Effects of electrodeposition conditions on the microstructures of co-based catalysts and their hydrogen generation characteristics in alkaline $NaBH_4$ solution

Abstract

Alkali metal borohydrides such as ＄NaBH_4＄, ＄LiBH_4＄, and ＄KBH_4＄ have received considerable attentions as excellent hydrogen storage materials. Among them, sodium borohydride (＄NaBH_4＄) provides the most economical and practical means to store and produce hydrogen. ＄NaBH_4＄ offers the high hydrogen storage density of up to 10.8 wt.% H, and produces hydrogen by the following hydrolysis and methanolysis reactions spontaneously: ＄NaBH_4＄ + ＄2H_2O＄ = ＄NaBO_2＄ + ＄4H_2＄ ＄NaBH_4＄ + ＄4CH_3OH＄ = ＄NaB(OCH_3)_4＄ + ＄4H_2＄ In addition, the overall hydrogen storage density of the ＄NaBH_4＄ methanolysis is lower than that of the hydrolysis because methanol is heavier than water, but the methanolysis of ＄NaBH_4＄ is adequate in a low temperature environment due to the low melting point of methanol, -97℃. Both reactions are strictly restrained by an addition of a base material such as NaOH, indicating that hydrogen can be stored safely in the alkaline ＄NaBH_4＄ solution at room temperature. For the hydrogen generation from the ＄NaBH_4＄ solution stabilized by NaOH, suitable catalysts such as Ru and Pt are needed to promote the hydrolysis reaction of ＄NaBH_4＄. Since the cost of such noble metal catalysts are very expensive, it is necessary to develop an economical alternative catalyst with an excellent efficiency for the hydrogen generation. Co and Co-P catalysts electrodeposited on Cu in sulfate based bath without or with an addition of ＄H_2PO_2^-＄ ions were developed for the hydrogen generation from alkaline ＄NaBH_4＄ aqueous solution. The microstructures of Co and Co-P catalysts and their hydrogen generation characteristics were analyzed as a function of cathodic current density and electrodeposition time. An amorphous Co-P electrodeposit with micro-cracks was formed in the sulfate based solution containing ＄H_2PO_2^-＄ ions. It was found that the amorphous Co-P catalyst formed at 0.01 A/㎠ for 1080 s showed the hydrogen generation rate of 954 ml/min.g-catalyst in 1 wt.%...