Ni ultrafine nanoparticles embedded into N,S

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Heterogeneous NiSe2/Ni ultrafine nanoparticles embedded into N,Scodoped carbon framework for pH-universal hydrogen evolution reaction Jing Yu, Meiling Zhang, Jingyuan Liu, Rongrong Chen, Rumin Li, Qi Liu, Limin Zhou, Peili Liu, and Jun Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05628 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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ACS Sustainable Chemistry & Engineering

Heterogeneous NiSe2/Ni ultrafine nanoparticles embedded into N,S-codoped carbon framework for pH-universal hydrogen evolution reaction Jing Yua, Meiling Zhanga, Jingyuan Liua, Rongrong Chena,c, Rumin Lia,b, Qi Liua, Limin Zhoub, Peili Liuc, Jun Wanga,c* a. Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, 145 Nantong Street, Harbin 150001, China. b. Hong Kong Polytechnic University, Interdisciplinary Division of Aeronautical and Aviation Engineering, Kowloon, Hong Kong, China. c. Institute of Advanced Marine Materials, Harbin Engineering University, 145 Nantong Street, Harbin 150001, China. *Corresponding author Email: [email protected]

ABSTRACT Developing noble-metal-free electrocatalysts with high efficiency and low cost in all-pH range is highly desired but challenging. Herein, we constructed the hybrid interface structure of NiSe2 and Ni nanoparticles, and embedded them into N,S-codoped carbon nanosheets framework (NiSe2/Ni-NSC). The partial selenization treatment endows the catalyst with abundant interface, resulting in optimal electronic structure and hydrogen adsorption energy due to the strong interaction between Ni metal and metallic NiSe2. The ultrafine nanoparticles size allows to fully expose the 1

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active edge sites and improve their utilization efficiency. Benefiting from the structure characteristics, interface engineering and high electroconductivity, the NiSe2/Ni-NSC exhibits superior and stable hydrogen evolution performances in pH-universal media, especially in acidic solution with an overpotential as low as 53 mV to achieve 10 mA cm-2 current density, compared favorably with most Pt-free catalysts. Our work may pave a novel pathway to design heterostructure catalysts with high performance toward hydrogen evolution in all-pH range. Keywords: ultrafine NiSe2/Ni nanoparticles, N,S-coped carbon, interface engineering, hydrogen evolution reaction, pH-universal solutions

INTRODUCTION Electrochemical water splitting to obtain hydrogen, which acts as a sustainable energy carrier with high energy density and environmental friendliness, has become one of the promising approaches to address the ongoing concerns on energy crisis and environmental pollution.1-4 To date, Pt-based catalysts still perform the most state-of-the-art performance for hydrogen evolution reaction (HER).5-7 Nevertheless, the scarcity and high cost hammer their wide application. Consequently, developing noble-metal-free electrocatalysts to reduce the energy barrier of HER is highly urgent but challenging. Recently, a range of transition metal selenides8-11 particularly nickel selenides12,13, have been intensively explored as attractive candidates for HER due to their intrinsically metallic behavior, owning fast charge transfer and high electrical conductivity. Xu et al.14 reported metallic Ni3Se2 nanoforest grown on Ni foam with hydrophilic and aerphobic surface, which exhibited excellent electrocatalytic 2


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performances toward bifunctional water splitting. It required an overpotential of 203 mV to reach a current density of 10 mA cm-2, while that of commercial Pt/C is about 70 mV. Although some achievements have been made, the HER performances of selenides are still unfavorable to compare with Pt-based materials. There is much space to improve the HER activity of selenides. Many strategies have been devoted to optimize the catalytic ability of initial material, such as morphological modulation15,16, alloying17-19, electrical structure engineering20,21 and interface engineering22,23. In view of the typical surface process for catalytic reaction, the rational design of the catalysts size to nanoscale enables to fully expose the superficial active sites, as well as to tune their surface configuration and electrical structure due to the quantum-size effect.24-26 Nevertheless, the increasing surface energy would lead to extreme instability and uncontrollable aggregation, making it difficult to reduce the diameter of nanoparticles.25,27,28 It is highly desirable to explore suitable carrier to support and disperse nanometer-sized particles. Furthermore, interface engineering is also an important avenue in improving electrocatalytic activity. The heterostructure catalysts provide the synergistic effect from multiple components, inducing the electron reconfigured interfaces and various active sites.29,30 The established heterointerfaces in heterogeneous nanostructures could facilitate rapid interfacial charge transport, thus leading to significantly enhanced catalytic performances. More important, the electronic interactions help to optimize the binding strength toward hydrogen adatoms, and then to obtain hydrogen adsorption Gibbs free energy close to 0.31,32 This is conducive to the moderate 3



