Improving the HER activity of Ni3FeN to convert the superior OER electrocatalyst to an efficient bifunctional electrocatalyst for overall water splitting by doping with molybdenum

Journal Pre-proof Improving the HER activity of Ni3FeN to convert the superior OER electrocatalyst to an efficient bifunctional electrocatalyst for overall water splitting by doping with molybdenum Xiaolei Liu, Xingshuai Lv, Peng Wang, Qianqian Zhang, Baibiao Huang, Zeyan Wang, Yuanyuan Liu, Zhaoke Zheng, Ying Dai PII:

S0013-4686(19)32360-6

DOI:

https://doi.org/10.1016/j.electacta.2019.135488

Reference:

EA 135488

To appear in:

Electrochimica Acta

Received Date: 8 April 2019 Revised Date:

2 December 2019

Accepted Date: 10 December 2019

Please cite this article as: X. Liu, X. Lv, P. Wang, Q. Zhang, B. Huang, Z. Wang, Y. Liu, Z. Zheng, Y. Dai, Improving the HER activity of Ni3FeN to convert the superior OER electrocatalyst to an efficient bifunctional electrocatalyst for overall water splitting by doping with molybdenum, Electrochimica Acta (2020), doi: https://doi.org/10.1016/j.electacta.2019.135488. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Improving the HER activity of Ni3FeN to convert the superior OER electrocatalyst to an efficient bifunctional electrocatalyst for overall water splitting by doping with molybdenum Xiaolei Liu a, Xingshuai Lv b, Peng Wang a,*, Qianqian Zhang a, Baibiao Huang a, Zeyan Wang a, Yuanyuan Liu a, Zhaoke Zheng a, Ying Dai b a

State Key Lab of Crystal Materials, Shandong University, Jinan 250100, China

b

School of Physics, Shandong University, Jinan 250100, China

*Corresponding authors e-mail address: [email protected] (P. Wang)

ABSTRACT: Developing efficient and earth-abundant bifunctional electrocatalyst for overall water splitting to produce hydrogen is very important. Ni3FeN is always used as an excellent electrocatalyst for the oxygen evolution reaction (OER), but its hydrogen evolution reaction (HER) performance is always negative and seldom concerned. Here, we improve the HER activity and maintain the excellent OER activity of Ni3FeN to realize the efficient overall water splitting by doping with molybdenum. The Mo-doped Ni3FeN shows a significantly improved HER activity with a low overpotential of 69 mV at 10 mA cm−2 as well as an excellent OER activity with a low overpotential of 250 mV at 20 mA cm−2. The improved HER activity can be ascribed to the favorable absorption/desorption process of H intermediate and improved conductivity, which was verified by the density functional theory calculation results of free energy of adsorbed H (∆GH*), the position of d-band center toward Fermi level and density of state near Fermi level. A two-electrode system that has Mo-doped Ni3FeN electrode as both cathode and anode requires only a low cell voltage of 1.554 V to reach the current density of 10 mA cm−2 and exhibits good stability for overall water splitting. The two-electrode system can also be powered by a commercial AA battery (~1.5 V), which is better than most of the bifunctional electrocatalysts.

1

Keywords： ：Mo-doped Ni3FeN; Overall water splitting; Electrocatalysis; Bifunctional electrocatalyst; DFT calculations

1. Introduction Electrochemical water splitting is a promising method to produce clean, high energy density and sustainable H2 fuel and has received extensive attention with increasing energy consumption and associated environmental pollution problems [1-3]. The process of water electrolysis involves cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER) [4-5]. Efficient electrocatalysts should accelerate both HER and OER and reduce the energy consumption for water splitting. To date, the state-of-art electrocatalyst is Pt for HER, and IrO2 or RuO2 for OER [6-7]. However, the high cost and scarcity of these materials seriously hinder the large-scale applications for water electrolysis. Over the past years, the low-cost and effective electrocatalysts have been explored for HER include transition metal sulfides, phosphides and carbides [8-13], those for OER include transition metal hydroxides, nitrides and oxides [14-19]. In general, HER is favorable in acidic solution, but OER is favorable in alkaline solution. These two tendencies are incompatible, which makes it difficult to realize the efficient overall water splitting in a same electrolyte [6]. Nevertheless, bifunctional electrocatalyst that enable both HER and OER in a same electrolyte would lower the cost for overall water splitting [20]. It is challenging to develop the bifunctional electrocatalyst with high-performance and excellent durability for overall water splitting. Transition-metal nitrides as a new class of electrocatalysts for water splitting have been extensively studied due to their high electrical conductivity, high chemical stability and superior water-splitting performance [21]. Previous studies showed that the Ni-Fe based nitrides are efficient OER catalysts in alkaline media. For examples, Jia et al. reported that Ni3FeN nanoparticles exhibited an extraordinarily high performance for OER with a low overpotential of 280 mV at 10 mA cm−2 and Tafel slope of 46 mV dec−1 [22], and Chen et al. reported that Ni3FeN nanoparticles showed 2

