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Letter

In-Situ Spectroscopic Identification of µ-OO Bridging on Spinel CoO Water Oxidation Electrocatalyst 3

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Hsin-Yi Wang, Sung-Fu Hung, Ying-Ya Hsu, Lulu Zhang, Jianwei Miao, Ting-Shan Chan, Qihua Xiong, and Bin Liu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b02147 • Publication Date (Web): 13 Nov 2016 Downloaded from http://pubs.acs.org on November 15, 2016

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The Journal of Physical Chemistry Letters

In-Situ Spectroscopic Identification of µ-OO Bridging on Spinel Co3O4 Water Oxidation Electrocatalyst Hsin-Yi Wang,† Sung-Fu Hung,‡ Ying-Ya Hsu,§ Lulu Zhang,∥ Jianwei Miao,† Ting-Shan Chan,§ Qihua Xiong,∥ and Bin Liu*,† †

School of Chemical and Biomedical Engineering, Nanyang Technological University, Block

N1.2, 62 Nanyang Drive, Singapore 637459, Singapore. ‡

Department of Chemistry, National Taiwan University, Taipei, 106, Taiwan, Republic of China

§

National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan, Republic of China

∥Division

of Physics and Applied Physics, School of Physical and Mathematical Sciences,

Nanyang Technological University, Singapore 637371, Singapore

ABSTRACT

The formation of µ-OO peroxide (Co-OO-Co) moieties on spinel Co3O4 electrocatalyst prior to the rise of the electrochemical oxygen evolution reaction (OER) current was identified by in-situ spectroscopic methods. Through a combination of independent in-situ X-ray absorption, grazingangle X-ray diffraction, and Raman analysis, we observed a clear coincidence between the formation of µ-OO peroxide moieties and the rise of the anodic peak during OER. This finding implies that a chemical reaction step could be generally ignored before the onset of OER current. 1 ACS Paragon Plus Environment


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More importantly, the tetrahedral Co2+ ions in the spinel Co3O4 could be the vital species to initiate the formation of the µ-OO peroxide moieties.

Table of Content (TOC) graphic:

Electrochemical water splitting provides an attractive strategy to produce hydrogen as a source of renewable energy.1-4 Ideally, water electrolyzer can produce hydrogen gas on the cathode and oxygen gas on the anode. Unfortunately, there is a significant energy penalty at the anode due to the thermodynamically unfavorable reaction in water oxidation that involves a stepwise fourelectron transfer. Spinel Co3O4 is a potent electrocatalyst for water oxidation composed of one Co2+ ion in the tetrahedral site (Co2+Td) and two Co3+ ions in the octahedral site (Co3+Oh).5-7 The spinel structure of Co3O4 will be transformed to layered oxyhydroxide (CoOOH) by means of oxidation of cobalt ions to higher valence states under positive bias.8-12 This step is crucial where the newly formed oxyhydroxide is considered as the active species for OER.13 However, in our previous study,14 the in-situ X-ray absorption (XAS) measurement revealed the valence states of cobalt ions in calcined Co3O4 during water oxidation were quite stable. More interestingly, the

