Typical SEM images of the hydrothermally prepared VO2(B) nanosheets. a) Galvanostatic charge-discharge curves of pH = 2, 6 and 9 equilibrated. VO2(B) nanosheets in 1M Na2SO4 electrolyte solution at different current densities. a) The XRD pattern and (b) SEM image of the hydrothermally prepared layer.
INTRODUCTION AND LITERATURE REVIEW
Fundamentals of electrochemical energy storage
In contrast, charge/discharge processes in a battery are rarely reversible, and different potential ranges are required for the oxidation and reduction of the active material. However, when charging a battery, as long as the two phases of active materials exist simultaneously, the thermodynamic potential is independent of the charge already added, so the potential difference is ideally constant.1.
Energy storage mechanisms of electrochemical supercapacitors
- Electrical double-layer capacitors
- Pseudocapacitors
- Extrinsic pseudocapacitors
One is in the network structure of the electrode surface, and the other is the solvated electrolyte ions of opposite polarity (which is called the outer Helmholtz layer). Faradaic pseudocapacitive reactions can take place both on the surface and in the bulk (usually near the surface) of the electrode.
Commonly used electrode materials for electrochemical supercapacitors
- Carbon-based materials
- Conductive polymers
- Transition metal oxides
- Ruthenium oxide
- Manganese oxide
- Vanadium oxide
- Other transition metal oxides
The ability to introduce positive or negative charge centers on electronically conjugated polymer chains through electrochemical redox reaction makes them viable candidate materials for supercapacitors.1 The conducting polymers can be positively or negatively charged, accompanied by ion insertion to balance the injected charges. price,39-42 more and more have been carried out to investigate the possibility of using conducting polymers as pseudocapacitor electrodes. a) The SEM image of PANI on stainless steel substrate and (b) cyclic voltammetry curves of the as-prepared PANI electrode under different scan rates. In addition, other commonly used conducting polymers serve as supercapacitor electrodes polythiophene (PTh), 45,46 poly(3,4-ethylene-dioxythiophene) (PEDOT), 47,48 etc. a) The SEM image of PPy film on polished graphite plate, and (b) cycling stability of PPy films obtained at 5 mA/cm 2 .
Fabrication methods for making supercapacitor electrodes
The galvanostatic charge-discharge studies show that a specific capacitance of 140 F/g can be achieved in 1 M H2SO4 solution. The electrochemical results show that the as-prepared electrode possesses a high specific capacitance of 320 F/g, as well as good rate capability and cycle stability.
Initiatives of using 2-D layered -MnO 2 and VO 2 (B) nanosheets as supercapacitor
- Advantages of using 2-D layered -MnO 2 and VO 2 (B) nanosheets as supercapacitor
- Commonly used approaches to fabricate 2-D layered -MnO 2 and VO 2 (B) nanosheets . 29
- Commonly used approaches to improve the electrode properties of transition metal
- A novel strategy to improve the charge storage properties of transition metal oxides
The results indicate that lower pH treatment of the reconstituted MnO2 nanostructures can lead to the formation of more Mn3+ in the MnO2 nanosheet. It can therefore be concluded that the formation of VO2(A) is not a main reason for the dramatic increase of the capacitance of Mn-doped samples.
Probable ways of intentionally incorporating structural defects into transition
Commonly used techniques for detecting the structural defects and alkali cation
- X-ray Absorption Spectroscopy (XAS)
- X-ray Scattering and Pair Distribution Function (PDF) analysis
- Raman Spectroscopy
- Other detection techniques
The atomic resolution TEM images shown in Figure 32 allow direct visualization of the titanium vacancies. Their results demonstrate the presence of hydroxyls and structural water in the delithiated sample, and the water may be located in the lamellar regions of the defect structure.
THE CRITICAL ROLE OF ACID TREATMENT INDUCED
Introduction
Manganese dioxide (MnO2) in its many forms is the subject of much study for electrochemical supercapacitor applications. In addition, nanostructuring has been employed to improve the surface area and capacitance, for example by growing nanoneals, 154 nanoflowers, 155 nanoparticles156 and so on. In recent years, deliberate creation of cation vacancies has been investigated to increase the charge storage capacity of transition metal oxides, where cation vacancies provide additional cation intercalation sites.119 The first studies correlating cation vacancies and charge storage properties were published in the mid-1980s. for mutual growth.
