Anion Engineering on 3D Ni3S2 Nanosheets Array toward Water

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Anion Engineering on 3D Ni3S2 Nanosheets Array toward Water Splitting Qilong Liu, Lingzhi Wei, Qiangchun Liu, Guilin Chen, and Xiangkai Kong ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00710 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 21, 2018

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ACS Applied Energy Materials

Anion Engineering on 3D Ni3S2 Nanosheets Array toward Water Splitting Qilong Liu,† Lingzhi Wei,‡ Qiangchun Liu,*, † Guilin Chen,# and Xiangkai Kong*, †,§ †

School of Physics and Electronic Information, Huaibei Normal University, Huaibei, Anhui 235000, P. R. China § High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei, Anhui 230031, P. R. China ‡ Center of Modern Experiment and Technology, Anhui University, Hefei, Anhui 230601, P. R. China # College of Physics and Energy, Fujian Normal University, Fuzhou, Fujian 350007, P. R. China

KEYWORDS: anion engineering, nanoarrays, electrolysis, hydrogen evolution, oxygen evolution

ABSTRACT: Anion engineering on transition-metal-based materials has been put forward as an important strategy to develop efficient and stable non-noble-metal electrocatalysts toward water electrolysis, including both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). On the basis of theoretical predictions, a three-dimensional (3D) integrated electrode constructed by P-decorated Ni3S2 nanosheet arrays (Ni3S2|P) has been prepared via a facile twostep method. The suitable controlled incorporation of P anions into the Ni3S2 matrix can have little influence on the crystal structure, and meanwhile can effectively modify the electronic structure, increase the concentration of charge carrier, supply more delocalized electrons,

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facilitate more active sites to be electrically activated, optimize the hydrogen adsorption Gibbs free-energy, strengthen the interaction for water molecules, and benefit the oxidation of Ni2+ to Ni3+ oxo-/hydroxides. As a result, the freshly achieved 3D Ni3S2|P electrode exhibits higher activity with lower overpotential toward HER and OER, in comparison with its pristine counterpart. Furthermore, when employed in an overall electrolytic cell as both cathode and anode, it can reduce the required overvoltage of 100 mV for reaching 10 mA cm-2 current density, verifying the great potential of anion engineering in the design of bifunctional materials for overall water splitting.

1. INTRODUCTION Development of high performance energy conversion and storage technologies represents one of the most urgent goals of contemporary research. Molecular hydrogen, a promising sustainable energy vector and important chemical building block, can be a suitable candidate as an ideal replacement for the exhausted fossil fuel, because of its zero carbon footprints and high gravimetric energy density.1 The traditional hydrogen production processes via catalytic steam reforming and coal gasification are complicated, and generally suffer from serious CO2 emission.2 To overcome this issue, water electrolysis has attracted a great deal of attention, which not only can provide rapid hydrogen generation, but also is environmentally benign.3 Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) constitute the two branches of water splitting. Currently, noble-metal-based catalysts have respective performance toward these two half reactions, such as Pt- and Ir/Ru-based materials for HER and OER, respectively.4 Unfortunately, their high cost and natural scarcity impose a big barrier on their


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widespread application in large-scale hydrogen production.5 As such, searching and designing earth-abundant materials with high activity, good stability and low-cost to replace the rare and expensive noble metal electrocatalysts to facilitate HER and OER, particularly for both, are urgently required. There exist multiple approaches to improve the performance of electrocatalysts. For instance, regulating specific morphology,6 optimizing chemical compositions,7 exposing high-index facets8 and heterogeneous structuring9 have been demonstrated to be efficient technologies to afford strong driving force and low the thermodynamic barrier for water splitting reactions. Specifically, composition regulation through introducing heteroatom is generally implemented to enhance the activity of various electrocatalysts, which can be explained by the increased active sites, improved conductivity and enriched free carrier density caused by the introduced heteroatom.10 Recently, anion-heteroatom-engineering has also opened up avenues to acquire extraordinary electrocatalysts. As an efficient technology, anion-modification would also provide effective regulation on the electronic structure of catalytic materials, with little influence on the crystal structure and catalytic active sites, and more benefits is in favor of optimizing the adsorption energy of reactive species to highly strengthen the electrocatalytic activity.11 Jaramillo et al. have introduced sulfur into the surface of molybdenum phosphide (MoP) to produce a molybdenum phosphosulfide (MoP|S) catalyst, which exhibited superb activity and stability for HER in acidic environment.12 Meanwhile, Jin’s group have synthesized cobalt phosphosulfide with a ternary structure (CoPS) featuring distinct (PS)3- anions, which could induce optimal binding energy for hydrogen atoms on the Co sites and thus generate high activity.13 In another report, Wang and his co-workers have systematically investigated the materials synthesis, solid state chemistry, surface structures, and electrocatalytic properties of


