Guanjie He

One pot synthesis of nickel foam supported self-assembly of NiWO4 and CoWO4 nanostructures that act as high performance electrochemical capacitor electrodes

In this work, we report a facile one-step hydrothermal approach to synthesize NiWO4 and CoWO4 nanostructures on nickel foam as binder-free electrodes for use as supercapacitors. The as-synthesized materials showed excellent electrochemical performance, with a high specific capacitance of 797.8 F g−1 and 764.4 F g−1 at a current density of 1 A g−1 after 3000 cycles. On increasing the current density by 20 times, the rate capabilities still maintained 55.6% and 50.6% of the original value for NiWO4/Ni foam and CoWO4/Ni foam, respectively. Moreover, both of these materials exhibited outstanding cycling stability, the 6000th cycle at 50 mV s−1 demonstrated 2.06 and 2.81 times better capacitance than the initial cycles for NiWO4/Ni foam and CoWO4/Ni foam, respectively. To our knowledge, this capacitance performance is better than any previously reported value for these materials and is a consequence of the highly evolved surface area/microstructure of the materials formed by this technique.

S, N‐Co‐Doped Graphene‐Nickel Cobalt Sulfide Aerogel: Improved Energy Storage and Electrocatalytic Performance

Metal sulfides are commonly used in energy storage and electrocatalysts due to their redox centers and active sites. Most literature reports show that their performance decreases significantly caused by oxidation in alkaline electrolyte during electrochemical testing. Herein, S and N co‐doped graphene‐based nickel cobalt sulfide aerogels are synthesized for use as rechargeable alkaline battery electrodes and oxygen reduction reaction (ORR) catalysts. Notably, this system shows improved cyclability due to the stabilization effect of the S and N co‐doped graphene aerogel (SNGA). This reduces the rate of oxidation and the decay of electronic conductivity of the metal sulfides materials in alkaline electrolyte, i.e., the capacity decrease of CoNi2S4/SNGA is 4.2% for 10 000 cycles in a three‐electrode test; the current retention of 88.6% for Co—S/SNGA after 12 000 s current–time chronoamperometric response in the ORR test is higher than corresponding Co—S nanoparticles and Co—S/non‐doped graphene aerogels. Importantly, the results here confirm that the Ni—Co—S ternary materials behave as an electrode for rechargeable alkaline batteries rather than supercapacitors electrodes in three‐electrode test as commonly described and accepted in the literature. Furthermore, formulas to evaluate the performance of hybrid battery devices are specified.

Graphene/nitrogen-doped porous carbon sandwiches for the metal-free oxygen reduction reaction: conductivity versus active sites

The oxygen reduction reaction (ORR) plays a critical role in sustainable energy systems. Among the most promising metal free ORR electrocatalysts, nitrogen-doped carbon materials have generated significant research interest. Nitrogen doping within a graphitic/turbostratic network of carbon atoms generates active sites for the ORR via C–N bond polarisation that induces a reduced energy barrier towards the ORR on the adjacent carbon atom. At the same time, nitrogen doping leads to an increased electrical conductivity due to electron excess in the delocalised π-system. Thus, the electrical conductivity and the number and the nature of the active sites are two important factors determining the performance of nitrogen-doped carbons in the ORR. Herein, N-doped nanocarbon/graphene composites were carefully designed, synthesized, characterized and tested as electrocatalysts in the ORR in order to decouple these two factors and investigate the underlying relationships between them. Chitosan was used as a nitrogen precursor for nanocarbon, while reduced graphene oxide was introduced to tune the electrical conductivity. Our results show that a low conductivity limits the exertion of active sites and results in a conductivity-dependent ORR activity. However, when the conductivity reaches a critical value, the active sites can be fully utilized and contribute to a positively correlated ORR activity.

Flexible and mechanically robust superhydrophobic silicone surfaces with stable Cassie–Baxter state

Durable non-wetting surfaces require high surface roughness on the nano- or micrometer scale, which is inherently fragile and easily removed by an external force. Elastic materials have potential advantages for constructing superhydrophobic surfaces with abrasion resistance since after friction or force deformation they often rebound to their original structure rather than undergoing degradation. Here we present a large-scale fabrication of free-standing silicone monoliths with a stable Cassie–Baxter state under mechanical stress cycles. The obtained elastic silicone retains excellent mechanical durability with constant super liquid-repellent after high external pressure, knife-scratch, and abrasion cycles with sandpaper. Furthermore the obtained silicone demonstrates high tolerance to continuous contact with extremely corrosive solutions, and also shows self-cleaning properties in air or under oil.

Synergistic relationship between the three-dimensional nanostructure and electrochemical performance in biocarbon supercapacitor electrode materials

A novel study presented herein correlates the multidimensional morphology with the electrochemical performance of activated bio-carbon materials, for supercapacitor devices over multiple length scales. The optimization of the potassium hydroxide (KOH)/cellulose ratio for supercapacitor electrode materials is related to morphological characteristics and corresponding electrochemical performance, as described in terms of porosity, specific surface area, specific capacitance and electrochemical impedance. KOH/cellulose samples with ratios 0.5 : 1 and 1 : 1 exhibited the best performance, characterized by a hierarchal porous network structure, high surface area and low cell resistance. Compared with the rest of the manufactured samples and commercial activated carbons, Ketjen Black (KB), Norit activated carbon (NAC) and bead-shaped activated carbon (BAC), the former two samples showed better results in three-electrode systems and coin cells, with specific gravimetric capacitances as high as 187 F g−1 at a current density of 1 A g−1. The high performance is attributed to the morphology of the samples that constituted a combination of micro-, meso- and macroporosity which consequently gave high specific surface area, high porosity, low cell resistance and high specific capacitance. This further corroborates the structure-performance relationship observed in the authors model KOH/cellulose system, highlighting that the work can be extended to other similar systems. It is clear that the three-dimensional nanostructure of a material must be understood in its entirety in order to optimize the electrochemical performance.

