Research On Synthesis Of Sodium Borohydride From Sodium Metaborate By Electrolysis

Sodium borohydride is not only a good reducing agent but also the excellent hydrogen source in fuel cell. The hydrogen-storage capacity of sodium borohydride is 10.8%; 1mol sodium borohydride with water can produces 4mol hydrogen and 1mol sodium metaborate. The theoretical open-circuit voltage of direct sodium borohydride fuel cell (DBFC) is up to 1.64V and has a high energy density 9.3Wh/g, which is higher than methanol fuel cell (6.1 Wh/g). But the price of sodium borohydride is very expensive which limit the wide application in DBFC. Therefore, research on synthesis of sodium borohydride from sodium metaborate by electrolysis possesses very important applicable value.At first, the electrochemical behavior of sodium metaborate alkaline solution on gold, nickel, lead, copper, (TiO2,ZnO,C)/Ni is studied by cyclic and linear sweep voltammetry. The results have shown that the CVs of sodium metaborate alkaline solution compare with sodium hydroxide solution, there is no new oxidation and reduction peaks, which indicates that metaborate ions is not directly involved in the cathode reduction processSecondly, the electrochemical behavior of sodium metaborate alkaline solution and sodium borohydride alkaline solution on gold is studied by cyclic and linear sweep voltammetry. The results have shown that there is an obviously oxidation peak at -0.4V in the CVs of sodium borohydride while there is no in sodium hydroxide and sodium metaborate. This can be used to analysis there is sodium borohydride or not in the electrolyte. In addition, the linear relationship was found between the current of oxidation peak and the concentrations of borohydride ions, the concentration from 10-4 mol/L to10-3 mol/L of borohydride can be monitored by this method and the relative error are 2.32% and 3.40%.Finally, the impact of current pulse, electrode materials, electrolysis time and thiourea (TU) additives are studied. The results have shown that metaborate ions can be effective attracted to the cathode to produce borohydride by current pulse. The best pulse is T1=2s and T2=3s. The most effective electrode materials for synthesis of sodium borohydride by electrolysis is found to be (ZnO,MnO2,C)/Ni, (ZnO,MnO2,C)/Ni also showed certain activity. The best time of the electrochemical reaction is 5 hours. The reduction of sodium metaborate by electrolysis can be improved when appropriate amount of TU is added. The conversion rate of synthesis of sodium borohydride form sodium metaborate by electrolysis is 8.3%, and the efficiency of current is 14%.