One-Pot Synthesis of NiCo2S4 Hollow Spheres via Sequential Ion

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Energy, Environmental, and Catalysis Applications

One-Pot Synthesis of NiCo2S4 Hollow Spheres via Sequential Ion-Exchange as an Enhanced Oxygen Bifunctional Electrocatalyst in Alkaline Solution Xueting Feng, Qingze Jiao, Huiru Cui, Mengmeng Yin, Qun Li, Yun Zhao, Hansheng Li, Wei Zhou, and Caihong Feng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08547 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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ACS Applied Materials & Interfaces

One-Pot Synthesis of NiCo2S4 Hollow Spheres via Sequential Ion-Exchange as an Enhanced Oxygen Bifunctional Electrocatalyst in Alkaline Solution Xueting Feng, † Qingze Jiao, †,‡ Huiru Cui, † Mengmeng Yin, † Qun Li, † Yun Zhao, † Hansheng Li, † Wei Zhou, § and Caihong Feng *,†. †

School of Chemistry and Chemical Engineering, Beijing Institute of Technology,

Zhongguancun South Street, Beijing 100081, China ‡

School of Materials and Environment, Beijing Institute of Technology, Jinfeng Road

No.6, Xiangzhou District, Zhuhai 519085, China

§

School of Chemistry, Beihang University, Xueyuan Road No.37, Haidian District,

Beijing 100191, China Corresponding Authors *Email: [email protected] (Caihong Feng) KEYWORDS: oxygen evolution reaction, oxygen reduction reaction, NiCo2S4 HSs, non-precious metal electrocatalysts, alkaline electrolyte.

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ABSTRACT

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are considered to be cornerstones of many energy conversion and storage technologies. It is difficult study high-performance non-precious materials as cost-effective bifunctional electrocatalysts for both the OER and ORR in future practical applications. In this study, NiCo2S4 hollow spheres (NiCo2S4 HSs) were fabricated via an effective and facile one-pot “green” approach in an N,N-dimethylformamide (DMF) - ethylene glycol (EG) binary solution. The obtained NiCo2S4 HSs had a high specific surface area as well as numerous active sites and showed a remarkable catalytic performance and durability towards both the OER and ORR in an alkaline electrolyte. For the ORR, NiCo2S4 HSs exhibited a positive half-wave potential of 0.80 V and demonstrated outstanding stability and enhanced methanol tolerance. For the OER, NiCo2S4 HSs presented a low overpotential (400 mV) at a current density of 10 mA cm-2, small Tafel slope and excellent stability in 0.1 M KOH. Moreover, regarding the overall electrocatalytic activity, the potential difference of NiCo2S4 HSs was 0.83 V, surpassing that of NiCo2S4 nanoparticles, binary counterparts (CoS, NiS) and most highly active bifunctional catalysts described in the literature. The superior catalytic performance of NiCo2S4 HSs is mainly ascribed to its unique hollow structure, which increases molecular diffusion and adsorption, as well as the synergistic effect of Ni and Co, which offers richer redox reaction sites. Importantly, this strategy may facilitate the design and preparation of excellent bifunctional non-precious metal electrocatalysts in various domains. 2


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1. INTRODUCTION The growing demands for clean, sustainable energy and overuse of fossil energy sources have stimulated intense research into the development of energy storage and conversion systems.1-2 It is well known that two key electrochemical processes, the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), play dominant roles in energy storage, such as in water splitting devices, rechargeable fuel cells and metal-air batteries.3-5 Moreover, bifunctional electrocatalysts for both the OER and ORR are essential for generating effective rechargeable fuel cells and metal-air batteries.6 However, practical application of these devices has been severely limited by kinetic and thermodynamic sluggishness, which are considered to be a major bottleneck owing to the required four-electron-transfer pathway.7 Therefore, the use of effective electrocatalysts is necessary to drastically improve the performance and efficiency of these devices. Presently, Pt-based electrocatalysts are widely used for the ORR, but lead to a poor OER performance. Iridium/ruthenium oxides are used as effective catalysts for the OER, but their ORR catalytic capacity is unsatisfactory.8-9 Additionally, the widespread and large-scale applications of these precious metal-based catalysts have been seriously hampered because of their relatively prohibitive cost, scarcity and poor durability over long periods of operation.10 Thus, it is urgent to develop effective, earthabundant and non-noble bifunctional electrocatalysts with high activity. Over the past few decades, mixed-valence transition-metal sulfides, such as nickel sulfides,11 cobalt sulfides,12 and manganese sulfides,13 have been considered to be

3



promising candidates because of their high abundance, low cost and outstanding stability in aqueous alkaline solutions. These sulfides also exhibit higher electrical conductivity and better redox chemistry than the previously mentioned metal oxide.14 Particularly, ternary metal sulfides have been widely studied due to their unique properties, such as the synergistic effect of two metal atoms and the presence of richer redox reaction sites than binary metal sulfides. Spinel compounds (AB2S4) built around a closely packed array of S2- have been investigated in a wide range of applications.1516

