N-Doped Sandwich-Structured Mo2C@C@Pt Interface with Ultralow

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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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N‑Doped Sandwich-Structured Mo2C@C@Pt Interface with Ultralow Pt Loading for pH-Universal Hydrogen Evolution Reaction Jing-Qi Chi,† Jing-Yi Xie,†,‡ Wei-Wei Zhang,† Bin Dong,*,†,‡ Jun-Feng Qin,† Xin-Yu Zhang,† Jia-Hui Lin,†,‡ Yong-Ming Chai,*,† and Chen-Guang Liu† †

State Key Laboratory of Heavy Oil Processing, Institute of New Energy and ‡College of Science, China University of Petroleum (East China), Qingdao 266580, P. R. China

ACS Appl. Mater. Interfaces Downloaded from pubs.acs.org by COLUMBIA UNIV on 01/17/19. For personal use only.

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ABSTRACT: Designing a unique electrochemical interface to exhibit Pt-like activity and good stability is indispensable for the efficient hydrogen evolution reaction (HER). Herein, we synthesize well-defined Mo2C@NC@Pt nanospheres with a sandwich-structured interface through a facile organic− inorganic pyrolysis and following reduction process. The obtained Mo2C@NC@Pt heterostructures with ultralow Pt loading are composed of well-dispersed Mo2C nanoparticles (NPs) inner layer, N-doped carbon layer, and ultrafine Pt NPs outer layer. Electrochemical measurements demonstrate that Mo2C@NC@Pt heterostructures not only exhibit superior HER activities than commercial Pt/C with small overpotentials of only 27, 47, and 25 mV to achieve a current density of 10 mA cm−2 in acidic, alkaline, and neutral media, respectively, but also possess favorable long-term stability in pH-universal solution. The improved reaction kinetics of Mo2C@NC@Pt heterostructures are mainly attributed to the unique sandwich-structured interface with well-defined Mo2C NPs encapsulated by carbon layers and Pt NPs well-dispersed on the carbon support, synergistic effects among Mo2C NPs, NC, and Pt NPs, high specific surface area, and N-doping into the catalysts. This facile approach not only provides a new pathway for preparing well-defined carbides but also gives insight into the development of low-Pt catalysts for the efficient HER. KEYWORDS: Mo2C@NC@Pt nanospheres, molybdenum carbides, low-Pt catalyst, sandwich-structured interface, hydrogen evolution reaction remain stable at pH-universal values.19,20 Given the above challenge, combining transition metals with less amount of Pt loading as cocatalysts has been attempted to improve the intrinsic activity.21 For example, Fu et al. reported the synthesis of Ni2P encapsulated by ultrathin graphite layers with low-Pt loading, displaying the enhanced oxygen reduction reaction performance in acid.22 Molybdenum carbides (Mo2C), as typical transition metal carbides (TMCs), have been widely employed as effective electrocatalysts for the HER due to the similar d-band states with Pt catalysts.23,24 Therefore, designing the heterostructures through contacting the Mo2C with trace of Pt loading may be an efficient strategy to realize intrinsically enhanced activity in different electrolytes, in which the electronic configuration of the Pt surface can be tuned by the underlying Mo2C, thus leading to improved HER activity.25,26 However, the unique Mo2C nanostructures are not easy to prepare because they are usually prepared under harsh thermal conditions, in which the sintering and aggregation of nanoparticles (NPs) are inevitable.27,28 Moreover, it is a challenge to synthesize well-defined nanostructures composed of interactional ultrafine Mo2C and Pt nanoparticles

1. INTRODUCTION Hydrogen (H2) has gained much attention as a promising alternative energy resource to traditional fossil fuels because of its renewability, free of greenhouse gases, and high-energy density.1−3 For efficient H2 production, electrochemical water splitting is a promising way, which requires effective electrocatalysts to lower the dynamic overpotential for the hydrogen evolution reaction (HER).4−7 So far, the state-of-the-art Ptbased noble materials have been recognized as the most efficient electrocatalysts for the HER owing to their optimum hydrogen adsorption Gibbs free energy (ΔGH*), which requires negligible overpotential to realize a high current density, however, their widespread implementation is hindered by the limited supply and high cost.8−10 Now, many efforts have been devoted to searching for numerous non-Pt catalysts, which are obtained from inexpensive and abundant materials, such as transition metal carbides (TMCs),11,12 transition metal sulfides,13,14 or transition metal phosphides,15−17 due to their Pt-like electrochemical behaviors. Although these transition metal alternatives display activities close to commercial Pt/C in acidic electrolytes, these catalysts still exhibit sluggish HER kinetics and inferior stability in alkaline or neutral electrolytes.18 Considering the practical use of electrochemical water splitting, an efficient electrocatalyst should function well and © XXXX American Chemical Society

