Hydrogen can be supplied to a fuel cell directly or may be obtained from natural gas, methanol or petroleum using a fuel processor, which converts the hydrocarbons into hydrogen and carbon dioxide through a catalytic chemical reaction.
There are just a couple of problems with that process.
1. If producing hydrogen is going to produce carbon dioxide, a greenhouse gas, it would be much more efficient to just burn the hydrocarbon in the first place (and besides, there's no real value to burning hydrogen, since you won't be reducing the production of greenhouse gases, you'll just be producing them somewhere besides in your automobile).
2. Energy has to be supplied to break the hydrocarbon into hydrogen and carbon dioxide - where do you get that energy, and how much pollution is produced in supplying it?
From the Ballard information page:
The core of the Ballard® fuel cell consists of a membrane electrode assembly (MEA), which is placed between two flow-field plates.
The MEA consists of two electrodes, the anode and the cathode, which are each coated on one side with a thin catalyst layer and separated by a proton exchange membrane (PEM). The flow-field plates direct hydrogen to the anode and oxygen (from air) to the cathode.
When hydrogen reaches the catalyst layer, it separates into protons (hydrogen ions) and electrons.
The free electrons, produced at the anode, are conducted in the form of a usable electric current through the external circuit. At the cathode, oxygen from the air, electrons from the external circuit and protons combine to form water and heat.
Maybe the electrolysis of water with electricity from tidal power generation would do the job.