High-frequency ultrasonic pyrolysis of 200 nm ultrafine Fe-doped NiO hollow spheres for efficient oxygen evolution catalysis

Abstract

The oxygen evolution reaction (OER) is essential for water splitting to produce H₂ gas, and its kinetically sluggish nature involving the transfer of four electrons hinders the energy-conversion efficiency and increases the cost; therefore, there is an urgent need to explore OER electrocatalysts with high efficiency and durability. The abundant internal cavities in hollow particles can provide a larger number of active sites and promote the electron/mass transport in the OER, and thus are highly desirable. However, their large-scale facile preparation, especially for small-sized particles, remains challenging. We present herein a scalable high-frequency ultrasonic pyrolysis strategy to prepare ultrafine Fe-doped NiO porous hollow spheres as OER catalysts. In this aerosol droplet-mediated method, a 3 MHz high-frequency ultrasonic wave ensures the formation of small sized droplets and oxide hollow products. The prepared Ni_0.9Fe_0.1O hollow spheres with a NiO phase exhibit a uniform and small size of ∼200 nm and a wall thickness of ∼10 nm, and they are highly porous with a specific surface area of about 50 m² g⁻¹. The fine and hollow structure increases the OER active site number, while the synergistic effect of Fe and Ni elements optimizes the intrinsic activity. As a result, the Ni_0.9Fe_0.1O hollow spheres have a low OER overpotential for achieving a current density of 10 mA cm⁻² (288 mV), suggesting an excellent OER performance. During potentiostatic testing, almost no decrease in the current density occurred upon catalyzing for 18 h, indicating the high stability of our catalyst, which may be because the well-crystallized NiO phase may suppress the rapid dissolution of the doped Fe in catalyzing the OER, thereby diminishing an appreciable activity loss. This synthesis strategy can be applied to other ultrafine hollow particles.