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Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 2
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Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy Sanaz Tajik a,b, Deepak P. Dubal b,c,*, Pedro Gomez-Romero b,**, Amir Yadegari d, Alimorad Rashidi e, Bahram Nasernejad a, Inamuddin f,g, Abdullah M. Asiri f,g a
Faculty of Chemical Engineering, Amirkabir University of Technology, Hafez Ave, P.O. Box 15875-4413, Tehran, Iran b Catalan Institute of Nanoscience and Nanotechnology (ICN2), and The Barcelona Institute of Science and Technology (CSIC-BIST), Campus UAB, Bellaterra, 08193 Barcelona, Spain c School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia d Department of Developmental Sciences, Marquette University School of Dentistry, Milwaukee, WI 53233, USA e Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), P.O. Box 14665-137, Tehran, Iran f Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia g Centre of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia
article info
abstract
Article history:
Exceptionally simple and cost-effective solid-state method is reported for the synthesis of
Received 6 January 2017
different mixed transition metal oxides (MTMOs) including FeCo2O4, MnCo2O4 and ZnCo2O4
Received in revised form
with unique nanostructures. The morphological analysis show that MTMOs possess
4 March 2017
distinct nanostructures such as tetragonal, spherical nanoparticles and hexagonal nano-
Accepted 19 March 2017
sheets. Furthermore, these MTMOs showed excellent supercapacitive properties with
Available online xxx
specific capacitances of 660e1263 F/g at current density of 2 A/g. Asymmetric capacitor was fabricated with FeCo2O4 as positive and activated carbon as negative electrode which ex-
Keywords:
hibits a specific capacitance of 88 F/g with energy density of 24 Wh/kg (1.1 mWh/cm3) and
Ternary metal oxides
cycle life (93%) over 5000 cycles.
Asymmetric supercapacitor
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
High energy density Specific capacitance
* Corresponding author. Catalan Institute of Nanoscience and Nanotechnology (ICN2), and The Barcelona Institute of Science and Technology (CSIC-BIST), Campus UAB, Bellaterra, 08193 Barcelona, Spain. Fax: þ34 936917640. ** Corresponding author. Fax: þ34 936917640. E-mail addresses: [email protected] (D.P. Dubal), [email protected] (P. Gomez-Romero). http://dx.doi.org/10.1016/j.ijhydene.2017.03.117 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Introduction The rise of concerns about environmental pollution, depletion of fossil fuels, and elevating global warming lead to significant researches on novel, low-cost, and environmentally friendly energy storage systems [1]. Among different technologies, supercapacitors, are categorized as one of the most promising energy storage devices not only due to their high power density and limited energy density value, but also by a remarkably long life time [2,3]. There are a variety of fascinating applications of supercapacitors which may be found in elevators and pallet trucks in the electric transportation, memory supplies in phones, computers, and so on [4,5]. However, developing supercapacitors with high energy density while maintaining their high power and long cycling life as competitive energy storage systems with current rechargeable batteries is still a main challenge [6]. Therefore, numerous efforts have been recently devoted to discover new materials for developing supercapacitors to enhance energy density [7,8]. Regarding the energy density issue, designing and developing asymmetric supercapacitors shows a great potential application owing to their wider potential window which leads to higher energy density versus symmetric supercapacitors [9]. Up to now, three considerable types of electrode materials including carbonaceous materials [10e12], metal oxides/hydroxides [13e16], and conducting polymers [17,18] have been intensively investigated for supercapacitors. Amongst, metal oxides, especially transition metal oxides, exhibit much higher specific capacitances than others [19,20]. For instance, ruthenium oxide shows high capacitances (500e1300 F/g), long cycle life, wide potential window, superior electric conductivity, descent rate capability, and excellent electrochemical reversibility [21e23]. Despite of the above-mentioned advantageous, using ruthenium oxide has been diminished due to its high cost and toxicity [24,25]. Hence, finding cost-effective, non-toxic, and environment friendly electrode materials are of particular interest. Recently, Mixed transition metal oxides (MTMOs), typically ternary metal oxides with two different metal cations, have received huge interest due to their promising performances in the field of energy storage [26e28]. Particularly, the MTMOs are known with the formula of AxB3-xO4 in a spinel structure, in which A and B stand for two different transition metals including Fe, Ni, Co, Mn, Zn, etc. The coupling of two metal species could render the MTMOs with rich redox reactions and improved electronic conductivity, which are beneficial to electrochemical energy storage applications [23,29]. Thus far, various techniques especially solution-based synthesis are being used as the most common approaches for preparation of MTMOs. For instance, Jiang et al. grown spinel (CoMn)3O4 nanostructures on nickel foam by employing hydrothermal method and further annealing as a high performance and binder free electrode material for supercapacitor [30]. The asprepared hierarchical electrode showed descent cycling life and high specific capacitance of 840 F/g at the current density of 10 A/g. Zhu et al. reported the synthesis of spinel NiCo2O4 through solegel method by applying three different organic acids as the chelating agents [31]. They showed that the synthesized spinel nanostructure possessed high specific surface
area and pore size which led to excellent rate capability, cycling life and ultrahigh specific capacitance of 1254 F/g at 2 A/g. All of the aforementioned techniques are among the most applicable and useful methods for the preparation of MTMOs. Nevertheless, they generally require sophisticated devices, high temperature treatment, time consuming procedures and solvent based approaches. Hence, developing an economic, simple, green, and solvent free rout for synthesis of MTMOs and their utilization, as the active material in supercapacitors is highly desirable. Here in, to our knowledge, a simple, efficient, solvent free and cost-effective solid-state reaction method for synthesis of various MTMOs including FeCo2O4, MnCo2O4 and ZnCo2O4 with unique nanostructures and remarkable electrochemical supercapacitive performances has been reported for the first time. All of the synthesized MTMOs possess high crystallinity with different morphologies among which FCO exhibits higher specific capacity and excellent electrochemical activity about 1263 F/g at 2 A/g. The assembled asymmetric supercapacitor with FCO was further investigated. The results showed that this work provides an opportunity for facile and large scale synthesis of MTMOs as a promising electrode material for developing asymmetric supercapacitors.
Experimental Synthesis of FCO, MCO and ZCO All the reagents in this study were used as received without further purification. MTMOs were successfully synthesized by solid state reaction method. Briefly, 1 mmol of FeCl2$4H2O, MnCl2$4H2O, and ZnCl2 each were separately mixed with 2 mmol of CoCl2$6H2O, in mortar and crushed. Afterwards, 6 mmol of KOH was added to each of the above mixtures without adding any solvent and the mixture was stirred for 30 min. After passage of time the KOH started to moisturize and thick slurry was formed which was further stirred for next 30 min. Subsequently, the products were washed with a mixture of deionized water and ethanol several times and centrifuged (3000 rpm) for 15 min. After that, the samples were dried overnight under vacuum at 80 C in order to remove any impurity. Finally, the obtained products were annealed at 300 C for 2 h in air. The as-prepared FeCo2O4, MnCo2O4 and ZnCo2O4 samples are hereafter denoted as FCO, MCO and ZCO, respectively.
