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Interface engineering of Ni3N@Fe3N heterostructure supported on carbon fiber for enhanced water oxidation Huawei Huang, Chang Yu, Xiaotong Han, Shaofeng Li, Song Cui, Changtai Zhao, Hongling Huang, and Jieshan Qiu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03351 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Industrial & Engineering Chemistry Research

Interface engineering of Ni3N@Fe3N heterostructure supported on carbon fiber for enhanced water oxidation Huawei Huang, a Chang Yu, *, a Xiaotong Han, a Shaofeng Li, a Song Cui, a Changtai Zhao, a Hongling Huang, a and Jieshan Qiu*, a, b a

State Key Lab of Fine Chemicals, School of Chemical Engineering, Liaoning Key Lab for

Energy Materials and Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China b

School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049,

China *Corresponding author: [email protected], [email protected], [email protected] Abstract Oxygen-evolution reaction (OER), a kinetically sluggish half-reaction involved in water splitting, generally needs large overpotentials to drive the catalytic process, leading to relatively low energy conversion efficiency. Therefore, the development of efficient, low-cost and stable electrocatalysts based on earth abundant elements is highly desired. Herein, we develop a novel method to construct Ni3N@Fe3N heterostructure anchored on carbon fiber (Ni3N@Fe3N/CF-6)

ACS Paragon Plus Environment

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consisting of Fe3N nanoparticles grafted on the metallic Ni3N nanosheets. The results show that of the as-synthesized electrocatalysts, the Ni3N@Fe3N/CF-6 features abundantly exposed interface and active sites, as well as open structure for intimate contact of electrolyte ions and easy release of generated gas. Hence, this Ni3N@Fe3N/CF-6 exhibits a great enhanced OER electrocatalytic performance, including overpotentials as low as 294 mV to achieve current densities of 10 mA cm-2, a small Tafel slope of 40 mV dec-1, and a superiorly stability at a large current density. 1. Introduction Electrochemical water splitting (H2O → H2 + 1/2 O2) is considered to be one of promising ways for sustainable hydrogen production, especially coupled with the renewable power (e.g., solar and wind).1-3 Nevertheless, the large-scale application of this technology is significantly limited by the kinetically sluggish oxygen evolution reaction (OER) at anode (2 H2O → 4 H+ + O2 + 4 e−, in acid; 4 OH− → 2 H2O + O2 + 4 e−, in alkali) owing to four-electron processes. Thus, low energy conversion efficiency in water splitting systems is delivered.4-6 It is believed that efficient, durable and cost-effective electrocatalysts can tackle this bottleneck issue to meet such a requirement. Currently, noble metal oxides (RuOx and IrOx) have been recognized as the stateof-the-art catalysts toward OER, but their scarcity and high cost limit their practical application at large-scale. As such, it is crucial and highly desirable to design efficient and low-cost electrocatalysts based on earth abundant elements (Fe, Ni, Co, Mn and etc.).7 Up to now, transition metal (oxy) hydroxides,8-9 oxides,10 nitrides,11-12 sulfides13 and phosphides14-15 have been developed and demonstrated superiorly catalytic performance as OER catalysts. Of these electrocatalysts, relatively poor conductivity results in Schottky barriers at substrate-catalysts


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and catalysts-electrolyte interfaces and further cause increased overpotentials.11,

