Fe-doped NiCoP porous nanosheet arrays as a highly efficient

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Fe-doped NiCoP porous nanosheet arrays as a highly efficient electrocatalyst for oxygen evolution reaction Qiong Zhang, Dafeng Yan, Zhenzhen Nie, Xiaobin Qiu, Shuangyin Wang, Jianmin Yuan, Dawei Su, Guoxiu Wang, and Zhenjun Wu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00143 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 25, 2018

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ACS Applied Energy Materials

Fe-doped NiCoP Porous Nanosheet Arrays as a Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

Qiong Zhang†, #, Dafeng Yan‡, #, Zhenzhen Nie†, Xiaobin Qiu†, Shuangyin Wang‡, Jianmin Yuan§, Dawei Su∥*, Guoxiu Wang∥* and Zhenjun Wu†*

†

College of Chemistry and Chemical Engineering, Hunan University, Changsha

410082, P. R. China. ‡

State Key Laboratory of Chem/Bio-Sensing and Chemometrics, College of

Chemistry and Chemical Engineering, Hunan University, Changsha 410082, P. R. China. §

College of Materials Science and Engineering, Hunan University, Changsha 410082,

P. R. China ∥

Centre for Clean Energy Technology, School of Mathematical and Physical

Sciences, Faculty of Science, University of Technology Sydney, NSW 2007, Australia.

ABSTRACT: There is a great challenge to employ an electrocatalyst with high-efficiency, earth-abundant and non-noble metals for oxygen evolution reaction (OER). Herein, we reported a low-cost and highly efficient OER catalyst, Fe-doped NiCoP nanosheet arrays in situ grown on nickel foam (NiCoFeP/NF), which was synthesized via a simple and mild hydrothermal and phosphorization method. In 1 M KOH solution, the as-prepared NiCoFeP/NF produces a larger current density of 200

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mA·cm-2 at a low overpotential of 271 mV and exhibits a low Tafel slope of 45 mV·dec-1, which is superior to commercial RuO2. The outstanding OER performance of the as-prepared NiCoFeP/NF can be attributed to the synergetic effects among Fe, Ni and Co elements, unique nanosheet arrays structure as well as the great intrinsic electrocatalytic activity. On the basis of above factors, the as-prepared NiCoFeP/NF may work as a promising OER electrocatalyst. KEYWORDS:

Fe-doped

NiCoP,

porous

nanosheet

arrays,

hydrothermal,

phosphorization, electrocatalyst, oxygen evolution reaction

INTRODUCTION Water splitting to generate H2 and O2 is one of the most promising approaches to get the environmental friendly and sustainable renewable energy. It is well known that the oxygen evolution reaction (H2O → O2) plays an important role on water splitting.1 However, this reaction demonstrates very sluggish kinetics because of its multistep proton-coupled electron transfer process. Therefore, the exploitation of the robust electrocatalyst with low overpotential, and high efficiency is still a big issue to be solved.2 Noble metal-based catalysts have been demonstrated as the most efficient catalysts for OER, such as IrO2 and RuO2.3 However, poor stability, high cost and low earth abundance etc. are the concerns for the wide use of noble metal-based OER catalysts. Therefore, it is desirable to develop non-noble metal-based OER electrocatalysts. Nowadays, transition metals (Fe, Co and Ni, etc.) and their derivatives, have received considerable interests as highly efficient, great stable, as well as low cost electrocatalysts for OER.4-15 Some researchers have reported that the OER activity of heterogeneous transition metals can be dramatically improved when another proper