the outstanding electrocatalytic OER activity with a low overpotential of 241 mV at 10 mA cm-2 and Tafel slope of 59 mV dec-1 [23]. However, the HER performance of Ni3FeN nanoparticles is not good compared with its superior OER performance. We note that Ni3N, MoN and NiMoN are good HER catalysts in alkaline media, but their OER performances are not satisfactory [24-26]. Active HER and OER capabilities can be combined on a same electrocatalyst by doping. For example, CoP nanoarrays are a good OER catalyst. The CoP nanoarrays doped with Mo become a superior catalyst for both HER and OER, and hence for overall water splitting [27]. NiFe-LDH is a good OER catalyst which, when doped with Ru, becomes a superior catalyst for both HER and OER, and hence for overall water splitting [28]. The above observations suggest that, when doped with Mo, the HER activity of Ni3FeN can be improved, Mo-doped Ni3FeN might become an efficient HER and OER bifunctional electrocatalyst. Our work shows that the Mo-doped Ni3FeN, Ni3FeN:Mo (5%), shows a superior HER activity compared with the undoped Ni3FeN. In 1 M KOH, Ni3FeN:Mo (5%) exhibits a low overpotential of 69 mV at 10 mA cm−2 and Tafel slope of 69.41 mV dec-1, while Ni3FeN shows a high overpotential of 185 mV at 10 mA cm−2 and Tafel slope of 112.72 mV dec-1. The improved HER activity can be ascribed to the enhanced electrochemically active surface areas, the favorable hydrogen intermediate adsorption/desorption and the increased density of state near Fermi level which can improve the conductivity and accelerate charge transfer. In addition, Ni3FeN:Mo (5%) shows a slightly enhanced OER activity (low overpotential of 250 mV at 20 mA cm−2 and Tafel slope of 52.39 mV dec-1 in 1 M KOH) than that of the undoped Ni3FeN. More importantly, a two-electrode overall water splitting system using Ni3FeN:Mo (5%) for both cathode and anode requires a low cell voltage of 1.554 V to reach the current density of 10 mA cm−2, and is very stable (e.g., the water splitting under this current density can be maintained for two days with little change in applied voltage), showing that Ni3FeN:Mo (5%) is better than most bifunctional electrocatalysts. This doping strategy can be expanded to other analogous electrocatalytic systems to make the overall water splitting more efficient. 3