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iron phosphosulfide (FePS) nanoparticles, and they discovered a positive correlation between the phosphorus content and their electrocatalytic activity for HER.14 Built on these understandings, it is anticipated that anion engineering, especially for S and P co-modification, has the capacity to optimize chemical and electronic environments of more transition metal-based materials, to further improve their catalytic activity. Heazlewoodite, one nickel-based compound with the formula of Ni3S2, has shown some inherent advantages over others due to its metallic nature with relatively low intrinsic electrical resistivity, which can facilitate rapid charge transfer during the electrochemical process.8,15-17 Some improvements have been achieved for HER and OER via cation V10 and Zn18 doping. However, there is little research on anion P and S co-decorated Ni material up to date. Henceforth, we hypothesize that anion-engineering through P modification can also enhance Ni3S2 activity toward water splitting. Structurely, three-dimensional (3D) electrodes with high active materials supported on the current collect substrate have been proved to be efficient catalysts. Sun’s group have constructed and reported a series of 3D integrated electrocatalysts toward water splitting, which can provide fast diffusion of relevant species in electrolyte, avoid the aggregation of catalyst arrays, and ensure good stability during electrocatalysis.19-23 Furthermore, they have demonstrated that heteratom doping is an effective strategy to enhance the water splitting performance.23-27 In this work, we elucidate our recent effort toward this direction. First-principle calculations have been carried out to address the advantages of P-decorated Ni3S2 and predict its behaviors in hydrogen evolution process. Inspired by the simulation results, we have tried to synthesize an integrated 3D electrode constructed by the desired anion-modified Ni3S2 nanosheet arrays through a facile two-step procedure. This 3D anion-engineered catalyst electrode behaves as a high-performance


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and durable catalyst electrode for both HER and OER. Furthermore, when implemented as both cathode and anode in a full cell, anion-processed catalyst needs of voltage of only 1.69 V , much smaller than 1.79 V for pristine Ni3S2, to drive a geometrical current density of 10 mA cm-2 in alkaline environment, as well as remarkable stability. All of these results make the as-prepared anion-modified 3D catalyst electrode promising for water splitting, and further offer a new way to optimize the catalytic materials for energy conversion and storage.

2. EXPERIMENTAL SECTION Calculations. All computations were carried out using DFT as implemented in VASP 5.2. The PBE-type GGA functional and the projector augmented wave technique were taken to treat the exchange-correlation energy and the ion-electron interaction. For geometric optimization, the atomic positions were relaxed until the forces on atoms were less than 0.02 eV Å-1 and the total energy change was less than 1.0 ×10-5 eV. A 2 × 2 surface unit cell with a six-layer slab model was used for the nanosheet structure. All of the models were calculated in a periodically repeated slab with a more than 12 Å vacuum thickness used in the vertical direction to separate the interaction between same periodic layers. The Brillouin zone was sampled using k-points with 3 × 3 ×1 in the Monkhorst-Pack scheme for geometry optimization. The Gibbs free-energy (∆GH*) was calculated as: ∆GH* =∆EH* +∆EZPE – T∆S, where ∆EH* was the binding energy of atomic hydrogen on the optimized catalyst substrate, ∆EZPE was the difference corresponding to zero point energy between the adsorbed hydrogen and hydrogen in the gas phase and ∆S was one hydrogen entropy between absorbed state and gas phase. As the entropy of hydrogen in absorbed state was negligible, ∆S could be written as -1/2 S0, where S0 was the entropy of H2 in the gas phase (1 bar, 300 K and PH=0). Therefore, the Gibbs free-energy could be simplified to: ∆GH*


