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Physical Chemistry

Physical Chemistry. §25−1 Electrode Polarization (1) Polarization & Overpotential Electrical energy Chemical energy Eg. in electrolysis of saturated NaCl, E = 3.5 V ( E reversible = 2.17 V ). Galvanic cell. Electrolytic cell.

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Physical Chemistry

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  1. Physical Chemistry Department of Chemistry, Fudan University

  2. §25−1 ElectrodePolarization (1) Polarization & Overpotential Electrical energy Chemical energy Eg. in electrolysis of saturated NaCl, E = 3.5 V ( Ereversible =2.17 V ) Galvanic cell Electrolytic cell Apply a voltage  Ereversible Department of Chemistry, Fudan University

  3. e.g.: (1) Pt，H2 | H2O | O2，Pt (+) Reaction at anode： H2→2H+ + 2e− Reaction at cathode： O2 + H2O + 2e− →2OH− The EMFs : In electrolysis, the theoretical voltage depends on the reversible potentials of the anode and the cathode; practically the voltage applied is much higher than the theoretical one. Department of Chemistry, Fudan University

  4. irreversible Practical voltages required for decomposing water in different solutions Polarization reflects the phenomenon that the potential deviation relative to the equilibrium (reversible) potential as the current passes through electrode Department of Chemistry, Fudan University

  5. Electrolyte composition：100 gdm3 NaOH & 190 gdm3 NaCl, pH14. provided then, DC power anode cathode Anode: Cl2 = Cl− - e− Cathode: H2 = H+ + e− Reversible potentials Department of Chemistry, Fudan University

  6. DC power app anode cathode anodic reaction：Cl− − e−→Cl2, cathodic reaction: H+ + e− →H2； The magnitude of the deviation of the electrode potential from the equilibrium value ris termed as the overpotential , defined as Department of Chemistry, Fudan University

  7. Additional potentials originate from (1) ohmic drops (2) polarization of electrodes In reality anode potential more positive; cathode potential more negative. Defining the overpotentials as a = a,ir  a,r > 0 c = c, r  c,ir > 0 Vapp = Er + IR + a +c Department of Chemistry, Fudan University

  8. Overpotentials may originate from: (1) diffusion (concentration) overpotential caused by concentration difference.  r =  0 - (RT/F) ln (1/C)  ir =  0 - (RT/F) ln (1/Cs) d =  ir - r = (RT/F) ln (C/Cs) if Cs < C, d> 0 （overpotential） Department of Chemistry, Fudan University

  9. (2) ohm overpotential caused by surface films formed on electrode (3) activation overpotential to overcome the activation energy of the rate determining step Usually, overpotentials are smaller for metal deposition, but larger for gas evolution. Department of Chemistry, Fudan University

  10. (2) Overpotential for hydrogen evolution Mechanisms for hydrogen evolution (1) Diffusion of H3O+ to electrode surface (2) M + H3O+ + e - M-H + H2O (3)-i M-H + M-H  2M + H2 -ii M-H + H3O+ + e - M + H2+ H2O (4) Desorption of H2 from electrode surface. Department of Chemistry, Fudan University

  11. Volmer reaction mechanism  Step (2) is the rate-determining step Hs+ + e - Hs r = k[H+]s = A exp(-Ea/RT) [H+]s Ea = Ea0 - F r = A [H+]sexp-(Ea0- F)/RT n = It/F dn/dt = I/F r = (1/A) dn/dt = (1/A) (I/F) = i / F i = r F = F A[H+] exp-(Ea0-F)/RT = i0 exp(F/RT) ln i = ln i0 + (F/RT)   = a + b ln i Department of Chemistry, Fudan University

  12. (三) Measurement & application of overpotentials Electrochemical analyzer Computer Salt bridge Electrolytic cell Department of Chemistry, Fudan University

  13. H2 的 O2的 Cl2的 Activation overpotentials for the evolution of H2,O2 & Cl2 on different metal electrodes Department of Chemistry, Fudan University

  14. a varies markedly for different metals; b rather constant Department of Chemistry, Fudan University

