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

Abstract

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 Co 4 (SO 4 ) 4 cluster within porphyrin‐based metal–organic frameworks (MOFs) and the electrocatalytic features of such Co 4 (SO 4 ) 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 Co 4 ‐M‐MOFs (M═Co, Cu, and Zn). Crystallographic studies revealed that these Co 4 ‐M‐MOFs have the same framework structure, having the Co 4 (SO 4 ) 4 clusters connected by metalloporphyrin units through Co─N pyridyl bonds. In the Co 4 (SO 4 ) 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 Co 4 ‐M‐MOFs are all active for electrocatalytic OER and ORR. Importantly, by regulating the activity of the metalloporphyrin units, it is confirmed that the Co 4 (SO 4 ) 4 cluster is active for oxygen electrocatalysis. With the use of Co porphyrins as connecting units, Co 4 ‐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 Co 4 (SO 4 ) 4 cluster in OER by facilitating the formation of O─O bonds. This work is of fundamental significance to present the construction of Co 4 (SO 4 ) 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.