A.J. Bard, L.R. Faulkner - Electrochemical methods - Fundamentals and Applications (794273), страница 5
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NHE)Figure 1.1.5 Schematic current-potential curve for the Hg electrode in the cell Hg/H + , Br (1M)/AgBr/Ag, showing the limiting processes: proton reduction with a large negative overpotentialand mercury oxidation. The potential axis is defined through the process outlined in the caption toFigure 1.1.4.With Hg, the anodic background limit occurs when Hg is oxidized to Hg2Br2 at a potential near 0.14 V vs.
NHE (0.07 V vs. Ag/AgBr), characteristic of the half-reactionHg 2 Br 2 + 2e«±2Hg2Br"(1.1.10)In general, the background limits depend upon both the electrode material and the solution employed in the electrochemical cell.Finally let us consider the same cell with the addition of a small amount of Cd 2 + tothe solution,Hg/H + ,Br"(l M), Cd 2+ (10" 3 M)/AgBr/Ag(1.1.11)The qualitative current-potential curve for this cell is shown in Figure 1.1.6. Note theappearance of the reduction wave at about -0.4 V vs. NHE arising from the reductionreactionCdBr|~ + 2e S Cd(Hg) + 4Br~(1.1.12)where Cd(Hg) denotes cadmium amalgam. The shape and size of such waves will be covered in Section 1.4.2.
If Cd 2 + were added to the cell in Figure 1.1.3 and a current-potential curve taken, it would resemble that in Figure 1.1.4, found in the absence of Cd 2 + . At aPt electrode, proton reduction occurs at less positive potentials than are required for thereduction of Cd(II), so the cathodic background limit occurs in 1 M HBr before the cadmium reduction wave can be seen.In general, when the potential of an electrode is moved from its open-circuit value toward more negative potentials, the substance that will be reduced first (assuming all possible electrode reactions are rapid) is the oxidant in the couple with the least negative (ormost positive) E®.
For example, for a platinum electrode immersed in an aqueous solutioncontaining 0.01 M each of F e 3 + , Sn 4 + , and N i 2 + in 1 M HC1, the first substance reducedwill be F e 3 + , since the E° of this couple is most positive (Figure 1.1.7a).
When the poten-1.1 IntroductionHg/I-Г, ВГ(1 М), Cd2+(1mM)/AgBr/AgCathodicOnset of Cd 2 'reductionAnodic l _Potential (V vs. NHE)Figure 1.1.6 Schematic current-potential curve for the Hg electrode in the cell Hg/H+,Br"(l M),Cd 2 + (l(T 3 M)/AgBr/Ag, showing reduction wave for Cd 2 + .tial of the electrode is moved from its zero-current value toward more positive potentials,the substance that will be oxidized first is the reductant in the couple of least positive (ormost negative) E°.
Thus, for a gold electrode in an aqueous solution containing 0.01 Meach of Sn 2 + and F e 2 + in 1 M HI, the Sn 2 + will be first oxidized, since the E° of this couple is least positive (Figure 1.1.7b). On the other hand, one must remember that these predictions are based on thermodynamic considerations (i.e., reaction energetics), and slowkinetics might prevent a reaction from occurring at a significant rate in a potential regionwhere the E° would suggest the reaction was possible. Thus, for a mercury electrode immersed in a solution of 0.01 M each of Cr 3 + and Zn 2 + , in 1 M HC1, the first reductionprocess predicted is the evolution of H 2 from H + (Figure 1.1.7c).
As discussed earlier,this reaction is very slow on mercury, so the first process actually observed is the reduc3+tion of Cr .1.1.2Faradaic and Nonfaradaic ProcessesTwo types of processes occur at electrodes. One kind comprises reactions like those justdiscussed, in which charges (e.g., electrons) are transferred across the metal-solution interface. Electron transfer causes oxidation or reduction to occur. Since such reactions aregoverned by Faraday's law (i.e., the amount of chemical reaction caused by the flow ofcurrent is proportional to the amount of electricity passed), they are called faradaicprocesses.
Electrodes at which faradaic processes occur are sometimes called chargetransfer electrodes. Under some conditions, a given electrode-solution interface willshow a range of potentials where no charge-transfer reactions occur because such reactions are thermodynamically or kinetically unfavorable (e.g., the region in Figure 1.1.5between 0 and —0.8 V vs. NHE). However, processes such as adsorption and desorptioncan occur, and the structure of the electrode-solution interface can change with changingpotential or solution composition. These processes are called nonfaradaic processes. Although charge does not cross the interface, external currents can flow (at least transiently)when the potential, electrode area, or solution composition changes. Both faradaic and10 • Chapter 1.
