Ultrathin 2D nitrogen-doped carbon nanosheets for high performance supercapacitors: insight into the effects of graphene oxides

Fabrication of dual heteroatom-doped graphitic carbon from waste sponge with "killing two birds with one stone" strategy for advanced aqueous zinc-ion hybrid capacitors

Emerging aqueous zinc-ion hybrid capacitors (AZICs) are considered a promising energy storage because of their superior electrochemical performance. The pore structure, suitable heteroatom content, and graphitization degree (GD) of carbon-based cathodes significantly influence the electrochemical performance of AZICs. The N, S dual-doped porous graphitic carbon materials (LC-750) with the combined characteristics of high GD (1.11) and large specific surface area (1678.38 m² g^-1) are successfully developed by a facile "killing two birds with one stone" strategy using K₃Fe(C₂O₄)₃·3H₂O as the activating and graphitizing agent, and waste sponge (WS) and coal tar pitch (CTP) as the heteroatom and carbon resource, respectively. Results show that the LC-750 cathode displays high capacities of 185.3 and 95.2 mAh g^-1 at 0.2 and 10 A g^-1. Specifically, the assembled LC-750//Zn capacitor can offer a maximal energy density of 119.5 Wh kg^-1, a power density of 20.3 kW kg^-1, and a capacity retention of 87.8% after 15,000 cycles at 10 A g^-1. Density functional theory simulations demonstrate that N and S dual-doping can promote the adsorption kinetics of Zn ions. This design strategy is a feasible and cost-effective method for the preparation of dual heteroatom-doped graphitic carbon electrodes, which enables recycling of WS and CTP into high-valued products.

MOF-Derived Hollow Carbon Supported Nickel-Cobalt Alloy Catalysts Driving Fast Polysulfide Conversion for Lithium-Sulfur Batteries

Transition-metal compounds can be used as electrocatalysts to expedite polysulfide conversion effectively in lithium-sulfur batteries. However, insufficient conductivity and tedious preparation process still limit their practical applications. In this work, NiCo alloy nanoparticles embedded in hollow carbon spheres (NiCo@HCS) are fabricated via a facile, template-free strategy from the NiCo-metal-organic framework (MOF) precursor and used as electrocatalysts for separator modification. NiCo@HCS can not only improve the adsorption capacity of polysulfides by d-band deviation to the Fermi level but also reduce the energy barrier in the process of catalytic polysulfide conversion. Due to favorable three-dimensional (3-D) morphology, improved adsorption, and promoted kinetics of NiCo@HCS, the battery containing the NiCo@HCS-modified separator gives a starting capacity of 1377 mAh g^-1 at 0.2C, which is retained by 72% over 500 charge/discharge operations at 1.0C current density. Moreover, the battery's start capacity reaches 1180 mAh g^-1 (5.9 mAh cm^-2) with a high sulfur content of 5.0 mg cm^-1 at 0.2C.

Iron oxide/hydroxide-nitrogen doped graphene-like visible-light active photocatalytic layers for antibiotics removal from wastewater

Hybrid layers consisting of Fe oxide, Fe hydroxide, and nitrogen doped graphene-like platelets have been synthesized by an eco-friendly laser-based method for photocatalytic applications. The complex composite layers show high photodecomposition efficiency towards degradation of antibiotic molecules under visible light irradiation. The photodecomposition efficiency was investigated as a function of relative concentrations of base materials, Fe oxide nanoparticles and graphene oxide platelets used for the preparation of target dispersions submitted to laser irradiation. Although reference pure Fe oxide/Fe hydroxide layers have high absorption in the visible spectral region, their photodecomposition efficiency is negligible under the same irradiation conditions. The high photocatalytic decomposition efficiency of the nanohybrid layer, up to 80% of the initial antibiotic molecules was assigned to synergistic effects between the constituent materials, efficient separation of the electron-hole pairs generated by visible light irradiation on the surface of Fe oxide and Fe hydroxide nanoparticles, in the presence of conducting graphene-like platelets. Nitrogen doped graphene-like platelets contribute also to the generation of electron-hole pairs under visible light irradiation, as demonstrated by the photocatalytic activity of pure, reference nitrogen doped graphene-like layers. The results also showed that adsorption processes do not contribute significantly to the removal of antibiotic molecules from the test solutions. The decrease of the antibiotic concentration under visible light irradiation was assigned primarily to photocatalytic decomposition mechanisms.

