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

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Electrochemical Camp 2012

Chapter 6Potential Sweep Method

Speaker : Yu-Yan LiAdvisor : Kuo-Chuan Ho

Aug, 6th, 2012

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

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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


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6.1 Introduction•What is Linear Sweep voltammetry (LSV)?


Cottrell eq. (i-t)

Voltage ramp

Limiting Current Plateau (i-E)

Linear sweep voltammetry (LSV)

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Linear Sweep Method (LSV)

•Why there is a peak?


Surface concentration

E→ Ep+

Co(0,t) → 0

E→ Ep-

Depletion effect results in small i

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Linear Sweep Method (LSV)


Reduction begins and current starts to flow

A +e- →A‧

Mass transfer of A reaches maximum rate

Nonfaradaic current flowCo approaches to zero and

diffuse layer grows

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Cyclic Voltammetry (CV)


Reverse the potential scan Reduction

Oxidation

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6.2 Nernstian (Reversible) Systems

•Scanning potential•Planar electrode


(5.4.2)(5.4.3)(5.4.4)(5.4.5)(5.4.6)

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Reversible Systems

•Time-dependent form

•Laplace transform → Convolution theorem


(6.2.2)

(6.2.3)

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Reversible Systems

•The current


Find the maximum

(6.2.17)

(6.2.16)

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Reversible Systems

•Peak current (at 25 )℃

•Peak potential

•Half-peak potential


ipA

A cm2

Co* mol/cm3

Docm2/sec

ν V/sec

(6.2.19)

(6.2.20)

(6.2.21)

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Reversible Systems


(6.2.22)

ip ∝ ν1/2

i ∝ ν1/2

Ep ≠Ep(ν)

For reversible rxn

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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

Chargingcurrent

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Detection limit of LSV•Capacitance of double layer


For DME

noise

signal

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Detection limit of LSV•Effect of double layer charging at different sweep

rate


ν ↑ → ic /ip ↑Co* ↓ → ic /ip ↑The noise grows!

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6.3 Totally Irreversible Systems

• Irreversible reaction•Boundary condition

•The current


(6.3.1)

(6.3.6)

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Totally Irreversible Systems

•Peak current

•Peak potential


ip ∝ ν1/2

i ∝ ν1/2

Ep =Ep(ν)

For totally irrev.

(6.3.8)

(6.3.10)

(6.3.11)

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6.4 Quasi-reversible Systems

•Boundary condition

•Parameter Λ


Do=DR=D

Λ ↑ , easy to reach equilibrium ( k0 ↑)

(6.4.4&5)

(6.4.2)

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Quasi-reversible Systems

•Peak current

•Peak potential


For quasi-rev

ip ∝ ν1/2 (6.4.7)

(6.4.7)(6.4.8)

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Summary

•Zone boundary for LSV


Reversible (Nernstian)Λ ≥ 15

k0 ≥0.3 ν1/2 cm/s

Quasi-reversible15 ≥ Λ ≥ 10-2(1+ )𝝰

0.3 ν1/2≥ k0 ≥2x10-5ν1/2 cm/s

Irreversible10-2(1+ ) 𝝰 ≥ Λ

k0 ≤ 2x10-5ν1/2 cm/s

Λ = Λ(ν)

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6.5 Cyclic Voltammetry

•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


(6.4.1)(6.4.2)

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Reversible System

•Find• ip,a & ip,c

• If no ip,a / ip,c


(6.5.4)

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Quasi-reversible System

•Wave shape & ΔEp f(∝ ν, α,k0,Eλ)

•Equivalent parameter

•For 0.3 <α< 0.7 the ΔEp is nearly

independent of α

•Ψ↑ as k0 ↑ or ν ↓ then ΔEp ↓


(6.5.5)

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6.6 Multicomponent Systems and Multistep Charge Transfers

•For independent reactions


O +ne- →RO’ +n’e- →R’

O +n1e- →R1

R1 +n2e- →R2

For stepwise reduction

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Peak Search


Method for obtaining baseline for measurement of ip’ of second wave

Method of allowing current of first wave to decay before scanning second wave

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Electrode reaction coupled with chemical reaction (Chapter 12)

1. O +ne- R 5. ⇌2. O +ne- →R3. 6. 4. 7. 8.

O +ne- R⇌R Z⇌

O +ne- R⇌R → Z

O +ne- R⇌R +Z →O

O +ne- → RR +Z →O

Z O ⇌O +ne- R⇌

Z O ⇌O +ne- → R

Er

Ei

CrEr

CrEi

ErCr

ErCi

ErCi’

EiCi’

Electro-Optical Materials Lab., Dept. Chem. Eng., NTUCatalytic

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CV Application –Diffusion control or Kinetic control

•Diffusion control

•Surface reaction control


(6.2.19)

(14.3.12)

For diffusion control ip ∝ ν1/2

For surface reaction controlip ∝ ν

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CV Application – Surface reaction control

•PEDOT/FAD Electrode (達人‘ s work)


0 40 80 120 160 200-300

-200

-100

0

100

200

300

Ipa

Y=1.15X, R2= 0.998 Ipc

Y=-1.34X, R2= -0.998

Curr

ent den

sity

(A

cm

-2)

Scan rate (mV s-1)-0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2

-0.5

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

3.125 to 200 mV s-1

3.125 to 200 mV s-1

Cur

rent

den

sity

(m

Acm

-2)

E (V vs. Ag/AgCl/sat'd KCl)

Modified electrode – surface reaction control

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CV Application – Diffusion control


Diffusion control

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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


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Reference1. 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!

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

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