Ternary active site Co3O4/NiO/MnO2 electrode with enhanced capacitive performances

Article abstract of DOI:10.1016/j.jallcom.2015.10.006

Composite electrodes with ternary active sites Co₃O₄/NiO/MnO₂ are employed to obtain higher capacitance than single component electrode. The dependence of capacitive performances on the composition in composite oxide electrodes is investigated. Binary Co₃O₄/NiO composite electrodes display better capacitive performances than pure Co₃O₄ and NiO single ones and the optimal Co₃O₄/NiO ratio is 2:3. Ternary Co₃O₄/NiO/MnO₂ composites are superior to Co₃O₄/NiO binary composite electrodes due to another redox reaction different from Co₃O₄ and NiO by introducing a third active material MnO₂ into the electrode. When the (Co₃O₄ + NiO)/MnO₂ ratio is 3:2, the ternary composite electrode delivers the discharge specific capacitance of 549 F g^-1 at 0.5 A g^-1, almost twice of Co₃O₄ and NiO single electrodes. Electrochemical impedance spectra show it comes from the decrease of the charge-transfer resistance and the total inner resistance.

Full text of DOI:10.1016/j.jallcom.2015.10.006

Accepted Manuscript
Ternary active site Co O /NiO/MnO electrode with enhanced capacitive
3
4
2
performances
Linghao Su, Yu Wang, Yongfang Sha, Mingwei Hao
PII:
S0925-8388(15)31262-7
10.1016/j.jallcom.2015.10.006
JALCOM 35561
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 8 August 2015
Revised Date: 1 September 2015
Accepted Date: 1 October 2015
Please cite this article as: L. Su, Y. Wang, Y. Sha, M. Hao, Ternary active site Co O /NiO/MnO
2
3
4
electrode with enhanced capacitive performances, Journal of Alloys and Compounds (2015), doi:
0.1016/j.jallcom.2015.10.006.
1
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ACCEPTED MANUSCRIPT
Ternary active site Co O /NiO/MnO electrode with
3
4
2
enhanced capacitive performances
Linghao Su, Yu Wang, Yongfang Sha, Mingwei Hao
College of Chemistry & Pharmaceutical Science, Qingdao Agricultural University,
Qingdao, Shandong, 266109, PR China
Tel.: +86-532-86080442;
Fax: +86-532-86080213;
E-mail address: qauslh@163.com
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Abstract
Composite electrodes with ternary active sites Co O /NiO/MnO are employed to
3
4
2
obtain higher capacitance than single component electrode. The dependence of
capacitive performances on the composition in composite oxide electrodes is
investigated. Binary Co O /NiO composite electrodes display better capacitive
3
4
performances than pure Co O and NiO single ones and the optimal Co O /NiO ratio
3
4
3
4
is 2:3. Ternary Co O /NiO/MnO composites are superior to Co O /NiO binary
3
4
2
3
4
composite electrodes due to another redox reaction different from Co O and NiO by
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4
introducing a third active material MnO2 into the electrode. When the
Co O +NiO)/MnO ratio is 3:2, the ternary composite electrode delivers the
(
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2
-1
-1
discharge specific capacitance of 549 F g at 0.5 A g , almost twice of Co O and
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4
NiO single electrodes. Electrochemical impedance spectra show it comes from the
decrease of the charge-transfer resistance and the total inner resistance.
Keywords: Composition dependence; Supercapacitor; Ternary oxides; Composite
electrode.
1
. Introduction
In the past decades, supercapacitor has become one of the hottest spots of intense
research in the energy-storage field due to the various advantages like high power
density, long cycle life and short charging time. Amongst the three typical active
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electrode materials (porous carbon, transition metal oxide and conducting polymer)
for supercapacitor, hydrous ruthenium oxide (RuO ) is regarded as the most
2
promising one with excellent electrical conductivity, reversible redox reactions and
large specific capacitance. Unfortunately the high cost, toxicity and scarcity prevent it
from being applied extensively. In order to overcome these shortcomings, a lot of
inexpensive metal oxides (or hydroxides) such as MnO , NiO, Co O , etc, were
2
3
4
exploited but these alternative materials have no capacitive features as good as RuO2.
