3D

Subscriber access provided by BUFFALO STATE

Article

Freestanding and Hierarchically Structured Au-Dendrites/ 3D-Graphene Scaffold Supports Highly Active and Stable Ni3S2 Electrocatalyst towards Overall Water Splitting Hong-Chi Tsai, Balaraman Vedhanarayanan, and Tsung-Wu Lin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00428 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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 44 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 Energy Materials

Freestanding and Hierarchically Structured Au-Dendrites/ 3DGraphene Scaffold Supports Highly Active and Stable Ni3S2 Electrocatalyst towards Overall Water Splitting Hong-Chi Tsai,† Balaraman Vedhanarayanan,† Tsung-Wu Lin†*

†Mr.

H. C. Tsai, Dr. B. Vedhanarayanan, and Prof. T. W. Lin

Department of Chemistry, Tunghai University, No.1727, Sec.4, Taiwan Boulevard, Xitun District, Taichung 40704, Taiwan E-mail: [email protected]

* Corresponding author. Tel.: +886(4)23590121 Ext. 32250; Fax: +886(4)23590426. E-mail address: [email protected]

1 ACS Paragon Plus Environment

ACS Applied Energy Materials 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 A free-standing three-dimensional (3D) electrocatalyst is demonstrated as an example of finetuning the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities through a simple electrodeposition method. The Ni3S2 nanosheets are electrodeposited onto the highly conducting Au dendrites that are grown over 3D-graphene (3DG) framework. The highest electrochemical surface area has been achieved to enhance exposure of active sites by the integration of hierarchical Au dendrites with a sufficient amount of Ni3S2 nanosheets. The optimized electrocatalyst (3DG-Au-Ni3S2-15c) delivers the current density of 10 mA cm−2 at an overpotential of 140 mV and a lowest Tafel slope of 93 mV dec-1 for HER. The higher HER activity is supported by the smaller charge transfer resistance of 1.09 , which confirms the faster interfacial electron-transfer between the electrode and electrolyte phases. The OER experiments suggest that 3DG-Au-Ni3S2 (15c) has a higher current density of 91.2 mA cm−2 at a potential of 1.6 V and a lower Tafel slope of 148 mV dec-1 in alkaline media. Furthermore, X-ray photoelectron spectroscopy reveals the conversion of Ni3S2 surface into the highly OER active hydrated nickel oxide during polarization experiment. Using 3DG-Au-Ni3S2 (15c) as bifunctional electrocatalysts, an alkali electrolyzer delivers the current density of 10 mA cm−2 at a low cell voltage of 1.63 V. Furthermore, this electrolyzer gives a smooth line with a very small deviation of 4% from the


Page 2 of 44

Page 3 of 44 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


initial potential value for 19 h, suggesting the excellent long-term stability of 3DG-Au-Ni3S2 (15c).

Keywords: Nickel Sulfide; 3D Graphene; Au Dendrite; Electrocatalyst; Hydrogen Evolution Reaction; Oxygen Evolution Reaction



1. Introduction The requirement of clean and green energy resources gradually becomes huge and crucial due to drastic climate change and the deficit of fossil fuels. Recently, hydrogen has been recognized as an efficient and green fuel. 1-3 Gasification of fossil fuels and natural gas conversion are the conventional more massive scale generation of hydrogen.4-5 However, high greenhouse gas emission and the harsh reaction conditions limit the applicability of these methods towards large hydrogen production. Usage of clean energy sources such as solar and wind has also been shown as inefficient due to the dynamic overpotentials in overall water splitting.6-7 Because of their high cost and low abundance, the noble-metal based electrocatalysts such as platinum, iridium and ruthenium oxides, etc., are not affordable. Alternatively, earth-abundant transition metal compounds have been demonstrated as a cheap and efficient electrocatalyst for either hydrogen or oxygen evolution reaction (HER or OER).8-12 Design and development of electrocatalyst with high activity towards HER as well as OER is an important topic of contemporary research in the field of overall water splitting.13-14 Nickel-based compounds such as NiS, NiS2, Ni3S2, etc., have been proved as a highly active electrocatalysts for HER and OER.15-18 For instance, NiS microspheres have been grown on nickel foam through direct sulphurization with sulphur powder and their HER and OER activities in the alkaline medium have been examined.16 The NiS microspheres deliver the current density of 20 4 ACS Paragon Plus Environment

Page 4 of 44

Page 5 of 44 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


mA cm-2 at the overpotential of 0.158 V for HER and afford the current density of 50 mA cm-2 at the overpotential of 0.335 V for OER. On the other hand, the effect of Ni3S2 structures on their electrocatalytic activity toward HER has been studied.17 It is reported that the Ni3S2 nanotubes exhibit a better activity than Ni3S2 nanosheets, which is attributed to the effective electrolyte diffusion in the nanotube structure. Combining Ni3S2 catalysts with other HER active materials is a novel method to further enhance the catalytic activities.18-19 Zhang et al. have reported that the numerous interfacial sites formed in MoS2/Ni3S2 heterostructures possess great chemisorption capability for HER and OER intermediates, which results in the excellent electrocatalytic performances in 1M KOH.20 Recently, some studies have suggested that the activity of electrocatalyst towards overall water splitting can be improved by enhancing the active surface area of the electrocatalyst. For example, a hybrid material composed of Ni0.9Fe0.3 alloy and carbon nanotubes has been investigated for the HER and OER activities.21 Although this electrocatalyst achieves a current density of 10 mA cm−2 for overall water splitting at a voltage of 1.58 V in the alkaline medium, a physical support such as Ni foam is required. On the other hand, the advancement in the activity of electrocatalysts can also be easily achieved by loading them on conductive carbon nanomaterials (CNs) such as carbon nanotubes and graphene. In our previous studies, we have successfully demonstrated the vital role of CNs in the HER and OER electrocatalysts.22-23 The CNs would 5 ACS Paragon Plus Environment


provide more physical and chemical stability to the electrocatalysts and also improve the transport of electrons in the overall water splitting reaction. Herein, we report a free-standing 3D electrocatalyst with low overpotential and high stability for overall water splitting application. For this purpose, we electrodeposit highly active and low-cost Ni3S2 catalyst over the Au dendrites grown on three-dimensional graphene (3DG). It is noteworthy that our composite electrode shows several advantages over the previously reported electrocatalysts in the literature. For example, we have adopted the electrochemical deposition to prepare the electrocatalyst, which is considered as a more convenient method for any material synthesis. We have demonstrated that the activity of the electrocatalyst can be easily controlled by adjusting the number of electrodeposition cycles. Furthermore, the 3DG/Au dendrites as the substrate for active materials are highly conductive and stable in the acidic as well as basic conditions. Due to the synergistic effect between Ni3S2 and 3DG/Au dendrites, this free standing and binder-free electrode displays the high activity towards HER and OER.

