Coupled Heterostructure of MoâFe Selenide Nanosheets Supported

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Energy, Environmental, and Catalysis Applications

Coupled heterostructure of Mo-Fe selenide nanosheets supported on carbon paper as an integrated electrocatalyst for efficient hydrogen evolution Yalan Chen, Jingtong Zhang, Peng Guo, Haijun Liu, Zhaojie Wang, Ming Liu, Tian Zhang, Shutao Wang, Yan Zhou, Xiaoqing Lu, and Jun Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08007 • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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ACS Applied Materials & Interfaces

Coupled heterostructure of Mo-Fe selenide nanosheets supported on carbon paper as an integrated electrocatalyst for efficient hydrogen evolution Yalan Chen,†, ‡ Jingtong Zhang,‡ Peng Guo,† Haijun Liu,† Zhaojie Wang,*† Ming Liu,‡ Tian Zhang,† Shutao Wang,† Yan Zhou,† Xiaoqing Lu,† Jun Zhang*‡ † College of Science, China University of Petroleum, Qingdao, 266580, P. R. China. ‡ College of Chemical Engineering, State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao, 266580, P. R. China.

ABSTRACT Hydrogen evolution reaction (HER) driven by high-performance and low-cost electrocatalysts has been well identified as one of the most promising technologies to explore sustainable power source. In this work, the authors demonstrated a series of Fe-Mo selenide composite nanomaterials with adjustable HER catalytic activities by a simple one-step hydrothermal reaction. The heterostructured catalyst exhibited significant enhancement in HER activity to the pristine MoSe2 and FeSe2 catalysts. It is found that the optimized Mo-Fe selenide can drive the HER at a current density of 10 mA cm-2 by overpotential of 86.9 mV in acidic solution. Also it possesses outstanding kinetics (Tafel slope of 57.7 mV dec-1) of the electrochemical reaction. Beside benefits from the synergistic effect of different chemical, the coupled Mo-Fe selenide achieved the electronic modulation of the heterointerface, where electrons were accelerated to transfer from dispersed FeSe2 to the active edges on 1T-MoSe2 and further actived the electrochemical performance.

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KEYWORDS: Mo-Fe selenide, self-supported electrocatalyst, hydrogen evolution, heterointerface, carbon fiber paper

1. INTRODUCTION Under the intense issue on fossil fuels and environment these years, electrochemical water splitting has been developed as one of the most appealing strategies to produce sustainable hydrogen for the merits of good output and nonpollution.1-5 The key factor of the integrate technology is to develop efficient durable and low-cost electrocatalysts for driving hydrogen evolution reaction (HER). Abundant researches have been reported to explore alternatives to Pt-group precious metals and metal oxides including transition metal phosphides,6-8 nitrides,9-11 carbides,12, 13 sulfides14 and selenides15-19 etc. Among them, few-layer transition metal dichalcogenides (TMDs) exhibit great advantages to HER catalysis, which has been widely approved both in experiments and theoretical calculations.20-22 MoSe2 nanosheets have been discovered as a promising electrocatalyst for HER because the few layer structure could expose rich active sites and powerful stability in acid.23-26 Moreover, the Gibbs free energy of hydrogen adsorption is close to zero on their edges which is favorable to accelerate HER.27 However, its poor conductivity and the serious aggregation or restacking during the fabrication of MoSe2 severely limited the catalytic application.28 Essentially, tuning its electric structure is an effective technique to improve the HER performance of MoSe2.29 On the one hand, the phase engineering of 2H→1T conversion can address this issue. MoSe2 was fabricated in two phases of 2H (trigonal