Solid-State Synthesis of Cobalt/NCS Electrocatalyst for Oxygen Reduction Reaction in Dual Chamber Microbial Fuel Cells

Due to the high cost of presently utilized Pt/C catalysts, a quick and sustainable synthesis of electrocatalysts made of cost-effective and earth-abundant metals is urgently needed. In this work, we demonstrated a mechanochemically synthesized cobalt nanoparticles supported on N and S doped carbons derived from a solid-state-reaction between zinc acetate and 2-amino thiazole as metal, organic ligand in presence of cobalt (Co) metal ions Zn_xCo_x(C₃H₄N₂S). Pyrolysis of the Zn_xCo_x(C₃H₄N₂S) produced, Co/NSC catalyst in which Co nanoparticles are evenly distributed on the nitrogen and sulfur doped carbon support. The Co/NSC catalyst have been characterized with various physical and electrochemical characterization techniques. The Co content in the Zn_xCo_x(C₃H₄N₂S) is carefully adjusted by varying the Co content and the optimized Co/NSC-3 catalyst is subjected to the oxygen reduction reaction in 0.1 M HClO₄ electrolyte. The optimized Co/NSC-3 catalyst reveals acceptable ORR activity with the half-wave potential of ~0.63 V vs. RHE in acidic electrolytes. In addition, the Co/NSC-3 catalyst showed excellent stability with no loss in the ORR activity after 10,000 potential cycles. When applied as cathode catalysts in dual chamber microbial fuel cells, the Co/NCS catalyst delivered satisfactory volumetric power density in comparison with Pt/C.

Polarity-dependent emission from hydroxyl-free carbon nanodots

Surface groups of carbon nanodots (CNDs) play a key role in modulating their photoluminescence (PL) properties. However, most of the as-prepared CNDs are complex mixtures of CNDs bearing different surface groups. Thus, the purification of CNDs is essential to reveal the PL mechanism of CNDs. Herein, we present a facile method to synthesize hydroxyl (-OH) free CNDs, followed by intelligently guided column chromatographic separation of CNDs with specific functional groups according to their degree of polarity. After systematic investigation of the separated non-polar CNDs (NP-CNDs) and polar CNDs (P-CNDs), it is revealed that radiative photon emission dominates in the NP-CNDs, which exhibits excitation wavelength-independent emissions. In contrast, an increase in the solvent polarity of P-CNDs improves Frank-Condon excited state stabilization to achieve excitation wavelength-dependent emissions. In particular, white-light emitting P-CNDs with CIE coordinates of (0.332, 0.336) are produced. These findings provide new insights into the nature of the PL mechanism for CNDs, which may pave the way towards the rational design of highly efficient and emission tunable CNDs for various applications.

A Review on Production and Surface Modifications of Biochar Materials via Biomass Pyrolysis Process for Supercapacitor Applications

Biochar (BC) based materials are solid carbon enriched materials produced via different thermochemical techniques such as pyrolysis. However, the non-modified/non-activated BC-based materials obtained from the low-temperature pyrolysis of biomass cannot perform well in energy storage applications due to the mismatched physicochemical and electrical properties such as low surface area, poor pore features, and low density and conductivity. Therefore, to improve the surface features and structure of the BC and surface functionalities, surface modifications and activations are introduced to improve its properties to achieve enhanced electrochemical performance. The surface modifications use various activation methods to modify the surface properties of BC to achieve enhanced performance for supercapacitors in energy storage applications. This article provides a detailed review of surface modification methods and the application of modified BC to be used for the synthesis of electrodes for supercapacitors. The effect of those activation methods on physicochemical and electrical properties is critically presented. Finally, the research gap and future prospects are also elucidated.

