VIETNAM JOURNAL OF CHEMISTRY VOL. 50(5) 619-623
OCTOBER 2012
CHARACTERIZATION OF MORPHOLOGY AND STRUCTURE OF Co AND Fe DOPED MANGANESE OXIDES FOR SUPERCAPACITIVE
APPLICATION
Le Thi Thu Hang'*, Nguyen Thi Lan Anh^ Mai Thanh Tung' Hanoi University of Science and Technology
Viet Tri University of Industry Received 27 September 2012
Manganese oxides doped by Co and Fe were prepared by anodic deposition at density current of 50mA/cm' using electrolyte contaming manganese sulfate and either cobalt sulfate or ferro sulfate. Surface morphology and crystal structure of oxides were studied using scanning electron microscope (SEM) and X-ray diffraction (XRD) Chemical compositions of materials were analyzed by using X - ray energy Dispersive Spectroscope (EDS). Average valence of manganese was estimated using iodometric titration and complexometric titration methods Results showed that the obtained doped manganese oxides were composed of nano-paticles sizing of 5-20 nm and remained amoiphous structure after heat treatmem at 100°C in 2 hours. After annealing at 400°C the crystalline Mn203 and MnOj phase were recognized obviously. The average valent of Mn increased fi-om +3.808 to +3.867 after doping Co and fi-om +3 808 to +3.846 after dopmgFe.
Keywords: Doped, manganese oxide, phases, supercapacitor.
1. INTRODUCTION
Electrochemical supercapacitors have attracted increasing attentions due to their interesting characteristics in terms of power and energy density and can be used for applications requiring a high power output and/or a high cycle capacity. The usual oxides to be used as the electrodes of supercapacitors are relatively expensive material, such as RuOj or Ir02 [1, 2]. Under the cost consideration, transition metal oxide that possesses multi-valence characteristics, easy to obtain, and relatively low cost is potential to be used as replacement. Manganese oxides, where the manganese exhibits many different valence states, are promising supercapacitor materials due to the low cost of raw materials and the fact that manganese is considered more environmental friendly than other noble metal oxides [3,4].
Recently, addition of other transition metal oxides such as: Ni, Mo, Fe and Co has been tried in order to further improve the pseudocapacitive performance of plain Mn oxide [5,6]. Doping transition metals makes morphology, structure of material as well as valence states of Mn change. This affects largely on supercapacitive performance of doped material.
In this paper, effect of M (Co, Fe) doping on the corresponding crystal structure, surface morphology, compositions and valence state of manganese oxides
for supercapacitive applications have been extensively examined.
2. EXPERIMENTAL
Co and Fe doped manganese oxides Mn(Co,Fe)0^ were deposited on graphite plates having an area of I cm by anodic deposition (M=
Co, Fe). Prior to electrodeposition, the graphite plates were etched ultrasonically in 0.2 M H2SO4 for 5 minutes and rinsed with distilled water. The electrodeposition was carried out in electrolyte containing 0.15 - 0.25 M MnS04, 0.05- 0.15 M sulfate salt of Co or Fe, 0.2 M ethylenediamine- tetraacetic acid (EDTA), pH = 6.5-7.0, t" = 80°C, i = 50 mA/cm . The concentration of M"* and Mn^"^
were varied so that the ratio [M"^]/[Mn^*] = 0/30;
5/25; 10/20; 15/15 and the total amount of cation was kept constant of 0.3 M ([M"^]+[Mn^*] = 0.3 M).
Thickness (d) of the obtained manganese oxides was controlled by deposition time so that d = 5- 6 (j,m. A three- electrode cell configuration which consists of a platinum counter electrode placed parallel graphite working electrode and a saturated calomel electrode (SCE) reference, was used for electrodeposition. The oxide electrodes obtained were dried in air for an hour. Then these electrodes were annealed at desired temperatures. The morphology and chemical compositions of the deposited oxides were examined 619
VJC, Vol, 50(5), 2012
wiih scanning electron microscope (SEM) and X-ray energy dispersive spectroscope (EDS), respectively.
Structures of the obtained oxides were confirmed by X-ray diffraction (XRD) analyses. Finally, average valence of manganese was estimated using iodometiic tilrpation and complexometric titration methods. Particularly, ekxtrodeposited manganese oxide materials consisting of MnO; and MnjOi after annealing in 2 hours were dissolved in a mixed solution of Kl and IL'SOj as followings:
M n 0 3 O I + 4 H ' - > M n ' + l 2 + 2 H 2 0 (I) Mn.Oj + 2 r + 6 H' ^ 2 Mn''+ I2 + 3 HjO (2) Derived products I2 and Mir were titrated by Na^S^Oi with an indicator of glucose and by cilnlcncdiamineletraacetic acid (EDTA) chelating agent uith an indicator of Eriochrome Black T, respectively, as following equations:
2 Na:S:0, + I2 -> 2 Nal + Na2S406 (3) Mn^' + EDTA""" -> MnEDTA^" (4) Thanks to consumed volume of h and EDTA, moles of MnOi and Mn203 in electrodeposited manganese oxide materials were calculated easily based on system of linear equation:
(6)
Le Thi Thu Hang, et al.
