Electronic structure and Crystalline Phase dual Modulation via Anion

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Electronic structure and Crystalline Phase dual Modulation via Anion-cation Co-doping for Boosting Oxygen Evolution with Long-term Stability under Large Current Density Jian Chen, jianpo chen, Hao Cui, and Chengxin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08060 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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ACS Applied Materials & Interfaces

Electronic Structure and Crystalline Phase Dual Modulation via Anion-cation Co-doping

for

Boosting

Oxygen

Evolution

with

Long-term Stability Under Large Current Density Jian Chen, Jianpo Chen, Hao Cui *, Chengxin Wang*

State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials Science and Engineering, The Key Laboratory of Low-Carbon Chemistry & Energy Conservation of Guangdong Province, Sun Yat-sen (Zhongshan) University, Guangzhou 510275, China

*Corresponding author: E-mail: [email protected]; [email protected]

Abstract The design of a state-of-the-art nonprecious OER electrocatalyst with ultralong stability under high current density (≥100 h under 1000 mA cm-2) is greatly desirable for the viable electrolysis of water. The synthesis of nanostructure catalysts is an effective method for improving the OER performance, but nanostructure-based catalysts are easily destroyed by mechanical force via the vigorous oxygen gas evolution process at high current density. Herein, we present a facile strategy of

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N-anion and Fe-cation dual doping to construct a 3D self-supported nickel selenide film-based catalyst via a one-step CVD process. The film exhibits outstanding OER activity with a small Tafel slope of 34.86 mV dec-1 and an overpotential of 267 mV at 100 mA cm-2 in 1 M KOH media. Impressively, the film-based catalyst can maintain this excellent catalytic activity over 100 h, even when operated at a high current density of 1 A cm-2, thus exhibiting the best reported OER stability under high current density so far. Further studies reveal that anion-cation co-doping can simultaneously modulate the electronic state and phase structure of nickel selenide, thereby promoting the in situ formation and transformation of oxygen vacancy-rich amorphous OER active species and resulting in the superior OER performance of the film-based catalyst.

Keywords: Anion-cation co-doping, OER electrocatalyst, nickel selenide film-based catalyst, long-term stability, large current density


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Introduction Electrochemical water-splitting (2H2O → 2H2 + O2) is one of the most promising technologies for hydrogen fuel production. The oxygen evolution reaction (OER, 4OH- → 2H2O + O2 + 4e- in base) is commonly considered a kinetically sluggish reaction due to the multistep proton-coupled electron transfer, 1 which limits the practical application of electrochemical water splitting. RuO2 are widely viewed as robust electrocatalysts for OER,

2-4

5-7

Currently, IrO2 and

but their scarcity and

high cost limit practical applications. Therefore, the synthesis of highly efficient, stable and low-cost OER electrocatalysts is greatly desirable for commercial electrochemical water-splitting devices. Transition metal-based catalysts are the most promising candidates for OER catalysts due to their high earth abundance, environmental friendliness and unique electronic structure.

8, 9

Among these catalysts, the selenides, especially nickel

selenide, have been considered as excellent OER electrocatalysts due to their excellent metallic features and special eg orbitals 8, 10 Thus far, great efforts have been devoted to the electrocatalysts with well-designed nanostructures.

11,

12

The

nanostructure plays an important role in enlarging the electrochemically active area (ECSA) and increasing the amount of active sites, which can dramatically promote the OER activity. However, the nanostructure suffers from complex synthesis steps and low yields. Besides, the electrocatalytic performance of these materials is still far from that required for industrial application in terms of both activity and stability, particularly for long-term stability under large current density (≥100 h under 1A



cm-2), because the nanostructures are easily destroyed by mechanical force during vigorous oxygen gas release at high current density. In contrast, the film-based catalysts have strong adhesion to the substrates and might satisfy the requirements of industrial application and operate well at a notably high current density (1 A cm−2) for a long period (>100 h). Thus, it is critical to realize large-scale application of bulk catalysts by enhancing their intrinsic activity. 13 Ion doping has been recognized as an efficient strategy for improving the intrinsic activity of catalysts, and interesting results have been reported in this area. 14, 15

For example, as anion dopants, selected nonmetallic elements (N, P and S) can tune

the electronic structure of catalysts and accelerate the formation of oxygen vacancies, resulting in OER activity promotion.

