Reversible insertion of a trivalent cation onto MnO2 leading to enhanced capacitance

Article abstract of DOI:10.1149/1.3565177

Supercapacitor properties of MnO₂ are studied generally in aqueous alkali metal salt solutions, often in a Na salt solution. During electrochemical discharge-charge processes, Na⁺ ions from the electrolyte get reversibly inserted/deinserted on the surface of MnO₂ particles, which leads to redox reaction between MnOONa and MnO₂. In the present study, it has been shown that MnO₂ exhibits enhanced capacitance behaviour in a rare earth metal salt solution, namely, La(NO ₃)₃ solution in comparison with NaNO₃ and Mg(NO₃)₂ aqueous solutions. The specific capacitance increases with an increase in charge on the solution cation (Na⁺, Mg²⁺ and La³⁺). It is proposed that the number of surface sites for adsorption of cations remains unaltered in all solutions. The surface insertion of cation facilitates the reduction of Mn⁴⁺ in MnO ₂ to Mn³⁺ equivalent to the charge present on the cation. As the specific capacitance is related to the quantity of charge that is exchanged between the solid MnO₂ and the aqueous solution, the trivalent cation (La³⁺) provides greater specific capacitance than in Mg(NO₃)₂ and NaNO₃ electrolytes. Accordingly, the number of Mn(IV)/Mn(III) redox pairs involved in the neighbourhood of the adsorption site is one, two and three when Na⁺, Mg²⁺ and La³⁺ ions, respectively, are adsorbed.

Full text of DOI:10.1149/1.3565177

Journal of The Electrochemical Society, 158 (5) A585-A591 (2011)
0013-4651/2011/158(5)/A585/7/$28.00 The Electrochemical Society
A585
C
V
Reversible Insertion of a Trivalent Cation onto MnO2 Leading
to Enhanced Capacitance
,z
*
Prasant Kumar Nayak and N. Munichandraiah
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Supercapacitor properties of MnO2 are studied generally in aqueous alkali metal salt solutions, often in a Na salt solution. During
electrochemical discharge-charge processes, Naþ ions from the electrolyte get reversibly inserted/deinserted on the surface of
MnO2 particles, which leads to redox reaction between MnOONa and MnO2. In the present study, it has been shown that MnO2
exhibits enhanced capacitance behaviour in a rare earth metal salt solution, namely, La(NO3)3 solution in comparison with NaNO3
and Mg(NO3)2 aqueous solutions. The specific capacitance increases with an increase in charge on the solution cation (Naþ, Mg2þ
and La3þ). It is proposed that the number of surface sites for adsorption of cations remains unaltered in all solutions. The surface
insertion of cation facilitates the reduction of Mn4þ in MnO2 to Mn3þ equivalent to the charge present on the cation. As the spe-
cific capacitance is related to the quantity of charge that is exchanged between the solid MnO2 and the aqueous solution, the triva-
lent cation (La3þ) provides greater specific capacitance than in Mg(NO3)2 and NaNO3 electrolytes. Accordingly, the number of
Mn(IV)/Mn(III) redox pairs involved in the neighbourhood of the adsorption site is one, two and three when Naþ, Mg2þ and La3þ
ions, respectively, are adsorbed.
C
V
2011 The Electrochemical Society. [DOI: 10.1149/1.3565177] All rights reserved.
Manuscript submitted November 18, 2010; revised manuscript received February 15, 2011. Published March 28, 2011.
