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Article Cite This: ACS Omega 2019, 4, 10347−10353

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Multifunctional Dicyandiamide Blowing-Induced Formation of Electrocatalysts for the Hydrogen Evolution Reaction Haiyan Zheng,† Xiao Li,‡ Na Xu,† Xinlong Wang,*,†,‡ Chunyi Sun,† and Zhongmin Su*,†,‡ †

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Key Lab of Polyoxometalate Science of Ministry of Education, National & Local United Engineering Laboratory for Power Battery Institution, Northeast Normal University, Changchun, Jilin 130024 China ‡ Jilin Provincial Science and Technology Innovation Center of Optical Materials and Chemistry, School of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun 130022 China S Supporting Information *

ABSTRACT: The development of cost-efficient, highly active, and stable electrocatalysts toward hydrogen evolution reaction (HER) is vital for energy conversion and clean hydrogen production. Herein, the novel electrocatalyst, porous carbon-coated Ni/β-Mo2C (Ni/β-Mo2C@PC), possesses favorable hydrogen production performance and stability in a wide range of pH via a facile dicyandiamideblowing and one-step synthesis method. The resultant Ni/βMo2C@PC displays pleasurable HER performance with low overpotentials of 123, 155, and 179 mV at 10 mA cm−2 in 1 M KOH, 0.5 M H2SO4, and 1 M phosphate buffer solution (PBS), respectively. Furthermore, Ni/β-Mo2C@PC retains high durability with quite little decay after 24 h stability test, suggesting the excellent activity and stability of Ni/β-Mo2C@PC served as HER electrocatalysts. Otherwise, it is advantageous compared with Ni@PC, Mo2C@PC, and most other nonprecious metal electrocatalysts. Benefiting from the synergistic effect of the two popular active species, uniform distribution, and porous carbon coating, these porous nanoelectrocatalysts showcase satisfactory electrocatalytic activity, improving the charge transfer and kinetics during the reaction. This work possesses significant universality, which can be applicable to the synthesis of diverse nanostructured noble metal-free catalysts.

1. INTRODUCTION With the increasing requirement in energy and environmental issues, renewable and green sources have received extensive attention in sustainable and clean energy resources, such as hydrogen.1,2 Hydrogen is supposed as a promising candidate in replacing traditional fossil fuels with its cleanest and renewable merits.3,4 Water electrolysis is an extremely efficient, fast, sustainable and squeaky-clean technology to produce highpurity H2.5 Electrochemical water splitting especially depends on electrocatalysts and electrolytes.6,7 Meanwhile, an ideal hydrogen evolution reaction (HER) electrocatalyst must satisfy four basic requirements: a low onset potential as much in accordance with the thermodynamic value as possible, an ultralow Tafel slope, a high current density, as well as longterm stability.8 As we all know, Pt and Pt-based materials are the benchmark catalysts for the HER;9−11 however, the low abundance and high cost efficiency greatly hinder their wide range of applications.12,13 Therefore, it is meaningful to develop nonprecious metal catalysts with superior catalytic activity, which are abundant around the earth and less costly to extract. Recently, early-transition carbides,14,15 borides16,17 nitrides,18,19 sulfides,20,21 phosphides,22,23 and selenides24 have been developed as typical nonprecious electrocatalysts with © 2019 American Chemical Society

excellent HER performance. Mo2C-based electrocatalysts, possessing remarkable electronic feature and tunable phases, are widely studied.25−27 However, the negative hydrogen bonding energy (ΔGH*) of Mo2C means that the intense Mo− H bond contains unoccupied d orbits of Mo with large density.26−28 To improve catalytic performance, we can introduce the group VIII transition metals to reduce the unoccupied d orbitals of Mo due to the large number of electrons in the dopants. Lately, 3d transition materials have been considered as noble metal-free electrocatalysts for the HER, which are low-cost and exhibit high intrinsic activity and fine stability.29−31 Among them, Ni-based catalysts have revealed optimal performance in alkaline solution because of the extremely excellent OH−Ni2+δ bond strength that is in favor of intermediate adsorption.32,33 Nevertheless, the Nibased catalysts are apt to the aggregate and dissolve after longterm operating environment where the activity easily deteriorates.34,35 To date, some effective strategies have been devoted to design splendid catalysts, such as porous nanostructures,36−38 complex multimetallic catalysts,39−41 and Received: March 27, 2019 Accepted: June 3, 2019 Published: June 14, 2019 10347

