- Email: [email protected]
Surfactant-free Pd–Fe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation Anni Feng, Jie Bai, Wenyao Shao, Wenjing Hong, Zhong-qun Tian**, Zongyuan Xiao* State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China
article info
abstract
Article history:
Herein, a novel surfactant-free nanocatalyst of PdeFe bimetallic nanoparticles (NPs) sup-
Received 14 March 2017
ported on the reduced graphene oxide (PdeFe/RGO) were synthesized using a two-step
Received in revised form
reduction in aqueous phase. Electrochemical studies demonstrate that the nanocatalyst
24 April 2017
exhibits superior catalytic activity towards the formic acid oxidation with high stability due
Accepted 29 April 2017
to the synergic effect of PdeFe and RGO. The optimized PdeFe/RGO (Pd:Fe ¼ 1:5) nano-
Available online xxx
catalyst possess an specific activity of 2.72 mA cm2 and an mass activity of 1.0 A mg1(Pd), which are significantly higher than those of Pd/RGO and commercial Pd/C catalysts.
Keywords:
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Fuel cells Reduced graphene oxide Palladium Bimetallic PdeFe NPs Formic acid oxidation Electrocatalysis
Introduction Direct formic acid fuel cells (DFAFCs) are expected to be one of the most promising power source due to its high energy density, low pollutant emission and low operating temperature [1,2]. To overcome the high cost and the poor poison tolerance of Pt-based catalysts in DFAFCs, Pt alloy, such as PtFe, has been developed to further reduce the usage of Pt and increase the CO poisoning tolerance [3,4]. On the other hand,
Pd-based catalysts are found to be more suitable than Pt for catalyzing the formic acid oxidation reaction owing to its higher abundance, better catalytic performance on small organic molecules and CO poisoning tolerance [5]. Based on similar strategy, a second component especially early transition metal (Cu, Ni, Co, Fe etc.) are incorporated into Pd structures to form alloys, which could maximize the activity and minimize the use of Pd for formic acid oxidation [6,7]. Graphene has been suggested to be an excellent catalytic support due to its large surface area, high electrical
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.-q. Tian), [email protected] (Z. Xiao). http://dx.doi.org/10.1016/j.ijhydene.2017.04.278 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
conductivity, and outstanding mechanical strength [8e10]. It is also found that the graphene scaffold improves the dispersion of nanoparticles and avoids the aggregation, and the residual oxygen functional groups on RGO can promote the CO poisoning tolerance [11]. More importantly, studies based on density function theory (DFT) calculations revealed that the binding of metal particles to the surface of graphene enhances the charge transfer across the metal-graphene interface, and changes the Fermi level of both the metal particles and graphene. This synergic effect leads to the significant improvement of their catalytic activity and stability [12], suggesting that the PdeFe NPs supported on reduced graphene oxide could be a promising electrocatalyst for the formic acid oxidation reaction in DFAFCs. In this communication, we report the preparation of a bimetallic PdeFe NPs with optimal size at around 4.5 nm and narrow distribution on RGO via a two-step aqueous phase reduction. It is highlighted that the formation of bimetallic nanoparticle dispersed well on the surface of RGO without surfactant, which avoid the blocking of the active sites by surfactant. It is found that the as-prepared PdeFe/RGO exhibited better catalytic activity towards the electro-oxidation of formic acid than conventional carbon supported catalysts, and the further optimization of Pd/Fe ratio in these catalyst achieved an optimal specific activity of 2.72 mA cm2 mg1(Pd) and an mass activity of 1.0 A mg1(Pd), which are significantly higher than those of Pd/RGO and commercial Pd/C catalysts.
Experimental PdeFe/RGO nanocatalyst was synthesized as follows: Firstly, 2.68 mL of 0.2 M FeSO4 solution was mixed with 20 mL GO (prepare by a modified Hummers method [13]) suspension (0.5 mg/mL) for 30 min. Then 4.02 mL of 0.8 M NaBH4 solution was gradually added and stirred for 20 min till the hydrogen was released completely, making sure no NaBH4 left in the solution. Finally, 1.41 mL of 40 mM PdCl2 was added by dropwise and stirred for 15 min. The product PdeFe/RGO nanocatalyst (feeding mass ratio, Pd:Fe ¼ 1:5) was washed with ultrapure water several times before freeze-drying. All the electrochemical measurements were performed in a nitrogen-saturated electrolytic solution and a threeelectrode cell including a platinum plate as counter electrode and a saturated calomel electrode (SCE) as the reference electrode was used. The working electrode was prepared as follows: catalyst of 4 mg mass equivalent Pd (active component) was dispersed in the solution composed of 1.87 mL ultrapure water, 0.93 mL isopropanol and 0.2 mL Nafion solution (5 wt%) to form the catalyst ink. Then 8.24 mL of the catalyst ink was casted on the pretreated GCE (5 mm in diameter) surface to get a Pd loading of ~56 mg metal cm2.
Results and discussion In the surfactant free synthesis, the PdeFe/RGO nanocatalyst was prepared by two steps. Firstly, Fe2þ dispersed on the surface of GO was reduced to form Fe/RGO. Secondly, Pd atoms were deposited at the Fe surface spontaneously because of the
much more positive equilibrium electrode potential with 2þ PdCl2 4 /Pd (0.729) V than that of Fe /Fe (0.669 V) [14]. Fig. 1(a) and (c), with the magnification of 60,000 and 49,000, respectively, show the TEM images of PdeFe/RGO (Pd:Fe ¼ 1:5) and Pd/RGO catalysts, respectively. The average particle size of PdeFe and Pd dispersed on the surface of RGO nanosheets is found to be 3.9±0.8 nm and 4.0±0.7 nm, respectively, which are most favorable for formic acid electro-oxidation [15]. And the shape of the nanoparticles are remain stable after electrochemical test as shown in Fig. S1. A very small lattice fringe spacing difference between Pd/RGO (0.224 nm) and PdeFe/RGO (0.223 nm) catalysts (shown as Fig. 1(b) and (d)) was determined by the XRD data of (111) plane, revealing that the Pd nanoclusters are highly crystalline and faceted at the surface of this PdeFe nanoparticles. However, the little lattice contraction of PdeFe nanoparticles is attributed to the dope of Fe atoms with different atom diameter and electro negativity, leading to the stress effect of the crystal structure [16,17]. The actual weight fractions of Pd:Fe determined by ICP-MS in the nanocatalysts before electrochemical test with feeding weight fraction of Pd:Fe ¼ 1:7, 1:6, 1:5, 1:4, 1:3 are 1:6.89, 1:5.74, 1:4.61, 1:3.79, 1:2.52, respectively, indicating the composition in the nanoparticles is controllable by varying the feeding of the metal precursors used in the synthesis. The Raman spectra of GO and PdeFe/RGO in Fig. 2(a) show two prominent peaks observed at 1351 cm1 (D band) and 1593 cm1 (G band). The significant increase of ID/IG for PdeFe/ RGO (1.01) compared with GO (0.88) suggests the enhanced conjugation during the reduction process [18]. The XRD patterns for different catalysts shown in Fig. 2(b) exhibit four characteristic diffraction peaks at 40.18 , 46.74 , 68.28 , 82.22 corresponding to (111), (200), (220) and (311) planes, respectively, which is similar to that of Pd/RGO catalyst synthesized with the same method. For PdeFe/RGO catalysts, the absence of XRD diffraction peaks of metal Fe or its oxides is attributed to the heavy atom effect from Pd, resulting in the enrich of Pd at the surface of these bimetallic nanoparticles [19]. As shown in the inset of Fig. 2(b), an enlarged view of the (111) diffraction peaks of PdeFe/RGO ¼ 40.18 is very close to Pd/RGO ¼ 40.05 , just a subtle shift to larger angle, and is far from the reported diffraction peak of Fe ¼ 44.5 [20]. As we know, If the PdeFe nanoparticles are nanoalloys, there would be a big shift of their diffraction peaks to the larger angle and located between those for Pd and Fe [21]. Therefore, these results demonstrate that PdeFe/RGO catalysts with PdeFe bimetallic nanoparticles of pseudo-core-shell structure were synthesized and Pd was in the outlayer. Moreover, it is also found that PdeFe/RGO (Pd:Fe ¼ 1:5) have the broadest (111) peak, suggesting that catalyst with Pd:Fe ¼ 1:5 is of smallest crystallite size according to Scherrer equation [22], and it became sharper with the little increase of the particle size after the electrochemical stability test (see in Fig. S2). XPS shown in Fig. 2(c) suggests that the graphite was extensively oxidized with large amount of oxygen functional groups [23] which are obviously decreased for PdeFe/RGO catalyst (see in Fig. 2(d)), indicating the formation of RGO [24]. The atomic percentage of boron was calculated to be <0.1% according to the B 1s region of PdeFe/RGO (shown as the insert in Fig. 2(d)). The absence of boron oxides or boron suggests that the boron oxide complexes could be removed along with the
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
3
Fig. 1 e TEM (a, c) images and HR-TEM (b, d) images of PdeFe/RGO (Pd:Fe ¼ 1:5) (a, b) and Pd/RGO (c, d) catalysts.
