Direct Observation of Structural Evolution of Metal Chalcogenide in

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Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation Ke Fan, Haiyuan Zou, Yue Lu, Hong Chen, Fusheng Li, Jinxuan Liu, Licheng Sun, Lianpeng Tong, Michael F. Toney, Manling Sui, and Jiaguo Yu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06312 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Direct Observation of Structural Evolution of Metal Chalcogenide in Electrocatalytic Water Oxidation

Ke Fan*,†,⊥ , Haiyuan Zou†, Yue Lu*,‡,⊥ , Hong Chen ◇ , Fusheng Li ┼ , Jinxuan Liu ┼ , Licheng Sun┼,║, Lianpeng Tong※, Michael F. Toney◇, Manling Sui‡, Jiaguo Yu*,†

†

State Key Laboratory of Advanced Technology for Materials Synthesis and

Processing, Wuhan University of Technology, Wuhan 430070, P. R. China ‡ Institute

of Microstructure and Properties of Advanced Materials, Beijing University

of Technology, Beijing 100124, P. R. China ◇

SSRL, SLAC National Accelerator Laboratory, Stanford University, Menlo Park,

California 94025, USA ┼

State Key Lab of Fine Chemicals, DUT-KTH Joint Education and Research Center

on Molecular Devices, Dalian University of Technology, Dalian 116024, P. R. China ║

Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm,

Sweden ※

School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou

510006, China *email: [email protected]; [email protected]; [email protected]

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ABSTRACT: As one of the most remarkable oxygen evolution reaction (OER) electrocatalysts, metal chalcogenides have been intensively reported due to their high OER activities during the past few decades. It has been reported that electron-chemical conversion of metal chalcogenides into oxides/hydroxides would take place after OER. However, the transition mechanism of such unstable structures, as well as the real active sites and catalytic activity during OER for these electrocatalysts, has not been understood yet, which urgently needs a direct observation for the electrocatalytic water oxidation process, especially at nano or even angstrom scale. In this research, by employing advanced Cs-corrected transmission electron microscopy (TEM), a step by step oxidational evolution of amorphous electrocatalyst CoSx into crystallized CoOOH in OER has been in situ captured: irreversible conversion of CoSx to crystallized CoOOH is initiated on the surface of electrocatalysts with a morphology change via Co(OH)2 intermediate during OER measurement, where CoOOH is confirmed as the real active species. Besides, this transition process has also been confirmed by multiple applications of X-ray photoelectron spectroscopy (XPS), in situ Fourier-transform infrared spectroscopy (FTIR) and other ex situ technologies. Moreover, based on this discovery, a high-efficiency electrocatalyst of a nitrogen-doped graphene foam (NGF) coated by CoSx has been explored through a thorough structure transformation of CoOOH. We believe this in situ and in-depth observation of structural evolution in OER measurement can provide insights into the fundamental understanding of the mechanism for OER catalysts, thus enabling the more rational design of low-cost and high-efficient electrocatalysts for water splitting. 2


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KEYWORDS

in situ TEM; water oxidation; cobalt chalcogenide; structural

evolution; XPS

Electrocatalytic water splitting is one of the convenient and environmentally benign routes for sustainable energy conversion and storage.1-4 The efficiency of overall water splitting is largely dependent on the half reaction of anodic oxygen evolution reaction (OER). However, the development of effective electrocatalysts toward OER still dominates the scientific challenge due to the complicated multiple electron-proton transfer process, and it is thus urgent to improve its sluggish kinetics, large overpotential (η) and consequently low efficiency.5-7 Within this context, it is highly required to understand the catalytically active site and electronic structure of electrocatalysts during OER, which can in turn guide the researchers for the rational design of more advanced electrocatalysts.8-11 During the past decades, low-cost electrocatalysts based on transition metal chalcogenides and phosphides, have been exploited for water oxidation, and some of them even outperform the precious Ru/Ir oxide benchmark catalysts.2,12-16 It is discovered that amorphous or highly disordered structure could govern the catalytically active site of these catalysts, thus rendering an enhanced OER activity relative to their crystalline counterparts.17-20 However, some literatures implied an irreversible species conversion of metal chalcogenides that could take place during OER. For example, it is found that the surface of cobalt sulfide will be oxidized to cobalt oxides irreversibly after OER test by spectroscopic technology.21 Likewise, using the post-catalytic analysis, Xu or Sternand et al. demonstrated that nickel selenides would be completely 3


