Insight into the structure and transport properties of pyrrolidinium-based geminal dicationic-organic ionic crystals: unravelling the role of alkyl-chain length.

The present study has been undertaken with an aim to design and develop safer and more efficient all solid-state electrolytes, so that the issues associated with the use of conventional room temperature ionic liquid-based electrolytes can be tackled. To fulfil this objective, a series of geminal di-cationic Organic Ionic Crystals (OICs), based on C3-, C6-, C8- and C9-alkylbridged bis-(methylpyrrolidinium)bromide are synthesized, and the structural features, thermal properties and phase behaviours of these as synthesized OICs have been investigated. Additionally, a number of electro-analytical techniques have been employed to assess their suitability as an efficient electrolyte composite (OIC:I2:TBAI) for all solid-state dye sensitised solar cells (DSSCs). The structural analysis has revealed that along with excellent thermal stability and well-defined surface morphology, all thsese OICs exhibit a well-ordered three-dimensional network of cations and anions that can serve as a conducting channel for the diffusion of iodide ions. Electrochemical investigations have shown that OICs with an intermediate length of alkyl bridge (C6- and C8-alkyl bridged) show better electrolytic performance than those that are based on OICs with a relatively shorter (C3-) or longer (C9-) alkyl-bridge chain. A careful analysis of the above data has essentially demonstrated that the length of the alkyl bridge chain plays a significant role in determining the structural organisation, morphology and eventually the ionic conductivity of OICs. Overall, the comprehensive knowledge on OICs that has been extracted from the current study is expected to be helpful to explore further new types of OIC-based all solid-state electrolytes with improved electrolytic performance for targeted applications.

Electrochemically activated Co-Prussian blue analogue derived amorphous CoB nanostructures: an efficient electrocatalyst for the oxygen evolution reaction.

The oxygen evolution reaction is a kinetically sluggish half-cell reaction which plays an important role in tuning the efficiency of various electrochemical energy conversion systems. However, this process can be facilitated by manipulating the composition and morphology of the electrocatalyst. Here, by tuning the annealing temperature, a series of cobalt borides (CoB@300, CoB@450, CoB@550 and CoB@650) were synthesized from a metal-organic framework Prussian blue analogue (PBA) following boronization. The resulting borides were characterized systematically and we explored their electrocatalytic activity towards the oxygen evolution reaction (OER). In an alkaline electrolyte, the in situ surface transformation of the boride working electrode to the corresponding metaborite and cobalt oxyhydroxide took place which thereafter acted as the active catalytic sites for the OER. Interestingly, the amorphous form of cobalt boride (i.e., CoB@300) shows many fold increased catalytic activity compared to those of crystalline CoB and commercial RuO2 requiring only 290 mV overpotential to reach the benchmarked 10 mA cm-2 current density and the trend follows the order as CoB@300 > CoB@450 > CoB@550 > CoB@650 > PBA. The dominant catalytic activity of the amorphous cobalt boride nanostructure is attributed particularly to its amorphous nature and synergy between the in situ formed catalytically active centres (meta-borites and cobalt oxyhydroxide).

Impact of Iron in Three-Dimensional Co-MOF for Electrocatalytic Water Oxidation.

The integration of iron (Fe) into a cobalt metal-organic framework (Co-MOF) tunes the electronic structure of the parent MOF as well as enhances their electrocatalytic characteristics. By using pyrazine and hydrofluoric acid, we have synthesized three-dimensional Co-MOF [CoFC4H4N2(SO4)0.5], (1), and Fe-MOF [FeFC4H4N2(SO4)0.5], (2), through a single-step solvothermal method. Further, a series of bimetallic (having both Co and Fe metal centers) MOFs [Co1-xFexFC4H4N2(SO4)0.5] were synthesized with variable concentrations of Fe, and their electrocatalytic performances were analyzed. The optimized amount of Fe significantly impacted the electrocatalytic behavior of the bimetallic MOF toward water oxidation. Particularly, the Co0.75Fe0.25-MOF needs only 239 and 257 mV of overpotential to deliver 10 and 50 mA/cm2 current density, respectively, in alkaline electrolytic conditions. The Co0.75Fe0.25-MOF shows a lower Tafel slope (42 mV/dec.) among other bimetallic MOFs and even the commercial RuO2, and it has excellent durability (with ∼8 mV increases in overpotential after 18 h of electrolysis) and 97.05% Faradaic efficiency, which further evident its catalytic excellency. These findings explore the intrinsic properties of MOF-based electrocatalysts and prospect the suitability for future water electrolysis.

