Rapid microwave synthesis of medium and high entropy oxides for outstanding oxygen evolution reaction performance†

Abstract

The development of efficient and durable catalysts for the oxygen evolution reaction (OER) is urgent for renewable and sustainable energy storage and conversion. High-entropy oxides (HEOs) have gained significant attention for OER electrocatalysis owing to their multielement synergy and tunable electronic structure. The presence of multiple cations and anions in HEOs’ crystal structure leads to a slow diffusion effect, lattice distortion, high configurational entropy, and cocktail effect. The high configurational entropy of HEOs reveals outstanding electrochemical activity due to the large number of active sites compared with their individual counterparts. Herein, a series of equimolar (quaternary, quinary, and senary) and non-equimolar HEOs were fabricated using a rapid microwave irradiation method. The crystal structure, morphology, elemental composition and oxidation states of the HEOs were explored via different physical characterizations. The OER activity of the HEOs was investigated through cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry, and electrochemical impedance spectroscopy (EIS). All the prepared HEOs demonstrated outstanding OER activity, where the optimum composition exhibited a low overpotential of 350 mV, Tafel slope of 49.4 mV dec⁻¹ at 10 mA cm⁻² and excellent stability for 3600 s. Other electrocatalytic parameters including high diffusion coefficient (D°) (2.2 × 10⁻⁸ cm² s⁻¹), mass transport coefficient (m_T) (2.9 × 10⁻⁴ cm s⁻¹), heterogeneous rate constant (k°) (5.85 × 10⁻⁴ cm s⁻¹), high active surface area (A) (0.0116 cm²), and turnover frequency (TOF) (1.388 s⁻¹) were observed for optimized composition. EIS analysis revealed low solution resistance and charge transfer resistance values. This outstanding performance is attributed to multiple cationic contribution due to the synergistic effect, high durability, improved conductivity, and high entropy stabilization. However, the electrochemical behavior of HEOs depends on each metal ion and its concentration on the catalyst's surface, thus providing new opportunities for tailoring their functional properties by simply changing their elemental composition for different electrochemical applications.