What is electrochemistry about? Electrochemistry in practice · A galvanic cell! The potential must...
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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 molecules Reactions Surface charge Capacitance Current/ potential source Electrochemistry at interfaces Charge transport via ions Charge transport via conduction electrons Electrode Solution φ x 0 0.1-10 nm φ M φ S Field strength: ∆E/∆x ≈ 1 V/ 1 nm ≈ ≈ MV-GV/m ! ∆E, ca -2 ... +2 V Potential drop at the interface φ M φ S 0 Adsorption influences properties such as capacitance and potential drop over the interface, as well as electrode reactions. We can take advantage of this to obtain information about the adsorbed layer!
Transcript of What is electrochemistry about? Electrochemistry in practice · A galvanic cell! The potential must...
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!
