The Figure [1] illustrates that water can be ‘split’ into oxygen and hydrogen (green arrows), and most of the energy used in this process can be released again when hydrogen reacts with oxygen. A carbon-based energy carrier system is also illustrated (blue arrows), but hydrocarbons are currently energy-consuming and difficult to produce from biomass or CO₂ from the atmosphere.

Notice that a sustainable future can only be created with closed-materials cycles for all chemicals and materials that we use. Today, most materials are unfortunately ‘single-use’ and not re-cycled. Likewise, CO₂ produced from fossil fuels is discarded into the atmosphere symbolized by personal vehicles and industrial emission ultimately leading to global warming (black arrow, picture row).  

[1] "Complex hydrides for hydrogen storage – new perspectives", Morten B. Ley, Lars H. Jepsen, Young-Su Lee, Young Whan Cho, José Bellosta von Colbe, Martin Dornheim, Masoud Rokni, Jens Oluf Jensen, Michael Sloth, Yaroslav Filinchuk, Jens Erik Jørgensen, Flemming Besenbacher, Torben R. Jensen, Mater. Today, 2014, 17(3), 122-128    

Link: 
www.sciencedirect.com/science/article/pii/S136970211400073X
(Open Access)