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In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance
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In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance M.M. Li, C.C. Yang*, W.T. Jing, B. Jin, X.Y. Lang, Q. Jiang Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, School of Materials Science and Engineering, Jilin University, Changchun 130022, China
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abstract
Article history:
Co3O4 is an effective additive to enhance the electrochemical performance of hydrogen
Received 5 January 2016
storage alloys. However, the low utilization efficiency has become a big challenge for direct
Received in revised form
adding Co3O4 powders into the alloys with mechanical mixing. Here, we report the in situ
27 January 2016
growth of Co3O4 on the alloy surface by using a facile and effective hydrothermal method.
Accepted 27 January 2016
Compared with bare hydrogen storage alloys, the fabricated composite shows larger
Available online xxx
maximum discharge capacity, 326.37 vs. 302.62 mAh g1, and enhanced high rate dischargeability with larger discharge capacity at a current density of 3000 mA g1, 59.01 vs.
Keywords:
40.88 mAh g1. These are contributed by the unique hybrid architecture of the composite:
Hydrogen storage alloy
(1) the in situ grown Co3O4 nanosheets improve the catalytic activity and utilization effi-
Co3O4
ciency of Co3O4 on the electrochemical reaction kinetics; and (2) the low-dimensional
Discharge capacity
Co3O4 coatings seamlessly integrated with hydrogen storage alloys decrease the internal
High rate dischargeability
resistance and polarization of the hybrid electrode. Such a simple and novel method can
Catalytic activity
also be extended to other energy storage devices.
Nickel metal hydride battery
Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The rechargeable nickel metal hydride (NieMH) batteries have been widely used in personal portable electronic devices, battery-powered/assisted vehicles, military equipment etc., owing to their superior safety, temperature adaptability, and environmental friendliness [1e4]. While with the development of advanced electrical energy storage devices (such as supercapacitors, lithium-ion batteries and fuel cell) [5e8] and increasing demands for power sources, it is essential to further improve the performance of NieMH batteries in energy density and power density [9,10]. AB5-type misch-metal based hydrogen storage alloys are the state-of-the-art
negative electrode materials in NieMH batteries [11], which dominate the electrochemical performance of NieMH batteries. Thus, how to enhance the performance of hydrogen storage alloys, such as the discharge capacity and high rate dischargeability (HRD), has attracted great attentions in recent years [9e11]. The study of modification of hydrogen storage alloys has been proceeding for decades and a number of methods have been proposed to improve the electrochemical performance of NieMH batteries. The elemental substitution, which could significantly change the microstructure and properties of alloys, has been proved to be an efficient approach to enhance the overall electrochemical properties of hydrogen storage alloys [12,13]. The annealing treatment and modified
* Corresponding author. Tel.: þ86 431 85095371; fax: þ86 431 85095876. E-mail address: [email protected] (C.C. Yang). http://dx.doi.org/10.1016/j.ijhydene.2016.01.129 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Li MM, et al., In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.129
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preparation methods have also been introduced to enable component homogeneity of alloys and thus to improve their cycle stability [14,15]. The increased surface area and higher conductivity of hydrogen storage alloys by surface treatments, such as electroless, acid, alkaline, or fluorination treatment, mechanical alloying etc., significantly enhance the electrochemical reaction kinetics [16e19]. Moreover, some conductive additives like transition metals, graphite, carbon nanotube and oxides were added into hydrogen storage alloys as current collector and binder, which are beneficial for the improvements of electrode properties [20,21]. A series of transition metal oxides (RuO2, Fe2O3, TiO2, MnO2 and Co3O4) have also been used as catalysts to improve the hydriding/dehydriding kinetics of hydrogen storage alloys [22,23]. Among these oxides, Co3O4 as an important technological material has been widely used in catalytic fields, hydrocracking process of crude fuels, treatment of waste gases, hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), glucose oxidation etc. [24e27]. Recently, it has also been considered as a potential negative electrode material for lithium-ion batteries owing to its high theoretical capacity (890 mAh g1), and as a promising catalyst for the alkaline secondary battery because of its excellent catalytic property [28,29]. Moreover, the performance of negative electrode of NieMH batteries could also be improved by modification the hydrogen storage alloys with Co3O4 [30]. It is known that for the MH electrode, the following processes are considered to take place at the electrode/electrolyte interface and in the alloy bulk during charging: (i) electrochemical reaction at the interface: H2O þ e / Hads þ OH (Volmer reaction); (ii) the transition between adsorbed hydrogen atoms and absorbed hydrogen atoms: Hads / Habs; and (iii) the diffusion of absorbed hydrogen into the bulk of alloy and the following phase transformation: MHa / MHb. Furthermore, there also exists the HER by two different reaction routes: the Volmer-Tafel route (Tafel reaction: 2Hads / H2) or the Volmer-Heyrovsky route (Heyrovsky reaction: Hads þ H2O þ e / H2 þ OH) [31,32]. During charging, the as-formed Hads at the electrode surface by Volmer reaction diffuse into the alloys, where the diffusion rate is proportional to the concentration gradient of H atoms between the electrode surface and the alloy bulk. In order to charge the electrode effectively, it is critical to keep a high concentration of H atoms at the electrode surface. Thus, the Volmer reaction should be accelerated while the Tafel or Heyrovsky reaction should be depressed. The experimental finding indicates that for the Co3O4-modified MH electrode, the rate of Volmer reaction is enhanced while the rate of Tafel or Heyrovsky reaction remains constant, which is beneficial for improvements of the electrode performance [31]. However, the Co3O4 in the electrode is only partially utilized by directly adding Co3O4 powders into hydrogen storage alloys through the mechanical mixing [30,33], leading to a moderate catalytic effect. In this work, Co3O4 nanosheet arrays are in situ grown on the surface of alloy particles via a simple hydrothermal method. In this case, the utilization efficiency of Co3O4 nanosheets could be highly enhanced and the performance of the hybrid electrode is thus noticeably increased. Compared with bare hydrogen storage alloys (or the master alloy), the maximum discharge capacity and the
discharge capacity at a current density of 3000 mA g1 of the composite increase from 302.62 mAh g1 to 326.37 mAh g1, and from 40.88 mAh g1 to 59.01 mAh g1, respectively. This facile and effective fabrication strategy of the composite could be extended to other alloy systems too for the electrochemical performance enhancements.
