The relation between structural features and

J Solid State Electrochem DOI 10.1007/s10008-013-2043-1

ORIGINAL PAPER

The relation between structural features and electrochemical activity of MnO2 nanoparticles synthesized from a polyol-made Mn3O4 precursor Cristiane de A. Dias & Henrique de Santana & Marcos A. L. Nobre & Mauro C. Lopes

Received: 18 December 2012 / Revised: 13 February 2013 / Accepted: 15 February 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract A simple hybrid synthesis processing method was developed to synthesize γ-MnO2 nanocrystalline particles. The polyol method was modified by the addition of nitric acid in order to allow the synthesizing of single-phase Mn3O4 in a large scale. In the sequence, the acid digestion technique was used to transform Mn3O4 into γ-MnO2. Structural and morphological characterization was carried out by X-ray diffractometry, Infrared and Raman spectroscopy, thermogravimetric analysis, nitrogen adsorption isotherm, scanning electron microscopy, and transmission electron microscopy. The electrochemical properties were investigated by cyclic voltammetry and galvanostatic charge–discharge measurements. The synthesized material exhibits a specific capacitance of 125.1 Fg−1 at a mass loading of 0.98 mgcm−2. The relation between structural features and electrochemical activity is discussed by comparing the synthesized material with commercial electrolytic manganese dioxide. Keywords Supercapacitors . De Wolff disorder . Microtwinning . Pseudocapacitance

C. de A. Dias : M. C. Lopes (*) Departamento de Química, Univ Estadual do Centro Oeste– UNICENTRO, Guarapuava, Paraná, C.P. 3010, 85040-080, Brazil e-mail: [email protected] H. de Santana Departamento de Química, Univ Estadual de Londrina–UEL, Londrina, Paraná 86051-990, Brazil M. A. L. Nobre Faculdade de Ciências e Tecnologia–FCT, Univ Estadual Paulista–UNESP, P.O. Box 467, Presidente Prudente, São Paulo 19060-900, Brazil

Introduction Manganese dioxide-based supercapacitors, despite their lower specific capacitance, are presently considered as a low-cost alternative to commercial organic-based electrochemical double-layer capacitors or RuO2-based acid systems [1]. Manganese dioxide exists in different polymorphs, each one of them being only dependent of a single basic structural unit, the MnO6 octahedron. It has been shown that the γ-form possesses the highest electrochemical activity among the different polytypes [2]. Also, it was recognized that the level of structural disorder of the γ-form determines morphological and surface properties of the material, affecting its charge storage capacity. However, the role of structural and morphological features in the charge storage mechanism is still unclear. Commercial MnO2 for alkaline batteries has been produced by electrolysis of an acid solution of MnSO 4 , resulting in MnO2 with high structural disorder and low specific capacitance. This has fuelled the search for chemical methods with a better structural and morphologic control. Several authors have obtained γ-MnO2 through a direct chemical synthesis by hydrothermal methods [3–9], coprecipitation [10, 11] and sonochemical methods [12], among others. Also, the use of indirect methods which combine the synthesis of a lower manganese oxide as a precursor and its oxidation to γ-MnO2 by acid treatment has been reported. Several methods have been suggested to prepare the precursors like sol-gel synthesis [13, 14], solidstate reaction [15], reduction of the MnO2 ore [16], thermal decomposition of the electrolytic manganese dioxide