adsorption and facile desorption of intermediates, thus providing an convenient proton-electron-transfer approach.33 For example, Li et al.34 synthesized core−shell metal−phosphide nanocomposite anchored on CNTs. In the Fe@FeP, the metal core could efficiently tune the surface electronic states of phosphide shell, as well as the related interactions with the intermediates during electrocatalytic process, causing optimized hydrogen adsorption energy, thus achieving superior HER activity. Inspired by the above concerns, we designed a novel NiSe2/Ni heterostructure with ultrafine particles size and embedded them into N,S-codoped carbon framework (NiSe2/Ni-NSC) by the partial selenization of the as-prepared Ni nanoparticles embedded N,S-codoped carbon framework precursor (Ni-NSC). The curved N,S-codoped carbon nanosheets 3D networks enable NiSe2/Ni particles to load uniformly, and prevent the particles from aggregation. The coupling of Ni metal and metallic NiSe2 creates an interface with regulated electronic interaction and optimized hydrogen adsorption free energy, benefiting the electrocatalysis. We accessed the HER performances of NiSe2/Ni-NSC in all-pH range. The resulting NiSe2/Ni-NSC exhibits outstanding catalytic activity and robust stability, which requires low overpotentials of 53, 168 and 116 mV to afford 10 mA cm-2 current density in acidic, neutral and basic media, respectively, representing one of the most excellent Pt-free catalysts for hydrogen evolution.

EXPERIMENTAL SECTION Synthesis of Ni nanoparticles embedded into N,S-codoped carbon framework Typically, 1.0 g polyvinylpyrrolidone (PVP), 2.0 g Ni(NO3)2·6H2O and 0.1 g 4


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thiourea were dissolved in 30 mL deionized water to form a clear solution. The mixture solution was dried at 90 °C overnight. And then the resultant solid was heated to 700 °C for 1 h in N2 atmosphere. Ni nanoparticles embedded into N,S-codoped carbon frameworks were obtained and named as Ni-NSC. Ni nanoparticles embedded into N-doped carbon frameworks (Ni-NC) were obtained by a similar process without the addition of thiourea. Synthesis of partially selenized Ni nanoparticles embedded into N,S-codoped carbon framework 50 mg Ni-NSC and 200 mg Se powder were placed into a tube furnace with the Se powder at the upstream. Subsequently, the furnace was heated to 350 °C with a ramp rate of 3 °C min-1 in N2 flow and kept for 2 h. The final black powder, namely partially selenized Ni nanoparticles embedded into N,S-codoped carbon framework, was collected and denoted as NiSe2/Ni-NSC. In contrast, completely selenized Ni nanoparticles embedded into N,S-codoped carbon framework (NiSe2-NSC) was obtained by heating Ni-NSC at 450 °C. Partially selenized Ni nanoparticles embedded N-doped carbon framework (NiSe2/Ni-NC) was prepared by heating Ni-NC at 350 °C. Material characterization The crystalline phase was recorded by XRD analysis on RigakuTTR-III X-ray diffractometer with a Cu Kα X-ray radiation (λ = 0.15418 nm). Scanning electron microscopy (SEM) was performed by using a FEI Quanta 200F scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution TEM 5



