Tailored Ni–Cu alloy hierarchical porous nanowire as a potential efficient catalyst for DMFCs

Abstract

An urchin-like Ni–Cu nanoalloy has been prepared for the first time by a combined urea precipitation-hydrothermal reaction and high-temperature decomposition approach, and has demonstrated better electrocatalytic activities towards the oxidation of methanol.

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Introduction

Nanoalloys, due to their synergistic effects and rich diversity of compositions, structures, and properties, have led to widespread applications in catalysis.^1–6 The tunable chemical and physical properties of nanoalloys, arising from their composition-dependent surface structure and atomic segregation behavior, show considerable promise for the development of new catalysts with enhanced activity and selectivity.^3–6 Among various alloys, Ni–Cu nanoalloys are particularly efficient catalysts to carbon nanotube formation and are electrocatalysts for a number of reactions including methane decomposition, methanol oxidation, and NaBH₄ hydrolysis to generate hydrogen.^7–12 Numerous publications have described the catalytic behavior of Ni–Cu alloys in carbon nanotube formations, since the alloying of the catalytically active Ni with the less active Cu can dramatically alter the reactivity of the catalyst.^13–14 Regarding the electrocatalysis of Ni–Cu nanoalloys, Zhang et al. recently made further progress, they demonstrated that near-monodisperse Cu_0.5Ni_0.5 nanoparticles have a high H₂ generation rate at 298 K in the hydrolysis of NaBH₄ because of the low activation energy.¹⁵

On the other hand, the electrocatalytic oxidation of methanol, which is particularly important in the development of direct methanol fuel cells (DMFCs), not only needs a high efficiency but also low cost for the potential industrial applications.^16–20 Non-noble transition metals, Ni–Cu alloys undoubtedly offer the possibility of enhanced electrocatalytic efficiency and long-term stability.^20–22 The fact that face-centered cubic (fcc) structures of Ni and Cu exhibit a reasonable lattice match to form a solid solution of Ni–Cu in the entire composition range, makes finding the appropriate composition of Ni–Cu alloy complex.^23,24 The work of Mahjani' group showed that a Ni_0.8Cu_0.2 modified electrode acts as an effective catalyst for the oxidation of methanol in an alkaline process, and further demonstrated that alloying Ni with Cu is a very effective method to suppress the formation of the disadvantageous γ-NiOOH species in the alkaline media.²⁵ Nevertheless, much more attention needs to be paid to develop well-defined Ni–Cu alloys for DMFCs development .

Several methods have been reported for the preparation of Ni–Cu nanoalloys including chemical vapor deposition process (CVD),²⁶ reduction of a mixture of Ni and Cu compounds under hydrogen,²⁷ electrochemical deposition,²⁸ evaporation of Ni–Cu alloy and co-condensation with organic solvents.²⁹ The above mentioned processes, however, show limited capability in controlling the alloy structure and morphology, which are the typical surfaces in high-performance electronic applications.^30,31 This limitation can be overcome by a sol–gel morphology control process and subsequent thermal treatment.³² Alternatively, our method is based on the hydrothermal synthesis of homogeneously aligned {Ni,Cu}₂(OH)₂CO₃ nanowires, which serve as the particular precursor to create metal alloy during the thermal treatment by self-decomposition. This method, however, has been curiously overlooked, which may be presumably related to the difficulty in handling a dual-source delivery process to adjust the precursor composition. For example, Katharina et al. synthesized the layered structure α-Ni_1−xCu_x(OH)₂ precipitation, but finally converted into Ni_1−xCu_xO (bunsenite), not a Ni–Cu alloy.³³

In this work, we report the synthesis of a hierarchical urchin-like Ni–Cu structure by a hydrothermal process and subsequent nanoscale decomposition related reaction within the {Ni,Cu}₂(OH)₂CO₃ nanowires during a 700 °C annealing. Compared with the electrochemically deposited Ni–Cu alloy, this special Ni–Cu alloy with small size and hierarchical structure, shows an improved electroactivity in the electrocatalytic oxidation of methanol in a 0.1 M NaOH aqueous solution. In particular, our synthesis system is novel and interesting, for the different intrinsic hydrolytic and complex chemistries of Cu(II) and Ni(II) would restrict the synthesis of the single {Ni,Cu}₂(OH)₂CO₃ coprecipitation to very narrow conditions. In general, the method attempted here, has great potential in synthesizing tailored Ni–Cu hierarchical alloy structure as a promising catalyst for DMFCs.