Characterization techniques The surface morphologies of the samples were thoroughly examined by transmission electron microscopy (Tecnai G2 F20 STWINHR(S) TEM, FEI) and field-emission scanning electron microscopy (FEI Quanta 650F Environmental SEM) attached with an energy-dispersive X-ray spectroscopy (EDS). The crystallinity of the samples was investigated by Powder Xray diffraction (XRD) using Panalytical X'Pert Pro-MRD instrument (Cu Ka source). The X-ray photoelectron spectra (XPS) analyses were obtained by X-ray photoelectron spectroscopy (XPS, SPECS Germany, PHOIBOS 150). N2 adsorption/ desorption was determined by BrunauereEmmetteTeller
Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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(BET) measurements using Micromeritics instrument (Data Master V4.00Q, Serial#:2000/2400).
Electrode preparation and electrochemical testing The pastes were prepared by mixing 80% of active material (FCO or MCO or ZCO), 10% of binder (PVDF) and 10% of conducting material (carbon black or SuperP) with N-methylpyrrolidone (NMP). Subsequently, the pastes were coated on the carbon cloth using doctor blade method and dried in vacuum oven overnight. The typical mass loading was found to be 0.9e1 mg/cm2 for MCO, FCO and ZCO samples. The supercapacitive performances of the materials were tested in 6 M KOH using cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) with conventional three electrode cell comprising of the MTMOs (FCO or MCO or ZCO) as a working electrode, platinum as a counter and Ag/AgCl as reference electrode, respectively. All the electrochemical measurements were carried out with a Biologic VMP3 potentiostat. Later, asymmetric capacitor cell was assembled with FCO (the reason chose FCO is relatively better electrochemical properties) as the positive electrode and activated carbon (AC) as the negative electrode using Swagelok cell. The mass of active material in both electrodes was 2.5 mg. The electrodes were separated by separator soaked with 6 M KOH electrolyte.
Results and discussion XRD patterns of MTMOs are shown in Fig. 1. The resultant diffraction peaks reveal the spinel structure and corroborate with the standard patterns [32,33] for MCO, ZCO and FCO, respectively. No unidentified peaks are present in the XRD patterns which indicate the purity of the formed products. The well-defined and broad diffraction peaks are also as an indicative of nanocrystalline nature of the sample. The lattice parameter (a0) for the cubic spinel structure can be calculated by following equation,
Fig. 1 e XRD patterns of FeCo2O4 (FCO), MnCo2O4 and ZnCo2O4 (ZCO) samples.
1=2 a0 ¼ d h2 þ k2 þ l2
3
(1)
where, d is the interplanar distance for crystal plane, and h, k, l are the Miller indices. The lattice parameters calculated for FCO, MCO and ZCO samples were 8.1623 A, 8.0958 A and 8.0680 A, respectively for (311) plane, which matches well with their corresponding standard values indicating the successful incorporation of Fe, Mn and Zn into the lattice [JCPDS:0231390]. The oxidation states of each element present in the spinel structure of FCO (Fe, Co), MCO (Mn, Co) and ZCO (Zn, Co) are investigated by X-ray photoelectron spectra (XPS) analysis. The Fe/Co, Mn/Co and Zn/Co atom ratio for FCO, MCO and ZCO samples is close to 1:2 as measured by XPS, which is in good agreement with the starting precursor ratio. Fig. 2(aec) shows the magnified XPS spectra of Fe2p, Mn2p and Zn2p of FCO, MCO and ZCO samples, respectively. As can be seen from Fig. 2(a), Fe2p3/2 and Fe2p1/2 lie about 711.8 and 725.2 eV, respectively. Peak positions and spin orbit separation of about 13.4 eV suggest the presence of Fe(III) in the oxide [34,35]. In case of MCO sample, the two main peaks of Mn2p3/2 at 641.3 eV and Mn2p1/2 at 652.8 eV deconvoluted into four subpeaks: the two at 641.5 and 653.1 eV are attributed to the presence of Mn2þ, whereas the other two at 643.2 and 654.1 eV originate from Mn3þ (Fig. 2b) [36e38]. Furthermore, as seen from Fig. 2c, two peaks with binding energy values of 1022.4 and 1045.5 eV can be ascribed to Zn2p3/2 and Zn2p1/2, indicating the Zn(II) oxidation state of ZCO sample (ZnCo2O4) [39]. The spectra observed for FCO, MCO and ZCO samples are same hence, we are presenting Co2p spectrum for FCO sample. The Co2p spectrum (Fig. 2d) was fitted with Gaussian fitting method under the assumption of eight species including two major signals, which are attributed to the Co2p3/2 (779.2 eV) and Co2p1/2 (794.4 eV) levels, respectively. The spineorbit splitting is 15.2 eV. Additionally, the two prominent peaks both possess weak shake-up satellites (denoted as “sat”) at 788.3 and 802.5 eV, showing binding energies of approximately 9 and 8 eV higher than the main signals, respectively. Therefore, the coexistence of Co2þ and Co3þ could be confirmed based on the positions of the main peaks and on the distance of satellites from their respective major signals [40]. All of these results suggest the existence of mixed-valence metal cations in the as-synthesized MTMOs samples, which are advantageous toward supercapacitors applications. The surface morphologies of MTMOs were investigated with different techniques such as SEM, TEM, HRTEM etc. Fig. 3(aef) shows FESEM images for FCO, MCO and ZCO samples at two different magnifications, respectively. The morphology of FCO sample revealed the coverage of homogeneous nanoparticles at low magnification (Fig. 3(a)) while at high magnified images showed the formation of tetragonal nanoparticles (see Fig. 3(b)). Similar appearance is also noticed for MCO sample (Fig. 3(c, d)). Further, it is seen that the shape of MCO nanoparticles are spherical, completely different than FCO. Interestingly, the ZCO sample exhibits totally different morphology than FCO and MCO as seen from Fig. 3(e, f). The surface morphology for ZCO is non-uniform consisting of different sized and shaped nanoparticles. Moreover, high magnified image reveals the presence of hexagonal
Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Fig. 2 e (aec) Magnified spectra of Fe, Mn and Zn for FCO, MCO and ZCO samples, respectively (d) Core level Co spectrum for FeCo2O4 (FCO) samples.
Fig. 3 e FE-SEM images of (a, b) FCO, (c, d) MCO and (e, f) ZCO samples at two different magnifications, respectively.
nanosheets with different sized spherical nanoparticles. In addition, the corresponding EDS mapping confirms the presence of Fe, Mn, Zn, Co and O elements on the entire surface of the materials (Supporting Information S1 and S2).
Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements were employed to investigate the microstructure and the
Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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crystalline phase of the MTMOs. TEM and HRTEM images with different magnifications of FCO, MCO and ZCO are displayed in Fig. 4. The formation of tetragonal nanoparticles for FCO sample is confirmed by TEM image shown in Fig. 4(a). Moreover, all the tetragonal nanoparticles are uniform in shape and size which further suggest the potential of this simple method for the preparation of controlled nanostructures. The size of these tetragonal nanoparticles is in the range of 80e100 nm. Similarly, Fig. 4(b) shows the uniform distribution of spherical nanoparticles of MCO sample. The TEM result clearly supports the SEM analysis. The size of spherical nanoparticles is in the range of 60e80 nm. On the other hand, ZCO sample exhibits completely different nanostructure than FCO and MCO. Interestingly, hexagonal nanosheet with homogeneous coverage of small nanoparticles is clearly seen (see Fig. 4(c)). Thus, it is clear that, the different nanostructures of different materials can be easily prepared with this simple method which holds a great promise of large-scale production of materials. HRTEM images taken from individual nanoparticles of FCO, MCO and ZCO samples are shown in Fig. 4(def), confirming that all the three MTMO samples are polycrystalline in nature. The clearly resolved lattice fringes were calculated to be about 4.71 A for FCO, 2.41 A and 2.32 A for MCO and 4.69 A and 2.43 A for ZCO samples which matches well with the (111), (311), (222) lattice planes of spinel MTMOs. So the interconnected nanoparticles morphology will facilitate the high transport rates for both electrons and electrolyte ions that simultaneously take part during Faradic reaction. In order to investigate, the surface areas of these MTMOs, BET analysis was carried out using N2 adsorption/desorption isotherms (Fig. 5). The shape of the isotherms acquired for all materials show the hysteresis loop in relative pressure (P/P0) between 0.6 and 1.0, showing characteristic of mesoporous materials [41]. The BET specific surface area obtained were 69, 54 and 49 m2g1 for FCO (FeCo2O4), MCO (MnCo2O4) and ZCO
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Fig. 5 e Nitrogen adsorption/desorption isotherms with corresponding pore-size distribution plots for FeCo2O4 (FCO), MnCo2O4 and ZnCo2O4 (ZCO) samples.