Experimental
Manganese oxides have been under intense investigations for
electrochemical energy storage and conversion because of their
high electrochemical activity, variable oxidation states, low cost
and environmental compatibility.1–3 MnO2 is useful for electro-
chemical supercapacitors,4 which are expected to meet high pulse
power demands. MnO2 is generally studied for its capacitance
properties in neutral aqueous solutions of an alkali salt, for
instance Na2SO4.4–7 When a MnO2 electrode is subjected to dis-
charge in Na2SO4 electrolyte, reduction of Mn4þ to Mn3þ occurs
with a simultaneous insertion of Naþ ion onto the oxide surface
leading to the formation of MnOONa. Non-diffusive reversible
transitions of surface MnO2 $ MnOONa are responsible for pro-
viding charge in pseudo-capacitance properties.7–9 It has been
established that the crystallographic structures of MnO2 which
possess sufficient gap or interlayer distance ( a- and d- MnO2) are
appropriate for insertion of Naþ ions into them.9 Although the
theoretical specific capacitance (SC) of MnO2, on the basis of
reduction of Mn4þ to Mn3þ, is 1100 F gꢀ1 over a potential win-
dow of 1.0 V, in practice this value is never realized for MnO2
powders. However, SC values equal to the theoretical value was
reported for a MnO2 thin film electrode.7 A value of 470 F gꢀ1 is
reported for nanocomposites consisting of several oxides of Mn.10
Nevertheless, for electrodes employing MnO2 powders, experi-
mental results from various laboratories indicate values of SC in
Nano-structured MnO2 was prepared by reduction of KMnO4
with ethylene glycol.16 In a typical preparation, 0.1 M of KMnO4
(3.16 g) was dissolved in 200 mL of doubly distilled water and then
5 mL of ethylene glycol was slowly added to the above solution
under stirring at ambient environment. The stirring was continued
for 3 h to ensure the completion of reaction. The resulting black pre-
cipitate was filtered and washed with distilled water several times
followed by ethanol and dried in air at 60ꢁC for overnight.
Powder X-ray diffraction (XRD) pattern of MnO2 was recorded
on a Bruker D8 Advance X-ray diffractometer using Cu Ka
˚
(k ¼ 1.5418 A) as the source. Raman scattering spectrum of MnO2
was collected by HORIBA Jobin Yvon Lab RAM HR100 Raman
instrument using argon ion laser of wavelength 514 nm. The mor-
phology of MnO2 sample was examined by a FEI Co. scanning elec-
tron microscope (SEM) model Sirion. The Brunauer-Emmet-Teller
(BET) surface area measurement was carried out using Micromerit-
ics surface area analyzer model ASAP 2020. X-ray photoelectron
spectroscopy (XPS) studies were conducted with XPS Thermo
Fisher Scientific, UK using X-ray Al anode (monochromatic Ka X-
rays at 1486.6 eV) as the source. The C 1S region was used as the
reference and was set at 284.6 eV.
For electrochemical characterization, electrodes were fabricated
on a high purity grade 304 stainless steel (SS) foil (thickness: 0.2
mm) as the current collector. For this, the SS was polished with suc-
cessive grades of emery and washed thoroughly with detergent,
etched in dilute HCl, rinsed with doubly distilled water and then
with acetone, and dried in air. MnO2 (70 wt %), Ketjen black EC-
600 JD (Akzo Noble Polymer Chemicals) (20 wt %) and poly(viny-
lidene fluoride) (Aldrich) (10 wt %) were mixed and ground in a
mortar. Several drops of n-methyl pyrrolidinone (Aldrich) were
added to make a syrup, which was coated on the pre-treated SS foil
of 1.0 cm2 area and dried at 100ꢁC under vacuum for 12 h. Coating
and drying steps were repeated to get a loading level of 0.5–0.6 mg
cmꢀ2. A Mettler Toledo electronic balance model AB265-S/FACT
with 0.01 mg sensitivity was used for weighing the electrodes. The
electrochemical studies were performed in a glass cell of about 50
mL volume, which had provision to introduce MnO2 coated SS as
the working electrode, Pt foil as the counter electrode and saturated
calomel electrode (SCE) as the reference electrode. The aqueous
electrolytes investigated include NaNO3, Mg(NO3)2 and La(NO3)3.
The concentration of each electrolyte was 0.1 M. The specific con-
ductivity values measured in 0.1 M solutions of NaNO3, Mg(NO3)2
and La(NO3)3 were 0.011, 0.018 and 0.025 Ohmꢀ1 cmꢀ1, respec-
tively. An Eco Chemie potentiostat/galvanostat model Autolab
PGSTAT 30 was used for all the electrochemical studies. Potential
a range from 100 to 250 F gꢀ1 11–13
The wide range of SC values
.