2. Experimental section 2.1. Materials Ni(NO3)2·6H2O (nickel nitrate hexahydrate), urea, Fe(NO3)3·9H2O (iron nitrate nonahydrate), (NH4)6Mo7O24·4H2O (ammonium molybdate tetrahydrate), and NH4F (ammonium fluoride) were purchased from Sinopharm Chemical Reagent Co. Ltd. Pieces of Ni foam (2 cm × 3.5 cm) were cleaned by consecutive sonication in acetone, ethanol, 3 M HCl and deionized water (10 min each). Deionized water was used in all the experiments. 2.2. Synthesis of Ni3N, Ni3FeN and Ni3FeN:Mo on nickel foam Ni3N, Ni3FeN and Ni3FeN:Mo were all prepared by a hydrothermal reaction followed by nitrogenation. To prepare nickel hydroxide, the precursor of Ni3N, 1.6 mmol Ni(NO3)2·6H2O, 8 mmol urea and 3.2 mmol NH4F were dissolved in 30 mL H2O and the solution was stirred for 30 min. A piece of Ni foam was immersed into the above solution and then transferred to a 40 mL Teflon-lined stainless steel autoclave and maintained at 120 °C for 6 h. To synthesize NiFe-based hydroxide, the precursor of Ni3FeN, 1.2 mmol Ni(NO3)2·6H2O, 0.4 mmol Fe(NO3)3·9H2O, 8 mmol urea and 3.2 mmol NH4F were dissolved in 30 mL H2O and the solution was stirred for 30 min. A piece of Ni foam was immersed into the above solution and then transferred to a 40 mL Teflon-lined stainless steel autoclave and maintained at 120 °C for 6 h. To prepare NiFeMo-based hydroxide, the precursor of Ni3FeN:Mo, 0.01143 mmol (NH4)6Mo7O24·4H2O, 1.14 mmol Ni(NO3)2·6H2O, 0.38 mmol Fe(NO3)3·9H2O, 8 mmol urea and 3.2 mmol NH4F were dissolved in 30 mL H2O and the solution was stirred for 30 min. A piece of Ni foam is immersed into the above solution and then transferred to a 40 mL Teflon-lined stainless steel autoclave and maintained at 120 °C for 6 h. With 1.6 mmol of all transition metal ions, the solution has the molar ratios of Ni:Fe = 3:1 and Mo:(Ni+Fe) = 0.05:0.95. After cooling to room temperature, the obtained products were washed with distilled water and dried at 70 °C. The precursor products were heated to 420 °C at 3.5 °C min-1 in a tube furnace under a constant flowing NH3 atmosphere (100 mL/min) and maintained for 120 min, and then cooled 4

down to room temperature. The obtained products are Ni3N, Ni3FeN and Mo-doped Ni3FeN. The above procedure leads to the Mo/(Ni+Fe) molar ratio of 0.0526 (namely, 5.26 % Mo-doping). By changing the the Mo/(Ni+Fe) molar ratio in the above preparation procedure, we obtained the Ni3FeN:Mo samples with 2.04 and 11.11 % Mo-doping (Mo:(Ni+Fe) = 0.02:0.98 and 0.1:0.9, respectively). For the convenience of our discussion in the following, the Ni3FeN:Mo samples with 2.04, 5.26 and 11.11 % Mo-doping

will

be

referred

as

Ni3FeN:Mo(2%),

Ni3FeN:Mo(5%),

and

Ni3FeN:Mo(11%), respectively. 2.3. Material characterization The morphologies and crystal structures of the products were examined by a Hitachi

S-4800

scanning

electron

microscopy

(SEM)

equipped

with

an

energy-dispersive spectrometer and transmission electron microscopy (TEM, JEOL JEM-2100F). The structures of the products were examined by X-ray diffraction (Bruker AXS D8 diffractometer equipped with Cu Kα radiation). The surface chemical compositions were analyzed by X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific Escalab 250 spectrometer). 2.4. Electrochemical measurements All electrochemical tests were performed on a CHI660E electrochemical workstation at room temperature. The HER and OER catalytic activity measurements were carried out in a standard three-electrode system in 1 M KOH solution (pH = 13.8), where the as-prepared catalysts on nickel foam were used as the working electrodes with area of 1 cm × 1 cm, a graphite rod as the counter electrode, and an Ag/AgCl (saturated KCl) electrode as the reference electrode. All potential values were measured with respect to the reversible hydrogen electrode (RHE) according to the equation: ERHE = EAg/AgCl + 0.059× pH + 0.1976. The working electrode needed pre-activation by 50 cyclic voltammetric sweeps from 0 to -0.4 V for HER, and from 1.1 to 1.65 V for OER, at a scan rate of 100 mV s-1. Electrolyte solution was saturated by argon for HER or oxygen for OER before the experiments. The polarization curves of both HER and OER were recorded by linear sweep voltammetry (LSV) with a scan 5

