Open-Circuit Potentials

or the rest potential of an electrode, is the potential of a freestanding electrode without electrical connection to any other conducting materials. Thus, at OCP there is no net current flow in or out of the electrode. OCP of an electrode is determined by the kinetic state of the electrode. It is the most easily measurable electrochemical parameter and at the same time is the most convoluted quantity as it is determined by aU the kinetic factors in the system. The electrode at OCP can be at an equilibrium state or a nonequiUbrium state depending on the nature of the particular electrode/electrolyte system and the reference time scale. 

When one of the redox couples is associated with the dissolution of the electrode, M + mh M + n- m)e, OCP is also called the corrosion potential and the net dissolution rate at OCP is the corrosion current, 4orr = 4a - 4c- This is generally the case with a silicon electrode at OCP in aqueous solutions because the thermodynamic poten- 

The change of the corrosion potential in either the anodic or the cathodic direction may correspond to a decrease or increase in the corrosion current. The variation of the corrosion potential and corrosion currents under various conditions can be generalized using schematic polarization curves in Fig. 1.26. The corrosion potential of an active electrode in a solution is Ecorr- -Ecorr. Escort, and Ecorr are the corrosion potentials under changed conditions. 

FIGURE 1.25. Illustration of corrosion potential, Econ. for p and n types of semiconductors when the corrosion current is limited by the minority limiting current, ii,m = ic . 

FIGURE 1.26. Schematic polarization curves illustrating coir and under various conditions (see text). 

Based on cathodic polarization curves, Dexter and Gao concluded that the increase of E for 316 stainless steel exposed to natural seawater was due to an increased rate of the cathodic reduction of oxygen at a given potential. It is not possible from Ecorr or polarization curves to decide whetlier the increase in Ecorr is due to thermodynamic effects, kinetic 

Both Linhardt and Dickinson et al. demonstrated that microbi-ologically deposited manganese oxide on stainless and mild steel coupons in fresh water (Fig. 4) caused an increase in Ecorr and increased cathodic current density at potentials above -200 mYscE- Biomineralization of 

The strong correlation between C and ennobled is shown in Fig. 7. Capacitance is expressed as a fraction of initial capacitance (Qnit) to account for variation in the surface area of the coupons. Data were expressed satisfactorily by the relationship  

The origin of the observed correlation was not established, and the relation was not interpreted as causal. It could be argued that a sustained elevated potential due to as-yet unknown microbial processes altered the passive film characteristics, as is known to occur for metals polarized at anodic potentials. If these conditions thickened the oxide film or decreased the dielectric constant to the point where passive film capacitance was on the order of double-layer capacitance (Cji), the series equivalent oxide would have begun to reflect the contribution from the oxide. In this scenario, decreased C would have appeared as a consequence of sustained elevated potential. 

An increase in reducible surface-bound material during ennoblement was demonstrated using galvanostatic reduction techniques to monitor potential as a stainless steel coupon was cathodically polarized. Coulombs of reducible material were calculated from the duration of regions of polarization rate lag that indicated reduction of surface-bound material. Longer exposure times and thicker biofouling were not sufficient to increase the abundance of reducible surface-bound material. The increase seemed to be associated with increased 

It can be shown that this approximation works well if the inequality e )g 1 holds. In PEM fuel cells, this inequality is fulfilled at vanishingly small currents )q 100 pA cm 

Differentiating Equation 5.102 over x, one obtains the disturbance of the cell current 

Separating the real and imaginary parts, one finally finds 

FIGURE 5.13 The Nyquist plot of a CCL, close to OCP, for the indicated values of parameter e. The curve for = 1 clearly shows the high-frequency 45 linear branch. When e = 10, in the scale of this plot, the linear branch is not seen. 

The term Ici/crp in Equation 5.110 is the proton resistivity of the CCL. This will be discussed in the section Finite but Small Current An Analytical Solution. Interestingly, if e is small. Red does not contain this term. Physically, if the reaction penetration depth is small (hydrogen electrode), the reaction runs close to the membrane and, therefore, protons do not need to be transported deep into the catalyst layer. Thus, the contribution of the proton transport to the cell resistivity does not appear as a separate term. The proton transport represented by ap is included into the total CL resistivity bf 2hap). 

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Open-arc furnaces Open-celled foam Open-circuit potential Open-mold processes Open-pit mining Operating line... 

Typical polarization curves for SOFds are shown in Fig. 27-67. As discussed earlier, the open-circuit potential of SOFds is less than 1 because of the high temperature, but the reaction overpotentials are... 

The electrode process at -500 mV on this potential scale is correlated to the growth of 250 20 pm high islands. They grow immediately upon a potential step from the open circuit potential to -500 mV (arrow in Figure 6.2-13). 

They form a monolayer that is rich in defects, but no second monolayer is observed. The interpretation of these results is not straightforward from a chemical point of view both the electrodeposition of low-valent Ge Iy species and the formation of Au-Ge or even Au Ge h compounds are possible. A similar result is obtained if the electrodeposition is performed from GeGl4. There, 250 20 pm high islands are also observed on the electrode surface. They can be oxidized reversibly and disappear completely from the surface. With Gel4 the oxidation is more complicated, because the electrode potential for the gold step oxidation is too close to that of the island electrodissolution, so that the two processes can hardly be distinguished. The gold step oxidation already occurs at -i-lO mV vs. the former open circuit potential, at h-485 mV the oxidation of iodide to iodine starts. 

The open-circuit potential of most metals in sea water is not a constant and varies with the oxygen content, water velocity, temperature and metallurgical and surface condition of the metal. 

Electrochemical aspects of the stress-corrosion behaviour have been investigated, mainly in neutral solutions. The open-circuit potential of Ti-8Al-lMo-l V is —800mV (v5. S.C.E.). The crack initiation load reaches... 

