Anodic Oxidation of Alkali Borohydrides Catalyzed by Nickel

Abstract

Borohydrides in alkaline media are potential fuels for fuel cells due to their high energy and power density. In this work, we studied the anodic oxidation of borohydrides on a nickel electrode. The open-circuit potential was found to be about 0.15-0.2 V more negative than the hydrogen potential, depending on the concentration of BH 4Ϫ . The results of polarization measurements indicated that a high power density can be achieved for the BH 4 Ϫ /Ni system. However, the coulombic efficiency was found to be 50% or less due to hydrogen evolution. A further investigation showed that hydrogen gas was not only generated from the hydrolysis reaction, but also from the electrochemical reaction. The actual anodic reaction of borohydride on the Ni electrode was proved to be a four-electron process rather than an eight-electron one.Fuel cells as efficient and clean power generation devices are attracting more and more public attention in recent years. Extensive effort is being made to apply them in various places including mobile appliances and vehicles. Although fuel cell technology has made considerable improvements in the last few decades, the industrial application of fuel cells will still take some time due to the problem of cost and some technical difficulties. For example, in some fuel cell types such as the polymer-electrolyte-membrane fuel cell ͑PEMFC͒, hydrogen with relatively high purity is required. However, the existing technologies for hydrogen storage are far from meeting the requirements. Lacking the infrastructure of fuel systems to a large extent blocks the way to the industrial application of these fuel cells.Besides gaseous hydrogen, some aqueous solutions containing compounds like hydrazine (N 2 H 4 ), 1,2 methanol (CH 3 OH), 3,4 and alkali borohydrides (MBH 4 , M ϭ Li, Na, K, etc.͒ 5,6 can also be used as the fuel. Among them, the direct methanol fuel cell ͑DMFC͒ using an aqueous solution of methanol is of considerable research interest recently. However, the low reactivity of methanol and its crossover to the cathode offer more challenges to researchers. Compared with methanol, alkali borohydrides (NaBH 4 , KBH 4 , etc.͒ are much more reactive and offer equally high energy densities.The anodic oxidation of BH 4 Ϫ was supposed to be as followsIn the above reaction, each BH 4 Ϫ ion releases eight electrons, resulting in very high electrochemical capacities for borohydrides (NaBH 4 5.67 Ah/g, KBH 4 3.98 Ah/g͒. The standard potential of Reaction 1 was calculated as Ϫ1.23 V ͑vs. NHE͒ by Pecsok 7 and Ϫ1.24 V by Stockmayer et al. 8 It is 0.4 V more negative than the hydrogen electrode in an alkaline medium. Also in the 1960s, some experimental efforts were made to employ alkali borohydrides as the fuel for fuel cells. 5,6,9 Although the observed potentials of BH 4Ϫ were less negative than the theoretical value, they proved to be promising fuels offering high potential and current density, except that they were expensive. Recently, borohydrides have been attracting attentions once again as a source of hydrogen...

Enclosure of Sodium Tetrahydroborate (NaBH4) in Solidified Aluminosilicate Gels and Microporous Crystalline Solids for Fuel Processing

“…New metal-organic framework compounds (MOFs) have been developed and proved to be outstanding hydrogen storage materials [6][7][8][9]. Besides these new materials also the well known hydride salt sodium tetrahydroborate (NaBH4) has recently been moved into new centre of interest as a possible hydrogen source according to its large hydrogen capacity, 5.3 wt% H2, which could be used to gain 2.4 l H2/g NaBH4 in the reaction with water [10][11][12][13].…”

Ammonia Borane Oxidation at Gold Microelectrodes in Alkaline Solutions

“…Boranes and borohydrides are also a nontoxic alternative to formaldehyde and hydrazine. There has also been an increased activity in the analysis of boranes [13,14] or borohydride materials as fuels for fuel cell systems [15][16][17][18][19]. In the optimisation of plating baths or catalysts for the fuel cell systems it is desirable to know the likely oxidation mechanism for the reducing agent or fuel material.…”

The preparation and performance of high activity Ni-Cr binary catalysts for direct borohydride fuel cells

“…It has been widely reported that the oxidation rate of BH -4 is increased by using anode electro-catalysts such as Ni, Au, Pt, or Pd despite their lower fuel efficiencies [2][3][4][5] (e.g. 50% for Ni [6], 75% for Pd [7]). Although Au can catalyze the complete 8e − oxidation of BH 4 − , its electrochemical activity is relatively low [8].…”