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C (M = Ru, Pt, Au) as anode catalysts for direct ethanol fuel cells
Journal of Alloys and Compounds 676 (2016) 390e396
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Surface noble metal modified PdM/C (M ¼ Ru, Pt, Au) as anode catalysts for direct ethanol fuel cells Han Mao, Tao Huang**, Aishui Yu* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200438, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2016 Received in revised form 21 March 2016 Accepted 23 March 2016 Available online 25 March 2016
In this article, we studied the surface noble metal modification on Pd nanoparticles, other than the homogeneous or core-shell structure. The surface modification will lead to the uneven constitution within the nanoparticles and thus more obvious optimization effect toward the catalyst brought by the lattice deformation. The surface of the as-prepared Pd nanoparticles was modified with Ru, Pt or Au by a moderate and green approach, respectively. XPS results confirm the interactive electron effects between Pd and the modified noble metal. Electrochemical measurements show that the surface noble metal modified catalysts not only show higher catalytic activity, but also better stability and durability. The PdM/C catalysts all exhibit good dispersion and very little agglomeration after long-term potential cycles toward ethanol oxidation. With only 10% metallic atomic ratio of Au, PdAu/C catalyst shows extraordinary catalytic activity and stability, the peak current reaches 1700 mA mg1 Pd, about 2.5 times that of Pd/C. Moreover, the PdAu/C maintains 40% of the catalytic activity after 4500 potential cycles. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ethanol oxidation Palladium-based catalysts Noble metal surface modification
1. Introduction Direct alcohol fuel cells are promising alternatives for portable power resources. Among the optional alcohols applied in fuel cells, direct ethanol alkaline fuel cells (DEAFCs) have become a research focus [1,2]. Ethanol has higher energy density and is less toxic than other alcohols, and can be largely produced from agricultural bioprocesses. Additionally, ethanol has been proven to have a lower crossover rate and affect cathode performance less severely than methanol [3e5]. In regard with the anode catalysts for DEAFCs, it's possible to use relatively cheap and abundant metals rather than platinum. It has been shown that Pd is more active and stable and poison tolerant than Pt for ethanol oxidation in alkaline media [6e8]. Researches about electrolytes show that alkaline electrolytes provide unique characteristics such as the enhanced electrochemical kinetics at low anodic overpotential for DEAFCs [9]. However, there are still many challenges about DEAFCs that need to be overcome, the primary challenge is the sluggish kinetics of the ethanol oxidation and the unsatisfied activity and stability of the anode catalysts [10]. To enhance the behavior of the Pd-based
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Huang), [email protected] (A. Yu). http://dx.doi.org/10.1016/j.jallcom.2016.03.200 0925-8388/© 2016 Elsevier B.V. All rights reserved.
catalysts, nanostructured Pd and its alloys with various morphologies and compositions have been synthesized. One main strategy to improve the performance of catalysts is to introduce a second element to modify the electron structure and chemical environment of Pd, and literature have reported the applications such as Pt, Ni, Ru, Rh, Sn, and Cu [11e17]. Among the modifying elements, noble metals seem to have better performance in the stabilization and activation for the catalysts [18,19]. In this work, we synthesized a Pd/C catalyst by a moderate and green method. The as-prepared Pd/C catalyst exposes complicated facets other than the conventional Pd-based catalysts with uniform (111) facet [20]. Considering the catalysts may suffer from degradation in the electrochemical process and may be affected by the adsorption of hydroxyl ions and the formation of an oxide layer on Pd surfaces, we decorated the Pd nanoparticles with noble metals such as Ru, Pt or Au on the surface. Thus the electron structures of the catalysts will be modified and the catalysts will be stabilized, leading to the enhancement of catalytic stability and activity toward the ethanol oxidation [21]. In this article we propose an innovative way of surface modification with noble metal on the Pd nanoparticles. The reason for the surface modification other than the homogeneous alloy is that we believe the surface noble metal can help reduce adsorption energy of CO, and the surface modification will lead to the uneven
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constitution within the nanoparticles and thus the optimization of the catalyst brought by the lattice deformation. Moreover, the economy factors are also considered that Pt and Au are more expensive than Pd. 2. Experimental section 2.1. Catalyst preparation The Pd nanoparticles were synthesized by a simple and green method. First, 240 mg (0.8 mmol) Pd(acac)2 and 80 mg polyvinylpyrrolidone (PVPK30) were dissolved in 160 mL ethylene glycol (EG) at 60 C under N2 atmosphere. The reaction solution was kept at 90 C for 1.5 h to get the solution of Pd nanoparticles. The
391
obtained Pd solution was equally divided into four parts, of which three parts were added with certain amount of chloride (RuCl3, K2PtCl6, or HAuCl4) separately at 80 C under N2 atmosphere to get the PdM (M ¼ Ru, Pt, Au) nanoparticles. Then we got four separate solution of Pd, PdRu, PdPt and PdAu nanoparticles. The obtained solution was first centrifuged at 12,000 rpm and then the precipitates were washed with the mixture of ethanol and acetone under sonication. After centrifugation four ethanol solutions were obtained, containing Pd, PdRu, PdPt and PdAu. Each dispersion was mixed with 70 mg Vulcan XC-72R and dried under stirring at 75 C, after the ethanol was evaporated, the resulting four catalysts were separately subjected to tube furnace and heated in N2 at 200 C for 3 h with a heating rate of 5 centigrade per minute. The obtained catalysts were denoted as Pd/C, PdRu/C, PdPt/C and PdAu/C,
Fig. 1. TEM images of (A)&(B) Pd/C, (C)&(D) PdRu/C, (E)&(F) PdPt/C, (G) &(H) PdAu/C.