2 ACS Paragon Plus Environment

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anodic peak originally ascribed to the oxidation of cobalt ions in the cyclic voltammetry (CV) scan completely vanished if the tetrahedral Co2+ ions were substituted by OER-inactive Zn2+ ions.14,15 This phenomenon has attracted our attention to understand the underlying chemical reaction of this anodic peak prior to the rise of the OER current. To instantaneously probe the variation of the chemical environment on the Co3O4 electrocatalyst, we adopted a metal-air battery cell configuration (Figure S1)16 as the platform for in-situ XAS (Figure S2) and grazing-angle XRD (Figure S3) studies. The Co3O4 powder (calcined at 800 oC) was drop-coated onto PTFE treated graphite paper as the cathode. The voltage versus current plot shows the discharging period where the oxygen reduction reaction (ORR) takes place and the charging period where OER takes place on Co3O4, represented by the negative and positive current regions, respectively (Figure 1a ). While charging the cell, a typical linear sweeping curve with an anodic peak at 1.89 V vs. Zn/Zn(OH)42- (or 1.45 vs. RHE, refer to the Supporting Information for the details of voltage conversion) and an OER current onset voltage at 2.01 V vs. Zn/Zn(OH)42- were developed. The declined performance at the high voltage (> 2.15 V vs. Zn/Zn(OH)42-) was due to the restricted amount of the reactants in terms of limited coverage of surface intermediates.17 The anodic peak prior to the rise of the OER current was generally attributed to the increase in valence state of cobalt ions (e.g., Co2+/Co3+ or Co3+/Co4+ redox couple) with an expected phase transformation (e.g., Co3O4(s) → CoOOH(s) or CoOOH(s) → CoO2)(s)).8-12,18,19 However, as depicted in Figure 1b, our in-situ grazing angle XRD results indicate that the intensities and positions of the main diffraction peaks of Co3O4 were nearly unaffected during the entire ORR and OER processes, suggesting a stable cubic-spinel structure of calcined Co3O4. Figure 1c displays the Fourier transformed extended X-ray absorption fine structure (EXAFS) spectra obtained from the k3-weighting EXAFS spectra (Figure S4a and b).



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It reveals that the interatomic distances of Co3+Oh and Co2+Td to their neighboring metal atoms in Co3O4 remain nearly unchanged in the voltage range between 0.9 to 2.3 V vs. Zn/Zn(OH)42-, where the intensity ratio between the Co-Co(Oh) peak, IOh, to the Co-Co(Td) peak, ITd, was estimated to be IOh/ITd = ~ 1.42 during the entire process. The corresponding Co K-edge X-ray absorption near edge structure (XANES) spectra (Figure S4c and d) give the edge-jumping positions, which show no obvious variation in cobalt valence states under different biases, yet a slightly expanded/compressed Co-O bond can be noticed during ORR/OER process (Figure 1c), suggesting some negative/positive charges are accumulated on the cobalt centers with increasing/decreasing bias. Furthermore, the high-resolution transmission electron microscopy (HRTEM) images indicate that nearly no layered CoOOH structure can be found on the Co3O4 surface after the XAS measurement (Figure S5).


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Figure 1. In-situ grazing-angle X-ray diffraction and X-ray absorption spectroscopy measurements. (a) The linear scanning voltammetry (LSV) curve of Co3O4, where the voltage at open circuit (VOC) was 1.52 V vs. Zn/Zn(OH)42- at the beginning. (b) X-ray diffraction patterns recorded following the sequence from 1→2 (ORR condition) then 3→4 (OER condition) as indicated in the LSV curve. (c) Co K-edge EXAFS spectra measured following the sequence from C1→C2→C3 (ORR condition) then A1→A2→A3→A4 (OER condition) as indicated in the LSV curve, in which the voltage is in the scale vs. Zn/Zn(OH)42-. The yellow star marks the position of oxidation peak of Co3O4 prior to the rise of OER current.



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Our in-situ XAS and XRD results seem incomparable with the current literature,8,9,20-22 in which the spinel Co3O4 should be converted to layered CoOOH based on the Co-H2O Pourbaix diagram. However, it is worth noting that the electrochemically deposited (ED) cobalt oxide (denoted as CoOx)21,22 did not go through the Co3O4 phase transition with bias increasing as indicated in the Co-H2O Pourbaix diagram, where Co3O4 should be present as the intermediate phase between cobalt hydroxide and cobalt oxyhydroxide.19 James B. et al. revised the Pourbaix diagram due to lack of a characteristic signal of tetrahedral Co2+ in their in-situ EPR spectra of the electrochemically deposited CoOx, suggesting that CoOx could not be electrochemically converted to Co3O4.22 To reconcile the stable structure of high-temperature calcined Co3O4 with the adjustable structure of CoOx, we propose that the properties of spinel Co3O4 should be distinct from CoOx. The Co-H2O Pourbaix diagram maps out the thermodynamic equilibrium of Co ions in an electrochemical aqueous system, in which only activities of free Co ions are considered.19 However, the Co ions in solid spinel Co3O4 are strongly bonded with O anions. Additionally, the Pourbaix diagram generally ignores the kinetics. Besides, the cobalt oxide prepared by the wet method such as ED8,20-22 or chemical bath deposition (CBD)9 without thermal annealing could contain some impurities (e.g., rock-salt CoO) or allow certain amounts of hydroxyl groups and/or water molecules to remain inside the bulk, making them flexible to undergo bulk redox activity and therefore phase transform.23 In a recent simulation work, the reaction of the intermediates toward OER was directly studied on a pure spinel Co3O4 surface where the pure spinel Co3O4 surface can still serve as a good model to represent the actual processes.24 In addition, the inserted crystal water is found to be critical for the structural transition from spinel structure, e.g., Co3O4, to layered oxyhydroxide.25 On the contrary, it