16

Recently,

Xie’s group revealed that the two dimensional (2D) Ni3N nanosheets feature intrinsically metallic properties because of being close to the Fermi level verified by density of states calculation, which is further verified by the superior electrical conductivity tested by resistivity measurements.16 Moreover, surface and interface engineering strategies are very effective in regulating the surface properties of electrocatalysts and can induce synergistic effects for greatly enhancing catalytic performance.13, 17 In previous studies, it is highly recognized that Fe species has a positive effect on Ni based OER catalysts, including NiOx,10, 18 NiOOH,8 NiS2,19 Ni2P,15 and so on. With this in mind, it is predictable and feasible to design a highly efficient OER catalyst based on metallic Ni3N via an interface modulation method (for example, grafting Fe compounds) yet without damaging the conductivity of Ni3N. Herein, for the first time, we report the interface engineering of Fe3N-coated Ni3N heterostructures supported on conductive carbon fiber paper (denoted as Ni3N@Fe3N/CF). Such Ni3N@Fe3N/CF architecture possesses an open structure constituted of 2D nanosheets with fully exposed active interface/sites vertically aligned on carbon fiber (CF), which is in favor of a fast mass transfer and rapid reaction kinetics. With the grafted Fe3N particles, the as-made heterostructure (Ni3N@Fe3N/CF-6) delivers a greatly enhanced OER catalytic activity, and only needs low potentials of 294 and 334 mV to deliver 10 and 100 mA cm-2, respectively, which is much lower than that of pure Ni3N/CF (392 mV@10 mA cm-2 and 514 mV@100 mA cm-2), indicative of highly synergistic effects between Fe3N and Ni3N. 2. Experimental section 2.1 Pretreatment of CF


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The CF is pretreated by acid oxidation according to the method described in previous work.9, 15 Typically, the CF was cut into pieces with a size of 2 × 4 cm and subsequently cleaned by sonication in acetone, ethanol and deionized water for 10 min, respectively. Then the CF was functionalized by acid oxidation treatment in a flask with 100 mL concentrated HNO3 (65 wt%) at 100 oC for 180 min. After cooling to room temperature, the resulting CF was washed by deionized water to neutral and dried at 60 oC in a vacuum oven. 2.2 Synthesis of Ni(OH)2/CF Precursor The Ni(OH)2/CF was fabricated by a facile hydrothermal method. For a typical run, nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 0.5 mmol), urea (CO(NH2)2, 2.5 mmol), and ammonium fluoride (NH4F, 1.0 mmol) were dissolved in 15 mL of deionized water. (Caution! NH4F is harmful to environment to some degree. Please use it carefully.) Subsequently, the resulting aqueous solution with a piece of pre-treated CF was transferred to 20 mL Teflon-lined autoclave, maintained at 150 oC for 90 min. After completion of the reaction, the as-made Ni(OH)2/CF were washed and dried at 60 oC. For comparison, Ni(OH)2/ Ni foam (NF) precursor was also prepared following the same procedure. 2.3 Synthesis of Ni3N@Fe3N/CF and Ni3N@Fe3N/NF Firstly, polydopamine (PDA) coated Ni(OH)2/CF was synthesized by immersing Ni(OH)2/CF into an aqueous solution (40 mL) containing 0.16 g of dopamine hydrochloride and 0.16 g of tris(hydroxymethyl)aminomethane for 24 h with continuous stirring at room temperature. For nucleation of Fe species, the as-prepared PDA coated Ni(OH)2/CF was added in 1 M FeCl2 and stirred for 3, 6, 9 h, respectively. Afterwards, the corresponding as-made samples were nitrided in NH3 atmosphere at 500 oC for 200 min in a tube furnace with a heating rate of 5 oC min-1, and


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cooled to room temperature naturally, yielding a series of corresponding samples including Ni3N@Fe3N/CF-3, Ni3N@Fe3N/CF-6 and Ni3N@Fe3N/CF-9. The Ni3N@Fe3N/NF was also fabricated following the same procedure with the deposition of Fe species for 6 h. Besides, the Ni3N/CF, Ni3N/NF, and Ni3N nanosheets/CF (Ni3N-NS/CF) were also prepared by nitriding the Ni(OH)2/CF, Ni(OH)2/NF and PDA coated Ni(OH)2/CF precursors following the same conditions, respectively. Furthermore, the Ni(OH)2/CF precursors with different PDA coating thickness were also synthesized at changing polymerization time for 12 h and 36 h, named as Ni(OH)2/CF-P12 and Ni(OH)2/CF-P36. Then, the corresponding samples were converted to Ni3N@Fe3N/CF-P12 and Ni3N@Fe3N/CF-P36 after Fe deposition and nitridation. For comparison, 0.5 M and 1.5 M of FeCl2 solutions were also used for the nucleation of Fe species for 6 h, and the as-obtained samples after nitridation are named as Ni3N@Fe3N/CF-0.5 M and Ni3N@Fe3N/CF-1.5 M, respectively. In addition, the Ni3N/CF-400 and Ni3N/CF-600 were also fabricated by nitriding Ni(OH)2/CF at 400 oC and 600 oC for 200 min in NH3 atmosphere. 2.4 Synthesis of Ni(OH)2/CF-HMT and Ni3N@Fe3N/CF-HMT In order to avoid the use of poisonous NH4F, an environmentally friendly and facile way has been proposed by choosing another alkaline source (hexamethylenetetramine, HMT). Typically, 2.5 mmol Ni(NO3)2·6H2O and 5 mmol HMT were dissolved in 15 mL of deionized water. Afterwards, the aqueous solution with a piece of pre-treated CF was transferred to 20 mL Teflonlined autoclave, kept at 100 oC for 10 h, yielding Ni(OH)2/CF-HMT. The Ni(OH)2/CF-HMT was converted to Ni3N@Fe3N/CF-HMT following the same treatment procedure as for the preparation of Ni3N@Fe3N/CF-6. 2.5 Morphology and Structure Characterization for the as-made Materials


