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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.30 No.5 pp.217-222
DOI : https://doi.org/10.3740/MRSK.2020.30.5.217

Binder-Free Synthesis of NiCo2S4 Nanowires Grown on Ni Foam as an Efficient Electrocatalyst for Oxygen Evolution Reaction

Komal Patil, Pravin Babar, Jin Hyeok Kim†
Optoelectronic Convergence Research Center, Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea
Corresponding author E-Mail : jinhyeok@chonnam.ac.kr (J. H. Kim, Chonnam Nat’l Univ.)
March 30, 2020 March 30, 2020 April 20, 2020

Abstract


The design and fabrication of catalysts with low-cost and high electrocatalytic activity for the oxygen evolution reaction (OER) have remained challenging because of the sluggish kinetics of this reaction. The key to the pursuit of efficient electrocatalysts is to design them with high surface area and more active sites. In this work, we have successfully synthesized a highly stable and active NiCo2S4 nanowire array on a Ni-foam substrate (NiCo2S4 NW/NF) via a two-step hydrothermal synthesis approach. This NiCo2S4 NW/NF exhibits overpotential as low as 275 mV, delivering a current density of 20 mA cm-2 (versus reversible hydrogen electrode) with a low Tafel slope of 89 mV dec-1 and superior long-term stability for 20 h in 1M KOH electrolyte. The outstanding performance is ascribed to the inherent activity of the binder-free deposited, vertically aligned nanowire structure, which provides a large number of electrochemically active surface sites, accelerating electron transfer, and simultaneously enhancing the diffusion of electrolyte.



초록


    Chonnam National University

    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    Increasing energy demands, ever-worsening environmental pollution, and depletion of fossil fuel are stimulating the search for eco-friendly, renewable and clean energy resources for the efficient generation of energy with new methods.1,2) Hydrogen, an ideal energy carrier considered as a promising resource to overcome the future energy crisis and meet the rising energy demands. Also, it has high energy density, excellent energy conversion efficiency and environmentally friendly nature.3,4) Among the various methods for hydrogen production, electrochemical water splitting is regarded as a promising way to provide carbon-free and sustainable energy source of hydrogen.5) Oxygen evolution reaction (OER) is the bottleneck in the water-splitting process due to its sluggish kinetics from the rigid O-O double bond formation and multiprotoncoupled electron transfer steps. Thus, it requires high overpotential (η), even using highly active noble metal catalysts such as IrO2 and RuO2.6-8) Also the scarcity and high cost of such noble metal-based catalysts always limit their large-scale industrial application. Consequently, it is of high desire to develop cost-effective, earthabundant and highly efficient electrocatalyst for replacing the state-of-the-art precious-metal-based catalysts.9-13)

    Nowadays, transition metal chalcogenides consisting of transition metal atoms (Ni, Fe, Co) and chalcogen atoms (S, Se, Te) have attracted significant attention, because of their low cost, high electrocatalytic activity, good electrical conductivity, low bandgap energy, and high stability.14,15) Amongst these chalcogenides, transition metal sulfides with spinel-type AB2S4 structure (i.e., thiospinel, A, B = Ni, Co, Fe, Cu, etc.) are widely studied as compared to their corresponding oxides due to their unique redox properties and surface abundant sulfur vacancies. NiCo2S4 is one of the bimetallic thiospinel with a normal spinel structure furnished more octahedral catalytic active sites of Co (III) cations compared to NiCo2O4 with inverse spinel structure.16-18) This helps to improve the electrochemical performance of NiCo2S4. It is well known that the reaction components, structural parameters, and oxidation states of cations highly influence the physicochemical properties of spinels which mostly depend on the synthesis conditions. Additionally, facile and viable routes to synthesize NiCo2S4 with nanostructured morphology enhances electrocatalytic performance.19-20)

    Herein, we report a simple and practical strategy for the fabrication of bimetallic nickel-cobalt sulfide nanowires (NiCo2S4 NW) on Ni-foam (NF) (labeled as NiCo2S4 NW/NF). This NiCo2S4 NW/NF electrode exhibits remarkable electrocatalytic activity for the OER, with low overpotential as 275 mV to obtain the current density of 20 mA cm−2 in 1 M KOH electrolyte. Benefitting from the high surface area, binder-free synthesis and porous conducting NF substrate, the as-prepared NiCo2S4 NW/NF electrode demonstrates excellent electrocatalytic activity and long-term stability for the OER. This work will provide a new prospect for the development of electrode material for various electrochemical conversion applications.

    2. Experimental

    2.1 Synthesis of Nickel cobalt oxide (NiCo2O4/NF)

    All reagents used were of analytical pure grade and used without further purification. Before synthesis, NF (1 × 1 cm) was cleaned with 1 M HCl to remove nickel oxide layers on the surface, then ultrasonicated in ethanol and water respectively for 5 min and dried in air. In a typical procedure, 1.0 mmol Nickel chloride hexahydrate (NiCl2.6H2O), 2.0 mmol Cobalt chloride hexahydrate (CoCl2.6H2O), 6.0 mmol NH4F and 15.0 mmol urea were dissolved in 35 mL distilled water and stirred to form a homogeneous solution. Then the pre-treated NF and the prepared homogeneous solution were transferred into Teflon-lined stainless-steel autoclave (100 mL). The autoclave was sealed and maintained at 120 oC for 3 h in a hot air oven and then allowed to cool to room temperature naturally. The obtained NF was then clean with distilled water and ethanol several times and then annealed at 350 °C for 2 h in air atmosphere, to obtain NiCo2O4/NF.