Meanwhile, composite oxide electrodes are recently constructed with binary or
ternary active materials because of more active sites and superior conductivity
compared to the single-component oxides.
-1
Owing to nickel oxide’s high theoretical specific capacitance (about 2573 F g )
and cobalt oxide’s high redox activity and reversibility, Co O /NiO composite
3
4
electrode[1-4] is most often fabricated and the electrochemical performances are
widely investigated. Despite different morphology (ultrathin nanosheet, nanorod array
or mesoporous architectures), the composite exhibits synergistic effect of pure oxides,
such as improved gravimetric capacitance, charge-discharge cycle stability and rate
capability even at high current densities. On the other side, nickel-cobalt oxides
composite has almost the same electrochemical potential as one-component oxides
because of the common redox reactions related to M-O/M-O-OH (M=Co or Ni).
Recently some researchers[5-9] have attempted to construct ternary Co-Ni-Al
composite electrode but Al element has no electrochemical activity and just acts as
structure stabilizer. Therefore it is extremely significant to fabricate novel ternary
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oxide composites via introducing a third active element to improve the electrical
conductivity and maximum the utilization of electrode materials.
Among various transition metal oxides, MnO is one of the most appealing
2
electrode materials for supercapacitor due to its high theoretical specific capacitance
−
1
(
nearly 1100 F g ) [10], low cost, abundance, and environmentally friendly nature.
Recently a few works[11-14] are reported on constructing ternary Co-Ni-Mn oxides
composite electrode for supercapacitor. Zhang et al[11] found nanostructured
Mn–Ni–Co oxide composites prepared by thermal decomposition exhibit good
electrochemical characteristics. Yuan[12] reported ternary Co–Ni–Mn oxide
nanowires via a hydrothermal route manifest excellent cycling stability at high rates.
Majid[14] synthesized Co–Ni–Mn oxide composite electrode on stainless steel by
galvanostatic electrodeposition method. Li et al[13] have prepared Co–Ni–Mn ternary
oxide nanowire array on nickel foam.
Although the investigations on Co–Ni–Mn oxide composite electrode for
supercapacitor have been conducted, to the best of our knowledge, there is no
systematic evaluation on the dependence of Co/Ni/Mn ratio on the capacitive
performance so far. In this paper, our interest is focused on the role of component in
the Co–Ni–Mn oxide composite. In addition, we notice that no Co–Ni–Mn solid
solution is formed in the most of previous works no matter what synthetic approaches
are applied, i.e., separate Co O , NiO, and MnO phase in the Co–Ni–Mn oxide
3
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2
composite, except that one paper[13] reported a spinel structure phase after annealing
the precursor film at 350 °C in argon gas. So in the present work, Co O , NiO, and
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MnO phase are independently synthesized and the composite electrode is made by
2
mixing the three oxides with the intention to have a good understanding on the effect
of Co/Ni/Mn ratio on the capacitive performances.
2
. Experimental
Co O and NiO materials were prepared by heating the precursors of cobalt
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4
hydroxide and nickel hydroxide at 300 ºC for 2 h, respectively. The precursor cobalt
hydroxide was synthesized as described in the previous report[15]. In a typical
procedure, 0.9515 g CoC1 ·6H O, 1.1706 g NaCl and 3.3085 g HMT were dissolved
2
2
in a mixture of deionized water (360 mL) and ethanol (40 mL). Then the mixed
solution was heated at 90 °C for 1 h under magnetic stirring. After filtering, washing
with distilled water and drying, the blue solid was collected. The precursor nickel
hydroxide was synthesized using the same procedure except replacing CoCl with
2
NiCl2.
MnO was synthesized by a hydrothermal method according to the literature[16].