are due to differences in methods of synthesis of MnO2, crystallo-
graphic structures, morphology, particle size, porosity, loading of
MnO2 on the substrate current collectors, methods of measure-
ment (cyclic voltammetry or galvanostatic charge-discharge cy-
cling), sweep rates used in cyclic voltammetry, current density
(or specific current, A gꢀ1) used in galvanostatic charge-discharge
cycling, etc. Nevertheless, it is noted that the maximum value of
SC reported is only about 30% of the theoretical value, assuming
that the manganese oxide is present as MnO2. Recently, it was
reported that the specific capacitance of MnO2 in Ca and Mg salt
solution is greater than that in Na salt solutions.14,15
The aim of the present investigation is to study the insertion of a
rare earth metal trivalent cation, namely, La3þ onto MnO2. Interest-
ingly, the specific capacitance measured in a La salt solution is
greater than the values measured in Na and Mg salt solutions under
similar experimental conditions.
*
Electrochemical Society Active Member.
z E-mail: muni@ipc.iisc.ernet.in
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A586
Journal of The Electrochemical Society, 158 (5) A585-A591 (2011)
Figure 1. Raman spectrum of MnO2.
values were reported with respect to SCE. All experiments were
conducted in an air-conditioned room at 22 6 1ꢁC.
Results and Discussion
Powder XRD pattern of MnO2 indicated that MnO2 is mainly in
amorphous state with a broad prominent peak at 2h ¼ 14ꢁ and
another small peak at 2h ¼ 30ꢁ, as reported previously.16 These two
peaks indicated that the sample is d-MnO2. The broad peaks indi-
cated nano-particle nature of the sample. It is understood that the
sample has short-range d-type structure although it is nearly X-ray
amorphous.16 d-MnO2 is a 2D layered compound with the interlayer
Figure 3. (A) N2 adsorption-desorption isotherm and (B) pore size distribu-
tion of MnO2.
9
˚
distance of 7.0 A. The Raman scattering spectrum shown in Fig. 1
is characterized by peaks at 566.2, 612.2 and 631.8 cmꢀ1. These
peaks are mainly attributed to the stretching vibrations of MnO6
octahedra.17 In SEM micrograph of MnO2 powder (Fig. 2), nearly
spherical nano-particles with diameter in the range 20–50 nm were
observed. The N2 adsorption-desorption isotherm is shown in Fig. 3.
A hysteresis loop of IV type at p/po between 0.70 and 0.99 is
observed, suggesting the existence of mesopores in the particles.
The BET surface area measured in the range p/po ¼ 0.05–0.25 was
Using the MnO2 coated stainless steel electrodes, electrochemi-
cal studies were carried out in 0.1 M aqueous electrolytes of
NaNO3, Mg(NO3)2 and La(NO3)3 under identical conditions.
Although it is known that the potential window appropriate for
MnO2 to exhibit capacitance behaviour in Na2SO4 electrolyte is
0.0–1.0 V, it is intended to examine the effect of the nature of the
electrolyte on potential window. Typical voltammograms recorded
at a sweep rate of 10 mV sꢀ1 are presented in Fig. 4. In Na2SO4
electrolyte (Fig. 4A), when the potential range is ꢀ1.0 to 1.4 V
(curve i), multiple current peaks are observed in both cathodic and
anodic scans. Current tends to increase at ꢀ1.0 V due to hydrogen
evolution reaction (HER). The increasing current at about 1.2 V is
due to oxygen evolution reaction (OER). When MnO2 electrode is
cycled between ꢀ0.5 and 1.4 V in Na2SO4 electrolyte (Fig. 4A,
curve ii) and also between 0.0 and 1.4 V (curve iii), current peaks
appear. However, if the potential range is limited from 0.0 to 1.0 V
(curve iv), the voltammogram tends to become rectangular in shape,
which is an indication for the capacitance behaviour of MnO2. In
Mg(NO3)2 and La(NO3)3 electrolytes, the voltammograms (Figs. 4B
and 4C) recorded in the potential ranges between ꢀ1.0 and 1.4 V;
ꢀ0.5 and 1.4 V; and 0.0 and 1.4 V (curves i, ii and iii) consist of
current peaks similar to the observations made in Na2SO4 (Fig. 4A).