Some or all of the tetrahedral and octahedral sites are occupied by A2+ and B3+ metal

cations. Among them, NiCo2S4 (normal spinel structure) has the most octahedral Co3+ cation active sites, demonstrating its remarkable bifunctional catalytic activity for the ORR/OER.17 Noticeably, the morphology and crystal structure of compounds are key to their catalytic activity and durability.18 Recently, NiCo2S4 compounds with several morphologies have been reported to enhance the electrocatalytic activity of including nanowires,19 nanotubes,20 nanosheets,21 hollow spheres,22 and so on. In particular, hollow architectures have drawn high research interest due to their intriguing advantages of (1) offering a large amount of electrochemically active sites, (2) facilitating mass transfer and (3) large electroactive surfaces/interfaces and a high surface-to-bulk ratio.23 Nonetheless, synthesis of compounds with a hollow architecture is complicated and the components are often complex. For example, Shen et al. prepared a NiCo2S4 ball-in-ball hollow structure via a two-step hydrothermal method using NiCo-glycerate as the precursor.24 These methods have a high cost, are time4


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consuming and have low reproducibility. Meanwhile, most of the preparation techniques for hollow structures require the use of templates, surfactants or special instruments. For instance, NiCo2S4 with reticulated hollow spheres was obtained using cetyltrimethyl ammonium bromide (CTAB) as a surfactant.25 Generally, the templates or surfactants introduce heterogeneous impurities, enhance the difficulty of postprocessing, cause an unfriendly environment and destroy the structural integrity. Despite some early successes on size- and morphology-controlled synthesis of NiCo2S4, most reported NiCo2S4 compounds were fabricated by a two-step method and various precursors or additives were needed, which may affect the electrocatalytic performance. Therefore, rational design and preparation of NiCo2S4 with a controllable hollow morphology in a mild environment are tremendous challenges. Herein, we focus on the use of NiCo2S4 HSs as bifunctional electrocatalysts of the ORR/OER in a 0.1 M KOH solution. Hollow structures with extensive nanoparticles were fabricated by a surfactant- and template-free method in an N,Ndimethylformamide (DMF) - ethylene glycol (EG) binary solution through a reflux route. This mild one-pot process is a “green” route and proceeds under atmospheric pressure, which offers greater flexibility and controllability than a two-step method. The formation mechanism of NiCo2S4 HSs was conjectured based on time-dependent experiments. Due to the benefit of the hollow architecture, large surface area, rational composition from NiCo2S4 and large amount of electrochemically active sites, the obtained NiCo2S4 HSs exhibited good ORR activity, with a 0.80 V half-wave potential (E1/2), which was only approximately 57 mV less positive than that of a commercial 5



Pt/C, and a lower overpotential (400 mV), achieving 10 mA cm-2 for the OER and excellent durability. For the overall electrocatalytic activity, NiCo2S4 HSs showed a low potential drop (△E) value (0.83 V) that was significantly superior to the performances of NiCo2S4 nanoparticles (NPs), CoS nanoparticles (CoS NPs), NiS nanoparticles (NiS NPs) and Pt/C. 2. EXPERIMENTAL SECTION 2.1 Synthesis of NiCo2S4 HSs. In a typical procedure, 2 mmol CoSO4•7H2O was dissolved into 80 ml mixed solvent with a volume ratio of VEG/VDMF = 1:4 under vigorous stirring in a 250 mL three-necked flask. The mixture was slowly heated to 145 °C under nitrogen ambience, and then 10 mL mixed solvent containing 10 mmol thiourea was added dropwise by syringe, continuously kept refluxing route for 5 h. After that, 10 mL mixed solvent containing 1 mmol Ni(OAc)2•4H2O was added dropwise to the reaction solution. Subsequently, the temperature was maintained at 170 °C for another 5 h. Finally, the dark precipitate was collected by centrifuge and washed by deionized water and ethanol several times, respectively and dried at 60 °C for 12 h in a vacuum oven. For comparison of catalytic activities, NiCo2S4 NPs, CoS NPs (synthesized with the same method of NiCo2S4 NPs just with the absence of Ni salt) and NiS NPs were prepared. (the details of preparation procedure can be found in supporting information) 2.2 Materials characterization. The phase compositions of the samples were identified using an Ultima IV X-ray diffractometer (XRD) (Rigaku, Japan) with Cu Kα 6


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radiation at 40 kV and 40 mA at the 2θ range from 10° to 80° with 0.02 per step. The morphologies of the samples were characterized by field-emission scanning electronic microscopy (FE-SEM, JEOL JSM-7500F) and transmission electronic microscopy (TEM, Hitachi HT7700). The compositions of the samples were analyzed by an energy dispersive X-ray spectrometer (EDS) attached to an SEM. X-Ray photoelectron spectroscopy (XPS, PHI QUANTERA-II SXM) was performed to examined the surface chemistry of the samples and perform elemental analysis. BET was used to analyze the surface area and pore size distribution of the samples. 2.3 Electrochemical measurements. A CHI potentiostat (CHI760e, CH Instruments) connected to a rotating disk electrode (RDE) system was used to test all of the electrochemical measurements for the ORR and OER. All measurements were conducted with a three-electrode system in 0.1 M KOH, a glass carbon (GC) electrode (5 nm diameter) decorated with catalyst as the working electrode, and a Hg/HgO (1 M KOH) electrode and platinum flake as the reference and counter electrode, respectively. It should be noted that the current density in this text was standardized to the geometrical surface area and that all of potential measurements were calibrated to a reversible hydrogen electrode (RHE) based on the following equation: ERHE = EHg/HgO + 0.0591 × pH + 0.098.

