- Email: [email protected]
Carbon supported Pt hollow nanospheres as anode catalysts for direct borohydride-hydrogen peroxide fuel cells
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Carbon supported Pt hollow nanospheres as anode catalysts for direct borohydride-hydrogen peroxide fuel cells Lanhua Yi, Yunfeng Song, Wei Yi, Xianyou Wang*, Hong Wang, Peiying He, Benan Hu Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Hunan 411105, PR China
article info
abstract
Article history:
The carbon supported Pt hollow nanospheres were prepared by employing cobalt nano-
Received 14 February 2011
particles as sacrificial templates at room temperature in aqueous solution and used as the
Received in revised form
anode electrocatalyst for direct borohydride-hydrogen peroxide fuel cell (DBHFC). The
6 April 2011
physical and electrochemical properties of the as-prepared electrocatalysts were investi-
Accepted 11 April 2011
gated by transmission electron microscopy (TEM), X-ray diffraction (XRD), cyclic voltam-
Available online 20 July 2011
metry (CV), chronoamperometry (CA), chronopotentiometry (CP) and fuel cell test. The results showed that the carbon supported Pt nanospheres were coreless and composed of
Keywords:
discrete Pt nanoparticles with the crystallite size of about 2.8 nm. Besides, it has been
Pt hollow nanospheres
found that the carbon supported Pt hollow nanospheres exhibited an enhanced electro-
Borohydride oxidation
catalytic performance for BH 4 oxidation compared with the carbon supported solid Pt
Anode electrocatalyst
nanoparticles, and the DBHFC using the carbon supported Pt hollow nanospheres as
Direct borohydride-hydrogen
electrocatalyst showed as high as 54.53 mW cm2 power density at a discharge current
peroxide fuel cell
density of 44.9 mA cm2. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Direct borohydride-hydrogen peroxide fuel cell (DBHFC), especially using sodium borohydride (NaBH4) aqueous solution as fuel, has been intensively studied [1e6]. As a nontoxic solid state fuel, sodium borohydride has many advantages over hydrogen, e.g., easily transport, convenient storage, high hydrogen content (weight content of 10.6%), high capacity of 5.7 Ah/g, high volumetric energy density (7314 Wh/L) as well as gravimetric energy density (7100 Wh/Kg). The anode electrocatalyst is the key component in advancing the application of DBHFC. In the past few years, different metals have been used as the anode electrocatalysts for DBHFC, such as Au [2,3], Pt [7], Ag [4,8,9], Ni [10], Zn [11] and Hydrogen storage alloys [12]. Among these metals, Pt and Au are more attractive than other metals as anode catalysts for
DBHFC. It has been conclusively established that the electrooxidation of BH 4 on Au electrode is virtually an eight-electron process because gold is an effective catalyst for BH 4 oxidation but not for BH 4 hydrolysis [7,13]. However, Au is supposedly inactive towards both hydrogen evolution and oxidation reactions [13e15], slow electrode kinetics of BH 4 on pure Au electrode results in the low current and power output, and thus degrades the electrochemical performance of the DBFC [6,16,17]. Currently, Pt and Os are regarded as highly active catalysts for the oxygen reduction reaction (ORR) [7,18]. In the application of fuel cell, Pt is also a very important metal because of it high activity toward the electrooxidation of fuels such as hydrogen, ethanol, methanol, formic acid and borohydride. However, Pt is a noble metal due to its limited nature storage and high cost, the usage of Pt in fuel cell application has to be reduced before this technology can be commercialized.
* Corresponding author. Tel.: þ86 731 5829060; fax: þ86 731 58292061. E-mail address: [email protected] (X. Wang). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.077
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Many efforts have been made to lower the cost of the catalyst, one way is to alloy Pt with transition metals such as Co [19,20], Ni [21,22], Fe [23,24], Sn [25,26] and Cr [27]. Although the Ptbased alloys almost demonstrated the similar electrocatalytic activities toward ORR as Pt catalyst [19e27], the alloy catalysts show poor long-term stability due to the dissolution of nonnoble metal [28,29], which hampers their final application. Recently, the research on hollow metal nanostructures has become a prevalent trend because of their large surface area, low density, saved material, low costs, and high catalytic activity. Although Pt hollow nanosphere catalysts have been used as the electrocatalyst for the oxidation of methanol [30,31] in DMFC, to our best knowledge, the electrooxidation of BH 4 on the carbon supported Pt hollow nanospheres was barely reported. In previous studies, we studied the electrochemical properties of Au hollow nanospheres as anodic electrocatalyst in DBFC and found that the Au hollow nanospheres showed much higher catalytic activity than common Au solid nanoparticles [2,3]. In this paper, the Pt hollow nanospheres were prepared by the replacement reaction between Co nanoions, and then Pt hollow nanospheres particles and PtCl2 6 were dispersed on Vulcan XC-72R carbon, finally the hollow nanosphere Pt/C composite (HNPt/C) was obtained. The electrooxidation behavior of BH 4 on HNPt/C electrode was investigated by electrochemical techniques. Moreover, the performance of the DBHFC employing HNPt/C as the anode catalyst and carbon supported Pt solid nanoparticles (SNPt/C) as the cathode catalyst was studied in detail.
2.
Experimental
2.1. Preparation of carbon supported Pt (Pt/C) electrocatalysts HNPt/C electrocatalyst was prepared by using cobalt nanoparticles as sacrificial templates [30,31]. In a typical synthesis, 2 ml of 0.1 M sodium citrate (Alfa Aesar, 99%) solution and 300 ml distilled water were added to a three-neck flask and the resulting solution was deoxygenated, then 1.2 ml of a freshly made 1 M sodium borohydride (Alfa Aesar, 98%) solution was added, and 0.3 ml of 0.4 M cobalt chloride (SigmaeAldrich, 99%) solution was added dropwise to the above solution with violently stirring. With immediate hydrogen evolution, the solution turned from colorless to brown, and was allowed to stir under argon for an additional 1 h so that the sodium borohydride to completely decompose. To avoid the oxidation of the prepared Co nanoparticles by atmospheric oxygen, high-purity argon was bubbled through the solution during the whole procedure. While maintaining argon flow, 15 ml of 0.005 M aqueous hexachloroplatinicacid (SigmaeAldrich) solution was added dropwise under magnetic stirring. The solution changed from brown to black, which showed that a trans-metalation reaction occurred according to Eq. (1) [30]: 2
2Co þ PtCl6 ¼ Pt þ 2Co2þ þ 6Cl
(1)
Upon completion of Pt addition, the argon flow was stopped and the vessel was opened at ambient conditions under
11513
stirring to oxidize any remaining cobalt metal left in solution. After 1 h, 58.5 mg Vulcan XC-72R carbon (Cabot, 240 m2 g1) were then added to the above solution. After stirring for 3 h, the final product carbon supported Pt hollow nanospheres were collected by filtration and washed several times with distilled water and alcohol. For comparison, SNPt/C was prepared by citrate reduction method [31]. The resulting catalysts dried for 12 h at 80 C in vacuum.
2.2.
Material characterization
The structure and morphology of the as-prepared HNPt/C electrocatalyst were examined by transmission electron microscopy (TEM), using a FEI Tecnai G2 microscope at 200 kV. For TEM analyses, samples were prepared by placing one or two drops of nanoparticles solution onto the carbon-coated copper grid and drying it in air at room temperature. A diffractometer (D/MAX-3C) was employed using Cu Ka ˚ ) and a graphite monochromator at radiation (l ¼ 1.54056 A 50 kV, 100 mA to obtain XRD spectra of the samples. The 2q angular regions between 10 and 80 were explored at a scan rate of 5 min1.
2.3.