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The morphologies of as-made samples were characterized by scanning electron microscopy (SEM, FEI NOVA NanoSEM 450 and QUANTA 450) and transmission electron microscopy (TEM, Tecnai F30). Furthermore, the crystal structure and electronic states of electrocatalysts were examined by powder X-ray diffraction (XRD, D/MAX-2400, Cu Kα, λ=1.5406 Å), and Xray photoelectron spectroscopy (XPS, Thermo ESCALAB 250). 2.6 Electrochemical Characterization As self-standing electrodes, the as-made samples were directly used and tested for electrochemical characterizations in a typical three-electrode system controlled by a CHI 760 D electrochemical workstation. Electrochemical tests were conducted in 1 M KOH alkaline solution, in which a graphite rod and a saturated Ag/AgCl electrode were employed as counter and reference electrode, respectively. The polarization curves for OER were measured from 0.2 to 0.8 V vs. Ag/AgCl at a scan rate of 5 mV s-1. Electrochemical impedance spectroscopy (EIS) was tested at potential of 1.55 V versus reversible hydrogen electrode (RHE) from 100,000 to 0.1 Hz with an amplitude of 5 mV in 1 M KOH. The electrical double layer capacitance (Cdl) of electrocatalysts was used to evaluated the electrochemical surface area (ECSA), which was measured by using cyclic voltammograms (CVs) in a no Faradaic reaction potential window (1.207 - 1.307 V vs. RHE) at the scan rates of 5, 15, 20, 25, and 30 mV s-1. The plot of the current density (△J = (Ja - Jc) at 1.257 V vs. RHE) against the different scan rates has a linear relationship and its slope is twice of the Cdl. The catalytic stability was evaluated by chronopotentiometry at a constant current density of 100 mA cm-2. All potentials measured against an Ag/AgCl electrode in the present study were converted to potential RHE according to E (RHE) = E (Ag/AgCl) + 0.197 V + 0.059 V × pH.


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3. Results and Discussion 3.1 Synthesis and Characterization of Electrocatalysts.

Scheme 1. Schematic process for the synthesis of the Ni3N@Fe3N/CF. The synthetic processes for a series of Ni3N@Fe3N/CF samples are illustrated in Scheme 1, for the detailed description, please refer to the experimental section. The pre-treated conductive CF substrate with well-enriched oxygen-containing functional groups is capable of providing a conductive matrix for the rapid transmission of electrons and also numerous nucleation sites for crystal seed, thus leading to strong bonding force between active species and substrate.15, 20-21 The Ni(OH)2 nanosheets are in-situ grown on CF matrix by a simple hydrothermal reaction, in which nickel nitrate react with the released ammonia from urea decomposition in the solution. Generally, the surface/interface engineering and modification of catalysts are crucial to improve the catalytic performance. It can be a facile yet efficient method to assemble iron hydroxides on the surface of Ni(OH)2 nanosheets forming heterostructures just through Fe2+ ion hydrolysis at room temperature. Nevertheless, it was noted that the Ni(OH)2 nanosheets will be damaged and partially dissolved in the 1 M FeCl2 solution (pH≈2), if such Ni(OH)2/CF (Figure 1 c and d)


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was directly immersed in 1 M FeCl2 for 3 h used for the nucleation of iron species (denoted as Fe-Ni(OH)2/CF, Figure 1 a and b). Furthermore, the nanosheets will also tend to agglomerate into clusters with few exposed surface and active sites after directly nitridation in NH3 atmosphere at 500 oC (denoted as Ni3N/CF, Figure 1 e and f).

