Low-temperature synthesis of high-entropy amorphous metal oxides (HEOs) for enhanced oxygen evolution performance.

The rational design of multiple metal ions into high-entropy oxide electrode material via a single-step hydrothermal process is applicable to the evolution of oxygen molecules (O2) through simple water electrolysis. Their cost-effectiveness, high performance, and durable nature are the key factors of non-precious high-entropy multiple metal-based electrocatalysts, which can be used as replaceable catalysts instead of precious ones. This article reports a low-temperature synthesis of the cauliflower-type morphology of high-entropy amorphous metal oxides, and their electrochemical performances towards the oxygen evolution reaction (OER) are investigated. The multiple metal ion (Mn2+, Fe3+, Co2+, Ni2+, Cu2+) oxide electrode material shows an acceptable oxygen evolution reaction (OER) with an overpotential of 290 mV at a current density of 10 mA cm-2 and a lower Tafel slope value of 85 mV dec-1, respectively. Moreover, the 20 h durability test with negligible change in overpotential shows the efficacy of the modified electrode material in harsh alkaline media. The observed electrochemical results towards the OER correspond to the amorphous nature of the active material that displayed a cauliflower-type morphology, having a large specific surface area (240 m2 g-1) and providing higher electrochemical active sites as well. Consequently, post-stability characterization studies (such as PXRD, FESEM, TEM, and XPS) provide more information for understanding the post-structural and morphological results of the high-entropy amorphous metal oxide.

Tetra germanium nonaselenide enwrapped with reduced graphene oxide and functionalized carbon nanotubes (Ge4Se9/RGO/FCNTs) hybrids for improved energy storage performances.

The development of multifunctional layered semiconductor materials and their carbonaceous hybrids as acceptable positive electrode materials for supercapacitor application is of key interest. Ternary germanium selenide (Ge4Se9) with reduced graphene oxide (RGO) and functionalized carbon nanotube (FCNT) hybrids were successfully synthesized by following a one-step hydrothermal approach, and their electrochemical energy storage performance toward supercapacitor (SC) applications was investigated. It was observed that the specific capacitance of Ge4Se9/RGO/FCNTs was 440 F g-1 at 1 A g-1 in an acidic (1 M H2SO4) medium. Further, the material showed 83% retention of its own initial value of capacitance with 98% coulombic efficiency after 5000 galvanostatic charge-discharge cycles. Considering the two-dimensional (2D) layered structures of MXenes with their greater stability, exceptional hydrophilicity, and pseudocapacitive behavior in aqueous electrolytes makes them an alternative for the fabrication of asymmetric SC devices. The above findings about MXenes suggest the design of an asymmetric device using MXene as the negative electrode material and as-prepared Ge4Se9/RGO/FCNTs as the positive electrode material in a similar electrolyte media. The fabricated Ge4Se9/RGO/FCNTs//MXenes displayed a higher specific capacitance of 102 F g-1 at 1 A g-1, with an acceptable energy density (E.D.) of 32 W h kg-1 and a power density (P.D.) of 1071 W kg-1. Furthermore, over long-term repeated 5000 GCD cycles the fabricated device retained 92% of its initial capacitance and good reversibility (96% coulombic efficiency), making the Ge4Se9/RGO/FCNT//MXenes assembly a preferable electrode material for enhancing asymmetric SC performance.

Metal-organic framework (MOF)-derived plate-shaped CoS1.097 nanoparticles for an improved hydrogen evolution reaction.

Metal-organic framework (MOF)-derived transition metal sulfides are viewed as reliable, cost-effective, and alternative hydrogen evolution reaction (HER)-efficient electrocatalysts. They have been used to replace platinum (and their alloys) for production of renewable energy carriers such as hydrogen. Progress towards development of non-precious transition-metal sulfides through different synthetic routes to obtain unique morphological nanostructures with enhanced HER activity is challenging. We introduced a transition-metal sulfide, cobalt sulfide (CoS1.097), derived from a cobalt MOF [Co-BPY-DDE] by following facile, one-step solvothermal sulfurization. By varying the sulfurization temperature (from 140 °C to 180 °C) during the solvothermal method, three cobalt-sulfide products were obtained: CoS1.097-140, CoS1.097-160, and CoS1.097-180, respectively. Temperature variation had a vital role in optimizing the HER activity of the electrocatalyst. Besides, notable plate-shaped cobalt sulfide nanoparticles (CoS1.097-160) required overpotential of 163 mV to deliver a current density of 10 mA cm-2 with a low Tafel slope of 53 mV dec-1, thereby demonstrating faster reaction kinetics during the evolution of molecular hydrogen. Furthermore, 25 h of long-term stability of the electrocatalyst reflected its practical applicability in acidic media. CoS1.097-160 had uniform plate-shaped morphology and large electrochemical active surface area, which contributed to enhanced electrochemical performance through water electrolysis.

NiSe2 Nanoparticles Encapsulated in N-Doped Carbon Matrix Derived from a One-Dimensional Ni-MOF: An Efficient and Sustained Electrocatalyst for Hydrogen Evolution Reaction.

The spherical-type NiSe2 nanoparticles encapsulated in a N-doped carbon (NC) matrix (NiSe2-T@NC, temperature (T) = 400-800 °C) are derived from a 1D Ni-MOF precursor of the formula [Ni(BPY)(DDE)] [(BPY = 2,2'-bipyridyl), (DDE = 4,4'-dicarboxy diphenyl ether)] via a facile solvothermal technique followed by annealing at different temperatures and selenylation strategies. The combined effect of a NC matrix and the Ni nanoparticles has been optimized during varied annealing processes with subsequent selenylation, leading to the formation of the series NiSe2-400@NC, NiSe2-500@NC, NiSe2-600@NC, NiSe2-700@NC, and NiSe2-800@NC, respectively. The variation of annealing temperature plays a vital role in optimizing the catalytic behavior of the NiSe2-T@NCs. Among different high-temperature annealed products, NiSe2-600@NC shows superior electrocatalytic performance because of the unique spherical-type morphology and higher specific surface area (57.95 m2 g-1) that provides a large number of electrochemical active sites. The synthesized material exhibits a lower overpotential of 196 mV to deliver 10 mA cm-2 current density, a small Tafel slope of 45 mV dec-1 for better surface kinetics, and outstanding durability in an acidic solution, respectively. Consequently, the post stability study of the used electrocatalyst gives insight into surface phase analysis. Therefore, we presume that the synthesized 1D MOF precursor derived NiSe2 nanoparticles encapsulated in a NC matrix has excellent potential to replace the noble-metal-based electrocatalyst for enhanced hydrogen evolution through simple 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.

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.

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.