During any sunny period, photovoltaic cells perform electrolysis on water, dividing it into its constituent elements -- oxygen and hydrogen. At some later time, you can feed these gases into the fuel cell to get electricity to do work (like power the motor on this little car). The byproduct of all this wizardry is just a bit of water!
Now you can have a fuel cell of your own with which to experiment. This is a big deal. Fuel cells are one of the most promising forms of environmentally-friendly energy on the horizon. Learn more about fuel cells from this article on Fuel Cell Basics.
Go for it, check out The Fuel Cell Car and Experiment Kit