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Hydrogen Storage Alloy and Carbon Nanotubes Mixed Catalyst in a Direct Borohydride Fuel Cell
J. Mater. Sci. Technol., 2011, 27(12), 1089-1093.
Hydrogen Storage Alloy and Carbon Nanotubes Mixed Catalyst in a Direct Borohydride Fuel Cell Sai Li1) , Xiaodong Yang1) , Haiyan Zhu2) , Yan Liu1) and Yongning Liu1)†
1) State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi an Jiaotong University, Xi an 710049, China 2) Department of Chemistry and Chemical Engineering, Weinan Normal University, Weinan 714000, China [Manuscript received April 26, 2011, in revised form July 16, 2011]
In this study, carbon nanotubes (CNTs) were mixed with AB5 -type hydrogen storage alloy (HSA), as catalyst for an anode in a direct borohydride fuel cell (DBFC). As comparision, a series of traditional carbon materials, such as acetylene black, Vulcan XC-72R, and super activated carbon (SAC) were also employed. Electrochemical measurements showed that the electrocatalytic activity of HSA was improved greatly by CNTs. The current density of the DBFC employing the HSA/CNTs catalytic anode could reach 1550 mA·cm−2 (at −0.6 V vs the Hg/HgO electrode) and the maximum power density of 65 mW·cm−2 for this cell could be achieved at room temperature. Furthermore, the life time test lasting for 60 h showed that the cell displayed a good stability. KEY WORDS: Direct borohydride fuel cell; Carbon nanotubes; Hydrogen storage alloy; Electrocatalytic activity
1. Introduction Fuel cells, which are efficient and environmentally friendly energy devices, have attracted much attention in recent years. They are expected to serve not only in large scale equipment but also in portable and mobile applications. Direct borohydride fuel cell (DBFC), a type of liquid fuel cell, which has been firstly proposed by Indig and Snyder[1] in the early 1960s, is regarded as a potential candidate for environmentally friendly energy devices, owing to its many advantages compared with other fuel cells. DBFCs employing noble metals (Pt, Au, Pd)[2–4] , transition metals (Ni)[5] , and AB5 - and AB2 -type hydrogen storage alloys[6,7] as anode catalysts have been reported. The activity of the catalysts was improved as the reaction surface area of the catalysts increased[8] . And the activity can be improved by two ways: first, by minimizing the size of the catalyst particles; second, by mixing the catalysts with high surface area materials (such as carbon materials). A suitable composite † Corresponding author. Prof., Ph.D.; Tel.: +86 29 82664602; Fax: +86 29 82663453; E-mail address: [email protected] (Y.N. Liu).
material should have a good compatibility between the catalyst and composite materials, high stability and high conductivity. Many kinds of carbon materials such as carbon black and Vulcan XC-72R were used in large-scale in fuel cells[2,5] . Carbon nanotubes (CNTs) have the tubular threedimensional morphology, which makes them unique among carbon materials. Compared with other carbon materials, they have a great many characteristics and advantages[8] such as good mechanical and chemical properties, high conductivity and high specific surface area, etc. So, CNTs have gradually substituted traditional carbon materials in fuel cells. Recent reports have discussed the application of CNTs in fuel cells as catalyst substrates[9–14] . Chien et al.[15] found the Pt/CNTs and Pt-Ru/CNTs catalysts showed high electrocatalytic activity. Viswanathan[16] observed that the Pt/CNTs catalysts gave rise to a better activity and stability for methanol oxidation when compared to traditional carbon materials. However, most of the studies highlighted above have been focused on the interaction between CNTs and noble metals (Pt, Au, Pd), but there was no report about the per-
