Spontaneous Delithiation under Operando Condition Triggers

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Spontaneous delithiation under operando condition triggers formation of amorphous active layer in spinel cobalt oxides electrocatalyst towards oxygen evolution Shuo Zhang, Songqi Gu, Yu Wang, Chao Liang, Yi Yu, Ling Han, Shun Zheng, Nian Zhang, Xiaosong Liu, Jing Zhou, and Jiong Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00928 • Publication Date (Web): 08 Jul 2019 Downloaded from pubs.acs.org on July 19, 2019

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Spontaneous delithiation under operando condition triggers formation of amorphous active layer in spinel cobalt oxides electrocatalyst towards oxygen evolution Shuo Zhang†,‡,*, Songqi Gu†, Yu Wang†, Chao Liang⊥, Yi Yu⊥, Ling Han†, Shun Zheng‖, Nian Zhang‖, Xiaosong Liu‖, Jing Zhou§,* and Jiong Li†,*

Shanghai Synchrotron Radiation Facility, Shanghai Advanced Research Institute,

†

Chinese Academy of Sciences, Shanghai 201204, P.R. China University of Chinese Academy of Sciences, Beijing 100049, P.R. China

‡

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai

§

201204, P.R. China ⊥School

of Physical Science and Technology, Shanghai Tech University, Shanghai

201210, P.R. China Center for Excellence in Superconducting Electronics, Shanghai Institute of

‖

Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P.R. China

KEYWORDS: electrocatalysts, oxygen evolution reaction, operando X-ray absorption spectroscopy, surface reconstruction, transition metal oxides 1 ACS Paragon Plus Environment

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ABSTRACT: Understanding the dynamic surface self-reconstruction of transition metal oxides (TMOs) under operando condition is the key for rational design of oxygen evolution reaction (OER) electrocatalysts. Herein, we aimed towards spinel Li2Co2O4, as a desirable system owing to the pure Co3+ ions in as-prepared sample, to clarify the origin of its enhanced activity during in-situ evolution. It was discovered that spontaneous extraction of lithium occurs upon the electrochemical condition to generate an amorphous active layer Li2-xCo2O4-δ(OH)δ (Co3+/Co4+), resulting in a progressively increased OER reactivity. Additionally, operando X-ray spectroscopy combined with ex-situ techniques unraveled that the appearance of Co4+ and oxidized oxygen ions (oxygen sites with electronic hole) is tightly accompanied by this surface reconstruction. Finally, density functional theory calculations further suggested a possible reconstruction pathway and addressed the importance of oxidized oxygen ions that are triggered by Co4+ ions. These new insights are helpful for understanding why high activity occurs in the system with Co4+ ions and thereby developing efficient OER electrocatalysts.

INTRODUCTION The discovery of new cost-effective and highly active catalysts for electrochemical energy conversion and storage is of prime importance for renewable energy 2 ACS Paragon Plus Environment

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production1. The oxygen evolution reaction (OER) is a critical step among the electrochemical

processes

but

is

kinetically

sluggish,

requiring

efficient

electrocatalyst to accelerate2. The usage of alkaline electrolytes creates the possibility to employ low-cost 3d transition metal (TM) hydroxides/oxides as catalysts3-5. Meanwhile, extensive efforts have been devoted to comprehending the OER mechanism in an attempt to capture the key parameters that dominate the OER activity6-7. The conventional understanding of OER catalysts on the basis of a well-defined electrode/electrolyte interface recognized that the activity of cationic redox centers substantially dominates the performance of catalysts8-12. On the other hand, anionic redox process had been proposed later, and highly-covalent TM-O bond,as well as the charge transfer energy, were assigned as the crucial parameters13-15. Recently, increasing studies have discovered the formation of amorphous surface in electrocatalysts that underwent OER process16-17. Specific to oxides, May and coworkers reported that the well-known Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF82) catalyst undergoes a quick amorphization on the surface, that is, the corner-sharing CoO6 octahedral structure transfers to the edge-sharing CoO6 octahedral structure18. Furthermore, by using advanced operando technique, Fabbri et al. unraveled a dynamic surface self-reconstruction of this system to form an amorphous (Co/Fe)OOH active layer19. Such transformation of surface structure was also uncovered in the Co3O420 and CoOx21. Importantly, Bergmann et al. recently exemplified CoOx(OH)y as an unified structural motifs for OER in various cobalt 3 ACS Paragon Plus Environment