Experimental

The {Ni,Cu}₂(OH)₂CO₃ precursor was firstly prepared by the coprecipitation-hydrothermal method: 0.32 g CuCl₂·2H₂O, 0.24 g NiCl₂·6H₂O and 0.7 g urea were first dissolved in 80 mL of distilled water. After stirring for 30 min at the room temperature, the obtained solution was further transferred into a Teflon-lined stainless steel autoclave, and kept at 120 °C for 24 h. After natural cooling, the resulting product was separated by centrifugation, washed several times with distilled water and ethanol, and dried at 60 °C for 5 h. Lastly, by a long annealing process at 700 °C for 5 h in a N₂ atmosphere, the as-made precursor was decomposed completely, giving rise to the novel Ni_0.7Cu_0.3 composite alloy.

Powder X-ray diffraction (XRD) (Bruker D8 A vance), scanning electron microscopy (SEM) (JSM-6700F, 5.0 kV), transmission electron microscopy (TEM and HRTEM, JEOL-2100F, 200 kV), and the X-ray photoelectron spectroscopy (XPS) (Kratos XSAM 800), were used to characterize the precursor and final alloy products. The obtained Ni–Cu nanoalloy was then employed to modify the glass carbon (GC) electrodes as follows: GC electrodes (3 mm diameter) were carefully polished with a diamond pad and a 3 μm polishing suspension, then rinsed with distilled water, ethanol and distilled water, finally dried under ambient nitrogen gas. Ni–Cu nanoalloy (5 mg) was dissolved in a mixture of 0.1 mL of Nafion perfluorosulfonated ion-exchange resin and 0.9 mL of distilled water. Approximately 5 min of rocking was necessary to obtain a uniformly dispersed Ni–Cu nanoalloy. After dropping 10 μL of the Ni–Cu nanocomposites solution onto the electrode surface, the electrode was dried in air. The electrocatalytic oxidation measurements of methanol were carried out in a CHI660C electrochemical workstation (Shanghai Chenhua, China). A saturated calomel electrode (SCE), a Pt wire and a Ni–Cu nanoalloy modified GC electrode were used as the reference, counter and working electrodes, respectively. The electrolyte was 10 ml of 0.1 M NaOH solution. All studies were carried out at 298 K.

Results and discussion

{Ni,Cu}₂(OH)₂CO₃ precursor

Fig. 1 depicts the 1D nanowire morphology and composition properties of the prepared precursor. The low-magnification SEM image (Fig. 1a) shows that the nanowires uniformly assembled into an urchin-like array over a large area. From a ruptured array in Fig. 1b, it can be clearly seen that the length of the organized nanowires is approximately 2 μm. And the XRD pattern shown in the Fig. 1c can be indexed as glaukosphaerite (JCPDS 27-0178),³⁴ no peaks corresponding to other possible crystalline structure nickel or copper solid phase were detected, demonstrating our precursor has a pure phase. The TEM image in Fig. 1e displays the needle morphology of an individual nanowire, in good agreement with the SEM results. Its corresponding high-resolution TEM (HRTEM) image shown in Fig. 1f reveals a single-crystalline structure, the spacing of the lattice fringes is calculated to be 5.05 Å, corresponding to the (120) plane of glaukosphaerite {Ni_xCu_1−x}₂(OH)₂CO₃. X-ray photoelectron spectroscopic (XPS) analysis further reveals that the atom molar ratio of Cu : Ni is about 1 : 1 (Fig. 1d), granting a high degree of metal ion intermixing (almost to an atomic level) in glaukosphaerite. The electron energy loss spectroscope (EELS) elemental mapping (Fig. 1g), as expected, clearly shows the homogeneous existence of C, O, Ni, and Cu in the single-crystalline structure; together with the above microscopy results, it unambiguously indicates the formation of high-quality {Ni,Cu}₂(OH)₂CO₃ nanowire. The simple precursor, in fact, controls the alloy formation in the subsequent annealing, and any other copper(II) or nickel(II)-containing hydroxide presence would effect the final conversion. Indeed, we found that shortening the hydrothermal reaction time, decreasing the hydrothermal reaction temperature, or changing the raw materials (such as Ni(NO₃)₂ or Cu(NO₃)₂), could lead to the introduction of other hydroxides in the {Ni,Cu}₂(OH)₂CO₃, and a failure in the conversion to a pure alloy, as a result of the accompanying metal oxide formation.

