A COMPARATIVE STUDY ON APPLICATION OF BIOGENIC … · The supercapacitors show distinct advantages...

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Christian Girginov, Svetlana Veleva, Stephan Kozhukharov, 557 A COMPARATIVE STUDY ON APPLICATION OF BIOGENIC HEMATITE AND MAGNETITE AS ELECTRODE MATERIALS IN HYBRID SUPERCAPACITORS Christian Girginov 1 , Svetlana Veleva 2 , Stephan Kozhukharov 1 , Antonia Stoyanova 2 , Elefteria Lefterova 2 , Mladen Mladenov 2 , Raicho Raicheff 2 1 University of Chemical Technology and Metallurgy 8 Kliment Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected] 2 Institute of Electrochemistry and Energy Systems Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria ABSTRACT In the present study activated carbon (AC) and two biogenic iron oxides (α-Fe 2 O 3 -hematite and Fe 3 O 4 -magnetite) were employed as electrode materials in a hybrid supercapacitor. The bio-iron oxides were produced by thermal treatment of biogenic α-FeOOH (goethite) and γ-FeOOH (lepidocrocite), respectively, obtained by laboratory cul- tivated Leptothrix/Sphaerotilus bacteria. The AC and biogenic oxides Fe 2 O 3 and Fe 3 O 4 were used to fabricate negative composite electrodes, while the positive electrode was made by AC. The electrodes were mounted in electrochemical coin-type cells. The electrolyte in both cases was 1M LiBF 4 with organic solvent mixture - ethylene carbonate/dimethyl carbonate (1:1). The electrode materials were characterized by X-ray diffraction and transmission electron microscopy. The electrochemical perfor- mance of the hybrid cells were studied by galvanostatic charge/discharge cycling tests and impedance spectroscopy. The investigations showed that the hybrid supercapacitor using biogenic Fe 2 O 3 in the composite electrode shows higher discharge capacity and charge/discharge efficiency, and more stable capacity behavior at prolonged cycling (above 1000 cycles) in comparison to the supercapacitor cell using biogenic Fe 3 O 4 as active electrode material. The analysis of the impedance spectra and calculated parameters shows the higher diffusion resistance of hybrid supercapacitors compared to the symmetric one. These results indicate the important role of the morphology of the electrode materials for the electrochemical characteristics of hybrid battery-supercapacitor systems. Keywords: hybrid supercapacitors, biogenic hematite/magnetite, electrode materials, galvanostatic charge/ discharge cycling, cycling voltammetry, impedance spectroscopy. Received 16 Septenber 2016 Accepted 10 January 2017 Journal of Chemical Technology and Metallurgy, 52, 3, 2017, 557-563 INTRODUCTION The supercapacitors show distinct advantages in comparison to conventional electrochemical power sources, such as batteries and fuel cells. Although they do not exhibit high energy density, the supercapacitors can provide much higher power capabilities, higher effi- ciency and they show exceptional cycling characteristics. Furthermore, in order to improve the energy density, hybrid electrochemical systems (e.g. asymmetric super- capacitors), where both electrodes are made of different materials,were introduced [1]. The common iron–oxy- gen compounds (Fe 2 O 3 , Fe 3 O 4 , FeO and FeOOH) are widely used as electrode materials in lithium-ion batter- ies and recently in supercapacitors [2, 3 - 5]. The hematite (a-Fe 2 O 3 ) is commonly used as elec- trode material for Li-ion batteries, because it possesses a large theoretical specific capacity. Its behavior is closely related to the chemical state variation between Fe 3+ and Fe 2+ on the surface of Fe 2 O 3 electrode during the charg- ing/discharging process [11]. The cycling performance of this material, however, is not satisfactory because of the partial electrode destruction that may take place upon repetitive cycling reactions between Fe 2 O 3 and Li-ions

Transcript of A COMPARATIVE STUDY ON APPLICATION OF BIOGENIC … · The supercapacitors show distinct advantages...

