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Heterogeneous Manganese Oxide‐Encased Carbon Nanocomposite Fibers for High Performance Pseudocapacitors

Authors:
HETEROGENEOUS MANGANESE OXIDE-ENCASED CARBON NANOCOMPOS1TE
FIBERS FOR HIGH PERFORMANCE PSEUDOCAPACITORS
Qiang Li1'2, Karen Lozano2, Yinong Lii3, Yuanbing Mao1'*
'Department of Chemistry, University of Texas - Pan American, Edinburg, TX 78539 USA
department of Mechanical Engineering, University of
Texas
- Pan American, Edinburg, TX
78539 USA
3Department of Materials Science and Engineering, Nanjing University of Technology, Nanjing,
Jiangsu 210009 China
ABSTRACT
The integration of transition metal oxide nanocrystals and one-dimensional (ID) conducting
carbon structures to generate their hybrids can create unpredictable new physical and chemical
properties in comparison with single phase components. Here we report the fabrication of
heterogeneous MnO nanocrystal (NC)-encased hierarchical carbon nanocomposite fibers
(MCNFs) via a novel and large-scale Forcespinning followed by low temperature carbonization.
Manganese nitrate containing polyvinylpyrrolidone (PVP) polymeric fiber was carbonized at
relatively low temperature, i.e. 500 °C, due to oxidant Mn2+ cations. Different inward and
outward ionic diffusion rates of Mn2+ cations concurrently result in congregating MnO NCs near
the surface region of the nanocomposites during thermolysis. After anodic and cyclic
voltammetric electrochemical oxidations, in situ phase transformation from electrochemically
inactive MnO NCs to pseudocapacitive MnOx counterparts occurs, which yields a MnOx
NC/carbon hybrid fiber network with MnOx NC-enriched functional surface. These NCs are
accessible to aqueous electrolyte ions for Faradic redox reactions. Therefore these unique
nanocomposites demonstrate a promising potential as pseudocapacitive electrode materials.
INTRODUCTION
The integration of transition metal oxide nanocrystals (TMONCs) and one-dimensional (ID)
carbon based skeletons has been exploited for various applications, such as chemical sensors,1"2
catalysis,3 nanoelectronics4 and electrochemical devices. "7 In view of the united advantages of
functionality of oxide nanocrystals and superior physical characteristics including electrical
conductivity, mechanical tolerance, thermal stability and surface area of carbonaceous
nanostructures, rationally constructing TMONC/carbon hybrids is prone to achieve
unprecedented physical and chemical features with respect to single component counterparts.8"9
Particularly for supercapacitor applications, two well-established synthetic strategies have
been widely carried out to fabricate ID TMONC/carbon hybrids: (1) pseudocapacitive TMONCs
are anchored onto conductive carbon-based backbones, such as single/multiple walled carbon
nanotubes, 1D conducting polymers and carbon cloth, via a variety of approaches including
hydrothermal method,10 electrochemical depositions,6 atomic layer deposition," precursor
hydrolysis.12 and so forth;5 and (2) ID TMO nanowires/nanoribbons with high aspect ratio are
surface-shielded by conducting polymers through monomer polymerizations.7- 13 For the first
protocol, hybrid electrodes would suffer from inevitably uncontrolled ionic and electronic
diffusions due to fast growth of active oxides during anchoring, capacitance fading caused by
unexpected reactions between electrode materials and electrolytes as well as dissolution of active
materials into electrolyte solutions, and pulverization problem because of particle aggregation
during charge and discharge cycles. On the other hand, the second protocol is unlikely to offer
41
Ceramic Materials for Energy Applications III.
Edited by Hua-Tay Lin,Yutai Katoh and Alberto Vomiero.
© 2014 The American Ceramic Society. Published 2014 by John Wiley & Sons, Inc.
