A study around the improvement of electrochemical activity of MnO2 as cathodic material in alkaline batteries

Article abstract of DOI:10.1016/j.electacta.2007.10.015

An optimized combination of reduction by methane and sulfuric acid digestion was developed to improve the electrochemical activity of manganese dioxide at a battery set. Chemical manganese dioxide, CMD, and electrolytic manganese dioxide, EMD, which have been destroyed after discharge cycling process in potential window of 900-1650 mV versus Hg/HgO, were reduced in a furnace with a flow of methane at 300 and 250 °C correspondingly. Thereafter, the reduced samples, CMDr and EMDr, were digested in a solution of sulfuric acid with optimized concentration and temperature. It was found that digested samples, CMDro and EMDro, typically show more stability in cycling, higher capacity and more reversible redox reaction. Alternatively, we reported about the effect of digestion temperature on electrochemical and structural properties of the samples. Digestion at temperatures 60 and 98 °C in 1.5 M sulfuric acid as superior concentration was preferred after comparative experiments in the range 40-98 °C. The samples which were digested in 60 °C (CMDro1 and EMDro1) showed superior electrochemical activity at the early stages of discharge cycling. By contrast, the samples which were obtained at 98 °C (CMDro2 and EMDro2) showed more stability and were superior to the former samples in final stages of discharge cycling process. Afterward, the electrochemical behavior of the pretreated samples was investigated by means of cyclic voltammetry technique and discharge cumulative capacity profiles. Also X-ray diffraction was employed to verify the responses of voltammetric methods. In XRD patterns, peak at 2θ = 28.6° which is due to β-MnO₂ type was the strongest signal as temperature 98 °C was selected for digestion. After digestion at 60 °C, the characteristic peaks at 2θ = 38° and 42° were amplified which are attributed to formation of γ-MnO₂. Interestingly enough, the results according to the XRD patterns were in good agreement with the electrochemical approaches.

Full text of DOI:10.1016/j.electacta.2007.10.015

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 3250–3256
A study around the improvement of electrochemical activity of
MnO2 as cathodic material in alkaline batteries
M. Ghaemia, A. Gholamia, R.B. Moghaddamb,∗
a Department of Chemistry, Science Faculty, Tarbiat Modares University, Tehran, Iran
b Department of Chemistry, K.N. Toosi University of Technology, 15875-4416 Tehran, Iran
Received 31 May 2007; received in revised form 4 September 2007; accepted 13 October 2007
Available online 22 October 2007
Abstract
An optimized combination of reduction by methane and sulfuric acid digestion was developed to improve the electrochemical activity of
manganese dioxide at a battery set. Chemical manganese dioxide, CMD, and electrolytic manganese dioxide, EMD, which have been destroyed
after discharge cycling process in potential window of 900–1650 mV versus Hg/HgO, were reduced in a furnace with a flow of methane at
300 and 250 ◦C correspondingly. Thereafter, the reduced samples, CMDr and EMDr, were digested in a solution of sulfuric acid with optimized
concentration and temperature. It was found that digested samples, CMDro and EMDro, typically show more stability in cycling, higher capacity and
more reversible redox reaction. Alternatively, we reported about the effect of digestion temperature on electrochemical and structural properties of
the samples. Digestion at temperatures 60 and 98 ◦C in 1.5 M sulfuric acid as superior concentration was preferred after comparative experiments in
the range 40–98 ◦C. The samples which were digested in 60 ◦C (CMDro1 and EMDro1) showed superior electrochemical activity at the early stages
of discharge cycling. By contrast, the samples which were obtained at 98 ◦C (CMDro2 and EMDro2) showed more stability and were superior to
the former samples in final stages of discharge cycling process. Afterward, the electrochemical behavior of the pretreated samples was investigated
by means of cyclic voltammetry technique and discharge cumulative capacity profiles. Also X-ray diffraction was employed to verify the responses
of voltammetric methods. In XRD patterns, peak at 2θ = 28.6◦ which is due to ␤-MnO2 type was the strongest signal as temperature 98 ◦C was
selected for digestion. After digestion at 60 ◦C, the characteristic peaks at 2θ = 38◦ and 42◦ were amplified which are attributed to formation of
␥-MnO2. Interestingly enough, the results according to the XRD patterns were in good agreement with the electrochemical approaches.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Manganese dioxide; Alkaline battery; Rechargeable battery; Digestion; Modification
