Tafel slope analysis

Electrochemical kinetic analysis yields different Tafel slope values for Co-N-C ( 59 mV dec ) and Fe-N-C ( 87 mV dec ) catalysts [52, 57], attesting to different ORR mechanism in the case of the two metals. Based on the Tafel slope analysis for the ORR on Pt catalysts by Coutanceau et al. [67], the diffusion of adsorbed oxygen intermediates during the ORR is a likely the rate-determining step... 

To determine the individual overpotentials of a fuel cell cathode, we followed the Tafel slope analysis given elsewhere in more detail (Gasteiger et al. 2003 Neyerlin et al. 2005 Schmidt 2006a, b Schmidt and Baurmeister 2008). However, for better understanding of the analyses presented in this chapter, a summary of the most important equations will be given. 

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

The use of the impedance technique in the study of polymer coated steel, has been thoroughly described elsewhere. The present paper compares this technique with that of harmonic analysis, originally proposed by Meszaros ). The authors have presented preliminary data using the latter technique(3) wherein the early stages of polymer breakdown have been studied. The current paper extends this work to polymers which have been immersed for a considerable period of time. The harmonic method gives information not available from the impedance technique in the Tafel slopes and the corrosion current are directly measurable. A brief summary of the harmonic method and the equations used are given below. 

In the above analysis, it has been considered that the transfer coefficient, a, was independent of temperature the temperature dependence of Tafel slopes is then given by... 

A key element in the analysis is the correspondence between the observed and the theoretical dependence of the current on the potential. In other words, one compares the theoretical Tafel slope for the postulated mechanisms, or the transfer coefficients which are derived from these, to the experimentally obtained values. 

It remains to consider whether oxidation of the adsorbed COadS or =C-OHads is taking place through H20 or OHads. This has proved a thorny issue, partly because of the sensitivity of the process to surface morphology and to the formation conditions of the adsorbed monolayer itself, but also because kinetic analysis has given ambiguous answers. In principle, both Tafel slope and pH dependence should differ for reactions (18.13) and (18.14) above, but whilst Bagotzky and Vassiliev s data [5] are most naturally interpreted in terms of reaction (14) above, the current equation derived from the studies of Inada et al. [93], i CMeOHcH 6 exp(0.6FE/RT)... 

Comparative anodic polarization data (Fig. 27) obtained by Conway and Liu (285-287) for chemically and anodically formed nickel oxide show a Tafel slope of 33 mV on anodically formed nickel oxide, lower than that for the chemically formed film (60 mV), with better activity than the former. Pseudocapacitance profiles obtained from an analysis of the potential relaxation data are shown in Fig. 28. The initial descending part of the C o versus rj profiles of Fig. 28a appears to be connected with the positive end of the well-known cyclic-voltammetry peak for Ni(II) - Ni(III) oxidation in nickel oxide. This peak goes into an ascending current versus potential line for O2... 

The measurement of Tafel slopes is an important means for elucidating electrochemical reaction mechanisms, and the keys to their analysis are the resulting transfer coefficients. The ability to gain insight into a mechanism from the experimentally obtained as arises from the potential dependence of the activities of reaction intermediates formed by electron-transfer steps before a rate-determining step (in either reaction direction). 

Fig. 7. Analysis of the experimental steady-state current-potential and impedance-potential data from E = - 1300 mV to E = — 600 mV for a titanium rotating-disc electrode (45 Hz) in a solution of 2 M hydrochloric acid, (a) Standard rate constant-potential curve calculated for the hydrogen evolution reaction on titanium assuming that DA = 7.5 x 10 5cm"1s 1 and E° = - 246 mV. The Tafel slope 6C = 211 mV and the measured ohmic resistance was 0.4 ohm cm2. The potentials are the "true potentials, (b) High-frequency double layer capacity-potential curve. The potentials are the measured potentials.

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