Electrochemical-reduction-assisted Assembly of Pd NPs/Polyoxometalates/ Graphene TernaryNanocomposite and Its Electrocatalytic Performance toward Formic Acid Oxidation①

LE Li-Jun ZHANG Xio-Feng HUANG Huo-Di ZHANG Yi LIN Shen

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Electrochemical-reduction-assisted Assembly of Pd NPs/Polyoxometalates/ Graphene TernaryNanocomposite and Its Electrocatalytic Performance toward Formic Acid Oxidation①

LE Li-Juana,bZHANG Xiao-FengaHUANG Huo-DiaZHANG YiaLIN Shena②

a(350007)b(350007)

A novel ternarynanocomposite, Pd nanoparticles (NPs)/polyoxometalates (POMs)/reduced graphene oxide (rGO), was prepared by a green, mild, electrochemical-reduction-assisted assembly. It is worth noting that the Keggin-type POM acts as an electrocatalyst as well as a bridging molecule. During the reduction process, POMs transfer the electrons from the electrode to GO, leading to a deep reduction of GO and the content of oxygen-containing groups is decreased to around 6.1%. Meanwhile, the strong adsorption effect between the POM clusters and rGO nanosheets induces the spontaneous assembly of POM on rGO in a uniformly dispersed state, forming a nanocomposite. The ternary Pd NPs/POMs/rGO nanocomposite exhibits higher electrocatalytic activities, better electrochemical stability, and higher resistance to CO poisoning than the Pd/C catalyst towards the formic acid oxidation(FAOR). Especially, the Pd/PW12/rGO exhibits the best electrocatalytic performance amongthree Pd/POMs/rGO composites (POMs = PW12, SiW12, PMo12).

electrochemical reduction, Pd, graphene, formic acid oxidation,polyoxometalates;

1 INTRODUCTION

Fuel cellsconvert chemical energy into electrical energy directlywithout combustion process. Re-cently, there has been significant demand for minia-turized fuel cells as battery replacements for stationary and portable electronic applications[1,2]. Direct formic acid fuel cells (DFAFCs) have emer-ged as a highly promising candidate for a com-mercially viable fuel cell feedstock owing to its higher energy densities, reduced toxicity, fast electro-oxidation kinetics, theoretical open-circuit potential, low operating temperature, and lower fuel crossover through polymer electrolyte membrane[3–7]. DFAFCs can avoid potential danger of explosive hydrogen and save cost for additional expenditure compared to hydrogen gas storage[2].

For anode catalysts applied in DFAFCs, pure Pdcatalysts have been extensively studied because of their lower cost and greater abundance than Pt, and higher catalytic activity for the formic acid electro-oxidation reaction (FAOR)[6-8]. However, recent progress shows that the adsorbed CO(COads) or a “CO-like” intermediate gradually build-up in the reaction will cause the deactivation of Pd catalysts. As a result, Pd catalysts possess a limited lifetime and require a periodic oxidative removal of these poisoning species to restore the full operating power, which has seriously restricted their practical appli-cation[9, 10]. Therefore, improving the resistance of Pd catalyst to the poisoning species is a preferable method for alleviating the deactivation of Pd catalyst compared with the oxidative treatment[9, 11].

The addition of antitoxic component as a coca-talyst improves the antitoxic ability of Pd catalyst. Polyoxometalates(POMs) can undergo a fast, rever-sible and stepwise multielectron transfer reaction while retaining an unchanged structure. Their multi-ple redox properties make them be attractive candi-dates for electrode modification, electrocatalysis and electroanalysis[12, 13]. Moreover, it is believed that Keggin-type POMs in an aqueous solution could effectively assist the electrochemical oxidation of carbon monoxide(CO) to carbon dioxide(CO2)[14-16]. However, the low specific surface area of POMs is a drawback for electrocatalytic activity, thus a suitable support material is required.

Recently, carbon materials have been proved as suitable matrices for the assembly of POMs owing to their excellent chemical stability and strong affinity for POMs.Reduced graphene oxide(rGO) not only is propitious to maximize the availability ofsurface area of supported nanoelectrocatalysts but also pro-vides efficient mass transport of reactants, products, and electrolytes[17]. Moreover, Pd NPs/rGO nano-composites arereported to have catalytic performa-nce in the FAOR[6, 18]. Although a large variety of conditions and protocols have been suggested for the synthesis of nanohybrids of metal NPs/rGO[19-21], it is still desirable to develop a green, mild and effective route that provides well-dispersed Pd NPs with good controllability and reproducibility without using hazardous reductants, e.g. hydrazine. POMs are con-sidered as novel green agents in many wet chemical processes due to their recyclability during the oxide-tion/reduction process.