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compounds22. Since that the in-situ generated amorphous surface layer is responsible for OER in 3d TM oxides, an emergent task is searching for the crucial parameter that affects this self-transformation. This is important for guiding rational design of highly-efficient electrocatalysts. A few efforts have already been devoted to this issue, e.g., Tong et al. reported that Jahn-Teller effect leads to surface amorphization in Mn-Ni system during OER23. However, this issue still calls for extended studies to elucidate the intrinsic mechanism and universal parameter. In the present work, we reported a comprehensive study on spinel Li2Co2O4 (Co3+) in an attempt to highlight the influence of the OER condition on the composition and surface structure, as well as the electronic configuration of cobalt ions by operando X-ray absorption spectroscopy (XAS) with ex-situ techniques. The spinel-type Li2Co2O4 (Co3+) can be synthesized at lower temperatures (below 400 °C) and possesses higher OER activity compared to its layered counterpart24-26. This material can be a desirable system for mechanism study, with well-defined crystalline structure, morphology and chemical state. We then clarified the origin of its enhanced activity during in-situ evolution and addressed the crucial role of Co4+ ions.

EXPERIMENTAL SECTION Electrodes Preparation. The preparation method of the working electrodes containing the investigated catalysts is stated as follows. Briefly, the electrocatalyst suspensions were prepared by sonicating a mixture of catalyst (5 mg), Nafion solution 4 ACS Paragon Plus Environment

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(5 wt %, 40 μL) and isopropanol (1 mL) for at least 2 h to generate a homogeneous ink. Next, 10 μL of the as-prepared catalyst ink was dropped on the commercial carbon fiber paper (CFP) to yield an approximate catalyst loading of 0.25 mg cm-2 and was left to dry for the OER tests. For the structural measurement of the cycled catalysts, the pristine Li2Co2O4 was drop casted onto commercial carbon fiber paper substrates. The as-prepared catalyst ink was dispersed on 22 cm2 carbon fiber paper. The electrode was treated using cyclic voltammetry (CV) cycled between -0.1 and 0.7 V (versus Hg/HgO) at a sweep rate of 5 mV s-1 for a given number of cycles in O2-saturated 1 M KOH. Electrochemical Measurement. All of the electrochemical tests are performed in a standard three-electrode setup. Carbon paper and Hg/HgO (1 M KOH) were used as the counter and reference electrodes, respectively. The potentials are referenced to a reversible hydrogen electrode (RHE) using the following equation: ERHE = EHg/HgO + 0.197 V + 0.059pH. The electrolyte was a 1 M KOH aqueous solution (99.99% metal purity, pH 13.5), which was saturated with O2 for ~ 30 min prior to each test and maintained under O2 atmosphere throughout the test. For operando XAS measurements, a Hg/HgO (1 M KOH) electrode and a platinum wire were used as the reference and counter electrodes, respectively. Linear sweep voltammetry (LSV) curves (also called polarization curves) were performed using a PGSTAT302N (Metrohm Autolab) electrochemical system with a scan rate of 5 mV s-1, and no activation was used before recording the polarization 5 ACS Paragon Plus Environment