Experimental The fabrication strategy of the composite by using a simple hydrothermal method [27] is briefly illustrated in Fig. 1a. With this approach, the Co3O4 nanosheets could be in situ grown on the surface of hydrogen storage alloys. Moreover, during the hydrothermal process, the loading amount and morphology of Co3O4 could be tailored by varying the concentration of Co(NO3)2. The electrode pellet used for the electrochemical measurements is shown in Fig. 1b. The experimental details are given below. The (LaCeY)(NiMnCoAl)5 alloy (master alloy) was prepared by induction melting the constituent metals, La, Ce, Y with the purity 99.5%, and Ni, Mn, Co, Al with the purity 99.9%, under a protective of Ar atmosphere. The sample was remelted for 5 times to improve the homogeneity of the alloy composition. Then, the fabricated ingot was annealed at 1000 C for 5 h under an atmosphere of high-purity Ar. The annealed alloy was crushed mechanically to powders of 50 mm in diameter, which was determined by Malvern Mastersizer 2000. The composite was fabricated by using a hydrothermal method [27]. The prepared alloy powder was put into the Teflon-lined stainless steel autoclave, which was filled with 6 mM Co(NO3)2, 2 mM cetyltrimethylammonium bromide (CTAB), 5 ml ultrapure water and 30 ml ethanol. In order to optimize the content of Co3O4 in the composite, 9 mM and 12 mM Co(NO3)2 were also used to fabricate the composite
Fig. 1 e A schematic illustration of the synthesis route of the composite. (a) The preparation process of the composite. (b) The photograph of an electrode pellet for electrochemical tests.
Please cite this article in press as: Li MM, et al., In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.129
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samples in this work. Then the sealed autoclave was heated in an electric oven at 180 C for 90 min for growing Co3O4 on the surface of alloy particles. After cooled to room temperature (25 C), the composite was filtered using filter paper and washed with deionized water and ethanol. Subsequently, the composite was dried in vacuum drying oven overnight. The fabricated three composite samples were denoted as CS1, CS2 and CS3, respectively, according to different Co(NO3)2 concentrations of 6 mM, 9 mM and 12 mM. The weight percent of Co3O4 in the composites was measured using inductively coupled plasma (ICP). The morphology and structure of the master alloy and composite were characterized with a field-emission scanning electron microscope (FESEM, JSM-6700F, JEOL, 15 keV) and a transmission electron microscope (TEM, Tecnai, F20, FEI, 200 keV). X-ray diffractometer (XRD) measurements were performed by using a D/max2500pc diffractometer (Cu-Ka radiation). The XRD data were collected over a scanning range of 2q from 10 to 80 with an increment of 1 /min. Raman spectrum analysis was performed by a micro-Raman spectrometer (Renishaw) with a laser of 532-nm excitation wavelength. To prepare the electrode for half-cell electrochemical measurements, 0.25 g active material (master alloy or composite) was mixed uniformly with 1.0 g carbonyl nickel powder and the mixture was pressed into a pellet of 15 mm in diameter under a pressure of 8 MPa. Electrochemical measurements were carried out by using an Arbin BT-2000 battery test system in a tri-electrode system, which includes a sintered Ni(OH)2 electrode (the counter electrode), a Hg/HgO electrode (the reference electrode) and as-prepared master alloy or composite electrode (the working electrode) with 30 wt% KOH as the electrolyte. The system was charged for 7.5 h and then discharged to a cut-off potential of 0.74 V (vs. Hg/HgO) for activation, which were carried out at a current density of 60 mA g1. The maximum discharge capacity Cmax was reached after 4-cycle activation. For HRD measurements, the charging current density is 300 mA g1 and discharging current densities are 300, 600, 900, 1200, 1500, 2400, and 3000 mA g1, respectively. The corresponding discharge capacity is named as Cd. The HRD property is evaluated by: HRD ¼ Cd =Cmax 100%:
(1)
The electrochemical curves were tested using an IVIUM electrochemical analyzer. The electrochemical impedance spectra (EIS) were measured with an amplitude of 5 mV [vs. open circuit potential (OCP)] over the frequency range from 100 kHz to 5 mHz at 50% depth of discharge (DOD). The linear polarization measurement was performed over the potential range from 5 to 5 mV (vs. OCP) with a scan rate of 0.05 mV s1 at the state of 50% DOD. The anodic polarization curves measurements were carried out by scanning the potential from 0 to 1.5 V (vs. OCP) at a rate of 5 mV s1 at 50% DOD. For the potentiostatic discharge experiment, the electrode with a state of 100% charge was polarized under a þ500 mV (vs. Hg/HgO) potential step for 4000 s. All above electrochemical measurements were performed at room temperature.