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formance of CNTs combined with HSA or transition metals in fuel cells. Concerning HSA, Wang et al.[17] investigated LaNi4.5 Al0.5 alloy as anodic catalyst in a DBFC. And then, Yang et al.[7] have reported that the LaNi4.5 Al0.5 alloy doped with Au supported on carbon black as the anodic material in a DBFC, which displayed a discharge current density of 250 mA·cm−2 . Liu et al.[18] have reported several Ni-based HSA as the anode materials for a DBFC with a current density of 500 mA·cm−2 (at −0.6 V vs the Hg/HgO electrode). The purpose of this work is to explore the catalytic effects of HSA mixed with CNTs as the anode materials in DBFCs, and compare it with other conventional carbon materials. It is expected to find a better compatible object for HSA. 2. Experimental 2.1 Materials The HSA (>250 mesh) with a composition of M mNi3.55 Co0.75 Mn0.4 Al0.3 (where M m represents Ce-rich mixed mischmetal composed of 50 wt% Ce, 30 wt% La, 5 wt% Pr and 15 wt% Nd) was prepared by inductive melting and then pulverized. The multiwalled CNTs ( 30–50 nm in outer diameter and 10–20 μm in length ) used in this research were purchased from Chendu Organic Chemicals Co. Ltd. Graphite, acetylene black, Vulcan XC-72R, and SAC were used as conventional carbon materials. Perovskitetype oxide (LaCoO3 ) was prepared following the sol-gel method described by our previous works[19] . Lanthanum nitrate (La(NO3 )3 ·6H2 O), cobalt nitrate (Co(NO3 )2 ·6H2 O), citric acid (C6 H8 O7 ·H2 O), and ammonia water (NH3 ·H2 O) were all of analytical grade purity. 2.2 Electrode preparation To prepare the anode, HSA powders (98 wt%) were mixed with CNTs (2 wt%) and moderate polyvinyl alcohol (PVA) solution. Then, the slurry was smeared onto a 1 cm×1 cm Ni-foam sheet (thickness=1.7 mm, porosity>95%). An experiment[20] using a blank sample in which there was no catalyst except for the Ni-foam proved that Ni-foam had no ◦ catalytic activity for BH− 4 . After drying at 80 C under vacuum for 2 h, the electrode was pressed under a pressure of 3 MPa. Before testing, the electrode was activated by immersion in a 6 mol/L KOH–0.8 mol/L KBH4 aqueous solution for 6 h. As comparison, commercial graphite, acetylene black, Vulcan XC-72R, and SAC were also mixed with the HSA under the same conditions for electrochemical measurements. The cathode was a sandwich construction consisting of a gas diffusion layer, an active layer and a current accumulating matrix. The active layer was prepared by mixing 30 wt% LaCoO3 and 45 wt% activated carbon with a 25 wt% polytetrafluoroethylene (PTFE) emulsion and then coated onto a Ni-foam. The gas diffusion layer was prepared by mix-
Fig. 1 Experimental design of the anode test system: 1— Pt wire counter electrode, 2—testing electrode, 3—Hg/HgO reference electrode, 4—Nafion 117 membranes, 5— 6 mol/L KOH electrolyte, 6— 0.8 mol/L KBH4 –6 mol/L KOH fuel
ing 60 wt% acetylene black and 40 wt% PTFE with ethanol into 0.3 mm thick film. The three-layer gas electrode was finished by pressing the coated Ni-foam and the gas diffusion layer at 3 MPa into a 0.6 mm thick sheet. 2.3 Characterisation of the catalysts The morphology and distribution of the prepared samples were characterized by means of scanning electron microscopy (SEM, JEOL-S2700). 2.4 Experimental set-up and procedure The polarisation was employed to characterize the electrocatalytic performance of anode by using a computer controlled Electrochemistry Workstation CHI650 (Chenhua, Shanghai, China) with a threeelectrodes configuration. Figure 1 shows the experimental design of the test system. The anode served as the working electrode. A Hg/HgO/6 mol/L KOH electrode was used as a reference electrode, and a Pt wire was used as a counter electrode. An anode with an active area of 1 cm2 was placed on a window on one side of the middle container. The reference electrode was connected to the body of the cell through a Luggin capillary, with the tip positioned close to the working electrode. The counter electrode (Pt wire) was placed on the other side of the container 2 cm away from the working electrode. The anode container was separated from the other containers by Nafion 117 membranes. The anolyte was 0.8 mol/L KBH4 –6 mol/L KOH or 0.8 mol/L NaBH4 –6 mol/L