1. Introduction
[2]. Moreover, the application of manganese oxides as an oxy-
gen storage component (OSC) for a three-way catalyst has been
proposed [3]. In all of the applications, the redox properties
of manganese oxides play a key role. In a practical applica-
tion, manganese dioxide can be used as a desirable catalyst for
oxidation of methane. The catalytic oxidation of methane over
manganese oxide is supposed to proceed through a Mars and van
Krevelen mechanism [2]. In principle, manganese oxides may
be able to oxidize methane in the absence of gas phase oxygen
while oxygen component of manganese dioxide should be con-
sumed during the process. During the reduction of manganese
oxide with methane, carbon dioxide and water were the main
products [4–6]. The oxidation of manganese oxides is reversible
up to Mn2O3, re-oxidation to MnO2 in pure oxygen only pro-
ceeds at pressures higher than 3000 bar while the most preferred
compound for pursuing the higher electrochemical activity is
MnO2 [7]. However, many attempts have been directed to pro-
Manganese dioxide is definitely the most widespread electro-
active cathode material used in primary batteries. Unique
combinationofbeneficialphysicochemical, electrochemicaland
economic properties of manganese dioxide cells is the most rea-
son for the superiority. In particular, manganese dioxide with
suitably high density and purity, as well as adequate electro-
chemical activity under a range of discharge conditions, can be
produced efficiently and relatively inexpensively on a commer-
cial scale [1]. Manganese oxides are known to be active catalysts
in several oxidation or reduction reactions and can be used as
catalysts for the oxidation of methane and carbon monoxide
∗
Corresponding author. Tel.: +98 21 2285 35 51; fax: +98 21 2285 36 50.
E-mail address: moghaddam@sina.kntu.ac.ir (R.B. Moghaddam).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.10.015

M. Ghaemi et al. / Electrochimica Acta 53 (2008) 3250–3256
3251
duce manganese dioxide with modified pattern for any variable
2.3. Materials preparation and nomination
conditions as temperature, pressure, concentration, etc. [8,9].
Therefore, any combination of these variables should lead to
deposition of ␥-MnO2 (the preferred structure for electrochemi-
cal application) while the variable ranges that produce a superior
material are very narrow indeed [10].
The scope of the present work is to develop a modified com-
bination of variables for reduction of inactive manganese oxides
by methane followed by oxidation of the reduced compounds by
digestion in a solution of sulfuric acid. This method was used for
recovery of destroyed cathodic material of a rechargeable alka-
linemanganesedioxidebattery. Also, wehavediscussedoverthe
role of temperature and concentration of digestion medium and
its influences on some defects in manganese dioxide crystalline
using potentiometric titration.
Two different kinds of manganese dioxide were used within
this study. Chemical manganese dioxide (CMD) was Merck
Sample and electrolytic manganese dioxide (EMD) which was
synthesized as previously stated. EMDd is obtained when
EMD in rechargeable alkaline MnO2–Zn battery is charged
and discharged after 70 consecutive cycles. Manganese diox-
ide, graphite and acetylene black which form the cathode of
the battery, were washed thoroughly with distilled water till the
washing solution became neutral. Thereafter, the sample was left
in quartz furnace. During the reduction of manganese oxide with
methane, carbon dioxide and water were the main byproducts.
Reduction was carried out at 300 ◦C for CMD and 250 ◦C for
EMDd with methane (100 ml/min) for 12 h. Finally the samples
were cooled down. Reduced CMD and EMDd are nominated
as CMDr and EMDr, respectively. Thus, the reduced MnO2
digests into H2SO4 solutions of different concentrations (up to
3 M) at various temperatures (up to 98 ◦C). For digestion, 0.5 g
of CMDr or EMDr sample was suspended in 50 cm3 of sulfu-
ric acid of a given concentration. Then, suspension was heated
in medium temperatures 40, 60, 70, 80, 98 ◦C for 24 h. After
the time, the suspension was filtered and washed with distilled
water. The resulting solids were dried in air. The samples which
were produced were typically nominated MDro. Generally, after
reduction by methane followed by digestion in sulfuric acid in
60 and 98 ◦C, the products were named as MDro1 and MDro2,
respectively.
2. Experimental
2.1. Reagents
Sulfuric acid, potassium hydroxide, potassium permanganate
and sodium pyrophosphate used in this work were Merck prod-
ucts of analytical grade and used as received. Methane was the
Iranian Gas Company origin. Doubly distilled water was used
throughout the experiment.