However, the potential range where MnO2 shows rectangular shape
of cyclic voltammograms in Mg(NO3)2 and La(NO3)3 is limited
between 0.2 and 1.0 V. Voltammograms were recorded at different
sweep rates varying from 5 to 50 mV sꢀ1 in the potential range of
0.2–1.0 V in 0.1 M aqueous electrolytes of NaNO3, Mg(NO3)2 and
La(NO3)3, and are shown in Fig. 5. At all sweep rates, the voltam-
mograms exhibit rectangular shape, suggesting capacitance behav-
iour of MnO2 in all electrolytes.
212 m2 gꢀ1
.
Typical charge-discharge curves of MnO2 electrodes at a current
density (c.d.) of 0.5 mA cmꢀ2 (1.0 A gꢀ1) in the potential range of
0.0–1.0 V in aqueous electrolytes of NaNO3, Mg(NO3)2 and
Figure 2. SEM micrograph of MnO2.
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Journal of The Electrochemical Society, 158 (5) A585-A591 (2011)
A587
Figure 4. Cyclic voltammograms at a
sweep rate of 10 mV sꢀ1 in different
potential ranges of (i) ꢀ1.0 to 1.4 V, (ii)
ꢀ0.5 to 1.4 V, (iii) 0.0–1.4 V and (iv)
0.0–1.0 V in 0.1 M aqueous electrolytes
of (A) NaNO3, (B) Mg(NO3)2 and (C)
La(NO3)3.
La(NO3)3 are shown in Fig. 6. There is a linear variation of potential
with time in NaNO3 electrolyte during charging and discharging
processes. However, in Mg(NO3)2 and La(NO3)3 electrolytes, the
variation of potential with time is linear in the potential range of
0.2–1.0 V. A plateau-like behaviour is observed in the potential
range between 0.2 and 0.0 V, which is also reflected in cyclic vol-
tammograms. The plateau-like behaviour during discharge of the
MnO2 electrode between 0.2 and 0.0 V is attributed to diffusion con-
trolled battery-like property in Mg(NO3)2 and La(NO3)3 electro-
lytes. Thus, capacitor behaviour in 1.0–0.2 V range and battery
behaviour in 0.2–0.0 are observed in Mg(NO3)2 and La(NO3)3 elec-
trolytes, although capacitor behaviour is observed in 1.0–0.0 V
range in NaNO3 electrolyte. Therefore, further charge-discharge
studies were carried out in 0.2–1.0 V range in all electrolytes for the
purpose of comparison. Charge-discharge data recorded for a few
cycles in NaNO3, Mg(NO3)2 and La(NO3)3 electrolytes in 0.2–1.0 V
range are presented in Fig. 7. The discharge SC of MnO2 was calcu-
lated using the following equation
tance of MnO2 in Ca(NO3)2 solution than in NaNO3 electrolyte was
attributed to divalent cationic species.14 Asymmetric capacitors of
activated carbon/MnO2 were studied in Mg(NO3)2, Ca(NO3)2 and
Ba(NO3)2 electrolytes.15 Mg(NO3)2 and Ca(NO3)2 electrolytes were
assessed to be better electrolytes than NaNO3 and Ba(NO3)2 electro-
lytes. The results of the present work in NaNO3 and Mg(NO3)2 elec-
trolytes agree well with the results reported earlier,15 although the
actual values of specific capacitance show some difference. The val-
ues of SC depend on several experimental conditions such as the
amount of active material per unit area, potential window, current
density used for charge-discharge cycling or sweep rate used for
cyclic voltammetry, etc., which are different in the present work as
compared with the previous reports.14,15 Nevertheless, the trend in
variation of SC with the Naþ and Mg2þ- solutions is essentially the
same to a previous report.15 In addition to the above two cations, the
present study involves La3þ ion. SC of MnO2 measured in
La(NO3)3 is found to be greater than the values measured in both
NaNaO3 and Mg(NO3)2 electrolytes.
The charge storage mechanism of MnO2 is scarcely discussed in
the literature.7–9 The capacitance behaviour of MnO2 is known due
to reversible redox process involving Mn4þ/Mn3þ in the solid MnO2
followed by insertion/deinsertion of cations
SC ¼ I t=ðDE mÞ
[1]
where I is the charge-discharge current, m is the mass of MnO2 pres-
ent in the electrode and t is the discharge time corresponding to the
voltage window, DE (¼ 0.8 V). The SC values of MnO2 electrodes
obtained at a c.d. of 0.5 mA cmꢀ2 (1.0 A gꢀ1) are 187, 211 and
257 F gꢀ1 in NaNO3, Mg(NO3)2 and La(NO3)3 electrolytes,
respectively.
