Facile Synthesis of Co9S8 Hollow Spheres as a High Performance

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Facile Synthesis of Co9S8 Hollow Spheres as a High Performance Electrocatalyst for the Oxygen Evolution Reaction Xueting Feng, Qingze Jiao, Tong Liu, Qun Li, Mengmeng Yin, Yun Zhao, Hansheng Li, Caihong Feng, and Wei Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03236 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Sustainable Chemistry & Engineering

Facile Synthesis of Co9S8 Hollow Spheres as a High Performance Electrocatalyst for the Oxygen Evolution Reaction Xueting Feng, † Qingze Jiao, †,‡ Tong Liu, § Qun Li, † Mengmeng Yin, † Yun Zhao, † Hansheng Li, † Caihong Feng*,† and Wei Zhou*,§ †

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)

*Email: [email protected] (Wei Zhou)

ABSTRACT:

Cobalt pentlandite (Co9S8) have recently attracted great attention as highly efficient and stable earth-abundant catalysts for the oxygen evolution reaction (OER) due to its 1

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electronic properties and intrinsic structural. However, it still remains a major challenging to rationally design and synthesize OER catalysts to further enhance the electrochemical activity and durability. Herein, a novel Co9S8 hollow spheres were successfully obtained through a template- and surfactant-free method and exhibited impressive electrocatalytic property for OER. Notably, the as-prepared material has superior OER performance with a low overpotential of 285 mV reached 10 mA cm-2, a Tafel slope of 58 mV decade-1 and improved stability in 1 M aqueous solution KOH. Impressively, the unique hollow structure of Co9S8 hollow spheres imparts large surface area and luxuriant active sites, resulting in the superior catalytic activity. Thus, this work provides a promising alternative cobalt sulfide-based oxygen evolution electrocatalysts to substitute for precious metal based electrocatalysts.

KEYWORDS: Cobalt pentlandite, hollow spheres, non-noble-metal electrocatalyst, oxygen evolution reaction, electrocatalysis, alkaline media INTRODUCTION Growing demand for sustainable and clean energy has sparked an extensive development of energy storage and conversion systems, such as fuel cells, water splitting devices and metal-air batteries.1, 2 In particular, photodriven water splitting into oxygen and hydrogen represents one of the most appealing systems for the production storage and conversion of largescale intermittent solar energy in the form of chemical bonds.3,

4

However, its critical challenge for practical application is

oxygen evolution reaction (OER) suffered from thermodynamically and kinetically sluggish as well as the high overpotential and poor cycle durability, which has been 2


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considered a major bottleneck in water splitting owing to its four-electron-transfer pathway.5, 6 To circumvent these difficulties, it is essential to find highly active and stable catalyst to meet the increasing demands of efficient, sustainable and clean energy supply.

Over the past years, state-of-the-art precious metal oxides (IrO2 and RuO2) are widely known as high-performance OER catalysts.7 However, the widespread commercial application of those catalysts has been severely limited due to relatively prohibitive cost and scarcity. Therefore, exploiting effective, earth-abundant and non-noble OER catalysts is central to the enormous research attempts.4, 6 Recently, a number of transition-metal-based compounds and their derivatives, including oxides,8 sulfides,9 phosphides,10 and layered double hydroxide (LDH),11 have emerged as an attractive alternative catalyst candidates towards OER. Especially, cobalt sulfide materials (including Co3S4,12 CoS2,13 Co9S8,9, 14 Co1-xS,15 CoxSy16 etc.) have gained noticeable attention as OER electrocatalysts owing to their wide stoichiometric composition, good stability and low overpotential.17 Shanmugam and co-workers discovered unprecedentedly striking oxygen electrode activity of CoS2(400)/N,S-GO, which showed comparable activity with precious catalysts.13 But it still exists challenges to exhibit remarkable OER performances with good electrode stability. By a facile one-step strategy, Xia et al. successfully synthesized uniform dispersed cobalt sulfide/N,S-codoped porous carbon with enhanced OER and ORR activities in alkaline media.16 However, a higher temperature was required in whole preparation process. Among these promising candidates, cubic Co9S8 have been demonstrated to 3



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be a promising candidate for OER electrocatalysts owing to its outstanding redox capability, relatively good durability in alkaline environment.18, 19 Unfortunately, the electrochemical performance of Co9S8 suffers from complicated synthetic procedures and low yield. In particular, the inferior micro/nanostructures may increase the difficulty of charge transport, which hampers their efficient dual functionality.20 Thus, it is highly challenging to rationally design and synthesize OER catalysts to further enhance the electrochemical activity and durability.

