Morphology Control of Carbon-Free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media

Enhance Water Electrolysis for Green Hydrogen Production with Material Engineering: A Review

Water electrolysis, a traditional and highly technology, is gaining significant attention due to the growing demand for renewable energy resources. It stands as a promising solution for energy conversion, offer substantial benefits in environmental protection and sustainable development efforts. The aim of this research is to provide a concise review of the current state-of-the-art in the field of water electrolysis, focusing on the principles of water splitting fundamental, recent advancements in catalytic materials, various advanced characterization methods and emerging electrolysis technology improvements. Moreover, the paper delves into the development trends of catalysts engineering for water electrolysis, providing insight on how to enhance the catalytic performance. With the advancement of technology and the reduction of costs, hydrogen production through water electrolysis is expected to assume a more significant role in future energy ecosystem. This paper not only synthesizes existing knowledge but also highlights emerging opportunities and potential advancements in this field, offering a clear roadmap for further research and innovation.

A Review of Surface Reconstruction and Transformation of 3d Transition-Metal (oxy)Hydroxides and Spinel-Type Oxides during the Oxygen Evolution Reaction

Developing efficient and sustainable electrocatalysts for the oxygen evolution reaction (OER) is crucial for advancing energy conversion and storage technologies. 3d transition-metal (oxy)hydroxides and spinel-type oxides have emerged as promising candidates due to their structural flexibility, oxygen redox activity, and abundance in earth's crust. However, their OER performance can be changed dynamically during the reaction due to surface reconstruction and transformation. Essentially, multiple elementary processes occur simultaneously, whereby the electrocatalyst surfaces undergo substantial changes during OER. A better understanding of these elementary processes and how they affect the electrocatalytic performance is essential for the OER electrocatalyst design. This review aims to critically assess these processes, including oxidation, surface amorphization, transformation, cation dissolution, redeposition, and facet and electrolyte effects on the OER performance. The review begins with an overview of the electrocatalysts' structure, redox couples, and common issues associated with electrochemical measurements of 3d transition-metal (oxy)hydroxides and spinels, followed by recent advancements in understanding the elementary processes involved in OER. The challenges and new perspectives are presented at last, potentially shedding light on advancing the rational design of next-generation OER electrocatalysts for sustainable energy conversion and storage applications.

The Doping of Al3+ at the Tetrahedral Site of Spinel Mn3O4 for Electrocatalytic Water Oxidation

Spinel oxides have attracted much attention for electrocatalytic water oxidation. Specially, the Mn-based spinel structures merits fundamental investigation, as Mn is involved in water oxidation in natural photosynthesis. Herein, Al-doped Mn3O4 spinel electrocatalyst was prepared for water oxidation. The Al cations selectively occupy the tetrahedral sites of Mn3O4 spinel structure, leaving Mn(III) at the catalytically critical octahedral sites. The substitution of Al3+ regulated the electronic structure of Mn3O4, induced lattice distortion and produced higher concentration of oxygen vacancy to promote the water oxidation performance.

Anchoring Platinum Nanoparticles onto Oxygen Vacancy-Modified Mixed Metal Oxides for Selective Oxidation Reaction of Aromatic Alcohols

Directed transformation of organic compounds under mild conditions, especially alcohol oxidation, presents great challenges in green chemistry. Herein, we report a platinum nanoparticle catalyst supported on zinc-gallium mixed metal oxides (denoted as Pt/ZnGa-MMOs), which displays superior catalytic activity for the selective oxidation reaction of benzyl alcohol to benzaldehyde (conversion: >99%; selectivity: >99%; reaction rate: 125 mmolbenzyl alcohol gPt-1 h-1). Both experimental studies [X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS)] and DFT calculations reveal the formation of an interfacial structure (Zn2+δ-Ov-Ga3-δ) on the ZnGa-MMOs support. Moreover, in situ Fourier transform infrared (FT-IR) spectroscopic analysis demonstrates that the Pt species acts as an intrinsic active center to promote the oxidation of the carbon-oxygen bond in the benzyl alcohol molecule, with the formation of the benzaldehyde. This work provides an effective strategy for the preparation of heterogeneous catalysts via constructing the support oxygen vacancy to anchor metal sites toward selective oxidation reactions.

