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Facile synthesis of Cu/NiCu electrocatalysts integrating alloy, core-shell and one-dimensional structures for efficient methanol oxidation reaction Dengfeng Wu, Wei Zhang, and Daojian Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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ACS Applied Materials & Interfaces

Facile

synthesis

of

Cu/NiCu

electrocatalysts

integrating alloy, core-shell and one-dimensional structures for efficient methanol oxidation reaction Dengfeng Wu, Wei Zhang, Daojian Cheng* Beijing Key Laboratory of Energy Environmental Catalysis, State Key Laboratory of OrganicInorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China

KEYWORDS Cu/NiCu, alloy, core-shell, one-dimensional, methanol oxidation reaction

ABSTRACT

The design and development of low-cost Pt-free, high-active and durable noble-metal-free electrocatalysts for methanol electrooxidation is highly desirable but remains a challenge. Herein, unique Cu/NiCu nanowires (NWs) integrating alloy, core-shell and one-dimensional structures are prepared by a facile one-pot strategy. It is found that Ni-Cu surface alloying structure can effectively change the charge distribution of atomic configuration, core-shell structure can be optimally the usage of Ni and Cu, and one-dimensional structure can effectively

ACS Paragon Plus Environment

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enhance the charge transfer between electrode surface and active sites, making the prepared NWs as promising electrocatalysts. Detailed catalytic investigations showed that the obtained Cu/NiCu NWs exhibit an enhanced electrocatalytic performance for methanol oxidation reaction (MOR). The optimized Cu/NiCu NWs in this work show the mass current densities of 867.1 mA mgmetal-1 at 1.55 V (vs. RHE) for MOR, which is far higher than those Ni-based electrocatalysts ever reported before. This work opens up a new pathway to the design and engineering of noblemetal-free alloy electrocatalysts with enhanced activity and durability.

INTRODUCTION Development of alternative energy sources is of great importance to relieve the global energy predicament. Direct methanol fuel cells (DMFCs) have been extensively considered as ideal energy converters that directly convert chemical energy of methanol to electrical energy.1-2 Enormous amount of attention has been focused on developing novel and efficient anode electrocatalysts since methanol oxidation reaction (MOR) is a critical reaction in DMFCs.3-5 Ptbased nanomaterials are mostly used as anode electrocatalysts for methanol oxidation in previous studies.6 However, the high cost, sluggish kinetics and low stability of the Pt-based electrocatalysts severely hamper the further commercialization of fuel cells.7-8 Among various Pt-free non-noble transition metal catalysts, Ni is considered to be a promising alternative due to its good surface oxidation properties, inexpensive price and abundant content in the earth.9 To this end, many researches on Ni-based catalysts toward methanol electro-oxidation have been reported.10-19 However, due to their poor electrical conductivity and durability, the performances of these catalysts are still unsatisfactory. Therefore, the exploration of novel Ni-based catalysts with both high conductivity and excellent catalytic activity is of great importance in this field.


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NiCu nanoalloy is considered particularly to be a potential alternative to Ni due to their synergistic effects.20-25 Previous studies have demonstrated that the MOR performance of NiCu nanoparticles is much better than that of pure Ni.26 In addition to the alloy effect, the shape and architecture of catalysts have significant influence on their catalytic properties.27-28 Previous investigations show that one-dimensional nanostructures, such as nanowires (NWs), are promising participants in elecrocatalysis process due to their high electrical conductivity and structural stability.29-31 Our previous studies indicate that Cu NWs are demonstrated to be an ideal participant in one-dimensional eletrocatalysts for fuel cells.32-34 Moreover, a core-shell heterostructure with conductive Cu-core and active NiCu-shell could maximize the utilization efficiency of Ni and Cu constituents. To the best of our knowledge, no report has been focused on unique Cu/NiCu NWs integrating alloying, one-dimensional and core-shell structures as electrocatalysts, which may be a promising way to improve the electrocatalytic activity. In this work, for the first time, unique Cu/NiCu NWs integrating alloy, core-shell and onedimensional structure types prepared by a facile one-pot method are used as Pt-free electrocatalysts. As shown in Scheme 1, Cu-Ni surface alloying structure can effectively change the charge distribution of atomic configuration, core-shell structure can be optimally the usage of Ni and Cu, and one-dimensional structure can effectively enhance the charge transfer between electrode surface and active sites, making the prepared NWs as promising electrocatalysts. Detailed catalytic tests showed that the obtained Cu/NiCu NWs exhibit an enhanced electrocatalytic performance for MOR. The optimized Cu/NiCu NWs catalyst in this work shows the mass current densities of 867.1 mA mgmetal-1 at 1.55 V (vs. RHE) for MOR, which is far higher than those Ni-based electrocatalysts ever reported before.