(HRTEM) were imaged through JEOL JEM-2100 transmission electron microscope. X-ray photoelectron spectroscopy (XPS) was carried out at PHI 5700 ESCA X-ray photoelectron spectrometer. Raman measurement was collected by Jobin Yvon Horiba LaBRAM HR800 spectrometer with a 532 nm laser source. Electrochemical measurement 3 mm glassy carbon electrode (GCE) loaded by active materials was using as working electrode in all electrochemical measurements. The preparation process of working electrode is presented as follows. 5 mg catalyst was added into 1 mL mixed solution containing 40 μL of 5 wt.% Nafion solution, 480 μL isopropanol and 480 μL water. The suspension was ultrasonicated to obtain a uniform ink. 5 μL catalyst ink was pipetted in 3 mm GCE, exhibiting a catalyst loading of 0.354 mg cm-2. The HER performances were studied by a three-electrode system on CHI 760 electrochemical workstation. A graphitic rod electrode was used as counter electrode. The saturated calomel electrode (SCE) was used as reference electrode in N2-saturated 0.5 M H2SO4 and 1 M PBS, while that in 1 M KOH was Hg/HgO electrode. All potentials in this report were converted to reversible hydrogen electrode (RHE). The polarization curves were corrected by 95% iR calibration. The scan rate for linear scan voltammetry (LSV) tests was 1 mV s-1. Electrochemical impedance spectroscopy (EIS) at the overpotential of 300 mV was recorded from 105 Hz to 10-2 Hz with 5 mV AC voltage. In order to estimate the electrochemically active surface area for the as-prepared catalysts, cyclic voltammograms (CVs) were performed at a series of scan rates from 20 to 200 mV s-1. Long-term stability of the catalyst was 6


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assessed by chronopotentiometry and accelerated CV cycles.

RESULTS AND DISCUSSION The synthesis of partially selenized ultrafine Ni nanoparticles embedded into N,S-codoped carbon nanosheets framework is illustrated in Scheme 1. First, the mixed solution containing PVP, nickel nitrate and thiourea was dried to obtain uniformly mixed solid. Notably, the mixtures would undergo large volume expansion during the evaporation of liquid compared with the solution of PVP and nickel nitrate, as depicted in Figure S1. By contrast, similar phenomenon was not observed in the solution of PVP and thiourea. These indicate the possible formation of Ni-S compounds by drawing Ni ions and partial S source, followed the accumulation to form clusters, and simultaneously inducing the volume expansion with gradually lost liquid. The large volume ensures the facile gas release because of the decomposition of PVP along with nickel nitrate and thiourea during high-temperature carbonization, preserving the Ni-S clusters to form the oval shape. Carbon matrix was blown up to form highly open and interlinked structure with N and S co-doped. Meanwhile, the ultrafine Ni nanoparticles are obtained and uniformly encapsulated into N,S-codoped carbon substrate without destroying the oval boundaries via in-situ carbothermal reduction reaction.35 Subsequently, the formed Ni-NSC composite was subjected for selenization in N2 atmosphere. Due to the low heating temperature, part of Ni-NSC was selenized and formed the hybrid of NiSe2/Ni-NSC with abundant interface. Simultaneously, the initial structure of Ni-NSC precursor was well remained. Yet, at higher temperature of 450 °C, Ni-NSC would be completely selenized to generate 7



NiSe2-NSC.

Scheme 1. Schematic illustration for the preparation of NiSe2/Ni-NSC hybrid. Figure S2a presents the powder XRD pattern of the as-prepared Ni-NSC. Three strong diffraction peaks at 2θ = 44.5, 51.8 and 76.3° are well defined, corresponding to (111), (200) and (220) facets of nickel metal (JCPDS no. 87-0712). SEM images (Figures. S2b and S3a) reveal the interconnected nanosheet framework with open macropores structure. Interestingly, numerous oval plates are deposited inside the cell, which is encircled by crosslinked carbon nanosheets, just like the eggs lie quietly in the nest. Enlarged SEM image in Figure S2c illustrates that these oval plates consist of numerous ultrafine nanoparticles, and no particles are observed outside the region of oval. As a contrast, Ni-NC exhibits the similar 3D framework architecture to Ni-NSC, while numerous nanoparticles are deposited on the curved nanosheets 8