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converted into nickel hydroxides in OER process, which were supposed as the final active form of the real catalyst in OER.22,23 Obviously, these inconsistent proposed mechanisms increase the difficulty of the understanding for structure-activity relationship of these catalysts. In this text, an in situ morphology and structural evolution of CoSx during OER has been captured by using transmission electron microscopies (TEM). Along with the in situ energy dispersive spectroscopy (EDS) mapping and electron energy loss spectra (EELS), we discern that the surface of amorphous CoSx is initially transformed to Co(OH)2 intermediate, and eventually leading to crystallized CoOOH irreversibly, a phase that affords catalytically active sites for efficient OER performance. This result is also supported by in situ Fourier transform infrared spectroscopy (FTIR) and multiple ex situ characterizations. Furthermore, the integration of CoSx with a three-dimensional (3D) N-doped graphene foam (NGF) yields, can take a complete transformation into CoOOH during OER, which enhances the activity of this electrocatalysts dramatically. The present work directly accesses and visualizes the detailed chemical-structure transformation process of catalysts under electrochemical water oxidation reaction, providing insights into the activity-structure relationship of metal chalcogenides for OER. We believe that the in situ and in-depth observation in this study can be extended to more systems, such as metal phosphides, thereby develop the rational design of lowcost and highly robust catalysts for water splitting.

RESULTS AND DISCUSSION The prototype of CoSx in this research was synthesized after hydrothermal treatment to convert ZIF-67 to amorphous CoSx (Figure S1)24, the morphology of rhombic 4


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dodecahedron is well preserved with an even size of ~ 500 nm. But interestingly, from the broken CoSx rhombic dodecahedrons, hollow interiors can be clearly observed (inset of Figure S1). Additionally, the high-resolution transmission electron microscopy (HRTEM) image of CoSx shows no obvious crystal lattice fringe (inset of Figure 1a), which further suggests the amorphous phase of CoSx (agree with XRD results in Figure S1). The electrocatalytic activity toward OER was firstly assessed on glassy carbon electrodes to reveal the potential electrocatalytic activity of CoSx. Figure 1a displays the linear sweep voltammetry (LSV) curves of CoSx and RuO2 (baseline) at a scan rate of 5 mV s-1. The required overpotential to achieve a catalytic current density of 10 mA cm-2 (a metric associated with solar fuel synthesis25) of CoSx is 396 mV, which is almost as same as that of the commercial RuO2 (398 mV). Moreover, the OER current density of CoSx increases sharply and surpasses RuO2 catalyst counterpart at higher overpotential, verifying the superior OER activity of CoSx. The Tafel slope of CoSx is 69 mV dec-1, lower than that of the commercial RuO2 (87 mV dec-1, Figure 1b), which means higher intrinsic activity of the active sites of CoSx. To better understand the OER performance of CoSx, electrochemical active surface areas (ECSA) were performed, which were obtained from cyclic voltammetry (CV) curves with different scan rates (Figure S2 and Figure 1c). As shown in Figure 1c, CoSx has a much larger ECSA of 10.6 mF cm-2 comparing with RuO2 (1.8 mF cm-2), meaning more electroactive sites of CoSx, which is believed as a contributor to its high OER performance. Meanwhile, electrochemical impedance spectra (EIS) (Figure S3) illustrates a smaller charge transfer resistance of CoSx than that of RuO2, which could be another contributor to the superior OER performance of CoSx. Meanwhile, the chronopotentiometry curve with an overpotential of CoSx at 10 mA cm-2 remains nearly constant under the long-term 5


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electrolysis reaction (Figure 1d), which exhibits very good stability for OER and exceeds that of RuO2. Such outstanding OER performance of CoSx qualifies it as the prototype for the following study.

Figure 1. (a) LSV curves of CoSx and RuO2 (insets show the TEM and HRTEM images of the as-prepared CoSx). (b) Tafel plots of CoSx and RuO2. (c) ΔJ (=Ja-Jc) of CoSx and RuO2 plotted against scan rates. (d) Stability measurement of CoSx and RuO2 with 10 mA cm-2 anodic current density in 1 M KOH electrolyte.

Normally, such remarkable durability of materials indicates the very stable chemicalelectronic structure of the catalysts in OER. Surprisingly, despite the stable OER performance of CoSx in Figure 1d, comparing with the one before OER, a significant morphological transformation is observed by SEM investigation on the post-OER CoSx (Figure 2a and 2b). After long-term OER, the initial rhombic dodecahedral CoSx particles almost disappear completely, instead, numerous nanoplates appear (Figure 2b). 6


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High angle annular dark-field images (HAADF) further confirm such morphology conversion after OER (Figure 2c and 2d). Energy-dispersive X-ray spectroscopy (EDS) mapping images show that plenty of elemental O can be detected after OER comparing with that of the as-prepared CoSx in Figure 2e and 2g, while the amorphous CoSx changes into a polycrystalline phase (Figure 2f and 2h). The above results imply that CoSx has been partially transformed into cobalt oxide and/or (oxy)hydroxide-based materials during OER process irreversibly (see the analyses follows), where the latter is very likely to be the real intrinsic active species for OER instead of the initial material of CoSx.