J Solid State Electrochem

(EMD) [17–19]. Other authors simply used commercially available Mn3O4 as precursor for acid digestion [20, 21]. The first attempt to prepare Mn3O4 via polyol method was made by Feldmann [22] envisaging magnetic and catalysis applications. A suspension of MnCl2 in ethylene glycol was heated at 180 °C for 2 h, yielding Mn3O4 spherical particles with average diameters of 30–40 nm. Recently, Sicard and coworkers applied polyol-made Mn3O4 as Fenton-like catalysts. In their synthesis, the authors used Mn(CH3COO)2 as a source of manganese which is soluble in diethylene glycol. After some time of heating, a rapid cooling of the solution allows the precipitation of hausmanite nanocrystals with diameters in the range of 8–13 nm [23, 24]. Liu and co-workers also used polyol synthesis with some additives to obtain Mn2O3, which is subsequently transformed in Mn3O4 by immersion in water [25]. To the best of our knowledge, the acid digestion of polyol-made Mn3O4 to obtain MnO2 was not attempted yet. We propose such a synthesis using a modified polyol method to obtain the Mn3O4. The proposed method uses manganese nitrate as a manganese source and nitric acid as oxidant reagent. Subsequently, the Mn3O4 is oxidized in acid solution to obtain γ-MnO2. This new route has the advantage of allowing an indirect control of the MnO2 particle size and morphology by controlling the synthesis of the precursor. Additionally, the method allows the preparation of large amounts of material in a short time from relatively cheap reagents. In the present paper, two main objectives are pursued. First, the synthesized material was characterized and electrodes prepared with it were tested in order to compare their electrochemical performance with similar materials in the literature. Second, a comparative study between the synthesized material and commercial EMD are performed. As EMD electrodes possess little or none pseudocapacitive behavior, the comparison provides a contrast through which the correlation between structural features and electrochemical behavior becomes apparent.

Experimental procedure Synthesis of the precursor Mn3O4 was synthesized by a polyol method which was modified by the addition of nitric acid. Manganese nitrate was used as a source of manganese and nitric acid was used as an oxidant reagent. Nitrate reduction promotes the release of gas which further inhibits particle aggregation. Manganese nitrate (Mn(NO3)2⋅4H2O) was dissolved in concentrated nitric acid (HNO3) with subsequent addition of the ethylene glycol (C2H6O2) under heating. The material formed was subjected to a pre-heat treatment at 150 °C for

2 h and calcined at 300 °C for 1 h under a nitrogen atmosphere. The remaining ash was crushed and sieved (325 mesh). With the resulting material we proceeded to the synthesis of γ-MnO2. Synthesis of γ-MnO2 The method of synthesis involved the digestion of Mn3O4 in a H 2SO 4 solution. One hundred cubic centimeters of 1.0 molL−1 H2SO4 was preheated at 60 °C in a beaker of 500 cm3. Ten grams of Mn3O4 was added to the solution of H2SO4 and was kept in the capped beaker under magnetic stirring at 80 °C for 10 h. After this time, the suspension formed was filtered and washed with distilled water. The resulting solids were dried in an oven at 60 °C with a constant air flux and, from now on, will be referred to as chemical manganese dioxide (CMD). It is believed that digestion of Mn3O4 in H2SO4 occurs via the following dismutation reaction [21]: Mn3 O4 þ 4Hþ ! MnO2 þ 2Mn2þ þ 2H2 O

ðIÞ

For comparison, EMD purchased from the company JB Química Indústria e Comércio LTDA was investigated in parallel with the synthesized material. Structural and morphological characterization The X-ray diffraction was performed on the powdered sample in an XRD-6000 diffractometer (Shimadzu) using Cu Kα radiation. Measurements were performed in a 2θ range from 10° to 80°. The average crystallite size was derived from the Scherrer equation [26] using the Jade 8 Plus software: D¼

kl b cos θ

ð1Þ

where D is the crystallite size, β is the broadening of the diffraction peak at half height, 1 is the wavelength of incident radiation (1.54060 Å) and k is a constant that depends on the reflection symmetry and generally taken as k=0.9 for powders. FTIR spectroscopy of prepared manganese oxides was carried out in a wavenumber range of 400 to 1,000 cm−1 in a spectrophotometer Excalibur FTS 3100 HE (Digilab). The Raman spectra were obtained from thin film samples, using a portable Raman Spectrometer DeltaNu with a 532-nm laser line and with the spectral resolution at 8 cm −1 . DeltaNu’s software, using baseline features, was used to remove background fluorescence. The surface morphology of the particles was determined using a scanning electron microscope (SHIMADZU SS 550) and a transmission electron microscope (JEOL EXII


1200). Surface areas were determined by nitrogen adsorption at 77 K, using a Micromeritics ASAP 2000 particle size analyzer. Before the measurements were taken, the samples were dried at 373 K and degassed to a pressure of less than 1 Pa. The surface areas were evaluated by the standard Brunauer Emmett Teller (BET) procedure.

where I stands for the charge current and t stands for the time of charge.

