Electrocatalysis factors

One factor contributing to the inefficiency of a fuel ceU is poor performance of the positive electrode. This accounts for overpotentials of 300—400 mV in low temperature fuel ceUs. An electrocatalyst that is capable of oxygen reduction at lower overpotentials would benefit the overall efficiency of the fuel ceU. Despite extensive efforts expended on electrocatalysis studies of oxygen reduction in fuel ceU electrolytes, platinum-based metals are stiU the best electrocatalysts for low temperature fuel ceUs. 

Electrodes. At least three factors need to be considered ia electrode selection as the technical development of an electroorganic reaction moves from the laboratory cell to the commercial system. First is the selection of the lowest cost form of the conductive material that both produces the desired electrode reactions and possesses stmctural iategrity. Second is the preservation of the active life of the electrodes. The final factor is the conductivity of the electrode material within the context of cell design. An ia-depth discussion of electrode materials for electroorganic synthesis as well as a detailed discussion of the influence of electrode materials on reaction path (electrocatalysis) are available (25,26). A general account of electrodes for iadustrial processes is also available (27). 

In the case of electrochemically promoted (NEMCA) catalysts we concentrate on the adsorption on the gas-exposed electrode surface and not at the three-phase-boundaries (tpb). The surface area, Ntpb, of the three-phase-boundaries is usually at least a factor of 100 smaller than the gas-exposed catalyst-electrode surface area Nq. Adsorption at the tpb plays an important role in the electrocatalysis at the tpb, which can affect indirectly the NEMCA behaviour of the electrode. But it contributes little directly to the measured catalytic rate and thus can be neglected. Its effect is built in UWr and [Pg.306]

Effective core potential, 269 Effective double layer characterization of, 189 isotherm, 306, 315 kinetic expressions, 316 observations of with STM, 259 stability of, 225, 351, 503 Effectiveness factor of promotion computation of, 505 definition of, 505 Electrocatalysis... 

The first In situ MBS Investigation of molecules adsorbed on electrode surfaces was aimed primarily at assessing the feasibility of such measurements In systems of Interest to electrocatalysis (18). Iron phthalocyanlne, FePc, was chosen as a model system because of the availability of previous situ Mossbauer studies and Its Importance as a catalyst for O2 reduction. The results obtained have provided considerable Insight Into some of the factors which control the activity of FePc and perhaps other transition metal macrocycles for O2 reduction. These can be summarized as follows ... 

The idea that the cathode potential with respect to ]lt(H20)/Pt-0Hads determines the value of the pre-exponential factor in the ORR rate expression was inspired by a comment by Andy Gewirth (Urbana) in his talk in Leiden, pointing to the value of Pourbaix diagrams for understanding ORR electrocatalysis. Indeed, the information on these ORR-mediating and facilitating M/M-OH surface redox systems is to be found in Pourbaix s Atlas. 

E. Yeager, M. Razaq, D. Gervasio, A. Razak and A. D. Tryk, "The electrolyte factor in 02 reduction electrocatalysis Proc. of Structural Effects on Electrocatalysis and Oxygen Electrochemistry, Cleveland, OH, 1991. 

Two are the main factors governing the activity of materials (i) electronic factors, related to chemical composition and structure of materials influencing primarily the M-H bond strength and the reaction mechanism, and (ii) geometric factors, related to the extension of the real surface area influencing primarily the reaction rate at constant electronic factors. Only the former result in true electrocatalytic effects, whereas the latter give rise to apparent electrocatalysis. 

Moreover, the conductivity, and hence the catalytic decomposition of hydrogen peroxide, has been observed to influence the stability of the oxygen electrode. The stability of phthalocyanine catalysts is a decisive factor for the practical applicability of organic catalysts in fuel cells operating in an acid medium. This is therefore a very important observation. The observed disturbance of the delocalization of the n electrons (rubiconjugation) in Fe-polyphthalocyanines, in addition to the correlation between conductivity on the one hand, and electrocatalysis and catalytic decomposition of hydrogen peroxide on the other, leads to a special model of the electroreduction of oxygen on phthalocyanines. The model... 

Additional information about this Fc GO preparation has been reported elsewhere (112). The intramolecular electron transfer rate constant kmirn calculated using Eq. (36) equals 40 s-1 and is by a factor of 50 higher than that for the randomly modified GO (104). The distance separating the ferrocene unit and FAD in Fc GO is believed to be ca. 19 A, by 2 A shorter than in the most effective electrically contacted enzyme generated by the random modification of GO by ferrocene units. This information supports the hypothesis about the key locations of ferrocene groups that play the dominant role in the electrocatalysis (104). 