rate of 2 mV s−1 and iR corrected by the electrochemical workstation. Tafel curves were obtained from the corresponding LSV data. Electrochemical impedance spectroscopy (EIS) was obtained at a specific overpotential with frequency ranging from 100 KHz to 0.1 Hz with an AC voltage of 10 mV. The electrochemical double layer capacitances (Cdl) were obtained by the cyclic voltammetry measurement in nonfaradaic region from 0.25 to 0.35 V vs. RHE with different scan rates from 10 to 50 mV s-1 to study the effective electrochemical active area. The stability tests for HER and OER were carried out by long-term cyclic voltammetry circulation measurement and I-T measurement at constant voltage (without iR correction). The overall water splitting measurement was carried out in a two-electrode system by using the same catalyst as both anode and cathode and the polarization curves were recorded without iR-compensation at a scan rate of 2 mV s−1. 2.5. Computational Methods To gain deeper insight into the reason for the enhanced HER activity of Ni3FeN after Mo doping, DFT calculations were carried out on the key reaction step (adsorption/desorption of H intermediate) for the alkaline HER. All first-principles spin-polarized calculations were performed by using the Vienna ab initio simulation package (VASP) [29,30]. The ion-electron interactions were described by the projector augmented wave method [31]. The generalized gradient approximation in the Perdew-Burke-Ernzerhof form and a cutoff energy of 450 eV were adopted [32]. The convergence criterion for the residual force and energy was set to 0.02 eV/Å and 10–5 eV, respectively. The Brillouin zones were sampled by a 5×5×1 Monkhorst-Pack k-point mesh. The Ni3FeN (111) surface was modeled by a 2×2 periodic slab and a vacuum slab of 15 Å was employed to avoid the interaction between periodic units. The hydrogen adsorption free energy, ∆GH*, was calculated using the equation: ∆GH* = E(surf+H) − E(surf) − 1/2Ε(Η2) + ∆ZPE − T∆S, where E(surf+H) and E(surf) represent the total energies of the surface with and without H adsorption. ∆ZPE and ∆S are the difference in the zeropoint energy and entropy between the adsorbed H atom and the gaseous phase H2, respectively. As the entropy 6

of hydrogen in absorbed state is negligible, ∆S can be calculated as −1/2 S0 (S0 is the entropy of H2 in the gas phase at standard conditions). At T = 300 K, ∆GH* can be calculated by ∆GH* = ∆EH* + 0.24 eV, ∆EH* = E(surf+H) − E(surf) − 1/2Ε(Η2) [33].

3. Results and discussion The morphologies of the as-prepared products were examined by the scanning electron microscopy (SEM). The SEM images of Fig. S1 show that the surface of the original nickel foam is very smooth. Fig. 1a and 1d show the SEM images of NiFe and Mo-doped NiFe hydroxide precursor, respectively. The nanosheet arrays cover the entire surface of Ni substrate tightly. The nanosheets of the Mo-doped NiFe hydroxide precursor are slightly smaller in size than those of the NiFe hydroxide precursor. Fig. 1b and 1c show the SEM images of Ni3FeN, and Fig. 1e and 1f show the SEM images of Ni3FeN:Mo(5%). Ni3FeN and Ni3FeN:Mo(5%) retain the sheet-like structure of their precursors after nitriding treatment, but their surfaces are rougher than those of their precursors. Fig. 1g-i show the energy dispersive x-ray spectroscopy (EDS) elemental mapping images of Ni3FeN:Mo(5%), which indicates a uniform distribution of Ni, Fe, N and Mo elements in the catalyst. The EDS result of the powder sample of Ni3FeN:Mo(5%) scratched off from the Ni foam is given in Fig. S3, the result shows the Mo/Fe molar ratio of ∼0.22 is close to the feed radio, and the Ni/Fe molar ratio of ∼3.3 is greater than the ideal ration of 3. The cause for the extra amount of Ni is due most likely to the scraping process from the Ni foam.