Results and discussion Structural characterization

Thermal analysis X-ray analysis The thermogravimetric curves of CMD and EMD samples were obtained in Exstar 6000 Seiko TG/DTA equipment using nitrogen atmosphere, in the temperature range of 17–700 °C at a heating rate of 5 °Cmin−1. Chemical analysis The amount of oxygen available in terms of %MnO2 was estimated by redox titration against KMnO4 [27]. The total content of manganese (%Mn) was determined by atomic absorption spectroscopy after dissolution in aqua regia. Fabrication and electrochemical measurements of MnO2 electrodes A mixture of MnO2/graphite powder/carbon black/polyv inylidene fluoride in the ratio of 70:12.5:12.5:5 was prepared and dissolved in N,N-dimethylformamide with ultrasonic agitation for 6 h, therefore obtaining a composite paste. A nickel foam (purchased from Marktech International) with apparent dimensions of 10×5 mm and previously treated with acetone and 3 molL−1 HCl was manually pasted to the composite material and then dried at 120 °C for 12 h. The composite electrodes were used as a working electrode in a single-compartment electrochemical cell, employing a titanium sheet as the counter electrode, Ag/AgCl as the reference electrode and 1.0 molL−1 Na2SO4 as the electrolyte. The cyclic voltammetry was performed in the potential range of 0.0 to 0.9 V vs Ag/AgCl at several scan rates. The specific capacitance (C) was calculated using the integrated area of the CV curve, as follows:

Figure 1 shows the X-ray diffraction patterns of CMD and EMD. The two investigated samples presented similar XRD patterns except for some differences that will be analyzed later in this paper. For both samples, no additional peaks could be observed, indicating the single-phase character of the studied materials. The CMD sample presented a typical XRD pattern, which matched with chemically synthesized manganese dioxide reported elsewhere [28], corresponding, therefore, to the γ-form. As mentioned by Walanda et al. [19], the γ-MnO2 structure was first described by De Wolff [29], who suggests that it consists of a microscopic intergrowth of pyrolusite (β-MnO2) and ramsdellite phases of manganese dioxide. This model was modified by Ruetschi [30], who proposed the existence of some cationic defects compensated by protons on adjacent oxide anions. Chabre and Pannetier [31] further improved this model by allowing it to have a successful correlation between structural features and surface properties. In this model, the degree of intergrowth, called the De Wolf disorder (Pr), was quantified by the shift of the (110) line. Additionally, a second type of growth defect in the ramsdellite lattice, identified as microtwinning (Tw), was assumed and used to correct the

R

C¼

IdV Q ; ¼ m v ΔV m ΔV

ð2Þ

where Q represents the charge, I represents the current, v represents the sweep velocity, m represents the mass of the active material (MnO2), and ΔV represents the width of the potential window. Galvanostatic charge–discharge curves were measured at different currents of 0.2, 0.5, 1.0 and 2.0 Ag−1. Galvanostatic capacitances were calculated using the following relation: C¼

It ; m ΔV

ð3Þ

Fig. 1 X-ray diffraction pattern of synthesized γ-MnO2 and commercial EMD


(110) line shift. The model leads to the classification of the different MnO2’s into four polytypes according to the Tw degree, and provides further explanation for its different electrochemical activity. Following the classification proposed by Chabre and Pannetier [31], the CMD sample can be categorized as being of the type II once the peaks corresponding to the planes (002) and (061) are clearly separated in the diffractogram, as can be seen in the insert of Fig. 1. According to the same authors, for type II structures, the amount of microtwinning was estimated by the following equation: Twð%Þ ¼ 100 25:20Δ2θ

