[23.6] storage and fuel cells

Question 1

what are the 2 types of storage cells?

Answer 1

• primary cells
• secondary cells

Question 2

describe primary cells

Answer 2

• non-rechargeable
• one use only
• low voltage
• most are alkaline based using KOH as electrolyte and Zn/MnO₂
• used for low current, long-storage devices eg. smoke detectors

Question 3

describe secondary cells

Answer 3

• rechargeable
• cell reaction can be reversed during recharging so chemicals in cell are regenerated and cell can be used again

Question 4

what are some common examples of secondary cells? (3)

Answer 4

• lead-acid batteries used in car batteries
• nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) are cylindrical batteries used in radios, torches etc.
• lithium-ion and litium-ion polymer used in modern electronic appliances eg. laptops, tablets etc.

Question 5

what direction is the flow during discharging / recharding in secondary cells?

Answer 5

• discharging = negative electrode to positive electrode
• recharging = positive electrode to negative electrode

Question 6

what are the 3 types of fuel cells?

Answer 6

• hydrogen fuel cell with acid electrolyte
• hydrogen fuel cell with alkali electrolyte
• methanol fuel clles

Question 7

how do fuel cells work?

Answer 7

• voltage is created rom the reaction of oxygen with a fuel
• fuel and oxygen flow into fuel cells
• products flow out
• electrolyte remains in cell

Question 8

what are the benefits of fuel cells?

Answer 8

• can operate continuously if fuel and oxygen are supplied
• don’t have to be recharged
• hydrogen fuel cells produce no carbon dioxide during combustion, water is only product

Question 9

what fuels can be used in fuel cells?

Answer 9

• hydrogen
• methane
• methanol
• ethanol

Question 10

what are the redox systems in an alkali hydrogen fuel cell?

Answer 10

• hydrogen + water in, oxygen out, alkaline electrolyte
• 2H₂O (l) + 2e⁻ ⇌ H₂ (g) + 2OH⁻ (aq)
• 0.5O₂ (g) + H₂O (l) + 2e⁻ ⇌ 2OH⁻ (aq)

Question 11

what are the redox systems in an acid hydrogen fuel cell?

Answer 11

• hydrogen in, oxygen + water out, acidic electrolyte
• 2H⁺ (aq) + 2e⁻ ⇌ H₂ (g)
• 0.5O₂ (g) + 2H⁺ (aq) + 2e⁻ ⇌ H₂O (l)

Question 12

what are the oxidation, reduction, and overall reactions in an alkali hydrogen fuel cell?

Answer 12

• oxidation = H₂ (g) + 2OH⁻ (aq) —> 2H₂O (l) + 2e⁻
• reduction = 0.5O₂ (g) + H₂O (l) + 2e⁻ —> 2OH⁻ (aq)
• overall = H₂ (g) + 0.5O₂ (g) —> H₂O (l)

Question 13

what are the oxidation, reduction, and overall reactions in an acid hydrogen fuel cell?

Answer 13

• oxidation = H₂ (g) —> 2H⁺ (aq) + 2e⁻
• reduction = 0.5O₂ (g) + 2H⁺ (aq) + 2e⁻ —> H₂O (l)
• overall = H₂ (g) + 0.5O₂ (g) —> H₂O (l)