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Results and discussion Fig. 2aed shows SEM images of CS1, CS2, CS3 and the master alloy, respectively. Compared with smooth surface of the master alloy, nanosheet-shaped Co3O4 grows vertically and uniformly on the surface of hydrogen storage alloys where the dimensions (length and width) of Co3O4 nanosheets decrease with increasing concentrations of Co(NO3)2. The lengths of Co3O4 nanosheets are 200e400, 100e250 and 60e100 nm while their widths are 15e25, 12e20 and 8e13 nm for CS1, CS2 and CS3, respectively. This is caused by the increase of Co3O4 nucleation sites on the alloy surface and the decrease of Co3O4 critical nucleation radius with increasing Co(NO3)2 concentrations, which benefit for the formation of smaller size of nanosheets [34]. Since the morphology of the Co3O4 coatings plays an important role in determining their catalytic effects [35], the above three composites should show different catalytic performances. Fig. 3a plots the XRD patterns of three composites and the master alloy. All samples retain CaCu5-type hexagonal structure [36] without peaks arising from Co3O4 due to tiny amounts of Co3O4 in the composites, which are 1.28, 1.46 and 2.03 wt% for CS1, CS2 and CS3, respectively, as measured by ICP. As shown in Fig. 3b, the characteristic peaks at 194, 482, 524, 619 and 691 cm1 in the Raman spectrum of the composite are assigned to F2g, Eg, F2g, F2g and A1g vibrational modes of Co3O4, respectively, corresponding to a spinel-type crystalline structure [27,37]. Fig. 3c presents a low-magnification TEM image of Co3O4, which shows a two-dimensional lamellar-like structure. The high resolution TEM (HRTEM) image (Fig. 3d) clearly reveals lattice spacings of 0.204, 0.246 and 0.288 nm for the as-prepared Co3O4, corresponding to (400), (311) (inset of Fig. 3d) and (220) planes of Co3O4, respectively. The typical discharge characteristic curves of the composite electrodes and the master alloy electrode are presented in Fig. 4a. These discharge curves show the variation in the potential between the reference electrode and the MH electrode with different hydrogen atom concentrations [38]. It is evident that all the composite electrodes present flatter and longer potential plateau than the master alloy electrode, indicating larger discharge capacities of the composite electrodes [39]. As illustrated in Table 1, the discharge capacities are 320.78, 326.37 and 324.04 mAh g1 for CS1, CS2 and CS3 electrodes, respectively, which are all larger than that of the master alloy electrode (302.62 mAh g1). This is caused by the catalytic activity of Co3O4 on the Volmer reaction on the alloy surface [31]. During charging, more hydrogen atoms are produced on the surface of composite electrodes thanks to the accelerated Volmer reaction while a constant rate of Tafel or Heyrovsky reaction. Thus, there is a larger concentration gradient of hydrogen atoms between the electrode surface and alloy bulk, which enables hydrogen atoms easier to enter the interstitial sites of alloys. This is beneficial for the availability of active materials (hydrogen storage alloys), leading to larger discharge capacity of the composite electrode. Moreover, different discharge properties of three composite electrodes are caused by their different morphologies, which affect the catalytic activity of Co3O4 and the penetration
Please cite this article in press as: Li MM, et al., In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.129
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Fig. 2 e Morphological characterization of the composites and the master alloy. SEM images of (a) CS1, (b) CS2, (c) CS3 and (d) the master alloy, respectively.
property of hydrogen atoms through the Co3O4 layer [35]. Compared with the mechanical mixing of hydrogen storage alloys and Co3O4 powders [30], the in situ growth of Co3O4 nanosheets on the alloys surface in our fabrication method significantly enhances the utilization efficiency of Co3O4, resulting in strong catalytic effect on the electrochemical reaction and thus superior electrochemical performance of the electrode. For example, the maximum discharge capacity of CS2 with only 1.46 wt% Co3O4 is higher than the master alloy by 7.85%. To reach such an enhancement effect, more than 6 wt% Co3O4 powder is required based on the mechanical mixing method [30]. From Fig. 4a, it can also be seen that the composite electrodes show higher discharge potential plateau than the master alloy electrode, implying smaller polarization (ohmic polarization and electrochemical polarization) during discharge processes and better dynamic properties of the composites [40]. As exhibited in Fig. 4b, the composite electrodes show superior HRD performance than the master alloy at different discharge current densities. The corresponding experimental data are listed in Table 1. At the discharge current density of 3000 mA g1 (10 C), the discharge capacities are 54.27, 59.01, 58.37 and 40.88 mAh g1 for CS1, CS2, CS3 and the master alloy, respectively, where CS2 shows the best performance. During the discharge process, the HRD properties of MH electrodes are determined by two steps: one is the diffusion of hydrogen atoms from inner part of MH to the surface and the other is the electrochemical reaction (Hads þ OH e / H2O)
at the electrodeeelectrolyte interface [11,32]. The former can be characterized by anodic polarization curves and discharge current-time curves while the latter can be represented by the exchange current density (I0) and charge transfer resistance (RCT). Fig. 4c shows the electrochemical impedance spectra (EIS) of the composite electrodes and the master alloy electrode. Each spectrum consists of two semicircles in the high frequency region and a straight line in the low frequency region: (1) the smaller semicircles are attributed to the contact resistance (RC) between current collector and alloy powders, or among alloy particles [11,41]; (2) the larger semicircles are ascribed to the charge transfer resistance (RCT) during electrochemical reaction; and (3) the straight line denotes the Warburg resistance corresponding to the diffusion. Using the equivalent circuit [42] in Fig. 4c, the RC and RCT values can be obtained, which are also listed in Table 1. It is clear that the RC and RCT values of the composite electrodes are all smaller than those of the master alloy electrode. For the composite electrodes, RC decreases (190, 189 and 170 mU) while RCT decreases at first and then increases (350, 329 and 346 mU) with increasing Co(NO3)2 concentrations. This may result from different morphologies of Co3O4 on hydrogen storage alloys. Fig. 4d illustrates linear polarization curves of the hybrid electrodes and master alloy electrode. It is evident that there exists a linear correlation between the current density and the overpotential when the variation of overpotential is smaller than 10 mV [11,30]. I0 is an important parameter for measuring the kinetics of electrochemical reaction, and the larger the
Please cite this article in press as: Li MM, et al., In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.129
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 e8
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Fig. 3 e Morphological and microstructure characterizations. (a) XRD patterns of the composites and master alloy powders. (b) Raman spectrum of the composite. (c) TEM image and (d) HRTEM images (including the inset) of Co3O4.
