Hydrogen gas is a crucial energy carrier in the renewable energy transition, as it has a high energy capacity of 122 MJ/kg. Nonetheless, it has a low density and thus a low volumetric energy capacity; at room temperature, 1 kg of H2 occupies a volume of 11 m3. Consequently, the storage of H2 is a key aspect of its large-scale application as a zero-emission energy carrier.
Interestingly, NaBH4 can act as a solid storage medium, releasing four equivalents of H2 upon its hydrolysis with >99% efficiency. A longtime challenge of the use of NaBH4 though is the formation of sodium metaborate, NaBO2, as so-called ‘spent fuel’. Yet if NaBO2 can be reused in the process, NaBH4 becomes a circular hydrogen storage material. Recently, we have developed a circular, waste-free method to regenerate NaBH4 through the electrochemical recycling of the NaBO2 spent fuel. This discovery lays the groundwork for the large-scale application of sodium borohydride as a sustainable hydrogen storage medium, and thus opens up exciting opportunities for importing hydrogen on a large, commercial scale.