Physicochemical and electrochemical behaviours of manganese oxide electrodes for supercapacitor application

Journal of Energy Storage 28 (2020) 101228

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Physicochemical and electrochemical behaviours of manganese oxide electrodes for supercapacitor application

T

Nitika devia,b, Manoj Goswamia,e, Mohit Sarafd, Bhupendra Singha, Shaikh M. Mobinc,d, ⁎ Rajesh Kumar Singhb, A. K. Srivastavaa,e, Surender Kumara,e, a

CSIR - Advanced Materials and Processes Research Institute, Bhopal - 462026, India Department of Physics, Central University Himachal Pradesh, Dharamshala - 176215, India c Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore 453552, India d Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore 453552, India e Academy of Scientific and Innovative Research (AcSIR), CSIR – Advanced Materials and Processes Research Institute, CSIR-AMPRI campus, Bhopal - 462026, India b

A R T I C LE I N FO

A B S T R A C T

Keywords: As-prepared MnO2 Physicochemical Supercapacitor Reducing agent

Objectives of this study are to probe the effect of reducing agents on physicochemical and electrochemical properties of manganese oxide. Manganese oxide is synthesized by chemical reduction of KMnO4 at room temperature using ethylene glycol, hydrazine hydrate, Na2S2O3, potassium iodide, formic acid, citric acid, and NaBH4. All as-prepared manganese oxide samples are analysed by powder XRD, FE-SEM and FT-IR. It is found that manganese oxide prepared using formic acid and sodium thiosulphate have nanorod or nanowire type morphology as confirmed by FE-SEM analysis. Electrochemical properties of samples are studied in aqueous medium (1 M Na2SO4) by cyclic voltammetry and galvanostatic charge-discharge techniques. Due to the rod type structure formic acid sample shows high surface area (117 m2g−1) and high porosity (0.1331 cc g−1), which results into high specific capacitance of 155 Fg−1 at 0.64 Ag−1.

1. Introduction Present energy crisis of world can be addressed through efficient energy storage and conversion devices [1,2]. Out of these possible devices, supercapacitors have been found to be efficient in providing good power density, and find the potential applications in automobile industry, smart phones, solar panels, electric drives, uninterrupted power supply systems, active filters, etc. [3,4]. Transition metal oxides (TMO) such as RuO2, Fe2O3,V2O5, Co3O4, MnO2, Mn3O4 etc., are explored as electrode material for charge storage, mainly by chemical reactions [5–7]. Out of these metal oxides, RuO2 was found to have a high theoretical and experimental specific capacitance value (1300–2200 Fg−1), but it is not a cost efficient material for commercial applications [8,9]. Manganese oxides also possess very good specific capacitance value with comparatively cheap price [10,11]. In crystalline Manganese oxides charge storage is mainly by reversible reaction because of its variable oxidation states [12,13]. Estimated specific capacitance of the MnO2 is 1100 Fg−1 for voltage window of one volt, it is found that specific capacitance of MnO2 directly dependent on the nature and concentration of metal cation in aqueous solution, with charge-discharge reaction being represented by Eq. (1).

⁎

(MnO2)surface + M n + + ne− ↔ (MnO2−M+n )surface

(1)

Generally, MnO2 exists in different crystallographic structures [14,15], out of these phases, α and δ forms are found to be more suitable for supercapacitor applications, as their adequate interlayer space and gap eases the insertion of ions [14]. Ragupathy et al. [16] reported different crystal structures of MnO2; synthesised by reduction of KMnO4 using aniline, to investigate the effect of crystal structure on electrochemical properties. It was also reported that size of electrolyte ions also effects ion-insertion in the electrode material, which ultimately affects the specific capacitance. Kumar et al. [17] dealt with the reversible insertion of trivalent cations onto MnO2. Specific capacitances of MnO2 powders prepared by different methods have a large difference in their specific capacitance ranging from 100–250 Fg−1. MnO2 prepared by electrochemical deposition method gave a capacitance value of 230 Fg−1 [18]. Electrochemical deposition was used for preparation of mesoporous MnO2-carbon aerogels which were found to have a large surface area 120 m2 g−1. Specific capacitance of this composite was 515.50 Fg−1, it was credited to synergetic effect of employed method of synthesis and its composite nature [19]. The capacitance of MnO2 can be improved by enhancing surface area and morphology [20]. Amorphous nature of the material

