FeOOH-enhanced baifunctionality in Ni3N nanotube arrays for water splitting

Journal Pre-proof FeOOH-enhanced Baifunctionality in Ni3 N Nanotube Arrays for Water Splitting Jielun Guan, Chengfei Li, Jiawei Zhao, Yanzhang Yang, Wen Zhou, Yi Wang, Gao-Ren Li

PII:

S0926-3373(20)30015-1

DOI:

https://doi.org/10.1016/j.apcatb.2020.118600

Reference:

APCATB 118600

To appear in:

Applied Catalysis B: Environmental

Received Date:

8 October 2019

Revised Date:

3 January 2020

Accepted Date:

4 January 2020

Please cite this article as: Guan J, Li C, Zhao J, Yang Y, Zhou W, Wang Y, Li G-Ren, FeOOH-enhanced Baifunctionality in Ni3 N Nanotube Arrays for Water Splitting, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118600

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FeOOH-enhanced Baifunctionality in Ni3N Nanotube Arrays for Water Splitting Jielun Guan,a[+] Chengfei Li,b[+] Jiawei Zhao,b Yanzhang Yang,b Wen Zhou,b Yi Wang,a* Gao-Ren Li b*

a

The Key Lab of Low-carbon Chemistry & Energy Conservation of Guangdong Province, School of Chemical Engineering and Technology, Sun Yat-sen University, Guangzhou 510275, P. R. China b

[+]

The first two authors contributed equally to this work

E-mail address of the corresponding author: [email protected]; [email protected]

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*

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MOE Laboratory of Bioinorganic and Synthetic Chemistry School of Chemistry, Sun Yat-sen University Guangzhou 510275, China

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Graphitical Abstract:

The schematic illustration of FeOOH as the promoter for promoting water dissociation

Highlights 

The functional FeOOH, as a cocatalyst with strong affinity for H2O/OH , induce excellent

bifunctionality of the Ni3N catalyst. 

The relatively vertical Ni3N nanotube arrays can facilitate the faster electrons transfer and mass transportation/diffusion.



FeOOH/Ni3N exhibits the highly efficient electrocatalytic activity toward HER and OER.

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Abstract: Here we report the functional FeOOH as a cocatalyst to enhance excellent bifunctionality of the Ni3N catalyst, which demonstrate remarkable electrocatalytic activity for hydrogen evolution reaction (HER) and oxygen evolution reduction (OER) compared to the pristine Ni3N sample. Experimental data and theoretical calcinations demonstrate that the FeOOH facilitates surface

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adsorption and cleavage of OH−/H2O species and decrease the d-band center of active Ni species for optimizing the Gibbs free energy of intermediates. Furthermore, relatively vertical Ni3N nanotubes can

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promote the mass transport and electrons transfer as a main catalyst. The unusual synergistic effect in hybrid system with high density of heterointerface is contributed to the improvement of HER/OER

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catalytic performance. Specifically, the obtained FeOOH/Ni3N hybrid catalysts display excellent OER/HER performance with an overpotential of 244 mV/67 mV at 10 mA cm-2 in 1.0 M KOH, along

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with an applied potential of 1.56 V to boost overall water splitting at 10 mA cm−2. This simple and effective strategy may provide a new path to design the bifunctional catalysts and improve the catalytic

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performance for water splitting.

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Keywords: FeOOH/Ni3N, synergistic effect, bifunctionality, OER/HER

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Introduction With the accelerating depletion of fossil fuels, water splitting is a promising strategy for producing the molecule hydrogen and oxygen (clean and renewable energy alternatives) to alleviate the energy crisis[1-3]. However, the preeminent efficiency of water splitting still remains a great challenge.