Electrochemical reduction is a clean, facile, and suitable method for mass production, and thus is a promising approach[22]. However, direct oxidation or reduction of some substances at conventional electro-des is irreversible and requires high overpo-tential. Using chemicallymodified electrodes (CMEs) is an effective solution to minimize overvoltage effects[23, 24]. In recent years, our group[25, 26]has syn-thesized Pd/POM/graphene nanocompositesusing electro-chemical self-assembly.

Herein, an electrochemical-reduction assisted assembly method was used for preparing Pd NPs/POMs/rGOnanocomposite (Scheme1). The POM was worked as both electrocatalyst and bri-dging molecule. When electrolysis occurs, the POM can be further reduced to HPB, and then adsorbed on the surface of rGO; thus, POM/rGO hybrids were formed in the electrode. After that, Pd particles were deposited on the surface of the POM/rGO composite modified GCE or ITO electrode in situ by cyclic voltammetry electrodeposition in H2PdCl4solution at a suitablepotential. Finally, Pd NPs/POMs/rGO nanocomposite formed. The ternary nanocomposite is expected to show high electrocatalytic activities toward formic acid oxidationand and high resistance to CO poisoning.

Scheme 1.Process of electrochemical-reduction-assisted assembly ofPd/POMs/rGO nanocomposites

2 EXPERIMENTAL

2.1 Reagents and materials

All chemicals were analytical purity and used without further purification. Graphite powder (-325 mesh, 99.9995%), Pd/C(10 wt.% loading, matrix activated carbon support) and Nafion (117 solution) were purchased from Sigma Aldrich. H2O2(30%), PdCl2, KMnO4, P2O5, K2S2O8, H3PW12O40·H2O (PW12), H3PMo12O40·H2O (PMo12), H3SiW12O40·H2O (SiW12), acetone, H2SO4, ethanol and formic acid were all purchased from Sinopharm Chemical Reagent Co.Ltd.(Shanghai, China). Indium tin oxide (ITO) glass was purchased from CSG Holding Co. Ltd. (Shenzhen, China). Doubly distilled water was exclusively used in all aqueous solutions and rinsing procedures.

2.2 Synthesis of nanocomposite of Pd NPs/POMs/rGO

Graphene oxide(GO) was synthesized by a modi-fied Hummers' method[27]. In a typical synthesis, an aqueous solution of POM (1 mL, 5.0 mM) was mixed with an aqueous solution of GO (9 mL, 1.0 mg×mL-1) under the assistance of sonication and stirring to form a homogeneous suspension. Prior to modification, a GCE (glass carbon electrode) was carefully polished successively with 1, 0.3 and 0.05 mm-Al2O3slurries and sonicated in deionized water for 5 min after each polishing step. Finally, the GCE was sonicated and washed with ethanol. After cleaning, this GC electrode was immersed in the mixed suspension of POM and GO as the electrolyte solution with N2purging for 20 min. Cyclic voltammetry (CV) curves were recorded in a potential ranging from –0.8 V to 0.4 V at a scan rate of 0.05 V/s for 100 cycles to clean the electrodesurface. After the electrochemical reduction was completed, Pd particles were deposited on the surface of the PW12/rGO composite modified GCE electrode by cyclic voltammetry electrodeposition in an aqueous solution of H2PdCl4(10 mL, 2 mM at ?0.6 to 0.4 V). After Pd electrodeposition, the ternary nanocomposite (Pd NPs/POMs/rGO) was prepared. The subsequent electrodeposition on the as-obtained Pd NPs/POMs/rGO electrode followed the same procedure described above, except the differences concentration of POM.

2.3 Characterization

Transmission electron microscopy (TEM) images were obtained on a TECNAI G2F20 field emission transmission electron microanalyzer (FEI, USA). X-ray photoelectron spectroscopy (XPS) was performed at room temperature with monochromatic Alradiation (1486.6 eV)using a Quantum 2000 system (PHI, USA). The actual amounts of Pd, P and Wloadingof the catalysts were determined by induc-tively coupled atomic emission spectroscopy (ICP-AES, ICAP6300, Thermo Scientific USA). Field emission scanning electron microscopy (FESEM) images were obtained on a JSM-7500F field emis-sion scanning electron microanalyzer (JEOL, Japan). Raman spectra were measured using a Renishaw-in-Via Raman micro-spectrometer equipped with a 514 nm diode laser excitation on a 300 lines mm-1grating.