2. Experimental Section 2.1. Growth of the Au-Dendrite/3D-Graphene


Page 6 of 44

Page 7 of 44 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


3DG has been grown on the surface of Ni-foam (1 cm 1 cm) using chemical vapor deposition (CVD) as described in the literature report.24 In brief, the oxide and impurity in the catalytic Ni substrate was firstly removed by heating it in 10% H2 at 1000 °C for 20 min. Next, CH4 (100 s.c.c.m.) as the carbon source was delivered to the CVD system where the pressure and temperature was 10 torr and 1000 °C, respectively. After the reaction time of 30 min, the sample was quickly cooled down in the carrier gas of 10% H2. Then, the Ni foam was etched in the mixed solution consisting of 50 mL of 3M HCl and 200 µL of 1M FeCl3.6H2O at 65°C for 7 h. Finally, the resultant 3DG was soaked into DI water for 30 min and then dried under vacuum. In the electrodeposition experiments, the electroplating solution was consisted of 0.5 M HAuCl4 (60 L), 10 mM cysteine (300 L), 18M H2SO4 (0.83 mL) and DI water (30 mL). The electrodeposition of dendritic gold structures on 3DG was performed using the three-electrode system where the 3DG, a saturated calomel electrode (SCE) and Pt were used as working, reference and auxiliary electrodes, respectively. In a typical experiment, the square-wave potential pulses with the step segments of 30000 and an each step time of 0.1 sec were applied to 3DG and the low as well as high potential of square-waves with respect to SCE were set as 0 and -0.8 V, to grow an uniform Au dendritic structures over the 3DG surface.25 After the electrodeposition, the resultant 3DG-Au was washed with DI water and then dried under vacuum.



2.2. Growth of Ni3S2 on the Au-Dendrite-3D-Graphene

For the electrodeposition of Ni3S2 on the 3DG-Au-dendrite substrate, the electroplating solution consisted of 0.05 mole of thiourea (3.8061 g), 4.5810-3 mole of NiCl2 (0.594 g) and DI water (50 mL). We adopted cyclic voltammetry (CV) to deposit Ni3S2 in the potential range from 0.2 to -1.2 V at the scan rate of 5 mV s-1 with the various numbers of cycles such as 5, 10, 15 and 20. The resultant nanocomposites were named as 3DG-Au-Ni3S2(5c), 3DG-Au-Ni3S2(10c), and so on. The 3DG-Au-Ni3S2 samples were washed with DI water and then dried under vacuum.

2.3. Material characterizations

Field-emission transmission electron microscope (TEM, JEOL JEM-2100F) equipped with an energy dispersive spectrometer (EDS) and scanning electron microscope (JEOL JSM-6510) were used to obtain the information on the chemical compositions and microstructures. X-ray powder diffraction (XRD) pattern of the composite was obtained from Philips X’Pert Pro MPD. XPS measurements were carried out by an X-ray photoelectron spectrometer (XPS, VG Scientific ESCALAB 250) with an Al Kα X-ray source. Electrical resistivity values of nanocomposite materials were measured by using Four-probe method with Keithley model 2636A Source Meter. The free-standing nanocomposite materials were pressed as thin-films and their thickness values were calculated by SEM. The current values of 10-150 μA were applied to the thin films and the 8 ACS Paragon Plus Environment

Page 8 of 44

Page 9 of 44 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


resultant potential values were measured and plotted. The linear fitting of the I-V curves yielded the corresponding resistance values. The measurements were repeated three times and the average resistance value was used for further calculation. The resistivity of the nanocomposite materials was calculated by using the following equation.

𝜌=

𝜋𝑡 ln 2

×𝑅

(1) where t is a thickness of the thin film (m) and R is resistance (Ohm).

2.4. HER and OER characterizations

All the electrochemical measurements were performed in a three-electrode configuration using Autolab PGSTAT-128N. The platinum wire and Ag/AgCl electrode were used as the counter and reference electrodes, respectively. Linear sweep voltammetry (LSV) was conducted at a scan rate of 1 mV s−1. The electrochemical impedance spectroscopy (EIS) spectra were recorded with a superimposed 5 mV sinusoidal voltage in the frequency range from 10-2 to 106 Hz. Detection of H2 and O2 produced in the overall water splitting reaction was performed by using the gas chromatography system (Agilent 6890 Series) equipped with a thermal conductivity detector and a 5 Å molecular sieve column. The calculations for Faraday efficiencies and the GC chromatograms

of produced H2 and O2 gases have been supplemented in the supporting information. 9 ACS Paragon Plus Environment


3. Results and discussion 3.1. Structural characterizations of 3D-Graphene-Au-Ni3S2 nanocomposite The 3DG sample exhibits a microporous three-dimensional network structure as shown in Figure 1a. When the Au is electrodeposited on the 3DG surface, it shows the highly branched dendritic structures with the stems of Au dendrites in the length of several micrometers (Figure 1b). Furthermore, many branches with the diameter of tens of nanometers are grown radially on the periphery of Au stems. It is noted that numerous leaf-like structures are densely populated along each branch. The resulting Au dendrites are enriched with the rough surface where the active materials can be easily deposited. More importantly, the electrical resistivity of 3DG-Au-dendrites (0.143 Ohm m) is lower than that of 3DG (0.599 Ohm m). Therefore, 3DG-Au-dendrites can serve as a more conductive and higher surface area support compared with 3DG. In addition to the conductivity enhancement, the use of Au substrate can significantly improve the geometric activity of OER catalysts because it enables the homogeneous formation of the electrodeposited catalysts with more electrochemically active sites and better adhesion.26-27 Figure 1c-f show the morphological changes of 3DG-Au-dendrites after the electrodeposition of Ni3S2. It is clearly observed that the loading amount of Ni3S2 on 3DG-Au-dendrites is increased with the increasing 10 ACS Paragon Plus Environment

