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Ultrafine amorphous Co–W–B alloy as the anode catalyst for a direct borohydride fuel cell
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 ) 2 8 8 4 e2 8 8 8
Available online at www.sciencedirect.com
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Ultrafine amorphous CoeWeB alloy as the anode catalyst for a direct borohydride fuel cell Sai Li a,*, Xiaodong Yang b, Haiyan Zhu b, Xiaozhu Wei b, Yongning Liu b a
School of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, PR China State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Eng., Xi’an Jiaotong University, Xi’an 710049, PR China b
article info
abstract
Article history:
The ultrafine amorphous CoeWeB alloy has been synthesized by chemical reduction and
Received 14 October 2012
used as anode catalyst in direct borohydride fuel cell. The results show that the maximum
Received in revised form
power output of the cell is 101 mW cm2 at 15 C, and the essential power density of this
24 November 2012
material can be up to 350 mW cm2 at 15 C and 500 mW cm2 at 60 C, respectively. The
Accepted 30 November 2012
cell has also a good durability, with no attenuation observed after one week of operation.
Available online 11 January 2013
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Keywords: Direct borohydride fuel cell Ultrafine amorphous CoeWeB catalyst Cell performance
1.
Introduction
Fuel cells, which can convert chemical energy into electrical energy with a high efficiency and low/zero-emission, are considered as promising power sources. It widely used in the field, such as mobile phones, portable computers, automobiles and power generators. Among various fuel cells, direct borohydride fuel cell (DBFC) which use KBH4 or NaBH4 aqueous solution as fuel [1] is one of the most exciting energy technologies. Compared with other liquid fuel cells, DBFC has many advantages [2e4] such as high theoretical open cell voltage (OCV) (1.64 V), high energy density (9300 Wh kg1), and low toxicity of borohydride. However, its development and commercialization is limited by some key issues like high cost. Platinum or platinum-based catalysts show good catalytic activity toward the electrochemical oxidation reaction, and
are wildly employed in DBFC. However the platinum resources are extremely limited [5], giving rise to the main technological obstacles in the development of DBFC. Lots of researchers have been therefore interested in exploring lower-cost substitutes, and have made some progresses. For examples, some transition metals (Ni and Cu) [6,7] and hydrogen storage alloys (AB5- and AB2-type alloys) [8e10] have been successful alternative platinum as anode in DBFCs. More recently, much effort has been engaged in developing binary alloy [3]. Such material is particularly important in the field of catalysts attributing to better catalytic properties than single component. For example, Geng et al. [11] prepared carbon-supported Ni and NiePt anodic catalyst. Electrochemical measurements showed that the binary alloy NiePt has a better electrocatalytic activity and stability than pure metal Ni catalyst [11]. Feng et al. [12] studied Ag and AgeNi alloy as anode catalyst and found that AgeNi alloy exhibited a higher
* Corresponding author. Tel.: þ86 29 8266 4602; fax: þ86 29 8266 3453. E-mail address: [email protected] (S. Li). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.148
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discharge voltage and larger capacity than Ag. In our previous studies [13], we found as well that the amorphous CoB alloy for catalytic oxidation reaction of BH 4 prepared by the combination of 3d transition metals Co with boron exhibited excellent electrochemical performance for DBFC. Based on our previous works, in this study, we explored the performance of multi-metal alloys materials and found by introducing W into the binary CoB alloy that the catalytic activity of the CoB is significantly improved.
2.
Experimental
2.1.
Catalyst preparation
The ultrafine amorphous CoeWeB catalyst was synthesized by chemical reduction of cobalt chloride (CoCl2) and sodium tungstate (Na2WO4) with potassium borohydride solution [14]. The different volume of CoCl2 (0.2 mol L1) and Na2WO4 (0.2 mol L1) solutions were mixed together to adjust the tungsten content in samples. Then 2.0 mol L1 KBH4/ 0.2 mol L1 KOH solution was added dropwise into the mixed solution at magnetic stirring. The B: (Co þ W) molar ratio was 3.0. The stirring needs to be held for 1 h without stop after the addition of potassium borohydride to release hydrogen. The resulting black precipitate was filtrated and washed with distilled water to neutrality, cut off from air by absolute alcohol and finally dried under vacuum at 60 C for 12 h. The cathode catalyst, perovskite-type oxide (LaNi0.9Ru0.1O3) was prepared following the solegel method described by previous paper [15]. Lanthanum nitrate (La(NO3)3 6H2O), nickel nitrate (Ni(NO3)2 6H2O), ruthenium chloride (RuCl3 nH20), citric acid (C6H8O7 H2O), and ammonia water (NH3 H2O) (25e28 wt.%) were used as raw materials. All the reagents are of analytical grade in purity (Beijing Jinkemei chemical product co. ltd.).
