Facile synthesis of Pd nanochains with enhanced electrocatalytic performance for formic acid oxidation

Electrochimica Acta 130 (2014) 446–452

Contents lists available at ScienceDirect

Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Facile synthesis of Pd nanochains with enhanced electrocatalytic performance for formic acid oxidation Jie-Ning Zheng a , Ming Zhang b , Fang-Fang Li b , Shan-Shan Li a , Ai-Jun Wang a,∗ , Jiu-Ju Feng a,b,∗ a b

College of Chemistry and Life Science, College of Geography and Environmental Science, Zhejiang Normal University, Jinhua, 321004, China School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, 453007, China

a r t i c l e

i n f o

Article history: Received 26 December 2013 Received in revised form 7 March 2014 Accepted 8 March 2014 Available online 22 March 2014 Keywords: Pd nanochains Octylphenoxypolyethoxyethanol Electrocatalysis Formic acid

a b s t r a c t In this report, a simple and rapid method was developed for the large-scaled synthesis of well-defined Pd nanochains based on one-pot wet-chemical reduction of PdCl2 with KBH4 , with the assistance of octylphenoxypolyethoxyethanol (OP-10) as a growth directing agent. The concentration of OP-10, the strength of a reducing agent and its amount, and the reaction temperature were demonstrated essential for preparation of the Pd nanocrystals. Furthermore, the as-prepared Pd nanochains exhibited a higher electrochemically active surface area and the enhanced electrocatalytic performance for formic acid oxidation in acid media, compared with the Pd nanoparticles. © 2014 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, morphology control synthesis of noble metal nanocrystals is the most attractive research topics especially in catalysis, because the catalytic activity and stability of a catalyst strongly depend on its size and shape. It has been manifested that intrinsic properties of a nanomaterial can be significantly improved by shape and structural variation [1–4]. One dimensional metal nanostructures such as chains[4–6], wires[7], belts[8,9], and tubes[10] have attracted significant interest because of their promising technological applications in catalysis, sensors, and surface enhanced Raman spectroscopy. To date, Pt is still the most commonly used electrocatalysts in fuel cells, owing to its good activity and stability. However, its high cost is one of the most important barriers and seriously limits its potential commercial applications [11–14]. Recent research has been found Pd nanostructures as Pt-alternative catalysts in direct formic acid fuel cells (DFAFCs) [15–19]. Formic acid electrooxidation usually occurs through two parallel pathways (i.e. direct and CO pathways, respectively)[20,21], where Pd catalysts display the improved catalytic activity for the oxidation of formic acid in comparison with Pt catalysts[22–26]. This is ascribed to their different catalytic oxidation mechanisms [21,27].

∗ Corresponding author. Tel.: +86 579 82282269; fax: +86 579 82282269. E-mail addresses: [email protected], [email protected] (A.-J. Wang), [email protected] (J.-J. Feng). http://dx.doi.org/10.1016/j.electacta.2014.03.054 0013-4686/© 2014 Elsevier Ltd. All rights reserved.

Currently, a variety of Pd nanostructures with different shapes have been prepared, including rods, wires, tubes, polyhedras, urchins, and dendrites[28–37]. Pd nanochains have attracted enormous interest, owing to their enlarged surface areas and more active sites available for reactant species. Many strategies were developed to prepare Pd nanostructures. Feng et al. synthesized short Pd nanochains by the one-step polyol process[5]. Zhou and coworkers fabricated the chains of Pd nanoparticles by incubating aged sodium tetrachloropalladate (II) with glucagon fibrils pre-deposited on a solid surface[38]. Lan’s group constructed Pd atomic chains in a hydrogen atmosphere[39]. The above successful examples are usually involved in toxic orgainic solvent, surfactant, high temperature, complicated synthetic procedures, and other harsh conditions. Therefore, it is still an urgent topic for developing a simple and reliable strategy for the large-scaled synthesis of Pd nanochains with well-defined shapes. Generally, sufficient surfactant molecules can self-assemble into micelles or vesicles to form interfaces with aqueous solutions, which are usually used as soft templates for preparation of advanced materials [40–42]. Octylphenoxy- polyethoxyethanol (OP-10), as a cheap non-ionic surfactant, is used for preparation of nanomaterials such as anatase TiO2 [43] and Ag nanoparticles [44]. Herein, OP-10 was used as a growth directing agent for synthesis of the Pd nanochains by a one-pot wet chemical route. The electrocatalytic activity and stability of the Pd nanocrystals were examined using the oxidation of formic acid as a model system.

