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High electrocatalytic activity of cobalt–multiwalled carbon nanotubes–cosmetic cotton nanostructures for sodium borohydride electrooxidation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7
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High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation Dongming Zhang, Ke Ye, Kui Cheng, Dianxue Cao, Jinling Yin, Yang Xu, Guiling Wang* Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China
article info
abstract
Article history:
Flexible and wearable cobalt electrode with a unique three-dimensional hierarchical-
Received 1 February 2014
network structure is prepared by electrodeposition of spherical Co particles onto multi-
Received in revised form
walled carbon nanotubes (MWNTs) which are assembled on the skeleton of cosmetic
10 April 2014
cotton (CC). The morphology and phase structure of the cobaltemultiwalled carbon
Accepted 15 April 2014
nanotubesecosmetic cotton (CoeMWNTseCC) electrode are characterized by scanning
Available online xxx
electron microscope, transmission electron microscope and X-ray diffraction spectrom-
Keywords:
tigated by means of cyclic voltammetry and chronoamperometry. Results show that the Co
Three-dimensional hierarchical-
eMWNTseCC electrode exhibits remarkably high catalytic activity and good stability for
network structure
NaBH4 electrooxidation. The oxidation current density reaches as high as 170 mA cm2 at
Multiwalled carbon nanotubes
0.7 V in 1.0 mol dm3 NaOH and 0.1 mol dm3 NaBH4, which is higher than the most-
Cosmetic cotton
related previous results.
NaBH4 electrooxidation
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
eter. The NaBH4 electrooxidation performance on the CoeMWNTseCC electrode is inves-
reserved.
High catalytic activity
Introduction Direct borohydride fuel cells (DBFCs) using NaBH4 directly as the fuel have attracted much attention recently. The advantages of NaBH4 as a fuel include a high energy density (9.3 Wh g1), high hydrogen contents (10.6 wt.%), high chemical stability in alkaline solution, non-toxic, and easy handling [1e5]. The complete electrooxidation of NaBH4 generates 8 electrons (Eq. (1)) [1e29]. During the past years, the catalytic electrooxidation of NaBH4 has been widely studied using various catalysts. Noble metals such as Pd, Pt, Os, Au, Ag, and their alloys exhibited high catalytic activity [9e19]. However,
they suffer a drawback of high cost. Transition metals such as Ni, Cu, Zn [20e24] and hydrogen storage alloys [24e29] are alternative low cost catalysts for NaBH4 electrooxidation but they have lower catalytic activity than precious metal catalysts, and thus they are always combined with noble metals to improve the catalytic activity of the catalyst, such as AueCu, AueNi, PteNi, LaNi4.5Al0.5-Au and so on [10,11,13,15,18,19,29]. Cobalt, another transition metal, is always employed to prepare alloy with noble metals to improve the performance, such as PteCo, AueCo [13,18,19]. M.J. Janik pointed out that the electrooxidation of BH 4 must result in the breaking of four BeH bonds [1]. D.M.F. Santos pointed out that Co has a stronger ability than other non-previous metals in breaking the BeH
* Corresponding author. Tel./fax: þ86 451 82589036. E-mail address: [email protected] (G. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.04.113 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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Fig. 1 e Fabrication process of CoeMWNTseCC electrode.
bond [5], so we believe the Co will obtain a higher catalytic activity. However, few of report are about the pure cobalt for NaBH4 electrooxidation. So improving the catalytic performance of the pure cobalt electrodes is significant for the development of low cost and high performance DBFCs. Besides, the study of the pure cobalt metal catalyst must be conducive to the study of the alloys containing cobalt for further. BH 4 þ 8OH /BO2 þ 4H2 O þ 8e
(1)
Recently, flexible electrodes have caught researchers’ attention due to their portability and high deformability [30e36]. Recently, we fabricated a flexible Ni@MWNTs/Sponge electrode for NaBH4 electrooxidation and achieved a high catalytic performance [36]. Carbon fiber cloth is usually used as the base structure of the flexible electrode due to its high conductivity and corrosion resistance [30e33]. Textile with carbon nanotubes (CNTs) was employed as the base of energy storage device [34], which was considered as a new flexible electrode material. However, few of these reports has been focus on DBFCs. Cosmetic cotton is another flexible material for fabrication of novel exciting and remarkable performance electrodes because cosmetic cotton is a deformable material with a hierarchicalnetwork nature. Such a three-dimensional (3D) network allows large contact area between the active material and electrolyte. Besides, it is readily available and quite inexpensive. In this paper, MWNTs, employed as a fine hydrogen storage material to improve the catalytic activity of LaNi5 alloy for NaBH4 electrooxidation in our previous work [26], were wrapped around the framework of CC via self-assembling to form a conductive hierarchical-netwok nanostructured substrate and then spherical Co particles were directly electrodeposited on the surface of MWNTs layer. The novel 3D nanostructured CoeMWNTseCC electrode shows remarkably high catalytic activity and stability for NaBH4 electrooxidation. The oxidation current density on CoeMWNTseCC electrode is much higher than the reported transition metals electrodes, such as Zn, Ni and hydrogen storage alloys [23e29].