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Scheme 1. Schematic of the mechanism of methanol electrocatalytic oxidation on the surface of Cu/NiCu NWs. EXPERIMENTAL SECTION Synthesis of Cu/NiCu nanowires (NWs) All chemicals in this work were used without any further purification. The synthesis of Cu/NiCu NWs is derived from our previous method to synthesize Cu NWs.32 In a typical synthesis, 0.8 mmol Cu(acac)2 (TCI Chmicals, >97%), 0.4 mmol NiCl2·6H2O (Aladdin Industrial Inc., 98%) and 0.5 mmol dimethyl distearylammonium chloride (D1821, Aladdin Industrial Inc., 97%) were dissolved in 7 mL oleylamine (Aladdin Industrial Inc., 80%-90%) in a 50 mL threeneck flask. The mixture was magnetically stirred at 85 °C for 20 min. Then, the temperature was controlled at 185 °C for 4 h. Finally, the mixture was aged at 210 °C (or 220 and 230 °C) for 1 h. The whole process was carried under continuous stirring in a high-purity N2 (Beijing RuYuanRuQuan Gas Company, 99.999%) atmosphere. After cooling to room temperature naturally, the products were centrifuged and washed by n-hexane (SCRC, AR) for three times. Products synthesized at different temperature were named Cu/NiCu NWs-210, Cu/NiCu NWs220 and Cu/NiCu NWs-230. Synthesis of NiCu NPs


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NiCu NPs were also synthesized similarly except for decreasing the Cu concentration. Briefly, 0.4 mmol Cu(acac)2 and 0.4 mmol NiCl2·6H2O were added to 7 mL oleylamine. The mixture was kept at 85 °C for 20 min under continuous stirring in high-purity N2 atmosphere, and then heated to 220 °C with a nonstop temperature rise for 1h. Preparation of carbon-supported Cu/NiCu NWs catalysts In a typical preparation of Cu/NiCu NWs/C catalysts, 10 mg washed Cu/NiCu NWs was redispersed in n-hexane by ultrasonication. 15 mg of XC-72R carbon was then added to the suspension. And the mixture was centrifuged after a 1-hour ultrasonic treatment. Finally, the catalysts were dried naturally in drying oven at room temperature and named Cu/NiCu NWs210/C, Cu/NiCu NWs-220/C and Cu/NiCu NWs-230/C, respectively. NiCu NPs/C and Cu NWs/C were also prepared for comparison. Characterizations The morphologies and microstructures were observed on an H-800 transmission electron microscopy (TEM) and a JEOL JEM-2100 instrument at 200 kV. The optical properties of materials were tested by a Tu-1901 Uv-vis spectrophotometer. X-ray diffraction (XRD) patterns were recorded at 298 K on a Bruker D8 diffractometer with a Cu Kα X-ray source (λ = 1.5405 Å) operated at 40 kV. EDS line-scan and mapping analysis was carried out on a JEOL JEM2010f with a high-angle annular dark-field scanning transmission electron microscopy (HAADFSTEM). The mass percentage of Ni or Cu in catalysts was determined by the inductively coupled plasma (ICP) on a Thermo Scientific iCAP6000 and energy dispersive spectrometer (EDS) on a Hitachi S-4800 field emission scanning electron microscope (SEM) operated at 20 kV. The


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surface properties of the samples were taken on a Thermo fisher ESCALAB 250 X-ray photoelectron spectroscopy (XPS) with a Mg-Ka source. Electrochemical measurements All electrochemical tests were taken on a CHI760e (ChenHua, China) electrochemical work station by a three electrode system at room temperature. The three electrode system consists of counter electrode (a platinum net), reference electrode (a saturated calomel electrode, SCE) and working electrode (a catalyst-modified glassy carbon electrode with diameter of 5 mm). All potentials presented in this paper were normalized to reversible hydrogen electrode (RHE). The catalyst inks were prepared by homogenously dispersing 6 mg of the self-prepared catalysts, 2 mL of deionized water, 1 mL of isopropanol (SCRC, AR) and 40 µL nafion solution (DuPont Company, 5 wt.%) in a 7 mL vial followed by ultrasonication for 2 h. Then, 10 µL of the catalyst ink was evenly deposited onto the polished glassy carbon electrode and dried naturally at room temperature. The loading of metal components on glassy carbon surface was about 0.04 mg cm-2. All electrochemical tests by Cyclic voltammetry (CV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS) were carried out with this catalyst-modified electrode in this work. RESULTS AND DISCUSSION Synthesis of Cu/NiCu core/shell nanowires A facile one-pot strategy containing two successive reacting stages was used to prepare Cu/NiCu core/shell nanowires (Cu/NiCu NWs) (see the Experimental Section for details). A typical synthesis process is illustrated in Figure 1a. In the first step, Cu NWs is formed in


