fuel cell

A fuel cell is basically a combustion that also produces current (i.e. it’s not really a battery since we need an input of fuel). The energy is clean because the only byproduct is water. Mechanical generators have 40-50% efficiency, while fuel cells have an efficiency of 80-100%.

$$\begin{array}{rlrl}(\times 2)\text{ Anode:}& & \mathrm{H}{}_2(\mathrm{g})+2\mathrm{OH}{}^{-}(\mathrm{aq})& \to 2\mathrm{H}{}_2\mathrm{O}(\mathrm{l})+2\mathrm{e}{}^{-}\\ \text{Cathode:}& & \mathrm{O}{}_2(\mathrm{g})+2\mathrm{H}{}_2\mathrm{O}(\mathrm{l})+4\mathrm{e}{}^{-}& \to 4\mathrm{OH}{}^{-}(\mathrm{aq})\\ \text{Overall:}& & 2\mathrm{H}{}_2(\mathrm{g})+\mathrm{O}{}_2(\mathrm{g})& \to 2\mathrm{H}{}_2\mathrm{O}(\mathrm{l})\end{array}$$