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curves. All potential values were iR-corrected to compensate for the effect of the solution resistance and were calculated by the following equation: EiR-corrected = E - iR, where i is the current, and R is the uncompensated ohmic electrolyte resistance (~0.33 Ω) measured via high frequency ac impedance in O2-saturated 1 M KOH. Electrochemical impedance spectroscopy (EIS) was recorded at 1.60 V vs. RHE with frequencies ranging from 100 kHz to 0.1 Hz under an AC voltage of 5 mV. Then, the samples underwent different CV cycles, labeled as LCO-1, LCO-10 and LCO-20, respectively, were collected for further ex-situ investigations. Characterizations. The crystalline phase of the Li2Co2O4 catalysts before and after the CV test were recorded by grazing incidence X-ray diffraction (XRD) on a D8-Advance Bruker-AXS diffractometer using Cu Kα irradiation (1° incident angle, 0.02° step size, 1 s/step). Raman spectroscopy was carried out using a Horiba XploRA confocal Raman microspectrometer with a 532 nm laser. The laser power was adjusted to a very low value (0.2 mW) to prevent the structural destruction, and the data were recorded with an acquisition time of 600 s. The morphology of the synthesized Li2Co2O4 samples was observed by high-resolution transmission electron microscopy (HRTEM) on a Tecnai G2 F20 S-TWIN microscope operating at 200 kV with a point-to-point resolution of 0.19 nm and high-angle annular-dark-field (HAADF)-STEM using aberration corrected Titan G2 60-300 microscope operated at 300 kV. The STEM convergence semi-angle was approximately 21.4 mrad. The spatial resolution of STEM is ∼0.07 nm, which offers an unprecedented opportunity to probe structures with a sub-angstrom resolution. The chemical composition of these 6 ACS Paragon Plus Environment

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materials was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) analysis using a Spectrometer-System Spectro Arcos SOP. X-ray absorption fine structure (XAFS) data were collected at BL14W1 beamline at the Shanghai Synchrotron Radiation Facility (SSRF) with a Si(111) double-crystal monochromator. The electron beam energy of the storage ring was 3.5 GeV, and the maximum current was approximately 250 mA. The energy calibration was performed using a Co foil (7709 eV). Co K-edge extended X-ray absorption fine structure (EXAFS) data were analyzed using the standard procedures in Demeter27. Theoretical EXAFS data were calculated using FEFF 9.028. For EXAFS data fitting, the Fourier transform (FT) window for the k space was of the Hanning type in the range of 3−13.6 Å-1 and for the R space of the Hanning type in the range of 1−3 Å. The amplitude reduction factor S02 was determined by fitting the Co foil. The soft X-ray absorption spectra measurements were performed at BL02B beamline of SSRF. All the spectra were recorded in the total electron yield (TEY) mode with the photon energy resolution of about 0.3 eV. First-principles calculations. The structure optimization and density-of-state (DOS) calculations were carried out using the Vienna ab initio simulation package (VASP)29.

The

Hubbard-U

model

was

applied

with

the

revised

Perdew-Buerke-Ernzerhof exchange-correlation functional (RPBE)30, and the value of Ueff (=U-J) for Co was set to 3.3 eV according to the previous work31. The electronic

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wave functions were expanded using a plane-wave basis set with an energy cutoff of 500 eV, and the Brillouin zone was sampled in a 5×5×5 mesh.

RESULTS AND DISCUSSION In-situ electrochemical activation and the evolution of OER performance of Li2Co2O4 as a function of lithium content. Powder samples of nanosized spinel Li2Co2O4 (LCO) were readily prepared by a sol-gel reaction approach using a previously published procedure24. The pristine LCO was dropped onto commercial carbon paper substrate as electrode, which was then exposed to CV scans in O2-saturated 1 M KOH. The samples that underwent different CV cycles were denoted as LCO-N (N is the cycle number), and were collected for further ex-situ investigations. With the aim of assessing the change in the electrochemical performance, LSV measurements were performed in O2-saturated 1 M KOH. The results were summarized in Table S1. The potential required to achieve a cathodic current density of 10 mA cm-2 continuously decreases during the first 20 CV cycles, as plotted in Figure 1A and Figure S1. In particular, LCO-20 requires an overpotential of 361 mV (vs. RHE), whereas the pristine sample requires 383 mV. The corresponding Tafel plots and EIS curves displayed in Figure S2 and Figure 1A, respectively, also reveal that LCO-20 possesses a smaller Tafel slope of 46 mV dec−1 and charge-transfer


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resistances (Rct) of 1.43  cm2 compared to those of the pristine sample. These observations demonstrate the enhanced activity along with the in-situ evolution.

Figure 1. (A) Polarization curves of Li2Co2O4 in 1 M KOH for LCO-1 and LCO-20. EIS recorded at 1.60 V vs. RHE are shown inset. (B) The Li/Co composition ratio (red) and overpotential at 10 mA cm-2 (blue) as a function of CV cycle numbers.

