Energy

Environmental impact

Infrared absorption

For molecules to absorb IR, the vibration of the molecule must lead to a change in dipole moment, which makes it IR active. For example, carbon dioxide is a non-polar molecule

The following vibration modes ares possible:

1. Symmetric stretching

   No change in dipole moment, this no IR absorption is noted.

2. Asymmetric stretching

   Change in dipole moment observed, this IR absorption is noted.

3. Symmetric bending

   Change in dipole moment, so IR active.

Electrochemical cells and Fuel cells

Electrochemical cells

Electrochemical cells that connect chemical energy to electrical energy through spontaneous redox reactions:

• ΔG < 0: free energy is utilised readily
• E_cell > 0: voltage/potential difference is produced

When a cell discharged, it is similar to a voltaic cell, and when it recharges, it is similar to an electrolytic cell. There are type types of electrochemical cells: primary cells and secondary cells.

Primary cells

Primary cells can only be discharged once as:

• The reaction is irreversible due to the anode being completely used up (dissolved) after the reaction
• Electrolyte poisoning: components of the electrolyte and anode are mixed during the reaction

Examples of primary cells include acidic batteries and alkaline batteries.

Secondary cells

Secondary cells can be discharged and recharged.

• Lead-acid battery
• Nickel-cadmium battery
• Lithium ion battery

Lead-acid battery

• Anode: Pb(s) converted to PbSO₄(s) during discharge
• Cathode: PbSO₂(s) converted to PbSO₄(s) during discharge
• Electrolyte: H₂SO₄(aq) converted to H₂O during discharge

Discharging reactions

• Anode: Pb(s) + SO₄^2-(aq) -> PbSO₄ + 2e-
• Cathode: PbO₂(s) + 4H⁺(aq) + SO₄^2-(aq) + 2e- -> PbSO₄(s) + 2H₂O
• Overall: Pb(s) + 2SO₄^2-(aq) + 4H⁺(aq) + PbSO₂(s) -> 2PbSO₄(s) + 2H₂O(aq)

After the battery is discharged, electricity must be supplied to remove the SO₄^2- coating from the anode and the cathode.

Advantages and disadvantages

Advantages

• Lasts a long time

Disadvantages

• Overcharging produces extra voltage which leads to electrolysis of H₂O into H₂(g) and O₂(aq); this is why non-sealed batteries require continuous top-up of H₂O
• Capacity decreases over time due to wear and tear

Nicked-cadmium battery

• Anode: Cd(s) converted to Cd(OH)₂ during discharge
• Cathode: NiO(OH) converted to Ni(OH)₂ during discharge
• Electrolyte: KOH(aq) (anything that can provide OH^-)

Discharging reactions

• Anode: Cd(s) + 2OH^- -> Cd(OH)₂(s) + 2e-
• Cathode: 2NiO(OH)(s) + 2H₂O(l) + 2e- -> 2Ni(OH)₂(s) + 2OH^-
• Overall:

Advantages and disadvantages

Advantages

• Low internal resistance due to a lower number of ions, so current/voltage can be discharged at a higher capacity
• Discharges larger amounts of voltage safely

Disadvantages

• Heavy metals are present in the battery: cadmium is highly toxic in nature
• Loses power easily due to high discharge rate: it needs to be charged continuously to maintain operation

Lithium ion battery

The lithium is embedded in a graphite matrix. During discharge, the lithium combines with the cathode.

• Anode: Li(s) converted to Li⁺ during discharge
• Cathode: CaO₂ converted LiCoO₂ converted to during discharge
• Electrolyte: non-aqueous electrolyte such as gel polymer

Discharging reactions

• Anode: Li(s) -> Li⁺(aq) + e-
• Cathode: Li⁺ + e- + CoO₂(s) -> LiCoO₂(s)
• Overall:

Advantages and disadvantages

Advantages

• Can be charged multiple times
• Discharged slower than nickel-cadmium battery, so suitable doe low-drain operations
• Holds onto charge better than other secondary cells
• No heavy metals present

Disadvantages

• Lithium is highly reactive; exposed battery or punctured batters leads to explosion
• Capacity reduced one time as some of the Li atoms do not return back into the graphite matrix, which reduced the battery’s potential

Fuel cells

Fuel cells are cells that produce electricity with supplies of fuel.