7

Fig. 1. SEM images of (a) NiFe hydroxide precursor, (b, c) Ni3FeN, (d) Mo-doped NiFe hydroxide precursor, and (e, f) Ni3FeN:Mo(5%). (g) SEM image of an area selected for the EDS element mapping for Ni3FeN:Mo(5%). (h) EDS combined element mapping for Ni, Fe, N and Mo. (i) EDS individual elemental mapping for Ni, Fe, N and Mo. The XRD patterns of the as-prepared samples of Ni3N, Ni3FeN and Ni3FeN:Mo(5%) are compared in Fig. 2a with the standard XRD patterns of Ni3N and Ni3FeN. The XRD pattern of as-prepared Ni3FeN shows three typical diffraction peaks at 41.5°, 48.3° and 70.8° indexed to the (111), (200) and (220) planes of the face-centered cubic Ni3FeN (JCPDS No. 50-1434) [23]. All the diffraction peaks of as-prepared Ni3FeN:Mo(5%) are the same as those of the un-doped Ni3FeN. Since Ni3FeN:Mo(5%) shows no other impurity peaks, it is clear that the Mo doping does not strongly influence the crystal structure. Transmission electron microscopy (TEM) was used to further characterize the morphology and structure of Ni3FeN:Mo(5%). The TEM images of Ni3FeN:Mo(5%) (Fig. 2b and 2c) show the nanosheet-like structure, which is in agreement with the SEM result. The lattice fringes with distance 8

of 0.217 nm shown in the high-resolution transmission electron microscope (HRTEM) image (Fig. 2d) correspond well with the d-spacing of the (111) plane of Ni3FeN.

Fig. 2. (a) XRD patterns of the as-prepared Ni3N, Ni3FeN and Ni3FeN:Mo(5%). (b, c) TEM images of Ni3FeN:Mo(5%). (d) HRTEM image of Ni3FeN:Mo(5%). X-ray photoelectron spectroscopy (XPS) measurement was carried out to characterize the surface chemical compositions and the element chemical valence states of the Ni3FeN and Ni3FeN:Mo(5%). The high resolution Ni 2p XPS spectra of Ni3FeN and Ni3FeN:Mo(5%) (Fig. 3a) are similar in peaks and peak positions; two Ni 2p peaks at 855.88 (2p3/2) and 873.88 eV (2p1/2) are assigned to Ni2+, and those at 852.98 (2p3/2) and 870.33 eV (2p1/2) to Ni0 [23,33]. In the Fe 2p XPS spectra of Ni3FeN and Ni3FeN:Mo(5%) (Fig. 3b), the two Fe 2p peaks at 711.78 (2p3/2) and 724.73 eV (2p1/2) are assigned to Fe3+, and the other two at 707.53 (2p3/2) and 720.23 eV (2p1/2) to Fe0 [34,35]. The N 1s peak occurs at 397.23 eV in the XPS spectra of Ni3FeN and Ni3FeN:Mo(5%) (Fig. 3c). The Mo 3d XPS spectrum of Ni3FeN:Mo(5%) 9

(Fig. 3d) shows the presence of Mo3+ (d3), Mo4+ (d2) and Mo6+ (d0) ions. The two peaks located at 229.03 eV and 232.13 eV can be ascribed to Mo3+ in the metal nitride and the peaks at 229.73, 232.98 and 235.33 eV can be attributed to Mo4+ and Mo6+, respectively. The occurrence of high valence of Mo is due most likely to the surface oxidation [26,36,37].

Fig. 3. Comparisons of the high-resolution XPS spectra of (a) Ni 2p, (b) Fe 2p and (c) N 1s in Ni3FeN and Ni3FeN:Mo(5%). (d) High-resolution XPS spectra of Mo 3d in Ni3FeN:Mo(5%). Two split peaks describe the 3d XPS spectrum of Mo3+ (d3) and Mo4+ (d2), and one peak the 3d XPS spectrum of Mo6+ (d0). The electrocatalytic HER performances of Ni foam, Ni3N, Ni3FeN and Mo-doped Ni3FeN were evaluated in 1.0 M KOH solution using a standard three-electrode system. Their iR-corrected linear sweep voltammogram (LSV) curves are presented in Fig. 4a. The HER activities of Ni3N and Ni3FeN are considerably higher than that of Ni foam. The HER activity of Ni3FeN is slightly lower than that of Ni3N, but the Mo-doping strongly enhances the activity so that all three doped 10

