Determination of Quantum Capacitance of Niobium Nitrides Nb2N and Nb4N3 for Supercapacitor Applications

none5siThe density of states and quantum capacitance of pure and doped Nb2N and Nb4N3 singlelayer and multi-layer bulk structures are investigated using density functional theory calculations. The calculated value of quantum capacitance is quite high for pristine Nb2N and decent for Nb4N3 structures. However for cobalt-doped unpolarized structures, significant increase in quantum capacitance at Fermi level is observed in the case of Nb4N3 as compared to minor increase in case of Nb2N. These results show that pristine and doped Nb2N and Nb4N3 can be preferred over graphene as the electrode material for supercapacitors. The spin and temperature dependences of quantum capacitance for these structures are also investigatedopenBharti; Gulzar Ahmed; Yogesh Kumar; Patrizia Bocchetta; Shatendra SharmaBharti, ; Ahmed, Gulzar; Kumar, Yogesh; Bocchetta, Patrizia; Sharma, Shatendr

Physical properties of heteroatom doped graphene monolayers in relation to supercapacitive performance

885-891Electrodes fabricated using graphene are quite promising for electric double layer capacitors. However graphene has the limitations of low ‘Quantum Capacitance (QC)’ near fermi level due to the presence of Dirac point that can be modified by doping graphenewith suitable dopant. The density functional theory DFT calculations are performed for doped graphene using Boron, Sulphur and phosphorus as dopants to improve the quantum capacitance of electrodes fabricated using graphene. The calculations are performed at temperatures of 233, 300 and 353 °K. From present calculations no significant temperature dependence of quantum capacitance is observed, however a marked increase in QC of value above 58μFcm-2 is seen. Forphosphorus and Sulphur doped graphene a significant energy gap shift of ~ 1.5 eV from the Fermi level is observed that significantly increases the QC at Fermi level to a high value of ~ 35 μFcm-2. With boron dopant as well, a shift of energy gap ~ 1.25eV from the Fermi level is observed. The shift in Dirac point increases quantum capacitance at Fermi level that in turn can increase the energy density of supercapacitor remarkably. The effect of increasing doping concentration on quantum capacitance is also investigated. These results suggest that doping of graphene may result in significant increase in QC near Fermi level, if the dopants are selected carefully depending upon the use of graphene as a positive or negative electrode. The results of these calculations reveal that the problem of low QC of graphene in the range of interest can be addressed by modifying itssurface and structure chemistry which may increase energy density in supercapacitors

Physical properties of heteroatom doped graphene monolayers in relation to supercapacitive performance

Electrodes fabricated using graphene are quite promising for electric double layer capacitors. However graphene has the limitations of low ‘Quantum Capacitance (QC)’ near fermi level due to the presence of Dirac point that can be modified by doping graphenewith suitable dopant. The density functional theory DFT calculations are performed for doped graphene using Boron, Sulphur and phosphorus as dopants to improve the quantum capacitance of electrodes fabricated using graphene. The calculations are performed at temperatures of 233, 300 and 353 °K. From present calculations no significant temperature dependence of quantum capacitance is observed, however a marked increase in QC of value above 58μFcm-2 is seen. Forphosphorus and Sulphur doped graphene a significant energy gap shift of ~ 1.5 eV from the Fermi level is observed that significantly increases the QC at Fermi level to a high value of ~ 35 μFcm-2. With boron dopant as well, a shift of energy gap ~ 1.25eV from the Fermi level is observed. The shift in Dirac point increases quantum capacitance at Fermi level that in turn can increase the energy density of supercapacitor remarkably. The effect of increasing doping concentration on quantum capacitance is also investigated. These results suggest that doping of graphene may result in significant increase in QC near Fermi level, if the dopants are selected carefully depending upon the use of graphene as a positive or negative electrode. The results of these calculations reveal that the problem of low QC of graphene in the range of interest can be addressed by modifying itssurface and structure chemistry which may increase energy density in supercapacitors

Study of effect of annealing on morphology of hydrothermally synthesized potassium titanate fibers

703-707The potassium titanate fibers have been synthesized using TiO2 and KOH as startup materials in one step hydrothermal method. The thickness of synthesized fibres ranges from 20 to 100 nm whereas the length varies from 200 nm to 200 microns. The effect of annealing at various temperatures on the synthesized nanofibers has also been investigated by characterization using XRD, SEM, TEM, Raman and FT Raman. The annealing at 800 °C has been found to yield a pure K2Ti6O13 phase when its XRD peaks have been compared with the standard XRD data. The growth of nanowires is along the (010) direction. However, when annealed at 400 °C, 600 °C, 800 °C and 1000 °C the samples show the transformation into several other structural phases. However, annealing above 800 °C causes all the structures of nanowires to transform into K2Ti6O13. The TEM image shows that the product formed contains a large quantity of fibres with almost uniform thicknesses but with varied lengths

Hydrothermal synthesis and characterization of nickel doped potassium titanate

453-460The potassium titanate nanowires doped with nickel using nickel chloride as dopant are synthesized by hydrothermal method. These synthesized structures are then characterized by XRD, SEM and TEM and the dopant concentrations are estimated by EDS. The XRD characterization of samples shows that the product formed is mainly K2Ti6O13. The nanowires/fibres of the K2Ti6O13 are formed having length up to 20 microns and thickness of about 20 nm. The TEM imaging confirms the formation of uniform internal structures of all nanowires however the inner structure of nickel doped samples are denser in comparison to the undoped samples. The annealing of synthesized nanowires at temperatures in the range of 600-1000 °C modifies their crystal structure. The EDS results have shown that the measured Ni doping concentration in all samples is about four times higher than the expected values. Therefore the possibility of chemical bonding or trapping of nickel atoms in the structure cannot be ruled out. The UV-Vis and Raman spectra of the doped samples are compared with that of un-doped samples. In present work, the use of nickel chloride as dopant in potassium titanate using hydrothermal method is reported for the first time

Experimental investigation of plasma instabilities by Fourier analysis in an electron cyclotron resonance ion source

The plasma instabilities play an important role in an electron cyclotron resonance (ECR) ion source for the production of intense heavy ion beams in high charge states for particle accelerators. The geometrical and operational constraints of ECR sources hinder the trapping of ions for a sufficient time to get fully ionized with maximum efficiency. This problem is looked at in detail by studying the plasma instabilities in ECR ion sources. The ECR environment is full of complex rearrangements of various electric and magnetic fields to define a sustainable trap for the ions. The maximum frequency of plasma instability has been observed to be of 122.5 kHz under a set of sustainable plasma parameters. However, this limit may be pushed further if the plasma is overdriven in terms of source parameters. The instabilities cover a full regime of few tens of Hz to few hundreds of kHz under various operating conditions of radio frequency (rf), negative bias voltage, rf power and injection gas pressure. The rigorous details of frequencies and amplitudes of plasma instabilities are being reported by studying the Fourier spectrum of extracted and analyzed beam intensity. The plasma instabilities are attributed as drift waves in an inhomogeneous ECR plasma generated by the application of radio-frequency fields