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3. Results and discussion
Fig. 2. XRD patterns of Pd/C, PdRu/C, PdPt/C and PdAu/C. The vertical lines are that of standard Pd crystal.
respectively. The bulk composition of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The precise metal compositions of Pd/C, PdRu/C, PdPt/C and PdAu/C are listed in the Supporting Information (Supporting Information, Page 1, Table 1).
2.2. Material characterization The as-prepared catalysts were characterized by X-ray diffraction (XRD) performed on a Bruker D8 Advance X-ray diffractometer using Cu Ka radiation with a k of 1.5406 Å at a scan rate of 4 min1. The morphologies were observed by transmission electron microscopy (TEM) (JEOL JEM-2010F UHR). A Kratos Axis Ultra X-ray photoelectron spectrometer (Al Ka source) was employed to record the XPS spectra.
2.3. Electrochemical measurement The working electrodes for electrochemical experiments were prepared by thin film electrode method. A polished glassy carbon (GC, F 5 mm) was used as the substrate. We controlled the Pd loading the same, on each electrode the mass of Pd is 8 mg, then we can calculate the amount of the catalyst needed to prepare the sample. The certain amount of sample was dispersed in 2 mL ethanol under sonication for twenty minutes, and then 25 mL suspension was pipetted onto a GC substrate under sonication. After ethanol evaporation, the deposited catalysts were covered by 5 mL Nafion solution (0.5 wt%, Dupont). All electrochemical measurements were carried out on a CHI 660A electrochemical workstation (Shanghai Chenhua apparatus corporation, China) using a conventional three-electrode glassy cell with a platinum sheet as the counter electrode. The reference electrode is an Hg/Hg2SO4, K2SO4 electrode (mercurous sulfate electrode, 0.615 V vs. NHE) in acidic solution, or an Hg/HgO/OH electrode (mercuric oxide electrode, 0.098 V vs. NHE) in alkaline solution. For each catalyst at least three parallel tests will be prepared and the error range should be within ±5% in these tests. We take the median value of each catalyst to do the comparison.
Fig. 1 shows the TEM images of Pd/C, PdRu/C, PdPt/C and PdAu/ C. For every catalyst we have checked about 200 metal nanoparticles to calculate the particle size and exposed facets. And we find the particle sizes of PdRu, PdPt and PdAu haven't changed much after the surface modification, the average size of the Pd, PdRu, PdPt and PdAu nanoparticles is ca. 8.0 nm, 7.8 nm, 8.0 nm, 8.5 nm, respectively. With the HR-TEM images of each catalyst, we can apply the fast fourier transform and calculate the d-spacing values. It's clear that the predominantly exposed facet of Pd/C, PdPt/ C and PdAu/C is complex, mainly exposed (200) facet and with small proportions of (111) and (220) facets as well. The complex exposed facet also indicates the roughness and unevenness of the particle surface. The Au surface modification results in some angular morphology and enlarged particles size of PdAu nanoparticles. As to PdRu/C (Fig. 1D), however, it seems that the addition of Ru changes the original complex facets into uniform (111) facets. We think this phenomenon may result from the different space groups of Pd (Fm-3m) and Ru (P63/mmc). Fig. 2 shows the XRD patterns of Pd/C, PdRu/C, PdPt/C and PdAu/ C catalysts. All the catalysts show similar broad peaks located around 25 , corresponding to the (002) plane of the carbon support Vulcan XC-72R. The Pd/C catalyst shows four peaks located around 40 , 47, 68 and 82 , which can be ascribed to the (111), (200), (220) and (311) planes of the face-centered cubic (fcc) structure of Pd. PdRu/C, PdPt/C and PdAu/C catalysts show similar fcc structures. The atomic proportions of decorated noble metal of PdRu, PdPt and PdAu are approximate, about 10% of the total metal (ranged from 9 to 12%), in this regard the characteristic of the nanoparticles will be Pd dominant, with a little distortion of Pd lattice due to the introduction of Ru, Pt or Au. The doping of larger radius Pt or Au makes the PdPt/C and PdAu/C shift to lower angle integrally compared to Pd/C, which is in accord with the Vegard's Law. As to PdRu/C, it seems that the doping of Ru makes negligible changes to the peak location, but only a slight asymmetry of the peaks which may reflect the Ru decoration. X-ray photoelectron spectroscopy (XPS) is a powerful tool for revealing the electron structures and valence states, Fig. 3 shows the typical XPS spectra of the catalysts. The different chemical states of an element can be analyzed by deconvolution of the original spectra. On the fitting curves, the Pd 3d signal of each catalyst consists of two doublets, which can be assigned to Pd(0) and Pd(II), respectively. In Fig. 3AeD and F, the binding energy for Pd0 3d5/2 of Pd/C, PdRu/C, PdPt/C and PdAu/C locate at 335.3 eV, 335.2 eV, 335.7 eV and 335.6 eV, respectively. Compared to the standard Pd0 3d5/2 located at 335.0 eV, it indicates all the asprepared catalysts shift to higher Pd core levels. In general, a positive core level shift of Pd can be interpreted as an electron loss of the Pd atoms, which will lead to a lower d-band center of Pd. A suitable downshift of Pd d-band center weakens the adsorption of surface oxygenated intermediates, and thus enhance the surface catalytic activity [22e24]. Besides, obviously shifted core levels of Ru, Pt and Au are also observed. The shifts of Ru, Pt and Au are approximate 0.5, 0.6 and 0.4 eV, respectively. The results indicate the existence of electron interaction between Pd and the modified noble metal. The existences of noble metals modify the electron structure of Pd, and may enhance the stability and activity of the catalysts. The electrochemical characterizations of Pd/C, PdRu/C, PdPt/C and PdAu/C catalysts were investigated. From the cathodic peaks around 0 V for the reduction of PdeO(H) species (Fig. 4A), it can be seen that the introduction of different noble metal leads to different peak position of oxide species' reduction. The calculated electrochemical surface active area (ESA) for Pd/C, PdRu/C, PdPt/C and
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Fig. 3. Core level XPS spectra for (A) Pd/C, (B)&(C) PdRu/C, (D)&(E) PdPt/C, (F) &(G) PdAu/C.
