Metal mass transport properties

In summary, there are many anion types which offer useftd properties for the creation of an electroplating medium. Choices must be made regarding electrochemical stability, relative hydrophobicity, the ability to coordinate metal salts and the mass transport properties of viscosity and conductivity. 

Before the hydrodynamic and mass-transport properties of the systems of interest are discussed, it is advantageous to outline first the sequence of events that occur at the metal/solution interface that leads to the development of damage. This is done so that the reader will have a greater appreciation of the role that fluid flow plays in each phase and how those parameters that are affected by fluid flow impact the nucleation, growth, and death phases of the damaging processes. 

The majority of todays membranes used in microfiitration, dialysis or ultrafiltration and reverse osmosis cire prepared from a homogeneous polymer solution by a technique referred to as phase inversion. Phase inversion can be achieved by solvent evaporation, non-solvent precipitation and thermcd gelation. Phase separation processes can not only be applied to a large number of polymers but also to glasses and metal alloys and the proper selection of the various process parameters leads to different membranes with defined structures and mass transport properties. In this paper the fundamentals of membrane preparation by phase inversion processes and the effect of different preparation parameters on membrane structures and transport properties are discussed, and problems utilizing phase inversion techniques for a large scale production of membranes are specified. 

Since the rate of all electrocatalytic reactions is strictly related to the active surface area, besides the surface chemistry, the morphology of the electrocatalyst needs to be tailored. Morphology is not only related to the metal-phase area but also to the presence of micro- and macro pores in the electrocatalyst support that could facilitate or hinder the mass transport properties. All these characteristics determine the cell performance even if the relative influence of each parameter is still not known in detail. It is thus necessary to select appropriate procedures for the optimization of these characteristics, i.e. composition, structure, particle size, porosity, etc. Generally a combination of physico-chemical and electrochemical analyses carried out on different electrocatalysts indicates the system that best suits the scope of application in a DMFC. 

Up-to-date non-noble metal catalysts only partially fulfill the demands in activity for a fuel cell application. To boost activity, the catalyst loading on the cathode can be increased. It was noticed, however, that above a certain layer thickness, mass transport properties hinder a further increase of the current density. For this reason, the specific volumetric activity has to be improved. 

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

Summing up this section, we would like to note that understanding size effects in electrocatalysis requires the application of appropriate model systems that on the one hand represent the intrinsic properties of supported metal nanoparticles, such as small size and interaction with their support, and on the other allow straightforward separation between kinetic, ohmic, and mass transport (internal and external) losses and control of readsorption effects. This requirement is met, for example, by metal particles and nanoparticle arrays on flat nonporous supports. Their investigation allows unambiguous access to reaction kinetics and control of catalyst structure. However, in order to understand how catalysts will behave in the fuel cell environment, these studies must be complemented with GDE and MEA tests to account for the presence of aqueous electrolyte in model experiments. 

The electrical transport properties of alkali metals dissolved in ammonia and primary amines in many ways resemble the properties of simple electrolytes except that the anionic species is apparently the solvated electron. The electrical conductance, the transference number, the temperature coefficient of conductance, and the thermoelectric effect all reflect the presence of the solvated electron species. Whenever possible the detailed nature of the interactions of the solvated electrons with solvent and solute species is interpreted by mass action expressions. 

What is dear from this introduction is that the journey into the area of metal deposition from ionic liquids has tantalizing benefits. It is also dear that we have only just begun to scratch the surface of this topic. Our models for the physical properties of these novel fluids are only in an early state of devdopment and considerably more work is required to understand issues such as mass transport, spedation and double layer structure. Nudeation and growth mechanisms in ionic liquids will be considerably more complex than in their aqueous counterparts but the potential to adjust mass transport, composition and spedation independently for numerous metal ions opens the opportunity to deposit new metals, alloys and composite materials which have hitherto been outside the grasp of electroplaters. 

The general approach for modelling catalyst deactivation is schematically organised in Figure 2. The central part are the mass balances of reactants, intermediates, and metal deposits. In these mass balances, coefficients are present to describe reaction kinetics (reaction rate constant), mass transfer (diffusion coefficient), and catalyst porous texture (accessible porosity and effective transport properties). The mass balances together with the initial and boundary conditions define the catalyst deactivation model. The boundary conditions are determined by the axial position in the reactor. Simulations result in metal deposition profiles in catalyst pellets and catalyst life-time predictions. 

Mass transport plays an important role in pulsed metal deposition. On the one hand it limits the maximum rate of deposition and influences the structure and properties of deposits. On the other hand it effects the macrothrowing and microthrowing power. Under dc conditions, the maximum deposition rate is given by the limiting current density, fg, where the metal ion concentration... 

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