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metal doping into.16-19 For example, Zhao and co-workers16 have observed that iron-cobalt-nickel alloy shows great electrocatalytic activity toward OER with low overpotential of 170 mV at 10 mA·cm−2 and a smaller Tafel slope of 37 mV·dec−1. Yang’s group has proven the better OER activity of FeNiCo LDH than FeNi LDH.18 On the other hand, transition metal phosphides have been demonstrated to have good efficiency and corrosion stability for the OER in alkaline electrolytes, which is related to a heterostructure where metal oxo/hydroxo species have formed on the surface of catalyst due to the surface oxidation of metal phosphides.1,20-23 Some related studies have been reported. Hu and co-workers have successfully proven the better OER activity of Ni2P than NiOx1. It was also reported that Fe-Ni phosphide exhibits outstanding OER activities than Fe-Ni hydroxide.24 Husam N. Alshareef et al. have shown that phosphorization is an important reason for NiCoP to obtain excellent electrocatalytic activity.25 In addition, not only the inherent nature of the catalyst, but also the catalytic structure plays an important role in improving the catalytic performance. For example, NiCo2S4 nanowires arrays can reach 100 mA·cm-2 at the overpotential of 340 mV in 1 M KOH solution.26 The Fe-doped NiSe nanoflake arrays on FeNi foam required the overpotential of 264 mV to achieve 100 mA·cm-2 in 1 M KOH solution.27 It is worth notice that most array structures are formed on the substrate, such as nickel foam (NF), carbon cloth, Ti mesh etc. Herein, NF obtains a lot of advantages, which not only has a three-dimensional porous structure with strong conductivity but also can provide a large space for the growth of catalyst and the increase of surface area. Furthermore, it is beneficial for the connection between NF and catalyst. It can thus take full advantage of the active sites and lifetime of catalyst.28



Based on the above factors, Ni-Co-Fe phosphides may hold prospect for OER activity compared with Ni-Co phosphides. Herein, we prepared Fe-doped NiCoP nanosheet arrays (NiCoFeP/NF) in situ grown on the nickel foam via a simple and mild method combined hydrothermal treatment with phosphorization. Due to the unique nanosheet arrays structure, fast electron transport as well as the synergetic effects among Fe, Ni and Co elements after doping with Fe, the as-prepared NiCoFeP/NF with 7% Fe shows excellent OER performance in alkaline electrolyte. With a low overpotential of 271 mV, it achieves a current density of 200 mA·cm-2 and shows a small Tafel slope of 45 mV·dec-1 in 1 M KOH solution. Therefore, the as-prepared NiCoFeP/NF is a low-cost, high efficient, robust stable competent catalyst for the oxygen evolution.

EXPERIMENT SECTION Chemicals. Co(NO3)2·6H2O, Fe(NO3)3·9H2O, NaH2PO2, Ni(NO3)2·6H2O, NH4F, HCl, KOH and CO(NH2)2 were purchased from Sinopharm Chemical Corp. RuO2 was purchased from Adamas Reagent. The source of P is NaH2PO2. All chemicals were used as received without further purification. Synthesis of NiCoFeP nanosheet arrays. NiCoFeP nanosheet arrays were synthesized by a hydrothermal phosphorization reaction. The synthesis process is as follows:22,29 nickel foam (NF, 1 cm×5 cm) was sonicated in acetone for 10 min, and immersed into 3 M HCl for 20 min to dissolve the inert NiO layer on the surface of NF, then washed for several times by deionized water and ethanol. 4 mmol of total metal precursors (including Ni, Co and Fe) with different molar ratios, 8 mmol NH4F and 10 mmol of urea were dissolved into a mixture of 70 mL DI water with magnetic stirring to form a homogeneous solution. The NF and obtained solution were transferred into a 100mL stainless-steel Teflon-lined autoclave and then kept at 100 ℃ for 8 h. After cooling


