IVf P u b i i a j m g I Vielnam Academy ol Science and Technology Advances in Natural Sciences: Nanoscience and Nanotechnology
Adv. Nai Ser Nanosci Nanotechnoi. 6 (2015) 025011 (7pp) doi.10.1008/2043-6262/6/2/025011
Synthesis of nanostructured manganese oxides based materials and application for supercapacitor
Trung Dung Dang, Thi Thu Hang Le, Thi Bich Thuy Hoang and Thanh Tung Mai
Department of Electrochemistry and Corrosion Protection, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hai Ba Trung District, Hanoi, Viemam
(D
E-mail' [email protected] and mng [email protected] vn Received 2 December 2014
Accepted for publication 24 January 2015 Published 20 Februaiy 2015
CrossMark Abstract
Manganese oxides are important materials with a variety of applications in different fields such as chemical sensing devices, magnetic devices, field-emission devices, catalysis, ion-sieves, rechargeable batteries, hydrogen storage media and microelectronics. To open up new applications of manganese oxides, novel morphologies or nanostructures are required to be developed. Via sol—gel and anodic electrodeposition methods, M (Co, Fe) doped manganese oxides were prepared. On the other hand, nanostructured (nanoparticles, nanorods and hollow nanotubes) manganese oxides were synthesized via a process mcluding a chemical reaction with carbon nanotubes (CNTs) templates followed by heat treatment. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), cyclic voltammetry (CV) and impedance spectroscopy (EIS) were used for characterization of the prepared materials. The influence of chemical reaction conditions, heat treatment and template present on fhe morphology, strucmre, chemical and electrochemical properties of the prepared materials were investigated.
Chronopotentiometry (CP) and CV results show high specific capacitance of 186.2 to 298.4 F g ~ ' and the charge/discharge stabihty of the prepared materials and the ideal pseudocapacitive behaviors were observed. These results give an opening and promismg apphcation of these materials in advanced energy storage applications.
Keywords: manganese oxide, nanostructure, supercapacitor Classification numbers: 4.00, 4.02, 4.06, 5.00
1. Introduction solution [6, 7], or oxidation by Mn04 [7], Oj, KaSaOg, and H2O2 [8] or by reduction of Mn04 using different routes [9].
Manganese oxides ( M n O J are natural components of soils. The reduction of KMnO^ in dilute aqueous H2SO4 produces aquifers and sediments, and are known to be strong adsor- manganese oxide nanoparticles witii hexagonal layer structure bents of metal ions. Synthetic manganese oxides with micro- [10]. As far as the synthesis of I D manganese oxide nano- and nano-structm^es are still attracting attention due to their materials is concerned, nanowires of a-MnOa are synthesized wide applications in different fields, such as catalysis, ion- using coordination-polymers [11-13], whereas single-crys- sieves, rechargeable batteries, chemical sensmg devices, talline ^-Mn02 nanorods are prepared by hydrothermal or magnetic devices, field-emission devices, hydrogen storage electrochemical methods [2, 3, 14, 15].
media and microelectronics [1-5]. Micro-, meso- and nano- Electrochemical capacitors (ECs), known as super- porous manganese oxides, as well as layered claylike man- capacitors, have attracted great interest as promismg energy ganese oxide nanomaterials are prepared via various routes, storage devices due to their high power energy density and There are many reports m the literature regarding synthetic long cycle performance in comparison with conventional processes tiiat include eitiier oxidation of Mn(II) in basic dielectric capacitors [16, 17]. ECs are being considered for 2043-6262/lS'0250l1-t-07$33.00 1 ©2015 Vietnam Academy ol Science & Technology
45
Adv. Nat SCI : Nanosci Nanotechnoi 6 (2015) 025011
different applications such as power sources for camera flash equipment, lasers, pulsed light generators, and as backup power source for computer memory [18]. They also became of interest in hybrid electric vehicles as an auxiliary power source in combination with battery. ECs can provide the peak power in such hybrid systems when accelerating, and thus the batteries can be optimized pnmarily for higher energy density and better cycle-life. Due to its satisfactory electrochemical performance, natural abundance and environmental compat- ibility, manganese oxide (Mn02) and especially, nanos- tructured manganese oxide is considered one of the most promising electrode materials for supercapacitor apptications [14, 15, 19-22].
