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Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization
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 6 ) 1 e9
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Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization Tahereh Gholami, Masoud Salavati-Niasari*, Shokufeh Varshoy Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box. 87317e 51167, Islamic Republic of Iran
article info
abstract
Article history:
In this work, CoAl2O4 pigments successfully were prepared via thermal decomposition
Received 18 January 2016
method by employing green tea extract as precipitating agent and capping agent. The ef-
Accepted 23 March 2016
fects of preparation parameters such as: calcination temperature, surfactants, solvent and
Available online xxx
pH were investigated to reach the optimum conditions. The biosynthesized cobalt alumina spinel was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM),
Keywords:
Fourier transform infrared spectroscopy (FT-IR), X-ray energy dispersive spectroscopy
Hydrogen storage
(EDS), ultravioletevisible (UVeVis) spectroscopy and vibrating sample magnetometer
Thermal decomposition
(VSM). The photocatalytic activity of CoAl2O4 nanostructures was compared by degradation
Nanostructures
of methylene blue in aqueous solution under UV light irradiation. Also the hydrogen
Green tea extract
storage capacity of CoAl2O4 was measured by chronopotentiometry method. The capacity
Capping agent
of CoAl2O4 in 1 mA current and 20 cycles obtained 1100 mh/g.
Chronopotentiometry
Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Spinel type oxides M'M2O4, where M0 and M stand for two different cations of comparable ionic sizes, are a class of thermally and chemically stable materials [1], which are suitable for a wide range of applications, such as catalysis, ceramics, magnetic materials [2,3] and humidity sensor [4]. In the spinel structure, the oxygen ions form cubic close packed structure, and the M0 and M cations occupying two different crystallographic sites tetrahedral (Td) and octahedral (Oh), respectively [5]. The distribution of M0 and M cations play an active part in physical properties of spinel type oxides adopting [6]. Among aluminate spinel materials, cobalt
aluminate (CoAl2O4) is a double oxide with a normal spinel type structure, in which Al3þ ions are in octahedral positions while Co2þ ions are accommodated in tetrahedral positions [7]. Cobalt aluminate (CoAl2O4) spinel, known as Thenard's blue, is widely used as catalyst, pigment layer on luminescent materials and color filter for automotive lamps [8]. Also, because of its chemical and thermal stability pigment of intense blue color and peculiar optical properties, Thenard's blue is used for the coloration of plastics, fibers, rubber, glass, glazes, ceramic bodies, paint and porcelain [9]. Several techniques such as combustion [10], hydrothermal [5], Pechini [11], precipitate, solegel [2,12], and oil-in-water [13] have been used for preparation of cobalt aluminate oxide. Among various techniques developed for the synthesis of cobalt aluminate
* Corresponding author. Tel.: þ98 31 55912383; fax: þ98 31 55913201. E-mail address: [email protected] (M. Salavati-Niasari). http://dx.doi.org/10.1016/j.ijhydene.2016.03.144 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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(CoAl2O4) spinel, thermal decomposition is a simple method to produce stable mono dispersed. In addition, synthesis cobalt aluminate (CoAl2O4) nanostructures utilizing chemical approaches could still lead to the presence of some hazardous chemical species being adsorbed on the surface of nanostructures. Green chemistry is the design and development of chemical products and processes to reduce or eliminate the use and generation of substances toxic chemical species to human health and the environment [14]. In this situation, synthesis of nanostructures utilizing green tea extract could be beneficial, in which bio templates can perform as both precipitating agent and capping agents during the reaction and resulted in nanostructures that are more biocompatible. Hence, this paper reports, preparation of CoAl2O4 pigment by thermal decomposition in the presence of green tea extract and investigation of hydrogen storage capacity and photocatalytic properties for the first time. Because of its high efficiency and nonpolluting nature, hydrogen is as ideal and the environmental friendly fuel for many energy converters [15]. one of the most environmentally sound methods for the production of electrical energy use of hydrogen for fuel cell applications [16] due high efficiency, easy synthesis and potential for implementation in a carbonfree emission cycle [17]. spinel compounds can be new compounds for hydrogen storage due to the porous.
Experimental Materials and physical measurements All chemical reagents in our experiments were used in analytical grade without further purification. X-ray diffraction (XRD) patterns was recorded by a Rigaku D-max C III, X-ray diffract meter using Ni-filtered Cu Ka radiation. Elemental analyses were obtained from Carlo ERBA Model EA 1108 analyzer. Scanning electron microscope (SEM) images were obtained on Philips XL-30ESEM equipped with an energy dispersive X-ray spectroscopy. Fourier transform infrared (FTIR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. The EDS analysis with 20 kV accelerated voltage was done. UVevis spectroscopy (diffuse reflectance) of the obtained was performed with a Shimadzu UV/3101 PC in a range between 200 and 700 nm. Magnetic 59 properties were measured using a vibrating sample magnetometer 60 (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran).