PdAu/C are 65, 60, 62.8 and 67.8 m2 g1. The area deviation of the other three is within 10%, which is much smaller than the deviation of the current density valued as mA mg1 Pd (Fig. 4B). In this account we think the difference toward activity is not just brought by the electrochemical active surface area, but also the interaction effect brought by the surface noble metal modification. The surface modification of noble metal has two effects on the ESA. One is
diminishing the ESA due to the surface Pd replacement by noble metal, the other is the enlarging the ESA due to the electron interaction. So we can find the ESA of PdPt/C and PdRu/C diminish and that of PdAu/C enlarges. The catalytic activities of the catalysts toward ethanol oxidation were measured by cyclic voltammetry (CV) in a solution of 0.5 M KOH and 0.5 M C2H5OH. The catalytic activity was evaluated as Pd
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Fig. 4. (A) CV curves in 0.5 M H2SO4 solution at a scan rate of 50 mV s1. (B) Ethanol electrooxidation curves in a solution of 0.5 M C2H5OH and 0.5 M KOH at a scan rate of 50 mV s1. (C) The comparison of current densities at lower potential. (D) Chronoamperometry curves polarizing at 0.15 V. (E) Chronopotentiometry curves with an anodic current of half of the peak current. (F) The long-term potential cycles in a solution of 0.5 M KOH and 0.5 M ethanol at 50 mV s1.
mass specific current. It should be mentioned that modified noble metals are not calculated in the current density, as neither Ru, Pt or Au shows catalytic activity in the potential range of our experiments. Fig. 4B shows the CV curves for Pd/C, PdRu/C, PdPt/C and PdAu/C. The forward anodic peak current density for prepared Pd/C,
PdRu/C, PdPt/C and PdAu/C is 707, 750, 1050 and 1700 mA mg1 Pd, respectively. PdAu/C shows the highest forward anodic peak current density, while the addition of Ru improves a little of the catalytic activity of PdRu/C even considering the effect of ESA, not as distinct as Pt and Au. Besides, we also compare the onset potential
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Fig. 5. TEM images of catalysts after long-term potential cycles in 0.5 M KOH þ EtOH. (A)&(B) Pd/C, (C)&(D) PdRu/C, (E) &(F) PdPt/C, (G) &(H) PdAu/C.
of these catalysts, the onset potential for Pd/C is 0.43 V, and PdRu/ C, PdPt/C and PdAu/C all show a down shift of onset oxidation potential by 10e20 mV. Lower onset potential means easier oxidation of ethanol in the same condition. Moreover, we also compare the current of the catalysts at lower potential (Fig. 4C). As it's practically difficult to operate the whole fuel cell at the peak potential after assembly, the current density at lower potential has an important practical meaning. In Fig. 4C we can see that the current densities at 0.3 V for Pd/C, PdRu/C, PdPt/C and PdAu/C are 270, 340, 350 and 550 mA mg1 Pd, respectively. It's clear that the noble metal surface modification further enhances the activity at lower potential, the PdAu/C still shows the highest current density of all, indicating the surface modified PdAu/C is even competitive in practical application.
The results of chronoamperometric and chronopotentiometric tests indicate that the surface modification by noble metal not only enhances the catalytic activity, but also improves the durability and stability of the catalysts. In Fig. 4D, PdAu/C shows the lowest degree of decay and highest current density at 1000 s in the chronoamperometry, PdPt/C takes the second place and Pd/C shows the worst. Chronopotentiometry is applied to study the poison resistance of the catalysts for ethanol oxidation, we set a constant current which is half of the peak current of each catalyst. Ethanol will be oxidized in the plateaus stage, due to catalyst poisoning, however, the potential must rise to achieve the pre-set current during this stage. When the catalyst are mostly poisoned, ethanol can no longer be oxidized, so the potential will suddenly rise to keep the pre-set current, in this stage the catalyst may be destroyed due to
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the high potential [3]. In this regard, the lower potential polarization and longer time before the potential jump are indicative of better anti-poisoning properties In the anti-poisoning test (Fig. 4E), the potential polarizations increase as PdAu/C < PdPt/C < PdRu/ C < Pd/C, indicating the same tendency of catalyst poisoning. With respect to the time for the potential jump, PdAu/C has the overwhelming superiority compared to the other catalysts. This result clearly suggests that practically less poisoning species can be formed and adsorbed on the PdAu/C surface in alkaline medium during ethanol oxidation process. The multiple cycles of the prepared catalysts in the same ethanol solution give a more intuitive impression (Fig. 4F). On the Pd/C electrode the current maintains 33% after 2000 cycles, while on the PdRu/C and PdPt/C electrodes, the catalytic activity maintains about 40% after 2000 cycles, indicating the improvement of the catalysts' duration after the surface modification. On PdAu/C electrode it shows the best stability and the duration, the catalytic activity maintains 40% after 4500 potential cycles, about twice of the other three. To observe the catalysts after the long-term test, we took the TEM images of cycled catalysts (Pd/C, PdRu/C, PdPt/C and PdAu/C). Fig. 5 shows that the degree of agglomeration of PdRu/C, PdPt/C and PdAu/C is less than that of Pd/C, and all the catalysts show clear lattice after cycling, which means the as-prepared catalysts haven't turn into the amorphous state as the catalysts usually will be. From Fig. 5 we can find that the complex exposed facet of Pd/C, PdPt/C and PdAu/C were worn and transformed to the preferential (111) facet after long-term electro-catalysis. We tend to believe that such complex facet exposure will favor the ethanol oxidation, thus we compare the electrochemical performance of commercial BASF Pd/ C and the as-prepared Pd/C. Although the as-prepared Pd/C