Received: November 16, 2018 Accepted: January 2, 2019

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DOI: 10.1021/acsami.8b20209 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic View of Synthesis Route of Mo2C@NC@Pt Nanospheres

polymer was washed with distilled water and dried at 60 °C in a vacuum. For the preparation of Mo2C@NC composites, the asobtained MoO42−@polymer was calcined at 800 °C for 4 h with the heating rate of 5 °C min−1 in an inert Ar atmosphere with an isothermal process at 500 °C for 2 h. After cooling down to room temperature naturally, the obtained sample was thoroughly washed with distilled water to remove the impurities, and dried at 60 °C in a vacuum, forming the final Mo2C@NC nanospheres. 2.2. Synthesis of Ultrafine and Well-Dispersed Pt Nanoparticles Supported on Mo2C@NC Nanospheres (Mo2C@NC@ Pt). 100 mg of Mo2C@NC composite was dispersed in 40 mL of distilled water and ultrasonicated for 0.5 h to form a well-distributed slurry. Then the slurry was added into 20 mL aqueous solution containing 20 mg of H2PtCl6·6H2O solution. Afterwards, excessive NaBH4 dissolved in 30 mL of aqueous solution was dropped into the mixed solution under magnetic stirring for 2 h. Then the solution was settled for 10 h and washed by centrifugation with water several times, and then dried in a vacuum. The as-prepared product was denoted as Mo2C@NC@Pt. For comparison and to deeply understand the interplay between the nanostructures and electrocatalytic activities of the Mo2C@NC@Pt catalysts, several contrastive samples were synthesized under identical conditions except that the amount of H2PtCl6·6H2O was 30 and 10 mg, and the mole ratio of H2PtCl6/ NaBH4 remained constant. 2.3. Synthesis of Reference Samples. NC nanospheres were synthesized through the calcination of polymer nanospheres under identical conditions as Mo2C@NC nanospheres. The NC@Pt control sample was prepared in the same way as Mo2C@NC@Pt but with NC replacing Mo2C@NC nanospheres. 2.4. Characterization. Scanning electron microscope (SEM) measurements equipped with an energy dispersive X-ray analyzer were carried out on a Hitachi S-4800 instrument. Transmission electron microscopy (TEM) experiments were performed by a FEI Tecnai G2 with a 200 kV accelerating voltage. Powder X-ray diffraction (XRD) patterns were recorded on an X’Pert PRO MPD diffractometer equipped with Cu Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) was carried out on a ThermoFisher Scientific II spectrometer with an Al Kα X-ray source. Raman spectroscopy was performed on a Renishaw inVia 2000 spectrometer with a He−Ne laser (514 nm). Brunauer−Emmett−Teller (BET) surface areas of samples were calculated with N2 adsorption− desorption isotherms using a tristar II Plus instrument at 77 K. 2.5. Electrochemical Measurements. All electrochemical measurements were performed on an electrochemical workstation (Gamry Reference 600) using a standard three-electrode setup. The as-obtained catalysts, a carbon rod, and a saturated calomel electrode (in 0.5 M H2SO4 and 1.0 M phosphate buffer saline (PBS)) or Hg/ HgO (1.0 M KOH) were used as a working, counter, and reference electrode, respectively. The potentials in the measurements were converted to potentials versus the reversible hydrogen electrode (RHE) according to the Nernst equation. The polarization curves were performed in a pH-universal solution with a scan rate of 5 mV s−1. The double-layer capacitances (Cdl) were measured by cyclic voltammetry (CV) scans with a different sweep rate ranging from 40 to 120 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a frequency ranging from 0.1 to 105 Hz with an amplitude of 5 mV at a potential of −0.01 V (vs RHE). The long-time durability was evaluated by continuous CV for

