Mesoporous Hollow Nested Nanospheres of Ni, Cu, Co-Based Mixed

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Mesoporous Hollow Nested Nanospheres of Ni, Cu, Co-Based Mixed Sulfides for Electrocatalytic Oxygen Reduction and Evolution Xiaoyu Wu, songmei li, Jianhua Liu, and Mei Yu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00891 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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ACS Applied Nano Materials

Mesoporous Hollow Nested Nanospheres of Ni, Cu,

Co-Based

Electrocatalytic

Mixed Oxygen

Sulfides

for

Reduction

and

Evolution Xiaoyu Wu, Songmei Li, Jianhua Liu, Mei Yu Key Laboratory of Aerospace Advanced Materials and Performance of Ministry of Education, School of Materials Science and Engineering, Beihang University, Beijing, 100191, P. R. China ABSTRACT: Nanostructured thiospinel-based transition metal sulfides with highdensity active sites hold great application potentials as non-noble electrocatalysts. In this paper, a high-performance bifunctional oxygen electrocatalysts of well-designed Ni, Cu, Co-based mixed sulfides, which combine two highly active thiospinels of NiCu2S4 and CuCo2S4, have been prepared successfully through solvothermal reaction, calcination and the following sulfurization. The products of Ni, Cu, Co-based mixed sulfides present unique hollow structures with mesopores, in which several nanoparticles-assembled hollow nanospheres of 30-50 nm in thickness are nested one by one, forming multi-shell morphologies. Benefiting from the synergistic effect from combined NiCu2S4 and CuCo2S4, and the well-designed hollow structures with large electroactive surfaces/interfaces and efficient mass transportation, the obtained Ni, Cu, Co-based mixed sulfides exhibit remarkable electrocatalytic activities and excellent



Corresponding author. Tel: +86 10 82317103; fax: +86 10 82317103. E-mail address: [email protected].

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long-term durability towards not only oxygen reduction reaction (ORR), but also oxygen evolution reaction (OER). Regarding the overall oxygen-based electrocatalytic performance, the mixed sulfides present an extremely low potential difference (0.73 V) between the ORR and OER in KOH electrolyte, delivering significant superiority to their oxide counterparts and the commercial catalyst of Pt/C, as well as most of the oxygen bi-catalysts reported recently. Therefore, the obtained Ni, Cu, Co-based mixed sulfides hold great promise as low-cost bifunctional catalysts with high efficiency for oxygen-based advanced energy storage systems. KEYWORDS: Bifunctional electrocatalyst, oxygen reduction reaction, transition metal sulfide, mesopore, hollow structure, oxygen evolution reaction.

1. INTRODUCTION Oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) based on coupled multi-electron and multi-proton processes are critical in advanced metal-air batteries (MABs) and fuel cells (FCs), which both have been considered as promising energy alternatives with clean, highly effective and economically sustainable features to meet the increasingly severe situation of energy crisis. 1-4 For example, due to the superiorities in high energy density and low emission, rechargeable MABs including Li, Mg, Al and Zn-air batteries etc. have received extensive research interests in promising energy applications for green electric vehicles (EVs) and hybrid EVs.

4,5

However, both the ORRs and OERs happened on the flexible oxygen electrode of MABs during the discharge/charge process, respectively, suffer inherently sluggish kinetics with high overpotential and poor efficiency, which becomes the major obstacle for the commercialization of MABs.

6-8

Meanwhile, FCs, which convert the potential

energy from hydrogen fuel into electricity through an electrochemical reaction with


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oxygen, are another promising sustainable electrochemical energy system. 2 However, due to the high overpotentials caused by the sluggish 4-electron charge transfer kinetics, inefficiencies are still existed in the oxygen electrodes of FCs based on the following electrochemical reactions: 9 4 𝑂𝐻 ―

𝑂𝐸𝑅 𝑂𝑅𝑅

2 𝐻2𝑂 + 𝑂2 +4 𝑒 ―

(1)

It is generally recognized that catalysts based on noble metals like Pt always exhibit the best ORR catalytic performance, and RuO2 is deemed as the most active catalyst for OER, but none of them can satisfyingly perform both reactions. 10 However, because of the inescapable structural degradation and the poisoning of catalytic centers, these noble metal-based catalysts always displayed limited durability during electrocatalysis processes.11 Also, their high cost and low reserve are serious obstacles for scale-up production.

12, 13

Therefore, a bifunctional low-cost, high-efficiency and

good-durability catalyst with active sites for both ORR and OER is still urgently required for rechargeable MABs and regenerative FCs. 14, 15 And it has been concluded that ideal bifunctional catalysts should achieved these key parameters including highdensity catalytic active sites, efficient transfer of mass and electrons, long-term durability, and low cost.

16

Accordingly, there have been three materials-design

approaches reported to fabricate promising bifunctional oxygen catalysts: 17 1) reducing the usage of noble metals by mixing or alloying with cheaper metals, such as Pt/Cu alloy, 11 Pt/Co alloy 18; (2) fabricating non-noble metals-based catalysts such as Fe, Co, Ni 19 and their compounds 20-22, especially transition metal chalcogenides (TMCs) such as Co3O4, 23 CoFe2O4, 5, 24 NiCo2O4, 25, 26 CuCo2O4, 27, 28 NiCo2S4 29-31; (3) synthesizing metal-free high-performance carbonaceous materials with heteroatomic doping, such as N-doped graphene,

32

N-doped carbon/g-C3N4 composites,

nanotube 34.


