When it comes to fuel cells, "The big market payoff is going to be vehicle propulsion systems," says Gregory M. Stoup, acting director at the Cleveland-based Center of Regional Economic Issues, The Weatherhead School of Management, Case Western Reserve University. "This is the biggest market with the biggest return." Indeed, many automakers are already gearing up for the fuel-cell future with such cars as General Motor Corp.'s HydroGen3, Nissan Motor Co.'s X-Trail, DaimlerChrysler's F-Cell and Honda Motor Co.'s FCX Fuel Cell Vehicle. But what exactly is a fuel-cell-powered vehicle? In essence, it's a vehicle that would use a hydrogen-rich fuel rather than gasoline. Also, rather than using a combustion engine, the cars would have fuel-cell stacks that would combine hydrogen and oxygen to form electricity, water and heat. The benefit: zero emissions. Unlike hybrid cars, which combine two forms of power such as electricity and gas (Honda Insight and Toyota Prius are just two examples), fuel-cell vehicles would rely only on the energy produced by the fuel cells.