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Iron-Nickel Nitride Nanostructures in-situ Grown on Surface-redox-etching Nickel Foam: Efficient and Ultrasustainable Electrocatalysts for Overall Water Splitting Bo Zhang, Chunhui Xiao, Sanmu Xie, Jin Liang, Xu Chen, and Yuhai Tang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02610 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016

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Chemistry of Materials

Iron-Nickel Nitride Nanostructures in-situ Grown on Surfaceredox-etching Nickel Foam: Efficient and Ultra-sustainable Electrocatalysts for Overall Water Splitting Bo Zhang, Chunhui Xiao*, Sanmu Xie, Jin Liang, Xu Chen, Yuhai Tang Department of Applied Chemistry, School of Science, Xi’an Jiaotong University, Xi’an, 710049, China ABSTRACT: Water splitting is widely considered to be a promising strategy for clean and efficient energy production. In this paper, for the first time we report an in-situ growth of iron-nickel nitride nanostructures on surface-redox-etching Ni foam (FeNi3N/NF) as a bifunctional electrocatalyst for overall water splitting. This method does not require a specially added nickel precursor nor an oxidizing agent, but achieves well-dispersed iron-nickel nitride nanostructures which are grown directly on the nickel foam surface. The commercial Ni foam in this work not only acts as a substrate, but also serves as a slow-releasing nickel precursor that is induced by redox-etching of Fe3+. FeCl2 is a preferable iron precursor than FeCl3 for no matter quality of FeNi3N growth or its electrocatalytic behaviors. The obtained FeNi3N/NF exhibits extraordinarily high activities for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) with low overpotentials of 202 and 75 mV at 10 mA cm-2, Tafel slopes of 40 and 98 mV dec-1, respectively. In addition, the presented FeNi3N/NF catalyst has an extremely good durability, reflecting in more than 400-hour consistent galvanostatic electrolysis without any visible voltage elevation.

Water splitting is widely considered to be a promising strategy for renewable, clean and efficient energy production.1 Electrocatalytic or photocatalytic water splitting into oxygen and hydrogen may potentially address the scale-up energy demand of future society. Currently, the state-of-art catalysts for electrocatalytic water splitting are IrO2 and Pt/C for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively.2 However, the high price and scarcity of these noble metals obstruct their large-scale applications. In recent years, enormous efforts have been devoted to efficient OER and HER catalysts with earth-abundant materials, such as transition metal-based oxides/hydroxides3-6, sulfides7-10, phosphides11-14, carbides15,16 selenides17,18 borate19, phosphate20 and their alloys (NiCo21, NiFe22-25). Notably, some of transition metal-based catalysts could be employed as bifunctional electrocatalysts for HER and OER, which simplifies the water splitting system and also reduces the product cost.26 As a new class of alternative electrocatalysts for water splitting, metal nitrides have been attracting great attention due to their advantages of excellent catalytic activities, high yield and easy operation.27 Theoretical calculations have already predicted that the metal nitrides allow for faster charge-carrier transportation and better electrical conductivity than their oxides.28 Wu’s group for the first time reported a synthesis of metallic Ni3N nanosheets with superior excellent catalytic performance toward

OER, and opens up a new path to highly efficient OER electrocatalyst which is based on metal nitride material.29 They shortly afterward developed Co4N porous nanowire arrays on flexible substrates as highly active OER electrocatalysts, which is considered the best OER performance among reported Co-based electrocatalysts.30,31 Very recently, Shalom’s group reported that Ni3N directly grown on nickel foam exhibits high electrocatalytic activity toward OER and HER, as well as oxygen reduction reaction (ORR).32 In a word, the high efficiency and universality of the metal nitrides toward general electrochemical reactions demonstrates their good operability for water splitting. Furthermore, bimetallic crystal nanomaterials are drawing much attention due to their better catalytic activities than related monometallic counterparts.33-37 The coordination of two or more metal species could provide richer active sites and improved electronic conductivity, which are beneficial to electrocatalytic applications.38 Besides, differential combination and tunable proportion of cations in bimetallic materials give tremendous opportunities to manipulate the physical/chemical properties in terms of valence and electronic state of the metal elements.39 Ni-Fe alloys comprising of two most widely used metals have been studied as high-performance electrocatalysts for water splitting.33,40,41 However, the research work about bimetallic nitrides is just at a start. More recently, Zhang et al. reported a synthesis of NiFe nitride nanopar-

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ticles by thermal ammonolysis of an ultrathin NiFe-LDH precursor and achieved a highly efficient electrocatalyst for overall water splitting.28 Despite significant progress, the incomplete contact between electrode substrate and active catalyst and the complexity of electrode fabrication process still restrict the performance boost of these materials particularly for their long-term durability. Direct growth of NiFe nitrides on electrode substrate could potentially address aforementioned issues, though very little work has been carried out in this field up to date.

