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AB5 composite hydrogen storage alloys and their catalytic properties for KBH4
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
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Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4 Jing Yan, Xiao Tian*, Xin-Yu Liu, Xuan Zhao, Rui Wang, Li-Juan Zhao, Xin Zhang Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, School of Physics and Electronic Information, Inner Mongolia Normal University, Hohhot, 010022, People’s Republic of China
highlights The CNTs/AB5 composite alloys are prepared by arc smelting and ball milling. Hydrogen storage alloy can be used as anode catalyst for direct borohydride fuel cell. The catalytic properties of the alloy are related to its electrochemical properties.
article info
abstract
Article history:
Searching for non-precious metal anode catalysts with high catalytic activity and capable
Received 18 July 2019
of inhibiting hydrolysis side reactions is very important for direct borohydride fuel cell
Received in revised form
(DBFC). In this work, the as-cast AB5 alloy powders are firstly mixed with CNTs in a ratio of
13 October 2019
1:9. Then the mixture of AB5 alloy and CNTs is ball milled in different milling time. Finally,
Accepted 20 October 2019
the CNTs/AB5 composite alloys are obtained. Not only the catalytic properties of the CNTs/
Available online xxx
AB5 composite alloys used as anode catalysts in DBFC, but also the electrochemical properties of the alloys have been investigated in detail. The research results indicate that,
Keywords:
as the ball milling time is extended, the electrochemical properties and catalytic properties
Direct borohydride fuel cell (DBFC)
on ВН 4 of the CNTs/AB5 composite alloys become better first and then worsen. The CNTs/
AB5-type hydrogen storage alloy
AB5 alloy milled 2 h exhibits the best electrochemical properties and catalytic properties.
Carbon nanotube (CNTs)
Furthermore, we predict that the electrochemical properties of the composite alloy are
Catalytic properties
positively correlated with the catalytic properties as anode catalyst for DBFC.
Electrochemical properties
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the depletion of traditional fossil fuels, the development of new energy sources has become an urgent problem to be solved today [1e4]. Hydrogen energy has attracted much attention because of its cleanliness, abundant reserves, and wide range of sources [1,2,5e9]. However, safety issues such
as storage and transportation problems during hydrogen utilization have been plagued [10e12]. Direct borohydride fuel cell (DBFC) is a new technology that combines hydrogen energy and fuel cells, because it can convert chemical energy, which stored in a borohydride (ВН 4 ), directly into electrical energy under the anode catalyst [13,14]. Borohydride is the fuel of the DBFC, because the solid borohydride itself contains
* Corresponding author. E-mail address: [email protected] (X. Tian). https://doi.org/10.1016/j.ijhydene.2019.10.158 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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very high hydrogen (10.6 wt% in solid state). The DBFC not only avoids the difficulty of hydrogen storage and transportation during hydrogen utilization, but also has a high theoretical output voltage and high power density, considered as an energy conversion device with development potential [15e19]. However, in the operation of this battery, hydrogen is generated due to the hydrolysis reaction at the anode, which greatly reduces the efficiency of the battery [20,21]. This can be found by a series of chemical reactions occurring during the energy conversion process of the DBFC. The chemical reaction that occurs when the DBFC is working: Anode: BH 4 þ 8OH /BO2 þ 6H2 O þ 8e o Ea ¼ 1:24 V vs: SHE
(1)
Cathode: O2 þ 4H2 O þ 8e /8OH Eoc ¼ 0:4 V vs: SHE
(2)
Overall: BH 4 þ 2O2 / BO2 þ 2H2 O
ðEo ¼ 1:64 VÞ
(3)
Ideally, an electrochemical reaction occurs when the DBFC is operating, and 8e can be released. However, due to the hydrolysis reaction occurring on the anode during the operation of the DBFC, the consumption of electrons generates hydrogen. Therefore, the number of electrons released is usually less than 8e, resulting in a great reduction in the energy efficiency of the fuel cell. The specific hydrolysis reaction equation: BH 4 þ 2H2 O/BO2 þ 4H2
(4)
Therefore, the actual anode reaction is as follows: BH 4 þ xOH /BO2 þ ðx 2ÞH2 O þ ð4 xÞH2 þxe
(5)
In Eq. (5), x means the number of electrons, which is released by each ВН 4 during the actual reaction. The less hydrogen produced, the more electrons released. It can be seen that the anode catalyst in DBFC is not only a key factor affecting the electrochemical oxidation reaction, but also a key factor affecting the hydrolysis reaction. Most of the previous studies used precious metals or complex precious metals [22e27] as anode catalysts for DBFC. However, precious metal catalysts are expensive and severely restrict their widespread use. It has also been studied to use transition metal or transition metal composites [28e32] as anode catalysts for DBFC. It has been found that transition metals have low catalytic activity for ВН 4 in the DBFC and are prone to hydrolysis. Considering the cost of the anode catalyst, the problems of catalytic oxidation and hydrolysis, researchers have proposed the idea that hydrogen storage material is used as anode catalyst of DBFC in recent years. If so, the anode catalyst cost is greatly reduced, but the hydrogen storage reaction can effectively inhibit the hydrolysis reaction in the anode catalysis process, the number of electrons transferred is increased, and improve the fuel efficiency of the battery [33e36]. Wang et al. [37] used the AB5type LmNi4.78Al0.22 alloy instead of the noble metal as anode catalyst for the DBFC. The experimental results show that the alloy has high electrochemical catalytic activity during
discharge process and the hydrogen generation process. And the utilization of the fuel cell increase with increasing discharge current density. Subsequently, Wang et al. [38] doped Si element in AB5-type LmNi4.78Mn0.22 hydrogen storage alloy by ball milling and heat treatment methods. They use the composite hydrogen storage alloy as anode catalyst for DBFC. The study shows that after the modification of the AB5type alloy, as anode catalyst for DBFC, its utilization efficiency is increased from 21.37% to 95.27% at low discharge current. Some researchers also use AB2 hydrogen storage alloy as anode catalyst for DBFC. Choudhury et al. [39] used Zr0.9Tio.1Mn0.6Cr0.05Co0.05Ni1.2 alloy as anode catalyst of DBFC. The study demonstrates that at a temperature of 70 C, the power density reaches a maximum of 150 mW cm2. The study of hydrogen storage material as anode catalyst of DBFC has gradually begun to be concerned. In order to explain the catalytic mechanism of the hydrogen storage alloy for ВН 4 in detail, we have designed a schematic diagram. Fig. 1 is schematic diagram οf catalytic mechanism of hydrogen storage alloy on ВН 4 . We can obtain the catalytic mechanism of the hydrogen storage alloy on ВН 4 through Fig. 1. During the oxidation process of ВН 4 , the hydrogen in the ВН 4 is oxidized from a negative monovalent to a positive valence, which is can divided into two processes: Firstly, the BeH bond of ВН 4 is broken, and the negative monovalent hydrogen is oxidized to zero valence. The surface-adsorbed hydrogen atom (Hads) (Such as process 1) is formed, and then the surface-adsorbed hydrogen atom (Hads) continues to be oxidized to a positive monovalent (Such as process 2), i.e., the hydrogen direct oxidation process, corresponding to the reaction equation (1). Secondly, the surfaceadsorbed hydrogen atom (Hads) is also combined to form H2 (Such as process 3), which is the side reaction (hydrolysis reaction) of the ВН 4 oxidation process, corresponding to the reaction equation (4). It can be seen that the excellent property of the anode catalyst requires high catalytic activity for the processes 1 and 2, but has a strong inhibitory effect on the side reaction (Such as process 3). Generally, noble metal catalysts have good catalytic activity for processes 1e2, but their side reactions are difficult to eliminate. For the hydrogen storage alloy, it has a strong binding force to the hydrogen atom (Hads) adsorbed on the surface (Such as process 4, the hydrogen absorption can be stored in the crystal lattice of the hydrogen storage alloy, it means that the hydrogen storage alloy realized hydrogen storage function). The inhibition of side reaction (Such as process 3) is achieved by the chemical reaction of
Fig. 1 e Schematic diagram of catalytic mechanism of hydrogen storage alloy on ВН¡ 4.
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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the process 4. Since the hydrogen storage alloy is reversible, the hydrogen absorbed into the hydrogen storage alloy crystal lattice can be released again, completing the process 5. Then, the surface-adsorbed hydrogen atom (Hads) continues to be oxidized to a positive monovalent (Such as process 6, similar to process 2). These will increase the number of electrons released, thereby improve fuel utilization. Corresponding to the following reaction equation: MHx þ xOH %M þ xH2 O þ xe
(6)
It can be found that the hydrogen storage material inhibits the hydrolysis side reaction and the re-oxidation process is closely relative to the hydrogen storage electrochemical property of the alloy. However, the relationship between the hydrogen storage electrochemical property of hydrogen storage material and the catalytic property of hydrogen storage material as anode catalysts for DBFC has rarely been reported systematically. The rare earth base AB5-type hydrogen storage alloy is the main negative electrode material of MH/Ni battery which has been commercialized, and its excellent comprehensive electrochemical performance has been recognized [40,41]. CNTs have excellent electrical and thermal conductivity, anti-alkali and anti-oxidation properties, and have good hydrogen storage function due to the special hollow structure of CNTs, and are inexpensive, which is an ideal additive for hydrogen storage anode alloy [42e44]. In order to improve the electrocatalytic activity and NaBH4 utilization, Zhang et al. [45] used a multi-walled carbon nanotubes (MWNTs, 2 wt%) modified AB5 hydrogen storage alloy (MmNi0.58Co0.07 Mn0.04Al0.02) as anode catalytic for the DBFC. The study found that the current density was twice that using the AB5/MWNTs (2 wt%). And they also discovered that use MWNTs to modify the working electrode of AB5 hydrogen storage alloy, the utilization efficiency of NaBH4 was 61.5% higher than that of the original. MWNTs not only enhance the electrocatalytic activity of the alloy and the fuel utilization, and italso absorbs hydrogen to reduce the release of hydrogen. In addition, Li et al. [46] used a mixture of carbon nanotubes (CNTs) and AB5-type hydrogen storage alloy as anode catalyst for DBFC, and they test at room temperature. The CNTs mixture hydrogen storage alloy has current density of up to 1550 mAcm2 and a maximum power density of 65 mWcm2 compared to a series of conventional carbon materials. And the DBFC could keep relatively good short-term performance stability. The CNTs/AB5 composite alloys are synthesized by ball milling method. The structure, electrochemical performance and catalytic performance for ВН 4 of CNTs/AB5 composite hydrogen storage alloys under different milling time were studied. The relationship of electrochemical properties of CNTs/ AB5 composite hydrogen storage alloys and their catalytic properties for borohydrides are speculated.
