High-Performance Hydrogen Evolution ... - ACS Publications

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High-Performance Hydrogen Evolution Electrocatalyst Derived from Ni3C Nanoparticles Embedded in a Porous Carbon Network Hao Wang,† Yingjie Cao,† Guifu Zou,*,† Qinghua Yi,† Jun Guo,‡ and Lijun Gao*,† †

College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China ‡ Testing and Analysis Center, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: In this letter, we report a facile self-foaming strategy to synthesize Ni3C nanoparticles embedded in a porous carbon network (Ni3C@PCN) by rationally incorporating a nickel salt precursor into the carbon source. As a novel hydrogen evolution reaction (HER) catalyst, the Ni3C@PCN shows superior catalytic activity with an onset potential of −65 mV, an overpotential of 262 mV to achieve 50 mA cm−2 current density, a Tafel slope of 63.4 mV/dec, and durability over 12 h in acidic media. The excellent performance of the novel 3D composite material along with its low-cost merits is suggestive of great potential for scalable electrocatalytic H2 production. KEYWORDS: hydrogen evolution, electrocatalyst, Ni3C, carbon network, composites

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catalytic activities.20,21 In this letter, we propose a templatefree self-foaming strategy for the fabrication of Ni3C nanoparticles embedded in a porous carbon network (Ni3C@PCN). As a novel HER catalyst, the Ni3C@PCN exhibits superior catalytic activity with an onset potential of −65 mV, an overpotential of 262 mV to achieve a current density of 50 mA cm−2, and a Tafel slope of 63.4 mV/dec in 0.5 M H2SO4. The core−shell nanostructure of the carbon-protected Ni3C delivers excellent stability over 12 h. Such a high HER performance makes Ni3C@PCN a promising catalyst for scalable electrochemical H2 production. The Ni3C@PCN was prepared by a controllable annealing process of citric acid monohydrate and nickel acetate tetrahydrate at 1000 °C in Ar/H2 atmosphere (see Supporting Information (SI) for the detailed synthetic process). The 3D porous carbon network (PCN) can be prepared by citric acid, and its morphology is described in Figure S1 in the SI. Thermogravimetric analysis of citric acid clearly reveals its evolution processes (Figure S2 in SI). During the heating process, citric acid was dehydrated and polymerized. The rapid releasing of H2O molecules leads to the formation of a 3D carbon foam network. The high-temperature treatment also caused some degree of graphitization in the 3D PCN. Note that the Ni3C@PCN can be obtained by rational addition of a trace amount of the Ni metal source. Figure 1a shows that the Ni3C@PCN has a 3D continuous structure, and it can stand on the stamen intact due to its ultralow density nature (inset of

ydrogen as a future energy carrier has been intensely explored in terms of its pureness and renewability.1 Widely recognized hydrogen production via water splitting by the hydrogen evolution reaction (HER) is the most promising and sustainable approach.2,3 Noble metals (e.g., platinum) have been demonstrated to be the most effective catalysts owing to their ultralow onset potential and high catalytic activity; however, their scarcity and high cost largely hinder their scalable use.4 Thus, development of highly active and durable catalysts to replace Pt is of great importance for HER. Recently, transition-metal-based materials (e.g., Mo, Co, Ni) have been intensively explored as HER catalysts,5−8 in which Ni-based catalysts have been demonstrated to be suitable for alkaline HER owing to the favorable H intermediate adsorption.9,10 However, the HER performance of Ni-based catalysts is unsatisfactory in acidic conditions due to the possible dissolution of Ni.11 Great efforts have been expended to enhance the durability of Ni-based electrocatalysts in acidic medium, for instance, through development of Ni compounds (e.g., Ni5P4,12 Ni2P,13 Ni3S2,14 NiSe15) and alloy composites (e.g., Fe−Ni−S,16 MoxC−Ni17); however, those catalysts still suffer from inferior durability in acidic solution. Most recently, Ni3C has garnered increasing attention owing to its superior catalytic activity and robust chemical stability.18 For instance, Fan et al. have developed Ni3C nanocrystals encased in graphene nanoribbons as an active and stable electrocatalyst for HER in acidic medium.19 Unfortunately, it remains a challenge to synthesize nanostructured Ni3C because the high-temperature reduction process could lead to the formation of a metallic Ni or Ni/Ni 3 C mixture and agglomeration of nanoparticles, which would deteriorate © 2016 American Chemical Society

Received: November 10, 2016 Accepted: December 22, 2016 Published: December 22, 2016 60

