Steady-State Polarization Measurements

The cell and connecting tubes are filled witfi a KCl solution of tfie chosen concentration. 

Similar designs are used for other REs on the basis of poorly soluble mercury compounds (1) the mercury-mercurous sulfate RE with H2SO4 or K2SO4 solutions saturated with Hg2S04, for which = 0.6151V and (2) the mercury-mercuric oxide RE, for measuring electrode potentials in alkaline solutions, with KOH solution saturated with HgO, for which = 0.098 V and E = 0.920 V. 

The silver-silver chloride RE consists of a small length of silver wire or piece of silver sheet coated with a thin layer of silver chloride (this layer can be deposited by anodic polarization of the silver in chloride-contaiifing solution) and dipping into HCl or KCl solutions of defined concentration its E° = 0.2224 V. 

In alkaline solutions, sometimes the cadmium-cadmium oxide RE is used its design is the same as that of the silver-silver chloride RE (a thin layer of cadmium oxide is formed on the surface of metallic cadmium). This electrode is quite simple to make and manipulate, but its potential is not very stable E = +0.013 V. 

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

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Correlation of the effect of particle size on the oxygen reduction reaction has been reviewed extensively by Kinoshita [4,5], Stonehart [8], and Mukerjee [9]. The general consensus, based on a large number of steady-state polarization measurements in several electrolytes, is that ORR exhibits a strong particle size effect in the... 

Steady-state polarization measurements, that is measurement of the current at constant potential or potential at the constant current provide current-potential curves from which a slope, that is, a polarization resistance, = dE/dj, can be determined. An example of such a curve for a fuel cell is displayed in Fig. 1.1. 

The charge acceptance of these nickel-metal hydride batteries at 70°C is over 96% at a charge/discharge rate of 1 C. These performance improvements are due to the increased oxygen evolution overpotential, slower oxygen evolution rate and lower electrochemical impedance, as indicated by CV, steady-state polarization measurements and EIS (114). 

Ohzuku T, Yamato R, Kawai T, Ariyoshi K (2008) Steady-state polarization measurements of lithium insertion electrodes for high-power lithium-ion batteries. J Solid State Electrochem... 

Steady-State Fluorescence Depolarization Spectroscopy. For steady state depolarization measurements, the sample is excited with linearly polarized lig t of constant intensity. Observed values of P depend on the angle between the absorption and emission dipole moment vectors. In equation 2 (9), Po is the limiting value of polarization for a dilute solution of fluorophores randomly oriented in a rigid medium that permits no rotation and no energy transfer to other fluorophores ... 

Weaver [40] studied alternate cathode materials at 650 °C, finding several that performed well. Steady-state polarization on Ni, Co and Fe porous electrodes operating as cathodes in a MCFC, with a standard (Li/K)2 C03 tile is shown in Figs. 30-32. Note that the oxidant gas fed to these cathodes is, in normal MCFC operation, the fuel, composed of 32.5% H2, 17.5% COz, 17.5% H20, the balance N2. Polarizations were first taken with this clean gas where the only reaction can be Eq. (35). After steady-state was attained, 0.65% H2S was added and sufficient time allowed for the electrode to convert to the sulfides. After 24 hours, the outlet H2S reached the inlet level and polarizations were measured. Note in Figs. 30-32, that the performance with H2S is significantly improved over the clean gas. (The Ni sample was a commercial (Gould) MCFC electrode the Co and Fe were pressed from powders. Each gas was 8 sq cm in superficial area). The improvement is probably due to a catalytic mechanism involving sulfur interactions with the electrode, as, for Co ... 

Figure 22. Steady-state polarization curves of aluminum in pure and mixed NaOH -f NaCl solutions , 4Af NaOH A, 4 M NaOH + 2 M NaCl O, 1 M NaOH + 2 M NaCl , 2 M NaCl (pH 1 to 13). Labels on the lines denote measured capacitances of the interface.

With the aid of steady-state polarization and electrochemical impedance measurements in an ammonia containing solution maintained at pH = 9-10.5, Touhami et al. proposed the following mechanism for the H2PO2 reaction utilizing an intermediate adion, Ni,[ds [72] generated in the two step reduction of Ni2+ ... 

Dr can be determined by time-resolved fluorescence polarization measurements, either by pulse fluorometry from the recorded decays of the polarized components I l and 11, or by phase fluorometry from the variations in the phase shift between J and I as a function of frequency (see Chapter 6). If the excited-state lifetime is unique and determined separately, steady-state anisotropy measurements allow us to determine Dr from the following equation, which results from Eqs (5.10) and (5.41) ... 

