2019-JMCA-2.5 V Salt-in-water supercapacitors based on alkali type double salt-carbon composite anode

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2.5 V Salt-in-water supercapacitors based on alkali type double
salt/carbon composite anode
Received 00th January 2019,
Accepted 00th January 2019
DOI: 10.1039/x0xx00000x
Published on 15 October 2019. Downloaded on 10/24/2019 4:58:28 AM.
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Tianfeng Qin,a Zhiyuan Xu, a Zilei Wang, a Shanglong Peng, a Deyan He *a
Due to the advantages of safe, cost-effective and environment friendly, aqueous high-voltage carbon/carbon
supercapacitors attract much attention but the poor cycling stability hinders their practical applications. In this work, by
electrodepositing Zn/Zn4SO4(OH)6·4H2O (ATDS) composite on micropore carbon cloth modified with basic functional groups
(OCC) as a negative electrode, we develop an aqueous carbon/carbon supercapacitor operated steady at a voltage as high
as 2.5 V. ATDS composite expands the negative potential window of carbon to -1.7 V vs SCE and thus makes the positive
carbon cloth (CC) operate in the stable potential range to inhibit the capacity degradation of the supercapacitor. A 2.1 V
beaker-shaped supercapacitor of ATDS@OCC//CC presents almost 100 % of capacity retention after 19000 cycles at 18 mA
cm-2. Moreover, the cycled 2.1 V ATDS@OCC//CC supercapacitor exhibits the improved energy density of 9 mWh cm-3 and
better retention of energy density. A 2.5 V ATDS@OCC//CC supercapacitor after 4900 cycles exhibits about 100% of capacity
retention and the ultrahigh volumetric energy density of 20 mWh cm-3 and the mass energy density of 29 Wh kg-1. The novel
strategy may pave the way for developing the safe, environment friendly and high-energy carbon/carbon supercapacitors.
1 Introduction
Rapid consumption of fossil fuel accompanying with the excessive
CO2 emission has already surpassed the self-recycling ability of
atmosphere, leading to a series of environment problems. 1
Correspondingly, clean energy technology develops so rapidly, which
is achieved by combining with effective energy storage systems.
Supercapacitors stand out among many energy storage systems due
to their high power density, fast charge/discharge rate and excellent
long-term cycling stability. 1, 2 However, the commercial
supercapacitors indicate lower energy density of 5 Wh kg-1 than 70100 Wh kg-1 of the traditional lithium ion batteries. 3, 4 Even though
organic electrolytes have high voltage windows of 2.5-2.7 V, their
disadvantages such as high cost and toxic nature make researcher
explore alternative electrolytes. 5 Aqueous electrolyte is extremely
desired to replace organic electrolyte for producing environmentally
friendly and safe energy storage devices.
The lower voltage window of aqueous electrolyte, which limits
enhancement of the energy density for supercapacitors according to
E=CV2/2, can be surpassed by increasing the decomposition potential
of aqueous electrolyte. 3, 6 Compared to KOH and H2SO4 aqueous
electrolyte, neutral electrolyte shows wider potential window due to
the higher di-hydrogen over-potential. 3 Among numerous electrode
materials, porous carbon materials are the most widely concerned
due to their high surface area, excellent electrical conductivity and
low cost, 6-8 and as well both positive and negative electrodes having
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education,
School of Physical Science and Technology, Lanzhou University, Lanzhou 730000,
P.R. China.
E-mail: [email protected].
† Electronic Supplementary Information (ESI) available:
the wider electrode potentials in neutral electrolyte. 9 For example,
F. Béguin et al. demonstrated that carbon/carbon supercapacitor
operates in Na2SO4 electrolyte with the wide voltage window of 1.6
V. 10 E. Frackowiak et al. manifested a 1.8 V of carbon/carbon
supercapacitor in Li2SO4 electrolyte by adjusting pH value at carbon
surface. 11 Gradual capacity decay of the aqueous carbon/carbon
supercapacitors during long-term cycles becomes significantly
pronounced especially at higher voltages close to or beyond 2 V,
accompanying with the increased resistances (e.g., equivalent series
resistance and charge transport resistance) and even the
supercapacitors failure at last. 3, 12, 13 Positive carbon materials
enduring high voltage of supercapacitors show the capacity decay
and resistance increase due to the surface electro-oxidation and the
pore blockage by the decomposers of electrolyte. 3, 13-15 F. Béguin et
al. prevented the positive carbon material from the electro-oxidation
by a controlled oxidation passivation strategy to realize the stable
operation of the aqueous carbon/carbon supercapacitor at 1.9 V
during long-term cycles. 3 But for all this, it is challenging and urgent
to obtain the stable aqueous high-voltage carbon/carbon
supercapacitors by inhibiting the capacity degradation of positive
carbon materials.