Recent studies of Xu et al, on capacitance properties of MnO2
involve aqueous electrolytes of Mg2þ, Ca2þ and Ba2þ ions over a
potential window of 0.9 V.14,15 Under similar experimental condi-
tions, MnO2 provided SC values of 320 and 194 F gꢀ1 in Ca(NO3)2
and NaNO3 electrolytes, respectively. The higher specific capaci-
nMnO2 þ Mnþ þ neꢀ $ ðMnOOÞnM
[2]
where Mnþ represents the cation. While this reaction is expected to
take place on or near surface region of MnO2 particles, it is also
reported by Kuo and Wu that bulk insertion/extraction of H3Oþ
occurs into/from a crystalline structure of electrochemically depos-
ited MnO2.6 In the present study, as the oxide is mainly amorphous
with short range crystallinity, it is likely that the insertion of cations
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Journal of The Electrochemical Society, 158 (5) A585-A591 (2011)
Figure 5. Cyclic voltammograms at sweep
rates of (i) 5, (ii) 10, (iii) 20, (iv) 30, (v) 40
and (vi) 50 mV sꢀ1 in the potential range
from 0.2 to 1.0 V in 0.1 M aqueous electro-
lytes of (A) NaNO3, (B) Mg(NO3)2 and (C)
La(NO3)3.
takes place predominantly at the surface of the MnO2 particles.11
This means, the cations entering the oxide particle concentrate on
the surface and they do not diffuse into the bulk of the particle. This
assumption is supported by the fact that the SC reported for MnO2
so far in the literature is substantially lower than the theoretical
value. Values of SC close to the theoretical value could have been
obtained if bulk MnO2 particles involve in the redox process.
Assuming that the number of adsorption sites available for cation
insertion remains the same irrespective of the valency of the cations,
the positive charge of the inserted cation is double in the case of
divalent cation and triple in the case of trivalent cation when com-
pared with monovalent cation. Accordingly, the amount of Mn4þ
getting reduced to Mn3þ in the solid becomes double and triple in
the neighbourhood of adsorption site when divalent and trivalent
cations get inserted in comparison with monovalent cation. Hence,
the value of SC in divalent cation and trivalent cation electrolytes is
expected to be double and triple, respectively to the value in mono-
valent cation electrolyte. However, it is seen that there is a substan-
tial increase in SC of MnO2 in Mg(NO3)2 electrolyte, but it is not
double to the SC measured in NaNO3 electrolyte. The differences in
SC of MnO2 in electrolytes of different cations may be attributed to
Figure 6. Charge-discharge cycle of MnO2 at a c.d. of 0.5 mA cmꢀ2 (1.0 A
g
ꢀ1) in the potential range of 0.0–1.0 V in 0.1 M aqueous electrolytes of (i)
NaNO3, (ii) Mg(NO3)2 and (iii) La(NO3)3.
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Journal of The Electrochemical Society, 158 (5) A585-A591 (2011)
differences in size of cations. Effective ionic radii of Naþ, Mg2þ
A589
and La3þ in aqueous solutions are 4, 8 and 9 A, respectively. The
ionic conductivity of the electrolyte is not expected to play a role in
the specific capacitance, and the three electrolytes used in the pres-
ent study have nearly the same specific conductivity (see Experi-
mental section). Nevertheless, it is interesting to note that MnO2
exhibits capacitance behaviour in an aqueous electrolyte containing
the trivalent ion, La3þ with increased SC in comparison with mono-
valent and divalent cations. A schematic picture of surface insertion
of different valency cations and reduction of Mn4þ to Mn3þ in
MnO2 is shown in Fig. 8.
18
˚
In order to identify the presence of the trivalent cation (La3þ) in
MnO2 after insertion into it, a MnO2 electrode was galvanostatically
discharged to 0.0 V and was held at this potential for 24 h, subse-
quently removed, washed, dried and subjected for recording of XPS
spectrum. As shown in Fig. 9, La is identified.19 This data confirms
that the La3þ ion gets inserted on to the surface of MnO2 particles.
