Simple Chemical Solution Deposition of Co3O4 Thin Film

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A simple chemical solution deposition of Co3O4 thin film electrocatalyst for oxygen evolution reaction Hyo Sang Jeon, Michael Shincheon Jee, Haeri Kim, Su Jin Ahn, Yun Jeong Hwang, and Byoung Koun Min ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b06189 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 25, 2015

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ACS Applied Materials & Interfaces

A simple chemical solution deposition of Co3O4 thin film electrocatalyst for oxygen evolution reaction Hyo Sang Jeon†, ‡, Michael Shincheon Jee†, Haeri Kim†, Su Jin Ahn†, §, Yun Jeong Hwang†, ‡ and Byoung Koun Min*†, ‡, §

†Clean

Energy Research Center, Korea Institute of Science and Technology, 39-1 Hawolgokdong, Seongbuk-gu, Seoul 136-791, Republic of Korea

‡Korea

University of Science and Technology, 176 Gajung-dong, 217 Gajungro Yuseong-gu, Daejeon 305-350, Republic of Korea

§Green

School, Korea University, Anam-dong Seongbuk-gu, Seoul 136-713, Republic of Korea

ABSTRACT Oxygen evolution reaction (OER) is the key reaction in electrochemical processes such as water splitting, metal-air batteries, and solar fuel production. Herein, we developed a facile chemical solution deposition method to prepare a highly active Co3O4 thin film electrode for OER, showing a low overpotential of 377 mV at 10 mA/cm2 with good stability. An optimal loading of ethyl cellulose additive in a precursor solution was found to be essential for the morphology control and thus its electrocatalytic activity. Our results also show that the distribution of Co3O4 nanoparticle catalysts on the substrate is crucial to enhance the inherent OER catalytic performance.

KEYWORDS : OER, water splitting, Co3O4, electrocatalyst, chemical solution deposition

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Introduction Development of electrocatalysts for oxygen evolution reaction (OER) has attracted much research attention due to their importance in electrochemical processes such as metal-air batteries, water electrolysis, and solar to fuel production.1-7 Among the various catalysts for OER, cobalt (Co)-based catalysts are considered one of the best performing materials; and the spinel oxides, namely Co3O4, have been reported to have high electrochemical activity with low overpotential and good chemical stability.8-19 However, like most materials, the OER activity of a Co3O4 electrode is highly dependent on the preparation methods. For example, Co3O4 electrode can be prepared by a two-step process in which the powder is synthesized and is later deposited onto a substrate surface with an additive material.20 Not only is this inconvenient for electrochemical testing, the extra contact resistance and reaction site obstruction from the additive has negative effects on the stability and catalytic activity.21,22 Therefore, developing an efficient, inexpensive process that directly synthesizes Co3O4 onto a substrate without compromising its intrinsic catalytic activity is critical before the catalyst can be marketed commercially. Up to now, various techniques have been developed to directly fabricate Co3O4 film onto substrates such as by using plasma sputtering and atomic layer deposition, but they need a high capital investment as well as maintenance due to the nature of vacuum equipments involved.23,24 Also, electrodeposition and hydrothermal method have been suggested to make the films, but they show lower OER catalytic activities.25,26 Meanwhile, solution-based methods for preparing electrodes allow the use of inexpensive processing techniques like spin coating and also allow a variety in synthetic approaches with a combination of precursor chemicals and additive materials.27,28 For the preparation of an electrode, a dispersant and an additive are need to


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formulate the paste for easier handling and storage.29 Also, the high surface areas of the porous structures resulting from these processes enhance their respective applications. Herein, we report a facile chemical solution deposition (CSD) method using a metal salt precursor to produce Co3O4 catalytic film on fluorine doped tin oxide (FTO) substrate for OER. In particular, the whole procedure to make the film was simplified by preparing Co precursor solution that simultaneously synthesizes and deposits Co3O4 onto the substrate. The catalytic activity of Co3O4 electrode shows an overpotential of 377 mV to achieve the current density of 10 mA/cm2, and the electrode was also tested to be stable over 10 h.