2.2. Synthesis of electrolytic manganese dioxide
(EMD)
2.4. Instrumentation
Synthesis was performed at a graphite substrate which has
been polished with emery paper prior to use. Electrolyte com-
position containing MnSO4 112 g l−1 in 0.10 M H2SO4 solution
was refluxed in bath temperature of 97–98 ◦C and at substrate
temperature 135 ◦C for 72 h [9]. Electrolysis was carried out
at a rather low current density (5 mA cm−2) to hold a lower
value of the anodic over-potential. This is to avoid defective
EMD structures caused by rapid growth at the electrode surface.
The precipitates obtained were removed mechanically from the
anode and washed several times with distilled water. The prod-
ucts were then ground with a mortar and pestle and neutralized
using 10% ammonia solution. Subsequently, the samples were
dried at 80 ◦C overnight. The overall reaction for deposition of
MnO2 is given by:
Electrochemical studies were carried out in a conventional
three electrode cell powered by a model 273A EG&G poten-
tiostat/galvanostat. The system was run by a PC through M270
commercial software. The working electrode potential moni-
tored against an Hg/HgO standard reference electrode and a
Pt plate formed the counter electrode. A model Philips Xpert
diffractometer and Cu K␣ radiation (λ = 0.15418 nm) were
employed to record X-ray diffraction patterns. Charging of the
samples was carried out at constant current 10 mA g−1
.
3. Results and discussion
3.1. Characterization of CMD and EMD
Mn2+ + 2H2O → MnO2 + 4H+ + 2e−
(1)
Fig. 1 presents a typical cyclic voltammogram of CMD in
9 M KOH at potential sweep rate of 0.25 mV s−1 in the range
−800–400 mV versus Hg/HgO. Two irreversible cathodic peaks
around −150 and −350 mV are observed which can be assigned
to reduction processes as following [12–14]:
Paul and Cartwright [11] proposed a model for the electrochem-
ical oxidation of Mn2+, i.e.,
Mn2+ → Mn3+ + e−
Mn3+ + 2H2O → MnOOH + 3H+
MnOOH → MnO2 + H+ + e−
(2)
(3)
(4)
MnO2 + H2O + e− → MnOOH + OH−
MnOOH + H2O + e− → Mn(OH)2 + OH−
(5)
(6)
The product was then filtered through a 100 ␮m mesh sieve
in order to be used in rechargeable alkaline manganese dioxide
(RAM) batteries.
Formation of Mn(OH)2 is believed to be irreversible and
restricts the reverse reaction in the anodic direction. Continu-
ous reduction of unreduced manganese dioxides in the reverse

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M. Ghaemi et al. / Electrochimica Acta 53 (2008) 3250–3256
Fig. 1. Cyclic voltammogram of CMD in 9 M KOH in the potential range
−800–400 mV at potential sweep rate of 0.25 mV s−1
.
Fig. 3. Typical CVs of EMD and EMDd in 9 M KOH in the range −800–400 mV
and at potential sweep rate of 0.25 mV s−1
.
direction results in the negative current of voltammogram in the
anodic sweep leading one to conclude that CMD is composed of
almost inactive manganese dioxides. Fig. 2 presents discharge
cumulative capacity of CMD as a function of cycle number dur-
ing 15 consecutive cycles in the potential range of 900–1650 mV
versus Hg/HgO at constant current 30 mA g−1. As can be seen,
the values of cumulative capacity in all stages of discharge are
not desirable. It can be predicted that ␣-MnO2 and ␤-MnO2 in
crystalline structure of CMD are the main features. These types
are structural compositions of manganese dioxide with relatively
low electrochemical activity.
Fig. 3 presents CVs of EMD before and after undergone 70
consecutive battery cycles within the same captions as Fig. 1.
The voltammogram obtained under these conditions is identical
to that given in the literature [15]. With taking into account that
the broad cathodic shape around −40 mV can be composed of
two overlapping peaks concerning the reduction of manganese
dioxide, it would be interesting to reveal that the surface and
inside manganese dioxides have been reduced in different poten-
tial values. Over-potential for the reduction of inside manganese
dioxides is supposed to be slightly more than that of on the sur-
face. Alternatively, two anodic peaks in the potentials −220 and
220 mV are probably assigned to the oxidation of the surface
and inside MnOOH. This further amplifies the argument that
the kinetics of Mn(OH)2 oxidation is poor and can reasonably
prevent the reversibility of the redox reactions. Additionally,
the plot of EMDd depicts that the cycling of EMD dramati-
cally decreases the capacity of the electro-active materials since
current is proportional to capacity.