Pitfalls in steady-state fluorescence measurements inner filter effects and polarization effects... 

At the present time, two methods are in common use for the determination of time-resolved anisotropy parameters—the single-photon counting or pulse method 55-56 and the frequency-domain or phase fluorometric methods. 57 59) These are described elsewhere in this series. Recently, both of these techniques have undergone considerable development, and there are a number of commercially available instruments which include analysis software. The question of which technique would be better for the study of membranes is therefore difficult to answer. Certainly, however, the multifrequency phase instruments are now fully comparable with the time-domain instruments, a situation which was not the case only a few years ago. Time-resolved measurements are generally rather more difficult to perform and may take considerably longer than the steady-state anisotropy measurements, and this should be borne in mind when samples are unstable or if information of kinetics is required. It is therefore important to evaluate the need to take such measurements in studies of membranes. Steady-state instruments are of course much less expensive, and considerable information can be extracted, although polarization optics are not usually supplied as standard. 

Solutions of many proteins, synthetic polypeptides, and nucleic acids show large increases in permittivity c (u>) over that of solvent, normally aqueous, at sufficiently low frequencies f = w/2ir of steady state AC measurements, but with dispersion and absorption processes which may lie anywhere from subaudio to megahertz frequencies. Although our interest here is primarily in counterion fluctuation effects as the origin of polarization of aqueous DNA solutions, we first summarize some relevant results of other models for biopolymers. 

Steady-state polarization curves, such as that presented in Figure 5.4(a), provide a means of identifying such important electrochemical parameters as exchange current densities, Tafel slopes, and diffusion coefficients. The influence of exchange current density and Tafel slopes on the steady-state current density can be seen in equations (5.17) and (5.18), and the influence of mass transfer and diffusivities on the current density is described in Section 5.3.3. Steady-state measmements, however, cannot provide information on the RC time constants of the electrochemical process. Such properties must be identified by using transient measurements. 

All impedance measurements should begin with measurement of a steady-state polarization curve. The steady-state polarization curve is used to guide selection of an appropriate perturbation amplitude and can provide initial hypotheses for model development. The impedance measirrements can then be made at selected points on the polarization curve to explore the potential dependence of reaction rate constants. Impedance measurements can also be performed at different values of state variables such as temperature, rotation speed, and reactant concentration. Impedance scans measured at different points of time can be used to explore temporal changes in system parameters. Some examples include growth of oxide or corrosion-product films, poisoning of catal5dic surfaces, and changes in reactant or product concentration. 

Figure 1 Correlation of specific activity for oxygen reduction reaction with particle size measured as steady-state polarization values with different electrolytes, (a) 98% H3PO4, 180 °C, (b) 0.5 M H2SO4, 25 °C, and (c) 97% H3PO4, 177 °C. Also superimposed is the surface-averaged distribution (SAD) of (100) sites (solid line). (From Ref. 4.)...

GOx/Pt PPI functionalized with 4, 8 and 32 octamethyl ferrocenyl groups (Dend-1, Dend-2 and Dend-3) Drop-coating Steady-state polarization at +0.1 V vs SCE 0.1 M Phosphate buffer with 0.1 M NaC104, pH 7 Detection limit (jaM) Dend-1 43 Dend-2 39 Dend-3 15 K M(mM) Dend-1 2.0 Dend-2 1.8 Dend-3 3.0 Stable response for intermittent measurements during 6 days Storage stability in air for 7 weeks Dend-1 0.17 Dend-2 0.20 Dend-3 0.32 pA mM 1 [121]... 

Oxygen Electrocatalytic Properties Oxygen Reduction. Figure 8 compares steady-state polarization curves for the electroreduction of Op on a typical pyrochlore catalyst, Pb2(Rui.42Pbo.53)06.5 15 w/o platinum on carbon. The latter was considered representative of conventional supported noble metal electrocatalysts. The activities of both catalysts are quite comparable. While the electrodes were not further optimized, their performance was close to the state of the art, considering that currents of 1000 ma/cm could be recorded, at a relatively moderate temperature (75 C) and alkali concentration (3M KOH). Also, the voltages were not corrected for electrolyte resistance. The particle size of the platinum on the carbon support was of the order of 2 nanometers, as measured by transmission electron microscopy. 

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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