33

B-doped carbon


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Among these new families of catalysts, TMCs, especially transition metal oxides (TMOs) and sulfides (TMSs), currently have drawn massive attentions due to the unique physicochemical properties, high abundant resources, low cost, environmental compatibility, and facile synthesis procedure.

35

And it has been demonstrated that

TMSs always exhibit enhanced electronic conductivity and improved redox activity compared to the corresponding oxides. 29, 36 Meanwhile, multiplex cations doping in catalysts has been reported as a promising way to modulate the electrocatalytic activity at atomic level.14 Owing to the synergistic effect from two kinds of metal cations, binary TMSs present higher electrochemical activities than single-metal counterparts.29, 36-38 In particular, thiospinels with large ionic character and good electronic conductivity, in which two different metal cations occupy the tetrahedral and octahedral vacancies of a closely-packed S anionic lattice respectively, provide a robust library of highperformance electrocatalytic materials.

39

Highly active NiCo2S4, with the most

octahedral Co3+ cation-active sites, is one of the most widely investigated bifunctional catalysts among thiospinel-typed binary TMSs.

29-31, 35, 40-43

Meanwhile, CuCo2S4,

which possesses a quite low resistivity (10−4 Ω) and exhibits unique oxidation states for Cu+1 and Co+2.4 ions different from normal thiospinel compounds, recently has emerged as a new promising alternative catalyst for ORR and OER.

29, 30

However, their

widespread applications are still impeded by the sluggish transport kinetics of mass and electron, as well as the unsatisfactory durability from structural change.35 It should be noted that the electrocatalytic activity and durability of catalysts are significantly influenced by their morphology and microstructure.44,45 In order to further improve their electrocatalytic performance, NiCo2S4 and CuCo2S4-based catalysts with well-designed microstructures of nanowires, 40 nanosheets, 39 nanorods, 46 nanotubes, 36 hollow spheres

29, 41

etc. have been fabricated and exhibited considerable catalytic


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activity. Recently, catalysts with hollow nanoarchitectures and mesoporous features have drawn increasing research interests due to their significant advantages in providing large electroactive surfaces/interfaces and plentiful exposed catalytic active sites, thus facilitating mass transport and improving the kinetics during reactions. 29, 36, 38 Other methods to improve the catalytic properties such as creating structural defects, controlling composition, tuning valence and electronic structure, using cocatalyst, etc. also have been proposed and applied recently. 8 Herein, combining NiCo2S4 and CuCo2S4, we fabricated a well-designed Ni, Cu, Co-based mixed sulfide with unique morphologies of mesoporous hollow nested nanospheres with a synthesis procedure involving solvothermal reaction, calcination and sulfurization treatments. By taking advantages of the enhanced synergetic effect from mixing different thiospinel compounds with rich electrochemical activities, as well as the rational-built hollow mesoporous structures, the obtained sulfides exhibit excellent ORR and OER catalytic performance. These low-cost, high-efficency and long-durability bifunctional oxygen electrocatalysts hold great promises for practical applications in rechargaeable MABs and regenerative FCs. 2. EXPERIMENTAL SECTION 2.1 Preparation of the Ni, Cu, Co-based mixed sulfides All the reagents used here are of analytical purity grade and provided by Beijing chemical Co., Ltd. Typically, nickel (II) nitrate hexahydrate of 0.15 mmol, copper (II) nitrate hydrate of 0.15 mmol, cobalt (II) nitrate hexahydrate of 0.6 mmol, isophthalic acid (H2IPA) of 0.5 mmol, dimethyl formamide (DMF) of 20 mL and acetone of 20 mL were mixed together by being stirred for 6 h. Then the obtained clear solution was subjected to solvothermal reaction in a Teflon-lined autoclave of 50 mL at 160 ˚C for 4 h to obtain the Ni, Cu, Co-coordinated polymer (NiCuCo-polymer) spheres. After



that, the Ni, Cu, Co-based mixed oxides (NiCuCo-oxides) were generated via a 500 ˚C calcination of the polymer for 10 min. Then the obtained NiCuCo-oxides were dispersed in a 0.1 M Na2S·9H2O through continuous stirring for 30 min. After sulfurization at 160 ˚C for 8 h and natural cooling, the products were taken out through suction filtration and rinsed by deionized (DI) water and absolute ethanol. Then the final products of Ni, Cu, Co-based mixed sulfides (NiCuCo-sulfides) were obtained after being dried overnight at 60 ˚C. 2.2 Materials Characterization To reveal the crystal characteristics, measurements of X-ray diffraction (XRD) were conducted in a Rigaku D/max-2200PC system in the angle range of 10-80°.38 Morphology and microstructure characterizations were carried out based on JEOL JSM-7500F scanning electron microscope (SEM), Titan F-20 transmission electron microscope (TEM) with commercial Cu TEM grid of 200 mesh. Energy-dispersive Xray spectroscope (EDX) equipped in TEM was used to analyze the elemental compositions. Measurements with an AXIS Ultra DLD X-ray photoelectron spectroscope (XPS) were also performed to further reveal the element and valence information. And N2 adsorption/desorption characteristics were measured by ASAP2010 surface area analyzer to calculate the specific surface area and pore size distributions. 2.3 Electrochemical measurements The electrocatalytic properties of NiCuCo-sulfides towards ORR and OER were investigated by a Princeton PARSTAT 2273 electrochemistry measurement system at room temperature in alkaline electrolyte of 0.1 M KOH based on a conventional threeelectrode configuration. For comparison, electrochemical measurements on the NiCuCo-oxides and commercially used Pt/C catalysts with 20 wt.% Pt were also