(1)

For the preparation of catalyst ink, 10 mg of the samples and 30 μL of Nafion solution (5 wt%) were dispersed into 1 mL of 3:7 v/v ethanol/water and sonicated for 1 h. Catalyst ink (10 µL) was deposited on a GC disk, resulting in a mass loading of 0.5 7



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mg/cm2. The contrastive samples, including NiCo2S4 NPs, CoS NPs, NiS NPs, commercial 20% Pt/C and IrO2 catalyst, were also investigated using the same mass loading. The ORR activity was first measured by cyclic voltammograms (CVs) from 0.16 to 1.16 V (vs RHE) with a sweep rate of 50 mV s-1 in O2-saturated 0.1 M KOH. Linear sweep voltammograms (LSVs) were examined with a scan rate of 5 mV s-1 at electrode rotation rates from 400-2000 rpm. A Tafel plot from the LSV curves was obtained to calculate the Tafel slopes. The chronoamperometric response was also determined to evaluate durability. The Koutecky-Levich equation was used to determine the electron transfer number as follows 1/J = 1/Jk + 1/JL = 1/Jk + 1/(B ω0.5)

(2)

B = 0.62 n F C0 D02/3 ν−1/6

(3)

Where J is the measured current densities, JL is the diffusion limiting current densities, Jk is the kinetic-limiting current density and ω is the angular velocity of the disk. In Equation (3), n is the electron transferring number, F is the Faradaic constant (96485 C mol-1), C0 is the bulk concentration in 0.1 M KOH (1.2×10-6 mol cm-3), D0 is the oxygen diffusion coefficient in 0.1 M KOH (1.90×10-5 cm2 s-1) and ν is the kinematic viscosity (0.01 cm2 s-1). For the rotating ring-disk electrode (RRDE) test, electrodes with a GC disk (0.2475 cm2 in area) and Pt ring (0.1866 cm2 in area) were used. The same method 8


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was used to prepare the catalyst ink with the same mass loading as the RDE. To determine the ORR reaction pathway, the ring potential was held constant at 1.36 V (vs RHE). The peroxide species and electron transfer number were calculated as follows:

(4) (5) where Ir and Id are the ring current and disk current, respectively, and N is the current collection efficiency (0.37). To evaluate the Faradaic efficiency, the ring potential was maintained at 0.40 V vs. RHE. ε = Ir / (Id N)

(6)

The OER activities were examined using linear sweep voltammograms (LSVs) at 1600 rpm with a scan rate of 5 mV s-1. The Tafel slopes, accelerated stability tests, chronoamperometric response, electrochemical impedance spectroscopy (EIS), turn over frequency (TOF), specific activity (mA cm-2) and mass activity (A g-1) were also performed according to our previous work in 0.1 M KOH.26 To reduce the impact of Fe on the OER performance, we followed the procedures illustrated by Burke et al. to scavenge Fe ions in the electrolyte solution.27 3. RESULTS AND DISCUSSION XRD was used to determine the composition and crystal structure of the prepared NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs and NiS NPs, as shown in Figures 1a and S1.

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NiCo2S4 HSs and NPs had the same diffraction peaks. The seven main broad peaks at 2θ values of 16.3°, 26.8°, 31.6°, 38.3°, 47.4°, 50.5°, and 55.3° were well-indexed to the cubic NiCo2S4 (JCPDS Card no. 20-0782) without any residues and contaminants, implying successful synthesis of NiCo2S4 materials. Likewise, according to the XRD pattern in Figure S1b, c, binary metal sulfides CoS (JCPDS Card no. 75-0605) and NiS (JCPDS

Card

no.

75-0613)

were

also

successfully

obtained.

Figure 1. (a) XRD pattern, (b) SEM image, (c) TEM image, (d) HRTEM image, (e) SAED image of NiCo2S4 HSs, and (f) EDS elemental mapping of Ni, Co and S. The structures and morphologies of the aforementioned NiCo2S4 HSs were further characterized by FESEM, TEM, HRTEM and selected area electron diffraction (SAED). As depicted in Figure 1b, the NiCo2S4 HSs exhibited highly dispersed morphologies, with a lateral size of 1 μm. The broken spheres in the insert picture of Figure 1b indicate the hollow structure of the products. In detail, it was noteworthy that the hollow spheres were composed of numerous irregularly assembled nano-primary particles and had 10