Hydrogen fuel cell

• Proton exchange membrane (PEM) fuel cell (with the membrane as an electrolyte)
• Acidic fuel cell (acid as the electrolyte)
• Basic fuel cell (base as the electrolyte)

PEM fuel cell

• Anode: H₂(g) -> 2H⁺ + 2e-
• Cathode: O₂(g) + 4e- -> 2O^-
• Overall: 2H₂(g) + O₂(g) -> 4H⁺ + 2O^2-, and 2H₂(g) + O₂(g) -> 2H₂O(l)

The PEM acts as an electrolyte and only allows H⁺ to pass though from the anode compartment to the cathode compartment.

The disadvantages of PEM fuel cells is that they are expensive as the catalyst used (Rhodium, Pd, or platinum) are expensive.

Acidic fuel cell

• Anode: H₂(g) -> 2H⁺(aq) + 2e-
• Cathode: 4H⁺(aq) + O₂(aq) + 4e- -> 2H₂O(aq)
• Overall: 2H₂(g) + O₂(g) -> 2H₂O(aq)

H₂O is proceed at the cathode, similar to the PEM fuel cell. H₃PO₄(aq) is used as the electrolyte as it provides H⁺(aq) ions and an acidic medium.

Alkaline fuel cell

• Anode: H₂(g) + 2OH^-(aq) -> 2H₂O + 2e-
• Cathode: H₂O(l) + 1/2O₂(g) + 2e- -> 2OH^-
• Overall: H₂(g) + 1/2O₂(g) -> H₂O(l)

KOH(aq) or NaOH(aq) are used as electrolyse as the contain OH^- ions which provide a basic medium.

Advantages and disadvantages of hydrogen fuel cells

Advantages

• No harmful by-products
• Use of simple gases as fuels

Disadvantages

• Required pure H₂(g) and O₂(g) supply, which are obtained from the electrolysis of H₂O or cracking of hydrocarbons
• H₂ is combustible gas, so dangerous
• Inefficient process are not much energy is released
• Required expensive catalysts

The alternative for hydrogen fuel cells are methanol and microbial fuel cells.

Methanol fuel cell

Methanol is used in place of H₂ to supply to H⁺ ions.

• Anode: CH₃OH(l) + H₂O(l) -> CO₂(g) + 6H⁺(aq) + 6e-
• Cathode: O₂(g) + 4H⁺(aq) + 4e- -> 2H₂O(aq)
• Overall: 2CH₃OH(l) + 3O₂(g) -> 2CO₂(g) + 4H₂O(l)

Microbial fuel cell

The aerobic oxidation of glucose produces carbon dioxide and water. Anaerobic respiration produced H⁺ ions and electrons, which can be harnessed at the anode, and the H⁺ ions permitted to diffuse through the PEM, where they reduce oxygen, forming water.

The aerobic oxidation is prevented by the PEM.

• Anode: C₆H₁₂O₆ + 6H₂O -> 6CO₂ + 24H⁺ + 24e- (anaerobic respiration)
• Cathode: O₂(g) + 4H⁺(aq) + 24e- -> 2H₂O(l)
• Overall: C₆H₁₂O₆ + 6O₂(g) -> 6CO₂(g) + 6H₂O(l)

The disadvantages is that specific types of bacteria are required.

Concentration cell

A concentration cell is cell that produced electricity based on differing concentrations of the same substance at the anode and cathode.

$$ E_\text{cell} = E^\theta_\text{cell} - \frac{RT}{nF}\ln Q $$