In addition to the intrinsic activity of OER catalysis, the factors including the crystal structure and morphology are also key to the catalyst electrochemical activity. Therefore, it is an essential strategy to further improve performance of OER catalytic activities by tailoring the morphologies of catalysts.20 In the past few years, the synthesis of novel micro/nanostructures have been pursued by considerable efforts, including one-dimensional (1-D) nanostructures,21,

22

hollow spheres,23,

microtube,25, 26 and complex hierarchical micro-/nanostructures.27,

28

24

hollow

Among them,

hollow micro/-delicate nanostructures have exhibited a unique set of advantages in subtle internal cavities and functional shell components, which can effectively accelerate

mass

transfer

and

electrocatalytic activity.29

boost

Through

a

self-templating strategy, Qiao and co-workers fabricated hollow Co3O4 microtube arrays with hierarchical porosity, which possessed high activity of overall water electrolysis.20 Jin and co-workers presented hydrothermal method and sintering treatment for the growth of NiFe2O4/C hollow spheres.30 In order to obtain hollow architectures, a variety of preparation techniques have been employed, such as 4


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template-based,31 surfactant-directed,32 or interface methods.33 Howerver, the use and removal of the templates are technically complicated and increase the difficulty of post-treatment,

hindering

their

further

applications.34

practical

The

surfactants-assisted synthesis process may introduce heterogeneous impurities and destroy the structural integrity by removing the surfactant, which may decrease the performance of electrocatalysts.35 Besides, most of these methods will cause the time-consuming, high cost and low reproducibility.22 In this regard, it is particularly desirable to explore new processes for the large-scale preparation of hollow architecture and composition without employing any additives. It is believed that the synthesis involving a template- and surfactant-free methods is a “green” route for the fabrication of various nanomaterials with controllable shape at large-scale, low-cost and moderate temperatures.

Herein, we reported a convenient solvothermal method for fabrication cubic Co9S8 hollow

spheres

with

well-structured

porosity

in

an

ethylene

glycol

(EG)-N,N-dimethylformamide (DMF) binary solution. The formation mechanism of Co9S8 hollow spheres and the electrochemical properties of as prepared Co9S8 electrocatalysts for OER was systematically investigated as well. It displayed superb activity on OER with many critical features such as low overpotentials at 10 mA cm-2, small Tafel slopes and good stability due to the unique structure. It indicated that Co9S8 hollow spheres were promising electrocatalysts for OER. EXPERIMENTAL SECTION Synthesis of Co9S8 hollow spheres： Co9S8 hollow spheres were fabricated via 5



simple one-step solvothermal reaction. In a typical experiment, CoSO4·7H2O (2.8 mmol) and thiourea (8.4 mmol) were mixed into 70 mL mixed solvent (volume ration of VEG/VDMF =1:4) and then formed a clear pink solution after constant stirring at ambient temperature. Then, the solution was transferred into a autoclave of 100 mL volume and kept at 160 ℃ for 12 h in blast oven. When the solution naturally cooling to ambient temperature, the dark solid sample was obtained by centrifuged, washed repeatedly with deionized water and then dealt with ethanol. Finally, the products were dried in vacuum at 60 ℃ for 6 h.

Flower-like Co9S8 was fabricated for comparison by a similar method except for a changing volume ratio of EG/DMF solvent (1:1).

Materials characterization. The X-ray diffraction (XRD) spectra of samples was identified by a Philips X’Pert Pro Multipurpose X-ray diffractometer, using Cu Kα radiation at 40 kV, 40 mA. The morphology was examined by field-emission scanning electronic microscopy (FE-SEM, JEOL JSM-7500F), transmission electronic microscopy (TEM, Hitachi HT7700) and the high-resolution TEM (HRTEM). Energy dispersive X-ray spectroscopy (EDS) was carried out to determine the compositions. X-ray photoelectron spectroscopy (XPS, PHI QUANTERA-II SXM) was performed to examine surface chemistry and elemental analysis. The pore size distribution and surface area was analyzed by Brunauer-Emmett-Teller (BET).

