In situ Surface Chemistry Engineering of Cobalt-Sulfide Nanosheets

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In situ Surface Chemistry Engineering of Cobalt- Sulfide Nanosheets for Improved Oxygen Evolution Activity Suxiao Ju, Yuanjun Liu, Hui Chen, Fujiang Tan, Aihua Yuan, Xiaoyun Li, and Guo-Xing Zhu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00687 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Applied Energy Materials

In situ Surface Chemistry Engineering of CobaltSulfide Nanosheets for Improved Oxygen Evolution Activity Suxiao Ju,† Yuanjun Liu,, † Hui Chen,† Fujiang Tan,† Aihua Yuan,, † Xiaoyun Li, ‡ and Guoxing Zhu, ‡ †School

of Environmental and Chemical Engineering, Jiangsu University of Science and

Technology, Zhenjiang 202018, China, *Email: [email protected]; [email protected] ‡School

of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China, *E-mail: [email protected]

ABSTRACT Large surface area and unique surface chemical environment are two critical factors for a catalyst, both of which can highly influence the catalytic activity. In this study, we present a small molecular amine mediated one-step approach to directly synthesize thin cobalt sulfide nanosheets by employing sulfur powder and CoCl2 as the reaction precursor. The surface chemical environment including cobalt oxidation state of the obtained cobalt sulfide materials can be in situ finely tuned by sulfur dosage. The as-prepared and optimized cobalt sulfide nanosheets with thickness of 5-12 nm have favorable surface chemical environment including amounts of highly active Co(III) species, easy transform of Co(II) to Co(III), richness of defects and higher electrochemical surface area, which induce enhanced

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catalytic activity for oxygen evolution reaction (OER) in alkaline media. When used as electrocatalyst, the optimized cobalt sulfide catalyst offers an overpotential of 312 mV at current density of 10 mA cm-2 and mass loading of 0.37-0.45 mg cm−2, which can remain constant for at least 36 h of OER operation in 1 M KOH electrolyte. This electrocatalytic activity is better than most of reported monometallic cobalt sulfide materials and commercial RuO2. KEYWORDS cobalt sulfide; oxygen evolution reaction; surface chemistry; electrocatalyst; Co-S-O species

Introduction Hydrogen is considered to be a promising energy carrier to replace fossil fuel. At present stage, hydrogen mainly comes from the by-products of petrochemical engineering and chlor-alkali industry, which often involves heavy environmental problems.1-4 One of the clean and green routes to obtain hydrogen is the (photo)electrochemical water splitting.5-6 During this process, both of the hydrogen evolution reaction at the cathode and the oxygen evolution reaction (OER) at the anode should be carefully considered, since higher overpotentials are needed to drive both of processes.1, 7-8 Compared to the needed over-potential for hydrogen evolution, which are about several tens of millivoltages for current density of 10 mA cm-2 with most reported catalysts,9-10 the oxygen evolution end needs much higher over-potentials (usually several hundreds of millivoltages for current density of 10 mA cm-2).6 In this respect, development of advanced oxygen evolution catalysts to decrease the overpotential of OER has much more room to improve the whole water splitting efficiency.11-15 On the other hand, OER is also involved in the charging process of


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various metal-air batteries (in the discharging process, oxygen reduction reaction (ORR) is involved).16-19 Thus, development of efficient electrocatalysts for OER is also of great significance in various affordable energy conversion and storage devices.20-24 Oxygen evolution from acidic or alkaline solution demands accumulating four electrons to form one oxygen molecular.25-26 It is believed that the whole process involves the following four main steps: the absorption of OH- or H2O on catalytic centers (M) forming M-OH, the transforming of M-OH to M=O species, the evolution of M=O to M-OOH, and the delivery of O2.25, 27 Note the energy gap between M-OH and M-OOH intermediates is certain;28 an excellent OER catalyst should have surface catalytic sites with suitable bonding strength to oxygen intermediates;20, 29-30 being too strong or too weak is unfavourable for the catalytic process. So, subtle modulation of the catalyst surface to an optimal state, making the surface catalytic centres have favourable bonding strength to oxygen intermediates is thus quite important.31 Precious metal oxides (RuO2 and IrO2) are the benchmark electrocatalysts for OER process, but their scarcity, high expense, and poor durability in alkaline solution hinder their widespread application.32-33 Recent years, transition metal-based OER catalysts such as double metal hydroxides, oxides, some sulfides and selenides were developed and have attracted more and more attention.34-36 Cobalt sulfide materials have got much attention when used as electrocatalysts for oxygen reduction.15,