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Because the rate of the hydrogen evolution reaction (HER) is relatively sluggish due to the high dissociation energy of water in alkaline media[4-6], and the quite sluggish kinetics of oxygen evolution

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reaction (OER) with multistep proton-electron transfer is more obvious [7-9]. Although the Pt metal and RuO2/IrO2 demonstrate much high catalytic active for HER and OER in alkaline media,

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respectively, the scarcity, high cost and poor stability have severely impeded their practical applications on a large scale[10-13]. Therefore, Great efforts should be devoted to develop highly

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active and durable non-noble metal catalysts to achieve high production rate of hydrogen/oxygen. Recently, there exist some intriguing non-precious catalysts with excellent catalytic performance for

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water splitting in alkaline electrolytes, including transition-metal oxides[14, 15], sulfides[16-18], selenides[19-21], phosphides[22, 23] and nitrides[24-26]. Among these electrocatalysts, the metal

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nitrides have emerged as the most optimistic class of water splitting catalysts owing to its metallic nature with excellent electro-conductivity, the favorable electronic structure modified by its nitride and high corrosion resistance[27-32]. Nevertheless, Considering the requirements of practical applications, the electrocatalytic activity of Ni3N catalyst for HER and OER still needs further enhancing. According to OER/HER mechanism, the significantly large overpotential is mainly ascribed to multiple steps of proton-coupled electron transfer, O−H bonds breaking, O−O bands formation and the cleavage of HO–H bonds in alkaline media [33-36] . Therefore, it is challenging to simultaneously

promote HER and OER catalytic activity of Ni3N catalyst. Herein, we reported the facile, direct growth of FeOOH on Ni3N nanotube arrays supported on conductive carbon cloth to serve as bifunctional catalysts (FeOOH/Ni3N/CC) by deposition methods. The modification of the Ni3N results in the following advantages: (1) The electronic interaction between FeOOH and Ni3N make the FeOOH positive charges resulting in easily adsorbing the OH- or H2O by the Fe-O bonds, while the negatively charged Ni3N adsorbs the H from H2O or OH- in alkaline media, thus, facilitating the water or OH- dissociation [37, 38]. (2) The FeOOH as promoter can not only promote the ability of water affinity, but also modulate electronic structure of active sites, along

vertical

Ni3N

nanotube

arrays

can

facilitate

the

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with in situ forming the heterointerface of FeOOH/NiOOH during OER process. (3) The relatively faster

electrons

transfer

and

mass

transportation/diffusion. As for the above advantages, the FeOOH/Ni3N/CC hybrid catalysts display excellent OER and HER performance with an overpotential of 244 mV and 67 mV at 10 mA cm-2 in

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1.0 M KOH, respectively. Moreover, the FeOOH/Ni3N/CC hybrid catalysts only require a cell voltage of 1.58 V to deliver the current density of 10 mA cm-2 with tremendous stability over 40 h for overall

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water splitting.

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Experimental method Synthesis of electrodes

All chemicals used in this report were analytical grade and used directly without further purification.

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The detailed preparation procedure is described below:

Preparation of Ni tubes/CC: ZnO nanorod array templates were first electrodeposited on carbon fiber

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(2.0 cm*0.5 cm) in solution of 0.01 M Zn(NO3)2 + 0.05 M NH4NO3 with a current density of 0.8 mA /cm-2 at 70 oC for 60 min, which were then taken out and washed by distilled water. ZnO@Ni

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core–shell array were fabricated via the electrodeposition of Ni nanoparticles on the surface of ZnO at a current density of 1.0 mA cm-2 in solution of 0.01 M Ni(Ac)2 + 0.05 M NH4Ac + 0.05 M H3BO3 for 60 min. The synthesized ZnO@Ni core-shell were then immersed in 2.5% NH3·H2O solution for 2 h to remove the ZnO template, and accordingly the Ni tubes were fabricated. The fabricated Ni tubes were washed. Preparation of Ni3N/CC: Ni tubes/CC was then placed into a porcelain boat, heated from temperature to 500 oC with heating rate of 1 oC per minute and was maintained for 3h in NH3 atmosphere. After

