Ammonia <-> Hydrogen+Nitrogen  storage has been studied in detail and is considered practical but unproven on a large scale.  Avoiding mixing with air has many advantages too (avoid contamination of the air and of the cells) and NH3 and N2 +H2 take up roughly the same volume*pressure allowing the same storage to be used for charged and discharged fuels (you can store H2 and N2 mixed together safely at room temp, they only burn with a catalyst or high temps).