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Greatly boosting electrochemical hydrogen evolution reaction over Ni3S2 nanosheets rationally decorated by Ni3Sn2S2 quantum dots
Applied Catalysis B: Environmental 267 (2020) 118675
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Greatly boosting electrochemical hydrogen evolution reaction over Ni3S2 nanosheets rationally decorated by Ni3Sn2S2 quantum dots
T
Shi-Yu Lua,d,*, Shengwen Lib, Meng Jinc, Jiechang Gaod, Yanning Zhangb,** a
The Beijing Innovation Center for Engineering Science and Advanced Technology (BIC-ESAT), College of Engineering, Peking University, Beijing 100871, China Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China c College of Chemistry & Chemical Engineering, Chongqing University, Chongqing 400044, China d Institute for Clean Energy & Advanced Materials, School of Materials and Energy, Southwest University, Chongqing 400715, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Heterointerfaces Electronic structure control Metal sulfides Earth-abundant hybrid catalyst Hydrogen evolution reaction
The electrode kinetics of hydrogen evolution reaction (HER) greatly relies on both strong water absorption and strong H atom desorption for fast electron transfer while prompting hydrogen evolution, but it is very challenging to achieve due to the tough trading off between water absorption and H-desorption ability of the catalyst. Herein, a unique high-surficial multi-heteroatomic catalytic process is realized by rationally design and tailor Ni3Sn2S2 dots-decorated thin Ni3S2 nanosheets to form sheets-on-sheets array self-supported electrode by simple hydrothermal process. The formed Ni3Sn2S2@Ni3S2-2 NF delivers a superior performance very close to the noble catalyst (Pt/C) at low current densities with an onset-potential of nearly 0 mV and overpotentials of 50.7 mV at 10 mA cm−2 while surprisingly surpassing Pt/C at high current densities. The outstanding HER performance of the catalyst can be ascribed that the rationally tuned multi-heterogeneous interfaces and electronic structure control can realize both strong water absorption and strong H atom desorption to not only significantly promotes fast electron transfer, but also greatly enhances the gas release toward efficient HER. This work holds a great promise to fabricate a non-noble HER catalyst for high-performance close to the noble catalysts such as Pt/C while shedding a light on fundamentals to guide construction of high-surficial heteroatomic multi-heterogeneous catalysts with superior performance.
1. Introduction Hydrogen as a clean energy source with high energy density and zero pollution is a highly desired alternative to the fast exhausted nonrenewable and pollute fossil fuels [1,2]. Electrolytic water splitting is an attractive eco-friendly and sustainable approach to produce pure hydrogen [3–5]. Up to date, platinum (Pt) and its alloys are still the best electrocatalysts for HER and hold the benchmark for HER [6–8]. However, the scarcity, poor stability and high expense of these noble metals catalysts greatly impede the large-scale and industrial water splitting process. Consequently, it is highly worthwhile to design and construct economic and high-performance alternative electrocatalysts. Recently, intensive endeavors have been focused on exploration of nanostructured catalysts based on transition metals and their alloys, such as sulfides, [9–11], selenide [12], carbides [13–15], nitrides [16,17] and phosphides [18–21], as potential noble-metal-free
electrocatalysts. In particular, heazlewoodite nickel sulfides have been widely studied in energy field due to their advantages of abundant reserves and good electron transport ability [22,23]. In most cases, nanoengineering approaches can synthesize materials as catalysts with high surface area and increased catalytic sites [24,25]. The mineral heazlewoodite has been synthesized with various morphologies to greatly improve catalytic activity toward HER in comparison to bulk heazlewoodite [26]. Nevertheless, the performance remains a great gulf from the noble metal-based catalysts. Fundamentally, in the crystal structure of Ni3S2 (101) Ni atoms are five-coordinated and SeHads bonds can readily form in an S-rich environment, but it could also cause a saturated coordination state of S atoms on the catalyst surface resulting in sluggish H2O adsorption. In an ideal HER process in an alkaline medium, a catalyst should have strong water absorption to be effectively activated to break OeH bonds and form H atoms by electrochemical reduction, and easily anchor H atoms on the catalytic sites
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Corresponding author at: The Beijing Innovation Center for Engineering Science and Advanced Technology (BIC-ESAT), College of Engineering, Peking University, Beijing, 100871, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (S.-Y. Lu), [email protected] (Y. Zhang). https://doi.org/10.1016/j.apcatb.2020.118675 Received 5 November 2019; Received in revised form 8 January 2020; Accepted 22 January 2020 Available online 23 January 2020 0926-3373/ © 2020 Elsevier B.V. All rights reserved.
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heazlewoodite Ni3S2 crystallizes can be unambiguously identified by side view of Ni3S2 crystallizes (Fig. 1d). Fig. 1e–g represent the front, top and side view of Ni3Sn2S2 supercell (2 × 2 × 1) crystallization, respectively. The electronegativity of Sn element is slightly more positive than Ni (1.96 vs 1.91) and more negative than S (1.96 vs 2.58). Owing to above natural characteristic, the particular characteristics of Sn element can cause two possible substitution in Ni3Sn2S2: 1) expressing non-metallic property, in which Sn substitutes a portion of Ni atoms and directly forms covalent bonding with S atoms (SneS). 2) Delivering metallic property, in which Sn replaces a part of S atoms to balance the electron density in crystal (NieSn). The substitution of Sn atoms in Ni3S2 plays in a curial role in tuning the electron distribution between Ni and S atoms. In addition, crystal structure of Ni3S2 and Ni3Sn2S2 can all belong to the space group of R-3m and thus can be classified to heazlewoodite, which also helps Sn atom easily being induced into Ni3S2 phase to form Ni3Sn2S2 phase without a crystal structure collapse. The optical images of electrodes (Fig. S1) reveal that the catalysts are uniformly covered on the nickel foam and the color of the electrode changes from light green to black with increasing the amount of SnCl2 introduced. The decoration amount of Ni3Sn2S2 quantum dots into the Ni3S2 nanosheets was well tuned by the quantity additions of SnCl2 (Experimental section). The micromorphology of the delicately tailored Ni3Sn2S2@Ni3S2 NF discerned from scanning electron microscope (SEM) and transmission electron microscope (TEM) with pure Ni3S2 NF as the baseline for comparison. It clearly shows that flexible and porous nanosheets uniformly grow over the nickel foam (Fig. S2) and are 3dimensionally interconnected with formed many void spaces when decorating Ni3Sn2S2 QDs in Ni3S2 in contrast to plain Ni3S2 (Fig. 2a and b). From the high magnification of SEM image (Fig. 2d), it can be unambiguously observed that ultrathin nanosheets are stacked sheet by sheet to assemble a porous 3-D sheets-on-sheets structure. This special nanostructure could be expected to produce coordinately unsaturated metal sites on exposed surfaces (ultrathin nanosheets) and could accelerate catalytic kinetics for H2 evolution (porous structure). Ni3S2 NF only displays continuous lattice fringes, corresponding to (101) crystallographic planes of Ni3S2 on the whole nanosheet (Fig. 2e and f). Less Ni3Sn2S2 QDs decorations (Ni3Sn2S2@Ni3S2-1 NF, Fig. 2h) results the distribution of QDs on the sheet is highly cohesive and uneven in size. An over-decoration (Ni3Sn2S2@Ni3S2-3 NF, Fig. 2j) only produces scattered on and off the Ni3S2 nanosheets caused by less interaction force between Ni3Sn2S2 and Ni3S2, which may lead to poor electrochemical stability of whole catalysis. The ultrathin nanosheets are proliferous (Fig. 2i) and many black spots with an average diameter of ∼4 nm (insert of Fig. 2i) uniformly disperse on the porous nanosheets, indicating very successful decoration of Ni3Sn2S2 on Ni3S2. The highresolution TEM (HRTEM) was measured to identify the specific components of nanosheets and quantum dot. The lattice fringes with interplanar distances of 0.408 nm at bright area can be assigned to (101) crystallographic planes of Ni3S2 (Fig. 2l), while the lattice fringes of quantum dot with interplanar space of 0.384 nm (Fig. 2g) can belong to (012) facet of Ni3Sn2S2, as well as the interface between Ni3S2 and Ni3Sn2S2 phases can be observed. Such abundant interfaces may not only improve the intrinsic electric properties of hybrids, but also further facilitate the mass and charge transfer between Ni3S2 and Ni3Sn2S2. The EDS elemental mapping (Fig. S3) and quantitative analysis (Fig. S4) shows uniform distribution of Ni, S and Sn elements throughout the entire electrode of Ni3Sn2S2@Ni3S2-2. The selected area electron diffraction (Fig. S5) represents distinct and clear diffraction rings, indicating a polycrystalline property of the nano-ligament for both Ni3S2 and Ni3Sn2S2. A perfect match of similar rhombohedral phases of Ni3S2 and Ni3Sn2S2 provide huge opportunities to form the hybrid nanocatalyst consist of Ni3Sn2S2 quantum dots incorporating on the Ni3S2 nanosheets and further generate abundant interfaces, which should also favor the stability of Ni3Sn2S2@Ni3S2 composite while improve the charge transport between Ni3Sn2S2 and Ni3S2.
