A Nanocomposite with Promoted Electrocatalytic Behavior Based on

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C: Energy Conversion and Storage; Energy and Charge Transport

A Nanocomposite with Promoted Electrocatalytic Behavior Based on Bimetallic Pd-Ni Nanoparticles, Manganese Dioxide and Reduced Graphene Oxide for Efficient Electrooxidation of Ethanol Saeed Shahrokhian, Sharifeh Rezaee, and Mohammad Kazem Amini J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01475 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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The Journal of Physical Chemistry

A Nanocomposite with Promoted Electrocatalytic Behavior Based on Bimetallic Pd-Ni Nanoparticles, Manganese Dioxide and Reduced Graphene Oxide for Efficient Electrooxidation of Ethanol Sharifeh Rezaeea, Saeed Shahrokhiana,b*, Mohammad K. Aminic a

Department of Chemistry, Sharif University of Technology, Tehran 11155–9516, Iran

b

Institute for Nanoscience and Technology, Sharif University of Technology, Tehran, Iran

c

Department of Chemistry, Isfahan University, Isfahan, Iran

Abstract In this work, a nanocomposite containing manganese dioxide (MnO2) modified reduced graphene oxide (rGO) supported bimetallic palladium-nickel (Pd-Ni) catalyst is prepared by electrodeposition method. The nanocomposite modifier film is prepared by forming a thin layer of graphene oxide (GO) via drop-casting of GO nanosheet dispersion on glassy carbon electrode (GCE), followed by electrochemical reduction of the film to provide rGO/GCE. Then, a two-step potential procedure is applied to deposit MnO2 nanoparticles on rGO/GCE. At the optimum deposition conditions, MnO2 nanoparticles with a thickness of 30-50 nm homogeneously covered the rGO surface (MnO2/rGO/GCE). Finally, the bimetallic Pd-Ni nanoparticles are electrodeposited on MnO2/rGO/GCE at a fixed potential to form a uniform dispersion with an average particle size of about 50 nm (Pd-Ni-MnO2/rGO/GCE). The morphology and crystalline structure of the prepared nanocomposites are characterized using XRD, SEM, EDX, FTIR, AFM, and Raman spectroscopy. The catalytic activity of different electrodes based on Pd/GCE, Pd/C/GCE, Pd/rGO/GCE, Pd-Ni/rGO/GCE and Pd-Ni/MnO2/rGO/GCE, for ethanol oxidation are compared using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy (EIS). The results revealed significantly higher electroactive surface area (ECSA), higher catalytic activity and better stability of Pd-Ni-MnO2/rGO/GCE towards the electrooxidation of ethanol compared to the other electrodes. The overall results corroborate the role of MnO2, Ni, and rGO as important constituents that significantly improve the electrocatalytic behavior, stability, and CO poisoning tolerance of Pd during the electrooxidation process. Thus, the prepared Pd-Ni-MnO2/rGO/GCE catalyst can be considered as a promising anode catalyst for alkaline direct ethanol fuel cells. *Corresponding author, Tel; +98-21-66165359; Fax: +98-21-66012983 E-mail address: [email protected]

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1.

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INTRODUCTION

Because of environmental necessities and energy crisis, fuel cells are being recognized as one of the most promising alternative energy power sources that can be used for various energy requirements. In this way, small organic molecules such as alcohols (methanol, ethanol, ethylene glycol) have been studied as model fuels 1. Ethanol as an appealing candidate, owing to its remarkable features including nontoxicity, easy storage, relatively low volatility, renewability and abundant availability, has attracted much attention

2

. Moreover, the complete

electrooxidation process of ethanol effectively delivers twelve electrons per molecule, making it a high energy density fuel 3. However, the electrooxidation of ethanol includes very complex reactions with slow reaction kinetics. Partial oxidation of ethanol leads to strongly adsorbed intermediates, which cause surface poisoning and catalyst deactivation 4,5, with the consequence of significantly lowering the fuel cell efficiency Therefore, it is of much interest to develop less expensive, more abundant and highly active electrocatalysts for development of more efficient direct alcohol fuel cells (DAFCs). Many investigations indicate that Pd can be considered as a suitable alternative for Pt-based catalysts because of its higher availability, lower cost, relatively high catalytic activity in alkaline solutions and better resistance to poisoning species 6. On the other hand, bimetallic catalysts due to their structural, geometric and electronic effects show much higher catalytic activity and stability for electrooxidation processes 7. Abundant researches have been conducted to fabricate bimetallic Pd-based catalysts with several transition metals such as Pd-Cu 8, Pd-Co 9, Pd-Sn

10

and etc. Compared to pure palladium, the bimetallic catalysts provide improved electrocatalytic activity and higher tolerance to poisoning intermediates. The reason for such enhancement is that the resulting bimetallic structure facilitates desorption of intermediate species, and therefore, provides more active sites for the desired electrochemical reaction 11. 2 ACS Paragon Plus Environment

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Some efficient Pd-based catalysts have been fabricated by decoration with some metal oxides such as WO3

12

, CeO2

13

, TiO2

14

, V2O5

15

, MoOx

16

, NiO

17

and Cu2O

18

. The related

results reveal that addition of such metal oxides with high microscopic surface area and surface functional groups provides strong interaction with Pd particles and efficiently enhances the performance of electrooxidation reactions

19

. Also, the presence of metal oxides increases the

concentration of OHads species and significantly reduces the onset potential for the ethanol electrooxidation

20

. Among the different types of oxides, MnO2 is one of the important

candidates on account of its low price, good electrochemical properties, excellent proton conductivity, high CO tolerance, and its synergistic effect on the metal catalysts. However, MnO2 suffers from poor electrical conductivity, which can be compensated by incorporating a highly conductive nanomaterial in its structure 21. In this regard, reduced graphene oxide (rGO) is considered as a good catalyst support due to its intrinsic properties like high electrical conductivity, high surface area, ease of preparation and low cost 22. Also, a graphitized two-dimensional plane structure with surface oxygen-containing functional groups on both basal planes and the edges of rGO provides anchoring places to facilitate distribution and stability of the surface immobilized catalysts 23. Moreover, rGO plays a vital role in decreasing catalyst poisoning effects by removing the CO-like intermediate species 24

. Up to now, some research studies have been focused on the use of MnO2 as the supporting

material to improve the electrocatalytic activity of different catalysts

19,20,25,26,27

. It is worth

noting that, the most important properties of catalysts including surface area, morphology, and structural distribution strongly depend on the synthesis procedure 18. Typical chemical routes for synthesis of MnO2 nanostructures presented in the literature are complicated and expensive 28,29.