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nanoporous structural features, which benefit molecular diffusion and adsorption, in turn, will improve the electrocatalytic properties (Figure 1c). Figure 1d shows a HRTEM image of NiCo2S4 HSs in which the measured interplanar d-spacings of 0.28 nm was indexed to plane (311). In addition, the well-defined rings also verified the polycrystalline nature of the NiCo2S4 HSs shown in Figure 1e, which were well assignable to planes of (111), (220), (311), (400), (422), (511) and (440). Importantly, the unique porous hollow structure provides benefits, as it provides efficient transport pathways and results in the enhancement of the electrochemical kinetics,28 is not only dependent on the surface of the materials but also on the interior of the hollow spheres.29 The morphologies and microstructures of the other samples were evaluated by SEM, TEM, HRTEM and SAED, and the results are shown in Figures S2-S4. It is clearly observed that both NiCo2S4 NPs and CoS NPs are composed of random inter-connected nanoparticles with a similar size (≈30 nm) and are inclined to aggregate. As shown in Figure S2c, the obvious lattice fringes with interplanar d-spacings of 0.28 nm were consistent with plane (311) of NiCo2S4. Meanwhile, the interlayer spacing of approximately 0.149 nm can be ascribed to the (103) plane of CoS shown in Figure S3c. Unlike the aforementioned sulfides, the NiS NPs were composed of spherical particles with a size of 200 nm. Also, the obvious interplanar distances were calculated to be approximately 0.258 nm, which corresponded to the (101) plane of NiS. The polycrystalline characteristics of those sulfides were confirmed by SAED images. Additionally, EDS was used to define the elemental composition of NiCo2S4 HSs. As shown in the elemental mapping (Figure 1f), a uniform and continuous distribution of 11



Ni, Co and S was observed, verifying the existence of Ni, Co and S in the products. Furthermore, XPS was used to identify the chemical valence states of each element. The survey XPS spectrum shown in Figure 2a revealed the appearance of Ni, Co and S for NiCo2S4 HSs, corresponding to the EDS results. C (as reference) and O emerged due to air exposure.30 Obviously, the Co XPS spectrum for NiCo2S4 HSs shown in Figure 2b indicates that the binding energies located at 778.2 and 793.4 eV correspond to the Co3+ oxidation state, while those at 781.8 and 797.6 eV are ascribed to the Co2+ oxidation state, indicating the coexistence of Co2+ and Co3+.31-32 Additionally, in regard to the Ni 2p region for NiCo2S4 HSs (Figure 2c), the fitted peaks at 852.9 and 871.0 eV are assigned to the Ni2+ state, and those situated at 856.2 and 874.6 eV correspond to the Ni3+ state.33-34 Moreover, Figure 2d shows the core-level spectrum of the S 2p region for NiCo2S4 HSs, in which the peaks at 161.3 and 162.5 eV represent 2p3/2 and 2p1/2, respectively. In detail, the component peak at 163.6 eV is ascribed to metal-sulfur bonds, while the peak at 162.5 eV is likely due to divalent sulfide ions (S2-) with a low coordination at the surface.35 Thus, all of the results confirmed that the chemical composition in the near-surface of NiCo2S4 HSs contained Co2+, Co3+, Ni2+, Ni3+ and S2-, coinciding with the NiCo2S4 results in the literature.36-37 Generally, the emergence of Ni3+ can offer extra electrons for n-type doping, whereas the existence of Co2+ can give rise to extra holes for p-type doping. Thus, more Ni3+ and Co3+ will lead to higher electrical conductivity.38-39 In our work, the atomic percentages of Ni3+ and Co3+ were assessed as high, at approximately 82.6% and 46.8%, demonstrating the enhanced electric conductivity of the NiCo2S4 HSs. 12


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Figure 2. (a) XPS full scan, (b) Co 2p XPS spectra, (c) Ni 2p XPS spectra and (d) S 2p XPS spectra of NiCo2S4 HSs. It is worth noting that the pore size distribution and surface area are important factors to enhance electrochemical performance. Thus, nitrogen adsorption-desorption measurements were performed to determine the surface characteristics of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs and NiS NPs. Figure S5 shows that all of the samples can be classified as type IV isotherms from 0.5-0.9 P/P0 with a hysteresis loop, implying the presence of a typical mesoporous structure.40 The results were further confirmed by the pore size distribution. The corresponding pore size distribution of NiCo2S4 HSs showed a predominant peak with a size of 2.6 nm. Furthermore, the pore volume and BET surface area were calculated to be 0.266 cm3 g-1 and 23.64 m2 g-1, respectively, higher 13



than those of NiCo2S4 NPs (0.069 cm3 g-1 and 13.15 m2 g-1), CoS NPs (0.078 cm3 g-1 and 11.83 m2 g-1) and NiS NPs (0.038 cm3 g-1 and 10.02 m2 g-1). Note that an abundant porosity and large surface areas are beneficial for mass transfer and provide more active sites.41 Thus, it is expected that the NiCo2S4 HSs may show a greatly improved electrochemical performance.

