The synergistic effect of nickel cobalt sulfide nanoflakes and sulfur-doped porous carboneous nanostructure as bifunctional electrocatalyst for enhanced rechargeable Li-O2 batteries

Journal Pre-proof The Synergistic Effect of Nickel Cobalt Sulfide Nanoflakes and Sulfur-doped Porous Carboneous Nanostructure as Bifunctional Electrocatalyst for Enhanced Rechargeable Li-O2 Batteries Suyeon Hyun, Byungrak Son, Hasuck Kim, Jakkid Sanetuntikul, Sangaraju Shanmugam

PII:

S0926-3373(19)31029-X

DOI:

https://doi.org/10.1016/j.apcatb.2019.118283

Reference:

APCATB 118283

To appear in:

Applied Catalysis B: Environmental

Received Date:

27 June 2019

Revised Date:

26 September 2019

Accepted Date:

11 October 2019

Please cite this article as: Hyun S, Son B, Kim H, Sanetuntikul J, Shanmugam S, The Synergistic Effect of Nickel Cobalt Sulfide Nanoflakes and Sulfur-doped Porous Carboneous Nanostructure as Bifunctional Electrocatalyst for Enhanced Rechargeable Li-O2 Batteries, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118283

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The Synergistic Effect of Nickel Cobalt Sulfide Nanoflakes and Sulfurdoped

Porous

Carboneous

Nanostructure

as

Bifunctional

Electrocatalyst for Enhanced Rechargeable Li-O2 Batteries

Suyeon Hyun†, Byungrak Son‡, Hasuck Kim†, Jakkid Sanetuntikul*,#, Sangaraju Shanmugam*,† Department of Energy Science & Engineering, and ‡Division of Energy Technology, Daegu Gyeongbuk Institute of Science & Technology (DGIST), Daegu 42988, Republic of Korea. # Faculty of Engineering and Technology, King Mongkut’s University of Technology North Bangkok (KMUTNB), Bankhai, Rayong, 21120, Thailand. E-mail: [email protected]; [email protected]

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*

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†

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Voltage (V)

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Graphical Abstract

Specific capacity (mAhg-1)

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Research Highlights  The NiCo2S4 sulfur-doped carbon nanostructure catalyst developed by the onestep method. 

The composite catalyst shows an excellent bifunctional activity for ORR and OER reactions.



The effects of NiCo2S4 and sulfur loading on the carbon surface are investigated.



A Li-O2 battery with NCS/S-3DPG cathode operates over 1704h with low

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overpotential.

Abstract

Herein, NiCo2S4 supported on sulfur-doped carbon with multi-porous nanostructures

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are developed to achieve excellent bifunctional catalytic activities. We present a comprehensive study on the composite effect between NiCo2S4 and sulfur-doped carbon as a bifunctional

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catalyst for oxygen reduction reaction and oxygen evolution reactions. Further, the high

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catalytic activities of NiCo2S4 modified sulfur-doped carbon composite is explored as an air cathode for rechargeable Li-O2 batteries. The sulfur-doped carbon surface modification with

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NiCo2S4 improves the reversibility and capacity due to its high specific surface area, exposing of large active sites, and the formation of smaller sized toroidal Li2O2 discharge product on

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cathode, which can be decomposed at a lower potential compared to a pristine carbon electrode. The Li-O2 battery constructed with the rationally designed composite air cathode delivers ultra-

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high discharge capacity of 14,173 mAhg-1 at 150 mAg-1 and superior cyclability for over 1704 hours without capacity fade due to the excellent surface properties.

Keywords: Bifunctional oxygen electrocatalyst; Nickel cobalt sulfide; Sulfur-doped carbon composites; 3D porous carbon; Li-O2 batteries. 2

1. Introduction The rechargeable Li-O2 batteries are considered as the attractive next-generation power sources due to their high theoretical energy density compared to that of conventional Li-ion batteries [1, 2]. However, some technical issues of Li-O2 batteries should be addressed before utilization in commercial devices due to their poor reversibility and low energy efficiency [3].

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For example, the issues regarding organic electrolyte decomposition and the corrosion of porous carbon electrode are deeply related with a high reactivity of peroxide and superoxide and their reversibility in Li-O2 batteries [4]. The oxygen cathode with open structure is the place where the oxygen reduction leads to the formation of insoluble lithium peroxide (Li2O2)

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discharge product, and oxidation of this product takes place; thus its structure plays a

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significant role in Li-O2 battery performance [5]. Therefore, a highly active and durable electrocatalyst towards the oxygen reduction reaction (ORR) and the oxygen evolution reaction

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(OER) is inevitable, hence it finally achieves much better Li-O2 battery performance [6, 7]. So far, various methods for designing new electrocatalysts have been explored to replace the most

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efficient electrocatalysts based on precious metals due to its uneconomical and unsustainable properties. In order to meet the criteria of high catalytic activity and durability, transition metal

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oxides/chalcogenides as a new group of non-precious metal-based electrocatalysts can be one of the best candidates, for example, MnFe2O4/NiCo2O4 [8], MnCo2S4 [9], CuCo2S4 [10], and

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ZnCo2S4 [5], owing to its low cost and excellent various physicochemical properties. Among them, nickel cobalt sulfide (NiCo2S4) has been considered as one of the most popular ternary sulfides in the field of supercapacitors and water splitting system as electrodes and electrocatalysts, respectively since it has intrinsic material aspects of high electrochemical

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activities originated from the rich redox reactions with multivalence states and comparatively high electrical conductivity [11-16]. On the other hand, the rationally designed novel nanostructured material effectively can improve the utilization of active materials due to their high surface area and short path for electron and ion transport [17, 18]. Based on this consideration, hierarchical three-dimensional (3D) multi-level porous carbon nanostructures synthesized from an ion-exchange resin-based

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technology by Shen et al. has been considered as a potential candidate of carbon substrate material [19, 20]. The hierarchical 3D porous graphene-like structures (3DPG) as catalyst substrate are composed of interconnected micro-, meso-, and sub-micrometer sized macropores. It can maximize the utilization of active materials by uniform dispersion on its surface, provide

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numerous channels to transfer the electrons and ions as well as large storage of the discharge

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products in macropores and keep the electrode stable from the strain effect given by the volume changes during discharge and charge. The high sulfur loading of 3DPG achieved a stable