Due to enduring a lower potential than that of the positive one
during long-term operation for supercapacitors, expanding the
electrode potential of the carbon materials as negative electrode
attracts much attention in recent years. In this case, the negative
carbon materials would work in the stable potential window and
show no capacity degradation even if the aqueous supercapacitor
operates in high voltage window. Unfortunately, the carbon
materials as negative electrode go through a risk of hydrogen
evolution when the applied potential is beyond the decomposition
potential of water. 13 Thus, effort has been made to inhibit the
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hydrogen evolution by enhancing the over-potential of di-hydrogen.
For example, an electroreduction strategy was employed to
introduce the basic oxygen-containing groups such as C=O and C-O
on the pore-rich carbon to expand the negative electrode potential
to -1.4 V (vs SCE) in Na2SO4. 16 Numerous basic functional groups can
be grafted on the carbon surface by using an aerobic pyrolysis, in
which the potential of the porous carbon negative electrode is also
up to -1.4 V (vs SCE). 17 The expanded potential range is due to 1) the
pore in carbon materials could reversibly store/release hydrogen
from the water reduction/oxidation, 3, 18 and 2) the basic functional
groups could inhibit the evolution of H2. 11, 19 For the carbon materials
as negative electrode, the decomposition of water generates
hydrogen and OH- simultaneously. Based on the above reason 2), the
negative potential range would be expanded if alkali OH- could be
anchored on the porous carbon as negative electrode in some form
such as alkali type double salt.
Recently, L. Dong et al. discovered the irreversible precipitation of
zinc sulfate hydroxide hydrate on Zn anode in aqueous Zn ion hybrid
capacitors due to the evolution of hydrogen gas near Zn anode. 20
The high pH value near Zn electrode results in the precipitation of
Zn4SO4(OH)6·5H2O on Zn anode, which could contribute to the
expanded potential range of porous carbon due to high di-hydrogen
overpotential according to the above reason 2). 21, 22 Besides,
Zn4SO4(OH)6·5H2O would precipitate (OH- is fixed in the
precipitation) only if pH value of the solution near the Zn electrode is
higher than that of ~5.3, and the precipitation would dissolve if pH
value of the solution around the Zn electrode decreases. 23 That is, if
hydrogen was stored in the modified pores by the basic functional
groups, the precipitation would form during the charge process, and
if hydrogen stored in pores could be oxidized into H+, the
precipitation would dissolve during the discharge process. These will
contribute to the realization of aqueous carbon/carbon
supercapacitor operating in high voltage window by introducing the
zinc sulfate hydroxide hydrate on porous carbon as negative
electrode.
Herein, for the first time, the composite of Zn and
Zn4SO4(OH)6·4H2O (ATDS) was grown on the micropore carbon cloth
modified with the basic functional groups (OCC) using a twoelectrode electrodeposition method proposed by ourselves.
Commercial micropore carbon cloth (CC) was first treated by a
modified electrochemical reduction to obtain OCC. Using the
prepared ATDS@OCC as negative electrode and the pristine CC as
positive electrode, an integrated beaker-shaped supercapacitor of
ATDS@OCC//CC in 1 M Na2SO4 aqueous electrolyte presents almost
100 % of the capacity retention after 19000 cycles at 18 mA cm-2
within the voltage window of 2.1 V, much higher than the contrast
OCC//CC capacitor which shows ~36 % of the capacity retention after
13000 cycles at the same test conditions. Surprisingly, the beaker
ATDS@OCC//CC supercapacitor still exhibits outstanding cycling
stability within the voltage windows of 2.3 V and 2.5 V, and the
capacity retention is about 100 % after 4900 cycles at 18 mA cm-2.
Moreover, the cycled 2.1 V ATDS@OCC//CC supercapacitor exhibits
increased energy density and better retention of energy density, the
cycled 2.5 V ATDS@OCC//CC supercapacitor shows the ultrahigh
volumetric energy density and excellent energy retention.
The stable
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voltage window of 2.5 V is comparable to the
reported
high-voltage
carbon/carbon supercapacitors, such as 2.5 V device using water-insalt aqueous electrolytes and 2.7 V device using organic electrolyte.
24, 25 The maximum mass energy density of the cycled 2.5 V
supercapacitor arrives at 29 Wh kg-1 based on the total mass of the
cycled negative and positive electrodes, which is also comparable to
the reported energy densities of carbon/carbon supercapacitors,
e.g., 24 Wh kg-1 for using water-in-salt electrolyte and 27 Wh kg-1 for
using organic electrolyte. 24, 25
2 Experimental
2.1 Fabrication of the modified CC with basic oxygen-containing
functional groups (OCC)
OCC was obtained by a cyclic voltammetry (CV) treatment to CC
(model number LS-CF-220, thickness 264 m. Shanghai Lishuo
Composite Materials Co. LTD, China). A three-electrode
configuration was used for the CV treatment. CC (11 cm2), Pt plate
(11 cm2) and saturated calomel electrode (SCE) were employed as
the working electrode, the counter electrode and the reference
electrode, respectively. Scan rate of 25 mV s-1 for 100 cycles was used
to start the CV treatment together with the subsequent scan rate of
10 mV s-1 for 300 cycles. Two cups of 60 mL saturated K2SO4 (AR.