Fig. 4 shows discharge cumulative capacity of test battery
containing EMD within the 70 consecutive cycles at constant
current of 30 mA g−1. The plot has been recorded at cycles 10,
20, 30, 40, 50, 60 and 70 which after any 10 cycles, cathodic
materials were extracted and washed with water and then dried
in air in order to fabricate a new battery set. Accordingly, dis-
charge cumulative capacity improved after the washing and
drying operation. This pathway is observed till the cycle 30.
After that, capacity greatly diminishes since the cathodic mate-
rials have been altered to the less active compounds. Formation
of undesirable types of manganese oxides can be responsible for
lowering of the capacity since these compounds are not properly
electro-active [9].
The basis for the currently accepted discharge mechanism
(Mn(IV) → Mn(III)) was proposed by Kozawa et al. [16–18],
and is constructed on proton and electron insertion into the man-
ganese dioxide structure. Typically, the cathode in an alkaline
Fig. 2. Discharge cumulative capacity of battery based on CMD as a function
of cycle number in the range 900–1650 mV recorded at initial 15 cycles and at
Fig. 4. Discharge cumulative capacity of battery based on EMD as a function of
cycle number in the range 900–1650 mV in 70 consecutive cycles recorded after
any 10 cycles; comparison between CMD and EMD at initial 15 cycles (inset).
constant current 30 mA g−1
.

M. Ghaemi et al. / Electrochimica Acta 53 (2008) 3250–3256
3253
Fig. 6. Discharge cumulative capacity of EMDro as a function of cycle number
at various medium temperatures; the other captions are the same as Fig. 4.
Fig. 5. X-ray diffraction patterns of CMD, EMD and EMDd.
3.2. Modification treatments
Zn/MnO2 cell essentially consists of an intimate mixture of elec-
trolytic manganese dioxide (EMD) and a small proportion of
graphite wetted with electrolyte. An electron from the external
circuit passes through the graphite into the manganese dioxide
structure where it reduces a Mn(IV) to Mn(III). For charge com-
pensation, a water molecule on the manganese dioxide surface
dissociates into a proton, which is inserted into the structure,
and an OH− ion, which remains in the electrolyte. Additional
capacity (other than that on the surface) can be extracted from
the manganese dioxide by the inserted protons and electrons dif-
fusing away from the surface into the bulk of the solid. In the
light of the previous study [1], it can be stated that within the
hexagonal close-packed framework of oxide anions, a fraction
of Mn(IV) ions have been replaced by Mn(III) (y), and that a
fraction of Mn(IV) ions are absent altogether (x). The positive
charge deficiency is compensated by an appropriate number of
protons located on adjacent oxide ions [1]:
Initially, CMD and EMDd were reduced in a flow of methane
(100 ml min−1) at temperatures 300 and 250 ◦C correspond-
ingly. Subsequently, the reduced samples were digested in a
solution of sulfuric acid in optimized medium temperature and
concentration. In order to determine the optimized tempera-
tures for digestion of CMDr and EMDr, a comparison was
performed in the range 40–98 ◦C in a 1.5 M medium of sul-
furic acid. Fig. 6 shows the cumulative capacity of EMDro
as a function of cycle number at constant current 30 mA g−1
recorded in various medium temperatures. According to the
measurements, it is originated in that digestion of samples at
higher temperatures results in formation of more stable struc-
tural types of manganese dioxide. After digestion at 60 ◦C,
the capacity slowly decreases within the seven initial cycles.
At the second half of the measurement, faster decrease of the
capacity indicates that EMDro1 is relatively unstable. After
digestion at 98 ◦C, the product (EMDro2) is more stable as
its cumulative capacity is superior to EMDro1 in final cycles.
This approach is in agreement with the Ohzuku et al. report
[22].
Fig. 7 presents comparative discharge cumulative capacity of
EMDro after digestion at various medium concentrations within
the 15 consecutive cycles. Meaningful decrease of capacity is
considered both in higher and lower sulfuric acid concentration
than 1.5 M. Increment of acid concentration leads to expected
increase in the oxidation rate. As a result of this, necessary time
for oxidizing of Mn(III) could not be provided with take it into
consideration that the rate of Mn(III) oxidation is believed to
be low [15]. Simultaneously, an electron transfer between two
neighbor Mn(III) is occurred.
