Polarization Curve Analysis

1) Voltage drop caused by mixed potential and fuel crossover. The mixed potential at electrodes is due to unavoidable parasitic reactions that tend to lower the equilibrium electrode potential. For a mixed cathode potential, a local-cell mechanism has been put forward to explain the Pt-02 reaction mechanism at the electrode in an 02-saturated acidic solution [20, 21], The mixed potential is composed of both the cathodic O2/H2O reaction potential (O2+ 4H -l- 4e 2H2O, 

El = 1.229 V (vs. NHE)), and the Pt/PtO anode reaction potential (Pt + H2O PtO + 2YC + 2e , E°p p,o = 0.88 V (vs. NHE)). The local electrochemical reaction on the Pt surface could create a PtO surface coverage of 30%, with 70% of the surface remaining as pure Pt. At steady-state mixed potential, a complete layer of Pt-0 can never be achieved in order to keep the reaction of Pt to PtO continuous, due to the diffusion of Pt-0 into the bulk metal. The reported mixed cathode potential is around 1.06 V (vs. NHE) at standard conditions [20, 21] with an O2 partial pressure dependence of 15 mV-atm Furthermore, the H2 that has crossed over through the membrane from anode to cathode can form a local half-cell electrochemical reaction on the cathode, such as H2 2H + 2e, resulting in a mixed cathode potential similar to that of the half-cell reaction (Pt -1- H2O PtO -1-2H + 2e ). The mixed potentials are the dominant sources of voltage losses at open circuits [13]. 

2) Activation loss (overpotential). Activation overpotential is caused by the kinetics of the electrode reactions. It is associated with both sluggish reaction kinetics and low catalyst activity. The effects of these losses are most pronounced at low current densities ( 0.1 A/cm ). Minimization of the activation overpotential thus requires maximization of the catalyst area available for reactions and improvement of catalyst activity. 

3) Ohmic loss (overpotential). Ohmic overpotential (% ,) is caused by ionic and electronic resistances. It consists of the ionic resistance of the membrane plus the bulk and contact electronic resistances of the consfruction materials. Because both the electrolyte and the fuel eell eleetrodes obey Ohm s law, the ohmie losses can be expressed by the equation 

In relative terms, the largest single source of ohmic losses is from the membrane. A simple way to determine the ohmic resistance is employ the impedance spectroscopic method. In a fuel cell impedance spectrum, the ohmic resistance is the real value of the impedance of the point for which the imaginary impedance is zero at the maximum frequency. The effects of these losses are most pronounced at intermediate current densities. Minimizing the ohmic losses requires effective water management in the membrane, excellent electron conductive materials, and minimal contact resistance. 

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A fuel cell is a typical electrochemical device. Thus, the electrochemical analysis methods developed for other electrochemical devices can be directly used or modified for diagnosis of fuel cell degradation, including catalyst layer degradation. Cyclic voltammetry and polarization curve analysis are very frequently used in failure analysis and degradation diagnosis. 

A polarization curve, i.e., an 1-V curve of a fuel cell stack under certain operating conditions, is frequently used to describe fuel cell performance and determine performance decay with time. Polarization curve analysis can provide the most important kinetic parameters, such as the Tafel slope, exchange current density, cell resistance, limiting current density, and specific activity of Pt. A single fuel cell potential can be described by ... 

The performance of a PEMFC can be expressed through the analytical formulation of the polarization curve. Simultaneous estimation of the parameters through analysis of the polarization curve can be helpful in diagnosis of PEMFC degradation and identification of degradation mechanism. Some models and testing methods have been developed to characterize PEMFC performance through polarization curve analysis [25,26]. 

A good method for studying passive film formation involves examining polarization curves. ° Theoretical anodic and cathodic polarization curves are shown in Figure 1. As the potential is shifted in the anodic (-I-) direction, the current (corrosion rate) increases. At a critical current density (/cnt), the current drops to a low value—indicating the onset of passivation. With the use of polarization curves, the solution conditions controlling passivation can be determined and described. Polarization curve analysis will be extensively used in the subsequent sections to describe passivation in chemical cleaning solutions. 

An analysis of Eq. (6.13) show that for n = 1 and P = 0.5 and for current densities less than 4% of t the polarization is very low (less than 1 mV) and can practically be neglected. The linear section of the polarization curve extends up to current densities which are 40% of f. At current densities higher than 4f, the semilogarith-mic polarization relation is observed. 

For an analysis of the polarization curves at low values of polarization (low overpotentials), we shall use the general polarization equation... 

The surface of the base metal is anodically polarized under the effect of local cells. For a graphical analysis of the phenomena, one must construct the polarization curves for the partial currents at the base metal as well as the overall anodic 4 vs. E curve reflecting the effective rate of dissolution of this metal under anodic polarization. The rate of the cathodic process, 4, at the inclusions is described by the corresponding cathodic polarization curve (since the surface areas of anodic and cathodic segments differ substantially, currents rather than current densities must be employed here). At open circuit the two rates are identical. 

For the sake of a practical analysis of the polarization curve the exponential dependence of the electrode reaction rate constant need not be assumed. Then the more general form of Eq. (5.4.22) can be written as... 