Chengdu Cologne Chemical Co. LTD, China) aqueous solution were
respectively used as electrolytes for the successive CV treatments.
Finally, the obtained OCC was washed with deionized water and
dried at room temperature for 12 h.
2.2 Growth of the Zn/Zn4SO4(OH)6·4H2O composite on OCC
(ATDS@OCC)
Growth of the ATDS composite on OCC was carried out by a
galvanostatic charge/discharge (GCD) method using a two-electrode
configuration. 1 M Na2SO4 powder (AR. Chengdu Cologne Chemical
Co. LTD) was firstly added into 60 mL aqueous solution stirring for 10
min. Subsequently, 0.05 M NaOH powder (AR. Xi'an Chemical
Reagent Factory, China) was added and stirred for 10 min. Then,
0.076 M ZnSO4 powder (AR. Chengdu Cologne Chemical Co. LTD ) was
added into the above mixed solution and stirred for another 10 min.
The final mixed solution containing Na2SO4, NaOH and ZnSO4 was
used as the electrodeposited solution. The prepared OCC and the
commercial CC were used as the negative and positive electrodes,
respectively. At the current density of 18 mA cm-2, the GCD process
was proceeded for 200 cycles with the voltage window of 0-2.1 V and
subsequent for 300 cycles with the voltage window of 0-2.5 V using
a Landian battery test system (CT3001A). The obtained ATDS@OCC,
in which the mass loading of ATDS is about 7 mg cm-2 (the mass
loading of Zn is 3.20 mg cm-2, and the determination method is
described in Note S2), was directly used as the negative electrode in
the beaker cell. The ATDS@OCC was washed with deionized water
and then dried at room temperature for 12 h for morphology and
structure characterizations. The ATDS@OCC electrode with a size of
4.5×4.5 cm2 was prepared to demonstrate the scalability of the
method (Fig. S1).
2.3 Morphology and structure characterizations
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Fig. 1 Schematics of the prepared processes for electrodes, OCC//CC and ATDS@OCC//CC supercapacitors.
Fig. 2 (a) XRD pattern and (b) Zn2p narrow XPS spectrum of ATDS@OCC.
Morphologies of the samples were observed by field emission
scanning electron microscopy (FE-SEM, Hitachi S-4800) and
transmission electron microscope (TEM, FEI, Tecnai G2 F30).
Nitrogen adsorption/desorption isotherm was carried out at 77 K on
ASAP 2027 specific surface area and pore diameter analyzer, the
samples were degassed at 200 °C for 6 h under vacuum prior to the
measurement. The chemical component of the samples was
analyzed by X-ray photoelectron spectroscope (XPS, PHI-5702, MgK irradiation, 1253.6 eV). The structure phase of the samples was
characterized by X-ray diffraction (XRD, SIEMENS D5000, Cu-K
irradiation, 0.154056 nm).
2.4 Electrochemical characterizations
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Fig. 3 SEM images of (a) OCC and (b, c) ATDS@OCC. (d, e) TEM images of the sheet-shaped ATDS. (f) High-resolution TEM image and (g) the
corresponding fast Fourier transform of ATDS, the red and blue texts represent Zn4SO4(OH)6·4H2O and Zn, respectively. (h) Energy dispersive
X-ray spectrum of ATDS.
Single-electrode electrochemical properties of the samples were
firstly tested in a three-electrode configuration at room
temperature, where 1 M of Na2SO4 aqueous solution was used as
electrolyte, Pt plate as counter and SCE as reference electrode. CV
and GCD of the single electrodes and the beaker cells were tested by
electrochemical station (CHI 660E). Electrochemical impedance
spectroscopy (EIS) measurements were conducted at the open
circuit potential with frequencies ranging from 100 kHz to 10 MHz at
an amplitude of 5 mV. The long-term cycle was carried out using
Landian battery test system (CT3001A). The capacities were obtained
from the recorded GCD curves according to the formula C=IΔt/S,
where I is the constant discharge current density, Δt the discharge
time, S the area of electrode and C the areal capacity. The energy
density was obtained according to E=18.94×CV/7200 (the thickness
of the electrode is 264 m), and the power density was obtained
according to P= E×3600/Δt. The mass energy density was calculated
based on E=CV2/7.2. The OCC//CC and ATDS@OI//CC
supercapacitors (beaker cells) were assembled using a columnar
shell, in which the 100 mL beakers were used with 60 mL 1 M Na2SO4
aqueous electrolyte, and the beakers were sealed using circular
plastic stoppers with two electrode wires.