Synthesis of pigment CoAl2O4 nanocrystals All the chemical reagents for the synthesis of Cobalt aluminate nanostructures such as Co (NO3)2.6H2O, Al (NO3)3.9H2O, methanol, cetyl tri methyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), poly vinyl pyrrolidone (PVP) with molecular weight of 25,000, poly ethylene glycol with molecular weight of 6000 (PEG), propylene glycol (PEG) and ethylene glycol (EG) were commercially available and employed without further purification. A mixed solution of metal nitrates was prepared by dissolving Co (NO3)2.6H2O, and Al (NO3)3.9H2O in water with the molar ratio of Co2þ/Al3þ ¼ 1:2. Separately 20 ml green tea extract was added. The previous
reaction mixture subsequent heating at 80 C with continuous stirring for 1 h on a hot plate let the ions react. The solution was evaporated on a hot plate above 120 C for dehydration and continued until powder was obtained and then was calcinated at temperature of 800 C for 5 h. Fig. 11 shows schematic flow chart for the synthesis of CoAl2O4 nanocrystalline spinel powder.
Photocatalytic experiment The photocatalytic activities of CoAl2O4 nanocrystal were determined by the degradation of aqueous methylene blue (MB) under UV light. About 2 mg of the sample was first inserted to a reactor that included 20 ppm of aqueous MB. The suspension was transferred into a self-designed glass reactor, and stirred in darkness to attain the adsorption equilibrium. In the research of photo degradation by UV light, a 125 W high-pressure mercury lamp with a water cooling cylindrical jacket was utilized. The concentration of MB was checked on the basis of its UVevisible absorption peak at 664 nm. The photocatalytic activity of CoAl2O4 nanostructure was tested by using methylene blue (MB) solution. The degradation reaction was carried out in a quartz photocatalytic reactor. The photocatalytic degradation was carried out with 0.02 g of MB solution containing 0.02 g of nanostructures. This mixture was aerated for 30 min to reach adsorption equilibrium. Then, the mixture was placed inside the photoreactor in which the vessel was 40 cm away from the UV. The quartz vessel and light sources were placed inside a black box equipped with a fan to prevent UV leakage. Aliquots of the mixture were taken at periodic intervals during the irradiation, and after centrifugation they were analyzed with the UVeVis spectrometer.
Electrochemical system For to measurement the hydrogen storage capacity of the electrodes was applied chronopotentiometry technique. The charge and discharge cycles was carried out in a threeelectrode system. The NieCoAl2O4, Pt and Ag/AgCl electrodes are as the working, counter and reference electrodes, respectively. The electrolyte solution was 6 M KOH dissolved in double distilled water. In this system, a constant current was applied between the working and counter electrodes, and was measured the potential difference between the working and the reference electrodes To prepare an electrode of CoAl2O4, a nickel foam of nano size porosities was used as a substrate for the CoAl2O4 (Fig. 13). The CoAl2O4 (homogeneously dispersed) was prepared in ethanol for 20 min. A pure nickel plate (1 2 cm2) is coated by a thin layer of CoAl2O4 powder at 100 C without using any of the glue or a binder. In the charging process, the electrolyte dissociates around the working electrode and hydrogen in the solution migrates to the cathode and is absorbed by the CoAl2O4. The opposite direction represents the discharging reaction; the hydrogen atom comes out of the cathode under alkaline circumstance.
Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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Fig. 2 e IR spectra of CoAl2O4 nanocrystals.
from 400 to 600, 800 and 900 C, leads to disappear Al (OH)3 peak and the appearance of pure CoAl2O4 phase. Pure CoAl2O4 pigment (2q ¼ 37.0496 ) was obtained at temperatures more than 800 C.The crystallite diameter (D) of the CoAl2O4 was calculated using the Debye-Scherrer formula: Fig. 1 e XRD patterns of CoAl2O4 powders (a) S4, (b) S5, (c) S6 and (d) S7.
Results and discussion CoAl2O4 pigment characterization prepared by thermal decomposition in the presence of green tea extract X-ray diffraction patterns Fig. 1 shows XRD patterns of the CoAl2O4 in the presence of green tea extract and calculated at 400e900 C for 5 h Fig. 1aed shows XRD patterns 0f samples S1eS4, respectively. When the sample S1 calculated at 400 C, the Al (OH) 3 diffraction peak at 2q ¼ 28.52 and 40.67 was appeared. Temperature increase
D¼
kl bcosq
K is the so-called shape factor, which usually takes a value of about 0.9. l b and q are, the wavelength of incident X-rays (1.5406 A for Cu Ka), the X-ray full width at half maximum height (FWHM) of the diffraction peak and the diffraction angle, respectively. The intensity of the diffraction peaks increases by increasing annealing temperature. Regarding to XRD data, the estimated crystallite size in the temperature range 400e900 C are shown in Table 1.
