Nanosheet Arrays on Nickel Foam for Methanol ... - ACS Publications

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Kinetics, Catalysis, and Reaction Engineering

Fabrication of Coral-like Pd based Porous MnO2 Nanosheet Arrays on Nickel Foam for Methanol Electro-Oxidation Yu Cheng, Meisong Guo, Yanan Yu, Miaomiao Zhai, Rui Guo, and Jingbo Hu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02059 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 27, 2018

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Industrial & Engineering Chemistry Research

Fabrication of Coral-like Pd based Porous MnO2 Nanosheet Arrays on Nickel Foam for Methanol Electro-Oxidation Yu Chenga, Meisong Guoa, Yanan Yua, Miaomiao Zhaia, Rui Guoa and Jingbo Hu*a, b a

College of Chemistry, Beijing Normal University, Beijing 100875, PR China.

b

Key Laboratory of Beam Technology and Material Modification of Ministry of Education,

Beijing Normal University, Beijing 100875, PR China *Corresponding author. E-mail addresses: [email protected] Telephone number: +86-010-58807843

ACS Paragon Plus Environment

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Abstract

Direct methanol fuel cells (DMFCs) have been seen as one of the desirable power sources and the performance always depends on the ability of electrode materials to oxidize methanol. In this work, we report a novel electrode composed of coral-like Pd and porous MnO2 nanosheet arrays on nickel foam (Pd-MnO2/NF). MnO2 and Pd are successively electrodeposited on NF with different concentrations of PdCl2, which decides the shape of Pd and catalytic properties. Compared with Pd/NF, Pd-MnO2/NF exhibits excellent catalytic performance and good stability for methanol oxidation in alkaline solution. The mass activity of Pd-MnO2/NF with low Pd loading is 197.7 mA mg-1, which is 2.3-fold higher than that of Pd/NF. The improvement of performance mainly comes from MnO2, which supplies a number of oxygen atoms to reduce intermediates and enhance anti-poisoning ability.

Keywords: coral-like palladium; manganese dioxide nanosheet; nickel foam; methanol electrooxidation; electrocatalysis


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1. Introduction In the face of the environmental pollution and energy crises, methanol has been considered as a clean and renewable fuel with high energy density.1 As power sources, direct methanol fuel cells (DMFCs) have advantages of low exhaust, low operational temperature, high efficiency and large energy density.1-4 In DMFCs, methanol oxidation reaction (MOR) is the key step and the performance of cells depends on the electrocatalytic materials. At present, Pt and Pt-based alloys have been reported as the best catalyst materials for MOR. However, the high price, low content, and poisoning effect of Pt lead us to find other alternatives. Pd-based materials have drawn wide attention owing to their lower price and higher content. More importantly, they can promote the oxidation of CO intermediates.5,

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A great

number of Pd-based binary, ternary and quaternary catalysts have been prepared for catalysis, such as Pd-Au,7 Pd-Ru,5 Pd-Ce,8 Pd-Cu,9 PdAuSn,10 PtPdRuTe11 and so on. They have shown the improved electrocatalytic performance. In addition, Pd can be modified by transition metal oxides. Transition metal oxides have been considered as promising materials in energy conversion and storage owing to their cost effectiveness, facile synthesis, stability and high electrocatalytic activity.2, 12-14 But most of all, metal oxides as a promoter can afford oxygen-containing species, which accelerate the oxidation of intermediates or reduce the poisoning species. The high anti-poisoning ability in the alkaline solution makes metal oxides popular in the field of electrocatalysis, especially combined with Pd. For example, MoOx on Pd/C,15 Pd/SmOx-C,16 CeO2 and SnO2,17 Eu oxide18 and so on. Among the metal oxides, MnO2 is an environmental-friendly and low cost material. It has been widely used in the urea oxidation,19 detections of heavy metals20 and hydrogen peroxide.21 Mesoporous architecture of materials is important to enhance catalytic performance,19, 22-25 which


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enlarges electroactive surface area and provides abundant open channels for electrolyte diffusion. In this work, porous MnO2 nanosheet arrays (MnO2 NSs) have been demonstrated a superior platform for Pd particles by promoting the catalytic activity. Traditionally, most catalysts were immobilized on electrodes by mixing with polymer binder or carbon, which increase resistance and hinder active sites.26, 27 Directly growing materials on active substrate is an excellent method, which can be used in the fabrication of electrodes for MOR.2,

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Furthermore, the construction of electrodes always requires harsh conditions and

complex operations. Electrodeposition as a facile and efficient method shows great promise in the direct decoration of electrocatalysts on substrates. Nickel foam (NF) with special porous 3D structure, mechanical flexibility and electrical conductivity is always used as the current collector, which increases specific surface areas and accelerates the release of gaseous products.29, 30 So far as we know, this is the first report on Pd as an electrocatalyst, porous MnO2 as a promoter and NF as a conductive substrate, simultaneously using for methanol oxidation. Herein, we have successfully synthesized a new Pd-MnO2/NF electrode via a facile two-step electrodeposition method. The catalytic performance of Pd-MnO2/NF has been discussed by oxidizing methanol in alkaline solution. Pd-MnO2/NF displays higher catalytic activity than Pd/NF owing to the special shape of Pd particles and abundant O atoms provided by porous MnO2 NSs. Furthermore, the shape of Pd particles and catalytic property are influenced by the concentration of Pd precursor. In brief, Pd-MnO2/NF is an ideal electrode for methanol oxidation. 2. Experimental section 2.1. Chemicals and materials


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Ni foam (30 mg cm-2, 0.1 mm-10 mm in pore size, 0.5 mm in thickness) was purchased from Shenzhen Lifeixin Reagent Co. Ltd. PdCl2 was purchased from Beijing InnoChem Science & Technology Co., Ltd. Manganese (II) chloride tetrahydrate (MnCl2·4H2O), sodium hydroxide (NaOH), methanol, ethanol, acetone, hydrochloric acid (HCl) were obtained from Beijing Chemical Reagents Co. Ltd. All the chemicals were of analytical reagent grade and used as received without further purification. Triple-distilled water was used throughout all experiments. 2.2. Preparation of the Pd-MnO2/NF electrodes Prior to use, the Ni foam was cleaned ultrasonically with acetone, 3 mol L-1 HCl solution and deionized water for 15 min, respectively. Then the as-cleaned NF substrate was rinsed with ethanol and dried by N2 before next step. The Pd-MnO2/Ni foam electrodes were prepared in two steps. Step one consisted of electrodeposition of porous MnO2 NSs on Ni foam by applying a cyclic voltammetry, which was achieved at a scan rate of 5 mV s-1 for 7.5 cycles and the voltage range of -1.2 V to 0.2 V (vs. Ag/AgCl).31 The electrodeposition solution contained 0.1 mol L-1 HCl and 10 mmol L-1 MnCl2. The surface of NF changed from silver to brown. Second step, Pd catalyst was electrodeposited onto the as-prepared MnO2 NSs precursor from 0.1 mol L-1 HCl solution containing different concentrations of PdCl2 (1 mmol L-1, 10 mmol L-1, 25 mmol L-1, 50 mmol L-1 and 100 mmol L-1), named as Pd-MnO2/NF-1, Pd-MnO2/NF-2, Pd-MnO2/NF-3, PdMnO2/NF-4 and Pd-MnO2/NF-5, respectively. The deposition potential was held at -0.5 V (vs. Ag/AgCl) for 10 seconds.32 The surface of MnO2/NF changed from brown to black. Pd/NF was also prepared for comparison using the same constant potential and solution but it was directly deposited on bare NF. The Pd loading of Pd-MnO2/NF-2 and Pd/NF-2 were 0.150 mg cm-2 and 0.125 mg cm-2, respectively. The mass activity was obtained by normalizing currents with loading mass of Pd.