J.-N. Zheng et al. / Electrochimica Acta 130 (2014) 446–452

447

40

2.2. Synthesis of Pd nanochains

60 2-Theta / degree

( 311)

( 222)

( 220)

( 200)

Palladium chloride (PdCl2 ), KBH4 , OP-10, and formic acid were obtained from Shanghai Aladdin Chemical Reagent Company. All the other chemicals were analytical grade and used without further purification. All the aqueous solutions were prepared with twicedistilled water.

Intensity / a.u.

2.1. Reagents

( 111)

2. Experimental

80

For typical preparation of the Pd nanochains, 3 mL of freshly prepared 0.1 M KBH4 solution was quickly put into 50 mL of OP-10 solution (0.2%, w/v) containing 1 mM PdCl2 under vigorous stirring at 0 ◦ C. After 5 min, the resulting solution was centrifuged and the precipitates were collected by thoroughly washing with water and ethanol, respectively. The final products were used for further characterization. Similar procedure was used for preparation of Pd nanoparticles in the absence of OP-10, while other conditions were kept unchanged.

modified electrodes. The working electrodes maintained at 0.1 V (vs. SCE), and excess CO in the electrolyte was removed by purging with nitrogen for 30 min. The CO-stripping voltammograms were recorded by oxidizing pre-adsorbed CO (COad ) in 0.5 M H2 SO4 at a scan rate of 50 mV s−1 . The amount of adsorbed CO was measured by integrating the COad stripping peak and correcting for the capacitance of the electric double-layer.

2.3. Characterization

3. Results and discussion

X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker-D8-AXS diffractometer. Transmission electron microscopy (TEM) and high-resolution TEM were performed on a JEOL JEM-2100F system at 200 kV accelerating voltage. Selectedarea electron diffraction (SAED) experiments were carried out to examine the crystallinity of the products.

3.1. Characterization

2.4. Electrochemical measurements Electrochemical experiments were carried out on a CHI 660D electrochemical workstation (CH Instruments, Chenhua Co., Shanghai, China) containing a traditional three-electrode cell, including a saturated calomel electrode (SCE) as reference electrode, a platinum wire as counter electrode, and a bare or modified glassy carbon electrode (GCE, 3 mm in diameter) as working electrode. For preparation of the Pd nanochains modified electrode, 2 mg of the Pd nanochains were dispersed into 1.0 mL water by ultrasonication (2 mg mL−1 ), followed by dropping 5 ␮L of the suspension onto the electrode surface and dried in air. The specific loading of the nanochains was calculated to be 0.142 mg cm−2 . After drying, 2 ␮L of Nafion solution (0.1 wt.%) was casted on the electrode surface to seal the sample in place, and dried in air. For comparison, the Pd nanoparticles modified electrode was prepared in a similar way. The CO stripping experiments were performed as follows: CO was firstly purged through 0.5 M H2 SO4 for 30 min to allow complete adsorption of CO onto the Pd nanochains and Pd nanoparticles

Fig. 2. XRD pattern of the typical Pd nanochains.

In this report, TEM image (Fig. 1A) of the sample reveals a lot of uniform interconnected Pd nanochains with the length of micrometers. HRTEM image (Fig. 1B) displays the fine structures of a single Pd nanochain with the average lattice spacing of ca. 0.23 nm. This value is consistent with the (111) planes of the face-centered cubic (fcc) Pd [45–48], implying that the nanoparticles are attached via the (111) planes. HRTEM images reveal that the lattice fringes of the Pd nanochains with different orientations (Fig. 1B), reflecting polycrystalline nature of the Pd nanochains, as demonstrated by the SAED pattern with several caliginous concentric rings (inset in Fig. 1B). XRD analysis was carried out to determine the crystalline structures of the Pd nanochains (Fig. 2). The representative diffraction peaks centered at 40.1◦ , 46.6◦ , 68.1◦ , 82.1◦ , and 86.6◦ are matched well with the (111), (200), (220), (311), and (222) planes of the fcc Pd (JCPDS No. 05–0681), respectively, while no any impurity peaks are detected, indicating high quality of the Pd nanostructures. Moreover, the peak intensity of the (111) planes is much stronger and sharper, compared with the other planes, revealing that the Pd nanoparticles are assembled via the (111) planes, as supported by the HRTEM analysis. This is due to no or very weak adsorption of OP-10 molecules on the (111) planes in the Pd nanochains. This observation is similar to the Pd nanoflowers[47] and Pd nanospheres [49].

Fig. 1. Morphological and structural characterizations of the Pd nanochains: TEM (A) and HRTEM (B) images. Inset shows the corresponding SAED pattern.

448

J.-N. Zheng et al. / Electrochimica Acta 130 (2014) 446–452

Fig. 3. TEM images of the Pd nanostructures obtained with hydrazine hydrate (A) and ascorbic acid (B).