DOI: 10.1021/acsami.6b14393 ACS Appl. Mater. Interfaces 2017, 9, 60−64

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ACS Applied Materials & Interfaces

of 10 nm (Figure 1d). The high-resolution TEM image (Figure 1e) reveals a lattice distance of 0.2 nm corresponding to the (113) facet of Ni3C.19 The typical energy-dispersive X-ray (EDX) spectrum of Ni3C@PCN (Figure 1f) confirms that the as-prepared material is composed of C, Ni, and O elements. Based on the XPS and EDX analysis, the Ni3C@PCN is determined to consist of 88.5% of C, 1.1% of Ni, and 10.4% of O (atomic content in inset of Figure 1f). The porous structure of the Ni3C@PCN materials was further characterized by N2 adsorption/desorption tests. As shown in Figure 2a, the N2 adsorption−desorption isotherm curve for PCN (close to the type I) indicates that the PCN has a structure with majority of micropores. The Barrett−Joyner− Halenda (BJH) pore size distribution plots (inset of Figure 2a) confirm the presence of micropores. It is interesting to see that the Ni3C@PCN exhibits the type IV isotherm curve with a hysteresis suggesting the existence of mesopores. The secondary larger pores (diameters >10 nm) in the Ni3C@ PCN lead to a rapid N2 uptake at the high-pressure range (P/P0 > 0.9). It is worth noting that the two samples have ultrahigh surface areas and pore volumes compared to those previously reported porous carbon materials (Table S1 in SI).22 Note that the surface area of Ni3C@PCN (2270 m2 g−1) is a little smaller than that of PCN (2830 m2 g−1), which should be attributed to the incorporation of Ni3C nanoparticles. Figure 2b shows the X-ray diffraction (XRD) patterns of PCN and Ni3C@PCN. The PCN pattern exhibits two broad peaks at 26° and 45°, which are indexed to the (002) and (100) reflections of graphite, respectively. The Ni3C@PCN pattern shows three additional peaks at 39.6°, 41.9°, and 45.1°, corresponding to the (110), (006), and (113) facets of hexagonal Ni3C (JCPDS no. 72-1467).23 The Raman spectra of PCN and Ni3C@PCN (Figure 2c) both exhibit peaks at 1335 cm−1, 1606 cm−1, 2661 cm−1, and 2916 cm−1, corresponding to D band, G band, 2D band, and D+G band from carbon, respectively. The results indicate the existence of defects and disorders in the two

Figure 1. SEM (a, b), TEM (c, d), and HRTEM (e) images of Ni3C@ PCN. Inset of (a) is the photograph of the Ni3C@PCN standing on top of a stamen. Inset of (c) is the corresponding size distribution of Ni3C nanoparticles. (f) EDX spectrum for Ni3C@PCN and the inset table provide atomic content of the elements.

Figure 1a). The enlarged scanning electron microscopy (SEM) (Figure 1b) reveals that the PCN consists of ultrathin nanoporous carbon sheets spatially stood by graphitic struts. The transmission electron microscopy (TEM) images (Figure 1c) reveal that the ultrasmall Ni3C nanoparticles are embedded in the carbon network. It can be seen from the size distribution in the inset of Figure 1c that the diameters of the Ni3C nanoparticles varied from 4 to 16 nm with an average diameter

Figure 2. (a) N2 adsorption−desorption isotherms, (b) XRD patterns, (c) Raman spectra, and (d) XPS full scans for PCN and Ni3C@PCN. XPS scan spectra of the PCN and Ni3C@PCN. Inset of (a) is the corresponding pore size distribution curves. Inset of (d) is the high-resolution XPS of Ni 2p for Ni3C@PCN. 61


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Figure 3. Polarization curves (a) with a scan rate of 5 mV s−1 and corresponding Tafel plots (b) in 0.5 M H2SO4 for the Ni3C@PCN, PCN, and Pt/ C. (c) Comparison of onset potential and Tafel slope of different HER catalysts in acidic solution. (d) Current density difference plotted against scan rates, which is obtained from Figure S6 in SI. (e) The polarization curves for the Ni3C@PCN before and after 1000 CVs between 0.1 V and −0.2 vs RHE with a scan rate of 0.1 V s−1 in acidic and basic solutions, respectively. (f) Time-dependent current density curves under η = 200 mV under acidic and η = 300 mV under basic conditions.

products.24 The higher value of ID/IG for Ni3C@PCN (1.36) than PCN (1.21) (Table S2 in SI) indicates that the incorporated Ni3C leads to the increases of sp3 C and surface/edge defects which is in agreement with the electron microscopy analyses. As shown in Figure 2d, the wide X-ray photoelectron spectroscopy (XPS) scans reveal the presence of C, O, and Ni in Ni3C@PCN. The detailed C 1s XPS scans (Figure S3 in SI) of Ni3C@PCN and PCN suggest similar carbon materials. The high-resolution XPS scan for the Ni region of Ni3C@PCN (inset of Figure 2d) shows the signals at 856.9 and 862 eV corresponding to the Ni 2p3/2 levels and peaks at 874.4 and 881.3 eV belonging to Ni 2p1/2 levels.7 The binding energy of the Ni 2p3/2 is higher than that of Ni0 (∼852.7 eV), which corresponds to Ni2+ in the sample.25 The as-obtained samples are evaluated as catalysts for HER in acidic media. Figure 3a shows the polarization curves of the PCN, Ni3C@PCN, Pt/C, and glassy carbon in 0.5 M H2SO4. The Pt/C catalyst has a superior HER activity with a near zero onset potential, while the Ni3C@PCN exhibits a small onset potential of −65 mV. In contrast, the value for PCN (∼200 mV) is much higher than that of Ni3C@PCN. To achieve current density of 20 mA cm−2, the Ni3C@PCN only needs an overpotential of 203 mV. The linear portions of the Tafel plots (Figure 3b) can be fitted to the Tafel equation (η = b log j + a, where η is the overpotential, j the current density, and b the Tafel slope).26 The b values of 125, 63.4, and 34 mV/decade

