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Palladium-Cobalt Nanowires Decorated with Jagged Appearance for Efficient Methanol Electrooxidation Chengwen Wang, Lijun Zheng, Rong Chang, Lingling Du, Chuhong Zhu, Dongsheng Geng, and Dachi Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06851 • Publication Date (Web): 07 Aug 2018 Downloaded from http://pubs.acs.org on August 8, 2018

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ACS Applied Materials & Interfaces

Palladium-Cobalt Nanowires Decorated with Jagged Appearance for Efficient Methanol Electrooxidation Chengwen Wang, Lijun Zheng, Rong Chang, Lingling Du, Chuhong Zhu, Dongsheng Geng *, § and † Dachi Yang *, †

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Department of Electronics, College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China ‡

Key Laboratory of Materials Physics, and Anhui Key Laboratory of Nanomaterials and Nanotechnology, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, China § Center for Green Innovation, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China g, Beijing 100083, China ABSTRACT: Inexpensive, active, stable and CO-tolerant non-platinum catalysts for efficient methanol electrooxidation are highly desirable to direct methanol fuel cells (DMFCs) technology, however, it is still challenging. In this study, we report palladium and cobalt nanowires with jagged appearance (Pd-Co J-NWs), synthesized via firstly anodic-aluminum-oxide (AAO) template-confined electrodeposition of Pd-Co regular nanowires (R-NWs), followed by a wet-chemical transformation. Benefiting from the “jagged” appearance and Co dopants, the mass and specific activities of Pd-Co J-NWs for methanol electrooxidation are evaluated ~ 3.2 times and ~ 2.1 times as high as those of Pd/C catalysts, respectively. After chronoamperometric measurements for 2000 sec, the catalytic stability of Pd-Co J-NWs is ~ 5.4 times higher compared with that of commercial Pd/C. Moreover, the onset potential of CO-stripping of Pd-Co J-NWs (0.5 V) is lower than that of Pd/C (0.7 V), suggesting CO anti-poisoning. Our approach to Pd-Co J-NWs catalysts provides an experimental guideline for designing other high-performance non-platinum catalysts, which is promising for future DMFCs industry.

Keywords: Palladium-Cobalt, Non-Platinum Catalysts, Jagged Appearance, Nanowires, Methanol Electrooxidation

■ INTRODUCTION Direct methanol fuel cells (DMFCs), which serve as low-temperature power conversation devices for portable cell, 1 electric vehicle 2 and other applications, 3 have currently received increasing attention. As essential components of DMFCs, Ptbased alloys are widely employed as anode catalysts, which are suffering from poisoning by CO species and deactivation in very short period. 2, 4 In addition, the widespread commercialization 5 of DMFCs is hampered by Pt’s high cost and low-abundance in nature. 6, 7 Thus, tremendous efforts have been contributed to develop low-cost non-Pt catalysts 8-11 for obtaining comparable or better electrocatalytic activity and anti-poisoning properties towards methanol electrooxidation. 12 So far, Pd alloys are deemed as the most promising substitute as the crystal structure and electronic properties of Pd are similar to those of Pt. Thus, Pd alloying non-precious metals (e.g., Fe, Ni and Co) is of great interest, which enable to assist desorbing molecule species and achieve more exposure of active sites. Accordingly, much effort has been devoted to exploring Pd alloy via various synthetic strategies to achieve both improved electrochemical performance and CO tolerance, such as Pd-Fe catalysts, 13 Pd-Cu catalysts, 14 Pd-Ni catalysts 15 and so on. 16, 17 However, Pd alloys catalysts usually take on compact and smooth appearance, and guest molecules species are not