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prismatic) and 1T (octahedral coordination) according to the arrangement of Se atoms. Pumera et al. concluded that MoSe2 is potential in HER catalysis if an efficient chemical exfoliation and consequently a larger portion of the metallic 1T phase is produced.30 Xu’s group also discussed the striking HER kinetic metrics of 1T-MoSe2 over 2H-MoSe2, which exhibited a low onset potential of only 60 mV and a small Tafel slope of 78 mV dec−1.31 On the other hand, hybridizing electrocatalysts has also been intensively investigated to overcome the barrier in pioneering works. Generally, interface of hybrids contributes synergistic effect of chemical and electronic couplings due to the different band structures of different semiconductors. Electrons can be easily transferred through the interface and bring about the electronic redistribution/modulation.14, 32 We thus expect that the HER performance of MoSe2 can be improved by coupling diverse conductive component (e.g. FeSe2) to form a hybrid electrocatalyst, which served as electronic transport corridors to enhance the conductivity as well.33, 34 Three-dimensional (3D) heterostructure obtained by nanoengineering was verified to be effective to expose more active sites and facilitate electrolyte and gas diffusion.35 Since it possesses high electrical conductivity, robust mechanical and chemical stability in electrolyte solution, carbon fiber paper has been utilized widely to assemble integrated electrocatalysts with self-supporting property.36 This kind of self-supporting heterostructure endows uniform dispersion of nanomaterials on the surface and prevents their aggregation, which avoids the addition of binders and the introduction of undesirable interface and extra resistance. Moreover, the protuberances on the carbon fiber surface tend to burst large hydrogen gas bubbles

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into smaller ones and get away quickly.37 Thus, the catalytic property of active materials worked with carbon fiber paper is also vital. In this work, coupled Mo-Fe selenide composite nanosheets were designed and prepared to improve the HER performance of 1T-MoSe2 on carbon fiber paper via a simple one-step hydrothermal method. During the reaction process, the simultaneous selenization of Fe and Mo was indispensable for the growth of Mo-Fe selenide composite nanosheets. The well-built 3D self-supported Mo-Fe selenide standing on carbon fiber paper (denoted as Mo/Fe(m/n)-Se-CP, where m/n represents the molar ratio of Mo and Fe precursor used in our experiment) with a rough surface contributed fast electron and electrolyte ions transfer during HER. Benefiting from the coupled heterointerfaces between 1T-MoSe2 and FeSe2 tight contact, the integrated electrode exhibited remarkable HER catalytic performance. Impressively, various ratio of Mo/Fe was tuned and the optimized coupled sample required relative low overpotential of 86.9 mV (vs. RHE) to drive 10 mA cm-2 in 0.5 M H2SO4, which is lower by 152.7 and 267.5 mV as compared to bare MoSe2 and FeSe2. A smaller Tafel slope of 57.7 mV dec-1 and excellent long-term durability with slight decay after continuous test for 24 h were also obtained. Our research provides a unique opportunity to enrich our knowledge in design and fabrication of novel HER electrocatalysts by coupled heterojunction.

2. EXPERIMENTAL 2.1 Chemicals and Materials Ferric chloride (FeCl3•6H2O), Sodium molydbate dehydrate (Na2MoO4·2H2O)

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and Sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Co., Ltd. Selenium powder was acquired from Aladdin. The carbon fiber paper was acquired from Shanghai Hesen Electric. Co., Ltd. in China. Nafion (5 wt%) and Pt/C (20%) were bought from Alfa Aesar. All chemicals reagents were used as received without any further purification. 2.2 Synthesis of Mo-Fe selenides One-step hydrothermal reaction was applied to fabricate Mo-Fe selenide nanocomposites. 40 mL of deionized water was purged of oxygen by bubbling nitrogen through for 20 minutes firstly. 2 mmol of NaBH4 and 1 mmol of Se powder were added to the beaker magnetically stirred until the solution was clear. 0.3 mmol of FeCl3•6H2O and 0.3 mmol of Na2MoO4 was dissolved in the above solution. The solution was stirred for another 10 min before being transferred into a Teflon-lined stainless autoclave containing a piece of pre-treated carbon paper (2×3 cm, it was treated by ultrasound in acetone, ethanol, hydrochloric adcid and water in turn for half an hour and calcined by muffle furnace at 500℃ for 24 h, the final carbon paper was sonicated in sulfuric acid and nitric acid in a radio of 1:3 for 2 hours.) and kept at 180℃ for 24 h. A series of hybrid electrodes on carbon paper in different composition were prepared by tuning the molar ratio of Mo and Fe precursor (1/5, 1/3, 1/1, 3/1, 5/1),