Page 10 of 44

Page 11 of 44 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


number of CV cycles. When the number of CV cycles is 5 in the electrodeposition experiment, a few Ni3S2 particles are dispersed and the morphology of Au dendrites can clearly be observed. When the 10 number of CV cycles are performed, it is found that Au dendrites are fully covered by Ni3S2 nanoparticles whereas a thick coating of Ni3S2 is observed on Au dendrites without changing the dendritic structure while increasing the number of CV cycles to 15. However, when the CV cycles are more than 20 cycles, Au dendrites structure is buried underneath the thick Ni3S2 coating. For comparison, the direct electrodeposition of Ni3S2 on 3DG support is shown in Figure 1g. The densely agglomerated micrometer-sized particles are observed and then merged together to form the thick film over the 3DG surface. This result further confirms that the presence of Au dendrites not only improves the conductivity of the support but also facilitates the formation of nano-sized Ni3S2 particles along the surface of Au dendrites.

Figure 1. SEM images of (a) 3DG, (b) 3DG-Au dendrites, (c-f) 3DG-Au-Ni3S2 (5,10,15, and 20 cycles, 11 ACS Paragon Plus Environment


respectively) and (g) 3DG-Ni3S2 (15c).

Figure 2a-b show the low and high magnification TEM images of 3DG-Au-Ni3S2 (15c), from which it is clearly found that the periphery of Au dendrites is densely covered with Ni3S2 nanosheets. This unique hierarchical structure provides easy access of electrolyte ions onto the surface of active material and increases active sites for HER and OER. The lattice fringes of Ni3S2 nanosheets are measured as 0.41, 0.29 and 0.18 nm, which corresponds to the crystallographic planes (101), (110) and (113), respectively. Figure 2d-f show the trace of individual atoms such as Au, Ni and S present in the 3DG-Au-Ni3S2 nanocomposite. EDS mapping data verify the presence of Au, Ni and S atoms in the nanocomposite. The electrical resistivity of nanocomposite material was measured by using the four-point-probe method. The nanocomposite of 3DG-Au-Ni3S2 (15c) exhibits the resistivity of 0.968 Ohm m, whereas 3DG-Ni3S2 (15c) possesses the resistivity of 2.52 ohm m. The lower resistivity of 3DG-Au-Ni3S2 (15c) is mainly attributed to the presence of Au dendritic nanostructures when compared to that of 3DG-Ni3S2 (15c).


Page 12 of 44

Page 13 of 44 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 2. (a and b) Low magnification, (c) high magnification TEM images of 3DG-Au-Ni3S2 (15c). (d-f) EDS elemental mapping of Au, Ni and sulfur, respectively.

As shown in Figure 3a, the XRD pattern of 3DG-Au displays that a peak at ca. 26.7° corresponds to the (002) plane of graphite (JCPDS No. 41-1487). Furthermore, the XRD peaks at 38.5°, 44.6°, 64.9° and 77.9° correspond to the (111), (200), (220), and (311) reflection planes of Au with an fcc structure (JCPDS file: 04-0784), respectively. On the other hand, the XRD pattern of 3DG-Ni3S2 exhibits the peaks at 21.7°, 31.2°, 48.6°, 50.3°, and 54.8°, which are attributed to (101), (110), (113), (211), and (122) reflection planes of Ni3S2 (JCPDS card No. 44-1418), respectively. In the XRD profile of 3DG-Au-Ni3S2 (15c) nanocomposite, the peaks corresponding 13 ACS Paragon Plus Environment


to the reflection planes of 3DG, Au, and Ni3S2 are present, which clearly shows that the nanocomposite has all the components in their pristine crystalline states without much change (Figure 3b). This data further confirms that the lattice fringes observed in Figure 2c correspond to the major crystallographic planes (101, 110 and 113) of Ni3S2 nanosheets. The chemical composition and valence states of 3DG-Au-Ni3S2 are further studied using XPS. The survey spectrum (Figure S1) reveals the presence of C, Ni, S, and Au elements in the composite material, and oxygen may be acquired from the environment. As shown in Figure 3c, the C1s core level spectrum can be deconvoluted into two peaks. The main peak at 284.6 eV is attributed to sp2hybridized graphite-like carbon atoms whereas the peak at 285.6 eV is assigned to sp3-hybridized carbon atoms.28 Furthermore, the XPS peaks at 84.1 and 87.8 eV correspond to the 4f7/2 and 4f5/2 peaks of metallic Au dendrites, respectively (Figure 3d).29 As shown in Figure 3e, the two prominent peaks at 856.7 and 874.4 eV correspond to the Ni 2p3/2 and Ni 2p1/2, which are associated with surface oxidation states.30 Furthermore, the satellite peaks are present at the higher binding energies. Due to the fact that the oxidation of electrode surface in air leads to the formation of Ni(OH)2, the characteristic peak of Ni3S2 at 852.5 eV disappears from the core level spectrum of Ni 2p.31 In the S 2p spectrum (Figure 3f), the peaks at 162 and 163.3 eV are assigned to the S 2p3/2 and 2p1/2, respectively, proving the presence of S in Ni3S2. Furthermore, it is noted that the peak at 168.7 eV is typical for the Ni–O–S species with a high oxidation state of S caused by 14 ACS Paragon Plus Environment

Page 14 of 44

Page 15 of 44 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


surface oxidation in air.32-33



Figure 3. (a) XRD profiles of 3DG-Au dendrites, 3DG-Ni3S2 (15c) and 3DG-Au-Ni3S2 (15c). (b) XRD profile of 3DG-Au-Ni3S2 (15c) with assigned diffraction planes. XPS spectra of (c) Carbon, (d) Au, (e) Ni and (f) Sulphur elements present in the as-synthesized 3DG-Au-Ni3S2 (15c) nanocomposite.