has larger particle size, which means the fewer ratio of surface Pd sites than the BASF Pd/C, the as-prepared Pd/C still shows obvious better activity and stability than the BASF Pd/C. Considering the two catalysts are only different in facet exposure and particle size, and the particle size is unfavorable for as-prepared Pd/C, we believe the better performance of the as-prepared Pd/C comes from the complex exposed facets like the more active (200) and (220) (Supporting Information, page 1e3, Fig. S1). However, the exact reason about the complex facet still remain unknown. Taking all the electrochemical results into consideration, the surface modification of noble metal can both enhance the catalytic activity and the stability, and Au is the best modified noble metal and Pt is the second best. 4. Conclusions In this article, we prepared the surface M (M ¼ Ru, Pt, Au) modified PdM/C catalysts in view of optimizing the Pd electron structure and enhancing the catalytic activity and stability. XPS results confirm the synergetic effect between Pd and M, and indicate a down shift of d-band center of Pd which is positive for ethanol oxidation. As to the decorated noble metals, Au is the most effective additive element, the PdAu/C catalyst demonstrates best catalytic activity. The forward peak current reaches 1700 mA mg1 Pd, about 2.5 times that of Pd/C. Moreover, PdAu/C
can maintain 40% of the catalytic activity after 4500 potential cycles. PdPt/C is the second best catalyst with a peak current of 1050 mA mg1 Pd. TEM images before and after electrochemical circulations show very little agglomeration of the prepared four catalysts occur after cycling, indicating the good durability of the catalysts after surface modification. In conclusion, the effect of modification with noble metals has been researched and the Au turns out to be the best additive which can greatly enhance the catalytic activity and stability towards DEAFCs. Acknowledgments The authors acknowledge funding supports from the National Key Basic Research Program of China (973 Program, 2015CB932303) and Science & Technology Commission of Shanghai Municipality (No. 08DZ2270500), China. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.03.200. References [1] R. Yue, H. Wang, D. Bin, J. Xu, Y. Du, W. Lu, J. Guo, J. Mater. Chem. A 3 (2015) 1077e1088. [2] X. Yu, A. Manthiram, Appl. Catal. B Environ. 165 (2015) 63e67. [3] W. Wang, Y. Yang, Y. Liu, Z. Zhang, W. Dong, Z. Lei, J. Power Sources 273 (2015) 631e637. ~ artu, C. Mascayano, C. Gutie rrez, Electrochim. Acta 165 (2015) [4] M.S. Ureta-Zan 232e238. [5] S. Abdullah, S.K. Kamarudin, U.A. Hasran, M.S. Masdar, W.R.W. Daud, J. Power Sources 262 (2014) 401e406. [6] L. Ma, D. Chu, R. Chen, Int. J. Hydrogen Energy 37 (2012) 11185e11194. [7] C.W. Xu, L.Q. Cheng, P.K. Shen, Y.L. Liu, Electrochem. Comm. 9 (2007) 997e1001. [8] V. Bambagioni, C. Bianchini, A. Marchionni, J. Filippi, F. Vizza, J. Teddy, P. Serp, M. Zhiani, J. Power Sources 190 (2009) 241e251. [9] H. Pramanik, S. Basu, Chem. Eng. Process 49 (2010) 635e642. [10] S.P.S. Badwal, S. Giddey, A. Kulkarni, J. Goel, S. Basu, Appl. Energy 145 (2015) 80e103. [11] Z. Yin, L. Lin, D. Ma, Catal. Sci. Technol. 4 (2014) 4116e4128. € nbeck, C. Liu, W. Xing, Chem[12] G. Li, L. Feng, J. Chang, B. Wickman, H. Gro SusChem 7 (2014) 3374e3381. [13] S. Shen, T. Zhao, J. Mater. Chem. A 1 (2013) 906e912. [14] T. Sheng, W.F. Lin, C. Hardacre, P. Hu, Phys. Chem. Chem. Phys. 16 (2014) 13248e13254. ger, F. Kadırgan, Appl. Catal. B Environ. 144 (2014) 66e74. [15] S. Beyhan, J.M. Le [16] H. Mao, T. Huang, A.S. Yu, J. Mater. Chem. A 2 (2014) 16378e16380. [17] T. Ramulifho, K.I. Ozoemena, R.M. Modibedi, C.J. Jafta, M.K. Mathe, Electrochim. Acta 59 (2012) 310e320. [18] X.L. Sun, D.G. Li, Y. Ding, W.L. Zhu, S.J. Guo, Z.L. Wang, S.H. Sun, J. Am. Chem. Soc. 136 (2014) 5745e5749. [19] S.S. Li, J.J. Lv, Y.Y. Hu, J.N. Zheng, J.R. Chen, A.J. Wang, J.J. Feng, J. Power Sources 247 (2014) 213e218. [20] J.F. Chang, L.G. Feng, C.P. Liu, W. Xing, X.L. Hu, Angew. Chem. Int. Ed. 53 (2014) 122e126. [21] L.Y. Chen, N. Chen, Y. Hou, Z.C. Wang, S.H. Lv, T. Fujita, J.H. Jiang, A. Hirata, M.W. Chen, ACS Catal. 3 (2013) 1220e1230. [22] H.L. Jiang, Q. Xu, J. Mater. Chem. 21 (2011) 13705e13725. [23] J. Zhao, K. Jarvis, P. Ferreira, A. Manthiram, J. Power Sources 196 (2011) 4515e4523. [24] K. Jiang, W.B. Cai, Appl. Catal. B Environ. 147 (2014) 185e192.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Surface noble metal modified PdM/C (M ¼ Ru, Pt, Au) as anode catalysts for direct ethanol fuel cells Han Mao, Tao Huang**, Aishui Yu* Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Institute of New Energy, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, Shanghai 200438, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2016 Received in revised form 21 March 2016 Accepted 23 March 2016 Available online 25 March 2016
In this article, we studied the surface noble metal modification on Pd nanoparticles, other than the homogeneous or core-shell structure. The surface modification will lead to the uneven constitution within the nanoparticles and thus more obvious optimization effect toward the catalyst brought by the lattice deformation. The surface of the as-prepared Pd nanoparticles was modified with Ru, Pt or Au by a moderate and green approach, respectively. XPS results confirm the interactive electron effects between Pd and the modified noble metal. Electrochemical measurements show that the surface noble metal modified catalysts not only show higher catalytic activity, but also better stability and durability. The PdM/C catalysts all exhibit good dispersion and very little agglomeration after long-term potential cycles toward ethanol oxidation. With only 10% metallic atomic ratio of Au, PdAu/C catalyst shows extraordinary catalytic activity and stability, the peak current reaches 1700 mA mg1 Pd, about 2.5 times that of Pd/C. Moreover, the PdAu/C maintains 40% of the catalytic activity after 4500 potential cycles. © 2016 Elsevier B.V. All rights reserved.