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cycling performance with outstanding reversibility of charge/discharge rate performance in LiS batteries [20]. Apart from the morphological aspects, doping heteroatom into the carbon

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matrix enables improvement in electron conductivity and catalytic activity. For example, doping of S atom could manipulate the charge state of neighbor carbon atoms [21-24]. An

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electronegativity of sulfur is 2.58 similar to that of carbon (2.55) shows that S atom is one of the ideal dopants for the modified carbon materials. Also, it is reported that –C-S-C- bonds in

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carbon frame are used as active sites for oxygen reduction reaction [25]. Likewise, by modifying the carbon nanostructures with various active materials, the resulted composite nanostructures with controlled physicochemical properties show improved ORR and OER performances such as Co9S8@carbon nanocages, NiS/graphene, FeCo/FeCoNi/N-doped carbon, PdNi/N,S co-doped carbon, CuGeO3/Graphene composites, and so on [26-32]. 4

Recently, Liu et al., developed the bifunctional electrocatalyst of NiCo2S4 synergized with sulfur-doped graphene nanosheets for zinc-air batteries with long term durability for over 100 hours operation without apparent decay and a narrow overall overpotential of 1.09 V [14]. However, to the best of our knowledge, no attempt has been constructed to develop cathode catalyst for high performance non-aqueous rechargeable Li-O2 batteries composed of nickel cobalt sulfide nanoflakes functionalized sulfur-doped porous carbon nanostructures.

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Therefore, in this work we focus on the rational design of composite air electrode consists of NiCo2S4 nanoflakes/multi-level pore sized S-doped graphene-like carbon nanostructures (NCS/S-3DPG) by optimizing the mass ratio and tailoring the architectures. Interestingly, the developed NCS/S-3DPG composites enhance the reversibility of Li2O2 formation and

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decomposition, and more importantly, NCS/S-3DPG based cathode achieve high discharge

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specific capacity of 14,173 mAhg-1 at 150 mAg-1 and preserve the battery performance for 1704 hours without any capacity fade. This excellent battery performance is on the basis of the

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following considerations: (i) plentiful multi-level pores from micro- to micrometer-sized macro- pores in the composite nanostructures provide a huge space for discharge products and

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for the incorporation of active materials, (ii) the hierarchically interconnected pores facile the charge and mass transport, (iii) nanoflakes morphology of NiCo2S4 with multi-valence state

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possess abundant active sites for oxygen redox reaction, (iv) the robust nature of electrode and the presence of NiCo2S4 onto the surface of sulfur-doped 3DPG substrate which minimizes the

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contact between carbon and the product during discharging helps to enhance cyclic performance, (v) S-doped carbon with graphene-like architecture can increase electric conductivity, ensuring the fast movement of electrons from inactive discharge products.

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2. Materials and methods 2.1. Chemical reagents Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) are procured from Alfa Aesar; Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), urea (CH4N2O), and sodium sulfide hydrate (Na2S·xH2O) are

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purchased from Sigma Aldrich; Three-dimensionally constructed graphene powder was obtained from the Sun Yat-sen University [19]. 2.2. Synthetic procedures

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The NCS/S-3DPG catalyst was synthesized by the hydrothermal method. In a typical synthesis, 0.136 g nickel nitrate, 0.276 g cobalt nitrate, and 0.084 g urea were added to 0.83

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mg mL-1 of 3D graphene aqueous solution under stirring. The mixture was sonicated until the

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mixture becomes homogeneously dispersed and then transferred into a 200 mL Teflon-lined autoclave, heated at 120 ℃ in an electric oven for 8 h. The intermediate product was collected,

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immersed in 0.2 M sulfide solution prepared by the solution dissolving sodium sulfide flakes in DI water, and sealed in an autoclave with maintaining at 160 ℃ for 6 h to gain the final

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product of NCS/S-3DPG. The obtained products were centrifuged and cleaned with ethanol and DI water for several times before using as catalysts. For comparison, NCS4/S-3DPG and

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NCS6/S-3DPG were also prepared by the same synthesis procedure, but the weight of Ni and Co salt precursors were increased up to 4 and 6 times compared to that of NCS/S-3DPG, respectively.

2.3. Material structure and physical properties characterization

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The phase of the catalysts was determined by X-ray diffraction (XRD, Rigaku, Miniflex 600). The morphology of the catalysts and the air electrodes before and after cycling was characterized by field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4800II, 3 kV) and a field-emission transmission electron microscope (FE-TEM, Hitachi, HF-3300). The oxidation state of the catalysts and the formation of discharge products were confirmed by Xray photoelectron spectroscopy (XPS, Thermo Scientific/ESCALAB 250Xi). The Raman

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spectra were obtained with an excitation wavelength of 532 nm (Thermo Scientific, Nicolet Almega XR). The TGA analysis of the catalysts was performed by a thermal gravimetric analyzer (Thermo plus EVO, TG8120), which was heated up to 900℃ with air and a heating rate of 5℃min-1. The Bruner-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH)

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models were used to determine the total specific surface area and pore size distribution

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(Micromeritics, ASAP 2020). The Fourier transform infrared (FT-IR) spectroscopy was also

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carried out in order to check the formation of byproducts due to a side reaction. 2.4. Preparation of air electrodes for Li-O2 batteries The electrodes were made by coating catalyst slurry on carbon fiber paper as a gas

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diffusion layer (GDL) with Ø12.7 mm. Typically as-synthesized catalyst and PVDF were mixed and grounded together in a mortar with a ratio of 9:1, and then added into N-Methyl-2-

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pyrrolidone (NMP) solvent. After grinding thoroughly, the slurry was coated on carbon fiber

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paper, and the coated GDL was dried at 110℃ for overnight in a vacuum oven to remove the absorbed water. The loading amount of final active material was about 0.7 mg cm-2. 2.5. Electrochemical measurements All of the half-cell experiments were evaluated using a three-electrode system connected to a potentiostat (Biologic, VSP). The CV curves were obtained between 2 and 4.5 V at a 7

voltage sweep rate of 1 mVs-1. The electrochemical impedance spectrum (EIS) measurements for the Li-O2 cell was carried out in the frequency range from 200 kHz to 100 mHz and an amplitude voltage of 10 mV and fitted to a simplified Randles circuit model to calculate the resistance. The Li-O2 cell was assembled in an argon filled glove box, comprised of an air cathode, lithium metal anode with Ø15.6 mm, Whatman-GF/A separator soaked in the electrolyte, which was saturated with 1 M lithium bis(trifluoromethane sulfonamide) (LiTFSI)

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in tetraethylene glycol dimethyl ether (TEGDME). The battery performance evaluation was performed on a MTI Korea battery analyzer, and the oxygen gas was maintained in the cell during the battery operation.

