C.6 Electrochemistry, rechargeable batteries and fuel cells
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