2.4 Electrochemistry experiments

All electrochemical experiments were conducted under 25℃ using a three-electrode cell by a CHI electrochemical workstation (CHI 660C, Shanghai Chenhua, China). A glass carbon electrode (GC with 3 mm in diameter) was used as a working electrode. A Pt wire electrode and an Ag/AgCl (3.0 M KCl) electrode were used as counter and reference electro-des, respectively. All potentials in this report were referred to Ag/AgCl. The electrolyte was saturated with high-purity nitrogen (N2) for at least 30 min and kept under a positive pressure of this gas during experiments. Pd/C modified electrode was prepared by pipetting 10 μL of a well-dispersed mixture (2 mg Pd/C catalyst dispersed in 1 mL ethanol with 50 μL 5% Nafion) on the polished GCE. Before all the measurements, the working electrodes were cycled in N2saturated 0.5 M H2SO4solution from-0.2 to 1.2 V at a scan rate of 50 mV×s-1in order to remove surfactant residues from the surface. The formic acid electrooxidation activity was also measured by CV scanning from ?0.2 to 1 V in a mixing N2saturated solution containing 1 mol×L-1HCOOH and 0.5 mol×L-1H2SO4at the same scan rate. To study the tolerance to CO poisoning, the CO stripping experiments were performed in a 0.5 mol×L-1H2SO4solution. At the beginning, the electrolyte was purged with N2, and then the electrode potential was fixed at-0.2 V vs. Ag/AgCl for CO adsorption along with the continuous CO bubbling for 15 min. At last, the stripping test was performed after purging the solution with N2for 10 min to remove the dissolved CO. In addition, CO voltammogram which came from the CO stripping experiments was also used to calculate the electrochemically active surface area (ECSA).To facilitate comparison of the catalytic activity across samples, the Pd loading on the working electrode for most of the samples was determined by ICP-AES measurements. Then, most of the electrochemical data were normalized to the mass of Pd.

3 RESULTS AND DISCUSSION

Raman spectroscopy is a useful technique for obtaining elaborate information about the structural properties of carbonaceous materials, including disorder and defect structures[28, 29]. For comparison, Raman spectra of GO and the as-prepared Pd/PW12/rGO, Pd/PMo12/rGO, Pd/SiW12/rGO were recorded. As shown in Fig.1, two fundamental vibrations attributed to the G (~1600 cm-1) and D (~1350 cm-1) bandsare observed for GO and Pd/POM/rGO. The G band corresponds to the scattering of the2gmode of2carbon atoms, while the D band originates from a breathing mode of a-pointphonon of1gsymmetry attributed to local defects anddisorder[30]. The intensity ratio of D and G bands,D/G, is a measure of disorder degree and average size of the2domains. It is shown that theD/Gratio is 1.06 (GO), 1.11 (Pd/PMo12/rGO), 1.24 (Pd/SiW12/rGO) and 1.33 (Pd/PW12/rGO), respec-tively, indicating the ascending degree of disorder due to the reductionof GO[31]. Therefore, the introduction of POMs is in favor ofthe more efficient reductionof graphene as shown in Fig.S1(Supporting Information).

Fig. 1. Raman spectra of Pd/PW12/rGO (a), Pd/SiW12/rGO (b), Pd/PMo12/rGO (c) and GO (d)

Fig. 2a and b show the C1XPS spectra of GO and Pd/PW12/rGO. As for GO (Fig. 2a), four types of XPS peaks ascribed to carbon with different chemical states are observed, namely,the peak at 284.8 eV is for graphite-like C, 286.8 eV for C–O, 287.8 eV for C=O and 288.8 eV for O–C=O, respectively[32-34]. After electroreduction, the content of C–O group decreases from the initial 52.6% to 6.1% for Pd/PW12(5mM)/rGO (see Table S1, Supporting Information), which reveals that the electroreduction can effectively eliminate the oxygen containing groups on GO. Meanwhile, the content of C–C/C=C group increased from 47.1% to 93.8%, indicating that significant3/2-hybridized carbon structures were restored. The oxidation state of Pd in NPs attached on rGO and the presence of W were also determined by XPS as shown in Fig. 2c～d. The Pd 3peak was split into 35/2and 33/2peaks. Distinct peaks located at around 335.5 and 340.6 eV areallocated to Pd0, while less intense doublets around 336.5, 337.9and 341.7, 342.2 eV correspond to 35/2and 33/2peaks of Pd2+andPd4+, respectively. The percentage of the Pd0, Pd2+and Pd4+species wascalculated by the relative areas of these peaks and it is 84.79%, 10.98% and 4.23%, respectively.This indicates that metallic Pd species in the compositeare mainly inzero-valent[33, 35]. The presence of W was also detected, andthe doublets with binding energies ofW47/2and W45/2are 35.61 and 37.89 eV[27], respectively, corresponding to W5+and W6+respectively.The XPS analysis above confirms the formation of tri-component hybrids of Pd/PW12/rGO.

