The voltage produced by a fuel cell is low. To increase this more fuel cells are Stacked together.
http://www.fueleconomy.gov/Feg/animation/images/fmodstack.gif
The efficiency of hydrogen fuel cells to generate electricity is about 80% which means that 80% of the energy from the Hydrogen is converted into electricity.
Fuel cells are being developed that will run on waste water. Microbes are employed to break down organic matter present in the waste water to releases hydrogen and electrons. This reduces the amount of organic material in the water leaving some heat and pure water.