Effect of oxidative surface treatments on charge
storage at titanium nitride surfaces for
supercapacitor applications†
Benjamin M. Gray,
a
Andrew L. Hector,
*
a
Marek Jura,
b
John R. Owen
a
and Joshua Whittam
a
The effects of surface oxidation on the capacitance of titanium nitride electrode surfaces, produced by
reaction of titanium foils with ammonia, are examined. Thermal oxidation and electrochemical oxidation
both increase the amount of redox active oxide at the surface, but electrochemical oxidation is found to
be more successful in increasing the capacitance.
Introduction
Supercapacitors store charge at the interface between an elec-
trode and an electrolyte. Initially they were used to provide
backup power to electronic components, but now nd appli-
cations ranging from cells used to produce small amounts of
energy in fast pulses, to fast recharging power tools and cells of
more than 100 kF to recover kinetic or potential energy in buses
and cranes.
1
Increasingly they will be used in vehicles, where
a battery or fuel cell will provide high density energy storage and
a supercapacitor the high power density required for hill
climbing and rapid acceleration.
2
Most research in super-
capacitors is concerned with devices that rely on high surface
area electrodes, typically carbon, and store charge in a thin
double layer of anions and cations very close to the surface.
There is an increasing interest in electrodes that can also
undergo surface redox processes and hence increase the
amount of charge that they can store. These devices are referred
to as redox supercapacitors, or pseudo capacitors, since they
combine the functions of capacitors with those of batteries.
1–3
Metal oxides and conducting polymers have been used exten-
sively as the active materials in redox supercapacitors.
4
The
current leading material is hydrous ruthenium oxide, which
combines good electronic conductivity with good transport of
protons in the hydrous surface layers and a theoretical charge
storage capacity of 1358 F g
1
.
5
Capacitances approaching this
theoretical value have been achieved in composites with 10–
20% RuO
2
loading on carbon,
6
and even at high rates of 1 V s
1
with structured electrodes such as nanotube arrays.
7
However,
these approaches suffer from fading capacity with increased
loading or a high processing cost, respectively, and the material
cost in use of RuO
2
is also an issue.
Metal nitrides including TiN,
8–14
VN,
15–22
NbN,
23
Nb
4
N
5
,
24
Mn
3
N
2
,
25
Mo
2
N,
26–29
RuN
30
and WN
31
have been investigated for
supercapacitor applications. Notably Choi et al. found that
nanocrystalline VN could deliver a specic capacity of 1340 F g
1
at 2 mV s
1
scan rate in aqueous KOH electrolyte.
15
The advan-
tage of metal nitrides in supercapacitor electrodes is that in most
cases they have much higher electronic conductivities than the
respective oxides. In VN the capacity increased over initial cycles
as a layer of oxide formed, and the high capacity was attributed to
redox processes in this surface oxide layer.
15
More recently this
principle of a conductive core metal nitride with a redox active
surface has been expanded with TiN nanostructures used to
provide current collectors for MnO
2
,
32
RuO
2
,
33
VN
34
and gra-
phene
35
redox materials.
However, TiN has also generated a growing interest in its use
without a separate redox component. Initial studies showed
that the capacitance of electrodes based on nanocrystalline TiN
decreased during cycling as surface oxide formed.
9
With larger
crystallites the capacitances were lower but were maintained
much better during cycling.
9,10
Recently the focus has moved to
structures with continuous electronic pathways, e.g. TiN nano-
pore structures produced by anodization methods have deliv-
ered a stable capacitance of 27.59 mF cm
2
,
12
mesoporous TiN
lms have achieved over 200 F g
1
capacitance,
14
and supported
TiN “corn-like” structures had high volumetric capacity of
1.5 mW h cm
3
.
11
Interestingly the latter study showed an
increase in capacitance during cycling that was attributed to
“surface activation”. Achour et al. were able to link the perfor-
mance of the native oxide surface directly to its nitrogen content
and show that nitrogen-rich TiN retains capacity more effec-
tively due to nitrogen doping of this surface oxide that enhances
its conductivity.
13
Titanium oxynitride nanostructures have also
shown good supercapacitor performance.