Fig. 2. C 1XPS spectra of (a) GO and (b) as-prepared Pd/PW12/rGO; XPS spectra of Pd 3(c) and (d) W 4in the as-prepared nanohybrids

The surface morphologies of Pd/PW12/rGOandPW12/rGO were investigated by SEM (Fig. 3). As shown in Fig. 3a, PW12/rGO presents wavy mor-phology. As seen in Fig. 3b, the Pd nanoparticles are very small and emerge more uniform distribution. Moreover, the characteristic wrinkled morphology of rGO nanosheets can be found.

Fig. 3. SEM images of PW12/rGO (a) and Pd/PW12/rGO (b)

The size and distribution of Pd NPs deposited on PW12/rGO have been examined by TEM and HRTEM. From Fig. S2a(Supporting Information), Pd nanoparticles are well dispersed on the surface of rGO in the Pd/PW12/rGO composite. Fig. 4a and b showHRTEM and HRTEM enlarged images of Pd/PW12/rGO. It can be clearly seen that the spacing of the lattice fringes is about 0.225 nm, corres-ponding to the (111) planes of face centered cubic (fcc) Pd[6]. SAED pattern (Fig. 4c) is also providing quick and easy crystal orientation information of the obtained Pd. The lattice spacing measured from the diffraction rings of Pdmatched well with the XRD results(Fig.S2, Supporting Information), which demonstrated that the Pd possessesa face-centered crystalline structure. The diameters of Pd NPs observed from Fig. 4d range from 1.5 to 5 nm (determined from a statistical study of 100 NPs) and the mean size calculated by Nano Measurer software is 2.6 nm (Fig. 4d). Therefore, Pd NPs are well dispersed on PW12/RGO with a much more uniform size distribution and their smaller size will contribute to the good electrocatalytic activity and stability for the FAOR.

Fig. 4. (a) HRTEM, (b) HRTEM enlarged image and (d) corresponding particle size distribution histograms of the Pd/PW12/rGO nanocomposite. (c) SAED pattern of a single Pd particle

Moreover, the concentration of POMs in the composite catalyst has an effect on the electroca-talytic performance,whichmay be attributed tothe differentsolution resistance and pH value[36].In Fig. 5a, all the CV curves show similar formic acid oxidation current peaks in the forward scans. The forward scan of the CV curves is characterized by a strong main current peak at ca. 0.2 V and a shoulder one at ca. 0.60 V corresponding to formic acid oxidation via the dehydrogenation reaction (HCOOH → CO2+ 2H++ 2e-) and the dehydration reaction (HCOOH→COads+ H2O→CO2+ 2H++ 2e-), respectively[37]. The main peak current densities of the five samples Pd/PW12(5mM)/rGO, Pd/PW12(2mM)/rGO, Pd/PW12(10mM)/rGO, Pd/PW12(20mM)/rGO and Pd/PW12(1mM)/rGO were deter-mined to be 4.53, 2.81, 2.31, 1.63 and 0.74 mA×cm-2, respectively, following the order of Pd/PW12(5mM)/rGO > Pd/PW12(2mM)/rGO > Pd/PW12(10mM)/rGO > Pd/PW12(20mM)/rGO > Pd/PW12(1mM)/rGO. By comparing the peak current densities of the five Pd/PW12/rGO catalystsabove, it can be deduced that the concentration of PW12added in Pd/PW12/rGO catalysts evidently affects their performance for formic acid electrooxidation. When the concentration of PW12is up to 5mM, the electrocatalytic activity of the Pd/PW12/rGO catalyst for formic acid electrooxidation is the highest among all the catalysts, which is in good agreement with the EIS results(Fig. 5b). If the PW12concentration is less than 5mM,the electrocatalysis of Pd/PW12/rGO declines due to the insufficient amount of PW12. When the PW12concentration is more than 5mM, the electrocatalysis of Pd/PW12/rGO also declines.It might be because the overdose of PW12will block the active sites of Pd and reduce the conductivity of the composite (Fig. 5b). Therefore, we chose the PW12(5mM)/rGO composite as the optimal subs-trate in the subsequent researches. It is also found that Pd/SiW12(5mM)/rGO and Pd/PMo12(5mM)/rGO have the best electrocatalytic activity for formic acid oxidation amongdifferent electrocata- lysts modified by different concentrations of POM(Fig. S3, Supporting Information).