Figure 2. SEM, TEM and EDS mapping images of the as-prepared (pristine) CoSx and the post-OER (after) CoSx. (a, b) SEM images for the as-prepared CoSx and the CoSx after OER. (c, d) HAADF images for the as-prepared CoSx and the CoSx after OER. (e) HAADF and EDS images for the initial CoSx. (f) HRTEM image for the initial CoSx, which is amorphous. (g) HAADF and EDS images for the CoSx after OER, which show

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a highly dispersed elemental O signal. (h) HRTEM image for the CoSx after OER, which shows a polycrystalline structure. In order to further confirm this transformation, X-ray photoelectron spectroscopy (XPS) was carried out to help understanding the surface-composition and chemicalstate transformation of CoSx by OER process. As shown in Figure 3a, the XPS spectrum of CoSx before OER measurement clearly shows the peaks at binding energies of 163, 227, 532 and 798 eV, which belong to the signals of S 2p, S 2s, O 1s and Co 2p, respectively. Significantly, the signals of S 2p and S 2s disappear completely after OER measurement, suggesting that sulfide is totally removed from the surface during the electrochemical oxidation process. A detailed analysis of the high-resolution spectra of S 2p for the as-prepared and post-OER CoSx can further confirm the removal of S (Figure 3b) from the surface of the sample. The S 2p spectrum in the as-prepared CoSx can be de-convoluted into two main peaks along with a shakeup satellite, in which the peaks located at 162.3 eV for S 2p3/2 and 164 eV for S 2p1/2 agree well with the Co-S bond.27 After long-term OER operation, no peak can be identified in S 2p spectra, clearly indicative of the entire loss of S from the surface. In Figure 3c, in the as-prepared CoSx, Co 2p spectrum can be fitted into two spin-orbit doublets of Co 2p3/2 and Co 2p1/2 with the main peaks located at 778.9 eV and 793.9 eV, respectively, which are attributed to the formation of Co-S bands in the composite.28,29 Notably, after OER measurement, the peak of Co 2p shifts to higher binding energies apparently compared to those of the pre-OER ones. The pair peaks for Co 2p3/2 and Co 2p1/2 shift to 780.3 eV and 795.6 eV, respectively, which accord very well with the formation of Co-O bond in the electrocatalyst and higher valence state Co3+ in CoOOH.6 Critically, in Figure 3d, the peaks in XPS spectrum of O 1s in the as-prepared CoSx are centered at 8


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approximately 532.2 eV and 534.0 eV, which are attributed to the adsorbed H2O (H2Oads) on the surface of CoSx and liquid phase water (H2Ogas), respectively.30,31 Apart from H2Oads, two other peaks at 531.4 eV and 530.2 eV can be detected in O 1s spectrum after OER, which are corresponded to OH- and Co-O bond,32 respectively. Therefore, the XPS analysis manifests the change of the elements of the materials via OER, where the surface of the as-prepared CoSx is almost completely transformed to cobalt oxide/(oxy)hydroxide (very likely CoOOH). In addition, near edge X-ray absorption spectrum (NEXAS) data were collected on cobalt K edge for both the cobalt metallic foil (Co0) and CoSx after long-term OER, as depicted in Figure S4. Compared to the cobalt foil with a zero oxidation valence, CoSx after catalytic reaction shows an absorption edge feature at E0 = 7719 eV, with the E0 shift of 10 eV respective to the metallic cobalt foil. This observation suggests that the oxidation state of the active species contains a lot of Co3+, which is consistent with the value reported in the literature,33 and also consistent with the evidences as we obtained from electron microscopy images, EDS mapping and XPS analysis.

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Figure 3. XPS of CoSx before and after OER measurement. (a) Survey. (b) S 2p. (c) Co 2p. (d) O 1s.