FT-IR spectroscopy and energy dispersive X-ray (EDS) spectrometry Fig. 2 a,b show IR spectra of sample S6 and S7. FT-IR spectra show the peaks corresponding to stretching vibration of OH by
Table 1 e Preparation conditions for the synthesis of CoAl2O4 nanostructures. Sample abbreviation S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
Surfactant
Annealing temperature ( n)
pH
Solution
Crystalline size (XRD)/nm
CTAB SDS PEG6000 PVP25000 e e e e e e e e
800 800 800 800 400 600 800 900 800 800 800 800
2.13 2.13 2.13 2.13 2.13 2.13 2.13 2.13 4.32 6.15 2.13 2.13
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O PEG EG
e e e e 23.68 29.58 29.62 23.65 e e e e
Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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absorbed water molecule at 3433 cm1. The absorption around 1645 cm1 is due to the bending vibration of water molecules. The bands around 562 and 676 cm1 are corresponding to CoO4 and AlO6 vibrations, respectively [18]. The bands at 1116 and 1238 cm1 are due to the stretch vibrations of NO3 [19]. Absorption peak at 2926 cm1 show the existence of the Al (OH)3 phase, which is formed as a precipitate [20]. With increasing temperature, all the peaks related to the organic species disappear and the characteristic absorption peaks of the sample related to metaleoxygen vibrations become much sharper. Fig. 3 shows the energy dispersive spectroscopy (EDS) spectrum of rod-particle CoAl2O4 (sample S7 in Table 1). EDS indicated the presence of cobalt, aluminum and oxygen from the sample S7. EDS pattern of sample confirms the presence of pure CoAl2O4.
Fig. 3 e EDS pattern of CoAl2O4 nanostructures.
Fig. 4 e SEM images of CoAl2O4 powders obtained with different surfactants (a) S1, (b) S2, (c) S3, (d) S4 and (e) S7.
Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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Fig. 5 e SEM images of CoAl2O4 powders of samples: (a) S5, (b) S6 and (c) S8.
Fig. 6 e SEM images CoAl2O4 nanostructures of samples: (a) S9 and (b) S10.
Fig. 7 e SEM images CoAl2O4 nanostructures of samples: (a) S11 and (b) S12. Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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Fig. 10 e Decolorization of MB dye of the CoAl2O4 sample S7 .
Fig. 8 e (a) UVeVis absorption spectrum of the CoAl2O4 sample S7, (b) Optical band gap of CoAl2O4 sample S7.
SEM image
it can be observed that the morphology of CoAl2O4 nanostructures without the addition of surfactant is so uniform and also they are separated from each other. The particle size is between 20 and 100 nm. Therefore, it can be concluded that the green tea extract act as both precipitating agent and capping agents. In Fig. 4e, it is observed; in addition to particle the product contains some rods. The effect of annealing temperature on morphologies of the obtained CoAl2O4 products is shown in Fig. 5 aec and Fig. 4 e. According to X-ray, CoAl2O4 nanocrystal unconstituted at
The effect of surfactant on morphologies of the obtained CoAl2O4 products is shown in Fig. 4. Fig. 4aee shows CoAl2O4 samples S1, S2, S3, S4 and S7 respectively. It is observed that in presence surfactants such as CTAB, SDS, PVP and PEG, the CoAl2O4 powder smaller particles were produced, but agglomeration was increased and uniformity was disappeared. With decreasing of the grain size due to increasing grain surface charge, agglomeration increased [21]. However,
Magnetization (emu/g)
2
1
0
-1
-2 -10000
-5000
0
Applid Field(Oe)
5000
10000
Fig. 9 e Magnetization versus applied magnetic field at room temperature for the pure CoAl2O4 (sample S7).
Fig. 11 e The schematic flow chart for the synthesis of CoAl2O4 nanocrystalline powder.
Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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400 and 600 C temperature and on the other hand, not suitable morphology. Optimum calcination temperature for synthesis of suitable and uniform CoAl2O4 nanostructure is 800 C By increasing the calcination temperature from 800 to 900 C agglomeration of the nanoparticles increased and nanostructure rods was disappeared. Fig. 6.shows SEM images of CoAl2O4 nanoparticles under different pH. Ammonia was used for increasing in pH. Optimum pH for synthesis of suitable and uniform CoAl2O4 nanostructure is 2.13 which were adjusted by green tea extract. It can be observed that by increasing pH from 2.13 to 4.43 and 6.15, the particle size decrease but agglomeration of the nanoparticles increase. Fig. 7 is the SEM image of CoAl2O4 nanostructure prepared with propylene glycol (PEG) and ethylene glycol (EG) as solvent. It is observed that changing the morphology of the particles to cubic, by changing the solvent from H2O to PEG and EG. On the other hand, particle size changes from the nano to microstructures.
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Fig. 13 e SEM image of nickel foam with nano scale porosities.
Optical studies To investigate the optical absorption properties of CoAl2O4, the UVeVis study was carried out (Fig. 8). The UVeVis spectrum of the CoAl2O4 (homogeneously dispersed) was prepared in absolute ethanol. There is a strong absorption peaks located at 540, 588 and 640 nm that this was compared with the absorption peak other articles. The maximum absorption was around 650 nm and the minimum absorption was around 500 nm [9]. It is shown that with decrease size, the absorption edge was shifted to the higher-energy region [22]. Thus, UVevis spectrum has a blue shift probably due to decrease size of nanoparticles. Therefore, it can be concluded that the green tea extract act as capping agents. Using the absorption data, the band gap was estimated by Tauc's relationship: a ¼ a0 hg Eg
n hg
Where a, hg, Eg, and a0, h are the absorption coefficient, the photon energy, the optical band gap of the material and the constants, respectively. The n depends on of the electronic transition and can be any value between ½ and 3 [23]. The CoAl2O4 nanostructures has a direct band gap and the value n is 2 [24].