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2.3. Materials characterization The morphologies of the as-prepared electrodes were characterized by a field emission scanning electron microscopy (FESEM, Hitachi S-8010, Japan) with an accelerating voltage of 10 kV. The FESEM was also equipped with an energy dispersive X-ray spectrometer (EDX) for elemental analysis. In order to reveal the microstructure of materials, the transmission electron microscopy (TEM) measurements were performed on a FEI Talos F200S electron microscope (FEI, USA) with an accelerating voltage of 200 kV. The phases of electrodes were recorded by X-ray diffraction patterns using a diffractometer (XRD, Rigaku Dmax-B, Japan) with Cu Kα source that was operated at 40 kV and 40 mA. X-ray photoelectron spectra (XPS) were determined by an AXIS-Ultra instrument (Kratos, UK) to analyze the chemical elements and its states. Pd-MnO2/NF-2 is seen as the best one to do further characterizations including EDX, TEM, XRD and XPS, which is decided by the results of FESEM and CVs. Pd-MnO2/NF-2 is also briefly written as Pd-MnO2/NF in this article. 2.4. Electrochemical measurements Electrochemical experiments were carried out with a three-electrode system on an electrochemical workstation (CHI660D, Shanghai Chenhua Instruments, Inc.). A platinum wire, Ag/AgCl, and bare or modified Ni foam (NF, 5 mm*15 mm) were used as the counter, reference, and working electrode, respectively. The electrochemical activity of the modified electrode was measured by cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). An aqueous NaOH solution was used as the electrolyte for the methanol electro-oxidation. All electrochemical measurements were carried out at room temperature. Pd-MnO2/NF-1 is not shown because the Pd loading is too low to have electroactivity.


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3. Results and discussion The schematic illustration for the preparation of electrodes is shown in Fig. 1. Firstly, MnO2 NSs were electrodeposited on NF by applying a cyclic voltammetry (Eqs. (1)) and formed a porous structure. Then Pd catalyst was electrodeposited on the surface of MnO2/NF with different concentrations of PdCl2 under a constant potential (Eqs. (2)). In contrast, Pd/NF was also prepared. Dramatically, the presence of MnO2 impacts the morphology of Pd particles. ‫݊ܯ‬ଶା + 2‫ܪ‬ଶ ܱ → ‫ܱ݊ܯ‬ଶ + 4‫ ܪ‬ା + 2݁ ି

(1)

ܲ݀ ଶା + 2݁ ି → ܲ݀

(2)

3.1. Morphology and structure of MnO2/NF substrate covered with Pd FESEM images of electrodes are shown in Fig. 2 and Fig. S1 (in the Supporting Information). Fig. 2A shows the image of bare NF with a three-dimensional (3D) porous framework and smooth surface, which can maximize the catalyst loading and facilitate the diffusion of reactants. After the electrodeposition of MnO2 NSs, a layered structure covers the NF surface and increases roughness of the substrate (Fig. 2B). Under the higher resolution (Fig. 2C), the deposited MnO2 NSs are vertically aligned nanosheet arrays with an average length about 1-1.5 µm. These uniform MnO2 NSs are intercrossed and form another porous architecture, which vastly enlarge the surface area of electrodes and provide abundant open channels for electrolyte penetration during electro-oxidation. In order to illustrate the point, we measure CV curves in the solution of 1 mmol L-1 K3[Fe(CN)6] in 0.5 mol L-1 KCl. The effective surface area is calculated by the Randles-Sevcik equation: ݅௣,௖ = 2.69 × 10ହ ݊

ଵൗ ଷൗ ଵ ଶ ‫ ܦ‬ଶ ‫ ݒܣ‬ൗଶ ܿ ைೣ

(3)

Where i is the cathode peak current (A), n is the gained or lost electrons, Dox is the diffusion coefficient of oxidation state (cm2 s-1), A is the effective surface area of electrode (cm-2), v is the


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scan rate (V s-1), c is the concentration of K3[Fe(CN)6] (mol L-1). The ESA of MnO2 (0.439 cm2) is larger than that of bare electrode (0.196 cm2), which confirms the large surface area of porous MnO2 NSs. In order to increase the electroactivity, the obtained MnO2/NF is further used as a substrate to electrodeposit Pd particles with different concentrations. It is clear that the morphologies of Pd particles are closely related to the concentrations of electrodepositing solution (Pd loading). The prepared Pd-MnO2/NF-2 is shown in Fig. 2D, special coral-like Pd particles are evenly dispersed on MnO2 NSs with the irregular surface, which can increase the amount of electroactive sites. In addition, uncovered MnO2 NSs surface is also captured. With closer observation shown in Fig. 2E, the coral is clearly seen with a length around 400nm and width around 30-50nm. The formation of coral may be caused by the structure of MnO2 NSs--that is to say, coral-like Pd particles probably grow along the vertical plane of nanosheet, which is just speculation without evidence. In this structure, the primary porous NF and secondary porous MnO2 NSs with large surface area provide enough space for the diffusion of reactant ions and fast escape of gaseous products, while the coral-like Pd particles with a high aspect ratio and uniform size provide abundant active sites for the electro-oxidation of methanol molecules. It is noteworthy that the MnO2 NSs also furnish large amount of oxygen atoms, which facilitate the electro-oxidation of the poison. This can be proved in the later research. In Fig. S1A, there are more coral-like Pd particles attaching to MnO2 NSs and fewer MnO2 NSs are exposed in Pd-MnO2/NF-3. Magically, when the concentration of PdCl2 is increased sequentially, the morphologies of Pd particles are changed from corals to spheres. The spheres are crowded together and overlapped in Pd-MnO2/NF-4 (Fig. S1B), while layered Pd with many humps cover on MnO2 NSs without interspace in Pd-MnO2/NF-5 (Fig. S1C). Apparently, the


