FeCo2S4 Nanosheet Arrays Supported on Ni Foam: An Efficient and

Subscriber access provided by - Access paid by the | UCSB Libraries

Article

FeCo2S4 Nanosheet Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Overall Water-Splitting Jingrui Hu, Yingqing Ou, Yanhong Li, Di Gao, Yunhuai Zhang, and Peng Xiao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01978 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 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

ACS Sustainable Chemistry & Engineering

FeCo2S4 Nanosheet Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Overall Water-Splitting Jingrui Hu,† Yingqing Ou,‡ Yanhong Li, † Di Gao,‡ Yunhuai Zhang,*, ‡, and Peng Xiao*, †

†

Chongqing Key Laboratory of Soft Condensed Matter Physics and Smart Materials, College of Physics, Chongqing University, No.55 Daxuecheng South Rd., Shapingba, Chongqing, 401331, China

‡

College of Chemistry and Chemical Engineering, Chongqing University, No.55 Daxuecheng South Rd., Shapingba, Chongqing, 401331, China

Corresponding Authors * Tel.: +86 13883077781. E-mail: [email protected]. * Tel.: +86 15823038874. E-mail: [email protected].

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

ABSTRACT Efficient and stable bifunctional electrocatalysts composed of earth-abundant elements are crucial to oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Herein, FeCo2S4 nanosheet arrays loaded on Ni foam (NF) are synthesized and firstly employed as a bifunctional electrocatalyst for full water-splitting. Remarkably, the self-assembled, binder-free, and cost-effective FeCo2S4/NF electrode shows high OER catalytic activity, which only need an overpotential of 270 and 290 mV to achieve current density of 50 and 100 mA cm-2, respectively. Moreover, FeCo2S4/NF electrode exhibits a considerable oxygen evolution reaction stability over 20 h on a static current density of 50 mA cm−2, with negligible potential change in alkaline electrolyte. While, when served as catalyst for hydrogen evolution reaction under alkaline conditions, it just requires an overpotential of 132 mV to deliver the current density of 10 mA cm-2. The structural investigation demonstrate the formation of Co(Fe)-(oxy)hydroxides layer on the catalyst surface during OER test, which could be the real active species. Furthermore, due to the high catalytic activity and stability of this bifunctional electrocatalyst, we prepared a high-performance overall water electrolyzer that could achieve a current density of 10 mA cm –2 at a cell voltage of 1.56 V.

KEYWORDS: FeCo2S4/NF, Bifunctional electrocatalyst, Oxygen evolution reaction, Hydrogen evolution reaction, Overall water-splitting

2


Page 2 of 37

Page 3 of 37 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


INTRODUCTION Increasing fossil fuel extraction and depletion have caused serious environmental problems. Many researchers are exploring new alternative clean energy to replace the diminishing fossil fuels.1,

2

Because of the high energy density, light weight,

earth-abundant, and environment friendliness, hydrogen is one of the best alternatives.3, 4 Electrochemical water electrolysis is regarded as the most effective method to produce hydrogen.5-7 One of the most important reason for performance loses of water electrolysis is the high overpotential and sluggish kinetics of oxygen and hydrogen evolution reaction (OER and HER) in the anode and cathode, respectively.8, 9 Electrocatalysts are very important for reducing the energy barrier and improving the kinetics of both OER and HER to facilitate the full water-splitting reaction.6 At present, the most advanced electrocatalyst for OER and HER are Ru-/Ir-based compounds and Pt, respectively.10, 11 However, the expensive price and scarcity restrict their practical applications.12, 13 Therefore, a great deal of work have been done in allusion to cost-efficient, earth-rich, and high activity electrocatalysts, which will serve as possible substitutes to precious Pt and Ru-/Ir- based catalysts, including transition metal oxides,14, 15 hydroxides,16 and oxyhydroxides17, 18 for OER and other non-oxide catalysts, such as metal chalcogenides19,

20, 21, 22

and metal

pnictides, 23, 24 for HER. OER and HER generally are operated in different media. In order to improve the efficiency of full water-splitting, simplify system, and lower the cost, the 3



Page 4 of 37

electrocatalyst of OER and HER should be operated in the same electrolyte (either alkaline or acid).25-27 Since the full decomposition of water under acidic conditions requires rare and expensive acid insoluble OER catalysts, alkaline water-splitting has become a superior method for large scale hydrogen production.20 Therefore, it is essential to develop highly efficient bifunctional electrocatalysts in alkaline electrolyte to assure both reactions,28, 29 among which, transition metal chalcogenides, phosphides, and selenides have been developed.20, 30, 31 Although the great progress have been made, the low electrical conductivity, limited active sites, and poor structural stability hinder their potential application in OER and HER. Thus, it is indispensable to develop the bifunctional electrocatalysts with higher activity, good stability, and cost-efficient. Recently, FeCo-based compounds have been extensively investigated as energy storage materials.32-35 It is know that Fe has a positive effect on the OER and HER activity of Ni/Co-based compounds, especially for OER.25,

36-38

Yan et al.39 coupled

FeCo2O4 and hollow reduced graphene oxide spheres (FCO/HrGOS) with enhanced electrocatalytic activities for oxygen reduction and oxygen evolution reaction. Liu et al.40 developed a hybrid of material of FeCo2O4 nanoparticles supported on hierarchical nitrogen-enriched porous carbon (FeCo2O4@NPC) for efficient water oxidation. Tang et al. reported that Fe replaces Co in CoP to optimize the hydrogen adsorption energy, thus enhancing HER activity. The large sizes anions can expose more active sites of cation with multivalent states to improve the electrical conductivity and catalytic activity of electrocatalysts. In addtion, due to the 4


Page 5 of 37 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


coordination environment of transition metal chalcogenides is similar to the active centre of hydrogenase and its analogues, which have been developed as promising HER electrocatalysts.27 Inspired by this, we have successfully prepared the FeCo2S4 nanosheet arrays supported on 3D Ni foam, which has highly catalytic activity of OER and HER under alkaline electrolyte. As far as we know, there are no reports of bifunctional electrocatalysts with Fe-Co-S based spinel structure for full water-splitting. The FeCo2S4/NF electrode exhibits high activity, achieving 50 and 100 mA cm-2 current density at low overpotential of 270 and 290 mV for OER, respectively. It also shows superior catalytic activity for HER and long-time stability for full water-splitting in 1M KOH electrolyte. Furthermore, the corresponding two-electrode alkaline water electrolyzer exhibit a low cell voltage of 1.56 V (with iR-corrected) at the current density of 10 mA cm−2, and remarkable stability over 20 h of continuous electrolysis (without iR-corrected, 1.63 V at 10 mA cm-2).