37-40

Although the excellent electrocatalytic performance for ORR is confirmed in many studies, the electrocatalytic activity of pristine cobalt sulfide materials for OER is considered to be poor.41 In order to improve their OER catalytic activity, second metal species are often introduced into cobalt sulfide materials to subtle modulate the electron structure of metal sites and so the binding between catalytic centres and



oxygen intermediates.42-43 Deka et al and Liu et al have introduced copper in cobalt species. It was found that the introduction of copper species can effectively improve the adsorption of -OH species, demonstrating enhanced electrocatalytic activity for OER.44-45 Similarly, Lu and his co-workers introduced iron sulfide into cobalt sulfide, inducing favourable adsorption of oxygen containing species and so enhancing OER with a over-potential of 0.41 V at 10 mA cm-2.38 As for monometallic cobalt sulfide materials, the present reports are mainly on the synthesis of various hierarchical microstructure with surfactants or templates and their application as catalysts for oxygen reduction reaction. Few studies are focused on the modulation of their surface state, although it is important for catalytic application. In addition, it should be noted that during OER process, the surface of sulfide OER catalysts usually evolves into oxygen-containing species under the bonding of OH- and the high overpotential environment in electrolytes. The oxidized cobalt sulfide species on the surface are the actual catalytic active phases, catalysing the reaction. The catalytic activity of the in situ derivative sulfur-containing oxide phase on the metal sulfide surface is dependent on the original sulfide materials. It is believed that the in situ derivative sulfurcontaining oxidized phase is responsible for the enhanced electrocatalytic performance of sulfides than the corresponding pristine oxides.46 Inspired by the above findings, here, we designed a new route to in situ modulate the surface state of cobalt sulfide materials and to improve the electrocatalytic activity. Thin nanosheet-like cobalt sulfide materials with thickness of 5-12 nm that are interconnected with each other were synthesized by a small molecular amine mediated process. The surface chemical environment can be easily in situ tuned by changing the dosage of sulfur powder. The modulated cobalt sulfide nanosheets have amounts of Co(III) species with highly active spin configuration of t2g4eg2 and richness of defects


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on the surface, presenting improved electrocatalytical activity and also long-term durability for OER, achieving a stable 10 mA cm−2 current density at an overpotential of 338 mV (pH 13.6) and mass loading of 0.17 mg cm−2. Increasing the mass loading to 0.37-0.45 mg cm−2 can further decrease this overpotential to 312 mV. This catalytic performance is over most of reported cobalt sulfide materials and some bimetal OER catalysts. It is believed that the unique surface state is responsible for the absorption of oxygen species and so the enhancement of electrocatalytical activity.

Experimental Section Materials Cobalt (II) chloride hexahydrate (CoCl2·6H2O), sulfur powder, and nbutylamine were purchased from J&K Scientific Ltd. Ethanol and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. All of the chemicals are of analytical grade and used as received without further purification. Synthesis of cobalt sulfide products In a typical procedure, 0.13 g of sulfur powder (4 mmol) was firstly dissolved in 4 mL of n-butylamine. Sulfur powder can be easily dissolved in n-butylamine (with an exothermic process), forming a clean light-brown solution. 0.48 g of CoCl2·6H2O (2 mmol) was then added into the above solution. The obtained mixture was vigorously stirred for 30 min. Afterwards, the reaction mixture was transferred into a 25 mL of Teflon-lined stainless-steel autoclave, and heated at 100 ° C for 12 h in a sealed autoclave. After being cool to room temperature, the formed solid product was washed several times with ethanol, deionized water and dried under vacuum at 60 °C overnight. The synthesized sulfide sample is denoted as Co-S-130. A series of contrast samples were prepared with the same procedure but changing sulfur dosage to 0.19 g (6 mmol), 0.15 g (4.7 mmol), 0.06 g (1.9 mmol) and



