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Probing Interfacial Electrochemistry on a CoO Water Oxidation Catalyst Using Lab-Based Ambient Pressure XPS Xueqiang Zhang, Yong-Siou Chen, Prashant V. Kamat, and Sylwia Ptasinska J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01012 • Publication Date (Web): 10 Apr 2018 Downloaded from http://pubs.acs.org on April 10, 2018

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The Journal of Physical Chemistry

Probing Interfacial Electrochemistry on a Co3O4 Water Oxidation Catalyst Using Lab-Based Ambient Pressure XPS

Xueqiang Zhang,1,2 Yong-Siou Chen,1,2 Prashant V. Kamat,1,2,3 and Sylwia Ptasinska1,4* 1

Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556, USA Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA 3 Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, USA 4 Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA 2

Abstract: The design and mechanistic understanding of efficient and low-cost catalysts for the oxygen evolution reaction (OER) are currently the focus of electrochemical water-splitting technology. Herein, we report the chemical transformations on the water-vapor/solid interface and catalytic performance of an OER catalyst consisting of Co3O4 nanoparticles on multi-walled carbon nanotubes (Co3O4-MWCNT). Using a specially constructed electrochemical cell incorporated to the lab-based ambient-pressure X-ray photoelectron spectroscopy (APXPS) to mimic operando conditions, we obtained experimental evidence for the formation of CoO(OH) as the catalytically active phase on a Co3O4-MWCNT OER catalyst. Under water and applied potential conditions, CoO(OH) is formed, enriching the surface of Co3O4 nanoparticles with subnanometer thickness, and oxidizing H2O into O2. However, immediately after the removal of the applied potential, the CoO(OH) phase is converted back to Co3O4. This back conversion from CoO(OH) to Co3O4 is likely driven by locally concentrated protons (H+) in water vapor, which shows the necessity of an electrochemical bias to preserve the catalytically active phase. These results reveal the surface chemical identities of the Co3O4-MWCNT OER catalyst, which are in agreement with those obtained from in-situ APXPS studies of liquid/solid interfaces consisting of Co3O4 catalyst and disagree with those obtained from ex-situ ultra-high vacuum (UHV) XPS. Thus, our results demonstrate the possibility of performing surface chemical analysis in simplified electrochemical

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systems and further reinforce the importance of performing mechanistic studies of electrochemical devices under in-situ conditions.

1. Introduction The electrolysis of water into molecular hydrogen, a clean and sustainable candidate with sufficient scale to replace fossil fuels, is one of the prevailing methods being considered for future energy supply.1-6. In an electrochemical cell used for water splitting via H2O→H2+1/2O2, two processes are involved, one in which water is oxidized to O2 by four-hole carriers at an anode and another in which it is reduced to H2 at a cathode by two electron carriers.6-10 The former process, called the oxygen evolution reaction (OER), is kinetically sluggish and considerably more complex. Consequently, extensive research has been focused on the development of available OER catalysts characterized by both high efficiency and stability. To date, the most active metal-oxide OER catalysts are noble and metal-based (e.g., RuO2 and IrO2) and are therefore not feasible for large-scale applications due to their high cost.11-16 Accordingly, considerable attention has been placed on the use of first-row spinel and perovskite metal oxides as electrocatalysts for the OER.7, 17-21 In particular, cobalt oxides with a spinel structure (Co3O4) exhibit excellent performance as OER catalysts, especially when they are loaded onto conductive supports, such as graphene, Ni foam, or Nafion, which enhance their conductivity and induce synergistic interactions.2,3,7,12,13,22-26 Despite cobalt oxide’s potential as a low-cost and efficient OER catalyst, our understanding of the electrolyte (H2O)/electrode (cobalt oxide) interfacial chemistry under operational conditions, particularly the identification of the catalytically active phase, remains incomplete and is the subject of ongoing research. Using electron paramagnetic resonance and Raman techniques, the Co4+ population was observed to increase and the intentional increase in the Co4+ components has led to improved OER performance for Co-Pi-based,3,27,28 and Au-supported cobalt oxidebased OER catalysts.9. However, other studies suggest that Co3+ within a CoO(OH) structure is the active phase that catalyzes the OER in alkaline solutions.1,10,29-31 Interestingly, when CoO nanoparticles were suspended in a neutral water solution, efficient water splitting performance was observed, but the catalytic activity is quenched when the CoO surface becomes corroded or oxidized, as observed using an ex-situ technique.32 All previous studies have demonstrated the 2 ACS Paragon Plus Environment