which

were

in

terms

of

Mo/Fe(1/5)-Se-CP,

Mo/Fe(1/3)-Se-CP,

Mo/Fe(1/1)-Se-CP, Mo/Fe(3/1)-Se-CP, and Mo/Fe(5/1)-Se-CP, respectively. The corresponding powder samples without carbon paper supporting were also collected. For comparison, pristine MoSe2-CP, FeSe2-CP and the corresponding powder samples

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were prepared as well. In the process, the total molar mass (0.6 mmol) of precursors were fixed in constant. The resultant was washed with water and ethanol for three times before being dried at 60℃ overnight. 2.3 Characterizations The X-ray diffraction (XRD) patterns were carried out on a Philips X-Pert diffractometer with Cu Kα radiation (λ = 0.15418 nm) at a voltage of 40 kV and a current of 40 mA. The scanning electron microscopy (SEM) and corresponding energy-dispersive spectroscopy (EDS) were obtained by a field emission scanning electron microscope (SEM, Hitachi S-480 equipped with energy dispersive X-ray spectroscopy). X-Ray photoelectron spectroscopy (XPS) spectra were performed on a VG ESCALABMK II spectrometer using an Al Kα (1486.6 eV) photon source. Transmission electron microscopy (TEM), high-resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) were conducted on a JEM-2100UHR transmission microscope (JEOL, Japan) operated at 200 kV. The elemental mapping were executed on a JEOL 2100F. Raman spectra were collected by a LabRAM HR Evolution (Horiba, Japan) instrument. 2.4 Electrochemical measurements The HER measurements were carried out in a typical three-electrode system on a CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai), where carbon rod and saturated calomel electrode were used as the counter electrode and reference electrode, respectively. Support electrode can be used directly as the working electrode, powders to be dropped to the glassy carbon electrode (GCE: 5mm

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in diameter, loading density 0.2 mg cm-2). The catalyst loading on carbon paper is about 0.19 mg cm-2. Linear sweep voltammetry (LSV), cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) were performed in N2-saturated 0.5 M H2SO4 solution.

3. RESULTS AND DISCUSSION The carbon paper supported electrodes with various compositions were prepared by one-step hydrothermal reaction. The morphology of the samples was first characterized by SEM and TEM. The commercial carbon paper in Figure 1a after pretreatment was examined for comparison. As for the single component catalysts, numerous FeSe2 nanoparticles (Figure 1b) and MoSe2 nanoparticles (Figure 1c) were deposited on the surface of carbon paper as embossment. A typical sample of Mo/Fe(1/1)-Se-CP were characterized and illustrated in Figure 1d. We can see the uniformly dispersed nanosheets with about 20 nm in thickness attached on the surface of bare carbon paper. It suggested that FeSe2 and MoSe2 would have interacted resulting in ultrathin morphology. These unstacked nanosheets were expected to expose more active sites than nanoparticles and facilitate the transfer of electrons by contacting with electrolyte ions in large area, indicating high performance in electrocatalysis. Similarly, it can be seen from Figure S1 that the other samples in various proportion of Mo/Fe also exhibited analogical morphologies. Generally, with the dosage of Mo source increasing, the amount of active materials deposited on carbon paper gradually decreased, revealing the controllability in the composition and catalytic activity. The advantage of this growth in-situ is that the substrate can