In tills study micro-, meso- and nano-porous structured manganese oxides and also M(Fe, Co) doped Mn02 were prepared via chemical and electrochemical methods Mor- phology, physical-chemical properties and electrochemical analysis were investigated to study imtially the super- capacitive behavior of prepared materials.
2. Experimental
Amorphous nanocluster manganese oxide (ANCMO) was synthesized via the reaction of diluted 0.1 M solution of potassium permanganate (Sigma-Aldrich), K M n 0 4 , with 6 M solution of hydrochloric acid (Sigma-Aldrich), HCl, at room temperature with ultrasonication. The formed solids are fil- tered and washed five times with de-ionized water, and then dried in air at room temperature for 24 h. Crystalline man- ganese oxide nanorods (CMONR) were prepared via heat treatment at 650 °C of unwashed ANCMO (high content of potassium ion) for 5 h The same reaction with the presence of carbon nanotubes (CNTs) templates forms CNTs coated Mn02 and after heat ti-eatment for 5 h at 400 and 650 °C, amorphous and crystalline manganese oxide hollow nano- tubes (AMONTs and CMONTs) were formed.
On the other hand, Co and Fe doped manganese oxides, Mn(M)Ox with M - C o , Fe, were electro-deposited on the graphite plates having 1 cm^ work surface area by anodic deposition. The electi:odeposibon was carried out in the elec- trolyte contauung 0.15-0.25 M MnSOj, 0.05-0.15 M sulfate salt of Co or Fe, 0.2 M EDTA, p H - 6 . 5 - 7 . 0 , ro = 800°C, / - 5 0 m A c m ~ ^ . The concentrations of Mn"^ ( M - C o , Fe) and Mn^"" were varied so fliat tiie ratio [Mn'"]/[Mn^'^ = 0/30; 5/25, 10/20; 15/15 and the total amount of cations was kept constant at 0.3 M ([Mn"']-H[Mn^"^ = 0.3M). The oxide elecbrodes obtained were dried at 100 "C in air for 2 h.
The morphology and chemical compositions of the deposited oxides were examined with scanning electron microscope (SEM), energy dispersive x-ray specti-oscopy (EDX), transmission electron microscopy (TEM) and che- mical analysis. The average valence of manganese was esti- mated using iodometric titration and complexometric titration methods, Electi-ochemical behavior and specific capacitance of the deposited oxides were characterized by cyclic vol- tammetry (CV) and impedance spectroscopy (EIS) tests in 2 M KCl solution at 25 °C.
3. Results and discussions
SEM and TEM images of the M n 0 2 nanostructures prepared by the above-mentioned chemical processes are shown in figure 1 As mentioned in the experimental section, ANCMOs are prepared according to the following reaction between KMn04 and HCl'
2KMn04 -I- 8HC1 ^ 2KC1 -I- 2Mn02 -t- 4H2O -I- 3CI2. (1) This figure shows that the nanosclusters are composed of Mn02 worm-like fibers aggregating on the surface of the nanosphere These fibers appear more tighly packed and slightiy smaller in size compared to the control sample (figure 1(a)), whereas the nanorods of the Mn02 which were prepared by heat treatment of ANCMO are well grown and uniform (figure 1(b)). Via the same chemical reaction, with the presence of CNTs templates and after heat treatment, amorphous and crystalline manganese oxide hollow nano- tubes (AMONTs and CMONTs) were successfully synthe- sized (figures 1(c) and (d)) The formation of hollow nanombes with porous worm-like morphology in amorphous nanotube and granular morphology in crystalline nanotubes was observed clearly.
On the other hand, electrolyzed manganese oxide and M (Co, Fe) doped manganese oxide were synthesized via ano- dizing process (figure (2)). It can be observed that morphol- ogy of manganese oxides has nanoscale fiber-like sfructure whose radius ranges between 15-20 nm and the surfaces were very porous. When Co^* and Fe^"^ were injected to the elec- trolyte, surfaces became more porous but porosities of sur- faces were different. This is attributed to the difference of electro-deposition rates which are shown in figure 3.