value of I0, the faster the electrochemical reaction rate on the electrode surface. It can be calculated by the expression [32]: I0 ¼ RTId =ðFhÞ
(2)
where R, T, Id, F and h are the ideal gas constant, absolute temperature, applied current density, Faraday constant and overpotential, respectively. By fitting the slopes of lines in Fig. 4d, the calculated I0 values are listed in Table 1. For the composite electrodes, I0 increases first and then decrease (121.08, 129.32 and 122.26 mA g1) with increasing Co(NO3)2 concentrations, which are all larger than that of the master alloy electrode (105.61 mA g1). Fig. 4e plots the anodic polarization curves of the composite electrodes and the master alloy electrode, where the current density increases and then decreases with increasing potential [43,44]. The corresponding peak value is named as the limiting current (IL), which represents the rate of hydrogen diffusion into the hydrogen storage alloys, and the larger the IL value, the faster the diffusion of hydrogen atoms. As listed in Table 1, the CS2 electrode owns the largest IL (3036.17 mA g1) value. Moreover, the hydrogen diffusion through the alloy bulk can also be characterized by the hydrogen diffusion coefficient DH, which can be determined by using a potentialstep measurement [45]. Fig. 4f gives the current-time
responses of the composite electrodes and the master alloy electrode. At first, the current densities of all electrodes decrease significantly with increasing discharge time due to the polarization. Then, a linear current response is observed, which is controlled by the diffusion of H atoms from the inner part of alloys to the electrode surface [46]. On the basis of a spherical diffusion model, the DH values can be calculated by using the slope of the linear part of the corresponding curves [39]:
log i ¼ log
6FDH p2 DH ðC0 Cs Þ t 2 da 2:303a2
(3)
where i is the current density, F the Faraday constant, d the material density, a the particle radius (a ¼ 25 mm here), C0 the initial hydrogen concentration in the alloy bulk, Cs the hydrogen concentration at the surface of alloy powders and t the discharge time. From Table 1, the DH values of the composite electrodes increase first and then decrease (1.39, 1.60 and 1.56 1010 cm2 s1) as Co(NO3)2 concentrations increase, which are all larger than that of the master alloy electrode (1.15 1010 cm2 s1). Summarizing the above results, the CS2 electrode owns the largest charge transfer rate on the electrode surface and the largest hydrogen diffusion rate within the alloy, and thus the best HRD performance.
Please cite this article in press as: Li MM, et al., In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.129
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Fig. 4 e Electrochemical measurement results of the composite electrodes and the master alloy electrode at room temperature. (a) The discharge curves at a discharge current density of 60 mA g¡1. (b) The HRD properties at different discharge current densities. (c) EIS at 50% DOD. (d) Linear polarization curves at 50% DOD. (e) Anodic polarization curves at 50% DOD. (f) Constant potential discharge curves at þ500 mV (vs. Hg/HgO) at 100% state of charge.
Table 1 e Electrochemical properties of the composite electrodes and the master alloy electrode. Samples CS1 CS2 CS3 Master alloy
Cmax (mAh g1)
C3000 (mAh g1)
RC (mU)
RCT (mU)
I0 (mA g1)
IL (mA g1)
DH (1010 cm2 s1)
320.78 326.37 324.04 302.62
54.27 59.01 58.37 40.88
190 189 170 192
350 329 346 375
121.08 129.32 122.26 105.61
2857.98 3036.17 2940.76 2701.38
1.39 1.60 1.56 1.15
Conclusions In summary, a simple method is reported to fabricate the composite with nanosheet-shaped Co3O4 in situ growing on the surface of hydrogen storage alloys. Due to the enhanced catalytic activity of Co3O4 and the decreased internal resistance, the hydrogen absorption/desorption ability and electrochemical reaction kinetics are improved. The discharge
capacity of CS2 electrode is 326.37 mAh g1, 23.75 mAh g1 larger than the master alloy electrode (302.62 mAh g1). At a discharge current density of 3000 mA g1, the discharge capacity of CS2 electrode is 59.01 mAh g1, which is 1.5 times that of the master alloy electrode (40.88 mAh g1). The simple and facile preparation method reported in this work is effective to improve the catalytic activity of Co3O4 and can also be extended to other alloy systems.
Please cite this article in press as: Li MM, et al., In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.129
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Acknowledgements We wish to thank ChangBai Mountain Scholars Program and Jilin University Basic Research Grants Program for financially supporting this project.
references
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[20] Hermann A, Ramakrishnan PA, Badri V, Mardilovich P, Landuyt W. Metal hydride batteries research using nanostructured additives. Int J Hydrogen Energy 2001;26:1295e9. [21] Rogulski Z, Dłubak J, Karwowska M, Krebs M, Pytlik E, Schmalz M, et al. Studies on metal hydride electrodes containing no binder additives. J Power Sources 2010;195:7517e23. [22] Tanaka T, Kuzuhara M, Watada M, Oshitani M. Effect of rare earth oxide additives on the performance of NiMH batteries. J Alloys Compd 2006;408e412:323e6. [23] Zhang P, Wei X, Liu Y, Zhu J, Guang Y. Effects of metal oxides addition on the performance of La1.3CaMg0.7Ni9 hydrogen storage alloy. Int J Hydrogen Energy 2008;33:1304e9. [24] Cao AM, Hu JS, Liang HP, Song WG, Wan LJ, He XL, et al. Hierarchically structured cobalt oxide (Co3O4): the morphology control and its potential in sensors. J Phys Chem B 2006;110:15858e63. [25] Casas-Cabanas M, Binotto G, Larcher D, Lecup A, Giordani V, Tarascon JM. Defect chemistry and catalytic activity of nanosized Co3O4. Chem Mater 2009;21:1939e47. [26] Liang Y, Li Y, Wang H, Zhou J, Wang J, Regier T, et al. Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction. Nat Mater 2011;10:780e6. [27] Lang XY, Fu HY, Hou C, Han GF, Yang P, Liu YB, et al. Nanoporous gold supported cobalt oxide microelectrodes as high-performance electrochemical biosensors. Nat Commun 2013;4:2169. [28] Li Y, Tan B, Wu Y. Mesoporous Co3O4 nanowire arrays for lithium ion batteries with high capacity and rate capability. Nano Lett 2008;8:265e70. [29] Li L, Wang Y, Wang Y, Han Y, Qiu F, Liu G, et al. Mesoporous nano-Co3O4: a potential negative electrode material for alkaline secondary battery. J Power Sources 2011;196:10758e61. [30] Zhang Q, Geng S, Li A, Liu K. Electrochemical performances of AB5-type hydrogen storage alloy modified with Co3O4. Nonferr Met Soc China 2011;6326:1428e34. [31] Iwakura C, Matsuoka M, Kohno T. Mixing effect of metal oxides on negative electrode reactions in the nickel-hydride battery. J Electrochem Soc 1994;141:2306e9. [32] Feng F, Geng M, Northwood DO. Electrochemical behaviour of intermetallic-based metal hydrides used in Ni/metal hydride (MH) batteries: a review. Int J Hydrogen Energy 