Highly dispersed carbon-supported Pd nanoparticles catalyst synthesized by novel precipitation–reduction method for formic acid electrooxidation

Electrochimica Acta 56 (2011) 4696–4702

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Highly dispersed carbon-supported Pd nanoparticles catalyst synthesized by novel precipitation–reduction method for formic acid electrooxidation Yan Liang a , Mengna Zhu a , Juan Ma b , Yawen Tang a , Yu Chen a,∗ , Tianhong Lu a a Jiangsu Key Laboratory of New Power Batteries, Laboratory of Electrochemistry, College of Chemistry and Materials Science, Nanjing Normal University, 1# Wenyuan Road, Nanjing 210046, PR China b Institute of Chemical Power Sources, Soochow University, Suzhou 215006, PR China

a r t i c l e

i n f o

Article history: Received 19 January 2011 Received in revised form 3 March 2011 Accepted 4 March 2011 Available online 12 March 2011 Keywords: Catalyst Precipitation–reduction method Formic acid Electrooxidation Electrocatalytic activity

a b s t r a c t The highly dispersed and ultrafine carbon-supported Pd nanoparticles (Pd/C) catalyst is synthesized by using an improved precipitation–reduction method, which involves in PdII → PdO·H2 O → Pd0 reaction path. In the method, palladium oxide hydrate (PdO·H2 O) nanoparticles (NPs) with high dispersion is obtained easily by adjusting solution pH in the presence of 1,4-butylenediphosphonic acid (H2 O3 P(CH2 )4 -PO3 H2 , BDPA). After NaBH4 reduction, the resulting Pd/C catalyst possesses high dispersion and small particle size. As a result, the electrochemical measurements indicate that the resulting Pd/C catalyst exhibits significantly high electrochemical active surface area and high electrocatalytic performance for formic acid electrooxidation compared with that prepared by general NaBH4 reduction method. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Palladium is one of the most used noble metals having technical applications in autocatalyst, dentistry, chemistry, and in the electronic industry [1–10]. Recently, noble Pd catalysts are found to possess superior performance for formic acid electrooxidation in direct formic acid fuel cells compared with Pt-based catalysts [11,12]. It has demonstrated that formic acid electrooxidation occurs mainly through the “direct pathway” on Pd instead of a dual pathway mechanism on Pt [13], in which formic acid oxidation occurs through both a “direct pathway” and a “CO pathway”. Thus, Pd-based catalysts can overcome the CO poisoning effect and thereby yield high performance for formic acid electrooxidation [14–16]. Moreover, Pd metal is considered less expensive than Pt metal. For example, the price of Pd is 1/4–1/5 as that of Pt [17,18]. Therefore, Pd nanoparticles (Pd–NPs) catalyst is a promising electrocatalyst for formic acid electrooxidation compared to the Pt–NPs catalysts. Recent progress in formic acid electrooxidation has revealed that the electrocatalytic performance of Pd–NPs for formic acid electrooxidation is strongly influenced by morphology and mean particle size of Pd–NPs [19–22]. In general, Pd–NPs with smaller average particle size can result in consistently higher electro-

∗ Corresponding author. Tel.: +86 25 85891651; fax: +86 25 83243286. E-mail address: [email protected] (Y. Chen). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.03.019

catalytic performance for formic acid electrooxidation due to area effect [20,23]. However, the particle size of Pd–NPs prepared by the general NaBH4 reduction method is difficult to control and it is usually larger than expected size [24]. Therefore, the controllable synthesis appears to be significantly desirable. To date, various novel preparation method, such as microemulsion/inverse microemulsion method [25,26], colloidal method [27], hydrothermal method [28], complexing-reduction method [29,30], electrospray [31], electrosynthesis [21] have been reported to prepare ultrafine Pd–NPs catalyst. Compared to above-mentioned preparation methods, the conventional chemical reduction method is very hopeful technique for catalyst preparation due to its simplicity and inexpensiveness. Thus, the further efforts should devote to improvement of the liquid phase chemical reduction method for the preparation of Pd–NPs catalyst. For the NaBH4 reduction method, the preparation process involved in a change process of soluble PdCl2 species to palladium oxide hydrate (PdO·H2 O) precipitation, which generally results in aggregation of Pd–NPs due to the trend of the agglomeration of PdO·H2 O–NPs. In this work, the highly dispersed Pd/C catalyst is prepared by controlling the dispersion of carbon supported PdO·H2 O–NPs. Briefly, Vulcan XC-72 carbon supported PdO·H2 O–NPs with high dispersion can been easily achieved in presence of 1,4-butylenediphosphonic acid (H2 O3 P-(CH2 )4 -PO3 H2 , BDPA) by adjusting solution pH. Then, PdO·H2 O–NPs are reduced by NaBH4 to generate Pd–NPs with high dispersion. As a result, the resulting Pd/C catalyst shows an