Corresponding author at: CSIR - Advanced Materials and Processes Research Institute, Bhopal - 462026, India.

https://doi.org/10.1016/j.est.2020.101228 Received 23 September 2019; Received in revised form 18 January 2020; Accepted 18 January 2020 2352-152X/ © 2020 Elsevier Ltd. All rights reserved.

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can also contribute some additional benefits to electrochemical properties. It has been found that amorphous or poorly crystalline MnO2 have good specific capacitance as compared to the purely crystalline materials [21–24]. Ren et al. [25] Synthesized tremella-like MnO2 in which the sample with the lowest crystallinity has shown the highest specific capacitance. Huang et al. [26] reviewed and discussed in detail the role of synthesis method, crystallinity, morphology etc. on the formation of various MnO2 nanostructures for supercapacitor applications. Li et al. [27] reported the synthesis and supercapacitor performance of MnO2 nanostructureshaving 3D morphology. These nanostructures synthesized by template-assisted hydrothermal process shows specific capacitance of 371.2 Fg−1 at a scan rate of 0.5 Ag−1 for melosira type MnO2. Other than MnO2, NiO [28], CuO [29] and other composites such as Cu0.27Co2.73O4/MnO2 [30] with different nanostructures are also found to be efficient materials for supercapacitor applications. Transition metal oxide/graphene composites are also promising composite materials for next generation energy storage devices. Composite of reduced graphene oxide (rGO) with MnO2 and Fe2O3 were synthesized by Saraf et al. [31,32] and found to be an excellent energy storage materials. The simplest synthesis procedure for MnO2 is to reduce the KMnO4 by chemical route. Some reducing agents like Potassium borohydride (KBH4), Sodium dithionite (Na2S2O4), Sodium hypophosphite (NaH2PO2) have been used by Jeong et al. [33]. The motive of this piece-of-work is to see effect of different reducing agents on physicochemical and electrochemical output of as-synthesized MnO2 nanostructures. In this work, we have synthesized the MnO2 by using reducing agents like ethylene glycol (EG), Potassium Iodide (KI), Na2S2O3 (NSO), Hydrazine hydrate (HH), NaBH4 (NBH), citric acid (CA), and formic acid (FA). Also, we have studied the effect of the surface area, structure and morphology on the electrochemical behaviour of all as prepared MnO2 samples. Generally, as-prepared samples shows better capacitance as compared to heated samples at various temperatures [34–36]. The main aim of this manuscript is to investigate physiochemical and electrochemical outcome of as-prepared MnO2 samples.

Fig. 1. Schematic of synthesis procedure.