Enhanced Oxygen Evolution Reaction with a Ternary Hybrid of Patronite-Carbon Nanotube-Reduced Graphene Oxide: A Synergy between Experiments and Theory.

This work reports the hybridization of patronite (VS4) sheets with reduced graphene oxide and functionalized carbon nanotubes (RGO/FCNT/VS4) through a hydrothermal method. The synergistic effect divulged by the individual components, i.e., RGO, FCNT, and VS4, significantly improves the efficiency of the ternary (RGO/FCNT/VS4) hybrid toward the oxygen evolution reaction (OER). The ternary composite exhibits an impressive electrocatalytic OER performance in 1 M KOH and requires only 230 mV overpotential to reach the state-of-the-art current density (10 mA cm-2). Additionally, the hybrid shows an appreciable Tafel slope with a higher Faradaic efficiency (97.55 ± 2.3%) at an overpotential of 230 mV. Further, these experimental findings are corroborated by the state-of-the-art density functional theory by presenting adsorption configurations, the density of states, and the overpotential of these hybrid structures. Interestingly, the theoretical overpotential follows the qualitative trend RGO/FCNT/VS4 < FCNT/VS4 < RGO/VS4, supporting the experimental findings.

In Situ Transformed Cobalt Metal-Organic Framework Electrocatalysts for the Electrochemical Oxygen Evolution Reaction.

The development of an active and efficient electrocatalyst for the oxygen evolution reaction remains indispensable for the smooth running of an electrolyzer. Herein, we have synthesized two cobalt metal-organic frameworks (Co-MOFs) with the formulas [C6H6CoN2O4] (compound 1) and [C12H10CoN2O4] (compound 2) using pyrazine and 4,4'-bipyridine as linkers in dimethylformamide medium by a solvothermal method. Both Co-MOFs shows strong antiferromagnetic interactions with Θp = -70 and -61 K for compounds 1 and 2, respectively. The in situ transformation of both compounds catalyzes the OER efficiently in alkaline medium, affording a current density of 10 mA/cm2 at overpotentials of 276 ± 3 and 302 ± 3 mV by compounds 1 and 2, respectively. Moreover, compound 1 shows a very high turnover frequency (15.087 s-1), lower Tafel slope (56 mV/dec), and greater Faradaic efficiency of 95.42% in comparison to compound 2. The transformations of the Co-MOFs have been accessed by employing powder X-ray diffraction (PXRD), high-resolution transmission electron microscopic (HRTEM) analysis, and X-ray photoelectron spectroscopy, which reveal the formation of uniform hexagonal Co(OH)2 plates. Therefore, the as-developed Co-MOF is found to be an efficient pre-electrocatalyst for the OER in alkaline medium. These results not only reveal the preparation of OER electrocatalysts from a Co-MOF but also establish a method to derive a potentially active electrocatalyst to substitute for the traditional noble-metal-based materials.

Three-dimensional NiCoP hollow spheres: an efficient electrode material for hydrogen evolution reaction and supercapacitor applications.

A binary metal phosphide (NiCoP) has been synthesized in a single-step hydrothermal method, and its energy conversion (hydrogen evolution reaction; HER) and energy storage (supercapacitor) performances have been explored. The physicochemical characterization of the NiCoP nanostructures show that they have a highly crystalline phase and are formed uniformly with a sphere-like surface morphology. In acidic electrolytic conditions, the NiCoP shows excellent HER performance, requiring only 160 and 300 mV overpotential to deliver 10 and 300 mA cm-2 current density, respectively. Interestingly, it follows the Volmer-Heyrovsky reaction pathway to execute the HER with robust durability (∼15 mV increase in overpotential even after 18 h of electrolysis). In an alkaline medium (5 M KOH), NiCoP shows specific capacitance of 960 F g-1 with higher energy density (33.3 W h kg-1) and power density (11.8 kW kg-1). Moreover, it shows better reversibility (∼97% coulombic efficiency) and long cycle life (∼95% capacitance retention after 10 000 repeated cycles). The unique surface morphology and phase purity of the binary metal phosphide avails more electroactive surface/redox centers, thereby showing better electrocatalytic as well as energy storage performances. Therefore, we presume that the NiCoP would be a suitable material for future energy conversion and storage systems.