(Volmer step) [27]. Further, the catalyst also should have a strong desorption ability to not strongly restrain Hads on catalytic sites (Heyrovsky step) otherwise H2 will be difficultly released to inhibit the whole HER process [28]. On a same electrocatalyst surface, it is very difficult to accomplish both strong absorption of water and strong desorption of H atoms. Tailoring chemical compositions to alter the electronic structure of the catalyst by doping 3d transitional metals (e.g. Fe, Co, Ni and Mn) can modulate the adsorption of water and desorption of hydrogen toward HER but it often requires trading off each ability for a balance catalytic process [29–31]. The hybridization of different components endows the HER catalyst ability exceeding the individual ones by contributions to a synergistic effect for interfacial stabilization, electronic regulation, atomic arrangement as well as the absorption/desorption abilities, of which the abundant interfaces between multi-constituents can accelerate the surface species binding, transforming and transporting [13,32]. Nevertheless, it is also lacking of a fundamental knowledge of doping non-conventional catalytic elements (especially no catalytic elements) toward water splitting process and their composite has not been systematically investigated. In this work, the stannum (Sn) element possesses a slightly electron positivity than Ni element (1.96 vs 1.91 of Ni) and a low electron negativity than S element (1.96 vs 2.58 of S), which can be a good candidate dopant to balance the chemical bonds (NieSneS) in Ni3S2. By decorating Ni3Sn2S2 QDs into Ni3S2 material may create effective heterogeneous multi-interfaces for the best balance between strong water absorption and strong H atomic desorption for both fast electron transfer and gas evolution. A high-density of Ni3Sn2S2 quantum dotsimpregnated ultrathin Ni3S2 nanosheets (abbreviated as Ni3Sn2S2@ Ni3S2-2) as a state-of-art robust non-precious metal electrocatalyst with high surface area is synthesized as a catalyst toward HER in alkaline media, exhibiting distinguished performance very close to the noble catalyst (Pt/C) at lower current density and transcending Pt/C at higher current density, which is very important for a large scale of hydrogen production. An optimal such catalyst, Ni3Sn2S2@Ni3S2-2 was selected as a model cathode material to systematically investigate by combined experimental and theoretical approaches for the multi-interfaced heterogeneous catalytic mechanism in electrolytic water splitting process. It reveals that the highly efficient catalytic process is mainly resulted from a high-surficial heteroatomic catalysis mechanism, in which the high electrocatalytic activity is generated by multi-heteroatoms-altered electronic structure and sufficient interfaces for both strong water absorption and strong H atoms desorption while producing highly porous sheet-by-sheet stacked nanostructure for high surface area from the synthesis decorating Ni3Sn2S2 onto Ni3S2. 2. Results and discussion 2.1. Characterization of Ni3Sn2S2@Ni3S2 catalyst The powder X-ray diffraction (XRD) patterns of hybrid nanocatalyst Ni3Sn2S2@Ni3S2 NF and Ni3S2 NF are presented in Fig. 1a to characterize the crystalline components. The sharp XRD peaks at 2θ = 21.7°, 31.1°, 37.8°, 49.7° and 55.1 are indexed to (101), (110), (003), (113) and (122) facets of heazlewoodite Ni3S2, respectively. The two diffraction peaks at 45.5° and 53.0° appear in both Ni3Sn2S2@Ni3S2 NF and Ni3S2 NF belonging to the (111) and (200) facets of metallic Ni, respectively. The relative weakly peaks at 19.9°, 23.1°, 32.7°,38.6°, 40.5° and 47.3° are affiliated to (101), (012), (110), (021) and (024) of Ni3Sn2S2, respectively. The diffraction peaks of Ni3Sn2S2@Ni3S2 NF contain the characteristics of all three Ni3S2 (JCDPS No. 71-1682), Ni3Sn2S2 (JCDPS No. 86-1828) and metallic nickel (JCPDS, No.650380) phases with no impurity peak. The Ni3S2 supercell (3 × 3 × 3) crystal structure can be clearly distinguished from the rhombohedral Ni3S2 by the nickel and sulfide atoms in a cubic-type structure, where nickel atoms tetrahedral are bonded to the body-centered cubic sulfur atoms (Fig. 1b and c). The network of NieNi bond paths in 2
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Fig. 1. (a) XRD pattern of Ni3S2 NF (violet line) and Ni3Sn2S2@Ni3S2 NF (blue line). Front (b), top (c) and side view (d) of heazlewoodite Ni3S2 crystallizes. Front (e), top (f) and side view (g) of heazlewoodite Ni3Sn2S2 crystallizes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
130.2, 165.8, 187.2, 192.3, 234.5 and 270.8 mV, respectively (insert of Figs. 3a and S6). As shown in Fig. 3b, the HER Tafel slope of Ni3Sn2S2@ Ni3S2-2 NF is only 73.2 mV dec−1, which is much smaller than those of Ni3Sn2S2@Ni3S2-1 NF (92.7 mV dec−1), Ni3Sn2S2@Ni3S2-3 NF (125.3 mV dec−1), Ni3S2 NF (138.1 mV dec−1) and close to those of Pt/ C NF (49.3 mV dec−1). Such small tafel slope of Ni3Sn2S2@Ni3S2-2 indicate of a Volmer-Heyrovsky HER mechanism. It is also discovered that the decoration amount of Ni3Sn2S2 quantum dots on Ni3S2 nanosheets plays an essential role in HER kinetics. The Tafel slope and overpotential (10 mA cm−2) of all electrocatalysis performance are summarized in Fig. 3c. Besides, the exchange current density (j0) of Ni3Sn2S2@Ni3S2 NFs and Pt/C NF were also calculated by Tafel plots (Fig. S7) and more detailed results were listed in Table S1. The
2.2. Electrocatalytic HER performance of Ni3Sn2S2@Ni3S2 NF Linear Scan Voltammetric (LSV) curves measured at extremely low scan rates such as 2 mV s−1 can be considered as steady-state responses as polarization curves. Ni3Sn2S2@Ni3S2 NFs, Ni3S2 NF, Pt/C NF and bare NF were tested in alkaline solution (1.0 M KOH) using a typical three-electrode system. The polarization curves unambiguously and surprisingly show that Ni3Sn2S2@Ni3S2-2 NF has a very comparable performance with Pt/C NF at low potential and even much superior to Pt/C NF after current density larger than 200 mA cm−2 (Fig. 3a). To deliver a current density of 10 mA cm−2, the Pt/C NF, Ni3Sn2S2@Ni3S22 NF, Ni3Sn2S2@Ni3S2-1 NF, NixSny NF, Ni3Sn2S2@Ni3S2-3 NF, SnS NF, Ni3S2 NF and bare NF catalysts require overpotential of 28.0, 53.2,
Fig. 2. SEM images of (a, b) Ni3S2 NF and (c, d) Ni3Sn2S2@Ni3S2-2 NF. (e, f) TEM and HRTEM images of Ni3S2 NF, TEM images of (h)Ni3Sn2S2@Ni3S2-1 NF, (i) Ni3Sn2S2@Ni3S2-2 NF (insert diameter-size histogram for quantum dots) and (j) Ni3Sn2S2@Ni3S2-3 NF. HRTEM images of the (g) quantum dots and (l) nanosheets parts of Ni3Sn2S2@Ni3S2-2 NF. 3
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Fig. 3. (a) Polarization curves of Ni3Sn2S2@Ni3S2 NFs, Ni3S2 NF, Pt/C NF and bare NF. (b) Tafel plots of Ni3S2 NF, Ni3Sn2S2@Ni3S2 NFs and Pt/C NF. (c) Tafel slopes (left) and overpotential at 10 mA cm−2 versus RHE (right). (d) EIS nyquist plots of the Ni3Sn2S2@Ni3S2 NFs and Ni3S2 NF (the insert depicts Rct value of Ni3S2 NF and Ni3Sn2S2@Ni3S2 NFs). (e) Polarization curves of Ni3Sn2S2@Ni3S2-2 NF before and after 20 h continuous HER reaction (the insert depicts chronopotentiometric test of Pt/C NF and Ni3Sn2S2@Ni3S2 -2 NF at a current density of 200 mA cm−2). (f) The comparison of HER overpotentials at 10 mA cm−2 and tafel slope of Ni3Sn2S2@ Ni3S2-2 NF and other recently reported electrocatalysts.
active sites on the surface.