(ZnCo2O4), respectively. Thus, the surface area of the FCO sample is relatively higher than that for MCO and ZCO samples. The pore size distribution curves for all these samples are shown in inset of Fig. 5. It is observed that, all these materials exhibits mesoporous nature. It is further confirmed that, the FCO sample exhibits relatively higher pore volume. The different surface areas for these materials might be attributed to their different surface morphologies. The high specific surface area is directly associated to higher supercapacitive properties of the materials, due to the mesoporous structures which enable the soaking of electrolyte, facilitating the ion diffusion and provide more electroactive sites for energy storage.
Fig. 4 e TEM images (aec) and HRTEM images (def) of FCO, MCO and ZCO samples, respectively. Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Electrochemical performance
The specific capacitances for the electrodes were calculated from CV measurements using the following equation:
The electrochemical performance of MTMO electrodes were investigated by cyclic voltammetry (CV) technique and results are displayed in Fig. 6. Thus, Fig. 6(aec) shows the CV curves for FCO, MCO and ZCO samples at different scan rates, respectively. The shapes of CV curves for FCO shows clear redox peaks suggesting the contribution from faradaic reactions while that for MCO and ZCO are nearly rectangular. It is further interesting to note that, the CV curves remain unchanged, as scan rate increases, thus indicating the excellent electrochemical reversibility and exceptional high-rate performance. It is also observed that, the area under curve increases with scan rate signifying that the current density is directly proportional to scan rate. The FCO electrode exhibits relatively highest current density, corresponding to high capacitance which might be attributed to its morphology and good conductivity. However, the redox peaks shifted towards lower potential, which may due to the polarization effect of the electrodes. The charge-storing mechanism in all these MTMOs samples can be explained with the following reactions, ðMÞCo2 O4 þ OH þ H2 O⇔ðMÞOOH þ 2CoOOH þ 2e
(2)
ðMÞOOH þ OH ⇔MO2 þ H2 O þ e
(3)
where, M is indicative of Fe or Mn or Zn.
C¼
1 mwðVc Va Þ
ZVc IðVÞdV
(4)
Va
where, C is the specific capacitance, m is the mass of active material, n represents the scan rate, Vc and Va refer to the high and low potential limit of the CV test, and I is the current recorded during CV measurements. The specific capacitances of all MTMO samples as a function of scan rate are displayed in Fig. 6(d). The specific capacitances decrease with increasing the scan rate which may be attributed to the presence of inner active sites that are unable to sustain the redox transition completely at high scan rates. The maximum specific capacitances calculated are 1182 F/g, 743 F/g and 844 F/g for FCO, MCO and ZCO samples, respectively. The high specific capacitance of FCO may be ascribed to the tetragonal nanoparticles surface morphology. Fig. 7(aec) manifests the GCD curves for MTMO samples at various current densities. It is fairly observed that, the GCD curves for all the samples are not ideal straight lines which confirms the inclusion of faradaic reactions during charging/ discharging processes. Obviously, the discharging time of FCO is much longer than that of other materials MCO and ZCO. The specific capacitance in the GCD process can be calculated according to the following equation:
Fig. 6 e CV curves at different scan rates (5e100 mV/s) of (a) FCO (b) MCO (c) ZCO in 6 mol/L KOH electrolyte. (d) The variation of specific capacitance of all electrodes with scan rate. Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Fig. 7 e GCD curves at different current densities (5e25 A/g) of (a) FCO (b) MCO (c) ZCO in 6 mol/L KOH electrolyte. (d) Comparison the specific capacitance of samples. Z I$ C¼
V$dt m$V2
(5)
where, I is applied the discharge current, m is the mass of the active material and V is the potential window. The maximum specific capacitance obtained were 1263, 660 and 800 F/g at 2 A/g for FCO, MCO and ZCO, respectively. Similar to the result obtained from CV tests, the FCO exhibits maximum specific capacitance as compared to that of other samples (see Fig. 7(d)). The stability of these three MTMOs was investigated at 6 A/g over 500 cycles and presented in Supporting Information S. I. 3. It is seen that all samples exhibits excellent cycling stability around 86e89% over 5000 cycles. Electrochemical impedance analysis was used to investigate the resistances involved in the overall system. The Nyquist plots for MTMO electrodes in 6 M KOH electrolyte at open circuit voltage (OCV) are shown in Fig. 8(a, b). The depressed semicircles are clearly observed at high frequency regime, which are related to charge transfer resistance at the electrode/electrolyte interfaces. The initial non-zero intercept at X-axis suggests electrochemical series resistance or electrolyte resistance (Rs). The electrolyte resistance (Rs) for FCO, MCO and ZCO are found to be 0.24, 0.11 and 1.28 U, respectively (Fig. 8b). Moreover, FCO and MCO exhibit smaller semicircles suggesting small charge transfer resistance. Thus, the impedance analysis suggests that, FCO provides relatively small charge transfer resistance which might be attributed to the intimate contact at electrolyte/electrode interface. Inset of
Fig. 8(a) is the circuit model and the parameters; Rs is the electrolyte resistance in the cell, Rct is the charge transfer resistance, CPE is a constant phase angle element, and Zw represent the Warburg diffusion. Overall electrochemical performances imply that FCO exhibits relatively excellent specific capacitance as well as good rate capability. The high capacitive performance of FCO might be ascribed to the unique tetragonal nanostructure that eases rapid transport of electrons between the active materials and the charge collecting substrate. In addition, these FCO tetragonal nanoparticles exhibit relatively high specific surface areas and porous features which could enlarge the efficient liquid/solid interfacial area, resulting in effective use of the active material by increasing the number of electroactive sites. Hence, FCO is used for further electrochemical characterizations.
