The gasoline-powered automobiles on which we currently depend have become such a fixture of modern life that it is difficult to imagine a world without them. Yet these vehicles threaten the health of our environment and our nation's energy security.
Burning fossil fuels such as gasoline results in unhealthy air and poor water quality, and produces heat-trapping gases that contribute to global warming. Furthermore, transportation—which accounts for two-thirds of U.S. petroleum consumption—is the key factor in our dependence on foreign oil. Fuel cell vehicles powered by hydrogen could be part of the solution to these problems by reducing or eliminating tailpipe emissions and replacing gasoline as our main transportation fuel.
William Grove, a British physicist, developed the initial concept for fuel cells more than 150 years ago, but little practical use was made of this technology until the 1960s, when the National Aeronautics and Space Administration (NASA) turned to fuel cells for its space program. The space shuttle, for example, uses fuel cells to generate electric power and drinking water. Fuel cells are now being investigated for motor vehicles, power plants, and as replacement batteries for laptop computers and other electronic equipment.
A Clean Machine
Fuel cell technologies are typically categorized by the material used for the electrolyte-the material at the core of the fuel cell. The most promising technology under development for use in passenger vehicles uses a proton exchange membrane (PEM) as its electrolyte. A PEM fuel cell uses hydrogen stored onboard the vehicle and oxygen from the surrounding air as fuel, and generates only electricity, water, and heat—no emissions.
 | A diagram of a proton exchange membrane
(PEM) fuel cell. |
Hydrogen (H2) enters one side of the fuel cell through a flow field plate, a layer of electron-conducting material containing small channels that allow hydrogen molecules into the fuel cell (see diagram). A gas diffusion layer of porous carbon disperses the hydrogen molecules as evenly as possible, and a catalyst, usually platinum, splits the molecules into positively charged hydrogen ions and negatively charged electrons.
The core of the fuel cell houses the PEM, which has the unique ability to allow hydrogen ions (but not electrons) to pass through it. The electrons are forced to travel around the PEM, creating an electric circuit that transfers power to the motor, propelling the vehicle.
Oxygen (O2) travels through the fuel cell in a similar fashion as hydrogen, with the catalyst splitting the oxygen molecules into two separate atoms. When these atoms come into contact with the hydrogen ions that have passed through the PEM and the electrons traveling around it, water molecules (H2O) are formed. The water is then directed out of the fuel cell.
Under peak operating conditions, approximately 40 to 60 percent of the energy contained in the hydrogen gas is converted into electrical energy. The remaining energy is expended as heat, which must be dissipated by a water- or air-cooling system, similar to the radiator in a conventional automobile. Studies suggest this process could be more than twice as fuel-efficient as the traditional internal combustion system used by the vast majority of cars today.
Is Clean Hydrogen in Our Future? Hydrogen is the most abundant element in the universe. However, since it is most commonly found in combination with other elements (in the form of water, plant material, coal, petroleum, and natural gas), it must be extracted for use in fuel cells. Which hydrogen source we choose will determine the environmental benefits we can ultimately achieve through fuel cell technology.
Ninety-five percent of the hydrogen gas produced in the United States is extracted from methane, a component of natural gas. Separating the hydrogen from methane also produces carbon dioxide, a heat-trapping gas that contributes to global warming. Electrolysis, the process by which hydrogen is separated from water, accounts for the remaining five percent of U.S. hydrogen production. Although electrolysis itself emits no heat-trapping gases, there is a dramatic difference in overall emissions depending on the source of the energy powering the process. Fuel cell vehicles whose hydrogen comes from coal-powered electrolysis, for example, ultimately release more heat-trapping gases into the atmosphere than gasoline-powered vehicles (see the figure below).
For a hydrogen infrastructure to be sustainable, the hydrogen must be produced from clean, renewable sources such as bioenergy, wind, and solar power.
Change in Heat-Trapping Gas Emissions
for Fuel Cell Vehicles Relative to
Gasoline Vehicles
SOURCE: UCS calculation based on GREET 1.6, including production, delivery, and tailpipe emissions.
Assumes a fuel cell vehicle economy factor 2.4 times greater than a gasoline vehicle. |
The Engineers' Challenge
A single PEM fuel cell produces only a small amount of voltage-less than a standard AAA battery. In order to produce enough power for a vehicle motor, hundreds of individual fuel cells are combined into a fuel cell stack.
This stack is only one component of a fuel cell vehicle. Other components include a hydrogen storage device, an air compressor or blower (to deliver oxygen to the fuel cell), and a cooling system. These components require power from the fuel cell stack, which lowers overall system efficiency. They also add to the system's complexity. Most major auto manufacturers are developing demonstration fuel cell vehicles and are working to overcome the durability, performance, and manufacturing issues that have thus far prevented fuel cell vehicles from becoming a cost-effective reality.
It Won't Happen Overnight
A successful transition to a hydrogen-based transportation system depends not only on fuel cell technology, but also on the availability and cost of hydrogen fuel (see sidebar). Past experience with alternative fuels has shown the difficulty of establishing a new fueling infrastructure before there is large demand for the fuel. Unfortunately, fuel cell vehicles will not be attractive to buyers unless a hydrogen fueling system is already in place. This Catch-22 will need to be overcome with a combination of government involvement, industry investment, and a public willingness to face the real environmental and security costs of our current fossil fuel addiction.
As the challenges of establishing a fuel cell vehicle market and hydrogen infrastructure are being addressed, we cannot neglect existing technologies that can increase fuel economy and reduce smog-forming pollution in today's conventional vehicles. Hybrid-electric vehicles can sharply reduce oil use in the near term while also helping automakers gain experience with the electric drive technology and advanced battery systems required in fuel cell vehicles. Improving fuel economy and reducing emissions today will give us the breathing room we need to build a successful and sustainable fuel cell future.
Don Anair is a vehicles engineer in the Clean Vehicles Program.
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