  15. Application of overpotentials metallic deposition and hydrogen overpotential 0.1 m ZnSO4，cathodic electrolysis of Cu，pH=7，H2 evolution or Zn depostion? 2H+ + 2e = H2 Zn2+ + 2e = Zn Deposition of Zn prefers (in the absence of overpotential, evolution of H2 occurs) Department of Chemistry, Fudan University

  16. Another important application of overpotential is the industrial fabrication of Na by taking use of the very high overpotential of H2 evolution on Hg electrode, the low overpotential for the formation of Na amalgam. Na+ discharges at a Hg electrode (0.2%Na amalgam) at -1.83V，the theoretical discharge potential for H+is -0.84V，the overpotential of H2 evolution on Hg is -1.35V, the practical potential for H2 evolution is -2.2V, indicates much higher difficulty in discharging H+ than Na+ on Hg electrode. After discharge, the resulting Na amalgam flows to a de-amalgaming chamber and reacts with H2O to form NaOH and H2, i.e., 2Na(Hg) + 2H2O → 2NaOH + H2 + 2Hg Department of Chemistry, Fudan University

  17. §25−2 Electrochemical measurements ㈠ Cyclic voltammetry (CV) CV is a controlled potential method in which a triangular potential waveform is imposed on a working electrode, while the current response is simultaneously recorded for the study of reduction and oxidation processes involved. current backward scan Forward scan First cycle potential time Department of Chemistry, Fudan University

  18. Ox + ne Red current potential Cyclic voltammogram recorded for K3Fe(CN)6 on Pt electrode Department of Chemistry, Fudan University

  19. Ox + ne Red current potential At 25℃ , Cyclic voltammogram recorded for K3Fe(CN)6 on Pt electrode Department of Chemistry, Fudan University

  20. Rich information on electrode reactions can be obtained based on the peak shapes, peak currents and peak potentials in a cyclic voltammogram, one of which is to judge the reversibility of electrode reactions. Two criteria for reversible reactions: Department of Chemistry, Fudan University

  21. (2) Chronoamperometry and Chronopotentiometry 1. Chronoamperometry Control potential step from an initial potential 1 to a second potential 2 (see left panel), record time-evolved current response( see right panel). Department of Chemistry, Fudan University

  22. For semi-finite linear diffusion controlled electrode process Ox + ne Red Cottrell Equation S –electrode area，DO-diffusion coefficient, CO0-bulk concentration of O species. Department of Chemistry, Fudan University

  23. Ox + ne Red 2. Chronopotentiometry Apply a constant current to a working electrode, and simultaneously record the potential vs. time plot. Department of Chemistry, Fudan University

  24. For semi-finite linear diffusion controlled electrode process =0 Sand Equation D0 can be obtained provided  is known Department of Chemistry, Fudan University

  25. (3) ac impedance Applying an ac potential (or current) to the cell forces the processes at the electrode surface also to oscillate with the applied frequency; the ac current (or potential) signal was simultaneously measured with a phase shift, hence the complex electrode impedance can be calculated. By fitting the complex impedance with a proposed equivalent circuit, useful kinetic parameters can be obtained. solution resistance double-layer capacitance Faradaic impedance, Rs-its resistive component, Cs-its capacitive component charge-transfer resistance Warburg impedance Department of Chemistry, Fudan University

  26. For with Recalling Department of Chemistry, Fudan University

  27. Applying a sinusoidal signal with a small amplitude to the cell Frequency dependence of Faradaic impedance Electrode reaction composed of a series processes including diffusion, interfacial charge transfer, etc. The impedance measurement may provide information regarding the nature of electrode reaction, diffusion coefficients, exchange current density as well as electron transfer number. Department of Chemistry, Fudan University

  28. §25−3 Applied electrochemistry ㈠ Electrolysis electrochemical reaction ( electrical energy  chemical energy ) objects: inorganic (metal, gas)organic merits: simple, green, and tunable process，mild conditions, oxidant and reductant - free. Department of Chemistry, Fudan University

  29. voltage efficiency= current efficiency = Electrical energy efficiency= Department of Chemistry, Fudan University

