Fuel cell: A device that converts chemical energy into electrical energy. In the hydrogenoxygen fuel cell, both cathode and anode are made of porous carbon with platinum or palladium. The electrolyte solution used is KOH or NaOH. What is the half reaction occurring at the anode? A: 2H2(g) + 4OH- (aq) → 4H2O (I) + 4eWhat is the half reaction occurring at the cathode? A: O2 (g) + 2H2O(I) + 4e- → 4OH- (aq) After cancelling, what is the overall reaction? A: 2H2 (g) + O2 (g) → 2H2O (I) You also need to know the half reactions in acidic conditions: Anode: 2H2(g) → 4H+(aq) + 4eCathode: O2 + 4H+ + 4e- → 2H2O Rechargeable batteries are like voltaic cells. Except that you can add electricity to make the reaction go the other way. Lead-acid batteries are used in cars. Sulfuric acid is the electrolyte. The anode is made of lead plates, the cathode is made of lead(IV) oxide plates. What is the reaction happening at the anode? A: Pb(s)+SO42-(aq) → PbSO4(s) + 2eWhat is the reaction happening at the cathode? PbO2(s) + 4H+(aq) + SO42-(aq) + 2e- → PbSO4(s) + 2H2O(l) Overall reaction? Pb(s) + PbO2(s) + H2SO4(aq) → 2PbSO4 + 2H2O The battery is then recharged using an alternator when the car is running. 2PbSO4(s) + 2H2O(l) → Pb(s) + PbO2(s) + 2H2SO4(aq) Nickel-cadmium batteries are used as lightweight batteries (AA, AAA, etc.) KOH is used as an electrolyte. The reaction at the anode? Cd(s) + 2OH-(aq) → Cd(OH)2(s) + 2eReaction at the cathode? NiO(OH)(s) + H2O(l) + e- → Ni(OH)2(s) + OH-(aq) Overall reaction? Cd(s) + 2NiO(OH)(s) + 2H2O(l) → Cd(OH)2(s) + Ni(OH)2(s) Cadmium is an extremely toxic heavy metal, so when disposing of rechargeable batteries, make sure you dispose of them (just like any battery) in a special drop-off for batteries. Lithium ion batteries are used in products such as cell phones and laptops. It typically contains a graphite anode, and a metal oxide cathode. The electrolyte is a lithium salt. Similarities Both devices convert chemical energy into electrical Both have redox reactions taking place at the anodes and cathodes. Differences Fuel cells convert energy and rechargeable batteries store energy. Fuel cell requires a constant supply of reactants. Batteries have stored chemical energy and provide energy until they are used up. Batteries can be recharged. A fuel cell does not need recharging, but needs a continuous supply of fuel. The electrodes in fuel cells are made of inert materials. Fuel cells are more expensive than batteries.