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P-Doped Iron-Nickel Sulfide Nanosheet Arrays for Highly Efficient Overall Water Splitting Caichi Liu, Dongbo Jia, Qiuyan Hao, Xuerong Zheng, Ying Li, Chengchun Tang, Hui Liu, Jun Zhang, and Xueli Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04528 • Publication Date (Web): 15 Jul 2019 Downloaded from pubs.acs.org on July 16, 2019

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ACS Applied Materials & Interfaces

P-Doped Iron-Nickel Sulfide Nanosheet Arrays for Highly Efficient Overall Water Splitting

Caichi Liu,† Dongbo Jia,† Qiuyan Hao,† Xuerong Zheng,§ Ying Li,† Chengchun Tang,†,‡ Hui Liu,* † Jun Zhang,* †,‡ and Xueli Zheng⊥ †School

of Material Science and Engineering, Hebei University of Technology,

Dingzigu Road 1, Tianjin 300130, P. R. China ‡Hebei

Key Laboratory of Boron Nitride Micro and Nano Materials, Guangrongdao

Road 29, Tianjin 300130, P. R. China §School

of Materials Science and Engineering, Tianjin University, Tianjin Haihe

Education Park, Tianjin 300072, PR China ⊥

Department of Materials Science and Engineering, Stanford University, Stanford, CA

94305, USA

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

KEYWORDS: bifunctional electrocatalysts; phosphorous-doping; iron nickel sulfide; water splitting; DFT calculation.

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ABSTRACT Iron-nickel sulfide ((Ni, Fe)3S2) is one of the most promising bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline media due to their metallic conductivity and low cost. However, the reported HER activity of (Ni, Fe)3S2 is still unsatisfactory. Herein, 3D self-supported phosphorus-doped (Ni, Fe)3S2 nanosheet arrays on Ni foam (P-(Ni, Fe)3S2/NF) are synthesized by simple one-step simultaneous phosphorization and sulfuration route, which exhibits dramatically enhanced HER activity as well as drives remarkable OER activity. The incorporation of P significantly optimized the hydrogen/water

absorption

free

energy

(ΔGH*/ΔGH2O*),

enhanced

electrical

conductivity and increase electrochemical surface area (ECSA). Accordingly, the optimal P-(Ni, Fe)3S2/NF exhibits relatively low overpotential of 98 mV and 196 mV at 10 mA cm-2 for HER and OER in 1 M KOH, respectively. Furthermore, an alkaline electrolyzer comprising the P-(Ni, Fe)3S2/NF electrodes needs a very low cell voltage of 1.54 V at 10 mA cm-2 and exhibits long-term stability, and outperforms most other state-of-the-art electrocatalysts. The reported electrocatalyst activation approach by anion doping can be adapted for other transition metal chalcogenides for water electrolysis, offering great promise for future application.


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INTRODUCTION Hydrogen is considered as one of the most ideal substitution of the traditional energy resources on account of its high energy-density about 282 KJ mol-1, clean combustion product (H2O) and high-efficient electric energy transformation via fuel cells.1-3 Electrocatalytic water splitting, especially powered by renewable energy involving wind and solar energy, has been considered as a promising way for producing clean hydrogen. Compared with hydrogen production by proton exchange membrane (PEM), water electrolysis in alkaline media has the advantages of avoiding usage of expensive noble metal catalysts.4 With the use of a solid alkaline electrolyte, i.e. anion exchange membranes, the defects in alkaline water electrolysis can be further eliminated.5 The critical factor in water electrolysis for future practical application is cell voltage, which contributes greatly to the overpotential of hydrogen evolution reactions (HER) in cathode and oxygen evolution reactions (OER) in anode. Currently, Pt and IrO2/RuO2 are considered as the state-of-the-art alkaline electrocatalysts for HER and OER, respectively.6-7 However, the lack and high cost of these noble metal-based electrocatalysts seriously hinder their commercial application in large-scale. Consequently, it is of great significance to explore efficient earth-abundant transition-metal-based compounds (TMCs) catalysts to replace precious metal-based materials for water splitting.2, 8-12. For the sake of further reduce the cost of manufacturing and simplify the system, developing a bifunctional electrocatalyst with high performance in alkaline electrolytes for both HER and OER is of great importance but still a challenge.13 Traditionally, Ni-based catalysts, such as Raney Ni and Ni alloys, have been widely used in alkaline water electrolyzer due to the low cost and high efficiency. 14-15

9,

Accordingly, a lot of research efforts have been paid to develop bifunctional

catalysts based on Ni-metal chalcogenides,16-17 phosphides,18-20 nitrides,21-22 carbides,23-24 and so on, due to their excellent activities both for HER and OER in alkaline media. Among these, nickel sulfide (Ni3S2) with a suitable d-electron configuration has been received extensive attention as a promising candidate due to its metallic conductivity. Recently, it has been well-established that Fe incorporation into



Ni3S2 electrocatalysts (Ni, Fe)3S2 can effectively boost the OER performance due to the synergistic effect of bimetallic atom, which are even better than that of optimal NiFeOx.25 However, comparing with their superior OER activity, the HER performance of (Ni, Fe)3S2 is far from satisfactory, correspondingly hindering the overall-water-splitting performance. The unsatisfactory HER catalytic activity of (Ni, Fe)3S2 should be attributed to the inadequate electronic structure, leading to very weak absorption of atomic H (H*) on the surface of catalysts, which also means positive H* absorption free energy ( Δ GH*>0).25-27 Hence, in order to enhance their HER performance, further optimizing the electronic structure of (Ni, Fe)3S2 electrocatalysts so as to shift ΔGH* to thermo neutral is essential. Recent studies indicate that the introduction of foreign nonmetal anion into TMCs can modify the electronic structure to shift the Δ GH* toward thermoneutral position.28 Typically, the incorporation of P anion into transition-metal sulfides (TMSs), such as MoS229 and CoS216,

