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Overall Water Splitting using Room-Temperature Synthesized NiFe Oxyfluoride Nanoporous Films Kun Liang, Limin Guo, Kyle Marcus, Shou-Feng Zhang, Zhenzhong Yang, Daniel E. Perea, Le Zhou, Yingge Du, and Yang Yang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02991 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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Overall Water Splitting using Room-Temperature Synthesized NiFe Oxyfluoride Nanoporous Films Kun Liang1,#, Limin Guo1,#, Kyle Marcus2,#, Shoufeng Zhang3,#, Zhenzhong Yang4, Daniel E. Perea5, Le Zhou2, Yingge Du4 & Yang Yang1, 2,* 1

NanoScience Technology Center, University of Central Florida, Orlando, FL 32826, USA.

2

Department of Materials Science and Engineering, University of Central Florida, Orlando, FL

32826, USA. 3

Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry,

Jilin University, Changchun 130023, P.R. China 4

Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory,

Richland, WA 99352, USA. 5

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory,

Richland, WA 99352, USA. #

These authors contributed equally to this work.

*

E-mail: [email protected]

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Abstract: A room-temperature synthesis of NiFe oxyfluoride (NiFeOF) holey film (HF) using electrochemical deposition and anodic treatments have been developed in this work. The developed room-temperature synthesis route can preserve the fine nanoporous structure inside the HF, providing high surface area and abundant reaction sites for electrocatalytic reactions. Both computational and experimental studies demonstrate that the developed NiFeOF HF with highly porous structure and metal residuals can be used as high-efficiency and bifunctional catalyst for overall water splitting. Simulation result indicates that the exposed Ni atom on the NiFeOF surface serves as the active site for water splitting. Fe-doping can improve the catalytic activity of the Ni active site due to the partial charge-transfer effect of Fe3+ on Ni2+. The electrochemical performance of NiFeOF catalyst can be experimentally further enhanced through the improved electrical conductivity by the residual NiFe alloy framework inside the HF. The synergistic combination of NiFeOF HF properties results in a highly efficient electrochemical catalyst, showing an overall water splitting.

Keywords: NiFe oxyfluoride; nanoporous; room-temperature synthesis; bifunctional catalyst; water splitting

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Energy crises and environmental issues are critical challenges, due to limitations of fossil fuel and population growth.1-2 To address these problems, renewable energy generation and storage systems are attracting more attention.3-6 Presently, water electrolysis is a clean and promising technology for generating pure hydrogen fuel gas from an abundant source at a low-cost. Although, the complex electron transfer process and sluggish kinetics by hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) has constrained large-scale application of water electrolysis because there is a lack of high-efficiency catalysts.7-8 Currently, noble metals and their oxides, such as Ru-, Ir- and Pt-based materials, still dominate as high-performance electrodes for such applications.6, 9-11 However, these materials are scarce and have a high-cost that ultimately hinders large scale applications. Meanwhile, powder materials are conventionally used to produce electrodes, even though they have several unaddressed issues: i) the use of binders and other organic additives may lead to increased internal resistance, reduced lifetime and undesirable side reactions within electrochemical systems; ii) electrochemically inactive current collectors used to support electrode materials reduce electrochemical performance of the catalysts; iii) powder materials can be difficult to recycle, further affecting the environment. To address these issues, additive-free and reusable thin-film materials are developed as electrodes for renewable energy applications. Carbon-based thin-films are widely investigated for renewable energy applications because they contain a considerable amount of active sites towards electrochemical reactions unlike conventional metal thin-films.10-12 Jia et al. investigated graphene films as tri-functional catalysts for hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).13 However, these carbon-based thin-film electrodes have limited applications that originate from their reliance on additives and binders that can lead to catalyst deactivation

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issues, when compared with metal compound thin-films14-15. Oxyfluoride compounds are a special composite of oxides and fluorides that display a narrow phonon spectrum, high luminescence quantum yield and long lifetime in a metastable state.

16-18

Additionally, fluorine and oxygen, due to the similar ionic radii, can be substituted

with one another more easily, consequently having dramatic effects. Metal oxyfluorides are present in a range of applications such as ceramic glasses, laser cooling systems, optical amplifiers and lithium-ion batteries. 19-21 However, it is difficult to prepare metal oxyfluorides at room temperature, due to the difficult task of stabilizing two hetero anions that potentially form highly non-equilibrium structures.