36
In view of these
studies and other recent work using TiN as the shell in core–
a
Chemistry, University of Southampton, Higheld, Southampton SO17 1BJ, UK.
E-mail: A.L.Hector@soton.ac.uk
b
ISIS, STFC, Harwell Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ta08308k
Cite this: J. Mater. Chem. A,2017,5,
4550
Received 24th September 2016
Accepted 2nd February 2017
DOI: 10.1039/c6ta08308k
rsc.li/materials-a
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shell supercapacitor structures,
37–39
there is a signicant need to
understand how the formation of surface oxide affects the
electrochemical behaviour of TiN surfaces. Herein we have
applied various oxidation treatments to low area titanium
nitride surfaces and linked their charge storage capacity to a
detailed characterisation of the electrode surfaces. The resul-
tant insights are intended to be transferable to high surface area
titanium nitride electrodes.
Experimental
TiN electrodes were prepared from titanium foil (99.96%, 0.25
mm thick, Goodfellow), which was cut into ag shapes with a 10
10 mm square plus a strip of metal protruding for electrical
connection. The electrodes were cleaned by sonication for 30
minutes each in acetone and deionized water and then blown
dry with nitrogen. These were heated under anhydrous
ammonia (BOC) in an alumina crucible contained in a quartz
furnace tube using a modied literature method,
40
taking care
to ensure that all metal surfaces were placed such that they were
well exposed to the gas environment. The electrodes were
heated at 550
C for 30 minutes then the temperature was raised
at 2
C min
1
to 1000
C, maintained for 10 hours and allowed
to cool naturally in the ammonia ow. Some TiN foils were
removed from the furnace at room temperature and stored in
air, these are referred to as “as prepared” or “untreated” foils.
Others were oxidised with the intention of producing a thicker
surface corrosion layer, either:
“Thermally oxidized” (TO) by heating in air for 24 h at
temperatures between 250 and 450
C.
“Cyclic voltammogram oxidized” (CVO) by ramping their
potential from 0 to 1.2 V vs. Hg/HgO and back 10 times at
100 mV s
1
in 6 mol dm
3
KOH. Samples were then rinsed with
deionised water, allowed to dry and stored in air.
“Potential step oxidized” (PSO) by maintaining a xed
potential in the range 0.5–2.0 V vs. Hg/HgO for a period of 50 to
200 s in 6 mol dm
3
KOH. Samples were then rinsed in
deionised water, allowed to dry and stored in air.
Grazing incidence X-ray diffraction data (XRD) were collected
using a Rigaku Smartlab diffractometer with a parallel beam of
Cu-K
a
X-rays, incidence angles between 0.5 and 5
and
a DTex250 1D detector. Rietveld renement was carried out
using the GSAS package.
41,42
Atomic force microscopy (AFM) was
conducted using a Veeco Dimension 3100 instrument in
tapping mode. Scanning electron microscopy (SEM) used a Jeol
JSM5910 microscope. Energy dispersive X-ray spectroscopy
(EDX) measurements were carried out with a Thermosher
Ultradry detector using a 15 kV accelerating voltage, and wave-
length dispersive X-ray spectroscopy (WDX) with a Thermo-
sher Magnaray detector and a 5 kV accelerating voltage, both
mounted on a Philips XL-30 ESEM.
X-ray photoelectron spectra (XPS) were recorded using a Sci-
enta ESCA300 with an Al K
a
(1486.7 eV) source and a 90
take-off
angle used in most cases. Some samples were etched using
argon ion bombardment for 20 minutes at 4 keV. The Ti 2p, N
1s, C 1s and O 1s spectra were collected. The CASA XPS 2.3.15
soware package was used for data analysis. Metal 2p doublets
were modelled with constrained relative intensities of the 2p
3/2
and 2p
1/2
peaks (3 : 1) and separation of these peaks based on
initial modelling of data for Ar
+
ion etched TiN. The asymmetric
peak shape was based on that reported in a previous study of
TiN.
43
Data was referenced to the adventitious carbon 1s peak,
which was assigned a binding energy of 284.8 eV, or to the argon
2p
3/2
at 241.6 eV aer etching where carbon 1s was not present.