Figure 3. (a) XRD of intermediate samples and SEM of NiCo2S4 HSs at different time intervals: (b) 2 h, (c) 5 h, (d) 6 h, (e) 8 h and (f) 10 h. Then, we investigated the formation mechanism of NiCo2S4 HSs by performing time14


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dependent experiments. Figure 3a shows the XRD pattern of intermediates at different times. The entire reaction process undergoes a transition from cobalt oxide to cobalt sulfide and eventually forms NiCo2S4. According to the XRD pattern shown in Figure 3a, cobalt oxides (CoO and Co3O4) were obtained at 2 h and were composed of random inter-connected nanoparticles (Figure 3b). When the reaction time was up to 5 h, the intermediates transformed into Co9S8 and the nanoparticles began to assemble into loosely hollow spheres with rough shells and distinguishable pores (Figure 3c). Prolonging the sulfidation time to 6 h and 8 h (Figure 3d-e) caused the hollow spheres to become compact and integrated, and the XRD pattern was indexed to the pure phase of NiCo2S4 but with low crystallinity. After reacting for 10 h (Figure 3f), highly dispersed and highly crystalline hollow spheres were obtained, and their composition was NiCo2S4. Based on the experimental results presented above, a sequential ionchange strategy was used to illustrate the possible formation mechanism of NiCo2S4 HSs in Scheme 1. In recent years, the sequential ion-change reaction has been considered to be an effective approach for the preparation of nanomaterials.42 For example, onion-like NiCo2S4 particles were synthesized via sequential ion-exchange reactions outlined by Lou et al.43 Lee et al. reported the generation of a Co9-xNixS8 ONC by a sequential ion-exchange strategy (sulfidation of CoO and a cation exchange reaction).44 Our previous work showed that Co2+ ions formed Co-EG complexes and [Co(NH3)n]x+ (x=2,3) at the beginning reaction stage.26 Then, the metastable complexes decomposed into CoO and Co3O4 nanoparticles. It is well known that thiourea can gradually hydrolyze and release H2S, CO2 and NH3 at high temperature, which act as 15



both sulfur and bubble sources.45 According to the hard-soft-acid-base principle, S2- has a higher affinity for Co2+ than O2-.44, 46 Therefore, CoO and Co3O4 nanoparticles can react with S2- and convert into Co9S8 by an anion-exchange reaction. Previously, Zhang et al. showed a similar phenomenon in ZnS shell formation based on the sulfidation of a ZnO template.47 Additionally, the generation of NH3 and CO2 bubbles can provide gaseous cavities for random moving nanoparticles. Afterward, these Co9S8 precursors can be further transformed into NiCo2S4 HSs via a cation-exchange reaction. This reaction can be ascribed to the smaller crystal ionic radius of Ni2+ (83 pm), which has a higher spin than that of Co2+ (88.5 pm).48 Meanwhile, compact shells with gas bubbles as their centers were formed due to the nanoparticle attachment and the further aggregation around the gas/liquid interface. Thus, it is interesting to note that the formation of NiCo2S4 HSs can be attributed to continuous aggregation and growth. However, when cobalt salt, nickel salt and sulfur source are added together at beginning of the reaction, the resulting product is NiCo2S4 NPs. Scheme 1. Schematic representation of the NiCo2S4 HSs.

Benefiting from the mixed valences, mesoporous features and high surface areas of the hollow structure, NiCo2S4 HSs had strong catalytic activity. Thus, we evaluated 16


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NiCo2S4 HSs as bifunctional catalysts for the OER and ORR and further compared them with NiCo2S4 NPs, binary metal sulfide (CoS NPs and NiS NPs), and commercial IrO2 and Pt/C. To assess the ORR activity of the synthesized samples, cyclic voltammograms (CVs) measurements of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs, and NiS NPs were performed in O2-saturated 0.1 M KOH at room temperature. As observed in Figure 4a, prominent cathodic peaks of all samples were observed in the O2-saturated electrolyte. Remarkably, it is interesting to note that the ORR peak and onset potential of NiCo2S4 HSs were more positive than those of NiCo2S4 NPs, CoS NPs and NiS NPs, revealing that NiCo2S4 HSs had enhanced electrocatalytic activity toward the ORR. Furthermore, linear sweep voltammetries (LSVs) of the NiCo2S4 HSs were performed at different rotation speeds, and the results are shown in Figure 4b. The curves had similar trends, except the reduction currents gradually increased with faster rotation rates, which were ascribed to faster oxygen transfer near the electrode surface.49 According to the K-L equations, the corresponding K-L plots at a series of potentials shown in Figure 4c were nearly linear and parallel, implying first-order reaction kinetics in relation to the concentration of dissolved oxygen and similar electron transfer numbers for the ORR at various potentials.6 Based on the slopes of the K-L plots, the average electron transferring number (n) of NiCo2S4 HSs was 3.6~3.9, indicating a four electron ORR pathway. Comparatively, a set of LSV curves at 1600 rpm of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs, NiS NPs and commercial Pt/C (20% wt%) are shown in Figure 4d, where the half-wave potential (E1/2) was paramount for evaluating the ORR activity. NiCo2S4 HSs presented a more positive value than those of NiCo2S4 NPs, CoS 17