In an attempt to comprehend how electrochemical treatment boosts the OER activity of the LCO, the representative samples were first examined by ICP-OES, as presented in Figure 1B (represented by red squares). The composition of the pristine sample is in good agreement with the chemical formula of Li2Co2O4. Nevertheless, for the sample that underwent CV scans, the Li/Co ratio gradually decreases with increasing cycle number. About half of the lithium ions disappear in the fully activated LCO-20 sample (Table S2). Further cycling does not change the composition. In addition, we also conducted the experiment by using different scan rate and found a similar delithiation process (Table S3). To expose the impact of 9 ACS Paragon Plus Environment

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lithium extraction on the electrochemical performance, we also plotted the overpotential at 10 mA cm-2, as presented by blue squares in Figure 1B. Obviously, within the 20 cycles, the change in the OER activity as a function of cycle number follows the same trend observed in the lithium content. In terms of these results, we established a close correlation between the enhancement of OER activity and decreased lithium content. Interestingly, it was found that the overpotential dramatically decreases in the initial delithiation stage and then slowly lowers when Li/Co ratio is below 0.8. We speculated two causes for this observation. First, the ICP-OES result shows the total change of lithium content includes both interior and surface region, whereas only the surface region plays the role of active layer for OER. Second, spontaneous delithiation possibly induces profound change of the electrocatalysts, i.e., the oxidation of cobalt ions and surface reconstruction. Therefore, there may be a more essential factor dominating the OER activity. In-situ evolution of local atomic and electronic structure. Attempting to track the evolution of local atomic structure and oxidation state of the Co ions under electrochemical conditions, we performed operando XAS. The electrochemical cell is displayed in Figure S3. We repeatedly collected Co K-edge data under a constant voltage of 1.60 V, as plotted in Figure 2. The main peak near 7730 eV, which corresponds to the electronic transition 1s-4p, shows a systematic positive shift and decreased intensity with increasing of reaction time. This suggests an increase in the oxidation of cobalt. The pre-edge peak near 7710 eV (Figure 2A), which is originated from transition from 1s core level to Co 3d states hybridized with Co 4p states, 10 ACS Paragon Plus Environment

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exhibits a positive shift accompanied by an increased intensity. This result clearly illustrates an increase in Co valence and a distortion of CoO6 octahedron32. This conclusion is further supported by the change of the feature at 7720 eV, which arises from an electric dipole-allowed transition along with a so-called “shakedown” (or simple core–hole screening) process of ligand to metal charge transfer giving as the final electronic state33. The slow disappearance of this screening reflects the increased distortion of the CoO6 octahedron. The local atomic structure of the Co ions could be assessed quantitatively from the Fourier-transformed EXAFS fitting (Table S4). The Co-O distance in the pristine Li2CoO4 sample is 1.92 Å. In contrast, the average Co-O distance decreases to 1.89 Å under the OER condition, implying the presence of cobalt ions with higher valence states. The second peak located at ~2.83 Å is consistent with the Co–Co distance for pairs of Co ions connected via di-μ-oxo bridges. In addition, the value of Debye-Waller factor (σ2Co-Co) increases, indicating that the structure of catalyst becomes disordered during the OER reaction. Overall, operando XAS captured an increase of the Co valence state and distortion of CoO6 octahedron along with increase of the reaction time. Combined with the finding of spontaneous delithiation, we found the “pre-catalyst” nature of spinel Li2Co2O4, and further exploring the active species is essential. More importantly, the corresponding chronoamperometry data with this operando XAS showed in Figure S4 indicates that the current density gradually increases within 1 h. This also links the enhanced OER activity with the raising of cobalt oxidation state and the change of local structure.


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Figure 2. (A) Co K-edge XANES of Li2Co2O4 electrodes with open circuit voltage (OCV) and at 1.60 V vs. RHE after 20 min and 60 min in 1 M KOH. The inset shows the pre-edge peaks. (B) Fourier transform of the corresponding Co K-edge EXAFS.