However, electrodeposition as an easy, inexpensive and low temperature method, provides a convenient route for preparation of MnO2 nanoparticles 30. In the present work, a new nanocomposite containing rGO, MnO2 and the bimetallic Pd-Ni nanoparticles was prepared via simple electrodeposition methods on the surface of GCE. First, an electrochemical reduction procedure was employed to reduce GO to obtain a film of rGO. In the next step, MnO2 nanoparticles were electrodeposited on rGO film using a two-step potential method. The prepared MnO2/rGO nanocomposite, which has been shown to provide synergistic effect, enhances the electrical conductivity of MnO2 and makes a unique supporting material for loading Pd-Ni nanoparticles. Finally, bimetallic Pd-Ni nanoparticles were deposited on MnO2/rGO film by applying a constant potential of -0.4 V, to afford Pd-Ni-MnO2/rGO/GCE. The electrooxidation of ethanol on Pd-Ni-MnO2/rGO/GCE has been studied by different electrochemical techniques. The obtained results reveal that, in comparison to other Pd-based electrocatalysts, the as-prepared Pd-Ni-MnO2/rGO/GCE has a remarkable electrocatalytic performance for ethanol electrooxidation. 2.

EXPERIMENTAL SECTION

2.1. Reagents and materials Graphene oxide was synthesized by the modified Hummers method 31. Graphite powder (average particle size, 300 nm) was purchased from Aldrich. Manganese (II) acetate, nickel (II) chloride, palladium (II) chloride, sodium nitrate, sulfuric acid, sodium sulfate, sodium hydroxide and ethanol were of analytical reagent grade purchased from Merck. The reagents used for electrochemical impedance studies, including K3Fe(CN)6, K4Fe(CN)6 and KCl (>99%), were also prepared from Merck. All aqueous solutions were prepared using ultra-pure deionized water (18.2 MΩ, Zolalan Sharif Company, Tehran, Iran). 4 ACS Paragon Plus Environment

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2.2. Instrumentation Electrochemical experiments were performed using a potentiostat/galvanostat from Sama Instrument (500-C Electrochemical analysis system, Sama, Iran) coupled with a PC through an interface with Sama-500 software version 2.7. Electrochemical impedance spectroscopy (EIS) data were obtained by an Autolab PGSTAT 204 with an FRA software version 4.9 (Eco Chemie, the Netherlands). EIS measurements were conducted by applying an AC voltage with an amplitude of 10 mV and the frequency of 0.01 Hz to 20 kHz at OCP (open circuit potential). Electrochemical measurements were performed with a three-electrode system consists of an Ag/AgCl as the reference electrode, a platinum wire as the auxiliary electrode and GCE (2 mm diameter, and 0.0314 cm2 geometric surface area) as the working electrode. Information about the crystalline structure (crystallinity) was obtained by an X-ray diffractometer (GBCMMA, Instrument) in the 2θ range from 0° to 90° using Cu Kα radiation. A scanning electron microscope (SEM, VEGA-Tescan) equipped with an energy-dispersive spectrometer (EDS) was used to determine the structure, morphology and quantitative elemental analysis. Atomic force microscopy (AFM) was performed in ambient conditions using DME Nanotechnology, GmbH. The Fourier transform infrared spectra (FT-IR) were recorded using an ABB Bomem MB-100 FT-IR spectrophotometer using KBr pellets. Raman spectra were collected using Raman spectrometer (Senterra-Bruker) with a 785 nm laser excitation. The Pd and Ni loadings on the modified electrode were determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Vista-Pro, Varian Australia) after dissolution of the sample in aqua regia. The CO stripping experiments were conducted in a solution of 0.5 M H2SO4 at a scan rate of 10 mV s-1 at 25 °C. Carbon monoxide was bubbled through the 5 ACS Paragon Plus Environment


electrolyte for 20 min to allow complete adsorption of CO onto the deposit. The amount of the COads was evaluated by integrating the COads stripping peak and correcting for the capacitance of the double electric layer. 2.4. Preparation of rGO/GCE, MnO2/rGO/GCE, and Pd-Ni/MnO2/rGO/GCE Before the surface modification, the GCE surface was polished gently on a felt pad with 0.3 µm alumina/water slurry until a mirror-like surface was established. The polished electrode was washed successively with double distilled water and sonicated in ethanol for a few minutes to remove the residual alumina particles. Then, 5 µL homogeneous suspension of GO (1 mg mL-1) was loaded onto the polished GCE using a microliter syringe, followed by drying in oven (60°C) to form GO/GCE. The electrochemical reduction of GO film was conducted at a fixed potential of -1.0 V (vs. Ag/AgCl) in 0.5 M NaNO3 for 300 s. The prepared electrode in this step is denoted as rGO/GCE. For uniform deposition of MnO2 nanoparticles, a two-step potential method was used, consisting of a short potential pulse followed by a long constant pulse. The anodic electrodeposition of MnO2 on the rGO/GCE surface was carried out in 0.5 M Mn (II) acetate. In the first potential step, which is the nucleation process, a high potential (Vnuc = 1.0 V) was applied for a very short time (0.5 s). As a result, a uniform coating of the MnO2 nuclei is well distributed on the surface of the rGO/GCE. In the second step, the electrochemical deposition of MnO2 nanoparticles was performed at a lower potential (Vdep = 0.3 V) for 100 s. Then, the electrode was rinsed thoroughly with double distilled water and then immersed in a freshly prepared solution of 0.1 M Na2SO4 (pH = 3.8) containing PdCl2 and NiCl2 (with a total concentration of 1 mM and Pd/Ni molar ratio of 70/30) for 10 min, to allow adsorption of the metal ions onto MnO2/rGO film. Afterward, the Pd-Ni nanoparticles were electro-reduced 6 ACS Paragon Plus Environment