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
stable and fast paths for ionic diffusion and electronic transportation due to the intrinsic
instability of conductive polymers and relatively large size of active materials. Even though in
situ carbonation of electrospun hybrid polymeric fibers can incorporate nanocrystals into
conducting carbon frameworks, the pseudocapacitive target oxides would be reduced into
electrochemically inactive metal or metal oxides with lower oxidation states of transition metals
during high temperature carbonations and the nature of homogeneously dispersed NCs in carbon
matrix can deteriorate the rate capacity when limited electrolyte ion penetration depth is taken
into consideration. Those are the primary reasons why carbonized hybrids have been explored
mostly as anode materials with enhanced stability of lithium ion batteries.14"16 Therefore,
establishing an efficient, facile and productive path to fabricate Faradic TMONCs into carbon
frameworks is still a tremendous challenge for excellent recyclability, high rate pseudocapacitors.
In this work, we present a novel strategy for heterogeneous MnOx NCs/carbon
nanocomposite fibers via high-yield Forcespinning® (FS) followed by carbonization in inert
atmosphere and combined with electrochemical oxidations by applying three-electrode
configuration. Acting as an updated substitution of electrospinning, FS eliminates the necessity
of taking solution/melt electrical conductivity, electrical field strength, surface charge and
ionization field in account by simply employing high speed centrifugal force and thus
significantly simplifies the fabrication process of fine nanofibers.17"18 Moreover, the high
production rate, ~lg/min, affords great opportunities to introduce nano-scaled ID materials into
commercial manufacturing in various areas. In this case, Mn(N03)2/PVP fibers (MPFs) were
spun onto current collectors directly without adding any polymer binder and conductive additive
for the first time to form intercross networks for facile electrolyte penetration and boosted
gravimetric capacitance. Subsequent calcinations led to chronologically uneven ionic diffusion
of Mn2+ ions, thermolysis and carbonization for forming hierarchical hybrid fibers. These fibers
consist of MnO NCs evenly distributed on the near surface regions but simultaneously shielded
by conducting carbon. The MnO NC/carbon hybrid carrying electrodes were further phase
transformed into pseudocapacitively promising MnOx NC/carbon hybrid fibers through anodic
and cyclic voltammetric electrochemical oxidations. Remarkably, the electrochemical evaluation
revealed that the final nanocomposite electrodes possess a maximum specific capacitance of
256.4 F g"1 when normalized to MnOx in 1 M Na2S04 electrolyte solution at a current density of
0.2 A g" , almost two orders of magnitude larger than the capacitance prior to oxidations, and
superior rate performance.
EXPERIMENTAL
Polyvinylpyrrolidone (PVP, average MW = 130,000) and Μη(Ν03)2·*Η20 (99.99%, x = 4-6)
were purchased from Sigma Aldrich. All reagents were of analytical grade and were used
without further purification. The Mn(N03)2-^H20/PVP fibers (MPFs) were prepared as follow.
PVP (3.7 g) was gradually dissolved into 8.7 mL of 18.7, 10, and 25.6 wt.% Mn(N03)2'xH20
aqueous solution by using both vortex mixing and sonication in 2-hour intervals for at least 48 h
for a homogeneous dissolution. The precursor solution was maintained statically in dark in a
vacuum oven at room temperature overnight to remove possible trapped air bubbles. To make
MPFs, the prepared spinning solutions were fed into proprietary designed spinneret with evenly
separated eight needles around its periphery. These needles have 0.29 mm inner orifice size and
are 13 mm long. FS was carried out at a rotational speed of 9,000 rpm for 30 s, a 6 cm needle-to-
collector distance with aluminum foil as collectors. The production rate in a lab scale unit was >1
g/min. Moreover, titanium substrates (0.25 mm thick, Aldrich), which were degreased
42 · Ceramic Materials for Energy Applications III
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
ultrasonically prior to collection in acetone and ethanol for 10 min, respectively, were directly
applied as current collectors. To directly deposit MPFs onto Ti substrate, Ti foil cut into 2 χ 0.5
cm was fastened onto alumina collectors with polytetrafluoroethylene (PTFE) tape prior to the
FS process. The MPFs were then calcined at 500 °C for 3 h in argon atmosphere with a heating
ramp rate of 2 °C/min. These calcined MPFs, namely manganese monoxide MnO/C
nanocomposite fibers (MCNFs), were oxidized using a Gamry reference 600 Potentiostat/
Galvanostat/ZRA workstation in a three-electrode cell system, in which 1 M Na2S04 aqueous
solution acted as electrolyte, and platinum gauze and Ag/AgCl were used as counter and
reference electrodes, respectively. More specifically, a constant anodic oxidation was initially
applied to the MCNFs at a current density of 10 μΑ cm"1 with a potential window from -0.6 to
0.9 V (vs. Ag/AgCl), and then a further oxidation was achieved after subsequent 1000 cyclic
voltammetric scans between 0 and 1 V at 50 mV s"1. After the oxidation processes, MCNFs
converted into the final MnOx NC-encased carbon nanocomposite fibers (EO-MCNFs).
