Instead of storing electrical energy received from an external source, fuel cells generate their own. A half way house between batteries and petrol driven vehicles, fuel cells produce the highest amount of energy for their size and weight but are expensive to produce.
Environmentally, the fuel cell is an improvement on the battery because, in consuming hydrogen, the only exhaust emission is harmless steam. The fuel cell concept predates both the IC engine and even the car itself, going back more than 150 years. Chemical energy is converted directly to electrical energy in a similar principle to the battery, but is continually fed with hydrogen stored in a pressurised tank on the vehicle.
Hydrogen passes over a platinum or nickel electrode containing a catalyst which strips electrons off the atoms. These pass through an external circuit while hydrogen ions pass through an electrolyte to a second electrode over which oxygen is passed. Water is formed at this electrode as a by-product and, because heat is generated, is discharged as steam. The harmful emission gases HC, CO, NOx and CO2 are completely eliminated.
Electrical voltage is generated between the two electrodes which is used to drive an electric motor on the vehicle. Fuel cells are highly efficient (about 60%) compared to batteries, but they are expensive to construct; they are also heavy and bulky, but improvements are on the way. Early fuel cells generated 1kW for every 20kg of weight (a typical IC engine might be 1kW/kg). Mercedes have a fuel cell working in its Necar II weighing only 6kg/kW generated and expects fuel cell cars to be competitive in about 15 years time. Ballard, a Canadian company, believes that life expectancy is 100,000 miles of operation.
Toyota recently announced its fuel cell electric car, the FCEV, a five door vehicle which uses the energy of reaction between oxygen and hydrogen to create electricity at a conversion efficiency of more than 60%. This compact cell used in the design overcomes the problem of bulk associated with earlier cells. It does this through the use of a new hydrogen absorbing alloy that stores large quantities of hydrogen at atmospheric conditions. It is claimed that this is double that of other hydrogen absorbing materials.
Each fuel cell consists of an electrolytic membrane placed between the positive and negative electrodes. Hydrogen is split into hydrogen ions and electrons by a catalyst on the negative electrode. While the smaller hydrogen ions can pass through the membrane, the electrons must travel through an external circuit thus generating electricity. The hydrogen ions and the electrons combine with the oxygen molecules to form water.
Although hydrogen does not pack the energy density of petrol and diesel, this is in part compensated for by the higher energy conversion efficiency.
Figure B: Advanced proton exchange membrane fuel cell developed by the Southwest Research Institute in San Antonio, Texas