Catalyst characterization
The particle size and morphology of the samples were observed by transmission electron microscopy (TEM). The structure of the CoeWeB powders was investigated with an X-ray diffractometer (D/MAX-3A, Japan) using a Cu Ka (l ¼ 1.5444 A) source. The elemental composition of the sample was analyzed by energy-dispersive X-ray spectroscopy (EDX).
2.3.
polytetrafluoroethylene (PTFE) emulsion and then coated onto a Ni-foam. The mass loading in the cathode was 7.5 mg cm2. The gas diffusion layer was prepared by mixing 60 wt.% acetylene black and 40 wt.% PTFE with ethanol and rolled into 0.3 mm thick film. The three-layer electrode was finished by pressing the coated Ni-foam and the gas diffusion layer at pressure of 3 MPa into a sheet with thickness of 0.6 mm.
2.4.
The cell performances evaluation
The cell performances were measured by a battery testing system (from Neware Technology Limited, Shenzhen, China). In the cell test system, the anode was placed inside of a container, the cathode was fixed 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 M KBH4e6 M KOH. And the structure of the DBFC has been described in our previous paper [16].
Electrodes preparation
To prepare the anode, CoeWeB powders (78 wt.%) were mixed together with 30% polytetrafluoroethylene (PTFE) solution (22 wt.%). The mixture was smeared onto a 1 cm 1 cm Ni-foam sheet (thickness ¼ 1.7 mm, porosity > 95%). An experiment [16] with a blank sample without catalyst on Ni-foam proved that Ni-foam has no catalytic activity for BH 4. The electrode is dried at 80 C under vacuum for 2 h, and was pressed under 3 MPa. The mass loading in the anode was 70 mg cm2. The cathode has 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.% LaNi0.9Ru0.1O3 and 45 wt.% carbon nanotubes with a 25 wt.%
intensity (a.u.)
2.2.
Fig. 1 e TEM images of the CoeWeB catalyst.
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2 theta (deg.) Fig. 2 e The XRD pattern of the CoeWeB catalyst.
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3.2.
Before the cell performance test, the polarization curves of both anode and cathode were tested, as depicted in Fig. 4(a). We found that the electrochemical dynamics of the cathode is dramatically inferior to that of the anode, according to their potential variation between the anode and the cathode. Concretely, when the current density changes from 0 to 100 mA cm2, the cathode potential decreases from 0 to 0.24 V while the anode potential increases from 1.12 to 1.1 V. The cathode, which is more polarized, has a controlling impact on the cell output voltage and power density. The same conclusion was also reached in our previous work [13]. Thus, to disclose the maximum power output of the anode material, or the essential power of this material [13], we must minimize the influence of cathode polarization and maximize the affect of anode, which is attained by reducing the anode electrode area while keeping normal the cathode area. Fig. 4(b) represents the performances of the designed cells assembled by the anodes of different sizes and the cathode of a constant size. The highest power density of 359 mW cm2 was achieved at 15 C with an anode of 0.25 cm2, 63% higher than binary CoB alloy, whose power density was 220 mW cm2 at the same temperature. This value should be represent the essential power of the CoeWeB material can output. Simultaneously, the peak power density was only 101 mW cm2 in the condition that the anode and the cathode are of same size, another proof of the controlling role of the cathode. Fig. 5 compares the performances of the DBFCs assembled by us and that reported in reference with noble metal Pt/C as catalyst for both anode and cathode [18]. The CoeWeB-catalyzed DBFC has a more considerable power density (101 mW cm2) than the Pt/C-catalyzed cell. The excellent performance of CoeWeB-catalyzed DBFC might be attributed to the amorphous structure of the CoeWeB alloy, as it owns more structural distortions and can create more active sites. The amorphous catalysts exhibit lower polarization and better performance than the crystalline ones [19,20], and may play an important role in the cell performance improvement [21].