being cooled to room temperature under NH3 naturally, the Ni3N/CC was collected for next preparation process and electrochemical measurements. Preparation of FeOOH/Ni3N /CC: 0.1 mol FeSO4·7H2O was dissolved in 15 ml distilled water and stirred to form a clear solution. Ni3N/CC was then fully immersed in 10 ml of a freshly prepared solution for 15 min, the adsorbed ferrous ion were gradually oxidized to FeOOH in the solution. After the in-situ growth of FeOOH, the sample were taken out and washed by distilled water. Preparation of FeOOH/CC: FeOOH were electrodeposited on carbon fiber (2.0 cm * 0.5 cm) in

/cm-2 at 25 oC for 30 min, taken out and washed by distilled water.

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solution of 0.01 M (NH4)2Fe(SO4)2·6H2O + 0.04 M CH3COONH4 with a current density of 1.0 mA

Physical characterizations: The powder X-ray diffraction (XRD) patterns of as-prepared samples were obtained from a Rigaku D/Max 2550 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å).

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The scanning electron microscopy (SEM) measurements were performed on a Zeiss Sigma field emission SEM (FE-SEM, JSM-6330F). Transmission electron microscopy (TEM) measurements were

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performed on a JEM-2010HR and high resolution TEM (HRTEM, 120 kV or 300 kV). X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCA Lab250 X-ray

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photoelectron spectrometer. The Raman tests of samples performed on a laser micro Raman spectrometer (Renishaw inVia) equipped with a He−Ne laser (wavelength =532 nm) and two objective lens (long working distance 20× and 50×).

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FTIR measurements: In order to guarantee the same conditions, the FeOOH/Ni3N, FeOOH and Ni3N catalysts firstly were maintained at 80 oC in oven with 0.1MPa for 24 h to remove moisture. After that,

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the all samples were immersed in 1M KOH solution for 10 min, and then transferred to room temperature for 48 h to remove the moisture from the surface. Lastly, the all samples were performed

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on the Fourier Transform infrared (FTIR) instrument (EQUINOX 55) under the same conditions. Electrochemical measurements: All the electrochemical measurements were performed in a three-electrode cell with an electrochemical station (CHI 760E), using the sample as working electrode, graphite rod as counter electrode and saturated calomel electrode (Hg/Hg2Cl2) as reference electrode. The linear sweep voltammograms (LSVs) and cyclic voltammograms (CVs) were measured at a scan rate of 5 mV s−1 in 1.0 M KOH solution, in order to study the real potential value of each electrode, here the potential values were corrected by compensating 90% iR drop. Electrochemical impedance

spectroscopy (EIS) measurements of each sample were carried out using above three electrode systems at 0.40 V vs Hg/Hg2Cl2. The frequency range was 100 K Hz to 0.1Hz, and the amplitude of the applied voltage was 5 mV.

Results and discussion Synthesis and characterizations of FeOOH/Ni3N/CC hybrid catalysts. The typical synthetic process of FeOOH/Ni3N/CC hybrid catalysts is shown in scheme 1. The carbon cloth (CC) is employed to the conductive substrates due to high electronic conductivity, no binder and high surface

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area. Firstly, the ZnO nanoarrays as the initial precursor were synthesized by the way of electrodeposition. The scanning electron microscope (SEM) image (Figure S1) displays the relatively vertical ZnO nanoarrays grown on the substrate along with uniform diameter size of approximately 250 nm. Subsequently, the nickel ions were reduced on the surface of ZnO nanoarrays. After that, the

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ZnO nanoarrays were dissolved by the ammonium hydroxide to prepare the nickel nanotubes. From the images, it further demonstrates that we successfully synthesized the nickel nanotubes with a

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thickness of the tube wall ranging from approximately 40 to 60 nm. Secondly, the nickel nanotubes were transformed into Ni3N nanotubes (Figure 1a) by calcination in ammonia at 500 oC for 2 h. After

