Heat-Up Colloidal Synthesis of Shape-Controlled Cu-Se-S Nanostructures-Role of Precursor and Surfactant Reactivity and Performance in N2 Electroreduction

Copper selenide-sulfide nanostructures were synthesized using metal-organic chemical routes in the presence of Cu- and Se-precursors as well as S-containing compounds. Our goal was first to examine if the initial Cu/Se 1:1 molar proportion in the starting reagents would always lead to equiatomic composition in the final product, depending on other synthesis parameters which affect the reagents reactivity. Such reaction conditions were the types of precursors, surfactants and other reagents, as well as the synthesis temperature. The use of 'hot-injection' processes was avoided, focusing on 'non-injection' ones; that is, only heat-up protocols were employed, which have the advantage of simple operation and scalability. All reagents were mixed at room temperature followed by further heating to a selected high temperature. It was found that for samples with particles of bigger size and anisotropic shape the CuSe composition was favored, whereas particles with smaller size and spherical shape possessed a Cu2-xSe phase, especially when no sulfur was present. Apart from elemental Se, Al2Se3 was used as an efficient selenium source for the first time for the acquisition of copper selenide nanostructures. The use of dodecanethiol in the presence of trioctylphosphine and elemental Se promoted the incorporation of sulfur in the materials crystal lattice, leading to Cu-Se-S compositions. A variety of techniques were used to characterize the formed nanomaterials such as XRD, TEM, HRTEM, STEM-EDX, AFM and UV-Vis-NIR. Promising results, especially for thin anisotropic nanoplates for use as electrocatalysts in nitrogen reduction reaction (NRR), were obtained.

Molten Salt Synthesized MgNiO₂ Micro/Nano-Particles for High Energy Density Supercapacitor and Electrocatalyst for Hydrogen Evolution Reaction in Alkaline Medium

In recent years, solid solutions have shown promising results as functional materials for different applications. These materials have tunable physiochemical properties and electronic properties, and are being intensively studied for next generation electrochemical charge storage as well as noble metal free low cost electrocatalyts. In the present work, Magnesium Nickel Oxide (MgNiO₂) solid solution is prepared by molten salt synthesis. MgNiO₂ particles having octahedron shaped morphology with size of 550 nm with an agglomerative behavior was observed through morphological studies. Raman studies revealed presence of three two-phonon modes as well as two one-phonon modes, which confirm the phase purity of MgNiO₂ sample. MgNiO₂ particles behaved as a promising supercapacitor candidate by exhibiting a large specific capacitance of 76 F/g. It also revealed electrochemical stability over an expansive potential range under the presence of 0.5 mol L^-1Sodium Sulfate (Na₂SO₄) electrolyte, having a high energy density of nearly 51 Wh/kg with a power density of nearly 825 w/kg. Further, MgNiO₂ particle showed improved electrocatalytic potential towards Hydrogen Evolution Reaction (HER) in 1 mol L^-1 Potassium Hydroxide (KOH) alkaline medium, by demonstrating an overpotential of 0.636 V with a Tafel slope of 0.22205 v/dec. Based on these observed promising results, it can be conclusively inferred that MgNiO₂ solid solution is a potential candidate for environmental friendly high voltage supercapacitor and HER electrocatalyst applications.

Engineering Co3O4/MnO2 nanocomposite materials for oxygen reduction electrocatalysis

Stable and active electrocatalysts preparation for the oxygen reduction reaction (ORR) is essential for an energy storage and conversion materials (e.g. metal-air batteries). Herein, we prepared a highly-active MnO₂ and Co₃O₄/MnO₂ nanocomposite electrocatalysts using a facial co-precipitation approach. The electrocatalytic activity was examined in alkaline media with LSV and CV. Additionally, the physicochemical characteristics of the MnO₂ and Co₃O₄/MnO₂ composite materials were studied via SEM, XRD, BET, UV-Vis, TGA/DTA, ICP-OES and FTIR. Morphological studies indicated that a pure MnO₂ has a spherical flower-like architecture, whereas Co₃O₄/MnO₂ nanocomposites have an aggregated needle-like structure. Moreover, from the XRD investigation parameters such as the dislocation density, micro-strain, and crystallite size were analyzed. The calculated energy bandgaps for the MnO₂, Co₃O₄/MnO₂-1-5, and Co₃O₄/MnO₂-1-1 nanocomposites were 3.07, 2.6, and 2.3 eV, correspondingly. The FTIR spectroscopy was also employed to study the presence of M-O bonds (M = Mn, Co). The thermal gravimetric investigation showed that the Co₃O₄/MnO₂ nanocomposite materials exhibited improved thermal stability, confirming an enhanced catalytic activity of ORR for MnO₂/Co₃O₄-1-1 composite materials for ORR. These results confirm that the prepared Co₃O₄/MnO₂ composite materials are promising air electrode candidates for the energy storage and conversion technologies.