Figure 1. SEM images of (a, b) Fe-Ni(OH)2/CF, (c, d) Ni(OH)2/CF and (e, f) Ni3N/CF.

As such, it is necessary to keep the open-ended nanosheets structure and be able to construct heterostructures by depositing Fe species without structural erosion. It was found that PDAcoated yet protective strategy is highly effective through a facile polymerization reaction at room temperature (for more details, please refer to the experimental section). The Ni(OH)2/CF is capable of maintaining the nanosheets structure after nitridation reaction (denoted as Ni3NNS/CF, Figure S1 a and b) without aggregation in the presence of the coated PDA protective


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layer. More importantly, the PDA layer coated on Ni(OH)2 with abundantly oxygen and nitrogen functional groups not only facilitates the adsorption and deposition of Fe species but also provides a protective layer to keep the structure from being corroded and destructed during the deposition and nitridation processes. To have an insight into the internal relationship between catalysts structure and catalytic performance, a series of Ni3N@Fe3N/CF (-3, 6, 9) heterostructures were fabricated by anchoring different stacking densities and sizes of Fe3N particles according to various Fe nucleation time (3, 6, and 9 h, respectively). As illustrated in Figure 2 a-f, it can be clearly noted that the Ni3N@Fe3N/CF-3, 6, 9 samples well inherited the open structures of Ni(OH)2/CF nanosheets arrays. Also, the stacking density and size of the Fe3N particles anchored on Ni3N nanosheets increase with an increase of Fe nucleation time. Compared with sparse and small convexs in the Ni3N@Fe3N/CF-3 (Figure 2 a and b), the asmade Ni3N@Fe3N/CF-6 (Figure 2 c and d) feature dense particles anchored on nanosheets surface, most of which have a diameter of 20 ~ 50 nm. For the as-made Ni3N@Fe3N/CF-9 (Figure 2 e and f), the nanosheets are anchored with bulky Fe3N particles and also cross-linked to each other, which may result in a decreased electrolyte accessible surface area and active sites for the catalytic processes. Compared with the regular and relatively smooth edge structure of Ni3N-NS/CF, the as-made Ni3N@Fe3N/CF-6 (Figure S2) features the heterostructure with nanoparticles decorated on the sheets. The outside nanoparticles with a lattice spacing of 0.30 nm and the inner layer with a lattice spacing of 0.20 nm are presented, which is corresponding to the (101) plane of Fe3N and (111) plane of Ni3N, respectively. Analogously, the Ni3N@Fe3N/NF (Figure S3 a-c) also shows a well-inherited sheet structure with numerous small particles coating.


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Figure 2. SEM images of (a, b) Ni3N@Fe3N/CF-3, (c, d) Ni3N@Fe3N/CF-6 and (e, f) Ni3N@Fe3N/CF-9. To get the crystal structure information and electronic states of as-made samples, the representative XRD and XPS spectra of the as-made samples are displayed in Figure 3 a-d. It can be noted from Figure 3 a that the Ni3N/CF and Ni3N-NS/CF show three sharp diffraction peaks at about 38.9, 42.1 and 44.5 o, which are indexed to (110), (002) and (111) planes of metallic Ni3N (JCPDS Card no.10-0280).16 For the Ni3N@Fe3N/CF-3, 6, 9, the intensity of the characteristic peaks corresponding to the metallic Ni3N becomes weaker. And, two new peaks corresponding to Fe3N (JCPDS Card no. 49-1662) are present and the intensity increase gradually at about 40.8 and 45.5 o, which is ascribed to the increase in the thickness of the surface deposited Fe3N layer. The Ni 2p XPS spectrum (Figure 3 b) of Ni3N@Fe3N/CF-6 shows two peaks at 869.5 (Ni 2p1/2) and 852.1 eV (Ni 2p3/2), which are consistent with the metallic Ni3N peaks, implying a superiorly electrical conductivity.11,

