Yunjun Ruan

NiCo2S4 porous nanotubes synthesis via sacrificial templates: high-performance electrode materials of supercapacitors

NiCo2S4 porous nanotubes are synthesised by a sacrificial template method based on the Kirkendall effect. The as-prepared NiCo2S4 nanotube electrode shows a specific capacitance of 1093 F g−1 at a current density of 0.2 A g−1 (933 F g−1 at 1 A g−1).

Hierarchical Configuration of NiCo2S4 Nanotube@Ni–Mn Layered Double Hydroxide Arrays/Three-Dimensional Graphene Sponge as Electrode Materials for High-Capacitance Supercapacitors

Three dimensional (3D) hierarchical network configurations are composed of NiCo2S4 nanotube @Ni-Mn layered double hydroxide (LDH) arrays in situ grown on graphene sponge. The 3D graphene sponge with robust hierarchical porosity suitable for as a basal growth has been obtained from a colloidal dispersion of graphene oxide using a simple directional freeze-drying technique. The high conductive NiCo2S4 nanotube arrays grown on 3D graphene shows excellent pseudocapacity and good conductive support for high-performance Ni-Mn LDH. The 3D NiCo2S4@Ni-Mn LDH/GS shows a high specific capacitance (Csp) 1740 mF cm(-2) at 1 mA cm(-2), even at 10 mA cm(-2), 1267.9 mF cm(-2) maintained. This high-performance composite electrode proposes a new and feasible general pathway as 3D electrode configuration for energy storage devices.

Hierarchical NiCo2S4@NiFe LDH Heterostructures Supported on Nickel Foam for Enhanced Overall-Water-Splitting Activity

Low-cost and highly efficient bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) are intensively investigated for overall water splitting. Herein, we combined experimental research with first-principles calculations based on density functional theory (DFT) to engineer the NiCo2S4@NiFe LDH heterostructure interface for enhancing overall water-splitting activity. The DFT calculations exhibit strong interaction and charge transfer between NiCo2S4 and NiFe LDH, which change the interfacial electronic structure and surface reactivity. The calculated chemisorption free energy of hydroxide (ΔEOH) is reduced from 1.56 eV for pure NiFe LDH to 1.03 eV for the heterostructures, indicating a dramatic improvement in OER performance, while the chemisorption free energy of hydrogen (ΔEH) maintains almost invariable. By the use of the facile hydrothermal method, NiCo2S4 nanotubes, NiFe LDH nanosheets, and NiCo2S4@NiFe LDH heterostructures are prepared on nickel foam, of which the corresponding experimental OER overpotentials are 306, 260, and 201 mV at 60 mA cm-2, respectively. These results are good agreement with the theoretical predictions. Meanwhile, the HER performance has little improvement, with an overpotential of about 200 mV at 10 mA cm-2. Due to the dramatic improvement in OER performance, there was an enhancement in the overall water-splitting activity of the NiCo2S4@NiFe LDH heterostructures, with a low voltage of 1.6 V.

Different charge-storage mechanisms in disulfide vanadium and vanadium carbide monolayer

Two-dimensional (2D) transition-metal (TM) compound nanomaterials, due to their high-surface-area and large potential charge capability of TM atoms, have been widely investigated as electrochemical capacitors. However, the understanding of charge-storage mechanisms of 2D transition-metal compounds as electrode materials is still limited. In this study, using density functional theory computations, we systematically investigate the electrochemical properties of monolayer VS2 and V2C. Their electronic structures show a significant electron storage capability of around 0.25 V, referenced to the standard hydrogen electrode, and indicate redox pseudocapacitance characteristics as cathodes. The different charge densities visually confirm that excess electrons tend to localize in the vanadium atoms nearby contact-adsorbed Li ions, corresponding to the redox of vanadium atoms. In contrast, only the electric double layer acts as a charge-storage mechanism in the V2C monolayer. However, the O saturation would induce redox pseudocapacitance in the V2C monolayer. Furthermore, the calculated metallic behavior and low Li ion diffusion barriers substantiate that V2C and VS2 monolayers would manifest low resistance in the charging process. Our findings provide insights for the different charge-storage mechanism of VS2 and V2C monolayers.

Nanostructured Ni compounds as electrode materials towards high-performance electrochemical capacitors

Integration of Ni compound-based nanomaterials into energy storage devices, especially electrochemical capacitors, offers opportunities to timely meet the ever increasing demands of fast charging/discharging processes, longer cycle life, and higher power density and energy density. Besides that, Ni compound-based nanomaterials are also found to bring such merits as high electrochemical activity, low price, and being environmentally friendly. Therefore, it is crucial to design nanostructured Ni compound-based nanomaterials towards high-performance electrochemical capacitors and understand the role of morphology in enhancing the electrochemical performance at the atomic or molecular scale. In this review, we focus on the rational design and synthesis of different structured Ni compound-based nanomaterials, such as nickel oxides, ternary Ni-based metal oxides, nickel sulfides, ternary Ni-based metal sulfides, Ni-based metal organic frameworks, and their composites. More importantly, their electrochemical performances are also discussed by virtue of the effects of morphology, structure, and composition of Ni compound-based materials. Finally, the main challenges as well as opportunities that researchers are facing now in the field of Ni compound-based nanomaterials for electrochemical energy storage are summarized. We hope that this review will shed light on the development of Ni compound-based nanostructured electrodes and exploitation of their energy storage mechanisms towards high-performance electrochemical capacitor applications.

