Ruthenium-Doping-Induced Amorphization of VS4 Nanostructures with a Rich Sulfur Vacancy for Enhanced Hydrogen Evolution Reaction in a Neutral Electrolyte Medium.

The generation of pure H2 from a neutral electrolyte solution represents a transformative route with low cost and environmentally friendly nature. However, the complex kinetics of hydrogen evolution reaction (HER) via water electrolysis make its practical application to be difficult. Herein, we have reported Ru-doping-induced formation of VS4 nanostructures with a rich S vacancy for neutral HER in a 0.2 M phosphate buffer solution. The Ru-doped VS4 demands an overpotential value of 160 mV at 10 mA/cm2 current density with a lower catalyst loading of 0.1 mg/cm2, while pristine VS4 demands a 374 mV overpotential with the same mass loading. 60 hours of chronoamperometric study reveals the excellent stability of Ru-doped VS4 materials, which is the highest amount of time ever reported for neutral HER. The marginal degradation of a catalyst under a long-term stability study was confirmed through inductively coupled plasma mass spectrometry (ICP-MS) analysis. The introduction of Ru to the VS4 lattice leads to a 4.35-fold increase in the turnover-frequency values compared to those of bare VS4 nanostructures. The higher HER activity of S-vacancy-enriched VS4 materials is thought to originate through effective water adsorption in S vacancy and Ru3+ sites followed by the dissociation of a H2O molecule, and S22- efficiently converts Had to H2. Also, post-HER characterization reveals that the transformation of some Ru3+ to Ru0 additionally favored the HER by providing a better H adsorption site under a static cathodic potential.

Revealing the pH-Universal Electrocatalytic Activity of Co-Doped RuO2 toward the Water Oxidation Reaction.

Electrocatalytic water splitting has gained vast attention in recent decades for its role in catalyzing hydrogen production effectively as an alternative to fossil fuels. Moreover, the designing of highly efficient oxygen evolution reaction (OER) electrocatalysts across the universal pH conditions was more challengeable as in harsh anodic potentials, it questions the activity and stability of the concerned catalyst. Generally, geometrical engineering and electronic structural modulation of the catalyst can effectively boost the OER activity. Herein, a Co-doped RuO2 nanorod material is developed and used as an OER electrocatalyst at different pH conditions. Co-RuO2 exhibits a lower overpotential value of 238 mV in an alkaline environment (1 M KOH) with a Tafel slope value of 48 mV/dec. On the other hand, in acidic, neutral, and near-neutral environments, it required overpotentials of 328, 453, and 470 mV, respectively, to attain a 10 mA/cm2 current density. It is observed that doping of Co into the RuO2 could synergistically increase the active sites with the enhanced electrophilic nature of Ru4+ to accelerate OER in all of the pH ranges. This study finds the applicability of earth-abundant-based metals like Co to be used in universal pH conditions with a simple doping technique. Further, it assured the stable nature in all pH electrolytes and needs to be further explored with other metals in the future.

Self-assembled cationic organic nanosheets: role of positional isomers in a guanidinium-core for efficient lithium-ion conduction.

The growing energy demand with the widespread use of smart portable electronics, as well as an exponential increase in demand for smart batteries for electric vehicles, entails the development of efficient portable batteries with high energy density and safe power storage systems. Li-ion batteries arguably have superior energy density to all other traditional batteries. Developing mechanically robust solid-state electrolytes (SSEs) for lithium-ion conduction for an efficient portable energy storage unit is vital to empower this technology and overcome the safety constraints of liquid electrolytes. Herein, we report the formation of self-assembled organic nanosheets (SONs) utilizing positional isomers of small organic molecules (AM-2 and AM-3) for use as SSEs for lithium-ion conduction. Solvent-assisted exfoliation of the bulk powder yielded SONs having near-atomic thickness (∼4.5 nm) with lateral dimensions in the micrometer range. In contrast, self-assembly in the DMF/water solvent system produced a distinct flower-like morphology. Thermodynamic parameters, crystallinity, elemental composition, and nature of H-bonding for two positional isomers are established through various spectroscopic and microscopic studies. The efficiency of the lithium-ion conducting properties is correlated with factors like nanostructure morphology, ionic scaffold, and locus of the functional group responsible for forming the directional channel through H-bonding in the positional isomer. Amongst the three different morphologies studied, SONs display higher ion conductivity. In between the cationic and zwitterionic forms of the monomer, integration of the cationic scaffold in the SON framework led to higher conductivity. Amongst the two positional isomers, the meta-substituted carboxyl group forms a more rigid directional channel through H-bonding to favor ionic mobility and accounts for the highest ion conductivity of 3.42 × 10-4 S cm-1 with a lithium-ion transference number of 0.49 at room temperature. Presumably, this is the first demonstration that signifies the importance of the cationic scaffold, positional isomers, and nanostructure morphologies in improving ionic conductivity. The ion-conducting properties of such SONs having a guanidinium-core may have significance for other interdisciplinary energy-related applications.