Excellent Bifunctional Oxygen Evolution and Reduction Electrocatalysts (5A1/5)Co2O4 and Their Tunability

Hastening the progress of rechargeable metal-air batteries and hydrogen fuel cells necessitates the advancement of economically feasible, earth-abundant, inexpensive, and efficient electrocatalysts facilitating both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a recently reported family of nano (5A1/5)Co2O4 (A = combinations of transition metals, Mg, Mn, Fe, Ni, Cu, and Zn) compositionally complex oxides (CCOs) [Wang et al., Chemistry of Materials, 2023,35 (17), 7283-7291.] are studied as bifunctional OER and ORR electrocatalysts. Among the different low-temperature soft-templating samples, those subjected to 600 °C postannealing heat treatment exhibit superior performance in alkaline media. One specific composition (Mn0.2Fe0.2Ni0.2Cu0.2Zn0.2)Co2O4 exhibited an exceptional overpotential (260 mV at 10 mA cm-2) for the OER, a favorable Tafel slope of 68 mV dec-1, excellent onset potential (0.9 V) for the ORR, and lower than 6% H2O2 yields over a potential range of 0.2 to 0.8 V vs the reversible hydrogen electrode. Furthermore, this catalyst displayed stability over a 22 h chronoamperometry measurement, as confirmed by X-ray photoelectron spectroscopy analysis. Considering the outstanding performance, the low cost and scalability of the synthesis method, and the demonstrated tunability through chemical substitutions and processing variables, CCO ACo2O4 spinel oxides are highly promising candidates for future sustainable electrocatalytic applications.

Stoichiometrically optimized eg orbital occupancy of Ni-Co oxide catalysts for Li-air batteries

Li-air battery (LAB) technology is making continuous progress toward its theoretical capacity, which is comparable to gasoline. However, the sluggish reaction at the cathode is still a challenge. We propose a simple strategy to optimize the surface eg occupancy by adjusting the stoichiometric ratios of transition metal-based spinel structures through a controlled hydrothermal synthesis. Three distinct stoichiometries of Ni-Co oxides were used to demonstrate the direct correlation between stoichiometry and catalytic performance. The groundsel flower-like structure having a 1 : 1.4 Ni : Co atomic ratio with high surface area, high defect density, and an abundance of Ni3+ at the surface with semi-filled eg orbitals was found to benefit the structure promoting high catalytic activities in aqueous and aprotic media. The assembled LAB cells employing this cathode demonstrate an exceptional lifespan, operating for 3460 hours and completing 173 cycles while achieving the highest discharge capacity of 13 759 mA h g-1 and low charging overpotentials. The key to this prolonged performance lies in the full reversibility of the cell, attributed to its excellent OER performance. A well-surface adsorbed, amorphous LiO2/Li2O2 discharge product is found to possess high diffusivity and ease of decomposition, contributing significantly to the enhanced longevity of the cell.

Cu-Modified Palladium Catalysts: Boosting Formate Electrooxidation via Interfacially OHad-Driven Had Removal

Direct formate fuel cells have gained traction due to their eco-friendly credentials and inherent safety. However, their potential is hampered by the kinetic challenges of the formate oxidation reaction (FOR) on Pd-based catalysts, chiefly due to the unfavorable adsorption of hydrogen species (Had). These species clog the active sites, hindering efficient catalysis. Here, we introduce a straightforward strategy to remedy this bottleneck by incorporating Pd with Cu to expedite the removal of Pd-Had in alkaline media. Notably, Cu plays a pivotal role in bolstering the concentration of hydroxyl adsorbates (OHad) on the surface of catalyst. These OHad species can react with Had, effectively unblocking the active sites for FOR. The as-synthesized catalyst of PdCu/C exhibits a superior FOR performance, boasting a remarkable mass activity of 3.62 A mg-1. Through CO-stripping voltammetry, we discern that the presence of Cu in Pd markedly speeds up the formation of adsorbed hydroxyl species (OHad) at diminished potentials. This, in turn, aids the oxidative removal of Pd-Had, leveraging a synergistic mechanism during FOR. Density functional theory computations further reveal intensified interactions between adsorbed oxygen species and intermediates, underscoring that the Cu-Pd interface exhibits greater oxyphilicity compared to pristine Pd. In this study, we present both experimental and theoretical corroborations, unequivocally highlighting that the integrated copper species markedly amplify the generation of OHad, ensuring efficient removal of Had. This work paves the way, shedding light on the strategic design of high-performing FOR catalysts.

Trimetallic Oxide Electrocatalyst for Enhanced Redox Activity in Zinc-Air Batteries Evaluated by In Situ Analysis

