Chemical energy ⇔ electric energy

Electrochemistry content in this course:

Ox + e- ⇔ Red

Charged surfaces

⇔

What is electrochemistry about? Electrochemistry in practice

Electrode

Electronics, Physics

Electrolyte (liquid or solid)

Chemistry

• Metal, carbon• Semiconductor• Membrane

• Ions• Molecules

Materials properties Chemical information

Adsorbed moleculesReactions

Surface chargeCapacitance

Current/potential source

Electrochemistry at interfaces

Charge transportvia ions

Charge transport viaconduction electrons

Electrode Solution

φ

x0

0.1-10 nm

φM

φS

Field strength: ∆E/∆x ≈ 1 V/ 1 nm ≈≈ MV-GV/m !

∆E, ca

-2 ... +2 V

Potential drop at the interface

φM

φS

0

Adsorption influences propertiessuch as capacitance and potentialdrop over the interface, as well aselectrode reactions.

We can take advantage of this toobtain information about theadsorbed layer!

Electrode currents

electrode solution

Red

Ox + e-

Electrochemical reactionElectric double layer

electrode solution

--

--

-

+++++

e-

Faradaic current

Current flows across the interfaceat a rate determined by the electrode reaction.

Non-Faradaic current

Transient current as the chargingproperties at the interface are changed,charging current.

No charge transfer across the interface!

2.3 µA

An electrical circuit – An electrochemical cell

A

A+

B

B-

e-

+ -

Electrodes

+

Anode Cathode

-

e-e-e-

Potential E [Volt]

Current i [Ampere]

(e- per unit time)

Charge Q=It [Coulomb]

Ohm’s law R=U/i [Ohm]

Electric power P=UI [Watt]

Kirchoff’s law What happens at the electrodes?

Charge transfer

M E+

e-

Oxidation

M E

φ e-

Reduction-

φ

Electrode (M) Electrolyte (E)φ+

-Energy level (EF)

at the electrode electrons.

VacantMO

OccupiedMO

By varying theelectrode potential,reactions with theelectrolyte becomepossible. Zn

ZnSO4 (aq)

In practice:

[ZnSO4]=“low”

Zn2+

Zn2+

Zn2+Zn2+

Zn2+-- - -

-- --

--

Potentialdifference betweenthe electrode andthe solution!

Electricdoublelayer

Electrode potentialsConsider theZinc reactionin Volta’s pile:

( Zn → Zn2+ + 2e- )

Zn2+ + 2e- → Zn

Zn → Zn2+ + 2e-

Zn2+ + 2e- → Zn

V

-----

∆E!

Potentialdifferencerelativeto what?!

A galvanic cell!

The potential must be measured relative to

another half cell!

Galvanic cells Standard (reduction) potentials, EPotentials relative aStandard Hydrogen Electrode(’Normalvätgaselektrod’)SHE, NHE (N=normal)

A more practical reference electrodeThe silver/silver chloride electrode

(Ag/AgCl)

AgCl salt onthe silver wire

(Saturated KCl)

Galvanic cell

Zn + Cu2+ → Zn2+ + Cu

Zn | Zn2+ (a=1) | | Cu2+ (a=1) | Cu

anode cathode

Ecell = Ec – Ea = 0.34 – (– 0.76) = 1.1 V

Half cell reactions:Zn2+ + 2e- → Zn E0 = – 0.76 V

Cu2+ + 2e- → Cu E0 = 0.34 V

+-

Electrolytic cell

Zn2+ + Cu → Zn + Cu2+

Cu | Cu2+ (a=1) | | Zn2+ (a=1) | Zn

anode cathode

Ecell = Ek – Ea = – 0.76 – 0.34 = – 1.1 V

Eapp = 1.1 V equilibrium!

Eapp > 1.1 V electrolysis!

+ -

Spontaneous reaction or not?

If the reaction is spontaneous,It is a galvanic cell (’battery’).

Cu

Cu2+

Cu2+

Cu2+Cu2+

Cu2+

Instru-

ment

Reference electrodeWorking electrode

-- - -

-- ----

Potential control Mass transport to the electrode - Diffusion[Ox] = 1 mM Initially: No reaction,

‘homogeneous’ concentration

Concentration gradient at the surface diffusion layer where [Ox] = 0 mM.

Electrode

e- Reduction: Ox + e- → RedDecreasing concentration at the surface!

e- Mass

transport

Large currents rapidly depletes [Ox], misrepresentating the potential ! Separate the reference function from the counter electrode function:

drive the current through a 3rd electrode!

Amperometry and Voltammetry• The working electrode potential

is varied to control

electrochemical reactions

at the electrode surface.

• The current is proportional to the

concentration of the analyte.

• Different substances are

oxidized/reduced at different

potentials.

• Inert working electrode (Pt, Au, C).

• Flow cells are common.

A

A+

B

B-

2.3 µA

e-

+ -

e-

Working electrode Auxiliary orcounterelectrode

Referenceelectrode

Potentiometry

The working electrode (indicator electrode) potential is measured using a reference electrode.

Ecell = Eind - Eref

Eind ∝ konc. according to Nernst’s

equation

High impedance volt meter, i ≈ 0

Ion selective electrode

Cu

Cu2+

Cu2+

Cu2+Cu2+

Cu2+

[Cu2+]=x

ErefEind

-- - -

-- ----

Ecell

Boundary capacitance

d

AC oεε=

Electronics:

+

-

dε

++ + ++ +

- -- - --

+

-- -- - --

+ + + + + +

Electrochemistry:

Electricaldouble layer

on clean gold:

~25 µF*cm-2

+

-- -- - --

+ + + +

d and εare changed

by a monolayer:

0.5-3 µF*cm-2

Q = C * E

ions

Capacitance [F]

Electrochemical surface analysis

e-

Red Ox

Red Ox

Cyclic Voltammetry

Reaction atdefects!

e-

i/µA*cm-2

pure Au

HS-C15H30-COOH

+

-

-60

-40

-20

0

20

40

60

-0.4 -0.2 0 0.2 0.4 0.6

E/V vs. Ag/AgCl

For these purposes we are not interested in the red-ox couple itself; We use any suitable redox pair and just look at the variation in reaction rates!