able to react with the internal surface, implying that atoms inside or below the surface are not fully utilized. To gain enhanced electro-activities of Pd-based catalysts, creating surface defects around the catalysts are considered as an effective way and may go beyond. On the one hand, catalysts with surface defects such as steps or kinks possess more exposure of active atoms. For instances, Duan’s group 18 recently reported superfine Pt NWs with jagged shape as high-performance oxygen reduction reaction (ORR) catalysts. Huang’s and Guo’s joint teams showed hierarchical and high-indexed Pt-Co NWs, platinum-rich planes, 19 in which surface defects significantly improve the exposure of active sites, contributing to ultrahigh electrochemically active surface area (ECSA) and to highly improved mass activity. On the other hand, the surface defects may weaken the Pd-CO bonding and thus result in enhanced CO tolerance. 20 For instance, crack-tips enriched ternary Pt-Pd-Cu nanodendrites 21 and binary Pt-Cu nanoflakes 22 have not only higher mass activity, but also more improved CO anti-poisoning than those of Pt/C. Motivated by these progress, generating high-density surface defects by creating creaks and tips 23 around Pd alloy catalysts may be feasible to enhance both electrochemical activity and CO tolerance for methanol electrooxidation. Here, Pd-Co NWs decorated with jagged appearance (Pd-Co JNWs) have been obtained via anodic-aluminum-oxide (AAO)

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Figure 1. The scheme on left panel shows the electrochemical transformation from Pd-Co R-NWs to J-NWs. (a) The SEM image of Pd-Co R-NWs arrays, and the inset (b) is the EDS analysis. (c) The SEM image of Pd-Co J-NWs arrays. (d)-(e) The elemental mapping corresponding to Pd and Co taken from the dashed box in (c) template 24 confined electrodeposition of Pd-Co regular NWs (R-NWs), followed by optimized chemical etching. The synthetic process is schematically described (Figure S1, Supporting information), and as-synthesized Pd-Co J-NWs show jagged appearance decorated with tips around the longitudinal axis of NWs and excellent crystallization. Benefiting from the synergistic effect of “jagged appearance” and “Co dopants”, the mass activity (1205 mA mgPd-1) and specific activity (3.584 mA cmPd2 ) of Pd-Co nanowires with jagged appearance (Pd-Co J-NWs) are ~ 3.2 times and ~ 2.1 times compared with those of Pd/C catalysts (376 mA mgPd-1; 1.723 mA cmPd-2), respectively. The electrocatalytic stability of Pd-Co J-NWs is more improved than that of Pd/C. Simultaneously, Pd-Co J-NWs show lower CO oxidation potential in comparison with commercial Pd/C catalysts, and present better CO tolerance.

■ RESULTS AND DISCUSSION Preparation and Characterization. In this synthesis, the PdCo J-NWs were obtained by first AAO template-confined electrochemical deposition of Pd-Co R-NWs, and subsequently excessive etching in phosphoric acid (Figure S1, Supporting information). To achieve desired Pd-Co J-NWs, the Pd-Co RNWs had been obtained via optimized electrodeposition. Pd-Co R-NWs arrays (Figure 1a) show smooth surface and uniform geometry. The diameter of the R-NWs is ~ 70 nm, agreeing with the channel diameter of the employed AAO template. After the wet-chemical etching, the SEM image in Figure 1c shows a bundle of Pd-Co J-NWs, which are observed uniform with massive production and jagged surface appearance. To verify the chemical components and distribution, the corresponding elemental mappings (Figure 1d-e) indicate that Pd and Co atoms distribute uniformly along the J-NWs. Furthermore, Co content is estimated ~ 10.36 at% from the EDS spectrum (Figure S2b, Supporting information) and lower than that (44.26 at%) of RNWs (Figure S2a, Supporting information), which confirms