3.2. Electrocatalytic activity of 3DG-Au-Ni3S2 toward HER

Figure 4a shows the polarization curves of various electrocatalysts for HER. It is found that 3DG exhibits the negligible activity toward HER. Furthermore, 3DG-Ni3S2(15c) electrode shows a higher activity when compared to 3DG-Au due to the intrinsic higher HER activity of Ni3S2 than that of Au.34-35 Among all the HER electrocatalysts, the 3DG-Au-Ni3S2 (15c) nanocomposite displays the highest HER activity due to the synergistic effects from Au dendrites and Ni3S2. For example, 3DG-Au-Ni3S2 (15c) achieves a current density of 10 mA cm-2 at a low overpotential of 140 mV. It should be noted that 3DG-Au-Ni3S2 (15c) show better HER activity than that of commercially available Pt/C (10 wt %). Pt/C showed the higher overpotential of 196 mV at a current density of 10 mA cm-2. In Figure 4b, the HER activities of 3DG-Au-Ni3S2 prepared at 5, 10 and 20 cycles are compared with that of 15 cycles. The corresponding Tafel plots of various electrocatalysts are shown in Figure 4c. The 3DG-Au-Ni3S2 (15c) exhibits the lowest Tafel slope of 93 mV dec-1 among the various 3DG-Au-Ni3S2 electrocatalysts prepared at different CV cycles. In Table S1, it is noteworthy that 3DG-Au-Ni3S2 (15c) outperforms the recently reported HER 16 ACS Paragon Plus Environment

Page 16 of 44

Page 17 of 44 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


electrocatalysts, e.g., Ni3S2 nanosheets/NF (107 mV dec-1)36 and Ni0.9Fe0.1/NC(111 mV dec-1).21 The small Tafel slope suggests that the rate of hydrogen generation is higher with the applied overpotential, which supports the higher current density in the polarization curve.37 As shown in Figure 4d, EIS measurements are carried out for all the nanocomposite electrocatalysts. At -0.3 V, 3DG-Au-Ni3S2 (15c) exhibits a much smaller charge-transfer resistance (Rct) of 1.09  than other 3DG-Au-Ni3S2 (5, 10 and 20c; Table S2). The smaller Rct is mainly due to the faster electrontransfer at the interface between electrode and electrolyte. The important parameters about HER performances of the various 3DG-Au-Ni3S2 nanocomposites are summarized in Table S2. The relationship between the surface area and HER activity of the various 3DG-Au-Ni3S2 electrocatalysts was further investigated with the help of CV experiments. According to the previous studies, the electrochemical surface area (ECSA) is proportional to the double-layer capacitance (Cdl), which can be derived from CV plot of the electrode.15, 18 As shown in Figure 4e and Figure S2, the CV plots of 3DG-Au-Ni3S2 electrodes measured at scan rate of 300 mVs-1 exhibit a quasi-rectangular shape that is typical behavior of electrical double-layer capacitance. The slopes shown in Figure 4f are used for the determination of Cdl values and the high Cdl value indicates the enriched active sites in 3DG-Au-Ni3S2 towards HER. It is noteworthy that 3DG-AuNi3S2 (15c) possesses the highest Cdl value of 37.98 mF cm-2 when compared with that of other 3DG-Au-Ni3S2 (5,10 and 20c) electrocatalysts. As evidenced by SEM and TEM images of 3DG17 ACS Paragon Plus Environment


Au-Ni3S2 (15c; Figure 1 and 2), the integration of hierarchical Au nanostructures with the sufficient amount of Ni3S2 nanosheets would improve the exposure of active sites towards HER. However, this kind of advantage is offset when the Au dendrites are overwhelmed with the excessive amount of Ni3S2 nanosheets. The aforementioned results suggest that the HER activity of the electrode is highly dependent on its ECSA.


Page 18 of 44

Page 19 of 44 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 4. (a) Polarization curves of various HER electrocatalysts. (b) Polarization curves, (c) Tafel and (d) Nyquist plots of 3DG-Au-Ni3S2 prepared by different Ni3S2 electroplating cycles. Inset of 4d shows the equivalent electrical circuit model used for EIS data fitting. (e) CV curves and (f) 2D plot of j vs scan rate of 3DG-Au-Ni3S2 prepared by different Ni3S2 electroplating cycles.

3.3. Electrocatalytic activity of 3DG-Au-Ni3S2 toward OER The electrocatalytic activity of 3DG-Au-Ni3S2 (15c) is further explored for OER in 1 M KOH electrolyte solution. For comparison, the polarization curves of 3DG, 3DG-Au, 3DG-Ni3S2 and 3DG-Au-Ni3S2 (5, 10, 15, and 20c) are also measured. In Figure 5a, it is found that 3DG and 3DGAu electrodes show a negligible OER activity whereas 3DG-Ni3S2 and 3DG-Au-Ni3S2 afford the higher current densities due to the high activity of Ni3S2. In the LSV curve of 3DG-Au-Ni3S2 (15c), there is a clearly visible peak at ca. 1.45 V versus RHE, which could be associated with the oxidation of Ni3S2 and Au dendrites.31, 38 The nanocomposite of 3DG-Au-Ni3S2 (15c) exhibits the best OER performance when compared to that of other composites. At 1.6 V, the current density of 3DG-Au-Ni3S2 (15c) is 91.2 mA cm-2, whereas the current densities of 3DG, 3DG-Au, 3DGNi3S2 (15c) and Pt/C are 1.1, 2.4, 14.7 and 3.6 mA cm-2, respectively. On the other hand, the current density of 3DG-Au-Ni3S2 (15c) is also higher than that of other 3DG-Au-Ni3S2 (5, 10, and



20c) nanocomposites (Figure 5b). Since the polarization curves of electrocatalysts showed the strong oxidation peaks around 1.45 V, the LSV curve of 3DG-Au-Ni3S2 (15c) was measured in the reverse sweep direction and compared with that of Pt/C. The overpotential values of 3DG-AuNi3S2 (15c) and Pt/C for OER were calculated as 1.51 and 1.69 V (vs. RHE) at the current density of 10 mA cm-2 (Figure S3). The results suggested that the nanocomposite electrocatalyst exhibited improved OER performance than that of Pt/C. Further, 3DG-Au-Ni3S2 (15c) possesses a lower Tafel slope of 106 mV dec-1, whereas 3DG-Au-Ni3S2 (5c, 10c, and 20c) show the higher Tafel slopes of 475, 243 and 240 mV dec-1, respectively (Figure 5c and Table S3). In Table S3, it is noteworthy that 3DG-Au-Ni3S2 (15c) surpasses the recently reported OER electrocatalysts, e.g., Ni3S2 leaves (150 mV dec-1),39 Ni3S2/NF (331 mV dec-1),40 Ni3S2 nanorods/NF (159 mV dec-1),41 and Ni3S2/N-doped carbon (196 mV dec-1).42 In Figure 5d, EIS measurements at 1.8 V exhibits the lower Rct value of 1.02 Ω for 3DG-Au-Ni3S2 (15c) being smaller than that of 3DG-Au-Ni3S2-5c (7.86 Ω), 10c (3.43 Ω) and 20 (2.50 Ω). The lower Tafel slope and Rct value suggest that the OER activity of 3DG-Au-Ni3S2 (15c) is superior to that of other nanocomposites. As evidenced in Figure 4f, 3DG-Au-Ni3S2 (15c) has the larger Cdl and ECSA than other electrocatalysts, which indicates the presence of enriched active sites towards the higher OER activity in an alkaline solution. The important parameters about OER performances of the various 3DG-Au-Ni3S2 nanocomposites are summarized in Table S4. 20 ACS Paragon Plus Environment

