Self-supported FeNi-P Nanosheets with Thin Amorphous Layers for

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Self-supported FeNi-P Nanosheets with Thin Amorphous Layers for Efficient Electrocatalytic Water Splitting Qing Yan, Tong Wei, Jun Wu, Xueying Yang, Min Zhu, Kui Cheng, Ke Ye, Kai Zhu, Jun Yan, Dianxue Cao, Guiling Wang, and Yue Pan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04743 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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ACS Sustainable Chemistry & Engineering

Self-supported FeNi-P Nanosheets with Thin Amorphous Layers for Efficient Electrocatalytic Water Splitting Qing Yan†, Tong Wei†, Jun Wu†, Xueying Yang†, Min Zhu†, Kui Cheng†, Ke Ye†, Kai Zhu†, Jun Yan†, Dianxue Cao†, Guiling Wang∗†, Yue Pan∗‡ †

Key Laboratory of Superlight Materials and Surface Technology of Ministry of

Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China ‡

State Key Laboratory Cultivation Base for Nonmetal Composites and Functional

Materials, Southwest University of Science and Technology, Mianyang 621010, China

Corresponding

authors:

[email protected]

(Guiling

Wang);

[email protected] (Yue Pan)

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT: Low-cost and efficient catalytic electrode toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is strongly attractive for overall water splitting. Herein, a 3D bifunctional electrode of FeNi phosphides nanosheets with a thin amorphous layers on Nikel foam (FeNi-P/NF) is designed via phosphorizing NiFe layered double hydroxides coated nickel foam (FeNi LDH/NF) in red phosphorous vapor at 500 °C. Benefiting from the unique architecture with increased exposure and accessibility of active sites, the electrode exhibits excellent electrocatalytic performance toward both HER and OER in 1.0 M NaOH, with 102 mV and 224 mV low overpotentials to achieve a catalytic current density of 10 mA cm-2, respectively. As a bifunctional electrode, the FeNi-P/NF can release a current density of 10 mA cm-2 at a low cell voltage of 1.57 V and 100 mA cm-2 at 1.82 V in an alkali electrolyzer, possessing a significantly durability over 100 h electrolysis at high current density of 100 mA cm-2.

KEYWORDS: Nanosheets, Thin Amorphous Layers, FeNi-P, Hydrogen Evolution Reaction, Oxygen Evolution Reaction

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INTRODUCTION

With the continuous rise of carbon dioxide (CO2) in the atmosphere, it becomes a severe problem for our climate. Developing alternatives to fossil fuels is more and more urgent.1 Hydrogen (H2) is widely treated as a sustainable energy carrier with a potential to solve current energy problem, owing to its high energy density and carbon-free emission upon releasing stored energy.2-3 To achieve global-scale sustainable hydrogen product, electrochemical splitting of water has been considered as a feasible strategy, which can produce pure H2 directly used to feed fuel cells without a risk of poisoning the anode catalysts.4 Electrochemical splitting of water contains two significant fundamental reaction, the hydrogen-evolution reaction (HER) and the oxygen-evolution reaction (OER). While, the reaction dynamics of HER and OER are extremely slow, especially OER. One feasible approach to solve this issue is using catalyst to lower the dynamic overpotential.5 Platinum (Pt) and iridium (Ir) based catalysts are optimal utilized for HER and OER, respectively, but they are very expensive and scarce in the earth. Therefore, it is a priority to develop highly active, earth-abundant, and steady catalysts.

Recently, transition metal phosphides are explored for HER with their outstanding activity and stability, especially Nickel based phosphides. A series of Ni-P catalysts have been reported like Ni12P5,6 Ni5P4,7 NiP28 and Ni2P.9 Liang10 reported that the addition of Fe could promote the catalytic activity performance of MoP toward HER, and Kwong11 reported that nickel-iron phosphide could attain -10 mA cm-2 at a

