The success of the electric vehicle is largely dependent on the realization of a battery with a large energy density (watt hours per liter) or the improvement of the fuel cell. The current electric vehicle in the market are faced with the challenge of range anxiety (Distance that can be covered with one full charge of the battery) for example the Nissan leaf have a rang less than 300km which is shorter than the journey from Nairobi to Mombasa, while the Tesla model S100D has the highest ever reported range of about 539 km per charge though its price is quite high. The price of electric vehicles is still high owing to the existing battery technologies and the high cost of setting up fuel cells technology infrastructure such as hydrogen filling stations. Charging time and charging infrastructure are also other challenges that have slowed the rolling out of the EV. To make the electric vehicle of more economic sense over the internal combustion engine vehicle the cost of batteries have to reduce significantly. Concerns on passengers’ safety in the case of a collision that can lead to contact of an anode and a cathode causing instantaneous release of energy resulting to explosion needs also be addressed. Most research in battery technology currently is focused on increasing energy density, reducing cost and increasing safety, while that on fuel cells is focused on reducing cost, operating temperature and making them operate at ambient conditions. This article gives an insight to the existing battery technologies from the lead acid battery to the futuristic lithium air battery and fuel cell technologies from polymer electrolyte membrane (PEM) to solid oxide (SOFC) cells.
Lead acid batteries.
This has remained as the most important widespread secondary battery since 1860 and its chemistry and electrochemistry has remained the same. There has been improvement such that hydrogen evolved at the negative electrode recombines with oxygen evolved at the positive electrode during recharging to replenish lost water hence maintenance free batteries. The competitive advantage of this battery in internal combustion engines is that it has high specific power (amount of power that can be delivered in short period of time) as hence its use in starting. Its major demerit is low specific energy (watt hours per kilogram) making it less attractive for use in EV. Currently very little research is being done on this battery. Other disadvantages include; slow recharging; poor performance at low temperature and lead is an environmental pollutant.
Nickel-cadmium cell (NiCad battery)
In this type of battery the electrochemistry is such that there is no net consumption of electrolyte and only species at constant activity are involved suggesting that the cell voltage remains constant. In practice this is not the case voltage drops from 1.35V to a cut off of 1.05V. Their average 1.2V compares poorly to 2 V in lead acid batteries, they have a superior specific energy. Due to the toxic nature of cadmium NiCad batteries require careful disposal making them unattractive for use in EVs.
Nickel-metal –hydride.
Compares in electrochemical reactions of nickel-cadmium cell at the anode and nickel hydrogen cell at the cathode. The metal halides are usually alloys of Zirconium (Zr), Titanium (Ti), Lanthanum (La) and misch metal (alloy of rare earth metals). These batteries are 70% batter than nickel-cadmium batteries, but their materials composition makes them expensive and un attractive for use in EVs.
Lithium Ion cell.
Need in portable consumer electronics has necessitated for a robust secondary battery that can be conveniently packaged in various sizes and shapes. This need has largely been met by the lithium ion cell. Lithium ions move from the cathode to the anode during discharge and back to the cathode during charging. Lithium cobalt oxide is usually used for the anode and graphite for the cathode. During charging the lithium ions intercalates between the graphite sheets and during discharging they intercalate in the metal oxide. Unlike in lithium-manganese primary cell in which metallic lithium is used in the cathode in the lithium ion battery use of metallic lithium results in dendrites formation resulting to internal short and thermal runaway as a result of instantaneous release of energy. This might be the scenario in reported cases of mobile phones battery explosion making it necessary for some manufactures to recall some phone models. The lithium ion battery have a cell voltage 4.2V3.0 V, specific power 75Whkg-1100 Whkg-1 and power density 180Whdm-1240Whdm-1 making the ideal choice for an Electric Vehicle. Currently all electric vehicles in the market are using lithium ion batteries in stacks although more research is needed on the type of electrolytes used such that the electrodes are well separated even in the worst case scenario of an accident, in this case more emphasis should be put on solid electrolytes.
Fuel cell
They are the most energy efficient devices for extracting power from fuels including hydrogen, natural gas and biogas. Fuel cells directly convert chemical energy in hydrogen to electricity with pure water and potentially useful heat as the only byproducts. Hydrogen powered fuel cells are pollution free and are more than two times efficient than traditional internal combustion engine. Hydrogen powered fuel cells have a conversion efficiency of approximately 60%. The fuel cell type that has already found application in the automobile industry is the polymer electrolyte membrane (PEM) type. The Toyota Mirai (future) is a typical example of a fuel cell vehicle with Japan being in the leading role of developing hydrogen filling stations and planning to have all teams that will participate in the 2020 Tokyo Olympics to be transported in fuel cell powered vehicles. In a single PEM fuel cell hydrogen gas flows through channels to the anode where a catalyst causes hydrogen molecule to separate into protons and electrons. The membrane allows only the protons to pass through it, the electrons flows through an external circuit to the cathode. This flow of electrons is electricity that is used to do work like powering an electric motor. Air flows through channels to the cathode, when electrons reach the cathode they combine with oxygen in air and the protons that moved through the membrane to form water in an exothermic reaction. The generated heat can be used externally. A single fuel cell produce 0.5 V to 1 V and thus several fuel cells are combined in series to form stacks for practical application.
Conclusion
The road to realty of competitively priced electric vehicle is still long until the challenges of producing a safe battery with high energy density at low cost are overcome. There have been several other technologies like the hybrids vehicle for examples the Toyota Prius as well as the plug in hybrid vehicle (Phev) which have helped in reducing emissions from vehicles to some degree, never the less these vehicles still pollute the environment as they still use the internal combustion engine. For benefits of electric and fuel cell vehicles in reducing emissions to be realize the source of the electric power have to be from green sources such as solar, wind, geothermal and hydro, otherwise it will be an excise of having a clean home and the problem of pollution at someone’s backyard especially when coal is used in generation. The electrolysis of water to generate hydrogen for fuel cell vehicle also needs to be done with electricity generated by green means.
Bibliography
- K.B. Oldham and J. C. Myland, Fundamentals of Electrochemical Science, Academic Press, Inc. (USA 1994) ISBN:0-12-525545-4
- Robert J. Naumann, Introduction to the Physics and Chemistry of Materials, CRC Press (2009).
- William D. Callister, Jr., Materials Science and Engineering an introduction, John Wiley & Sons, Inc. (USA 1985).