30,

have dramatically enhanced their HER

catalytic activities. DFT calculations further demonstrated both of their Δ GH* are greater than thermo neutral (ΔGH*>0) and the P doping could modify the electronic structure and shift d band center, resulting in the optimization of Δ GH*, possibly because of the more negative electronegativity and larger atomic radius of P than that of S. Note that the ΔGH* of (Ni, Fe)3S2 is also greater than zero, which is similar with that of MoS2 and CoS2, we rationally speculate the P doping in (Ni, Fe)3S2 may also optimize the electronic structure and then promote its HER performance. In addition, the introduction of P not only optimize the intrinsic activities for HER, but also enhances the intrinsic conductivity compared to that of the parent TMSs identified by the DFT calculations and increases active sites due to the distortion of TMSs lattices with different atom radius.28 The improved electrical conductivity and the increased number of catalytic site are also beneficial for the OER process. Encouraged by the aforementioned researches, we infer that the introduction of P into (Ni, Fe)3S2 may


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further improve both the HER and OER performance, ultimately, possibly leading to a low voltage of water electrolysis in alkaline media. Moreover, arrays construction of adhesive-free nanosheets on 3D foam through direct growth electrocatalysts on the current collector can further improve the conductivity and offer multiple active sites, which are also beneficial for improving the electrocatalytic activities. However, to our best knowledge, no attentions have been paid to develop P-doped (Ni, Fe)3S2 nanosheets arrays on 3D Ni foam for electrocatalytic water splitting. Herein, we present a self-supported electrode of P-doped iron-nickel sulfide nanosheet arrays grown on 3D Ni foam (P-(Ni, Fe)3S2/NF) by simple one-step simultaneous phosphorization and sulfuration route on NiFe LDH precursor, exhibiting excellent electrocatalytic activities towards not only HER but also OER under alkaline electrolytes for the first time. DFT calculations and electrochemical tests show that the P-(Ni, Fe)3S2/NF has more favorable H/H2O absorption free energy, better conductivity, faster charge transfer and larger electrochemical surface area than that of (Ni, Fe)3S2/NF. Inspiringly, the P-(Ni, Fe)3S2/NF electrodes acted as both cathode and anode assembled in an alkaline water electrolyzer exhibit a relatively low cell voltage of 1.54 V to obtain 10 mA cm-2 and a high durability under large current density (15 h@ ~100 mA cm-2), which is comparable to most of the state-of-the-art bifunctional electrocatalysts.

EXPERIMENTAL SECTION Materials. Ni foam, Ni(NO3)2·6H2O, FeSO4·7H2O, NH4F, ethyl alcohol, urea and 2-Mercaptoethanol were bought from Macklin. Nafion (5wt %) was purchased from Aladdin. 20wt% Pt/C and IrO2 were purchased from Alfa Aesar. Synthesis of NiFe LDH/NF and Ni(OH)2/NF. The NiFe LDH/NF was prepared via hydrothermal reaction similar to the previous reported procedure.31 Typically, 4 mmol NH4F, 10 mmol urea, 2 mmol of Ni(NO3)2·6H2O and 0.25 mmol of FeSO4·7H2O were first dissolved into 40 ml deionized water (DIW) under stirring. A piece of cleaned NF (1×3 cm2) was placed into an autoclave which contains the



aforementioned solution. The sealed autoclave was heated at the temperature of 120°C for 16 h. The as-prepared NiFe LDH/NF was washed thoroughly with DIW, followed by drying at 80°C under vacuum. Ni(OH)2/NF was prepared via using same procedure except of the FeSO4·7H2O replaced by equal molar of Ni(NO3)2·6H2O. Synthesis of (Ni, Fe)3S2/NF and Ni3S2/NF. Typically, 2.5 mL 2-Mercaptoethanol was well mixed in 25ml of ethanol and transferred into an autoclave, and then a piece of NiFe LDH/NF was dropped into the solution. The autoclave was then heated at the temperature of 150°C for 5 h. In addition, the Ni3S2/NF was obtained by the same process just using a piece of Ni(OH)2/NF as source. Synthesis of P-(Ni, Fe)3S2/NF. In order to obtain P-(Ni, Fe)3S2/NF, one-step simultaneous phosphorization and sulfuration of NiFe LDH process was performed. First, NaH2PO2·H2O with different amount (0.03g, 0.06 g and 0.09 g) was dissolved in 25 ml of hot ethanol and then 2.5 ml of 2-Mercaptoethanol was mixed. Then a piece of NiFe LDH/NF was placed into an autoclave which contains the aforementioned solution. The sealed autoclave was heated at the temperature of 150°C for 5 h. Then, the P-(Ni, Fe)3S2/NF with different P content were obtained, which is defined as Px-(Ni, Fe)3S2. The doping amount of P (x) is defined as: x=n(P)/(n(P)+n(S)) × 100%, where n(P) is the mole amount of P elementary and n(S) represents the mole amount of S elementary. The mole amount is determined from the ICP-AES data. Characterizations. The crystalline structure of the samples was obtained through X-ray diffraction XRD (Rigaku DMax-γA) diffractometer equipped with Cu Kα radiation (λ =1.5406 Å). Scanning electron microscopy (SEM) and EDS (FEI Quanta-450 FEG) were utilized to determine the morphology and elemental composition of the samples. X-ray Photoelectron Spectrum (XPS) were processed on an ESCALAB 250Xi X-ray photoelectron spectroscopy. ICP-AES was performed on Varian 710-ES analyzer. Transmission electron microscope (TEM) was tested on a JEM-2010. Electrocatalytic Measurements. All electrochemical measurements were