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presence of nickel chloride and capping agent (D1821) in oleylamine medium at 185 °C. Notably, Ni2+ plays an important role in the formation of Cu NWs at specific reaction temperature.35 Ni2+ would be reduced into Ni0 atoms by oleylamine at first, but crystal nuclei would not be formed from these Ni0 atoms due to the galvanic replacement between Cu2+ and Ni0, resulting that the color of reaction solution is changing from bluish green to brown (Figure 1b and 1c). The final morphology of Cu NWs is depended on the selective chemical adsorption of Cl- and surfactant (D1821) on the different facets of crystal nuclei during the growth phase.32, 36

The reaction solution would turn red along with the increase of reaction time (Figure 1d),

indicating the formation of Cu NWs. As the reaction solution raised to higher temperature in the second reaction stage, Cu2+ and Ni2+ will be co-reduced to Cu0 and Ni0 on Cu NWs to form core-shell nanostructures, due to that plenty of Cu2+ had been reduced after the complete formation of the Cu NWs. Therefore, highly active Ni atoms would partially deposit on the surface of the Cu NWs as well as Cu atoms. As a result, NiCu alloy shells can be formed on Cu NWs to produce Cu/NiCu core/shell nanowires and the reaction solution turns black (Figure 1e). In addition to the intuitive color changes from Cu NWs dispersion to Cu/NiCu NWs dispersion (insets in Figures 1d and 1e), the differences exhibit also in their optical and magnetic properties (see Figures S1 and S2) before further physical characterizations. The surface plasmon resonance (SPR) band of self-synthesized Cu NWs is located at 576 nm (see Supporting Information, Figure S1), which is in accordance to the results in previous investigations.37 However, no obvious SPR band can be observed for Cu/NiCu NWs-220. This should be attributed to the damping effect of Ni.38-40 The significant difference between Cu NWs and Cu/NiCu NWs on UV-Vis extinction spectra indicates the change of surface composition from pure Cu to NiCu alloy. As shown in Figure S2, a simple


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magnetic response test was performed to illustrate the magnetic properties of Cu/NiCu NWs when an external magnetic field is used.

Figure 1. (a) Schematic illustration for the synthesis of Cu/NiCu NWs. The reaction solution at (b) 85 °C and (c) 185 °C. (d) The colloidal suspension of Cu NWs after reacting at 185 °C for 4 h. (e) The colloidal suspension of Cu/NiCu NWs-220 after reacting at 220 °C for 1 h. Insets in Figures (d) and (e) is clean-washed Cu NWs and Cu/NiCu NWs-220 dispersion in n-hexane. TEM images of (f) Cu NWs and (g) Cu/NiCuNWs-220. Scale bars: 50 nm. Morphology and Composition The representative morphologies of self-synthesized nanowires were characterized by TEM. As shown in Figure 1f, the Cu NWs synthesized in the first reacting stage have one dimensional (1D) nanostructure with high slenderness ratio and smooth surfaces, which is in line with our previous results.33-34 And the 1D nanostructure is retained very well for Cu/NiCu NWs by the


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two-step strategy (see Figures 1g and S3). For comparison, the TEM image of products synthesized via a none-stop temperature rising displays irregular rather than a well-defined 1D nanostructures (Figure S4). The interface between NiCu-shell and Cu-core is fuzzy in low magnification TEM images, which should be attributed to the near atomic number between Ni (28) and Cu (29) and one-pot method. With increasing reaction temperature, the average diameter of Cu/NiCu NWs increases from 31.96 nm for Cu/NiCu NWs-210, to 32.82 nm for Cu/NiCu NWs-220 and 33.48 nm for Cu/NiCu NWs-230 (Figure S5). The increased average shell thicknesses from 1.9 nm for Cu/NiCu NWs-210, to 2.33 nm for Cu/NiCu NWs-220 and to 2.66 nm for Cu/NiCu NWs-230, indicate the stepped-up deposition rate of metal atoms on Cu NWs (28.16 nm) by rising temperature. XRD was implemented to examine the crystal structure and structural evolution of Cu/NiCu NWs (Figures 2). Figure 2a shows the wide-range XRD curves of Cu NWs, Cu/NiCu NWs and NiCu NPs. For Cu NWs, peaks around 43.3°, 50.4°, and 74.1° can be assigned to the (111), (200) and (220) facets of pure Cu (PDF#04-0836) and no Ni (PDF#04-0850) peaks can be observed, indicating that a well-defined face-center cubic (fcc) Cu crystal structure has been formed. For Cu/NiCu NWs, peaks around 43.9°, 50.8°, and 74.7° can be assigned to the (111), (200) and (220) facets of NiCu alloy with fcc crystal structure, respectively. Pure Cu peaks can be found, but pure Ni peaks cannot be found, which further validates the formation of NiCu alloy shell on Cu NWs. In contrast, very low Cu peaks next to NiCu alloy peaks can be observed on the XRD pattern of NiCu NPs, suggesting that pure Cu can be well formed at a relatively low reacting temperature. As shown in Figure 2b, the (111) peaks of the Cu NWs and Cu/NiCu NWs are selected out to analyze the detailed information on crystal structure. It can be seen that the (111) peak of Cu/NiCu NWs can be divided into two peaks, corresponding to the NiCu alloy and pure


