3D coral-like Ni3S2 on Ni foam as a bifunctional electrocatalyst for

Subscriber access provided by Kaohsiung Medical University

Energy, Environmental, and Catalysis Applications

3D coral-like Ni3S2 on Ni foam as a bifunctional electrocatalyst for overall water splitting Jianfang Zhang, Yang Li, Tianyu Zhu, Yan Wang, Jiewu Cui, Jingjie Wu, Hui Xu, Xia Shu, Yongqiang Qin, Hongmei Zheng, Pulickel M. Ajayan, Yong Zhang, and Yu-Cheng Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09361 • Publication Date (Web): 23 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 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 30 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 Applied Materials & Interfaces

3D coral-like Ni3S2 on Ni foam as a bifunctional electrocatalyst for overall water splitting Jianfang Zhanga, Yang Lia, Tianyu Zhua, Yan Wanga, e, f *, Jiewu Cuia, *, Jingjie Wub, Hui Xuc, Xia Shua, Yongqiang Qina, e, Hongmei Zhenga, f, Pulickel M Ajayand, e, f, Yong Zhanga, e, f, g, Yucheng Wua, e, f, g *

a

School of Materials Science and Engineering, Hefei University of Technology, Hefei

230009, China b

Department of Chemical and Environmental Engineering, University of Cincinnati,

Cincinnati, OH 45221, United States c

School of Chemistry and Chemical Engineering; Institute for Energy Research,

Jiangsu University; Zhenjiang 212013, P. R. China d

Department of Material Science and NanoEngineering, Rice University, Houston,

Texas 77005, United States e

Base of Introducing Talents of Discipline to Universities for Advanced Clean Energy

Materials and Technology, Hefei 230009, China f

China International S&T Cooperation Base for Advanced Energy and Environmental

Materials, Hefei 230009, China g

Key Laboratory of Advanced Functional Materials and Devices of Anhui Province,

Hefei 230009, China.

*Corresponding authors: Email: [email protected]; [email protected]; [email protected]

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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: To explore high performance noble-metal-free electrocatalysts for overall water splitting is extremely desired but still has huge challenge. Herein, we report a novel one-step electrochemical anodic oxidation and cathodic deposition strategy to in situ fabricate a three-dimensional (3D) coral-like Ni3S2 on Ni foam for electrocatalytic overall water splitting. In a typical two-electrode cell, Ni foam (NF) acts as both cathodic and anodic electrodes with a thiourea aqueous solution as the electrolyte. The nickel ions from anodic oxidation of NF are directly used as nickel sources to form 3D coral-like Ni3S2/NF by cathodic deposition method simultaneously. The optimal 3D Ni3S2/NF-4 electrode shows high electrocatalytic activity for HER and OER with low overpotentials of 89 and 243 mV to afford 10 mA cm−2, respectively. When the as-obtained Ni3S2/NF-4 is used as bifunctional electrocatalyst in an electrolyzer, a low applied voltage of 1.577 V is needed to reach 10 mA cm-2, with an extremely long durability. This work focuses on a rational design of unique structures as efficient non-noble-metal based electrocatalysts, which holds great potential for practical applications in electrocatalytic water splitting. Key words: Cathodic deposition; Ni3S2/NF; Hydrogen evolution; Oxygen evolution; Bifunctional electrocatalyst


Page 2 of 30

Page 3 of 30 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


1. Introduction Exploring clear and renewable energy is a promise route to resolve the rising issue of energy shortage and environmental pollution.1-2 Electrochemical water splitting is deemed to be a high efficient, eco-friendly and economical scheme to produce high-purity Hydrogen (H2).3-5 Despite the excellent catalytic performance of noble-metal based electrocatalysts, the high cost, poor stability and low abundance impede their large-scale application.6-8 Currently, many researchers have made efforts to develop low cost, earth-abundant and high efficient hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) electrocatalysts and great results have been achieved.9-18 In order to work well for the potential application, HER electrocatalysts and OER electrocatalysts should work in the same condition to lower the overpotential. However, the mismatch in electrolyte for HER and OER often leads to a poor activity for overall water splitting.19 Therefore, it is still a big challenge to develop bifunctional electrocatalysts in the same condition for overall water splitting. So far, many scientists have made efforts to develop highly efficient non-noblemetal-based bifunctional electrocatalysts, such as transition-metal oxides, hydroxides, chalcogenides, sulfides, carbides, nitrides, phosphides and so on.20-29 Among them, heazlewoodite Ni3S2, as a promising electrocatalysts for water splitting, have received considerable attention due to its high conductivity, earth-abundance and ecofriendly.30-32 Nevertheless, the electrocatalytic activity of Ni3S2 materials is still relatively lower than noble-metal materials due to the less surface active sites and poor durability.31, 33 In this regard, many efforts have been developed to regulate the morphology of Ni3S2 materials with 3D nanostructures, especially those grown on conductive substrates, which can expose more surface active sites and accelerate electron transfer rate.34-35 Therefore, developing a facile synthetic method to prepare



Page 4 of 30

3D nanostructured Ni3S2 with more active sites on conductive substrate is an effective route to build a high efficiency electrocatalyst for overall water splitting. Recently, nickel sulfides have been directly supported on conductive substrates as electrocatalytic materials using various methods such as hydrothermal or solvothermal reaction, chemical vapor deposition, electrodeposition and so on. For example, NiS2 nanosheets were developed from Ni(OH)2 on carbon cloth through in situ sulfidization process.36 Ni3S2 nanorods decorated Ni foam were fabricated by hydrothermal reaction.37 NiS microsphere films were grown on Ni foam by a sulfurization process.38 Hierarchically nanoporous Ni3S2 films on copper were in situ fabricated by galvanic replacement reaction method.39 Ni-S alloy was obtained by electrodeposition

method.40

Amorphous

potentiodynamic

deposition

method.41

Ni-S

films

Compared

were with

fabricated other

via

methods,

electrodeposition appears as a cheap, efficient, convenient and less time consuming approach for the quick and scalable synthesis of electrocatalysts. However, the electrocatalysts synthesized by electrodeposition are always amorphous film structures with relatively poor catalytic performance. It is highly desired to expand the scope of electrodeposition approach for the development of unique micro- and nanostructured non-noble-metal-based bifunctional electrocatalysts. Herein, we report the fabrication of 3D coral-like Ni3S2 nanostructure on Ni foam by a facile electrochemical anodization and cathodic deposition strategy. It is worth to mention that our electrochemical anodization and cathodic deposition method is performed on a DC voltage constant-current power with a two-electrode cell, which is different from the ordinary three-electrode electrodeposition process. The relatively high applied voltage and long deposition time guarantee the successful formation of Ni2S3 on Ni foam. In the synthetic process, Ni cations derived from the