Enhancement of the OER Kinetics of the Less-Explored α-MnO2 via Nickel Doping Approaches in Alkaline Medium.

Development of a low-cost transition metal-based catalyst for water splitting is of prime importance for generating green hydrogen on an industrial scale. Recently, various transition metal-based oxides, hydroxides, sulfides, and other chalcogenide-based materials have been synthesized for developing a suitable anode material for the oxygen evolution reaction (OER). Among the various transition metal-based catalysts, their oxides have received much consideration for OER, especially in lower pH condition, and MnO2 is one of the oxides that have widely been used for the same. The large variation in the structural disorder of MnO2 and internal resistance at the electrode-electrolyte interfaces have limited its large-scale application. By considering the above limitations of MnO2, here in this work, we have designed Ni-doped MnO2 via a simple wet-chemical synthetic route, which has been successfully applied for OER application in 0.1 M KOH solution. Doping of various quantities of Ni into the MnO2 lattices improved the OER properties, and for achieving 10 mA/cm2 current density, the Ni-doped MnO2 containing 0.02 M of Ni2+ ions (coined as MnO2-Ni0.002(M)) demands only 445 mV overpotential, whereas the bare MnO2 required 610 mV overpotential. It has been proposed that the incorporation of nickel ions into the MnO2 lattices leads to an electron transfer from the Ni3+ ions to Mn4+, which in turn facilitates the Jahn-Teller distortion in the Mn-O octahedral unit. This electron transfer and the creation of a structural disorder in the Mn sites result in the improvization of the OER properties of the MnO2 materials.

Temperature-Controlled Structural Variations of Meticulous Fibrous Networks of NiFe-Polymeric Zeolite Imidazolate Frameworks for Enhanced Performance in Electrocatalytic Water-Splitting Reactions.

Developing non-noble, earth-ample, and stable electrocatalysts are highly anticipated in oxygen-evolution reaction (OER) and hydrogen-evolution reaction (HER) at unique pH conditions. Herein, we have synthesized bimetallic (nickel and iron) zeolite imidazolate framework (ZIF)-based nanofibrous materials via a simple electrospinning (ES) process. The structural stability of the fibrous material is subjected to various calcination conditions. We have elaborated the structural importance of the one-dimensional (1D) fibrous materials in electrocatalytic water-splitting reactions. As a result, NiFe-ZIF-NFs (Nanofibers)-RT (Room Temperature) have delivered a small overpotential of 241 mV at 10 mA cm-2 current density in OER and 290 mV at a fixed current density of 50 mA cm-2 in HER at two different pH conditions with 1 M KOH and 0.5 M H2SO4, respectively. Furthermore, it exposes the actual surface area of 27.270 m2 g-1 and a high electrochemical active surface area (ECSA) of 50 µF in OER and 55 µF in HER, which is responsible for the electrochemical performance with better stability. This exceptional activity of the materials is mainly attributed to the structural dependency of the fibrous network through the polymeric architecture.

Electrospun Cobalt-Incorporated MOF-5 Microfibers as a Promising Electrocatalyst for OER in Alkaline Media.