In situ transmission electron microscopy study of individual nanostructures during lithiation and delithiation processes

Direct observation of the nanostructural evolution of electrode materials is critical to understanding lithiation and delithiation processes during cycling of batteries. Due to its real-time monitoring and high spatial resolution, in situ transmission electron microscopy (TEM) plays an important role in understanding the reaction mechanism and dynamic processes in battery materials. This paper reviews the recent progress in using in situ TEM to study individual nanostructures in battery materials using an open-cell design, including for anode materials and cathode materials in lithium ion batteries, and Li–S batteries. Through in situ TEM, the fundamental science and reaction mechanisms, including phase transformations, electrode degradation, size effects, evolution of a solid electrolyte interphase (SEI) and nanostructures, and electrolyte decomposition of nanomaterial-based electrodes were observed during lithiation and delithiation processes. These characteristics will be very useful to the development of basic guidelines for the rational design of high-performance batteries. Finally, the challenges and perspectives of observing individual nanostructures using in situ TEM during electrochemical processes still need to be discussed and addressed.

SnS nanosheets for efficient photothermal therapy

A novel photothermal agent based on PEGylated SnS nanosheets is developed via a simple solvothermal route and the subsequent exfoliation is carried out using an ultrasonication method. The PEGylated SnS nanosheets exhibit much higher extinction coefficient and photothermal conversion efficiency than bulk SnS. With the irradiation of the NIR light, cancer cells in vitro can be efficiently killed by the photothermal effects of the SnS nanosheets. The findings reported here show promising potential for further exploration of 2D nanomaterials as a nanoplatform for cancer therapy.

Synthesis and characterization of omniphobic surfaces with thermal, mechanical and chemical stability

A durable slippery liquid-infused porous surface (SLIPS) omniphobic coating was developed that can be simply applied to various substrates to repel water, coffee, red wine and cooking oils. The coatings retain their slippery properties and omniphobicity even after extreme thermal, mechanical and chemical tests, including rapid cooling down to liquid nitrogen temperatures (−196 °C), heating 200 °C, knife cutting, 850 kPa newton meter pressing, Scotch tape peeling, exposure to acid, base and corrosive salt solutions (pH from 0 to 14), and neutralization on the SLIPS surface. This simple, low-cost and “ready to use” surface is believed to be useful for self-cleaning, anti-fouling, and anti-corrosion purposes potentially for large scale of industrial applications.

A general method for boosting the supercapacitor performance of graphitic carbon nitride/graphene hybrids

Graphitic carbon nitride (g-C3N4) contains a high C/N ratio of 3/4; however, utilizing nitrogen atoms in pseudocapacitive energy storage systems remains a challenge due to the limited number of edge nitrogen atoms and inherent poor electrical conductivity of this semi-conductor material. 3D oxidized g-C3N4 functionalized graphene composites (GOOCN24), in which reduced graphene oxide providing high electron conductivity acts as a skeleton and hybridises with oxidized g-C3N4 segments, were synthesized using a facile two-step solution-based method. Due to the pre-oxidation treatment of g-C3N4, which breaks the polymeric nature of g-C3N4 and increases in the proportion of edge nitrogen atoms and the subsequent solubility in water, the GOOCN24 composites used as electrodes for supercapacitors show a specific capacitance as high as 265.6 F g−1 in acid electrolyte and 243.8 F g−1 in alkaline electrolyte in three-electrode configuration at a current density of 1 A g−1. In addition, low internal resistance, excellent rate performance of over 74% capacitance retention (over a 50-fold increase in current density), and outstanding cycling stability of over 94% capacitance retention after 5000 cyclic voltammetry cycles in both alkaline and acid electrolytes was attained. This translated into excellent energy density with appropriate power density when demonstrated in a symmetrical device.

Sulfur-Deficient Bismuth Sulfide/Nitrogen-Doped Carbon Nanofibers as Advanced Free-Standing Electrode for Asymmetric Supercapacitors

The use of free-standing carbon-based hybrids plays a crucial role to help fulfil ever-increasing energy storage demands, but is greatly hindered by the limited number of active sites for fast charge adsorption/desorption processes. Herein, an efficient strategy is demonstrated for making defect-rich bismuth sulfides in combination with surface nitrogen-doped carbon nanofibers (dr-Bi2 S3 /S-NCNF) as flexible free-standing electrodes for asymmetric supercapacitors. The dr-Bi2 S3 /S-NCNF composite exhibits superior electrochemical performances with an enhanced specific capacitance of 466 F g-1 at a discharge current density of 1 A g-1 . The high performance of dr-Bi2 S3 /S-NCNF electrodes originates from its hierarchical structure of nitrogen-doped carbon nanofibers with well-anchored defect-rich bismuth sulfides nanostructures. As modeled by density functional theory calculation, the dr-Bi2 S3 /S-NCNF electrodes exhibit a reduced OH- adsorption energy of -3.15 eV, compared with that (-3.06 eV) of defect-free bismuth sulfides/surface nitrogen-doped carbon nanofiber (df-Bi2 S3 /S-NCNF). An asymmetric supercapacitor is further fabricated by utilizing dr-Bi2 S3 /S-NCNF hybrid as the negative electrode and S-NCNF as the positive electrode. This composite exhibits a high energy density of 22.2 Wh kg-1 at a power density of 677.3 W kg-1 . This work demonstrates a feasible strategy to construct advanced metal sulfide-based free-standing electrodes by incorporating defect-rich structures using surface engineering principles.