Se-Doping Activates FeOOH for Cost-Effective and Efficient

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Se-Doping Activates FeOOH for Cost-Effective and Efficient Electrochemical Water Oxidation Shuai Niu, Wen-Jie Jiang, Zengxi Wei, Tang Tang, Jianmin Ma, Jin-Song Hu, and Li-Jun Wan J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 01 Apr 2019 Downloaded from http://pubs.acs.org on April 1, 2019

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Journal of the American Chemical Society

Se-Doping Activates FeOOH for Cost-Effective and Efficient Electrochemical Water Oxidation Shuai Niu,†,§,∇ Wen-Jie Jiang,†,§,∇ Zengxi Wei,‡ Tang Tang,†,§ Jianmin Ma,‡,∥ Jin-Song Hu,*,†,§ Li-Jun Wan†,§ †

Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Science Beijing 100190, China ‡ School of Physics and Electronics, Hunan University, Changsha 410082, China § University of the Chinese Academy of Sciences, Beijing 100049, China ∥ Key Laboratory of Materials Processing and Mold (Ministry of Education), Zhengzhou University, Zhengzhou 450002, China KEYWORDS: Oxygen evolution, Electrocatalysts, Electrolysis, Water splitting, Iron selenide ABSTRACT: Ni or Co is commonly required in efficient electrocatalysts for oxygen evolution reaction (OER). Although Fe is much more abundant and cheaper, full-Fe or Fe-rich catalysts suffer from insufficient activity. Herein, we discover that Se-doping can drastically promote OER on FeOOH and develop a facile on-site electrochemical activation strategy for achieving such Se-doped FeOOH electrode via FeSe pre-catalyst. Theoretical analysis and systematic experiments prove that Se-doping enables FeOOH as an efficient and low-cost OER electrocatalyst. By optimizing the electrode structure, an industrial-level OER current output of 500 mA cm-2 is secured at a low overpotential of 348 mV. The application of such a Fe-rich OER electrode in practical solar-driven water splitting system demonstrates a high and stable solar-to-hydrogen efficiency of 18.55%, making the strategy promising for exploring new cost-effective and highly active electrocatalysts for clean hydrogen production.

1.

Introduction

Oxygen evolution reaction (OER) is considered as the bottleneck in water splitting as it involves multiple proton coupled electron transfer steps with high-energy barriers.1-2 In view of the scalable and practical application of these techniques, developing OER catalysts that are highly efficient and durable at large output, especially made of low-cost and earth-abundant materials, is imperative but still challenging.3-6 Transition metal-based oxides, metal phosphides, and chalcogenides are extensively investigated as OER catalysts over past years.7-13 Among them, nickel or cobalt is commonly needed for efficient OER electrocatalysts due to their appropriate electronic states for binding intermediates. Bimetal compounds are further developed to balance the adsorption/desorption of reaction species for advancing OER activity.14-15 However, the state-of-the-art OER electrocatalysts are still Ni-rich or Co-rich.16-17 Although Fe is tens to hundreds of times cheaper than Ni or Co and the second most earthabundant metal (nearly 5.0% in earth's crust, Figure S1) behind aluminum, Fe-based materials still exhibit low OER activity compared with Ni or Co-based counterparts.18 Understanding the underlying limits and exploring highly efficient and stable all-Fe or Fe-rich OER electrocatalysts are highly desirable and will advance the field.19-21 Metal sulfides or selenides usually exhibit higher OER activity than oxides/(oxy)hydroxides, although they are believed to be completely or partly converted to oxides/(oxy)hydroxides during OER process, leading to the

similar catalytically active centers in these catalysts, i.e. metal centers in form of oxides/(oxy)hydroxides.22-24 What causes the difference in OER activity is still unclear. Despite the fact that Se-doped NiP2 and MoS2 have demonstrated the enhanced electrocatalytic activity for hydrogen evolution,25-26 less attention has been paid on the role of sulfur or selenium in these OER catalysts. Among Fe-based OER electrocatalysts, FeOOH commonly acts as the real active center and is stable during OER operation although its OER activity still far lags behind Ni- or Co-based electrocatalysts. Therefore, understanding the roles of chalcogen elements in highly efficient chalcogenide-derived OER electrocatalysts and developing feasible strategy to significantly improve the OER activity of Fe-based electrocatalysts will advance the knowledge in the field and the application of low-cost Fe-based materials in water splitting. In this work, we focus on understanding whether and how non-metal elements promote the OER activity of FeOOH. Theoretical calculation is firstly conducted to understand the underlying reasons for low catalytic activity of FeOOH and discovers that Se-doping can drastically lower the energy barrier of rate-determining step during OER. Bearing such highly active site in mind, we develop a facile on-site electrochemical activation strategy from FeSe precatalyst for achieving such Sedoped FeOOH electrode. Electrochemical conversion in OER condition not only makes electrode preparation simpler and easier for mass production, but also can achieve 3D vertical nanosheet-like Se-doped FeOOH structure, where Se-doping boosts the intrinsic OER activity of FeOOH while nanosheet