Fig. 5. CV(a) and Nyquist plots(b) of different PW12concentration modified electrodes at a scan rate 50 mV×s-1in 0.5 M H2SO4containing 1 M HCOOH: (1) Pd/PW12(5mM)/rGO, (2) Pd/PW12(2mM)/rGO, (3) Pd/PW12(10mM)/rGO, (4) Pd/PW12(20mM)/rGO, (5)Pd/PW12(1mM)/rGO

As indicated in Fig. 6a, the highest anodic peak current(4.53 mA×cm-2) is obtained with regard to Pd/PW12/rGO in the formic acid electrooxidation process, which is 1.91 times that of the Pd/C(2.35 mA×cm-2), 1.58 times that of the Pd/PMo12/rGO(2.87 mA×cm-2) and 1.34 times that of the Pd/SiW12/rGO(3.39 mA×cm-2). The current density(mA×cm-2) represents a specific current density and means the current divided by the electro active surface area. The higher activity in electrochemical performance observed here can probably be attributed to superior electric conductivity of graphene-based support and better dispersion of Pd NPs on the support. It is also related to the reversible multi-electron redox properties of POMs. Moreover, the composite containing PW12displays the best electrocatalytic activity as compared with the others containing PMo12or SiW12. These results indicate that the Pd NPs/POM/rGO composite catalysts, especially the Pd/PW12/rGO, possess good electrocatalytic activity for FAOR, and excellent performance in removal of the oxidative intermediates (CO, etc.) as compared with Pd/C because of the synergistic effect between POM and Pd. Moreover, Nyquist plots of Pd/PW12/rGO, Pd/SiW12/rGO, Pd/PMo12/rGO and Pd/C were recorded under open circuit voltage in order to investigate the interfacial properties of the modified electrode. The impedance measurements were made with frequencies ranging from 0.01 Hz to 105Hz and an amplitude voltage of 0.1 V. The impedance data can be fitted by an equivalent electrical circuit composed by one series circuit of a resistance (ct) and capacitor (d) in parallel[38]. Usually, the high frequency semicircle diameter is equal to the charge transfer resistance (ct), which is resulted from the charge transfer process at the interface of electrode/electrolyte[39]. The results including the equivalent circuit used to model the impedance data are shown in Fig. 6b. Fig. 6b shows that the semicircle diameter for Pd/PW12/rGO electrode in the high medium-frequency region is much smaller than that for Pd/C electrode, suggesting that Pd/PW12/rGO electrode possesses lower contact and charge-transfer resistances. The value of charge-transfer resistance (ct) is 180? for Pd/PW12/rGO electrode,which is significantly lower than the Pd/C electrode (666?). This indicates that the conductivity of Pd/PW12/rGOelectrode is enhanced, thus leading to a significant improvement in the electrocatalytic activity. The results are con-sistent with Fig. 6a.

Fig. 6. CV (a) and Nyquist plots (b) of different modified electrodes at a scan rate 50 mV×s-1in 0.5 M H2SO4containing 1 M HCOOH: (1) Pd/PW12/rGO, (2) Pd/SiW12/rGO, (3) Pd/PMo12/rGO, (4) Pd/C

The stability of the electrocatalysts is very im-portant for their real applications in DFAFC. In order to investigate the durability of Pd/PW12/rGO and Pd/C catalysts, the CV curves after 100 cycles in 0.5 mol×L-1H2SO4containing 1.0 mol×L-1HCOOH solutions at a scan rate of 50 mV×s-1and room temperature are measuredas shown in Fig. 7a. It can be found that Pd/PW12/rGO presents the highest main current peak and least drop in the peak current density among all composite catalystafter 100 cyclic voltammetry measurement process.To further compare their catalytic stability, the amperometric i-t curves were performed in 0.5 mol×L-1H2SO4containing 1.0 mol×L-1HCOOH solutions at 0.2 V for 3000s as illustrated in Fig. 7b. The polarizationcurrentsof Pd/C and Pd/PW12/rGOmodified electrodes decrease sharply at the initial stage due to the formation of intermediates during the formic acid oxidation.Then, their polarizationcurrentsgradually decrease and maintain at a steady state with increasing time. After 3000 s, the current value of Pd/PW12/rGO modified electrode is higher than that of the Pd/C catalyst, which confirms that the Pd/PW12/rGO catalyst possesses better stability as compared with Pd/C.