Electrochemical measurements were conducted using Bio-
Logic VMP2 or SP-150 potentiostats. The aqueous 1 mol dm
3
electrolytes were deoxygenated by bubbling nitrogen through
them and all potentials are quoted relative to Hg/HgO in 0.1 M
NaOH, which has a potential of 0.10 V vs. SHE.
44
Capacitance
measurements used cyclic voltammograms collected with
a potential window chosen (aer initial experiments) to avoid
solvent decomposition. A sequence of scan rates was applied,
with 10 scans each at 1000, 1, 5, 10, 50, 100 and 500 mV s
1
and
then 100 scans at 1000 mV s
1
. The middle (5
th
or 50
th
) scan at
each rate was used for capacitance calculations and plotted CVs.
Areal specic capacitance was calculated as half of the integral
of the CV (mA V) trace divided by (scan rate (mV s
1
) the
width of the potential window (1.1 V) area of the lm (cm
2
)
exposed to the electrolyte). Galvanostatic experiments were
performed aer 180 scans of cyclic voltammetry as described
above, with currents of 0.033, 0.1, 0.33, 1.0 and 3.3 mA and xed
potential limits. Electrochemical impedance spectroscopy (EIS)
measurements were made at the open circuit potential aer all
other electrochemical treatments, with a frequency range of 1
MHz to 10 mHz and an amplitude of 10 mV, and EIS data were
tted using ZView.
45
Results and discussion
Electrodes were initially prepared by heating ag-shaped pieces
of titanium foil in ammonia. Aer nitridation the electrodes
exhibit the golden colour usually seen in TiN lms produced by
physical vapour deposition and retain metallic conductivity,
with a resistance of 2.5 mU measured along the 2 cm length of
a ag-shaped electrode (2-point resistance measurement, so
this value includes the resistance of the leads and contacts).
Grazing incidence X-ray diffraction measurements were
carried out to determine which phases were present at the
surface of the electrodes. Absorption of 50% of photons will
occur at a path length of 8 mm in TiN,
46
and this path length is
equivalent to a depth normal to the surface of 0.7 or 0.07 mm
with an incident angle of 5 or 1
, respectively. The as-prepared
metal nitride foils exhibited strong peaks corresponding to
rocksalt-structured TiN (Fig. 1). Rietveld re nement of the data
collected at 5
incidence resulted in lattice parameters of
4.22857(13)
˚
A for TiN (Fig. S1†). This is close to the literature
values, which range between 4.23–4.27
˚
A for TiN.
47
Further
peaks are also observed, although these are weaker at lower
incidence angles relative to the TiN peaks indicating that they
are due to phases present below the surface. These secondary
peaks also showed strong texturing (spotty diffraction rings
when collected with an area detector) suggesting large crystal-
lites. Phase matching identied these peaks as due to “TiN
0.3
”
and “Ti
2
N”,
48
which are both disordered alloy phases with the
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hexagonal titanium structure. A survey of the phases in the ICSD
and PDF databases showed that these phases show a clear
Vegard trend in the cell volume with nitrogen content
(Fig. S2†),
47,48
and from this the two sub-surface phases can be
estimated to have compositions of TiN
0.29
(volume ¼ 37.155(4)
˚
A
3
) and TiN
0.47
(volume ¼ 38.357(8)
˚
A
3
).
Scanning electron microscopy shows fairly smooth metallic
surfaces on the as-prepared TiN foils (Fig. S3†), with no obvious
changes on this length scale in those that had undergone
surface treatment. EDX measurements showed 51% N and 49%
Ti, with oxygen contents <1%. This sampling depth for EDX is
around 1 mm, so this supports the evidence from variable inci-
dence angle measurements (Fig. 1) that the nitrogen decient
phases observed in the XRD are below the surface of the foils.
AFM data collected to observe the roughness on shorter length
scales show some surface roughness on the as-prepared TiN
foils and allow the real surface area of the electrodes to be
measured (Fig. 2).
