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AB5 alloy composite and its electrochemical hydrogen absorbing properties
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 3 8 ( 2 0 1 3 ) 8 9 0 3 e8 9 0 8
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Preparation of nickel modified activated carbon/AB5 alloy composite and its electrochemical hydrogen absorbing properties Xiaofeng Li*, Tongchi Xia, Huichao Dong, Qing Shang, Yanghua Song Henan Provincial Key Laboratory of Surface & Interface, Zhengzhou University of Light Industry, Zhengzhou 450002, China
article info
abstract
Article history:
Due to the good catalysis of metal nickel for the electrochemical reduction/oxidation re-
Received 8 December 2012
actions between H2O and H in alkaline electrolyte, it is separately introduced into activated
Received in revised form
carbon (AC) by ball milling (named AC-1), liquid phase reduction (AC-2) and hydrothermal
27 March 2013
(AC-3) methods. The results of scanning electron microscopy (SEM) show that different to
Accepted 7 May 2013
the aggregation of metal nickel in AC-1 and the coating of metal nickel on the surface of
Available online 2 June 2013
AC-2, metal nickel introduced by the hydrothermal method is uniformly dispersed among the AC-3 particles. The AC-3/AB5 alloy (90 wt%) composite shows the best electrochemical
Keywords:
hydrogen absorbing activity. Its capacity at a 0.2 C rate reaches to 318 mA h/g. Compared to
Activated carbon
the AB5 alloy electrode, its discharge capacity separately increases 16% at a 1 C rate and
AB5 alloy
59% at a 3 C rate, which is particularly suitable for the high-power nickel metal-hydride (Ni
Composite electrode
eMH) battery.
Metal nickel
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Electrochemical hydrogen absorbing
reserved.
property
1.
Introduction
Recently, the hydrogen absorbing properties of carbon materials (such as supper-activated carbon, carbon nanotube and graphite nanofiber) have attracted great interests since their high stability in cycling operation and fast hydrogen sorption kinetics [1e5]. Hydrogen absorption in carbon materials is based on weak van der Waals force between H2 and the surface of the materials [6], and due to insufficient binding between H2 molecules and adsorbent surface [7,8], these physisorption based storage processes have low hydrogen capacity especially under normal temperature and pressure (<1 wt %) [9e12]. On the other hand, hydrogen absorbing alloys, such as MmNi5xyzCoxAlyMnz (AB5, where Mm is misch metal) alloy,
are mature in technology and have been widely used in nickel metalehydride (NieMH) battery. As hydrogen absorption in the alloys is based on the formation of metalehydride in their crystal lattice, these storage processes have good hydrogen capacity at normal temperature and pressure. But compared to other hydrogen absorbing materials, it has some shortcomings such as high price, poor cyclic adsorption performance and slow hydrogen absorption/desorption kinetics. It was reported that some metal particles could assist the dissociation of hydrogen molecules to hydrogen atoms, thereby allowing atomic hydrogen to adsorb chemically at the defective sites of carbon materials [13,14]. The effects of these metal particles, such as Pd, Pt, Ni, V, Ti and Fe on the hydrogen storage properties of carbon materials have been studied by many researchers [10,15e21]. Park and his co-workers [15]
* Corresponding author. Tel./fax: þ86 371 86609676. E-mail address: [email protected] (X. Li). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.05.039
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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 3 8 ( 2 0 1 3 ) 8 9 0 3 e8 9 0 8
reported that the hydrogen storage capacity of platinumsupported multi-walled carbon nanotubes (MWCNTs) reached 3.72 wt% at 298 K and 100 bar when the content of Pt was 5 wt%. V. Jime´nez and his co-workers [10] reported that with 20 wt% nickel modification, the hydrogen storage capacity of amorphous carbon was 4.42 wt% at 77 K and 10 bar. H. Kim and his co-workers [16] studied the hydrogen storage properties of Pd-coated porous carbon nanofibers and reported that the reversible capacity was 0.35 wt% at 298 K and 0.1 MPa. A. Reyhani and his co-workers [21] studied the electrochemical hydrogen storage on raw, oxidized, purified and Fe-doped MWCNTs and they realized that, Fe-doped one produced the highest capacity. The combination of carbon materials and hydrogen absorbing alloys is a practical way to produce a composite with better hydrogen absorbing activity at normal temperature and pressure. In our previous study [22], commercial graphite was modified with metal nickel powder by a simple ball milling process, and then it was mixed with AB5 alloy to form a graphite/AB5 alloy composite. The composite showed higher electrochemical hydrogen absorption/desorption activity especially discharging at large currents. In this paper, another cheap carbon material, activated carbon (AC) was modified with metal nickel by different processes and the electrochemical hydrogen absorption/desorption activity of the AC/AB5 alloy composite was studied in details.