The charge-discharge cyclings of MnO2 electrodes were carried
out at different current densities in the potential range from 0.2 to
1.0 V in the three electrolytes. The SC values obtained using Eq. 1
are plotted against the charge-discharge c.d. and shown in Fig. 10.
In general, there is a decrease in SC with an increase in c.d. in all
electrolytes. However, a higher SC is obtained in aqueous
Figure 8. Schematic picture of surface insertion of cations, Naþ, Mg2þ and
La3þ followed by reduction of Mn4þ to Mn3þ on MnO2 particles.
Figure 7. Charge-discharge cycling of
MnO2 at a c.d. of 0.5 mA cmꢀ2 (1.0 A
g
ꢀ1) in the potential range from 0.2 to 1.0
V in 0.1 M aqueous electrolytes of (i)
NaNO3, (ii) Mg(NO3)2 and (iii) La(NO3)3.
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Journal of The Electrochemical Society, 158 (5) A585-A591 (2011)
Figure 11. Cycle life test of MnO2 at a charge-discharge c.d. of 0.5 mA
cmꢀ2 (1.0 A gꢀ1) in 0.1 M aqueous electrolytes of (i) NaNO3, (ii) Mg(NO3)2
and (iii) La(NO3)3 in the potential window from 0.2 to 1.0 V.
The cycle-life of MnO2 electrodes were tested by galvanostatic
charge-discharge cycling at a current density of 0.5 mA cmꢀ2 (1.0
A gꢀ1) in the potential range of 0.2–1.0 V in all the three electro-
lytes (Fig. 11). In NaNO3, an initial discharge SC of 185 F gꢀ1 is
obtained, which decreases to 163 F gꢀ1 after 20 cycles. Then the SC
becomes stabilized and a discharge SC of 160 F gꢀ1 is obtained after
600 cycles. In Mg(NO3)2, an initial discharge SC of 228 F gꢀ1 is
obtained, which rapidly falls to a value of 202 F gꢀ1 after 20 cycles.
Then it slowly decreases and a discharge SC of 166 F gꢀ1 is
obtained after 600 cycles. In La(NO3)3, an initial discharge SC of
272 F gꢀ1 is obtained, which slowly increases to a value of 281 F
gꢀ1 for first 100 cycles. Then it starts to decrease slowly and a SC
of 202 F gꢀ1 is obtained after 600 cycles. Thus the SC obtained after
600 cycles in La(NO3)3 is greater in comparison to that in NaNO3
and Mg(NO3)2 electrolytes. There are about 13, 27 and 26%
decrease in SC in NaNO3, Mg(NO3)2 and La(NO3)3 electrolytes,
respectively, after 600 charge-discharge cycles. The factors influ-
encing cycle-life of MnO2 electrodes are not understood, corrosion
of the substrate metal could be a possibility.20 It is interesting to
note that SC measured in La(NO3)3 electrolyte is greater than the
SC values measured in Mg(NO3)2 and NaNO3 electrolytes through-
out the cycle-life test (Fig. 11).
Figure 9. (A) XPS spectrum showing the presence of La on MnO2 and (B)
the expanded 3d energy level of La.
electrolyte of La(NO3)3 in comparison to aqueous electrolytes of
NaNO3 and Mg(NO3)2 at all charge-discharge c.d.s from 0.5 to 20.0
mA cmꢀ2 (1.0–40.0 A gꢀ1). Thus, it is shown in the present work
that in addition to the insertion of alkali and alkaline earth cations in
MnO2, La3þ ions can also be inserted. The reversible insertion/dein-
sertion of La3þ ions results in an increased specific capacitance of
MnO2.
Conclusions
It has been shown that MnO2 exhibits superior supercapacitor
behaviour in La(NO3)3 electrolyte in comparison with Mg(NO3)2
and NaNO3 electrolytes. The specific capacitance increases with an
increase in charge present on the cation of the electrolyte. It is pro-
posed that insertion of the trivalent cation, La3þ on the surface of
MnO2 particles leads to an increase in Mn4þ ions undergoing reduc-
tion to Mn3þ ions.
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