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potentiostatically on the surface of MnO2/rGO/GCE by keeping the electrode potential at -0.40 V (vs. Ag/AgCl) for 300 s. The charge resulted from the complete reduction of the precursor salts was 3176 mC cm-2. This value is equivalent to 1.75 mg cm-2 Pd if it is considered as the only metal ion deposited. After completion of this step, the electrode, denoted as PdNi/MnO2/rGO/GCE, was rinsed thoroughly with double distilled water. For comparison, we also prepared Pd/GCE, Pd/C/GCE, Pd/rGO/GCE and Pd-Ni/rGO/GCE under the same conditions.

3.

RESULTS AND DISCUSSION

3.1. Surface characterizations Morphological features of the as-fabricated electrodes were investigated by SEM technique and the obtained micrographs are presented in Fig. 1. Trace (A) shows the surface of the bare GCE after smooth polishing on alumina slurry. As can be seen in this figure, there is nearly a smooth surface with some minor defects. Trace (B) depicts the SEM image of GCE surface after coating with rGO nanosheets. The rGO nanosheets with wrinkled structure can be clearly seen in this micrograph. Trace (C), exhibits the SEM image of MnO2 nanoparticles, indicating that they are uniformly distributed on the surface of rGO/GCE with a diameter of 30-50 nm. Fig. 1D displays SEM image after Pd-Ni electrodeposition on the surface of MnO2/rGO. This micrograph shows that Pd-Ni nanoparticles with an average size of 50 nm totally covered the electrode surface, with some interconnected aggregates.

In order to obtain more information about the modifier film, EDX analysis was used as a technique to determine the chemical composition of Pd-Ni/MnO2/rGO/GCE, and the corresponding spectrum is shown in Fig. 2A. The obtained data demonstrate the presence of Pd, 7 ACS Paragon Plus Environment


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Ni, and Mn in the prepared nanocomposite. The carbon (C) peak comes from the rGO film, and the oxygen signal can be related to MnO2 nanoparticles and the partially reduced GO. The elemental analysis data from EDS analysis are presented in the inset of Fig. 2A. It is evident from the recorded data that the atomic composition of the Pd-Ni catalyst is roughly consistent with the corresponding precursor solutions. Fig. 2B shows the elemental mapping of PdNi/MnO2/rGO/GCE, which reveals the distributions of C, Mn, Pd and Ni throughout the entire nanocomposite. In order to determine the Pd and Ni loadings on the modified electrode, ICP-OES analysis was performed after dissolution of the catalyst in aqua regia. The results indicated the presence of 0.053 mg Pd in Pd/C, Pd/GCE and Pd/rGO/GCE catalysts; the concentration of Pd and Ni were 0.036 and 0.014 mg, respectively, in Pd-Ni/rGO/GCE and Pd-Ni/MnO2/rGO/GCE catalysts.

The FT-IR spectra of GO and MnO2/rGO are presented in Fig. 3A. Absorption bands observed at 3427, 2924 and 2854 cm−1 are attributed to the stretching vibrations of O-H, -CH3 and -CH2, respectively. The peaks at 1720 and 1589 cm-1 are identified as the stretching vibrations of carbonyl C=O and aromatic C=C. Also, absorption peaks at 1383 cm-1 and 1088 cm-1 can be assigned to typical stretching vibrations of C-OH and C-O-C. Clearly, all these peaks exhibit abundant oxygen-containing groups in the GO structure. Moreover, after the electrochemical reduction of GO to rGO and electrodeposition of MnO2, the intensities of the oxygen-containing functional groups considerably decrease and a new peak appears at 560 cm-1, which corresponds to the Mn-O stretching vibration in the MnO2 phase

32

. Fig. 3B shows the

Raman spectra of GO and rGO/MnO2. The two characteristic peaks around 1310 and 1580 cm-1


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are related to D band and G band. The ID/IG ratio for MnO2/rGO (1.05) is higher than that of GO (0.8), which may be attributed to the localized sp3 defects in the sp2 carbon network upon the electrochemical reduction. Moreover, MnO2/rGO shows characteristic stretching vibrations of Mn-O around 645 cm-1

33

. These results further indicate the presence of rGO and MnO2 in the

composite. The phase structures of the samples were examined by XRD measurements. Fig. 3C shows the corresponding XRD patterns of MnO2/rGO (a) and Pd-Ni/MnO2/rGO (b) on stainless steel (SS). The presence of a sharp peak at 2θ near 43.5° in Fig (a) can be related to SS substrate. The appearance of a broad peak around 25° can be ascribed to the (002) plane of rGO. Moreover, the peaks located at 2θ = 37° (211), 49.8° (411), 55.9° (600), 61° (521) and 65.2° (002), can be assigned to α-MnO2 in the nanocomposite 34. The main characteristic peaks observed in Fig. (b) at 40.1°, 46.6°, 68.1°, and 82.0° can be attributed, respectively, to (111), (200), (220) and (211) crystalline plane diffraction peaks of face-centered cubic Pd metal (JCPDS No. 46-1043). Also, diffraction peaks at 2θ values of 44.5°, 51.8°, and 76.4° are assigned to planes (111), (200) and (220) for Ni, indicating the face-centered cubic (fcc) arrangements (JCPDS No. 04-0850) 35.