EXPERIMENTAL SECTION Materials and Chemicals. Cobalt nitrate hexahydrate (Co(NO3)2•6H2O), ferrous chloride tetrahydrate (FeCl2•4H2O), ammonium fluoride (NH4F), urea (CO(NH2)2), potassium hydroxide (KOH), anhydrous ethanol (CH3CH2OH), hydrochloric acid (HCl), and bare Ni foam (thickness: 1.6mm) were purchased from Chuan Dong Ltd. Sodium sulfide nonahydrate (Na2S•9H2O), ruthenium dioxide (RuO2), and Pt/C nanoparticles were obtained from Aladdin Reagent Co., Ltd. All chemicals used in the 5



experiment were of analytical grade and used directly without further purification. Before it was used as a substrate, the bare Ni Foam (2 cm × 3 cm) was clean through consecutive sonication in dilute hydrochloric acid and deionized (DI) water (20 min each) to remove the oxides on the surface. DI water (18.25 MΩ, Aquapro AWL-1002-B) was used for preparation of solutions and washing. Synthesis of FeCo2S4/NF. FeCl2•4H2O (1 mmol), Co(NO3)2•6H2O (2 mmol), CO(NH2)2 (5 mmol), and NH4F (2 mmol) were dissolved in 30 mL DI water to obtain a pink solution. The mixed solution and a piece of cleaned Ni foam were transferred into Teflon-lined stainless-steel autoclave (45 mL) and then it was sealed and kept for 7 h at 140 °C. Subsequently, the uniformly grown Fe-Co precursor was removed from the solution and immersed into DI water to remove loosely attached residues with the assistance of slight ultrasonication. Finally, the FeCo-precursor was annealed at 400℃ for 2 h in air atmosphere to thoroughly transform the structure into FeCo2O4. To obtain FeCo2S4/NF, 0.1M Na2S•9H2O solution (30mL) was prepared and transferred to Teflon-lined stainless-steel autoclave (45 mL) together with FeCo-precursor, which was maintained at 160℃ for 8h in an electric oven. After the reaction was finished, the FeCo2S4/NF was washed several times with ethanol and DI water and then dried at 60℃ for 10 hours in the vacuum oven. Characterization. The crystalline phase of catalysts was identified by powder X-ray diffraction (XRD) (Spectris Pte. Ltd, PANalytical X’Pert Powder, Cu Kα, λ = 1.5418 Å). Field-emission scanning electron-microscope (FE-SEM) (Nova 400 Nano-SEM) was used to analyze the morphology and structure of the catalysts. The chemical 6


Page 6 of 37

Page 7 of 37 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


composition and elements states of the catalysts were distinguished using X-ray photoelectron spectroscopy (XPS), (Thermo-Scientific, ESCALAB 250Xi). All binding energies of the sample were corrected by referencing the C 1s peak at 284.8 eV. Raman spectroscopy measurements was conducted on a confocal Raman microscopy system (HORIBA Jobin Yvon S.A.S, LabRAM HR Evolution) with the excitation laser of 532 nm. The actual ration of elements of the FeCo2S4/NF was measured

by

Inductively

coupled

plasma

mass

spectrometry

(ICP-MS),

(Thermo-Scientific iCAP 6300 Duo). Electrochemical measurements. All electrochemical tests were performed at room temperature (25℃) on an electrochemical workstation (CHI660D, CH Instruments, Shanghai, China). The Ag/AgCl (sat. KCl) was employed as the reference electrode. Pt foil (for OER) and graphite rod (for HER) were used as the counter electrode. To prepare the RuO2 loaded electrode, RuO2 (10 mg) and Nafion (30 uL) were dispersed in water/ethanol (1 mL, 1:1 v/v) solvent mixture with the assistance of ultrasonication for 30 minutes. Then the above ink (100 uL) was loaded on a Ni foam. The mass loading of the catalyst is～1 mg cm-2. Pt/C loaded electrode were prepared by the same method. The electrochemical tests were conducted in 1M KOH solution, saturated by hydrogen (for HER) and oxygen (for OER) before and during the whole experiments. Linear sweep voltammogram (LSV) was measured from -0.2 to 0.9 V and -0.9 to -1.5 V vs. Ag/AgCl (sat. KCl) at a scan rate of 2 mV s−1 and 5 mV s−1, respectively, for OER and HER. The potential was corrected to reversible hydrogen electrode (RHE) according to the following equation:Eୖୌ୉ = E୅୥/୅୥ୡ୪ + 0.197 + 7



Page 8 of 37

0.0592 × PH, the current density in this paper was normalized to the geometric area of the electrode. The electrochemical durability of the as-prepared electrocatalyst in an alkaline environment was determined by chronopotentiometry method. Cyclic voltammetry (CV) was conducted at different scan rates to evaluate the electrochemical

double-layer

capacitance

(Cdl).

Electrochemical

impedance

spectroscopy (EIS) was performed in 1 M KOH electrolyte at different potentials (0 and 0.5 V), and the frequency was swept from 100 KHz to 10 KHz. All LSV curves were iR-corrected. The full water-splitting test was carried out in a two-electrode system, and two symmetrical catalyst electrodes were used as anode and cathode.

RESULTS AND DISCUSSION Figure 1 shows a schematic diagram representative of the in-suit growth of the FeCo2S4/NF using a two-step hydrothermal method. Firstly, uniform Fe-Co precursor nanosheet arrays were grown on a 3D conductive Ni foam through hydrothermal treatment in the presence of urea and NH4F at 140℃ for 7 h (Figure 1b), where NH4F has been demonstrated to be vital in activating the substrates and producing more active sites for nucleation.41 In the second step, the Fe-Co precursor was completely converted into FeCo2S4/NF by a subsequent hydrothermal sulfuration. The S2- ions released from Na2S•9H2O solution replaced CO32− and OH− anions of the Fe-Co precursor, forming hierarchical FeCo2S4 nanosheet arrays. The surface of the Ni foam was black without any patches, suggesting the uniform formation of FeCo2S4 nanosheet, which was confirmed by the FE-SEM images (Figure 1c). Figure 2a-c 8


Page 9 of 37 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


present the FE-SEM images of FeCo-precursor, from which we can see that the surface of Ni foam was evenly covered by FeCo-precursor nanosheet arrays (Figure 2c). Figure 2d-f display the images of the FeCo2S4/NF. The results showed that the FeCo2S4 nanosheets grew almost vertically on the substrate and they were interconnected with each other, forming a 3D-networked structure with plenty of void spaces. FeCo2S4 nanosheet has a thickness of ~20 to 30 nm and a length of ~2 µm along the direction of growth. Additionally, these nanosheets attached to the substrate closely and such structure is in favor of efficient electron transport and long-term stability of catalysts. Similarly, the morphology of FeCo-precursor was maintained after heat treatment and uniformly grown FeCo2O4 nanosheets were obtained. The phase of FeCo-precursor was presented in Figure S1, which matches well with Co(CO3)0.5OH•0.11H2O (JCPDS:48-0083). Figure 3a displays the XRD patterns of FeCo2O4/NF and FeCo2S4/NF. The well-defined diffraction peaks of FeCo2O4/NF at values of 30.1°, 35.5°, 37.3°, 43.5°, 57.2°, and 62.7°, are ascribed to the (220), (311), (222), (400), (511), and (440) planes of FeCo2O4 (JCPDS: 03-0864), respectively. The XRD pattern of the as-grown FeCo2S4/NF can be well matched with Co3S4 (JCPDS: 42-1448), since FeCo2S4 can be regarded as partial replacement of Co ion by Fe ion in crystal lattice of Co3S4 with a little changes in lattice parameter.41 The diffraction peaks at values of 31.4°, 38°, 50.2°, and 55° can be indexed to the (311), (400), (511), and (440) planes, respectively, while the three strong characteristic peaks at values of 44.5°, 51.8°, and 76.4° belong to Ni foam (JCPDS: 04-0850). The peak at 21.7° (marked with a dot) corresponds to Ni3S2 that might be formed when the Ni foam was 9