Experimental Preparation of samples According to MmNi3.55Co0.75Mn0.4Al0.3 (Mm is composed of a variety of rare earth elements, of which Ce is 65.25%, La is
3
25.20%, Pr is 8.43%, Nd is 1.12%), the as-cast AB5-type MmNi3.55Co0.75Mn0.4Al0.3 alloy was prepared by vacuum arc furnace. CNTs (purity >99.9%) were produced by Shanghai Yao Tian Nano Material Co., Ltd. The CNTs and AB5 hydrogen storage alloy were mechanically ball milled using a pulverisette-6 single bowl star high energy ball mill manufactured by FRITSCH, Germany. Firstly, the CNTs was mixed with the AB5-type MmNi3.55Co0.75Mn0.4Al0.3 alloy at the mass ratio of 1:9 [47]. The mixture of the CNTs/AB5-type alloy powders are placed in a 400 ml ball mill jar. In order to prevent the experimental sample from oxidizing during the ball milling process, the ball milling jar containing the samples was placed in a glove box, flushed with argon gas and vacuumed repeatedly. Ball milling conditions: ball-to-power weight ratio is 4:1, rotation speed of 200 r/min, ball milling time of 0.5 h, 1 h, 2 h, 3 h, 4 h and 5 h.
Characterization of microstructure The phase structure analysis of alloy sample was performed by X-ray diffractometer (XRD). The model of the X-ray diffractometer is PHILIPS PW1830 type. The measured voltage is 30 KV, the current is 30 mA, the scanning method is a step scan method and the step size is 0.200. The XRD data was analyzed using Jade 6.0 software. The microstructure of the sample was observed by scanning electron microscope (SEM), which model is ZEISS Sigma 500 model.
Preparation of electrodes Firstly, the carbonyl nickel powders were mixed with the CNTs/AB5 composite alloy powders in a mass ratio of 3:1. The mixture having a total mass of about 1.0 g was poured into a steel mold with a diameter of 17 mm and was pressed into a disc. The mixture was pressed with a pressure of 11 tons for 25 min. Then, the disc was completely wrapped with foamed nickel and welded the nickel strip. Finally, the experimental electrode to be tested is obtained.
Measurement of electrochemical performance The measurement of electrochemical performance was carried out using an open three-electrode system at room temperature. The three electrodes are the electrode to be tested, the NiOOH/Ni(OH)2 counter electrode and the Hg/HgO reference electrode, respectively. The electrolyte used in the test was a 6 M KOH solution. Electrochemical performance test instrument and the charging and discharging conditions used in the test were the same with our previous researches [48,49].
Measurement of catalytic performance The measurement of catalytic performance was carried out using an open three-electrode system at room temperature. The three electrodes consist of the electrode to be tested, the nickel foam auxiliary electrode and the Hg/HgO reference electrode. The auxiliary electrode is a 3 cm 3 cm foamed nickel electrode. The electrolyte used in the test was 2 M KOH þ0.1 M KBH4 aqueous solution and 2 M KOH solution, respectively. The experimental electrode used as the DBFC
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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a
b
Fig. 2 e (a) - XRD patterns of the CNTs, AB5 alloy and CNTs/AB5 composite alloys. Fig. 2 (b) - Magnified view of XRD patterns of the AB5 alloy and CNTs/AB5 composite alloys. anode in the electrochemical performance test, the charging and discharging system and test equipment used were the same as the above section of “measurement of electrochemical performance”. The catalytic performance of the anode catalyst
in DBFC was tested by Shanghai Chenhua’s CHI660E B14637 electrochemical workstation. Cyclic volt-ampere characteristics test conditions: scanning speed of 0.05 V/s, sweeping range of 0.6 Ve0.6 V, test temperature is room temperature.
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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Table 1 e The positions of main diffraction peaks of AB5 alloy and CNTs/AB5 composite alloys. Samples
(200)
(111)
(002)
2T (cal) 2T (cor) 2T (cal) 2T (cor) 2T (cal) 2T (cor) AB5 alloy 0.5 h 1h 2h 3h 4h 5h
42.089 42.084 42.017 42.086 42.016 41.965 e
42.054 42.071 42.091 42.111 42.016 42.040 42.062
42.802 42.840 42.721 42.769 42.719 42.693 e
42.823 42.849 42.750 42.765 42.721 42.715 42.426
44.887 e 44.779 44.768 44.774 44.819 e
44.892 45.049 44.780 44.773 44.779 44.882 44.791
“2T(cal)” is the value of 2Theta tested and denotes the diffraction peak position. “2T(cor)”is the value of 2Theta calculated by Jade software and denotes the diffraction peak position.
Chronoamperometrv curves test condition: the constant voltage is 0.95 V.
Results and discussion The XRD patterns of the CNTs, AB5 alloy and CNTs/AB5 composite alloys at different ball milling time are shown in Fig. 2. It can be found from Fig. 2 that the XRD spectra of CNTs only appears two CNTs characteristic diffraction peaks at 26.1 and 43.2 , respectively. The AB5 alloy consists of a single LaNi5 with a hexagonal type CaCu5 (P6/mmm space group). Moreover, all CNTs/AB5 composite alloys also consist of a single LaNi5. It is worth noting that in the XRD patterns of the CNTs/ AB5 composite alloys at different ball milling time, there is no diffraction peak of CNTs. This may be mainly due to the following points: 1) In this work, the mass ratio of CNTs to AB5 alloy in the composite alloys is 1:9. The diffraction peaks of CNTs are not observed in the XRD patterns of the composite alloys, probably due to the small content of CNTs in the composite alloy. 2) The partial diffraction peak position of the AB5 alloy is close to the characteristic peak position of the CNTs, for example, the diffraction peak of CNTs at 43.2 and the (111) diffraction peak of AB5 alloy. When the AB5 alloy and CNTs are subjected to ball milling, their diffraction peaks are broadened, resulting in a severe superposition of diffraction peaks, so that the diffraction peak of CNT is not seen in the CNTs/AB5 composite alloys. Furthermore, the XRD diffraction peaks of CNTs/AB5 composite hydrogen storage alloys shift slightly to low angles
Table 2 e Lattice parameters, unit cell volumes and grain sizes of the AB5 alloy, CNTs/AB5 composite alloys at different ball milling time. Samples
a (nm)
c (nm)
V (nm3)
D (nm)
AB5 alloy milled 0.5 h milled 1 h milled 2 h milled 3 h milled 4 h milled 5 h
0.4955 0.4958 0.4959 0.4960 0.4956 0.4962 0.4959
0.4034 0.4023 0.4041 0.4044 0.4057 0.4054 0.4044
0.0857 0.0856 0.0861 0.0862 0.0862 0.0862 0.0861
553 534 488 475 420 403 417
5
with the extension of ball milling time. For more convenient observation, we made an enlarged XRD pattern [Fig. 2(b)]. At the same time, we have listed positions for the main XRD diffraction peak of AB5 alloy and CNTs/AB5 composite alloys (Table 1). It is speculated that the lattice parameters of alloys tend to increase with the extension of ball milling time. Table 2 shows the lattice parameters, unit cell volumes and grain sizes of CNTs/AB5 composite hydrogen storage alloys at different ball milling time. It can be seen from Table 2, the lattice parameters and unit cell volumes of the composite alloys increase with the extension of the ball milling time. The grain size of alloy phase is roughly estimated by the Scherrer equation. The grain size of the alloy phase significantly reduces with the extension of the ball milling time. In addition, with the extension of the ball milling time, the XRD diffraction peak of the CNTs/AB5 composite hydrogen storage alloy obviously widens, which is caused by the grain refinement or the defect of the alloy caused by ball milling [50,51]. The SEM images of CNTs and CNTs/AB5 composite hydrogen storage alloys are shown in Fig. 3. As shown in Fig. 3(a), the CNTs are elongated tubular structures that are randomly tangled like balls of loose twine and few fractures. It can be seen from Fig. 3(b, d) that the CNTs adhere to the surface of the alloy to form a coating effect on the hydrogen storage alloy. In addition, the elongated CNTs are broken, the length is obviously shortened, and many cocked broken ends and defects appear. Fractures and defects of the CNTs can be clearly seen through Fig. 3(c). Broken and defective CNTs caused by ball milling cover the surface of the alloy, forming a three-dimensional pore structure at the microscopic level, increasing the specific surface area of the alloy. Moreover, with the increase of the ball milling time, the particles of the alloys gradually become smaller, the fracture and defect of CNTs are also gradually increasing, the hollow structure is destroyed. When the ball milling time reached 5 h, it can be seen from Fig. 3(e and f) that the CNTs agglomerate to form a cluster, resulting in the disordered superposition of CNTs and AB5 alloy. In order to analyze the constitution of the CNTs/AB5 composite alloy at different ball milling time, we selected the regions of the CNTs/AB5 composite alloy that were not covered by CNTs for Energy Dispersive Spectrometer (EDS) analysis. The SEM images and EDS patterns of the CNTs/AB5 composite alloy are shown in Fig. 4 and the corresponding data of EDS patterns are listed in Table 3. As shown in Fig. 4, the surface composition of the alloy is uniform, implying that the alloy is composed of one phase. In addition, the data of Table 3 was analyzed by semi-quantitative method. It can be found that the ratio of rare earth to Ni (Co, Mn, Al) is about 1:5. This indicates that the alloy phase in the CNTs/AB5 composite alloy has a single LaNi5 phase. It can be seen that the alloy phase in the composite alloy is only LaNi5 phase. It is consistent with the previous XRD analysis results. Fig. 5 shows the discharge characteristics of CNTs, AB5 alloy and the CNTs/AB5 composite alloys in 6 M KOH solution. The discharge capacities of the alloy electrodes largely determine the width of the discharge platform and the discharge time. The uniformity of the electrode and the activity of the surface determine the flatness of the discharge platform [52]. It can be seen from Fig. 5 that the discharge
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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Fig. 3 e The SEM images of CNTs and CNTs/AB5 composite alloys: (a) CNTs; (b) milled 2 h; (c) the fractures of the CNTs for 2 h; (d) milled 3 h; (e) milled 5 h; (f) CNTs have agglomerated for 5 h.
