Saturday 4 December 2010

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+SO42-→PbSO4+2e- and PbO2+4H++SO42-+2e-
→PbSO4+H2O at cathode. Overall reaction: Pb+SO42-+PbO2+4H++SO42-→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)

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