(Mn4+
)
1−x−y(Mn3+)y(ꢀ)x(O2−
)
2−4x−y(OH−)4x+y
(7)
where ꢀ represents a cation vacancy. Typical ranges for x and
y are 0.06–0.08 and 0.04–0.12, respectively. It is these protons
that constitute structural water within ␥-MnO2.
To guarantee the accuracy of the voltammetric studies, X-ray
diffraction patterns, Fig. 5, was employed. XRD plots of CMD
are composed of a strong peak in 2θ = 28.6◦ attributed to ␣-
MnO2 and/or ␤-MnO2, and two weak peaks at 2θ = 38◦ and 42◦
which are assigned to the presence of ␥-MnO2 [19,20]. Appar-
ently, weakness of the characteristic peaks around 2θ = 38◦ and
42◦ in this pattern, is due to the existing of the low concentration
of ␥-MnO2. Consequently as stated before, ␣-MnO2 and ␤-
MnO2 seem to be the main types in a CMD sample. Strong peaks
around 2θ = 38◦ and 42◦ and a weak peak around 2θ = 28.6◦ are
the main features in XRD pattern of EMD. This clearly shows
␥-MnO2 is the dominant type in EMD. Appearance of a strong
signal at around 2θ = 33◦ in XRD of EMDd is indexed to the
formation of Mn2O3 [21]. On the basis of the results, it can be
assumed that pyrolusite is the main type forming CMD while
EMD is mostly composed of ramsdellite.
Without going into details, the mechanism of the interac-
tion can be proposed via a dissolution process followed by a
disproportionation–precipitation step [1,23]:
• Dissolution:
Mn2O3 + 6H+ ↔ 2Mn3+ + 3H2O
(8)

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M. Ghaemi et al. / Electrochimica Acta 53 (2008) 3250–3256
Table 2
Cation vacancy fraction of EMDro as a function of medium concentration at
temperature 60 ◦C
Sample
H2SO4 Conc. (M)
Temperature (◦C)
Cation vacancy
EMDro
EMDro
EMDro
EMDro
EMDro
0.5
1
1.5
2.5
3
60
60
60
60
60
0.05
0.1
0.13
0.18
0.19
Fig. 7. Discharge cumulative capacity of EMDro as a function of cycle number
at various medium concentrations within the 15 consecutive cycles in the range
900–1650 mV and at constant current 30 mA g−1
.
• Disproportionation–precipitation:
2Mn3+ + 2H2O ↔ MnO2 + Mn2+ + 4H+
• Overall reaction:
(9)
Mn2O3 + 2H+ ↔ MnO2 + Mn2+ + H2O
(10)
At the final stages of this interaction, formed Mn(II) leaves
the crystalline structure and accordingly, a cation vacancy left-
overs. Therefore, the amount of vacancies increases rapidly as
acid concentration is raised. Generally, at discharge cycle in
alkaline cells, an electron can diffuse via the conductor towards
the surface manganese dioxides. Reduction of inside manganese
dioxides is carried out through the diffusion of a proton into the
structure [23]. Increment of cation vacancies leads to provide
more available sites for proton substitution and this can proba-
bly increase the rate of proton diffusion. By contrast, decrease
of manganese dioxide concentration of structure as a result of
overproduction of cation vacancy, may dramatically decrease
the capacity values. Consequently, at higher concentration of
sulfuric acid than 1.5 M, the capacity diminishes gradually.
Potentiometric titration was performed to calculate the cation
vacancy fraction as well as Mn(III) fraction (y) in order to shed
more light on prevailing mechanism. The method of calculation
has been formerly explained by Walanda et al. [1]. According to
the literature, the fraction of Mn(III) ions in the MnO2 structure
(y) can be quite readily determined by potentiometric titration.
Table 1 shows the quantity of Mn(III) fraction in MnO2 struc-
ture in relation to medium concentration. The Ruetschi cation
Fig. 8. Cyclic voltammograms of CMD, CMDro1 and CMDro2 in the potential
window of −800–400 mV in 9 M KOH; potential sweep rate was 0.25 mV s−1
.
vacancy fraction is also an indicator of the level of structural
defects within ␥-MnO2 crystalline [24]. Table 2 shows cation
vacancy as a function of medium concentration at temperature
60 ◦C.