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effectively control the development of its topography and enhance catalytic activity on account of supernal conductivity. The active materials were separated from the substrate of carbon paper for TEM test. As shown in Figure 1e, the typical Mo/Fe(1/1)-Se-CP seems like nanoflowers assembled by ultrathin nanosheets. A closer look at the nanosheet attached to the carbon paper (Figure 1f) reveals the incorporated FeSe2 (011) and MoSe2 (102) and (002) with the lattice distance of 0.305, 0.261 and 0.580 nm, respectively. However, it is difficult to identify each phase or the boundary of different components, indicating a high degree of consistency. The coaxial rings illustrated in the selected area electron diffraction (SAED) pattern reveals the polycrystalline structure of the powder. FeSe2 (121), FeSe2 (211) and MoSe2 (102) of can be indexed, which is consistent with the XRD spectrum. Figure 1h present the elemental mapping tests of Mo-Fe selenide powder separated from Mo/Fe(1/1)-Se-CP. All elements of Fe, Mo and Se were successfully identified and have uniform distributions. The actual atomic ratios of Fe and Mo obtained from adding different precursors were collected and listed in Table S1. As expected, the amount of Mo tested increased with the more Mo precursors used. For comparison, the morphologies of pure MoSe2 and FeSe2 were characterized as shown in Figure S3. According to the SEM images of MoSe2 and FeSe2 stripped from carbon paper, MoSe2 nanosheets are reunited and have a rough surface, while FeSe2 were smooth nanosheets and stacked together. The thickness of the FeSe2 nanosheets is about 20 nm, and the lateral dimension is 50-150 nm. The TEM images

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of the MoSe2 and FeSe2 powders are consistent with the SEM images. The deep color parts indicate the overlap of the nanosheets. In addition, because of the poor crystallinity of MoSe2, the crystal surface is not obvious in Figure S3e, while FeSe2 in Figure S3f shows a lattice spacing of 0.248 nm representing FeSe2 (120).

Figure 1. SEM images of (a) carbon paper, (b) FeSe2-CP, (c) MoSe2-CP, and (d) Mo/Fe(1/1)-Se-CP. (e) TEM and (f) HRTEM images of Mo-Fe selenides separated from Mo/Fe(1/1)-Se-CP. (g) SAED pattern and (h) elemental mapping distribution of Mo-Fe selenides separated from Mo/Fe(1/1)-Se-CP.

The crystalline structure of the resultant samples was characterized by XRD. As shown in Figure 2a, most of the diffraction peaks were assigned to FeSe2 (JCPDS no. 82-0269) except the two peaks at 26.54º and 54.66º, which corresponded to (002) and (004) planes of graphite carbon. However, no typical peaks of MoSe2 were observed possibly due to its poor crystallinity. For comparison, the powder samples were prepared without carbon paper supporting and characterized as shown in Figure S4. With the increase of Mo precursor used, the crystallinity of the products decreased. 9



However, both FeSe2 and MoSe2 could be carefully identified from their typical diffraction peaks. The diffraction angle of 15.51º and 34.24º correspond to the (002) and (102) crystal faces of MoSe2 (JCPDS no. 17-0887) respectively while other diffraction peaks belong to FeSe2 in Mo/Fe(1/1)-Se powder. Here the (002) crystal face of MoSe2 has been angularly offset. It can be calculated that the interlayer spacing offset angle is 0.573 nm according to the Prague formula: 2d sin ߠ = nλ (therein, d represents the interplanar spacing, θ represents the diffraction angle of one-half and nλ is a constant for a test instrument). This result is in keeping with the value of HRTEM approximately. It can be seen that two peaks of MoSe2 intensified as the addition of Sodium molybdenate dehydrate increasing from Mo/Fe(1/5)-Se to Mo/Fe(1/3)-Se and then to Mo/Fe(1/1)-Se and the crystallinity of the samples deteriorate when the molybdenum precursor used is more than the iron precursor. Pristine MoSe2 can also be obtained when no iron precursor was used and interestingly it is in metallic 1T phase. The structure and composition was further confirmed by Raman spectrum presented in Figure 2b. All the peaks obtained from Mo/Fe(1/1)-Se-CP can be assigned to the typical peaks obtained from pristine FeSe2 and MoSe2. Also the peaks at 350 cm-1 and 485 cm-1 as well as those of MoSe2-CP further confirmed the metastable metallic 1T phase of MoSe2 in our products. The peak in FeSe2-CP observed at 215 cm-1 appears due to shakings between Se-Se or stretching vibrations of them or both while the peaks at 276 cm-1 and 393 cm-1 show absorption which high surface activity of nanoparticles of FeSe2-CP affected by amount.38