Keywords: Ethanol oxidation Palladium-based catalysts Noble metal surface modification
1. Introduction Direct alcohol fuel cells are promising alternatives for portable power resources. Among the optional alcohols applied in fuel cells, direct ethanol alkaline fuel cells (DEAFCs) have become a research focus [1,2]. Ethanol has higher energy density and is less toxic than other alcohols, and can be largely produced from agricultural bioprocesses. Additionally, ethanol has been proven to have a lower crossover rate and affect cathode performance less severely than methanol [3e5]. In regard with the anode catalysts for DEAFCs, it's possible to use relatively cheap and abundant metals rather than platinum. It has been shown that Pd is more active and stable and poison tolerant than Pt for ethanol oxidation in alkaline media [6e8]. Researches about electrolytes show that alkaline electrolytes provide unique characteristics such as the enhanced electrochemical kinetics at low anodic overpotential for DEAFCs [9]. However, there are still many challenges about DEAFCs that need to be overcome, the primary challenge is the sluggish kinetics of the ethanol oxidation and the unsatisfied activity and stability of the anode catalysts [10]. To enhance the behavior of the Pd-based
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (T. Huang), [email protected] (A. Yu). http://dx.doi.org/10.1016/j.jallcom.2016.03.200 0925-8388/© 2016 Elsevier B.V. All rights reserved.
catalysts, nanostructured Pd and its alloys with various morphologies and compositions have been synthesized. One main strategy to improve the performance of catalysts is to introduce a second element to modify the electron structure and chemical environment of Pd, and literature have reported the applications such as Pt, Ni, Ru, Rh, Sn, and Cu [11e17]. Among the modifying elements, noble metals seem to have better performance in the stabilization and activation for the catalysts [18,19]. In this work, we synthesized a Pd/C catalyst by a moderate and green method. The as-prepared Pd/C catalyst exposes complicated facets other than the conventional Pd-based catalysts with uniform (111) facet [20]. Considering the catalysts may suffer from degradation in the electrochemical process and may be affected by the adsorption of hydroxyl ions and the formation of an oxide layer on Pd surfaces, we decorated the Pd nanoparticles with noble metals such as Ru, Pt or Au on the surface. Thus the electron structures of the catalysts will be modified and the catalysts will be stabilized, leading to the enhancement of catalytic stability and activity toward the ethanol oxidation [21]. In this article we propose an innovative way of surface modification with noble metal on the Pd nanoparticles. The reason for the surface modification other than the homogeneous alloy is that we believe the surface noble metal can help reduce adsorption energy of CO, and the surface modification will lead to the uneven
H. Mao et al. / Journal of Alloys and Compounds 676 (2016) 390e396
constitution within the nanoparticles and thus the optimization of the catalyst brought by the lattice deformation. Moreover, the economy factors are also considered that Pt and Au are more expensive than Pd. 2. Experimental section 2.1. Catalyst preparation The Pd nanoparticles were synthesized by a simple and green method. First, 240 mg (0.8 mmol) Pd(acac)2 and 80 mg polyvinylpyrrolidone (PVPK30) were dissolved in 160 mL ethylene glycol (EG) at 60 C under N2 atmosphere. The reaction solution was kept at 90 C for 1.5 h to get the solution of Pd nanoparticles. The
391
obtained Pd solution was equally divided into four parts, of which three parts were added with certain amount of chloride (RuCl3, K2PtCl6, or HAuCl4) separately at 80 C under N2 atmosphere to get the PdM (M ¼ Ru, Pt, Au) nanoparticles. Then we got four separate solution of Pd, PdRu, PdPt and PdAu nanoparticles. The obtained solution was first centrifuged at 12,000 rpm and then the precipitates were washed with the mixture of ethanol and acetone under sonication. After centrifugation four ethanol solutions were obtained, containing Pd, PdRu, PdPt and PdAu. Each dispersion was mixed with 70 mg Vulcan XC-72R and dried under stirring at 75 C, after the ethanol was evaporated, the resulting four catalysts were separately subjected to tube furnace and heated in N2 at 200 C for 3 h with a heating rate of 5 centigrade per minute. The obtained catalysts were denoted as Pd/C, PdRu/C, PdPt/C and PdAu/C,
Fig. 1. TEM images of (A)&(B) Pd/C, (C)&(D) PdRu/C, (E)&(F) PdPt/C, (G) &(H) PdAu/C.