electrodeposition of Co on conductive MWNTs surface. To prepare the MWNTseCC substrate, a piece of CC (HAOLING Daily necessily & Cosmetics Co. Ltd.) was washed by acetone and ethanol for several times to get rid of greasy dirt and then dried at 373.15 K in a vacuum oven for 2 h prior to use. The cleaned CC was dipped in a MWNTs suspension, which consists of 2.6 mg cm3 MWNTs (>50 nm in outer diameter and 10e20 mm in length, Shenzhen Nanotech Port Co. Ltd.) and 10 mg cm3 of sodium dodecyl benzene sulfonate (SDBS) and 50 mL ultrapure water (Milli-Q, 18 MU cm). The suspension was prepared by sonicating the mixture for 12 h at ambient temperature. After dipping for 30 s, the CC was removed from the suspension and dried at 373.15 K for 2 h. The dipedry cycle was repeated once again to obtain the MWNTseCC substrate, which were further washed with a large amount of ultrapure water to remove SDBS. The MWNTseCC has a sheet resistance of 1U/square, measured by four points probe technique. Electrodeposition of Co on substrates was performed using Autolab PGSTAT302 (Eco Chemie) electrochemical work station in a conventional three-electrode electrochemical cell with a saturated Ag/AgCl, KCl reference electrode and Pt foil counter electrode. The electrodeposition was carried out at a constant current of 3 mA cm2 for 4 h in the solution containing 2.5 mol dm3 KCl, 0.4 mol dm3 NH4Cl, 0.5 mol dm3 H3BO3 and 1.0 mol dm3 CoCl2$6H2O. NaBH4 electrooxidation was also performed in the same three-electrode electrochemical cell using the 1 cm2 CoeMWNTseCC electrode. The morphology of the electrodes was determined using a scanning electron microscope (SEM, JEOL JSM-6480) and a transmission electron microscope (TEM, FEI TeccaiG2S-Twin, Philips). The structure was analyzed by a powder X-ray diffractometer (XRD, Rigaku TTR- III) equipped with Cu Ka radiation (l ¼ 0.15406 nm).
Results and discussion Fabrication process of CoeMWNTseCC electrode
Experimental The CoeMWNTseCC electrodes were prepared by selfassembling of MWNTs layer on CC followed by
The fabrication process of the CoeMWNTseCC electrodes and corresponding photos of the samples are shown in Fig. 1. The pure CC substrate is made up of cotton fibers which have a hierarchical structure with complicated surface morphology,
Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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Fig. 2 e SEM images of the MWNTseCC (a, b, c) and CoeMWNTseCC (d, e, f) electrode at different magnification and TEM image of Co nanoparticle.
functional groups such as hydroxyl groups, and high porosity [34,35]. Besides, each cotton fiber is comprised of multiple individual cotton fibrils (Fig. 1(d)). Due to the mechanical flexibility of MWNTs and strong van der Waals interactions between the nanowire CC cellulose and MWNTs, the MWNTs can be easily coated onto the skeleton of a CC and the white CC substrate turned to black after dipping in MWNTs suspension and became red after Co electrodeposition (Fig. 1(a) and (b)). The mass of the pure CC substrate, MWNTs and Co on the CoeMWNTseCC are w1.00, 0.80 and 10 mg cm2, respectively. It is obvious that the multiple individual cotton fibrils are surrounded by MWNTs, which could lead a high electronic conductivity (Fig. 1(e) and (f)). Besides, NaBH4 can be stored in the CC substrate due to its high aqueous absorption, which will lower the concentration polarization.
Material characterization The SEM images of the MWNTseCC substrate, CoeMWNTseCC electrode at different magnification and the TEM image of the deposited Co nanoparticle are shown in Fig. 2. Fig. 2(a) and (d) shows the frames of the MWNTseCC and CoeMWNTseCC. Clearly, at low magnification, both the MWNTseCC and CoeMWNTseCC electrodes exhibit 3D hierarchical-network structure and the cosmetic cotton lines are still maintained without merging into a film. Some condensates, which are the MWNTs and Co respectively, can be found in both of the two electrodes. Fig. 2(b) and (e) shows the single fiber of the MWNTseCC and CoeMWNTseCC. The diameter of single fiber is about 15 mm. It is obvious that the fiber of the CC was completely covered by the layer of the MWNTs (Fig. 2(b)), which will lead to a high electric conductivity. Compared with the MWNTseCC, the fiber of the CoeMWNTseCC was wrapped by Co particles (Fig. 2(e)). The 3D hierarchical-network allows easy transportation of reactants to the entire surface of Co active material. At the same time, the high water absorption capacity of the CC keeps amount of reactants resident in the Co surface, which will reduce the concentration polarization during the reaction process. Fig. 2(c) and (f) shows the high magnification SEM of the MWNTseCC and CoeMWNTseCC electrodes. As seen, the MWNTs were coated on the surface of the CC and formed a
hairy surface (Fig. 2(c)), which may survive as a high hydrogen adsorbent during the electrooxidation reaction [26]. Fig. 2(f) indicated that the Co film on the MWNTseCC skeletons is composed of spherical Co particles with diameters below around 1 mm. It is obvious that there are some gaps among the Co particles, which allow the NaBH4 solution diffuses into the CC and keeps the CC wet. Besides, the surface of the Co particles is not smooth, but very rough. It will remarkably increase the surface area of Co and the catalytic activity of Co surface active sites owing to the defects and the high surface energy. Fig. 2(g) shows the TEM image of a spherical Co particle. The Co particle exhibits an evenly distributed spiny edge, which could be one of the main reasons that the CoeMWNTseCC electrode has a high catalytic activity. The XRD patterns of CC, MWNTseCC and CoeMWNTseCC are shown in Fig. 3. Three broad diffraction peaks centers at 18 , 23 and 25 can be observed on the three materials, which can be attributed to carbon. Compared with the CC and MWNTseCC, the peaks positioned at 42 , 44 , 47 and 76 matched well with the (1 0 0), (0 0 2), (1 0 1) and (1 1 0) planes of Co, respectively, according to the standard crystallographic spectrum of Co (JCPDS card NO. 05e0727), indicating that Co presents in metallic state instead of oxides or hydroxides [33]. So, the XRD results demonstrate that metallic cobalt was deposited on MWNTseCC.