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3. Results and discussion

3.1. The synthesis of NiCo2S4 modified sulfur-doped carbon composite catalysts

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The NiCo2S4 with S-3DPG composite nanostructures are prepared via a two-step synthesis

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hydrothermal procedure. During the hydrothermal process, NiCo2S4 nanoflakes are in-situ grown on the porous carbon substrates, and the ratio between NiCo2S4 and S-3DPG is controlled by tailoring the amount of NiCo2S4 precursors. Based on an increase in the weight

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of metal salt precursor compared to NCS/S-3DPG by multiplying certain value, the controlled sample name is denoted as NCS4/S-3DPG and NCS6/S-3DPG, respectively. Besides, the

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sulfur atom is incorporated into the carbon matrix when sulfurization occurred during the

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second hydrothermal process, indicating it does not need any additional step for the doping of sulfur. To obtain an efficient composite air electrode for Li-O2 cells, we have investigated the coupling effect of NiCo2S4 and sulfur-doped porous carbon and confirmed the appropriate combination ratio between them. 3.2. Morphology analysis 8

The structure and morphology of the catalysts are analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and presented in Fig. 1. The SEM images of the pristine 3DPG material show numerous micrometer-sized voids with about 200 nm thick rigid walls, and the pores are hierarchically interconnected with each other, generating three-dimensional porous carbon networks (Fig. 1a). The TEM image further confirms the presence of interconnected meso-macroporous architectures with many voids from hundreds

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of nanometer to tens of micrometers, and the thin walls of around 5 nm thickness with a large degree of graphitization (Fig. 1d). The presence of numerous meso- and macro-porous structures in carbon can provide big advantages as an air cathode, specifically an ideal substrate for accommodating the generating products during battery operation and encapsulating the

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active materials into the carbon matrix to prevent from dissolution. The Fig.1b shows the FE-

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SEM images after incorporating the NiCo2S4 into carbon, showing the uniform distribution of NiCo2S4 nanoflakes in the range of 200-300 nm, which are vertically anchored on the carbon

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substrate. Based on the mapping and the XRD results of NCS/S-3DPG composite indicates that the nanoflakes formed on the carbon surface can be determined to be NiCo2S4

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(Fig. S1, Fig. 2a). The TEM images of NCS/S-3DPG composite catalyst shows the successful deposition of NiCo2S4 nanoflakes onto the three-dimensional meso-macroporous graphene-like

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structures (Fig. 1e,f). Meanwhile, even though the excess NiCo2S4 is deposited on the carbon network, the carbon walls can be seen from the FE-SEM and TEM images of NCS6/S-3DPG

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catalyst, indicating that the meso-macropores structure of carbon is well preserved (Fig. S2). However, the aggregated NiCo2S4 nanorods almost fully covered the carbon substrate and formed hybrid composite structures. The high-resolution TEM image (Fig. 1c) presents the highly crystalline of NiCo2S4 with the d-spacing values of 0.283 nm and 0.166 nm, correspond to the (311) and (440) planes, respectively. 9

3.3. Physico-chemical characterization The indexed X-ray diffraction (XRD) patterns of the pristine 3DPG, NCS/S-3DPG, NCS4/S-3DPG and NCS6/S-3DPG are shown in Fig. 2a. The XRD peaks at 26.4° and 44.4° are related to the standard pattern of graphene (ICSD No.00-041-1487). Further modification of carbon substrate with NiCo2S4, the peak at 26.4° is shifted to the higher 2θ value of 26.7°. Also, the new four peaks presented at 2θ values of 31.6°, 38.3°, 50.5°, and 55.3° are consistent

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with the (311), (400), (511), and (440) planes of the NiCo2S4, confirming the formation of NiCo2S4 in the composite (ICSD No.01-073-1704). Meanwhile, it is required to maximize the mass activity of active materials, which affects to enlarge the overall battery performance concerning specific capacity and overpotentials. The increase in mass activities can reduce the

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total weight of cathode while it still preserves its good electrochemical properties [33]. So, the

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fabrication of homogeneously dispersed active materials with fully exposed surface area is needed to utilize the maximum catalytic sites for the electrochemical reaction. Based on FE-

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TEM images, we developed uniform-sized microspheres composed of numerous NiCo2S4 nanoflakes and are grown onto the carbon surface. The TGA curves of all catalysts are shown

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in Fig. S3. Below 200℃, dehydration of crystal water in the lattice leads to the weight loss of ~15%. The weight loss from 200 to 420℃ is due to the decomposition of carbon substrate as

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well as the oxidation of nickel cobalt sulfide to sulfates. Further, the oxidation of nickel cobalt

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sulfates to oxides can be observed over 420℃. The TGA results obtained under air atmosphere present that the mass of carbon in the NCS/S-3DPG composite is 65%, and the active material of NiCo2S4 makes up 25 wt% of the total mass. The NCS4/S-3DPG and NCS6/S-3DPG contain 42 and 50 wt% of NiCo2S4, respectively.

Even though 1.5 times higher amount of metal salt

precursor is added in the preparation step, there is only 8% difference in a weight of NiCo2S4 in the NCS4/S-3DPG and NCS6/S-3DPG composite. Hence, we precede the electrochemical 10

experiments based on NCS/S-3DPG, NCS6/S-3DPG, and 3DPG catalysts, which will discuss further below. The Raman spectra of all catalysts are presented in Fig. 2b. The D- and G-band are located at ≈1350 cm−1 and ≈1570 cm−1, and the ratio between D- and G-band is related to the degree of disorder and defects in the carbon. The ID/IG ratio of 0.87 is obtained for 3DPG. The ID/IG ratio for NCS/S-3DPG, NCS4/S-3DPG, and NCS6/S-3DPG are calculated to be 0.89, 0.93, and 1.01, respectively. It indicates that the formation of considerable disorder and defects on

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the carbon substrates are attributed to sulfur doping in the carbon matrix, as confirmed by XPS. Further increasing the amount of NiCo2S4 in composites produces more defects with high D band intensity, leading a higher ID/IG ratio of composites compared to that of pristine 3DPG.