During galvanostatic deposition, the obtained relation- ship between potential and time was recorded and results are displayed in figure 3. These curves include two zones' (i) unstable zone, which is tiie first thirty seconds, is character- ized by the variation of potentials with time. This zone represents the appearance of manganese oxide on graphite;
(ii) stable zone, which begins from the thirtieth second, cor- responds to the deposition of manganese oxide on the first dense manganese oxide layer. The value of potential in this zone was almost constant. Despite the deposition at the same density current, the potentials of Fe doped samples were higher than tiiose of Co doped samples. This suggested that the current of tiie deposition of Fe-doped manganese oxide was smaller than that of pure manganese oxide and much smaUer than the one with Co-dopmg.
The composition of fhe nanostmctured manganese oxides (ANCMOs, CMONRs, AMONTs, and CMONTs) which was determined by EDX and chemical analysis consists of man- ganese and oxygen only but the compositions of the elec- ttolized manganese oxide and M-doped manganese oxide are different and are summarized in table 1. It can be observed that atomic ratio M/(M + Mn) (M = Co, Fe) increases with increasing [M""^]/[Mn^'^ ratio. This behavior can be explained by the fact that cobalt oxides and ferro oxides were co- deposited with manganese oxides during the process of electiolysis. Oxygen element contents of the obtained
anosci Nanotechnoi. 6(2015)025011
Figure 1. SEM and TEM (inset) images and of nanostmctured manganese oxide (a) ANCMOs, (b) CMONRs, (c) AMONTs and (d) CMONTs.
=^-pif
/ ( . ' . „ .
Figure 2. SEM unages of doped manganese oxides deposited from different electrolytes (a) [Mn^*] = 0.3M; (b) [Co^*]/[Mn^*l = 15/15 and (c)[Fe^*l/[Mn=n= 15/15.
Adv. Nat. SCI Nanosci. Nanotechnoi 6(2015)02501
Figure 3. Galvanostatic curves of doped manganese oxides from different electrolytes.
manganese oxides are higher than 70%, mdicating that hydrated crystal structures were formed during deposition.
These can be explained by tiie co-electrodeposition of Co and Fe with manganese oxides by the following reactions:
M n 2 0 j -I- M2O3 + 2H2O -» 2Mn02 -1- 2M(OH)2, (2)
* 2Mn02-»-2M(OH)2, M n i O j -I- 2 M 0 0 H -I- H2O -
(M = Co, Fe). (3)
The elecfi-ochemical performance of the electrolyzed manganese oxide and M(M = Co,Fe)-doped manganese oxide was evaluated by cychc voltammetry tests in the 2 M KCl electrolyte. The voltammograms measm^ed at various poten- tial scan rates of doped manganese oxides obtained from different electrolytes are shown in figure 4. It can be observed that all the CV curves of electrodes coated with Co- and Fe- doped manganese oxides were symmetric in anodic and cathodic areas. The CV responsive current remained nearly constant dunng forward and backward scans. It is important to note that the rectangular shapes and mirror image char- acteristics of these CV curves represent the idea] pseudoca- pacitive behavior. These results suggested that M(M = Co, Fe)-doped manganese oxides are promising electrode mate- rials for supercapacitive applications. In this figure, although
tfie shapes of CV curves of tiiese materials are similar, Co- doped manganese oxides had a smaller enclosed area tiian pure manganese oxides. This reflects their degraded charge storage performance. Meanwhile the addition of Fe into materials had more enlarged area which corresponds to their ; mcreased charge storage.
However, in order to consider the quantifiable influence of doping Co and Fe on this manganese oxide matenal, variations of the specific capacitance of samples with different potential scan rates and dopant contents were calculated fi-om CV. Results are illustrated in figure 5. At first glance, it is important to see that doping Co increased tiie specific capa- citances for manganese oxide at high sweep rates. After doping Fe the specific capacitances of materials increased at all sweep rates. The maximum specific capacitances achieved at tiie ratio Fe/(Mn-(-Fe)= 14.6% (sample obtained from solution [Fe^*]l[Mn^*] = 2QnO). This can be explained as the addition of Co and Fe in materials making the average valence of Mn element increase. Moreover, both Co- and Fe- doped manganese oxides had specific capacitances decayed with increasing potential sweep rate. This is attributed to the fact that at high scan rates cations do not keep up widi dif- fusing deeply into layer structure of materials, so the Faradic reactions were prevented.