Page 1: A COMPARATIVE STUDY ON APPLICATION OF BIOGENIC … · The supercapacitors show distinct advantages in comparison to conventional electrochemical power sources, such as batteries and

Christian Girginov, Svetlana Veleva, Stephan Kozhukharov, Antonia Stoyanova, Elefteria Lefterova, Mladen Mladenov Raicho Raicheff

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A COMPARATIVE STUDY ON APPLICATION OF BIOGENIC HEMATITE AND MAGNETITE AS ELECTRODE MATERIALS IN HYBRID SUPERCAPACITORS

Christian Girginov1, Svetlana Veleva2, Stephan Kozhukharov1, Antonia Stoyanova2, Elefteria Lefterova2, Mladen Mladenov2, Raicho Raicheff2

1 University of Chemical Technology and Metallurgy 8 Kliment Ohridski, 1756 Sofia, Bulgaria E-mail: [email protected] Institute of Electrochemistry and Energy Systems Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

ABSTRACT

In the present study activated carbon (AC) and two biogenic iron oxides (α-Fe2O3-hematite and Fe3O4-magnetite) were employed as electrode materials in a hybrid supercapacitor. The bio-iron oxides were produced by thermal treatment of biogenic α-FeOOH (goethite) and γ-FeOOH (lepidocrocite), respectively, obtained by laboratory cul-tivated Leptothrix/Sphaerotilus bacteria.

The AC and biogenic oxides Fe2O3 and Fe3O4 were used to fabricate negative composite electrodes, while the positive electrode was made by AC. The electrodes were mounted in electrochemical coin-type cells. The electrolyte in both cases was 1M LiBF4 with organic solvent mixture - ethylene carbonate/dimethyl carbonate (1:1). The electrode materials were characterized by X-ray diffraction and transmission electron microscopy. The electrochemical perfor-mance of the hybrid cells were studied by galvanostatic charge/discharge cycling tests and impedance spectroscopy.

The investigations showed that the hybrid supercapacitor using biogenic Fe2O3 in the composite electrode shows higher discharge capacity and charge/discharge efficiency, and more stable capacity behavior at prolonged cycling (above 1000 cycles) in comparison to the supercapacitor cell using biogenic Fe3O4 as active electrode material. The analysis of the impedance spectra and calculated parameters shows the higher diffusion resistance of hybrid supercapacitors compared to the symmetric one. These results indicate the important role of the morphology of the electrode materials for the electrochemical characteristics of hybrid battery-supercapacitor systems.

Keywords: hybrid supercapacitors, biogenic hematite/magnetite, electrode materials, galvanostatic charge/discharge cycling, cycling voltammetry, impedance spectroscopy.

Received 16 Septenber 2016Accepted 10 January 2017

Journal of Chemical Technology and Metallurgy, 52, 3, 2017, 557-563

INTRODUCTION

The supercapacitors show distinct advantages in comparison to conventional electrochemical power sources, such as batteries and fuel cells. Although they do not exhibit high energy density, the supercapacitors can provide much higher power capabilities, higher effi-ciency and they show exceptional cycling characteristics. Furthermore, in order to improve the energy density, hybrid electrochemical systems (e.g. asymmetric super-capacitors), where both electrodes are made of different materials,were introduced [1]. The common iron–oxy-

gen compounds (Fe2O3, Fe3O4, FeO and FeOOH) are widely used as electrode materials in lithium-ion batter-ies and recently in supercapacitors [2, 3 - 5].

The hematite (a-Fe2O3) is commonly used as elec-trode material for Li-ion batteries, because it possesses a large theoretical specific capacity. Its behavior is closely related to the chemical state variation between Fe3+ and Fe2+ on the surface of Fe2O3 electrode during the charg-ing/discharging process [11]. The cycling performance of this material, however, is not satisfactory because of the partial electrode destruction that may take place upon repetitive cycling reactions between Fe2O3 and Li-ions

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[12]. It has been shown [13], that goethite (α-FeOOH) obtained by laboratory cultured Leptothrix bacteria, can be used as a precursor for synthesis of electrochemically active nanosized α-Fe2O3. The nanoparticles of bio-α-Fe2O3can be easily applied for fabrication of composite electrodes with activated carbon matrices. The results prove the possibility of application of bio-Fe2O3 as electrochemically active material for hybrid lithium battery-supercapacitor systems [14, 15].

The electrochemical evaluation results have shown that magnetite (Fe3O4) can act as a rechargeable con-version electrode material that reacts with Li-ions with a theoretical specific capacity of about 926 mAh g-1. Fe3O4 features higher electric conductivity compared with other transition metal oxides [6 - 8]. Varieties of nanostructures (nanocomposites) with different matri-ces have recently been explored and achieve improved electrochemical properties. Among these materials, carbonaceous materials are usually selected as matrices to anchor Fe3O4 nanoparticles to prepare novel nano-composites [6, 7, 9, 10]. Here, the carbon phase can act as a volume buffer to absorb the volume changes and improve structural stability of the electrode and increase the electrical conductivity.