functional groups of GO in concentrated NaBH4, and the boron acid and residual NaBH4 could be eliminated during washing [25,26]. As shown in Fig. 2(e), there are two pairs of doublets for Pd/RGO: the first obvious pair of doublet fixed at 340.98 eV and 335.72 eV corresponding to Pd 3d3/2 and Pd 3d5/2 of Pd and the second weaker doublet fixed at 342.30 eV and 337.04 eV corresponding to Pd 3d3/2 and Pd 3d5/2 of PdO, respectively. For Pd/ RGO, there is about 22.3% of Pd2þ attributed to the oxidation of the surface Pd upon exposure to ambient atmosphere. Interestingly, the splitting pattern of Pd 3d for PdeFe/RGO (Pd:Fe ¼ 1:5) revealed a shift to higher binding energy with 19.9% of Pd2þ (as shown in Fig. 2(f)). This shift could be attributed to the electron loss from surface Pd atoms, suggesting an enhanced PdePd strength and a decreased bond length which increases the lattice strain and decreases the Pd d-band center, and therefore weakens the adsorption of inhibiting reaction intermediates [27e34], which enhance the electrocatalytic performance for formic acid oxidation [35,36]. Besides, the oxidation of Fe atoms on the surface of the nanoparticle was estimated from the Fe 2p region (shown as the insert in Fig. 2(f)) to be 91.6% of Fe2þ. The increase of Pd0 active sites in PdeFe/ RGO is due to the incorporation of Fe leading to a lower propensity of Pd to surface oxidation [37]. The relative surface composition determined by the XPS analysis of this PdeFe/ RGO (Pd:Fe ¼ 1:5) is found to be Pd:Fe ¼ 1:1.52, significantly higher than the result of Pd:Fe ¼ 1:4.61 determined by ICP-MS, suggesting that Pd enriched on the surface of the NPs, which confirms the hypothesis raised by XRD.
The electrochemical behavior of different catalysts is shown in Fig. 3. The electrochemical active surface area (ECSA) was calculated using the following equation: ECSA ¼ Q/lS, where Q is obtained by integrating the charge associated with PdO reduction region (as shown in Fig. 3(a)), l is the Pd loading on the electrode and S (424 mC cm2) is the palladium's conversion factor [38e46]. Fig. 3(c, d) illustrate the specific activities normalized by ECSA and mass activities normalized by the amount of Pd loaded onto the electrode surface of the formic acid oxidation, respectively. In the forward scan, only one formic acid oxidation peak (0.1e0.2 V) exists for each of the PdeFe/RGO catalysts, suggesting the direct formate pathway [47], and a volcano-like trend is obtained when comparing the current peak with Fe content. The best performance was obtained at the Pd:Fe ¼ 1:5, of which the specific activity (2.72 mA cm2) is much higher than those of reported Pd NPs/C (0.45 mA cm2), Pd NPs/RGO (1.05 mA cm2) [48] and get close to Cu/Pd porous structure (3.0 mA cm2) [49], and is about 1.3 times of the as-prepared Pd/RGO catalyst (2.04 mA cm2) and 2.5 times of the commercial Pd/C catalyst (1.09 mA cm2), respectively. Meanwhile, the optimized mass activity (1.00 A mg1 (Pd)) is superior ), Pd than that of Pd/MWCNT (0.21 A mg1 (Pd) 2Ir1/MWCNT 1 (0.30 A mg1 (Pd)) [7] and Cu/Pd porous structure (0.82 A mg ) [49] reported recently. We also tested the long-term stability of the catalysts (see in Fig. 3(b)). PdeFe/RGO (Pd:Fe ¼ 1:5) exhibited the best stability at 3000 s (0.12 mA cm2), which is about 1.7 times of Pd/RGO (0.07 mA cm2) and 4.0 times of
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
Fig. 2 e (a) Raman spectra of GO and PdeFe/RGO nanocomposites; (b) A comparison of XRD patterns for different catalysts; (b) inset: an enlarged view of the (111) diffraction peaks of different catalysts; XPS spectra of C1s regions for GO (c) and PdeFe/RGO (d); (d) inset: B1s region of PdeFe/RGO; Pd 3d regions for Pd/RGO (e) and PdeFe/RGO (f); (f) inset: Fe2p region of PdeFe/RGO.
commercial Pd/C (0.03 mA cm2), respectively, and is even better than a newly developed PdeP catalyst that its percentage current intensity remained 17% comparing with 7% for PdeP after 1200s [50]. The gradually enhanced electrochemical activity with the increasing of the Fe may be due to the formation of pseudo-core-shell structure, which would result in an increased ratio of surface Pd atoms, leading to a high atom utilization efficiency and changing the microscopic electron structure on the nanoparticle surface with the bimetal synergy effect [35e37], and then the interactions between the surface and the adsorbed small molecules are also changed, making an enhancement of the catalytic performance [51,52]. However, with the further increase in Fe
content within the catalysts, the catalytic activity decreased gradually due to the decreasing of the adsorption strength between the adsorbate and the active site on the catalysts. Moreover, the large surface area of the RGO is highly accessible to the formic acid molecules activated by the adjacent nanoparticles, and the enhanced CO-poisoning tolerance ability would inherit from the residual oxygen groups on RGO [11]. Interestingly, we find that the content of Pd2þ on the surface of the PdeFe/RGO (Pd:Fe ¼ 1:5) was increased after 3000 s stability test (see in Fig. S3), indicating the generation of a PdO oxidation film, which may prevent the dissolution of Fe nanoparticles in the acidic solution, further enhance the stability [53e58].
