Facile synthesis of Pd/PDDA-GN/PMo₁₁Co composite and its enhanced catalytic performance for formic acid oxidation

Abstract

A novel Pd-based catalyst, Pd/poly(diallyldimethylammonium chloride) functionalized graphene (PDDA-GN)/transition-metal-substituted polyoxometalate H₇PMo₁₁CoO₄₀·xH₂O (PMo₁₁Co), for direct formic acid fuel cells (DFAFC) has been prepared by a layer by layer (LBL) electrostatic assembly method combined with electro-deposition of Pd particles in situ. The morphology, particle size and composition of the as-prepared Pd/PDDA-GN/PMo₁₁Co composite were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). The results reveal that Pd particles are uniformly deposited on the surface of PDDA-GN/PMo₁₁Co. Electrochemical analysis shows that the Pd/PDDA-GN/PMo₁₁Co composite exhibits higher electrocatalytic activity, better electrochemical stability, and higher resistance to CO poisoning than the Pd/PDDA-GN/PMo₁₂ and Pd/C catalysts for the formic acid oxidation reaction (FAOR). The current density of the anodic peak at the Pd/PDDA-GN/PMo₁₂ modified electrode in the formic acid oxidation process is up to 639.3 mA mg⁻¹, which is 4.2 times that of the Pd/C and 1.2 times that of the Pd/PDDA-GN/PMo₁₂. The experimental investigations indicate that PDDA-GN as a support preserves superior electric conductivity of graphene sheets of the composite, and the introduction of electroactive PMo₁₁Co into the composite contributes to convert intermediate species CO into CO₂. The synergistic effects of PDDA-GN and PMo₁₁Co remarkably enhance the electrocatalytic activity and stability of nano-Pd regarding formic acid oxidation.

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1. Introduction

The direct formic acid fuel cell (DFAFC) is a kind of electrochemical device which can convert chemical energy into electrical energy and heat without a combustion process as the intermediate process. Compared with the direct methanol fuel cell (DMFC), the DFAFC offers a broader range of advantages, such as higher energy densities, reduced toxicity, fast electro-oxidation kinetics and low fuel crossover through a Nafion membrane.^1–3

In order to develop the high performance of DFAFCs, more interest has been paid to fabricate highly efficient electrocatalysts towards formic acid oxidation. Among the metal-based catalysts, both Pt and Pd-based materials have been widely expected to be the most efficient catalysts for electrochemical oxidation of formic acid. In view of the catalytic efficiency, Pd-based catalysts have attracted more and more attentions, owing to its advantages towards formic acid electrochemical oxidation mostly through the direct pathway.⁴ However, pure Pd suffers from the catalyst poisoning during the formic acid electro-oxidation reaction more or less.⁵ Therefore, developing new Pd-based catalysts with better resistance to CO poisoning is highly desirable.⁶

Furthermore, to maximize the electrocatalytic activity of Pt/Pd nanoparticles (NPs), a suitable carbon support is required to disperse these NPs. Graphene nanosheets (GN), a two-dimensional nanomaterial with one or several atomic layers of carbon, has been considered to be a promise due to its superior stability, excellent mechanical strength and unique electronic properties, especially the high specific surface area and excellent electric conductivity.^7–11 GN used in electrochemistry have commonly been prepared by the oxidative exfoliation of pristine graphite to get the aqueous dispersion of graphene oxide (GO) and the subsequent chemical reduction of graphene oxide.^12–19 However, in the process of reduction, the chemical reduced graphene oxide tends to agglomerate or precipitate and can not be redispersed in water by ultrasonication. To solve this problem, modification of graphene oxide with poly(diallyldimethylammonium chloride) has to be introduced to improve their properties and extend their functions. Non-covalent functionalization is preferable for catalyst support applications, because it enables attachment of molecules or charged polyelectrolytes through π–π stacking or hydrophobic interactions, and thus preserves the intrinsic electronic and structural properties of graphene. High electronic conductivity is very important for catalyst support applications.²⁰

