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Crystalline Cobalt Oxide Films for Sustained Electrocatalytic Oxygen Evolution under Strongly Acidic Conditions Jared S. Mondschein, Juan F. Callejas, Carlos G. Read, Jamie Y. C. Chen, Cameron F. Holder, Catherine K. Badding, and Raymond E. Schaak Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02879 • Publication Date (Web): 28 Dec 2016 Downloaded from http://pubs.acs.org on December 29, 2016

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Chemistry of Materials

Crystalline Cobalt Oxide Films for Sustained Electrocatalytic Oxygen Evolution under Strongly Acidic Conditions Jared S. Mondschein,‡ Juan F. Callejas,‡ Carlos G. Read, Jamie Y.C. Chen, Cameron F. Holder, Catherine K. Badding, and Raymond E. Schaak* Department of Chemistry and Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 ABSTRACT: Earth-abundant materials capable of catalyzing the electrochemical decomposition of water into molecular hydrogen and oxygen are necessary components of many affordable water-splitting technologies. However, water oxidation catalysts that facilitate sustained oxygen evolution at device-relevant current densities in strongly acidic electrolytes have been limited almost exclusively to precious metal oxides. Here, we show that nanostructured films of cobalt oxide (Co3O4) on fluorine-doped tin oxide (FTO) substrates, made by first depositing Co onto FTO and heating in air at 400 °C to produce films having a robust electrical and mechanical Co3O4/FTO interface, function as active electrocatalysts for the oxygen evolution reaction (OER) in 0.5 M H2SO4. The Co3O4/FTO electrodes evolve oxygen with near-quantitative Faradaic yields and maintain a current density of 10 mA/cm2 for over 12 h at a moderate overpotential of 570 mV. At lower current densities that require lower overpotentials, sustained oxygen production for several days and weeks can be achieved.

INTRODUCTION The development of efficient, affordable, and carbon-neutral platforms for fuel generation is a critical global challenge.1 Devices that facilitate water splitting, including electrolyzers and photoelectrochemical cells (PECs), are powerful technologies that have the potential to renewably generate clean fuels, including hydrogen.2,3 Achieving the complete dissociation of water, however, requires electrocatalysts capable of efficiently carrying out the corresponding cathodic and anodic half reactions, which consist of the hydrogen evolution reaction (HER)4 and the oxygen evolution reaction (OER).5,6 Practical HER and OER catalysts must function for long periods of time under the operational conditions of the devices into which they are integrated, which range from mild, pH-neutral buffers to highly acidic or alkaline aqueous solutions that are chemically harsh and corrosive.2,3,7 Additionally, HER and OER catalysts should be comprised of elements that are inexpensive and abundant in the Earth’s crust to minimize cost and facilitate widespread utilization on a global scale.1,2 Earth-abundant catalysts for the HER under acidic, neutral, and alkaline aqueous conditions are well known and include metal alloys,8,9 chalcogenides,10-12 phosphides,13-16 and carbides,17,18 as well as metal- and heteroatom-substituted graphitic materials.19 In contrast, investigations of Earth-abundant materials as catalysts for the OER, which is regarded as the bottleneck in overall water splitting, have largely focused on oxide materials operating under alkaline20-23 and near-neutral2325 conditions due to their tendency to rapidly dissolve in acidic environments.26-28 Robust and inexpensive OER catalysts operating under acidic conditions, however, are important for expanding the scope of fuels that can be generated using PECs as well as for improving the overall compatibility of device components.29,30 Catalysts capable of sustained oxygen production at a PEC-relevant current density31 of 10 mA/cm2 in strongly acidic solutions have been limited exclusively to