Metal-organic framework (MOF)-based materials have attracted attention in recent times owing to their remarkable properties such as regulatable pore size, high specific surface area, and elasticity in their network topology, geometry, dimension, and chemical functionality. It is believed that the incorporation of a MOF network into a fibrous matrix results in the improvement of the electrocatalytic properties of the material. Herein, we have synthesized a Co-incorporated MOF-5-based fibrous material by a simple wet-chemical method, followed by an electrospinning (ES) process. The as-prepared Co-incorporated MOF-5 microfibers were employed as an electrocatalyst for the oxygen evolution reaction (OER) in 1 M KOH electrolyte. The catalyst demands a lower overpotential of 240 mV to attain a current density of 10 mA/cm2 with a lower Tafel slope value of 120 mV/dec along with a charge transfer resistance value of 2.9 Ω from electron impedance spectroscopy (EIS) analysis. From these results, it has been understood that the incorporation of Co metal into the MOF-5 microfibrous network has significantly improved the OER performance, which made them a potential entrant in other energy-related applications also.

Prospects in interfaces of biomolecule DNA and nanomaterials as an effective way for improvising surface enhanced Raman scattering: A review.

Surface Enhanced Raman Scattering (SERS) is a field of research that has shown promising application in the analysis of various substrate molecules by means of rough metallic surfaces. In directing the enhancement of substrate molecules in micro and nano-molar concentrations, plasmonic coupling of metal nanoparticles (NPs), morphology of metal NPs and the closely arrangement of rough metal surfaces that produces 'hot spots' can effectively increase the so-called enhancement factor (EF) that will be applicable in various fields. As the mechanistic aspects are still not clear, research has been triggered all over the world for the past two decades to have a clear understanding in chemical and electromagnetic effects. As the reproducibility of intensity of signals at low concentrations of probe molecules is of a big concern, metal NPs with various scaffolds were prepared and recently bio-molecule, DNA has been studied and showed promising advantages. This review first time highlights metal NPs with DNA interface as an effective rough metallic surface for SERS with high intensity and also with better reproducibility. Based on this review, similar kinds of scaffolds like DNA can be used to further analyze SERS activities of various metal NPs with different morphologies to have high intense signals at low concentrations of probe molecules.

Electrospun Fe-Incorporated ZIF-67 Nanofibers for Effective Electrocatalytic Water Splitting.

The explorations of earth-abundant, noble metal-free, highly efficient electrocatalysts for water-splitting reactions have been considered as highly significant for imperishable energy production. Though the metal organic framework (MOF)-based materials are highly promising candidates in the area of material chemistry, the combined properties associated with MOFs and the one-dimensional (1D) fibrous matrix, which can lead to better electrocatalytic performance, have been less explored. Herein, we ascertain a fabrication method for ZIF-67 (zeolite imidazolate framework) nanofibers (NFs), Fe-ZIF NFs, and Fe-ZIF-67 NFs via the wet chemical combined electrospinning (ES) approach. The as-synthesized catalysts were utilized for the electrochemical reaction, which showed a high efficiency toward the oxygen evolution reaction (OER). Compared to other catalysts, the Fe-ZIF-67 NF catalyst showed a very less overpotential of 278 mV at a fixed current density of 10 mA cm-2. The obtained Tafel slope and Rct values are 77 mV dec-1 and 1.2 Ω, respectively. The post-X-ray photoelectron spectroscopy (XPS) analysis revealed the transformation of FeOOH during the OER study along with Co3+ states in mixed Fe-ZIF-67 NFs. In an alkaline electrolyzer, Fe-ZIF-67 NFs were utilized as the anode and a Pt wire as the cathode in 1 M KOH solution, which required a cell voltage of 1.68 V at 10 mA cm-2 current density with astonishing stability. Hence, this work should open a new path for the exploration of efficient non-noble metal catalysts for energy-related applications.

Surface Decoration of DNA-Aided Amorphous Cobalt Hydroxide via Ag+ Ions as Binder-Free Electrodes toward Electrochemical Oxygen Evolution Reaction.