On the other hand, polyoxometalate has been applied to various kinds of acid-catalyzed oxidation reactions due to their electrochemical oxidizing abilities.^21–23 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).^24–30 Especially, transition metal-substituted polyoxometalates (TMSP) are expected to act as promoters in the formic acid electro-oxidation process on fuel cell anode due to their superior redox ability. Moreover, the existence of Co also likely facilitates the effective CO removal according to bifunctional mechanism.³¹

In this paper, Pd/PDDA-GN/PMo₁₁Co composite was prepared by layer by layer (LBL) electrostatic assembly method and were used for electro-deposition of Pd particles in situ (Scheme 1). The CV results indicate that the Pd/{PDDA-GN/PMo₁₁Co}₂ as a electrocatalyst shows the excellent electrocatalytic performance for formic acid oxidation. Three advantages of the developed Pd/PDDA-GN/PMo₁₁Co required for fuel cell catalysts have been found: first, graphene sheets can not only offer a high surface area for the deposition of noble metal NPs, but also allow for good electrical conductivity and stability in the ternary composite. Second, the catalyst can be used for formic acid oxidation with significant tolerance of carbon monoxide poisoning effects due to electrochemical oxidizing abilities of TMSPs. Third, the small-sized and highly-dispersed Pd NPs supported on graphene sheets take advantage of their catalytic function, which is conducive to a notable enhancement of the electrocatalytic activity. It can be demonstrated that Pd/PDDA-GN/PMo₁₁Co nanoparticles exhibit enhanced electrocatalytic activity and stability for formic acid oxidation compared to Pd supported on carbon (Pd/C).

Scheme 1 The synthetic procedures of the Pd/{PDDA-GN/TMSPs}_n catalyst.

2. Experimental section

2.1. Chemicals and materials

Graphite powder (−325 mesh, 99.9995%), poly(diallyldimethylammonium chloride) (PDDA, M_w 100 000–200 000, 20%), Nafion (117 solution), Pd/C (10 wt% loading, matrix activated carbon support), were purchased from Sigma-Aldrich. PdCl₂, KMnO₄, H₂O₂ (30%), hydrazine hydrate (80%), NaBH₄, CoSO₄, KCl, K₂S₂O₈, P₂O₅, H₃PMo₁₂O₄₀·xH₂O (PMo₁₂), H₂SO₄, HCl, HNO₃, NaOH, acetone, formic acid and ethanol were all purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China) and used without further purification. Indium tin oxide (ITO) glass was purchased from CSG Holding Co. Ltd. (Shenzhen, China). Doubly distilled water is used throughout the whole experiments.

2.2. Preparation

Graphene oxide (GO) was synthesized from natural graphite powder by a modified Hummer's method.³² The preparation of PDDA-GN is based on the reference.³³ The 1 : 11 metal-substituted derivative of the keggin polyoxometalate PMo₁₁Co was prepared as follows:³⁴

Phosphomolybdic acid (H₄PMo₁₂O₄₀) and equimolar amounts of CoSO₄·7H₂O were added together to 50 mL of deionized water. After the pH of the mixed solution was adjusted to 4 with the NaHCO₃, the temperature of the solution was kept at 50 °C for 5 h. The product was filtered under vacuum, washed with ethanol and water and dried under vacuum for 24 h. Finally, the brown precipitate was obtained after recrystallization.