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down to room temperature, the precursor was washed for several times by deionized water and dried at 60 ℃ for 10 h in vacuum oven. The dried NiCoFe precursor and 100 mg NaH2PO2 were placed at the downstream and the upstream side of the tube furnace and heated at 300 ℃ for 2 h in the Ar atmosphere to achieve a massloading of 3 mg/cm2. In particular, nFe + nNi + nCo=4 mmol (nNi = nCo), n was the number of moles of the respective metal precursors, the content of Fe (CFe) was calculated by CFe = nFe/4. The different content of Fe-doping (3%, 5%, 7%, 10% and 13%) was denoted as NiCoFe3P/NF, NiCoFe5P/NF, NiCoFeP/NF, NiCoFe10P/NF, NiCoFe13P/NF (the percentage composition of Ni:Co:Fe: NiCoFe3P/NF: 1.94: 1.94: 0.12 mmol; NiCoFe5P/NF: 1.9: 1.9: 0.2 mmol; NiCoFe7P/NF: 1.86: 1.86: 0.28 mmol; NiCoFe10P/NF: 1.8: 1.8: 0.4 mmol; NiCoFe13P/NF: 1.74: 1.74: 0.52 mmol.). The NiCoP were synthesized as the reference materials, following the same procedure. Materials characterizations. X-ray diffraction patterns (XRD) were recorded on an X-ray diffraction (Bruker D8 Advance diffractometer, Cu Kα1). The morphologies of synthesized materials were observed using Scanning electron microscopy (SEM) system (Hitachi, S-4800). Samples grown on NF were directly used for SEM observation. The Transmission electron microscopy (TEM) system (FEI Tecnai G20), high-resolution transmission electron microscopy (HRTEM) system and selected-area electron diffraction (SAED) were used to characterize the microstructures of the samples in order to confirm the size and lattice parameters of the nanoparticles. For TEM, HRTEM, SAED observation, the samples grown on NF were immersed into ethanol and sonicated for 40 min. The X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, K-Alpha 1063) was conducted to characterize the element states of the samples. Binding energies of all XPS spectra were calibrated by the C 1s binding energy of adventitious carbon contamination which was taken to be 284.8 eV.



Electrochemical measurements. Electrochemical workstation (CHI660e, Shanghai) was used to carry out electrochemical measurements in 1.0 M KOH solution. All electrochemical tests were conducted on a three-electrode system, the saturated calomel electrode (SCE) as the reference electrode, the graphite rod as the counter electrode and the as-prepared catalysts as the working electrode (1 cm×1 cm). The preparation of working electrode with commercial catalyst is as follows: 5 mg of RuO2 was dispersed into 1 mL ethanol with the ultrasonication for 30 min, and then 50 µL Nafion solution was added into the above solution with the assistance of ultrasonication for 30 min to form a homogeneous catalyst ink. The catalyst electrodes were obtained by doping approximately 3 mg commercial catalyst on 1 cm ×1 cm NF. Linear sweep voltammetry (LSV) was carried on 0 to 0.6 V vs. SCE which kept the scan rate at 5 mV s-1. Cyclic voltammogram (CV) was conducted from 0.81 to 0.886 V vs. RHE by changing scan rates (10, 20, 30, 40 and 50 mV s-1), which can estimate the double-layer capacitances (Cdl) of all catalyst. The electrochemical impedance spectroscopy (EIS) test was executed at 1.5 V vs. RHE over the frequency range from 100 KHz to 10-1 Hz in 1 M KOH.

RESULTS AND DISCUSSION A typical synthetic approach to a series of NiCoFeP/NF is schematically shown in Scheme 1: we prepared Fe-doped NiCoP nanosheet arrays (NiCoFeP/NF) in situ grown on the nickel foam via a simple and mild method of hydrothermal and phosphorization. The NiCoP served as the reference materials, following the same synthesis procedure. After hydrothermal reaction, the color of the Ni-Co precursor covered on NF is pink, and the color turned to yellow after doped with Fe (Ni-Co-Fe precursor) and after phosphorization to form black NiCoP/NF and NiCoFeP/NF (Figure. S1).


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Scheme 1. Schematic illustration of the NiCoFeP synthetic process.