Page 20 of 44

Page 21 of 44 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 5. (a) Polarization curves for the various OER electrocatalysts (b) Polarization curves, (c) Tafel and (d) Nyquist plots of 3DG-Au-Ni3S2 prepared by different electroplating cycles of Ni3S2.

To get the deep insight into the origin of OER activity, the nanocomposite electrode of 3DGAu-Ni3S2 (15c) after OER experiment is investigated by XPS analysis. As displayed in Figure 6a, the main peak in Ni 2p3/2 spectrum shifts to higher binding energy (863.4 eV) and its spectrum profile is obviously changed when compared to that of the as-synthesized electrode (Figure 4e). In this region, the appearance of a small peak at 856.3 eV indicates the presence of trace amount of Ni(OH)2.43-44 and the more intense peak at 859 eV can be attributed to NiOOH.33, 45 The spin-orbit 21 ACS Paragon Plus Environment


splitting energy between the two Ni 2p peaks is also found as 17.5 eV. As shown in Figure 6b, no S 2p peak can be detected in the electrode surface after OER experiment. The data above support that Ni3S2 surface undergoes oxidative phase conversion under electrochemical polarization in alkaline solution.41 More importantly, the hydrated nickel oxide is considered as the active electrocatalyst for OER because the preferred absorption of water on its surface plays a critical role in the OER performance.46

Figure 6. XPS profiles of (a) Ni and (b) Sulphur elements present in the 3DG-Au-Ni3S2 (15c) nanocomposite after OER experiment. In order to demonstrate the ability of the free-standing 3D electrocatalyst for overall water splitting, the two-electrode cell system consisting of 3DG-Au-Ni3S2 (15c) as an anode as well as a cathode is tested. As shown in Figure 7a, it affords a current density of 10 mA cm−2 in 1.0 M KOH 22 ACS Paragon Plus Environment

Page 22 of 44

Page 23 of 44 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


at a cell voltage of 1.63 V. Such activity is comparable to the nickel dichalcogenide based bifunctional electrocatalysts (Table S5) e.g., Ni3Se2/NF (1.61 V),47 Co-doped NiSe2/Ti mesh (1.62 V),48 NiS/NF (1.64V),16 Ni3S2‐NGQDs/NF (1.58 V)49 and even better than Ni3S2 nanosheets (1.76 V)15 and NiCo2S4 nanowires/carbon cloth (1.68 V).50 In order to test the electrode stability, the water splitting experiment is further extended to 19 hours. The average voltage applied for 19 h water splitting is measured as 1.65 V and the deviation of the potential is less than 4%, which demonstrates that 3DG-Au-Ni3S2 (15c) exhibits excellent stability. As shown in Figure 7b and Movie S1, the electrolyzer is able to generate large volumes of H2 and O2 gases on its respective 3DG-Au-Ni3S2 electrodes. Finally, the amount of H2 and O2 produced in 62 minutes of overall water splitting reaction at the current density of 20 mA cm-2 were detected as 3.85 × 10-4 mol and 1.87 × 10-4 mol, respectively (Figure S4). It is noted that the molar ratio of produced H2 to O2 under the same current density is close to 2:1. The calculated faraday efficiencies of H2 and O2 produced at cathode and anode were 99 and 97 %, respectively. The purity of produced gases was also analyzed by GC studies, which revealed that the generated H2 and O2 are pure and free from the other gaseous by-products (Figure S5 and Table S6). The excellent faraday efficiencies and purity of produced gases further support the superior electrocatalytic performance of free-standing 3DG-Au-Ni3S2 (15c) hybrid electrode towards overall water splitting application.



Figure 7. (a) 2D plot showing the voltage variations with time for symmetrical two-electrode cell assembled from 3DG-Au-Ni3S2(15c) nanocomposite. (b) Photograph showing the evolution of hydrogen and oxygen gases as bubbles from the anode and cathode.

The superior catalytic performance of 3DG-Au-Ni3S2 (15c) can be attributed to the following facts. (i) Ni3S2 is intrinsically high activity toward HER because it is located at the apex of theoretically predicted volcano plot.34 (ii) OER has been considered as the rate-limiting step for the entire water splitting reaction due to four electrons transfer. Our XPS data has proved that Ni3S2 is oxidized to NiOOH in the OER process. The resulting NiOOH has been experimentally recognized as a highly active and stable electrocatalyst toward OER.51 (iii) The sufficient Ni3S2 nanosheets decorated on Au dendrites/3DG support ensures that 3DG-Au-Ni3S2 has more exposure 24 ACS Paragon Plus Environment

Page 24 of 44

Page 25 of 44 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 active sites for catalytic reactions. This is corroborated by the fact that 3DG-Au-Ni3S2 (15c) has the highest ECSA among other electrocatalysts. (iv) In addition to enhancing the stability of 3DGAu-Ni3S2(15c), the intimate contact between Ni3S2 nanosheets and Au dendrites/3DG support facilitates interfacial electron transport between them. A rapid electron transfer in 3DG-AuNi3S2(15c) has been evidenced by its lower Rct value compared with other nanocomposite electrocatalysts.

