Kaiyuan Shi

Cellulose Nanocrystal Aerogels as Universal 3D Lightweight Substrates for Supercapacitor Materials

Chemically cross-linked cellulose nanocrystal aerogels represent a versatile and universal substrate on which to prepare lightweight hybrid materials. In situ incorporation of polypyrrole nanofibers, polypyrrole-coated carbon nanotubes, and manganese dioxide nanoparticles in the aerogels gives flexible 3D supercapacitor devices with excellent capacitance retention, low internal resistance, and fast charge-discharge rates.

Polypyrrole nanofiber–carbon nanotube electrodes for supercapacitors with high mass loading obtained using an organic dye as a co-dispersant

Polypyrrole (PPy) nanofibers and composites, containing PPy and multiwalled carbon nanotubes (MWCNTs), were used for the fabrication of electrodes of electrochemical supercapacitors (ES). It was found that cationic malachite green (MG) dye adsorbed on PPy and MWCNTs and allowed their efficient electrosteric dispersion in water. Sedimentation tests, electrophoretic deposition (EPD) experiments, Fourier transform infrared spectroscopy and UV-Vis spectroscopy data showed strong adsorption of MG on PPy and MWCNTs. The important finding was the possibility of efficient dispersion of PPy and MWCNTs using MG dye as a co-dispersant. The new approach allowed colloidal processing of individual materials and composites. This method is suitable for the cathodic EPD of electrically neutral PPy and MWCNTs. The ES electrodes with high material loading in the range of 10–30 mg cm−2 were prepared from colloidal suspensions of PPy nanofibers and MWCNTs using Ni foam current collectors. The composite PPy–MWCNT electrodes, prepared using MG, showed superior electrochemical performance compared to pure PPy and composite electrodes prepared without MG. The composite PPy–MWCNT electrodes with high mass loadings showed excellent capacitance retention at high scan rates and over a wide range of frequencies, and excellent cycling stability. The highest capacitance of 4.62 F cm−2 was obtained at a material loading of 30 mg cm−2. The PPy nanofiber–MWCNT composites, prepared using MG as a dispersant, are promising electrodes for ES.

Electrophoretic nanotechnology of graphene–carbon nanotube and graphene–polypyrrole nanofiber composites for electrochemical supercapacitors

Thin films of multiwalled carbon nanotubes (MWCNT), graphene and polypyrrole (PPy) nanofibers were prepared by cathodic electrophoretic deposition (EPD) from aqueous suspensions, containing safranin (SAF) as a new dispersant. The results of Fourier transform infrared spectroscopy, UV-Vis spectroscopy studies and sedimentation tests, coupled with deposition yield and electron microscopy data showed that SAF adsorbed on MWCNT, graphene and PPy, provided their dispersion and charging in the suspensions and allowed efficient EPD. The deposition yield can be controlled by the variation of SAF concentration in the suspensions and deposition time. The use of SAF as a co-dispersant for MWCNT, graphene and PPy, allowed controlled EPD of composite graphene-MWCNT and graphene-PPy films. The proposed approach for the deposition of PPy paves the way for EPD of neutral polymers using organic dyes as dispersing and charging agents. The composite films were investigated for application in electrochemical supercapacitors (ES). The graphene-MWCNT and graphene-PPy films showed significant increase in capacitance, decrease in resistance and increase in capacitance retention at high charge-discharge rates compared to the films of individual components. The analysis of electrochemical testing results and electron microscopy data provided an insight into the influence of composite microstructure on electrochemical performance. The composites, prepared by EPD are promising materials for electrodes of ES.

Fabrication of polypyrrole-coated carbon nanotubes using oxidant-surfactant nanocrystals for supercapacitor electrodes with high mass loading and enhanced performance.