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3. Results and discussion
Fig. 2. XRD patterns of Pd/C, PdRu/C, PdPt/C and PdAu/C. The vertical lines are that of standard Pd crystal.
respectively. The bulk composition of the catalysts were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The precise metal compositions of Pd/C, PdRu/C, PdPt/C and PdAu/C are listed in the Supporting Information (Supporting Information, Page 1, Table 1).
2.2. Material characterization The as-prepared catalysts were characterized by X-ray diffraction (XRD) performed on a Bruker D8 Advance X-ray diffractometer using Cu Ka radiation with a k of 1.5406 Å at a scan rate of 4 min1. The morphologies were observed by transmission electron microscopy (TEM) (JEOL JEM-2010F UHR). A Kratos Axis Ultra X-ray photoelectron spectrometer (Al Ka source) was employed to record the XPS spectra.
2.3. Electrochemical measurement The working electrodes for electrochemical experiments were prepared by thin film electrode method. A polished glassy carbon (GC, F 5 mm) was used as the substrate. We controlled the Pd loading the same, on each electrode the mass of Pd is 8 mg, then we can calculate the amount of the catalyst needed to prepare the sample. The certain amount of sample was dispersed in 2 mL ethanol under sonication for twenty minutes, and then 25 mL suspension was pipetted onto a GC substrate under sonication. After ethanol evaporation, the deposited catalysts were covered by 5 mL Nafion solution (0.5 wt%, Dupont). All electrochemical measurements were carried out on a CHI 660A electrochemical workstation (Shanghai Chenhua apparatus corporation, China) using a conventional three-electrode glassy cell with a platinum sheet as the counter electrode. The reference electrode is an Hg/Hg2SO4, K2SO4 electrode (mercurous sulfate electrode, 0.615 V vs. NHE) in acidic solution, or an Hg/HgO/OH electrode (mercuric oxide electrode, 0.098 V vs. NHE) in alkaline solution. For each catalyst at least three parallel tests will be prepared and the error range should be within ±5% in these tests. We take the median value of each catalyst to do the comparison.
Fig. 1 shows the TEM images of Pd/C, PdRu/C, PdPt/C and PdAu/ C. For every catalyst we have checked about 200 metal nanoparticles to calculate the particle size and exposed facets. And we find the particle sizes of PdRu, PdPt and PdAu haven't changed much after the surface modification, the average size of the Pd, PdRu, PdPt and PdAu nanoparticles is ca. 8.0 nm, 7.8 nm, 8.0 nm, 8.5 nm, respectively. With the HR-TEM images of each catalyst, we can apply the fast fourier transform and calculate the d-spacing values. It's clear that the predominantly exposed facet of Pd/C, PdPt/ C and PdAu/C is complex, mainly exposed (200) facet and with small proportions of (111) and (220) facets as well. The complex exposed facet also indicates the roughness and unevenness of the particle surface. The Au surface modification results in some angular morphology and enlarged particles size of PdAu nanoparticles. As to PdRu/C (Fig. 1D), however, it seems that the addition of Ru changes the original complex facets into uniform (111) facets. We think this phenomenon may result from the different space groups of Pd (Fm-3m) and Ru (P63/mmc). Fig. 2 shows the XRD patterns of Pd/C, PdRu/C, PdPt/C and PdAu/ C catalysts. All the catalysts show similar broad peaks located around 25 , corresponding to the (002) plane of the carbon support Vulcan XC-72R. The Pd/C catalyst shows four peaks located around 40 , 47, 68 and 82 , which can be ascribed to the (111), (200), (220) and (311) planes of the face-centered cubic (fcc) structure of Pd. PdRu/C, PdPt/C and PdAu/C catalysts show similar fcc structures. The atomic proportions of decorated noble metal of PdRu, PdPt and PdAu are approximate, about 10% of the total metal (ranged from 9 to 12%), in this regard the characteristic of the nanoparticles will be Pd dominant, with a little distortion of Pd lattice due to the introduction of Ru, Pt or Au. The doping of larger radius Pt or Au makes the PdPt/C and PdAu/C shift to lower angle integrally compared to Pd/C, which is in accord with the Vegard's Law. As to PdRu/C, it seems that the doping of Ru makes negligible changes to the peak location, but only a slight asymmetry of the peaks which may reflect the Ru decoration. X-ray photoelectron spectroscopy (XPS) is a powerful tool for revealing the electron structures and valence states, Fig. 3 shows the typical XPS spectra of the catalysts. The different chemical states of an element can be analyzed by deconvolution of the original spectra. On the fitting curves, the Pd 3d signal of each catalyst consists of two doublets, which can be assigned to Pd(0) and Pd(II), respectively. In Fig. 3AeD and F, the binding energy for Pd0 3d5/2 of Pd/C, PdRu/C, PdPt/C and PdAu/C locate at 335.3 eV, 335.2 eV, 335.7 eV and 335.6 eV, respectively. Compared to the standard Pd0 3d5/2 located at 335.0 eV, it indicates all the asprepared catalysts shift to higher Pd core levels. In general, a positive core level shift of Pd can be interpreted as an electron loss of the Pd atoms, which will lead to a lower d-band center of Pd. A suitable downshift of Pd d-band center weakens the adsorption of surface oxygenated intermediates, and thus enhance the surface catalytic activity [22e24]. Besides, obviously shifted core levels of Ru, Pt and Au are also observed. The shifts of Ru, Pt and Au are approximate 0.5, 0.6 and 0.4 eV, respectively. The results indicate the existence of electron interaction between Pd and the modified noble metal. The existences of noble metals modify the electron structure of Pd, and may enhance the stability and activity of the catalysts. The electrochemical characterizations of Pd/C, PdRu/C, PdPt/C and PdAu/C catalysts were investigated. From the cathodic peaks around 0 V for the reduction of PdeO(H) species (Fig. 4A), it can be seen that the introduction of different noble metal leads to different peak position of oxide species' reduction. The calculated electrochemical surface active area (ESA) for Pd/C, PdRu/C, PdPt/C and
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Fig. 3. Core level XPS spectra for (A) Pd/C, (B)&(C) PdRu/C, (D)&(E) PdPt/C, (F) &(G) PdAu/C.
