One-Step Synthesis of Monodispersed Mesoporous Carbon Nanospheres for High-Performance Flexible Quasi-Solid-State Micro-Supercapacitors.

Cost-effective synthesis of carbon nanospheres with a desirable mesoporous network for diversified energy storage applications remains a challenge. Herein, a direct templating strategy is developed to fabricate monodispersed N-doped mesoporous carbon nanospheres (NMCSs) with an average particle size of 100 nm, a pore diameter of 4 nm, and a specific area of 1093 m2 g-1 . Hexadecyl trimethyl ammonium bromide and tetraethyl orthosilicate not only play key roles in the evolution of mesopores but also guide the assembly of phenolic resins to generate carbon nanospheres. Benefiting from the high surface area and optimum mesopore structure, NMCSs deliver a large specific capacitance up to 433 F g-1 in 1 m H2 SO4 . The NMCS electrodes-based symmetric sandwich supercapacitor has an output voltage of 1.4 V in polyvinyl alcohol/H2 SO4 gel electrolyte and delivers an energy density of 10.9 Wh kg-1 at a power density of 14014.5 W kg-1 . Notably, NMCSs can be directly applied through the mask-assisted casting technique by a doctor blade to fabricate micro-supercapacitors. The micro-supercapacitors exhibit excellent mechanical flexibility, long-term stability, and reliable power output.

Unlocking Maximum Synergy: Screen-Printing Fabrication of Heterostructured Microsupercapacitor Stacks.

A cost-effective and scalable approach for the fabrication of heterostructured microsupercapacitors (MSCs) employing screen-printing followed by sequential electrochemical and microspray deposition techniques has been demonstrated. The microsupercapacitor electrode (MSC) that composed of stacked layers of mesoporous carbon, polyaniline (PANI), and MXene hold significant promise for wearable electronics. By adjusting the deposition and spray cycles, the MSC can be readily coated with PANI and MXene. The sequentially stacked two layers of MXene and PANI on the mesoporous carbon spheres (PMPM-MSC) yielded a specific capacitance of 1003 mF cm-2 at 0.5 mA cm-2, surpassing the performance of PANI/mesoporous carbon electrode by 1.6 times (771 mF cm-2). After 10,000 cycles of charge and discharge, PMPM-MSCs retained more than 86% of their initial capacitance. In-situ Raman spectroscopy confirmed the synergistic effects between MXene and PANI within the heterostructured stacked PMPM-MSC electrodes, including enhanced electronic conductivity and improved electrolyte ion dissociation, which aligned with the electrochemical measurement results, such as fast charge/discharge rates and reduced internal and mass transport resistance. This study demonstrates the potential of screen-printed heterostructured MSC stacks with maximum electrochemical synergy for portable and wearable energy storage devices.

Atomic-level tungsten doping triggered low overpotential for electrocatalytic water splitting.

The design of electrocatalysts with lower overpotential is of great significance for water splitting. Herein, cobalt hydroxide carbonate (CCH) has been used as a model to demonstrate the boost of its oxygen evolution reaction (OER) activity by atomic doping of W6+ (W-CCH). The 5 at % W doping reduced the OER overpotential of CCH by 95.3 mV at 15 mA cm-2, and increased the current density by 2.8 times at 1.65 V. 5%W-PCCH || 5%W-CCH-based electrolyzer only required a potential of 1.65 V to afford 10 mA cm-2 for full water splitting. The W6+ in CCH are active sites for O2- adsorption and induced an incesaed electron density near the Fermi level, which facilitates the charge transfer during electrocatalysis. The W6+ doping has been validated as an efficient booster for transition-metal carbonate hydroxides-based electrocatalysts, which has half or more than half-filled d-bands.

A TiS2/Celgard separator as an efficient polysulfide shuttling inhibitor for high-performance lithium-sulfur batteries.

The rapid capacity loss caused by the shuttling effect of polysulfides is one of the great challenges of Li-S batteries. In this work, we adopted a simple solid-phase sintering method to synthesize titanium disulfide (TiS2) and further demonstrated it as a superior modifier of separators for Li-S batteries. Two commonly adopted modification processes of separators, including vacuum filtration (VF) and slurry casting (SC) have been used to prepare TiS2/Celgard separators. TiS2-VF/Celgard can better restrain the polysulfide shuttling effect compared with TiS2-SC/Celgard. A TiS2-VF/Celgard-based Li-S battery has a reversible capacity of 771.6 mA h g-1, with a capacity retention of 645.6 mA h g-1 after 500 cycles at 2.0 C, corresponding to a capacity fading rate of ∼0.033% per cycle. This study has shown the potential of TiS2 as a multifunctional modifier of separators for high performance and long cycle life Li-S batteries.