16, 17

Certain researchers have reported that the

incorporation of cations could also enhance OER activity by increasing conductivity and regulating the electronic state.

18, 19

It is worth noting that due to the synergistic

effect of dopants, dual doping has a notable advantage in improvement of catalytic performance.

11, 20

Recently, Xu et al. stated that metal and nonmetal co-doping can

induce lattice irregularity, optimize the electronic configuration of catalysts, and boost HER activity.

21

Similarly, co-doping of anions and cations may enhance the activity

of OER electrocatalysts by modulating the electronic structure. So far, there are only rare reports of anion and cation co-doping into non-noble metal OER electrocatalysts. Furthermore, doping can also achieve phase transition. Modulation of phase structure is a key step for the design of a catalyst. A preferred phase is favorable for enhancing the intrinsic activity of catalysts. 19, 22, 23


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Inspired by the above discussions, we present a facile strategy of anion and cation co-doping to boost the intrinsic OER activity of nickel selenide film by simultaneously optimizing the electronic state and phase structure. Herein, nitrogen-anion and iron-cation dual-doped 3D self-supported nickel selenide films were synthesized by the direct selenization of 3D nickel-iron alloy foam (NIF) with urea（as nitrogen source）via a one-step chemical vapor deposition (CVD) process. The as-obtained film electrode exhibits outstanding OER activity with a small Tafel slope of 34.86 mV dec-1 and an overpotential of only 267 mV; this film also reaches a current density of 100 mA cm-2 in alkaline media, which is superior to that of most nanostructure-based catalysts. Impressively, the film-based catalyst can maintain its excellent catalytic activity over a period of 100 h even if it is operated at a high current density of 1 A cm-2. To the best of our knowledge, this is the first time that ultralong stability is reported under a high current density of 1000 mA cm-2. Further studies reveal that anion-cation dual doping can simultaneously modulate the electronic state and phase structure of nickel selenide and thereby promote the in situ formation and transformation of oxygen vacancy-rich amorphous OER active species, resulting in superior OER performance of the film-based catalyst. Our finding offers a novel insight into the design of a state-of-the-art OER electrocatalyst for industrial application with large current density and ultralong stability.

Results and Discussion The N,Fe-NiSe@NIF electrode was prepared via a one-step CVD process with Se



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powder serving as the Se source and urea serving as the N source on 3D NIF (Supporting Information, Figure S1). To observe the influence of dual doping, N-doped NiSe (N-NiSe@NF) and Fe-doped NiSe (Fe-NiSe@NIF) were also prepared via the same CVD process (detailed information is presented in the Experimental Section). Figure 1a shows the XRD pattern of the N,Fe-NiSe@NIF electrode. The diffraction peaks at 27.9°, 32.8°, 44.4°, 49.9°, 59.6°, 61.2°, 69.1° and 70.4° can be indexed to the crystalline planes of (100), (101), (102), (110), (103), (201), (202) and (004), respectively, corresponding to hexagonal NiSe (JCPDS No. 04-0892). The remainder of the diffraction peaks belong to nickel foam (JCPDS No. 04-0850).

24

Notably, as shown in Figure 1b, the diffraction peaks of the N,Fe-NiSe@NIF electrode are slightly shifted to higher angles, indicating a decrease of the interplanar spacing, which might be caused by the dual doping of nitrogen and iron.

