Enhanced-electrocatalytic activity of Ni_1−xFe_x alloy supported on polyethyleneimine functionalized MoS₂ nanosheets for hydrazine oxidation

Abstract

A high-performance Ni_1−xFe_x on polyethyleneimine (PEI)-functionalized molybdenum disulfide (MoS₂) electrocatalyst has been synthesized by an electroplating in situ growth approach. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirm the successful functionalization of MoS₂ with PEI. The empty orbitals of Ni²⁺ and Fe²⁺ (Ni and Fe precursors) coordinated with the donated lone pairs of nitrogen atoms in PEI-mediated MoS₂ and then the Ni²⁺ and Fe²⁺ were in situ reduced at a negative potential. Transmission electron microscope images and X-ray diffraction reveal that Ni₈₅Fe₁₅ nanoparticles with an average size of 2.25 nm are uniformly dispersed on the PEI–MoS₂ sheets. The Ni_1−xFe_x/PEI–MoS₂ catalyst exhibits unexpectedly high activity towards the hydrazine oxidation reaction, which can be attributed to highly homogeneous dispersed Ni_1−xFe_x alloy. It also shows enhanced electrochemical stability due to the structural integrity of PEI–MoS₂. Finally, the Ni₈₅Fe₁₅/PEI–MoS₂ catalyst is proved to be very valuable for applications in hydrazine fuel cells, as compared with Ni₉₀Fe₁₀/PANi–MoS₂ and Ni₈₅Fe₁₅/MoS₂ catalysts.

---

Introduction

Since 2004,¹ graphene, a 2D single layer of carbon atoms with an hexagonal packed structure, has evoked wide interest due to its excellent physicochemical properties such as mechanical strength, electrical conductivity and ease of functionalization.^1–3 Considerable efforts have been successfully devoted to modifying graphene with metal nanoparticles (NPs) and to measure the properties of the graphene-based composites.^4–7 Besides well-known graphene, graphene-like species formed by layered inorganic materials such as molybdenum disulfide (MoS₂₎ have also sparked increasing attention in recent years.^8–18 In particular, the unique physical, optical, electrical properties of single-layered MoS₂, and its potential for applications (e.g., in topological insulators, transistors, and lithium ion batteries)^15–17 has generated an accelerating pace of research relative to that on bulk MoS₂.^11,12 Unfortunately, limited studies are available on the preparation of an alloy supported on graphene-like MoS₂ as a synergistic catalyst in fuel cells.

Hydrazine (N₂H₄), as a high-energy fuel molecule, has remarkable advantages including a superior theoretical standard equilibrium potential and a high power density.¹⁹ Numerous researchers have focused on studying the high power density of direct hydrazine–air fuel cells.²⁰ Since the 1960s, the electrooxidation of hydrazine in alkaline media has been investigated extensively at platinum, palladium, gold, nickel, and silver electrodes.^21–25 However, the high cost limits the use of noble metals as catalysts for direct alkaline hydrazine fuel cell anodes. Previous studies indicated that Ni-based alloy exhibits excellent electrocatalytic activity for hydrazine electrooxidation.¹⁹ Importantly, the catalytic activity depends not only on the metal NPs, but also on the nature of the support.^26,27 For supported metal catalysts, the activity and selectivity are strongly influenced by the amount of metal, the size of the dispersed metal particles, the preparation method, and the support composition.²⁸ With regard to support materials, single-layered MoS₂ has abundant active edges^13,14 and a large specific surface area^10,15 so that it may be a good catalytic support to load metal NPs. Zhong et al. have electroplated Ni–Fe alloy on graphene-like MoS₂ and applied the material in hydrazine oxidation. However, an obvious weakness exists in MoS₂, which has poor conductivity.¹⁸ In order to enhance the electrical conductivity of single-layered MoS₂ and to obtain a better dispersed active phase in the catalyst, the introduction of conducting polymers (CPs) to single-layered MoS₂ is a novel idea. As a matter of fact, CPs are at present studied in view of their high electrical conductivity and they have extensive technological applications such as in electronic devices and sensors.^29,30 Single-layered MoS₂ functionalized with CPs exhibit a large surface-to-volume ratio, together with good chemical stability and a broad electrochemical window, rendering them attractive electrode materials.^31,32 Meanwhile, CPs have also been explored for the dispersal of metallic particles.³³

CPs include various types, mainly electron-conducting and ion-conducting. These polymers have different cross-linked structures, which may affect the electrocatalytic performance. One of the most traditional electro-conducting polymers, polyaniline (PANi), has been widely reported in the fields of nanotechnology.^34–36 Nowdays, there has been much attention on PANi-based materials because of their good environmental stability and ease of synthesis.^35,36 Among the ion-conducting polymers, polyethyleneimine (PEI) has also attracted extensive attention from experimental-scientific communities because of its relatively high conductivity and high-water dispersibility.^37,38 Many studies on the advantages of PEI have focused on the physical adsorption of PEI on multiwalled carbon nanotubes (CNTs) in order to promote the solubility, biological compatibility and electrical conductance of CNTs.^37,39,40 Moreover, PEI contains one of the highest density of amino groups among all the polymers, donating electrons that help to anchor metal ions.^41,42