Through the aforementioned ex situ measurements, we have preliminarily proven that CoSx is not the real catalyst for OER performance, which has been converted to cobalt(III) oxide and/or (oxy)hydroxide (very likely CoOOH) as the true catalytically active species. However, the information obtained from these ex situ measurements is limited, and the mechanism in this conversion process is still not clear. In order to understand the actual mechanism and active phase of CoSx reasonably, in situ technologies are highly required34,35. Toward this end, we performed a pseudo-in situ TEM experiment on the carbon grid (with the as-prepared CoSx deposited as the anode) to identify the chemical changes of CoSx during OER test (Experimental section). After 10


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anodization with a current of 10 nA in an aqueous alkali for different times, the CoSx deposited carbon grid was dried out and imaged immediately in the TEM, as shown in Figure 4a-4d. Initially, the material of CoSx exhibits a regular-shaped hollow morphology with an amorphous structure (Figure 4a), as observed previously in Figure 2. As water oxidation measurement for 2.5 min (Figure 4b), the thickness of the surface shell has an obvious increase from 45 nm to 115 nm, while the EDS mapping demonstrates an inhomogeneous distribution of the oxygen element at the inner shell of the hollow structured material. The concentration of oxygen increases continuously from 0 at.% to 60 at.% (Figure 4e) in the whole process. Meanwhile, electron energy loss spectra (EELS) clearly show a decrease for the intensity ratio of the Co-L3/Co-L2 peaks (Figure 4f), which indicates an increase of the valence state of Co during the OER measurement.36 Interestingly, the analysis of the in situ high-resolution TEM images (HRTEM) in the identical location also shows an amorphous structure of the initial material (Figures 5a and 5d), but a highly dispersed nucleation of nanocrystals in the amorphous CoSx hollow shell after 2.5 min anodization (Figure 5b and 5e). With the elongation of the electrolysis time to 12.5 min, clear lattice fringes of CoOOH emerge (Figures 5c and 5f). The detailed analysis of SAED patterns for the evolution of CoSx during OER test for 0 and 12.5 min is exhibited in Figures 5g and 5h, which is consistent with the above results, indicating the structural evolution from initial amorphous CoSx to crystalline CoOOH via Co(OH)2 intermediate.

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Figure 4. In situ TEM observation for the evolution of CoSx during the OER test. (a-d) HAADF images for the morphology evolution of CoSx after the water oxidation for different times. The corresponding color images represent the EDS elementary mapping of the electrocatalyst. (e) Evolution for the percentage of oxygen, sulfur and cobalt dependent on water oxidation measurement time. (f) EELS for CoSx during OER test.

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The OER mechanism in alkaline condition has been well recognized, which typically takes place via four elementary steps37: OH－+ * → OH* + e－

(1)

OH* + OH－ → O* + H2O + e－

(2)

O* + OH－ → OOH* + e－

(3)

OOH* + OH－ → O2 + H2O + e－

(4)

Here, metal ions play as the active sites (*) on the electrode surface.38 From the above reaction steps, the final product of oxygen is always taken along with the generation of OH* or OOH*. Due to the limited volume of the cavity in the initial CoSx hollow structure, the emission and accumulation of the oxygen gas can increase the inner concentration of the intermediate of OOH*, which erodes the inner shell of CoSx continuously to form the Co(OH)2 or CoOOH nanocrystals. And this is why we could find such a thick oxygen-rich inner shell (Figure 4b) during the oxygen evolution reactions. Finally, the shell of the particles cannot sustain the high pressure of the encapsulated oxygen gas,39 and the shell will be pierced (Figure 4d) or even crushed, eventually leading to the final nanosheet-shaped catalyst as shown in Figure 2. Combining these in situ TEM investigations and the CoOOH species finally presented in the previous sections, it is inferred that the transformation process of the species should be CoSx → Co(OH)2 → CoOOH (Figure 5a-5f), where CoOOH is the catalytic species responsible for water oxidation. This two-step transition process for CoSx into CoOOH is also confirmed by the analysis of chronopotentiometry curve of CoSx at a low anodic current density of 0.5 mA cm-2, as shown in Figure 5i. This measurement was operated directly without any catalyst activation treatment or preelectrochemical test. Significantly distinct from the stable chronopotentiometry curve 13


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at 10 mA cm-2 current density (Figure 1d), in Figure 5i, the required potential in this case increases continuously in the initial ~ 1.1 h with two plateaus, while after ~ 1.1 h the curve shows a considerable stability. Thus, the whole process can be divided to three parts: the first part (~ 0-0.6 h) corresponds to the electrochemical oxidation of CoSx to Co(OH)2 intermediate, after which the reaction process goes into the second part (~ 0.6-1.1 h), suggesting that Co(OH)2 intermediate undergoes an ion intercalation process and completes the conversion to CoOOH phase prior to reaching OER conditions. Finally, in the third part (> ~ 1.1 h s), the resulted CoOOH on the surface acts as the true catalytic species and participates the water oxidation with a very stable performance. This curve is apparently contradictory to the stable OER performance observed in chronopotentiometry curve at 10 mA cm-2 in Figure 1d. We ascribe such contradict to two plausible reasons: 1) in the stable curve of CoSx in Figure 1d, before the electrochemical test was performed, one or a few electrochemical scan cycle(s) had been done to stabilize the system (see Experimental Section), in which the transformation to CoOOH has been complete, therefore afterward the OER performance appears very stable; 2) 10 mA cm-2 current density is too high for the stable test in Figure 1d, which causes the conversion to CoOOH is too fast to observe in the chronopotentiometry curve. Therefore, we suggest that it should be necessary to re-consider the reported “stable” OER performance of cobalt sulfides in alkaline electrolyte at a relatively high current density (such as 10 mA cm-2) in literatures.