samples Ni3FeN:Mo(2%), Ni3FeN:Mo(5%), Ni3FeN:Mo(11%) have a significantly higher HER activity than those of Ni3N and Ni3FeN. Among the three Mo-doped samples, Ni3FeN:Mo(5%) shows the optimal HER with the lowest overpotentials of 69 mV and 195 mV at 10 and 100 mA cm−2, respectively, which are lower than those of Ni3FeN (184 mV and 294 mV at 10 and 100 mA cm−2, respectively) or Ni3N (145 mV and 264 mV at 10 and 100 mA cm−2, respectively). To gain insight into the catalytic kinetics of the HER, the Tafel plots were obtained from the iR-corrected LSV curves, as shown in Fig. 4b. Since the Tafel slope for Ni3FeN:Mo(5%), 69.41 mV dec-1, is the smallest compared with those of Ni3FeN, Ni3N and Ni foam (112.72, 118.75 and 159.82 mV dec-1, respectively), indicating that Ni3FeN:Mo(5%) is more favorable for the HER kinetics [37,38]. Ni3FeN:Mo(5%) has a much better HER activity than other reported metal nitrides (Table S1). Electrochemical impedance spectroscopy (EIS) was carried out to study the electron-transfer kinetics at the electrode/electrolyte interface at the bias voltage of -140 mV. The Nyquist plots of Ni3FeN and Ni3FeN:Mo(5%) in Fig. 4c show that the charge transfer resistance of Ni3FeN:Mo(5%) is much smaller than that of Ni3FeN, indicating a much faster electron transfer process [39]. To test the durability of Ni3FeN:Mo(5%), the LSV measurement was repeated 5000 times scanning between 0 to -0.35 V. Fig. 4d shows that almost no change in the polarization curve relative to the initial curve. The long-term HER stability of Ni3FeN:Mo(5%) was also tested at a fixed bias of -80 mV vs. RHE (without iR-correction), which shows the current density remains unchanged. The results both indicate that Ni3FeN:Mo(5%) electrode has a good HER stability. To help understand the improved HER performance of Mo-doped Ni3FeN, we measured the electrochemical double-layer capacitances (Cdl) to evaluate the electrochemically active surface areas (ECSA), which are linearly proportional to the number of reaction active sites. As shown in Fig. S4 and 5, the calculated Cdl values of Ni3FeN and Ni3FeN:Mo(5%) are 25.84 and 38.06 mF cm-2, respectively. The enhanced Cdl of Ni3FeN:Mo(5%) suggests it has more effective active sites for the HER. Our DFT calculations were carried out to gain insight into how Mo dopant affects the HER activity of Ni3FeN, the result shows that it is energetically favorable for the doped Mo 11

atom to replace the Fe atom in Ni3FeN (Fig. S6). A key descriptor for the alkaline hydrogen evolution is the adsorption free energy of H intermediate [39,40]. As summarized in Fig. 4e, the free energy (∆GH*) for the adsorbed H on the surface of Ni3FeN is 0.13 eV, indicating that it is unfavorable for the adsorption of H. On Ni3FeN:Mo, however, ∆GH* = −0.03 eV, which is near to ideal 0 eV suggesting that the Mo doping makes the adsorption and desorption of H intermediate more favorable. In addition, the electronic interaction can be deemed as the coupling between the adsorbate valence states and the transition-metal d states, leading to the formation of fully filled bonding and partially filled antibonding states. In this respect, the occupancy of antibonding states will determine the bonding strength between the adsorbate and the catalysts. Since the antibonding states are always above the d states, the d band center model can therefore act as an efficient descriptor of the adsorbate-metal interaction [39]. As shown in Fig. 4f, the d-band center of Ni3FeN and Ni3FeN:Mo relative to the Fermi level are examined to be -2.43 and -2.39 eV, respectively, demonstrating that the d-band center is closer to the Fermi level after Mo doping, therefore decreasing the occupancy of antibonding states. As a result, the decreased occupancy of antibonding states will enhance the interaction between H intermediate and the catalyst, which explains the negative ∆GH of Ni3FeN:Mo than un-doped Ni3FeN. Also, the increased density of states of Mo-doped Ni3FeN near Fermi level can accelerate the charge transfer and increase the conductivity, which is benefit for HER [41].