Result and discussion
3.1.
Catalyst characterization
The morphology of the sample was observed by TEM. Fig. 1 shows the image and corresponding selected area diffraction (SAD) pattern of the CoeWeB. Obviously, the alloy is in a well-dispersed spherical shape with the average size of about 25 nm. And the selected-area diffraction (SAD) displays amorphous character of the CoeWeB powders. Fig. 2 presents the XRD pattern of the prepared sample. It was found that the catalyst was typical of an amorphous structure without any sharp crystalline peaks. It is in accordance with the SAD results. The energy-dispersive X-ray spectroscopy (EDX) was used to determine the chemical composition of the catalysts. Fig. 3 shows the sample energy dispersive spectra corresponding to a randomly selected zone in the sample indicated in the inset. The EDX analysis showed that the nanoparticles consisted of the three elements studied, namely Co, W and B, The atomic ratio of Co, B and W is 44:25:3. Besides, the signal of the element C in the spectra can be attributed to the conductive adhesive introduced during the sample preparation. And that of the element O might come from the adsorbed oxygen or H2O [17].
1.2
(b)
(a)
-0.2
1.0
-0.4
0.8
400 350 300
Cell voltage / V
Potential / V (vs. Hg/HgO)
0.0
Power density / mW cm -2
Fig. 3 e Energy-dispersive X-ray spectroscopy of the CoeWeB catalyst.
3.
The cell performance evaluation
cathode -0.6 -0.8
anode
-1.0 -1.2
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2 2 Anode 0.5cm Cathode 1cm 2 2 Anode 0.25cm Cathode 1cm
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0 1000
Discharge current / mA cm-2
Fig. 4 e Comparison of the polarization curves between anode and cathode (a). Performances of the DBFCs using different anode area (CoeWeB), and the cathode (LaNi0.9Ru0.1O3) area is 1 cm2. (b). Anode loading: 1 cm2 is 70 mg cmL2, 0.5 cm2 is 35 mg cmL2, 0.25 cm2 is 17.5 mg cmL2. Cathode loading: 7.5 mg cmL2. Electrolyte fuel is 0.8 M KBH4e6 M KOH and temperature is 15 C.
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60 0.6 [ref.18]
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Pt/C at 30oC
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Size: 0.25cm o 15 C o 30 C o 45 C o 60 C
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1.0
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Co-W-B at 15oC
0
Discharge current/mA cm-2
Fig. 5 e Performances of the DBFCs using CoeWeB anode and Pt/C [18] as both anode and cathode catalysts, respectively. Anode: CoeWeB loading is 70 mg cmL2. Cathode: LaNi0.9Ru0.1O3 loading is 7.5 mg cmL2. Electrolyte fuel is 0.8 M KBH4e6 M KOH.
Fig. 7 e Performances of the DBFC at different operating temperatures. Anode loading: 0.25 cm2 is 17.5 mg cmL2. Cathode loading: 1 cm2 is 7.5 mg cmL2. Electrolyte fuel is 0.8 M KBH4e6 M KOH.
3.3. Influence of tungsten content on the cell performance
3.4. Influence of operating temperatures on the cell performance
In order to investigate the influence of tungsten content on catalytic activity we prepared the CoeWeB catalyst with different ratios of W to Co, noted cW. Fig. 6 displays the power densities of the cells using CoeWeB catalyst of different cW. The cW varies from 0 to 0.4. The inset in Fig. 6 shows the plot of maximum power densities on function of cW. The CoeWeB alloy catalyzed cell exhibits higher peak power density than binary CoB catalyzed ones. And the power density reaches its summit of 350 mW cm2 when cW ¼ 0.2. So, in the process of the whole research, we choose cW ¼ 0.2 to evaluate the material property.
