I know that they produce electricity and are an alternative to conventional batteries.
Hydrogen fuel cells are enormously expensive and don't last long.? This is a far bigger problem than their effluent. Water vapor is the biggest greenhouse gas (on average) by weight, but only accounts for about half the total greenhouse effect (mostly in the tropics, very little at the poles - this is why AGW warms the poles much more than the rest of the earth).? Also, water vapor only lasts about 9 days in the atmosphere before it precipitates out as rain or snow.? Adding or subtracting water vapor has a very small influence on climate; the movers and shakers are the things which drive the amount of water which stays, how much forms high clouds vs. low clouds vs. vapor, etc.
Fuel cells work with continuous supply of fuel
