1. CH5715 Energy Conversion and Storage

3. Impedance Spectroscopy
Electrode Reactions
Charge Transfer
Impedance Spectroscopy
Texts
Solid State Electrochemistry, Cambridge - P. G. Bruce
A First Course in Electrode Processes
Electrochemical Consultancy - D Pletcher
Electrochemical Methods
Bard and Faulkner
Electrocatalysis chapter
Irvine, proofs in notes

4. Electrochemical characterisation
Three variables Current, Time, Voltage
A range of variants give rise to the measurement techniques
Voltammetry - Current voltage (different scan rates)
Chronopotentiometry - fix current Potential versus time
Impedance AC V/I measured at different frequencies etc

5. Types of voltammetry Linear sweep voltammetry Staircase voltammetry
Squarewave voltammetry
Cyclic voltammetry - A voltammetric method that can be used to determine diffusion coefficients and half cell reduction potentials.
Anodic stripping voltammetry - A quantitative, analytical method for trace analysis of metal cations.
Cathodic stripping voltammetry .
Adsorptive stripping voltammetry –
Alternating current voltammetry
Polarography - working electrode is a dropping mercury electrode (DME),
Rotating electrode voltammetry Normal pulse voltammetry
Differential pulse voltammetry
Chronoamperometry

6. A typical electrochemical measurement

7. an example LSCM/GDC composite cathode Xiangling Yue
0.8
0.2
1.0V
0.5
I-V curves of the cell with LSCM/GDC cathode for CO2 electrolysis and for H2O-CO2 co-electrolysis
Impedance responses of LSCM/GDC cathode at different voltages at 900oC for H2O-CO2 co-electrolysis
Linear sweep voltammogram Impedance plots at potential H2O-CO2 co-electrolysis, 20% H2O in 5% H2/Ar at 10ml/min plus CO2/CO 70/30 at 20ml/min were used as feed gas.

8. Electrochemical Cells and Reactions
There are two types of current flow:
1. Faradaic – charge transferred across the electrified interface as a result of an electrochemical reaction.
2. Non-faradaic – charge associated with movement of
electrolyte ions, reorientation of solvent dipoles,
adsorption/desorption, etc. at the electrode-electrolyte interface. This is the background current in voltammetric measurements.

9. Ideally polarizable electrode (IPE) – no charge transfer across the interface. Ions move in and out of the interfacial region in response to potential changes. The interface behaves as a capacitor (charge storage device).
Excess electrons on one
plate and a deficiency on
the other.
Changing the potential, E, causes the charge stored, Q, to change according to the relationship:
Q(Coulombs) = C(Farads) x E(Volts)

12. The solution side of the interface consists of a compact layer (inner and outer Helmholtz layers) plus a diffuse layer.
Diffuse layer extends from the OHP to the bulk solution. Ionic distribution influenced by ordering due to coulombic forces and disorder caused by random thermal motion.

13. Bockris/Devantha/ Müller Model
Schematic representation of a double layer on an electrode
1. Inner Helmholtz plane, (IHP), 2. Outer Helmholtz plane (OHP), 3. Diffuse layer, 4. Solvated ions (cations) 5. Specifically adsorbed ions (redox ion, which contributes to the pseudocapacitance), 6. Molecules of the electrolyte solvent

14. Non-Polarisable Interfaces
Exhibit charge transfer
Divide electrodes into 3 types:
Redox st Kind 2nd Kind
Fe3+/2+ at Pt electrode Ag+/Ag AgCl, Cl /Ag+
e transfer e + ion transfer e + ion transfer
Redox Electrodes, Fe3+/2+
The electrode reaction involves e tunnelling between the ions in
solution within 30Å from the electrode, and with the electrode
itself.
We will consider Fe2+/3+, i.e. a one electron transfer

17. Introduce Pt electrode into aqueous solution of Fe(NO3)2 e- transfer very fast 10-16s - solution vib. and rot. frozen during e- transfer. Tunnelling only occurs between levels of equal energy on the metal and the ion. If only Fe2+ in solution to begin with, then at equilibrium e- transfer very slow. few levels on metal for Fe2+ - Fe3+ + e- few Fe3+ ions
Fe3+

18. Position of e- energy level in metal determined by ions in solution
Position of e- energy level in metal determined by ions in solution. Increase [Fe3+]/[Fe2+] pushes e-level down i.e. more +ve potentials. Compare with Nernst equation:

19. The Electrochemical Series
Position of e (and hence voltage) depends on ions in solution.
When you have equal concentrations of reduced and oxidised forms
then position of electron energy level in metal depends on ion type i.e. redox couple
Co has higher effective nuclear charge than Fe, therefore e on Co ion which transfers to/from metal lower in energy than e on Fe
lower Fermi level (more positive potential EO)

21. Butler Volmer equation -
No diffusion limitations Charge Transfer
"Butler volmer equation graph nl" by Jakobswiki - Own work. Licensed under CC BY 3.0 via Wikimedia Commons -

22. The Butler–Volmer equation describes how the electrical current on an electrode depends on the electrode potential.

23. Leads to Tafel Plot
See also re corrosion but generally useful materials resources