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.% NaOH + 10 wt.% ＄NaBH_4＄ solution at $30^\circC$, which was 18 times higher than that of the polycrystalline Co catalyst. This is attributed to the fact that P codeposited into the Co catalyst significantly reduces the charge transfer resistance for hydrogen evolution reaction in alkaline solution.
In order to improve the hydrogen generation efficiency of the Co-P catalyst in alkaline ＄NaBH_4＄ aqueous solution, electrodeposition process to form microporous Co-P catalysts on Cu substrate directly was explored. In a chloride based Co-P bath containing glycine, the microstructure of the Co-P electrodeposits changed from nonporous to microporous with increasing the cathodic current density from 0.01 to 0.05 A/㎠, and their corresponding hydrogen generation rate significantly increased from 150 to 2290 ml/min.g-catalyst. Furthermore, the microporous Co-P electrodeposits grew gradually with an increase in electrodeposition time at a cathodic current density of 0.05 A/㎠, which additionally increased the hydrogen generation rate in ＄NaBH_4＄ solution. This is attributed to the increase in the number of surface catalytic site as well as the catalytic activity caused by a decrease in P concentration in the Co-P electrodeposits. It was found that the hydrogen generation of Co-P catalyst largely depended on the concentration of NaOH in alkaline ＄NaBH_4＄ solution; as it was increased from 1 to 10 wt.%, the hydrogen generation rate of the Co-P catalyst abruptly increased from 2290 to 6050 ml/min.g-catalyst.
It was revealed that the electrodeposited Co-P catalyst with microporous structure exhibits excellent catalytic activity for methanolysis as well as hydrolysis of ＄NaBH_4＄. The microporous Co-8.7 at.% P catalyst electrodeposited on Cu in the chloride based Co-P bath containing ＄H_2PO_2^-＄ ions at cathodic current density of 0.05 A/㎠ for 5 min showed the highest hydrogen generation rate of 470 ml/min.g-catalyst in 10 wt.% NaOH + methanol solution containing 0.2 g ＄NaBH_4＄ at$30^\circC$, which was much faster than that of the Ru catalyst, 170 ml/min.g-catalyst. The catalytic efficiency of the Co-P catalyst decreased gradually with increasing the cycle number due to the growth of ＄NaB(OCH_3)_4＄ layer on its surface. However, at the sixth cycle a very small amount of the Co-P particles dropped from the catalyst, new bare surface of the Co-P catalyst appeared and finally the catalytic efficiency increased abruptly. After the twentieth cycle the maximum hydrogen generation rate was 207 ml/min.g-catalyst, which was 44 % of that at the first cycle, but it was still higher than that of Ru at the first cycle.