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excellent electrocatalytic performance for formic acid electrooxidation. 2. Experimental 2.1. Reagents and chemicals Vulcan XC-72 carbon was obtained from Cabot Company (Boston, USA). Palladium chloride and formic acid were purchased from Sinopharm Chemical Reagent Co., Ltds. (Shanghai, China). 1,4-Butylenediphosphonic acid (H2 O3 P-(CH2 )4 -PO3 H2 , BDPA) was prepared according to the literatures [32,33]. All other reagents were of analytical grade and used without further purification. All the aqueous solutions were prepared with Millipore water having a resistivity of 18.2 M (Purelab Classic Corp., USA). The solution pH was adjusted by the addition of dilute NaOH or HCl solution. 2.2. Preparation of Pd/C catalysts Pd/C catalyst with 20.0 wt% metal Pd was prepared by following procedure. 3.0 mL 0.045 M PdCl2 , 3.0 mL 0.045 M BDPA and 60 mg Vulcan XC-72 carbon were added into 10 mL water. After the suspension was sonicated for 30 min, the pH of suspension was slowly adjusted to 6, 9 and 12 using NaOH solution under strongly continuous stirring condition, respectively. After the resulting suspensions were mechanically stirred for 1 h, the appropriate amounts of NaBH4 solution was added to the suspension and stirred for an additional 1 h. Finally, the mixture was washed, filtered and dried in a vacuum oven at 50 ◦ C for 12 h. Consequently, Pd/C catalysts prepared at pH 6, 9 and 12 were referred to as Pd/C-6, Pd/C-9 and Pd/C-12 catalysts, respectively. As comparison, Pd/C-NaBH4 catalyst was also prepared by the conventional NaBH4 liquid phase reduction method at pH 9 in the absence of BDPA, and all other procedures were the same as those for the preparation of Pd/C catalysts above. 2.3. Physical characterizations X-ray diffraction (XRD) measurements of catalysts were performed with Model D/max-rC diffractometer using Cu-K␣ radiation ( = 0.15406 nm) and operating at 45 kV and 100 mA. The morphology and particle size of Pd/C catalyst were investigated using a JEOL JEM-2010 transmission electron microscope (TEM) operated at 200 kV accelerating potential. UV–vis spectra were recorded at room temperature on a UV3600 spectrophotometer (Shimadzu, Japan) equipped with 1.0 cm quartz cells. Fourier transform infrared measurements (FT-IR) were carried out on a Nicolet 520 SXFTIR spectrometer using KBr pellets. FT-IR spectra were collected in the wave number range between 400 and 4000 cm−1 over 128 scans at a resolution of 4 cm−1 . The metal loading in catalyst was accurately determined with a Leeman inductively coupled plasma atomic emission spectrometry (ICP-AES). Zeta potential analysis was performed with a PALS Zeta potential analyzer, Version 3.43 (Brookhaven Instruments Corp.). The pH measurements were carried out using a METTLER TOLEDO DETTA320 Digital pHmeter. 2.4. Electrochemical measurements Electrochemical measurements were performed by using a CHI 600C electrochemical analyzer (CH Instruments, Shanghai Chenghua Co.) in a conventional three-electrode electrochemical cell. A Pt plate auxiliary electrode and a saturated calomel reference electrode (SCE) were used. All potentials in this study were reported with respect to SCE. For the preparation of working electrode, the typical process followed the previous procedure reported [34]. 4 mg

Scheme 1. Formation mechanism of Pd/C catalyst.