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that the Co atoms were etched off the R-NWs and it’s consistent with the wet-chemical process. In addition, jagged nanowires in large-scale (Figure 2a-b; Figure S3, Supporting information) further confirm that most of as-prepared nanowires present jagged morphology in our approach. To further observe the morphology and understand the crystallization, we select dual Pd-Co J-NWs for TEM characterization. Figure 2c exhibits a representative TEM image, from which “jagged” shape arrays along the axis direction of NWs are identified. “Defects” and “steps” generated from “jagged” shape (shown in Figure S4c in Supporting information) may offer more Pd atoms exposure than those of Pd-Co R-NWs (Figure 1a) and which will be descripted in below text. Furthermore, from an enlarged TEM image, “jagged” surface appearance is observed decorating along the axis of NWs (Figure 2d). However, the geometrical parameters of the “jagged” motifs are not uniform. A high resolution TEM (HR-TEM) image is displayed in Figure 2e, from which the lattice spacing is measured ~ 0.225 nm belonging to Pd (111). However, Co lattice fringe was not seen in this “root” region of Pd enrichment. Another J-NWs with more sharp “jagged tip” (Figure S4, Supporting information) was testified, from which both Pd and Co lattice fringes were observed. The formation of jagged-shaped NWs may be ascribed to the interaction between the Pd2+, Co2+ and the AAO channel wall, and excessively etching in acidic solution. 25 At initial electrodeposition, both Pd2+ and Co2+ are deposited inside the AAO nanochannels. Simultaneously, both Pd2+ and Co2+ interact with AAO surface wall in the form of [PdCl]+ and [CoO]+, and move towards the AAO channel wall vertical to the axis during electrodeposition. In such condition, both Pd and Co are unevenly deposition, leading to Pd and Co elemental enrichments, which may contribute to jagged appearance formation. In addition, the concentration of Pd2+ and Co2+, pH value, applied voltage may also affect the formation of jagged appearance. In the process of optimized wet-chemical etching, a “galvanic corrosion” system is formed with Pd, Co and H3PO4 media. The reaction can be illustrated as follows, and the process is schematically shown in Figure S1 (d-g) of the Supporting information. Co – 2e → Co2+ 2H+ + 2e → H2 In this case, the “jagged” shape forms around the NWs, where Co as the anode is oxidized to Co2+ and is then partially etched off, while Pd as the cathode is protected, and thus leave where it is. The subsequent etching and chemical components of J-NWs is essential to the “jagged” shape formation. Otherwise, either PdCo NWs with smooth (Figure 1a) or rough surface (Figure S5, Supporting information) were obtained. For example, when 2M NaOH electrolyte was utilized for dissolving the AAO template without further chemically etching, uniform Pd-Co R-NWs with smooth surface can be achieved with the diameter of ~ 70 nm in Figure 1a. The corresponding Co content inside NWs is of~ 44.36 at% from the EDS analysis (Figure S2a, Supporting information). Moreover, 10 wt% H3PO4 was also tried as the etching solution, and NWs with rough surface have been obtained under the condition of 50 oC for 40 min (Figure S5, Supporting information). It should be mentioned that if the Co atoms were fractionally removed by tuning the etching parameters, the NWs with irregularly rough surface (TEM and HRTEM; Figure S5, Supporting information) had been obtained,


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Figure 2. (a)-(b) TEM and enlarged TEM images of PdCo J-NWs arrays. (c) A representative TEM image. (d) The enlarged TEM image is taken from the dashed rectangle in (c). (e) The HR-TEM image and inset SAED are acquired from the dashed box in (d) and circle (c), respectively. of which the morphologies were different from the J-NWs descripted above. The EDS spectrum (Figure S5, Supporting information) confirms the corresponding Co atomic content (~ 24.57 at%) in the NWs, indicating that only a small fraction of the Co atoms are etched. The crystalline structure of Pd-Co J-NWs was studied by the XRD analysis (Figure S6, Supporting information). The pattern with peak positions are accorded with Pd fcc crystalline (PDF#46-1043). However, it can be noticed that a slight positive shift of four peaks to higher 2θ angles is observed with the corresponding indexed (111), (200), (220) and (311) planes of Pd. Accordingly, Bragg's law is demonstrated below. d = nλ / 2sinθ (n = 1, 2, 3, 4….) Where d, λ, θ are atomic spacing, wavelength of X-rays and scattering angle, respectively. Such positive angle shifts suggest the alloy formed with Pd and Co resulting in a lattice contraction, which is possibly due to incorporating Co into the Pd fcc structure. Electrocatalytic evaluation of Pd-Co J-NWs towards methanol electrooxidation. To evaluate the electrocatalytic performance of Pd-Co J-NWs towards methanol electrooxidation, we