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Fig. 3 Polarisation curves of the HSA-catalyzed anode using different carbon supports in 0.8 mol/L KBH4 –6 mol/L KOH solution (the current density at −0.6 V vs the Hg/HgO electrode was annotated)
Fig. 2 SEM images of HSA/CNTs with an HSA-loading of 0.14 g·cm−2
NaOH. The electrolyte in the outside compartments was either 6 mol/L KOH or 6 mol/L NaOH. The cell performances were measured by a battery testing system (from Neware Technology Limited, Shenzhen, China). The structure of this DBFC has been described in a previous paper[20] . The anode was placed inside a container, the cathode was sandwiched on a square window of the container wall and the area of the window was 1 cm2 . The gas diffusion layer of the cathode was exposed to air, whereas the active layer was in contact with the electrolyte. The anode was 2 cm away from the cathode. The electrolyte fuel was 0.8 mol/L KBH4 –6 mol/L KOH. All experiments were carried out at room temperature and under atmospheric pressure. 3. Results and Discussion 3.1 Physical characterization Figure 2 shows the micrograph of the HSA particles scattered on the CNTs. As seen from the image, unlike Pt, Au etc. precious metals dispersed on CNTs[21] (these precious metals nanoparticles could be supported on CNTs due to their small size), the CNTs appear as reticular, tangled morphology, like a fishing net, then the HSA particles are caught (Fig. 2(a)) or embedded in them (Fig. 2(b)). 3.2 Electrochemical properties Figure 3 shows the polarisation curves of various carbon-HSA mixed electrodes. Clearly, HSA mixed with graphite, Vulcan XC-72R, acetylene black, and
Fig. 4 Cyclic voltammograms on the HSA electrodes in the 0.1 mol/L KBH4 –6 mol/L KOH solution with different carbon composite materials, scan rate: 10 mV·s−1
SAC exhibits moderate performances. However, the catalytic performance is improved remarkably by using the HSA/CNTs as catalyst. The polarisation current density of the HSA/CNTs electrode reaches about 1550 mA·cm−2 (at −0.6 V vs the Hg/HgO electrode), which is much higher than that of other carbon-mixed electrodes. It is also much higher than that reported in literature, which reached 500 mA·cm−2[18] and 250 mA·cm−2[7] . The probable reason for this outcome will be discussed later. Figure 4 shows the cyclic voltammetry curves of the HSA mixed with different carbon materials at a scan rate of 10 mV·s−1 in 0.1 mol/L KBH4 –6 mol/L KOH solution. It is clear that the HSA/CNTs electrode possesses the highest anodic peak at −0.52 V vs Hg/HgO, which is close to the report[7] . The anodic peak could be ascribed to the oxidation of hydrogen absorbed on the surface of the electrode[7,22] . It also illustrates that the HSA/CNTs catalyst has excellent electrocatalytic activity on the oxidization of BH−1 4 . To further explicate the influence of CNTs on the
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Fig. 5 Polarisation curves of HSA-catalyzed anode in 6 mol/L KOH–0.8 mol/L KBH4 solution with or without CNTs
Fig. 7 Performance stability of the DBFC at the galvanostatic discharging of 20 mA·cm−2 in 0.8 mol/L KBH4 –6 mol/L KOH under ambient conditions. Anode: HSA/CNTs loading is 0.14 g·cm−2 . Cathode: LaCoO3 loading is 7.5 g·cm−2
help to promote the performance of the HSA/CNTs electrodes. Besides, the tubular three-dimensional morphology of the CNTs could make the fuel and product diffusion easier, whereas mostly carbon supports (as the Vulcan XC-72R) contain randomly distributed pores which make fuel diffusion difficult[8] . 3.3 Cell performances evaluation
