Microwave-assisted conversion of biomass wastes to pseudocapacitive mesoporous carbon for high-performance supercapacitor

Developing an efficient approach of transforming biomass waste to functional carbon-based electrode materials applied in supercapacitor offers an important and high value-added practical application due to the abundance and considerable low price of biomass wastes.Herein,a hierarchical carbon functionalized with electrochemical-active oxygen-containing groups was fabricated by microwave treatment from the biomass waste of camellia oleifera.The obtained mesoporous carbon(MAC)owns nanosheet morphology,rich mesoporosity,large surface area(1726 m2/g)and very high oxygenic functionalities(16.2 wt%)with pseudocapacitive activity.Prepared electrode of supercapacitor and tested in 2.0 M H2 SO4,the MAC exhibits an obvious pseudocapacitive activity and achieved a superior supercapacitive performance to that of directly activated carbon(DAC-800)including high specific capacitance(367 F/g vs.298 F/g)and better rate performance(66%vs.44%).The symmetrical supercapacitor based on MAC shows a high capacity of275 F/g,large energy density of 9.55 Wh/kg(at power density of 478 W/kg)and excellent cycling stability with 99%capacitance retention after 10000 continuous charge-discharge,endowing the obtained MAC a promising functional material for electrochemical energy storage.

Full-faradaic-active nitrogen species doping enables high-energy-density carbon-based supercapacitor

Nitrogen doping is usually adopted in carbon based supercapacitor to enhance its relatively low energy density by providing extra pseudocapacity.However,the improvement of energy density is normally limited because the content of the introduced nitrogen species is not high and meanwhile only part of them is electrochemically active.Herein,we designed and fabricated a class of hierarchical nitrogen-rich porous carbons(HNPCs)possessing not only very high nitrogen content(up to 21.7 atom%)but also fully electrochemically active nitrogen species(i.e.,pyridinic N,pyrrolic N and oxidized N).Especially,in the synthesis of HNPCs,graphitic carbon nitride(g-C3N4)was used in situ not only as a nitrogen source but also as a catalyst to facilitate the polymerization of phenol and formaldehyde(as carbon precursor)and as a template to create the hierarchical porous structure.As electrodes for aqueous symmetric supercapacitor,the HNPCs with full faradaic-active nitrogen functionalities exhibit excellent supercapacitor performance:high energy density of 36.8 Wh/kg at 2.0 kW/kg(maintaining 25.7 Wh/kg at 38 kW/kg),superior rate capability with 78%capacitance retention from 1.0 to 20 A/g and excellent cycling stability with over95%capacitance retention after 10000 cycles,indicating their promising application potential in electrochemical energy storage.This novel carbon material with high-content and full electrochemically active nitrogen species may find extensive potential applications in the energy storage/conversion,catalysis,adsorption,and so on.

High-temperature treatment to engineer the single-atom Pt coordination environment towards highly efficient hydrogen evolution

Development of high-performance and cost-effective catalysts for electrocatalytic hydrogen evolution reaction(HER)play crucial role in the growing hydrogen economy.Recently,the atomically dispersed metal catalysts have attracted increasing attention due to their ultimate atom utilization and great potential for highly cost-effective and high-efficiency HER electrocatalyst.Herein,we propose a hightemperature treatment strategy to furtherly improve the HER performance of atomically dispersed Ptbased catalyst.Interestingly,after appropriate high-temperature treatment on the atomically dispersed Pt0.8@CN,the Pt species on the designed N-doped porous carbon substrate with rich defect sites can be re-dispersed to single atom state with new coordination environment.The obtained Pt0.8@CN-1000 shows superior HER performance with overpotential of 13 m V at 10 m A cm^(-2)and mass activity of 11,284 m A/mgPtat-0.1 V,much higher than that of the pristine Pt0.8@CN and commercial Pt/C catalyst.The experimental and theoretical investigations indicate that the high-temperature treatment induces the restructuring of coordination environment and then the optimized Pt electronic state leads to the enhanced HER performances.This work affords new strategy and insights to develop the atomically dispersed high-efficiency catalysts.