Electrochemical performance
A conventional three-electrode cell was used to perform the electrochemical tests of the anode catalysts employing a CHI660A Electrochemistry Workstation. The HNPt/C and SNPt/C were used as working electrode, a Ni foam mesh with 3 5 cm2 as counter electrode and an Ag/AgCl, KClstd as the reference electrode. The electrolyte was 0.5 M H2SO4 or 0.1 M NaBH4 þ 3.0 M NaOH. The working electrode was prepared as follows: 10 mg of HNPt/C or SNPt/C powder was dispersed by ultrasonic for 2 h in 1 ml blending solution of 0.25 ml 5 wt% Nafion solution and 0.75 ml de-ionized water. Then 5 ml of slurry was pasted on the surface of the glassy carbon (GC) electrode (3 mm in diameter) which was polished to mirror by 0.5 mm alumina and sonicated 15 min prior to use. The dispersed catalyst on the GC surface was dried for 5 h at ambient temperature and the loading mass of catalyst was 0.7 mg cm2.
2.4.
Fuel cell test
The catalysts ink was made by mixing isopropyl alcohol with 7 wt.% of nafion solution and carbon supported catalysts. Then the ink was coated onto stainless steel gauze resulting in a 4.0 mg cm2 loading. The catalyst electrodes were pressed at 10 MPa for 1 min after drying at 50 C in vacuum for 8 h. The cell performance of DBHFC was tested against a SNPt/C cathode and a HNPt/C or SNPt/C anode. A schematic diagram of the experimental set-up was shown in our previous studies [2,3,32e34]. An activated nafion 117 membrane was used to separate the anolyte and catholyte. Anolyte composed of 1 M NaBH4 þ 3 M NaOH, and catholyte composed of 2 M H2O2 þ 0.5 M H2SO4. The fresh anolyte and catholyte were continuously supplied and withdrawn from the cell at 0.7 ml min1 during the test process. The electrochemical testing of the cell was performed using a battery testing
11514
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Fig. 1 e Schematic illustration of the experimental procedure which generates Pt hollow nanosphere by templating against Co nanoparticles.
system (Qingtian, Guangzhou, China). Power densities were calculated from the applied current and steady state potential.
3.
Results and discussion
3.1.
Physical characterizations
For trans-metalation reaction in Eq. (1), the standard reduction potential of the PtCl2 6 /Pt redox pair (þ0.744 V vs. the standard hydrogen electrode (SHE)) is higher than that of the Co2þ/Co redox pair (0.227 V vs. SHE), thus Co nanoparticles can be immediately oxidized into Co2þ when the solution containing Co nanoparticles is added to the H2PtCl6 solution due to the big difference between the potential of two redox couples. The schematic mechanism of Pt hollow nanospheres is shown in Fig. 1. Fig. 2 (a) and (b) are typical TEM and HRTEM images of HNPt/C. It was found that many nanoparticles assembled together and formed the hollow spherical shell. Besides, it can be seen from Fig. 2(a) and (b) that the centers of the Pt nanospheres were brighter than the edges, indicating that the nanosphere was actually a hollow spherical structure. The average diameter of the hollow Pt nanosphere was statistically estimated to be 20 2 nm, while the average particle size of Pt nanoparticles was about 2e3 nm. In addition, the shell of the hollow Pt nanosphere was incomplete and porous, and this porous and incomplete shell structure of hollow Pt nanosphere allows the interior surface to occur also catalytic
reaction, thus the hollow Pt nanospheres will be much more the specific surface area for catalytic reaction than solid Pt nanparticles and result in a much higher catalytic activity. The XRD patterns of the as-prepared HNPt/C and SNPt/C catalysts were shown in Fig. 3. The wide diffraction peak at 2q ¼ 25.0 is attributed to Vulcan XC-72R carbon (002) crystal face, which matches well with the standard C peaks (JCPDS No. 75-1621). The strong diffraction peaks positioned at 2q ¼ 39.7 , 46.2 , and 67.4 can be indexed to the (111), (200) and (220) crystalline planes of face-centered cubic (fcc) Pt, which are in good agreement with the standard card of cubic Pt (JCPDS No. 04-0802). The diffraction peak for Pt (200) is used to estimate the Pt particle size by the Scherrer’s equation [22,35]:
D¼
0:9l Bcos q
(2)
where D is average particle size, nm, the wavelength l is equal to 0.154056 nm, q is the angle of Pt (200) peak and B is the full width at half-maximum in radians (FWHM). The calculated average particle size of the hollow Pt nanoparticles and solid Pt nanoparticles dispersed on carbon is 2.8 nm and 4.4 nm, respectively, which is consistent with the TEM results.
3.2.
Electrochemically active surface area estimation
Electrochemically active surface area (ECSA) of the electrocatalysts could be estimated from the coulombic charge for the hydrogen adsorption and desorption (QH) in the cyclic voltammograms. Cyclic voltammograms were recorded from 0.2 V to 1.4 V at a scan rate of 50 mV s1 in a 0.5 M H2SO4 solution, high-purity argon gas was used during the experiments and argon gas was passed for 30 min to eliminate oxygen (Fig. 4). The value of QH is calculated as the mean value between the amounts of charge transfer during the electroadsorption and desorption of H2 on Pt sites [36,37]:
ECSA ¼
QH ½Pt 0:21
Fig. 2 e TEM (a) and HRTEM (b) images of Pt hollow nanospheres dispersed on carbon.
(3)
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Fig. 3 e XRD patterns of Pt hollow nanospheres dispersed on carbon and Pt solid nanoparticles dispersed on carbon.
11515
Fig. 5 e Changes of electrochemical active surface area of HNPt/C and SNPt/C electrodes with different cycle number.
electrocatalysts have good long-term running stability, especially HNPt/C presents much better stability.
where [Pt] is the platinum loading (mg cm2) in the electrode, QH is the charge for hydrogen desorption (mC cm2) and 0.21 is the charge required to oxidize a monolayer hydrogen adsorption on bright Pt (mC cm2) [38]. The calculated ECSA of the hollow Pt nanospheres dispersed on carbon is 224.32 cm2 mg1, which is much higher than that of solid Pt nanoparticles dispersed on carbon (77.38 cm2 mg1), and the high ECSA of electrocatalysts is attributed to the small particle size of Pt nanoparticles loaded on the carbon [39]. The stabilizing effect of both HNPt/C and SNPt/C was investigated under a continuous 2000 potential cycles in the potential range of 0.2 V to 1.4 V in 0.5 M H2SO4 solution [39]. The changes of ECSA with the increasing cycle numbers were shown in Fig. 5. As seen in Fig. 5, the ECSA of both electrocatalysts is decreased after the 2000 cycles, and HNPt/C retains 81.12% of the initial ECSA, which is higher than that of SNPt/C (60.43%). The above results indicate that both
The cyclic voltammograms recorded using HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 þ 3.0 M NaOH solution at a scan rate of 20 mV s1 in the potential range of 1.2 V to 0.6 V vs. Ag/ AgCl, KClstd are showed in Fig. 6. The HNPt/C and SNPt/C electrodes show a similar catalytic behavior for the electrooxidation of BH 4 by comparing the shapes of two CV curves between 1.2 and 0.6 V vs. Ag/AgCl. In the CV curves, the electrochemical behavior of BH 4 is fairly complex characterized by a number of oxidation peaks. During the forward sweep, at a rate of 20 mV s1, a well-defined oxidation peak occurs at 0.83 V (a1 or a10 ), followed by a broad hump anodic peak (a2 or a20 ) was observed. During the reverse sweep,
Fig. 4 e Cyclic voltammograms of HNPt/C and SNPt/C electrodes in Ar-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV sL1.
Fig. 6 e Cyclic voltammograms of HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 D 3.0 M NaOH solution at a scan rate of 20 mV sL1.
