Steady-State Polarization Curves

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.

B) Steady-state polarization curves of carbon paste electrodes in the presence of 33.3 pM glucose. (C) Variation of the steady-state current of carbon paste electrodes with glucose concentration (at +350 mV vs. SCE). 

Fig. 7 Experimental steady state polarization curves of Zn(ll) electroreduction for various electrolytes (SI) additive-free bath (1.6 M ZnCh -E5.3 M KCl) (S2) industrial bath (1.6 M ZnCh -E 5.3 M KCl + long-chain polymer + pH buffer of pH 4.7) solution (S3) (1.6 M ZnCh -f 5.3 M KCl + long-chain polymer with the same additive at the concentration of 10 M in volume) [218] ...

Figure 3. Steady state polarization curves in KF-2HF at 90 C O layer plane of pyrolytic graphite, edge plane of pyrolytic graphite...

Assuming negligible anode polarization, the steady-state polarization curves can be described by a semi-empirical equation ... 

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 11 (A) Stripping voltammetry (20 m Vs at 55 °C) of CO layers on humidified PEM fuel-cell anodes (1) platinum catalyst (2) platinum/molybdenum catalyst. Voltammetry in the absence of adsorbed CO on the platinum/molybdenum catalyst is shown in (3). Molybdenum-mediated electro-oxidation of adsorbed CO takes place on the alloy catalyst in the peak at 0.45 V and at lower overpotentials [79]. (B) Steady-state polarization curves of PEM fuel-cell anode at 85 °C for platinum (squares) and platinum/molybdenum catalysts in the presence of 100 ppm CO (filled points) and pure H2 (unfilled points). (From Ref 79.)... 

Figure 3.47 Upper panels steady state polarization curves recorded with a Pt OPG RRDE (co=100rpm) for coatings (6 x 10-8 mol citT2) of FeTPP (A), FePPIX (B) and FeTPyP (C) (see solid circles) deposited on the OPG disk in aqueous 02-saturated 0.1 M HCIO4 + 0.1 M NaCI04, where each ofthe points was collected for a freshly prepared coating. The data in empty circles are ring currents recorded...

The effects of pyrolysis on the electrocatalytic properties of the same three macrocycles dispersed in Vulcan XC-72 for 02 reduction were examined using thin porous electrodes applied to the carbon disk of an RRD E as sembly. Steady state polarization curves obtained at co = 2500 rpminO.l M NaOH and 0.05 M H2S04are shown in Figures 3.70 and 3.71. As is evident from the results presented therein, the activity of H2TMPP in both media (see solid squares) before and after pyrolysis was much lower than that of the metallated counterparts and fairly close to that of... 

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. 

Figure 8. Steady-state polarization curves for O9 reduc. - ...

LiCI, 0.5 mol/1 glassy carbon room temperature HMPA Steady state polarization curves on rotating disc electrode and on a One wave 1976... 

Pt,5 C Steady state polarization curves in a stirred solution up to ... 

Steady-state polarization curves on a rotating disc electrode... 

HMPA Steady-state polarization curves Single Tafel line with a slope of Electron thermoemission 1972... 

It was pointed out earlier that oscillations in NDR oscillators are linked to three features of the electrochemical system (1) an N-shaped steady-state polarization curve (2) a resistance in series with the working electrode, which must not be too large and (3) a slow recovery of the electroactive species, in most cases due to slow mass transport. Hence, for every system that was discussed in the context of the possible origin of N-shaped characteristics, conditions can be estabhshed under which stable limit cycles exist, and for most of the systems mentioned, oscillations were in fact observed. This unifying approach was first put forth by Koper and Sluyters, and numerous experimental examples of electrochemical oscillations that can be deduced according to this mechanism are discussed in Ref. 60. 

When the system is operated under galvanostatic conditions or when a sufficiently large series resistance is added, pronounced oscillations are observed around the positively sloped branch in the steady-state polarization curve. A cyclic voltammogram exhibiting current oscillations is reproduced in Fig. 12. Typical time series under galvanostatic control, close to the onset of oscillatory behavior, are shown in Fig. 13(A). Their... 

Figure 49. (a) N-shaped steady-state polarization curve and different load lines referring to the different effective local electrolyte resistances. The intersections between the polarization curve and load line are stationary states at a certain radial position when the spatial coupling vanishes, (b) Coexisting radial profiles of the double-layer potential at a disk electrode for an electrochemical system with a bistable reaction part. ... 

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]. 

Fig. 2.23 (a) Comparison of the CO stripping of the three colloidal catalysts, (b) Steady-state polarization curves along with simulated curves for methanol oxidation of the three different 30wt.% PtRu/Vulcan XC-72 catalysts (symbols) at two different temperatures (22 and 60°C). Conditions fixed delay of 5 min, methanol concentration 1 M in the working electrode compartment, flow rate 10Lh . ... 

The importance of knowing the exact value of the ohmic drop or uncompensated resistance in an electrochemical system has been pointed out by many workers. In studies of the kinetics of electrode processes by potentiostatic techniques, the ohmic potential drop produces a distortion of the steady state polarization curve which, if uncorrected, will yield erroneous values of the characteristic parameters (Tafel slope, reaction orders) of the electrode reactions (Fig. 6.2). 

This steady state polarization curve showed that the current decreased smoothly and the voltage increased smoothly as the load resistance was increased from 0.2 to 11 fl. When the load resistance was increased from 11 to 12 n both the steady state current and voltage decreased abruptly, but the fuel cell did not extinguish. The resistance was then increased stepwise from... 

The polarization curves in Figure 3.10 show a hysteresis loop, where the steady state current and voltage depend on the direction of approach. It was possible to go around the hysteresis loop shown in Figure 3.10 reproducibly many times. When the load resistance was between 5 and 110, the steady state current and voltage were dependent on the direction of approach. These multi-valued steady states were stable the current and voltage at any point on the steady state polarization curve was steady for periods of >24 h. 

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