Fig. 12 e The schematic reaction mechanism of photocatalytic degradation of MB dye over CoAl2O4 (S7) nanocrystalline under UV light irradiation.
The energy gap of the sample S7 have been determined by extrapolating the linear portion of the plot of (ahg)2 against hg to the energy axis (Fig. 8b). The value is estimated 2.38 eV for S7 sample.
VSM studies The magnetic property of CoAl2O4 nanocrystsls (sample S7) has been measured (Fig. 9). The hysteresis loop at room temperature shows that CoAl2O4 nanocrystals are ferromagnetic. Maximum saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) of sample S7 also was obtained from VSM. The saturation magnetization (Ms) is about 1.87 emu/g which is higher than spinel CoAl2O4 microstructures (Ms ¼ 43.26 103 emu/g) [8]. The coercivity field (Hc) and remanent magnetization (Mr) of CoAl2O4 sample S7 are about 668.5Oe, 0.534emu/g, respectively. Also, remanence Mr ) was calculated about 0.285. ration (M s
Photocatalytic activity of CoAl2O4 nanocrystals The photocatalytic activity of the CoAl2O5 nanocrystals was estimated by monitoring the degradation of methylene blue (MB) as water pollutant in an aqueous solution, under UV light
Fig. 14 e Diagram of nickel foam discharge capacity in absence of CoAl2O4 (S7).
Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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irradiation (Fig. 10). The decolorization efficiency (%) was calculated as decolorization: A0 A 100 A0
Where A0 is initial absorbance of methylene blue solution and A is absorbance of methylene blue solution after degradation [25]. According to photocatalytic calculations, 37% MB degraded after 80 min for CoAl2O4 nanocrystals. Fig. 12 shows schematic of photocatalytic degradation of MB dye over CoAl2O4 nanocrystals under UV light irradiation. The possible mechanism of the photocatalytic degradation of MB dye can be described as follows: CoAl2 O4 þ hg/CoAl2 O*4 $ hþ VB þ eCB
I = 1mA 1000
Discharge Capacity (mAh/g)
D% ¼
1200
800
600
400
200
0
0
e þ
O2 /Oo 2
hþ þ H2 O/OHo
5
10
15
20
Number of Cycle Fig. 16 e Cycling performance of the NieCoAl2O4 (S7) at a current density of 1 mA.
o Oo 2 þ OH þ MB/degradation products
In this paper, for the first time photocatalytic activity of CoAl2O4 nanocrystals is reporter.
Hydrogen storage capacity As shown in Fig. 14, the amount of discharge capacity of nickel foam empty without CoAl2O4 is less than 0.1 mAh/g. This curve shows that high capacity of Nie CoAl2O4 electrode is due to the presence CoAl2O4 in the alkaline medium. Fig. 15 shows the discharge behavior of the Nie CoAl2O4 electrode over the 20 cycles, which mainly composed of two conjugated plateaus at approximately 0.1 V during discharging. It is observed that the discharge profile of the Nie CoAl2O4 electrode in the third cycle looks different from that in the first cycle, but the differences in the shape and potentials between the third cycle and the following ones (third to twentieth) were relatively small. The galvanostatic charge/discharge cycling performances of Nie CoAl2O4 electrodes on the amount of storage
Fig. 15 e Discharge curves of NieCoAl2O4 (S7) electrode at a current density of 1 mA.
capacity at 1 mA are shown in Fig. 16. The results showed that discharge capacity increased by repeating the cycles, and after 20 cycles, it reached a constant and stabilized amount. The initial discharge capacity of Nie CoAl2O4 electrode is around 460 mA h/g. The discharge capacity increased to 900 mA h/g in the third cycle and 1100 mA h/g over 20 cycles (corresponds to z 3.735wt% hydrogen). The main reason of this increasing discharge capacity can be related to the existence of different hydrogen adsorption sites in the working electrode. The results of galvanostatic discharge experiments show that the addition the storage capacity of the CoAl2O4 electrode depends greatly on the amount of current on the cycles. Fig. 17 shows a comparison between the discharge capacities for currents of 1 and 2 mA during the first cycle in NieCoAl2O4 electrode.
Fig. 17 e Comparison between discharge capacities for currents of 1 and 2 mA during the first cycles (S7).
Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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Conclusions In summary, CoAl2O4 pigments were prepared via simple thermal decomposition using green tea extract. The effects of preparation parameters such as: calcination temperature, surfactants, solvent and pH were investigated to reach the optimum conditions. It was shown that the green tea extract can be used as capping, precipitating agent, and for adjusted pH. In addition, the optical, magnetic properties, hydrogen storage capacity and photocatalytic activity are studied.
