Mengdi Zhang

A superhydrophilic “nanoglue” for stabilizing metal hydroxides onto carbon materials for high-energy and ultralong-life asymmetric supercapacitors

Coupling electroactive species with carbon supports to fabricate hybrid electrodes holds promise for high-performance supercapacitors. Nevertheless, the poor compatibility and weak bonding between carbon substrates and electroactive species remain a bottleneck to be tackled. Herein, we present a superhydrophilic “nanoglue” strategy for stabilizing NiCo-layered double hydroxide (NiCo-LDH) nanosheets on inert carbon cloth (CC) by employing a nitrogen-doped (N-doped) carbon layer as the structure/interface coupling bridge to make hybrids (denoted as CC–NC-LDH) for supercapacitors. Such a “nanoglue” on a CC substrate results in the formation of a superhydrophilic surface/interface, which is favorable for the robust and uniform growth of NiCo-LDH on the CC, and helps effectively tune the electronic structural states and results in a strong coupling interaction between the CC and NiCo-LDH nanosheets. Benefiting from these integrated merits, asymmetric supercapacitors fabricated with the CC–NC-LDH hybrids as the positive electrode and typical activated carbon as the negative electrode deliver a high energy density of 69.7 W h kg−1 at a power density of 0.8 kW kg−1, with an ultra-low average capacitance fade rate of ∼0.00065% per cycle within 20 000 cycles at a current density of 10 A g−1. This superhydrophilic “nanoglue” strategy can also be extended to assemble other kinds of active species on different inert substrates, and holds the potential for creating efficient and robust electrode materials for energy-related devices.

Freeze-drying for sustainable synthesis of nitrogen doped porous carbon cryogel with enhanced supercapacitor and lithium ion storage performance

A chitosan (CS) based nitrogen doped carbon cryogel with a high specific surface area (SSA) has been directly synthesized via a combined process of freeze-drying and high-temperature carbonization without adding any activation agents. The as-made carbon cryogel demonstrates an SSA up to 1025 m(2) g(-1) and a high nitrogen content of 5.98 wt%, while its counterpart derived from CS powder only shows an SSA of 26 m(2) g(-1). Freeze-drying is a determining factor for the formation of carbon cryogel with a high SSA, where the CS powder with a size of ca. 200 μm is transformed into the sheet-shaped cryogel with a thickness of 5-8 μm. The as-made carbon cryogel keeps the sheet-shaped structure and the abundant pores are formed in situ and decorated inside the sheets during carbonization. The carbon cryogel shows significantly enhanced performance as supercapacitor and lithium ion battery electrodes in terms of capacity and rate capability due to its quasi two-dimensional (2D) structure with reduced thickness. The proposed method may provide a simple approach to configure 2D biomass-derived advanced carbon materials for energy storage devices.

Ultrahigh Rate and Long‐Life Sodium‐Ion Batteries Enabled by Engineered Surface and Near‐Surface Reactions

To achieve the high-power sodium-ion batteries, the solid-state ion diffusion in the electrode materials is a highly concerned issue and needs to be solved. In this study, a simple and effective strategy is reported to weaken and degrade this process by engineering the intensified surface and near-surface reactions, which is realized by making use of a sandwich-type nanoarchitecture composed of graphene as electron channels and few-layered MoS2 with expanded interlayer spacing. The unique 2D sheet-shaped hierarchical structure is capable of shortening the ion diffusion length, while the few-layered MoS2 with expanded interlayer spacing has more accessible surface area and the decreased ion diffusion resistance, evidenced by the smaller energy barriers revealed by the density functional theory calculations. Benefiting from the shortened ion diffusion distance and enhanced electron transfer capability, a high ratio of surface or near-surface reactions is dominated at a high discharge/charge rate. As such, the composites exhibit the high capacities of 152 and 93 mA h g-1 at 30 and 50 A g-1 , respectively. Moreover, a high reversible capacity of 684 mA h g-1 and an excellent cycling stability up to 4500 cycles can be delivered. The outstanding performance is attributed to the engineered structure with increased contribution of surface or near-surface reactions.

High‐Stacking‐Density, Superior‐Roughness LDH Bridged with Vertically Aligned Graphene for High‐Performance Asymmetric Supercapacitors

The high-performance electrode materials with tuned surface and interface structure and functionalities are highly demanded for advanced supercapacitors. A novel strategy is presented to conFigure high-stacking-density, superior-roughness nickel manganese layered double hydroxide (LDH) bridged by vertically aligned graphene (VG) with nickel foam (NF) as the conductive collector, yielding the LDH-NF@VG hybrids for asymmetric supercapacitors. The VG nanosheets provide numerous electron transfer channels for quick redox reactions, and well-developed open structure for fast mass transport. Moreover, the high-stacking-density LDH grown and assembled on VG nanosheets result in a superior hydrophilicity derived from the tuned nano/microstructures, especially microroughness. Such a high stacking density with abundant active sites and superior wettability can be easily accessed by aqueous electrolytes. Benefitting from the above features, the LDH-NF@VG can deliver a high capacitance of 2920 F g-1 at a current density of 2 A g-1 , and the asymmetric supercapacitor with the LDH-NF@VG as positive electrode and activated carbon as negative electrode can deliver a high energy density of 56.8 Wh kg-1 at a power density of 260 W kg-1 , with a high specific capacitance retention rate of 87% even after 10 000 cycles.