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Also, spectra showed two characteristic peaks at 496 and 664 cm-1 for all composite materials,

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which are attributed to the F2g and A1g stretching modes of NiCo2S4, respectively [34, 35]. As can be seen from the spectra, the intensity of these two peaks increased as the amount of

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NiCo2S4 increased for the NCS6/S-3PPG sample.

To analyze the chemical composition and the states on NCS/S-3DPG hybrid structure,

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XPS measurements are conducted and the results are shown in Fig. 2. The Ni 2p spectrum demonstrates two spin-orbit doublets of Ni2+ and Ni3+. The binding energies of 853.4 (Ni2p3/2)

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and 870.6 eV (Ni2p1/2) correspond to Ni2+ species, and 856.9 (Ni2p3/2) and 874.8 eV (Ni2p1/2) are related to Ni3+, respectively (Fig. 2c) [11, 36]. It indicates that both Ni2+ and Ni3+ exist in

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the samples. However, the high intensity peaks of Ni2+ species and the strong satellite peak signals indicate that Ni is mostly presented as lower valence state of Ni2+ in the catalyst. Similarly, both Co3+ (778.6 and 793.7 eV) and Co2+ (780.7 and 799.4 eV) are presented in the catalysts, and Co3+ is the major oxidation state in the cobalt element by confirming the weak signal of satellite peaks and high intensities of Co3+ (Fig. 2d) [11, 13, 36]. Particularly, the 11

deconvolution of the XPS spectrum of carbon (C 1s) shows a C-S peak located at 286.4 eV, confirming that the most of oxygen functional groups are removed and the sulfur atom is successfully doped into the graphene framework by generating C-S covalent bonds (Fig. 2e) [25]. Based on previous reports, the covalent C-S bond is believed to be formed through the reaction between oxygen-containing functional groups of graphene and sulfur atom [37]. During the sulfurization process happened in second step synthesis procedure, the creation of

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more defective sites makes more possibilities for sulfur atoms to bind with carbon hosts. Finally, the C 1s spectrum confirms that the S atom has been successfully substituted into the graphene framework. Also, based on the S2p spectrum, it indicates that the peak at 164.2 eV belongs to C-S-C bond, confirming S atom is successfully doped into the carbon support. The binding

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energy values observed at 161.6 (S2p3/2) and 162.8 eV (S2p1/2) correspond to the M-S bond of

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the NiCo2S4, and the peak at 170.2 eV is assigned to satellite peak of S 2p (Fig. 2f) [36]. 3.4. Surface area analysis

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It is well known that the surface area and pore structures are considered as important factors which affect a large fraction of electrochemically active catalytic sites for the

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electrocatalysis [38]. To understand the effect of pore properties and confirm the adsorption of active materials on carbon surface, N2 adsorption-desorption isotherms and their characteristics

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are studied and revealed in Fig. 3. The BET surface area of 3DPG is 1221 m2g-1 with a large

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BJH pore volume of 0.37 cm3g-1, indicating the favorable condition for the uniform dispersion of active materials of sulfides nanoflakes to provide sufficient active sites. The total pore volume is mostly contributed from the micro/meso pores less than 4 nm, with an average of 2.6 nm. After incorporating NiCo2S4 nanoflakes into 3DPG substrate, the isotherm of NCS/S3DPG composite still maintain type IV isotherm with a hysteresis loop, but the specific surface area decreased which is due to the increment in the loading of NiCo2S4 on the 3DPG surface. 12

When the BET surface area decreased, the micropore volume is gradually reduced and finally almost disappeared, demonstrating the impregnation of NCS nanoflakes into micropores at the initial time. Importantly, the macropores and mesopores of the NCS/S-3DPG composites are sustained well despite the presence of sulfides on the carbon surface (Fig. 3). It was already confirmed that the morphology and the specific area greatly affect the specific capacity and rate capability [39]. Especially, the mesopores formed in the composite nanostructures are

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advantageous for facile lithium-ion transportation and improvement in discharge specific capacity [9, 40, 41]. The BET surface area values of NCS/S-3DPG, NCS4/S-3DPG, and NCS6/S-3DPG are found to be 461, 222, and 158 m2g-1, respectively, with a range of

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mesopores (avg. 2.9, 6.6, and 12 nm sized for each sample). 3.5. Electrocatalytic performance

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Fig. 4a presents the CV curves of the catalysts obtained from the three-electrode configuration system at a scan rate of 0.1 mVs-1 in non-aqueous 1 M LiTFSI/TEGDME

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electrolyte. The electrode represents a typical ORR current when the potential sweep to the cathodic side during a negative scan and reversely a peak associated with OER is observed.

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The cathodic peaks at 2.5 and 2.3 V are attributed to the formation of discharged products of Li2O2 and LiOH during the oxygen reduction on the electrode surface. The peak at a lower

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overpotential region of around 2.5 V is due to the one-electron reduction of O2 to form LiO2,

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and subsequently, it transformed to Li2O2 either by the electrochemical reduction or chemical disproportionation process. The second shoulder peak at 2.3 V (for LiOH) is linked to the presence of H2O, which is observed in the 1st CV scan and diminished in the 2nd cycle. The initial anodic peak around ~3.2 V corresponds to the oxidation of Li2O2 discharged product followed by the chemical disproportionation reaction [42]. The subsequent higher potential peak at 3.74 V could be ascribed to the oxidation of interior bulk Li2O2. Notably, the 2nd CV 13

scan NCS/S-3DPG can still oxidize the discharge products at the same anodic potential without any potential loss, indicating NCS/S-3DPG electrode can facilitate the oxygen evolution reaction effectively compared with the pristine 3DPG catalyst. Also, the 2nd CV scan the OER onset potential for NCS/S-3DPG occurs at 3.16 V while the first anodic peak of pristine 3DPG is observed at 3.45 V, indicating much improvement in OER catalytic activity of NCS/S-3DPG composite after hybridization.

