Hydrogen generation from NaBH4 solutions using a molecular mixed-valence nickel-substituted K7[NiIIINiII(H2O)W11O39]·15H2O catalyst

Abstract

In the exploration for clean and sustainable energy, hydrogen has emerged as an important alternative energy carrier. In this study, K₇[Ni^IIINi^II(H₂O)W₁₁₁O₃₉]·15H₂O (Ni-POM), a Keggin-type mixed valence polyoxometalate (POM) compound, was synthesised, and its catalytic effect on hydrogen production by hydrolysis of NaBH₄ was investigated. The Ni-POM complex was characterised by UV-visible spectroscopy (UV-vis), FT-IR spectroscopy (FT-IR), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), cyclic voltammetry (CV) and atomic force microscopy (AFM) methods. The characteristic absorption band observed at 265 nm in the UV-vis was attributed to ligand–metal charge transfer originating from the Ni(III) centres. XRD analyses confirmed the presence of a highly crystalline Keggin-type structure. TEM images identified its nanosized morphology dispersed in the range of 5–15 nm. The effects of temperature, substrate and catalyst concentrations were determined, which gave highhydrogen production rate, a conversion frequency (TOF) of 806.38 s⁻¹ and an activation energy (E_a) of 22.26 kJ mol⁻¹ at the optimum conditions for the Ni-POM catalytic activity studies. Results showed that Ni-POM is a stable and promising homogeneous catalyst capable of showing high catalytic activity in hydrogen production via hydrolysis of NaBH₄. The findings suggest that mixed-valence POMs may open opportunities for innovative applications in hydrogen energy technologies.