(NPs) to ensure the intimate synergistic effect between Mo2C and Pt NPs. The use of the heteroatom-doped carbon support with unique nanostructures can not only optimize the electronic structures of the catalyst but also disperse and prevent NPs from corrosion in all kinds of harsh environments.29,30 Moreover, optimizing the carbon support to acquire high specific surface areas with abundant mesopores is also vital for exposing rich active sites for the HER.31,32 Polyaniline and pyrrole, as typical N-doped organic matrices, have served as excellent catalyst supports owing to their good conductivity and unique structures after pyrolysis.33,34 Organic matrix reduction in high temperature can also promote partial carbon decomposing in the form of carbon oxides, thus producing a porous structure.35,36 Therefore, combining the organic polyaniline and pyrrole with inorganic Mo species can not only realize the partial removal of organic species but also promote the reconstruction of ultrafine Mo2C NPs to produce encapsulation heterostructures.37 Meanwhile, the carbon matrix with a porous structure can serve as an ideal support to disperse Pt NPs and promote the intimate contact between Mo2C and Pt NPs, forming an active electrochemical interface. So, it is highly desirable to construct a well-defined sandwichstructured interface comprising Mo2C NPs, the carbon matrix, and Pt NPs for the promoted catalytic performance. Herein, we have prepared well-defined Mo2C@NC@Pt nanospheres with a sandwich-structured interface through a facile organic−inorganic pyrolysis and following reduction process. This strategy depends on the in-situ restricted carbonization and reduction process between the polyaniline/pyrrole and Mo and Pt species. Then the obtained Mo2C@NC@Pt heterostructures are composed of welldispersed Mo2C NPs inner layer, N-doped carbon layers intermediate later, and ultrafine Pt NPs outer layer. Benefiting from the well-defined sandwich-structured interface with Mo2C NPs encapsulated by carbon layers and Pt NPs welldispersed on the carbon support, synergistic effects among Mo2C NPs, NC, and Pt NPs, high specific surface area, and Ndoping into the catalysts, the obtained Mo2C@NC@Pt heterostructures exhibit superior HER performance with ultrasmall overpotentials than commercial Pt/C and maintain long-time durability in acidic, alkaline, and neutral conditions.

2. EXPERIMENTAL SECTION 2.1. Synthesis of N-Doped Mo2C Encapsulated by Carbon Shells (Mo2C@NC). For the synthesis of the MoO42−@polymer nanospheres precursor, 1.2 mmol of (NH4)6Mo7O24·4H2O, 3.5 mmol of aniline and pyrrole, and 0.4 mmol of surfactant (Triton X-100) were dissolved in 60 mL distilled water under magnetic stirring and sonication for 1 h to form a homogeneous solution. Afterwards, 8.0 mmol of (NH4)2S2O8 dissolved in 20 mL distilled water was poured into the above homogeneous solution and the polymerization reaction is maintained at 0 °C for 24 h. Finally, the as-obtained MoO42−@ B

DOI: 10.1021/acsami.8b20209 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) SEM, (b, c) TEM, and (d) HRTEM images of Mo2C@NC@Pt. TEM images of (e) Mo2C@NC and (f) NC@Pt. (g) XRD pattern, (h) Raman spectrum, and (i) N2 adsorption−desorption isotherms of Mo2C@NC@Pt (insets: pore size distribution of Mo2C@NC@Pt). 1000 cycles from −0.2 to 0.1 V (vs RHE) at a sweep rate of 100 mV s−1 or amperometric i−t tests for 10 h.

framework. The Mo2C@NC nanosphere-supported ultrafine Pt NPs with a low-Pt loading of 7.49 wt % are fabricated through inductively coupled plasma atom emission spectrometry (ICP-AES) measurements. 3.2. Characterization of Mo2C@NC@Pt Nanospheres. The structures and compositions of all as-prepared samples are first characterized by SEM and high-resolution TEM ((HR)TEM). As shown in Figure S1a,b, the MoO42−@polymer composite consisted of well-distributed nanospheres with smooth surfaces and the average diameter is estimated to be 120 nm. After the carbonization process at high temperature, the SEM image displays that the as-prepared Mo2C@NC structures retain the spherical-like morphology intactly but with a shrunken diameter of ∼90 nm (Figure S2a,b). For the as-prepared Mo2C@NC@Pt hybrids, Figure 1a shows that the these nanospheres are also preserved intactly, but the surfaces become more rough compared to the Mo2C@NC composite. For comparison, the SEM images of NC (Figure S3a,b) and NC@Pt (Figure S4a,b) composites show that these two samples also exhibit spherical-like morphologies, but NC@Pt nanospheres possess rougher surfaces than NC nanospheres, indicating that the Pt nanoparticles are well-distributed on the NC@Pt surfaces after going through the reduction process. The TEM image in Figure 1b reveals that the Mo2C@NC@Pt