Acknowledgments The authors are grateful to University of Kashan for supporting this work by Grant No. (159271/375).
references
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Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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ScienceDirect journal homepage: www.elsevier.com/locate/he
Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization Tahereh Gholami, Masoud Salavati-Niasari*, Shokufeh Varshoy Institute of Nano Science and Nano Technology, University of Kashan, Kashan, P. O. Box. 87317e 51167, Islamic Republic of Iran
article info
abstract
Article history:
In this work, CoAl2O4 pigments successfully were prepared via thermal decomposition
Received 18 January 2016
method by employing green tea extract as precipitating agent and capping agent. The ef-
Accepted 23 March 2016
fects of preparation parameters such as: calcination temperature, surfactants, solvent and
Available online xxx
pH were investigated to reach the optimum conditions. The biosynthesized cobalt alumina spinel was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM),
Keywords:
Fourier transform infrared spectroscopy (FT-IR), X-ray energy dispersive spectroscopy
Hydrogen storage
(EDS), ultravioletevisible (UVeVis) spectroscopy and vibrating sample magnetometer
Thermal decomposition
(VSM). The photocatalytic activity of CoAl2O4 nanostructures was compared by degradation
Nanostructures
of methylene blue in aqueous solution under UV light irradiation. Also the hydrogen
Green tea extract
storage capacity of CoAl2O4 was measured by chronopotentiometry method. The capacity
Capping agent
of CoAl2O4 in 1 mA current and 20 cycles obtained 1100 mh/g.
Chronopotentiometry
Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Spinel type oxides M'M2O4, where M0 and M stand for two different cations of comparable ionic sizes, are a class of thermally and chemically stable materials [1], which are suitable for a wide range of applications, such as catalysis, ceramics, magnetic materials [2,3] and humidity sensor [4]. In the spinel structure, the oxygen ions form cubic close packed structure, and the M0 and M cations occupying two different crystallographic sites tetrahedral (Td) and octahedral (Oh), respectively [5]. The distribution of M0 and M cations play an active part in physical properties of spinel type oxides adopting [6]. Among aluminate spinel materials, cobalt
aluminate (CoAl2O4) is a double oxide with a normal spinel type structure, in which Al3þ ions are in octahedral positions while Co2þ ions are accommodated in tetrahedral positions [7]. Cobalt aluminate (CoAl2O4) spinel, known as Thenard's blue, is widely used as catalyst, pigment layer on luminescent materials and color filter for automotive lamps [8]. Also, because of its chemical and thermal stability pigment of intense blue color and peculiar optical properties, Thenard's blue is used for the coloration of plastics, fibers, rubber, glass, glazes, ceramic bodies, paint and porcelain [9]. Several techniques such as combustion [10], hydrothermal [5], Pechini [11], precipitate, solegel [2,12], and oil-in-water [13] have been used for preparation of cobalt aluminate oxide. Among various techniques developed for the synthesis of cobalt aluminate
* Corresponding author. Tel.: þ98 31 55912383; fax: þ98 31 55913201. E-mail address: [email protected] (M. Salavati-Niasari). http://dx.doi.org/10.1016/j.ijhydene.2016.03.144 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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(CoAl2O4) spinel, thermal decomposition is a simple method to produce stable mono dispersed. In addition, synthesis cobalt aluminate (CoAl2O4) nanostructures utilizing chemical approaches could still lead to the presence of some hazardous chemical species being adsorbed on the surface of nanostructures. Green chemistry is the design and development of chemical products and processes to reduce or eliminate the use and generation of substances toxic chemical species to human health and the environment [14]. In this situation, synthesis of nanostructures utilizing green tea extract could be beneficial, in which bio templates can perform as both precipitating agent and capping agents during the reaction and resulted in nanostructures that are more biocompatible. Hence, this paper reports, preparation of CoAl2O4 pigment by thermal decomposition in the presence of green tea extract and investigation of hydrogen storage capacity and photocatalytic properties for the first time. Because of its high efficiency and nonpolluting nature, hydrogen is as ideal and the environmental friendly fuel for many energy converters [15]. one of the most environmentally sound methods for the production of electrical energy use of hydrogen for fuel cell applications [16] due high efficiency, easy synthesis and potential for implementation in a carbonfree emission cycle [17]. spinel compounds can be new compounds for hydrogen storage due to the porous.
Experimental Materials and physical measurements All chemical reagents in our experiments were used in analytical grade without further purification. X-ray diffraction (XRD) patterns was recorded by a Rigaku D-max C III, X-ray diffract meter using Ni-filtered Cu Ka radiation. Elemental analyses were obtained from Carlo ERBA Model EA 1108 analyzer. Scanning electron microscope (SEM) images were obtained on Philips XL-30ESEM equipped with an energy dispersive X-ray spectroscopy. Fourier transform infrared (FTIR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. The EDS analysis with 20 kV accelerated voltage was done. UVevis spectroscopy (diffuse reflectance) of the obtained was performed with a Shimadzu UV/3101 PC in a range between 200 and 700 nm. Magnetic 59 properties were measured using a vibrating sample magnetometer 60 (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran).
