Dian Denga,
Byoung-Suhk Kimb,
Mayakrishnan Gopiraman*a and
Ick Soo Kim*a
aNano Fusion Technology Research Lab, Institute for Fiber Engineering (IFES), Division of Frontier Fibers, Interdisciplinary Cluster for Cutting Edge Research (ICCER), National University Corporation, Shinshu University, Ueda, Nagano 386-8567, Japan. E-mail: kim@shinshu-u.ac.jp; gopiramannitt@gmail.com
bDepartment of Organic Materials & Fiber Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea
First published on 17th September 2015
Bio-waste material, human hair, was used as a source to prepare activated carbon (ACs) by a simple NaOH activation method. MnO2 with a needle like structure was successfully decorated on ACs by a wet impregnation method. TEM images confirmed the needle-like morphology of MnO2 on ACs. The chemical state of Mn in MnO2/ACs was found to be +4, as confirmed by XPS analysis. Other physicochemical properties of MnO2/ACs were investigated by means of SEM-EDS, XRD, Raman and XPS analyses. After complete characterization, the resultant nanocomposites with different MnO2-loading [MnO2/ACs(X:
Y)] were employed as electrode materials for supercapacitors. Interestingly, the MnO2/ACs nanocomposites showed an excellent capacitance in three different electrolytes (1.0 M H2SO4, 1.0 M KOH and 1.0 M Na2SO4). The MnO2/ACs(1
:
12) achieved a maximum capacitance of 410 F g−1, 345 F g−1 and 291 F g−1 in 1.0 M H2SO4, 1.0 KOH and 1.0 Na2SO4, respectively. To the best of our knowledge, this is the first MnO2-based carbon nanocomposite to show good capacitance performance in three different kinds of electrolytes. Moreover, the MnO2/ACs showed a capacitance of ∼300 F g−1 even after 500 cycles in 1.0 M H2SO4.
Modification or functionalization of porous ACs with metal oxides such as MnOx, RuOx, NiOx and CuOx is one of the promising approaches to achieve superior electrode performance.11,12 Among them, manganese oxide (MnO2) is often preferred as an electrode material for supercapacitors due the low cost, high theoretical surface area and non-toxic.13 Moreover, the MnO2 can exhibit larger capacitance values and energy density due to its fast surface redox reactions. The MnO2 has showed a maximum specific capacitance of 1370 F g−1.14 However, the pseudocapacitive reaction of MnO2 is known to be a surface reaction that can occur only on the surface. Hence, the MnO2 is often combined with conductive materials such as graphene, CNTs and carbon fibers to improve its performance.15 Yan et al.,16 prepared MnO2/graphene and employed as supercapacitor electrode material. The MnO2/graphene demonstrated a very high capacitance of 310 F g−1. MnO2 nanorods were supported on ACs and employed as electrode material for supercapacitor.17 It showed a moderate capacitance of 165 F g−1 in Na2SO4 electrolyte. In case of MnO2/activated carbon composites, the performance is mainly dependent on five factors:18,19 (a) surface area of the composites, (b) structure of the MnO2 and ACs, (c) conductivity of the materials, (d) large pore volume and pore size, and (e) presence of heteroatom such as O, N and S. Recently, human hair has been used to prepare ACs with very high surface area and excellent pore structure. In addition, the ACs found to consist of various heteroatoms such as O, N and S, and has exhibited very high specific capacitance in 6 M KOH electrolyte.5 We believe that the functionalization of MnO2 with unique structure on ACs (derived from human hair) would show superior capacitance performance in various electrolytes.
Herein, we prepared needle-like MnO2/ACs nanocomposites with different MnO2 loading by a simple wet impregnation method. The ACs was derived from human hair by NaOH activation. The prepared composite materials were characterized by various spectroscopic and microscopic methods such as HR-TEM, SEM-EDS, XRD, Raman and XPS analyses. After complete characterization, the resultant nanocomposites MnO2/ACs(X:
Y)] were employed as electrode materials for supercapacitor in three different electrolytes, namely, 1.0 M H2SO4, 1.0 M KOH and 1.0 M Na2SO4. Conductivity and cycle life of the nanocomposites were also studied.