3.2. Effects of the experimental parameters The reaction rate strongly depends on the strength of reducing agents, which is associated with the morphology (i.e. shape and size) of the final product[50,51]. In control experiments, replacing KBH4 with hydrazine hydrate (Fig. 3A) produces poor quality of the branched nanochains with severe aggregation, whereas the

existence of ascorbic acid (Fig. 3B) induces the formation of numerous spherical Pd particles with an average particle size of 20 nm, rather than the Pd nanochains. These observations are different from that using KBH4 as a strong reducing agent (Fig. 1A). It indicates that fast reducing rate facilitates the anisotropic growth of Pd nanocrystals with specific shapes, in which thermodynamic effects are predominated, rather than kinetic effects. This phenomenon is similar to the Au nanochains[52], but different from the flower-like Au nanochains previously prepared in our group[50]. Most importantly, varying the concentrations of KBH4 from 1 to 10 mM yields the Pd products with different shape and size (Fig. 4), while the other conditions kept unchanged. Specifically, shorter and thicker nanochains are assembled by irregular Pd nanoparticles with 1 mM KBH4 (Fig. 4A). Increasing the KBH4 concentrations to 2 mM (Fig. 4B) and 4 mM (Fig. 4 C) produce large-scaled immature Pd nanochains with improved quality, accompanied with the extended chain length. And best Pd nanochains are generated with 6 mM KBH4 (Fig. 1A). Conversely, much higher concentration of KBH4 such as 10 mM (Fig. 4D) leads to the Pd nanochains with poor quality. This is attributed to the fact that extra KBH4 would react with water and produce a lot of small hydrogen gas bubbles, which would inhibit the fusion and attachment of newly generated Pd nanocrystals, along with the transformation of Pd nanoparticles into nanochains. These variations reveal the critical role of the KBH4 concentrations in the present synthesis. Impressively, the reaction temperature has great effects on the Pd nanochains. When the reaction temperature increases to 25 ◦ C (Fig. 5A) and 50 ◦ C (Fig. 5B), the nanochains are gradually shortened, accompanied with the increase of the agglomerated nanostructures and the emergence of some irregular Pd nanoparticles, compared with those obtained at 0 ◦ C (Fig. 1A). Increasing the reaction temperature causes the decrease of the solution viscosity, but the increase of the molecular mobility (or diffusion rate). As a result, the micellar structures of OP-10 become disordered in some degree, inducing serious desorption of the adsorbed OP-10 from the Pd surface. It means that lower temperature promotes the formation of the Pd nanochains. When the concentration of a surfactant is higher than its critical micelle concentration, micelle is non-sphericity including ellipsoidal, oblate, acetabuliform, and claviform shapes[42], as also

Fig. 4. TEM images of the Pd nanostructures obtained with different concentrations of KBH4 solutions: 1 mM (A), 2 mM (B), 4 mM (C), and 10 mM (D).

J.-N. Zheng et al. / Electrochimica Acta 130 (2014) 446–452

449

decreased by further increasing the amount of OP-10 up to 1.0% (Fig. 6D), and even some of them are ruptured simultaneously. It is known that the surface energies of the fcc metal crystals decrease in the order ␥(110) > ␥(100) > ␥(111)[4,53]. As expected, low concentrations of OP-10 possibly induces incomplete coverage the (110) and (100) planes, whereas higher concentrations of OP-10 (i.e. 1 wt.%) cover all the planes and block the oriented attachment of Pd nanoparticles into nanochains [53]. These results confirm the critical role of OP-10 as a structure directing agent and a stabilizing agent. 3.3. Formation mechanism of the Pd nanochains

Fig. 5. TEM images of the Pd products fabricated at different reaction temperature: 25 ◦ C (A) and 50 ◦ C (B).

demonstrates by this work (Fig. 6). The absence of OP-10 produces numerous Pd nanoparticles (Fig. 6A). In contrast, the presence of OP-10 (ie, 0.05%, Fig. 6B) yields a lot of immature Pd nanochains. And the quality of the Pd nanochains is gradually improved with the increase of OP-10 to 0.2% (Fig. 1A) and 0.5% (Fig. 6 C), respectively. Nevertheless, the quality of the Pd nanochains is reversely