Fig. 6 Performances of the DBFC using HSA/CNTs anode. Cathode: the LaCoO3 loading is 7.5 mg·cm−2 . The electrolyte fuel is 0.8 mol/L KBH4 –6 mol/L KOH
HSA, the polarisation of pure HSA, pure CNTs, and HSA/CNTs mixed electrodes have been investigated, and the results are shown in Fig. 5. Pure CNTs show hardly any electrocatalytic activity. And the pure HSA displays only limited electrocatalytic activity. The largest polarisation current density of the pure HSA electrode is about 500 mA·cm−2 (at −0.7 V vs the Hg/HgO electrode). However, when HSA mixed with CNTs, the largest polarisation current density approaches 1500 mA·cm−2 . This well confirms that CNTs help to promote the electrocatalytic activity of the HSA catalyst. According to above electrochemical experiments, the HSA mixed with CNTs have better electrocatalytic activity than the HSA mixed with other carbon materials or pure HSA. It can be inferred from Fig. 2 that the combination of CNTs with HSA improves the conductivity of the electrode. The specific crystalline nature of CNTs makes it act as a good conductive substrate. As reported, the electrical conductivity is about 0.25 S·cm−1 for common carbon supports, but CNTs possess conductivities of about 102 – 103 S·cm−1[21] . The high conductivity of CNTs will
A test cell was constructed using the HSA/CNTs as anode catalyst and LaCoO3 as cathode catalyst. The performances of the cell at different discharge current densities are shown in Fig. 6. The peak power density of 65 mW·cm−2 can be achieved at room temperature. The good cell performance should be attributed to the combination of CNTs and HSA. Furthermore, the cell performance stability was investigated. Figure 7 gives the DBFC performance stability. A continuous life test was performed by monitoring the cell voltage change during the constantcurrent discharging of 20 mA·cm−2 of the DBFC for a period of 65 h at room temperature. As shown in Fig. 7, despite some slight fluctuations, attenuation is not observed, which suggests that the cell using HSA/CNTs as the anode catalyst has good performance stability. As mentioned in literature, multiwalled CNTs were more stable than other carbon materials, the tubular morphology could be more durable and their use could extend the service life of a device compared with conventional carbon morphology[8] . 4. Conclusion In summary, CNTs were used instead of commom carbon materials to mix with HSA and their electrochemical properties for use as the anode catalyst in a DBFC were investigated. Compared with traditional carbon materials, the CNTs had a promoted function to HSA catalytic effect. The largest polarisation cur-
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rent density approached 1550 mA·cm (at −0.6 V vs the Hg/HgO electrode). And a peak power density of 65 mW·cm−2 was obtained at room temperature. Furthermore, the DBFC could keep relatively good short-term performance stability. REFERENCES [1 ] M.E. Indig and R.N. Snyder: J. Electrochem. Soc., 1962, 109, 1104. [2 ] X.Y. Geng, H.M. Zhang, W. Ye, Y.W. Ma and H.X. Zhong: J. Power Sources, 2008, 185, 627. [3 ] S.C. Amendola, P. Onnerud, M.T. Kelly, P.J. Petillo, S.L. Sharp-Goldman and M. Binder: J. Power Sources, 1999, 84, 130. [4 ] J.A. Gardiner and J.W. Collat: J. Am. Chem. Soc., 1965, 87, 1692. [5 ] B.H. Liu, Z.P. Li and S. Suda: Electrochim. Acta, 2004, 49, 3097. [6 ] L.B. Wang, C.A. Ma, X.B. Mao, J.F. Sheng, F.Z. Bai and F. Tang: Electrochem. Commun., 2005, 7, 1477. [7 ] Z.Z. Yang, L.B. Wang, Y.F. Gao, X.B. Mao and C.A. Ma: J. Power Sources, 2008, 184, 260. [8 ] E. Antolini: Appl. Catal. B, 2009, 88, 1. [9 ] V. Selvaraj and M. Alagar: Electrochem. Commun., 2007, 9, 1145.