Asymmetric capacitors using FCO as the positive electrode and activated carbon (AC) as the negative electrode Considering the excellent electrochemical properties of FCO electrode and the fast ion transport property of activated carbon (AC) material, an asymmetric capacitor was successfully fabricated using these materials as the positive and negative electrodes, respectively. The CV curves for AC and FCO electrodes in three electrode configuration at scan rate of 10 mV/s in 6 M KOH are measured (Supporting Information S.I.4). The potential windows of FCO electrode and AC electrode are 0.5 to 0.3 V and 1.0 to 0 V (vs. Ag/AgCl), respectively; indicating that AC and FCO are greatly stable in
Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Fig. 8 e (a) Nyquist plots of FCO, MCO and ZCO samples with corresponding equivalent circuit, (b) Magnified view of Nyquist plots.
different potential windows. Therefore, the total cell voltage can be expressed as the sum of the potential range for FCO and AC electrode for asymmetric configuration. It should be noted that, to approach the highest cell voltage, the charges stored in both electrodes must be balanced by adjusting the mass loading of each of the active electrode materials [42,43]. The AC to FCO mass ratio was calculated by the following equation [44]: mþ C DE ¼ m Cþ DEþ
(6)
where, C is the specific capacitance and DE is the potential range for positive (þ) and negative () electrodes. On the basis of the specific capacitance values found for FCO and AC, the optimal mass ratio between the electrodes should be m (FCO)/ m (AC) ¼ 0.34:1 in the asymmetric capacitor cell. In present investigation, the weight of active material in both electrodes was 2.5 mg (0.89 mg of FeCo2O4 and 1.61 mg of AC). Asymmetric capacitor, AC//FCO, was assembled by using aqueous 6 M KOH electrolyte. The CV curves of the AC//FCO
asymmetric cell measured at various scan rates from 5 to 100 mV/s between 0 and 1.4 V and are shown in Fig. 9(a). The shape of the CV remains unchanged even at a high scan rate of 100 mV/s, implying an excellent rate capability of the cell. Fig. 9(b) shows the GCD curves of asymmetric cell and corresponding potential distribution across the electrodes. During the charge/discharge, the positive electrode (FCO) is swinging between 0 and 0.4 V vs. Ag/AgCl, while AC between 0 and -1 V vs. Ag/AgCl and the cell between 0 and 1.4 V at 0.8 A/g. Further, GCD curves of asymmetric cell were recorded at various current densities and are presented in Fig. 9(c). Accordingly, a nonlinear relation of the charge/discharge potentials with time was found. In addition, the initial voltage loss (i.e., iR drop) observed in the discharge curves is small even at high current densities; this is an indicative of fast IeV response and low internal resistance of the supercapacitors. Additionally, from the typical GCD curves, it can be observed that the discharge curve is nearly symmetric with its corresponding charging counterpart, demonstrating the excellent electrochemical reversibility and good coulombic efficiency. The calculated specific and volumetric capacitance for AC//FCO asymmetric cell at different current densities is shown in Fig. 9(d). The maximum capacitance obtained for cell is 88.1 F/ g (3.07 F/cm3, considering volume of device 0.0678 cm3) at current density of 0.8 A/g. Since it is vital to retain high specific capacitances (or energy density) at high current density, we estimated the energy and power density, which are key factors for the supercapacitor applications. The Ragone plots are presented in Fig. 10(a, b). The energy density of the AC//FCO asymmetric cell based on the total mass of the active electrode materials (including AC and FCO) reaches to 24.0 Wh/kg (1.1 mWh/cm3) at a power density of 560 W/kg (25.5 mW/cm3), and still remains 19.0 Wh/kg (0.9 mWh/cm3) at a power density of 5600 W/kg (255.4 mW/cm3). Interestingly, the maximum energy density obtained for AC//FCO asymmetric capacitor with a cell voltage of 1.4 V is much higher than those of symmetric AC/AC supercapacitor (<10 Wh/kg) [45], graphene/graphene (9.1 Wh/kg) [46], CNT/CNT supercapacitor (<10 Wh/kg) [47,48]. Additionally, the proposed AC//FCO asymmetric supercapacitor shows higher energy density versus other reported asymmetric supercapacitors with aqueous electrolyte solutions, such as NieCo oxyhydroxide// AC (17.8 Wh/kg) [49], NiCo2O4-RGO//AC (12 and 23.3 Wh/kg) [49,50], NiCo2O4//AC (15.32 and 17.72 Wh/kg) [51,52], NiCo2O4graphene//AC (7.6 Wh/kg) [53], FeCo2O4//AC (23 Wh/kg) [54], Co3O4@MnO2//AC (17.7 Wh/kg) [55]. Therefore, the considerably high energy density derived by this simple AC//FCO asymmetric cell has important industrial applications in the field of supercapacitors [56e60]. The cycling life test over 5000 cycles for the AC//FCO asymmetric capacitor was carried out by repeating the charging/discharging test between 0 and 1.4 V at a current density of 4 A/g. The cycle performance of the asymmetric capacitor charged at 1.4 V as a function of the cycle number is shown in Fig. 10(c). The asymmetric cell exhibits excellent electrochemical stability of 94% over 5000 cycles with Coulombic efficiency more than 90%.The results demonstrate that AC//FCO asymmetric cell displays a high specific capacitance and excellent cycling stability. EIS of AC// FCO asymmetric capacitor was measured at open circuit potential and the Nyquist plot is shown in Fig. 10(d) with inset
Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Fig. 9 e (a) CV curves of AC//FCO asymmetric cell within operating voltage window of 1.4 V at various scan rates, (b) GCD curves of FCO, AC and AC//FCO asymmetric cell, (c) GCD curves of AC//FCO asymmetric cell at different current densities (0.8e8.0 A/g), (d) Plots of volumetric and specific capacitances with current density for AC//FCO asymmetric cell.
Fig. 10 e (a, b) Plots of energy density versus power density of AC//FCO asymmetric cell in Ragone plot, (c) Variation of specific capacitance with number of cycles measured at 4 A/g over 5000 cycles, (d) Nyquist plot and inset shows Bode plot of AC//FCO cell. Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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representing the Bode plot. As can be seen in Fig. 10(d), the vertical rise in impedance value, at low frequency region, indicates good capacity behavior. Moreover, a small depressed semicircle which represented the small charge transfer resistance at the electrode/electrolyte interface can be detected at high frequency region. The Bode plot for asymmetric capacitor shows phase angle z 80 at 1 Hz, which is close to that of an ideal capacitor, verifying that the AC//FCO asymmetric capacitor exhibits a good capacitive behavior.
Conclusions In summary, different mixed transition metal oxides such as FeCo2O4, MnCo2O4 and ZnCo2O4 were successfully synthesized by simple solid-state reaction method. The physiochemical characterizations show that three different nanostructures including tetragonal and spherical nanoparticles and hexagonal nanosheets are formed for FCO, MCO and ZCO, respectively. Moreover, all these MTMO samples are intrinsically polycrystalline and hold spinel crystal structure. All the synthesized nanomaterials show excellent supercapacitive properties with specific capacitance in the range of 660e1263 F/g. Furthermore, an asymmetric capacitor based on FCO as positive electrode and AC as negative electrode (AC//FCO) was successfully cycled up to 1.4 V. The AC//FCO cell exhibits a specific capacitance of 88.06 F/g (3.07 F/cm3) with high specific energy of 24 Wh/kg (1.1 mWh/cm3) and 93% capacitance retention after 5000 cycles. The outcomes demonstrate that the proposed solid-state reaction method has a great potential application for large scale synthesis of MTMOs as an applicable electrode material for asymmetric supercapacitors.
Acknowledgment The authors would like to acknowledge the Catalan Institute of Nanoscience and Nanotechnology (ICN2) for their support and warm collaboration. D.P. Dubal and P. Gomez-Romero acknowledge AGAUR (Generalitat de Catalunya) for Project NESTOR (Nanomaterials for Energy STORage) 2014_SGR_1505.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.117.
