Open circuit reactions

The variation of the reaction rate between the two extremes, i.e., the open-circuit reaction rate, o, and the steady-state promoted reaction rate, A st, was fully reversible. A possible irreversible contribution to the promotion effect (permanent NEMCA) was avoided by limiting the polarizing current to 10 (tA. The current dependence of the steady-state promoted catalytic reaction rate was also investigated. The total increase in reaction rate, was a continuously increasing function... 

For enhancement factors higher than nnity the reaction is called non-Faradaic. The rate enhancement factor, p, is defined as the ratio of the promoted catalytic rate, r, to the initial open-circuit reaction rate, and it is a measure of the level of promotion ... 

It should be pointed out that external polarization differs from the unbiased (open circuit) case in that after application of, say, an anodic voltage only the oxidation reaction takes place on the metal, whereas the cathodic reaction (H — H2) occurs at the external counter-electrode. 

The electrons, Hberated at the anode, travel by electrical cable through the external load, such as an electric motor, to the cathode. If the external circuit is open the reaction is stopped, no fuel is consumed, and no power is generated. The electrolytic reaction, then, is controlled by the load connected to the cell. The overall fuel cell reaction is... 

In the thermodynamic treatment of electrode potentials, the assumption was made that the reactions were reversible, which implies that the reactions occur infinitely slowly. This is never the case in practice. When a battery deUvers current, the electrode reactions depart from reversible behavior and the battery voltage decreases from its open circuit or equiUbrium voltage E. Thus the voltage during battery use or discharge E is lower than the voltage measured under open circuit or reversible conditions E by a quantity called the polari2ation Tj. 

To calculate the open circuit voltage of the lead—acid battery, an accurate value for the standard cell potential, which is consistent with the activity coefficients of sulfuric acid, must also be known. The standard cell potential for the double sulfate reaction is 2.048 V at 25 °C. This value is calculated from the standard electrode potentials for the (Pt)H2 H2S04(yw) PbS04 Pb02(Pt) electrode 1.690 V (14), for the Pb(Hg) PbS04 H2S04(yw) H2(Pt) electrode 0.3526 V (19), and for the Pb Pb2+ Pb(Hg) 0.0057 V (21). 

When these half-reactions are summed, there is no net reaction. Thus the material balance of the cell is not altered by overcharge. At open circuit, equation 19 at the negative electrode is the sum of a two-step process, represented by equation 15 and... 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

At open-circuit, the current in the cell is 2ero, and species in adjoining phases are in equilibrium. Eor example, the electrochemical potential of electrons in phases d and P are identical. Furthermore, the two electrochemical reactions are equilibrated. Thus,... 

Thus, because the standard cell potential for reaction 15 is positive, the reaction proceeds spontaneously as written. Consequendy, to produce chlorine and hydrogen gas, a potential must be appHed to the cell that is greater than the open-circuit value. This then becomes an example of an electrolytic process. 

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... 

When lithium ions become sufficiently mobile due to heating, they migrate from the anode to the cathode with the reactions shown in Fig. 5.24 and produce open circuit voltages of about 2.5 V under ideal conditions. In... 

For simplicity a cell consisting of two identical electrodes of silver immersed in silver nitrate solution will be considered first (Fig. 1.20a), i.e. Agi/AgNOj/Ag,. On open circuit each electrode will be at equilibrium, and the rate of transfer of silver ions from the metal lattice to the solution and from the solution to the metal lattice will be equal, i.e. the electrodes will be in a state of dynamic equilibrium. The rate of charge transfer, which may be regarded as either the rate of transfer of silver cations (positive charge) in one direction, or the transfer of electrons (negative charge) in the opposite direction, in an electrochemical reaction is the current I, so that for the equilibrium at electrode I... 

Fig. 1.40 Schematic anodic polarisation curve for a passivatable metal (solid line), shown together with three alternative cathodic reactions (broken line). Open-circuit corrosion potentials are determined by the intersection between the anodic and cathodic reaction rates. Cathode a intersects the anodic curve in the active region and the metal corrodes. Cathode b intersects at three possible points for which the metal may actively corrode or passivate, but passivity could be unstable. Only cathode c provides stable passivity. The lines a, b and c respectively could represent different cathodic reactions of increasing oxidizing power, or they could represent the same oxidizing agent at increasing concentration.

Because the film growth rate depends so strongly on the electric field across it (equation 1.115), separation of the anodic and cathodic sites for metals in open circuit is of little consequence, provided film growth is the exclusive reaction. Thus if one site is anodic, and an adjacent site cathodic, film thickening on the anodic site itself causes the two sites to swap roles so that the film on the former cathodic site also thickens correspondingly. Thus the anodic and cathodic sites of the stably passive metal dance over the surface. If however, permanent separation of sites can occur, as for example, where the anodic site has restricted access to the cathodic component in the electrolyte (as in crevice), then breakdown of passivity and associated corrosion can follow. 

The corrosion rate is greatest close to the reversible Pb02/PbS04 potential as a result of a solid state reaction between PbOj and the underlying lead surface". This corresponds to the rest or open circuit condition. 

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. 

At open-circuit voltage, no anodic current flow through the positive electrode occurs that can oxidize the PbO (or PbO x) layer, but the corrosion reaction... 

In the latter case one would like to know the length Apb of the metal-solid electrolyte-gas three-phase-boundaries (tpb) (in m or in metal mols, for which we use the symbol Ntpb throughout this book) and the value of the exchange current I0, where (W2F) expresses the value of the (equal and opposite under open-circuit conditions) forward and reverse rates of the charge-transfer reaction 4.1. 

Table 4.2 lists the same catalytic systems but now grouped in terms of different reaction types (oxidations, hydrogenations, reductions and others). In this table and in subsequent chapters the subscript D denotes and electron donor reactant while the subscript A denotes an electron acceptor reactant. The table also lists the temperature and gas composition range of each investigation in terms of the parameter Pa/Pd which as subsequently shown plays an important role on the observed r vs O global behaviour. Table 4.3 is the same as Table 4.2 but also provides additional information regarding the open-circuit catalytic kinetics, whenever available. Table 4.3 is useful for extracting the promotional rules discussed Chapter 6. 

In Table 4.3 we had classified all published electrochemical promotion studies on the basis of the catalytic reaction and had provided the observed global r vs behaviour together with the observed r vs po and r vs pA open-circuit kinetic behaviour. We had then invited the reader to use Table 4.3 in order to derive the rules of promotion. As a further step we present here in Table 6.1 the same information given in Table 4.3 with only one difference In Table 6.1 the 58 catalytic reactions are grouped in terms of their global r vs [Pg.285]

The kinetics and mechanism of this reaction have been studied for years on Pt films deposited on doped Zr02.15 It has been found that at temperatures above 280°C the open-circuit catalytic kinetics can be described quantitatively by the rate expression... 

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