Fig. 7 shows the performance of the DBFC at different temperatures ranging from 15 C to 60 C. The peak power densities apparently increase with the rising of temperature. The essential power densities achieved are 351 mW cm2, 376 mW cm2, 450 mW cm2 and 500 mW cm2 at 15 C, 30 C, 45 C, and 60 C, respectively. According to the literature, the performances of the cell were sensitive to the operating temperature. One probable explanation of this phenomenon is that the mass transfer of the reactants and the kinetics of borohydride oxidation are more active at high temperature [22].
0.9 0.8
300 250
[Ref.13]
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0.9 0.8 0.7
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50 0.1
Discharge current / mA cm-2
0.2 X
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Fig. 6 e Performance of the DBFCs using CoeWeB catalysts with different molar ratio of W and Co. Anode loading: 0.25 cm2 is 17.5 mg cmL2. Cathode loading: 1 cm2 is 7.5 mg cmL2. Electrolyte fuel is 0.8 M KBH4e6 M KOH and temperature is 15 C.
0.5
0
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80 100 120 140 160 180 Time / h
Fig. 8 e Durability of the DBFC at ambient atmosphere. Applied current density: 20 mA cmL2. Turning point 1: fuel run out. Turning point 2: fuel change.
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3.5.
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Durability of the cell
Durability is one of most important factors in the cell performances evaluation. The DBFC short-term stability was tested by monitoring the voltage changes on galvanostatic mode. Fig. 8 shows a lifetime test of the cell at 20 mA cm2 for one week (about 170 h). The voltage and time data were recorded after each change of new fuel. The turning point 1 represents the run out of fuel, and the turning point 2 represents the addition of new fuel. It is evident that the voltage could reach the initial value (1.0 V) soon after refueling. No attenuation was observed after a week’s operation. This result confirms that the DBFC using CoeWeB catalyzed anode has fairly good stability at room temperature.
4.
Conclusions
An ultrafine amorphous CoeWeB powder has been prepared for anode catalyst in a DBFC. A peak power density of 101 mW cm2 was obtained at 15 C in equal areas at both electrodes. And the essential power output of this anode material could reach 350 mW cm2 at 15 C and 500 mW cm2 at 60 C, respectively, when anode area is reduced to a quarter of the cathode. The DBFC remained stable after one week’s test. Although its catalytic activity needs further improvements, the CoeWeB alloy is still a promising anode catalyst for the application of DBFC with lower catalyst price.
references
[1] Ma JF, Liu YN, Zhang P, Wang J. A simple direct borohydride fuel cell with a cobalt phthalocyanine catalyzed cathode. Electrochem Commun 2008;10:100e2. [2] Celik C, San FGB, Sarac HI. Effects of operation conditions on direct borohydride fuel cell performance. J Power Sources 2008;185:197e201. [3] Ma J, Choudhury NA, Sahai Y. A comprehensive review of direct borohydride fuel cells. Renew Sustain Energy Rev 2010; 14:183e99. [4] Deshmukh K, Santhanam KSV. New borohydride fuel cell with multiwalled carbon nanotubes as anode: a step towards increasing the power output. J Power Sources 2006;159:1084e8. [5] Miyazaki K, Sugimura N, Matsuoka K, Iriyama Y, Abe T, Matsuoka M, et al. Perovskite-type oxides La1xSrxMnO3 for cathode catalysts in direct ethylene glycol alkaline fuel cells. J Power Sources 2008;178:683e6.