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nitrogen doping.42,43 Recently, a facile sugar-blowing technique has been reported that uses NH4Cl as a gas template and blows glucose-derived polymers into large bubbles to generate porous carbon-coated nanostructures which showed benign HER performance. The specific surface area is further increased. The synthetic method contributes to an obvious increase of the surface area.44 Based on the above discussion, we speculate that taking advantage of the blowing technology to prepare high-surface Ni/Mo2C nanocatalysts will possess an excellent hydrogen production effect. In this work, we reported a novel Ni/β-Mo2C@PC nanoelectrocatalyst synthesized through a facile blowing technique combined with carburization treatment. The asprepared porous Ni/β-Mo2C@PC nanoparticles are evaluated as electrocatalysts for the HER, which is cost-efficient, longterm stable, and highly active. Meanwhile, the composite catalyst has low overpotentials of 123, 155, and 179 mV for reaching a current density of 10 mA cm−2 in 1.0 M KOH, 0.5 M H2SO4, and 1 M PBS, respectively. These results prove that the reasonable design and rapid and effective synthesis of electrocatalysts depend on low-cost Ni and Mo2C, which effectively prevents the agglomeration and uneven distribution. In addition, the synthetic method contributes to an obvious increase of the surface area and in the acceleration of electron transfer from Ni to Mo2C. It is important to highlight that compared with Ni@PC and Mo2C@PC, Ni/β-Mo2C@PC suggests a much lower overpotential during the hydrogen evolution test at all pH values. The remarkably catalytic activities can be attributed to the synergetic effect of Ni and Mo, the nanoparticles uniformly dispersed in graphitized carbon as well as the well-defined porous structure with the large surface area.

Figure 1. XRD patterns of Ni/β-Mo2C@PC at different NiMoO4 nanorods, NH4Cl, and DCA mass ratio from 1:1.5:1.5 to 1:14:14.

MoC phase (JCPDS no. 65-8765) can be clearly observed at the mass ratio of 1:1.5:1.5. When the mass ratio increased to 1:4:4, the phase of MoC became appreciably weaker. This MoC phase almost disappears as the mass ratio further increases to 1:8:8, while a new hexagonal β-Mo2C phase (JCPDS no. 35-0787) can be clearly observed. The Mo2C phase still remains when the mass ratio reaches 1:14:14. Based on these results, the formation of the Ni/Mo2C@PC composite with an increasing carbon content can be reasonably speculated to be the reaction described in the following equation:46 NiMoO4 + C → Ni + MoC → Ni + Mo2C + MoC → Ni + Mo2C

To demonstrate the function of the DCA precursor, we used glucose and urea instead of the carbon source for mechanical grinding with NH4Cl followed by further carbonization, the Xray diffraction (XRD) spectrum as demonstrated in Figure S3. The PXRD patterns exhibit the phases of Ni (JCPDS no. 650380) and MoC (JCPDS no. 65-8765). No corresponding peak of β-Mo2C phase (JCPDS no. 35-0787) was observed. The DCA plays a vital multifunctional role in the synthetic process; DCA was used as a reducing and partitioning agent, which promotes the deoxidization of the NiMoO4 precursor and prevents the aggregation of the Ni/β-Mo2C@PC nanomaterials. At the same time, in the synthetic system, DCA not only causes the in situ generation of the N-doped carbon layer to cover the Ni/β-Mo2C@PC catalyst but also enhances the surface of the catalyst and improves the stability of the nanocatalyst.47 The linear sweep voltammetry (LSV) curves in 0.5 M H2SO4, 1 M KOH, and 1 M PBS are shown in Figure S9. Also, the mixture with urea and glucose showed poor HER activity in 0.5 M H2SO4, 1 M KOH, and 1 M PBS solutions. Based on the PXRD patterns and LSV curves, we reasoned that the DCA precursor contributes to the formation of graphitized carbon and highly active Ni and Mo2C particles, which further leads to effective HER performance. It is worth noticing that the five diffraction peaks of the Mo2C in Ni/β-Mo2C@PC are located around 34.4°, 37.9°, 52.1°, 61.9°, and 69.7°, in comparison with pure Mo2C, there appears a slight shift to a higher diffraction angle. The result indicates that the shrinkage of the Mo2C cell due to the radius of nickel is smaller than molybdenum. Moreover, the strong metallic Ni phase presents in all samples from 1:1.5:1.5 to 1:14:14. Typical transmission microscopy (TEM) images, as presented in Figure 2, reveal the diameter of Ni/β-Mo2C