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Fig. 4. Polarization curves of Ni foam, Ni3N, Ni3FeN, Ni3FeN:Mo(2%), Ni3FeN:Mo(5%), Ni3FeN:Mo(11%) for HER (a), Tafel slopes of Ni foam, Ni3N, Ni3FeN and Ni3FeN:Mo(5%) (b), Nyquist plots of Ni3FeN and Ni3FeN:Mo(5%) at the bias voltage of -140 mV (c), LSV curves of Ni3FeN:Mo(5%) before and after 5000 CV cycles from 0 to -0.35 V vs. RHE and the inserted time-dependent current density curve of Ni3FeN:Mo(5%) at the overpotential of 80 mV (d), free energy diagram for the HER on Ni3FeN and Ni3FeN:Mo (e) and The density of states (DOS) of Ni3FeN and Ni3FeN:Mo, the d-band centers are highlighted in the DOS curves (the Fermi level is set to zero). 13

The electrocatalytic OER activities of Ni foam, Ni3N, Ni3FeN and Ni3FeN:Mo(5%) were evaluated in 1.0 M KOH solution using a standard three-electrode system. Fig. 5a shows the corresponding iR-corrected LSV curves. Both Ni foam (346 and 441 mV at 20 and 100 mA cm−2, respectively) and Ni3N (358 and 431 mV at 20 and 100 mA cm−2, respectively) have a negative OER activity. Ni3FeN shows a good OER activity with overpotentials of 255 mV and 293 mV at 20 and 100 mA cm−2 and Ni3FeN:Mo(5%) shows a little better OER activity than Ni3FeN with the overpotentials of 250 mV and 288 mV at 20 and 100 mA cm−2, which means that Mo dopant has little influence on the OER activity. The OER kinetics is evaluated by the Tafel plots obtained from the corresponding polarization curves. As shown in Fig. 5b, Ni3FeN:Mo(5%) shows the smallest Tafel slope of 52.39 mV dec-1 than those of Ni foam (128.75 mV dec-1), Ni3N (93.66 mV dec-1) and Ni3FeN (53.49 mV dec-1). A lower Tafel slope of Ni3FeN:Mo(5%) means a more favorable OER kinetics [33], and a comparison of Ni3FeN:Mo(5%) with other reported electrocatalysts (Table S2) indicates that its OER activity is superior to most of them. Electrochemical impedance spectroscopy (EIS) was carried out to study the electron-transfer kinetics at the electrode/electrolyte interface during the OER at the overpotential of 250 mV. The Nyquist plots of Ni3N, Ni3FeN and Ni3FeN:Mo(5%) in Fig. 5c show a lower charge transfer resistance for Ni3FeN:Mo(5%) than for those of Ni3N and Ni3FeN, indicating a much faster electron transfer process due to the Mo doping. Furthermore, the good OER stability is important for the practical application. So the durability test of Ni3FeN:Mo(5%) was carried out, as shown in Fig. 5d. The polarization curve after continuous 3000 CV cycles scanning from 1 to 1.65 V shows almost no obvious change compared with the initial curve. The time-dependent current density profile of the Ni3FeN:Mo(5%) measured at the overpotential of 270 mV for 12 h (inset of Fig. 5d) shows only a slight attenuation in the current density. Both indicate that the Ni3FeN:Mo(5%) electrode exhibits a good OER stability. The SEM images after the HER and OER stability test (Fig. S7 and S8) show that the sheet-like morphologies are well preserved, exhibiting a good stability of the structure. 14

Fig. 5. Polarization curves of Ni foam, Ni3N, Ni3FeN and Ni3FeN:Mo(5%) for OER (a), Tafel slopes of Ni foam, Ni3N, Ni3FeN and Ni3FeN:Mo(5%) (b), Nyquist plots of Ni3N, Ni3FeN and Ni3FeN:Mo(5%) at the overpotential of 250 mV (c) and LSV curves of Ni3FeN:Mo(5%) before and after 3000 CV cycles from 1 to 1.65 V vs. RHE and the inserted time-dependent current density curve of Ni3FeN:Mo(5%) at the overpotential of 270 mV (d). In view of the excellent HER and OER activities and durability of Ni3FeN:Mo(5%), we use this electrode as a bifunctional catalyst to further evaluate its activity of the overall water splitting in a two-electrode system at ambient environment. As shown in Fig. 6a (inset), the produced hydrogen and oxygen can be easily observed from the anode and cathode, respectively. Fig. 6a also shows the polarization curves of the Ni3N//Ni3N, Ni3FeN//Ni3FeN and Ni3FeN:Mo(5%)// Ni3FeN:Mo(5%) electrodes in 1.0 M KOH. The Ni3FeN:Mo(5%)//Ni3FeN:Mo(5%) electrolyzer requires only 1.554 V and 1.644 V cell voltage to reach 10 mA cm−2 and 20 mA cm−2 without iR-compensation, respectively, which are much lower than those of Ni3FeN//Ni3FeN electrolyzer (1.63 V and 1.771 V, respectively) or Ni3N//Ni3N 15