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NPs, and NiS NPs and was only approximately 57 mV less positive than that of the commercial Pt/C. The trend of the onset potential from the LSV curves was similar to that of the CVs. Additionally, the inherent ORR activities of various electrocatalysts were corroborated by Tafel plots. As is evident from Figure 4e, NiCo2S4 HSs had a much smaller Tafel slope values (48.6 mV dec-1) than NiCo2S4 NPs (49.6 mV dec-1), CoS NPs (59.7 mV dec-1), NiS NPs (115.6 mV dec-1) and the commercial Pt/C (70.0 mV dec-1), further demonstrating its efficient kinetics for the ORR. The RRDE was determined to confirm the ORR pathway of NiCo2S4 HSs by monitoring intermediate peroxide species (HO2-). The ring current is shown in Figure S6. Figure 4f shows that the HO2- yields of NiCo2S4 HSs were below 4% from 0.2 V to 0.9 V and that n was nearly 4. These results were consistent with the RDE measurement and revealed the complete four-electron-transfer pathway for NiCo2S4 HSs. Mechanism diagrams of NiCo2S4

for

the

ORR

are

illustrated

18


in

Figure

S7.

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Figure 4. (a) CV curves of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs and NiS NPs in 0.1 M KOH. (b) LSVs of NiCo2S4 HSs at various rotation rates. (c) Kouteckye Levich curves of the NiCo2S4 HSs at various potentials. (d) Comparison of the ORR test for NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs, NiS NPs and Pt/C at 1600 rpm. (e) Tafel plots of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs, NiS NPs and Pt/C. (f) The peroxide yield (top) and electron transfer number (n) (bottom) of NiCo2S4 HSs and Pt/C. For practical application, we also assessed the durability of NiCo2S4 HSs. As shown 19



in Figure 5a, NiCo2S4 HSs were durable with a high current retention, while Pt/C showed a high attenuation of 54.7% over 10 h of operation. Additionally, the tolerance to methanol crossover was evaluated, and the result is shown in Figure 5b. After introduction of a 3 M aqueous methanol solution, a small change was observed in NiCo2S4 HSs, whereas the current of the Pt/C catalyst dropped dramatically. It has been suggested that NiCo2S4 HSs possess a strong immunity to methanol crossover.50 The superior electrocatalytic properties of NiCo2S4 HSs are mainly ascribed as follows. On the one hand, NiCo2S4 offers more redox reaction sites than CoS NPs and NiS NPs because of the synergistic effect of Ni and Co. Additionally, Ni cations can effectively activate Co cations and promote the penetration and absorption of oxygen molecules onto the pore network of the hollow spheres through electrostatic interactions, leading to a highly efficient ORR.51-52 On the other hand, the hollow structure can lead to abundant active sites and a larger surface area compared to nanoparticles and promote electrical contact and facile electrolyte accessibility. Thus, these results showed that NiCo2S4 HSs have a greatly improved electrochemical performance for the ORR.

Figure 5. (a) Current-time responses and (b) methanol-tolerance evaluation of NiCo2S4

20


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HSs and Pt/C for the ORR at 0.7 V. Furthermore, the OER activity was also examined by LSVs in 0.1 M KOH solution with a sweep rate of 5 mV s-1. As shown in Figure 6a, the bare GC electrode showed a negligible OER activity and the OER performance was mainly contributed to the catalysts. By contrast, NiCo2S4 HSs had the lowest onset potential (1.49 mV), which was lower than those of NiCo2S4 NPs (1.61 mV), CoS NPs (1.62 mV) and NiS NPs (1.61 mV) and was the same compared to that of the commercial IrO2 (1.49 mV), indicating its excellent OER performance. Moreover, as shown in Figure 6b, the overpotential at 10 mA cm-2 for NiCo2S4 HSs was 400 mV, and the value was smaller than those of NiCo2S4 NPs (510 mV), CoS NPs (550 mV), and NiS NPs (680 mV) and similar to that of IrO2 (400 mV). A smaller Tafel slope (93.7 mV dec-1) of the NiCo2S4 HSs shown in Figure 6c was observed, which was significantly lower than those of NiCo2S4 NPs (96.4 mV dec-1), CoS NPs (161.6 mV dec-1) and NiS NPs (244.4 mV dec1

) according to the OER polarization curves. This result suggested that NiCo2S4 HSs

had efficient kinetics. The greater OER activity of NiCo2S4 HSs was illustrated by the positive onset potential, lower overpotential and Tafel slope compared with those of the other metal sulfides. Additionally, EIS was carried out to investigate the ion transport kinetics of the catalysts at the open circuit voltage, and the corresponding results are presented in Figure 6d. Clearly, this result showed that all of the catalysts consist of semicircles. The EIS data were fitted with the equivalent circuit shown in Figure S8.53 The fitting impedance parameters, Rct, Rs and CdI, denoted the charge transfer resistance, uncompensated series resistance and accompanying double-layer capacitance. It is 21



interesting to note that the similar Rs values indicated a consistent experimental configuration. The Rct values follow the order of NiCo2S4 HSs < NiCo2S4 NPs < CoS NPs < NiS NPs. The smallest Rct value indicated a fast charge transfer rate of the OER at the catalyst interface,54 which was due to the superior electronic conductivity and abundant active sites of NiCo2S4 HSs.