The crystal structures of catalysts were characterized by XRD, as shown in Figure. 1a. The characteristic peaks of NiCoP are observed at 41.0°, 47.6°, and 54.5°, which are indexed to the (111), (210), and (300) crystal planes, respectively. It is consistent with the standard pattern of the NiCoP (JCPDS No. 71-2336). It should be noted that two peaks at 44.5° and 51.8° are assigned to the NF substrate (JCPDS No. 65-2865).30 Meanwhile, it is interesting that the characteristic diffraction peaks of NiCoFeP with different Fe concentration (3%, 5%, 7%, 10% and 13%) are completely coincident with NiCoP, and there are no other impurity peaks appearing (Figure. S2). The HRTEM image of NiCoFeP (Figure. 2b) shows that the lattice spacing of 0.22 nm which can be assigned to (111) interplanar spacing of NiCoP. That is because the element radius of Ni, Co and Fe is similar and Fe is embedded into the lattice of NiCoP,17,31,32 replacing nickel and cobalt to produce lattice defects. To a certain extent,



those lattice defects can make more active sites exposed, which facilitate the progress of OER.33

Figure. 1 (a) XRD patterns of the NF, NiCoP/NF, NiCoFeP/NF, SEM images of (b), (c) the NiCoFeP/NF and (d) the NiCoP/NF.

As shown in SEM images (Figure. S3a and b), the NF has a porous architecture and smooth surface. The porous structure not only plays an important role in providing a large space for the growth of the catalyst, but also can be used as a good channel for bubble release to prevent decline of catalytic performance due to the accumulation of bubbles.22 After hydrothermal reaction and phosphorization, a series of catalysts are uniformly coating on the skeleton of NF (Figure. S3c and d). As shown in Figure. 1b, c and d, nanosheets were uniformly grown on the NF to form the nanosheet arrays. It is interesting that nanoporous layer has been formed, because these nanosheets are mutually interconnected and this structure ensures a strong


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interaction among nanosheets, indicating good electrical conductivity and mechanical robustness. Furthermore, these nanosheets are vertical grown on the NF rather than piling up with each other, which provide sufficient effective and stable channels for mass transfer and decrease electrical contact resistance. Meanwhile, the nanosheet of NiCoFeP/NF is thinner and more crowded than that of NiCoP/NF (Figure. 1c and d). In addition, the trace iron element is successfully doped into the catalyst and the elements (Ni, Co, Fe and P) are evenly distributed in the catalyst, as shown by the energy dispersive X-ray spectroscopy (EDX) elemental mapping images (Figure. S4a). These elements are homogeneous distribution although the content of iron is very small, which is highly accordant with the corresponding EDX spectrum (Figure. S4b). These phenomena show that iron doping can effectively change the morphology,24,34 which are beneficial for forming more nanopores, obtaining larger surface area, and presenting more active sites. Moreover, the nanoporous structure can provide channels for charge transfer and more efficient contact between the catalyst and the electrolyte.

Figure. 2

(a) TEM images of the NiCoFeP/NF, (b) HRTEM and SAED images of the

NiCoFeP/NF.

TEM and HRTEM images were performed to further insight into the morphologies and sizes of the as-prepared materials. From the TEM (Figure. 2a), it