immersed in the Na2S•9H2O solution.30, 31 To further characterize the elemental composition and chemical state on the near-surface of the as-prepared FeCo2S4/NF, XPS was carried out. Figure 3b-d represent the Fe 2p, Co 2p, and S 2p XPS spectra of the FeCo2S4/NF. The Fe 2p spectrum (Figure 3b) was well fitted into two main peaks at the binding energy of 712.7 eV and 724.3 eV, corresponding to the spin-orbit peaks of the Fe 2p3/2 and Fe 2p1/2, respectively, indicating the main presence of Fe2+.42 Two “shoulder” weak satellite peaks (signed as “Sat.”) at binding energy of 718.7 eV and 733.4eV can be observed, confirming the existence of Fe3+ in the material.33 As presented in the Figure 3c, the Co 2p spectrum was fitted into two spin-orbit doublets at 781.2 eV and 796.4 eV, representative of Co 2p3/2 and Co 2p1/2, respectively, which can be indexed to the Co3+ species, while, the binding energy at 786.3eV and 798.5 eV are assigned to Co2+.43 The weak satellite peaks at 791.1 eV and 802.6 eV indicate the main valence of Co is +3.35 In the XPS of S 2p spectrum, the peaks at 161.6 eV and 164.7 eV, respectively, correspond to the S 2p3/2 and S 2p1/2. The peak at 162.6 eV indicated the presence of S2- in a low coordination state on the surface, and the other peak at 163.8 eV stands for metal-sulfur (M-S) bond. The peak at higher energy of 168.7 eV is ascribed to the sulfur at the surface and/or edges with highly oxidized state.27, 30, 44 The full spectrum of XPS was recorded in Figure S2. The elements of Fe, Co, S, O, and C can be identified and the atom ratio of Fe, Co, and S is calculated to be 1:2.12:3.67. Moreover, the ICP-MS result demonstrated an approximate atomic ratio of 1:2:4 for Fe:Co:S (Table S1). The result is very consistent with the ratio of starting 10


Page 10 of 37

Page 11 of 37 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


materials and the stoichiometric ratio of FeCo2S4. Therefore, the XPS results manifested that the FeCo2S4 nanosheet arrays consist of Fe2+, Fe3+, Co3+, Co2+, and S2-. Together with XRD results, the results confirm the successful preparation of FeCo2S4. The electrocatalytic OER activity of the FeCo2S4/NF was evaluated (FeCo2S4 loading: 1.02 mg cm-2) in 1M KOH solution with a scan rate of 2 mV s-1 using a standard three-electrode system (Figure 4). As a contrast, the OER activities of FeCo2O4/NF, bare NF, and RuO2/NF catalysts were also investigated under identical conditions. Figure 4a shows the LSV curves of FeCo2S4/NF, FeCo2O4/NF, bare Ni foam, and RuO2/NF. Compared with FeCo2O4/NF, bare Ni foam, and RuO2/NF, the FeCo2S4/NF electrode exhibits much higher current density and lower onset potential. The FeCo2S4/NF electrode requires an overpotential of 270 mV to deliver the current density of 50 mA cm-2, much lower than that of FeCo2O4/NF (350 mV), RuO2/NF (420 mV) and bare Ni foam (490 mV), and it needs only 290 mV to achieve 100 mA cm-2 (Figure 4b). Tafel slope (Figure 4c) indicates how fast the current density increases against overpotential, reflecting corresponding OER kinetics.28 The Tafel slope for FeCo2S4/NF is 59 mV dec-1, less than FeCo2O4/NF (79 mV dec-1), Ni foam (149 mV dec-1) and RuO2/NF (97 mV dec-1), which means that the OER rate of FeCo2S4/NF electrode is more faster. The FeCo2S4/NF also only need an overpotential of 390 mV to deliver the current density of 50 mA cm-2 in 0.1 M KOH (Figure S3a). The outstanding catalytic performance of FeCo2S4/NF is better than previously reported

non-precious

metal

OER

electrocatalysts.

Information). 11


(Table

S2,

Supporting


In addition to the catalytic activity, the stability is also a critical indicator for catalysts. The stability of FeCo2S4/NF electrode was explored by CV with a voltage in the range of 0 to 0.6 V vs. Ag/AgCl (sat. KCl) (Figure S4a). FeCo2S4/NF electrode exhibits only a slight increase in overpotential after continuous CV test for 3000 cycles, implying its excellent stability. We also probed the long-time electrochemical durability of the FeCo2S4/NF electrode for OER in 1 M KOH at a static current density of 50 mA cm-2 (Figure 4d). As observed, the electrode exhibited an overpotential of 1.56 V, which stayed stable after 20 h, indicating its excellent OER catalytic activity. The galvanostatic measurements of FeCo2S4/NF were also performed at 100 and 200 mA cm-2, respectively. Figure 4d shows that the overpotential of FeCo2S4/NF maintained stable during long-term tests at each current density, demonstrating an ultra-high electrochemical durability. Moreover, the high mechanical stability is also verified by SEM results after long-term OER test (Figure S5), which shows a slight change in surface morphology and microstructure of the FeCo2S4/NF electrode. We analyzed the electrocatalytic HER performance of the FeCo2S4/NF electrode (FeCo2S4 loading: 1.02 mg cm-2) in 1M KOH solution with a scan rate of 5 mV s-1 by a standard three-electrode system (Figure 5). All of our data were iR-corrected. It is obvious that FeCo2S4/NF electrode only need an overpotential of 132 mV to achieve a current denstiy of 10 mA cm –2 (Figure 5b), indicating its outstanding catalytic activity for HER in strong basic solution, which is superior to representative non-precious electrocatalysts in alkaline solution (Table S3). Figure S3b displays the HER curve of 12


Page 12 of 37

Page 13 of 37 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


FeCo2S4/NF in 0.1 M KOH, and it requires an overpotential of 258 mV to drive 10 mA cm-2. The overpotential of FeCo2O4/NF, bare Ni foam, and Pt/C on Ni foam are 215 mV, 195 mV, and 35 mV, respectively, to afford the current density of 10 mA cm-2. The Tafel slope (Figure 5c) of FeCo2S4/NF is 164 mV dec-1, smaller than FeCo2O4/NF (186 mV dec-1) and bare Ni foam (184 mV dec-1). The HER stability of the FeCo2S4/NF electrode was investigated by cyclic voltammetry between +0.2 V and -0.2 V vs. RHE. After 3000 cycles, the overpotential only increases by 2 mV at the current density of 10 mA cm-2 (Figure S4b). The long-time electrochemical stability of FeCo2S4/NF electrode was tested and the operating potential was very stable for 20 h, suggesting its excellent electrochemical stability (Figure 5d). Furthermore, no obvious changes in the XRD patterns (Figure 6c) and FE-SEM image (Figure S5d) of the FeCo2S4/NF were observed after 20 h HER test, indicating its robust mechanical stability. Furthermore, compared with the initially recorded, the XPS spectra of FeCo2S4/NF after HER test (Figure S7) showed similar peaks in the Fe 2p, Co 2p, and S 2p spectra, suggesting only very small changes in the valence state. The above result indicated good durability of FeCo2S4/NF during HER in strong basic electrolyte. In order to further evaluate the intrinsic activity of catalyst, the turnover frequency (TOF) was calculated, which is defined as the number of moles of O2/H2 evolved per second per active site. The TOF of OER and HER of FeCo2S4/NF at corresponding given overpotential are 0.37 s-1 and 0.15 s-1, respectively, outperforming FeCo2O4/NF and NF (Table S4). However, it should be noticed that there is a great decline in HER electrocatalytic 13