PdAu/C are 65, 60, 62.8 and 67.8 m2 g1. The area deviation of the other three is within 10%, which is much smaller than the deviation of the current density valued as mA mg1 Pd (Fig. 4B). In this account we think the difference toward activity is not just brought by the electrochemical active surface area, but also the interaction effect brought by the surface noble metal modification. The surface modification of noble metal has two effects on the ESA. One is
diminishing the ESA due to the surface Pd replacement by noble metal, the other is the enlarging the ESA due to the electron interaction. So we can find the ESA of PdPt/C and PdRu/C diminish and that of PdAu/C enlarges. The catalytic activities of the catalysts toward ethanol oxidation were measured by cyclic voltammetry (CV) in a solution of 0.5 M KOH and 0.5 M C2H5OH. The catalytic activity was evaluated as Pd
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Fig. 4. (A) CV curves in 0.5 M H2SO4 solution at a scan rate of 50 mV s1. (B) Ethanol electrooxidation curves in a solution of 0.5 M C2H5OH and 0.5 M KOH at a scan rate of 50 mV s1. (C) The comparison of current densities at lower potential. (D) Chronoamperometry curves polarizing at 0.15 V. (E) Chronopotentiometry curves with an anodic current of half of the peak current. (F) The long-term potential cycles in a solution of 0.5 M KOH and 0.5 M ethanol at 50 mV s1.
mass specific current. It should be mentioned that modified noble metals are not calculated in the current density, as neither Ru, Pt or Au shows catalytic activity in the potential range of our experiments. Fig. 4B shows the CV curves for Pd/C, PdRu/C, PdPt/C and PdAu/C. The forward anodic peak current density for prepared Pd/C,
PdRu/C, PdPt/C and PdAu/C is 707, 750, 1050 and 1700 mA mg1 Pd, respectively. PdAu/C shows the highest forward anodic peak current density, while the addition of Ru improves a little of the catalytic activity of PdRu/C even considering the effect of ESA, not as distinct as Pt and Au. Besides, we also compare the onset potential
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Fig. 5. TEM images of catalysts after long-term potential cycles in 0.5 M KOH þ EtOH. (A)&(B) Pd/C, (C)&(D) PdRu/C, (E) &(F) PdPt/C, (G) &(H) PdAu/C.
of these catalysts, the onset potential for Pd/C is 0.43 V, and PdRu/ C, PdPt/C and PdAu/C all show a down shift of onset oxidation potential by 10e20 mV. Lower onset potential means easier oxidation of ethanol in the same condition. Moreover, we also compare the current of the catalysts at lower potential (Fig. 4C). As it's practically difficult to operate the whole fuel cell at the peak potential after assembly, the current density at lower potential has an important practical meaning. In Fig. 4C we can see that the current densities at 0.3 V for Pd/C, PdRu/C, PdPt/C and PdAu/C are 270, 340, 350 and 550 mA mg1 Pd, respectively. It's clear that the noble metal surface modification further enhances the activity at lower potential, the PdAu/C still shows the highest current density of all, indicating the surface modified PdAu/C is even competitive in practical application.
The results of chronoamperometric and chronopotentiometric tests indicate that the surface modification by noble metal not only enhances the catalytic activity, but also improves the durability and stability of the catalysts. In Fig. 4D, PdAu/C shows the lowest degree of decay and highest current density at 1000 s in the chronoamperometry, PdPt/C takes the second place and Pd/C shows the worst. Chronopotentiometry is applied to study the poison resistance of the catalysts for ethanol oxidation, we set a constant current which is half of the peak current of each catalyst. Ethanol will be oxidized in the plateaus stage, due to catalyst poisoning, however, the potential must rise to achieve the pre-set current during this stage. When the catalyst are mostly poisoned, ethanol can no longer be oxidized, so the potential will suddenly rise to keep the pre-set current, in this stage the catalyst may be destroyed due to
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the high potential [3]. In this regard, the lower potential polarization and longer time before the potential jump are indicative of better anti-poisoning properties In the anti-poisoning test (Fig. 4E), the potential polarizations increase as PdAu/C < PdPt/C < PdRu/ C < Pd/C, indicating the same tendency of catalyst poisoning. With respect to the time for the potential jump, PdAu/C has the overwhelming superiority compared to the other catalysts. This result clearly suggests that practically less poisoning species can be formed and adsorbed on the PdAu/C surface in alkaline medium during ethanol oxidation process. The multiple cycles of the prepared catalysts in the same ethanol solution give a more intuitive impression (Fig. 4F). On the Pd/C electrode the current maintains 33% after 2000 cycles, while on the PdRu/C and PdPt/C electrodes, the catalytic activity maintains about 40% after 2000 cycles, indicating the improvement of the catalysts' duration after the surface modification. On PdAu/C electrode it shows the best stability and the duration, the catalytic activity maintains 40% after 4500 potential