22,23-26

Numerous studies have demonstrated that Ni and Fe

corresponding materials can be employed as catalysts to achieve high OER/HER performance, such as NiFe layered double hydroxide27, NiFe hydroxides11, NiFe alloyed nanoparticles28, NiFe oxyhydroxide29. Although it is evident Fe is essential to improve the electrochemical activity, the roles of Fe in catalysts are still under debate. It is not clear what degree these structural changes are due to unintentional Fe incorporation and how structural changes affect activity in the absence of Fe. Previous works have been performed to find the effect of catalyst composition on electrochemical performance. Friebel et al. achieved a 500-fold OER activity enhancement using mixed (Ni, Fe) oxyhydroxides by experimental and computational method30. Trotochaud et al. found that the absorption of Fe impurities was responsible for the dramatic increase in OER activity31. In order to deep understanding the role of Fe in NiFeOF catalyst, a systematic research was performed by control the Fe concentration. In this work, a facile electrochemical technique was employed to develop NiFe oxyfluoride (NiFeOF) holey film (HF), which can be directly utilized as a binder-free catalyst for full water splitting. As far as we know, this is the first report to

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investigate electrocatalytic performance using binary metal oxyfluoride. The as-prepared HF retained a vast interconnected holey structure, which provided greater electrochemically active surface area and abundant reaction sites, enhancing the electrochemical performance. The residual NiFe alloy framework improved electrical conductivity of the catalyst, facilitating electron transfer and kinetic diffusion. The synergistic combination of NiFeOF HF properties results in a high-efficient electrochemical catalyst, showing that it is a promising alternative for water electrolysis. A typical NiFeOF HF fabrication process is schematically illustrated in Fig. 1a (for more details see Experimental Section): (i) NiFe thin-film with a thickness of 2 µm was electrochemically deposited in a plating electrolyte onto a conductive substrate. (ii) A NiFe thinfilm was achieved after removing the substrate. (iii) Freestanding NiFeOF HF was obtained by electrochemical anodic treatment. As shown in Fig. 1b and c, the cross-sectional and top-view scanning electron microscopy images present a holey structure with pore size distributed in a range between 20-50 nm, revealing a hierarchically mesoporous HF. The morphology and structure of the HF were further examined by transmission electron microscopy (TEM) as shown in Figure S1. A highly porous structure similar with the surface image morphology can be observed. High-resolution TEM (HRTEM) images in Figure 2a and b clearly show lattice fringes of the NiFe alloy and an amorphous NiFeOF phase was well distributed (NiFe alloy/NiFeOF boundaries are highlighted by yellow dashed lines). The NiFe alloy phase forms an interconnected framework that directly contacts the amorphous NiFeOF, facilitating proper catalyst conductivity. Atom probe tomography (APT) has emerged as an excellent characterization technique, providing 3D nanoscale models with sub-nanometer resolution and equally sensitivity for

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elemental composition. In Figure 2c, the reconstructed 3D compositional map is shown, where each colored sphere represents the relative position of a corresponding element. From the 3D composition map, two compositionally-distinct regions are clearly observed: 1) the NiFeOF cap which is due to anodization treatments;32-33 2) the NiFe framework (Note: the cap dimension is not accurate. This could be due to the APT cap possibly being damaged by FIB process, therefore, reducing the oxidized region size. More details see Figure S2 and Movie 1-2). The NiFe framework is found to be present within the NiFeOF HF interior, consistent with TEM results. F and Fe enriched layers were isolated as isoconcentration surfaces from the reconstructed 3D compositional map, as shown in Figure 2d, which to our understanding, is the first time such isolation layers are observed. The oxyfluorides region exhibited complex morphologies with finger-like protrusions that integrated into the HF and Fe enriched regions, forming a distinct continuous layer, creating more defects and enhancing conductivity.34-36 The amorphous NiFeOF HF feature was also determined by X-ray diffraction (XRD) and Raman testing, as displayed in Figure S3. It is apparent that the as-prepared sample presents only strong peaks contributed by the deposited NiFe alloy, further confirming that the NiFeOF regions are amorphous. The Ni-Fe oxides NPL composition was investigated by X-ray photoelectron spectroscopy (XPS). As presented in Figure S4, Ni, Fe, O and F were detected, indicating that a NiFeOF composite was obtained. Further XPS deep profile analysis was carried out to identify the chemical distribution from top to bottom. It is noted that Ni(OH)2 covered the top layer of the porous film, though NiFeOF and NiFe alloy regions mainly contributed to the deeper layers (for more details see XPS analysis). It is important to note that a greater amount of the Ni phase is observed at deeper interior layers (Figure S4f), which provides a greater interconnected conductive framework throughout the entire HF to enhance conductivity.