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(Figure S5). The formation of oval plates may be ascribed to the coupling of Ni source and partial S source during the evaporation process, and subsequent carbonization treatment preserves the initial structure of Ni-S seed. TEM analysis further confirms the oval shape of Ni-NSC constituted by lots of nanoparticles. However, many nanoparticles are found besides oval region, which is due to the strong ultrasound during the sample preparation for TEM measurement, inducing the ultrafine nanoparticles to redeposit on the surface of carbon nanosheets. High-magnification TEM image clearly shows that these ultrafine nanocrystals with the size of several nanometers are randomly anchored within N,S-codoped carbon nanosheets. HRTEM observation presents a well-defined lattice fringes of 0.206 nm, assigned to the (111) plane of Ni metal. In addition, graphitic carbon shell is found to surround the metallic Ni core. The as-prepared Ni-NSC with unique configuration is employed as the precursor to fabricate the partially selenized NiSe2/Ni-NSC.

Figure 1. (a) XRD patterns of NiSe2-NSC and NiSe2/Ni-NSC. (b, c) SEM, (d, e) 9



TEM and (f) HRTEM images of NiSe2/Ni-NSC. The yellow species in (c) is marked as NiSe2/Ni nanoparticles.

The existence of Ni species gives the feasibility to tailor the initial Ni-NSC by appropriate chemical conversion. After low-temperature selenization, the resultant shows the mixed phases of Ni metal and metallic NiSe2, as illustrated by the XRD pattern in Figure 1a, indicating the occurrence of partial selenization of Ni-NSC precursor. SEM analysis displays 3D framework structure with oval plates located in the cell, similar to Ni-NSC precursor. Meanwhile, no nanoparticles are found outside the oval range. It is worth noting that in the SEM images of NiSe2/Ni-NC (Figure S5c, d), large particles are observed on the surface of carbon framework, and even many particles are accumulated to form clusters, which is not favorable to the charge/mass transfer, thus restricting the efficient electrocatalysis. Consequently, we surmise the oval frame of Ni-NSC would enable to protect the inner ultrafine nanoparticles from the external atmosphere to some degree, leading to moderate selenization procedure. When the heat treatment temperature is increased to 450 °C, Ni-NSC would be completely selenized to obtain cubic NiSe2 (JCPDS no. 89-7161). The oval frame is destroyed to form large selenides particles due to high-energy Se vapour (Figure S6). TEM measurements of NiSe2/Ni-NSC further verify the shape of Ni-NSC is well maintained. Nevertheless, the HRTEM image in Figure 1f exhibits two clear lattice fringes with the distances of 0.196 and 0.263 nm, in agreement with the reflection of (111) plane of Ni and (210) facet of NiSe2, further confirming the partial selenization 10


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of Ni-NSC precursor. Figure S7 gives the Raman spectrum of NiSe2/Ni-NSC. The presence of strong D band at 1354 cm-2 reveals the obvious structural defects occurred in the graphitic architecture. The intensity ratio of ID/IG is about 0.95, indicating the rich disordered structure in carbon framework because of N,S codoping.20 The weight percent of NiSe2, Ni, and N,S-codoped carbon in NiSe2/Ni-NSC is calculated to be 45.35%, 23.95%, 30.70% according to the TG analysis (Figure S8). The highly open frameworks with curved nanosheets of NiSe2/Ni-NSC enable to own a large surface area, providing a convenient electron transfer and ions diffusion pathway, as well as benefiting the exposure of loaded active catalyst. The ultrafine nanoparticles limited in the ovals make the active edge sites to be utilized fully and efficiently. The surface chemical composition and valence states of NiSe2/Ni-NSC were further examined by XPS spectra. As displayed in Figure 2a, the XPS survey spectrum of NiSe2/Ni-NSC exhibits all expected elements, such as C, O, N, S, Ni and Se. The C 1s spectrum contains two signals at 284. 8 and 285.6 eV, corresponding to sp2 hybridized carbon species and C-N/C-S, respectively.36 In Figure 2c, the N 1s spectrum are split into three peaks centered at 398.6, 399.4 and 400.9 eV, related to three types N species, namely pyridinic N, pyrrolic N and graphitic N.37 The presence of pyridinic N enables to enhance the electroconductivity of carbon framework by donating a p electron to the aromatic π system of carbon material, and graphitic N could affect the chemisorption route to intermediate by inducing charge delocalization, thus facilitating the electrocatalytic process.36,38 In S 2p region, S 2p3/2 and 2p1/2 signals are observed at 161.1 and 162.2 eV, associated with the surface oxidized S species. The 11