Spatial Confinement Strategy for Micelle-Size-Mediated Modulation of Mesopores in Hierarchical Porous Carbon Nanosheets with an Efficient Capacitive Response

Commercial supercapacitors using available carbon products have long been criticized for the under-utilization of their prominent specific surface area (SSA). In terms of carbonaceous electrode optimization, excessive improvement in SSA observed in the gaseous atmosphere might have little effect on the final performance because cracked/inaccessible pore alleys considerably block the direct electrolyte ion transport in a practical electrochemical environment. Herein, mesopore-adjustable hierarchically porous carbon nanosheets are fabricated based on a micelle-size-mediated spatial confinement strategy. In this strategy, hydrophobic trimethylbenzene in different volumes of the solvent can mediate the interfacial assembly with a carbon precursor and porogen segment through π-π bonding and van der Waals interaction to yield micelles with good dispersity and the diameter varying from 119 to 30 nm. With an increasing solvent volume, the corresponding diffusion coefficient (3.1-14.3 m2 s-1) of as-obtained smaller micelles increases, which makes adjacent micelles gather rapidly and then grow along the radial direction of oligomer aggregates to eventually form mesopores on hierarchically porous carbon nanosheets (MNC150-4.5). Thanks to the pore-expansion effect of trimethylbenzene, the mesoporous volume can be adjusted from 28.8 to 40.0%. Mesopores on hierarchically porous carbon nanosheets endow MNC150-4.5 with an enhanced electrochemically active surface area of 819.5 m2 g-1, which gives rise to quick electrolyte accessibility and a correspondingly immediate capacitive response of 338 F g-1 at 0.5 A g-1 in a three-electrode system. Electrolyte transport through pathways within MNC150-4.5 ultimately enables the symmetric cell to deliver a high energy output of 50.4 Wh kg-1 at 625 W kg-1 in a 14 m LiOTF electrolyte and 95% capacitance retention after 100,000 cycles, which show its superior electrochemical performance over representative carbon-based supercapacitors with aqueous electrolytes in recent literature.

Nanosheet-assembled NiCo-LDH hollow spheres as high-performance electrodes for supercapacitors

Layered double hydroxides (LDHs) have been considered as favorable pseudocapacitive electrode materials for supercapacitors due to their tunable layered structure/compositions and low cost. Here, we report the NiCo-LDH hollow spheres prepared with Co-glycerate as the sacrificial template and cobalt source. The hollow spheres are assembled with frizzy NiCo-LDH nanosheets, where the hollow structure can inhibit agglomeration of the LDH nanosheets to expose more active sites and shorten the diffusion path of electrolyte ions. The prepared NiCo-LDH hollow spheres show a high specific capacitance of 1962 F g^-1 at 1 A g^-1 and good capacitance retention rate of 66.4 % at 30 A g^-1. The asymmetric supercapacitors fabricated using NiCo-LDH hollow spheres as positive electrode yields a large energy density 62.9 Wh kg^-1 at the power density of 0.8 kW kg^-1. This research may develop a facile synthesis way to prepare LDH hollow spheres for supercapacitors.

Hydrogen-bonded frameworks crystals-assisted synthesis of flower-like carbon materials with penetrable meso/macropores from heavy fraction of bio-oil for Zn-ion hybrid supercapacitors

The application of biomass-based carbon materials in electrode materials are usually subject to their deficient adsorption sites as well as sluggish diffusion of electrolyte ions. Herein, flower-like carbons are obtained from the heavy fraction of bio-oil with the auxiliary of Hydrogen-bonded frameworks (HOFs) crystals. During the co-carbonization of the both, the HOFs crystals are removed on account of its poor stability, which directs the formation of flower-like morphology and generates the penetrable meso/macropores across petal-like carbon nanosheets. In addition, the pyrolysis gases serve as the agents for activation to enrich the active sites without the further activation. The degree of graphitization and the contents of pyridine nitrogen for carbon materials could be flexibly adjusted with the contents of HOFs. Owing to the beneficial 3D flower-like structure, high specific surface area (1076 m²/g), large pore volume (2.59 cm³/g), and rational N species, the assembled Zn//BH-4 hybrid supercapacitor reaches a superior energy density of 117.5 Wh/kg at 890 W/kg and maintains 60.7 Wh/kg even at 16.2 kW/kg.