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arrays provide a large quantity of accessible active sites and prompt gas release. As a result, such full-Fe electrocatalysts exhibits a high catalytic activity, competing with or even outperforming those traditional Ni or Co based counterparts. The systematic investigations and control experiments reveal that Se-doping does greatly enhance the OER catalytic activity of FeOOH. Furthermore, by optimizing the electrode structure, an industrial-level OER current output of 500 mA cm-2 is secured at a low overpotential of 348 mV. The application of such OER electrode in a water-alkali electrolyzer as well as a practical solar-driven water splitting system demonstrates a high and stable solar-to-hydrogen efficiency of 18.55%, making the present strategy promising for exploring new earthabundant, cost-effective and highly active electrocatalysts for clean hydrogen production. 2.

Experimental Section

Chemicals and Materials: All reagents are of analytical grade and were used without further purification. Iron foam (IF) and nickel foam (NF) were purchased from Kunshan Kuangxun Electrical Co Ltd. Sodium borohydride, nickel(II) chloride hexahydrate, ammonium molybdate tetrahydrate, nickel nitrate hexahydrate, iron chloride hexahydrate, sodium chloride, carbon disulfide, potassium hydroxide, and commercial IrO2 nanoparticles were obtained from Alfa Aesar. Se powder was purchased from Acros Organics. Ethanol and acetone were purchased from Beijing Chemical Works. Milli-Q water (18.2 MΩ cm at 25 oC) was used for experiments. Preparation of FeSe/IF: A piece of IF (20 × 30 mm, 1.9 mm in thickness) was ultrasonically cleaned in ethanol, acetone, and then deionized water for 30 min prior to use. For the preparation of NaHSe solution, 7.5 mmol Se powder was added into deionized water (15 mL) containing 17.2 mmol NaBH4 under N2 flow. After gentle stirring for several minutes, a clear NaHSe solution was obtained. Then the solution was transferred into 25 mL Teflonlined stainless steel autoclave with a piece of pretreated IF and maintained at 140 oC for 12 h in an oven. After the autoclave cooled down slowly to room temperature, the sample was taken out and washed with water and ethanol thoroughly, followed by vacuum drying. Preparation of FeOOH(Se)/IF: The FeOOH(Se)/IF was obtained by in-situ electrochemical oxidizing FeSe/IF in 1 M KOH solution at a constant current density of 10 mA cm-2 for 4 h. Preparation of FeOOH/CC: A piece of carbon cloth (20 × 30 mm) was ultrasonically cleaned in ethanol, acetone, and deionized water for 30 min prior to use. A clean carbon cloth was then put into a Teflon-lined stainless autoclave (50 mL) with 35 mL aqueous solution containing 3.6 mmol FeCl3·6H2O and 19.8 mmol NaCl. The autoclave was maintained at 120 oC for 12 h and then cooled down to the room temperature naturally. The product was taken out and washed with water and ethanol thoroughly, followed by vacuum drying. Preparation of FeOOH(Se)/CC: A piece of as-prepared FeOOH/CC and 2.5 mmol Se powder were placed at separate positions in a ceramic boat containing Se powder at the upstream side. After flowing with Ar, Se powder was elevated to 300 oC at a ramping rate of 10 oC min-1. FeOOH/CC was simultaneously heated at 150 oC with a heating rate of 5 oC min-1 and kept for 10 min. After cooling down slowly to room temperature, the sample was taken out and washed with carbon disulfide, water, and ethanol thoroughly, followed by vacuum drying.