Fig. 7. Positive-going cyclic voltammograms (a) and Chronoamperometric curves (b) of different electrocatalysts in 0.5 M H2SO4solution containing 1 M HCOOH with a scan rate of 50 mV×s-1:(1)Pd/PW12/rGO, (2)Pd/C

CO stripping voltammograms of the Pd/PW12/rGO, Pd/SiW12/rGO, Pd/PMo12/rGO catalysts and Pd/C are measured in order to evaluate the tolerance ability to CO poisoning, as depicted in Fig. 8. Weak anodic peak is observed at the Pd/C catalyst, indicating that little CO can be further oxidized to CO2in this catalyst. In contrast, the Pd/PW12/rGO catalysts show stronger anodic oxidation peak of the adsorbed CO and larger peak area, which reveals that the addition of PW12contributes to the adsorption of COand its further oxidation to prevent the accumulation of poisoning intermediates. Additionally, for the Pd/PW12/rGO catalyst electrode, the anodic oxidation peak of the adsorbed CO is located at 0.855 V, which is more negative than that at other catalyst electrodes, especially 49 mV more negative than that at the Pd/C catalyst electrode. It indicates that the oxidization of CO to CO2is much easier on the Pd/PW12/rGO catalyst than on others. Therefore, more active sites are available for the formic acid electro-oxidation, resulting in a remarkable enhancement of the activity.

The ECSA can be used to determine the number of active sites on catalyst surface. Besides, it is also a significant parameter to compare different electro-catalytic supports by accounting for the conductive path which is available for electron transfer[5, 40]. CO voltammogram of Pd/PW12/rGO, Pd/SiW12/rGO, Pd/PMo12/rGO and Pd/C (Fig. 8) are tested in 0.5 mol×L-1H2SO4solution at a scan rate of 50 mV×s-1and room temperature to calculate their ECSA. According to the Coulombic amount (CO) associa-ted with the peak area, the ECSA area can be cal-culated using the equation ECSA =CO/(= 0.42 mC×cm-2) of reference[41]. The ECSA results are listed in Table S2, which is consistent with the area of hydrogen desorption peak in the above CV test. The Pd/PW12/rGO catalyst exhibitsthe highest ECSA value corresponding to better catalytic activity in FAOR among the four samples. This larger ECSA of Pd/PW12/rGO contributes to its higher electro-catalytic activity for FAOR.

Fig. 8. CO-stripping voltammograms of different modified electrodes in 0.5 M H2SO4solution at a scan rate of 50 mV×s-1: (a) Pd/PW12/rGO, (b) Pd/SiW12/rGO, (c) Pd/PMo12/rGO, (d) Pd/C

4 CONCLUSION

In summary, the POM/rGO nanocomposite was successfully fabricated by a green and facile electrochemical-reduction-assisted method and used as a support for electrodeposition of Pd nanoparticles in situ, in which the POM served as an electro-catalyst as well as a bridging molecule. Comparative studies of different POMs modified composites as electrocatalysts for formic acid oxidation indicate that the Pd/PW12/rGO nanocomposite shows higher electrocatalytic activity, improved CO tolerant ability and better electrochemical stability than Pd/SiW12/rGO and Pd/PMo12/rGO. More importantly, the Pd/PW12/RGO catalyst givesmuch higher electro-oxidation current density (4.53 mA×cm-2Pd) at0.2 V vs. Ag/AgCl than that (2.35 mA×cm-2Pd) of Pd/C.The superior electrochemical performance of Pd/PW12(5mM)/rGO can be attributed to the synergistic effects between Pd NPs and PW12(5mM)/rGO.Moreover, large surface area, good conductivity of graphene matrices and superior redox property of POM can result in higher stability of catalytic system.Thus, the Pd/PW12(5mM)/rGO nanocomposite is expected to be a substantial electrocatalyst in DFAFC.

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10.14102/j.cnki.0254-5861.2011-1746

5 June 2017;

24 July 2018

① This project was financially supported by the National Natural Science Foundation of China (No. 21571034), the Natural Science Foundation of Fujian Province (No.2014J01033) and a Key Item of Education Department of Fujian Province (No. JA13085)