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conducted. Glassy carbon rotating disk electrode (RDE) of 5 mm in diameter acted as working electrode, on which the catalysts loaded. Graphite rod and Ag/AgCl electrode were used as counter and reference electrodes, respectively. During the typical working electrode preparing process, the prepared catalyst of 2 mg, Nafion solution (5 wt.%) of 21.2 μL, solvent consist of DI water and absolute ethanol (3:1, v:v) of 1 mL were mixed homogeneously by continuous ultrasonication to form a catalyst ink. Then using a microsyringe, the prepared ink of 10 μL was dipped carefully onto the RDE surface and dried naturally, yielding a loading of 99.80 μg cm-2 for NiCuCo-sulfides and oxides, and a Pt loading of 19.96 μg cm-2 for Pt/C catalyst. In addition, it should be noted that all potentials mentioned in this paper are relative to the reversible hydrogen electrode (RHE). To revealing the ORR catalytic activities, cyclic voltammetry (CV) characterizations were carried out in electrolyte with saturated N2 and O2, respectively, with a voltage rate of 50 mV s-1 in 0.0-1.2 V.38 Basing on the RDE technique, linear sweep voltammetry (LSV) experiments with a sweeping rate of 5 mV s-1 were conducted in O2-saturated electrolyte under rotation rates varied from 100 to 2500 rpm. The final LSV curves were calibrated by excluding the measured current backgrounds in N2-saturated electrolyte with the same manner. And we also conducted the rotating ring disk electrode (RRDE) experiments on similar glassy carbon disk equipped with Pt ring at a bias of 1.45 V, during which the rotation rate was set as 1600 rpm. Meanwhile, N2-saturated electrolyte was used for OER catalysis measurements. The OER polarization curves were obtained in the potential range of 1.0-2.0 V at a 1600 rpm rotation rate. And the CV curves measured in non-faradic voltage region of 1.101.20 V at different scan rates of 10-80 mV s-1 were also obtained to calculate the electrochemical active surface area (ECSA). Electrochemical impedance spectroscopy



(EIS) characterizations in frequency range of 105-0.1 Hz at 1.45 V, and the long-term durability tests based on chronoamperometric techniques were also carried out on these catalysts. 3. RESULTS AND DISCUSSION

Scheme 1 sketches the main fabrication steps of the NiCuCo-sulfides nanospheres, including the solvothermal coordination, calcination and final sulfurization. And XRD characterizations were performed on the products after calcination and the final sulfurization treatments, respectively, to obtain the detailed phase compositions and crystal features. Firstly, the NiCuCo-polymer precursors with typical spherical shapes were prepared via a facile solvothermal reactions between Ni2+, Cu2+, Co2+ ions and the H2IPA. After calcinated in air, the NiCuCo-polymer precursors were oxidized into yolk-shell structured NiCuCo-oxides with hollow cavity, which were caused by the heterogeneous contraction during the heating process of the polymer precursor. 47 The obtained XRD results of the NiCuCo-oxides in Figure 1(a) approximately correspond to the standard patterns of spinel NiCo2O4 (JCPDS No.73-1702) and CuCo2O4 (JCPDS No.01-1155). Finally, the NiCuCo-oxides were converted into NiCuCo-sulfides via a sulfurization treatment of S2- based on anion exchange reaction, forming nested hollow nanospheres. As shown in Figure 1(b), the obtained NiCuCo-sulfides show typical XRD diffraction peaks at values of 2θ of 16.3°, 26.8°, 31.6°, 38.3°, 50.5°, and 55.3°, corresponding to the (111), (022), (113), (004), (224), (115) and (044) planes of the cubic-type NiCo2S4 (JCPDS No. 43-1477), while the obvious diffraction peaks at about 16.1°, 26.6°, 31.2°, 37.9°, 46.7°, 50.0°, and 54.8° can be well indexed to (111), (022), (113), (004), (224), (115) and (044) planes in CuCo2S4 (JCPDS No. 42-1450). 43, 48-50 These XRD results confirm that the obtained NiCuCo-oxides are mainly composed of mixed NiCo2S4 and CuCo2S4. And no peaks of NiCo2O4 and CuCo2O4 can be observed,


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indicating the complete sulfurization of the NiCuCo-oxides after the treatment using S2- anions as sulfur sources. The sulfurization mechanism can be expressed as follows 51, 52:

NiCo2O4 + S2- + H2O → NiCo2S4 + OH-

(2)

CuCo2O4 + S2- + H2O → CuCo2S4 + OH-

(3)

Characterizations based on SEM were conducted to reveal the nanosphere-shaped morphologies and detailed microstructures of the NiCuCo-oxides and sulfides. As shown in Figure 2 (a), the NiCuCo-oxides obtained after calcination exhibit regular sphere-shape morphology with the approximate size range of 500-900 nm in diameter. The magnified SEM images in Figure 2 (b, c) present the rough morphologies of the surface of NiCuCo-oxides nanospheres, which seem to be assembled with massive stacked building blocks instead of integral seamless structures. The formation of rough non-dense surface morphology of the oxides nanospheres may be ascribed to the oxidation reaction of NiCuCo-precursor accompanying with the release of H2O. And the broken nanosphere in Figure 2 (c) shows an obvious cavity, indicating the possible hollow structures of the nanospheres. TEM characterizations further provide detailed structural information of the NiCuCo-oxides nanospheres. As displayed in Figure 2 (df), the nanospheres exhibit obvious yolk-shell nested structures, in which the outside nanospheres show a uniform shell with a thickness of about 100 nm and an obvious hollow cavity with a smaller nanosphere is nested inside the hollow sphere. Meanwhile, the obviously observed nanoparticles in the surface of the nanospheres (Figure 2 (f)) also confirm their non-dense and building blocks-assembled shell structures. As reported before, the formation of the unique yolk-shell nested microstructure is resulted from the large volume shrinkage from the NiCuCo-polymer precursors to the oxides when calcinated in air, during which the obvious radial temperature gradient inside the



precursors leads to the gradual oxidation transformation and causes a spallation when a critical thickness of the surface oxide layer is reached. 47 And as observed in Figure 2(g), the two sets of clear lattice fringes in the HRTEM image can be well indexed to the (220) plane of NiCo2O4 (JCPDS No. 73-1702) and (311) plane of CuCo2O4 (JCPDS No.01-1155), respectively, agreeing well with the XRD results. Additionally, the EDX results of the oxides in Figure 2(h-l) indicate the Ni, Cu, Co and O elements are uniformly existed in the oxides products, thus confirming the uniform dispersion of the NiCo2O4 and CuCo2O4. According to the XRD analysis, the NiCuCo-oxides are completely converted into the corresponding sulfides composed of mixed NiCo2S4 and CuCo2S4 after sulfurized by Na2S. As shown in Figure 3(a-c), the obtained NiCuCo-sulfides maintain the basic morphologies of the oxides with spherical shape in nanoscale with diameters below 1000 nm. And the NiCuCo-sulfides nanospheres also present rough surface morphologies assembled with smaller building blocks of nanoparticles, showing porous features as shown in Figure 3(c). Specifically, the broken nanosphere in Figure 3 (c) exhibits a much thinner shell than that of the oxides shown in Figure 2(c), implying the existence of structural changes after sulfurization. According to the TEM results presented in Figure 3 (d), the yolk-shell nanospheres are converted into more complex and incompact structures after reacting with the Na2S. More specifically, it is revealed clearly in the magnified TEM images (Figure 3(e)) that the obtained NiCuCo-sulfides are multi-shell hollow nanospheres, which are formed by several decrescent hollow nanospheres nested inside one by one. And the NiCuCo-sulfides present much thinner shells (approximately 30-50 nm in thickness) than these of the oxides, in good agreement with SEM results. Furthermore, as shown in Figure 3(f), the shells of all the nested hollow nanospheres are confirmed to be assembled with massive nanoparticles


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of about 10 nm in size, forming mesopores between the adjacent nanoparticles. Compared to the oxides, the much looser structures even with multi-shell nested hollow nanospheres of the NiCuCo-sulfides could be originated from the anion exchange reaction with pseudo Kirkendall effect during sulfurization process with anion exchange and cation diffusion in different diffusion rate. 51, 52 In details, S2- anions in the Na2S aqueous solution firstly exchange with the hydrolyzed anions from the NiCuCo-oxides, forming a thin sulfide layer on the oxide surface and providing obstacles for the direct reactions between the outside S2- ions and inner oxides. And the following reactions depend on the spontaneous ionic diffusion from oxides to the outside nanosphere surface through the newly formed sulfide layers. The unequal diffusion procedure originated from the much faster outward diffusion of the Ni, Cu and Co sources than the inward transfer of S2- ions leads to the formation of voids in the center of the nanospheres, finally resulting in the multiple shells and looser structures.37,38 And the mesoporous features and incompact hollow structures of the NiCuCo-sulfides nanospheres could not only provide high-density catalytic active sites, but also effectively facilitate the transport of reactant species during reactions, thus playing a significant role on enhancing ORR and OER catalysis.