of the catalysts as-prepared and 1.6 mL H2 O were thoroughly mixed by sonication for 30 min to generate an evenly distributed suspension, and 4 ␮L of the resulting suspension was laid on the surface of the pre-cleared glassy carbon electrode (3 mm diameter, 0.07 cm2 ). After drying at room temperature, 3 ␮L Nafion 5.0 wt% solution was covered on the surface of the catalyst electrode and allowed to dry again. Thus, the working electrode was obtained, and the specific loading of Pd metal on the electrode surface was about 28 ␮g cm−2 . Prior to the electrochemical measurements, N2 was bubbled through the solution for 10 min to remove the dissolved O2 . During experiments, a continuous N2 flow was maintained over the solution. All the electrochemical measurements were carried out at 30 ± 1 ◦ C. 3. Results and discussion 3.1. Formation mechanism of the Pd–NPs Carbon supported Pd–NPs catalyst is prepared through a PdII → PdO·H2 O → Pd0 reaction path in the presence of BDPA. Scheme 1 schematically depicts the formation mechanisms of Pd–NPs. The method is essentially a two-step process. The purpose of the first step is adjusting pH value of PdCl2 solution to obtain phosphonic acid-functionalized PdO·H2 O–NPs in the presence of BDPA. In this step, one phosphonate group of BDPA molecule adsorb on the PdO·H2 O–NPs surface by P–O–Pd coordination, while the other phosphonate group stabilize PdO·H2 O–NPs through electrostatic repulsion between the negatively charged phosphonate groups. In the second step, the formed phosphonic acid-functionalized PdO·H2 O–NPs are reduced to Pd–NPs by using NaBH4 as reducing agent, and the adsorbed phosphonate anion are removed by using 0.1 M NaOH solution. For PdCl2 aqueous solution, [PdCl4 ]2− species easily transform into the highly insoluble red hydroxides with increasing solution pH due to hydrolysis [35–43]. After BDPA was added into PdCl2 solution, the UV–vis characteristic absorption peaks of PdCl2 solution kept constant (Fig. 1), indicating coordination interaction between BDPA with PdCl2 molecules do not occur under the present experimental conditions. As expected, the yellow color BDPA/PdCl2 mixture solution rapidly changes to red color by increasing pH value of BDPA/PdCl2 mixture solution to 9 (Fig. 2A), which is likely attributed to appearance of hydroxides colloid. After the red color colloid solution is kept in air for 72 h, the red precipitation appears gradually (Fig. 2B). According to XRD data of red precipitation and JCPDS standard 09-0254 (PdO·H2 O) (Fig. 2C), we can confirm that the red precipitation is PdO·H2 O species. Since the general NaBH4 reduction method involves in a change process of soluble PdCl2 species to PdO·H2 O precipitation during the adjustment of solution pH, the dispersion of PdO·H2 O–NPs on the carbon surface is the vital premise for preparation of Pd/C catalyst with high dispersion. The previous studies have demonstrated that the phosphonic acid groups (–PO3 H2 ) have a strong affinity to metal oxide through stable M–O–P bonds, such as ZrO2

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16 14

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Wavelength/nm Fig. 1. UV–vis absorption spectra of (a) 1.0 × 10−4 M PdCl2 solution (pH 4.6) and (b) the mixture solution of 1.0 × 10−4 M BDPA and 1.0 × 10−4 M PdCl2 (pH 4.6).

Fig. 2. (A) Photograph of BDPA/PdCl2 mixture solution at pH 9. (B) Photograph of red flocculent precipitation, which is obtained by keeping the resulting BDPA/PdCl2 mixture solution (pH 9) in air for 72 h. (C) XRD pattern of the red flocculent precipitation obtained in B. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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Fig. 3. FT-IR spectra of (a) PdO·H2 O precipitation obtained in the absence of BDPA, (b) PdO·H2 O precipitation obtained in the presence of BDPA, and (c) pure BDPA sample.

