Equilibrium open-circuit electrode potential

It should also be recalled that, for many electrodes in various solutions, the open circuit electrode potential, i.e. the electrode potential in the absence of current flow, is not the thermodynamic equilibrium but rather a mixed potential, discussed in Section 5.8.4. 

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).

A baseline potential pulse followed each current pulse in order to strip extracted ions from the membrane phase and, therefore, regenerated the membrane, making it ready for the next measurement pulse. This made sure that the potentials are sampled at discrete times within a pulse that correspond to a 6m that is reproducible from pulse to pulse. This made it possible to yield a reproducible sensor on the basis of a chemically irreversible reaction. It was shown that the duration of the stripping period has to be at least ten times longer than the current pulse [53], Moreover the value of the baseline (stripping) potential must be equal to the equilibrium open-circuit potential of the membrane electrode, as demonstrated in [52], This open-circuit potential can be measured prior to the experiment with respect to the reference electrode. 

One of the simplest electrochemical systems to analyse is that of a rapid reaction at its equilibrium, open circuit potential Eg h with no net current flowing. Under a.c. excitation (and in this and subsequent examples it is assumed that the excitation of potential of the working electrode is small, which means ideally less than 5 mV peak-to-peak) the net current is, of course, still zero, but the current-potential relationship is approximately linear rather than exponential. 

When chemical species come into equilibrium with an electrode in an open circuit, the potential between the electrode and a reference electrode is related to the potential difference of the half reaction occurring at the electrode. If no... 

FIGURE 1.8 Potential distribution in a fuel cell with planar Pt electrodes, (a) Equilibrium (open-circuit) conditions (b) under load, assuming infinite membrane conductivity, and (c) under load, assuming finite membrane conductivity. In cases (b) and (c), the anode polarization is assumed to remain constant, implying a negligible shift of the anode electrode potential. 

Here, represents the crossover effect of fiiel/oxidizer through the electrolyte to the opposite electrode or internal short circuits in the cell that is responsible for more departure of theoretical equilibrium open-circuit potential from Nernst equilibrium voltage. 

Step 5 Calculate the equilibrium (open circuit) potential, E, of the electrochemical cell as difference between the right-hand, E, and left-hand, electrode potentials ... 

Weaver calculated the open circuit potentials of these and other possible reactions that might occur under open circuit conditions, finding agreement between measured potentials and the potentials calculated from thermodynamic tables (Weaver et al, 1979). Hemmes and Cassir (2004) recalculated the cell open circuit potentials. They determined the equilibrium concentrations and electrode potentials in a system comprised of carbon, carbonate, CO2, CO, O ", and electrons, using the phase rule modified for electrochemical systems by Coleman and White (1996). Hemmes expressed the half-cell potentials of the anode reactions (3) and (4) referenced to an idealized cathode reaction (unit oxygen and CO2 partial pressures) ... 

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

Corrosion of the positive grid [Eq. (28)1 occurs equivalent to about 1 mA/lOOAh at open-circuit voltage and intact passivation layer. It depends on electrode potential, and is at minimum about 40-80mV above the PbS04/Pb02 equilibrium potential. The corrosion rate depends furthermore to some extent on alloy composition and is increased with high anti-monial alloys,... 

NonequiUbrium Open-Circuit Potentials Different reasons exist for lack of equilibrium at electrode-electrolyte interfaces even in the absence of an electric current ... 

Equilibrium potentials can be calculated thermodynamically (for more details, see Chapter 3) when the corresponding electrode reaction is known precisely, even when they cannot be reached experimentally (i.e., when the electrode potential is nonequilibrium despite the fact that the current is practically zero). The open-circuit voltage of any galvanic cell where at least one of the two electrodes has an nonequilibrium open-circuit potential will also be nonequilibrium. Particularly in thermodynamic calculations, the term EMF is often used for measured or calculated equilibrium OCV values. 

The value of polarization defined by Eq. (2.21) is referred to a calculated value of equilibrium potential of the reaction, rather than to the electrode s effective open-circuit potential, when the latter is not the equilibrium potential. Sometimes a thermodynamic calculation of the equilibrium potential is not possible for instance, when several electrode reactions occur simultaneously. In this case one either uses, conditionally, the concept of a polarization which via Eq. (2.21) refers to the effective open-circuit potential, or (since the latter is often irreproducible) one simply discusses electrode potentials at specified current densities rather than the potential shift away from some original value. 

The electrode s open-circuit potential (steady potential) , depends on the relative values of the exchange CD of both reactions and also on the slopes of the polarization curves. When the exchange CD and slopes are similar, the open-circuit potential will have a value, the mixed (or compromise ) potential, which is intermediate between the two equilibrium potentials (Fig. 13.2a). However, when the exchange CD for one of the reactions is much higher than that for the other, the open-circuit potential will practically coincide with the equilibrium potential of this reaction (Fig. 13.2b). 