electronic structure of carbon framework could be tuned efficiently by the codoping of N and S atoms, thus improving the inherent catalytic activity.39 With respect to Ni 2p3/2, the signal located at 852.8 eV is originated from Ni0, which is in accordance with metallic Ni in Ni-NSC. The peak at 853.9 eV is attributed to Ni2+ from selenides. The slightly positive shift in binding energy (0.5 eV) compared with that of NiSe2-NSC indicates the higher partial positive charge due to the bonding with metallic Ni, thus leading to the promoted electron transfer ability.19 Meanwhile, the peak at 855.4 eV is derived from Ni oxides owing to surface oxidation when exposed to air. Thanks to the coexistence of Ni0 and Ni2+ with various valence states, the hybrid containing metallic element and related selenides is favorable to improve their electrochemical performance.40 In the Se 3d region, the binding energies at 54.8 and 55.7 eV suggest the typical reflections from Se 3d5/2 and 3d3/2 in selenides, and the peak at 58.9 eV reveals the presence of superficial Se with a high oxidation state.41 Overall, these results suggest the successful preparation of practically selenized nanocomposites composed of ultrafine NiSe2/Ni nanoparticles embedded within N,S-codoped 3D carbon framework, encouraging NiSe2/Ni-NSC as the promising HER electrocatalyst in all-pH range.

12


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Figure 2. (a) XPS survey spectrum, (b) C 1s, (c) N 1s and (d) S 2p spectra of NiSe2/Ni-NSC. (e) Ni 2p3/2 spectra of Ni-NSC, NiSe2/Ni-NSC and NiSe2-NSC. (f) S 3d spectra of NiSe2/Ni-NSC and NiSe2-NSC.

We first assessed the HER performance of NiSe2/Ni-NSC in 0.5 M H2SO4. For comparison, Ni-NC, Ni-NSC, NiSe2-NSC and NiSe2/Ni-NC were also measured. Figure 3a presents the iR corrected polarization curves. The commercial Pt/C catalyst possesses the best electrocatalytic HER ability. Ni-NSC exhibits obviously better catalytic activity than Ni-NC with an overpotential of 200 mV to deliver a current density of 10 mA cm-2, while that of Ni-NC is 260 mV, indicating the positive effect of S doping on the catalytic ability. After partial selenization, NiSe2/Ni-NSC possesses significantly enhanced catalytic performance, which only requires the overpotential of 53 mV at 10 mA cm-2, much lower than that of Ni-NSC, completely selenized NiSe2-NSC (188 mV) and NiSe2/Ni-NC (92 mV). Due to the similar morphology of NiSe2-NSC and NiSe2/Ni-NC (Figures. S4 and S5), the more superior 13



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catalytic activity of NiSe2/Ni-NC is ascribed to the abundant interface between Ni and NiSe2, benefiting to the electrocatalytic reactions. Besides the interface engineering, the unique structure features of NiSe2/Ni-NSC also contribute favorably to the HER performance. The uniformly dispersed ultrafine nanoparticles provide accessible active surface for hydrogen adsorption. Even if compared with the recently reported state-of-the-art catalysts in acid media, NiSe2/Ni-NSC is also placed at the top of noble-metal-free

materials

(Table

S2),

e.g.