Catalysts confined inside CNTs derived from 2D metal-organic frameworks for electrolysis.

Two-dimensional metal-organic framework (MOF) nanosheets have attracted considerable research interest as electrocatalysts, and thermal annealing is important to boost their conductivity. The effect of annealing atmosphere on the electrochemical performance of 2D MOFs and their catalytic center structure have been investigated. The Co-MOF/H2 synthesized by annealing of 2D MOF under a H2 atmosphere has shown a significantly enhanced catalytic activity compared with those annealed under an Ar atmosphere (Co-MOF/Ar). The Co-MOF/H2 has 2-3 graphitic layers of graphitic carbon coating and presents a large amount of high valent Co2+. H2 annealing leads to a fast reduction of Co-MOF to Co/CoOx nanoparticles and catalyzes the growth of CNTs with MOF feed as carbon source. The Co-MOF/H2 shows a high electrocatalytic activity which requires an overpotential of 312 mV to reach a current density of 10 mA cm-2. A Co-MOF/H2-based water electrolyzer requires a potential of 1.619 V to reach a current density of 10 mA cm-2 for overall water splitting in 1.0 M KOH. After 25 h of continuous operation for water electrolysis, the Co-MOF/H2-based cell has shown a negligible increase in the overpotential, indicating its superior durability compared to the 2D Co-MOF.

Co2P@N,P-Codoped Carbon Nanofiber as a Free-Standing Air Electrode for Zn-Air Batteries: Synergy Effects of CoNx Satellite Shells.

Here, a free-standing electrode composed of cobalt phosphides (Co2P) supported by cobalt nitride moieties (CoNx) and an N,P-codoped porous carbon nanofiber (CNF) in one-step electrospinning of environmentally friendly benign phosphorous precursors is reported. Physiochemical characterization revealed the symbiotic relationship between a Co2P crystal and surrounding nanometer-sized CoNx moieties embedded in an N,P-codoped porous carbon matrix. Co2P@CNF shows high oxygen reduction reaction and oxygen evolution reaction performance owing to the synergistic effect of Co2P nanocrystals and the neighboring CoNx moieties, which have the optimum binding strength of reactants and facilitate the mass transfer. The free-standing Co2P@CNF air-cathode-based Zn-air batteries deliver a power density of 121 mW cm-2 at a voltage of 0.76 V. The overall overpotential of Co2P@CNF-based Zn-air batteries can be significantly reduced, with low discharge-charge voltage gap (0.81 V at 10 mA cm-2) and high cycling stability, which outperform the benchmark Pt/C-based Zn-air batteries. The one-step electrospinning method can serve as a universal platform to develop other high-performance transition-metal phosphide catalysts benefitting from the synergy effect of transition nitride satellite shells. The free-standing and flexible properties of Co2P@CNF make it a potential candidate for wearable electronic devices.

Tailored synthesis of Zn-N co-doped porous MoC nanosheets towards efficient hydrogen evolution.

Developing non-precious metal catalysts with both high efficiency and long-term stability is the top priority for hydrogen evolution reactions (HER). Herein, we present a facile two-step method to synthesize Zn, N co-doped molybdenum carbide nanosheets (Zn-N-MoC-H NSs) by using bi-metal oxides of ZnMoO4 as a unique precursor. Zn not only serves as a template to form a porous structure on MoC nanosheets during volatilizing at high temperatures, but also acts as a doping source for Zn doping in MoC. The N-containing carbon source realizes N doping of MoC. Benefitting from Zn, N co-doping and the porous nanosheet structure with a large electrochemical surface area, Zn-N-MoC-H NSs lead to enhanced HER activity in an acidic electrolyte (0.5 M H2SO4) with a low onset potential of -66 mV vs. RHE (1 mA cm-2), overpotential of 128 mV (10 mA cm-2), small Tafel slope of 52.1 mV dec-1 and persistent long-term stability. Density functional theory calculations reveal that Zn, N co-doping can synergistically weaken the strong Mo-H bonding, improve absorbed hydrogen atom (Hads) desorption and lead to faster HER kinetics. This study provides new insights into the use of Zn as a template and electronic regulator toward efficient catalysis and applications in energy storage and conversion.