Ni3Sn2S2@Ni3S2-2 processed extremely larger exchange current density (j0) of 2.624 mA cm−2 than those of Ni3Sn2S2@Ni3S2 NFs and Ni3S2 NF. Such value was 5–15 times larger than the other three counterparts and very close to the Pt/C NF (3.258 mA cm−2), suggesting a larger number of active sites and the excellent activity of Ni3Sn2S2@Ni3S2-2 NFs for HER catalysis. Nyquist plots of all electrocatalysts (Fig. 3d) display well-defined semicircles over the high frequency range resulting from the charge transfer resistance (Rct) at the electrode-electrolyte interface [33–35]. The calculated Rct value for the Ni3Sn2S2@Ni3S2-2 NF is 5.79 Ω, which is much smaller than that of Ni3S2 NF (20.93 Ω). Furthermore, the Rct value for Ni3Sn2S2@Ni3S2-2 is also smaller than that of Ni3Sn2S2@ Ni3S2-1 (12.49 Ω) and Ni3Sn2S2@Ni3S2-3 (18.46 Ω), confirming that Ni3Sn2S2 has much higher catalytic activity toward HER than Ni3S2 and Ni3Sn2S2@Ni3S2-2 is the best among all Ni3Sn2S2 QDs-decorated composites. Nyquist plots (Fig. 3d) results demonstrates that rich Ni3Sn2S2@Ni3S2 interfaces feature can endow Ni3Sn2S2@Ni3S2-2 with large electrochemical surface areas and fast electron transport, which promotes the HER performance. Chronopotentiometic experiments were performed at a continuous current density of 200 mA cm−2 in alkaline solution to evaluate the long-time stability toward HER for Pt/ C and Ni3Sn2S2@Ni3S2-2 (insert depicts of Fig. 3e). Pt/C NF exhibits a sharp drop of performance in the beginning few hours and is extremely worse than Ni3Sn2S2@Ni3S2-2 after 20 h. Ni3Sn2S2@Ni3S2-2 demonstrates that the polarization curves of Ni3Sn2S2@Ni3S2-2 NF measured before and after the 20 h stability tests are almost overlapped, thus indicating excellent stability of Ni3Sn2S2@Ni3S2-2 NF (Fig. 3e). The above results undoubtedly confirm that the stability of Ni3Sn2S2@ Ni3S2-2 NF is much superior to Pt/C NF at high current density. In addition, Ni3Sn2S2@Ni3S2-2 NF is also outstanding among the recent electrocatalysts toward HER as shown in Fig. 3f and Table S2 [36–47]. The electrochemical double layer capacitances (Cdl) were also measured to evaluate the electrochemical surface area of the as-prepared catalysts (Fig. S8). The measured Cdl values for Ni3S2 NF, Ni3Sn2S2@ Ni3S2-1 NF, Ni3Sn2S2@Ni3S2-2 NF, and Ni3Sn2S2@Ni3S2-3 NF are 1.03, 2.12, 4.01 and 1.56 m F cm−2, respectively (Fig. S9). The highest Cdl value of Ni3Sn2S2@Ni3S2-2 NF indicates there would be more catalytic
2.3. Mechanism investigations To further understand the effect of the heteroatomic catalysis effect on the electrode kinetics of HER, systematic density functional theory (DFT) calculations were performed by constructing the correlative theoretical models on (101) facets of Ni3S2 and (012) facets of Ni3Sn2S2, respectively. By comparing the computed surface energies of possible exposed terminations, we obtained Ni3S2 (101)-Ni1 and Ni3Sn2S2 (012)NiSn are stable in S-poor condition, and Ni3S2 (101)-S and Ni3Sn2S2 (012)-S2 are stable in S-rich condition (Figs. S10 and S11). Using DFT computations, the activity of the correlative surfaces toward HER was also evaluated. Some possible adsorption sites of H* are described in Figs. S12 and S13. Generally, the adsorption free energy of H* (ΔG(H*)) can be serve as a good measure of the activity of a catalytic site toward HER. The smaller the ΔG(H*) absolute value of the site, the higher the catalytic activity toward HER. Under the S-poor condition as shown in Fig. 4a and e, the Ni site is found to have a much small ΔG(H*) value, and in particular the Ni site in Ni3Sn2S2 (012)-NiSn possesses the lowest ΔG(H*) value (−0.099 eV), indicating the Ni site of Ni3Sn2S2 (012)NiSn is more catalytically active (Table S2) than others. Additionally, there are two types of Ni sites on Ni3S2 (101)-Ni1 facet, the five-coordinated Ni atom (NiA site) with three S atoms and two Ni atoms around and seven-coordinated Ni atom (NiB site) with four S atoms and three Ni atoms around. Moreover, the Ni site on the Ni3Sn2S2 (012)NiSn facet is seven-coordinated with two Ni atoms as well as four Sn atoms and two S atoms around. Thus, it is discovered that the Ni atoms with less S atoms around are mainly responsible for rising the HER activity. As shown in Fig. 4b and f, the HER activity on the Ni3S2 (101)S and Ni3Sn2S2 (012)-S2 facets are evaluated. There are two types S sites, the two-coordinated (S2 site) and four coordinated (S1 site), and the latter is more catalytically active (−0.418 eV). However, the Ni site on the Ni3S2 (101)-S is seven-coordinated with large steric effect, which makes Ni site not good for HER. For Ni3Sn2S2 (012)-S2, the ΔG(H*) value is too small and not beneficial on dissociation of hydrogen. Using Ni3S2 (101)-Ni1 and Ni3Sn2S2 (012)-NiSn surfaces as examples, the 4
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Fig. 4. (a) The adsorption of H atom on the most stable Ni3S2 (101)-Ni1 and Ni3Sn2S2 (012)-NiSn surfaces under S-poor conditions. The light blue sphere represents H atom. (b) The adsorption of H atom on the most stable Ni3S2 (101)-S and Ni3Sn2S2 (012)-S2 surfaces under S-rich conditions. (c and d) The partial density of states (PDOS) of Ni3S2 (101)-Ni1 surfaces and Ni3Sn2S2 (012)-NiSn surfaces. Zero energy is the Fermi energy. (e) H adsorption free energy profiles of Ni3S2 (101)-Ni1 facet and Ni3Sn2S2 (012)-NiSn facet at the different adsorption sites. (f) H adsorption free energy profiles of Ni3S2 (101)-S facet and Ni3Sn2S2 (012)-S2 facet at the different adsorption sites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(Ni3Sn2S2@Ni3S2-2) has the enhancement effect on HER. Raman and XPS spectra were measured to prove the electronic interaction between Ni3Sn2S2 and Ni3S2. As in Fig. 5a, the characteristic Raman band peaks at 185.7, 220.0, 245.4, 300.5, 321.2 and 347.5 cm−1 correspond to NieS bonds of Ni3Sn2S2@Ni3S2 NF, which show red-shifts compared with the bare Ni3S2 NF. This result obviously indicates that the state of NieS bonds changes due to electronic interaction between Ni3Sn2S2 and Ni3S2. Chemical states information of Ni3Sn2S2@Ni3S2 NF and Ni3S2 NF was further examined by X-ray photoelectron spectroscopy (XPS, Fig. 5b). For Ni3S2 NF, the high-resolution XPS spectrum of Ni 2p exhibits two main peaks situated at 856.2 and 874.0 eV, which can be assigned to Ni 2p3/2 and Ni 2p1/2, respectively. Meanwhile, the satellite peaks at 862.0 and 880.1 eV can be ascribed to shakeup type peaks of Ni. The peaks of Ni 2p3/2 and Ni 2p1/2 of Ni3Sn2S2@Ni3S2-2 shows negative shifts of ∼0.36 and 0.53 eV, respectively. Meanwhile, two new small peaks located at the binding energy of 852.3 and 869.3 eV were detected, indicating that the state of Ni0 exist in Ni3Sn2S2@Ni3S22 NF. It can dope out that the lower valence state of Ni can be generated by Sn introduction, which is greatly facile for HER. In S 2p region, the
interactions between H and surface through the projected density of states (PDOS) are illustrated in Fig. 4c and d. The NiA-d orbitals of Ni3S2 (101)-Ni1 is partially unoccupied with hybridized NiA-dx2-y2 and S-px in-plane states near the Fermi energy (EF). The H-s states that locate in the tail of both occupied and unoccupied NiA-d orbitals mainly interact with NiA-d states, and also induce obvious disturbance on S-p states. In contrast, the NiA-d orbitals of Ni3Sn2S2 (012)-NiSn are more localized and its d-band center is relatively close to the Fermi level, implying a better catalytic activity of Ni site. The Sn-p states are rather delocalized and have no hybridization with H-s states. The most active site on the Ni3Sn2S2 (012)-NiSn for HER can be attributed the following three reasons. (i) the Ni sites have a negligible steric effect from Ni3Sn2S2 (012)-NiSn facet, (ii) the Sn atoms in Ni3Sn2S2 (012) replace the S atoms around Ni reducing the number of S atoms around, which makes the Ni site more active for electron transfer toward HER, and (iii) Sn 3d orbital can restrain the adsorption of H induced hybridization of S-dz2 orbitals for strong desorption ability. After individually investigating the surface state of Ni3Sn2S2 and Ni3S2, the next interesting things is how the complex structure 5
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Fig. 5. (a) Raman spectra of Ni3Sn2S2@ Ni3S2-2 NF and Ni3S2 NF at 180–405 cm−1. (b) Ni2p XPS spectra of Ni3S2 NF and Ni3Sn2S2@Ni3S2-2 NF. (c) Raman spectra of fresh catalysts (dots line) and that at current density of (solid lines) at 200 mA cm−2 2000–2900 cm−1 (d) Ni 2p XPS spectra of Ni3Sn2S2@Ni3S2-2 before and after HER for 20 h.