Synthesis of pigment CoAl2O4 nanocrystals All the chemical reagents for the synthesis of Cobalt aluminate nanostructures such as Co (NO3)2.6H2O, Al (NO3)3.9H2O, methanol, cetyl tri methyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS), poly vinyl pyrrolidone (PVP) with molecular weight of 25,000, poly ethylene glycol with molecular weight of 6000 (PEG), propylene glycol (PEG) and ethylene glycol (EG) were commercially available and employed without further purification. A mixed solution of metal nitrates was prepared by dissolving Co (NO3)2.6H2O, and Al (NO3)3.9H2O in water with the molar ratio of Co2þ/Al3þ ¼ 1:2. Separately 20 ml green tea extract was added. The previous
reaction mixture subsequent heating at 80 C with continuous stirring for 1 h on a hot plate let the ions react. The solution was evaporated on a hot plate above 120 C for dehydration and continued until powder was obtained and then was calcinated at temperature of 800 C for 5 h. Fig. 11 shows schematic flow chart for the synthesis of CoAl2O4 nanocrystalline spinel powder.
Photocatalytic experiment The photocatalytic activities of CoAl2O4 nanocrystal were determined by the degradation of aqueous methylene blue (MB) under UV light. About 2 mg of the sample was first inserted to a reactor that included 20 ppm of aqueous MB. The suspension was transferred into a self-designed glass reactor, and stirred in darkness to attain the adsorption equilibrium. In the research of photo degradation by UV light, a 125 W high-pressure mercury lamp with a water cooling cylindrical jacket was utilized. The concentration of MB was checked on the basis of its UVevisible absorption peak at 664 nm. The photocatalytic activity of CoAl2O4 nanostructure was tested by using methylene blue (MB) solution. The degradation reaction was carried out in a quartz photocatalytic reactor. The photocatalytic degradation was carried out with 0.02 g of MB solution containing 0.02 g of nanostructures. This mixture was aerated for 30 min to reach adsorption equilibrium. Then, the mixture was placed inside the photoreactor in which the vessel was 40 cm away from the UV. The quartz vessel and light sources were placed inside a black box equipped with a fan to prevent UV leakage. Aliquots of the mixture were taken at periodic intervals during the irradiation, and after centrifugation they were analyzed with the UVeVis spectrometer.
Electrochemical system For to measurement the hydrogen storage capacity of the electrodes was applied chronopotentiometry technique. The charge and discharge cycles was carried out in a threeelectrode system. The NieCoAl2O4, Pt and Ag/AgCl electrodes are as the working, counter and reference electrodes, respectively. The electrolyte solution was 6 M KOH dissolved in double distilled water. In this system, a constant current was applied between the working and counter electrodes, and was measured the potential difference between the working and the reference electrodes To prepare an electrode of CoAl2O4, a nickel foam of nano size porosities was used as a substrate for the CoAl2O4 (Fig. 13). The CoAl2O4 (homogeneously dispersed) was prepared in ethanol for 20 min. A pure nickel plate (1 2 cm2) is coated by a thin layer of CoAl2O4 powder at 100 C without using any of the glue or a binder. In the charging process, the electrolyte dissociates around the working electrode and hydrogen in the solution migrates to the cathode and is absorbed by the CoAl2O4. The opposite direction represents the discharging reaction; the hydrogen atom comes out of the cathode under alkaline circumstance.
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Fig. 2 e IR spectra of CoAl2O4 nanocrystals.
from 400 to 600, 800 and 900 C, leads to disappear Al (OH)3 peak and the appearance of pure CoAl2O4 phase. Pure CoAl2O4 pigment (2q ¼ 37.0496 ) was obtained at temperatures more than 800 C.The crystallite diameter (D) of the CoAl2O4 was calculated using the Debye-Scherrer formula: Fig. 1 e XRD patterns of CoAl2O4 powders (a) S4, (b) S5, (c) S6 and (d) S7.
Results and discussion CoAl2O4 pigment characterization prepared by thermal decomposition in the presence of green tea extract X-ray diffraction patterns Fig. 1 shows XRD patterns of the CoAl2O4 in the presence of green tea extract and calculated at 400e900 C for 5 h Fig. 1aed shows XRD patterns 0f samples S1eS4, respectively. When the sample S1 calculated at 400 C, the Al (OH) 3 diffraction peak at 2q ¼ 28.52 and 40.67 was appeared. Temperature increase
D¼
kl bcosq
K is the so-called shape factor, which usually takes a value of about 0.9. l b and q are, the wavelength of incident X-rays (1.5406 A for Cu Ka), the X-ray full width at half maximum height (FWHM) of the diffraction peak and the diffraction angle, respectively. The intensity of the diffraction peaks increases by increasing annealing temperature. Regarding to XRD data, the estimated crystallite size in the temperature range 400e900 C are shown in Table 1.