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conducted using the electrochemical workstation (CHI604E). Three-electrodes and two-electrodes system were respectively utilized to test the HER/OER performance and overall water splitting. On the base of Nernst equation, all the potentials were adjusted to the reversible hydrogen electrode (RHE) in 1 M KOH. In electrochemical tests we used 1cm×1cm chopped P-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF and Ni3S2/NF as working electrodes. All measurements were carried out in 1 M KOH electrolyte, and N2 or O2 was blew before and during the measurement of HER or OER. After cyclic voltammetry (CV) scanning at a speed of 100 and 20 mV/s for several times to activate the working electrode, the linear sweep voltammetry (LSV) with 90% iR-compensation was scanned at a speed of 2 mV/s. Stability testing was tested by CV scanning at 100 mV/s (-0.3 to 0.1 V for HER; 1.2-1.6 V for OER. vs RHE) for 3000 cycles firstly and the final LSV polarization curves was recorded at 2 mV/s. We measured the electrochemical impedance spectroscopy (EIS) by the electrochemical workstation CHI604E using the A.C. Impedance technology. The impedance experiments were scanned from 1 MHz to 10 mHz at -200 mV (vs RHE) for HER and 250 mV (vs RHE) for OER, respectively, while the amplitude voltage was 5 mV. The electrochemical surface area (ECSA) was evaluated by the electrochemical double-layer capacitance (Cdl), which was obtained through cyclic voltammetry (CV) measurement. The CV was scanned at different scan rates ranged from 5 mV/s to 300 mV/s in a potential window with no faradic processes, of which scope is -0.98- -0.88 V (vs RHE) for HER and 0.1- 0.2 V (vs RHE) for OER. We conducted IR compensation for all the data throughout the paper. In addition, the loading amount of different catalysts is shown in Table S1. Theroetical Calculations. All the density functional theory (DFT) calculations calculated by using Vienna Ab-initio Simulation Package32-33 (VASP), employing the Projected Augmented Wave34 (PAW) method. As has been proved effective, the revised Perdew-Burke-Ernzerhof (RPBE) functional was used to treat the exchange and correlation effects. In all the cases, a 500 eV cutoff energy was chosen. The (210) surface of Ni3S2 was simulated to represent the catalytic interface. The



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Monkhorst-Pack grids35 were set to be 4×4×1 for all the energy calculations. The vacuum layer was set to be 15 Å, which is enough to prevent the vertical interactions between slabs. The system proposed by Norskov et al., where the hydrogen adsorption free energy (∆GH*) was used to describe the HER activity of a given material, was adopted in this work to test the HER performance on Ni3S2.36 In this system, HER is considered as a two-step reaction and contains an adsorbed hydrogen intermediate. The free energy of this intermediate is defined as:

where ∆EH is the binding energy of hydrogen atom, ∆EZPE is the difference in zero point energy between the adsorbed hydrogen and the gaseous hydrogen gas, and T∆S is the entropy difference for the two states. Based on previous studies36, we have chosen a 0.24 eV value in this work to stand for the contribution of zero point energy and entropy. DFT calculations were further performed to discover the nature of the enhanced OER activities of the P-(Ni, Fe)3S2 and (Ni, Fe)3S2. The OER usually involves four reaction steps, which can be described as follows. H2O + *→ HO* +H+ +e- ,

(1)

HO * → O *+H+ +e- ,

(2)

O *+H2O→ HOO* +H+ +e-

,

(3)

HOO*→ + O2+H+ +e- ,

(4)

where the “*” represents the active site. In our calculation, Ni is active site.

RESULTS AND DISCUSSION Electrocatalysts Characterizations The 3D self-supported P-doped (Ni, Fe)3S2 nanosheet arrays were obtained by topotactical

transformation

from

NiFe

LDH

nanosheets

through

one-step

simultaneous phosphorization and sulfuration process using NaH2PO2·H2O and 2-Mercaptoethanol as P and S sources, respectively (Figure S1, Supporting


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Information). First, the NiFe precursor arrays were directly grown on Ni foam with typical Ni/Fe feed ratio by a hydrothermal method. The XRD, EDS, and SEM results (Figure S2) have demonstrated the synthesis of NiFe LDH/NF nanosheets arrays. After simultaneous sulfuration and phosphorization using solvothermal technique, the materials can change its color from dark yellow to black (Figure S3). For comparison, Ni3S2/NF, (Ni, Fe)3S2/NF, and P-doped (Ni, Fe)3S2/NF with different P/(P+S) mole ratios were also fabricated by similar strategy, of which detailed process was described in Experimental Section. XRD results show that no obvious changes of the diffraction peaks for the as-synthesized Ni3S2/NF, (Ni, Fe)3S2/NF and P-(Ni, Fe)3S2/NF can be observed, as illustrated in Figure 1a, and all of them are in a heazlewoodite crystal phase of Ni3S2 (JCPDS NO.44-1418). In addition, no other diffraction peaks of impure phase containing Fe or P compounds for P-(Ni, Fe)3S2/NF can be detected even with P content up to 13.76%, demonstrating the possible doping state of Fe and P in the latter of P-(Ni, Fe)3S2/NF. The morphology of as-prepared P9.03%-(Ni, Fe)3S2/NF was characterized by SEM in Figure 1b. It shows that the product is composed of the well-aligned nanosheets arrays covering the entire Ni foam with a similar morphology to the precursor and other products (Figure S2 (c, d) and Figure S4). EDS (Figure S5) and ICP-AES (Table S2) results demonstrate the existence of elementary Ni, Fe, S and P in the product, and the variation tendency of the P/S molar ratio in the P-(Ni, Fe)3S2/NF is consistent with the P/S precursor ratio, as shown in Figure S5 and Table S3. Moreover, the elemental mapping images (Figure 1c) results also indicate the existence and uniform distribution of elements Ni, Fe, S and P in the P-(Ni, Fe)3S2/NF. The porous nanosheet of P9.03%-(Ni, Fe)3S2 can be seen from the transmission electron microscope (TEM) image (Figure 1d). The corresponding HRTEM image in Figure 1e shows distinct lattice fringes with interplanar spaces of 0.23, 0.24 and 0.29 nm, corresponding to the (021), (003) and (110) plane of Ni3S2. The angle between (021) and (003) crystalline planes is about 70.7°, indicating the exposed crystalline plane is (210). We note that the high-index



(210) planes of Ni3S2 have stronger catalytic activities than that of low-index planes for HER and OER,37 leading to the highly efficient performance for water splitting.

Figure 1. (a) XRD spectra of PX%-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF and Ni3S2/NF. (x=4.12, 9.03 and 13.76), (b) SEM, (c) EDS elemental mapping, (d) TEM, and (e) HRTEM images of P9.03%-(Ni, Fe)3S2.