Synthesis of Mesoporous TiO2-B Nanobelts with Highly Crystalized Walls toward Efficient H2 Evolution.

Mesoporous TiO2 is attracting increasing interest due to properties suiting a broad range of photocatalytic applications. Here we report the facile synthesis of mesoporous crystalline TiO2-B nanobelts possessing a surface area as high as 80.9 m2 g-1 and uniformly-sized pores of 6-8 nm. Firstly, P25 powders are dissolved in NaOH solution under hydrothermal conditions, forming sodium titanate (Na2Ti3O7) intermediate precursor phase. Then, H2Ti3O7 is successfully obtained by ion exchange through acid washing from Na2Ti3O7 via an alkaline hydrothermal treatment. After calcination at 450 °C, the H2Ti3O7 is converted to a TiO2-B phase. At 600 °C, another anatase phase coexists with TiO2-B, which completely converts into anatase when annealed at 750 °C. Mesoporous TiO2-B nanobelts obtained after annealing at 450 °C are uniform with up to a few micrometers in length, 50-120 nm in width, and 5-15 nm in thickness. The resulting mesoporous TiO2-B nanobelts exhibit efficient H2 evolution capability, which is almost three times that of anatase TiO2 nanobelts.

Electrochemically Synthesis of Nickel Cobalt Sulfide for High-Performance Flexible Asymmetric Supercapacitors.

A lightweight, flexible, and highly efficient energy management strategy is highly desirable for flexible electronic devices to meet a rapidly growing demand. Herein, Ni-Co-S nanosheet array is successfully deposited on graphene foam (Ni-Co-S/GF) by a one-step electrochemical method. The Ni-Co-S/GF composed of Ni-Co-S nanosheet array which is vertically aligned to GF and provides a large interfacial area for redox reactions with optimum interstitials facilitates the ions diffusion. The Ni-Co-S/GF electrodes have high specific capacitance values of 2918 and 2364 F g-1 at current densities of 1 and 20 A g-1, respectively. Using such hierarchical Ni-Co-S/GF as the cathode, a flexible asymmetric supercapacitor (ASC) is further fabricated with polypyrrple(PPy)/GF as the anode. The flexible asymmetric supercapacitors have maximum operation potential window of 1.65 V, and energy densities of 79.3 and 37.7 Wh kg-1 when the power densities are 825.0 and 16100 W kg-1, respectively. It's worth nothing that the ASC cells have robust flexibility with performance well maintained when the devices were bent to different angles from 180° to 15° at a duration of 5 min. The efficient electrochemical deposition method of Ni-Co-S with a preferred orientation of nanosheet arrays is applicable for the flexible energy storage devices.

Micro-supercapacitors based on oriented coordination polymer thin films for AC line-filtering.

Reported herein is a facile solution-processed substrate-independent approach for preparation of oriented coordination polymer (Co-BTA) thin-film electrodes for on-chip micro-supercapacitors (MSCs). The Co-BTA-MSCs exhibited excellent AC line-filtering performance with an extremely short resistance-capacitance constant, making it capable of replacing aluminum electrolytic capacitors for AC line-filtering applications.

Interdiffusion Reaction-Assisted Hybridization of Two-Dimensional Metal-Organic Frameworks and Ti3C2Tx Nanosheets for Electrocatalytic Oxygen Evolution.

Two-dimensional (2D) metal-organic framework (MOF) nanosheets have been recently regarded as the model electrocatalysts due to their porous structure, fast mass and ion transfer through the thickness, and large portion of exposed active metal centers. Combining them with electrically conductive 2D nanosheets is anticipated to achieve further improved performance in electrocatalysis. In this work, we in situ hybridized 2D cobalt 1,4-benzenedicarboxylate (CoBDC) with Ti3C2Tx (the MXene phase) nanosheets via an interdiffusion reaction-assisted process. The resulting hybrid material was applied in the oxygen evolution reaction and achieved a current density of 10 mA cm-2 at a potential of 1.64 V vs reversible hydrogen electrode and a Tafel slope of 48.2 mV dec-1 in 0.1 M KOH. These results outperform those obtained by the standard IrO2-based catalyst and are comparable with or even better than those achieved by the previously reported state-of-the-art transition-metal-based catalysts. While the CoBDC layer provided the highly porous structure and large active surface area, the electrically conductive and hydrophilic Ti3C2Tx nanosheets enabled the rapid charge and ion transfer across the well-defined Ti3C2Tx-CoBDC interface and facilitated the access of aqueous electrolyte to the catalytically active CoBDC surfaces. The hybrid nanosheets were further fabricated into an air cathode for a rechargeable zinc-air battery, which was successfully used to power a light-emitting diode. We believe that the in situ hybridization of MXenes and 2D MOFs with interface control will provide more opportunities for their use in energy-based applications.