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loading of Pd particles on MnO2 NSs increases with the concentrations of precursor. In contrast, the Pd /NF-2 is also characterized with FESEM. It is clear that the Pd particles are shown as spheres with different diameters and nonuniform distribution (Fig. 2F). Comparing the PdMnO2/NF-2 with Pd/NF-2, the introduction of MnO2 NSs enhances the interaction between Pd and substrate, which increases the Pd loading. It is meaningful to reduce the dosage of precursor in the practical application. The chemical composition and elemental distribution of the Pd-MnO2/NF and MnO2/NF are characterized by energy-dispersive X-ray spectroscopy (EDX). The elemental mappings indicate the uniform dispersion of elements O, Mn, Ni, Pd (Fig. 2G) and elements O, Mn, Ni (Fig. S2) on surface. Since the surface MnO2 NSs are porous structure, the Ni atoms are still exposed and probed through the pores of MnO2 NSs. The EDX spectra are shown in Fig. S3. No other peaks are found that confirm the absence of impurity elements. To establish the microstructure of Pd-MnO2/NF, the TEM and high resolution (HR)TEM were carried out. Before characterization, the Pd-MnO2 and MnO2 were scratched off from the Ni foam ultrasonically. As shown in Fig. 3A, the MnO2 NSs are ultra-thin and look like graphene, providing bigger surface area. It is clear that some lumps exist on the MnO2 with different shapes. From the HRTEM image (Fig. 3B) of selected lump, well-defined lattice fringes are observed. The fringe spacing of 0.222 nm can be indexed to the (111) plane of the cubic Pd (JCPDS card No.65-6174) , which illustrates the lumps are crystalline Pd. From the TEM image of MnO2 in Fig. 3C, many flakes overlap and numerous pores are distributed uniformly (blue rectangles). Fig. 3D shows the HRTEM of marked circle (red) in Fig. 3C, which displays the amorphous nature of MnO2. In fact, the morphology of MnO2 and Pd-MnO2 are both changed after stripping from Ni foam ultrasonically. The MnO2 is broken into small pieces and then stack together. Pd particles


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are also destroyed into different shapes. At the same time, some magnetic Ni particles are stripped from Ni foam, which can be seen from the TEM images (Fig. S4). The lattice spacing matches well with the plane of Ni. Because of the particularity of Ni foam as a substrate, the TEM characterization cannot truly reflect the structure of materials, but can illustrate the crystalline Pd and amorphous nature of MnO2. To further determine the structure and composition of the electrodes, we have carried out XRD measurement, as shown in Fig. S5. Owing to the strong peaks of NF and a small amount of deposited Pd and MnO2, the diffraction peaks of Pd and MnO2 are not found in patterns of Pd/NF and MnO2/NF. Only three diffraction peaks of Pd are probed in pattern of Pd-MnO2/NF. Then we deposit Pd-MnO2 on ITO electrode under the same condition (Fig. 4A). Three weak but identifiable diffraction peaks of face-centered cubic (FCC) Pd are probed in pattern of Pd-MnO2 and Pd. The peaks located at 2θ values of 40.0°, 46.6° and 68° correspond to the (111), (200) and (220) planes of Pd (JCPDS No.46-1043). The weak peaks illustrate that the amount of Pd is little by electrodepositing. There is no diffraction peak of MnO2, demonstrating the amorphous structure of it. Furthermore, the existence of a broad peak at 2θ=20°-30° illustrates the modification of Pd or MnO2 on electrodes. Specially, in the pattern of Pd-MnO2, the broad peak is more obvious and sharp, which may be caused by the interaction between Pd and MnO2. The other peaks are corresponding to the ITO substrate. XPS measurement is carried out to confirm the chemical composition and valence states. The survey spectra shown in Fig. 4B clearly depict the scans of the Pd/NF, MnO2/NF and PdMnO2/NF, which illustrate the existence of Mn, Pd, Ni, O and C elements. An atomic Mn/O ratio of approximately 2:5 is determined and the extra O atoms may come from the surface oxidation


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of Pd and Ni. More details explanation can be explained by the high resolution of Ni 2p, Pd 3d, Mn 2p and O 1s XPS scan. As shown in Fig. 4C, the banding energies of Ni 2p1/2 and Ni 2p3/2 at 873.6 eV and 855.8 eV with satellite peaks, respectively, reveal the presence of Ni2+. The presence of Pd and MnO2 can enhance the intensity of Ni 2p. The XPS spectra of Pd 3d for Pd/NF and Pd-MnO2/NF are compared in Fig. 4D. In Pd-MnO2/NF, Pd has Pd 3d3/2 and Pd 3d5/2 peaks at 340.6 and 335.2 eV, respectively. Both the Pd 3d5/2 and Pd 3d3/2 signals can be further split into two pairs of overlapping Lorentzian-Gaussian curves that are ascribed to Pd (0) and Pd (II). The peaks at 341.4 and 336.0 eV are in the form of oxide Pd, while the rest peaks are attributed to metallic Pd. The Pd (II) may come from the partial oxidation during XPS analysis. In comparison with the Pd/NF, the Pd 3d binding energies of Pd-MnO2/NF are negatively shifted by 0.5 eV relative to the binding energies of Pd (0) and Pd (II), indicating that Pd get electrons from his neighbors.33 The Mn 2p spectra of MnO2/NF and Pd-MnO2/NF are shown in Fig. 4E. The Mn 2p of MnO2/NF shows two distinguishable peaks at 653.2 and 641.5 eV corresponding to Mn 2p1/2 and Mn 2p3/2, respectively. A spin orbit coupling value of 11.7 eV is in good accordance with previous works,19, 34, 35 which indicates the four-valence of Mn element (Mn4+) and the form of MnO2. After the electrodeposition of Pd, both the peaks of Mn 2p1/2 and Mn 2p3/2 exhibit a positive shift of 0.7 eV, which is the opposite direction compared to the shift of Pd 3d. The results imply a strong electronic effect between the elements. Possibly, the electron affinity of Pd induces the partial electron transfer between Pd and MnO2, thereby enhancing the electronic interaction.33 For the XPS profile of O 1s in MnO2/NF as shown in Fig. 4F, the fitting peaks at binding energy of 529.9 eV and 531.4 eV can be assigned to oxygen-metal bonds in MnO2 and surface


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adsorbed OH-,19, 36, 37 respectively. After the electrodeposition of Pd, the peak of OH- disappears, which indicates the absence of physically adsorbed oxygen. The peak of oxygen-metal bonds moves to 530.9 eV, which can be attributed to the interaction between Pd and MnO2. The results reveal that the MnO2 in Pd-MnO2/NF can provide rich O atoms, which contribute to the fast electro-oxidation of intermediates. 3.2. Electrochemical characterization of as-prepared electrodes The electrochemical properties of Pd-MnO2/NF without methanol are first investigated. In contrast, Pd/NF, MnO2/NF and bare NF are also studied. Fig. 5A depicts the CV curves of Pd/NF-2, Pd-MnO2/NF-2, MnO2/NF and bare NF in a potential range of -1.2-0.8 V (vs. Ag/AgCl) under a scan rate of 50 mV s-1 in 1 mol L-1 NaOH. The MnO2/NF and bare NF have no other peaks except peaks of Ni2+/Ni3+. In Fig. 5B, the peaks at about -0.56V can be attributed to the oxidation of Pd, which confirm the formation of a palladium oxide layer on the surface of the materials. The oxygen may be from the MnO2 and chemisorbed OH-, then Pd is transformed into PdO.38, 39 Comparatively, the peaks at around -0.33V come from the reduction of PdO. All the electrodes show oxidation peaks at about 0.5-0.6 V and reduction peaks at around 0.28-0.35 V. The pair of redox peaks are ascribed to the redox reactions of Ni2+/Ni3+ in the alkaline electrolyte. The shift of peaks may be caused by the interaction between components. The possible mechanism can be described as following:38, 40-43 ‫ܱ݊ܯ‬ଶ + ‫ܪ‬ଶ ܱ ↔ ‫ ܪܱܱ݊ܯ‬ା + ܱ‫ି ܪ‬

(4)