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ure 1a shows a typical Ni foam framework with a pore size of ~250 μm, on which a large amount of NiFe(OH)x nanosheets with a thickness of ~20 nm are closely assembled (as shown in Figure 1b). In addition, deep etching marks of either micro cracks or cavities are clearly observed at the exposed area of nickel foam, while hardly seen on pristine one (images shown in Figure S2). They come from the redox-etching on Ni foam substrate by the added iron precursor where NiFe bimetal hydroxide nanostructures were fabricated simultaneously.

Scheme 1. Schematic illustration of the formation of FeNi3N/NF. In this paper, we report an in-situ growth of hierarchical iron-nickel nitride nanostructures on surfaceredox-etching Ni foam (FeNi3N/NF) as a bifunctional electrocatalyst for overall water splitting. As shown in Scheme 1, the FeNi3N/NF electrode was fabricated by thermal ammonolysis of NiFe hydroxide nanosheets (NiFe(OH)x) which are in-situ grown on Ni foam via a simple chemical precipitation reaction. Nickel foam has widely been used as an electrode support for water electrocatalysis owing to its excellent porous ﬂexibility, high specific and electrochemically accessible surface area.7,42,43 Herein, the commercial Ni foam not only acts as an electrode substrate, but also serves as a slow-releasing nickel precursor that is induced by redox-etching of iron precursor (Fe3+) during the precipitation process. The method does not require a specially added nickel precursor nor an oxidizing agent, but achieves well-dispersed iron-nickel nitride nanostructures which are grown directly on the nickel foam surface. Benefiting from collaborative advantages of bimetallic composite, metallic nitrides and unique electrode fabrication, FeNi3N/NF electrode exhibits extraordinary behaviors for both OER and HER in terms of low overpotentials of 202 and 75 mV at 10 mA cm-2, Tafel slopes of 40 and 98 mV dec-1, respectively, as well as extremely good durability (more than 400-hour consistent galvanostatic electrolysis without any visible voltage elevation). The successful growth of NiFe(OH)x and FeNi3N insitu grown on Ni foam was evidenced by the apparent difference of nickel foam color, as shown in Figure S1. Micro-morphologies of NiFe(OH)x and FeNi3N on Ni foam was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig-

Figure 1. SEM images of a, b) NiFe(OH)x/NF and c, d, e) FeNi3N/NF. f) HRTEM image of FeNi3N/NF (upper inset: the corresponding FFT pattern and lower inset: TEM image). g, h) EDX characterization and i) XRD pattern and atomic structure model (inset) of FeNi3N/NF. Figure 1c-e show the SEM images of NiFe hydroxidedecorated Ni foam after thermal ammonolysis. The regular nanosheets of NiFe hydroxide are transformed into porous ones that comprises of numerous nanorods. Their diameter is ~50 nm (TEM images shown in the inset of Figure 1f and Figure S3), which is obviously thicker than the original thickness of nanosheets prior to ammonolysis. Similar morphological changes derived from thermal ammonolysis have also been reported in other work.44 The unique architecture of hierarchical FeNi3N/NF electrode is expected to increase specific surface area and mass transfer rate, which are beneficial to the latter electrochemical testing. High-resolution TEM (HRTEM) image at an individual FeNi3N nanorod shows a wellresolved lattice fringe of 0.215 nm (Figure 1f), indexed to the (111) facet diffraction in FeNi3N crystal. The corresponding FFT (Fast Fourier Transformation) pattern (inset of Figure 1f) further confirms the existence of (111) facets of FeNi3N. As shown in Figure 1g, the chemical composition of FeNi3Nwas characterized by Energydispersive X-ray spectroscopy (EDX) at a specific zone. It classifies the peaks corresponding to Ni and Fe, respectively, from which the atom ratio of Ni/Fe was close to be ~3:1 (shown in Figure 1h), verifying the final product of FeNi3N rather than Fe2Ni2N, another FeNi bimetallic nitride. Additionally, electron energy loss spectroscopy