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complexity of the chemistry on cobalt oxide surfaces, which depends strongly on the experimental conditions. Previous in-situ or quasi in-situ studies using synchrotron radiation (e.g., EXAFS)33-35 have revealed structural changes in the bulk of the cobalt oxide and surface crystallinity. Most recent operando ambient pressure X-ray photoelectron spectroscopy (APXPS), also using synchrotron radiation, have shown that the Co3O4/Co(OH)2 biphasic electrocatalyst undergoes chemical and structural transformation with a partial conversion of Co3O4 and a complete conversion of Co(OH)2 to CoO(OH).36,37 In classical heterogeneous catalysis, the chemistry occurs at the reactant/catalyst interface. As such, a surface-sensitive technique, such as X-ray photoelectron spectroscopy (XPS), has specific advantages over other methodologies because it probes the surface chemistry and excludes most of the bulk contribution.38-40 Since the chemical identity of the catalyst surface can differ before, during, and after the catalytic reaction, it is important to monitor the catalyst’s surface under in-situ and operando conditions.41-53 For example, the study of a water-splitting device under such conditions can provide valuable information about the electrolyte (H2O)/electrode (cobalt oxide) interfacial chemistry during device operation.14,30,34 The development of APXPS offers the ability to perform operando studies with electrochemical water-splitting devices using a synchrotron facility.36 In this study, our aim was to probe the working mechanism of a Co3O4-MWCNT catalyst by tracking the H2O/Co3O4-MWCNT interfacial chemistry using mainly lab-based APXPS, which allows the study of surface chemistry below water pressures of a few mbars. Despite the simplified electrochemical conditions in our experiment in which we investigated interactions of the Co3O4 catalyst with water vapor instead of liquid water, our chemical identification of the interfacial components is in agreement with other synchrotron-based studies. Moreover, as we also showed with respect to other water-splitting systems,54 APXPS studies are more relevant to realistic conditions performed under ultra-high vacuum conditions. However, we are aware of different electrochemical potentials in the case of bulk water and water vapor, as revealed in our study.

2. Experimental section 2.1. Synthesis and characterization of Co3O4 nanoparticles and Co3O4-MWCNT nanocomposites. We synthesized Co3O4 nanoparticles as previously described.55 Briefly, we 3 ACS Paragon Plus Environment


dissolved 0.5 g of cobalt acetate in 25 mL of ethanol, followed by the dropwise addition of 2.5 mL ammonia (25%) with vigorous stirring. We then stirred the solution for 10 min before transferring it into a 48-mL Teflon-lined autoclave. We sealed the reaction vessel and heated it in an oven at 150 ºC for 3 h, then allowed it to cool to room temperature naturally. We collected the as-synthesized Co3O4 nanoparticles and washed them with de-ionized (DI) water three times via centrifugation redispersion, then finally dried them at 60 ºC for 4 h. Next, we pretreated multi-walled carbon nanotubes (MWCNTs) in 8 M HNO3 and 8 M H2SO4 under sonication at 60 °C for 3 h. Then, we collected the chemically treated MWCNTs and washed them with DI water five times via centrifugation redispersion and finally dried them at 60 °C for 4 h. To synthesize Co3O4-MWCNT nanocomposites, we sonicated the chemically treated MWCNTs in an ethanol solution containing cobalt acetate for 30 min at room temperature. The remaining procedures were the same as those described for the synthesis of the Co3O4 nanoparticles. We collected the as-synthesized Co3O4-MWCNT nanocomposites and washed them with DI water three times via centrifugation redispersion, then finally them dried at 60 °C for 4 h. We employed 70 wt.% catalyst loading (nominally) for all of the samples in this study. We recorded the XRD patterns using a Bruker D8 Advance Davinci Powder X-Ray Diffractometer. The scanning range was from 2ϴ = 20º to 90º with 0.02º per step, with each step acquired over 8 s. We obtained TEM images using an FEI Titan 80-300 microscope at 300 kV with a Gatan Image Filter. 2.2. Electrochemical Characterization of the Co3O4-MWCNT nanocomposites. To characterize the activity of Co3O4 and Co3O4-MWCNTs in the OER, we employed a typical three-electrode system using the Princeton Applied Research model PARSTAT 2263 potentiostat. To do so, via sonication, we suspended 1 mg of Co3O4 or 1.5 mg of Co3O4-MWCNTs in 1 mL of isopropanol with 0.1 mg of Nafion (10% to catalyst). We then dropped-cast the catalyst suspension onto a 1 cm x 1 cm square of carbon paper and dried the catalyst-loaded carbon paper, which served as the working electrode, overnight at 60 ºC in a vacuum oven. We used platinum gauze as the counter electrode and an Hg/HgO electrode (1 M KOH) as the reference electrode. We placed the platinum gauze in a secondary glass cell, which was in contact with the electrolyte through a porous glass frit to separate the gas products. Then, we used 1 M KOH as the 4 ACS Paragon Plus Environment