[6] Liu BH, Li ZP, Suda S. Electrocatalysts for the anodic oxidation of borohydrides. Electrochim Acta 2004;49: 3097e105. [7] Liu BH, Li ZP, Suda S. Anodic oxidation of alkali borohydrides catalyzed by nickel. J Electrochem Soc 2003;150:A398e402. [8] 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. [9] Liu BH, Suda S. Hydrogen storage alloys as the anode materials of the direct borohydride fuel cell. J Alloys Compd 2008;454:280e5. [10] 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. [11] Geng XY, Zhang HM, Ye W, Ma YW, Zhong HX. NiePt/C as anode electrocatalyst for a direct borohydride fuel cell. J Power Sources 2008;185:627e32. [12] Feng RX, Dong H, Cao YL, Ai XP, Yang HX. Agni-catalyzed anode for direct borohydride fuel cells. Int J Hydrogen Energy 2007;32:4544e9. [13] Li S, Yang XD, Zhu HY, Chen YZ, Liu YN. Investigation of amorphous CoB alloy as the anode catalyst for a direct borohydride fuel cell. J Power Sources 2011;196:5858e62. [14] Dai HB, Liang Y, Wang P, Yao XD, Rufford T, Lu M, et al. Highperformance cobaltetungsteneboron catalyst supported on Ni foam for hydrogen generation from alkaline sodium borohydride solution. Int J Hydrogen Energy 2008;33: 4405e12. [15] Wei XZ, Yang XD, Li S, Chen YZ, Liu YN. LaNi0.9Ru0.1O3 as a cathode catalyst for a direct borohydride fuel cell. Electrochim Acta 2011;56:7523e9. [16] Li S, Liu YN, Liu Y, Chen YZ. Study of CoO as an anode catalyst for a membraneless direct borohydride fuel cell. J Power Sources 2010;195:7202e6. [17] Dai HB, Liang Y, Wang P. Effect of trapped hydrogen on the induction period of cobaltetungsteneboron/nickel foam catalyst in catalytic hydrolysis reaction of sodium borohydride. Catal Today 2011;170:27e32. [18] Cheng H, Scott K, Lovell K. Material aspects of the design and operation of direct borohydride fuel cells. Fuel Cells 2006;6: 367e75. [19] Yang JS, Xu JJ. Nanoporous amorphous manganese oxide as electrocatalyst for oxygen reduction in alkaline solutions. Electrochem Commun 2003;5:306e11. [20] Zhu KN, Qin HY, Liu BH, Li ZP. Carbon nanotubes supported Fe-based compounds as the electrode catalyst for oxygen reduction reaction. J Power Sources 2011;196: 182e5. [21] Ramos-Sa´nchez G, Pierna AR, Solorza-Feria O. Amorphous Ni59Nb40PtxM1x (M ¼ Ru, Sn) electrocatalysts for oxygen reduction reaction. J Non-Cryst Solids 2008;354:5165e8. [22] Ma JF, Liu YN, Liu Y, Yan Y, Zhang P. A membraneless direct borohydride fuel cell using LaNiO3-catalysed cathode. Fuel Cells 2008;6:394e8.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Ultrafine amorphous CoeWeB alloy as the anode catalyst for a direct borohydride fuel cell Sai Li a,*, Xiaodong Yang b, Haiyan Zhu b, Xiaozhu Wei b, Yongning Liu b a
School of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710054, PR China State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Eng., Xi’an Jiaotong University, Xi’an 710049, PR China b
article info
abstract
Article history:
The ultrafine amorphous CoeWeB alloy has been synthesized by chemical reduction and
Received 14 October 2012
used as anode catalyst in direct borohydride fuel cell. The results show that the maximum
Received in revised form
power output of the cell is 101 mW cm2 at 15 C, and the essential power density of this
24 November 2012
material can be up to 350 mW cm2 at 15 C and 500 mW cm2 at 60 C, respectively. The
Accepted 30 November 2012
cell has also a good durability, with no attenuation observed after one week of operation.
Available online 11 January 2013
Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Keywords: Direct borohydride fuel cell Ultrafine amorphous CoeWeB catalyst Cell performance
1.