The crystalline structure and morphology of activated catalyst. The ex-situ XRD was employed to examine whether there are changes in the crystalline structure during the CV scans, as shown in Figure S5. The diffraction pattern of the pristine sample can be well indexed to a standard Fd-3m symmetry (JCPDS No. 01-080-2159). For the LCO-10 and LCO-20 samples, the patterns of both are consistent with that of the pristine material, and no peaks of impurity phases were observed except for the reflections from the carbon paper substrate. As illustrated, the overall crystalline phase of Li2Co2O4 remains unchanged after electrochemical treatment, although delithiation occurs. To rule out the impurity phase, such as Co3O4 and layered LiCoO2, which have been found in the sample exposed to certain electrochemical processes, we measured the Raman spectra. As shown in Figure S6, the data obtained from all 12 ACS Paragon Plus Environment

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samples display four Raman peaks at 450, 487, 590, and 608 cm−1, corresponding to the A1g+Eg+2F2g active modes of the spinel structure34. No impurity phase was detected. TEM together with HAADF-STEM images provide direct evidence of changes in the morphology in the near-surface regions of the sample after CV scans. As revealed by the TEM images in Figure 3A, the as-synthesized nanoparticles present a cubic shape, and the average size is approximately 20 nm. Figure 3B-3D plot the HAADF-STEM images of pristine Li2Co2O4, LCO-10 and LCO-20, viewed down the [110] zone axis. The atomic arrangement in the as-prepared nanoparticles matches the expectation for the spinel lattice with the space group of Fd-3m, where Co and Li ions occupy the 16d and 16c octahedral sites (inset of Figure 3C). The HRTEM data (Figure S7) and HAADF-STEM images (Figure 3D) reveal an amorphization of surface region upon full activation. The BET measurement indicated that the specific surface area of the electrocatalysts nearly remained after CV treatment (Figure S8). Consequently, we could conclude that the enhancement of OER activity arises from the increased intrinsic activity, which is directly associated with this amorphous layer.


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Figure 3. (A) TEM image of pristine Li2Co2O4. HAADF-STEM images and the inset fast Fourier transforms (FFTs) of (B) pristine Li2Co2O4, (C) LCO-10 and (D) LCO-20 viewed down the [110] zone axis. Intensity profile along the arrowed line is inset in (B) (the Co-Co and Co-Li layers alternately). The unit cell Li2Co2O4 arrangements are overlaid on the image in (C). The blue, purple and red atoms are Co, Li and O, respectively.

The electronic structure of activated catalyst. The oxidation state of cobalt ions at surface was examined by the postmortem soft XAS experiment at the Co L2,3-edge, which has a typical probing depth of 2-5 nm35. Figure 4A displays the spectra of LCO, LCO-10 and LCO-20, as well as those of CoO, LaCoO3 and SrCoO3 serving as the 14 ACS Paragon Plus Environment

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divalent, trivalent and tetravalent references, respectively. The spectra are dominated by the Co 2p core-hole spin-orbit coupling, which splits the spectrum roughly into two parts, namely, the L3 (hν≈782 eV) and L2 (hν≈796 eV) white lines regions. From CoO to LaCoO3 and further to SrCoO3, the spectral weight center shift to the higher energy by more than 1 eV, reflecting the Co L2,3-edge XAS is very sensitive to the Co valence state. The lowest energy of peak A in CoO is the spectral characteristics of Co2+ ions. The absence of feature A in LCO, LCO-10 and LCO-20 excludes Co2+ impurity. The same energy position of the main peak and the very similar multiplet spectral feature of Li2Co2O4 as those of LaCoO3 indicate a Co3+ state. The sharp peak C at lower energy is the spectral characteristics of Co4+ ions and is related to the transition from the 2p core level to one hole in the t2g state. The positive energy shift of the main peak B and the appearance of peak C in the LCO-10 and LCO-20 samples, as seen in Figure 4B, show the occurrence of Co4+ ions in these samples. Although previous investigations showed the formation of Co3O4 on the surface under the OER condition through Raman spectroscopy26, our work using a surface-sensitive technique did not probe the Co2+ ion signal. The average valence state in LCO-20 was determined to be ~+3.3 by the fitting of Co L2,3-edge data (Figure S9). Overall, the Co L2,3-edge XAS revealed that the content of Co4+ ions increases along with the CV scans, agreeing well with the operando XAS result.


