Download Cells and Batteries File

Cells and Batteries
Engineering Chemistry
CHM 406
Common voltaic cells








Zinc – carbon dry cell (Leclanché cell)
Alkaline dry cell
Lithium – iodine cell
Lead storage cell (car battery)
Nickel – cadmium battery
Nickel – metal hydride battery
Lithium – ion battery
Fuel cell
Design and components

Half reactions
◦ Electrodes
◦ Oxidants and reductants
◦ Electrolytes

Purpose: voltage, current, and duration
required.
◦
◦
◦
◦
Cell design
Internal resistance: aqueous solution or paste
Interface between half cells
Size and shape
Zinc – carbon dry cell
Zinc – carbon dry cell (contd.)

Anode: Zn(s) | ZnCl2, NH4Cl aq. paste
Zn → Zn2+ + 2 e-

Cathode: C (graphite) | MnO2(s), C powder,
NH4Cl
2 NH4+ + 2 MnO2 + 2 e→ Mn2O3 + H2O + 2 NH3

Ecell = 1.5 V, quickly reduced as reaction
proceeds or in cold weather, not reversible in
practice.
Alkaline cell
Same electrodes as the Leclanché cell, but
with conc. aq. KOH as the electrolyte instead
of NH4Cl.
 Anode: Zn(s) + 2 OH- → Zn(OH)2(s) + 2 e

Cathode: MnO2(s) + H2O + e→ MnO(OH) (s) + OH-
Longer lasting, less temperature sensitive.
 Ecell = 1.5 V

Lithium – iodine cell






Anode: Li → Li+ + eCathode: I2 + 2 e- → 2 IElectrolyte is solid crystalline LiI; allows
slow migration of Li+ ions from anode to
cathode.
High internal resistance, very low current,
stable voltage, long lasting ( 8-10 years).
Used for medical devices such as
pacemakers.
Ecell = 2.8 V.
Nickel cadmium (nicad) cell
Similar to alkaline cell, but reversible; can
be recharged.
 Anode: Cd(s) + 2 OH- → Cd(OH)2(s) + 2 e

Cathode: NiO(OH)(s) + H2O + e→ Ni(OH)2(s) + OH-
Ecell = 1.3 V
 Recharging reverses above reactions.
 Disadvantages: Cd is heavy (low charge
density) and toxic.

Nickel – metal hydride cell
Also rechargeable.
 Anode: OH- + “MH” → H2O + “M” + e
M is a metal alloy capable of absorbing H atoms,
and MH is the metal - hydrogen complex.
Cathode: same as in a nicad battery.
 Ecell = 1.2 V
 Charge density much higher than for nicad
batteries; also less environmental pollution
as no Cd is used.

Lead – acid storage battery
Lead – acid storage battery (contd.)

Anode: Pb(s) | 65% aq. H2SO4
Pb(s) + HSO4- → PbSO4(s) + H+ + 2 e-

Cathode: Pb(s) | PbO2(s) | 65% aq. H2SO4
PbO2(s) + HSO4- + 3 H+ + 2 e→ PbSO4(s) + 2 H2O

Recharging reverses these reactions.
 Ecell = 2 V. The battery contains 6 cells
connected in series, hence 12 V.
Lithium – ion battery


These are the rechargeable batteries commonly
found in consumer electronic devices (cell phones,
laptops, etc.); now also used in vehicles such as
hybrid cars.
Anode: Li(s) intercalated with C(graphite) | Li+
cations in non-aqueous (organic) solvent
Li(s) (C) → Li+ + C + e-

Cathode: CoO2(s) | Li+ cations in non-aqueous
(organic) solvent
Li+ + CoO2 + e- → LiCoO2

Li cations migrate between the two
electrodes. Ecell = 3.6 V
Fuel cell
Fuel cell (contd.)



Reaction of a fuel, e.g., H2, with atmospheric O2,
separated into half cells.
Chemical energy is directly converted into electricity
– high efficiency (40 – 60% but up to 85% in some
cases).
Catalysts are required at both anode and cathode.
◦ Anode: Pt, Pt/Ru mixtures (expensive)
◦ Cathode: Ni can be used.

Operates at high temperatures, depending on the
electrolyte, e.g.,
◦ Conc. H3PO4 - ~200oC
◦ Molten Na2CO3 or K2CO3 - 600oC
◦ H+ exchanging polymer - 80oC
Fuel cells (contd.)

Anode: reaction will depend on the fuel and
the electrolyte. Typically
H2(g) → 2H+ + 2 eCH3OH(l) + H2O → CO2(g) + 6 H+ + 6 e-
(Methanol fuel cell)
 The H+ ions are transported to the cathode
by the electrolyte.
 Cathode: O2(g) + 4 H+ + 4 e- → 2 H2O

Used in cars, spacecraft, back-up generators,
even electronic devices.