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Figure 5. (a-f) In situ HRTEM analyses for the evolution of CoSx into alpha phase CoOOH during OER. (d), (e) and (f) correspond to SAED patterns of (a), (b) and (c), respectively. (g and h) SAED patterns for the evolution of CoSx during OER test for 0 and 12.5 min. (i) Chronopotentiometry curve of CoSx at a low anodic current density of 0.5 mA cm-2. Note in this case, the as-prepared CoSx was directly used for chronopotentiometry test, no pre-electrochemical or activation treatment was applied.

Furthermore, we recorded the in situ FTIR spectra of CoSx at various time of OER (0-1000 s) at a constant catalytic current of 1 mA. As shown in Figure S5, after 200 s, 15


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the bands at 3350 cm-1 and 1630 cm-1 are increased and then stabilized as the reaction time going on. These bands can be assigned to the stretching and bending vibration of the hydrogen-bonded hydroxyl group on the surface of the catalyst, respectively,40 indicating the enhanced absorption of H2O on the surface of the catalyst during the electrocatalytic process. The increased band located at 892 cm-1 is ascribed to O-H bending vibrations for the structural hydroxyls,40 suggesting the formation of hydroxide species on the surface of the catalyst during OER. Moreover, after OER measurement for 1000 s, we removed the bias to stop the reaction and examined the FTIR again. Obviously, the bands at 3350 cm-1, 1630 cm-1 and 892 cm-1 still exist, which indicates that the structural and composition transformation of the catalyst is irreversible after OER measurement. Significantly, the enhanced absorption of H2O on the surface (the bands at 3350 cm-1 and 1630 cm-1) and the formation of hydroxides (the band located at 892 cm-1) of the catalyst during OER indicated by in situ FTIR can be ascribed to elementary steps of (1) and (3), matching well with the above investigations. Previously, in the field of supercapacitor, Zhang et al. revealed the mutual transformation between Co(OH)2 and structural-similar CoOOH in KOH electrolyte for charging/discharging process by in situ X-ray absorption spectra (XAS), which enables a battery-mimic mechanism, resulting in high specific pseudocapacitance and long cycling life.41 This investigation clearly coincides the observation in our study, although without the participation of CoSx. Importantly, in their report, the in situ XAS confirmed the increased valence state of Co during the charging process. Considering the almost same electrochemical environment (identical KOH electrolyte and similar applied potential), this result could be a strong support for our direct observation of Co(OH)2 to CoOOH during OER by CoSx. 16


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Meanwhile, CoOOH was directly synthesized6 to compare with the above CoSxderived CoOOH. The directly synthesized CoOOH has an amorphous phase (XRD, Figure S6a) with a nanosheet-assembled sphere morphology (SEM, Figure S6b). It shows good OER performance, as shown in Figure S7, where the required overpotential to achieve 10 mA cm-2 catalytic current density is only 315 mV, obviously better than that of CoSx in our case (396 mV overpotential). The better OER performance of the directly synthesized CoOOH is very likely due to its larger quantity of active CoOOH than that derived from CoSx. Interestingly, the Tafel slope of the directly synthesized CoOOH (61 mV dec-1, inset of Figure S7) is similar to that of CoSx (69 mv dec-1, Figure 1b), indicating the same type of the active species.42 Because the only supposed active species in the former is CoOOH, the similar Tafel slope can be another support for the conclusion of CoOOH as the true active species in CoSx. To extend this research to metal chalcogenides, CoSe2 was prepared according to the report.43 The XRD pattern matches well with CoSe2 with Powder Diffraction File (PDF) no. 09-0234 (Figure S8a). The as-prepared CoSe2 shows a morphology of severalhundred-nanometer sized spheres (SEM, Figure S8b), however, after OER measurement, these spheres are transformed to ~ 100 nm sized particles (SEM, Figure S8c). Importantly, the XPS data show that the Se signal disappears after OER measurement (Figure S9a), instead, the O signal increases significantly (Figure S9b), and Co 2p peaks shift to the higher binding energies belonging to Co-O bonds (Figure S9c), which phenomena are similar to CoSx in OER, indicating the transformation of CoSe2 to CoOOH on the surface during OER. It is also noted that the Tafel slope of CoSe2 (68 mV dec-1, inset of Figure S9d) is very close to that of CoSx (69 mV dec-1, Figure 1b) and directly synthesized CoOOH (61 mV dec-1, inset of Figure S7), indicating the same active species for OER again,42 i.e., CoOOH. Therefore, we can 17