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0.016 g (0.05 mmol) under constant other conditions. The corresponding products are named as Co-S-190, Co-S-150, Co-S-60 and Co-S-16, respectively. Materials Characterization Phase identification of the obtained samples were characterized

via

powder

X-ray

diffraction

(XRD,

Shimadzu

XRD-6000

diffractometer, Cu-Kα radiation, λ = 0.15406 nm). The morphologies and microstructures were observed via scanning electron microscopy (SEM, Hitachi S4800 microscope) and transmission electron microscopy (TEM, Tecnai G2 F30 S-Twin, FEI, Netherlands). Energy dispersive spectrum (EDS) and element mapping analysis were performed with an Oxford X-ray detector that is set on the above TEM. The chemical composition and oxidation state of the samples were investigated with X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, Japan UlVAC-PHI Company). Fourier transform infrared spectra (FT-IR) were carried out in the range of 400-4000 cm-1 on an ATR spectrometer. Raman spectra were obtained with a 532 nm laser source with exposure time of 2 s. The cobalt contents in the sulfides were also determined by an inductively coupled plasma optical emission spectrometer (ICPOES, Vista-MPX). Electrochemical measurements The electrochemical characterization was carried out on an Ivium electrochemical workstation (Vertex.one, v54810) with a typical threeelectrode system and 1 M KOH aqueous solution as the electrolyte. Hg/HgO was used as reference electrode. To avoid the possible contamination from the usually used Pt counter electrode, a carbon rod was used as counter electrode in our experiment. The working electrode is a glassy carbon electrode (GCE, sectional area of 0.0707 cm-2) loaded with catalyst ink. To prepare the catalyst ink, 12 mg of the catalyst powder and 21 mg of acetylene black were dispersed in 60 μL of Nafion solution (5 wt%) and 6 ml of ethanol. The obtained mixture was sonicated for 40 min to form a catalyst ink. 6 μL


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of the catalyst ink was dropped onto GCE that was allowed to be dry in air. The catalyst loading density on GCE was determined to be 0.17 mg cm-2. For the electrochemical performance testing, cyclic voltammetry scanning was firstly performed at a scanning rate of 500 mV s-1 for 600 cycles to activate the catalysts. Then, cyclic voltammogram test with different scanning rate (20, 40, 60, 80, 100, and 120 mV/s) were conducted in the potential range of 0.3-0.4 V (vs. Hg/HgO) without redox reaction to check the electrochemical active surface area of the electrodes. Polarization curves (LSVs) were then collected with a sweep rate of 5 mV s-1. Tafel slopes were derived from the LSV curves by plotting overpotential (η) against log(J). As for the electrochemical impedance spectroscopy (EIS) test, the frequency range is 105-0.1 Hz with an added potential of 1.55 V (vs RHE). The catalytic stability of the catalyst was checked with chronoamperometry method. Unless otherwise stated, all potential values relative to the Hg/HgO reference electrode are converted to potentials relative to the reversible hydrogen electrode (RHE) as follows: ERHE = EHg/HgO + EϴHg/HgO + 0.059 pH (in volts). To check the Faradic efficiency of the catalyst for OER, the evolved O2 was collected by a water drainage method, the molar amount of which was then estimated by ideal gas law.

Results and Discussion Cobalt sulfide nanosheet products were synthesized with CoCl2 and sulfur powder as the raw materials (Figure 1a). Small molecular n-butylamine was used as the solvent and also the activate agent for element sulfur.47 Sulfur powder can be easily dissolved in n-butylamine forming a light-brown solution. During the dissolving and the following heating process, the nucleophilic attack of nitrogen atom from n-butylamine molecules on S8 rings will cause sulfur ring to break.48-49 Chain polysulfide ions are



thus formed in the form of nBuNH3+-nBuNH-S8-, which is active with relatively weaker S-S bond.48 The open chain polysulfide ions will further react with excess nbutylamine, forming of thiobutylamide.48 During this process, H2S is formed. The formation of H2S can be easily confirmed with a wet Pb(Ac)2 testing paper. In the nbutylamine system, CoCl2 can well be dissolved forming a clear solution. The obtained solution shows improved conductivity than that of n-butylamine, suggesting that CoCl2 is ionized. The added cobalt ions would be coordinated by amounts of organic amine, forming sheet-like units, which act as a soft template for the formation of cobalt sulfide nanosheets.50 The active sulfur species, H2S, reacts with cobalt ions forming cobalt sulfide and by-products of butyl-ammonium chloride. It is proposed that the activation of sulfur by organic amines is an abrupt process, which will cause the formation of cobalt sulfide in an instant, like a “hot-injection” process.51 The crystallinity of the synthesized cobalt sulfide products was firstly characterized by XRD. No obvious peaks are observed in the entire XRD pattern suggesting the synthesized cobalt sulfide products only have weak crystalline. The formation of weak crystallized sulfides would relate to the amine-mediated activation process, which produces active sulfur species in an instant and causes the rapid nucleation and growth.


