1.   Introduction

As the conventional energy sources consume increasingly daily, methanol is considered as a green and renewable energy resource. Direct methanol fuel cells (DMFCs) have gained intensive interests in power energy field because of their high energy density, low operating temperature, and environmental friendliness [1–5]. During the process of DMFCs, methanol oxidation reaction (MOR) is the bottleneck in improving the overall performance of DMFCs, and thus, highly activity electrocatalysts for MOR are urgently demanded.

Platinum (Pt)-based nanocomposites have attracted great attention due to their excellent electrocatalytic performance in MOR [6–10]. The present hot spots are focusing on the promotion of MOR electrocatalytic activity by accurately controlling the shape and composition of Pt-based nanocomposites. However, the electrocatalytic activity of Pt-based catalysts degrades quickly on account of CO poisoning, where CO comes from the decomposition of methanol molecules during the electrocatalytic methanol oxidation reaction. According to the previous research [11–15], modified ruthenium on Pt-based catalysts has been recognized as one of the best candidates to solve CO poisoning that was interpreted as Watanabe-Motoo mechanism. Among various nano-substrates, the multi-walled carbon nanotubes (MWCNT) have been recognized as highly desirable one-dimensional carbon materials due to their low business cost, exceptional electrical conductivity, and electron transporting feature, as well as good synergistic effect with metal nanoparticles [16–18]. Thus, a noticeable performance improvement can be obtained by using MWCNT as efficient carrier for nanocomposites. In addition, shape engineering and substrate modulation for Pt–Ru bimetallic nanoparticles have been used to enhance electrocatalytic performances. The Pt–Ru bimetallic decorated carbon nanotubes (CNT) caught much interest. However, the previous nanocomposite designs were normally based on van der Waals (vdW) heterostructures, where the interactions between Pt-based nanoparticles and the nano-substrates are mainly vdW interactions [19–20]. The relatively weak interactions could limit the electrochemical activity and electron-transfer properties of nanocomposites [21–22]. In addition, a variety of methods including electrostatic adsorption [23–24], reverse microemulsion method [25], electrochemistry deposition [26–27], and in-situ reduction have been attempted to synthesize Pt-decorated-CNT nanocomposites [28–30]. Among these strategies, in-situ reduction on the MWCNT surface has drawn much attention by researchers because of the improved loading for catalyst nanoparticles. However, various templates, reducing agents, and additives are needed in the in-situ reduction method, such as inorganic reductants [28], organic reductants [12,29–30], surfactants, etc., which result in resources wasted and environmental pollution.

In this paper, instead of vdW heterostructures, we started from functionalized multi-walled CNTs (F-MWCNTs) and reported the synthesis of a novel nanocomposite, Pt–Ru bimetallic nanoparticles covalently bonded on MWCNTs (Pt–Ru@MWCNT), through a facile and environment-friendly procedure without adding additional reducing agent. The Ru atom was introduced to Pt-based nanocomposite to enhance the resistance to CO poisoning. The microstructure of the as­prepared nanocomposites exhibits that the surface of MWCNTs are homogeneously decorated with covalently-bonded metallic nanoparticles of the size around 2–3 nm. The electrocatalytic activities towards MOR of these nanocomposites have been investigated systematically. The results reveal that the covalently-bonded Pt@MWCNT nanocomposite possesses much enhanced electrocatalytic performance than that of the commercial 20wt% Pt@C. More importantly, the covalently-bonded Pt–Ru@MWCNT nanocomposite shows the highest electrocatalytic activity and durability for MOR among the tested Pt-based catalysts at all applied potentials.

