ORIGINAL PAPER

Electrooxidation of formic acid catalyzed by PdNanoparticles supported on multi-walled carbonnanotubes with sodium oxalate

Zhengyu Bai & Huiying Yan & Feifei Wang & Lin Yang &

Kai Jiang

Received: 12 June 2012 /Revised: 30 June 2012 /Accepted: 12 July 2012 /Published online: 8 August 2012# Springer-Verlag 2012

Abstract This paper repots a highly catalytic palladiumnanoparticle catalyst dispersed on the purified multi-walledcarbon nanotubes (P-MWCNTs) for the electrooxidation offormic acid, in which sodium oxalate is employed as both adispersant and a coordination agent. The nanostructuredcatalysts have been characterized by X-ray diffraction tech-nique and transmission electron microscopy. It is found thatthe as-prepared face-centered cubic crystal Pd nanoparticlesare uniformly dispersed on the surface of MWCNTs with anaverage particle size of 5.6 nm. Fourier transform infraredspectroscopy and thermogravimetric analysis revel that so-dium oxalate is a tractable ligand with the aid of a suitablesolution. Cyclic voltammetry and chronoamperometry testsdemonstrate that the obtained Pd/P-MWCNT catalyst fromtypical experiment has better catalytic activity and stabilityfor formic acid electrooxidation than acid-oxidationtreatment MWCNT (AO-MWCNT)-supported Pd catalystfrom the control experiment. Therefore, the as-preparedPd/P-MWCNTs would be a potential candidate as an anodeelectrocatalyst in direct formic acid fuel cells.

Keywords Sodium oxalate . Palladium . Formic acidoxidation

Introduction

Fuel cells are attractive option for power generation due totheir high efficiency and little or no pollution. Amongvarious types of fuel cells, direct formic acid fuel cell(DFAFC) is considered to be a promising system for auto-motive and portable electronic applications owing to its highenergy density and low operating temperature [1–3].DFAFC has advantages over the direct methanol fuel cellbecause it can achieve a higher power density and formicacid is nontoxic, although the energy density of methanol ishigher than that of formic acid [4, 5]. Considering efficiencyand cost requirements [6, 7], one major challenge is arisingfrom the electrocatalyst of the DFAFCs. To solve theseproblems, many investigations in this field focused on theexploration of less expensive, more abundant non-platinumcatalysts that can offer acceptable performance. Recently,Pd catalyst was found to possess superior performancesin formic acid oxidation compared with Pt-based catalysts[8, 9].

It is well-accepted that fabricating catalyst nanoparticlesimmobilized on a suitable support are an ideal goal formaximizing the utilization rate of Pd catalyst, and thereby,the choice of a suitable support is one of the key factorsaffecting the performance of the catalysts [10, 11]. In thisregard, multi-walled carbon nanotubes (MWCNTs) are con-sidered to be the ideal electrocatalyst support because theypossess a large surface area, good thermal and chemicalstability, as well as great electrical conductivity [12]. Be-cause of the chemical inertia of CNTs, it is necessary toactivate the graphitic surface of the nanotubes in order toanchor and deposit catalyst nanoparticles [13]. Hitherto, themost popular means for the activation of CNTs is harshoxidative treatment; however, it is too complex and mayeven impair the mechanical properties of MWCNTs, such as

Ionics (2013) 19:543–548DOI 10.1007/s11581-012-0779-8

Z. Bai :H. Yan : F. Wang : L. Yang (*) :K. JiangCollege of Chemistry and Environmental Science,Henan Normal University,Xinxiang 453007, Chinae-mail: yanglin1819@163.com

Z. Bai :H. Yan : F. Wang : L. Yang :K. JiangKey Laboratory of Green Chemical Media and Reactions,Ministry of Education,Xinxiang 453007, China

electrical conductivity and corrosion resistance [14]. Toovercome the disadvantages, some new strategies are car-ried out, which have involved the utilization of surfacemodification with surfactants [15] and polymers [16] asstabilizing agents. For example, 1-aminopyrene and ionic-liquid polymer have been used to functionalize theMWCNTs by introducing the functional groups onto thesurface of MWCNTs instead of harsh oxidative treatment[16, 17]. However, the surfactants may not be readily re-moved from the system, and an important change in thecharacter of the surfactants would take place under hightemperature conditions during the work process of DFAFCs.To address these issues, we attempt to develop a simplemethod to control particle size of catalyst and increase thePd loading on the supports via a facile strategy, using atractable ligand with the aid of a suitable solution.