FT-IR spectroscopy and energy dispersive X-ray (EDS) spectrometry Fig. 2 a,b show IR spectra of sample S6 and S7. FT-IR spectra show the peaks corresponding to stretching vibration of OH by
Table 1 e Preparation conditions for the synthesis of CoAl2O4 nanostructures. Sample abbreviation S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12
Surfactant
Annealing temperature ( n)
pH
Solution
Crystalline size (XRD)/nm
CTAB SDS PEG6000 PVP25000 e e e e e e e e
800 800 800 800 400 600 800 900 800 800 800 800
2.13 2.13 2.13 2.13 2.13 2.13 2.13 2.13 4.32 6.15 2.13 2.13
H2O H2O H2O H2O H2O H2O H2O H2O H2O H2O PEG EG
e e e e 23.68 29.58 29.62 23.65 e e e e
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absorbed water molecule at 3433 cm1. The absorption around 1645 cm1 is due to the bending vibration of water molecules. The bands around 562 and 676 cm1 are corresponding to CoO4 and AlO6 vibrations, respectively [18]. The bands at 1116 and 1238 cm1 are due to the stretch vibrations of NO3 [19]. Absorption peak at 2926 cm1 show the existence of the Al (OH)3 phase, which is formed as a precipitate [20]. With increasing temperature, all the peaks related to the organic species disappear and the characteristic absorption peaks of the sample related to metaleoxygen vibrations become much sharper. Fig. 3 shows the energy dispersive spectroscopy (EDS) spectrum of rod-particle CoAl2O4 (sample S7 in Table 1). EDS indicated the presence of cobalt, aluminum and oxygen from the sample S7. EDS pattern of sample confirms the presence of pure CoAl2O4.
Fig. 3 e EDS pattern of CoAl2O4 nanostructures.
Fig. 4 e SEM images of CoAl2O4 powders obtained with different surfactants (a) S1, (b) S2, (c) S3, (d) S4 and (e) S7.
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Fig. 5 e SEM images of CoAl2O4 powders of samples: (a) S5, (b) S6 and (c) S8.
Fig. 6 e SEM images CoAl2O4 nanostructures of samples: (a) S9 and (b) S10.
Fig. 7 e SEM images CoAl2O4 nanostructures of samples: (a) S11 and (b) S12. Please cite this article in press as: Gholami T, et al., Investigation of the electrochemical hydrogen storage and photocatalytic properties of CoAl2O4 pigment: Green synthesis and characterization, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/ j.ijhydene.2016.03.144
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Fig. 10 e Decolorization of MB dye of the CoAl2O4 sample S7 .
Fig. 8 e (a) UVeVis absorption spectrum of the CoAl2O4 sample S7, (b) Optical band gap of CoAl2O4 sample S7.
SEM image
it can be observed that the morphology of CoAl2O4 nanostructures without the addition of surfactant is so uniform and also they are separated from each other. The particle size is between 20 and 100 nm. Therefore, it can be concluded that the green tea extract act as both precipitating agent and capping agents. In Fig. 4e, it is observed; in addition to particle the product contains some rods. The effect of annealing temperature on morphologies of the obtained CoAl2O4 products is shown in Fig. 5 aec and Fig. 4 e. According to X-ray, CoAl2O4 nanocrystal unconstituted at
The effect of surfactant on morphologies of the obtained CoAl2O4 products is shown in Fig. 4. Fig. 4aee shows CoAl2O4 samples S1, S2, S3, S4 and S7 respectively. It is observed that in presence surfactants such as CTAB, SDS, PVP and PEG, the CoAl2O4 powder smaller particles were produced, but agglomeration was increased and uniformity was disappeared. With decreasing of the grain size due to increasing grain surface charge, agglomeration increased [21]. However,
Magnetization (emu/g)
2
1
0
-1
-2 -10000
-5000
0
Applid Field(Oe)
5000
10000
Fig. 9 e Magnetization versus applied magnetic field at room temperature for the pure CoAl2O4 (sample S7).
Fig. 11 e The schematic flow chart for the synthesis of CoAl2O4 nanocrystalline powder.
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400 and 600 C temperature and on the other hand, not suitable morphology. Optimum calcination temperature for synthesis of suitable and uniform CoAl2O4 nanostructure is 800 C By increasing the calcination temperature from 800 to 900 C agglomeration of the nanoparticles increased and nanostructure rods was disappeared. Fig. 6.shows SEM images of CoAl2O4 nanoparticles under different pH. Ammonia was used for increasing in pH. Optimum pH for synthesis of suitable and uniform CoAl2O4 nanostructure is 2.13 which were adjusted by green tea extract. It can be observed that by increasing pH from 2.13 to 4.43 and 6.15, the particle size decrease but agglomeration of the nanoparticles increase. Fig. 7 is the SEM image of CoAl2O4 nanostructure prepared with propylene glycol (PEG) and ethylene glycol (EG) as solvent. It is observed that changing the morphology of the particles to cubic, by changing the solvent from H2O to PEG and EG. On the other hand, particle size changes from the nano to microstructures.
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Fig. 13 e SEM image of nickel foam with nano scale porosities.
Optical studies To investigate the optical absorption properties of CoAl2O4, the UVeVis study was carried out (Fig. 8). The UVeVis spectrum of the CoAl2O4 (homogeneously dispersed) was prepared in absolute ethanol. There is a strong absorption peaks located at 540, 588 and 640 nm that this was compared with the absorption peak other articles. The maximum absorption was around 650 nm and the minimum absorption was around 500 nm [9]. It is shown that with decrease size, the absorption edge was shifted to the higher-energy region [22]. Thus, UVevis spectrum has a blue shift probably due to decrease size of nanoparticles. Therefore, it can be concluded that the green tea extract act as capping agents. Using the absorption data, the band gap was estimated by Tauc's relationship: a ¼ a0 hg Eg
n hg
Where a, hg, Eg, and a0, h are the absorption coefficient, the photon energy, the optical band gap of the material and the constants, respectively. The n depends on of the electronic transition and can be any value between ½ and 3 [23]. The CoAl2O4 nanostructures has a direct band gap and the value n is 2 [24].