XPS is inherently surface sensitive as photoelectrons can
only escape from within a few nm of the surface, and hence is
particularly relevant in understanding the layer of material that
will be in direct contact with the electrolyte. The Ti 2p XPS
spectrum of as-prepared TiN foils showed three overlapping
doublets (Fig. 3). Based on the work of Saha et al.,
49
those with
binding energies of 455 and 461 eV (2p
3/2
and 2p
1/2
respectively)
were assigned as TiN (i.e. roughly Ti
3+
) and those at 459 and
464.5 eV as TiO
2
(Ti
4+
), whereas broader peaks in between were
assigned as Ti(O,N) species (Table 1), with the greater breadth of
these intermediate signals attributed to a range of possible
environments.
43
Other recent studies of TiN for supercapacitor
applications have found very similar surface environment
distributions and used similar assignments of the peaks.
11,13,36
Reduction of the take-off angle results in reduction of the TiN
Fig. 1 Grazing incidence XRD patterns for titanium nitride foils
collected at incidence angles as shown. The Miller indices of the cubic
rocksalt-type TiN
47
peaks are labelled, and the remaining peaks are due
to hexagonal titanium nitride phases.
Fig. 2 AFM images of titanium nitride foil surfaces, as prepared or with
oxidation treatments as shown. Surface areas are in cm
2
per cm
2
of
electrode area. AFM images were collected on a 1 1 mm region of the
electrode surface and the images are displayed with a 1 1 1 mm
scale.
Fig. 3 The Ti 2p region of the XPS spectra for un-etched TiN foils as-
prepared and after application of various oxidative treatments as
indicated. The TiN peaks are highlighted in green, Ti(O,N) in red and
TiO
2
in purple. The yellow region in the as-prepared sample is a shake-
up peak.
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signal and enhancement of the TiO
2
signal, suggesting that
even within this thin surface lm there is some variation in the
composition with the most oxide-rich component closest to the
surface. Ar
+
-ion etching reduced the contribution from Ti(O,N)
from 26% to 3% and the TiO
2
contribution from 33% to an
undetectable level, suggesting that the majority of the oxide was
present in a thin passivation layer. A strong shake up peak
43
was
also observed in these heavily etched samples, coincident with
the position of the Ti(O,N) 2p
3/2
peak.
Characterisation of TiN surfaces aer surface oxidation
Surface oxidations were carried out either by heating the TiN foils
in air or by ramping their potential to values positive of those at
which the aqueous KOH electrolyte started to oxidise. Previously
we showed that the interface resistance at a TiN surface increases
above 350
C,
50
so the expectation was that a very thin oxide
layer would be produced at 300
C. The effect of surface
oxidation during cyclic voltammetry within the potential window
in which the electrolyte was stable (typically 0.8 to +0.3 V vs. Hg/
HgO herein) will be observed during cycling of the as-prepared
electrodes. To produce thicker electrochemical oxidation layers
the potential was raised to +1.2 V (vs. Hg/HgO), either in 10 CV
cycles up to this potential or by holding for a period of time. No
changes to the surfaces were observable by XRD or SEM, but AFM
of electrodes oxidised using these thermal and electrochemical
oxidation procedures showed that in all cases the surfaces
became a little smoother (Fig. 2). These AFM measurements have
been used to calculate areal specic capacitances based on the
real surface areas of the electrodes.
XPS studies of oxidised electrodes were carried out without
etching to obtain a direct comparison between the surfaces aer the
various surface treatments, focussing on the Ti 2p spectra so that the
data reected the surface composition rather than adsorbed species.
Fig. 3 shows the tte d data and Table 1 lists the compositions in
terms of the various surface species observed. Thermal oxidation
increases the amount of Ti(O,N) and TiO
2
at the surface with
a matching reduction in the TiN content, but these are relatively
subtle changes compared with those observed in electrochemical
oxidation. Both the potential step and the cyclic voltammetry
oxidation processes result in the surface being dominated by species
with binding energies consistent with TiO
2
(or Ti
4+
).
