High-Entropy Spinel Oxide: A Prolific System of Panoramic Functional Properties

Here, we report the spectacular evolution of the magnetic, supercapacitor, and oxygen evolution reaction (OER) activities of high-entropy-stabilized spinels with slight tuning in chemical composition. (Cr0.6Mn0.6Fe0.6Co0.6Ni0.6)O4, (Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)Cr2O4, (Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)Fe2O4, and (Mn0.2Co0.2Ni0.2Mg0.2Zn0.2)Fe2O4 were synthesized and characterized using powder X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM) imaging, energy dispersive X-ray analyses (EDX), magnetization, and electrochemical studies. Interestingly, all of the compositions exhibit long-range magnetic ordering. Except (Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)Cr2O4, which is ferrimagnetic with TN ∼ 140 K, other compositions show magnetic ordering above room temperature. This indicates the trivial influence of competing interaction between multiple cations on magnetic ordering in the spinel structure. However, the magnetic dilution effect in (Mn0.2Co0.2Ni0.2Mg0.2Zn0.2)Fe2O4 caused by replacing the A-site Fe and Cu of (Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)Fe2O4 is clearly reflected in the magnetization value. We observed a high sensitivity of electrochemical behavior on the composition and possibly on the synthesis process or morphology in these high-entropy oxide (HEO) spinels. A very uncommon electrical double-layer capacitance (EDLC) behavior is observed in (Cr0.6Mn0.6Fe0.6Co0.6Ni0.6)O4, whereas (Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)Cr2O4 exhibits combined EDLC and pseoducapacitance-type mechanisms of charge-discharging. The particle size of these two spinels is in the nanodimension. The supercapacitor behavior is related to the morphology of the samples. On the other hand, the bulk ferrites (Mn0.2Fe0.2Co0.2Ni0.2Cu0.2)Fe2O4 and (Mn0.2Co0.2Ni0.2Mg0.2Zn0.2)Fe2O4 exhibit OER activity at overpotentials of 295 and 315 mV, respectively, at a current density of 10 mA cm–2, suggesting the superior performance of the former one as an electrocatalyst. The increase in the number of active sites by reducing the particle size is not followed in the present system. The differential OER activity of high-entropy ferrite spinel is possibly related to the specific activity of iron and surface oxygen vacancies. The exotic magnetic property, EDLC and pseudocapacitance-type supercapacitor behavior, and potential OER electrocatalytic activity in the HEO spinels require further experimental as well as theoretical exploration.