Batteries and Electric Cars
As a nation, we have come to rely on the convenience of battery-operated tools and toys. On this page is a brief introduction to the theory behind battery operation (how do they work) and a look at some possible future trends.
Common types of Batteries
- Alkaline - The common and cheap "throw-away" battery (Energizer, Duracell, ...). These batteries principle advantage is their cost and convenience. Their primary drawback is that they cannot be recharged, so after use they become "trash".
- Lithium Ion - Batteries based on lithium have very high energy density and low weight, making them ideal for mobile applications (like cell phones and laptop computers). Their primary disadvantage includes the fact that lithium metal is very reactive, and will react violently with water. (In the fall of 2006, several major computer and electronics makers announced "massive battery recalls" due to defective Li-ion batteries).
- Nickel Ion - This category includes the 'older' nickel-cadmium (NiCd) batteries and the more recent nickel metal hydrides (NiMH). Both of these are rechargable with reasonable power densities and work well under "high drain" conditions, such as required in digital cameras. In addition, they do not appear to have as several "memory" problems as lithium batteries (although the memory problem of Li has improved significantly). On average, these batteries tend to be slightly more expensive than lithium batteries.
- Silver Oxide - These batteries are common in small devices (such as hearing aids). While these batteries have excellent properties (long life, low weight, and good discharge characteristics), the cost of silver makes these impractical for use in high energy demand applications.
- Lead Acid - The most common type of automobile battery. Because these batteries contain large amounts of lead (a toxic metal) and concentrated sulfuric acid, these batteries are typically "recycled". While not lightweight, these batteries can be recharged multiple times and provide adequate power density.
- Solar Cells (Photovoltaic) - The most environmentally-friendly "battery", solar cells obviously require sunlight, which limits their potential for many applications. The major problems associated with solar cells are their manufacturing costs and energy efficiency ( 5-19%).
- Fuel Cells - Fuel cells have the potential of dramatically changing both the battery and the automotive industry. Fuel cells differ from batteries in that fuel is constantly required for the battery to operate rather than being "stored" in the battery. The advantage of this approach is that fuel cells do not need to be recharged. In theory, 'burning' fuel in a fuel cell could lead to much higher efficiency than is possible in a combustion engine. The current disadvantage of fuel cells is that the technology has not yet advanced to the point of widespread commercial production.
Battery Components
- Cathode - reduction (gain of electrons) occurs here.
- Anode - oxidation (loss of electrons) occurs here.
- Electrolyte - Allow charge to flow, but can NOT allow anode and cathode reactions to "short circuit".
- External Connection - Completes the curcuit. This provides access to the electrical energy of the cell.
Chemical Reactions in batteries
Alkaline Cell Batteries
| Cathode | |||
| 2 MnO2 + H2O + 2 e- | ® | Mn2O3 + 2 OH- | |
| Anode | |||
| Zn + 2 OH- | ® | Zn(OH)2 + 2 e- | |
| Electrolyte | |||
| KOH(aq) | |||
Alkaline Cell Cutaway
See also
Energizer
site
Lithium Metal Batteries
| Cathode | |||
| MnO2 + Li+ + e- | ® | LiMnO2 | |
| Anode | |||
| Li | ® | Li+ + e- | |
| Electrolyte | |||
| Often propylene carbonate/dimethoxyethane (Must avoid water). | |||
Structure of Lithium metal cells
Nickel/Cadmium Batteries
| Cathode | |||
| NiO2 + 2H2O + 2 e- | ® | Ni(OH)2 + 2 OH- | |
| Anode | |||
| Cd + 2 OH- | ® | Cd(OH)2 + 2 e- | |
| Electrolyte | |||
| KOH(aq) | |||
Nickel/Metal-Hydride Batteries
| Cathode | |||
| Ni(O)(OH) + H2O + e- | ® | Ni(OH)2 + OH- | |
| Anode | |||
| MH + OH- | ® | M + H2O + e- | |
| Metal (M) is complex alloy (such as LaNi5) | |||
| Electrolyte | |||
| KOH(aq) | |||
Battery Limitations
Batteries provide energy, but where does energy come from? Processing of electrode materials typically requires more energy than battery supplies. Electric Cars may be less polluting, but energy is stilled required to charge them.
Solar Energy
Energy in sunlight is converted into electricity. Solar energy is practical on small scale (calculators, small lights, ...). However, except for special circumstances, solar energy is generally too inefficient and too expensive for widespread application (although the economics depends on the price of energy).
Fuel Cells
In fuel cells, a "fuel" must be continuously supplied to the electrodes rather than being stored in the battery. The advantage of this approach is that the battery does not need to be recharged. At least in theory, the direct conversion of "fuel" into electricity could be much more efficient than the conversion of fuel into energy by combustion (burning). The fact that high temperatures and pressures are not required means that the fuel cells should be less polluting and should not emit nitrogen oxides (NOx) and/or carbon monoxide.
While fuel cells have several potential advantages, technological limitations currently prevent the wide-spread implementation of fuel cells. In addition, it should be noted that all fuel cells (except those using H2 as the fuel) still release CO2.
One of the most obvious unanswered questions regarding fuel cells is "Which fuel should be used?". Among many others, potential fuels include ethanol, methanol, and hydrogen. In all cases, the net reaction for these cells is:
fuel + O2 ® H2O + CO2
Hydrogen has a number of potential advantages. Most importantly, "burning" of H2 produces water as the only product, so hydrogen is the ideal "clean" fuel. The limitations of H2 are (1) lack of a convenient source for the fuel and (2) potentially explosive nature of the gas (which may be more of a public relationships problem than a real issue). The dream is to somehow convert water (H2O) into H2 and O2. The problem with this is that it requires at least as much energy for this reaction as can be produced in the fuel cell. This limitation could be overcome if a "cheap" source of energy (such as a solar process) could be used.
Reactions of fuel cells
H2 Fuel cells
| Cathode | O2 + 4 H+ + e- | ® | 2 H2O | |
| Anode | 2 H2 | ® | 4 H+ + 4 e- | |
| Net Reaction | 2 H2 + O2 | ® | 2 H2O |
Methanol Fuel cells
| Cathode | 3 O2 + 12 H+ + 12 e- | ® | 6 H2O | |
| Anode | 2 CH3OH + 2 H2O | ® | 2 CO2 + 12 H+ + 12 e- | |
| Net Reaction | 2 CH3OH + 3 O2 | ® | 2 CO2 + 4 H2O |
Electric Cars
One of the most anticipated breakthroughs in the use of batteries is the widespread production/sales of electric cars. Shown below are links to current production plans for a selection of manufacturers.- Mitsubishi's concept car.
- Zap Xebra - A
car that is currently in production.
(Video) - Tesla Motors - Expect to begin sales in limited markets by summer 2007.
- Altairnano - Maker of a
next-generation Li battery. Teamed with Phoenix and Boshart to
demonstrate and SUV.
(Press release)