53, 54

The HRTEM

image of the NiCuCo-sulfides (Figure 3(g)) also agrees well with the XRD results discussed above, which shows obvious lattice fringes of the NiCo2S4 (220) plane (JCPDS No. 43-1477) and CuCo2S4 (311) plane (JCPDS No.42-1450), respectively. The EDX spectrum presented in Figure S1 (Supporting Information) confirms the elemental compositions of Ni, Cu, Co and S. And as presented in Figure 3(i-l), remarkable uniformities are displayed in all the EDX mapping images of these elements, further indicating the formation of uniformly dispersed NiCo2S4 and CuCo2S4. In order to further verify the mesoporous features of the NiCuCo-sulfides



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nanospheres, we measured the full N2 adsorption/desorption characteristics of the products. As shown in Figure 4(a), the typical IV-type isotherm of the NiCuCo-sulfides nanospheres with an obvious hysteresis loop implies the existence of mesoporous structure. 36, 55 The inserted pore size distribution curve in Figure 4(a) further confirms this result, in which the sharp peak at 2-5 nm is resulted from the mesoporous gaps between the nanoparticles in the shells and the peak positioned in the wide range of 1050 nm may be attributed to the hollow cavities between the nested nanospheres. Furthermore, calculated from the N2 adsorption/desorption results, the NiCuCo-sulfides nanospheres exhibit a much higher specific surface area (43.32 m2 g-1) than that of the NiCuCo-oxides (18.63 m2 g-1, as presented in Figure S2 (Supporting Information)). These results are consistent with the looser microstructures of the sulfides as shown in the TEM images. It is noteworthy that the mesoporous features and large surface areas are beneficial for oxygen-related catalysis, which provides additional, low-resistance mass transport pathways, guarantees faster interfacial charge transfer and efficient ion diffusion, facilitates the convenient infiltration of electrolytes and the fast emission of oxygen gas bubbles.

53, 54

Thus, it is expected that the NiCuCo-sulfides nanospheres

may present a superior electrochemical activity. Moreover, the detailed chemical valence states of the nanospheres are analyzed by XPS measurements. And fitted by Gaussian method, the obtained typical Ni 2p, Cu 2p and Co 2p XPS spectra are presented in Figure 4(b-d). The obvious peaks at 855.1 and 872.2 eV presented in Figure 4(b) correspond to the typical electronic configurations of Ni 2p3/2 and 2p1/2, respectively, verifying the existence of the Ni2+ and Ni3+.

36, 56-59

And the existed

shakeup satellites peaks are resulted from the high-spin excitation of Ni2+ and Ni3+ ions. 60, 61 Meanwhile, as shown in Figure 4(c), two sets of spin-orbit doublets at 932.9, 952.5 eV and 935.5, 954.2 eV, respectively, are displayed in the Cu 2p spectrum,


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demonstrating the coexistence of both Cu+ and Cu2+.62 And in the Co 2p spectrum of Figure 4(d), the spin-orbit doublets at 779.4 eV (2p3/2 peak) and 794.9 eV (2p1/2 peak) correspond to Co3+, and the obvious peaks at 781.4 and 796.8 eV are characteristics of Co2+.

57, 59, 62, 63

According to the proposed catalysis mechanism for sulfides, these

mixed Ni, Cu, Co cations play a crucial role in providing highly active sites during catalysis process.14 Besides, the XPS spectrum for S 2p in Figure S3 (Supporting Information) exhibits typical 2p3/2 and 2p1/2 peaks located at ~161.6 and 162.8 eV, respectively, corresponding to S2- ions. And the peak at 164.2 eV is typical of metalsulfur bond. These results agree well with the result of XRD and EDX analysis, further confirming the formation of NiCuCo-sulfides. It has been reported that both NiCo2S4 and CuCo2S4 exhibit remarkable electrochemical activities and are efficient electrocatalysts for ORR and OER reactions. 31, 43, 49

And benefiting from the uniformly mixed and highly active components, unique

structures of hollow nested nanospheres with mesoporous features and high surface areas, the as-prepared NiCuCo-sulfides are expected to present enhanced catalytic activities. And the obtained catalysts with a loading amount of 99.80 μg cm-2 on glassy carbon electrodes were used to conduct electrochemical tests for catalytic performance characterizations. And SEM results in Figure S4 (Supporting Information) verified that the NiCuCo-sulfides nanospheres are dispersed uniformly on the glassy carbon electrodes without obvious aggregation and collapse. In order to reveal the catalytic activities towards ORR, CV curves of the NiCuCo-sulfides and NiCuCo-oxides catalysts in 0.0-1.2 V were measured in O2 or N2-saturated electrolyte with a rate of 50 mV s-1. According to Figure 5(a), in stark contrast to the no-peak CV curves measured in N2-saturated electrolyte, obvious reduction peaks are presented in the CV curves of NiCuCo-sulfides and NiCuCo-oxides catalysts with O2. This result indicates the