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Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy Sanaz Tajik a,b, Deepak P. Dubal b,c,*, Pedro Gomez-Romero b,**, Amir Yadegari d, Alimorad Rashidi e, Bahram Nasernejad a, Inamuddin f,g, Abdullah M. Asiri f,g a
Faculty of Chemical Engineering, Amirkabir University of Technology, Hafez Ave, P.O. Box 15875-4413, Tehran, Iran b Catalan Institute of Nanoscience and Nanotechnology (ICN2), and The Barcelona Institute of Science and Technology (CSIC-BIST), Campus UAB, Bellaterra, 08193 Barcelona, Spain c School of Chemical Engineering, The University of Adelaide, Adelaide, South Australia 5005, Australia d Department of Developmental Sciences, Marquette University School of Dentistry, Milwaukee, WI 53233, USA e Nanotechnology Research Center, Research Institute of Petroleum Industry (RIPI), P.O. Box 14665-137, Tehran, Iran f Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia g Centre of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia
article info
abstract
Article history:
Exceptionally simple and cost-effective solid-state method is reported for the synthesis of
Received 6 January 2017
different mixed transition metal oxides (MTMOs) including FeCo2O4, MnCo2O4 and ZnCo2O4
Received in revised form
with unique nanostructures. The morphological analysis show that MTMOs possess
4 March 2017
distinct nanostructures such as tetragonal, spherical nanoparticles and hexagonal nano-
Accepted 19 March 2017
sheets. Furthermore, these MTMOs showed excellent supercapacitive properties with
Available online xxx
specific capacitances of 660e1263 F/g at current density of 2 A/g. Asymmetric capacitor was fabricated with FeCo2O4 as positive and activated carbon as negative electrode which ex-
Keywords:
hibits a specific capacitance of 88 F/g with energy density of 24 Wh/kg (1.1 mWh/cm3) and
Ternary metal oxides
cycle life (93%) over 5000 cycles.
Asymmetric supercapacitor
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
High energy density Specific capacitance
* Corresponding author. Catalan Institute of Nanoscience and Nanotechnology (ICN2), and The Barcelona Institute of Science and Technology (CSIC-BIST), Campus UAB, Bellaterra, 08193 Barcelona, Spain. Fax: þ34 936917640. ** Corresponding author. Fax: þ34 936917640. E-mail addresses: [email protected] (D.P. Dubal), [email protected] (P. Gomez-Romero). http://dx.doi.org/10.1016/j.ijhydene.2017.03.117 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Introduction The rise of concerns about environmental pollution, depletion of fossil fuels, and elevating global warming lead to significant researches on novel, low-cost, and environmentally friendly energy storage systems [1]. Among different technologies, supercapacitors, are categorized as one of the most promising energy storage devices not only due to their high power density and limited energy density value, but also by a remarkably long life time [2,3]. There are a variety of fascinating applications of supercapacitors which may be found in elevators and pallet trucks in the electric transportation, memory supplies in phones, computers, and so on [4,5]. However, developing supercapacitors with high energy density while maintaining their high power and long cycling life as competitive energy storage systems with current rechargeable batteries is still a main challenge [6]. Therefore, numerous efforts have been recently devoted to discover new materials for developing supercapacitors to enhance energy density [7,8]. Regarding the energy density issue, designing and developing asymmetric supercapacitors shows a great potential application owing to their wider potential window which leads to higher energy density versus symmetric supercapacitors [9]. Up to now, three considerable types of electrode materials including carbonaceous materials [10e12], metal oxides/hydroxides [13e16], and conducting polymers [17,18] have been intensively investigated for supercapacitors. Amongst, metal oxides, especially transition metal oxides, exhibit much higher specific capacitances than others [19,20]. For instance, ruthenium oxide shows high capacitances (500e1300 F/g), long cycle life, wide potential window, superior electric conductivity, descent rate capability, and excellent electrochemical reversibility [21e23]. Despite of the above-mentioned advantageous, using ruthenium oxide has been diminished due to its high cost and toxicity [24,25]. Hence, finding cost-effective, non-toxic, and environment friendly electrode materials are of particular interest. Recently, Mixed transition metal oxides (MTMOs), typically ternary metal oxides with two different metal cations, have received huge interest due to their promising performances in the field of energy storage [26e28]. Particularly, the MTMOs are known with the formula of AxB3-xO4 in a spinel structure, in which A and B stand for two different transition metals including Fe, Ni, Co, Mn, Zn, etc. The coupling of two metal species could render the MTMOs with rich redox reactions and improved electronic conductivity, which are beneficial to electrochemical energy storage applications [23,29]. Thus far, various techniques especially solution-based synthesis are being used as the most common approaches for preparation of MTMOs. For instance, Jiang et al. grown spinel (CoMn)3O4 nanostructures on nickel foam by employing hydrothermal method and further annealing as a high performance and binder free electrode material for supercapacitor [30]. The asprepared hierarchical electrode showed descent cycling life and high specific capacitance of 840 F/g at the current density of 10 A/g. Zhu et al. reported the synthesis of spinel NiCo2O4 through solegel method by applying three different organic acids as the chelating agents [31]. They showed that the synthesized spinel nanostructure possessed high specific surface
area and pore size which led to excellent rate capability, cycling life and ultrahigh specific capacitance of 1254 F/g at 2 A/g. All of the aforementioned techniques are among the most applicable and useful methods for the preparation of MTMOs. Nevertheless, they generally require sophisticated devices, high temperature treatment, time consuming procedures and solvent based approaches. Hence, developing an economic, simple, green, and solvent free rout for synthesis of MTMOs and their utilization, as the active material in supercapacitors is highly desirable. Here in, to our knowledge, a simple, efficient, solvent free and cost-effective solid-state reaction method for synthesis of various MTMOs including FeCo2O4, MnCo2O4 and ZnCo2O4 with unique nanostructures and remarkable electrochemical supercapacitive performances has been reported for the first time. All of the synthesized MTMOs possess high crystallinity with different morphologies among which FCO exhibits higher specific capacity and excellent electrochemical activity about 1263 F/g at 2 A/g. The assembled asymmetric supercapacitor with FCO was further investigated. The results showed that this work provides an opportunity for facile and large scale synthesis of MTMOs as a promising electrode material for developing asymmetric supercapacitors.
Experimental Synthesis of FCO, MCO and ZCO All the reagents in this study were used as received without further purification. MTMOs were successfully synthesized by solid state reaction method. Briefly, 1 mmol of FeCl2$4H2O, MnCl2$4H2O, and ZnCl2 each were separately mixed with 2 mmol of CoCl2$6H2O, in mortar and crushed. Afterwards, 6 mmol of KOH was added to each of the above mixtures without adding any solvent and the mixture was stirred for 30 min. After passage of time the KOH started to moisturize and thick slurry was formed which was further stirred for next 30 min. Subsequently, the products were washed with a mixture of deionized water and ethanol several times and centrifuged (3000 rpm) for 15 min. After that, the samples were dried overnight under vacuum at 80 C in order to remove any impurity. Finally, the obtained products were annealed at 300 C for 2 h in air. The as-prepared FeCo2O4, MnCo2O4 and ZnCo2O4 samples are hereafter denoted as FCO, MCO and ZCO, respectively.