The surface morphology of nickel modified AC was tested by using scanning electron microscopy (SEM). The powder Xray diffraction (XRD) was used to analyze its phase structure. The negative composite electrodes were prepared by filling a nickel foam substrate with a mixture of AC (untreated or nickel modified), commercial MmNi3.55Co0.75Al0.2Mn0.5 alloy and polytetrafluoroethylene (PTFE) binder. The percentage composition of PTFE binder in the mixture is 8 wt%. The pasted electrodes were then dried at 65 C and pressed to a thickness of 0.30 mm. The electrochemical properties of the composite electrodes were investigated by using a ’sandwich-like’ simulation battery in a 7 mol l1 KOH solution at 25 C. Two sintered Ni(OH)2 electrodes obtained from a commercial supplier were placed on both sides and the composite electrode was positioned at the center. A cycling test was performed on the batteries by charging at a 0.2 C (60 mA/g) rate for 7 h and discharging at the same rate to a cut-off voltage 1.0 V for stabilizing their capacity. The electrodes were then charged at a 0.5 C (150 mA/g) rate for 2.6 h and discharged separately at a 1 C (300 mA/g) or 3 C (900 mA/g) rate to a cut-off voltage 0.8 V for high-power performance tests. Electrochemical impedance spectroscopy (EIS) tests were performed on the electrodes by using a Solartron Electrochemical Interface model SI1287.
3. 2.
Experimental
Mesoporous AC (specific surface area: 1660 m2 g1) was used in this paper. It was separately modified with metal nickel by three methods. Firstly, a mixture of AC and metal nickel powder (nickel carbonyl, purity 99.5 wt%, particle size about 1.5 mm) was added into a 50 ml-capacity stainless steel pot with 50 g stainless steel balls. The weight of the mixture was 2 g and 2 ml ethanol was also added into the spot as a grinding aid. Ball milling was performed by using a planetary ball mill under a revolution speed of 300 r min1 and milling time of 2 h. The obtained powder (named AC-1) was washed repeatedly with distilled water, and then dried at 50 C for use. Next, a liquid phase reduction method was used to prepare nickel modified AC. Nickel sulfate (NiSO4$6H2O) was resolved in a three-neck flask with 15 ml distilled water, and 2 g AC was dispersed in the solution under stirring. A 2 mol l1 sodium hydroxide solution was slowly added into it till the pH of the solution in the flask was 14. Then, 30 ml hydrazine hydrate solution (80 wt%) was slowly added into the flask in 1 h, and the reduction reaction temperature was kept at 80 C. The obtained powder (named AC-2) was washed repeatedly with distilled water, and then dried at 50 C for use. Finally, a hydrothermal method was used to prepare nickel modified AC. NiSO4$6H2O, NaOH and 2 g AC was added into 25 ml ethylene alcohol in a beaker. The mole ratio between NiSO4$6H2O and NaOH was 1:5. After being stirred for 30 min, the suspension was transferred into a Teflon-lined autoclave. The autoclave was heated at 160 C for 12 h, and then cooled naturally in air. The obtained powder (named AC-3) was washed repeatedly with distilled water, and then dried at 50 C for use.
Results and discussion
Firstly, the hydrogen storage property of the AC electrode was tested and Fig. 1 shows its chargeedischarge curves. It is clear that the AC electrode has poor electrochemical hydrogen absorbing performance, since it has no stable discharge plateau and its capacity only is 58 mA h/g. Then, AC was mixed with AB5 alloy to form a composite and its chargeedischarge curves are shown in Fig. 2. Compared to the AB5 alloy electrode, the battery with the composite electrode has higher charge voltage and lower capacity. Excluding the AB5 alloy’s capacity (297 mA h/g), the capacity of AC in the composite reaches 101 mA h/g. This result indicates that the combination of AC with AB5 alloy could obviously improve the
Fig. 1 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate (20 mA/g) with the AC electrode.
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 3 8 ( 2 0 1 3 ) 8 9 0 3 e8 9 0 8
Fig. 2 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate (60 mA/g) with (a) the AC/AB5 alloy (90 wt%) composite electrode and (b) the AB5 alloy electrode.
former’s electrochemical hydrogen storage capacity. However, as the electrochemical reduction of H2O molecule to H atom is difficult on the surface of the AC particles, and there is insufficient binding between H2 molecules (or H atoms) and the surface, the composite electrode results in a lower discharge capacity than the AB5 alloy electrode. As mentioned above, commercial graphite was modified with metal nickel by ball milling and the graphite/AB5 alloy composite showed higher electrochemical hydrogen absorption/desorption activity than AB5 alloy in our previous study. Here, AC was modified with 20 wt% metal nickel powder by ball milling and the electrochemical properties of AC-1/AB5 alloy composite are shown in Fig. 3. Contrary to the authors’
Fig. 3 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate (60 mA/g) with (a) the AB5 alloy electrode, (b) the AC-1/AB5 alloy (90 wt%) composite electrode, (c) the AC-2/AB5 alloy (90 wt%) composite electrode, and (d) the AC-3/AB5 alloy (90 wt%) composite electrode.