can be seen that many holes with different size disperse in the nanosheet. The formation of holes can be ascribed to the transforming of metal carbonate and the CO3- into CO2 during the phosphorization process.35 The macroporous structure of the NF combines with the nanoporous layer formed by nanosheets. The SAED patterns (Figure. 2b) exhibit several bright rings which can be indexed to the (111), (210) and (211) planes of NiCoP. XPS was utilized to investigate the surface composition of the NiCoFeP/NF and NiCoP/NF. As shown in Figure. S5, it presents the elements of Ni, Co, Fe and P in the NiCoFeP, and Ni, Co and P element in NiCoP/NF, respectively, indicating that Fe is successfully incorporated into catalyst to form NiCoFeP/NF. The elements of C and O may come from ambient environment and raw materials.24 For the Ni 2p region (Figure. 3a), five peaks are observed in the NiCoFeP/NF. The binding energies of 856.5 (Ni 2p3/2) and 874.5 eV (Ni 2p1/2) are related to the Ni-PO, while the binding energies of 852.8 eV is attributed to the Ni-P. The peaks at 861.3 and 879.7 eV can be assigned to satellite peaks. The binding energy of 852.8 and 856.5 eV are assigned to the Ni 2p3/2 and the 874.5 eV belongs to Ni 2p3/2 energy levels.36,37 It's interesting that the peak intensity of Ni-P (852.8eV) in the XPS spectrum of NiCoP/NF is weaker than that of NiCoFeP/NF. This feature proves that the content of metal phosphide (M-P) on the surface of NiCoP/NF is lower than that of NiCoFeP/NF. In the high resolution Co 2p spectrum of NiCoFeP/NF (Figure. 3b), three main peaks are observed ( 779.2 corresponding to CoP (Co 2p3/2) 38, 782.2 (Co 2p3/2) and 798 eV (Co 2p1/2) corresponding to oxidized Co species like Co-PO20,31) and there are two satellite peaks. The peaks for Co in NiCoFeP are shifted toward positive binding energies compared with that of NiCoP, suggesting strong electron interactions after Fe doping. The high-resolution Fe 2p spectrum (Figure. 3c) shows four peaks which are divided


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into 711.7 of Fe 2p3/2, 724.9 of Fe 2p1/2 and two satellite peaks at 715.3 and 735.6 eV,20,24,27,39 respectively. There are no characteristic peaks of FeP in the Fe 2p region, indicating that the iron is successfully doped into the catalyst. The P 2p spectrum of NiCoFeP/NF (Figure. 3d) shows the P 2p1/2 binding energy at 130.2 eV indicating the presence of M-P, while the centered peaks at 134.9 and 133.5eV are assigned to oxidized phosphorous species (M-PO). The P 2p spectrum of NiCoP/NF shows the peak at 130.4 eV, which should be related to the M-P, while the peak at 134.5 eV can be attributed to the oxidized phosphorous species. Note that a new peak at 133.5 eV is appearing and the content of metal phosphide species increased after Fe doping. These features indicate strong electron interactions with doped iron.36,40,41

Figure. 3 XPS spectra of (a) Ni 2p, (b) Co 2p, (c) Fe 2p and (d) P 2p of NiCoFeP/NF and NiCoP/NF.



The electrocatalytic OER performance of NiCoFeP/NF was conducted on a three-electrode system in 1 M KOH solution. As comparison, the NF, NiCoP/NF and commercial RuO2/NF catalyst were tested under the same condition. As shown in Figure. 4a, to obtain the intrinsic electrocatalytic performance of catalysts and eliminate the influence of ohmic resistance, the polarization curves were measured by IR-corrected linear sweep voltammograms (LSV) with the scan rate of 5 mV·s-1 (see IR-uncorrected linear sweep voltammetry (LSV) in Figure S6). The blank NF did not exhibit obvious OER activity. Indeed, the blank NF producing negligible anodic current below 1.55 V vs. RHE. The current density of 10 mA·cm-2 is reached at an overpotential of 392 mV for the NF. In contrast, the NiCoFeP/NF electrode exhibits a fast increase of current density above 1.25V. The polarization curves of all samples exhibit a distinct anodic peak which is attributed to the oxidation of Ni (II) to Ni (III) and Co (II) to Co (III) .11,25 The CV cycling test with different scanning rates (0.002, 0.003, 0.004, 0.005 mV/s) reveals that the oxidation process is reversible (Figure. S7). It is interesting that the redox potential of NiCoFeP/NF shifted toward positive direction compared with NiCoP/NF, suggesting that it is more difficult to oxidize Ni (II) to Ni (III) with the presence of Fe, which needs more oxidizing energy, and thus possibly strengthen OER kinetics.35 As the commercial catalyst, RuO2 can generate a current density of 100 mA cm-2, 200 mA·cm-2 at the overpotential of 384 mV and 416 mV, which are significantly larger than those of NiCoP/NF (336 mV and 381 mV, respectively). An overpotential of 271 mV is required for NiCoFeP/NF to produce a catalytic current density of 200 mA·cm-2, which is comparable and even superior to commercial RuO2 and NiCoP/NF. This is also much better than the previously reported OER catalysts under alkaline conditions (Table S1). To reach 200 mA·cm-2, the IR-uncorrected overpotentials for RuO2/NF, NiCoP/NF, NiCoFeP/NF are 577,