and TiO2 . [44–47]. Compared to pure PdO·H2 O sample prepared in the absence of BDPA (Fig. 3 curve a), PdO·H2 O sample obtained in the presence of BDPA shows the characteristic peaks of phosphonic acid groups located at 950–1300 cm−1 [48] (Fig. 3 curve b), which is consistent with that of pure BDPA (Fig. 3 curve c). In each of FT-IR spectra curve, the peaks between 1260–1700 cm−1 and 3500 cm−1 correspond to the bending and the stretching vibrations peaks of hydroxyl [49]. Thus, FT-IR spectra measurements indicate that BDPA molecules absorb on the PdO·H2 O–NPs surface. It is well known that NPs stabilization likely involves both charge stabilization by surface-adsorbed anions such as polyoxoanions and citrate, and steric stabilization by polymers such as poly(vinylpyrrolidone) [50–52]. Since phosphonic acid (–PO3 H2 ) group is a dibasic acid, the electrostatic repulsion between –PO3 2− species is expected to be much stronger than that between –COO− or –SO3 − groups according to Coulomb’s law [53]. Moreover, –PO3 H2 groups are extremely hydrophilic [54]. Such a strong electrostatic repulsion together with the excellent hydrophilic property of the –PO3 2− groups may facilitate the dispersion of the PdO·H2 O–NPs [53]. As expected, PdO·H2 O precipitation generate immediately once adjusting pH value of single-component PdCl2 solution to 9 (data not shown). As comparison, the obtained phosphonic acid-functionalized PdO·H2 O–NPs shows a good colloidal stability in aqueous solution after the pH value of BDPA/PdCl2 mixture solution is adjusted to 9 (Fig. 2A). Considering the strong electrostatic interaction between phosphonic acid-functionalized PdO·H2 O–NPs, it is reasonably expected that the phosphonic acidfunctionalized PdO·H2 O–NPs can be highly dispersed uniformly on the carbon support surface during the preparation of Pd/C catalyst. As confirmed by TEM images (Fig. 4), the spherical phosphonic acidfunctionalized PdO·H2 O–NPs are highly dispersed on the carbon surface, while the dispersion of pure PdO·H2 O–NPs on the carbon surface is uneven with heavy agglomeration. Although Pd–NPs with very narrow size and shape distribution were made easily in the presence of some macromolecular polymer ligands, the residual macromolecules adsorbed on the NPs surface generally undermined the overall electrocatalytic activity of Pd–NPs due to the lack of a “clean”, polymer-free NPs surface [12,55,56]. Compared to macromolecular polymer ligands, micromolecular ligands could be removed readily, leaving a clean Pd–NPs surface for catalytic evaluation [12,56]. In the present work, BDPA was chosen as stabilizer because of its other particular chemical property. The previous reports indicated that M–O–P bond between phosphonic acid groups and metal oxide broke in solution with high

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(2 0 0), (2 2 0) and (3 1 1) planes of face-centered cubic crystalline Pd, respectively [JCPDS standard 05-0681(Pd)], indicating that the Pd–NPs in each Pd/C catalyst present the face centered cubic structure. In order to avoid disturbance of the diffraction peak of the carbon support, the average size of the Pd–NPs are calculated from the half peak widths of the Pd (2 2 0) peak according to Scherrer equation [57,58]. The average sizes of the Pd–NPs in the Pd/C-6, Pd/C-9, Pd/C-12 and Pd/C-NaBH4 are calculated to be 3.1, 2.5, 3.3 and 3.5 nm, respectively.

pH value [45]. After NaBH4 reduction, the residual BDPA adsorbed on the Pd–NPs surface may be removed by rinse with excess 0.1 M NaOH solution. Fig. 5 shows the characteristic peaks of phosphonic acid groups (850–1200 cm−1 ) almost is not observed at the Pd–NPs sample compared to FT-IR spectrum of phosphonic acidfunctionalized PdO·H2 O–NPs, confirming that BDPA adsorbed on the Pd–NPs surface is efficiently removed by washing with 0.1 M NaOH solution. 3.2. Physicochemical characterizations of Pd/C catalysts 3.2.1. XRD characterization The exact Pd loadings of catalysts were evaluated by ICP-AES analysis. All Pd/C catalysts contained about 21.0 wt% Pd, nearly the same as the pre-set values, indicating that PdII precursor was successfully reduced to form metallic Pd in our synthesis. The crystalline nature of the Pd–NPs on carbon surface was confirmed by recording XRD (Fig. 6). In each of XRD patterns, the peaks at about 25◦ is attributed to the C(0 0 2) characteristic diffraction peak of XC-72 carbon, whereas the other four peaks are indexed as (1 1 1),

Pd(111) C(002) Pd(200)

d

Intensity/a.u

Fig. 4. TEM images of XC-72 carbon-supported PdO·H2 O–NPs prepared (a) in the absence of BDPA and (b) in the presence of BDPA.