Little research has been done to date on the formation of cation vacancies in -MnO2 nanosheets, but extensive research on the important role of birnessite-like MnO2 in photosynthesis, ion sorption and other bio- and geochemical processes provides rich literature on its behavior in aqueous environments.157 Recently, building on much previous work on the Mn oxidation state in K-birnessites, Manceau et al,134 systematically investigated the effects of pH on cation vacancy content in phyllomanganate nanoparticles.
Materials and Methods
- Chemicals and reagents
- Fabrication of -MnO 2 nanostructures
- Characterization of the samples
- Electrochemical measurements
Thus, porous 3-D MnO2 nanostructures assembled from ultrathin 2-D -MnO2 nanosheets with controlled defect content were obtained. Energy dispersive spectroscopy (EDS) was performed using an FEI EDAX system equipped with a silicon-drift detector. Transmission electron microscopy and high-resolution SEM were performed using a Hitachi HF-3300 TEM/STEM (Images courtesy of Jane Y. Howe at Hitachi High-Technologies Canada, Inc.).
The thickness and crystallite dimensions of the exfoliated MnO2 nanoplates were examined with a multimode atomic force microscope (AFM, Bruker Dimension Icon®) in tapping mode using antimony-doped silicon tips (images taken by Trevyn Hey of Binghamton University).
Results and Discussion
- Phases and microstructures of parent K x MnO 2 and protonated H x MnO 2
- Characterization of the exfoliated -MnO 2 nanosheets
- Investigation of the -MnO 2 nanosheets flocculation/reassembly processes
- Phases and microstructures of the reassembled -MnO 2 nanostructures
- Oxidation states determination of the protonated and reassembled -MnO 2
- Defects structure characterization of the protonated and reassembled -MnO 2
- Investigation of supercapacitor electrodes preparation methods for obtaining
- Electrochemical measurements of the protonated and reassembled -MnO 2
The effects of different cationic species (H+, Li+ and Na+) on the surface morphologies of the reassembled -MnO2 nanostructures have also been investigated, with the results shown in Figure 38. The shifts of basal reflections result from increased water content in the reassembled nanosheet flocs are compared to the proton exchange form. To determine the oxidation states of Mn ions in all samples (protonated MnO2, pH = 2 and 4 reassembled equilibrated MnO2), we first used X-ray photoelectron spectroscopy (XPS).
The specific capacitances of the three samples obtained at different current densities are summarized in Figure 56(e).
Summary: effects of defects on capacitance
INVESTIGATION OF THE CHARGE STORAGE
Introduction
In situ XANES measurements show a significantly lower reduction rate of iron cations compared to the theoretical one. Thus, in situ studies reveal that the presence of a large amount of Fe vacancies leads to intercalation of lithium ions without structural changes and reduction of iron cations. Therefore, the preparation of electrodes without carbon and binder is necessary for in situ X-ray scattering measurements.
Finally, the in-situ XANES spectrum clearly shows the reversible reduction/oxidation of Mn upon charge/discharge, confirming that the Faradaic redox reaction is a key charge storage mechanism in the damaged MnO2 nanosheet system.
Materials and Methods
- Chemicals and reagents
In-situ high-energy X-ray diffraction and scattering measurements of defected -MnO2 nanosheets on glassy carbon electrodes under electrochemical cycling were performed at the 11-ID-B APS beamline at Argonne National Laboratory using our custom electrochemical cell. . The thickness of the electrolyte between the Kapton® window and the glassy carbon working electrode is ~140 μm, which is thin enough to minimize the effect of electrolyte sputtering. Absorption edge energies were taken as half the edge step height for all samples.
The loading of the active material on the working electrode was typically controlled in the range of 0.4-0.5 mg/cm2.
Results and Discussion
- Investigation of the electrophoretic deposition process to make carbon and binder
- Characterization of the EPD prepared binder-free electrodes charge/discharged with
- Electrochemical comparison of the EPD prepared -MnO 2 nanosheet electrodes that
- Ex-situ Raman spectroscopy of charge/discharged -MnO 2 nanosheet electrodes
- In-situ XANES spectroscopy of -MnO 2 nanosheet electrodes equilibrated in
- In-situ X-ray diffraction, scattering and PDF analysis of pH = 2 equilibrated -
Specific capacitances of the EPD-prepared binder-free MnO2 nanosheet electrodes on Ni foil substrates with different deposition time. Energy dispersive spectra of the MnO2 nanosheet flakes electrophoretically on Ni foil substrate (a) fully charged and (b) fully discharged with potassium. Finally, we compared the specific capacitance of the EPD-prepared binder-free MnO2 nanosheet electrodes equilibrated in different pH solutions.