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The composition and Fe/Ni atomic ratio was examined by energy dispersive spectrometer (EDS) in the HF and can be easily controlled by adjusting the plating bath compositions (Figure S5, at% Fe in plating bath was used to denote different samples in this paper unless otherwise specified). The Fe content in the HF did not increase linearly with increasing Fe bath concentration. A sudden increase was observed when Fe content reached higher than 15 at%, which is attributed to the suppression of Ni deposition by ferrous hydroxide, owing to increased local alkalinity (hydroxide suppression).37 To investigate the HF electrochemically accessible surface area, roughness factor (RF, Figure S6-7) and surface wettability (contact angle, Figure S8-9) measurements were employed. The RF presents a nonlinear variation with increasing bath Fe-content and a maximum value ~ 15 is achieved for 15 at% Fe. Moreover, the HF with 15 at% Fe shows excellent hydrophilicity due to a hydrated surface (See Figure S4d) and optimum wettability properties. Water electrolysis activity using the NiFeOF HF film was first investigated by OER activity in O2-saturated 1 M NaOH aqueous electrolyte using a three-electrode system: freestanding NiFeOF HF as working electrode, Hg/HgO as reference electrode (potentials were converted to reversible hydrogen electrode, RHE, for comparison) and Pt wire as counter electrode. OER testing of the HF with different Fe-contents was performed by iR-corrected linear sweep voltammetry (LSV) curves (Figure 3a and Figure S10a) at a scan rate of 5 mV s-1.

An

oxidization peak with an anodic sweep can be found from the HF, which is due to the oxidation of NiII to NiIII .38 The HF with 15 at% Fe exhibited a remarkably higher current density and lower onset overpotential (ηonset) of 207 mV compared to other HF with different Fe-contents (Figure S11). The (15 at% Fe) HF requires an overpotential of only 295 mV to reach 10 mA cm-2, which is lower than that of IrO2/C and most state-of-the-art OER catalysts (for details see Table S1).39

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Tafel plots (Figure 3b and Figure 10b) show a slope of 38 mV decade-1 for 15 at% Fe, which again is lower than most state-of-the-art OER catalysts and indicates excellent electron transfer facilitation within the freestanding HF. To investigate the amorphous contribution to electrochemical performance, the as-prepared NiFeOF HF was annealed at 250 ºC and 350 ºC. As shown in Figure 3e, the amorphous NiFeOF HF displayed a much lower ηonset and higher current density than crystalline HF, indicating that the amorphous NiFeOF offers significantly enhanced OER activity. Chronopotentiometric testing was used to test long-term stability of the HF electrode at a constant current density of 10 mA cm-2 for 10k seconds (Figure S12). The potential required to deliver 10 mA cm-2 current density maintained at 1.55 V, demonstrating excellent durability of the HF when serving as an OER catalyst. Electrochemical impedance spectroscopy (EIS) analysis performed at η of 350 mV was employed to provide further insight to the OER kinetics (Figure S13, more details see Supporting Information). The HF exhibits a low charge transfer resistance, owing to enhanced conductivity from the residual HF metal-framework. Additionally, this as-prepared binder-free catalyst electrode provides an efficient pathway for electron transport that is evident throughout the entire electrode material. The highly porous structure is also an important contributor to the low resistance by providing sufficient pathways for ion and gas diffusion.40 The small wettability contact angle for the 15 at% Fe HF provides insight of improved electrolyte affinity at the electrode/electrolyte interface and increases the likelihood of ion permeation.39, 41-44 The NiFeOF HF electrocatalytic activity towards HER was investigated by testing LSV in 1 M NaOH electrolyte (Figure 3c and Figure S14a). The ηonset and η to reach 10 mA cm-2 are estimated to be 56 mV and 253 mV, respectively, in the HF with 17.5 at% Fe, offering a distinguishable reduction in overpotentials to that of other HF (Figure S15). Moreover, the HF

