!? THINGS THAT WE ARE FAMILIAR WITH : !? Ohm’s law ( and Kirchoff’s…) (ABC... electrical circuits) U

1. !? THINGS THAT WE ARE FAMILIAR WITH : !? Ohm’s law ( and Kirchoff’s…) (ABC... electrical circuits)   U = I  R , R = L / S Faraday’s law (ABC... electrolysis)   m = k  Q , k = M / n  F ( F = ?) Fick’s laws (ABC... diffusion)   J =  D  dC/dx C/t = D 2C/x2 Electrical properties of condensed phases – conducting electrical current (metals , semiconductors) Ionic compounds , properties of solutions, ionic conductivity Redox reactions ( np. 2Cr3+ +3 H2O2 + 10 OH- = 2 CrO4- + 8H2O ) Phase boundary electrolyte - electrode  ELECTROCHEMISTRY INTRO

2. Me+ transport Me Me+ electrode electrolyte Charge transfer Oxidation – reduction reaction rate Diffusion transport rate ELECTROCHEMISTRY INTRO

3. Eachstagecandeterminetheoverallreactionrate 1. „obligatory” stages charge transfer Transport (diffusion, convection , migration) 2. otherpossiblestages Chemical reactionbeforeorafter (c t) Crystallisation of newphases Adsorptionattheelectrode ELECTROCHEMISTRY INTRO

4. = Reaction rate v = ∆ cA / ∆t , ( or v = kA-B·cA ) ( cA - volumetric concentration – we must do something about it) In electrode kinetics the transferred charge is a measure of reaction rate Following Faraday’s law: mA = kF ·I·∆t = k ·Q (here k – electrochem equivalent, not reaction rate constant) And back to general reaction rate formula mA =cA·V or cA surface·S = kF·I·∆t v = k·I·/ S v ( mol·s-1m) = kF (mol/C)·j , j – current density (A/m2) ELECTROCHEMISTRY INTRO

5. CURRENT DENSITY = MEASURE OF ELECTRODE PROCESS RATE And whatmakesthereactionhappenatall?? Equilibrium – no products ( isanything happening?) Deviationfromequilibrium - energy impuls needed Reaction – transformation to newequilibrium state Whatmight be an energy impuls? ELECTROCHEMISTRY INTRO

6. energy state of a particle – chemical potential • μi= μo + RT ( ai) • Charged particle - electrochemical potential , possible responce to electrical field • φ= φo + RT/nF ln ( ai(n+) ) • Equilibrium - equal potentials of a particle in two phases (electrode –electrolyte) • E = E0 + RT/nF ln ( aelectrode / aelectrolyte ) Changeinconcentration, temperature ENERGY IMPULS Overpotential applied to theelectrode ELECTROCHEMISTRY INTRO

7. Atequilibrium Redoxtransitions on molecularscale Identicaloverall charge for oxidation and reduction jk = ja, overallcurrentdensityjk - ja = 0 Atoverpotential∆E j = jk - ja ≠ 0 as measure for reactionrate, so j/nF = v = krr × C ELECTROCHEMISTRY INTRO

8. Reactionrateconstant - overpotential: krr = ksexp[ α n F ΔE / RT] (one equilibrium – twoconstants: anodic and cathodic) Combining v = .. And k = …. (To getcurrent-overpotentialdependence) i = nF S ks[ cutlexp(-αn F ΔE / RT ) – cred (βn F ΔE / RT)] where ks- standard reactionrateconstant α i βcoefficients for energy barriersymmetry ΔE overpotential ELECTROCHEMISTRY INTRO

9. Electrode process – heterogenous, charge transfer at phase boundary + transport Electrode = element of electrical circuit Measurement = two electrodes form a cell Circuit – measurable : voltage and current Difference in V/I response for a.c and d.c. ELECTROCHEMISTRY INTRO

10. Transport properties • Structure of electrolytes, dissociation • Movement of ionic species • Mobility, velocity of part i vi = E·ui • Conductivity  = e·Ni·zi·ui • Transference number ELECTROCHEMISTRY INTRO

11. Cell voltage or electrode potential • Equilibrium at the electrode – Nernst pot. • Overpotential – driving force for the reaction • Current – electrical measure of reaction rate • Voltage – measure of potential difference ! • For kinetics – we must know the potential of a single electrode! ELECTROCHEMISTRY INTRO

12. 3 –electrode cell • 3-electrode cells : WE and CE - „working circuit” • reactions at electrodes , current flow • WE- RE - measuring circuit , high input impedance • on RE no current flow • Reference electrodes : very precise potential, • examples : Hg/ Hg2Cl2 , Ag/ AgCl , • Quasi-reference : W, Ta, other non-reactive metals • idea : stable potential, easy assembly, • Function current-potential – diffusion and kinetics in the • cell must be described electroanalysis ELECTROCHEMISTRY INTRO

13. I RE CE E Ez Potentiostat Cell Electrolytic cell and potentiostat Electrodes: Meas. Parameters: CE - counter E - WE potential RE - reference Ez - applied potential WE - working I - current in CE-WE circuit WE ELECTROCHEMISTRY INTRO

14. EIS • Electrotechnical aspect : a.c.circuit • Electrochemical aspect : approximation of electrode process with circuit elements Charge transfer Conductivity Resistance of layer Resistances R Z = R ELECTROCHEMISTRY INTRO

15. Double layer capacity Capacity of layers Capacities C Z = -j/C Constant phase element Admittance Y = Yo (j)n for n=0 resistance For n=1 capacity Diffusion phenomena Roughness of surface Inhomogenity of layer Corrosion processes (many reactions and equilibria) Inductance L Z = j L ELECTROCHEMISTRY INTRO

16. Equivalent circuits • Electrical model of electrode • Connections in series and parallel – interpretation of consecutive or simultaneous reactions / phenomena • Physical sense vs. numerical possibilities ELECTROCHEMISTRY INTRO

17. Our lab sessions EIS – dr Regina Borkowska ( 5h basic electrode kinetics) Voltammetry – dr Regina Borkowska 5h Conducting polymers dr M. Siekierski 5h Batteries – dr Marek Marcinek (5h basic cells + 5h Li- cells) Transference numbers – Msc Michał Piszcz(5h diffusion coefficient + 5h transference numbers in Li systems) Ion associations – Dr Leszek Niedzicki (5h Fuoss-Kraus formalism – electrochemical approach) Corrosion dr Andrzej Królikowski 5h Instructions and auxillary materials: download from http://pirg.ch.pw.edu.pl/ ELECTROCHEMISTRY INTRO