characteristic of CNTs is relatively poor, its discharge time is short and the discharge median potential (Vmid) value is low. Compared with AB5 alloy, the discharge characteristic of the CNTs/AB5 composite alloy is significantly improved. Furthermore, the ball milling time has a significant effect on the discharge characteristic of the CNTs/AB5 composite alloy. With the increase of the ball milling time, the discharge characteristic of the CNTs/AB5 composite alloy electrode becomes better first and then deteriorates significantly. When the ball milling time reaches 2 h, the discharge plateau of the alloy is the widest, the discharge time is the longest, and the Vmid value is relatively higher. The proper ball milling time (2 h), the slender CNTs are cut, the fractures and defects are increased, which is beneficial to the hydrogen in and out. Hydrogen adsorbed on the surface of CNTs is easier to diffuse to the surface of the AB5 hydrogen storage alloy and then enter the interior of the alloy, thereby increasing the rate of charge transport on the electrode surface and ameliorating of hydrogen storage capacity [53]. In addition, the CNTs overlying of the alloy not only may prevent oxidation and corrosion of the alloy particles, but also facilitate the contact between the hydrogen and the alloy particles, play a conductive and catalytic role, and contribute to the
improvement of the discharge capacity and discharge time. However, as the ball milling time is further extended (3 h), the discharge performance of the alloy electrode is obviously deteriorated, as in the previous XRD and SEM analysis, mainly owing to the destruction of hollow tubular structure of the carbon nanotube caused by long-time ball milling, and the agglomeration phenomenon occurs. The CNTs cannot be uniformly dispersed and coated in the AB5-type alloy. The CNTs does not promote the hydrogen storage of the hydrogen storage alloy, and thus the discharge characteristics of the alloy remarkably deteriorated. The discharge characteristic curve of CNTs, AB5 alloy and CNTs/AB5 composite alloys in a solution of 2 M KOH þ 0.1 M KBH4 are displayed in Fig. 6 and the corresponding electrochemical properties for the experimental alloys are listed in Table 4. The CNTs have almost no discharge platform, and the potential reaches a minimum in a short time and the potential drops rapidly, indicating that it has almost no catalytic oxidation ability to potassium borohydride. However, the AB5 alloy and the CNTs/AB5 composite alloys exhibit better discharge performance in a solution of 2 M KOH þ0.1 M KBH4. Compared with the discharge time of the AB5 alloy and the CNTs/AB5 composite alloys in 6 M KOH solution (As displayed in Fig. 5),
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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Fig. 4 e SEM images and EDS patterns of the CNTs/AB5 composite alloys: (a) 2 h; (b) 3 h; (c) 4 h; (d) 5 h; (e) 2 h; (f) 3 h; (g) 4 h; (h) 5 h.
the discharge time of the AB5 alloy and the CNTs/AB5 composite alloys in the 2 M KOH þ 0.1 M KBH4 mixed solution was increased by about 6 times. This hints that the AB5 alloy and the CNTs/AB5 composite hydrogen storage alloys have obvious catalytic oxidation ability on KBH4. In addition, it can also be found from Fig. 6 that the discharge characteristic curve of the alloy is greatly affected by the ball milling time. As the extension of the ball milling time, the discharge platform of the alloys first becomes wider and then narrow, and the discharge median potential (Vmid) of the alloys increases first and then decreases, and the slope of the discharge characteristic curve of the alloys first decreases and then increases. When the ball
milling time reaches 2 h, the CNTs/AB5 composite alloy exhibits the best discharge characteristic. Comparing Fig. 5 with Fig. 6, it can be seen that the law of the discharge characteristic of the CNTs/AB5 composite alloys in 6 M KOH solution is almost consistent with that in the 2 M KOH þ0.1 M KBH4 solution. This speculated that the CNTs/AB5 composite alloys with good hydrogen storage electrochemical performance have better catalytic oxidation ability on KBH4. As mentioned in Fig. 1 above, this is closely related to the hydrogen storage alloy which can effectively inhibit the hydrolysis reaction. Fig. 7 displays the cyclic voltammetry (CV) curves of CNTs, AB5 alloy and CNTs/AB5 composite alloys in 2 M KOH þ 0.1 M
Table 3 e The corresponding data of EDS patterns of the CNTs/AB5 composite alloys. Samples
2 3 4 5
h h h h
Elements (at.%)
(Ni, Co, Mn, Al):(La, Ce)
Ce
La
Ni
Co
Mn
Al
12.33 11.64 11.17 11.50
3.79 3.92 3.68 3.24
59.32 58.05 60.62 58.92
10.29 13.10 13.54 10.95
6.27 11.61 9.64 6.49
7.99 1.68 1.35 8.91
5.20 5.42 5.73 5.78
: : : :
1 1 1 1
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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Discharge potential ( V vs.-Hg/HgO)
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1.0
0.9
0.8
CNTs AB5 alloy 0.5 h 1h 2h 3h 4h 5h
0.7
0.6 0
200
400
600
800
1000
1200
1400
1600
Time (min) Fig. 5 e Discharge characteristics of the CNTs, AB5 alloy and CNTs/AB5 composite alloys at different ball milling time in 6 M KOH.
KBH4 solutions. The peak potential of the oxidation peak and the corresponding current density of the alloy are listed in Table 5. From Fig. 7 and Table 5 that the CV curve of pure CNTs has no oxidation peak, indicating that pure CNTs has almost no catalytic oxidation ability on ВН 4 . This is consistent with the analysis in Fig. 6. However, the CV curves of the AB5 alloy and the CNTs/AB5 composite alloys have obviously oxidation peak. The CV curves of the CNTs/AB5 composite alloys with different ball milling time are similar, obvious oxidation peaks appear during the scanning process, and all of them are irreversible oxidation peaks, indicating that the AB5 alloy and the CNTs/AB5 composite alloys have an effective catalytic effect on KBH4. The catalytic mechanism of the CNTs/AB5 composite alloys is similar, which also shows that the oxidation reaction of ВН 4 on the surface of the composite alloys is also similar. However, the peak potential and the peak current density in the CV curves of the AB5 alloy and the CNTs/AB5 composite alloys are different. It can be seen from Table 5 that the oxidation peak potential of the alloys decreases first and then increases, and the current density of oxidation peak of the alloys first increases and then decreases with the increase of the ball milling time. Generally speaking, the higher the oxidation peak (peak current density), the more negative the corresponding peak potential, indicating that the catalytic performance of the catalyst is better [54,55]. In this work,
Table 4 e The main electrochemical properties for the experimental alloys. Samples
The maximum discharge time (min)
Vmid (V)
CNTs AB5 alloy 0.5 h 1h 2h 3h 4h 5h
212 1216 1198 1217 1534 1356 1123 876
0.8643 0.8767 0.8668 0.8955 0.9040 0.9090 0.8885 0.7406
Fig. 6 e Discharge characteristics of the CNTs, AB5 alloy and the CNTs/AB5 composite alloys in 2 M KOH þ0.1 M KBH4 mixed solution.