  30. ㈡ Metal electrodeposition 1. Electrodeposition of metals in the presence of H+ More positive reduction electrode potential for a metal, easier its electrodepostion. (1) Provided the reversible electrode potential for M/Mn+ is far more positive than that for H2/H+ , and the overpotential for the former is very small, then only metal can be electrodeposited. (2) Provided the reversible electrode potential for M/Mn+ is far more negative than that for H2/H+ , and the overpotential for the former is large, then only hydrogen evolution occurs. Department of Chemistry, Fudan University

  31. 第二十五章 电化学动力学及其应用 (3) the reversible electrode potential for M/Mn+ is much more positive than that for H2/H+ , but with a large overpotential. At a certain current density, the cathodic potential can be more negative than the reversible potential of H2/H+, hence the hydrogen evolves. (4) the reversible electrode potential for M/Mn+ is very negative with a moderate overpotential for its deposition and accompanied with a very high overpotential for hydrogen evolution, At a high current density favors metal deposition more than hydrogen evolution. Department of Chemistry, Fudan University

  32. 2. Codeposition of metals (1) Close standard electrode potentials with small overpotentials facilitate the codeposition of two metals from their salt solution, only a slight adjustment of ion activities is needed. (2) Difference between two standard electrode potentials is large but can be compensated by the discharge overpotentials of their cations. (3) Differences in standard electrode potentials and the discharge overpotentials are to be compensated by changing the activities of metal ions. Department of Chemistry, Fudan University

  33. Ag(CN)+ e− → Ag + 2CN− Ag(CN)→ Ag+ + 2CN− Ag++ e− → Ag Department of Chemistry, Fudan University

  34. (3) Metal corrosion and anticorrosion Chemical corrosion is due to the direct chemical reaction btw metal surface and its ambient media. Electrochemical corrosion is due to the electrochemical reaction btw metal surface and its ambient media producing a corrosion current. anode: Fe → Fe2+ + 2e− cathode: O2 + 4H+ + 4e− → 2H2O 4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3 Department of Chemistry, Fudan University

  35. 第二十五章 电化学动力学及其应用 Three preconditions for electrochemical corrosion： (1) Local potential difference on a metal surface, or potential difference on two metals in contact; (2) the two electrodes on a metal (or metals) are short-circuited； (3) Both electrodes immersed in an electrolyte. Department of Chemistry, Fudan University

  36. Anticorrosion 1. Metal plating Forming a protective layer on a desired metal by (electro)chemical plating a second metal or alloy. 2. Anode protection Maintaining the passivation of metal by anodizing it with a very small current Passivation Department of Chemistry, Fudan University

  37. 3. Cathode protection Keep cathodic polarization of the desired metal by sacrificing anode. The negatively shifted potential minimizes the electrochemical corrosion. Department of Chemistry, Fudan University

  38. 4. Corrosion inhibitors Addition of some inhibitors (eg. organic surfactants) may greatly reduce the corrosion rates of a metal in corrosive media. Department of Chemistry, Fudan University

  39. (4) Chemical power sources 1. Terms and calcution methods used in chemical power sources (1) primary battery (2) secondary battery (3) open circuit voltage (4) working voltage (5) discharge (6) charge (7) discharge rate (8) battery capacity (9) energy density (10) specific power Department of Chemistry, Fudan University

  40. 第二十五章 电化学动力学及其应用 2. Brief introduction to batteries (1) Zn-MnO2 primary battery (−) Zn | NH4Cl | MnO2 | C (+) EMF: 1.5 V. anode： Zn + H2O →ZnO+ 2H+ + 2e− cathode： 2MnO2 + 2H+ + 2e−→ 2MnOOH overall： Zn + 2MnO2 + H2O → ZnO + 2MnOOH Department of Chemistry, Fudan University

  41. 第二十五章 电化学动力学及其应用 (2) Lead-acid battery (−) Pb| H2SO4 | PbO2 (+) EMF: 2.0 V. anode：Pb + H2SO4 → PbSO4 + 2H+ + 2e− cathode：PbO2 + 2H+ + H2SO4 + 2e− → PbSO4 + 2H2O overall：Pb + PbO2 + 2H2SO4 →2PbSO4 + 2H2O In charge, cathode：PbSO4 + 2H+ + 2e → Pb + H2SO4− anode：PbSO4 + 2H2O → PbO2 + 2H+ + H2SO4 + 2e− overall： 2PbSO4 + 2H2O → Pb + PbO2 + 2H2SO4 Department of Chemistry, Fudan University

