Boosting the Energy Density of Bowl-Like MnO2@Carbon Through Lithium-Intercalation in a High-Voltage Asymmetric Supercapacitor with "Water-In-Salt" Electrolyte.

Highly concentrated "'water-in-salt"' (WIS) electrolytes are promising for high-performance energy storage devices due to their wide electrochemical stability window. However, the energy storage mechanism of MnO2 in WIS electrolytes-based supercapacitors remains unclear. Herein, MnO2 nanoflowers are successfully grown on mesoporous bowl-like carbon (MBC) particles to generate MnO2/MBC composites, which not only increase electroactive sites and inhibit the pulverization of MnO2 particles during the fast charging/discharging processes, but also facilitate the electron transfer and ion diffusion within the whole electrode, resulting in significant enhancement of the electrochemical performance. An asymmetric supercapacitor, assembled with MnO2/MBC and activated carbon (AC) and using 21 m LiTFSI solution as the WIS electrolyte, delivers an ultrahigh energy density of 70.2 Wh kg-1 at 700 W kg-1, and still retains 24.8 Wh kg-1 when the power density is increased to 28 kW kg-1. The ex situ XRD, Raman, and XPS measurements reveal that a reversible reaction of MnO2 + xLi+ + xe-↔LixMnO2 takes place during charging and discharging. Therefore, the asymmetric MnO2/MBC//AC supercapacitor with LiTFSI electrolyte is actually a lithium-ion hybrid supercapacitor, which can greatly boost the energy density of the assembled device and expand the voltage window.

Distinguished Roles of Nitrogen-Doped Sp2 and Sp3 Hybridized Carbon on Extraordinary Supercapacitance in Acidic Aqueous Electrolyte.

The acidic aqueous supercapacitors have been found to deliver appealing capacitive properties due to fast ion diffusion caused by the applied smallest size of hydrion. However, their practical applications are largely inhibited by the narrow electrochemical stability window of water (1.23 V). Herein, A nitrogen-enriched porous carbon materials (RNOPCs) is reported, consisting of varied nitrogen doping bonded on sp2 and sp3 carbon sites, which are capable of stimulating a wider potential window up to 1.4 V and thus resulting in a great enhancement of capacitive performance in aqueous acidic electrolytes. Together with the improved electrical conductivity and preferable hydrion diffusion, RNOPCs exhibit an ultrahigh volumetric capacitance (1084 F cm-3) in 0.5 M H2SO4. Besides, a fully packed RNOPCs-based symmetrical supercapacitor can deliver a high gravimetric and volumetric energy density of 31.8 Wh Kg-1 and 54.3 Wh L-1 respectively, approaching those of lead acid batteries (25-35 Wh Kg-1). The first-principles calculations reveal that the lone pair electrons of the doped nitrogen can be delocalized on its neighboring carbon atoms, improving charge uptakes and overpotentials. Such facile and scale-up production of carbon-based supercapacitors can bridge the gap of energy density between traditional supercapacitors and batteries in aqueous electrolytes.

Dual-Silica Template-Mediated Synthesis of Nitrogen-Doped Mesoporous Carbon Nanotubes for Supercapacitor Applications.

1D carbon nanotubes have been widely applied in many fields, such as catalysis, sensing and energy storage. However, the long tunnel-like pores and relatively low specific surface area of carbon nanotubes often restrict their performance in certain applications. Herein, a dual-silica template-mediated method to prepare nitrogen-doped mesoporous carbon nanotubes (NMCTs) through co-depositing polydopamine (both carbon and nitrogen precursors) and silica nanoparticles (the porogen for mesopore formation) on a silica nanowire template is proposed. The obtained NMCTs have a hierarchical pore structure of large open mesopores and tubular macropores, a high specific surface area (1037 m2 g-1 ), and homogeneous nitrogen doping. The NMCT-45 (prepared at an interval time of 45 min) shows excellent performance in supercapacitor applications with a high capacitance (373.6 F g-1 at 1.0 A g-1 ), excellent rate capability, high energy density (11.6 W h kg-1 at a power density of 313 W kg-1 ), and outstanding cycling stability (98.2% capacity retention after 10 000 cycles at 10 A g-1 ). Owing to the unique tubular morphology, hierarchical porosity and homogeneous N-doping, the NMCT also has tremendous potential in electrochemical catalysis and sensing applications.