Electrochemical measurements. The OER test was evaluated on a CHI potentiostat (CHI660e, CH Instruments) at room temperature. A typical three 6


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electrode system was used for all the tests, with strongly alkaline medium (1 M KOH) as electrolyte, including glassy carbon (GC) electrode (5 mm diameter, 0.196 cm2 area) as the base working electrode (the detail saw Supporting Information), a KCl saturated Ag/AgCl as reference electrode and platinum wire as counter electrode. All potential measurements were vs. RHE.

ERHE = EAg/AgCl+ 0.0591 × pH + 0.197

(1)

All potential values were corrected for the iR compensation in the electrochemical workstation itself due to the influence of the solution resistance. The potential values with IR-correction were obtained as follows:36

EiR-corrected = E-iR

(2)

Here i denotes the corresponding current, R denotes the ohmic electrolyte resistance (~5.36Ω) obtained from EIS plots.

During the all electrochemical experiments, continuous O2 flow was bubbled over the O2-saturated electrolyte to ensure the O2/H2O equilibrium (1.23 V vs. RHE). Cyclic voltammograms (CVs) were performed with sweeping rate of 100 mV s-1 for at least 30 cycles until the signals were stabilized. Afterwards, the linear sweep voltammograms (LSVs) were examined from 0.2 V to 0.8 V with the sweeping rate of 5 mV s-1 at 1600 rmp. A slow scan rate was applied to achieve steady-state at the electrode surface.7 According to Tafel plots in the beginning of the linear region, the onset potentials were collected.37 Tafel slopes based on plot overpotential (η) - log (J) 7



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were calculated. The accelerated stability tests were measured by CV for 1000 cycles at sweep rate of 100 mV s-1. The chronoamperometric response was also conducted for evaluating durability. Calculation method: the calculation of overpotential (η), mass activity (A g-1), specific activity (mA cm-2) and turn over frequency (TOF) were shown as follows:

η = E (vs. RHE) - 1.23 V

(3)

Mass activity = J / m

(4)

Specific activity = J / (10 SBET m)

(5)

TOF = J S / (4 F n)

(6)

Where, J (mA cm-2) denotes the current density at η = 0.29 V; m (0.1 mg cm-2) denotes the catalyst loading; SBET (m2 g-1) and S (0.196 cm2) is the BET and RDE surface area, respectively; F denotes Faraday constant (96485.3 C mol-1); n is the number of Co ions molar assumed all the Co ions are active for OER. RESULTS AND DISCUSSION The crystallinity and composition of aforesaid Co9S8 were confirmed by using X-ray diffractometer (XRD). As depicted in Figure 1a, the as-synthesized sample was Co9S8. The diffraction peaks at 15.5°, 17.9°, 25.4°, 29.8°, 31.2°, 36.2°, 39.6°, 47.6°, 52.1°, 61.2°, 62.0°, 73.2° and 76.8° can be indexed to planes (111), (200), (220), (311), (222), (400), (533), (622), (731) and (800) of Co9S8 (JCPDS No. 86-2273), respectively. There were no extra diffraction peaks in the XRD pattern apart from 8


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Co9S8 nanocrystals, manifesting highly purified products. In addition, the sample of flower-like Co9S8 in Figure S1a showed similar diffraction peaks to Co9S8 hollow spheres, that well indexed to Co9S8 (JCPDS Card no. 86-2273).

The surface morphology of Co9S8 was examined by scanning electron microscope (SEM). The Co9S8 samples in Figure 1b presented that the size of hollow spheres was about 2 µm and the surface of hollow spheres was smooth. The insert picture in Figure 1b showed a broken Co9S8 hollow spheres, which clearly indicated the hollow structure of Co9S8. This hollow microstructure is considered to be critical to acquire a high specific surface area and excellent electrocatalytic performance.38 More details of Co9S8 hollow spheres were investigated by transmission electronic microscopy (TEM) and the high-resolution TEM (HRTEM). The shell of Co9S8 hollow spheres structure consisted of numerous randomly assembled nano-primary particles, depicted in Figure 1c. The lattice fringes were observed from the high magnification microscope elucidated in Figure 1d, and the lattice spacing was 0.29 nm, assigning to the cubic Co9S8 (222) plane.39 This was consistent with the results of XRD. In comparison, SEM images of flower-like Co9S8 product was shown in Figure S1b,c. It was clearly seen that a typical flower-like microstructure was formed with a size of 6 µm in Figure S1b. Moreover, higher magnification SEM images demonstrated that the microstructures were composed of many nanoplates, as presented in Figure S1c.

