Highly Active Oxygen Evolution on Carbon Fiber Paper Coated with

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Highly Active Oxygen Evolution on Carbon Fiber Paper Coated with Atomic Layer-deposited Cobalt Oxide Hyung Jong Choi, Gwon Deok Han, Kiho Bae, and Joon Hyung Shim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19064 • Publication Date (Web): 25 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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ACS Applied Materials & Interfaces

Highly Active Oxygen Evolution on Carbon Fiber Paper Coated with Atomic Layer-deposited Cobalt Oxide Hyung Jong Choi1, Gwon Deok Han1, Kiho Bae1,2, and Joon Hyung Shim1,*

1School

of Mechanical Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul

02841, South Korea 2Department

of Mechanical Engineering, Stanford University, 440 Escondido Mall, Stanford,

CA 94305, U.S.A.

KEYWORDS Atomic layer deposition, thin film catalysts, cobalt oxide, oxygen evolution, surface chemistry

ACS Paragon Plus Environment

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ABSTRACT

In this work, we evaluated the oxygen evolution performance of cobalt oxide (CoOx)-coated carbon fiber paper in electrochemical water splitting. For uniform coating of the CoOx layers along the carbon fiber paper, the atomic layer deposition (ALD) technique was applied. We achieved uniform and conformal coating of atomic-layer-deposited-CoOx (ALD-CoOx) on the carbon fiber paper. The overpotential for oxygen evolution measured for the optimized ALDcoated carbon fiber paper was as low as 343 mV at 10 mA cm-2, which is competitive with the activity of state-of-the-art CoOx prepared on electrodes with large surface areas. Oxygen evolution is not enhanced after a critical thickness, about 28 nm in our study, is reached. The optimal thickness of the ALD-CoOx film is dependent on two competing effects; the high oxidation state of cobalt ions in thicker CoOx helps oxygen evolution, whereas the introduction of a thick oxide coating decelerates the rate of charge transfer at the surface.


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INTRODUCTION Electrochemical water splitting to make hydrogen and oxygen enables storage of electrical energy as chemical energy. This process is especially attractive for storing electricity produced from renewable resources such as wind or solar energy, since there is no reliance on the carbon material and the energy required for transportation is relatively low due to high gravimetric energy density (142 MJ/kg for hydrogen gas vs. 55.6 MJ/kg for methane based on the higher heating value (HHV)).1 The rate of electrochemical water splitting is led by the oxygen evolution reaction (OER) at the anode due to the sluggish reaction kinetics of the multielectron charge transfer process with respect to the hydrogen evolution reaction at the cathode.2-3 Therefore, finding a good anodic catalyst for the OER is the key. However, many practically useful high-performance materials are very expensive. For example, currently, the most actively investigated electrocatalysts for the water-splitting OER are IrOx (US$ 32.79 g-1 as of March 23rd, 2018) and RuO2 (US$ 6.75 g-1 as of March 23rd, 2018).4 In this respect, developing a novel catalyst that can perform equivalently well as IrOx or RuO2 or outperforming these materials, but with a cheaper price is very important. So far, transition metal- (Mn, Ni, Co and Fe etc.)-based compounds have exhibited potentials as alternatives.5-7 Recently, coating of thin OER-active layers on current-collecting supports has been proven as an effective strategy to enhance catalytic performance.8-10 Thin film-based electrocatalysts grown directly on a current collector have the advantage of superior adhesion and greater surface-to-volume ratio due to nanostructuring.11 Furthermore, as the thickness of the OER layer is very small, energy loss from the charge transport of the film can be minimized. This advantage is especially attractive when the electrical conductivity of the OER layer is relatively low while surface catalysis is very active. Furthermore, the use of nanoporous materials or entangled nanowires as the coating support and current collector can maximize the catalysis surface.12-14


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Therefore, OER-active films are ideally coated as thin as possible, uniform along the nanosupport. In this respect, atomic layer deposition (ALD) can be a good option for coating. ALD has attracted attention due to its ability to achieve excellent step coverage on high-aspectratio structures such as nanotrenches or pores.15 For this reason, recent studies have actively exploited the ALD process to develop high-performance OER catalysts. For example, 50 nmthick ALD MnOx prepared on a glassy carbon substrate showed activity comparable to the best MnOx catalysts reported in the literature.16 In addition, other studies have successfully produced high-density nanoscale ALD NiOx thin films (~18 nm) and have been able to improve OER activity by adding Fe in-situ using Fe-rich KOH electrolytes.17 Another advantage of the ALD process other than conformality is its ability to synthesize multi-component films in a very thin layer simply by mixing ALD cycles of component materials. ALD can control composition and thickness on the scale of one or less than one atom layer, in principle. Using this advantage of ALD, composite films comprising MnOx/TiO2 have been fabricated with thicknesses of 1.4-2.8 nm as OER catalysts, successfully demonstrating the effects of the change in composition on catalytic activity.9 In this study, we evaluated the effect of OER catalysis on atomic layer-deposited CoOx (ALD-CoOx) on carbon fiber paper. CoOx-based catalysts have been reported to be one of the non-precious catalysts for OER with low overvoltage and high corrosion stability, as well as much lower cost than RuO2 or IrO2.18 Despite the potential of ALD to allow conformal deposition of nanoscale thin-film catalysts on large-surface-area electrodes, little research has been reported on CoOx using ALD for electrochemical water splitting. In addition, whilst many studies on the catalytic activity of CoOx use simple planar electrodes such as glassy carbon1920

and fluorine-doped tin oxide (FTO),21-22 we used ALD to directly grow nanoscale CoOx films

on self-supporting carbon fiber paper electrodes. Taking advantage of the excellent step coverage of the ALD process, we were able to prepare carbon fiber paper composed of complex