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reduction reaction occurred just in the presence of O2 but not in N2, thus confirming the oxygen electrocatalytic activity of the obtained catalysts. And the NiCuCo-sulfides exhibit a ORR onset potential of ~ 0.89 V, where 10% of the current value at the peak potential is reached, and a cathodic peak positioned at 0.73 V. 64 In comparison, the NiCuCo-oxides displays a much lower peak potential of 0.65 V, suggesting the enhanced ORR electrocatalytic activities of the sulfides. Meanwhile, in order to reveal the catalytic kinetics of the as-prepared products, LSV results at different rotation speeds were obtained. Obviously, all the LSV curves of the NiCuCo-sulfides in Figure 5(c) show similar profiles, in which the limiting current densities gradually increase with the growing rotation rates due to the accelerated surface oxygen transfer at higher rotation speeds. 29 And the NiCuCo-sulfides show a one-step pathway in LSV curves, implying a four-electron reaction. 64 Meanwhile, the Koutecky-Levich (K-L) plots of the NiCuCo-sulfides describe the relationship between the current density (j, mA cm-2) measured in experiments and the rotation rate of electrode ( , rpm), which are obtained from the LSV results based on K-L equations as follows: 36, 65 1 j

=

1 jL

1

+ jK =

1 Bω

1

12

2

+ jK

B = 0.2nFC𝑂(D𝑂) 3

-1 6

(4) (5)

Noted that the jK and jL in equation (4) refer to the kinetic and diffusion-limited current densities, respectively. And B is calculated according to the relationship with the number of transferred electron per oxygen molecule (n), Faraday’s constant (F), the concentration of dissolved oxygen (C𝑂), the oxygen diffusion coefficient (D𝑂) and kinetic viscosity () presented in equation (5). The K-L plots of various potentials in Figure 5(d) shows linear and almost parallel profiles, implying that the ORR processes at different potentials exhibit similar electron


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transfer numbers and the ORR reaction kinetics are related to the concentration of dissolved oxygen. The average values of n in equation (5) for the sulfides were calculated to be 3.6-3.9 according to the K-L slopes, as presented in the inset of Figure 5(d), verifying the four-electron ORR pathway. Figure 5(b) compares the LSV curves of the NiCuCo-sulfides, NiCuCo-oxides and the commercial Pt/C catalysts. Obviously, the NiCuCo-sulfides present a more positive half-wave potential (E1/2) than the oxides. Compared to that of the Pt/C, the E1/2 value of the sulfides (0.84 V) is just a little less (~20 mV) positive, and their onset potentials are also very close to each other. Figure S5 (Supporting Information) displays the Tafel plots converted from the kinetic current densities in equation (4). 38 The NiCuCo-sulfides show a Tafel slope as 89 mV dec-1, which is obviously smaller than that of NiCuCo-oxides (102 mV dec-1), but not far away from that of the Pt/C (82 mV dec-1), demonstrating their efficient kinetics and high intrinsic ORR catalytic activities. Furthermore, electrochemical measurements based on RRDE techniques were also conducted to confirm the catalytic pathways and calculate the HO2- formation yields during ORR catalysis, and the results are presented in Figure 5 (e, f).

66

Remarkably, all these catalysts display low HO2- formation

yields over a wide potential range. Especially, it can be observed that the HO2formation yields of the NiCuCo-sulfides (~ 4%) is similar to that of Pt/C. And agreeing well with the K-L results discussed above, the calculated electron transfer numbers (n) for these catalysts are close to 4, further verifying the typical four-electron ORR catalysis process. Chronoamperometric measurements were conducted to compare the durability for ORR catalysis of the NiCuCo-sulfides and NiCuCo-oxides, as well as the Pt/C. According to the current-time curves in Figure 6(a), the NiCuCo-sulfides exhibit the



most remarkable stability among them. Different from the severe current loss after 36 000 s in Pt/C (34.9%), the NiCuCo-sulfides present a much higher current retention of 88.1%, which is also higher than that of the NiCuCo-oxides (81.6%). Moreover, the Pt/C catalyst suffers an obvious current drop when adding 3.0 M methanol into the electrolyte in Figure 6(b). However, disregarding some unavoidable slight disturbance, there are no noticeable response observed on the NiCuCo-sulfides and NiCuCo-oxides, suggesting that the as-prepared NiCuCo-based catalysts possess higher tolerance to methanol and exhibit more superiority in practical application than commercial Pt/C catalyst. Electrocatalytic properties of the as-prepared NiCuCo-sulfides and the counterparts of NiCuCo-oxides and Pt/C catalysts toward OER were examined with a similar RDE technique in N2-saturated electrolyte. According to the LSV curves (Figure 7(a)), the NiCuCo-sulfides possess the lowest onset potential and the highest current density, verifying their outstanding OER catalytic activities among the three samples. More importantly, it can be clearly observed that the potential at 10 mA cm-2 for the NiCuCo-sulfides is 1.57 V, much lower than that of NiCuCo-oxides (1.68 V) and Pt/C (1.97 V). And the enhanced OER catalysis performance of NiCuCo-sulfides is further supported by its significantly smaller Tafel slope (96 mV dec-1), as shown in Figure 7(b). Furthermore, the LSV profile of NiCuCo-sulfides (Figure 7(c)) during the first cycle is almost overlapped with that of the continuous potential cycles, even after 2000 cycles, confirming their great stability for OER. Meanwhile, Figure 7(d) presents the OER time-dependent current density curves of these catalysts measured at 1.45 V. Compared to the obvious current density decades in the Pt/C and NiCuCo-oxides catalysts, only slight decrease in current density of the NiCuCo-sulfides was observed even after 60,000 s, indicating the superior durability of the NiCuCo-sulfides for OER