Researchers are investigating innovative composite materials for renewable energy and energy storage systems. The major goals of this studies are i) to develop a low-cost and stable trimetallic oxide catalyst and ii) to change the electrical environment of the active sites through site-selective Mo substitution. The effect of Mo on NiCoMoO4 is elucidated using both in situ X-ray absorption spectroscopy and X-ray diffraction analysis. Also, density functional theory strategies show that NiCoMoO4 has extraordinary catalytic redox activity because of the high adsorption energy of the Mo atom on the active crystal plane. Further, it is demonstrated that hierarchical nanoflower structures of NiCoMoO4 on reduced graphene oxide can be employed as a powerful bifunctional electrocatalyst for oxygen reduction/evolution reactions in alkaline solutions, providing a small overpotential difference of 0.75 V. Also, Zn-air batteries based on the developed bifunctional electrocatalyst exhibit outstanding cycling stability and a high-power density of 125.1 mW cm-2 . This work encourages the use of Zn-air batteries in practical applications and provides an interesting concept for designing a bifunctional electrocatalyst.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Starch-based porous carbon microsphere composited NiCo2O4 nanoflower as bifunctional electrocatalyst for zinc-air battery

It is significant to explore and design outstanding bifunctional oxygen electrocatalysts to promote the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in zinc-air batteries. Herein, a novel porous carbon microspheres (CMS₂) modified by NiCo₂O₄ nanoflower (CMS₂-NiCo₂O₄) has been prepared as an ORR and OER catalyst. The hierarchical porous structure of CMS provides high conductivity and abundant active sites for ORR, whereas the synergistic effect of NiCo₂O₄ nanosheets and a small amount of FeZn oxides act as the positive phase for OER. The efficient oxygen catalytic activity is gained by creating a coupling interface between NiCo₂O₄ and CMS. The optimized CMS₂-NiCo₂O₄ shows a half-wave potential of 0.82 V toward ORR and an overpotential of 392 mV toward OER. Particularly, CMS₂-NiCo₂O₄ also exhibits an excellent peak power density (175.5 mW cm^-2) as a catalyst for zinc-air batteries, which is superior to the commercial Pt/C + RuO₂ catalyst (120.5 mW cm^-2), and it also demonstrates a remarkable stability even after the charge-discharge cycles of 167 h. The prepared CMS₂-NiCo₂O₄ is promising for the application of the bimetallic oxide catalyst for zinc-air battery.

Rod-Like Nanostructured Cu-Co Spinel with Rich Oxygen Vacancies for Efficient Electrocatalytic Dechlorination

Dichloromethane (CH₂Cl₂) hydrodechlorination to methane (CH₄) is a promising approach to remove the halogenated contaminants and generate clean energy. In this work, rod-like nanostructured CuCo₂O₄ spinels with rich oxygen vacancies are designed for highly efficient electrochemical reduction dechlorination of dichloromethane. Microscopy characterizations revealed that the special rod-like nanostructure and rich oxygen vacancies can efficiently enhance surface area, electronic/ionic transport, and expose more active sites. The experimental tests demonstrated that CuCo₂O₄-3 with rod-like nanostructures outperformed other morphology of CuCo₂O₄ spinel nanostructures in catalytic activity and product selectivity. The highest methane production of 148.84 μmol in 4 h with a Faradaic efficiency of 21.61% at -2.94 V (vs SCE) is shown. Furthermore, the density function theory proved oxygen vacancies significantly decreased the energy barrier to promote the catalyst in the reaction and O_v-Cu was the main active site in dichloromethane hydrodechlorination. This work explores a promising way to synthesize the highly efficient electrocatalysts, which may be an effective catalyst for dichloromethane hydrodechlorination to methane.

Thermally constructed stable Zn-doped NiCoOx-z alloy structures on stainless steel mesh for efficient hydrogen production via overall hydrazine splitting in alkaline electrolyte

Hydrogen has a high energy density of approximately 120 to 140 MJ kg^-1, which is very high compared to other natural energy sources. However, hydrogen generation through electrocatalytic water splitting is a high electricity consumption process due to the sluggish oxygen evolution reaction (OER). As a result, hydrogen generation through hydrazine-assisted water electrolysis has recently been intensively investigated. The hydrazine electrolysis process requires a low potential compared to the water electrolysis process. Despite this, the utilization of direct hydrazine fuel cells (DHFCs) as portable or vehicle power sources necessitates the development of inexpensive and effective anodic hydrazine oxidation catalysts. Here, we prepared oxygen-deficient zinc-doped nickel cobalt oxide (Zn-NiCoO_x-z) alloy nanoarrays on stainless steel mesh (SSM) using a hydrothermal synthesis method followed by thermal treatment. Furthermore, the prepared thin films were used as electrocatalysts, and the OER and hydrazine oxidation reaction (HzOR) activities were investigated in three- and two-electrode systems. In a three-electrode system, Zn-NiCoO_x-z/SSM HzOR requires -0.116 V (vs RHE) potential to achieve a 50 mA cm^-2 current density, which is dramatically lower than the OER potential (1.493 V vs RHE). In a two-electrode system (Zn-NiCoO_x-z/SSM(-)∥Zn-NiCoO_x-z/SSM(+)), the overall hydrazine splitting potential (OHzS) required to reach 50 mA cm^-2 is only 0.700 V, which is dramatically less than the required potential for overall water splitting (OWS). These excellent HzOR results are due to the binder-free oxygen-deficient Zn-NiCoO_x-z/SSM alloy nanoarray, which provides a large number of active sites and improves the wettability of catalysts after Zn doping.