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systematically confirm the structural evolution to CoOOH for metal chalcogenides (sulfides and selenides) in OER process. On the other sides, in order to investigate the effect of active sites on the transformation of CoSx into CoOOH, here, a hybrid catalyst based on CoSx and 3D Ndoped graphene foam (NGF) (denoted as CoSx/NGF) is designed to tailor the bare CoSx OER performance (Figure S10, see details in Experimental Section). The SEM and TEM images of CoSx/NGF before and after OER are investigated. Before OER, as shown in Figure S11, the morphology of rhombic dodecahedral CoSx particles homogeneously anchors on the 3D tube-like graphene scaffolds to form hybrid CoSx/NGF (Figure S11d). After OER, similar to the bare CoSx, the polyhedron CoSx particles on NGF are also converted to nanoplates and homogeneously distributed on the graphene sheets (Figure 6a and 6b). The HAADF (Figure 6c) and EDS mapping images before OER confirm the homogeneously dispersed elements of C, N, Co, O and S in CoSx/NGF (with no oxygen segregation on CoSx). After OER, the uniform-spatial distribution of Co, O, C and N elements is well demonstrated, but almost no S segregation (with oxygen segregation) on CoSx can be detected (Figure 6d). This phenomenon is very different from the ones in bare CoSx, which illustrates that the increasing for the active sites (due to the support of NGF) can enhance the transition of CoSx into CoOOH to be much more completely. The comparison of XPS of CoSx/NGF before and after OER measurement also shows the removal of S from the surface (Figure 7e) and high valence state of Co (~ 3+) of CoSx/NGF after OER (Figure S12), supporting the conclusion of CoSx to CoOOH species by OER measurement. The enhanced transition of CoSx into CoOOH induces a great improvement of OER performance with the hybrid NGF/CoSx as shown in Figure 7. Previous reports have shown that the integrating of carbon additives and cobalt sulfide can effectively 18


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increase the electroactive sites on catalyst during OER.44-46 Obviously, in all the tested samples in this study, the hybrid material of CoSx/NGF has the best OER response (Table S1) by rendering the smallest overpotential of 330 mV at 10 mA cm-2 catalytic current density (Figures 7a and 7b), highest current density of 21.5 mA cm-2 at 350 mV overpotential (Figure 7c), lowest Tafel slope of 62 mV dec-1 (inset of Figure 7a and Figure 7d), largest ECSA of 45.5 mF cm-2 (Figure 7e and Figure S13) and optimized resistance illustrated by EIS (Figure S14. More analyses and discussion on EIS can be found in Figure S15 and Table S2). CoSx/NGF also exhibits a remarkable stability with no apparent overpotential increase over 40000 s of continuous operation at 10 mA cm2

catalytic current density (Figure S16). These results reveal the positive interplay of

CoSx and NGF in CoSx/NGF for OER. Moreover, according to the inductively coupled plasma atomic emission spectrometry (ICP-AES), the content of Co in CoSx/NGF is confirmed to be ~ 11 wt.%. Assuming all the Co sites are electrochemically active, the turnover frequency (TOF) is calculated to be 0.10 s-1 at an overpotential of 300 mV, which is 1-2 orders of magnitude higher than the TOFs of the listed catalysts in Table S1 (note the TOF of CoSx/NGF in our case is underestimated because some of the Co sites cannot be electrochemically accessible). By the comparison in Table S1, indeed, CoSx/NGF exhibits comparable or even better OER performance than the most of the reported state-of-the-art Co-based OER catalysts, demonstrating its remarkable electrocatalytic activity and becomes one of the best Co-based electrocatalysts for OER.

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Figure 6. SEM and HAADF images of CoSx/NGF before and after OER. (a) SEM image of the as-prepared (pristine) CoSx/NGF. (b) SEM image of the post-OER CoSx/NGF. (c) HAADF image and EDS mapping of the as-prepared CoSx/NGF. (d) HAADF image and EDS mapping of the post-OER CoSx/NGF. (e) XPS of S in CoSx/NGF before (blue line) and after OER measurement (red line).

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Figure 7. OER performance of NGF, CoSx, CoSx/NGF and RuO2. (a) LSV curves, inset: Tafel plots. (b) Required overpotentials to achieve 10 mA cm-2 catalytic current density. NA: not available. (c) The catalytic current densities at 350 mV overpotential. (d) Tafel slopes. (e) ECSAs.

