The only emission is water
Fuel cells are electrochemical cells that convert hydrogen and oxygen into water whilst producing electricity. Fuel cells can produce electricity continuously as long as they have a supply of hydrogen and oxygen.
The key components are the anode, cathode and the electrolyte (similar to a battery). Proton Exchange Membrane (PEM) fuel cells use a proton conducting polymer membrane as the electrolyte and can operate at higher power densities than other fuel cell types. This makes them ideal for use in vehicles.
The only by-product other than water is heat. This waste heat can be captured and used as a useful source of heating. A single fuel cell is too small for most applications so they are combined into “stacks” that can consist of hundreds of cells. The output voltage of the stack is determined by the number of cells.