NiSe2/Ni

(143

mV)13,

CoSe2-MoSe2/rGO-C (195 mV)42, Ni0.33Co0.67Se2 (65 mV)43, WS2(1–x)Se2x/NiSe2 (88 mV)44, holey NiCoP NS (80 mV)45, Ni2P@NPCNFs (63.2 mV)46, Co9S8/1L MoS2 (97 mV)47, Fe/P/C (256 mV)48, and so on. Subsequently, the cathodic current densities of NiSe2/Ni-NSC present a fast increasing response with the increasing potentials. The HER kinetics were evaluated by Tafel plots derived from the corresponding LSV curves. As shown in Figure 3b, NiSe2/Ni-NSC exhibits favorable hydrogen evolution kinetics with a Tafel slope of 68 mV dec-1, revealing a Volmer-Heyrovsky mechanism, in which hydrogen desorption is considered as the determining step for HER. The Tafel value is superior to that of other counterparts, such as Ni-NC (227 mV dec-1), Ni-NSC (139 mV dec-1), NiSe2-NSC (174 mV dec-1) and NiSe2/Ni-NC (87 mV dec-1). The smaller Tafel slope of NiSe2/Ni-NSC enables to induce a more rapid growth of cathodic current under a larger overpotential, thus arising a higher catalytic efficiency. The superior HER performance is not only attributed to the unique structure of NiSe2/Ni-NSC, but also the strong interaction between Ni and NiSe2.

14


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Figure 3. HER performances of the as-prepared samples in 0.5 M H2SO4. (a) Polarization curves, (b) corresponding Tafel slopes, (c) Nyquist plots at -0.3 V, (d) capacitive current at 0.1 V (vs RHE) as a function of scan rate. (e) Polarization curves of NiSe2/Ni-NSC at 1st and 2000th cycles. (f) Chronopotentiometric measurements of long-term durability upon NiSe2/Ni-NSC at the current densities of 10 and 100 mA cm-2.

To better illustrate the physical origin of the outstanding HER activity of NiSe2/Ni-NSC, electrochemical impedance spectroscopy (EIS) was performed at -0.3 V and was well-fitted by the equivalent circuit in the inset of Figure 3c. Obviously, NiSe2/Ni-NSC owns the lower charge transfer resistance (Rct) compared to NiSe2-NSC and NiSe2/Ni-NC, revealing the faster electron transfer rate and more favorable reaction kinetics, thus resulting in rapider hydrogen evolution rate. The Rct for NiSe2/Ni-NSC, NiSe2/Ni-NC and NiSe2-NSC are calculated to be 46, 71 and 84 Ω, respectively. This indicates the strong interaction between ultrafine NiSe2/Ni 15



nanoparticles and N,S-codoped carbon matrix, paving a good electron transfer avenue. According to foregoing analysis, ultrafine NiSe2/Ni nanoparticles of several nanometers are uniformly embedded within N,S-codoped carbon with highly open 3D framework, which would endow the hybrid with high surface area. Accordingly, we appraise the electrochemically active surface area (ECSA) by double-layer capacitance (Cdl) tests, arised from the CV curves at different scan rates (Figure S12). As expected, NiSe2/Ni-NSC exhibits the largest Cdl value of 12.1 mF dec-1, which is 1.7, 2.5, 4.3 and 5.3 times that of NiSe2/Ni-NC, NiSe2-NSC, Ni-NSC and Ni-NC, respectively, demonstrating the highest ESCA of NiSe2/Ni-NSC. This could be ascribed to the unique configuration of NiSe2/Ni-NSC, thus inducing fast ionic exchange and maximized exposure of active edge sites. By contrast, the unavoidable aggregation and poor particles dispersion of NiSe2/Ni-NC and NiSe2-NSC hinder their active sites. Durability is also a vital benchmark to evaluate electrocatalysts. In this work, we carried out accelerated CV cycles and chronopotentiometry to examine the long-term stability of NiSe2/Ni-NSC electrode. As depicted in Figure 3e, the polarization curve at 2000th cycle presents the almost identical trend with the initial curve along with a negligible current delay. Moreover, the electrode also shows slight potentials degradation to maintain 10 and 100 mA cm-2 current densities for more than 24 h, indicating the robust long-term durability of NiSe2/Ni-NSC. In addition, Ni element wasn’t defected in the electrolyte after HER test according to ICP analysis, revealing the good structure stability of NiSe2/Ni-NSC. Taken together, NiSe2/Ni-NSC is 16


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highlighted as a promising candidate with remarkable electrocatalytic performances toward hydrogen evolution in acid solution.