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that, the Ni3N nanotubes were immersed in FeSO4 solution to obtain the FeOOH/Ni3N hybrid catalysts, as shown in Figure 1b-d. Obviously, the inset of Figure 1b exhibits the FeOOH nanosheets anchoring

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on the Ni3N nanotubes with high degree of surface roughness. The TEM image (Figure 1c) further demonstrates the hollow nanotubes, and the edges are partially located in a leaf-like structure of FeOOH nanosheets with clearly distinguishable hierarchy. Furthermore, the high-resolution TEM

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image of FeOOH/Ni3N hybrid catalysts (Figure 1d) clearly demonstrates the heterointerface between FeOOH and Ni3N. Moreover, the interplanar distances of 0.204 and 0.214 nm are assigned to the (111)

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and (002) planes of the Ni3N, respectively, which are confirmed by the following XRD results (Figure S2), and the characteristic lattice fringes of (120) and (020) planes in FeOOH with interplanar distances of 0.202 and 0.221 nm can also be observed, further verifying the formation of the FeOOH phase[39-41], in consisting with the XRD (Figure S2) and Raman results (Figure S3). The bright diffused halo of the selected-area electron diffraction (SAED) patterns (Figure 1e and f) can clearly exhibit the existence of FeOOH and Ni3N phase, in which the relative crystal face is well in agreement with the above analysis results. Elemental mapping analysis of TEM (Figure 1g–k) showed that Ni, Fe,

N and O are homogeneously distributed in FeOOH/Ni3N hybrid catalysts, suggesting successfully nitriding and depositing and meanwhile showing effective removal of zinc element, consisted with the XPS results (Figure S4). Further analysis of ICP-MAS elucidates that the atomic ratio of Ni and Fe is 4.4: 1.8 (Table S1). In-depth characterizations were performed to exhibit the electronic properties and chemical valence states of the FeOOH/Ni3N hybrid catalysts as shown in Figure 2. To identify the electronic interactions between Ni3N and FeOOH, the electronic binding energy was investigated by the XPS measurement. Figure 2a represents the Ni 2p XPS spectrum of FeOOH/Ni3N with three main peaks at 853.2 eV

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(Ni-N), 856.8 eV (Ni 2p3/2) and 874.5 eV (Ni 2p1/2), along with their shakeup satellites. Compared with the peak position of Ni 2p3/2 of Ni3N, the negative shift of ~1.0 eV of FeOOH/Ni3N can be obviously observed, which elucidates the charge transfer from Fe to Ni in FeOOH/Ni3N hybrid catalysts resulting in the negatively charged Ni3N catalysts. Furthermore, the binding energy of Fe 2p3/2 and Fe 2p1/2 at

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712.2 and 725.6 eV with two shakeup satellites (Figure 2b), respectively, which is higher than those of Fe 2p3/2 and Fe 2p1/2 in pure FeOOH, demonstrating the electronic interaction between them. In order

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to further show the pathway of the electrons transfer, the main peaks of metal-O bonds in the O 1s spectrum was shifted from 530.7 eV in FeOOH/Ni3N to 530.2 eV in FeOOH(Figure 2c), exhibiting the

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Ni3N attracts more electrons through the pathway of Ni-O-Fe bonds as shown in supplementary information Figure S5. Moreover, the UV-Vis spectra of all samples display the significant difference

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of bandgap absorption edge owing to the electron interaction between the Ni3N and FeOOH (Figure 2d), in good agreement with the above analysis results[42, 43]. In addition, according to the previous reports[42, 43], the positively charged FeOOH can easily match with OH- and H2O with high

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electronegativity/polarization, while the negatively charged Ni3N can easily adsorb the H atoms from OH- or H2O. Therefore, the FeOOH can be used as a promoter to facilitate the cleavage of H-OH or