The AFM as a powerful technique was used to image the topography of the as prepared electrodes to obtain structural information. Fig. 4 depicts the AFM topology of the rGO/GCE and Pd-Ni/MnO2/rGO/GCE surfaces, corresponding to 2D (A) and 3D (B) images recorded over an area of 10 µm × 10 µm. As can be seen, the wrinkled surface structure of rGO could be clearly observed on the surface of rGO/GCE. On the other hand, in comparison with rGO, PdNi/MnO2/rGO/GCE shows a more rough surface, which can be ascribed to the deposited nanoparticles on the entire surface of wrinkled graphene.



3.2. Electrochemical measurements for the ethanol oxidation Cyclic voltammetry is probably the most widely used technique for understanding the electrochemical behavior of the prepared modified electrodes. In this way, cyclic voltammetry was conducted on Pd/GCE (a), Pd/C/GCE (b), Pd/rGO/GCE (c), Pd-Ni/rGO/GCE (d), and PdNi/MnO2/rGO/GCE (e) in a solution containing 0.5 M NaOH and 0.5 M H2SO4 and the typical cyclic voltammograms (CVs) are depicted in Figs. 5 A and B, respectively. The voltammograms show quite similar features, regardless of the nature of the electrode material. In both figures, the potential region (I) is attributed to the adsorption-desorption of hydrogen, and typical peaks (II) and (III) are associated with the formation/reduction of surface oxides of Pd. Obviously, the observed difference in current due to Pd oxides, can be ascribed to the difference in the microscopic surface area. The results represent higher surface area of the surface-confined Pd-Ni particles in the presence of MnO2/rGO, which may arise from better dispersion of the catalyst on the electrode surface. Also, a closer look at Fig. 5B reveals that both the initial oxidation potential of the metal films and the reduction peak potential of the metal oxide show negative shift with the successive incorporation of rGO, Ni, and MnO2 into the composite. This phenomenon may be due to partial charge transfer from rGO, Ni and MnO2 to Pd and spillover of OH from MnO2 to the Pd surface, leading to increased strengthened oxophilicity of the Pd surface 20. It is worth noting that, ethanol oxidation pathways require the participation of OHads, so the slightly enhanced oxophilicity of Pd surface is expected to facilitate the ethanol oxidation reaction (EOR) in alkaline medium. Fig. 5C shows the CO-stripping voltammograms of the prepared catalysts in 0.5 M H2SO4 at a scan rate of 10 mV s-1 8. Also, the enlarged plots in Fig. 5D present a closer look at the onset 10 ACS Paragon Plus Environment

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potential of CO oxidation on the prepared Pd-based catalysts. According to the obtained results, Pd-Ni/MnO2/rGO/GCE shows the most negative onset potential for CO oxidation. This phenomenon indicates that rGO, Ni and MnO2 increase the OHads on Pd surface at lower potential, and consequently facilitate the CO oxidation, so that Pd-Ni/MnO2/rGO/GCE catalyst has less opportunity to be poisoned by CO 20. It is known that the activity of a catalyst is not only controlled by the catalytic properties but also by its geometrical characteristics, and electrochemical surface area (ECSA). Thus, for comparison of the active sites available on different modified electrodes, the ECSA of the Pd component was estimated based on the consumed charges for oxidizing the CO monolayer from the CO-stripping measurements. Thus, the ECSA of the Pd-based catalysts was determined using Eq. (1): =

(1)

×

where Q is the charge of CO desorption-electrooxidation (µC), m is the total amount of the metal (mg) on the electrode surface, and 420 is the charge required to oxidize a monolayer of adsorbed CO on the catalyst surface (µC cm-2) 36. The calculated data are summarized in Table 1.

< Table 1> In order to evaluate the electrocatalytic activities of various modified electrodes, CVs of Pd/GCE (a), Pd/C/GCE (b), Pd/rGO/GCE (c), Pd-Ni/rGO/GCE (d) and Pd-Ni/MnO2/rGO/GCE (e) were recorded in a solution containing 0.5 M of both ethanol and NaOH at a scan rate of 50 mV s-1 (Fig. 6A). In the presented CV curves, there are two well-defined peaks observed in the forward and backward scans. Obviously, the anodic peak in the positive scan (I) is ascribed to



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oxidation of chemisorbed species coming from adsorption of ethanol. The other peak in the negative scan (II) also comes from removal of the incompletely oxidized carbonaceous species formed in the forward scan

18

. The performance of these electrodes for ethanol oxidation was

assessed by comparing four basic parameters. First, the anodic peak current density (jf) and the ratio of the forward peak current density to the reverse one (jf/jr), obtained by normalizing the oxidation peak current to ECSA, were used to evaluate the electrocatalytic activity and tolerance of the prepared electrocatalyst to carbonaceous intermediates

37

. It was found that Pd-

Ni/MnO2/rGO/GCE with higher jf and jf/jr,, exhibited remarkably enhanced catalytic activity and anti-poisoning ability. The obtained results are consistent with the results of CO stripping measurements. Also, the onset potential (Eonset) and peak potential (Ep) in the forward scan can be considered as indicators for comparing and determining the activity of the prepared electrocatalysts

38

. Thus, negative shifts in Eonset and Ep values reveal that Pd-

Ni/MnO2/rGO/GCE can effectively improve the reaction kinetics and decrease the overpotential in the electrooxidation of ethanol. Different electrocatalytic parameters such as Eonset, Ef, jf, Eb, jb and jf/jb obtained from CVs of the modified electrodes are presented in Table 2. < Table 2> Several factors may be considered to explain the enhanced ethanol oxidation reaction on the surface of Pd-Ni/MnO2/rGO/GCE. First, it has been found that the introduced rGO as a suitable support plays a key role for the dispersion of the metal nanoparticles, leading to smaller size, and better uniform distribution, as well as preventing the immobilized catalyst particles from agglomeration

39

.

