Mass power density

A trend in electrode preparation is to reduce the catalyst layer thickness to improve the mass transfer efficiency at the interface, such as the efficient movement of protons, electrons, and dissolved reactants in the reaction zone. In addition, a thinner electrode will be beneficial to reduce catalyst loading and increase mass power density. The deposition technique is an effective way to achieve a thinner electrode through depositing a nano-scale catalyst film on the substrate. Deposition methods include chemical vapor deposition, physical or thermal vapor deposition, sputtering deposition, electrochemical deposition, chemical deposition, as well as ion beam deposition. The following sections will focus on electrodes fabricated with these various deposition methods. 

For a fuel ceU stack, mass- and volume-specific power densities are more useful parameters. The mass power density is the ratio of power to stack weight, measured in kilowatts per kilogram. Clearly, light-weight stack materials are required to increase the mass power density of a stack. Volume power density is the ratio of power to stack volume, measured in kilowatts per liter or kilowatts per cubic meter. Thus, for increasing the volume power density, one requires to reduce the stack size and simplify the stack system. 

The extremely high peak power densities available ia particle beams and lasers can heat the small amounts of matter ia the fuel capsules to the temperatures required for fusion. In order to attain such temperatures, however, the mass of the fuel capsules must be kept quite low. As a result, the capsules are quite small. Typical dimensions are less than 1 mm. Fuel capsules ia reactors could be larger (up to 1 cm) because of the iacreased driver energies available. 

For a while now, the problem of flow and heat transfer in heated capillaries has attracted attention from a number of research groups, with several applications to engineering. The knowledge of the thermohydrodynamic characteristics of capillary flow with evaporative meniscus allows one to elucidate the mechanism of heat and mass transfer in porous media, to evaluate the efficiency of cooling system of electronic devices with high power density, as well as to optimize MEMS. 

The processes in a cooling system of electronic devices with high power density can be modeled as follows. The coolant with temperature T2.0 and pressure F2.0 enters into the micro-channel from the tank (5) (Fig. 10.2). The mass capacity of the liquid in the tank (5) is large enough, therefore the heat flux from the micro-channel... 

From the practical point of view, this is the discharge of a SC device under constant power conditions that is normally of the most interest. That is why the present work is aimed at determining the optimum electrode thickness that enables one to obtain the maximum energy, E, output (referred to as unit of volume or mass) if the discharge with a fixed power takes place. For the sake of simplicity we will speak about the energy density (E) and power density (p), but all the expressions derived below can easily be transformed to obtain the specific energy or power, if the volume is substituted by mass. 

J. Srmivason, et al., "High Energy Efficiency and High Power Density Proton Exchange Membrane Fuel Cells - Electrode Kinetics and Mass Transport," Journal of Power Sources, p. 36, 1991. 

In a PEMFC, the power density and efficiency are limited by three major factors (1) the ohmic overpotential mainly due to the membrane resistance, (2) the activation overpotential due to slow oxygen reduchon reaction at the electrode/membrane interface, and (3) the concentration overpotential due to mass-transport limitations of oxygen to the electrode surfaced Studies of the solubility and concentration of oxygen in different perfluorinated membrane materials show that the oxygen solubility is enhanced in the fluorocarbon (hydrophobic)-rich zones and hence increases with the hydrophobicity of the membrane. The diffusion coefficient is directly related to the water content of the membrane and is thereby enhanced in membranes containing high water content the result indicates that the aqueous phase is predominantly involved in the diffusion pathway. ... 

Since high current density at the maximum power density and the cost of the noble metals are important parameters for the commercialization of DMECs, H-CNE-supported Pt-Ru alloys maybe classified among the most efficient and cost-effective anode catalysts. It is also worth mentioning that the CNF-supported catalysts feature superior catalytic activity at the high temperatures where the mass transfer of methanol and oxygen is more favorable due to the fibrous network of CNEs. 

Figure 9.56 Cluster ion formation in laser plasma in dependence of laser power density . (C. Seifert, j. S. Becker and H. J. Dietze, Int.J. Mass Spectrom. Ion Proc., 184, 121(1988). Reproduced by permission of Elsevier)...

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