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overpotential of 101 mV by controlling the ratio of Fe and Ni. These results show that the introduction of Fe could significantly improve the HER activity of transition-metal catalysts. For OER catalysts, Fe, Co and Ni based materials3, 12 have attracted great attention because of their outstanding chemisorption of OH- and oxygen-containing intermediates. To further improve the catalytic activity, lots of researchers settle down to modifying these metals into their oxides,13 sulfides,14 phosphides,15 selenides,16 nitrides,17 and so on. S.W. Boettcher and his cooperators proved that the addition of Fe element could improve the OER performance of Ni based catalysts.18 Qiu and his partners reported Fe-tuned Ni2P electrocatalysts enhanced the OER catalytic performance.19 Du group prepared the novel FeNiP solid-solution nanoplate arrays and applied it as an active catalyst for high-performance water-oxidation catalysis.20 Hu and his partners prepared a class of bulk amorphous NiFeP materials with metallic bonds and found they can be applied as excellent OER catalysts.21 Li and his partners developed a controllable approach to engineer a bimetallic Ni-Fe phosphide nanocomposite through the topological conversion of the flowerlike precursor of Ni-Fe layered double hydroxides, achieving a pretty low overpotential of 233 mV to deliver a current density of 10 mA cm-2.22 Jiang group adopted PCN-600-Ni, integrating with graphene oxide (GO), to afford bimetallic iron-nickel phosphide/reduced graphene oxide composite and achieved a low overpotential of 240 mV at 10 mAcm-2.23 The overpotential of OER has been attained below 250 mV at 10 mA cm-2 based on the researchers` unremitting efforts.

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To simplify the system and lower the cost of water splitting electrolyzers, using a bifunctional electrode toward both OER and HER in alkaline solution is a rather effective way. Because the overpotential of the OER much higher than that of the HER, almost nonprecious OER catalysts are unstable in acidic solution, and electrolyte conductivity is much lower in neutral electrolytes than acid and alkaline solutions, so it is highly attractive for both OER and HER in strongly basic media. 24 The bifunctional electrode can combine the advantages of the HER and OER catalysts and dramatically improce the overall electrochemical water-splitting efficiency. In the latest literatures，MxPy (M = Cu, Co, Ni, Fe and so on) grown on different matrix as electodes for splitting got considerable achievement. Du25 adopted crystalline copper phosphide nanosheets developed on nickel foam as an efficient Janus catalyst for overall water splitting, which attained a current density of 10 mA cm-2 at 1.67 V. Liu and Wang26 employed one-step electro-deposition followed by phosphorization to get the multiphase Ni-P composite on carbon cloth, by which the water electrolysis can be achieved

at

a

relatively

low

voltage.

Xiao

group

established

a

partial-sacrificial-template synthesis of Fe/Ni phosphides as the befuctional catalyst for water splitting.27

Inspired by the researches above, we synthesized a 3D bifunctional electrode of FeNi phosphides nanosheets with thin amorphous layers supported on Ni foam (FeNi-P/NF) via a facile hydrothermal synthesis method followed by a convenient one-step phosphorization treatment in phosphorus vapor at 500 °C. The catalytic performance of as-fabricated FeNi-P/NF was investigated by linear sweep 5



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voltammetry and chronopotentiometry in a standard three-electrode system. FeNi-P/NF exhibited excellent electrocatalytic performance toward both HER and OER in 1.0 M NaOH solution, with low overpotentials of 102 mV and 224 mV to attain a current density of 10 mA cm-2, respectively. And it showed fast kinetics with a small Tafel slope of 82 mV dec-1 (HER) and 77 mV dec-1 (OER). The alkali electrolyzer with FeNi-P/NF as bifunctional electrodes released a current density of 10 mA cm-2 at a low cell voltage of ca. 1.57 V, which was lower than many state-of-the-art water-splitting catalysts, including NiCoP/NF,28 MoS2/Ni3S2/NF,29 FeP,30 Ni-P/CP31 and VOOH.32 In addition, the electrolyzer possessed a stable durability for long-time electrolysis.

EXPERIMENTAL SECTION

Chemicals and materials

Red phosphorous (P), iron (II) sulfate heptahydrate (FeSO4·7H2O) and urea (CO(NH2)2) were purchased from Tianli Chemical Reagent Co., Ltd. (Tianjin, China). Nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and ammonium fluoride (NH4F) were obtained from Fuchen Chemical Reagent Factory (Tianjin, China). All the chemicals were of analytical grade and used without further purification. All the solutions were prepared with Milli-Q water (18.2 MΩ cm). Nickel foam (thickness：1.5 mm，110 PPI, 320 g m-2) was ordered from Changsha force yuan new materials co., Ltd. (Changsha, China).