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Figure. 4 (a) LSV curves of NF, RuO2/NF, NiCoP/NF, NiCoFeP/NF for OER, (b) Tafel plots of RuO2/NF, NiCoP/NF, NiCoFeP/NF for OER, (c) Nyquist plots at 1.50 V (vs. RHE) of NiCoP/NF, NiCoFeP/NF for OER, (d) Double-layer capacitances of NiCoP/NF, NiCoFeP/NF, (e) Stability test of NiCoFeP/NF, (f) Time dependent-current density of NiCoFeP/NF for the OER.

510, 375 mV, respectively. This result is consistent with the previously reported literature: the catalyst performance can be significantly improved with iron doping, even in trace amount.34 Therefore, the improved OER performance of NiCoFeP/NF



may be attributed to the synergistic coupling effect among these ions Fe, Ni and Co, which can change the electronic structure. On the other hand, doped Fe can improve the electron transport of catalysts, and the iron with multiple valence states suggests the possibility that Fe is the active site.17 Furthermore, the catalysts with different content of Fe (3%, 5%, 7%, 10% and 13%) are also tested under the same conditions for the OER. As shown in Figure. S8, the overpotentials corresponding to 200 mA·cm-2 for NiCoFe3P/NF, NiCoFe5P/NF, NiCoFe10P/NF, NiCoFe13P/NF are 313, 298, 290 and 305 mV, respectively, confirming NiCoFeP/NF with 7% Fe shows the best performance. The IR-uncorrected overpotentials reached 200 mA·cm-2 for NiCoFe3P/NF, NiCoFe5P/NF, NiCoFe10P/NF, NiCoFe13P/NF are 444, 384, 387, 424 mV, respectively. The mass loading of those catalyst layer was approximately 3.0 mg/cm2, determined by using a high precision microbalance. Mass activity = j/m (where j is the current density at the overpotential of 271 mV in 1 cm−2, m is the loading mass of the active materials in g·cm−2). The mass activity of NF, RuO2/NF, NiCoP/NF, NiCoFeP/NF is 0.07, 2.88, 28.82, 68.00A/g, respectively. Tafel slope plays an important role in evaluating the catalysis kinetics for OER. Figure. 4b shows that the NiCoFeP/NF displays a small Tafel slope of 45 mV·dec-1 which is lower than that of RuO2/NF (111 mV·dec-1) and NiCoP/NF (150 mV·dec-1). In order to further study the kinetics of these catalysts, the electrochemical impedance spectroscopy (EIS) measurements of the NiCoFeP/NF, NiCoP/NF were performed at 1.50 V vs. RHE in 1.0 M KOH (Figure. 4c). The radius of NiCoFeP/NF is far smaller than NiCoP/NF, indicating that the charge transfer resistance of NiCoFeP/NF is lower than that of NiCoP/NF. Meanwhile, the results of Tafel slope and EIS curves reveal doped iron in the NiCoFeP/NF can reduce the Tafel slope and promote the improvement of reaction kinetics of NiCoFeP/NF, which is beneficial for OER.17