Morphology-Dependent Electrocatalytic Behavior of Cobalt Chromite toward the Oxygen Evolution Reaction in Acidic and Alkaline Medium

Exploiting an affordable, durable, and high-performance electrocatalyst for the oxygen evolution reaction (OER) under lower pH condition (acidic) is highly challengeable and much attractive toward the hydrogen-based energy technologies. A spinel CoCr₂O₄ is observed as a potential noble-metal-free candidate for OER in alkaline medium. The presence of Cr further leads to electronic structure modulation of Co₃O₄ and thereby greatly increases the corrosive resistance toward OER in acidic environment. Herein, a typical CoCr₂O₄ with three different morphologies was synthesized for the very first time and employed as an electrocatalyst for OER in alkaline (1 M KOH) and acidic (0.5 M H₂SO₄) medium. Moreover, different morphologies display a different intrinsic exposed active site and thereby display different electrocatalytic activities. Likewise, the CoCr₂O₄ Mic (synthesized by the microwave heating method) displays a higher catalytic activity toward OER and delivers a low overpotential of 293 and 290 mV to attain 10 mA/cm² current density and smaller Tafel slope values of 40 and 151 mV/dec, respectively, in alkaline and acidic environment than the synthesized CoCr₂O₄ Wet (wet-chemically synthesized) and CoCr₂O₄ Hyd (hydrothermally synthesized). Moreover, CoCr₂O₄ Mic exhibits a long-term durability of 24 h (1 M KOH) and 10.5 h (0.5 M H₂SO₄). The optimized Co-O bond energy in OER condition makes the CoCr₂O₄ Mic superior than the CoCr₂O₄ Hyd and CoCr₂O₄ Wet. Moreover, the substitution of Cr induces the electron delocalization around the Co active species and thereby, positive shifting of the redox potential leads to providing an optimal binding energy for OER intermediates. Also, interestingly, this work represents a catalytic activity trend by a simple experimental result without any complex theoretical calculation. The morphology-dependent electrocatalytic activity obtained in this work will provide a new strategy in the field of electrochemical conversion and energy storage application.

Cerium-Doped CoMn2O4 Spinels as Highly Efficient Bifunctional Electrocatalysts for ORR/OER Reactions

Low-cost and highly efficient electrocatalysts for oxygen reactions are highly important for oxygen-related energy storage/conversion devices (e.g., solar fuels, fuel cells, and rechargeable metal-air batteries). In this work, a range of compositionally-tuned cerium-doped CoMn2O4 (Ce-CMO-X) spinels were prepared via oxidizing precipitation and subsequent crystallization method and evaluated as electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The Ce modification into the CMO spinels lead to the changes of surface electronic structure. And Ce-CMO-X catalysts display better electrochemical performance than that of pristine CMO spinel. Among them, Ce-CMO-18% shows the best activity. The Ce-CMO-18% processes a higher ratio of Co3+/Co2+, Mn4+/Mn3+, which is beneficial to ORR performance, while the higher content of oxygen vacancies in Ce-CMO-18% make for better OER performance. Thus, the Ce-doped CMO spinels are potential candidates as bifunctional electrocatalysts for both ORR and OER in alkaline environments. Then, the hybrid Ce-CMO-18%/MWCNTs catalyst was also synthesized, which shows further enhanced ORR and OER activities. It displays an ORR onset potential of 0.93 V and potential of 0.84 V at density of 3 mA cm−2 (at 1600 rpm), which is comparable to commercial Pt/C. The OER onset potential and potential at a current density 10 mA cm-2 are 183 mV and 341 mV. The superior electrical conductivity and oxygen functional groups at the surface of MWCNTs can facilitate the interaction between metal oxides and carbon, which promoted the OER and ORR performances significantly.