when the ball milling time reaches 2 h, the current density of the oxidation peak of the alloy reaches a maximum of 82.533 mA cm2, and the oxidation peak potential reaches a minimum of 0.065 V (vs Hg/HgO), indicating that it has the best catalytic oxidation ability for ВН 4 . This further verified that the hydrogen storage alloys with good hydrogen storage electrochemical performance has better catalytic oxidation ability on KBH4. Combined with the previous analysis, when the composite alloy milled 2 h with good electrochemically performance participates in the electrochemical reaction, it has a strong binding force to the surface-adsorbed hydrogen atom Hads (Such as process 4), thereby inhibiting the hydrolysis side reaction (Such as process 3). Then, after the process 5 and the process 6, the hydrogen is oxidized again, so that the number of electrons released is increased, which can be effectively improved the utilization rate of the fuel. The comprehensive comparison shows that the catalytic oxidation ability of the composite alloys on ВН 4 is ranked as: 2 h > 1 h > 3 h > 0.5 h > 4 h > 5 h > AB5 alloy > CNTs. In order to comparing the catalytic properties of the CNTs, AB5 alloy and CNTs/AB5 composite alloys to borohydride, we tested the CV of all the samples in 2 M KOH. Fig. 8 shows the cyclic voltammetry (CV) curves of CNTs, AB5 alloy and CNTs/ AB5 composite alloys in 2 M KOH solutions. For the sake of clarity, the peak potential of the oxidation peak and the corresponding current density of the samples are listed in Table 5. From Fig. 8 and Table 5, we can see that both CNTs and AB5 alloy have no oxidation peaks, while the CNTs/AB5 composite alloys have oxidation peaks, indicating that CNTs and AB5 alloy have no catalytic effect in 2 M KOH solution, while the CNTs/AB5 composite alloys have catalytic effect in 2 M KOH solution. And with the increase of ball milling time, the peak potential of the CNTs/AB5 composite alloy has no obvious law, while the peak current density of the CNTs/AB5 composite alloy decreases first and then increases. Overall, the CNTs/AB5 composite alloy milled 2 h exhibits the smaller peak potential and the highest peak current density. Hinting
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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a b c d e f g h
100 b
Current density (mA cm-2)
80 e 60 d
40
f c
h
g
20
a
CNTs AB5 alloy 0.5 h 1h 2h 3h 4h 5h
0 -20 -40 -60 -80 -100 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Potential (V vs.-Hg/HgO) Fig. 7 e CV curves of the CNTs, AB5 alloy and the CNTs/AB5 composite alloys in 2 M KOH þ0.1 M KBH4 mixed solution. that the CNTs/AB5 composite alloy milled 2 h has the best catalytic performance. Furthermore, comparing with the CV curves of all samples in 2 M KOH solution (Fig. 8) and in 2 M KOH þ 0.1 M KBH4 mixed solution (Fig. 7), we find that CNTs have no oxidation peaks in both solutions, while the AB5 alloy has an oxidation peak in the 2 M KOH þ 0.1 M KBH4 and no oxidation peak in 2 M KOH solution. The CNTs/AB5 composite alloys have oxidation peaks in both solutions. This conclusion is the same as Zhang et al. [45]. Comparing with 2 M KOH solution, the CNTs/AB5 composite alloy exhibits the smaller peak potential and the higher peak current density. It indicated that the CNTs/AB5 composite alloys have catalytic effect in 2 M KOH solution, but they have the better catalytic effect in 2 M KOHþ0.1 M KBH4 solution. Moreover, the compounding of CNTs and AB5 alloy can effectively improve the catalytic activity of AB5 alloy. In general, the CNTs/AB5 composite alloy milled 2 h has the best
catalytic activity not only in 2 M KOH solution but also in 2 M KOHþ0.1 M KBH4 solution. Generally, the chronoamperometry (CA) is a relationship of the oxidation current of borohydride as a function of time. The CA curve can reflect the stability of the catalyst during the catalytic process. The number of transferred electrons during electrocatalytic oxidation can also be calculated from the CA curve and the Cottrell equation [55]. Fig. 9 is CA curves of CNTs, AB5 alloy and CNTs/AB5 composite alloys in 2 M KOH þ0.1 M KBH4 with a constant voltage of 0.95 V. Corresponding to Fig. 9, the steady current density and the number of transferred electron of the alloy samples are listed in Table 6. The higher current density exhibits the better catalytic performance [55,56]. As can be found from Fig. 9 and Table 6, the current density of the CNTs begins to decrease rapidly and then tends to stabilize. While the composite alloy, after activation, has a rapid decrease in current density and then tends to be stable. In addition, as the ball milling time extends, the current density of the composite alloy increases first and then decreases. The CNTs/AB5 composite alloy milled 2 h has the highest current density value of 5.8194 mA cm2. It is shown that the CNTs/AB5 composite alloy milled 2 h has the best catalytic performance. It can be seen from Table 6 that the number of electrons transferred increases first and then decreases with the extension of ball milling time. When the ball milling time is 2 h, the number of electrons transferred is the most, which is 1.31. This result is comparable to the results of the literature [55]. When the noble metal Au/C was used as anode catalyst for DBFC in the literature [55], the number of electrons transferred is about 1.1 during the catalytic oxidation process. It indicated that the catalytic performance of the CNTs/AB5 composite hydrogen storage alloy milled for 2 h on ВН 4 can be compared with that of precious metal Au/C. Moreover, it is inferred that the better the electrochemical performance of the hydrogen storage alloy, the more electrons are transferred. By comparing the electrochemical performance and catalytic performance of CNTs/AB5 composite alloy on ВН 4 , it can be found that the better the hydrogen storage performance, the better the catalytic performance of the alloy for
Table 5 e Peak potential and peak current density of the CNTs, AB5 alloy and CNTs/AB5 composite alloys. Peak current density (mA cm2)
2 M 2 M KOH þ 0.1 M 2 M 2 M KOH þ 0.1 M KOH KBH4 KOH KBH4 CNTs AB5 alloy milled 0.5 h milled 1h milled 2h milled 3h milled 4h milled 5h
e e 0.175
e 0.135 0.095
e e 67.599
e 57.449 64.207
0.245
0.065
78.887
72.533
0.185
0.065
79.427
82.533
0.225
0.105
64.559
72.137
0.335
0.255
26.079
62.321
0.215
0.115
31.189
61.120
100
Current density (mA cm-2)
Samples Peak potential (V vs Hg/ HgO)
120
80 60 40 20
a b c d e f g h
CNTs AB5 alloy 0.5 h 1h 2h 3h 4h 5h
e
d
c f g
0 -20 a
-40
b
-60 -80 -100 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Potantial / (V vs.-Hg/HgO) Fig. 8 e CV curves of CNTs, AB5 alloy and CNTs/AB5 composite alloys in 2 M KOH solution.
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
10
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borohydride, that means that the electrochemical performance of the hydrogen storage alloy is positively correlated with the catalytic performance as anode catalyst in a DBFC.
Conclusions In this work, the CNTs/AB5 composite hydrogen storage alloys at different milling time are prepared. The electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys under different milling time were studied. And we also discussed the catalytic properties of CNTs/AB5 composite hydrogen storage alloys used as anodic catalysts in DBFC. Through the experimental conclusion, we analyzed the correlation between electrochemical properties of CNTs/AB5 composite hydrogen storage alloys and their catalytic properties on KBH4. The main results obtained were as following:
Fig. 9 e (a) - CA curves of the CNTs, AB5 alloy and the CNTs/ AB5 composite alloys in 2 M KOH þ 0.1 M KBH4 mixed solution. Fig. 9 (b) - Partial enlargement of CA curves of the CNTs, AB5 alloy and the CNTs/AB5 composite alloys in 2 M KOH þ 0.1 M KBH4 mixed solution.
1) The CNTs/AB5 composite hydrogen storage alloys have a single CaCu5 type LaNi5 hexagonal crystal structure. As the ball milling time extends, the diffraction peak of the composite alloy gradually shifts to a low angle. On the whole, the unit cell volumes gradually increase and the grain sizes decrease. The CNTs were elongated tubular structures. When the ball milling time reaches 2 h, the CNTs adhere to the surface of the AB5-type alloy to form a coating. While with the further extension of the milling time, the AB5 alloy particles become smaller, and the hollow tubular structure of the CNTs is gradually damaged, agglomeration occurs, and amorphous carbon is formed, and the two are distributed in disorder. 2) The electrochemical property of CNTs/AB5 composite alloys get better and then get worse with the extension of ball milling time. At the same time, when the CNTs/AB5 composite hydrogen storage alloy is used as anode catalyst for DBFC, the catalytic performance on KBH4 also get better and then get worse with the extension of ball milling time. When ball milling time reaches 2 h, the CNTs/AB5 composite alloys show the best electrochemical performance and catalytic performance on KBH4. From experimental data we know that the better the electrochemical performance of hydrogen storage alloy, the better the catalytic performance on KBH4.
Acknowledgements Table 6 e The steady current densities and the number of transferred electrons of the CNTs, AB5 alloy and the CNTs/AB5 composite alloys. Samples CNTs AB5 alloy milled 0.5 h milled 1 h milled 2 h milled 3 h milled 4 h milled 5 h
Current density（mA cm2）
Number of electrons (n)
0.0092 0.0096 þ3.4031 þ4.3062 þ5.8194 þ4.2202 þ4.0242 þ3.6674
e e 0.77 0.97 1.31 0.94 0.91 0.83
This work is supported by Inner Mongolia Autonomous Region Higher Education Science and Technology Research Foundation (NJZZ17040), Natural Science Foundation of Inner Mongolia Autonomous Region (2019MS05002), National Natural Science Foundation of China (51661027).