X-ray photoelectron spectroscopy (XPS) was utilized to survey not only the surface chemical compositions and valence states of the P-(Ni, Fe)3S2 nanosheets, but also the change in electronic structure after P-doping. First, the XPS spectra confirm the presence of Ni, Fe, S and P in the product (Figure S6a). Figure 2a shows the high-resolution Ni 2p spectra of P9.03%-(Ni, Fe)3S2/NF and (Ni, Fe)3S2/NF. The peaks located at 873.5 eV and 855.8 eV in P9.03%-(Ni, Fe)3S2/NF are assigned to Ni 2p1/2 and Ni 2p3/2 respectively, characterizing the presence of Ni3+ ion, while another two peaks located at 870.0 eV and 853.0 eV are ascribed to Ni 2p1/2 and Ni 2p3/2 of Ni2+ ion, respectively. Meanwhile, the two satellite peaks located at 879.5 eV and 861.6 eV are belonged to the shakeup type peaks of Ni.38 Note that the Ni 2p peaks are slightly shifted to higher binding energies compared with (Ni, Fe)3S2/NF, indicating the decrease of electron density around Ni cations by P-doping. Correspondingly, the decrease of electron density can produce more empty d orbitals to promote the adsorption of hydrogen species, thereby boosting the electrocatalytic HER properties.39-40 In addition, by calculating the peak areas, it can be clearly observed that the Ni3+/Ni2+ ratio of P9.03%-(Ni, Fe)3S2/NF (7.56) is obvious larger than that of (Ni, Fe)3S2/NF (2.25). The increase of Ni3+ numbers may result in an enhanced OER activity for P9.03%-(Ni, Fe)3S2/NF, which is attributed to a stronger binding energy of the absorbed OH- species on the Ni3+ sites than that of on Ni2+ sites due to a higher


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electron affinity and more electron configuration of Ni3+ (t2g6eg1).41-43 For the XPS spectrum of Fe 2p in P9.03%-(Ni, Fe)3S2/NF (Figure 2b), two prominent peaks located at 725.6 eV and 711.9 are assigned to Fe 2p1/2 and Fe 2p3/2, respectively, while the presence of other two satellite peaks located at 715.6 and 735.8 eV indicates that the Fe is present in the form of Fe3+ ion in the products.44 Two main peaks are slightly shifted to lower binding energies compared with (Ni, Fe)3S2/NF, indicating that the electronic environments of Fe species was modulated by the P introduction. Such negative shift phenomenon is also observed in the spectrum of S 2p for P-(Ni, Fe)3S2/NF (Figure 2c). Two characteristic peaks in P9.03%-(Ni, Fe)3S2/NF located at the binding energy of 163.5 eV and 162.3 are assigned to S 2p1/2 and S 2p3/2 belonged to S2−, while another small peak located at 168.3 eV is assigned to the sulfate species possibly generated by the oxidation of sulfides in air.45 Moreover, a weak peak at 161.3 eV can be assigned to the Fe-S bond.46 The core-level spectrum of P 2p is shown in Figure 2d. It can be observed that the two peaks at 130.9 eV and 130.3 eV can be assigned to P 2p1/2 and P 2p3/2, characterizing the formation of metal-phosphorus bond.46 Meanwhile, two strong peaks located at 132.9 eV and 133.7 eV are ascribed to phosphate, which may origin from the surface oxidation in air.47 In addition, the O 1s XPS spectrum peaks (Fig. S6b) could be assigned to adsorbed O, P-O, O=P and O from the surface of absorbed H2O, respectively.47-48 Meanwhile，no signal of metal-oxygen (M-O) bond can be observed, implying no formation of metal oxides or hydroxides. The aforementioned XPS results demonstrated the successful incorporation of P into the (Ni, Fe)3S2, resulting in an effective regulation of the electronic structure of (Ni, Fe)3S2, which may boost both the HER and OER catalytic activities as an outstanding bifunctional catalyst for electrochemical water splitting.



Figure 2. XPS spectra of (a) Ni 2p, (b) Fe 2p, (c) S 2p, and (d) P 2p of P9.03%-(Ni, Fe)3S2 and (Ni, Fe)3S2.

HER Catalytic Performance The electrocatalytic activity of the as-synthesized 3D P-(Ni, Fe)3S2/NF for HER was measured in 1 M KOH under a typical three-electrodes system, while NF, Ni3S2/NF, (Ni, Fe)3S2/NF and 20 wt% Pt/C deposited on NF were also evaluated for comparison in Figure 3a and S7. It can be seen that all of the three P-(Ni, Fe)3S2/NF sample show better HER activity than that of NF, Ni3S2/NF and (Ni, Fe)3S2/NF, getting close to that of Pt/C. Such result suggests that the P-doping is of great importance for enhancing the HER catalytic performance. In addition, among all of the LSV curves of with different P content, the P9.03%-(Ni, Fe)3S2/NF displays the best HER performance with the overpotential of 98 mV, 126 mV and 218 mV at 10, 20 and 100 mA cm − 2 (Figure 3b), respectively, which is competitive to most of the reported Ni3S2-based and other HER electrocatalysts (Table S4). Moreover, the HER kinetics of P-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF, NF and 20 wt% Pt/C were also evaluated by Tafel plots, as shown in Figure 3c. We have measured the LSV curves of every samples with different phosphorus doping for 6 times and took the LSV curves which is closed to average value. The error bar of the HER performance (Figure S8) show that repeatability is very well. The P9.03%-(Ni, Fe)3S2/NF electrode


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displays a Tafel slope of 88 mV dec-1, which is lower than that of NF (184 mV dec-1), Ni3S2/NF (125 mV dec-1) and (Ni, Fe)3S2/NF (120 mV dec-1), indicating a more efficient kinetics for hydrogen evolution. The Tafel slopes of NF, Ni3S2/NF and (Ni, Fe)3S2/NF suggest that the process for HER may be dominated by Volmer step, while Volmer-Heyrovsky reaction involves in P-(Ni, Fe)3S2/NF.49-50 The result suggests that the reaction kinetics has been enhanced by P-doping during HER process, in which both the water dissociation and hydrogen adsorption rates have been accelerated.51 In order to evaluate the durability of the as-prepared P9.03%-(Ni, Fe)3S2/NF electrode, the LSV curve after scanning of 3000 cycles were measured, which was almost no decay, demonstrating the strong durability of P9.03%-(Ni, Fe)3S2/NF electrode for HER in 1 M KOH, as shown in Figure 3d. Inset of Figure 3d shows that the electrode was able to operate at an overpotential of -250 mV (vs RHE) with negligible degradation after 15 h test, indicating that P9.03%-(Ni, Fe)3S2 own excellent stability for HER in alkaline media. In addition, the post-HER measured electrode materials were characterized by XRD, SEM, TEM and XPS, as shown in Figure S9 and S10. The results indicate that the sample is stable under alkaline testing.

