Nickel Sulfide Heterostructure Directly Grown on Nickel

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Energy, Environmental, and Catalysis Applications

Cobalt sulfide/nickel sulfide heterostructure directly grown on nickel foam: an efficient and durable electrocatalyst for overall water splitting application Subhasis Shit, Suman Chhetri, Wooree Jang, Naresh Chandra Murmu, Hyeyoung Koo, Pranab Samanta, and Tapas Kuila ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Jul 2018 Downloaded from http://pubs.acs.org on July 25, 2018

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ACS Applied Materials & Interfaces

Cobalt sulfide/nickel sulfide heterostructure directly grown on nickel foam: an efficient and durable electrocatalyst for overall water splitting application Subhasis Shit,† Suman Chhetri, †,‡ Wooree Jang, # Naresh C. Murmu, †,‡ Hyeyoung Koo,# Pranab Samanta, †,‡ and Tapas Kuila†,‡,* †

Surface Engineering & Tribology Division, Council of Scientific and Industrial Research-

Central Mechanical Engineering Research Institute, Durgapur -713209, India ‡

Academy of Scientiﬁc and Innovative Research (AcSIR), CSIR-CMERI Campus, Durgapur -

713209, India #

Soft Innovative Materials Research Centre, Institute of Advanced Composite Materials, Korea

Institute of Science and Technology (KIST), Jeonbuk - 565905, South Korea KEYWORDS: Hydrogen Evolution Reaction, Oxygen Evolution Reaction, Bifunctional Electrocatalyst, Overall Water Splitting, Heterostructure.

ABSTRACT: Fabrication of high performance noble-metal-free bifunctional electrocatalyst for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in water is promising strategy towards future carbon-neutral economy. Herein, one-pot hydrothermal synthesis of cobalt sulfide/nickel sulfide heterostructure supported by nickel foam

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(CoSx/Ni3S2@NF) was performed. Ni foam acted as 3D conducting substrate as well as the source of nickel for Ni3S2. The formation of CoSx/Ni3S2@NF was confirmed by X-Ray diffraction and X-ray photoelectron spectroscopy. The formation of CoSx/Ni3S2@NF facilitated easy charge transport and showed synergistic electrocatalytic effect towards HER, OER and overall water splitting in alkaline medium. Remarkably, CoSx/Ni3S2@NF showed catalytic activity comparable with benchmarking electrocatalysts Pt/C and RuO2. For CoSx/Ni3S2@NF overpotential of 204 and 280 mV were required to achieve 10 and 20 mA cm-2 current density for HER and OER, respectively in 1.0 M KOH solution. The two electrode system was formulated for overall water splitting reaction, which showed 10 and 50 mA cm-2 current density at 1.572 and 1.684 V, respectively. The prepared catalyst exhibited excellent durability in HER and OER catalyzing condition and also in overall water splitting operation. Therefore, CoSx/Ni3S2@NF could be a promising noble-metal-free electrocatalyst for overall water splitting application.

1. INTRODUCTION The harness and storage of renewable energy are challenging yet promising approach to confront with the two major problems of contemporary time - energy depletion and environmental degradation. Central to this challenge, hydrogen has great merit to be used as clean energy carrier because it has a high energy density of 283 kJ mol-1 and produces only water (H2O) as a product after combustion.1 Therefore, electrochemical water splitting using energy derived from sources like solar, wind etc. to produce hydrogen at cathode and oxygen at the anode, shows great commitment.2,3 However, both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have sluggish kinetics, thus employment of an effective catalyst is


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necessary to narrow the gap between theoretically calculated and practically required potential.4,5 The state-of-the-art catalysts used for HER are Pt-based materials and for OER are Ru and Irbased oxides.6,7 However, their low abundance, high cost, and short-term durability impedes their large-scale application as electrocatalyst.8,9 In addition, the efficiency of overall water splitting depends on both HER and OER rate kinetics. Therefore, the development of a material which can concurrently catalyze both the HER and OER in the same electrolytic medium could be advantageous, which might further simplify the development process of electrolyzer device.1013

Over the last few years, extensive researches are being carried out to develop earth-abundant, noble-metal-free bi-functional electrocatalysts. Several materials such as transition metal chalcogenides, pnictides, borides, etc. were developed as electrocatalyst for water splitting reaction.14-22 However, these types of materials suffer from low electrical conductivity and less specific surface area. Direct growing of the nanostructured catalyst materials on conducting substrates like carbon cloth, metal foam, metal mesh, metal foil etc. have proven effective in overcoming those lacuna.23-27 To the present, nickel sulfide has been extensively studied and been proposed to be active towards electrocatalytic water splitting in alkaline medium,12,16,28 but its catalytic activity towards HER is not up to the mark.11,13 In order to augment the catalytic activity, Ni3S2 has been hybridized with highly efficient electrocatalyst, like MoS2, which exhibited improved catalytic activity.11,29-30 Although MoS2 has good catalytic activity towards HER in acidic medium, its activity is not up to the mark in alkaline medium.31 Prior studies revealed that sulfides of cobalt, such as., Co9S8, CoS2, CoS, etc. have better catalytic activity towards both HER and OER in alkaline medium and those have excellent chemical stability and electrical conductivity.32-35 Furthermore, the previous studies revealed that the free energy of


