Fuel cells and Batteries

Properties of primary cells

• non re chargeable - due to the products slowly migrating away from the electrodes or are consumed by side reactions occurring in cell

• alkaline cells

• they contain no fluids

• simple and light

• cheap

• short lasting

• lower self discharge

• higher energy density

Properties of secondary cells

• Rechargeable- done by reversing reaction through attaching cell to an electrical charge which has a potential difference a little greater than that of the cell. Positive electrode of the charger to positive electrode of cell and negative electrode of the charger to negative electrode.

• electrical energy is converted to chemical in cell, in order for it to work products formed must stay in contact with electrodes

• These are made up of wet cells (flooded and liquid cells) and molten salt (liquid cells with different composition)

• complex and heavy

• expensive

• long lasting

• can withstand higher electrical currents

energy transformation in secondary cells

• when cell discharges it acts as a galvanic cell converting chemical energy into electrical

• when cell is recharged it acts as an electrolytic cell converting electrical to chemical energy

properties of fuel cells

• cells that are constructed with a continuous flow of reactants allowing for a constant production of chemical energy

• typically use hydrogen

• transform chemical energy into electrical energy

• continuous electricity

• efficient

• a fuel cell using hydrogen produces electricity, water, heat and small amounts of nitrogen dioxide

anode of a leclanche cell

zinc case: Zn → Zn2+ + 2e

cathode of leclanche cell

carbon rod surrounded by paste: 2MnO2 + 2NH4+ + 2e → Mn2O3 + 2NH3 +H2O

lead-acid cell anode and cathode

anode: Made of lead and surrounded by sulfuric acid Pb + SO42- → PbSO4 +2e

Cathode: made of solid lead oxide: PbO2 + 4H+ + SO42- +2e → PbSO4 + 2H2O + 2e

wet corrosion

• at anodic site, electrons are released when iron atoms are oxidised to iron (II) ions

• at cathodic site the electrons reduce oxygen gas and water to produce hydroxide ions

• these combine to form insoluble iron (II) hyrdoxide

• iron (II) hydroxide is readily oxidised by more oxygen in the air to form iron (III) hydroxide

• Iron (III) hydroxide dehydrates to form rust, Fe2O3*H2O

types of methods to prevent corrosion

surface protection, galvanising, sacrificial anode and cathodic protection

Surface protection

• prevents iron from coming into contact with oxygen and water

• oil, grease, paint, plastic, other metals

• advantages: cheap and easy, can be flexible, effective for as long as coat remains in tact

• Disadvantages: coating must be reaplied when it deteriorates any scratch will leave metal susceptible

• For less reactive metal coating: any scratch will mean the more reactive iron will act as the anode and will corrode faster

Galvanising

• more reactive metal e.g. zinc is used as a coating

• more reactive metal will corrode quickly and form a thin layer of zinc oxide which protects the iron

• will protect the iron even if scratched since zinc is more reactive

Sacrificial anode

• force iron to become the cathode by electrically connecting it to a more reactive metal which will act as the anode

• Sacrificial anode will need to be replaced

Cathodic protection

• Use of sacrificial anode may be improved by use of a DC electrical current

• The (-) terminal of the DC power supply is electrically connected to iron making it the cathode of a galvanic cell

• the (+) terminal is connected to the sacrificial anode, which could be made of iron in this scenario but typically a more reactive metal

Electrolysis

Forcing a non-spontaneous to occur reaction by using electricity

Rechargeable batteries act as a voltaic cell when discharging and a electrolytic cell when recharging