Characterization techniques The surface morphologies of the samples were thoroughly examined by transmission electron microscopy (Tecnai G2 F20 STWINHR(S) TEM, FEI) and field-emission scanning electron microscopy (FEI Quanta 650F Environmental SEM) attached with an energy-dispersive X-ray spectroscopy (EDS). The crystallinity of the samples was investigated by Powder Xray diffraction (XRD) using Panalytical X'Pert Pro-MRD instrument (Cu Ka source). The X-ray photoelectron spectra (XPS) analyses were obtained by X-ray photoelectron spectroscopy (XPS, SPECS Germany, PHOIBOS 150). N2 adsorption/ desorption was determined by BrunauereEmmetteTeller
Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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(BET) measurements using Micromeritics instrument (Data Master V4.00Q, Serial#:2000/2400).
Electrode preparation and electrochemical testing The pastes were prepared by mixing 80% of active material (FCO or MCO or ZCO), 10% of binder (PVDF) and 10% of conducting material (carbon black or SuperP) with N-methylpyrrolidone (NMP). Subsequently, the pastes were coated on the carbon cloth using doctor blade method and dried in vacuum oven overnight. The typical mass loading was found to be 0.9e1 mg/cm2 for MCO, FCO and ZCO samples. The supercapacitive performances of the materials were tested in 6 M KOH using cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) with conventional three electrode cell comprising of the MTMOs (FCO or MCO or ZCO) as a working electrode, platinum as a counter and Ag/AgCl as reference electrode, respectively. All the electrochemical measurements were carried out with a Biologic VMP3 potentiostat. Later, asymmetric capacitor cell was assembled with FCO (the reason chose FCO is relatively better electrochemical properties) as the positive electrode and activated carbon (AC) as the negative electrode using Swagelok cell. The mass of active material in both electrodes was 2.5 mg. The electrodes were separated by separator soaked with 6 M KOH electrolyte.
Results and discussion XRD patterns of MTMOs are shown in Fig. 1. The resultant diffraction peaks reveal the spinel structure and corroborate with the standard patterns [32,33] for MCO, ZCO and FCO, respectively. No unidentified peaks are present in the XRD patterns which indicate the purity of the formed products. The well-defined and broad diffraction peaks are also as an indicative of nanocrystalline nature of the sample. The lattice parameter (a0) for the cubic spinel structure can be calculated by following equation,
Fig. 1 e XRD patterns of FeCo2O4 (FCO), MnCo2O4 and ZnCo2O4 (ZCO) samples.
1=2 a0 ¼ d h2 þ k2 þ l2
3
(1)
where, d is the interplanar distance for crystal plane, and h, k, l are the Miller indices. The lattice parameters calculated for FCO, MCO and ZCO samples were 8.1623 A, 8.0958 A and 8.0680 A, respectively for (311) plane, which matches well with their corresponding standard values indicating the successful incorporation of Fe, Mn and Zn into the lattice [JCPDS:0231390]. The oxidation states of each element present in the spinel structure of FCO (Fe, Co), MCO (Mn, Co) and ZCO (Zn, Co) are investigated by X-ray photoelectron spectra (XPS) analysis. The Fe/Co, Mn/Co and Zn/Co atom ratio for FCO, MCO and ZCO samples is close to 1:2 as measured by XPS, which is in good agreement with the starting precursor ratio. Fig. 2(aec) shows the magnified XPS spectra of Fe2p, Mn2p and Zn2p of FCO, MCO and ZCO samples, respectively. As can be seen from Fig. 2(a), Fe2p3/2 and Fe2p1/2 lie about 711.8 and 725.2 eV, respectively. Peak positions and spin orbit separation of about 13.4 eV suggest the presence of Fe(III) in the oxide [34,35]. In case of MCO sample, the two main peaks of Mn2p3/2 at 641.3 eV and Mn2p1/2 at 652.8 eV deconvoluted into four subpeaks: the two at 641.5 and 653.1 eV are attributed to the presence of Mn2þ, whereas the other two at 643.2 and 654.1 eV originate from Mn3þ (Fig. 2b) [36e38]. Furthermore, as seen from Fig. 2c, two peaks with binding energy values of 1022.4 and 1045.5 eV can be ascribed to Zn2p3/2 and Zn2p1/2, indicating the Zn(II) oxidation state of ZCO sample (ZnCo2O4) [39]. The spectra observed for FCO, MCO and ZCO samples are same hence, we are presenting Co2p spectrum for FCO sample. The Co2p spectrum (Fig. 2d) was fitted with Gaussian fitting method under the assumption of eight species including two major signals, which are attributed to the Co2p3/2 (779.2 eV) and Co2p1/2 (794.4 eV) levels, respectively. The spineorbit splitting is 15.2 eV. Additionally, the two prominent peaks both possess weak shake-up satellites (denoted as “sat”) at 788.3 and 802.5 eV, showing binding energies of approximately 9 and 8 eV higher than the main signals, respectively. Therefore, the coexistence of Co2þ and Co3þ could be confirmed based on the positions of the main peaks and on the distance of satellites from their respective major signals [40]. All of these results suggest the existence of mixed-valence metal cations in the as-synthesized MTMOs samples, which are advantageous toward supercapacitors applications. The surface morphologies of MTMOs were investigated with different techniques such as SEM, TEM, HRTEM etc. Fig. 3(aef) shows FESEM images for FCO, MCO and ZCO samples at two different magnifications, respectively. The morphology of FCO sample revealed the coverage of homogeneous nanoparticles at low magnification (Fig. 3(a)) while at high magnified images showed the formation of tetragonal nanoparticles (see Fig. 3(b)). Similar appearance is also noticed for MCO sample (Fig. 3(c, d)). Further, it is seen that the shape of MCO nanoparticles are spherical, completely different than FCO. Interestingly, the ZCO sample exhibits totally different morphology than FCO and MCO as seen from Fig. 3(e, f). The surface morphology for ZCO is non-uniform consisting of different sized and shaped nanoparticles. Moreover, high magnified image reveals the presence of hexagonal
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Fig. 2 e (aec) Magnified spectra of Fe, Mn and Zn for FCO, MCO and ZCO samples, respectively (d) Core level Co spectrum for FeCo2O4 (FCO) samples.
Fig. 3 e FE-SEM images of (a, b) FCO, (c, d) MCO and (e, f) ZCO samples at two different magnifications, respectively.
nanosheets with different sized spherical nanoparticles. In addition, the corresponding EDS mapping confirms the presence of Fe, Mn, Zn, Co and O elements on the entire surface of the materials (Supporting Information S1 and S2).
Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) measurements were employed to investigate the microstructure and the
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crystalline phase of the MTMOs. TEM and HRTEM images with different magnifications of FCO, MCO and ZCO are displayed in Fig. 4. The formation of tetragonal nanoparticles for FCO sample is confirmed by TEM image shown in Fig. 4(a). Moreover, all the tetragonal nanoparticles are uniform in shape and size which further suggest the potential of this simple method for the preparation of controlled nanostructures. The size of these tetragonal nanoparticles is in the range of 80e100 nm. Similarly, Fig. 4(b) shows the uniform distribution of spherical nanoparticles of MCO sample. The TEM result clearly supports the SEM analysis. The size of spherical nanoparticles is in the range of 60e80 nm. On the other hand, ZCO sample exhibits completely different nanostructure than FCO and MCO. Interestingly, hexagonal nanosheet with homogeneous coverage of small nanoparticles is clearly seen (see Fig. 4(c)). Thus, it is clear that, the different nanostructures of different materials can be easily prepared with this simple method which holds a great promise of large-scale production of materials. HRTEM images taken from individual nanoparticles of FCO, MCO and ZCO samples are shown in Fig. 4(def), confirming that all the three MTMO samples are polycrystalline in nature. The clearly resolved lattice fringes were calculated to be about 4.71 A for FCO, 2.41 A and 2.32 A for MCO and 4.69 A and 2.43 A for ZCO samples which matches well with the (111), (311), (222) lattice planes of spinel MTMOs. So the interconnected nanoparticles morphology will facilitate the high transport rates for both electrons and electrolyte ions that simultaneously take part during Faradic reaction. In order to investigate, the surface areas of these MTMOs, BET analysis was carried out using N2 adsorption/desorption isotherms (Fig. 5). The shape of the isotherms acquired for all materials show the hysteresis loop in relative pressure (P/P0) between 0.6 and 1.0, showing characteristic of mesoporous materials [41]. The BET specific surface area obtained were 69, 54 and 49 m2g1 for FCO (FeCo2O4), MCO (MnCo2O4) and ZCO
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Fig. 5 e Nitrogen adsorption/desorption isotherms with corresponding pore-size distribution plots for FeCo2O4 (FCO), MnCo2O4 and ZnCo2O4 (ZCO) samples.