Figure 6. (a) OER LSV curves of NiCo2S4 HSs (A), NiCo2S4 NPs (B), CoS NPs (C), NiS NPs (D), Pt/C (E), IrO2 (F) and bare GC (G) in 0.1 M KOH. (b) Corresponding overpotentials and (c) Tafel plots of NiCo2S4 HSs (A), NiCo2S4 NPs (B), CoS NPs (C), NiS NPs (D), Pt/C (E) and IrO2 (F). (d) Nyquist plot of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs and NiS NPs. To evaluate the intrinsic catalytic performance, the mass activity, specific activity and turn over frequency (TOF) were also measured. As revealed in the Table 1, the 22


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values of the mass activity and specific activity for NiCo2S4 HSs were 20.00 A g-1 and 0.085 mA cm-2 , respectively, at an overpotential η = 0.4 V, which are much higher than those of NiCo2S4 NPs, CoS NPs and NiS NPs. Also, the TOF was assessed by assuming that all of the Ni and Co sites were catalytically active. The highest TOF value (0.0053) of NiCo2S4 HSs was calculated, demonstrating its significant improvement for OER activity. To investigate the electrochemically active surface area (ECSA), CdI was measured by CVs under different scan rates in a non-faradaic region (Figure S9). As observed in Figure 7a, CdI increased in the order of NiCo2S4 HSs (2.67 mF mA-2) > NiCo2S4 NPs (2.21 mF mA-2) > CoS NPs (0.65 mF mA-2) > NiS NPs (0.14 mF mA-2). Thus, NiCo2S4 HSs had more effective active sites compared that the NiCo2S4 NPs, CoS NPs and NiS NPs benchmarks. In stark contrast, the ternary metal sulfide NiCo2S4 had a higher CdI value than the binary metal sulfide, which revealed that the effect of two metals can lead to more redox reaction sites. Moreover, the faradaic efficiency of the OER activity was investigated by measuring the RRDE. The ring potential was set 0.4 V (vs. RHE) to decrease the produced O2 and ensure a consecutive OER and ORR process.55 As shown in Figure 7b, the disk current was 0.55 mA (blue curve) and molecular O2 was simultaneously produced from the surface of NiCo2S4 HSs. The produced O2 diffused to the Pt ring electrode and was quickly reduced to H2O2. As a result, a constant ring current (0.197 mA) was observed (black curve), revealing that the detected oxidation current was attributed to the OER process. The faradaic efficiency was 96-98%.

23



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Table 1. Electrochemical results of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs and NiS NPs. mass activity

specific activity

TOF

at η = 0.4 V

at η = 0.4 V

at η = 0.4 V

[A g-1]

[mA cm-2]

[s-1]

NiCo2S4 HSs

20.00

0.085

0.0053

NiCo2S4 NPs

3.00

0.023

0.0009

CoS NPs

2.04

0.017

0.0005

NiS NPs

2.46

0.024

0.0006

catalyst

Figure 7. (a) Plots of ΔJ/2 vs. the scan rates of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs and NiS NPs at 1.03 V. (b) Ring and disk currents of NiCo2S4 HSs on RRDE. The insert is the faradaic efficiency of NiCo2S4 HSs. Additionally, catalytic stability plays an important role in practical applications. On the one hand, the electrocatalytic durability was evaluated using accelerated degradation tests at 100 mV s-1 over 1000 cycles. Figure 8a shows that the overpotential at 10 mA cm-2 only increased by 8 mV. On the other hand, the amperometry curves (it) were obtained to confirm the long-term stability of the HSs. After 10 h at a constant potential, the current density of NiCo2S4 HSs was almost unchanged, while IrO2 24


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presented a remarkable attenuation of 40% (Figure 8b). As presented in Figure S10a, the hollow spherical morphology of NiCo2S4 HSs was maintained, while some nanosheets emerged on the surface after the 10 h OER test. The detailed morphology was further confirmed by TEM, and the results are shown in Figure S10b. It is interesting to note that the surfaces of the particles were surrounded by various nanosheets, consistent with previous reports. For example, Chen et al. prepared a Ni1.5Co1.5S4 sample and showed that the surface was covered with various nanosheets after long-term cycling.56 These newly formed nanosheets can not only provide rich electroactive sites but also offer a fast diffusion rate and high electrochemical accessibility, which are beneficial for electrocatalytic activity.56 These results certified the significant catalytic durability. Furthermore, we also evaluated the conversion of NiCo2S4 after the OER durability test by EDS, Raman microscopy and XPS. Notably, EDS elemental mapping displayed a homogeneous distribution of Ni, Co, S and O, and Figure S10 shows the oxidation of NiCo2S4 during OER test. In the Raman spectra (Figure S11), the peaks at 523.5 and 671.2 cm-1 are assigned to NiCo2S4.57 After the 10 h stability test, two bands at 465.0 and 593.0 cm-1 are observed. The broad Raman band located at 593 cm-1 is due to amorphous CoOx, while we attribute the poorly resolved shoulder at 465 cm-1 to NiOOH and CoO2 in the hexagonal delafossite structure.58 XPS analysis was used to evaluate the surface chemical changes, and the results are shown in Figure S12. In Co 2p curves, the peaks at 780.8 and 796.3 eV can be ascribed to cobalt hydroxides/oxides.59-60 Likewise, the two peaks at 856.0 and 873.7 eV in the Ni 2p region demonstrated the presence of Ni3+, suggesting the formation of nickel 25



