Chem Y12 - Redox Investigation - Primary/Secondary/Fuel Cells

Primary Cell

A cell that can only be used once. Have a fixed amount of oxidant and reductant. Non-rechargeable.

Secondary Cell

an electric cell that can be recharged

Recharging

process of electrolysis where electrical energy is used to force the spontaneous discharge reactions to be reversed and regenerate the oxidant and reductant originally present.

Fuel Cell

- oxidant and reductant is constantly fed into the cell.
- reductant is typically a fuel (H2, methane etc)
- oxidant is usually oxygen gas
- can run forever as long as oxidant and reductant are fed to cell

Dry Cell (electrodes, process in cell, other materials and why, pros and cons, uses)

- zinc anode and manganese oxide/ammonium chloride cathode paste
- electrons are conducted from zinc anode to surface of manganese oxide particles through the graphite.

- ammonium chloride acts as a salt bridge and provides protons since ammonium is a weak acid. Graphite powder and rod conduct electrons through cathode paste.
- pros: inexpensive, non-hazardous waste
- cons: low energy density, can't be recharged
- uses: flashlights, calculators

Alkaline Cell (electrodes, process in cell, other materials and why, pros and cons, uses)

- powdered zinc (Zn) and KOH anode, manganese oxide, carbon and KOH cathode paste
- electrons are conducted towards the cathode paste where reduction can occur

- KOH is a better replacement to ammonium chloride since it is not acidic in nature and does not cause the breakdown of the zinc anode over time.
- pros: long shelf life (then dry cell), can sustain higher current
- cons: no economic process for recycling
- used in devices that require higher current flow. e.g toys, portable radios

Lead Acid Cell (electrodes, process in cell, other materials and why, pros and cons, uses)

- anode grid is spongy lead. cathode grid is lead packed with lead oxide
- during discharge lead at anode is oxidised, forming lead ions. This reacts with sulfate ions in electrolyte to precipitate solid lead sulfate onto electrode. Lead oxide at cathode is reduced to form lead sulfate which also precipitates onto electrode.

- Sulfuric acid as the electrolyte mixture in order to supply H+ and sulfate ions required for oxidation and reduction to produce high cell potentials. Powdered nature of lead and lead oxide increases surface area, largely increasing rate of reaction.
- pros: able to produce high current, low cost
- cons: low energy density, heavy, hazardous toxic waste
- uses: when large currents are needed for long time. e.g motors of cars, trucks, wheelchairs, forklifts

Lithium Ion Cell (electrodes, process in cell, other materials and why, pros and cons, uses)

- anode is porous graphite with lithium ions intercalated in layers. Cathode is porous lithium metal oxide (e.g LiCoO2)
- during discharge, lithium ions migrate out of graphite anode to lithium metal oxide cathode. Electrons flow from anode through external circuit to cathode. Lithium intercalates with cobalt to form lithium cobalt oxide. Lithium balances charge buildup.

- Graphite is a stable storage space for lithium. Electrolyte allows lithium ions to migrate without allowing electrons to move through it. Immersed in a non-aqueous organic electrolyte since lithium is very reactive with aqueous species. Porous separator located between the circuit.
- pros: very high energy density, good shelf life
- cons: can easily pose danger is overcharged or damaged
- uses: laptops, ipads, cameras, mobile phones

Proton Exchange Membrane Fuel Cell (PEMFC) (electrodes, process in cell, other materials and why, pros and cons, uses)

- both anode and cathode consist of nanoparticles of platinum impregnated onto porous carbon
- pure hydrogen gas is pumped on one side where it is oxidised at anode. Electrons flow to cathode through external circuit generating a potential difference. H+ ions move through solid electrolyte towards cathode. Oxygen gas molecules are reduced and combine with H+ to form water.

- Solid polymer proton exchange membrane (PEM) acts as the electrolyte and electrode separator and allows for compact, flexible fuel cell design. Platinum acts as a catalyst. Carbon conducts electrons away from electrode surface.
- pros: carbon neutral, only require hydrogen and oxygen which are abundant, simple
- cons: hydrogen takes more space to store then fossil fuels, making hydrogen fuel is energy consuming
- uses: produce drinking water for astronauts and generate energy

Phosphoric Acid Fuel Cell (PAFC) (electrodes, process in cell, other materials and why, pros and cons, uses)

- both electrodes consist of porous carbon and platinum catalyst particles
- hydrogen gas diffuses into porous C/Pt electrode and is oxidised forming H+ ions and electrons. Electrons are conducted out of cell and to cathode, where oxygen gas is reduced and combine with H+ ions to form water

- Phosphoric acid electrolyte contained in ceramic matrix of silicon carbide increases conductivity of phosphoric acid at higher temperatures. Porous carbon and platinum catalyst allow for fast reaction rate of oxidation and reduction.
- pros: quiet, maintain god air quality, can run on hydrogen gas produced from traditional hydrocarbon fuels
- cons: costly method, catalyst poisoning is common, low efficiency
- uses: emergency power for banks, hotels, hospitals and police stations