A Review of Electrochemical Reduction of Sodium Metaborate

The recycling of sodium borohydride poses a huge challenge to the drive towards a hydrogen economy. Currently, mechano-chemical, thermo-chemical and electrochemical are the only reported methods of recycling sodium metaborate into sodium borohydride. Much attention has been devoted to the mechano-chemical and thermo-chemical methods of reduction, but little focus has been devoted to electrochemical methods. This review describes the electrochemical behaviour of borohydride (BH4−) and metaborate (BO2−) anions in alkaline solutions. The BH4− is stabilized in highly concentrated alkaline solutions, while the electro-oxidation of BH4− is dependent on the type of electrode material. The attempts to electro-reduce the BO2− into BH4− is reviewed and the challenges, suggestions and future outlook of electro-reduction for the recycling of BO2− into BH4− is highlighted.