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Cu phases. As the temperature going up, the relative intensity of NiCu(111) and Cu(111) (INiCu(111)/ICu(111)) increases from 0.95 for Cu/NiCu NWs-210 to 1.25 for Cu/NiCu NWs-220 and to 1.44 for Cu/NiCu NWs-230 (Table S1), suggesting the accumulation of NiCu alloy phase on Cu NWs. Moreover, the slight positive shift of NiCu alloy peaks from 43.8° for Cu/NiCu NWs210 to 43.9° Cu/NiCu NWs-230 indicates the increase of Ni proportion in the shell region according to the Vegard’s law.41

Figure 2. (a) Wide-range XRD patterns of Cu NWs, Cu/NiCu NWs-210, Cu/NiCu NWs-220, Cu/NiCu NWs-230 and NiCu NPs. (b) XRD peak of (111) plane in Cu NWs, Cu/NiCu NWs210, Cu/NiCu NWs-220 and Cu/NiCu NWs-230. Figures 3a and 3b show the high-resolution TEM (HRTEM) images of selected areas on single Cu NW and Cu/NiCu NW. The clear and neat lattice fringes suggest that both Cu NWs and Cu/NiCu NWs are of well-ordered crystalline structures. For Cu NWs, the interplanar spacing is 2.08 Å (Figure 3a), corresponding to the (111) facets of typical fcc Cu crystal. While, the HRTEM image of Cu/NiCu NWs-220 (Figure 3b) displays an obvious lattice contraction from core region (2.08 Å) to shell region (2.06 Å), indicating that Cu atoms has been replaced by Ni atoms in the unit cell of shell region. Moreover, this lattice contraction in shell region would


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be more and more obvious with the increase of reacting temperature (inserts in Figure S3). To identify the elemental distribution visually on Cu/NiCu NWs, elemental mapping image and linescan profile of the Cu/NiCu NWs-220 were obtained as shown in Figures 3c and 3d. The asprepared NWs contain dominant Cu (green), showing almost uniform spatial distribution over the selected detection range. In contrast, Ni (red) only distributes in the shell region.

Figure 3. HRTEM images of (a) Cu NW and (b) Cu/NiCu NWs-220. (c) HAADF-STEM image and corresponding elemental mapping images for Ni and Cu of Cu/NiCu NWs-220. (d) EDS linescan profile of the Cu/NiCu NWs-220. Scale bars: 50 nm, 2 nm (HRTEM images). XPS was performed to examine the chemical state and surface composition ratio of the Cu and Ni in these Cu/NiCu NWs (Figures 4 and S6). The fitting peaks around 852.4 eV and 872.1 eV are indexed to metallic Ni, and the fitting peaks at 855.4 eV and 873.6 eV are indexed to Ni oxides (Figure 4a). The satellite peaks at around 861.4 eV and 879.8 eV are two shake-up type peaks of nickel at the high binding energy side of the Ni 2p3/2 and Ni 2p1/2 edge. Ni 2p XPS spectra show that even though Cu/NiCu NWs samples are dominated by metallic state Ni, most of Ni atoms on the surface or near-surface have been oxidized. The proportion of Ni oxide would


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decrease with the increase of Ni content (Table S1). In contrast, Cu 2p XPS spectra show that only a bit of Cu atoms have been oxidized to Cu2+ (Figure 4b) and the proportion of Cu oxide would increase with the decrease of Cu content. Moreover, metallic Ni peaks on Cu/NiCu NWs shift to higher binding energy with the increase of Ni content on the surface, while the Cu peaks shift to lower binding energy, indicating the alloy effect in NiCu shell. The selective oxidation of Ni atoms could be attributed to the fact that the standard redox potential of Ni/Ni2+ (- 0.24 V) is much lower than that of Cu/Cu2+ (+ 0.34 V), while the corresponding peak shift of Ni and Cu may be due to the charge transfer from Ni to Cu, indicating that Cu-Ni surface alloying structure can effectively change the charge distribution of atomic configuration.42 The role of Ni in shell layer is crucial to enhance the oxidation resistance of the CuNi core-shell nanowires, and thus enhance the stability of the electrical conductivity of core region.29 However, the electrical conductivity of the 1 D nanostructure may be encumbered by high content of Ni due to the inferior electrical conductivity of metal Ni to Cu.30 As shown in Table S2, the bulk compositions of Cu/NiCu NWs synthesized at different temperatures were determined by EDS coincided with the ICP results. Moreover, the Ni atomic ratios in bulk Cu/NiCu NWs can be raised along with the increase of reaction temperature. The surface Ni/Cu ratios of Cu/NiCu NWs determined by XPS are higher than that of EDS and ICP results, indicating the enriched Ni on the surface of Cu/NiCu NWs.