Page 5 of 30 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


anodic oxidation of Ni foam at anode serve as a nickel source to form the Ni3S2 samples supported on the Ni foam at cathode by cathodic deposition at almost the same time as the anodic oxidation process. After optimizing the synthesis time, optimal 3D coral-like Ni3S2/NF prepared by anodization and cathodic deposition time of 4 h (Ni3S2/NF-4) shows high electrocatalytic performance toward HER and OER in 1.0 M KOH solution. Moreover, when Ni3S2/NF-4 works as bifunctional electrocatalyst for overall water splitting in a two-electrode system electrolyzer, it needs a low overall water splitting voltage of 1.577 V to deliver 10 mA cm-2, indicating the construction of 3D coral-like Ni3S2/NF nanostructure could be a prospective route towards efficient catalysts for overall water splitting. 2. Experimental section 2.1 Preparation of coral-like Ni3S2 on Ni foam The coral-like Ni3S2 were synthesized by electrochemical anodization and cathodic deposition method in two electrode cell with two pieces of Ni foam (2 cm × 3 cm) as the anodic and cathodic electrodes. Ni foam was successively ultrasonic cleaned in acetone, ethanol and deionized water for 20 min, respectively. A complex aqueous solution containing 1.0 M NH4F and 2.0 M thiourea was used as the electrolyte. The electrochemical anodization and cathodic deposition process was carried out at a constant potential of 4 V on a DC voltage constant-current power supply for various time of 1, 2, 3, 4, and 5 h. The samples prepared by different time were denoted as Ni3S2/NF-1, Ni3S2/NF-2, Ni3S2/NF-3, Ni3S2/NF-4, and Ni3S2/NF-5, respectively. The masses of loaded Ni3S2 are about 2.0, 2.3, 2.5, 2.8 and 3.3 mg cm-2 for Ni3S2/NF-1, Ni3S2/NF-2, Ni3S2/NF-3, Ni3S2/NF-4, and Ni3S2/NF-5, respectively. 2.2 Characterization



The morphologies of as-obtained samples were checked by an SU8020 fieldemission scanning electron microscopy (FESEM, Hitachi, Japan) and a field-emission transmission electron microscopy (FETEM, JEOL, Japan). The phase structures were investigated by X-ray powder diffraction (XRD, Rigaku, Japan). N2 adsorptiondesorption test was carried out by Brunauer−Emmett−Teller (BET, Autosorb-IQ3, American) mehod. X-ray photoelectron spectra (XPS) were recorded on ESCALAB250 (Thermo, American) with a monochromatic Al Kα X-ray as the excitation source. All the XPS spectra were corrected by using the C1s peak at 284.5 eV as a reference. 2.3 Electrochemical measurements The electrocatalytic activities were carried out using a CHI760E electrochemical workstation with three-electrode cell where a saturated calomel electrode (SCE) and graphite rod were used as reference and counter electrodes, respectively. The Ni3S2/NF (1 cm×1 cm) was directly used as the working electrode. For the Pt/C/NF and RuO2/NF, 4 mg of 20% Pt/C or RuO2 was dissolved in 1 ml of the mixture of NMP and H2O (volume ratio is 1:1), and 20 ul of 5% PVDF solution was added in the mixture with ultrasonic treatment to obtain homogenous ink. And then 0.07 ml ink was loaded onto a piece of 1 cm2 Ni foam for 10 times by drop-coating method (loading mass: 2.8 mg cm-2). 1.0 M KOH electrolyte was used for the electrochemical tests. All of the potential values versus the reversible hydrogen electrode (RHE) were calculated by the equation: E(RHE)=E(SCE)+0.0591pH+0.2415 V. Linear sweep voltammetry (LSV) curves were collected at a scan rate of 2 mV s-1. A 90% iRcorrection was applied for all the LSV curves. The electrochemical impedance spectroscopy (EIS) was carried out from 100 kHz and 0.1 Hz. Cyclic voltammetry (CV) tests were carried out with various scan rates ranging from 10 to 200 mV s-1 to


Page 6 of 30

Page 7 of 30 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


estimate the electrochemical active surface areas. The CV potential window for HER is 0.1-0.2 V vs. RHE and for OER is 1.05-1.15 vs. RHE. Amperometric curves and chronopotentiometric curves were conducted to explore the stability of the Ni3S2/NF samples. The multi-potential step (from 100 to 800 mV vs. RHE) and multi-current step (from 10 to 100 mA cm-2) were also measured for HER and OER process of the Ni3S2/NF samples, respectively. The Faradic efficiency of overall water splitting of Ni3S2/NF-4 was estimated by measuring the experimentally generated gas versus the theoretically calculated at 1.65 V for 120 min. The amounts of gas were determinded by a gas chromatography (Fuli, 9790II) connected with a two-electrode cell. 3. Results and discussion 3.1 Synthesis and structural characterization of the 3D Ni3S2/NF The formation mechanism of the three-dimensional (3D) Ni3S2/NF is illustrated in Scheme 1. In the typical electrochemical anodization and cathodic deposition process, the Ni cations (Ni2+) were generated from the anodization of Ni foam (NF) at anode, and then the as-produced Ni2+ were transfered to the cathode under the electric field effect. For cathode, the coral-like Ni3S2 were deposited on NF through a reaction of Ni2+ with thiourea and H2O in the electrolyte. It is worth noting that the anodization and cathodic deposition process occured almost at the same time. The corresponding photographs of the NF at both anode and cathode after different anodization and cathodic deposition time from 1 to 5 h are shown in Figure S1 (Supporting Information). The size of NF on anode became smaller and thinner when the time increased, indicating that the Ni2+ was generated and dissolved into electrolyte, while the color of NF on cathode changed from light grey into dark black, indicating the formation of Ni3S2. The NF on anode provided nickel sources and on cathode acted as



current collector to fabricate the 3D Ni3S2/NF. The possible mechanism can be described by equations as follows: Anode Cathode