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b) Co-S-16

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co-S-60

Co-S-130

Co-S-150 Co-S-190

20

30

40

50

60

70

80

2theta (degree)

Figure 1. a) Illustration of the nanosheet formation, b) XRD patterns of the synthesized cobalt sulfide products. SEM image of the typical sample, Co-S-130, is shown in Figure 2a. Spherical-like particles with size of several microns were observed during SEM observation. As shown by the SEM image with higher magnification, the spherical particles are actually composed by sheet-like units (Figure 2a). The sheet-like units present curved edges. Their microstructure and size of these sheet-like uints were then further examined by TEM. It can be clearly seen that the sample is composed of thin sheetlike units with featured wrinkle structure (Figure 2b). The nanosheets are connected with each other. Some of the nanosheets show curved edges, from which the thickness can be determined to be 5-12 nm (Figure 2c). It seems that the nanosheets have rough



surface and are composed by tiny particles (Figure 2c). Figure 2d shows the highresolution TEM image of the nanosheet. Only smaller crystal domain was observed; most of the nanosheet region is composed by amorphous structure (or weak crystallized domain) with amounts of defects. The spacing of crystal fringes is measured to be 0.276 nm, which can be attributed to (200) crystal planes of cubic phase CoS2. This indicates that CoS2 is formed with this synthesis route. EDS spectrum obtained with an EDS equipment set on the TEM indicates that the sample has a composition of CoS1.72 that is generally consistent with CoS2 phase. The element mapping of the Co-S-130 product is shown in Figure 2e-h, suggesting the uniform distribution of cobalt, sulfur, and oxygen. It is reasonable that oxygen element is present in the sample, since the sulfide surface is easily oxidized in air; and during the synthesis, oxygen will also enter the sample. The microstructures of other cobalt sulfide products were also observed by TEM. As shown in Figure S1 (see Supporting Information (SI)), all of the obtained cobalt sulfide products show similar sheet-like microstructure.

Figure 2. a) SEM and b-d) TEM images of the typical sample Co-S-130. e-h) Elemental mapping of the corresponding sample for cobalt, sulphur, and oxygen.


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Figure 3a illustrates the Raman spectra of the five cobalt sulfide products, demonstrating five clear and typical bands at 188, 465, 511, 600, 663 cm-1 because of the different types of vibration modes (Eg mode, A1g mode and three modes of vibrations).45 According to the literature the peaks at 465, 511 and 600 cm-1 are attributed to vibrational modes of Co-S bond and the one at 663 cm-1 is due to the involved S-S stretching vibration, representing the formation of cobalt sulfide phase.45, 52-53

Especially, the products of Co-S-60 and Co-S-130 show relatively bigger full

width at half maximum (FWHM) of ~27 cm-1 for the strongest band at ~663 cm-1, which is higher than those of other products with FWHM of ~20 cm-1. This implies more structural disorders or defects in these sulfide products especially the products of Co-S-60 and Co-S-130, since for nanocrystals free of defects the FWHM should be of the order of ~8.0 cm-1.54 As shown in Figure S2 (see SI), the strong FT-IR bands centered at ~3424 and 3733 cm-1 are assigned to the stretching vibration of structural O-H adsorbed on the surface or O-H in adsorbed water molecules. In the FT-IR spectrum of sample Co-S-16, the sharp peak at 3557 cm-1 is attributed to the stretching vibration of -NH2, suggesting the bonding of amine molecules on the surface. The product of Co-S-190 shows an FT-IR band of 1097 cm-1 related to the S=O bond, indicating the surface oxidation of the product.55



Figure 3. a) Raman and b-d) detailed XPS spectra of the cobalt sulfide products, b) Co 2p region, c) S 2p region, and d) O 1s region.

X-ray photoelectron spectra (XPS) were explored to identify the surface chemical compositions and chemical environment of the obtained sulfide products (Figure 3bd). The Co 2p spectrum of Co-S products show two wide bands at 778-790 and 796806 eV, corresponding to the Co 2p3/2 and Co 2p1/2, respectively.56-58 The Co 2p XPS bands at 781.3 and 797.1 eV in the sample of Co-S-16 with spin-orbit splitting of ~15.8 eV indicate the presence of Co(II) species.36 As for the samples of Co-S-60, CoS-130, and Co-S-150, the wide XPS band of Co 2p3/2 can be fitted into two bands at 779.3 and ~781.8 eV, which are characteristics of Co(III) and Co(II) states, respectively, suggesting the co-existence of Co(III) and Co(II) species in these three products.59-60 It is proposed that the Co(III) species on the surface presents with a high-spin state (t2g4eg2), which should be beneficial for OER activity.45 In contrast, the XPS band of Co-S-190 didn’t present obvious XPS band assigned to Co(III) species; the two XPS bands at 781.5 and 783.5 eV suggest the quite different surface chemical 12 Environment ACS Paragon Plus

