Synthesis and functionality of boron-, nitrogen- and oxygen-doped shaped carbon-based nanomaterials and titania nanocomposites in electrochemical capacitors
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Date
2017
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Abstract
Energy is a global fundamental sector and major concerns are inclusive of; making renewable
power economical, reliable and accessible to all, maintain and improve power quality, voltage and
frequency, amongst others. There is need for development of intelligent energy storage systems
(ESS) that maximise and provides durable storage of electrical power generated. This is a suitable
approach towards reducing gas emissions, lowering electricity bills, meet power needs at any time
and for lowering excess power fluctuations. Much advancement is required on ESS to shift their
optimum working regions towards preferred limits with both high justifiable power and
energy. Advancement of ESS need to be sought through developing effective electrode materials.
Shaped carbon nanomaterials (SCNMs) are suitable for ESS in the Smart Grids with potential better
cost effective and scalable standards. The investigation of related physicochemical properties of
SCNMs, modification of nano-structural parameters and development of appropriate strategies that
would enhance their functionality in ESS is key
in this regard.
In this study, various ESS were reviewed with more focus on development of electrochemical
capacitors (ECs) with a bias towards the use of SCNMs as electrodes. The work was aimed at
understanding the influence of reagent ratio in the physicochemical properties of
N-doped multiwalled carbon nanotubes (N-MWCNTs) and graphene oxide (GO). Also, it focused on
modifying the functionality of MWCNTs, N-MWCNTs and reduced graphene oxide (RGO) in ECs via
introduction and control of heteroatoms such as nitrogen and its functional moieties or
introduction of oxygen-containing groups. Thirdly, the work investigated the effect of
composite synthesis on the performances of individual components via control of wt.% ratios.
Characterisation techniques used include transmission and scanning electron microscopies,
atomic force microscopy, textural characteristics, thermogravimetric analysis, elemental
analysis, cyclic voltammetry, electrochemical impedance spectroscopy, X-ray photoelectron
spectroscopy, X-ray diffraction, ultraviolet-visible spectrophotometry, Raman and Fourier
transform infra-red spectroscopies.
N-MWCNTs were synthesized from N,N’-dimethyl formamide and acetonitrile as sp3 and sp hybridized
nitrogen sources, respectively, as materials for ECs. The combination of ferrocene carboxaldehyde,
N,N’-dimethyl formamide and acetonitrile in N-MWCNTs synthesis was a novel approach. Mixing the
sp3 and sp sources in 1:3 ratio enhanced nitrogen content to 9.38%
from that of both sp3 (5.87%) and sp (3.49%). The physical properties such as number of
concentric shells were tailored by varying synthesis temperature. Pyrrolic N-doping was
achieved as the main constituent of nitrogen moieties.
Furthermore, GO was synthesized as a preliminary step for further N-doping. The effect of
graphite: Na2NO4 reagent ratio in the synthesis of GO was studied to elucidate the influence of the
initial step in GO synthesis, via modified Hummer’s method, and to develop novel
strategies towards controllable products. The physicochemical properties such as content of
oxygen-containing groups on GO and the surface areas were increased from 0% and 2 m2 g-1 to 30%
and 188 m2 g-1, respectively, by increasing the proportion of Na2NO4 in reagents. The manipulation
of the initial step was a novel means of tailoring the associated physicochemical properties of GO.
Also, this study determined, for the first time, the most effective group one sulfate electrolyte
at fixed concentrations. This aided the selection of the electrolyte used in the application of
the SCNMs in this thesis. Oxygen moieties were introduced, by ultra-sonic waterbath
treatment, onto MWCNT surfaces using various reagents namely; HCl, HNO3, H2O2 and HNO3/
HCl solutions. The study highlighted how the various reagents, commonly used to purify MWCNTs
after synthesis, modify associated physicochemical properties and alter charge storage
characteristics. Oxygen-containing groups increased capacitance of pristine MWCNTs and
introduced pseudo charge storage mechanism via oxygen functionalities. HNO3 treated MWCNTs had a
77- and 2.5-fold upgrading from pristine using Li2SO4 and Na2SO4, respectively, whilst HNO3/ HCl
was the best, 5 times better, in K2SO4. The oxygen-modified MWCNTs performance was highest and of
best quality in Na2SO4.
The effectiveness of common GO reductants, namely; ascorbic acid, hydrazine hydrate and sodium
borohydride were practically investigated. This was done to select a reductant for the current
work. This study also provided a viable novel chemical tuning approach for nitrogen moieties and
content as well as to introduce boron, with sodium borohydride. Thirdly, under this particular
study, the effect of heteroatoms, boron and nitrogen, as well as nitrogen moieties on
physicochemical characteristics of RGO was also explored. Hydrazine hydrate was the most
effective reductant and was associated with highest surface area and N-content of 390.55 m2 g-1 and
4.07 at.%, respectively. The nitrogen groups of RGO reduced by means of ascorbic acid, hydrazine
hydrate and pristine were pyrrolic, pyridinic and sp3 N-C, respectively. N- doped RGO,
particularly pyrrolic moieties, were 76-fold better than B-doped. A further
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thermal reduction, of RGO from hydrazine hydrate, increased surface area from c.a. 391 to c.a.
600 m2 g-1 at 750 ℃.
The effect of oxygen-containing groups was then investigated in composites of titania with GO, RGO
and cellulose reduced graphene oxide (CRG). The wt.% ratios of titania were varied; i.e., 5, 10,
20 and 40%. Based on earlier deductions in this thesis, reductant chosen was
hydrazine hydrate. Titania enabled better exfoliation of GO but at higher wt.%, it culminated in
larger agglomerates which in turn increased diffusion path-length. RGOTi at 5 wt.% titania
increased surface area from 136.89 to 434.24 m2 g-1. The study generally showed that
capacitance was better at lower wt.% titania in RGOTi and that cellulose surface area increase was
outweighed by associated insulating effect. The present data infers that the impact of
oxygen moieties on capacitance of SCNMs was subject to specific structures; MWCNTs, GO and RGO.
Capacitance of titania and GO were improved by composite synthesis.
Graphenated N-MWCNTs were targeted, as a means, to lessen agglomeration, without the use of
surfactants, and to generate 3-D scaffolds for better electrical conductivity channels. Also,
better physicochemical characteristics for higher capacitance were obtained via sol-gel than CVD
method. The ratios of sp3- and sp-hybridized nitrogen in reagent mixtures, in this thesis, was
effectively used to tune the composition of pyrrolic nitrogen moieties. Pyrrolic
composition of N-MWCNTs was uniquely aimed because studies of typical moieties on RGO deduced
pyrrolic to be better than pyridinic groups. The increase of pyrrolic nitrogen
composition; 35, 45 and 60%, culminated in capacitance deterioration. Composite synthesis reduced
Warbug length and amplified associated capacitance.
The physicochemical properties of RGO, GO, MWCNTs and N-MWCNTs were positively tuned from reagent
ratios, conditions and composite syntheses. The conjectured strategies could modulate
their overall capacitance via manipulation of heteroatom content and functional groups, amongst
others listed herein. Several traits that linked physicochemical properties and capacitance were
successfully elucidated. This affirms the hypothesized potential of SCNMs in ESS through
understanding and control of both nano-structural parameters and
physicochemical properties.
Description
Doctor of Philosophy in Higher Education. University of KwaZulu-Natal,Westville,2020