activity of FeCo2S4/NF after the OER test (Figure S8). It may be ascribed to the formation of a HER-inactive layer on the surface of catalyst during OER stability test, though the crystal phase (Figure S6b) did not change obviously. In order to understand the catalytic mechanism, XPS and Raman spectra of FeCo2S4 before and after OER measurements were performed. The wake satellite at approximately 719.2 eV belongs to Fe 2p3/2 of FeCo2S4 after OER (Figure S9a), manifesting its main component is Fe3+. The Co 2p spectrum in the Figure S9b reveals the increased proportion of Co3+ after the OER measurement. It can be seen that the peak at 786.3 eV disappeared after OER test, implying the transformation of FeCo2S4 to metal oxyhydroxides on the surface,45, 46 which is confirmed by the weaker intensity of S 2p (Figure S9d) and remarkably enhanced intensity of O 1s spectrum at 531.5 eV (Co(Fe)-O) (Figure S9c). The peaks at 529.8 eV and 533.3 eV are assigned to lattice oxygen (O2-) and surface hydroxyls, respectively.26, 45, 47 Moreover, Raman spectra of the FeCo2S4 sample before and after the OER were performed to further verify the structure transformation (Figure S10). The Raman band at 555 cm-1 can be indexed to Co(Fe)-OOH, while the peaks at 650-720 cm-1 may originate from iron oxides and oxyhydroxides.37,

48

Due to the similar structure of iron oxide and oxyhydroxide

phases, coupled with the impact of substrate (Ni foam), it is quite difficult to accurately identify these bands. Taking into account the chemical properties of the sample and the results of XPS, we believed that the near surface species should be Co(Fe)-(oxy)hydroxides. The above results suggest that the surface of FeCo2S4 was transformed into Co(Fe)-(oxy)hydroxides active species for OER, which has been 14


Page 14 of 37

Page 15 of 37 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


demonstrated previously by similar structural composition evolution on catalysts during OER process.26, 48, 49 The structure of (oxy)hydroxiodes shell and sulfide core is conducive to increase active sites and conductivity, and the structure of (oxy)hydroxiodes shell can protect the sulfide core from oxidation, thus further enhancing the catalytic activity and stability. 50, 51 EIS was used to further explore the electrode kinetics during OER and HER electrocatalysis. The Nyquist plots at 0.5 V and open circuit voltage (OCV) were presented in Figure 6a and Figure S11, respectively. We obtained the ions and charge transfer resistances by fitting the Nyquist plots through an equivalent circuit (inset in Figure 6a). Rs is the resistance of electrolyte. Cdl and Rct are the electrochemical double-layer capacitance (EDLC) and charge transfer resistance of the various electrocatalysts in 1M KOH solution, respectively.30 The intercept on abscissa of Nyquist plot in the high frequency region revealed the Rs of these catalysts. From the Z-fitted curve (Fig. 6a), we obtained the Rs value of 1.6 Ω, it was comparable for all of the catalysts at 0.5V and OCV. The diameter of the semicircle represents the Rct, and the smaller the diameter, the faster the charge transfer. The Rct of FeCo2S4/NF is 0.5 Ω, much smaller than that of FeCo2O4/NF (5 Ω) and NF (45 Ω). The EIS results indicated that FeCo2S4/NF had a lower resistance, suggesting an increased electrocatalytic reaction kinetics. The electrochemically active surface area (ECSA) of these catalysts was predicted through Cdl, which can be estimated by measuring cyclic voltammograms at various scan rates in a non-faradic region (Figure S12). The ECSA is twice as much as Cdl. In 15



Page 16 of 37

figure 6b, the Cdl of the FeCo2S4/NF, FeCo2O4/NF, and bare Ni foam are 90.4 mF cm-2, 64.9 mF cm-2, and 2 mF cm-2, respectively. Therefore, the predominant OER and HER activities of the FeCo2S4/NF are attributed to the significantly enhanced ECSA, thanks to the improved anion exchangeability between the catalytic active sites and electrolyte.45, 52 Encouraged by the superior catalytic performance of FeCo2S4/NF for OER and HER, we constructed an alkaline water electrolyzer using FeCo2S4/NF as anode and cathode to simulate real water electrolysis. For comparison, we prepared the electrolyzers with the structure of FeCo2O4/NF//FeCo2O4/NF and NF//NF. Figure 7a displays

the

OER

LSV

curves

of

all

the

electrocatalysts.

The

FeCo2S4/NF//FeCo2S4/NF electrolyzer exhibited the lowest cell voltage of 1.63 V, superior to the electrolyzers of FeCo2O4/NF//FeCo2O4/NF (1.78 V) and NF//NF (1.82 V), and even is comparable to the recently reported literature (Table S5). In 0.1 M KOH, it also performed well, requiring 1.82 V to drive the current density of 10 mA cm-2

(Figure

S3c).

Furthermore,

the

long-time

stability

of

the

FeCo2S4/NF//FeCo2S4/NF electrolyzer was evaluated (Figure 7b). The potential to deliver the current density of 10 mA cm-2 stayed almost unchanged after 20 h continuous electrolysis. The superior catalytic performance of FeCo2S4/NF in overall water-splitting system are mainly attributed to the following advantages: (ⅰ) ternary compounds are usually better than related binary compounds in structural stability and catalysis activity, owing to the synergistic effects between different components;27, 16


53

(ⅱ) spinel

Page 17 of 37 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


nanostructures are more stable under alkaline condition during electrochemical processes;54 (ⅲ) incorporating catalysts on various substrates (such as Ni foam, carbon cloth, graphene, and N-rich carbon) can ensure uniform growth of stereo nanostructures to improve the ECSA, thus increasing the number of active sites, facilitating the diffusion of ions and electron transport, maintaining the integrity of morphology, and suppressing the aggregation of active species during the electrolysis, which is an effective method to improve the catalysis performance of water electrolysis. 19, 55-58

CONCLUSIONS In summary, 3D FeCo2S4/NF electrode have been successfully prepared through a facile two-step hydrothermal method as an efficient and durable bifunctional electrocatalyst for overall water-splitting in strong alkaline solution. The 3D FeCo2S4/NF shows excellent activity for oxygen evolution reaction and requires a low overpotential of 270 and 290 mV to afford the current density of 50 mA cm-2 and 100 mA cm-2, respectively. Furthermore, when used as both anode and cathode in an alkaline water electrolyzer, the FeCo2S4/NF electrode requires 1.56 V (with iR-corrected) to deliver the current density of 10 mA cm-2. The predominant catalytic activity and excellent stability of this 3D structure suggest that it could be a promising electrode material for overall water-splitting for large scale hydrogen generation.