Experimental Preparation of Co3O4 electrocatalytic film by chemical solution deposition (CSD) A metal precursor solution was spin-coated onto a FTO substrate. A precursor mixture solution was prepared by dissolving Co(NO3)2·6H2O (Aldrich, 1.0 g) in ethanol (5.0 mL), followed by adding an ethanol solution (10.0 mL) mixed with ethyl-cellulose (EC) (Aldrich, 1.0 g) and alpha-terpineol (Aldrich, 5 mL). The solution mixture was thoroughly stirred for 30 min prior to spin coating on a clean FTO substrate. Finally, the resulting film was processed under thermal deposition and was allowed to anneal at 500 oC for 1 h in air to form the Co3O4 thin film electrocatalyst. Preparation of conventional Co3O4 electrocatalytic film by a doctor-blade coating Co3O4 nanoparticle (~50 nm, Aldrich) was dispersed in isopropanol (J. T. Baker) and a Nafion solution (20 wt%, DuPont) was added to prepare a paste, which was subsequently coated on a FTO substrate by doctor-blade coating. After coating the paste on the FTO substrates, the samples were placed into an oven or/and furnace for casting at 70oC for 30 min in air.


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Electrochemical measurements for OER activity All of the electrocatalytic measurements were conducted by using a potentiostat (Ivium, Iviumtechnology) in a one-compartment electrochemical cell made of polyether ether ketone (PEEK). All measurements were carried out in 1 M NaOH (≥ 99.99%, Aldrich) electrolyte (pH = 13.7) at a scan rate of 0.5 mV/s. A platinum counter electrode and a Hg/HgO reference electrode in 1 M NaOH were used in a three-electrode configuration to characterize the catalytic activities of the Co3O4 nanoparticle film. All potential values were converted in terms of reversible hydrogen electrode (RHE) using E (vs. RHE) = E (vs. Hg/HgO) + 0.140 + 0.059 V × pH. Solution resistance (Rs) was measured by circle fitting from electrochemical impedance spectroscopy (EIS) measurement. Potential values were compensated for iR loss and our electrochemical cell had Rs = ~ 20 Ω in 1 M NaOH. Roughness factor (Rf) was obtained to determine the electrochemical surface area (ECSA) of the Co3O4 films by measuring the double-layer capacitance (Cdl) in a non-faradaic potential region from 0.20 to 0.25 V vs. Hg/HgO. The Cdl values were calculated from the slope of the current vs. the scan rate and divided by the value of 60 uF/cm2 (the capacitance constant for planar Co3O4) to obtain the Rf. Products analysis for H2 and O2 production The products analysis for hydrogen and oxygen during water splitting were conducted in a three-electrode configuration in a gas-tight one compartment electrochemical cell. The gas products were quantified by gas chromatography (Younglin 6500 GC) equipped with a pulsed discharge detector (PDD) using ultra high purity (UHP) He (99.9999 %) as the carrier gas. GC was directly connected to the electrochemical cell and the gaseous samples were injected by a six-port valve. The electrochemical cell contained 40 mL of electrolyte and 15 mL of headspace


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and He gas was bubbled through the electrolyte at an average rate of 1 mL/sec. The Faradaic efficiencies (F.E.) of H2 and O2 gaseous products were calculated from the areas of GC chromatogram as:

where V(H , O ) is volume concentration of H2 or O2 based on calibration of the GC, Q is a flow 2