2. RESULTS AND DISCUSSION The method of synthesizing Ni/β-Mo2C@PC by one step is shown in Scheme 1. First, NiMoO4 nanorods were prepared by Scheme 1. Synthesis of Ni/β-Mo2C@PC Catalysts via a Facile One-Step Synthetic Method

a hydrothermal reaction.45 Next, under the Ar atmosphere, the mixture of NiMoO4 nanorods, dicyandiamide (DCA), and NH4Cl was annealed at 900 °C for 2 h and then gradually cooled down to room temperature to obtain porous carboncoated Ni/β-Mo2C@PC. Powder X-ray diffraction (PXRD) was carried out to characterize Ni/β-Mo2C@PC samples annealed at different mass ratios of NiMoO4, NH4Cl, and DCA from 1:1.5:1.5 to 1:14:14 (Figure 1). Interestingly, the phase of Mo was changed from MoC to Mo2C versus the increasing amount of DCA. The strong metallic Ni phase (JCPDS no. 65-0380) and pure 10348

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Mo2C@PC is 0.23 m−3 g−1 measured at p/p0 = 0.9961. Ni/βMo2C@PC nanoparticles have a mesoporous structure (3.7 nm) as indicated by the pore size distribution data. The presence of mesopores may be due to the accumulation of nanoparticles. The N2 adsorption−desorption isotherms and the single-point total pore volume of the comparative samples are placed in the Supporting Information (Figure S2). In contrast to the comparative sample, the Ni/β-Mo2C@PC has a large BET specific surface area. However, the large BET area and pore size not only expose more active sites to ensure that reactants and electrolytes are more accessible but also provide good buffering capacity for volume expansion in higher HER activity. We tested the electrocatalytic HER activity of Ni/β-Mo2C@ PC using a standard three-electrode system in 1 KOH, 0.5 M H2SO4, and 1 M PBS aqueous solution in the protection of N2, respectively. We first optimized the mass loading of Ni/βMo2C@PC nanoparticles with 0.85 mg cm−2, as shown in Figure 3. Moreover, we also explored the annealing temperature and time and the mass ratio of the precursor of the HER (Figures S5−S8). Ultimately, the optimal sample Ni/βMo2C@PC was obtained at 900 for 2 h, and the weight percentage of NiMoO4, NH4Cl, and DCA was 1:8:8 for optimum HER activity. The favorable HER activity of catalysts under the above conditions can be attributed to the excellent balance of the synergetic effect of Ni and Mo2C, the conductivity, and porosity. The HER polarization curves of Ni@PC, Mo2C@PC, Ni/β-Mo2C mixtures, Ni/β-Mo2C@PC, and commercial Pt/C (20 wt %) were tested and are also shown in Figure 3. By contrast, the overpotential of the Pt electrode is almost at least close to zero. Meantime, Ni/βMo2C@PC give the lowest overpotential of 123, 155, and 179 mV to achieve 10 mA cm−2 in alkaline, acidic, and neutral solutions, which are much smaller than that of Ni@PC, Mo2C@PC, and Ni/β-Mo2C@PC mixtures. Meanwhile, the η10 values of Ni/β-Mo2C@PC are advantageous compared to similar nonprecious metal catalysts described in a plethora of documented literature studies (Tables S1−S3). Figure 3b,d,f reveals the Tafel slopes of 87, 91, and 106 mV dec−1 for Ni/β-Mo2C@PC in 1 M KOH, 0.5 M H2SO4, and 1 M PBS, respectively, which are lower than Ni/β-Mo2C mixture. In addition, the Tafel slopes obtained by processing the polarization curve are also another important parameter for estimating the electrocatalytic performance. The smaller slopes mean that the HER rate increases greatly with the increase of the overpotential. Meanwhile, the Tafel slope values mean that Volmer step-water in alkaline media (H2O + e− ↔ H + OH−) and acidic media (H2 + e−↔ H) is the rate-determining step. The neutral medium is similar to the alkaline medium.49 The electrochemically active area is estimated via electrochemical double-layer capacitances (Cdl), which is proportional to the effective electrochemical surface area, as marked in Figures S10−S13. The Cdl values of Ni/β-Mo2C@PC are 19.5, 16.1, and 14.2 mF cm−2 in 1 M KOH, 0.5 M H2SO4, and 1 M PBS, respectively. The Cdl of Ni/β-Mo2C@PC was 2−5 times larger than that of other comparative catalysts, indicating that more active sites are available for the Ni/β-Mo2C@PC catalyst. Electrochemical impedance spectroscopy (EIS) was carried out to study the charge transfer kinetics in the hydrogen generation process of the catalysts (Figure S14). The EIS test was carried out at overpotentials of 50, 80, and 100 mV in alkaline, acidic, and neutral solutions, respectively. At the same time, the EIS spectra of Ni/β-Mo2C mixtures, Mo2C@PC, and Ni@PC were

