Sol-Gel Synthesized High Entropy Metal Oxides as High-Performance Catalysts for Electrochemical Water Oxidation

Abstract

Hexanary high-entropy oxides (HEOs) were synthesized through the mechanochemical sol-gel method for electrocatalytic water oxidation reaction (WOR). As-synthesized catalysts were subjected to characterization, including X-ray diffraction (XRD), Fourier transforms infrared (FTIR) analysis, and scanning electron microscopy (SEM). All the oxide systems exhibited sharp diffraction peaks in XRD patterns indicating the defined crystal structure. Strong absorption between 400–700 cm−1 in FTIR indicated the formation of metal-oxide bonds in all HEO systems. WOR was investigated via cyclic voltammetry using HEOs as electrode platforms, 1M KOH as the basic medium, and 1M methanol (CH3OH) as the facilitator. Voltammetric profiles for both equiatomic (EHEOs) and non-equiatomic (NEHEOs) were investigated, and NEHEOs exhibited the maximum current output for WOR. Moreover, methanol addition improved the current profiles, thus leading to the electrode utility in direct methanol fuel cells as a sequential increase in methanol concentration from 1M to 2M enhanced the OER current density from 61.4 to 94.3 mA cm−2 using NEHEO. The NEHEOs comprising a greater percentage of Al, ([Al0.35(Mg, Fe, Cu, Ni, Co)0.65]3O4) displayed high WOR catalytic performance with the maximum diffusion coefficient, D° (10.90 cm2 s−1) and heterogeneous rate constant, k° (7.98 cm s−1) values. These primary findings from the EC processes for WOR provide the foundation for their applications in high-energy devices. Conclusively, HEOs are proven as novel and efficient catalytic platforms for electrochemical water oxidation.

Synthetic Strategies toward High Entropy Materials: Atoms-to-Lattices for Maximum Disorder

“…Sol–gel synthesis is another interesting atom-up approach toward the synthesis of a multitude of size- and shape-controlled nanomaterials. The sol–gel process can roughly be defined as the conversion of a precursor solution into an inorganic solid by chemical means, − and has been used as a synthetic approach to obtaining both high-entropy alloys and oxides. − The synthesis of HE alloys can be conducted by initial dissolution of the metal salts, followed by addition of citric acid to form precursor complexes in situ . The solvent is then evaporated at 90 °C for 48 h to form a homogeneously dispersed gel, and the dried gels calcinated at 300 °C under an argon or flowing hydrogen atmosphere to obtain the alloys .…”

Denary High-Entropy Oxide Nanoparticles Synthesized by a Continuous Supercritical Hydrothermal Flow Process

“…Since the initial report by Rost et al, various studies of HEOs have been conducted. In the majority of these, the solid-state or sol–gel methods have been used as the synthetic method. − The synthesis of HEMs is often performed under high-temperature conditions to leverage the large configurational entropy, which leads to larger particle sizes by the sintering process. Accordingly, the majority of the fabricated HEOs are in bulk size; in contrast, the production of HEO NPs first reported in 2019 remains limited.…”

Nickel-doped lanthanum cerate nanomaterials as highly active electrocatalysts

“…OER is the anodic reaction in electrolyzers and metal–air batteries. Hydrolysis of water via the electrochemical (EC) pathway to produce high-purity hydrogen (hydrogen evolution reaction; HER) is a sustainable and cost-effective approach toward meeting the world’s clean energy demand ( Rossmeisl et al, 2005 ; Zhou et al, 2018 ; Asim et al, 2022 ). Electrochemical water splitting is comprised of two half-reactions, an anodic reaction in which oxygen is produced (OER) and a cathodic reaction in which hydrogen is produced (HER).…”