Figure 4. (a-c) HER performances of the as-prepared samples in 1 M KOH: (a) Polarization curves, (b) corresponding Tafel slopes, (c) polarization curves of NiSe2/Ni-NSC at 1st and 2000th cycles. (d-f) HER performances of the as-prepared samples in 1 M PBS: (d) Polarization curves, (e) corresponding Tafel slopes, (f) polarization curves of NiSe2/Ni-NSC at 1st and 2000th cycles.

Significantly,

NiSe2/Ni-NSC

also

possesses

superior

hydrogen

evolution

performances in alkaline and neutral media, which are close to the commercial Pt/C catalyst. Figure 4a exhibits the polarization curves of the as-prepared samples in 1 M KOH (pH 14). It is revealed that NiSe2/Ni-NSC owns much lower overpotential of 116 mV to afford 10 mA cm-2 current density than that of NiSe2-NSC (184 mV) and NiSe2/Ni-NC (143) mV. The Tafel slope of NiSe2/Ni-NSC is calculated to be 87 mV dec-1, suggesting a fast reaction kinetics dominated by the Heyrovsky mechanism.9 17



NiSe2/Ni-NSC is also proved to have the lowest charge-transfer resistance compared to NiSe2-NSC and NiSe2/Ni-NC, as shown in Figure S13. To test the durability of NiSe2/Ni-NSC in basic solution, 2000 continuous CV cycles were performed. The LSV curve after 2000 CV (Figure 4c) is compared with the initial one, in which no polarization curve shift is observed. Furthermore, the time-dependent potential curve indicates that NiSe2/Ni-NSC could preserve its HER activity only with an ignored potential increase after the continuous operation for 24 h (Figure S14). These observations encourage NiSe2/Ni-NSC as excellent HER catalyst in basic condition. In the neutral media of 1 M PBS (pH 7), NiSe2/Ni-NSC also has considerable hydrogen evolution activity and good long-term stability. As observed in Figure 4d, an overpotential of 168 mV is required at 10 mA cm-2, while that of NiSe2-NSC is 396 mV and NiSe2/Ni-NC is 224 mV, respectively. Moreover, NiSe2/Ni-NSC owns a smaller Tafel slope of 144 mV dec-1 compared with that of NiSe2-NSC and NiSe2/Ni-NC, indicating the more favorable HER kinetics. The LSV polarization curve at 2000th CV cycle shows no observable change compared to the 1st curve (Figure 4f). In addition, the potential to keep a static current density of 10 mA cm-2 only exhibits a slight degradation after 12 h (Figure S15), suggesting the outstanding long-term stability. In current work, N,S-codoped carbon is mainly used as the substrate to load NiSe2/Ni nanoparticles. In order to further reveal the interface effect between Ni and NiSe2 in the electrocatalytic process, we used density functional theory (DFT) calculations to access the surface adsorption energy of atomic hydrogen (ΔGH*) on 18


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NiSe2/Ni surface, which is well-established as the descriptor to evaluate the HER ability.31 The ideal H atom binding on the surface of catalyst should be moderate, namely not too strong or too weak, presenting a ΔGH* value of 0 eV, which would lead to efficient proton-electron-transfer process.49 To simulate the typical interface characteristics of NiSe2/Ni, as depicted in Figure 5a, we set up a model interface structure containing NiSe2 layers along 210 direction and Ni layers along 111 direction. Figure 5b indicates the calculated ΔGH* value on NiSe2/Ni is about -0.15 eV, which is closer to thermo-neutral (0 eV) than that of NiSe2 (-0.36 eV) and Ni (-0.82 eV), inducing facilitated proton reduction and hydrogen desorption rate.31 The small ΔGH* indicates the reduced reaction barrier on NiSe2/Ni, which is in agreement with the experimental results. In the neutral or basic conditions, the Tafel slopes of NiSe2/Ni-NSC are also much lower than those counterparts, revealing the favorable reaction kinetics of NiSe2/Ni-NSC in all-pH media. The HER activities in neutral or basic solutions are mainly determined by the adsorption of H2O.11 As shown in Figure S16, the interface effect of NiSe2/Ni may lead to the promotion role in H2O adsorption, and then the H-OH bond would be weakened due to the electronic effects from the interaction of Ni center-O atom and dangling Se atom-H atom. The surface Ni sites can facilitate water dissociation. It has already been reported that Ni is favorable to the desorption of OH-.50 The synergistic effect between Ni metal and NiSe2 would result in the reduced energy barriers for the adsorption of both H and H2O, leading to the improved HER performances in all-pH media.

