Electrodeposited Nanostructured CoFe2O4 for Overall Water Splitting and Supercapacitor Applications

Ligand-selective turn-off sensing, harvesting and post-adsorptive use of Dy(III) and Yb(III) by intrinsically fluorescent flower-shaped Gum Acacia-grafted hydrogels

Rare earth metals (REMs), such as Dysprosium (Dy) and Ytterbium (Yb), have experienced unprecedented demand in recent times due to their applications in high-end technologies. REMs are found only in select geographic locations placing tremendous economic constraints on their use. In this work, we have developed Gum Acacia-grafted hydrogels (GmAc-FluoroTerPs) that are capable of selective detection and capture of Dy and Yb. The intrinsically blue fluorescent polymer hydrogel GmAc-FluoroTerP has been optimized for Dy(III) and Yb(III) specific quenching, enabling limit of detection of the REMs at 0.13 nM and 60.8 pM, respectively. A comprehensive structural characterization of the fluorescent hydrogel has been performed via NMR, FTIR, XPS, EPR, TGA, XRD, TEM, SEM, EDX, TCSPC, and DLS. In addition to an in situ generated fluorophore, GmAc-FluoroTerP displays a distinctive aggregation induced emission enhancement in mixed solvents. The complexation of Dy(III)/Yb(III) with GmAc-FluoroTerP hydrogel has been characterized by XPS, TCSPC, and logic gate analyses, and the adsorptive capacity for Dy(III) and Yb(III) are found to be best reported till date as 125.57 mg g-1 and 102.27 mg g-1, respectively. Desorption at acidic pH allows recovery of the REMs. We also report semiconducting behaviour of the native fluorescent hydrogel, that is enhanced upon adsorptive capture of Dy(III) and Yb(III), with calculated band gaps at 1.37, 0.77, and 0.49 eV, respectively. The convergent sensing, capture, and reuse of Dy(III) and Yb(III) presented in this work promises a hitherto unreported template for application on other REMs.

Electrochemical preparation of nano/micron structure transition metal-based catalysts for the oxygen evolution reaction

Electrochemical water splitting is a promising technology for hydrogen production and sustainable energy conversion, but the existing electrolytic cells lack a sufficient number of robust and highly active anodic electrodes for the oxygen evolution reaction (OER). Electrochemical synthesis technology provides a feasible route for the preparation of independent OER electrodes with high utilization of active sites, fast mass transfer, and a simple preparation process. A comprehensive review of the electrochemical synthesis of nano/microstructure transition metal-based OER materials is provided. First, some fundamentals of electrochemical synthesis are introduced, including electrochemical synthesis strategies, electrochemical synthesis substrates, the electrolyte used in electrochemical synthesis, and the combination of electrochemical synthesis and other synthesis methods. Second, the morphology and properties of electrochemical synthetic materials are summarized and introduced from the viewpoint of structural design. Then, the latest progress regarding the development of transition metal-based OER electrocatalysts is reviewed, including the classification of metals/alloys, oxides, hydroxides, sulfides, phosphides, selenides, and other transition metal compounds. In addition, the oxygen evolution mechanism and rate-determining steps of transition metal-based catalysts are also discussed. Finally, the advantages, challenges, and opportunities regarding the application of electrochemical techniques in the synthesis of transition metal-based OER electrocatalysts are summarized. This review can provide inspiration for researchers and promote the development of water splitting technology.

Microwave-Assisted Auto-Combustion Synthesis of Binary/Ternary Co x Ni1-x Ferrite for Electrochemical Hydrogen and Oxygen Evolution