Introduction
Fuel cells, which can convert chemical energy into electrical energy with a high efficiency and low/zero-emission, are considered as promising power sources. It widely used in the field, such as mobile phones, portable computers, automobiles and power generators. Among various fuel cells, direct borohydride fuel cell (DBFC) which use KBH4 or NaBH4 aqueous solution as fuel [1] is one of the most exciting energy technologies. Compared with other liquid fuel cells, DBFC has many advantages [2e4] such as high theoretical open cell voltage (OCV) (1.64 V), high energy density (9300 Wh kg1), and low toxicity of borohydride. However, its development and commercialization is limited by some key issues like high cost. Platinum or platinum-based catalysts show good catalytic activity toward the electrochemical oxidation reaction, and
are wildly employed in DBFC. However the platinum resources are extremely limited [5], giving rise to the main technological obstacles in the development of DBFC. Lots of researchers have been therefore interested in exploring lower-cost substitutes, and have made some progresses. For examples, some transition metals (Ni and Cu) [6,7] and hydrogen storage alloys (AB5- and AB2-type alloys) [8e10] have been successful alternative platinum as anode in DBFCs. More recently, much effort has been engaged in developing binary alloy [3]. Such material is particularly important in the field of catalysts attributing to better catalytic properties than single component. For example, Geng et al. [11] prepared carbon-supported Ni and NiePt anodic catalyst. Electrochemical measurements showed that the binary alloy NiePt has a better electrocatalytic activity and stability than pure metal Ni catalyst [11]. Feng et al. [12] studied Ag and AgeNi alloy as anode catalyst and found that AgeNi alloy exhibited a higher
* Corresponding author. Tel.: þ86 29 8266 4602; fax: þ86 29 8266 3453. E-mail address: [email protected] (S. Li). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.148
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discharge voltage and larger capacity than Ag. In our previous studies [13], we found as well that the amorphous CoB alloy for catalytic oxidation reaction of BH 4 prepared by the combination of 3d transition metals Co with boron exhibited excellent electrochemical performance for DBFC. Based on our previous works, in this study, we explored the performance of multi-metal alloys materials and found by introducing W into the binary CoB alloy that the catalytic activity of the CoB is significantly improved.
2.
Experimental
2.1.
Catalyst preparation
The ultrafine amorphous CoeWeB catalyst was synthesized by chemical reduction of cobalt chloride (CoCl2) and sodium tungstate (Na2WO4) with potassium borohydride solution [14]. The different volume of CoCl2 (0.2 mol L1) and Na2WO4 (0.2 mol L1) solutions were mixed together to adjust the tungsten content in samples. Then 2.0 mol L1 KBH4/ 0.2 mol L1 KOH solution was added dropwise into the mixed solution at magnetic stirring. The B: (Co þ W) molar ratio was 3.0. The stirring needs to be held for 1 h without stop after the addition of potassium borohydride to release hydrogen. The resulting black precipitate was filtrated and washed with distilled water to neutrality, cut off from air by absolute alcohol and finally dried under vacuum at 60 C for 12 h. The cathode catalyst, perovskite-type oxide (LaNi0.9Ru0.1O3) was prepared following the solegel method described by previous paper [15]. Lanthanum nitrate (La(NO3)3 6H2O), nickel nitrate (Ni(NO3)2 6H2O), ruthenium chloride (RuCl3 nH20), citric acid (C6H8O7 H2O), and ammonia water (NH3 H2O) (25e28 wt.%) were used as raw materials. All the reagents are of analytical grade in purity (Beijing Jinkemei chemical product co. ltd.).
Catalyst characterization
The particle size and morphology of the samples were observed by transmission electron microscopy (TEM). The structure of the CoeWeB powders was investigated with an X-ray diffractometer (D/MAX-3A, Japan) using a Cu Ka (l ¼ 1.5444 A) source. The elemental composition of the sample was analyzed by energy-dispersive X-ray spectroscopy (EDX).
2.3.
polytetrafluoroethylene (PTFE) emulsion and then coated onto a Ni-foam. The mass loading in the cathode was 7.5 mg cm2. The gas diffusion layer was prepared by mixing 60 wt.% acetylene black and 40 wt.% PTFE with ethanol and rolled into 0.3 mm thick film. The three-layer electrode was finished by pressing the coated Ni-foam and the gas diffusion layer at pressure of 3 MPa into a sheet with thickness of 0.6 mm.
2.4.
The cell performances evaluation
The cell performances were measured by a battery testing system (from Neware Technology Limited, Shenzhen, China). In the cell test system, the anode was placed inside of a container, the cathode was fixed 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 M KBH4e6 M KOH. And the structure of the DBFC has been described in our previous paper [16].