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Figure 2. (a) XRD and (b) Raman patterns of Mo/Fe(1/1)-Se-CP, MoSe2, FeSe2.

XPS spectra were employed to verify the composition and surface state of the aimed product. C, O, Se, Mo, and Fe elements can be identified in survey spectrum of Figure S5. In addition, Figure. 3a and b present the high-revolution Fe 2p and Mo 3d in XPS spectra of Mo/Fe(1/1)-Se-CP together with pristine FeSe2-CP and MoSe2-CP samples, respectively. The Fe 2p spectrum of pristine FeSe2-CP can also be deconvoluted into Fe 2p3/2 and Fe 2p1/2 at 706.4 and 719.2 eV.39 The pristine MoSe2-CP sample show double peaks at binding energies of 228.3 and 231.3 eV for Mo 3d5/2 and Mo 3d3/2, corresponding to Mo4+ of 1T-MoSe2 respectively.28 Notably, the Fe 2p binding energy of Mo/Fe(1/1)-Se-CP shifted to the other direction compared with that of pristine FeSe2-CP,40 while the Mo 3d binding energy of Mo/Fe(1/1)-Se-CP decreased to lower value slightly by 0.3 eV compared with that of pristine MoSe2-CP. The changes in the binding energies for Mo/Fe(1/1)-Se-CP indicates that there are electrons transferred from FeSe2 to MoSe2 in the coupled hybrid. It also brought about the change in binding energy of Se 3d as shown in Figure 3c.

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Figure 3. XPS high-revolution scans of (a) Fe 2p in Mo/Fe(1/1)-Se-CP and bare FeSe2-CP , (b) Mo 3d in Mo/Fe(1/1)-Se-CP and bare MoSe2-CP, and (c) Se 3d in Mo/Fe(1/1)-Se-CP, bare FeSe2-CP and bare MoSe2-CP.

The electrocatalytic HER performances of the samples in different composition were measured in a standard three electrode system in 0.5M H2SO4 with Nitrogen saturation. The loading densities of the free-standing Mo/Fe-based samples were listed in Table S2 in Supporting Information. As shown in Figure 4a, commercial Pt/C exhibited good activity with overpotential near zero from the polarization curve while the bare carbon paper exhibited poor catalytic property with current changing till it was operated at a high potential. The integrated electrodes decorated with selenides in different compositions showed apparent HER activity. The coupled Mo/Fe(1/1)-Se-CP could drive acidic HER with a small onset potential of only 50 mV, which was superior to that of FeSe2-CP (225 mV), MoSe2-CP (128 mV) and many other non-precious metal electrocatalysts in acid. Typically, the overpotential at 10 mA cm-2 and 100 mA cm-2 was collected for comparison in Figure 4b. The enhanced HER catalytic activity was regulated by verifying the ratio of Mo/Fe precursor used, and specially, Mo/Fe(1/1)-Se-CP produced significant enhancement beyond others (Figure S6).