NaBH4 electrooxidation at CoeMWNTseCC electrode The catalytic activity of the CoeMWNTseCC electrode was further investigated by varying the NaBH4 concentration in 1 mol dm3 NaOH. As seen from the CV (Fig. 4(a)), the NaBH4 electrooxidation starts at around 1.20 V, which is more positive than the standard potential of Eq. (1) (1.24 V versus SHE and thus 1.43 V versus Ag/AgCl) [1]. This implies that the observed potential was accompanied by the hydrolysis reaction (Eq. (2)) [22] and the incomplete electrooxidation of NaBH4 (Eqs. (3) and (4)) [3,22]. The CoeMWNTseCC electrode exhibits a shoulder oxidation peak at around 0.9 V in NaBH4 solution, which is due to the oxidation of hydrogen on the CoeMWNTseCC electrode (Eq. (4)) [18]. With the increase of the NaBH4 concentration to 0.2 and 0.3 mol dm3, the hydrolysis of the NaBH4 get serious and the peaks of the hydrogen
Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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number of electrons released by the direct electrooxidation of NaBH4 is more than that by the electrooxidation of hydrogen. No corresponding reduction peaks were observed, suggesting that the electrooxidation of NaBH4 in alkaline media is an irreversible process. In addition, the current density increases with increasing the NaBH4 concentration during the overall test region (1.2 V to 0.2 V).
Fig. 3 e XRD patterns of the pristine CC, MWNTseCC, and CoeMWNTseCC.
oxidation disappeared, which demonstrated that the electrooxidation of hydrogen is controlled by the diffusion. It is obvious that the CoeMWNTseCC exhibits an oxidation peak that is around 0.73 V and a reduction peak that is around that is around 1.12 V in bare NaOH solution, the two peaks can be attributed the conversion of Co to Co(OH)2 and Co(OH)2 to Co, respectively [33] (inset in Fig. 4(a)). However, the oxidation current density in the solution containing NaBH4 is much higher than in bare NaOH, which demonstrated that the oxidation peak around 0.7 V should be attributed to the electrooxidation of NaBH4 and similar NaBH4 oxidation potential occurs in hydrogen storage alloy electrode according to our previous work [26]. Increasing the NaBH4 concentration (0.05e0.2 mol dm3), the oxidation peaks move positively and the oxidation current density increase obviously, which can be attributed to the diffusion of NaBH4 [26]. Both of the oxidation peaks of the hydrogen and NaBH4 disappeared when the NaBH4 concentration increased to 0.3 mol dm3, which indicated that the concentration polarization of both hydrogen and BH 4 decreased with the increase of NaBH4 concentration. Besides, it is obvious that the current density at 0.7 V is higher than that at 0.9 V, indicating that the
BH 4 þ2H2 O/BO2 þ4H2
(2)
BH 4 þ xOH /BO2 þ ðx 2ÞH2 O þ ð4 x=2ÞH2 þ xe
(3)
H2 þ2OH /2H2 O þ 2e
(4)
The catalytic performances of the Ni, AB5-type hydrogen storage alloys and CoeMWNTseCC electrodes are compared in Table 1. The oxidation current density at 0.6 V on CoeMWNTseCC in 0.1 mol dm3 NaBH4 reached to 19 mA (cm2 mg)1, which is almost 15 times than that on Ni powder [24] and LmNi4.78Mn0.22-Ti/Zr [28], 30 times than other reported hydrogen storage alloys [25e27], which indicated that the CoeMWNTseCC exhibits remarkably higher catalytic activity and lower cost than the most-related previous results. The high catalytic activity of the CoeMWNTseCC electrode can be attributed that the large surface of nanostructured Co and the high water absorption capacity, which remarkably lower the concentration polarization. Besides, the existence of MWNTs may enhance the performance of hydrogen electrooxidation and improve the catalytic performance [26]. Fig. 4(b) shows the CA curves measured at a constant potential of 0.7 V with 1 mol dm3 NaOH and variable NaBH4 concentration, respectively. It is obvious that the oxidation current density increased remarkably from 30 to 230 mA cm2 with the increase of NaBH4 concentration from 0.05 to 0.30 mol dm3. These results are consistent with that of CV measurements. The effect of NaOH concentration on the catalytic performance of the CoeMWNTseCC electrode for NaBH4 electrooxidation is shown in Fig. 5. Cyclic voltammograms (CVs, Fig. 5(a)) of the CoeMWNTseCC electrodes were measured in the solution containing 0.1 mol dm3 NaBH4 and x (x ¼ 1, 2 and 3) mol dm3 NaOH with the scan rate of 10 mV s1. Around 0.9 V, the CoeMWNTseCC electrode in the solution
Fig. 4 e Cyclic voltammograms (Scan rate: 10 mV sL1, a) and chronoamperometric curves (L0.7 V) of the CoeMWNTseCC recorded in 1 mol dmL3 NaOH D x mol dmL3 NaBH4 (x [ 0.05, 0.1, 0.2, 0.3). Inset in Fig. 4(a) is the CV in bare NaOH solution (1 mol dmL3). Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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Table 1 e Current density of BHL 4 electrooxidation on Ni and hydrogen storage alloys. Electrode Ni powder MmNi4.03Co0.42Mn0.31Al0.24 (AB5-type hydrogen storage alloy) LaNi5/MWNTs LmNi4.78Mn0.22/Si LmNi4.78Mn0.22-Ti/Zr LaNi4.5Al0.5-Au Ni@MWNTs/sponge CoeMWNTseCC
3 BH 4 concentration [mol dm ]
Potential (V)
Performance [mA/(cm2 mg)]
0.9 0.1
0.6 0.6
1.20 0.80
0.1 1.0 1.0 1.0 0.1 0.1
0.6 0.6 0.6 0.6 0.6 0.6
0.13 0.35 1.20 0.59 22.35 18.83
containing 1 mol dm3 NaOH and 0.1 mol dm3 NaBH4 exhibits a stronger oxidation peak that in 2 and 3 mol dm3 NaOH. The current density reached to 180 mA cm2 for the CoeMWNTseCC electrode at 0.7 V (around the second oxidation peak) in the solution containing 1 mol dm3 NaOH and 0.1 mol dm3 NaBH4, which is higher than that in the solution containing 2 and 3 mol dm3 NaOH. Increasing the NaOH concentration, the flux ratio of [OH/BH 4 ] increased and the amount of NaBH4 diffused to the electrode surface decreases and the hydrogen on the surface of the CoeMWNTseCC electrodes released from the hydrolysis and electrooxidation of NaBH4 decreased. Accordingly, the CoeMWNTseCC electrode exhibits a more positive oxidation peak potential for NaBH4 electrooxidation. The oxidation peak positions are almost overlap when in 2 and 3 mol dm3 NaOH at 0.7 V, which is likely caused by the balance of the hydrolysis and diffusion of the NaBH4. Fig. 5(b) shows the chronoamperometric curves (CA) at the potential 0.7 V. The current densities reached to a steady state at w125, 70 and 60 mA cm2 after a few seconds with the NaOH concentrations of 1, 2 and 3 mol dm3, respectively. As we know, the stoichiometry of NaBH4 electrooxidation is linked to the following reaction (Eq. (3)) [22]. When the NaBH4 concentration remains the same, the [OH/BH 4 ] increases with increasing the NaOH concentration, and the mount of the BH 4 in the surface of the CoeMWNTseCC electrode will decrease at the same time, leading to a decrease of the current density. The effect of temperature on the electrocatalytic activity of the CoeMWNTseCC electrode for NaBH4 electrooxidation was
