Cobalt in NaBH4 hydrolysis

Cobalt-catalyzed hydrolysis of sodium borohydride (NaBH4) has attracted great attention since the hydride is believed to be promising hydrogen storage material. Cobalt is an efficient metal catalyst and has already proven to be a potential alternative to noble metals. Nevertheless it is not stable. Indeed it transforms into a Co- and B-based material when on contact with NaBH4. Through ex situ characterizations (e. g. ICP, XRD, XPS and SEM), the Co- and B-based material has been supposed to be either a cobalt boride CoxB (with x from 1 to 3) or a Co-B alloy. This contradiction is the topic of the present paper. Herein, the literature dedicated to the Co-catalyzed NaBH4 hydrolysis is exhaustively surveyed. The results of the ex situ characterizations are largely discussed, for example that: (i) the ex situ characterized CoxB or Co-B might be different from the in situ formed Co- and B-based catalyst; (ii) there is no clear evidence of the formation of either CoxB or Co-B; (iii) the in situ formed catalyst would change in accordance with a cycle in the course of the hydrolysis; and (iv) in situ characterizations are clearly required but their setting up is a challenge. These conclusions, among others, are argued.