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Transcript

CH5715 Energy Conversion and Storage http://jtsigroup.wp.st-andrews.ac.uk/ch5715-energyconversion-and-storage/ Double layer illustration http://www.doitpoms.ac.uk/tlplib/aqueous_corrosion/double_layer.php 3 electrode setup (1) working electrode; (2) auxiliary electrode; (3) reference electrode 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 +0.244 V vs. SHE at 25 °C St Andrews geometry Impedance Spectroscopy Introduction to EIS and process speciation 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 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 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 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 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 Questions Electrode Processes