scarce precious metal oxides of iridium and ruthenium.2,27,28 Indeed, recent benchmarking studies revealed no Earthabundant materials that are capable of reaching the target metric for short-term acid stability, which was defined as two hours of operation at 10 mA/cm2.28 Since then, electrodeposited films of manganese oxide (MnOx) were found to be functionally stable, with approximately 20% of the film dissolving after 2 hours at 10 μA/cm2 in pH 2.0 electrolyte.32 The activity of these MnOx films was enhanced by potential cycling, demonstrating galvanostatic stability for up to 8 h in 0.5 M H2SO4 at a current density of 0.1 mA/cm2,33 which is two orders of magnitude lower than the PEC target values.28,31 TiO2decorated MnO2 films were also shown to exhibit improved stability in 0.5 M H2SO4, decreasing Mn dissolution by roughly 50%.34 While progress is being made toward achieving greater acid stability under device-relevant conditions for manganese oxide OER catalysis, cobalt-based OER catalysts are known to be more active than manganese systems under various pH conditions,35 motivating complementary studies on cobalt oxide materials. As with manganese oxides, cobalt oxides are widely considered to be unstable in strongly acidic solutions because of their tendency to rapidly corrode, especially at the high applied potentials required to achieve oxygen evolution catalysis.26,27 Prior reports of cobalt oxide OER catalysts have demonstrated high catalytic performance in alkaline and nearneutral pH solutions.36,37 At lower pH values, however, most cobalt oxide OER catalysts exhibit high overpotentials, low Faradaic efficiencies, and rapid dissolution, in some cases leading to considerably less active homogeneous catalyst species.28,38,39 Achieving a current density of 10 mA/cm2 for even a few hours with existing base-metal OER catalysts, including both cobalt and manganese oxides, requires aqueous solutions having pH > 2.5, which is significantly less acidic than 0.5 M H2SO4 and therefore less desirable for applications such as

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PECs that benefit from high proton concentrations for maximum efficiency.2,32,33,38,39 Given these considerations, it is important to identify and understand synthetic pathways and key materials characteristics that lead to optimal OER catalysis under strongly acidic conditions. This is especially true for cobalt oxides, which are already known to be active for the OER under acidic conditions but with rapidly degrading performance as pH decreases below 2.5. Here, we report that thin films of nanostructured, highly-crystalline cobalt oxide on fluorinated tin oxide (FTO) electrodes made through a multi-step deposition/annealing pathway are able to catalyze the OER in 0.5 M H2SO4 while maintaining a PEC-relevant current density of 10 mA/cm2 for over 12 h at a moderate overpotential of 570 mV. Such performance metrics, which surpass the threshold for short-term acid stability defined by recent benchmarking protocols, are unmatched by other known base metal OER catalysts in strongly acidic aqueous solutions.28 In addition to characterizing the catalytic performance of the cobalt oxide films in 0.5 M H2SO4, we also correlate the catalytic performance and dissolution rate with pH, film thickness, applied potential, and current density. EXPERIMENTAL SECTION Materials. Cobalt powder [99.8%, 1.6 micron, Lot #D24R033] (Alfa-Aesar), cobalt nanopowder [99.8%, 25-30 nm, Lot #P27A041] (Alfa-Aesar), cobalt(II,III) oxide [99.7%, Lot #C11I14] (Alfa-Aesar), cobalt(II) oxide [99.998%, Lot #J28W012] (Alfa-Aesar), cobalt(II) nitrate [≥98%, Lot #MKBH0419V] (Sigma Aldrich), sulfuric acid [99.999%, Lot #SHBF5690V] (Sigma Aldrich), sodium sulfate, anhydrous (EMD, Lot #47158734), cobalt(III) acetylacetonate [Lot #MKBG1260V] (Sigma Aldrich), iridium(III) chloride hydrate [99.9%] (Alfa-Aesar), ruthenium(III) chloride hydrate [Lot #MKBB0272V] (Sigma Aldrich), isopropyl alcohol [GR ACS, Lot #54315] (EMD Millipore), acetone [GR ACS, Lot #54287] (EMD Millipore), and Alconox detergent powder [Lot #A2B4] (Alconox Inc.) were used as received without further purification. Nanopure water (18 MΩ) was obtained from a Barnstead Nanopure Analytical Ultrapure water system. The cobalt (99.95%, Part# EVMCO35QXQ), gold (99.99%, Part# EVMAU40QXQ), manganese (99.95%, Part# EVMMNM1034), and iron (99.95%, Part# EVMFE35QXQ) targets used for electron-beam evaporation were obtained from Kurt J. Lesker as pellets. TEC-7 (7 Ω/sq) fluorine-doped tin oxide (FTO) slides (1 × 2.5 cm, precut) were purchased from the Hartford Glass Co., Inc. Electrode Preparation. FTO substrates were cleaned as previously described.40 The as-received substrates were sequentially sonicated in solutions of Alconox detergent, acetone, isopropyl alcohol, and nanopure water for 15 minutes each and dried under a gentle stream of air. Kapton tape was then applied to the conductive side of the FTO substrates such that only 1-cm2 areas were exposed. Cobalt films with a thickness of approx. 75 nm, 150 nm, and 210 nm were subsequently deposited onto the FTO by electron-beam evaporation using a Kurt J. Lesker Lab-18 Thin Film Deposition System or a Semicore Evaporator, with a 1.0 Å/s deposition rate at 14.1% power using 761 mA of current. 157-nm iron thin films were deposited with a 1.0 Å/s deposition rate at 5.9 percent power using 29.3 mA of current. 165-nm manganese films were deposited using a 1.0 Å/s deposition rate at 1.9% power using 8.6 mA of current, and 150-nm films of gold were deposited with a 1.0 Å/s deposition rate at 20.3% power using 101 mA of current. During a typical deposition, the pressure and temperature inside the chamber were 7.30 × 10-7 torr and 22 °C, respectively. Film thicknesses were verified using a KLA Tencor P16+ profilometer. Films of iridium and ruthenium oxides were prepared by dropcasting powder/ethanol suspensions onto FTO substrates. Colloidal cobalt nanoparticles were synthesized as previously described and the resulting nanoparticle/hexanes suspension was dropcast onto the FTO electrode.41 While dropcasting is preferable for controlling loading densi-