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Further confirming the synergistic effect between NiCo2S4 and S-doped 3DPG on electrocatalytic activities (OER and ORR) are evaluated by the RDE in aqueous 1M KOH electrolyte solution. The pristine 3DPG catalyst reveals the lowest OER activity compared with the composite structures of NCS/S-3DPG and NCS6/S-3DPG (Fig. S4). The pristine 3DPG

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requires an overpotential of 460 mV to generate 10 mAcm-2 current density whereas

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NCS/S-3DPG and NCS6/S-3DPG need 300 and 400 mV, respectively, suggesting that the hybridizing with NiCo2S4 material improves the catalytic performance towards OER. The ORR

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behavior of NCS/S-3DPG catalyst presents an almost similar trend of pristine 3DPG, as the half-wave potential (E1/2) for NCS/S-3DPG is 0.828 V, while 0.823 V is obtained for pristine

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3DPG, suggesting that the coupling between NiCo2S4 and S-3DPG is successfully carried out with preserving the good ORR characteristics of pristine 3DPG. However, the E1/2 for NCS6/S-

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3DPG is 0.796 V with 32 mV potential difference from NCS/S-3DPG catalyst. The threeelectrode experiment results presented in Fig. 4a and Fig. S4 indicate that NCS/S-3DPG

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catalyst has potential as an efficient catalytic cathode material with the largest current density and the lowest onset potential at which the oxidation of discharge product occurs. Particularly, we measure the intrinsic OER activity by calculating the turnover frequency (TOF) value for each catalyst by assuming NiCo2S4 as an active material at overpotential of 400 mV [43]. The TOF values reveal the amount of O2 production per active site of material per unit time. The 14

NCS/S-3DPG catalyst has a TOF value of 0.22 s-1, whereas the NCS6/S-3DPG has a TOF of 0.17 s-1. This result clearly shows that NCS/S-3DPG has excellent intrinsic OER catalytic activity. 3.6. Performance of primary and rechargeable Li-O2 batteries The as-prepared catalysts are employed as oxygen electrodes for Li-O2 batteries (0.7 mgcm-2), and the Li-O2 cells are fully discharged and charged between 2.2 V and 4.4 V at an applied current density of 150 mAg-1, which corresponds to the current gravimetric density

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of 150 mAg-1. The NCS/S-3DPG can deliver a remarkable specific capacity of 14,071 mAhg-1 during the discharge process, and the coulombic efficiency reaches 89.4% with a charge capacity of 12,581 mAhg-1 at the 1st deep discharge-charge cycle as shown in Fig. 4b.

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The NCS6/S-3DPG electrode with high loading of NiCo2S4 shows inferior discharge capacity

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compared to the NCS/S-3DPG electrode, mainly due to an insufficient specific surface area for copious Li2O2 deposition. Also, active catalytic sites are not well exposed in NCS6/S-3DPG

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(high loading NCS) due to the agglomeration of NiCo2S4 nanoflakes on the surface, which might lead to a decreased efficiency for Li2O2 formation and the decomposition. Meanwhile,

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the NCS4/S-3DPG electrode shows the highest discharge capacity of 15,600 mAhg-1. The reason for observing such a high discharge capacity is attributed to the large average pore

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diameter of the NCS4/S-3DPG cathode compared to other electrodes is one of major factors to store ten to hundred nanometer-sized discharged Li2O2 toroidal particles. In addition, based on

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the EIS results, the fresh NCS4/S-3DPG electrode exhibits the lowest charge transfer resistance among all other electrodes, which indicates its fast electron transfer. However, it shows lower charge capacity of 11,216 mAhg-1, indicating the lowest coulombic efficiency (72%) of NCS4/S-3DPG electrode among all composite electrodes which may due to its unstable structure. The pristine 3DPG electrode delivers a discharge capacity of 6,804 mAh g-1 which 15

is comparable with the Super P based electrode (7,436 mAhg-1), but the polarization is almost 100 mV lower. When compared with recent reports, the specific discharge capacity, as well as cycle durability of NCS/S-3DPG electrode, is much higher or comparable to other highperformance electrodes reported (Table S1) [44-51] which further confirms the high electrocatalytic activity of NCS/S-3DPG. The rate performance of NCS/S-3DPG composite electrode can maintain high specific discharge capacities of 6127 and 3802 mAhg-1 and charge capacities of 5531 and 3227 mAhg-1, respectively at higher current densities of 450 and

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750 mAg-1 (Fig. 4c). As the oxygen diffusion length is decreased at a higher current density; therefore, the discharge products deposited in a thinner layer near the electrode surface, which resulted in a lower specific capacity [52]. Meanwhile, the pristine 3DPG itself also possess

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high rate capability at high current density even though the coulombic efficiency of the 3DPG

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electrode (82.5%) becomes lower than that of NCS/S-3DPG electrode (84.9%) at 750 mAg-1 (Fig. 4d). Thanks to the three-dimensioned hierarchical porous nanostructures of NCS/S-3DPG,

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O2 and Li+ ion can diffuse rapidly; hence, the high rate capability is achieved. Further evaluation of reversibility of air cathodes is conducted by operating the battery

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between 2.0 V and 4.4 V at a restricted capacity of 1000 mAhg-1 and a current density of 150 mAg-1. The initial cycle of discharge-charge profiles of various catalysts is shown in Fig. 4e.

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The initial overall overpotential of NCS/S-3DPG is 1.27 V which are combined with 1.01 and 0.26 V at charging and discharge stage (1.01/0.26 V) with 68% round-trip efficiency, while the

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pristine 3DPG based cell needs 1.53 V overpotential for the initial cycle (1.24/0.29 V). Meanwhile, NCS6/S-3DPG shows the smallest overpotential of 1.24 V (1.06/0.27 V) based on a first charge-discharge profile. We have further evaluated the Li-O2 cell performance using a high catalyst loading of 3 mgcm-2 compared with the initial loading of 0.7 mgcm-2. The charge overpotential is 16

significantly decreased below 4.0 V after increasing the amount of NCS/S-3DPG (Fig. S6), indicating that the NCS/S-3DPG acts as an OER catalyst during the charging process. Furthermore, by introducing the NiCo2S4 on the cathode, smaller-sized toroidal nanoparticles of Li2O2 are formed, whereas the much larger-sized toroid morphologies of Li2O2 are observed on the pristine 3DPG electrode surface (Fig. 6d,g). The morphology change is attributed to the strong O2 adsorption via decoration of NiCo2S4 nanoflakes on the porous carbon, which is well