2001;26:725e34. [33] Su G, He Y, Liu K. Effects of Co3O4 as additive on the performance of metal hydride electrode and Ni-MH battery. Int J Hydrogen Energy 2012;37:11994e2002. [34] Frens G. Controlled nucleation for the regulation of the particle size in monodisperse gold suspensions. Nat Phys Sci 1973;241:20e2. [35] Trasatti S. Physical electrochemistry of ceramic oxides. Electrochim Acta 1991;36:225e341. [36] Kong L, Chen B, Young K, Koch J, Chan A, Li W. Effects of Aland Mn-contents in the negative MH alloy on the selfdischarge and long-term storage properties of Ni/MH battery. J Power Sources 2012;213:128e39. [37] Hadjiev VG, Iliev MN, Vergilov IV. The Raman spectra of Co3O4. J Phys C Solid State Phys 1988;21:L199e201. [38] Zhao X, Ma L, Ding Y, Shen X. Effect of particle size on the electrochemical properties of MmNi3.8Co0.75Mn0.4Al0.2 hydrogen storage alloy. Int J Hydrogen Energy 2009;34:3389e94. [39] Zhang W, Han S, Hao J, Li Y, Bai T, Zhang J. Study on kinetics and electrochemical properties of low-Co AB5-type alloys for high-power Ni/MH battery. Electrochim Acta 2009;54:1383e7. [40] Ouyang LZ, Cao ZJ, Li LL, Wang H, Liu JW, Min D, et al. Enhanced high-rate discharge properties of
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La11.3Mg6.0Sm7.4Ni61.0Co7.2Al7.1 with added graphene synthesized by plasma milling. Int J Hydrogen Energy 2014;39:12765e72. [41] Slepski P, Darowicki K, Janicka E, Sierczynska A. Application of electrochemical impedance spectroscopy to monitoring discharging process of nickel/metal hydride battery. J Power Sources 2013;241:121e6. [42] Valøen LO, Lasia A, Jensen JO, Tunold R. The electrochemical impedance of metal hydride electrodes. Electrochim Acta 2002;47:2871e84. [43] Iwakura C, Fukuda K, Senoh H, Inoue H, Matsuoka M, Yamamoto Y. Electrochemical characterization of MmNi4.0-
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In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance M.M. Li, C.C. Yang*, W.T. Jing, B. Jin, X.Y. Lang, Q. Jiang Key Laboratory of Automobile Materials (Jilin University), Ministry of Education, School of Materials Science and Engineering, Jilin University, Changchun 130022, China
article info
abstract
Article history:
Co3O4 is an effective additive to enhance the electrochemical performance of hydrogen
Received 5 January 2016
storage alloys. However, the low utilization efficiency has become a big challenge for direct
Received in revised form
adding Co3O4 powders into the alloys with mechanical mixing. Here, we report the in situ
27 January 2016
growth of Co3O4 on the alloy surface by using a facile and effective hydrothermal method.
Accepted 27 January 2016
Compared with bare hydrogen storage alloys, the fabricated composite shows larger
Available online xxx
maximum discharge capacity, 326.37 vs. 302.62 mAh g1, and enhanced high rate dischargeability with larger discharge capacity at a current density of 3000 mA g1, 59.01 vs.
Keywords:
40.88 mAh g1. These are contributed by the unique hybrid architecture of the composite:
Hydrogen storage alloy
(1) the in situ grown Co3O4 nanosheets improve the catalytic activity and utilization effi-
Co3O4
ciency of Co3O4 on the electrochemical reaction kinetics; and (2) the low-dimensional
Discharge capacity
Co3O4 coatings seamlessly integrated with hydrogen storage alloys decrease the internal
High rate dischargeability
resistance and polarization of the hybrid electrode. Such a simple and novel method can
Catalytic activity
also be extended to other energy storage devices.
Nickel metal hydride battery
Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The rechargeable nickel metal hydride (NieMH) batteries have been widely used in personal portable electronic devices, battery-powered/assisted vehicles, military equipment etc., owing to their superior safety, temperature adaptability, and environmental friendliness [1e4]. While with the development of advanced electrical energy storage devices (such as supercapacitors, lithium-ion batteries and fuel cell) [5e8] and increasing demands for power sources, it is essential to further improve the performance of NieMH batteries in energy density and power density [9,10]. AB5-type misch-metal based hydrogen storage alloys are the state-of-the-art
negative electrode materials in NieMH batteries [11], which dominate the electrochemical performance of NieMH batteries. Thus, how to enhance the performance of hydrogen storage alloys, such as the discharge capacity and high rate dischargeability (HRD), has attracted great attentions in recent years [9e11]. The study of modification of hydrogen storage alloys has been proceeding for decades and a number of methods have been proposed to improve the electrochemical performance of NieMH batteries. The elemental substitution, which could significantly change the microstructure and properties of alloys, has been proved to be an efficient approach to enhance the overall electrochemical properties of hydrogen storage alloys [12,13]. The annealing treatment and modified
* Corresponding author. Tel.: þ86 431 85095371; fax: þ86 431 85095876. E-mail address: [email protected] (C.C. Yang). http://dx.doi.org/10.1016/j.ijhydene.2016.01.129 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Li MM, et al., In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.129
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preparation methods have also been introduced to enable component homogeneity of alloys and thus to improve their cycle stability [14,15]. The increased surface area and higher conductivity of hydrogen storage alloys by surface treatments, such as electroless, acid, alkaline, or fluorination treatment, mechanical alloying etc., significantly enhance the electrochemical reaction kinetics [16e19]. Moreover, some conductive additives like transition metals, graphite, carbon nanotube and oxides were added into hydrogen storage alloys as current collector and binder, which are beneficial for the improvements of electrode properties [20,21]. A series of transition metal oxides (RuO2, Fe2O3, TiO2, MnO2 and Co3O4) have also been used as catalysts to improve the hydriding/dehydriding kinetics of hydrogen storage alloys [22,23]. Among these oxides, Co3O4 as an important technological material has been widely used in catalytic fields, hydrocracking process of crude fuels, treatment of waste gases, hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), glucose oxidation etc. [24e27]. Recently, it has also been considered as a potential negative electrode material for lithium-ion batteries owing to its high theoretical capacity (890 mAh g1), and as a promising catalyst for the alkaline secondary battery because of its excellent catalytic property [28,29]. Moreover, the performance of negative electrode of NieMH batteries could also be improved by modification the hydrogen storage alloys with Co3O4 [30]. It is known that for the MH electrode, the following processes are considered to take place at the electrode/electrolyte