ܲ݀ + ‫ ܪܱܱ݊ܯ‬ା + 2ܱ‫ ݀ܲ ↔ ି ܪ‬− ܱ‫ ܪ‬+ ‫ ܱ݊ܯ‬− ܱ + ‫ܪ‬ଶ ܱ + ݁ ି

(5)

ܲ݀ − ܱ‫ ܪ‬+ ܲ݀ − ܱ‫ ݀ܲ ↔ ܪ‬− ܱ + ‫ܪ‬ଶ ܱ

(6)

ܲ݀ − ܱ‫ ܪ‬+ ܱ‫ ݀ܲ ↔ ି ܪ‬− ܱ + ‫ܪ‬ଶ ܱ + ݁ ି

(7)

ܲ݀ + ܱ‫ ݀ܲ ↔ ି ܪ‬− ܱ‫ܪ‬௔ௗ௦ + ݁ ି

(8)


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ܲ݀ − ܱ‫ܪ‬௔ௗ௦ + ܱ‫ ݀ܲ ↔ ି ܪ‬− ܱ + ‫ܪ‬ଶ ܱ + ݁ ି

(9)

ܲ݀ − ܱ‫ܪ‬௔ௗ௦ + ܲ݀ − ܱ‫ܪ‬௔ௗ௦ ↔ ܲ݀ − ܱ + ‫ܪ‬ଶ ܱ

(10)

ܲ݀ − ܱ + ‫ܪ‬ଶ ܱ + 2݁ ି ↔ ܲ݀ + 2ܱ‫ି ܪ‬

(11)

ܰ݅ሺܱ‫ܪ‬ሻଶ + ܱ‫ ܪܱܱ݅ܰ ↔ ି ܪ‬+ ‫ܪ‬ଶ ܱ + ݁ ି

(12)

In contrast, the redox peaks of Pd in Pd-MnO2/NF-2 exhibit higher current densities than that of Pd/NF-2, which can be contributed to the presence of MnO2 NSs. The porous MnO2 NSs provide a bigger surface area for the deposition of Pd and increase the Pd loading. Furthermore, MnO2 can also supply more oxygen which promote the inter-conversion of Pd to PdO. Fig. S6 shows the CV curves of electrodes prepared from 25, 50 and 100 mmol L-1 PdCl2 solutions. The peak current densities of Pd/PdO in Pd-MnO2/NF are higher than that of Pd/NF as well. But the peak current densities of Pd-MnO2/NF-5 are similar to that of Pd/NF-5. The current densities of Pd/PdO in Pd-MnO2/NF are not higher than that of the Pd alone when the concentration is 100 mmol L-1. In this concentration, large amounts of Pd entirely cover on MnO2 without interspace and then hinder the interaction between Pd and MnO2. Therefore, the MnO2 has a significant impact when the PdCl2 concentration (Pd loading) is low. Fig. 6 depicts the CV curves of electrodes in solutions containing 1 mol L-1 NaOH and 0.5 mol L-1 CH3OH with the potential scanning from -0.6 to 0.4 V (vs. Ag/AgCl) at a scan rate of 50 mV s-1. In Fig. 6A and B, no catalytic activity toward the electro-oxidation of methanol is observed in MnO2/NF and bare NF, while Pd-MnO2/NF and Pd/NF show distinct catalytic activities. Fresh methanol is first oxidized to CO2, CO and other carbonaceous intermediates on Pd-based electrodes at an onset potential (~-0.34 V) during the forward scan, with continual increase of the current density until anodic peaks appear at around -0.04 V and -0.10 V, corresponding to PdMnO2/NF and Pd/NF, respectively. Then the current density is reduced owing to the coverage of


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carbonaceous intermediates on the active sites and Pd oxides are formed at a higher potential. The shift of the first oxidation peak position may be due to the electronic interaction between the Pd and MnO2. During the backward scan, the electrode surface is re-activated with the reduction of Pd oxide, then the adsorbed carbonaceous species are further oxidized to CO2, corresponding to the second oxidation peak at about -0.21 V (Pd-MnO2/NF) and -0.23 V (Pd/NF). The oxidation can be described by the following reactions: ܲ݀ + ‫ܪܥ‬ଷ ܱ‫ ܪ‬+ 4ܱ‫݀ܲ → ି ܪ‬ሺ‫ܱܥ‬௔ௗ௦ ሻ + 4‫ܪ‬ଶ ܱ + 4݁ ି

(13)

ܲ݀ሺ‫ܱܥ‬௔ௗ௦ ሻ + 2ܱ‫ ݀ܲ → ି ܪ‬+ ‫ܪ‬ଶ ܱ + ‫ܱܥ‬ଶ + 2݁ ି

(14)

ܲ݀ሺ‫ܱܥ‬௔ௗ௦ ሻ + ‫ ܪܱܱ݊ܯ‬ା + 3ܱ‫ ݀ܲ → ି ܪ‬+ ‫ܱ݊ܯ‬ଶ + 2‫ܪ‬ଶ ܱ + ‫ܱܥ‬ଶ + 2݁ ି

(15)

The mass activity and the peak potential in the forward scan of Pd-MnO2/NF and Pd/NF are employed to compare their catalytic activity. Though the peak potential of Pd-MnO2/NF-2 (-0.04 V) is more positive than that of Pd/NF-2 (-0.10 V), the mass activity of Pd-MnO2/NF-2 (197.7 mA mg-1) is 2.3-fold higher than that of Pd/NF-2 (59.7 mA mg-1). The same conclusions can be drawn in Fig. 6C and D. The mass activity of Pd-MnO2/NF-3 (169.7 mA mg-1, 0 V) is 1.9-fold higher than that of Pd/NF-3 (56.8 mA mg-1, -0.07 V), while the mass activity of Pd-MnO2/NF-4 (156.5 mA mg-1, 0.037V) is 1.1-fold higher than that of Pd/NF-4 (75 mA mg-1, -0.025 V). Overall, The Pd-MnO2/NF has a better catalytic activity than Pd/NF. Dramatically, the mass activity and the peak potential of Pd-MnO2/NF-5 are close to those of Pd/NF-5 as shown in Fig. 6E. It is clear that the difference between Pd/NF and Pd-MnO2/NF decreases as the increase of Pd concentration until they reach an equal level. This may be caused by the extremity of Pd loading, which is in accord with the CVs without methanol. Fig. 6F exhibits the CVs of PdMnO2/NF-2, 3, 4 and 5. The first anodic peak potential of Pd-MnO2/NF is positively shifted with the increase of Pd concentration, which demonstrates that the oxidation of methanol is more