CONCLUSIONS Overall, we employed a prototype of amorphous CoSx to probe the structural evolution of metal chalcogenides in OER measurement. It is concluded that the initial CoSx is converted irreversibly to CoOOH via Co(OH)2 intermediate on the surface, which is the true intrinsic active species for the water oxidation in the OER measurement. Such transformation is well confirmed by various ex situ and in situ characterizations. Particularly, the structural evolution is clearly captured by in situ TEM in nanoscale resolution, visualizing the process of the electrocatalyst activation 21


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for OER. Furthermore, we used carbon additive of NGF to fabricate the hybrid material of CoSx/NGF, which shows much improved catalytic activity. It is observed the transformation of CoSx to CoOOH occurs more thoroughly in this hybrid material than the ones in bare cobalt sulfide, which indicates the active sites have an enhancement for the transformation process during OER. We believe this research benefits the fundamental understanding of the underlying OER mechanism, and enables further development of water splitting by low-cost electrocatalysts.

EXPERIMENTAL SECTION Preparation of ZIF-67. In a typical synthesis,24 4 mmol Co(NO3)2·6H2O was dissolved in a 40 mL methanol solution. Then 16 mmol 2-methylimidazole was dissolved in another 40 mL methanol, and dropped into the above solution followed by stirring for 30 min. After aged 20 h, the solid sample was rinsed with ethanol and dried in an oven for 12 h to obtain ZIF-67. Preparation of CoSx. In a typical synthesis process,24 10 mg of the as-prepared ZIF67 and 30 mg thioacetamide were mixed in 30 ml ethanol solution, and kept stirring for 6 h. The mixture was then transferred into an autoclave, and heated at 120 oC for 6 h. After cooling to the room temperature, the sample was collected by centrifugation and rinsed with ethanol and water for six times, and dried in vacuum at 60 oC for 12 h. Preparation of NGF. A pre-cleaned commercial polyurethane (PU) sponge was immersed into the 2 mg mL-1 graphene oxide (GO) suspension (Vwater : Vethanol = 2 : 8). The sponge was then transferred on an ethanol flame and annealed for few seconds to remove the PU sponge.47 22


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Preparation of ZIF-67/NGF. The procedure is as same as the preparation of ZIF-67, except adding ~ 2 mg NGF to the Co(NO3)2·6H2O solution. Preparation of CoSx/NGF. The procedure is as same as the preparation of CoSx except using ZIF-67/NGF instead of ZIF-67. Preparation of CoOOH. CoOOH was directly synthesized according to the previous report.6 Preparation of CoSe2. CoSe2 was synthesized according to the previous report.41 Structure and surface characterization. The morphologies of all the prepared materials were characterized by a JSM-7500 field emission scanning electron microscopy (FESEM, JEOL, Japan) and a transmission electron microscope (TEM, JEM-2100F, Japan). X-ray diffraction (XRD) patterns of the samples were conducted on a D/MAX-RB X-ray diffractometer. The X-ray photoelectron spectroscopy (XPS) measurements were conducted on an ESCALAB250xi electron spectrometer using Al Kα source (1486.6 eV) as radiation source, and the binding energies were referenced to the C 1s peak at 284.8 eV from adventitious carbon. For the post-OER XPS measurement, the materials were deposited on relatively larger conductive fluorinedoped tin oxide (FTO)-glass substrates, and the powders after OER were collected from the FTO-glass for XPS measurement, except that CoSx/NGF on FTO-glass after OER was directly used for XPS measurement with the FTO-glass substrate. The inductively coupled plasma atomic emission spectrometry (ICP-AES) was carried out on an Optima 4300 DV spectrometer (Perkin-Elmer). In situ Fourier transform infrared spectroscopy (FITR) was recorded on Bruker VERTEX 80v instrument, with a glassy carbon 23