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prediction, its discharge capacity (290 mA h/g) at a 0.2 C rate is lower than the AB5 alloy electrode (297 mA h/g) and it has higher voltage at the latter charge stage, which indicates larger chargeedischarge polarization. Fig. 4 shows the SEM images of the AC particles before and after being modified with 20 wt% metal nickel. Obviously, after ball milling, the metal nickel powders are aggregated and unevenly dispersed in the AC-1 particles; on the contrary, the powders could be uniformly dispersed in the graphite particles [22]. This may be because the flake graphite particles could be a good lubricant to prevent the metal nickel powders from aggregation. Since metal nickel is a good catalyst for the electrochemical reduction/oxidation reactions between H2O and H on the surface of hydrogen absorbing materials, better dispersion of nickel in the materials should result in higher electrochemical activity of the hydrogen absorbing electrodes. In order to improve the dispersion of metal nickel in the AC particles, the other two methods were used to prepare nickel modified AC. Fig. 5 shows XRD patterns of the modified AC particles. The diffraction peaks near 44 , 52 and 76 correspond to {111}, {200} and {220} crystal planes of metal nickel, respectively (PDF number 04-0850). This result indicates that metal nickel can be successfully introduced into the AC particles by both the liquid phase reduction and hydrothermal methods. But as shown in Fig. 4, the AC-2 particles are coated with metal nickel and their surface is no longer smooth; on the other hand, tiny metal nickel is uniformly dispersed among the AC-3 particles. On the same time, as shown in Fig. 3, the discharge capacity of the AC-3/AB5 composite electrode reaches to 318 mA h/g, which is obviously larger than the AB5 alloy electrode (297 mA h/g) and the AC-2/AB5 composite electrode (300 mA h/g). These results indicate that although metal nickel coated on the AC-2 particles could benefit the electrochemical reduction/oxidation reactions between H2O and H, the defective sites for the binding between hydrogen and their surface might decrease, therefore resulting in a lower capacity than the AC-3/AB5 composite. Since the hydrothermal method is the best one among the three methods for the introduction of metal nickel into the AC particles, the AC-3/AB5 alloy (90 wt%) composites with different nickel modification amounts were prepared and their chargeedischarge curves are shown in Fig. 6. Obviously, after being modified with metal nickel, all the discharge capacity of the composite electrodes exceeds the AB5 electrode, which indicates the good catalysis of metal nickel for the electrochemical reduction/oxidation reactions on the surface of AC. The experimental results show that the appropriate modification amount is 20 wt%. Otherwise, there would be insufficient catalytic sites at lower amount or excessive nonactive material in the electrode at higher amount. Next, different composite electrodes of AC-3/AB5 alloy (95 wt%, 90 wt% or 80 wt%) were prepared and their electrochemical properties were tested. As shown in Fig. 7, the composite electrode with 10 wt% AC-3 has the maximum discharge capacity. This result indicates that there should be a synergistic effect between AC and AB5 alloy during the hydrogen storage process. The physi-sorption of hydrogen in AC and chemi-sorption of it in AB5 alloy are mutually improved under a proper ratio between AC and AB5 alloy in the composite. The discharge capacity of the composite
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Fig. 4 e SEM images of AC particles (a) untreated, (b) AC-1, (c) AC-2 and (d) AC-3, where AC-1, AC-2 and AC-3 are all modified with 20 wt% metal nickel.
electrode with 20 wt% AC-3 decreases obviously, which might suggest the insufficient enhancement of binding between H2 molecules (or H atoms) and the AC surface by AB5 alloy. Finally, high-power performance of the composite was tested and the results are shown Fig. 8. Compared to the simulated battery with the AB5 alloy electrode, the battery with the composite electrode of AC-3/AB5 alloy (90 wt%) has better high-power performance. The discharge capacity of the latter increases 16% at a 1 C rate and 59% at a 3 C rate, respectively. At the same time, the discharge voltage of the latter obviously exceeds the former. This result suggests the
Fig. 5 e XRD patterns of the modified AC particles by a hydrothermal method (a) and a liquid phase reduction method (b).
AC-3/AB5 alloy (90 wt%) composite is particularly suitable for the high-power NieMH battery. Fig. 9 shows the impedance plots for the negative electrodes discharged to 50% depth of discharge (DOD) in the 7 mol l1 KOH electrolytes. The plots exhibit three arcs in the whole frequency range. For a MH electrode, the semicircle in high-frequency region is attributed to the contact resistance between the alloy particle and the current collector; the other
Fig. 6 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate with (a) AB5 alloy electrode and (b, c, d) AC-3/AB5 alloy (90 wt%) composite electrodes, where AC-3 is modified with (b) 10 wt% (c) 20 wt% and (d) 30 wt% metal nickel.
