Fe-modulated NH₂-CoFe MOF nanosheet arrays on nickel foam by cation exchange reaction for an efficient OER electrocatalyst at high current density in alkaline water/seawater

Electrocatalysts that perform well at high current densities play a key role in the industrial implementation of electrochemical water splitting. Recently, bimetallic metal-organic frameworks (MOFs) with abundant active sites have become promising catalysts for efficient hydrogen generation. Herein, we designed Fe-modulated NH₂-CoFe MOF nanosheet arrays on nickel foam fabricated by a hydrothermal method and cation exchange reaction for highly efficient oxygen evolution reaction (OER). The cation exchange reaction is a simple and facile strategy for preparing bimetallic MOFs, which allows for the tuning of additional metals while maintaining the structure of the monometallic MOF precursor. In particular, the chemical composition and electrochemical performance of an NH₂-CoFe MOF were confirmed to be strongly dependent on the valence state of the Fe precursor. The NH₂-CoFe MOF exhibited excellent electrochemical performance and durability in alkaline media, and it showed excellent OER activity at high current density in an alkaline electrolyte, requiring a low overpotential of 358 mV at a current density of 200 mA cm⁻². The NH₂-CoFe MOF also showed excellent stability and durability for 100 h of continuous operation, even at 100 mA cm⁻². Furthermore, we confirmed that the NH₂-CoFe MOF showed a small overpotential and great stability under real seawater conditions, proving that it is also applicable to seawater electrolysis. This work not only provides an effective strategy for synthesizing bimetallic MOFs, but also shows that the valence state of Fe ions in the cation exchange reaction affects the electrocatalytic performance of the resulting MOFs.