The European Union 2050 Strategy for Carbon Neutral Economy requires:
Strong need for an interconvertible (electricity ↔ storage) energy carrier to fill the energy and power gap left by fossil fuels
How much energy does it contain?
How much does it weigh? (Gravimetric energy density)
How much space does it occupy? (Volumetric energy density)
The conversion reaction is where work is extracted from the fuel. Is this a fast or slow process? (Power Density)
What does it become?
How much space does it occupy? How much does it weigh? (Reaction product density)
Can it be converted back into the original material easily? (Ease of regeneration)
Is this energy carrier easily adopted? (Cost of implementation)
Store alkali metal as a solid
Store reaction products in most compact form