CHARACTERIZATION
Field-emission scanning electron microscope (FESEM) characterizations were carried out on
Carl Zeiss Sigma VP at 2 or 7.5 kV, equipped with backscatter electron detector (BSD). High
resolution transmission electron microscopy (HRTEM) characterization was conducted on
JEOL JEM-2010UHR coupled with selected area electron diffraction (SAED). The charging
effect during SEM imaging on precursor MPFs was eliminated by coating with an approximately
100 A thick Au/Pd layer using a Denton Desk II TSC turbo-pumped sputter coater. Fourier
transform infrared (FTIR) spectra were recorded from 4000 to 450 cm"1 with a 4 cm"1 spectral
resolution on a Thermal Nicolet Nexus 470 spectrometer with a DTGS detector by signal-
averaging 32 scans. Differential scanning calorimetry (DSC) of the MPFs was run between 25 °C
and 300 °C at a heating rate of 5 °C/min on a TA Instruments Q100 DSC in nitrogen
environment. Thermogravimetic Analysis (TGA) was performed from 25 °C to 800 °C at
5 °C/min on a TA Instruments Q500 Thermogravimetric Analyzer for the MPFs and
corresponding calcined MCNFs in nitrogen and air fluxes, respectively. The structural
information of the calcined MCNFs were determined by X-ray powder diffraction (XRD) on a
Rigaku Miniflex II with Cu Ka radiation=
1.5418
A) between 5° and 80°.
ELECTROCHEMICAL EVALUATIONS
All the electrochemical measurements were also conducted using the Gamry reference 600
Potentiostat/Galvanostat/ZRA workstation combined with PWR 800 software suit. Cyclic
voltammetry (CV) were performed at a potential window of
0
to 1.0 V (vs. Ag/AgCl) with scan
rates ranging from 2 mV s"1 to 100 mV s'1. Galvanostatic charge/discharge testing was conducted
between 0 and 1.0 V (vs. Ag/AgCl) at current densities ranging from 0.2 to 1.0 A g"1.
RESULTS AND DISCUSSION
Figure 1 shows the SEM images of MPFs with low and high magnifications. MPFs were
successfully spun into interconnected networks with no beads exhibiting via the FS method for
different manganese nitrate contents. Bead-free feature can improve gravimetric performances
when subsequently MPFs thermally decompose and phase transform into pseudocapacitive
nanohybrids. For convenience, these MPFs are denoted as
MPF-1,
MPF-2 and MPF-3 with the
increasing concentration of manganese nitrate. With increasing ratio of manganese nitrate to
PVP,
the as-spun MPFs exhibit more wrinkled surfaces. Additionally, these relatively randomly
Ceramic Materials for Energy Applications III · 43
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
oriented MPFs form an intercrossing network. The average diameters of these MPFs vary from
400 nm to 1 μπι for
MPF-1,
MPF-2 and MPF-3. According to previous FS studies, the fiber
diameter is determined by multiple factors, such as spinning solution viscosity, solution surface
tension, spinning rotational speed and spinneret needle to fiber collector distance.17"18 Owing to
the increases in MnCNOsV-xFhO concentrations, the precursor solution viscosity increases,
which is mainly responsible for the variations of diameters of the MPFs.19"20 The rotational force
and polymeric wettability are creatively utilized to cement MPFs onto current collectors without
introducing any polymer binder for electrode fabrications. Particularly noteworthy is that
elimination of inactive binder and porous feature of network are critical to constructions of high
rate devices in energy storage.