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H-O* bonds for promoting HER or OER catalytic performance in alkaline solution. Furthermore, to further evaluate the ability of H-OH and O-H- dissociation and adsorption, the FTIR measurement has been carried out as shown in Figure 2e. Compared with Ni3N and FeOOH, the FeOOH/Ni3N hybrid catalysts display the much higher amount of OH

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and H2O, demonstrating the positively charged

FeOOH can promote the adsorption, in good agreement with the static contact angle measurements as shown in Figure 2f, where the FeOOH/Ni3N becomes superhydrophilic with a contact angle of 0º after

introducing the superhydrophilic FeOOH nanosheets, beneficial for electrolyte penetration and gas adsorption/desorption kinetics. Interestingly, the FTIR results also demonstrate that the peak positions -

of H2O and OH in FeOOH/Ni3N negatively shift from 3363 to 3342, from 643 to 618 compared with FeOOH, respectively, further demonstrating the synergistic interaction and water dissociation as schematic illustration in Figure 4d[44, 45]. To gain further insights into the heterointerface effect between Ni3N and FeOOH, the DFT calculations has been conducted as shown in Figure 2g-i. Figure 2g displays the theoretical model of heterostructure of FeOOH/Ni3N, in which the different dotted boxes represent the different electronic effect of Ni and O atoms from the first layer and second layer

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on the interface. From the density of state of O 2p (Figure 2h), obviously, the pseudogap of first oxygen layer is wider than that of the second oxygen layer, suggesting the covalence is stronger on Ni-O-Fe bonds favored tailoring adsorption energy of the intermediate species[30], and also further

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verifying that the stronger interaction exists at the heterointerface, which is different from the bulk phase. Furthermore, the FeOOH/Ni3N shows metallic behavior with the density of states (DOS)

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crossing over the Fermi level (Figure 2i). In addition, the DOS of Ni 3d states near the Fermi level at the interface are located in the middle of the Ni 3d states in bulk phase, resulting in charge transfer

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from FeOOH to Ni3N[44], which is consistent with the XPS results. Evaluation of electrocatalytic OER performance. Inspired by the novel heterostructure with electronic effect and excellent superhydrophilicity, the OER performance of as-prepared samples has

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been evaluated by a typical three-electrode system in 1 m KOH aqueous solution at room temperature. Impressively, the FeOOH/Ni3N hybrid catalysts demonstrate a greatly enhanced OER performance

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with an overpotential of 244 mV at the current density of 10 mA cm−1 compared with other employed electrodes (The Ni tubes, Ni3N and FeOOH catalysts show the overpotentials of 415, 379 and 440 mV,

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respectively) (Figure 3a and c), outperforming most of Ni-based OER catalysts reported to date (Table S2, Supporting Information). The excellent OER performance FeOOH/Ni3N hybrid catalysts may be attributed to the following reasons: (1) the relatively vertical Ni3N nanotube arrays have a fast electron transfer and mass diffusion; (2) the FeOOH as promoter not only facilitate the adsorption of OH-, but also decrease the d-band center of Ni in this hybrid system beneficial for optimizing the Gibbs free energy of intermediates during OER process; and (3) the strong chemical and electronic coupling between Ni3N and FeOOH. Furthermore, to further exhibit the fastest kinetics during OER process, the

Tafel slopes (Figure 3 b and c) of the as-synthesized samples were fitted according to the LSV polarization curves. Compared with that of the Ni tubes (88 mV dec−1), Ni3N (85 mV dec−1) and FeOOH (130 mV dec−1), the modified FeOOH/Ni3N hybrid catalysts feature smaller Tafel slopes of 65 mV dec−1, indicating fast reaction kinetics attributing to the synergistic effect and its high conductivity of metallic character. Moreover, the electrochemical impedance of as-prepared electrocatalysts was further evaluated to verify the superior OER reaction kinetics of FeOOH/Ni3N hybrid catalysts. The charge transfer resistance (Rct) value of 1.7 Ω is much lower than those of Ni3N (3.6 Ω) and Ni tubes (9.8 Ω) and FeOOH (10.7 Ω) (Figure 3d), facilitating the faster charge transfer, well in agreement with