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
5
Fig. 3 e Cyclic voltammograms of PdeFe/RGO (Pd:Fe ¼ 1:7e1:3), Pd/RGO and commercial Pd/C in N2-saturated 0.1 M H2SO4 solution (a) and 0.5 M HCOOH þ 0.1 M H2SO4 solution (c, d) at a scan rate of 50 mV s¡1; (b)The chronoamperometric curves of 0.1 M H2SO4 þ 0.5 M HCOOH solution on Pd/RGO, PdeFe/RGO (Pd:Fe ¼ 1:7e1:3), and commercial Pd/C catalysts, respectively, at a fixed potential of 0.16 V vs. SCE.
Conclusions
Appendix A. Supplementary data
In summary, PdeFe/RGO nanocomposite was fabricated by a surfactant free two step chemical route. The electrochemical studies showed that the optimized PdeFe/RGO (Pd:Fe ¼ 1:5) catalyst possesses excellent electrocatalytic activity and stability towards the formic acid oxidation. The significant enhancement of the catalytic activity of PdeFe/RGO can be attributed to large surface area of the introduced RGO, which not only improves the dispersion of nanoparticles during the synthesis process and promote the stability of the composites, but also facilitates the diffusion of the formic acid molecules in the electrolyte. Moreover, the doping of Fe could increase the ratio of surface Pd atoms, leading to a high atom utilization efficiency, and modify the microscopic electron structure on the surface of nanoparticles due to the bimetal synergy effect, making the less expensive PdeFe/RGO nanocatalyst a highly promising strategy for the future design and synthesis of electrocatalysts towards the oxidation of formic acid.
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.04.278.
Acknowledgments The authors appreciate the financial support from the National Natural Science Foundation of China (Grant No. 51576168).
references
[1] Bauskar AS, Rice CA. Impact of anode catalyst layer porosity on the performance of a direct formic acid fuel cell. Electrochim Acta 2012;62:36e41. [2] Ohkubo Y, Shibata M, Kageyama S, Seino S, Nakagawa T, Kugai J, et al. Carbon-supported AuPd bimetallic nanoparticles synthesized by high-energy electron beam irradiation for direct formic acid fuel cell. J Mater Sci 2013;48:2142e50. [3] Wang X-M, Xia Y-Y. FexC-C hybrid material as a support for Pt anode catalyst in direct formic acid fuel cell. Electrochem Commun 2009;11:28e30. [4] Chumillas S, Buso-Rogero C, Solla-Gullon J, Vidal-Iglesias FJ, Herrero E, Feliu JM. Size and diffusion effects on the oxidation of formic acid and ethanol on platinum nanoparticles. Electrochem Commun 2011;13:1194e7. [5] Mazumder V, Sun SH. Oleylamine-mediated synthesis of Pd nanoparticles for catalytic formic acid oxidation. J Am Chem Soc 2009;131:4588e9. [6] Xu CX, Liu YQ, Wang JP, Geng HR, Qiu HJ. Nanoporous PdCu alloy for formic acid electro-oxidation. J Power Sources 2012;199:124e31.
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
[7] Hao Y, Shen J, Wang X, Yuan J, Shao Y, Niu L, et al. Facile preparation of PdIr alloy nano-electrocatalysts supported on carbon nanotubes, and their enhanced performance in the electro-oxidation of formic acid. Int J Hydrogen Energy 2016;41:3015e22. [8] Rao CNR, Subrahmanyam KS, Matte H, Abdulhakeem B, Govindaraj A, Das B, et al. A study of the synthetic methods and properties of graphenes. Sci Technol Adv Mater 2010;11:15. 054502. [9] Hong WJ, Xu YX, Lu GW, Li C, Shi GQ. Transparent graphene/ PEDOT-PSS composite films as counter electrodes of dyesensitized solar cells. Electrochem Commun 2008;10:1555e8. [10] Zhang X, Zhu J, Tiwary CS, Ma Z, Huang H, Zhang J, et al. Palladium nanoparticles supported on nitrogen and sulfur dual-doped graphene as highly active electrocatalysts for formic acid and methanol oxidation. ACS Appl Mater Inter 2016;8:10858e65. [11] Sharma S, Ganguly A, Papakonstantinou P, Miao XP, Li MX, Hutchison JL, et al. Rapid microwave synthesis of CO tolerant reduced graphene oxide-supported platinum electrocatalysts for oxidation of methanol. J Phys Chem C 2010;114:19459e66. [12] Li YJ, Li YJ, Zhu EB, McLouth T, Chiu CY, Huang XQ, et al. Stablilization of high-performance oxygen reduction reaction Pt electrocatalyst supported on reduced graphene oxide/carbon black composite. J Am Chem Soc 2012;134:12326e9. [13] Lerf A, He HY, Forster M, Klinowski J. Structure of graphite oxide revisited. J Phys Chem B 1998;102:4477e82. [14] Baranova EA, Miles N, Mercier PHJ, Page YL, Pararachao B. Formic acid electro-oxidation on carbon supported PdxPt1-x (0x1) nanoparticles synthesized via modified polyol method. Electrochim Acta 2010;55:8182e8. [15] Erikson H, Lusi M, Sarapuu A, Tammeveski K, Solla-Gullon J, Feliu JM. Oxygen electroreduction on carbon-supported Pd nanocubes in acid solutions. Electrochim Acta 2016;188:301e8. [16] Wang W, Huang Q, Liu J, Zou Z, Zhao M, Vogel W, et al. Surface and structure characteristics of carbon-supported Pd3Pt1 bimetallic nanoparticles for methanol-tolerant oxygen reduction reaction. J Power Sources 2009;266:156e63. [17] Flores WH, Teixeira SR, Geshev J, da Cunha JBM, Schilling PJ, Traverse A, et al. Structural evolution and magnetic behavior of Co/Ag multilayers submitted to thermal annealing. J Magn Magn Mater 1998;188:17e29. [18] Xu CH, Sun J, Gao L. Direct growth of monodisperse SnO2 nanorods on graphene as high capacity anode materials for lithium ion batteries. J Mater Chem 2012;22:975e9. [19] Teng XW, Black D, Watkins NJ, Gao YL, Yang H. Platinummaghemite core-shell nanoparticles using a sequential synthesis. Nano Lett 2003;3:261e4. [20] Zhang XB, Yan JM, Han S, Shioyama H, Xu Q. Magnetically recyclable Fe@Pt core-shell nanoparticles and their use as electrocatalysts for ammonia borane oxidation: the role of crystallinity of the core. J Am Chem Soc 2009;131:2778e9. [21] Qin YH, Jia YB, Jiang Y, Niu DF, Zhang XS, Zhou XG, et al. Controllable synthesis of carbon nanofiber supported Pd catalyst for formic acid electrooxidation. Int J Hydrogen Energy 2012;37:7373e7. [22] Liu ZL, Hong L, Tham MP, Lim TH, Jiang HX. Nanostructured Pt/C and Pd/C catalysts for direct formic acid fuel cells. J Power Sources 2006;161:831e5. [23] Blyth RIR, Buqa H, Netzer FP, Ramsey MG, Besenhard JO, Golob P, et al. XPS studies of graphite electrode materials for lithium ion batteries. Appl Surf Sci 2000;167:99e106. [24] Dong Q, Zhao Y, Han X, Wang Y, Liu MC, Li Y. Pd/Cu bimetallic nanoparticles supported on graphene nanosheets: facile synthesis and application as novel electrocatalyst for