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[10] K.T. Jeng, C.C. Chien, N.Y. Hsu, W.M. Huang, S.D. Chiou and S.H. Lin: J. Power Sources, 2007, 164, 33. [11] K. Deshmukh and K.S.V. Santhanam: J. Power Sources, 2006, 159, 1084. [12] R. Kannan, M. Parthasarathy, S.U. Maraveedu, S. Kurungot and V.K. Pillai: Langmuir, 2009, 25, 8299. [13] Z.Z. Zhu, Z. Wang and H.L. Li: J. Power Sources, 2009, 186, 339. [14] H. Su, B. Zhang and L.J. Chen: J. Mater. Sci. Technol., 2010, 26, 529. [15] C.C. Chien and K.T. Jeng: J. Mater. Chem. Phys., 2006, 99, 80. [16] B. Viswanathan: Catal. Today, 2009, 141, 52. [17] L.B. Wang, G.B. Wu, Z.Z. Yang, Y.F. Gao, X.B. Mao and C.A. Ma: J. Mater. Sci. Technol., 2011, 27, 46. [18] B.H. Liu and S. Suda: J. Alloy. Compd., 2008, 454, 280. [19] Y. Liu, J.F. Ma, J.H. Lai and Y.N. Liu: J. Alloy. Compd., 2009, 488, 204. [20] S. Li, Y.N. Liu, Y. Liu and Y.Z. Chen: J. Power Sources, 2010, 195, 7202. [21] H.Y. Du, C.H. Wang, H.C. Hsu, S.T. Chang, U.S. Chen, S.C. Yen, L.C. Chen, H.C. Shih and K.H. Chen: Diamond Relat. Mater., 2008, 17, 535. [22] S.A. Gamboa, P.J. Sebastian, F. Feng, M. Geng and D.O. Northwood: J. Electrochem. Soc., 2002, 149, A137.
Hydrogen Storage Alloy and Carbon Nanotubes Mixed Catalyst in a Direct Borohydride Fuel Cell Sai Li1) , Xiaodong Yang1) , Haiyan Zhu2) , Yan Liu1) and Yongning Liu1)†
1) State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi an Jiaotong University, Xi an 710049, China 2) Department of Chemistry and Chemical Engineering, Weinan Normal University, Weinan 714000, China [Manuscript received April 26, 2011, in revised form July 16, 2011]
In this study, carbon nanotubes (CNTs) were mixed with AB5 -type hydrogen storage alloy (HSA), as catalyst for an anode in a direct borohydride fuel cell (DBFC). As comparision, a series of traditional carbon materials, such as acetylene black, Vulcan XC-72R, and super activated carbon (SAC) were also employed. Electrochemical measurements showed that the electrocatalytic activity of HSA was improved greatly by CNTs. The current density of the DBFC employing the HSA/CNTs catalytic anode could reach 1550 mA·cm−2 (at −0.6 V vs the Hg/HgO electrode) and the maximum power density of 65 mW·cm−2 for this cell could be achieved at room temperature. Furthermore, the life time test lasting for 60 h showed that the cell displayed a good stability. KEY WORDS: Direct borohydride fuel cell; Carbon nanotubes; Hydrogen storage alloy; Electrocatalytic activity
1. Introduction Fuel cells, which are efficient and environmentally friendly energy devices, have attracted much attention in recent years. They are expected to serve not only in large scale equipment but also in portable and mobile applications. Direct borohydride fuel cell (DBFC), a type of liquid fuel cell, which has been firstly proposed by Indig and Snyder[1] in the early 1960s, is regarded as a potential candidate for environmentally friendly energy devices, owing to its many advantages compared with other fuel cells. DBFCs employing noble metals (Pt, Au, Pd)[2–4] , transition metals (Ni)[5] , and AB5 - and AB2 -type hydrogen storage alloys[6,7] as anode catalysts have been reported. The activity of the catalysts was improved as the reaction surface area of the catalysts increased[8] . And the activity can be improved by two ways: first, by minimizing the size of the catalyst particles; second, by mixing the catalysts with high surface area materials (such as carbon materials). A suitable composite † Corresponding author. Prof., Ph.D.; Tel.: +86 29 82664602; Fax: +86 29 82663453; E-mail address: [email protected] (Y.N. Liu).