3.3.
Cyclic voltammetry study
11516
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
a sharp anodic spike (c1 or c10 ) was observed. Similar anodiccathodic peak pattern in cyclic voltammetry has reported by Gyenge [7] and Concha [40]. The first anodic peak (a1 or a10 ) can be assigned to borohydride hydrolysis followed by the electrooxidation of H2, the second oxidation peak (a2 or a20 ) is attributed to the direct oxidation of BH 4 in absence of H2 electroxidation, and the peak (c1 or c10 ) is due to the oxidation of absorbed intermediate oxidation products of BH3OH (Eq. (5)), but on the partially oxidized Pt surface [7,40]. BH 4 þ H2 O/BH3 OH þ H2
(4)
3 BH3 OH þ 3OH /BO 2 þ H2 þ 2H2 O þ 3e 2
(5)
The peak a2 on HNPt/C electrode occurs at 0.255 V, which is much more negative than that of peak a20 on SNPt/C electrode (0.062 V), and the current density of peak a2 on HNPt/C electrode is 26.21 mA cm2, it is also much higher than that of peak a20 on SNPt/C electrode (21.88 mA cm2). Apparently, the catalytic activity of HNPt/C electrode for electrooxidation of BH 4 will be higher than that of SNPt/C electrode. During the reverse sweep, the current density of peak c1 on HNPt/C electrode and SNPt/C electrode is almost similar; however, the potential of peak c10 on SNPt/C electrode is about 0.398 V, and the potential of peak c1 on HNPt/C electrode shifts positively to 0.294 V. Therefore, the electrocatalytic activity to BH 4 hydrolysis reaction on HNPt/C electrode is clearly less than that on SNPt/C electrode. As a result, it can be concluded that HNPt/C will be much more favorable to DBHFC.
3.4.
Chronopotentiometry
By analogy with the constant current operation of a direct borohydride fuel cell, chronopotentiometry could provide further relevant information. Fig. 7 depicts the chronopotentiometry of BH 4 oxidation on the HNPt/C and SNPt/C electrodes at a current density of 8.5 mA cm2 in the solution of 0.1 M NaBH4 þ 3.0 M NaOH. After 120 s, the operating potential of HNPt/C electrode was more negative (about
Fig. 7 e Chronopotentiometry curves of HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 D 3.0 M NaOH solution. Current step: from 0 to 8.5 mA cmL2.
Fig. 8 e Chronoamperometry curves of HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 D 3.0 M NaOH solution. Potential step: from L1.2 to L0.2 V vs. Ag/AgCl.
26 mV) than that of the SNPt/C electrode. This is attributed to the microstructure of Pt hollow nanospheres. The porous and hollow structure will provide much more active center for electrocatalytic reaction due to big specific surface area and is more favorable for mass transfer of liquid fuel toward active site, and thus the kinetics of BH 4 electrooxidation can be improved [41].
3.5.
Chronoamperometry
Chronoamperometric technique is an effective method to evaluate the electrocatalytic activity and stability of catalyst material. Fig. 8 shows the chronoamperometric responses of the HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 þ 3.0 M NaOH solution from 1.2 to 0.2 V vs. Ag/AgCl, KClstd. It can be found from Fig. 8 that two samples show current decay during
Fig. 9 e Cell voltage and power density vs. current density for DBHFCs with different anodes at 20 C: (a) with HNPt/C (0.98 mg Pt cmL2) as anode catalyst; (b) with SNPt/C (1.08 mg Pt cmL2) as anode catalyst. Anolyte: 1 M NaBH4 D 3 M NaOH; catholyte: 2 M H2O2 D 0.5 M H2SO4.
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Table 1 e Characteristic parameters of DBHFCs using HNPt/C and SNPt/C as anode catalysts at 20 C. Example
Open circuit voltage OCP (V)
Current density j (mA cm2)
Peak power density P (mW cm2)
1.728
44.9
54.53
1.656
34.8
37.95
DBHFC with HNPt/C as anode DBHFC with SNPt/C as anode
the electrooxidation of BH 4 . After 2 s, the HNPt/C electrode delivers higher current density (38.7 mA cm2) than the SNPt/ C electrode (27.4 mA cm2). This result also confirms that the HNPt/C electrode has a higher electrocatalytic activity than the SNPt/C electrode.
3.6.
Fuel cell performance measurement
Fig. 9 shows the changes of the cell voltage and power density as a function of current density at 20 C using the HNPt/C anode catalysts in comparison with SNPt/C anode catalysts. The cathode catalysts of both DBHFC systems were SNPt/C. The anolyte was 3 M NaOH þ 1 M NaBH4 solution. The catholyte was 0.5 M H2SO4 þ 2 M H2O2 solution according to our previous studies [34]. The open circuit voltage (OCP) of the cell is about 1.728 V, which lower than the standard cell potential for the DBHFC in acid electrolyte. The low value is probably caused by mixed potential at the anode and cathode from simultaneous oxidation of BH 4 ions and hydrogen at the anode and reduction of H2O2 and O2 at the cathode [42]. The behavior of the cell voltage vs. current density obtained using HNPt/C as anode catalysts shows a similar linear behavior experienced with SNPt/C anode catalysts. However, the cell voltage dropped more slowly as the current density increased. For comparison, the characteristic parameters of DBHFCs were tabulated in Table 1. It can be seen from Table 1 that the maximum power density is 54.53 mW cm2 at a discharge current density of 44.9 mA cm2 at 20 C. Apparently, the DBHFC using HNPt/C as anode catalysts exhibits a clearly superior performance to the SNPt/C as the anode catalysts, although the loading amount (0.98 mg cm2) of actual Pt in HNPt/C is less than that in SNPt/C (1.08 mg cm2).
4.
Conclusions
The Pt hollow nanospheres dispersed on carbon were successfully prepared at room temperature in a homogeneous solution with Co nanoparticles as sacrificial templates. The HNPt/C catalyst shows much higher electrochemical activity than SNPt/C for BH 4 oxidation in the alkaline solution. The DBHFC using HNPt/C as the anode catalyst exhibits excellent electrochemical performance compared with SNPt/C as the anode catalyst, which is ascribed to the porous, hollow microstructure and higher active surface area of Pt nanospheres. The maximum power density of the DBHFC employing HNPt/C as the anode catalyst and SNPt/C as the cathode catalyst was 54.53 mW cm2 at 20 C. Therefore, the HNPt/C
11517
catalyst with high performance and low cost will be a promising anode catalyst for the application of DBHFCs.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 20871101 and 51072173), Doctoral Fund of Ministry of Education of China (Grant No. 20094301110005), the Science and Technology Plan Project of Hunan Province (Grant No. 2010GK3181) and Natural Science Foundation of Xiangtan University (Grant No.10XZX07).
