One-pot hydrothermal synthesis of platinum nanoparticle-decorated three-dimensional nitrogen-doped graphene aerogel as a highly efficient electrocatalyst for methanol oxidation

Abstract

Controllable integration of platinum nanoparticles (PtNPs) and a carbonaceous material is a promising strategy to obtain cost-effective and highly efficient nano-catalysts. Three-dimensional (3D) graphene-based aerogel is considered as an ideal catalyst support because it not only possesses the superior properties of graphene, but also can provide a high loading volume with hierarchical 3D porous architectures. In this paper, PtNP-decorated 3D nitrogen-doped graphene aerogel was prepared through a one-pot hydrothermal route for the first time. Compared to PtNP-decorated 3D graphene aerogel or PtNP-decorated 2D graphene, this nanocomposite shows excellent electrocatalytic activity because of the nitrogen doping and 3D porous structure.

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1. Introduction

Noble-metals, such as platinum (Pt), have been widely used as excellent catalysts, while the scarcity and high price of Pt have become key barriers for its broad development.¹ With the development of nanotechnology, the high specific surface areas of nanomaterials are in favor of providing more active sites, which bring better catalytic properties and effectively reduce the usage amount.² The catalytic performances of various nanoparticles have been extensively explored.³ But the high surface energy of nanoparticles results in easy aggregation, which seriously affects their catalytic performance and hinders practical applications.⁴ Thus, diverse novel supports have been developed to obtain stable and well-dispersed nano-catalysts.⁵ Appropriate support materials can not only maximize the availability of the surface area for electron transfer but also provide better mass transport.⁶

In recent years, graphene has opened a new avenue for utilizing carbonaceous material to support catalyst. However, graphene sheets are easily to re-stack together because of the strong π–π interaction. Although this irreversible stacking may be partly alleviated by the loading of nanoparticles,⁷ it still cause the blocking of active sites and the increased resistance for the mass transport, which seriously affected the catalytic performance.⁸ Assembling 2D planar graphene into three-dimensional (3D) structures, such as 3D graphene-based aerogel (3DGA), is regarded as a promising strategy to prevent restacking. 3DGA with a cross-linked porous structure not only maintain the excellent characteristics of graphene but also has an exceptionally large void volume, high corrosion resistance and high electrical conductivity. Meanwhile, this material could be tightly packed without significantly decreasing the accessible surface area.⁹ Particularly, nitrogen doping is an effective method to intrinsically improve the electrical properties of carbon materials.¹⁰ Hence, the special 3D structures of N-doped graphene-based composite show superior electrical conductivity, thermal stability, and specific surface area.^11–13 Many previous works studied the fabrication process of three-dimensional N-doped graphene-based composite. Typically, existing methods require multi-steps to prepare functional graphene aerogels modified with noble metals or metal oxides. Chen et al.¹⁴ prepared 3DNGA/MnO nanoparticle hybrids prepared through incorporating covalent assembly and noncovalent metal oxide loading method. Liu's group⁴ developed Pt nanoparticle decorated a robust 3D nitrogen-doped porous graphene (PtNPs/R-3DNG) material. Firstly, they prepared the R-3DNG product by hydrothermal treatment. Then the dried product was added into the homogeneous mixture of chloroplatinic acid and ethylene glycol to obtain PtNPs/R-3DNG after another steps of hydrothermal treatment, rinsing and drying. This synthetic route is relatively complicate. Thus, developing more efficient and simplified approaches to prepare 3D graphene-based composites is still challenging.

The electrochemical oxidation of methanol has potential applications in direct methanol fuel cells (DMFCs), which have been considered as green power sources.¹⁵ In this work, PtNPs decorated 3D nitrogen-doped graphene (NG) aerogel (3DNGA) were prepared via a one-pot hydrothermal treatment for the first time. And the resulting PtNPs decorated 3DNGA (PtNPs/3DNGA) were successfully applied in the electrochemical oxidation of methanol. Compared to previous reports, we developed a novel method that integrated the formations of aerogel and PtNPs into one pot, which effectively simplified the process and shorten the time. The 3D structure composited of covalent linked graphene sheets could be well preserved. Moreover, the hierarchical porous architecture and abundant N-doped sites of the aerogel could provide abundant anchoring sites for the formation of homogeneous dispersed PtNPs.¹⁶ As an electrocatalyst for methanol oxidation, PtNPs/3DNGA exhibited substantially enhance electro-catalytic activity and stability.