material should have a good compatibility between the catalyst and composite materials, high stability and high conductivity. Many kinds of carbon materials such as carbon black and Vulcan XC-72R were used in large-scale in fuel cells[2,5] . Carbon nanotubes (CNTs) have the tubular threedimensional morphology, which makes them unique among carbon materials. Compared with other carbon materials, they have a great many characteristics and advantages[8] such as good mechanical and chemical properties, high conductivity and high specific surface area, etc. So, CNTs have gradually substituted traditional carbon materials in fuel cells. Recent reports have discussed the application of CNTs in fuel cells as catalyst substrates[9–14] . Chien et al.[15] found the Pt/CNTs and Pt-Ru/CNTs catalysts showed high electrocatalytic activity. Viswanathan[16] observed that the Pt/CNTs catalysts gave rise to a better activity and stability for methanol oxidation when compared to traditional carbon materials. However, most of the studies highlighted above have been focused on the interaction between CNTs and noble metals (Pt, Au, Pd), but there was no report about the per-
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formance of CNTs combined with HSA or transition metals in fuel cells. Concerning HSA, Wang et al.[17] investigated LaNi4.5 Al0.5 alloy as anodic catalyst in a DBFC. And then, Yang et al.[7] have reported that the LaNi4.5 Al0.5 alloy doped with Au supported on carbon black as the anodic material in a DBFC, which displayed a discharge current density of 250 mA·cm−2 . Liu et al.[18] have reported several Ni-based HSA as the anode materials for a DBFC with a current density of 500 mA·cm−2 (at −0.6 V vs the Hg/HgO electrode). The purpose of this work is to explore the catalytic effects of HSA mixed with CNTs as the anode materials in DBFCs, and compare it with other conventional carbon materials. It is expected to find a better compatible object for HSA. 2. Experimental 2.1 Materials The HSA (>250 mesh) with a composition of M mNi3.55 Co0.75 Mn0.4 Al0.3 (where M m represents Ce-rich mixed mischmetal composed of 50 wt% Ce, 30 wt% La, 5 wt% Pr and 15 wt% Nd) was prepared by inductive melting and then pulverized. The multiwalled CNTs ( 30–50 nm in outer diameter and 10–20 μm in length ) used in this research were purchased from Chendu Organic Chemicals Co. Ltd. Graphite, acetylene black, Vulcan XC-72R, and SAC were used as conventional carbon materials. Perovskitetype oxide (LaCoO3 ) was prepared following the sol-gel method described by our previous works[19] . Lanthanum nitrate (La(NO3 )3 ·6H2 O), cobalt nitrate (Co(NO3 )2 ·6H2 O), citric acid (C6 H8 O7 ·H2 O), and ammonia water (NH3 ·H2 O) were all of analytical grade purity. 2.2 Electrode preparation To prepare the anode, HSA powders (98 wt%) were mixed with CNTs (2 wt%) and moderate polyvinyl alcohol (PVA) solution. Then, the slurry was smeared onto a 1 cm×1 cm Ni-foam sheet (thickness=1.7 mm, porosity>95%). An experiment[20] using a blank sample in which there was no catalyst except for the Ni-foam proved that Ni-foam had no ◦ catalytic activity for BH− 4 . After drying at 80 C under vacuum for 2 h, the electrode was pressed under a pressure of 3 MPa. Before testing, the electrode was activated by immersion in a 6 mol/L KOH–0.8 mol/L KBH4 aqueous solution for 6 h. As comparison, commercial graphite, acetylene black, Vulcan XC-72R, and SAC were also mixed with the HSA under the same conditions for electrochemical measurements. The cathode was a sandwich construction consisting of a gas diffusion layer, an active layer and a current accumulating matrix. The active layer was prepared by mixing 30 wt% LaCoO3 and 45 wt% activated carbon with a 25 wt% polytetrafluoroethylene (PTFE) emulsion and then coated onto a Ni-foam. The gas diffusion layer was prepared by mix-
Fig. 1 Experimental design of the anode test system: 1— Pt wire counter electrode, 2—testing electrode, 3—Hg/HgO reference electrode, 4—Nafion 117 membranes, 5— 6 mol/L KOH electrolyte, 6— 0.8 mol/L KBH4 –6 mol/L KOH fuel