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reported for similarly modified cathodes [53, 54]. It has been established that the increased O2 adsorption sites provide more Li2O2 nucleation sites on the surface rather than the solution. Also, the uniform distributions of nucleation site on the surface suppress the Li2O2 migration, leading the formation of smaller toroidal particles. The smaller-sized discharge product needs

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lower charging overpotential to be decomposed compared with the large Li2O2 toroids. Thus,

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the NCS/S-3DPG can be considered as one of appropriate catalysts for Li-O2 batteries, thanks to the high specific surface area, excellent catalytic activities, and the formation of small-sized

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discharge products by the surface modification with NiCo2S4 nanoflakes. The electrochemical impedance spectra (EIS) analysis for the Li-O2 cells with various

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air electrodes is investigated before and after cycling at a current density of 450 mAg-1 in the frequency range of 0.01-103 Hz described in Fig. 4f. The high-frequency segment of the

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semicircle is originated by an ohmic resistance (Rs), corresponding to the ionic resistance from electrolyte and separator, electrical resistance from both electrodes and current collectors, and

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the contact resistance between the electrode and the current collector [52]. The Nyquist plots show that the ohmic resistance is 10.9 Ω for both pristine 3DPG and NCS/S-3DPG, and 8.2 Ω for NCS4/S-3DPG and 8.9 Ω for NCS6/S-3DPG electrode which are lower than that of bare GDL (12.2 Ω) and Super P (11.7 Ω). After the 1st charge, the Rct value of NCS6/S-3DPG is the smallest among pristine 3DPG and NCS/S-3DPG composite samples because of its good redox 17

kinetics and higher electrical conductivity (Fig. S5). Also, the Rs for the 3DPG cathode remarkably increased especially after 20th cycling due to an increase in deficient of Li+ concentrations in the electrolyte attributed by the surface-adsorbed LiO2 during discharge and the instability of electrolyte in this system, corroborating to the increment of overpotential towards OER [52, 55]. The Rs value shows the similar trend in the composite samples, however, it is not increased after 20th cycles, indicating better stability of the electrolyte in contact with

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metal sulfide/carbon composite than a pristine 3DPG. The continuous production and decomposition of insulating solid discharge products on the cathode surface affects the chargetransfer resistance (Rct), which corresponds to the kinetic reaction at the surface of the air cathode. The corresponding Rct value of the 3DPG electrode significantly increased after

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20 cycles compared to that of NCS/S-3DPG and NCS6/S-3DPG, indicating poor cycling

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performance of the 3DPG air cathode.

The long-term cycling performance of all electrodes with a limited specific capacity of

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1000 mAhg-1 and a current rate of 150 mAg-1 is evaluated and shown in Fig. 5. In the case of NiCo2S4 nanoflakes electrode without 3DPG carbon support is also prepared to mix with super

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P carbon and PVDF binder as a conventional preparation method with a weight ratio of 7:2:1 to keep the same ratio of other 3DPG supported electrodes as shown in Fig. S8a,b. Surprisingly,

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the NCS/S-3DPG based electrode shows superior durability with almost similar voltage profiles and low potential gap of 1.32 V (1.06/0.26 V) for subsequent cycling. Also, the same

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overall overpotential values of 1.41 V with 66% round trip efficiency and 100% coulombic efficiency is achieved after 85 cycles compared to the initial cycle (Fig. 5b). The charge profile of NCS/S-3DPG reveals a stepwise delithiation process; first OER stage occurs up to 3.8 V which attributed to the delithiation of the outer part of the Li2O2 forming LiO2-like species (Li2O2  LiO2 + Li+ + e-), which chemically disproportionate to evolve O2 (LiO2 + LiO2  18

Li2O2 + O2), yielding an overall 2e-/O2 OER process (Li2O2  2Li+ + O2 + 2e-). The second stage occurs until the end of the charging process of 4.4 V ascribed to the bulk Li2O2 oxidation process [56]. However, in case of 3DPG electrode, the discharge-charge overpotential dramatically increased from 1.53 V (1.24/0.29 V) to 1.89 V (1.42/0.47 V) with 33.5% loss of coulombic efficiency in 50 cycles (Fig. 5a). It is mainly due to the increase in Rct, which is consistent with the observation from post mortem FE-SEM images that will be further shown

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in Fig. S12. Besides, another reason in the capacity fading is due to decomposition of the electrolyte upper 4.3 V high charge voltage, thereby forming such as RCOOLi on the electrode surface which decreases Li+ ion concentration in an electrolyte. On the contrary, the Li-O2 cell assembled with NCS/S-3DPG cathode catalyst can sustain over 101 cycles (1704 hours) even

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though the terminal discharge voltage shows 2.48 V and then gradually decreased. In the case

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of NCS6/S-3DPG electrode, the voltage gap is gradually increased, and the charge capacity is decreased to 887 mAhg-1 at the 78th cycle (Fig. 5c). This phenomenon is occurred due to an

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aggregation of NiCo2S4 on carbon substrate and unstable architecture. Similarly, the NCS4/S3DPG shows a specific charge capacity loss from 40 cycles (Fig. S7). The NiCo2S4 nanoflakes

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electrode without supports also exhibits the capacity fading during the 61st charge state, indicating the importance of optimal substrates with porous nanostructures (Fig. 5d). Based on

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the above results, we confirm that the cycling performance of NCS/S-3DPG composite is remarkably extended when compared to the controlled pristine 3DPG and NiCo2S4 electrodes.