Environmental tests have been combined with conventional electrochemical measurements by Smallen et al. [131] and by Novotny and Staud [132], The first electrochemical tests on CoCr thin-film alloys were published by Wang et al. [133]. Kobayashi et al. [134] reported electrochemical data coupled with surface analysis of anodically oxidized amorphous CoX alloys, with X = Ta, Nb, Ti or Zr. Brusic et al. [125] presented potentiodynamic polarization curves obtained on electroless CoP and sputtered Co, CoNi, CoTi, and CoCr in distilled water. The results indicate that the thin-film alloys behave similarly to the bulk materials [133], The protective film is less than 5 nm thick [127] and rich in a passivating metal oxide, such as chromium oxide [133, 134], Such an oxide forms preferentially if the Cr content in the alloy is, depending on the author, above 10% [130], 14% [131], 16% [127], or 17% [133], It is thought to stabilize the non-passivating cobalt oxides [123], Once covered by stable oxide, the alloy surface shows much higher corrosion potential and lower corrosion rate than Co, i.e. it shows more noble behavior [125]. 

Little is known about the fluorescence of the chla spectral forms. It was recently suggested, on the basis of gaussian curve analysis combined with band calculations, that each of the spectral forms of PSII antenna has a separate emission, with Stokes shifts between 2nm and 3nm [133]. These values are much smaller than those for chla in non-polar solvents (6-8 nm). This is due to the narrow band widths of the spectral forms, as the shift is determined by the absorption band width for thermally relaxed excited states [157]. The fluorescence rate constants are expected to be rather similar for the different forms as their gaussian band widths are similar [71], It is thought that the fluorescence yields are also probably rather similar as the emission of the sj tral forms is closely approximated by a Boltzmann distribution at room temperature for both LHCII and total PSII antenna [71, 133]. 

The generation and analysis of polarization curve data has become increasingly straightforward with the increasing computerization of electrochemical instru-... 

The open circuit Ecm values for each metal are the entries in the traditional galvanic series. Kinetic information is also available via analysis of the polarization curves. The 4ouPie can be used to calculate the increased corrosion rate of Metal 2. Because of the coupling to Metal 1, the dissoultion rate has increased from 4cathodic kinetics on Metal 1 must now be satisfied. In addition to determining the increase in the corrosion... 

EIS has the ability to distinguish between influences from different processes, especially when the system involves multiple-step reactions, parallel reactions, or additional processes such as adsorption. Generally speaking, the measurements and analysis of the EIS for a PEMFC are complicated compared with those of the polarization curve. However, the results from both methods are not insular, and some relationships exist between the complicated impedance spectrum and the simple polarization curve [22],... 

D. Outka et al. have studied the orientation of several Langmuir-Blodgett monolayers on oxidized Si(111) (16). By very detailed curve fitting and polarization dependence analysis they determined that arachidic acid (CH3(CH2)lgC02H) was not ordered, Cd arachidate was ordered normal to the substrate surface, and Ca arachidate was tilted 33° from the surface normal. 

M. Stem and A. L. Geary, "Electrochemical Polarization I. A Theoretical Analysis of the Shape of Polarization Curves," Journal of The Electrochemical Society, 104 (1957) 56-63. 

A kinetic analysis is not complete without determination of the temperature effects and activation energies. Figure 6 summarizes some of the polarization curves for the ORR recorded at 333 K and 298 K for details, see [41]. Clearly, results obtained at 333 K are qualitatively similar to the curves recorded at room temperature, and the order of activity remains the same as at room temperature, i.e., Pt(lll)elevated temperatures in both the mixed diffusion-kinetic potential region and the hydrogen adsorption potential region. These higher currents reflect the temperature dependence of the chemical rate constant, which is approximately proportional to jRT where is the apparent enthalpy of activation at the reversible... 

Polarization Analysis. Polarization curves were obtained for pure iron, 1080 carbon steel, ASAI 316 type and 304 type stainless steel near the critical point of water. Passivation was indicated based... 

A completely new approach to the analysis of experimental data is introduced by the use of the complete polarization curve equation and by the method of digital simulation. It was possible in this way to elucidate the polarization behavior of the partially covered inert... 

In the electrochemical benchmark monograph by Bockris and Reddy (B R) (Ref. 3, p. 1001), these authors developed, based upon the quasiequilibrium approximation, transfer coefficients, as, in terms of mechanistic parameters. Their analysis demonstrated how such as, obtained from experimental polarization curves, can give information directly, enabling elucidation of reaction mechanisms. Their transfer coefficients are written as... 

Equation (50) forms the basis upon which v can be evaluated (e.g. (1) by the radioactive tracer method to evaluate simultaneously and ), (2) by comparing i values at appropriate potentials for different reactant activities (3) coupling information from high and low overpotential regions of steady-state polarization curves " (extrapolated io and charge-transfer resistance, Rcr, respectively) (4) or by back-reaction correction analysis. 2 qqie first two methods involve determination of v at any single potential while the latter two procedures must assume that the same mechanism (and hence v) applies at different potentials (at which individual measurements are required) and that the reverse reaction occurs by the same path and has the same transition state and thus rate-determining step [for both forward (cathodic) and reverse reactions]. 

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