Mo2C Nanoparticles Dispersed on Hierarchical Carbon Microflowers for Efficient Electrocatalytic Hydrogen Evolution Yang Huang, Qiufang Gong, Xuening Song, Kun Feng, Kaiqi Nie, Feipeng Zhao, Yeyun Wang, Min Zeng, Jun Zhong, and Yanguang Li* Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China S Supporting Information *
ABSTRACT: The development of nonprecious metal based electrocatalysts for hydrogen evolution reaction (HER) has received increasing attention over recent years. Previous studies have established Mo2C as a promising candidate. Nevertheless, its preparation requires high reaction temperature, which more than often causes particle sintering and results in low surface areas. In this study, we show supporting Mo2C nanoparticles on the three-dimensional scaffold as a possible solution to this challenge and develop a facile two-step preparation method for ∼3 nm Mo2C nanoparticles uniformly dispersed on carbon microflowers (Mo2C/NCF) via the self-polymerization of dopamine. The resulting hybrid material possesses large surface areas and a fully open and accessible structure with hierarchical order at different levels. MoO42− was found to play an important role in inducing the formation of this morphology presumably via its strong chelating interaction with the catechol groups of dopamine. Our electrochemical evaluation demonstrates that Mo2C/NCF exhibits excellent HER electrocatalytic performance with low onset overpotentials, small Tafel slopes, and excellent cycling stability in both acidic and alkaline solutions. KEYWORDS: hydrogen evolution reaction, molybdenum carbide, three-dimensional hierarchical structure, hybrid electrocatalyst, polydopamine
H
of 190−230 mV is still needed in order to achieve a cathodic current of 10 mA/cm2.16 Further improving its HER activity requires engineering this material at the nanoscale to increasingly expose its catalytically active sites. Unfortunately, the high reaction temperature (>700 °C) of transition metal carbides more than often induces extensive particle sintering, leading to products with low surface areas that are not suitable for practical applications (Figure 1a).17,18 In order to mitigate their sintering, Mo2C nanoparticles were usually dispersed on conductive supports such as carbon nanotubes or graphene nanosheets.19−24 Nevertheless, these one-dimensional (1D) or two-dimensional (2D) carbonaceous supports themselves are prone to entangle or aggregate, which may risk negating all the advantages associated with the small Mo2C nanoparticle size (Figure 1b). The controllable synthesis of small-sized, welldispersed, and electrochemically accessible Mo2C nanoparticles therefore remains challenging at present.
ydrogen evolution reaction (HER) is the key to electrolytic or photocatalytic water splitting.1,2 It proceeds via the reduction of protons (in acid) or water (in base) with the generation of molecular hydrogen as the final product.3,4 Pt group metals are the most active HER electrocatalysts currently. However, their large-scale application is essentially held back by their limited natural abundance and high cost. Increasing research efforts have now been concentrated on the pursuit of their low-cost alternatives with comparable activity and stability.3−10 Among many potential candidates, early transition metal carbides hold special promise.11 Theoretical calculations of these materials suggest that the incorporation of carbon atoms into the lattice interstitials affords them with d-band electronic structures resembling that of the Pt benchmark.12 As one of the most representative carbides, Mo2C has been investigated for a range of traditional catalytic reactions including desulfurization, hydrogenation, and the water gas shift reaction since the early 1980s.13−15 It has recently been demonstrated that commercially available Mo2C is also HER active in both acidic and basic solutions, even though a relatively large overpotential © 2016 American Chemical Society
Received: September 29, 2016 Accepted: November 22, 2016 Published: November 22, 2016 11337
DOI: 10.1021/acsnano.6b06580 ACS Nano 2016, 10, 11337−11343
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areas are electrochemically accessible. They should also have sufficient mechanical robustness so as to maintain the structural integrity during prolonged electrocatalysis. In addition, the preparation method should also be economical and scalable. To this end, we report here a two-step process to prepare ultrasmall Mo2C nanoparticles uniformly dispersed on 3D Ndoped carbon microflowers (Mo2C/NCF) using dopamine as the precursor. The hierarchical micro- and nanostructure of the final product is thoroughly investigated via multiple characterization techniques. When evaluated for HER performance, Mo2C/NCF exhibits small onset overpotentials and great longterm cycling stability in both acidic and alkaline electrolytes.
RESULTS AND DISCUSSION We chose dopamine as the carbon precursor based on the following considerations. As a nontoxic and sustainable biomolecule, dopamine is known to spontaneously selfpolymerize under weakly alkaline conditions and conformally coat substrates (if there is any) or form self-supported structures.25,26 It also contains rich catechol groups that have the capability to chelate and adsorb a variety of metal ions during self-polymerization.26,27 Here in our experiment, Mo2C/ NCF was prepared in two steps (see Experimental Methods for more details). In the first step, dopamine was polymerized together with MoO42− in an alkaline mixture of water and ethanol. Thus, derived Mo-polydopamine (denoted as MoPDA) compound was subsequently carburized at 750 °C for 3 h to form nanosized Mo2C and partially graphitic support. Figure 2a shows the typical powdered X-ray diffraction (XRD) pattern of Mo2C/NCF. Most peaks can be well indexed to hexagonal β-Mo2C, which has been suggested as the most HER-active phase of molybdenum carbides.28 The two weak signals at 26° and 54° are assignable to the (002) and (004) planes of graphite. In addition, there are no discernible impurities such as molybdenum metal, oxides, or other carbides. A scanning electron microscopy (SEM) image of a Mo-PDA intermediate shows that it consists of microflowers
Figure 1. Schematic showing the different situations when supported or nonsupported carbide materials are annealed at high temperatures. (a) Nonsupported carbide particles always experience extensive coarsening. (b) Carbide particles supported on 1D or 2D substrates can retain their small sizes, but the substrates themselves are prone to entangle or restack. (c) Carbide particles supported on 3D scaffolds may preserve their large surface areas and possess fully electrochemical accessibility.