Synthesis of Ge4Se9 nano plates and a reduced graphene oxide composite of Ge4Se9 for electrochemical energy storage application.

In this study, we introduced a facile and single-step synthetic protocol for the scalable synthesis of tetra germanium nonaselenide (Ge4Se9) and it's composite with reduced graphene oxide (RG). The physicochemical properties of the samples were studied systematically, and their electrochemical performances for energy storage in supercapacitors were explored. Herein, the weight percentage of graphene oxide in the composite played a vital role in the enhancement of charge storage. Among other composites, the Ge4Se9/RG1 composite showed an enhanced specific capacitance (220 F g-1) with a higher specific energy (12 W h kg-1) as well as power (4.6 kW kg-1). Moreover, the composite showed excellent cycling stability (with 91% of capacitance retention after 10 000 repeated cycles) and reversibility (∼98% coulombic efficiency). Furthermore, the robustness of the composite was accessed via the post-stability analysis.

Synthesis of a 3D free standing crystalline NiSex matrix for electrochemical energy storage applications.

The electrochemical performance for energy storage of three-dimensional (3D) self-supported heterogeneous NiSex cubic-orthorhombic nanocrystals grown by a facile one-step chemical vapour deposition (CVD) approach on Ni foam substrates has been explored. NiSex shows a high specific capacitance of 1333 F g-1 with ultra-high energy (105 W h kg-1) and power (54 kW kg-1) densities. Furthermore, by integrating the as-grown NiSex as the anode and reduced graphene oxide as the cathode, a hybrid supercapacitor (HSC) prototype with a coin cell configuration has been fabricated. The device shows better capacitance (40 F g-1) with high energy (22 W h kg-1) and power (5.8 kW kg-1) densities and robust cycling durability (∼88% capacitance retention after 10 000 repeated cycles). For practical reliability of the as-fabricated HSC, a red LED has been illuminated by connecting it with two charged coin cells. These outstanding performances of the HSC prove to be promising for applications in high energy storage systems.

A cobalt metal-organic framework (Co-MOF): a bi-functional electro active material for the oxygen evolution and reduction reaction.

The oxygen electrocatalysis, i.e. the oxygen reduction and evolution reactions, is traditionally executed using noble metal and metal oxide-based nanostructures. However, they are associated with many disadvantages such as high cost, lower durability/selectivity and detrimental environmental effects; this motivates researchers to develop new electroactive materials. In this study, we presented the synthesis of a Co-containing metal-organic framework (Co-MOF) and explored its electrocatalytic application towards the oxygen electrocatalysis (i.e. the oxygen reduction reaction and oxygen evolution reaction). The Co-MOF efficiently catalyzes the ORR with a lower onset (0.85 V vs. RHE)/reduction potential and higher reduction current density by a four-electron reduction path. Moreover, the MOF shows higher durability with >70% performance retention after 25 hours of reaction and tolerance towards methanol; this demonstrates its potential for application in direct methanol fuel cells (DMFCs); furthermore, due to the availability of more active sites and accessible surface area, the Co-MOF performs well towards the OER with lower onset potential and small Tafel slope as compared to the commercial RuO2 nanoparticles. Moreover, it needs only 280 mV overpotential to deliver the state-of-the-art current density of 10 mA cm-2 and robust stability. It shows the high TOF value of 93.21 s-1 at the overpotential of 350 mV as compared to the reported MOF/nanoparticle-based electrocatalysts and the state-of-the-art RuO2. Therefore, we believe that the as-developed Co-MOF holds the potential to be used as both a cathode and an anode electrocatalyst in the future energy storage and conversion systems.

VS2: an efficient catalyst for an electrochemical hydrogen evolution reaction in an acidic medium.

In view of preparing efficient electrocatalysts for energy conversion applications, we have developed an eco-friendly, cost effective, single step method for the scalable synthesis of VS2 and its reduced graphene oxide composite VS4/rGO. Furthermore, the electrocatalytic performances of the catalysts have been studied toward the hydrogen evolution reaction in an acid medium (0.1 M H2SO4). Presumably, the large exposed electrochemical active surface area (27.7 cm2) and hexagonal crystal lattice of VS2 result in its dominating catalytic performance over that of the linear VS4/rGO composite. Also, a VS2 modified electrode was demonstrated to have better stability (with a negligible change in the overpotential even after 10 h and 43 h of continuous electrolysis) with a notably low Tafel slope (36 mV dec-1, close to that of commercial Pt/C) and onset potential (15 mV vs. RHE) with robust durability for long term application. A preliminary study on the photoelectrochemical activities of VS2 showed a significant decrease in the charge transfer resistance upon illumination of light on the electrode surface.