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(EELS) elemental mapping images of FeNi3N in Figure S4 demonstrates a uniform spatial distribution of Ni, Fe, N. The wide-angle XRD pattern confirms the successful synthesis of FeNi3N product. As shown in Fig. 1i, the diffraction peaks appeared at 41.52o, 48.33o, 70.81o and 85.61o match well with the (111), (200), (220) and (311) planes of face-centered cubic FeNi3N (JCPDS card no. 50-1434).45,46

Figure 2. XPS spectra of a) survey scan, b) Ni 2p, c) Fe 2p, and d) N 1s peaks of NiFe(OH)x and FeNi3N. X-ray photoelectron spectroscopy (XPS) was performed to classify the elemental compositions and chemical valences of samples. Figure 2 compares the XPS spectra of as-prepared NiFe(OH)x and Ni3FeN in detail. Carbon was likely from the conductive adhesive carbon tape which was used to fix the samples during the XPS measurement (Figure 2a). For NiFe(OH)x, two core-level peaks of Ni centered at 855.5 and 873.1 eV are ascribed to Ni2+ 2p3/2 and Ni2+ 2p1/2, respectively,47,48 accompanying with two satellite peaks located at 861.5 and 879.2 eV (Figure 2b).48 In Figure 2c, the peak at binding energies of 711.2 and 724.5 eV should be assigned to Fe 2p3/2 and Fe 2p1/2, respectively, indicating the presence of Fe element is in the state of +3.49 After thermal treatment in NH3 atmosphere, the product retains the nature of Ni2+ that corresponds to two Ni 2p peaks at 855.7 (2p3/2) and 873.8 (2p1/2) eV, respectively . Note that the other two peaks at 853.0 (2p3/2) and 870.5 (2p1/2) eV suggest a metallic state of nickel,27 indicating that Ni2+ ions were partially reduced by the thermal treatment of NH3. For the other component, Fe element with a state of +3 was maintained in FeNi3N, reflecting in binding peaks at 711.7 and 724.5 eV derived from Fe 2p3/2 and Fe 2p1/2, respectively. Similarly with Ni element, newly emerged binding peaks such as 707.3 (2p3/2) and 720.6 eV (2p1/2) are related to Fe0. The metallic characteristics typically offer rapid electron transportation, high hardness as well as high tensile strength, by virtue of which efficient and sustainable electrocatalyst would be obtained. Moreover, the appearance of strong N 1s peak (397.8 eV) while not observed in

NiFe(OH)x once again confirms the successful preparation of metal nitride by ammonolysis (Figure 2d).50 According to the results given above, we could speculate the possible reactions arising in the wet synthesis of NiFe(OH)x which is illustrated in Scheme 1. Firstly, Fe2+ in a heating aqueous solution could not react with Ni foam directly because of its lower EƟFe2+/Fe (-0.440 V), but is slowly oxidized to Fe3+ with assistance of water and oxygen (equation 1). Secondly, nickel foam is easily redoxetched by the newly-generated Fe3+ and thereby releases nickel ions slowly (equation 2), owing to much higher EƟFe3+/Fe2+ (0.771 V) than EƟNi2+/Ni (-0.257 V). It is worth noting that the slow-releasing nickel ions are very likely to stay around at the pore surroundings for the next precipitation process, since we used a very low stirring (200 rpm) in this case. At the same while, Ni2+ together with Fe3+ tends to precipitate on the sacrificial Ni foam to form NiFe hydroxide nanosheets (equation 3). Similar shapedirecting synthesis for various metal hydroxide nanostructures has been extensively applied.51-54 Moreover, the growth of NiFe(OH)x on Ni foam from 0.5 h to 6 h was monitored by SEM with images shown in Figure S5 and Figure 1. Large-scale growth did not occur until three hours later, which proves the slow but efficient synthesis. 4Fe2+ +O2 +2H2 O↔4Fe3+ +4OH3+

2+

2+

2Fe +Ni→2Fe +Ni 2+

3+

(1) (2)