3. RESULTS AND DISCUSSION 3.1. Preparation of Mo2C@NC@Pt Nanospheres. The Mo2C@NC@Pt nanospheres with a sandwich-structured interface is prepared via a high-temperature carbonization method and a following NaBH4 reduction process. The synthetic procedure for Mo2C@NC@Pt nanospheres is displayed in Scheme 1. The possible formation mechanism for the Mo2C@NC@Pt nanospheres is as follows: for the first step of forming MoO42−@polymer nanospheres, aniline and pyrrole flock at different locations of the Triton X-100 micelles because of their different hydrophobic nature and the (NH4)6Mo7O24·4H2O crosslinks with the well-dispersed aniline and pyrrole. After adding initiator ((NH4)2S2O8), aniline and pyrrole polymerize at the interface of micelle and water, and the (NH4)6Mo7O24·4H2O, which is cross-linked with aniline and pyrrole, are diffused along with the aniline and pyrrole simultaneously, thus leading to the formation of the spherical-like MoO42−@polymer. For the following harsh hightemperature carbonization and NaBH4 reduction process, the intact spherical morphology is still well retained, indicating the stable structure of the MoO42−@polymer organic−inorganic C

DOI: 10.1021/acsami.8b20209 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) XPS high-resolution scans for Mo2C@NC@Pt. (b) Mo 3d XPS spectrum of Mo2C@NC and Mo2C@NC@Pt. (c) The Pt 4f XPS spectrum of NC@Pt and Mo2C@NC@Pt. (d, e) N 1s and C 1s XPS spectrum of Mo2C@NC@Pt.

(Figures S8 and S9). The crystalline phase structure of Mo2C@NC@Pt nanospheres is further characterized by XRD. The XRD pattern of the Mo2C@NC@Pt composite (Figure 1g) exhibits the characteristic diffraction peaks at 37.9, 39.4, 61.6, and 74.6°, corresponding to the (002), (101), (110), and (112) facets of the hexagonal β-Mo2C (PDF No. 00-35-0787), respectively. Moreover, an additional broad peak is observed at 26°, corresponding to the (002) facets of graphitic carbon. However, no characteristic diffraction peaks of Pt species are observed due to the ultrafine and the ultralow loading of Pt nanoparticles. It can be seen from Figure S10 that the peak intensity of β-Mo2C for Mo2C@NC@Pt nanospheres reduces compared with Mo2C@NC, indicating that the Pt NPs supported on the Mo2C@NC surface result in the reduction of peak intensity. For the NC composite, the broad peak observed at 26° is attributed to the (002) planes of the graphitic carbon. Moreover, the degrees of graphitization of Mo2C@NC@Pt nanospheres is further investigated by Raman spectroscopy (Figure 1h). Two main peaks at 1350 and 1590 cm−1 are related to the D and G bands, respectively. The value of ID/IG is estimated to be 1.10, implying the partial graphitization of the carbon matrix, which is beneficial for electron transfer for the Mo2C@NC@Pt hybrids.38 The BET surface area of Mo2C@NC@Pt nanospheres is calculated to be 128.4 m2 g−1 and the large number of nanopores mainly centers at 3.63 nm (Figure 1i). The existence of mesoporous structures is beneficial for exposing more catalytic sites and accelerate the mass transfer rate between the catalysts and electrolytes. XPS is performed to further obtain elemental compositions and electronic interaction of the as-prepared catalysts. As depicted in Figure 2a, the Mo2C@NC@Pt nanospheres composed of Mo, Pt, C, N, and O elements are clearly observed. The surface oxidation in the carbonization process leads to the occurrence of the O element. As shown in Figure 2b, the Mo 3d profiles of Mo2C@NC and Mo2C@NC@Pt

composite displays a typical pitaya-like morphology with a number of nanoparticles distributed throughout the whole carbon matrix. Under high magnification, Figure 1c exhibits that some of the nanoparticles in the darker contrast were encapsulated in an ultrathin carbon shell for every nanosphere and other nanoparticles with ultrafine size are homogeneously distributed on the carbon surfaces. The HRTEM image (Figure 1d) clearly discloses that these nanoparticles inside the carbon shells are well-crystallized and the lattice fringes with an interplanar distance of 0.229 nm is attributed to the (101) plane of hexagonal β-Mo2C. Moreover, the ultrafine nanoparticles supported on carbon layers are also highly crystalline with lattice fringes consistent with the (111) plane of Pt. So, the results powerfully demonstrate that the Mo2C@NC@Pt nanospheres present a typical sandwich-structured interface. It is worth noting that the β-Mo2C and Pt nanoparticles tightly cross-linked with the carbon matrix would promote the electronic interaction effectively, thus leading to the fast charge transfer rate during the electrocatalytic process. To further confirm the sandwich nanostructures of Mo2C@NC@ Pt nanospheres, the TEM images of Mo2C@NC and NC@Pt nanospheres are also presented as comparison. TEM and HRTEM images of Mo2C@NC (Figures 1e and S5) reveal that the well-dispersed nanoparticles with the lattice fringes of 0.229 nm are encapsulated by an ultrathin carbon shell for every nanosphere, which indicates that the inside nanoparticles are composed of β-Mo2C. Figures 1f and S6 display that the ultrafine Pt nanoparticles are homogeneously supported on the carbon surfaces for the NC@Pt nanospheres, further confirming the sandwich-structured-shell of Mo2C@NC@Pt nanospheres. In addition, the SEM mapping result demonstrates that the Mo2C@NC@Pt composites consisted of Mo, Pt, C, O, and N elements and these elements are welldispersed (Figure S7). As a comparison, SEM mapping images of the as-obtained NC and Mo2C@NC nanospheres are also presented to verify the uniform distribution of these elements D