In the present study, Pd electrocatalysts with smaller parti-cle size and better dispersion were prepared in the presence ofsodium oxalate through a facile process at room temperature.The research results showed that sodium oxalate could makethe Pd nanoparticles homogeneously anchored onto theMWCNTs, and it could be removed easily in the subsequentprocess. All of these may make Pd nanoparticles highly dis-persed on the surface of MWCNTs with smaller particle size.The results of cyclic voltammetry and chronoamperometrytests demonstrated that the catalyst exhibited high catalyticactivity and good stability in the oxidation of acid oxidation.

Experimental section

Materials

Potassium borohydride (KBH4), sodium oxalate, ethanol,formic acid, sulfuric acid, and nitric acid were purchasedfrom China National Pharmaceutical Group Corp. PdCl2was obtained from Alfa Aesar, and the pristine MWCNTswere provided by research group of Prof. Zhang HB inXiamen University. All chemical reagents used in this ex-periment were of analytical grade and were used as receivedwithout further purification. Double-distilled water (DDwater) was used in all of the experiments.

Catalyst preparation

Typical experiment: preparation of a Pd/P-MWCNTelectrocatalyst in the presence of sodium oxalate

Briefly, MWCNTs was pretreated in 1.0 M HCl underconstant stirring for 2 h then washed with DD water anddried at 40 °C in vacuum condition for 6 h. One hundredmilligrams of sodium oxalate and 40 mg of purifiedMWCNTs were dispersed in 80 mL of ethanol and DD

water (v/v01:1) solution, and then, an appropriate amountof H2PdCl4 solution was added. After the pH was adjustedto near 9 using 0.1 M NaOH, the system was ultrasonicatedfor 20 min to obtain a homogeneous suspension. Subse-quently, a freshly prepared solution of KBH4 (80 mg in80 mL DD water) was added dropwise into the abovesystem under stirring, and the formed suspension was stirredfor another 2 h to make sure the complete reduction of Pdions. Finally, the product was collected by filtration andwashed several times with DD water and ethanol then driedat 40 °C under vacuum condition for 6 h. The obtained Pdcatalyst on P-MWCNTs was labeled as Pd/P-MWCNTs.

Control experiment: preparation of a Pd/AO-MWCNTelectrocatalyst

MWCNTs were pretreated at 80 °C in mixed acid solution(H2SO4/HNO3 in 1:1v/v ratio) for 8 h, then washed with DDwater and dried at 40 °C in vacuum condition for 6 h. Forcomparison, Pd/acid-oxidation treatment MWCNT (AO-MWCNT) catalyst was prepared by using the AO-MWCNTsas a support under the same conditions described as the typicalexperiment. In both of the catalysts, a total metal loading of20 wt% of Pd was obtained by controlling the PdCl2 solution.

Characterization of Pd/MWCNT electrocatalyst

The morphology of the catalysts was determined by JEOL-100CX high resolution transmission electron microscopyoperated at 200 kV. Powder X-ray diffraction (XRD) patternwas recorded on D/max-2200/PC X-ray diffractometer withCuKα radiation source. UV–Vis absorption spectra (TU-1901, China) were employed to analyze the interactionsbetween Pd ions and oxalate. The Fourier transform infrared(FT-IR) spectra were recorded on a Bio-Rad FTS-40 FT-IRspectrometer in the wave number range of 4,000–400 cm−1,and the thermogravimetric analyses (TGA) were performedon an NETZSCH STA 449C instrument.

Electrochemical behavior of Pd/MWCNT electrocatalyst

Cyclic voltammetry measurements were carried out in athree-electrode cell by using Solartron 1287 electrochemicaltest system (Solartron Analytical, England). A glassy carbondisk (3 mm o.d.) coated with catalyst was used as theworking electrode, a platinum foil (1 cm−2) as the counter-electrode, and an Ag/AgCl electrode as the reference. Themetal loading of the as-prepared catalysts on glassy carbonelectrode is 0.386 mg cm−2. H2SO4 aqueous solution(0.5 M) was used as electrolyte for hydrogen oxidationmeasurements and 0.5 M HCOOH containing 0.5 MH2SO4 for formic acid oxidation measurements, respectively.High-purity N2 was bubbled into the electrolyte during the

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experiments. All electrochemical measurements were per-formed at 25±1 °C.