Porous spinel-type transition metal oxide nanostructures as emergent electrocatalysts for oxygen reduction reactions

Porous spinel-type transition metal oxide (PS-TMO) nanocatalysts comprising two kinds of metal (denoted as A_xB_3-xO₄, where A, B = Co, Ni, Zn, Mn, Fe, V, Sm, Li, and Zn) have emerged as promising electrocatalysts for oxygen reduction reactions (ORRs) in energy conversion and storage systems (ECSS). This is due to the unique catalytic merits of PS-TMOs (such as p-type conductivity, optical transparency, semiconductivity, multiple valence states of their oxides, and rich active sites) and porous morphologies with great surface area, low density, abundant transportation paths for intermediate species, maximized atom utilization and quick charge mobility. In addition, PS-TMOs nanocatalysts are easily prepared in high yield from Earth-abundant and inexpensive metal precursors that meet sustainability requirements and practical applications. Owing to the continued developments in the rational synthesis of PS-TMOs nanocatalysts for ORRs, it is utterly imperative to provide timely updates and highlight new advances in this research area. This review emphasizes recent research advances in engineering the morphologies and compositions of PS-TMOs nanocatalysts in addition to their mechanisms, to decipher their structure-activity relationships. Also, the ORR mechanisms and fundamentals are discussed, along with the current barriers and future outlook for developing the next generation of PS-TMOs nanocatalysts for large-scale ECSS.

3D hierarchical ZnCo2S4@Ni(OH)2 nanowire arrays with excellent flexible energy storage and electrocatalytic performance

It is essential for energy storage and conversion systems to construct electrodes and electrocatalysts with superior performance. In this work, ZnCo₂S₄@Ni(OH)₂ nanowire arrays are synthesized on nickel foam by hydrothermal methods. As a supercapacitor electrode, the ZnCo₂S₄@Ni(OH)₂ structure exhibits a specific capacitance of 1,263.0C g^-1 at 1 A g^-1. The as-fabricated ZnCo₂S₄@Ni(OH)₂//active carbon device can achieve a maximum energy density of 115.4 Wh kg^-1 at a power density of 5,400 W kg^-1. As electrocatalysts, the ZnCo₂S₄@Ni(OH)₂ structure delivers outstanding performance for oxygen evolution reaction (an overpotential of 256.3 mV at 50 mA cm^-2), hydrogen evolution reaction (141.7 mV at 10 mA cm^-2), overall water splitting (the cell voltage of 1.53 V at 50 mA cm^-2), and a high stability for 13 h.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Tuning Two-Electron Oxygen-Reduction Pathways for H2 O2 Electrosynthesis via Engineering Atomically Dispersed Single Metal Site Catalysts

The hydrogen peroxide (H₂ O₂ ) generation via the electrochemical oxygen reduction reaction (ORR) under ambient conditions is emerging as an alternative and green strategy to the traditional energy-intensive anthraquinone process and unsafe direct synthesis using H₂ and O₂ . It enables on-site and decentralized H₂ O₂ production using air and renewable electricity for various applications. Currently, atomically dispersed single metal site catalysts have emerged as the most promising platinum group metal (PGM)-free electrocatalysts for the ORR. Further tuning their central metal sites, coordination environments, and local structures can be highly active and selective for H₂ O₂ production via the 2e^- ORR. Herein, recent methodologies and achievements on developing single metal site catalysts for selective O₂ to H₂ O₂ reduction are summarized. Combined with theoretical computation and advanced characterization, a structure-property correlation to guide rational catalyst design with a favorable 2e^- ORR process is aimed to provide. Due to the oxidative nature of H₂ O₂ and the derived free radicals, catalyst stability and effective solutions to improve catalyst tolerance to H₂ O₂ are emphasized. Transferring intrinsic catalyst properties to electrode performance for viable applications always remains a grand challenge. The key performance metrics and knowledge during the electrolyzer development are, therefore, highlighted.

Anisole hydrodeoxygenation over Ni–Co bimetallic catalyst: a combination of experimental, kinetic and DFT study†

Catalytic hydrodeoxygenation (HDO) of anisole was performed with a series of Ni and Co containing catalysts with different weight ratios on activated carbon (AC) for cyclohexanol production. The catalytic activities of various catalysts revealed that Ni₅Co₅-AC was the best catalytic system. Structural analysis obtained from XRD, TPR, XPS, and TEM evidently demonstrates that Ni₅Co₅-AC sample consists of a distorted metal alloy spinel structure and optimum particle size, enhancing its catalytic performance. Kinetics were investigated to identify cyclohexanol production rate, activation energy, and reaction pathway. Structural, experimental, kinetics and density functional simulations suggested that high amount of distorted metallic alloy in Ni₅Co₅-AC, presence of water, high adsorption efficiency of anisole, and low adsorption tendency of cyclohexanol on metallic alloy surface were the critical factors for HDO of anisole to cyclohexanol.

High reducible distorted bimetallic sites with medium size in Ni₅Co₅-AC promoted the production of cyclohexanol by hydrogenation of anisole and subsequent cleavage of C₆H₁₁O–CH₃ bond.

A low-cost and facile electrochemical sensor for the trace-level recognition of flutamide in biofluids using large-area bimetallic NiCo2O4 micro flowers

Nickel–cobalt-based bimetallic oxide compound (NiCo2O4) as a highly sensitive and selective platform for the detection of flutamide in biological fluids.