25

The

diffraction peaks of the N-NiSe@NF electrode (Supporting Information, Figure S2) are similar to those of the N,Fe-NiSe@NIF electrode with hexagonal NiSe. The XRD pattern of Fe-NiSe@NIF is shown in Figure 1a. The peaks in the XRD pattern belong to rhombohedral Ni3Se2 (JCPDS No. 19-0841). Notably, the XRD pattern of the Fe-NiSe@NIF electrode is completely different from those of the N,Fe-NiSe@NIF and N-NiSe@NF electrodes. The results indicate that the introduction of the N source can induce a phase transition from rhombohedral Ni3Se2 to hexagonal NiSe. Similar phenomena have been reported.26 Figure 1c, d show SEM images of the N,Fe-NiSe@NIF electrode, which possesses a film morphology. Compared with pure NIF film (Supporting Information, Figure S3), N,Fe-NiSe@NIF has a rougher surface


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consisting of lager particles with a size of ~2μm. Moreover, N-NiSe@NF and Fe-NiSe@NIF also display a similar film morphology (Supporting Information, Figure S4 and Figure S5). Figure 1e and Figure S6 (Supporting Information) show the TEM image of N,Fe-NiSe. The HRTEM lattice fringes (Figure 1f) reveal that the interplanar spacings are 3.12 Å and 2.65 Å, corresponding to the (100) and (101) planes of hexagonal NiSe, respectively. The interplanar spacings of (100) and (101) planes are slightly smaller than that of standard NiSe (3.2 Å and 2.73 Å), a result that agrees well with the XRD analysis.

27

The HRTEM and SAEF images of Fe-NiSe

were also measured and confirmed the existence of Ni3Se2 (Supporting Information, Figure S7). The TEM-EDX element mapping observation of N,Fe-NiSe (Figure 1g) reveals that the elements of Ni, Se, Fe and N are uniformly distributed on N,Fe-NiSe. Based on above analyses, we can safely conclude that the nitrogen anions and iron cations have been successfully doped into the NiSe film and that the N,Fe-NiSe@NIF electrode has been obtained. The X-ray photoelectron spectra (XPS) is investigated to further examine the chemical compositions on the surface. The XPS survey spectrum of the N,Fe-NiSe@NIF electrode (Supporting Information, Figure S8) confirms the presence of Ni, Se, Fe and N. As shown in Figure 2a, the bonding energies of Ni0 appear at 852.2 and 869.5 eV. The peaks located at 855.7 and 873.3 eV correspond to the 2p1/2 and 2p3/2 spin-orbital of Ni2+ with two satellite peaks appearing at 860.9 and 879.3 eV. 10, 28, 29. The Se 3d peaks at 54.3 and 55.1 eV are assigned to Se2- (Figure 2b). The existence of Ni2+ and Se2- indicates the successful formation of NiSe. 29, 30 The single



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peak at 55.7 eV can be attributed to the Fe 3p, which overlaps with the XPS scope of Se 3d.

31

As shown in Figure 2c, the bonding energies at 711.0 and 724.4 eV can be

attributed to 2p3/2 and 2p1/2 of Fe3+, indicating the successful incorporation of the Fe3+ into NiSe. The satellite peak at 718.7 eV belongs to Fe3+.

18, 32

The N 1s peaks for

N,Fe-NiSe@NIF (Figure 2d) can be split into three peaks at 398.4, 399.4 and 400.7 eV, corresponding to the N-Ni bond, pyrrolic N and graphitic N, respectively.