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4 Jing Yan, Xiao Tian*, Xin-Yu Liu, Xuan Zhao, Rui Wang, Li-Juan Zhao, Xin Zhang Inner Mongolia Key Laboratory for Physics and Chemistry of Functional Materials, School of Physics and Electronic Information, Inner Mongolia Normal University, Hohhot, 010022, People’s Republic of China
highlights The CNTs/AB5 composite alloys are prepared by arc smelting and ball milling. Hydrogen storage alloy can be used as anode catalyst for direct borohydride fuel cell. The catalytic properties of the alloy are related to its electrochemical properties.
article info
abstract
Article history:
Searching for non-precious metal anode catalysts with high catalytic activity and capable
Received 18 July 2019
of inhibiting hydrolysis side reactions is very important for direct borohydride fuel cell
Received in revised form
(DBFC). In this work, the as-cast AB5 alloy powders are firstly mixed with CNTs in a ratio of
13 October 2019
1:9. Then the mixture of AB5 alloy and CNTs is ball milled in different milling time. Finally,
Accepted 20 October 2019
the CNTs/AB5 composite alloys are obtained. Not only the catalytic properties of the CNTs/
Available online xxx
AB5 composite alloys used as anode catalysts in DBFC, but also the electrochemical properties of the alloys have been investigated in detail. The research results indicate that,
Keywords:
as the ball milling time is extended, the electrochemical properties and catalytic properties
Direct borohydride fuel cell (DBFC)
on ВН 4 of the CNTs/AB5 composite alloys become better first and then worsen. The CNTs/
AB5-type hydrogen storage alloy
AB5 alloy milled 2 h exhibits the best electrochemical properties and catalytic properties.
Carbon nanotube (CNTs)
Furthermore, we predict that the electrochemical properties of the composite alloy are
Catalytic properties
positively correlated with the catalytic properties as anode catalyst for DBFC.
Electrochemical properties
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the depletion of traditional fossil fuels, the development of new energy sources has become an urgent problem to be solved today [1e4]. Hydrogen energy has attracted much attention because of its cleanliness, abundant reserves, and wide range of sources [1,2,5e9]. However, safety issues such
as storage and transportation problems during hydrogen utilization have been plagued [10e12]. Direct borohydride fuel cell (DBFC) is a new technology that combines hydrogen energy and fuel cells, because it can convert chemical energy, which stored in a borohydride (ВН 4 ), directly into electrical energy under the anode catalyst [13,14]. Borohydride is the fuel of the DBFC, because the solid borohydride itself contains
* Corresponding author. E-mail address: [email protected] (X. Tian). https://doi.org/10.1016/j.ijhydene.2019.10.158 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
2
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very high hydrogen (10.6 wt% in solid state). The DBFC not only avoids the difficulty of hydrogen storage and transportation during hydrogen utilization, but also has a high theoretical output voltage and high power density, considered as an energy conversion device with development potential [15e19]. However, in the operation of this battery, hydrogen is generated due to the hydrolysis reaction at the anode, which greatly reduces the efficiency of the battery [20,21]. This can be found by a series of chemical reactions occurring during the energy conversion process of the DBFC. The chemical reaction that occurs when the DBFC is working: Anode: BH 4 þ 8OH /BO2 þ 6H2 O þ 8e o Ea ¼ 1:24 V vs: SHE
(1)
Cathode: O2 þ 4H2 O þ 8e /8OH Eoc ¼ 0:4 V vs: SHE
(2)
Overall: BH 4 þ 2O2 / BO2 þ 2H2 O
ðEo ¼ 1:64 VÞ
(3)
Ideally, an electrochemical reaction occurs when the DBFC is operating, and 8e can be released. However, due to the hydrolysis reaction occurring on the anode during the operation of the DBFC, the consumption of electrons generates hydrogen. Therefore, the number of electrons released is usually less than 8e, resulting in a great reduction in the energy efficiency of the fuel cell. The specific hydrolysis reaction equation: BH 4 þ 2H2 O/BO2 þ 4H2
(4)
Therefore, the actual anode reaction is as follows: BH 4 þ xOH /BO2 þ ðx 2ÞH2 O þ ð4 xÞH2 þxe
(5)
In Eq. (5), x means the number of electrons, which is released by each ВН 4 during the actual reaction. The less hydrogen produced, the more electrons released. It can be seen that the anode catalyst in DBFC is not only a key factor affecting the electrochemical oxidation reaction, but also a key factor affecting the hydrolysis reaction. Most of the previous studies used precious metals or complex precious metals [22e27] as anode catalysts for DBFC. However, precious metal catalysts are expensive and severely restrict their widespread use. It has also been studied to use transition metal or transition metal composites [28e32] as anode catalysts for DBFC. It has been found that transition metals have low catalytic activity for ВН 4 in the DBFC and are prone to hydrolysis. Considering the cost of the anode catalyst, the problems of catalytic oxidation and hydrolysis, researchers have proposed the idea that hydrogen storage material is used as anode catalyst of DBFC in recent years. If so, the anode catalyst cost is greatly reduced, but the hydrogen storage reaction can effectively inhibit the hydrolysis reaction in the anode catalysis process, the number of electrons transferred is increased, and improve the fuel efficiency of the battery [33e36]. Wang et al. [37] used the AB5type LmNi4.78Al0.22 alloy instead of the noble metal as anode catalyst for the DBFC. The experimental results show that the alloy has high electrochemical catalytic activity during
discharge process and the hydrogen generation process. And the utilization of the fuel cell increase with increasing discharge current density. Subsequently, Wang et al. [38] doped Si element in AB5-type LmNi4.78Mn0.22 hydrogen storage alloy by ball milling and heat treatment methods. They use the composite hydrogen storage alloy as anode catalyst for DBFC. The study shows that after the modification of the AB5type alloy, as anode catalyst for DBFC, its utilization efficiency is increased from 21.37% to 95.27% at low discharge current. Some researchers also use AB2 hydrogen storage alloy as anode catalyst for DBFC. Choudhury et al. [39] used Zr0.9Tio.1Mn0.6Cr0.05Co0.05Ni1.2 alloy as anode catalyst of DBFC. The study demonstrates that at a temperature of 70 C, the power density reaches a maximum of 150 mW cm2. The study of hydrogen storage material as anode catalyst of DBFC has gradually begun to be concerned. In order to explain the catalytic mechanism of the hydrogen storage alloy for ВН 4 in detail, we have designed a schematic diagram. Fig. 1 is schematic diagram οf catalytic mechanism of hydrogen storage alloy on ВН 4 . We can obtain the catalytic mechanism of the hydrogen storage alloy on ВН 4 through Fig. 1. During the oxidation process of ВН 4 , the hydrogen in the ВН 4 is oxidized from a negative monovalent to a positive valence, which is can divided into two processes: Firstly, the BeH bond of ВН 4 is broken, and the negative monovalent hydrogen is oxidized to zero valence. The surface-adsorbed hydrogen atom (Hads) (Such as process 1) is formed, and then the surface-adsorbed hydrogen atom (Hads) continues to be oxidized to a positive monovalent (Such as process 2), i.e., the hydrogen direct oxidation process, corresponding to the reaction equation (1). Secondly, the surfaceadsorbed hydrogen atom (Hads) is also combined to form H2 (Such as process 3), which is the side reaction (hydrolysis reaction) of the ВН 4 oxidation process, corresponding to the reaction equation (4). It can be seen that the excellent property of the anode catalyst requires high catalytic activity for the processes 1 and 2, but has a strong inhibitory effect on the side reaction (Such as process 3). Generally, noble metal catalysts have good catalytic activity for processes 1e2, but their side reactions are difficult to eliminate. For the hydrogen storage alloy, it has a strong binding force to the hydrogen atom (Hads) adsorbed on the surface (Such as process 4, the hydrogen absorption can be stored in the crystal lattice of the hydrogen storage alloy, it means that the hydrogen storage alloy realized hydrogen storage function). The inhibition of side reaction (Such as process 3) is achieved by the chemical reaction of
Fig. 1 e Schematic diagram of catalytic mechanism of hydrogen storage alloy on ВН¡ 4.
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the process 4. Since the hydrogen storage alloy is reversible, the hydrogen absorbed into the hydrogen storage alloy crystal lattice can be released again, completing the process 5. Then, the surface-adsorbed hydrogen atom (Hads) continues to be oxidized to a positive monovalent (Such as process 6, similar to process 2). These will increase the number of electrons released, thereby improve fuel utilization. Corresponding to the following reaction equation: MHx þ xOH %M þ xH2 O þ xe
(6)
It can be found that the hydrogen storage material inhibits the hydrolysis side reaction and the re-oxidation process is closely relative to the hydrogen storage electrochemical property of the alloy. However, the relationship between the hydrogen storage electrochemical property of hydrogen storage material and the catalytic property of hydrogen storage material as anode catalysts for DBFC has rarely been reported systematically. The rare earth base AB5-type hydrogen storage alloy is the main negative electrode material of MH/Ni battery which has been commercialized, and its excellent comprehensive electrochemical performance has been recognized [40,41]. CNTs have excellent electrical and thermal conductivity, anti-alkali and anti-oxidation properties, and have good hydrogen storage function due to the special hollow structure of CNTs, and are inexpensive, which is an ideal additive for hydrogen storage anode alloy [42e44]. In order to improve the electrocatalytic activity and NaBH4 utilization, Zhang et al. [45] used a multi-walled carbon nanotubes (MWNTs, 2 wt%) modified AB5 hydrogen storage alloy (MmNi0.58Co0.07 Mn0.04Al0.02) as anode catalytic for the DBFC. The study found that the current density was twice that using the AB5/MWNTs (2 wt%). And they also discovered that use MWNTs to modify the working electrode of AB5 hydrogen storage alloy, the utilization efficiency of NaBH4 was 61.5% higher than that of the original. MWNTs not only enhance the electrocatalytic activity of the alloy and the fuel utilization, and italso absorbs hydrogen to reduce the release of hydrogen. In addition, Li et al. [46] used a mixture of carbon nanotubes (CNTs) and AB5-type hydrogen storage alloy as anode catalyst for DBFC, and they test at room temperature. The CNTs mixture hydrogen storage alloy has current density of up to 1550 mAcm2 and a maximum power density of 65 mWcm2 compared to a series of conventional carbon materials. And the DBFC could keep relatively good short-term performance stability. The CNTs/AB5 composite alloys are synthesized by ball milling method. The structure, electrochemical performance and catalytic performance for ВН 4 of CNTs/AB5 composite hydrogen storage alloys under different milling time were studied. The relationship of electrochemical properties of CNTs/ AB5 composite hydrogen storage alloys and their catalytic properties for borohydrides are speculated.