decoration of Ni3Sn2S2 in Ni3S2, especially Sn element plays a critical role in the significant performance improvement. For Ni3S2, the water molecule (H2O) is first absorbed on S of the NieS bonds with electronic strength to form SeHeOH, then can be weakened and finally be cleaved releasing electron for H atom and OH−. Apparently, the OH− favors to take up the Ni (δ+) center and the generated H atom transfers from S centers onto the neighboring vacant Ni cluster forming the absorbed H intermediate (Hads, Volmer reaction), followed by forming and releasing of H2 from the absorbed H intermediates while removing OH− from Ni (δ+) centers (Heyrovsky reaction). Optimal Ni3Sn2S2@ Ni3S2 hybrid can reduce energy of SeHads bond in SeHeOH to facilitate H2O molecule cleaving, while reducing electron density around Ni for sufficient empty d orbitals binding the generated H atom for facilitating the kinetics of Volmer reaction. In particular, the Sn-p states are rather delocalized and have no hybridization with H-s states and thus in Ni3Sn2S2 (012) the Sn atoms replace the S atoms around Ni reducing the number of S atoms around to make the Ni sites more active for electron transfer toward HER, while Sn 3d orbital can restrain the adsorption of H induced hybridization of S-dz2 orbitals for strong desorption ability for fast Heyrovsky reaction. Consisting of optimize chemical electronic structure regulation induced by Sn element and abundant interface rising from hybrid components, the nanocatalyst accomplishes both strong adsorption ability of water and strong desorption of H atoms unlike most of HER catalysts requiring trade out each other for a balance to deliver better performance, which is the beauty of this multiheterocatalysis mode and also provide a new strategy to resolve the problem of high overpotential and poor durability existing in noblemetal-free electrocatalysts.
peaks at 162.4 eV and 161.0 eV originate from S 2p1/2 and S 2p3/2, respectively. The peak at around 168.9 eV might be due to the residual S2O32− on the surface of both Ni3S2 NF and Ni3Sn2S2@Ni3S2-2 NF (Fig. S14) [48]. As expected, the S 2p1/2 and S 2p3/2 of Ni3Sn2S2@Ni3S2-2 NF move toward higher binding energy areas compared with that of Ni3S2 NF, which agrees with XPS spectrum of Ni 2p. To better understand the catalytic mechanism of Ni3Sn2S2@Ni3S2-2, water adsorption and activation for HER was further investigated by Raman. As depicted in Fig. 5c, Raman spectra of Ni3Sn2S2@Ni3S2-2 NF and Ni3S2 NF before and after HER at the current density of 200 mA cm−2 for 20 h were employed to analyze the bonding states of SeHads intermediate on the surface of Ni3Sn2S2@Ni3S2-2 NF and Ni3S2 NF electrocatalysts. For both fresh Ni3Sn2S2@Ni3S2 NF and Ni3S2 NF, no peak can be detected. However, after HER reaction, two peaks at ∼2556 cm−1 and ∼2584 cm−1 were detected for Ni3Sn2S2@Ni3S2-2 NF and Ni3S2 NF, respectively, which can be attributed to SeHads bonds forming on the surface of catalysts during HER process. The Raman spectrum of SeHads for Ni3Sn2S2@Ni3S2-2 NF has a red shift of ∼28 cm−1 versus that of Ni3S2 NF, which could indicate that the SeHads bonds in Ni3Sn2S2@ Ni3S2-2 NF are weaker than those in Ni3S2 NF to promote hydrogen evolution. The Raman spectrum greatly correspond with computed result, which confirm the fact that SeHads can be effectively regulated by Sn doping and forming hybrid, further achieving an optimal balanced strong H (water) absorption/H desorption toward efficient HER. After HER tests, the Ni3Sn2S2@Ni3S2-2 NF shows similar XPS spectra compared fresh electrocatalyst, only exhibit that the amount of Ni0 derived from Ni2+/3+ and a part of Sn0 comes from Sn2+/4+ are increased after electrolytic process in alkaline media (Figs. 5d and S15). It is observed that the Ni3Sn2S2 dots also steadily embed on the Ni3S2 nanosheets and 3-dimentsionally structures are remained after HER test (Figs. S16 and S17). According to our experimental results and theoretical calculations discussed above, a high-surficial multi-heteroatomic catalysis mechanism is schematically shown in Fig. 6, in which the rational
3. Conclusion In summary, high surficial multi-heteroatomic Ni3Sn2S2 dots-decorated thin Ni3S2 nanosheets was tailored toward highly efficient HER. In particular, Sn atom delocalizes the unpaired electron in the NiA-d 6
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Fig. 6. Schematic illustration of multi-heteroatomic catalysis mechanism toward hydrogen evolution process for Ni3Sn2S2@Ni3S2 NF in alkaline media.
orbitals of Ni3Sn2S2 for its d-band center close to the Fermi level while delocalizing Sn-p states to avoid hybridizing with H-S states. Rich interfaces and artful electronic structure adjustment in Ni3Sn2S2@Ni3S2-2 achieves both strong H absorption and desorption to accomplish fast electron transfer and efficient H2 releasing. This work not only open up a new path to develop novel metal sulfides to replace noble-based metal for practical high-performance industrialized hydrogen evolution, but also provide fundamental insights to design multi-heteroatomic catalysts for an energy conversion process thus possessing universal scientific significance.
[4]
[5] [6]
[7]
CRediT authorship contribution statement
[8]
Shi-Yu Lu: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing, Funding acquisition. Shengwen Li: Formal analysis, Software. Meng Jin: Investigation. Jiechang Gao: Investigation. Yanning Zhang: Conceptualization, Writing - review & editing, Funding acquisition.
[9]
[10]
[11]
Declaration of Competing Interest [12]
The authors declare no competing financial interest.
[13]
Acknowledgements
[14]
This work is financially supported by National Nature Science Foundation of China (11874005) and engineering science youth scholars fund program of BIC-ESAT.
