Cyclic Trinickel(II) Clusters in a Metal-Azolate Framework for Efficient Overall Water Splitting

Abstract

Herein, a stable metal-azolate framework with cyclic trinickel(II) clusters, namely [Ni₃ (μ₃ -O)(BTPP)(OH)(H₂ O)₂ ] (Ni-BTPP, H₃ BTPP=1,3,5-tris((1H-pyrazol-4-yl)phenylene)benzene), achieved a current density of 50 mA cm^-2 at a cell voltage of 1.8 V in 1.0 M KOH solution, while the current density of 20%Pt/C@NF||IrO₂ @NF is just 35.8 mA cm^-2 at 2.0 V under the same condition. Moreover, no obvious degradation was observed over 12 hours of continuous operation at a large current density of 50 mA cm^-2 . Theoretical calculations revealed that the μ₃ -O atom in the cyclic trinickel(II) cluster serves as hydrogen-bonding acceptor to facilitate the dissociation of a H₂ O molecule adsorbed on the adjacent Ni(II) ion, giving a lower energy barrier of H₂ O dissociation compared with Pt/C; meanwhile, the μ₃ -O atom can also participate in the water oxidation reaction to couple with the adjacent *OH adsorbed on Ni(II) ion, providing a low-energy coupling pathway, thus Ni-BTPP achieves a high performance for overall water splitting.

Keywords: Metal-azolate framework; Nickel(II) cluster; Non-precious metal cluster; Overall water splitting.