On the basis of these observations, in this work, we report a novel work on the synthesis of Ni_1−xFe_x/PEI–MoS₂ catalyst by using PEI as a functionalization and anchoring agent (Scheme 1). The template single-layered MoS₂ was functionalized in a synergistic coupling method using PEI, to form PEI–MoS₂.⁴³ Composites based on MoS₂ and PEI form a “sandwich” organization. We then electroplated Ni_1−xFe_x alloy NPs on the novel support material to produce the electrocatalyst, this was a facile and rapid process, and proceeded at low temperature. Owing to the amino-rich structure of PEI, the enhanced nucleation was favorable to anchor Ni_1−xFe_x NPs. And their interaction was suggested to originate from the special nickel/iron–nitride (Ni/Fe–N) with PEI that bridged between the Ni_1−xFe_x NPs and MoS₂ sheets. To the best of our knowledge, there have been few reports on using PEI for the preparation of MoS₂-based hybrid nanomaterials. The present report is the first on the electrodeposition of Ni_1−xFe_x alloy on PEI–MoS₂ sheets with different mass ratios of Ni and Fe, and the study of their use for the electrocatalysis of hydrazine oxidation. In order to vary the composition of bimetallic NPs, the mass concentration ratio of NiSO₄·6H₂O and FeSO₄·7H₂O was changed as some test examples. In addition, the electrocatalytic activities of the Ni_1−xFe_x/PEI–MoS₂ catalyst were further compared with the Ni_1−xFe_x/PANi–MoS₂ catalyst in which MoS₂ was wrapped physically by PANi. The advantage of this work is that PEI plays a role in mediated functionalization. It not only improves the catalytic performance of single-layered MoS₂ for hydrazine oxidation, but also leads to homogeneous distribution of Ni_1−xFe_x NPs.

Scheme 1 The process for the preparation of the Ni_1−xFe_x/PEI–MoS₂.

Experimental

Materials

Molybdenum disulfide (99%), polyethyleneimine (PEI, M_n = 600) were purchased from Aladdin-reagent Co. Ltd. Aniline, hydrogen chloride, ammonium persulfate ((NH₄)₂S₂O₈), de-ionized water (18.2 MΩ) were commercially available and used throughout the experiments. All the other commercial chemicals were used without further purification.

MoS₂ preparation

The preparation of MoS₂ has been reported previously.⁸ The details of the process are described as follows: 30 mg of MoS₂ powder was added to 25 mL flasks. 10 mL of ethanol–water with an EtOH volume of 45% was added as the dispersion solvent. The sealed flask was sonicated for 16 h, and then the dispersion was centrifuged at 5000 rpm for 10 min to remove the aggregates. The supernatant was collected and the concentration of MoS₂ in 45% ethanol–water was estimated to be 0.02 mg·mL⁻¹. The prepared dispersion of graphene-like MoS₂ was centrifuged to 0.2 mg mL⁻¹.

Preparation of PEI–MoS₂

The 0.1 wt% PEI was dispersed in the prepared MoS₂ (100 mg) aqueous solution. The suspension was sonicated in an ultrasonic bath and refluxed at 60 °C for 24 h and washed with deionized water twice. Then PEI–MoS₂ was obtained after drying the powder at 80 °C.

Preparation of PANi–MoS₂

PANi–MoS₂ composites were synthesized by the polymerization of aniline in the presence of MoS₂ suspension. An HCl solution (2 M, 150 mL) containing a prepared MoS₂ (100 mg) aqueous solution was sonicated at room temperature for about half an hour and then mixed with 0.1 mL aniline monomer. Followed by the addition of 0.26 g of (NH₄)₂S₂O₈ (dissolved in 10 mL of deionized water) that served as the oxidant. The reaction mixture was constantly stirred in an ice bath (temperature range of 0–5 °C) for 1 h and then reacted at room temperature for 12 h. The mixing was filtered and washed with distilled water–ethanol until the filtrate became colorless. PANi–MoS₂ was obtained after drying the powder at 80 °C.

Details of the direct current (DC) electroplating

An electrochemical cell with a two-electrode configuration was used for the experiments. A copper plate (50 mm × 10 mm × 1 mm) was used as the cathode and a glass carbon electrode (GCE) to which the materials (PEI–MoS₂ and PANi–MoS₂) were loaded was used as the anode. The two electrodes were placed into a cell containing NiSO₄·6H₂O and FeSO₄·7H₂O as the plating solution at 60 °C. The mass ratio of Ni NPs to Fe NPs was controlled by varying the mass concentration of NiSO₄·6H₂O (x = 250 g L⁻¹) and FeSO₄·7H₂O (y = 2.7, 9.7, 25.6, 30.5, 32, 39 g L⁻¹). This kind of electroplating is an anomalous type of codeposition. The nickel iron electroplating formula and DC electroplating experimental factors are given in Table S1 and S2 (see ESI†).