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=∆EH* + 0.24 eV. The water adsorption energies were taken as: Eads = Ecatalyst + EH2O – Esystem, where Eads expressed the water adsorbed energy, Ecatalyst, EH2O, and Esystem were the energies of free catalysts, the isolated water molecule and the total energy of the adsorbed H2O on the catalyst surface, respectively. Synthesis of the 3D Ni3S2 nanosheet array. A piece of NF (1 cm × 3 cm) was carefully sonicated in concentrated HCl solution for 10 min to remove the oxides and impurity on surface, and then washed with water, acetone, and ethanol for several times to ensure its surface clean. After that, it was immersed into a 20 ml solution containing 1 mM nickel chloride and 1.5 mM thiourea. After being stirred for 10 min, the solution with NF was transferred into a 25 ml polytetrafluoro-ethylene Teflon-lined stainless steel autoclave and maintained at 200 ℃ for 20 hours. The system was allowed to cool down to room temperature naturally after the reaction. The final product was washed with deionized water and ethanol for several times, and dried under vacuum overnight. Synthesis of the 3D Ni3S2|P nanosheet array. A piece of the obtained 3D Ni3S2 material and 100 mg sodium hypophosphite were put at two separate positions in two porcelain boats in the center of one quartz tube, with sodium hypophosphite at the upstream side of the furnace. N2 was introduced for 20 min before heating to remove the oxygen in the quartz tube and then was kept flowing during the whole heating process. The temperature was increased to 300 ℃ within 30 min and maintained at that temperature for different time (2, 5, and 20 min). Then, the system was cooled to the room temperature immediately. The as-prepared samples were named Ni3S2|P(2), Ni3S2|P, and Ni3S2|P(20), respectively. Characterizations. The samples were characterized by taking TEM images using a JEOL JEM2100 microscope with an accelerating voltage of 200 kV and through a JSM-6610LV SEM. The


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main constituent elements of typical products were also determined by means of energydispersive X-ray spectrometry (EDS, X-Max, Oxford Instruments) attached to the SEM. X-ray diffraction (XRD) patterns were obtained on Panalytical Empyrean with Cu Kα radiation (λ=1.5406 Å). The peeled off Ni3S2|P materials were deposited onto a freshly Si substrate and the thickness of the sample was measured by tapping-mode using an atomic force microscope (AFM) (Multimode 8, Bruker). X-ray photoelectron spectra were recorded using a ESCALAB 250Xi. Raman spectra were measured on a confocal laser microRaman spectrometer (inViaReflex). Electrochemical Measurements. All the electrochemical measurements were carried out in a standard three-electrode system on an electrochemical workstation (CHI 760e, CH Instruments, Inc.). The electrochemical performance were studied in 0.5 M H2SO4 and 1.0 M KOH, respectively, using Ag/AgCl (in 3 M KCl solution) electrode as the reference electrode, a platinum wire as the counter electrode, and the prepared 3D integrated electrode as the working electrode. Pure Ar and O2 were used for purging for HER and OER tests, respectively. The potentials were calibrated and reported respect to reversible hydrogen electrode (RHE). Linear sweep voltammetry (LSV) was conducted with a scan rate of 5 mV/s in all studies. The Nyquist plots were performed with frequencies ranging from 100 kHz to 0.05 Hz at an overpotential of 400 mV in HER measurement. The durability measurement was performed via the chronopotentiometry method to main 10 mA cm-2. The water splitting experiments were conducted in an electrolytic setup, which had cathode and anode with catalysts in two separated glass cells connected with a membrane.

3. RESULTS AND DISCUSSION


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Density functional theory (DFT) calculations were conducted on the pristine and P-incorporated Ni3S2 nanocrystals (denoted as Ni3S2|P), to elucidate the anion-engineering influence on the electronic structure. Figure 1a displays the optimized structure of Ni3S2|P, which almost keeps the configuration of its pristine counterpart (seen in Figure S1 in the Supporting Information), suggesting little influence caused on the crystal structure and catalytic active sites via anion

Figure 1. DFT calculation results for Ni3S2|P and Ni3S2: a) The optimized Ni3S2|P structure. b) Density of states, with the Fermi level set to be 0 eV. c) Charge density distribution with black lines representing the contour lines of the charge density. d) HER free energy diagram calculated at the equilibrium potential, with Ni and S sites marked in Figure 1a taken as the active sites, respectively, which are denoted as “-X”. e) Adsorption energies for molecular H2O adsorbed at the Ni active site.