Figure 1. SEM images and diameter distributions of MPFs with different Mn(N03)2'.xH20
concentrations. (A, B, C)
MPF-1,
(D, E, F) MPF-2, and (G, H, I) MPF-3.
Figure 2 shows the FTIR spectra from the three MPF samples. The sharp absorption peak at
1383 cm"1 is attributed to N-0 asymmetric stretching vibrations from NO3" group. ' Two
remarkable broad peaks originating from 3400 to 3480 cm"1 and from 1668 to 1630 cm"1
correspond to O-H stretching and bending bands, respectively. They are derived from the
residual water solvent, the hydrate water from manganese nitrate and absorbed moisture on the
hygroscopic manganese nitrate and PVP. The fingerprint absorption modes of
PVP
are distinctly
identified with prominent peaks at about 1277-1294, 1321, 1424-1468, 1500
cm"1
and 2916-2960
cm"1, which are assigned to N-C stretching of N-CH2, C-H2 wagging, C-H2 scissoring, N-C
44 · Ceramic Materials for Energy Applications III
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
stretching of N-C=0, and symmetric C-H stretching, respectively. " The primary absorption
mode at 1653 cm"1 corresponding to the carbonyl group C=0 stretching vibration of PVP is
probably superimposed with strong O-H bending vibration. The FTIR analyses indicate that the
MPFs consist of nitrate anions and PVP polymer as well as water molecules. Along with the
increasing manganese nitrate, the intensities of N-0 stretching vibrations are apparently
increased.
=
«
E
m
C
E
~i—■—i—'—i—'—i—'—i—■—i—■—p
4000 3500 3000 2500 2000 1500 1000 500
Wavelength (cm "*)
Figure 2. FTIR spectra of (a)
MPF-1,
(b) MPF-2, and (c) MPF-3.
Thermal experiments of the as-spun MPFs as well as PVP fibers were performed by DSC
under nitrogen atmosphere, as showed in Figure 3. Each DSC thermogram from the three MPFs
exhibits a distinct exothermic peak and an endothermic peak, even though the peak positions are
shifting depending on different Mn(N03)2JcH20 content. The endothermic peak located in
relatively low temperature region is associated with the dehydration of PVP in the MPFs.24 The
upshifted degradation temperature is presumably owing to increasing MniNChVxFkO content as
well as the interactions between Mn(N03)2*H20 and PVP.25"27 The presence of manganese
nitrate gives rise to steric hindrance to the mobility of polymeric PVP backbone. Coordination
complexes formed through van der Waals forces, polar attraction and stabilization from p-bond
overlap reduce the polymeric PVP flexibility and dehydration rate while improve its thermal
stability in the lower temperature region. On the other hand, the exothermic peaks that appear in
the region of 210-283 °C is downshifting with increasing Mn(N03)2^H20 content. According to
the XPS study toward the residual product from PVP degradation at different temperatures by
Chen et
a/.,28
the exothermic peaks can be attributed to the breaking of the C-N bonds linking
pyrrolidone rings to the polymeric backbone during the decomposition of
PVP.
The premature
degradation of PVP as shown by the downshifted exothermic peak with increasing
Μη(Νθ3)2·χΗ2θ content possibly results from the interaction between Mn2+ ions and carbonyl
groups of
PVP.
This interaction hinders the formation of free radicals and subsequently that of
Ceramic Materials for Energy Applications III · 45
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
their intermolecular termination reactions and thus accelerates the degradation of PVP.