33, 34

The existence of N-Ni bond further confirms the incorporation of N-anion. To investigate the effects of anion-cation dual-doping on OER performance, we performed electrochemical measurements. Figure 3a shows the linear sweep voltammetry (LSV) curves for different samples with a scan rate of 1 mV s-1. For the N,Fe-NiSe@NIF electrode, it only requires a low overpotential of 232 mV to reach 10 mA cm-2. In contrast, that data for Fe-NiSe@NIF, N-NiSe@NF and pure NIF are 249, 292 and 286 mV, respectively, demonstrating that the N,Fe-NiSe@NIF electrode has the best catalytic activity among all samples. The outstanding OER catalytic activity of N,Fe-NiSe@NIF electrode is also superior to that of many other nonprecious metal catalysts in alkaline solution (Table S1). In the insert of Figure 3a, the oxidation peak located at 1.37 V vs. RHE is assigned to the transformation of Ni2+ to Ni3+. Of note, this redox peak is larger than that of other samples, indicating more active sites (Ni3+) were formed on the N,Fe-NiSe@NIF electrode surface. 8 The Tafel slope is calculated by using the equation η = a + b*log [j].

9

The Tafel slope values (Figure 3b) of the

N,Fe-NiSe@NIF, Fe-NiSe@NIF, N-NiSe@NF, and NIF electrodes are 34.86, 38.13, 52.17 and 46.13 mV dec-1, respectively, suggesting that the N,Fe-NiSe@NIF


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electrode has faster OER dynamics than other references. These analyses for the electrochemical measurements illustrate that N-anion and Fe-cation co-doping can bring significant improvements for OER activity. The electrochemical impedance spectroscopy (EIS) was performed to study the charge transfer process on the N,Fe-NiSe@NIF electrode during OER. As shown in Figure 3c, the N,Fe-NiSe@NIF electrode exhibits a smaller Rct than that of other samples. This means that the N,Fe-NiSe@NIF electrode has favorable reaction kinetics and charge transfer properties. In addition, the electrochemically active surface areas (ECSA) of all samples are also investigated via double-layer capacitance (Cdl) measuring (Supporting Information, Figure S9). The ECSA of the N,Fe-NiSe@NIF electrode is the largest one among all samples, which reveals that the N,Fe-NiSe@NIF electrode has more surface active sites than the other references. Notably, due to the film morphology, the Cdl values for all samples are relatively smaller than those of the common nanostructure-based nickel selenide catalysts due to the film structure.

35-37

Generally, the specific activity of electrocatalysts, which is

calculated with the current normalized to its real ECSA, can reliably indicate its intrinsic electrocatalytic activity. The specific activities of N,Fe-NiSe@NIF, Fe-NiSe@NIF and N-NiSe@NF are obtained (Supporting Information, Figure S10). The results illustrate that the N,Fe-NiSe@NIF electrode still possesses the highest specific activity among our samples. We also determined the chronopotentiometric measurements using the N,Fe-NiSe@NIF electrode as a working electrode in 1 M KOH solution. As shown in



Figure 3e, the N,Fe-NiSe@NIF electrode shows excellent stability at constant current densities of 10 and 100 mA cm-2 for 100 h. We consider that the catalysts with film morphology have strong adhesion to the electrode, which makes the catalyst have good stability at high current. The Faradic efficiency (FE) of OER was calculated by comparing the amount of experimentally quantified gas with the theoretically calculated gas. The FE values (Figure 3f) are approximately 100%, indicating that no other side reactions occur during OER. We further explored the effects of ions doping during OER process. The effects of different Fe contents on OER performance by using the NIFs with different Fe ratio (10, 20, 40, 50 wt%, purchased from company) were shown in Figure S11. The sample with 10 wt% Fe has the best OER performance among all the products. The result indicates that appropriate incorporation of Fe into Ni based catalysis can produce synergistic effect and improve the OER performance. To investigate the effects of different N contents on the OER process, a serious of experiments have been conducted. We change the content of N in the product by changing the amount of urea and measure the corresponding content of N by XPS. According to the result, the N content in the product increased with the amount of urea (Table S2). As shown in and Figure S12, when the amount of urea is in the range of 1~10mg, the OER activity of the catalyst enhanced with the increase of urea amount. However, when the amount of urea added exceeds 10 mg, the OER performance is almost no longer improved. Thus,10 mg of urea are needed to prepare N,Fe-NiSe@NIF electrode with outstanding OER activity. As previous reports, anion doping might accelerate the


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formation of oxygen vacancies, resulting in OER activity promotion.