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Figure 4. XPS spectra of (a) Ni 2p and (b) Cu 2p in Cu/NiCu NWs-220. Elecrtrocatalytic Properties for Methanol Oxidation Reaction Previous investigations have shown that both MOR and oxygen evolution reaction (OER) may occur in a similar potential range after the formation of Ni3+ (NiOOH) on Ni-based catalysts in alkaline medium.43 Thus, a contrast test was implemented firstly to make out the MOR performance in Cu/NiCu NWs-220/C electrode (Figure 5a). Comparing the cyclic voltammograms (CVs) in 1 M KOH solution in presence and absence of a low concentration of methanol (0.2 M) in potentials range of 1.0~1.8 V, it is found that the electrocatalytic oxidation of methanol on Cu/NiCu NWs-220/C electrode can be clearly observed from 1.4 V to 1.65 V after the appearance of the oxidation peak of Ni2+. In contrast, the OER process in Cu/NiCu NWs-220/C electrode is occurred over 1.6 V with a large overpotential, indicating that Cu/NiCu NWs are not good electrocatalysts for OER. Moreover, the best performance of our catalysts for MOR is around 1.55 V according to the difference of current densities with and without methanol. Therefore, the potential range in the following tests is intended to be 1 ~ 1.55 V to evaluate the MOR performance of Cu/NiCu NWs. The above results indicate that our catalysts are more suitable for MOR compared with OER at a relatively low potential (< 1.6 V).


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The effect of methanol concentration on CVs in Cu/NiCu NWs-220 electrode was also investigated in 1 M KOH electrolyte (Figure 5b). It can be seen that the Ni2+ oxidation peak is gradually submerged in the current density of MOR with the increase of methanol concentration from 0.2 to 2 M. Figure S7a shows that the MOR current density at 1.55 V increases drastically and steadily with increasing methanol concentration up to 1.4 M, after that the MOR current density increases laxly. The results indicate that our catalysts can tolerate the high concentration of methanol even though the active sites get saturated at ultrahigh concentration of methanol (> 1.4 M). In addition, the MOR on Cu/NiCu NWs-220/C is estimated to be first order reaction with the methanol concentration blow 1.4 M (Figure S7b). Based on above results, the concentration of methanol is intended to be 1 M to evaluate the MOR performance of Cu/NiCu NWs in the following tests.

Figure 5. (a) CVs of Cu/NiCu NWs-220/C in 1 M KOH solution with and without 0.2 M methanol in 1 M KOH solution in different potential ranges at scan rate of 50 mV s-1. (b) CVs of


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Cu/NiCuNWs-220/C electrode in 1 M KOH solution with different methanol concentrations from 0.2 to 2.0 M at a scan rate of 50 mV s-1. (c) CVs and Jap ~ v1/2 relationship of Cu/NiCu NWs-220/C in the presence of 1 M KOH at various scan rates of 5, 10, 15, 20, 30, 40, 50, 60, 80 and 100 mV s-1. (d) CVs of bare GC, Cu NWs/C and Cu/NiCu NWs/C electrodes in 1 M KOH at scan rate of 50 mV s-1. According to the above results and previous reports, mechanism of electrocatalytic methanol oxidation on Ni-based catalysts can be described as following equations: 9, 44-45 Ni(OH)2 + OH- ↔ NiOOH + H2O + e NiOOH + CH3OH → Ni(OH)2 + Products NiOOH is the mainly active species for the oxidation of adsorbed methanol molecules on the surface of catalysts and would be reduced to Ni(OH)2 at the same time. As displayed in above equation, the potential products of methanol oxidation on Ni-based catalysts are carbonate, formaldehyde and formic acid.46 Electrocatalytic oxidation of methanol at the surface of Cu/NiCu NWs may via a possible mechanism presented in Scheme 1, in which only a few of NiCu alloy layers on the surface of nanowire involve in the electrocatalysis process. Especially, Cu atoms play an important roles for modifying the charge distribution of Ni sites and thus enhance the instruct activity of the catalysts. Moreover, both intimal NiCu alloy and Cu in the core region play curial roles in electrons transfer during catalysis. Figure 5c shows the typical CVs of Cu/NiCu NWs-220/C electrode in 1 M KOH solution at different scan rates. Distinct pairs of redox peaks corresponding to the Ni2+/Ni3+ redox couple according to equation mentioned above can be observed in both the anodic and cathodic sweeps.