Figure 3. (a) The LSV curves of the HER performance of PX%-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF and Pt/C/NF. (x=4.12, 9.03 and 13.76). (b) The comparison of overpotential at 10, 20 and 100 mA cm−2. (c) Tafel plots derived from (a). (d) The LSV curves of P9.03%-(Ni, Fe)3S2/NF after the 1st, 3000th CV (Inset: Long-term stability test of P9.03%-(Ni, Fe)3S2/NF at 250 mV (vs RHE)).



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To realize why the introduction of P into the (Ni, Fe)3S2/NF is beneficial for the improvement

of

HER

performance,

electrochemical

tests

involving

the

electrochemical impedance spectroscopy (EIS) and electrochemical surface area (ECSA) were performed. First, to explore the electrode HER kinetics, EIS measurement was implemented, which provided more details about the interfacial reactions and electrode kinetics behavior of the catalysts. As shown in Figure 4a and Table S5, the Nyqusit plots reveal that the P9.03%-(Ni, Fe)3S2/NF possesses the smallest charge-transfer resistance (Rct) of 1.56 Ω, indicating a faster charge-transfer kinetics between the P9.03%-(Ni, Fe)3S2/NF and electrolyte during the HER process. The P introduction in P9.03%-(Ni, Fe)3S2/NF may account for the highest evolution kinetics, which is consistent with the smallest Tafel slope. Furthermore, the ECSA was also utilized to realize the HER catalytic activity, which is generally assessed by the double layer capacitance (Cdl) of the catalysts due to their positive correlation. Figure 4b and Table S5 show that the P9.03%-(Ni, Fe)3S2/NF has the largest Cdl among all the samples, indicating the P9.03%-(Ni, Fe)3S2/NF has more catalytic activity sites, which is beneficial for HER process. Moreover, to exclude the contribution of ECSA for HER performance, we defined the normalized HER current density as j , which is equal to the HER current density (j) divided by its Cdl, and the j (j/Cdl) can be used to evaluate the intrinsic activity of a catalyst.52 The result shown in Figure S11 indicates that the P9.03%-(Ni, Fe)3S2/NF has larger j value than that of other samples, demonstrating that the enhanced HER activity is not only attributed to the increased ECSA, but also the improved intrinsic activity of a catalyst due to P dopant. In order to acquire a deep understanding of the improved intrinsic HER activity of P-(Ni, Fe)3S2, we carried out the DFT calculations and first-principle on the (Ni, Fe)3S2 and P-(Ni, Fe)3S2. As is well known, the adsorption free energy of H* on the surface of catalysts has been widely accepted as a good descriptor to evaluate the intrinsic catalytic activities toward HER. According to the Sabatier principle, the absorbed ability of H* for an ideal HER catalyst should be neither too strong nor too weak, suggesting the idea Δ GH* is thermoneutral.53 Herein, we performed DFT to


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calculate the ΔGH* of (Ni, Fe)3S2 before and after incorporation of P. In consideration of the HRTEM result (Figure 1e), which demonstrates that the (210) and (001) planes are exposed surface, we choose the (210) planes as the basal surface of H* adsorption for DFT calculation, because the catalytic activities of (210) planes is higher than that of (001) planes.37 The crystal structure model of (Ni, Fe)3S2 and P-(Ni, Fe)3S2 for DFT calculation are shown in Figure 4c. The Ni, S and P atomic sites other than Fe site are considered as the possible adsorption sites of H* during HER process.52 The DFT calculations (Figure 4d) show that the ΔGH* of (Ni, Fe)3S2 are 1.34 and 0.92 eV for Ni and S sites, respectively. After dopant of P into (Ni, Fe)3S2, the ΔGH* for Ni and S sites is further optimized to 1.26 and 0.76 eV, respectively. Particularly, for the P sites of P-(Ni, Fe)3S2, the ΔGH* is reduced to only -0.1 eV, which is close to the thermoneutral site, demonstrating the superior intrinsic HER catalytic activities of P-(Ni, Fe)3S2. Moreover, for most of HER catalysts, their catalytic activities toward HER in alkaline media are more inferior than that of those in acid media, which is ascribed to different Volmer step in acid media (H3O+ + e- → H* + OH-) and alkaline media (H2O + e- → H* + OH-). Therefore, the catalysts in alkaline media require to break the stronger covalent bond of H-OH prior to adsorb H*, resulting in poorer catalytic activity for most of catalysts.54 Therefore, the water activation process for most of the HER catalysts in alkaline media is a rate-determining step, which can be evaluated by the adsorption free energy of water.52 Figure S12 shows the calculated adsorption energy of H2O for (Ni, Fe)3S2 and P-(Ni, Fe)3S2. It can be observed that the H2O adsorption energy of P-(Ni, Fe)3S2 (-0.83 eV) is much lower than that of (Ni, Fe)3S2 (-0.15 eV), suggesting an easier water activation process for P-(Ni, Fe)3S2, which is beneficial for the improvement of HER catalytic activities in alkaline condition. In addition, Figure 4e presents the density of states (DOS) of the (Ni, Fe)3S2 and P-(Ni, Fe)3S2. It can be clearly seen that the sequential DOS around the Fermi level for both of them demonstrates their intrinsic metallic properties. Obviously, the P-(Ni, Fe)3S2 has stronger DOS near the Fermi level than that of (Ni,



Fe)3S2, suggesting more carrier concentration for P-(Ni, Fe)3S2, which will enhance its electrical conductivity. The increased electrical conductivity for the electrocatalysts is beneficial for either HER or OER performance.27 Based on the above results, the superior HER activity of the P-(Ni, Fe)3S2/NF can be attributed to the following reasons: (1) The reaction kinetics has been enhanced by P introduction; (2) The P-(Ni, Fe)3S2/NF electrode possess larger active surface area; (3) P-dopant optimizes the Δ GH* and adsorption free energy of water, which increase the intrinsic activity; (4) The electrical conductivity has been enhanced to facilitate the electron transfer.

