Cobalt Oxides as Cobalt Boride Catalyst Precursors

Cobalt boride (Co2B) is an inexpensive catalyst for the hydrolysis of sodium borohydride (NaBH4) to generate hydrogen (H2) for PEM fuel cells. Preparation of Co2B in situ in the H2 generation vessel as well as in an external reactor was studied using cobalt (III) oxide (Co3O4) and lithium cobalt oxide (LiCoO2) as precursors. The oxide precursors were characterized by XRD and the Co2B catalysts by XRD and ICP. Co2B formation from the oxide precursors depends on the crystallinity of the oxides as well as the concentration of NaBH4 in the solution. During reduction of the oxides to CoB2, Co(s) metal, Co(BO2)2 may also form from competing side reactions, however crystalline oxides react faster leading to higher Co2B yield and better H2 generation efficiency. Crystallinity of Co3O4 was improved by preparing it at a higher temperature. Co3O4 prepared at 600 °C reacts faster leading to enhanced Co2B formation than Co3O4 prepared at lower temperatures (200 °C and 400 °C). The activation energy for the hydrolysis of NaBH4 by Co2B formed in situ from Co3O4 produced at 600 °C was calculated to be 77.90 kJ mol-1. This activation energy value is found to be slightly higher than that of Pt, Ru-based catalysts.

Hydrogen generation profiles of in situ formed Co2B from Co3O4 precursor: (a) Co3O4 prepared at various temperatures, 20 wt.% NaBH4 (5 wt.% NaOH) and (b) cumulative hydrogen generation; catalyst quantity 50 mg.

Thin-Film CoB Catalyst Templates

Thin-film CoB alloy catalysts were prepared on Ni-foam substrates using electroless as well as electroplating techniques. Electroless plating was carried out using cobalt (II) sulfate as the source of Co2+, sodium succinate as the complexing agent, and dimethyamine borane as the source of boron as well as the reducing agent. Electroplating was carried out using cobalt (II) sulfate and cobalt (II) chloride as the sources of cobalt, and boric acid as the source of boron. The thin-film CoB/Ni-foam templates were characterized using ICP, XRD and SEM techniques. The normalized B content was in the range of 1.0–1.30 and 0.20–0.60 wt.% for electroless and electroplated templates, respectively. The B content is less than that required for stoichiometric alloy formation, which indicates the simultaneous deposition of the Co metal along with CoB alloy. An optimum condition of 0.100 M L-1 each of cobalt (II) sulfate heptahydrate Co(SO4)·7H2O, sodium succinate (Na2C4H4O4) and dimethylamine borane (CH3)2NHBH3, at 60 °C with the pH value of 4–5 and a plating time of 1 h was identified for the preparation of the catalyst templates by electroless plating. Where as, 0.125 M L-1 each of cobalt (II) chloride hexahydrate (CoCl2·6H2O), Co(SO4)·7H2O, 0.125 M L-1 of boric acid at the current density range of 160–320 mA cm-2 and a temperature of 60 °C was identified as the optimum condition for the electroplating method. Maximum H2 generation rates of 1.64 and 0.30 L min-1 g-1 of catalyst were obtained with electroless and electroplated thin-film CoB/Ni-foam templates, respectively. The suitability of the electroless plated CoB/Ni-foam catalyst template for extended duration of hydrogen generation from NaBH4 was studied up to 60 h. Activation energies of 44.47 and 54.89 kJ mol-1 were calculated for electroless and electroplated CoB/Ni-foam catalyst templates, respectively.

H2 generation using electroless plated CoB/Ni-foam template. (a) Repetitive use test, 5 wt.% NaBH4 (5 wt.% NaOH); (b) extended durability test 10 wt.% NaBH4 (5 wt.% NaOH).
Krishnan P., Advani S.G., and Prasad A.K., "Thin-film CoB catalyst templates for the hydrolysis of NaBH4 solution for hydrogen generation," Applied Catalysis B: Environmental, Vol. 86, pp. 137-144, February 23, 2009. doi:10.1016/j.apcatb.2008.08.005
Krishnan P., Advani S.G., and Prasad A.K., "Cobalt oxides as Co2B catalyst precursors for the hydrolysis of sodium borohydride solutions to generate hydrogen for PEM fuel cells," International Journal of Hydrogen Energy, Vol. 33, pp. 7095-7102, November 12, 2008. doi:10.1016/j.ijhydene.2008.09.026
Krishnan P., Prasad A.K., Advani S.G., "Plated cobalt-boron catalyst on high surface area templates for hydrogen generation from sodium borohydride," US patent application 20090196821, serial no. 12/367089 (filed February 6, 2009, provisional application filed February 2008).