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 3 8 ( 2 0 1 3 ) 8 9 0 3 e8 9 0 8
Fig. 7 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate with (a) AB5 alloy electrode and (b, c, d) AC-3/AB5 alloy composite electrodes, where the mass percent of AB5 alloy in the composites is (b) 95 wt%, (c) 90 wt% and (d) 80 wt%, respectively and AC-3 is modified with 20 wt% metal nickel.
semicircle in low-frequency region is attributed to a charge transfer process on the electrode surface, and the slope is caused by the diffusion of hydrogen in the alloy [23]. According to an equivalent circuit for the frequency response of the negative electrode [22], the results of a curve fitting show that as the electrochemical reaction activity on the surface of AC-3 is lower than AB5 alloy, the charge transfer resistance of the composite electrode (0.754 U) is larger than the AB5 alloy electrode (0.498 U). But compared to the latter, the Warburg admittance of hydrogen diffusion in the former increases from 18.9S to 35.1S, which suggests improved hydrogen diffusion ability. As hydrogen diffusion in the bulk of the alloy
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Fig. 9 e Impedance plots of the negative electrodes (discharged to 50% depth of discharge) in 7 mol lL1 KOH electrolytes at a frequency range from 105 to 10L2 Hz: (a) AB5 alloy electrode and (b) AC-3/AB5 alloy (90 wt%) composite electrode, where AC-3 is modified with 20 wt% metal nickel.
is a limiting step in the reaction mechanisms of the metal hydride electrode, more rapid diffusion would result in better electrochemical properties of the electrode, especially its discharge performance at large currents.
4.
Conclusion
(1) The AC electrode has poor electrochemical hydrogen absorbing performance, and the untreated AC/AB5 alloy (90 wt%) composite electrode also shows a lower capacity and higher charge voltage than the AB5 alloy electrode. (2) Metal nickel was separately introduced into AC by three methods. The results of scanning SEM show that the metal nickel powders are aggregated and unevenly dispersed in the AC-1 particles after ball milling. The AC-2 particles are coated with metal nickel by the liquid phase reduction method, and metal nickel introduced by the hydrothermal method is uniformly dispersed among the AC-3 particles. (3) With the good catalysis of metal nickel for the electrochemical reduction/oxidation reactions on the surface of AC, the composite electrode of AC-3/AB5 alloy (90 wt%) has a 0.2 C capacity of 318 mA h/g. Compared to the AB5 alloy electrode, its discharge capacity separately increases 16% at a 1 C rate and 59% at a 3 C rate, which should benefit from more rapid H diffusion in it.
Acknowledgments Fig. 8 e Typical chargeedischarge curves of the simulated batteries at a 1 C or 3 C rate with (a) AB5 alloy electrode and (b) AC-3/AB5 alloy (90 wt%) composite electrode, where AC3 is modified with 20 wt% metal nickel.
Funding for this work is being provided by Education Department of Henan Province, China through a Leading Young Teacher Training Grant in Colleges and Universities (2011GGJS-101).
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references [13] [1] Yu¨ru¨m Y, Taralp A, Veziroglu TN. Storage of hydrogen in nanostructured carbon materials. Int J Hydrogen Energy 2009;34:3784e98. [2] Kojima Y, Kawai Y, Koiwai A, Suzuki N, Haga T, Hioki T, et al. Hydrogen adsorption and desorption by carbon materials. J Alloys Comp 2006;421:204e8. [3] Akasaka H, Takahata T, Toda I, Ono H, Ohshio S, Himeno S, et al. Hydrogen storage ability of porous carbon material fabricated from coffee bean wastes. Int J Hydrogen Energy 2011;36:580e5. [4] Babel K, Janasiak D, Jurewicz K. Electrochemical hydrogen storage in activated carbons with different pore structures derived from certain lignocellulose materials. Carbon 2012;50:5017e26. [5] Lee S-Y, Park S-J. Synthesis of zeolite-casted microporous carbons and their hydrogen storage capacity. J Colloid Interf Sci 2012;384:116e20. [6] Shindoa K, Kondo T, Arakawa M, Sakurai Y. Hydrogen adsorption/desorption properties of mechanically milled activated carbon. J Alloys Comp 2003;359:267e71. [7] Rashidi AM, Nouralishahi A, Khodadadi AA, Mortazavi Y, Karimi A, Kashefi K. Modification of single wall carbon nanotubes (SWNT) for hydrogen storage. Int J Hydrogen Energy 2010;35:9489e95. [8] Carpetis C, Peschka W. A study on hydrogen storage by use of cryoadsorbents. Int J Hydrogen Energy 