Impressively, the self-assembly of the Pd nanochains is performed without any linking reagents such as dithiol and amino thiol compounds in the literature[4]. The optimal amount of OP-10 is 0.2%, much higher than its critical micelle concentration (0.04%), resulted into the formation of the Pd nanochains, as confirmed by the control experiments of OP-10 (Fig. 6). Although OP-10 has a certain adsorption capacity, the accumulation speed of Pd nanocrystals plays an important role for the final morphology. As illustrated in Fig. 7, numerous Pd atoms are quickly formed upon the addition of KBH4 at the very early stage, owing to the extremely fast reduction rate between the adjacent Pd2+ ions and KBH4 . The newly generated Pd atoms are transformed to Pd nuclei based on Ostwald ripening, wherein the smaller Pd nuclei start to dissolve into the solution and add onto the larger particles (2∼3 nm in size). The as-prepared Pd particles have relatively high surface energy and low surface charge. The (111) planes have the lowest energy, the greatest bonding ability and chemical reactivity that induces the self-assembly between Pd particles with the fcc crystalline structure. As a result, the residual Pd atoms preferentially diffuse onto the (111) planes, the (111) planes easily fuse with one another, and form the Pd nanochains based on the lowest energy principle. In addition, the adsorption of OP-10 is affected by the reaction temperature, as confirmed above (Fig. 5). During the synthetic process, OP-10 acts as a directing agent, which can rapidly and selectively adsorb on the specific Pd crystal planes. Meanwhile, the phenyl ring groups in OP-10 have weak

Fig. 6. TEM images of the Pd products prepared with different amounts of OP-10 (w/v): 0 (A), 0.05% (B), 0.5% (C), and 1.0% (D).

450

J.-N. Zheng et al. / Electrochimica Acta 130 (2014) 446–452

j / mA cm-2

6

a

A

3

b

0 -3 0.0 0.3 0.6 Potential / V (vs. SCE)

j / mA cm-2

6

Fig. 7. The formation mechanism of the Pd nanochains.

j / mA cm-2

10

a

B

3 0

0.9

b

a

-3 0.0 0.3 0.6 Potential / V (vs. SCE)

b

0.9

0 Fig. 9. CO-stripping voltammograms of the Pd nanochains (A) and Pd nanoparticles (B) modified electrodes with (curve a) and without (curve b) COad in 0.5 M H2 SO4 at 50 mV s−1 .

-10

-0.3

0.0 0.3 0.6 0.9 Potential / V ( vs. SCE )

Fig. 8. Cyclic voltammograms of the Pd nanochains (curve a) and Pd nanoparticles (curve b) modified electrodes in 0.5 M H2 SO4 at 50 mV s−1 .

In general, bulk Pd can absorb hydrogen, and thereby the electrochemically active surface area (EASA) can be estimated by CO stripping measurements[57,58]. Fig. 9 displays the CO-stripping voltammograms of the Pd nanochains (Fig. 9A) and Pd nanoparticles (Fig. 9B) modified electrodes in 0.5 M H2 SO4 , respectively. The EASA of Pd catalysts is obtained by the following formula [48,59]: EASA =

␲–d interactions with the Pd planes, which facilitate the attachment and coalescence of Pd crystals as nuclei and growth along the Pd (111) planes[50,53], leading to the formation of the best Pd nanochains. This formation mechanism is similar to those of Ag and Pt nanochains[4,54], unlike those of Pd nanowires via polyol synthesis route[37] and metal nanowires with the assistance of Triton X-114[53]. 3.4. Electrochemical experiments Fig. 8 shows the typical cyclic voltammograms of the Pd nanochains (curve a) and Pd nanoparticles (curve b) modified electrodes in 0.5 M H2 SO4 . In the hydrogen region from −0.20 to 0.15 V, there are two pairs of peaks (−0.16 and −0.19 V; −0.05 and −0.15 V), which are assigned to the hydrogen adsorption and desorption for the ␣ and ␤ hydride phases[48], respectively. In the Pd oxidation region, the onset potential occurs at 0.52 V for producing Pd surface oxide in the anodic scan, while the reduction of Pd oxides is detected at 0.64 V and reaches the maximum at 0.43 V in the reverse scan. These observations are similar to those on the bulk polycrystalline Pd electrodes under the identical conditions[48]. The peak at–0.15 V is ascribed to the hydrogen adsorption on the Pd nanochains modified electrode [7,55]. Meanwhile, a part of the desorption peak is also detected at–0.05 V[56]. These electrochemical results demonstrate the formation of the Pd product in the present synthesis.