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3 Results and discussion
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3.1 Characterizations of CC, OCC and ATDS@OCC
Commercial O/N-containing CC with abundant micropores and
large surface area (Fig. S2a and b, Fig. S3, and Table S1) was
electrochemically treated in saturated K2SO4 aqueous solution at
room temperature. The obtained OCC was employed as negative
electrode to assemble the OCC//CC supercapacitors with the pristine
CC, in which the characterizations of OCC were carried out and the
results are shown in Figs. S2c, S3 and S4, and Table S1. The typical
OCC//CC supercapacitor presents a poor cycling stability as shown in
Fig. 4a. Thus, ATDS was in-situ deposited on OCC and used as
negative electrode to assemble ATDS@OCC//CC supercapacitors
with the pristine CC to obtain durable high-voltage supercapacitors.
The prepared processes of the electrodes and supercapacitors are
illustrated in Fig. 1. Satisfactory, the representative ATDS@OCC//CC
supercapacitor shows almost 100 % of the capacity retention.
The typical XRD spectrum shown in Figure 2a is firstly employed to
investigate the structure phase of the prepared ATDS@OCC sample.
The peak at ~25° is attributed to the substrate of OCC. The other
diffraction peaks are indexed to triclinic zinc sulfate hydroxide
hydrate (Zn4SO4(OH)6·4H2O, PDF#44-0673) and hexagonal Zn metal
(PDF#04-0831), indicating that the composite of Zn and
Zn4SO4(OH)6·4H2O was electrodeposited on OCC. The formation of
metallic Zn is due to the reduction of Zn2+ in solution, which could
enhance the electronic conductivity of the composite. The layershaped Zn4SO4(OH)6·4H2O in ATDS consists of water molecules
sandwiched by Zn(OH)2 sheets and the corresponding
crystallographic model, which is beneficial to ionView
transport
in
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DOI: 10.1039/C9TA08490H
electrolyte. 26 XPS spectrum reveals the valence
states of Zn in the
prepared ATDS@OCC. As shown in Fig. 2b, the narrow spectrum of
Zn2p3/2 can be fitted in two peaks at 1021.5 eV for Zn0 and 1022 eV
for Zn2+, demonstrating the existence of Zn and Zn4SO4(OH)6·4H2O in
ATDS. 27, 28
The SEM morphologies of OCC and ATDS@OCC were presented in
Fig. 3a-c. The inset in Fig. 3a indicates that OCC is consists of
interwoven carbon fibers bundles. The carbon fibers show smooth
surface with a diameter of ~ 10 m. After electrodepositing ATDS on
OCC, the carbon fibers become rough surface with a diameter of ~ 40
m. The interwoven nanosheets were observed and vertically
anchored on the carbon fibers from the enlarged image of the carbon
fiber shown in Fig. 3c. TEM characterizations were carried out to
further uncover the micro-morphology of the prepared ATDS
nanosheet with the irregular shape (Fig. 3d). The nanoparticles were
observed on the irregular ATDS nanosheet shown in Fig. 3e. The
lattice fringes with the spacings of 0.27 nm and 0.21 nm can be
observed in Fig. 3f (also shown in Fig. S5 with a more clear lattice
image), which are consistent with the (-107) plane of
Zn4SO4(OH)6·4H2O and the (101) plane of Zn, respectively. The
corresponding fast Fourier transform shown in Fig. 3g indicates the
polycrystalline nature of the prepared ATDS sheets, and the energy
dispersive X-ray spectrum shown in Fig. 3h confirms the existence of
the Zn, S and O elements.
3.2 Electrochemical properties of the 2.1 V beaker-shaped
supercapacitors
Fig. 4 Electrochemical properties of the OCC//CC and ATDS@OCC//CC aqueous supercapacitors with the voltage window of 0-2.1 V. (a) Cycling stability of 13000 cycles
for OCC//CC and 19000 cycles for ATDS@OCC//CC at a current density of 18 mA cm-2. (b) CV curves at a scan rate of 10 mV s-1. (c) GCD curves at 1 and 15 mA cm-2. (d)
Capacity as a function of current density. (e) Ragone plots. (f) Nyquist curves of the two supercapacitors after 13000 cycles.