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environment for cobalt sites in this product, possibly due to the involved amounts of sulfur in the product (the composition of CoS3.55 is shown by XPS result). In addition, there are two corresponding shakeup satellites cantered at ~786.6 and ~803.0 eV for all samples.61 Figure 3c presents the XPS band of S 2p for all samples. The band at 161-164 eV can be assigned to the sulfur species in Co-S, while that at 166-171 eV suggests the presence of S-O species.36 For sulfide nanoparticles synthesized with water, the oxidation of surface sulfur species is difficult to be avoided, thus causing the presence of S-O species on the surface. In our case, the crystallized water molecules in cobalt salt and the possible oxidation during sample treatment process are responsible for the formation of S-O species on the surface. Note that the relative content of oxidized sulfur to sulfur on the sulfide surface varies for different samples due to the different sulfur powder used for the sample preparation route. To further check the involved oxygen species on the surface, XPS spectra of O 1s is compared for all samples (Figure 3d). The O 1s bands can be fitted into three different oxygen species. In the samples of Co-S-16 and Co-S-60, the band at ~530.6 eV corresponds to lattice Co-O species.62 The peak at 531.3 eV was found for all samples and can be indexed into oxygen species at high defect sites with a low oxygen or sulfur coordination.63-65 In addition, the O 1s bands corresponding to adsorbed oxygen including the surface S-O, absorbed OH-, or molecularly adsorbed water locates at the range 531.8-533.5 eV.63-65 It should be noted that the sample of Co-S-130 shows stronger band at 531.5 eV suggesting the high concentration of surface defects and relatively stronger band at 532.8 eV implying the strong absorption ability for water. The high concentration of surface defects and the favourable surface absorption for water are beneficial for the electrocatalytic process in oxygen evolution.66-67 These



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XPS results clearly confirm that the obtained cobalt sulfide materials actually have a surface layer containing Co-S-O species (oxidized cobalt sulfide) and the chemical environment of the surface layer can be finely tuned by changing of sulfur dosage. The semi-quantitative XPS analysis suggest that Co:S atomic ratio in the samples are changed with the sulfur dosage (Table S1, see SI). During the cobalt sulfide growth process, if less sulfur is involved (such as samples of Co-S-16), the final product contains less sulfur (the composition of CoS0.13 is shown by XPS result, Table S1, see SI); some oxygen species coming from the crystallized water will enter the sample, possibly forming some oxide species, causing higher cobalt content of this sample (53.8

wt%,

higher than the theoretical value of 47.9

wt%

for CoS2). This is

further suggested by the fact of stronger XPS bands of S-O and Co-O species (Figure 3c and 3d). The entered oxygen species will induce the blue-shift of sulfur XPS band to higher binding energy. When enough sulfur was involved for the synthesis, the obtained samples have surfaces with more lattice sulfur species (composition of CoS2.5-3.0 is demonstrated for samples of Co-S-60, Co-S-130, Co-S-160, Table S1, see SI), showing stronger XPS band at 161-164 eV corresponding to S2- (Figure 3c). In this case, sulfur species is enough, thus only less oxygen species will be involved into the sulfide lattice; more oxygen species would locate on sulfide surface due to the oxidation of surface sulfur species. Oxygen species will coordinate on the surface defect sites (such as samples of Co-S-130), which are proposed to be the catalytic active sites. During the reaction process, the generated poly-sulfides with certain oxidation ability would oxidize Co(II) to Co(III) species. While if amounts of sulfur is used for the sample preparation, the excess sulfur species (including polysulfide species) would heavily cover the surface of the formed cobalt sulfide (such as the sample of Co-S-190, the composition of CoS3.55 is shown by XPS result). The surface


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amounts of sulfur species provide cobalt sites a quite different chemical environment (Figure 3b-d). In addition, the amounts of surface sulfur species will be easily oxidized forming more S-O species on the surface (Figure 3c), inducing higher oxygen content on the surface (Table S1, see SI). Due to the presence of surface oxygen species, these sulfide products shows relatively lower cobalt content of 32-37 wt %.