ing 60 wt% acetylene black and 40 wt% PTFE with ethanol into 0.3 mm thick film. The three-layer gas electrode was finished by pressing the coated Ni-foam and the gas diffusion layer at 3 MPa into a 0.6 mm thick sheet. 2.3 Characterisation of the catalysts The morphology and distribution of the prepared samples were characterized by means of scanning electron microscopy (SEM, JEOL-S2700). 2.4 Experimental set-up and procedure The polarisation was employed to characterize the electrocatalytic performance of anode by using a computer controlled Electrochemistry Workstation CHI650 (Chenhua, Shanghai, China) with a threeelectrodes configuration. Figure 1 shows the experimental design of the test system. The anode served as the working electrode. A Hg/HgO/6 mol/L KOH electrode was used as a reference electrode, and a Pt wire was used as a counter electrode. An anode with an active area of 1 cm2 was placed on a window on one side of the middle container. The reference electrode was connected to the body of the cell through a Luggin capillary, with the tip positioned close to the working electrode. The counter electrode (Pt wire) was placed on the other side of the container 2 cm away from the working electrode. The anode container was separated from the other containers by Nafion 117 membranes. The anolyte was 0.8 mol/L KBH4 –6 mol/L KOH or 0.8 mol/L NaBH4 –6 mol/L
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Fig. 3 Polarisation curves of the HSA-catalyzed anode using different carbon supports in 0.8 mol/L KBH4 –6 mol/L KOH solution (the current density at −0.6 V vs the Hg/HgO electrode was annotated)
Fig. 2 SEM images of HSA/CNTs with an HSA-loading of 0.14 g·cm−2
NaOH. The electrolyte in the outside compartments was either 6 mol/L KOH or 6 mol/L NaOH. The cell performances were measured by a battery testing system (from Neware Technology Limited, Shenzhen, China). The structure of this DBFC has been described in a previous paper[20] . The anode was placed inside a container, the cathode was sandwiched on a square window of the container wall and the area of the window was 1 cm2 . The gas diffusion layer of the cathode was exposed to air, whereas the active layer was in contact with the electrolyte. The anode was 2 cm away from the cathode. The electrolyte fuel was 0.8 mol/L KBH4 –6 mol/L KOH. All experiments were carried out at room temperature and under atmospheric pressure. 3. Results and Discussion 3.1 Physical characterization Figure 2 shows the micrograph of the HSA particles scattered on the CNTs. As seen from the image, unlike Pt, Au etc. precious metals dispersed on CNTs[21] (these precious metals nanoparticles could be supported on CNTs due to their small size), the CNTs appear as reticular, tangled morphology, like a fishing net, then the HSA particles are caught (Fig. 2(a)) or embedded in them (Fig. 2(b)). 3.2 Electrochemical properties Figure 3 shows the polarisation curves of various carbon-HSA mixed electrodes. Clearly, HSA mixed with graphite, Vulcan XC-72R, acetylene black, and
Fig. 4 Cyclic voltammograms on the HSA electrodes in the 0.1 mol/L KBH4 –6 mol/L KOH solution with different carbon composite materials, scan rate: 10 mV·s−1
SAC exhibits moderate performances. However, the catalytic performance is improved remarkably by using the HSA/CNTs as catalyst. The polarisation current density of the HSA/CNTs electrode reaches about 1550 mA·cm−2 (at −0.6 V vs the Hg/HgO electrode), which is much higher than that of other carbon-mixed electrodes. It is also much higher than that reported in literature, which reached 500 mA·cm−2[18] and 250 mA·cm−2[7] . The probable reason for this outcome will be discussed later. Figure 4 shows the cyclic voltammetry curves of the HSA mixed with different carbon materials at a scan rate of 10 mV·s−1 in 0.1 mol/L KBH4 –6 mol/L KOH solution. It is clear that the HSA/CNTs electrode possesses the highest anodic peak at −0.52 V vs Hg/HgO, which is close to the report[7] . The anodic peak could be ascribed to the oxidation of hydrogen absorbed on the surface of the electrode[7,22] . It also illustrates that the HSA/CNTs catalyst has excellent electrocatalytic activity on the oxidization of BH−1 4 . To further explicate the influence of CNTs on the
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Fig. 5 Polarisation curves of HSA-catalyzed anode in 6 mol/L KOH–0.8 mol/L KBH4 solution with or without CNTs
