Comparison of specific energy and energy density of the energy carriers used in the proposed storage technology with currently used technologies and gasoline.

	Energy density [kW h L−1]	Specific energy [kW h kg−1]
Elemental Fe (calculated based on Atkins and de Paula 2010 (ref. 42))	11.3 (Fe oxidation with HCl yielding H2)	1.4 (Fe oxidation with HCl yielding H2)
	16.1 (burning Fe powder to Fe2O3)	2.1 (burning Fe powder to Fe2O3)
Gasoline (Mazloomi et al. 2012 (ref. 43))	9.5	12.9
Hydrogen gas (atmospheric pressure)	∼0.003 (Lanz et al.44)	33.3 (LHV)
Compressed hydrogen gas (700 bar)	1.3 (Hirscher & Hirose 2010 (ref. 45))	39.4 (HHV)
Liquid hydrogen (T =< −253 °C)	2.2 (Hirscher & Hirose 2010 (ref. 45))	(Lanz et al.44)
Hydrogen in hydrides (Abe et al. 2019,46 Klopčič et al. 2023 (ref. 47))	1.6–6.2	0.4–6.2
Commercially available Li-batteries (Li-ion, Li-polymer) (Choi and Aurbach 2016,48 Hamerr and van Niekerk 2015 (ref. 49))	0.2–0.7	0.1–0.3
Emerging battery technologies (Li–S, Li–air, Mg–air) (Hagen et al. 2015,50 Kim et al. 2019 (ref. 51))	0.4–1	0.35–1.5
Flywheels (Ibrahim et al. 2008 (ref. 52))	∼0.02
Compressed air energy storage (Hameer and van Niekerk 2015 (ref. 49))	∼0.01
Pumped hydropower energy storage (Ibrahim et al. 2008 (ref. 52))	∼0.001