Triggering Redox Active Sites Through Electronic Structure Modulation in rGO Encapsulated Mixed Transition Metal Oxides Hybrid for Alkaline Hydrogen Evolution

Designing and developing noble-metal-free catalysts are of current interest in clean hydrogen generation via water splitting. As carbonaceous species are ideal choices as templates for various electrocatalysis, an improved synthetic route and an in-depth understanding of their electrochemical performance are essential. Herein, we have investigated the catalytic performance of rGO-encapsulated Mn and V mixed oxide hybrid structures (MVG) on a NiFeP matrix, focusing on their potential for catalyzing hydrogen evolution in an alkaline environment. The hierarchical MVG hollow microspheres hybrids are synthesized via a simple one-step in situ solvothermal method and MVG/NiFeP coatings are developed by facile electroless plating technique. As evidenced from the X-ray photoelectron spectroscopy, the multiple redox active sites in the 3d-band of Mn and V in MVG hybrid structural coatings serve as electron pumps, and rGO facilitates electronic conductions during catalytic reactions. The modulated electronic structure and strong synergistic effects between NiFeP and MVG facilitate rapid electron transfer kinetics, and the hybrids demonstrate superior HER performance. Consequently, the structural hybrid coatings possess an enhanced electronic conducting path (lower RCT = 545.3 Ω) and large ECSA values with a lower overpotential of 85 mV at 10 mA cm-2 and a reduced Tafel slope of 64.1 mV dec-1 with Volmer-Heyrovsky mechanism in alkaline media.

One-Pot Synthesis of Ruthenium-Based Nanocatalyst Using Reduced Graphene Oxide as Matrix for Electrochemical Synthesis of Ammonia

Conventional ammonia production consumes significant energy and causes enormous carbon dioxide (CO₂) emissions globally. To lower energy consumption and mitigate CO₂ emissions, a facile, environmentally friendly, and cost-effective one-pot method for the synthesis of a ruthenium-based nitrogen reduction nanocatalyst has been developed using reduced graphene oxide (rGO) as a matrix. The nanocatalyst synthesis was based on a single-step simultaneous reduction of RuCl₃ into ruthenium-based nanoparticles (Ru-based NPs) and graphene oxide (GO) into rGO using glucose as the reducing agent and stabilizer. The obtained ruthenium-based nanocatalyst with rGO as a matrix (Ru_nano-based/rGO) has shown much higher catalytic activity at lower temperatures and pressures for ammonia synthesis than conventional iron catalysts. The rGO worked as a promising promoter for the electrochemical synthesis of ammonia due to its excellent electrical and thermal conductivity. The developed Ru_nano-based/rGO nanocatalyst was characterized using transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), ultraviolet-visible (UV-vis) absorption spectroscopy, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), dynamic light scattering (DLS), inductively coupled plasma mass spectrometry (ICP-MS), and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the size of the Ru-based NPs on the surface of rGO was 1.9 ± 0.2 nm and the ruthenium content was 25.03 wt %. Bulk electrolysis measurements were conducted on thin-layer electrodes at various cathodic potentials in a N₂-saturated 0.1 M H₂SO₄ electrolyte at room temperature. From the chronoamperometric measurements, the maximum faradic efficiency (F.E.) of 2.1% for ammonia production on the nanostructured Ru_nano-based/rGO electrocatalyst was achieved at a potential of -0.20 V vs reversible hydrogen electrode (RHE). This electrocatalyst has attained a superior ammonia production rate of 9.14 μg·h^-1·mg_cat.^-1. The results demonstrate the feasibility of reducing N₂ into ammonia under ambient conditions and warrant further exploration of the nanostructured Ru_nano-based/rGO for electrochemical ammonia synthesis.

