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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.31 No.3 pp.115-121
DOI : https://doi.org/10.3740/MRSK.2021.31.3.115

# Optimal Porous Structure of MnO2/C Composites for Supercapacitors

Shinichiroh Iwamura1, Ryotaro Umezu2, Kenta Onishi2, Shin R. Mukai1
1Faculty of Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan
2Graduate School of Chemical Sciences and Engineering, Hokkaido University
Corresponding author E-Mail : iwamura@eng.hokudai.ac.jp (S. Iwamura, Hokkaido Univ.)
December 21, 2020 February 17, 2021 February 17, 2021

## Abstract

MnO2 can be potentially utilized as an electrode material for redox capacitors. The deposition of MnO2 with poor electrical conductivity onto porous carbons supplies them with additional conductive paths; as a result, the capacitance of the electrical double layer formed on the porous carbon surface can be utilized together with the redox capacitance of MnO2. However, the obtained composites are not generally suitable for industrial production because they require the use of expensive porous carbons and/or inefficient fabrication methods. Thus, to develop an effective preparation procedure of the composite, a suitable structure of porous carbons must be determined. In this study, MnO2/C composites have been prepared from activated carbon gels with various pore sizes, and their electrical properties are investigated via cyclic voltammetry. In particular, mesoporous carbons with a pore size of around 20 nm form a composite with a relatively low capacitance (98 F/g-composite) and poor rate performance despite the moderate redox capacitance obtained for MnO2 (313 F/g-MnO2). On the other hand, using macro-porous carbons with a pore size of around 60 nm increases the MnO2 redox capacitance (399 F/g-MnO2) as well as the capacitance and rate performance of the entire material (203 F/g-composite). The obtained results can be used in the industrial manufacturing of MnO2/C composites for supercapacitor electrodes from the commercially available porous carbons.

## 초록

This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

## 1. Introduction

Redox capacitors can be used as next-generation electronic devices, which possess larger capacitance than that of the conventional electrical double-layer (EDL) capacitors. It is well known that their electrodes can be fabricated from various materials such as RuO2, MnO2, and NiO1,2). Among these compounds, MnO2 is most suitable for industrial applications because of its low cost and large natural supplies. However, the use of MnO2 as a material for redox capacitor electrodes requires significant improvement of its electrochemical reaction rate because the corresponding redox reaction progresses much slower than the physical adsorption occurring on the surface of EDL capacitors. Thus, decreasing the MnO2 size to the nano-level can serve as an effective method for shortening the electron diffusion length in the MnO2 layers or particles and increasing its contact area with the electrolyte. Further, the addition of an electrical conductive material (such as carbon) can also improve the electrical conductivity of MnO2. In previous studies, various MnO2/C nanocomposites have been prepared and roughly categorized into the following two groups. The first group contained MnO2 layers deposited on nonporous carbon substrates, such as carbon nanofibers3,4) and graphene-derived carbons5-8). To achieve relatively high capacitance of these composites, large amounts of the deposited MnO2 particles are required because the supporting carbons act not as active materials, but only as conductive ones. In addition, the utilized carbons have to be homogeneously distributed among thin MnO2 layers or small MnO2 nanoparticles to effectively form electrical conductive paths. The preparation of such composites generally requires the use of expensive nanocarbons and/or laborious preparation methods. The second group of MnO2/C nanocomposites consists of porous carbons impregnated with MnO29-17). Their preparation requires a lower MnO2 content because the EDL capacitance originated from the presence of porous carbons can also be used to a certain degree. During composite preparation, it is difficult to homogeneously deposit MnO2 particles inside the pores because MnO2 typically bonds to the outer surface of the porous carbons. To improve the diffusivity of Mn species, nanocarbons with a tailored pore structure are often used.9-17) However, such materials are generally expensive and not efficient, and, therefore, not very suitable for industrial production. The main problem related to the fabrication of MnO2/C composites is that the optimal pore size of the porous carbons utilized for MnO2 deposition has not been established yet. Hence, the determination of the optimal pore size of cheap porous carbons would make the resulting MnO2/C nanocomposites more feasible for the industrial manufacturing of redox capacitors.