Fig. 8 presents cyclic voltammograms of CMD, CMDro1
and CMDro2 in the potential window of −800–400 versus
Hg/HgO in 9 M KOH and Fig. 9 presents CVs of EMDd,
EMDro1 and EMDro2. After thermal treatment on the destroyed
cathodic materials, typical mechanism of redox reactions of
EMD seems to be unchanged since the overall pattern of voltam-
mogramsremainsidentical. Incontrast, irreversibilityofCMDis
Table 1
Mn(III) fraction of EMDro as a function of medium concentration at temperature
60 ◦C
Sample
H2SO4 Conc. (M)
Temperature (◦C)
Mn(III) fraction
EMDro
EMDro
EMDro
EMDro
EMDro
0.5
1
1.5
2.5
3
60
60
60
60
60
0.12
0.1
0.09
0.07
0.04
Fig. 9. Cyclic voltammograms of EMDd, EMDro1 and EMDro2 in the range
−800–400 mV in 9 M KOH at potential sweep rate of 0.25 mV s−1
.

M. Ghaemi et al. / Electrochimica Acta 53 (2008) 3250–3256
3255
Fig. 10. Comparative discharge cumulative capacity of CMD, CMDro1 and
CMDro2 as a function of cycle number in the range 900–1650 mV recorded at
Fig. 12. X-ray diffraction patterns of CMDro1, CMDro2, EMDro1 and
EMDro2.
first 15 cycles and at constant current 30 mA g−1
.
vations evidently show that the sample which has been digested
at temperature 60 ◦C shows better electrochemical activity and
less stability than the sample which has been digested in
98 ◦C.
XRD plots of CMDro1, EMDro1, CMDro2 and EMDro2,
Fig. 12, further verify the accuracy of the results. At Fig. 12, sig-
nals around 2θ = 38◦, 42◦ are stronger in CMDro1 and EMDro1
in comparison with the other samples. This obviously shows that
␥-MnO2 is dominant crystalline type as digestion is performed
in 60 ◦C. In contrast, a peak in 2θ = 28.6◦, which is attributed
to ␤-MnO2, is found in XRD pattern of CMDro2 and EMDro2
[1,21].
Fig. 13 compares the efficiency of the treatment on CMD
and EMDd within 15 consecutive cycles. The records indicate
that while the recovery process was efficient for both CMD and
EMDd, there was a stronger impact on EMDd than on CMD.
This can be assigned to different structure of the destroyed
samples and/or existing of more inactive manganese oxides in
destroyed CMD crystalline than in EMDd.
believed to be changed to a more reversible redox system. Inter-
estingly, the treatment has definitely recovered the destroyed
manganese dioxide both in CMD and EMD. Moreover, the
current densities have been dramatically increased. Without
much speculation, it is obviously unveiled that reduction by
methane followed by digestion in a solution of sulfuric acid has
expanded the integrated region of faradic and non-faradic charge
transfer.
Cumulative capacities of CMD, CMDro1 and CMDro2 have
been compared in Fig. 10. As previously demonstrated and on
the basis of this plot, it is resulted that the capacities of CMDro1
and CMDro2 are dramatically enhanced compared to CMD at
all discharge steps. On the other hand, capacity of CMDro1,
which has been obtained of digestion in 60 ◦C, is superior to
CMDro2 in the early stages of discharge process. By contrast,
its capacity decays more rapidly than CMDro2 as discharge pro-
ceeds. Similarly, these results are repeated typically in EMDs,
Fig. 11, accompanied by the fact that the capacity of EMDro is
quantitatively larger in comparison with CMDro. These obser-
Fig. 11. Comparative discharge cumulative capacity of EMD, EMDro1 and
EMDro2 as a function of cycle number in the range 900–1650 mV recorded at
Fig. 13. Efficiency of the recovery process (reduction in an atmosphere of
methane followed by digestion in sulfuric acid) for both CMD and EMDd within
15 consecutive cycles.
first 15 cycles and at constant current 30 mA g−1
.

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M. Ghaemi et al. / Electrochimica Acta 53 (2008) 3250–3256
4. Conclusion
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Financial support of this work by Tarbiat Modares Univer-
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extend gratitude to Mr. Davoud Moghaddam for his useful com-
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