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catalysis. And according to the SEM image in Figure S6(a) (Supporting Information), the NiCuCo-sulfides maintain complete spherical morphologies without obvious collapse after long-term durability test. And the XRD and XPS results of the NiCuCosulfides after cycles (Figure S6, Supporting Information) indicate their great compositional stability, in which the decreased intensities of the peak for S2- in the XPS spectra of S 2p region can be ascribed to the conversion of sulfides into M-OOH (M is the metal cations as catalytic active sites) and SO42- during OER catalysis, consistent with the proposed catalysis mechanism of sulfides.46, 67 And characterizations based on EIS techniques were conducted to reveal the OER kinetics of these catalysts, and the obtained Nyquist plots are presented in Figure 7(e). In order to analyze the resistance characteristics of the catalysts, the inset of Randles equivalent circuit in Figure 7(e) was used to fitting the measured Nyquist plots, in which Rs is the electrolyte resistance, Cdl refers to the double-layer capacitance and Rct is charge transfer resistance.68-70 Benefiting from the enhanced electronic conductivity and abundant active sites, the derived Rct of NiCuCo-sulfides (3.5 Ω) is smaller than that of NiCuCo-oxide (5.9 Ω), suggesting the faster electron transfer rate for NiCuCo-sulfides catalysts.70 These results confirm the great potentials of the obtained NiCuCo-sulfides as bifunctional oxygen catalysts. Additionally, the ECSA were calculated according to the following equation: 33 ECSA =Cdl/Cs

(6)

where Cs (40 mF cm-2) refers to the atomically smooth planar surface capacitance, and the double-layer capacitance of Cdl is calculated from the CV curves (Figure S7, Supporting Information) in a non-faradic region of 1.1-1.2 V. And the obtained values of Cdl are presented in Figure 7(f). Then the ECSA of the NiCuCo-sulfides is calculated to be 104.75 cm2, superior to that of the NiCuCo-oxides (94.50 cm2) and Pt/C catalysts



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(57.75 cm2), indicating the remarkable catalytic activity of the sulfides originated from the mixed highly-active components with synergistic effect and the rational-designed mesoporous hollow nested structures. Furthermore, the potential difference ΔE (ΔE = EOER - EORR) is applied to evaluate the overall catalytic activity, in which the EOER is the OER potential at 10 mA cm-2 (Ej=10), while the EORR refers to the ORR half-wave potential (E1/2).33, 35 As shown in Figure 8, the obtained NiCuCo-sulfides exhibit the smallest potential difference of only 0.73 V compared to NiCuCo-oxides (0.89 V) and Pt/C (1.13 V). Impressively, the ΔE value of the NiCuCo-sulfides also shows superiority to most of the reported highperformance bifunctional catalysts, such as catalysts with noble metals (Ir/C, 0.92 V 7), transition-metal oxide/sulfides (hierarchical nanostructures NiCo2O4, 0.84 V;

26

NiCo2O4 nanowires, 0.97 V; 25 CuCo2S4 nanosheets, 0.835 V; 39 NiCo2S4/N-CNT, 0.80 V; 30 NiCo2S4 hollow spheres, 0.83 V 29), advanced carbonaceous catalysts (P-doped gC3N4, 0.94 V; 72 S-doped graphene/CNT, 1.00 V; 73 N, S co-doped carbon nanosheets, 0.88 V

74),

as shown in Figure S8 (Supporting Information). These results further

confirm the outstanding bifunctional activity of the obtained NiCuCo-sulfides. 4. CONCLUSIONS In summary, high-performance and low-cost Ni, Cu, Co-based mixed sulfides as bifunctional non-noble electrocatalyst for ORR and OER have been designed and developed via a facile solvothermal route. The obtained sulfides are composed of two highly active thiospinels of NiCu2S4 and CuCo2S4, and exhibiting unique hollow structures assembled with multi-shell nested nanospheres of high surface area and mesoporous features. Compared with the oxide counterparts and commercially used Pt/C catalysts, the obtained Ni, Cu, Co-based mixed sulfides exhibit excellent overall electrocatalytic activities towards both ORR and OER with an extremely low potential


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difference of 0.73 V and display excellent long-term durability and remarkable methanol tolerance. More importantly, the bifunctional catalytic activities of the sulfides are also superior to most of the proposed bifunctional oxygen catalysts, which can be ascribed to two main reasons: (1) the synergistic effect from combined NiCu2S4 and CuCo2S4 of high electrochemical activities, excellent electronic conductivities and different electronic structures and oxidation states, (2) the well-designed hollow mesoporous microstructures with large electroactive surfaces/interfaces, high-density active sites and efficient reaction kinetics. These results indicate that the synergistical mixing highly active catalysts with rational nanostructures can be considered as a promising designing strategy to improve the electrocatalytic properties of the existing bifunctional catalysts and developing high-performance novel advanced alternatives for ever-growing application requirements in advanced energy systems of metal-air batteries and sustainable fuel cells.

