Fuel processors also have drawbacks, including pollution and overall fuel efficiency.
Although fuel processors can provide hydrogen gas to a fuel cell while producing much less pollution than an internal combustion engine, they still produce a significant amount of carbon dioxide (CO2). Although this gas is not a regulated pollutant, it is suspected of contributing to global warming.
If pure hydrogen is used in a fuel cell, the only byproduct is water (in the form of steam). No CO2 or any other gas is emitted. But because fuel-cell-powered cars that use fuel processors emit small amounts of regulated pollutants, such as carbon monoxide, they will not qualify as zero emissions vehicles (ZEVs) under California's emissions laws. Right now, the main technologies that qualify as ZEVs are the battery-powered electric car and the hydrogen-powered fuel-cell car.
Instead of trying to improve fuel processors to the point where they will emit no regulated pollutants, some companies are working on novel ways to store or produce hydrogen on the vehicle. Ovonic is developing a metal hydride storage device that absorbs hydrogen somewhat like a sponge absorbs water. This eliminates the need for high-pressure storage tanks, and can increase the amount of hydrogen that can be stored on a vehicle.
Powerball Technologies wants to use little plastic balls full of sodium hydride, which produce hydrogen when opened and dropped into water. The byproduct of this reaction, liquid sodium hydroxide, is a commonly used industrial chemical.
Another downside of the fuel processor is that it decreases the overall efficiency of the fuel-cell car. The fuel processor uses heat and pressure to aid the reactions that split out the hydrogen. Depending on the types of fuel used, and the efficiency of the fuel cell and fuel processor, the efficiency improvement over conventional gasoline-powered cars can be fairly small. See this comparison of the efficiencies of a fuel-cell-powered car, a gasoline-powered car and an electric car.