Fig. 12 e The schematic reaction mechanism of photocatalytic degradation of MB dye over CoAl2O4 (S7) nanocrystalline under UV light irradiation.
The energy gap of the sample S7 have been determined by extrapolating the linear portion of the plot of (ahg)2 against hg to the energy axis (Fig. 8b). The value is estimated 2.38 eV for S7 sample.
VSM studies The magnetic property of CoAl2O4 nanocrystsls (sample S7) has been measured (Fig. 9). The hysteresis loop at room temperature shows that CoAl2O4 nanocrystals are ferromagnetic. Maximum saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) of sample S7 also was obtained from VSM. The saturation magnetization (Ms) is about 1.87 emu/g which is higher than spinel CoAl2O4 microstructures (Ms ¼ 43.26 103 emu/g) [8]. The coercivity field (Hc) and remanent magnetization (Mr) of CoAl2O4 sample S7 are about 668.5Oe, 0.534emu/g, respectively. Also, remanence Mr ) was calculated about 0.285. ration (M s
Photocatalytic activity of CoAl2O4 nanocrystals The photocatalytic activity of the CoAl2O5 nanocrystals was estimated by monitoring the degradation of methylene blue (MB) as water pollutant in an aqueous solution, under UV light
Fig. 14 e Diagram of nickel foam discharge capacity in absence of CoAl2O4 (S7).
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irradiation (Fig. 10). The decolorization efficiency (%) was calculated as decolorization: A0 A 100 A0
Where A0 is initial absorbance of methylene blue solution and A is absorbance of methylene blue solution after degradation [25]. According to photocatalytic calculations, 37% MB degraded after 80 min for CoAl2O4 nanocrystals. Fig. 12 shows schematic of photocatalytic degradation of MB dye over CoAl2O4 nanocrystals under UV light irradiation. The possible mechanism of the photocatalytic degradation of MB dye can be described as follows: CoAl2 O4 þ hg/CoAl2 O*4 $ hþ VB þ eCB
I = 1mA 1000
Discharge Capacity (mAh/g)
D% ¼
1200
800
600
400
200
0
0
e þ
O2 /Oo 2
hþ þ H2 O/OHo
5
10
15
20
Number of Cycle Fig. 16 e Cycling performance of the NieCoAl2O4 (S7) at a current density of 1 mA.
o Oo 2 þ OH þ MB/degradation products
In this paper, for the first time photocatalytic activity of CoAl2O4 nanocrystals is reporter.
Hydrogen storage capacity As shown in Fig. 14, the amount of discharge capacity of nickel foam empty without CoAl2O4 is less than 0.1 mAh/g. This curve shows that high capacity of Nie CoAl2O4 electrode is due to the presence CoAl2O4 in the alkaline medium. Fig. 15 shows the discharge behavior of the Nie CoAl2O4 electrode over the 20 cycles, which mainly composed of two conjugated plateaus at approximately 0.1 V during discharging. It is observed that the discharge profile of the Nie CoAl2O4 electrode in the third cycle looks different from that in the first cycle, but the differences in the shape and potentials between the third cycle and the following ones (third to twentieth) were relatively small. The galvanostatic charge/discharge cycling performances of Nie CoAl2O4 electrodes on the amount of storage
Fig. 15 e Discharge curves of NieCoAl2O4 (S7) electrode at a current density of 1 mA.
capacity at 1 mA are shown in Fig. 16. The results showed that discharge capacity increased by repeating the cycles, and after 20 cycles, it reached a constant and stabilized amount. The initial discharge capacity of Nie CoAl2O4 electrode is around 460 mA h/g. The discharge capacity increased to 900 mA h/g in the third cycle and 1100 mA h/g over 20 cycles (corresponds to z 3.735wt% hydrogen). The main reason of this increasing discharge capacity can be related to the existence of different hydrogen adsorption sites in the working electrode. The results of galvanostatic discharge experiments show that the addition the storage capacity of the CoAl2O4 electrode depends greatly on the amount of current on the cycles. Fig. 17 shows a comparison between the discharge capacities for currents of 1 and 2 mA during the first cycle in NieCoAl2O4 electrode.
Fig. 17 e Comparison between discharge capacities for currents of 1 and 2 mA during the first cycles (S7).
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Conclusions In summary, CoAl2O4 pigments were prepared via simple thermal decomposition using green tea extract. The effects of preparation parameters such as: calcination temperature, surfactants, solvent and pH were investigated to reach the optimum conditions. It was shown that the green tea extract can be used as capping, precipitating agent, and for adjusted pH. In addition, the optical, magnetic properties, hydrogen storage capacity and photocatalytic activity are studied.
Acknowledgments The authors are grateful to University of Kashan for supporting this work by Grant No. (159271/375).
references
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