Prior to modification, a GCE (glass carbon electrode) was carefully polished successively with 1, 0.3 and 0.05 mm α-Al₂O₃ slurries and sonicated in deionized water for 5 min after each polishing step. Finally, the GCE was sonicated and washed with ethanol. After cleaning, it was dipped into 1 mg mL⁻¹ PDDA-GN solution for 30 min, rinsed with distilled water, and dried with nitrogen stream. Then, it was immersed in 2 mM PMo₁₁Co for 30 min. The pre-coated substrates were alternately immersed in PDDA-GN and TMSPs solutions, respectively. After each immersion, the substrates were rinsed with distilled water and dried with nitrogen gas. Composite films were formed on the GCE by repeating the above steps in a cyclic fashion. An indium tin oxide (ITO) glass and a quartz slide were ultrasonically cleaned with ethanol, isopropyl alcohol, and water subsequently and dried over N₂. Then, the assembly of composite films on ITO followed the same procedure as that for the GCE. Pd particles were deposited on the surface of the {PDDA-GN/TMSPs}_n composite film modified GCE or ITO electrode in situ {PDDA-GN/TMSPs}_n by potentiostatic electrodeposition in a 0.5 M H₂SO₄ solution containing 2 mM PdCl₂ at −0.2 V. For the purpose of comparison, Pd/{PDDA-GN/PMo₁₂}₂ was also prepared under the same conditions. 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.

2.3. Characterization

The chemical structure and elemental analysis of the synthesized Pd/PDDA-GN/PMo₁₁Co were characterized by a FT-IR (Nicolet Magna-IR 750, America) with the scanned area from 400 to 2000 cm⁻¹, UV-vis absorption spectra were recorded on a quartz slide using TU-1810DPC spectrophotometer (PUXI, Beijing, China). The bulk composition of the catalysts was evaluated by energy dispersive spectra (EDS) on a JEOL 7500F scanning electron microscope. X-ray diffraction (XRD) patterns were obtained on an X'pert Pro diffractometer (Philips, USA), using Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed at room temperature with monochromatic Al Kα radiation (1486.6 eV) using a Quantum 2000 system (PHI, USA). The actual amounts of Pd, P, Mo, Co and Cu loadings of the catalysts were determined by inductively coupled atomic emission spectroscopy (ICP-AES, ICAP6300, Thermo Scientific USA). Transmission electron microscopy (TEM) images were obtained on a TECNAI G2F20 field emission transmission electron microanalyzer (FEI, USA).

2.4. Electrochemical measurements

Electrochemical experiments were carried out at the CHI 660C electrochemical work station (Chenhua, Shanghai, China). A conventional three electrode system was used, with a composite film coated electrode as the working electrode, a platinum wire as the counter electrode and an Ag/AgCl (3.0 M KCl) (Chenhua, Shanghai, China).

3. Results and discussion

The FT-IR spectra of as-prepared PMo₁₁Co compound shows four characteristic peaks ascribed to polyoxometalate with Keggin structure at the range of 1100–700 cm⁻¹ (Fig. 1A). Namely, four characteristic vibration bands of ν_as(P–O), ν_as(Mo–O), ν_as(Mo–O_b–Mo) and ν_as(Mo–O_c–Mo) of PMo₁₁Co are observed at 1048.9, 944.1, 872.1 and 798.6 cm⁻¹ respectively. The corresponding characteristic bands of H₄PMo₁₂O₄₀ appear at 1062.1, 960.5, 865.5 and 783.6 cm⁻¹. It is found by comparison that the vibrational frequencies of P–O and Mo–O are red shifted, while that of Mo–O_b–Mo and Mo–O_c–Mo are blue shifted, indicating the introduction of cobalt atom.³⁷ In addition, the peak attributed to Co–O vibration is found at 732.6 cm⁻¹ in the FT-IR spectra of as-prepared PMo₁₁Co compound.³⁸ It further proves that the as-synthesized compound is PMo₁₁Co. The peak at 1615.9 cm⁻¹ arises from the flexural vibration of H–O–H. ICP measurement indicates that the actual mass fractions of elements P, Mo, Co in PMo₁₁Co is 1.73%, 58.84% and 3.72%, respectively, which is close to the theoretical values: 1.73%, 58.89% and 3.29%. Therefore, the corresponding formula is PMo₁₁Co. EDS spectrum of the Pd/PDDA-GN/PMo₁₁Co composite (Fig. 1B) also shows the characteristic peaks assigned to Co, P, Mo, Pd, O and C elements, indicating existence of PMo₁₁Co in the composite.

