The only byproducts of the electricity generated by hydrogen fuel cells are heat and water, which makes them an extremely desirable source of power, environmentally speaking. But hydrogen fuel cell kits currently exist only as limited-release concept cars or in miniature for RC cars or in hands-on science experiments for kids. A couple of big roadblocks stand in the way of wide deployment of fuel cells in the world's transportation fleets, namely cost and net energy yield.
Fuel cell technology for powering electric cars is currently more expensive than conventional engines. Plus, it can take more energy to make and run the cell than the cell can in turn put out. So, in spite of its overall cleanliness, fuel cell technology isn't always efficient.
Interestingly, those same RC car and science experiment kits available may provide a sense of how future fuel cells will gain energy. Some model car kits use solar panels to draw the energy to power electrolysis. Perhaps as solar power becomes more ubiquitous, so too will hydrogen fuel cells.