The CV tests were firstly carried out to evaluate the charge storage
characteristic of OCC with a gradual negative shift of potential as
shown in Fig. S6a. When the potential is below -0.59 V vs SCE, the
water solvent in electrolyte will be reduced, and the double-layer
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would be produced accompanying with the pseudo-capacitance
originated from the reversible sorption of hydrogen in the pores of
carbon cloth. 3, 29 Upward shifts are obviously observed during the
anodic sweep due to the electrochemical oxidation of hydrogen
stored in the pores. 3, 18 Thus, the reduction/oxidation of hydrogen
may contribute the pseudo-capacitance to the total capacitance of
OCC. The basic functional groups such as C-O and C=O on the in-wall
of the pores shown in Fig. S4 can inhibit the evolution of H2, 11, 19
which is benefit for the expansion of the negative potential range. A
short video was recorded to verify the above conclusion whether the
evolution of hydrogen gas was inhibited or not. Correspondingly, the
video (Video 1) and screenshot at 12th second (charging to -1.3 V, Fig.
S7) showed that no bubbles were observed on the surface of black
OCC as negative electrode, indicating the inhibition of hydrogen gas
evolution. The positive CC electrode with high specific surface area is
shown in Fig. S3, and the rich O functional groups could make the
electrode operate in the potential range from 0 to 0.8 V vs SCE as
shown in Fig. S6b. Finally, the potential window can be estimated to
be about 2.1 V for the OCC//CC aqueous supercapacitor.
Even though the OCC//CC aqueous supercapacitor could operate
in the estimated potential range of 2.1 V, the long-term cycling
stability should be evaluated firstly. As shown in Fig. 4a,
unfortunately, the aqueous supercapacitor of OCC//CC cycled in the
voltage window of 2.1 V presents a rapid capacity degradation, just
with about 36 % of the capacity retention after 13000 cycles at 18
mA cm-2. In contrast, the aqueous supercapacitor of ATDS@OCC//CC
shows an excellent cycling stability with about 100 % of the initial
capacity retention after 19000 cycles at the same current density.
The electrochemical properties of the two supercapacitors after
long-term cycles were evaluated within the voltage window of 2.1 V.
Compared to OCC//CC, the CV curve of ATDS@OCC//CC in Fig. 4b
shows the typical rectangular shape and the large curve closure area,
indicating the ideal capacitive behaviors and the outstanding charge
storage ability. The GCD curves of ATDS@OCC//CC shown in Fig. 4c
exhibit the longer charge/discharge time and the smaller voltage
drop than those of OCC//CC. At a current density of 1 mA cm-2, the
capacity of ATDS@OCC//CC is up to 1638 mC cm-2, higher than that
(1236 mC cm-2) of OCC//CC as displayed in Fig. 4d. When the current
density increases to 15 mA cm-2, the aqueous supercapacitor of
ATDS@OCC//CC still delivers a capacity as high as 1206 mC cm-2,
significantly higher than that of OCC//CC. Ragone plots shown in Fig.
4e were used to assess the energy storage ability of the two
supercapacitors. Compared to OCC//CC, at a power density of 20 mW
cm-3, the aqueous supercapacitor of ATDS@OCC//CC shows the
higher volumetric energy density of 9 mWh cm-3 and the better
energy retention, superior to those of the reported literatures shown
in Fig. S17. The results demonstrate that the long-term cycling
performance of the aqueous high-voltage supercapacitor is
dramatically improved by depositing ATDS on OCC. Nyquist plots
shown in Fig. 4f reveal the resistance variation of the two
supercapacitors after long-term cycles. The linear components of
both the ATDS@OCC//CC and OCC//CC supercapacitors show an
inclination angles near 90°, demonstrating an ideal capacitive
behavior. For ATDS@OCC, the equivalent series resistance Res is 7.10
ohm, smaller than 9.98 ohm for OCC//CC. In addition,
ATDS@OCC//CC only shows a negligible semicircle compared with
the large semicircle of OCC//CC, indicating the smaller charge
transfer resistance Rct. It can be seen that the rapid
capacity
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degradation of OCC//CC is due to the increased
Res and Rct. The
deposition of ATDS on OCC inhibits the increases of Res and Rct.
The electrochemical tests of the cycled electrodes were carried
out in order to find out the capacity degradation reason of OCC//CC.
For the two negative electrodes cycled in the supercapacitors, the
electrochemical performances basically show no difference as shown
in Fig. S8. That is to say, the origin of the capacity degradation for
OCC//CC is not the cycled negative electrodes. Successively, the
electrochemical properties of the cycled CC positive electrodes in the
two supercapacitors were tested as shown in Fig. S9, in which CC
(OCC//CC) represents the cycled CC positive electrode in OCC//CC.
The CV curve of CC (OCC//CC) shown in Fig. S9a presents a fusiform
shape with smaller integral area compared to that of CC
(ATDS@OCC) with the rectangular shape. The areal capacity of CC
(OCC//CC) only delivers 273 mC cm-2 at a current density of 3 mA cm2, significantly lower than that (1098 mC cm-2) of CC
(ATDS@OCC//CC) at the same current density (Fig. S9b). At the
current density of 5 mA cm-2, the areal capacity of CC (OCC//CC)
decreases to 8 mC cm-2, far less than that (1040 mC cm-2) of CC
(ATDS@OCC//CC). Nyquist plots were employed to further reveal the
resistance variations of the cycled CC electrodes as shown in Fig. S9c.