Figure 4. (a) Nyquist plots of PX%-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF. (b) Calculated electrochemical double-layer capacitance for PX%-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF. (x=4.12, 9.03 and 13.76). (c) Top and side view of the (2̅10) surface of (Ni, Fe)3S2/NF and P-(Ni, Fe)3S2/NF. (d) The free energy diagram of HER on different sites of (Ni, Fe)3S2/NF and P-(Ni, Fe)3S2/NF. (e) The projected Density of States (DOS) on the (2̅10) surface of (Ni, Fe)3S2/NF and P-(Ni, Fe)3S2/NF.

In order to further realize the surface change of the post-HER catalyst, many characterization methods including XRD, SEM, TEM, EDS and XPS were performed. As shown in Figure S9, no obvious change for the XRD pattern of post-HER


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P9.03%-(Ni, Fe)3S2/NF can be observed, demonstrating no new crystalline phase generated even under high cathodic potential. The SEM result (Figure S9b) indicate the maintaining of nanosheets arrays after HER. However, we carefully observed the HRTEM images of the post-HER catalyst (Figure S9c) and found the edge of the nanoplate became more disorder than that of the catalyst before HER (Figure S9d). The disorder area is composed of small polycrystal and amorphous region. The lattice distance in polycrystal region are 0.23 and 0.24 nm, respectively, corresponding to the (021) and (003) planes of Ni3S2. We also use XPS to study the surface chemical state of the catalyst after HER, as shown in Figure S10. In contrast to the XPS of the catalyst before HER (Figure 2), the core-level spectrum of Ni 2p after HER shows obvious change (Figure S10a). But the core-level spectrum of Fe 2p after HER is very different (Figure S10b). Two new peaks located at the binding energy of 876.2 and 857.9 eV are emerging in the spectrum of Ni 2p of the catalyst after HER, which can be ascribed to nickel oxides or hydroxides, respectively.55 Meanwhile, we found a disordered structure constituted of small polycrystal and amorphous region on the surface. This consistent with the above results. In addition, It can be seen that two new peaks located at 722.4 and 707.3 eV appear, which are assigned to 2p1/2 and 2p3/2 of Fe2+ ion.56 And the peak intensity of Fe3+ 2p decrease rapidly, demonstrating the Fe3+ ion is reduced to Fe2+ ion under cathodic reduction potential. The observed disorder structure including amorphous region and small polycrystal observed in Figure S9c may be attributed to the formation of nickel oxides or hydroxides and transformation from Fe3+ to Fe2+. In the spectrum of S 2p (Figure S10c), it can be obviously seen that the peak of sulfate disappears after HER, implying that it is reduced under high cathodic reduction potential. Furthermore, no big difference of the core-level spectrum of P 2p before and after HER shows the stable existence of P anion in crystalline lattice even under high cathodic potential. And the O 1s XPS spectrum peaks (Fig. S10e) could be assigned to M-O, M-OH, and absorbed O, respectively. The above results are confirmed about the form of nickel oxides or hydroxides.



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OER Catalytic Performance The catalytic performance of P-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF, NF and IrO2/NF toward OER was also displayed in Figure 5 and S7. The redox peaks at ~1.32 V (vs. RHE) is attributed to the reduction of Ni3+ to Ni2+.57 Figure 5a shows that the introduction of Fe into Ni3S2/NF can obviously improve the OER activity of Ni3S2/NF, which agrees well with the reported works.25, 58 Further introduction of P into (Ni, Fe)3S2/NF regardless of the P contents can efficiently enhance the OER activity of (Ni, Fe)3S2/NF. Especially, the P9.03%-(Ni, Fe)3S2 electrode exhibits the best OER activity among the P-(Ni, Fe)3S2 electrodes with different P content, only requiring low OER overpotentials (ηOER) of 196, 202 and 246 mV to drive the current density of 10, 20 and 100 mA cm-2 (Figure 5b), respectively, which is better than that of as-synthesized (Ni, Fe)3S2/NF (224 ， 239 and 277 mV), Ni3S2/NF (303, 328 and 396 mV) NF (490, 540 and 670 mV) and benchmark OER catalyst of IrO2/NF (324， 345 and 450 mV). Meanwhile, the Tafel slope of the as-synthesized P9.03%-(Ni, Fe)3S2/NF is 30 mV dec-1, which is not only smaller than that of (Ni, Fe)3S2/NF (41 mV dec-1), Ni3S2 /NF (78 mV dec-1), NF (170 mV dec-1), but also superior to the benchmark OER catalyst of IrO2/NF (69 mV dec-1) (Figure 5c). The smaller Tafel slope suggests favorable OER kinetics over the P9.03%-(Ni, Fe)3S2/NF. To our best knowledge, the OER activity of our as-synthesized P9.03%-(Ni, Fe)3S2 is comparable or better than most of the optimal OER catalysts (Table S6), such as N-Ni3S2/NF (ηOER(100mA -2 cm )=282

-2 cm )=330

mV, Tafel slope =79 mV dec-1),27 Fe-Ni3S2/FeNi (ηOER(10

mA

mV, Tafel slope =54 mV dec-1),45 Ni1.5Fe0.5P/CF (ηOER(100mA cm-2)=293 mV,

Tafel slope =55 mV dec-1),59 Ni0.75Fe0.125V0.125 LDHs/NF (ηOER(10

-2 mA cm )=231

mV,

Tafel slope =39.4 mV dec-1),60 MoS2/Ni3S2 (ηOER(10 mA cm-2)=218 mV, Tafel slope=88 mV dec-1)61 and NiFeLDH@NiCoP/NF (ηOER(10

-2 mA cm )=220

mV, Tafel slope=48.6

mV dec-1)47. Cyclic voltammogram (CV) scan for 3000 cycles was used to evaluate the stability of the as-synthesized P9.03%-(Ni, Fe)3S2/NF electrode, shown in Figure 5d. There was almost no decay of the LSV curves for OER, demonstrating the outstanding stability of the as-synthesized P9.03%-(Ni, Fe)3S2/NF electrode in alkaline solution. As shown in inset of Figure 5d, the OER current density of this electrode


