Hydrogen storage is the bottleneck for the implementation of a so-called “hydrogen economy”, which could replace the fossil fuels used today with hydrogen generated by renewable energy. Sodium borohydride (NaBH₄) is a promising hydrogen storage material for some applications. It releases hydrogen during hydrolysis, forming sodium metaborate hydrate (NaBO₂·x H₂O) as a byproduct.

Recycling NaBO₂ and regenerating NaBH₄ usually involves annealing dehydrated NaBO₂ and MgH₂ at high temperatures and hydrogen pressures. This process is costly, energy-intensive, and hazardous, which prevents the practical use of NaBH₄ as a hydrogen storage material.

Michael Felderhoff, Max Planck Institute for Coal Research, Mülheim an der Ruhr, Germany, Min Zhu, South China University of Technology, Guangzhou, and colleagues have developed an energy-efficient process to recycle the NaBO₂·x H2O formed during hydrolysis of NaBH₄. The team ball-milled a mixture of either NaBO₂·2 H₂O or NaBO₂·4 H₂O with Mg to regenerate NaBH₄ and achieved yields of 68.55 % and 64.06 %, respectively.

The approach avoids the energy-extensive dehydration of NaBO₂·x H₂O as well as the need for expensive MgH₂ as a reactant. Instead, MgH₂ is formed as an intermediate during the ball milling process. According to the researchers, a cost analysis for NaBH₄ regenerated by this method shows it to be promising as a practical hydrogen supply for vehicles.

• Enhancing the Regeneration Process of Consumed NaBH₄ for Hydrogen Storage,
  Liuzhang Ouyang, Wei Chen, Jiangwen Liu, Michael Felderhoff, Hui Wang, Min Zhu,
  Adv. Energy Mater. 2017.
  DOI: 10.1002/aenm.201700299