    2.2 Synthesis of Nickel cobalt sulfide (NiCo2S4 NW/ NF)

    3.0 mmol Na2S·9H2O was dissolved in 35 mL distilled water and stirred to form a clear solution and transferred into a Teflon-lined stainless-steel autoclave (100 mL). Then the prepared NiCo2O4/NF was immersed into the above solution and maintained at 80 °C for 12 h and then cooled to room temperature. After the reactor was cooled to room temperature, NiCo2S4 NW/NF was obtained by rinsing it with distilled water and ethanol several times.

    2.3 Structural characterization

    The powder X-ray diffraction (XRD) measurements were recorded on a Rigaku Dmax/Ultima IV diffractometer with monochromatized Cu Kα radiation (λ = 1.54 A ͦ ). The morphology of the deposited samples was examined by field-emission scanning electron microscopes (FESEM, FEI Nova Nano SEM 450). X-ray photoelectron spectroscopy (XPS) measurements were recorded on a Perkin-Elmer PHI 5000C spectrometer using monochromatized Al Kα excitation. All binding energies were calibrated by using the contaminant carbon (C1S = 284.6 eV) as a reference.

    2.4 Electrochemical measurements

    All the electrochemical measurements were performed on an electrochemical workstation (CHI 760E, CH Instruments Inc., Shanghai) using a conventional threeelectrode system with an electrolyte solution of 1 M KOH, a Pt wire as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode and freshly prepared NiCo2S4 NW/NF was directly used as working electrode. All potentials measured were converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation: ERHE = ESCE + (0.24 + 0.059 pH). Before the electrochemical measurement, the electrolyte was degassed by bubbling oxygen for at least 30 min to ensure the H2O/O2 equilibrium at 1.23 V (versus the RHE). Linear sweep voltammetry (LSV) was measured at the scan rate of 5 mV s−1. The double-layer capacitance (Cdl) of the electrode was obtained by cyclic voltammetry (CV) and measured in the region of 0.20 ~ 0.25 V vs SCE with scan rates of 4, 6, 8, and 10 mV s−1, respectively. The long-term stability was measured using chronopotentiometry at a constant current density of 20 mA cm−2 for 20 h.

    3. Results and discussion

    3.1 Morphological and structural study

    The morphological characterization of prepared NiCo2S4 film is shown in Fig. 1(a-c). Fig. (a) shows the low-magnification FE-SEM image of the NiCo2S4 NW/ NF. It shows the vertically aligned NiCo2S4 nanowires uniformly formed on the entire NF surface. Such vertically aligned 1D nanowire structure can afford high surface area with abundant active sites.

    The crystal phase of NiCo2S4 was investigated using the XRD method. As shown in Fig. 2(a) XRD pattern reveals the crystal structure and phase purity of the NiCo2S4 nanowires. The strong diffraction peaks at 44.8°, 52.2°, and 76.8° are attributed to the (111), (200), and (220) planes of the NF substrate, respectively. Additionally, the peaks at 31.2°, 37.8°, 50.3°, and 55.3° are attributed to the (311), (400), (511), and (440) planes of cubic phase NiCo2S4 (JCPDS: 00-020-0782), respectively.17) The XPS was employed to study the detailed chemical composition and electronic states of the NiCo2S4 NW/NF. Fig. 2(b) demonstrate high-resolution XPS spectrum of Ni 2p. The Ni 2p spectrum shows a pair of peaks at 855.4 eV and 872.8 eV corresponds to Ni 2p3/2 and Ni 2p1/2 respectively along with their satellite peaks, which confirms the existence of Ni2+ and Ni3+ oxidation state. For, Co 2p spectrum [Fig. 2(c)], the two main peaks Co 2p3/2 and Co 2p1/2 were at 781.0 and 796.1 eV respectively, which suggests the coexistence of Co2+, Co3+. Fig. 2(d) shows the spectrum of S 2p region, the main peak located at binding energies of 162.2 eV is attributed to the sulfur ion in low coordination at the surface. The XPS analysis clearly indicates the NiCo2S4 has a composition of Ni2+, Ni3+, Co2+, Co3+, and S2- which is in good agreement with previous reports.20)

