Electrocatalysis mass transport

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. 

Chen SL, Kucemak A. 2004a. Electrocatalysis under conditions of high mass transport rate oxygen reduction on single submicrometer-sized Pt particles supported on carbon. J Phys Chem B 108 3262-3276. 

The choice of immobilization strategy obviously depends on the enzyme, electrode surface, and fuel properties, and on whether a mediator is required, and a wide range of strategies have been employed. Some general examples are represented in Fig. 17.4. Key goals are to stabilize the enzyme under fuel cell operating conditions and to optimize both electron transfer and the efficiency of fuel/oxidant mass transport. Here, we highlight a few approaches that have been particularly useful in electrocatalysis directed towards fuel cell applications. 

Mediated electrocatalysis at a polymer-modified electrode charge and mass transport processes. 

This brief review attempts to summarize the salient features of chemically modified electrodes, and, of necessity, does not address many of the theoretical and practical concepts in any real detail. It is clear, however, that this field will continue to grow rapidly in the future to provide electrodes for a variety of purposes including electrocatalysis, electrochromic displays, surface corrosion protection, electrosynthesis, photosensitization, and selective chemical concentration and analysis. But before many of these applications are realized, numerous unanswered questions concerning surface orientation, bonding, electron-transfer processes, mass-transport phenomena and non-ideal redox behavior must be addressed. This is a very challenging area of research, and the potential for important contributions, both fundamental and applied, is extremely high. 

Figures 5.4 and 5.5 summarize results of a recent study of P. versicolor laccase electrochemistry based on cyclic and rotating disk voltammetry [60]. Figure 5.4 shows unequivocally that this laccase is voltammetrically active and gives a kinetically controlled, unpromoted four-electron peak at edge-plane pyrolytic graphite. Electrochemical reduction of 02 catalyzed by an immobilized laccase monolayer is close to reversible, and unrestricted by mass transport. The electrocatalysis follows, moreover, a Michaelis-Menten pattern (Fig. 5.5). Finally, there is a characteristic bell-shaped functional pH-profile with a pronounced maximum at pH 3.1.

As already shown in Fig. 1, a general feature of electrocatalysis is that the current passing through an electrode-electrolyte interface depends exponentially on overpotential, as described by the Butler-Volmer equation discussed in Sect. 2.4.1, so that logi versus r] U — C/rev) gives straight lines, termed Tafel plots (Fig. 1). On this basis, one would expect an exponential-type dependence of current on overpotential in Fig. 12 (curve labeled 7ac). Such a curve would correspond to pure activation control, that is, to infinitely fast mass-transport rates of reactants and products to and from the two electrodes. 

RRDE is significantly simpler than with conventional cyclic voltammetry data in quiescent solutions [88, 89]. As such, these forced convection systems have been widely used in the study of electrocatalysis in general. Of special interest are situations where the rate determining step is chemical (a) or electrochemical (B) (Scheme 3.7) [60], In particular, for an RDE at steady state, the rate at which the reactant is depleted at the interface must be equal to the rate at which it is replenished from the solution via convective mass transport. For a reaction first order in dioxygen this relationship reads ... 

Phenomena that arise in these materials include conduction processes, mass transport by convection, potential field effects, electron or ion disorder, ion exchange, adsorption, interfacial and colloidal activity, sintering, dendrite growth, wetting, membrane transport, passivity, electrocatalysis, electrokinetic forces, bubble evolution, gaseous discharge (plasma) effects, and many others. 

The principal problem as far as electrocatalysis is concerned is the relation of the current density using a porous electrode to that using a planar electrode, i.e., the rate unaffected by mass transport and diffusion. 

CPs and their composites are utilized in the fields ofelectrochemistiy, electroanalysis, electrocatalysis, batteries and capacitors, etc as electrode. In addition to the conductivity and electroactivity of CPs, small ions and molecules can diffuse into the CP matrices, providing further improvement compared to the conventional electrode materials. Efficiently using all the active sites and enhancing mass transport during the electrode process, the thickness of the CP film can be reduced to allow the ion diffusion in the CP matrices. By these properties CP nanomaterials exhibit better performances, due to their larger specific surface areas and small dimensions. Additionally, nanostructures of CPs may produce new surface properties and better functionalities. 

Chen, S. and Kucemak, A. (2004) Electrocatalysis under conditions of high mass transport investigation of hydrogen oxidation on single submicron Pt particles supported on carbon. The Journal of Physical Chemistry B, 108,13984-13994. Li, X. (2006) Principles of fuel cells. Platinum Metals Review, 50,200. Srinivasan, S., EnayetuUah, MA., Somasimdaram, S., Swan, D.H., Manko,... 

Several important technological applications, such as battery devices and electrocatalysis, need a very large effective surface of contact between the electrode and the electrolyte. This expanded surface can be developed on porous electrode surfaces. The complexity of the random structure of the porous electrode and various experimental situations related to mass-transport impedance in the pores, coupled with interfacial kinetics inside the pores, led investigators initially to investigate simple single-pore models. Of the possible shapes modeled, the cylindrical pore with a length I and a radius r has been... 

Chen S and Kucemak A (2004) Electrocatalysis under conditions of High Mass Transport Investigation of Hydrogen Oxidation on Single Submicron Pt Particles Supported on Carbon, J. Phys. Chem. B, 108, pp. 13984-13994. 

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