19



Figure 5. (a) Schematic model of NiSe2/Ni heterostructure. (b) DFT calculation of hydrogen adsorption energy on the surface of Ni, NiSe2 and NiSe2/Ni heterostructure.

The above-mentioned results suggest NiSe2/Ni-NSC could be utilized as a pH-universal electrocatalyst with outstanding HER performance and robust stability, which can be ascribed to the cooperative effect from several factors. (1) The strong coupling effect between Ni metal and metallic NiSe2 contribute a lot to the high catalytic activity. The coexistence of Ni and NiSe2 phases in NiSe2/Ni-NSC hybrid induces the formation of NiSe2/Ni heterojunctions, creating abundant available sites at the heterogeneous interface and grain boundaries.41 The electronic structure is moderately tuned to significantly enhance the initial electroconductivity, leading to high-efficient charge transfer. DFT calculations reveal the optimal hydrogen adsorption energy on the surface of NiSe2/Ni. This reduces the reaction energy barriers significantly, thus improving the intrinsic catalytic ability. (2) The oval frames helps to maintain the initial structure of Ni-NSC precursor, forming the ultrafine NiSe2/Ni nanoparticles embedded uniformly into N,S-codoped carbon 20


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framework. The ultrafine sizes enable the catalytic reaction to occur efficiently with the highly utilized edge sites. Furthermore, the coat of carbon shell could protect the inner NiSe2/Ni core from the corrosion of electrolyte, benefiting the long-term stability. (3) The unique 3D framework with curved and interconnected N,S-codoped carbon nanosheet walls endow the hybrid with large surface area, enabling to efficiently load the active transition metal compounds, as well as exposing abundant active sites. The open space encourages the fast charge and electrolyte transfer, as well as quick escape of the generated bubbles from the hybrid surface, thus releasing the occupied active sites. Overall, these effects render NiSe2/Ni-NSC remarkable HER performances in all the acidic, neutral and basic media.

CONCLUSIONS In conclusion, a novel partially selenized Ni particles with ultrafine sizes were embedded into N,S-codoped 3D carbon framework by high-temperature carbonization treatment and subsequent selenization process. The as-prepared NiSe2/Ni-NSC hybrid possesses outstanding catalytic activities and robust stability toward HER in all-pH range, which requires the overpotentials of 53, 168 and 116 mV to deliver 10 mA cm-2 in acidic, neutral and alkaline solutions, respectively. Experimental and theoretical results reveal the strong interaction due to the coupling of metallic Ni and NiSe2. In addition, the unique structure features endow NiSe2/Ni-NSC with more accessible active sites and more convenient change transfer approach. The current research not only provides a pH-universal electrocatalyst for efficient energy storage, but also opens new insights to design heterointerfaces and fabricate ultrafine and 21



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uniformly dispersed nanoparticles.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 51603053 and 51872057), Fundamental Research Funds of the Central University, the Application

Technology

Research

and

Development

Projects

of

Harbin

(2015RAQXJ038), and Defense Industrial Technology Development Program (JCKY2016604C006).

Supporting Information Available: The additional details of chemical structure characterization and HER electrocatalytic performance of the as-prepared samples. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author * [email protected] (J. Wang)

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Table of Contents

The heterogeneous NiSe2/Ni nanoparticles embedded into N,S-codoped carbon exhibit superior HER performances in all-pH media.

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Ni ultrafine nanoparticles embedded into N,S

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