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Moreover, the HER kinetics of the above electrocatalysts were investigated by extracting the slopes from the Tafel plots in Figure 4c. The Tafel plots were fitted to the Tafel equation (η = a + blogj), in which η is the overpotential, j is the current density, and b is the Tafel slopes.41-44 The results of Tafel slopes followed the same trends to those of polarization curves. The Tafel slope were 57.7, 83, 109.7 and 201.1 mV dec-1 for Mo/Fe(1/1)-Se-CP, MoSe2-CP, FeSe2-CP and bare CP, respectively. According to previous reports, the Tafel slopes were in the range of 50-120 mV dec−1, indicating the HER may proceed via a Volmer–Heyrovsky mechanism.45, 46 The small Tafel slope value of Mo/Fe(1/1)-Se-CP revealed not only a favorable electron transfer kinetics but also a HER catalytic mechanism of Volmer-Heyrovsky mechanism, and the rate-limiting step is the electrochemical desorption step. The electrochemical active area was calculated according to the CV curves in the potential window from 0.562 to 0.762 V vs. RHE (Figure S8), which had a liner relationship with the electrochemical double layer capacitance (Cdl). The Cdl of Mo/Fe(1/1)-Se-CP was 0.672 F cm-2 (Figure 4d), which was much higher than that of FeSe2-CP (0.245 F cm-2), MoSe2-CP (0.389 F cm-2) and other Mo/Fe-Se-CP electrocatalysts (Figure S6d). The small Tafel slope and large Cdl suggested that Mo/Fe(1/1)-Se-CP had superior HER catalytic kinetics and activity. Electrochemical impedance spectroscopy (EIS) measurements are used to survey the interface reactions and electrode kinetics in the HER process. Figure S10 displays the Nyquist plots of different catalysts. By fitting the experimental date to equivalent circuit medel (Figure S10, inset), the serious resistance (Rs) and charge transfer resistance (Rct) can be determined. The

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Mo/Fe(1/1)-Se-CP, MoSe2-CP and FeSe2-CP possess Rs values of 1.94, 2.49 and 2.97 Ω, and Rct values of 5.3, 10.2 and 17.3 Ω, respectively. The smaller Rs and Rct values indicate that the Mo/Fe(1/1)-Se-CP has better electronic conductivity and a quicker charge transfer. Thus, electrons can be delivered directly and efficiently between electrocatalyst and electrolyte in the HER process, that contributes to a better HER activity.

Figure 4. (a) LSV curves and (b) Comparison of the overpotential at current densities of 10 mA cm-2 and 100 mA cm-2 obtained from 20% Pt/C, Mo/Fe(1/1)-Se-CP, MoSe2-CP, FeSe2-CP and bare CP. (c) The corresponding Tafel plots of Mo/Fe(1/1)-Se-CP, MoSe2-CP, FeSe2/CP and CP (d)The calculated double-layer capacitances (Cdl) of Mo/Fe(1/1)-Se-CP, MoSe2-CP and FeSe2-CP.

The theoretical and experimental values of Faradaic efficiency were tested by means of water drainage and gas collection. The experimental result is close to the theoretical value, and the Faradaic efficiency is calculated to be 93.1% in Figure S11. A long-term stability test is critical to evaluate a HER electrocatalyst. As shown in the 14


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chronopotentiometry response curves in Figure 5a, the potential increased along with the increase of current density while changed slightly in hours on each current density step. When the current density returned to the initial value, the potential came back to the initial step as well. Mo/Fe(1/1)-Se-CP, MoSe2-CP and FeSe2-CP electrodes lost 8.4%, 11.3% and 13.7% activity in potential collected from the same current density of 10 mA cm-2 at 2 hours and 22 hours, respectively, indicating the coupled hybrid achieved a higher stability. In order to have a more intuitive understanding, the polarization curves were measured before and after the 24 h test. Mo/Fe(1/1)-Se-CP showed negligible changes in LSV after 24 h of testing while the overpotential for FeSe2-CP and MoSe2-CP increased obviously at the same current density. In order to further understand its stability, we characterized the sample collected after stability test by SEM, TEM and XPS measurements. No obvious morphology (Figure S12) and composition state (Figure S13) changes were observed after long-term HER catalysis. As the experimental results mentioned above, it can be concluded that the superior HER catalytic properties of Mo/Fe(1/1)-Se-CP originated from the ultra-thin nanosheets array architecture, large surface area, and unique incorporation of FeSe2 and MoSe2. Firstly, homogeneous nanosheets structure of Mo/Fe(1/1)-Se-CP standing on carbon paper can be constructed by adjusting a proper ratio of Mo/Fe, which would expose more active edge sites. It can also provide short diffusion pathways to facilitate mass and charge transport to boost the HER. To make it clear, the Mo/Fe-based active materials in powder (drop-casted on glass carbon electrode with the same loading amount) were performed the electrochemical tests in the same