-

xNi +Fe +(2x+3)OH →Nix FeOH2x+3

(3)

Figure 3. iR-corrected linear sweep voltammetry (LSV) curves for a) OER and b) HER measured at a scan rate of 5 mV s-1 and c, d) corresponding Tafel plots of different ma-



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terials. EIS Nyquist plots and fitting curves at e) 1.6 V and f) -0.2V for various electrocatalysts. The electrocatalytic activity of FeNi3N for OER was evaluated in an O2-saturated 1 M KOH solution. For comparison, NiFeOx/NF (pyrolysis product of NiFe(OH)x/NF sample in N2 atmosphere), NiFe(OH)x/NF, Ni foam and commercial RuO2 electrodes were also studied under the same conditions. As displayed in Figure 3a, the FeNi3N/NF electrode exhibits an early onset potential of ~1.43 V vs. RHE and the lowest overpotential (202 mV) at 10 mA cm-2 and absolutely higher current density than that of others during the whole polarization range. Furthermore, the Tafel slope at FeNi3N/NF electrode is fitted to be merely 40 mV dec-1 (Figure 3c), which is smaller than the NiFeOx/NF (54 mV dec-1), NiFe(OH)x/NF (54 mV dec-1), Ni foam (74 mV dec-1) and RuO2 (58 mV dec-1). As per our best knowledge, such a low Tafel slop value accompanying with a low overpotential has rarely been reported for transition metal-based electrocatalysts, indicating the outstanding OER catalytic properties of Fe Ni3N/NF. Electrochemical impedance spectroscopy (EIS) results are depicted in Figure 3e accompanied with circuit model fitting analysis, and the equivalent circuit is consisted of a solution resistance (Rs), a constant phase element (CPE) and a charge transfer resistance (Rct). The observed Rct value (0.3 Ω) of FeNi3N/NF is smaller than NiFeOx/NF (0.4 Ω), NiFe(OH)x/NF (0.6 Ω), Ni foam (1.2 Ω), suggesting a much faster electron transfer during the electrochemical reaction. Additionally, to reveal the influence of electrochemically active surface areas (EASA) and structure-activity relationship, we calculated the doublelayer capacitances by conducting cyclic voltammograms for the electrodes (Figure S6).55 The capacitance of NiFe(OH)x/NF is larger than that of Ni foam, indicating the result of corrosion by Fe3+. The capacitance of FeNi3N/NF is 2.5 times as high as that of NiFe(OH)x/NF, which can be attributed to the porous nanosheets stucture after thermal ammonolysis. Therefore, good electrocalytic behaviors of FeNi3N/NF is related to its large electrochemical surface area. Design of a bifunctional electrocatalyst with enhanced performance for both OER and HER simplifies water splitting systems and lowers the electrolyze cost, as only single catalyst would be required. Therefore, we assessed the electrocatalytic properties of the FeNi3N/NF for HER in the same basic media (Figure 3b). As expected, Pt/C shows detached HER activity with a negligible overpotential with only 50 mV for achieving current density of 10 mA cm-2, while that for FeNi3N/NF is 75 mV. However, the distinctions between these two electrodes are gradually decreased with the increase of current density (the overpotential for Pt/C and FeNi3N/NF are 220 mV and 230 mV at 150 mA cm-2, respectively). Considering the fact that high voltage is commonly applied for practical electrolyzers (1.8 V to 2.0 V),56 the as-prepared FeNi3N/NF could act an ideally good catalyst for HER with comparable performance to Pt/C. Meanwhile, comparing with