DOI: 10.1021/acsami.8b20209 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. SEM, TEM, and HRTEM images of Mo2C@NC@Pt with the Pt loading of (a−c) 30 mg and (d−f) 10 mg. (g) XRD patterns, (h) Raman spectrum, and (i) N2 adsorption−desorption isotherm of Mo2C@NC@Pt with the Pt loading of 30, 20, and 10 mg, respectively.

corresponding to the pyridinic-N and pyrrolic-N, respectively.45,46 Another peak at 397.0 eV can be ascribed to the N− Mo bonding of molybdenum nitride.47 Therefore, the XPS results indicate the successful doping of N into carbon materials and Mo2C. The N dopants with strong electronwithdrawing features and large electronegativity can induce the electron-transfer process and the pyridinic-N is conductive to improve the conductivity of catalysts.48,49 The C 1s spectrum is split into three kinds of peaks at 283.4, 284.0, and 284.6 eV, corresponding to graphite C, carbides, and C−C species, respectively.50,51 In summary, XPS analysis reveals the successful synthesis of the Mo2C@NC@Pt heterostructures. To reveal the interplay between nanostructures and activities of Mo2C@NC@Pt hybrids, series of interrelated samples are synthesized by varying the amount of Pt species for the reduction reaction, and their structures and composites are investigated in detail (Figure 3). As shown in Figure 3a−f, SEM and TEM images of Mo2C@NC@Pt (30 mg) and Mo2C@NC@Pt (10 mg) composites also show the pitaya-like morphology with sandwich nanostructures, in which Mo2C NPs are encapsulated by carbon shells and Pt NPs are uniformly dispersed on the carbon surfaces, but the external Pt NPs for Mo2C@NC@Pt (30 mg) aggregate severely compared

nanospheres are displayed. For Mo2C@NC@Pt nanospheres, two peaks at 228.7 and 232.1 eV are ascribed to Mo2+ of Mo2C.39,40 Moreover, according to the XPS peak fitting results, the amount of Mo2C NPs encapsulated in Mo2C@NC@Pt accounts for the weight percentage of 21.6% of total compounds, and the other peaks at 232.6 and 235.7 eV correspond to the oxidized phases of the Mo2C@NC@Pt catalyst.41,42 However, the binding energy of the Mo 3d region in Mo2C@NC@Pt nanospheres shifts to higher values compared with Mo2C@NC nanospheres, indicating the electronic interaction between Mo2C and Pt in the Mo2C@ NC@Pt composite.43 The Pt 4f XPS high-resolution spectrum of NC@Pt and Mo2C@NC@Pt nanospheres are displayed in Figure 2c. The calculated weight percentage of Pt NPs in Mo2C@NC@Pt nanospheres is 7.5%, which is highly consistent with that of ICP-AES measurements. As observed, the Pt 4f peaks for the Mo2C@NC@Pt composite shifts to lower values compared to that of the NC@Pt sample.44 These results powerfully demonstrate the electronic interaction of Mo2C and Pt and the binding energy shift can be ascribed to the electron-transfer effect from Mo2C to Pt. In the N 1s region (Figure 2d), the XPS spectrum is deconvoluted into two kinds of N species at 398.3 and diameter 399.6 eV, E

DOI: 10.1021/acsami.8b20209 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. (a) Polarization curves of all samples in 0.5 M H2SO4. (b) The corresponding Tafel plots. (c) CV polarization curves of Mo2C@NC@Pt in the nonfaradaic region with scan rates from 40 to 120 mV s−1. (d) The double-layer capacitance (Cdl) and (e) Nyquist plots of NC, Mo2C@NC, NC@Pt, and Mo2C@NC@Pt. (f) Comparison of the polarization curves of Mo2C@NC@Pt before and after 1000 cycles (inset: time-dependent current density curves of Mo2C@NC@Pt).