2.   Experimental

2.1   Synthesis of Pt–Ru@MWCNT nanocomposite

The functionalized MWCNTs were synthesized according to previous reports [31–32]. The covalently-bonded Pt–Ru@MWCNT nanocomposite was synthesized through a new way. First, 6 mg F-MWCNTs were dispersed in 30 mL of ultrapure water, and sonicated for 30 min at 25°C. The dispersed mixture was then added to 200 μL of 0.1 g·mL⁻¹ H₂PtCl₆ and 101 μL of 0.01 g·mL⁻¹ RuCl₃ aqueous solution. The reaction solution was heated for 10 h at 100°C under vigorous stirring. When the reaction was over, the Pt–Ru@MWCNT nanocomposite was obtained through centrifugation, washing, and vacuum drying. For comparison, the nanocomposites of Pt@MWCNT and Ru@MWCNT were synthesized using the similar procedure by reduction of 0.1 g·mL⁻¹ H₂PtCl₆ (200 μL) and 0.01 g·mL⁻¹ RuCl₃ (101 μL) aqueous solution, respectively. In addition, Pt₁@MWCNT was also synthesized using NaBH₄ as the reducing agent as shown in previous reports [28].

2.2   Electrochemical measurements

The cyclic voltammetry (CV), MOR, and CO stripping measurements of covalently-bonded Pt@MWCNT and Pt–Ru@MWCNT were all carried out on an electrochemical station (CHI 660D). Pt₁@MWCNT and commercial 20wt% Pt@C were also used as controls. The preparations of the working electrodes and the details for the electrochemical measurements were carried out according to the procedures described in supplementary information.

3.   Results and discussion

3.1   Characterization of Pt–Ru@MWCNT nanocomposite

The Pt–Ru@MWCNT nanocomposite was synthesized through a new two-step procedure. First, the MWCNTs were partially oxidized by the mixed acids of sulfuric acid and nitric acid, forming F-MWCNTs. The large amount of hydroxyl, carbonyl, and carboxylic groups has been formed on the surface of F-MWCNTs (Figs. S1 and S2), which could obviously increase the dispersibility of F-MWCNTs in water and serve as the anchor sites for further reaction. During the second step, the Pt and Ru precursors were in-situ reduced directly by F-MWCNTs without adding additional templates and reducing agents, generating Pt–Ru@MWCNT nanocomposite. It is interesting that the partially oxidized MWCNTs is the reducing agent rather than oxidation agent in the second step, maybe because of the intact inner walls in MWCNTs. Transmission electron microscopic (TEM) images in Fig. 1 show that the nanoparticles are homogeneously distributed on the surface of MWCNTs, and the microstructure of MWCNTs were characterized though the high-resolution TEM (HRTEM) (Fig. S3). In contrast, when pristine MWCNTs (untreated by mixed acids) are used, only few metals nanoparticles could be loaded on the MWCNT surface (Fig. S4).

Figure  1.  Morphological and nanostructural characterizations of Pt–Ru@MWCNT nanocomposite: (a) TEM images of Pt–Ru@MWCNT nanocomposite; (b) Pt and Ru nanoparticles loaded on the single MWCNT surface; (c) HRTEM images of Pt–Ru@MWCNT nanocomposite; (d) lattice image of Pt and Ru nanoparticles.

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The HRTEM images in Fig. 1 clearly show the cluster comprising many nanoparticles. The lattice spacings are 0.223 and 0.206 nm for nanoparticles in Fig. 1(d), which can be corresponding to the metallic Pt (111) and Ru (111) lattice spacings, respectively. The HRTEM images in Fig. 1(c) and (d) also demonstrate that the lattice fringes of the decorated nanoparticles fit well with those of metallic Pt, but have a little enlarged interlayer spacing. This result implies that Ru atoms have been incorporated into the Pt lattice, forming Pt–Ru bimetallic nanoparticles on MWCNTs. The morphologies and microstructures of as-prepared Pt@MWCNT, Ru@MWCNT, and Pt₁@MWCNT nanocomposites have also been investigated by HRTEM, as shown in Fig. S5, Fig. S6, and Fig. S7, respectively. It can be clearly seen that Pt@MWCNT and Ru@MWCNT fabricated using the same strategy as that of Pt–Ru@MWCNT also show homogenous distribution of nanoparticles on the MWCNT surface, while in terms of Pt₁@MWCNT nanocomposites fabricated using NaBH₄ reduction method, Pt have large particle size and serious aggregation. The statistical analysis of the particle sizes in Fig. S8 shows that the average nanoparticle sizes of Pt–Ru@MWCNT, Pt@MWCNT, and Ru@MWCNT are (2.11 ± 0.59), (1.94 ± 0.51), and (1.42 ± 0.41) nm, respectively.