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with 17.5 at% Fe also exhibits a lower Tafel slope of 96 mV decade-1, considerably outperforming other HER catalysts used in alkaline electrolytes (See Table S2). EIS analysis (Figure S16) indicates that the HF with 17.5 at% Fe shows smaller polarization and chargetransfer resistance owing to a facilitated HER kinetics (more details see Supplementary Information). The HF long-term electrochemical stability toward HER (Figure S17) was further investigated in 1 M NaOH electrolyte, showing a stable performance at 10 mA cm-2 for 10k seconds. The bi-functional activities (OER and HER) of the HF for full water splitting were investigated using the freestanding NiFeOF HF thin-films with Fe-contents of 15 at%. As shown in Figure 3f, potential of 1.80 V is required to deliver 10 mA cm-2 for overall water splitting using the HF as working electrode in a three-electrode system. The mixed phases of NiFeOF within the HF attribute to high electrocatalytic performance, enabling these freestanding HF to operate bi-functionally for OER and HER. Incorporating Fe into Ni-mixed phases could create more active sites and improve the electrochemical activity. In addition, residual metal-framework enhances electrode conductivity and the porous structure provides abundant active sites for water electrolysis. Strong association of mix phases with metal-framework further facilitated charge transport, improving catalytic performance. Here, to reveal the enhancement effect of Fe as well as F doping, the free energy diagrams of OER/HER processes have been investigated. For OER, we consider the following fourelectron reaction model based on the associative mechanism under alkaline condition: ∆G1

M + OH MOH + e-

(1)

∆G2

MOH + OH MO + H O + e∆G3

MO + OH MOOH + e-

(2) (3)

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∆G4

MOOH + OH M + O2 + H O + e-

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(4)

where M is the adsorbed active site on the catalyst surface and ∆Gj is the Gibbs free energy for reaction sub-step j. Notably, all sub-steps of OER are endergonic and hence the sub-step with the largest ∆Gj should be the rate-determining step. The related limiting potential can be defined as: = ∆G1 , ∆G2 , ∆G3 , ∆G4 /e − 1.23 V

(5)

Herein, the lower should have better catalytic OER performance. Theoretical calculations show that the Ni atom on the oxide surface should be the active site of OER (Figure 4a). For NiO, NiFe oxides and NiFeOF catalysts, the rate determining steps all are the third step (MO → MOOH) with the calculated limiting potentials of 1.074 V, 0.481 V and 0.364 V respectively. With much smaller limiting potential, the Ni atoms connected with Fe in NiFe oxides catalyst are more active than Ni atoms in NiO which should be due to the partial charge-transfer effect of Fe3+ on Ni2+. The most stable doping structures of NiFe oxides demonstrate that the Fe atoms can occupy the previous sites of Ni atoms. Consequently, the efficiency of NiFe oxides is increasing first as the increasing of Fe component because more Ni atoms are activated. On the ideal point that all Ni atoms are activated (15 at% Fe), the catalyst has the best performance of OER. After such a turning point, the previous active sites on the catalyst surface will be replaced by passivated Fe atoms and hence the efficiency will be decreasing gradually due to the low density of active sites. Moreover, the further smaller limiting potential after the introduction of F atoms signifies that the role of F is to increase the number of active sites due to its strong electronwithdrawing effect which renders more partial charge-transfer. For HER, the reaction model can be defined as follows with Volmer-Heyrovsky mechanism in alkaline conditions: ∆GH

M + H O + e MH + OH-

10


(6)

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∆GH

MH + H O + e M + H + OH-

(7)