mechanism via a two-step process, including the first step of Volmer reaction (eq 1) and the second step of Heyrovsky (eq 2) or Tafel reaction (eq 3).52 As shown in Figure 4b, the Tafel slopes of NC, Mo2C@NC, NC@Pt, Mo2C@NC@Pt, and Pt/ C are measured to be 198, 73, 30, 28, and 29 mV dec−1, respectively. The Tafel slopes of NC@Pt, Mo2C@NC@Pt, and Pt/C electrodes are nearly the same, suggesting a possible Volmer−Tafel mechanism, in which recombination of absorbed H is the rate-limiting step,53 and the lowest Tafel slope of the Mo2C@NC@Pt electrode indicates the favorable kinetics for the HER.

with Mo2C@NC@Pt (10 mg) and Mo2C@NC@Pt (20 mg) nanospheres. SEM mapping images in Figure S11a,b show that Mo, Pt, C, N, and O for Mo2C@NC@Pt series samples are well-dispersed throughout the carbon spheres. The XRD patterns for Mo2C@NC@Pt series samples are consistent with the hexagonal β-Mo2C (PDF No. 00-35-0787) phase but with different peak intensities (Figure 3g). The peak intensities of Mo2C@NC@Pt series samples decrease with the increasing Pt content due to the different coverages of Pt NPs, which may be the reason for Mo2C@NC@Pt series samples to exhibit different HER performances. The Raman results of Mo2C@ NC@Pt series samples exhibit that the ID/IG values are nearly the same with little difference, indicating the partial graphitization and good conductivity of the carbon matrix for all samples (Figure 3h). Moreover, the BET surface areas of Mo2C@NC@Pt series samples decrease with the increasing Pt content, which can be explained that severe aggregation of a large amount of Pt species results in the decreasing of BET surface areas. Therefore, choosing an appropriate Pt content is of vital importance for enhanced HER performance. To evaluate the electrocatalytic HER activities of the Mo2C@NC@Pt electrode, we first investigate the polarization curves of all samples in acid (0.5 M H2SO4), and commercial 20 wt % Pt/C electrodes are also tested for comparison. As shown in Figure 4a, the Mo2C@NC@Pt electrode exhibits unexpected superior electrocatalytic activities than those of contrastive samples, and even Pt/C electrodes, with a small overpotential of only 27 mV to achieve 10 mA cm−2, which is lower than those of Mo2C@NC (230 mV) and NC@Pt (32 mV). To achieve a current density of 100 mA cm−2, Mo2C@ NC@Pt needs an overpotential of 60 mV less than that of the 20 wt % Pt/C electrode, which is lower than those of many recently reported active HER electrocatalysts in acidic solution (Table S1). In addition, Tafel slopes are calculated by the Tafel equation (η = b log j + a, where a is the constant, b is the slope, and j is the current density) to interpret the possible reaction

H3O+ + e− → Hads + H 2O

(1)

Hads + H3O + e− → H 2 + H 2O

(2)

Hads + Hads → H 2

(3)

To evaluate the electrochemically active surface area of the solid−liquid interface of the Mo2C@NC@Pt electrode, the capacitance of the double-layer (Cdl) is calculated through a cyclic voltammetry (CV) method. The CV curve is tested at various scanning rates ranging from 40 to 120 mV s−1 in a nonfaradaic region (Figure 4c). The Cdl of the Mo2C@NC@Pt electrode is estimated to be 3.8 mF cm−2, which is larger than those of contrastive electrodes (Figure 4d). Such high Cdl value indicates more exposure of active sites, thus promoting the electrocatalytic process. EIS measurements are also presented to study the possible electrode kinetics and interfacial properties of the Mo2C@NC@Pt electrode (Figure 4e). As shown in the Nyquist plot and equivalent circuit in Figure 4e, the Mo2C@NC@Pt electrode displays a smaller diameter of semicircle and a lower charge transfer resistance (Rct) value of 11.38 Ω (Table S2), implying good electron transfer ability, which can be ascribed to the partial graphitization (ID/IG = 1.10) of the carbon matrix, thus leading to a decreased Rct at the interface of the catalyst and electrolyte. F

DOI: 10.1021/acsami.8b20209 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. Polarization curves of all samples in (a) 1.0 M KOH and (d) 1.0 M PBS. The corresponding Tafel plots in (b) 1.0 M KOH and (e) 1.0 M PBS. Comparison of the polarization curves of Mo2C@NC@Pt before and after 1000 cycles in (c) 1.0 M KOH and (f) 1.0 M PBS (inset: timedependent current density curves of Mo2C@NC@Pt in (c) 1.0 M KOH and (f) 1.0 M PBS).

