Unusual Defective Dicubane Azide Bridge Tetranuclear Ni^II₄-Cluster: Highly Active and Stable Electrocatalyst for Hydrogen Evolution Reaction

Designing highly efficient, environmentally friendly, sustainable, and durable catalysts for water electrolysis is vital for advancing renewal energy systems due to the continued reliance on fossil fuels leading to energy shortages and rapid environmental pollution. In this work, we have synthesized a tetranuclear centrosymmetric, unusual azide-bridged (end-on) Ni^II₄N₄O₂ defective dicubane core with all hexacoordinated nickel(II) atoms via a simple slow evaporation method. [C₄₂H₅₀Ni₄N₁₆O₁₀] (Ni₄-cluster) has an orthorhombic crystal system with space group Pbca, distorted octahedral of each nickel center to the formation of a tetranuclear nickel cluster. Single-crystal X-ray diffraction (SCXRD) analyses reveal that the cluster is tetranuclear with μ₂-(1,1-azido) and μ₃-(1,1,1-azido) bridges. The Ni₄-cluster shows a highly active and stable electrocatalyst for hydrogen evolution reaction (HER) in a 0.25 M acetate buffer electrolyte (pH = 3.8), achieving a low overpotential of only 287 mV at 10 mA cm^-2 current density, with high double-layer capacitance (C_dl) (0.34 mF cm^-2) and low charge-transfer resistance (17.6 Ω). The intrinsic parameters such as Tafel slope (316 mV dec^-1), turnover frequency (TOF) (1.82 s^-1 at 350 mV), and exchange current density (0.002 A cm^-2) correlate to the fair electrocatalytic activity of the Ni₄-cluster cathode for the HER. The electrode remained stable for over 25 h at a constant current density of −10 mA cm^-2. The Ni₄-cluster delivered over 92% Faradaic efficiency for hydrogen production in the 0.25 M acetate buffer electrolyte. Compared all the electrochemical analyses, the Ni₄-cluster reported that it is more active, efficient, and sustainable for H₂ production and makes it an auspicious candidate in the field of energy conversion application.