Figure 2. (a) TEM image of Ni/β-Mo2C@PC (the inset shows the particle size distribution); (b−d) HRTEM images of Ni/β-Mo2C@ PC; (f−h) EDX elemental mapping of Mo and Ni in Ni/β-Mo2C@ PC.

nanoparticles is about 2−4 nm, encapsulated by graphitic carbon shells. The high-resolution TEM (HRTEM) images (Figure 2b) display distinct lattice fringes with an interplanar spacing of 0.35 nm, corresponding to the (002) plane of graphite. Furthermore, HRTEM photos of Ni/β-Mo2C@PC are presented in Figure 2c,e, in which d-spacing distances of 0.20 and 0.24 nm, consistent with the (111) plane of Ni (JCPDS no. 65-0380) and (002) plane of Mo2C (JCPDS no. 35-0787), can be clearly seen, respectively. It is worth mentioning that the majority of Ni nanoparticles are surrounded by Mo2C nanoparticles, implying that there may be election transfer between them. The elemental mapping of Mo and Ni, for Ni/β-Mo2C@PC as shown in Figure 2g−h, reveals the homogenous distribution of Ni and Mo2C nanoparticles. The materials of Ni/β-Mo2C@PC are further confirmed the carbon component by Raman spectroscopy (Figure S1). All samples show that two clear peaks were located at around 1350 and 1580 cm−1, assigned to the D and G bands of graphitic carbon, respectively. The value of ID/IG (the relative intensity ratio of the D-band to the G-band) correlates with the existence of the defects and the degree of graphitization.48 For Ni/β-Mo2C@PC, the value of ID/IG is about 0.96, suggesting a higher degree of graphitization than Ni@PC, Mo2C@PC, and Ni/β-Mo2C@PC mixtures, meaning that more imperfection and defect sites exist in the carbon lattice of Ni/β-Mo2C@PC. To further shed light on the porous structure of Ni/β-Mo2C@PC, the N2 adsorption−desorption isotherms are obtained (Figure S2). The observed isotherm characteristics are similar to Type I and Type IV isotherms with H3 hysteresis loops. The Brunauer−Emmett−Teller (BET) specific surface area of the obtained sample is 104.94 m2 g−1. In addition, the single-point total pore volume of Ni/β10349

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Figure 3. (a−f) Polarization curves of HER at 5 mV s−1 and corresponding Tafel plots in 1 M KOH, 0.5 M H2SO4, and 1 M PBS.

Figure 4. XPS spectra of the Ni/β-Mo2C@PC electrocatalyst: (a) high-resolution spectra of Ni 2P; (b) high-resolution spectra of Mo 3d.

Another important factor is excellent durability for outstanding catalysts. The long-term durability of Ni/β-Mo2C@ PC was examined by optimized mass loading on Ni foam (Figure S16). There was only very little decay at 1 M KOH and 1 M PBS electrolyte after long-term continuous CV scanning for 2000 sweeps and chronoamperometry testing at certain overpotentials. Moreover, in the 0.5 H2SO4, there was almost no loss of activity in the current density measured and chronoamperometry measured, revealing satisfactory stability under all pH conditions.

acquired (Figure S15). Two time constants are shown in the Nyquist plots of Ni/β-Mo2C@PC, where the Rct of Ni/βMo2C@PC is 4.2, 4.7, and 11.2 Ω in alkaline, acidic, and neutral measurements, respectively, at an overpotential of 100 mV. This result means that Ni/β-Mo2C@PC has the smallest Rct value compared to Ni/β-Mo2C mixtures and Mo2C@PC. The smaller Rct values mean the faster charge transfer at the Ni/β-Mo2C@PC electrodes, which are attributed to the porous composite structure. 10350