Enormous efforts have been dedicated to engineering low-cost and efficient electrocatalysts for both hydrogen evolution and oxygen evolution reactions (HER and OER, respectively). For this, the current contribution reports the successful synthesis of binary/ternary metal ferrites (Co x Ni1-x Ferrite; x = 0.0, 0.1, 0.3, 0.5, 0.7, and 1.0) by a simple one-step microwave technique and subsequently discusses its chemical and electrochemical properties. The X-ray diffraction analysis substantiated the phase purity of the as-obtained catalysts with various compositions. Additionally, the morphology of the nanoparticles was identified via transmission electron microscopy. Further, the vibrating sample magnetometer justified the ferromagnetic character of the as-prepared products. The electrochemical measurements revealed that the as-prepared materials required the overpotentials of 422-600 and 419-467 mV for HER and OER, respectively, to afford current densities of 10 mA cm-2. In the general sense, Ni cation substitution with Co influenced favorably toward both HER and OER. Among all synthesized electrocatalysts, Co0.9Ni0.1Ferrite displayed the highest performance in terms of OER in 1 M KOH solution, which is related to the synergistic effect of multiple parameters including the optimal substitution amount of Co, the highest Brunauer-Emmett-Teller surface area, the smallest particle size among all samples (26.71 nm), and the lowest charge transfer resistance. The successful synthesis of ternary ferrites carried out for the first time via a microwave-assisted auto-combustion route opens up a new path for their applications in renewable energy technologies.

Low temperature scalable synthetic approach enabling high bifunctional electrocatalytic performance of NiCo2S4 and CuCo2S4 thiospinels

Ternary metal sulfides are currently in the spotlight as promising electroactive materials for high-performance energy storage and/or conversion technologies. Extensive research on metal sulfides has indicated that, amongst other factors, the electrochemical properties of the materials are strongly influenced by the synthetic protocol employed. Herein, we report the electrochemical performance of uncapped NiCo₂S₄ and CuCo₂S₄ ternary systems prepared via solventless thermolysis of the respective metal ethyl xanthate precursors at 200 and 300 °C. The structural, morphological and compositional properties of the synthesized nanoparticles were examined by powder X-ray diffraction (p-XRD), transmission electron microscopy (TEM), high-resolution TEM, scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDX) techniques. Electrochemical studies indicate that NiCo₂S₄ nanoparticles synthesized at 300 °C exhibit superior energy storage characteristics with a high specific capacitance of ca. 2650 F g⁻¹ at 1 mV s⁻¹, as compared to CuCo₂S₄ nanoparticles, which showcased a specific capacitance of ca. 1700 F g⁻¹ at the same scan rate. At a current density of 0.5 A g⁻¹, NiCo₂S₄ and CuCo₂S₄ nanoparticles displayed specific capacitances of 1201 and 475 F g⁻¹, respectively. In contrast, CuCo₂S₄ nanoparticles presented a higher electrocatalytic activity with low overpotentials of 269 mV for oxygen evolution reaction (OER), and 224 mV for the hydrogen evolution reaction (HER), at 10 mA cm⁻². The stability of the catalysts was examined for 2000 cycles in which a negligible change in both OER and HER activities was observed.

A scalable solventless approach is employed to prepare NiCo₂S₄ and CuCo₂S₄ with bare surface for enhanced supercapacitance and water splitting. The particles exhibit good energy storage and electrocatalytic activity as well as stability.

Spinel ferrites (MFe2O4): Synthesis, improvement and catalytic application in environment and energy field

To develop efficient catalysts is one of the major ways to solve the energy and environmental problems. Spinel ferrites, with the general chemical formula of MFe₂O₄ (where M = Mg²⁺, Co²⁺, Ni²⁺, Zn²⁺, Fe²⁺, Mn²⁺, etc.), have attracted considerable attention in catalytic research. The flexible position and valence variability of metal cations endow spinel ferrites with diverse physicochemical properties, such as abundant surface active sites, high catalytic activity and easy to be modified. Meanwhile, their unique advantages in regenerating and recycling on account of the magnetic performances facilitate their practical application potential. Herein, the conventional as well as green chemistry synthesis of spinel ferrites is reviewed. Most importantly, the critical pathways to improve the catalytic performance are discussed in detail, mainly covering selective doping, site substitution, structure reversal, defect introduction and coupled composites. Furthermore, the catalytic applications of spinel ferrites and their derivative composites are exclusively reviewed, including Fenton-type catalysis, photocatalysis, electrocatalysis and photoelectro-chemical catalysis. In addition, some vital remarks, including toxicity, recovery and reuse, are also covered. Future applications of spinel ferrites are envisioned focusing on environmental and energy issues, which will be pushed by the development of precise synthesis, skilled modification and advanced characterization along with emerging theoretical calculation.