Polarise all cathodic areas to open circuit potential of most active anode areas. 

It follows from the above that, for an anode material to offer sacrificial protection, it must have an open-circuit potential that is more negative than that of the structure itself (the cathode). The extent of protection experienced by the cathode will depend on the potential it achieves. This is dependent on the electrochemical properties of the anode which in turn are governed by its composition and the environment to which it is exposed. 

The operating potential of an anode material is its potential when coupled to a structure (i.e. the closed-circuit potential). Since all commercial anode materials are formulated to suffer only slight polarisation under most conditions of exposure, the operating potential approximates to the open-circuit potential. Indeed, any substantial difference (>50mV) between these two potentials will call into question the suitability of the anode in the particular environment. 

The simplest procedure in studying galvanic corrosion is a measurement of the open-circuit potential difference between the metals in a couple in the environment under consideration. This will at least indicate the probable direction of any galvanic effect although no information is provided on the rate. A better procedure is to make similar open-circuit potential measurements between the individual metals and some appropriate reference electrode, which will yield the same information and will also permit obser-... 

Measurements of open-circuit potentials relative to some reference electrode have been assumed on occasion to provide a means of rating metals as to their relative resistance to corrosion on the basis that the more negative the measured potential, the higher will be the rate of corrosion, but this assumption is obviously invalid, since it disregards polarisation of the anodic and cathodic areas. 

Legault, Mori and Leckie have used open-circuit potential vs. time measurements and cathodic reduction of rust patinas for the rapid laboratory evaluation of the performance of low-alloy weathering steels. The steel specimens are first exposed for 48 h to the vapour of an 0-(X)l mol dm sodium bisulphite solution maintained at 54°C (humid SOj-containing atmosphere) to stimulate corrosion under atmospheric conditions. They are then subjected to two types of test (tt) open-circuit potential-time tests for periods up to 3 000 s in either distilled water or 0 -1 mol dm Na2S04 and... 

In order to evaluate the tests determinations were carried out on the steels that had been exposed to the atmosphere for 1,2, 3, 4 and 6-month periods. It was established that the initial open-circuit potential and the decrease in potential (more negative) with time varied with the nature of the steel and the time of exposure to the atmosphere, and the maximum negative potential was taken as a measure of corrosion resistance the more negative the... 

Fig. 19.16 Schematic E — I diagrams of local cell action on stainless steel in CUSO4 + H2SO4 solution showing the effect of metallic copper on corrosion rate. C and A are the open-circuit potentials of the local cathodic and anodic areas and / is the corrosion current. The electrode potentials of a platinised-platinum electrode and metallic copper immersed in the same solution as the stainless steel are indicated by arrows, (a) represents the corrosion of stainless steel in CUSO4 -I- H2 SO4, (b) the rate when copper is introduced into the acid, but is not in contact with the steel, and (c) the rate when copper is in contact with the stainless steel...

Open-circuit Potential the potential of an electrode (relative to a reference electrode) from which no net current flows, so that the anodic and cathodic reactions occur at an equal rate. 

See also Corrosion Potential, Electrode Potential, Equilibrium Potential, Flade Potential, Open-circuit Potential, Passivation Potential, Protection Potential, Redox Potential.)... 

In an Evans diagram 89> the mixed potential can easily be found and also be verified by measuring the open circuit potential of a zinc-amalgam electrode in a Cu2+-ion solution. Even the complication by the simultaneous presence of another reducible species, e.g., Pbz+ can be graphically demonstrated for different limiting conditions... 

Figure 9 shows the first and second cycle of a cyclic voltammogram of a 0.2 molal (mol kg"1) solution of lithium bis[2,2 biphenyldiolato(2-)-0,0 ]borate in PC at a stainless steel electrode. The sweep covers the potential range from open circuit potential ER versus a lithium reference electrode up to 4500 mV versus Li and back to ER. The first cycle shows... 

Figure 1. Sketch of an electrochemical cell whose equilibrium (open circuit) potential difference is AE. (a) Conventional configuration and (b) short-circuited configuration with an air gap. M and R are the electrodes, S is the solvent (electrolyte solution). Cu indicates the cables connecting the two electrodes to a measuring instrument (or to each other).

F/gwre 5 JO, (a) Complex impedance spectra (Nyquist plots) of the CH4,02) Pd YSZ system at different Pd catalyst potentials. Open circuit potential U R =-0.13 V. Dependence on catalyst potential of the individual capacitances, C4i (b) and of the corresponding frequencies, fmii, at maximum absolute negative part of impedance (c).54 Reprinted with permission from Elsevier Science. 

Figure 7.8. (a) Dependence of (Ptr RfAg) on potential UWR for the system Pt(W)-Ag(R) exposed to H2-He mixtures (open-symbols, Ph2 varying between 0.53 and 0.024 kPa) and H2-02 mixtures (filled symbols, p02=12 kPa, Ph2 varying between 0.28 and 7.8 Pa) open-circuit operation, T=673 K, Au counter electrode, (b) Work function of working (W) and reference (R) electrode as a function of open-circuit potential UWR.21 Symbols and conditions as in figure 7.8a. Diamonds show the literature41 values of 0,w(pt) and o,R(Ag)- Reprinted with permission from The Electrochemical Society. 

As shown on this Figure and also in Fig. 8.29 increasing Uwr and O above their open-circuit potential values leads to a local "volcano , i.e. the rate goes through a maximum. This is consistent with the global promotional rule G3 and the observed rate dependence on pco/po2 (Fig- 8.30) where it is interesting to observe that the rate maximum is only moderately affected by the applied potential. 

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