Ni

Ni2++2e-

3Ni2+ + 2NH2CSNH2 + 4H2O + 6e-

Ni3S2 + 4NH3 + 2CO2 + 2H2

The microstructures of the 3D Ni3S2/NF samples were studied by FESEM. Figure 1 shows the FESEM images of the 3D Ni3S2/NF prepared by different deposition durations ranging from 1 to 5 h. At the reaction time of 1 h, the interconnected Ni3S2 nanosheets were deposited on the NF and only a small partial of Ni3S2 nanospheets agglomerated into nanospheres. As the time increasing to 2 h, most of Ni3S2 nanosheets gathered into the nanospheres. When prolonged the time to 3 h, the structure of the Ni3S2 became different from the previous two samples. The Ni3S2 nanospheres formed the coral-like morphology and connected between each other with some interspaces. The size of the coral-like Ni3S2 nanospheres became smaller and the interspaces became larger when the time reached to 4 h. This unique corallike structure with some interspaces can provide more active reaction interfaces and facilitate the infiltration of electrolyte and release the generated gas bubbles, which is expected to enhance the electrocatalytic activity. When the time reached to 5 h, the amount of the Ni3S2 nanospheres increased accordingly and became more closely interconnect with each other, leaving less interspaces. The microstructures of the NF on anode after anodization for different time were also investigated by FESEM, as shown in Fig. S2. The surface of NF became more and more rough with the formation of nanoparticles and the size of the nanoparticles decreased with the increasing of time, which suggests the NF was dissolved during the anodization process. The specific surface area and BJH pore size distribution of the 3D Ni3S2/NF samples were measured by the BET method. The N2 adsorption-desorption curves of the 3D


Page 8 of 30

Page 9 of 30 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


Ni3S2/NF samples show the typical IV isotherm with hysteresis loop at a P/P0 from 0.15 to 0.9 (Fig. S3). The specific surface areas of the Ni3S2/NF-1, Ni3S2/NF-2, Ni3S2/NF-3, Ni3S2/NF-4 and Ni3S2/NF-5 are detected to be 13.28, 23.89, 23.91, 29.03 and 24.34 m2 g-1, respectively. The BJH pore size distribution curves illustrate the mesoporous structure of the 3D Ni3S2/NF samples with the main distribution at 3.42, 3.41, 3.44, 3.82 and 3.81 nm for the Ni3S2/NF-1, Ni3S2/NF-2, Ni3S2/NF-3, Ni3S2/NF-4 and Ni3S2/NF-5, respectively (Fig. S4). The mesoporous structure will accelerate the ion transport, thus improving the electrocatalytic activity. TEM images were employed to further demonstrate that the Ni3S2/NF-4 exhibits the coral-like morphology with interconnected nanospheres (Fig. 2a). The amplified TEM image in Fig. 2b shows the nanospheres are composed of nanosheets with crumpled structure. The HRTEM image of the Ni3S2/NF-4 (Fig. 2c) shows obvious lattice fringes with a spacing of 0.20 nm corresponding to (202) planes of Ni3S2. Moreover, the chemical components and the spatial distribution of the corresponding elements in the Ni3S2/NF-4 were investigated by the elemental mapping analysis, as shown in Fig. 2d. The mapping results demonstrate the uniformly distributed Ni and S components in the Ni3S2/NF-4. Figure 3a shows the XRD pattern of 3D Ni3S2/NF prepared by different time, from which similar diffraction characteristics can be observed. All the diffraction peaks can be indexed to the planes of Ni3S2 (JCPDS no.44-1418) except for the three additional peaks at 44.5°, 51.8°, and 76.4°corresponding well to (111), (200) and (220) planes of the NF substrate (JCPDS no.04-0850), respectively. In order to further investigate the structural composition of Ni3S2, we then studied the Raman spectra of these Ni3S2/NF samples. As shown in Fig. 3b, five distinct peaks locating at 199, 221, 302, 323 and 348 cm-1 are observed for all samples, consistent with the Raman peaks



of Ni3S2 reported in the literature.42 To obtain the surface information of the Ni3S2/NF, we further employed the XPS measurement on Ni3S2/NF-4. The survey XPS spectrum (Fig. S5a) indicates that the Ni3S2/NF-4 consists of Ni and S. The C signal can be attributed to adventitious carbon from the XPS instrument itself. The O signal can be divided into two peaks at 531.3 eV and 532.4 eV by Gaussian fitting (Fig. S5b), which are assigined to the hydroxyl group and absorbed H2O, respectively.43-44 There is no obvious peak of lattice oxygen loaded at about 529.9 eV in the O 1s spectrum, demonstrating the presence of pure Ni3S2 in the samples without further oxidized.44 A Gaussian fitting of the Ni 2p spectrum in Fig. 3c shows that the Ni 2p3/2 peak is divided into three peaks at 853.7, 856.5, and 860.2 eV and the Ni 2p1/2 peak also reveals the three peaks at 870.9, 874.6, and 879.5 eV. The peaks located at 853.7 and 870.9 eV can be assigned to Ni 2p3/2 and 2p1/2, respectively, indicating the existence of Ni2+. The peaks located at 856.5 and 874.6 eV can be attributed to the 2p3/2 and 2p1/2 of Ni3+, respectively.45 This demonstrates the state of Ni in Ni3S2 is mixed phase of Ni2+ and Ni3+. Besides, the peaks at 860.2 and 879.5 eV are the shake-up satellite peaks.46 Fig. 3d displays the S 2p spectrum with the peaks of 161.3 and 162.6 eV corresponding to the S 2p3/2 and 2p1/2 in Ni3S2, respectively.46 3.2 HER activities of the 3D Ni3S2/NF The electrocatalytic properties of the 3D Ni3S2/NF for various reactions were then investigated. First, the electrocatalytic performance of the 3D Ni3S2/NF samples for HER was evaluated in a three-electrode cell in 1.0 M KOH. For comparision, HER performance of bare NF and 20% Pt/C were also studied. The polarization curves of the various Ni3S2/NF samples, Pt/C/NF and bare NF with 90% iR-correction were measured at 2 mV s-1. As displayed in Fig. 4a, the bare NF exhibits the lowest catalytic activity towards the HER whereas the Pt/C/NF displays the best catalytic