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ty, bulk cobalt and cobalt oxide powders were not sufficiently dispersible and therefore spin-coating was used. Ethanolic suspensions (0.33 mg/mL) were sonicated for at least 5 minutes prior to use and kept agitated immediately before spin coating onto FTO substrates having a 1-cm2 area. The spin-coating process was carried out at 2000 rpm for 15 seconds and used 200 µL of the powder/ethanol suspension at 20 µL increments with 3 minutes of drying on a 100 ºC hotplate between each coating. All films were then annealed in air at 400 °C for 2 hours using a 20 °C/min ramp rate in a Thermo Scientific Thermolyne muffle furnace. Electrochemical measurements. All electrochemical measurements were performed using a Gamry Instruments Reference 600 potentiostat. A saturated calomel electrode (SCE) was used as the reference electrode and a porous graphitic rod was used as the counter electrode for all electrochemical measurements. Measurements at pH 0.3 were performed in high purity 0.5 M H2SO4 (99.99%) using a onecompartment, three-neck glass cell. The pH of the electrolyte was adjusted by altering the concentration of H2SO4 and a constant ionic strength was maintained by adding the appropriate concentration of Na2SO4. The cell was purged with O2 for approximately 15 minutes prior to each set of experiments. During all electrochemical measurements, the solution was not stirred and was kept blanketed under an atmosphere of O2. To minimize capacitive current, two cyclic voltammograms were collected at a sweep rate of 10 mV/s before obtaining linear sweep voltammograms at a sweep rate of 2 mV/s. Solution resistance was measured and compensated by the potentiostat using the current interrupt method. Uncompensated resistance values (Ru) typically ranged between 13-18 Ω. Electrochemical surface area was determined as previously described.42,43 Cyclic voltammetry was performed on different Co3O4 catalyst films in a region where no redox activity was observed (0.9 – 1.0 V vs. SCE). CV measurements were collected in 0.5 M H2SO4 (pH 0.3) electrolyte before electrolysis and at 2, 4, and 8 hours during a 10 mA/cm2 choronopotentiometry experiment. Varying the scan rate resulted in different currents (Figure S1, averaged at ±15 mV of midpoint potential), and the slope of the j - v plot (Figure S2) gave the capacitance of the catalysts. While literature values for specific capacitance of metal oxides varies significantly, relative roughness can be determined (in our case reported relative to bare FTO) and compared between different catalysts and at different times during electrolysis. The electrochemical stabilities of the annealed films on FTO were examined using cyclic voltammetry and galvanostatic measurements. Cyclic voltammograms were collected at a sweep rate of 50 mV/s and a step size of 50 mV. For galvanostatic measurements, the current density was held at 10 mA/cm2, 1 mA/cm2 and 0.1 mA/cm2, as described in the main text. All galvanostatic measurements were iR corrected using the obtained Ru values. Tafel data were collected starting from 2 V vs. SCE and decreased by 25 mV increments until the observed current fell below 10-6 A. Current was recorded at each applied potential until steady state was reached. The current was averaged over the last 30 seconds of the measurement and used to construct the Tafel plot. Faradaic yield measurements were performed in 0.5 M H2SO4 using a custom-made, two-compartment, airtight electrochemical cell. The cell was directly connected to an online gas chromatograph (Shimadzu GC-2014). Prior to the experiment, the cell was purged with Ar for ~15 min to ensure the removal of air. An Ar flow of 10 mL/min was maintained during testing. Working electrodes were then held at a current density of 20 mA/cm2 for more than 10 hours, while gaseous reaction products were monitored at 12 min intervals using a thermal conductivity detector. The activity of a homogeneous cobalt species was studied by immersing a blank FTO electrode in solutions containing 3.5 ppm Co2+ ions [cobalt(II) nitrate hexahydrate] and 3.5 ppm Co3+ [cobalt(III) acetylacetonate], corresponding to five times the aqueous Co concentration that would be found in solution if all of the cobalt contained in the entire Co3O4/FTO electrode had dissolved. Linear sweep voltammograms were acquired as described above. The concentration of dissolved cobalt ions as a function of time for multiple samples was determined by removing electrolyte aliquots at various time points during chronopotentiometric experiments under