ð4Þ

where Δ2θ stands for the separation in degrees between the (002) and (061) peaks in the 2θ scale. In the diffractogram shown in Fig. 1, the peaks corresponding to planes (002) and (061) are located, respectively, at 65.73° and 69.05°. Therefore, the peaks separation, Δ2θ, is 3.32 which leads to 16.34 % of microtwinning through Eq. 4. However, Chabre and Pannetier established empirically that the value of Tw calculated by Eq. 4 is about 80 % of the actual value. Thus, we assume Tw=20 %. The X-ray diffraction pattern of commercial EMD exhibits only two broad lines in the range 54°80

Pr 0.39 0.46

D (nm) 15±4 6±3

According to the classification suggested by Chabre and Pannetier [31]

Fig. 3 Raman spectra of (A) γ-MnO2 and (B) EMD solids, at a 532nm laser line with an 8-cm−1 resolution

J Solid State Electrochem Fig. 4 SEM micrographs (a, b) and TEM images (c, d) of γMnO2 (a, c) and commercial EMD (b, d)

location of the three major bands (designated by v1, v2 e v3) of the CMD are consistent with those reported in literature [34] for a sample of γ-MnO2, to which the pyrolusite content is similar to our synthesized γ-MnO2. It is possible to quantitatively determine the structural disorder in the MnO2 by Raman scattering spectroscopy. According to the authors who have made such elucidation [34], MnO2 Raman data should be treated by a local environment model, which takes into account the relationship between the wavenumber of the band and the intergrowth of pyrolusite. According to the authors, among the three main bands of ramsdellite, the v1 and v2 are shifted to higher wavenumbers due to the increase of pyrolusite content, while the v3 band remains almost at the same position.

For the CMD sample, the bands corresponding to v1, v2, and v3 are centered at 756, 644, and 575 cm−1, respectively, while for the EMD sample, these bands are centered at 773, 651, and 577 cm−1, respectively. Therefore, the Raman data

Table 2 Comparison between BET results and microtwinning Sample BET surface area (m2 g−1) CMD EMD

53.54 40.03

Average pore diameter (nm) 14.67 6.05

Total pore Tw volume (cm3 g−1) (%) 0.20 0.06

20 >80

Fig. 5 TG curves of CMD and EMD samples from 17 to 700 °C

J Solid State Electrochem Table 3 Weight loss in different temperature ranges

go in agreement with what was found by calculating the Chabre and Pannetier model and the FTIR analysis: the EMD has a higher amount of pyrolusite in its structure than the CMD.

while the EMD sample presents a more continuous weight loss when brought up to 400 °C. The weight loss in the first step, ranging from ambient temperature to 150 °C can be attributed to the loss of physically bound water and in the second step, ranging from 150 to 400 °C, can be attributed to the loss of chemically bound water [30]. The weight loss in the third step ranging from 400 to 600 °C can be attributed to the decomposition of MnO2 to Mn2O3 [16]. The percentual losses of mass in the different temperature ranges are listed in Table 3, showing that the amount of physically bound water is greater for CMD while the amount of chemically bound water is greater for EMD.

Morphology of nano-manganese oxides

Chemical composition and formulae

A set of scanning electron microscopy (SEM) micrographs and transmission electron microscopy (TEM) images of both CMD and commercial EMD powders is shown in Fig. 4. The micrographs of characterized samples are very similar, revealing globular aggregates of nanoparticles. TEM images show that the CMD particles are rod-shaped with about 10 nm in diameter and about 30 nm in length, while EMD particles present a plate-like morphology. Table 2 gives the results of the BET analysis for the two investigated materials. As shown, the γ-MnO2 has a much greater porosity but with averagely larger pore diameters, which results in a 30 % greater specific surface area than the EMD. It is interesting to compare the BET-specific surface area with the amount of microtwinning already calculated. The results shown in Table 2 are consistent with the results found by Prélot et al. [36]. The authors studied in detail the influence of structural defects in the surface properties of several types of MnO2 and concluded that the BET-specific surface area is correlated to the Tw and does not depend on Pr. Up to Tw=0.35 (S BET=40.4 m2 g−1) a positive correlation was observed between the BET-specific surface and the Tw, but for EMD samples, with Tw=1, the BET-specific surface area decreases (S BET=35.4 m2 g−1). Our data also follow this trend but for higher values of BET-specific surface area.