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Preparation of NiFeSe/IF: A piece of as-prepared FeSe/IF was immersed into 25 mL nickel chloride solution (15 mM) and reacted at room temperature for 10 min. The bulk material was then taken out and washed with water and ethanol several times, then dried at room temperature. Preparation of FeNiOOH(Se)/IF: The FeNiOOH(Se)/IF was obtained by in-situ electrochemical oxidizing NiFeSe/IF in 1 M KOH solution at a constant current density of 10 mA cm-2 for 4 h. Preparation of MoNi4/MoO2/NF: A piece of nickel foam (10 × 30 mm, 1.9 mm in thickness) was ultrasonically cleaned in ethanol, acetone, and deionized water for 30 min prior to use. A piece of pretreated nickel foam was immersed into 15 mL of aqueous solution containing 0.04 M, Ni(NO3)2·6H2O and 0.01 M (NH4)6Mo7O24·6H2O in a Teflon autoclave. The autoclave was sealed and maintained at 150 oC for 6 h in an oven to obtain NiMoO4 cuboids on nickel foam (NiMoO4/NF). Finally, NiMoO4/NF were annealed at 500 oC for 2 h in H2/Ar (v/v, 10/90) atmosphere to achieve MoNi4/MoO2/NF. Characterizations: Field-emission scanning electron microscopic (FE-SEM) images were taken on a Hitachi S-4800 at an acceleration voltage of 10 kV. Transmission electron microscopic (TEM) images and corresponding elemental distribution analyses were performed on a JEM-2100F equipped with an energy dispersive X-ray spectrometer, working on an acceleration voltage of 200 kV. The surface elemental information was analyzed by Xray photoelectron spectroscopy (XPS) performed on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. The binding energies for all spectra were calibrated with respect to C 1s line at 284.8 eV. Powder X-ray diffraction (XRD) patterns were recorded on a Regaku D/Max-2500 diffractometer equipped with a Cu Kα1 radiation (λ=1.54056 Å). Electrochemical Measurements: Oxygen evolution reaction (OER) and hydrogen evolving reaction (HER). All electrochemical measurements were carried out on an electrochemical workstation (Autolab PGSTAT 302N, Metrohm, The Netherlands) using a conventional three-electrode cell in 1.0 M KOH electrolyte. The as-prepared electrodes, Hg/HgO electrode (1 M NaOH), and carbon rod were used as working electrode, reference electrode, and counter electrode, respectively. All measured potentials were reported versus reversible hydrogen electrode (RHE) according to the equation: ERHE=EHg/HgO + 0.098 + 0.059 × pH The linear sweep voltammetry (LSV) curves were recorded at a scan rate of 2 mV s-1 with 85% iR-compensation except for overall water splitting. The long-term durability test was performed using chronopotentionmetry method at a constant current density. The Cdl values for as-prepared working electrodes were determined from the cyclic voltammogram (CV) in the double layer region (without faradaic processes) at different scan rates. In order to exclude the influence of ECSA on the performance comparison, the OER curves were normalized by ECSAs. The ECSA-normalized current density for as-prepared catalysts was calculated by: ECSA-normalized current density = current density × Cs/Cdl where Cs is the specific capacitance. In this work, 0.04 mF cm-2 was adopted as the value of Cs based on previously reported OER catalysts in alkaline solution.15 Overall water splitting measurements were performed in a twoelectrode system consisting of our FeNiOOH(Se)/IF as anode and MoNi4/MoO2/NF as cathode. The LSV curve for overall water splitting was recorded in 1.0 M KOH at a rate 2 mV s-1 without iRcompensation. All the electrocatalysts and electrochemical tests


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have been repeated for at least five times to make sure that all reported results are reproducible and reliable. Theoretical calculations: The model structure of FeOOH was derived from the previous report.27 In this work, the spin-polarized density functional theory (DFT) were performed by using the Vienna Ab Initio Simulation Package (VASP) code.28-30 The cut-off energy was set at 400 eV. The projector augmented wave (PAW) method was used to describe the ionic cores.31 The electronic exchange-correlation energy was described by the Perdew-BurkeErnzerhof (PBE) functional within the generalized gradient approximation (GGA).32 A Monkhorst-Pack 3 3 1 k-point grid was used to sample the Brillouin zone. The convergence criterion for the electronic structure iteration was set to 10-4 eV, and 0.02 eV/Å for the force. The optimized geometry contained a 2 2 unit cells with 15 Å vacuum in the z direction. The following mechanisms for oxygen evolution reaction (OER) were considered in our calculations:33 (1) *+ 2H2O → OH* + H2O + H+ + e(2) OH* + H2O + H+ + e- → O* + H2O + 2(H+ + e-) (3) O* + H2O + 2(H+ + e-) → OOH* + 3(H+ + e-) (4) OOH* + 3(H+ + e-) →OO* + 4(H+ + e-) where * refers to a possible active site in the specific model. The standard Gibbs free energy change was shown in the following equation: ΔG = ΔE + ΔZPE – TΔS (5) where the ΔE, ΔZPE, ΔS are for the reaction energy, the change in zero point energy, and the change in entropy, respectively. The entropy and zero point energy were obtained by frequency calculation at 300K.