references
[1] Wu HJ, Wang C, Liu ZX, Mao ZQ. Influence of operation conditions on direct NaBH4/H2O2 fuel cell performance. Int J Hydrogen Energy 2010;35:2648e51. [2] Wei JL, Wang XY, Wang Y, Guo J, He PY, Yang SY, et al. Carbon-supported Au hollow nanospheres as anode catalysts for direct borohydride-hydrogen peroxide fuel cells. Energy Fuels 2009;23:4037e41. [3] Wei JL, Wang XY, Wang Y, Chen QQ, Pei F, Wang YS. Investigation of carbon-supported Au hollow nanospheres as electrocatalyst for electrooxidation of sodium borohydride. Int J Hydrogen Energy 2009;34:3360e6. [4] Sanli E, Celikkan H, Uysal BZ, Aksu ML. Anodic behavior of Ag metal electrode in direct borohydride fuel cell. Int J Hydrogen Energy 2006;31:1920e4. [5] Miley GH, Luo N, Mather J, Burton R, Hawkins G, Gu LF, et al. Direct NaBH4/H2O2 fuel cells. J Power Sources 2007;165: 509e16. [6] 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(2): 1084e8. [7] Gyenge EL. Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells. Electrochim Acta 2004;49:965e78. [8] Sanli E, Uysal BZ, Aksu ML. The oxidation of NaBH4 on electrochemicaly treated silver electrodes. Int J Hydrogen Energy 2008;33:2097e104. [9] Concha BM, Chatenet M. Direct oxidation of sodium borohydride on Pt, Ag and alloyed PteAg electrodes in basic media: Part II. Carbon-supported nanoparticles. Electrochim Acta 2009;54:6130e9. [10] Ma J, Sahai Y, Buchheit RG. Direct borohydride fuel cell using Ni-based composite anodes. J Power Sources 2010;195: 4709e13. [11] Santos DMF, Sequeira CAC. Zinc anode for direct borohydride fuel cells. J Electrochem Soc 2010;157:B13e9. [12] Wang GL, Wang XY, Miao RR, Cao DX, Sun KN. Effects of alkaline treatment of hydrogen storage alloy on electrocatalytic activity for NaBH4 oxidation. Int J Hydrogen Energy 2010;35:1227e31. [13] Mirkin MV, Yang H, Bard AJ. Borohydride oxidation at a gold electrode. J Electrochem Soc 1992;139:2212e7. [14] Okinaka Y. An electrochemical study of electroless golddeposition reaction. J Electrochem Soc 1973;120:739e44. [15] Dong H, Feng RX, Ai XP, Cao YL, Yang HX, Cha CS. Electrooxidation mechanisms and discharge characteristics of borohydride on different catalytic metal surfaces. J Phys Chem B 2005;109:10896e901.
11518
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
[16] Elikkana HC, Ahin MS, Aksu ML, Veziroglu TN. The investigation of the electrooxidation of sodium borohydride on various metal electrode in aqueous basic solutions. Int J Hydrogen Energy 2007;32:588e93. [17] Cheng H, Scott K. Determination of kinetic parameters for borohydride oxidation on a rotating Au disk electrode. Electrochim Acta 2006;51:3429e33. [18] Vincent WSL, Gyenge EL. High-performance osmium nanoparticle electrocatalyst for direct borohydride PEM fuel cell anodes. J Electrochem Soc 2008;155:B1155e60. [19] Kadirgan F, Kannan AM, Atilan T, Beyhan S, Ozenler SS, Suzer S, et al. Carbon supported nano-sized PtePd and PteCo electrocatalysts for proton exchange membrane fuel cells. Int J Hydrogen Energy 2009;34:9450e60. [20] Santiago EI, Varanda LC, Villullas HM. Carbon-supported PteCo catalysts prepared by a modified polyol process as cathodes for PEM fuel cells. J Phys Chem C 2007;111:3146e51. [21] 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. [22] Wang GJ, Gao YZ, Wang ZB, Du CY, Wang JJ, Yin GP. Investigation of PtNi/C anode electrocatalysts for direct borohydride fuel cell. J Power Sources 2010;195:185e9. [23] Li WZ, Xin Q, Yan YS. Nanostructured PteFe/C cathode catalysts for direct methanol fuel cell: the effect of catalyst composition. Int J Hydrogen Energy 2010;35:2530e8. [24] Baglio V, Arico AS, Stassi A, Urso CD, Blasi AD, Lunab AMC, et al. Investigation of PteFe catalysts for oxygen reduction in low temperature direct methanol fuel cells. J Power Sources 2006;159:900e4. [25] Jiang L, Hsu A, Chu D, Chen R. Ethanol electro-oxidation on Pt/C and PtSn/C catalysts in alkaline and acid solutions. Int J Hydrogen Energy 2010;35:365e72. [26] Song SQ, Zhou WJ, Zhou ZH, Jiang LH, Sun GQ, Xin Q, et al. Direct ethanol PEM fuel cells: the case of platinum based anodes. Int J Hydrogen Energy 2005;30:995e1001. [27] Yang H, Alonso-Vante N, Leger JM, Tailoring CL. Structure and activity of carbon-supported nanosized PteCr alloy electrocatalysts for oxygen reduction in pure and methanolcontaining electrolytes. J Phys Chem B 2004;108:1938e47. [28] Colon-Mercado HR, Kim H, Popov BN. Durability study of Pt3Ni1 catalysts as cathode in PEM fuel cells. Electrochem Commun 2004;6:795e9. [29] Colon-Mercado HR, Popov BN. Stability of platinum based alloy cathode catalysts in PEM fuel cells. J Power Sources 2006;155:253e63.
[30] Liang HP, Zhang HM, Hu JS, Guo YG, Wan LJ, Bai CL. Pt hollow nanospheres: facile synthesis and enhanced electrocatalysts. Angew Chem Int Ed 2004;43:1540e3. [31] Zhao J, Chen WX, Zhang YF, Li X. Novel carbon supported hollow Pt nanospheres for methanol electrooxidation. J Power Sources 2006;162:168e72. [32] He PY, Wang Y, Wang XY, Pei F, Wang H, Liu L, et al. Investigation of carbon supported AueNi bimetallic nanoparticles as electrocatalyst for direct borohydride fuel cell. J Power Sources 2011;196:1042e7. [33] Pei F, Wang Y, Wang XY, He PY, Chen QQ, Wang XY, et al. Performance of supported AueCo alloy as the anode catalyst of direct borohydride-hydrogen peroxide fuel cell. Int J Hydrogen Energy 2010;35:8136e42. [34] Wei JL, Wang XY, Wang H, Yang SY, Dai CL, Pei F. Performance of nanosized Au particle as a cathode catalyst for direct borohydride-hydrogen peroxide fuel cell. Acta Chim Sinica 2008;66:2675e80. [35] Niu JJ, Wang JN. Activated carbon nanotubes-supported catalyst in fuel cells. Electrochim Acta 2008;53:8058e63. [36] Wang JJ, Yin GP, Zhang J, Wang ZB, Gao YZ. High utilization platinum deposition on single-walled carbon nanotubes as catalysts for direct methanol fuel cell. Electrochim Acta 2007; 52:7042e50. [37] Pozio A, Francesco MD, Cemmi A, Cardellini F, Giorgi L. Comparison of high surface Pt/C catalysts by cyclic voltammetry. J Power Sources 2002;105:13e9. [38] Maillard F, Martin M, Gloaguen F, Le´ger JM. Oxygen electroreduction on carbon-supported platinum catalysts. Particle-size effect on the tolerance to methanol competition. Electrochim Acta 2002;47:3431e40. [39] Kou R, Shao YY, Wang DH, Engelhard MH, Kwak JH, Wang J, et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction. Electrochem Commun 2009;11:954e7. [40] Concha BM, Chatenet M. Direct oxidation of sodium borohydride on Pt, Ag and alloyed PteAg electrodes in basic media. Part I: bulk electrodes. Electrochim Acta 2009;54:6119e29. [41] Kim JH, Kim HS, Kang YM, Song MS, Rajendran S, Han SC. Carbon-supported and unsupported Pt anodes for direct borohydride liquid fuel cells. J Electrochem Soc 2004;151: A1039e43. [42] Ponce de leo´n C, Walsh FC, Patrissi CJ, Medeiros MG, Bessette PR, Reeve RW, et al. A direct borohydride-peroxide fuel cell using a Pd/Ir alloy coated microfibrous carbon cathode. Electrochem Commun 2008;10:1610e3.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Carbon supported Pt hollow nanospheres as anode catalysts for direct borohydride-hydrogen peroxide fuel cells Lanhua Yi, Yunfeng Song, Wei Yi, Xianyou Wang*, Hong Wang, Peiying He, Benan Hu Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, School of Chemistry, Xiangtan University, Hunan 411105, PR China
article info
abstract
Article history:
The carbon supported Pt hollow nanospheres were prepared by employing cobalt nano-