take the Pd NWs catalysts (Figure S7, Supporting information) and commercial 10 wt% Pd/C (Figure 3) for comparison. The CVs of Pd-Co J-NWs and Pd/C in Ar-saturated 1.0 M KOH solutions at 50 mV s-1 are shown in Figure 3a, from which the ECSA of Pd-Co J-NWs is calculated to be 33.68 m2 gpd-1, and is apparently larger than those of Pd NWs (23.85 m2 gpd-1) and commercial Pd/C catalysts (27.26 m2 gPd-1). Generally, the ECSA is estimated with the PdO reduction peak instead of the hydrogen desorption peak in 1.0 M KOH, according to the previous publication. 26 Figure 3b shows the electrocatalytic activities of the Pd-Co JNWs towards methanol oxidation compared with commercial Pd/C catalysts which were carried out in 1.0 M KOH + 1.0 M CH3OH at 50 mV s-1. We concluded that the mass activity of Pd-Co J-NWs (1205 mA mgPd-1) is of ~ 2.0 times and of~ 3.2 times as high as those of Pd NWs (606 mA mgPd-1, Figure S7, Supporting information) and commercial Pd/C catalyst (376 mA mgPd-1), respectively. Obviously, Pd-Co J-NWs catalysts show better mass activity for methanol electrooxidation. Moreover, when the catalytic activity was normalized into the ECSA as mentioned above, the specific activity of Pd-Co J-NWs (3.584 mA cm-2, Figure 3c) is obtained of~ 1.2


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times and ~ 2.1 times compared with those of Pd NWs (2.805 mA cm-2; Figure S7c, Supporting information) and commercial Pd/C (1.723 mA cm-2) catalysts (Figure 3c), respectively, indicating the higher specific activity. For other Pd-based catalysts, PdCu/carbon catalysts 14 and Pd/PPy-graphene catalysts 27 towards methanol oxidation have been reported with a mass activity of 220 mA mgPd-1 and 370 mA mgPd-1, respectively. In contrast, our Pd-Co JNWs catalysts exhibit much higher mass activity (1205 mA mgPd-1), implying lower catalyst usage being able to maintain higher catalytic activity. Meanwhile, the forward-scanning peak potential towards methanol oxidation of the Pd-Co J-NWs (0.82 V, Figure 3b) is lower than those of Pd NWs (0.92 V, Figure S7b, Supporting information) and Pd/C (0.89 V, Figure 3b), respectively, which further indicates that the Pd-Co J-NWs are capable to efficiently decrease the overpotential of methanol oxidation. More importantly, the onset potential of Pd-Co J-NWs is obviously much lower than those of Pd NWs and Pd/C catalysts, indicating it is more favorable for methanol electrooxidation. Next, the electrocatalytic stability of Pd-Co J-NWs catalysts towards methanol oxidation was performed by the chronoamperometric (CA) technique at a potential of 0.80 V in 1.0 M KOH + 1.0 M CH3OH solution for 2000 s and Figure 3d shows the related results. The mass current density (24.5 mA mg-1) after 2000 sec of Pd-Co J-NWs catalysts is ~ 2.0 times and ~ 6.4 times as high as those of Pd NWs (12.3 mA mg-1, Figure S7d, Supporting information) and Pd/C (3.8 mA mg1 ), indicating Pd-Co J-NWs show the highest stability. It should be mentioned that the catalysts display a quick current