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electrolyte, which we purged with high-purity oxygen gas for 30 min prior to and during the experiments. We first calibrated the Hg/HgO electrode with a Pt/H2 electrode before measurement. We used a cyclic voltammogram to acquire the electrochemical activity by scanning the voltage between 0.3 and 1.0 V against an Hg/HgO reference electrode at a scan rate of 5 mV/s. We calibrated the resulting current–voltage curve by compensating for the voltage loss from the electrolyte. We measured the resistance of the electrolyte by performing an electrochemical impedance analysis with a frequency sweep ranging from 0.1 MHz to 100 MHz with an amplitude of 10 mV at 1.55 V vs. RHE. We extracted the series resistance from the Nyquist plot by applying a simplified Randles equivalent circuit and identified a typical 1–2 ohm in our system. 2.3. Fabrication of the Co3O4-MWCNT/Nafion/Pt electrochemical cell. To perform in-situ APXPS chemical analysis of the Co3O4-MWCNT surface, we constructed a simplified model electrochemical cell and incorporated it into a reaction chamber of our APXPS instrumentation (Scheme 1). The cell consisted of a Nafion membrane (Fuel Cell Technology) coated on one side with 3 mg/cm2 of Pt catalyst. Initially, we suspended 4.3 mg of the Co3O4-MWCNT (3 mg of Co3O4) via sonication in 1 mL isopropanol with 0.3 mg Nafion (10% to catalyst). We then dropcast the solution onto the Nafion surface. This arrangement enabled the application of a bias voltage via metal wires and the exposure of all the cell components to water vapor. We connected metal wires to each electrode surface using silver paste and then covered them with epoxy (Hysol 1C). 2.4. In-situ study of the Co3O4-MWCNT/Nafion/Pt electrochemical cell. We recorded highresolution photoemission spectra using a lab-based APXPS apparatus (SPECS Surface Nano Analysis GmbH, Berlin, Germany). We generated an Al Kα X-ray beam with a photon energy of 1486.7 eV from an aluminum anode in a microfocus X-ray (XR-MF) source. The XR-MF source was also equipped with a quartz crystal mirror, which we used to monochromatize the photon beam. A membrane (Si3N4) window allowed the X-ray beam to pass through to a reaction chamber and this beam was incident to the Co3O4-MWCNT surface of the electrochemical cell. We directly introduced the electrochemical cell to the XPS reaction chamber without any pretreatment and then pumped it out at a background pressure of 10-9 mbar. A PHOIBOS 150 Hemispherical Energy Analyzer (HEA) with a nine-single-channel electron multiplier (MCD-9) 5 ACS Paragon Plus Environment