1980;5:539e54. [9] Xu W-C, Takahashi K, Matsuo Y, Hattori Y, Kumagai M, Ishiyama S, et al. Investigation of hydrogen storage capacity of various carbon materials. Int J Hydrogen Energy 2007;32:2504e12. [10] Jime´nez V, Ramı´rez-Lucas A, Sa´nchez P, Valverde JL, Romero A. Improving hydrogen storage in modified carbon materials. Int J Hydrogen Energy 2012;37:4144e60. ez S. Hydrogen adsorption on Pd[11] Dı´az E, Leo´n M, Ordo´n modified carbon nanofibres: influence of CNF surface chemistry and impregnation procedure. Int J Hydrogen Energy 2010;35:4576e81. [12] Be´nard P, Chahine R, Chandonia PA, Cossement D, DorvalDouville G, Lafi L, et al. Comparison of hydrogen adsorption
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on nanoporous materials. J Alloys Comp 2007;446e447:380e4. Yoo E, Gao L, Komatsu T, Yagai N, Arai K, Yamazaki T, et al. Atomic hydrogen storage in carbon nanotubes promoted by metal catalysts. J Phys Chem B 2004;108:18903e7. Kim H-S, Lee H, Han K-S, Kim J-H, Song M-S, Park M-S, et al. Hydrogen storage in Ni nanoparticle-dispersed multiwalled carbon nanotubes. J Phys Chem B 2005;109:8983e6. Park S-J, Lee S-Y. Hydrogen storage behaviors of platinumsupported multi-walled carbon nanotubes. Int J Hydrogen Energy 2010;35:13048e54. Kim H, Lee D, Moon J. Co-electrospun Pd-coated porous carbon nanofibers for hydrogen storage application. Int J Hydrogen Energy 2011;36:3566e73. Wu H, Wexler D, Liu H. Effects of different palladium content loading on the hydrogen storage capacity of double-walled carbon nanotubes. Int J Hydrogen Energy 2012;37:5686e90. Reyhani A, Mortazavi SZ, Akhavan O, Moshfegh AZ, Lahooti SH. Effect of Ni, Pd and Ni-Pd nano-islands on morphology and structure of multi-wall carbon nanotubes. Appl Surf Sci 2007;253:8458e62. Brieno-Enriquez KM, Ledesma-Garcia J, Perez-Bueno JJ, Godinez LA, Terrones H, Angeles-Chavez C. Bonding titanium on multi-walled carbon nanotubes for hydrogen storage: an electrochemical approach. Mater Chem Phys 2009;115:521e5. Suttisawat Y, Rangsunvigit P, Kitiyanan B, Williams M, Ndungu P, Lototskyy MV, et al. Investigation of hydrogen storage capacity of multiwalled carbon nanotubes deposited with Pd or V. Int J Hydrogen Energy 2009;34:6669e75. Reyhani A, Mortazavi SZ, Moshfegh AZ, Golikand AN, Amiri M. Enhanced electrochemical hydrogen storage by catalytic Fe-doped multi-walled carbon nanotubes synthesized by thermal chemical vapor deposition. J Power Sources 2009;188:404e10. Li X, Wang L, Dong H, Song Y, Shang H. Electrochemical hydrogen absorbing properties of graphite/AB5 alloy composite electrode. J Alloys Comp 2012;510:114e8. Yang H, Chen Y, Tao M, Wu C, Shao J, Deng G. Low temperature electrochemical properties of LaNi4.6xMn0.4Mx (M ¼ Fe or Co) and effect of oxide layer on EIS responses in metal hydride electrodes. Electrochim Acta 2010;55:648e55.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Preparation of nickel modified activated carbon/AB5 alloy composite and its electrochemical hydrogen absorbing properties Xiaofeng Li*, Tongchi Xia, Huichao Dong, Qing Shang, Yanghua Song Henan Provincial Key Laboratory of Surface & Interface, Zhengzhou University of Light Industry, Zhengzhou 450002, China
article info
abstract
Article history:
Due to the good catalysis of metal nickel for the electrochemical reduction/oxidation re-
Received 8 December 2012
actions between H2O and H in alkaline electrolyte, it is separately introduced into activated
Received in revised form
carbon (AC) by ball milling (named AC-1), liquid phase reduction (AC-2) and hydrothermal
27 March 2013
(AC-3) methods. The results of scanning electron microscopy (SEM) show that different to
Accepted 7 May 2013
the aggregation of metal nickel in AC-1 and the coating of metal nickel on the surface of
Available online 2 June 2013
AC-2, metal nickel introduced by the hydrothermal method is uniformly dispersed among the AC-3 particles. The AC-3/AB5 alloy (90 wt%) composite shows the best electrochemical
Keywords:
hydrogen absorbing activity. Its capacity at a 0.2 C rate reaches to 318 mA h/g. Compared to
Activated carbon
the AB5 alloy electrode, its discharge capacity separately increases 16% at a 1 C rate and
AB5 alloy
59% at a 3 C rate, which is particularly suitable for the high-power nickel metal-hydride (Ni
Composite electrode
eMH) battery.
Metal nickel
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
Electrochemical hydrogen absorbing
reserved.
property
1.