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electrode (diameter 10 mm) loaded by the catalyst as the tested sample, Pt wire as the counter electrode and Ag/AgCl as the reference electrode. 1 mA anodic current was applied to the tested sample in 1 M KOH for 1000 s and the FTIR spectra were recorded every 200 s in situ in the process with the subtraction of the signal from the electrolyte. Electrochemical measurements. Electrochemical experiments were performed on an electrochemical work station (CHI660C Instruments, Shanghai Chenhua Instrument Crop., China) in a standard three-electrode system, where the catalysts deposited on a glassy carbon electrode (GCE) as the working electrode, a Pt plate as the counter electrode and a Ag/AgCl (saturated KCl) as the reference electrode. 1 M KOH solution was used as electrolyte for electrochemical measurements. All the applied potentials were converted to reversible hydrogen electrode (RHE), ERHE (V) = EAg/AgCl + 0.059pH + 0.197, and the overpotential was calculated according to the equation of η = ERHE – 1.23V. Working electrode was prepared as follow: 4 mg of the as-prepared catalysts were dispersed in the mixture of 0.8 mL distilled water, 0.2 mL ethanol and 50 μL Nafion solution (5 wt% in ethanol). The suspension was sonicated for 1 h to get a homogeneous ink. Then, 5 μl catalyst ink was dropped onto a freshly polished glass carbon electrode (3 mm diameter) and dried at 50 oC to obtain the working electrode. The amount of loaded catalyst was 0.21 mg cm-2. Linear sweep voltammetry (LSV) curves were firstly performed for several times until the stable curves were obtained to active the catalysts, after which the other electrochemical measurements were allowed. Polarization curves were obtained from the stable LSV with a scan rate of 5 mV s-1. Tafel slopes were obtained by plotting overpotential η against log (J) via LSV curves. 24


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Electrochemical impedance spectroscopy (EIS) was measured under bias of 350 mV with the frequency range of 0.01 Hz-105 Hz. Electrochemical active surface areas (ECSA) were determined by cyclic voltammetries at the potential window of 0.2-0.3V, and the scan rates were 20, 40, 60, 80, 100 and 120 mV s-1. The double-layer capacitance (Cdl) was evaluated by plotting the difference of charging current density (ΔJ = Ja - Jc) at 0.25 V vs. Ag/AgCl against scan rate, and the slope which is twice of Cdl is used to represent ECSA. The stability of catalysts was achieved by the chronopotentiometry at a current density of 10 mA cm-2. The turnover frequency (TOF) values of the obtained catalysts were calculated by the equation: TOF 

JA 4Fm

where J is the current density at a given overpotential η, in our cases η = 300 mV, A is the surface area of the electrode (0.071 cm-2), F is the Faraday constant (96485 C mol1),

and m is the mole number of the electrocatalyst. In our cases, since Co is more active

for OER catalysis than NGF, we assume that the Co atoms in electrocatalysts are the active sites for electrochemical reaction. X-ray adsorption spectra (XAS) measurements XAS data were acquired at Stanford Synchrotron Radiation Lightsource on beam line 2-2 with an energy range of 2-13 KeV in transmission mode. Helium filled ion chamber was used to monitor the signal. Co foil was used as an energy standard. Each sample was measured three times. The data were processed with the software EXASFPACK.

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The extracted absorption oscillations χ(E) were converted to k-space and then Fourier transformed from k-space to real-space using a k-window from 2-13 Å-1. In situ TEM The CoSx alcoholic suspension was firstly dropped on the carbon grid to prepare the TEM samples. Then the sample was clamped as the anode to perform the OER experiment in an alkaline solution of pH = 12, a continuous current was maintained at 10 nA as shown in Figure S17. After different times, the samples were dried out to take the TEM experiments. All of the in situ TEM experiments were carried out on a Titan G2 60-300 Cs-corrected TEM with the electron voltage of 300 keV, which was also equipped with a Gatan Enfina EELS system and super X-EDS system.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] *Email: [email protected] ORCID Ke Fan: 0000-0003-2269-4042 Jiaguo Yu: 0000-0002-0612-8633

Author contributions 26


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H. Zou and K. Fan. designed the experiments, H. Zou, Y. Lu, M. Sui, H. Chen, L. Tong and K. Fan co-wrote the paper. H. Zou synthesized the materials and performed the electrochemical measurements. Y. Lu and M. Sui carried out the in situ TEM and analyzed the data. H. Chen and M. F. Toney carried out the XAS measurements and discussed the results. F. Li, J. Liu and L. Sun helped with the measurement of in situ FTIR. Y. Lu, J. Yu and K. Fan led this project. All of the authors have revised the manuscript.

⊥These

authors contributed equally to the study.

The authors declare no competing financial interests.

ACKNOWLEDGMENTS We thank Chenghuan Gong and Wenlong Li in Dalian University of Technology for the help on in situ FTIR measurement. This study was partially supported by NSFC (51320105001, 21433007, 51772234 and 21573170). Furthermore, this work was financially supported by the Natural Science Foundation of Hubei Province of China (2015CFA001), the Fundamental Research Funds for the Central Universities (WUT: 2017 IVA 092), and Innovative Research Funds of SKLWUT (2017-ZD-4). Y.L. thanks the support of the National Natural Science Foundation of China (Grants No. 11704015), and L.S. thanks the K&A Wallenberg Foundation for financial support. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

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Supporting Information Available: Typical SEM observations, electrochemical, XPS, XAS and in situ FTIR characterizations; This material is available free of charge via the Internet at http://pubs.acs.org.

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