Received 14 February 2011
particles as sacrificial templates at room temperature in aqueous solution and used as the
Received in revised form
anode electrocatalyst for direct borohydride-hydrogen peroxide fuel cell (DBHFC). The
6 April 2011
physical and electrochemical properties of the as-prepared electrocatalysts were investi-
Accepted 11 April 2011
gated by transmission electron microscopy (TEM), X-ray diffraction (XRD), cyclic voltam-
Available online 20 July 2011
metry (CV), chronoamperometry (CA), chronopotentiometry (CP) and fuel cell test. The results showed that the carbon supported Pt nanospheres were coreless and composed of
Keywords:
discrete Pt nanoparticles with the crystallite size of about 2.8 nm. Besides, it has been
Pt hollow nanospheres
found that the carbon supported Pt hollow nanospheres exhibited an enhanced electro-
Borohydride oxidation
catalytic performance for BH 4 oxidation compared with the carbon supported solid Pt
Anode electrocatalyst
nanoparticles, and the DBHFC using the carbon supported Pt hollow nanospheres as
Direct borohydride-hydrogen
electrocatalyst showed as high as 54.53 mW cm2 power density at a discharge current
peroxide fuel cell
density of 44.9 mA cm2. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Direct borohydride-hydrogen peroxide fuel cell (DBHFC), especially using sodium borohydride (NaBH4) aqueous solution as fuel, has been intensively studied [1e6]. As a nontoxic solid state fuel, sodium borohydride has many advantages over hydrogen, e.g., easily transport, convenient storage, high hydrogen content (weight content of 10.6%), high capacity of 5.7 Ah/g, high volumetric energy density (7314 Wh/L) as well as gravimetric energy density (7100 Wh/Kg). The anode electrocatalyst is the key component in advancing the application of DBHFC. In the past few years, different metals have been used as the anode electrocatalysts for DBHFC, such as Au [2,3], Pt [7], Ag [4,8,9], Ni [10], Zn [11] and Hydrogen storage alloys [12]. Among these metals, Pt and Au are more attractive than other metals as anode catalysts for
DBHFC. It has been conclusively established that the electrooxidation of BH 4 on Au electrode is virtually an eight-electron process because gold is an effective catalyst for BH 4 oxidation but not for BH 4 hydrolysis [7,13]. However, Au is supposedly inactive towards both hydrogen evolution and oxidation reactions [13e15], slow electrode kinetics of BH 4 on pure Au electrode results in the low current and power output, and thus degrades the electrochemical performance of the DBFC [6,16,17]. Currently, Pt and Os are regarded as highly active catalysts for the oxygen reduction reaction (ORR) [7,18]. In the application of fuel cell, Pt is also a very important metal because of it high activity toward the electrooxidation of fuels such as hydrogen, ethanol, methanol, formic acid and borohydride. However, Pt is a noble metal due to its limited nature storage and high cost, the usage of Pt in fuel cell application has to be reduced before this technology can be commercialized.
* Corresponding author. Tel.: þ86 731 5829060; fax: þ86 731 58292061. E-mail address: [email protected] (X. Wang). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.077
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Many efforts have been made to lower the cost of the catalyst, one way is to alloy Pt with transition metals such as Co [19,20], Ni [21,22], Fe [23,24], Sn [25,26] and Cr [27]. Although the Ptbased alloys almost demonstrated the similar electrocatalytic activities toward ORR as Pt catalyst [19e27], the alloy catalysts show poor long-term stability due to the dissolution of nonnoble metal [28,29], which hampers their final application. Recently, the research on hollow metal nanostructures has become a prevalent trend because of their large surface area, low density, saved material, low costs, and high catalytic activity. Although Pt hollow nanosphere catalysts have been used as the electrocatalyst for the oxidation of methanol [30,31] in DMFC, to our best knowledge, the electrooxidation of BH 4 on the carbon supported Pt hollow nanospheres was barely reported. In previous studies, we studied the electrochemical properties of Au hollow nanospheres as anodic electrocatalyst in DBFC and found that the Au hollow nanospheres showed much higher catalytic activity than common Au solid nanoparticles [2,3]. In this paper, the Pt hollow nanospheres were prepared by the replacement reaction between Co nanoions, and then Pt hollow nanospheres particles and PtCl2 6 were dispersed on Vulcan XC-72R carbon, finally the hollow nanosphere Pt/C composite (HNPt/C) was obtained. The electrooxidation behavior of BH 4 on HNPt/C electrode was investigated by electrochemical techniques. Moreover, the performance of the DBHFC employing HNPt/C as the anode catalyst and carbon supported Pt solid nanoparticles (SNPt/C) as the cathode catalyst was studied in detail.
2.
Experimental
2.1. Preparation of carbon supported Pt (Pt/C) electrocatalysts HNPt/C electrocatalyst was prepared by using cobalt nanoparticles as sacrificial templates [30,31]. In a typical synthesis, 2 ml of 0.1 M sodium citrate (Alfa Aesar, 99%) solution and 300 ml distilled water were added to a three-neck flask and the resulting solution was deoxygenated, then 1.2 ml of a freshly made 1 M sodium borohydride (Alfa Aesar, 98%) solution was added, and 0.3 ml of 0.4 M cobalt chloride (SigmaeAldrich, 99%) solution was added dropwise to the above solution with violently stirring. With immediate hydrogen evolution, the solution turned from colorless to brown, and was allowed to stir under argon for an additional 1 h so that the sodium borohydride to completely decompose. To avoid the oxidation of the prepared Co nanoparticles by atmospheric oxygen, high-purity argon was bubbled through the solution during the whole procedure. While maintaining argon flow, 15 ml of 0.005 M aqueous hexachloroplatinicacid (SigmaeAldrich) solution was added dropwise under magnetic stirring. The solution changed from brown to black, which showed that a trans-metalation reaction occurred according to Eq. (1) [30]: 2
2Co þ PtCl6 ¼ Pt þ 2Co2þ þ 6Cl
(1)
Upon completion of Pt addition, the argon flow was stopped and the vessel was opened at ambient conditions under
11513
stirring to oxidize any remaining cobalt metal left in solution. After 1 h, 58.5 mg Vulcan XC-72R carbon (Cabot, 240 m2 g1) were then added to the above solution. After stirring for 3 h, the final product carbon supported Pt hollow nanospheres were collected by filtration and washed several times with distilled water and alcohol. For comparison, SNPt/C was prepared by citrate reduction method [31]. The resulting catalysts dried for 12 h at 80 C in vacuum.
2.2.
Material characterization
The structure and morphology of the as-prepared HNPt/C electrocatalyst were examined by transmission electron microscopy (TEM), using a FEI Tecnai G2 microscope at 200 kV. For TEM analyses, samples were prepared by placing one or two drops of nanoparticles solution onto the carbon-coated copper grid and drying it in air at room temperature. A diffractometer (D/MAX-3C) was employed using Cu Ka ˚ ) and a graphite monochromator at radiation (l ¼ 1.54056 A 50 kV, 100 mA to obtain XRD spectra of the samples. The 2q angular regions between 10 and 80 were explored at a scan rate of 5 min1.
2.3.