Effect of thermal oxidation on capacitance
Cyclic voltammograms of the as-prepared TiN electrode in
aqueous KOH electrolyte showed a typical capacitive “ letter-
box” shape at high scan rates (Fig. 4), with an exponential decay
behaviour at the current reversal regions that is characteristic of
series resistance. Aqueous KOH has frequently been used in
charge storage with TiN
8–10
and herein was found to give higher
capacitances than acidic or neutral electrolytes. Superimposed
onto this shape the current is seen to gradually tail downwards
at low potential and a broad, low peak is observed on re-
oxidation, with the voltage envelope broadened due to the
reliance of the (presumably Ti
3+
/Ti
4+
) redox chemistry on proton
diffusion through the surface layer. It may also be the case that
the currents are generally higher at low potentials because
reduction of some of the titanium ions at the surface enhances
the electronic conductivity in this region. The electrochemical
Table 1 Binding energies (Ti 2p
3/2
only quoted) and relative percentages of different titanium environments obtained by modelling the 2p region
of the XPS spectra of un-etched TiN foils as-prepared and after application of various oxidative treatments
Sample BE TiN (eV) % BE Ti(O,N) (eV) % BE TiO
2
(eV) %
TiN as prepared
a
455.1 38 457.4 26 458.9 33
TiN TO 300
C 10 h 454.8 21 456.5 37 458.4 42
TiN CVO 10 0 to 1.2 V 453.9 3 456.5 10 458.3 87
TiN PSO 1.2 V 100 s 454.6 3 456.6 8 458.6 89
a
TiN shake-up
43
peak (3%) also included in as-prepared data t.
Fig. 4 (a) Cyclic voltammograms in 1 mol dm
3
KOH (5
th
cycle at
1000 mV s
1
) of as-prepared and thermally oxidised TiN samples, and
(b) areal specific capacitance values at the 5
th
cycle at each of the
sequential scan rates. TO250, TO300, TO350, TO400 and TO450
represent oxidation at 250, 300, 350, 400 and 450
C respectively.
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stability window was originally assessed by running CVs over
various potential ranges to avoid solvent decomposition, and
the 1.1 V range is wider than in many other studies
9,13,36
and
closer to the limits of the water window (0.93 to +0.30 V vs. Hg/
HgO based on standard electrode potentials
51
). At high potential
a small upturn in the current is observed in the positive sweep,
which may contain a small solvent oxidation contribution. The
bulk of this upturn is likely to be due to electrode oxidation
since with further cycling (Fig. S4–S9†) the upturn becomes
minimal. Note that the potential hysteresis in this region of the
CV is narrower than the average across the CV, so this feature
does not result in an increase in the calculated capacitance
relative to that measured with a smaller potential window.
Based on the AFM-measured surface area, the areal specic
capacitance of the as-prepared TiN electrode in the 5
th
cycle at
1000 mV s
1
was 30.7 mFcm
2
(oen such values are quoted
based on the electrode area without a correction for the surface
area of the material, and on this basis the areal specic
capacitance would be 215 mFcm
2
). CVs were recorded at
a series of rates and the capacitance calculated at the middle
scan at each rate, with the largest capacitance of 915 mFcm
2
found in the 5
th
scan at the slowest scan speed of 1 mV s
1
.Aer
further sets of 10 scans each at 5, 10, 50, 100 and 500 mV s
1
(Fig. S4–S9†) the capacitance in the 50
th
scan at 1000 mV s
1
(Fig. S10†) was 35.1 mFcm
2
. A small increase in capacitance
has thus been observed during electrochemical cycling of TiN
within the electrochemical window in which the aqueous 6 mol
dm
3
KOH electrolyte was stable, but this is quite limited over
the 170 cycles at various scan rates carried out here.
Thermal oxidation was carried out at temperatures between
250 and 450
C, over which range the interface resistance of TiN
has previously been shown in organic electrolyte to increase
from a few ohms to megaohms.