This work is aimed to prepare novel iron oxides based electrode materials for supercapacitors with highenergy and high-power densities and long cycle life. Activated carbon and two biogenic iron oxides - bio-α-Fe2O3 (hematite) and bio-Fe3O4 (magnetite) are em-ployed in composite electrodes for hybrid supercapaci-tors. The electrochemical properties of these composites are investigated and the capacitance behavior of the hybrid supercapacitor systems developed are compared with those of a symmetric carbon-based supercapacitor.

EXPERIMENTALSynthesis of electrode materialsThree types of electrode materials are used for the

assembly of supercapacitor cells: activated carbon (AC) and bio-iron oxides - a-Fe2O3 (hematite) and Fe3O4 (magnetite). The АС with large surface area (about 1600 m2 g-1) is a product of TDA Research (USA).The nano-sized bio-hematite and bio-magnetite are produced by thermal treatment of biogenic α-FeOOH (goethite) and γ-FeOOH (lepidocrocite), respectively, obtained by labo-ratory cultivated Leptothrix/Sphaerotilus bacteria [8, 9].

Structural and morphological characterization of the electrode materials

The morphology of the electrode materials is ex-amined by Transmission electron microscopy (TEM), using a JEOL Superprobe 733 [16, 17].

The biogenic iron (oxide/hydroxide) precursor and the obtained oxide materials are structurally characterized by X-diffraction (XRD) method using a Bruker D8 Advance diffractometer with Cu Kα radiation. The phases were identified and the mean crystallite size was determined.

Supercapacitor cellsTwo types asymmetric supercapacitor cells were

assembled with a composite electrode using hematite or magnetite as electrochemically active compounds and an activated carbon electrode. The composite (negative) electrodes are produced of activated carbon matrix with addition of 50 % of bio-hematite or bio-magnetite. The positive electrode is made from the same activated carbon.

The activated carbon was previously teflonised (with 10 % polytetrafluorethylene used as Aldrich PTFE binder, 60 % suspension in water), then heated at 70oC and dried. By adding a natural graphite NG-7 (5 %) to the electrode materials as conductive material a paste was formed, which was glued to Al foil substrate (in the form of disk with surface area of 1,75 cm2). Further, the formed sheet electrodes were dried and pressed under pressure of 200 kg cm-2. Thus obtained electrodes were soaked in organic electrolyte under vacuum and then mounted in a coin-type cell with Glassmat separator and the cell was filled with electrolyte. For a reference cell a symmetric supercapacitor composed of two identical АС-electrodes was used. In all three types of super-capacitor cells, the organic electrolyte was 1M LiBF4 dissolved in a mixture of ethylene carbonate/dimethyl carbonate (EC/DMC) in ratio 1:1 [17].

Electrochemical testsThe elctrochemical studies on the three types of

supercapacitor cells were performed by galvanostatic charge/discharge cycling test at different current load (10 - 600 mA g-1) at room temperature, i. e. the cells:

- symmetric supercapacitor cell: (+)(AC)/(LiBF4- EC/DMC)/(AC)(–);

- hybrid lithium battery-supercapacitor cell: (+)(AC)/(LiBF4- EC/DMC)/(AC+bio-Fe2O3)(–);

- hybrid lithium battery-supercapacitor cell: (+)(AC)/(LiBF4- EC/DMC)/(AC+bio- Fe3O4)(–).

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Fig. 1. XRD powder patterns of bio-Fe2O3 (a) and bio-Fe3O4 (b).

Fig. 2. TEM image of bio-Fe2O3 (a) and bio-Fe3O4 (b).

The tests were performed by an Arbin Instrument Sys-tem BU-2000, thus obtaining dependencies of the discharge capacity on the current load and the number of cycles [16, 17]. Electrochemical impedance spectroscopy (EIS) measurements were also carried out, using Phase Sensitive Multimeter Psimetri Q-PSM1700 equipped with Newtons Impedance Analysis Interface in the frequency range 1MHz - 1mHz, with a sampling rate of 10 points per decade and an amplitude of the AC perturbation signal of 10 mV.

