Iron-Based Metal–Organic Framework System as an Efficient Bifunctional Electrocatalyst for Oxygen Evolution and Hydrogen Evolution Reactions

The fabrication of highly efficient and sustainable electrocatalysts used for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is exceedingly challenging and warranted for overall water splitting. In this work, we successfully synthesized a series of metal-organic frameworks (MOFs), namely, as Fe2M-MOF (M = Fe, Co, Ni, Zn, Mn; H4L = 3,3,5,5'-azoxybenzenetetracarboxylic acid) under a simple and mild condition, in which the Fe3 cluster as a basic building unit was replaced by the second kind of metal center; at the same time, a redox-active organic linker was adopted. The Fe2M-MOF system as a multifunctional catalyst realizes great improvement of the OER and HER performances. Among of them, the Fe2Co-MOF catalyst exhibits an extremely low overpotential of 339 mV at a current density of 10 mA cm-2 and a very small Tafel slope of 36.2 mV dec-1 in an alkaline electrolyte for OER. This result has far exceeded the commercial catalyst IrO2. Meanwhile, Fe2Zn-MOF manifests excellent HER activity with a small overpotential of 221 mV at 10 mA cm-2 and a low Tafel slope of 174 mV dec-1. In addition, the good long-term stability for these catalysts can be evaluated under working conditions. Systematic investigations are used to explain the enhanced electrocatalytic mechanism. In conclusion, we provide a simple and effective strategy for the preparation of multifunctional catalysts for energy conversion applications based on a pristine MOF material with redox-active metal centers and organic linkers.