[15]
Appendix A. Supplementary data [16]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2020.118675. [17]
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Greatly boosting electrochemical hydrogen evolution reaction over Ni3S2 nanosheets rationally decorated by Ni3Sn2S2 quantum dots
T
Shi-Yu Lua,d,*, Shengwen Lib, Meng Jinc, Jiechang Gaod, Yanning Zhangb,** a
The Beijing Innovation Center for Engineering Science and Advanced Technology (BIC-ESAT), College of Engineering, Peking University, Beijing 100871, China Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, Sichuan, China c College of Chemistry & Chemical Engineering, Chongqing University, Chongqing 400044, China d Institute for Clean Energy & Advanced Materials, School of Materials and Energy, Southwest University, Chongqing 400715, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Heterointerfaces Electronic structure control Metal sulfides Earth-abundant hybrid catalyst Hydrogen evolution reaction
The electrode kinetics of hydrogen evolution reaction (HER) greatly relies on both strong water absorption and strong H atom desorption for fast electron transfer while prompting hydrogen evolution, but it is very challenging to achieve due to the tough trading off between water absorption and H-desorption ability of the catalyst. Herein, a unique high-surficial multi-heteroatomic catalytic process is realized by rationally design and tailor Ni3Sn2S2 dots-decorated thin Ni3S2 nanosheets to form sheets-on-sheets array self-supported electrode by simple hydrothermal process. The formed Ni3Sn2S2@Ni3S2-2 NF delivers a superior performance very close to the noble catalyst (Pt/C) at low current densities with an onset-potential of nearly 0 mV and overpotentials of 50.7 mV at 10 mA cm−2 while surprisingly surpassing Pt/C at high current densities. The outstanding HER performance of the catalyst can be ascribed that the rationally tuned multi-heterogeneous interfaces and electronic structure control can realize both strong water absorption and strong H atom desorption to not only significantly promotes fast electron transfer, but also greatly enhances the gas release toward efficient HER. This work holds a great promise to fabricate a non-noble HER catalyst for high-performance close to the noble catalysts such as Pt/C while shedding a light on fundamentals to guide construction of high-surficial heteroatomic multi-heterogeneous catalysts with superior performance.
1. Introduction Hydrogen as a clean energy source with high energy density and zero pollution is a highly desired alternative to the fast exhausted nonrenewable and pollute fossil fuels [1,2]. Electrolytic water splitting is an attractive eco-friendly and sustainable approach to produce pure hydrogen [3–5]. Up to date, platinum (Pt) and its alloys are still the best electrocatalysts for HER and hold the benchmark for HER [6–8]. However, the scarcity, poor stability and high expense of these noble metals catalysts greatly impede the large-scale and industrial water splitting process. Consequently, it is highly worthwhile to design and construct economic and high-performance alternative electrocatalysts. Recently, intensive endeavors have been focused on exploration of nanostructured catalysts based on transition metals and their alloys, such as sulfides, [9–11], selenide [12], carbides [13–15], nitrides [16,17] and phosphides [18–21], as potential noble-metal-free
electrocatalysts. In particular, heazlewoodite nickel sulfides have been widely studied in energy field due to their advantages of abundant reserves and good electron transport ability [22,23]. In most cases, nanoengineering approaches can synthesize materials as catalysts with high surface area and increased catalytic sites [24,25]. The mineral heazlewoodite has been synthesized with various morphologies to greatly improve catalytic activity toward HER in comparison to bulk heazlewoodite [26]. Nevertheless, the performance remains a great gulf from the noble metal-based catalysts. Fundamentally, in the crystal structure of Ni3S2 (101) Ni atoms are five-coordinated and SeHads bonds can readily form in an S-rich environment, but it could also cause a saturated coordination state of S atoms on the catalyst surface resulting in sluggish H2O adsorption. In an ideal HER process in an alkaline medium, a catalyst should have strong water absorption to be effectively activated to break OeH bonds and form H atoms by electrochemical reduction, and easily anchor H atoms on the catalytic sites
⁎
Corresponding author at: The Beijing Innovation Center for Engineering Science and Advanced Technology (BIC-ESAT), College of Engineering, Peking University, Beijing, 100871, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (S.-Y. Lu), [email protected] (Y. Zhang). https://doi.org/10.1016/j.apcatb.2020.118675 Received 5 November 2019; Received in revised form 8 January 2020; Accepted 22 January 2020 Available online 23 January 2020 0926-3373/ © 2020 Elsevier B.V. All rights reserved.
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heazlewoodite Ni3S2 crystallizes can be unambiguously identified by side view of Ni3S2 crystallizes (Fig. 1d). Fig. 1e–g represent the front, top and side view of Ni3Sn2S2 supercell (2 × 2 × 1) crystallization, respectively. The electronegativity of Sn element is slightly more positive than Ni (1.96 vs 1.91) and more negative than S (1.96 vs 2.58). Owing to above natural characteristic, the particular characteristics of Sn element can cause two possible substitution in Ni3Sn2S2: 1) expressing non-metallic property, in which Sn substitutes a portion of Ni atoms and directly forms covalent bonding with S atoms (SneS). 2) Delivering metallic property, in which Sn replaces a part of S atoms to balance the electron density in crystal (NieSn). The substitution of Sn atoms in Ni3S2 plays in a curial role in tuning the electron distribution between Ni and S atoms. In addition, crystal structure of Ni3S2 and Ni3Sn2S2 can all belong to the space group of R-3m and thus can be classified to heazlewoodite, which also helps Sn atom easily being induced into Ni3S2 phase to form Ni3Sn2S2 phase without a crystal structure collapse. The optical images of electrodes (Fig. S1) reveal that the catalysts are uniformly covered on the nickel foam and the color of the electrode changes from light green to black with increasing the amount of SnCl2 introduced. The decoration amount of Ni3Sn2S2 quantum dots into the Ni3S2 nanosheets was well tuned by the quantity additions of SnCl2 (Experimental section). The micromorphology of the delicately tailored Ni3Sn2S2@Ni3S2 NF discerned from scanning electron microscope (SEM) and transmission electron microscope (TEM) with pure Ni3S2 NF as the baseline for comparison. It clearly shows that flexible and porous nanosheets uniformly grow over the nickel foam (Fig. S2) and are 3dimensionally interconnected with formed many void spaces when decorating Ni3Sn2S2 QDs in Ni3S2 in contrast to plain Ni3S2 (Fig. 2a and b). From the high magnification of SEM image (Fig. 2d), it can be unambiguously observed that ultrathin nanosheets are stacked sheet by sheet to assemble a porous 3-D sheets-on-sheets structure. This special nanostructure could be expected to produce coordinately unsaturated metal sites on exposed surfaces (ultrathin nanosheets) and could accelerate catalytic kinetics for H2 evolution (porous structure). Ni3S2 NF only displays continuous lattice fringes, corresponding to (101) crystallographic planes of Ni3S2 on the whole nanosheet (Fig. 2e and f). Less Ni3Sn2S2 QDs decorations (Ni3Sn2S2@Ni3S2-1 NF, Fig. 2h) results the distribution of QDs on the sheet is highly cohesive and uneven in size. An over-decoration (Ni3Sn2S2@Ni3S2-3 NF, Fig. 2j) only produces scattered on and off the Ni3S2 nanosheets caused by less interaction force between Ni3Sn2S2 and Ni3S2, which may lead to poor electrochemical stability of whole catalysis. The ultrathin nanosheets are proliferous (Fig. 2i) and many black spots with an average diameter of ∼4 nm (insert of Fig. 2i) uniformly disperse on the porous nanosheets, indicating very successful decoration of Ni3Sn2S2 on Ni3S2. The highresolution TEM (HRTEM) was measured to identify the specific components of nanosheets and quantum dot. The lattice fringes with interplanar distances of 0.408 nm at bright area can be assigned to (101) crystallographic planes of Ni3S2 (Fig. 2l), while the lattice fringes of quantum dot with interplanar space of 0.384 nm (Fig. 2g) can belong to (012) facet of Ni3Sn2S2, as well as the interface between Ni3S2 and Ni3Sn2S2 phases can be observed. Such abundant interfaces may not only improve the intrinsic electric properties of hybrids, but also further facilitate the mass and charge transfer between Ni3S2 and Ni3Sn2S2. The EDS elemental mapping (Fig. S3) and quantitative analysis (Fig. S4) shows uniform distribution of Ni, S and Sn elements throughout the entire electrode of Ni3Sn2S2@Ni3S2-2. The selected area electron diffraction (Fig. S5) represents distinct and clear diffraction rings, indicating a polycrystalline property of the nano-ligament for both Ni3S2 and Ni3Sn2S2. A perfect match of similar rhombohedral phases of Ni3S2 and Ni3Sn2S2 provide huge opportunities to form the hybrid nanocatalyst consist of Ni3Sn2S2 quantum dots incorporating on the Ni3S2 nanosheets and further generate abundant interfaces, which should also favor the stability of Ni3Sn2S2@Ni3S2 composite while improve the charge transport between Ni3Sn2S2 and Ni3S2.