Introduction
Recently, the hydrogen absorbing properties of carbon materials (such as supper-activated carbon, carbon nanotube and graphite nanofiber) have attracted great interests since their high stability in cycling operation and fast hydrogen sorption kinetics [1e5]. Hydrogen absorption in carbon materials is based on weak van der Waals force between H2 and the surface of the materials [6], and due to insufficient binding between H2 molecules and adsorbent surface [7,8], these physisorption based storage processes have low hydrogen capacity especially under normal temperature and pressure (<1 wt %) [9e12]. On the other hand, hydrogen absorbing alloys, such as MmNi5xyzCoxAlyMnz (AB5, where Mm is misch metal) alloy,
are mature in technology and have been widely used in nickel metalehydride (NieMH) battery. As hydrogen absorption in the alloys is based on the formation of metalehydride in their crystal lattice, these storage processes have good hydrogen capacity at normal temperature and pressure. But compared to other hydrogen absorbing materials, it has some shortcomings such as high price, poor cyclic adsorption performance and slow hydrogen absorption/desorption kinetics. It was reported that some metal particles could assist the dissociation of hydrogen molecules to hydrogen atoms, thereby allowing atomic hydrogen to adsorb chemically at the defective sites of carbon materials [13,14]. The effects of these metal particles, such as Pd, Pt, Ni, V, Ti and Fe on the hydrogen storage properties of carbon materials have been studied by many researchers [10,15e21]. Park and his co-workers [15]
* Corresponding author. Tel./fax: þ86 371 86609676. E-mail address: [email protected] (X. Li). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.05.039
8904
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 3 8 ( 2 0 1 3 ) 8 9 0 3 e8 9 0 8
reported that the hydrogen storage capacity of platinumsupported multi-walled carbon nanotubes (MWCNTs) reached 3.72 wt% at 298 K and 100 bar when the content of Pt was 5 wt%. V. Jime´nez and his co-workers [10] reported that with 20 wt% nickel modification, the hydrogen storage capacity of amorphous carbon was 4.42 wt% at 77 K and 10 bar. H. Kim and his co-workers [16] studied the hydrogen storage properties of Pd-coated porous carbon nanofibers and reported that the reversible capacity was 0.35 wt% at 298 K and 0.1 MPa. A. Reyhani and his co-workers [21] studied the electrochemical hydrogen storage on raw, oxidized, purified and Fe-doped MWCNTs and they realized that, Fe-doped one produced the highest capacity. The combination of carbon materials and hydrogen absorbing alloys is a practical way to produce a composite with better hydrogen absorbing activity at normal temperature and pressure. In our previous study [22], commercial graphite was modified with metal nickel powder by a simple ball milling process, and then it was mixed with AB5 alloy to form a graphite/AB5 alloy composite. The composite showed higher electrochemical hydrogen absorption/desorption activity especially discharging at large currents. In this paper, another cheap carbon material, activated carbon (AC) was modified with metal nickel by different processes and the electrochemical hydrogen absorption/desorption activity of the AC/AB5 alloy composite was studied in details.
The surface morphology of nickel modified AC was tested by using scanning electron microscopy (SEM). The powder Xray diffraction (XRD) was used to analyze its phase structure. The negative composite electrodes were prepared by filling a nickel foam substrate with a mixture of AC (untreated or nickel modified), commercial MmNi3.55Co0.75Al0.2Mn0.5 alloy and polytetrafluoroethylene (PTFE) binder. The percentage composition of PTFE binder in the mixture is 8 wt%. The pasted electrodes were then dried at 65 C and pressed to a thickness of 0.30 mm. The electrochemical properties of the composite electrodes were investigated by using a ’sandwich-like’ simulation battery in a 7 mol l1 KOH solution at 25 C. Two sintered Ni(OH)2 electrodes obtained from a commercial supplier were placed on both sides and the composite electrode was positioned at the center. A cycling test was performed on the batteries by charging at a 0.2 C (60 mA/g) rate for 7 h and discharging at the same rate to a cut-off voltage 1.0 V for stabilizing their capacity. The electrodes were then charged at a 0.5 C (150 mA/g) rate for 2.6 h and discharged separately at a 1 C (300 mA/g) or 3 C (900 mA/g) rate to a cut-off voltage 0.8 V for high-power performance tests. Electrochemical impedance spectroscopy (EIS) tests were performed on the electrodes by using a Solartron Electrochemical Interface model SI1287.
3. 2.
Experimental
Mesoporous AC (specific surface area: 1660 m2 g1) was used in this paper. It was separately modified with metal nickel by three methods. Firstly, a mixture of AC and metal nickel powder (nickel carbonyl, purity 99.5 wt%, particle size about 1.5 mm) was added into a 50 ml-capacity stainless steel pot with 50 g stainless steel balls. The weight of the mixture was 2 g and 2 ml ethanol was also added into the spot as a grinding aid. Ball milling was performed by using a planetary ball mill under a revolution speed of 300 r min1 and milling time of 2 h. The obtained powder (named AC-1) was washed repeatedly with distilled water, and then dried at 50 C for use. Next, a liquid phase reduction method was used to prepare nickel modified AC. Nickel sulfate (NiSO4$6H2O) was resolved in a three-neck flask with 15 ml distilled water, and 2 g AC was dispersed in the solution under stirring. A 2 mol l1 sodium hydroxide solution was slowly added into it till the pH of the solution in the flask was 14. Then, 30 ml hydrazine hydrate solution (80 wt%) was slowly added into the flask in 1 h, and the reduction reaction temperature was kept at 80 C. The obtained powder (named AC-2) was washed repeatedly with distilled water, and then dried at 50 C for use. Finally, a hydrothermal method was used to prepare nickel modified AC. NiSO4$6H2O, NaOH and 2 g AC was added into 25 ml ethylene alcohol in a beaker. The mole ratio between NiSO4$6H2O and NaOH was 1:5. After being stirred for 30 min, the suspension was transferred into a Teflon-lined autoclave. The autoclave was heated at 160 C for 12 h, and then cooled naturally in air. The obtained powder (named AC-3) was washed repeatedly with distilled water, and then dried at 50 C for use.