In Situ Activation of Nitrogen-Doped Graphene Anchored on Graphite Foam for a High-Capacity Anode.

We report the fabrication of a three-dimensional free-standing nitrogen-doped porous graphene/graphite foam by in situ activation of nitrogen-doped graphene on highly conductive graphite foam (GF). After in situ activation, intimate "sheet contact" was observed between the graphene sheets and the GF. The sheet contact produced by in situ activation is found to be superior to the "point contact" obtained by the traditional drop-casting method and facilitates electron transfer. Due to the intimate contact as well as the use of an ultralight GF current collector, the composite electrode delivers a gravimetric capacity of 642 mAh g(-1) and a volumetric capacity of 602 mAh cm(-3) with respect to the whole electrode mass and volume (including the active materials and the GF current collector). When normalized based on the mass of the active material, the composite electrode delivers a high specific capacity of up to 1687 mAh g(-1), which is superior to that of most graphene-based electrodes. Also, after ∼90 s charging, the anode delivers a capacity of about 100 mAh g(-1) (with respect to the total mass of the electrode), indicating its potential use in high-rate lithium-ion batteries.

Engineering the electronic structure of graphene.

Graphene exhibits many unique electronic properties owing to its linear dispersive electronic band structure around the Dirac point, making it one of the most studied materials in the last 5-6 years. However, for many applications of graphene, further tuning its electronic band structure is necessary and has been extensively studied ever since graphene was first isolated experimentally. Here we review the major progresses made in electronic structure engineering of graphene, namely by electric and magnetic fields, chemical intercalation and adsorption, stacking geometry, edge-chirality, defects, as well as strain.

Preparation of supercapacitor electrodes through selection of graphene surface functionalities.

In order to investigate the effect of graphene surface chemistry on the electrochemical performance of graphene/polyaniline composites as supercapacitor electrodes, graphene oxide (G-O), chemically reduced G-O (RG-O), nitrogen-doped RG-O (N-RG-O), and amine-modified RG-O (NH(2)-RG-O) were selected as carriers and loaded with about 9 wt % of polyaniline (PANi). The surface chemistry of these materials was analyzed by FTIR, NEXAFS, and XPS, and the type of surface chemistry was found to be important for growth of PANi that influences the magnitude of increase of specific capacitance. The NH(2)-RG-O/PANi composite exhibited the largest increase in capacitance with a value as high as 500 F g(-1) and good cyclability with no loss of capacitance over 680 cycles, much better than that of RG-O/PANi, N-RG-O/PANi, and G-O/PANi when measured in a three-electrode system. A NH(2)-RG-O/PANi//N-RG-O supercapacitor cell has a capacitance of 79 F g(-1), and the corresponding specific capacitance for NH(2)-RG-O/PANi is 395 F g(-1). This research highlights the importance of introducing -NH(2) to RG-O to achieve highly stable cycling performance and high capacitance values.

Influences of graphene oxide support on the electrochemical performances of graphene oxide-MnO2 nanocomposites.

MnO2 supported on graphene oxide (GO) made from different graphite materials has been synthesized and further investigated as electrode materials for supercapacitors. The structure and morphology of MnO2-GO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Nitrogen adsorption-desorption. As demonstrated, the GO fabricated from commercial expanded graphite (denoted as GO(1)) possesses more functional groups and larger interplane gap compared to the GO from commercial graphite powder (denoted as GO(2)). The surface area and functionalities of GO have significant effects on the morphology and electrochemical activity of MnO2, which lead to the fact that the loading amount of MnO2 on GO(1) is much higher than that on GO(2). Elemental analysis performed via inductively coupled plasma optical emission spectroscopy confirmed higher amounts of MnO2 loading on GO(1). As the electrode of supercapacitor, MnO2-GO(1) nanocomposites show larger capacitance (307.7 F g-1) and better electrochemical activity than MnO2-GO(2) possibly due to the high loading, good uniformity, and homogeneous distribution of MnO2 on GO(1) support.