3.2.2. TEM characterization Fig. 7 shows TEM images and corresponding particle size distribution histograms of all Pd/C catalysts. As observed, the dispersion of Pd–NPs in the Pd/C-NaBH4 catalyst is uneven with heavy agglomeration, while the dispersion of Pd–NPs at Pd/C catalysts prepared in PdCl2 /BDPA system is much better than that of Pd/C-NaBH4 catalyst. According to distribution histogram of catalyst, the average sizes of the Pd–NPs in the Pd/C-6, Pd/C-9, Pd/C-12 and Pd/CNaBH4 catalysts are 3.0, 2.3, 3.2 and 3.6 nm, respectively. The statistical TEM measurement results indicate that the improved precipitation–reduction procedure used in this work is very efficient for the preparation of carbon-supported Pd–NPs. During

Pd(220)

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agglomeration of PdO·H2 O–NPs [23,55,59–62]. Thus, the improvement of the dispersion of PdO·H2 O–NPs will restrain the serious agglomeration of resulting metallic Pd–NPs. As mentioned above, the presence of BDPA can result in the generation of the phosphonic acid-functionalized PdO·H2 O–NPs with high colloidal stability due to strong electrostatic interaction. Consequently, the dispersion of Pd–NPs at Pd/C catalysts prepared in PdCl2 /BDPA system is much better than that of Pd/C-NaBH4 catalyst. Furthermore, TEM images show that the particle size of Pd–NPs in Pd/C catalysts obtained from PdCl2 /BDPA system changes with increasing reaction solution pH. As can be seen from Fig. 7E, the dependence of the particle size of NPs on reaction solution pH shows a V-shaped form. The particle size of Pd–NPs decrease with increasing reaction solution pH, goes through a minimum, and then increase as the solution pH increase. The minimum particle size seems to be obtained at pH 9. Moreover, it is observed from Fig. 7 that the spherical Pd–NPs on Pd/C-9 catalyst are highly dispersed on the carbon support, whereas Pd–NPs on Pd/C-6 and Pd/C-12 catalysts appear a little aggregation. Since particle size and dispersion of Pd/C catalyst depend on the particle size and dispersion of the PdO·H2 O–NPs, the effect of reaction solution pH on the particle size and dispersion of the PdO·H2 O–NPs should be responsible for the dependence of reaction solution pH on particle size and dispersion of Pd/C catalysts. With the increase of reaction solution pH, three factors likely influence the formation of PdO·H2 O–NPs. (i) The speed of pure PdO·H2 O–NPs growth increases with solution pH in the absence of BDPA. Thus, a high reaction solution pH is disadvantageous for the formation of PdO·H2 O–NPs with small particle size. (ii) The dissociation degree of phosphonate groups of BDPA is highly pH dependent and increases with the increasing solution pH. Consequently, the colloidal stability of phosphonic acid-functionalized PdO·H2 O–NPs increases with increasing pH due to increase of electrostatic repulsion, which is favorable for the formation of PdO·H2 O nanoparticles with small particle size. (iii) When solution pH reaches certain high pH, the BDPA adsorbed on PdO·H2 O–NPs surface may occur desorption due to the break of Pd–O–P bonds [45], which is unfavorable for the dispersion of the PdO·H2 O–NPs. The surface charge of phosphonic acid-functionalized PdO·H2 O–NPs depends on dissociation degree of phosphonate groups and amount of adsorbed phosphonate. Zeta potential measurements show the zeta potentials of phosphonic acid-functionalized PdO·H2 O–NPs in pH 6, 9, 12 are −8.1 mV, −24.8 mV and −12.8 mV, respectively, which confirms desorption of BDPA occurs in high pH solution. Therefore, the dispersion and particle size of the PdO·H2 O–NPs on the carbon support surface is dependent on the coherent contributions from three factors mentioned above, which may be responsible for in V-shaped dependence of the particle size of Pd/C catalyst on reaction solution pH. 3.3. Electrocatalytic activity of Pd/C catalysts for formic acid oxidation

Fig. 7. TEM images and corresponding size distribution histograms of (A) Pd/CNaBH4 , (B) Pd/C-6, (C) Pd/C-9, and (D) Pd/C-12 catalysts; (E) dependence of the particle size of Pd/C catalyst on reaction solution pH.