was coupled to a differentially pumping electrostatic lens system, which we used to collect photoemission spectra. We recorded high-resolution spectra with a pass energy of 20 eV, given a full-width at half maximum (FWHM) of ~0.5 eV for the Au 4f7/2 peak. We calibrated the binding energy (BE) of the photoemission spectra with reference to C 1s at 284.5 eV. Due to the technical limitations of our experimental setup, which allows the introduction of gas pressure up to 20 mbar, we cannot perform studies with bulk water on the catalyst surface as is performed in synchrotron facilities.36,37 The pressure in the reaction chamber operates under a continuous-flow mode, which we fixed at 7 mbar by adjusting the flow rate of the water vapor through the cell. We also performed a chemical analysis of the Co3O4-MWCNT surface in 4 mbar H2O and 2 mbar O2 pressure by adjusting the partial pressures for both gases. We used a residual gas analyzer (RGA)/mass spectrometer mounted to one of the differentially pumped stages downstream of the reaction chamber nozzle to monitor any reactant or product coming from the reaction chamber. To ensure that the cell was electrically isolated with respect to the grounding potential, we assembled the electrochemical water splitting cell on a retractable sample stage using nonconductive material (glass). We connected two metal wires to the cathode and anode sides of the cell using silver paste and then covered them with epoxy. We connected the other ends of the metal wires to the thermocouple on the sample stage to allow for the control and measurement of the electric current. The separation between the cell cathode and the sample stage by the glass allowed access for the reactant, H2O, and the anode side, which consisted of the Co3O4-MWCNT nanocomposite as the OER catalyst, faced upward towards the energy analyzer for the collection of photoelectrons (Scheme 1).


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Scheme 1. Configuration of the model electrochemical cell incorporated into the reaction chamber of the APXPS instrumentation for the in-situ interfacial chemical, catalytic, and electrical characterization of Co3O4-MWCNT nanocomposites as a working electrode in the water splitting cell. 3. Results and discussions First, we characterized the structural, surface chemical, and electrical properties of the Co3O4 nanoparticles and the Co3O4-MWCNT nanocomposite. As shown in the high-resolution transmission electron microscopy (HR-TEM) image, the highly crystalline Co3O4 nanoparticles with an average size of 3–5 nm are evenly distributed on the surface of the MWCNT (Figure 1a). The large surface area of the MWCNT creates a platform for the dispersion of Co3O4 nanoparticles and the electron conductivity of MWCNT allows for the direct transport of charges to further enhance the overall catalytic activity.13,56 To identify the chemical species possibly formed on the Co3O4 surface due to H2O interactions in our study, we also synthesized the standards of Co(OH)2, Co3O4, and CoO(OH) (Figure S1 in the Supporting Information (SI)). Based on the HR-TEM, X-ray diffraction (XRD), and XPS results (Figures 1a, 1b, S1, S2, and S3), we can confirm that the as-synthesized nanoparticles are solely Co3O4 and no other cobalt species can be found. 7 ACS Paragon Plus Environment


Figure 1c shows the current density–voltage curve recorded with the working electrodes of Co3O4 nanoparticles and Co3O4-MWCNT loaded onto carbon papers. We used a three-electrode system comprising 1 M KOH as an electrolyte, platinum gauze as a counter electrode, and Hg/HgO (1 M KOH) as a reference electrode. The bare carbon-paper electrodes show no catalytic activity toward the OER (Figure 1c), whereas the increase in the anodic current at potentials greater than 1.5 V suggests that the as-synthesized Co3O4 nanoparticles are electrochemically active towards the OER. The Co3O4-MWCNT catalyst exhibits a greater OER performance than do the Co3O4 nanoparticles alone. We observed an overpotential of ~0.35 V for the Co3O4 nanoparticles at a current density of 10 mA/cm2, and the kinetic analysis of the J– V curve reveals a Tafel slope of 75 mV/decade (Figure 1d). In comparison, a Co3O4-MWCNT catalyst has an overpotential of 0.32 V at 10 mA/cm2 and a Tafel slope of 69 mV/decade (Figures 1c and 1d), which suggests a higher OER activity. We believe that the enhanced catalytic performance of the Co3O4-MWCNT nanocomposite arises from the improved catalyst dispersion on the MWCNT, which in turn improves the overall electron collection efficiency. A synergistic effect has also been proposed to explain the observed high catalytic activity of carbon-supported Co3O4 nanoparticles.2,13


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Figure 1. Characterization of Co3O4 nanoparticles and Co3O4-MWCNT nanocomposite: (a) HRTEM image of the Co3O4-MWCNTs; (b) Co 2p photoemission spectra of Co3O4 nanoparticles and Co3O4-MWCNTs; (c) J–V characteristics recorded in an oxygen saturated 1M KOH aqueous solution with a scan rate of 5 mV/s; and (d) their corresponding Tafel plots.