3.

Results and Discussion

Figure 1. (a) Structures of FeOOH and various Se-doped FeOOH models. Fe atom (orange), O atom (red), Se atom (green). (b) The free energy profile at 1.23 V for OER pathway on bare FeOOH and various Se-doped FeOOH models.

Theoretical calculation is firstly carried out to investigate the influence of Se-doping on the catalytic activity of FeOOH for OER. Se-doping is simulated by replacing O on the top surface and in the subsurface of FeOOH with Se, which are denoted as FeOOH-Se1 and FeOOH-Se2, respectively. During the OER process, the various intermediates such as M-OH, M-O, MOOH, or M-OO are formed. The optimized structures for Sedoped FeOOH and intermediates on catalysts are presented in Figure 1a and S2, respectively. The energy-change profile in Figure 1b for each intermediate on FeOOH, FeOOH-Se1, and

FeOOH-Se2 shows the energy barrier for each reaction step. The step M-O to M-OOH exhibits the largest energy barrier for three models (Table S1), indicating it is the rate-determining step (RDS) for OER. This is consistent with previous reports.34 For pristine FeOOH, The energy barrier of RDS is 3.20 eV. This value is substantially reduced to 1.45 eV for FeOOH-Se1 and 1.83 eV for FeOOH-Se2, suggesting Se-doping on FeOOH significantly facilitate OER process. It is also seen that Sedoping can lower the energy barrier of the step M-OH to M-O. These results predict that Se-doping on FeOOH should be able to boost its OER activity. Besides the intrinsic activity of catalytic sites, rational design of electrode structure to have abundant accessible active sites, sufficient conductivity and mass transfer are other requisites to achieve highly efficient and stable water electrolysis.35 We report here a new strategy of fabricating Se-doping FeOOH nanosheet arrays on low-cost iron foam (IF) by on-site electrochemically conversion of iron selenide (FeSe) precatalyst to fulfill these requirements. FeSe is an intrinsically conductive layered material (Figure S3), which can be easily prepared from Fe metal in form of sheet-like structure.36 In brief, FeSe nanosheet arrays on IF (FeSe/IF) were firstly prepared by using commercial IF as Fe source to react with NaHSe under hydrothermal condition. After reaction, the metal luster of IF substrate changes into black (Figure S4). The X-ray diffraction (XRD) pattern of FeSe/IF in Figure 2a shows a set of diffraction peaks at 28.5o, 37.3o, 48.1o, and 70.5o etc. from tetragonal FeSe phase (JCPDS No. 03-0533) except for the peaks at 44.7o and 65.0o from Fe foam (JCPDS No. 06-0696), suggesting the formation of FeSe on IF. The intensities of FeSe diffraction peaks increase with the synthetic temperature (Figure S5), indicating higher synthetic temperature favors the growth of FeSe. The product prepared at 140 oC presents uniform FeSe nanosheet arrays (Figure 2b and Figure S6). The nanosheet thickness is about 15 nm. The morphology and structure of FeSe are confirmed by transmission electron microscopy (TEM) as shown in Figure S7. The high-resolution TEM (HRTEM) image demonstrates a lattice fringe spacing of 0.54 nm (figure 2c), which agrees with the distance of (001) planes of FeSe. The fast Fourier transform (FFT) pattern further confirms the product is FeSe (Figure S8). Energy-dispersive X-ray spectrum and mapping images (EDS, Figure S9) indicate that Se and Fe are distributed uniformly in the nanosheet with an atomic ratio of Fe to Se of ~1:1, corroborating the formation of FeSe. X-ray photoelectron spectroscopy (XPS) measurement was further conducted. As shown in Figure S10, the Fe 2p3/2 XPS spectrum shows a main peak at 710.5 eV, which is assigned to Fe2+ in FeSe.37-38 Moreover, Se 3p XPS spectrum exhibits the typical Se2- signal in metal selenides.39-40 These evidences together with XRD and SEM results evidence the formation of FeSe. Electrochemical conversion was on-site carried out in OER condition by applying a constant current density at 10 mA cm-2 for 4 h. XRD pattern (Figure 2a) indicates that the diffraction peaks of FeSe disappear after the conversion although the nanosheet morphology is well preserved as indicated by SEM image (Figure S11). High-resolution TEM image (Figure 2d) displays that the well-crystallized layered structure of FeSe turns into disordered structure. The lattice fringes in a very short range distinguishable in few regions give a spacing of 0.25 nm, corresponding to the distance of (031) planes of FeOOH.