Figure 6. (a, d, g) Polarization curves of Mo2C@NC@Pt with the Pt amount of 30, 20, and 10 mg, (b, e, h) the corresponding Tafel plots, and (c, f, i) the double-layer capacitance (Cdl) in (a−c) 0.5 M H2SO4, (d−f) 1.0 M KOH, and (g−i) 1.0 M PBS.

G

DOI: 10.1021/acsami.8b20209 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Mo2C@NC@Pt (20 mg), and Mo2C@NC@Pt electrodes (10 mg) are 32, 27, and 29 mV in 0.5 M H2SO4 (Figure 6a), respectively. The corresponding Tafel slopes of Mo2C@NC@ Pt (30 mg), Mo2C@NC@Pt (20 mg), and Mo2C@NC@Pt electrodes (10 mg) are calculated to be 28, 28, and 30 mV dec−1 (Figure 6b), respectively, indicating the favorable HER performance of the Mo2C@NC@Pt (20 mg) catalyst with the Pt loading of 7.49%. Figures 6c, S19, and Table S3 show that the Mo2C@NC@Pt (20 mg) electrode exhibit larger Cdl of 3.8 mF cm−2 and a lower Rct value of 11.38 Ω than those of Mo2C@NC@Pt (30 mg) and Mo2C@NC@Pt (10 mg) electrodes. In addition, the Mo2C@NC@Pt (20 mg) electrode also exhibits superior electrocatalytic HER performance than those of Mo2C@NC@Pt (30 mg) and Mo2C@NC@Pt (10 mg) electrodes in alkaline and neutral media, which are in agreement with the results obtained in acidic solution (Figures 6d−i, S20, and S21), indicating the favorable HER performance of the Mo2C@NC@Pt (20 mg) catalyst. The Mo2C@ NC@Pt (20 mg) catalyst exhibits superior electrocatalytic performance for the HER than the contrastive catalysts, mainly because of the optimal ratio of Pt/Mo2C NPs, and also related to the well-dispersed Pt NPs, partial graphitization of the carbon matrix, and the large BET surface area. However, due to the low-Pt loading in the Mo2C@NC@Pt (10 mg) electrode, the electrocatalytic activity is inferior to that of the Mo2C@NC@Pt (20 mg) electrode. In contrast, a high-Pt loading in the Mo2C@NC@Pt (30 mg) electrode leads to the aggregation of Pt NPs, which may result in the decreasing surface area and inferior HER performance than that of Mo2C@NC@Pt (20 mg). These results reveal that the amount of Pt NPs plays an indispensable role in influencing the HER performance. On the basis of the aforementioned experiments and analysis, the remarkable HER activities and durability of Mo2C@NC@Pt nanospheres in a pH-universal solution may originate from the following reasons: (1) the conductive carbon matrix can promote electron transfer and prevent active Mo2C and Pt NPs from agglomeration during pyrolysis and reduction process. (2) High exposure of active sites from the uniform N-doped carbon matrix, Mo2C, and Pt NPs originate from the uniform sandwich-structured interface. (3) The synergistic effects among NC, Mo2C, and Pt NPs boost excellent catalytic activities of Mo2C@NC@Pt nanospheres, making it as effective HER catalysts. (4) Heteroatoms Ndoped into the carbon matrix and Mo2C NPs can optimize the electronic configurations, thus improving the intrinsic activities.

