Harnessing Transformation of Metal-Ligand Coordination in Dinuclear Ni(II)-Schiff Base Coordination Polymer for...

One of the major challenges in overall water splitting is improving the sluggish kinetics of the anodic oxygen evolution reaction (OER). Various 3d transition metal oxides, hydroxides, and complexes have been investigated as cost-effective OER electrocatalysts. Herein, a novel type of Ni-based coordination polymer, [Ni₂(tmd)₂]_n, containing six-coordinate Ni centers (two Ni─O and four Ni─N bonds) was synthesized. It acts as a pre-catalyst for alkaline OER, undergoing structural transformation in KOH into [Ni₂(tmd)₂]_n-OH with five-coordinate Ni centers, and further into the catalytically active state [Ni₂(tmd)₂]_n-CV after 150 cycles of cyclic voltammetry (CV). Under Fe-depleted alkaline conditions (KOH, pH = 13.8), [Ni₂(tmd)₂]_n-CV exhibited outstanding OER performance, with overpotentials of η₁₀ = 222 mV and η₅₀ = 385 mV, a Tafel slope of 94.15 mV/dec, and a charge-transfer resistance (R_ct) of 3.1 Ω, lower than those of NiPc, RuO₂, and NiO. The catalyst also manifested excellent durability, with only a 6.1% drop in current density after 100 h of operation. X-ray absorption and emission spectroscopies revealed its structural changes at different applied potentials. These changes are attributed to adsorption of oxygen species at the five-coordinate Ni sites, potentially associated with Ni-OOH intermediates forming during OER.