(Volmer step) [27]. Further, the catalyst also should have a strong desorption ability to not strongly restrain Hads on catalytic sites (Heyrovsky step) otherwise H2 will be difficultly released to inhibit the whole HER process [28]. On a same electrocatalyst surface, it is very difficult to accomplish both strong absorption of water and strong desorption of H atoms. Tailoring chemical compositions to alter the electronic structure of the catalyst by doping 3d transitional metals (e.g. Fe, Co, Ni and Mn) can modulate the adsorption of water and desorption of hydrogen toward HER but it often requires trading off each ability for a balance catalytic process [29–31]. The hybridization of different components endows the HER catalyst ability exceeding the individual ones by contributions to a synergistic effect for interfacial stabilization, electronic regulation, atomic arrangement as well as the absorption/desorption abilities, of which the abundant interfaces between multi-constituents can accelerate the surface species binding, transforming and transporting [13,32]. Nevertheless, it is also lacking of a fundamental knowledge of doping non-conventional catalytic elements (especially no catalytic elements) toward water splitting process and their composite has not been systematically investigated. In this work, the stannum (Sn) element possesses a slightly electron positivity than Ni element (1.96 vs 1.91 of Ni) and a low electron negativity than S element (1.96 vs 2.58 of S), which can be a good candidate dopant to balance the chemical bonds (NieSneS) in Ni3S2. By decorating Ni3Sn2S2 QDs into Ni3S2 material may create effective heterogeneous multi-interfaces for the best balance between strong water absorption and strong H atomic desorption for both fast electron transfer and gas evolution. A high-density of Ni3Sn2S2 quantum dotsimpregnated ultrathin Ni3S2 nanosheets (abbreviated as Ni3Sn2S2@ Ni3S2-2) as a state-of-art robust non-precious metal electrocatalyst with high surface area is synthesized as a catalyst toward HER in alkaline media, exhibiting distinguished performance very close to the noble catalyst (Pt/C) at lower current density and transcending Pt/C at higher current density, which is very important for a large scale of hydrogen production. An optimal such catalyst, Ni3Sn2S2@Ni3S2-2 was selected as a model cathode material to systematically investigate by combined experimental and theoretical approaches for the multi-interfaced heterogeneous catalytic mechanism in electrolytic water splitting process. It reveals that the highly efficient catalytic process is mainly resulted from a high-surficial heteroatomic catalysis mechanism, in which the high electrocatalytic activity is generated by multi-heteroatoms-altered electronic structure and sufficient interfaces for both strong water absorption and strong H atoms desorption while producing highly porous sheet-by-sheet stacked nanostructure for high surface area from the synthesis decorating Ni3Sn2S2 onto Ni3S2. 2. Results and discussion 2.1. Characterization of Ni3Sn2S2@Ni3S2 catalyst The powder X-ray diffraction (XRD) patterns of hybrid nanocatalyst Ni3Sn2S2@Ni3S2 NF and Ni3S2 NF are presented in Fig. 1a to characterize the crystalline components. The sharp XRD peaks at 2θ = 21.7°, 31.1°, 37.8°, 49.7° and 55.1 are indexed to (101), (110), (003), (113) and (122) facets of heazlewoodite Ni3S2, respectively. The two diffraction peaks at 45.5° and 53.0° appear in both Ni3Sn2S2@Ni3S2 NF and Ni3S2 NF belonging to the (111) and (200) facets of metallic Ni, respectively. The relative weakly peaks at 19.9°, 23.1°, 32.7°,38.6°, 40.5° and 47.3° are affiliated to (101), (012), (110), (021) and (024) of Ni3Sn2S2, respectively. The diffraction peaks of Ni3Sn2S2@Ni3S2 NF contain the characteristics of all three Ni3S2 (JCDPS No. 71-1682), Ni3Sn2S2 (JCDPS No. 86-1828) and metallic nickel (JCPDS, No.650380) phases with no impurity peak. The Ni3S2 supercell (3 × 3 × 3) crystal structure can be clearly distinguished from the rhombohedral Ni3S2 by the nickel and sulfide atoms in a cubic-type structure, where nickel atoms tetrahedral are bonded to the body-centered cubic sulfur atoms (Fig. 1b and c). The network of NieNi bond paths in 2
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Fig. 1. (a) XRD pattern of Ni3S2 NF (violet line) and Ni3Sn2S2@Ni3S2 NF (blue line). Front (b), top (c) and side view (d) of heazlewoodite Ni3S2 crystallizes. Front (e), top (f) and side view (g) of heazlewoodite Ni3Sn2S2 crystallizes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
130.2, 165.8, 187.2, 192.3, 234.5 and 270.8 mV, respectively (insert of Figs. 3a and S6). As shown in Fig. 3b, the HER Tafel slope of Ni3Sn2S2@ Ni3S2-2 NF is only 73.2 mV dec−1, which is much smaller than those of Ni3Sn2S2@Ni3S2-1 NF (92.7 mV dec−1), Ni3Sn2S2@Ni3S2-3 NF (125.3 mV dec−1), Ni3S2 NF (138.1 mV dec−1) and close to those of Pt/ C NF (49.3 mV dec−1). Such small tafel slope of Ni3Sn2S2@Ni3S2-2 indicate of a Volmer-Heyrovsky HER mechanism. It is also discovered that the decoration amount of Ni3Sn2S2 quantum dots on Ni3S2 nanosheets plays an essential role in HER kinetics. The Tafel slope and overpotential (10 mA cm−2) of all electrocatalysis performance are summarized in Fig. 3c. Besides, the exchange current density (j0) of Ni3Sn2S2@Ni3S2 NFs and Pt/C NF were also calculated by Tafel plots (Fig. S7) and more detailed results were listed in Table S1. The
2.2. Electrocatalytic HER performance of Ni3Sn2S2@Ni3S2 NF Linear Scan Voltammetric (LSV) curves measured at extremely low scan rates such as 2 mV s−1 can be considered as steady-state responses as polarization curves. Ni3Sn2S2@Ni3S2 NFs, Ni3S2 NF, Pt/C NF and bare NF were tested in alkaline solution (1.0 M KOH) using a typical three-electrode system. The polarization curves unambiguously and surprisingly show that Ni3Sn2S2@Ni3S2-2 NF has a very comparable performance with Pt/C NF at low potential and even much superior to Pt/C NF after current density larger than 200 mA cm−2 (Fig. 3a). To deliver a current density of 10 mA cm−2, the Pt/C NF, Ni3Sn2S2@Ni3S22 NF, Ni3Sn2S2@Ni3S2-1 NF, NixSny NF, Ni3Sn2S2@Ni3S2-3 NF, SnS NF, Ni3S2 NF and bare NF catalysts require overpotential of 28.0, 53.2,
Fig. 2. SEM images of (a, b) Ni3S2 NF and (c, d) Ni3Sn2S2@Ni3S2-2 NF. (e, f) TEM and HRTEM images of Ni3S2 NF, TEM images of (h)Ni3Sn2S2@Ni3S2-1 NF, (i) Ni3Sn2S2@Ni3S2-2 NF (insert diameter-size histogram for quantum dots) and (j) Ni3Sn2S2@Ni3S2-3 NF. HRTEM images of the (g) quantum dots and (l) nanosheets parts of Ni3Sn2S2@Ni3S2-2 NF. 3
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Fig. 3. (a) Polarization curves of Ni3Sn2S2@Ni3S2 NFs, Ni3S2 NF, Pt/C NF and bare NF. (b) Tafel plots of Ni3S2 NF, Ni3Sn2S2@Ni3S2 NFs and Pt/C NF. (c) Tafel slopes (left) and overpotential at 10 mA cm−2 versus RHE (right). (d) EIS nyquist plots of the Ni3Sn2S2@Ni3S2 NFs and Ni3S2 NF (the insert depicts Rct value of Ni3S2 NF and Ni3Sn2S2@Ni3S2 NFs). (e) Polarization curves of Ni3Sn2S2@Ni3S2-2 NF before and after 20 h continuous HER reaction (the insert depicts chronopotentiometric test of Pt/C NF and Ni3Sn2S2@Ni3S2 -2 NF at a current density of 200 mA cm−2). (f) The comparison of HER overpotentials at 10 mA cm−2 and tafel slope of Ni3Sn2S2@ Ni3S2-2 NF and other recently reported electrocatalysts.
active sites on the surface.