Fig. 1 (A) FT-IR spectrum of PMo₁₁Co (a), PMo₁₂ (b) and (B) EDS spectrum of the Pd/PDDA-GN/PMo₁₁Co.

The XRD patterns of Pd/PDDA-GN/PMo₁₁Co, Pd/PMo₁₁Co, Pd/PDDA-GN/PMo₁₂ catalyst are shown in Fig. 2. The characteristic peaks at 30.6°, 35.6°, 50.8°, 60.5° belong to the ITO and the diffraction peaks at 39.9°, 46.2° can be indexed to (111) and (200) lattice planes of Pd face-centered cubic crystalline according to JCPDS standard 01-1201 (Pd).^35,36

Fig. 2 XRD patterns of ITO (a), Pd/PMo₁₁Co (b), Pd/PDDA-GN/PMo₁₁Co (c), Pd/PDDA-GN/PMo₁₂ (d).

Fig. 3A shows the XPS full spectrum of the Pd/PDDA-GN/PMo₁₁Co. The signals of Pd3d, Co2p, C1s, O1s, P2p, Mo3d, N1s indicate that the compound is composed of Pd, Co, C, O, P, Mo and N elements. Fig. 3C and D show the C1s XPS spectra of GO and Pd/PDDA-GN/PMo₁₁Co composites, respectively. Four types of carbon with different chemical states are observed: 284.6 eV (sp²C), 286.7 eV (C–O), 287.7 eV (C=O) and 288.8 eV (O–C=O). As depicted in Fig. 3C, the C–O peak intensities significantly decrease indicating the percentage of surface oxygen groups in PDDA-GN is reduced obviously after treatment with hydrazine in the presence of PDDA. The Pd3d spectra of the Pd/PDDA-GN/PMo₁₁Co are displayed in Fig. 3B. Apparently, the two distinct peaks located at 335.8 eV and 341.2 eV can be assigned to the Pd3d_3/2 (high-energy band) and Pd3d_5/2 (low-energy band) spin–orbit states of zero-valent Pd, and the weak intensity doublets around 338.0 eV and 343.7 eV can be ascribed to Pd3d_3/2 and Pd3d_5/2 peaks of PdO.⁴⁰

Fig. 3 XPS spectra of (A) full spectrum, (B) Pd3d of Pd/PDDA-GN/PMo₁₁Co, (C) C1s of Pd/PDDA-GN/PMo₁₁Co, (D) C1s of GO.

Surface morphology of Pd/PDDA-GN/PMo₁₁Co and Pd/PDDA-GN/PMo₁₂ were characterized by scanning electron microscope (SEM), as shown in Fig. 4. PDDA-GN shows characteristic wrinkled morphology, the Pd NPs are deposited on PDDA-GN/PMo₁₁Co (Fig. 4A). Compared with Pd/PDDA-GN/PMo₁₂, it can be noticed that the Pd nanoparticles are well dispersed on the surface of graphene in the Pd/PDDA-GN/PMo₁₁Co.

Fig. 4 SEM images of Pd/PDDA-GN/PMo₁₁Co (A), Pd/PDDA-GN/PMo₁₂ (B).