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The existence of NiOOH in Cu/NiCu NWs-220/C electrode has been confirmed by XPS analyses after CV treatment (Figure S8), in which only Ni3+ and Ni2+ peaks can be observed on the Ni 2p XPS spectrum. Moreover, the position of anodic peak shifts positively to higher potential while the position of cathodic peak shifts negatively to lower potential with the increase of scan rate, and the currents of both anodic and cathodic peaks rise up. The same situation can also be observed in Cu/NiCu NWs-210/C and Cu/NiCu NWs-230/C electrodes (see Figure S9). The shift of these peaks could be attributed to the electrochemical polarization and the limitation of reaction kinetics, resulting in the formation of NiOOH species would be insufficient at higher scan rates.42-43 Therefore, it can be inferred that the total redox transition of Ni(OH)2/NiOOH on Cu/NiCu NWs is a diffusion-controlled process. Previous investigations indicated that the relative diffusion capacity of OH- on catalysts can be revealed by comparing the slope values of the linear plots of the anodic peak currents (Jap) versus the square roots of scan rate (v1/2).11, 44 Particularly, a larger slope value means that OH- has a higher diffusion capacity and also more electroactive NiOOH species can be formed on the catalysts. The slope for Cu/NiCu NWs210/C, Cu/NiCu NWs-220/C and Cu/NiCu NWs-230/C is 0.327, 0.425 and 0.451, respectively (insets in Figures 5c and S9), indicating that the Ni sites in Cu/NiCu NWs-230/C is more accessible to OH- compared with Cu/NiCu NWs-210/C and Cu/NiCu NWs-220/C. Figures 5d and S10 show the CVs of Cu/NiCu NWs/C electrodes in 1 M KOH solution with scan rate of 50 mV s-1. CVs of bare GC, Cu NWs/C and NiCu NPs/C electrodes tested in the same condition are also added for comparison. The current was normalized to the geometric surface area of the glass carbon (GC) electrode. The half wave redox potential of Ni2+ and Ni3+ (E1/2) could reveal the capability of catalysts to produce NiOOH, and the potential difference of redox peaks (∆Ep) could reveal the rate of the electron transfer kinetics between the electrode


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surface and the active centers.43 The E1/2 of Cu/NiCu NWs-210/C, Cu/NiCu NWs-220/C and Cu/NiCu NWs-230/C are calculated to be 1.300, 1.293 and 1.291 V, respectively, suggesting that the capability of producing NiOOH would be enhanced with the increase of Ni content. However, the E1/2 of NiCu NPs/C (1.289 V) is less than that of Cu/NiCu NWs/C, which may be attributed to the fact that specific surface area of zero-dimensional nanomaterials is larger than that of one-dimensional nanomaterials. Furthermore, the surface concentration of active species NiOOH (Г) can be calculated by following equation:43, 49 Г=Q/(nFA) where Q is the charge under NiOOH peak, n is the number of electrons transferred from Ni(OH)2 to NiOOH (assumed to be 1 according to chemical equation), F is Faraday’s constant and A is the geometric surface area of the GC electrodes. The results show that NiOOH surface concentration increases from 1.374 × 10-9 mol cm-2 in Cu/NiCu NWs-210/C electrode, to 1.925 × 10-9 mol cm-2 in Cu/NiCu NWs-210/C electrode and to 2.623 × 10-9 mol cm-2 in Cu/NiCu NWs210/C electrode, which is in line with the results of E1/2. The ∆Ep of NiCu NPs/C catalyst is 0.193 V, which is larger than that of Cu/NiCu NWs-220/C (0.162 V), suggesting that the electron transfer kinetics on Cu/NiCu NWs is much faster than that on NiCu NPs. Moreover, the ∆Ep of Cu/NiCu NWs-210/C and Cu/NiCu NWs-230/C are calculated to be 0.129 and 0.197 V, respectively, indicating that the rate of electron transfer kinetics would be slowed with the increase of Ni content in the surface of Cu/NiCu NWs. This should be attributed to the inferior electronic conductivity of Ni-rich shell according to the conclusion mentioned above. Figure 6a shows the CVs of bare GC electrode, Cu NWs/C, Cu/NiCu NWs-220/C, and NiCu NPs/C in 1 M KOH solution with 1 M methanol. The current density in vertical axis was


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normalized by the surface area of electrode (0.196 cm2). It can be seen that the resulting curve of Cu/NiCu NWs-220/C shows a robust rise, while curves of Cu NWs/C and NiCu NPs/C electrodes display mildly ascending after the anodic peak of Ni2+/Ni3+ in presence of 1 M methanol. The current density of Cu/NiCu NWs-220/C (34.9 mA cm-2) is about 8.9 times higher than that of Cu NWs/C (3.9 mA cm-2) and about 2.1 times higher than that of NiCu NPs-210/C (16.5 mA cm-2). The results show that Cu/NiCu NWs-220 display a remarkably higher electrocatalytic activity in comparison with NiCu NPs for MOR. Moreover, with the increase of Ni/Cu atomic ratio on the surface of NWs, the current response of the resulted Cu/NiCu NWs/C catalysts increases firstly and then decreases (Figure S11), indicating that only an optimized Ni/Cu atomic ratio could display the best catalytic activity.