Res and Rct of CC (OCC//CC) are 7.14 and 41.24 ohm, respectively,
much larger than those of CC (ATDS@OCC//CC). Thus, it can be
concluded that the rapid capacity degradation during cycles and the
resistance increases after 13000 cycles for the OCC//CC
supercapacitor are due to the gradual capacity degradation and the
resistance increases of the CC positive electrodes during the frequent
charges/discharges. Positive carbon material enduring the high
voltage of supercapacitor shows a capacity decay and resistance
increases due to the electro-oxidation and the pore blockage by the
decomposers of electrolyte. 3, 13-15
Note that the deposition of ATDS on OCC can dramatically enhance
the cycling stability of the 2.1 V aqueous carbon/carbon
supercapacitors by avoiding the capacity gradation of CC positive
electrode as shown in Figs. 4, S8 and S9. The large amount of ATDS
(~ 7 mg cm-2) as alkali type double salt on OCC inhibits the evolution
of H2 to expand the potential range as described in Introduction. 11, 19
In this case, the carbon materials as positive electrodes would work
within the stable potential window and thus show no capacity
degradation when the aqueous supercapacitor works in high voltage
window. H2O molecules in alkali type double salt determines the
spacing between Zn(OH)2 sheets and the corresponding
26
crystallographic
model.
Like
as
Zn4SO4(OH)6·5H2O,
Zn4SO4(OH)6·4H2O in ATDS would precipitate (OH- is fixed in the
precipitation) only if pH value of the solution near the negative
electrode is higher than ~5.3. The precipitation would dissolve if pH
value of the solution around the negative electrode decreases
according to Eq. (1). 23
4Zn2++6OH-+SO42-+xH2O ⇆ Zn4SO4(OH)6·xH2O
(1)
For ATDS@OCC in the charging ATDS@OCC//CC supercapacitor, due
to the water reduction generating hydrogen and OH-, hydrogen is
stored in micropores of OCC as shown in Fig. S6a, 3, 18 and the
increased pH value (OH-) results in the precipitation of
Zn4SO4(OH)6·4H2O on OCC. The measured pH value of 1 M Na2SO4
electrolyte is about 6, which guarantees the precipitation of
Zn4SO4(OH)6·4H2O on OCC. For ATDS@OCC in discharging
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ATDS@OCC//CC supercapacitor, the hydrogen stored in micropores
of OCC is oxidized into H+ as shown in Fig. S6a. 3, 18 SEM and XRD
characterizations of the cycled ATDS@OCC in ATDS@OCC//CC were
conducted to verify the reversible precipitation/dissolution based on
Eq. (1). Compared to the SEM images of the as-prepared ATDS@OCC
shown in Fig. 3b and c, the larger and thicker nanosheets in Fig. S10
were observed on the surface of the cycled ATDS@OCC, which is due
to the dissolution/precipitation based on Eq. (1). For the XRD pattern
shown in Fig. S11, diffraction peaks are indexed to Zn (PDF#04-0831)
and Zn4SO4(OH)6 (PDF#35-0910) as well as 6Zn(OH)2ZnSO4·4H2O
(PDF#11-0280), indicating the phase transition during the cycles.
However, there still exists Zn4SO4(OH)6, in which the absence of H2O
molecules decreases the spacing between the Zn(OH)2 sheets and
the corresponding crystallographic model. 26 The alkali type double
salt of Zn4SO4(OH)6 and 6Zn(OH)2ZnSO4·4H2O on OCC also inhibits the
evolution of H2 and thus expand the potential window as described
in Introduction. 11, 19
The cycling stability during the long-term
cycles was significantly enhanced by electrodepositing ATDS on OCC
as shown in Fig. 4a, where if the ATDS deposition contributes
the
View Articleto
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DOI:
10.1039/C9TA08490H
capacity or not. The control experiments were
carried
out to uncover
the doubt. Oxygen-modified carbon cloth with low surface area (HLCC) was used as negative electrode to assemble the beaker-shaped
supercapacitor of HL-CC//CC as shown in Fig. S12. ATDS@HL-CC//CC
was obtained by replacing OCC with HL-CC. As shown in Fig. S13a,
compared to HL-CC//CC, the CV curve of ATDS@HL-CC//CC exhibits
a lower polarization current, indicating that the deposition of ATDS
could decrease the water oxidation to some extent due to the
expanded potential window of ATDS@OCC. ATDS@HL-CC//CC shows
a negligible discharge time (Fig. S13b) and ~100 % of capacity
retention (Fig. S13c). Thus, the electrodeposition of ATDS on OCC can
dramatically enhance the cycling stability of 2.1 V aqueous
carbon/carbon supercapacitors by expanding the potential window
of the negative OCC to avoid the capacity gradation of the positive
CC.