the MnO2 were explained and discussed [14]. It can be seen from XRD patterns that as-synthesised MnO2 material is mostly in amorphous phase, which belongs to α-MnO2 [14]. In our case, the samples prepared by NBH, HH, CA found to have some crystalline peaks as compared to samples prepared by NSO, KI, FA, EG. These additional diffraction peaks could be seen when strong reducing agents like NBH and HH were used. It has been found that these crystalline peaks can be designated to Hausmannite structure of Mn3O4 according to the JCPDS number 89-4837 [37]. Identification of the attached functional groups was done by using FT-IR analysis. Fig. 3 gives FT-IR spectrum for as-synthesized powder of MnO2 using various reducing agents. Peaks in range of 3200–3400 and 1300–1650 cm−1 resemble to OeH bond vibration and CeC bond vibration. Peaks in 457–1054 cm−1 range can be assigned to vibration of bonds between Mn and oxygen. The transmission peaks in 457–485 cm−1 belongs to vibration of νeOeMneO symmetric bond and those at 563 and 594 cm−1correspond to υ(MneO) symmetric stretching and δ(MneO) bending, respectively. Peaks representing symmetric stretching of υ(MneO), δ(OeMneOH) bending vibrations are at 614 and 1052–1075 cm−1 [38], respectively. Thus, FT-IR study concludes that KMnO4 has been reduced to manganese oxide. Morphological study was carried out using FE-SEM for various samples of manganese oxide. FE-SEM images of all the samples are shown in Fig. 4a to g, belong to samples reduced via EG, HH, NSO, KI, FA, NBH, CA, respectively. The samples prepared using FA, NSO and KI are found to be nanorods, nanowires and non-uniform small size nanosphere. Particles are porous and of different sizes in manganese oxide samples prepared using EG (Fig. 4a) HH (Fig. 4b), NBH (Fig. 4f), and CA (Fig. 4g), and this porosity in some samples result into high surface area such as EG and NBH samples. This difference in morphology may arise due to the difference in surface activity and reaction kinetics of various reducing agents. The nanostructures formation includes steps like nucleation, aggregation and then growth, different rate of chemical reactions can result into different type of nanostructures. A study by Chodankar et al. [39] concludes that a slow reaction rates results into more fibrous and high surface area materials in comparison to fast reaction rates which result into aggregated nanostructured materials. As studied NSO, FA, EG have slow reaction rates and results in fibrous nanorods, nanowires and porous nanostructures in comparison to NBH, HH, CA and KI reducing agents. As observed, MnO2 samples (Fig. 4a, c, d, e), were more efficient than other samples in term of BET surface area, wire type nanostructure and porous nanostructure provide high surface area in comparison to other nanoparticles morphologies. BET

2. Experimental MnO2 was prepared by chemical reduction of KMnO4 employing different reducing agents that are EG, KI, NSO, HH, NBH, CA, and FA. 3.16 gm of KMnO4 was taken in 200 ml water and 5.0 ml of EG is slowly mixed for reducing 0.1 M solution of KMnO4. The reaction was completed after 3 h stirring and resulting precipitates were separated out and cleaned several times with water and finally with ethanol. Then MnO2 precipitates were dried for 12 h in hot air oven at 60 °C. The same process was followed for the each reducing agent, as shown schematically in Fig. 1. 3. Electrochemical measurements Electrochemical studies were conducted in 1 M Na2SO4 electrolyte with loaded 12.5 μg material on glassy carbon. Cyclic voltammetry was performed at a sweep rate of 10–500 mV s−1. The charge-discharging was done at current density from 0.64–4.0 Ag−1. The Retention of specific capacitance was studied for 1000 cycles and the stability of capacitance was checked for first 25 cycles. 4. Results and discussions 4.1. Physicochemical characterisations Powder XRD patterns for as-synthesised MnO2 samples using different reducing agents are recorded and presented in Fig. 2. Diffraction patterns are analysed and peaks are reported according to the JCPDS number 44-0141. Earlier different forms of MnO2 and the effect of different crystallographic structure on the electrochemical properties of 2

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Fig. 2. Powder XRD pattern of Manganese oxide samples.

measurements result showed that samples EG (170 m2 g−1), NSO (175 m2 g−1), FA (117 m2 g−1), NBH (113 m2 g−1) found to have a significant values of BET surface area. Surface area and pore volume of all MnO2 samples are given in Table 1. The N2 adsorption-desorption graphs for surface area and pore volume are given in supplementary information as figure S4 and figure S5. Also, FE-SEM images of HH (b),

NBH (f), and CA (g) which appear less porous have surface area of the order 65, 1.398, and 9.756 m2 g−1, respectively. Overall, manganese dioxide prepared using potassium iodide has the lowest surface area among all the samples.