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In order to prepare efficient electrocatalysts, the active material of NiCo2S4 should be smallsized and highly dispersed on carbon, so that it can form more active sites for the OER involving Li2O2. At the same time, maintaining highly porous architecture of carbon with good mechanical/electrochemical properties is also important. Based on these aspects, we conclude that below 25 wt% of NiCo2S4 content in the composite is appropriate to maximize each 19

advantageous characteristic of NiCo2S4 and S-doped 3DPG substrate for oxygen reduction and evolution reactions. The proper combination of the NiCo2S4 active material and S-doped 3DPG substrate synergistically enables Li2O2 to generate and decompose and reduced voltage polarization during ORR and OER. To study the electrochemical reversibility of air electrodes, cathodic products at different stages are characterized by XRD, and FE-SEM measurements and results are presented in

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Fig. 6. First, the change of the morphology and structure of NCS/S-3DPG and 3DPG electrodes at initial, fully discharged, and fully charged state are studied. The XRD results depicted in Fig. 6a and Fig. 6b show that the main component of the discharge products of NCS/S-3DPG electrode is Li2O2. The low intensity peak at 32.5° corresponds to LiOH, which is derived from

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a trace amount of moisture present in the LiTFSI/TEGDME electrolyte (Fig. 6a). In the fully

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charged state, Li2O2 or LiOH peaks are not detected for the NCS/S-3DPG electrode, whereas the XRD peak of Li2CO3 appears for the pristine 3DPG cathode. The content of water in the

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electrolyte is 150 ppm, so the discharge product of LiOH is detected in the fully discharged state. Commonly, the solvating property of the water content promotes the Li2O2 solution

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growth mechanism; hence, it helps to increase the cell discharge capacity [57]. However, it can also change the Li2O2 discharge product to LiOH or LiOOH and result in an increment in the

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charging potential leading low rechargeability. Due to the possibility of complex reaction triggered by H2O (undesirable parasitic reactions), it seems that the water content might induce

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a negative effect in this system. Fig. S9a shows the Li 1s XPS spectra for the fully discharged NCS/S-3DPG electrode,

from that which can be confirmed that lithium peroxides (Li2O2) appear at 55.7 eV as the dominant discharge product in the electrochemical reactions. The FTIR spectra show three peaks at 530 cm-1, 567 cm-1, and 613 cm-1 are mainly attributed to those of Li-O stretching in 20

Li2O2 (Fig. S9b) [58]. At the same time, the IR observations also demonstrate the undesired byproducts during the ORR process. The peaks at 862 cm-1 and 1411 cm-1 can be assigned to the out-of-plane deformation mode of CO3 and the COO stretching modes from Li2CO3, respectively. The additional peak at 670 and 1104 cm-1 is attributed to the SO4 antisymmetric stretch in Li2SO4, indicating the decomposition of electrolyte as well as the small portion of NiCo2S4. The spectral region between 1000 and 1200 cm-1 is attributed to the presence of

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residual solvents on the electrode surface. The FE-SEM images show that numerous toroidal shaped Li2O2 with 200 nm diameter size are formed on the NCS/S-3DPG cathode surface. After increasing the voltage up to 4.5 V for charging, most of the toroidal Li2O2 products are not visible on the cathode surface, indicating

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the complete decomposition of Li2O2 discharge product (Fig. 6c-e). Meanwhile, similar toroid

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shaped with crystallized discharge products are formed on the pristine 3DPG electrode, and few of them are still exist on the surface of the electrode at full charging state up to 4.5 V

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(Fig. 6f-h). Based on the XRD results, the major discharge product for the pristine 3DPG electrode is also Li2O2, however, the occurred Li2CO3 during charge is accumulated on the

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carbon surface. Due to the aggregation of accumulated discharge products, the joined particles grow larger and form the nanorod morphology, revealed in Fig. 6h.

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In comparison with the properties with less crystalline discharge products of NCS/S-3DPG electrode can easily be decomposed on charging, which leads to lower charge

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overpotential [38]. Also, it is known that Li2CO3 product requires larger charging potential than Li2O2 for the decomposition, which significantly hinders the electrochemical reaction for battery cycling. Hence, we believe that the OER/ORR electrocatalytic activities for the NCS/S-3DPG composite electrode can be much improved compared with the pristine 3DPG electrode by the hybridization with NiCo2S4 active materials. 21

In order to confirm the electrode stability of NiCo2S4 nanoflakes after cycling on the S-doped 3DPG substrate, the post XPS analysis was conducted. As shown in Fig. S10a,b, the Co 2p and Ni 2p spectra of the cycled NCS/S-3DPG electrode are similar to those obtained for the pristine electrode with a small shift of Ni2+/3+ and Co 2+/3+ species. On the other hand, the S2p spectra indicate that -C-S-C- structure and the metal-sulfur bond in the NCS/S-3DPG disappeared over the cycling and finally the oxidized sulfur (inactive sulfates) was observed

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after discharge/charge cycling test (Fig. S10c). This XPS data suggested the surface of cathode is transformed into Ni-Co-Ox composite during cycling, and the degradation of the battery with the loss of sulfur during cycling provides strong evidence that the active sites occupied by sulfur atoms are indeed involved in the electrochemical redox reaction of Li-O2 batteries, as

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similarly reported by Han et al. [4]. However, the active site of sulfur is at least partially

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responsible for the degradation of the NCS/S-3DPG electrode during the discharge/charge cycles. We also investigate the reversibility of the discharged products obtained for

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NCS/S-3DPG and pristine 3DPG electrodes before and after Li-O2 battery cycling tests. The XRD patterns of NCS/S-3DPG are shown in Fig. S11a for the pristine, 10th charge and

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101st charged cathode. The XRD patterns depict the absence of peaks at 32.8° and 34.9° (2), which correspond to the (100) and (101) planes of Li2O2, indicating the good reversibility of

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the NCS/S-3DPG cell for a high number of cycling operation. The reversible formation and decomposition of Li2O2 are also confirmed by FE-SEM images (Fig. S12b). The overall

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morphology of NCS/S-3DPG electrode shows well-recovered surface compared with that of the pristine state and maintains its original structure, confirming an excellent redox reaction of the formation and oxidation of discharge product at 50th cycles. The NiCo2S4 material behaves as a nucleus for LiO2 growth, and consequently it nucleates and transforms into Li2O2 on the surface, due to the weakening of the binding of superoxide (O2- or LiO2*) to the substrate [27]. 22

Thus, NiCo2S4 nanoflakes could significantly improve cycling durability by minimizing the interface between LiO2 and the carbon surface and further delaying the formation of Li2CO3 byproducts. Also, the sulfur-doped carbon might help to sustain longer cycling due to enhanced reaction kinetics of Li2O2 oxidation and the relatively high stability of S dopants [59]. Meanwhile, after 101 discharge/charge cycles, the electrode surface is almost fully covered by insoluble discharge products of Li2O2 and Li2CO3 confirmed by XRD spectra

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(Fig. S11). The additional peak observed at ~22° for 101 cycled electrodes corresponds to the Li2CO3. It can be explained with an occurrence of inevitable parasitic reactions related to the Li2O2 induced electrolyte instability and the oxidation of electrolyte and catalysts at high OER voltages during long-term cycle ability test [60]. Likewise, only after 50 cycles, the pristine

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3DPG electrode surface is fully deposited by insoluble discharge products such as Li2CO3 and

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Li2O2, leading to the continuous decrease of the oxygen concentration inside the air electrode and blockage of the accessible pores (Fig. S11b, S12a). In detail, it reveals the pinch-off (closed

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out) of the porous diffusion channel near the air side which leads oxygen deficiency in the air cathode as well as the inaccessibility of active catalytic sites [52]. Therefore, it is

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understandable that the cell voltage is rapidly increased and finally could not able to reach the desired specific capacity in the 3DPG electrode after the 50th charge, which is described in

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Fig. 5a. These cycling performance results verify that the rationally organized composite nanostructures composed of the optimal loading of NiCo2S4 incorporated with sulfur-doped

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porous carbon can improve the cyclability compared to that of pristine NiCo2S4 or 3DPG electrode.

