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

Herein, a stable metal-azolate framework with cyclic trinickel(II) clusters, namely [Ni 3 (μ 3 -O)(BTPP)(OH)(H 2 O) 2 ] (Ni-BTPP, H 3 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||IrO 2 @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 μ 3 -O atom in the cyclic trinickel(II) cluster serves as hydrogen-bonding acceptor to facilitate the dissociation of a H 2 O molecule adsorbed on the adjacent Ni(II) ion, giving a lower energy barrier of H 2 O dissociation compared with Pt/C; meanwhile, the μ 3 -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.