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adsorption of H and O containing species decreased with the incorporation of Co-atoms on the edge sites of other transition metal chalcogenides. Moreover, charge transfer within the electrocatalyst material via S-linkages as well as between electrocatalyst interface with H and O containing species increased synergistically. As a consequence, the HER and OER catalyzing activity increased significantly.36-38 Therefore, integration of cobalt sulfide with nickel sulfide may be promising towards electrocatalytic water splitting application. Herein, cobalt sulfide anchored nickel sulfide on Nickel foam (CoSx/Ni3S2@NF) were fabricated for the first time via one-step hydrothermal process, using nickel foam (NF) itself as nickel precursor. Prepared CoSx/Ni3S2@NF electrode showed excellent performance as an electrocatalyst towards overall water splitting. It exhibited comparable catalytic activity with that of Pt and Ru-based electrocatalysts used for HER and OER. An overpotential of 204 and 280 mV was required to achieve 10 and 20 mA cm-2 current density, respectively for HER and OER in 1.0 M KOH solution. In order to ensure its overall water splitting competency, a twoelectrode system was constructed using CoSx/Ni3S2@NF as both cathode and anode in 1.0 M KOH solution. 2. EXPERIMENTAL SECTION 2.1 Materials CoCl2.6H2O, thiourea, KOH pellets and HCl (~35%) were purchased from Merck Specialties Pvt. Ltd. (Mumbai), India. Nickel foam was bought from Shanghai Winfay New Material Co. Ltd., China. Absolute ethanol (99.9%) was obtained from Honyon International Inc., China. Pt on activated carbon (10 wt%) and RuO2 were purchased from Sigma-Aldrich. Mentioned


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chemicals were of analytical grade and thus used without further purification unless specifically mentioned. 2.2 Preparation of electrocatalyst Synthesis of CoSx/Ni3S2@NF, Ni3S2@NF and CoSx@NF: Three pieces of NF with dimensions of 2×3 cm2 were cut. Then, those pieces were cleaned sequentially with HCl solution (3M), ethanol and distilled water mixture by water bath sonication to remove the oxide layers. At first, 0.1 mmol CoCl2.6H2O and 1 mmol thiourea were dissolved in 40 mL distilled water separately. Then, one piece of cleaned NF, Co(II) solution, and thiourea solution were transferred into a 100 mL Teflon-lined stainless-steel autoclave reactor. The autoclave reactor was locked properly and placed inside a preheated oven at 120 °C for 12 h. After that, the autoclave reactor was cooled down to room temperature and the prepared CoSx/Ni3S2@NF was taken out and rinsed thoroughly with distilled water. Thereafter, it was dried inside a vacuum oven at 60 °C for 12 h. The change in mass was determined with the help of high-precision balance and the average mass loading was ~2.83 mg cm-2. Two other materials were prepared according to the similar procedure where CoCl2.H2O was taken as 0.05 and 0.2 mmol, which were denoted as C0.05NS and C0.2NS, respectively. For comparison, Ni3S2@NF was prepared according to the same procedure as mentioned earlier without using cobalt precursor. CoSx was also prepared following the same procedure without putting NF in the hydrothermal reactor. After preparation of CoSx, a catalyst ink was prepared and loaded on to the pre-treated NF to fabricate CoSx@NF. Synthesis of Pt/C@NF and RuO2@NF: About 71 mg of Pt/C (10 wt% Pt content) and RuO2 was taken in two different sample tubes each containing ~5 mL of N, N-dimethylformamide


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(DMF). As binder material, ~7 mg of polyvinylidene fluoride (PVDF) was added to each dispersion. The catalyst inks were then sonicated for ~30 minutes. After that, the calculated amount of catalyst ink was deposited on two pre-cleaned NF piece. The loading of the catalysts was ~2.83 mg cm-2 in each case. 2.3 Physicochemical characterization Field-emission scanning electron microscopy (FE-SEM) and energy-dispersive X-ray spectroscopic (EDS) elemental mapping were performed using Σigma HD, Carl Zeiss, Germany. High-resolution transmission electron microscopy (HR-TEM), selective area electron diffraction (SAED) pattern and EDS were recorded at 200 kV using a JEM 2100, JEOL, Japan. Powder XRay diffraction (PXRD) patterns were recorded using Cu Kα radiation (λ = 0.15418 nm) by D2 PHASER, Bruker, Germany. X-ray photoelectron spectroscopy (XPS) was carried out using Thermal ScientificTM equipped with a monochromatic Al-Kα X-ray source (1486.6 eV). 2.4 Electrochemical measurement All the electrochemical measurements were carried out using PARSTAT 4000 (Princeton Applied Research, USA) electrochemical workstation. About 1.0 M KOH solution was used as electrolyte throughout the experiments. In three electrode electrochemical system, catalyst loaded NFs, Pt wire, and Ag/AgCl/saturated KCl were used as working, counter and reference electrode, respectively. The potentials recorded were converted into reversible hydrogen electrode (RHE) scale according to the following equation: E = E/ / . + 0.059pH + 0.197

(Eq. 1)


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The current densities reported in this current work were normalized as per the geometric area.39 Prior to recording the polarization curves, for HER and OER, several voltammetric cycles were performed at the scan rate of 100 mV s-1. The linear sweep voltammetry (LSV) scans were performed at the scan rate of 5 mV s-1 to minimize the double layer charging current. All the above data were 100% iR corrected to eliminate the influence of uncompensated resistance. The Tafel plot was derived from the LSV curves and the analysis was carried out according to the following equation: η = a + b log j

(Eq. 2)

Where η is the overpotential, j is the current density and b is Tafel slope. The η was calculated according to the following equations: For HER,

η = 0 − E

(Eq. 3)

For OER,

η = E − 1.229

(Eq. 4)

The electrochemically active surface area (ECSA) and roughness factor (RF) is calculated from electrochemical double layer capacitance (Cdl) value following the equations: ECSA =

RF =

&' -. () *+ , /01 ,.2345

(Eq. 5)

809,01: 10 9;

Nickel Sulfide Heterostructure Directly Grown on Nickel

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