(ZnCo2O4), respectively. Thus, the surface area of the FCO sample is relatively higher than that for MCO and ZCO samples. The pore size distribution curves for all these samples are shown in inset of Fig. 5. It is observed that, all these materials exhibits mesoporous nature. It is further confirmed that, the FCO sample exhibits relatively higher pore volume. The different surface areas for these materials might be attributed to their different surface morphologies. The high specific surface area is directly associated to higher supercapacitive properties of the materials, due to the mesoporous structures which enable the soaking of electrolyte, facilitating the ion diffusion and provide more electroactive sites for energy storage.
Fig. 4 e TEM images (aec) and HRTEM images (def) of FCO, MCO and ZCO samples, respectively. Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Electrochemical performance
The specific capacitances for the electrodes were calculated from CV measurements using the following equation:
The electrochemical performance of MTMO electrodes were investigated by cyclic voltammetry (CV) technique and results are displayed in Fig. 6. Thus, Fig. 6(aec) shows the CV curves for FCO, MCO and ZCO samples at different scan rates, respectively. The shapes of CV curves for FCO shows clear redox peaks suggesting the contribution from faradaic reactions while that for MCO and ZCO are nearly rectangular. It is further interesting to note that, the CV curves remain unchanged, as scan rate increases, thus indicating the excellent electrochemical reversibility and exceptional high-rate performance. It is also observed that, the area under curve increases with scan rate signifying that the current density is directly proportional to scan rate. The FCO electrode exhibits relatively highest current density, corresponding to high capacitance which might be attributed to its morphology and good conductivity. However, the redox peaks shifted towards lower potential, which may due to the polarization effect of the electrodes. The charge-storing mechanism in all these MTMOs samples can be explained with the following reactions, ðMÞCo2 O4 þ OH þ H2 O⇔ðMÞOOH þ 2CoOOH þ 2e
(2)
ðMÞOOH þ OH ⇔MO2 þ H2 O þ e
(3)
where, M is indicative of Fe or Mn or Zn.
C¼
1 mwðVc Va Þ
ZVc IðVÞdV
(4)
Va
where, C is the specific capacitance, m is the mass of active material, n represents the scan rate, Vc and Va refer to the high and low potential limit of the CV test, and I is the current recorded during CV measurements. The specific capacitances of all MTMO samples as a function of scan rate are displayed in Fig. 6(d). The specific capacitances decrease with increasing the scan rate which may be attributed to the presence of inner active sites that are unable to sustain the redox transition completely at high scan rates. The maximum specific capacitances calculated are 1182 F/g, 743 F/g and 844 F/g for FCO, MCO and ZCO samples, respectively. The high specific capacitance of FCO may be ascribed to the tetragonal nanoparticles surface morphology. Fig. 7(aec) manifests the GCD curves for MTMO samples at various current densities. It is fairly observed that, the GCD curves for all the samples are not ideal straight lines which confirms the inclusion of faradaic reactions during charging/ discharging processes. Obviously, the discharging time of FCO is much longer than that of other materials MCO and ZCO. The specific capacitance in the GCD process can be calculated according to the following equation:
Fig. 6 e CV curves at different scan rates (5e100 mV/s) of (a) FCO (b) MCO (c) ZCO in 6 mol/L KOH electrolyte. (d) The variation of specific capacitance of all electrodes with scan rate. Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Fig. 7 e GCD curves at different current densities (5e25 A/g) of (a) FCO (b) MCO (c) ZCO in 6 mol/L KOH electrolyte. (d) Comparison the specific capacitance of samples. Z I$ C¼
V$dt m$V2
(5)
where, I is applied the discharge current, m is the mass of the active material and V is the potential window. The maximum specific capacitance obtained were 1263, 660 and 800 F/g at 2 A/g for FCO, MCO and ZCO, respectively. Similar to the result obtained from CV tests, the FCO exhibits maximum specific capacitance as compared to that of other samples (see Fig. 7(d)). The stability of these three MTMOs was investigated at 6 A/g over 500 cycles and presented in Supporting Information S. I. 3. It is seen that all samples exhibits excellent cycling stability around 86e89% over 5000 cycles. Electrochemical impedance analysis was used to investigate the resistances involved in the overall system. The Nyquist plots for MTMO electrodes in 6 M KOH electrolyte at open circuit voltage (OCV) are shown in Fig. 8(a, b). The depressed semicircles are clearly observed at high frequency regime, which are related to charge transfer resistance at the electrode/electrolyte interfaces. The initial non-zero intercept at X-axis suggests electrochemical series resistance or electrolyte resistance (Rs). The electrolyte resistance (Rs) for FCO, MCO and ZCO are found to be 0.24, 0.11 and 1.28 U, respectively (Fig. 8b). Moreover, FCO and MCO exhibit smaller semicircles suggesting small charge transfer resistance. Thus, the impedance analysis suggests that, FCO provides relatively small charge transfer resistance which might be attributed to the intimate contact at electrolyte/electrode interface. Inset of
Fig. 8(a) is the circuit model and the parameters; Rs is the electrolyte resistance in the cell, Rct is the charge transfer resistance, CPE is a constant phase angle element, and Zw represent the Warburg diffusion. Overall electrochemical performances imply that FCO exhibits relatively excellent specific capacitance as well as good rate capability. The high capacitive performance of FCO might be ascribed to the unique tetragonal nanostructure that eases rapid transport of electrons between the active materials and the charge collecting substrate. In addition, these FCO tetragonal nanoparticles exhibit relatively high specific surface areas and porous features which could enlarge the efficient liquid/solid interfacial area, resulting in effective use of the active material by increasing the number of electroactive sites. Hence, FCO is used for further electrochemical characterizations.