Some metals are thermodynamically unstable in aqueous solutions because their equilibrium potential is more negative than the potential of the reversible hydrogen electrode in the same solution. At such electrodes, anodic metal dissolution and cathodic hydrogen evolution can occur as coupled reactions, and their open-circuit potential (OCP) will be more positive than the equilibrium potential (see Section 13.7). 

Potentiometry, which measures the open-circuit equilibrium potential of an indicator electrode, for which the substance being examined is potential determining... 

If A is placed in an open circuit electrochemical cell containing no A+ ions, A will donate electrons to the metal electrode, forming A+ ions. Rather quickly, the electrode will be unable to accept additional electrons and the system will reach equilibrium. This equilibrium potential is a reversible electrode potential. If the electrode potential is made more positive, the electrode will again be able to accept electrons and additional A+ will be produced. Conversely, if the electrode is made more negative, A+ will accept electrons from the electrode. The rate at which A is oxidized is proportional to the current density, i (typically in units of A/cm2), by the relation... 

An important property of the solution to be investigated is the rest or open-circuit potential Er. This is the potential that the working electrode develops in the solution at equilibrium, that is, when no current flows through the electrode. The value of r depends on the components of the solution and the electrode itself. 

As to what, exactly, might be the fluorinating agent in the proposed Radical Mechanism we have seen how some workers favour that of fluorine absorbed on or in the nickel fluoride layer, whereas others prefer that of high valence fluorides. In this respect, it is interesting to note how different workers interpret what are apparently very similar phenomena upon opening the electrical circuit of a previously conditioned ECF anode, e.g., Watanabe [169], and the decay of electrode potential, as an example of the first, and Sartori et al. [186], and the persistence of chemical activity, as an example of the second. Perhaps, in reality, the dynamic equilibrium which relates fluoride ion, nickel fluorides, and atomic... 

At the equilibrium potential, both anodic and cathodic processes of a single electron transfer reaction take place at the same exchange rate (exchange current density) and no net current is observed through the external circuit. The exchange rate reflects the kinetics of the overall reaction and, in many cases, the electrocatalytic properties of the electrode surface. The open circuit potential, in this case, is the equilibrium potential and is therefore a thermodynamic quantity independent of kinetic factors and is related to the activities in solution through the Nemst law. 

Consider two electrode reactions O/R and OVR with transfer of n and ri electrons, respectively, taking place at a given electrode surface simultaneously with equilibrium potentials E f and E2. Inspection of Fig. 13(a), shows that, at open circuit, the mixed potential, EM, adopts an intermediate value E < EM equilibrium potential of the faster component reaction. 

Figure 3.37 illustrates the Nernst diffusion layer in terms of concentration-distance profiles for a solution containing species O. As pointed out previously, the concentration of redox species in equilibrium at the electrode-solution interface is determined by the Nernst equation. Figure 3.37A illustrates the concentration-distance profile for O under the condition that its surface concentration has not been perturbed. Either the cell is at open circuit, or a potential has been applied that is sufficiently positive of Eq R not to alter measurably the surface concentrations of the 0,R couple. 

The SOFC consists of cathode, electrolyte and anode collectively referred to as the PEN - positive electrode, electrolyte, negative electrode. A single cell operated with hydrogen and oxygen provides at equilibrium a theoretical reversible (Nernst) or open circuit voltage (OCV) of 1.229 V at standard conditions (STP, T = 273.15 K. i> = 1 atm). With the standard electrode potential E°, universal gas constant R. temperature T. Faraday s constant F, molar concentration x and pressure p, the OCV is given by... 

As mentioned in Sect. 4.1, in DDPV technique initial equilibrium conditions are reestablished before the application of each double potential pulse, for example, by renewal of the electrode (mercury drop electrode) or by open circuiting the working electrode for a waiting period. Lovric shows [49], and it is proved by numerical simulations in [5] that for a reversible process a waiting period of < 5tj is long enough to achieve this condition at spherical electrodes the smaller the electrode radius, the shorter the waiting period required. So, the expression for DDPV response, for which t>. is imme-... 

Corrosion — Corrosion current density — Figure. Polarization curves of a metal/metal ion electrode and the H2/H+ electrode including the anodic and cathodic partial current curves, the Nernst equilibrium electrode potentials E(Me/Mez+) and (H2/H+), their exchange current densities / o,M> o,redox and related overpotentials Me) and 77(H), the rest potential r, the polarization n and the corrosion current density ic at open circuit conditions (E = Er) [i]... 

Open-circuit potential (OCP) — This is the - potential of the - working electrode relative to the - reference electrode when no potential or - current is being applied to the - cell [i]. In case of a reversible electrode system (- reversibility) the OCP is also referred to as the - equilibrium potential. Otherwise it is called the - rest potential, or the - corrosion potential, depending on the studied system. The OCP is measured using high-input - impedance voltmeters, or potentiometers, as in - potentiometry. OCP s of - electrodes of the first, the second, and the third kind, of - redox electrodes and of - ion-selective membrane electrodes are defined by the - Nernst equation. The - corrosion po-... 

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