4. Conclusions In summary, the nanocomposites of 3DG-Au-Ni3S2 have been prepared by using the electroplating method with a varying number of cycles and their performance as free-standing electrocatalysts in HER, OER and overall water splitting have been investigated. Among the various nanocomposites, 3DG-Au-Ni3S2 (15c) exhibits the best catalytic performance due to its highest ECSA. Furthermore, 3DG-Au-Ni3S2 (15c) shows the current density of 10 mA cm−2 at an overpotential of 140 mV for HER and 91.2 mA cm−2 at a potential of 1.6 V for OER in alkaline media. As for overall water splitting, a cell voltage of only 1.63 V is required for the current density of 10 mA cm−2 and the deviation of the potential is less than 4%. Such superior catalytic performance is attributed to the effective exposure of active sites in hierarchical nanostructures, 25 ACS Paragon Plus Environment


and the facilitated electron transport along 2D Ni3S2 nanosheets which are anchored over the Au dendrites. This work will open up a way to develop advanced bifunctional electrocatalysts with low cell voltage and higher stability for water splitting.

Associated Content

Supporting Information

The Supporting Information is available on free of charge in the ACS Publications website at DOI:

Comparison of HER and OER performances with the reported electrocatalyst in the literature; HER and OER performances of 3DG-Au-Ni3S2 prepared at different number of CV cycles; XPS survey spectra of 3DG-Au-Ni3S2 (15c) before and after OER; CV measurements of of 3DG-Au-Ni3S2 nanocomposites; Polarization curves of 3DG-AuNi3S2 (15c) and Pt/C in the reverse sweep direction; Measurements and calculation of mole number and faraday efficiencies of H2 and O2 produced by 3DG-Au-Ni3S2 (15c) under two-electrode configuration; GC chromatograms and their reports of H2 and O2 produced in the over-all water splitting;

Author Information

(H.C.T) Email: [email protected] 26 ACS Paragon Plus Environment

Page 26 of 44

Page 27 of 44 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


(B.V.) Email: [email protected]

*(T-W.L.) Email: [email protected]. Tel.: +886(4)23590121 Ext. 32250; Fax: +886(4)23590426

ORCiD

Dr. Balaraman Vedhanarayanan: 0000-0002-7785-136X

Prof. Tsung-Wu Lin: 0000-0003-0641-9139

Notes

Authors declare no conflict of interest.

Acknowledgements

This research was supported by the Ministry of Science and Technology, Taiwan (MOST- 1072113-M-029-004, MOST- 106-2632-M-029-001, and MOST-104-2628-M-029-001-MY3).

References (1) Cook, T. R.; Dogutan, D. K.; Reece, S. Y.; Surendranath, Y.; Teets, T. S.; Nocera, D. G., Solar Energy Supply and Storage for the Legacy and Nonlegacy Worlds. Chem. Rev. 2010, 110, 6474-6502. 27 ACS Paragon Plus Environment


(2) Dresselhaus, M. S.; Thomas, I. L., Alternative Energy Technologies. Nature 2001, 414, 332. (3) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S., Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catal. 2016, 6, 8069-8097. (4) Rosen, M. A.; Scott, D. S., Comparative Efficiency Assessments for a Range of Hydrogen Production Processes. Int. J. Hydrog. Energy 1998, 23, 653-659. (5) Trommer, D.; Noembrini, F.; Fasciana, M.; Rodriguez, D.; Morales, A.; Romero, M.; Steinfeld, A., Hydrogen Production by Steam-Gasification of Petroleum Coke Using Concentrated Solar Power—I. Thermodynamic and Kinetic Analyses. Int. J. Hydrog. Energy 2005, 30, 605-618. (6) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Grätzel, M., Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and Earth-Abundant Catalysts. Science 2014, 345, 1593. (7) Zou, X.; Zhang, Y., Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44, 5148-5180. (8) Morales-Guio, C. G.; Stern, L.-A.; Hu, X., Nanostructured Hydrotreating Catalysts for Electrochemical Hydrogen Evolution. Chem. Soc. Rev. 2014, 43, 6555-6569. (9) Maiyalagan, T.; Jarvis, K. A.; Therese, S.; Ferreira, P. J.; Manthiram, A., Spinel-Type Lithium Cobalt Oxide as a Bifunctional Electrocatalyst for the Oxygen Evolution and Oxygen 28 ACS Paragon Plus Environment

Page 28 of 44

Page 29 of 44 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


Reduction Reactions. Nat. Commun. 2014, 5, 3949. (10) Yang, X.; Lu, A.-Y.; Zhu, Y.; Hedhili, M. N.; Min, S.; Huang, K.-W.; Han, Y.; Li, L.-J., CoP Nanosheet Assembly Grown on Carbon Cloth: A Highly Efficient Electrocatalyst for Hydrogen Generation. Nano Energy 2015, 15, 634-641. (11) Smith, A. J.; Chang, Y.-H.; Raidongia, K.; Chen, T.-Y.; Li, L.-J.; Huang, J., Molybdenum Sulfide Supported on Crumpled Graphene Balls for Electrocatalytic Hydrogen Production. Adv. Energy Mater. 2014, 4, 1400398. (12) Chang, Y.-H.; Lin, C.-T.; Chen, T.-Y.; Hsu, C.-L.; Lee, Y.-H.; Zhang, W.; Wei, K.-H.; Li, L.-J., Highly Efficient Electrocatalytic Hydrogen Production by MoSx Grown on GrapheneProtected 3D Ni Foams. Adv. Mater. 2013, 25, 756-760. (13) Li, J.; Wang, Y.; Zhou, T.; Zhang, H.; Sun, X.; Tang, J.; Zhang, L.; Al-Enizi, A. M.; Yang, Z.; Zheng, G., Nanoparticle Superlattices as Efficient Bifunctional Electrocatalysts for Water Splitting. J. Am. Chem. Soc. 2015, 137, 14305-14312. (14) 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. (15) Feng, L.-L.; Yu, G.; Wu, Y.; Li, G.-D.; Li, H.; Sun, Y.; Asefa, T.; Chen, W.; Zou, X., HighIndex Faceted Ni3S2 Nanosheet Arrays as Highly Active and Ultrastable Electrocatalysts for Water 29 ACS Paragon Plus Environment