2
A mixed solution was formed by adding 0.9060 g KMnO and 2 mL HCl (37%) into
4
8
0 mL deionized water under continuous stirring for 20 min at room temperature, then
transferred into a Teflon-lined stainless steel autoclave of 100 mL and heated at 100
ºC for 12 h. Finally, a brown powder was collected after filtering, washing with
distilled water and drying at 60 ºC.
The structure and morphology of Co O , NiO, and MnO materials were
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2
characterized with D8 advance X-ray diffractometer using Cu Kα radiation (λ =
0
.15406 nm) and JEOL 7500F scanning electron microscopy operating at 10 kV,
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respectively.
The electrochemical performances in 6 M KOH aqueous solution were studied
with a CHI660D electrochemical workstation in a conventional three-electrode
2
system equipped with a Pt foil of 1 cm and a saturated calomel electrode (SCE) as
counter and reference electrodes, respectively. The work electrode was prepared by
mixing active material (Co O , NiO or MnO ), acetylene black and
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2
polyterafluoroethylene with a weight ratio of 75: 15: 10 uniformly, coating and
2
pressing on a piece of foamed nickel grid of about 1 cm (15 Mpa), respectively.
Binary Co O /NiO composite electrodes were fabricated using a similar procedure
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4
just varying Co O /NiO weight ratio from 1:4 to 4:1 while keeping the total mass of
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4
active materials unchanged (10 mg). The ternary Co O /NiO/MnO composite
3
4
2
electrodes were constructed by varying the (Co O +NiO)/MnO ratio while keeping
3
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2
the total mass of active materials (10 mg) and the Co O /NiO ratio (2:3) in the
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4
electrodes unchanged. Electrochemical impedance spectra were measured at 0.4 V vs.
SCE, varying frequency from 0.01 to 100 000 Hz with an AC signal of 0.005 V.
3
. Results and discussion
The all as-prepared materials were characterized by X-ray powder diffraction and
field-emission scanning electron microscopy. In Fig. 1(a), the XRD pattern is assigned
to cubic NiO phase (JCPDS No.: 89-5881). From the SEM image shown in Fig. 1(b),
it is found the NiO material is composed of microsphere/nanoparticle hierarchical
nanostructures. The formation of Co O is confirmed by the XRD spectrum in Fig.
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o
o
o
o
o
1
(c), in which the diffractions at the 2θ values of 31.1 , 36.8 , 44.8 , 59.1 and 65.3
are ascribed to the (220), (311), (400), (511) and (440) planes of cubic phase (JCPDS
No.: 78-1969), respectively. The Co O material reveals irregular sheet-like
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4
morphology (Fig. 1(d)). As for MnO , its XRD pattern in Fig. 1(e) can be perfectly
2
indexed to tetragonal phase (JCPDS No.: 72-1982). The SEM image in Fig. 1(f)
exhibits that the MnO material has 1-D tube-like morphology, typically 2-3 ꢀm in
2
length and below 100 nm in diameter. Interestingly, it is clearly observed in the inset
that each MnO tube has a square open end.
2
In the beginning, the electrochemical performances of one-component oxide were
investigated by cyclic voltammetry and galvanostatic charge/discharge tests. Fig. 2(a)
provides the CV curves of NiO, Co O and MnO single electrodes at a scan rate of 2
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2
-1
mV s . NiO and Co O display similar profiles and a pair well-defined redox waves
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between 0 to 0.5 V vs SCE, corresponding to the reaction of M-O/M-O-OH (M=Co or
Ni). As far as MnO is concerned, its CV profile is different from NiO and Co O .
2
3
4
Its cathodic and anodic peaks appear at 252 and 418 mV and the peak current is lower
than the first two. Meanwhile the current in the whole potential range from -0.4 to
0
.55 V is more uniform, indicating better pseudocapacitance property.