Figure 4. a) LSV curves of the synthesized cobalt sulfide products in 1 M KOH aqueous solution at scanning rate of 5 mV s-1. b) The corresponding Tafel plots derived from the LSV curves. c) Mass activity normalized by the loading mass at overpotential of 350 mV and the overpotentials for 10 mA cm-2. d) Double layer capacitance (Cdl) values obtained by plotting of the current density, (ja-jc)/2, vs scanning rate.

As for an electrocatalyst, amounts of exposed surface metal sites, richness of defects, and suitable electronic structure are favourable factors for the improvement of



electrocatalytic activity. The electrocatalytic activity of the obtained cobalt sulfide products with different surface chemical environment were then investigated in 1 M KOH solution with a typical three-electrode system. The polarization curves were collected without iR compensation and were shown in Figure 4a. The Co-S-130 product shows the lowest onset potential. To achieve a current density of 10 mA cm-2, which is a typical value for the comparison of various electrocatalysts, the Co-S-130 product requires only an overpotential of 338 mV, which is much smaller than the other contrast samples (400 mV for Co-S-190, 390 mV for Co-S-150, 380 mV for CoS-60, and 390 mV for Co-S-16). It can be seen that with the increase of sulfur dosage in the preparation process, the overpotential of the catalyst towards current density of 10 mA cm-2 firstly decreases and then increases. Be noted that the bare glassy carbon electrode and the electrode modified by pristine acetylene black show similar higher overpotential of about 510 mV to achieve a current density of 10 mA cm-2, indicating the involved acetylene black used for the preparation of catalyst ink is inert for the catalytic process (Figure S3, see SI). To check the kinetics of the formed cobalt sulfide catalysts, Tafel plots of the catalyst were then plotted from the obtained LSV curves shown in Figure 4a (Figure 4b). Tafel slope of the optimal cobalt sulfide product Co-S-130 is 69 mV dec-1, suggesting the four-electron transfer reaction pathway.68 In contrast, the other catalysts show much higher Tafel slopes, for example, 82 mV dec-1 for Co-S-150, 80 mV dec-1 for Co-S-60. More detailed comparison of the electrocatalytic performance among the cobalt sulfide catalysts is based on the mass activity (Figure 4c). The mass activity of the catalysts was calculated based on the catalyst mass loading on the glassy carbon electrode and the current density at over-potential of 350 mV. The optimized catalyst


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Co-S-130 shows a mass activity of 76.0 A g-1, which is 3.8 times of Co-S-190 (20 A g1)

and 2.7 times of Co-S-60 (28 A g-1). This suggests the effective mass transport and

faster reaction kinetics on the Co-S-130 catalyst for OER. In order to estimate the apparent electrochemical activity specific surface area (ECSA) and to further describe the activity of the electrode, cyclic voltammetry curves were tested with different scanning rates to measure the double layer capacitance (Cdl), which is positive correlation to the ECSA and is often used to represent the ECSA.69 Figure S4 shows the corresponding CV curves (see SI). The Cdl values for the product are estimated by plotting the (ja-jc)/2 at 1.25 V (vs. RHE) against the scanning rate (Figure 4d). Among of the investigated five catalysts, the Co-S-130 catalyst shows the highest Cdl value of 36 mF cm-2, much higher than other cobalt sulfide catalysts (2.510.7 mF cm-2). This suggests that on Co-S-130 catalyst surface, there are more accessible exposed active sites.



Figure 5. a) Cyclic voltammetry curves of the cobalt sulfide products in 1 M KOH electrolyte. b) Nyquist plots of the cobalt sulfide catalysts measured in the frequency range of 105-0.1 Hz with a potential of 1.55 V (vs RHE). c) LSV curves of the Co-S130 catalyst with different mass loading in the range of 0.17-0.51 mg/cm2. d) Overpotentials for 10 mA cm-2 and current density at overpotential of 350 mV for the electrodes with different mass loading.