16

As shown in Figure 3 c, the


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electron binding energy peaks of Fe 2p3/2 and Fe 2p1/2 are found to be at 710.7 and 723.1 eV, respectively.22-23 In the Figure 3 d, it can be noted that a strong peak appears at 397.7 eV, which is in agreement with the N region in metal nitride.22, 24

Figure 3. (a) XRD patterns of as-made samples. XPS spectra of elements (b) Ni 2p, (c) Fe 2p, and (d) N 1s for the as-synthesized Ni3N@Fe3N/CF-6. 3.2 Electrocatalytic OER Performance of the as-made Samples The OER catalytic activities of as-made samples and pre-treated CF are evaluated in 1 M KOH aqueous electrolyte with a three-electrode system, of which the obtained results are illustrated in Figure 4. For comparison, commercial RuO2 dropped on carbon fiber (RuO2-CF) with a mass loading of 1.0 mg cm-2 was also measured. It can be observed from the polarization curves (Figure 4 a) and Table 1 that the catalytic activities of Ni3N-NS/CF (374 mV@ 10 mA cm-2) with regular nanosheet arrays structure (Figure S1) outperform the Ni3N/CF (392 mV@ 10 mA


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cm-2) with aggregate structure (Figure 1 e and f), which results from the more exposed active edges and atoms, and the open structure for intimate contact of electrolyte ions and easy release of generated oxygen. After decorating with nanostructured Fe3N grains, the Ni3N@Fe3N/CF-3, 6, 9 exhibit a greatly enhanced OER performance, which only need overpotentials of 318, 294 and 312 mV to deliver a current density of 10 mA cm-2, respectively. It can also be noted from Figure 4 a that the Ni3N@Fe3N/CF-3 features a similar behavior to the Ni3N@Fe3N/CF-9 at low current densities but have an obvious superiority when current density is over 100 mA cm-2. The possible reason is that for the Ni3N@Fe3N/CF-3, small-sized nanoparticles are anchored on nanosheets for easy release of generated oxygen gas. Whereas, in the case of Ni3N@Fe3N/CF-9, the open structure and active interface/sites would be blocked to some degree due to the crosslinked large-sized Fe3N particles. This is also further proved by the fluctuation of polarization curve at high current densities caused by the non-smooth of gas diffusion.25 Remarkably, the Ni3N@Fe3N/CF-6 delivers the best catalytic activity at both low and high current densities. The numerous small-sized Fe3N particles coupled with Ni3N nanosheets synergistically lead to more exposed interface and active sites, and the radially aligned nanosheet arrays structure is also well reserved. These integrated features facilitate the electrolyte transport and gas release for rapid catalytic processes.9,

15

As illustrated in Figure 4 b and Table 1, the overpotential of

Ni3N@Fe3N/CF-6 (334 mV) at 100 mA cm-2 is much lower than that of Ni3N@Fe3N/CF-3 (362 mV), Ni3N@Fe3N/CF-9 (367 mV), RuO2-CF (380 mV), Ni3N-NS/CF (491 mV), and Ni3N /CF (514 mV), indicative of a superiorly practical potential at high current densities. More importantly, it is worth noting that the as-made Ni3N@Fe3N/NF only needs overpotentials as low as 234 and 271 mV to achieve current densities of 10 and 100 mA cm-2, respectively, which is superior to that of Ni3N/NF (300 mV @10 mA cm-2 and 418 mV @100 mA cm-2) and


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comparable to most of the state-of-the-art catalysts reported in literature (for more detailed comparison please refer to Table S1). In other words, the methodology of synthetic heterojunction structure reported in this paper is universal to different synthesis systems, and the constructed heterojunction structure feature a greatly enhanced catalytic performance.