Ref. 24 25 26 27 28 29 36 This work
investigated. Fig. 6(a) shows the CA of the catalytic performances at different temperatures. With the increase of the temperature from 298.15 K to 343.15 K, the oxidation current density increases from about 120 mA cm2 to 250 mA cm2. However, when the temperatures increase to 328.15 K and 343.15 K, the currentetime curves become noisier and decrease with the reaction time, likely due to the release of massive hydrogen gas caused by the hydrolysis of BH 4 (Eq. (2)) [26] at higher temperatures. Besides, the current density exhibits a little decrease during the process of the reaction, which may be caused by the decrease of the NaBH4. The logarithm of current densities (ln j) at 0.7 V were plotted against the reciprocal of absolute temperatures (1/T) (insert in Fig. 6(b)). The activation energy for the electrooxidation of NaBH4 on CoeMWNTseCC electrode was calculated to be 11.95 kJ mol1 obtained from the Arrhenius relationship (Eq. (5)) [37]. Where j is the current density; T is the thermodynamic temperature; R is the molar gas constant; and Ea is the activation energy. The value is lower than the electrooxidation of N2H4 [38]. It demonstrated that NaBH4 is a promising fuel with fast oxidation kinetics allowing the use of the novel CoeMWNTseCC electrode. Fig. 6(b) shows the corresponding vInj Ea ¼ 2 vT RT
(5)
Fig. 7 shows effects of the oxidation potential on the electrocatalytic activity. As seen, the current density after reaction for 600 s in the solution containing 0.10 mol dm3 NaBH4 þ 1.0 mol dm3 NaOH increased from 30 to
Fig. 5 e Cyclic voltammograms (Scan rate: 10 mV sL1, a) and chronoamperometric curves (L0.7 V) of the CoeMWNTseCC recorded in the solution containing 0.1 mol dmL3 NaBH4 D x mol dmL3 NaOH (x [ 1, 2, 3). Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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Fig. 6 e Chronoamperometric curves (L0.7 V, a) of the CoeMWNTseCC recorded in 0.1 mol dmL3 NaBH4 D 1 mol dmL3 NaOH at different temperatures. Arrhenius plot of the current densities at L0.7 V for NaBH4 electrooxidation on the CoeMWNTseCC electrode (b). 120 mA cm2 when the potential increased from 1.1 to 0.7 V. The current density is quite stable at all of the different potentials. This result implies that the CoeMWNTseCC electrode has good catalytic stability for NaBH4 electrooxidation in alkaline medium.
Acknowledgments We gratefully acknowledge the finance supported by the Fundamental Research Funds for the Central Universities (HEUCF201403018) and the Heilongjiang Postdoctoral Fund (LBH-Z13059).
Conclusions This work demonstrated that the CoeMWNTseCC electrode, simply fabricated by the “dipping and drying” and electrodeposition techniques, exhibited a high electrocatalytic performance and superior stability for NaBH4 electrooxidation. The enhanced performance is ascribed to the 3D hierarchicalnetwork structure, which ensures the massive nano-scale Co particle connect with NaBH4 and make full use of it in the limited area. At the same time, the MWNTs increase the adsorption and electrooxidation of hydrogen during the reaction. The excellent catalytic activity, facile synthesis and low cost, making it a promising material for future energy system.
Fig. 7 e Chronoamperometric curves of the CoeMWNTseCC in 1.0 mol dmL3 NaOH D 0.1 mol dmL3 NaBH4 at different potentials.
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activity for NaBH4 oxidation. Int J Hydrogen Energy 2010;35:1227e31. Zhang DM, Wang GL, Cheng K, Huang JC, Yan P, Cao DX. Enhancement of electrocatalytic performance of hydrogen storage alloys by multi-walled carbon nanotubes for sodium borohydride oxidation. J Power Sources 2014;245:482e6. Wang LB, Ma CA, Mao XB. LmNi4.78Mn0.22 alloy modified with Si used as anodic materials in borohydride fuel cells. J Alloys Compd 2005;397:313e6. Wang LB, Ma CA, Mao XB, Sun YM, Suda S. AB5-type hydrogen storage Alloy modified with Ti/Zr used as anodic materials in borohydride fuel cell. J Mater Sci Technol 2005;21:831e5. Yang ZZ, Wang LB, Gao YF, Mao XB, Ma CA. LaNi4.5Al0.5 alloy doped with Au used as anodic materials in a borohydride fuel cell. J Power Sources 2008;184:260e4. Liu CH, Ko TH, Kuo WS, Chou HK, Chang HW, Liao YK. Effect of carbon fiber cloth with different structure on the performance of low temperature proton exchange membrane fuel cells. J Power Sources 2009;186:450e4. Yang F, Cheng K, Mo YH, Yu LQ, Yin JL, Wang GL, et al. Direct peroxide-peroxide fuel cell-Part 1: the anode and cathode catalyst of carbon fiber cloth supported dendritic Pd. J Power Sources 2012;217:562e8. Milczarek G, Ciszewski A, Stepniak I. Oxygen-doped activated carbon fiber cloth as electrode material for electrochemical capacitor. J Power Sources 2011;196:7882e5. Yang F, Cheng K, Xiao X, Yin JL, Wang GL, Cao DX. Nickel and cobalt electrodeposited on carbon fiber cloth as the anode of direct hydrogen peroxide fuel cell. J Power Sources 2014;245:89e94. Hu LB, Chen W, Xie X, Liu N, Yang Y, Wu H, Yao Y, Pasta M, Alshareef HN, Cui Y. Symmetrical MnO2 carbon nanotube textile nanostructures for wearable pseudocapacitors with high mass loading. ACS Nano 2011;5:8904e13. Hu LB, Pasta M, Mantia FL, Cui LF, Jeong S, Deshazer HD, et al. Stretchable, porous, and conductive energy textiles. Nano Lett 2010;10:708e14. Zhang DM, Cheng K, Shi NN, Guo F, Wang GL, Cao DX. Nickel particles supported on multi-walled carbon nanotubes modified sponge for sodium borohydride electrooxidation. Electrochem Commun 2013;35:128e30. Cao DX, Sun LM, Wang GL, Lv YZ, Zhang ML. Kinetics of hydrogen peroxide electroreduction on Pd nanoparticles in acidic medium. J Electroanal Chem 2008;621:31e7. Liu R, Jiang X, Guo F, Shi NN, Yin JL, Wang GL, et al. Carbon fiber cloth supported micro- and nano-structured Co as the electrode for hydrazine oxidation in alkaline media. Electrochim Acta 2013;94:214e8.
Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation Dongming Zhang, Ke Ye, Kui Cheng, Dianxue Cao, Jinling Yin, Yang Xu, Guiling Wang* Key Laboratory of Superlight Materials and Surface Technology of Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, PR China
article info
abstract
Article history:
Flexible and wearable cobalt electrode with a unique three-dimensional hierarchical-
Received 1 February 2014
network structure is prepared by electrodeposition of spherical Co particles onto multi-
Received in revised form
walled carbon nanotubes (MWNTs) which are assembled on the skeleton of cosmetic
10 April 2014
cotton (CC). The morphology and phase structure of the cobaltemultiwalled carbon
Accepted 15 April 2014
nanotubesecosmetic cotton (CoeMWNTseCC) electrode are characterized by scanning
Available online xxx
electron microscope, transmission electron microscope and X-ray diffraction spectrom-
Keywords:
tigated by means of cyclic voltammetry and chronoamperometry. Results show that the Co
Three-dimensional hierarchical-
eMWNTseCC electrode exhibits remarkably high catalytic activity and good stability for
network structure
NaBH4 electrooxidation. The oxidation current density reaches as high as 170 mA cm2 at
Multiwalled carbon nanotubes
0.7 V in 1.0 mol dm3 NaOH and 0.1 mol dm3 NaBH4, which is higher than the most-
Cosmetic cotton
related previous results.
NaBH4 electrooxidation
Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
eter. The NaBH4 electrooxidation performance on the CoeMWNTseCC electrode is inves-
reserved.
High catalytic activity
Introduction Direct borohydride fuel cells (DBFCs) using NaBH4 directly as the fuel have attracted much attention recently. The advantages of NaBH4 as a fuel include a high energy density (9.3 Wh g1), high hydrogen contents (10.6 wt.%), high chemical stability in alkaline solution, non-toxic, and easy handling [1e5]. The complete electrooxidation of NaBH4 generates 8 electrons (Eq. (1)) [1e29]. During the past years, the catalytic electrooxidation of NaBH4 has been widely studied using various catalysts. Noble metals such as Pd, Pt, Os, Au, Ag, and their alloys exhibited high catalytic activity [9e19]. However,
they suffer a drawback of high cost. Transition metals such as Ni, Cu, Zn [20e24] and hydrogen storage alloys [24e29] are alternative low cost catalysts for NaBH4 electrooxidation but they have lower catalytic activity than precious metal catalysts, and thus they are always combined with noble metals to improve the catalytic activity of the catalyst, such as AueCu, AueNi, PteNi, LaNi4.5Al0.5-Au and so on [10,11,13,15,18,19,29]. Cobalt, another transition metal, is always employed to prepare alloy with noble metals to improve the performance, such as PteCo, AueCo [13,18,19]. M.J. Janik pointed out that the electrooxidation of BH 4 must result in the breaking of four BeH bonds [1]. D.M.F. Santos pointed out that Co has a stronger ability than other non-previous metals in breaking the BeH
* Corresponding author. Tel./fax: þ86 451 82589036. E-mail address: [email protected] (G. Wang). http://dx.doi.org/10.1016/j.ijhydene.2014.04.113 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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Fig. 1 e Fabrication process of CoeMWNTseCC electrode.
bond [5], so we believe the Co will obtain a higher catalytic activity. However, few of report are about the pure cobalt for NaBH4 electrooxidation. So improving the catalytic performance of the pure cobalt electrodes is significant for the development of low cost and high performance DBFCs. Besides, the study of the pure cobalt metal catalyst must be conducive to the study of the alloys containing cobalt for further. BH 4 þ 8OH /BO2 þ 4H2 O þ 8e
(1)
Recently, flexible electrodes have caught researchers’ attention due to their portability and high deformability [30e36]. Recently, we fabricated a flexible Ni@MWNTs/Sponge electrode for NaBH4 electrooxidation and achieved a high catalytic performance [36]. Carbon fiber cloth is usually used as the base structure of the flexible electrode due to its high conductivity and corrosion resistance [30e33]. Textile with carbon nanotubes (CNTs) was employed as the base of energy storage device [34], which was considered as a new flexible electrode material. However, few of these reports has been focus on DBFCs. Cosmetic cotton is another flexible material for fabrication of novel exciting and remarkable performance electrodes because cosmetic cotton is a deformable material with a hierarchicalnetwork nature. Such a three-dimensional (3D) network allows large contact area between the active material and electrolyte. Besides, it is readily available and quite inexpensive. In this paper, MWNTs, employed as a fine hydrogen storage material to improve the catalytic activity of LaNi5 alloy for NaBH4 electrooxidation in our previous work [26], were wrapped around the framework of CC via self-assembling to form a conductive hierarchical-netwok nanostructured substrate and then spherical Co particles were directly electrodeposited on the surface of MWNTs layer. The novel 3D nanostructured CoeMWNTseCC electrode shows remarkably high catalytic activity and stability for NaBH4 electrooxidation. The oxidation current density on CoeMWNTseCC electrode is much higher than the reported transition metals electrodes, such as Zn, Ni and hydrogen storage alloys [23e29].