Electrochemical CO stripping voltammograms wereobtained by oxidizing preadsorbed CO (COad) in 0.5 MH2SO4 at a scan rate of 20 mV s−1. CO was purged through0.5 M H2SO4 for 30 min to allow complete adsorption ofCO onto the catalyst. The working electrode was stayed at0.1 V (vs. Ag/AgCl), and excess CO in the electrolyte wasremoved by purging with high-purity N2 for 30 min. Theamount of COad was evaluated by integrating the COad-stripping peak and correcting for the capacitance of theelectric double layer. The activity of the catalysts in theoxidation of formic acid was evaluated in a solution con-taining 0.5 M H2SO4 and 0.5 M HCOOH, and cyclic vol-tammetry measurement was performed by applying a linearpotential scan at a sweep rate of 50 mV s−1.

Results and discussion

In this work, Pd nanoparticles supported on P-MWCNTswere prepared through a two-step facile process at room

temperature. Firstly, the Pd2+ was mixed and coordinatedwith oxalate. Subsequently, the Pd2+ was in situ reduced toPd nanoparticles with KBH4 as the reducing agent. Herein,the stabilizing effect of the sodium oxalate may be attributedto its carboxyl anions, which can coordinate with Pd2+.

Figure 1 shows the transmission electron microscopy(TEM) images and the size frequency curve of the resultingsample from the typical experiment. As shown in the Fig. 1a,it can be seen that P-MWCNTs are covered with a continuousPd nanoparticle, which are well-separated from each other andextend the overall length of MWCNTs. The nanoparticlediameters from the amplificatory TEM image (Fig. 1b) rangefrom 3.8 to 7.8 nm and the mean size calculated by thelognormal distribution is 5.6 nm (Fig. 1c). To investigate theinfluence of sodium oxalate on the formation of Pd nano-particles, a control experiment was carried out under the sameconditions described as the typical experiment, apart fromacid-treated MWCNTs as a support and in the absence ofsodium oxalate. Figure 2 shows the TEM image of the Pdnanoparticles deposited on AO-MWCNTs. However, the dis-persion of Pd nanoparticles on AO-MWCNTs is characterizedby a poor distribution with some aggregates, instead of gooddispersion of the typical sample. The results demonstrate thatthe presence of sodium oxalate is a key factor in controlling

Fig. 1 a–c TEM images and corresponding size distribution of the synthesized Pd/P-MWCNT catalysts

Fig. 2 TEM image of the synthesized Pd/AO-MWCNT catalysts Fig. 3 XRD patterns of the as-prepared Pd/P-MWCNTs

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the size of the Pd nanoparticles. The stabilizing property ofsodium oxalate may be rendered by its two carboxyl anions.During the reaction process, the two carboxyl anions mayadsorb the metal particles and stabilize them, then preventthem from growth and agglomeration. Therefore, it may con-tribute to the synthesis of Pd nanoparticles with a smallparticle size.

Figure 3 shows the XRD patterns of the as-prepared Pd/P-MWCNTs from the typical experiment. Five peaks at39.8°, 46.3°, 67.6°, 81.9°, and 86.1° are characteristics offace-centered cubic crystalline Pd, which are correspondingto the facets (111), (200), (220), (311), and (222), respec-tively, except for the (002) plane characteristic peak of theMWCNTs support around 25°.

The UV–Visible spectroscopy analysis indicates that thePd2+ can coordinate with oxalate easily, while the effectbetween oxalate and P-MWCNTs is negligible. Figure 4displays the UV–Visible spectra of H2PdCl4, oxalate–Pd

2+,oxalate–Pd2+/P-MWCNTs, and sodium oxalate in ethanoland DD water (v/v01:1) solution, respectively. As observed

in curve a, the aqueous solution containing [PdCl4]2− ions

shows two characteristic peaks at ca. 208 and 237 nm (seecurve a), which are assigned to the ligand-to-metal chargetransfer transitions [18]. Compared with curve a, the char-acteristic adsorption peak at ca. 208 nm shifts to 225 nm,and the weak peak of 237 nm disappears (see curve b).Therefore, a red shift in the characteristic adsorption peaksis detected after addition of aqueous sodium oxalate to thesolution. These may due to the presence of coordinationinteraction between Pd2+ and carboxyl anions of sodiumoxalate, which might contribute to the formation of the Pdnanoparticles with a much more uniform size and distribu-tion. Obviously, the existence of MWCNTs has no signifi-cant influence on the coordination interaction between Pd2+

and carboxyl anions of sodium oxalate (see curve c). Asobserved in curve d, the sodium oxalate is characterized by amajor peak at ca. 195 nm assigned to carboxyl anions.