9



Figure 1. (a) XRD pattern (b) SEM images (c) TEM and (d) High-magnification TEM of as-grown Co9S8 hollow spheres. Energy dispersive X-ray spectra (EDS) was conducted to investigate the element distribution and composition of Co9S8 hollow spheres. The elemental mappings of Co9S8 hollow spheres (Figure S2a) in Figure S2b and c showed that Co and S elements were uniformly and continuously distributed throughout the Co9S8 hollow spheres. Besides, as illustrated in Figure S2d, the atomic ratio of Co to S was 9:8.3, which was approximately consistent with the theoretical stoichiometry of 9:8, confirming the formation of Co9S8 compound.

Furthermore, X-ray photoelectron spectra (XPS) was carried out using a Gaussian fitting method to investigate the bonding configurations, such as identifing the surface 10


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elements and chemical states of samples. As was evident from Figure 2a, it ascertained the presence of Co and S, consistent with the EDS analyses. Owing to exposed in air, the surface of product could be adsorbed with oxygen or slight oxidized.40, 41 The Co XPS spectrum was well fitted with two couples of shake-up satellites (identified as “Sat.”) and two spin-orbit doublets, presented in Figure 2b. The fitting peaks of Co 2p at 778.2 and 793.0 eV and another fitting peaks of Co 2p at 781.4 and 796.9 eV were corresponded to Co3+ and Co2+ oxidation state,16, 42, 43 while the spectrum at 778.2 and 781.4 eV could be associated with Co-S and Co-NH binding energy, respectively.44, 45 Figure 2c illustrated that the intensive peaks at 161.0 and 162.0 eV were indexed to the S 2p3/2 and 2p1/2 peaks of Co9S8, respectively, while binding energy of 162.8 eV can be related to the sulphur ion at the surface in low coordination.46

Figure 2. (a) XPS survey spectrum of Co9S8 hollow spheres; High-resolution XPS survey spectra of Co 2p (b) and S 2p (c) for Co9S8 hollow spheres. Meanwhile, time-dependent experiments were conducted to illustrate the growth mechanism of Co9S8 hollow spheres fabricated by a surfactant-and template-free method. The XRD pattern in Figure 3a showed the component of intermediate production at different time. As shown in Figure 3b, amounts of nanoparticles were 11



obtained after 1.5 h, while two diffraction peaks in agreement with CoO and Co3O4 exhibited according to XRD pattern (Figure 3a), respectively. Increasing the reaction time to 2 h (Figure 3c), the nanoparticles transformed into micronspheres of 1 µm in diameter and the same crystallographic structure as the intermediate products obtained in 1.5 h. When the reaction has carried out for 4 h, the quantity of microspheres has evolved into core-shell structures, which could be further confirmed by TEM image in Figure 3d. Further increasing to 8 h (Figure 3e), the core-shell structure tended to form the hollow sphere with rough thick wall, but most of them were incomplete. With the sulfidation reaction increasing, the core continuously evacuated and finally those core could be diminished. Upon continuing the reaction time to 12 h, the uniform hollow spheres with smooth surface were finally generated in Figure 3f. In addition, the XRD pattern in Figure 3a revealed the phase in aggerment with cubic Co9S8 and the intensity of the peaks became stronger.

12


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Figure 3. (a) XRD patterns of intermediate production and SEM images of the Co9S8 spheres at different time (b) 1.5 h, (c) 2 h, (d) 4 h, (e) 8 h, (f) 12 h. Accordingly, on the basis of experimental proof, Figure 4 schematically illustrated the growth mechanism of Co9S8 hollow spheres, and involved the following steps. Generally, it is considered that the hydrolyzation of thiourea can be divided into two steps.47 The formula (Equation (7), Equation (8) and total reaction Equation (9) ) is as follows: 13



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NH2CSNH2⇋CH2N2+H2S

(7)

CH2N2+2H2O⇋2NH3+CO2

(8)

NH2CSNH2+H2O⇋H2S+2NH3+CO2

(9)