Results and discussion
Firstly, the hydrogen storage property of the AC electrode was tested and Fig. 1 shows its chargeedischarge curves. It is clear that the AC electrode has poor electrochemical hydrogen absorbing performance, since it has no stable discharge plateau and its capacity only is 58 mA h/g. Then, AC was mixed with AB5 alloy to form a composite and its chargeedischarge curves are shown in Fig. 2. Compared to the AB5 alloy electrode, the battery with the composite electrode has higher charge voltage and lower capacity. Excluding the AB5 alloy’s capacity (297 mA h/g), the capacity of AC in the composite reaches 101 mA h/g. This result indicates that the combination of AC with AB5 alloy could obviously improve the
Fig. 1 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate (20 mA/g) with the AC electrode.
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Fig. 2 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate (60 mA/g) with (a) the AC/AB5 alloy (90 wt%) composite electrode and (b) the AB5 alloy electrode.
former’s electrochemical hydrogen storage capacity. However, as the electrochemical reduction of H2O molecule to H atom is difficult on the surface of the AC particles, and there is insufficient binding between H2 molecules (or H atoms) and the surface, the composite electrode results in a lower discharge capacity than the AB5 alloy electrode. As mentioned above, commercial graphite was modified with metal nickel by ball milling and the graphite/AB5 alloy composite showed higher electrochemical hydrogen absorption/desorption activity than AB5 alloy in our previous study. Here, AC was modified with 20 wt% metal nickel powder by ball milling and the electrochemical properties of AC-1/AB5 alloy composite are shown in Fig. 3. Contrary to the authors’
Fig. 3 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate (60 mA/g) with (a) the AB5 alloy electrode, (b) the AC-1/AB5 alloy (90 wt%) composite electrode, (c) the AC-2/AB5 alloy (90 wt%) composite electrode, and (d) the AC-3/AB5 alloy (90 wt%) composite electrode.
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prediction, its discharge capacity (290 mA h/g) at a 0.2 C rate is lower than the AB5 alloy electrode (297 mA h/g) and it has higher voltage at the latter charge stage, which indicates larger chargeedischarge polarization. Fig. 4 shows the SEM images of the AC particles before and after being modified with 20 wt% metal nickel. Obviously, after ball milling, the metal nickel powders are aggregated and unevenly dispersed in the AC-1 particles; on the contrary, the powders could be uniformly dispersed in the graphite particles [22]. This may be because the flake graphite particles could be a good lubricant to prevent the metal nickel powders from aggregation. Since metal nickel is a good catalyst for the electrochemical reduction/oxidation reactions between H2O and H on the surface of hydrogen absorbing materials, better dispersion of nickel in the materials should result in higher electrochemical activity of the hydrogen absorbing electrodes. In order to improve the dispersion of metal nickel in the AC particles, the other two methods were used to prepare nickel modified AC. Fig. 5 shows XRD patterns of the modified AC particles. The diffraction peaks near 44 , 52 and 76 correspond to {111}, {200} and {220} crystal planes of metal nickel, respectively (PDF number 04-0850). This result indicates that metal nickel can be successfully introduced into the AC particles by both the liquid phase reduction and hydrothermal methods. But as shown in Fig. 4, the AC-2 particles are coated with metal nickel and their surface is no longer smooth; on the other hand, tiny metal nickel is uniformly dispersed among the AC-3 particles. On the same time, as shown in Fig. 3, the discharge capacity of the AC-3/AB5 composite electrode reaches to 318 mA h/g, which is obviously larger than the AB5 alloy electrode (297 mA h/g) and the AC-2/AB5 composite electrode (300 mA h/g). These results indicate that although metal nickel coated on the AC-2 particles could benefit the electrochemical reduction/oxidation reactions between H2O and H, the defective sites for the binding between hydrogen and their surface might decrease, therefore resulting in a lower capacity than the AC-3/AB5 composite. Since the hydrothermal method is the best one among the three methods for the introduction of metal nickel into the AC particles, the AC-3/AB5 alloy (90 wt%) composites with different nickel modification amounts were prepared and their chargeedischarge curves are shown in Fig. 6. Obviously, after being modified with metal nickel, all the discharge capacity of the composite electrodes exceeds the AB5 electrode, which indicates the good catalysis of metal nickel for the electrochemical reduction/oxidation reactions on the surface of AC. The experimental results show that the appropriate modification amount is 20 wt%. Otherwise, there would be insufficient catalytic sites at lower amount or excessive nonactive material in the electrode at higher amount. Next, different composite electrodes of AC-3/AB5 alloy (95 wt%, 90 wt% or 80 wt%) were prepared and their electrochemical properties were tested. As shown in Fig. 7, the composite electrode with 10 wt% AC-3 has the maximum discharge capacity. This result indicates that there should be a synergistic effect between AC and AB5 alloy during the hydrogen storage process. The physi-sorption of hydrogen in AC and chemi-sorption of it in AB5 alloy are mutually improved under a proper ratio between AC and AB5 alloy in the composite. The discharge capacity of the composite
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Fig. 4 e SEM images of AC particles (a) untreated, (b) AC-1, (c) AC-2 and (d) AC-3, where AC-1, AC-2 and AC-3 are all modified with 20 wt% metal nickel.