ASSOCIATED CONTENT 17



Supporting information XRD patterns; XPS and Raman spectra; FE-SEM images of FeCo2S4/NF; cycle voltammograms; LSV curves; Nyquist plots; Table S1, S2, S3, S4,and S5.

AUTHOR INFORMATION Corresponding Authors * Tel.: +86 13883077781. E-mail: [email protected]. * Tel.: +86 15823038874. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was supported by the fundamental research funds for the Central Universities (No.2018CDJDWL0011).

REFERENCES (1) Chow, J.; Kopp, R. J.; Portney, P. R. Energy resources and global development. Science 2003, 302, 1528-1531. (2) Bard, A. J.; Fox, M. A. ARTIFICIAL PHOTOSYNTHESIS - SOLAR SPLITTING OF WATER TO HYDROGEN AND OXYGEN. Acc. Chem. Res. 1995, 28, 141-145. (3) Hou, W.; Yu, B.; Qi, F.; Wang, X.; Zheng, B.; Zhang, W.; Li, Y.; Chen, Y.

18


Page 18 of 37

Page 19 of 37 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


Scalable synthesis of graphene-wrapped CoSe2-SnSe2 hollow nanoboxes as a highly efficient and stable electrocatalyst for hydrogen evolution reaction. Electrochim. Acta 2017, 255, 248-255. (4) Wang, B.; Wang, X.; Zheng, B.; Yu, B.; Qi, F.; Zhang, W.; Li, Y.; Chen, Y. NiSe2 nanoparticles embedded in CNT networks: Scalable synthesis and superior electrocatalytic activity for the hydrogen evolution reaction. Electrochem. Commun. 2017, 83, 51-55. (5) Wang, X.; Zheng, B.; Yu, B.; Wang, B.; Hou, W.; Zhang, W.; Chen, Y. In situ synthesis of hierarchical MoSe2-CoSe2 nanotubes as an efficient electrocatalyst for the hydrogen evolution reaction in both acidic and alkaline media. J. Mater. Chem. A 2018, 6, 7842-7850. (6) Liu, T.; Xie, L.; Yang, J.; Kong, R.; Du, G.; Asiri, A. M.; Sun, X.; Chen, L. Self-Standing CoP Nanosheets Array: A Three-Dimensional Bifunctional Catalyst Electrode for Overall Water Splitting in both Neutral and Alkaline Media. ChemElectroChem 2017, 4, 1840-1845. (7) Yu, B.; Hu, Y.; Qi, F.; Wang, X.; Zheng, B.; Liu, K.; Zhang, W.; Li, Y.; Chen, Y. Nanocrystalline Ni0.85Se as Efficient Non-noble-metal Electrocatalyst for Hydrogen Evolution Reaction. Electrochim. Acta 2017, 242, 25-30. (8) Ji, X.; Zhang, R.; Shi, X.; Asiri, A. M.; Zheng, B.; Sun, X. Fabrication of hierarchical CoP nanosheet@microwire arrays via space-confined phosphidation toward high-efficiency water oxidation electrocatalysis under alkaline conditions. Nanoscale 2018, 10, 7941-7945. 19



(9) Yu, B.; Qi, F.; Wang, X.; Zheng, B.; Hou, W.; Hu, Y.; Lin, J.; Zhang, W.; Li, Y.; Chen,

Y.

Nanocrystalline

Co0.85Se

as

a highly

efficient non-noble-metal

electrocatalyst for hydrogen evolution reaction. Electrochim. Acta 2017, 247, 468-474. (10) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399-404. (11) Tian, J.; Liu, Q.; Asiri, A. M.; Sun, X. Self-Supported Nanoporous Cobalt Phosphide Nanowire Arrays: An Efficient 3D Hydrogen-Evolving Cathode over the Wide Range of pH 0-14. J. Am. Chem. Soc. 2014, 136, 7587-7590. (12) Liu, Y.; Wang, H.; Lin, D.; Liu, C.; Hsu, P.-C.; Liu, W.; Chen, W.; Cui, Y. Electrochemical tuning of olivine-type lithium transition-metal phosphates as efficient water oxidation catalysts. Energy Environ. Sci. 2015, 8, 1719-1724. (13) McCrory, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, 4347-4357. (14) Xiong, X.; Ji, Y.; Xie, M.; You, C.; Yang, L.; Liu, Z.; Asiri, A. M.; Sun, X. MnO2-CoP3 nanowires array: An efficient electrocatalyst for alkaline oxygen evolution reaction with enhanced activity. Electrochem. Commun. 2018, 86, 161-165. (15) Xiong, X.; You, C.; Liu, Z.; Asiri, A. M.; Sun, X. Co-Doped CuO Nanoarray: An Efficient Oxygen Evolution Reaction Electrocatalyst with Enhanced Activity. ACS 20


Page 20 of 37

Page 21 of 37 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


Sustainable Chem. Eng. 2018, 6, 2883-2887. (16) Zhu, Y. P.; Guo, C.; Zheng, Y.; Qiao, S.-Z. Surface and Interface Engineering of Noble-Metal-Free Electrocatalysts for Efficient Energy Conversion Processes. Acc. Chem. Res. 2017, 50, 915-923. (17) Chen, Z.; Kronawitter, C. X.; Yeh, Y.-W.; Yang, X.; Zhao, P.; Yao, N.; Koel, B. E. Activity of pure and transition metal-modified CoOOH for the oxygen evolution reaction in an alkaline medium. J. Mater. Chem. A 2017, 5, 842-850. (18) Zhang, B.; Zheng, X.; Voznyy, O.; Comin, R.; Bajdich, M.; Garcia-Melchor, M.; Han, L.; Xu, J.; Liu, M.; Zheng, L.; de Arquer, F. P. G.; Dinh, C. T.; Fan, F.; Yuan, M.; Yassitepe, E.; Chen, N.; Regier, T.; Liu, P.; Li, Y.; De Luna, P.; Janmohamed, A.; Xin, H. L.; Yang, H.; Vojvodic, A.; Sargent, E. H. Homogeneously dispersed multimetal oxygen-evolving catalysts. Science 2016, 352, 333-337. (19) Amiinu, I. S.; Pu, Z.; Liu, X.; Owusu, K. A.; Monestel, H. G. R.; Boakye, F. O.; Zhang, H.; Mu, S. Multifunctional Mo-N/C@MoS2 Electrocatalysts for HER, OER, ORR, and Zn-Air Batteries. Adv. Funct. Mater. 2017, 27, 1702300. (20) Zhang, J.; Wang, Y.; Zhang, C.; Gao, H.; Lv, L.; Han, L.; Zhang, Z. Self-Supported Porous NiSe2 Nanowrinkles as Efficient Bifunctional Electrocatalysts for Overall Water Splitting. ACS Sustainable Chem. Eng. 2018, 6 , 2231-2239. (21) Yu, B.; Qi, F.; Zheng, B.; Hou, W.; Zhang, W.; Li, Y.; Chen, Y. Self-assembled pearl-bracelet-like CoSe2-SnSe2/CNT hollow architecture as highly efficient electrocatalysts for hydrogen evolution reaction. J. Mater. Chem. A 2018, 6, 1655-1662. 21