Typical CV curves for MnO2 nanosheet flocks deposited on Au-coated Kapton® tapes and equilibrated at (a) pH = 2 and (b) pH = 4 solution for 24 h.
Summary: investigation of the charge storage mechanism of defective -MnO 2
The electrodes equilibrated at a lower pH show greater Mn oxidation state changes, according to the XANES data, indicating that the defects may help to promote the ion intercalation and charge transfer, thereby promoting the Faradic redox reactions and reduction of Mn facilitate. When comparing the data obtained from XANES and calculated from CV loops, a smaller change of Mn oxidation states during loading was observed for the MnO2 nanosheets with higher defect content, implying that a large amount of Mn -vacancies in the MnO2. Other than a small fraction of double layer capacitance, hydroxyls or protonated oxygen sites around Mn vacancies can also help to accumulate the charges during K+ ion intercalation without affecting the oxidation state of Mn, according to several recently published papers.
The in situ XRD and PDF data also reveal reversible expansion/contraction of nanosheet layers upon charge/discharge, as well as unchanged interlayer spacing during cycling, consistent with in situ XANES data.
MN DOPED VO 2 (B) NANOSHEETS
Introduction
VO2(B) nanosheets218 can be easily obtained, thus making it an excellent method to fabricate the VO2(B) nanosheets. The electrochemical results also indicate improved discharge capacities with much improved cyclic stability for the Mn-doped samples. In this chapter, pure and Mn-doped VO2(B) nanosheets were successfully prepared by means of a simple one-step hydrothermal reaction.
The surface morphologies, chemical and crystal structures, and sodium ion intercalation properties of pure and Mn-doped VO2(B) nanosheets have been systematically investigated.
Materials and Methods
- Chemicals and reagents
- Preparation of VO 2 (B) and Mn-doped VO 2 (B) nanosheets
- Preparation of VO 2 (A) nanobelts
- Characterization of the samples
- Electrochemical measurements
The composition of the samples was characterized by energy-dispersive spectroscopy (EDS) using an FEI EDAX system equipped with a silicon drift detector (built-in SEM). The local chemical environment of the samples was probed with a PHI Quantera X-ray photoelectron spectrometer (XPS) equipped with Al K radiation. Ex-situ total X-ray scattering and PDF measurements were performed at F2 Cornell High Energy Synchrotron Source (CHESS), Cornell University, with X-ray energy 61.33 KeV Å) at room temperature.
Structural modeling with fitting of the G(r) function over the 0–90 Å r range was performed using the PDFgui software.221.
Results and Discussion
- Investigation of the hydrothermal reaction conditions to prepare the VO 2 (B)
- Effects of different pH equilibration on structures, morphologies and charge storage
- Phases and microstructures of the hydrothermally prepared pure and Mn-doped
- Determination of oxidation states of the hydrothermally prepared pure and Mn-
- Phase composition and local structure characterization of the hydrothermally
- Electrochemical measurements of the hydrothermally prepared pure and Mn-doped
The SEM images of hydrothermally prepared pure and Mn-doped VO2(B) nanoplates are shown in Figure 81. Comparison of the experimental pair distribution functions (PDF) G(r) over low r regions (1 – 7 Å) of pure and Mn -doped VO2(B) nanosheets. With increasing scan rate, the capacitance of Mn-doped VO2(B) decreases a little faster than that of pure VO2(B) (62% capacitance retention for Mn-doped VO2(B), vs.
Cycling stability of pure and Mn-doped VO2(B) nanosheets at constant current density of 5 A/g between 0 and 0.8 V.
Summary: Mn doped VO 2 (B) nanosheets
Kim, “Synthesis of a novel mesoporous carbon and its application to electrochemical double-layer capacitors,” Chem. Miura, “Electrochemical Deposition of Nanostructured Indium Oxide: High-Performance Electrode Material for Redox Supercapacitors,” Chem. 34; Investigation of the charge storage mechanism of a pseudocapacitive MnO2 electrode using Operando Raman spectroscopy", Chem.
Balakrishnan, “Chemical and Structural Stability of Nio Nanowire Porous Thin Film Based Electrodes for Supercapacitors,” Chem.