Mutually beneficial Co3O4@MoS2 heterostructures as a highly efficient bifunctional catalyst for electrochemical overall water splitting

Designing low-cost and highly efficient bifunctional electrocatalysts for compatible integration with the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) for overall water splitting is critical but challenging. Herein, mutually beneficial Co3O4@MoS2 heterostructures were adopted to efficiently balance both HER and OER performance by improving the sluggish kinetics. These heterostructures synergistically favoured the reduction of the energy barrier of the initial water dissociation step and optimization of the subsequent H adsorption/desorption for MoS2 in alkaline HER. Moreover, the adsorption of oxygen intermediates was enhanced for Co3O4 in the OER process. As a result, the Co3O4@MoS2 heterostructures showed excellent overall water splitting performance with a low overpotential and Tafel slope.

A Universal Method to Engineer Metal Oxide–Metal–Carbon Interface for Highly Efficient Oxygen Reduction

Oxygen is the most abundant element in the Earths crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes and energy converting/storage systems. Developing techniques toward high-efficiency ORR remains highly desired and a challenge. Here, we report a N-doped carbon (NC) encapsulated CeO2/Co interfacial hollow structure (CeO2-Co-NC) via a generalized strategy for largely increased oxygen species adsorption and improved ORR activities. First, the metallic Co nanoparticles not only provide high conductivity but also serve as electron donors to largely create oxygen vacancies in CeO2. Second, the outer carbon layer can effectively protect cobalt from oxidation and dissociation in alkaline media and as well imparts its higher ORR activity. In the meanwhile, the electronic interactions between CeO2 and Co in the CeO2/Co interface are unveiled theoretically by density functional theory calculations to justify the increased oxygen absorption for ORR activity improvement. The reported CeO2-Co-NC hollow nanospheres not only exhibit decent ORR performance with a high onset potential (922 mV vs RHE), half-wave potential (797 mV vs RHE), and small Tafel slope (60 mV dec-1) comparable to those of the state-of-the-art Pt/C catalysts but also possess long-term stability with a negative shift of only 7 mV of the half-wave potential after 2000 cycles and strong tolerance against methanol. This work represents a solid step toward high-efficient oxygen reduction.

Construction of MoO2 Quantum Dot–Graphene and MoS2 Nanoparticle–Graphene Nanoarchitectures toward Ultrahigh Lithium Storage Capability

Herein, MoO2 quantum dots (QDs; <5 nm) are synthesized through a one-step solvothermal process. MoO2 QD-bonded graphene sheets (MoO2-QDs@RGO) are facilely produced and can be further converted through sulfidation into MoS2 nanoparticle-bonded graphene sheets (MoS2-NPs@RGO). The novel MoO2-QDs@RGO electrodes demonstrate exceptionally attractive lithium storage capability (e.g., 1257 mA h g-1 at 100 mA g-1, being close to the highest values ever reported for a MoO2-based lithium ion battery electrode), rate capability, and cycle stability. Moreover, the MoS2-NPs@RGO delivered a superior capacity (1497 mA h g-1 at 100 mA g-1) with outstanding rate retention and cycling stability. The superior lithium storage capabilities are ascribed to the synergetic effects of the high-surface-area graphene sheets, the well-dispersed MoS2 nanoparticles, and their strong bonding with each other, which effectively prevents aggregation of MoS2 while the composite architecture allows fast transport of electrons and ions.

Water-Activated VOPO4 for Magnesium Ion Batteries

Rechargeable Mg batteries, using high capacity and dendrite-free Mg metal anodes, are promising energy storage devices for large scale smart grid due to low cost and high safety. However, the performance of Mg batteries is still plagued by the slow reaction kinetics of their cathode materials. Recent discoveries demonstrate that water in cathode can significantly enhance the Mg-ion diffusion in cathode by an unknown mechanism. Here, we propose the water-activated layered-structure VOPO4 as a novel cathode material and examine the impact of water in electrode or organic electrolyte on the thermodynamics and kinetics of Mg-ion intercalation/deintercalation in cathodes. Electrochemical measurements verify that water in both VOPO4 lattice and organic electrolyte can largely activate VOPO4 cathode. Thermodynamic analysis demonstrates that the water in the electrolyte will equilibrate with the structural water in VOPO4 lattice, and the water activity in the electrolyte alerts the mechanism and kinetics for electrochemical Mg-ion intercalation in VOPO4. Theoretical calculations and experimental results demonstrate that water reduces both the solid-state diffusion barrier in the VOPO4 electrode and the desolvation penalty at the interface. To achieve fast reaction kinetics, the water activity in the electrolyte should be larger than 10-2. The proposed activation mechanism provides guidance for screening and designing novel chemistry for high performance multivalent-ion batteries.