The cyclic voltammetry test of various cobalt sulfides was performed and compared in 1 M KOH (Figure 5a). The clear anodic peak corresponding to the oxidation of Co(II) to Co (III) species locates at 1.17 V for Co-S-130 product,59 which shifts to 1.27 V for the samples of Co-S-150 and Co-S-190. This shift indicates that the oxidation of Co(II) into Co(III) is easy in the product of Co-S-130 and implies the different surface chemical environment of them. Obviously, the easy transform of


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Co(II) to highly active Co(III) species would play crucial roles for the catalytic process, since it is believed that the catalytic active centers of cobalt-based compounds are the cobalt sites with high oxidation state.55 In addition, the relatively strong anodic peak on the catalyst of Co-S-130 also indicates that it contains more electroactive sites on the surface comparison to other counterparts. To get a further insight into the different OER performance of the obtained cobalt sulfide catalyst, electrochemical impedance spectroscopy (EIS) tests were then conducted in the range of 0.1-105 Hz with a potential of 1.55 V to investigate the charge transfer ability. The obtained Nyquist plots of the catalysts are depicted in Figure 5b. The semi-circular proportion of the Nyquist plots presents the information of charge transfer resistance (Rct). A small semi-circular indicates a lower Rct value, that is, a faster charge transfer process between the electrolyte and the electrocatalysts. As shown by Figure 5b, the product of Co-S-130 shows the smallest semi-circular, indicating the lowest Rct value. According to the fitting results of the Nyquist plots, the Rct value of the Co-S-130 is estimated to be 138 Ω, much smaller than those of other Co-S catalysts (571 Ω for Co-S-190, 301 Ω for Co-S-150, 228 Ω for Co-S-60, 429 Ω for Co-S-16), suggesting that the Co-S-130 product have a relatively faster charge transfer rate for the OER in comparison to other counterparts. Absolutely, the faster charge transfer rate is favourable of the electrocatalytic performance. Considering the potential applications, we further optimized the mass loading of the catalyst on the electrode. The results are shown in Figure 5c and d. It can be clearly seen that with the increasing of mass loading from 0.17 to 0.51 mg cm-2, the catalytic current firstly increases, keeps constant, and then decreases. This change trend is consistent

to

the

electrodeposited

thin-film

catalyst

of

transition

metal

(oxy)hydroxides.70 At lower catalyst loadings, almost all of the active sites can be



accessible for the catalytic process. Thus, the catalytic current increases with the increase of catalyst loading density. Beyond one value, the catalyst film becomes thicker than that the electrolyte can access to. At this stage, the catalytic current will get its maximum. The values obtained in this region may be considered as a true indicator of the intrinsic activity of the materials. For our optimized cobalt sulfide catalyst, catalytic current of 30 mA cm-2 at overpotential of 350 mV and overpotential of 312 mV for current density of 10 mA cm-2 can be obtained in this region. At even higher mass loadings, a decrease in activity will be observed because of the increased resistance due to the thicker catalyst film on the electrode. In general, RuO2 was used as a benchmark for the OER. In our previous study, the OER activity of the commercial RuO2 electrocatalyst was tested in the same electrolyte,41 which displays an overpotential of 380 mV at a current density of 10 mA cm−2. This overpotential is much higher than that of this Co-S-130 catalyst. This fact indicates that the prepared cobalt sulfide catalyst with finely tuned surface chemistry environment has better electrocatalytic activity for OER than the state of the art RuO2 electrocatalyst. We further compared the catalytic performance of our catalyst with other reported cobalt sulfide materials. As shown in Table S2 (see SI), the catalytic activity of our Co-S-130 catalyst is better than most of reported cobalt sulfide OER electrocatalysts and some bimetal OER catalysts. The improved electrocatalytic activity of Co-S-130 should be due to the unique surface chemical environment. On the one hand, the abundant highly active Co(III) species on the surface of Co-S-130 is favourable for OER activity enhancement.55 The easy transform of Co(II) to Co(III) for the product of Co-S-130 further strengths this factor. On the other hand, the product of Co-S-130 has a high electrochemical activity specific surface area. As shown by the investigation of Raman spectrum (Figure 3a), a


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clear widen peak can be observed, indicating the presence of amounts of defects on the surface of the Co-S-130, which is also confirmed by the O1s XPS spectrum. These Co(III) species and defects can create more electrochemically active sites for the absorption of oxygen species, act as catalytic centers and thus enhance the electrocatalytic activity for OER.59 What is more, as shown by the result of EIS, the unique surface chemical environment with sheet-like microstructure can yield easier mass transport and so faster charge transfer rate.71 All of these factors contribute to the improved OER kinetics of the Co-S-130 product.

Figure 6. a) OER polarization curves of the Co-S-130 products before and after 2000 CV cycles and b) the chronoamperometry (i-t) curve was recorded at a constant applied overpotential of 338 mV to evaluate the stability of the Co-S-130 catalyst.