Fig. 3. FT-IR spectra of Manganese oxide samples. 3

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Fig. 4. FESEM images of MnO2 samples; (a) EG, (b) HH, (c) NSO, (d) KI, (e) FA, (f) NBH, and (g) CA. Table 1 Comparison table of all the MnO2 samples. Sample name

Morphology

Surface area (m2/g)

Pore volume (CC/g)

Specific capacitance (F) by CV at 10 mV s−1

Specific capacitance (F) by GCD at 0.64 Ag−1

EG HH NSO KI FA NBH CA

Nanoparticle Nanoparticle Nanowire Spherical Nanoparticle Nanorod Nanoparticle Nanoparticle

171 65 175 2 117 113 10

0.2402 0.1048 0.4045 0.0117 0.1331 0.2816 0.0564

64 43 88 23 103 83 24

102 69 121 20 155 127 19

5. Electrochemistry of MnO2 samples

Specific capacitance =

Electrochemistry data for all samples at different sweep rate and current densities are presented in supplementary information as a figure S1. A comparison is drawn in Fig. 5, cyclic voltammetry (CV) and galvanostatic charge-discharge curves (GCD) for all samples have been plotted at sweep rate of 10 mVs−1 and current density of 0.64 Ag−1. CV and GCD curves show similar type of behaviour as reported in literatures [40,41]. All CV curves are showing a rectangular type graph (capacitive behaviour) which is reversible in nature. Specific capacitance values were calculated from both CV and GCD analysis. Specific capacitance of manganese oxide samples was determined by calculating integral area under the curve. For calculating specific capacitance from CV, Eq. (2) is used.

Specific capacitance =

I dt × m dv

(3)

Here, I (A/g) is current density, m is loaded material and dt is the dv slope calculated from discharging curve. The calculated values of specific capacitance for samples EG, HH, NSO, KI, FA, NBH, and CA are 102, 69, 121, 20, 155, 127 and 19 Fg−1, respectively. Here, capacitance values are reported at 10 mV s−1 scan rate and 0.64 Ag−1 current density. Specific capacitance at other sweep rate and current densities are given in supplementary information as table S2 and table S3. It has been found that specific capacitance is high at low scan rate, which may be due to the reason that at low scan rate, ions has sufficient time for adsorption and desorption. But with increasing scan rate and current density, there is a corresponding decrease in specific capacitance. This may be because of insufficient time for adsorption-desorption of ions. In few samples, there are slight signature of pseudocapacitance i.e. oxidation and reduction peaks arise with increasing scan rate (supplementary information S1). Also graphs between peak current vs square root of scan rate is given in supplementary information as figure S6. There is shift in the oxidation and reduction peak current which give evidence of pseudocapacitance in some samples. This is found in case of samples prepared by NBH, HH, CA which shows crystalline nature as compared to NSO, KI, FA, EG as seen from Fig. 2. In these samples capacitance is sum of pseudocapacitance and adsorption-desorption of ions. But overall adsorption-desorption of ions phenomenon is

Integrated area under curve Scan rate × potential window × Sample weight (2) −1

Specific capacitances of 64, 43, 88, 23, 103, 83, and 24 Fg , which is corresponding to sample EG, HH, NSO, KI, FA, NBH and CA, respectively. The triangular shaped graphs for all samples of manganese oxide in GCD also showed that samples have good reversibility. The specific capacitance is calculated from GCD curves using Eq. (3). 4

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Fig. 5. CV and GCD curves for all MnO2 samples; (a) CV curves at 10 mV s−1 scan rate, (b) GCD curves at 0.64 Ag−1 current density, (c) Retention behaviour of specific capacitance for all MnO2 samples and (d) Potential stability for first 25 cycles for formic acid sample.