the preparation of Pd/C-NaBH4 , the corresponding reaction process involved in a change process of soluble PdCl2 species to PdO·H2 O precipitation in the course of the adjustment of solution pH. The highly insoluble hydroxides generally resulted in the serious agglomeration of metallic Pd–NPs due to the tendency towards

Fig. 8 shows the cyclic voltammograms of 0.5 M H2 SO4 solution at four different Pd/C catalysts. The features in the “hydrogen region” (multiple peaks), in the region between −0.25 and 0.05 V, were obtained at all Pd/C catalysts. In the positive scan direction, the first peak below −197 mV corresponds to the oxidation of absorbed hydrogen and the second one in the potential region −160 to 50 mV corresponds to the oxidation of adsorbed hydrogen [43]. Recently, it has been shown that electrochemically active surface area (ECASA) of Pd/C catalyst can also be calculated from the integrated reduction charge of surface palladium oxide besides CO-stripping method [63,64]. According to the Coulombic amount (Q) associated with the reduction peak area of palladium oxide, the ECASA values of Pd/C-NaBH4 , Pd/C-6, Pd/C-9 and Pd/C-12 catalysts are estimated to

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be 23.5, 68.9, 107.2 and 45.8 m2 g−1 Pd, respectively. The largest electrochemical specific surface may be due to the high dispersion and small particle size of Pd/C-9 catalyst [65], which is in accord with the result of TEM described above. Fig. 9A shows the cyclic voltammograms of 0.5 M HCOOH + 0.5 M H2 SO4 solution at four different Pd/C catalysts. Before recording the cyclic voltammograms, the electrodes were soaked in the test solution for 10 min to allow the system to reach a stable state under open circuit conditions [66,67]. In the positive scan direction, the two oxidative peaks of formic acid are located at 0.18 V and 0.53 V for all electrodes, which correspond with the two pathways of the formic acid oxidation (i.e., the direct pathway and the CO pathway), respectively [14]. Moreover, it is observed that the formic acid oxidation current at 0.18 V for the Pd/C-NaBH4 , Pd/C-6, Pd/C-9 and Pd/C-12 catalysts are 332.5, 1228.3, 1426.5 and 1091.1 A g−1 Pd, respectively, indicating that Pd/C catalysts prepared by the present precipitation–reduction method exhibit much better electrocatalytic performance than the counterpart catalyst prepared by the general NaBH4 reduction method. For example, the formic acid oxidation current at Pd/C-9 catalyst is about four times larger than that of Pd/C-NaBH4 catalyst, which is attributed to the small particle size and excellent dispersion of Pd/C-9 [68]. Previous reports have demonstrated specific size effect of Pd–NPs on formic acid electrooxidation, and have given the optimal particle size which possessed better specific activity (normalized to electrochemically active surface area) [20,23,52,69]. To further study the electrocatalytic performance of the different Pd/C catalysts in detail, the specific activity of the catalysts are calculated from the mass activity and ECASA, which are provided in Fig. 9B and Table 1. As evident, the specific catalytic activity of the 2.3 nm Pd–NPs did not exceed that of the 3.0 nm and 3.2 nm Pd–NPs, which better rationalizes the effect of Pd particle size on formic acid electrooxidation. Recently, Xia et al. indicated that the mass activity of Pd/C catalyst is a more preferential factor for direct formic acid fuel cells application compared to the specific activity of Pd/C catalyst [50].

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E vs SCE/V Fig. 9. (A) Cyclic voltammograms of (a) Pd/C-NaBH4 , (b) Pd/C-6, (c) Pd/C-9 and (d) Pd/C-12 catalysts in 0.5 M HCOOH + 0.5 M H2 SO4 solution at the scan rate of 50 mV s−1 . (B) Corresponding cyclic voltammograms normalized using the Pd electrochemically active surface area (A m−2 Pd). The electrochemically active surface areas of the Pd in catalysts are estimated from the integrated reduction charge of surface palladium oxide in Fig. 8.

