B-Site Effect on High-Entropy Perovskite Oxide as a Bifunctional Electrocatalyst for Rechargeable Zinc–Air Batteries

Abstract

High-entropy perovskite oxides (HEPOs) combine the advantageous characteristics of both high-entropy oxides and perovskite oxides such as robust long-term crystal and electronic structure stability and finely adjustable physicochemical properties. HEPOs hold significant promise as bifunctional catalysts for oxygen electrocatalytic reactions in alkaline environments. In this study, we introduce a series of HEPOs with distinct cation ratios, denoted as La­(Fe x Co y Mn z Cr0.2Zn0.2)­O3 (with x, y, z = 0.3, 0.2, 0.1, alternating). HEPOs were synthesized through a rapid joule synthesis. The main objective is to explore the influence of the cation composition while maintaining a consistent crystal structure across all synthesized HEPOs. La­(Fe0.2Co0.3Mn0.1Cr0.2Zn0.2)­O3−δ (referred to as La5M-Co/Mn) HEPO exhibits an abundance of oxygen vacancies due to the disparity in net charge resulting from the specific cation ratios. As a result of this unique composition, the La5M-Co/Mn electrocatalyst demonstrates an impressively low overpotential of 296 mV at 10 mA cm–2 for the oxygen evolution reaction (OER) and boasts a bifunctional index (BI) of 1.042 V in alkaline media, signifying exceptional oxygen catalytic activity. When incorporated as electrocatalysts in the air cathode of zinc–air batteries, the La5M-Co/Mn HEPO outperforms the equimolar HEPO electrocatalyst (La5M), showcasing a higher peak power density, capacity, and cyclic stability. Our findings underscore the feasibility of synthesizing HEPOs with identical crystal structures but varying cation ratios. Furthermore, the results highlight that adjusting cation ratios induces lattice structure distortions and electronic charge imbalances, ultimately leading to an augmented presence of oxygen vacancies and enhancing the bifunctionality of the electrocatalyst. This approach holds promise as a fundamental strategy to tailor cation ratios within the same crystal structure, yielding substantial improvements in the rechargeable aqueous zinc–air battery performance.

High-Entropy Oxides as Electrocatalysts for the Oxygen Evolution Reaction: A Mini Review

“…Further, high configuration entropy can promote the generation of surface oxygen vacancies since the formation of oxygen vacancies in the plane site ( x 2 -y 2 ) of CoO 6 octahedral (1st layer) in HEOs surface (−0.53 eV) is thermodynamically better than binary surface (−0.23 eV) as shown in Figure d and e . Besides, abundant oxygen vacancies can be generated by differences in the net charge induced by specific cation ratios . The rate of oxygen-vacancy diffusion is also associated with the OER activity.…”

“…43 115 Further, high configuration entropy can promote the generation of surface oxygen vacancies since the formation of oxygen vacancies in the plane site (x 2 -y 2 ) of CoO 6 octahedral (1st layer) in HEOs surface (−0.53 eV) is thermodynamically better than binary surface (−0.23 eV) as shown in Figure 3d and e. 61 Besides, abundant oxygen vacancies can be generated by differences in the net charge induced by specific cation ratios. 62 The rate of oxygenvacancy diffusion is also associated with the OER activity. This is because the participation of the lattice oxygen is directly related to the diffusion rate of oxygen ions, which is equal to that of oxygen vacancies in the opposite direction; thus, the oxygen vacancy diffusion coefficient (D V ) can be measured as follows:…”