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processing. Density of states (DOS) have been calculated and shown in Figure 1b. Both materials possess metallic characteristics, guaranteeing rapid electron transport on these electrocatalysts. It is worth noting that the electron contribution to DOS near the Fermi level are different. In detail, more states are induced around the Fermi level after P incorporation, indicating that anion modification can lead to further increased concentration of charge carrier and higher electronic conductivity, which will be in favor of the improvement of electron transfer capacity, and then strengthen the electrocatalytic activities.28 Beside this, the calculated static charge density shown in Figure 1c demonstrates Ni3S2|P materials have a higher charge density and the new formed electron clouds around Ni-P bonds are more delocalized. This is advantageous for electrochemical catalysis, since it can activate and enrich the electrically connected active sites, and further accelerate the water electrolysis process.29 Energy diagrams obtained from the calculated Gibbs free energy change for hydrogen adsorption have been examined. Ni and S atoms are selected as the adsorption sites, respectively, as marked in Figure 1a. It is well accepted, |∆GH*| is a good descriptor for estimating HER, and the optimal value should be close to 0 as revealed by Parsons.30 Figure 1d depicts the free energy diagrams. Remarkably, Ni site exhibits an obviously lower |∆GH*| compared with S adsorbed site on both structures, implying Ni should be the active site for hydrogen evolution, which is in agreement with previous reports.31 The ∆GH* value is calculated to be 0.197 eV on Ni3S2|P, which is significantly less negative than 0.600 eV on its pristine counterpart. Meanwhile, molecular H2O adsorption has also been considered. As seen in Figure 1e, P modification provides higher water molecule binding energy compared with the pristine case, which is beneficial to the water decomposition reaction. As a result, anion engineering on Ni3S2 electrocatalyst via P incorporation can be expected to show enhanced water splitting activities.


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Based on the above DFT hypothesis, Ni3S2|P was prepared using a facial two-step method. In brief, nickel chloride and thiourea were dissolved in deionized water with one piece of tailored nickel foam (NF) immerged in the solution. After the hydrothermal treatment at 180 ℃, Ni3S2 nanosheets were grown on the NF surface to make up a 3D arrays configuration. Then, postcalcination with sodium hypophosphite as the anion precursor was conducted to introduce P into the Ni3S2 matrix, while keeping its original 3D array structure. The detailed preparation can be found in the experimental section, and the fabrication process was illustrated in Figure S2 in the Supporting Information. Figure S3 presents the scanning electron microscopy (SEM) image of bare NF, demonstrating the cross-linked grid configuration with smooth surface. The hydrothermal growth will result in vertically standing Ni3S2 nanosheets on the NF surface (Figure S4 in the Supporting Information). After anion-engineering treatment, the entire NF is fully coated with nanosheets arrays, which are interconnected with each other to form a 3D open network-like structure (Figure 2a). The asprepared Ni3S2|P nanosheets are carefully peeled off from the integrated electrode and transmission electron microscopy (TEM) characterization is performed as shown in Figure 2b, to address its two-dimensional (2D) sheet-like configuration with clear edges. Considering that Ni3S2 belongs to the trigonal crystal system with R32 space group, the Ni3S2|P corners commonly exhibit special angle as marked in the corresponding TEM image, verifying the little influence of anion engineering on crystal structures. The peeled nanosheets are then deposited on a freshly cleaved silicon substrate for thickness measurement using atomic force microscopy (AFM). The height profiles illustrate that the 2D anion-modified nanosheets range from about 30-40 nm in thickness for individual ones (Figure S5 in the Supporting Information), which are consistent with the observation in SEM image. Figure 2c shows the representative high-


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resolution (HRTEM) image of Ni3S2|P with good crystallinity, which will facilitate fast electron transport and contribute to the stability. The measured d spacing for the parallel lattice fringes is 0.204 nm, matching that for the (202) planes of Ni3S2, once again confirming the small influence on crystal structure caused by anion processing. In order to investigate the elemental composition, scanning TEM (STEM) and energy-dispersive X-ray spectroscopy mapping analysis are conducted (Figure 2d), from which, the homogeneous spatial distributions of nickel, sulfur and phosphorus in the as-prepared Ni3S2|P sample can be clearly observed. This not only verifies the successful introduction of P element as designed into the Ni3S2 matrix, but also suggests the robust method for the anion engineering on inorganic functional materials.


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Figure 2. Characterizations of the as-synthesized Ni3S2|P: a) SEM image, b) TEM image, and c) HRTEM image. d) STEM image and corresponding elemental mapping images of Ni, S, and P for Ni3S2|P.