Based on the above studies, the properties which improve the overall electrochemical

reactions during battery operation are summarized. First, the introduction of NiCo2S4 nanoflakes in porous carbon structure provides high catalytic activity due to large surface23

active sites. Also, the S-doped carbon via modification with NiCo2S4 nanoflakes controls the Li2O2 growth pathways; thus the surface-based growth is favored, and smaller-sized Li2O2 toroids are formed on the NCS/S-3DPG composite surface which requires a lower overpotential for decomposition. Also, NiCo2S4 nanoflakes could minimize the contact between Li2O2 discharge product and the carbon surface, preventing the byproduct formation, resulting in improving cycle durability. Secondly, the hierarchical porous nanostructures with

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large pore volume and specific surface area enhance the discharge capacity and rate capability. Besides, the nanostructures can facilitate the diffusion of oxygen gas and electrolyte ions, improving mass transport. Thirdly, the NCS/S-3DPG composite with an optimal ratio of NiCo2S4 active materials and sulfur-doped 3DPG substrate improves the catalytic activity and

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electrode stability. Finally, doping of sulfur atoms into the graphene lattice changes the

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electronic structure of carbon; hence it can enhance the catalytic activities as well as the electrical conductivity. We believe that the further improvement of cycle stability with NCS/S-

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3DPG cathode catalyst can be achieved with the help of other parts, by employing an efficient

4. Conclusions

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electrolyte and the separators to be practically applicable in energy devices or vehicles.

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We develop novel composite material consisting of NiCo2S4 nanoflakes supported on a three-dimensional S-doped meso/macroporous carbon and explore as cathode catalyst for

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Li-O2 batteries. The Li-O2 cell with NCS/S-3DPG cathode bifunctional catalyst has a high specific capacity and long cycling durability. The NCS/S-3DPG air electrode exhibits a remarkable specific capacity of 14,173 mAh g-1 at 150 mAg-1 and stable more than 1704 hours with a limited capacity of 1000 mAhg-1. Based on the structural and electrochemical investigation, the improvement in cell performance is attributed to the coupling effect of 24

NCS/S-3DPG composite tailored by the composition ratio and the porous nanostructure with the presence of sulfur doping into the carbon. This study offers a promising strategy to make improvements in Li-O2 battery performance by tailoring the surface properties of the air cathode.

Declaration of Interest Statement:

The submitted work described in this article has not been published previously in any form of

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an abstract or as part of an academic thesis. And it is not under consideration for publication elsewhere. All authors have approved tacitly or explicitly of the submission and the work has been carried out with approval of all authors. We confirm that the submission if accepted, it will not be published elsewhere in the same form, in English or in any other language, without

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the written consent of the Publisher.

This work was financially supported by the DGIST R&D Program of the Ministry of Science,

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ICT and Future Planning of Korea (19-IT-02) and the King Mongkut's University of

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Technology North Bangkok (KMUTNB-62-KNOW-35).

Acknowledgements

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This work was supported by the DGIST R&D Program of the Ministry of Science, ICT and

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Future Planning of Korea (19-IT-02) and the King Mongkut's University of Technology North Bangkok (KMUTNB-62-KNOW-35). Additionally, we thank the DGIST-Center for Core Research Facilities (CCRF) for providing facilities for sample analysis.

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Fig. 1 SEM images of the (a) pristine 3DPG and (b) NCS/S-3DPG composite electrode, (c) HR-TEM image of NCS/S-3DPG catalyst, and the TEM images of (d) pristine 3DPG with high

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magnification (inset) and (e, f) NCS/S-3DPG composite electrode.

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Fig. 2 (a) XRD patterns for pristine 3DPG and NCS/S-3DPG, NCS4/S-3DPG, and NCS6/S-

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3DPG composites, (b) Raman spectra, and the high-resolution XPS spectra of 3DPG and NCS/S-3DPG composite electrodes of (c) Ni2p, (d) Co2p, (e) C1s, and (f) S2p.

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Fig. 3 N2 adsorption-desorption isotherms and the corresponding pore size distributions of the

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(a, a’) pristine 3DPG, (b, b’) NCS/S-3DPG, and (c, c’) NCS6/S-3DPG.

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Fig. 4 (a) CVs of pristine 3DPG and NCS/S-3DPG at a scan rate of 0.1 mVs-1 (b) Comparison

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of the initial discharge/charge profiles of the Li-O2 batteries over a voltage range of 2.2-4.4 V for the 3DPG, NCS/S-3DPG, NCS6/S-3DPG, and Super P cathodes at 150 mAg-1, (c) Rate capability of NCS/S-3DPG and (d) 3DPG cathode at different current densities, (e) comparison of the first scan discharge/charge profiles with a limited capacity of 1000 mAhg-1, (f) Nyquist plots of the all electrodes before cycling.

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Fig. 5 The discharge and charge profiles of (a) 3DPG, (b) NCS/S-3DPG, (c) NCS6/S-3DPG,

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and (d) NiCo2S4 with different cycles at 150 mA g-1 and a limited capacity of 1000 mAh g-1.

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Fig. 6 XRD patterns of the (a) NCS/S-3DPG and (b) 3DPG cathodes at deep discharge or

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charge state; FE-SEM images of the pristine NCS/S-3DPG electrode (c) at initial, (d) after full discharge and (e) charge; the pristine 3DPG electrode FE-SEM images (f) at initial state,

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(g) after full discharge and (h) charge.

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