From the study of FT-IR spectroscopy and TGA curves,it indicates that there is no sodium oxalate existing in theresulting sample. Figure 5a displays the FT-IR spectra ofpure sodium oxalate (curve 1) and the as-prepared Pd/P-MWCNTs (curve 2). As observed in curve 1, pure sodiumoxalate is characterized by major peaks at 1,633 and1,407 cm−1 assigned to the unsymmetrical stretching vibra-tion and stretching vibration of COO−, and peaks at 1,335and 785 cm−1 are due to C–C stretching vibrations. Theabsorption at 514 cm−1 shows the presence of oxygen metalbonds [19]. However, there are almost no obvious peaks ofsodium oxalate in the FT-IR spectra of as-prepared Pd/P-MWCNT catalysts (see curve 2). It is evident that redundantsodium oxalate can be efficiently removed by washing withwater. The TGA curves shown in Fig. 5b compare thethermal–decomposition curves of pure sodium oxalate(curve 1) and the as-prepared Pd/P-MWCNTs (curve 2)under the protection of N2 at a heating rate of 10°/min from30 to 700 °C. From the TGA curve of pure sodium oxalate,

Fig. 4 The UV–Visible spectroscopy curves of H2PdCl4 (a), oxalate–Pd2+ (b), oxalate–Pd2+/P-MWCNTs (c), and sodium oxalate (d)

Fig. 5 a FT-IR spectroscopy and b TGA curves of pure sodium oxalate and the as-prepared Pd/P-MWCNT catalysts

546 Ionics (2013) 19:543–548

one-step weight loss attributed to the decomposition of puresodium oxalate can be observed obviously, which starts ataround 500 °C and ends at about 580 °C. Compared withcurve 1, the curve 2 exhibits little weight loss, indicating theabsence of sodium oxalate in the resulting sample. In con-clusion, sodium oxalate is a tractable stabilizer, which issuperior to macromolecule stabilizers.

The electrochemical behavior of different catalystswas recorded by cyclic voltammetry (CV) measurement,which was performed in 0.5 M H2SO4 electrolyte at a scanrate of 20 mV s−1. Figure 6 shows the cyclic voltammo-grams obtained with CO adsorbed onto the catalysts (solidcurves) and without adsorbed CO (dashed curves), of theas-prepared Pd/P-MWCNTs and Pd/AO-MWCNTs, respec-tively. Obviously, the hydrogen adsorption peak of the Pd/P-MWCNTs is larger than that of the Pd/AO-MWCNTs,indicating that the Pd/P-MWCNTs have a higher electro-catalytic activity. The corresponding electrochemically

active surface (EAS) of the catalyst was obtained fromEq. (1) [20]:

EAS ¼ Q

G� 420ð1Þ

where Q is the charge of CO desorption–electrooxidation inmicrocoulomb, G represents the total amount of Pd (inmicrogram) on the electrode, and 420 is the charge requiredto oxidize a monolayer of CO on the catalyst in micro-coulomb per square centimeter. The EAS value of the Pd/MWCNT catalyst (48.6 m2 g−1) is much larger than that ofthe Pd/AO-MWCNT catalyst (35.7 m2 g−1), and it also hashigher electrocatalytic activity than that of the previouslyreported Pd catalysts [21]. The larger EAS of the Pd/P-MWCNT catalyst might be attributed to the smaller sizeand better dispersion of the Pd nanoparticles.

The electrocatalytic activities for the formic acid oxida-tion of the as-prepared electrocatalysts were analyzed by CV

Fig. 6 Cyclic voltammograms for the oxidation of preadsorbed CO ofthe as-prepared catalysts from a Pd/P-MWCNTs and b Pd/AO-MWCNTs on glassy carbon electrode in 0.5 M H2SO4 aqueous

solution with a scan rate of 20 mV s−1 at 25 °C. Dashed curves wereCVs for these electrodes without COad

Fig. 7 Cyclic voltammograms of a Pd/P-MWCNTs and b Pd/AO-MWCNTs on glassy carbon electrode in 0.5 M H2SO4+1 M HCOOHaqueous solution with a scan rate of 50 mVs−1 at 25 °C

Fig. 8 Chronoamperometric curves of a Pd/P-MWCNTs and b Pd/AO-MWCNTs on glassy carbon electrode in 0.5 M H2SO4+1 MHCOOH aqueous solution at a potential of 0.1 V

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measurement in 0.5 M H2SO4+1 M HCOOH aqueous so-lution. The CV curves of the Pd/P-MWCNTs and Pd/AO-MWCNT catalyst electrodes are displayed in Fig. 7. Twomain peaks for the formic acid oxidation in both positiveand negative direction are observed at the both electrodes.Clearly, the maximum peak current density of the Pd/P-MWCNT catalyst is much higher than that of the Pd/AO-MWCNT catalyst, demonstrating further the higher catalyticactivity of the Pd/P-MWCNT catalyst than that of thePd/AO-MWCNT catalyst for the formic acid oxidation.