Catalyst synthesis 6


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Typically, NiFe layered double hydroxides coated nickel foam (FeNi LDH/NF) was fabricated as: Ni(NO3)2·6H2O (2.0 mmol, 582 mg), FeSO4·7H2O (0.5 mmol, 139 mg), NH4F (10 mmol, 370 mg) and urea (25 mmol, 1.501 g) were dissolved in deionized H2O (40 ml), then the obtained solution and one piece of nickel foam (10 × 30 mm) were sealed together in a 45 ml Teflon-lined stainless steel autocalve followed by heating at 120 °C in an electric oven for 16 h. After being washed thoroughly with distilled water and absolute ethanol, the preliminary coated nickel foam was dried at 60 °C overnight in an oven. The preparation of FeNi-P/NF can be described as: firstly, the preliminary coated nickel foam was loaded into a ceramic boat, with ~1 g of red P placed 2 cm away on the upstream side. Secondly, the ceramic boat was heated in a tube furnace at 500 °C for 2 h in Ar atmosphere. After cooling down to 250 °C, the chamber was held at this temperature for another 3 h to convert the residual toxic and pyrophoric yellow P to red P. Finally, the furnace was naturally cooled down to room temperature to get FeNi-P/NF. For comparison, the FeNi-O/NF was also prepared by heating the FeNi LDH/NF in air at 450 °C for 2 h at a heating rate of 2 °C per minute.

Characterization

The morphology of all the samples were examined by scanning electron microscopy (SEM, JEOL JSM-6480) and transmission electron microscopy (TEM, FEI Teccai G2 S-Twin, Philips). The phase composition of FeNi-P samples was characterized by X-ray diffraction (XRD, Rigaku TTR III) using Cu Kα radiation (λ = 0.17889 nm) in

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a 2θ range of 10°-90° with a scan rate of 1° min-1. The IR spectrum of FeNi-P was obtained on a Fourier spectrometer (Perkin Elmer SP-100 spectrometer). The sample for recording of IR spectra were prepared by the standard procedure as KBr pellets. Raman spectra of the sample was recorded on a Jobin-Yvon HR800 Raman spectrometer with 532 nm wavelength incident laser light. The valence states of Fe, Ni and P in the composite were investigated by X-ray photoelectron spectroscopy using Al Kα radiation (XPS, Thermo ESCALAB 250). The chemical formula of FeNi-P were measured by inductively coupled plasma mass spectroscopy (ICP-MS, Thermo X Series II).

Electrochemical measurements

The electrochemical performance of the electrocatalyzer in a standard three-electrode system was tested with cyclic voltammetry, linear sweep voltammetry and chronopotentiometry (CP) controlled by a computerized potentiostat (Autolab PGSTAT 302, Eco Chemie company in Holland). The catalysts (FeNi LDH/NF, FeNi-O/NF, FeNi-P/NF or NF) served as the working electrode, a Ag/AgCl (saturated KCl) electrode served as the reference electrode, and a carbon rod served as the counter electrode (carbon rod was used as the counter electrode instead of a Pt wire/coil to avoid contamination from dissolved Pt which will positively contribute to the HER). All tests were carried out in 1M NaOH solution. Before every HER (OER) test, the electrolyte was kept bubbling with high-purity H2 (O2) for 2 h to ensure the system was H2-saturated (O2-saturated). Electrochemical impedance spectroscopy 8


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(EIS) measurements were accomplished in the frequency range of 105 Hz to 0.01 Hz. For overall water splitting, a symmetrical full electrolyzer was constructed using two identical FeNi-P/NF electrodes as cathode and anode, respectively. Overall water splitting performance was evaluated using LSV and CP in a two-electrode configuration in 1.0 M NaOH. The LSV curves measured for both OER and HER were IR-corrected. All potentials are reported versus reversible hydrogen electrode (RHE) by converting the potentials measured vs. Ag/AgCl according to the following equation: E(RHE) = E(Ag/AgCl) + 0.197+ 0.0592*pH.

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RESULTS AND DISCUSSION

FeNi-P/NF was prepared through two steps shown in scheme 1. The FeNi layered double hydroxide precursor was grown on nickel foam (FeNi LDH/NF) via a hydrothermal method,33 then the precursor was converted into phosphides by an one-step phosphorization treatment presented by Liu.26 NF was chosen as the current collector because of three-dimensional network structure, high electrical conductivity, good malleability and excellent resistance to corrosion in alkaline solution. After the hydrothermal reaction, the original silver NF changed into yellow green shown in Fig. S1, with densely-packed precursor nanosheets uniformly coated on the surface (Fig. S2). After phosphorization, the NF changed into black indicating the production of FeNi-P/NF. The final product FeNi-P/NF mainly kept the morphology of uniformly nanosheets array shown in Fig.1b and c. These nanoplates are interconnected tightly with each other forming a wall-like secure structure, which is benefit for electronic transmission. From Fig. 1b, the thickness of the wall is about 45 nm by measurement. The crystallographic structure and phase purity of the FeNi-P samples (scraped from nickel foam surface) were determined by powder X-ray diffraction (PXRD). According to the result shown in Fig. 1a, the FeNi-P sample shows phosphide structure, which is the same as hexagonal Ni5P4 (PDF card no. 01-089-2588) with almost diffraction peaks. The main peaks at 15.1º, 16.1º, 28.7º, 30.4º, 31.5º, 36.1º, 43.9º, 45.1º, 47.1º, 47.8º, 53.0º and 54.0º match well with (100), (002), (103), (200), (201), (104), (212), (204), (301), (213), (303) and (220) planes of Ni5P4. Though the 10