I-2H. 'A'nA - "M„'- - "F.DTA
The average valence of manganese x was estimated by:
X = ^-^
3. RESULTS AND DISCUSSION
(7)
During galvanostatic deposition, obtained relationship between potential and time was recorded and results are displayed in Fig. 1. These curves include two zones: (i) Unstable zone, which begins first thirty seconds, this zone is characterized by the varying of potentials with time. This zone represents for appearance of manganese oxide on graphite, (ii) Stable zone, which begins from thirtieth second, corresponds to the deposition of manganese oxide on the first dense manganese oxide layer. The value of potential in this zone was almost constant. Despite depositing at the same density current, the potentials of Fe doped samples were higher than of Co doped samples. This suggested that deposition current of Fe doped manganese oxide was smaller than that of pure manganese oxide and much smaller than that of Co doped manganese oxide.
Fig. 2 displays SEM images of the manganese oxides from electrolyte without doping (Fig. 2a), with addition of Co^* cation (Fig. 2b) and with
1,0 O 08
CO
g 0 6
UJ
0.4 02
1 = SOmAcm'
-IFt-MMn'-l-IS/IS / ' /.IMn'j'OW //ZlCo'-lf[Mn'-j.W5 / / / / lCo"l'lMn''|.)snS
/ / / /
/•
'
^•"""''^ unstable
/
"""'
100 200 300 400 500 600 t{8)
Fig 1: Galvanostatic curves of doped manganese oxides deposited from different electrolytes.
addition of Fe^' cation (Fig. 2c). It can be observed that morphology of manganese oxides all were kinds of nano-scalc llber-likc structure whose radian ranges between 15-20 nm and the surfaces were very porous. When Co^*. Fe^' were injected to the electrolyte, surfaces became more porous but porosity of each surface was different. This is attributed to the difference of electro-deposition rates.
Fig. 3 shows EDS spectra of Mn(Co,Fe)0, layers deposited in different electrolytes. As observed the specific peaks of Mn. Co, Fe and 0 elements appeared in these spectra, indicating thai the Co^' and le^* were deposited inside the layers.
The corresponding composition of Fe and Co using EDS and chemical analysis are summarized in Tab. 1. Il can be observed that atomic ratio M/(M+Mn) (M = Co, Fe) increases with increasing [M"']/[Mn"'] ratio. This behavior can be explained by fact that cobalt oxides and ferro oxides were co- deposited with manganese oxides during the process of electrolysis. Oxygen element contents of the obtained manganese oxides are higher than 70%, indicating that hydrated crystal structures were formed during deposition. Mn average valence of the materials was performed by using chemical method described in experimental part. It is note that average valence of Mn in doped materials increased with Co and Fe content in materials. These can be explained that the coelectrodepositing of Co and Fe with manganese oxides by the following reactions:
MniOa + M2O3 + 2H2O -> 2Mn02 + 2M(OH)2 (8) MnzOj + 2 M 0 0 H + HjO -> 2Mn02 + 2M(0Hh (9)
( M = Co, Fe) Post-heat treatment on prepared manganese oxides was also carried out in this study. It was found that material properties and pseudocapacitive performance of manganese oxide as a function of
VJC, Vol. 50(5), 2012 Characterization of morphology and slruciur
Fig. 2: SEM images of doped manganese oxides deposited from different electrolytes:
a) [Mn^*] = 0.3M b)[Co^*]/[Mn'*]= 15/15 c)[Fe'*]/[Mn^*]= 15/15
\L
i a * » , * < » . jilnwn;w,M
Fig. 3: EDS spectroscopy of manganese oxides obtained from various electrolytes:
a) [Mn'*] = 0.3 M b) [Co'*]/[Mn'*]= 15/15 c) [Fe'*]/[Mn'*] =15/15
annealing temperature varied with the preparation treated at various temperatures for 2 hours and their process adopted. Thus, in this study manganese XRD patterns are shown in F,g. 4.
oxide samples with different dopants were heat
VJC.Vol. 50(5), 2012 Le Thi Thu Hang, el al Table I: Chemical composition of the various oxides deposited from solutions with different
[M°*]/[Mn''] ratios, (M= Co, Fe).