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other electrodes, the FeNi3N/NF displays visibly larger current density during the whole polarization range (later elaborated). As shown in Figure 3d, the linear relationship of Tafel plot gives FeNi3N/NF a slope of 98 mV dec-1, which is much lower than that of NiFeOx/NF (128 mV dec1 ), NiFe(OH)x/NF (130 mV dec-1) and Ni foam (196 mV dec1 ), respectively. All four samples exhibited classical Volmer-Heyrovsky behavior (ca. 120 mV dec-1), similar to most of Ni-based materials in alkaline water electrolysis.27,32,57,58 The good catalytic performance of FeNi3N/NF towards HER was reconfirmed by the following EIS measurements (Figure 3f). FeNi3N/NF displays the smallest Rct in Nyquist plots (2.6 Ω) compared with NiFeOx/NF (5.1 Ω), NiFe(OH)x/NF (6.7 Ω) and Ni foam (17.2 Ω), suggesting the smallest charge transfer resistance among these catalysts. In addition, OER and HER were also conducted without iR-correction as shown in Figure S7. As we can see from that, FeNi3N/NF still shows the best catalytic activities towards OER and HER. Table S1 lists some important parameters for evaluation of catalysts developed in the past few years. FeNi3N/NF exhibits an extraordinarily efficient catalytic activity toward water splitting particularly for OER. Both overpotential at a current density of 10 mA cm-2 (202 mV) and Tafel slop (40 mV dec-1) are one of the best OER performance among reported transition metal-based electrocatalysts in an alkaline medium to date. Taking into account its relatively good catalysis toward HER, we believe that it is exceptionally competitive as a class of bifunctional water electrolysis catalyst. The effect of iron precursor on the growth of FeNi3N/NF as well as electrochemical property has been investigated. In the case of Fe3+ precursor, a large amount of nanoclusters grew on the nickel foam, yet still in the midst of the framework with a large area of exposed substrate (Figure S8a-b). It is very hard to observe from that using Fe2+ as precursor as shown in Figure 1, also can bring about color depth to differ (Figure S8c). The high concentration of Fe3+ causes the rapid dissolution of nickel metal. If it is greater than the deposition rate of hydroxide, the surface-growth of FeNi3N on nickel foam would be inhibited and thus switches to the precipitation in solution. Utilization of Fe2+ enables to a slow-release of Fe3+ and Ni2+, and a uniform-precipitation on Ni foam. XRD patterns in Figure S8d reveal a better crystallinity of products obtained by Fe2+ owing to its “slow-growth”. Moreover, corresponding electrochemical results including polarization curves and Tafel plots for both OER and HER (Figure S9) confirm the superiority of Fe2+ precursor. Since pyrolysis temperature allows for great influence on the structure and electrochemical property of a catalyst, electrocatalysts of various pyrolysis temperatures were compared (SEM images and XRD patterns seen in supporting information Figure S10 and S11). The ammonolysis reaction does not occur at temperature of 300 oC, because the features of NiFe(OH)x including morphology


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and amorphous nature are still well preserved. However, it is totally different from the cases above 400 oC. At first, all of the XRD patterns from the pyrolyzed products match well with standard FeNi3N, demonstrating that the ammonolysis reaction is carried out. As we can see from that the nanosheets appear crowded holes on surface and gather to thicker ones. With the increase of nitridation temperatures, their morphology transformations are more and more dramatic, and even complete pulverization arises during the ammonolysis of 700 oC. The catalyst prepared by 500oC stands out among these samples (Figure S12) especially for HER, owing to its unique poroussheet structure and moderate particle size. To investigate the essential role of Ni foam substrate, samples decorated with FeNiNx and FexN on carbon cloth (CC) were also studied, respectively. Sparse distribution of FexN particles could be seen on CC with a diameter of ~50 nm (Figure S13). On account of overlap from the huge diffraction peaks of carbon material, it is difficult to observe the characteristic peaks from the XRD patterns. Pristine CC electrode exhibits insignificant response toward two basic reactions, as shown in Figure S14. Although a great promotion occurs when modified with FexN, especially with FeNiNx, their performances including HER and OER are still behind FeNi3N/NF, verifying our aforementioned hypothesis on synthesis process and superiority of Ni foam substrate.

Figure 4. a) Chronopotentiometric curves of FeNi3N/NF for OER and HER at 50 mA cm-2 and at 100 mA cm-2 (successive testing) without iR-correction. b) XRD patterns of FeNi3N/NF after corresponding tests. Stability evaluation of FeNi3N/NF for both OER and HER were carried out under a constant current density of 50 mA cm-2 and -50 mA cm-2, respectively with curves shown in Figure 4a. Notably, FeNi3N/NF electrode shows excellent performance in successive 30 h of OER (100 mA cm-2) and 30 h of HER (-100 mA cm-2) at large current density using the same piece of electrode. Negligible degradation could be observed in the stable corresponding current densities after continuous testing. The outstanding stability of FeNi3N/NF is confirmed by the XRD patterns (Figure 4b), from which not much of a difference is observed after the corresponding tests. The durability of the FeNi3N/NF for the OER and HER were further examined by successive CV at a scan rate of 50 mV s-1 for 1000 cycles (1.0-1.45 V and -0.1-0.3 V), respectively. FeNi3N/NF for catalysis of either OER or HER displays negligible decrease after 1000-cycle scanning (Figure S15).