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Figure 2. (a) XRD patterns of FeSe/IF and FeOOH(Se)/IF. (b) SEM image of FeSe/IF. (c) HRTEM image of FeSe/IF. (d) HRTEM image of FeOOH(Se)/IF (inset: FFT pattern). (e) Raman spectrum and (f-i) TEM image and corresponding elemental mapping images of FeOOH(Se)/IF. The FFT pattern shows barely discernible diffraction rings (inset in Figure 2d), indicating the poor polycrystalline nature of the nanosheet. Raman spectrum in Figure 2e exhibits five main peaks at 249, 310, 379, 422, and 473 cm-1, all of which can be well assigned to the typical vibration peaks of FeOOH.41-42 These results suggest that the crystalline FeSe nanosheets are converted into FeOOH nanosheets in low crystallinity via electrochemical oxidation. The sample is thus denoted as FeOOH(Se)/IF. Energy-dispersive X-ray spectroscopic (EDS) mapping images in Figure 2f-i displays the uniform distribution of Fe, O, and Se elements. However, in contrast to 1 : 1 ratio of Fe and Se in FeSe, EDS spectrum (Figure S12) clearly shows after electrochemical conversion the sample it is mainly composed of Fe (37.7 at%) and O (60.8 at%) with a small amount of Se (1.5 at%). The result corroborates the transformation of FeSe to Se-doped FeOOH. X-ray photoelectron spectroscopic (XPS) spectra was further recorded (Figure S13). The shift of Fe 2p3/2 XPS peak from 710.5 to 711.3 eV implies the oxidation of Fe2+ in FeSe into Fe3+ in FeOOH.37 The weak Se2- peak at 161.0 eV is consistent with Se doping in FeOOH. The electrochemical surface areas (ECSA) of samples are evaluated by measuring the electrochemical double layer capacitances (Cdl) in 1 M KOH via cyclic voltammetry at different scan rates (Figure S14). The Cdl of FeOOH(Se)/IF is 1.22 mF cm-2, higher than 0.87 mF cm-2 for FeSe/IF and 0.54 mF cm-2 for IF, indicating the electrochemical conversion of crystalline FeSe into Se-doped FeOOH indeed increases the number of active sites. As expected, the catalytic activity is significantly improved after electrochemical activation even with ECSA normalization (Figure S15). The OER polarization

curves in Figure 3a show that FeOOH(Se)/IF exhibits a high catalytic activity with low overpotentials of 287 and 364 mV at 10 and 100 mA cm-2, respectively, outperforming commercial IrO2 on IF (IrO2/IF) and bare IF (Figure 3a). It also displays high reaction kinetics with a Tafel slope of 54 mV dec-1, lower than 60 mV dec-1 of IrO2/IF and 76 mV dec-1 of bare IF (inset in Figure 3a). This indicates the Se-doped FeOOH nanosheets should be responsible for the high electrocatalytic activity of FeOOH(Se)/IF. As an unary Fe-based OER catalyst without Ni or Co element, FeOOH(Se)/IF can compete with or even outperform those traditional Ni or Co based counterparts as well as other previously reported unary Fe-based materials (Figure 3b and Table S2). Besides the activity, the durability of FeOOH(Se)/IF is evaluated by chronopotentiometric measurement at 10 mA cm-2. FeOOH(Se)/IF exhibits almost unchanged overpotential after 15 h test (Figure 3c). Multi-step chronopotentiometric curve (Figure S16) at a varying current density from 20 to 100 mA cm-2 further confirms the excellent durability of FeOOH(Se)/IF. The XRD, SEM, TEM, Raman, and EDS results suggest that the morphology and composition of FeOOH(Se)/IF are well maintained after durability test, explaining its stability (Figure S17). In order to directly support the activity enhancement of FeOOH by Se doping, pure FeOOH nanorods on carbon cloth (CC) are prepared and post-doped by Se, denoted as FeOOH/CC and Se-FeOOH/CC, respectively. XRD and SEM results in Figure S18 show that no impurities and morphology change are found after Se doping. XPS results reveal the successful doping of Se (1.9 at%) in FeOOH nanorods (Figure S19). Electrochemical measurements indicate that Se-doped FeOOH/CC shows a 42 mV lower overpotential than undoped