WS2-WC-WO3 nano-hollow spheres as an efficient and durable catalyst for hydrogen evolution reaction

Transition metal dichalcogenides (TMDs), transition metal carbides (TMCs), and transition metal oxides (TMOs) have been widely investigated for electrocatalytic applications owing to their abundant active sites, high stability, good conductivity, and various other fascinating properties. Therefore, the synthesis of composites of TMDs, TMCs, and TMOs is a new avenue for the preparation of efficient electrocatalysts. Herein, we propose a novel low-cost and facile method to prepare TMD-TMC-TMO nano-hollow spheres (WS2-WC-WO3 NH) as an efficient catalyst for the hydrogen evolution reaction (HER). The crystallinity, morphology, chemical bonding, and composition of the composite material were comprehensively investigated using X-ray diffraction, Raman spectroscopy, field emission scanning electron microscopy, and X-ray photoelectron spectroscopy. The results confirmed the successful synthesis of the WS2-WC-WO3 NH spheres. Interestingly, the presence of nitrogen significantly enhanced the electrical conductivity of the hybrid material, facilitating electron transfer during the catalytic process. As a result, the WS2-WC-WO3 NH hybrid exhibited better HER performance than the pure WS2 nanoflowers, which can be attributed to the synergistic effect of the W-S, W-C, and W-O bonding in the composite. Remarkably, the Tafel slope of the WS2-WC-WO3 NH spheres was 59 mV dec-1, which is significantly lower than that of the pure WS2 NFs (82 mV dec-1). The results also confirmed the unprecedented stability and superior electrocatalytic performance of the WS2-WC-WO3 NH spheres toward the HER, which opens new avenues for the preparation of low-cost and highly effective materials for energy conversion and storage applications.

Novel In Situ Synthesis of Freestanding Carbonized ZIF67/Polymer Nanofiber Electrodes for Supercapacitors via Electrospinning and Pyrolysis Techniques

Zeolitic imidazolate framework-67 (ZIF67) has been regarded as an effective energy storage material due to its high surface area and electroactive cobalt center. Carbonizing ZIF67 can enhance electrical conductivity by converting 2-methylimidazole (2-melm) to carbon with cobalt doping. In this work, a novel in situ electrospinning is proposed to fabricate carbonized ZIF67 on carbon fiber (C67@PAN-OC) as a freestanding supercapacitor electrode. Polyacrylonitrile solution containing a cobalt precursor is used for electrospinning, and produced fibers are immersed in 2-melm to form ZIF67. Individually grown carbonized ZIF67 on carbon fiber is obtained using the in situ electrospinning method, while the one-body mixed carbon electrode is formed using the ex situ electrospinning method. A highest specific capacitance (C_F) of 386.3 F/g at 20 mV/s is obtained for the in situ synthesized C67@PAN-OC electrode due to the largest electrochemical surface area and the smallest resistance, while the ex situ synthesized electrode only shows a C_F value of 27.7 F/g. A symmetric supercapacitor (SSC) assembled using the optimized C67@PAN-OC electrodes and gel electrolytes shows a maximum energy density of 9.64 kWh/kg at 0.55 kW/kg and a C_F retention of 59.5% after 1000 times charge/discharge process. A C_F retention of 75.6% after bending 100 times is also obtained for SSC.

Synergistic Electrocatalytic Hydrogen Evolution in Ni/NiS Nanoparticles Wrapped in Multi-Heteroatom-Doped Reduced Graphene Oxide Nanosheets

Hydrogen production is a key driver for sustainable and clean fuels used to generate electricity, which can be achieved through electrochemical splitting of water in alkaline solutions. However, the hydrogen evolution reaction (HER) is kinetically sluggish in alkaline media. Therefore, it has become imperative to develop inexpensive and highly efficient electrocatalysts that can replace the existing expensive and scarce noble-metal-based catalysts. Herein, we report on the rational design of nonprecious heterostructured electrocatalysts comprising a highly conductive face-centered cubic nickel metal, a nickel sulfide (NiS) phase, and a reduced graphene oxide (rGO) doped with phosphorous (P), sulfur (S), and nitrogen (N) in one ordered heteromaterial named Ni/NiS/P,N,S-rGO. The Ni/NiS/P,N,S-rGO electrode shows the best performance toward HER in 1.0 M KOH media among all materials tested with an overpotential of 155 mV at 10.0 mA cm^-2 and a Tafel slope of 135 mV dec^-1. The performance is comparable to the herein used Pt/C-20% benchmark catalyst examined under the same experimental conditions. The chronoamperometry and chronopotentiometry measurements have reflected the high durability of the Ni/NiS/P,N,S-rGO electrode for technological applications. At the same time, the current catalyst showed a high robustness and structure retention after long-term HER performance, which is reflected by SEM, XRD, and XPS measurements.