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usually requires a high-temperature treatment to convert the cobalt based hydrates to the pure spinel Co3O4.26,27 To gain in-depth understanding of the anodic peak prior to the OER onset, a series of electrochemical experiments were thus carried out. Nickel oxide (NiOx) was selected as the control to demonstrate the electrochemical behavior of the phase-changeable electrocatalyst. NiOx exhibits a reversible wave in the CV scan resulting from the Ni redox process (Ni2+/Ni3+) in an alkaline solution along with phase transformation from rock-salt structure to layered Ni(OH)2/NiOOH structure.23,28 The NiOx and Co3O4 grown on FTO (F:SnO2) substrate were directly used as the electrode (Figure S6). Notably, a long-time voltammetric cycling was required to achieve complete NiOx → Ni(OH)2 transformation until nearly all of the Ni centers were electrochemically active23 (Figure S7). Generally, the oxidation peak current (IP) obeys the power-law relation with scan rate (ν): Ip = aνb where IP is the peak current, a and b are adjustable parameters, and the value of b can be estimated from the slope of the log IP vs. log ν plot. There are two well-defined conditions: (i) if b is close to 0.5, the current is controlled by semi-infinite linear charge diffusion, indicative of a faradaic redox process in the bulk, and (ii) if b is close to 1.0, it indicates that the oxidation peak is a surface reaction limited response.29 As revealed in Figure 2a, b was determined to be ~ 0.56 for NiOx during anodic scan, suggesting a charge diffusion limited bulk redox process.30 On the contrary, spinel Co3O4 showed a surface reaction controlled behavior as its b value was ~ 0.93 (Figure 2b). Besides, the redox wave observed on spinel Co3O4 is intrinsically different from the redox wave of Co(OH)2/CoOOH, where the redox position of Co(OH)2/CoOOH is around 1.05



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V vs. RHE, and an irreversible reaction in terms of the changes of the redox wave area can be identified.31 For Co3O4, the redox wave prior to the OER onset remained almost unchanged between the first and the second CV cycle as shown in Figure S8.

Figure 2. Electrochemical properties of the anodic peak. CV curves with scan rate at 1 mV/s, 3 mV/s, 5 mV/s, 8 mV/s, 10 mV/s, and 20 mV/s of (a) NiOx and (b) Co3O4 electrode. From which, parameter b can be determined by plotting log(IP, anodic peak current) versus log(ν, scan rate). CV curves with scan rate at 5 mV/s in 0.1 M, 0.2 M, 0.5 M, 1.0 M, and 2.0 M KOH of (c) NiOx and (d) Co3O4 electrode. It is noted that the position of oxidation peak shifts toward the lower voltage vs. SCE with increase of KOH concentration due to the fact of pH increase.