Facile ZnO-based nanomaterial and its fabrication as a supercapacitor electrode: synthesis, characterization and electrochemical studies

In recent times, tremendous efforts have been devoted to the efficient and cost-effective advancements of electrochemically active metal oxide nanomaterials. Here, we have synthesized a facile nanomaterial of ZnO@PdO/Pd by employing extracted fuel from E. cognata leaves following a hydrothermal route. The phyto-fueled ZnO@PdO/Pd nanomaterial was fabricated into a supercapacitor electrode and was scrutinized by galvanostatic charge–discharge, electrochemical impedance spectroscopy and cyclic voltammetry to evaluate its energy storage potential, and transport of electrons and conductivity. Substantial specific capacitance i.e., 178 F g⁻¹ was obtained in the current study in aKOH electrolyte solution. A specific energy density of 3.7 W h Kg⁻¹ was measured using the charge–discharge data. A high power density of 3718 W Kg⁻¹ was observed for the ZnO@PdO/Pd electrode. Furthermore, the observed low internal resistance of 0.4 Ω suggested effective electron- and ion diffusion. Thus, the superb electrochemical behavior of the ZnO@PdO/Pd nanocomposite was exposed, as verified by the significant redox behavior shown by cyclic voltammetry and galvanostatic charge–discharge.

In recent times, tremendous efforts have been devoted to the efficient and cost-effective advancements of electrochemically active metal oxide nanomaterials.

Editorial—Special Issue “Catalysis for Energy Production”

The rapid increase in anthropogenic greenhouse gas concentrations in the last several decades means that the effects of climate change are fast becoming the familiar horsemen of a planetary apocalypse. Catalysis, one of the pillars of the chemical and petrochemical industries, will play a critical role in the effort to reduce the flow of greenhouse gases into the atmosphere. This Special Issue is timely, as it provides a collection of high-quality manuscripts in a diverse range of topics, which include the production of green hydrogen via water electrolysis, the steam reforming of ethanol, propane or glycerol, the dry reforming of methane, and the autothermal reforming of diesel surrogate fuel. The topic of the transformation of biomass waste to chemicals is also well represented as is the tackling of CO2 emissions via novel utilization technologies. The Editors are grateful to all authors for their valuable contributions and confident that this Special Issue will prove valuable to scholars, university professors and students alike.

Crystalline Multi-Metallic Compounds as Host Materials in Cathode for Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) battery is one of the most promising next-generation rechargeable batteries. Lots of fundamental research has been done for the problems during cycling like capacity fading and columbic efficiency reducing owing to severe diffusion and migration of polysulfide intermediates. In the early stage, a wide variety of carbon materials are used as host materials for sulfur to enhance electrical conductivity and adsorb soluble polysulfides. Beyond carbon materials, metal based polar compounds are introduced as host materials for sulfur because of their strong catalytic activity and adsorption ability to suppress the shuttle effect. In addition, relatively high density of metal compounds is helpful for increasing volumetric energy density of Li-S batteries. This review focuses on crystalline multi-metal compounds as host materials in sulfur cathodes. The multi-metal compounds involve not only transition metal composite oxides with specific crystalline structures, binary metal chalcogenides, double or complex salts, but also the metal compounds doped or partially substituted by other metal ions. Generally, for the multi-metal compounds, microstructure and morphologies in micro-nano scale are very significant for mass transfer in electrodes; moreover, adsorption and catalytic ability for polysulfides make fast kinetics in the electrode processes.

