Electrochemistry: Electricity-Driven Chemistry

1. ELECTROCHEMISTRY • Electricity-driven Chemistry • or • Chemistry-driven Electricity • Electricity:charge flow • (electrons,holes,ions) • Chemistry (redox):reduction = electron uptake • oxidation = electron loss

2. ELECTROCHEMICAL CELL ELECTROLYTIC CELL  Power Source GALVANIC CELL Electrical Load  e-  C A T H O D E Porous Diaphragm or Membrane A N O D E Red1 Ox2 e- e- Anions, X- Cations, M+ Ox1 Red2 Electrolyte MX

3. Anode:the site of oxidations • (Positiveelectrodeof an electrolyticcell • negativeelectrode of agalvaniccell) • Cathode:the site of reductions • (Negativeelectrodeof an electrolyticcell • positiveelectrode of agalvaniccell)

4. ELECTROCHEMICAL APPLICATIONS • (Conversion of Chemical to Electrical Energy) • Batteries • (for electronic devices, automotion etc) • Fuel Cells • (for automotion, power stations etc) • Electroanalysis • (potentiometric electronalytical techniques and sensors, e.g. • ion selective electrodes, gas sensors etc)

5. ELECTROCHEMICAL APPLICATIONS • (Conversion of Electrical to Chemical Energy) • Electrolysis • (e.g. chloralkali industry, hydrogen production) • Electrosynthesis • (e.g. adiponitrile production Νylon 66) • Electroplating and Metal Processing • (e.g. decorative metal plating, elctrochemical machining) • Cathodic corrosion protection of metals and metal composites • (e.g. bridge and ship protection) • Waste treatment • (e.g. metal ion removal and recovery, organics oxidation etc) • Electroanalysis and Elecrochemical Sensors • (e.g. determination of heavy metals, organic contaminants • and biological compounds; glucose, oxygen, ethanol sensors)

6. Cell or electrode potential: E • Current or current density: Ι ori=Ι/Α • Concentration of electroactive species • in the bulk (homogeneous) solution: Cb • time: t PARAMETERS OF AN ELECTROCHEMICAL PROCESS i=f(E, Cb, t) orE=g(i, Cb, t )

7. Cell potential: • Electrode potential (cathode/anode): • Equilibrium electrode potentials: • Overpotentials of cathode and anode reactions:

8. GENERAL STEPS OF AN ELECTRODE PROCESS • Mass transferof reactants/products to/from the electrode. • Surface reactions(e.g. adsorption, phase transitions etc). • Charge transfer(heterogeneous electron or hole exchange)at the electrode surface. • Homogeneous chemical reactionsin the bulk solution.

9. ELECTRODE e- Red (bulk) Red (surf) Mass transfer T r a n s f e r C h a r g e BULK SOLUTION Mass transfer Ox (surf) Ox (bulk) ELEMENTARY STEPS OF AN ELECTRODE PROCESS

10. where: • km = mass transfer coefficient • = f(diffusion/flow rateand cell geometry ) • ke = charge transfer coefficient • = f(electrode reaction, electrode material, electrode potential) CURRENT DENSITY-ELECTRODE REACTION RATE

11. slow charge transfer+small overpotential •  (electrode) kinetic control • fast charge transfer+high overpotential  mass transfer control  CURRENT DENSITY-ELECTRODE REACTION RATE Multi-step reaction:the slowest step determines the rate of the overall reaction (rate determining step, rds)

12. Mass transfer control iL=f(km) independent of E Limiting current C u r r e n t D e n s i t y Mass transfer control Mass transfer control Steady state i Non-steady state kinetic control Electrode potential, E CURRENT-POTENTIAL CURVES

13. cathodic anodic Cathodic process:j<0, E<Eeq Equilibrium:j=0, E=Eeq Anodic process:j>0, E>Eeq THERMODYNAMICS AND KINETICS OF ELECTRODE REACTIONS

14. Total current: • Equilibrium potential of cathode: Ox2 + ne- Red2 (Nernst potential) • Equilibrium potential of anode: Ox1 + ne- Red1 (Nernst potential) Ox2 + Red1  Red2 + Ox1 • Equilibrium potential of cell: ELECTROCHEMICAL CELL AT EQUILIBRIUM

15. Free-Gibbs Energy (of the overall reactionOx2 + Red1  Red2 + Ox1 in the electrochemical cell): ELECTROCHEMICAL CELL AT EQUILIBRIUM • (Eeq)cell > 0 ΔG < 0 •  spontaneous process (galvanic cell) • (Eeq)cell < 0 ΔG > 0 •  non spontaneous process (electrolytic cell)

16. slow charge transfer: • kinetically controlled current: (η=Ε-Εeq) Butler-Volmer equation KINETICS OF AN IRREVERSIBLE-SLOW ELECTRODE REACTION

17. TRANSFER PHENOMENA IN ELECTROCHEMICAL PROCESSES Mass transfer (Heterogeneous) Charge transfer Heat transfer Convection Molecular diffusion Ion migration Forced Convection Natural Convection Convective diffusion

18. Mass flow: ionic migration diffusion flow • Concentration variation: MASS TRANSFER EQUATIONS

19. Diffusion to a planar electrode Plane parallel to the electrode Elementary volume Flux Fick’s 2nd Law Fick’s 1st Law Linear semi-infinite diffusion to a planar electrode in a stationary solution

20. linear profile Cb true profile Nernst diffusion layer model All mass transfer modes and corresponding concentration profiles can bereplaced by linear diffusion through the stagnant layer of an equivalent linear profile.

21. δ δ Bulk solution concentration, Cb solution or gas electrode electrode rotation membrane solutionflow increasing t, increasing δ, decreasing km, decreasing i δ solution δ cathode anode Distance fromelectrode , x solution Microelectrode (<50 μm) insulator thin layer cell Diffusion to a planar electrode from a stagnant solution δand ivariation with time  non-steady state Diffusion barrier of constantδ  i constant with time  steady state Non-steady state and steady state mass transfer