The galvanostatic charge/discharge test confirms the difference of capacitance
property among the three materials. In Fig. 2(b), NiO and Co O manifest almost
3
4
same charge/discharge curves with a clear potential plateau while the
charge/discharge curves of MnO are more linear except for the IR drop which comes
2
from the poor electrical conductivity of the electrode and is also observed in the cases
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of NiO and Co O , indicating that the electrochemical nature of MnO is
3
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2
pseudocapacitance mechanism while NiO and Co O prefer to battery property. For
3
4
clarity, Tab. 1 lists the discharge specific capacitance of NiO, Co O and MnO single
3
4
2
electrodes at various current density. The specific capacitance of NiO is comparable
to Co O ’s and both are larger than MnO .
3
4
2
Based on the above investigation, Co O /NiO composite electrodes were
3
4
fabricated to understand their synergistic effect. Keeping the total mass of active
materials unchanged, the Co O /NiO weight ratio varied from 1:4 to 4:1. Seen from
3
4
Fig. 3(a), all Co O /NiO composite electrodes display similar CV profiles, in which
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4
the peak currents for the ratio of 2:3 are the largest. Compared to NiO and Co O
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single electrodes, the difference between anode and cathode peak potentials reduces,
suggesting better electrochemical reversibity for Co O /NiO composite electrodes.
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The typical constant current charge/discharge curves are compared in Fig. 3(b).
Among the five Co O /NiO ratios, the case at the ratio of 2:3 displays the longest
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4
charge and discharge times. In Tab. 2, the specific capacitance for the case at the
Co O /NiO ratio of 2:3 is larger than the others. Thus, according to these facts, a
3
4
conclusion can be drawn that the optimal ratio for binary Co O /NiO composite is 2:3.
3
4
The ternary Co O /NiO/MnO composite electrodes were constructed by varying
3
4
2
the (Co O +NiO)/MnO ratio while keeping the total mass of active materials (10 mg)
3
4
2
and the Co O /NiO ratio (2:3) in the electrodes unchanged. Fig. 4(a) shows the CV
3
4
-1
profiles of the ternary Co O /NiO/MnO composite electrodes at 2 mV s . It is found
3
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2
the (Co O +NiO)/MnO ratio exerts great effect on the anodic and cathodic current of
3
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2
the CV curves. Increasing the amount of MnO in the ternary Co O /NiO/MnO
2
2
3
4
composite electrode, the anodic and cathodic current increases. When the
Co O +NiO)/MnO ratio reaches to 3:2, the result is the best. However further
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2
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increase results in the decrease of the current due perhaps to the reduce of the amount
of Co O and NiO. Another feather in the CV curves is the two anodic peaks. For
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4
clarity, the CV curve of binary Co O /NiO electrode with the ratio of 2:3 is also
3
4
plotted in Fig. 4(b). For binary Co O /NiO electrode, only a pair of redox peaks are
3
4
detected, which arises from the common redox potential related to the similar
reactions of M-O/M-O-OH (M=Co or Ni). For the ternary Co O /NiO/MnO
3
4
2
composite electrode, another anodic peak appears, confirming MnO ’s taking part in
2
the electrode process. Namely, the ternary Co O /NiO/MnO composite electrode
3
4
2
exhibits synergistic effect of pure oxides.
The influence of (Co O +NiO)/MnO ratio is also observed in electrochemical
3
4
2
impedance spectra (EIS). In Fig. 5, all EIS spectra displays common shapes: two
quasi-semicircles in the high-frequency and low-frequency regions. The diameter and
the intersection with the real axis of the quasi-semicircle in the high-frequency region
represent charge-transfer resistance (R ) related to the process occurring at the
ct
electrode/electrolyte interface and the total inner resistance (R ) including bulk
i
electrolyte, the electrode and the connection between current collector and the
electrode, respectively. The quasi-semicircle in the low-frequency region can be
described by the capacitive component of the diffusion process of ions in bulk
electrolyte. The transition in the mid-frequency region is attributed to the Warburg
element. The sub-real-axis part in the high-frequency region stands for an inductive
impedance. Seen from Fig. 5, when (Co O +NiO)/MnO ratio is 3:2, both R and R
3
4
2
ct
i
are the smallest among the five ternary composite electrodes, indicating the reduced
resistance and more facile charge transfer.