Solventless synthesis of nanospinel Ni1−xCoxFe2O4 (0 ≤ x ≤ 1) solid solutions for efficient electrochemical water splitting and supercapacitance

The formation of solid solutions represents a robust strategy for modulating the electronic properties and improving the electrochemical performance of spinel ferrites. However, solid solutions have been predominantly prepared via wet chemical routes, which involve the use of harmful and/or expensive chemicals. In the present study, a facile, inexpensive and environmentally benign solventless route is employed for the composition-controlled synthesis of nanoscopic Ni_1−xCo_xFe₂O₄ (0 ≤ x ≤ 1) solid solutions. The physicochemical characterization of the samples was performed by p-XRD, SEM, EDX, XPS, TEM, HRTEM and UV-Vis techniques. A systematic investigation was also carried out to elucidate the electrochemical performance of the prepared nanospinels towards energy generation and storage. Based on the results of CV, GCD, and stability tests, the Ni_0.4Co_0.6Fe₂O₄ electrode showed the highest performance for the supercapacitor electrode exhibiting a specific capacitance of 237 F g⁻¹, superior energy density of 10.3 W h kg⁻¹ and a high power density with a peak value of 4208 W kg⁻¹, and 100% of its charge storage capacity was retained after 4000 cycles with 97% coulombic efficiency. For HER, the Ni_0.6Co_0.4Fe₂O₄ and CoFe₂O₄ electrodes showed low overpotentials of 168 and 169 mV, respectively, indicating better catalytic activity. For OER, the Ni_0.8Co_0.2Fe₂O₄ electrode exhibited a lower overpotential of 320 mV at a current density of 10 mA cm⁻², with a Tafel slope of 79 mV dec⁻¹, demonstrating a fast and efficient process. These results indicated that nanospinel ferrite solid solutions could be employed as promising electrode materials for supercapacitor and water splitting applications.

The formation of solid solutions represents a robust strategy for modulating the electronic properties and improving the electrochemical performance of spinel ferrites.

Phyto-inspired and scalable approach for the synthesis of PdO-2Mn2O3: a nano-material for application in water splitting electro-catalysis

A modified co-precipitation method has been used for the synthesis of a PdO-2Mn₂O₃ nanocomposite as an efficient electrode material for the electro-catalytic oxygen evolution (OER) and hydrogen evolution reaction (HER). Palladium acetate and manganese acetate in molar ratio 1 : 4 were dissolved in water, and 10 ml of an aqueous solution of phyto-compounds was slowly added until completion of precipitation. The filtered and dried precipitates were then calcined at 450 °C to obtain a blackish brown colored mixture of PdO-2Mn₂O₃ nanocomposite. These particles were analyzed by ultra violet visible spectrophotometry (UV-vis), infrared spectroscopy (FTIR), powder X-ray diffractometry (XRD), scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS) for crystallinity, optical properties, and compositional and morphological makeup. Using Tauc's plot, the direct band gap (3.18 eV) was calculated from the absorption spectra. The average crystallite sizes, as calculated from the XRD, were found to be 15 and 14.55 nm for PdO and Mn₂O₃, respectively. A slurry of the phyto-fabricated PdO-2Mn₂O₃ powder was deposited on Ni-foam and tested for electro-catalytic water splitting studies in 1 M KOH solution. The electrode showed excellent OER and HER performance with low over-potential (0.35 V and 121 mV) and Tafel slopes of 115 mV dec^-1 and 219 mV dec^-1, respectively. The outcomes obtained from this study provide a direction for the fabrication of a cost-effective mixed metal oxide based electro-catalyst via an environmentally benign synthesis approach for the generation of clean energy.

Stabilizing Oxygen Vacancy in Entropy-Engineered CoFe2O4-Type Catalysts for Co-prosperity of Efficiency and Stability in an Oxygen Evolution Reaction

We used entropy engineering to design a series of CoFe₂O₄-type spinels. Through microstructural characterization, electrochemical measurements, and X-ray photoelectron spectroscopy, we demonstrated that the entropy-stabilized oxide (Co_0.2Mn_0.2Ni_0.2Fe_0.2Zn_0.2)Fe₂O₄ has a single-phase spinel structure and exhibits both efficient and stable catalytic oxygen evolution. This is attributable to disordered occupation of multivalent cations, which induces severe lattice distortion and increases configurational entropy, thereby facilitating formation of structurally stable, high-density oxygen vacancies on the exposed surface of the spinel. Thus, more catalytic sites on the surface are activated and retained over the course of long-duration testing for oxygen evolution. Entropy engineering expands researchers' access to catalysts that link entropy-stabilized structures to useful properties.

