1. Engineering Chemistry CHM 406
Cells and Batteries
Engineering Chemistry
CHM 406

2. 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

3. 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

4. Zinc – carbon dry cell

5. Zinc – carbon dry cell (contd.)
Anode: Zn(s) | ZnCl2, NH4Cl aq. paste
Zn → Zn e-
Cathode: C (graphite) | MnO2(s), C powder, NH4Cl
2 NH MnO e-
→ Mn2O3 + H2O NH3
Ecell = 1.5 V, quickly reduced as reaction proceeds or in cold weather, not reversible in practice.

6. 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

7. Lithium – iodine cell Anode: Li → Li+ + e- Cathode: I2 + 2 e- → 2 I-
Electrolyte 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.

8. 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.

9. 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.

10. Lead – acid storage battery

11. Lead – acid storage battery (contd.)
Anode: Pb(s) | 65% aq. H2SO4
Pb(s) + HSO4- → PbSO4(s) + H e-
Cathode: Pb(s) | PbO2(s) | 65% aq. H2SO4
PbO2(s) + HSO H e-
→ PbSO4(s) H2O
Recharging reverses these reactions.
Ecell = 2 V. The battery contains 6 cells connected in series, hence 12 V.

12. 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

13. Fuel cell

14. 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 K2CO oC
H+ exchanging polymer oC

15. Fuel cells (contd.)
Anode: reaction will depend on the fuel and the electrolyte. Typically
H2(g) → 2H e-
CH3OH(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.