General Synthesis of Composition-Tunable High-Entropy Amorphous Oxides Toward High Efficiency Oxygen Evolution Reaction

High-entropy materials have attracted much attention in the electrocatalysis field due to their unique structure, high chemical activity, and compositional tunability. However, the harsh and complex synthetic methods limit the application of such materials. Herein, a universal non-equilibrium liquid-phase synthesis strategy is reported to prepare high-entropy amorphous oxide nanoparticles (HEAO-NPs), and the composition of HEAO-NPs can be precisely controlled from tri- to ten-component. The non-equilibrium synthesis environment provided by an excessively strong reducing agent overcomes the difference in the reduction potentials of various metal ions, resulting in the formation of HEAO-NPs with a nearly equimolar ratio. The oxygen evolution reaction (OER) performance of HEAO-NPs is further improved by adjusting the composition and optimizing the electronic structure. The Fe16Co32Ni32Mn10Cu10BOy exhibits a smaller overpotential (only 259 mV at 10 mA cm-2) and higher stability in OER compared with commercial RuO2. The amorphous high-entropy structure with an optimized concentration of iron makes the binding energy of CoNi shift to a higher direction, promotes the generation of high-valence active intermediates, and accelerates the OER kinetic process. The HEAO-NPs have promising application potential in the field of catalysis, biology, and energy storage, and this work provides a general synthesis method for composition-controllable high-entropy materials.

Phase tunable nickel doped Mn3O4 nanoparticle synthesis by chemical precipitation: kinetic study on dye degradation

Nickel (Ni) doped Mn3O4 nanoparticles (NPs) were synthesized by a quick and facile chemical precipitation technique to investigate their performance in the degradation of methylene blue (MB) in the absence of light. XRD, FESEM, TEM, AAS, XPS, and FT-IR were used for the investigation of the structural, surface morphological, and elemental composition of Ni doped Mn3O4 NPs. XRD confirms the formation of a tetragonal phase structure of pure Mn3O4 and 1% and 3% Ni doped Mn3O4 NPs. However, mixed phases were found in the case of 5 to 10% Ni doped Mn3O4 NPs. Well-defined spherical-shaped morphology was presented through FESEM. Particle sizes decreased linearly (58.50 to 23.68 nm) upon increasing the doping concentration from 0% (pure Mn3O4) to 7% respectively, and then increased (48.62 nm) in the case of 10% doping concentration. TEM further confirmed spherical shaped 32 nm nanoparticles for 7% Ni doped Mn3O4. The elemental composition and oxidation state of the prepared NPs were confirmed by using XPS spectra. Mixed valence Mn2+ and Mn4+ states were found in pure Mn3O4 and 1% and 3% Ni doped Mn3O4 NPs in the ratio of 2MnO-MnO2. In addition, three different oxidation states Mn2+, Mn3+, and Mn4+ were found in 5 to 10% Ni doped Mn3O4 NPs. Moreover, as a dopant Ni exists as Ni2+ and Ni3+ states in all Ni doped Mn3O4 NPs. The synthesized NPs were then applied as potent oxidants for the degradation of MB at pH 3. With the increase of doping concentration to 7%, the degree of degradation was increased to 79% in the first 10 min and finally, it became about 98%. The degradation of MB follows the pseudo-first-order linear kinetics with a degradation rate of 0.0342 min-1.

Solution combustion synthesis of Ni-based hybrid metal oxides for oxygen evolution reaction in alkaline medium

Oxygen evolution reaction (OER) has arisen as an outstanding technology for energy generation, conversion, and storage. Herein, we investigated the synthesis of nickel-based hybrid metal oxides (Ni x M1-x O y ) and their catalytic performance towards OER. Ni x M1-x O y catalysts were synthesized by solution combustion synthesis (SCS) using the metal nitrates as oxidizer and glycine as fuel. Scanning electron microscope (SEM) micrographs display a porous morphology for the hybrid binary Ni x M1-x O y , the common feature of combusted materials. X-ray diffraction (XRD) of Ni x M1-x O y depicted well-defined diffraction peaks, which confirms the crystalline nature of synthesized catalysts. The particle size of as-synthesized materials ranges between 20 and 30 nm with a mesoporous nature as revealed by N2-physisorption. The electrocatalytic performance of the as-prepared materials was evaluated towards OER in alkaline medium. Among them, Ni x Co1-x O y showed the best catalytic performance. For instance, it exhibited the lowest overpotential at a current density of 10 mA cm-2 (404 mV), onset potential (1.605 V), and Tafel slope (52.7 mV dec-1). The enhanced electrocatalytic performance of Ni x Co1-x O y was attributed to the synergism between cobalt and nickel and the alteration of the electronic structure of nickel. Also, Ni x Co1-x O y afforded the highest Ni3+/Ni2+ when compared to other electrocatalysts. This leads to higher oxidation states of Ni species, which promote and improve the electrocatalytic activity.