Q G × 420

where Q is the charge of CO desorption–electrooxidation in microcoulomb (␮C), G represents the total amount of Pd (␮g) on the electrode, and 420 is the charge required to oxidize a monolayer of CO on the catalyst in ␮C cm−2 . The EASA on the Pd nanochains modified electrode is 15.80 m2 g−1 , which is larger than that on the Pd nanoparticles modified electrode (7.06 m2 g−1 ). The larger EASA of the Pd nanochains catalyst might be attributed to the special nanostructures [47,48]. Therefore, the Pd nanochains might have high electrocatalytic activity, compared with the Pd nanoparticles under the similar conditions, if equal amount of metallic Pd employed. The electrochemical oxidation of formic acid is investigated as a model system to examine the catalytic activity of the Pd nanochains and Pd nanoparticles modified electrodes in the absence and presence of 0.5 M formic acid, respectively (Fig. 10). Clearly, the peak current density is distinctly larger in the range of 0.1∼0.2 V (Fig. 10, curve a) than those from 0.5 to 0.6 V, indicating the oxidation of formic acid mainly follows the direct pathway, rather than the CO pathway[60]. The oxidation peak current density is 40.20 mA cm−2 for the Pd nanochains, which is significantly larger than the Pd nanoparticles with a value of 15.90 mA cm−2 . Meanwhile, the mass activity of the Pd nanochains is 283.81 mA mg−1 , which is larger than that of the Pd nanoparticles (112.25 mA mg−1 ). These findings illustrate that the enhanced catalytic performance of the Pd nanochains is possibly due to the larger EASA of the Pd nanochains.

J.-N. Zheng et al. / Electrochimica Acta 130 (2014) 446–452

Acknowledgments

45

j / mA cm-2

a

This work has been financially supported by the NSFC (Nos. 21175118 and 21275130), and Zhejiang province university young academic leaders of academic climbing project (No. pd2013055).

30

b 15

References

0

c -15 -0.3

0.0

0.3

0.6

0.9

Potential / V ( vs. SCE ) Fig. 10. Cyclic voltammograms of the Pd nanochains (curves a and c) and Pd nanoparticles (curve b) modified electrodes in the absence (curve c) and presence (curves a and b) of 0.5 M formic acid in 0.5 M H2 SO4 at 50 mV s−1 .

9

j / mA cm-2

451

6

a

3

b 0

0

1000 2000 3000 4000 5000 6000 Time / s

Fig. 11. Chronoamperometric curves of the Pd nanochains (curve a) and Pd nanoparticles (curve b) modified electrodes in 0.5 M H2 SO4 containing 0.5 M formic acid by applying a constant potential of 0.12 V.

Moreover, the stability of the Pd nanochains modified electrode was examined by chronoamperometric measurements in 0.5 M H2 SO4 containing 0.5 M formic acid (Fig. 11). The applied potential at 0.12 V was chosen based on the maximum of the catalytic current (Fig. 10). In each case, the current density exponentially decreases at the initial stage. The decay of the current density is slower for the Pd nanochains (Fig. 11, curve a), compared with that of the Pd nanoparticles (Fig. 11, curve b). When the time extends to 6000 s, the current densities and mass activities of the Pd nanochains and Pd nanoparticles modified electrodes are 1.798 and 0.558 mA cm−2 , 12.694 and 3.939 mA mg−1 , respectively. Additionally, the chronoamperometric current is still above 90% of its initial value within a storage period of 5 weeks under the same conditions. These results indicate that the Pd nanochains have the improved catalytic performances.

4. Conclusions In summary, uniform Pd nanochains were prepared by a simple one-pot solution method, in which OP-10 was used as a growth directing agent. The reaction temperature, OP-10 and KBH4 , as well as their concentrations have great effects on the final morphology and size. The as-prepared Pd nanochains showed the enhanced electrocatalytic performance for formic acid oxidation in acidic media, which can be applied as electrocatalysts in fuel cells.