Experimental Preparation of samples According to MmNi3.55Co0.75Mn0.4Al0.3 (Mm is composed of a variety of rare earth elements, of which Ce is 65.25%, La is
3
25.20%, Pr is 8.43%, Nd is 1.12%), the as-cast AB5-type MmNi3.55Co0.75Mn0.4Al0.3 alloy was prepared by vacuum arc furnace. CNTs (purity >99.9%) were produced by Shanghai Yao Tian Nano Material Co., Ltd. The CNTs and AB5 hydrogen storage alloy were mechanically ball milled using a pulverisette-6 single bowl star high energy ball mill manufactured by FRITSCH, Germany. Firstly, the CNTs was mixed with the AB5-type MmNi3.55Co0.75Mn0.4Al0.3 alloy at the mass ratio of 1:9 [47]. The mixture of the CNTs/AB5-type alloy powders are placed in a 400 ml ball mill jar. In order to prevent the experimental sample from oxidizing during the ball milling process, the ball milling jar containing the samples was placed in a glove box, flushed with argon gas and vacuumed repeatedly. Ball milling conditions: ball-to-power weight ratio is 4:1, rotation speed of 200 r/min, ball milling time of 0.5 h, 1 h, 2 h, 3 h, 4 h and 5 h.
Characterization of microstructure The phase structure analysis of alloy sample was performed by X-ray diffractometer (XRD). The model of the X-ray diffractometer is PHILIPS PW1830 type. The measured voltage is 30 KV, the current is 30 mA, the scanning method is a step scan method and the step size is 0.200. The XRD data was analyzed using Jade 6.0 software. The microstructure of the sample was observed by scanning electron microscope (SEM), which model is ZEISS Sigma 500 model.
Preparation of electrodes Firstly, the carbonyl nickel powders were mixed with the CNTs/AB5 composite alloy powders in a mass ratio of 3:1. The mixture having a total mass of about 1.0 g was poured into a steel mold with a diameter of 17 mm and was pressed into a disc. The mixture was pressed with a pressure of 11 tons for 25 min. Then, the disc was completely wrapped with foamed nickel and welded the nickel strip. Finally, the experimental electrode to be tested is obtained.
Measurement of electrochemical performance The measurement of electrochemical performance was carried out using an open three-electrode system at room temperature. The three electrodes are the electrode to be tested, the NiOOH/Ni(OH)2 counter electrode and the Hg/HgO reference electrode, respectively. The electrolyte used in the test was a 6 M KOH solution. Electrochemical performance test instrument and the charging and discharging conditions used in the test were the same with our previous researches [48,49].
Measurement of catalytic performance The measurement of catalytic performance was carried out using an open three-electrode system at room temperature. The three electrodes consist of the electrode to be tested, the nickel foam auxiliary electrode and the Hg/HgO reference electrode. The auxiliary electrode is a 3 cm 3 cm foamed nickel electrode. The electrolyte used in the test was 2 M KOH þ0.1 M KBH4 aqueous solution and 2 M KOH solution, respectively. The experimental electrode used as the DBFC
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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a
b
Fig. 2 e (a) - XRD patterns of the CNTs, AB5 alloy and CNTs/AB5 composite alloys. Fig. 2 (b) - Magnified view of XRD patterns of the AB5 alloy and CNTs/AB5 composite alloys. anode in the electrochemical performance test, the charging and discharging system and test equipment used were the same as the above section of “measurement of electrochemical performance”. The catalytic performance of the anode catalyst
in DBFC was tested by Shanghai Chenhua’s CHI660E B14637 electrochemical workstation. Cyclic volt-ampere characteristics test conditions: scanning speed of 0.05 V/s, sweeping range of 0.6 Ve0.6 V, test temperature is room temperature.
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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Table 1 e The positions of main diffraction peaks of AB5 alloy and CNTs/AB5 composite alloys. Samples
(200)
(111)
(002)
2T (cal) 2T (cor) 2T (cal) 2T (cor) 2T (cal) 2T (cor) AB5 alloy 0.5 h 1h 2h 3h 4h 5h
42.089 42.084 42.017 42.086 42.016 41.965 e
42.054 42.071 42.091 42.111 42.016 42.040 42.062
42.802 42.840 42.721 42.769 42.719 42.693 e
42.823 42.849 42.750 42.765 42.721 42.715 42.426
44.887 e 44.779 44.768 44.774 44.819 e
44.892 45.049 44.780 44.773 44.779 44.882 44.791
“2T(cal)” is the value of 2Theta tested and denotes the diffraction peak position. “2T(cor)”is the value of 2Theta calculated by Jade software and denotes the diffraction peak position.
Chronoamperometrv curves test condition: the constant voltage is 0.95 V.
Results and discussion The XRD patterns of the CNTs, AB5 alloy and CNTs/AB5 composite alloys at different ball milling time are shown in Fig. 2. It can be found from Fig. 2 that the XRD spectra of CNTs only appears two CNTs characteristic diffraction peaks at 26.1 and 43.2 , respectively. The AB5 alloy consists of a single LaNi5 with a hexagonal type CaCu5 (P6/mmm space group). Moreover, all CNTs/AB5 composite alloys also consist of a single LaNi5. It is worth noting that in the XRD patterns of the CNTs/ AB5 composite alloys at different ball milling time, there is no diffraction peak of CNTs. This may be mainly due to the following points: 1) In this work, the mass ratio of CNTs to AB5 alloy in the composite alloys is 1:9. The diffraction peaks of CNTs are not observed in the XRD patterns of the composite alloys, probably due to the small content of CNTs in the composite alloy. 2) The partial diffraction peak position of the AB5 alloy is close to the characteristic peak position of the CNTs, for example, the diffraction peak of CNTs at 43.2 and the (111) diffraction peak of AB5 alloy. When the AB5 alloy and CNTs are subjected to ball milling, their diffraction peaks are broadened, resulting in a severe superposition of diffraction peaks, so that the diffraction peak of CNT is not seen in the CNTs/AB5 composite alloys. Furthermore, the XRD diffraction peaks of CNTs/AB5 composite hydrogen storage alloys shift slightly to low angles
Table 2 e Lattice parameters, unit cell volumes and grain sizes of the AB5 alloy, CNTs/AB5 composite alloys at different ball milling time. Samples
a (nm)
c (nm)
V (nm3)
D (nm)
AB5 alloy milled 0.5 h milled 1 h milled 2 h milled 3 h milled 4 h milled 5 h
0.4955 0.4958 0.4959 0.4960 0.4956 0.4962 0.4959
0.4034 0.4023 0.4041 0.4044 0.4057 0.4054 0.4044
0.0857 0.0856 0.0861 0.0862 0.0862 0.0862 0.0861
553 534 488 475 420 403 417
5
with the extension of ball milling time. For more convenient observation, we made an enlarged XRD pattern [Fig. 2(b)]. At the same time, we have listed positions for the main XRD diffraction peak of AB5 alloy and CNTs/AB5 composite alloys (Table 1). It is speculated that the lattice parameters of alloys tend to increase with the extension of ball milling time. Table 2 shows the lattice parameters, unit cell volumes and grain sizes of CNTs/AB5 composite hydrogen storage alloys at different ball milling time. It can be seen from Table 2, the lattice parameters and unit cell volumes of the composite alloys increase with the extension of the ball milling time. The grain size of alloy phase is roughly estimated by the Scherrer equation. The grain size of the alloy phase significantly reduces with the extension of the ball milling time. In addition, with the extension of the ball milling time, the XRD diffraction peak of the CNTs/AB5 composite hydrogen storage alloy obviously widens, which is caused by the grain refinement or the defect of the alloy caused by ball milling [50,51]. The SEM images of CNTs and CNTs/AB5 composite hydrogen storage alloys are shown in Fig. 3. As shown in Fig. 3(a), the CNTs are elongated tubular structures that are randomly tangled like balls of loose twine and few fractures. It can be seen from Fig. 3(b, d) that the CNTs adhere to the surface of the alloy to form a coating effect on the hydrogen storage alloy. In addition, the elongated CNTs are broken, the length is obviously shortened, and many cocked broken ends and defects appear. Fractures and defects of the CNTs can be clearly seen through Fig. 3(c). Broken and defective CNTs caused by ball milling cover the surface of the alloy, forming a three-dimensional pore structure at the microscopic level, increasing the specific surface area of the alloy. Moreover, with the increase of the ball milling time, the particles of the alloys gradually become smaller, the fracture and defect of CNTs are also gradually increasing, the hollow structure is destroyed. When the ball milling time reached 5 h, it can be seen from Fig. 3(e and f) that the CNTs agglomerate to form a cluster, resulting in the disordered superposition of CNTs and AB5 alloy. In order to analyze the constitution of the CNTs/AB5 composite alloy at different ball milling time, we selected the regions of the CNTs/AB5 composite alloy that were not covered by CNTs for Energy Dispersive Spectrometer (EDS) analysis. The SEM images and EDS patterns of the CNTs/AB5 composite alloy are shown in Fig. 4 and the corresponding data of EDS patterns are listed in Table 3. As shown in Fig. 4, the surface composition of the alloy is uniform, implying that the alloy is composed of one phase. In addition, the data of Table 3 was analyzed by semi-quantitative method. It can be found that the ratio of rare earth to Ni (Co, Mn, Al) is about 1:5. This indicates that the alloy phase in the CNTs/AB5 composite alloy has a single LaNi5 phase. It can be seen that the alloy phase in the composite alloy is only LaNi5 phase. It is consistent with the previous XRD analysis results. Fig. 5 shows the discharge characteristics of CNTs, AB5 alloy and the CNTs/AB5 composite alloys in 6 M KOH solution. The discharge capacities of the alloy electrodes largely determine the width of the discharge platform and the discharge time. The uniformity of the electrode and the activity of the surface determine the flatness of the discharge platform [52]. It can be seen from Fig. 5 that the discharge
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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Fig. 3 e The SEM images of CNTs and CNTs/AB5 composite alloys: (a) CNTs; (b) milled 2 h; (c) the fractures of the CNTs for 2 h; (d) milled 3 h; (e) milled 5 h; (f) CNTs have agglomerated for 5 h.