2. Results and discussion

As illustrated in Fig. 1, the PtNPs/3DNGA was synthesized by one-pot hydrothermal route. The detailed experimental procedure were provided in the ESI.† GO lost oxygenated functional groups under high temperatures and pressure and could be self-assembled into a 3D hydrogel structure by π–π stacking interactions between the GO sheets.¹⁷ During the hydrothermal process, the chloroplatinic acid were also reduced to PtNPs and deposited on the nitrogen-doped 3D graphene surface. Meanwhile, the introduction of glycine could provide N-source to form strong interactions with Pt atoms. Pt catalysts had been widely used as electrocatalysts in DMFCs.¹⁸ Methanol molecules would be adsorbed on the surface of Pt and dehydrogenated to form CO. As the intermediate during the methanol electro-oxidation in acid electrolytes, the adsorption of CO on Pt catalysts would reduce the active surface area and affect catalytic activity.¹⁹ But the prepared PtNPs/3DNGA had remarkable CO poison tolerance because of the synergetic effects between the doped N atoms and the PtNPs. Thus, the fabricated PtNPs/3DNGA catalyst displayed superior performance in the electrochemical oxidation of methanol.

Fig. 1 Illustration for the synthesis process of PtNPs/3DNGA composites and its application in the oxidation of methanol.

The morphology and structure of prepared PtNPs/3DNGA were carefully studied. As shown in Fig. 2A, scanning electron microscope (SEM) images revealed that this composites exhibited a well 3D porous structure. And the high-resolution SEM (Fig. 2B) and transmission electron microscopy (TEM) (Fig. S2A†) image clearly indicated PtNPs around 2–3 nm were uniformly anchored onto the 3DNGA surface, which demonstrated that the successful preparation of nanoparticle decorated 3D nitrogen-doped porous graphene. The high-resolution TEM images of PtNPs/3DNGA (Fig. S2B†) revealed the highly crystalline features of PtNPs, which indicated lattice spacing of the adjacent fringe for the Pt nanocrystals are 0.22 nm. As a comparison, the morphologies of PtNPs/3DGA and PtNPs/2DGR were also observed using TEM, as shown in Fig. S2C and S2D.† For PtNPs/2DGR, although the PtNPs are also homogeneously dispersed, and have a uniform size distribution, the graphene sheets are thick and opaque, indicating the graphene sheets are restacked together. Raman spectra is widely recognized as an important tool to characterize the quality and structure of carbon-based nanomaterials.²⁰ As shown in Fig. 2C, the PtNPs/3DNGA displayed a strong D band peak because of the existence of defects caused by N-doping in the graphene sheet (curve c).²⁰ The ratio of relative intensity for D and G band (I_D/I_G) could be used to measure the extents of defects in graphene-based materials. The PtNPs/3DNGA revealed higher intensity ratio of I_D/I_G (1.15) than 3DNGA (1.03) and GO (0.89).²¹

Fig. 2 The low-resolution (A) and high-resolution (B) SEM images of PtNPs/3DNGA. (C) Raman spectra of GO (a), 3DNGA (b) and PtNPs/3DNGA (c). (D) XRD patterns of PtNPs/3DNGA.

X-ray diffraction (XRD) patterns of PtNPs/3DNGA were shown in Fig. 2D. It displayed three strong diffraction peaks. It displayed four strong diffraction peaks centered at 39.65°, 46.1°, 67.2° and 80.9°, which could be indexed to the (111), (200), (220), and (311) planes of the fcc structure of the platinum crystal (PDF no. 87-0642). Furthermore, PtNPs/3DNGA composites all showed a broad diffraction peak at 26.2°, corresponding to the (002) reflection of the graphene sheets.⁴ X-ray photoelectronic spectroscopy (XPS) was used to investigate the chemical composition of PtNPs/3DNGA composites. As expected, Pt, C, N, O peaks were observed clearly (see the ESI†). The surface areas of PtNPs/3DNGA, PtNPs/3DGA and PtNPs/2DGR were estimated Brunauer–Emmett–Teller (BET) gas adsorption, which were 144.3 m² g⁻¹, 125.6 m² g⁻¹ and 1.58 m² g⁻¹, respectively (Fig. S3†). As well, the content (mass%) of C, N and Pt for different catalysts was provided in the Table S1.†

The electrochemical impedance spectra (EIS) responses for the GCE modified with different nanomaterial were investigated (Fig. 3). After PtNPs/3DNGA was immobilized on the electrode surface, it showed much lower electrochemical impedance (curve a) compared with those of PtNPs/3DGA (curve b) and PtNPs/2DGR (curve c). The decreased impedance may be attributed to the superior properties of the hybrid material that served as an ideal conducting substrate to provide sufficient electronic conductive channels.