According to the model proposed by Coeffier and Brenet [37], the chemical composition of the manganese oxides can be expressed by

Sample

Weight loss (%)

Temp. range (°C)

17–150

150–400

400–600

4.45 2.59

2.78 4.51

6.44 7.71

CMD EMD

Thermal analysis TG curves of CMD and EMD samples are shown in Fig. 5. The TG curve for the CMD has three well-defined steps Table 4 Chemical composition obtained by chemical analysis

ðMnO2 Þ2n3 ðMnOOHÞ42n mH2 O where m is the number of neutral water molecules and n is the degree of oxidation of manganese. The parameter m is calculated as a function of the chemically bound water (w) which can be measured in the thermogravimetric analysis by the weight loss in the temperature range between 150 and 400 °C. The parameter n is calculated as a function of the ratio of reducible Mn to the total Mn (z) measured respectively by redox titration and atomic absorption spectroscopy. Results from the chemical analysis are summarized in Table 4. Thus, the chemical formulae for CMD and EMD are (MnO2)0.878⋅(MnOOH)0.122⋅0.076H2O and (MnO2)0.923⋅(MnOOH)0.076⋅0.189H2O, respectively. Electrochemical activity Figure 6a shows the cyclic voltammogram (CV) for both CMD and EMD electrodes in 1.0 molL−1 Na2SO4 electrolyte, at several scan rates. The CV profile clearly indicates the capacitive behavior with an almost ideal rectangular shape, which also indicates that both prepared electrodes exhibit good electrochemical reversibility between 0.0 and 0.9 V. At the end of the cathodic sweep, a current increase is observed, probably due to the MnO2 reduction. The onset of an anodic wave is observed for potentials more positive than 0.8 V. This can be compared with an anodic irreversible

Sample

MnO2 (%)

Mn (%)

z em MnO(1+z)

Chemically bound water (%)

Neutral water molecules

Deg. of oxidation (n)

CMD EMD

83.92±0.53 92.22±0.89

60.40±0.2 63.14±0.3

0.878 0.923

2.78 4.51

0.076 0.189

1.939 1.962

J Solid State Electrochem Fig. 6 CMD and EMD voltammetric behavior. a Cyclic voltammograms at several scan rates. b Specific capacitance as a function of the scan rate

wave observed by Toupin et al. [38] in the potential range of 0.5 to 0.65 V vs Hg/Hg2SO4 reference electrodes. The specific capacitances at different scan rates for both CMD and EMD electrodes are shown in Fig. 6b. As the scan rate increases, the specific capacitance for both samples decreases, which is characteristic of a limited diffusion process. At high scan rates, diffusion limitation slows the accessibility of the ions to the inner part of the material, and only the most accessible active surface contributes to the capacitance. Clearly, EMD capacitance is less dependent on scan rate. Such large capacitance retention is an indicative of the predominance of a pure capacitive behavior. The interfacial capacitance of EMD can be estimated from the potential interval 0.45–0.55 V in the cathodic sweep where the voltammograms are flat and practically independent of the scan rate. It corresponds to a specific capacitance of 21 F g−1 or, using the BET area, a geometric capacitance density of 52.5 μFcm−2. Figure 7a and b show the plots of 1/q vs v1/2 and q vs v−1/2, respectively. Both plots are very similar to that obtained by other authors [38] showing linear dependence to v1/2, which is characteristic of limited diffusion processes. By extrapolating q to v→∞ from the q vs v−1/2 plot (Fig. 7b) we obtain the outer charge q0, which is the charge on the most accessible active surface. The extrapolation of q to v→0 from the q−1 vs v1/2 plot (Fig. 7a) gives the total charge qT, related to the whole active surface. The less accessible active surface is obtained from the difference qT−q0. Table 5 compares the values of q0, qT, and the total geometric capacitance density (CdT) for both EMD and CMD samples. As indicated, the charge storage capacity of the chemically synthesized γ-MnO2 is larger than the EMD by a factor greater than 4. The comparison between q0 and qT points out to the significant differences in the charge storage mechanisms. For CMD the pseudocapacitive contribution associated to the inner charge represents 60 % of the total