Fig. 6. (a) Plot between specific capacitance versus scan rate (a) ,and specific capacitance versus current density (b), for FA sample.

case of NBH sample, in spite of having additional Mn3O4 phase in material, it shows good specific capacitance. It is because of crystallinity of Mn3O4 with porous microstructure. Also, as reported in literature, Mn3O4 is conductive in nature due to the formed tetragonal crystal structure at room temperature with varying +2/+3 valence states [43]. Due to which it has a comparable value of specific capacitance with amorphous MnO2. It was found that the highest specific capacitance corresponds to the FA sample, can be explained by surface area and highly fibrous nature of material, as can be seen from surface area value and FE-SEM image. FA sample has a highest specific capacitance in comparison to EG or NSO samples, probably this may be due to the fact that FA sample has large pore size. HH and KI samples have

predominant for all samples. However, this capacitance is sum of two processes but amorphous samples exhibit high specific capacitance. GCD triangular curves for all samples again showed a good stability and reversible nature. It is found that the samples with high surface area, EG, FA, NSO, and NBH, have high value of specific capacitance. Also FA, EG, NSO, NBH have high pore volume in comparison to other samples as given in Table 1. This high pore volume is also responsible for the high specific capacitance of these samples. This increase in specific capacitance is because of increase in available surface for adsorption –desorption of ions. Natalia et al. [42] study on electrochemical behaviour of activated carbon also concludes that high pore volume and surface area result into high specific capacitance [42]. In 5

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Table 2 Comparison of this work with some of the litrature reported data. S. No. 1.

Synthesis Method Hydrothermal synthesis

Structure

Morphology & Surface area

tetragonal phase of β-MnO2

2.

Hydrothermal synthesis

Birnesite-type MnO2

3. 4. 5. 6.

Dual- templating method Sol–gel template synthesis Hydrothermal Synthesis Precipitation reactions

α-MnO2 α-MnO2 α-MnO2 δ-MnO2

7.

Simple reduction of KMnO4

Amorphous

8.

Simple reduction of KMnO4

Amorphous

Nanowire (120 m2 g−1) Nanowire (76.3 m2 g−1) Nanowire Nanowire Nanord Nanorod (135 m2 g−1) Nanorod (117 m2 g−1) Nanowire (175 m2 g−1)

Capacitance 279 Fg 191 Fg

−1

−1

at 1 Ag

References −1

at 1 mV s

−1

[45] [46]

493 Fg−1 at 4 Ag−1 165 Fg−1 71.6 Fg−1 201 Fg−1

[47] [48] [49] [50]

155 Fg−1at 0.64 Ag−1

Our work

121 Fg−1at 0.64 Ag−1

Our work

CRediT authorship contribution statement

low specific capacitance because of low surface area values. Table 1 gives a complete overview of specific capacitance and surface area for all samples. Fig. 5(c) shows specific capacitance retention curve versus number of cycles. As it can be seen from graphs, CA, KI, NSO samples deliver good specific capacitance retention and no evidence of decreasing capacitance. Whereas in samples FA, NBH and HH, there is a decrease in specific capacitance after ~500 cycles, which in case of FA sample is regained to the earlier constant value after some cycles. This decrease in specific capacitance may be because of dissolution of manganese oxide ions in electrolyte [44]. In case of EG sample, there is monotonic increase and decrease of specific capacitance, which shows that it is quite unstable with increase in numbers of cycles. Potential stability has been calculated for first 25 cycles and stable value of specific capacitance was found for all the samples given in supplementary information figure S1.1- figure S1.7. Fig. 5(d) shows the potential stability of FA sample for first 25 cycles. Fig. 6 showed change of specific capacitance with scan rate and current density for FA sample. It was noticed that specific capacitance deceases linearly with increase of scan rate and current density, which is because of lesser penetration of electrode by electrolyte ions. This is a typical feature associated with the electrochemical supercapacitors. Table 2 shows the comparative data of specific capacitance with the other reported works. From this table, one can conclude that our work involves a simple procedure which provides good specific capacitance as compared to the other sophisticated techniques employed for manganese oxide preparation.