Out of various available methods, generation of hydrogen by electrocatalytic water splitting is the most accepted one which consists of two half-cell reactions, viz, oxygen evolution reaction (OER) at the anode and hydrogen evolution reaction at the cathode. OER is a complex four-electron transfer process, and to sustain the spontaneous generation of hydrogen at the cathode, it is urgent to develop some earth-abundant, low-cost electrode materials. Recently, use of cobalt-based hydroxide as the electrode substrate has taken much consideration and has been fabricated over various substrates. Because of various structural disorders, internal resistance, and dependence on the electrode, the binder substrate makes their applications limited. Here, in this work, to remove structural disorder and to increase electrical conductivity, we have incorporated silver ions into amorphous Co(OH)2, which turns to be a highly active OER electrocatalyst. Also, for the first time, we have developed hydroxide-based materials by using DNA as a stabilizer, and most importantly, using DNA gives an immense opportunity to run long-term OER applications without using an external binder such as nafion. Moreover, for the first time, these DNA-based materials were coated on nickel foam mainly to eliminate the low conductive nature of Ag2O. The synthesized catalyst showed a very high OER activity, and to reach 50 mA/cm2 current density, it needs only 260 mV as overpotential. The amorphous nature of hydroxide-based materials gives a higher opportunity toward the electrolyte to bind on the surface of a catalyst to run the OER with less applied overpotentials.

Enabling and Inducing Oxygen Vacancies in Cobalt Iron Layer Double Hydroxide via Selenization as Precatalysts for Electrocatalytic Hydrogen and Oxygen Evolution Reactions.

Production of hydrogen by water electrolysis is an environment-friendly method and comparatively greener than other methods of hydrogen production such as stream reforming carbon, hydrolysis of metal hydride, etc. However, sluggish kinetics of the individual half-cell reactions hinders the large-scale production of hydrogen. To minimize this disadvantage, finding an appropriate, competent, and low-cost catalyst has attracted attention worldwide. Layer double hydroxide (LDH)-based materials are promising candidates for oxygen evolution reaction (OER) but not fruitful and their hydrogen evolution reaction (HER) activity is very poor, due to the lack of ionic conductivity. The inclusion of chalcogenide and generation of inherent oxygen vacancies in the lattice of LDH lead to improvement of both OER and HER activities. The presence of rich oxygen vacancies was confirmed using both the Tauc plot (1.11 eV, vacancy induction) and the photoluminescence study (peak at 426 nm, photoregeneration of oxygen). In this work, we have developed vacancy-enriched, selenized CoFe-LDH by the consequent wet-chemical and hydrothermal routes, respectively, which was used for OER and HER applications in 1 M KOH and 0.5 M H2SO4 electrolytes, respectively. For OER, the catalyst required only 251 mV overpotential to reach a 50 mA/cm2 current density with a Tafel slope value of 47 mV/dec. For HER, the catalyst demanded only 222 mV overpotential for reaching a 50 mA/cm2 current density with a Tafel slope value of 126 mV/dec. Hence, generating oxygen vacancies leads to several advantages from enhancing the exposed active sites to high probability in obtaining electrocatalytically active species and subsequent assistance in oxygen and hydrogen molecule cleavage.

Green and sustainable route for oxidative depolymerization of lignin: New platform for fine chemicals and fuels.

Depolymerization of lignin biomass to its value-added chemicals and fuels is pivotal for achieving the goals for sustainable society, and therefore has acquired key interest among the researchers worldwide. A number of distinct approaches have evolved in literature for the deconstruction of lignin framework to its mixture of complex constituents in recent decades. Among the existing practices, special attention has been devoted for robust site selective chemical transformation in the complex structural frameworks of lignin. Despite the initial challenges over a period of time, oxidation and oxidative cleavage process of aromatic building blocks of lignin biomass toward the fine chemical synthesis and fuel generation has improved substantially. The development has improved in terms of cost effectiveness, milder reaction conditions, and purity of compound individuals. These aforementioned oxidative protocols mainly involve the breaking of C-C and C-O bonds of complex lignin frameworks. More precisely in the line with environmentally friendly greener approach, the catalytic oxidation/oxidative cleavage reactions have received wide spread interest for their mild and selective nature toward the lignin depolymerization. This mini-review aims to provide an overview of recent developments in the field of oxidative depolymerization of lignin under greener and environmentally benign conditions. Also, these oxidation protocols have been discussed in terms of scalability and recyclability as catalysts for different fields of applications.