Electrochemical performance
A conventional three-electrode cell was used to perform the electrochemical tests of the anode catalysts employing a CHI660A Electrochemistry Workstation. The HNPt/C and SNPt/C were used as working electrode, a Ni foam mesh with 3 5 cm2 as counter electrode and an Ag/AgCl, KClstd as the reference electrode. The electrolyte was 0.5 M H2SO4 or 0.1 M NaBH4 þ 3.0 M NaOH. The working electrode was prepared as follows: 10 mg of HNPt/C or SNPt/C powder was dispersed by ultrasonic for 2 h in 1 ml blending solution of 0.25 ml 5 wt% Nafion solution and 0.75 ml de-ionized water. Then 5 ml of slurry was pasted on the surface of the glassy carbon (GC) electrode (3 mm in diameter) which was polished to mirror by 0.5 mm alumina and sonicated 15 min prior to use. The dispersed catalyst on the GC surface was dried for 5 h at ambient temperature and the loading mass of catalyst was 0.7 mg cm2.
2.4.
Fuel cell test
The catalysts ink was made by mixing isopropyl alcohol with 7 wt.% of nafion solution and carbon supported catalysts. Then the ink was coated onto stainless steel gauze resulting in a 4.0 mg cm2 loading. The catalyst electrodes were pressed at 10 MPa for 1 min after drying at 50 C in vacuum for 8 h. The cell performance of DBHFC was tested against a SNPt/C cathode and a HNPt/C or SNPt/C anode. A schematic diagram of the experimental set-up was shown in our previous studies [2,3,32e34]. An activated nafion 117 membrane was used to separate the anolyte and catholyte. Anolyte composed of 1 M NaBH4 þ 3 M NaOH, and catholyte composed of 2 M H2O2 þ 0.5 M H2SO4. The fresh anolyte and catholyte were continuously supplied and withdrawn from the cell at 0.7 ml min1 during the test process. The electrochemical testing of the cell was performed using a battery testing
11514
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Fig. 1 e Schematic illustration of the experimental procedure which generates Pt hollow nanosphere by templating against Co nanoparticles.
system (Qingtian, Guangzhou, China). Power densities were calculated from the applied current and steady state potential.
3.
Results and discussion
3.1.
Physical characterizations
For trans-metalation reaction in Eq. (1), the standard reduction potential of the PtCl2 6 /Pt redox pair (þ0.744 V vs. the standard hydrogen electrode (SHE)) is higher than that of the Co2þ/Co redox pair (0.227 V vs. SHE), thus Co nanoparticles can be immediately oxidized into Co2þ when the solution containing Co nanoparticles is added to the H2PtCl6 solution due to the big difference between the potential of two redox couples. The schematic mechanism of Pt hollow nanospheres is shown in Fig. 1. Fig. 2 (a) and (b) are typical TEM and HRTEM images of HNPt/C. It was found that many nanoparticles assembled together and formed the hollow spherical shell. Besides, it can be seen from Fig. 2(a) and (b) that the centers of the Pt nanospheres were brighter than the edges, indicating that the nanosphere was actually a hollow spherical structure. The average diameter of the hollow Pt nanosphere was statistically estimated to be 20 2 nm, while the average particle size of Pt nanoparticles was about 2e3 nm. In addition, the shell of the hollow Pt nanosphere was incomplete and porous, and this porous and incomplete shell structure of hollow Pt nanosphere allows the interior surface to occur also catalytic
reaction, thus the hollow Pt nanospheres will be much more the specific surface area for catalytic reaction than solid Pt nanparticles and result in a much higher catalytic activity. The XRD patterns of the as-prepared HNPt/C and SNPt/C catalysts were shown in Fig. 3. The wide diffraction peak at 2q ¼ 25.0 is attributed to Vulcan XC-72R carbon (002) crystal face, which matches well with the standard C peaks (JCPDS No. 75-1621). The strong diffraction peaks positioned at 2q ¼ 39.7 , 46.2 , and 67.4 can be indexed to the (111), (200) and (220) crystalline planes of face-centered cubic (fcc) Pt, which are in good agreement with the standard card of cubic Pt (JCPDS No. 04-0802). The diffraction peak for Pt (200) is used to estimate the Pt particle size by the Scherrer’s equation [22,35]:
D¼
0:9l Bcos q
(2)
where D is average particle size, nm, the wavelength l is equal to 0.154056 nm, q is the angle of Pt (200) peak and B is the full width at half-maximum in radians (FWHM). The calculated average particle size of the hollow Pt nanoparticles and solid Pt nanoparticles dispersed on carbon is 2.8 nm and 4.4 nm, respectively, which is consistent with the TEM results.
3.2.
Electrochemically active surface area estimation
Electrochemically active surface area (ECSA) of the electrocatalysts could be estimated from the coulombic charge for the hydrogen adsorption and desorption (QH) in the cyclic voltammograms. Cyclic voltammograms were recorded from 0.2 V to 1.4 V at a scan rate of 50 mV s1 in a 0.5 M H2SO4 solution, high-purity argon gas was used during the experiments and argon gas was passed for 30 min to eliminate oxygen (Fig. 4). The value of QH is calculated as the mean value between the amounts of charge transfer during the electroadsorption and desorption of H2 on Pt sites [36,37]:
ECSA ¼
QH ½Pt 0:21
Fig. 2 e TEM (a) and HRTEM (b) images of Pt hollow nanospheres dispersed on carbon.
(3)
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Fig. 3 e XRD patterns of Pt hollow nanospheres dispersed on carbon and Pt solid nanoparticles dispersed on carbon.
11515
Fig. 5 e Changes of electrochemical active surface area of HNPt/C and SNPt/C electrodes with different cycle number.
electrocatalysts have good long-term running stability, especially HNPt/C presents much better stability.
where [Pt] is the platinum loading (mg cm2) in the electrode, QH is the charge for hydrogen desorption (mC cm2) and 0.21 is the charge required to oxidize a monolayer hydrogen adsorption on bright Pt (mC cm2) [38]. The calculated ECSA of the hollow Pt nanospheres dispersed on carbon is 224.32 cm2 mg1, which is much higher than that of solid Pt nanoparticles dispersed on carbon (77.38 cm2 mg1), and the high ECSA of electrocatalysts is attributed to the small particle size of Pt nanoparticles loaded on the carbon [39]. The stabilizing effect of both HNPt/C and SNPt/C was investigated under a continuous 2000 potential cycles in the potential range of 0.2 V to 1.4 V in 0.5 M H2SO4 solution [39]. The changes of ECSA with the increasing cycle numbers were shown in Fig. 5. As seen in Fig. 5, the ECSA of both electrocatalysts is decreased after the 2000 cycles, and HNPt/C retains 81.12% of the initial ECSA, which is higher than that of SNPt/C (60.43%). The above results indicate that both
The cyclic voltammograms recorded using HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 þ 3.0 M NaOH solution at a scan rate of 20 mV s1 in the potential range of 1.2 V to 0.6 V vs. Ag/ AgCl, KClstd are showed in Fig. 6. The HNPt/C and SNPt/C electrodes show a similar catalytic behavior for the electrooxidation of BH 4 by comparing the shapes of two CV curves between 1.2 and 0.6 V vs. Ag/AgCl. In the CV curves, the electrochemical behavior of BH 4 is fairly complex characterized by a number of oxidation peaks. During the forward sweep, at a rate of 20 mV s1, a well-defined oxidation peak occurs at 0.83 V (a1 or a10 ), followed by a broad hump anodic peak (a2 or a20 ) was observed. During the reverse sweep,
Fig. 4 e Cyclic voltammograms of HNPt/C and SNPt/C electrodes in Ar-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV sL1.
Fig. 6 e Cyclic voltammograms of HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 D 3.0 M NaOH solution at a scan rate of 20 mV sL1.
3.3.