Fig. 7 Variation of the voltammetric charge (q) with respect to the sweep rates v: a 1/q vs v1/2 plot gives the total charge qT, b q vs the v−1/2 plot gives the outer charge q0


charge. For EMD, the charge storage mechanism is predominantly capacitive and the inner charge represents only 32 % of the total charge. It is worth noting that the specific capacitance estimated from q0 (20.4 Fg−1) is practically the same as the specific capacitance estimated from the scan rate independent regions in the voltammograms (21.0 Fg−1), which confirms the interfacial nature of the EMD capacitance. The total specific capacitance and the total geometric capacitance density are indicated in the two last columns of Table 5. The magnitude of the total capacitance density obtained for CMD (233.6 μFcm−2) is about ten times higher than the classical double layer capacitance (~20 μFcm−2) and four times higher than the double layer capacitance (52.5 μFcm−2) estimated for EMD, which shows that the contribution of the MnO2 pseudo-capacitance is determinant for its storage capacity. A widely accepted mechanism to explain the MnO2 pseudocapacitive behavior implies on the cathodic intercalation of protons (H+) or alkali metal cations (M+) in the bulk of the material followed by the anodic deintercalation [39], as shown below: MnO2 þ Hþ þ e ! MnOOH

ðIIÞ

MnO2 þ Mþ þ e ! MnOOM

ðIIIÞ

where M+ can be equal to: Na+, K+, Li+. The intercalation process is facilitated in the CMD due to the large porosity and the higher content of physically bound water [40]. On the other hand, the higher defect content in the EMD increases the electronic conductivity favoring the interfacial charge storage [30]. Though it is instructive to compare the behaviors of EMD and CMD to enlighten the relationship between structure and electrochemical activity, it is evident that EMD is not an appropriate capacitance benchmark material. In Table 6, the specific capacitance of the material synthesized in this work is compared with chemically synthesized MnO2 reported in the literature. The entries in Table 6 refer to electrodes made solely of MnO2 and conventional additives, namely carbon black and binder, without any additive that contributes to the capacitance itself, as carbon nanofibers or nanotubes, and therefore are suitable to be compared with our material. It is observed that even at relatively high mass loading the γ-MnO2 synthesized in the present work has a comparable capacitance value with those reported in the literature.

Table 6 Comparison between capacitances of the EMD and γ-MnO2 reported in the literature Reference C (F g−1)

Scan rate Potential Mass loading (mg (mVs−1) window (V) cm−2)

CMDa

102.8

0.98

2

[0–0.9]b

EMDa

28.0

1.33

2

[0–0.9]b

[2]

65

0.11

200

[11]

95.5

8.1

5

[0–1.0]c

[−0.7–0.6]c

[10]

125

–

1

[−0.1 to 0.9]c

[9]

122

–

2

[0–0.9]c

[4]

107

0.5

20

[0–1.0]c

a

Electrolyte

1.0 M Na2SO4 1.0 M Na2SO4 1.0 M KOH 1.0 M Na2SO4 1.0 M Na2SO4 0.5 M Na2SO4 0.1 M Na2SO4

This work

b

Potential vs Ag/AgCl

c

Potential vs SCE

A further improvement on the electrochemical activity can be expected by tailoring the synthesis process and optimizing the electrode construction. It is also important to consider the galvanostatic charge– discharge performance of the electrodes prepared with the synthesized material. Figure 8 shows the capacitance retention as a function of the number of cycles at different current densities. The inset shows charge–discharge plots for the second cycle of each current density. The specific capacitances calculated from the charge–discharge curves are

Table 5 Comparison of charge storage capacity between EMD and CMD Sample q0/C g−1 qT/C g−1 CMD EMD