Figure 6. (a) CVs of bare GC, Cu NWs/C, Cu/NiCu NWs-220/C and NiCu NPs/C electrodes 1 M KOH electrolyte with 1 M methanol at a scan rate of 50 mV s-1. (b) Mass current densities in Cu/NiCu NWs/C electrodes prepared at different temperature (210, 220 and 230 °C) at 1.55 V vs. RHE. Compared with the rapid increase of oxidation current densities, reduction peaks in these electrodes exhibit different changes in absence and presence of 1 M methanol. It can be seen that Cu/NiCu NWs-220/C shows the minimum cathodic peak current density, indicating that more


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active sites (NiOOH) are involved in MOR according to chemical equation with a high catalytic efficiency. Especially for Cu/NiCu NWs-230/C, a large number of catalytic active sites may be covered by the reaction intermediates and thus seriously hindered the further catalytic process. Moreover, the inferior electronic conductivity of NiCu shell on Cu/NiCu NWs-230/C should also be responsible for its lower catalytic efficiency. For Cu/NiCu NWs-210/C, the inferior activity should be attributed to the low concentration of active sites. As a result, Cu/NiCu NWs-220/C is the best eletrocatalyst for MOR in this work. As shown in Figure 6b, the mass current density of Cu/NiCu NWs-220/C normalized by the mass of metal components in the modified electrode is 867.1 mA mgmetal-1, which is much superior to that of Cu NWs/C (98.5 mA mgmetal-1), NiCu NPs (410.5 mA mgmetal-1), Cu/NiCu NWs-210/C (478.3 mA mgmetal-1), and Cu/NiCu NWs-230/C (527.6 mA mgmetal-1) at 1.55 V. A catalytic performance comparison between Cu/NiCu NWs220/C and other reported Ni-based catalysts for MOR is listed in Table 1, which the catalyst of Cu/NiCu NWs-220/C stands out from state-of-the-art Ni-based catalysts. It is noted that the mass activity of Cu/NiCu NWs-220/C is far higher than those Ni-based electrocatalysts ever reported before (see Table 1) suggesting that Cu/NiCu NWs-220/C has a really high efficiency and economic value for MOR. In addition, the MOR performance of commercial Pt/C (Figure S12) was also estimated in the same condition for comparison. Although the mass activity of Cu/NiCu NWs-220/C is much higher than that of commercial Pt/C (710.0 mA mgpt-1), the onset potential of Cu/NiCu NWs-220/C toward MOR is still fall behind to that of commercial Pt/C, indicating that Cu/NiCu NWs-220/C is an excellent MOR catalysts but still need to be improved. Based on the results described above, the excellent MOR performance of Cu/NiCu NWs-220 should be attributed to mainly three points: a) NiCu alloy structure in the shell can effectively change the charge distribution of atomic configuration and thus change the oxidation properties of Ni. b)


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One-dimensional structure can effectively enhance the charge transfer between electrode surface and active sites. c) The usage of Ni and Cu can be optimized in this unique core-shell structure. Table 1. Electrocatalytic performance comparison between the Cu/NiCu NWs-220/C electrodes and other Ni-based nanomaterials in previous reports. Activity Catalysts

(mA cm-2)

Scanning rate

(mA mg-1)

(mV s-1)

Condition

Cu/NiCuNWs-220/C

34.9

867.1

50

1 M KOH + 1 M CH3OH

Ni-Cu50

1.5

--

10

1 M NaOH + 0.5 M CH3OH

~21.4

~30

10

0.1 M NaOH + 0.2 M CH3OH

89

~60

10

1 M KOH + 0.5 M CH3OH

NiMoO4/C18

49

~16.3

50

1 M KOH + 2 M CH3OH

NiCo2O4-rGO52

16.6

4.8

50


Urchin-like NiCo2O453

13.5

192.7

50

0.1 M KOH + 0.5 M CH3OH

Ni/TiO2NTs54

28.3

--

50

1 M NaOH + 0.5 M CH3OH

21.0

~29.4

50


30

--

10


Ni-Cu-P56

17

--

10

0.1 M KOH + 0.5 M CH3OH

Ni-P/RGO19

16.4

117

10


Ni-Cu51 15

NiO NS@NW/NF

Ni-beta-SDS/GC NiNPs

10

55

The stability of Cu/NiCu NWs/C electrodes was evaluated by chronoamperometry (CA) in 1 M KOH solution containing 1 M methanol at 1.55 V for 10000s (see Figures 7a and S13). It is found that the current density in Cu/NiCu NWs-220/C electrode is much greater than that in NiCu NPs/C electrode all the way. Moreover, the current densities in Cu/NiCu NWs-220/C almost do not change while NiCu NPs/C suffer a rapid loss as time changes (inset in Figure 7a), indicating that the Cu/NiCu NWs-220/C catalyst possesses higher tolerance against the intermediates formed in the methanol oxidation compared with NiCu NPs/C. The stability of