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conditions detailed in the Results and Discussion section. These samples were then diluted with 2% HNO3 and analyzed via inductively coupled plasma mass spectrometry (ICP-MS) using a Thermo Fisher Scientific X Series 2 ICP-MS with Collision Cell Technology. Materials Characterization. The experimental powder X–ray diffraction (XRD) patterns were collected at room temperature with a PANalytical Empyrean Series 2 diffractometer using Cu Kα (λ = 1.5405 Å) radiation and a PIXcel3D detector. Scanning electron microscopy (SEM) images were obtained using a FEI Helios NanoLab 660 at an accelerating voltage of 5.0 keV and a working distance of 2.5 mm. EDS data and elemental mapping were collected at an accelerating voltage of 10.0 keV and a working distance of 2.5 mm. Highresolution TEM (HRTEM) images and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images were collected on an FEI Titan G2 S/TEM equipped with spherical aberration correctors on the image and probe-forming lenses at an accelerating voltage of 200 kV. STEM-EDS maps were acquired in the FEI Titan using the Super-X EDX quad detector system at a current of 0.1 nA. Standardless Cliff-Lorimer quantification was performed on the deconvoluted EDS line intensity data using the Bruker Esprit software. X-ray photoelectron spectra were collected on a PHI VersaProbe II spectrometer, equipped with a scanning monochromatic Al-Kα X-ray source (hν = 1486.6 eV). The X-ray gun was set at high-power mode. All spectra were acquired with electron and ion neutralizers active with the analytical chamber pressures in the mid-10-6 Pa range during data acquisition. Survey and high-resolution scans were recorded at pass energies of 117.4 eV, and 93.9 eV, respectively, and the binding energy values were referenced to C 1s at 284.8 eV. All samples were electrically isolated from the platen, and fastened to it with 3M double-sided tape. ToF-SIMS analysis was performed on a Physical Electronics nanoToF II system equipped with a 30 keV bismuth liquid metal ion gun (LMIG) selected for Bi32 + clusters. Analysis was conducted on a 100 × 100 µm2 area with a primary ion dose of 5.0 × 1011 ions/cm2. Charge neutralization with 15 eV electrons and 10 eV Ar+ was utilized.

the Co3O4 film is conformal and uniform and that Co and O are present in the Co3O4 film and Sn and O are present in the FTO substrate. EDS data, X-ray photoelectron spectra (XPS), and time-of-flight secondary ion mass spectrometry (TOFSIMS) all demonstrate the absence of noble metal contaminants, including Ir and Ru (Figure S3, S4).

Figure 1. SEM images of (A) the as-deposited Co/FTO electrode and (B,C) the annealed Co3O4 electrode.