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It is well known that electrochemically active surface area (ECSA) is another important criterion for evaluating active sites for catalyst.23 Meanwhile, there is a positive linear correlation between the ECSA value and the double-layer capacitances (Cdl) value. Therefore, Cdl was performed in 1 M KOH solution via cyclic voltammetry test with different scanning rates (10 mV·s-1, 20 mV·s-1, 30 mV·s-1, 40 mV·s-1 and 50 mV·s-1) (Figure. S9). The effective Cdl of NiCoP/NF and NiCoFeP/NF are shown in Figure. 4d. It can be seen that the Cdl of the NiCoP/NF is 5.2 mF·cm-2, lower than that of NiCoFeP/NF (7.7 mF·cm-2). This is related to the unique hierarchical pore of NiCoFeP/NF with uniform thin nanosheet arrays and porous structure, leading to more crowded and thinner than NiCoP/NF nanosheet arrays. Therefore, the more active sites of NiCoFeP/NF can improve the performance of OER. Furthermore, after the Fe-doping, the increased active surface area is due to varied physical structure.24,34 The durability is another crucial criterion to appraise the performance of catalyst during the OER process. Figure. 4e shows that the LSV of NiCoFeP/NF demonstrates no obvious change after 2000 continuous sweeping cycles between 1.40 and 1.60 V vs. RHE at a scanning rate of 100 mV·s-1. Meanwhile, the SEM image (Figure. S10) reveals that the uniform nanosheet arrays and nanoporous layer of NiCoFeP/NF are well retained after stability test, which may be due to the ultrathin and mutually interconnected nanosheets and the close affiliation between catalysts and the substrate. By applying a stable overpotential (1.485V), the catalytic current density remains at around 100 mA·cm−2 after 10 h of electrolysis. These results indicate the NiCoFeP/NF possesses prominent durability in 1M KOH (Figure. 4f). In addition, to further explore the change of the catalyst during the electrocatalytic OER process, we investigated the XRD patterns after cyclic



voltammetry. As shown in Figure. S11, some XRD peaks disappear and the intensity of the peaks is weaken. We further carried out HRTEM to observe the change of NiCoFeP/NF. It can be seen from Figure. S12, the crystal lattices are disordered. These phenomena implies that more and more amorphous structure of NiCoFeP have been formed along with the progress of OER, which is consistent with the previous results.17 It is known that amorphous structure has a lot of advantages to promote the process of OER. Furthermore, the flexible structure presents more surface area in electrolyte.7,42 To further explore the OER mechanism, we carried out the XPS to study the change of NiCoFeP/NF after OER test. The Ni 2p spectrum (Figure. S13a) shows that the binding energy of 852.8 eV (Ni-P) disappear, and the intensity at about 856 eV (Ni-PO) was increased, indicating that the Ni-P on the surface of particles is oxidized to form Ni-PO. The peaks for Co in NiCoFeP after OER are shifted toward lower binding energies compared with the ones in NiCoP before OER, which may imply the decrease of electron transfer after OER, but it is not particularly noticeable(Figure. 4e). The state of Fe has no significant change (Figure. S13b and c). It is worth noting that the intensity at about 130.1 eV of P 2p was also decreased (Figure. S13d ) and oxidized P species formed at about 132.9 eV.20 In O 1s XPS spectrum (Figure. S13e and f), there are two peaks of NiCoFeP/NF before OER test, which distribute at 532.4 and 531.3 eV, and can be ascribed to H2O and metal oxyhydroxides (M-OH), respectively. As for NiCoFeP/NF after OER test, two peaks at 531.7 and 530.4eV can be ascribed to M-OH and metal oxide (M-O),43 respectively. At the same time, the intensity of M-OH was increased after OER test. Based on above analysis, phosphides are oxidized to phosphate and the M-OH and M-O were gradually produced during the OER process. The oxidized P species, M-OH and M-O


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Figure. 5 The proton-coupled electron transfer at the high equivalency of 4.