Durability is also an important factor for evaluating the catalytic properties of a catalyst for the HER. The durability of the Mo2C@NC@Pt electrode is tested by measuring continuous CV for 1000 cycles between −0.2 and 0.1 V (vs RHE) at a scan rate of 100 mV s−1. As displayed in Figure 4f, the Mo2C@NC@Pt electrode exhibits only a slight negative shift after 1000 sweeps in acid, demonstrating the good stability of the Mo2C@NC@Pt electrode for the HER. The morphology and composition information of the Mo2C@ NC@Pt electrode after continuous CV are investigated by the SEM (Figure S12a), TEM (Figure S12b), and EDS spectrum (Figure S12c). As expected, Mo2C@NC@Pt maintains an intact spherical-like shape with ultrafine Mo2C and Pt NPs well-dispersed. Moreover, chronoamperometry measurement of the Mo2C@NC@Pt electrode at an overpotential of 30 mV (vs RHE) indicates that the current density of the Mo2C@ NC@Pt electrode shows a negligible decrease after 10 h. These results demonstrate that the Mo2C@NC@Pt electrode possesses remarkable long-term durability in acid. The HER performance of the Mo2C@NC@Pt electrode in alkaline (1.0 M KOH) and neutral (1.0 M PBS) media are further investigated. As expected, the as-prepared Mo2C@ NC@Pt electrode also exhibits Pt-like activities with an overpotential of only 47 mV to afford 10 mA cm−2 (Figure 5a) in alkaline media, which only require an overpotential of 10 mV larger than that of the commercial Pt/C electrode. As exhibited in Figure 5b, the Tafel slope of the Mo2C@NC@Pt electrode is 57 mV dec−1, implying that the catalyzed HER process complies with the Volmer-mechanism. The Cdl of the Mo2C@NC@Pt electrode is estimated to be 8.0 mF cm−2, which is larger than those of NC, Mo2C@NC, and NC@Pt electrodes (Figure S13a,b), indicating more exposure of active species for the HER. The Mo2C@NC@Pt electrode also displays a lower Rct (33.58 Ω) value than those of NC (39 660 Ω), Mo2C@NC (99.23 Ω), and NC@Pt electrodes (49.09 Ω), indicating a fast charge transfer rate of the Mo2C@NC@Pt electrode (Figure S14 and Table S2). The continuous CV for 1000 cycles or chronoamperometry measurement demonstrate the high stability of the Mo2C@NC@Pt electrode in 1.0 M KOH (Figure 5c). Moreover, Figure S15a−c show that the structure and well distribution of Mo2C@NC@Pt nanospheres remain unchanged after continuous CV tests. In 1.0 M PBS, the Mo2C@NC@Pt electrode exhibits higher HER activities with an overpotential of 8 mV less than that of the commercial Pt/C electrode to achieve a current density of 10 mA cm−2 (Figure 5d), a smaller Tafel slope of 33 mV dec−1 (Figure 5e), a larger Cdl of 11.02 mF cm−2 (Figure S16a,b), and a lower Rct of 37.86 Ω (Figure S17 and Table S2). The overpotential of the Mo2C@NC@Pt electrode is lower than those of many recently reported active HER catalysts in alkaline and neutral media (Table S1). In addition, the Mo2C@NC@Pt electrode also retains high long-term stability after the continuous CV or chronoamperometry test (Figure 5f) and the spherical-like morphology is preserved intactly (Figure S18). Overall, the smaller overpotentials, lower Tafel slope, larger Cdl, and lower Rct value demonstrate the excellent activities of the Mo2C@ NC@Pt catalyst for the HER in a pH-universal solution. To further reveal the interplay between the nanostructures and activities of Mo2C@NC@Pt hybrids, the HER performance of the series of Mo2C@NC@Pt heterostructures by varying the amount of Pt loading are also investigated in a pHuniversal solution. To achieve a current density of 10 mA cm−2, the overpotentials for Mo2C@NC@Pt (30 mg),

4. CONCLUSIONS In summary, well-defined Mo2C@NC@Pt nanospheres with a sandwich-structured interface are synthesized as a highly active electrocatalyst for the HER via an organic−inorganic carbonization and following reduction process. The precursors consisting of polyaniline/pyrrole can efficiently hinder the obtained Mo2C and Pt NPs from aggregation during the carbonization and reduction process. The as-prepared Mo2C@ NC@Pt nanospheres exhibit a sandwich-structured interface composed of well-dispersed Mo2C NPs inner layer, N-doped carbon intermediate layer, and ultrafine Pt NPs outer layer. Benefiting from the unique sandwich-structured interface and synergistic effect among Mo2C NPs, NC, and Pt NPs, the Mo2C@NC@Pt nanospheres with ultralow Pt loading exhibit improved catalytic activity compared to the commercial Pt/C H

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catalyst. Therefore, this work provides a feasible method for the rational design of the low-Pt HER catalyst by combining with inexpensive materials and via a facile synthesis route.

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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.8b20209. SEM images; TEM images; SEM mapping; XRD patterns; CV polarization; double-layer capacitance and Nyquist plots (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.D.). *E-mail: [email protected] (Y.-M.C.). ORCID

Bin Dong: 0000-0002-4817-6289 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Natural Science Foundation of China (21776314), Shandong Provincial Natural Science Foundation (ZR2017MB059), the Major Program of Shandong Province Natural Science Foundation (ZR2018ZC0639), and the Fundamental Research Funds for the Central Universities (18CX05016A).

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