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rate (L/sec) measured by a universal flow meter (Agilent Technologies, ADM 2000) at the exit of the electrochemical cell. itotal is a steady-state current, F is the Faraday constant (96485 A·s/mol), p0 is pressure, T0 is temperature (273.15 K), and R is the ideal gas constant (8.314 J·mol-1·K-1). A stability test was performed for 10 hours in 1 M NaOH solution by chronoamperometry at 1.6 V vs. RHE while checking the Faradaic efficiency of H2 and O2 at 10 min intervals. Characterizations The surface morphologies were imaged by field emission gun scanning electron microscopy (FEG-SEM, FEI Inc., Inspect F), transmission electron microscopy (TEM, FEI, Titan) and grazing incidence x-ray diffraction (GI-XRD, Rigaku corporation, D/Max 2500) with Cu-Kα radiation (0.15406 nm) were used to characterize the crystal structure of the Co3O4. X-ray photoelectron spectroscopy (XPS, VG Scientific, Sigma probe) with monochromated aluminium Kα radiation was conducted to analyze the elemental composition. The C1s core level at 284.6 eV was used as a peak reference. The detailed surface morphology of the Co3O4 electrode was measured by using an atomic force microscope (AFM, Park System Co., XE-100).


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Results and Discussion The procedure for the Co3O4 film synthesis is illustrated in Figure 1. A precursor solution with Co precursor (Co(NO3)2·6H2O) and ethyl cellulose (EC) additive was spin-coated onto a FTO glass substrate followed by annealing at 500oC in air. EC was used to improve wetting of the precursor solution on the substrate, which influences the quality of the film (e.g. suppression of film cracks).29 In addition, viscosity is proportional to the EC amount which determines the thickness of the spin-coated film, and thus controls the thickness of the Co3O4 film. In order to find optimum precursor solution condition for Co3O4 OER catalyst, we first conducted linear sweep voltammetry (LSV) measurement to compare OER activities of various Co3O4 films from precursor solutions added with different amounts of EC: 0, 0.25, 0.50 and 0.75 g, which are denoted as Co3O4_EC_0, Co3O4_EC_0.25, Co3O4_EC_0.50 and Co3O4_EC_0.75, respectively (Figure 2). The Co3O4 film electrodes present lower overpotentials and higher current densities than the bare FTO substrate, indicating the OER catalytic activity of Co3O4 films. As with the conventional metric in academia and industry, the overpotentials were determined at 10 mA/cm2. At this current, the overpotential of Co3O4_EC_0.50 is 377 mV while Co3O4 film electrode fabricated without EC (Co3O4_EC_0) shows slightly higher overpotential, 433 mV. Co3O4_EC_0.75 rather showed an increased overpotential up to 401 mV. These results confirm that Co3O4_EC_0.50 showed the best performance, and hence our investigation was focused on this catalyst. Notably, we also investigated the Co3O4 film prepared in an absence of EC as a comparison. With scanning electron microscopy (SEM) images (Figure 3a and b), we observed that a ~180 nm thick layer of ~20 nm particles was uniformly synthesized on the FTO substrate over the deposited area. The thickness of the films increased as the amount of EC was increased (see


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supporting information, Figure S1).

EC in the precursor solution seems to suppress the

formation of large cracks of a few hundred nanometers formed during the drying process as seen in Co3O4_EC_0 (see supporting information, Figure S1a). In the high resolution transmission electron microscopy (HR-TEM) images clearly showed the lattice fringes of d220 (0.285 nm) and d311 (0.244 nm), indicating the cubic spinel structure of Co3O4 (Figure 3c). This crystal structure is well matched with grazing incidence X-ray diffraction (GI-XRD) measurement (Figure 3d). All of the XRD patterns were solely associated with the spinel Co3O4 crystal structure (JCPDS 09-0418) without other cobalt oxide phases such as CoO or Co2O3. X-ray photoelectron spectroscopy (XPS) revealed the presence of only the elements Co, O, and C in the Co3O4 film (Figure 4a). In the Co 2p region, the main peaks at 780.0 and 795.6 eV can each be assigned to Co 2p3/2 and Co 2p1/2 binding energies (BE).30 When the 2p3/2 peak was deconvoluted, the peaks at 781.6 and 780.0 eV can be assigned to Co2+ and Co3+ states, respectively, while 789.6 and 786.0 eV correspond to their respective satellite lines (Figure 4b).31 As mentioned earlier, we could confirm that Co3O4_EC_0.50 had the best OER catalytic activity with an overpotential of 377 mV at 10 mA/cm2 (Figure 1). Such performance was comparable to that of the best pure Co3O4 OER catalysts that have been reported previously (Table S1). When we measured the Tafel slopes (Figure 5) in order to investigate the kinetics of OER on Co3O4 electrode, the lower Tafel slope for Co3O4_EC_0.50 of 58.1 mV/dec compared to that of Co3O4_EC_0 (69.7 mV/dec) suggest its beneficial effect in accelerating OER kinetics. This value (approximately 60 mV/dec) indicated that the rate-determining step is the formation of the OH– adsorbate on the active sites in alkaline solution.32 In order to prepare catalytic films for OER, it is more commonly practiced to use additive materials such as Nafion and polyvinylidene fluoride (PVDF) as physical adhesive after