characteristic of CNTs is relatively poor, its discharge time is short and the discharge median potential (Vmid) value is low. Compared with AB5 alloy, the discharge characteristic of the CNTs/AB5 composite alloy is significantly improved. Furthermore, the ball milling time has a significant effect on the discharge characteristic of the CNTs/AB5 composite alloy. With the increase of the ball milling time, the discharge characteristic of the CNTs/AB5 composite alloy electrode becomes better first and then deteriorates significantly. When the ball milling time reaches 2 h, the discharge plateau of the alloy is the widest, the discharge time is the longest, and the Vmid value is relatively higher. The proper ball milling time (2 h), the slender CNTs are cut, the fractures and defects are increased, which is beneficial to the hydrogen in and out. Hydrogen adsorbed on the surface of CNTs is easier to diffuse to the surface of the AB5 hydrogen storage alloy and then enter the interior of the alloy, thereby increasing the rate of charge transport on the electrode surface and ameliorating of hydrogen storage capacity [53]. In addition, the CNTs overlying of the alloy not only may prevent oxidation and corrosion of the alloy particles, but also facilitate the contact between the hydrogen and the alloy particles, play a conductive and catalytic role, and contribute to the
improvement of the discharge capacity and discharge time. However, as the ball milling time is further extended (3 h), the discharge performance of the alloy electrode is obviously deteriorated, as in the previous XRD and SEM analysis, mainly owing to the destruction of hollow tubular structure of the carbon nanotube caused by long-time ball milling, and the agglomeration phenomenon occurs. The CNTs cannot be uniformly dispersed and coated in the AB5-type alloy. The CNTs does not promote the hydrogen storage of the hydrogen storage alloy, and thus the discharge characteristics of the alloy remarkably deteriorated. The discharge characteristic curve of CNTs, AB5 alloy and CNTs/AB5 composite alloys in a solution of 2 M KOH þ 0.1 M KBH4 are displayed in Fig. 6 and the corresponding electrochemical properties for the experimental alloys are listed in Table 4. The CNTs have almost no discharge platform, and the potential reaches a minimum in a short time and the potential drops rapidly, indicating that it has almost no catalytic oxidation ability to potassium borohydride. However, the AB5 alloy and the CNTs/AB5 composite alloys exhibit better discharge performance in a solution of 2 M KOH þ0.1 M KBH4. Compared with the discharge time of the AB5 alloy and the CNTs/AB5 composite alloys in 6 M KOH solution (As displayed in Fig. 5),
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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Fig. 4 e SEM images and EDS patterns of the CNTs/AB5 composite alloys: (a) 2 h; (b) 3 h; (c) 4 h; (d) 5 h; (e) 2 h; (f) 3 h; (g) 4 h; (h) 5 h.
the discharge time of the AB5 alloy and the CNTs/AB5 composite alloys in the 2 M KOH þ 0.1 M KBH4 mixed solution was increased by about 6 times. This hints that the AB5 alloy and the CNTs/AB5 composite hydrogen storage alloys have obvious catalytic oxidation ability on KBH4. In addition, it can also be found from Fig. 6 that the discharge characteristic curve of the alloy is greatly affected by the ball milling time. As the extension of the ball milling time, the discharge platform of the alloys first becomes wider and then narrow, and the discharge median potential (Vmid) of the alloys increases first and then decreases, and the slope of the discharge characteristic curve of the alloys first decreases and then increases. When the ball
milling time reaches 2 h, the CNTs/AB5 composite alloy exhibits the best discharge characteristic. Comparing Fig. 5 with Fig. 6, it can be seen that the law of the discharge characteristic of the CNTs/AB5 composite alloys in 6 M KOH solution is almost consistent with that in the 2 M KOH þ0.1 M KBH4 solution. This speculated that the CNTs/AB5 composite alloys with good hydrogen storage electrochemical performance have better catalytic oxidation ability on KBH4. As mentioned in Fig. 1 above, this is closely related to the hydrogen storage alloy which can effectively inhibit the hydrolysis reaction. Fig. 7 displays the cyclic voltammetry (CV) curves of CNTs, AB5 alloy and CNTs/AB5 composite alloys in 2 M KOH þ 0.1 M
Table 3 e The corresponding data of EDS patterns of the CNTs/AB5 composite alloys. Samples
2 3 4 5
h h h h
Elements (at.%)
(Ni, Co, Mn, Al):(La, Ce)
Ce
La
Ni
Co
Mn
Al
12.33 11.64 11.17 11.50
3.79 3.92 3.68 3.24
59.32 58.05 60.62 58.92
10.29 13.10 13.54 10.95
6.27 11.61 9.64 6.49
7.99 1.68 1.35 8.91
5.20 5.42 5.73 5.78
: : : :
1 1 1 1
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Discharge potential ( V vs.-Hg/HgO)
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1.0
0.9
0.8
CNTs AB5 alloy 0.5 h 1h 2h 3h 4h 5h
0.7
0.6 0
200
400
600
800
1000
1200
1400
1600
Time (min) Fig. 5 e Discharge characteristics of the CNTs, AB5 alloy and CNTs/AB5 composite alloys at different ball milling time in 6 M KOH.
KBH4 solutions. The peak potential of the oxidation peak and the corresponding current density of the alloy are listed in Table 5. From Fig. 7 and Table 5 that the CV curve of pure CNTs has no oxidation peak, indicating that pure CNTs has almost no catalytic oxidation ability on ВН 4 . This is consistent with the analysis in Fig. 6. However, the CV curves of the AB5 alloy and the CNTs/AB5 composite alloys have obviously oxidation peak. The CV curves of the CNTs/AB5 composite alloys with different ball milling time are similar, obvious oxidation peaks appear during the scanning process, and all of them are irreversible oxidation peaks, indicating that the AB5 alloy and the CNTs/AB5 composite alloys have an effective catalytic effect on KBH4. The catalytic mechanism of the CNTs/AB5 composite alloys is similar, which also shows that the oxidation reaction of ВН 4 on the surface of the composite alloys is also similar. However, the peak potential and the peak current density in the CV curves of the AB5 alloy and the CNTs/AB5 composite alloys are different. It can be seen from Table 5 that the oxidation peak potential of the alloys decreases first and then increases, and the current density of oxidation peak of the alloys first increases and then decreases with the increase of the ball milling time. Generally speaking, the higher the oxidation peak (peak current density), the more negative the corresponding peak potential, indicating that the catalytic performance of the catalyst is better [54,55]. In this work,
Table 4 e The main electrochemical properties for the experimental alloys. Samples
The maximum discharge time (min)
Vmid (V)
CNTs AB5 alloy 0.5 h 1h 2h 3h 4h 5h
212 1216 1198 1217 1534 1356 1123 876
0.8643 0.8767 0.8668 0.8955 0.9040 0.9090 0.8885 0.7406
Fig. 6 e Discharge characteristics of the CNTs, AB5 alloy and the CNTs/AB5 composite alloys in 2 M KOH þ0.1 M KBH4 mixed solution.