Thus, at the initial reaction stage, the Co2+ ions coordinated with NH3 pyrolyzed by thiourea and EG in the solvothermal environment to form [Co(NH3)n]x+ (x=2,3) and Co-EG complexes, which controlled the nucleation rate of nanocrystals by decreasing the free Co2+ concentration.48, 49 However, the complex was metastable, resulting in further decomposition to CoO, Co3O4 and then reacted with S2- pyrolyzed by thiourea through an ion-exchange process.47 Simultaneously, there existed the dominated inward flow of S2- ions and outward diffusion of cobalt ions.50, 51 Because of the faster outward transport rate of cobalt ions, a thin layer of Co9S8 on the surface was formed, which generated a well-defined gap and determined the core-shell structure in the intermediates, consistent with the illustration in Figure 3d.42 It could be accounted for the nanoscale Kirkendall effect, resulting in porosity or deformations due to the unequel diffusivities in a diffusion couple.51, 52

Figure 4. Schematic illustration for preparing Co9S8 hollow spheres. 14


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The specific surface area and pore structure are key to the OER catalysis. The higher surface area not noly shortens the diffusion pathway of mass and electron but also greatly enhances the electrocatalytic activity through exposing more luxuriant active sites.20 Therefore, the pore size distribution and surface area of Co9S8 hollow spheres and flower-like Co9S8 were determined by nitrogen adsorption-desorption measurements. As shown in Figure S3, they both showed a type IV isotherm at a range of 0.5-0.9 (P/P0), which suggested that the Co9S8 hollow spheres and flower-like Co9S8 were mesoporous materials.53 The hollow spheres showed a sharp pore distribution with size of 4.3 nm. The total pore volume and surface area of the hollow spheres were recorded to be 0.085 cm3 g-1 and 65.81 m2 g-1, which was larger than that of flower-like Co9S8 (0.059 cm3 g-1, 20.00 m2 g-1). The higher surface area are beneficial to mass transfer.18 And it can be foresaw that the as-prepared Co9S8 hollow spheres will show much better OER performance.

The electrochemical characterization of various catalysts, including flower-like Co9S8, NiFe layered double hydroxide (LDH) nanosheet and commerical IrO2, towards the OER was assessed using a rotating disk electrode technique in alkaline environment. (see the detail synthesis of compound NiFe LDH in the Supporting Information) As seen in Figure 5a, the typical linear sweep voltammograms (LSVs) of the diverse catalysts with iR-compensation presented different activities in OER. It was interesting to note that the anodic current recorded on Co9S8 hollow spheres posessed superior behavior with a sharp onset potential (~1.43V vs RHE), which rendered a slightly more negative one compared with IrO2, NiFe LDH nanosheet and 15



flower-like Co9S8. Generally, it is commonly compared the overpotential values reaching 10 mA cm-2 to evaluate the OER performance because it represents figures-of-merit in solar fuel synthesis.4 In addition, a smaller overpotential at 10 mA cm-2 is suggested to possess better catalytic performance.14 Thus, as exihibited in Figure 5a, the Co9S8 hollow spheres exhibited a significantly improved OER catalytic activity with relatively smaller OER overpotential of 285 mV than that of flower-like Co9S8 with overpotential of 380 mV, NiFe LDH nanosheet with overpotential of 390 mV and even better than commerical IrO2 catalyst with overpotential of 390 mV. Interestingly, compared with the previous literature, this overpotential was also significantly lower or similar to that of researched materials showed in Table S1, such as core-oxidized amorphous CoxPy (Ej = 287 mV at 10 mA cm-2 in 1 M KOH),54 Fe3O4@Co9S8/rGO nanoparticle (Ej = 320 mV at 10 mA cm-2 in 1 M KOH).14 It has to be noted that the hollow architectures are conductive to expediting the electrochemical performance. The hollow structure could provide luxuriant electroactive sites for oxygen reversible adsorption on the catalyst surface and remarkably short the diffusion pathway of the electrolyte, which is beneficial for electrocatalytic reactions.20 Consequently, it is deduced that Co9S8 hollow spheres exhibit outstanding catalytic performance.

In addition, the OER kinetics of various electrocatalysts were also corroborated in Figure 5b. A low Tafel slope value of 58.0 mV decade-1 for Co9S8 hollow was smaller than that of Co9S8 flower-like structure (75.5 mV decade-1), NiFe LDH nanosheet (96.0 mV decade-1) and commercial IrO2 catalyst (76.6 mV decade-1), demonstrating 16


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its more efficient kinetics for OER. It is indicated that similar Tafel slopes for the OER corresponds to analogous reaction pathways and rate-determining steps.55 In general, OH- electrodeposited on the catalyst surface indicates the beginning of the oxygen evolution reaction and a high affinity for adsorbed OH- intermediates is one marker of an efficient catalytic activity in the OER.56 As such, the Co9S8 hollow spheres catalysts with lower Tafel slopes resulted in a prominent kinetic process for the OER activity.