Cyclic voltammetry study
11516
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
a sharp anodic spike (c1 or c10 ) was observed. Similar anodiccathodic peak pattern in cyclic voltammetry has reported by Gyenge [7] and Concha [40]. The first anodic peak (a1 or a10 ) can be assigned to borohydride hydrolysis followed by the electrooxidation of H2, the second oxidation peak (a2 or a20 ) is attributed to the direct oxidation of BH 4 in absence of H2 electroxidation, and the peak (c1 or c10 ) is due to the oxidation of absorbed intermediate oxidation products of BH3OH (Eq. (5)), but on the partially oxidized Pt surface [7,40]. BH 4 þ H2 O/BH3 OH þ H2
(4)
3 BH3 OH þ 3OH /BO 2 þ H2 þ 2H2 O þ 3e 2
(5)
The peak a2 on HNPt/C electrode occurs at 0.255 V, which is much more negative than that of peak a20 on SNPt/C electrode (0.062 V), and the current density of peak a2 on HNPt/C electrode is 26.21 mA cm2, it is also much higher than that of peak a20 on SNPt/C electrode (21.88 mA cm2). Apparently, the catalytic activity of HNPt/C electrode for electrooxidation of BH 4 will be higher than that of SNPt/C electrode. During the reverse sweep, the current density of peak c1 on HNPt/C electrode and SNPt/C electrode is almost similar; however, the potential of peak c10 on SNPt/C electrode is about 0.398 V, and the potential of peak c1 on HNPt/C electrode shifts positively to 0.294 V. Therefore, the electrocatalytic activity to BH 4 hydrolysis reaction on HNPt/C electrode is clearly less than that on SNPt/C electrode. As a result, it can be concluded that HNPt/C will be much more favorable to DBHFC.
3.4.
Chronopotentiometry
By analogy with the constant current operation of a direct borohydride fuel cell, chronopotentiometry could provide further relevant information. Fig. 7 depicts the chronopotentiometry of BH 4 oxidation on the HNPt/C and SNPt/C electrodes at a current density of 8.5 mA cm2 in the solution of 0.1 M NaBH4 þ 3.0 M NaOH. After 120 s, the operating potential of HNPt/C electrode was more negative (about
Fig. 7 e Chronopotentiometry curves of HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 D 3.0 M NaOH solution. Current step: from 0 to 8.5 mA cmL2.
Fig. 8 e Chronoamperometry curves of HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 D 3.0 M NaOH solution. Potential step: from L1.2 to L0.2 V vs. Ag/AgCl.
26 mV) than that of the SNPt/C electrode. This is attributed to the microstructure of Pt hollow nanospheres. The porous and hollow structure will provide much more active center for electrocatalytic reaction due to big specific surface area and is more favorable for mass transfer of liquid fuel toward active site, and thus the kinetics of BH 4 electrooxidation can be improved [41].
3.5.
Chronoamperometry
Chronoamperometric technique is an effective method to evaluate the electrocatalytic activity and stability of catalyst material. Fig. 8 shows the chronoamperometric responses of the HNPt/C and SNPt/C electrodes in 0.1 M NaBH4 þ 3.0 M NaOH solution from 1.2 to 0.2 V vs. Ag/AgCl, KClstd. It can be found from Fig. 8 that two samples show current decay during
Fig. 9 e Cell voltage and power density vs. current density for DBHFCs with different anodes at 20 C: (a) with HNPt/C (0.98 mg Pt cmL2) as anode catalyst; (b) with SNPt/C (1.08 mg Pt cmL2) as anode catalyst. Anolyte: 1 M NaBH4 D 3 M NaOH; catholyte: 2 M H2O2 D 0.5 M H2SO4.
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
Table 1 e Characteristic parameters of DBHFCs using HNPt/C and SNPt/C as anode catalysts at 20 C. Example
Open circuit voltage OCP (V)
Current density j (mA cm2)
Peak power density P (mW cm2)
1.728
44.9
54.53
1.656
34.8
37.95
DBHFC with HNPt/C as anode DBHFC with SNPt/C as anode
the electrooxidation of BH 4 . After 2 s, the HNPt/C electrode delivers higher current density (38.7 mA cm2) than the SNPt/ C electrode (27.4 mA cm2). This result also confirms that the HNPt/C electrode has a higher electrocatalytic activity than the SNPt/C electrode.
3.6.
Fuel cell performance measurement
Fig. 9 shows the changes of the cell voltage and power density as a function of current density at 20 C using the HNPt/C anode catalysts in comparison with SNPt/C anode catalysts. The cathode catalysts of both DBHFC systems were SNPt/C. The anolyte was 3 M NaOH þ 1 M NaBH4 solution. The catholyte was 0.5 M H2SO4 þ 2 M H2O2 solution according to our previous studies [34]. The open circuit voltage (OCP) of the cell is about 1.728 V, which lower than the standard cell potential for the DBHFC in acid electrolyte. The low value is probably caused by mixed potential at the anode and cathode from simultaneous oxidation of BH 4 ions and hydrogen at the anode and reduction of H2O2 and O2 at the cathode [42]. The behavior of the cell voltage vs. current density obtained using HNPt/C as anode catalysts shows a similar linear behavior experienced with SNPt/C anode catalysts. However, the cell voltage dropped more slowly as the current density increased. For comparison, the characteristic parameters of DBHFCs were tabulated in Table 1. It can be seen from Table 1 that the maximum power density is 54.53 mW cm2 at a discharge current density of 44.9 mA cm2 at 20 C. Apparently, the DBHFC using HNPt/C as anode catalysts exhibits a clearly superior performance to the SNPt/C as the anode catalysts, although the loading amount (0.98 mg cm2) of actual Pt in HNPt/C is less than that in SNPt/C (1.08 mg cm2).
4.
Conclusions
The Pt hollow nanospheres dispersed on carbon were successfully prepared at room temperature in a homogeneous solution with Co nanoparticles as sacrificial templates. The HNPt/C catalyst shows much higher electrochemical activity than SNPt/C for BH 4 oxidation in the alkaline solution. The DBHFC using HNPt/C as the anode catalyst exhibits excellent electrochemical performance compared with SNPt/C as the anode catalyst, which is ascribed to the porous, hollow microstructure and higher active surface area of Pt nanospheres. The maximum power density of the DBHFC employing HNPt/C as the anode catalyst and SNPt/C as the cathode catalyst was 54.53 mW cm2 at 20 C. Therefore, the HNPt/C
11517
catalyst with high performance and low cost will be a promising anode catalyst for the application of DBHFCs.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 20871101 and 51072173), Doctoral Fund of Ministry of Education of China (Grant No. 20094301110005), the Science and Technology Plan Project of Hunan Province (Grant No. 2010GK3181) and Natural Science Foundation of Xiangtan University (Grant No.10XZX07).