16, 17

We

conducted the XPS analysis on N,Fe-NiSe@NIF and Fe-NiSe@NIF electrode after 10 and 50h chronoamperometry test under 10 mA cm-2 (Supporting Information, Figure S13). The peaks at 530.8, 531.6 and 532.6 eV can contribute to the Ni-O-Ni, Ni-O-H bond and oxygen vacancies, respectively.

38

The results showed that the contents of

oxygen vacancy on N,Fe-NiSe@NIF were 16.14 and 38.52% respectively, which were higher than those of the Fe-NiSe@NIF electrode (0 and 28.08%), indicating N incorporation may be beneficial to the formation of oxygen vacancy. The formation of oxygen vacancies can improve electronic conductivity and create new defect states resulting in higher activity for OER electrocatalysts. 17, 39 Then, due to N-doping play a key role to the phase transition from rhombohedral Ni3Se2 to hexagonal NiSe. Thus, the XRD pattern of these sample were also determined to further study the effect of N contents on the phase transition of catalyst. As shown in Figure S14, in the absence of urea, only a single phase of rhombohedral Ni3Se2 is present. When 1 mg of urea is added, the phase transition occurs and both phases of rhombohedral Ni3Se2 and hexagonal NiSe are present in the product. And when the amount of urea added exceeds 10 mg, only the hexagonal NiSe phase exists in the product. The amount of N source is critical for phase transitions from rhombohedral Ni3Se2 to hexagonal NiSe. In our experiments, hexagonal NiSe formed by N doping is easier to transform into amorphous NiOOH, which is the actual active material of OER. Previous studies have demonstrated that in situ electrochemical conversion occurs from the nickel-based catalyst to nickel hydroxide/(oxy)hydroxide during the OER process.



The in situ formed nickel hydroxide/(oxy)hydroxide catalysts are the actual active species for OER in basic media.

19, 22, 40

In our experiments, hexagonal NiSe formed

by N doping is easier to transform into amorphous NiOOH, compareing to Fe-NiSe@NIF. To reveal the reason for the high activity, we also study the electrochemical conversion of the N,Fe-NiSe@NIF electrode after OER stability with a constant current density of 10 mA cm-2 for 50 h. As shown in SEM, the 3D-supported film structure with a rough surface is still well maintained after OER (Figure 4a), confirming the strong adhesion to the substrate. However, the XRD pattern (Figure 4b) reveals that the diffraction peaks of the NiSe disappear after OER. Furthermore, the HRTEM image (Figure 4c) shows a large number of amorphous phases and a rare crystal phase. The interplanar spacings of crystal phases are approximately 2.17 and 1.55 Å, corresponding to (104) and (1010) planes of NiOOH (JCPDS No. 06-0075). This indicates the N,Fe-NiSe was transformed from hexagonal NiSe to amorphous NiOOH during the OER process. In contrast, the XRD pattern of the Fe-NiSe@NIF electrode (Supporting Information, Figure S15) exhibits obvious diffraction peaks corresponding to a nickel selenide crystalline structure. Thus, the preferable crystalline phase for the N,Fe-NiSe@NIF electrode could promote the in situ formation and transition of active amorphous species. These in situ formed amorphous species in N,Fe-NiSe@NIF are the key of the outstanding OER activity. 22, 23, 41

Figure 4d presents the TEM-EDX element mapping, which reveals the

existence of Ni, Se, Fe, O. Notably, there were no N elements detected in N,Fe-NiSe after OER. We suppose that the N elements will be oxidized to NO2 during OER,


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which easily soluble in water and desorbed from electrode. (details see the caption of Table S3). Since amorphous NiOOH is the actual active material in samples during the OER, the desorption of N may not affect OER activity. XPS spectrum were displayed to further analyze the chemical composition of post-OER samples. Ni 2p spectrum reveal that two peaks at 855.7 and 873.3 eV are responsible for Ni2+ (Figure 5a). The bonding energies which belong to Ni0 have disappeared due to the oxidation process during the OER.

