For the fuelcells themselves? Chemical and mechanical engineers. For the fuel (hydrogen) preparation and storage? Ditto. On the power output end, electrical engineers are designing the inverters to use the power generated, but otherwise aren't all that much involved.
Do what you want, but personally I wouldn't bet on fuelcells as a long-term career option. Until we've derived all our stationary power needs from renewable resources, hydrogen as a fuel source for transportation won't be sensible. It makes no energetic sense to generate hydrogen from a fossil fuel source merely because you can use it in a fuelcell- there's just not enough efficiency gain in the fuelcell to compensate for all the energy losses in the process of making, distributing and storing the hydrogen.
Fuelcells are a useful but massively OVER-HYPED technology. Right now the "industry" makes no profit and is entirely fueled by government subsidy and stock market speculation, with some car company money thrown in as a distraction. Far easier to tout the dollars spent on this research and "development" than to spend the money convincing customers to actually pay more for less powerful, lighter and smaller but more fuel-efficient IC hybrid vehicles- much less to tax fuel consumption and spend the money on better public transit so that more cars can be parked.
Fuelcells will be a future industry for sure, but not the size of the present internal combustion engine industry. Don't bet your future locking yourself up as a specialist in this area.