Furthermore, Mn2+ ions can act as an oxidant to lower the minimum oxidation temperature of
PVP.29"30
Similar behaviors have been reported in literatures from nanocomposites. In addition,
the heat released from the decomposition of PVP in the MPF-2 is much sharper than that from
the other two MPFs. As it is well-known, thermogram study can be interpreted to structural
variations. The degradation of PVP contains multiple stages, such as chain scission, cross-linking,
and side-chain cyclization. The intensified peak may be affected by complex combination of
thermally exothermic processes.14
5=
1
*»
T
c
=
50 100 150 200 250 300
Temperature (°C)
Figure 3. DSC thermograms of (a) pure PVP NFs, (b)
MPF-1,
(c) MPF-2, and (d) MPF-3 in
nitrogen atmosphere.
Figure 4A shows the TGA thermograms of the MPFs and PVP fibers under nitrogen
atmosphere. The first weight loss occurring around 50 - 110 °C is attributed to the evaporation
of physically adsorbed water molecules. After that, the thermal degradation curves of MPFs
show two decays whereas pure PVP fibers show only one decay, which is in accordance with
other observations.31 The degradation with the onset of 250 °C correlates with the exothermic
peaks in DSC curves and corresponds to the breakage of pyrrolidone pendant groups from the
PVP backbone. The second weight drop stretching up to 480
°C
is attributed to the degradation of
the main hydrocarbon chains of
PVP.
The differential of the weight loss (Figure 4B) highlights
the two stages of PVP degradation more clearly. Interestingly no weight loss related to
decomposition of anhydrous manganese nitrate from the MPNFs is distinguished in the range of
200 - 230 °C in DTG associated with DSC pattern, unlike thermal decomposition behavior of
the pristine manganese nitrate. It is inferred that Mn(N03)2 decomposition is postponed and
occurs accompanying the degradation of the side chain of PVP due to the interaction between the
manganese nitrate and PVP. The interaction may increase the activation energy of
the
formation
of manganese oxide from manganese nitrate in PVP. The simultaneous degradation of
(d>
(c)
(b)
(a)
' ~\
i
46 Ceramic Materials for Energy Applications III
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
manganese nitrate and PVP in the MPFs is supported by the fact that the decomposition
percentage in the range of
263
- 315 °C increases proportionally with increasing the manganese
nitrate content. Another interesting fact is that there is a slow but slight weight loss (~2 %) taking
place after 480 °C from all three MPFs, in contrast to the pure PVP fibers. The decomposition of
pure PVP completes at 480 °C. Thereby, the 2 % weight loss after 480 °C is tentatively ascribed
to the reduction of MnCh, the direct decomposition product of Mn(N03)2, to lower valent
manganese oxide by surrounding reductive carbon or carbon-rich compounds generated during
carbonization.15 In addition, about 18.1, 19.7, 22.6 and 3.4 wt.% residuals are eventually present
from the calcination to 800 °C in nitrogen of the
MPF-1,
-2, -3 and pure PVP fibers, respectively.
The higher residual percentage from MPFs than that from pure PVP fibers indicates the
formation of manganese oxide/carbon composites.
10Q
200 300 400 500 600 700 800
Temperature (°C)
100
200 300 400 500 600 700 800
Temperature (°C)
Figure 4. (A) TGA thermograms and (B) related DTG of (a) pure PVP fibers, (b)
MPF-1,
(c)
MPF-2,
and (d) MPF-3 in nitrogen atmosphere.
Ceramic Materials for Energy Applications III · 47
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
MPFs are thermally transformed into heterogeneous MnO NC-encased carbon
nanocomposite fibers through controlled calcinations outlined in the experimental section. Figure
5 displays morphological and structural information of the typical MCNF-2 sample initially
originating from MPF-2. Amazingly, the averaged diameter of MCNF-2 does not shrink
apparently with respect to MPF-2 in spite of about 80 wt.% weight loss during the calcinations
and this phenomenon is tentatively ascribed to formed carbon skeleton transformed from
unprecedented heavily loaded polymer in FS recipe in contrast to electrospinning ingredients.