interface and in the alloy bulk during charging: (i) electrochemical reaction at the interface: H2O þ e / Hads þ OH (Volmer reaction); (ii) the transition between adsorbed hydrogen atoms and absorbed hydrogen atoms: Hads / Habs; and (iii) the diffusion of absorbed hydrogen into the bulk of alloy and the following phase transformation: MHa / MHb. Furthermore, there also exists the HER by two different reaction routes: the Volmer-Tafel route (Tafel reaction: 2Hads / H2) or the Volmer-Heyrovsky route (Heyrovsky reaction: Hads þ H2O þ e / H2 þ OH) [31,32]. During charging, the as-formed Hads at the electrode surface by Volmer reaction diffuse into the alloys, where the diffusion rate is proportional to the concentration gradient of H atoms between the electrode surface and the alloy bulk. In order to charge the electrode effectively, it is critical to keep a high concentration of H atoms at the electrode surface. Thus, the Volmer reaction should be accelerated while the Tafel or Heyrovsky reaction should be depressed. The experimental finding indicates that for the Co3O4-modified MH electrode, the rate of Volmer reaction is enhanced while the rate of Tafel or Heyrovsky reaction remains constant, which is beneficial for improvements of the electrode performance [31]. However, the Co3O4 in the electrode is only partially utilized by directly adding Co3O4 powders into hydrogen storage alloys through the mechanical mixing [30,33], leading to a moderate catalytic effect. In this work, Co3O4 nanosheet arrays are in situ grown on the surface of alloy particles via a simple hydrothermal method. In this case, the utilization efficiency of Co3O4 nanosheets could be highly enhanced and the performance of the hybrid electrode is thus noticeably increased. Compared with bare hydrogen storage alloys (or the master alloy), the maximum discharge capacity and the
discharge capacity at a current density of 3000 mA g1 of the composite increase from 302.62 mAh g1 to 326.37 mAh g1, and from 40.88 mAh g1 to 59.01 mAh g1, respectively. This facile and effective fabrication strategy of the composite could be extended to other alloy systems too for the electrochemical performance enhancements.
Experimental The fabrication strategy of the composite by using a simple hydrothermal method [27] is briefly illustrated in Fig. 1a. With this approach, the Co3O4 nanosheets could be in situ grown on the surface of hydrogen storage alloys. Moreover, during the hydrothermal process, the loading amount and morphology of Co3O4 could be tailored by varying the concentration of Co(NO3)2. The electrode pellet used for the electrochemical measurements is shown in Fig. 1b. The experimental details are given below. The (LaCeY)(NiMnCoAl)5 alloy (master alloy) was prepared by induction melting the constituent metals, La, Ce, Y with the purity 99.5%, and Ni, Mn, Co, Al with the purity 99.9%, under a protective of Ar atmosphere. The sample was remelted for 5 times to improve the homogeneity of the alloy composition. Then, the fabricated ingot was annealed at 1000 C for 5 h under an atmosphere of high-purity Ar. The annealed alloy was crushed mechanically to powders of 50 mm in diameter, which was determined by Malvern Mastersizer 2000. The composite was fabricated by using a hydrothermal method [27]. The prepared alloy powder was put into the Teflon-lined stainless steel autoclave, which was filled with 6 mM Co(NO3)2, 2 mM cetyltrimethylammonium bromide (CTAB), 5 ml ultrapure water and 30 ml ethanol. In order to optimize the content of Co3O4 in the composite, 9 mM and 12 mM Co(NO3)2 were also used to fabricate the composite
Fig. 1 e A schematic illustration of the synthesis route of the composite. (a) The preparation process of the composite. (b) The photograph of an electrode pellet for electrochemical tests.
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samples in this work. Then the sealed autoclave was heated in an electric oven at 180 C for 90 min for growing Co3O4 on the surface of alloy particles. After cooled to room temperature (25 C), the composite was filtered using filter paper and washed with deionized water and ethanol. Subsequently, the composite was dried in vacuum drying oven overnight. The fabricated three composite samples were denoted as CS1, CS2 and CS3, respectively, according to different Co(NO3)2 concentrations of 6 mM, 9 mM and 12 mM. The weight percent of Co3O4 in the composites was measured using inductively coupled plasma (ICP). The morphology and structure of the master alloy and composite were characterized with a field-emission scanning electron microscope (FESEM, JSM-6700F, JEOL, 15 keV) and a transmission electron microscope (TEM, Tecnai, F20, FEI, 200 keV). X-ray diffractometer (XRD) measurements were performed by using a D/max2500pc diffractometer (Cu-Ka radiation). The XRD data were collected over a scanning range of 2q from 10 to 80 with an increment of 1 /min. Raman spectrum analysis was performed by a micro-Raman spectrometer (Renishaw) with a laser of 532-nm excitation wavelength. To prepare the electrode for half-cell electrochemical measurements, 0.25 g active material (master alloy or composite) was mixed uniformly with 1.0 g carbonyl nickel powder and the mixture was pressed into a pellet of 15 mm in diameter under a pressure of 8 MPa. Electrochemical measurements were carried out by using an Arbin BT-2000 battery test system in a tri-electrode system, which includes a sintered Ni(OH)2 electrode (the counter electrode), a Hg/HgO electrode (the reference electrode) and as-prepared master alloy or composite electrode (the working electrode) with 30 wt% KOH as the electrolyte. The system was charged for 7.5 h and then discharged to a cut-off potential of 0.74 V (vs. Hg/HgO) for activation, which were carried out at a current density of 60 mA g1. The maximum discharge capacity Cmax was reached after 4-cycle activation. For HRD measurements, the charging current density is 300 mA g1 and discharging current densities are 300, 600, 900, 1200, 1500, 2400, and 3000 mA g1, respectively. The corresponding discharge capacity is named as Cd. The HRD property is evaluated by: HRD ¼ Cd =Cmax 100%:
(1)
The electrochemical curves were tested using an IVIUM electrochemical analyzer. The electrochemical impedance spectra (EIS) were measured with an amplitude of 5 mV [vs. open circuit potential (OCP)] over the frequency range from 100 kHz to 5 mHz at 50% depth of discharge (DOD). The linear polarization measurement was performed over the potential range from 5 to 5 mV (vs. OCP) with a scan rate of 0.05 mV s1 at the state of 50% DOD. The anodic polarization curves measurements were carried out by scanning the potential from 0 to 1.5 V (vs. OCP) at a rate of 5 mV s1 at 50% DOD. For the potentiostatic discharge experiment, the electrode with a state of 100% charge was polarized under a þ500 mV (vs. Hg/HgO) potential step for 4000 s. All above electrochemical measurements were performed at room temperature.