The long-term stability is another crucial metric in commercial applications for OER. It has been widely reported that the benchmark IrO2 under strongly alkaline condition exhibited unstable state, due to the process of highly oxidation.57 Afterwards, a durability test of hollow spheres catalyst was performed on accelerated degradation tests using continuous CV for 1000 cycles at rate of 100 mV s-1. As illustrated in Figure 5c, it demonstrated that the OER catalytic activity for the Co9S8 hollow spheres showed similar to the initial analysis after 1000 cycles, the overpotential at 10 mA cm-2 only decreased 11 mV, revealing the enhanced durability of Co9S8 hollow spheres in this harsh oxidizing environment. Additionally, the amperometry (i-t) response measurements were assessed at a constant potential for 10 h to determine the elctrochemical durability. As can be seen in Figure 5d, Co9S8 hollow spheres apparently presented only a slight current attenuation of 15% after 10 h while IrO2 electrode (35%) and NiFe LDH nanosheet (31%) over the same period of time, verifying that Co9S8 hollow spheres can maintain an improve long-term stability. Hollow spheres kept the original feature very well while a few broken hollow spheres 17



were emerged after 10 h OER test. Obviously, electrochemical impedance spectroscopy (EIS) measurement (Figure S5) illustrated that the electron-transfer resistance (Rct) of Co9S8 hollow spheres (5.36 Ω) was smaller than Co9S8 flower-like structure (5.59 Ω), revealing the upper charge transport efficiency under OER operating conditions.

Figure 5 (a) Polarization curves recorded in 1 M aqueous KOH, (b) Tafel plots, (c) The accelerated stability of the Co9S8 electrode after 1000 cycles (d) The amperometry plots of the Co9S8 at the overpotentials of 285 mV. To obtain further insight into OER processes, the specific activity, mass activity, and turn over frequency (TOF) were also calculated to reflect the intrinsic catalytic performance at a given overpotential.58 As presented in the Table 1, it was obviously noted that the specific activity and mass activity of Co9S8 hollow spheres were calculated 0.180 mA cm-2 and 118.3 A g-1 in 1 M KOH, respectively, surpassing Co9S8 18


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flower-like structure catalysts. As well as, the TOF values of Co9S8 hollow spheres were also estimated through assuming all Co ions to possess catalytically active. It was noteworthy that Co9S8 hollow spheres exhibited significant improvement as OER electrocatalyst with the highest TOF value of ~0.0268 at 0.29 V, which is ~16 times higher than Co9S8 flower-like structure (0.0016). Morever, the electrochemical active surface area (ECSA) as an addition point was estimated by calculating the electrical double-layer capacitance (CdI) that determined using simple CV methods. Therefore, CVs measurements with various scan rates were performed in 1 M KOH shown in Figure S6a,b. Derived from Figure S6a,b, the CdI of Co9S8 hollow spheres was calculated to be a 27.9 mF cm-2, higher than that of Co9S8 flower-like structure (24.7 mF cm-2), as shown in Fig. S6c, suggesting a larger electrochemically active surface area (ECSA). This also indicated the enhanced OER activity of Co9S8 hollow spheres. Therefore, we speculated that morphology of materials had a great impact on their catalytic performance. Table 1. OER activity data for Co9S8 catalysts. η at -2

Mass activity

Specific activity

TOF at

J=10 mA cm

at η=0.29 V

at η=0.29 V

η=0.29 V

[mV]

[A g-1]

[mA cm-2]

[s-1]

Co9S8 hollow spheres

285

118.3

0.180

0.0268

Co9S8 flower-like structure

380

7.1

0.035

0.0016

Catalysts

To confirm the observed current derived from the OER process, an rotating ring-disk electrode (RRDE) with a ring (Pt) potential of 0.4 V (vs RHE) was employed to reduce generated O2. It could detect the molecular oxygen though rendering a simultaneous OER (disk electrode) and ORR (oxygen reduction reaction, 19



ring electrode) process.37 As presented in Figure 6, the disk current of 0.448 mA was held (black line) while molecular O2 generated from Co9S8 hollow spheres surface on the disk electrode. Those O2 molecules swept across the surrounding Pt ring electrode and were rapidly reduced. Consequently, a content ring current of ∼0.088mA (collection efficiency 0.2) was observed (blue curve), confirming the detected oxidation current could be ascribed to OER process. The faradaic efficiency of OER was 95-99%.