electrodeposition of Co on conductive MWNTs surface. To prepare the MWNTseCC substrate, a piece of CC (HAOLING Daily necessily & Cosmetics Co. Ltd.) was washed by acetone and ethanol for several times to get rid of greasy dirt and then dried at 373.15 K in a vacuum oven for 2 h prior to use. The cleaned CC was dipped in a MWNTs suspension, which consists of 2.6 mg cm3 MWNTs (>50 nm in outer diameter and 10e20 mm in length, Shenzhen Nanotech Port Co. Ltd.) and 10 mg cm3 of sodium dodecyl benzene sulfonate (SDBS) and 50 mL ultrapure water (Milli-Q, 18 MU cm). The suspension was prepared by sonicating the mixture for 12 h at ambient temperature. After dipping for 30 s, the CC was removed from the suspension and dried at 373.15 K for 2 h. The dipedry cycle was repeated once again to obtain the MWNTseCC substrate, which were further washed with a large amount of ultrapure water to remove SDBS. The MWNTseCC has a sheet resistance of 1U/square, measured by four points probe technique. Electrodeposition of Co on substrates was performed using Autolab PGSTAT302 (Eco Chemie) electrochemical work station in a conventional three-electrode electrochemical cell with a saturated Ag/AgCl, KCl reference electrode and Pt foil counter electrode. The electrodeposition was carried out at a constant current of 3 mA cm2 for 4 h in the solution containing 2.5 mol dm3 KCl, 0.4 mol dm3 NH4Cl, 0.5 mol dm3 H3BO3 and 1.0 mol dm3 CoCl2$6H2O. NaBH4 electrooxidation was also performed in the same three-electrode electrochemical cell using the 1 cm2 CoeMWNTseCC electrode. The morphology of the electrodes was determined using a scanning electron microscope (SEM, JEOL JSM-6480) and a transmission electron microscope (TEM, FEI TeccaiG2S-Twin, Philips). The structure was analyzed by a powder X-ray diffractometer (XRD, Rigaku TTR- III) equipped with Cu Ka radiation (l ¼ 0.15406 nm).
Results and discussion Fabrication process of CoeMWNTseCC electrode
Experimental The CoeMWNTseCC electrodes were prepared by selfassembling of MWNTs layer on CC followed by
The fabrication process of the CoeMWNTseCC electrodes and corresponding photos of the samples are shown in Fig. 1. The pure CC substrate is made up of cotton fibers which have a hierarchical structure with complicated surface morphology,
Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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Fig. 2 e SEM images of the MWNTseCC (a, b, c) and CoeMWNTseCC (d, e, f) electrode at different magnification and TEM image of Co nanoparticle.
functional groups such as hydroxyl groups, and high porosity [34,35]. Besides, each cotton fiber is comprised of multiple individual cotton fibrils (Fig. 1(d)). Due to the mechanical flexibility of MWNTs and strong van der Waals interactions between the nanowire CC cellulose and MWNTs, the MWNTs can be easily coated onto the skeleton of a CC and the white CC substrate turned to black after dipping in MWNTs suspension and became red after Co electrodeposition (Fig. 1(a) and (b)). The mass of the pure CC substrate, MWNTs and Co on the CoeMWNTseCC are w1.00, 0.80 and 10 mg cm2, respectively. It is obvious that the multiple individual cotton fibrils are surrounded by MWNTs, which could lead a high electronic conductivity (Fig. 1(e) and (f)). Besides, NaBH4 can be stored in the CC substrate due to its high aqueous absorption, which will lower the concentration polarization.
Material characterization The SEM images of the MWNTseCC substrate, CoeMWNTseCC electrode at different magnification and the TEM image of the deposited Co nanoparticle are shown in Fig. 2. Fig. 2(a) and (d) shows the frames of the MWNTseCC and CoeMWNTseCC. Clearly, at low magnification, both the MWNTseCC and CoeMWNTseCC electrodes exhibit 3D hierarchical-network structure and the cosmetic cotton lines are still maintained without merging into a film. Some condensates, which are the MWNTs and Co respectively, can be found in both of the two electrodes. Fig. 2(b) and (e) shows the single fiber of the MWNTseCC and CoeMWNTseCC. The diameter of single fiber is about 15 mm. It is obvious that the fiber of the CC was completely covered by the layer of the MWNTs (Fig. 2(b)), which will lead to a high electric conductivity. Compared with the MWNTseCC, the fiber of the CoeMWNTseCC was wrapped by Co particles (Fig. 2(e)). The 3D hierarchical-network allows easy transportation of reactants to the entire surface of Co active material. At the same time, the high water absorption capacity of the CC keeps amount of reactants resident in the Co surface, which will reduce the concentration polarization during the reaction process. Fig. 2(c) and (f) shows the high magnification SEM of the MWNTseCC and CoeMWNTseCC electrodes. As seen, the MWNTs were coated on the surface of the CC and formed a
hairy surface (Fig. 2(c)), which may survive as a high hydrogen adsorbent during the electrooxidation reaction [26]. Fig. 2(f) indicated that the Co film on the MWNTseCC skeletons is composed of spherical Co particles with diameters below around 1 mm. It is obvious that there are some gaps among the Co particles, which allow the NaBH4 solution diffuses into the CC and keeps the CC wet. Besides, the surface of the Co particles is not smooth, but very rough. It will remarkably increase the surface area of Co and the catalytic activity of Co surface active sites owing to the defects and the high surface energy. Fig. 2(g) shows the TEM image of a spherical Co particle. The Co particle exhibits an evenly distributed spiny edge, which could be one of the main reasons that the CoeMWNTseCC electrode has a high catalytic activity. The XRD patterns of CC, MWNTseCC and CoeMWNTseCC are shown in Fig. 3. Three broad diffraction peaks centers at 18 , 23 and 25 can be observed on the three materials, which can be attributed to carbon. Compared with the CC and MWNTseCC, the peaks positioned at 42 , 44 , 47 and 76 matched well with the (1 0 0), (0 0 2), (1 0 1) and (1 1 0) planes of Co, respectively, according to the standard crystallographic spectrum of Co (JCPDS card NO. 05e0727), indicating that Co presents in metallic state instead of oxides or hydroxides [33]. So, the XRD results demonstrate that metallic cobalt was deposited on MWNTseCC.