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the Tafel results. To assess the exposed active sites, the CV curves of Ni tubes, Ni3N, FeOOH and the FeOOH/Ni3N hybrid catalysts were measured at different scan rates (5, 10, 20, 40, 60, 80 mV s-1) to calculate their double-layer capacitance for determining the electrochemical active surface area (Figure S6 and Figure 3e). Obviously, the FeOOH/Ni3N hybrid catalysts display the largest ECSA, which is

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about 1.7, 2.6 and 14.6 fold than those of Ni3N, Ni tubes and FeOOH catalysts, respectively, suggesting the FeOOH/Ni3N hybrid catalysts have the most catalytic active sites for OER. Besides

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high OER activity, the durability of FeOOH/Ni3N hybrid catalysts in alkaline solutions is also very important for practical application. Thus, the chronoamperometric measurement at 10 mA cm-2 was

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carried out as shown in Figure 3f, which apparently illustrates the applied voltage maintains almost unchanged even after 50 h chronoamperometry running, indicating the fascinating structural stability

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of FeOOH/Ni3N hybrid catalysts for OER. Furthermore, The Faradaic efficiency was evaluated by comparing the theoretical amount of evolved gas with the experimental value. The anode shows stable oxygen evolution rates that match the theoretical values well (Figure S7), implying nearly 100%

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Faradaic efficiency of the FeOOH/Ni3N-based electrolyzer and stable OER electrocatalyst. In addition, to further elucidate the robust stability and intrinsically active species of FeOOH/Ni3N hybrid catalysts,

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their crystal structure, chemical composition and morphology was conducted by the XRD, XPS, SEM and TEM measurements (Figure S8-S12) after durability tests. The XRD result exhibits that apart from the characteristic peaks of Ni3N, the new peaks of NiOOH (JCPD No: 060075) can be obviously observed, suggesting the Ni3N partially converted into the NiOOH as active species, which is consisted with the most reported works with Ni3+ as active centers for OER[46-48]. The Ni 2p XPS spectra further demonstrates that there is a presence of characteristic peak of Ni3+ (857.5 eV) derived from NiOOH[41, 49-51]. In addition, compared with the Fe 2p XPS of FeOOH, the downshift about 0.2 eV

of the peak positions of the Fe 2p of Ni3N/NiOOH/FeOOH can be apparently observed, demonstrating the interaction between them in hybrid system. The SEM and TEM results display the catalysts maintain original morphology features. Interestingly, the hybrid catalysts are comprised of the inner conductive substrate (Ni3N) as the pathway of electron transfer, active center (NiOOH), and cocatalyst (FeOOH) to improve catalytic activity of the main catalyst by optimizing the adsorption/desorption energy of intermediates during OER process. Therefore, the above excellent OER performance may be attributed to their superior hybrid system.

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Evaluation of electrocatalytic HER performance. To gain insight into the role of FeOOH as the cocatalyst For HER, the electrocatalytic activities of Ni tubes, Ni3N, FeOOH and FeOOH/Ni3N catalysts were measured as shown in Figure 4a. Notably, the FeOOH/Ni3N hybrid catalysts exhibit the best catalytic activity among the three electrodes, delivering a current density of 10 mA cm-2 with only

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overpotential of 67 mV, which was 130, and 30 mV lower than those of Ni tubes and Ni3N, respectively. The Tafel slope is also an important parameter to evaluate the effect of H-OH dissociation

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by the HER kinetics. The FeOOH/Ni3N exhibits the lowest Tafel slope of 82 mV dec−1, which is much lower than those of Ni3N (200 mV dec−1) and Ni tubes (256 mV dec−1) (Figure 4b), suggesting that the