It is known that the nearly zero free energy describes the optimal adsorption conditions for catalysis. As shown in Figure 4b, the hydrogen bonding energy are negative for all systems. Compared to NiO (−∆G = 0.275 !), the free energy of adsorption for NiFe oxides (17.5 at% Fe) becomes much close to the optimum value (−∆G = 0.192 !) and hence is more amenable for HER. This clearly demonstrates that Fe could enhance the activity of Ni active sites. The introduction of F has further suppressing effect on adsorption energy at −∆G = 0.158 ! , indicating that the higher efficiency can be obtained. It is similar to the OER process that the best performance of HER is the 17.5 at% Fe sample, which is in good agreement with the experimental results. A two-electrode water electrolyzer was developed for full water splitting using NiFeOF HF as both anode and cathode in 1 M NaOH aqueous solution. A voltage of 1.83 V was required to deliver a current density of 10 mA cm-2 for water-splitting with vigorous gas evolution on HF electrodes (Figure 5a and Movie S3), showing more favorable activity than precious metal catalyst. As presented in Figure 5c, the Faradaic efficiency (FE) is 100% for both H2 and O2 evolution, calculated by using the amount of experimentally measured gas divided by the amount of theoretically calculated gas. From the Figure 5d, the HF shows 10k seconds long-term stability in two-electrode water electrolyzer, which is promising for practical applications. Above all, the freestanding NiFeOF HF exhibits excellent electrochemical performance as a catalyst for water splitting, due to following reasons: (i) The vast interconnected holey structure provides greater electrochemically active surface area and abundant reaction sites, enhancing electrochemical performance. (ii) The residual NiFe alloy framework improves electric conductivity of the catalyst, effectively facilitating electron transfer and kinetic diffusion. (iii)

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The anionic substitution of O2- and F- is beneficial for improving electrochemical catalytic performance. The synergistic combination of NiFeOF HF properties results in a high-efficient electrochemical catalyst, showing that it is a promising alternative for water electrolysis. In summary, a room-temperature synthesis was developed to fabricate freestanding NiFeOF holey film in a facile manner. Both computational and experimental studies demonstrates that superior electrocatalytic activities towards both H2 and O2 evolution reactions can be achieved using bifunctional NiFeOF catalysts through tuning the Ni:Fe ratios. Further improved with excellent electric conductivity and high surface area in the vast holey structure, the developed NiFeOF HF presents excellent electrochemical behavior and high-efficiency for practical overall water splitting.

Experimental section Fabrication of NiFeOF HF: All analytical-grade chemicals were purchased from Fisher Scientific and used without further purification. The electrolyte bath was prepared in an aqueous plating solution of 0.3 M NiSO4, 0.01 to 0.1 M FeSO4, 1.0 M H3BO3, 0.05 M NaCl, and 0.010.05 M Saccharin. Electrochemical deposition of NiFe alloy layer was performed with cathode current density of 25 mA cm-2 for 10 min using stainless steel as substrate. Subsequent anodic treatments were performed with a constant voltage of 20 V for 10 min in an electrolyte of 0.1 M NH4F with 1 M deionized water in ethylene glycol. The NiFeOF HF was obtained after washing with deionized water and ethanol, then dried under air gas flow. Characterization: A ZEISS ULTRA 55 scanning electron microscope (SEM) and an FEI TecnaiF30 high resolution transmission electron microscope (HRTEM) was used to analyze the top-view and cross-sectional morphology. A LEAP 4000X-HR atom probe equipped with a 355

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nm UV picosecond pulsed laser from Cameca Instruments, Inc. was employed for Atom Probe Tomography (APT) analysis. X-ray diffraction (XRD) analysis was performed by a PANalytical Empyrean diffractometer (PANalytical B.V.) with a Cu Kα radiation. Raman spectra were recorded with a Renishaw Raman RE01 scope (Renishaw, Inc.) using a 532 nm excitation argon laser. Chemical compositions were checked by X-ray photoelectron spectroscope (XPS, Physical Electronics) with low-resolution survey and high-resolution region scans at the binding energy of, respectively. Wettability (contact angle) property was conducted with a Ramé Hart contact angle goniometry. Electrochemical measurements: Electrochemical performances in 1 M NaOH electrolyte were performed by a CHI 760E electrochemical workstation (CH Instruments) with a three-electrode cell system. The nanoporous thin-film with geometric area of 0.785 cm2 was used as the working electrode, a Hg/HgO electrode was used as the reference electrode and Pt wire as the counter electrode. The electrolyte was saturated with H2 and O2 before/during HER and OER tests, respectively. All polarization curves were converted from linear sweep voltammetry (LSV) measured at 5 mV s-1. Note that all currents presented are corrected against ohmic potential drop and current density are based on projected geometric area of an electrode. Long-term stability tests were performed using galvanostatic method (10 mA cm-2). The potentials reported herein were converted to reversible hydrogen electrode (RHE), and ERHE = EHg/HgO + 0.0591pH + 0.098. The electrochemical impendence spectroscopy (EIS) at different overpotentials was collected by AC impedance spectroscopy in 1 M NaOH aqueous solution. The amplitude of the alternating voltage was 5 mV and the frequency range was from 10 mHz to 100 kHz.