Page 10 of 30

Page 11 of 30 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


activity with a near-zero onset potential. For Ni3S2/NF samples, the cathodic currennt densities are singnificantly improved compared to bare NF. Among the Ni3S2/NF samples, the Ni3S2/NF-4 exhibits the smallest onset potential of 32 mV (-2 mA cm-2), among all five Ni3S2/NF samples. Besides, the Ni3S2/NF-4 requires the overpotential of 89 and 186 mV to deliver -10 and -100 mA cm-2, respectively, which is much lower than other Ni3S2/NF counterparts and some recently reported HER electrocatalysts (Fig. S6, Table S1). The electrocatalytic kinetics of HER is investigated by the Tafel plots. Fig. 4b displays the Tafel plots of various Ni3S2/NF samples. Evidently, the Ni3S2/NF-4 exhibits a much lower Tafel slope of 85 mV dec-1 in comparison of Ni3S2/NF-1 (136 mV dec-1), Ni3S2/NF-2 (128 mV dec-1), Ni3S2/NF-3 (110 mV dec-1), Ni3S2/NF-5 (103 mV dec-1) and bare NF (151 mV dec-1), and only slight larger than the benchmark Pt/C/NF(42 mV dec-1), demonstrating the higher HER kinetics reaction indeed taken place on the surface of Ni3S2/NF-4. To further explore the kinetics of the Ni3S2/NF samples under HER processes, EIS tests were recorded in a frequency from 100 kHz to 0.1 Hz at an overpotential of 200 mV vs. RHE. As shown in Fig. 4c, all the control Ni3S2/NF catalysts provide the semicircles in the plane plots. Generally, smaller diameter of the semicircle represents the lower charge transfer resistance (Rct) at the interface between catalyst and electrolyte. The Rct is known to be related with the catalytic kinetics and its lower value represents the faster electron transfer rate.47 Apparently, the Ni3S2/NF-4 exhibits the minimum Rct value compared to that of other control electrocatalysts, indicating the more rapid electron transfer rate of Ni3S2/NF-4. The lower Tafel slop and Rct value of the Ni3S2/NF-4 suggest that it possesses high HER performce.



Page 12 of 30

The apparent HER performce of the electrocatalysts can be ascribed to electrochemical active surface areas (ECSA), which could be obtained from the double layer capacitance (Cdl) using a simple CV test (Fig. S7). The Ni3S2/NF-4 possesses the largest Cdl of 98.4 mF cm-2 among various Ni3S2/NF electrocatalysts, as shown in Fig. 4d, indicating a much larger ECSA and more active reaction sites of Ni3S2/NF-4. The calculated ECSA of the Ni3S2/NF samples are 942.5, 1672.5, 1832.5, 2460, 2307.5 cm2, respectively (details in the experimental section of Supporting Information). It is noted that the increased ECSA and active reaction sites of Ni3S2/NF-4 are attributed to the unique 3D coral-like Ni3S2 nanospheres structrue as revealed by the SEM results. Fig. 4e shows multi-step chronoamperometric curves of the various Ni3S2/NF samples recorded under different overpotentials ranging from 100 to 800 mV. Upon increasing the overpotential, the current densities of all the Ni3S2/NF samples rise accordingly and remain steady for 500 s, indicating the excellent mass transport properties of the Ni3S2/NF catalysts.48 The current densities delivered from Ni3S2/NF4 are much larger than those of other Ni3S2/NF electrocatalysts under the same overpotential,

revealing

the

enhanced

HER

activity

of

Ni3S2/NF-4.

The

chronoamperometric test was carried out to inspect the stability of Ni3S2/NF-4. As shown in Fig. 4f, there is a negligible regression in the polarization curve after chronoamperometric test for 40 h and the current density remians almost a straight line under an overpotential of 150 mV for up to 40 h, indicating a long-term extraordinary durability of Ni3S2/NF-4. The concentration of Ni dissolved in the electrolyte after the chronoamperometric test was measured by inductively coupled plasma mass spectrometry (ICP-MS), and the dissolution rate of Ni was about 0.242 µg g-1 h-1. The SEM, XRD, and XPS (Fig. S8) results further demonstrate that the


Page 13 of 30 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


morphology, composition and chemical state of the post-Ni3S2/NF-4 remain unchanged after HER tests. All above results reveal the excellent stability of the Ni3S2/NF-4 for HER. 3.3 OER activities of the 3D Ni3S2/NF Besides working well as an electrocatalyst for the HER, the electrocatalytic properties of obtained Ni3S2/NF-4 towards the OER in 1.0 M KOH were investigated. The polarization curves of the various Ni3S2/NF samples with 90% iR-correction were performed on a three-electrode cell with a scan rate of 2 mV s-1. The bare NF and RuO2/NF as two references were also evaluated for comparison. As displayed in Fig. 5a, the bare NF shows the sluggish electrocatalytic activity towards the OER, whereas the RuO2/NF shows superior electrocatalytic activity with an onset potential of 209 mV (2 mA cm-2). The Ni3S2/NF-4 shows an onset potential of 202 mV (2 mA cm-2) and requires overpotentials of 242 mV and 318 mV to afford 10 and 100 mA cm-2, respectively, which are smaller than those of other control Ni3S2/NF samples (Fig. S9). Of note, the electrocatalytic activity of Ni3S2/NF-4 for OER also surpasses the RuO2/NF (259 mV at 10 mA cm-2). All these results suggest that Ni3S2/NF-4 possesses high electrocatalytic performance towards the OER. These overpotentials for Ni3S2/NF-4 compare favorably to that of most other high-performance metal sulfides (Table S2). On the other hand, the Tafel slopes are determined to be 177, 147, 140, 108, 76, 103 and 106 mV dec-1 for the bare NF, Ni3S2/NF-1, Ni3S2/NF-2, Ni3S2/NF-3, Ni3S2/NF-4, Ni3S2/NF-5 and RuO2/NF, respectively (Fig. 5b). Similar as surveyed for HER, Ni3S2/NF-4 exhibits the best OER performance due to its unique coral-like structure with faster electron transfer rate and higher electrocatalytic kinetics. This is due to the improved electron transfer rate and the larger ECSA as



indicated respectively by the comparision of corresponding Rct and Cdl of the Ni3S2/NF samples (Fig. 5c, d, and Fig. S10)33. The multi-step chronopotentiometric tests were carried out under different current densities in the range of 10-100 mA cm-2 for the various Ni3S2/NF samples, as shown in Fig. 5e. In the begining of the chronopotentiometric tests, the potential rapidly responds and gets stabilized for the rest 500 s, and the similar results are also reflected by othe steps, indicating good conductivity, mass transport properties and mechanical robustness of the Ni3S2/NF catalysts.3, 49 The overpotentials required for Ni3S2/NF-4 to maintain the given current densities are lower than those of other Ni3S2/NF samples. The electrochemical stabilty of Ni3S2/NF-4 for the OER was also checked by chronoamperometric test at an overpotential of 250 mV. The polarization curve of Ni3S2/NF-4 shown in Fig. 5f displays negligible regression after the chronoamperometric test and the current density remians almost no change under a constant overpotential of 250 mV for up to 40 h, indicating the excellent durability of the Ni3S2/NF-4. The concentration of Ni dissolved in the electrolyte was also evaluated by ICP-MS, and the dissolution rate of Ni was about 0.196 µg g-1 h-1. The structrue and composition of the Ni3S2/NF-4 was further examed by characterization the post-Ni3S2/NF-4 with XRD, SEM, EDS, and XPS (Fig. S11). After OER tests, the microstructure of the Ni3S2/NF-4 observed from the SEM image has a subtle changed, which the nanospheres assemble together but still remain the coral-like shapes with some interspaces. The changed microstructure can be attributed to the slight oxidation of the Ni3S2/NF-4 to form the nickel (oxy)hydroxides species on the surface. However, the nickel (oxy)hydroxides species have not been detected by XRD, which is due to the small amount of nickel (oxy)hydroxides on the surface. The formation of nickel (oxy)hydroxides on the surface of Ni3S2/NF-4 can be further confirmed by the XPS