Pseudocapacitance multiporous vanadyl phosphate/graphene thin film electrode for high performance electrochemical capacitors

Supercapacitors are being considered as promising electricity storage devices with green sustainable energy conversion. To efficiently develop and optimize pseudocapacitive material of vanadyl phosphate, herein, multiporous vanadyl phosphate/graphene (denoted as MP-VOPO₄@rGO) is fabricated for the first time with phytic acid as a phosphorus source by extremely simple sol-gel and drop coating methods, and used as the free binder thin film electrode of supercapacitors. The smart combination of honeycomb-like architecture and graphene incorporation results in more active sites and low internal resistance, significantly improving energy storage performance. The effect of introducting polystyrene (denoted as PS) template and rGO on the performance of the nanocomposite is systematically analyzed by comparing the performance of the corresponding thin film electrodes. The MP-VOPO₄@rGO thin film electrode delivers superior pseudocapacitive performance of 672 F g^-1 at 1 A g^-1 as well as a remarkable rate capability of 552 F g^-1 at 5 A g^-1, and it presents a remarkable longterm cycling stability, with a capacitance retention of 83.5% after 5000 cycles. Very interestingly, the results of surface capacitance contribution dominance clearly demonstrates its rapid capacitive response. In addition, based on MP-VOPO₄@rGO thin film as positive and negative electrodes, the corresponding assembled symmetric supercapacitors exihibits outstanding energy density of 26.3 Wh kg^-1 at power density of 249.9 W kg^-1. This investigation can not only provide a versatile strategy to design other thin film electrode materials but also open up a new insight into the development of polyanion phosphate composites for next-generation high performance energy storage systems.

Hydrated vanadium pentoxide/reduced graphene oxide composite cathode material for high-rate lithium ion batteries

As well-known, hydrated vanadium pentoxide (V₂O₅·nH₂O) has a larger layer spacing than orthogonal V₂O₅, which could offer more active sites to accommodate lithium ions, ensuring a high specific capacity. However, the exploration of V₂O₅·nH₂O cathode is limited by its inherently low conductivity and slow electrochemical kinetics, leading to a significant decrease in capability. Herein, we prepared V₂O₅·nH₂O/reduced graphene oxide (rGO) composite with low rGO content (8 wt%) via a simple yet effective dual electrostatic assembly strategy. When used as the cathode material for lithium-ion batteries (LIBs), V₂O₅·nH₂O/rGO manifests a high reversible capacity of 268 mAh g^-1 at 100 mA g^-1 and especially an excellent rate capability (196 mAh g^-1 at 1000 mA g^-1 and 129 mA h g^-1 at 2000 mA g^-1), surpassing those of the V₂O₅/carbon composites reported in the literatures. Notably, the remarkable performance should be referable to the synergetic effects between one-dimensional V₂O₅·nH₂O nanobelts and two-dimensional rGO nanosheets, which provide a short transport pathway and enhanced electrical conductivity. This strategy opens a new opportunity for designing high-performance cathode material with excellent rate performance for advanced LIBs.

Hierarchical NiMoO4@Co3V2O8 hybrid nanorod/nanosphere clusters as advanced electrodes for high-performance electrochemical energy storage

Herein, a synergistic strategy to construct hierarchical NiMoO₄@Co₃V₂O₈ (denoted as NMO@CVO) hybrid nanorod/nanosphere clusters is proposed for the first time, where Co₃V₂O₈ nanospheres (denoted as CVO) have been uniformly immobilized on the surface of the NiMoO₄ nanorods (denoted as NMO) via a facile two-step hydrothermal method. Due to the surface recombination effect between NMO and CVO, the as-prepared NMO@CVO effectively avoids the aggregation of CVO nanosphere clusters. The unique hybrid architecture can make the most of the large interfacial area and abundant active sites for storing charge, which is greatly beneficial for the rapid diffusion of electrolyte ions and fast electron transport. The optimized NMO@CVO-8 composite nanostructure displays battery-like behavior with a maximum specific capacity of 357 C g^-1, excellent rate capability (77.8% retention with the current density increasing by 10 times) and remarkable cycling stability. In addition, an aqueous asymmetric energy storage device is assembled based on the NMO@CVO-8 hybrid nanorod/nanosphere clusters and activated carbon. The device shows an ultrahigh energy density of 48.5 W h kg^-1 at a power density of 839.1 W kg^-1, good rate capability (20.9 W h kg^-1 even at 7833.7 W kg^-1) and excellent cycling stability (83.5% capacitance retention after 5000 cycles). More notably, two charged devices in series can light up a red light-emitting diode (LED) for 20 min, demonstrating its potential in future energy storage applications. This work indicates that the hierarchical NiMoO₄@Co₃V₂O₈-8 hybrid nanorod/nanosphere clusters are promising energy storage materials for future practical applications and also provides a rational strategy for fabricating novel nanostructured materials for high-performance energy storage.