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For overall water splitting tests, NiFe oxyfluoride HF was used as both anode and cathode in a two-electrode cell. Polarization curves were determined using LSV at a scan rate of 5 mV s-1. All data for the two-electrode electrolyzer was collected without iR compensation. Computational methods: The DFT calculations were performed with CASTEP program embedded in MaterialStudio package. The interactions between ionic-cores and the valence electrons are described by the projector-augmented wave (PAW) pseudopotentials. The exchange-correlation interactions are treated using the Pedrew-Burke-Ernzerhof (PBE) functional with the kinetic energy cutoff of 300 eV. All intermediates for OER/HER catalysis were simulated on the (100)-face of 2 × 2 × 1 supercells for all systems in this study with 5 × 5 × 1 k points.

Acknowledgements This work was financially supported by the University of Central Florida through a start-up grant (No. 20080741). Y.D. acknowledges the support by the Office of Science Early Career Research Program through the U.S. Department of Energy, Materials Science and Engineering Division. A portion of the research was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research. We greatly thank Dr. Matt Schneider, Dr. Yong-Ho Sohn and Dr. Nina Orlovskaya for characterization help in EELS, TEM and Raman. Conflict of Interest The authors declare potential conflict of interest with a patent in application.

Supporting Information TEM, ATP, XPS, EASA, EIS analysis are available free of charge on ACS Publications website.

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Figure 1. (a) Schematic illustration outlining holey film (HF) fabrication. (b-c) Cross-sectional and top-view images of HF (15.0 at% Fe). The scale bars in (b) and (c) denote 500 nm and 200 nm, respectively.

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Figure 2. (a) Cross-sectional TEM image. Yellow dashed areas are amorphous NiFeOF. (b) Bright field image derived from white dashed box in (a). Interface boundary highlighted by yellow dash. (c) 3D compositional map of HF determined by Atom Probe Tomography (APT) analysis. Bounding box dimensions are 85.4×83.7×60 nm3. Ni is in green, Fe is in purple, O is in white, F is in red. (d) Isolation of F and Fe-rich layer.

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Figure 3. Electrochemical activity of the NiFe oxyfluorides HF. (a-d) iR-corrected LSV curves for OER and HER measured at 5 mV s-1 in 1 M NaOH aqueous solution and corresponding Tafel slopes, respectively. (e) LSV curves of 15.0 at% HF measured with different annealing temperature in a three-electrode system. (f) Bifunctional water electrolysis tested at 5 mV s-1 in 1 M NaOH aqueous solution.

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Figure 4. (a) Reaction free energy diagrams as well as the rate-limiting steps of OER process catalyzed by NiO, NiFe oxides, and NiFeOF, respectively. The optimized geometry structures of intermediates for NiFeOF (100) catalysis (M), such as M-OH, M-O and M-OOH are shown along the reaction coordinates. (b) Reaction free energy diagrams of HER process catalyzed by NiO, NiFe oxides, and NiFeOF, respectively. The inset shows the active sites of Ni-edge on NiFeOF (100) surface.

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Figure 5. Two-electrode electrochemical activity of the NiFe oxyfluorides HF. (a) Bifunctional water electrolysis tested at 5 mV s-1 in 1 M NaOH aqueous solution. (b) Digital image of twoelectrode water splitting. (c) The amount of gas theoretically calculated and experimentally measured curves for overall water splitting. (d) Chronopotentiometric curve water electrolysis with constant current density of 10 mA cm-2.

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Overall Water Splitting with Room-Temperature ... - ACS Publications

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