are obtained for PCN, Ni3C@PCN, and Pt/C, respectively. The b is an inherent property of the catalysts determined by the rate-determining step (RDS) (Note S1 in SI).26 That the b of Ni3C@PCN is much smaller than that of PCN can be interpreted by the change of RDS during HER processes. The Pt/C clearly follows the Tafel recombination RDS step with b of 34 mV dec−1. The b values from polarization experiments (Figure 3b) indicate that the RDS of PCN is determined by the Volmer step, while that of Ni3C@PCN may be determined by the Volmer−Heyrovsky steps. Although the HER mechanism of the Ni3C@PCN is still inconclusive, the much smaller Tafel slope indicates its better kinetics of HER processes compared to that of PCN. By extrapolating the Tafel plots to the j axis at overpotential of zero, the exchange current density for Ni3C@ PCN is estimated to be 0.03 mA cm−2 (Figure S4 in SI), which is much higher than that of PCN (0.011 mA cm−2). Figure 3c gives the onset potential and Tafel slope value for Ni3C@PCN together with those of previously reported nonprecious metal HER catalysts, including Ni−Mo−S nanosheets, 5 CoSe2 nanowires,6 Ni foam,7 MoS2 spheres,8 Ni-doped graphene,11 and N,S-doped graphene.27 It is clear that such a good performance for Ni3C@PCN makes it one of the best HER catalysts. In addition, the Ni3C@PCN also exhibits high HER performance in basic condition with a low onset potential of ∼80 mV, a Tafel slope of 149 mV/decade, and an exchange current density of 0.219 mA cm−2 (Figure S5 in SI). 62


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ACS Applied Materials & Interfaces The inferior HER performance of PCN implies that the ultrahigh surface area is not the most important factor toward HER activity. It is confirmed by the double-layer capacitance (Cdl) measurements for Ni3C@PCN and PCN (Figure 3d and Figure S6 in SI).28 Although Ni3C@PCN has a little smaller surface area, it has a larger Cdl (37.5 mF cm−2) compared to that of PCN (33.8 mF cm−2), which may be attributed to the higher pore volume, but the difference in Cdl values of Ni3C@ PCN and PCN is insignificant, corresponding to the similar electrochemical surface areas. This suggests that the superior HER activity of Ni3C@PCN is mainly dominated by the active sites from incorporated Ni3C nanoparticles. Nevertheless, the large surface area of the structured PCN with good conductivity does benefit the HER performance as well, which facilitates the mass transfer of ions in electrolyte. Long-term stability is another important criterion indicating viability of the Ni3C@PCN. As shown in Figure 3e, the polarization curves after 1000 cyclic voltammograms (CVs) show negligible loss of current density compared to the initial curve. It illustrates the excellent stability of Ni3C@PCN in both acidic and basic conditions. The Ni3C nanoparticles embedded in the PCN can probably be protected by surrounding carbon from corrosion in the solution.19 The morphology of Ni3C@ PCN after the continuous HER operation (Figure S7 in SI) shows little change which further confirms its endurance in the acid. Besides, the time-dependent current density curves at certain overpotential for Ni3C@PCN are shown in Figure 3f. The curves are in typical serrate shape attributing to the alternate processes of bubble accumulation and release (Figure S8 in SI).29 After continuous operation for more than 12 h, the current density only loses by 1 mA cm−2 (5%) in acid and 1.2 mA cm−2 (13%) in base. The inferior stability in basic condition might result from the corrosion of carbon by alkaline solution.30 The good stability coupled with superior catalytic activity ensures the Ni3C@PCN is a promising candidate for electrocatalytic hydrogen production. In summary, this study proposes a template-free self-foaming strategy to synthesize Ni3C nanoparticles embedded in the porous carbon network. The Ni3C@PCN exhibits excellent catalytic activity in both acidic and basic solutions, which is mainly attributed to the rich active sites from Ni3C nanoparticles for electrocatalytic reactions and conductive 3D continuous structure of the porous carbon network for electronic and ionic transfer. The Ni3C@PCN also demonstrates good long-term stability in acid and basic solutions. This work not only provides an efficient HER electrocatalyst but also opens up an applicable strategy to facile synthesis of nanostructured materials for electrocatalysis and beyond.

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Lijun Gao: 0000-0002-2304-102X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (U1401248), “973 Program - the National Basic Research Program of China” Special Funds for the Chief Young Scientist (2015CB358600), and the Excellent Young Scholar Fund from National Natural Science Foundation of China (21422103).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b14393. Detailed synthetic procedures, SEM, TEM, TG, XPS, and electrochemical analysis (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guifu Zou: 0000-0002-8342-7768 63


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