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ICP analysis suggests nearly a 0.73:4.96:4 for atomic ratio for Fe:Ni:P, giving a stoichiometric formula of Fe0.73Ni4.96P4, there is no prominent diffraction peaks associated with iron species, indicating that the iron atoms might be amorphous forms or dopants in the Ni5P4 lattice.27 Generally, we should defined the formula the material we prepared as Fe0.73Ni4.96P4, considering we want to compare the catalytic performance among these three catalysts (FeNi LDH/NF, FeNi-O/NF and FeNi-P/NF), the expression of FeNi-P/NF can be more clear. From the TEM images, the thin microstructure of the fabricated FeNi-P nanosheets can be further studied (Fig. 1d). Taken from selected-area electron diffraction (SAED) over this nanosheet, a well-resolved spotted pattern corresponding to the diffraction along the [0001] zone axis of the Ni5P4 hexagonal structure can be observed, indicating the single-crystalline nature of this nanosheet from Fig. 1e.34 It can be further identified by HRTEM investigation (Fig. 1f), in which well-resolved crystal lattices with an interplanar spacing of 0.59 nm can be clearly distinguished. This value is slightly larger than that of (100) crystal planes of hexagonal Ni5P4, due to the substitution of Ni by Fe in FeNi-P. In addition, the small crystalline FeNi-P is surrounded by thin amorphous layers (several nanometers), suggesting the surface of FeNi-P may be oxidized to phosphates or oxides by air.

To better demonstrate the ingredient of thin amorphous layers, the IR and Raman spectra were employed shown in Fig. 2. It is well known that the peaks in the region between 1200 and 500 cm-1 correspond to the symmetric and asymmetric vibrations of the phosphate groups.35 The spectra of all catalysts exhibited a broad band at 1046 11



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cm-1 due to asymmetric stretching mode of the PO4 group. The spectra showed shoulders at about 740 cm-1, which could be due to the symmetric stretching mode of P-O-P bond.36 The IR band from 600 to 500 cm-1 corresponds to the characteristic vibration of Ni-P stretching and bending vibration of P-O bond.37-38 The peak at 465 cm-2 in IR is closer to the Ni-O in nickel oxides (450 cm-1) comparing with that in nickel phosphates (613 cm-1), which proves there is Ni(Fe) oxides in the layers.39-40 The peaks at 1176, 986 and 751 cm-1 in Raman spectra were accounting for the surface phosphates as well.35, 41 The peak at 448 cm-1 can be O-P-O bond. The bands in the range 400 to 100 cm-1 are owing to the translation vibrations of cations and the translations and rotations of phosphorus tetrahedra.35 The IR peaks of H2O and O-H usually appear at 1631 and 3421 cm-1 respectively, and there is no obvious peak in the range from 4000 to 1500 cm-1, which indicates that the catalyst does not contain water molecules or hydroxyl groups.42 Therefore, there isn’t FeNi hydroxides in the amorphous layers. It can be consist of FeNi phosphates and a small amount of FeNi oxides. Combining with the PXRD results (Fig. 1a) and HRTEM image (Fig. 1e), it can be discriminated that the crystalline FeNi-P with thin amorphous layers electrodes were successfully synthesized. This structure could protect the inner FeNi-P from being seriously etched during the catalysis and improve the catalytic activity by facilitating the electron transfer from the metallic inner FeNi-P to the outer amorphous layers. 28

The surface chemistry of FeNi LDH and FeNi-P catalysts was further investigated through X-ray photoelectron spectroscopy (XPS). The data was corrected by C1s 12