Page 14 of 30

Page 15 of 30 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


analysis. The Ni 2p spectrum of Ni3S2/NF-4 after OER tests displays the increasing intensity of the Ni3+ peak, indicating the existence of surface oxidation state of Ni species (nickel (oxy)hydroxides) in Ni3S2/NF-4.50-51 The formation of nickel (oxy)hydroxides species on the surface was regarded as the active sites to promote the OER performance.50, 52 Furthermore, an additional peak of SO42- appeared at 168.5 V belongs to the oxidation of S at the Ni3S2/NF-4 surface under alkaline media for long time.53 Additionally, the O 1s spectrum reveals the existing of lattice oxygen at the peak of 529.9 eV except for the hydroxyl group and absorbed H2O in the Ni3S2/NF-4, indicating the formation of nickel (oxy)hydroxides on the surface.44 The selfformation of nickel (oxy)hydroxides leads to a remarkable electrocatalytic performance and durability for the OER. 3.4 Overall water-splitting activities of the 3D Ni3S2/NF Finally, noticing the Ni3S2/NF-4 exhibits good electrocatalytic activity towards both HER and OER, a bifunctional catalyst was expected for overall water splitting. An electrolyzer with two symmetrical Ni3S2/NF-4 as the anode and the cathode was investigated in 1 M KOH solution. Meanwhile, compared electrolyzers comprising 20% Pt/C/NF with RuO2/NF (Pt/C/NF//RuO2/NF) as well as a pair of bare NFs, were also fabricated. Fig. 6a displays the polarization curves of Ni3S2/NF-4//Ni3S2/NF-4, NF//NF, and Pt/C/NF//RuO2/NF. A high applied voltage of 1.884 V is needed for the NF//NF to deliver a current density of 10 mA cm-2. The Pt/C/NF//RuO2/NF shows better electrocatalytic performance with an applied voltage of 1.556 V to afford 10 mA cm-2. The Ni3S2/NF-4//Ni3S2/NF-4 displays favorable comparable activity to that of Pt/C/NF//RuO2/NF, which requires an applied voltage of 1.577 V to reach the same current density. The remarkable electrocatalytic performance of the Ni3S2/NF4//Ni3S2/NF-4 is better than or quite close to most of the reported bifunctional



electrocatalysts (Table S3). The multi-step chronopotentiometric curve was also carried out to investigate the capability of the Ni3S2/NF-4//Ni3S2/NF-4 electrolyzer under varying current densities ranging from 5 to 100 mA cm-2 (Fig. 6b). The applied voltage quickly responds and levels off at 1.51 V at 5 mA cm-2 and then it gets stabilized quickly as the current density increases. Furethermore, the stability of Ni3S2/NF-4//Ni3S2/NF-4 electrolyzer was investigated by both chronopotentiometric and chronoamperometric tests as illustrated in Fig. 6c and d, respectively. Impressively, a nearly constant applied voltage could be kept at 1.72 V under 20 mA cm-2 for 40 h and obvious H2 and O2 bubbles were evolved continuously at its respective Ni3S2/NF-4 electrode (inset of Figure 6c). The current density shows only a slight degradation under the applied voltage of 1.65 V for 40 h, revealing good electrocatalytic stability upon long-term test. The Faradaic efficiency was estimated by measuring the amount of H2 and O2 gas at 1.65 V for 120 min via a gas chromatography (Fuli, 9790II). As shown in Figure S12, the amount of experimentally generated gas fits well with that of theoretically calculated, illustating a near-100% Faradaic efficiency. Based on the above results, the excellent electrocatalytic performance and durability of the Ni3S2/NF-4 electrocatalyst could be attributed to the following four factors: firstly, the Ni3S2 nanospheres assembled with ultrathin nanosheets expose more active sites on the surfaces, which accelerate the transmission of the active species and improve the surface electrochemical reactions. Secondly, the unique 3D coral-like structure with with some interspaces can prvide more active reaction interfaces, which facilitates the infiltration of electrolyte and releases the generated gas bubbles. Thirdly, the Ni3S2 nanospheres directly deposited on Ni foam without adding extra adhesive possess high electrical conductivity that accelerate charge transport in the HER and OER. Lastly, the slight surface oxidization


Page 16 of 30

Page 17 of 30 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


of Ni3S2 can provide the active species to promote the OER performance. Thus, the unique 3D coral-like Ni3S2/NF-4 electrocatalyst exhibits a superior water splitting performance. 4. Conclusion In summary, we reported a facile electrochemical anodization and cathodic deposition strategy to synthesize the 3D coral-like Ni3S2/NF used directly as a highperformance bifunctional electrocatalyst for overall water splitting in alkaline condition. We demonstrated that the synthesis time influences the morphological structure of the Ni3S2/NF samples, which has significant effects on their electrocatalytic activity. Due to the unique 3D coral-like structure and the intrinsic catalytic ability of nickel sulfide catalysts, the optimized Ni3S2/NF-4 shows the highest electrocatalytic performances and excellent long-term durability toward both HER and OER in alkaline condition. Moreover, the as-prepared Ni3S2/NF-4 can be employed as bifunctional electrocatalyst for overall water splitting.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Photographs and SEM images, XPS spectra, CV curves, SEM images, XRD patterns and XPS spectra of the Ni3S2/NF-4 after HER and OER test, Faradic efficiency for overall water splitting, comparison of the HER, OER, and overall water splitting performances (PDF). AUTHOR INFORMATION



Corresponding Author *E-mail: [email protected]; [email protected]; [email protected] ORCID

Notes The authors declare no competing financial interest. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51772072, 51502071, 51402081), the Natural Science Foundation of Anhui Province (1708085ME100, 1508085ME97, 1608085QE105), the Fundamental Research Funds for the Central Universities (JZ2017HGTB0203, JZ2016HGTB0719, 201610359018, 2018CXCYS254). We also would like to thank the financial support from the 111 Project (B18018).