Asymmetric capacitors using FCO as the positive electrode and activated carbon (AC) as the negative electrode Considering the excellent electrochemical properties of FCO electrode and the fast ion transport property of activated carbon (AC) material, an asymmetric capacitor was successfully fabricated using these materials as the positive and negative electrodes, respectively. The CV curves for AC and FCO electrodes in three electrode configuration at scan rate of 10 mV/s in 6 M KOH are measured (Supporting Information S.I.4). The potential windows of FCO electrode and AC electrode are 0.5 to 0.3 V and 1.0 to 0 V (vs. Ag/AgCl), respectively; indicating that AC and FCO are greatly stable in
Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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Fig. 8 e (a) Nyquist plots of FCO, MCO and ZCO samples with corresponding equivalent circuit, (b) Magnified view of Nyquist plots.
different potential windows. Therefore, the total cell voltage can be expressed as the sum of the potential range for FCO and AC electrode for asymmetric configuration. It should be noted that, to approach the highest cell voltage, the charges stored in both electrodes must be balanced by adjusting the mass loading of each of the active electrode materials [42,43]. The AC to FCO mass ratio was calculated by the following equation [44]: mþ C DE ¼ m Cþ DEþ
(6)
where, C is the specific capacitance and DE is the potential range for positive (þ) and negative () electrodes. On the basis of the specific capacitance values found for FCO and AC, the optimal mass ratio between the electrodes should be m (FCO)/ m (AC) ¼ 0.34:1 in the asymmetric capacitor cell. In present investigation, the weight of active material in both electrodes was 2.5 mg (0.89 mg of FeCo2O4 and 1.61 mg of AC). Asymmetric capacitor, AC//FCO, was assembled by using aqueous 6 M KOH electrolyte. The CV curves of the AC//FCO
asymmetric cell measured at various scan rates from 5 to 100 mV/s between 0 and 1.4 V and are shown in Fig. 9(a). The shape of the CV remains unchanged even at a high scan rate of 100 mV/s, implying an excellent rate capability of the cell. Fig. 9(b) shows the GCD curves of asymmetric cell and corresponding potential distribution across the electrodes. During the charge/discharge, the positive electrode (FCO) is swinging between 0 and 0.4 V vs. Ag/AgCl, while AC between 0 and -1 V vs. Ag/AgCl and the cell between 0 and 1.4 V at 0.8 A/g. Further, GCD curves of asymmetric cell were recorded at various current densities and are presented in Fig. 9(c). Accordingly, a nonlinear relation of the charge/discharge potentials with time was found. In addition, the initial voltage loss (i.e., iR drop) observed in the discharge curves is small even at high current densities; this is an indicative of fast IeV response and low internal resistance of the supercapacitors. Additionally, from the typical GCD curves, it can be observed that the discharge curve is nearly symmetric with its corresponding charging counterpart, demonstrating the excellent electrochemical reversibility and good coulombic efficiency. The calculated specific and volumetric capacitance for AC//FCO asymmetric cell at different current densities is shown in Fig. 9(d). The maximum capacitance obtained for cell is 88.1 F/ g (3.07 F/cm3, considering volume of device 0.0678 cm3) at current density of 0.8 A/g. Since it is vital to retain high specific capacitances (or energy density) at high current density, we estimated the energy and power density, which are key factors for the supercapacitor applications. The Ragone plots are presented in Fig. 10(a, b). The energy density of the AC//FCO asymmetric cell based on the total mass of the active electrode materials (including AC and FCO) reaches to 24.0 Wh/kg (1.1 mWh/cm3) at a power density of 560 W/kg (25.5 mW/cm3), and still remains 19.0 Wh/kg (0.9 mWh/cm3) at a power density of 5600 W/kg (255.4 mW/cm3). Interestingly, the maximum energy density obtained for AC//FCO asymmetric capacitor with a cell voltage of 1.4 V is much higher than those of symmetric AC/AC supercapacitor (<10 Wh/kg) [45], graphene/graphene (9.1 Wh/kg) [46], CNT/CNT supercapacitor (<10 Wh/kg) [47,48]. Additionally, the proposed AC//FCO asymmetric supercapacitor shows higher energy density versus other reported asymmetric supercapacitors with aqueous electrolyte solutions, such as NieCo oxyhydroxide// AC (17.8 Wh/kg) [49], NiCo2O4-RGO//AC (12 and 23.3 Wh/kg) [49,50], NiCo2O4//AC (15.32 and 17.72 Wh/kg) [51,52], NiCo2O4graphene//AC (7.6 Wh/kg) [53], FeCo2O4//AC (23 Wh/kg) [54], Co3O4@MnO2//AC (17.7 Wh/kg) [55]. Therefore, the considerably high energy density derived by this simple AC//FCO asymmetric cell has important industrial applications in the field of supercapacitors [56e60]. The cycling life test over 5000 cycles for the AC//FCO asymmetric capacitor was carried out by repeating the charging/discharging test between 0 and 1.4 V at a current density of 4 A/g. The cycle performance of the asymmetric capacitor charged at 1.4 V as a function of the cycle number is shown in Fig. 10(c). The asymmetric cell exhibits excellent electrochemical stability of 94% over 5000 cycles with Coulombic efficiency more than 90%.The results demonstrate that AC//FCO asymmetric cell displays a high specific capacitance and excellent cycling stability. EIS of AC// FCO asymmetric capacitor was measured at open circuit potential and the Nyquist plot is shown in Fig. 10(d) with inset
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Fig. 9 e (a) CV curves of AC//FCO asymmetric cell within operating voltage window of 1.4 V at various scan rates, (b) GCD curves of FCO, AC and AC//FCO asymmetric cell, (c) GCD curves of AC//FCO asymmetric cell at different current densities (0.8e8.0 A/g), (d) Plots of volumetric and specific capacitances with current density for AC//FCO asymmetric cell.
Fig. 10 e (a, b) Plots of energy density versus power density of AC//FCO asymmetric cell in Ragone plot, (c) Variation of specific capacitance with number of cycles measured at 4 A/g over 5000 cycles, (d) Nyquist plot and inset shows Bode plot of AC//FCO cell. Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117
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representing the Bode plot. As can be seen in Fig. 10(d), the vertical rise in impedance value, at low frequency region, indicates good capacity behavior. Moreover, a small depressed semicircle which represented the small charge transfer resistance at the electrode/electrolyte interface can be detected at high frequency region. The Bode plot for asymmetric capacitor shows phase angle z 80 at 1 Hz, which is close to that of an ideal capacitor, verifying that the AC//FCO asymmetric capacitor exhibits a good capacitive behavior.
Conclusions In summary, different mixed transition metal oxides such as FeCo2O4, MnCo2O4 and ZnCo2O4 were successfully synthesized by simple solid-state reaction method. The physiochemical characterizations show that three different nanostructures including tetragonal and spherical nanoparticles and hexagonal nanosheets are formed for FCO, MCO and ZCO, respectively. Moreover, all these MTMO samples are intrinsically polycrystalline and hold spinel crystal structure. All the synthesized nanomaterials show excellent supercapacitive properties with specific capacitance in the range of 660e1263 F/g. Furthermore, an asymmetric capacitor based on FCO as positive electrode and AC as negative electrode (AC//FCO) was successfully cycled up to 1.4 V. The AC//FCO cell exhibits a specific capacitance of 88.06 F/g (3.07 F/cm3) with high specific energy of 24 Wh/kg (1.1 mWh/cm3) and 93% capacitance retention after 5000 cycles. The outcomes demonstrate that the proposed solid-state reaction method has a great potential application for large scale synthesis of MTMOs as an applicable electrode material for asymmetric supercapacitors.
Acknowledgment The authors would like to acknowledge the Catalan Institute of Nanoscience and Nanotechnology (ICN2) for their support and warm collaboration. D.P. Dubal and P. Gomez-Romero acknowledge AGAUR (Generalitat de Catalunya) for Project NESTOR (Nanomaterials for Energy STORage) 2014_SGR_1505.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.03.117.
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Please cite this article in press as: Tajik S, et al., Nanostructured mixed transition metal oxides for high performance asymmetric supercapacitors: Facile synthetic strategy, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/ j.ijhydene.2017.03.117