Furthermore, the stability of the optimal catalyst was then tested. The long-term stability of the Co-S-130 catalyst was firstly explored by CV cycles in the range of 1.05-1.75 V vs RHE with scanning rate of 100 mV s-1. The LSV curve after 2000 CV cycles was compared with the first LSV curve. It can be found that only slight increase of the overpotential after 2000 CV cycles, implying its excellent stability (Figure 6a). The catalyst stability was also tested with chronoamperometry method under a constant overpotential of 338 mV for 36 h. The slight current oscillation originates



from the accumulation and delivery of O2 bubbles on the electrode surface. As observed in Figure 6b, the electrode exhibited a catalytic current density of 10 mA cm2,

which basically remains stable for the whole testing period, implying the high

stability of the electrocatalysis. The Faradic efficiency for oxygen evolution during the catalytic process was also checked in i-t testing by water-gas displace method, which shows Faradaic efficiency >90%, suggesting that the electron transfer during the process is dominated by oxygen evolution. Combining the above electrochemical results, it is concluded that this surface engineered cobalt sulfide nanosheets can afford high electrocatalytic activity and durability, outperforming the benchmark RuO2 electrocatalyst. Noted that the surface of sulfide material will be oxidized during the OER process with higher over-potential and strong alkaline environment. To check the possible change of the catalyst during OER process, the Co-S-130 catalyst after 36 h of OER operation was then further characterized. SEM image of the Co-S-130 catalyst after 36 h of OER operation shows relatively smaller sheet-like units (Figure S5a, see SI). The corresponding TEM image indicates that the original sheet-like units are partially broken (Figure S5b, see SI). Especially, on some nanosheets, there are some rod-like units formed on the surface (Figure S5c, see SI), HRTEM image of which shows lattice fringes with spacing of 0.23 nm that can be attributed to (012) crystal planes of CoOOH (Figure S5d, see SI). This indicates that the sulfide surface is oxidized during the OER process. The catalyst after OER operation was also characterized by XPS, the corresponding results are shown in Figure S6 (see SI). After long time of OER operation, the cobalt sulfide species are oxidized showing a composition of CoS0.1O2.7 determined by XPS spectrum. ICP analysis of the catalyst after OER provides a cobalt content of 36.4 wt% (Table S1, see SI), lower than that calculated from composition of


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CoS0.1O2.7 (55.9

wt%).

This indicates that only the surface is oxidized. After OER

testing, the Co(II) band in XPS spectrum disappears but only showing bands at 779.5 and 781.1 eV, which can be corresponded to Co(III) in CoOOH (Figure S6a, see SI). The formation of CoOOH is further confirmed by the XPS spectrum of O 1s, which shows peaks at 529.1 and 530.6 eV corresponding to lattice oxygen and OH species (Figure S6b, see SI). Most of sulfur on the cobalt sulfide surface in S-Co units is lost during the process. However, there are still some oxidized sulfur and S-Co species in the sample (Figure S6c, see SI). It is believed that the involved sulfur species in the oxidized cobalt sulfide layer can remain in the surface layer for a long time and would be responsible for the enhanced catalytic activity of sulfide catalysts than those of pristine oxides.46

Conclusions In summary, we have highlighted a small molecular amine mediated strategy for the one-step synthesis and surface modulation of thin cobalt-sulfide nanosheets by employing sulfur powder and cobalt chloride as raw materials. Our approach is facile and efficient, and can easily tune the surface chemical environment of the obtained cobalt sulfide nanosheets, which improves the electrocatalytic activities for OER. When used as an electrocatalyst for OER, the optimized cobalt sulfide nanosheets with amounts of Co(III) species and richness of defects on the surface display superior electrocatalytic activity in comparison to their counterparts due to the special surface chemical environment. Meanwhile, the obtained cobalt sulfide affords high electrocatalytic activity and durability, outperforming the commercial benchmark RuO2 electrocatalyst, showing their promising application.



ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.xxxxx. TEM images, IR spectra, and cyclic voltammetry curves of various cobalt sulfide products. SEM images, TEM images, and XPS spectra of the catalyst after long time OER operation.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] *E-mail: [email protected]

ORCID Yuanjun Liu: 0000-0003-3881-4256 Guoxing Zhu: 0000-0002-0756-7451

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for National Natural Science Foundation of China (No.21776115) and Jiangsu Natural Science Foundation (No. BK20161343). Six talent peaks project in Jiangsu Province (XCL-2018-017).

REFERENCES


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In situ Surface Chemistry Engineering of Cobalt-Sulfide Nanosheets

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