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difficult. Furthermore, the mass activity of Pd-MnO2/NF-2, 3 and 4 decrease as the increase of Pd loading. Magically, the mass activity of Pd-MnO2/NF-5 is higher than that of Pd-MnO2/NF-3 and 4, but it has a more positive potential. Therefore, Pd-MnO2/NF-2 has the strongest ability to enhance the electroactivity. The ratio of forward peak current density (If) to backward peak current density (Ib) can evaluate the tolerance limit of the catalysts toward poisoning by the adsorbed carbonaceous species.44, 45 A higher ratio represents a stronger ability to resist poison, which is favorable to the entire oxidation of methanol.11 The ratios of If/Ib for Pd-MnO2/NF-2, 3, 4 and 5 are calculated to be 2.79, 2.67, 2.40 and 2.42, respectively. This indicates that the Pd-MnO2/NF electrodes have a good tolerance of poisoning, while the Pd-MnO2/NF-2 is the best one. Therefore, the oxidation of carbonaceous intermediates formed in the forward scan is more effective under a lower Pd loading. In order to research the generation of intermediates, we conduct another CV measurement as shown in Fig. 7.44 In cycle 1, the CV is scanned in 1 mol L-1 NaOH without CH3OH, which exhibits the nonexistence of oxidation peak. In cycle 2, 0.5 mol L-1 methanol is rapidly added with stirring at the ending of the forward scan. The intermediates have not been generated at this point, therefore, the backward peak is entirely caused by the fresh methanol. And then the cycle 3 is carried on. In comparison with the second backward peak, the extra area of the third backward peak is induced by intermediates produced during the third forward scan. The percent of extra areas are calculated as about 38.5 % and 63.1 %, corresponding to Pd-MnO2/NF-2 and Pd/NF-2, respectively. The result shows that Pd-MnO2/NF generates fewer intermediates than Pd/NF, which indicates a better catalytic ability in methanol oxidation. Therefore, the methanol can be oxidized quickly under the presence of MnO2.


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Base on the measurement results, some points are listed as follows. First, in NaOH solution without CH3OH, the presence of MnO2 NSs increases the Pd loading and supplies oxygen atoms to promote the conversion of Pd/PdO. The current densities of Pd-MnO2/NF are higher than that of Pd/NF. However, the former is equal to the latter when the concentration of PdCl2 increases to 100 mmol L-1. Second, in the solution containing NaOH and CH3OH, though the peak potentials of Pd-MnO2/NF are more positive than that of Pd/NF, Pd-MnO2/NF has better catalytic performance because of much higher mass activity. Third, when the Pd loading is lower, PdMnO2/NF has higher tolerance limit, more negative potential, higher-fold increase of mass activity and fewer intermediates. However, Pd-MnO2/NF-1 has no catalysis because of a little amount of Pd, which illustrates the importance of suitable concentration. Hence, Pd-MnO2/NF-2 is the best one. Forth, Pd particles exhibit special shape of coral by optimizing PdCl2 concentration. The coral-like Pd provides lots of active sites to catalyze methanol oxidation, meanwhile, MnO2 NSs supply abundant oxygen atoms for the oxidation of intermediates. A small amount of Pd covers the MnO2 NSs with enough interspace, which is in favor of the delivery of O atoms and accelerates the interaction between Pd and MnO2. In order to observe the stability of Pd-MnO2/NF towards methanol oxidation, the best one is compared with Pd/NF by chronoamperometric curves (CA) as shown in Fig. 8A, with a constant potential of -0.05 V (vs. Ag/AgCl) in 1 mol L-1 NaOH containing 0.5 mol L-1 CH3OH for 6000s. In the initial stage, the high concentration of adsorbed methanol lead to a high mass activity. Afterwards, the mass activity decays sharply owing to the generation of intermediates and concentration polarization. Then the mass activity remains stable for a long period. The PdMnO2/NF shows a higher mass activity than Pd/NF, which illustrates a better stability for CH3OH oxidation, in other words, the incorporation of MnO2 can enhance the activity of


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methanol oxidation.38 The durability of Pd-MnO2/NF is evaluated by cycling 500 cycles in 1 mol L-1 NaOH with 0.5 mol L-1 CH3OH at scan rate of 50 mV s-1. The first and 500th cycles are shown in the inset image of Fig. 8A. The mass activity of Pd-MnO2/NF remain 80.7% after 500 cycles, proving the good durability. Electrochemical impedance spectroscopy (EIS) is further conducted to investigate the electrochemical properties of electrodes. The Fig. 8B displays the Nyquist plots of Pd-MnO2/NF and MnO2/NF in the frequency ranging from 100 KHz to 1 Hz in 1 mol L-1 NaOH with 0.5 mol L-1 CH3OH. At the low frequency, a straight line indicates the Warburg impedance (W). The curve of Pd-MnO2/NF exhibits a larger slope than that of Pd/NF, suggesting that Pd-MnO2/NF offers faster electrolyte ion transport and diffusion. At the high frequency, the intersection of the curve at the real axis corresponds to the internal resistance (Rs), while the diameter of semicircle represents the charge transfer resistance (Rct). From the enlarged image (Fig. 8B inset), the Rs value of Pd-MnO2/NF is higher than that of Pd/NF, which can be attributed to the poor conductivity of MnO2. The nearly negligible semicircle indicates rapid charge transfer. PdMnO2/NF exhibits a smaller Rct value than Pd/NF, indicating faster charge-transfer kinetics at the interface of electrode and electrolyte. CV measurements of Pd-MnO2/NF at different scan rates (a-g) are conducted in 1 mol L-1 NaOH with 0.5 mol L-1 CH3OH. As shown in Fig. 8C, the mass activity increases with the increase of the scan rate and the peak potential displays a positive shift which indicates a kinetically limited process. A linear relationship is got on the mass activity versus the square root of the scan rate, which suggests that the electro-oxidation of methanol is a diffusion-controlled process on the surface of the Pd-MnO2/NF. The correlation coefficient is 0.9797 as shown in Fig. 8D.


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Fig. 9A displays the CV plots in the absence (a) and presence of different concentrations of methanol (b-i) in 1 mol L-1 NaOH for Pd-MnO2/NF. In the absence of methanol, no oxidation peak is observed. An obvious peak appears when the concentration of methanol is over 0.05 mol L-1. The mass activity positively increases with the increase of methanol concentration (Fig. 9B), which indicates a fast process of methanol oxidation. Furthermore, the If/Ib ratios remain nearly constant when the concentrations of methanol are changed from 0.10 to 1.00 mol L-1. The results indicate a good methanol tolerance and abundant active sites for the adsorption and conversion of methanol.11 Fig. 9C shows the CV curves in 0.5 mol L-1 CH3OH with different concentrations of NaOH (a-e) for Pd-MnO2/NF. The oxidation current density increases with the increase of NaOH concentrations (Fig. 9D), which illustrates the important role of OH- during methanol oxidation reaction. The catalytic parameters of Pd-MnO2/NF electrode and the results from previously published literatures are compared and summarized in Table 1, among which Pd-MnO2/NF has comparable mass activity, current density, peak potential and If/Ib. It is noteworthy that the Pd loading of this work is comparatively low. The outstanding performance of Pd-MnO2/NF for MOR mainly attributes to following features. First, 3D NF and secondary porous MnO2 NSs with large surface areas accelerate the diffusion of reactant ions and escape of gaseous products. Second, the special coral-like Pd particles with a high aspect ratio and uniform size provide abundant active sites. Third, MnO2 NSs increase Pd loading and supply a large number of oxygen atoms, which reduce the production of intermediates and accelerate their oxidation. Forth, when the Pd loading is low, the electronic interaction between Pd and MnO2 contribute to the improvement of performance. 4. Conclusions


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In the present study, coral-like palladium based porous manganese dioxide nanosheet arrays on nickel foam have been successfully synthesized using a simple electrodeposition method. During methanol electro-oxidation, the as-synthesized Pd-MnO2/NF exhibits much higher catalytic activity and stability than Pd/NF, which derive from the double porous structure of MnO2/NF, the special shape of Pd, a large number of oxygen atoms from MnO2 and the electronic interaction between Pd and MnO2. Furthermore, the results illustrate that suitable concentration of Pd precursor (10 mmol L-1) deposited on MnO2/NF makes the special shape of coral and better catalytic property. In a word, the new Pd-MnO2/NF electrode would be a promising catalyst material for alcohol oxidation, meanwhile, it reduces the amount of Pd used in practical application.