Figure 5A&B show the micro/nanostructures of the nanocomposite fibers. The well-inherited
intercrossing network of nanocomposites ensures great percolation of electrolyte and sufficient
mechanical stability for volumetric expansion during battery-like pseudocapacitive TMOs
performing. In the meanwhile, MCNFs exhibit undulate surface topography which is favorable to
offer more accessible electrochemically active sites for Faradic redox reactions on the interface
of electrolyte and electrode materials. The Figure 5C shows a cross-section image, taken under
back-scatter detector, of an individual MCNF. Compared to lightweight carbon atoms,
manganese with larger atomic number is chosen to be detected under backscattered electron
imaging and apparently congregates on the surface or near surface regions of MCNFs,32"33 unlike
other homogeneous ID hybrids.16,34 The formation of heterogeneous structures with TMONC
enriched surface is essential for high rate pseudocapacitors because it is widely accepted that the
pseudocapacitive reactions only effectively occur in the first dozens of nanometers at high
current densities.35"36 In other words, the utilization efficiency of pseudocapacitive materials in
ID configuration will decrease along the radical direction from exterior surface to interior axis.
Heterogeneous 1D nanocomposites with active material-enriched surface or near surface utilize
active materials most thoroughly. TEM studies of MCNF-2 were conducted to offer a further
comprehension of structural features. In Figure D&E, the MCNFs display small, dark domains
resulting from nanocrystals and relatively translucent carbon matrix, which indicates that the
manganese oxide nanoparticles with about 10 nm size are evenly distributed and incorporated
into near surface terrains of carbon frameworks and shielded by the surrounding meandering-
featured carbon.16 The meandering-featured carbon is believed to possess a graphitic-like
structure that formed in relatively low temperature carbonization due to Mn2+ as oxidant
catalyzing the carbonation of polymer.37"39 The carbon protection can prevent oxide domains
from further growth during material synthesis and undesired aggregation during long term cycles
of charge and discharge, which is one of key structural characteristics of these unique
nanocomposites to overcome the devastating capacitance fading in current designs of
pseudocapacitors. The XRD pattern in Figure 5F indicates that manganosite-type MnO (JCPDS
card #75-0626; space group: Fm3m (225); a=4.4435 A) is readily identified by the emergence of
five characteristic peaks (2Θ = 34.94, 40.58, 58.72, 70.20 and 73.82°), marked by their
corresponding Miller indices ((111), (200), (220), (311), and (222)).40 This result is further
confirmed by two diffraction rings, ascribed to (111), and (220) planes of
MnO,
in S AED pattern
and the well-resolved lattice fringes with an interplanar spacing of 0.26 nm of the (111) plane of
cubic MnO in HRTEM image (Figure 5E). The broad peak centered at about 25° is assigned to
amorphous or graphitic-like carbon.37"39
48 Ceramic Materials for Energy Applications III
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
2 e(degree)
Figure 5. (A, B) Low- and high-magnification SEM images, (C) BackScatter image with
inset of corresponding InLens image, (D, E) TEM and HRTEM images with inset of related
SAED pattern in (E), and (F) XRD pattern of MnO nanocrystal-encased carbon nanocomposite
fibers (MCNF-2).