Characterization

The morphologies of Ni_1−xFe_x/PEI–MoS₂ and Ni_1−xFe_x/PANi–MoS₂ were observed using a TECNAI G²Tf²⁰ transmission electron microscope, TEM (FEI American). The Fourier transform infrared (FT-IR) spectra were obtained by the KBr disc technique and recorded on a NEXUS 670 instrument (Nicolet American). X-Ray photoelectron spectroscopy (XPS) was recorded on a PHI-5702 instrument. X-ray diffraction (XRD) measurements were performed on a Rigaku D/max-2400 diffractometer, using Cu-Kα radiation as the X-ray source in the range of 10–90 °C. The amount of PEI and PANi loaded on MoS₂ were determined by thermogravimetric analysis (TGA) (Linseis German). The content of Ni_1−xFe_x alloy in the Ni_1−xFe_x/PEI–MoS₂ and Ni_1−xFe_x/PANi–MoS₂ was analyzed by inductively coupled plasma (ICP) spectroscopy using an IRIS Advantage analyzer.

Electrochemical measurement

Electrochemical measurements of Ni_1−xFe_x/PEI–MoS₂ (PANi–MoS₂) hybrid were performed with a CHI660C electrochemical analyzer (CHI Instrument Corp. Shanghai) using a three-electrode system at 60 °C throughout this work. The concentration of the Ni_1−xFe_x/PEI–MoS₂ (Ni_1−xFe_x/PANi–MoS₂) in water–ethanol–Nafion (volume ratio = 2 : 1 : 1) solution was 1 mg mL⁻¹. A GCE electrode loaded with 10 μL of the prepared Ni_1−xFe_x/PEI–MoS₂ (PANi–MoS₂) solution served as the working electrode, a platinum wire and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The electrolyte was 0.1 M hydrazine hydrate–0.015 M NaOH which had been purged with N₂ for 10 min prior to the experiment.

Results and discussion

TEM and XRD analysis of PEI–MoS₂ and PANi–MoS₂

Fig. 1 shows the dispersion and morphology together with the particle size distribution of the Ni₈₅Fe₁₅/PEI–MoS₂ and Ni₉₀Fe₁₀/PANi–MoS₂ hybrids . It can be seen clearly that the PEI–MoS₂ sheets were successfully decorated with many well dispersed Ni_1−xFe_x NPs (Fig. 1a). Electrochemical deposition of bimetallic Ni_1−xFe_x NPs resulted in the appearance of small particles on the surface of PEI–MoS₂. Systematic counting revealed that the size of about 60% of the NPs was 2.25 nm. Further analysis of the Ni_1−xFe_x NPs by ICP spectroscopy showed a composition of 85% : 15% (mass ratio). Fig. 1c reveals that the Ni_1−xFe_x alloy NPs were also stabilized on PANi–MoS₂. However, the dispersed metal particles on the PANi–MoS₂ support were not uniform in size. Therefore, it can be confirmed that PEI played an important role in the formation of the Ni_1−xFe_x/PEI–MoS₂ composite materials. The PEI coupling on the MoS₂ sheets produced a uniform distribution of the amino groups which served as condense centers for the immobilization of Ni and Fe NPs.

Fig. 1 TEM images of (a) Ni₈₅Fe₁₅/PEI–MoS₂ and (c) Ni₉₀Fe₁₀/PANi–MoS₂; HRTEM images of (b) Ni₈₅Fe₁₅/PEI–MoS₂ and (d) Ni₉₀Fe₁₀/PANi–MoS₂.

As a comparison, the TEM image of the graphene like MoS₂ and the Ni₈₅Fe₁₅/MoS₂ hybrid are illustrated (see ESI,† Fig. S1). Fig. S1b† shows the TEM image of the Ni₈₅Fe₁₅ NPs directly deposited on the surface of MoS₂, it is obvious that the metal NPs aggregate readily and the size of the metal NPs isn't remarkably uniform.

In addition, energy dispersive X-ray (EDX) analysis was utilized to determine the chemical composition of the Ni₈₅Fe₁₅/PEI–MoS₂ and Ni₉₀Fe₁₀/PANi–MoS₂ catalysts. Peaks for Ni, Fe, Mo, S and N can be observed in Fig. 2 and indicate the composition of the catalysts. The peak of Cu arises from the copper grid in TEM analysis.

Fig. 2 EDX images of (a) Ni₈₅Fe₁₅/PEI–MoS₂ and (b) Ni₉₀Fe₁₀/PANi–MoS₂.

Fig. 3 shows the XRD patterns of the Ni₈₅Fe₁₅/PEI–MoS₂ and Ni₈₅Fe₁₅/PEI–MoS₂ hybrids to further investigate the crystalline structure. The primary (002) diffraction peak can be observed at 2θ = 14.3°, which is in good agreement with the hexagonal structure of MoS₂.⁴⁴ Moreover, it is seen that the crystal structure of Ni_1−xFe_x is face-centered cubic (fcc), diffraction peaks at 2θ values of 44.5°, 51.9° and 76.4° can be assigned to the (111), (200), and (220) crystal planes of the Ni_1−xFe_x alloy, respectively.⁴⁵ Thus, the successful electroplating of Ni–Fe ions to Ni–Fe NPs can be confirmed. In addition, it is shown that the Ni₉₀Fe₁₀/PANi–MoS₂ hybrid has a crystalline structure similar to that of Ni₈₅Fe₁₅/PEI–MoS₂.