MXenes induced formation of Ni-MOF microbelts for high-performance supercapacitors

For the sake of developing new energy storage devices for satisfying the energy needs of the modern society, we herein report an innovative MXene-induced strategy to synthesize Ti₃C₂T_x MXenes/Ni based metal-organic framework composites (Ti₃C₂T_x/Ni-MOFs) for high-performance supercapacitors. The two-dimensional (2D) MXenes with oxygen-containing groups on the surface can be used as structure-directing agents to tune the Ni-MOFs into 2D microbelts. The presence of MXenes cannot only improve conductivity of the composite but also provide additional electric double layer capacitance and faradaic pseudocapacitance. The 2D Ni-MOF microbelts can offer rich activity sites for the faradaic redox reactions and shorten the ion transport path. Taking advantages of synergistic effects of Ni-MOF microbelts and Ti₃C₂T_x MXenes, the prepared Ti₃C₂T_x/Ni-MOFs electrode shows a good electrochemical performance with 1124 F g^-1 at the current density of 1 A g ^-1 and 62% rate capability at 20 A g ^-1. This work can offer a new insight to design 2D MOF belts as high-performance electrode materials for supercapacitors.

Porous 3D graphene aerogel co-doped with nitrogen and sulfur for high-performance supercapacitors

Heteroatom-doped carbon materials with a high specific area, a well-defined porous structure is important to high-performance supercapacitors (SCs). Here, S and N co-doped three-dimensional porous graphene aerogel (NS-3DPGHs) have been synthesized in a facile and efficient self-assembly process with thiourea acting as the reducing and doping agent solution. Operating as a SC electrode, fabricated co-doping graphene, i.e. the sample of NS-3DPGH-150 exhibits the highest specific capacitance of 412.9 F g^-1 under 0.5 A g^-1 and prominent cycle stabilization with 96.4% capacitance retention in the back of 10 000 cycles. Furthermore, based on NS-3DPGH-150, the symmetrical supercapacitor as-prepared in 6 M KOH displays a superior energy density of 12.9 Wh kg^-1 under the power density of 249 W kg^-1. Hence, NS-3DPGHs could be considered as an excellent candidate for SCs.

Covalent Functionalization of Graphene Oxide with Fructose, Starch, and Micro-Cellulose by Sonochemistry

In this work, we report the synthesis of graphene oxide (GO) nanohybrids with starch, fructose, and micro-cellulose molecules by sonication in an aqueous medium at 90 °C and a short reaction time (30 min). The final product was washed with solvents to extract the nanohybrids and separate them from the organic molecules not grafted onto the GO surface. Nanohybrids were chemically characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy and analyzed by thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray diffraction (XRD). These results indicate that the ultrasound energy promoted a chemical reaction between GO and the organic molecules in a short time (30 min). The chemical characterization of these nanohybrids confirms their covalent bond, obtaining a grafting percentage above 40% the weight in these nanohybrids. This hybridization creates nanometric and millimetric nanohybrid particles. In addition, the grafted organic molecules can be crystallized on GO films. Interference in the ultrasound waves of starch hybrids is due to the increase in viscosity, leading to a partial hybridization of GO with starch.

Ingenious preparation of N/NiOx co-doped hierarchical porous carbon nanosheets derived from chitosan nanofibers for high-performance supercapacitors

It is still a key challenge to find suitable materials and methods to obtain super capacitors (SCs) with high specific capacitance due to the difficulty of balancing low theoretical capacity of conventional carbon materials and the poor ion transportability of transition metal oxide. In this work, N Ni dual-doped porous carbon nanosheets (NiO_x-NCNSs) are prepared through a novel way using original N-doped chitosan nanofibers and nickel nitrate to form honeycomb layered structure constructed by 2D fiber network as precursors. NiO_x-NCNS exhibits a high specific surface area of 1847.4 m² g^-1 with a rich N content of up to 5.16 wt% and shows a higher specific capacitance of 614.6 F g^-1 at a current density of 1 A g^-1 than that of carbon nanosheets of undoped Ni (NCNS) (427.5 F g^-1). More specifically, the SC assembled using NiO_x-NCNS electrodes achieves a high energy density of 20.3 W h kg^-1 at a power density of 240.9 W kg^-1, which is superior to most of the CNS-based SCs that have been reported. After 10 000 cycles, the symmetric SC retains approximately 85.61% of the initial capacitance. The strategy provided in this work probes the feasibility of high-performance SCs.