Graphene supported single metal atom catalysts for the efficient hydrogen oxidation reaction in alkaline media

Single Pt and Ni atoms anchored on the divacancy graphene exhibit both high activity and superior antioxidant capacity for the hydrogen oxidation reaction in alkaline fuel cells.

Interfacial Engineering of CuCo2S4/g-C3N4 Hybrid Nanorods for Efficient Oxygen Evolution Reaction

Altering the morphology of electrochemically active nanostructured materials could fundamentally influence their subsequent catalytic as well as oxygen evolution reaction (OER) performance. Enhanced OER activity for mixed-metal spinel-type sulfide (CuCo₂S₄) nanorods is generally done by blending the material that has high conductive supports together with those having a high surface volume ratio, for example, graphitic carbon nitrides (g-C₃N₄). Here, we report a noble-metal-free CuCo₂S₄ nanorod-based electrocatalyst appropriate for basic OER and neutral media, through a simple one-step thermal decomposition approach from its molecular precursors pyrrolidine dithiocarbamate-copper(II), Cu[PDTC]₂, and pyrrolidine dithiocarbamate-cobalt(II), Co[PDTC]₂ complexes. Transmission electron microscopy (TEM) images as well as X-ray diffraction (XRD) patterns suggest that as-synthesized CuCo₂S₄ nanorods are highly crystalline in nature and are connected on the g-C₃N₄ support. Attenuated total reflectance-Fourier-transform infrared (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy studies affirm the successful formation of bonds that bridge (Co-N/S-C) at the interface of CuCo₂S₄ nanorods and g-C₃N₄. The kinetics of the reaction are expedited, as these bridging bonds function as an electron transport chain, empowering OER electrocatalytically under a low overpotential (242 mV) of a current density at 10 mA cm^-2 under basic conditions, resulting in very high durability. Moreover, CuCo₂S₄/g-C₃N₄ composite nanorods exhibit a high catalytic activity of OER under a neutral medium at an overpotential of 406 mV and a current density of 10 mA cm^-2.

Metal-Organic Framework-Derived Trimetallic Nanocomposites as Efficient Bifunctional Oxygen Catalysts for Zinc-Air Batteries

Transition-metal-based multifunctional catalysts have attracted increasing attention owing to high possibilities of substituting the expensive noble-metal-based catalysts in various scenarios. Multivariate metal-organic frameworks (MTV-MOFs) are ideal precursors to prepare multimetallic nanocomposites with high catalytic activity since the uniform distribution and precise regulation of mixed metal centers, as well as the consequent strong synergistic effect, could be readily achieved. Herein, a Mn/Co/Ni trimetallic catalyst (MnCoNi-C-D) with a hollow rhombic dodecahedron shape was synthesized via pyrolysis of the corresponding trimetallic-based MTV-MOF. The catalyst shows outstanding electrochemical activity toward the oxygen reduction reaction including a half-wave potential of 0.82 V and superior tolerance against methanol as well as high stability in an alkaline medium, and its oxygen evolution reaction activity also surpasses a RuO₂ catalyst. Moreover, primary and rechargeable zinc-air batteries based on MnCoNi-C-D delivered preferable performances compared with commercial Pt/C-RuO₂, including higher peak power density (116.4 mW cm^-2), higher specific capacity (841.3 mAh g^-1), higher open-circuit potential (OCV) (1.46 V), and better stability for more than 180 h. A comprehensive comparison was also conducted to prove the necessity of employing the MTV-MOF as the precursor and investigate the intrinsic superiority of the catalyst.

Hierarchical tube brush-like Co3S4@NiCo-LDH on Ni foam as a bifunctional electrocatalyst for overall water splitting

The Co3S4@NiCo-LDH/NF nanocomposite exhibits outstanding electrocatalytic performances toward both the HER and OER at high current densities, along with a remarkable durability.

Advanced non-noble materials in bifunctional catalysts for ORR and OER toward aqueous metal-air batteries

The catalyst in the oxygen electrode is the core component of the aqueous metal-air battery, which plays a vital role in the determination of the open circuit potential, energy density, and cycle life of the battery. For rechargeable aqueous metal-air batteries, the catalyst should have both good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance. Compared with precious metal catalysts, non-precious metal materials have more advantages in terms of abundant resource reserves and low prices. Over the past few years, great efforts have been made in the development of non-precious metal bifunctional catalysts. This review selectively evaluates the advantages, disadvantages and development status of recent advanced materials including pure carbon materials, carbon-based metal materials and carbon-free materials as bifunctional oxygen catalysts. Preliminary improvement strategies are formulated to make up for the deficiency of each material. The development prospects and challenges facing bifunctional catalysts in the future are also discussed.

