 Fuel cell: A device that converts chemical energy into electrical energy.  In the hydrogen- oxygen 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: 2H 2 (g) + 4OH - (aq) → 4H 2 O (I) + 4e -  What is the half reaction occurring at the cathode?  A: O 2 (g) + 2H 2 O(I) + 4e - → 4OH - (aq)  After cancelling, what is the overall reaction?  A: 2H 2 (g) + O 2 (g) → 2H 2 O (I)

 You also need to know the half reactions in acidic conditions:  Anode: 2H 2 (g) → 4H + (aq) + 4e -  Cathode: O 2 + 4H + + 4e - → 2H 2 O

 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)+SO 4 2- (aq) → PbSO 4 (s) + 2e -  What is the reaction happening at the cathode?  PbO 2 (s) + 4H + (aq) + SO 4 2- (aq) + 2e - → PbSO 4 (s) + 2H 2 O(l)  Overall reaction?  Pb(s) + PbO 2 (s) + H 2 SO 4 (aq) → 2PbSO 4 + 2H 2 O

 The battery is then recharged using an alternator when the car is running.  2PbSO 4 (s) + 2H 2 O(l) → Pb(s) + PbO 2 (s) + 2H 2 SO 4 (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) + 2e-  Reaction at the cathode?  NiO(OH)(s) + H 2 O(l) + e - → Ni(OH) 2 (s) + OH - (aq)  Overall reaction?  Cd(s) + 2NiO(OH)(s) + 2H 2 O(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.