RESULTS AND DISCUSSION

Cobalt oxide films on FTO were prepared using a multi-step film deposition and oxidation process. Films of elemental cobalt with thicknesses of approx. 150 nm were first deposited onto FTO electrodes using electron-beam evaporation (Figure 1A). The as-deposited Co/FTO films were then annealed at 400 °C in air for 2 h, oxidizing Co to Co3O4 (Figure 1B). The annealing step facilitated a strong electrical and mechanical interface between the catalyst film and the FTO support, which is important for achieving high activity and stability.13,27,44,45 The scanning electron microscopy (SEM) images in Figures 1A and 1B indicate that the Co and Co3O4 films are conformal on the FTO substrate, and the highermagnification SEM image in Figure 1C shows a nanostructured polycrystalline surface with randomly oriented grains. The powder X-ray diffraction (XRD), shown in Figure 2, is consistent with the formation of highly crystalline Co3O4. The (220) and (440) reflections exhibit higher relative intensities than the simulated reference pattern, suggesting the presence of some preferred orientation. A high-resolution TEM (HRTEM) image of a cross section of the film (Figure 3) confirms that the Co3O4 is polycrystalline, with highly interconnected ~25 nm grains that are consistent with different orientations of Co3O4. The inset in Figure 3 shows the (220) plane of Co3O4. The Co3O4 film is approx. 300 nm thick, which is consistent with the volume expansion expected upon oxidation of a ~150-nm Co film. Element maps of a cross section of the Co3O4/FTO electrode (Figures 4A-C), obtained using scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDS), further confirm that

Figure 2. Powder XRD pattern for the Co3O4/FTO electrode (red), along with the simulated XRD pattern for Co3O4 (black) and the locations of the reflections from the FTO.



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Figure 3. HRTEM images of a cross section of the subsurface region of the Co3O4/FTO film.

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is at least 4 times higher than the bare FTO surface (Table S1), reproducibly achieve a current density of 10 mA/cm2 at overpotentials near 570 mV. For comparison, IrO2/FTO and RuO2/FTO electrodes require overpotentials of 330 and 370 mV, respectively, to achieve the same current density. Comparable Mn3O4/FTO and Fe2O3/FTO electrodes exhibit considerably lower catalytic performance than Co3O4/FTO, reaching current densities of less than 0.3 mA/cm2 (close to that of the bare FTO substrate) at similar overpotentials. While the 570mV overpotential of the Co3O4/FTO electrode at 10 mA/cm2 is higher than benchmark IrO2/FTO and RuO2/FTO systems, it is significantly improved relative to other base metal OER catalysts in acid, which either are not reported at pH values as low as 0.3 or do not reach the target current density of 10 mA/cm2. Galvanostatic experiments demonstrated that the Co3O4/FTO electrodes are capable of sustained oxygen production for over 12 h at the PEC-relevant current density of 10 mA/cm2 (Figures 5B and S6) with only negligible changes in the recorded overpotentials. O2 detection by gas chromatography shows that a >95% Faradaic efficiency was achieved and that the rate of O2 production was approximately 3.1 mmol O2/min (Figure S7). As expected, benchmark IrO2/FTO and RuO2/FTO electrodes are also capable of evolving oxygen over the same length of time and with minimal degradation. In contrast, Mn3O4/FTO and Fe2O3/FTO electrodes are unable to sustain oxygen production at 10 mA/cm2. The observed overpotentials for the Mn3O4/FTO and Fe2O3/FTO electrodes almost immediately increase to that of the bare FTO substrate, consistent with the complete dissolution of the films observed after a few cyclic voltammetry cycles in 0.5 M H2SO4. Accelerated degradation studies via cyclic voltammetry (CV) also confirmed that the Co3O4/FTO electrodes are capable of sustained oxygen evolution in 0.5 M H2SO4 (Figure S8). After 500 CV cycles, the overpotential at 10 mA/cm2 increased by only 30 mV.

Figure 4. (A) HAADF-STEM image of a cross-section of the Co3O4/FTO electrode, along with STEM-EDS element maps (B,C) showing the co-localization of Co (red) and O (cyan) in the Co3O4 film, as well as Sn (green) and O (cyan) in the FTO substrate.

The activity of the Co3O4/FTO working electrode for electrocatalytic water oxidation was assessed in 0.5 M H2SO4 (pH 0.3). Figure 5A shows plots of current density vs. potential for Co3O4/FTO, along with IrO2/FTO and RuO2/FTO as benchmark OER catalysts28 and other identically prepared first row transition metal oxides (Mn3O4, Fe2O3) for comparison. The formation of these crystalline phases was verified by XRD (Figure S5). Under these conditions, the Co3O4/FTO electrodes, which have an electrochemical surface roughness that

Figure 5. (A) Linear sweep voltammograms of a Co3O4/FTO electrode, benchmark IrO2/FTO and RuO2/FTO electrodes, and


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Mn3O4/FTO, Fe2O3/FTO, Co2+(aq), Co3+(aq), and bare FTO for comparison. A bare FTO electrode immersed in an aqueous cobalt solution is also shown. (B) Galvanostatic measurements at 10 mA/cm2 for Co3O4/FTO, IrO2/FTO, and RuO2/FTO electrodes.