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(284.8 eV). Fig. 2b shows the XPS comparison of Ni 2p, Fe 2p and P 2p between FeNi-P and the precursor (FeNi LDH). The peaks at 856.2 and 874.2 eV of FeNi LDH are assigned to Ni (2p3/2) and Ni (2p1/2) of Ni2+,43 and the peaks shift to higher binding energies by about 0.8 and 0.9 eV respectively after phosphorization. Therefore, the oxidation of Ni2+ to a higher valence state may be facilitated,28, 44 further benifitting for the catalysis process of HER and OER. The peaks at 854.2 and 871.5 are appeared only in FeNi-P relating to Niδ﹢in Ni-P, and the value of δ is close to 0.45 However, compared with FeNi LDH, the peaks of Fe 2p1/2 (726.9 eV) and Fe 2p3/2 (713.5 eV) negatively shift about 0.3 and 0.9 eV successively, confirming the strong electron interactions involving Ni and Fe in FeNi-P electrode.46 Another obvious peak at 707.0 eV accounts for the formation of Fe-P. In the P 2p spectrum, the peaks at 128.6 and 129.5 eV are attributed to P 2p3/2 in phosphides and the peak at about 134.5 eV represents P-O in phosphate group. The binding energy of 129.5 and 128.6 eV is slightly lower than that of elemental P (130.0 eV),28 suggesting that P element is partially negatively charged (Pδ‑ ). Therefore, P element can act as base to trap positively charged protons during the electrocatalysis.

The FeNi-P/NF was directly used as an electrode to catalyze the OER in a typical three-electrode setup in 1.0 M O2-saturated NaOH solution with a scan rate of 1 mV s-1. All polarization curves are IR-corrected (i.e., electrolyte resistance compensated, the values of Rs listed in Table S3). Three controlled samples of NF, FeNi LDH/NF, and FeNi-O/NF were also investigated under the same conditions. The FeNi-O/NF was synthesized according to the procedure reported.47 The linear sweep voltammetry 13



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(LSV) polarization curves (Fig. 3a) show that the FeNi-P/NF need the overpotential of 224 mV to drive the OER at 10 mA cm-2, which is lower than FeNi LDH/NF (280 mV), FeNi-O/NF (326 mV) and NF (380 mV). All catalysts show a distinct anodic peak between 1.3 V and 1.5 V versus RHE in Fig. 3a (The partial enlarged figure of Fig. 3a can be seen from Fig. S8), which should be associated with the oxidation of Ni or Fe (or both) species.

30, 48

The FeNi-P/NF is a pretty good catalyst comparing with

those reported latest (see Table S2). Interestingly, the FeNi-P/NF electrode only needs 320 mV to get 100 mA cm-2, indicating its favorable prospect of industrialization application. Furthermore, from Fig. 3a, at the overpotential of 300 mV, the FeNi-P/NF attains a current density of 72 mA cm-2, which is 3.7 times than FeNi LDH/NF (19 mA cm-2) and 15 times than FeNi-O/NF (4.8 mA cm-2). To ascertain the reaction kinetics, Tafel analysis was shown in Figure 3b. The Tafel slopes of 72 mV/dec, 86 mV/dec, 93 mV/dec and 106 mV/dec were yielded for FeNi-P/NF, FeNi LDH/NF, FeNi-O/NF and NF, respectively. Analysis of the linear regression analysis for corresponding electrocatalysts can be seen from Table S1.The Tafel slope of FeNi-P/NF is the smallest, indicating its reaction kinetics process is easier to drive than the others. We also calculated the the exchange current densities (J0) of these catalysts by the extrapolation of the Tafel plot. The FeNi-P/NF has the highest J0 of 0.0062 mA cm-2, larger than FeNi LDH/NF (0.0056 mA cm-2), FeNi-O/NF (0.0032 mA cm-2) and NF (0.0027 mA cm-2), suggesting that FeNi-P/NF suffers the highest kinetics of charge transfer reactions under OER. Next, electrochemical impedance spectroscopy (EIS) investigations were performed at the overpotential of 300 mV as 14