Fig. 1 (a–b) SEM images with different magnification of the precursor. (c) Corresponding XRD pattern and (d) XPS result. (e) TEM image and (f) HR-TEM image of an individual nanowires. (g) Elemental mapping of C, O, Ni, Cu.

Ni_0.7Cu_0.3 nanocrystals

Annealing under a flow of N₂ at 700 °C, the {Ni,Cu}₂(OH)₂CO₃ nanowires were converted to hierarchical Ni–Cu nanoalloy arrays, accompanied by a color change from virescent to argenteous. XRD profile in Fig. 2a, shows the single face-centered cubic (fcc) structure of the bimetallic nanoalloy. The diffraction peaks centered between the standard copper (Fm3m, PDF 04-0836) and nickel (Fm3m, PDF 04-0850)²⁶ degrees suggest the mutual insertion of the copper and nickel atoms. Again, no evidence of impurities is found in the XRD pattern. Fig. 2b is a large-scale observation of the hierarchical Ni–Cu nanoalloy. It demonstrates that the product preserves the urchin-like structures of the untreated precursor, where the nanowire arrays still stand perpendicular to the side surface of the sphere as multiple rows as shown in Fig. 1a. Herein, we need to emphasize that the surface of the nanowires appear damaged as the magnified SEM shown in Fig. 2d. It seems that the porous nanowires are packed with 20 nm nanoparticles, which should be attributed to the gradual decomposition of precursor and the release of gases (H₂O, CO₂) as the temperature increased from room temperature to 700 °C. Also, we found that the hollow spherical diameters of the urchin-like array decrease from the initial 3.6 μm (shown in Fig. 1b) to a final 1.2 μm (shown in Fig. 2c), no collapse appears. It’s thought that the removal of OH⁻ and CO₃²⁻ constitutes a loss of mechanical support for the whole hierarchical configuration, which naturally compacted in order to reduce the free volume generated.³⁵ Further HRTEM analysis reveals that secondary nanoparticles are of high single crystallinity (Fig. 3a), with exposed (111) faces with a lattice spacing of 2.1 Å, in agreement with the XRD result. The X-ray line scan profile along the line indicated in the TEM image further suggests that the bimetallic nanoparticles are Ni rich in the core region and Cu is uniform in the whole particle (Fig. 3b). Thus, the overall results demonstrate the phase-pure and highly crystalline nanowire Ni–Cu alloy obtained by delicate control of {Ni,Cu}₂(OH)₂CO₃ growth and heat treatment. Herein, we emphasize that this porous nanowire structure of Ni–Cu alloy is the first to be reported, and very is different to previous reports.^25–29 The common methods such as pulsed-spray evaporation (PSE) CVD, can not control the alloy structure and morphology, generally leading to the agglomeration of the alloy particles, while the hierarchical nanowire structure in our work overcomes this disadvantage. It is noteworthy that the energy dispersive X-ray (EDX) analysis of a single urchin-like indicates that the Ni–Cu alloy is 30 atom% Cu and 70 atom% Ni (Fig. 3c), i.e. the Cu atom molar ratio is drastically lower than in the precursors. It has been proposed that in the decomposition process, due to the mild formation of anion vacancies which will facilitate lattice diffusion as follows:³⁶

NiO → [O]′′ + 1/2O₂ + Ni[Ni]′′

CuO → [O]′′ + 1/2O₂ + Cu[Cu]′′

the mutual insertion of the generated copper and nickel nanoclusters occurs and forms the Ni–Cu alloy, while part of the copper nanoclusters could evaporate with the N₂ flow due to their relatively low melting point.