Controllable Synthesis of Fe-Doped NiCo2 O4 Nanobelts as Superior Catalysts for Oxygen Evolution Reaction

As one of the promising clean and renewable technologies, water splitting has been a hot topic, especially the half-reaction of oxygen evolution reaction (OER) due to its sluggish and complex kinetics. Hence, Fe-doped NiCo₂ O₄ nanobelts were designed and prepared as catalysts toward OER. By increasing the Fe amount, the catalytic performances of the as-synthesized products went up and then decreased. Profiting from the synergistic effect between Fe atom and NiCo₂ O₄ , all the Fe-NiCo₂ O₄ catalysts exhibited superior catalytic activities to the corresponding NiCo₂ O₄ . In addition, the characteristic nanobelt architecture facilitates the conduction of electrons and the exposure of active sites. With the optimal Fe content, the 9.1 % Fe-NiCo₂ O₄ yielded the smallest overpotential and Tafel slope among the catalysts, distinctly lower than that of RuO₂ .

Spinel of Nickel-Cobalt Oxide with Rod-Like Architecture as Electrocatalyst for Oxygen Evolution Reaction

The renewable energy technologies require electrocatalysts for reactions, such as the oxygen and/or hydrogen evolution reaction (OER/HER). They are complex electrochemical reactions that take place through the direct transfer of electrons. However, mostly they have high over-potentials and slow kinetics, that is why they require electrocatalysts to lower the over-potential of the reactions and enhance the reaction rate. The commercially used catalysts (e.g., ruthenium nanoparticles—Ru, iridium nanoparticles—Ir, and their oxides: RuO₂, IrO₂, platinum—Pt) contain metals that have poor stability, and are not economically worthwhile for widespread application. Here, we propose the spinel structure of nickel-cobalt oxide (NiCo₂O₄) fabricated to serve as electrocatalyst for OER. These structures were obtained by a facile two-step method: (1) One-pot solvothermal reaction and subsequently (2) pyrolysis or carbonization, respectively. This material exhibits novel rod-like morphology formed by tiny spheres. The presence of transition metal particles such as Co and Ni due to their conductivity and electron configurations provides a great number of active sites, which brings superior electrochemical performance in oxygen evolution and good stability in long-term tests. Therefore, it is believed that we propose interesting low-cost material that can act as a super stable catalyst in OER.

One-Pot Efficient Catalytic Oxidation for Bio-Vanillin Preparation and Carbon Isotope Analysis

Vanillin (4-hydroxy-3-methoxybenzaldehyde) is one of the most widely used food spices. Aimed at bio-vanillin green production, the natural materials were directly catalytically oxidized efficiently in one pot under low O₂ pressure (0.035 MPa) in the presence of a non-noble metal oxidation combined catalyst (NiCo₂O₄/SiO₂ nanoparticles), which showed remarkable advantages of a short synthetic route and less industrial waste. The catalytic system showed good universality to many natural substrates with nearly 100% conversion and 86.3% bio-vanillin yield. More importantly, carbon isotope ratio investigations were employed to verify the origin of the organic matter. One hundred percent ¹⁴C content of the obtained vanillin was detected, which indicated that it was an efficient method to distinguish the vanillin from biomass or fossil materials. Furthermore, the ¹³C isotope examination showed effective distinguishing ability for the vanillin from a particular biomass source. The C isotope detection provides an effective method for commercial vanillin identification.

Mechanistic Investigation into Efficient Water Oxidation by Co-Ni-Based Hybrid Oxide-Hydroxide Flowers

Oxides are envisioned as promising catalysts to facilitate water oxidation, and the benign presence of hydroxide moieties can further enhance the catalyst performance. However, the nature of synergy between oxides and hydroxides remains elusive. In this study, we have designed a one-pot solution growth technique for the synthesis of flower-shaped N-doped-C-enveloped NiCo₂O₄/Ni_xCo_(1-x)(OH)_y catalysts with varying oxide and hydroxide contents and investigated their water oxidation behavior. The correlation between performance-determining parameters involved in water oxidation, such as the onset potential and overpotential with oxide and/or hydroxide content, oxidation states (oxides), and elemental composition (Co/Ni content), and the possible ways to achieve their optimal values are discussed in detail. Our observations conclude that the onset potential and overpotential are minimal for the hybrid oxide-hydroxide bimetallic system compared with pristine hydroxide or oxide. The optimal hybrid catalyst shows excellent current density, low Tafel slope (82 mV/dec), and low onset potential (281 mV at 2 mA/cm²) and overpotential (348 mV at 10 mA/cm²), besides enduring operational stability in alkaline medium. The low Tafel slope suggests the preferable kinetics for water oxidation, and the poisoning study reveals the direct involvement of metal as active sites. The overall study unveils the synergy in the Co-Ni-based binary transition-metal oxide-hydroxide hybrid, which makes it a potential candidate for water oxidation catalysts, and hence, it is expected that the hybrid will find applications in energy conversion devices, such as electrolyzers.