Page 18 of 30

Page 19 of 30 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


References (1) M. S. Dresselhaus and I. L. Thomas, Alternative Energy Technologies Natrue, 2001, 414, 332-337. (2) P. M. Grant, Hydrogen Lifts Off-with a Heavy Load, Natrue, 2003, 424, 129-130. (3) Tang, C.; Cheng, N.; Pu, Z.; Xing, W.; Sun, X. NiSe Nanowire Film Supported on Nickel Foam: An Efficient and Stable 3D Bifunctional Electrode for Full Water Splitting. Angew. Chem. Int. Ed. 2015, 54, 9351-9355. (4) Zhu, Y.-P.; Liu, Y.-P.; Ren, T.-Z.; Yuan, Z.-Y. Self-Supported Cobalt Phosphide Mesoporous Nanorod Arrays: A Flexible and Bifunctional Electrode for Highly Active Electrocatalytic Water Reduction and Oxidation. Adv. Funct. Mater. 2015, 25, 7337-7347. (5) Ray, C.; Lee, S. C.; Sankar, K. V.; Jin, B.; Lee, J.; Park, J. H.; Jun, S. C. Amorphous PhosphorusIncorporated Cobalt Molybdenum Sulfide on Carbon Cloth: An Efficient and Stable Electrocatalyst for Enhanced Overall Water Splitting over Entire pH Values. ACS Appl. Mater. Interfaces 2017, 9, 3773937749. (6) Kong, D.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 nanoparticles grown on carbon fiber paper: an efficient and stable electrocatalyst for hydrogen evolution reaction. J. Am. Chem. Soc. 2014, 136, 4897-900. (7) Ming, F.; Liang, H.; Shi, H.; Xu, X.; Mei, G.; Wang, Z. MOF-derived Co-doped nickel selenide/C electrocatalysts supported on Ni foam for overall water splitting. J. Mater. Chem. A 2016, 4, 1514815155. (8) Wang, Z.; Zeng, S.; Liu, W.; Wang, X.; Li, Q.; Zhao, Z.; Geng, F. Coupling Molecularly Ultrathin Sheets of NiFe-Layered Double Hydroxide on NiCo2O4 Nanowire Arrays for Highly Efficient Overall Water-Splitting Activity. ACS Appl. Mater. Interfaces 2017, 9, 1488-1495. (9) Chen, W.; Liu, Y.; Li, Y.; Sun, J.; Qiu, Y.; Liu, C.; Zhou, G.; Cui, Y. In Situ Electrochemically Derived Nanoporous Oxides from Transition Metal Dichalcogenides for Active Oxygen Evolution Catalysts. Nano Lett. 2016, 16, 7588-7596. (10) Zhao, Y.; Jia, X.; Chen, G.; Shang, L.; Waterhouse, G. I.; Wu, L. Z.; Tung, C. H.; O'Hare, D.; Zhang, T. Ultrafine NiO Nanosheets Stabilized by TiO2 from Monolayer NiTi-LDH Precursors: An Active Water Oxidation Electrocatalyst. J. Am. Chem. Soc. 2016, 138, 6517-6524. (11) Voiry, D.; Salehi, M.; Silva, R.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Conducting MoS2 Nanosheets as Catalysts for Hydrogen Evolution Reaction. Nano Lett. 2013, 13, 6222-6227. (12) Ma, C. B.; Qi, X.; Chen, B.; Bao, S.; Yin, Z.; Wu, X. J.; Luo, Z.; Wei, J.; Zhang, H. L.; Zhang, H. MoS2 Nanoflower-Decorated Reduced Graphene Oxide Paper for High-Performance Hydrogen Evolution Reaction. Nanoscale 2014, 6, 5624-5629. (13) Voiry, D.; Yang, J.; Chhowalla, M. Recent Strategies for Improving the Catalytic Activity of 2D TMD Nanosheets Toward the Hydrogen Evolution Reaction. Adv. Mater. 2016, 28, 6197-6206. (14) Zhao, X.; Zhu, H.; Yang, X. Amorphous Carbon Supported MoS2 Nanosheets as Effective Catalysts for Electrocatalytic Hydrogen Evolution. Nanoscale 2014, 6, 10680-10685. (15) Liu, T.; Liu, Q.; Asiri, A. M.; Luo, Y.; Sun, X. An Amorphous CoSe Film Behaves as an Active and Stable Full Water-Splitting Electrocatalyst under Strongly Alkaline Conditions. Chem. Commun. 2015, 51, 16683-16686. (16) Li, H.; Chen, S.; Jia, X.; Xu, B.; Lin, H.; Yang, H.; Song, L.; Wang, X. Amorphous Nickel-Cobalt Complexes Hybridized with 1T-Phase Molybdenum Disulfide via Hydrazine-Induced Phase Transformation for Water Splitting. Nat. Commun. 2017, 8, 15377. (17) Li, K.; Zhang, J.; Wu, R.; Yu, Y.; Zhang, B. Anchoring CoO Domains on CoSe2 Nanobelts as Bifunctional Electrocatalysts for Overall Water Splitting in Neutral Media. Adv. Sci. 2016, 3, 1500426. (18) Zhou, X.; Liu, Y.; Ju, H.; Pan, B.; Zhu, J.; Ding, T.; Wang, C.; Yang, Q. Design and Epitaxial Growth of MoSe2–NiSe Vertical Heteronanostructures with Electronic Modulation for Enhanced Hydrogen Evolution Reaction. Chem. Mater. 2016, 28, 1838-1846. (19) Shi, H.; Liang, H.; Ming, F.; Wang, Z. Efficient Overall Water-Splitting Electrocatalysis Using Lepidocrocite VOOH Hollow Nanospheres. Angew. Chem. Int. Ed. 2017, 56, 573-577. (20) Fan, X.; Liu, Y.; Peng, Z.; Zhang, Z.; Zhou, H.; Zhang, X.; Yakobson, B. I.; Goddard, W. A., 3rd; Guo, X.; Hauge, R. H.; Tour, J. M. Atomic H-Induced Mo2C Hybrid as an Active and Stable Bifunctional Electrocatalyst. ACS Nano 2017, 11, 384-394. (21) Zhang, L.; Xie, L.; Ma, M.; Qu, F.; Du, G.; Asiri, A. M.; Chen, L.; Sun, X. Co-Based Nanowire Films as Complementary Hydrogen- and Oxygen-Evolving Electrocatalysts in Neutral Electrolyte. Catal. Sci. Technol. 2017, 7, 2689-2694.