Acknowledgements This research work was supported by the Fundamental Research Funds for the Central Universities and the National Natural Science Foundation of China (Grant No. 21590801).

Supporting Information FESEM images of Pd-MnO2/NF-3, Pd-MnO2/NF-4 and Pd-MnO2/NF-5; Elemental mappings of MnO2/NF; EDX spectra of Pd-MnO2/NF and MnO2/NF; TEM and HRTEM images of Ni stripped from Ni foam ultrasonically; XRD patterns of NF, Pd/NF, MnO2/NF and Pd-MnO2/NF; CV curves of Pd/NF and Pd-MnO2/NF prepared from different concentrations of PdCl2.

References (1)

Niu, X.; Xiong, Q.; Li, X.; Zhang, W.; He, Y.; Pan, J.; Qiu, F.; Yan, Y., Incorporating Ag

into Pd/Ni Foam via Cascade Galvanic Replacement to Promote the Methanol Electro-Oxidation Reaction. J. Electrochem. Soc. 2017, 164, F651-F657.


19


(2)

Page 20 of 39

Jadhav, H. S.; Roy, A.; Chung, W.-J.; Seo, J. G., Growth of urchin-like ZnCo2O4

microspheres on nickel foam as a binder-free electrode for high-performance supercapacitor and methanol electro-oxidation. Electrochim. Acta 2017, 246, 941-950. (3)

Bangyang, J.; Tang, H.; Pan, M., Well-ordered sulfonated silica electrolyte with high

proton conductivity and enhanced selectivity at elevated temperature for DMFC. Int. J. Hydrogen Energy 2012, 37, 4612-4618. (4)

Das, A. K.; Layek, R. K.; Kim, N. H.; Jung, D.; Lee, J. H., Reduced graphene oxide

(RGO)-supported NiCo(2)O(4) nanoparticles: an electrocatalyst for methanol oxidation. Nanoscale 2014, 6, 10657-65. (5)

Lu, X.; Zheng, L.; Zhang, M.; Tang, H.; Li, X.; Liao, S., Synthesis of Core-shell

Structured Ru@Pd/C Catalysts for the Electrooxidation of Formic Acid. Electrochim. Acta 2017, 238, 194-201. (6)

Garin, F., Environmental catalysis. Catal. Today 2004, 89, 255-268.

(7)

Nagaiah, T. C.; Schafer, D.; Schuhmann, W.; Dimcheva, N., Electrochemically deposited

Pd-Pt and Pd-Au codeposits on graphite electrodes for electrocatalytic H2O2 reduction. Anal. Chem. 2013, 85, 7897-903. (8)

Alvi, M. A.; Akhtar, M. S., An effective and low cost Pd Ce bimetallic decorated carbon

nanofibers as electro-catalyst for direct methanol fuel cells applications. J. Alloys Compd. 2016, 684, 524-529. (9)

Xu, C.; Liu, A.; Qiu, H.; Liu, Y., Nanoporous PdCu alloy with enhanced electrocatalytic

performance. Electrochem. Commun. 2011, 13, 766-769. (10)

Geraldes, A. N.; da Silva, D. F.; e Silva, L. G. d. A.; Spinacé, E. V.; Neto, A. O.; dos

Santos, M. C., Binary and ternary palladium based electrocatalysts for alkaline direct glycerol


20

Page 21 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fuel cell. J. Power Sources 2015, 293, 823-830. (11)

Ma, S. Y.; Li, H. H.; Hu, B. C.; Cheng, X.; Fu, Q. Q.; Yu, S. H., Synthesis of Low Pt-

Based Quaternary PtPdRuTe Nanotubes with Optimized Incorporation of Pd for Enhanced Electrocatalytic Activity. J. Am. Chem. Soc. 2017, 139, 5890-5895. (12)

Jadhav, H. S.; Pawar, S. M.; Jadhav, A. H.; Thorat, G. M.; Seo, J. G., Hierarchical

Mesoporous 3D Flower-like CuCo2O4/NF for High-Performance Electrochemical Energy Storage. Sci. Rep. 2016, 6, 31120. (13)

Jadhav, H. S.; Kalubarme, R. S.; Park, C. N.; Kim, J.; Park, C. J., Facile and cost

effective synthesis of mesoporous spinel NiCo2O4 as an anode for high lithium storage capacity. Nanoscale 2014, 6, 10071-6. (14)

Wang, S.; Pu, J.; Tong, Y.; Cheng, Y.; Gao, Y.; Wang, Z., ZnCo2O4 nanowire arrays grown

on nickel foam for high-performance pseudocapacitors. J. Mater. Chem. A 2014, 2, 5434-5440. (15)

Feng, L.; Cui, Z.; Yan, L.; Xing, W.; Liu, C., The enhancement effect of MoOx on Pd/C

catalyst for the electrooxidation of formic acid. Electrochim. Acta 2011, 56, 2051-2056. (16)

Wang, K.; Wang, B.; Chang, J.; Feng, L.; Xing, W., Formic acid electrooxidation

catalyzed by Pd/SmOx-C hybrid catalyst in fuel cells. Electrochim. Acta 2014, 150, 329-336. (17)

Monyoncho, E. A.; Ntais, S.; Brazeau, N.; Wu, J. J.; Sun, C. L.; Baranova, E. A., Role of

the metal-oxide support in the catalytic activity of Pd nanoparticles for ethanol electrooxidation in alkaline media. ChemElectroChem 2016, 3, 218-227. (18)

Feng, L.; Yang, J.; Hu, Y.; Zhu, J.; Liu, C.; Xing, W., Electrocatalytic properties of

PdCeOx/C anodic catalyst for formic acid electrooxidation. Int. J. Hydrogen Energy 2012, 37, 4812-4818. (19)

Xiao, C.; Li, S.; Zhang, X.; MacFarlane, D. R., MnO2/MnCo2O4/Ni heterostructure with


21


Page 22 of 39

quadruple hierarchy: a bifunctional electrode architecture for overall urea oxidation. J. Mater. Chem. A 2017, 5, 7825-7832. (20)

Hao, S.; Li, J.; Li, Y.; Zhang, Y.; Hu, G., Facile synthesis of a 3D MnO2 nanowire/Ni

foam electrode for the electrochemical detection of Cu(ii). Anal. Methods 2016, 8, 4919-4925. (21)

He, S. J.; Zhang, B. Y.; Liu, M. M.; Chen, W., Non-enzymatic hydrogen peroxide

electrochemical sensor based on a three dimensional MnO2 nanosheets/carbon foam composite. RSC Adv. 2014, 4, 49315-49323. (22)

Xiao, C.; Li, Y.; Lu, X.; Zhao, C., Bifunctional Porous NiFe/NiCo2O4/Ni Foam

Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting. Adv. Funct. Mater. 2016, 26, 3515-3523. (23)

Lu, X.; Zhao, C., Electrodeposition of hierarchically structured three-dimensional nickel-

iron electrodes for efficient oxygen evolution at high current densities. Nat. Commun. 2015, 6, 6616. (24)

Xiao, C.; Zhang, X.; Mendes, T.; Knowles, G. P.; Chaffee, A.; MacFarlane, D. R., Highly

Ordered Hierarchical Mesoporous MnCo2O4 with Cubic Iα3d Symmetry for Electrochemical Energy Storage. J. Phys. Chem. C 2016, 120, 23976-23983. (25)

Xu, C.; Li, Z.; Yang, C.; Zou, P.; Xie, B.; Lin, Z.; Zhang, Z.; Li, B.; Kang, F.; Wong, C.