Organic template-assisted green synthesis of CoMoO4 nanomaterials for the investigation of energy storage properties

Transitional metal oxide nanomaterials are considered to be potential electrode materials for supercapacitors. Therefore, in the past few decades, huge efforts have been devoted towards the sustainable synthesis of metal oxide nanomaterials. Herein, we report a synergistic approach to synthesize spherical-shaped CoMoO₄ electrode materials using an inorganic-organic template via the hydrothermal route. As per the synthesis strategy, the precursor solution was reacted with the organic compounds of E. cognata to tailor the surface chemistry and morphology of CoMoO₄ by organic species. The modified CoMoO₄ nanomaterials revealed a particle size of 23 nm by X-ray diffraction. Furthermore, the synthesized material was scrutinized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy and energy dispersive spectroscopy. The optical band gap energy of 3.6 eV was calculated by a Tauc plot. Gas chromatography-mass spectrometry identified cyclobutanol (C₄H₈O) and octodrine (C₈H₁₉N) as the major stabilizing agents of the CoMoO₄ nanomaterial. Finally, it was revealed that the bioorganic framework-derived CoMoO₄ electrode exhibited a capacitance of 294 F g^-1 by cyclic voltammetry with a maximum energy density of 7.3 W h kg^-1 and power density of 7227.525 W kg^-1. Consequently, the nanofeatures and organic compounds of E. cognata were found to enhance the electrochemical behaviour of the CoMoO₄-fabricated electrode towards supercapacitor applications.

Oxygen Evolution Reaction at IrO2/Ir(Ni) Film Electrodes Prepared by Galvanic Replacement and Anodization: Effect of Precursor Ni Film Thickness

IrO2/Ir(Ni) film electrodes of variable Ni content have been prepared via a galvanic replacement method, whereby surface layers of pre-deposited Ni are replaced by Ir, followed by electrochemical anodization. Electrodeposition of Ni on a glassy carbon electrode support has been carried out at constant potential and the charge of electrodeposited Ni controlled so as to investigate the effect of precursor Ni layer thickness on the electrocatalytic activity of the corresponding IrO2/Ir(Ni)/GC electrodes for the oxygen evolution reaction (OER). After their preparation, these electrodes were characterized by microscopic (SEM) and spectroscopic (EDS, XPS) techniques, revealing the formation of Ir deposits on the Ni support and a thin IrO2 layer on their surfaces. To determine the electroactive surface area of the IrO2 coatings, cyclic voltammograms were recorded in the potential range between hydrogen and oxygen evolution and the charge under the anodic part of the curves, corresponding to Ir surface oxide formation, served as an indicator of the quantity of active IrO2 in the film. The electrocatalytic activity of the coatings for OER was investigated by current-potential curves under steady state conditions, revealing that the catalysts prepared from thinner Ni films exhibited enhanced electrocatalytic performance.

Engineering Ternary Copper-Cobalt Sulfide Nanosheets as High-performance Electrocatalysts toward Oxygen Evolution Reaction

The rational design and development of the low-cost and effective electrocatalysts toward oxygen evolution reaction (OER) are essential in the storage and conversion of clean and renewable energy sources. Herein, a ternary copper-cobalt sulfide nanosheets electrocatalysts (denoted as CuCoS/CC) for electrochemical water oxidation has been synthesized on carbon cloth (CC) via the sulfuration of CuCo-based precursors. The obtained CuCoS/CC reveals excellent electrocatalytic performance toward OER in 1.0 M KOH. It exhibits a particularly low overpotential of 276 mV at current density of 10 mA cm−2, and a small Tafel slope (58 mV decade−1), which is superior to the current commercialized noble-metal electrocatalysts, such as IrO2. Benefiting from the synergistic effect of Cu and Co atoms and sulfidation, electrons transport and ions diffusion are significantly enhanced with the increase of active sites, thus the kinetic process of OER reaction is boosted. Our studies will serve as guidelines in the innovative design of non-noble metal electrocatalysts and their application in electrochemical water splitting