C.6 Electrochemistry, rechargeable batteries and fuel cells

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What does internal resistance depend on?

1. Ion mobility 2. Electrolyte conductivity 3. Electrode SA (bigger SA = higher conductivity) 4. Temperature

What are the signs for the anode/cathode during recharge/discharge?

CHARGE (voltaic cell) Anode (-) Cathode (+) DISCHARGE (electrolytic cell) Anode (+) Cathode (-)

Define *fuel cell*

Cell that can be used to convert chemical energy directly into electrical energy Fuel and an oxidizing agent react in the presence of an electrolyte to produce electrical energy

Define *secondary cell*

Cells that involve redox reactions that *can be reversed using electricity*

Define *primary cell*

Cells where the electrochemical reaction is *irreversible*

Recall the oxidation (-) reaction for a *nickel-cadmium battery* during discharge

cadmium metal → oxidized to cadmium hydroxide Cd(s) + 2OH-(aq) → Cd(OH)2(s) + 2e- Reactions reverse when battery is recharged

Recall the reduction (+) reaction for a *nickel-cadmium battery* during discharge

nickel (III) oxide hydroxide cathode → reduced to nickel (II) hydroxide 2NiO(OH)2(s) + 2H2O(l) + 2e- → 2Ni(OH)2(s) + 2OH-(aq) Reactions reverse when battery is recharged

Recall the electrolyte used in a *lead-acid battery*

sulfuric acid (dilute during discharge)

Recall the oxidation (-) reaction for a *lithium-ion battery* during discharge

Li(s) → Li+ + e- Embedded atoms lose an e- to the external circuit and Li+ ions migrate to the cathode Reversed when battery is recharged.

Recall the reduction (+) reaction for a *lithium-ion battery* during discharge

Li+ + e- + CoO2(s) → LiCoO2(g) Reversed when battery is recharged.

What does the voltage of a cell depend on?

Materials the cathode and anode are made of - greater the difference in electrode potentials = more voltage Placing the cells in a *series* increases voltage

Recall the oxidation (-) reaction for a *lead-acid battery* during discharge

Pb + HSO4- → PbSO4 + H+ + 2e-

Recall the overall reaction for a *lead-acid battery*

Pb + PbO2 + 2H+ + 2HSO4- → 2PbSO4 + 2H2O

Recall the reduction (+) reaction for a *lead-acid battery* during discharge

PbO2 + 3H+ + HSO4- + 2e- → PbSO4 + 2H2O

Define *power of a battery*

Total work that can be obtained from the cell, and therefore the rate at which it can deliver energy (measured in J s^-1)

How does temperature affect internal resistance?

High T = low internal resistance because ion mobility increases HOWEVER self-discharge rate increases, therefore storing batteries at high temperatures is not advisable

Define *battery*

Portable electrochemical source made up of one or more voltaic cells connected in a series.

What are the materials used at the anode and cathode of a *lead-acid battery* during recharge/discharge?

RECHARGE Both use PbSO4 DISCHARGE Anode (-) uses Pb(s) Cathode (+) uses PbO2(s)

What does internal resistance limit?

The *maximum current* a battery can provide


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