The compositions of all the nanocomposites were analyzed by scanning TEM (STEM), energy dispersive X-ray spectroscopy (EDX) spectrum, and elemental mapping. As shown in Fig. 2, the components of C, O, Pt, and Ru elements for Pt–Ru@MWCNT were homogeneously dispersed and overlapped, which further verifies that Pt–Ru bimetallic nanoparticles have been decorated on MWCNTs, and the Pt and Ru elemental contents from EDX mapping were 15.94wt% and 1.51wt%. STEM images and EDX elemental mapping images of Pt@MWCNT and Ru@MWCNT were also displayed in Figs. S9 and S10, and the corresponding elemental contents from EDX mapping were described in Table S1. Furthermore, the quantitative elemental analysis of Pt and Ru elements loaded on MWCNTs were investigated by the inductively coupled plasma atomic emission spectroscopy (ICP-AES). The contents of Pt for Pt–Ru@MWCNT, Pt@MWCNT, and Pt₁@MWCNT were measured as 14.1269wt%, 15.4583wt%, and 15.3663wt%, while the contents of Ru for Pt–Ru@MWCNT and Ru@MWCNT were measured as 1.7942wt% and 4.9248wt%, respectively. The mass ratio of Pt : Ru for Pt–Ru@MWCNT is approximately 8:1, which is in accordance with the EDX elemental mapping results. Obviously, the elemental contents of all the nanocomposites measured by ICP-AES and EDX mapping were approximately the same.

Figure  2.  STEM images and EDX elemental mapping images of Pt–Ru@MWCNT.

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X-ray diffraction (XRD) measurement was used to characterize the phases for the as-prepared nanocomposites. As shown in Fig. S11(a), the black line is the XRD pattern of MWCNTs, and the three typical diffraction peaks at 25.9°, 44.5°, and 51.9° correspond to the lattice planes of the graphitic carbon (002), (100), and (004), respectively. The XRD pattern of Pt–Ru@MWCNT (blue line) displays four typical diffraction peaks (2θ) at 39.8°, 46.3°, 67.7°, and 81.3°, which can be assigned to (111), (200), (220), and (311) lattice planes of the face-centered cubic structure of Pt [33]. The red line is the XRD pattern of the Pt@MWCNT. It is interesting that the main peaks for the metallic phase of the Pt–Ru@MWCNT shift slightly to the right (Fig. S11(b)), as compared to those of Pt@MWCNT, suggesting the formation of Pt–Ru bimetallic nanoparticles on the surface of MWCNT. This is consistent with the results in Fig. 1(d) and Fig. 2. The chemical states of the surface of the as-fabricated nanocomposites were further investigated by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 3. As shown in Fig. S2(b) by C 1s spectrum, the clear peak appeared at 286.8 and 291.4 eV can be assigned to carbon–oxygen bond (–C–O) and carbonyl (–C=O) respectively, which is mainly due to the functionalization by hydroxide and carbonyl groups. The functional groups on MWCNTs was also demonstrated by fourier transform infrared (FT-IR) spectoscopy in Fig. S1. The characteristic peaks of the MWCNTs at 3443.82, 1621.52, and 1098.34 cm⁻¹ are thought to –C=O, –OH, and –C–O–, respectively. As displayed in Fig. 3(b) by C 1s spectrum, there is a new peak at around 282.5 eV for metal–C bond in compared with that in Fig. S2(b) [34–35]. Both Fig. S2(b) and Fig. 3(b) suggest the formation of covalent bonds between Pt–Ru bimetallic nanoparticles on the MWCNTs surface. In addition, the XPS spectra of O 1s in Fig. 3(c) show a strong M–O peak coupled with C–O peak. Three valence states of Pt can be identified on the basis of doublet binding energy of Pt 4f_7/2 and Pt 4f_5/2 in Fig. 3(d). Specifically, the binding energies at 71.32 and 74.01 eV were considered to be zero-valent metallic Pt [36], the binding energies at 72.90 and 76.62 eV can be attributed to Pt²⁺ species, and the binding energies at 75.12 and 78.37 eV can be assigned to Pt⁴⁺ species [37–38]. Furthermore, the peaks at 463.3 and 484.2 eV in Fig. 3(e) correspond to zero-valent metallic Ru [12]. These results reveal that the Pt–Ru bimetallic nanoparticles have oxidized surface that connects the metallic phase and MWCNTs, forming M–O–C covalent bonds. Therefore, the covalently-bonded Pt–Ru@MWCNTs nanocomposite has been successfully obtained from a novel green hydrothermal method.