The constant current charge/discharge test of the ternary composite electrode with
-1
the (Co O +NiO)/MnO ratio of 3:2 was carried out at different current. At 0.5 A g ,
3
4
2
the ternary composite electrode delivers the discharge specific capacitance of 549 F
-1
-1
-1
g . Increasing the current to 1.5 and 2.0 A g , the C value is 431 and 422 F g ,
s
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respectively, almost twice of Co O and NiO single electrodes (see Tab. 1). Fig. 6
3
4
provides the long-term charge/discharge cyclibility of the ternary composite electrode
-1
at the current of 2.0 A g . After continuous 600 cycles, its capacitance retention is
7
6%, lower than those of Co O and NiO single electrodes (95% and 80%), but the C
3
4
s
-1
-1
value is still 322 F g , much higher than the last two (207 and 194 F g ).
4
. Conclusions
The electrochemical capacitance of oxides electrode depends on the composition.
For binary Co O /NiO electrodes, their capacitive performances are better than Co O
3
4
3
4
or NiO single electrode and the optimal ratio of Co O /NiO is 2:3. When introducing
3
4
MnO to construct ternary Co O /NiO/MnO composite electrodes, more active sites
2
3
4
2
and novel redox reaction are provided. At the (Co O +NiO)/MnO ratio of 3:2, the
3
4
2
ternary composite electrode delivers twice capacitance of pure Co O and NiO single
3
4
electrodes.
Acknowledgement
This work was supported by National Training Program of Innovation and
Entrepreneurship for Undergraduates (No. 201310435008) and Scientific Research
Foundation for Advanced Scholars of Qingdao Agricultural University (No.
6
31015).
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Figure captions and Tables
Fig. 1 The XRD patterns and SEM images of the as-prepared materials. (a) and (b):
NiO; (c) and (d): Co O ; (e) and (f): MnO .
3
4
2
Fig. 2 (a) The cyclic voltammograms and (b) galvanostatic charge/discharge curves of
the NiO, Co O and MnO single electrodes.
3
4
2
Fig. 3 (a) The cyclic voltammograms and (b) galvanostatic charge/discharge curves of
the Co O /NiO composite electrodes.
3
4
Fig. 4 (a) The cyclic voltammograms of the ternary Co O /NiO/MnO composite
3
4
2
electrodes; (b) the comparison of CV curves of binary Co O /NiO and ternary
3
4
Co O /NiO/MnO composite electrodes.
3
4
2
(
Co O +NiO)/MnO ratio: 1#, 4:1; 2#, 3:2; 3#, 1:1; 4#, 2:3; 5#, 1:4.
3 4 2
Fig. 5 The electrochemical impedance spectra of the ternary Co O /NiO/MnO
2
3
4
composite electrodes.
Co O +NiO)/MnO ratio: 1#, 4:1; 2#, 3:2; 3#, 1:1; 4#, 2:3; 5#, 1:4.
(
3
4
2
-1
Fig. 6 The comparison of long-term cyclibility at 2 A g of NiO, Co O and the
3
4
ternary composite electrode with the (Co O +NiO)/MnO ratio of 3:2.
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4
2
1
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Tab. 1 The discharge specific capacitance of NiO, Co O and MnO single electrodes
3
4
2
-1
at various current density (F g ).
-1
J / A g
NiO
248
244
240
234
Co O
MnO2
120
116
88
3
4
0
1
1
2
.5
.0
.5
.0
236
224
219
207
78
Tab. 2 The discharge specific capacitance of Co O /NiO composite electrodes at
3
4
-1
various current density (F g ).
-1
-1
Co O /NiO weight ratio
0.5 A g
1 A g
292
294
248
243
275
3
4
1:4
2:3
1:1
3:2
4:1
336
361
300
240
307
1
4

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Highlights
Co O /NiO/MnO composite electrode provides ternary electrochemically active sites.
3
4
2
The composite electrode delivers twice capacitance of Co O or NiO single one.
3
4
The optimal (Co O +NiO)/MnO weight ratio is 3:2.
3
4
2