Electrodes preparation
To prepare the anode, CoeWeB powders (78 wt.%) were mixed together with 30% polytetrafluoroethylene (PTFE) solution (22 wt.%). The mixture was smeared onto a 1 cm 1 cm Ni-foam sheet (thickness ¼ 1.7 mm, porosity > 95%). An experiment [16] with a blank sample without catalyst on Ni-foam proved that Ni-foam has no catalytic activity for BH 4. The electrode is dried at 80 C under vacuum for 2 h, and was pressed under 3 MPa. The mass loading in the anode was 70 mg cm2. The cathode has 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.% LaNi0.9Ru0.1O3 and 45 wt.% carbon nanotubes with a 25 wt.%
intensity (a.u.)
2.2.
Fig. 1 e TEM images of the CoeWeB catalyst.
10
20
30
40
50
60
70
2 theta (deg.) Fig. 2 e The XRD pattern of the CoeWeB catalyst.
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3.2.
Before the cell performance test, the polarization curves of both anode and cathode were tested, as depicted in Fig. 4(a). We found that the electrochemical dynamics of the cathode is dramatically inferior to that of the anode, according to their potential variation between the anode and the cathode. Concretely, when the current density changes from 0 to 100 mA cm2, the cathode potential decreases from 0 to 0.24 V while the anode potential increases from 1.12 to 1.1 V. The cathode, which is more polarized, has a controlling impact on the cell output voltage and power density. The same conclusion was also reached in our previous work [13]. Thus, to disclose the maximum power output of the anode material, or the essential power of this material [13], we must minimize the influence of cathode polarization and maximize the affect of anode, which is attained by reducing the anode electrode area while keeping normal the cathode area. Fig. 4(b) represents the performances of the designed cells assembled by the anodes of different sizes and the cathode of a constant size. The highest power density of 359 mW cm2 was achieved at 15 C with an anode of 0.25 cm2, 63% higher than binary CoB alloy, whose power density was 220 mW cm2 at the same temperature. This value should be represent the essential power of the CoeWeB material can output. Simultaneously, the peak power density was only 101 mW cm2 in the condition that the anode and the cathode are of same size, another proof of the controlling role of the cathode. Fig. 5 compares the performances of the DBFCs assembled by us and that reported in reference with noble metal Pt/C as catalyst for both anode and cathode [18]. The CoeWeB-catalyzed DBFC has a more considerable power density (101 mW cm2) than the Pt/C-catalyzed cell. The excellent performance of CoeWeB-catalyzed DBFC might be attributed to the amorphous structure of the CoeWeB alloy, as it owns more structural distortions and can create more active sites. The amorphous catalysts exhibit lower polarization and better performance than the crystalline ones [19,20], and may play an important role in the cell performance improvement [21].
Result and discussion
3.1.
Catalyst characterization
The morphology of the sample was observed by TEM. Fig. 1 shows the image and corresponding selected area diffraction (SAD) pattern of the CoeWeB. Obviously, the alloy is in a well-dispersed spherical shape with the average size of about 25 nm. And the selected-area diffraction (SAD) displays amorphous character of the CoeWeB powders. Fig. 2 presents the XRD pattern of the prepared sample. It was found that the catalyst was typical of an amorphous structure without any sharp crystalline peaks. It is in accordance with the SAD results. The energy-dispersive X-ray spectroscopy (EDX) was used to determine the chemical composition of the catalysts. Fig. 3 shows the sample energy dispersive spectra corresponding to a randomly selected zone in the sample indicated in the inset. The EDX analysis showed that the nanoparticles consisted of the three elements studied, namely Co, W and B, The atomic ratio of Co, B and W is 44:25:3. Besides, the signal of the element C in the spectra can be attributed to the conductive adhesive introduced during the sample preparation. And that of the element O might come from the adsorbed oxygen or H2O [17].
1.2
(b)
(a)
-0.2
1.0
-0.4
0.8
400 350 300
Cell voltage / V
Potential / V (vs. Hg/HgO)
0.0
Power density / mW cm -2
Fig. 3 e Energy-dispersive X-ray spectroscopy of the CoeWeB catalyst.
3.