In order to compare the electrochemical stability of theas-prepared catalysts for formic acid oxidation, chronoam-perometry tests were carried out in 0.5 M H2SO4+1 MHCOOH aqueous solution at 0.1 V for 6,000 s (shown inFig. 8). Evidently, the Pd/P-MWCNT catalyst showsmuch higher anodic currents and much slower degrada-tion in currents, demonstrating better activity and stabilitythan that of the Pd/AO-MWCNT catalyst under the sameconditions.

Conclusions

In conclusion, the Pd/P-MWCNT catalyst for the oxidationof formic acid has been prepared through a facile chemicalreduction method at room temperature. Herein, sodium ox-alate makes a strong impact on the electrocatalytic activityof the catalyst through the formation of well-dispersed Pdnanoparticles with small size on the surface of unimpairedMWCNTs. In addition, sodium oxalate is a tractable stabi-lizer, which is superior to macromolecule stabilizers. There-fore, the dispersivity and the ESA of the Pd nanoparticlesare obviously enhanced in the presence of sodium oxalate,resulting in better electrocatalytic activity and utilizationefficiency of the catalyst. The above information impliedthat the as-prepared Pd/P-MWCNTs should be a good can-didate catalyst for DFAFC.

Acknowledgments This work was financially supported by the Na-tional Natural Science Foundation of China (grant no. 21171051 andgrant no. 61176004), the Science and Technology Program of HenanProvince (grant no.112102210005), the Science and Technology Foun-dation of He’nan Educational Committee (grant no. 12A150013) and thedoctor start-up fund of Henan Normal University.

References

1. Lim B, Jiang MJ, Camargo PHC, Cho EC, Tao J, Lu XM, ZhuYM, Xia YN (2009) Science 324:1302

2. Zhu YM, Ha SY, Masel RI (2004) J Power Sources 130:83. Kang SJ, Lee J, Lee JK, Chung SY, Tak Y (2006) J Phys Chem B

110:72704. Zhu YM, Khan Z, Masel RI (2005) J Power Sources 139:155. Jayashree RS, Spendelow JS, Yeom J, Rastogi C, Shannon MA,

Kenis PJA (2005) Electrochimica Acta 50:46746. Zhao J, Wang P, Chen WX, Liu R, Li X, Nie QL (2006) J Power

Sources 160:5637. Wang RF, Li H, Feng HQ, Wang H, Lei ZQ (2010) J Power

Sources 195:10998. Wang XM, Xia YY (2009) Electrochim Acta 54:75259. Bai ZY, Yang L, Li L, Lv J, Wang K, Zhang J (2009) J Phys Chem

C 113:1056810. Uchida M, Aoyama Y, Tanabe M, Yanagihara N, Eda N, Ohta A

(1995) J Electrochem Soc 142:257211. Rajesh B, Karthik V, Karthikeyan S, Thampi KR, Bonard JM,

Viswanathan B (2002) Fuel 81:217712. Britto PJ, Santhanam KSV, Ajayan PM (1996) Bioelectrochem

Bioenerg 41:12113. Lin JF, Kamavaram V, Kannan AM (2010) J Power Sources 195:46614. Hsin YL, Hwang KC, Yeh CT (2007) J Am Chem Soc 129:999915. Karousis N, Tagmatarchis N, Tasis D (2010) Chem Rev 110:536616. Wang SY, Wang X, Jiang SP (2008) Langmuir 24:1050517. Wu BH, Hu D, Kuang YJ, Liu B, Zhang XH, Chen JH (2009)

Angew Chem Int Ed 48:475118. Zhao SY, Chen SH, Wang SY, Li DG, Ma HY (2002) Langmuir

18:331519. Parekh BB, Vyas PM, Vasant SR, Joshi MJ (2008) Bull Mater Sci

31:14320. Zhao Y, Yang X, Tian J, Wang F, Zhan L (2010) J Power Sources

195:463421. Bai ZY, Guo YM, Yang L, Li L, Hu CG (2011) J Power Sources

196:6232

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