    3.2 Electrochemical activity towards OER

    The electrocatalytic performances for the OER of asprepared NiCo2S4 nanowires and NiCo2O4 electrocatalysts were investigated in 1 M KOH electrolyte in a typical three-electrode configuration at a scan rate of 5 mV s−1. To understand the contribution of the NF substrate in the performance of NiCo2S4 NW/NF we also studied LSV of bare NF. Fig. 3(a) shows the OER polarization curves of all electrodes and it is observed that NiCo2S4 NW/NF requires a low overpotential of only 275 mV to reach a current density of 20 mA cm−2, which is much lower than that of NiCo2O4/NF (380 mV), and NF (410 mV) indicating the excellent catalytic performances of NiCo2S4 NW/NF. Tafel plots are demonstrated in Fig. 3(b). The Tafel slopes of NiCo2S4 NW/NF is 89 mV dec−1, lower than that of NiCo2O4/NF (96 mV dec−1) and NF (113 mV dec−1), which means a higher electrocatalytic reaction rate and higher catalytic activity. This may be due to the faster rate of electron transfer owing to the good electrical conductivity of NiCo2S4 NW/NF. A comparative plot of overpotential and Tafel of three catalysts are displayed in Fig. 3(c). Besides catalytic performance, we also inspected the electrochemical durability of NiCo2S4 NW/NF by using chronopotentiometry. As displayed in Fig. 3(d), after 20 h of constant electrolysis at a current density of 20 mA cm−2 there was a negligible loss in current density, demonstrating the excellent stability of the NiCo2S4 NW/NF. The high stability of the NiCo2S4 NW/NF electrode was also confirmed by the results of LSV measurements after 20 h. The LSV curves of NiCo2S4 NW/NF before and after stability are almost identical [inset in Fig. 4(d)].

    The electrochemical double-layer capacitance (ECSA) is estimated from Cdl, where the Cdl of catalysts was measured by simple CV tests at different scan rates of 4, 6, 8, and 10 mV s−1 as shown in Fig. 4(a-b). In Fig. 4(c) it can be clearly seen that the Cdl (4.36 mF cm−2) for NiCo2S4 NW/NF is higher than that of NiCo2O4/NF (2.18 mF cm−2). The high value of Cdl indicates a higher electrochemically active surface area. Further, to get an insight into the OER kinetics, the electrochemical impedance analysis (EIS) was carried out. The Nyquist plots of NiCo2S4 NW/NF and NiCo2O4/NF are shown in Fig. 4(d). The Nyquist plot indicates the low charge transfer for NiCo2S4 NW/NF electrode than NiCo2O4/ NF. This suggested the high electron transfer kinetics of NiCo2S4 NW/NF during OER. In light of all the above experimental results and analysis, we would like to attribute the enhancement in the OER activity of the NiCo2S4 NW/NF to several factors. (1) First of all, synergism between nickel and cobalt increases the number of their exposed active cites promotes OER activity.17) (2) Large S2− anions (than O2− anions) with closely packed array structure together with Ni and Co metal cations in different valence states present in the tetrahedral and octahedral sites respectively, of the spinel crystal structure, resulting in more octahedral active sites of Co(III) cations for efficient OER activity at a very low cell voltage. (3) Vertically aligned nanowire structure of NiCo2S4 provides a large specific area and 3D NF substrate promotes the fast electron transfer channel. (4) Strong binding between NiCo2S4 nanowires and NF substrate, ensuring good electrical contact and efficient charge transfer resistance between the catalyst and the current collector.15)

    4. Conclusion

    In summary, we have successfully prepared onedimensional NiCo2S4 nanowire arrays on the NF substrate via facile hydrothermal synthesis. The prepared NiCo2S4 electrode showed an excellent OER performance in 1 M KOH electrolyte. The NiCo2S4 electrode needs low overpotential of 275 mV at a current density of 20 mA cm-2 and showing excellent stability of 20 h. ESCA and EIS analysis indicate excellent catalytical activity of the NiCo2S4 due to high surface area and superior reaction kinetics compared to another electrode. The uniformly grown nanowire structure of NiCo2S4 enhances charge transport, and highly exposed active sites. The above results demonstrate that NiCo2S4 NW/NF electrode is a promising, highly active and stable electrode for the OER in alkaline electrolyte conditions.

    Acknowledgment

    This study was financially supported by Chonnam National University (Grant number: 2017-2842).

    Figure

    MRSK-30-5-217_F1.gif

    FE-SEM images of NiCo2S4 NW/NF at different magnifications.

    MRSK-30-5-217_F2.gif

    (a) XRD patterns of NiCo2S4 NW/NF. XPS spectra of (b) Ni 2p, (c) Co 2p, (d) S 2p for NiCo2S4 NW/NF.

    MRSK-30-5-217_F3.gif

    (a) OER LSV polarization curves, (b) the corresponding Tafel plots, (c) Comparative plot of overpotential and Tafel of NiCo2S4 NW/NF, NiCo2O4/NF and bare Ni-foam (d) Chronopotentiometry stability test of NiCo2S4 NW/NF over 20 h at a constant current density of 20 mA cm-2 (inset shows the polarization curve before and after the 20 h stability test).

    MRSK-30-5-217_F4.gif

    CVs of (a) NiCo2S4 NW/NF, (b) NiCo2O4/NF in 1 M KOH solution at different scan rates. (c) The capacitive current densities of NiCo2S4 NW/NF and NiCo2O4/NF plotted against the scan rate. (d) Nyquist plots of NiCo2S4 NW/NF and NiCo2O4/NF.

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