3.3 Electrochemical properties of ATDS@OCC//CC with the voltage
windows beyond 2.1 V
Fig. 5 Electrochemical properties of the ATDS@OCC//CC aqueous supercapacitors with the voltage windows of 2.3 V, 2.5 V, 2.7 V and 3 V. (a) Cycling stability at the
current density of 18 mA cm-2. (b) CV curves at the scan rate of 10 mV s-1. (c) Capacity as a function of current density. (e) Ragone plots after 4900 cycles.
Inspired by the significantly enhanced cycling stability of
ATDS@OCC//CC mentioned above, the cycling stabilities of
ATDS@OCC//CC at the voltage windows beyond 2.1 V were
examined. Fig. 5a shows the cycling profiles at a current density of
18 mA cm-2 with different voltage windows of 2.3 V, 2.5 V, 2.7 V and
3 V. Compared to the 2.7 V and 3 V supercapacitors, the 2.3 V and
2.5 V devices present excellent capacity retention of about 100%.
After 4900 cycles, 1011 mC cm-2 and 1072 mC cm-2 of capacity are
retained for the 2.3 V and 2.5 V devices, respectively. Previous
literatures reported that the capacity degradation of the aqueous
high-voltage carbon supercapacitors during long-term cycles is
derived from the capacity decay and electrochemical oxidation of the
positive carbon material. 3, 14 One can conclude that a more severe
surface oxidation of positive carbon electrodes occurs when the
potential of 2.5 V is applied on the aqueous carbon supercapacitor,
resulting in the severe loss of the capacitance in the initial cycles.
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Thus, water-in-salt electrolyte is considered to inhibit the
electrochemical oxidation of the positive carbon electrodes due to
the reduced number of free water molecules and the decreased
chemical activity of H2O. 30, 31 The further work will be done in the
following work. The electrochemical performances were tested for
the cycled supercapacitors with the voltage windows of 2.3 V, 2.5 V,
2.7 V and 3 V. In Fig. 5b, CV curves of the supercapacitors cycled in
the voltage windows of 2.3 V, 2.5 V and 2.7 V show the typical
rectangular shape, demonstrating the ideal capacitive behaviors.
When the voltage window limit of the supercapacitor is up to 3 V, CV
curve shows a fusiform, indicating the poor capacitive behavior. GCD
curves of the ATDS@OCC//CC supercapacitor cycled at the voltage
windows beyond 2.1 V in Figs. S14 and S15 show the triangle waves.
And the cycled 2.5 V supercapacitor (Fig. 5c) demonstrates excellent
capacities compared to the other cycled devices, especially the 2.7 V
and 3 V devices. Ragone plots were plotted in Fig. 5d to evaluate the
energy storage ability of the cycled supercapacitors. Compared to
the other cycled supercapacitors, the cycled 2.5 V supercapacitor
shows the highest volumetric energy density of 20 mWh cm-3 at a
power density of 24 mW cm-3, which is higher than those of the
reported literatures as shown in Fig. S17. Especially, Zhao at al.
reported a solid-state EDLC using a renewable mesoporous cellulose
membrane sandwiched with activated carbon as an electrode,
demonstrating a high capacitance and excellent cycling stability.32
Chen et al. presented a biodegradable supercapacitor using low
tortuosity and all-wood membrane as a separator sandwiched with
activated wood carbon anode and MnO2/wood carbon cathode.33
The works could represent a popular trend for the renewable energy
storage with biocompatibility and provide an inspiration for our
future work. The stable voltage window of 2.5 V is comparable to the
reported high-voltage carbon/carbon supercapacitors, such as 2.5 V
devices using water-in-salt aqueous electrolytes and 2.7 V devices
using organic electrolyte. 24, 25 The maximum mass energy density of
the 2.5 V supercapacitor arrives at 29 Wh kg-1 based on the mass (37
mg) of the cycled negative and positive electrodes, which is also
comparable to the reported energy densities of carbon/carbon
supercapacitors, e.g., 24 Wh kg-1 for using water-in-salt and 27 Wh
kg-1 for using organic electrolyte. 24, 25 And OCC//CC was also cycled
at the voltage window of 2.5 V with the same current density to
manifest the advantages as shown in Fig. S16. Compared to the
cycled OCC//CC, the cycled ATDS@OCC//CC shows better
electrochemical properties, such as the significantly improved cycling
stability, the increased capacity and energy density as well as the
decreased resistances. Also, CV curves of the cycled ATDS@OCC in
ATDS@OCC//CC were recorded in Fig. S18. It indicates that
ATDS@OCC could charge/discharge with a wide potential window
from 0 V to -1.7 V (Fig. S18d). The results further demonstrate the
great advantage of ATDS in-situ deposited on OCC enhancing the
long-term cycling performance of the aqueous high-voltage
supercapacitors.
