Storage cells and fuel cells

Question 1

a storage cell (sometimes called secondary cells or batteries) is a cell that

Answer 1

→that can be recharged by passing a current through it in the opposite direction to the flow of current generated by the cell

Question 2

one of the first storage cells used ………………for the positive electrode, ……………….. for the negative electrode and …………………. as the electrolyte

Answer 2

NiO(OH) for the +tive
cadmium, Cd, for the -tive
aqueous potassium hydroxide as the electrolyte

Question 3

the two half-cell reactions for the nickel-cadmium storage cell are:

Answer 3

Cd(OH)2(s) + 2e- ⇌ Cd(s) + 2OH-(aq)
E⦵ = -0.88V
NiO(OH)(s) + H2O(l) + e- ⇌ Ni(OH)2(s) + OH-(aq)
E⦵ = +0.52V

the E⦵ value of the Cd equilibrium is more negative than the Ni equilibrium, hence the equilibrium position for this reaction shifts to the left when the cell is generating a flow of electrons

Question 4

the advantages and disadvantages of using a cadmium-nickel cell are:

Answer 4

advantages:
-can be recharged when an external potential difference is applied that reverses the reaction
disadvantages:
-cadmium is very toxic

Question 5

a fuel cell produces a voltage through

Answer 5

a chemical reaction between a fuel and oxygen

Question 6

examples of fuel cells are:

how they work:

Answer 6

• hydrogen-oxygen fuel cell
• methanol/ethanol-oxygen fuel cell

the fuel is supplied eternally (H2 as a gas) and the cell operates as long as the fuel supply is maintained

Question 7

in the hydrogen-oxygen fuel cell, both electrodes are coated with

Answer 7

platinum which catalyses the reation

Question 8

in the hydrogen-oxygen fuel cell, the both reactions takes place in the presence of an ………………electrolyte

Answer 8

acid electrolyte

Question 9

the reactions at the positive and negative electrodes of the hydrogen-oxygen fuel cell using an acidic electrolyte are:
positive electrode:
negative electrode:

Answer 9

positive electrode:
H2(g) → 2H+ (aq) + 2e-
negative electrode:
1/2O2(g) + 2H+(aq) + 2e- → H2O(l)

the H+ ions (protons) pass through the proton exchange membrane which allows them to enter the compartment containing the positive electrode where the H+ can react with oxygen

Question 10

the reactions at the positive and negative electrodes of the hydrogen-oxygen fuel cell using an alkaline electrolyte are:
positive electrode:
negative electrode:

Answer 10

positive electrode:
H2(g) + 2OH-(aq) → 2H2O(l) + 2e-
negative electrode:
1/2O2(g) + 2H+(aq) + 2e- → H2O(l)

Question 11

advantages of hdrogen-oxygen fuel cells are:

Answer 11

• offer an alternative to direct use of fossil fuels such as petrol or diesel
• unlike fossil fuels, these cells do not include pollutants such as CO, CO2 or NOx
• they are lighter and more efficient than engines that use fossil fels

Question 12

disadvantages of hydrogen-oxygen fuel cells:

Answer 12

• H2 is explosive, and compressing the gas then transporting it as high pressure is hazardous
• the H2(g) has to adsorb onto the surface of the catalyst, and so far no firm conclusions have been reached as to the best metal to use for adsorbing H2(g)
• a suitable material for storing the H2 for release is also needed. Although metal hydrides absorb hydrogen, they only release it at high temperatures
• the supply of hydrogen itself is also not optimal. at the moment, most H2 is produced from CH4, a finite resource. Integrated wind-to-hydrogen plants using the electrolysis of water, are exploring techniques to produce H2 on mass at low cost, but so far the method is expensive