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Figure 4. (A) Co L2,3-edge XAS spectra of Li2Co2O4 samples. The references CoO (high spin, Co2+), LaCoO3 (low spin, Co3+) and SrCoO3 (low spin, Co4+) spectra are shown for comparison. Comparison of (B) the Co L3-edge XAS and (C) O K-edge XAS spectra of the pristine and cycled Li2Co2O4 samples.

The O K-edge XAS directly reflects the unoccupied 2p states and hence illustrates the oxygen valence state. The intensity of the pre-edge peaks in the O K-edge XAS greatly increases, with a decrease of as much as ~2 eV in the energy position of the pre-edge peaks representing the lattice oxygen oxidation. Actually, the oxidized oxygen species have been proposed to play a significant role in OER. For instance, the previous work speculated the presence of peroxo-like (O2)n- species with O-O bond13. Our O K-edge XAS data suggest an alternative insight. It is well known that 16 ACS Paragon Plus Environment

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the pre-edge features arise from electronic transitions from the O 1s core level to the unoccupied O 2p levels hybridized with the transition metal 3d levels36,37. Accordingly, this oxygen 2p hole is ascribed to the covalent bond of Co-O. Since that this hole is associated with ligand metal ions, it could be denoted as ligand oxygen hole (LOH), fundamentally differing from that in the O-O bond. We could see that this LOH depends on the valence state of cobalt ion. For instance, in the pristine Li2Co2O4, a symmetric and intense absorption peak γ represents the hybridization of unoccupied oxygen 2p level with the unoccupied Co3+ 3d-eg orbital. Furthermore, the intensity of the γ peak decreases abruptly after the CV scans, and two additional well-resolved absorption peaks (α and β) appear in the region lower than the threshold energy. These peaks are assigned to transitions to the O 2p states mixed with the t2g and eg orbitals of the Co4+ ions, respectively. As a result, we identified that the oxidized oxygen is originated from the increased LOH. The underlying mechanism that high-valence transition metal ions enhance the LOH will be discussed below. Possible reconstruction pathway to produce amorphous active layer. Our experimental data conclusively linked a spontaneous delithiation with the generation of amorphous active layer and in turn the enhanced OER activity. In fact, the in-situ generation of amorphous layer in TMOs during OER has been repeatedly reported and has been identified as (oxy)-hydroxides22. With respect to our sample (LCO-20), the similar observation can be revealed by two surface-sensitive techniques. First, the features at high-energy region at O K-edge XAS arises from the photoelectron multiple-scattering effect38, reflecting the information of local structure. As shown in 17 ACS Paragon Plus Environment

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Figure S10 and Figure S11, the feature ε located at 552.5 eV presents in the LCO-20, showing that the surface of these catalysts contains hydroxyl. Second, the O 1s X-ray photoelectron spectroscopy (XPS) shows the presence of hydroxyl on the surface, as displayed in Figure S12. Through these combined characterizations, we confirmed the amorphous layer as LixCo2O4-δ(OH)δ. In terms of these findings, we further studied the possible underlying mechanism for generation of this amorphous (oxy)-hydroxides layer. Fabbri et al. recognized that the lattice oxygen evolution reaction (LOER) accounts for the dynamic surface self-reconstruction of BSCF8219. Moreover, Grimaud et al. experimentally found that the oxygen in O2 gas is derived from lattice oxygen by using in-situ

18O

isotope

labelling mass spectrometry39. These observations clearly suggest that the loss of lattice oxygen is key step for generation of amorphous layer. Given the strong correlation between appearance of amorphous layer and delithiation, we employed the density functional theory (DFT) calculations to study the formation energy of oxygen vacancies as a function of lithium content, as plotted in Figure 5. The finding illustrate that the formation energy of oxygen vacancies rapidly decreases along with the decreasing of lithium content. In particular, this value decreases to ~1 eV when the lithium ions were fully extracted. This result confirms that spontaneous formation of oxygen vacancies in the low-lithium phase is energy favorable. Thereby, we deduced a possible reconstruction pathway. The lithium ions are dissolved into the electrolyte and consequently result in oxygen vacancies. Afterwards, electro-driven filling of these vacancies by hydroxyl leads to the surface layer containing hydroxyl, which acts 18 ACS Paragon Plus Environment