The results of microscopy and XRD investigation are combined to propose a formation
mechanism through which the heterogeneous MCNFs arise. The MniNChVPVP composite
MPFs become more and more viscous and start to plasticize with increasing temperature from
room temperature to dehydration temperature of Mn(N03)2'xH20. As the temperature continues
increasing, the anhydrous Mn(N03)2 starts to incline to diffuse outward due to the premature
rigid shell of composite fibers causing different inward and outward ionic diffusion rates and
subsequently decomposes into MnC>2 or other possible manganese compounds in argon with the
simultaneous breaking down of the pendants on the PVP backbone.41 After that, burst-nucleation
Ceramic Materials for Energy Applications III · 49
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
of
MnC>2
or other possible manganese compounds occurs once their concentration exceeds a
critical concentration at certain temperature. And then, they continue to grow following the
Ostwald ripening mechanism. During this process, however, the polymer matrix prevents them
from further freely growing and aggregating. Finally, during the carbonization of PVP, MnO
NCs are obtained through the reduction of manganese oxide with higher oxidation states by the
surrounding abundant carbon. The collapse of the skin of carbon matrix and the phase separation
of MnO NCs and carbon matrix are possible reasons for the formation of undulate surface
morphology of these MCNFs.42 To our best knowledge, it is the first time to form heterogeneous
MnO NC-encased carbon nanocomposite fibers with the undulate surface morphology. In
contrast, for electrospinning, in order to have relative low viscous and high dielectric spinning
solution, it is not possible to have such high content of polymer, which is important to construct
hierarchical nanocomposite carbon fibers.
In order to perform these heterogeneous structures as promising pseudocapacitive materials,
in situ electrochemical oxidations are conducted in three-electrode cell system to phase transform
electrochemically inactive MnO to pseudocapacitive Mn02/MnOx by applying one cycle of
anodic charge on the MCNFs carrying electrodes at a current density of
10
μΑ cm"1 between -0.6
to 0.9 V for approximate 5000 s, and then 1000 cycles of voltammetric scans with a potential
window of 0 - 1 V (vs Ag/AgCl). According to Messaoudi et
a/.,43
the transition sequence is
described by the following stoichiometric reaction equations with slight modifications, even
though no particular transition would happen exclusively and thoroughly in each stage.
3MnO + H20
«-
Mn304 + 2H+ + 2e"
Μη304·2Η20 +
OH"
<-
2MnOOH + Mn(OH)3 + e"
4MnOOH + 2Mn(0H)3 + 30H" «· 6Mn02 + 5H20 + 3H+ + 6e"
Figure 6A exhibits the evolutions of CV curves and remarkable uphill leap in higher potential
regions and expanded CV integrated areas are observed during the first three cyclic voltammetric
oxidations, which is an evident indication of electrochemical oxidations of MnO and increased
specific capacitances.44"45 Figure 6B compares the CV curves of MCNF-2 before electrochemical
oxidations and after anodic oxidation and 1000 cycles of CV oxidations, respectively. Before
electrochemical oxidations, the specific capacitance (Cs) of MCNF-2 is estimated to be 3.4 F g"1
from the calculation of integrated area of CV curve. Partial contribution of the Cs can be
attributed to the double-layer energy storage that arises from electrostatic adsorption of
electrolyte ions when hierarchical carbon skeleton with enhanced surface area is taken into
account. Particularly noteworthy is that after anodic oxidation at an areal current density of 10
μπι
cm"1
and oxidations of 1000 CV potential cycles, the Cs is dramatically boosted to 100.4 and
128.3 F g"1, respectively. This can be explained by the phase transformation from inactive MnO
to pseudocapacitive Mn02/MnOx which activates pseudocapacitive energy storage on the surface
or near surface of nanocomposites and eventually converts these unique heterogeneous
nanostructures into pseudocapacitive electrode materials. Galvanostatic charge and discharge
measurements are carried out to further investigate nanocomposites at a variety of current
densities. The excellent symmetry of charge and discharge times and the pyramidal waves
indicate a superior reversibility of the nanocomposites. The potential plateaus at about 0.7 V and
0.4 V on pyramids are well consistent with the anodic and cathodic peaks in CV studies (Figure
6B) and validate the Faradic redox reactions.46"47 Figure 6D shows the Cs, estimated from
discharging times, as a function of the different discharging current densities. The
nanocomposites deliver a maximum Cs of 257.9 F g"1 at a current density of
0.2
A g"1. Also, the
50 Ceramic Materials for Energy Applications III
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
heterogeneous nanocomposite fibers maintain a Cs of 214.2 F g"1 at a high current density of
1.0 A g"1, which is about 83.1 % of the value at 0.2 A g"1. The fiber-twined network with
kinetically unlimited ion diffusion is mainly responsible for the extraordinary rate capacity. The
slope of linear correlation between IR drops and discharging current density can be applied to
evaluate the internal resistance of electrode materials.4 It is interesting to notice that the
nanocomposites exhibit a low internal resistance without employing conductive and polymeric
additives, which is owing to the firm attachment between nanocomposite electrode materials and
current collector. Making use of high speed centrifugal force and wettability of PVP plays an
important role in fabrication of mechanically stable electrodes.