[1] M. H. Shao, J., Odell, M., Humbert, T. Y. Yu Y. N. Xia, Electrocatalysis on shapecontrolled palladium nanocrystals: Oxygen reduction reaction and formic acid oxidation. J. Phys. Chem. C 117 (2013) 4172. [2] Y. Xia, Y. Xiong, B. Lim, S.E. Skrabalak, Shape-controlled synthesis of metal nanocrystals: Simple chemistry meets complex physics? Angew. Chem. Int. Edit. 48 (2009) 60. [3] M. Crespo-Quesada, A. Yarulin, M. Jin, Y. Xia, L. Kiwi-Minsker, Structure sensitivity of alkynol hydrogenation on shape- and size-controlled palladium nanocrystals: Which sites are most active and selective? J. Am. Chem. Soc. 133 (2013) 12787. [4] X. Chen, J. Xie, J. Hu, X. Feng, A. Li, EDTA-directed self-assembly and enhanced catalytic properties of sphere-constructed platinum nanochains, J. Phys. D: Appl. Phys. 43 (2010) 115403. [5] C. Feng, L. Guo, Z. Shen, J. Gong, X.-Y. Li, C. Liu, S. Yang, Synthesis of short palladium nanoparticle chains and their application in catalysis, Solid State Sci. 10 (2008) 1327. [6] F. Lan, D. Wang, S. Lu, J. Zhang, D. Liang, S. Peng, Y. Liu, Y. Xiang, Ultra-low loading Pt decorated coral-like Pd nanochain networks with enhanced activity and stability towards formic acid electrooxidation, J. Mater. Chem. A 1 (2013) 1548. [7] A. Czerwinski, The adsorption of carbon oxides on a palladium electrode from acidic solution, J. Electroanal. Chem. 379 (1994) 487. [8] J.W. Zhu, D.F. Xue, Crystallography and interfacial kinetic controlled ultra-uniform single crystal silver nanobelts and their optical properties, CrystEngComm 16 (2014) 642. [9] G. Chen, P. Zhang H. W. Liu, Analysis of Pd-Ni nanobelts melting process using molecular dynamics simulation. J. Nanomater. 2013 (2013) 1. [10] M.F. Sun, F. Xiao, C. Deng, Near-ideal strength in metal nanotubes revealed by atomistic simulations, Appl. Phys. Lett. 103 (2013) 4. [11] Y. Wang, C. Zhao, M. Wu, W. Liu, T. Ma, Highly efficient and low cost Pt-based binary and ternary composite catalysts as counter electrode for dye-sensitized solar cells, Electrochim. Acta 105 (2013) 671. [12] W. Chen, J. Kim, S. Sun, S. Chen, Composition effects of FePt alloy nanoparticles on the electro-oxidation of formic acid, Langmuir 23 (2007) 11303. [13] T.C. Deivaraj, W. Chen, J.Y. Lee, Preparation of PtNi nanoparticles for the electrocatalytic oxidation of methanol, J. Mater. Chem. 13 (2003) 2555. [14] Y. Shao, G. Yin, Y. Gao, Understanding and approaches for the durability issues of Pt-based catalysts for PEM fuel cell, J. Power Sources 171 (2007) 558. [15] T. Tsujiguchi, S. Hirano, T. Iwakami, N. Nakagawa, The performance degradation of a passive direct formic acid fuel cell and its improvement by a hydrophobic filter, J. Power Sources 223 (2013) 42. [16] Y. Zhou, J. Liu, J. Ye, Z. Zou, J. Ye, J. Gu, T. Yu, A. Yang, Poisoning and regeneration of Pd catalyst in direct formic acid fuel cell, Electrochim. Acta 55 (2010) 5024. [17] L. Zhang, T. Lu, J. Bao, Y. Tang, C. Li, Preparation method of an ultrafine carbon supported Pd catalyst as an anodic catalyst in a direct formic acid fuel cell, Electrochem. Commun. 8 (2006) 1625. [18] X.W. Yu, P.G. Pickup, Recent advances in direct formic acid fuel cells (DFAFC), J. Power Sources 182 (2008) 124. [19] X.W. Yu, P.G. Pickup, Deactivation/reactivation of a Pd/C catalyst in a direct formic acid fuel cell (DFAFC): Use of array membrane electrode assemblies, J. Power Sources 187 (2009) 493. [20] L. Lu, H. Li, Y. Hong, Y. Luo, Y. Tang, T. Lu, Improvement of electrocatalytic performance of carbon supported Pd anodic catalyst in direct formic acid fuel cell by ethylenediamine-tetramethylene phosphonic acid, J. Power Sources 210 (2012) 154. [21] K. Kim, J.-K. Yu, H. Lee, J. Choi, S. Noh, S. Yoon, C.-S. Lee, T.-S. Hwang, Y. Rhee, Preparation of Pt-Pd catalysts for direct formic acid fuel cell and their characteristics, Korean J. Chem. Eng. 24 (2007) 518. [22] L. Shen, H. Li, L. Lu, Y. Luo, Y. Tang, Y. Chen, T. Lu, Improvement and mechanism of electrocatalytic performance of Pd-Ni/C anodic catalyst in direct formic acid fuel cell, Electrochim. Acta 89 (2013) 497. [23] Z.-Y. Zhou, X. Kang, Y. Song, S. Chen, Butylphenyl-functionalized palladium nanoparticles as effective catalysts for the electrooxidation of formic acid, Chem. Commun. 47 (2011) 6075. [24] Y. Luo, H. Li, T. Chen, C. Ge, Y. Tang, Y. Chen, T. Lu, Effect of Pd ions in electrolyte on electrocatalytic performance of carbon supported Pd catalyst for oxidation of formic acid, Electrochim. Acta 87 (2013) 839. [25] H. Huang, X. Wang, Pd nanoparticles supported on low-defect graphene sheets: for use as high-performance electrocatalysts for formic acid and methanol oxidation, J. Mater. Chem. 22 (2012) 22533. [26] J. Ge, W. Xing, X. Xue, C. Liu, T. Lu, J. Liao, Controllable synthesis of Pd nanocatalysts for direct formic acid fuel cell (DFAFC) application:From Pd hollow nanospheres to Pd nanoparticles, J. Phys. Chem. C 111 (2007) 17305.