To investigate the structural information, X-ray diffraction (XRD) patterns of these samples are compared as shown in Figure 3a. Obviously, two new diffraction peaks appear on the integrated electrode after the hydrothermal growth (compared with the bare NF, with the Ni signals marked by “#”), matching well with that of reference Ni3S2 crystal (PDF 44-1418).32 It is worth noting that anion treatment has little effect on the XRD pattern, with the Ni3S2 peaks basically remain during the P introduction, especially for the (003) peak at 37.8º. In a control experiment, NF has been directly processed with sodium hypophosphite under the same condition for comparison, denoted as Ni|P. However, no diffraction peaks can be identified except the metallic NF, revealing the validity and little crystal structure influence of P incorporation. Meanwhile, time-dependent experiments have been carried out. Figure 3b and Figure S6 summarize the energy dispersive spectrometer (EDS) results, including the determined elemental ratio change with different anion processing time. The anionic P/S ratio will increase from 0.28 to 0.54 and then to 1.18 with the calcination time extending from 2 to 5 and then to 20 min, supporting the gradual P incorporation into the Ni3S2 matrix.


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Figure 3. a) XRD patterns, b) P/S atomic ratios obtained from EDS spectra, c) Raman spectra, and d) XPS survey specra for the prepared materials. Ni3S2|P(2), Ni3S2|P, and Ni3S2|P(20) standing for the integrated electrodes processed by P engineering for 2, 5, and 20 min, respectively.

To better understand the products, Raman spectra are compared in Figure 3c. Both Ni3S2 and Ni3S2|P possess the characteristic peaks labeled by “*” at 187, 201, 223, 304, 323 and 350 cm-1, which are assigned to Ni3S2 bands.33 However, if the reaction time extends to 20 min, all these peaks will disappear, and the obtained Ni3S2|P(20) exhibits additional Raman bands in the region around 145 and 270 cm-1, indicating the anion engineering is required to be highly controlled to


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avoid serious damage on the Ni3S2 crystal structure. Furthermore, X-ray photoelectron spectroscopy (XPS) experiments are conducted to investigate the surface state change during the P incorporating process. As seen in Figure 3d, the two major peaks ascribing to Ni 2p signals locate around 857 and 875 eV, which are the characteristics of 2p3/2 and 2p1/2, respectively. Besides, the two satellite peaks labeled as 2p3/2 (S) and 2p1/2 (S) are present around.34 Evidently, all the peaks become relatively broader after the anion treatment, and notably, a new peak (as marked in Figure 3d) will gradually evolve at 855 eV with longer phosphorization time, which should be caused by the new formed Ni-P bond.35 In addition, the charge density on active Ni site will turn less positive owing to the smaller electronegativity of P (χ = 2.19) than that of S (χ = 2.58), evidenced by the negative shift of Ni signal. As H adsorption is the key point that influences the activity of HER catalyst, the reduced positive charge on active site can strengthen the interaction between H and catalyst. These results suggest that anion-engineering is effective to modify the electronic structure and it has the large application potential in regulating active sites in the field of high-efficient water-splitting. In order to verify the catalytic benefits of the anion-engineered materials, electrochemical measurements of the as-prepared 3D integrated electrodes are carried out, with the Ni3S2|P mass loading of ca. 0.92 mg cm-2. Techniques of linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) are performed to estimate the hydrogen evolution properties. Figure 4a shows the LSV curves in 0.5 M H2SO4 on the RHE scale using different catalyst materials. A slow scan rate of 5 mV s-1 is applied to reduce the capacitive current. It is clearly seen that Ni3S2|P behaves the best, exhibiting high current density at low overpotential and possessing the lowest onset potential compared to others. It requires an overpotential of 301 mV to reach 100 mA cm-2, much smaller than those for Ni3S2 (357 mV) and