P., An Ultralong, Highly Oriented Nickel-Nanowire-Array Electrode Scaffold for HighPerformance Compressible Pseudocapacitors. Adv. Mater. 2016, 28, 4105-10. (26)

Luo, Q.; Peng, M.; Sun, X.; Asiri, A. M., Hierarchical nickel oxide nanosheet@nanowire

arrays on nickel foam: an efficient 3D electrode for methanol electro-oxidation. Catal. Sci. Technol. 2016, 6, 1157-1161. (27)

Wu, J. B.; Li, Z. G.; Huang, X. H.; Lin, Y., Porous Co3O4/NiO core/shell nanowire array


22

Page 23 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with enhanced catalytic activity for methanol electro-oxidation. J. Power Sources 2013, 224, 1-5. (28)

Qian, L.; Gu, L.; Yang, L.; Yuan, H.; Xiao, D., Direct growth of NiCo2O4 nanostructures

on conductive substrates with enhanced electrocatalytic activity and stability for methanol oxidation. Nanoscale 2013, 5, 7388-96. (29)

Wang, M. Y.; Zhu, W.; Ma, L.; Ma, J. J.; Zhang, D. E.; Tong, Z. W.; Chen, J., Enhanced

simultaneous detection of ractopamine and salbutamol--Via electrochemical-facial deposition of MnO2 nanoflowers onto 3D RGO/Ni foam templates. Biosens. Bioelectron. 2016, 78, 259-66. (30)

Guo, M.; Yu, Y.; Hu, J., Enhanced Electrooxidation of Methanol via Ion Implantation of

Co Nanoparticles onto 3D Ni Foam Templates. J. Electrochem. Soc. 2017, 164, H198-H202. (31)

Yu, M.; Li, X.; Ma, Y.; Liu, R.; Liu, J.; Li, S., Nanohoneycomb-like manganese cobalt

sulfide/three dimensional graphene-nickel foam hybid electrodes for high-rate capability supercapacitors. Appl. Surf. Sci. 2017, 396, 1816-1824. (32)

Chorbadzhiyska, E.; Mitov, M.; Nalbandian, L.; Hubenova, Y., Effect of the support

material type on the electrocatalytic activity of Pd–Au electrodeposits in neutral electrolyte. Int. J. Hydrogen Energy 2015, 40, 7329-7334. (33)

Li, J.; Wang, S.; Zhang, B.; Wang, W.; Feng, L., Highly efficient methanol

electrooxidation catalyzed by co-action of Pd-Y2O3 in alkaline solution for fuel cells. Int. J. Hydrogen Energy 2017, 42, 12236-12245. (34)

Liu, Y.; Hu, M.; Zhang, M.; Peng, L.; Wei, H.; Gao, Y., Facile method to prepare 3D

foam-like MnO2 film/multilayer graphene film/Ni foam hybrid structure for flexible supercapacitors. J. Alloys Compd. 2017, 696, 1159-1167. (35)

Liu, D.; Zhang, Q.; Xiao, P.; Garcia, B. B.; Guo, Q.; Champion, R.; Cao, G., Hydrous

Manganese Dioxide Nanowall Arrays Growth and Their Li+ Ions Intercalation Electrochemical


23


Page 24 of 39

Properties. Chem. Mater. 2008, 20, 1376-1380. (36)

Liang, J.; Fan,

Z.; Chen, S.; Ding, S.; Yang, G., Hierarchical NiCo2O4

Nanosheets@halloysite Nanotubes with Ultrahigh Capacitance and Long Cycle Stability As Electrochemical Pseudocapacitor Materials. Chem. Mater. 2014, 26, 4354-4360. (37)

Shackery, I.; Patil, U.; Pezeshki, A.; Shinde, N. M.; Im, S.; Jun, S. C., Enhanced Non-

enzymatic amperometric sensing of glucose using Co(OH)2 nanorods deposited on a three dimensional graphene network as an electrode material. Microchim. Acta 2016, 183, 2473-2479. (38)

Mavrokefalos, C. K.; Hasan, M.; Khunsin, W.; Schmidt, M.; Maier, S. A.; Rohan, J. F.;

Compton, R. G.; Foord, J. S., Electrochemically modified boron-doped diamond electrode with Pd and Pd-Sn nanoparticles for ethanol electrooxidation. Electrochim. Acta 2017, 243, 310-319. (39)

Grden, M.; Kotowski, J.; Czerwinski, A., The study of electrochemical palladium

behavior using the quartz crystal microbalance. J. Solid State Electrochem. 2000, 4, 273-278. (40)

Kannan, R.; Karunakaran, K.; Vasanthkumar, S., Poly(aniline)/MnO2 Supported

Palladium—A Facile Nanocatalyst for the Electrooxidation of Methanol. Mater. Focus 2013, 2, 267-271. (41)

Liang, Z. X.; Zhao, T. S.; Xu, J. B.; Zhu, L. D., Mechanism study of the ethanol oxidation

reaction on palladium in alkaline media. Electrochim. Acta 2009, 54, 2203-2208. (42)

Cheng, K.; Cao, D.; Yang, F.; Zhang, D.; Yan, P.; Yin, J.; Wang, G., Pd doped three-

dimensional porous Ni film supported on Ni foam and its high performance toward NaBH4 electrooxidation. J. Power Sources 2013, 242, 141-147. (43)

Xing, W.; Qiao, S.; Wu, X.; Gao, X.; Zhou, J.; Zhuo, S.; Hartono, S. B.; Hulicova-

Jurcakova, D., Exaggerated capacitance using electrochemically active nickel foam as current collector in electrochemical measurement. J. Power Sources 2011, 196, 4123-4127.