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FeOOH facilitates the dissociation of H-OH bonds and then accelerates the HER reaction kinetics in alkaline media. Furthermore, the Nyquist plot of FeOOH/Ni3N exhibits a very smaller charge transfer

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resistance (3.7 Ω) than those of Ni3N (8.1 Ω) and Ni tubes (10.49 Ω), suggesting much faster charge-transfer kinetics (Figure 4c), which is highly consistent with the result of the Tafel slope. The superior catalytic performance of FeOOH/Ni3N hybrid catalysts can arise from the following reasons:

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as we know, the polar molecule is consisted of two positively charged H atoms and a negatively charge oxygen atom. Thus, the FeOOH with positive charge prefer to adsorb the O atom, while the Ni3N with

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negative charge prefer to bond the H atom. Accordingly, the synergistic effect will activate the water molecule and facilitate the cleavage of H-OH bond. In addition, the relatively vertical Ni3N nanotubes will provide the fast rate of electrons transfer. The long-term durability of FeOOH/Ni3N hybrid catalysts was also evaluated because it is the important process parameters for practical application. As shown in Figure 4e, the performance of FeOOH/Ni3N hybrid catalysts for HER have no obvious attenuation after 50 h continuous testing, demonstrating much better stability of FeOOH/Ni3N hybrid catalysts.

We also investigate the effect of cover density of FeOOH nanosheets on the electrocatalytic performance. When the time of FeOOH deposition increases from 5 to 30 min, the FeOOH nanosheets on the surface of Ni3N can be obviously observed (Figure S13). The contents of FeOOH on the surface of Ni3N were further confirmed by the ICP-AES measurement as shown in Table S3, suggesting the cover density of FeOOH nanosheets increases. Electrochemical tests indicate that the FeOOH/Ni3N-15 shows much high catalytic activity as compared to other samples (Figure 4f), which can be attributed to the presence of more heterointerface. This result also elucidates high density of heterointerface of FeOOH/Ni3N hybrid catalysts play important role for promoting HER performance because of more

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synergistic sites for the H-OH bonds dissociation. Meanwhile, the highest amounts of FeOOH nanosheets in FeOOH/Ni3N-30 hybrid catalysts result in the worst catalytic activity due to the active site blocked in Ni3N, further demonstrating the Ni3N is the main catalyst, while the FeOOH is the cocatalyst. Thus, these above phenomena provide a useful guidance for design of the effective

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electrocatalysts.

Evaluation of electrocatalytic water splitting performance. As excellent OER and HER catalysts,

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the FeOOH/Ni3N hybrid catalysts were simultaneously employed to the anode and cathode for over water splitting in 1M KOH solution at room temperature. Impressively, the electrodes exhibit relatively

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excellent activity with only requiring 1.58 V of the cell voltage to drive a current density of 10 mA cm−2 compared with Ni3N electrodes (1.72 V) (Figure 5a). In addition, the electrodes imply the

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superior stability with a negligible loss of operating voltage in two-electrode system after 40 h chronoamperometry running (Figure 5b). In order to further investigate strong long-term stability of hybrid catalyst, the higher current density and cycling time have been carried out as shown in Figure

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S14. Obviously, the current density of the electrolyzer remains stable during the 80h test at 50 mA cm-2, illustrating the great potential of FeOOH/Ni3N hybrid catalysts for water splitting for commercial

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utilization to replace noble metal materials. Conclusions

In summary, we synthesized the Ni3N nanotube arrays modified with FeOOH nanosheets as an effective bifunctional catalyst, which displayed high OER and HER activity with only requiring 244 and 67 mV overpotential to drive the current density of 10 mA cm−2 for water splitting, respectively. The anchoring of FeOOH with metal nature of Ni3N not only can improve the intrinsic activity but

also enhance the cycling stability. The XPS, FTIR, UV results and DFT calculations further revealed high OER, HER activity and stability mainly arising from strong synergetic electron coupling between Ni3N and FeOOH, and the FeOOH as the cocatalyst for facilitating adsorption/dissociation of water. These findings could open up new opportunities in rational design of bifunctional catalysts for energy conversion-related applications.