electrode with 20 wt% AC-3 decreases obviously, which might suggest the insufficient enhancement of binding between H2 molecules (or H atoms) and the AC surface by AB5 alloy. Finally, high-power performance of the composite was tested and the results are shown Fig. 8. Compared to the simulated battery with the AB5 alloy electrode, the battery with the composite electrode of AC-3/AB5 alloy (90 wt%) has better high-power performance. The discharge capacity of the latter increases 16% at a 1 C rate and 59% at a 3 C rate, respectively. At the same time, the discharge voltage of the latter obviously exceeds the former. This result suggests the
Fig. 5 e XRD patterns of the modified AC particles by a hydrothermal method (a) and a liquid phase reduction method (b).
AC-3/AB5 alloy (90 wt%) composite is particularly suitable for the high-power NieMH battery. Fig. 9 shows the impedance plots for the negative electrodes discharged to 50% depth of discharge (DOD) in the 7 mol l1 KOH electrolytes. The plots exhibit three arcs in the whole frequency range. For a MH electrode, the semicircle in high-frequency region is attributed to the contact resistance between the alloy particle and the current collector; the other
Fig. 6 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate with (a) AB5 alloy electrode and (b, c, d) AC-3/AB5 alloy (90 wt%) composite electrodes, where AC-3 is modified with (b) 10 wt% (c) 20 wt% and (d) 30 wt% metal nickel.
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Fig. 7 e Typical chargeedischarge curves of the simulated batteries at a 0.2 C rate with (a) AB5 alloy electrode and (b, c, d) AC-3/AB5 alloy composite electrodes, where the mass percent of AB5 alloy in the composites is (b) 95 wt%, (c) 90 wt% and (d) 80 wt%, respectively and AC-3 is modified with 20 wt% metal nickel.
semicircle in low-frequency region is attributed to a charge transfer process on the electrode surface, and the slope is caused by the diffusion of hydrogen in the alloy [23]. According to an equivalent circuit for the frequency response of the negative electrode [22], the results of a curve fitting show that as the electrochemical reaction activity on the surface of AC-3 is lower than AB5 alloy, the charge transfer resistance of the composite electrode (0.754 U) is larger than the AB5 alloy electrode (0.498 U). But compared to the latter, the Warburg admittance of hydrogen diffusion in the former increases from 18.9S to 35.1S, which suggests improved hydrogen diffusion ability. As hydrogen diffusion in the bulk of the alloy
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Fig. 9 e Impedance plots of the negative electrodes (discharged to 50% depth of discharge) in 7 mol lL1 KOH electrolytes at a frequency range from 105 to 10L2 Hz: (a) AB5 alloy electrode and (b) AC-3/AB5 alloy (90 wt%) composite electrode, where AC-3 is modified with 20 wt% metal nickel.
is a limiting step in the reaction mechanisms of the metal hydride electrode, more rapid diffusion would result in better electrochemical properties of the electrode, especially its discharge performance at large currents.
4.
Conclusion
(1) The AC electrode has poor electrochemical hydrogen absorbing performance, and the untreated AC/AB5 alloy (90 wt%) composite electrode also shows a lower capacity and higher charge voltage than the AB5 alloy electrode. (2) Metal nickel was separately introduced into AC by three methods. The results of scanning SEM show that the metal nickel powders are aggregated and unevenly dispersed in the AC-1 particles after ball milling. The AC-2 particles are coated with metal nickel by the liquid phase reduction method, and metal nickel introduced by the hydrothermal method is uniformly dispersed among the AC-3 particles. (3) With the good catalysis of metal nickel for the electrochemical reduction/oxidation reactions on the surface of AC, the composite electrode of AC-3/AB5 alloy (90 wt%) has a 0.2 C capacity of 318 mA h/g. Compared to the AB5 alloy electrode, its discharge capacity separately increases 16% at a 1 C rate and 59% at a 3 C rate, which should benefit from more rapid H diffusion in it.
Acknowledgments Fig. 8 e Typical chargeedischarge curves of the simulated batteries at a 1 C or 3 C rate with (a) AB5 alloy electrode and (b) AC-3/AB5 alloy (90 wt%) composite electrode, where AC3 is modified with 20 wt% metal nickel.
Funding for this work is being provided by Education Department of Henan Province, China through a Leading Young Teacher Training Grant in Colleges and Universities (2011GGJS-101).
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