Fig. 3 XRD patterns of the Ni₉₀Fe₁₀/PANi–MoS₂ hybrid, the Ni₈₅Fe₁₅/PEI–MoS₂ hybrid and MoS₂.

FT-IR analysis of PEI–MoS₂ and PANi–MoS₂

FT-IR spectra of MoS₂, pure PEI, PEI–MoS₂, Ni₈₅Fe₁₅/PEI–MoS₂, PANi–MoS₂, and Ni₉₀Fe₁₀/PANi–MoS₂ are illustrated in Fig. 4. As can be seen from the spectrum of MoS₂, the band at 467 cm⁻¹ could be assigned to stretching of the Mo–S vibrations.⁴⁶

Fig. 4 FT-IR spectra of (a) MoS₂, (b) PEI, (c) PEI–MoS₂, (d) Ni₈₅Fe₁₅/PEI–MoS₂, (e) PANi–MoS₂, (f) Ni₉₀Fe₁₀/PANi–MoS₂.

In the spectrum of PEI, two bands at 3350 and 3285 cm⁻¹ arise from NH₂ stretching vibrations, the strong peaks at 2938 and 2823 cm⁻¹ that can be assigned to asymmetric and symmetric vibrations of the CH₂ group, respectively, and the peak at 1459 cm⁻¹ which corresponds to inplane bending of CH₂. Peaks for the bending vibration of the NH group and the stretching vibration of the C–N groups of PEI can be seen at 1598 cm⁻¹ and 1120 cm⁻¹, respectively.⁴⁷ However, there are subtle differences at the same positions between the IR spectrum of the PEI–MoS₂ complex and that of MoS₂ or PEI alone. The stretching vibrations of NH₂ groups, NH groups and C–N groups shift to higher frequencies (3380 cm⁻¹, 1616 cm⁻¹, 1142 cm⁻¹) for the PEI–MoS₂ hybrid material, which clearly proves our hypothesis that synergistic coupling occurs between the amino-rich PEI and MoS₂ sheets. Overall, the IR spectrum of PEI–MoS₂ shows peaks that are characteristic of their main functional groups. When depositing Ni_1−xFe_x alloy on the PEI–MoS₂ sheets, the stretching vibrations of the main functional groups also shift to higher frequencies, which is thought to originate from the interaction of nickel/iron–nitride (Ni/Fe–N).

For the IR spectrum of PANi–MoS₂,^48,49 the band at 3440 cm⁻¹ is attributable to N–H stretching vibration. The main peaks at 1575 and 1492 cm⁻¹ can be assigned to the stretching vibrations of quinone and benzene rings, respectively. The bands at 1297 and 1240 cm⁻¹ are attributed to C–N and C=N stretching mode, while the band at 1136 cm⁻¹ is assigned to the in-plane bending of C–H. A band at about 800 cm⁻¹ could be attributed to the out-of-plane bending of C–H, while the many low-intensity peaks ranging from 780 to 580 cm⁻¹ can be assigned to the vibrations of the C–H bonds in the benzene rings. The spectrum clearly indicates that PANi has been wrapped onto the surface of MoS₂. Moreover, the spectrum of Ni₉₀Fe₁₀/PANi–MoS₂ is shown in Fig. 4f. The stretching vibrations of the main characteristic groups shifting to lower frequency indicate the formation of Ni_1−xFe_x alloy on PANi–MoS₂.

XPS analysis of N1s spectrum of PEI and PEI–MoS₂

In conjunction with FT-IR spectroscopy, XPS has increasingly been used to study the interactions between metal ions and electron-donating ligands.^50,51 As is shown in Fig. 5a and b, the N1s spectrum was used to determine the nitrogen configurations. In the research work on PEI, the N1s spectrum can be deconvoluted to several individual peaks which are assigned to =N– (398.51 eV), –NH– (399.23 eV), and –NH₂ (399.84 eV). However, the peak position of these nitrogen types varies as shown in Fig. 5b after MoS₂ was modified by PEI. The binding energies were 398.71, 399.85, and 400.88 eV for =N–, –NH–, and –NH₂, respectively. The peak shift of –NH₂ is as large as 1 eV, which is explained by the synergistic effect of Mo ions and the donating electrons of –NH₂.⁴² The charge of nitrogen and its neighbouring atoms and the electron redistribution all influence the precise position of different nitrogen types. Thus, the binding energy of =N– and –NH– results in little peak shift.

Fig. 5 N1s spectra of (a) PEI and (b) PEI–MoS₂.

TGA analysis of PEI–MoS₂ and PANi–MoS₂

TGA analysis (Fig. 6) was also used to characterize the surface modification of the MoS₂. The weight loss stage below 200 °C is a result of the evaporation of physically adsorbed water and residual solvent. As is shown in Fig. 6b, there is a sharp decrease at about 400 °C and this could be due to the decomposition of PEI. Comparing the TGA curves of MoS₂ and PEI–MoS₂, the weight loss of PEI in PEI–MoS₂ is 20.91%. In Fig. 6c, a rapid decrease begins at about 200 °C, and there is a 18.74% weight loss of PANi. It is worth mentioning that the PEI–MoS₂ hybrid has a much better thermal stability than that of PANi–MoS₂.