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Author contributions

Jielun Guan, Chengfei Li, Yi Wang, and Gao-Ren Li designed the experiments and performed the analysis of the whole data and wrote the paper. Jiawei Zhao performed DFT calculations and analysis

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of the data. Yanzhang Yang and Wen Zhou conducted relative measurements and data analysis.

Competing financial interests

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The authors declare no competing financial interests.

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgments

This work was supported by National Basic Research Program of China (2015CB932304 and 2016YFA0202603), NSFC (91645104), Science and Technology Program of Guangzhou (201704030019), Natural Science Foundation of Guangdong Province (2016A010104004 and 2017A010103007), and Guangdong Science and Technology Innovation Leading Talent Fund (2016TX03N187). References

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Figure captions

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Figure 1. a) The SEM image of Ni3N nanotube arrays; b) The SEM image of FeOOH/Ni3N hybrid catalysts. c) The TEM image of FeOOH/Ni3N hybrid catalysts; d) The high-resolution TEM images of

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FeOOH/Ni3N hybrid catalysts, showing heterointerface between FeOOH and Ni3N. The SAED pattern (e and f) taken from FeOOH/Ni3N hybrid catalysts; g–k) The TEM images of FeOOH/Ni3N hybrid

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catalysts and corresponding elemental mappings.

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Figure 2. a) XPS spectra of Ni 2p; b) Fe 2p and c) O 1s of Ni3N, FeOOH and FeOOH/Ni3N; d) UV-Vis spectra of Ni3N, FeOOH and FeOOH/Ni3N; e) The FTIR patterns of FeOOH/Ni3N catalysts; f)

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Contact angle measurements of Ni3N, FeOOH and FeOOH/Ni3N, respectively; g) Calculation model

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of FeOOH/Ni3N; h and i) The density of states of FeOOH/Ni3N.

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Figure 3. Comparison with OER electrocatalytic performance. a) IR-corrected polarization plots for OER processes in 1 M KOH electrolyte at the scan rate of 5 mV s−1; b) Tafel curves for OER process;

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c) Corresponding overpotentials and Tafel slops; d) Nyquist plots obtained by EIS at 1.32 V vs RHE for Ni tubes Ni3N, FeOOH and FeOOH/Ni3N; e) Cdl measurements for OER process; f)

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Chronoamperometric measurements recorded on FeOOH/Ni3N hybrid catalysts for 50 h at the steady

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current density of 10 mA/cm2.

Figure 4. Comparisons with HER electrocatalytic performance. a) IR-corrected polarization plots for HER processes in 1 M KOH electrolyte. Scan rate: 5 mV s−1; b) Tafel curves for HER processes showing the reaction kinetics; c) Nyquist plots obtained by EIS at 0.07 V vs RHE for the Ni tubes

Ni3N and FeOOH/Ni3N; d) The schematic illustration of FeOOH as the promoter for water dissociation; e) Chronoamperometric measurements recorded on FeOOH/Ni3N hybrid catalysts for 50 h at the steady current density of 10 mA/cm2; f) IR-corrected polarization plots of FeOOH/Ni3N-5,

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FeOOH/Ni3N-15 and FeOOH/Ni3N-30.

Figure 5. a) Polarization plot of FeOOH/Ni3N in 1 M KOH electrolyte for over water splitting. Scan rate: 5 mV s−1; b) Chronopotentiometry curve of the FeOOH/Ni3N at constant current densities of 10

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mA cm−2.

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Scheme captions

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Scheme 1. Schematic illustration of the synthesis routes of the FeOOH/Ni3N hybrid catalysts.