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Cu/NiCu NWs-220/C was also evaluated by continuous CV scanning. As shown in Figure 7b, the electrode losses only 12% of the anodic current density at 1.55 V after 1000 cycles. These results suggest that Cu/NiCu NWs-220/C has a good long-term stability as an anode electrocatalysts in DMFCs.

Figure 7. (a) Chronoamperometric responses of Cu/NiCu NWs-220/C and NiCu NPs/C electrodes at 1.55 V (vs. RHE) in 1 M KOH containing 1 M methanol. (b) CVs of Cu/NiCu NWs-220/C electrode in 1 M KOH electrolyte with 1 M methanol at a scan rate of 50 mV s-1 before and after 1000 cycles. Electrochemical impedance spectroscopy (EIS) was employed to characterize the electrochemical properties of the electrode. Figure 8 shows the Nyquist plots of Cu/NiCu NWs220/C and NiCu NPs/C electrodes at 1.4 V in 1 M KOH with 1 M methanol. The semicircle with well-defined high and low frequencies indicates that the electron transfer rate plays an important role in the oxidation of methanol. The electrical equivalent circuit used for fitting the impedance data is shown in inset of Figure 8a, where Rs, Rct and CPE represent electrolyte resistance, charge transfer resistance and constant phase element of the non-ideal electrical double-layer. According to the fitting, the Rs value is estimated to be about 4 Ω, indicating that the Ohmic loss is small in


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the electrolyte. A small Rct value is beneficial for the fast electron transfer in the electrode at the electrode-electrolyte interface. The Rct values in NiCu NPs/C and Cu/NiCu NWs-220/C electrodes are 55 and 19 Ω, respectively, indicating that the reaction kinetics in Cu/NiCu NWs220/C electrode is much faster than that in NiCu NPs/C for methanol electro-oxidation. Moreover, the Rct values in Cu/NiCu NWs-220/C electrode toward methanol oxidation decreases with the rise of methanol concentration (Figure 8b), suggesting that the rate of the charge transfer is much depended on the rate of methanol oxidation on the catalysts. Although the conductivity of Cu/NiCu NWs is mainly depended on the Ni content in shell region, the value of Rct in Cu/NiCu NWs-220/C electrode is smaller than those of Cu/NiCu NWs-210/C (26 Ω, Figure S14) and Cu/NiCu NWs-230/C (31 Ω). This could be attributed to the fact that the effective active sites on Cu/NiCu NWs-220/C electrode are more active and numerous than those on Cu/NiCu NWs-210/C or Cu/NiCu NWs-230/C electrode.

Figure 8. (a) Nyquist plots of Cu/NiCu NWs-220/C and NiCu NPs/C electrodes at 1.4 V (vs. RHE) in 1 M KOH electrolyte with 1 M methanol. Inset: the equivalent circuit used for fitting the Nyquist plots. (b) Nyquist plots of Cu/NiCu NWs-220/C electrodes in 1 M KOH electrolyte with various concentration of methanol. CONCLUSION


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In summary, a unique Cu/NiCu electrocatalyst integrating alloy, core-shell and onedimensional structure types is prepared by a facile one-pot method. Detailed catalytic investigations showed that the obtained Cu/NiCu NWs exhibit an enhanced electrocatalytic performance for methanol oxidation reaction (MOR). The optimized Cu/NiCu NWs catalyst in this work shows the mass current densities of 867.1 mA mgmetal-1 at 1.55 V (vs. RHE) for MOR, , which is far higher than those Ni-based electrocatalysts ever reported before. Causally, the excellent MOR performance of Cu/NiCu NWs should be attributed to the structural effects, in which alloy structure can effectively change the charge distribution of atomic configuration, and one-dimensional structure can effectively enhance the charge transfer between electrode surface and active sites, and core-shell structure can be optimally the usage of Ni and Cu. Our results demonstrate that Cu/NiCu NWs possess enhanced MOR activity and durability and also provide a promising pathway to the design and synthesis of noble-metal-free electrocatalysts for fuel cells. ACKNOWLEDGEMENTS This work is supported by the National Natural Science Foundation of China (21576008, 91634116, 91334203), the Fundamental Research Funds for the Central Universities (PYCC1705) and PetroChina Innovation Foundation (2016D-5007-0505). Corresponding Author *E-mail addresses: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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(15) Luo,

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Asiri,

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NiCu electrocatalysts ... - ACS Publications

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