Additional control experiments further validate the importance of the multi-step Co/FTO → Co3O4/FTO deposition/annealing process for achieving the high crystallinity and robust interface that are putatively important for achieving sustained oxygen evolution in 0.5 M H2SO4. Figure 6 shows plots of current density vs. potential for a variety of bulk and nanostructured cobalt and cobalt oxide materials, all annealed on FTO substrates. Using elemental cobalt as the precursor – including bulk Co powder from multiple suppliers and Co nanoparticles synthesized colloidally and also purchased as powders from commercial vendors – yielded Co3O4/FTO electrodes that were similarly active and stable over several hours. Electrodes having Co3O4 films ranging from 160 – 500 nm showed comparable activities, indicating that the OER activity is independent of film thickness (Figure S9). Co3O4 nanoparticles obtained from annealing colloidally synthesized Co nanoparticles on FTO were more active than the Co3O4/FTO thin films, producing a current density of 10 mA/cm2 at an overpotential of just 490 mV (Figures 6 and S10). This is likely due to the higher surface area of the Co3O4 nanoparticle film, as evident by the higher roughness determined by electrochemical surface area measurements (Table S1). In contrast, Co3O4/FTO electrodes prepared directly from cobalt oxide powders and not through the cobalt → cobalt oxide deposition/annealing process yielded electrodes that had negligible activities approaching that of the bare FTO substrate, despite analogous annealing protocols. Additionally, a ~150-nm Co film deposited onto Au-coated FTO and subsequently annealed at 400 °C in air to form Co3O4/Au/FTO shows similar sustained oxygen evolution activity (Figure S11), suggesting that tin or fluorine doping of the Co3O4 film is not responsible for the observed catalytic activity and that Co3O4 interfaced with conductive substrates other than FTO are also viable.

mately 80 mV/decade for the Co3O4/FTO electrodes in 0.5 M H2SO4. The Tafel slope of the Co3O4/FTO electrode is comparable to the approximately 60 mV/decade reported for various heterogeneous cobalt oxide electrocatalysts studied at higher pH values, including slightly acidic and strongly alkaline electrolytes.38,46 The deviation from a Tafel slope of 60 mV/decade is potentially a consequence of slow dissolution of the Co3O4 film over the duration of the catalytic testing.47 The significant difference in Tafel slope between Co3O4/FTO and that expected for homogeneous catalysis by soluble Co species (> 120 mV/decade),38 along with the lack of any significant catalytic activity of 60 mM Co2+(aq) and 60 mM Co3+(aq) at pH 0.3 (Figure 5A), indicates that soluble Co2+ or Co3+ species do not contribute significantly to the observed OER activity. Assuming the complete dissolution of Co3O4 from the Co3O4/FTO electrode and concomitant release of stoichiometric O2(g), the volume of O2(g) produced would be only ~0.037 mL, which accounts for approximately 0.1% of the observed 27.4 mL of O2(g) produced by the Co3O4 film over 12 h. Taken together, the data indicate that Co3O4 film dissolution and homogeneous cobalt species do not contribute significantly to the observed catalysis. However, the ability of the Co3O4/FTO film to sustain oxygen evolution in 0.5 M H2SO4 at a current density of 10 mA/cm2 is remarkable, given that cobalt oxide is widely considered to undergo rapid dissolution in strongly acidic solutions at anodic potentials.26,27 Our observation that a ~300-nm Co3O4 film on FTO remains capable of sustained oxygen production for more than 12 h at 10 mA/cm2 indicates that dissolution of the Co3O4 film in 0.5 M H2SO4 occurs very slowly To quantify the dissolution process as a function of pH, film thickness, applied potential, and current density, timedependent dissolution data were obtained by inductively coupled plasma mass spectrometry (ICP-MS) analysis of soluble cobalt species in solution at time points taken over 12 h. The average Co dissolution rate for the 300-nm Co3O4 film held galvanostatically at 10 mA/cm2 was 100 ng/min (Figure 7). SEM images after 10 min and 4 h of galvanostatic testing at 10 mA/cm2 reveal a surface that has smaller features than the starting film, along with the formation of pits (Figure S12). The dissolution rate for the 160-nm Co3O4 film was 90 ng/min (Figure S13), which is comparable to that of the 300-nm film and suggests that film thickness has little effect on dissolution rate. However, the 300-nm Co3O4 films at 10 mA/cm2 dissolve at rates of 100 ng/min and 50 ng/min at pH 1 and pH 2, respectively (Figure S14). As pH increases, dissolution becomes slower, as expected.48 For comparison, ruthenium oxides in strongly acidic solutions and FeOOH in strongly alkaline solutions were reported to dissolve at rates of 540 ng/min39, 40 and 1200 ng/min, respectively.51