(22) 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-WaterSplitting Activity. ACS Appl. Mater. Interfaces 2017, 9, 15364-15372. (23) S. Chen, S. Z. Qiao, Hierarchically Porous Nitrogen-Doped Graphene–NiCo2O4 Hybrid Paper as an Advanced Electrocatalytic Water-Splitting Material. ACS Nano 2013, 7, 10190-10196.. (24) Zheng, J.; Zhou, W.; Liu, T.; Liu, S.; Wang, C.; Guo, L. Homologous NiO//Ni2P Nanoarrays Grown on Nickel Foams: a Well Matched Electrode Pair with High Stability in Overall Water Splitting. Nanoscale 2017, 9, 4409-4418. (25) Yu, Z. Y.; Duan, Y.; Gao, M. R.; Lang, C. C.; Zheng, Y. R.; Yu, S. H. A One-Dimensional Porous Carbon-Supported Ni/Mo2C Dual Catalyst for Efficient Water Splitting. Chem. Sci. 2017, 8, 968-973. (26) Li, J.; Yan, M.; Zhou, X.; Huang, Z.-Q.; Xia, Z.; Chang, C.-R.; Ma, Y.; Qu, Y. Mechanistic Insights on Ternary Ni2−xCoxP for Hydrogen Evolution and Their Hybrids with Graphene as Highly Efficient and Robust Catalysts for Overall Water Splitting. Adv. Funct. Mater. 2016, 26, 6785-6796. (27) Zhang, Q.; Wang, Y.; Wang, Y.; Al-Enizi, A. M.; Elzatahry, A. A.; Zheng, G. Myriophyllum-Like Hierarchical TiN@Ni3N Nanowire Arrays for Bifunctional Water Splitting Catalysts. J. Mater. Chem. A 2016, 4, 5713-5718. (28) 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. (29) Xiao, X.; Huang, D.; Fu, Y.; Wen, M.; Jiang, X.; Lv, X.; Li, M.; Gao, L.; Liu, S.; Wang, M.; Zhao, C.; Shen, Y. Engineering NiS/Ni2P Heterostructures for Efficient Electrocatalytic Water Splitting. ACS Appl. Mater. Interfaces 2018, 10, 4689-4696. (30) 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-WaterSplitting Activity. Angew. Chem. Int. Ed. 2016, 55, 6702-6707. (31) Feng, L. L.; Yu, G.; Wu, Y.; Li, G. D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X. High-Index Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14023-14026. (32) Qu, Y.; Yang, M.; Chai, J.; Tang, Z.; Shao, M.; Kwok, C. T.; Yang, M.; Wang, Z.; Chua, D.; Wang, S.; Lu, Z.; Pan, H. Facile Synthesis of Vanadium-Doped Ni3S2 Nanowire Arrays as Active Electrocatalyst for Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2017, 9, 5959-5967. (33) Lv, J. J.; Zhao, J.; Fang, H.; Jiang, L. P.; Li, L. L.; Ma, J.; Zhu, J. J. Incorporating Nitrogen-Doped Graphene Quantum Dots and Ni3S2 Nanosheets: A Synergistic Electrocatalyst with Highly Enhanced Activity for Overall Water Splitting. Small 2017, 13, 1700264. (34) Tang, C.; Pu, Z.; Liu, Q.; Asiri, A. M.; Luo, Y.; Sun, X. Ni3S2 Nanosheets Array Supported on Ni Foam: A Novel Efficient Three-Dimensional Hydrogen-Evolving Electrocatalyst in both Neutral and Basic Solutions. Int.J. Hydrogen Energy 2015, 40, 4727-4732. (35) Tong, M.; Wang, L.; Yu, P.; Tian, C.; Liu, X.; Zhou, W.; Fu, H. Ni3S2 Nanosheets in Situ Epitaxially Grown on Nanorods as High Active and Stable Homojunction Electrocatalyst for Hydrogen Evolution Reaction. ACS Sustainable Chem. Eng. 2018, 6, 2474-2481. (36) Tang, C.; Pu, Z.; Liu, Q.; Asiri, A. M.; Sun, X. NiS2 Nanosheets Array Grown on Carbon Cloth as an Efficient 3D Hydrogen Evolution Cathode. Electrochim. Acta 2015, 153, 508-514. (37) Zhou, W.; Wu, X.-J.; Cao, X.; Huang, X.; Tan, C.; Tian, J.; Liu, H.; Wang, J.; Zhang, H. Ni3S2 Nanorods/Ni foam Composite Electrode with Low Overpotential for Electrocatalytic Oxygen Evolution. Energ. Environ. Sci. 2013, 6 (10), 2921-2924. (38) Zhu, W.; Yue, X.; Zhang, W.; Yu, S.; Zhang, Y.; Wang, J.; Wang, J. Nickel Sulfide Microsphere Film on Ni Foam as an Efficient Bifunctional Electrocatalyst for Overall Water Splitting. Chem. Commun. 2016, 52, 1486-1489. (39) Yang, C.; Gao, M. Y.; Zhang, Q. B.; Zeng, J. R.; Li, X. T.; Abbott, A. P. In-Situ Activation of Self-Supported 3D Hierarchically Porous Ni3S2 Films Grown on Nanoporous Copper as Excellent pHUniversal Electrocatalysts for Hydrogen Evolution Reaction. Nano Energy 2017, 36, 85-94,. (40) Han, Q. A Study on The Electrodeposited Ni–S Alloys as Hydrogen Evolution Reaction Cathodes. Int.J. Hydrogen Energy 2003, 28, 1207-1212. (41) Jiang, N.; Bogoev, L.; Popova, M.; Gul, S.; Yano, J.; Sun, Y. Electrodeposited Nickel-Sulfide Films as Competent Hydrogen Evolution Catalysts in Neutral Water. J. Mater. Chem. A 2014, 2, 19407-19414. (42) Zhang, Z.; Liu, X.; Qi, X.; Huang, Z.; Ren, L.; Zhong, J. Hydrothermal Synthesis of Ni3S2/Graphene Electrode and Its Application in a Supercapacitor. RSC Adv. 2014, 4, 37278-37283.