ASSOCIATED CONTENT Supporting Information More details of additional figures depicting experiment results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHER INFORMATION Corresponding Authors *E-mail: [email protected] Notes The authors declare no competing financial interest.



ACKNOWLEDGMENTS Prof. Songmei Li received funding from the National Natural Science Foundation of China (Grant No. 51271012). Dr. Xiaoyu Wu are financially supported by China Scholarship Council for overseas studies (CSC No. 201706020015).

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Table of Contents


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Scheme 1 Schematic of the fabrication of NiCuCo-sulfides nanospheres. 360x179mm (96 x 96 DPI)



Figure 1 XRD patterns of the (a) NiCuCo-oxides nanospheres, (b) NiCuCo-sulfides nanospheres. 160x56mm (300 x 300 DPI)


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Figure 2 (a-c) Typical SEM images of the NiCuCo-oxides nanospheres with different magnifications; (d-f) Typical TEM images of the NiCuCo-oxides nanospheres with different magnifications; (g) HRTEM image of the NiCuCo-oxides nanospheres; (h) HADDF-STEM image of a single NiCuCo-oxides nanosphere and the corresponding EDX mapping images of (i) Ni, (j) Cu, (k) Co and (l) O elements of the nanosphere. 239x174mm (300 x 300 DPI)



Figure 3(a-c) Typical SEM images of the NiCuCo-sulfides nanospheres with different magnifications; (d-f) Typical TEM images of the NiCuCo-sulfides nanospheres with different magnifications; (g) HRTEM image of the NiCuCo-sulfides nanospheres; (h) HADDF-STEM image of a single NiCuCo-sulfides nanosphere and the corresponding EDX mapping images of (i) Ni, (j) Cu, (k) Co and (l) S elements of the nanosphere. 239x173mm (300 x 300 DPI)


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Figure 4 (a) N2 adsorption-desorption isotherms of the NiCuCo-sulfides nanospheres and the inset is the plot of the pore size distribution; XPS spectra of (b) Ni 2p, (c) Cu 2p and (d) Co 2p for the NiCuCo-sulfides nanospheres. 160x112mm (300 x 300 DPI)



Figure 5 (a) CV curves of NiCuCo-sulfides and NiCuCo-oxides in O2/N2-saturated 0.1 M KOH aqueous electrolyte, respectively; (b) Comparison of ORR LSV curves of the NiCuCo-sulfides, NiCuCo-oxides, and Pt/C in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm; (c) ORR LSV curves of the NiCuCo-sulfides in O2saturated 0.1 M KOH at different rotating speeds; (d) K-L plots of the NiCuCo-sulfides based on their LSV curves of (c), and the inset is the corresponding number of transferred electrons during ORR of the NiCuCosulfides; (e) RRDE disk and ring currents and of the NiCuCo-sulfides, NiCuCo-oxides, and Pt/C with a rotation rate of 1600 rpm in O2-saturated 0.1 M KOH; (f) 〖"HO" 〗_"2" ^"-" formation yields and electron numbers of NiCuCo-sulfides, NiCuCo-oxides, and Pt/C at various potentials based on the corresponding RRDE data. 160x168mm (300 x 300 DPI)


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Figure 6 (a) ORR current-time chronoamperometric response of NiCuCo-sulfides, NiCuCo-oxides, and Pt/C at o.5 V (vs. RHE) in O2-saturated 0.1 M KOH solution at a rotation rate of 1600 rpm; (b)ORR current-time chronoamperometric response of NiCuCo-sulfides, NiCuCo-oxides, and Pt/C before and after addition of 3 M methanol, in which the arrow indicate the introduction of O2 and methanol. 160x56mm (300 x 300 DPI)



Figure 7 (a) OER LSV curves of the NiCuCo-sulfides, NiCuCo-oxides, and Pt/C in N2-saturated 0.1 M KOH at a rotation rate of 1600 rpm; (b) Comparison of Tafel plots of the NiCuCo-sulfides, NiCuCo-oxides, and Pt/C obtained from the OER LSV curves at 1600 rpm; (c) OER LSV plots of the NiCuCo-sulfides in the beginning and after 100, 2000 cycles; (d) OER current-time chronoamperometric response of NiCuCo-sulfides, NiCuCooxides, and Pt/C at 1.45 V (vs. RHE); (e) EIS spectra of the NiCuCo-sulfides and NiCuCo-oxides, and the inset is the Randles equivalent circuit used for fitting the impedance results; (f) capacitive currents (j) vs. scan rates plots of the the NiCuCo-sulfides, NiCuCo-oxides, and Pt/C, the capacitive currents were measured at 1.15 V (vs. RHE) from cyclic voltammetry curves measured in a potential range (1.1-1.2 V vs. RHE). 160x168mm (300 x 300 DPI)


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Figure 8 The overall LSV curves of the NiCuCo-sulfides, NiCuCo-oxides, and Pt/C in the potential range of 02.0 V, ΔE (Ej=10 - E1/2) is a metric for bifunctional ORR and OER activity. 139x56mm (300 x 300 DPI)


Mesoporous Hollow Nested Nanospheres of Ni, Cu, Co-Based Mixed

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