Electrical systems can be developed around battery in preparation for emergent fuel cell technologies...
Intelligent energy is up to around 500W/kg using proton exchange membrane fuel cells, so only just behind Gaia batteries at 1.5kW/kg. Although Gaia batteries are only at the 0.47 MJ/kg mark, combinations of Li (just below H on periodic table) with other oxidisers may push towards H2/O2 of 16MJ/kg.
Fuel cell plane in aviation first - Intelligent Energy
Aerospace systems - Intelligent Energy
For comparison a tank of Kerosine is around the 45MJ/kg mark because the oxidiser does not need to be carried. Fuel cells can also be air breathing.