Fig. 3 Electrochemical impedance spectra of PtNPs/3DNGA (a), PtNPs/3DGA (b), PtNPs/2DGR (c) composites and bare GCE (d) in 0.1 M KCl containing 5 mM Fe(CN)₆^3−/4−. Inset: the magnification of curve (a).

Cyclic voltammetry (CV) was a convenient and efficient tool used to estimate the catalytic performances. Fig. 4A showed the cyclic voltammetry (CV) to estimate the electrochemical active surface area (ECSA) of Pt catalyst. In the potential region of −0.2 to 0 V (vs. Ag/AgCl), the typical H⁺ ions were reduced and the hydrogen atoms were absorbed. In the reverse scan the adsorbed hydrogen was desorbed resulting in the generation of anodic current. Hydrogen adsorption/desorption peaks were usually used to evaluate ECSA of the catalyst.^22,23 As expected, the ECSA of PtNPs/3DNGA was 42.17 m² g⁻¹, whereas those of PtNPs/3DGA and PtNPs/2DGR were 19.38, and 12.05 m² g⁻¹. The higher ECSA demonstrates PtNP/3DNGA could provide more accessible active sites and a better intrinsic electrocatalytic activity for methanol oxidation.

Fig. 4 (A) Cyclic voltammograms of the PtNPs/3DNGA (a), PtNPs/3DGA (b), and PtNPs/2DGR (c) composites in a 0.5 M H₂SO₄ solution. (B) CVs for methanol oxidation reaction catalyzed by PtNPs/3DNGA (a), PtNPs/3DGA (b), and PtNPs/2DGR (c) composites in the mixture solution containing 0.5 M H₂SO₄ and 1 M CH₃OH.

Fig. 4B showed the CV curves of methanol electro-oxidation over the different catalysts. CV curves of all catalysts showed two well-defined peaks at the forward and backward scans. The forward peak around 0.65 V was attributed to the oxidation of methanol molecules while the backward peak around 0.45 V was related to the oxidation of intermediates (mainly CO).²⁴ Among these catalysts, PtNPs/3DNGA had the highest forward anodic peak current density (9.32 mA cm⁻²), which implied the best electro-catalytic activity. It also could be seen that the PtNPs/3DNGA catalyst exhibited a lower onset potential of methanol oxidation. In principle, the onset potential was related to the breaking of C–H bonds and subsequent removal of CO intermediates by the oxidation with OH supplied by Pt–OH sites or other sources.²⁵ So the lower potential indicated that the oxidative removal of carbon-based intermediates would easily occur on PtNPs/3DNGA catalyst, which may be ascribed to the synergetic effects between the doped N atoms and the PtNPs. The nitrogen atoms could activate a large number of neighboring carbon atoms and accelerate the formation of OH by water dissociation, thus promoting the oxidative removal.²⁴ And stronger Pt-support interactions in the PtNPs/3DNGA catalyst would also contribute to the promotion of C–H breaking and CO adsorption tolerance.²⁵ Also, the significantly higher methanol oxidation currents density in both forward and subsequently reversed scans clearly demonstrated a better activity of PtNPs/3DNGA. In addition, the ratio of the forward scan peak current (I_f) to the backward scan peak current (I_b), I_f/I_b, was a critical parameter for evaluating accumulated CO on electrode surface.²⁶ The I_f/I_b ratios of PtNPs/3DNGA, PtNPs/3DGA and PtNPs/2DGR composites were 2.1, 1.8, and 1.7, which also revealed that methanol can be more effectively oxidized on PtNPs/3DNGA.

3. Conclusions

In summary, PtNPs/3DNGA with a porous structure and uniformly distributed PtNPs was successfully fabricated by one-pot hydrothermal synthesis of treatment. This facile prepared material exhibited unprecedented activity, remarkable CO tolerance and good stability towards methanol oxidation as anode catalyst in DMFC owing to the 3D porous architecture, nitrogen doping and uniform distribution of the PtNPs. This composite is highly desirable for the development of high-performance energy conversion devices. We believe that 3DNGA is a promising support for catalysts. And the developed one-pot hydrothermal method effectively simplified previous reported approaches and can be further extended to accommodate other nanoparticles, which is a promising pathways in various fields.

Acknowledgements

This work was supported by National Natural Science Foundation of China (No. 21175061, 21375050, and 21505055), Science Foundation of Jiangsu Province (No. BK20150486. No. BK20130498), Foundation of Jiangsu University (15JDG145), Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (No. PAPD-2014-37) and Qing Lan Project.

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, Fig. S1–S3 and Table S1. See DOI: 10.1039/c6ra12562j

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