The morphology, structure, particle size of Pd/PMo₁₁Co, and Pd/PDDA-GN/PMo₁₁Co were examined by TEM and HRTEM. Fig. 5e and f display TEM images and the corresponding size distribution histograms of the Pd/PDDA-GN/PMo₁₁Co and Pd/PMo₁₁Co composites, respectively. The size distributions were obtained by measuring sizes of 100 randomly selected particles in the magnified TEM images. As shown in Fig. 5e and f, Pd NPs on the PDDA-GN/PMo₁₁Co support are uniform and exhibit narrow size distribution with an average particle size of ca. 3.2 nm (Fig. 5f). In contrast, Pd NPs in the Pd/PMo₁₁Co composite are dispersed in a relatively wide size distribution, and the average particle size is approximately 3.5 nm (Fig. 5e). This result indicates that the introduction of PDDA-GN for Pd/PDDA-GN/PMo₁₁Co composite is beneficial to dispersion and stabilization of Pd NPs. The typical high magnification TEM images of Pd/PMo₁₁Co, Pd/PDDA-GN/PMo₁₁Co (Fig. 5c and d) reveal that the d-spacing of adjacent fringes for the Pd NPs is 0.194 nm and 0.223 nm, respectively, corresponding to the (200) and (111) planes of face-centered-cubic (fcc) Pd NPs.³⁹ It indicates that Pd NPs can be effectively synthesized with uniform shape, excellent size distribution and well dispersed on the unique 2D PDDA-GN supports.

Fig. 5 TEM images, HRTEM images and corresponding particle size distribution histograms of the Pd/PMo₁₁Co composite (a, c and e) and the Pd/PDDA-GN/PMo₁₁Co composite (b, d and f).

The performance of the composites with different layers toward formic acid oxidation reaction in acid medium was evaluated by cyclic voltammetry. Fig. 6 shows the CVs of the electrodes coated with Pd/{PDDA-GN/PMo₁₁Co}_n (n = 1–4) composites in a solution containing 0.5 M H₂SO₄ and 1 M HCOOH at a scan rate of 50 mV s⁻¹. The electrode modified by Pd/{PDDA-GN/PMo₁₁Co}₂ shows the highest peak current intensity, suggesting the highest catalytic activity for formic acid electro-oxidation. Therefore, the Pd/{PDDA-GN/PMo₁₁Co}₂ composite film was chosen as the optimal substrate in the subsequent researches.

Fig. 6 The CV of different lays modified electrodes: at scan rate 50 mV s⁻¹ in 0.5 M H₂SO₄ containing 1 M HCOOH. (a) Pd/{PDDA-GN/PMo₁₁Co}₄, (b) Pd/{PDDA-GN/PMo₁₁Co}₃, (c) Pd/{PDDA-GN/PMo₁₁Co}₂, (d) Pd/{PDDA-GN/PMo₁₁Co}.

Fig. 7 shows the linear polarization curves of the electrodes coated with Pd/{PDDA-GN/PMo₁₁Co}₂ and Pd/PMo₁₁Co composites in a solution containing 0.5 M H₂SO₄ and different formic acid concentrations at a scan rate of 50 mV s⁻¹. The peak potential (E_p) and the peak current density (j) are shown in Table 1 below. From Fig. 7A and B, it can be clearly seen that formic acid concentrations have obvious influence on electrocatalytic activity of different composite as a catalyst. When the formic acid concentrations increased (≥2 M HCOOH) or decreased (≤0.5 M HCOOH), the peak current density of formic acid oxidation decreased. In addition, the peak potentials of HCOOH (1 M) oxidation shifts negatively compared to those of HCOOH (2 M) and HCOOH (5 M).⁴¹ The higher peak current density and the more negative peak potentials of formic acid oxidation indicate the higher catalytic activity of Pd/{PDDA-GN/PMo₁₁Co}₂ composite.

Fig. 7 The CV of different modified electrodes at scan rate 50 mV s⁻¹ in 0.5 M H₂SO₄ containing χ M HCOOH: (A) Pd/PMo₁₁Co, (B) Pd/{PDDA-GN/PMo₁₁Co}₂. HCOOH concentrations: (a) 0.5 M, (b) 1 M, (c) 2 M, (d) 5 M.