To analyze the catalytic behavior of the OER catalyst, we conducted a post-operational characterization of the Co3O4-MWCNT working electrode using XPS. We note that the Co3O4based working electrodes, after their operation, were exposed to ambient air prior to the UHVXPS characterizations (hereafter denoted as the ex-situ study). We observed no changes in the spectral features in either the Co 2p or O 1s spectra before or after the cell operation (Figure 2), which suggests that no changes in the catalytic phase had occurred.



Figure 2. Ex-situ study: photoemission spectra of (a) Co 2p and (b) O 1s obtained for the Co3O4MWCNT electrode of the Co3O4-MWCNT/Nafion/Pt water-splitting cell recorded before and after its operation.

Several factors could influence the outcome of the characterization while using ex-situ techniques, including the exposure to ambient air (H2O and O2) and the removal of the electrolyte and bias potential. As has been well established, ex-situ characterization can mislead the interpretation of results and generate strong debate about whether the chemical identity of the surface cobalt species changes under the operational conditions. Therefore, to capture and correctly identify the surface chemistry of the OER catalysts, we investigated the electrolyte (H2O)/electrode (Co3O4-based catalyst) interface under operando conditions using a synchrotron facility.36,37 Similarly, in our laboratory, to determine the chemical identity of Co3O4-MWCNT under operational-like conditions (i.e., under 7 mbar H2O pressure), we constructed an electrochemical cell that was compatible with our lab-based APXPS to enable us to also perform an in-situ study. Scheme 1 shows the configuration of the cell incorporated into the APXPS system. We used a polymer electrolyte membrane (PEM) and a commercially available Nafion membrane, with Pt and Co3O4-MWCNT coated on two different sides of the membrane to serve as the cathode and anode, respectively. We used the Nafion membrane for proton transportation between the two electrodes. Note that when Nafion is exposed to KOH, H+ protons are substituted with K+, thus decreasing the local pH toward a more neutral value. Although the


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experimental conditions do not exactly mimic typical electrode/electrolyte (KOH) conditions, they provide a means to investigate electro-oxidation close to neutral pH.14 Figure 3 shows the results from our in-situ studies of the Co3O4-MWCNT/Nafion/Pt electrochemical water splitting cell, for which we observed an onset voltage of 1.43 V and a current of 150 µA at 2 V potential (Figure 3a). We obtained a ratio of 2:1 for H2:O2 production from the mass spectrometric (MS) data under operational conditions (Figure 3b), which is consistent with the stoichiometry of these water splitting products. By switching the applied potential between the electrodes on and off every 2 minutes for 10 cycles, we established the catalytic response of the working cell. Prior this study, we investigated the correlation between an operating current of the electrochemical cell and H2O pressure (Figure S4). By reaching a compromise between the electric current and a reasonable signal-to-noise ratio of the photomemission spectra, we found 7 mbar of H2O vapor to be the optimum pressure for this insitu study. Figures 3c and 3d show the photoemission spectra of Co 2p and O 1s recorded at an H2O pressure of 7 mbar, with and without the application of a 2 V potential. We observed no appreciable change in the Co 2p photoemission spectra following the introduction of 7 mbar of H2O, which suggests the preservation of a Co3O4 phase (without the application of a bias, i.e., 0 V). In the O 1s spectra, however, the component at ~532 eV slightly increased and shifted towards higher BEs (Figure 3d) in comparison to the peak observed for the Co3O4-MWCNT electrode (Figure 2), which suggests water absorption onto the Co3O4 nanoparticles. Upon the application of a 2 V potential, we can clearly observe a shoulder at the BE of ~780.6 eV in the Co 2p photoemission spectra (Figure 3c), which indicates the formation of a new component in the surface region of the Co3O4 nanoparticle. In the O 1s photoemission spectra, we can see that the Co-O contribution at 530 eV decreases considerably in respect to the component at ~532 eV. Based on previous XPS studies57,64,65 and our synthesized standards (Figures S1 and S2), we can identify the formation of CoO(OH) on the Co3O4-MWCNT catalyst surface. Under operational-like conditions (7 mbar, 2 V), the BEs of Co 2p peaks match well with those of CoO(OH). The increased OH-related components (~532 eV) and the decreased Co-O components (~530 eV) in the O 1s photoemission spectrum (Figure 3d) are consistent with the conversion from Co3O4 to CoO(OH). The formation of pure Co(OH)2 can 11 ACS Paragon Plus Environment