5 «
—·
Firai CV scan
* Second CV ican
—* Thifd CV »can
0 0 0.2 0 4 0.8 0.8 10
Potential (V vs. Ag/AgCI)
3
<
*
i
c
&
c
u
U
J0
?0
0
?'l
Before electrocJiemKal oxidation
-j » After enochc oxidation
—*- After 1000 cycles of CV ojttdattcn^*1
4
1
0
ns
Ub
i j J
0.2
B.O
"i*
' a / *
*T£]/jl
h
Ύ φ
A
^rv *
r ψ %
\ \
\\
-•-1.0 Ala
0 8 Ag
-J-O.SAig
» 0.3 Alg
. * 0.2 Aifl
0.D3S-
et
-t f T < 1 » V
o.o 02 0.4 on o.a 1.0
Potential {V vs. Ag/AgCI)
0.4 ο.β ο.β i.o
Current Density (Aig)
0 500 1000 1500 2000
Time (s)
Figure 6. (A) The first three cyclic voltammetric cycles (CV) of MCNF-2 at a scan rate of
100 mV s'1. (B) CV evolutions before electrochemical oxidations and after anodic and 1000
cycles of CV oxidations at a scan rate of
50
mV s"1. (C) Galvanostatic charge and discharge
curves at various current densities, and (D) specific capacitances and IR drops as a function of
the current densities of electrochemically oxidized MCNF-2 (EO-MCNF-2).
CONCLUSION
In summary, we have successfully fabricated nanoarchitectured heterogeneous MnOx NC-
encased carbon nanocomposite fibers with MnOx NC-enriched surface via a novel FS method
followed by calcination and electrochemical oxidations. The controllable ionic diffusion during
Ceramic Materials for Energy Applications III · 51
Heterogeneous Manganese Oxide-Encased Carbon Nanocomposite Fibers
thermolysis and phase transformation from electrochemically inactive MnO NCs to
pseudocapacitive MnOx NCs by anodic and CV scan oxidations has near surface regions of
nanocomposite fibers incorporated with MnOx NCs accessible to Faradic redox reactions that
occur on the surface/near surface. The nanocomposite electrode with electrochemically oxidative
enhancements can achieve boosted Cs of 128.3 F g"1 from initial value of 3.4 F g" before
oxidations at a scan rate of 100 mV s"1. The Mn2+ cations as oxidizing agent can accelerate
carbonation of polymeric precursor and make carbonation take place at relatively low
temperature condition. The fiber-twined network enables a high rate capacity in view of ideal
pathways for ion percolation and electron transportation without kinetic limitations. When
current density is up to 1.0 A g"1, the Cs still maintain about 83.1 % of the maximum Cs obtained
at 0.2 A g"1. Owing to functional oxide nanoparticles thoroughly shielded by thin layer
conductive carbon, the unique heterogeneous nanocomposite fibers are expected to resolve the
uncontrollable aggregation and pulverization problems of oxide nanoparticles and become one of
the best electrode designs for high performance pseudocapacitors.
ACKNOWLEDGEMENT
The authors appreciate the support from the University of Texas-Pan American (startup for
YM),
American Chemical Society-Petroleum Research Fund #51497 (YM), and the National
Science Foundation under DMR grant # 0934157 (PREM-UTPA/UMN-Science and Engineering
of Polymeric and Nanoparticle-based Materials for Electronic and Structural Applications) and
DMR MRI grant #1040419.
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