The hydrogen evolution seems severe when the applied voltage is
larger than 2 V as shown in Fig. 5b. Among the several voltage
windows, 2.5 V was chosen as an example to elucidate whether the
hydrogen evolution occurs or not. CV curves of the cycled
ATDS@OCC in ATDS@OCC//CC and the cycled OCC in OCC//CC in the
voltage window of 2.5 V were recorded as shown in Fig. S19.
Compared to the cycled OCC in OCC//CC, CV curve of the cycled
ATDS@OCC in ATDS@OCC//CC shows the significantly
decreased
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10.1039/C9TA08490H
current response in the scan range from -0.8DOI:
to -1.5
V, demonstrating
an inhibition of the hydrogen evolution to some extent. H2O
molecules decompose into H+ and OH- due to the high voltage
window, where H+ is reduced into H atom adsorbed to the surface of
negative electrode through Volmer reaction (H+ + e- → H*, * is an
adsorption site on the electrode). 34, 35 Then, H2 is produced from the
electrode surface through Heyrovsky reaction (H* + H+ + e- → H2 + *)
or Tafel reaction (2H* → H2 + *). 34, 35 In our case, the theoretical
potential in 1 M Na2SO4 aqueous solution is -0.59 V vs SCE, e.g., the
reduction potential of water (the calculation is illustrated in Note S1).
As the applied potential is below -0.59 V vs SCE, the water solvent in
electrolyte would be reduced through Volmer reaction. For CV curves
shown in Fig. S19, the obvious down-warping of CV curves is
observed during the cathodic sweep from 0 to -1.5 V, which is
corresponding to Volmer reaction that H+ is reduced into H atom and
stores into the pores of OCC by adsorption. 3, 29 Moreover, the
upward of CV curves at about -0.1 V appears during the anodic sweep
from -1.5 to 0 V, demonstrating the oxidation of hydrogen. 3, 18 Much
effort has been made to inhibit the hydrogen evolution by
introducing the basic oxygen-containing functional groups (e.g., C-O
and C=O) on the surface of the electrode according to Le Chatelier
principle (2H2O + 2e- ↔ H2 + 2OH-, OH- represent the basic functional
groups).11 Due to ATDS on OCC, the more basic ATDS inhibit the
hydrogen evolution (Heyrovsky reaction or Tafel reaction) according
to Le Chatelier principle.
4 Conclusions
It is for the first time that, by electrodepositing Zn/Zn4SO4(OH)6·4H2O
on the micropores carbon cloth modified with basic functional
groups, we successfully achieved the aqueous high-voltage
carbon/carbon supercapacitors stably working at the voltage
windows of 2.1 V, even 2.3 V and 2.5 V. The alkali type double salt of
Zn/Zn4SO4(OH)6·4H2O expands the potential window of OCC to -1.7
V vs SCE, which makes the positive CC electrode operate in a stable
potential range to inhibit its capacity degradation and the
corresponding supercapacitor cycling in high voltage windows. A 2.1
V beaker-shaped supercapacitor of Zn/Zn4SO4(OH)6·4H2O@OCC//CC
with 1 M Na2SO4 aqueous electrolyte presents almost 100 % of the
capacity retention after 19000 cycles at a current density of 18 mA
cm-2, much higher than that (~36 % after 13000 cycles) of OCC//CC.
Surprisingly,
the
beaker
supercapacitor
of
Zn/Zn4SO4(OH)6·4H2O@OCC//CC still exhibits outstanding cycling
stability within the voltage windows of 2.3 V and 2.5 V, i.e., about
100 % of the capacity retention after 4900 cycles at a current density
of
18
mA
cm-2.
Moreover,
the
cycled
2.1
V
Zn/Zn4SO4(OH)6·4H2O@OCC//CC supercapacitor exhibits the
improved energy density of 9 mWh cm-3 as well as better retention
of energy density. The cycled 2.5 V Zn/Zn4SO4(OH)6·4H2O@OCC//CC
supercapacitor shows an ultrahigh volumetric energy density of 20
mWh cm-3 and a high mass energy density of 29 Wh kg-1. The novel
strategy proposed in this work may pave the way for the
development of the safe, environment friendly and high-energy
carbon/carbon supercapacitors.
Acknowledgement
8 | Journal of Materials Chemistry A, 2016, 00, 1-3
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This work was supported by the National Natural Science Foundation
of China [Grant No. 11674138] and the Fundamental Research Funds
for the Central Universities [No. lzujbky-2019-it23].
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