To investigate the pore size effects on the capacitor properties of MnO2/C composites, porous carbons with a tunable pore structure can be utilized. For industrial applications, it is more preferable to use commercially available carbon materials rather than relatively expensive nanocarbons. Carbons gels (CGs) represent an example of such materials, whose pore structure can be tuned by varying the gelation conditions of the precursor resin.18-21) Moreover, the surface area of CGs can be easily increased by activation, leading to a higher EDL capacitance.22) In this study, MnO2/C nanocomposites have been prepared from activated CGs with various pore sizes using a simple deposition method, and their capacitor performance was evaluated via cyclic voltammetry. Using the obtained results, the effects of the pore size on the capacitor properties were discussed, and the optimal pore size of the carbon framework corresponding to the highest capacitance of the produced MnO2/C composites was established.

## 2. Experimental

### 2.1 Preparation of CG/MnO2 nanocomposites

Resorcinol (R, 99.0 %), formaldehyde (F, 37 wt% solution stabilized by 7.5 wt% of methanol), sodium carbonate (C, 99.8 %), tert-butyl alcohol (TBA, 99.0 %), potassium permanganate (99.9 %), and sodium sulfite (99.9 %) were purchased from Wako Pure Chemical Industries Ltd.

Activated carbon gels were prepared according to the method described in detail elsewhere.22) Briefly, R, F, C and water (W) were mixed at ratios of R/C = 200-1000 (mol/mol), R/W = 0.5 (g/mL), and R/F = 0.5 (mol/mol). The resulting solution was stored at 30 °C for 2 h and at 60 °C for 12 h to obtain an RF gel. This gel was freezedried after the solvent exchange with TBA and carbonized at 1,000 °C for 4 h under N2 flow. The obtained carbon sample was crushed into powder and then activated at 1,000 °C under CO2 flow until its weight decreased to 50 % of the initial weight. The activated CGs were denoted according to their R/C ratios (for example, CG1000).

MnO2 was introduced into the activated CGs using the following procedure. 200 mg of the activated CG powder was stirred in an excess amount of 1 M KMnO4 solution for 5 min to ensure the adsorption of Mn species (i.e. $MnO 4 -$) inside the pores. After that, the mixture was washed with water and dried at 120 °C for 5 h under vacuum, where the adsorbed $MnO 4 -$ were reduced to MnO2. The obtained activated CG/MnO2 composites were labeled according to their R/C ratios (for example, CG1000/MnO2). For comparison purposes, MnO2 was also deposited on the surface of activated carbon (AC, MSP-20, Kansai Coke and Chemicals Co. Ltd.) via the same method (the resulting sample was denoted as AC/ MnO2).

### 2.2 Characterizations

The porous structure of the prepared samples was analyzed via N2 adsorption experiments using a BELSORP-mini II (Microtrack, BEL LTD.). Before the experiments, the samples were dried at 120 °C under N2 flow for 12 h. The MnO2 content in the samples was calculated from the weight decrease measured after carbon combustion, which was performed using a thermogravimetric (TG) apparatus (TGA-50H, Shimadzu Co. LTD.). Sample microstructures were observed using a field-emission scanning electron microscope (SEM; JSM-7500F, JEOL LTD.) and a field-emission transmission electron microscope (TEM; JEM-2010, JEOL LTD.). The elemental mapping of the sample was conducted using a scanning transmission electron microscope equipped with an energy dispersive X-ray spectroscopy (STEM-EDS; HD-2000, Hitachi).

To evaluate the electrochemical performance of the prepared samples, they were mixed with carbon black (Denka Black, Denki Kagaku Kougyo LTD.) and polytetrafluoroethylene (PTFE 6J, Du Pont-Mitsui Fluorochemicals Company, Ltd.) at a weight ratio of 8:1:1 and pressed on a nickel mesh (φ0.1 mm, 100 mesh, Nilaco Co.). Note that 2 mg of the sample was loaded in this electrode. Using the resulting sample electrode as a working electrode, beaker-type three-electrode cells containing Pt counter electrodes (φ0.2 mm Pt wire, Nilaco Co.), Ag/Ag+ reference electrodes (RE-1C, BAS Inc.), and 1 M NaSO4 electrolyte solution (99.0%, Wako Pure Chemical Industries Ltd.) were assembled. Cyclic voltammetry measurements were conducted by connecting the fabricated cells to a potentiostat (HSV-100, Hokuto Denko LTD.), and the corresponding sample capacitances (C [F/g]) were calculated according to the following equation:

$C = Σ ( | I | Δ t ) / ( 2 m V w )$
(1)

where Σ (|I| Δt), m, and Vw represent the area under the plot of the current I [A] versus time t [s], sample weight [g], and potential window [V], respectively.

