electrochemistry 1 the basic of the basic

Electrochemistry 1

1

The Basic of the basic

1. Interface;

2. Thermodynamics & Kinetics;

3. Overpotential.

2

Electrochemistry 1

2

1. Interface

Electrochemistry is the study of reactions in whichcharged particles (ions and/or electrons) cross theinterface between two phases of matter, such as theinterface between a solid and a liquid (=electrode/electrolyte).

3

Electrochemistry 1

3

1. Interface

4

Electrochemistry 1

4

1. InterfaceElectrode processes takeplace within an electricdouble layer.

Electric double layer that is atransition region between twophases consists of (1) an innermonomolecular layer and (2)an outer diffuse region.Between the inner molecularlayer and outer diffuse layer,and (3) a layer intermediatebetween inner molecularlayer and outer diffuse layerexists.

(1) (3)

(2)

5

Electrochemistry 1

5

1. Interface

(1) an inner monomolecularlayer of adsorbed moleculesor ions in which a very largepotential gradient isproduced (e.g. 1 volt across1 angstrom thatcorresponds to 100MV/cm).

(1) (3)

(2)

6

Electrochemistry 1

6

1. Interface(2) an outer diffuse regionthat compensates for anylocal charge unbalance thatgradually merges into thecompletely randomarrangement of the bulksolution (charge unbalance,namely, the violation ofelectronuetrality, can betemporarily/locallyproduced but it will benuetralized/compensated.).

(1) (3)

(2)

7

Electrochemistry 1

7

1. Interface

(3) a layer intermediatebetween inner molecularlayer and outer diffuse layerexists. In this intermediatelayer, excess charges aresolvated or weakly bondedwith counter ions.

(1) (3)

(2)

8

Electrochemistry 1

8

2. Electrode thermodynamics and kinetics

Thermodynamics:

Thermodynamic equation ∆G=-nFE refers to themovement of n moles of charge across the cell potentialE. The value of ∆G expresses the maximum useful energythat a system can give the surroundings. This quantity canonly be perfectly extracted from the system under thelimiting conditions of a reversible change, which implieszero current. The more rapidly the cell operates, the lesselectrical energy it can supply.

9

Electrochemistry 1

9

2. Electrode thermodynamics and kinetics

Kinetics:

(1) If the redox reaction steps of an electrode reaction are rapid enough, then its potential is equal to the equlibriumpotential (the electrode will be non-polarizable).

(2) If, on the other hand, an equlibrium is established onlyslowly due to a kinetic inhibition of steps involved in anelectrode reaction, then the electrode will be polarizable:in order to induce the reaction to proceed in a givendirection, the kinetic inhibition of the reaction must beovercome by applying a high overpotential.

10

Electrochemistry 1

10

3. OverpotentialWithin the theory of thermodynamic, the Nernstequation should predict what electrode reaction will takeplace. According to the Nernst equation, the hydrogenevolution potential as a function of the concentration ofproton [H+] is E=0.000-0.059*log(1/[H+]).

At pH=7, E=-0.414 V. Therefore, only metals whosereduction potentials are less negative than -0.41 V shouldbe reduced and plate out at the cathode in theelectrolysis of aqueous solution of electrolytes. Thismeans that it should not be possible to reduce metal ionssuch as Zn2+ (E0=-0.76 V) from aqueous solution.However, some such metals including Zn do plate out ofaqueous solution of electrolytes.

11

Electrochemistry 1

11

3. OverpotentialThe Nernst equation is a thermodynamic equation thattells nothing about kinetics. For example, the evolution ofH2 at some cathode surfaces in some aqueous solutionsof electrolytes are too slow to occur at the potentialsgiven by the Nernst equation and only take place athigher voltages (it needs overpotentials). Activationoverpotentials for the evolution of H2 on Zn, graphite,and glassy carbon electrodes are -0.77 V, -0.62 V, andmore negative than -0.62 V, respectively. Carbon fibersare also sp2 carbons, and its activation overpotentials areusually more negative than that of graphite but lessnegative than those of glassy carbons.

