Engineering Chemistry CHM 406 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+ + 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.

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 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.