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shown in Fig. 3c. For a reasonable analysis of EIS, we have quantitatively treated all the data using an equivalent circuit model with use of the CNLS fitting procedure. The data under OER were fitted using the modified Randle equivalent circuit model shown in Fig. 3c, which exhibits that those four electrode materials owned similar resister (Rs) which is the resistor between the reference and working electrode. There are two time constants, of which the first at high frequencies is related to the surface porosity or the redox reaction of catalysts and the second at low frequencies corresponds to the charge transfer (Rct) process. It is obvious that FeNi-P/NF electrode had the lowest Rct, the specific values can be seen in Table S3. The EIS results are consistent with their OER catalytic performance. We further measured the double layer capacitance (Cdl) using cyclic voltammetry (CV) method at different scan rates (Fig. S9) to estimate the electrochemically active surface area (ECSA) in the potential range from 1.05 to 1.15 V versus RHE close to OER condition, and it was founded that FeNi-P/NF possesses the highest Cdl of 8.8 mF cm-2, followed by FeNi LDH/NF and FeNi-O/NF (3.1 and 2.6 mF cm-2, respectively, in Fig. 3d). The FeNi-P/NF electrode has the highest Cdl, meaning the highest electrocatalytically relevant surface area and more exposed active sites, which could be mainly ascribed to the nanosheet structure. When normalized to the ECSA (Fig. 3e), the FeNi-P/NF also displays the highest current density, followed by FeNi LDH/NF and then FeNi-O/NF, indicating the FeNi-P/NF has the best intrinsic OER activity. These results indicates the FeNi-P/NF is a good catalyst for OER. Durability an important indicator for HER catalysts was evaluated by accelerated degradation test (ADT) and 15



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chronopotentiometry (CP). The ADT was performed continuously for 1000 cycles in a potential range from 1.2 to 1.8 V versus RHE at the rate of 100 mV s-1.The inset in Fig. S11 shows the polarization curves, there is a minor positive shift in overpotential before and after cycling. From another aspect of time dependence, the CP was tested at the current density of 10 mA cm-2 for 10 hours as shown in Fig. S11, and the potential was almost at a static state. Furtherly, we tested the catalyst at a larger current at 20 mA cm-2 and 100 mA cm-2 (See Fig. 3f). Though the curves wasn’t very smooth, the potential only increased from 1.478 V to 1.5 V after 20 h’ reaction at 20 mA cm−2, increasing by 1.5%. Suffering from another 20 h at 100 mA cm−2, the potentials were mainly at the range from 1.55 V to 1.56 V. The results above shows FeNi-P/NF electrode has good stability.

After OER measurements, several new peaks appeared in the Ni 2p XPS spectrum (Fig. S12 a). The peaks at ~856.4 eV and ~873.7 can be ascribed to Ni(OH)2, and the peak emerged at ∼875.1 eV is due to the formation of NiOOH, which indicated that at least the surface of FeNi-P has been transformed to metal oxides/hydroxides during the OER process. Furtherly, the evolution of O peaks at 530.5 eV and 531.7 eV are ascribed to Ni(Fe)-O and 531.7 eV O-H in the O 1s XPS spectrum (Fig. S12 d), which is more solid evidence of the formation of NiOOH and Ni(OH)2 layers. Fig. S12 b showed the Fe 2p XPS, and we can clearly see the signal at 707.0 eV almost disappeared, which stated that the surface changed during OER process. Combined the chemical nature of the pristine FeNi-P/NF sample, it is believed that the newly

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formed surface layer should be Ni(Fe)OOH and Ni(Fe)(OH)2. Therefore, the real active species during the OER process should be Ni(Fe)OOH and Ni(Fe)(OH)2.49

Afterwards, the electrocatalytic performance toward HER was evaluated in H2-saturated 1.0 M NaOH solution. Fig. 4a shows the polarization curve of FeNi-P/NF cathode. A current density of -10 mA cm-2 can be achieved at the overpotential as low as 102 mV on FeNi-P/NF, while the current densities of 203, 225, and 253 mV on the FeNi LDH/NF, FeNi-O/NF, and NF electrodes are needed respectively to drive the same current density. These HER overpotentials are comparable to some catalysts reported lately (see Table S5). Tafel plots of the FeNi-P/NF, FeNi LDH/NF, FeNi-O/NF and raw NF are shown in Fig. 4b. The FeNi-P/NF shows a Tafel slope of 82 mV/dec, which is superior to those of FeNi LDH/NF (99 mV/dec), FeNi-O/NF (105 mV/dec) and NF (113 mV/dec). From Table S4, the detail analysis of the linear regression analysis for corresponding electrocatalysts can be obtained, and the correlation coefficients of the linear regression analysis are all larger than 0.99, which indicated that the liner fitting is pretty accurate. These slope values all decrease within the range of 38~116 mV/dec, suggesting the HER taking place on their surface would obey a Volmer-Heyrovsky mechanism.50 Furthermore, the exchange current densities (J0) are obtained by the intercept of the Tafel plot. The FeNi-P/NF has the highest J0 of 0.66 mA cm-2, much larger than FeNi LDH/NF (0.088 mA cm-2), FeNi-O/NF (0.067 mA cm-2) and NF (0.051 mA cm-2), suggesting that FeNi-P/NF suffers the highest kinetics of charge transfer reactions. EIS was applied at an overpotential of 200 mV (See Fig. 4c), 17