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shows slightly change after 15 h test at an overpotential of 1.525 V (vs RHE), indicating its excellent stability of P9.03%-(Ni, Fe)3S2/NF electrode for OER in 1 M KOH. To understand the excellent OER catalytic activity of P-(Ni, Fe)3S2/NF, the EIS and the Cdl measurements were also performed. The EIS results (Figure 5e and Table S5) display that the P9.03%-(Ni, Fe)3S2/NF electrode has the lowest charge transfer resistance of (1.12 Ω ) among all the samples, indicating a faster charge-transfer kinetics during the OER process. Furthermore, as shown in the Figure 5f and Table S5, the P9.03%-(Ni, Fe)3S2/NF reveals the highest Cdl of 18.7 mF cm-2 among all the samples, indicating the electrocatalyst has more catalytic activity sites, which is beneficial for the improvement of OER catalytic activity. In addition, the higher valence state of the Nin+ sites benefit the electrostatic adsorption of OH- ions to increase the intrinsic activity for OER process, which has been identified by above XPS results (Figure 2). Figure S13(a-h) shows the crystal structure model after being optimized for (Ni, Fe)3S2 and P-(Ni, Fe)3S2.The (Ni, Fe)3S2 and P-(Ni, Fe)3S2 (210) surface with 10 layers was selected for the OER steps and the bottom six layers were fixed. Figure 13(i-j) displays the adsorption free energies for the four reaction steps of OER catalyzed by (Ni, Fe)3S2 and P-(Ni, Fe)3S2 at different potentials. It can be seen that there exist endothermic reactions in both case of 0 V and 1.23 V. Until the applied potential increases to 3.50 V and 2.59 V, the free energies for all steps of (Ni, Fe)3S2 and P-(Ni, Fe)3S2 run downhill, respectively. This indicates that based on the normal hydrogen electrode model, the overpotential of P-(Ni, Fe)3S2 is less than (Ni, Fe)3S2 by 0.91 V. All of the above results demonstrate that P-(Ni, Fe)3S2/NF has better intrinsic OER activity than (Ni, Fe)3S2/NF. To further ensure the actual active species of P9.03%-(Ni, Fe)3S2/NF during OER process, the electrode after OER measured was characterized by XRD, SEM, TEM and XPS, as shown in Figure S14 and S15. Comparing with the initial P9.03%-(Ni, Fe)3S2/NF, no additional peaks could be observed from the XRD patterns for the post-OER electrocatalyst (Figure S14a), revealing that no new crystal phase was formed after OER. The SEM result (Figure S14b) demonstrates the maintaining of



nanosheets arrays after OER. The HRTEM image of the catalyst after OER (Figure S14d) shows the surface of the nanoplate has an emerging amorphous layer with thickness of 5 ～ 8 nm, which is obviously different from the sample before OER (Figure S14c). XPS (Figure S15) was also utilized to reveal the change of the surface of the post-OER catalyst in terms of chemical composition and valence state. In comparison with the core-level spectrum of Ni 2p shown in Figure 2a, three new peaks located at the binding energy of 877.2, 875.3 and 856.8 eV are emerging in the spectrum of Ni 2p of the catalyst after OER (Figure S15a), which can be ascribed to (Ni, Fe)OOH and (Ni, Fe)(OH)2 respectively.56 In the spectrum of S 2p (Figure S15c), it can be obviously seen that the intensity ratio of sulfate/metal-S peaks is increasing after OER, implying that the surface S anion is oxidized under high anodic oxidation potential. Hardly any peaks of P 2p can be found in Figure S15d, also indicating the surface oxidation. This result is also consistent with the composition content analysis characterized by XPS, of which P mole amount is 9.03% before and 0.42% after. In addition, the strong peak assigned to Ni(Fe)-O in the O 1s spectrum (Figure S15e) also demonstrated the surface of the post-OER P9.03%-(Ni, Fe)3S2/NF is mainly composed of nickel iron oxides. Based on the aforementioned discussion, we realized the surface of post-OER P9.03%-(Ni, Fe)3S2/NF electrode is composed of amorphous (Ni, Fe)OOH and (Ni, Fe)(OH)2. Note that similar surface oxidation of bifunctional electrocatalysts during OER have been largely reported in recent years including transition-metal chalcogenides, phosphides and nitrides, and the formed oxides/hydroxides on the surface of bifunctional electrocatalysts promote to their excellent OER activity during OER.62


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Figure 5. (a) The LSV curves of the OER performance of PX%-(Ni, Fe)3S2/NF, (Ni,Fe)3S2/NF, Ni3S2/NF and IrO2/NF. (b) The comparison of overpotential at 10, 20 and 100 mA cm−2. (c) Tafel plots derived from (a). (d) The LSV curves of P9.03%-(Ni, Fe)3S2/NF after the 1st, 3000th CV (Inset: Long-term stability test of P9.03%-(Ni, Fe)3S2/NF at 295 mV (vs RHE)). (e) Nyquist plots of PX%-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF. (f) Calculated electrochemical double-layer capacitance for PX%-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF. (x=4.12, 9.03 and 13.76)

Overall Water Splitting Owing to the its excellent electrocatalytic activity and stability for both HER and OER process, the as-synthesized P9.03%-(Ni, Fe)3S2/NF electrode has potential to act as an ideal bifunctional electrocatalyst for overall water splitting. Two-electrodes system with two pieces of P9.03%-(Ni, Fe)3S2/NF working as both cathode and anode in 1 M KOH was utilized to evaluate the electrocatalytic performance for full water splitting. As shown in Figure 6a, to deliver a current density of 10, 20 and 100 mA cm-2, it only needs the cell voltage of 1.54, 1.58 and 1.72 V. From inset of Figure 6a, obvious gas evolution could be watched at both electrodes. The low potential of the



Page 22 of 35

as-synthesized P9.03%-(Ni, Fe)3S2/NF electrode for full water splitting is better than that of (Ni, Fe)3S2/NF (1.62, 1.67 and 1.80 V), Ni3S2 /NF (1.71, 1.76 and 1.97 V) and most of the recently reported bifunctional electrocatalysts for overall water splitting (Table S7). Meanwhile, the long-term stability test of the electrolyzer was also measured

by

the

chronoamperometry,

shown

in

Figure

6b.

From

the

chronoamperometry curve of the electrolyzer for 15 h test, it can operate with negligible degradation under high current density (~100 mA cm-2), implying its superior stability. Moreover, the faradaic efficiency of P9.03%-(Ni, Fe)3S2/NF electrode are almost 100% for both HER and OER with the 2:1 molar ratio of H2 and O2.( Figure S16). Finally, a commercial dry battery (1.5 V) was utilized to drive the electrolyzer using P9.03%-(Ni, Fe)3S2/NF as both anode and cathode in 1 M KOH, and continuous H2 and O2 bubbles can be seen on both of electrodes (Figure S17 and Video S1). These results demonstrate that novel P-(Ni, Fe)3S2/NF is promising candidate catalyst in efficient and durable alkaline water electrolysis.