The cell performance evaluation
cathode -0.6 -0.8
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Discharge current / mA cm-2
Fig. 4 e Comparison of the polarization curves between anode and cathode (a). Performances of the DBFCs using different anode area (CoeWeB), and the cathode (LaNi0.9Ru0.1O3) area is 1 cm2. (b). Anode loading: 1 cm2 is 70 mg cmL2, 0.5 cm2 is 35 mg cmL2, 0.25 cm2 is 17.5 mg cmL2. Cathode loading: 7.5 mg cmL2. Electrolyte fuel is 0.8 M KBH4e6 M KOH and temperature is 15 C.
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Co-W-B at 15oC
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Discharge current/mA cm-2
Fig. 5 e Performances of the DBFCs using CoeWeB anode and Pt/C [18] as both anode and cathode catalysts, respectively. Anode: CoeWeB loading is 70 mg cmL2. Cathode: LaNi0.9Ru0.1O3 loading is 7.5 mg cmL2. Electrolyte fuel is 0.8 M KBH4e6 M KOH.
Fig. 7 e Performances of the DBFC at different operating temperatures. Anode loading: 0.25 cm2 is 17.5 mg cmL2. Cathode loading: 1 cm2 is 7.5 mg cmL2. Electrolyte fuel is 0.8 M KBH4e6 M KOH.
3.3. Influence of tungsten content on the cell performance
3.4. Influence of operating temperatures on the cell performance
In order to investigate the influence of tungsten content on catalytic activity we prepared the CoeWeB catalyst with different ratios of W to Co, noted cW. Fig. 6 displays the power densities of the cells using CoeWeB catalyst of different cW. The cW varies from 0 to 0.4. The inset in Fig. 6 shows the plot of maximum power densities on function of cW. The CoeWeB alloy catalyzed cell exhibits higher peak power density than binary CoB catalyzed ones. And the power density reaches its summit of 350 mW cm2 when cW ¼ 0.2. So, in the process of the whole research, we choose cW ¼ 0.2 to evaluate the material property.
Fig. 7 shows the performance of the DBFC at different temperatures ranging from 15 C to 60 C. The peak power densities apparently increase with the rising of temperature. The essential power densities achieved are 351 mW cm2, 376 mW cm2, 450 mW cm2 and 500 mW cm2 at 15 C, 30 C, 45 C, and 60 C, respectively. According to the literature, the performances of the cell were sensitive to the operating temperature. One probable explanation of this phenomenon is that the mass transfer of the reactants and the kinetics of borohydride oxidation are more active at high temperature [22].
0.9 0.8
300 250
[Ref.13]
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50 0.1
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0.2 X
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Fig. 6 e Performance of the DBFCs using CoeWeB catalysts with different molar ratio of W and Co. Anode loading: 0.25 cm2 is 17.5 mg cmL2. Cathode loading: 1 cm2 is 7.5 mg cmL2. Electrolyte fuel is 0.8 M KBH4e6 M KOH and temperature is 15 C.
0.5
0
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40
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80 100 120 140 160 180 Time / h
Fig. 8 e Durability of the DBFC at ambient atmosphere. Applied current density: 20 mA cmL2. Turning point 1: fuel run out. Turning point 2: fuel change.
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Durability of the cell
Durability is one of most important factors in the cell performances evaluation. The DBFC short-term stability was tested by monitoring the voltage changes on galvanostatic mode. Fig. 8 shows a lifetime test of the cell at 20 mA cm2 for one week (about 170 h). The voltage and time data were recorded after each change of new fuel. The turning point 1 represents the run out of fuel, and the turning point 2 represents the addition of new fuel. It is evident that the voltage could reach the initial value (1.0 V) soon after refueling. No attenuation was observed after a week’s operation. This result confirms that the DBFC using CoeWeB catalyzed anode has fairly good stability at room temperature.
4.
Conclusions
An ultrafine amorphous CoeWeB powder has been prepared for anode catalyst in a DBFC. A peak power density of 101 mW cm2 was obtained at 15 C in equal areas at both electrodes. And the essential power output of this anode material could reach 350 mW cm2 at 15 C and 500 mW cm2 at 60 C, respectively, when anode area is reduced to a quarter of the cathode. The DBFC remained stable after one week’s test. Although its catalytic activity needs further improvements, the CoeWeB alloy is still a promising anode catalyst for the application of DBFC with lower catalyst price.
references
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