Fuel Cells & Rechargeable Batteries

By Anisha Kesarwani2013

• Combustion of hydrocarbons release large quantities of the greenhouse gas carbon dioxide into the atmosphere.

• Thus, electrochemistry is an important field of technological development because it offers a cleaner way of producing energy.

Background

Fuel CellsFuel Cells – electrochemical device for converting chemical energy stored in fuels directly

to usable electrical energy (i.e. basic voltaic cell)

Example - Hydrogen Fuel Cell

This is a redox reaction in which electrons are being transferred from the hydrogen to the oxygen. To produce an electric current, the reactants have to be physically separated. Once this is done, it creates the foundation for a fuel cell in which the reactants are continuously supplied to the electrodes.

Hydrogen- Oxygen Fuel Cell

• Operates with either an acidic or alkaline electrolyte

2 2 21

( ) ( ) ( )2

H g O g H O l 286kJ

Hmol

Hydrogen – Oxygen Fuel Cell with an Alkaline Electrolyte

The potassium hydroxide (KOH) provides the OH- that are transferred across the cell.

Hydrogen – Oxygen Fuel Cell with an Alkaline Electrolyte

Overall Reaction is the sum of the oxidation and reduction half reactions:

The fuel cell will function as long it has a continuous supply of hydrogen and oxygen. The electrodes are made of porous carbon with added transition metals such as nickel.

• The proton exchange membrane fuel cell has a membrane usually made from the strong & durable plastic Teflon which allows H+ ions to move from the anode to the cathode. Both electrolytes are coated with tiny particles of platinum to catalyze the reaction.

Hydrogen – Oxygen Fuel Cell with an Acidic Electrolyte

Anode Reaction:2H2 (g) 4H+ + 4e-

Cathode Reaction:4H+(aq) + O2(g) + 4e- 4H2O(l)

Hydrogen – Oxygen Fuel Cell with an Acidic Electrolyte

Overall Reaction is the sum of the oxidation and reduction half reactions:

A good animation to better understand the concept of hydrogen-oxygen fuel cell is:

http://www.lanl.gov/orgs/mpa/mpa11/animation.htm

Problems with the Hydrogen Fuel Cell

• Hydrogen gas is rarely found in its elemental form in nature and thus, has to be extracted from other sources.

• Hydrocarbons, including fossil fuels, & biomass can be broken down into hydrogen & carbon dioxide.

• Another option is to electrolyze water.

Rechargeable Batteries

• Primary Cells/Battery – electrons that are transferred in a spontaneous redox reaction produce electricity. Non-rechargeable. e.g. Lithium ion battery

• Secondary Cells/Battery – rechargeable cells & have a longer life in comparison the lives of primary cells. e.g. Lead acid battery, Nickel cadmium battery

Lead Acid Battery

• Can deliver a high current for short periods of time.

• Relies on the ability of lead to exist in two oxidation states: +2 & +4 and the insolubility of lead (II) sulfate: PbSO4

• Each cell contains two lead plates & the negative electrodes are filled with a paste of lead (IV) oxide

• Electrolyte - sulfuric acid

• Anode – Pb (s)

• Cathode – PbSO2 (s)

• When the lead- acid is discharging, it behaves like a voltaic cell: the anode is negative & the cathode is positive.

• Anode Reaction:

Pb(s) + SO42-(aq) PbSO4(s) + 2e-

Lead is oxidized to lead (II) sulfate.

• Cathode Reaction:

PbSO2(s) + 4H+(aq) + SO42-(aq) + 2e- PbSO4(s) + H2O(l)

Lead (IV) oxide is reduced to lead (II) sulfate

• Overall Reaction

• PbO2(s) + Pb(s) + 2H2SO4 PbSO4(s) + 2H2O(l)

Discharging a Lead Acid Battery

• To recharge the battery, the equation is reversed because the lead (II) sulfate produces is insoluble and it cannot be dispersed into the electrolyte.

• When it is recharging, the battery behaves like an electrolytic cell, the anode is now positive & the cathode is negative.

• Process: Of the two lead (II) sulfate on both the electrodes, one of them is oxidized back to lead (IV) oxide & the other is reduced to lead by electrolysis.

• Anode Reaction:

PbSO4(s) + 2H2O(l) + PbSO2(s) + 4H+(aq) + SO42-(aq) + 2e-

Lead (II) sulfate is oxidized to lead (IV) oxide.

• Cathode Reaction:

PbSO4(s) + 2e- Pb(s) + SO42-(aq)

Lead(II) sulfate is reduced to lead.

Charging a Lead Acid Battery

Charging a Lead Acid Battery

During the charging process, some water may be lost. The lead acid battery needs to be topped up with water at intervals to make up for the loss. This is an efficient method for testing the state of a batter, as the density decrease as the sulfuric acid is used up.

Nickel Cadmium Batteries

• Used in electronics & toys. • Discharging

- Positive Electrode: Nickel hydroxide- Negative Electrode: Cadmium hydroxide- Electrolyte: Potassium hydroxide (aq)Overall Reaction:

2NiO(OH)(s) + Cd(s) + 2H2O(l) 2Ni(OH)2(s) + Cd(OH)2(s)• Charging

- Anode Reaction:Ni(OH)2(s) + OH-(aq) 2NiO(OH)(s) + H2O(l) + e- Nickel(II) hydroxide is oxidixed to Ni3+ in the form of NiO(OH)- Cathode Reaction:

Cd(OH)2(s) + 2e- Cd(s) + 2OH-(aq)

Cadmium(II) hyrdroxide is reduced to the element.

• The reactions can be reversed because both metal hydroxides are insoluble.

• These batteries have a discharge memory. If their normal cycle of use involves short periods of discharge followed by periods of recharge, they can be discharged for longer periods.

Nickel Cadmium Batteries

Lithium Ion Battery

• Beneficial because of lithium’s low density & high reactivity.

• Can store a lot of electrical energy per unit mass.

• Two kinds of electrode used.– Transition metal compound ex. Manganese dioxide

– Graphite

• Used on laptops, cell phones, & other handheld devices.

• Anode: C graphite electrode

• Cathode: metal compounds (e.g. MnO2, CoO2, NiO2)

• The half-reactions are reversed when the battery is recharged.

Lithium Ion Battery

Negative Electrode:Li Li+ (polymer) + e-

Lithium is oxidized.

Positive Electrode:Li+ (polymer) + MnO2(s) + e- Li MnO2(s)

Assuming Li is present as Li+, it is the MN which is reduced.

Similarities & Difference B/w Fuel Cells & Rechargeable Batteries

Fuel Cell/Battery Advantages Disadvantages

Fuel Cell More efficient than direct combustion as more chemical energy is converted to useful energy; no pollution; low density

Hydrogen is a potentially explosive gas. It must be stored & transported in large/heavy containers, expensive, technical problems due to catalytic failures, leaks & corrosion

Lead Acid Can deliver large amts. Of energy over short periods.

Heavy mass, lead & sulfuric acid can cause pollution

Cadmium Nickel Longer life than lead acid batteries

Toxic, produces a low voltage, expensive

Lithium Ion Small density, high voltage, no toxic heavy metal

Expensive, limited life span