## 3. Results and Discussion

To investigate the effect of the porous structure of the carbon substrate on the MnO2 deposition, N2 adsorption experiments were conducted before and after MnO2 deposition, as shown in Fig. 1. Brunauer–Emmett–Teller (BET) surface areas (SBET), micropore volumes (Vmicro), mesopore volumes (Vmeso), and average pore diameters (dp) were calculated from the obtained isotherms (see Table 1), while the related MnO2 contents were estimated via the TG analysis. Note that the values presented in Fig. 1 and Table 1 were the values based on the carbon weights to discuss the change of the porous structure of the carbon substrates. The obtained mesopore size depended on the utilized R/C ratio as well as previous studies.22) SBET of the activated CGs was around 2000 m2/g, which was almost equal to that of AC, owing to the introduction of a significant number of micropores during the activation procedure. Although the MnO2 composites were prepared from the porous carbons through the same method, their MnO2 content depended on the utilized carbon type because the porous structure of the carbons affected the adsorbed amount of Mn species. During deposition, a fraction of porous carbons acted as reducing agents, which converted Mn(VII) to Mn(IV), and the deposition process of MnO2 was accompanied by the consumption of the carbon framework. For mesoporous carbons (CG200 and CG500 samples), the adsorption volumes measured at P/P0 = 0.7-0.8 and around P/P0 = 0.9, respectively, were increased after MnO2 deposition, suggesting that the corresponding carbon frameworks were partially consumed and the pores constructed from them were expanded. For CG1000, which contained both micropores and macropores, Vmicro decreased, while the value of Vmeso increased after deposition, indicating that its micropores were partially expanded to mesopores. In contrast, Vmicro and SBET measured for the AC (which contained only micropores) simultaneously increased after MnO2 deposition since the deposition of MnO2 on the outer surface of AC particles consumed a fraction of the carbon framework while maintaining its inner porous structure. In addition, the deposited MnO2 also possessed some surface area; as a result, the apparent N2 adsorption amount obtained for the MnO2-covered AC was higher than that measured for pristine AC.

To investigate the morphology of the deposited MnO2 particles on the activated CGs, the CG1000 sample was observed by SEM and TEM before and after MnO2 deposition. The obtained SEM images [Figs. 2(a) and (c)] show that the sample outer surface was not changed after MnO2 deposition, indicating that MnO2 species were deposited onto its internal side. The corresponding TEM images [Figs. 2(b) and (d)] revealed that the carbon framework was not significantly affected by the MnO2 deposition process; however, the formation of rod-shaped particles with diameters of several nanometers inside the pores was observed. These particles corresponded to MnO2 crystals, which often assumed the shape of the rod.23) To investigate the deposition state of MnO2 in CG1000, elemental mapping of CG1000/MnO2 was conducted. Fig. 2(e-g) shows the result of STEM-EDX observation, indicating that the elemental mapping of Mn was significantly similar to that of carbon. This result concluded that MnO2 was homogeneously deposited in CG1000.

Since MnO2 particles in the produced sample were connected to the electrical conductive paths of the carbon support, their electrochemical properties were investigated as well. Fig. 3 shows the cyclic voltammograms (CVs) recorded for CG1000 and CG1000/MnO2. For CG1000/ MnO2, the anodic peak was detected at around 0.8 V (where Mn4+ species were reduced to Mn3+), indicating that the MnO2 species in CG1000/MnO2 were electrochemically connected and participated in the redox reaction. The capacitance of CG1000/MnO2 (calculated from the corresponding CV curve) was equal to 205 F/g, which was higher than that of CG1000 (150 F/g). Therefore, the capacitance of the original carbon framework was retained after MnO2 deposition and significantly increased due to the presence of MnO2 species.