Figure 4. Electrochemical OER performance of as-made electrocatalysts: (a) polarization curves with iR corrected; (b) comparison of the needed overpotentials to deliver a current density of 100 mA cm-2 and (c) the Tafel plots for the as-made electrocatalysts; (d) long-term stability test of Ni3N@Fe3N/CF-6 at a constant current density of 100 mA cm-2. Table 1. The needed overpotentials to achieve current densities of 10 and 100 mA cm-2, Tafel slopes, and Rct for different electrocatalysts.


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Samples Overpotential @10 mA cm-2, mV Overpotential @100 mA cm-2, mV Tafel slope, mV dec-1 Charge transfer resistance (Rct), Ω

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Ni3N/CF

Ni3N-NS/CF

Ni3N@Fe3N/CF-3

Ni3N@Fe3N/CF-6

Ni3N@Fe3N/CF-9

392

374

318

294

312

514

491

362

334

367

70

71

41

40

43

159.72

93.78

3.93

3.85

5.32

In order to study the intrinsic reaction kinetics process involved in OER process, the Tafel slopes (Figure 4 c) of the as-synthesized samples were fitted according to Tafel plots derived from polarization curves by Tafel equation (η = b log j + a), where η refers to overpotential, b represents the Tafel slope, and j is the current density. Low Tafel slope is generally suggested rapid catalytic kinetics involved in electrocatalytic reaction.26-27 Compared with that of the Ni3N/CF (70 mV dec-1), Ni3N-NS/CF (71 mV dec-1) and RuO2-CF (141 mV dec-1), the as-made Ni3N@Fe3N/CF-6 feature small Tafel slopes of 40 mV dec-1, being close to that of Ni3N@Fe3N/NF(39 mV dec-1), Ni3N@Fe3N/CF-3 (41 mV dec-1) and Ni3N@Fe3N/CF-9 (43 mV dec-1) and indicating fast reaction kinetics. Furthermore, the electrochemical impedance of asprepared electrocatalysts was evaluated and the recorded EIS plots and fitted results are shown in Figure S4 and Table 1. The charge transfer resistance (Rct) values of Ni3N@Fe3N/CF-3, 6, 9 are just 3.93, 3.85, and 5.32 Ω, respectively, which is much lower than that of Ni3N-NS/CF (93.78 Ω) and Ni3N/CF (159.72 Ω). Another important parameter to have an insight into the reaction mechanism, ECSA evaluated in terms of Cdl, was estimated by cyclic voltammetry test at a range of scan rates.28-29 Figure 5 shows that the Ni3N-NS/CF possesses the largest ECSA, which is 1.9-


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fold than that of Ni3N/CF and about 2.5-fold than that of Ni3N@Fe3N/CF-3, 6, 9. The increased ECSA of Ni3N-NS/CF is mainly attributed to uniformly assembled nanosheets arrays with more exposed surface area. In the case of the Ni3N@Fe3N/CF-3, 6, 9, the deposited iron species on nanosheet surface would block the accessibility of OH- to some active sites on the electrode surface.30 Therefore, it can be concluded that the highly electrocatalytic activity of Ni3N@Fe3N/CF-6 is not caused by the increased ECSA, but the constructed active interface/sites, fast reaction kinetics and decreased Rct (as listed in Table 1). The long-term operational stability at a high current density of 100 mA cm-2 for Ni3N@Fe3N/CF-6 was also measured to evaluate the potential for commercial applications. As shown in Figure 4 d, the operating potential is almost unchanged even after 12 hours of constant operation. In addition, the morphology and crystal structure are also well preserved (Figure S5 a-c), indicative of a high stability of Ni3N@Fe3N/CF-6 for water splitting.

