CH5715 Energy Conversion and Storage http://jtsigroup.wp.st-andrews.ac.uk/ch5715-energy-conversion-and-storage/

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

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

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

A typical electrochemical measurement

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.

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.

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)

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.

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

Non-Polarisable Interfaces Exhibit charge transfer Divide electrodes into 3 types: Redox 1st 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

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+

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:

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)

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 - http://commons.wikimedia.org/wiki/File:Butler_volmer_equation_graph_nl.png#mediaviewer/File:Butler_volmer_equation_graph_nl.png

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

Leads to Tafel Plot See also re corrosion but generally useful materials resources http://www.doitpoms.ac.uk/tlplib/aqueous_corrosion/tafel_plot.php