Double self-reinforced coordination modulation constructing stable Ni4+ for water oxidation

Abstract

High-valence transition metal species can overcome the restriction of linear scaling relationships to expedite reaction kinetics and offer high intrinsic activity for the oxygen evolution reaction (OER). However, their formation and stabilization are thermodynamically unfavorable according to Hund's rule. Here, the stoichiometric nitrate-coordinated Ni hydroxide with Co doping (Co-NiNH) has been fabricated. Physical characterization and electrochemical measurements demonstrate that the double self-reinforced coordination modulation of Co-NiNH is instrumental in the formation of stable high-density Ni⁴⁺. The rich nitrate ligands, serving as proton transfer relay, lower the energy barrier of Ni²⁺ to Ni³⁺/Ni⁴⁺ by promoting rapid proton diffusion in Ni (oxy)hydroxides, enabling the smooth evolution of Ni²⁺ → Ni³⁺ → Ni⁴⁺. More intriguingly, the coordinated nitrate can build an electrocatalytic stable configuration that invokes electron-absorbing bridging hydroxyl moieties to stabilize the generated Ni⁴⁺. The abundant Ni⁴⁺ subsequently impels holes into the oxygen ligand to activate direct intermolecular oxygen coupling, engaging lattice oxygen in the OER process at the Co–Ni dual-site. As a verification, Co-NiNH exhibits remarkable OER activity with an ultralow overpotential of 115.8 mV at 100 mA cm⁻². The turnover frequency and mass activity at an overpotential of 200 mV were up to 2.41 s⁻¹ and 15.77 A mg_metal⁻¹, respectively. Besides, the assembled anion exchange membrane water electrolyzer delivers 1.0 A cm⁻² at only 1.99 V_cell in 1.0 M KOH, superior to other reported water electrolyzers constructed from Ni-based catalysts.