oxyhydroxide (NiOOH).61 The metal-sulfur bond in the S 2p disappeared. Moreover, the O 1s spectrum at 530.6 and 531.3 eV can be attributed to cobalt oxides/hydroxides and NiOOH.62 It is well known that NiOOH is an active phase for Ni-based materials in OER electrocatalysis.63-64 Zhang et al. illustrated that highly oxidative NiIII/IV is an active site for OER.65 Therefore, the NiCo2S4 core and cobalt hydroxides/oxides as well as the NiOOH shell were the active phases for the OER.66 It is noteworthy that the NiCo2S4 core can serve as a conducting support to provide an effective electron pathway to the less conductive shell, which effectively facilitates a catalytic OER.67 The OER catalytic mechanism can be seen in Figure S13.

Figure 8. (a) Accelerated stability of NiCo2S4 HSs. (b) Amperometry plots of NiCo2S4 HSs at an overpotential of 400 mV. Considering practical applications, the overall electrocatalytic activity is usually estimated by the variance matrices △ (Ej=10 mA cm-2 - E1/2) between the OER and ORR.32 The smaller the potential drop (△E) value, the closer the catalysts are to the ideal reversible oxygen electrode. Compared to NiCo2S4 NPs, binary metal sufides and the commercial Pt/C, NiCo2S4 HSs had the smallest value of 0.83 V, as shown in Figure 9, 26


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demonstrating the enhanced bifunctional oxygen electrode activity. A comparison with previously reported electrocatalysts is provided in Table S1. NiCo2S4 HSs showed similar or better bifunctional oxygen electrode performance.

Figure 9. ORR and OER curves of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs, NiS NPs and Pt/C with a sweep rate of 5 mV s-1 at 1600 rpm in 0.1 M KOH. The desirable ORR and OER catalytic capabilities of NiCo2S4 HSs was mainly assigned to their mixed valences and unique hollow structure. Mixed valences can offer acceptor-donor chemisorption sites for OH- and oxygen reversible adsorption at a low activation potential.68 The higher conductivities of Co and Ni were beneficial for electron transfer to the substrate. The excellent porous structure was conductive to decreasing the mass transport resistance of ionic species and could provide more active sites. Moreover, the hollow space and abundant mesopores between these nanoparticles can serve as a reservoir for electrolytes, facilitating the internal reflex space for fast ion transmission and related reactions.69 Thus, it is noteworthy that critical factors, including the elemental valence, strong structural integrity, chemical composition and 27



crystal structure, affect the ORR and OER catalytic capabilities. 4. CONCLUSIONS In summary, we successfully developed an effective and facile approach for a onepot synthesis of NiCo2S4 HSs with a size of 1 μm. This strategy is a “green” route and does not use a template or surfactant under atmospheric pressure. The formation mechanism of hollow nanostructures is attributed to continuous aggregation and growth on the gas/liquid interface via a sequential ion-exchange approach. The hollow structure generates NiCo2S4 HSs that have large number of active sites and a specific surface area. The synergistic effect of Co and Ni can offer richer redox reaction sites. Benefiting from those unique features, NiCo2S4 HSs possess excellent bifunctional electrocatalytic activity and enhanced diffusion kinetics for the OER/ORR in 0.1 M KOH, and their methanol tolerance and long-term durability outperform those of Pt/C. Additionally, NiCo2S4 HSs exhibit the smallest △E between the ORR and OER compared to the other samples evaluate in our work and a commercial Pt/C. The present work provides a new viewpoint for the rational design and preparation of highly active non-noble metal catalysts with a controllable morphology for use as bifunctional catalysts for energy conversion applications. ASSOCIATED CONTENT Supporting Information Synthesis of NiCo2S4 NPs, CoS NPs and NiS NPs; XRD, SEM, TEM and HRTEM of NiCo2S4 NPs, CoS NPs and NiS NPs; BET, equivalent circuit and CVs measurement 28


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with various scan rates of NiCo2S4 HSs, NiCo2S4 NPs, CoS NPs and NiS NPs; Ring current of NiCo2S4 HSs and Pt/C; ORR mechanism on NiCo2S4 catalyst; SEM, TEM, EDS, Raman spectra and XPS analysis of NiCo2S4 HSs after 10 h stability test; OER reaction process based on the NiCo2S4 HSs; comparison of bifunctional oxygen electrode activities. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] (Caihong Feng) Author Contributions All authors have contributed to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgements Yun Zhao and Caihong Feng received funding from National Natural Science Foundation of China (21376029). Hansheng Li received funding from Program for Innovative and Entrepreneurial team in Zhuhai (ZH01110405160007PWC). Wei Zhou received funding from National Natural Science Foundation of China (51622204, 51472014) and the Beijing Nova Program (Z171100001117071).

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One-Pot Synthesis of NiCo2S4 Hollow Spheres via Sequential Ion

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