NaBH4 electrooxidation at CoeMWNTseCC electrode The catalytic activity of the CoeMWNTseCC electrode was further investigated by varying the NaBH4 concentration in 1 mol dm3 NaOH. As seen from the CV (Fig. 4(a)), the NaBH4 electrooxidation starts at around 1.20 V, which is more positive than the standard potential of Eq. (1) (1.24 V versus SHE and thus 1.43 V versus Ag/AgCl) [1]. This implies that the observed potential was accompanied by the hydrolysis reaction (Eq. (2)) [22] and the incomplete electrooxidation of NaBH4 (Eqs. (3) and (4)) [3,22]. The CoeMWNTseCC electrode exhibits a shoulder oxidation peak at around 0.9 V in NaBH4 solution, which is due to the oxidation of hydrogen on the CoeMWNTseCC electrode (Eq. (4)) [18]. With the increase of the NaBH4 concentration to 0.2 and 0.3 mol dm3, the hydrolysis of the NaBH4 get serious and the peaks of the hydrogen
Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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number of electrons released by the direct electrooxidation of NaBH4 is more than that by the electrooxidation of hydrogen. No corresponding reduction peaks were observed, suggesting that the electrooxidation of NaBH4 in alkaline media is an irreversible process. In addition, the current density increases with increasing the NaBH4 concentration during the overall test region (1.2 V to 0.2 V).
Fig. 3 e XRD patterns of the pristine CC, MWNTseCC, and CoeMWNTseCC.
oxidation disappeared, which demonstrated that the electrooxidation of hydrogen is controlled by the diffusion. It is obvious that the CoeMWNTseCC exhibits an oxidation peak that is around 0.73 V and a reduction peak that is around that is around 1.12 V in bare NaOH solution, the two peaks can be attributed the conversion of Co to Co(OH)2 and Co(OH)2 to Co, respectively [33] (inset in Fig. 4(a)). However, the oxidation current density in the solution containing NaBH4 is much higher than in bare NaOH, which demonstrated that the oxidation peak around 0.7 V should be attributed to the electrooxidation of NaBH4 and similar NaBH4 oxidation potential occurs in hydrogen storage alloy electrode according to our previous work [26]. Increasing the NaBH4 concentration (0.05e0.2 mol dm3), the oxidation peaks move positively and the oxidation current density increase obviously, which can be attributed to the diffusion of NaBH4 [26]. Both of the oxidation peaks of the hydrogen and NaBH4 disappeared when the NaBH4 concentration increased to 0.3 mol dm3, which indicated that the concentration polarization of both hydrogen and BH 4 decreased with the increase of NaBH4 concentration. Besides, it is obvious that the current density at 0.7 V is higher than that at 0.9 V, indicating that the
BH 4 þ2H2 O/BO2 þ4H2
(2)
BH 4 þ xOH /BO2 þ ðx 2ÞH2 O þ ð4 x=2ÞH2 þ xe
(3)
H2 þ2OH /2H2 O þ 2e
(4)
The catalytic performances of the Ni, AB5-type hydrogen storage alloys and CoeMWNTseCC electrodes are compared in Table 1. The oxidation current density at 0.6 V on CoeMWNTseCC in 0.1 mol dm3 NaBH4 reached to 19 mA (cm2 mg)1, which is almost 15 times than that on Ni powder [24] and LmNi4.78Mn0.22-Ti/Zr [28], 30 times than other reported hydrogen storage alloys [25e27], which indicated that the CoeMWNTseCC exhibits remarkably higher catalytic activity and lower cost than the most-related previous results. The high catalytic activity of the CoeMWNTseCC electrode can be attributed that the large surface of nanostructured Co and the high water absorption capacity, which remarkably lower the concentration polarization. Besides, the existence of MWNTs may enhance the performance of hydrogen electrooxidation and improve the catalytic performance [26]. Fig. 4(b) shows the CA curves measured at a constant potential of 0.7 V with 1 mol dm3 NaOH and variable NaBH4 concentration, respectively. It is obvious that the oxidation current density increased remarkably from 30 to 230 mA cm2 with the increase of NaBH4 concentration from 0.05 to 0.30 mol dm3. These results are consistent with that of CV measurements. The effect of NaOH concentration on the catalytic performance of the CoeMWNTseCC electrode for NaBH4 electrooxidation is shown in Fig. 5. Cyclic voltammograms (CVs, Fig. 5(a)) of the CoeMWNTseCC electrodes were measured in the solution containing 0.1 mol dm3 NaBH4 and x (x ¼ 1, 2 and 3) mol dm3 NaOH with the scan rate of 10 mV s1. Around 0.9 V, the CoeMWNTseCC electrode in the solution
Fig. 4 e Cyclic voltammograms (Scan rate: 10 mV sL1, a) and chronoamperometric curves (L0.7 V) of the CoeMWNTseCC recorded in 1 mol dmL3 NaOH D x mol dmL3 NaBH4 (x [ 0.05, 0.1, 0.2, 0.3). Inset in Fig. 4(a) is the CV in bare NaOH solution (1 mol dmL3). Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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Table 1 e Current density of BHL 4 electrooxidation on Ni and hydrogen storage alloys. Electrode Ni powder MmNi4.03Co0.42Mn0.31Al0.24 (AB5-type hydrogen storage alloy) LaNi5/MWNTs LmNi4.78Mn0.22/Si LmNi4.78Mn0.22-Ti/Zr LaNi4.5Al0.5-Au Ni@MWNTs/sponge CoeMWNTseCC
3 BH 4 concentration [mol dm ]
Potential (V)
Performance [mA/(cm2 mg)]
0.9 0.1
0.6 0.6
1.20 0.80
0.1 1.0 1.0 1.0 0.1 0.1
0.6 0.6 0.6 0.6 0.6 0.6
0.13 0.35 1.20 0.59 22.35 18.83