RESULTS AND DISCUSSION

Physicochemical characterization of the electrode materials

Fig. 1 represents the powder X-ray diffraction pat-terns of the biogenic a-Fe2O3 (а) and biogenic Fe3O4 (b). From Fig. 1a it can be seen that the bio-a-Fe2O3 is a

single phase hematite with unit cell parameters: (a) = 5.033 Å and (c) = 13.778 Å. The mean crystallite size of the bio-a-Fe2O3 is estimated to be about 25 - 30 nm. The unit cell parameter of bio-Fe3O4 is (a) = 8.394 Å and the mean crystallite size (14 nm) is lower than that of the biogenic hematite.

The TEM observations (Fig. 2a,b) show that bio-Fe2O3 particles are long stretched (i. e. like nanorods) with specific cavities and irregular shape, while the bio-Fe3O4 consists of nanoparticles in a similar form, arranged in a row.

The examination of the samples of activated carbon have shown that the specific surface area is 1520 m2 g-1, the total volume of the pores - 0.68 cm3 g-1, the volume of micropores is 0.55 cm3 g-1 (i.e. 80 % of the total pore volume) and the volume of mesopores 0.13 cm3 g-1 [14].

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Electrochemical performance of the supercapaci-tor cells

The dependence of the discharge capacity of the supercapacitor cells on the current load was investigated by charge-discharge cycling test [16, 17]. The value of the specific capacitance (C) is obtained from the charge/discharge cycling measurements according to the equa-tion [18]: (C = 4I∆t/m∆V), where I, ∆t, ∆V and m are the constant current, discharge time, voltage interval of the charge (or discharge) and the total mass of the active electrode material, respectively.

The galvanostatic charge/discharge profiles of the two types hybrid and the symmetric supercapacitor cells are compared in the potential window between 0.6 and 2.4 V. The hybrid lithium battery-supercapacitor cells show reproducible charge/discharge behavior and stable and high specific capacity values at relatively high current loads. The value of the discharge current affects much more strongly the capacity of the hybrid supercapacitors.

A comparison of the results for the discharge per-formance of the hybrid and symmetric supercapacitors is illustrated in Fig. 3. The results demonstrate higher capacity values of the hybrid supercapacitors, especially those with composite electrode with bio-Fe2O3 at cur-rent load up to about 300 mA g-1. The processes on the composite anode are obviously lithiation/delithiation of Fe2O3 or Fe3O4 (i.e. a typical Faradaic process) to-gether with the adsorption/desorption of Li-ion on the

Fig. 3. Dependence of the discharge capacity of hybrid supercapacitors: (AС/bio-Fe2O3), (AC/bio-Fe3O4) and symmetric capacitor (AC/AС) on the current load.

Fig. 4. Dependence of the discharge capacity of hybrid supercapacitors: (AС/bio-Fe2O3), (AC/bio-Fe3O4) and symmetric capacitor (AC/AС) on the number of cycles at constant current load of 60 mA g-1.

AC matrix of the electrode, while on the AC cathode only the process of electrostatic adsorption/desorption of BF4− takes place [14]. The higher capacity of the su-percapacitor with composite electrode with addition of Fe2O3 is due to its morphology, which favors to a greater extent the electrochemical reaction [2].

The dependence of discharge capacity on the number of cycles was also studied. The results show that the hybrid supercapacitors and the symmetric supercapacitor have significantly different capacity behavior. Fig. 4 dis-plays the above dependencies at a constant current load of 60 mA g-1. It is worth noting that the symmetric cell represents a typical electric double-layer supercapaci-tor with stable discharge capacity behavior at prolong cycling. The AС/bio-Fe2O3 supercapacitor demonstrates higher efficiency and a much higher stability of its capac-ity during prolong cycling, when compared to the AC/bio-Fe3O4 supercapacitor.

The EIS investigations provide additional informa-tion about the electrolyte resistance (Rel), charge-transfer resistance (Rct), diffusion resistance, etc. The electro-chemical impedance spectra (EIS) of the supercapacitors under investigation are shown in Fig. 5. The Nyquist plot of AC/AC cell is typical for supercapasitor with well-defined three frequency regions, clearly shown in zoomed plots (Fig. 5b). In high frequency range (1) the small depressed semicircle is connected to internal resist-ance of the electrode (Ri). The beginning of the semicir-cle line (left-intercept of Z” at the Z’ axis) represents the

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Fig. 5. Nyquist plot of the EIS spectra of symmetric and hybrid supercapacitors: (a) EIS spectra in full frequency range (1mHz - 1MHz); (b) zoomed EIS spectra and equivalent scheme. The symbols denote experimental data, while the lines - fitted spectra.

electrolyte resistance (Rel). In middle frequency range (2) the line with slope ≤ 45o represents diffusion (W).The fast growing nearly vertical line at low frequency region (3) is due to the capacitive behavior (C).