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condition. Noticeably, the performance of all samples in powder seriously fell behind to those supported on carbon paper, respectively (Figure S7). Secondly, higher conductivity of the hybrids contributes to the enhancement for HER. MoSe2 nanosheets are considered as a promising electrocatalyst for the HER due to the Gibbs free energy of hydrogen adsorption on MoSe2 edges is close to zero. The activity could be further improved by means of higher conductivity. According to the EIS analysis in Figure S10, The smaller Rs and Rct values indicate that the Mo/Fe(1/1)-Se-CP has better electronic conductivity and a quicker charge transfer. Moreover, the aggregation and re-stacking of 2D hybrid nanosheets may be suppressed in size, and more active sites could be exposed in the electrolyte to participate in the HER. Next, Mo/Fe(1/1)-Se-CP with the biggest Cdl of 0.672 F cm-2 is most effective in enlarging the electrocatalytic active surface area in comparison to others. In other words, better exposure and enhanced utilization of electroactive sites on the large active surface of Mo/Fe(1/1)-Se-CP contribute to its significant HER performance. Last but not least, the incorporation of FeSe2 in the coupled hybrid facilitates electrons transferring from FeSe2 to 1T-MoSe2. Consequently, FeSe2 is slightly positively charged together with 1T-MoSe2 being slightly negatively charged.47 The 1T-MoSe2 with injected electrons will be more active in HER by a faster electron transport between electrocatalyst-electrolyte interfaces.48

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Figure 5. (a) Long-term stability test of Mo/Fe(1/1)-Se-CP, MoSe2-CP and FeSe2-CP at different current densities. (b) LSV curves of Mo/Fe(1/1)-Se-CP, MoSe2-CP and FeSe2-CP for HER before and after 24 hours.

4. CONCLUSION In summary, we successfully constructed the coupled heterostructure of Mo-Fe selenides on carbon paper and demonstrated as efficient electrocatalysts for HER via a one-step hydrothermal reaction. The composition, structure and activity can be easily tuned by varying the molar ratio of Mo and Fe precursors used. The hybrid Mo/Fe(1/1)-Se-CP exhibits outstanding HER catalytic performance with low overpotential and small Tafel slope. To drive HER with a current density of 100 mA cm-2, the overpotential of 158.5 mV is required for the Mo/Fe(1/1)-Se-CP, which is much lower to 1T-MoSe2 (227.9 mV) and FeSe2 (363.7 mV). The advanced HER activities of Mo-Fe selenides on carbon paper is attributed to the heterostructure as a integrated electrode, metallic phase of MoSe2, and the interaction at the interface of MoSe2 and FeSe2 with electronic modulation. This work offers promising features of constructing semiconductor heterointerface for potential application as non-precious metal water-splitting electrocatalysts.

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ASSOCIATED CONTENT Supporting Information Available: TEM images, XRD patterns, CV curves and Raman analysis of Mo/Fe(1/1)-Se-CP and other control samples. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors

* E-mail: [email protected]; [email protected] Notes

The authors declare no competing financial interest. ACKOWLEDGEMENTS The authors gratefully acknowledge the financial support by National Natural Science Foundation

of

China

(ZR2017MA024

and

(21471160),

Shandong

ZR2017QB015),

Natural

PetroChina

Science

Foundation

Innovation

Foundation

(2016D-5007-0401) and the Fundamental Research Funds for the Central Universities (18CX07002A, 18CX05011A, 18CX02042A and 15CX08010A).

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Coupled Heterostructure of MoâFe Selenide Nanosheets Supported

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