Anchoring γ-MnO2 within β-NiCo(OH)2 as an Interfacial Electrode Material for Boosting Power Density in an Asymmetric Supercapacitor Device and for Oxygen Evolution Catalysis

The great challenge is to improve the high-competence electrochemical supercapacitor (ES) and oxygen evolution reaction (OER) electrocatalyst with earth-abundant transition metals rather than using limited noble metals. Herein, we developed a facile strategy to introduce two different phases such as α-MnO₂ or γ-MnO₂ on porous hexagonal bimetallic β-NiCo(OH)₂-layered double hydroxide (LDH) nanosheets for an enhanced bifunctionality and to ease out interfacial redox reaction kinetics. Due to the rational intend of LDH morphology and well-retained starlike γ-MnO₂ nanostructures, the bifunctional LDHs exhibit commendable activities toward ESs and in the OER study. Importantly, the γ-MnO₂ phase loaded at β-NiCo(OH)₂ LDHs shows superior ESs or electrocatalytic OER performance in comparison with the α-MnO₂ phase on LDHs. Besides, the assembled fabricated asymmetric supercapacitor (FASC) device possesses convincing energy (24.43 W h/kg) and power densities (5312 W/kg) and enabled us to glow a 1.4 V light-emitting diode for 45 s. Accordingly, three-/two-electrode systems or the solid-state FASC device has exhibited high efficiency in ESs. Also, the optimized γ-MnO₂ phase on β-NiCo(OH)₂ LDHs with the detailed mass ratio of Ni and Co has displayed the OER performance comparable to commercial RuO₂. The electrochemical studies and structural classifications offer in-depth analysis on the electrochemical behaviors, especially the stability in both ES and OER studies, signifying a promising aspirant in the alternative energy field.

One step microwave-hydrothermal synthesis of rGO–TiO2 nanocomposites for enhanced electrochemical oxygen evolution reaction

rGO–TiO₂ nanocomposites exhibited greater electrocatalytic activity for H₂O oxidation in neutral and alkaline medium as compared with pure TiO₂.

Recent Progresses in Electrocatalysts for Water Electrolysis

Abstract

The study of hydrogen evolution reaction and oxygen evolution reaction electrocatalysts for water electrolysis is a developing field in which noble metal-based materials are commonly used. However, the associated high cost and low abundance of noble metals limit their practical application. Non-noble metal catalysts, aside from being inexpensive, highly abundant and environmental friendly, can possess high electrical conductivity, good structural tunability and comparable electrocatalytic performances to state-of-the-art noble metals, particularly in alkaline media, making them desirable candidates to reduce or replace noble metals as promising electrocatalysts for water electrolysis. This article will review and provide an overview of the fundamental knowledge related to water electrolysis with a focus on the development and progress of non-noble metal-based electrocatalysts in alkaline, polymer exchange membrane and solid oxide electrolysis. A critical analysis of the various catalysts currently available is also provided with discussions on current challenges and future perspectives. In addition, to facilitate future research and development, several possible research directions to overcome these challenges are provided in this article.

Graphical Abstract

Supported Mn3O4 Nanosystems for Hydrogen Production through Ethanol Photoreforming

Photoreforming promoted by metal oxide nanophotocatalysts is an attractive route for clean and sustainable hydrogen generation. In the present work, we propose for the first time the use of supported Mn₃O₄ nanosystems, both pure and functionalized with Au nanoparticles (NPs), for hydrogen generation by photoreforming. The target oxide systems, prepared by chemical vapor deposition (CVD) and decorated with gold NPs by radio frequency (RF) sputtering, were subjected to a thorough chemico-physical characterization and utilized for a proof-of-concept H₂ generation in aqueous ethanolic solutions under simulated solar illumination. Pure Mn₃O₄ nanosystems yielded a constant hydrogen production rate of 10 mmol h^-1 m^-2 even for irradiation times up to 20 h. The introduction of Au NPs yielded a significant enhancement in photocatalytic activity, which decreased as a function of irradiation time. The main phenomena causing the Au-containing photocatalyst deactivation have been investigated by morphological and compositional analysis, providing important insights for the design of Mn₃O₄-based photocatalysts with improved performances.

Controllable vapor phase fabrication of F:Mn3O4thin films functionalized with Ag and TiO2

The first example of vapor phase fabrication of Mn₃O₄(hausmannite) thin films chemically modified with fluorine and functionalized with Ag and TiO₂, resulting in high purity composites with an intimate constituent contact.

Supercritical fluid processing for the synthesis of NiS2 nanostructures as efficient electrocatalysts for electrochemical oxygen evolution reactions

A facile supercritical fluid process was demonstrated for the synthesis of cubic NiS₂ nanostructures for efficient electrochemical oxygen evolution reactions.