Such frequency ac respond could be described with the equivalent circuit, presented by inset in Fig. 5b [19, 20] and used for fitting of the impedance spectra. Indeed the high frequency part consists of two overlapped semi-circles and hence Ri is the sum of the two component Rp and Rct. Rp is connected with increased immobility of the electrolyte ions in the electrode pores [19], and Rct is resistivity of the charge transfer through double layer with capacitance Cdl on the electrolyte/electrode boundary. The leakage resistance (RL) in parallel to the capacitance (C) should be added at an inclination of the capacitive line, due to the leakage current [21].

The capacitance (C1mHz) at 1 mHz was calculated from the EIS data in Fig. 5a using relationship C = -/(2 π f Z”)-1 and presented in Table 1 together with data from fitting procedure and specific capacities (Csp = C/m, where m is average mass of the electrodes). The analysis of the impedance spectra and calculated parameters shows the relationships Ri(AC/AC) < Ri(AC/bio-hematite) < Ri(AC/bio-magnetite)

and C(AC/bio-hematite) > C(AC/AC) > C(AC/bio-magnetite). The highest specific capacity possesses (AC/bio-hematite), but the curvature of the low frequency line to the real axis on impedance of (AC/bio hematite) means the lower leak-age resistance. The diffusion behavior of the hybrid supercapacitors occurs at a lower frequencies and this means higher diffusion resistance compared to the sym-metric (AC/AC) supercapacitor.

The supercapacitor cells were subjected to cyclic voltammetry measurements. All measurements were carried out with a gradual increase in the speed of the applied potential (0.001, 0.010, 0.025 and 0.050 V s-1). The starting potential at all measurements was the open circuit potential (OCP) of the investigated samples and the scan was performed at potentials up to ОСР + 2.40 V. The results obtained in all speeds of the applied poten-tial showed very good reproducibility. Furthermore, it is worth noting that the hybrid supercapacitor (+)(AC)/(LiBF4-EC/DMC)/(AC+bio-Fe3O4)(–) has a signifi-cantly higher internal resistance compared with (+)(AC)/(LiBF4-EC/ DMC)/(AC+bio-Fe2O3)(–). These results are consistent with data obtained by the еlectrochemical impedance spectroscopy measurements (cf. Table 1).

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CONCLUSIONS

On the basis of the results in the present study, the following conclusions can be made:

• The nanosized hematite (bio-Fe2O3) and magnetite (bio-Fe3O4) could be employed as electrode materials for the assembly of hybrid lithium battery-supercapacitors.

• The hybrid supercapacitor based on activated carbon and bio-Fe2O3 demonstrates high efficiency and specific capacity as well as good reproducibility of the discharge processes. Furthermore it exhibits a very stable cycleability at prolonged cycling.

• The analysis of the impedance spectra and calcu-lated electrochemical parameters confirm the results of charge/discharge cycling tests and show the higher diffusion resistance of the hybrid lithium battery-super-capacitors compared to the symmetric supercapacitor with activated carbon as electrode material.

Further structural and electrochemical investigations are needed in order to confirm that the presence of bio-iron oxides in the electrode materials leads to formation of structures with regularly distributed pores (in nano-tubular form), suitable for supercapacitor applications.

AcknowledgementsThe authors are indebted to the BNSF project

DFNI Е02/18-2014 for the financial support of this investigation.

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Cell

Rel

Rp

Rct

Ri

CPEdl Wcpe C

Csp

RL

C1mHZ

Csp1mHZ T P T P

[Ω] [Ω] [Ω] [Ω] [mFsp-1] [Ω-1 s-p] F Fg-1 [Ω] F Fg-1

AC/AC

5.8 8.6 8.8 17.4 0.107 0.65 0.079 0.30 0.75 40 2143 0.68 36

AC/bio-hematite

10.7 3.4 29.3 32.7 0.084 0.63 0.049 0.32 1.44 45 206 1.19 37

AC/bio- magnetite

4.8 10.9 37.2 48.1 3.586 0.51 0.025 0.47 0.57 37 1648 0.30 19

Table 1. Calculated parameters from fitting procedure and EIS spectra.

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