So much, then, for two essential cases in which the adsorptive bond between an atom of the substrate acts differently, depending on the nature of the rds. This bonding, and how it affects electrocatalysis, is called the electronic factor, in electrocatalysis. 

So, in describing factors in electrocatalysis that can be understood without resorting to quantal concepts, it can be said in summary that an electrode catalyst can be rationally chosen only if one knows what the rds is in the electrode reaction. Then... 

Electrocatalysis is, in the majority of cases, due to the chemical catalysis of the chemical steps in an electrochemical multi-electron reaction composed of a sequence of charge transfers and chemical reactions. Two factors determine the effective catalytic activity of a technical electrocatalysts its chemical nature, which decisively determines its absorptive and fundamental catalytic properties and its morphology, which determines mainly its utilization. A third issue of practical importance is long-term stability, for which catalytic properties and utilization must occasionally be sacrificed. 

The search for new electrode materials is expected to be guided by the fundamental understanding of the factors governing the activity. In electrochemistry, this branch of the discipline is known by the name of electrocatalysis . Strictly speaking, electrocatalysis is the science devoted to the relationship between the properties of materials and the electrode reaction rate. The scope of electrocatalysis as a science is to establish a predictive basis for the design and the optimization of electrocatalysts. 

As the particle size decreases, the ratio between the number of atoms at the surface to those in the bulk increases with a parallel decrease in the average coordination number for the metal atom, which is also expected to be a factor of electrocatalysis. It has been calculated for Pt that the minimum size of a crystallite (cluster) for all atoms to be on the surface is 4 nm, corresponding to a specific surface area of 280 m2g-1 [322] (note that this is larger than the critical particle size where absorption of H atoms disappears on Pd) [333]. It is also interesting that dispersed catalysts can in turn influence the electronic properties of the support so that an interesting combination of sites with varied properties can result [330]. At low catalyst loadings, spillover of intermediates is also possible. 

Some predictions beyond the theory of electrocatalysis for pure metals seem indeed possible. It is, however, necessary to stress again that the applicability of a cathode depends on the impact of many factors, the most outstanding ones being the intrinsic stability and the resistance to poisoning. This is probably still the weak point of cathodes. Their life-time appears to be lower than for anodes, although the deactivation process for cathodes is slower and less abrupt than for anodes. 

The redox potential of the surface-bound redox couple is an important factor in achieving electrocatalysis, or more correctly, mediated reduction of the analyte. In... 

Guerrini E, Trasatti S. Recent developments in understanding factors of electrocatalysis. Russian Journal of Electrochemistry. 2006 42(10) 1017—1025. 

In all experimental situations, some judgment must be made concerning the quality of the results.10 In this, it is important to understand the validity of the measurements, and the limits of error in the analytical determinations. In the case of electrocatalysis, the reaction rate on a catalyst surface is the most significant factor. Measurements of the reaction rate can easily be lower than the true reaction rate value but never higher. For this reason, the highest values must be given greater consideration than the lowest values. 

The electrochemical techniques do not differ significantly with respect to time resolution. Pseudo first order rate constants ranging from about 0.1 to 10 S can be measured by techniques which monitor the response of the intermediate and LSV and electrocatalysis can give estimates of rate constants as high as 10 s . In the opinion of the author, the factors of most importance to be considered in selecting a measurement method of the first style are (i) the selectivity of the response, (//) the ease of obtaining reliable data, and (ill) the kinetic or thermodynamic information content of the data. Another factor of utmost importance to the non-specialist is (iv) the availability of instrumentation. 

Electrocatalysis is manifested when it is found that the electrochemical rate constant, for an electrode process, standardized with respect to some reference potential (often the thermodynamic reversible potential for the same process) depends on the chemical nature of the electrode metal, the physical state of the electrode surface, the crystal orientation of single-crystal surfaces, or, for example, alloying effects. Also, the reaction mechanism and selectivity 4) may be found to be dependent on the above factors in special cases, for a given reactant, even the reaction pathway [4), for instance, in electrochemical reduction of ketones or alkyl halides, or electrochemical oxidation of aliphatic acids (the Kolbe and Hofer-Moest reactions), may depend on those factors. 

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