24

Page 25 of 39 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(44)

Qi, Z.; Xiao, C.; Liu, C.; Goh, T. W.; Zhou, L.; Maligal-Ganesh, R.; Pei, Y.; Li, X.;

Curtiss, L. A.; Huang, W., Sub-4 nm PtZn Intermetallic Nanoparticles for Enhanced Mass and Specific Activities in Catalytic Electrooxidation Reaction. J. Am. Chem. Soc. 2017, 139, 47624768. (45)

Kang, Y.; Pyo, J. B.; Ye, X.; Gordon, T. R.; Murray, C. B., Synthesis, Shape Control, and

Methanol Electro-oxidation Properties of Pt-Zn Alloy and Pt3Zn Intermetallic Nanocrystals. ACS nano 2012, 6, 5642-5647. (46)

Xu, M.-W.; Gao, G.-Y.; Zhou, W.-J.; Zhang, K.-F.; Li, H.-L., Novel Pd/β-MnO2

nanotubes composites as catalysts for methanol oxidation in alkaline solution. J. Power Sources 2008, 175, 217-220. (47)

Liu, R.; Zhou, H.; Liu, J.; Yao, Y.; Huang, Z.; Fu, C.; Kuang, Y., Preparation of Pd/MnO2-

reduced graphene oxide nanocomposite for methanol electro-oxidation in alkaline media. Electrochem. Commun. 2013, 26, 63-66. (48)

Zeng, D.; Liu, R.; Xie, C.; Xu, Y.; Zhou, H.; Huang, Z.; Kuang, Y., Preparation of

Pd/MgO-reduced graphene oxide hybrid catalyst and enhanced activity for methanol electrooxidation. J. Solid State Electrochem. 2014, 18, 2549-2553. (49)

Zhang, K.-F.; Guo, D.-J.; Liu, X.; Li, J.; Li, H.-L.; Su, Z.-X., Vanadium oxide nanotubes

as the support of Pd catalysts for methanol oxidation in alkaline solution. J. Power Sources 2006, 162, 1077-1081. (50)

Li, J.; Ren, J.; Yang, G.; Wang, P.; Li, H.; Sun, X.; Chen, L.; Ma, J.-T.; Li, R., Simple and

efficient deposition of Pd nanoparticles on Fe3O4 hollow nanospheres: A new catalytic system for methanol oxidation in alkaline media. Mater. Sci. Eng.: B 2010, 172, 207-212. (51)

Wang, L.; Wang, Y.; Li, A.; Yang, Y.; Wang, J.; Zhao, H.; Du, X.; Qi, T., Electrocatalysis


25


Page 26 of 39

of carbon black- or chitosan-functionalized activated carbon nanotubes-supported Pd with a small amount of La2O3 towards methanol oxidation in alkaline media. Int. J. Hydrogen Energy 2014, 39, 14730-14738. (52)

Song, Y.; Zhang, X.; Yang, S.; Wei, X.; Sun, Z., Electrocatalytic performance for

methanol oxidation on nanoporous Pd/NiO composites prepared by one-step dealloying. Fuel 2016, 181, 269-276.


26

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Figure and Table Captions Fig. 1. Schematic illustration of the preparation of Pd-MnO2/NF and Pd/NF electrodes. Fig. 2. FESEM images of (A) bare NF, (B-C) low (×300) and high (×10.0 k) magnification images of MnO2/NF, (D-E) low (×10.0 k) and high (×200.0 k) magnification images of PdMnO2/NF-2, (F) Pd/NF-2. (G) Elemental mappings of Pd-MnO2/NF-2. Fig. 3. (A) TEM and (B) HRTEM images of Pd-MnO2 stripped from Ni foam ultrasonically. (C) TEM and (D) HRTEM images of MnO2 stripped from Ni foam ultrasonically. Fig. 4. (A) XRD patterns of ITO, Pd/ITO, MnO2/ITO and Pd-MnO2/ITO. XPS spectra of Pd/NF, MnO2/NF and Pd-MnO2/NF: (B) survey spectra, (C) Ni 2p, (D) Pd 3d, (E) Mn 2p and (F) O 1s spectra. Fig. 5. (A) CV curves of Pd/NF-2, Pd-MnO2/NF-2, MnO2/NF and bare NF. (B) CV curves of Pd/NF-2 and Pd-MnO2/NF-2 (10 mmol L-1 PdCl2) in 1 mol L-1 NaOH at scan rate of 50 mV s-1. Fig. 6. (A) CV curves of Pd/NF, Pd-MnO2/NF, MnO2/NF and bare NF electrodes. CV curves of Pd/NF and Pd-MnO2/NF prepared from different concentrations of PdCl2: (B) 10 mmol L-1, (C) 25 mmol L-1, (D) 50 mmol L-1 and (E) 100 mmol L-1. (F) Comparison among different concentrations for Pd-MnO2/NF electrodes in 1 mol L-1 NaOH containing 0.5 mol L-1 CH3OH at scan rate of 50 mV s-1. Fig. 7. Study of intermediates for (A) Pd/NF-2 and (B) Pd-MnO2/NF-2 in a potential range of 0.6-0.4 V (vs. Ag/AgCl) at scan rate of 50 mV s-1. Fig. 8. (A) Chronoamperometric curves for Pd/NF-2 and Pd-MnO2/NF-2 in 1 mol L-1 NaOH containing 0.5 mol L-1 CH3OH at -0.05 V; the inset shows the first and 500th curves of Pd-


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MnO2/NF in 1 mol L-1 NaOH containing 0.5 mol L-1 CH3OH at scan rates of 50 mV s-1. (B) Nyquist plots of Pd-MnO2/NF and Pd/NF in 1 mol L-1 NaOH with 0.5 mol L-1 CH3OH; the inset shows the enlarged image. (C) CVs of Pd-MnO2/NF in 1 mol L-1 NaOH containing 0.5 mol L-1 CH3OH at scan rates of: (a-g) 10, 30, 50, 100, 150, 200 and 300 mV s-1. (D) Mass activity versus the square roots of the scan rate. Fig. 9. (A) CV plots of Pd-MnO2/NF in 1 mol L-1 NaOH in the absence (a) 0 mol L-1 and presence of different concentrations of methanol (b-i): 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 2.0 and 3.0 mol L-1. (B) Relationship among mass activity, peak potential and methanol concentration. (C) CV plots of Pd-MnO2/NF in 0.5 mol L-1 CH3OH containing different concentrations of NaOH (a-e): 0.3, 0.5, 1.0, 1.5 and 2.0 mol L-1. (D) Relationship among mass activity, peak potential and NaOH concentration. Table 1. Summary of literature catalytic parameters in MOR of various Pd-based electrocatalysts.


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Figure 1


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Figure 2


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Figure 3


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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Figure 8


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Figure 9


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Table 1

Electrode

Mass or specific activity

Peak potential (V)

If/Ib

Reference

Pd/β-MnO2/GC

--

0.035 (vs. Ag/AgCl)

5.87

46

Pd/MnO2-RGO/GC

20.4 mA cm-2

-0.2 (vs. SCE)

4.3

47

Pd/MgO-RGO/GC

814.3 mA mg-1

-0.2 (vs. SCE)

2.65

48

Pd/VOx-NTs/GC

--

0.072 (vs. SCE)

3a

49

Pani/MnO2/Pd/GC

13.5 mA cm-2 a

-0.245 (vs. Ag/AgCl)

7

40

Pd/Fe3O4/GC

110 mA mg-1 a

0.06 (vs. SCE) a

--

50

Pd-La2O3/CS-

0.0025 A cm-2 a

-0.1 (vs. Hg/HgO) a

2.68

51

Pd-Y203/C-20%

145 mA cm-2

-0.1 (vs. Ag/AgCl)

1.45 a

33

np-Pd/NiO/GC

0.28 mA cm-2 a

-0.15 (vs. Ag/AgCl)

--

52

Pd-MnO2/NF-2

197.7 mA mg-1

-0.04 V(vs. Ag/AgCl)

2.79

This work

aCNTs/GC

a

The values were estimated from CV curves in the literatures.


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graphic abstract 351x140mm (96 x 96 DPI)


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