Fig. 6 TGA curves of MoS₂, PEI–MoS₂ and PANi–MoS₂.

Hydrazine oxidation on the hybrid electrodes in the 0.1 M hydrazine hydrate/0.015 M NaOH solution

The electrochemical activity of the hydrazine oxidation reaction has been evaluated using cyclic voltammetry. Cyclic voltammograms of hydrazine on a GCE (in Fig. 7) and on a MoS₂/GCE were first recorded. On both electrodes, no oxidation peaks were seen. When PEI–MoS₂/GCE and PANi–MoS₂/GCE were applied, a similar oxidation peak appeared at −0.2 V, while on a PEI–MoS₂/GCE the oxidation current was higher and the capacitive current was bigger than that on a PANi–MoS₂/GCE, confirming that the PEI–MoS₂ material has a higher effective surface area and a better synergistic catalytic performance for hydrazine oxidation.

Fig. 7 Cyclic voltammetric measurements of the PEI–MoS₂ hybrid, the PANi–MoS₂ hybrid, the MoS₂ and naked electrode at a scan rate of 50 mV s⁻¹.

Then the electrocatalytic oxidation of hydrazine with bimetallic Ni_1−xFe_x NPs on the PEI–MoS₂/GCE and PANi–MoS₂/GCE were studied. The mass ratios of Ni to Fe were varied in order to obtain the optimum content for hydrazine electrocatalysis. These NPs were produced by varying the mass concentration ratios of [NiSO₄·6H₂O]/[FeSO₄·7H₂O] during the electrodeposition process. The final mass ratio of Ni to Fe was analyzed by ICP spectroscopy (see ESI,† Table S3). Fig. 8 records a series of Ni_1−xFe_x mass ratios ranging from 95 : 5 to 70 : 30 on the PEI–MoS₂/GCE and PANi–MoS₂/GCE, respectively. In Fig. 8a, the Ni_1−xFe_x/PEI–MoS₂ catalyst presents a high peak of anodic oxidation current. With an increase of the amount of Ni NPs, it leads to a positive shift of the oxidation potential as well as an enhancement of the anodic current. Nevertheless, the amount of Ni NPs doesn't solely influence the electrocatalytic activity. Our previous studies reported that nickel is the major active catalyst in the oxidation of hydrazine, while iron is the synergistic one.⁵² It is interesting to note that different metal ratios give a minor variation in the oxidation potential. Analyzed from a theoretical view (Fig. 8a), a Ni₈₅Fe₁₅/PEI–MoS₂ is better for voltammetric detection of hydrazine since a higher oxidation current is recorded. This results in better sensitivity and a lower detection limit. While for energy and environmental related applications, a Ni₇₀Fe₃₀/PEI–MoS₂ is more promising since the oxidation potential is reduced, leading to higher efficiencies.^53,54 The oxidation of Ni₈₅Fe₁₅ in Ni₈₅Fe₁₅/PEI–MoS₂ occurs at more positive potentials compared to Ni₇₀Fe₃₀/PEI–MoS₂, suggesting that there may be some relation between hydrazine oxidation reason (HOR) and Ni_1−xFe_x alloy.⁵⁵ How to select a preferable ratio of Ni to Fe in the Ni_1−xFe_x/PEI–MoS₂ for hydrazine oxidation is an important issue. Ye et al.⁵⁶ electrodeposited Au–Pt alloy particles with various Pt/Au molar ratios (including 4 : 1, 1 : 1, 1 : 4) on indium tin oxide (ITO) substrates. Then they applied the catalyst to investigate the reaction of methanol oxidation. Eventually, Ye et al. concluded that the electrocatalytic performance of Pt₄Au₁ was better than that of Pt although the oxidation peak potential shifted positively by about 160 mV with Pt₄Au₁ compared to that with Pt. In our study, the oxidation potential of Ni₈₅Fe₁₅/PEI–MoS₂ shifted positively by just about 100 mV compared to that with Ni₇₀Fe₃₀/PEI–MoS₂ and the oxidation current of Ni₈₅Fe₁₅/PEI–MoS₂ was obviously 2 times higher than that of Ni₇₀Fe₃₀/PEI–MoS₂. Thus, Ni₈₅Fe₁₅/PEI–MoS₂ is considered to be a preferable electrocatalyst for hydrazine oxidation. A similar finding is observed in Fig. 8b, where it can be seen that the electrocatalytic performance of Ni₉₀Fe₁₀/PANi–MoS₂ is better for hydrazine oxidation. The maximum oxidation current peaks for Ni_1−xFe_x/PEI–MoS₂ and Ni_1−xFe_x/PANi–MoS₂ correspond to different mass ratios of Ni to Fe (85 : 15 and 90 : 10), suggesting that the interactions between PEI–MoS₂ (PANi–MoS₂) and Ni_1−xFe_x alloy are distinguishable.