High Pseudocapacitance-Driven CoC2 O4 Electrodes Exhibiting Superior Electrochemical Kinetics and Reversible Capacities for Lithium-Ion and Lithium-Sulfur Batteries.

In this study, cuboid-like anhydrous CoC2 O4 particles (CoC2 O4 -HK) are synthesized through a potassium citrate-assisted hydrothermal method, which possess well-crystallized structure for fast Li+ transportation and efficient Li+ intercalation pseudocapacitive behaviors. When being used in lithium-ion batteries, the as-prepared CoC2 O4 -HK delivers a high reversible capacity (≈1360 mAh g-1 at 0.1 A g-1 ), good rate capability (≈650 mAh g-1 at 5 A g-1 ) and outstanding cycling stability (835 mAh g-1 after 1000 cycles at 1 A g-1 ). Characterizations illustrate that the Li+ -intercalation pseudocapacitance dominates the charge storage of CoC2 O4 -HK electrode, together with the reversible reaction of CoC2 O4 +2Li+ +2e- →Co+Li2 C2 O4 on discharging and charging. In addition, CoC2 O4 -HK particles are also used together with carbon-sulfur composite materials as the electrocatalysts for lithium-sulfur (Li-S) battery, which displays a gratifying sulfur electrochemistry with a high reversibility of 1021.5 mAh g-1 at 2 C and a low decay rate of 0.079% per cycle after 500 cycles. The density functional theory (DFT) calculations show that CoC2 O4 /C can regulate the adsorption-activation of reaction intermediates and therefore boost the catalytic conversion of polysulfides. Therefore, this work presents a new prospect of applying CoC2 O4 as the high-performance electrode materials for rechargeable Li-ion and Li-S batteries.

Controllable and Scale-Up Synthesis of Nickel-Cobalt Boride@Borate/RGO Nanoflakes via Reactive Impingement Mixing: A High-Performance Supercapacitor Electrode and Electrocatalyst.

Large-scale synthesis of graphene-based nanomaterials in stirred tank reactor (STR) often results in serious agglomeration because of the poor control during micromixing process. In this work, reactive impingement mixing is conducted in a two-stage impinging jet microreactor (TS-IJMR) for the controllable and scale-up synthesis of nickel-cobalt boride@borate core-shell nanostructures on RGO flakes (NCBO/RGO). Benefiting from the good process control and improved micromixing efficiency of TS-IJMR, NCBO/RGO nanosheet provides a large BET surface area, abundant of suitable mesopores (2-5 nm), fast ion diffusion, and facile electron transfer within the whole electrode. Therefore, NCBO/RGO electrode exhibits a high specific capacitance of 2383 F g-1 at 1 A g-1, and still retains 1650 F g-1 when the current density is increased to 20 A g-1, much higher than those of nickel boride@borate/RGO (NBO/RGO) and cobalt boride@borate/RGO (CBO/RGO) synthesized in TS-IJMR, as well as NCBO/RGO-S synthesized in STR. In addition, an asymmetric supercapacitor (NCBO/RGO//AC) is constructed with NCBO/RGO and activated carbon (AC), which displays a high energy density of 53.3 W h kg-1 and long cyclic lifespan with 91.8% capacitance retention after 5000 charge-discharge cycles. Finally, NCBO/RGO is used as OER electrocatalyst to possess a low overpotential of 309 mV at a current density of 10 mA cm-2 and delivers a good long-term durability for 10 h. This study opens up the potential of controllable and scale-up synthesis of NCBO/RGO nanosheets for high-performance supercapacitor electrode materials and OER catalysts.

Polymer-Inorganic Thermoelectric Nanomaterials: Electrical Properties, Interfacial Chemistry Engineering, and Devices.