43.6 18.4

112.6 27.1

qT q0 qT

100 CT/F g−1 CdT/μF cm−2

61 32

125.1 30.1

233.6 75.2

Fig. 8 Capacitance retention of CMD electrodes as a function of cycle number measured at several current densities. The inset shows charge– discharge profiles of the second cycle for the same current densities. The electrodes were prepared with a mass loading of 0.81±0.01 mgcm−2


102.8, 104.5, 76.2, and 56.9 Fg−1 for the current densities of 0.2, 0.5, 1.0, and 2.0 A g−1, respectively. The value of 102.8 Fg−1 obtained for 0.2 Ag−1 is similar to that obtained voltammetrically at 2 mVs−1. At a high rate discharge of 1.0 Ag−1, the electrodes retain 74 % of the capacitance at 0.2 Ag−1, showing a high power performance comparable with similar materials reported in the literature [15, 41]. Under the discharge rate of 2.0 A g−1, any capacitance decrease was observed along 1,000 cycles. Under the discharge rate of 1.0 Ag−1, the electrodes show a capacity retention greater than 85 % over 1,000 cycles. Under lower discharge rates, the capacitance decreased faster, since a higher utilization of the material causes a faster deterioration of their properties. Electrochemical impedance spectroscopy measurements were performed after every 50 cycles of charge/discharge at current density of 0.5 A g−1 and after every 200 cycles of charge/discharge at current density of 2 A g −1 . The typical Nyquist plots (not shown) obtained in both cases are similar, presenting three distinct regions, namely a semicircle at high frequencies followed by a diagonal line at intermediate frequencies and an almost vertical line at low frequencies. An equivalent circuit represented by [Rs([RFCpW] Cdl)] was used to fit the experimental curves. In this notation, elements enclosed in square brackets are arranged in series and elements enclosed in rounded brackets are arranged in parallel. This circuit is similar to that proposed by Conway [42] and used for various authors [43, 44] to describe porous supercapacitors, but with an added Warburg element. RS is the solution resistance, RF corresponds to the charge transfer resistance through the pseudocapacitive processs, CDL is the double layer capacitance and C P arises from the pseudocapacitance. The Warburg element accounts for the semi-infinite diffusion of the electrolyte’s cations in the electrode. In the two sets of experiments performed, no significant variation of the RF was noted, indicating that the charge transfer resistance remains constant along the cycles. Consistent with galvanostatic measurements, the value of CP remains constant over 1,000 cycles under 2.0 Ag−1 and decreases by about 10 % after 200 cycles under 0.5 Ag−1. The most remarkable effect is the variation of the Warburg diffusion resistance upon cycling. Under 2.0 A g−1, the diffusion resistance increases by about 50 % after 1,000 cycles, while under 0.5 Ag−1, the diffusion resistance increases by about 120 % after 200 cycles. This suggests some electrochemically driven structural changes, hindering the cation transport through the electrode. At this point, the exact nature of this structural transformation is still unclear, and further investigations are needed to disclose the degradation mechanism occurring on the studied material.

Conclusions In this paper we show the possibility of using polyol-made Mn 3 O 4 as precursor for the preparation of γ-MnO 2 nanocrystals. A slightly modified polyol method followed by acid digestion was used to obtain rod-shaped nanoparticles about 10 nm in diameter and about 30 nm in length. The prepared oxide exhibits a lower degree of spontaneous microtwinning (20 %) and slightly lower content of pyrolusite in comparison with the commercial EMD. According to Chabre's and Pannetier's classification, γMnO2 belongs to the type II class material. The chemically synthesized material presents a specific capacitance of 125.1 Fg−1 at a mass load of 0.98 mgcm−2 which is comparable to similar materials reported in the literature. The microstructure appears to be determinant for the higher surface area and higher electrochemical activity of the synthesized material. The intercalation process is facilitated in the CMD due to the large porosity and the higher content of physically bound water, resulting in a greater pseudocapacitance. The higher defect content in the EMD increases the electronic conductivity favoring the interfacial charge storage.

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