Fig. 8 Cyclic voltammetric measurements of Ni_1−xFe_x/PEI–MoS₂ (a) and Ni_1−xFe_x/PANi–MoS₂ (b) electrode with different Ni/Fe mass ratios at a scan rate of 50 mV s⁻¹. Insets: mass activities at respective peaks.

As a comparison, the Ni_1−xFe_x alloy NPs were also deposited on MoS₂ with different Ni_1−xFe_x mass ratios at the same mass loading under the same conditions (see ESI,† Fig. S2).

Fig. 9 compares the cyclic voltammograms for the oxidation of hydrazine on a Ni₈₅Fe₁₅/PEI–MoS₂/GCE, a Ni₉₀Fe₁₀/PANi–MoS₂/GCE, a Ni₈₅Fe₁₅/MoS₂/GCE, a Ni₈₅Fe₁₅/GCE, a PEI–MoS₂/GCE, a PANi–MoS₂/GCE, a MoS₂/GCE. The maximum current of the hydrazine hydrate oxidation peaks for Ni₈₅Fe₁₅/PEI–MoS₂ and Ni₉₀Fe₁₀/PANi–MoS₂ hybrids were higher than that of the Ni₈₅Fe₁₅/MoS₂ catalyst, which indicates the obvious advantage of modifying the single-layer MoS₂ sheets (e.g. larger active area) with CPs. Remarkably, the oxidation current on a Ni₈₅Fe₁₅/PEI–MoS₂/GCE is higher than that on a Ni₉₀Fe₁₀/PANi–MoS₂/GCE and on a Ni₈₅Fe₁₅/GCE, confirming the enhancement effect of PEI–MoS₂ as the support. On a Ni₈₅Fe₁₅/PEI–MoS₂/GCE, Ni₉₀Fe₁₀/PANi–MoS₂/GCE and Ni₈₅Fe₁₅/MoS₂/GCE the oxidation occurs at quite similar potentials. According to these results, we found that PEI–MoS₂ supported metal NPs are promising platforms for electrochemical oxidation of hydrazine.

Fig. 9 Cyclic voltammetric measurements of the Ni₈₅Fe₁₅/PEI–MoS₂ hybrid, the Ni₉₀Fe₁₀/PANi–MoS₂ hybrid, the Ni₈₅Fe₁₅/MoS₂ hybrid, the Ni₈₅Fe₁₅ alloy hybrid, the PEI–MoS₂ hybrid, the PANi–MoS₂ hybrid, and MoS₂ at a scan rate of 50 mV s⁻¹. Inset: mass activities at respective peaks.

The stability of catalysts is important for the application of fuel cells. Chronoamperometric measurements were carried out to appraise the durability of the catalysts. Fig. 10 shows the i–t curves of the Ni₈₅Fe₁₅/PEI–MoS₂ hybrid and the Ni₉₀Fe₁₀/PANi–MoS₂ at a working potential of −0.2 V. In the initial period, the decay current density decreased rapidly for all the catalysts. This may be due to the formation of the intermediate species (such as N_2ads, NH_3ads) during the hydrazine oxidation reaction.¹⁹ However, during the whole time, the larger residual currents for the Ni₈₅Fe₁₅/PEI–MoS₂ hybrid compared with those of the Ni₉₀Fe₁₀/PANi–MoS₂ hybrid indicate that PEI–MoS₂ offers ideal support for the stability of the catalyst.

Fig. 10 Chronoamperometric measurement in 0.1 M hydrazine hydrate and 0.015 M NaOH at −0.2 V. (a) The Ni₈₅Fe₁₅/PEI–MoS₂ hybrid; (b) the Ni₉₀Fe₁₀/PANi–MoS₂ hybrid; (c) the Ni₈₅Fe₁₅/MoS₂ hybrid.

Mechanism of formation of Ni_1−xFe_x/PEI–MoS₂ catalyst

The following mechanism for the Ni_1−xFe_x/PEI–MoS₂ hybrid which has superior electrocatalytic activity for hydrazine electrooxidation is proposed. It is well known that MoS₂ is a good electron acceptor and PEI on the other hand is a very good electron donor.⁴³ PEI has an amino-rich structure with polar groups, and the donated lone pairs of nitrogen atoms in the –NH₂ polar groups of a PEI unit may attach to Mo atoms. Meanwhile, other functional groups, such as –NH– and =N–, which can act as nucleation centers for Ni_1−xFe_x NPs. In addition, the exposed sulfur edges can play an important role in anchoring Ni_1−xFe_x NPs as well. Given such a background, the surface of the Ni_1−xFe_x NPs becomes electron-rich when the potential gradually changes from positive to negative, which can promote the dissociation of N₂H₄ and adsorbed hydrogen atom on the metal surface.^12,57 The PEI–MoS₂ hybrid presents more homogeneous active sites to anchor more Ni_1−xFe_x NPs. The increasing number of activated regions can enhance the catalytic reaction directly. Therefore, the Ni_1−xFe_x/PEI–MoS₂ catalyst shows distinguished electrocatalytic activity for hydrazine oxidation.