28

Two peaks at 857.3 and

875.6 eV suggest the existence of Ni3+ and the formation of NiOOH on the surface, which is the active species for OER. 19, 39, 42 Figure 5b shows XPS peaks at 712.2 and 725.6 eV belong to Fe3+ and a single peak at 720 eV is responsible for the satellite peak of Fe3+. In fact, Fe acts as a dopant into the amorphous nickel hydroxide/(oxy)hydroxide can not only increase the conductivity of active species, but also modify the NiOOH electronic configuration, especially the Ni3+ centers.

19

The spectra of Se 3d (Figure 5c) reveal that the Se has completely transformed to SeOx in the N,Fe-NiSe@NIF electrode, whereas the Se2- still can be observed in the Fe-NiSe@NIF electrode after OER. This result might demonstrate that the preferable crystalline phase of the N,Fe-NiSe@NIF electrode, which is induced by the incorporation of nitrogen anions, can accelerate the formation of SeOx. Chen et al. have revealed that the metal-Se compounds transforming to SeOx might be positive to the adsorption of H2O, which might be the key of an improved water splitting rate. 43 Finally,

we

explore

the

possibility

of

industrialized

application

of

N,Fe-NiSe@NIF. The chronopotentiometric measurements with high current density



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were executed in 30 wt% KOH (This concentration is commonly applied in the industrial electrochemical water-splitting process). The polarization curve operated in 30wt% KOH is shown in Figure 6a. For the N,Fe-NiSe@NIF electrode, it needs only 251 and 290 mV to reach the current density of 500 and 1000 mA cm-2, respectively. As shown in Figure 6b, the ultralong stability of the N,Fe-NiSe@NIF electrode is investigated under high current densities of 500 and 1000 mA cm-2. From the figure we can see that only a slight loss happened after 100 h measurement. To the best of our knowledge, reports about such long-term stability are rare.

We thought our

samples are most likely utilized for industrial applications among present electrochemical water-splitting catalysts.

Conclusion In summary, we present a facile strategy of anion-cation dual doping to enhance the OER performance of a nickel selenide film-based catalyst by simultaneously modulating the electronic state and phase structure. The N,Fe-NiSe film is successfully prepared via a one-step CVD process and exhibits outstanding OER performance exceeding that of other references in alkaline solution. Unexpectedly, the N,Fe-NiSe@NIF electrode shows ultralong stability under high current densities of 500 and 1000 mA cm-2. The superior activity of the N,Fe-NiSe@NIF electrode could be attributed to the following: Ⅰ) the film-based structure is able to survive the mechanical force produced by vigorous oxygen gas at


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high current density and supply ultralong stability; Ⅱ) the 3D self-supported structure can offers an unhindered path for electron transfer; Ⅲ) the N-anion and Fe-cation dual-doping optimizes the electronic states of the Ni3+ centers and crystalline phase, which can accelerate the formation of oxygen vacancy-rich amorphous OER active species. Our investigation offers a novel insight into design of advanced OER catalysts via a facile co-doping strategy, and this catalyst might satisfy the ultralong stability and high current density requirements of industrial application.

4. Experimental Section

4.1. Chemicals and Materials Selenium powder and urea were purchased from Aladdin. Hydrochloric acid (HCl) and ethanol were purchased from Guangzhou Chemical Reagent Factory with analytic grade (AR). All chemical reagents were used as received without any purification. Ultrapure water with 18.2M Ω at room temperature was used at all experiments. Nickel-iron alloy foam (NIFs) (thickness: 1.5mm, area: 1 cm × 3 cm) was employed as the substrate. In order to wipe off the impurities and oxidation layer on the surface, NIFs were washed by ultrasonication treatment in 0.5M HCl solution for 3 min, followed by rinsing with ultrapure water for 5 min, washing with ethanol and drying in air.