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as a catalytically active surface for OER. This self-reconstruction could be described by the equation (1):

𝐿𝑖2𝐶𝑜2𝑂4 + 𝛿𝑂𝐻 ― → (2 ― 𝑥)𝐿𝑖 + + 𝐿𝑖𝑥𝐶𝑜2𝑂4 ― 𝛿(𝑂𝐻)𝛿 +

𝛿

― 2𝑂2 + (2 + 𝛿 ― 𝑥)𝑒

(1)

Figure 5. The formation energy of oxygen vacancies (red) and the content of unoccupied oxygen 2p states (blue) as a function of lithium content in LixCo2O4.

The role of Co4+ ions in surface reconstruction and OER activity. The DFT calculation indicated that delithiation could result in oxygen vacancies. In this regard, we further calculated the projected density of states (PDOS) to clarify the underlying correlation. As shown in Figure 6B, it was found that the significant unoccupied oxygen 2p states in the full delithiation state Li0Co2O4. This indicates that delithiation-induced holes also occupy the O 2p orbitals rather than being completely 19 ACS Paragon Plus Environment

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charge compensated via oxidation of cobalt ions, leading to oxidized oxygen ions. This occurrence is similar to the recent observation on the high-capacity Li/Na ion battery, and available results show that the excess oxygen holes will result in thermal instability of the system and could be eliminated via the release of O240-42. This finding is consistent with the previous observation that moving O 2p band center close to the Fermi level is necessary to induce amorphization18,43. On the other hand, the primary effect of delithiation is generating Co4+ ions, which should contribute to this self-reconstruction. This can also be seen in the previous observations, e.g., BSCF82 shows the remarkable amorphous layer, while LaCoO3 exhibits well-defined surface after OER18. As shown in Figure 6B, it is worth noting that the overlap of Co 3d and O 2p states near the Fermi level remarkably enhances in Li0Co2O4, which definitely accounts for the appearance of O 2p holes. This can be interpreted with the scheme suggested by Zunger et al.44, as shown in Figure 6D. The interaction between Co 3d and O 2p orbitals induces bonding levels below the valence band maximum (VBM) and antibonding levels above the Fermi level. The relative energy position between cobalt and oxygen can be deduced from the charge transfer energy. When Co 3d level is above the O 2p level, the bonding level has strong O 2p character; meanwhile, the antibonding level is dominated by the Co 3d orbital component. In contrast, the scenario for the Co4+ orbital is reversed because it shifts down below the O 2p level due to the negative charge transfer energy 45. As a result, the O 2p level will be pushed up resulting in an upshift of the O 2p center and an increase in oxygen holes. Combining DFT calculation with previous experimental 20 ACS Paragon Plus Environment

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findings, we suggested that the role of Co4+ ions in production of amorphous layer is activating oxidized oxygen ions. Our experimental data identified a LixCo2O4-δ(OH)δ (Co3+/Co4+) active layer towards OER, consistent with the recent reports that high-valence metal ions have notable contributions to OER activity46-48. Accordingly, we constructed a H1.4Co2O4 model to simulate the surface active species and calculated its electronic structure, as potted in Figure 6C. It was found a high-covalent Co-O bond and significant O 2p state near the Fermi level. This electronic structure agrees well with the requirement of anionic redox process15,49,50, and accordingly implies that Co4+-O atomic motif potentially acts as active sites in consistent with previous investigation51.

Figure 6. Projected density of states of (A) Li2Co2O4, (B) Li0Co2O4 and (C) H1.4Co2O4. (D) Schematic illustration of energy level for Co-O interaction. (E) 21 ACS Paragon Plus Environment

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Relationship between ligand oxygen holes and the Co–O covalent bond. The red and blue semicircles represent the O 2p and Co 3d states, respectively. As the oxidation state of Co is increased through delithiation, the Fermi level decreases into the Co 3d/O 2p band, creating ligand holes. Oxygen is released from the system resulting in oxygen vacancies.

