1. CH5715 Energy Conversion and Storage

2. Double layer illustration

3. 3 electrode setup
(1) working electrode; (2) auxiliary electrode; (3) reference electrode

4. A reference electrode is an electrode which has a stable electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each of the participants of the redox reaction. Eg saturated calomel electrode V vs. SHE at 25 °C

5. St Andrews
geometry

7. Impedance Spectroscopy
Introduction to EIS and process speciation

8. Introduction
Why use a spectroscopic technique for electrical characterisation?
dc measurements give "total" or "composite" response:
Grain boundaries
Electrode-sample interface
Film-substrate interface
Defect/compositional gradients
Space charge effects
Charge transfer
diffusion
Impedance spectroscopy allows separation of different electroactive regions
i.e. Electrical Microstructure deconvolution of bulk behaviour from other effects

10. Basics
V =V0 sin ωt
I = lo sin(ωt +Φ)
The impedance is given by Z =V/I
Impedance is a vector quantity and can be defined using complex numbers
Z* = Z’ – jZ”
Complex real imaginary
Note that this defines Z" as a positive value for a
capacitive reactance
Z' + jZ" also often used

11. No phase difference between current (lt) and voltage (Vt):
Circuit Elements
Impedance, Z (essentially ac version of Ohm’s law):
Resistor, R:
No phase difference between current (lt) and voltage (Vt):
Capacitor, C:
Current (lt) leads voltage (Vt) by 90o
Inductor, L:
Current (lt) lags voltage (Vt) by 90o

14. What is Impedance Spectroscopy?
Apply an ac (sinusoidal) voltage of varying frequency:
(typically in the range 10-2 to 107 Hz)
Measure the current response
Determine IZI(ω) and Ɵ(ω),
or Z*(ω) = Z’(ω)-jZ"(ω):
Z and Ɵ (and Z', Z") vary with frequency
- varying R, C (and L) contributions
Separation in frequency domain of physical processes
giving rise to each type of behavior
electrical "composition" or microstructure

15. Physical origin of circuit elements
Capacitive: recoverable displacement of charge
elastic process
Dielectric constant (permittivity, e’) Polarisability/ polarsisation processes
Resistive: transfer of charge, energy loss inelastic process
Conductivity, dc or ac (semi-, conductors) (long range migration of charge (electrons/ ions)
Inductance : storage of energy in magnetic field generated by current – mainly conductors
tan δ (dielectrics/ insulators), ac only
ε”/ ε’ (dissipation, transient leakage)
Materials response to an ac field depends on frequency
Response can be represented by a combination of various R. C and L => Equivalent circuit

27. Questions

28. Electrode Processes