containing 1 mol dm3 NaOH and 0.1 mol dm3 NaBH4 exhibits a stronger oxidation peak that in 2 and 3 mol dm3 NaOH. The current density reached to 180 mA cm2 for the CoeMWNTseCC electrode at 0.7 V (around the second oxidation peak) in the solution containing 1 mol dm3 NaOH and 0.1 mol dm3 NaBH4, which is higher than that in the solution containing 2 and 3 mol dm3 NaOH. Increasing the NaOH concentration, the flux ratio of [OH/BH 4 ] increased and the amount of NaBH4 diffused to the electrode surface decreases and the hydrogen on the surface of the CoeMWNTseCC electrodes released from the hydrolysis and electrooxidation of NaBH4 decreased. Accordingly, the CoeMWNTseCC electrode exhibits a more positive oxidation peak potential for NaBH4 electrooxidation. The oxidation peak positions are almost overlap when in 2 and 3 mol dm3 NaOH at 0.7 V, which is likely caused by the balance of the hydrolysis and diffusion of the NaBH4. Fig. 5(b) shows the chronoamperometric curves (CA) at the potential 0.7 V. The current densities reached to a steady state at w125, 70 and 60 mA cm2 after a few seconds with the NaOH concentrations of 1, 2 and 3 mol dm3, respectively. As we know, the stoichiometry of NaBH4 electrooxidation is linked to the following reaction (Eq. (3)) [22]. When the NaBH4 concentration remains the same, the [OH/BH 4 ] increases with increasing the NaOH concentration, and the mount of the BH 4 in the surface of the CoeMWNTseCC electrode will decrease at the same time, leading to a decrease of the current density. The effect of temperature on the electrocatalytic activity of the CoeMWNTseCC electrode for NaBH4 electrooxidation was
Ref. 24 25 26 27 28 29 36 This work
investigated. Fig. 6(a) shows the CA of the catalytic performances at different temperatures. With the increase of the temperature from 298.15 K to 343.15 K, the oxidation current density increases from about 120 mA cm2 to 250 mA cm2. However, when the temperatures increase to 328.15 K and 343.15 K, the currentetime curves become noisier and decrease with the reaction time, likely due to the release of massive hydrogen gas caused by the hydrolysis of BH 4 (Eq. (2)) [26] at higher temperatures. Besides, the current density exhibits a little decrease during the process of the reaction, which may be caused by the decrease of the NaBH4. The logarithm of current densities (ln j) at 0.7 V were plotted against the reciprocal of absolute temperatures (1/T) (insert in Fig. 6(b)). The activation energy for the electrooxidation of NaBH4 on CoeMWNTseCC electrode was calculated to be 11.95 kJ mol1 obtained from the Arrhenius relationship (Eq. (5)) [37]. Where j is the current density; T is the thermodynamic temperature; R is the molar gas constant; and Ea is the activation energy. The value is lower than the electrooxidation of N2H4 [38]. It demonstrated that NaBH4 is a promising fuel with fast oxidation kinetics allowing the use of the novel CoeMWNTseCC electrode. Fig. 6(b) shows the corresponding vInj Ea ¼ 2 vT RT
(5)
Fig. 7 shows effects of the oxidation potential on the electrocatalytic activity. As seen, the current density after reaction for 600 s in the solution containing 0.10 mol dm3 NaBH4 þ 1.0 mol dm3 NaOH increased from 30 to
Fig. 5 e Cyclic voltammograms (Scan rate: 10 mV sL1, a) and chronoamperometric curves (L0.7 V) of the CoeMWNTseCC recorded in the solution containing 0.1 mol dmL3 NaBH4 D x mol dmL3 NaOH (x [ 1, 2, 3). Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
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Fig. 6 e Chronoamperometric curves (L0.7 V, a) of the CoeMWNTseCC recorded in 0.1 mol dmL3 NaBH4 D 1 mol dmL3 NaOH at different temperatures. Arrhenius plot of the current densities at L0.7 V for NaBH4 electrooxidation on the CoeMWNTseCC electrode (b). 120 mA cm2 when the potential increased from 1.1 to 0.7 V. The current density is quite stable at all of the different potentials. This result implies that the CoeMWNTseCC electrode has good catalytic stability for NaBH4 electrooxidation in alkaline medium.
Acknowledgments We gratefully acknowledge the finance supported by the Fundamental Research Funds for the Central Universities (HEUCF201403018) and the Heilongjiang Postdoctoral Fund (LBH-Z13059).
Conclusions This work demonstrated that the CoeMWNTseCC electrode, simply fabricated by the “dipping and drying” and electrodeposition techniques, exhibited a high electrocatalytic performance and superior stability for NaBH4 electrooxidation. The enhanced performance is ascribed to the 3D hierarchicalnetwork structure, which ensures the massive nano-scale Co particle connect with NaBH4 and make full use of it in the limited area. At the same time, the MWNTs increase the adsorption and electrooxidation of hydrogen during the reaction. The excellent catalytic activity, facile synthesis and low cost, making it a promising material for future energy system.
Fig. 7 e Chronoamperometric curves of the CoeMWNTseCC in 1.0 mol dmL3 NaOH D 0.1 mol dmL3 NaBH4 at different potentials.
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Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 4 ) 1 e7
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Please cite this article in press as: Zhang D, et al., High electrocatalytic activity of cobaltemultiwalled carbon nanotubesecosmetic cotton nanostructures for sodium borohydride electrooxidation, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/j.ijhydene.2014.04.113