on the surface of NiCoFeP/NF can create a heterostructure, the NiCoFeP/NiCoFeO (or NiCoFeOOH, NiCoFePO).22,25,28 NiCoFeP is the pre-catalysts in OER and acts as the precursors to form the oxide/hydroxide OER catalysts.44 Boettcher et al. believe that the poor electrical conductivity of FeOOH results in low apparent activity of OER, but NiFeOOH and CoFeOOH show high conductivity to enhance OER activity.45 And they also reported that the distance of Fe−O bond in NiFeOOH is shorter than that of γ-FeOOH and NiFeOOH has the optimal adsorption energy for OER, which suggests that Fe-doping affects the local electronic structure of the NiOOH. They further found that Fe incorporation increased the OER activity of CoOOH.11 Xiong et al. reported that Fe as the active sites are most likely the hydroxyl acceptors, which accelerate the OER through discharging and desorption.46 Therefore, Fe-doped NiCoP is beneficial for electron transfer between the layers of NiCoFeP and



NiCoFeO (or NiCoFeOOH) and correlating the electrical structure of NiCoP. As shown in Figure. 5, the proton-coupled electron transfer at a high equivalency of 4.46 At the same time, the metallic phosphates can accelerate proton transfer to enhance the OER kinetics during water oxidation catalysis due to its lone-pair electrons in 3p orbitals and vacant 3d orbitals46,47 The interface of NiCoFeP/NiCoFeO (or NiCoFeOOH) helps for better carrier transportation from the NiCoFeP to the NiCoFeO (or NiCoFeOOH).48 On the other hand, the P and its oxides have high affinity toward water in all pH values which can accelerate the coordination of water to the electroactive site, and then bring down the overpotential of OER and improve the performance of OER.49 These features are likely to be desirable reasons for high OER activity of the NiCoFeP/NF.

CONCLUSIONS In summary, we have synthesized NiCoFeP/NF nanosheet arrays via a simple and mild hydrothermal and phosphorization method. The products exhibit remarkable activity for OER: only 271 mV overpotentials is required to achieve a current density of 200 mA·cm-2. It also shows small Tafel slope of 45 mV·dec-1. The superior performances of NiCoFeP/NF may be attributed to the following factors: firstly, the transition metal phosphides with high inherent electrocatalytic performance, and Fe-doping can improve electron transport and correlate the electrical structure, the synergetic effects among Ni, Co and Fe elements can change the electronic structure; secondly, iron doping can effectively change the morphology of catalyst to obtain the unique nanostructure with thin nanosheet arrays in situ grown on the NF, forming hierarchical porous structure, resulting in a large specific channel and surface area to expose more active sites and improve mass transfer; finally, metal oxyhydroxides and metal oxide can be produced with the progress of OER, which can create a


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heterostructure with NiCoFeP/NF to promote the performance of OER. In general, this simple and mild method and the accessible strategy produce multiple transition metal compounds with economic, earth-abundant and high efficiency, which may be applicable to other catalytic fields.

ASSOCIATED CONTENT Supporting Information XRD patterns, optical photograph, EDX spectrum, XPS survey scans, LSV curves, Electrochemical cyclic voltammetry curves, HRTEM images.

AUTHOR INFORMATION Corresponding Author * Email: [email protected] * E-mails: [email protected] * E-mails: [email protected] Author Contributions # contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This original research was supported by the China Scholarship Council, Natural Science Foundation of Hunan (No.: 2017JJ2040), the Australia Research Council and the University of Technology Sydney (UTS) through the Discovery Early Career



Researcher Award (DECRA DE170101009), ARC Discovery Project (DP170100436), UTS Chancellor's Postdoctoral Research Fellowship project (PRO16-1893), and UTS Early Career Researcher Grants ECRGS (PRO16-1304).

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Fe-doped NiCoP porous nanosheet arrays as a highly efficient

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