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4. EXPERIMENTAL SECTION 4.1. Materials and Methods. Commercial chemicals used in reactions were purchased without purification. Elemental analyses were performed with a PerkinElmer 240C elemental analyzer. PXRD patterns were gathered by a Siemens D5005 diffractometer with Cu Kα (λ = 1.5418 Å) radiation ranging from 5° to 80° at 293 K with a rate of 5 min−1. A JY LabRam HR 800 was utilized for the Raman spectra. A SU8000 ESEM FEG microscope was utilized to obtain the interrelated energy dispersive X-ray detector (EDX) spectra. The N2 sorption tests were measured on automatic volumetric adsorption equipment (Belsorp mini II). TEM was accomplished on a JEOL-2100F transmission electron microscope. 4.2. Synthesis of NiMoO4 Nanorods. The NiMoO4 nanorods were obtained via a hydrothermal method. Ni(NO3)2·6H2O (4 mmol) and Na2MoO4·2H2O (4 mmol) were separately dissolved in 10 mL of water and then added with Na2MoO 4·2H2O solution under vigorous stirring. The resulting solution was added to 40 mL of ethanol. Finally, the mixture was placed in to a 100 mL Teflon reactor after stirring at room temperature for 24 h and further sealed, heating at 150 °C for 6 h. The precipitate was collected by centrifugation, washed, and vacuum-dried at 80 °C. 4.3. Preparation of Ni/β-Mo2C@PC, Ni/β-Mo2C Mixtures, Mo2C@PC, and Ni@PC. Mechanochemical synthetic reactions were carried out in a ball mill using a 50 mL stainless steel grinding jar and 5 steel balls with 10 mm diameter. NiMoO4 nanorods (0.5 g), DCA (4 g), and NH4Cl (4 g) were added into the jar. Then, the mixtures were ground for 30 min at 40 Hz. The Ni/β-Mo2C@PC was obtained via placing samples in a tube furnace with the ramping rate at 5 °C min−1, subsequently, maintained at 900 °C for 2 h and naturally cooled down to 25 °C under Ar flow. The preparations of Ni@ PC, Mo2C@PC, and Ni/β-Mo2C mixtures were similar to that of Ni/β-Mo2C@PC. The Ni@PC was obtained by replacing NiMoO4 nanorods with Ni (NO3)2·6H2O (0.5 g), and then DCA (4 g) and NH4Cl (4 g) were added into the jar subsequently annealed under the same conditions as above. The Mo2C@PC was obtained by replacing NiMoO4 nanorods with (NH4)6Mo7O24·4H2O (0.3 g), and then DCA (4 g) and NH4Cl (4 g) were added into the jar subsequently annealed under the same conditions as above. The Ni/β-Mo2C mixture is prepared by mixing Ni (NO3)2·6H2O (0.5 g) and (NH4)6Mo7O24·4H2O (0.3 g) in the same situation. 4.4. Electrochemical Measurements. We used an electrochemical workstation (CHI 660e, China) to conduct the electrochemical measurements with a three-electrode system. The working electrode is a GC electrode (d = 3 mm), a graphite rod is the counter electrode, and the reference electrode is an Ag/AgCl electrode. For Ni/β-Mo2C@PC, 6 mg of the samples was dispersed in the mixture of 100 μL of 0.5 wt % Nafion solution, 0.2 mL of water and 0.2 mL of ethanol, followed by ultrasonication for 30 min to make an even suspension. After ultrasonication, 5 μL of the suspension was pipetted onto the GC electrode. Then, the working electrode was dried naturally under ambient temperature. The final mass loading of the catalysts was about 0.85 mg cm−2. The samples of Ni/β-Mo2C mixtures, Mo2C@PC, and Ni@PC were prepared with the same procedure with a mass loading of 0.85 mg cm−2. The commercial Pt/C electrocatalyst with the same preparation was conducted with the same loading with Ni/β-Mo2C@PC. The LSV curves were measured deaerated

