Second coordination sphere modulation during water oxidation with metal-hydroxide organic frameworks

Abstract

Herein, we have explored a NiFe-hydroxide organic framework (bridging ligand = chloranilic acid) for the electrochemical oxygen evolution reaction (OER). The anodic activation of the precatalyst forms the active catalyst (AC-2) consisting of Fe-doped α-Ni(OH)₂–γ-Ni(O)OH having a second coordination sphere interaction with the ligand chloranilic acid. The spectroscopic studies established that AC-2 has a different electronic and local atomic structure compared to the active catalyst (AC-3) obtained from NiFe-layered double hydroxide (NiFe-LDH) by anodic activation. The short Ni–O bond, its optimized covalency and the presence of high valent Ni³⁺ and Fe³⁺ in AC-2 facilitated the O–O bond formation. Interestingly, the ligand chloranilic acid tuned the second coordination sphere (SCS) of the AC-2 to improve the charge transfer kinetics. AC-2 delivered 50 mA cm⁻² current density for the OER at only 254 mV overpotential (η⁵⁰_OER) – far better than the other studied catalysts. The kinetic study reveals a primary kinetic isotope effect (KIE) in the rate-determining step (RDS) involving proton-coupled electron transfer (PCET) for AC-2.