when the ball milling time reaches 2 h, the current density of the oxidation peak of the alloy reaches a maximum of 82.533 mA cm2, and the oxidation peak potential reaches a minimum of 0.065 V (vs Hg/HgO), indicating that it has the best catalytic oxidation ability for ВН 4 . This further verified that the hydrogen storage alloys with good hydrogen storage electrochemical performance has better catalytic oxidation ability on KBH4. Combined with the previous analysis, when the composite alloy milled 2 h with good electrochemically performance participates in the electrochemical reaction, it has a strong binding force to the surface-adsorbed hydrogen atom Hads (Such as process 4), thereby inhibiting the hydrolysis side reaction (Such as process 3). Then, after the process 5 and the process 6, the hydrogen is oxidized again, so that the number of electrons released is increased, which can be effectively improved the utilization rate of the fuel. The comprehensive comparison shows that the catalytic oxidation ability of the composite alloys on ВН 4 is ranked as: 2 h > 1 h > 3 h > 0.5 h > 4 h > 5 h > AB5 alloy > CNTs. In order to comparing the catalytic properties of the CNTs, AB5 alloy and CNTs/AB5 composite alloys to borohydride, we tested the CV of all the samples in 2 M KOH. Fig. 8 shows the cyclic voltammetry (CV) curves of CNTs, AB5 alloy and CNTs/ AB5 composite alloys in 2 M KOH solutions. For the sake of clarity, the peak potential of the oxidation peak and the corresponding current density of the samples are listed in Table 5. From Fig. 8 and Table 5, we can see that both CNTs and AB5 alloy have no oxidation peaks, while the CNTs/AB5 composite alloys have oxidation peaks, indicating that CNTs and AB5 alloy have no catalytic effect in 2 M KOH solution, while the CNTs/AB5 composite alloys have catalytic effect in 2 M KOH solution. And with the increase of ball milling time, the peak potential of the CNTs/AB5 composite alloy has no obvious law, while the peak current density of the CNTs/AB5 composite alloy decreases first and then increases. Overall, the CNTs/AB5 composite alloy milled 2 h exhibits the smaller peak potential and the highest peak current density. Hinting
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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a b c d e f g h
100 b
Current density (mA cm-2)
80 e 60 d
40
f c
h
g
20
a
CNTs AB5 alloy 0.5 h 1h 2h 3h 4h 5h
0 -20 -40 -60 -80 -100 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Potential (V vs.-Hg/HgO) Fig. 7 e CV curves of the CNTs, AB5 alloy and the CNTs/AB5 composite alloys in 2 M KOH þ0.1 M KBH4 mixed solution. that the CNTs/AB5 composite alloy milled 2 h has the best catalytic performance. Furthermore, comparing with the CV curves of all samples in 2 M KOH solution (Fig. 8) and in 2 M KOH þ 0.1 M KBH4 mixed solution (Fig. 7), we find that CNTs have no oxidation peaks in both solutions, while the AB5 alloy has an oxidation peak in the 2 M KOH þ 0.1 M KBH4 and no oxidation peak in 2 M KOH solution. The CNTs/AB5 composite alloys have oxidation peaks in both solutions. This conclusion is the same as Zhang et al. [45]. Comparing with 2 M KOH solution, the CNTs/AB5 composite alloy exhibits the smaller peak potential and the higher peak current density. It indicated that the CNTs/AB5 composite alloys have catalytic effect in 2 M KOH solution, but they have the better catalytic effect in 2 M KOHþ0.1 M KBH4 solution. Moreover, the compounding of CNTs and AB5 alloy can effectively improve the catalytic activity of AB5 alloy. In general, the CNTs/AB5 composite alloy milled 2 h has the best
catalytic activity not only in 2 M KOH solution but also in 2 M KOHþ0.1 M KBH4 solution. Generally, the chronoamperometry (CA) is a relationship of the oxidation current of borohydride as a function of time. The CA curve can reflect the stability of the catalyst during the catalytic process. The number of transferred electrons during electrocatalytic oxidation can also be calculated from the CA curve and the Cottrell equation [55]. Fig. 9 is CA curves of CNTs, AB5 alloy and CNTs/AB5 composite alloys in 2 M KOH þ0.1 M KBH4 with a constant voltage of 0.95 V. Corresponding to Fig. 9, the steady current density and the number of transferred electron of the alloy samples are listed in Table 6. The higher current density exhibits the better catalytic performance [55,56]. As can be found from Fig. 9 and Table 6, the current density of the CNTs begins to decrease rapidly and then tends to stabilize. While the composite alloy, after activation, has a rapid decrease in current density and then tends to be stable. In addition, as the ball milling time extends, the current density of the composite alloy increases first and then decreases. The CNTs/AB5 composite alloy milled 2 h has the highest current density value of 5.8194 mA cm2. It is shown that the CNTs/AB5 composite alloy milled 2 h has the best catalytic performance. It can be seen from Table 6 that the number of electrons transferred increases first and then decreases with the extension of ball milling time. When the ball milling time is 2 h, the number of electrons transferred is the most, which is 1.31. This result is comparable to the results of the literature [55]. When the noble metal Au/C was used as anode catalyst for DBFC in the literature [55], the number of electrons transferred is about 1.1 during the catalytic oxidation process. It indicated that the catalytic performance of the CNTs/AB5 composite hydrogen storage alloy milled for 2 h on ВН 4 can be compared with that of precious metal Au/C. Moreover, it is inferred that the better the electrochemical performance of the hydrogen storage alloy, the more electrons are transferred. By comparing the electrochemical performance and catalytic performance of CNTs/AB5 composite alloy on ВН 4 , it can be found that the better the hydrogen storage performance, the better the catalytic performance of the alloy for
Table 5 e Peak potential and peak current density of the CNTs, AB5 alloy and CNTs/AB5 composite alloys. Peak current density (mA cm2)
2 M 2 M KOH þ 0.1 M 2 M 2 M KOH þ 0.1 M KOH KBH4 KOH KBH4 CNTs AB5 alloy milled 0.5 h milled 1h milled 2h milled 3h milled 4h milled 5h
e e 0.175
e 0.135 0.095
e e 67.599
e 57.449 64.207
0.245
0.065
78.887
72.533
0.185
0.065
79.427
82.533
0.225
0.105
64.559
72.137
0.335
0.255
26.079
62.321
0.215
0.115
31.189
61.120
100
Current density (mA cm-2)
Samples Peak potential (V vs Hg/ HgO)
120
80 60 40 20
a b c d e f g h
CNTs AB5 alloy 0.5 h 1h 2h 3h 4h 5h
e
d
c f g
0 -20 a
-40
b
-60 -80 -100 -0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
Potantial / (V vs.-Hg/HgO) Fig. 8 e CV curves of CNTs, AB5 alloy and CNTs/AB5 composite alloys in 2 M KOH solution.
Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158
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borohydride, that means that the electrochemical performance of the hydrogen storage alloy is positively correlated with the catalytic performance as anode catalyst in a DBFC.
Conclusions In this work, the CNTs/AB5 composite hydrogen storage alloys at different milling time are prepared. The electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys under different milling time were studied. And we also discussed the catalytic properties of CNTs/AB5 composite hydrogen storage alloys used as anodic catalysts in DBFC. Through the experimental conclusion, we analyzed the correlation between electrochemical properties of CNTs/AB5 composite hydrogen storage alloys and their catalytic properties on KBH4. The main results obtained were as following:
Fig. 9 e (a) - CA curves of the CNTs, AB5 alloy and the CNTs/ AB5 composite alloys in 2 M KOH þ 0.1 M KBH4 mixed solution. Fig. 9 (b) - Partial enlargement of CA curves of the CNTs, AB5 alloy and the CNTs/AB5 composite alloys in 2 M KOH þ 0.1 M KBH4 mixed solution.
1) The CNTs/AB5 composite hydrogen storage alloys have a single CaCu5 type LaNi5 hexagonal crystal structure. As the ball milling time extends, the diffraction peak of the composite alloy gradually shifts to a low angle. On the whole, the unit cell volumes gradually increase and the grain sizes decrease. The CNTs were elongated tubular structures. When the ball milling time reaches 2 h, the CNTs adhere to the surface of the AB5-type alloy to form a coating. While with the further extension of the milling time, the AB5 alloy particles become smaller, and the hollow tubular structure of the CNTs is gradually damaged, agglomeration occurs, and amorphous carbon is formed, and the two are distributed in disorder. 2) The electrochemical property of CNTs/AB5 composite alloys get better and then get worse with the extension of ball milling time. At the same time, when the CNTs/AB5 composite hydrogen storage alloy is used as anode catalyst for DBFC, the catalytic performance on KBH4 also get better and then get worse with the extension of ball milling time. When ball milling time reaches 2 h, the CNTs/AB5 composite alloys show the best electrochemical performance and catalytic performance on KBH4. From experimental data we know that the better the electrochemical performance of hydrogen storage alloy, the better the catalytic performance on KBH4.
Acknowledgements Table 6 e The steady current densities and the number of transferred electrons of the CNTs, AB5 alloy and the CNTs/AB5 composite alloys. Samples CNTs AB5 alloy milled 0.5 h milled 1 h milled 2 h milled 3 h milled 4 h milled 5 h
Current density（mA cm2）
Number of electrons (n)
0.0092 0.0096 þ3.4031 þ4.3062 þ5.8194 þ4.2202 þ4.0242 þ3.6674
e e 0.77 0.97 1.31 0.94 0.91 0.83
This work is supported by Inner Mongolia Autonomous Region Higher Education Science and Technology Research Foundation (NJZZ17040), Natural Science Foundation of Inner Mongolia Autonomous Region (2019MS05002), National Natural Science Foundation of China (51661027).
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Please cite this article as: Yan J et al., Correlation between electrochemical properties of the CNTs/AB5 composite hydrogen storage alloys and their catalytic properties for KBH4, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.158