Electrode ohmic correction

If the reference electrode is correctly placed in the same equipotential as the working electrode, ohmic drop in the electrolyte may be negligible, but this may not be the situation in the bulk of the electrode. Quantitative determination of the electrode resistance is difficult when using voltammetry only. It is much easier using galvanostatic cycling. 

Fig. 13. Stationary ruthenium dioxide/titanium dioxide electrode (UN5) in 5.13 M NaCl solution, (a) Standard rate constant-potential curve assuming a constant Tafel slope of 70 mV. DC) = 5 x 10 6cm s Z)C 2 = 7 x 10"6 cm s1, E = 1050 mV SCE, no ohmic correction, (b) Standard rate constant-potential curve assuming a constant Tafel slope of 70 mV. Z)C1 = 5 x 10"ecm s-1, Dc,2 = 7 x 10 6cm s, ° = 1050mV SCE, and R = 1.7 ohm cm2, (c) Standard rate constant-potential curve assuming a constant Tafel slope of 40 mV. DCI = 5 x 10-6cm s-1, Da = 7 x 10 9cm s->,fi° = 1050mV SCE, and ft = 1.7 ohm cm2, (d) Common experimental and calculated current-potential curve using the parameters of Fig. 13(c). The broken curve refers to the calculated "reversible curve.

Hi) Frequency response methods. This method superimposes low-amph-tude, high-frequency (about 1000 Hz) AC signals over the DC potential supplied to the electrode. The response of the resulting AC component of current to changes in the frequency is analyzed to give information on a variety of electrode parameters including the ohmic correction. For details of the principle and its application, consult Refs. 24 and 25. 

Figure 4.23 Experimental Tafel plot of cell voltage versus current, corrected for fuel cell ohmic and other losses, so that only cathode polarization losses are remaining. The results are normalized to platinum loading. Results with open circles are with humidified oxygen, and closed circles are with humidified air. The dashed line represents the Tafel slope behavior. Note that for all loadings the Tafel slope for oxygen reduction on platinum is the same but deviates from this behavior under mass-limiting behavior. Also note that the vertical axis is ohmic corrected fuel cell voltage, not electrode overpotential, so the voltage falls with increasing current density. (Reproduced with permission from [9].)...

There are two difficulties with this method. The first one is due to the fact that in reality the potentiostat keeps the potential between the working and the reference electrode constant there is an ohmic resistance Rq between the tip of the Luggin capillary (see Chapter 2) and the working electrode, giving rise to a potential drop I R-n (7 is the current). Since I varies in time, so does the potential drop by which ry is in error. However, modern potentiostats can correct for this to some extent. The second difficulty is more serious. Immediately after the... 

Fig. 2.6 Current-potential curves for (A) p-InP, (B) p-GaAs, and (C) p-Si electrodes in 0.3 M TBAP in methanol (40 atm C02) (b) in the dark and (a, c) under illumination. Curves a and c correspond to the behavior corrected and uncorrected for ohmic losses, respectively. Curve d was obtained for a metallic Cu electrode. (QRE stands for quasi-reference electrode).

Hg/HgO is often the electrode of choice in alkaline aqueous medium, silver/silver acetate in many nonaqueous medium such as acetic acid, etc. When the experiment requires large currents to flow between the working and the counterelectrodes, a particular attention must be paid to place the reference electrode at an equipotential line close to the working electrode or to make an appropriate ohmic drop correction. 

For potential measurements without any current flow, the IR drop is zero. In all other cases, beside the electrode placement mentioned above, IR drop should be estimated and eliminated from the measured potential It can be either compensated during potential control, or one can correct for it to obtain the real electrode potential. The IR drop can be straightaway determined by fast current interruption measurements Shortly, e.g., 1 ps, after a current shut-down, the potential is already decreased by all ohmic voltage drop while other overvoltages remain at their stationary values. 

Figure 6.20. Nyquist plots for the electrodes fabricated according to the same preparation procedure [19]. Note NSGA stands for novel silica gel additive, and TNPA stands for traditional Nafion polymer additive. The values in parentheses are the ohmic drop corrected cell potential. (Reproduced from Wang C, Mao ZQ, Xu JM, Xie XF. Preparation of a novel silica gel for electrode additive of PEMFCs. Journal of New Materials for Electrochemical Systems 2003 6(2) 65-9, with permission from JNMES.)...

The magnitude, presented in Figure 17.8(b), tends toward Re as frequency tends toward oo and toward oo as frequency tends toward zero. The slope of the line at low frequencies has a value of —a for the blocking electrode. Slopes with values smaller than unity could provide an indication of a blocking electrode with a distribution of characteristic time constants. The slope of the modulus corrected for Ohmic resistance is equal to —a for all frequencies. 

Figure 20.9 Comparison of the measurement model to impedance data obtained for reduction of ferricyanide on a Pt rotating disk electrode a) Ohmic-resistance-corrected phase angle and b) log-imaginary impedance.

Fig. 17.2 Tafel plots for the (normalized, dimensionless) current, yjy, that accompanies hydrogen evolution in a solution containing 3.4 mM HCl + 1.0 M KCl, corrected for diffuse-double-layer effects, mass transport controlled kinetics and ohmic potential drop, measured at three temperatures (5, 45, 75°C all results fall on the same line of this reduced plot) at a dropping mercury electrode. The slope obtained from this plot is 0.52, independent of temperature. (Based on data from E. Kirowa-Eisner, M. Schwarz, M. Rosenblum, and E. Gileadi, J. Electroanal. Chem. 381, 29 (1995) and reproduced by the authors.)...

It is assumed that the capacity measured, C, is not distorted due to the leakage effect at the interface, a finite value of the ohmic resistance of the electrode and electrolyte, etc. A correct allowance for these obstacles is an individual problem, which is usually solved by using an equivalent electrical circuit of an electrode where the quantity in question, Csc, appears explicitly. Several measurement techniques and methods of processing experimental data have been suggested to find the equivalent circuit and its elements (see, e.g.. Ref. 40). 

The graphs [Fig. 17(a) and (b)] show the large increase in standard rate constant as the potential goes negative, suggesting that the palladium electrode is much more active for the deposition reaction at potentials less than about 100 mV. This effect is also reflected in Fig. 17(c) and (d) in which the double layer capacity-potential curves are reproduced. These show that the double layer capacity sharply increases with negative potential. The main reason for this effect is, undoubtably, an area increase as palladium metal is deposited. Figure 17(e) and (f) show the associated log current-potential curves (corrected for ohmic resistance). These curves are also reproduced by calculation from the measured impedance-potential curves. 

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