Electiochemical stability of the doped oxide electrodes was also evaluated by repeating the CV tests for 500 cycles Figure 6 shows the variations of the specific capacitance after scanning 500 cycles. As observed, M-doped manganese oxides had the higher charge-discharge stabihty compared to pure material. The pure manganese oxide retains only 70% of its initial specific capacitance after cycling, while tiie capa- citance retained ratios gam about 84% for Co-doped and 83%
for Fe-doped manganese oxides. Higher cychc stability for Co-doped manganese oxides was also reported by otiier authors [23, 24]. The decay of specific capacitances can be explained as follows:
(i) The first is due to corrosion of materials during the active process. If so, doping Co and Fe diminished Table 1. Chemical composition of the doped manganese oxidesdeposited from solutions with different [M'''^/[Mn^] ratios, (M = Co, Fe),
Indeed Co (%at) Mn (%at) 0 (%a() Co/(Co + Mn) Average value of Mn
[Co ^/[Mn^*] = 0/30 19.18
—
80.82 0.00 3.808
[Co'*]/lMn'*] = 5/25 0 24 24.55 75.21 0.95 3.849
Co-doped materials [Co2+l/[Mn^']= 10/20
0.88 28.54 70.58 3.00 3 852
[Co2+]/[Mn^*]^ 15/15 0.79 14.98 84.22 5.27 3.867 Fe-doped materials
Indeed Fe (% at) Mn (% at) 0 (% at) Fe/(Fe + Mn) Average value of Mn
[Fe *)/[Mn'*] = 0/30 19.18
—
80.82 0.00 3 808
[Fe'*J/[Mn^*| = 5/25 2.32 21.89 75.79 9.58 3.819
[Fe3-l-l/[Mn'-| = 10720 4.48 27.06 68.46 14.20 3.846
[Fe3-fJ/[Mn'T = I5/I5 3.82 23.39 72.79 17.02 3.834
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b). [F»'-lr[Mn 1 = 5/25 Cl (F«'7|Mn'T = i a r M If)-IFe'TlMfi-l= 15(15
0.0 0 2 0.4 0 6 Oa 10 0 0 0.2 0 4 0 6 OS 10
E(V)/SCE E(V)/SCE Figure 4. CycHc voltammetry curves of doped manganese oxides, (a) Co-doped matenals, (b) Fe-doped matenals
F e / ( F e + M n ) a t o m i c ratio
Figure 5. Dependences of the specific capacitance of materials on the potential scan rates and dopant contents, (a) Co-doped materials and (b) Fe-doped materials.
(b) 160
140
- 120
100
80
Fe doping O p i H R y o f i m l
, ^ * * * , ^ ^ *FaitDiHnerningiiH l u m n g n t i p K i n e
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Figure 6. Fading of the specific capacitance of different manganese oxides with CV cycles to clarify tiiose explanations electrochemical impedance spectroscopies were carried out. (a) Co-doped materials, (b) Fe- doped materials.
defects of lattice structure making materials stable and inert,
(ii) The latter is due to the intercalation or deintercalation of H* and K^ ion in the bulk of manganese oxides:
M n 0 2 MnOz
H+ -t^ e - -<- MnOOH, K+ -)- e - -» MnOOC.
If the intercalation or deintercalation happens easily and the change of volume of oxides is negligible, tiie specific
capacitances of these oxides decay more slowly after 500 cycles.
Figures 7 and 8 display Nyquist plots and the equivalent circuit model of manganese oxides electrode. The Nyquist plots of impedance of the manganese oxides electrode con- sists of an arc and a spike. The arc at higher frequency region should be associated with the charge transfer process at electrode/electrolyte interface [25], and the next part at lower frequency region should be ascribed to the diffusion of cations m the bulk of manganese oxides and the inner contact
Adv. Nat Sci.; Nanosci Nanolechnol 6(2015)025011
• • - As-prepaced . . • - A f l e r l O C V
• A - A f e f l O O C V
- • - A s - p i c p a c E d - ^ A n » 10 CV - ^ A T e r 100 CV
Z.C'cin') z„(ncm-l
/
1=0 3M
-A-Aner100CV
(e)
200 400 600 BOO l^incrri')
Figure 7. Nyquist of manganese oxides obtained ftom different electrolytes (a) [Co^*]/[Mn^•^ = 5/25. (b) [Fe^*l/[Mn^*] = 5/25, (c) [Co^T/
[Mn-*] = 15/15. (d) [Fe'']/[Mn-*] = 15/15, (e) Mn^T = 0.3M.
