Chapter 7 Electrochemistry

1. Chapter 7 Electrochemistry §7.11 Polarization of electrode

2. Why do we concern the irreversible electrochemical processes Reversible cell correlates electrochemistry with thermo-dynamics and has great theoretical importance. However, either electrolytic cell or galvanic cell always works in an irreversible way.

3. 1.229 O2  / V 0.401 H2O 0.000 H2 H2SO4 -0.828 0 2 4 6 8 12 14 pH 10 7.11.1. Decomposition voltage and overvoltage Electrolysis of water Water decomposition does not depends on pH. At what voltage can water undergo decomposition?

4. I / A 1.70 V 1.229 V 0.0 1.0 2.0 E / V Decomposition voltage: the minimum potential difference which must be applied between electrodes before decomposition occurs and a current flows. The reversible electromotive force of the cell (Theoretical decomposition voltage) is 1.229 V, while theeffective decomposition voltageis 1.70 V. A discrepancy of ca. 0.5 V, which is named asovervoltage, exist.

5. 7.11. 2 Thermodynamics of irreversible cell For reversible cell: Wre = nFEre; For irreversible cell: Wir = nFEir For electrolytic cell: Ere < Eir ; E = Eir - Ere > 0 E = (a, ir-c, ir) - (a, re - c, re) = (a, ir - a, re) + (c, re - c, ir) (a, ira, re ) = a (c, rec, ir ) = c E = c + a

6. For galvanic cell: Ere > Eir; E = EreEir > 0 E = (c, re a, re)( c, ira, ir) = (c, rec, ir) + (a, ira,re) (c, rec, ir ) = c (a, ira, re ) = a E = c + a

7. Under irreversible conditions, electrode potential differs from its reversible value, this phenomenon is defined as polarization. The discrepancy between reversible potential and irreversible potential is termed as overpotential (). By definition, overpotential always has positive value.

8. I / A I / A a c a c Eir Eir a, ir a, ir c, ir c, ir Ere Ere c, re a, re a, re  / V c, re  / V The irreversible potential and the irreversible electromotive force of cell depend on the current density imposed. Polarization cause decrease in electromotive force of galvanic cell and increase in decomposition voltage of electrolytic cell.

9. 7.11.3 Origin of overpotential 1) Resistance overpotential (R) 2) Concentration overpotential (C) 3) Activation overpotential (a)  = r + d + a 1) Resistance overpotential (R) Electrode, electrode/solution interface, solution and separator all have resistance. R = IR Elimination: lower the inner resistance

10. Cu2+ Cu2+ Cu2+ if Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ ib Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ c c c cb if > ib Cu  Cu2+ + 2e ir > re if < ib Cu2+ + 2e  Cu ir < re c cb d d 2) Concentration overpotential (C) Cu = Cu2+ + 2e­ i0 = ib = if elimination: 1) stir the solution in electroplating and in space battery; 2) discharge the battery with intervals

11. e e Fe3+ e e e Fe2+ e e 3) Activation overpotential (a) If the removal of electron from the electrode is not fast enough, excess charge will accumulate on the electrode’s surface, which results in shift of electrode potential i.e., electrochemical / activation polarizaiton. Chemical species that can undergo oxidation or reduction on the electrode surface can slow the shift of electrode potential. depolarizer, depolarization

12. potentiostat potentiostat R.E. W.E. C.E. H2SO4 Conventional three-electrode cell 7.11.4 Measurement of overpotential Polarization circuit Measurement circuit C.E.: Counter/auxiliary electrode W.E.: Working electrode R.E.: Reference electrode

14. e e H+ j / Am-2 e 10000 e Black Pt e Hg C e H Au e bright Pt Ag 6000 2000 0.4 0.0 1.2 0.8  / V 7.11.5. Hydrogen overpotential 1) Hydrogen polarization and Tafel plot Polarization curve If H+ acts as depolarizer 2H+ + 2e  H2

15. log j / A m-2 0.0 E / V In 1905, Tafel reported the log J ~  curves of hydrogen evolution on different metal surfaces. At higher polarization > 118 mV, a linear relation exists: Tafel equation a and b are empirical constant, which can be obtained from the Tafel plot. Tafel plot

16. Values of a and b of different metals

17. 2) Classification of metal according to a value

18. 7.11.6. Theories of hydrogen overpotential The discharge of hydrogen ions on metal surface comprises five steps. • diffusion: H+ diffuses from bulk solution to the vicinity of the double layer • Foregoing step: H+ transfers across the double layer and undergoes configuration changes such as dehydration etc. • Electrochemical step: H3O+ + M + e M-H + H2O, Volmer reaction, forms adsorbed H atom • Desorption of H atom:

19. Combination desorption (catalytic reaction): 2 M-H  2M + H2 (Tafel reaction) Electrochemical desorption: M-H + H3O+ + e- H2 + M (Heyrovsky reaction) 5) Succeeding step: diffusion, evolution. The slowest step will control the overall rate of the electrochemical reaction. The theories of hydrogen overpotential: 1) The slow discharge theory 2) the slow combination theory

20. 7.11.7. Application of hydrogen overpotential 1) The Way to reduce hydrogen overpotential Discussion: According to Tafel equation, how can we lower hydrogen overpotential ? How can we reduce overpotential of an electrode?

21. (1) Use materials with low a as electrode For electrolysis of water, in laboratory, we use Pt (a = 0.05) as cathode, while in industry, we use iron (a = 0.7). Now, Ni-S alloy is used for evolution of hydrogen. For evolution of oxygen, we now use RuO2 as anodic catalyst. Electrocatalysis and electrocatalyst Pt nanoparticles loaded on carbon.

22. (2) Enlarge effective surface area: porous electrode 1) Why do we use platinized platinum electrode? Its effective area is more than 1000~3000 times larger than that of bright platinum. 2) Porous electrode. In lead-acid battery, porous lead electrode and porous lead dioxide electrode was adopted. SEM photograph of porous electrode. The particle is in fact aggregate of nanoparticles.

23. (3) Take advantage of hydrogen overpotential 1) Electroplating of active metal from aqueous solution (Pb, Zn, Sn). 2) Corrosion protection: zinc- or tin-plated iron 3) In battery: Pb negative electrode; amalgamated zinc negative electrode in dry-battery. (homogeneity, tension, overpotential) 4) Use lead or lead alloy as cathode materials in electrosynthesis to improve current efficiency.