  42. 第二十五章 电化学动力学及其应用 (3) Ag-Zn battery (−) Zn | KOH (40%) | Ag2O (+) EMF: 1.8 V. anode：Zn + 2OH− → Zn(OH)2+ 2e− cathode：Ag2O + H2O + 2e− → 2Ag + 2OH− overall：Ag2O + Zn + H2O → 2Ag + Zn(OH)2 Department of Chemistry, Fudan University

  43. (4) Ni-H battery (−) MH KOH NiOOH(+) EMF: 1.32 V。 anode ：MH + OH− → M + H2O + e− cathode：NiOOH + H2O + e− → Ni(OH)2 + OH− overall：MH + NiOOH → Ni(OH)2 + M Advantages compared with Pb-H2SO4 & Ni-Cd batteries: (1) stable composition in electrolyte. (2) dendrites –free, no problem of short-circuit. (3) no pollution, higher capacity. Department of Chemistry, Fudan University

  44. (5) Li-ion battery Based on the intercalation and de-intercalation of Li+ in cathode and anode. cathode：LiCoO2 Li1-xCoO2 + xLi+ + xe− anode：6C + xLi+ + xe− LixC6 overall：6C + LiCoO2 Li1-xCoO2 + LixC6 High working voltage(3.6 V)；high specific energy；durable cycling life；safety，no pollution，no memory effect；small self-discharge；wide working temperature. Department of Chemistry, Fudan University

  45. 第二十五章 电化学动力学及其应用 (6) fuel cells fuel cell is a device that directly converts the chemical energy of fuel combustion to electrical energy . H2-O2 fuel cell (PEMFC) anode： H2  2H+ + 2e- cathode： 1/2O2+2H++ 2e-  H2O overall： H2 + 1/2O2 H2O Direct methanol fuel cell (DMFC) anode： CH3OH + H2O  CO2 + 6H+ + 6e- cathode： 6H+ + 3/2O2+ 6e-  3H2O overall： CH3OH + 3/2O2 CO2 + 2H2O Department of Chemistry, Fudan University

  46. §25−4 Selected topics in modern electrochemistry (1) Surface electrochemistry Applying various methodologies used in surface science, such as spectroscopies and scanning probes to in situ investigate the structure and properties at electrode/electrolyte interface at molecular and atomic levels. eg H adsorption on poly- and single- crystal Pt electrodes is surface sensitive. Department of Chemistry, Fudan University

  47. current current current potential CVs for a Pt(poly) electrode in H2SO4 Anodic segments in CVs for Pt(111) & Pt(100) electrodes in H2SO4 Department of Chemistry, Fudan University

  48. SERS for pyridine (Py) on an ORC-roughened Ag electrode Cathodic polarization curve for O2 reduction on Pt(111)和(100) (a): Liquid Py； (b): 0.05 M Py； (c)～(h) at different potentials，(c) 0.0V；(d) -0.2V; (e) –0.4V; (f) –0.6V; (g) –0.8V; (h) –1.0V Department of Chemistry, Fudan University

  49. 第二十五章 电化学动力学及其应用 SERS for Py on different electrodes Department of Chemistry, Fudan University

  50. unconventional techniques used in in-situ surface electrochemistry: Surface-enhanced Raman spectroscopy (SERS), infrared reflection absorption spectroscopy (IR-RAS), surface-enhanced IR absorption spectroscopy (SEIRAS), surface plasmon resonance (SPR), electro-reflectance spectroscopy, electron spin resonance (ESR), electrochemical mass spectrometry (EC-MS), second harmonic generation (SHG), extended X-ray absorption fine structure (EXAFS), electrochemical quartz crystal microbalance (EQCM), scanning tunneling microscopy (STM), Atomic forces microscopy (AFM), scanning electrochemical microscopy (SECM). Pyridine adsorbed on a roughened Ag electrode produces giant enhancement (105106) in surface Raman scattering. （Surface Enhanced Raman Scattering） Department of Chemistry, Fudan University

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