Figure 5. Electrochemical double-layer capacitance measurements: cyclic voltammetry curves from 1.207 to 1.307 V vs. RHE at scan rates ranging from 5, 10 ,15, 20, 25 to 30 mV s-1 for (a)


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Ni3N/CF,

(b)

Ni3N-NS/CF,

(c)

Ni3N@Fe3N/CF-3,

(d)

Ni3N@Fe3N/CF-6,

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and

(e)

Ni3N@Fe3N/CF-9; (f) charging current density differences (∆J = Ja − Jc) plotted against scan rates of as-made samples and the corresponding slopes. Moreover, the effect of process parameters on the catalytic performance of final catalysts are also considered and optimized. We have conducted a series of experiments to explore the influence of the thermal treatment temperature, thickness of PDA layer, and the concentration of FeCl2 solution. Firstly, a series of Ni3N samples supported on CF were synthesized by thermal treatment of Ni(OH)2/CF at 400 oC and 600 oC, resulting in Ni3N/CF-400 and -600, respectively. For the Ni3N/CF (treated at 500 oC) and Ni3N/CF-600, the corresponding XRD spectra (Figure S6 a) show the three typical diffraction peaks at about 38.9, 42.1 and 44.5 o, corresponding to (110), (002) and (111) planes of metallic Ni3N (JCPDS Card no.10-0280), respectively. In the case of the as-made Ni3N/CF-400, the two wide peaks at 37.3 o and 43.3 o, which are indexed to (111) and (200) planes of NiO (JCPDS Card no.47-1049), are observed. That is to say that the nitridation procedure at 400 oC is not able to convert the precursor into nitride. Furthermore, the catalytic performance of as-made samples are also tested and compared. As shown in Figure S6 b and c, the Ni3N/CF delivers better catalytic activities and lower Tafel slope than that of Ni3N/CF-600 and Ni3N/CF-400. Therefore, the thermal treatment at 500 oC is a relatively good nitridation temperature in present work. Furthermore, we also synthesize samples with different PDA coating thickness by controlling the polymerization time, in which the Ni(OH)2/CF-P12, P24, and -P36 represents the samples at polymerization time for 12 h, 24 h, and 36 h, respectively. As shown in Figure S7 a-c, the thickness of PDA coating layer increases from 20 nm to 47 nm with the extension of polymerization time from 12 h to 36 h. After deposition of Fe species and nitridation treatment, the as-made corresponding samples are named as


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Ni3N@Fe3N/CF-P12 and -P36. It can be observed from the polarization curves illustrated in Figure S7 d and e that the Ni3N@Fe3N/CF-P12 delivers a smaller onset potential than that of Ni3N@Fe3N/CF-6 (the polymerization time is 24 h) and Ni3N@Fe3N/CF-36. Nevertheless, the catalytic performance of Ni3N@Fe3N/CF-6 exceeds that of Ni3N@Fe3N/CF-P12 as the voltage increases, which is mainly due to the fast reaction kinetics demonstrated by the lowest Tafel slopes (40 mV dec-1). Also, the catalytic performance of Ni3N@Fe3N/CF-36 with an increased thickness of PDA coating layer quickly decreases, which may ascribe to the blocked formation of heterogeneous junctions. Besides, the different FeCl2 concentration solutions (0.5 M and 1.5 M) were used for the nucleation of Fe species for 6 h following the same experimental procedure described in manuscript, and the resulting catalysts are denoted as Ni3N@Fe3N/CF-0.5 M and 1.5 M, respectively. It can be noted from Figure S8 a-c that the Ni3N@Fe3N/CF-0.5 M, Ni3N@Fe3N/CF-6 (deposition of Fe species in 1.0 M FeCl2) and Ni3N@Fe3N/CF-1.5 M feature the sheet-shaped structure decorated with lots of particles, and there is a slight increase in the number of particles on the sheet surface as the concentration increases. Notably, as shown in Figure S8 d and e, these three samples deliver relatively close onset potentials (1.52 V) and Tafel slopes (about 40 mV dec-1) for the OER catalytic activities. Nevertheless, the Ni3N@Fe3N/CF1.5 M delivers some superiority when potential is over 1.57 V. It can be concluded that the concentration of FeCl2 solution has a slight influence on the final performance of catalysts in the present work.


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Figure 6. (a, b) SEM images of Ni3N@Fe3N/CF-HMT; (c) polarization curves and corresponding (d) Tafel plots of the as-made Ni3N/CF-HMT and Ni3N@Fe3N/CF-HMT.