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Figure 3. (a) LSV curves of as-prepared catalysts (inset: Tafel plots). (b) Overpotential comparison of FeOOH(Se)/IF and state-of-the-art unary metal-based OER electrocatalysts. (c) Chronopotentiometric curve of FeOOH(Se)/IF. (d) ECSA-normalized LSV curves.

FeOOH/CC at 10 mA cm-2 (Figure S20), suggesting that Sedoping significantly improves OER activity. To compare the intrinsic activity, the LSV curves are normalized by ECSAs (Figure S21). It can be seen from Figure 3d that the catalytic activity of FeOOH is much improved by Se doping. This result directly supports that Se-doping in FeOOH is an effective method to improve its catalytic for OER, providing a new option for designing efficient OER catalysts without using Ni or Co. The resulting highly active and stable FeOOH(Se)/IF could also serve as an active electrode to couple with other elements towards more active catalysts for OER. It has been reported that the bimetal or multi-metal catalysts generally exhibit enhanced performance compared with the unary metal catalysts for OER due to the interplay of additional metals in finely tuning the electronic properties and the adsorption energies of the intermediates.43-44 Thus, the asprepared FeSe/IF is further coupled with second metal such as Ni but in a trace amount. It is done by a facile partial cation exchange of FeSe/IF in Ni2+ solution at room temperature (denoted as NiFeSe/IF), followed by the same electrochemical activation process to achieve Ni doping in FeOOH(Se) (denoted as FeNiOOH(Se)/IF). Similar to FeOOH(Se)/IF, XRD pattern (Figure S22) suggests the low crystallinity of FeNiOOH(Se) since no clear diffraction peak is observed except for the ones at 44.7o and 65.0o for Fe foam substrate (JPCDS No. 06-0696). SEM image shows that the morphology of nanosheets is preserved after Ni doping (Figure 4a). The lattice fringes could hardly be seen in HRTEM image,

corroborating the low crystallinity (Figure S23). The typical peak of Ni3+ at 856.0 eV in Ni 2p3/2 XPS spectrum (Figure S24) confirms the incorporation of Ni. The Ni doping content is about 3.3 at%. The OER activity of FeNiOOH(Se)/IF is evaluated in 1 M KOH as shown in Figure 4b. Compared with FeOOH(Se)/IF, it exhibits greatly enhanced catalytic performance in terms of low overpotentials of 222, 261, and 279 mV at 10, 50, and 100 mA cm-2, which is 65, 73, and 85 mV lower than FeOOH(Se)/IF, respectively. To exclude the influence of ECSA on analysing the catalytic activity, the ECSA-normalized LSV curves are plotted in Figure S25. It is clear that the intrinsic activity of FeOOH(Se)/IF is much improved by Ni doping. Such performance of our Fe-rich catalyst (3.3 at% Ni) are comparable to or outperform recently developed Ni-rich NiFe composites (Figure 4c and Table S3), such as Ni2Fe1O (η50=273 mV),45 NiFeOx/CFP (η50=265 mV),46 Sandwich-like NiFe/C (η50=275 mV),47 NiFe-LDH/DG (η50=310 mV),48 49 Ni1.5Fe0.5P/CF (η50=300 mV), HG-NiFe (η50=350 mV),50 NiFeP (η50=285 mV),44 NiFe/NF (η50=265 mV),51 Ni3FeNNPs (η50=360 mV),12 and Ni-Fe-Se cages (η50=265 mV).52 Besides high OER activity, chronopotentiometric test was performed to check the stability of FeNiOOH(Se)/IF. Almost unchanged overpotential is observed during consecutive current output at 100 mA cm-2 for 100 h (Figure 4d). Moreover, the Ni doping content also influences the OER activity. As shown in Figure S26, OER activity of FeNiOOH(Se)/IF increases as the Ni doping amount increases from 2.2 to 3.3 at%



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Figure 4. (a) SEM images of FeNiOOH(Se)/IF. (b) LSV curves of FeNiOOH(Se)/IF. (c) Overpotential comparison of FeNiOOH(Se)/IF and state-of-the-art OER catalysts. (d) Chronopotentiometric curve of FeNiOOH(Se)/IF.