(22) Zheng, B.; Chen, Y.; Qi, F.; Wang, X.; Zhang, W.; Li, Y.; Li, X. 3D-hierarchical MoSe2 nanoarchitecture as a highly efficient electrocatalyst for hydrogen evolution. 2D Mater. 2017, 4, 025095. (23) Jiang, M.; Li, Y.; Lu, Z.; Sun, X.; Duan, X. Binary nickel-iron nitride nanoarrays as bifunctional electrocatalysts for overall water splitting. Inorg. Chem. Front. 2016, 3, 630-634. (24) Jia, X.; Zhao, Y.; Chen, G.; Shang, L.; Shi, R.; Kang, X.; Waterhouse, G. I. N.; Wu, L.-Z.; Tung, C.-H.; Zhang, T. Ni3FeN Nanoparticles Derived from Ultrathin NiFe-Layered Double Hydroxide Nanosheets: An Efficient Overall Water Splitting Electrocatalyst. Adv. Energy Mater. 2016, 6, 1502585. (25) Wang, P.; Pu, Z.; Li, Y.; Wu, L.; Tu, Z.; Jiang, M.; Kou, Z.; Arniinu, I. S.; Mu, S. Iron-Doped Nickel Phosphide Nanosheet Arrays: An Efficient Bifunctional Electrocatalyst for Water Splitting. ACS Appl. Mater. Interfaces 2017, 9, 26001-26007. (26) Song, B.; Li, K.; Yin, Y.; Wu, T.; Dang, L.; Caban-Acevedo, M.; Han, J.; Gao, T.; Wang, X.; Zhang, Z.; Schmidt, J. R.; Xu, P.; Jin, S. Tuning Mixed Nickel Iron Phosphosulfide Nanosheet Electrocatalysts for Enhanced Hydrogen and Oxygen Evolution. Acs Catalysis 2017, 7, 8549-8557. (27) Xue, Y.; Zuo, Z.; Li, Y.; Liu, H.; Li, Y. Graphdiyne-Supported NiCo2S4 Nanowires: A Highly Active and Stable 3D Bifunctional Electrode Material. Small 2017, 13,

1700936.

(28) Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. NiSe Nanowire Film Supported 22


Page 22 of 37

Page 23 of 37 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


on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting. Angew. Chem. Int. Ed. 2015, 54, 9351-9355. (29) Arniinu, I. S.; Pu, Z.; He, D.; Monestel, H, G, R.; Mu, S. Scalable cellulose-sponsored functionalized carbon nanorods induced by cobalt for efficient overall water splitting. Carbon 2018, 137, 274-281. (30) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661-4672. (31) Liu, J.; Wang, J.; Zhang, B.; Ruan, Y.; Lv, L.; Ji, X.; Xu, K.; Miao, L.; Jiang, J. Hierarchical NiCo2S4@NiFe LDH Heterostructures Supported on Nickel Foam for Enhanced Overall-Water-Splitting Activity. ACS Appl. Mater. Interfaces 2017, 9, 15364-15372. (32) Zhu, B.; Tang, S.; Vongehr, S.; Xie, H.; Meng, X. Hierarchically MnO2-Nanosheet Covered Submicrometer-FeCo2O4-Tube Forest as Binder-Free Electrodes for High Energy Density All-Solid-State Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 4762-4770. (33) Liu, L.; Zhang, H.; Mu, Y.; Yang, J.; Wang, Y. Porous Iron Cobaltate Nanoneedles Array on Nickel Foam as Anode Materials for Lithium-Ion Batteries with Enhanced Electrochemical Performance. ACS Appl. Mater. Interfaces 2016, 8, 1351-1359. (34) Tang, S.; Zhu, B.; Shi, X.; Wu, J.; Meng, X. General Controlled Sulfidation 23



toward Achieving Novel Nanosheet-Built Porous Square-FeCo2S4-Tube Arrays for High-Performance Asymmetric All-Solid-State Pseudocapacitors. Adv. Energy Mater. 2017, 7, 1601985. (35) Zhu, J.; Tang, S.; Wu, J.; Shi, X.; Zhu, B.; Meng, X. Wearable High-Performance Supercapacitors Based on Silver-Sputtered Textiles with FeCo2S4-NiCo2S4 Composite Nanotube-Built Multitripod Architectures as Advanced Flexible Electrodes. Adv. Energy Mater. 2017, 7, 1601234. (36) Xie, M.; Xiong, X.; Yang, L.; Shi, X.; Asiri, A. M.; Sun, X. An Fe(TCNQ)2 nanowire array on Fe foil: an efficient non-noble-metal catalyst for the oxygen evolution reaction in alkaline media. Chem. Commun. 2018, 54, 2300-2303. (37) Louie, M. W.; Bell, A. T. An Investigation of Thin-Film Ni-Fe Oxide Catalysts for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc.2013, 135, 12329-12337. (38) Zhao, J.; Li, X.; Cui, G.; Sun, X. Highly-active oxygen evolution electrocatalyzed by an Fe-doped NiCr2O4 nanoparticle film. Chem. Commun. 2018, 54, 5462-5465. (39) Yan, W.; Yang, Z.; Bian, W.; Yang, R. FeCo2O4/hollow graphene spheres hybrid with enhanced electrocatalytic activities for oxygen reduction and oxygen evolution reaction. Carbon 2015, 92, 74-83. (40) Liu, J.; Nan, Y.; Chang, X.; Li, X.; Fang, Y.; Liu, Y.; Tang, Y.; Wang, X.; Li, R.; Ma, J. Hierarchical nitrogen-enriched porous carbon materials derived from Schiff-base networks supported FeCo2O4 nanoparticles for efficient water oxidation. 24


Page 24 of 37

Page 25 of 37 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


Int. J. Hydrogen Energy 2017, 42, 10802-10812. (41) Liu, S.; Jun, S. C. Hierarchical manganese cobalt sulfide core-shell nanostructures for high-performance asymmetric supercapacitors. J. Power Sources 2017, 342, 629-637. (42) Mohamed, S. G.; Tsai, Y.-Q.; Chen, C.-J.; Tsai, Y.-T.; Hung, T.-F.; Chang, W.-S.; Liu, R.-S. Ternary Spinel MCo2O4 (M = Mn, Fe, Ni, and Zn) Porous Nanorods as Bifunctional Cathode Materials for Lithium-O2 Batteries. ACS Appl. Mater. Interfaces 2015, 7, 12038-12046. (43) Tang, C.; Gan, L.; Zhang, R.; Lu, W.; Jiang, X.; Asir, A. M.; Sun, X.; Wang, J.; Chen, L. Ternary FexCo1-xP Nanowire Array as a Robust Hydrogen Evolution Reaction Electrocatalyst with Pt-like Activity: Experimental and Theoretical Insight. Nano Lett. 2016, 16, 6617-6621. (44) Ma, L.; Hu, Y.; Chen, R.; Zhu, G.; Chen, T.; Lv, H.; Wang, Y.; Liang, J.; Liu, H.; Yan, C.; Zhu, H.; Tie, Z.; Jin, Z.; Liu, J. Self-assembled ultrathin NiCo2S4 nanoflakes grown on Ni foam as high-performance flexible electrodes for hydrogen evolution reaction in alkaline solution. Nano Energy 2016, 24, 139-147. (45) Dai, Z.; Geng, H.; Wang, J.; Luo, Y.; Li, B.; Zong, Y.; Yang, J.; Guo, Y.; Zheng, Y.; Wang, X.; Yan, Q. Hexagonal-Phase Cobalt Monophosphosulfide for Highly Efficient Overall Water Splitting. Acs Nano 2017, 11, 11031-11040. (46) Yang, J.; Liu, H.; Martens, W. N.; Frost, R. L. Synthesis and Characterization of Cobalt Hydroxide, Cobalt Oxyhydroxide, and Cobalt Oxide Nanodiscs. J. Phys. Chem. C 2010, 114, 111-119. 25