Figure 6. Disk current (Idisk) and ring current (Iring) of Co9S8 hollow spheres using RRDE measurements. The inset shows the Faradaic efficiency of Co9S8 hollow spheres for OER.

CVs were performed to reveal the oxidation of Co9S8 hollow spheres during OER. According to Figure S7, obvious electro-transfer peaks were observed between 1.0 and 1.3 V, which could be assigned to the reversible redox reaction: Co(OH)2 + OH↔ CoOOH + H2O + e. The progressive increase in intensity with cycling resulted in

20


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successive formation of hydroxide/oxyhydroxide layer on the Co9S8 surface.59 Moreover, the CoOOH could be further oxidized to CoO2 at higher potential and the derived cobalt oxides/hydroxides exhibited excellent OER performance.60 Thus, it indicated that Co9S8 core with a thin cobalt oxide/hydroxide shell was the active phases during OER process.61 It was similar to other cobalt compound based catalysts occurred in OER.62

To verify the conversion of Co9S8, SEM, EDS and XPS were carried out after OER stability test. As shown in Figure S8a, hollow spheres kept the original feature while few nanosheets on the surface appeared. According to the EDS elemental mapping, Co9S8 electrode was mainly composed by Co and O after OER test for 10 h (Figure S8). In addition, XPS was performed to better understand the electronic state changes of Co, O, and S in Co9S8 electrocatalyst after 10 h stability test. The intensive peaks at 780.2 and 795.8 eV (Figure S9a) contributed to cobalt oxides/hydroxides, revealing the surface oxidation of Co9S8 during the OER.63, 64 Moreover, the S 2p (Figure S9b) characteristic peaks indexed to Co-S bond partially disappeared after OER stability test while the SO42- was emerged due to the sulfur partly oxidized on surface of Co9S8.42 The O 1s spectrum (Figure S9c) at 531.3 eV was the oxygen from cobalt oxides/hydroxides.65 Based on those results, it verified that the Co9S8 was converted to cobalt oxides/hydroxides during the OER process. According to all above results, it is noteworthy that Co9S8 hollow spheres are a promising OER electrocatalyst for future application.

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CONCLUSIONS

In summary, a novel Co9S8 hollow spheres with an average diameter of 2 µm were successfully fabricated by a surfactant- and template-free method and found to be an excellent electrocatalyst for OER. Notably, it was observed that the performance of Co9S8 hollow spheres with low overpotential (285 mV), tafel slop (58 mV decade-1) and good durability in 1 M KOH, which obviously surpassed most non-precious metal OER catalysts and commerical IrO2. The remarkable activities could be ascribed to hollow structure and large-surface-area that provided more luxuriant active sites and enhanced mass transport. Our study indicated great potential to attain a series of materials with well-defined structures and controllable compositions through the readily scalable synthesis. Therefore, it is a promising alternative for Co9S8 to replace precious metal based electrocatalysts.

ASSOCIATED CONTENT

Supporting Information

Synthesis of NiFe LDH; preparation of working electrode; XRD, SEM of Co9S8 flower-like; EDS of Co9S8 hollow spheres; BET and EIS of Co9S8 flower-like and Co9S8 hollow spheres; comparison of the OER electrocatalytic performance; CVs measurement with various scan rates and plots of ∆J/2 vs. scan rate for Co9S8 flower-like and Co9S8 hollow spheres; CVs of Co9S8 hollow spheres; SEM, EDS analysis and XPS of Co9S8 electrocatalyst after 10 h stability test.

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AUTHOR INFORMATION

Corresponding Authors

*Email: [email protected] (Caihong Feng)

*Email: [email protected] (Wei Zhou)

Author Contributions

All authors have contributed to the final version of the manuscript.

Notes The authors declare no competing financial interest.

Acknowledgements

The project was financially supported by National Natural Science Foundation of China (no. 21376029).

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For Table of Contents Use Only:

The hollow structure endow Co9S8 with impressive OER performance, which can applied to energy storage and conversion technologies.

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Facile Synthesis of Co9S8 Hollow Spheres as a High Performance

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