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The underlying properties of surface reaction could be further identified by observing the influence of the alkaline concentration on the maximum intensity of the anodic peak. As NiOx went through Ni2+ → Ni3+ or even higher valent nickel peroxide32 transition with nearly all Ni centers being electrochemically active, increasing OH- concentration could enhance the kinetics of nickel oxidation,32 leading to an enhanced peak current intensity as shown in Figure 2c. However, the peak current remained nearly constant for spinel Co3O4 (Figure 2d), indicating that the reaction kinetics was mainly controlled by the number of exposed active sites on surface. The examination of single-crystal Co3O4 nanocubes, which were synthesized by reflux method without undergoing calcination,9 were also carried out to verify our assumptions and methodology. Owing to the wet synthesis process, impurities, hydroxyl groups, and/or water molecules could be unavoidably introduced into the structure of Co3O4 nanocubes, and thus the surface of the nanocubes could be partially oxidized and transformed to layered CoOOH9 during OER (Figure S9). Moreover, the estimated b value was shifted to 0.82 (Figure S10), suggesting that the anodic peak current was partially charge-diffusion controlled in terms of bulk oxidation. The bulk oxidation of Co3O4 nanocubes was further verified by the in-situ XAS measurement using our metal-air cell. As depicted in Figure S11a, the in-situ XANES spectra reveal a slow positive shift of cobalt valence states with increase of bias. The Fourier transformed EXAFS spectra (Figure S11c) obtained from the k3-weighting EXAFS spectra (Figure S11b) indicate the peak intensity ratio (IOh/ITd) shifted from 1.46 to 1.78 due to the progressive formation of layered CoOOH with the increase of bias, in which the Co3+ ion is surrounded by six oxygen atoms in the octahedral geometry. The aforementioned results indicate that except for the structural transformation, there should be more reactions on the surface of calcined Co3O4 hiding



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in the anodic peak current. In-situ Raman measurement (Figure S12 and Figure 3) was then carried out to probe the potential-resolved variation of the intermediates on Co3O4 surface.

Figure 3. In-situ Raman measurements. Raman spectra of Co3O4 recorded in the voltage range of 1.4 to 2.2 V vs. Zn/Zn(OH)42-, where VOC was around 1.32 V vs. Zn/Zn(OH)42-. As shown in Figure 3, four Raman-active phonon modes of cubic spinel Co3O4, Eg (487 cm-1), F2g (529 cm-1), F2g (624 cm-1), and A1g (692 cm-1)33 were clearly observed, which were nearly unaffected with increase in applied bias. Meanwhile, no other structural Raman signals including Co(OH)2 and layered CoOOH could be detected.34 Under anodic sweeping, a new Raman peak at 1068 cm-1 gradually emerged. The position of this newly formed peak is lower than adsorbed molecular O2 species (~ 1556 cm-1)35 and free superoxide species (1070–1200 cm-1).36 Although the Raman peak in the wavenumber region of 800–1150 cm-1 was assigned to the “active 10 ACS Paragon Plus Environment

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oxygen” species on Ni oxyhydroxide37 and Ni-Fe oxyhydroxide,38 this “active oxygen” assignment may not be appropriate in our case as the crystal structure of spinel Co3O4 was well maintained during the electrochemical conditioning. Besides, the nature and formation mechanism of the “active oxygen” species still cannot be equivocally specified.38 Koper et. al., recently corrected the assignment of the band in the range of 800-1150 cm-1 based on the

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O-

labeling in-situ surface enhanced Raman spectroscopy (SERS) experiments, which revealed a superoxo-type surface species possessing a redshift characteristic as a superoxide moiety.39 The redshift to lower frequencies by ca. 26 cm-1 in H218O electrolyte is also observed for the new Raman peak at 1068 cm-1 (Figure S13) in our experiment, which is identical to the characteristics of the superoxo (OO) species.36,39 Therefore, we attribute the newly evolved Raman peak to the superoxide moieties that could be produced by a 3-electron oxidation process of the surface adsorbed H2O intermediates. With further increase of bias, another new peak at 931 cm-1 starts emerging at the voltage of 1.8 V vs. Zn/Zn(OH)42- (or 1.36 V vs. RHE), which is in line with the onset of the anodic peak (Figure 1a and Figure S14) in the LSV scan. The position of this peak is higher than the Co(IV)=O vibrational mode at around 840 cm-1.36 A possible assignment of the peak at 931 cm-1 is the peroxide moiety as the OO stretching modes of peroxide generally fall in the range of 640– 970 cm-1.40 Besides, this Raman peak also exhibits a redshift in the

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O-labeled electrolyte

(Figure S15), suggesting that the surface moieties possess the nature of OO bond.41,42 However, since the peroxide is a 2-electron oxidation intermediate during the H2O oxidation to O2, this later formed species cannot be directly produced from the early formed superoxide moieties under a positive bias due to the fact that the peroxide (O22-) is the product of 1-electron reduction of superoxide (O2-). Nevertheless, it seems that the first emerging Raman peak at 1068 cm-1 can