BaMnO3 nanostructures: Simple ultrasonic fabrication and novel catalytic agent toward oxygen evolution of water splitting reaction

In the current paper, the main aim is to fabricate the BaMnO₃ nanostructures via the sonochemical route. The various factor, including precursors, reaction time and power of sonication can affect the shape, size, and purity of the samples. We utilized X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), and X-ray energy dispersive spectroscopy (EDS) to characterize the BaMnO₃ nanostructures. The optical property of BaMnO₃ nanostructures was explored by Ultraviolet-visible spectroscopy (UV-vis) and the energy gap was suitable for catalytic activity (about 2.75 eV). Changing the precursor can affect the size, nanoparticle shape, architectures, and uniformity of the samples. We employed the BaMnO₃ nanostructures for O₂ evolution reaction as catalysts. It can observe that increasing the homogeneity of the catalysts can increase the efficiency of the Oxygen evolution reaction. The maximum amount of the O₂ evolution and the highest TOF and TON are related to nanoplate disc using barium salicylate as a precursor of barium. As a result, we can nominate the BaMnO₃ nanostructures as an effective and novel catalyst for water-splitting reaction.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.

High performance supercapacitor electrodes based on spinel NiCo2O4@MWCNT composite with insights from density functional theory simulations

In this work, we demonstrated the supercapacitor performance of pristine and composites of spinel NiCo₂O₄ with a multi-walled carbon nanotube (MWCNT) assembled in a two-electrode cell configuration. Spinel NiCo₂O₄ and NiCo₂O₄@MWCNT composites were synthesized via a facile hydrothermal method. The supercapacitive performance of as-synthesized NiCo₂O₄ and NiCo₂O₄@MWCNT fabricated on Ni-foam was studied in a 0.5M K₂SO₄ electrolyte using electrochemical measurement techniques. The symmetric cell configuration of NiCo₂O₄@MWCNT delivers high specific capacitance (374 F/g at 2 A/g) with high energy density and power density (95 Wh/kg and 3 964 W/kg, respectively) compared to that of pristine NiCo₂O₄ electrodes (137 F/g at 0.6 A/g). Furthermore, the energy storage performance of the asymmetric cells of NiCo₂O₄//MWCNT and NiCo₂O₄@MWCNT//MWCNT was studied to enhance cycling stability (retention of 74.85% over 3000 cycles). We have also theoretically studied the supercapacitance performance of pristine NiCo₂O₄ and NiCo₂O₄@SWCNT hybrid structures through its structural and electronic properties using density functional theory predictions. The higher specific capacitance of the NiCo₂O₄@SWCNT hybrid system with high power density and energy density is supported by the enhanced density of states near the Fermi level and increased quantum capacitance of the hybrid structure. We have theoretically computed the diffusion energy barrier of K⁺ ions of the K₂SO₄ electrolyte in the NiCo₂O₄ layer and compared it with the diffusion barrier for Na⁺ ions. The lesser diffusion energy barrier for K⁺ ions in the NiCo₂O₄ layer contributes toward higher energy storage capacity. Thus, owing to superior electrochemical performance of NiCo₂O₄ composites with MWCNTs, it can serve as a high-performance electrode material for supercapacitor applications.

Multifunctional Electrocatalysis on a Porous N-Doped NiCo2O4@C Nanonetwork

Developing a multifunctional electrocatalyst with eminent activity, strong durability, and cheapness for the hydrogen/oxygen evolution reaction (HER/OER) and oxygen reduction reaction (ORR) is critical to overall water splitting and regenerative fuel cells. Herein, a nitrogen-doped nanonetwork assembled by porous and defective NiCo₂O₄@C nanowires grown on nickel foam (N-NiCo₂O₄@C@NF) is crafted via biomimetic mineralization and following carbonization of phase-transited lysozyme (PTL)-coupled NiCo₂O₄. The as-obtained N-NiCo₂O₄@C@NF electrocatalysts exhibit an exceptional catalytic activity with ultralow overpotentials for the HER (42 mV) and OER (242 mV) to afford 10 mA cm^-2 while maintaining good stability in alkaline media. Meanwhile, the N-NiCo2O4@C electrocatalysts presents a superior catalytic activity for ORR and a favorable four-electron pathway. The unprecedented catalytic performance arises from a highly porous structure and abundant defects and synergistic effects of components. This work may offer a new possibility in the exploration of multifunctional electrocatalysts for various energy-related electrocatalytic reactions.