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synthesizing the catalysts separately, which counterproductively lowers catalytic activity by restricting contact between the catalyst and the electrode.26 When comparing the OER activities of electrodes made by conventional methods using commercial Co3O4 nanoparticle catalyst and Nafion as an additive, the electrode synthesized with Co3O4_EC_0.50 exhibited higher performance (see supporting information, Figure S2). This may be mainly due to the fact that the catalyst is in good contact with the electrode which translates to lower resistance in electron transfer. One consideration that must be made before fully acknowledging the enhanced OER catalytic activity is the Co3O4 catalysts loading on the substrates varied by different amount of EC added to the precursor solution. To verify the origin of the enhanced current density, the surface areas of Co3O4_EC_0 and Co3O4_EC_0.50 electrodes were measured electrochemically.33 When comparing Co3O4_EC_0 and Co3O4_EC_0.50 electrodes, the ECSA of Co3O4_EC_0.50 (Rf: 6.2) had approximately twice that of Co3O4_EC_0 (Rf: 3.3). When the specific current densities were recalculated using Rf and the geometric current densities (Jspecific = Jgeometric / Rf), we could confirm that the electrodes made with EC in the precursor solution yielded higher currents than the electrodes without EC, which respectively were 1.20 and 0.48 mA/cm2 at 1.6 V vs. RHE (Figure 6). These results validate the enhanced intrinsic OER activity of Co3O4 films when EC was used in the precursor solution. A contributing factor for enhanced OER activity may arise from the morphological characteristics of the uniformly synthesized Co3O4 particle on the substrate. In order to observe a detailed surface morphology, atomic force microscope (AFM) images were obtained. (Figure 7) We could confirm similar morphologies observed from SEM images above but some unnoticed features were more discernible in terms of porosity and particle size. Co3O4_EC_0.50 had


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uniformly distributed ~20 nm particles with pores of a few nanometers. On the other hand, a densely packed catalyst with very low degree of porosity was observed in Co3O4_EC_0 with relatively larger particle sizes than that of Co3O4_EC_0.50. In case of the densely packed film, less active sites are expected to be exposed for the electrochemical surface reaction. To count the number of active sites on the Co3O4 film, an oxidation peak of Co3+ to Co4+ around 1.40 V vs. RHE in the LSV data (see supporting information, Figure S4) was carefully observed because the Co3+ cation is thought to be the active sites for OER in Co3O4 catalysts.15 The anodic peak areas of Co3+/Co4+ were used by integrating oxidation peak areas.34 It was found that Co3O4_EC_0.50 had almost two times higher number of active sites (2.57×1012) than that for Co3O4_EC_0 (1.41×1012), which is in good agreement with our specific current density data (Figure 6). The enhanced activity for OER could be sourced from the dynamic change of the chemical composition of the catalytic surface. However, interestingly, the Co2+/Co3+ ratios were both approximately two as determined by deconvoluting the Co 2p region of the XPS data, meaning that EC does not change the composition of Co3O4 (see supporting information, Figure S5). It can be deduced that the more porous structure of Co3O4_EC_0.50 films may facilitate higher probability for hydroxyl ions to interact with the Co3+ sites to generate oxygen while the agglomeration of particles in Co3O4_EC_0 may be actually obstructing the reaction sites. Thus, the distribution of nanoparticles on the substrate contributed more to the catalytic activity than its chemical composition. The long-term stability of the synthesized Co3O4 was investigated with chronoamperometry at 1.6 V vs. RHE. The current density was maintained at almost a constant level over the 10 h duration (Figure 8a). The Faradaic efficiencies for hydrogen and oxygen evolution were 101.6 ± 2.5 and 101.6 ± 2.9%, respectively, and a ratio close to two was confirmed (Figure 8b). This