Furthermore,

the

enhanced

electrooxidation

performance

of

Pd-

Ni/MnO2/rGO/GCE can be attributed to the presence of Ni in the Pd-based catalysts. The Ni role can be satisfactorily explained with two main effects. First, as far as the ligand effect is


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concerned, the electronic interaction between the second metal and Pd is helpful in weakening the adsorption strength of CO intermediates, resulting in less carbonaceous material accumulation and more tolerance of the electrode toward poisoning species

10

. Second, on the

basis of a bi-functional mechanism, the second metal is believed to provide oxygen-containing species for CO oxidation at lower potentials than Pd. So, these surface adsorbed species, in fact, accelerate the elimination of poisoning species from the catalyst surface according to the following reactions 40: Ni + H2O → Ni-OHads + H+ + e-

(2)

Ni-OHads + Pd-COads → CO2 + Pd + Ni + H+ + e-

(3)

Overall, the obtained results exhibit that using MnO2/rGO as the support for Pd-Ni nanoparticles enhance the catalytic activity of the catalyst. In fact, MnO2/rGO nanoparticles are a good support which improve the dispersion of Pd-Ni nanoparticles and provide a highly active large surface area

25

. The highest electrocatalytic activity of Pd-Ni/MnO2/rGO/GCE, compared to the other

catalytic systems investigated, is ascribed to a synergistic effect between Pd, Ni, and MnO2. Moreover, the oxophilic nature of the transition metal oxide, MnO2, facilitates the formation of OHads and its spillover to the Pd-Ni surface. This phenomenon is an important step in the removal of COads poisoning from the active sites, which, in turn, releases the active sites on Pd surface for continuation of the desired electrochemical reaction, i.e., rapid adsorptiondissociation of ethanol

41

. The ethanol oxidation on Pd-Ni in the presence of MnO2 in alkaline

media can be expressed by the following reactions. In this mechanism MnO2 nanoparticles facilitate the ethanol oxidation by removal of CH3COads and oxidation of acetaldehyde into acetic acid 18.



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Pd + CH3CH2OH → Pd-(CH3CH2OH)ads

(4)

Pd-(CH3CH2OH)ads + 3OH- → Pd-(CH3CO)ads + 3H2O + 3e-

(5)

-

-

MnO2 + OH → MnO2-OHads + e

(6)

Pd-(COCH3)ads + MnO2-OHads → Pd-CH3COOH + MnO2

(7)

The electrocatalytic activity of Pd-Ni/MnO2/rGO/GCE in terms of the kinetic parameters, are listed in Table 3, and the values are compared with some previous works. < Table 3> The catalyst stability and resistance to surface poisoning effects was investigated by performing chronoamperometric experiments in 0.5 M NaOH containing 0.5 M ethanol at peak potential values for a period of 3600 s. The results, presented in Fig. 6B for Pd/GCE (a), Pd/C/GCE (b), Pd/rGO/GCE (c), Pd-Ni/rGO/GCE (d) and Pd-Ni/MnO2/rGO/GCE (e), indicate highest oxidation current density for Pd-Ni/MnO2/rGO/GCE over the test period. This can be attributed to highest electrocatalytic activity of this electrode towards the electrooxidation of ethanol, in agreement with the cyclic voltammetry results. The results suggest that, in comparison to other modified electrodes, Pd-Ni/MnO2/rGO/GCE displays improved stability and higher tolerance to carbonaceous species like COads generated during ethanol oxidation. Chronopotentiometry is another useful method to evaluate the stability and poisoning– tolerance ability of the electrocatalysts for ethanol oxidation. In this technique, the potential increases with the polarization time and finally shifts to a higher potential value, which indicates poisoning of the catalysts. Thus, the time at which the electrode potential jumps to a higher potential is taken as an estimate of the catalyst tolerance to COads poisoning 47. Fig. 6C shows the chronopotentiograms of Pd/GCE (a), Pd/C/GCE (b), Pd/rGO/GCE (c), Pd-Ni/rGO/GCE (d) and Pd-Ni/MnO2/rGO/GCE (e) in 0.5 M NaOH containing 0.5 M ethanol at a current density of 0.5


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mA cm−2. It can be seen that Pd-Ni/MnO2/rGO/GCE operates at considerably longer times compared to the other ones, as a result of better electrocatalytic stability and antipoisoning ability. Moreover, the stabilities of Pd-Ni/rGO/GCE and Pd-Ni/MnO2/rGO/GCE were verified by measuring their responses to ethanol oxidation over 300 CV cycles in 0.5 M ethanol and 0.5 M NaOH solution (Fig. 6D). The increased peak current density during the initial cycles can be attributed to effective elimination of the contaminants from the active sites, which in turn, facilitates the access of ethanol molecules to the active sites. Then, after the initial cycles, a gradual decrease in the anodic current density is observed for both electrocatalysts. This behavior can be related to the catalyst surface poisoning and change of the catalyst structure as a result of potential perturbation during successive scans. However, after completion of the cycles, PdNi/rGO/GCE and Pd-Ni/MnO2/rGO/GCE retain 51% and 83% of their maximum peak current density, observed at 54th and 83th scans, respectively. These results again suggest less poisoning and better long-term stability of Pd-Ni/MnO2/rGO/GCE. 3.3. Optimization of the Pd/Ni ratio on the electrocatalytic activity of the nanoparticles The effect of Ni content of the co-deposited Pd-Ni nanoparticles on the electrooxidation of ethanol was investigated by varying the Pd/Ni molar ratio of the deposition solution, keeping the total concentration constant at 1 mM in all cases. The CVs were recorded, and the three key parameters including jf, Eonset and jp,f/jp,b ratio were monitored as indices to find the optimum conditions. The results, presented in Table 4, indicate that, introduction of the second metal (Ni) has a significant effect on the catalytic performance of the catalyst. Clearly, all the catalysts with bimetallic Pd-Ni nanoparticles display higher jp,f, jpf/jp,b and lower Eonset values, compared to Pd alone. The results in Table 4 indicate that the catalytic performance enhances significantly with increasing Ni content and reaches a maximum at 70/30 (Pd/Ni) molar ratio. In general, the effect 15 ACS Paragon Plus Environment