452

J.-N. Zheng et al. / Electrochimica Acta 130 (2014) 446–452

[27] S.M. Baik, J. Han, J. Kim, Y. Kwon, Effect of deactivation and reactivation of palladium anode catalyst on performance of direct formic acid fuel cell (DFAFC), Int. J. Hydrogen. Energ 36 (2011) 14719. [28] S.-P. Ju, M.-H. Weng, W.-C. Huang, Potential applications of two ultrathin Pd nanowires to the hydrogen economy, J. Mater. Chem. 22 (2012) 20319. [29] Z.C. Zhang, X. Zhang, Q.Y. Yu, Z.C. Liu, C.M. Xu, J.S. Gao, J. Zhuang, X. Wang, Pd cluster nanowires as highly efficient catalysts for selective hydrogenation reactions, Chem. Eur. J. 18 (2012) 2639. [30] K. Kim, M. Kim, S.M. Cho, Pulsed electrodeposition of palladium nanowire arrays using AAO template, Mater. Chem. Phys. 96 (2006) 278. [31] Y. Yoo, I. Yoon, H. Lee, J. Ahn, J.-P. Ahn, B. Kim, Pattern-selective epitaxial growth of twin-free Pd nanowires from supported nanocrystal seeds, ACS Nano 4 (2010) 2919. [32] R. Inguanta, S. Piazza, C. Sunseri, Synthesis of self-standing Pd nanowires via galvanic displacement deposition, Electrochem. Commun. 11 (2009) 1385. [33] M. Jin, H. Zhang, Z. Xie, Y. Xia, Palladium nanocrystals enclosed by {100} and {111} facets in controlled proportions and their catalytic activities for formic acid oxidation, Energy Environ. Sci. 5 (2012) 6352. [34] K.J. Jeon, M. Jeun, E. Lee, J.M. Lee, K.I. Lee, P. von Allmen, W. Lee, Finite size effect on hydrogen gas sensing performance in single Pd nanowires, Nanotechnology 19 (2008) 495501. [35] M.N. Nadagouda, R.S. Varma, Green synthesis of Ag and Pd nanospheres, nanowires, and nanorods using vitamin B: Catalytic polymerisation of aniline and pyrrole, J. Nanomater. 2008 (2008) 1. [36] E. Sennik, N. Kilinc, Z.Z. Ozturk, Temperature-dependent H2 gas-sensing properties of fabricated Pd nanowires using highly oriented pyrolytic graphite, J. Appl. Phys. 108 (2010) 054317. [37] Y. Wang, S.-I. Choi, X. Zhao, S. Xie, H.-C. Peng, M. Chi, C.Z. Huang, Y. Xia, Polyol synthesis of ultrathin Pd nanowires via attachment-based growth and their enhanced activity towards formic acid oxidation, Adv. Funct. Mater. 19 (2013) 1. [38] X. Zhou, L. Zheng, R. Li, B. Li, S. Pillai, P. Xu, Y. Zhang, Biotemplated fabrication of size controlled palladium nanoparticle chains, J. Mater. Chem. 22 (2012) 8862. [39] M. Kiguchi, K. Hashimoto, Y. Ono, T. Taketsugu, K. Murakoshi, Formation of a Pd atomic chain in a hydrogen atmosphere, Phys. Rev. B 81 (2010) 195401. [40] Y. Liu, C. Wang, Y. Wei, L. Zhu, D. Li, J.S. Jiang, N.M. Markovic, V.R. Stamenkovic, S. Sun, Surfactant-induced postsynthetic modulation of Pd nanoparticle crystallinity, Nano lett. 11 (2011) 1614. [41] G.S. Attard, J.M. Corker, C.G. Goltner, S. Henke, R.H. Templer, Liquid-crystal templates for nanostructured metals, Angew. Chem. Int. Ed. 36 (1997) 1315. [42] Z.-X. Cheng, X.-B. Dong, Q.-Y. Pan, J.-C. Zhang, X.-W. Dong, Preparation and characterization of In2 O3 nanorods, Mater. Lett. 60 (2006) 3137. [43] J. Yi, C.L. Tan, W.S. Li, J.F. Lei, L.S. Hao, Preparation of anatase TiO2 with assistance of surfactant OP-10 and its electrochemical properties as an anode material for lithium ion batteries, Rare Metals 29 (2010) 505. [44] X. Wu, T. Wang, H. Wu, Facile Preparation and formation mechanism of uniform silver nanoparticles using OP-10 as emulsifier in reverse microemulsion, J. Wuhan Univ. Technol. 26 (2011) 38.