Page 26 of 37

(47) Xu, X.; Song, F.; Hu, X. A nickel iron diselenide-derived efficient oxygen-evolution catalyst. Nat. Commun. 2016, 7, 12324. (48) Konkena, B.; Masa, J.; Botz, A. J. R.; Sinev, I.; Xia, W.; Kossmann, J.; Drautz, R.;

Muhler,

M.;

Schuhmann,

W.

Metallic

NiPS3@NiOOH

Core-Shell

Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction. ACS Catal. 2017, 7, 229-237. (49) Mabayoje, O.; Shoola, A.; Wygant, B. R.; Mullins, C. B. The Role of Anions in Metal Chalcogenide Oxygen Evolution Catalysis: Electrodeposited Thin Films of Nickel Sulfide as "Pre-catalysts". ACS Energy Lett. 2016, 1, 195-201. (50) Li, X.; Han, G.-Q.; Liu, Y.-R.; Dong, B.; Hu, W.-H.; Shang, X.; Chai, Y.-M.; Liu, C.-G. NiSe@NiOOH Core-Shell Hyacinth-like Nanostructures on Nickel Foam Synthesized by in Situ Electrochemical Oxidation as an Efficient Electrocatalyst for the Oxygen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 20057-20066. (51) Zhang, J.; Wang, T.; Pohl, D.; Rellinghaus, B.; Dong, R.; Liu, S.; Zhuang, X.; Feng, X. Interface Engineering of MoS2/Ni3S2 Heterostructures for Highly Enhanced Electrochemical Overall-Water-Splitting Activity. Angew. Chem. Int. Ed. 2016, 55, 6702-6707. (52) Chen, W.; Wang, H.; Li, Y.; Liu, Y.; Sun, J.; Lee, S.; Lee, J.-S.; Cui, Y. In Situ Electrochemical Oxidation Tuning of Transition Metal Disulfides to Oxides for Enhanced Water Oxidation. ACS Cent. Sci. 2015, 1, 244-251. (53) Liu, D.; Lu, Q.; Luo, Y.; Sun, X.; Asiri, A. M. NiCo2S4 nanowires array as an efficient bifunctional electrocatalyst for full water splitting with superior activity. 26


Page 27 of 37 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


Nanoscale 2015, 7, 15122-15126. (54) Cheng, H.; Su, Y.-Z.; Kuang, P.-Y.; Chen, G.-F.; Liu, Z.-Q. Hierarchical NiCo2O4

nanosheet-decorated

carbon

nanotubes

towards

highly

efficient

electrocatalyst for water oxidation. J. Mater. Chem. A 2015, 3, 19314-19321. (55) Wu, N.; Lei, Y.; Wang, Q.; Wang, B.; Han, C.; Wang, Y. Facile synthesis of FeCo@NC core-shell nanospheres supported on graphene as an efficient bifunctional oxygen electrocatalyst. Nano Res. 2017, 10, 2332-2343. (56) Xiao, C.; Li, Y.; Lu, X.; Zhao, C. Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting. Adv. Funct. Mater. 2016, 26, 3515-3523. (57) Hou, Y.; Lohe, M. R.; Zhang, J.; Liu, S.; Zhuang, X.; Feng, X. Vertically oriented cobalt selenide/NiFe layered-double-hydroxide nanosheets supported on exfoliated graphene foil: an efficient 3D electrode for overall water splitting. Energy Environ. Sci 2016, 9, 478-483. (58) Li, X.; Niu, Z.; Jiang, J.; Ai, L. Cobalt nanoparticles embedded in porous N-rich carbon as an efficient bifunctional electrocatalyst for water splitting. J. Mater. Chem. A 2016, 4, 3204-3209.

ABSTRACT GRAPHIC

27



200

Current density (mA cm-2)

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

180 160 140 120 100

(a) NF // NF FeCo2O4/NF // FeCo2O4/NF FeCo2S4/NF // FeCo2S4/NF FeCo2S4/NF // FeCo2S4/NF (iR-corrected)

80 60 40 20 0 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

Potential (V)

SYNOPSIS We have developed a new type of electrocatalyst for water electrolysis to facilitate the development of sustainable chemical technologies.

28


Page 28 of 37

Page 29 of 37 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


Figure Captions

Figure 1. Schematic illustration of the formation of FeCo2S4 nanosheet arrays on Ni foam and their corresponding morphology. a) bare Ni foam substrate; b) in situ growth of FeCo precursor nanosheet arrays on Ni foam (1st step); c) anion exchange reaction with complete growth of hierarchical FeCo2S4 nanosheet arrays on Ni foam (2nd step) using hydrothermal method.

Figure 2. FE-SEM images of a–c) FeCo precursor nanosheet; d–f) FeCo2S4/NF; g–i) FeCo2O4/NF.

Figure 3. a) XRD patterns of FeCo2O4/NF and FeCo2S4/NF; XPS spectra of b) Fe 2p; c) Co 2p; d) S 2p for FeCo2S4/NF (Sat. means shake-up satellites).

Figure 4. a) OER polarization curves (iR-corrected) of FeCo2S4/NF, FeCo2O4/NF, bare Ni foam, and RuO2 on Ni foam with a scan rate of 2 mV s-1; b) The required overpotential to achieve a current density of 50 and 100 mA cm-2 for different electrocatalysts; c) Corresponding OER Tafel plots; d) Chronopotentiometric measurements of long-term stability of FeCo2S4/NF at various current densities of 50 mA cm-2, 100 mA cm-2, and 200 mA cm-2.

1



Figure 5. a) HER polarization curves (iR-corrected) of FeCo2S4/NF, FeCo2O4/NF, bare Ni foam, and Pt/C on Ni foam with a scan rate of 5 mV s-1; b) The required overpotential to achieve a current density of 10 mA cm-2 for different electrocatalysts; c) Correspoding Tafel Plots of HER; d) Chronopotentiometric measurements of long-term stability of FeCo2S4/NF at a current density of 10 mA cm-2.

Figure 6. a) Nyquist plots of bare Ni Foam, FeCo2O4/NF, and FeCo2S4/NF electrodes recorded at an applied potential of 0.5 V with a frequency range of 100 kHz to 10 mHz in 1 M KOH; b) Corresponding capacitive currents at the specific potential vs Ag/AgCl as a function of scan rate (2, 4, 6, 8, 10 mV s-1).