Figure 6. Linear sweep voltammograms of various cobalt oxide materials on FTO. Electrodes of bulk CoO and bulk Co3O4 deposited directly onto FTO, as well as bare FTO, are also shown.

Tafel analysis (Figure S7), which can provide insights into the mechanism of OER catalysis, yields a slope of approxi-



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strongly acidic solutions at oxidizing potentials, 26,27 the Co3O4 films on FTO dissolve at rates of approx. 1-100 ng/min, depending on the pH and applied potential, which leads to 300nm films being able to sustain oxygen evolution catalysis for 12 h at 10 mA/cm2 and several days at 1 mA/cm2 and 0.1 mA/cm2 Additional electrode engineering, composition modulation, and corrosion protection strategies33,52-53 may help to decrease the dissolution rate to produce even longer-lasting catalyst films. Furthermore, the identification of an electrode preparation method that maximizes activity and stability for Co3O4/FTO may be portable to other systems and also help facilitate the discovery of new, acid-stable OER catalysts comprised of Earth-abundant elements.

ASSOCIAT ED CONT ENT Figure 7. ICP data characterizing the time-dependent dissolution of a 300-nm Co3O4 film on FTO during chronopotentiometric experiments at 10 mA/cm2, 1 mA/cm2, and 0.1 mA/cm2. At lower current densities that require lower overpotentials, the Co dissolution rates decrease significantly. At 1 mA/cm2, the dissolution rate was 20 ng/min while at 0.1 mA/cm2, the dissolution rate was 1 ng/min (Figure 7). As a result, significantly longer operation times can be achieved at lower current densities. Figure 8 shows additional galvanostatic measurements at current densities of 1 mA/cm2 and 0.1 mA/cm2. At these lower current densities, which better match those used previously to evaluate the performances of other Earthabundant OER catalysts in acidic solutions,33 the Co3O4/FTO electrode is highly durable in 0.5 M H2SO4 and is capable of catalyzing sustained oxygen production for over 50 hours at 1 mA/cm2 and for more than 10 days at 0.1 mA/cm2. Its ability to facilitate sustained oxygen production on the scale of hours to weeks at operationally relevant current densities, including a moderate 570-mV overpotential at 10 mA/cm2 in 0.5 M H2SO4, places the Co3O4/FTO catalyst in a category unmatched by other Earth-abundant OER catalysts (Fig. S15).

Supporting Information Experimental details and additional XRD, XPS, EDS and electrochemical data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected]

Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT This work was supported by the U.S. National Science Foundation Center for Chemical Innovation in Solar Fuels (CHE1305124). HRTEM, SEM, XRD, ToF-SIMS, and XPS data were acquired through the Materials Characterization Laboratory at the Penn State University Materials Research Institute. The authors thank Greg Barber, Vince Bojan, William Drawl, Jordan Lerach, Matthew Gonzales, Frank Dorman, and Paulina Piotrowski for technical support.

REFERENCES

Figure 8. Galvanostatic measurements of a Co3O4/FTO electrode at (A) 1 mA/cm2 and (B) 0.1 mA/cm2, showing sustained electrocatalytic oxygen production for 50 h and 10 days, respectively.

CONCLUSIONS In summary, we have shown that Co3O4 films on FTO made by first depositing Co and then annealing at 400 °C in air to produce a robust catalyst/substrate interface are highly crystalline and capable of sustained oxygen evolution at operationally relevant current densities in strongly acidic solutions. While cobalt oxides are widely considered to dissolve quickly in

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