12

Electrochemistry 1

12

3. Overpotential

The cell overpotential isconsidered to be composedof a number of independentcontributions: (1) ohmicdrop; (2) activationoverpotential; and (3)diffusion overpotential.

13

Electrochemistry 1

13

3. Overpotential

(1) Ohmic drop betweenelectrodes results from thefact that the electrolytesolution has a finiteconductivity;

14

Electrochemistry 1

14

3. Overpotential

(2) activation overpotentialat one or both electrodesarising from kineticinhibition of one of thesteps involved in theelectrode reaction(desolvation of the reactiveion, chemisorption of thereaction product, etc.);

15

Electrochemistry 1

15

3. Overpotential

(3) diffusion overpotentialat one or both electrodesdue to the presence ofconcentration gradients inthe vicinity of the electrodesurface. As a result ofelectrochemical reaction,the concentration at theelectrode surface no longerhave their equilibriumvalues.

16

Electrochemistry 1

16

3. Overpotential

(3) (diffusion overpotential.continued) If migrationthrough the electric doublelayer is very rapid, thendiffusion from the bulk ofthe solution towards theelectrode will be unable toreplenish the ions at thedouble layer quickly enoughand a concentrationgradient will result.

17

Electrochemistry 1

17

3. Overpotential

18

Electrochemistry 1

18

3. Overpotential

Detailed information

(1) Ohmic (IR) drop

Polarization measurements include a so-called ohmicpotential drop through a portion of the electrolytesurrounding the electrode, through a metal- reactionproduct film on the surface, or both.

An ohmic potential drop always occurs between theworking electrode and the reference electrode. Thiscontribution to polarization is equal to IR , where I is thecurrent density, and R is the resistance.

19

Electrochemistry 1

19

3. Overpotential

Detailed information

(1) Ohmic (IR) drop

If copper is made cathode in a solution of dilute CuSO4 inwhich the activity of cupric ion is represented by α(Cu+2),then the potential φ1, in absence of external current, isgiven by the Nernst equation,

φ1=0.34+(0.059/2)*log[α(Cu+2)].

20

Electrochemistry 1

20

3. OverpotentialDetailed information

(2) Activation overpotential

Activation polarization is caused by a slow electrodereaction. The reaction at the electrode requires anactivation energy in order to proceed. The mostimportant example is that of hydrogen ion reduction at acathode, H++e−→0.5H2 . For this reaction, the polarizationis called hydrogen overpotential. Overpotential is definedas the polarization (= potential change) of an equilibriumelectrode that results from current flow across theelectrode/solution interface. Hydrogen overpotential canvary with metal, current density, etc.

21

Electrochemistry 1

21

3. OverpotentialDetailed information

(3) Diffusion overpotential

When current flows, copper is deposited on theelectrode, thereby decreasing surface concentration ofcopper ions to an activity α(Cu+2)s. The potential φ2 of theelectrode becomes,

φ2=0.34+(0.059/2)*log[α(Cu+2)s].

Since α(Cu+2)s is less than α(Cu+2), the potential of thepolarized cathode is less positive than in the absence ofexternal current. The difference of potential, φ2−φ1, is theconcentration polarization, equal to

φ1-φ2=(0.059/2)*log[α(Cu+2)s/α(Cu+2)].

22

Electrochemistry 1

22

3. Overpotential

Detailed information

(3) Diffusion overpotential

The larger the current, the smaller the surfaceconcentration of copper ion, or the smaller the value ofα(Cu+2)s, thus the larger the corresponding polarization.

23

Electrochemistry 1

23

3. Overpotential

Note

The product, IR, decays simultaneously with shutting offthe current, whereas concentration polarization andactivation polarization usually decay at measurable rates.

Concentration polarization decreases with stirring,whereas activation polarization and IR drop are notaffected significantly with stirring.