Electrocatalyst kinetics, current density

Figure 9.16 ORR activity of two mixed-metal monolayer electrocatalysts supported on Pd(l 11), expressed as the kinetic current density at 0.85 V as a function of the M Pt ratio in the Pd-supported Pt-M monolayer. (Reproduced with permission from Zhang et al. [2005b].)...

Koslowski UI, Abs-Wurmbach I, Fiechter S, Bogdanoff P (2008) Nature of the catalytic centers of porphyrin-based electrocatalysts for the ORR a correlation of kinetic current density with the site density of Fe-N-4 Centers. J Phys Chem C 112(39) 15356-15366... 

Figure 5.8. Kinetic current densities at 0.80 V as a function of the calculated interaction energy between two OHs, or OH and O in a 2x2 nnit cell, relative to the corresponding interaction on PtMLd d(l 1 1) for several mixed monolayers at a Pt/M ratio of 80/20. Positive (negative) energies indicate more (less) repnlsive interaction between adsorbed OHs, respectively, compared to PtML/Pd(l 1 1) [64]. (Reprinted from Electrochimica Acta, 52(6), Viikmirovic MB, Zhang J, Sasaki K, NUekar AU, Uribe F, Mavrikakis M, et al. Platinum monolayer electrocatalysts for oxygen rednction, 2257-63, 2007 with permission from Elsevier.)...

As discussed earlier, it is generally observed that reductant oxidation occurs under kinetic control at least over the potential range of interest to electroless deposition. This indicates that the kinetics, or more specifically, the equivalent partial current densities for this reaction, should be the same for any catalytically active feature. On the other hand, it is well established that the O2 electroreduction reaction may proceed under conditions of diffusion control at a few hundred millivolts potential cathodic of the EIX value for this reaction even for relatively smooth electrocatalysts. This is particularly true for the classic Pd initiation catalyst used for electroless deposition, and is probably also likely for freshly-electrolessly-deposited catalysts such as Ni-P, Co-P and Cu. Thus, when O2 reduction becomes diffusion controlled at a large feature, i.e., one whose dimensions exceed the O2 diffusion layer thickness, the transport of O2 occurs under planar diffusion conditions (except for feature edges). 

Figure 6.20. Experimental linear sweep voltammogram of carbon-supported high surface area nanoparticle electrocatalyst in oxygen-saturated perchloric acid electrolyte (room temperature). Solid curve pure Pt dashed curve Pt50Co50 alloy electrocatalyst. Inset a blow up of the kinetically controlled ORR regime. Inset b comparison of the specific (Pt surface area normalized) current density of the Pt and the Pt alloy catalyst for ORR at 0.9 V.

In conclusion, the presence of both poisoning species (mainly CO) and intermediate reaction products (AAL, AA) decreases correspondingly the useful energy density of the fuel, and also the power density, since the oxidation current densities are lower than those obtained with the oxidation of methanol and above all of hydrogen. To improve the kinetics of ethanol oxidation would require the development of new electrocatalysts able to break the C-... 

Electrocatalysts are produced in different ways, on different substrates, and for different purposes,10,64-72 but almost in all cases the electrochemical characterization was performed by using the cyclic voltammetry observations. In this way, it was not possible to analyze the effects of the mass-transfer limitations on the polarization characteristics of electrochemical processes. As shown recently,7,9 the influence of both kinetic parameters and mass-transfer limitations can be taken into account using the exchange current density to the limiting diffusion current density ratio, jo/ju for the process under consideration. Increased value of this ratio leads to the decrease of the overpotential at one and the same current density and, hence, to the energy savings. 

The reported work by Nocera and coworkers concerning the dissolution and redeposition of MnO to form self-healing stable electrocatalysts under acidic, neutral, and basic conditions not only supported that manganese(iii) underwent disproportionation in acidic medium, whereas the one-electron-one-proton PCET (proton-coupled electron transfer) pathway is dominant under alkaline conditions, but resulted in a Tafel slope of 60 mV dec in alkaline pH regime.Apart from the usually discussed overpotential of electrochemical water oxidation, the kinetics of the reaction has also been considered for an efficient electrocatalyst. It is well known that the higher the Tafel slope value, the higher the energy needed to reach maximum current density. 

Figure 3.6a depicts the linear scan voltammograms (LSVs) of the ORR on 20 % Pd/C without ethylene glycol at co = 400, 800, 1,200 and 1,600 rpm. The LSVs show a clear dependence of the ORR current densities with potential and rotation rate. The ORR on the Pd electrocatalyst seems to be under kinetic and mixed control in the potential scanned, not reaching a well defined limiting current. Levich-Koutecky plots 1/j vs. at different potentials corresponding to the experimen-... 

Carbon-supported Pt nanoparticles (e.g. Pt/C) are currently the most efficient electrocatalysts for the HOR in PEM fuel ceUs. If pure hydrogen is used as the fuel, the overpotential of the HOR at the Pt anode is small due to the fast electrode kinetics of the reaction. Normally, the overpotential is <20 mV, even at a current density of 1.0 A cm . The mechanism and kinetic behavior of the HOR on a Pt electrode or Pt/Nation interface has been smdied extensively in both acidic and alkaline media [9,16-20]. 

Galvanostatic versus potentiostatic measurements have a special importance in alloy deposition. This has already been discussed in Section 4.2.1 where it was pointed out that galvanostatic measurements are unique in chemical kinetics, in the sense that the total rate of the reaction (which is proportional to the current density) is held constant by the electronic circuit, and the overpotential is allowed to reach the value corresponding to the applied current density. In other words, the reaction rate is independent of the heterogeneous rate constant, the concentration, the nature of the electrocatalyst and the temperature. The effect of these variables is expressed by the overpotential developed. 

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