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also fit in the range of 1015–1160 cm-1 as assigned for the µ-superoxo (µ-OO) species bridging on two metal centers in CoO–MgO solid solution,43,44 and the superoxide with µ-OO structure could be produced from the particular peroxide with µ-OO feature by 1-electorn oxidation process. This assignment provides a probable explanation to the formation of the second evolved Raman peak at 931 cm-1. As it has been revealed that the µ-OO coupling process on two metal centers is kinetically slow,45 an individual observation of µ-OO peroxide (Co-OO-Co) under a higher positive bias should be possible. Because the Co3O4 surface is saturated with OHads, the neighboring OHads moieties could connect to each other undergo a 2-electron oxidation process, leading to the surface moieties with µ-OO structure, in which the µ-OO peroxide moieties can thus be produced. A similar bridging scenario of direct µ-OO coupling on the dicobalt edge sites has been probed using a dinuclear cobalt complex.45 As previously reported,14 we found that tetrahedral zinc (Zn2+Td) substituted Co3O4 (ZnCo2O4) showed a vanishing anodic peak feature (Figure S14), which now may be comprehended due to the missing characteristic of µ-OO peroxide moieties (Figure 4a). The low intensity of superoxide at 1065 cm-1 and the vanishing Raman peak at around 926 cm-1 indicates the crucial role of Co2+Td to initiate the µ-OO bridging process, and thus, the OER catalyzed by ZnCo2O4 is rate-limited at the beginning step where the coverage of surface intermediate is low (Tafel slope of 113 mV/dec14 close to the featured Tafel slope of 120 mV/dec46). Though octahedral aluminum (Al3+Oh) substituted Co3O4 (CoAl2O4) with a small Tafel slope (60 mV/dec)

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exhibited a µ-OO peroxide stretching band at 928 cm-1 along with the characteristic CoAl2O4 Raman peaks47 (Figure 4b), the required bias to form the µ-OO peroxide moieties is too high at 2.0 V vs. Zn/Zn(OH)42-. Therefore, the anodic peak current could be overlapped with its OER current in which the onset is ~ 2.05 V vs. Zn/Zn(OH)42- (Figure S14).


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Figure 4. In-situ Raman measurements. Raman spectra of (a) ZnCo2O4 and (b) CoAl2O4 recorded in the voltage range of 1.4 V to 2.2 V versus Zn/Zn(OH)42- by using the designed in-situ Raman cell, where VOC was around 1.35 V versus Zn/Zn(OH)42- for ZnCo2O4 and 1.29 V vs. Zn/Zn(OH)42- for CoAl2O4. The characteristic Raman peak positions of ZnCo2O4 and CoAl2O4 in the range < 800 cm-1 fitted well with the reposted data in the literature.47,48 However, due to the inevitable fluorescence-effect for CoAl2O4 sample, some feature peaks were superimposed on the fluorescence board peaks that appeared from the beginning.



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In summary, we have revealed the underlying origin of the oxidation peak on Co3O4 in water oxidation catalysis, which could involve the oxidation of surface adsorbed intermediates to form µ-OO peroxide moieties. Our work disclosed the hiding chemical step reaction under the anodic peak current where Co2+Td should be responsible for the initiation of the reaction. Though we did not find a clear phase transformation on the calcined Co3O4, the layered CoOOH strucutre can still be observed on the Co3O4 syntheized by wet method. Besides, the thermodynamically stable hydrated oxide products could be limited by the slow kinetics. Thus, we reserve the possibility that the hydrated oxide layer can be formed under a long-term OER as such “corrosion phenomenon” is commonly observed on metal oxide based electrocatalyst.49

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Experimental details, the devices of in-situ measurements, TEM images, XRD spectra, and additional XAS spectra, CV, and Raman spectra.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work was supported by the Nanyang Technological University startup grant: M4080977.120, Singapore Ministry of Education Academic Research Fund (AcRF) Tier 1: RG111/15, Singapore A*Star Science and Engineering Research Council − Public Sector Funding (PSF): M4070232.120 and the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) program. The authors thank Sunny Hy from Tesla for reading the manuscript.

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