Fig. 2 (a) XRD pattern and (b–d) SEM images with different magnification of the Ni–Cu urchin-like nanoalloy.

Fig. 3 (a) TEM image of secondary nanoparticles. (b) TEM image and X-ray line scan profile along the line indicated in this image. (c) EDX spectrum on an Au grid.

Electrocatalyst properties

Successful synthesis of such porous Ni–Cu nanowires would increase the electrolyte-material contact area, adsorption ability, and enhance ion diffusion which are critical to electrocatalyst efficiency. As an example, we herein investigate its electro-catalytic property for the oxidation of methanol. The electrochemical reaction mechanism of a Ni–Cu alloy has been well studied and can be described in eqn (1) and (2).^36–39

Ni³⁺ + methanol → Ni²⁺ + intermediate (1)

Ni³⁺ + intermediate → Ni²⁺ + products (2)

where Ni³⁺ ions are an active surface for methanol oxidation, and are regenerated by the redox transition of nickel species fabricated in the electrode Ni(II) ↔ Ni(III) + e⁻ (3); The role of copper in the alloy is to inhibit the volume expansion of the Ni(II) phase during the methanol oxidation in NaOH.³⁷ Typically, Cu clusters have a stabilizing effect on the Ni surface under highly oxidizing conditions, which originates from the synergetic effect presented in a two-metal nanostructure system.^36,37

The behavior of the CVs curves at various potential sweep rates of 10–700 mVs⁻¹ (Fig. 4a) for our Ni–Cu electrode in 0.1 M NaOH solution is similar to that reported before. The peaks in the curves can be ascribed to the redox processes:

Ni(OH)₂ + OH⁻ ↔ NiOOH + H₂O + e⁻ (3)

Both the redox peak currents vary linearly with the square root of scan rate (ν^1/2), as shown in Fig. 4b and Fig. 4c, signifying a typical diffusion-controlled kinetics. In addition, the peak-to-peak separations (ΔE_p) also increase gradually with the increase of scan rate, so the electrochemical parameters of the electrode reaction are calculated according to the Laviron's equations:⁴⁰where α is the charge transfer coefficient, n is the number of electrons transferred and κ_s is the apparent heterogeneous electron transfer rate constant. Two straight lines are got with the equations as E_pa = 0.0385 lnυ + 0.7286 (γ = 0.995) and E_pc = −0.026 lnυ + 0.428. (γ = 0.992). Then the values of α and κ_s are calculated as 0.59 and 0.7 s⁻¹ respectively, which reveal a higher electron transfer rate in the redox process.

Fig. 4 (a) Typical cyclic voltammograms of a Ni–Cu electrode in 0.1 M NaOH in the potential sweep rates of 10, 20, 50, 100, 200, 300, 500, 700 mV s⁻¹, (b) The proportionality of cathodic and anodic (c) peak currents to the square roots of sweep rates.

Fig. 5a shows cyclic voltammograms of Ni–Cu electrode in 0.1 M NaOH solution in the presence of various concentrations of methanol at a potential sweep rate of 10 mV s⁻¹. As can be seen, the anodic current passes through a maximum as the potential is anodically swept, owing to the fact that the number of sites for methanol adsorption tends to decrease; In the reverse half cycle, the oxidation continues and its corresponding current goes through a maximum due to the regeneration of active adsorption sites for methanol as a result of the removal of adsorbed intermediates and products.²⁵ It is noteworthy that the electrode reveals a better desorption capacity to the products and intermediates, due to the smaller number of vacant d-levels in Ni–Cu alloy, which can lower the energy of the activated complex.⁴¹ Moreover, the anodic current in the positive sweep is proportional to the bulk concentration of methanol and any increase in the concentration of methanol causes an almost proportional linear enhancement of the anodic current (Fig. 5b). So, catalytic electro-oxidation of methanol on a Ni–Cu alloy electrode seems to be certain.