In addition to CG1000 and CG1000/MnO2, the other activated CGs with various porous structures as well as AC were electrochemically analyzed before and after MnO2 deposition. Fig. 4 shows their capacitances calculated from the corresponding CVs recorded at various scan rates. Thus, the capacitances of the CG200 and CG500 samples were smaller than that of CG1000, which was consistent with the results of our previous study [Fig. 4(a)].22) At the measurement conditions, the capacitance of AC (estimated at a low scan rate of 5 mV/s) was almost identical to that of CG500. However, it significantly decreased with increasing scan rate, indicating that the rate performance of AC was poorer than that of CGs. For all samples, MnO2 deposition increased their capacitances by 20-50 F/g at a scan rate of 5 mV/s, confirming that the deposited MnO2 was electrochemically active and exhibited some redox capacitance. Nevertheless, the capacitances of the produced composites decreased with an increase in the scan rate (especially the capacitances of the composites containing small pores). At a scan rate of 200 mV/s, the calculated capacitances of all composite samples (except for CG1000/MnO2) were below 20 F/g. The obtained results suggest that the capacitance at a high scan rate mainly depended on accessibility to the MnO2 deposited inside pores of CGs and the macropores with diameters of 50 nm enable efficient electrolyte diffusion. In addition, the redox capacitances of the deposited MnO2 layers were calculated from the capacitances of the porous carbons before and after MnO2 deposition and the corresponding MnO2 contents. As a result, the highest MnO2 capacitance (400 F/g) was obtained for CG1000/MnO2 at a scan rate of 5 mV/s, while the capacitances of the deposited MnO2 in the remaining composites were equal to 190-310 F/g. At a high scan rate of 200 mV/s, the MnO2 layers of CG1000/MnO2 retained almost half of their capacitance measured at a low scan rate of 5 mV/s. However, the MnO2 capacitances of the other composites were extremely small (around 200 mV/s). Hence, the MnO2 deposited inside the pores with diameters of around 20 nm exhibited sufficient redox capacitances at low rates, but not at high rates. On the other hand, the MnO2 particles deposited inside the macropores with diameters of around 60 nm exhibited not only high redox capacitances at low scan rates, but also remarkable rate performance, indicating the importance of maintaining a sufficient number of diffusion paths for electrolyte molecules. Therefore, it can be concluded that macroporous carbons with pore-sizes of around 60 nm can be effectively used for fabricating MnO2/porous carbon composites for redox capacitors. Finally, the cycling measurement of CG1000/MnO2 was conducted to evaluate the stability of the deposited MnO2. Fig. 4(d) shows its capacitance retention measured at 5 mV/s until the 300th cycle. The capacitance slightly increased with the charge/discharge cycling, suggesting that the MnO2 which did not react at an initial charge/discharge cycle was gradually reacted. Because the decrease in the capacitance of MnO2 was not observed until the 300th cycle, it is expected that the deposited MnO2 was stable and CG1000/MnO2 can be used for a long cycle.

## 4. Conclusions

In this study, porous carbons with tunable pores were used as model materials to obtain the optimal pore size for the MnO2 deposition and related fabrication of redox capacitor electrodes. In spite of the simplicity of the utilized method, MnO2 species were homogeneously deposited inside mesoporous and macroporous carbons while preserving their BET surface areas. The produced composites exhibited both the MnO2 redox capacitance and EDL capacitance of the carbon surface. In particular, the macroporous composite with a pore size of around 60 nm exhibited both high capacitance and remarkable rate performance, suggesting that its macropores effectively enabled the electrolyte diffusion to the inner composite part. Furthermore, the capacitance of the deposited MnO2 could be used for a long cycle. The described strategy can be potentially used for developing MnO2/porous carbon composites with practical applications in the industrially manufactured high-performance supercapacitors.

## Acknowledgments

This work was partially supported by the 2016 Feasibility Study Program of the Frontier Chemistry Center, Faculty of Engineering, Hokkaido University.

## Figure

N2 adsorption isotherms obtained for the activated CG and AC samples before and after MnO2 deposition (a: CG200, b: CG500, c: CG1000, d: AC).

(a,c) SEM images, (b,d) TEM images and (e-g) elemental mappings of (a,b) CG1000 and (c-g) CG1000/MnO2. (e: secondary electron image, f: carbon, g: manganese)

CVs recorded for CG1000 and CG1000/MnO2 at a scan rate of 10 mV/s.

Capacitances of the prepared CG and AC samples measured at various scan rates (a) before and (b) after MnO2 deposition. (c) Capacitances per MnO2 weight estimated using the data presented in panels (a) and (b) and related MnO2 contents. (d) Capacitance retention of CG1000/MnO2 measured at 5 mV/s.

## Table

Textural properties of the activated CG and AC samples evaluated before and after MnO2 deposition.

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