[25]
[26]
[27]
[28]
[29]
[30]
[31] [32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
ethanol oxidation in alkaline media. Int J Hydrogen Energy 2014;39:14669e79. Shin H-J, Kim KK, Benayad A, Yoon S-M, Park HK, Jung I-S, et al. Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance. Adv Funct Mater 2009;19:1987e92. Okamoto Y, Nitta Y, Imanaka T. Surface characterization of nickel boride and nickel phosphide catalysts by X-ray photoelectron spectroscopy. Faraday Trans 1979;75:2027e39. Xu CX, Liu AH, Qiu HJ, Liu YQ. Nanoporous PdCu alloy with enhanced electrocatalytic performance. Electrochem Commun 2011;13:766e9. Stamenkovic VR, Mun BS, Mayrhofer KJJ, Ross PN, Markovic NM. Effect of surface composition on electronic structure, stability, and electrocatalytic properties of Pttransition metal alloys: Pt-skin versus Pt-skeleton surfaces. J Am Chem Soc 2006;128:8813e9. Strasser P, Koh S, Anniyev Y, Greeley J, More K, Yu C, et al. Lattice-strain control of the activity in dealloyed core-shell fuel cell catalysts. Nat Chem 2010;2:454e60. Chen M, Kumar D, Yi CW, Goodman DW. The promotional effect of gold in catalysis by palladium-gold. Science 2005;310:291e3. Mavrikakis M, Hammer B, Norskov JK. Effect of strain on the reactivity of metal surfaces. Phys Rev Lett 1998;81:2819e22. Stamenkovic VR, Fowler B, Mun BS, Wang G, Ross PN, Lucas CA, et al. Improved oxygen reduction activity on Pt3Ni(111) via increased surface site availability. Science 2007;315:493e7. Kibler LA, El-Aziz AM, Hoyer R, Kolb DM. Tuning reaction rates by lateral strain in a palladium monolayer. Angew Chem Int Ed 2005;44:2080e4. Zhang J, Vukmiovic MB, Xu Y, Mavrikakis M, Adzic RR. Controlling the catalytic activity of platinum-monolayer electrocatalysts for oxygen reduction with different substrates. Angew Chem Int Ed 2005;44:2132e5. Hu S, Scudiero L, Ha S. Electronic effect on oxidation of formic acid on supported Pd-Cu bimetallic surface. Electrochim Acta 2012;83:354e8. Ren M, Zhou Y, Tao F, Zou Z, Akins DL, Yang H. Controllable modification of the electronic structure of carbon supported core-shell Cu@Pd catalysts for formic acid oxidation. J Phys Chem C 2014;118:12669e75. Liao MY, Hu Q, Zheng JB, Li YH, Zhou H, Zhong CJ, et al. Pd decorated Fe/C nanocatalyst for formic acid electrooxidation. Electrochim Acta 2013;111:504e9. Erikson H, Sarapuu A, Alexeyeva N, Tammeveski K, Solla n J, Feliu JM. Electrochemical reduction of oxygen on Gullo palladium nanocubes in acid and alkaline solutions. Electrochim Acta 2012;53:329e35. n J, Erikson H, Sarapuu A, Tammeveski K, Solla-Gullo Feliu JM. Enhanced electrocatalytic activity of cubic Pd nanoparticles towards the oxygen reduction reaction in acid media. Electrochem Commun 2011;13:734e7. Santasalo-Aarnio A, Kwon Y, Ahlberg E, Kontturi K, Kallio T, Kopper MTM. Comparison of methanol, ethanol and iso-propanol oxidation on Pt and Pd electrodes in alkaline media studied by HPLC. Electrochem Commun 2011;13:466e9. M, Łukaszewskia M, Jerkiewic G, Czerwin ski A. Grden Electrochemical behavior of palladium electrode: oxidation, electrodissolution and ionic adsorption. Electrochim Acta 2008;53:7583e98. Eriksona H, Kasikovb A, Johansc C, Kontturic K, Tammeveskia K, Sarapuua A. Oxygen reduction on Nafioncoated thin-film palladium electrodes. J Electroanal Chem 2011;652:1e7.
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
[43] Pattabiraman R. Electrochemical investigations on carbon supported palladium catalysts. Appl Catal A Gen 1997;153:9e20. [44] Trasatti S, Petrii OA. Real surface area measurements in electrochemistry. Pure Appl Chem 1991;63:711e34. [45] Woods R. Chemisorption at electrodes. Hydrogen and oxygen on noble metals and their alloys. In: Bard AJ, editor. Electroanalytical chemistry, a series of advances, vol. 9. New York: Basel: Marcel Dekker; 1976. p. 119e25. [46] Zhang LY, Zhao ZL, Li CM. Formic acid-reduced ultrasmall Pd nanocrystal on graphene to provide superior electrocatalytic activity and stability toward formic acid oxidation. Nano Energy 2015;11:71e7. [47] Zhang L, Xie Z, Gong J. Shape-controlled synthesis of Au-Pd bimetallic nanocrystals for catalytic applications. Chem Soc Rev 2016;45:3916e34. [48] Jin T, Guo SJ, Zou J-L, Sun SH. Synthesis and assembly of Pd nanoparticles on graphene for enhanced electrooxidation of formic acid. Nanoscale 2013;5:160e3. [49] Ojani R, Abkar Z, Hasheminejad E, Raoof J-B. Rapid fabrication of Cu/Pd nano/micro-particles porous-structured catalyst using hydrogen bubbles dynamic template and their enhanced catalytic performance for formic acid electrooxidation. Int J Hydrogen Energy 2014;39:7788e97. [50] Poon KC, Khezri B, Li Y, Webster RD, Su H, Sato H. A highly active Pd-P nanoparticle electrocatalyst for enhanced formic acid oxidation synthesized via stepwise electroless deposition. Chem Commun 2016;52:3556e9.
7
[51] Yu S, Zhang L, Zhao ZJ, Gong J. Structural evolution of concave trimetallic nanocubes with tunable ultra-thin shells for oxygen reduction reaction. Nanoscale 2016;8:16640e9. [52] Zhang L, Yu S, Zhang J, Gong J. Porous single-crystalline AuPt@Pt bimetallic nanocrystals with high mass electrocatalytic activities. Chem Sci 2016;7:3500e5. [53] Appleby AJ. Oxygen reduction and corrosion kinetics on phase-oxide-free palladium and silver electrodes as a function of temperature in 85% orthophosphoric acid. J Electrochem Soc 1970;117:1373e8. [54] Perdomo F, Avaca LLA, Aegerter MA, De Lima-Neto P. Oxygen-free deposition of ZrO2 sol-gel films on mild steel for corrosion protection in acid medium. J Mater Sci Lett 1998;17:295e8. [55] Tiwari SK, Tripathi M, Singh R. Electrochemical behavior of zirconia based coatings on mild steel prepared by sol-gel method. Corros Sci 2012;63:334e41. [56] Zakorchemna I, Carmona N, Zakroczymski T. Hydrogen permeation through sol-gel-coated iron during galvanostatic charging. Electrochim Acta 2008;53:8154e60. [57] Harkonen E, Diaz B, Swiatowska J, Maurice V, Seyeux A, Vehkamaki M, et al. Corrosion protection of steel with oxide nanolaminates grown by atomic layer deposition. J Electrochem Soc 2011;158:C369e78. [58] Tiwari SK, Sahu RK, Pramanick AK, Singh R. Development of conversion coating on mild steel prior to sol gel nanostructured Al2O3 coating for enhancement of corrosion resistance. Sur Coat Tech 2011;205:4960e7.
