Chemistry : Chemical Cell II

Redox in primary cells

-          Oxidation occurs at anode, and reduction occurs in cathode. Since anode gains e^- and cathode loses e^-, we can deduce that the e^- flow is from anode to cathode in external circuit and cathode to anode in the internal circuit.

-          Zinc carbon cell: There’re four layers of chemical carbon rod (cathode), MnO₂, moist paste of NH₄Cl (electrolyte) and Zn case (anode).

Oxidation at anode: Zn→Zn²⁺+2e^-, Reduction at cathode: 2NH₄⁺+2e^-→2NH₃+H₂. Since H₂ is explosive, they’re oxidized by MnO₂: 2MnO₂+H₂→Mn₂O₃+H₂O. NH₃ form complexes with the ions: Zn²⁺+2NH₃+2Cl^-→Zn(NH₃)₂Cl₂, so the overall equation is  2MnO₂+2NH₄Cl+Zn→Zn(NH₃)₂Cl₂+Mn₂O₃+H₂O

Uses: low voltage supply (small radios, remotes)

Disadvantage: leakage since Zn is consumed directly; voltage drops significantly since gas is accumulated in the cell.

-          Alkaline Manganese cell: The layers are: MnO₂ and carbon powder(cathode), KOH (electrolyte) and Zn (anode). In this case, Zn + 2OH^-→ZnO+H₂O+2e^-,

2MnO₂+H₂O+2e^-→Mn₂O₃+2OH^-, the overall reaction is Zn +2MnO₂→ZnO+Mn₂O₃

Uses: higher capacity like camera and motors.

Disadvantage: The leakage of alkaline contents is irritate/corrosive, may form crystal and harm electrical component.

-          Silver oxide cells (“Button” cells): AgO(cathode), KOH(electrolyte), Zn powder(anode)

Uses: portable small devices such as pacemakers, calculators

Secondary Cells

-          Lithium ion cell: Li⁺ migrate between cathode and anode between graphite sheets during charging (cathode to anode) and discharging (anode to cathode). It has a large applications like laptops since it’s light and has high energy density.

-          Nickel metal hydride (NiMH): It has metal hydride as anode and Ni(OH)₂ as cathode, KOH as the electrolyte. It has a quite high energy density and used as cells like Zinc-carbon cells and alkaline manganese cells. Currently it’s replacing by NiCd (nickel-cadmium) cell with more reliable and low temperature performance.

-          Lead-acid accumulator (known as car battery): Pb as anode and PbO₂ as cathode, while H₂SO₄ is the electrolyte. At anode, Pb+SO₄^2-→PbSO₄+2e^- and PbO₂+4H⁺+SO₄^2-+2e^-

→PbSO₄+H₂O at cathode. Overall reaction: Pb+SO₄^2-+PbO₂+4H⁺+SO₄^2-→2PbSO₄+2H₂O

The reaction is reversible; equilibrium goes to RHS when discharging and LHS when charging. Note that after some discharging, both anode and cathode becomes PbSO₄, which cause dropping in voltage.

Theoretically it is eternally rechargeable, but in reality PbSO₄ may fall off from the Pb plates which can’t be recharge again.

-          Fuel cell: change fuel’s chemical energy to electrical energy. A fuel and oxidant (usually O₂) is used. There’re many type of fuel cell such as alkaline fuel cell, phosphoric acid fuel cell, molten carbonate fuel cell, solid oxide fuel cell, proton exchange membrane fuel cell and direct methanol cell.

Alkaline fuel cell: insert H₂ and O₂, OH^- as electrolyte, at anode: H₂+2OH^-→2H₂O+2e^-, at cathode, O₂+2H₂O+4e^-→4OH^-, overall reaction: 2H₂+O₂→2H₂O

Direct methanol fuel cell: at anode: CH₃OH+H₂O→CO₂+6H⁺+6e^-, at anode: 3O₂+12H⁺+12e^-→6H₂O, overall reaction: 2CH₃OH+3O₂→2CO₂+4H₂O

Advantage: highly efficient and environmental friendly (CO₂ and H₂O as the only product)