of Pd-Ni composition on the catalytic activity can be attributed to variation of the structural parameters and surface area of the nanoparticles The catalytic activity decreases at higher Ni concentrations (> 30 mole%), possibly due to agglomeration or cluster formation, which lowers the surface area of the catalyst. Therefore, the electrocatalytic activity of Pd-Ni/MnO2/rGO/GCE. depends on the ratio of Pd/Ni nanoparticles as well as on MnO2/rGO/GCE as a suitable substrate which provides a high electrochemically accessible surface areas and high stability. < Table 4> 3.4. The effect of switching potential and potential scan rates The effect of upper limit potentials on the ethanol electrooxidation at Pd-Ni/MnO2/rGo/GCE was studied and the obtained CVs are shown in Fig. 7A. As can be seen in this figure, by increasing the switching potential (Eλ), jf (I) remains constant but jb (II) decreases (inset A). This observation reveals that the current density ratios of the two peaks (jf/jb) increase by changing Eλ to more positive values (inset C). Indeed, by extending the potential window to the positive direction in the forward scan, conversion of Pd to PdO is accelerated which decreases the oxidation current density in the backward scan. Thus, the ratio of jf/jb can be considered as a criterion for the electrocatalytic activity of the catalyst and its tolerance to the poisonous intermediates. Additionally, in the case of peak potentials, by shifting the Eλ to more positive values, the peak potential for peak (I) remains almost invariable, but for peak (II) shifts to the negative direction (inset B), resulting in increase of the difference Ep,f-Ep,b (inset C). It can be concluded that, by preventing PdO formation and keeping the electrode surface relatively clean, the rate of oxidation of intermediates increases and the peak (II) appears at more positive potentials.


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The kinetic aspects of the reaction was investigated by studying the influence of the potential scan rate on the electrocatalytic behavior of Pd-Ni/MnO2/rGO/GCE toward ethanol electrooxidation. Fig. 7B exhibits the CVs of ethanol oxidation on Pd-Ni/MnO2/rGO/GCE at different scan rates from 0.03 to 0.4 V s-1. In inset A, jp is plotted as a function of scan rate (υ) and square root of the scan rate (υ1/2) (curves a and b, respectively). As can be seen, with increasing the scan rate, jp shifts to higher values. Observation of a linear relationship between jp and υ1/2 suggests that the electrocatalytic oxidation of ethanol on the surface of the modified electrode is a diffusion-controlled process

46

. Moreover, inset (B) shows that by increasing the

scan rate, Ep in the forward scan shifts to more positive potentials, and a linear relationship is observed between Ep and log υ. According to these findings, it can be concluded that the electrooxidation of ethanol at Pd-Ni/MnO2/rGO/GCE is an irreversible oxidation process

53

. On

the other hand, the jp for the reverse anodic oxidation process decreases by increasing the scan rate. Inset (C) shows that Ip,f/Ip,b ratio increases by increasing the scan rate, indicating enhanced ethanol oxidation during the forward scan and less accumulation of oxidizable species on the electrocatalyst surface 54.

3.5. EIS studies Electrochemical impedance spectroscopy (EIS) as a valuable electrochemical technique was performed to study the mechanism of ethanol oxidation. The impedance behavior of the catalysts is strongly dependent on the electrode potential. In this way, we attempted to use EIS as a dynamic method for the mechanism discrimination of ethanol electrooxidation at different potentials. The Nyquist plots for Pd/GCE (a), Pd/rGO/GCE (b), Pd-Ni/rGO/GCE (c) and PdNi/MnO2/rGO/GCE (d) in 0.5 M NaOH containing 0.5 M ethanol solution at -0.3 V are shown in



Fig. 8. Also, the equivalent circuit, shown in inset of Fig. 8, was used to simulate the EIS data. By comparison of the results, semicircle diameters were found to be in the order of a > b > c > d. It can be seen that the semicircle diameters considerably diminish upon incorporation of rGO, Ni and MnO2 into the composite. The results clearly indicate higher electrocatalytic activity and faster kinetics for ethanol oxidation reaction at Pd-Ni/MnO2/rGO/GCE. This improvement is partially related to MnO2 which provides oxygen-containing species for the adsorbed CO to form CO2, and can be considered as a kind of bi-functional mechanism 41.

Fig. 9 includes the Nyquist complex impedance plots obtained for the oxidation of ethanol at different potentials on the surface of Pd-Ni/MnO2/rGO/GCE (Panels A, B and C) and Pd-Ni/rGO/GCE (Panels D, E and F). The appearance of different impedance behaviors at various potentials indicates that ethanol electrooxidation mechanism changes at various potentials. These observations are generally similar to the impedance behavior observed previously in the electrooxidation of ethanol on Pt and formic acid on Pd 47,50. It can be seen that for Pd-Ni/MnO2/rGO/GCE (Panel A), the impedance arcs start to decrease with further increase of the electrode potential to -0.20 V. This behavior is ascribed to the oxidative removal of the adsorbed CO intermediates which are generated during ethanol dehydrogenation at lower potentials 51. Moreover, as shown in Panel B at potential of -0.1 V, a sudden change occurs in the impedance plot and transit from the first quadrant to the second quadrant to negative values. The negative impedance is attributed to rate-determining step changes from the ethanol dehydrogenation to the oxidative removal of the adsorbed CO intermediates via chemisorbed hydroxyl species 52. In the potential range of -0.1 V to 0.05 V, the arc only appears in the second


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quadrant. Finally, with increasing the potential to values greater than 0.1 V, the impedance arcs return to the first quadrant to positive values (Panel C). Finally, the arc diameter decreases with further increase in potential. This behavior is due to the adsorbed OH groups which completely cover the surface of the catalyst and inhibit the oxidation of ethanol

53

. For comparison, the

impedance plots of Pd-Ni/rGO/GCE are shown in Panels D-F. As can be seen in these plots, PdNi/rGO/GCE shows similar impedance features with somewhat different. In general, the comparison between the impedance profiles shows that, impedance arc diameters on PdNi/rGO/GCE are smaller than those of Pd-Ni/MnO2/rGO/GCE. It can be concluded that, PdNi/MnO2/rGO/GCE has much smaller charge transfer resistance, which suggests that it has a higher surface area and more active sites for ethanol oxidation reaction.