Crystalline Free-Standing Two-Dimensional Zwitterionic Organic Nanosheets for Efficient Conduction of Lithium Ions.

Crystalline two-dimensional organic nanosheets (2D-ONs) having atomic or near-atomic thickness with infinite lateral dimensions are of crucial significance for their possible application as a material for energy storage. The presence of nanofluidic channels with a designed array of molecular interlayers in such 2D-ONs, for a favorable lithium-ion transport, has special significance for improving the efficacy of lithium-ion batteries. However, the rational design of crystalline 2D-ONs remains a challenge because of the lack of appropriate monomers and convenient preparation methods. Herein, we report a unique lithium-ion conducting behavior of zwitterionic 2D-ONs, formed through self-assembly of a small organic molecule AM-1. Different microscopic studies confirm the near-atomic thickness (∼3.5 nm) of these 2D-ONs. Results of the single-crystal X-ray diffraction studies confirm the presence of a one-dimensional (1D) channel in crystalline 2D-ONs, which was generated during the self-assembly process of the zwitterionic monomer scaffold. The presence of immobilized ionic centers with well-defined directional channels in the 2D-ONs favors the transportation of lithium ions with a room-temperature lithium-ion conductivity of 5.14 × 10-5 S cm-1, which is rather unique for self-assembled 2D-ONs.

Intervening Bismuth Tungstate with DNA Chain Assemblies: A Perception toward Feedstock Conversion via Photoelectrocatalytic Water Splitting.

An advanced approach with DNA-mediated bismuth tungstate (Bi2WO6) one-dimensional (1-D) nanochain assemblies for hydrogen production with 5-fold enhanced photoelectrochemical (PEC) water splitting reaction is presented. The creation of new surface states upon DNA modification mediates the electron transfer in a facile manner for a better PEC process. The UV-Vis-DRS analysis results a red shift in the optical absorption phenomenon with the interference of DNA modification on Bi2WO6, and, thus, the band gap was tuned from 3.05 eV to 2.71 eV. The applied bias photon-to-current efficiency (ABPE) was calculated and shows a maximum for the Bi2WO6@DNA-2 (25.22 × 10-4%), compared to pristine Bi2WO6 (7.76 × 10-4%). Furthermore, the idea of practical utility of produced hydrogen from PEC is established for the first time with photocatalytic feedstock conversion to platform chemicals using cinnamaldehyde, 2-hydroxy-1-phenylethanone, and 2-(3-methoxyphenoxy)-1-phenylethanone in large scale by hydrogenation and/or hydrogenolysis reactions under eco-friendly green conditions with external hydrogen pressure in an aqueous mixture. Also, the recyclability experiment delivered good yields, which further confirm the robustness of the developed catalyst.

Tuning Cu Overvoltage for a Copper-Telluride System in Electrocatalytic Water Reduction and Feasible Feedstock Conversion: A New Approach.

Highly efficient and earth-abundant elements capable of water reduction by electrocatalysis and are attractive for the sustainable generation of fuels. Among the earth-abundant metals, copper is one of the cheapest but often the most neglected choice for the hydrogen evolution reaction (HER) due to its high overvoltage. Herein, for the first time we have tuned the overpotential of copper by tellurizing it by two different methodologies, viz. hydrothermal and wet chemical methods, which form copper telluride nanochains and aggregates. The application of copper telluride as an electrocatalyst for the HER gave fruitful results in terms of both activity and stability. The hydrothermally synthesized catalyst Cu2-xTe/hyd shows a low overpotential (347 mV) at 10 mA cm-2 toward the HER. In addition, the catalyst showed a very low charge transfer resistance (Rct) of 24.4 Ω and, as expected, Cu2-xTe/hyd exhibited a lower Tafel slope value of 188 mV/dec in comparison to Cu2-xTe/wet (280 mV/dec). A chronoamperometry study reveals the long-term stability of both catalysts even up to 12 h. The Faradaic efficiency of Cu2-xTe/hyd was calculated and found to be 95.06% by using gas chromatographic (GC) studies. Moreover, with the idea of utilizing produced hydrogen (H2) from electrocatalysis, for the first time we have carried out feedstock conversion to platform chemicals in water under eco-friendly green conditions. We have chosen cinnamaldehyde, 2-hydroxy-1-phenylethanone, 4-(benzyloxy)benzaldehyde, and 2-(3-methoxyphenoxy)-1-phenylethanone (ß-O-4) as model compounds for feedstock conversion by hydrogenation and/or hydrogenolysis reactions in aqueous medium using external hydrogen pressure. This protocol could also be scaled up for large-scale conversion and the catalyst is likely to find industrial application since it requires an inexpensive catalyst and an easily available, mild reducing agent. The robustness of the developed catalyst is proven by recyclability experiments and its possibility of use in real-life applications.