Tailor-made graphene aerogels with inbuilt baffle plates by charge-induced template-directed assembly for high-performance Li–S batteries

Graphene as a host material has attracted intense interest to accommodate the sulfur for lithium–sulfur (Li–S) batteries. Nevertheless, there is still a major challenge on how to modulate the nanostructure of graphene architectures to further enhance the electrochemical performance. Herein, self-closure graphene aerogels with inbuilt baffle plates (SGA) were prepared by a combined strategy involving electrostatic assembly, hydrothermal fixing, polydopamine (PDA) coating, and annealing. The electrostatic assembly between graphene oxide (GO) and polystyrene sphere@polydopamine (PS@PDA) is the key factor to form the self-closure aerogels and the graphene sheets wrapped onto the PS@PDAs are responsible for the formation of the baffle plates. When employed as the host material for Li–S batteries, the as-made SGA can contribute to promotion of the transport of electrons, increasing the sulfur loading, confining the dissolution and diffusion of lithium polysulfides, and accommodating the volume expansion. As a result, the as-made SGA–sulfur composite can deliver an outstanding cycling stability of 509 mA h g−1 after 400 cycles at 1C. The present work will provide a simple and effective approach to tuning the assembly of graphene and further configuring the tailor-made host materials for high-performance Li–S batteries.

An ionic liquid template approach to graphene–carbon xerogel composites for supercapacitors with enhanced performance

Graphene–carbon xerogel composites with tailored pore structure and morphology are synthesized by a facile yet effective method with ionic liquids as templates, showing improved supercapacitor performance.

Nitrogen-doped tubular/porous carbon channels implanted on graphene frameworks for multiple confinement of sulfur and polysulfides

Graphene has excellent potential as a sulfur host in a lithium–sulfur (Li–S) battery owing to its outstanding electrical conductivity and robust mechanical properties. However, graphene itself cannot effectively confine sulfur and suppress polysulfide diffusion, leading to severely fast capacity decay. Herein, nitrogen-doped tubular/porous carbon channels were implanted on graphene sheets (NTPC–G) via a double-template method, with graphene sheets as the shape-directed agents and NiCo–carbonate hydroxide nanowires as the guides of tubular channels. The resultant one-dimensional hollow tubular carbon and two-dimensional graphene nanosheets were wrapped by nitrogen-doped porous carbon layers to construct the unique three-dimensional sandwich-type architectures. The adopted graphene sheets functioned as conductive networks and robust frameworks; moreover, the nitrogen-doped tubular/porous carbon channels comprising hollow tubular carbon and porous carbon coating layers implanted on graphene frameworks served as the sulfur-confined space and polysulfide reservoirs. On integrating these fascinating benefits into one electrode material, sulfur and NTPC–G composites (S@NTPC–G) delivered high rate capability (563 mA h g−1 at 6 C) and good cycle stability up to 600 cycles. This rational construction of tubular/porous carbon channels on nanosheet materials with comprehensive advantages could be promising and applicable in rechargeable Li–S batteries and other advanced energy storage devices.

Template-free synthesis of interconnected carbon nanosheets via cross-linking coupled with annealing for high-efficiency triiodide reduction

Counter electrodes (CEs) play critical roles in the reduction and regeneration of triiodide/iodide redox couple in dye-sensitized solar cells (DSSCs). Compared to commercial Pt, cost-efficient CEs with excellent electrocatalytic activity and superior electrochemical stability are highly desired. Herein, we report a facile, template- and active agents-free fabrication strategy for the synthesis of carbon nanosheets (CNSs) via annealing of small molecular precursors. This process was achieved by a combined strategy, including a low-temperature solid-phase cross-linking reaction and a subsequent high temperature annealing. When employed as metal-free CEs for DSSCs, the as-obtained CNSs demonstrated an annealing temperature-dependent electrochemical behavior. Owing to the superior electrical conductivity and electrocatalytic activity, the CNSs obtained by annealing at 1200 °C exhibit the best electrochemical performance with a power conversion efficiency of 8.71%, which is superior to that of Pt CE (7.24%), thus being attractive alternatives to precious metal Pt CEs. This study presents a simple and effective strategy to configure nanostructured carbonaceous materials for high-performance energy storage and conversion.