Fig. 7 Performance stability of the DBFC at the galvanostatic discharging of 20 mA·cm−2 in 0.8 mol/L KBH4 –6 mol/L KOH under ambient conditions. Anode: HSA/CNTs loading is 0.14 g·cm−2 . Cathode: LaCoO3 loading is 7.5 g·cm−2
help to promote the performance of the HSA/CNTs electrodes. Besides, the tubular three-dimensional morphology of the CNTs could make the fuel and product diffusion easier, whereas mostly carbon supports (as the Vulcan XC-72R) contain randomly distributed pores which make fuel diffusion difficult[8] . 3.3 Cell performances evaluation
Fig. 6 Performances of the DBFC using HSA/CNTs anode. Cathode: the LaCoO3 loading is 7.5 mg·cm−2 . The electrolyte fuel is 0.8 mol/L KBH4 –6 mol/L KOH
HSA, the polarisation of pure HSA, pure CNTs, and HSA/CNTs mixed electrodes have been investigated, and the results are shown in Fig. 5. Pure CNTs show hardly any electrocatalytic activity. And the pure HSA displays only limited electrocatalytic activity. The largest polarisation current density of the pure HSA electrode is about 500 mA·cm−2 (at −0.7 V vs the Hg/HgO electrode). However, when HSA mixed with CNTs, the largest polarisation current density approaches 1500 mA·cm−2 . This well confirms that CNTs help to promote the electrocatalytic activity of the HSA catalyst. According to above electrochemical experiments, the HSA mixed with CNTs have better electrocatalytic activity than the HSA mixed with other carbon materials or pure HSA. It can be inferred from Fig. 2 that the combination of CNTs with HSA improves the conductivity of the electrode. The specific crystalline nature of CNTs makes it act as a good conductive substrate. As reported, the electrical conductivity is about 0.25 S·cm−1 for common carbon supports, but CNTs possess conductivities of about 102 – 103 S·cm−1[21] . The high conductivity of CNTs will
A test cell was constructed using the HSA/CNTs as anode catalyst and LaCoO3 as cathode catalyst. The performances of the cell at different discharge current densities are shown in Fig. 6. The peak power density of 65 mW·cm−2 can be achieved at room temperature. The good cell performance should be attributed to the combination of CNTs and HSA. Furthermore, the cell performance stability was investigated. Figure 7 gives the DBFC performance stability. A continuous life test was performed by monitoring the cell voltage change during the constantcurrent discharging of 20 mA·cm−2 of the DBFC for a period of 65 h at room temperature. As shown in Fig. 7, despite some slight fluctuations, attenuation is not observed, which suggests that the cell using HSA/CNTs as the anode catalyst has good performance stability. As mentioned in literature, multiwalled CNTs were more stable than other carbon materials, the tubular morphology could be more durable and their use could extend the service life of a device compared with conventional carbon morphology[8] . 4. Conclusion In summary, CNTs were used instead of commom carbon materials to mix with HSA and their electrochemical properties for use as the anode catalyst in a DBFC were investigated. Compared with traditional carbon materials, the CNTs had a promoted function to HSA catalytic effect. The largest polarisation cur-
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rent density approached 1550 mA·cm (at −0.6 V vs the Hg/HgO electrode). And a peak power density of 65 mW·cm−2 was obtained at room temperature. Furthermore, the DBFC could keep relatively good short-term performance stability. REFERENCES [1 ] M.E. Indig and R.N. Snyder: J. Electrochem. Soc., 1962, 109, 1104. [2 ] X.Y. Geng, H.M. Zhang, W. Ye, Y.W. Ma and H.X. Zhong: J. Power Sources, 2008, 185, 627. [3 ] S.C. Amendola, P. Onnerud, M.T. Kelly, P.J. Petillo, S.L. Sharp-Goldman and M. Binder: J. Power Sources, 1999, 84, 130. [4 ] J.A. Gardiner and J.W. Collat: J. Am. Chem. Soc., 1965, 87, 1692. [5 ] B.H. Liu, Z.P. Li and S. Suda: Electrochim. Acta, 2004, 49, 3097. [6 ] L.B. Wang, C.A. Ma, X.B. Mao, J.F. Sheng, F.Z. Bai and F. Tang: Electrochem. Commun., 2005, 7, 1477. [7 ] Z.Z. Yang, L.B. Wang, Y.F. Gao, X.B. Mao and C.A. Ma: J. Power Sources, 2008, 184, 260. [8 ] E. Antolini: Appl. Catal. B, 2009, 88, 1. [9 ] V. Selvaraj and M. Alagar: Electrochem. Commun., 2007, 9, 1145.
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