Figure 5. a) Polarization curve of water electrolysis for FeNi3N/NF||FeNi3N/NF with a scan rate of 5 mV s-1 in 1.0 M KOH without iR-correction. b) Chronopotentiometric curve of water electrolysis for FeNi3N/NF||FeNi3N/NF with a constant current density of 10 mA cm-2 at room temperature. Considering the easy-fabrication, high-efficiency as well as stability of this FeNi3N/NF electrode toward OER and HER, we tested the possibility of utilizing it as anode and cathode (FeNi3N/NF||FeNi3N/NF) in a two-electrode system for overall water splitting. It was very clear to observe the evolution of hydrogen and oxygen from the anode and cathode, respectively. Figure 5a shows the polarization curve of the FeNi3N/NF||FeNi3N/NF electrolyzer in 1.0 M KOH. Only 1.62 V of cell voltage is needed when achieving a current density of 10 mA/cm2 even without iRcompensation. Although this value is still larger than that of Pt/C/NF||RuO2/NF (~1.53 V),59 it is smaller than the values required by electrolyzers using other transitionmetal-based bifunctional catalysts such as NiFe LDH/NF||NiFe LDH/NF (~1.70 V),60 NiCo2S4 61 NA/CC||NiCo2S4 NA/CC (~1.68 V), NiS/NF||NiS/NF (~1.64 V),59 as well as commercial electrolyzers (1.8 V to 2.0 V).56 We also tested the long-term durability of this fabricated electrolyzer (Figure 5b), which is particularly impressive. A constant voltage was well maintained for more than 400 h without any visible elevation but a slight decrease, when setting a current density at 10 mA cm-2. This result is entirely up to even more than we expected for this in-situ grown catalyst. In summary, a straightforward in-situ growth of ironnickel nitride nanostructures on surface-redox-etching Ni foam (FeNi3N/NF) has been reported as a highly efficient and ultra-sustainable electrocatalyst for overall water splitting. Ni foam is partially redox-etched by Fe(III) ion, on which the slow-releasing nickel ions together with Fe3+ co-precipitate to form NiFe(OH)x nanosheets, and thermal ammonolysis of NiFe(OH)x nanosheet-decorated Ni foam leads to the subsequent identification of FeNi3N/NF. FeCl2 is a preferable iron precursor than FeCl3 for no matter quality of FeNi3N growth or its electrocatalytic behaviors, owing to its “slow-growth” process. The outstanding catalytic performance of the FeNi3N/NF derives from the strong interaction between electrode patches and active substances, the intrinsic metallic character and unique electronic structure of FeNi3N. The easy-fabrication, highefficiency and durability (without any visible elevation after 400-hour sustained electrolysis) of this FeNi3N/NF