4. Conclusions In this study, MnO2 nanoparticles were covered on the surface of rGO/GCE by a simple and convenient electrodeposition method (MnO2/rGO/GCE). The MnO2/rGO/GCE was used to support Pd-Ni nanoparticles as a catalyst for electro-oxidation of ethanol. The EDX, XRD, AFM and SEM were used to characterize the surface morphology and the catalyst composition. Electrochemical performance of Pd-Ni/MnO2/rGO/GCE nanocomposite was investigated for the electrooxidation of ethanol in alkaline medium by means of cyclic voltammetry, chronoamperometry, chronopotentiometry and electrochemical impedance spectroscopy. The obtained results indicate that, with respect to various electrochemical parameters including ESCA, jf, jf/jb, Eonset and Ep, Pd-Ni/MnO2/rGO/GCE presents high electrochemical active surface area, high electrocatalytic activity, outstanding stability and excellent anti-poisoning behavior toward the electrooxidation of ethanol. The enhanced electrocatalytic performance is mainly due 19 ACS Paragon Plus Environment


to synergic effects between Ni, Pd, MnO2 and rGO. It has been found that rGO plays a key role for the dispersion of metal nanoparticles, leading to smaller size and better uniform nanostructures. Moreover, the MnO2 nanoparticles enhance the dehydrogenation of ethanol oxidation and increase the formation of OHads species, leading to elimination of the poisoning intermediates. The remarkable catalytic performance also can be attributed to the effect of Ni on the electronic structure of Pd nanoparticles. Therefore, in view of its high catalytic performance, according to the obtained results, Pd-Ni/MnO2/rGO/GCE is a great promising candidate as anode catalyst for direct ethanol fuel cells.

Acknowledgment The authors gratefully acknowledge the support of this work by the Research Council and the Center of Excellence for Nanostructures of the Sharif University of Technology, Tehran, Iran.


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(43) Xu, C.; Tian, Z.; Shen, P.; Jiang, S.P. Oxide (CeO2, NiO, Co3O4 and Mn3O4)-Promoted Pd/C Electrocatalysts for Alcohol Electrooxidation in Alkaline Media. Electrochim. Acta 2008, 53, 2610-2618. (44) Wen, Z.; Yang, S.; Liang, Y.; He, W.; Tong, H.; Hao, L. The Improved Electrocatalytic Activity of Palladium/Graphene Nanosheets Towards Ethanol Oxidation by Tin Oxide. Electrochim. Acta 2010, 56, 139-144. (45) Maiyalagan, T.; Scott, K. Performance of Carbon Nanofiber Supported Pd-Ni Catalysts for Electro-Oxidation of Ethanol in Alkaline Medium. J. Power Sources 2010, 195, 5246-5251. (46) Raoof, J.B.; Hosseini, S.R.; Rezaee, S. Preparation of Pt/poly (2-Methoxyaniline)-Sodium Dodecyl Sulfate Composite and Its Application for Electrocatalytic Oxidation of Methanol and Formaldehyde. Electrochim. Acta 2014, 141, 340-348. (47) Shahrokhian, S.; Rezaee, S. Vertically Standing Cu2O Nanosheets Promoted Flower-Like PtPd Nanostructures Supported on Reduced Graphene Oxide for Methanol ElectroOxidation. Electrochim. Acta 2017, 259, 36-47. (48) Yang, S.; Zhang, X.; Mi, H.; Ye, X. Pd Nanoparticles Supported on Functionalized MultiWall Carbon Nanotubes (MWCNTs) and Electrooxidation for Formic Acid. J. Power Sources 2008, 175, 26-32. (49) Chatterjee, A.; Chatterjee, M.; Ghosh, S.; Basumallick, I. Electrooxidation of Isopropanol on to Pt Loaded Carbon Felt Surface Modified by Polyaniline. Int. J. Emerg. Sci. 2012, 2, 123-133. (50) Hasheminejad, E.; Ojani, R.; Raoof, J.B. A Rapid Synthesis of High Surface Area PdRu Nanosponges: Composition-Dependent Electrocatalytic Activity for Formic Acid Oxidation. J Energy Chem. 2017, 26, 703-711. (51) Zhou, W.; Du, Y.; Ren, F.; Wang, C.; Xu, J.; Yang, P. High Efficient Electrocatalytic Oxidation of Methanol on Pt/Polyindoles Composite Catalysts. Int. J. Hydrogen Energy 2010, 35, 3270-3279. (52) Zhang, F.; Zhou, D.; Zhang, Z.; Zhou, M.; Wang, Q. Preparation of Rh/C and its High Electro-Catalytic Activity for Ethanol Oxidation in Alkaline Media. RSC. Adv 2015; 5, 91829-91835.