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shows that our synthesis method could be used to produce highly efficient and durable electrocatalysts for the production of clean fuels from the electrolysis of water. Furthermore, this method is inexpensive because the simple, one-step annealing process of the deposited precursor solution allow the simultaneous synthesis and deposition of catalyst directly on a substrate as an attractive aspect for industry. Lastly, it should be noted that there are many possibilities to improve the OER catalytic activity of the undecorated Co3O4 further. For example, many researchers have reported that the OER catalytic activity of Co3O4 could be improved with transition metal dopants such as Ni, Mn, and Fe by increasing its electrical conductivity.35-39 Decorating Co3O4 with Au or C materials such carbon nanotube and graphene have also been reported to enhance OER catalytic performance and stability.40-43 We could expect that this method is easily applicable for such material combination by adding Ni, Mn, Fe, Au and C materials in the precursor solution. Therefore, we suggest that OER catalytic performance of Co3O4 combined with other elements to further improve other features, and is now under investigation. We also expect that this method could be applied to other electrochemical processes such as oxygen reduction reaction (ORR), hydrogen evolution reaction (HER) and carbon dioxide reduction reaction (CO2RR) as engineered distribution of the nanoparticle catalysts on substrates remains to be a crucial issue to obtain the inherent catalytic performances.

Conclusion In this study, we developed a facile and simple precursor solution based method to produce Co3O4 catalytic film for OER. The overpotential of Co3O4_EC_0.50 is 377 mV at 10 mA/cm2 and has the long term stability over 10 h. Our results show that high activity of our synthetic


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method for OER is attributed to the unique feature in the distribution of nanoparticles on the substrate that facilitate a higher probability for hydroxyl ions to interact with active sites to generate oxygen. In addition, our CSD method is further applicable beyond the pure Co3O4 electrode to improve the OER catalytic activity.


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ASSOCIATED CONTENT Supporting Information Figure S1-S5 and Table S1 are included in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author * B.K. Min: Tel.: +82 2 958 5853; Fax: +82 2 958 5809; E-mail: [email protected] ACKNOWLEDGMENT This work was supported by the program of the Korea Institute of Science and Technology (KIST) and partly by the University-Institute cooperation program of the National Research Foundation of Korea Grant funded by the Ministry of Science, ICT and Future Planning. REFERENCES 1. Gong, M.; Li, Y.; Wang, H.; Liang, Y.; Wu, J. Z.; Zhou, J.; Wang, J.; Regier, T.; Wei, F.; Dai, H. An Advanced Ni–Fe Layered Double Hydroxide Electrocatalyst for Water Oxidation. J. Am. Chem. Soc. 2013, 135, 8452-8455. 2. Kudo, A.; Miseki, Y. Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. 3. Lu, Y.-C.; Xu, Z.; Gasteiger, H. A.; Chen, S.; Hamad-Schifferli, K.; Shao-Horn, Y. Platinum−Gold