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Results and discussion Fig. 2aed shows SEM images of CS1, CS2, CS3 and the master alloy, respectively. Compared with smooth surface of the master alloy, nanosheet-shaped Co3O4 grows vertically and uniformly on the surface of hydrogen storage alloys where the dimensions (length and width) of Co3O4 nanosheets decrease with increasing concentrations of Co(NO3)2. The lengths of Co3O4 nanosheets are 200e400, 100e250 and 60e100 nm while their widths are 15e25, 12e20 and 8e13 nm for CS1, CS2 and CS3, respectively. This is caused by the increase of Co3O4 nucleation sites on the alloy surface and the decrease of Co3O4 critical nucleation radius with increasing Co(NO3)2 concentrations, which benefit for the formation of smaller size of nanosheets [34]. Since the morphology of the Co3O4 coatings plays an important role in determining their catalytic effects [35], the above three composites should show different catalytic performances. Fig. 3a plots the XRD patterns of three composites and the master alloy. All samples retain CaCu5-type hexagonal structure [36] without peaks arising from Co3O4 due to tiny amounts of Co3O4 in the composites, which are 1.28, 1.46 and 2.03 wt% for CS1, CS2 and CS3, respectively, as measured by ICP. As shown in Fig. 3b, the characteristic peaks at 194, 482, 524, 619 and 691 cm1 in the Raman spectrum of the composite are assigned to F2g, Eg, F2g, F2g and A1g vibrational modes of Co3O4, respectively, corresponding to a spinel-type crystalline structure [27,37]. Fig. 3c presents a low-magnification TEM image of Co3O4, which shows a two-dimensional lamellar-like structure. The high resolution TEM (HRTEM) image (Fig. 3d) clearly reveals lattice spacings of 0.204, 0.246 and 0.288 nm for the as-prepared Co3O4, corresponding to (400), (311) (inset of Fig. 3d) and (220) planes of Co3O4, respectively. The typical discharge characteristic curves of the composite electrodes and the master alloy electrode are presented in Fig. 4a. These discharge curves show the variation in the potential between the reference electrode and the MH electrode with different hydrogen atom concentrations [38]. It is evident that all the composite electrodes present flatter and longer potential plateau than the master alloy electrode, indicating larger discharge capacities of the composite electrodes [39]. As illustrated in Table 1, the discharge capacities are 320.78, 326.37 and 324.04 mAh g1 for CS1, CS2 and CS3 electrodes, respectively, which are all larger than that of the master alloy electrode (302.62 mAh g1). This is caused by the catalytic activity of Co3O4 on the Volmer reaction on the alloy surface [31]. During charging, more hydrogen atoms are produced on the surface of composite electrodes thanks to the accelerated Volmer reaction while a constant rate of Tafel or Heyrovsky reaction. Thus, there is a larger concentration gradient of hydrogen atoms between the electrode surface and alloy bulk, which enables hydrogen atoms easier to enter the interstitial sites of alloys. This is beneficial for the availability of active materials (hydrogen storage alloys), leading to larger discharge capacity of the composite electrode. Moreover, different discharge properties of three composite electrodes are caused by their different morphologies, which affect the catalytic activity of Co3O4 and the penetration
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Fig. 2 e Morphological characterization of the composites and the master alloy. SEM images of (a) CS1, (b) CS2, (c) CS3 and (d) the master alloy, respectively.