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similarly as OER, all the impedance spectra were fitted using the modified Randle equivalent circuit model shown in Figure 4c, including Rs, R1, Rct and two Constant Phase Element (Q), which could be seen more clearly in Fig. S13. In addition, it can be obtained from Table S6 that FeNi-P/NF electrode has a lower Rct value than FeNi LDH/NF, FeNi-O/NF and NF, indicating its faster electron transfer kinetics of HER. This can be due to the direct growth of the unique structure of crystalline FeNi-P with thin amorphous layers on the surface of NF, ensuring excellent electron contact to the NF substrate. Meanwhile, Cdl was explored from 0 to 0.1 V where there is no Faraday reaction versus RHE close to the HER condition (See Fig. S14), and the polarization curve was normalized as shown in Fig. 4e. The FeNi-P/NF also exhibits the best HER performance, which indicates the FeNi-P/NF is intrinsically more active than FeNi LDH/NF and FeNi-O/NF. The stability was performed by ADT and CP as well. The ADT was applied continuously for 1000 cycles in a potential range from 0 to -0.6 V versus RHE at the rate of 100 mV s-1 (See Fig. S15). There was minor change before and after the ADT test. Though FeNi-P/NF cathode was detected by CP under a current density of -10 mA cm-2 for 10 h, the potential was extremely stable (Fig. S15). Furthermore, FeNi-P/NF electrode was tested in -20 mA cm-2 -100 mA cm-2 for every 20 h as shown in Fig. 4f. Though the catalytic activity weakened with long-time reaction, the increasing range of potential needed wasn’t very large. Therefore, the catalyst of FeNi-P was stable towards HER.

Inspired by the good catalytic performances in both HER and OER, FeNi-P/NF was used as a bifunctional electrocatalyst for overall water splitting in a two-electrode 18


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device (see the inset of Fig. 5a) in 1.0 M NaOH solution. First, the LSV experiment was employed to observe the electrolysis performance of the alkaline electrolyzer. With continuous increase of the cell voltage, dense bubbles could be observed clearly generated on the electrode surfaces. A stable current density of 10 mA cm-2 was obtained at about 1.57 V, which was superior to FeNi LDH/NF || FeNi LDH/NF (1.67 V), FeNi-O/NF || FeNi-O/NF (1.77 V) and comparable to the reported state-of-the-art bifunctional electrocatalysts (see Table S7). The stability of this electrolyzer was tested at different current density every step for 10 h shown in Figure 5b, the cell voltage changed from 1.53 V to 1.57 V in the first 10 h at 10 mA cm-2 and changed slightly at the following 20, 50 and 100 mA cm-2. It only needed a cell voltage of 1.82 V for FeNi-P/NF || FeNi-P/NF to drive a current density of 100 mA cm-2. When the current density decreased back to 10 mA cm-2, the cell voltage returned to 1.57 V. To further verify the stability of FeNi-P/NF, a 100 h’s test at 100 mA cm-2 was performed, the cell voltage were stable and basically lower than 1.85 V, suggesting the FeNi-P/NF could be a tolerant bifunctional electrode to electrocatalytic water splitting.

The superior electrocatalytic performance of FeNi-P may be related to its crystalline structure with thin amorphous layers. This structure could facilitate the electron transfer from the metallic inner FeNi-P to the amorphous layers, which was supported by low Rct values of EIS results both under HER and OER. From the ECSA, it can be determined that FeNi-P has larger area offering more active sites. The positive shifts of Ni 2p measured by XPS indicate that phosphatization may facilitate Ni2+ to a 19



Page 20 of 42

higher valence state, which is efficient for catalysis process of HER and OER. In addition, the use of nikel foam as substrate could further enhance the conductivity and electron transfer rate.

CONCLUSIONS

In summary, the self-supported crystalline FeNi-P nanosheets with thin amorphous layers have been synthesized by a facile hydrothermal process and an one-step low temperature phosphorization. The crystalline FeNi-P structure with thin amorphous layers can greatly increase its active sites as well as electron transfer rate. The existence of Fe element has altered the crystal texture of Ni5P4, further optimizing the performance of FeNi-P/NF. The FeNi-P/NF exhibits excellent electrocatalytic activity and durability toward both HER and OER in alkaline electrolyte. Thus, a noble-metal-free alkaline electrolyzer was constructed using two same FeNi-P/NF electrodes as both cathode and anode. The current density of 10 mA cm-2 and 100 mA cm-2 with a cell voltage of 1.57 V and 1.82 V can be obtained respectively, making FeNi-P/NF into one of the most active bifunctional catalysts for electrocatalytic water splitting. This work would be helpful for further exploring better electrocatalysts for other energy storage and conversion devices.