be excluded because the Co 2p spectrum under in-situ conditions (Figure 3c) exhibits different BE values and a different spectral shape from those of Co(OH)2 (Figure S1a).

Figure 3. In-situ studies of the interfacial chemistry and electrical properties of the Co3O4MWCNT/Nafion/Pt water-splitting cell incorporated into the APXPS instrument under 7 mbar H2O: (a) current–voltage characteristics, (b) H2 and O2 signal intensities under operational conditions (at a potential of 3.5 V) obtained using mass spectrometry (MS); photoemission spectra of (c) Co 2p and (d) O 1s obtained without (0 V) and with applied voltage (2 V).

In addition, as was reported recently, in the case of Co3O4/Co(OH)2 biphasic electrocatalyst, a complete conversion of Co(OH)2 to CoO(OH) was observed under operando conditions.36 In a 12 ACS Paragon Plus Environment

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recent synchrotron-based study, a spectral simulation and multiplet fitting were performed to resolved photoemission spectra, but in general three spectral components, i.e., Co-O bonds, adsorbed OH, and adsorbed H2O, were used to assign interfacial species.36 In our study, in consideration of the limitations in the resolution of our spectra, we resolved two components. We found that the use of an additional third peak did not significantly influence the qualitative analysis of our study. Thus, in the O 1s spectrum, two peak maxima are primarily resolved, in which we attribute the lower BE maximum (~530 eV) to the Co-O bond30,57-59 and the higher BE maximum (531-532 eV) to the OH group due to the dissociative adsorption of H2O, oxygen components of Nafion, and Co3O4, and adsorbed molecular water (Figure S1b).44, 59-63 Based on the fitting of the Co 2p photoemission spectrum into Co3O4 and CoO(OH) (Figure 3c), we can also obtain the surface structural information on the Co3O4-MWCNT catalyst under operational conditions. We estimate that ~40% of the surface Co atoms are in the form of CoO(OH), and this percentage continues to increase to ~50% under prolonged operation of the working electrode. Additionally, the amount of catalytically active species depends on the extent of electrochemical polarization at the anode. As we can see in Figure S5, the Co 2p photoemission spectra show enhanced formation of CoO(OH) species as the applied voltage increases from 0 V to 4 V. Since the Co3O4 feature is still partially preserved, the surface region of the Co3O4-MWCNT catalyst is enriched with CoO(OH) under these operational conditions. The Co3O4 signal observed in the Co 2p photoemission spectra (Figure 3c) originates either from islands of newly-formed CoO(OH) species or from the surface region due to the formation of the CoO(OH)-Co3O4 nanocomposite. Assuming that the Co3O4 nanoparticle is covered by a uniform layer of CoO(OH), we estimate the thickness of this CoO(OH) shell to be ~0.5 nm. However, we cannot exclude another scenario in which the catalytically active CoO(OH) forms a CoO(OH)Co3O4 nanocomposite on the surface of the OER catalyst. The chemical identity of Co3O4-MWCNT under operational-like conditions (Figure 3) differs from that detected by ex-situ study (Figure 2). As noted above, this discrepancy has led to inconsistency in the literature regarding the identity of the catalytically active phase, which is likely caused by the exposure of the working electrode to ambient conditions, when an applied bias is removed. As a reactant and environmental component in an electrochemical device, H2O plays a vital role in governing the chemical state of an OER catalyst. The reversible conversion 13 ACS Paragon Plus Environment