50
No increases in capacitance
relative to the as-prepared samples were observed in the ther-
mally oxidised samples during the initial cycles at any scan rate
(Fig. 4). This observation persisted in the slower scans, apart
from in the sample produced at the lowest temperature of
250
C, in which the areal specic capacitance gradually
increased during cycling to a larger capacitance value than the
as-prepared sample of 92.2 mFcm
2
in the nal 1000 mV s
1
cycles. Here a signicant imbalance in the oxidation vs. reduc-
tion charge passed during the cyclic voltammetry (e.g. 0.0012
and 0.0009C in the 5
th
cycle at 5 mV s
1
) suggests some further
oxidation of the surface is linked to the development of further
capacity. By the end of cycling these values come close to
convergence, with a small excess oxidation charge probably
attributable to the observed water oxidation at the highest
potentials. The XPS data in Fig. 3 show that an increase in the
oxide content of the surface is produced by thermal oxidation.
This surface oxide could increase the capacitance by providing
a dielectric layer as found in electrolytic capacitors
1
or by
undergoing redox reactions. It is noteworthy that in the as-
prepared sample and those with thicker oxide layers the log
capacitance vs. log scan rate plot (Fig. 4) has a slope close to
1/2 at low scan rate suggesting capacity is diffusion limited
52
(i.e. it has a signicant redox component reliant on solid state
diffusion) and is atter at high scan rate suggesting mainly
double layer capacitance. The samples produced at 250 and
300
C have a much atter prole overall, presumably due to the
changing capacitance as the surface changes as described
above.
The reduction in capacity and increase in resistance seen in
most thermally oxidised samples could suggest that the thin
(according to XPS) oxide layer produced under these conditions
is dense with low ionic and electronic conductivity (this could
contribute to capacitance by providing a surface dielectric
layer). The development of higher capacitance values during
cycling in the thinnest of these could be due to ion doping of the
layer and further oxidation as the potential is cycled in the
aggressive aqueous KOH electrolyte.
Effect of electrochemical oxidation on capacitance
Initial cyclic voltammetry studies based on the samples used for
XPS and AFM showed that potential step oxidation had a larger
effect on the capacitance than cyclic voltammetry up to the
same potential, presumably because the time spent at the
maximum potential was greater. The XPS and AFM showed both
techniques produced similar surfaces. Hence a more detailed
assessment of the benets of electrochemical oxidation was
carried out using the potential step method.
Potential step oxidations were carried out in 6 mol dm
3
aqueous KOH at 1.2 V (vs. Hg/HgO) for periods between 50 and
200 s, and also for 100 s at potentials between 0.5 and 2.0 V (vs.
Hg/HgO). Cyclic voltammograms (Fig. S11–S17† and 5) were
similar in shape to those observed aer thermal oxidation, and
were recorded over the same series of scan rates (10 scans each at
1000,1,5,10,50,100,500mVs
1
then 100 scans at 1000 mV s
1
).
In the initial scans at 1000 mV s
1
most of the oxidised electrodes
had no increase in areal specic capacitance relative to the as-
prepared TiN. However, in the scans at 1 mV s
1
and in subse-
quent scans signicant changes in capacitance were observed.
The samples produced at 1.2 V vs. Hg/HgO all had higher
capacitance, with 2170 mFcm
2
in the sample oxidised for 150 s
(11 100 mFcm
2
based on geometric electrode area). These
increases could be due to thickening of the oxide layer or to further
ion doping. At fast scan rates aer cycling the increase in capaci-
tance observed in one of the thermally oxidised samples was
observed in all of the potential step oxidised samples. The largest
capacitances at 1000 mV s
1
were found in the samples that had
been produced at less positive potentials, with 83.1 mFcm
2
in the
sample oxidised at 1.0 V vs. Hg/HgO for 100 s (424 mFcm
2
based
on geometric electrode area), increasing from a value of 34.9 mF
cm
2
before cycling. An extended cycling study (5000 cycles) on
one such electrode at 1000 mV s
1
showed that the capacitance
gradually increased by a further 13% over the rst 1200 cycles,
then gradually decayed to a value 3% higher than the value at the
start of the 5000 cycles by the end of the 5000 cycles (Fig. S18†).
Hence these electrodes show good stability.
Galvanostatic charge/discharge measurements were per-
formed on three electrodes aer the cyclic voltammograms
shown in Fig. 5 (CVs were carried out rst to ensure full devel-
opment of the oxide surface). This process was carried out with
an untreated electrode and with electrodes that had originally
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