references
[1] Wu HJ, Wang C, Liu ZX, Mao ZQ. Influence of operation conditions on direct NaBH4/H2O2 fuel cell performance. Int J Hydrogen Energy 2010;35:2648e51. [2] Wei JL, Wang XY, Wang Y, Guo J, He PY, Yang SY, et al. Carbon-supported Au hollow nanospheres as anode catalysts for direct borohydride-hydrogen peroxide fuel cells. Energy Fuels 2009;23:4037e41. [3] Wei JL, Wang XY, Wang Y, Chen QQ, Pei F, Wang YS. Investigation of carbon-supported Au hollow nanospheres as electrocatalyst for electrooxidation of sodium borohydride. Int J Hydrogen Energy 2009;34:3360e6. [4] Sanli E, Celikkan H, Uysal BZ, Aksu ML. Anodic behavior of Ag metal electrode in direct borohydride fuel cell. Int J Hydrogen Energy 2006;31:1920e4. [5] Miley GH, Luo N, Mather J, Burton R, Hawkins G, Gu LF, et al. Direct NaBH4/H2O2 fuel cells. J Power Sources 2007;165: 509e16. [6] 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(2): 1084e8. [7] Gyenge EL. Electrooxidation of borohydride on platinum and gold electrodes: implications for direct borohydride fuel cells. Electrochim Acta 2004;49:965e78. [8] Sanli E, Uysal BZ, Aksu ML. The oxidation of NaBH4 on electrochemicaly treated silver electrodes. Int J Hydrogen Energy 2008;33:2097e104. [9] Concha BM, Chatenet M. Direct oxidation of sodium borohydride on Pt, Ag and alloyed PteAg electrodes in basic media: Part II. Carbon-supported nanoparticles. Electrochim Acta 2009;54:6130e9. [10] Ma J, Sahai Y, Buchheit RG. Direct borohydride fuel cell using Ni-based composite anodes. J Power Sources 2010;195: 4709e13. [11] Santos DMF, Sequeira CAC. Zinc anode for direct borohydride fuel cells. J Electrochem Soc 2010;157:B13e9. [12] Wang GL, Wang XY, Miao RR, Cao DX, Sun KN. Effects of alkaline treatment of hydrogen storage alloy on electrocatalytic activity for NaBH4 oxidation. Int J Hydrogen Energy 2010;35:1227e31. [13] Mirkin MV, Yang H, Bard AJ. Borohydride oxidation at a gold electrode. J Electrochem Soc 1992;139:2212e7. [14] Okinaka Y. An electrochemical study of electroless golddeposition reaction. J Electrochem Soc 1973;120:739e44. [15] Dong H, Feng RX, Ai XP, Cao YL, Yang HX, Cha CS. Electrooxidation mechanisms and discharge characteristics of borohydride on different catalytic metal surfaces. J Phys Chem B 2005;109:10896e901.
11518
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 6 ( 2 0 1 1 ) 1 1 5 1 2 e1 1 5 1 8
[16] Elikkana HC, Ahin MS, Aksu ML, Veziroglu TN. The investigation of the electrooxidation of sodium borohydride on various metal electrode in aqueous basic solutions. Int J Hydrogen Energy 2007;32:588e93. [17] Cheng H, Scott K. Determination of kinetic parameters for borohydride oxidation on a rotating Au disk electrode. Electrochim Acta 2006;51:3429e33. [18] Vincent WSL, Gyenge EL. High-performance osmium nanoparticle electrocatalyst for direct borohydride PEM fuel cell anodes. J Electrochem Soc 2008;155:B1155e60. [19] Kadirgan F, Kannan AM, Atilan T, Beyhan S, Ozenler SS, Suzer S, et al. Carbon supported nano-sized PtePd and PteCo electrocatalysts for proton exchange membrane fuel cells. Int J Hydrogen Energy 2009;34:9450e60. [20] Santiago EI, Varanda LC, Villullas HM. Carbon-supported PteCo catalysts prepared by a modified polyol process as cathodes for PEM fuel cells. J Phys Chem C 2007;111:3146e51. [21] 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. [22] Wang GJ, Gao YZ, Wang ZB, Du CY, Wang JJ, Yin GP. Investigation of PtNi/C anode electrocatalysts for direct borohydride fuel cell. J Power Sources 2010;195:185e9. [23] Li WZ, Xin Q, Yan YS. Nanostructured PteFe/C cathode catalysts for direct methanol fuel cell: the effect of catalyst composition. Int J Hydrogen Energy 2010;35:2530e8. [24] Baglio V, Arico AS, Stassi A, Urso CD, Blasi AD, Lunab AMC, et al. Investigation of PteFe catalysts for oxygen reduction in low temperature direct methanol fuel cells. J Power Sources 2006;159:900e4. [25] Jiang L, Hsu A, Chu D, Chen R. Ethanol electro-oxidation on Pt/C and PtSn/C catalysts in alkaline and acid solutions. Int J Hydrogen Energy 2010;35:365e72. [26] Song SQ, Zhou WJ, Zhou ZH, Jiang LH, Sun GQ, Xin Q, et al. Direct ethanol PEM fuel cells: the case of platinum based anodes. Int J Hydrogen Energy 2005;30:995e1001. [27] Yang H, Alonso-Vante N, Leger JM, Tailoring CL. Structure and activity of carbon-supported nanosized PteCr alloy electrocatalysts for oxygen reduction in pure and methanolcontaining electrolytes. J Phys Chem B 2004;108:1938e47. [28] Colon-Mercado HR, Kim H, Popov BN. Durability study of Pt3Ni1 catalysts as cathode in PEM fuel cells. Electrochem Commun 2004;6:795e9. [29] Colon-Mercado HR, Popov BN. Stability of platinum based alloy cathode catalysts in PEM fuel cells. J Power Sources 2006;155:253e63.
[30] Liang HP, Zhang HM, Hu JS, Guo YG, Wan LJ, Bai CL. Pt hollow nanospheres: facile synthesis and enhanced electrocatalysts. Angew Chem Int Ed 2004;43:1540e3. [31] Zhao J, Chen WX, Zhang YF, Li X. Novel carbon supported hollow Pt nanospheres for methanol electrooxidation. J Power Sources 2006;162:168e72. [32] He PY, Wang Y, Wang XY, Pei F, Wang H, Liu L, et al. Investigation of carbon supported AueNi bimetallic nanoparticles as electrocatalyst for direct borohydride fuel cell. J Power Sources 2011;196:1042e7. [33] Pei F, Wang Y, Wang XY, He PY, Chen QQ, Wang XY, et al. Performance of supported AueCo alloy as the anode catalyst of direct borohydride-hydrogen peroxide fuel cell. Int J Hydrogen Energy 2010;35:8136e42. [34] Wei JL, Wang XY, Wang H, Yang SY, Dai CL, Pei F. Performance of nanosized Au particle as a cathode catalyst for direct borohydride-hydrogen peroxide fuel cell. Acta Chim Sinica 2008;66:2675e80. [35] Niu JJ, Wang JN. Activated carbon nanotubes-supported catalyst in fuel cells. Electrochim Acta 2008;53:8058e63. [36] Wang JJ, Yin GP, Zhang J, Wang ZB, Gao YZ. High utilization platinum deposition on single-walled carbon nanotubes as catalysts for direct methanol fuel cell. Electrochim Acta 2007; 52:7042e50. [37] Pozio A, Francesco MD, Cemmi A, Cardellini F, Giorgi L. Comparison of high surface Pt/C catalysts by cyclic voltammetry. J Power Sources 2002;105:13e9. [38] Maillard F, Martin M, Gloaguen F, Le´ger JM. Oxygen electroreduction on carbon-supported platinum catalysts. Particle-size effect on the tolerance to methanol competition. Electrochim Acta 2002;47:3431e40. [39] Kou R, Shao YY, Wang DH, Engelhard MH, Kwak JH, Wang J, et al. Enhanced activity and stability of Pt catalysts on functionalized graphene sheets for electrocatalytic oxygen reduction. Electrochem Commun 2009;11:954e7. [40] Concha BM, Chatenet M. Direct oxidation of sodium borohydride on Pt, Ag and alloyed PteAg electrodes in basic media. Part I: bulk electrodes. Electrochim Acta 2009;54:6119e29. [41] Kim JH, Kim HS, Kang YM, Song MS, Rajendran S, Han SC. Carbon-supported and unsupported Pt anodes for direct borohydride liquid fuel cells. J Electrochem Soc 2004;151: A1039e43. [42] Ponce de leo´n C, Walsh FC, Patrissi CJ, Medeiros MG, Bessette PR, Reeve RW, et al. A direct borohydride-peroxide fuel cell using a Pd/Ir alloy coated microfibrous carbon cathode. Electrochem Commun 2008;10:1610e3.