Chapter 6 Potential Sweep Method

1. Electrochemical Camp 2012 Chapter 6Potential Sweep Method Speaker : Yu-Yan Li Advisor : Kuo-Chuan Ho Aug, 6th, 2012 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

2. Outline • Introduction to Linear Sweep Method (LSV) and Cyclic Voltammetry (CV) • Three cases (rev, quasi-rev, irrev) • Detection limit of LSV • Multi-component system • Electrode reaction coupled with chemical reaction (Chapter 12) • Application Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

3. 6.1 Introduction • What is Linear Sweep voltammetry (LSV)? Cottrell eq. (i-t) Voltage ramp Linear sweep voltammetry (LSV) Limiting Current Plateau (i-E) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

4. Linear Sweep Method (LSV) Surface concentration • Why there is a peak? E→ Ep+ Co(0,t) → 0 E→ Ep- Depletion effect results in small i Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

5. Linear Sweep Method (LSV) Reduction begins and current starts to flow Mass transfer of A reaches maximum rate A +e-→A‧ Co approaches to zero and diffuse layer grows Nonfaradaic current flow Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

6. Cyclic Voltammetry (CV) Reverse the potential scan Reduction Oxidation Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

7. 6.2 Nernstian (Reversible) Systems • Scanning potential • Planar electrode (5.4.2) (5.4.3) (5.4.4) (5.4.5) (5.4.6) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

8. Reversible Systems • Time-dependent form • Laplace transform → Convolution theorem (6.2.2) (6.2.3) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

9. Reversible Systems (6.2.17) • The current (6.2.16) Find the maximum Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

10. Reversible Systems • Peak current (at 25℃) • Peak potential • Half-peak potential (6.2.19) (6.2.20) (6.2.21) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

11. Reversible Systems (6.2.22) For reversible rxn ip∝ ν1/2 i∝ ν1/2 Ep≠Ep(ν) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

12. Detection limit of LSV • [Definition] The ratio of charging to Faradaic current • <Assumption> Absence of adsorption/desorption influence either double layer or Faradaic process Faradaic current Charging current Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

13. Detection limit of LSV • Capacitance of double layer For DME noise signal Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

14. Detection limit of LSV • Effect of double layer charging at different sweep rate ν ↑ → ic/ip↑ Co* ↓ → ic/ip↑ The noise grows! Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

15. 6.3 Totally Irreversible Systems • Irreversible reaction • Boundary condition • The current (6.3.1) (6.3.6) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

16. Totally Irreversible Systems For totally irrev. ip∝ ν1/2 i∝ ν1/2 Ep =Ep(ν) • Peak current • Peak potential (6.3.8) (6.3.10) (6.3.11) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

17. 6.4 Quasi-reversible Systems • Boundary condition • Parameter Λ (6.4.2) Do=DR=D (6.4.4&5) Λ ↑ , easy to reach equilibrium ( k0↑) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

18. Quasi-reversible Systems For quasi-rev (6.4.7) ip∝ν1/2 • Peak current • Peak potential (6.4.7) (6.4.8) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

19. Summary Λ = Λ(ν) • Zone boundary for LSV Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

20. 6.5 Cyclic Voltammetry (6.4.1) • Scanning potential • Two parameters • 1. Ep,a – Ep,c 2. ip,a / ip,c • ΔEp=|Ep,c – Ep,a| Formal potential • approaching to E0’= (Ep,c + Ep,a)/2 (6.4.2) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

21. Reversible System • Find • ip,a & ip,c • If no ip,a / ip,c (6.5.4) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

22. Quasi-reversible System • Wave shape & ΔEp∝ f(ν, α,k0,Eλ) • Equivalent parameter • For 0.3 <α< 0.7 the ΔEpis nearly independent of α • Ψ↑ as k0 ↑ or ν↓ then ΔEp↓ (6.5.5) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

23. 6.6 Multicomponent Systems and Multistep Charge Transfers O +ne-→R O’ +n’e-→R’ • For independent reactions For stepwise reduction O +n1e-→R1 R1 +n2e-→R2 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

24. Method for obtaining baseline for measurement of ip’ of second wave Peak Search Method of allowing current of first wave to decay before scanning second wave Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

25. Electrode reaction coupled with chemical reaction (Chapter 12) Er O +ne-⇌ R R ⇌ Z ErCr Ei 1. O +ne-⇌R 5. 2. O +ne-→R 3. 6. 4. 7. 8. Z ⇌ O O +ne-⇌R O +ne-⇌ R R → Z CrEr ErCi Z ⇌ O O +ne-→ R CrEi O +ne-⇌ R R+Z →O ErCi’ O +ne-→ R R+Z →O EiCi’ Catalytic Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

26. CVApplication –Diffusion control or Kinetic control • Diffusion control • Surface reaction control (6.2.19) For diffusion control ip∝ ν1/2 For surface reaction control ip∝ ν (14.3.12) Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

27. CVApplication – Surface reaction control • PEDOT/FAD Electrode(達人‘s work) Modified electrode – surface reaction control Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

28. CVApplication – Diffusion control Diffusion control Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

29. Homework • 1. Bard Ch6 Problem 6.6 • 2. Search a paper including the CV method (1) Introduce the materials briefly (2) Explain the graph ( ex: redox reaction, formal potential, reversible/irreversible……) TPTA and BP system Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

30. Reference 1. J. Bard and L. R. Faulkner, Electrochemical methods: fundamentals and applications, 2nd ed., John Wiley & Sons, Inc., New York (2001). 2. Applied Electrochemistry Notes (嘉筠、仲偉) 3. Handout of Ch 6 (瑋翰) 4.Joseph Wang, Analytical Electrochemistry, 3rd ed., John Wiley & Sons, Inc., (2006) 5. 達人學長’s work 6. M. Y. Yen et al., RSC Adv., 2012, 2, 2725-2728. Thanks for your attention! Electro-Optical Materials Lab., Dept. Chem. Eng., NTU