Highly Active 2D Layered MoS 2 -rGO Hybrids for Energy Conversion and Storage Applications.

The development of efficient materials for the generation and storage of renewable energy is now an urgent task for future energy demand. In this report, molybdenum disulphide hollow sphere (MoS2-HS) and its reduced graphene oxide hybrid (rGO/MoS2-S) have been synthesized and explored for energy generation and storage applications. The surface morphology, crystallinity and elemental composition of the as-synthesized materials have been thoroughly analysed. Inspired by the fascinating morphology of the MoS2-HS and rGO/MoS2-S materials, the electrochemical performance towards hydrogen evolution and supercapacitor has been demonstrated. The rGO/MoS2-S shows enhanced gravimetric capacitance values (318 ± 14 Fg-1) with higher specific energy/power outputs (44.1 ± 2.1 Whkg-1 and 159.16 ± 7.0 Wkg-1) and better cyclic performances (82 ± 0.95% even after 5000 cycles). Further, a prototype of the supercapacitor in a coin cell configuration has been fabricated and demonstrated towards powering a LED. The unique balance of exposed edge site and electrical conductivity of rGO/MoS2-S shows remarkably superior HER performances with lower onset over potential (0.16 ± 0.05 V), lower Tafel slope (75 ± 4 mVdec-1), higher exchange current density (0.072 ± 0.023 mAcm-2) and higher TOF (1.47 ± 0.085 s-1) values. The dual performance of the rGO/MoS2-S substantiates the promising application for hydrogen generation and supercapacitor application of interest.

Urea-Assisted Room Temperature Stabilized Metastable ß-NiMoO4: Experimental and Theoretical Insights into its Unique Bifunctional Activity toward Oxygen Evolution and Supercapacitor.

Room-temperature stabilization of metastable ß-NiMoO4 is achieved through urea-assisted hydrothermal synthesis technique. Structural and morphological studies provided significant insights for the metastable phase. Furthermore, detailed electrochemical investigations showcased its activity toward energy storage and conversion, yielding intriguing results. Comparison with the stable polymorph, α-NiMoO4, has also been borne out to support the enhanced electrochemical activities of the as-obtained ß-NiMoO4. A specific capacitance of ∼4188 F g-1 (at a current density of 5 A g-1) has been observed showing its exceptional faradic capacitance. We qualitatively and extensively demonstrate through the analysis of density of states (DOS) obtained from first-principles calculations that, enhanced DOS near top of the valence band and empty 4d orbital of Mo near Fermi level make ß-NiMoO4 better energy storage and conversion material compared to α-NiMoO4. Likewise, from the oxygen evolution reaction experiment, it is found that the state of art current density of 10 mA cm-2 is achieved at overpotential of 300 mV, which is much lower than that of IrO2/C. First-principles calculations also confirm a lower overpotential of 350 mV for ß-NiMoO4.

Au Nanowire-Striped Cu3P Platelet Photoelectrocatalysts.

A stripy pattern of continuous epitaxial growth of thin Au nanowires on plasmonic Cu3P platelets is reported. The obtained Au-Cu3P heterostructures retain their wide area interfacial heterojunction, which is typically not observed in metal-semiconductor heterostructures. This is performed by phosphine-mediated in situ reduction of Au ions on specific facets of Cu3P platelets. The intriguing stripy movements of nanowires are regulated by strong surface binding ligands. Because this is a dual plasmon heterostructure with wide visible absorption window, these are further explored as a photoelectrocatalyst for efficient hole transfer and sensing of an important biomolecule, nicotinamide adenine dinucleotide (NADH). The observed anodic photocurrent was 30 times higher in the presence of NADH, and this proves that the heterostructured material is an ideal photosenser and an efficient catalyst for solar energy conversion.

Good's buffer derived highly emissive carbon quantum dots: excellent biocompatible anticancer drug carrier.