ASSOCIATED CONTENT

Supporting Information. Text and figures giving details of NF, FeNi LDH/NF, FeNi-O/NF

and

FeNi-P/NF

morphological,

structural,

and

electrochemical

characterization data, and tables of the linear regression analysis, EIS analysis and 20


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performance comparison with recently reported electrocatalysts. This Supporting Information is available free of charge via the Internet at“http://pubs.acs.org.”

AUTHOR INFORMATION Corresponding Author * E-mail for Guiling Wang and Yue Pan: [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We gratefully acknowledge Li Kong, Ping Yang and Zheng Liu for helpful discussions. This research was supported by the National Natural Science Foundation of China (51572052 and 21403044). REFERENCES 1.

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Scheme 1. Schematic representation for the preparation of FeNi-P/NF by two steps and the catalytic diagram of FeNi-P nanosheets.

30


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Figure 1. Morphology and structure of FeNi-P electrodes. (a) XRD patterns of FeNi-P samples. (b, c) SEM images of FeNi-P nanosheets array. Inset: the skeleton of FeNi-P/NF. (d) TEM image of FeNi-P nanosheet. (e) SAED pattern and (f) HRTEM image taken from a single FeNi-P nanosheet.

31



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Figure 2. (a) Vibrational IR (blue) and Raman (black) spectra of FeNi-P samples. (b) XPS spectra of FeNi LDH (red) and FeNi-P (green) samples.

32


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Figure 3. OER performance of FeNi-P/NF, FeNi LDH/NF, FeNi-O/NF and NF. (a) IR-corrected polarization curves of FeNi-P/NF anode at a scan rate of 1 mV s-1, compared with FeNi-O/NF, FeNi LDH/NF, and raw NF. (b) Polarization curves-derived Tafel slopes for corresponding electrocatalysts. (c) Nyquist plots of corresponding electrocatalysts under OER at an overpotential of 300 mV. Inset: The data were fitted using the modified Randle equivalent circuit model. (d) Difference in current density plotted against the scan rate for the determination of the double-layer capacitance (Cdl). (e) Polarization curves normalized by ECSA. (f) Chronopotentiostatic curves of FeNi-P/NF at a constant current density of 20 mA cm−2 for 20 h and 100 mA cm-2 for another 20 h.

33



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Figure 4. HER performance of FeNi-P/NF, FeNi LDH/NF, FeNi-O/NF and NF. (a) IR-corrected polarization curves of FeNi-P/NF cathode at 1 mV s-1 compared with FeNi-O/NF, FeNi LDH/NF, and raw NF. (b) Polarization curves-derived Tafel slopes for corresponding electrocatalysts. (c) Nyquist plots of corresponding electrocatalysts at the overpotential of 200 mV. (d) Difference in current density plotted against the scan rate for the determination of the double-layer capacitance (Cdl). (e) Polarization curves normalized by ECSA. (f) Chronopotentiostatic curves of FeNi-P/NF at a constant current density of -20 mA cm−2 for 20 h and -100 mA cm-2 for another 20 h.

34


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Figure 5. (a) The LSV plot of overall water splitting with FeNi-P/NF, FeNi LDH/NF, FeNi-O/NF and NF as bifunctional electrodes; inset: the photograph show the released bubbles during overall water splitting catalyzed by FeNi-P/NF. (b) The stability of the two-electrode electrolyzer of electrolysis at different geometry current density in 1.0 M NaOH. (c) Long-time stability test of the FeNi-P/NF || FeNi-P/NF electrolyzer recorded at a current density of 100 mA cm-2.

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SYNOPSIS TOC

Self-supported FeNi-P nanosheets for as a bifunctional electrode for efficient electrocatalytic water splitting in 1M NaOH. Here shown the polarization curve of the FeNi-P/NF || FeNi-P/NF along with along with FeNi LDH/NF || FeNi LDH/NF, FeNi-O/NF || FeNi-O/NF and NF || NF.

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300x189mm (72 x 72 DPI)



700x415mm (72 x 72 DPI)


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583x227mm (72 x 72 DPI)



508x617mm (72 x 72 DPI)


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494x617mm (72 x 72 DPI)



318x249mm (72 x 72 DPI)


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Self-supported FeNi-P Nanosheets with Thin Amorphous Layers for

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