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Ni|P (368 mV). Meanwhile, samples obtained from different anion processing times have also been tested for comparison (seen in Figure S7 in the Supporting Information). However, both of them demand higher overpotentials, which can be explained by the less amount of P incorporation for Ni3S2|P(2) and the serious damage on crystal structure for Ni3S2|P(20). Therefore, the anion engineering can be ajusted, and to get a super catalyst, well controlled with suitable P modification should be paid much attention to. For the sake of catalysis, 10 mA cm-2 current desnity is another pivotal criteria, as it is the approximate current density expected for a solar-tofuels conversion device with 10% overalll efficiency under 1 sun illumination.36 Figure 4b diplayes the required overpotenal for these materials to achieve 10 mA cm-2. Among them, Ni3S2|P needs the lowest ovepotential of 222 mV, which is favorably comparable with Co0.85Se2/NiFe-LDH/graphene (η10 = 360 mV)37, nitrogen-phosphorus doped surface-etched stainless steel mesh (η10 = 330 mV)38, manganese doped nickel-superstructure (η10 = 360 mV)39, and cobalt phosphide/cobalt phosphate thin film (η10 = 380 mV)40, confirming its enhanced activity and verifing our expection. Tafel slop is an inherent property of elctrocatalysts and is useful for analyzing elctrochemcial reaction mechanisms. All these electrodes exhibite slops close to or higher than 60 mV dec-1, suggesting their similar surface chemistry and the Volmer adsorption is the rate-limiting step during hydrogen evolution.41 As depicted in Figure 4c, the Tafel plot obtained from the polarization curves demonstrate the lowest slope of 79 mV dec-1 for Ni3S2|P, inferring its favorable catalytic kinetics for hydrogen generation. The much enhanced performance is also supported by the EIS spectra (Figure 4d), which measures a dramatically smaller interfacial charge transfer resistance (Rct) of anion-modified Ni3S2|P (14.6 Ω) and thus much promoted HER kinetics comparing to the pristine Ni3S2 (26.2 Ω) and Ni|P (27.1 Ω).


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Figure 4. Electrochemical characterizations in 0.5 M H2SO4 electrolyte with pH = 0. a) LSV, b) Overpotentials for reaching 10 mA cm-2, c) Tafel plots, d) EIS at potential of -0.40 V vs. RHE, and e) the calculated double-layer capacitance for the materials.

The electrochemically active surface area (ECSA) is obtained by runing CV at different rates and thus determined by extracting the current difference against the potential scan rate (Figure S8 in the Supporting Information). Figure 4e illustrates small ECSAs on Ni3S2 and Ni|P. As expected, P incorporation has a higher ECSA, indicating more fraction of active sites are electrically activated. The excellent hydrogen generating kinetics can be ascribed to the following factors: 1) anion P can optimize the electronic states of Ni3S2 with little influence on its crystal structure. This will strengthen the H adosorption on suface active sites, and is benefit to overcome the ratedeterming step barrier; 2) Both the metallic NF substrate and its supported Ni3S2|P possess enough conductivity to afford fast electron transfer during the electrocatalysis process, and also it


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can greatly reduce the resistance between the current collector and the loaded catalysts; 3) 3D open structure makes easy access to the electrocatalytic relevant species in electrolyte to the active sites, and facilitates the quick release of formed gas bubbles from the catalyst surface, continuously exposing of active sites to the reaction; 4) the integrated electrode prepared by directly growing catalysts on NF can avoid the use of polymer binder and prevent the catalyst agglomeration, such as for free-standing nanosheets and nanoparticles, which guarantees improved catalytic performance and is in favor of its good stability. Significantly, the anion engineering can also deliver outstanding HER behavior in alkaline environment. Figure 5a records the polarization curves of 3D Ni3S2|P, Ni3S2, and Ni|P measured in 1.0 M KOH solution. It is revealed that Ni3S2|P demands overpotentials of 173 and 298 mV to reach 10 and 100 mA cm-2, respectively. These needed overpotentials are clearly smaller compared to those for Ni3S2 (η10 = 230 mV and η100 = 390 mV) and Ni|P (η10 = 240 mV and η100 = 400 mV). Furthermore, Tafel slop calculations address that the hydrogen evolution process in alkaline should also be assigned to Volmer mechanism, with the water adsorption taken as its rate-limiting step (Figure 5b). Ni3S2|P possesses the smallest slope among these catalysts, suggesting its more facorable reaction kinetics in the alkaline HER process. Meanwhile, we have assessed the performance of anion engineering in OER. Recent research have disclosed that the transion metal sulfides42 and phosphides43 can be electrochemically activated in alkaline media to form oxo-/hydroxide layers on surface, accounting for the observed excellent OER activity. Figure 5c displays their polarization curves during water electro-oxidation. As expected, Ni3S2|P


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Figure 5. Electrochemical characterizations in 1.0 M KOH electrolyte with pH = 14. a) LSV and b) Tafel plots for HER measurements. c) LSV and d) Tafel plots in OER tests. e) Current density – voltage polarization curves (inset is the photo image of the full cell setup). f) Durability test at 10 mA cm-2 for the full cell with 3D Ni3S2|P as both cathode and anode.