To further investigate chemical composition, X-ray photoelectron spectroscopy of Ni/β-Mo2C@PC was performed, which demonstrated the existence of Ni, Mo, C, N, and O elements, as observed in Figure S18. The high-resolution Mo 3d spectrum in Ni/β-Mo2C@PC is shown in Figure 4, which deconvoluted into two peaks consistent with Mo2+ (228.3, 231.4 eV) and Mo6+ (232.4, 235.2 eV). In these Mo valences, the Mo6+ species is derived from surface oxidation, which has been shown to be inactive for the HER.50−52 Among them, Mo2+ can be attributed to molybdenum carbides (Mo2C). It has been confirmed by seniors that Mo2C was the active substance of the HER.25,26 In contrast, the Mo of Ni/βMo2C@PC has lower binding energy than Mo2+ (228.4, 231.5 eV) and Mo6+ (232.4, 235.6 eV) in Ni/β-Mo2C mixtures as well as Mo2+ (228.4, 231.6 eV) and Mo6+ (232.4, 235.7 eV) in Ni/β-Mo2C@PC, indicating that the Mo valence is lower in Ni/β[email protected] Meanwhile, the Ni 2p XPS spectrum peaks around at 852.7, 856.0, 861.5, and 854.5 eV are assigned to metallic state Ni, Niδ+, and Ni2+, respectively.53,54 It is worth mentioning that most of the surface Ni is oxidized to the +2 oxidation state.2,33 Ni/β-Mo2C@PC showed higher activity by comparing the Ni/β-Mo2C mixtures, Mo2C@PC, and Ni@PC. The EIS spectrum showed that the Rct value of Ni/β-Mo2C@ PC was the smallest, indicating that electron transfer was faster. In addition, the peaks of Ni2+ are positively shifted by 0.2 and 0.1 eV in Ni/β-Mo2C@PC compared with Ni/β-Mo2C mixtures and Ni@PC, which means that Ni has a higher valence state in Ni/β-Mo2C@PC. According to these insights, we proposed that the synergetic effect of Ni and Mo2C enables electron transfer from Ni to Mo2C, resulting in higher Ni valence and lower Mo valence and thus further explain the reasons for improving HER performance.46 The Ni/Mo2C@PC catalyst was thoroughly characterized after HER measurements. After the HER test, the morphology of the particles hardly changed compared to the original topography. The lattice fringes of Ni and Mo2C can still be clearly seen (Figure S24). The XRD patterns (Figure S23) also demonstrate that the structures of the nanoparticles Ni and Mo2C were retained after the three media reactions, except that the intensity of the peak is slightly reduced compared to the original sample. The XPS spectra (Figures S25−S26) show that there was no significant change in the 3d pattern of Mo. However, the intensity of Mo6+ is slightly increased compared to the original sample, which was attributable to the oxidation of the surface of the Mo2C nanoparticle. There were no significant changes in the N 2p region after the HER test.55

3. CONCLUSIONS In summary, a novel and facile strategy was used to synthesize Ni-doped Mo2C nanoparticles for efficient electrocatalytic hydrogen production. Ni/β-Mo2C@PC has overpotentials of 123, 155, and 179 mV at 10 mA cm−2, which is superior to single Ni and Mo2C in acidic, alkaline, and neutral solutions. Moreover, the catalyst demonstrates robust stability after CV2000 and stability test. The excellent performance of this composite catalyst can be ascribed to the synergistic effect of Ni and Mo2C, porous structure, high surface area, outstanding conductivity, and uniform distribution. This work implies the opportunity to effectively synthesize novel nanomaterials via a facile method that may have broader applications in different areas such as green energy conversion and storage. 10351

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with nitrogen with a scan rate of 5 mV s−1 in 0.5 M H2SO4, 1 M KOH, and 1 M PBS aqueous solutions at 25 °C prior to the HER. The catalysts were activated using 20 cyclic voltammetry scans with a scan rate of 100 mV s−1 before recording the electrochemical performance. All the potentials were referenced to a reversible hydrogen electrode (RHE) computed by the Nernst equation: ERHE = EAg/AgCl + 0.059 pH + 0.197 V. All data displayed here were treated without iR compensation. The CV measurements were taken with scan rates of 10, 30, 50, 80, 100, 150, and 200 mV s−1 between 0.10 and 0.30 V, respectively. EIS spectra were operated on a PARSTAT 2273 electrochemical system (Princeton Applied Research Instrumentation, USA) with the frequencies in the range of 0.1−100 000 Hz at an amplitude of 5 mV.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.9b00844. Experimental section; physical characterization of Ni/βMo2C@PC including Raman spectra, N2 adsorption− desorption isotherms, PXRD curves, and TEM images; additional electrochemical experiments of Ni/β-Mo2C@ PC including HER polarization and CV curves and Nyquist plots; XPS spectra of Ni/β-Mo2C@PC electrochemical tests; and comparison of HER parameters of different non-Pt catalysts (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.W.). *E-mail: [email protected] (Z.S.). ORCID

Xinlong Wang: 0000-0002-5758-6351 Zhongmin Su: 0000-0002-3342-1966 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC of China (nos. 21771035 and 21671034). REFERENCES

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DOI: 10.1021/acsomega.9b00844 ACS Omega 2019, 4, 10347−10353