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the Ni Particle-Dihydroterpineol Interface. J. Am. Ceram. Soc. 2006, 89, 3050-3055. 30. Zhang, L.; He, W.; Xiang, X.; Li, Y.; Li, F. Roughening of Windmill-Shaped Spinel Co3O4 Microcrystals Grown on a Flexible Metal Substrate by a Facile Surface Treatment to Enhance Their Performance in the Oxidation of Water. RSC Adv. 2014, 4, 43357-43365. 31. Xiang, X.; Zhang, L.; Hima, H. I.; Li, F.; Evans, D. G. Co-Based Catalysts from Co/Fe/Al Layered Double Hydroxides for Preparation of Carbon Nanotubes. Appl. Clay Sci. 2009, 42, 405-409. 32. Bockris, J. O.; Otagawa, T. Mechanism of Oxygen Evolution on Perovskites. J. Phys. Chem. 1983, 87, 2960-2971. 33. McCrory, C. C. L.; Jung, S.; Peters, J. C.; Jaramillo, T. F. Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 1697716987. 34. Liu, X.; Chang, Z.; Luo, L.; Xu, T.; Lei, X.; Liu, J.; Sun, X. Hierarchical ZnxCo3–xO4 Nanoarrays with High Activity for Electrocatalytic Oxygen Evolution. Chem. Mater. 2014, 26, 1889-1895. 35. Yang, Y.; Fei, H.; Ruan, G.; Xiang, C.; Tour, J. M. Efficient Electrocatalytic Oxygen Evolution on Amorphous Nickel–Cobalt Binary Oxide Nanoporous Layers. ACS Nano 2014, 8, 9518-9523. 36. Xiao, C.; Lu, X.; Zhao, C. Unusual Synergistic Effects upon Incorporation of Fe and/or Ni into Mesoporous Co3O4 for Enhanced Oxygen Evolution. Chem. Commun. 2014, 50, 1012210125. 37. Li, Y.; Hasin, P.; Wu, Y. NixCo3−xO4 Nanowire Arrays for Electrocatalytic Oxygen Evolution. Adv. Mater. 2010, 22, 1926-1929. 38. Lee, D. U.; Kim, B. J.; Chen, Z. One-pot Synthesis of a Mesoporous NiCo2O4 Nanoplatelet and Graphene Hybrid and Its Oxygen Reduction and Evolution Activities as an Efficient Bifunctional Electrocatalyst. J. Mater. Chem. A 2013, 1, 4754-4762. 39. Kim, T. W.; Woo, M. A.; Regis, M.; Choi, K.-S. Electrochemical Synthesis of Spinel Type ZnCo2O4 Electrodes for Use as Oxygen Evolution Reaction Catalysts. J. Phys. Chem. Lett. 2014, 5, 2370-2374. 40. Yeo, B. S.; Bell, A. T. Enhanced Activity of Gold-Supported Cobalt Oxide for the Electrochemical Evolution of Oxygen. J. Am. Chem. Soc. 2011, 133, 5587-5593.


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Table of Contents


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Figure 1. Schematic of the procedure for the solution-based Co3O4 electrode synthesis.


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Figure 2. Linear sweep voltammetry (LSV) measurements of the FTO substrate and Co3O4_EC_0, Co3O4_EC_0.25, Co3O4_EC_0.50 and Co3O4_EC_0.75


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Figure 3. Characterization of Co3O4_EC_0.50 electrode: (a) and (b) SEM images electrode, (c) a TEM image, (d) GI-XRD patterns


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Figure 4. XPS spectra of Co3O4_EC_0.50 electrode with (a) wide scan and (b) narrow scan of Co 2p .


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Figure 5. Tafel slope of Co3O4_EC_0.0 and Co3O4_EC_0.50 electrode.


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Figure 6. Specific current density of Co3O4_EC_0 and Co3O4_EC_0.50 electrode recalculated from the geometric current density (Figure 1) with roughness factor (Figure. S3) consideration (Jspecific = Jgeometric / Rf).


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Figure 7. AFM images of (a) Co3O4_EC_0 and (b) Co3O4_EC_0.50 showing detailed surface morphology and greater interparticular distance for Co3O4_EC_0.50.


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Figure 8. (a) Long term stability test at 1.6 V vs. RHE, (b) Faradaic efficiency of H2 and O2 with their ratio.


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Simple Chemical Solution Deposition of Co3O4 Thin Film

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