4.2. Synthesis of Fe-NiSe@NIF



In a typical procedure, 25 mg selenium powder was added into a quartz tube (2 cm × 25 cm)，follow by adding a piece of cleaned NIF (1 cm × 3 cm) into the quartz tube. The quartz tube added selenium powder and NIF was blocked by quartz stopper via local heating and the pressure of tube was maintained at vacuum during the local heating process. Then, the quartz tube stated above was placed in the tube furnace (GSL-1500X). The tube furnace heated the quartz tube from 30℃ to 400℃ for 74 min under an air atmosphere. And the quartz tube was maintained at 400 ℃ for 120 min and cooled naturally to room temperature. Then, the Fe-NiSe@NIF electrode was obtained from the quartz tube. The mass loading of catalyst is 18.92 mg/cm2.

4.3. Synthesis of N,Fe-NiSe@NIF N,Fe-NiSe@NIF was prepared by a facile CVD similar to Fe-NiSe@NIF. 25 mg selenium powder and 10 mg urea were added into a quartz tube at the same time and other steps followed are the same with the synthesis process of Fe-NiSe@NIF. In our experiment, the original iron nickel foam is purchased from company with the mass ratio of Fe to Ni is 1:9, which corresponds to the atomic ratio of 1:8.56. The atomic ratio of Fe to Ni in N,Fe-NiSe@NIF were also determined via XPS analysis. The XPS results show that the average atomic ratio of Fe to Ni in the product is 1:11.76 (Table S4). The mass loading of material is 15.32 mg/cm2.

4.4. Synthesis of N-NiSe@NF All steps to obtain N-NiSe@NF were the same as the preparation of


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N,Fe-NiSe@NIF, excepting for replacing NIF with NF. The mass loading of catalyst is 18.92 mg/cm2.

4.5. Materials characterization The crystalline structure of samples was examined by X-ray powder diffraction (XRD) on Japan Rigaku D-MAX 2200 VPC X-ray diffractometer at 40 kV and 26 mA. The surface morphologies of products were inspected by SEM (Zeiss Auriga-4532) at an acceleration voltage of 5 kV. TEM, high-resolution TEM (HRTEM) and energy-dispersive spectrum (EDX) were obtained from FEI Tecnai G2 F30 microscope operated at 300kV. The information of XPS was collected from ECSA Lab250 photoelectron spectrometer with Al Kα (1486.6 eV) as the radiation source to

identify the surface chemical binding energy.

4.6. Electrochemical performance measurements All electrochemical measurements were performed at 298K using Autolab Potentiostat-Golvanostat

electrochemical

workstation

linked

with

a

typical

three-electrode electrolyzer. The samples were served as the working electrode (WE), directly, and placed in 1M KOH electrolyte solution saturated with oxygen. Hg/HgO (1 M KOH) electrode served as a reference electrode (RE) and graphite rod was chosen as a counter electrode (CE). The polarization curves of samples were adopted after several linear sweep voltammetry (LSV) was executed. All the potentials were converted into the reversible hydrogen electrode (RHE), which were obtained



according to the equation followed: ERHE = EHg/HgO + 0.059 * pH+ 0.098 where EHg/HgO means the experimental potential against the CE. Besides the potentials were compensated with 90% iR which is measured via EIS which was tested from 100 kHz to 10-2 Hz at 1.45 V vs.RHE. Linear sweep voltammetry (LSV) was recorded with a scan rate of 1 mV s-1.

