Commercial Voltaic Cells

Voltaic cells provide a convenient, safe and portable supply of electrical energy.

• The Industrial Revolution marked the development of heat engines, and other devices, that utilized the energy released from combustion reactions in the form of heat
• The Digital Age (Electronic Revolution?) revolves around devices that require energy in the form of electrical energy. Such devices require voltaic cells and redox chemistry (as opposed to gasoline and combustion chemistry)
• The future??? This might include devices that depend upon chemical reactions that release light.

Technology in transition

• The automobile industry has relied upon combustion reactions (the internal combustion engine). But these pollute, causing acid rain and global warming. Also, fossil fuels are a limited resource.
• Electric cars promise lower pollution, and are essentially silent (an important feature in a population-dense world). A few electric cars have been developed, but they have problems:
• Range is limited
• The electrical systems are heavy
• They are expensive

• Hybrid combustion/voltaic systems have been developed and can be purchased
• These designs combine a small internal combustion (IC) engine with an electric motor and voltaic cells
• The IC engine may power an electric generator for the electric subsystem. The IC engine can therefore run at its most efficient rpm at all times (even when the car is not moving). Alternatively, the IC engine can be drive the wheels directly during situations when extra power is needed (starting, passing, going uphill, etc.). All other times, the car is powered by the electric motor.
• These cars still have problems related to the size/weight/storage capacity of voltaic cells

Other stuff

• Steam engines in trains used heat (from the combustion of coal) to cause a phase transition in water, this resulted in gaseous expansion of the steam, this in turn was used to do mechanical work.
• After steam engines, diesel electric trains came into being. These are like some of the hybrid electric cars: the diesels were used not to turn the wheels, but to power electric generators. The generators powered electric motors that drove the wheels
• Purely electric trains don't run by voltaic cells that they carry; they are typically powered by overhead wires, or electrified tracks. Electric trains are quiet and don't pollute, and can be used in crowded cities without causing too much disturbance.
• Electric powered airplanes? The history of flight is that almost all ideas are tested first on small models. The bane of all aircraft is weight, and unfortunately, one of the most successful types of voltaic cells is based on lead! (lead acid battery). However, with careful design, some model aircraft have flown powered by electric motors and lead acid batteries. However, they were at the limits of their design - often they could fly once airborne, but didn't have the ability to actually take off. Also, lifting the weight of the battery required the entire lifting capacity of the plane, so they could carry no useful load. More recent success in electric flight has come with a combination of new types of batteries (particularly nickel cadmium), light but strong construction methods (carbon fiber and composite type construction) and developments in stronger magnets for the motors (which weigh a heck of a lot also). Nickel cadmium batteries have a higher storage capacity for their weight than lead acid batteries. They also can discharge with high current, and maintain their voltage right up until the end of their discharge - great features for an electric airplane. In conjunction with powerful rare-earth magnets in the electric motors, electric model airplanes have demonstrated remarkable speed, duration, and useful lifting capacity. What about full-size aircraft? While it is not unreasonable to expect that some day full size electric aircraft may be developed, for the foreseeable future the gas-turbine engine will be the powerplant of choice for commercial aircraft. A man-carrying electric plane has successfully flown using solar cells, instead of batteries, for the voltaic power.

The redox half-reactions in a lead-storage (lead acid) battery are as follows:

• Cathode:

PbO₂(s) + SO₄^2-(aq) + 4H⁺(aq) + 2e^- ® PbSO₄(s) + 2H₂O(l)

• Anode:

Pb(s) + SO₄^2-(aq) ® PbSO₄(s) + 2e^-

• Overall redox reaction:

PbO₂(s) + Pb(s) + 4H⁺(aq) + 2SO₄^2-(aq) ® 2PbSO₄(s) + 2H₂O(l)

• Sulfuric acid provides the protons and sulfate ions: 2H₂SO₄(aq) ® 2SO₄^2-(aq) + 4H⁺(aq)
• The solid electrodes (Pb and PbO₂) do not react in their respective redox reactions to produce soluble ions. In both cases, PbSO₄(s) is formed and remains attached as a solid to the electrode(s). Thus, ions do not diffuse from one half-cell to the other. Therefore, the two electrodes can be placed in the same container of acid. Its pretty remarkable.
• Water is produced and sulfuric acid is consumed during the reaction.
• The EMF per "cell" under standard conditions is:

E⁰_cell = E⁰_red (cathode) - E⁰_red (anode) = (+1.685 V) - (-0.356 V) = 2.041 V

• 6 cells can be combined end-to-end (i.e. in series) to produce about 12 V (what you find in a typical car)
• This is a reversible reaction. If electrical current is applied in the opposite direction (this is the job of a generator or alternator in your car) the electrodes are regenerated

2PbSO₄(s) + 2H₂O(l) ® Pb(s) + PbO₂(s) + 4H⁺(aq) + 2SO₄^2-(aq)

These are your basic (not alkaline) type battery. The 6V battery in your emergency flashlight is most likely a dry cell type. It was invented in 1866. The reactions are curiously rather complex. A simple version of the half-reactions is as follows:

• Cathode:

2NH₄⁺(aq) + 2MnO₂(s) + 2e^- ® Mn₂O₃(s) + 2NH₃(aq) + H₂O(l)

• Anode:

Zn(s) ® Zn²⁺(aq) + 2e^-

• The construction consists of a zinc electrode (for the anode). The cathode is a bit weird - it is an inert support of graphite is immersed in a paste of ammonium chloride and manganese dioxide. The dimanganese trioxide solid precipitates out on the surface of the inert graphite
• This is not reversible, so the battery cannot be recharged (the electrode reaction products diffuse throughout the cell)
• In an "alkaline" type battery the ammonium chloride is replace by potassium hydroxide (KOH). This provides more useable voltage and greater capacity than the typical dry cell

• Cathode:

NiO₂(s) + 2H₂O(l) + 2e^- ® Ni(OH)₂(s) + 2OH^-(aq)

• Anode:

Cd(s) + 2OH^-(aq) ® Cd(OH)₂(s) + 2e^-

This type of cell uses a cadmium anode and a nickel dioxide cathode

• The solid products of the respective electrode reactions adhere to the electrodes and do not diffuse throughout the cell. Thus, the redox reaction is reversible (i.e. like the lead acid cell, the nickel cadmium cell is reversible)
• No gases are produced, so the cell can be sealed

Combustion reactions produce heat that, in turn, can be used to produce electricity. However, typically less than 40% of the heat energy is converted to electrical energy - the rest is "wasted" as heat.

Combustion reactions are actually redox reactions: diatomic oxygen (0 oxidation number) is reduced to carbon dioxide (-2 oxidation number) or water (-2 oxidation number). Direct production of electricity from redox chemistry, instead of combustion, for these reactions could result in higher efficiency of production of electrical energy. Voltaic cells that perform this type of redox reaction for conventional fuels (such as hydrogen or methane) are called fuel cells

• A common reaction being utilized in fuel cells is the reduction of oxygen by hydrogen
• Cathode:

O₂(g) + 2H₂O(l) + 4e^- ® 4OH^-(aq)

• Anode:

2H₂(g) + 4OH^-(aq) ® 4H₂O(l) + 4e^-

• Overall reaction:

2H₂(g) + O₂(g) ® 2H₂O(l)

This is currently an very expensive way to generate energy, but is extremely efficient and compact. It's broadest application to date has been to provide electricity (and drinking water) for spacecraft.