decay in the period of 0-500 s, which can be ascribed to the presence of intermediate species such as CH3O, CH2O, CHO, and CO. 28 Alternatively, we have also performed multiple CVs (300 cycles) to testify the catalytic stability of the Pd-Co J-NWs catalysts (Figure S8, Supporting information). The commercial Pd/C catalysts showed 215 mA mg-1 decay in peak current densities over the cycling period. In contrast, PdCo J-NWs has only 158 mA mg-1 of loss, possibly indicating higher stability of the catalyst. CO tolerance is another criterion towards methanol oxidation. To further investigate the CO tolerance performance of the Pd-Co J-NWs catalysts, CO stripping measurements are performed. In Figure 4, we observed that the peak potential of CO oxidation (curves in red) on the Pd-Co J-NWs catalysts is 0.78 V, which is of ~ 30 mV smaller than that on the commercial Pd/C catalysts (0.81 V) in the initial cycle. Moreover, the onset potential of CO oxidation for Pd-Co J-NWs (0.50 V, the dashed line on the left) is much more negative than that of commercial Pd/C (0.70 V), suggesting that the adsorbed CO is easier to be removed from the surface of Pd-Co J-NWs catalysts. The significant negative shifts for both the peak potential and onset potential indicate that Pd-Co J-NWs have more improved CO anti-poisoning than that of the commercial Pd/C. In fact, these improved electrocatalytic performances of PdCo J-NWs catalysts may be ascribed to the synergistic effect of jagged surface appearance and of doping Co atoms. From Figure 2, the jagged surface appearance may offer more Pd atoms exposure to improve its utility than those of smooth surfaced Pd-Co NWs. Furthermore, the jagged surface appearance decorated with massive cracks and tips could


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Figure 4. The CO stripping measurements of (a) Pd-Co JNWs and (b) Pd/C in 1.0 M KOH at 50 mV s-1, respectively. generate surface defects (such as steps and kinks) to improve both activity and CO tolerance. 21, 22 In addition, the electronic structure of Pd was modified by the dopant Co, from which the electrons transfer from Co to Pd atoms, leading to the Pd d-band center shift downwards. In this case, the electrocatalytic activity could be greatly improved. Meanwhile, the modified Pd electronic structure would also facilitate C-C bond scission and increase the tolerance over CO species’ poisoning. 29 To gain insight into the electronic structure of Pd by Co atoms’ modification, XPS spectra of Pd-Co J-NWs in Pd 3d regions was conducted with Pd/C as control sample. In Figure 5, we can see that a doublet with the binding energies located at 335.8 and 341.0 eV corresponding to Pd 3d 5/2 and Pd 3d 3/2, respectively. Obviously, both the Pd 3d 5/2 and Pd 3d 3/2 peaks shift to the lower binding energy by ~ 0.4 eV compared with those of commercial Pd/C, suggesting that electrons transfer from Co to Pd atoms, and thus lead to Pd 3d band centered shift downs. As described above, the oxygen affinity of Pd atoms in our Pd-Co J-NWs catalysts would decrease more than commercial Pd/C, which should assist CO desorption from the active sites and enhance the CO tolerance. Thus, the Pd-Co J-NWs catalysts display excellent catalytic activity and CO tolerance.

In summary, Pd-Co NWs decorated with jagged appearance have been tailored via the combination arts of first AAO template-confined electrodeposition and then wet-chemical etching. The jagged shape is formed as the result of the “galvanic corrosion” system including Pd, Co and H3PO4 acid media. And this synthetic strategy could be extended to the Pd-based, and Pt-based catalysts with similar unique nanostructures. The synergetic effect of “jagged appearance” and “Co dopants” has been introduced into Pd-Co J-NWs to enhance the electrocatalytic performance, and Pd-Co J-NWs catalysts show possibility as efficient anode catalysts for future DMFCs. Moreover, the Pd-Co J-NWs with high exposure of Pd atoms and functional dopants are expected to explore both highly sensitive and stable hydrogen sensors in our future investigations.

■ EXPERIMENTAL METHODS Synthesis of Pd-Co catalysts. The AAO templates were prepared similar to previous studies. 30, 31 The electrolyte for the electrodeposition of Pd-Co R-NWs contained 0.01 M PdCl2, 0.02 M CoSO4·7H2O and 0.01 M H3BO3, which was further buffered to pH = ~ 3 with 0.1 M HCl solution. A thick layer of Au was evaporated onto either AAO surface side to fully cover the channels as working electrode. The electrodeposition of the Pd-Co R-NWs was carried out at 1.6 ~ 2.5 V, utilizing a graphite plate for the counter electrode. As-synthesized Pd-Co NWs were finally released from the AAO templates by using 10 wt % H3PO4 solution at 50 °C for 80 min, and then conducted thoroughly rinsing in deionized water, in which cobalt atoms were partially dissolved. Characterizations. The samples were characterized using Xray diffraction (XRD), field-emission scanning electron microscope (FE-SEM, JEOL JSM-7500F, at 2 kV) with energy dispersive X-ray spectroscopy (EDS, OXFORD), transmission electron microscope (TEM, JEOL-2010, at 200 kV), high resolution TEM (HR-TEM, JEOL-2010, at 200 kV) and