-%-jH
Figure 8. Equivalent circuit model of the manganese oxide electrodes. R^. solution resistance; CPEi: constant phase representing double layer; R,: charge Uansfer resistance; (R2CPE2)W: diffuse impedance of cation into pores of elecmxle at low frequency; Rj: contact resistance between Pt with manganese oxide; C3: capacity between Pt surface and manganese oxide.
of the films. The equivalent circuit model shown in flgure 8 addition of Co and Fe decreased diffuse resistance, charge consists of tiiree parts representing three processes: charge transfer resistance and contact resistance of the films. The transfer, diffuse and contact. Data obtained by fitiing with the addition of Co and Fe also increased electiic double layer circuit model are summarized in table 2, showing that the capacitance and capacitor response ftequency. These caused
'lanoscL Nanolechnol 6 (2015) 025011
Table 2. Selected data obtained by fitting the experimental impedance data Eo flie equivalent cux;uit.
Oxides obtamed fttim electrolytes CPEI (fiF) Rl (i2cm ) Rj (i3cm ^) R3 (i2cm"T f^-45° (Hz) [Mn'*) = 0.3M
[Co'*)/[Mn=*] = 5/25 [Co'*l/(Mn'*] = 15/15 [Fe'*l/[Mn'l = 5/25 [Fe'*l/[Mn^*J = 15/15
as prepaied after lOOCV as prepared after lOOCV as prepared after lOOCV as prepared after lOOCV as prepared after lOOCV
2.201 3.72 2.38 3.81 12.42 10.45 10.32 7.54 9.48 8.84
3.76 4.47 5.23 3.43 5.37 2.25 3.36 4.01 4 53 3.45
1.62 4 95 3.61 3.23 2 15 3.84 4.61 2.56 2.64 3.48
6.27 9 32 5 80 8.65 4.68 7.65 5.32 7 93 6 12 8 98
1.22 0.52 0.967 2.11 4.27 8 21,526,5715 53.5, 1600
0 967 0 032. 6760 10200
the specific capacitance and stabihty of manganese oxides doped with Co and Fe to increase
The electrochemical performance of the prepared nanos- tmctured manganese oxides are ongoing and will be reported soon but their nanoporous stincture, which offers very high specific surface area, promises good apphcation not only in supercapacitors but also in other energy storage systems.
4. Conclusions
Nanostructures of manganese oxides and also M(Fe, Co)- doped manganese oxides were successfully prepared via chemical and electrochemical methods. SEM and TEM ana- lyses confirm the formation of nanocluster, nanorod, nanofi- ber and hollow nanotubes with porous worm-like morphology in amorphous and granular morphology in crystalline struc- ture. The electrochemical performance of the electrolyzed manganese oxides and also M(Fe, Co)-doped manganese oxides was smdied. The highest specific capacitance of 186.2 F g " ' and stability of 84% obtained wifli flie Co-doped manganese dioxide. For Fe- doped manganese dioxide, those parameters were 2 9 8 . 4 F g " ' and 83%, respectively. The Co", Fe~doped nanostructured manganese dioxide film-coated electiodes showed ideal pseudocapacitive behaviors. The addition of Co and Fe decreased diffuse resistance and tiansfer resistance of the films. The electric double layer capacitance and capacitor response frequency is also increased. The successful fabncation of amorphous and crystalline nanostrucmred manganese oxide and also M(Fe, Co)-doped nanostracmred manganese oxides give an opening and promising application of these matenals in supercapacitor applications.
Acknowledgments
This work is funded by Vietnam's National Foundation for Science and Technology Development (NAFOSTED) (Pro- ject Nr. 103.02-2013.76), The World Academy of Sciences (TWAS) Research Grant (No: 14-107RG/CHE/AS_G-
Unesco FR:324028603) and Nippon Sheet Glass Foundation for Materials Science and Engineering (NSG Foundation).
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