Small fuel cells electrodes

S. Koh, J. Leisch, M.F. Toney, P. Strasser, Probing size and composition distribution dynamics of alloy nanoparticles in fuel cell electrodes using anomalous Small Angle X-ray Scattering (ASAXS), ECS fall meeting, Cancun, Mexico, 2006. 

M. S. Wilson and S. Gottesfeld, J. Appl. Electrochem. 22 1 (1992). Making fuel cell electrodes with small Pt loading. 

High r factors are, however, not without some other complications since they imply porosity of materials. Porosity can lead to the following difficulties (a) impediment to disengagement of evolved gases or of diffusion of elec-trochemically consumable gases (as in fuel-cell electrodes 7i2) (b) expulsion of electrolyte from pores on gas evolution and (c) internal current distribution effects associated with pore resistance or interparticle resistance effects that can lead to anomalously high Tafel slopes (132, 477) and (d) difficulties in the use of impedance measurements for characterizing adsorption and the double-layer capacitance behavior of such materials. On the other hand, it is possible that finely porous materials, such as Raney nickels, can develop special catalytic properties associated with small atomic metal cluster structures, as known from the unusual catalytic activities of such synthetically produced polyatomic metal clusters (133). 

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. 

Small fuel cells are considered a strong candidate for the next generation of portable power supplies. A microfluidic fuel cell is defined as a fuel cell with fluid delivery and removal, reaction sites and electrode structures all confined to microfluidic channels. This type of fuel cell can use both metallic and biological catalysts and normally operates without a physical barrier, such as a membrane, to separate the electrodes. Other types of small fuel cells, such as microstructured polymer electroljde membrane-based fuel cells, will not be covered in this entry. 

Sealing means must be provided at the edges of the cells to prevent reactants from escaping to the atmosphere from porous elements of the electrodes. Also, since reactant manifolding is usually internal to the stack in small fuel cells (for the sake of compactness), sealing around manifold holes is required to prevent the mixing of reactants between manifolds and electrodes the considerations are the same for internal tie-bolt holes. 

Interest in fuel cells has stimulated many investigations into the detailed mechanisms of the electrocatalytic oxidation of small organic molecules such as methanol, formaldehyde, formic acid, etc. The major problem using platinum group metals is the rapid build up of a strongly adsorbed species which efficiently poisons the electrodes. 

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