cycles, about twice of the other three. To observe the catalysts after the long-term test, we took the TEM images of cycled catalysts (Pd/C, PdRu/C, PdPt/C and PdAu/C). Fig. 5 shows that the degree of agglomeration of PdRu/C, PdPt/C and PdAu/C is less than that of Pd/C, and all the catalysts show clear lattice after cycling, which means the as-prepared catalysts haven't turn into the amorphous state as the catalysts usually will be. From Fig. 5 we can find that the complex exposed facet of Pd/C, PdPt/C and PdAu/C were worn and transformed to the preferential (111) facet after long-term electro-catalysis. We tend to believe that such complex facet exposure will favor the ethanol oxidation, thus we compare the electrochemical performance of commercial BASF Pd/ C and the as-prepared Pd/C. Although the as-prepared Pd/C has larger particle size, which means the fewer ratio of surface Pd sites than the BASF Pd/C, the as-prepared Pd/C still shows obvious better activity and stability than the BASF Pd/C. Considering the two catalysts are only different in facet exposure and particle size, and the particle size is unfavorable for as-prepared Pd/C, we believe the better performance of the as-prepared Pd/C comes from the complex exposed facets like the more active (200) and (220) (Supporting Information, page 1e3, Fig. S1). However, the exact reason about the complex facet still remain unknown. Taking all the electrochemical results into consideration, the surface modification of noble metal can both enhance the catalytic activity and the stability, and Au is the best modified noble metal and Pt is the second best. 4. Conclusions In this article, we prepared the surface M (M ¼ Ru, Pt, Au) modified PdM/C catalysts in view of optimizing the Pd electron structure and enhancing the catalytic activity and stability. XPS results confirm the synergetic effect between Pd and M, and indicate a down shift of d-band center of Pd which is positive for ethanol oxidation. As to the decorated noble metals, Au is the most effective additive element, the PdAu/C catalyst demonstrates best catalytic activity. The forward peak current reaches 1700 mA mg1 Pd, about 2.5 times that of Pd/C. Moreover, PdAu/C
can maintain 40% of the catalytic activity after 4500 potential cycles. PdPt/C is the second best catalyst with a peak current of 1050 mA mg1 Pd. TEM images before and after electrochemical circulations show very little agglomeration of the prepared four catalysts occur after cycling, indicating the good durability of the catalysts after surface modification. In conclusion, the effect of modification with noble metals has been researched and the Au turns out to be the best additive which can greatly enhance the catalytic activity and stability towards DEAFCs. Acknowledgments The authors acknowledge funding supports from the National Key Basic Research Program of China (973 Program, 2015CB932303) and Science & Technology Commission of Shanghai Municipality (No. 08DZ2270500), China. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.03.200. References [1] R. Yue, H. Wang, D. Bin, J. Xu, Y. Du, W. Lu, J. Guo, J. Mater. Chem. A 3 (2015) 1077e1088. [2] X. Yu, A. Manthiram, Appl. Catal. B Environ. 165 (2015) 63e67. [3] W. Wang, Y. Yang, Y. Liu, Z. Zhang, W. Dong, Z. Lei, J. Power Sources 273 (2015) 631e637. ~ artu, C. Mascayano, C. Gutie rrez, Electrochim. Acta 165 (2015) [4] M.S. Ureta-Zan 232e238. [5] S. Abdullah, S.K. Kamarudin, U.A. Hasran, M.S. Masdar, W.R.W. Daud, J. Power Sources 262 (2014) 401e406. [6] L. Ma, D. Chu, R. Chen, Int. J. Hydrogen Energy 37 (2012) 11185e11194. [7] C.W. Xu, L.Q. Cheng, P.K. Shen, Y.L. Liu, Electrochem. Comm. 9 (2007) 997e1001. [8] V. Bambagioni, C. Bianchini, A. Marchionni, J. Filippi, F. Vizza, J. Teddy, P. Serp, M. Zhiani, J. Power Sources 190 (2009) 241e251. [9] H. Pramanik, S. Basu, Chem. Eng. Process 49 (2010) 635e642. [10] S.P.S. Badwal, S. Giddey, A. Kulkarni, J. Goel, S. Basu, Appl. Energy 145 (2015) 80e103. [11] Z. Yin, L. Lin, D. Ma, Catal. Sci. Technol. 4 (2014) 4116e4128. € nbeck, C. Liu, W. Xing, Chem[12] G. Li, L. Feng, J. Chang, B. Wickman, H. Gro SusChem 7 (2014) 3374e3381. [13] S. Shen, T. Zhao, J. Mater. Chem. A 1 (2013) 906e912. [14] T. Sheng, W.F. Lin, C. Hardacre, P. Hu, Phys. Chem. Chem. Phys. 16 (2014) 13248e13254. ger, F. Kadırgan, Appl. Catal. B Environ. 144 (2014) 66e74. [15] S. Beyhan, J.M. Le [16] H. Mao, T. Huang, A.S. Yu, J. Mater. Chem. A 2 (2014) 16378e16380. [17] T. Ramulifho, K.I. Ozoemena, R.M. Modibedi, C.J. Jafta, M.K. Mathe, Electrochim. Acta 59 (2012) 310e320. [18] X.L. Sun, D.G. Li, Y. Ding, W.L. Zhu, S.J. Guo, Z.L. Wang, S.H. Sun, J. Am. Chem. Soc. 136 (2014) 5745e5749. [19] S.S. Li, J.J. Lv, Y.Y. Hu, J.N. Zheng, J.R. Chen, A.J. Wang, J.J. Feng, J. Power Sources 247 (2014) 213e218. [20] J.F. Chang, L.G. Feng, C.P. Liu, W. Xing, X.L. Hu, Angew. Chem. Int. Ed. 53 (2014) 122e126. [21] L.Y. Chen, N. Chen, Y. Hou, Z.C. Wang, S.H. Lv, T. Fujita, J.H. Jiang, A. Hirata, M.W. Chen, ACS Catal. 3 (2013) 1220e1230. [22] H.L. Jiang, Q. Xu, J. Mater. Chem. 21 (2011) 13705e13725. [23] J. Zhao, K. Jarvis, P. Ferreira, A. Manthiram, J. Power Sources 196 (2011) 4515e4523. [24] K. Jiang, W.B. Cai, Appl. Catal. B Environ. 147 (2014) 185e192.