Developments in DNA metallization strategies for water splitting electrocatalysis: A review.

The biomolecule DNA with the presence of different functionalities found to interact with different kinds of metal ions and show relatively higher stability over a long period of time when optimized appropriately. With the presence of A-T and G-C pairs, sugar moieties, phosphate functional groups and the double-helical structure, it can assemble both cationic and anionic species and forms a perfect metal-DNA self-assembly. Depending upon the aspect ratio of metal-DNA self-assemblies, metal content and their morphological outcomes, they could deliver variance in the catalytic activities. Such differences can be brought out by varying the synthesis reaction parameters focusing on a specific electrocatalytic application. In this review, recent developments in DNA metallization is elaborated first highlighting the underlying interactions between DNA and cationic/anionic species of various metals following which application of metal-DNA assemblies in electrocatalytic water oxidation and reduction are discussed critically. Knowledge provided in this review thus acts as the guide to various DNA metallization strategies and their subsequent application to water electrolysis for hydrogen generation.

Transition-Metal-Based Zeolite Imidazolate Framework Nanofibers via an Electrospinning Approach: A Review.

Zeolite imidazolate frameworks (ZIFs) are a subclass of metal organic frameworks (MOFs) and have been considered as a special finding in the current platform of the research arena. ZIFs have been comprised of metal ions with imidazolate linkers. In recent times, ZIFs have been predominately utilized for various applications. This excellent feature is because of its fascinating properties. During the evolution of the materials era, one-dimensional (1-D) fibrous materials are also considered as an important area of research. In order to make the fibrous materials, electrospinning (ES) is considered as a more reliable way for their synthesis. 1-D material has also been utilized for various applications owing to their abnormal physicochemical properties. In this mini-review, the recent developments with various processes have been followed for the synthesis of ZIF materials and 1-D fibrous materials. We elaborated their advantages over their applications in the past years which are discussed and reviewed. More importantly, we have proposed a new area for the incorporation of transition-metal-based ZIF materials into the 1-D fibrous materials, which confers the new direction to the research community to explore its use in various applications.

In Situ Modified Nitrogen-Enriched ZIF-67 Incorporated ZIF-7 Nanofiber: An Unusual Electrocatalyst for Water Oxidation.

One-dimensional (1D) based carbonaceous materials have wider applicability owing to flexibility over their properties. Electrospinning is one of the pioneering techniques that is being utilized predominantly for the synthesis of fibers. Fibrous material has their own advantages like, high porosity over its surface, mechanical stability and fascinating physicochemical properties. With these many advantages, herein we have synthesized a distinctive nanofiber from a zeolite imidazolate framework (ZIF) based material. This powder has been mixed with PAN (polyacrylonitrile) which act as both a heteroatom source and a precursor material in the making of a fibrous material via electrospinning. The synthesized nitrogen-enriched ZIF-67 incorporated ZIF-7 microfibers were prepared under two different conditions (room temperature and N2 atmosphere) and labeled as CoZn-ZIF-RT and CoZn-ZIF-500, respectively. The newly developed materials were assessed for the first time in electrocatalytic oxygen evolution reaction (OER) studies have been carried out in a 1 M KOH electrolyte. CoZn-ZIF-500 delivered superior activity at a current density of 10 mA cm-2 with a kinetically facile Tafel slope and explicitly showed good long-term stability.