Unit 3 AOS 2 - Chapter 6: Electrolysis

Energy conversion in electrolysis cells

Electrical energy to chemical energy (non-spontaneous)

using the electrochemical series

the strongest oxidizing agent has to be below the strongest reducing agent

Anode in electrolysis

Site of oxidation

Is positive as it its connected to the positive terminal of the power supply

Electrons are forced away from the anode by the power supply

Cathode in electrolysis

Site of reduction

Connected to the negative side of power supply, thus negative

Electrons are sent towards the cathode by the power supply

Electrodes in electrolysis

Have to be electrically conductive

Can be inert (pt or grahite) to prevent unwanted reactions form happening or reactive if the electrode is required in the reaction

In the same vessel as the reactions are non-spontaneous, however any products that are formed must be kept seperate as they are likely to spontaneously react

Electrolyte in electrolysis (aqueous)

Allows the flow of ions, completing the internal circuit. An aqueous electrolyte is usually used in favour of molten. If it is aqueous, an ionic salt is used and water must be considered as a reactant. If specific oxidants and reductants want to be used, they must be stronger agents respectively than water and the electrolyte.

Molten electrolytes in electrolysis

(L) is used to symbolise its molten. Water does not have to be considered. However they are more expensive and as they require extremely high temperatures, they are very dangerous.

determining the required voltage of a electrolytic cell

required energy > oxidation - reduction

Competitive electrolytical reactions

If 2 or more oxidants or reductants are present, as the concentration of the oxidant and reducing that is easiest to discharge decrease, the substances that are the next easiest (the equation just below the oxidant and the equation above the reductant) will continue the reaction.

Additives

Species that can be added to the cell that do not interfere but optimise the cell.

- make it safer to run

- cheaper

- more efficient

Electroplating

an electrolytic process in which a metal ion is reduced and a solid metal is deposited on a surface.

- metal ions from electrolyte are deposited on to the cathode and a metal anode is selected such that the concentration of metal ions remains constant (the same metal in the anode and electrolyte)

- alters appearance of metal, or is used to protect strong reducing agents with a coating of a metal that is likely to oxidise.

Secondary Cells

Electrochemical cell that can be recharged

What is discharge

The secondary cell behaves as a galvanic cell (chem to electrical energy)

What is recharge

The secondary cell behaving as an electrolytic cell (requires a power source)

When can a secondary cell be recharged?

Cell must be connected to a power supply that provides voltage greater than the required voltage

products of the discharge half reaction must remain in contact with the electrode at which they react

What happens as time goes on in a secondary cell?

As the battery undergoes numerous cycles, a sufficient proportion of products may not be able to remain in contact with the electrode for the cells to operate as required

Green Hydrogen

A form of hydrogen fuel that is produced through the process of electrolysis using renewable energy sources. It is considered green due to its association with renewable energy sources. It does not produce any emissions or release any greenhouse gas.

polymer electrolyte membrane (PEM)

Connects two half cells by allowing them to exchange protons between 2 electrodes, but prevents the mixing of the contents of 2 chambers.

Green Hydrogen Electrolyser cell design features:

- Electrodes are typically covered with a platinum catalyst, increases the rate of production of gases.

- Gas diffusion layers made up of gold-lined, titanium and carbon paper help distribute the reactant gases evenly across the surface of the electrodes.

- Bipolar plates prevent build of up current and provide mechanical support.

PEM design features:

- It is a plastic polymer that is selectively permeable to protons but blocks the other ions (oxide or impurities) and electrons.

- Acts as a barrier, blocking the passage of water, H and O molecules.

- Overall it ensures H and O gases are produced separately which increases the efficiency and lowers the possibility of contamination or mixing gases.

What are the equations in the PEM

Oxidation: 2H2O —> 4H+ + O2 + 4e-

Reduction: 4H+ + 4e- —> 2H2

Overall: 2H2O —> 2H2 + O2

- electrical current is passed through water, splitting it into hydrogen and oxygen gas

What green chemistry principles are associated with PEM cell?

Use of catalysts, designing safer chemicals, prevention of wastes and use of renewable feedstock

Artificial photosynthesis

Process that utilises human made materials to capture sunlight and split water molecules to create hydrogen and oxygen. Typically done using a Hydrogen ion conduction membrane and catalysts on electrodes

What are the 4 steps of artificial photosynthesis?

1. Light harvesting system: Materials that are able to absorb light promote electrons to a higher energy state to produce around 2V, the amount of energy req. to drive reactions.

2. Oxidation using catalysts: 2H2O —> 4H+ + O2+ 4e-

3. Reduction using catalysts: Reduction: 4H+ + 4e- —> 2H2

Overall: 2H2O —> 2H2 + O2

What are the uses of hydrogen created from artificial photosynthesis?

- A liquid energy source for fuel cells

- A fuel source in combustion engines

- reduce CO2 to methanoic acid w/ catalysts (HCOOH)

- reduce CO2 to methanol and water w/ catalysts (CH3OH)

- reduce CO2 to methane and water w/ Catalysts

- To reduce nitrogen gas to ammonia w/ catalysts (NH3)

(N2 + H2 —> NH3)

These compounds are a store of chemical energy and can be used as industrial fuels or starting materials for other consumer products.

What are the advantages of creating fuels through artificial photosynthesis?

- the process does not create green houses gases

- the process does not require the use of fossil fuels

_ can remove CO2 from atmosphere

- Produces Oxygen

- Can create and store energy in more than one type of hydrocarbon fuel

- can create ammonia which is used to manufacture fertiliser.

What are the challenges with using Green hydrogen?

Energy efficiency: Using green hydrogen can be very energy intensive, especially when using renewable energy sources that are costly to implement. This can reduce overall efficiency of the process and increase costs.

- Infrastructue: Currently limited infrastructure for producing, storing and transporting hydrogen making it challenging to scale up the use of hydrogen as a fuel source.

- Cost: Currently, the cost of green hydrogen production is higher than that of other conventional fuels. This can limit its use especially in developing countries.

- Water resources: Hydrogen production requires a significant amount of water which can be challenging in areas facing water scarcity or drought