and oxygen consumption techniques. Polymer Degradation and Stability, 82(1), 25-35, 2003. 

L. Matisova-Rychla and J. Rychly, Inherent relations of chemiluminescence and thermooxidation of polymers, In R.L. Clough, N.C. Billingham and K.T. Gillen (Eds.), Advances in Chemistry, Series 249 Polymer Durability, Degradation, Stabilization and Lifetime Prediction. American Chemical Society, Washington, DC, 1996, p. 175. 

Once a database is established, it is made available to other laboratories through the company s secured intranet, so that the information therein can be updated, retrieved and reviewed. The resulting structural library can be referenced throughout the lifetime of the drug for rapid identification of impurities, degradants, and metabolites. 

Artificial satellites, which are now used for communication, broadcast, weather forecast, etc., are equipped with a variety of semiconductor devices, which are often exposed to the high levels of radiation found in space. Such energetic particles, called cosmic rays, cause the degradation and malfunction of semiconductor devices, which lowers both the mission lifetime and reliability of satellites. Using ion beam irradiation facilities at TIARA, which have been uniquely adapted for simulating the radiation environment of space, we have... 

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