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Figure captions Fig. 1. The SEM images of: (A) bare GCE, (B) rGO/GCE, (C) MnO2/rGO/GCE, (D) PdNi/MnO2/rGO/GCE. Fig. 2. (A) Energy dispersive spectra of Pd-Ni/MnO2/rGO/GCE. (B) Elemental mapping images for C, Mn, Pd and Ni. Fig. 3. (A) FTIR spectra and (B) Raman spectra of GO and rGO/MnO2. (C) XRD patterns of (a) MnO2/rGO and (b) Pd-Ni/MnO2/rGO. Fig. 4. (A) 2D and (B) 3D AFM images of rGO/GCE and Pd-Ni/MnO2/rGO/GCE surfaces. Fig. 5. Cyclic voltammograms of (a) Pd/C, (b) Pd/GCE, (c) Pd/rGO/GCE, (d) Pd-Ni/rGO/GCE and (e) Pd-Ni/MnO2/rGO/GCE in 0.5 M NaOH (A) and 0.5 M H2SO4 (B) solutions at a scan rate of 50 mV s-1. (C) CO stripping voltammograms in 0.5 M H2SO4 at scan rate of 10 mV s−1; (D) zoomed current vs. potential responses in the onset stripping region of CO on the prepared catalysts. Fig. 6. (A) Cyclic voltammograms, (B) chronoamperometric curves, (C) chronopotentiometric curves and (D) consecutive cyclic voltammograms of (a) Pd/C, (b) Pd/GCE, (c) Pd/rGO/GCE, (d) Pd-Ni/rGO/GCE and (e) Pd-Ni/MnO2/rGO/GCE in 0.5 M NaOH + 0.5 M ethanol solution. The cyclic voltammograms were recorded at 50 mV s−1. Chronoamperograms were recorded at peak potential values. Chronopotentiograms were recorded at 0.5 mA cm−2. Fig. 7. (A) Effect of upper limit potential on the oxidation of 0.5 M ethanol on PdNi/MnO2/rGO/GCE in 0.5 M NaOH solution at υ = 50 mV s−1: (a) -0.1 V, (b) 0.0 V, (c) 0.1 V, (d) 0.2 V and (e) 0.3 V. Insets: (A) plot of anodic peak current density in the forward (jf) () and backward scans (jb) (); (B) variation of anodic peak potential in the forward () and backward () scans; (C) plot of the ratio of jf/jb and difference between Epf-Epb as a function final potential. (B) CVs of Pd-Ni/MnO2/rGO/GCE in 0.5 M NaOH + 0.5 M ethanol solution at various scan 27 ACS Paragon Plus Environment


rates in V s−1 (a) 0.03, (b) 0.05, (c) 0.08, (d) 0.1, (e) 0.2, (f) 0.4. The insets: (A) relationship between forward anodic peak current density vs scan rate (curve a) and vs. square root of scan rate (curve b); (B) relationship between forward anodic peak potential and logarithm of scan rates; (C) relationship between jf/jb ratios and scan rates. Fig. 8. Nyquist plots for ethanol electrooxidation in 0.5 M ethanol + 0.5 M NaOH solution on (a) Pd/GCE, (b) Pd/rGO/GCE, (c) Pd-Ni/rGO/GCE and (d) Pd-Ni/MnO2/rGO/GCE at electrode potential of -0.3 V. Fig. 9. Nyquist plots of ethanol electrooxidation on Pd-Ni/MnO2/rGO/GCE (A–C) and PdNi/rGO/GCE (D-F) in 0.5 M ethanol + 0.5 M NaOH at the potential range of -0.6 – 0.4 V.


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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5



Figure 6


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Figure 7



Figure 8


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Figure 9



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Table 1. Surface parameters of Pd deposited on different modified GCEs.

ESCA (m2g-1Pd )

Eonset of CO stripping (V)

EP,PdO reduction (V)

Pd/GCE

17.48

0.574

0.473

Pd/C/GCE

23.64

0.563

0.458

Pd/rGO/GCE

28.13

0.557

0.452

Pd-Ni/rGO/GCE

41.23

0.451

0.427

Pd-Ni/MnO2/rGO/GCE

53.72

0.423

0.368

Electrocatalyst


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Table 2. Electrochemical characteristics for ethanol oxidation on different modified electrodes in

0.5 M NaOH and 0.5 M ethanol at 50 mV s-1. Eonset

jf

(V)

-2 ( mAcm )

(V)

-2 ( mAcm )

Pd/GCE

-0.39

4.2

-0.062

11.98

-0.16

0.35

Pd/C/GCE

-0.41

11.4

-0.071

17.81

-0.17

0.54

Pd/rGO/GCE

-0.42

13.8

-0.073

19.24

-0.19

0.71

Pd-Ni/rGO/GCE

-0.55

54.4

-0.142

39.37

-0.28

1.38

Pd-Ni/MnO2/rGO/GCE

-0.72

115.2

-0.217

47.76

-0.39

2.2

Electrocatalyst

Epf

jb


Epb

jf/jb

(V)


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Table 3. Comparison of the parameters for electrocatalytic oxidation of ethanol at PdNi/MnO2/rGO/GCE with several chemically modified electrodes.

Modified electrode

Eonset (V)

Epf (V)

Pd/Cu2O/MWCNT Pd/MnO2/GNR

-0.71 -0.77

-0.21 -0.18

2.67 0.86

(21) (23)

Pt/MnOx-CNTs

-0.15

0.36

1.17

(48)

Pd-NiO/C

-0.72

-0.18

1.90

(49)

Pd-CeO2/C

-0.70

-0.19

0.91

(49)

PdPd-Co3O4/C

-0. 69

-0. 24

1.02

(49)

Pd/SnO2/graphene

-0.60

-0.05

1.67

(50)

Pd-Ni/CNF

-0.70

-0.17

0.75

(51)

PdNPs/SWCNT/CCE

-0.60

0.058

1.84

(52)

Pd-Ni/MnO2/rGO/GCE

-0.72

-0.21

2.2

This work


jf/jb

Ref.

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Table 4. Comparison of the electrochemical activities of the electrodes prepared with different Pd-Ni ratio on the surface of MnO2/rGO/GCE. Pd-Ni ratio*

jf (mAcm-2)

Eonset (V)

jf/jb

Pd (100)

43.1

-0.51

0.82

Pd-Ni (90:10)

91.8

-0.61

0.88

Pd-Ni (80:20)

109.4

-0.67

1.67

Pd-Ni (70:30)

115.2

-0.72

2.20

Pd-Ni (60:40)

114.3

-0.70

1.96

Pd-Ni (50:50)

107.5

-0. 66

1.82

Pd-Ni (40:60)

89.7

-0.62

1.53

Pd-Ni (30:70)

85.9

-0.57

1.25

Pd-Ni (20:80)

77.6

-0.55

0.90

* total concentration of two salts equal to 1 mM



"TOC Graphic"


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A Nanocomposite with Promoted Electrocatalytic Behavior Based on

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