property of hydrogen atoms through the Co3O4 layer [35]. Compared with the mechanical mixing of hydrogen storage alloys and Co3O4 powders [30], the in situ growth of Co3O4 nanosheets on the alloys surface in our fabrication method significantly enhances the utilization efficiency of Co3O4, resulting in strong catalytic effect on the electrochemical reaction and thus superior electrochemical performance of the electrode. For example, the maximum discharge capacity of CS2 with only 1.46 wt% Co3O4 is higher than the master alloy by 7.85%. To reach such an enhancement effect, more than 6 wt% Co3O4 powder is required based on the mechanical mixing method [30]. From Fig. 4a, it can also be seen that the composite electrodes show higher discharge potential plateau than the master alloy electrode, implying smaller polarization (ohmic polarization and electrochemical polarization) during discharge processes and better dynamic properties of the composites [40]. As exhibited in Fig. 4b, the composite electrodes show superior HRD performance than the master alloy at different discharge current densities. The corresponding experimental data are listed in Table 1. At the discharge current density of 3000 mA g1 (10 C), the discharge capacities are 54.27, 59.01, 58.37 and 40.88 mAh g1 for CS1, CS2, CS3 and the master alloy, respectively, where CS2 shows the best performance. During the discharge process, the HRD properties of MH electrodes are determined by two steps: one is the diffusion of hydrogen atoms from inner part of MH to the surface and the other is the electrochemical reaction (Hads þ OH e / H2O)
at the electrodeeelectrolyte interface [11,32]. The former can be characterized by anodic polarization curves and discharge current-time curves while the latter can be represented by the exchange current density (I0) and charge transfer resistance (RCT). Fig. 4c shows the electrochemical impedance spectra (EIS) of the composite electrodes and the master alloy electrode. Each spectrum consists of two semicircles in the high frequency region and a straight line in the low frequency region: (1) the smaller semicircles are attributed to the contact resistance (RC) between current collector and alloy powders, or among alloy particles [11,41]; (2) the larger semicircles are ascribed to the charge transfer resistance (RCT) during electrochemical reaction; and (3) the straight line denotes the Warburg resistance corresponding to the diffusion. Using the equivalent circuit [42] in Fig. 4c, the RC and RCT values can be obtained, which are also listed in Table 1. It is clear that the RC and RCT values of the composite electrodes are all smaller than those of the master alloy electrode. For the composite electrodes, RC decreases (190, 189 and 170 mU) while RCT decreases at first and then increases (350, 329 and 346 mU) with increasing Co(NO3)2 concentrations. This may result from different morphologies of Co3O4 on hydrogen storage alloys. Fig. 4d illustrates linear polarization curves of the hybrid electrodes and master alloy electrode. It is evident that there exists a linear correlation between the current density and the overpotential when the variation of overpotential is smaller than 10 mV [11,30]. I0 is an important parameter for measuring the kinetics of electrochemical reaction, and the larger the
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Fig. 3 e Morphological and microstructure characterizations. (a) XRD patterns of the composites and master alloy powders. (b) Raman spectrum of the composite. (c) TEM image and (d) HRTEM images (including the inset) of Co3O4.
value of I0, the faster the electrochemical reaction rate on the electrode surface. It can be calculated by the expression [32]: I0 ¼ RTId =ðFhÞ
(2)
where R, T, Id, F and h are the ideal gas constant, absolute temperature, applied current density, Faraday constant and overpotential, respectively. By fitting the slopes of lines in Fig. 4d, the calculated I0 values are listed in Table 1. For the composite electrodes, I0 increases first and then decrease (121.08, 129.32 and 122.26 mA g1) with increasing Co(NO3)2 concentrations, which are all larger than that of the master alloy electrode (105.61 mA g1). Fig. 4e plots the anodic polarization curves of the composite electrodes and the master alloy electrode, where the current density increases and then decreases with increasing potential [43,44]. The corresponding peak value is named as the limiting current (IL), which represents the rate of hydrogen diffusion into the hydrogen storage alloys, and the larger the IL value, the faster the diffusion of hydrogen atoms. As listed in Table 1, the CS2 electrode owns the largest IL (3036.17 mA g1) value. Moreover, the hydrogen diffusion through the alloy bulk can also be characterized by the hydrogen diffusion coefficient DH, which can be determined by using a potentialstep measurement [45]. Fig. 4f gives the current-time
responses of the composite electrodes and the master alloy electrode. At first, the current densities of all electrodes decrease significantly with increasing discharge time due to the polarization. Then, a linear current response is observed, which is controlled by the diffusion of H atoms from the inner part of alloys to the electrode surface [46]. On the basis of a spherical diffusion model, the DH values can be calculated by using the slope of the linear part of the corresponding curves [39]:
log i ¼ log
6FDH p2 DH ðC0 Cs Þ t 2 da 2:303a2
(3)
where i is the current density, F the Faraday constant, d the material density, a the particle radius (a ¼ 25 mm here), C0 the initial hydrogen concentration in the alloy bulk, Cs the hydrogen concentration at the surface of alloy powders and t the discharge time. From Table 1, the DH values of the composite electrodes increase first and then decrease (1.39, 1.60 and 1.56 1010 cm2 s1) as Co(NO3)2 concentrations increase, which are all larger than that of the master alloy electrode (1.15 1010 cm2 s1). Summarizing the above results, the CS2 electrode owns the largest charge transfer rate on the electrode surface and the largest hydrogen diffusion rate within the alloy, and thus the best HRD performance.
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Fig. 4 e Electrochemical measurement results of the composite electrodes and the master alloy electrode at room temperature. (a) The discharge curves at a discharge current density of 60 mA g¡1. (b) The HRD properties at different discharge current densities. (c) EIS at 50% DOD. (d) Linear polarization curves at 50% DOD. (e) Anodic polarization curves at 50% DOD. (f) Constant potential discharge curves at þ500 mV (vs. Hg/HgO) at 100% state of charge.
Table 1 e Electrochemical properties of the composite electrodes and the master alloy electrode. Samples CS1 CS2 CS3 Master alloy
Cmax (mAh g1)
C3000 (mAh g1)
RC (mU)
RCT (mU)
I0 (mA g1)
IL (mA g1)
DH (1010 cm2 s1)
320.78 326.37 324.04 302.62
54.27 59.01 58.37 40.88
190 189 170 192
350 329 346 375
121.08 129.32 122.26 105.61
2857.98 3036.17 2940.76 2701.38
1.39 1.60 1.56 1.15
Conclusions In summary, a simple method is reported to fabricate the composite with nanosheet-shaped Co3O4 in situ growing on the surface of hydrogen storage alloys. Due to the enhanced catalytic activity of Co3O4 and the decreased internal resistance, the hydrogen absorption/desorption ability and electrochemical reaction kinetics are improved. The discharge
capacity of CS2 electrode is 326.37 mAh g1, 23.75 mAh g1 larger than the master alloy electrode (302.62 mAh g1). At a discharge current density of 3000 mA g1, the discharge capacity of CS2 electrode is 59.01 mAh g1, which is 1.5 times that of the master alloy electrode (40.88 mAh g1). The simple and facile preparation method reported in this work is effective to improve the catalytic activity of Co3O4 and can also be extended to other alloy systems.
Please cite this article in press as: Li MM, et al., In situ grown Co3O4 on hydrogen storage alloys for enhanced electrochemical performance, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.01.129
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Acknowledgements We wish to thank ChangBai Mountain Scholars Program and Jilin University Basic Research Grants Program for financially supporting this project.
references
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