Hollow Spherical High-Entropy Perovskite Oxide for Supercapacitor and OER Applications

High-entropy oxides (HEOs) have attracted much attention in the field of electrochemistry due to their distinctive structural characteristics and unique properties. The multiple-principal elements in HEOs offer the multiple redox pairs and multiple possible active sites, which can enhance the energy storage capacity and the electrocatalytic performance. Although the notable progress is achieved in the development of HEOs electrodes, their electrochemical properties should be further improved to meet the requirements of high-performance supercapacitors and OER electrocatalysts. The abundant active sites for the Faradic redox reactions and short pathways for charge transportation could be constructed through the design of novel HEOs with advanced microstructures, thus improving the electrochemical properties. As advanced microstructures, a hollow structure has a great promise for energy storage and conversion because it can provide more accessible storage sites, more catalytic centers and a larger electrode/electrolyte contact area. It is thus expected that the construction of hollow structure is an alternative route to significantly promote the electrochemical properties of HEOs electrode materials. However, it is difficult to prepare the HEOs with a hollow structure due to the complexity of the high-entropy system.

In this work, a hollow spherical high-entropy perovskite oxide of La(Cr_0.2Mn_0.2Fe_0.2Ni_0.2Cu_0.2)O₃(HS-HEPs) was prepared by microwave solvothermal process and subsequent calcination treatment. The as-prepared HS-HEPs exhibited the excellent electrochemical performance when used as an electrode material for supercapacitors and OER electrocatalysts due to the advantages resulted from the combination of high-entropy perovskite and special hollow structure.

HS-HEPs were prepared by microwave solvothermal process and subsequent calcination treatment. Typically, 0.134 mmol Cr(NO₃)₃·6H₂O, 0.134 mmol Mn(NO₃)₂·4H₂O, 0.134 mmol Fe(NO₃)₃·9H₂O, 0.134 mmol Ni(NO₃)₂·6H₂O, 0.134 mmol Cu(NO₃)₂·3H₂O, and 0.5 mmol La(NO₃)₃·6H₂O were dissolved in 30 mL ethanol under stirring for 1 h to obtain the homogeneous solution. Afterwards, 60 mg of carbon spheres were added in the solution under ultrasonic treatment for 30 min. The resulting mixture was transferred to a 50 mL microwave digestion vessel. The vessel was heated in a microwave oven at a power of 210 W for 10 min. Subsequently, the obtained mixture was centrifuged, washed with deionized water, and dried in a vacuum drying oven at 70 ℃ for 12 h. Finally, the obtained precursor powder was calcinated in a tube furnace with a heating rate of 3 ℃/min at 650 ℃ for 2 h to acquire HS-HEPs.

The crystalline structure of the sample was determined by X-ray diffraction (XRD, D8 Davinci, Bruker Co., Germany). The morphology and microstructure of sample were characterized by field-emission scanning electron microscopy (FESEM, S-4800, Hitachi Co. Ltd., Japan) equipped with energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM, 2100F, JEOL Co., Japan). The X-ray photoelectron spectra were obtained by a X-ray photoelectron spectrometry (XPS, ESCALab 250, Thermo VG Co., USA). The supercapacitor and OER performance of the sample were measured on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., China).

The as-prepared samples display a cubic perovskite crystalline structure and a hollow sphere morphology. According to the XPS analysis, the variable oxidation states of Cr, Fe and Mn present in the HS-HEPs, which benefits the Faradaic redox reactions and increases the capacitance. In addition, the existence of high concentration of oxygen vacancies in HS-HEPs is beneficial to enhancing the capacitance and OER activity. Based on the GCD curve, the specific capacitance of HS-HEPs is estimated to be 406 F/g at 1 A/g. After GCD cycles of 5000 at a current density of 5 A/g, 65% capacitance is retained, implying a good long-term electrochemical stability. An asymmetric supercapacitor device (HS-HEPs//AC) with a two electrode configuration is assembled. A maximum energy density of 39.4 W·h/kg is achieved at power density of 746 W/kg. The OER activity of HS-HEPs is evaluated by a linear sweep voltammetry (LSV) polarization curve in 1 mol/L KOH aqueous solution using a standard three-electrode system. The overpotential of HS-HEPs is identified as 347 mV versus RHE for achieving a current density of 10 mA/cm², which is smaller than that of commercial IrO₂ (372 mV). The HS-HEPs possess the excellent electrochemical performance, which can be ascribed to the high specific surface area, abundant active sites, and high oxygen vacancy content, resulting from the combination of high-entropy perovskite and special hollow structure.

High-entropy La(Cr_0.2Mn_0.2Fe_0.2Ni_0.2Cu_0.2)O₃ hollow spheres with a perovskite crystalline structure were prepared by microwave solvothermal process and subsequent calcination treatment. The HS-HEPs possessed the excellent electrochemical performance, which could be ascribed to the high specific surface area, abundant active sites, and high oxygen vacancy content, resulting from the combination of high-entropy perovskite and special hollow structure. Based on the electrochemical performance, HS-HEPs could be used as supercapacitor electrode material and OER electrocatalysts. This work could provide a strategy to design and prepare high-entropy oxides with a hollow sphere structure, having promising applications in energy storage and conversion.