Figure  3.  XPS patterns of Pt–Ru@MWCNTs: (a) full scan spectrum; (b) C 1s; (c) O 1s; (d) Pt 4f; (e) Ru 3p.

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3.2   Electrochemical measurements of all the catalysts

The electrochemical activity towards MOR for Pt–Ru@MWCNT, Pt@MWCNT, and Pt₁@MWCNT nanocomposites were investigated through cyclic voltammetry (CV) tests by using a saturated calomel electrode reference electrode (SCE), and benchmarked against the commercial 20wt% Pt@C. As shown in Fig. 4(a), an obvious positive shift for the reduction peak potential on the surface of Pt–Ru@MWCNT electrode implies that the desorption rate of OH adsorbed on the surface of Pt–Ru@MWCNT nanocomposites is relatively faster, and the adsorbed intermediates are more easily oxidized compared with commercial 20wt% Pt@C electrode [39]. The electrochemical active surface area (ECSA) of Pt–Ru@MWCNT, Pt@MWCNT, Pt₁@MWCNT, and commercial Pt@C were calculated to be 110.4, 69.85, 27.63, and 41.37 m²·g⁻¹ through the area of hydrogen adsorption/desorption, and the specific data are summarized in Table 1. Evidently, the ECSA of Pt–Ru@MWCNT far exceed those of Pt@MWCNT, commerciall 20wt% Pt@C, and Pt₁@MWCNT. The outstanding electrochemical activity of Pt–Ru@MWCNT can be attributed to the synergistic effect from the highly uniform monodispersed and ultrasmall particle size of bimetallic Pt–Ru nanostructure and the MWCNTs, bridged by M–O–C covalent bonds.

Figure  4.  Comparison of electrocatalytic activity of different nanocomposites: (a) CV of the different nanocomposites recorded in 0.5 M H₂SO₄ solution at a scan rate of 100 mV·s⁻¹; (b) mass-normalized CVs of the electrooxidation of methanol in a mixture of 0.5 M H₂SO₄ and 1.0 M CH₃OH at a scan rate of 100 mV·s⁻¹; (c) mass-normalized chronoamperometric curves (recorded at 0.60 V for 8000 s) for methanol oxidation reactions in a mixture of 0.5 M H₂SO₄ and 1.0 M CH₃OH at a scan rate of 100 mV·s⁻¹; (d) LSV of the different nanocomposites catalyst for methanol oxidation reactions in a mixture of 0.5 M H₂SO₄ and 1.0 M CH₃OH at a scan rate of 100 mV·s⁻¹.

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Table  1.  Electrocatalytic activity characteristics of Pt–Ru@MWCNT, Pt@MWCNT, Pt₁@MWCNT, and commercial 20wt% Pt@C

Catalyst	ECSA / (m2·g−1)	Mass activity / (mA·mg−1)	If/Ib
Pt–Ru@MWCNT	110.4	720.23	1.24
Pt@MWCNT	69.85	425.36	1.05
Pt1@MWCNT	27.63	261.92	0.91
Commercial Pt@C (20wt%)	41.37	327.50	0.97