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation Anni Feng, Jie Bai, Wenyao Shao, Wenjing Hong, Zhong-qun Tian**, Zongyuan Xiao* State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China
article info
abstract
Article history:
Herein, a novel surfactant-free nanocatalyst of PdeFe bimetallic nanoparticles (NPs) sup-
Received 14 March 2017
ported on the reduced graphene oxide (PdeFe/RGO) were synthesized using a two-step
Received in revised form
reduction in aqueous phase. Electrochemical studies demonstrate that the nanocatalyst
24 April 2017
exhibits superior catalytic activity towards the formic acid oxidation with high stability due
Accepted 29 April 2017
to the synergic effect of PdeFe and RGO. The optimized PdeFe/RGO (Pd:Fe ¼ 1:5) nano-
Available online xxx
catalyst possess an specific activity of 2.72 mA cm2 and an mass activity of 1.0 A mg1(Pd), which are significantly higher than those of Pd/RGO and commercial Pd/C catalysts.
Keywords:
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Fuel cells Reduced graphene oxide Palladium Bimetallic PdeFe NPs Formic acid oxidation Electrocatalysis
Introduction Direct formic acid fuel cells (DFAFCs) are expected to be one of the most promising power source due to its high energy density, low pollutant emission and low operating temperature [1,2]. To overcome the high cost and the poor poison tolerance of Pt-based catalysts in DFAFCs, Pt alloy, such as PtFe, has been developed to further reduce the usage of Pt and increase the CO poisoning tolerance [3,4]. On the other hand,
Pd-based catalysts are found to be more suitable than Pt for catalyzing the formic acid oxidation reaction owing to its higher abundance, better catalytic performance on small organic molecules and CO poisoning tolerance [5]. Based on similar strategy, a second component especially early transition metal (Cu, Ni, Co, Fe etc.) are incorporated into Pd structures to form alloys, which could maximize the activity and minimize the use of Pd for formic acid oxidation [6,7]. Graphene has been suggested to be an excellent catalytic support due to its large surface area, high electrical
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Z.-q. Tian), [email protected] (Z. Xiao). http://dx.doi.org/10.1016/j.ijhydene.2017.04.278 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
2
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
conductivity, and outstanding mechanical strength [8e10]. It is also found that the graphene scaffold improves the dispersion of nanoparticles and avoids the aggregation, and the residual oxygen functional groups on RGO can promote the CO poisoning tolerance [11]. More importantly, studies based on density function theory (DFT) calculations revealed that the binding of metal particles to the surface of graphene enhances the charge transfer across the metal-graphene interface, and changes the Fermi level of both the metal particles and graphene. This synergic effect leads to the significant improvement of their catalytic activity and stability [12], suggesting that the PdeFe NPs supported on reduced graphene oxide could be a promising electrocatalyst for the formic acid oxidation reaction in DFAFCs. In this communication, we report the preparation of a bimetallic PdeFe NPs with optimal size at around 4.5 nm and narrow distribution on RGO via a two-step aqueous phase reduction. It is highlighted that the formation of bimetallic nanoparticle dispersed well on the surface of RGO without surfactant, which avoid the blocking of the active sites by surfactant. It is found that the as-prepared PdeFe/RGO exhibited better catalytic activity towards the electro-oxidation of formic acid than conventional carbon supported catalysts, and the further optimization of Pd/Fe ratio in these catalyst achieved an optimal specific activity of 2.72 mA cm2 mg1(Pd) and an mass activity of 1.0 A mg1(Pd), which are significantly higher than those of Pd/RGO and commercial Pd/C catalysts.
Experimental PdeFe/RGO nanocatalyst was synthesized as follows: Firstly, 2.68 mL of 0.2 M FeSO4 solution was mixed with 20 mL GO (prepare by a modified Hummers method [13]) suspension (0.5 mg/mL) for 30 min. Then 4.02 mL of 0.8 M NaBH4 solution was gradually added and stirred for 20 min till the hydrogen was released completely, making sure no NaBH4 left in the solution. Finally, 1.41 mL of 40 mM PdCl2 was added by dropwise and stirred for 15 min. The product PdeFe/RGO nanocatalyst (feeding mass ratio, Pd:Fe ¼ 1:5) was washed with ultrapure water several times before freeze-drying. All the electrochemical measurements were performed in a nitrogen-saturated electrolytic solution and a threeelectrode cell including a platinum plate as counter electrode and a saturated calomel electrode (SCE) as the reference electrode was used. The working electrode was prepared as follows: catalyst of 4 mg mass equivalent Pd (active component) was dispersed in the solution composed of 1.87 mL ultrapure water, 0.93 mL isopropanol and 0.2 mL Nafion solution (5 wt%) to form the catalyst ink. Then 8.24 mL of the catalyst ink was casted on the pretreated GCE (5 mm in diameter) surface to get a Pd loading of ~56 mg metal cm2.
Results and discussion In the surfactant free synthesis, the PdeFe/RGO nanocatalyst was prepared by two steps. Firstly, Fe2þ dispersed on the surface of GO was reduced to form Fe/RGO. Secondly, Pd atoms were deposited at the Fe surface spontaneously because of the
much more positive equilibrium electrode potential with 2þ PdCl2 4 /Pd (0.729) V than that of Fe /Fe (0.669 V) [14]. Fig. 1(a) and (c), with the magnification of 60,000 and 49,000, respectively, show the TEM images of PdeFe/RGO (Pd:Fe ¼ 1:5) and Pd/RGO catalysts, respectively. The average particle size of PdeFe and Pd dispersed on the surface of RGO nanosheets is found to be 3.9±0.8 nm and 4.0±0.7 nm, respectively, which are most favorable for formic acid electro-oxidation [15]. And the shape of the nanoparticles are remain stable after electrochemical test as shown in Fig. S1. A very small lattice fringe spacing difference between Pd/RGO (0.224 nm) and PdeFe/RGO (0.223 nm) catalysts (shown as Fig. 1(b) and (d)) was determined by the XRD data of (111) plane, revealing that the Pd nanoclusters are highly crystalline and faceted at the surface of this PdeFe nanoparticles. However, the little lattice contraction of PdeFe nanoparticles is attributed to the dope of Fe atoms with different atom diameter and electro negativity, leading to the stress effect of the crystal structure [16,17]. The actual weight fractions of Pd:Fe determined by ICP-MS in the nanocatalysts before electrochemical test with feeding weight fraction of Pd:Fe ¼ 1:7, 1:6, 1:5, 1:4, 1:3 are 1:6.89, 1:5.74, 1:4.61, 1:3.79, 1:2.52, respectively, indicating the composition in the nanoparticles is controllable by varying the feeding of the metal precursors used in the synthesis. The Raman spectra of GO and PdeFe/RGO in Fig. 2(a) show two prominent peaks observed at 1351 cm1 (D band) and 1593 cm1 (G band). The significant increase of ID/IG for PdeFe/ RGO (1.01) compared with GO (0.88) suggests the enhanced conjugation during the reduction process [18]. The XRD patterns for different catalysts shown in Fig. 2(b) exhibit four characteristic diffraction peaks at 40.18 , 46.74 , 68.28 , 82.22 corresponding to (111), (200), (220) and (311) planes, respectively, which is similar to that of Pd/RGO catalyst synthesized with the same method. For PdeFe/RGO catalysts, the absence of XRD diffraction peaks of metal Fe or its oxides is attributed to the heavy atom effect from Pd, resulting in the enrich of Pd at the surface of these bimetallic nanoparticles [19]. As shown in the inset of Fig. 2(b), an enlarged view of the (111) diffraction peaks of PdeFe/RGO ¼ 40.18 is very close to Pd/RGO ¼ 40.05 , just a subtle shift to larger angle, and is far from the reported diffraction peak of Fe ¼ 44.5 [20]. As we know, If the PdeFe nanoparticles are nanoalloys, there would be a big shift of their diffraction peaks to the larger angle and located between those for Pd and Fe [21]. Therefore, these results demonstrate that PdeFe/RGO catalysts with PdeFe bimetallic nanoparticles of pseudo-core-shell structure were synthesized and Pd was in the outlayer. Moreover, it is also found that PdeFe/RGO (Pd:Fe ¼ 1:5) have the broadest (111) peak, suggesting that catalyst with Pd:Fe ¼ 1:5 is of smallest crystallite size according to Scherrer equation [22], and it became sharper with the little increase of the particle size after the electrochemical stability test (see in Fig. S2). XPS shown in Fig. 2(c) suggests that the graphite was extensively oxidized with large amount of oxygen functional groups [23] which are obviously decreased for PdeFe/RGO catalyst (see in Fig. 2(d)), indicating the formation of RGO [24]. The atomic percentage of boron was calculated to be <0.1% according to the B 1s region of PdeFe/RGO (shown as the insert in Fig. 2(d)). The absence of boron oxides or boron suggests that the boron oxide complexes could be removed along with the
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
3
Fig. 1 e TEM (a, c) images and HR-TEM (b, d) images of PdeFe/RGO (Pd:Fe ¼ 1:5) (a, b) and Pd/RGO (c, d) catalysts.
