Dimension-Tunable Medium-to-High-Entropy All-Inorganic Perovskites for Methanol Oxidation and Oxygen Evolution Reactions

Abstract

Halide and organic linker-free, all-inorganic cubic (ABO3) and Ruddlesden–Popper perovskites (RPPs) with the general formula An+1BnO3n+1 are less explored in the field of alcohol fuel cells. Subsequently, the impact of variation in the synthesis approach leading to phase transformation with the change of nanoarchitecture is also not known, especially for compositional high-entropy-based Ruddlesden–Popper perovskites (HERPPs). At the same time, the literature is very silent with regard to the application of 2D and 3D phases of such perovskites for the methanol oxidation reaction (MOR) in an alkaline medium and the oxygen evolution reaction (OER). The present manuscript thus brings into light the influence of the transformation of RP perovskites from 2D layered to 3D cubic perovskite (ABO3) phases, along with the change of compositional entropy and how structural dimensionality governs the balance between active site density and intrinsic activity of the perovskites in electrocatalytic oxidation of methanol. The structural information on these synthesized materials was analyzed through synchrotron XRD, and other parameters were evaluated through various other analytical tools. During the MOR study, at a scan rate of 100 mV s–1, the 2D RPPs exhibited an onset potential of 1.59 V vs RHE with a maximum current density of 79.06 mA cm–2 and an overpotential of 310 mV. In contrast, at the same scan rate, the 3D cubic perovskite phase displayed a maximum current density of 168.79 mA cm–2 at a lower onset potential of 1.32 V vs RHE and an overpotential of 110 mV. OER showed a similar trend with improved kinetics. The Tafel slope also showed a decrease in value from 197 to 22 mV dec–1 in MOR and from 361 to 217 mV dec–1 in OER. The compositional high-entropy-based 3D cubic perovskite (ABO3) phase having minor impurity of the “n” = 4 RP phase with a diamond-like morphology showed the highest MOR and OER activity compared to medium-entropy-based RPPs with “n” = 1, 2 and “n” = 1, 3. The MOR activity of the materials was found to be the same until 18 h under alkaline conditions. Further, from the theoretical analysis, it was evident that the compositional high-entropy-based perovskites showed metallic and semimetallic nature on phase transformation.