Figure 6. (a) The LSV curve of water electrolysis using P9.03%-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF. (b) Long-term stability test of P9.03%-(Ni, Fe)3S2/NF for water splitting at 1.8 V.

CONCLUSIONS In conclusion, a novel, self-supported P-(Ni, Fe)3S2 nanosheets arrays grown on 3D NF as a bifunctional electrocatalyst was reported, exhibiting superior catalytic activities for both HER and OER in alkaline media. Based on systemic experiments and DFT calculation, it revealed that the introduction of P into (Ni, Fe)3S2/NF will increase the ECSA, enhance the electrical conductivity, optimize (ΔGH*) and (ΔGH2O*),

leading to an improved HER performance. Moreover, the P doping could

increase the ratio of Ni3+/Ni2+ in (Ni, Fe)3S2/NF, resulting in stronger OH- absorption,


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which efficiently enhanced the OER activity of (Ni, Fe)3S2/NF. Furthermore, it was found that the surface of P-(Ni, Fe)3S2/NF was converted to (Ni, Fe)OOH and (Ni, Fe)(OH)2 during OER, which were actual OER activity sites. P-(Ni, Fe)3S2/NF can be utilized as both anode and cathode during alkaline water splitting, exhibits a low cell voltage of 1.54 V at 10 mA cm-2 and a high durability under large current density (15 h@ ~100 mA cm-2) in 1 M KOH, which is superior to most of the state-of-the-art bifunctional electrocatalysts. This work will not only provide an in-depth understanding of the anion doping effects on transition metal dichalcogenides but also shed light on the rational design of efficient bifunctional electrocatalysts for clean energy applications.

ASSOCIATED CONTENT Supporting Information The schematic illustration of the synthesis process for the P-(Ni, Fe)3S2 grown on Ni foam (Figure S1); XRD pattern, EDS spectrum and SEM images of NiFe LDH/NF(Figure S2); Photographic images of Ni foam (NF), NiFe LDH/NF and P-(Ni, Fe)3S2/NF(Figure S3); SEM image of P4.12%-(Ni, Fe)3S2/NF, P13.76%-(Ni, Fe)3S2/NF

Ni3S2/NF and (Ni, Fe)3S2/NF(Figure S4); EDS elemental mapping of

P4.12%-(Ni, Fe)3S2/NF, P9.03%-(Ni, Fe)3S2/NF, P13.76%-(Ni, Fe)3S2/NF (Figure S5); Survey and O 1s XPS spectra of P9.03%-(Ni, Fe)3S2/NF (Figure S6); HER and OER polarization curves for NF and Tafel plots(Figure S7); The error bar of the HER performance of PX%-(Ni, Fe)3S2/NF(Figure S8); XRD pattern, SEM, HRTEM image of the edge of P9.03%-(Ni, Fe)3S2/NF after HER, HRTEM image of the edge of P9.03%-(Ni, Fe)3S2/NF before HER, and EDS elemental mapping of P9.03%-(Ni, Fe)3S2/NF after HER. (Figure S9); Ni 2p, Fe 2p, S 2p, P 2p and O 1s XPS spectra of



P9.03%-(Ni, Fe)3S2/NF(Figure S10); Comparison of j between P9.03%-(Ni, Fe)3S2/NF and (Ni, Fe)3S2/NF at different overpotential (Figure S11); The water adsorption energy of (Ni, Fe)3S2/NF and P-(Ni, Fe)3S2/NF(Figure S12); The theoretical models of OER steps on (Ni, Fe)3S2/NF and

P-(Ni, Fe)3S2/NF involved the adsorption of

HO*, O*, HOO*. Free energy profiles for

(Ni, Fe)3S2/NF and P-(Ni, Fe)3S2/NF at

zero potential (U = 0 V), equilibrium potential for oxygen evolution (U = 1.23 V), and minimum potential where all steps are downhill (Figure S13). XRD pattern, SEM, HRTEM and EDS image of P9.03%-(Ni, Fe)3S2/NF after OER (Figure S14); Ni 2p, Fe 2p, S 2p, P 2p and O 1s XPS spectra of P9.03%-(Ni, Fe)3S2/NF and (Ni, Fe)3S2 after OER (Figure S15); Faradic efficiency measurement of P9.03%-(Ni, Fe)3S2/NF showing the theoretically calculated and experimentally measured gas with time (Figure S16); Photograph of overall water splitting powered by 1.5 V battery (Figure S17); The loading amount of different catalysts (Table S1); ICP-AES data of P4.12%-(Ni, Fe)3S2/NF, P9.03%-(Ni, Fe)3S2/NF, P13.76%-(Ni, Fe)3S2/NF, P9.03%-(Ni, Fe)3S2/NF after HER and P9.03%-(Ni, Fe)3S2/NF after OER(Table S2); S/P Feed Ratio and Atomic Ratio of the P4.12%-(Ni, Fe)3S2/NF, P9.03%-(Ni, Fe)3S2/NF, P13.76%-(Ni, Fe)3S2/NF, P9.03%-(Ni, Fe)3S2/NF after HER and P9.03%-(Ni, Fe)3S2/NF after OER (Table S3); Comparison of the HER performance of P9.03%-(Ni, Fe)3S2/NF with other well-performed electrocatalysts (Table S4); EIS and Cdl results of PX%-(Ni, Fe)3S2/NF, (Ni, Fe)3S2/NF, Ni3S2/NF (Table S5); Comparison of the OER performance of P9.03%-(Ni, Fe)3S2/NF with other well-performed electrocatalysts (Table S6); Comparison of the Overall water splitting performance of P9.03%-(Ni, Fe)3S2/NF with


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other well-performed electrocatalysts (Table S7); AUTHOR INFORMATION Corresponding Authors E-mail:

[email protected] (Hui Liu)

E-mail:

[email protected] (Jun Zhang)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 51402085), Nature Science Foundation of Hebei Province (B2016202213) and Nature Science Foundation of Tianjin City (No. 16JCYBJC17500).



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