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

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 2 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU

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

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

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

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

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

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

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

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

Reversible Systems 11 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU (6.2.22) i p ∝ ν 1/2 i ∝ ν 1/2 E p ≠E p (ν) i p ∝ ν 1/2 i ∝ ν 1/2 E p ≠E p (ν) For reversible rxn

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

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

Detection limit of LSV Effect of double layer charging at different sweep rate 14 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU ν ↑ → i c /i p ↑ C o * ↓ → i c /i p ↑ The noise grows! ν ↑ → i c /i p ↑ C o * ↓ → i c /i p ↑ The noise grows!

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

Totally Irreversible Systems Peak current Peak potential 16 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU i p ∝ ν 1/2 i ∝ ν 1/2 E p =E p (ν) i p ∝ ν 1/2 i ∝ ν 1/2 E p =E p (ν) For totally irrev. (6.3.8) (6.3.10) (6.3.11)

6.4 Quasi-reversible Systems Boundary condition Parameter Λ 17 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU D o =D R =D Λ ↑, easy to reach equilibrium ( k 0 ↑) (6.4.4&5) (6.4.2)

Quasi-reversible Systems Peak current Peak potential 18 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU For quasi-rev i p ∝ ν 1/2 (6.4.7) (6.4.8)

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

6.5 Cyclic Voltammetry Scanning potential Two parameters 1. E p,a – E p,c 2. i p,a / i p,c ΔEp=| E p,c – E p,a | Formal potential approaching to E 0’ = (E p,c + E p,a )/2 20 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU (6.4.1) (6.4.2)

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

Quasi-reversible System Wave shape & ΔE p ∝ f(ν, α,k 0,E λ ) Equivalent parameter For 0.3 <α< 0.7 the ΔE p is nearly independent of α Ψ↑ as k 0 ↑ or ν ↓ then ΔE p ↓ 22 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU (6.5.5)

6.6 Multicomponent Systems and Multistep Charge Transfers For independent reactions 23 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU O +ne - →R O’ +n’e - →R’ O +n 1 e - →R 1 R 1 +n 2 e - →R 2 For stepwise reduction

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

Electrode reaction coupled with chemical reaction (Chapter 12) 1. O +ne - ⇌ R O +ne - →R O +ne - ⇌ R R ⇌ Z O +ne - ⇌ R R → Z O +ne - ⇌ R R +Z →O O +ne - → R R +Z →O Z ⇌ O O +ne - ⇌ R Z ⇌ O O +ne - → R ErEr EiEi CrErCrEr CrEiCrEi ErCrErCr ErCiErCi ErCi’ErCi’ EiCi’EiCi’ Electro-Optical Materials Lab., Dept. Chem. Eng., NTU Catalytic

CV Application –Diffusion control or Kinetic control Diffusion control Surface reaction control 26 Electro-Optical Materials Lab., Dept. Chem. Eng., NTU (6.2.19) ( ) For diffusion control i p ∝ ν 1/2 For surface reaction control i p ∝ ν For diffusion control i p ∝ ν 1/2 For surface reaction control i p ∝ ν

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

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

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

Reference 1. J. Bard and L. R. Faulkner, Electrochemical methods: fundamentals and applications, 2 nd 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, 3 rd ed., John Wiley & Sons, Inc., (2006) 5. 達人學長 ’s work 6. M. Y. Yen et al., RSC Adv., 2012, 2, Electro-Optical Materials Lab., Dept. Chem. Eng., NTU Thanks for your attention!