CONCLUSIONS In conclusion, we reported on the compositional and structural evolution of crystalline spinel-type Li2Co2O4 (Co3+) material during the OER by using advanced operando X-ray spectroscopy combined with ex-situ techniques. It was discovered that along with the increasing number of CV cycles, the OER activity gradually enhances accompanied by a spontaneous delithiation. Importantly, our experimental data conclusively linked this delithiation with the generation of amorphous active layer and thereby the OER activity. Then, we tried to search for the fundamental electronic parameter accounting for this surface reconstruction. It was found that concurrent appearance of Co4+ and oxidized oxygen ions with amorphous active layer, indicating their intrinsic correlation. The DFT calculation definitely indicated that delithiation remarkably decrease the formation energy of oxygen vacancies. In terms of these experimental and theoretical findings, we suggest a possible reconstruction pathway including: (1) delithiation leads to Co4+ and oxidized oxygen ions; (2) O2 evolution derived from these oxidized lattice oxygen leaves oxygen vacancies; (3) 22 ACS Paragon Plus Environment

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electro-driven filling of these oxygen vacancies by hydroxyl from electrolyte. The further calculation revealed the critical role of Co4+ in this process is activating oxidized oxygen through highly-covalent Co4+-O bond.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Additional electrochemical data, XRD, Raman spectra, TEM, SEM, additional ex-situ XAS data of Co K-edge, Co L-edge and O K-edge, and the fitting results of EXAFS. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grants 11575280, 11775296, 11305250 and 11405252), the Joint Funds of the National Natural Science Foundation of China (Grant U1232117) and Youth Innovation Promotion Association of the Chinese Academy of Sciences (2015212 and 2017310). REFERENCES (1) Chu, S.; Majumdar, A., Opportunities and challenges for a sustainable energy future. Nature 2012, 488, 294-303. (2) Koper, M. T. M., Hydrogen electrocatalysis: A basic solution. Nat. Chem. 2013, 5, 255-256. (3) Suen, N.-T.; Hung, S.-F.; Quan, Q.; Zhang, N.; Xu, Y.-J.; Chen, H. M., Electrocatalysis for the oxygen evolution reaction: recent development and future perspectives. Chem. Soc. Rev. 2017, 46, 337-365. (4) Faber, M. S.; Jin, S., Earth-abundant inorganic electrocatalysts and their nanostructures for energy conversion applications. Energy Environ. Sci. 2014, 7, 3519-3542. (5) Chen, D.; Chen, C.; Baiyee, Z. M.; Shao, Z.; Ciucci, F., Nonstoichiometric Oxides as Low-Cost and Highly-Efficient Oxygen Reduction/Evolution Catalysts for Low-Temperature Electrochemical Devices. Chem. Rev. 2015, 115, 9869-9921. (6) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Nakanishi, H.; Goodenough, J. B.; Shao-Horn, Y., Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal–air batteries. Nat. Chem. 2011, 3, 546-550. (7) Urmimala Maitra, B. S. N., A. Govindaraj, and C. N. R. Rao, Importance of trivalency and the eg1 configuration in the photocatalytci oxidation of water by Mn and Co oxides. Proc. Natl. Acad. Sci. 2013, 119, 11704-11707. (8) Zhou, S.; Miao, X.; Zhao, X.; Ma, C.; Qiu, Y.; Hu, Z.; Zhao, J.; Shi, L.; Zeng, J.: Engineering electrocatalytic activity in nanosized perovskite cobaltite through surface spin-state transition. Nat. Commun. 2016, 7, 11510. (9) Chen, J.; Selloni, A.: Water Adsorption and Oxidation at the Co3O4 (110) Surface. J. Phys. Chem. Lett. 2012, 3, 2808-2814.


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Table of Contents.

Operando X-ray absorption spectroscopy captured a spontaneous de-lithiation and the formation of amorphous active layer in spinel cobalt oxides electrocatalyst towards oxygen evolution.


Spontaneous Delithiation under Operando Condition Triggers

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