We reason that dispersing Mo2C nanoparticles on threedimensional (3D) scaffolds may represent an effective solution to this challenge (Figure 1c). The ideal support materials should have large surface areas, high porosity, and hierarchical order at different levels, so that a large fraction of their surface
Figure 2. Structural characterizations of Mo2C/NCF. (a) XRD pattern, (b, c) SEM images at different magnifications, (d, e) TEM images at different magnifications, (f) STEM image, and (g−i) corresponding EDS elemental mapping of Mo2C/NCF. 11338
DOI: 10.1021/acsnano.6b06580 ACS Nano 2016, 10, 11337−11343
Article
ACS Nano
Figure 3. Spectroscopic characterizations of Mo2C/NCF. (a) Raman spectrum, (b) Mo 3d XPS spectrum, (c) N 1s XPS spectrum, and (d) C K-edge XANES spectrum of Mo2C/NCF. The data of NCS are also included for comparison in (d).
having diameters of approximately 2.5−3 μm. After the carburization, Mo2C/NCF retains the 3D architecture (Figure 2b). Its microflower consists of numerous thin nanoflakes with an average thickness of ∼10 nm, all radiating from the center and loosely stacked together (Figure 2c). Such a feature is reminiscent of the many petals of a certain chrysanthemum flower. More interestingly, when viewed under transmission electron microscopy (TEM), each nanoflake is found to be uniformly decorated with dark contrast nanoparticles with a size of ∼3 nm (Figure 2d,e). Many nanoparticles exhibit obvious lattice fringes with a spacing of 0.23 nm, in good agreement with the d-spacing of the β-Mo2C (101) plane. Energy dispersive spectroscopy (EDS) analysis under scanning transition microscopy (STEM) indicates the coexistence of C, N, and Mo. The N species is inherited from the amine functionality of dopamine molecules. EDS mapping over a 20 nm × 20 nm area evidences that all three constituent elements have a correlated spatial distribution (Figures 2f−i). The amount of Mo2C in the final product is found to be ∼53 wt % based on the thermogravimetric analysis (TGA) in air (Figure S1). In addition, Mo2C/NCF is also measured to have a BET specific surface area of 83 m2/g (Figure S2), an impressive value considering the high density of Mo2C (∼9.1 g/cm3), which is impossible if it was not for the fully open structure of the microflowers. More insights into the bonding environment and structure of Mo2C/NCF were gained via multiple spectroscopic characterizations. Its Raman spectrum displays fingerprint bands characteristic of β-Mo2C at 660, 812, and 987 cm−1,29 as well as broad D and G bands between 1300 and 1600 cm−1 (Figure 3a). The G to D band intensity ratio of >1 attests that the carbon support is partially graphitic. X-ray photoelectron spectroscopy (XPS) analysis detects that the surface is composed of Mo, N, C, and O (Figure S3). Peak deconvolution of the Mo 3d XPS spectrum divulges the contribution from Mo2C (d5/2 at 228.3 eV), MoO2 (d5/2 at 229.2 eV), and MoO3 (d5/2 at 232.3 eV) (Figure 3b).23,28 The presence of a significant amount of superficial oxides is not surprising and seems to be unavoidable for carbide materials due to their
gradual oxidation at the surface upon exposure to air.28,30 The N 1s XPS spectrum shows the predominant contribution from pyridinic nitrogen and some minor contribution from pyrollic nitrogen (Figure 3c). Compared to N-doped carbon spheres (NCSs) formed in the absence of MoO42− under otherwise identical conditions, the C K-edge X-ray absorption near-edge structure (XANES) spectrum of Mo2C/NCF exhibits typical features of Mo2C (Figure 3d). The two intense resonances at 285 and 288 eV are due to the transitions of C 1s electrons to the p−d(t2g) and p−d(eg) hybridized orbitals of carbides, respectively.31 We identified several key experimental parameters to the successful preparation of 3D hierarchically ordered structure. First, the introduction of MoO42− anions proves decisive. Our control experiment shows that NCS prepared in the absence of any MoO42− consists of only relatively smooth carbon spheres with a diameter of ∼200 nm and lower surface areas (Figure S4). 3D hierarchical microflowers can form when the Mo-todopamine molar ratio is controlled over the range of 1:4 to 1.5:1 (Figure S5). Second, we note that a slow heating rate (2 °C/min) is essential to ensure the formation of β-Mo2C. If the heating rate is too fast, MoO42− decomposes to oxides and directly vaporizes before its possible transformation to carbide. Last, the choice of reaction temperature is also of critical importance: higher temperatures (>800 °C) cause the extensive coarsening and the collapse of the 3D structure, and lower temperatures (