between sub-nanometer CoO(OH) and Co3O4 is consistent with previous reports, which showed that a reversible phase formation depends on the application of a potential between the electrodes.35,36 This back conversion from CoO(OH) to Co3O4 involves a change in the octahedral coordination in Co3+ to tetrahedral and Td ions (Td, tetrahedral coordination by four oxygen atoms) in Co3O4,35 and the extraction of oxygen atoms by a reducing reagent. Therefore, H2O surroundings, independent of its being in liquid or gas phase, has an indispensable role in the reduction of CoO(OH) to form Co3O4. We hypothesize that the conversion from CoO(OH) to Co3O4 likely occurs via a local proton–electron-driven reduction reaction: 3CoO(OH) + H+ + e → Co3O4 + 2H2O. (1) Under operational conditions, protons are formed due to the OER and can be driven to the other side of the electrochemical cell by an applied potential and then converted into H2, whereas protons left near the CoO(OH)-Co3O4 catalyst form a local acidic environment. On the other side of the reaction cell (cathode), with no supply of electrons, H2 can adsorb onto a Pt surface, which is a highly efficient H2 dissociation catalyst, and eventually form H2O by reaction with environmental oxygen, thereby releasing two electrons. Therefore, driven by instantaneously concentrated protons and electrons in the local environment of a CoO(OH)-Co3O4 surface region, the catalytically active phase, CoO(OH), is reduced back to Co3O4. Although H2O is not directly involved in the reaction, it provides a local environment that facilitates the reaction by forming a hydrogen-bond network on the surface of the CoO(OH)-Co3O4-MWCNT catalyst, which enables efficient proton and electron transfer.54,66 The autocatalytic back conversion of CoO(OH) into Co3O4 explains the observation of Co3O4 alone using ex-situ XPS (Fig. 2), during which the OER catalyst was exposed to the ambient environment (H2O moisture in air). Note that O2, as a reaction product and a component in ambient air, also exists in the local environment of the H2O/OER catalyst. Therefore, we briefly investigated the surface chemistry of the Co3O4-MWCNT electrode in the presence of O2 and under operational conditions. As shown in Figure S6, in a 4mbar H2O and 2 mbar O2 environment with an applied potential of 2 V, we also detected a CoO(OH) component, which suggests that the formation of CoO(OH) is independent of the presence of both H2O and O2 under in-situ conditions.


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4. Conclusions In summary, we designed, synthesized, and performed mechanistic characterization of a Co3O4MWCNT OER catalyst in a simplified electrochemical cell incorporated into a lab-based APXPS system. The application of MWCNT facilitates the overall electrocatalytic performance of Co3O4 nanoparticles in the OER. Our study results demonstrate the formation of CoO(OH) as the catalytically active phase with subnanometer thickness on the surface of the Co3O4-MWCNT OER catalyst. The CoO(OH) exists only under operational conditions and is rapidly converted back into Co3O4 when no voltage is applied. The back reaction from CoO(OH) to Co3O4 is governed by the presence of H2O, which can facilitate the hopping of instantaneously formed protons on the catalyst surface. Moreover, we observed this reaction independently of the H2O phase. Although, the electrochemical performance of our cell differs from a real electrochemical system, the surface chemistry reflects well that occurs at the liquid/electrode interface. Thus, in this study, we took advantage of the surface-sensitive feature of the lab-based APXPS technique and captured the catalytically active phase of Co3O4-MWCNT OER catalysts under operandolike conditions.

Associated Content Supporting Information: Characterization of synthesized standards, HR-TEM, photoemission spectra and XRD of Co3O4 nanoparticles and Co3O4-MWCNTs, current/H2O pressure relationship under different in situ conditions, Co 2p photoemission spectra obtained under different applied potentials.

Author information Corresponding Author *E-mail: [email protected] (Sylwia Ptasinska) Note. The authors declare no competing financial interest.

ACKNOWLEDGMENTS



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