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shows high response, with a small onset overpotential of 270 mV, and an anode current density of 100 mA cm-2 at an overpotential of 420 mV, both of which are lower than those of the contrative samples. The anodic peak around 1.5 V of Ni3S2 material associates with the oxidation of Ni2+ to Ni3+, indicating it can be easily oxidized to NiOOH that is important for high activities in the water oxidation reaction.44 Figure S9 and S10 reveal the corresponding structure after OER test, and the formation of NiOOH on surface can further be confirmed by the Raman spectra measurement (Figure S11 in the Supporting Information) and XPS analysis (Figure S12 in the Supporting Information), which are in line with Sun’s results.45 This peak will shift negatively after anion engineering, inferring its alternation on electronic structures, which makes the catalyst be more easily electroactivated. Moreover, the weak Ni oxidation peak on Ni|P reveals its limited active sites on surface, demonstrating the structural merit of Ni3S2|P nanoarrays. Furthermore, Ni3S2|P exhibites a significantly smaller tafel slope in comparison with Ni3S2, supporting the improved OER activity induced by anion engineering (Figure 5d). To demonstrate the practical application of this integrated catalyst electrode, a “H-type” twoelectrode electrolytic cell is fabricated using 3D Ni3S2|P as both cathode and anode in alkaline environment, and is investigated for the full water splitting (inset in Figure 5e). The current density – cell voltage polarization curves are recorded. As shown in Figure 5e, the cell requires significantly less voltage (η10 = 460 mV) than those assembled by Ni3S2 without P incorporation (η10 = 560 mV) and Ni|P (η10 = 570 mV) for achieving a same current density, and is also comparable to the values of recent reported Ni5P4 (η10 = 470 mV)46, NiSe2//Ni(OH)2 (η10 = 540 mV)47, NiFe oxyfluoride nanoporous film (η10 = 570 mV)48, and Co9S8@MoS2 on carbon nanofibers (η10 = 620 mV)49, indicating great potential of anion-engineering for improving catalytic performance (see detail in Table S1). The commercial catalysts with Pt/C and RuO2 as


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cathode and anode (the same mass loading with Ni3S2|P) have also been tested for comparison as seen in Figure 5e, in which, although the noble metal catalysts require a lower voltage to reach 10 mA cm-2, they will show a weakness under high voltages and large current densities. This can be explained by the structural advantage of the in-situ grown 3D electrode. In addition, 3D Ni3S2|P electrolytic cell shows only gradual current density change after 10 h operation (Figure 5f), suggesting the good durability of the catalyst.

4. CONCLUSIONS In summary, based on DFT calculations and predictions, we have described an anion engineering strategy for the synthesis of 3D Ni3S2|P nanosheets arrays material by using a simple, green, and low-cost method. Compared with its pristine counterpart, this integrated 3D electrode will exhibit good stability and improved catalytic activities for both HER and OER under the same condition. Suitable control on the phosphorization is important for introducing foreign anion element into the Ni3S2 matrix, while maintaining its original crystal structure. Such performance improvement is ascribed to the increased charge carrier concentration, the induced more delocalized electrons, newly electrically activated active sites, the optimized hydrogen adsorption Gibbs free-energy, the strengthened interaction for water molecules, as well as the easier oxidation of Ni2+ to Ni3+ oxo-/hydroxides. Furthermore, the unique 3D configuration can provide superior stability during HER and OER processes. These effects confirm the advances of anion engineering, which can be expected to explore the rational design of advanced bifunctional materials for the further application in water splitting system.


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ASSOCIATED CONTENT Supporting Information. Calculations, SEM, AFM and EDS of the materials (PDF) AUTHOR INFORMATION Corresponding Author *E-mail for Q. Liu: [email protected]. *E-mail for X. Kong: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The work was supported by the National Natural Science Foundation of China (51602116), Natural Science Foundation of Anhui Province (1708085QB40), and China Postdoctoral Science Foundation (2016M600492) (to X. K.). The calculations were completed on the supercomputing system in the Supercomputing Center of USTC.

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Anion Engineering on 3D Ni3S2 Nanosheets Array toward Water

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