functional groups of GO in concentrated NaBH4, and the boron acid and residual NaBH4 could be eliminated during washing [25,26]. As shown in Fig. 2(e), there are two pairs of doublets for Pd/RGO: the first obvious pair of doublet fixed at 340.98 eV and 335.72 eV corresponding to Pd 3d3/2 and Pd 3d5/2 of Pd and the second weaker doublet fixed at 342.30 eV and 337.04 eV corresponding to Pd 3d3/2 and Pd 3d5/2 of PdO, respectively. For Pd/ RGO, there is about 22.3% of Pd2þ attributed to the oxidation of the surface Pd upon exposure to ambient atmosphere. Interestingly, the splitting pattern of Pd 3d for PdeFe/RGO (Pd:Fe ¼ 1:5) revealed a shift to higher binding energy with 19.9% of Pd2þ (as shown in Fig. 2(f)). This shift could be attributed to the electron loss from surface Pd atoms, suggesting an enhanced PdePd strength and a decreased bond length which increases the lattice strain and decreases the Pd d-band center, and therefore weakens the adsorption of inhibiting reaction intermediates [27e34], which enhance the electrocatalytic performance for formic acid oxidation [35,36]. Besides, the oxidation of Fe atoms on the surface of the nanoparticle was estimated from the Fe 2p region (shown as the insert in Fig. 2(f)) to be 91.6% of Fe2þ. The increase of Pd0 active sites in PdeFe/ RGO is due to the incorporation of Fe leading to a lower propensity of Pd to surface oxidation [37]. The relative surface composition determined by the XPS analysis of this PdeFe/ RGO (Pd:Fe ¼ 1:5) is found to be Pd:Fe ¼ 1:1.52, significantly higher than the result of Pd:Fe ¼ 1:4.61 determined by ICP-MS, suggesting that Pd enriched on the surface of the NPs, which confirms the hypothesis raised by XRD.
The electrochemical behavior of different catalysts is shown in Fig. 3. The electrochemical active surface area (ECSA) was calculated using the following equation: ECSA ¼ Q/lS, where Q is obtained by integrating the charge associated with PdO reduction region (as shown in Fig. 3(a)), l is the Pd loading on the electrode and S (424 mC cm2) is the palladium's conversion factor [38e46]. Fig. 3(c, d) illustrate the specific activities normalized by ECSA and mass activities normalized by the amount of Pd loaded onto the electrode surface of the formic acid oxidation, respectively. In the forward scan, only one formic acid oxidation peak (0.1e0.2 V) exists for each of the PdeFe/RGO catalysts, suggesting the direct formate pathway [47], and a volcano-like trend is obtained when comparing the current peak with Fe content. The best performance was obtained at the Pd:Fe ¼ 1:5, of which the specific activity (2.72 mA cm2) is much higher than those of reported Pd NPs/C (0.45 mA cm2), Pd NPs/RGO (1.05 mA cm2) [48] and get close to Cu/Pd porous structure (3.0 mA cm2) [49], and is about 1.3 times of the as-prepared Pd/RGO catalyst (2.04 mA cm2) and 2.5 times of the commercial Pd/C catalyst (1.09 mA cm2), respectively. Meanwhile, the optimized mass activity (1.00 A mg1 (Pd)) is superior ), Pd than that of Pd/MWCNT (0.21 A mg1 (Pd) 2Ir1/MWCNT 1 (0.30 A mg1 (Pd)) [7] and Cu/Pd porous structure (0.82 A mg ) [49] reported recently. We also tested the long-term stability of the catalysts (see in Fig. 3(b)). PdeFe/RGO (Pd:Fe ¼ 1:5) exhibited the best stability at 3000 s (0.12 mA cm2), which is about 1.7 times of Pd/RGO (0.07 mA cm2) and 4.0 times of
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
4
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
Fig. 2 e (a) Raman spectra of GO and PdeFe/RGO nanocomposites; (b) A comparison of XRD patterns for different catalysts; (b) inset: an enlarged view of the (111) diffraction peaks of different catalysts; XPS spectra of C1s regions for GO (c) and PdeFe/RGO (d); (d) inset: B1s region of PdeFe/RGO; Pd 3d regions for Pd/RGO (e) and PdeFe/RGO (f); (f) inset: Fe2p region of PdeFe/RGO.
commercial Pd/C (0.03 mA cm2), respectively, and is even better than a newly developed PdeP catalyst that its percentage current intensity remained 17% comparing with 7% for PdeP after 1200s [50]. The gradually enhanced electrochemical activity with the increasing of the Fe may be due to the formation of pseudo-core-shell structure, which would result in an increased ratio of surface Pd atoms, leading to a high atom utilization efficiency and changing the microscopic electron structure on the nanoparticle surface with the bimetal synergy effect [35e37], and then the interactions between the surface and the adsorbed small molecules are also changed, making an enhancement of the catalytic performance [51,52]. However, with the further increase in Fe
content within the catalysts, the catalytic activity decreased gradually due to the decreasing of the adsorption strength between the adsorbate and the active site on the catalysts. Moreover, the large surface area of the RGO is highly accessible to the formic acid molecules activated by the adjacent nanoparticles, and the enhanced CO-poisoning tolerance ability would inherit from the residual oxygen groups on RGO [11]. Interestingly, we find that the content of Pd2þ on the surface of the PdeFe/RGO (Pd:Fe ¼ 1:5) was increased after 3000 s stability test (see in Fig. S3), indicating the generation of a PdO oxidation film, which may prevent the dissolution of Fe nanoparticles in the acidic solution, further enhance the stability [53e58].
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
5
Fig. 3 e Cyclic voltammograms of PdeFe/RGO (Pd:Fe ¼ 1:7e1:3), Pd/RGO and commercial Pd/C in N2-saturated 0.1 M H2SO4 solution (a) and 0.5 M HCOOH þ 0.1 M H2SO4 solution (c, d) at a scan rate of 50 mV s¡1; (b)The chronoamperometric curves of 0.1 M H2SO4 þ 0.5 M HCOOH solution on Pd/RGO, PdeFe/RGO (Pd:Fe ¼ 1:7e1:3), and commercial Pd/C catalysts, respectively, at a fixed potential of 0.16 V vs. SCE.