Fig. 5 (a) Cyclic voltammograms of the Ni–Cu electrode in 0.1M NaOH solution in the presence of 0 M (0); 0.001 M (1); 0.01 M (2); 0.02 M (3); 0.03 M (4); 0.1 M (5); 0.2 M (6) methanol in the solution. Potential sweep rate is 10 mV s⁻¹. (b) Dependency of the anodic peak current on the concentration of methanol in solution.

The measurement of the catalytic rate constant as well as the diffusion coefficient of methanol is further performed under a chronoamperogram regime. Fig. 6a presents double step choronoamperograms for the Ni–Cu electrode in the absence (curve 1) and presence (curves 2–6) of methanol over a concentration range of 0.01–0.2 M with an applied potential step of 650 and 0 mV, respectively. No significant current is observed when the potential is stepped down to 0 mV, indicating the irreversibility of the methanol oxidization process. The plot of the net current versus t^−0.5 obtained by removing the background current presents a linear dependency (Fig. 6b). This indicates that the transient current is controlled by a diffusion process. Using the slope of this line in the Cottrell equation⁴²

I = nFAD^0.5C*π^−0.5t^−0.5 (7)

the diffusion coefficient of methanol has been obtained to be 2.59 × 10⁻⁶ cm² s⁻¹. Choronoamperometry also can be used for the evaluation of the catalytic rate constant according to Pariente et al.⁴³where I_catal and I_L are the currents in the presence and absence of methanol and λ = kC*t is the argument of the error function. k is the catalytic rate constant, C* is bulk concentration of methanol and t is elapsed time (s). For λ > 1.5, erf(λ^0.5) almost equals unity and the above equation reduces toFrom the slope of the I_catal/I_Lvs. t^0.5 plot, presented in Fig. 6c, the value of k for the concentration range of 0.01–0.2 M of methanol is found to be as high as 9.4 × 10⁶ cm³ mol⁻¹ s⁻¹. Such excellent electrocatalytic activities towards methanol oxidation is proposed to be related to the regularly porous nanostructure. Firstly, the homogeneous distribution of Cu clusters in the alloy ensures very efficient inhibition for volume expansion of the Ni phase. On the other hand, a large surface to volume ratio in the 1D porous nanostructure is also able to improve its adsorption capability to methanol and obviously shortens the pathway of ion and electron transport, providing more active redox sites to the electrode.^44–45

Fig. 6 (a) Double step chronoamperograms of the Ni–Cu electrode in 0.1 M NaOH solution with different concentrations of methanol: (1) 0 M, (2) 0.01 M, (3) 0.03 M, (4) 0.05 M, (5) 0.1 M and (6) 0.2 M. Potential steps are 650 and 0 mV, respectively. (b) Dependency of transient current on t^−1/2. (c) Dependence of I_catal/I_L on t^1/2 derived from the data of the chronoamperograms of 1 and 2 in panel (a).

Conclusions

We have developed a novel low-cost method to produce a large area of perfectly-ordered Ni–Cu nanoalloy arrays by a combined urea precipitation and high-temperature decomposition approach. The key mechanism for the formation of these composite architectures is the delicate control of the solvent's coordinating behavior which renders the Cu²⁺ and Ni²⁺ of similar radii and precipitation of the single-phase solubility product, {Ni,Cu}₂(OH)₂CO₃. The mechanism opens up new opportunities for processing novel metal alloy or hydroxide materials based on a similar growth mechanism to that of {Ni,Cu}₂(OH)₂CO₃. Importantly, the tailored Ni_0.7Cu_0.3 alloy, has manifested improved electrocatalytic activities towards methanol oxidation, attributed to the regular porous nanostructure. Furthermore, the different atom ratios in the Ni_xCu_1−x alloy may lead to diverse catalytic properties, our future work will focus on the high-temperature decomposition process to control the atom ratio of the alloy.

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