Table 1 The plot of the peak potentials and the peak current density versus formic acid concentration

HCOOH concentration (M)	0.5		1		2		5
	Ep/V	j/mA mg^−1	Ep/V	j/mA mg^−1	Ep/V	j/mA mg^−1	Ep/V	j/mA mg^−1
Pd/PMo11Co	0.25	106.13	0.29	406.59	0.35	339.45	0.43	294.15
Pd/{PDDA-GN/PMo11Co}2	0.25	153.96	0.30	641.36	0.35	405.75	0.45	362.98

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Fig. 8 presents the CVs of the Pd/{PDDA-GN/PMo₁₁Co}₂, Pd/C and Pd/{PDDA-GN/PMo₁₂}₂ as electrocatalysts for formic acid oxidation in 0.5 M H₂SO₄ + 1 M HCOOH solution. The peak current density of Pd/{PDDA-GN/PMo₁₁Co}₂ is 641.36 mA mg⁻¹ compared to 521.13 mA mg⁻¹ for Pd/{PDDA-GN/PMo₁₂}₂, whereas 405.75 mA mg⁻¹ for Pd/PMo₁₁Co and 155.93 mA mg⁻¹ for Pd/C. Obviously, Pd/{PDDA-GN/PMo₁₁Co}₂ exhibits the highest electrocatalytic activity toward formic acid oxidation compared with Pd/C, Pd/PMo₁₁Co and Pd/{PDDA-GN/PMo₁₂}₂. The better performance of Pd/{PDDA-GN/PMo₁₁Co}₂ can be attributed to the following three major factors: (1) poly(diallyldimethylammonium chloride) (PDDA) has excellent binding capability with graphene so as to increase the solubility and decrease agglomeration of graphene and maintain the electronic structure of graphene. PDDA-GN with 2D flat planes exhibit substantial advantages with regard to mass and charge transport for both electronic and ionic transport, a higher electrode/electrolyte contact area,^20,34 (2) the well dispersed Pd NPs with small size are beneficial to their electrocatalytic activities, stability, CO toleration and (3) most importantly, the existence of PMo₁₁Co in the hybrids may be helpful for the electrocatalytic activities and anti-poisoning properties of the catalysts. TMSPs are crystalline materials with very high proton conduction. They can act as redox mediators for the electrochemical oxidation of CO.⁴² Compared with PMo₁₂, electrodes modified with PMo₁₁Co catalyst showed better effect on the electrocatalytic oxidation of formic acid, resulting from the high charge density, and the excellent ability of electron donating of PMo₁₁Co.⁴³

Fig. 8 The CV of different modified electrodes at scan rate 50 mV s⁻¹ in 0.5 M H₂SO₄ containing 1 M HCOOH: (a) Pd/C, (b) Pd/PMo₁₁Co, (c) Pd/{PDDA-GN/PMo₁₂}₂, (d) Pd/{PDDA-GN/PMo₁₁Co}₂.

The electrocatalytic stability of different composites was evaluated by the chronoamperometry measurements performed in 0.5 M H₂SO₄ containing 1 M HCOOH at a fixed potential of 0.2 V as shown in Fig. 9. Although the current density of all composites decayed rapidly at the initial stage, the decrease in the oxidation current densities on the Pd/{PDDA-GN/PMo₁₁Co}₂ electrode was significantly slower than that on Pd/{PDDA-GN/PMo₁₂}₂, Pd/PMo₁₁Co and commercial Pd/C, indicating its better durability towards formic acid oxidation. At the end of 3600 s, the residual current density of Pd/{PDDA-GN/PMo₁₁Co}₂ was also higher than that of the other three composites. These results indicate that the Pd/{PDDA-GN/PMo₁₁Co}₂ composite exhibits the slowest current decay over time, confirming the excellent catalytic stability, which is consistent with the above CV results (Fig. 8).

Fig. 9 Chronoamperometric curves of different modified electrodes in 0.5 M H₂SO₄ containing 1 M HCOOH at 0.2 V for 3600 s: (a) Pd/C, (b) Pd/PMo₁₁Co, (c) Pd/{PDDA-GN/PMo₁₂}₂, (d) Pd/{PDDA-GN/PMo₁₁Co}₂.