Here, a facile one-step approach has been developed for the synthesis of carbon quantum dots (CQDs) from Good's buffer. The as-synthesized CQDs emit a bright greenish blue coloured fluorescence after exposure to a 365 nm UV-lamp illumination. The bright fluorescence nature of the CQDs has proven them to be excellent probes for cellular imaging. The CQDs are highly biocompatible in nature, which has been validated by an MTT assay test. The in vitro MTT assay demonstrates a more than 95% survival rate when HEK293 (human embryonic kidney) and H357 (human oral squamous carcinoma) cells were treated with CQDs. The low cytotoxicity of Good's buffer derived CQDs opens the door to biomedical applications. The anticancer drug doxorubicin (DOX) was successfully loaded on the CQDs and their delivery efficiency to the target cells via in vitro treatment of cancerous cells was explored. The CQDs supported DOX showed a higher killing rate of the cancer cells compared to bare DOX due to its ease of internalization and efficient pH-triggered release inside the cells.

Simple Growth of Faceted Au-ZnO Hetero-nanostructures on Silicon Substrates (Nanowires and Triangular Nanoflakes): A Shape and Defect Driven Enhanced Photocatalytic Performance under Visible Light.

A simple single-step chemical vapor deposition (CVD) method has been used to grow the faceted Au-ZnO hetero-nanostructures (HNs) either with nanowires (NWs) or with triangular nanoflakes (TNFs) on crystalline silicon wafers with varying oxygen defect density in ZnO nanostructures. This work reports on the use of these nanostructures on substrates for photodegradation of rhodamine B (RhB) dyes and phenol under the visible light illumination. The photoluminescence measurements showed a substantial enhancement in the ratio of defect emission to band-edge emission for TNF (ratio ≈ 7) compared to NW structures (ratio ≤ 0.4), attributed to the presence of more oxygen defects in TNF sample. The TNF structures showed 1 order of magnitude enhancement in photocurrent density and an order of magnitude less charge-transfer resistance (R(ct)) compared to NWs resulting high-performance photocatalytic activity. The TNFs show enhanced photocatalytic performance compared to NWs. The observed rate constant for RhB degradation with TNF samples is 0.0305 min(-1), which is ≈5.3 times higher compared to NWs case with 0.0058 min(-1). A comparison has been made with bulk ZnO powders and ZnO nanostructures without Au to deduce the effect of plasmonic nanoparticles (Au) and the shape of ZnO in photocatalytic performance. The results reveal the enhanced photocatalytic capability for the triangular nanoflakes of ZnO toward RhB degradation with good reusability that can be attracted for practical applications.

A facile approach for in situ synthesis of graphene-branched-Pt hybrid nanostructures with excellent electrochemical performance.

A facile and green approach for the synthesis of highly electroactive branched Pt nanostructures well dispersed on graphene has been developed by in situ reduction of graphene oxides and Pt(iv) ions in an aqueous medium. The as-synthesized branched Pt and graphene hybrid nanomaterials (GR-BPtNs) were thoroughly characterized using Transmission Electron Microscope (TEM), UV-Visible spectroscopy, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and Raman spectroscopy. This report clearly exploits the decisive role of the graphene support, the pH of the solution and the stabiliser on shaping the branched morphology of the Pt nanostructures well dispersed on graphene. Cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy (EIS) measurements were employed to investigate the electrocatalytic performance and durability of GR-BPtNs towards methanol oxidation and oxygen reduction. The results reveal that the synergetic effect of the graphene support and the branched morphology triggers electrocatalytic performance and robust tolerance to surface poisoning of GR-BPtNs.

A bioinspired approach for shaping Au nanostructures: the role of biomolecule structures in shape evolution.

A new approach for shaping Au nanostructures by tuning the molecular structure of biomolecules has been explored. Different molecules, such as catechol, rutin, and quercetin, which have structural similarity to the catechol ring, were used to induce Au nanostructures under similar conditions. The as-synthesized nanostructures are characterized by using TEM, XPS, XRD, and UV/Vis spectral measurements. The growth mechanism for the formation of these noble metal shapes and the role of the molecular structure of the stabilizing/reducing agent were investigated by using TEM and UV/Vis spectral measurements. The structure and functional groups of the reducing/stabilizing agent play a vital role in the shape evolution of nanostructures. The electrocatalytic activity of different nanostructures in the reduction of oxygen was investigated and was found to be shape-dependent.