Splitting. J. Am. Chem. Soc. 2015, 137, 14023-14026. (16) 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. (17) Jinlong, L.; Tongxiang, L., The Effect of Urea on Microstructures of Ni3S2 on Nickel Foam and Its Hydrogen Evolution Reaction. J. Solid State Chem. 2016, 243, 106-110. (18) 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. (19) Wu, Y.; Liu, Y.; Li, G.-D.; Zou, X.; Lian, X.; Wang, D.; Sun, L.; Asefa, T.; Zou, X., Efficient Electrocatalysis of Overall Water Splitting by Ultrasmall NixCo3−xS4 Coupled Ni3S2 Nanosheet Arrays. Nano Energy 2017, 35, 161-170. (20) 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. (21) Zhang, X.; Xu, H.; Li, X.; Li, Y.; Yang, T.; Liang, Y., Facile Synthesis of Nickel– Iron/Nanocarbon Hybrids as Advanced Electrocatalysts for Efficient Water Splitting. ACS Catal. 2016, 6, 580-588. 30 ACS Paragon Plus Environment

Page 30 of 44

Page 31 of 44 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


(22) Lin, T.-W.; Liu, C.-J.; Lin, J.-Y., Facile Synthesis of MoS3/Carbon Nanotube Nanocomposite with High Catalytic Activity Toward Hydrogen Evolution Reaction. Appl. Catal. B Environ. 2013, 134-135, 75-82. (23) Lin, T.-W.; Liu, C.-J.; Dai, C.-S., Ni3S2/Carbon Nanotube Nanocomposite as Electrode Material for Hydrogen Evolution Reaction in Alkaline Electrolyte and Enzyme-Free Glucose Detection. Appl. Catal. B Environ. 2014, 154-155, 213-220. (24) Lin, T.-W.; Dai, C.-S.; Tasi, T.-T.; Chou, S.-W.; Lin, J.-Y.; Shen, H.-H., High-Performance Asymmetric Supercapacitor Based on Co9S8/3D Graphene Composite and Graphene Hydrogel. Chem. Eng. J 2015, 279, 241-249. (25) Lai, Y.-H.; Kuo, S.-C.; Hsieh, Y.-C.; Tai, Y.-C.; Hung, W.-H.; Jeng, U. S., Electrochemically Fabricated Gold Dendrites with Underpotential Deposited Silver Monolayers for a Bimetallic SERS-Active Substrate. RSC Adv. 2016, 6, 13185-13192. (26) Chakthranont, P.; Kibsgaard, J.; Gallo, A.; Park, J.; Mitani, M.; Sokaras, D.; Kroll, T.; Sinclair, R.; Mogensen, M. B.; Jaramillo, T. F., Effects of Gold Substrates on the Intrinsic and Extrinsic Activity of High-Loading Nickel-Based Oxyhydroxide Oxygen Evolution Catalysts. ACS Catal. 2017, 7, 5399-5409. (27) Ng, J. W. D.; García-Melchor, M.; Bajdich, M.; Chakthranont, P.; Kirk, C.; Vojvodic, A.; Jaramillo, T. F., Gold-Supported Cerium-Doped NiOx Catalysts for Water Oxidation. Nat. Energy 31 ACS Paragon Plus Environment


2016, 1, 16053. (28) Tai, S.-Y.; Liu, C.-J.; Chou, S.-W.; Chien, F. S.-S.; Lin, J.-Y.; Lin, T.-W., Few-Layer MoS2 Nanosheets Coated onto Multi-Walled Carbon Nanotubes as a Low-Cost and Highly Electrocatalytic Counter Electrode for Dye-Sensitized Solar Cells. J. Mater. Chem. 2012, 22, 24753-24759. (29) Cuche, A.; Viarbitskaya, S.; Sharma, J.; Arbouet, A.; Girard, C.; Dujardin, E., Modal Engineering of Surface Plasmons in Apertured Au Nanoprisms. Sci. Rep. 2015, 5, 16635. (30) Du, X.; Yang, Z.; Li, Y.; Gong, Y.; Zhao, M., Controlled Synthesis of Ni(OH)2/Ni3S2 Hybrid Nanosheet Arrays as Highly Active and Stable Electrocatalysts for Water Splitting. J. Mater. Chem. A 2018, 6, 6938-6946. (31) Ho, T. A.; Bae, C.; Nam, H.; Kim, E.; Lee, S. Y.; Park, J. H.; Shin, H., Metallic Ni3S2 Films Grown by Atomic Layer Deposition as an Efficient and Stable Electrocatalyst for Overall Water Splitting. ACS Appl. Mater. Interfaces 2018, 10, 12807-12815. (32) Dueso, C.; Izquierdo, M. T.; García-Labiano, F.; de Diego, L. F.; Abad, A.; Gayán, P.; Adánez, J., Effect of H2S on the Behaviour of An Impregnated NiO-Based Oxygen-Carrier for Chemical-Looping Combustion (CLC). Appl. Catal. B Environ. 2012, 126, 186-199. (33) Zhu, W.; Yue, Z.; Zhang, W.; Hu, N.; Luo, Z.; Ren, M.; Xu, Z.; Wei, Z.; Suo, Y.; Wang, J., Wet-Chemistry Topotactic Synthesis of Bimetallic Iron–Nickel Sulfide Nanoarrays: An Advanced 32 ACS Paragon Plus Environment

Page 32 of 44

Page 33 of 44 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


and Versatile Catalyst for Energy Efficient Overall Water and Urea Electrolysis. J. Mater. Chem. A 2018, 6, 4346-4353. (34) Ledendecker, M.; Schlott, H.; Antonietti, M.; Meyer, B.; Shalom, M., Experimental and Theoretical Assessment of Ni-Based Binary Compounds for the Hydrogen Evolution Reaction. Adv. Energy Mater. 2016, 7, 1601735. (35) Mahmood, N.; Yao, Y.; Zhang, J.-W.; Pan, L.; Zhang, X.; Zou, J.-J., Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes: Mechanisms, Challenges, and Prospective Solutions. Adv. Sci 2018, 5, 1700464. (36) 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. Hydrog. Energy 2015, 40, 4727-4732. (37) Benson, J.; Li, M.; Wang, S.; Wang, P.; Papakonstantinou, P., Electrocatalytic Hydrogen Evolution Reaction on Edges of a Few Layer Molybdenum Disulfide Nanodots. ACS Appl. Mater. Interfaces 2015, 7, 14113-14122. (38) Nakamura, M.; Nakajima, Y.; Kato, K.; Sakata, O.; Hoshi, N., Surface Oxidation of Au(111) Electrode in Alkaline Media Studied by Using X-ray Diffraction and Infrared Spectroscopy: Effect of Alkali Metal Cation on the Alcohol Oxidation Reactions. J. Phy. Chem. C 2015, 119, 2358623591. 33 ACS Paragon Plus Environment