Figure 7. a) Two electrode OER polarization curves of FeCo2S4/NF//FeCo2S4/NF, FeCo2O4/NF//FeCo2O4/NF, and NF//NF with a scan rate 2 mV s-1; b) Two-electrode cell durability of FeCo2S4/NF//FeCo2S4/NF electrode image with gas evolution (inset) in 1 M KOH. The size of both the electrodes is 1 cm × 1 cm.

2


Page 30 of 37

Page 31 of 37 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


Figure 1. Schematic illustration of the formation of FeCo2S4 nanosheet arrays on Ni foam and their corresponding morphology. a) bare Ni foam substrate; b) in situ growth of FeCo precursor nanosheet arrays on Ni foam (1st step); c) anion exchange reaction with complete growth of hierarchical FeCo2S4 nanosheet arrays on Ni foam (2nd step) using hydrothermal method.

3



Figure 2. FE-SEM images of a–c) FeCo precursor nanosheet; d–f) FeCo2S4/NF; g–i) FeCo2O4/NF.

4


Page 32 of 37

Page 33 of 37

(a)

(b)

FeCo2S4/NF

Fe 2p3/2

712.7 eV

JCPDS:42-1448

Intensity (a.u)

(511)

(440)



(440)





(511)

(311) (222) (400)

(400)

(220) 

(311)

Intensity (a.u)

FeCo2O4/NF

Fe 2p

Fe 2p1/2 724.3 eV 733.4 eV Sat.

718.7 eV Sat.

JCPDS:04-0850

20

30

40

50

60

70

80

700 705 710 715 720 725 730 735 740

2 Theat (Degree)

Binding Energy (eV) (d)

Co 2p

(c) Co 2p3/2 Co 2p1/2 796.4 eV

786.3 eV

168.7 eV Sat.

S 2p

162.6 eV 802.6 eV Sat.

791.1 eV Sat. 798.5 eV

Intensity (a.u)

781.2 eV

Intensity (a.u)

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


163.8 eV S 2p1/2 164.7 eV S 2p3/2 161.6 eV

160

775 780 785 790 795 800 805 810

165

170

175

Binding Energy (eV)

Binding Energy (eV)

Figure 3. a) XRD patterns of FeCo2O4/NF and FeCo2S4/NF; XPS spectra of b) Fe 2p; c) Co 2p; d) S 2p for FeCo2S4/NF (Sat. means shake-up satellites).

5



1200

180

(b)

(a)

NF FeCo2O4/NF

160 140

FeCo2S4/NF RuO2/NF

120 100 80 60 40 20 0 1.1

800 600 400 200 0

1.2

1.3

1.4

100 mA cm-2 50 mA cm-2

1000

Overpotential (mV)


200

1.5

1.6

1.7

1.8

Potential (V vs RHE) (c) 149 mV dec-1

FeCo2S4/NF RuO2/NF

1.65

97 mV dec-1

1.60

2.4

Ni foam FeCo2O4/NF

79 mV dec-1

1.55

59 mV dec-1

1.50

Potential (V vs RHE)

1.70

1.45 1.0

1.2

1.4

1.6

1.8

Log i (mA cm-2)

RuO2/NF

FeCo2O4/NF FeCo2S4/NF

Ni foam

Catalysts

1.75


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

Page 34 of 37

(d)

50 mA cm-2 100 mA cm-2

2.2

200 mA cm-2

2.0 1.8 1.6 1.4 1.2

2.0

0

5

10

15

20

25

Time (h)

Figure 4. a) OER polarization curves (iR-corrected) of FeCo2S4/NF, FeCo2O4/NF, bare Ni foam, and RuO2 on Ni foam with a scan rate of 2 mV s-1; b) The required overpotential to achieve a current density of 50 and 100 mA cm-2 for different electrocatalysts; c) Corresponding OER Tafel plots; d) Chronopotentiometric measurements of long-term stability of FeCo2S4/NF at various current densities of 50 mA cm-2, 100 mA cm-2, and 200 mA cm-2.

6


-20

400 NF FeCo2O4/NF

Overpotential (mV)

0 (a)

FeCo2S4/NF Pt/C

-40 -60 -80 -100 -0.5

(b)

300 215

195

200

132

100 35

0 -0.4

-0.3

-0.2

-0.1

0.0

Ni foam

FeCo2O4/NF FeCo2S4/NF

-0.8

0.00

(c)

NF FeCo2O4/NF

78 mV dec-1

-0.08

-0.9

FeCo2S4/NF Pt/C

-0.12 -0.16 184 mV dec-1

-0.20

164 mV dec-1

-0.24 -0.28

186 mV dec-1

(d)

-1.0


-0.04

Pt/C

Catalysts



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



Page 35 of 37

-1.1 -1.2 -1.3 -1.4 -1.5 -1.6 -1.7 -1.8

-0.32 0.8

1.0

1.2

1.4

1.6

1.8

Log i (mA cm-2)

2.0

0

2.2

5

10

15

20

25

Time (h)

Figure 5. a) HER polarization curves (iR-corrected) of FeCo2S4/NF, FeCo2O4/NF, bare Ni foam, and Pt/C on Ni foam with a scan rate of 5 mV s-1; b) The required overpotential to achieve a current density of 10 mA cm-2 for different electrocatalysts; c) Correspoding Tafel Plots of HER; d) Chronopotentiometric measurements of long-term stability of FeCo2S4/NF at a current density of 10 mA cm-2.

7



1.0

(a)


25

NF FeCo2O4/NF

20

FeCo2S4/NF

15

5 4

10

-lm (Z) (Ohm)

-lm (Z) (Ohm)

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

5

3 2 1 0 0

2

4

6

8

10

Re (Z) (Ohm)

0 0

10

20

30

40

50

0.8

(b) NF FeCo2O4/NF

Page 36 of 37

90.4 mF cm-2

FeCo2S4/NF

0.6

64.9 mF cm-2

0.4 0.2 2 mF cm-2

0.0 0.000 0.002 0.004 0.006 0.008 0.010

Scan Rate (V s-1)

Re (Z) (Ohm)

Figure 6. a) Nyquist plots of bare Ni Foam, FeCo2O4/NF, and FeCo2S4/NF electrodes recorded at an applied potential of 0.5 V with a frequency range of 100 kHz to 10 mHz in 1 M KOH; b) Corresponding capacitive currents at the specific potential vs Ag/AgCl as a function of scan rate (2, 4, 6, 8, and 10 mV s-1).

8


Page 37 of 37

180 160 140 120 100

4

(a) NF // NF FeCo2O4/NF // FeCo2O4/NF FeCo2S4/NF // FeCo2S4/NF FeCo2S4/NF // FeCo2S4/NF (iR-corrected)

80 60 40 20


200


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


0 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9

(b)

3

2

1

0 0

5

10

15

20

25

Time (h)

Potential (V)

Figure 7. a) Two electrode OER polarization curves of FeCo2S4/NF//FeCo2S4/NF, FeCo2O4/NF//FeCo2O4/NF, and NF//NF with a scan rate 2 mV s-1; b) Two-electrode cell durability of FeCo2S4/NF // FeCo2S4/NF electrode image with gas evolution (inset) in 1 M KOH. The size of both the electrodes is 1 cm × 1 cm.

9


FeCo2S4 Nanosheet Arrays Supported on Ni Foam: An Efficient and

Recommend Documents