Conclusion

In this paper, we have demonstrated a novel and simple in situ growth method to prepare the Ni_1−xFe_x/PEI–MoS₂ composite material. This synthetic route involved a mild heat-treatment process, which induced the synergistic coupling between PEI and MoS₂. The introduction of PEI to MoS₂ enhanced the interaction between Ni_1−xFe_x alloy and MoS₂, so as to improve the stability of the catalyst. More importantly, PEI-functionalized MoS₂ provided more homogeneous active sites to attach Ni_1−xFe_x NPs. By electrochemical testing, the Ni₈₅Fe₁₅/PEI–MoS₂ catalyst for hydrazine oxidation exhibits unexpected, surprisingly pronounced electrocatalytical activity in alkaline solutions, comparable to Ni₉₀Fe₁₀/PANi–MoS₂ and Ni₈₅Fe₁₅/MoS₂ catalyst but far exceeding in durability. Various methods for the characterization of the composites were presented, which prove that the Ni₈₅Fe₁₅/PEI–MoS₂ catalyst is very promising for portable applications in hydrazine fuel cells.

Acknowledgements

This research was supported by the National Science Foundation of China (No. 21345003), the Key Laboratory of Catalytic engineering of Gansu Province and Resources Utilization, Gansu Province for financial support, the Fundamental Research Funds for the Central Universities (Lzujbky-2013-67).