(39) Zhu, T.; Zhu, L.; Wang, J.; Ho, G. W., In-Situ Chemical Etching of Tunable 3D Ni3S2 Superstructures for Bifunctional Electrocatalysts for Overall Water Splitting. J. Mater. Chem. A 2016, 4, 13916-13922. (40) Ouyang, C.; Wang, X.; Wang, C.; Zhang, X.; Wu, J.; Ma, Z.; Dou, S.; Wang, S., Hierarchically Porous Ni3S2 Nanorod Array Foam as Highly Efficient Electrocatalyst for Hydrogen Evolution Reaction and Oxygen Evolution Reaction. Electrochim. Acta 2015, 174, 297301. (41) 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. Energy Environ. Sci. 2013, 6, 2921-2924. (42) Barman, B. K.; Nanda, K. K., A Noble and Single Source Precursor for The Synthesis of Metal-Rich Sulphides Embedded in an N-Doped Carbon Framework for Highly Active OER Electrocatalysts. Dalton Trans. 2016, 45, 6352-6356. (43) Yan, B.; Krishnamurthy, D.; Hendon, C. H.; Deshpande, S.; Surendranath, Y.; Viswanathan, V., Surface Restructuring of Nickel Sulfide Generates Optimally Coordinated Active Sites for Oxygen Reduction Catalysis. Joule 2017, 1, 600-612. (44) Zhu, D.; Guo, C.; Liu, J.; Wang, L.; Du, Y.; Qiao, S.-Z., Two-Dimensional Metal-Organic Frameworks with High Oxidation States for Efficient Electrocatalytic Urea Oxidation. Chem. 34 ACS Paragon Plus Environment

Page 34 of 44

Page 35 of 44 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


Commun. 2017, 53, 10906-10909. (45) She, Y.; Lu, Z.; Fan, W.; Jewell, S.; Leung, M. K. H., Facile Preparation of PdNi/rGO and Its Electrocatalytic Performance Towards Formic Acid Oxidation. J. Mater. Chem. A 2014, 2, 3894-3898. (46) Kim, B.; Oh, A.; Kabiraz, M. K.; Hong, Y.; Joo, J.; Baik, H.; Choi, S.-I.; Lee, K., NiOOH Exfoliation-Free Nickel Octahedra as Highly Active and Durable Electrocatalysts Toward the Oxygen Evolution Reaction in an Alkaline Electrolyte. ACS Appl. Mater. Interfaces 2018, 10, 10115-10122. (47) Xu, R.; Wu, R.; Shi, Y.; Zhang, J.; Zhang, B., Ni3Se2 Nanoforest/Ni Foam as a Hydrophilic, Metallic, and Self-Supported Bifunctional Electrocatalyst for Both H2 and O2 Generations. Nano Energy 2016, 24, 103-110. (48) Liu, T.; Asiri, A. M.; Sun, X., Electrodeposited Co-Doped NiSe2 Nanoparticles Film: a Good Electrocatalyst for Efficient Water Splitting. Nanoscale 2016, 8, 3911-3915. (49) Lv, J.-J.; Zhao, J.; Fang, H.; Jiang, L. P.; Li, L. L.; Ma, J.; Zhu, J. J., Incorporating NitrogenDoped Graphene Quantum Dots and Ni3S2 Nanosheets: A Synergistic Electrocatalyst with Highly Enhanced Activity for Overall Water Splitting. Small 2017, 13, 1700264. (50) 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. Nanoscale 2015, 7, 35 ACS Paragon Plus Environment


15122-15126. (51) Cheng, Y.; Jiang, S. P., Advances in Electrocatalysts for Oxygen Evolution Reaction of Water Electrolysis-from Metal Oxides to Carbon Nanotubes. Pro. Nat. Sci-Mater. 2015, 25, 545-553.


Page 36 of 44

Page 37 of 44 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


Table of Contents Graphic:



Figure 1. SEM images of (a) 3DG, (b) 3DG-Au dendrites, (c-f) 3DG-Au-Ni3S2 (5,10,15, and 20 cycles, respectively) and (g) 3DG-Ni3S2 (15c). 424x161mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 38 of 44

Page 39 of 44 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 2. (a and b) Low magnification, (c) high magnification TEM images of 3DG-Au-Ni3S2 (15c). (d-f) EDS elemental mapping of Au, Ni and sulfur, respectively. 378x212mm (300 x 300 DPI)



Figure 3. (a) XRD profiles of 3DG-Au dendrites, 3DG-Ni3S2 (15c) and 3DG-Au-Ni3S2 (15c). (b) XRD profile of 3DG-Au-Ni3S2 (15c) with assigned diffraction planes. XPS spectra of (c) Carbon, (d) Au, (e) Ni and (f) Sulphur elements present in the as-synthesized 3DG-Au-Ni3S2 (15c) nanocomposite. 288x377mm (300 x 300 DPI)


Page 40 of 44

Page 41 of 44 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 4. (a) Polarization curves of various HER electrocatalysts. (b) Polarization curves, (c) Tafel and (d) Nyquist plots of 3DG-Au-Ni3S2 prepared by different Ni3S2 electroplating cycles. Inset of 4d shows the equivalent electrical circuit model used for EIS data fitting. (e) CV curves and (f) 2D plot of j vs scan rate of 3DG-Au-Ni3S2 prepared by different Ni3S2 electroplating cycles. 317x341mm (300 x 300 DPI)



Figure 5. (a) Polarization curves for the various OER electrocatalysts (b) Polarization curves, (c) Tafel and (d) Nyquist plots of 3DG-Au-Ni3S2 prepared by different electroplating cycles of Ni3S2. 325x235mm (300 x 300 DPI)


Page 42 of 44

Page 43 of 44 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 6. XPS profiles of (a) Ni and (b) Sulphur elements present in the 3DG-Au-Ni3S2 (15c) nanocomposite after OER experiment. 341x148mm (300 x 300 DPI)



Figure 7. (a) 2D plot showing the voltage variations with time for symmetrical two-electrode cell assembled from 3DG-Au-Ni3S2(15c) nanocomposite. (b) Photograph showing the evolution of hydrogen and oxygen gases as bubbles from the anode and cathode. 261x140mm (300 x 300 DPI)


Page 44 of 44

3D

Recommend Documents