Unit 5 Section 2 Electrochemical Cells

• non-rechargeable cells use irreverseable reactions i.e. a dry cell alkaline battery. 
• used for: remotes, alarms and other small devices that don't need a lot of power and, are only used for a short amount of time.
• the half equations for this are not reverseable because it is not practical to reverse them in a battery.
• batteries can be made to run backwards but this can cause it to leak or explode. The zinc casing becomes thinner as it is oxidised in this way.
• also the ammonium ions would produce hydrogen gas which would escape making it impossible for the ammonia to reform by reversing the reactions.

• Rechargeable cells use reverseable reactions
• They are found in lots of devices like: phones, laptops and cars.

Lead-acid cells are used in car batteries. They normally consist of 6 cells connected in series. Each cell is made up of a lead anode and a lead(IV)dioxide cathode immersed in a sulfuric acid elctrolyte. Both electrodes end up coated lead(II)sulfate.

• Other types of rechargeable battery are NiCad(nickel-cadmium) and L ion(lithium ion).
• To recharge these batteries a current is supplied to force electrons to flow in the opposite direction around the circuit and reverse the reactions.
• This is possible because none of the substances in a rechargeable battery escape or are used up.

• Cost: non-rechargeable batteries are cheaper for the short-term but in the long-term rechargeable batteries are cheaper as they only require one purchase.
• Lifetime: non-rechargeable batteries last longer but have to be thrown away at the end where rechargeable batteries do not.
• Power: rechargeable batteries can supply more power than non-rechargeable batteries, so are used more in power hungry devices like laptops and phones.
• Use of resources and waste: more non-rechargeable batteries are produced which uses more resources and creates more waste than rechargeable batteries. Both types can be recycled but they instead often end up in landfills.
• Toxicity: non-rechargeables are less likely to contain toxic lead and cadmium(might have some mercury though), so they're less hazardous in landfills if they leak.

• In most cells the chemicals that generate electricity are congtained in the electrodes and the electrolyte that form the cell.
• In a fuel cell the chemcials are stored seperately and used only when electricity is needed.
• An example is a hydrogen-oxygen fuel cell, which can be used to power electric vehicles.

Pros

• They don't need electrical recharging, as long as hydrogen and oxygen are supplied the cell will continue to produce electricity.
• Only waste product is water, no harmful chemicals or gaseous emissions from the cell as a bi-product.

Cons

• Energy is needed to produce a supply of hydrogen and oxygen from electrolysis from the waste water.
• The initial energy for hydrolysis is mostly likely obtained from fossil fuels so the process is not usually carbon-neutral.
• Hydrogen is also highly flammable so needs to be handled carefully.