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The electrocatalytic behaviors of Pt–Ru@MWCNT nanocomposite for MOR were measured and compared to other fabricated nanocomposites and commercial Pt@C in mixed solution of 0.5 M H₂SO₄ and 1.0 M CH₃OH at a sweep rate of 100 mV·s⁻¹. As shown in Fig. 4(b), the measured results distinctly demonstrate that the Pt–Ru@MWCNT nanocomposite possesses the highest mass activity towards MOR among the four catalysts. The mass current density of Pt–Ru@MWCNT nanocomposite towards MOR was measured as 720.23 mA·mg⁻¹, which is 1.69, 2.20, and 2.75 times higher than those of Pt@MWCNT (425.36 mA·mg⁻¹), commerciall 20wt% Pt@C (327.50 mA·mg⁻¹), and Pt₁@MWCNT (261.92 mA·mg⁻¹), respectively. Obviously, the order of mass current density is consistent with that of ECSA. Meanwhile, the onset potential and potential at maximum current intensity of Pt–Ru@MWCNT towards MOR are the lowest among the four nanocomposites, which indicate that the oxidation of methanol is much easier to be happened on the surface of Pt–Ru@MWCNT, and the methanol is oxidized on the surface of Pt–Ru@MWCNT with lower energy [40]. Fig. S12(a) shows the CV curves of Pt–Ru@MWCNT for MOR at different concentrations of CH₃OH (ranging from 0.5–2.5 mol). With the increase of concentrations, the current density increase positively as well as the peak potential shift. As shown in Fig. S12(b), it has a good linear relationship between the concentrations of CH₃OH and the forward peak current density (j_m). The long term electrocatalytic durability of four nanocomposites were measured though chronoamperometric (CA). The corresponding mass current density–time curves were recorded in Fig. 4(c). The mass current density for Pt–Ru@MWCNT nanocomposite is highest among the four nanocomposites under the certain time ranges. At the same time, Pt–Ru@MWCNT nanocomposite shows the slowest mass current density decay among the four nanocomposites, indicating an obviously improved electrocatalytic durability. Furthermore, the stability experiment of the Pt–Ru@MWCNT and commercial 20wt% Pt@C electrodes for MOR were also studied and displayed in Fig. S13. After 1000 CV cycling, the Pt–Ru@MWCNT still keep as high as 65.27% of the initial mass current activity, which is much better than commercial 20wt% Pt@C (47.72%). The morphological difference of Pt–Ru@MWCNT by long-term cycles was also characterized by TEM. As exhibited in Fig. S14, the Pt and Ru particles show slightly aggregation compared to the initial morphology. The Pt–Ru@MWCNT nanocomposite possesses the excellent electrocatalytic performance and durability, probably due to a large amount of interspaces, uniform micromorphology, and steady interconnection among particles.

Fig. 4(d) displays the linear sweep voltammetry (LSV) curves of those catalysts. Among the four catalysts at all applied potentials, the corresponding current density of four nanocomposites follow the order: Pt–Ru@MWCNT > Pt@MWCNT > commercial Pt@C > Pt₁@MWCNT, implying the excellent electrocatalytic activity of Pt–Ru@MWCNT nanocomposite for the MOR.

It is widely acknowledged that carbon monoxide as intermediate product is adsorbed on the active sites of the Pt-containing electrocatalysts in process of MOR, leading to the poisoning of Pt-containing electrocatalysts. However, the addition of Ru to an electrocatalyst can effectively mitigate the poisoning of Pt-containing electrocatalysts [41–43]. The ratio of the forward oxidation peak to the backward peak (I_f/I_b) is a crucial parameter to assess the tolerance of electrocatalyst to carbon monoxide poisoning. As displayed in Table 1 by calculating from Fig. 4(b), the Pt–Ru@MWCNT possesses the highest I_f/I_b ratio (1.24) compared with that of Pt@MWCNT (1.05), Pt₁@MWCNT (0.91), and commercial 20wt% Pt@C (0.97) catalysts. Evidently, the addition of Ru to Pt@MWCNT nanocomposite can effectively remove carbon monoxide molecule from the active sites of surface, resulting in Pt–Ru@MWCNT nanocomposite possessing a better resistance for carbon monoxide poisoning. In order to provide more information on the surface mobility of carbon monoxide, carbon monoxide stripping experiments were initiated and the carbon monoxide stripping voltammetry of Pt–Ru@MWCNT and commercial 20wt% Pt@C catalysts were displayed in Fig. 5. The Pt–Ru@MWCNT nanocomposite possesses a much lower onset potential (0.43 V) than commercial 20wt% Pt@C (0.67 V), suggesting that the carbon monoxide oxidation is triggered at a lower potential. The lower initial potential for Pt–Ru@MWCNT, the easier removal of carbon monoxide from the surface active sites of catalyst, which would improve its electrocatalytic activity towards MOR. Compared to Pt-containing electrocatalysts, the addition of Ru in Pt@MWCNT nanocomposite can easily adsorbed oxygen-containing intermediates at a lower potential to generate (Ru–OH) intermediates, which serve to effectively oxidize CO adsorbed on the surface of Pt–Ru@MWCNT nanocomposites, and thus alleviate the poisoning of Pt-containing electrocatalysts. In addition, the Pt–Ru@MWCNT may undergo a bifunctional mechanism under the electrocatalytic process due to the addition of Ru. Specifically, Pt serves as the catalytic active site to facilitate the dissociation of CH₃OH, and Ru is accountable for facilitating the activation of low-potential water molecules, which leading to the formation of oxygen-containing species. These maybe result in its excellent electrocatalytic activity towards MOR than other Pt-containing electrocatalysts under all applied conditions. Recently, some bimetallic or multimetal Pt-based nanocomposites have been developed [44–53], and the electrocatalytic activity characteristics are summarized in Table 2. Evidently, Pt–Ru@MWCNT shows superior electrocatalytic activity and lower oxidation potential compared with other reported Pt-containing electrocatalysts.

