Basic Concepts of Electrochemistry

Size: px

Start display at page:

Download "Basic Concepts of Electrochemistry"

Dinah Ross
6 years ago
Views:

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

2 ELECTROCHEMICAL CELL ELECTROLYTIC CELL Power Source GALVANIC CELL A N O D E e - Red 1 Electrical Load Porous Diaphragm or Membrane e - Anions, X - Cations, M + Ox 2 Ox 1 Red 2 Electrolyte MX e - C A T H O D E

3 Anode: the site of oxidations Red1 Ox1 + n e (Positive electrode of an electrolytic cell negative electrode of a galvanic cell) Cathode: the site of reductions Ox 2 + n e Red 2 (Negative electrode of an electrolytic cell positive electrode of a galvanic cell)

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 PARAMETERS OF AN ELECTROCHEMICAL PROCESS Cell or electrode potential: Current or current density: E Ι or i=ι/α Concentration of electroactive species in the bulk (homogeneous) solution: time: C b t i=f(e, C b, t) or E=g(i, C b, t )

7 Cell potential : E cell = E C E A IR cell Electrode potential (cathode/anode): EC EA = (EC) eq = (EA ) eq + ηc ηa Equilibrium electrode potentials: (E (E C A ) ) eq eq = E = E 0 C 0 A + (RT / nf)ln[(c + (RT / nf)ln[(c Ox1 Ox2 ) s ) s /(C /(C Red1 ) Red2 Overpotentials of cathode and anode reactions: s ) s η η )] )] C A = f(i) = g(i)

8 GENERAL STEPS OF AN ELECTRODE PROCESS Mass transfer of 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 reactions in the bulk solution.

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

10 CURRENT DENSITY-ELECTRODE REACTION RATE i = 1 A dq dt = nf A dn dt reaction rate i = f(km,k where: k m = mass transfer coefficient = f(diffusion/flow rate and cell geometry ) k e = charge transfer coefficient = f(electrode reaction, electrode material, electrode potential) e,c) αnf E E 0 eq ke = ks exp( ) RT

11 CURRENT DENSITY-ELECTRODE REACTION RATE Multi-step reaction: the slowest step determines the rate of the overall reaction (rate determining step, rds) i = slow charge transfer + small overpotential k e pp km i = nfc bke fast charge transfer + high overpotential k e ff k m i = nfcb ke km nfc bk m (electrode) kinetic control mass transfer control

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

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

14 ELECTROCHEMICAL CELL AT EQUILIBRIUM Total current: i = 0 Equilibrium potential of cathode: RT [(C Ox ) s ] ( E 0 eq ) C = (E eq ) C + ln nf [(C Red ) s ] 2 2 (Nernst potential) Ox 2 +ne - Red 2 Equilibrium potential of anode : Ox 1 +ne - Red 1 RT [(C Ox ) s ] 1 ( E 0 eq ) A = (E eq ) A + ln nf [(C Red ) s ] (Nernst potential) Equilibrium potential of cell : ( E 0 0 eq ) cell = [(E eq ) C (E eq ) A] + RT nf Ox 2 + Red 1 Red 2 + Ox 1 [(C Ox ) s ] 1[(C Red ) s ] 2 ln [(C Red ) s ] [(C Ox ) s ] 1 1 2

15 ELECTROCHEMICAL CELL AT EQUILIBRIUM Free-Gibbs Energy (of the overall reaction Ox 2 + Red 1 Red 2 + Ox 1 in the electrochemical cell): ΔG = nf(e eq ) cell (E eq ) cell > 0 ΔG < 0 spontaneous process (galvanic cell) (E eq ) cell < 0 ΔG > 0 non spontaneous process (electrolytic cell)

16 KINETICS OF AN IRREVERSIBLE-SLOW ELECTRODE REACTION slow charge transfer: ke i = pp km nfcbke kinetically controlled current: s i = j r (A) j(c) αa n αf α η c n αf η i = i e RT i e RT 0 0 Butler-Volmer equation (η=ε-ε eq )

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

18 MASS TRANSFER EQUATIONS Mass flow: r dn Adt = DgradC ucgrad Ψ + r C υ diffusion ionic migration flow Concentration variation: dc = D 2 C dt u gradψ gradc r + υgradc

19 Linear semi-infinite diffusion to a planar electrode in a stationary solution Diffusion to a planar electrode Plane parallel to the electrode Elementary volume dc i = nf( ) x = 0 dx Flux Fick s 1 st Law Fick s 2 nd Law

20 Nernst diffusion layer model linear profile Cb i = nfd C x C (b) x 0(s) δ true profile k m = D δ i = nfkm (Cx (b) Cx 0(s) ) All mass transfer modes and corresponding concentration profiles can be replaced by linear diffusion through the stagnant layer of an equivalent linear profile.

21 Non-steady state and steady state mass transfer δ δ Bulk solution concentration, C b solution or gas increasing t, increasing δ, decreasing k m, decreasing i Distance from electrode, x Diffusion to a planar electrode from a stagnant solution δ and i variation with time non-steady state electrode rotation δ solution flow anode cathode solution thin layer cell electrode Microelectrode (<50 μm) Diffusion barrier of constant δ i constant with time steady state δ membrane solution insulator

Overview of electrochemistry

Overview of electrochemistry 1 Homogeneous Heterogeneous Equilibrium electrochemistry (no current flows) Thermodynamics of electrolyte solutions: electrolytic dissociation thermodynamics and activities