Notes and references

1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666–669.
2. A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov and A. K. Geim, Rev. Mod. Phys., 2009, 81, 109–162.
3. S. Stankovich, D. A. Dikin, G. H. B. Dommett, K. M. Kohlhaas, E. J. Zimney, E. A. Stach, R. D. Piner, S. T. Nguyen and R. S. Ruoff, Nature, 2006, 442, 282–286.
4. S. Anandan, A. Manivel and M. Ashokkumar, Fuel Cells, 2012, 12, 956–962.
5. U. Lange, T. Hirsch, V. M. Mirsky and O. S. Wolfbeis, Electrochim. Acta, 2011, 56, 3707–3712.
6. C. Nethravathi, M. Rajamathi, N. Ravishankar, L. Basit and C. Felser, Carbon, 2010, 48, 4343–4350.
7. S. Bhaviripudi, X. T. Jia, M. S. Dresselhaus and J. Kong, Nano Lett., 2010, 10, 4128–4133.
8. K.-G. Zhou, N.-N. Mao, H.-X. Wang, Y. Peng and H.-L. Zhang, Angew. Chem., Int. Ed., 2011, 50, 10839–10842.
9. X. S. Zhou, L. J. Wan and Y. G. Guo, Chem. Commun., 2013, 49, 1838–1840.
10. C. N. R. Rao and A. Nag, Eur. J. Inorg. Chem., 2010, 4244–4250.
11. Z. Y. Zeng, Z. Y. Yin, X. Huang, H. Li, Q. Y. He, G. Lu, F. Boey and H. Zhang, Angew. Chem., Int. Ed., 2011, 50, 11093–11097.
12. Z. Y. Yin, H. Li, L. Jiang, Y. M. Shi, Y. H. Sun, G. Lu, Q. Zhang, X. D. Chen and H. Zhang, ACS Nano, 2012, 6, 74–80.
13. T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 2007, 317, 100–102.
14. B. Hinnemann, P. G. Moses, J. Bonde, K. P. Jorgensen, J. H. Nielsen, S. Horch, I. Chorkendorff and J. K. Norskov, J. Am. Chem. Soc., 2005, 127, 5308–5309.
15. K. K. Liu, W. J. Zhang, Y. H. Lee, Y. C. Lin, M. T. Chang, C. Su, C. S. Chang, H. Li, Y. M. Shi, H. Zhang, C. S. Lai and L. J. Li, Nano Lett., 2012, 12, 1538–1544.
16. H. Li, G. Lu, Z. Y. Yin, Q. Y. He, Q. Zhang and H. Zhang, Small, 2012, 8, 682–686.
17. K. Chang, W. X. Chen, L. Ma, H. Li, F. H. Huang, Z. D. Xu, Q. B. Zhang and J. Y. Lee, J. Mater. Chem., 2011, 21, 6251–6257.
18. X. Zhong, H. Yang, S. Guo, S. Li, G. Gou, Z. Niu, Z. Dong, Y. Lei, J. Jin, R. Li and J. Ma, J. Mater. Chem., 2012, 22, 13925.
19. J. Sanabria-Chinchilla, K. Asazawa, T. Sakamoto, K. Yamada, H. Tanaka and P. Strasser, J. Am. Chem. Soc., 2011, 133, 5425–5431.
20. N. V. Rees and R. G. Compton, Energy Environ. Sci., 2011, 4, 1255–1260.
21. V. Rosca and M. T. M. Koper, Electrochim. Acta, 2008, 53, 5199–5205.
22. N. Maleki, A. Safavi, E. Farjami and F. Tajabadi, Anal. Chim. Acta, 2008, 611, 151–155.
23. M. Hosseini, M. M. Momeni and M. Faraji, J. Mol. Catal. A: Chem., 2011, 335, 199–204.
24. R. Wojcieszak, M. Zieliński, S. Monteverdi and M. M. Bettahar, J. Colloid Interface Sci., 2006, 299, 238–248.
25. M. Hosseini and M. M. Momeni, J. Mater. Sci., 2010, 45, 3304–3310.
26. M. Valden, X. Lai and D. W. Goodman, Science, 1998, 281, 1647–1650.
27. G. Budroni and A. Corma, Angew. Chem., Int. Ed., 2006, 45, 3328–3331.
28. D. G. Mustard and C. H. Bartholomew, J. Catal., 1981, 67, 186–206.
29. G. Inzelt, M. Pineri, J. W. Schultze and M. A. Vorotyntsev, Electrochim. Acta, 2000, 45, 2403–2421.
30. S. Nambiar and J. T. W. Yeow, Biosens. Bioelectron., 2011, 26, 1825–1832.
31. C. Y. Wang, D. Li, C. O. Too and G. G. Wallace, Chem. Mater., 2009, 21, 2604–2606.
32. S. R. C. Vivekchand, C. S. Rout, K. S. Subrahmanyam, A. Govindaraj and C. N. R. Rao, J. Chem. Sci., 2008, 120, 9–13.
33. G. Wu, L. Li, J. H. Li and B. Q. Xu, Carbon, 2005, 43, 2579–2587.
34. X. F. Lu, W. J. Zhang, C. Wang, T. C. Wen and Y. Wei, Prog. Polym. Sci., 2011, 36, 671–712.
35. J. Yan, T. Wei, B. Shao, Z. J. Fan, W. Z. Qian, M. L. Zhang and F. Wei, Carbon, 2010, 48, 487–493.
36. S. Chen, Z. Wei, X. Qi, L. Dong, Y.-G. Guo, L. Wan, Z. Shao and L. Li, J. Am. Chem. Soc., 2012, 134, 13252–13255.
37. X. G. Hu, T. Wang, X. H. Qu and S. J. Dong, J. Phys. Chem. B, 2006, 110, 853–857.
38. X. Fang, H. Ma, S. L. Xiao, M. W. Shen, R. Guo, X. Y. Cao and X. Y. Shi, J. Mater. Chem., 2011, 21, 4493–4501.
39. X. Y. Cao, J. J. Chen, S. H. Wen, C. Peng, M. W. Shen and X. Y. Shi, Nanoscale, 2011, 3, 1741–1747.
40. Y. Liu, D.-C. Wu, W.-D. Zhang, X. Jiang, C.-B. He, T. S. Chung, S. H. Goh and K. W. Leong, Angew. Chem., Int. Ed., 2005, 117, 4860–4863.
41. L. Bai, H. Zhu, J. S. Thrasher and S. C. Street, ACS Appl. Mater. Interfaces, 2009, 1, 2304–2311.
42. P. L. Kuo, W. F. Chen, H. Y. Huang, I. C. Chang and S. A. Dai, J. Phys. Chem. B, 2006, 110, 3071–3077.
43. H. Zhang, K. P. Loh, C. H. Sow, H. R. Gu, X. D. Su, C. Huang and Z. K. Chen, Langmuir, 2004, 20, 6914–6920.
44. K. Chang and W. Chen, Chem. Commun., 2011, 47, 4252.
45. S. Bai, X. P. Shen, G. X. Zhu, Z. Xu and J. Yang, CrystEngComm, 2012, 14, 1432–1438.
46. T. Weber, J. C. Muijsers, H. vanWolput, C. P. J. Verhagen and J. W. Niemantsverdriet, J. Phys. Chem., 1996, 100, 14144–14150.
47. T. Yang, A. Hussain, S. Bai, I. A. Khalil, H. Harashima and F. Ahsan, J. Controlled Release, 2006, 115, 289–297.
48. N. A. Kumar, H. J. Choi, Y. R. Shin, D. W. Chang, L. M. Dai and J. B. Baek, ACS Nano, 2012, 6, 1715–1723.
49. X. M. Feng, G. Yang, Q. Xu, W. H. Hou and J. P. Zhu, Macromol. Rapid Commun., 2006, 27, 31–36.
50. S. Deng and Y.-P. Ting, Water Res., 2005, 39, 2167–2177.
51. S. B. Deng, R. B. Bai and J. P. Chen, Langmuir, 2003, 19, 5058–5064.
52. H. D. Yang, X. Zhong, Z. P. Dong, J. Wang, J. Jin and J. T. Ma, RSC Adv., 2012, 2, 5038–5040.
53. K. Asazawa, K. Yamada, H. Tanaka, A. Oka, M. Taniguchi and T. Kobayashi, Angew. Chem., Int. Ed., 2007, 119, 8170–8173.
54. Q. Wan, Y. Liu, Z. Wang, W. Wei, B. Li, J. Zou and N. Yang, Electrochem. Commun., 2013, 29, 29–32.
55. M. Khosravi and M. K. Amini, Int. J. Hydrogen Energy, 2010, 35, 10527–10538.
56. W. Ye, H. Kou, Q. Liu, J. Yan, F. Zhou and C. Wang, Int. J. Hydrogen Energy, 2012, 37, 4088–4097.
57. Q. J. Xiang, J. G. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134, 6575–6578.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra42757a

---