Samples | BET analysis | EDS (wt%) | |||||
---|---|---|---|---|---|---|---|
SBET (m2 g−1) | Daver (nm) | Vpore (cm3 g−1) | C | O | S | Mn | |
Pure ACs | 1318.4 | 2.40 | 0.79 | 85.69 | 12.05 | 2.06 | — |
MnO2/ACs(1![]() ![]() |
106.8 | 7.14 | 0.19 | 42.12 | 21.78 | 1.94 | 34.17 |
MnO2/ACs(1![]() ![]() |
636.0 | 3.41 | 0.39 | 45.51 | 21.58 | 2.48 | 30.43 |
MnO2/ACs(1![]() ![]() |
600.2 | 2.60 | 0.54 | 60.62 | 23.52 | 2.37 | 13.49 |
MnO2/ACs(1![]() ![]() |
1355.7 | 2.38 | 0.81 | 72.44 | 15.38 | 2.07 | 10.11 |
MnO2/ACs(1![]() ![]() |
1519.3 | 2.51 | 0.95 | 77.33 | 15.65 | 2.46 | 4.57 |
MnO2/ACs(1![]() ![]() |
1560.9 | 2.35 | 0.92 | 77.87 | 14.23 | 2.72 | 5.18 |
MnO2/ACs(1![]() ![]() |
1597.4 | 2.55 | 1.02 | 79.73 | 12.38 | 3.17 | 4.72 |
MnO2/ACs(1![]() ![]() |
1705.3 | 2.68 | 1.14 | 81.73 | 12.12 | 2.28 | 3.87 |
MnO2/ACs(1![]() ![]() |
1641.4 | 2.00 | 0.82 | 85.12 | 10.16 | 2.53 | 2.19 |
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Fig. 2 (i) SEM image of MnO2/ACs(1![]() ![]() ![]() ![]() |
![]() | ||
Fig. 3 HR-TEM images of (i) ACs, (ii and iii) MnO2/ACs(1![]() ![]() ![]() ![]() ![]() ![]() ![]() ![]() |
The HR-TEM images were taken to study the microstructure of the ACs [Fig. 3(i)], MnO2/ACs(1:
2) [Fig. 3(ii and iii)] and MnO2/ACs(1
:
12) [Fig. 3(iv–vi)]. As seen from HR-TEM images of pure ACs in Fig. 3(i), the architecture of ACs with two-dimensional (2D) nanosheet morphology and the average thickness of the sheets were calculated to be ∼65 nm. In addition, meso/micropores channels can be clearly seen on the surface of the nanosheets. In Fig. 3, the MnO2/ACs(X
:
Y) composite showed that the needle like MnO2 was homogeneously anchored on the 2D nanosheet. The average diameter and average length of the MnO2 needles are found to be ∼14 nm and ∼110 nm, respectively. Interestingly, the average thickness of the nanosheets was dramatically decreased from 65 to 41 nm compared to pure ACs. In general, the inter-layers of carbon stacked by van der Waals forces have interaction energy of ∼2 eV nm−2 and typically a very weak ∼300 nN lm−2 magnitude of force is required to break this energy.21,22 In the present study, the exfoliation of the carbon sheets might have achieved during the preparation of nanocomposite (stirring and sonication process). In addition, the formed MnO2 may be stabilized or prevented the carbon nanosheets from the further regeneration. The HR-TEM result agrees well with the BET results. Moreover, it was found that the loading of KMnO4 has greater influence on the structure of the MnO2. At higher loading of MnO2, the surface morphology of the nanocomposites was quite different and no needle-like MnO2 structures were observed on the carbon nanosheets. Instead, the MnO2 was fully covered the carbon sheets at higher loading of KMnO4. The needle-like nanostructure of MnO2 can help to an easy ionic charge transport at electrode/electrolyte interfaces.23
XRD and Raman spectra were recorded to further investigate the graphitic structure [ordered and disordered (defect sites) nature] of pure ACs and nanocomposites (Fig. 4, 5, S3 and S4†). Fig. 4 shows the XRD patterns of pure ACs and MnO2/ACs(1:
12). Two characteristic peaks were seen at around 2θ = 22.3° corresponding to (002) and 2θ = 43.8° (a weak peak) corresponding to (101) plane.24 The XRD peak at around 2θ = 22.3° (002) attributed to a well-defined graphitic stacking. Alike, the weak peak at 2θ = 43.8° supports to the higher degree of interlayer condensation of carbon. Moreover, the clear observation of these XRD peaks indicating that the pure ACs and MnO2/ACs(1
:
12) consist of small domains of prearranged graphene sheets. These results support that the samples are highly conductive in nature.25 Raman spectra of both ACs and MnO2/ACs(1
:
12) showed two characteristic bands at 1325 (D band) and 1597 cm−1 (G band).26 The G-band was invented from the in-plane vibration of sp2 carbon atoms, which attributed to the graphitic carbon. The D-band line was related to the amount of disorder, indicating the presence defect sites in the carbon network. Generally, the D/G ratio of band intensities is often used to study defects concentration in carbon.27 At first, the D band and G band were fitted as the sum of a Gaussian function and then the IG/ID values were calculated (Fig. 5). The IG/ID values of ACs and MnO2/ACs(1