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X-ray photoelectron spectroscopy (XPS, PHI 5000). The elemental loadings were evaluated by an inductively coupled plasma-atomic emission spectroscopy (ICP-AES, Thermo, IRIS Advantage). Electrocatalytic evaluation towards MOR. The electrochemical measurements were conducted using an electrochemical workstation (VersaSTAT 4, AMETEK Princeton) with a rotating disk electrode system (RDE; ATA-1B, Jiangfen Instruments) and a three-electrode system including a glassy carbon (GC) (working electrode), a platinum wire (counter electrode) and an Hg/HgO electrode (reference electrode), respectively. The GC electrode loaded with the catalyst ink was handled similar to the routine reported previously. 32 In our cases, 2 mg Pd-Co J-NWs catalysts were ultrasonicated for 30 min in 1.5 mL isopropanol, 0.5 mL deionized water and 0.1 mL nafion solution (5 wt%, Aldrich) at temperature lower than 30 oC, then as-prepared catalyst ink was loaded onto the GC electrode, and was finally dried at ambient temperature. The Pd loading onto the GC electrode was controlled ~ 20 µgPd cm-2 by ICP analysis determination. Similarly, the commercial 10 wt% Pd/C catalyst with equivalent Pd loading was used for comparison. Before the cyclic voltammetry (CV) tests, the working electrode was scanned in Ar-saturated 1 M KOH solution for consecutive 50 cycles at a sweep rate of 50 mV s-1 between 0 V and 1.2 V for activation. Then the CVs towards methanol oxidation was tested in 1.0 M KOH and 1.0 M CH3OH between the potential of 0 ~ 1.2 V at 50 mV s-1. The stability was evaluated by chronoamperometry (CA) curves with the potential at 0.80 V. Before the CO stripping measurement, high-purity CO (99.99% v/v) was bubbled in 1.0 M KOH solution for about 30 min to guarantee the full coverage of CO on the Pd surface. Then excessive dissolved CO was eliminated via bubbling Ar gas for 30 min. All electrochemical evaluations were carried out at 25 oC, and all potential values were converted into the corresponding reversible hydrogen electrode (RHE) potential.

■ ASSOCIATED CONTENT Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website at DOI: Synthetic process of the Pd-Co J-NWs. The EDS analysis of the Pd-Co J-NWs after being etched with the Co atomic content of ~ 10.36 % with 10 wt% H3PO4 at 50 °C for 80 minutes. Large-scale synthesis of Pd-Co J-NWs with SEM characterization. A typical TEM image of Pd-Co J-NWs. The SEM and TEM images of the Pd-Co NWs with wet-chemical modification by 10 wt% H3PO4 at 50 °C for 40 minutes the EDS analysis. The XRD pattern of the Pd-Co J-NWs catalysts. The electrocatalytic performance of Pd-Co J-NWs towards MOR compared with Pd NWs and commercial 10 wt% Pd/C. Multiple CVs of Pd-Co J-NWs, Pd NWs and commercial Pd/C with increasing cycles.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

Notes

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■ ACKNOWLEDGMENT This work was financially supported by the Natural Science Foundation of China (Grant No. 21473093), Fundamental Research Funds for the Central Universities and Tianjin Research Program of Application Foundation and Advanced Technology (Grant No. 14JCYBJC41300).

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The authors declare no competing financial interest.


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Table of Contents Peak Potential Pd/C Pd-Co J-NWs

20 nm

1000 800

1500

MA SA

600 400 200

0.82 V

4

Pd/C Pd-Co J-NWs

1200

mA mg-1


-1

1200

3

900 2 600

mA cm-2

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

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0.89 V

1

300 0

0

Mass Activity

Specific Activity

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

E (vs. RHE)

8 ACS Paragon Plus Environment

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