Combination of fuel cells and electric motors - whether it has been used as part of an electric pump-fed cycles? gives a short summary of fuel cells and lists the following energy-containing fuels that can be used to produce electrical power in cells which are optimized for them; I don't think you can just pour any fuel into any cell.
Fuel cells are very versatile in terms of fuels they can use (hydrogen, methane, hydrocarbons, hydrogen peroxide, hydrazine etc.)
Hydrogen fuel cells (together with batteries) were used for electrical power in the Apollo program, where it was recombined with oxygen on board to produce electricity.
Question: Beyond H2 + 2O2 → 2H2O, which fuels have been "celled" to produce electrical power in space? Successful proof of principle demonstrations would certainly count.
I noticed that energetic materials like peroxide and hydrazine are mentioned above, so please include a simple summary of the reaction; hydrogen and hydrocarbons would require comparable quantities of an oxidizer, where as the others might be "monofuels" in the same way that they can perform as monopropellants.