Based on CO-stripping voltammogram studies (Fig. 10), it is found that the electrochemically surface area of the Pd/C, Pd/{PDDA-GN/PMo₁₂}₂, Pd/PMo₁₁Co and Pd/{PDDA-GN/PMo₁₁Co}₂ are 10.33, 16.40, 18.76 and 20.11 m² g⁻¹, respectively. The oxidation peak potential of CO oxidation at the Pd/{PDDA-GN/PMo₁₁Co}₂ is negatively shift compared to that at the Pd/C, Pd/PMo₁₁Co and Pd/{PDDA-GN/PMo₁₂}₂. This difference can be explained that the PMo₁₁Co ions facilitate the electrooxidation of intermediate species such as CO_ads or the CO-like species during formic acid electrooxidation due to its superior redox quality.⁴ Therefore, PMo₁₁Co ions as an additive into the Pd/{PDDA-GN/PMo₁₁Co}₂ composite should have a positive effect on the removal of CO_ads in the formic acid oxidation. The above results demonstrate that the Pd/{PDDA-GN/PMo₁₁Co}₂ catalyst exhibits better electrocatalytic properties and better resistance to poisoning than the Pd/C, Pd/PMo₁₁Co and Pd/{PDDA-GN/PMo₁₂}₂ catalysts.

Fig. 10 CO-stripping voltammograms of different modified electrodes in 0.5 M H₂SO₄ solution at scan rate of 50 mV s⁻¹: (a) Pd/C, (b) Pd/{PDDA-GN/PMo₁₂}₂, (c) Pd/PMo₁₁Co, (d) Pd/{PDDA-GN/PMo₁₁/Co}₂.

Electrochemical impedance spectrum (EIS) was performed to investigate the intrinsic role of polymer modified graphene (PDDA-GN) in the multilayer composites.^13,44–46 The impedance measurements were made with frequencies ranging from 0.01 Hz to 10⁵ Hz 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 (R_ct) and capacitor (C_d) in parallel. Usually, the high frequency semicircle diameter is equal to the charge transfer resistance (R_ct), which is resulted from the charge transfer process at the interface of electrode/electrolyte.³⁹ As shown in Fig. 11, R_ct decreases remarkably with the introduction of PDDA-GN into multilayer films, which means that PDDA-GN/PMo₁₁Co helps enhance electron transfer obviously on the electrode interface. At the same time, with the preparation of LBL and electrodeposition, electrons transfer faster on the smooth and thin films than on the rough surface of Pd/C through dropping on an electrode.

Fig. 11 Nyquist plots of the different composites in 0.5 M H₂SO₄ under open circuit voltage with frequencies ranging from 10⁵ Hz to 0.01 Hz: (a) Pd/{PDDA-GN/PMo₁₁Co}₂, (b) Pd/{PDDA-GN/PMo₁₂}₂, (c) Pd/{PDDA-GN}, (d) Pd/PMo₁₁Co, (e) Pd/C.

4. Conclusions

In summary, the {PDDA-GN/PMo₁₁Co}_n multilayer films were successfully fabricated by LBL method and used as support for electrodeposition of Pd nanoparticles in situ. It is worthy of note that Pd is uniformly distributed on the surface of {PDDA-GN/PMo₁₁Co}₂ film. Furthermore, due to the superior redox property of PMo₁₁Co, the Pd/PDDA-GN/PMo₁₁Co composite using as a electrocatalyst exhibits higher electro-catalytic activity, improved CO tolerant ability and better electrochemical stability for formic acid oxidation compared with Pd/PDDA-GN/PMo₁₂ and commercial Pd/C. Thus, the Pd/PDDA-GN/PMo₁₁Co composite is expected to be a substantial electrocatalyst in DFAFC.

Acknowledgements

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).

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