Constructing Co₄(SO₄)₄ Clusters within Metal-Organic Frameworks for Efficient Oxygen Electrocatalysis.

Abstract

Multinuclear metal clusters are ideal candidates to catalyze small molecule activation reactions involving the transfer of multiple electrons. However, synthesizing active metal clusters is a big challenge. Herein, on constructing an unparalleled Co₄(SO₄)₄ cluster within porphyrin-based metal-organic frameworks (MOFs) and the electrocatalytic features of such Co₄(SO₄)₄ clusters for the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is reported. The reaction of Co^II sulfate and metal complexes of tetrakis(4-pyridyl)porphyrin under solvothermal conditions afforded Co₄-M-MOFs (M═Co, Cu, and Zn). Crystallographic studies revealed that these Co₄-M-MOFs have the same framework structure, having the Co₄(SO₄)₄ clusters connected by metalloporphyrin units through Co─N_pyridyl bonds. In the Co₄(SO₄)₄ cluster, the four Co^II ions are chemically and symmetrically equivalent and are each coordinated with four sulfate O atoms to give a distorted cube-like structure. Electrocatalytic studies showed that these Co₄-M-MOFs are all active for electrocatalytic OER and ORR. Importantly, by regulating the activity of the metalloporphyrin units, it is confirmed that the Co₄(SO₄)₄ cluster is active for oxygen electrocatalysis. With the use of Co porphyrins as connecting units, Co₄-Co-MOF displays the highest electrocatalytic activity in this series of MOFs by showing a 10 mA cm^-2 OER current density at 357 mV overpotential and an ORR half-wave potential at 0.83 V versus reversible hydrogen electrode (RHE). Theoretical studies revealed the synergistic effect of two proximal Co atoms in the Co₄(SO₄)₄ cluster in OER by facilitating the formation of O─O bonds. This work is of fundamental significance to present the construction of Co₄(SO₄)₄ clusters in framework structures for oxygen electrocatalysis and to demonstrate the cooperation between two proximal Co atoms in such clusters during the O─O bond formation process.