Nitika devi: Investigation, Methodology, Data curation, Writing original draft, Writing - review & editing. Manoj Goswami: Investigation, Methodology, Validation. Mohit Saraf: Supervision, Data curation. Bhupendra Singh: Investigation, Methodology, Writing original draft. Shaikh M. Mobin: Software, Formal analysis. Rajesh Kumar Singh: Supervision, Validation. A. K. Srivastava: Project administration, Resources. Surender Kumar: Conceptualization, Project administration, Resources, Supervision. Declaration of Competing Interest The author(s) declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.est.2020.101228. References [1] P. Thounthong, S. Raël, B. Davat, Energy management of fuel cell/battery/supercapacitor hybrid power source for vehicle applications, J. Power Sources 193 (2009) 376–385, https://doi.org/10.1016/j.jpowsour.2008.12.120. [2] H.D. Yoo, E. Markevich, G. Salitra, D. Sharon, D. Aurbach, On the challenge of developing advanced technologies for electrochemical energy storage and conversion, Mater. Today 17 (2014) 110–121, https://doi.org/10.1016/j.mattod.2014.02. 014. [3] P. Simon, Y. Gogotsi, B. Dunn, Where do battries end and supercapacitors begin ? Science 343 (2014) 1210–1211, https://doi.org/10.1126/science.1249625. [4] N.R. Dywili, A. Ntziouni, C. Ikpo, M. Ndipingwi, N.W. Hlongwa, A.L.D. Yonkeu, M. Masikini, K. Kordatos, E.I. Iwuoha, Graphene oxide decorated nanometal-poly (anilino-dodecylbenzene sulfonic acid) for application in high performance supercapacitors, Micromachines 10 (2019) 115–135, https://doi.org/10.3390/ mi10020115. [5] M. Vangari, T. Pryor, L. Jiang, Supercapacitors: review of materials and fabrication methods, J. Energy Eng. 139 (2013) 72–79, https://doi.org/10.1061/(ASCE)EY. 1943-7897.0000102. [6] A. González, E. Goikolea, J.A. Barrena, R. Mysyk, Review on supercapacitors: technologies and materials, Renew. Sustain. Energy Rev. 58 (2016) 1189–1206, https://doi.org/10.1016/j.rser.2015.12.249. [7] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797–828, https://doi.org/10.1039/ c1cs15060j. [8] Y. Wang, J. Guo, T. Wang, J. Shao, D. Wang, Y.W. Yang, Mesoporous transition metal oxides for supercapacitors, Nanomaterials 5 (2015) 1667–1689, https://doi. org/10.3390/nano5041667. [9] F. Ran, H. Fan, L. Wang, L. Zhao, Y. Tan, X. Zhang, L. Kong, L. Kang, A bird nest-like manganese dioxide and its application as electrode in supercapacitors, J. Energy Chem. 22 (2013) 928–934, https://doi.org/10.1016/S2095-4956(14)60274-6. [10] F. Ran, Y. Yang, L. Zhao, X. Niu, D. Zhang, L. Kong, Y. Luo, L. Kang, Preparation of nano-PANI@MnO2/ by surface initiated polymerization method using as a nanotubular electrode material: the amount effect of aniline on the microstructure and electrochemical performance, J. Energy Chem. 24 (2015) 388–393, https://doi.org/

6. Conclusions Nanostructured manganese oxide synthesised by reduction of KMnO4 using various reducing agents. XRD study results revealed that some sample contained additional Mn3O4 peaks, especially in samples prepared using strong reducing agent. Manganese oxide prepared by using FA, EG, NSO and KI having nanorods, highly porous nanopartices, nanowires and naosphere type structures which were confirmed by FESEM and other remaining samples also having nanoparticles type structures morphology. BET surface area measurements showed good porosity and high surface area, especially in sample prepared using ethylene glycol, Na2S2O3 and formic acid. Specific capacitances corresponding to FA is 155 Fg−1 with 117 m2g−1 surface area. All samples show high stability and in some cases 100% retention of the specific capacitance is seen. These results showed that reducing agent like formic acid, ethylene glycol, and Na2S2O3 have great efficiency toward synthesizing high surface area MnO2 materials. Also, Mn3O4 sample obtained using NaBH4 reducing agent has good specific capacitance of 127 Fg−1 at 0.64 Ag−1. Finally, this study concludes that optimised morphologies with high surface area of MnO2 are capable to act as an efficient electrode for supercapacitor. 6

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