A conceptually new approach to the fabrication of polypyrrole (PPy)-coated multiwalled carbon nanotubes (MWCNT) for application in electrodes of electrochemical supercapacitors (ES) is proposed. Cetrimonium persulfate (CTA)2S2O8 in the form of nanocrystals is used as an oxidant for the chemical polymerization of PPy. Ponceau S (PS) dye is investigated as a new anionic dopant. Testing results show that PS allows reduced PPy particle size and improved electrochemical performance, whereas (CTA)2S2O8 nanocrystals promote the formation of PPy nanofibers. We demonstrate for the first time that MWCNT can be efficiently dispersed using (CTA)2S2O8 nanocrystals. The analysis of the dispersion mechanism indicates that (CTA)2S2O8 dissociation is catalyzed by MWCNT. This new finding opens a new and promising strategy in MWCNT dispersion for colloidal processing of nanomaterials and electrophoretic nanotechnology. Uniformly coated MWCNT are obtained using (CTA)2S2O8 as a dispersant for MWCNT and oxidant for PPy polymerization and utilizing advantages of PS as an efficient dopant and nanostructure controlling agent. The analysis of the testing results provides an insight into the influence of PS molecular structure on PPy nanostructure and electrochemical properties. The PPy-coated MWCNT show superior electrochemical performance compared to PPy nanoparticles. The proof-of-principle is demonstrated by the fabrication of ES electrodes with excellent electrochemical performance at high active material loadings, good capacitance retention at high charge-discharge rates, and excellent cycling stability.

Anionic dopant–dispersants for synthesis of polypyrrole coated carbon nanotubes and fabrication of supercapacitor electrodes with high active mass loading

A conceptually new approach has been developed for the fabrication of composite polypyrrole (PPy)–multiwalled carbon nanotube (MWCNT) electrodes for electrochemical supercapacitors (ESs). The approach is based on the use of pyrocatechol violet (PV), eriochrome cyanine R (ECR) and acid fuchsin (AF) dyes as dispersants for MWCNTs and dopants for PPy polymerization. Testing results showed excellent electrochemical performance of the composite electrodes at high active mass loadings. The composite electrodes showed superior capacitance retention at high scan rates, compared to pure PPy electrodes. The comparison of the experimental data for the ES electrodes, prepared using different dyes, provided insight into the influence of their structure and functional groups on the composite microstructure and electrochemical performance. The use of ECR as a dispersant for MWCNTs and dopants for PPy allowed the fabrication of PPy coated MWCNTs. The fabrication method is simple and suitable for mass production. This new finding opens up a new and promising strategy for the fabrication of efficient ES electrodes and devices. The PPy coated MWCNTs were used for the fabrication of electrodes with a specific capacitance of 2.430–4.798 F cm−2 in the scan rate range of 2–100 mV s−1 for active mass loading of 18 mg cm−2. The ES cells showed high capacitance at different charge discharge rates and good cycling stability. The ES cells and modules showed promising performance for practical applications.

Supercapacitor devices for energy storage and capacitive dye removal from aqueous solutions

Multiwalled carbon nanotubes (MWCNT) are coated with nitrogen doped activated carbon (NC) for applications in electrochemical supercapacitor electrodes and devices. The capacitive behavior of the electrodes and devices is tested in solutions of organic dyes. The process involves energy accumulation in electrical double layers. The accumulated energy may be further utilized to remove dyes with a similar scheme via discharge of the supercapacitor, also fully regenerating the electrode materials for further use. Proof of concept investigations involve the testing of anionic and cationic dyes and analysis of the influence of dye concentration, charge as well as charge to mass ratio on the capacitance, impedance, specific power and energy of the devices. Cyclic voltammetry and chronopotentiometry data, obtained in different voltage windows, are used for the optimization of device performance. The discussion of quartz crystal microbalance data provides an insight into the factors which govern the charge–discharge behavior. The devices show good cyclic stability. This method offers the advantage of saving energy and applicability to a variety of different dyes.

Azopolymer triggered electrophoretic deposition of MnO2-carbon nanotube composites and polypyrrole coated carbon nanotubes for supercapacitors

Poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) exhibits a number of unique physical properties, which are important for diverse applications of this functional polymer in photonics, optoelectronics, memory devices and sensors. A conceptually new strategy has been utilized for the fabrication of thin films of PAZO using electrophoretic deposition (EPD). The deposition kinetics and mechanism have been investigated and the advantages of EPD have been discussed. Our new findings in the surface and interface chemistry allowed for the development of surface modification methods, which were utilized for the electrosteric dispersion and EPD of MnO2 nanofibers, multiwalled carbon nanotubes (MWCNT), polypyrrole (PPy) nanoparticles and PPy coated MWCNT. New method has been developed for the fabrication of PPy coated MWCNT, using bromothymol blue sodium salt as a dopant for PPy and dispersant for MWCNT. The aromatic PAZO monomers, containing chelating salicylate ligands provided multiple adsorption sites for PAZO adsorption on different materials and allowed for their efficient electrosteric dispersion. Another major finding was the possibility of efficient deposition of composites, using PAZO as a co-dispersant for MnO2 nanofibers and MWCNT. The MnO2 nanofibers, MWCNT, PPy nanoparticles, PPy coated MWCNT and composites, deposited by EPD, were used for energy storage in electrodes of electrochemical supercapacitors. Testing results showed beneficial effect of PAZO for the dispersion and EPD of advanced supercapacitor materials. The results of this investigation paved the way for EPD of other composites utilizing properties of different functional materials and unique physical properties of PAZO.

Electrodeposition of Carbon Nanotubes Triggered by Cathodic and Anodic Reactions of Dispersants

The combination of advanced functionalization methods for surface modification and dispersion of carbon nanotubes (CNTs) and electrochemically triggered strategies for coagulation and film formation allowed efficient electrodeposition of CNTs by cathodic and anodic methods. The electrochemical methods involved the application of cationic basic fuchsin or anionic fluorescein dyes. The dyes adsorbed on CNTs provided electrosteric dispersion. The film forming and binding properties of the dyes and their charge neutralization in electrode reactions promoted deposit formation. The proposed methods allowed controlled electrodeposition of CNT films.

Surface modification and cathodic electrophoretic deposition of ceramic materials and composites using celestine blue dye

A new method has been developed for the surface modification of inorganic particles, which allowed their efficient electrostatic dispersion and cathodic electrophoretic deposition (EPD). The approach is based on the use of cationic celestine blue (CB) dye as a charging and dispersing agent. The key advantages of this approach are related to its applicability to different materials and strong adsorption of CB to the inorganic surfaces, which is of critical importance for efficient particle dispersion. Proof-of-concept studies involved the EPD of thin films of various materials, such as TiO2, MnO2, Mn3O4, BaTiO3, halloysite nanotubes, zirconia and yttria. The results of the deposition rate measurements, Fourier transform infrared spectroscopy, UV-vis and quartz crystal microbalance studies provided an insight into the mechanism of CB adsorption, which involved the interactions of the OH groups of the catechol ligand of CB and metal atoms on the particle surface. It was demonstrated that CB can be used as an efficient dispersing agent for the nanoparticle synthesis by chemical precipitation methods. The feasibility of EPD of various oxide materials paved the way to the EPD of various composites using CB as a co-dispersant for the individual components. Thin films of individual oxides and composites were investigated by electron microscopy and X-ray diffraction methods. The benefits of cathodic EPD for nanotechnology were demonstrated by the formation of nanostructured MnO2 films on commercial high surface area current collectors for energy storage in electrochemical supercapacitors. Testing results showed that the method allowed the fabrication of efficient electrodes with high capacitance and excellent capacitance retention at high charge–discharge rates. The new method paves the way for the deposition of other functional materials and composites for advanced applications.

Surface Treatment of 45 Steels by Plasma Beam Alloying and Plasma Surface Quenching

Plasma beam is currently advisable and advantageous in surface modification for several reasons; for instants, high energy, portability and low cost. In this study, plasma beam was employed to deposit pre-paste, and also used for surface quenching. The results are shown that a uniformed harden layer free of crack and porosities with a thickness of 1.8-2.2mm can be produced by both two methods. After plasma beam alloying, the overlayer with obvious two interfaces can achieve a maximum hardness of 1100HV0.2. This improvement is the mainly contribution not only by land-like TiC reinforced particles and Fe2.5Ti0.5O4, a product of reaction between the pre-paste and molten matrix, with dispersive distribution at the surface, but also alloying elements enriched at the top of molten pool. The experiment suggested that the hardness of the surface was also highly affected by the parameters of plasma beam process alloying; namely, the higher velocity plasma beam moves and the more TiC particles contained, the higher hardness can be obtained. In contrast with the former process, after plasma surface quenching, the overlayer with one interface has a maximum hardness of 600HV0.2.