Conclusions
Appendix A. Supplementary data
In summary, PdeFe/RGO nanocomposite was fabricated by a surfactant free two step chemical route. The electrochemical studies showed that the optimized PdeFe/RGO (Pd:Fe ¼ 1:5) catalyst possesses excellent electrocatalytic activity and stability towards the formic acid oxidation. The significant enhancement of the catalytic activity of PdeFe/RGO can be attributed to large surface area of the introduced RGO, which not only improves the dispersion of nanoparticles during the synthesis process and promote the stability of the composites, but also facilitates the diffusion of the formic acid molecules in the electrolyte. Moreover, the doping of Fe could increase the ratio of surface Pd atoms, leading to a high atom utilization efficiency, and modify the microscopic electron structure on the surface of nanoparticles due to the bimetal synergy effect, making the less expensive PdeFe/RGO nanocatalyst a highly promising strategy for the future design and synthesis of electrocatalysts towards the oxidation of formic acid.
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.04.278.
Acknowledgments The authors appreciate the financial support from the National Natural Science Foundation of China (Grant No. 51576168).
references
[1] Bauskar AS, Rice CA. Impact of anode catalyst layer porosity on the performance of a direct formic acid fuel cell. Electrochim Acta 2012;62:36e41. [2] Ohkubo Y, Shibata M, Kageyama S, Seino S, Nakagawa T, Kugai J, et al. Carbon-supported AuPd bimetallic nanoparticles synthesized by high-energy electron beam irradiation for direct formic acid fuel cell. J Mater Sci 2013;48:2142e50. [3] Wang X-M, Xia Y-Y. FexC-C hybrid material as a support for Pt anode catalyst in direct formic acid fuel cell. Electrochem Commun 2009;11:28e30. [4] Chumillas S, Buso-Rogero C, Solla-Gullon J, Vidal-Iglesias FJ, Herrero E, Feliu JM. Size and diffusion effects on the oxidation of formic acid and ethanol on platinum nanoparticles. Electrochem Commun 2011;13:1194e7. [5] Mazumder V, Sun SH. Oleylamine-mediated synthesis of Pd nanoparticles for catalytic formic acid oxidation. J Am Chem Soc 2009;131:4588e9. [6] Xu CX, Liu YQ, Wang JP, Geng HR, Qiu HJ. Nanoporous PdCu alloy for formic acid electro-oxidation. J Power Sources 2012;199:124e31.
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
6
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
[7] Hao Y, Shen J, Wang X, Yuan J, Shao Y, Niu L, et al. Facile preparation of PdIr alloy nano-electrocatalysts supported on carbon nanotubes, and their enhanced performance in the electro-oxidation of formic acid. Int J Hydrogen Energy 2016;41:3015e22. [8] Rao CNR, Subrahmanyam KS, Matte H, Abdulhakeem B, Govindaraj A, Das B, et al. A study of the synthetic methods and properties of graphenes. Sci Technol Adv Mater 2010;11:15. 054502. [9] Hong WJ, Xu YX, Lu GW, Li C, Shi GQ. Transparent graphene/ PEDOT-PSS composite films as counter electrodes of dyesensitized solar cells. Electrochem Commun 2008;10:1555e8. [10] Zhang X, Zhu J, Tiwary CS, Ma Z, Huang H, Zhang J, et al. Palladium nanoparticles supported on nitrogen and sulfur dual-doped graphene as highly active electrocatalysts for formic acid and methanol oxidation. ACS Appl Mater Inter 2016;8:10858e65. [11] Sharma S, Ganguly A, Papakonstantinou P, Miao XP, Li MX, Hutchison JL, et al. Rapid microwave synthesis of CO tolerant reduced graphene oxide-supported platinum electrocatalysts for oxidation of methanol. J Phys Chem C 2010;114:19459e66. [12] Li YJ, Li YJ, Zhu EB, McLouth T, Chiu CY, Huang XQ, et al. Stablilization of high-performance oxygen reduction reaction Pt electrocatalyst supported on reduced graphene oxide/carbon black composite. J Am Chem Soc 2012;134:12326e9. [13] Lerf A, He HY, Forster M, Klinowski J. Structure of graphite oxide revisited. J Phys Chem B 1998;102:4477e82. [14] Baranova EA, Miles N, Mercier PHJ, Page YL, Pararachao B. Formic acid electro-oxidation on carbon supported PdxPt1-x (0x1) nanoparticles synthesized via modified polyol method. Electrochim Acta 2010;55:8182e8. [15] Erikson H, Lusi M, Sarapuu A, Tammeveski K, Solla-Gullon J, Feliu JM. Oxygen electroreduction on carbon-supported Pd nanocubes in acid solutions. Electrochim Acta 2016;188:301e8. [16] Wang W, Huang Q, Liu J, Zou Z, Zhao M, Vogel W, et al. Surface and structure characteristics of carbon-supported Pd3Pt1 bimetallic nanoparticles for methanol-tolerant oxygen reduction reaction. J Power Sources 2009;266:156e63. [17] Flores WH, Teixeira SR, Geshev J, da Cunha JBM, Schilling PJ, Traverse A, et al. Structural evolution and magnetic behavior of Co/Ag multilayers submitted to thermal annealing. J Magn Magn Mater 1998;188:17e29. [18] Xu CH, Sun J, Gao L. Direct growth of monodisperse SnO2 nanorods on graphene as high capacity anode materials for lithium ion batteries. J Mater Chem 2012;22:975e9. [19] Teng XW, Black D, Watkins NJ, Gao YL, Yang H. Platinummaghemite core-shell nanoparticles using a sequential synthesis. Nano Lett 2003;3:261e4. [20] Zhang XB, Yan JM, Han S, Shioyama H, Xu Q. Magnetically recyclable Fe@Pt core-shell nanoparticles and their use as electrocatalysts for ammonia borane oxidation: the role of crystallinity of the core. J Am Chem Soc 2009;131:2778e9. [21] Qin YH, Jia YB, Jiang Y, Niu DF, Zhang XS, Zhou XG, et al. Controllable synthesis of carbon nanofiber supported Pd catalyst for formic acid electrooxidation. Int J Hydrogen Energy 2012;37:7373e7. [22] Liu ZL, Hong L, Tham MP, Lim TH, Jiang HX. Nanostructured Pt/C and Pd/C catalysts for direct formic acid fuel cells. J Power Sources 2006;161:831e5. [23] Blyth RIR, Buqa H, Netzer FP, Ramsey MG, Besenhard JO, Golob P, et al. XPS studies of graphite electrode materials for lithium ion batteries. Appl Surf Sci 2000;167:99e106. [24] Dong Q, Zhao Y, Han X, Wang Y, Liu MC, Li Y. Pd/Cu bimetallic nanoparticles supported on graphene nanosheets: facile synthesis and application as novel electrocatalyst for
[25]
[26]
[27]
[28]
[29]
[30]
[31] [32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