Ni3Sn2S2@Ni3S2-2 processed extremely larger exchange current density (j0) of 2.624 mA cm−2 than those of Ni3Sn2S2@Ni3S2 NFs and Ni3S2 NF. Such value was 5–15 times larger than the other three counterparts and very close to the Pt/C NF (3.258 mA cm−2), suggesting a larger number of active sites and the excellent activity of Ni3Sn2S2@Ni3S2-2 NFs for HER catalysis. Nyquist plots of all electrocatalysts (Fig. 3d) display well-defined semicircles over the high frequency range resulting from the charge transfer resistance (Rct) at the electrode-electrolyte interface [33–35]. The calculated Rct value for the Ni3Sn2S2@Ni3S2-2 NF is 5.79 Ω, which is much smaller than that of Ni3S2 NF (20.93 Ω). Furthermore, the Rct value for Ni3Sn2S2@Ni3S2-2 is also smaller than that of Ni3Sn2S2@ Ni3S2-1 (12.49 Ω) and Ni3Sn2S2@Ni3S2-3 (18.46 Ω), confirming that Ni3Sn2S2 has much higher catalytic activity toward HER than Ni3S2 and Ni3Sn2S2@Ni3S2-2 is the best among all Ni3Sn2S2 QDs-decorated composites. Nyquist plots (Fig. 3d) results demonstrates that rich Ni3Sn2S2@Ni3S2 interfaces feature can endow Ni3Sn2S2@Ni3S2-2 with large electrochemical surface areas and fast electron transport, which promotes the HER performance. Chronopotentiometic experiments were performed at a continuous current density of 200 mA cm−2 in alkaline solution to evaluate the long-time stability toward HER for Pt/ C and Ni3Sn2S2@Ni3S2-2 (insert depicts of Fig. 3e). Pt/C NF exhibits a sharp drop of performance in the beginning few hours and is extremely worse than Ni3Sn2S2@Ni3S2-2 after 20 h. Ni3Sn2S2@Ni3S2-2 demonstrates that the polarization curves of Ni3Sn2S2@Ni3S2-2 NF measured before and after the 20 h stability tests are almost overlapped, thus indicating excellent stability of Ni3Sn2S2@Ni3S2-2 NF (Fig. 3e). The above results undoubtedly confirm that the stability of Ni3Sn2S2@ Ni3S2-2 NF is much superior to Pt/C NF at high current density. In addition, Ni3Sn2S2@Ni3S2-2 NF is also outstanding among the recent electrocatalysts toward HER as shown in Fig. 3f and Table S2 [36–47]. The electrochemical double layer capacitances (Cdl) were also measured to evaluate the electrochemical surface area of the as-prepared catalysts (Fig. S8). The measured Cdl values for Ni3S2 NF, Ni3Sn2S2@ Ni3S2-1 NF, Ni3Sn2S2@Ni3S2-2 NF, and Ni3Sn2S2@Ni3S2-3 NF are 1.03, 2.12, 4.01 and 1.56 m F cm−2, respectively (Fig. S9). The highest Cdl value of Ni3Sn2S2@Ni3S2-2 NF indicates there would be more catalytic
2.3. Mechanism investigations To further understand the effect of the heteroatomic catalysis effect on the electrode kinetics of HER, systematic density functional theory (DFT) calculations were performed by constructing the correlative theoretical models on (101) facets of Ni3S2 and (012) facets of Ni3Sn2S2, respectively. By comparing the computed surface energies of possible exposed terminations, we obtained Ni3S2 (101)-Ni1 and Ni3Sn2S2 (012)NiSn are stable in S-poor condition, and Ni3S2 (101)-S and Ni3Sn2S2 (012)-S2 are stable in S-rich condition (Figs. S10 and S11). Using DFT computations, the activity of the correlative surfaces toward HER was also evaluated. Some possible adsorption sites of H* are described in Figs. S12 and S13. Generally, the adsorption free energy of H* (ΔG(H*)) can be serve as a good measure of the activity of a catalytic site toward HER. The smaller the ΔG(H*) absolute value of the site, the higher the catalytic activity toward HER. Under the S-poor condition as shown in Fig. 4a and e, the Ni site is found to have a much small ΔG(H*) value, and in particular the Ni site in Ni3Sn2S2 (012)-NiSn possesses the lowest ΔG(H*) value (−0.099 eV), indicating the Ni site of Ni3Sn2S2 (012)NiSn is more catalytically active (Table S2) than others. Additionally, there are two types of Ni sites on Ni3S2 (101)-Ni1 facet, the five-coordinated Ni atom (NiA site) with three S atoms and two Ni atoms around and seven-coordinated Ni atom (NiB site) with four S atoms and three Ni atoms around. Moreover, the Ni site on the Ni3Sn2S2 (012)NiSn facet is seven-coordinated with two Ni atoms as well as four Sn atoms and two S atoms around. Thus, it is discovered that the Ni atoms with less S atoms around are mainly responsible for rising the HER activity. As shown in Fig. 4b and f, the HER activity on the Ni3S2 (101)S and Ni3Sn2S2 (012)-S2 facets are evaluated. There are two types S sites, the two-coordinated (S2 site) and four coordinated (S1 site), and the latter is more catalytically active (−0.418 eV). However, the Ni site on the Ni3S2 (101)-S is seven-coordinated with large steric effect, which makes Ni site not good for HER. For Ni3Sn2S2 (012)-S2, the ΔG(H*) value is too small and not beneficial on dissociation of hydrogen. Using Ni3S2 (101)-Ni1 and Ni3Sn2S2 (012)-NiSn surfaces as examples, the 4
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Fig. 4. (a) The adsorption of H atom on the most stable Ni3S2 (101)-Ni1 and Ni3Sn2S2 (012)-NiSn surfaces under S-poor conditions. The light blue sphere represents H atom. (b) The adsorption of H atom on the most stable Ni3S2 (101)-S and Ni3Sn2S2 (012)-S2 surfaces under S-rich conditions. (c and d) The partial density of states (PDOS) of Ni3S2 (101)-Ni1 surfaces and Ni3Sn2S2 (012)-NiSn surfaces. Zero energy is the Fermi energy. (e) H adsorption free energy profiles of Ni3S2 (101)-Ni1 facet and Ni3Sn2S2 (012)-NiSn facet at the different adsorption sites. (f) H adsorption free energy profiles of Ni3S2 (101)-S facet and Ni3Sn2S2 (012)-S2 facet at the different adsorption sites. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(Ni3Sn2S2@Ni3S2-2) has the enhancement effect on HER. Raman and XPS spectra were measured to prove the electronic interaction between Ni3Sn2S2 and Ni3S2. As in Fig. 5a, the characteristic Raman band peaks at 185.7, 220.0, 245.4, 300.5, 321.2 and 347.5 cm−1 correspond to NieS bonds of Ni3Sn2S2@Ni3S2 NF, which show red-shifts compared with the bare Ni3S2 NF. This result obviously indicates that the state of NieS bonds changes due to electronic interaction between Ni3Sn2S2 and Ni3S2. Chemical states information of Ni3Sn2S2@Ni3S2 NF and Ni3S2 NF was further examined by X-ray photoelectron spectroscopy (XPS, Fig. 5b). For Ni3S2 NF, the high-resolution XPS spectrum of Ni 2p exhibits two main peaks situated at 856.2 and 874.0 eV, which can be assigned to Ni 2p3/2 and Ni 2p1/2, respectively. Meanwhile, the satellite peaks at 862.0 and 880.1 eV can be ascribed to shakeup type peaks of Ni. The peaks of Ni 2p3/2 and Ni 2p1/2 of Ni3Sn2S2@Ni3S2-2 shows negative shifts of ∼0.36 and 0.53 eV, respectively. Meanwhile, two new small peaks located at the binding energy of 852.3 and 869.3 eV were detected, indicating that the state of Ni0 exist in Ni3Sn2S2@Ni3S22 NF. It can dope out that the lower valence state of Ni can be generated by Sn introduction, which is greatly facile for HER. In S 2p region, the
interactions between H and surface through the projected density of states (PDOS) are illustrated in Fig. 4c and d. The NiA-d orbitals of Ni3S2 (101)-Ni1 is partially unoccupied with hybridized NiA-dx2-y2 and S-px in-plane states near the Fermi energy (EF). The H-s states that locate in the tail of both occupied and unoccupied NiA-d orbitals mainly interact with NiA-d states, and also induce obvious disturbance on S-p states. In contrast, the NiA-d orbitals of Ni3Sn2S2 (012)-NiSn are more localized and its d-band center is relatively close to the Fermi level, implying a better catalytic activity of Ni site. The Sn-p states are rather delocalized and have no hybridization with H-s states. The most active site on the Ni3Sn2S2 (012)-NiSn for HER can be attributed the following three reasons. (i) the Ni sites have a negligible steric effect from Ni3Sn2S2 (012)-NiSn facet, (ii) the Sn atoms in Ni3Sn2S2 (012) replace the S atoms around Ni reducing the number of S atoms around, which makes the Ni site more active for electron transfer toward HER, and (iii) Sn 3d orbital can restrain the adsorption of H induced hybridization of S-dz2 orbitals for strong desorption ability. After individually investigating the surface state of Ni3Sn2S2 and Ni3S2, the next interesting things is how the complex structure 5
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Fig. 5. (a) Raman spectra of Ni3Sn2S2@ Ni3S2-2 NF and Ni3S2 NF at 180–405 cm−1. (b) Ni2p XPS spectra of Ni3S2 NF and Ni3Sn2S2@Ni3S2-2 NF. (c) Raman spectra of fresh catalysts (dots line) and that at current density of (solid lines) at 200 mA cm−2 2000–2900 cm−1 (d) Ni 2p XPS spectra of Ni3Sn2S2@Ni3S2-2 before and after HER for 20 h.
