Ultrasonic-assisted preparation of Fe-MOF with rich oxygen vacancies for efficient oxygen evolution

Abstract

Metal-Organic Frameworks (MOFs) materials with unique properties have attracted much attention in various fields. However, the limited number of active sites and the time-consuming synthesis process hinder their widespread application. Herein, this work demonstrates the first preparation of highly efficient and stable MOF (Fe-MOF-U) catalysts with enhanced specific surface area, morphology and active sites, through ultrasonic-assisted solvothermal method for oxygen evolution reaction (OER) electrocatalysis. The results show that the crystal size of Fe-MOF-U decreases significantly, and the specific surface area and pore size increase. In addition, many oxygen vacancies are produced in the crystal, making the reaction intermediates easier to form and thus improving the catalyst's oxygen evolution performance. The overpotential of Fe-MOF-U catalyst at 10 mA cm⁻² is 221 mV, which is lower than that of the conventional solvothermal synthesized Fe-MOF-S (237 mV). Furthermore, the turnover frequency of the Fe-MOF-U catalyst is 3.32 s⁻¹ at the overpotential of 300 mV, which is 5 times higher than that of Fe-MOF-S (0.70 s⁻¹). In situ Raman and methanol molecular probe tests indicate a weakened adsorption of *OH on the Fe-MOF-U surface. DFT calculations are consistent with the characterization results, both proving that the ultrasonic-assisted solvothermal method can generate specific oxygen vacancy (μ₃-O defect) in Fe-MOF-U which improved OER performance. This study provides a new idea for the morphology controlling and defect introduction of MOF and a new approach for its application.