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electrode demonstrate its feasibility and practical prospect for overall water splitting. Moreover, the proposed method prompts further investigations into the direct growth of active species on metal substrate for electrocatalysis and other applications. Experimental Section Chemicals: The Pores Per Linear Inch (PPI) value of Ni foam is 110 obtained from Changde Liyuan Material Company (China). Trisodium citrate and FeCl2·4H2O and FeCl3·6H2O were purchased from Tianjin Zhiyuan Chemical Reagent Company (China). Pt/C catalyst (20 wt% Pt on Vulcan XC-72R) was obtained from ETEK Inc (USA). RuO2 was purchased from Alfa Aesar (China). All reagents were of analytical grade and used as received. All the aqueous solutions used were prepared by the ultra-pure water (>18 MΩ cm) from a Millipore system. Synthesis of FeNi3N/NF: Nickel foams were cleaned by 3 M HCl solution, ethanol and ultrapure water with assistance of ultrasonication for several minutes. The purified Ni foams were transferred into a 100 mL flask containing a homogeneous solution of 1 mL FeCl2 (0.4 mmol), 1 mL trisodium citrate (0.025 mmol) and 38 mL ultrapure water to form a light green solution. The resulting solution was then heated to 90 oC in an oil bath for 6 h with a slow stirring (200 rpm). Until the samples were cooled to ambient temperature, the brown products were taken out, rinsed several times with ultrapure water and ethanol and dried at 60 oC under vacuum. The brown nickel foam plates denoted as NiFe(OH)x/NF were then calcinated at 500 oC with a heating rate of 10 oC min-1 for 2 h in a tube furnace under a flowing NH3 atmosphere (160 sccm). After that, samples were cooled to ambient temperature in the atmosphere of ammonia. The black nickel-foam pieces are denoted as FeNi3N/NF. ICP measurement has been used to quantify the accurate mass of iron and nickel in the composite. After calculation, the content of active material was calculated to be ~15.9 %. For control samples, Pt/C catalyst was prepared by casting commercial available 20 wt% Pt/C (4 mg/mL in mixture of water and ethanol (4:1 v/v)) onto glassy carbon electrode (GCE, 5 mm in diameter) with a loading of 0.815 mg cm-2. NiFeOx/NF (XRD shown in Figure S16) was prepared by calcination of NiFe(OH)x/NF at 500 oC under N2 for 2h. FeNiNx/CC was prepared via the similar procedure with FeNi3N/NF sample except adding Ni(NO3)2·6H2O with a Fe:Ni precursor atom ratio of 1:3. Iron nitride grown on carbon cloth (FexN/CC) and the other FeNi3N/NF replacing precursor of FeCl2 by FeCl3 were prepared via the similar procedure with abovementioned FeNi3N/NF sample. Material characterization:

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The morphology of the samples was examined by using field-emission scanning electron microscopy (FESEM; JEOL, JSM-7000F) and transmission electron microscopy (TEM; JEOL, JEM-2100F). TEM samples were pretreated by strong ultrasonic process in ethanol solution. Electron energy loss spectroscopy (EELS) were performed on FEIG2F30. The accurate mass of iron and nickel in the composite was measured by direct-reading inductively coupled plasma emission spectrometer(ICPE-9000). The crystallographic and composition information were characterized by powder X-ray diffraction measurement (XRD; SHIMADZU, Lab X XRD-6000). The X-ray photoelectron spectroscopy (XPS) measurement were performed on a monochromatic Al Ka radiation (Axis Ultra, Kratos, 150 W, 15 kV and 1486.6 eV), and the binding energies were calibrated by referring the carbon 1s peak to 284.6 eV. Electrochemical measurements: All electrochemical measurements were all conducted on a CHI 660D electrochemical workstation (Chenhua, Shanghai, China) connecting with a three-electrode electrochemical cell with modified Ni foam electrode as working electrode directly, Ag/AgCl electrode as reference electrode and graphite rod electrode as counter electrode, repectively. Electrolyte of 1 M KOH aqueous solution was saturated by oxygen (for OER) or nitrogen (for HER) bubbles at least 30 min prior to experiment and maintained above the liquid level during all the polarizations. Both OER and HER polarization curves were obtained by linear sweep voltammetry (LSV) with a scan rate of 5 mV s-1. Polarization curves unless otherwise specified were corrected for an ohmic drop (iR) tested by electrochemical impedance spectroscopy (EIS). Long-term durability test was performed under fixed current densities such as 100, 50 and 10 mA cm-2 without iR-correction. The EIS measurements were set at operate potential of 0.6 V (OER) and -0.2 V (HER) with an amplitude of 5 mV, respectively. The frequency range is 100 K-0.1 Hz. The potential was calibrated with respect to reversible hydrogen electrode (RHE) according to procedure reported in literature.62 In 1 M KOH, ERHE= EAg/AgCl + 1.003 V (Figure S17). All the potentials mentioned in our manuscript are against RHE unless otherwise specified. All electrochemical measurements were carried out at room temperature.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Additional SEM, TEM, optical and EELS mapping images, XRD patterns, double-layer capacitance measurement for determining electrochemically active surface area, LSV curves, calibration of the reference electrode, and Table S1 for a brief comparison.

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]

ACKNOWLEDGMENT Financial support from the National Natural Science Foundation of China (21405119), Natural Science Basic Research Plan in Shanxi Province of China (2015JQ2046), China Postdoctoral Science Foundation (2014M562392) and Fundamental Research Funds for the Central Universities (No. 08143099) is gratefully acknowledged.

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