In order to verify the heterostructure universality of our synthetic strategy for enhancing the electrochemical performance, we have also synthesized Ni(OH)2 grown on carbon fiber (denoted as Ni(OH)2/CF-HMT) in the presence of HMT instead of NH4F and urea (for more details please refer to the experimental section in the revised manuscript), and the as-made Ni(OH)2/CF-HMT also feature sheet-shaped and open structure (Figure S9 a and b). Afterwards, the Ni(OH)2/CFHMT was treated by the deposition of iron species and nitridation, yielding Ni3N@Fe3N/CFHMT. As shown in Figure 6 a and b, the Ni3N@Fe3N/CF-HMT well inherited the sheet-shaped and open structure, and it can also be noted that numerous little particles are anchored on the surface of sheets (inset picture illustrated in Figure 6 b). From polarization curves and Tafel plots


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illustrated in Figure 6 c and d, it can be noted that the as-made Ni3N@Fe3N/CF-HMT delivers a greatly enhanced catalytic performance and reaction kinetics (251 mV@10 mA cm-2, 298 mV@100 mA cm-2, and Tafel slope of 47 mV dec-1) compared with that of the pure and nonheterostructured Ni3N/CF-HMT (274 mV@10 mA cm-2, 357 mV@100 mA cm-2, and Tafel slope of 78 mV dec-1). In other words, the methodology of synthetic heterojunction structure reported in this paper is universal to different synthesis systems, and the constructed heterojunction structure feature a greatly enhanced catalytic performance. 4. Conclusion In conclusion, we have reported a facile yet protective strategy to construct heterostructures with abundantly active interface/sites by grafting Fe3N nanoparticles on Ni3N nanosheets. After decorating with Fe species, the OER electrocatalytic activities are enhanced greatly, indicative of the highly synergistic effects between Ni3N and Fe3N. The as-made Ni3N@Fe3N/CF-6 with numerous exposed interface and active sites delivers current densities of 10 and 100 mA cm-2 at low overpotentials of 294 and 334 mV, respectively, and features fast reaction kinetics (b = 40 mV dec-1) and a low charge transfer resistance (Rct = 3.85 Ω). Furthermore, the Ni3N@Fe3N/CF6 also demonstrates a superior durability at a high current density for long-term operation. More importantly, this synthetic strategy for constructing the heterostructure to enhance the electrochemical performance is universal to different substrates (such as Ni foam) and precursors obtained by different synthesis systems (like Ni(OH)2/CF-HMT). This also provides an innovative approach to prepare high-performance heterostructure electrocatalysts for energy related catalysis.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected], [email protected] ACKNOWLEDGMENT This work was partly supported by the by the National Natural Science Foundation of China (Nos. 21522601, U1508201, 21361162004) and the National Key Research Development Program of China (2016YFD0200502). ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication websites. SEM images of Ni3N-NS/CF and Ni3N@Fe3N/NF; TEM image of Ni3N-NS/CF; TEM and HR-TEM images of Ni3N@Fe3N/CF-6, EIS plots, SEM images and XRD pattern of Ni3N@Fe3N/CF-6 after long-term stability test; XRD patterns, polarization curves and Tafel plots of Ni3N/CF, Ni3N/CF-400, and -600; TEM images of Ni(OH)2/CF-P12, -P24, and -P36; polarization curves and Tafel plots of the as-made Ni3N@Fe3N/CF-6, Ni3N@Fe3N/CF-P12 and P36; SEM images, polarization curves and Tafel plots of Ni3N@Fe3N/CF-6, Ni3N@Fe3N/CF-0.5 M, and -1.5 M; SEM images of Ni(OH)2/CF-HMT; the OER performance comparison. REFERENCES (1) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z., Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions. Chem. Soc. Rev. 2015, 44, 2060-2086. (2) Tang, C.; Wang, H.-S.; Wang, H.-F.; Zhang, Q.; Tian, G.-L.; Nie, J.-Q.; Wei, F., Spatially confined hybridization of nanometer-sized NiFe hydroxides into nitrogen-doped graphene frameworks leading to superior oxygen evolution reactivity. Adv. Mater. 2015, 27, 45164522.


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