Page 20 of 30

Page 21 of 30 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


(43) Xiao, C.; Zhang, X.; Li, S.; Suryanto, B. H. R.; MacFarlane, D. R., In Situ Synthesis of Core– Shell-Ni3Fe(OH)9/Ni3Fe Hybrid Nanostructures as Highly Active and Stable Bifunctional Catalysts for Water Electrolysis. ACS Appl. Energy Mater. 2018, 1, 986-992. (44) Wu, Z.; Wang, Z.; Geng, F., Radially Aligned Hierarchical Nickel/Nickel-Iron (Oxy)hydroxide Nanotubes for Efficient Electrocatalytic Water Splitting. ACS Appl. Mater. Interfaces 2018, 10, 85858593. (45) Li, X.; Wu, H.; Elshahawy, A. M.; Wang, L.; Pennycook, S. J.; Guan, C.; Wang, J., Cactus-Like NiCoP/NiCo-OH 3D Architecture with Tunable Composition for High-Performance Electrochemical Capacitors. Adv. Funct.Mater. 2018, 28, 1800036. (46) Yang, Y.; Zhang, K.; Lin, H.; Li, X.; Chan, H. C.; Yang, L.; Gao, Q. MoS2–Ni3S2 Heteronanorods as Efficient and Stable Bifunctional Electrocatalysts for Overall Water Splitting. ACS Catal. 2017, 7, 2357-2366. (47) Wu, C.; Yang, Y.; Dong, D.; Zhang, Y.; Li, J. In Situ Coupling of CoP Polyhedrons and Carbon Nanotubes as Highly Efficient Hydrogen Evolution Reaction Electrocatalyst. Small 2017, 13, 1602873. (48) Wu, Y.; Li, G.-D.; Liu, Y.; Yang, L.; Lian, X.; Asefa, T.; Zou, X. Overall Water Splitting Catalyzed Efficiently by an Ultrathin Nanosheet-Built, Hollow Ni3S2-Based Electrocatalyst. Adv. Funct. Mater. 2016, 26, 4839-4847. (49) Huang, H.; Yu, C.; Zhao, C.; Han, X.; Yang, J.; Liu, Z.; Li, S.; Zhang, M.; Qiu, J. Iron-Tuned Super Nickel Phosphide Microstructures with High Activity for Electrochemical Overall Water Splitting. Nano Energy 2017, 34, 472-480. (50) Xi, W.; Ren, Z.; Kong, L.; Wu, J.; Du, S.; Zhu, J.; Xue, Y.; Meng, H.; Fu, H., Dual-Valence Nickel Nanosheets Covered withThin Carbon as Bifunctional Electrocatalysts for Full Water Splitting. J. Mater. Chem. A 2016, 4, 7297-7304. (51) Wang, J.; Ma, X.; Qu, F.; Asiri, A. M.; Sun, X., Fe-Doped Ni2P Nanosheet Array for HighEfficiency Electrochemical Water Oxidation. Inorg. Chem. 2017, 56, 1041-1044. (52) Chen, G.-F.; Ma, T. Y.; Liu, Z.-Q.; Li, N.; Su, Y.-Z.; Davey, K.; Qiao, S.-Z. Efficient and Stable Bifunctional Electrocatalysts Ni/NixMy (M = P, S) for Overall Water Splitting. Adv. Funct. Mater. 2016, 26, 3314-3323. (53) Zhang, G.; Feng, Y.-S.; Lu, W.-T.; He, D.; Wang, C.-Y.; Li, Y.-K.; Wang, X.-Y.; Cao, F.-F., Enhanced Catalysis of Electrochemical Overall Water Splitting in Alkaline Media by Fe Doping in Ni3S2 Nanosheet Arrays. ACS Catal. 2018, 8, 5431-5441.



Page 22 of 30

Figure Captions Scheme 1 Schematic illustration of the synthesis of the 3D coral-like Ni3S2/NF samples. Fig. 1

SEM images of various Ni3S2/NF samples prepared by anodization and

cathodic deposition at different time: (a) 1h, (b) 2h, (c) 3 h, (d) 4h, and (e) 5 h. Fig. 2 (a,b) TEM, (c) HRTEM, and (d) EDS mapping images of Ni3S2/NF-4 sample. Fig. 3 Structural and chemical compositional information of the Ni3S2/NF samples. (a,) XRD spectra of Ni3S2/NF samples, (b) Raman spectral of Ni3S2/NF samples (c) Ni 2p spectrum of Ni3S2/NF-4, and (d) Ni 2p spectrum of Ni3S2/NF-4. Fig. 4 Electrocatalytic HER activities of the Ni3S2/NF samples in 1.0 M KOH. (a) polarization curves with iR correction; (b) Tafel plots; (c) Nyquist plots; (d) Cdl; (e) multi-step chronoamperometric curves. The overpotential increases from 100 to 800 mV; and (f) The time-depencent current density curve of Ni3S2/NF-4 under a static overpotential of 150 mV for 40 h and the polarization curves before and after stability tests. Fig. 5 Electrocatalytic OER activities of the Ni3S2/NF samples in 1.0 M KOH. (a) polarization curves with iR correction; (b) Tafel plots; (c) Nyquist plots; (d) Cdl; (e) multi-step chronopotentiometric curves. The current density increases from 10 to 100 mA cm-2; and (f) The time-depencent current density curve of Ni3S2/NF-4 under a static overpotential of 250 mV for 40 h and the polarization curves before and after stability tests. Fig. 6 Overall water splitting activities of the Ni3S2/NF//Ni3S2/NF. (a) Polarization curves of NiF//NiF, Pt/C/NF//Pt/C/NF and Ni3S2/NF//Ni3S2/NF; (b) multi-step chronopotentiometric curves of Ni3S2/NF//Ni3S2/NF. The current density increases from 5 to 100 mA cm-2; (c) Chronopotentiometric curve of Ni3S2/NF//Ni3S2/NF at 20 mA cm-2 for 40 h. The inset photograph shows the H2 and O2 bubbles evolved on the cathode

and

anode,

respecdtively,

during

chronopotentiometric

test

Chronoamperometric curve of Ni3S2/NF//Ni3S2/NF at a voltage of 1.65 V for 40 h.


(d)

Page 23 of 30 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


Scheme 1



Fig. 1


Page 24 of 30

Page 25 of 30 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


Fig. 2



Fig. 3


Page 26 of 30

Page 27 of 30 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


Fig. 4



Fig. 5


Page 28 of 30

Page 29 of 30 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


Fig. 6



Graphical abstract


Page 30 of 30

3D coral-like Ni3S2 on Ni foam as a bifunctional electrocatalyst for

Recommend Documents