[45] V. Mazumder, S. Sun, Oleylamine-mediated synthesis of Pd nanoparticles for catalytic formic acid oxidation, J. Am. Chem. Soc. 131 (2009) 4588. [46] X. Huang, N. Zheng, One-pot, high-yield synthesis of 5-fold twinned Pd nanowires and nanorods, J. Am. Chem. Soc. 131 (2009) 4602. [47] A.-J. Wang, F.-F. Li, J.-N. Zheng, H.-X. Xi, Z.-Y. Meng, J.-J. Feng, Green synthesis of porous flower-like palladium with high electrocatalytic activity towards methanol oxidation, RSC Adv. 3 (2013) 10355. [48] A.-J. Wang, F.-F. Li, Z. Bai, J.-J. Feng, Large-scale electrosynthesis of Pd nanodendrites and their improved electrocatalytic properties for methanol oxidation, Electrochim. Acta 85 (2012) 685. [49] Y.-H. Chien, C.-C. Huang, S.-W. Wang, C.-S. Yeh, Synthesis of nanoparticles: sunlight formation of gold nanodecahedra for ultra-sensitive lead-ion detection, Green Chem. 13 (2011) 1162. [50] A.-J. Wang, S.-F. Qin, D.-L. Zhou, L.-Y. Cai, J.-R. Chen, J.-J. Feng, Caffeine assisted one-step synthesis of flower-like gold nanochains and their catalytic behaviors, RSC Adv. 3 (2013) 14766. [51] M.C. Daniel, D. Astruc, Gold nanoparticles: Assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology, Chem. Rev. 104 (2004) 293. [52] H. Jia, X. Bai, L. Zheng, One-step synthesis and assembly of gold nanochains using the Langmuir monolayer of long-chain ionic liquids and their applications to SERS, CrystEngComm 14 (2012) 2920. [53] R. Liu, J.F. Liu, G.B. Jiang, Use of Triton X-114 as a weak capping agent for onepot aqueous phase synthesis of ultrathin noble metal nanowires and a primary study of their electrocatalytic activity, Chem. Commun. 46 (2010) 7010. [54] X. Feng, F. Ruan, R. Hong, J. Ye, J. Hu, G. Hu, Z. Yang, Synthetically directed self-assembly and enhanced surface-enhanced Raman scattering property of twinned crystalline Ag/Ag homojunction nanoparticles, Langmuir 27 (2011) 2204. [55] A. Czerwinski, R. Marassi, S. Zamponi, The absorption of hydrogen and deuterium in thin palladium electrodes: Part I. Acidic solutions, J. Electroanal. Chem. 316 (1991) 211. [56] A.N. Correia, L.H. Mascaro, S.A.S. Machado, L.A. Avaca, Active surface area determination of Pd-Si alloys by H-adsorption, Electrochim. Acta 42 (1997) 493. [57] H.-X. Zhang, C. Wang, J.-Y. Wang, J.-J. Zhai, W.-B. Cai, Carbon-supported Pd-Pt nanoalloy with low pt content and superior catalysis for formic acid electrooxidation, J. Phys. Chem. C 114 (2010) 6446. [58] P.-P. Fang, S. Duan, X.-D. Lin, J.R. Anema, J.-F. Li, O. Buriez, Y. Ding, F.-R. Fan, D.-Y. Wu, B. Ren, Z.L. Wang, C. Amatore, Z.-Q. Tian, Tailoring Au-core Pd-shell Ptcluster nanoparticles for enhanced electrocatalytic activity, Chem. Sci. 2 (2011) 531. [59] Z. Bai, Y. Guo, L. Yang, L. Li, W. Li, P. Xu, C. Hu, K. Wang, Highly dispersed Pd nanoparticles supported on 1,10-phenanthroline-functionalized multi-walled carbon nanotubes for electrooxidation of formic acid, J. Power Sources 196 (2011) 6232. [60] S. Ha, R. Larsen, Y. Zhu, R.I. Masel, Direct Formic Acid Fuel Cells with 600 mA/cm2 at 0.4 V and 22 (C, Fuel Cells 4 (2004) 337.