Figure  5.  Carbon monoxide stripping voltammetry of the as-prepared Pt–Ru@MWCNT and commercial 20wt% Pt@C catalysts in 0.5 M H₂SO₄ solution at a scan rate of 100 mV·s⁻¹.

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Table  2.  Comparison of the electrocatalytic activity of various catalysts for MOR in reported literatures

Catalysts	Electrolyte	Scan rate / (mV·s−1)	ECSA / (m2·g−1)	Mass activity / (mA·mg−1)	Onset CO oxidation potential / V	Ref.
Pt–Ru@MWCNT	A	100	110.4	720.23	0.43	This work
PtNW/PDDA-Ti3C2Tx	B	50	61	607.6	0.52	[44]
PtFeCu concave octahedron	B	50	34.88	622.0	0.512	[45]
Pt/PANI-CB-1:0.3	B	50	35.29	431.8	0.45	[46]
Pt nanowire	B	20	45.6	500	0.567	[47]
PteNi/CNSs	A	50	29.69	393	0.52a	[48]
Pt/G3-(CN)7	C	20	69	612.8	0.62a	[49]
Mn-PtCo-3	A	50	73.97	595.5	0.58	[50]
PtCo/MWCNT	B	50	—	616.7	0.51	[51]
PtNW-GO	A	50	8.45	609.44	—	[52]
PtPdAu HNAs-2	D	50	80.56	263.5	—	[53]
Note: “—” in the table indicates that this value was not explicitly stated in this paper; “a” is obtained from the figure in the literature; “A” represents that the electrolyte of the electrochemical test system is 0.5 M H2SO4 + 1.0 M CH3OH; “B” represents that the electrolyte is 0.5 M H2SO4 + 0.5 M CH3OH; “C” represents that the electrolyte is 1 M H2SO4 + 2 M CH3OH; “D” represents that the electrolyte is 0.1 M H2SO4 + 0.1 M CH3OH.

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4.   Conclusion

In summary, we report a novel and green hydrothermal synthesis method to prepared Pt–Ru@MWCNT and Pt@MWCNT nanocomposites by using partial oxidized multi-walled carbon nanotubes as the reducing agents. The ultrasmall size (with size around 2 nm) bimetallic Pt–Ru nanoparticles are homogeneously loaded onto the surface of F-MWCNTs bridged by the chemical bonding. The Pt–Ru@MWCNT nanocomposite is regard as an excellent and durable catalyst towards MOR reaction due to their special microstructure. In comparison with the 20wt% commercial Pt@C and Pt₁@MWCNT, the Pt–Ru@MWCNT nanocomposite exhibits excellent electrocatalytic activity with the ECSA of 110.4 m²·g⁻¹ and the mass current density of 720.23 mA·mg⁻¹ for Pt towards MOR. These results are expected to contribute to the development of a new strategy for the structural design of highly-efficient electrocatalys in boosting MOR towards real applications.