electrolyzer (1.692 V and 1.813 V, respectively). It shows that Mo-doped Ni3FeN as a bifunctional catalyst has a superior overall water splitting performance; a comparison of Ni3FeN:Mo(5%) with other reported bifunctional electrocatalysts (Table S3) indicates that its overall water splitting performance is superior to most of them. In addition, the long-term stability test is further carried out at a constant current density of 10 mA cm−2 for over water splitting in a two-electrode configuration. As shown in Fig. 6b, the overpotential of Ni3FeN:Mo(5%)//Ni3FeN:Mo(5%) electrolyzer shows only a slight increase after the 48 h continuous testing. These results indicate that Mo-doped Ni3FeN can be a promising candidate to realize the efficient alkaline water splitting in practical application. Even a commercial AA battery (with a voltage of ~1.5 V) can be used to power the Ni3FeN:Mo(5%) based electrolyzer (Fig. S9 and Supporting Video). The obvious gas bubbles can be observed from the cathode and anode, indicating the Mo-doped Ni3FeN catalyst has the good prospect for practical application.

Fig. 6. (a) Polarization curves of Ni3N//Ni3N, Ni3FeN//Ni3FeN and Ni3FeN:Mo(5%)// Ni3FeN:Mo(5%) electrodes for overall water splitting in a two-electrode system. (b) Chronopotentiometric curve of Ni3FeN:Mo(5%)//Ni3FeN:Mo(5%) electrolyzer with a constant current density of 10 mA cm−2 for 48 h in a two-electrode system. 4. Conclusion In summary, our work demonstrated that Mo-doped Ni3FeN is a bifunctional electrocatalyst for efficient overall water splitting. Through Mo doping, the HER activity of Ni3FeN can be improved with a low overpotential of 69 mV at 10 mA cm−2 16

and Tafel slope of 69.41 mV dec-1, the superior OER activity of Ni3FeN can be maintained and even get a little improvement with a low overpotential of 250 mV at 20 mA cm−2 and Tafel slope of 52.39 mV dec-1. The improved HER activity can be ascribed to the enhanced electrochemically active surface area, the favorable hydrogen intermediate adsorption/desorption and the increased density of states near Fermi level which can accelerate charge transfer and improve the conductivity. A two-electrode system, consisting of Ni3FeN:Mo(5%) electrodes as both cathode and anode, needs only a low cell voltage of 1.554 V to reach the current density of 10 mA cm−2 and exhibits good durability for overall water splitting. The two-electrode system can also be powered by a commercial AA battery (~1.5 V), which is better than most of bifunctional catalysts indicating the Mo-Ni3FeN catalyst has the good prospect for practical application.

Conflict of interest There are no conflicts to declare.

Acknowledgments This work was financially supported by the research grants from the National Natural Science Foundation of China (No. 21832005, 51602179, 51972195, 21972078 and U1832145), Recruitment Program for Young Professionals, China and Taishan Scholar Foundation of Shandong Province, China.

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The HER activity of Ni3FeN can be greatly improved by doping with Mo.




DFT calculation results demonstrate that the favorable ∆GH* and improved conductivity can be obtained for Mo-doped Ni3FeN.




Mo-doped Ni3FeN retains the good OER activity of Ni3FeN.




The bifunctional Mo-doped Ni3FeN electrocatalyst can realize the efficient overall water splitting.

Author Contributions Section Xiaolei Liu and Prof. Peng Wang took part in the main experimental design and operation; Xingshuai Lv, Qianqian Zhang and Prof. Ying Dai participated in the density functional theory calculation and analysis for the experiment; Prof. Zeyan Wang and Yuanyuan Liu gave some valuable suggestions for the mechanism analysis; Prof. Baibiao Huang and Zhaoke Zheng helped to revise the manuscript.

Declaration of Interest Statement 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.