ethanol oxidation in alkaline media. Int J Hydrogen Energy 2014;39:14669e79. Shin H-J, Kim KK, Benayad A, Yoon S-M, Park HK, Jung I-S, et al. Efficient reduction of graphite oxide by sodium borohydride and its effect on electrical conductance. Adv Funct Mater 2009;19:1987e92. Okamoto Y, Nitta Y, Imanaka T. Surface characterization of nickel boride and nickel phosphide catalysts by X-ray photoelectron spectroscopy. Faraday Trans 1979;75:2027e39. Xu CX, Liu AH, Qiu HJ, Liu YQ. Nanoporous PdCu alloy with enhanced electrocatalytic performance. Electrochem Commun 2011;13:766e9. Stamenkovic VR, Mun BS, Mayrhofer KJJ, Ross PN, Markovic NM. Effect of surface composition on electronic structure, stability, and electrocatalytic properties of Pttransition metal alloys: Pt-skin versus Pt-skeleton surfaces. J Am Chem Soc 2006;128:8813e9. Strasser P, Koh S, Anniyev Y, Greeley J, More K, Yu C, et al. Lattice-strain control of the activity in dealloyed core-shell fuel cell catalysts. Nat Chem 2010;2:454e60. Chen M, Kumar D, Yi CW, Goodman DW. The promotional effect of gold in catalysis by palladium-gold. Science 2005;310:291e3. Mavrikakis M, Hammer B, Norskov JK. Effect of strain on the reactivity of metal surfaces. Phys Rev Lett 1998;81:2819e22. Stamenkovic VR, Fowler B, Mun BS, Wang G, Ross PN, Lucas CA, et al. Improved oxygen reduction activity on Pt3Ni(111) via increased surface site availability. Science 2007;315:493e7. Kibler LA, El-Aziz AM, Hoyer R, Kolb DM. Tuning reaction rates by lateral strain in a palladium monolayer. Angew Chem Int Ed 2005;44:2080e4. Zhang J, Vukmiovic MB, Xu Y, Mavrikakis M, Adzic RR. Controlling the catalytic activity of platinum-monolayer electrocatalysts for oxygen reduction with different substrates. Angew Chem Int Ed 2005;44:2132e5. Hu S, Scudiero L, Ha S. Electronic effect on oxidation of formic acid on supported Pd-Cu bimetallic surface. Electrochim Acta 2012;83:354e8. Ren M, Zhou Y, Tao F, Zou Z, Akins DL, Yang H. Controllable modification of the electronic structure of carbon supported core-shell Cu@Pd catalysts for formic acid oxidation. J Phys Chem C 2014;118:12669e75. Liao MY, Hu Q, Zheng JB, Li YH, Zhou H, Zhong CJ, et al. Pd decorated Fe/C nanocatalyst for formic acid electrooxidation. Electrochim Acta 2013;111:504e9. Erikson H, Sarapuu A, Alexeyeva N, Tammeveski K, Solla n J, Feliu JM. Electrochemical reduction of oxygen on Gullo palladium nanocubes in acid and alkaline solutions. Electrochim Acta 2012;53:329e35. n J, Erikson H, Sarapuu A, Tammeveski K, Solla-Gullo Feliu JM. Enhanced electrocatalytic activity of cubic Pd nanoparticles towards the oxygen reduction reaction in acid media. Electrochem Commun 2011;13:734e7. Santasalo-Aarnio A, Kwon Y, Ahlberg E, Kontturi K, Kallio T, Kopper MTM. Comparison of methanol, ethanol and iso-propanol oxidation on Pt and Pd electrodes in alkaline media studied by HPLC. Electrochem Commun 2011;13:466e9. M, Łukaszewskia M, Jerkiewic G, Czerwin ski A. Grden Electrochemical behavior of palladium electrode: oxidation, electrodissolution and ionic adsorption. Electrochim Acta 2008;53:7583e98. Eriksona H, Kasikovb A, Johansc C, Kontturic K, Tammeveskia K, Sarapuua A. Oxygen reduction on Nafioncoated thin-film palladium electrodes. J Electroanal Chem 2011;652:1e7.
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e7
[43] Pattabiraman R. Electrochemical investigations on carbon supported palladium catalysts. Appl Catal A Gen 1997;153:9e20. [44] Trasatti S, Petrii OA. Real surface area measurements in electrochemistry. Pure Appl Chem 1991;63:711e34. [45] Woods R. Chemisorption at electrodes. Hydrogen and oxygen on noble metals and their alloys. In: Bard AJ, editor. Electroanalytical chemistry, a series of advances, vol. 9. New York: Basel: Marcel Dekker; 1976. p. 119e25. [46] Zhang LY, Zhao ZL, Li CM. Formic acid-reduced ultrasmall Pd nanocrystal on graphene to provide superior electrocatalytic activity and stability toward formic acid oxidation. Nano Energy 2015;11:71e7. [47] Zhang L, Xie Z, Gong J. Shape-controlled synthesis of Au-Pd bimetallic nanocrystals for catalytic applications. Chem Soc Rev 2016;45:3916e34. [48] Jin T, Guo SJ, Zou J-L, Sun SH. Synthesis and assembly of Pd nanoparticles on graphene for enhanced electrooxidation of formic acid. Nanoscale 2013;5:160e3. [49] Ojani R, Abkar Z, Hasheminejad E, Raoof J-B. Rapid fabrication of Cu/Pd nano/micro-particles porous-structured catalyst using hydrogen bubbles dynamic template and their enhanced catalytic performance for formic acid electrooxidation. Int J Hydrogen Energy 2014;39:7788e97. [50] Poon KC, Khezri B, Li Y, Webster RD, Su H, Sato H. A highly active Pd-P nanoparticle electrocatalyst for enhanced formic acid oxidation synthesized via stepwise electroless deposition. Chem Commun 2016;52:3556e9.
7
[51] Yu S, Zhang L, Zhao ZJ, Gong J. Structural evolution of concave trimetallic nanocubes with tunable ultra-thin shells for oxygen reduction reaction. Nanoscale 2016;8:16640e9. [52] Zhang L, Yu S, Zhang J, Gong J. Porous single-crystalline AuPt@Pt bimetallic nanocrystals with high mass electrocatalytic activities. Chem Sci 2016;7:3500e5. [53] Appleby AJ. Oxygen reduction and corrosion kinetics on phase-oxide-free palladium and silver electrodes as a function of temperature in 85% orthophosphoric acid. J Electrochem Soc 1970;117:1373e8. [54] Perdomo F, Avaca LLA, Aegerter MA, De Lima-Neto P. Oxygen-free deposition of ZrO2 sol-gel films on mild steel for corrosion protection in acid medium. J Mater Sci Lett 1998;17:295e8. [55] Tiwari SK, Tripathi M, Singh R. Electrochemical behavior of zirconia based coatings on mild steel prepared by sol-gel method. Corros Sci 2012;63:334e41. [56] Zakorchemna I, Carmona N, Zakroczymski T. Hydrogen permeation through sol-gel-coated iron during galvanostatic charging. Electrochim Acta 2008;53:8154e60. [57] Harkonen E, Diaz B, Swiatowska J, Maurice V, Seyeux A, Vehkamaki M, et al. Corrosion protection of steel with oxide nanolaminates grown by atomic layer deposition. J Electrochem Soc 2011;158:C369e78. [58] Tiwari SK, Sahu RK, Pramanick AK, Singh R. Development of conversion coating on mild steel prior to sol gel nanostructured Al2O3 coating for enhancement of corrosion resistance. Sur Coat Tech 2011;205:4960e7.
Please cite this article in press as: Feng A, et al., Surfactant-free PdeFe nanoparticles supported on reduced graphene oxide as nanocatalyst for formic acid oxidation, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.278