Introduction and Overview of Electrode Processes0PossibleoxidationreactionsPossiblereductionreactions0E°(V w. NHE)(V vs. NHE)-0.25N i 2 + + 2e -> Ni02 H + + 2e - » H 9+0.15Approximatepotential forzero currentS n 4 + + 2e -> S n 2 +(Au)- - +0.15\2 + 2e<-2£+0.54<- F e 2 ++0.773+ +(Pt)--- 0eApproximate potentialfor zero current©+1.23(a)A u 3 + + 3e <- Au0- - +1.500-0.76 - --0.41 - 0 --(Kineticallyslow)(Hg)Approximate potentialfor zero currentE°(V vs.
NHE)©(c)Figure 1.1.7 (a) Potentials for possible reductions at a platinum electrode, initially at ~ 1 V vs.NHE in a solution of 0.01 M each of Fe 3 + , Sn 4+ , and Ni 2+ in 1 M HCL (b) Potentials for possibleoxidation reactions at a gold electrode, initially at ~0.1V vs. NHE in a solution of 0.01 M each ofSn 2+ and Fe 2+ in 1 M HI. (c) Potentials for possible reductions at a mercury electrode in 0.01 MCr 3+ and Zn 2+ in 1 M HCL The arrows indicate the directions of potential change discussed in thetext.nonfaradaic processes occur when electrode reactions take place.
Although the faradaicprocesses are usually of primary interest in the investigation of an electrode reaction (except in studies of the nature of the electrode-solution interface itself), the effects of thenonfaradaic processes must be taken into account in using electrochemical data to obtaininformation about the charge transfer and associated reactions. Consequently, we nextproceed by discussing the simpler case of a system where only nonfaradaic processesoccur.1.2 Nonfaradaic Processes and the Nature of the Electrode-Solution Interface111.2 NONFARADAIC PROCESSES AND THE NATURE OF THEELECTRODE-SOLUTION INTERFACE1.2.1The Ideal Polarized ElectrodeAn electrode at which no charge transfer can occur across the metal-solution interface, regardless of the potential imposed by an outside source of voltage, is called an ideal polarized (or ideal polarizable) electrode (IPE).
While no real electrode can behave as an IPEover the whole potential range available in a solution, some electrode-solution systemscan approach ideal polarizability over limited potential ranges,. For example, a mercuryelectrode in contact with a deaerated potassium chloride solution approaches the behaviorof an IPE over a potential range about 2 V wide. At sufficiently positive potentials, themercury can oxidize in a charge-transfer reaction:Hg + С Г -> |Hg 2 Cl 2 + e(at ~ +0.25 V vs.
NHE)(1.2.1)+and at very negative potentials K can be reduced:,HgK + + e -> K(Hg)(at ~ -2.1 V vs. NHE)(1.2.2)In the potential range between these processes, charge-transfer reactions are not significant. The reduction of water:H 2 O + e -> | H 2 + OH"(1.2.3)is thermodynamically possible, but occurs at a very low rate at a mercury surface unlessquite negative potentials are reached. Thus, the only faradaic current that flows in this region is due to charge-transfer reactions of trace impurities (e.g., metal ions, oxygen, andorganic species), and this current is quite small in clean systems.
Another electrode thatbehaves as an IPE is a gold surface hosting an adsorbed self-assembled monolayer ofalkane thiol (see Section 14.5.2).1.2.2Capacitance and Charge of an ElectrodeSince charge cannot cross the IPE interface when the potential across it is changed, thebehavior of the electrode-solution interface is analogous to that of a capacitor. A capacitor is an electrical circuit element composed of two metal sheets separated by a dielectricmaterial (Figure 1.2.1a).
Its behavior is governed by the equation| =Сe(1.2.4)^-eBattery —_ Capacitor++ ++©e(b)Figure 1.2.1 (a) A capacitor, (b)Charging a capacitor with a battery.12Chapter 1. Introduction and Overview of Electrode ProcessesMetalSolution(a)MetalSolution(b)Figure 1.2.2 The metal-solutioninterface as a capacitor with acharge on the metal, qM, (a)negative and (b) positive.where q is the charge stored on the capacitor (in coulombs, С), Е is the potential across thecapacitor (in volts, V), and С is the capacitance (in farads, F). When a potential is appliedacross a capacitor, charge will accumulate on its metal plates until q satisfies equation1.2.4.
During this charging process, a current (called the charging current) will flow. Thecharge on the capacitor consists of an excess of electrons on one plate and a deficiency ofelectrons on the other (Figure 1.2.1b). For example, if a 2-V battery is placed across a 10/л¥ capacitor, current will flow until 20 /лС has accumulated on the capacitor plates. Themagnitude of the current depends on the resistance in the circuit (see also Section 1.2.4).The electrode-solution interface has been shown experimentally to behave like a capacitor, and a model of the interfacial region somewhat resembling a capacitor can begiven. At a given potential, there will exist a charge on the metal electrode, qM, and acharge in the solution, qs (Figure 1.2.2).
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