:
12) were calculated to be 1.0167 and 1.0601, respectively. The Raman IG/ID values confirm a highly graphitized ACs and MnO2/ACs(1
:
12), offering an excellent electric conductivity, which is consistent with the XRD results.28
Further, the XPS spectra were recorded for pure ACs and MnO2/ACs(1:
12). Fig. 6 shows the high-resolution XPS spectra of C 1s, N 1s, O 1s, S 2p, and Mn 2p. It was confirmed that the chemical compositions of pure ACs and nanocomposites are consist of C, N, O and S. The narrow and intense C 1s XPS peak was observed for both ACs and MnO2/ACs(1
:
12), indicating an improved degree of graphitic order. Alike, the O 1s peak located at B.E. = 533.5 eV shows the presence of oxygen atoms. In fact the oxygen functional groups can enhance wettability of the carbon materials.29 A strong and broad peak at B.E. = 400 eV (N 1s XPS peak) was observed which reveal the presence of N species in the form of pyridinic, pyrrolic, N-oxide and quaternary nitrogen.30 Similarly, the S appears at approximately B.E. = 165 eV. In fact the presence of heteroatom such as S, N and O can contribute greatly to the pseudocapacitance. Two obvious peaks at B.E. = 642.0 eV and B.E. = 653.8 eV are observed in the Mn 2p XPS spectrum, corresponding to Mn 2p3/2 and Mn 2p1/2 peak, respectively. The B.E. values of Mn 2p peaks confirmed the +4 oxidation state of Mn in MnO2/ACs(1
:
12).31
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Fig. 6 (i) Survey XPS spectra, (ii) S 2s peaks, (iii) C 1s peaks, (iv) N 1s peaks and, (v) O 1s peaks of ACs (a) and MnO2/ACs(1![]() ![]() ![]() ![]() |
To evaluate the specific capacitance (Cs) of the prepared ACs and nanocomposites, CV curves were recorded at different scan rates in three different electrolytes such as 1.0 M KOH, 1.0 M H2SO4, and 1.0 M Na2SO4; the results are presented in Fig. 8 and 9. As can be seen in Fig. 7, the shape of the CV curves is slightly distorted from the rectangular shape. The polarization resistance of the samples is the main reason for the distortion of CV curves.32 In addition, the presence of functional groups contributes to the pseudocapacitance behavior and the appearance of wide redox peak.33 Fig. 8 shows the calculated Cs values of ACs and nanocomposites at scan rate of 5 mV s−1. The superior capacitance performance of MnO2/ACs(X:
Y) was realized from the higher Cs values in all the three different electrolytes. At the scan rate of 5 mV s−1, the MnO2/ACs(1
:
12) achieved a maximum Cs of 410 F g−1, 345 F g−1 and 291 F g−1 in 1.0 M H2SO4, 1.0 KOH and 1.0 Na2SO4, respectively. However, under same conditions, the pure ACs showed the Cs values of 275 F g−1, 171 F g−1 and 22 F g−1 in 1.0 M H2SO4, 1.0 KOH and 1.0 Na2SO4, respectively. In comparison to pure ACs, the better Cs of MnO2/ACs(1
:
12) is due to the presence of MnO2. In fact, the charge can store on both MnO2 and ACs support, and therefore, the better Cs has been achieved.34 To the best of our knowledge, this is the first MnO2-based carbon nanocomposite to show good Cs in three kinds of electrolytes. Effect of MnO2 loading on electrochemical performance was also investigated. Fig. 8 represents the Cs values of nanocomposites with different MnO2 loading. As can be seen in Fig. 8, the loading of MnO2 has a profound effect on the electrochemical performance of the MnO2/ACs(1
:
12). At higher loading of MnO2 [such as MnO2/ACs(1
:
2)], the MnO2 has played a negative role by blocking the carbon surface and hinder the transport of ions.35 In addition, the blocking of interconnected pores of the carbon support by the MnO2 leads to a lower surface area of the nanocomposites and, therefore, the lower Cs values are obtained. Alike, very low MnO2 loadings also exhibited low Cs values which may be due to the insufficient amount of MnO2.36 This result agrees well with the BET results.