Though solar cells are one of the promising technologies to address the energy crisis, this technology is still far from commercialization. Thermoelectric materials offer a novel opportunity to convert energy between thermal and electrical aspects, which show the feasibility to improve the performance of solar cells via heat management and light harvesting. Polymer-inorganic thermoelectric nanocomposites consisting of inorganic nanomaterials and functional organic polymers represent one kind of advanced hybrid nanomaterials with tunable optical and electrical characteristics and fascinating interfacial and surface chemistry. During the past decades, they have attracted extensive research interest due to their diverse composition, easy synthesis, and large surface area. Such advanced nanomaterials not only inherit low thermal conductivity from polymers and high Seebeck coefficient, and high electrical conductivity from inorganic materials, but also benefit from the additional interface between each component. In this review, we provide an overview of interfacial chemistry engineering and electrical feature of various polymer-inorganic thermoelectric hybrid nanomaterials, including synthetic methods, properties, and applications in thermoelectric devices. In addition, the prospect and challenges of polymer-inorganic nanocomposites are discussed in the field of thermoelectric energy.

Enhanced Potassium Storage Capability of Two-Dimensional Transition-Metal Chalcogenides Enabled by a Collective Strategy.

Potassium-ion batteries (PIBs) have been considered as a promising alternative to lithium-ion batteries due to their merits of high safety and low cost. Two-dimensional transition-metal chalcogenides (2D TMCs) with high theoretical specific capacities and unique layered structures have been proven to be amenable materials for PIB anodes. However, some intrinsic properties including severe stacking and unsatisfactory conductivity restrict their electrochemical performance, especially rate capability. Herein, we prepared a heterostructure of high-crystallized ultrathin MoSe2 nanosheet-coated multiwall carbon nanotubes and investigated its electrochemical properties with a view to demonstrating the enhancement of a collective strategy for K storage of 2D TMCs. In such a heterostructure, the constructive contribution of CNTs not only suppresses the restacking of MoSe2 nanosheets but also accelerates electron transport. Meanwhile, the MoSe2 nanosheets loaded on CNTs exhibit an ultrathin feature, which can expose abundant active sites for the electrochemical reaction and shorten K+ diffusion length. Therefore, the synergistic effect between ultrathin MoSe2 and CNTs endows the resulting nanocomposite with superior structural and electrochemical properties. Additionally, the high crystallinity of the MoSe2 nanosheets further leads to the improvement of electrochemical performance. The composite electrode delivers high-rate capacities of 209.7 and 186.1 mAh g-1 at high current densities of 5.0 and 10.0 A g-1, respectively.

Rapid and Controllable Synthesis of Nanocrystallized Nickel-Cobalt Boride Electrode Materials via a Mircoimpinging Stream Reaction for High Performance Supercapacitors.

Nickel-cobalt borides (denoted as NCBs) have been considered as a promising candidate for aqueous supercapacitors due to their high capacitive performances. However, most reported NCBs are amorphous that results in slow electron transfer and even structure collapse during cycling. In this work, a nanocrystallized NCBs-based supercapacitor is successfully designed via a facile and practical microimpinging stream reactor (MISR) technique, composed of a nanocrystallized NCB core to facilitate the charge transfer, and a tightly contacted Ni-Co borates/metaborates (NCBi ) shell which is helpful for OH- adsorption. These merits endow NCB@NCBi a large specific capacity of 966 C g-1 (capacitance of 2415 F g-1 ) at 1 A g-1 and good rate capability (633.2 C g-1 at 30 A g-1 ), as well as a very high energy density of 74.3 Wh kg-1 in an asymmetric supercapacitor device. More interestingly, it is found that a gradual in situ conversion of core NCBs to nanocrystallized Ni-Co (oxy)-hydroxides inwardly takes place during the cycles, which continuously offers large specific capacity due to more electron transfer in the redox reaction processes. Meanwhile, the electron deficient state of boron in metal-borates shells can make it easier to accept electrons and thus promote ionic conduction.