Figure 5. (a) LSV curve and photo of the electrolyzer. (b) Long-term durability of overall water splitting at a current density of 100 mA cm-2. (c) J-V curves of the solar-driven electrolyzer. (d) Time-dependent current output without external bias


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but does not improve much when it further increases to 5.1%. Such excellent OER performance inspires us to assemble a water-alkali electrolyzer to evaluate its potential for practical overall water splitting. A previously reported MoNi4 on MoO2 cuboids (denoted as MoNi4/MoO2/NF) is synthesized for hydrogen evolution reaction (HER).53 As shown in Figure S27, MoNi4/MoO2/NF requires a very low overpotential of 15 mV to deliver 10 mA cm-2 for HER. A two-electrode water electrolyzer is assembled using MoNi4/MoO2/NF as cathode electrode and FeNiOOH(Se)/IF as anode electrode. Its optical photo during the operation at 100 mA cm-2 is shown in the inset of Figure 5a. The polarization curve shows that the electrolyzer needs a very small cell voltage of 1.55 V and 1.62 V to deliver a current density of 20 mA cm-2 and 50 mA cm-2, respectively (Figure 5a). Such a low overpotential for overall water splitting is impressive and outperforms most of the recent state-of-theart materials (Table S4). Moreover, the continuous operation at 100 mA cm-2 indicates that the cell voltage shows negligible loss in 50 h, suggesting the superior durability (Figure 5b). Furthermore, we integrate such water splitting electrolyzer with a commercial GaAs solar cell to demonstrate a solar-driven water splitting system. The J-V curves in Figure 5c present a high solar-to-hydrogen efficiency of 18.55% with continuous hydrogen and oxygen bubbles released (Movie S1). Such efficiency is higher than most of other reported catalysts (Table S5). The solar driven current output shows no decay in consecutive 3600 s operation (Figure 5d), demonstrating the feasibility of the developed Fe-rich OER electrode in a practical unbiased solar water splitting system. 4.

Conclusion

In summary, we discovered that non-metal element Se doping could enhance the OER activity of FeOOH by theoretical analysis and developed a facile on-site electrochemical activation method to achieve such Se-doped FeOOH electrode for efficient OER. By pre-preparing FeSe nanosheet arrays on IF and subsequently converting FeSe into amorphous Se-doping FeOOH during OER process while keeping the morphology of 3D vertical nanosheet array for prompt gas release, a highly active low-cost full-Fe OER electrode was achieved. The systematic investigations and control experiments revealed that Se-doping did greatly enhance the catalytic activity of FeOOH. Such strategy worked also on Ni-doped FeSe pre-catalyst, giving an electrode capable of delivering an industrial-level OER current density of 500 mA cm-2 at a low overpotential of 348 mV at a cost of only 3.3 at% Ni doping. This electrode was further used in a water-alkali electrolyzer, presenting the stable hydrogen production with a small cell voltage of 1.62 V at 50 mA cm-2. Finally, a practical solar-driven water splitting system was demonstrated to steadily convert solar energy to hydrogen with a high efficiency of 18.55%, suggesting that the developed strategy may open up an avenue for the exploration of new earth-abundant, cost-effective, and highly efficient electrocatalysts for clean hydrogen production.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

This file includes Experimental section, Figure S1-S27, and Table S1-S5.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions ∇These

authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the financial support from the National Key Project on Basic Research (2015CB932302), the National Natural Science Foundation of China (91645123 and 21773263), the National Postdoctoral Program for Innovative Talents (BX2001700250). We thank Dr. Fen Liu, Dr. Zhijuan Zhao, Xiaoyu Zhang for their help in XPS analysis, Yang-Sun for XRD analysis, Dr. Bo Guan and Ji-Ling Yue for SEM.

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