Shape-controlled synthesis of Ni-based metal-organic frameworks with albizia flower-like spheres@nanosheets structure for high performance supercapacitors

Metal organic frameworks (MOFs) are considered as very promising positive electrode materials for supercapacitors. To achieve good electrochemical performance, in this work, we report a mixed-ligand approach to prepare modified Ni-MOF by using trimesic acid (BTC) as the modulator to partially replace the terephthalic acid (PTA) ligands. The introduction of BTC can induce the formation of nanosheets with inserted albizia flower-like spheres, where the nanowires on the albizia flower-like spheres can provide rich redox reaction sites and the "spacer" spheres between the layers can hinder the aggregation of the 2D nanosheets to provide fast transport pathways. Moreover, adsorption simulation shows that the adsorption energy of OH^- on the mixed organic ligands is increased after introducing the BTC ligands, which may improve the reversible redox reaction kinetics in the electrode materials. The as-obtained albizia flower-like spheres@nanosheets structured Ni-MOF with the optimized amount of BTC exhibits a high capacitance of 920 F g^-1 at 1 A g^-1, good rate capability of 61% at 20 A g^-1, and an excellent cycling stability in 6 M KOH electrolyte. This work may provide helpful guidance for controlling the structure and surface property of MOFs to improve the electrochemical performance for supercapacitors.

Graphene Oxide-Assisted Morphology and Structure of Electrodeposited ZnO Nanostructures

In this paper, ZnO electrodeposition was studied with the presence of graphene oxide (GO) exploited as a possible structure-directing agent. The effect of deposition potential and duration on the morphology and structure of ZnO was analyzed. The morphology and structure of the hybrids was analyzed by Raman spectroscopy, X-ray diffraction (XRD), and Scanning Electron Microscopy (SEM). The Raman results indicate a successful modification of ZnO with GO sheets and a hybridization threshold of 10 mg L⁻¹ by the evolution of the defect related band of ZnO at 580 cm⁻¹. The morphology results show that a low GO content only slightly influences the morphology and orientation of ZnO nanostructures while a high content as 10 mg L⁻¹ changes the morphology in nanoplates and growth orientation to lateral. The results show that while GO participated in the deposition reaction, it has a two-fold role, also by structure-controlling ZnO, indicating that the approach is valid for the use of GO as a structure-directing agent for the fabrication of ZnO nanostructures by electrodeposition with varying morphologies and orientations.

Scalable fabrication of high-performance micro-supercapacitors by embedding thick interdigital microelectrodes into microcavities

Micro-supercapacitors (MSCs) with thick interdigital microelectrodes of carbon-based materials exhibit excellent electrochemical performance and hold tremendous promise for applications in microscale energy storage devices. Here, a scalable strategy to fabricate thick embedded multiwalled carbon nanotubes (MWCNTs) as interdigital microelectrodes for MSCs has been developed and investigated. To this end, sufficient MWNCT inks are firstly cast onto pre-patterned microcavity surfaces and then more MWCNT materials are embedded into the microcavities by rapid solvent evaporation. After removal of residual materials from the surfaces by a doctor-blading process, thick interdigital MWCNT microelectrodes with heights up to 190 μm are obtained. These embedded microelectrodes simplify the device structure and improve the mechanical flexibility by acting as both active materials and current collectors. Using interdigital microelectrodes with a width of 250 μm and an interspace of 50 μm, the fabricated MSCs exhibit outstanding electrochemical performance with a high capacitance of 19.5 mF cm^-2 and an energy density of 2.48 μW h cm^-2 at a power density of 24.7 μW cm^-2. On the other hand, four light emitting diodes (LEDs) are successfully powered by three series of MSCs, indicating that MSCs can be connected in series and parallel to yield suitable operating voltages and currents for practical applications.