Table 1 Comparison of electrocatalytic performance of Pd/C-NaBH4 , Pd/C-6, Pd/C-9 and Pd/C-12 catalysts for formic acid oxidation. Catalysts

ECASA, m2 g−1 Pd

Mass activity, A g−1 Pd

Specific activity, A m−2 Pd

Pd/C-N Pd/C-6 Pd/C-9 Pd/C-12

23.5 68.7 107.2 45.8

332.5 1228.3 1426.5 1091.1

14.2 17.9 13.3 23.8

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Thus, the considerable efforts should be focused on improvement of the noble metal utilization, namely the mass activity of Pd/C catalyst. To further evaluate the electrocatalytic performance of Pd/C catalysts, the chronoamperometry tests were conducted. Fig. 10 shows the chronoamperometric curves of 0.5 M HCOOH + 0.5 M H2 SO4 solution at the different Pd/C catalysts at a potential of 0.15 V. As revealed, formic acid oxidation current at the all Pd/C catalysts decayed rapidly in the initial period of the experiments, suggesting that all catalysts are easily poisoned. In addition, it is observed that formic acid oxidation current at the Pd/C-NaBH4 , Pd/C-6, Pd/C9 and Pd/C-12 catalyst electrodes at 1000 s are 49.4, 108.4, 144.2 and 92.0 A g−1 Pd, respectively, further confirming that the electrocatalytic performance of Pd/C-9 catalyst is the best among all Pd/C catalysts. 4. Conclusions Since the general NaBH4 reduction method involved in a change process of soluble PdCl2 species to PdO·H2 O precipitation in the course of the adjustment of solution pH, we propose a novel precipitation–reduction method to synthesize Pd/C catalyst. In the resulting PdII → PdO·H2 O → Pd0 reaction path, PdO·H2 O–NPs with high dispersion and small particle size can easily be obtained in the presence of BDPA, which results in the formation of Pd/C catalyst with high dispersion and small particle size. TEM characterization confirmed that the average particle size of Pd/C-9 catalyst was 2.3 nm and without visible agglomerations. Electrochemical tests further indicated that the electrocatalytic performance of Pd/C9 catalyst for formic acid electrooxidation was 4 times greater than that of Pd/C-NaBH4 catalyst prepared by the conventional liquid phase reduction method. This significant improvement was attributed to larger surface areas of Pd/C-9 catalyst due to the smaller particle size and uniform size dispersion. Acknowledgments The authors are grateful for the financial support of National Natural Science Foundation of China (20873065, 21073094, 21005039), Natural Science Foundation of Jiangsu Higher Education Institutions of China (10KJB150007), and The Priming Scientific Research Foundation for Advanced Talents in Nanjing Normal University (2010103XGQ0057). References [1] V.D. Noto, E. Negro, S. Lavina, S. Gross, G. Pace, Electrochim. Acta 53 (2007) 1604. [2] J. Ge, X. Chen, C. Liu, T. Lu, J. Liao, L. Liang, W. Xing, Electrochim. Acta 55 (2010) 9132. [3] R.S. Jayashree, J.S. Spendelow, J. Yeom, C. Rastogi, M.A. Shannon, P.J.A. Kenis, Electrochim. Acta 50 (2005) 4674. [4] H. Qiao, H. Shiroishi, T. Okada, Electrochim. Acta 53 (2007) 59. [5] C.V. Rao, B. Viswanathan, Electrochim. Acta 55 (2010) 3002. [6] S. Thanasilp, M. Hunsom, Electrochim. Acta 56 (2011) 1164. [7] S. Tominaka, T. Momma, T. Osaka, Electrochim. Acta 53 (2008) 4679. [8] S. Wang, S.P. Jiang, T.J. White, X. Wang, Electrochim. Acta 55 (2010) 7652. [9] J. Zhao, A. Sarkar, A. Manthiram, Electrochim. Acta 55 (2010) 1756. [10] Y. Zhou, J. Liu, J. Ye, Z. Zou, J. Ye, J. Gu, T. Yu, A. Yang, Electrochim. Acta 55 (2010) 5024. [11] Z. Liu, L. Hong, M.P. Tham, T.H. Lim, H. Jiang, J. Power Sources 161 (2006) 831. [12] V. Mazumder, S. Sun, J. Am. Chem. Soc. 131 (2009) 4588. [13] C. Rice, S. Ha, R. Masel, P. Waszczuk, A. Wieckowski, T. Barnard, J. Power Sources 111 (2002) 83. [14] R. Larsen, S. Ha, J. Zakzeski, R.I. Masel, J. Power Sources 157 (2006) 78.

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