decoration of Ni3Sn2S2 in Ni3S2, especially Sn element plays a critical role in the significant performance improvement. For Ni3S2, the water molecule (H2O) is first absorbed on S of the NieS bonds with electronic strength to form SeHeOH, then can be weakened and finally be cleaved releasing electron for H atom and OH−. Apparently, the OH− favors to take up the Ni (δ+) center and the generated H atom transfers from S centers onto the neighboring vacant Ni cluster forming the absorbed H intermediate (Hads, Volmer reaction), followed by forming and releasing of H2 from the absorbed H intermediates while removing OH− from Ni (δ+) centers (Heyrovsky reaction). Optimal Ni3Sn2S2@ Ni3S2 hybrid can reduce energy of SeHads bond in SeHeOH to facilitate H2O molecule cleaving, while reducing electron density around Ni for sufficient empty d orbitals binding the generated H atom for facilitating the kinetics of Volmer reaction. In particular, the Sn-p states are rather delocalized and have no hybridization with H-s states and thus in Ni3Sn2S2 (012) the Sn atoms replace the S atoms around Ni reducing the number of S atoms around to make the Ni sites more active for electron transfer toward HER, while Sn 3d orbital can restrain the adsorption of H induced hybridization of S-dz2 orbitals for strong desorption ability for fast Heyrovsky reaction. Consisting of optimize chemical electronic structure regulation induced by Sn element and abundant interface rising from hybrid components, the nanocatalyst accomplishes both strong adsorption ability of water and strong desorption of H atoms unlike most of HER catalysts requiring trade out each other for a balance to deliver better performance, which is the beauty of this multiheterocatalysis mode and also provide a new strategy to resolve the problem of high overpotential and poor durability existing in noblemetal-free electrocatalysts.
peaks at 162.4 eV and 161.0 eV originate from S 2p1/2 and S 2p3/2, respectively. The peak at around 168.9 eV might be due to the residual S2O32− on the surface of both Ni3S2 NF and Ni3Sn2S2@Ni3S2-2 NF (Fig. S14) [48]. As expected, the S 2p1/2 and S 2p3/2 of Ni3Sn2S2@Ni3S2-2 NF move toward higher binding energy areas compared with that of Ni3S2 NF, which agrees with XPS spectrum of Ni 2p. To better understand the catalytic mechanism of Ni3Sn2S2@Ni3S2-2, water adsorption and activation for HER was further investigated by Raman. As depicted in Fig. 5c, Raman spectra of Ni3Sn2S2@Ni3S2-2 NF and Ni3S2 NF before and after HER at the current density of 200 mA cm−2 for 20 h were employed to analyze the bonding states of SeHads intermediate on the surface of Ni3Sn2S2@Ni3S2-2 NF and Ni3S2 NF electrocatalysts. For both fresh Ni3Sn2S2@Ni3S2 NF and Ni3S2 NF, no peak can be detected. However, after HER reaction, two peaks at ∼2556 cm−1 and ∼2584 cm−1 were detected for Ni3Sn2S2@Ni3S2-2 NF and Ni3S2 NF, respectively, which can be attributed to SeHads bonds forming on the surface of catalysts during HER process. The Raman spectrum of SeHads for Ni3Sn2S2@Ni3S2-2 NF has a red shift of ∼28 cm−1 versus that of Ni3S2 NF, which could indicate that the SeHads bonds in Ni3Sn2S2@ Ni3S2-2 NF are weaker than those in Ni3S2 NF to promote hydrogen evolution. The Raman spectrum greatly correspond with computed result, which confirm the fact that SeHads can be effectively regulated by Sn doping and forming hybrid, further achieving an optimal balanced strong H (water) absorption/H desorption toward efficient HER. After HER tests, the Ni3Sn2S2@Ni3S2-2 NF shows similar XPS spectra compared fresh electrocatalyst, only exhibit that the amount of Ni0 derived from Ni2+/3+ and a part of Sn0 comes from Sn2+/4+ are increased after electrolytic process in alkaline media (Figs. 5d and S15). It is observed that the Ni3Sn2S2 dots also steadily embed on the Ni3S2 nanosheets and 3-dimentsionally structures are remained after HER test (Figs. S16 and S17). According to our experimental results and theoretical calculations discussed above, a high-surficial multi-heteroatomic catalysis mechanism is schematically shown in Fig. 6, in which the rational
3. Conclusion In summary, high surficial multi-heteroatomic Ni3Sn2S2 dots-decorated thin Ni3S2 nanosheets was tailored toward highly efficient HER. In particular, Sn atom delocalizes the unpaired electron in the NiA-d 6
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Fig. 6. Schematic illustration of multi-heteroatomic catalysis mechanism toward hydrogen evolution process for Ni3Sn2S2@Ni3S2 NF in alkaline media.
orbitals of Ni3Sn2S2 for its d-band center close to the Fermi level while delocalizing Sn-p states to avoid hybridizing with H-S states. Rich interfaces and artful electronic structure adjustment in Ni3Sn2S2@Ni3S2-2 achieves both strong H absorption and desorption to accomplish fast electron transfer and efficient H2 releasing. This work not only open up a new path to develop novel metal sulfides to replace noble-based metal for practical high-performance industrialized hydrogen evolution, but also provide fundamental insights to design multi-heteroatomic catalysts for an energy conversion process thus possessing universal scientific significance.
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[5] [6]
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CRediT authorship contribution statement
[8]
Shi-Yu Lu: Conceptualization, Investigation, Methodology, Writing - original draft, Writing - review & editing, Funding acquisition. Shengwen Li: Formal analysis, Software. Meng Jin: Investigation. Jiechang Gao: Investigation. Yanning Zhang: Conceptualization, Writing - review & editing, Funding acquisition.
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Declaration of Competing Interest [12]
The authors declare no competing financial interest.
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Acknowledgements
[14]
This work is financially supported by National Nature Science Foundation of China (11874005) and engineering science youth scholars fund program of BIC-ESAT.
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Appendix A. Supplementary data [16]
Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcatb.2020.118675. [17]
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