4.7 Mass loading calculation method: The mass loading of different catalysts on NIF is determined by weighing of the NIF before and after reaction with semi-micro balance (METTLER TOLEDO MS105DU). For N,Fe-NiSe@NIF and N-NiSe@NIF electrodes, the calculation formula of mass loading is as follows: 𝑚total loading,NiSe =

MNiSe MSe

× ∆𝑚𝑆𝑒

𝑚loading,NiSe = 𝑚total loading,NiSe ÷ 𝐴 where 𝑚total loading,NiSe is the total mass loading of catalyst on NIF. MNiSe and MSe are the molar mass of NiSe and Se, respectively. ∆𝑚𝑆𝑒 is the mass change of NIF substrate before and after reaction. 𝐴 is the area of NIF substrate and 𝑚loading,NiSe is the mass loading of catalytic deposition per unit area. For the Fe-NiSe@NIF electrodes (with the formula Ni3Se2), the calculation method is as follows: 𝑚total loading,Ni3𝑆𝑒2 =

M𝑁𝑖3𝑆𝑒2 2MSe

× ∆𝑚𝑆𝑒


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𝑚loading,NiSe = 𝑚total loading,NiSe ÷ 𝐴 where M𝑁𝑖3𝑆𝑒2 is the molar mass of Ni3Se2. The mass loading date were shown in Table S5 (Supporting Information)

Acknowledgment This work was financially supported by the National Natural Science Foundation of China (Grants No. 51772338). Pearl River S&T Nova Program of Guangzhou (Grant Nos. 201610010085).

Supporting Information: More experimental details (XRD, SEM, TEM, XPS and CV); N contents in N,Fe-NiSe@NIF with different OER times at 10 mA cm-2; the mass loading of samples.



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Figure 1. (a) XRD pattern of the N,Fe-NiSe@NIF and Fe-NiSe@NIF electrode. (b) Partial XRD pattern of N,Fe-NiSe@NIF electrode. (c, d) SEM images of N,Fe-NiSe@NIF electrode. (e, f) TEM and HRTEM images of N,Fe-NiSe. The insert in f is the corresponding fast-Fourier transform image. g) EFTEM and TEM-EDX element mapping of N,Fe-NiSe with Ni, Se, Fe and N.


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Figure 2. XPS spectrum of the N,Fe-NiSe@NIF electrode. (a) Ni 2p. (b) Se 3d. (c) Fe 2p. (d) N 1s.



Figure 3. (a) LSV curves of the N,Fe-NiSe@NIF, Fe-NiSe@NIF, N-SeNi@NF and NIF electrodes with 90% iR-compensation at scan rates of 1 mV s-1. Insert in a exhibits a detailed drawing of the polarization curves. (b, c) Tafel plots and EIS of as-prepared samples. The insert in c exhibits a detailed drawing of EIS. (d) ECSA for all samples. (e) Chronopotentiometric curves for the N,Fe-NiSe@NIF electrode at a constant current density of 10 and 100 mA cm-2. (f) Faraday efficiency plots of the N,Fe-NiSe@NIF electrode.


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Figure 4. (a) SEM image of the N,Fe-NiSe@NIF electrode after OER. (b) XRD pattern of the N,Fe-NiSe@NIF electrode before and after OER. (c, d) HRTEM and TEM-EDX mapping images of N,Fe-NiSe@NIF after OER.



Figure 5. High-resolution XPS spectrum for the N,Fe-NiSe@NIF electrode (upside) and the Fe-NiSe@NIF electrode (bottom) after OER. (a) Ni 2p. (b) Fe 2p. (c) Se 3d.


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Figure 6. (a) LSV curve of the N,Fe-NiSe@NIF electrode with a scan rate of 1 mV s-1. Insert in a is an optical photograph of the N,Fe-NiSe@NIF electrode at a current density of 1000 mA

cm-2. (b) Chronopotentiometric curves for the N,Fe-NiSe@NIF electrode at a constant current density of 500 and 1000 mA cm-2. All measurements were performed in 30wt% KOH.



TOC


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Electronic structure and Crystalline Phase dual Modulation via Anion

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