Indeed
% atom (EDS analyses)
Co Mn 0 Co/(Co+Mn) Mn chemical state (chemical analyses) Chemical formulation
Co doped materials (Mn"]=0,3M
19.18 80.82 0.00 3.808 MnOi«M
[Co"]/[Mn"']-5/25 0.24 24.55 75.21 0.95 3.849 Mnow)COo,(lMOi,924
[Co'*)/[Mn"]-IO/20 0.88 28.54 70.58 3.00 3.852 Mno97oCOooJoO[M6
[Co'*]/[Mn'*]-l5/l5 0.79 14.98 84.22 5.27 3.867 Mnow7COooilO|93j Fe doped materials
Indeed
% atom (EDS analyses)
Fc Mn 0 Fe/(Fe+Mn) Mn chemical state (chemical analyses) Chemical formulation
tMn"j-0.3M 19.18 80.82 0.00 3.808 MnOi904
(Fc"]/|Mn'*J-5/25 2.32 21.89 75,79 9.58 3.819 Mno9o4Feoo960i9io
[Fe"J/[Mn'*]-10/20 4.48 27.06 68.46 14.20 3.846 Mnog5gFeoi420|923
|Fe'*]/[Mn'>l5/l5 3.82 23.39 72.79 17.02 3.834 MnogjoFeoi7oOi9i7
UOIS
0 ) t -
^
ctlU _ . _^
'"™»Kn," '°°°'^
(a) Gfaphile (bKMn'>0 3M (c) [Co"l/[Mn"l=15/15
«Jl[Fe"l/lMn'>15/15
^
A
f i
! f
T.n,..*., =^00°C (•ltMn'>03M (b)(Co")'[Mn'>15/1S tC)[FB'-HMr.'>iaiS
* GnphU M»02
i 1 "•'°'
E 1
I I I 1
A , A,,,A, ,— - . -,-,..1 .n«... '•'
I I I ! 1
Diffractive angle ( 26) Diffractive angle ( 26) Fig. 4: XRD patterns of the doped manganese oxides deposited from different electrolytes after heat
treatment at I OO^C and 400''C for 2 hours Table 2- Data obtained from XRD analyses
Sample [Mn" ] - 0.3M (Co'*]/[Mn"] =15/15 [Fe'*]/[Mn"]-15/15
Oxides phase Mn,Oj MnOj MnO, MnjO]
MnOj
Orient facets (222), (044) (211), (310), (200), (110) (211),(310),(200),(1I0)
(222) (211)
d ( ^ ) (211) 2.3931 2.4012 2.3969
-
(222) 2.7I8I
- -
2.7192
-
Results show that after annealing at 1 OO^C doped material samples deposited from various solutions have amorphous structures and/or hydrous structure.
This shows a similar trend as reported in the
literature [7]. All strong diffi-action peaks are associated with the graphite substrate. When the annealing temperature increased to 4O0''C temperature, formation of highly crystalline MniOj
VJC, Vol. 50(5), 2012
and MnO; phases were recognized. The oriental crystalline plans that predominate are (222) and (211), respectively. Additionally, crystalline orients many other facets with less reflective peak intension such as (310), (200), (110), (044). In presence of Co dopant, crystalline peaks of Mn203 disappear.
Meanwhile those of Mn02 remain. Information obtained from XRD analyses in table 2 also illustrates that distance of (211) orient facets of MnOi phase increases from 2.3931 A to 2.4012 A after doping Co. Similarly, with doping Fe, (211) distance increases from 2.3931 A to 2.3969 A and (222) one climbs from 2.7181 A to 2.7192 A. So doping M (M = Co, Fe) makes the crystalline structure of previous manganese oxide explain. In material respect, this valuable phenomenon suggests that it is a positive sign for pseudocapacitive performance. These further investigations should be conducted.
Overall, it is interesting to note that the different crystalline oxide phases of M (M = Co, Fe) were not recognized in these doping cases despite high annealing temperature. This can be explained by the fact that the amount of M is too small comparing with that of Mn.
4. CONCLUSIONS
Detailed characterizafion of Co doped manganese oxides was investigated using EDS, SEM, XRD and chemical analysis. The obtained results su^ested that all manganese oxides prepared by anodic deposition in the solutions with various [M"*]/[Mn^'^ ratios were composed of nano-paticles sizing of 5-20 nm. Co content of the obtained materials ranged from 0-5.27% while Fe content
Characterization of morphology and structure...
ranged from 0-17.02%. These oxide films have the amorphous and/or hydrous natures after annealing at I OO^C but have crystalline structure after annealing at 400°C. The chemical state of Mn increased from +3.808 to +3.867 after doping Co and from +3.808 to +3.846 after doping Fe as well.
Acknowledgements. We thank the Vietnam's National Foundation for Science and Technology Development (NAFOSTED). Project Nr 104.02.98.09) for the financial support of this work
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4. Ming-Tsung Lee, Jeng-Kuei Chang, Yao-Tsung Hsieh, Wen-Ta Tsai. Annealed Mn-Fe binary oxides for supercapacitor applications, Joumal of Power Sources, 185(2), 1550-1556 (.2008).
5. H. Kim, B. N. Popov. Synthesis and Characterization of Mn02-Based Mixed Oxides as Supercapacitor, Journal of Electrochemical Society, 150(3), D56-D62 (2003).
6. Ming-Tsung Lee, Jeng-Kuei Chang, Wen-Ta Tsai, Chung-Kwei Lin. In situ X-ray absorption spectroscopic studies of anodically deposited binary Mn-Fe mixed oxides with relevance to pseudocapacitance, Joumal of Power Source, 178(1), 476-482 (2008).
Corresponding author: Le Thi Thu Hang
Department of Electrochemistry and Corrosion Protection,
School of Chemical Engineering, Hanoi University of Science and Technology 1 Dai Co Viet, Hai Ba Trung, Hanoi Vietnam
Email: [email protected] Tel: 0978792108.
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