Further, the Cs of pure ACs and MnO2/ACs(1:
12) at different scan rates was measured in 1.0 M H2SO4; the results are presented in Fig. 9. It was noticed that the Cs values are gradually decreased with scan rate for both ACs and MnO2/ACs(X
:
Y). At the scan rate of 500 mV s−1, the ACs showed nearly zero capacitance, whereas, the MnO2/ACs(1
:
12) maintained a maximum Cs of about 50 F g−1. The EIS spectrum [Fig. 10(i)] of ACs and MnO2/ACs(1
:
12) presented a depressed semicircle and a smaller interfacial charge-transfer resistance, representing good conductivity of the materials and high ion transfer speed across interfaces between the electrolyte and electrode.37 Fig. 10(ii) represents the cycle stability of pure ACs and MnO2/ACs(1
:
12) at scan rate of 5 mV s−1 in 1.0 M H2SO4.
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Fig. 9 Specific capacitance of (a) pure ACs and MnO2/ACs(X![]() ![]() |
It was calculated that about 70% of the Cs was maintained by the MnO2/ACs(1:
12) after 500 cycles. The result confirms the good capacitance retention of the electrode material. However, after 500 cycles, the Cs of pure ACs was decreased to 59%. Overall, the better performance of the MnO2/ACs(X
:
Y) is due five obvious reasons (i) the high specific surface area, (ii) high average pore size and mean pore volume, (iii) the presence of heteroatom such as O, S and N, (iv) conductivity of carbon and (v) needle-like MnO2.
The Cs performance of the present MnO2/ACs(1:
12) is better or comparable to many other MnO2-based electrode materials (see Table 2) including graphene–MnO2, MnO2/activated CNTs, C/MnO2 nanorods, MnO2/ACs, MnO2/aniline, MnO2–VACNF, MnO2/diamond, flexible carbon cloth based MnO2 nanosheets, ternary MnO2/graphene nanosheets/CNTs composites, and MnO2 nanocrystals/carbon spheres.
Nanocomposites | Cs F g−1 | Scan rate mV s−1 | Reference |
---|---|---|---|
MnO2/ACs(1![]() ![]() |
410 (1.0 H2SO4) | 5 | This work |
345 (1.0 KOH) | |||
291 (1.0 Na2SO4) | |||
Graphene–MnO2 | 310 | 2 | 16 |
MnO2/activated CNTs | 250 | 10 | 38 |
C/MnO2 nanorods | 165 | 5 | 17 |
MnO2/ACs | 62 | — | 39 |
MnO2/aniline | 626 | 10 | 40 |
MnO2–VACNF | 437 | 1 | 41 |
MnO2/diamond | 326 | 10 | 42 |
Flexible carbon cloth based MnO2 nanosheets | 683.73 | — | 43 |
Ternary MnO2/graphene nanosheets/CNTs composite | 367 | 20 | 44 |
MnO2/carbon spheres | 412 | 2 | 45 |
Footnote |
† Electronic supplementary information (ESI) available: SEM images, EDS spectra, BET isotherm curves and XRD spectra are provided. See DOI: 10.1039/c5ra16624a |
This journal is © The Royal Society of Chemistry 2015 |