Adjacent acid sites cooperatively catalyze fructose to 5-hydroxymethylfurfural in a new, facile pathway

To study the effect of adjacent hydroxyl to the active sites, several acid catalysts, i.e. substituted benzoic acids with adjacent carboxyl are employed in the fructose dehydration to 5-hydroxymethylfurfural(HMF).Experimental results reveal that Br?nsted acid sites with adjacent carboxyl present higher catalytic ability than isolated ones. Computational results suggest that the adjacent sites lead to co-interaction on fructose, corresponding more stable transition state and faster HMF formation rate. Based on the enhancement from the adjacent sites, a novel ordered mesoporous carbon(OMC) full of carboxyls in surface is prepared and turns out to be an effective solid catalyst for HMF production from fructose derived from biomass.

High-temperature deoxygenation-created highly porous graphitic carbon nanosheets for ultrahigh-rate supercapacitive energy storage

Developing carbon-based supercapacitors with high rate capability is of great importance to meet the emerging demands for devices that requires high energy density as well as high power density.However,it is hard to fabricate a nanocarbon with high electro-active surface area meanwhile maintaining superior conductivity to ensure the high rate capability since excellent conductivity is usually realized by high temperature graphitization,which would lead to the structural collapse and sintering resulting in low surface area.Herein,we reported a highly porous graphitic carbon nanosheet with an unprecedented rate capability of 98%of its initial capacitance from 0.5 to 50 A/g for ultrahigh-rate supercapacitive energy storage.These hierarchical mesoporous carbon nanosheets(HMCN)were fabricated by a template induced catalytic graphitization approach,in which sheet-like Mg(OH)_(2) was employed as catalytic template in situ catalytically polymerizing of catechol and formaldehyde and catalytically graphitizing of the formed carbon skeleton.Upon the co-effect of template(avoiding the sintering)and the deoxygenation(creating the pores)during the high temperature graphitization process,the obtained HMCN material possesses nanosheet morphology with highly porous graphitic microstructure rich in mesoporosity,large in surface area(2316 m^(2)/g),large in pore volume(3.58 cm^(3)/g)and excellent in conductivity(109.8 S/cm).In 1.0 M TEABF_(4)/AN,HMCN exhibits superior supercapacitive performance including large energy density of 52.2 Wh/kg at high power density of 118 k W/kg,long-cycling stability and excellent rate capability,making HMCN a promising electrode material for supercapacitor devices.

Facile growth of homogeneous Ni（OH）2 coating on carbon nanosheets for high-performance asymmetric supercapacitor applications

The growth of a Ni（OH）2 coating on conductive carbon substrates is an efficient way to address issues related to their poor conductivity in electrochemical capacitor applications. However, the direct growth of nickel hydroxide coatings on a carbon substrate is challenging, because the surfaces of these systems are not compatible and a preoxidation treatment of the conductive carbon substrate is usually required. Herein, we present a facile preoxidation-free approach to fabricate a uniform Ni（OH）2 coating on carbon nanosheets （CNs） by an ion-exchange reaction to achieve the in situ transformation of a MgO/C composite to a Ni（OH）2/C one. The obtained Ni（OH）2/CNs hybrids possess nanosheet morphology, a large surface area （278 m2/g）, and homogeneous elemental distributions. When employed as supercapacitors in a three-electrode configuration, the Ni（OH）JCNs hybrid achieves a large capacitance of 2,218 F/g at a current density of 1.0 A/g. Moreover, asymmetric supercapacitors fabricated with the Ni（OH）2/CNs hybrid exhibit superior supercapacitive performances, with a large capacity of 198 F/g, and high energy density of 56.7 Wh/kg at a power density of 4.0 kW/kg. They show excellent cycling stability with 93% capacity retention after 10,000 cycles, making the Ni（OH）2/CNs hybrid a promising candidate for practical applications in supercapacitor devices.