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3D structured carbon fibers decorated by the uniform nanoscale CoOx thin-film electrocatalyst (Fig. 1). To characterize the ALD-CoOx electrocatalytic activity on carbon fiber paper, we analyzed the OER performance of ALD-CoOx of various thicknesses in alkaline solution. Finally, we studied the thickness limitations of ALD-CoOx to optimize OER activity, considering the microstructure and surface chemistry.

Figure 1. Schematic diagram of carbon fiber paper coated with atomic layer-deposited cobalt oxide as an electrocatalyst. EXPERIMENTAL SECTION The deposition of a thin CoOx film on CP was performed using an ICOT-MINI ALD system (ICOT Inc.) equipped with a tubular stainless-steel chamber. Details of the hardware are well documented in previous reports.23 We used the bis(1,4-diiso-propyl-1,4-diazabutadiene)cobalt [C16H32N4Co,Co(dpdab)2] (UP Chemical Co.) precursor for the deposition of CoOx. Ozone was selected as the oxidant for the ALD reaction. Prior to deposition, CP was ultrasonically cleaned with acetone, methanol, and DI water. The chamber pressure was maintained at 0.1 Torr using


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a rotary pump. The chamber and cobalt precursor were heated to 200 and 112.5 °C, respectively. During the deposition, high-purity nitrogen gas (3 sccm) was flowed through the chamber to purge by-products and excess chemicals. One cycle of ALD-CoOx on CP consisted of N2 purge (40 s)/Co(dpdab)2 pulses (5 s)/N2 purge (40 s)/O3 pulse (10 s). The growth rate of ALD-CoOx was 0.4 A/cycle. Deposition of ALD-CoOx was carried out with different numbers of cycles (50, 100, 350, and 700). The morphology and chemical composition of the ALD-CoOx thin films were analyzed by scanning electron microscopy (SEM; Hitachi S-4800) using energy-dispersive X-ray spectroscopy. The detailed microstructure and chemical composition were characterized by a transmission electron microscope (TEM; TECNAI G2 F30 operating at 300 kV) equipped with an electronic energy loss spectrometry (EELS) detector. X-ray photoelectron spectroscopy (XPS, ULVAC-PHI X-TOOL) was used to analyze the surface electronic state and chemical composition of the sample. Electrochemical measurements including linear swept voltammetry (LSV) and electrical impedance spectroscopy (EIS) were performed using an electrochemical analyzer (Gamry Reference 3000 Potentiostat/Galvanostat/ZRA). The LSV was measured at 0-0.8 V (Ag/AgCl) and the potential scale was converted to reversible hydrogen electrode (RHE). EIS measurements were performed from 0.5 to 106 Hz at a potential of 0.6 V (vs. Ag/AgCl). For the electrochemical analysis of water oxidation, a three-electrode cell consisting of a graphite rod (counter), a saturated Ag/AgCl electrode, and a carbon fiber paper (working area 1 × 1 cm2) was adopted. KOH (1 M) was used as an aqueous electrolyte.

RESULTS AND DISCUSSION ALD-CoOx was grown on carbon fiber paper using various numbers of ALD cycles: 50, 100, 350, and 700 cycles were used, and the corresponding samples are denoted CP-Co50, CP-


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Co100, CP-Co350, and CP-Co700, respectively. Their thickness corresponds to 2, 4, 14, and 28 nm, respectively, depending on the growth rate during ALD (0.4 Å/cycle). Figure 2 shows the microstructure of ALD-CoOx deposited on carbon fiber paper. Untreated carbon fiber paper shows a smooth surface (Fig. 2b), but when 50 cycles of ALD are performed to grow CoOx, cobalt oxide particles begin to appear on the surface (Fig. 2c). A dense film composed of CoOx particles can be observed as the number of ALD cycles increases (Fig. 2d-f). Cobalt and oxygen elements were detected by SEM energy-dispersive X-ray spectroscopy (EDS) as shown in Figs. S1 and S2.

Figure 2. SEM images of (a) carbon fiber paper (CP) and (b) magnified version. (c-f) SEM images of ALD-CoOx on CP (CP-Co#, where # indicates the number of the ALD cycles). (c) CP-Co50, (d) CP-Co100, (e) CP-Co350, and (f) CP-Co700. Transmission electron microscopy (TEM) analysis of CP-Co350 was performed to confirm the microstructure of ALD-CoOx deposited on carbon fiber paper. The image in Fig. 3a shows the uniform thickness of the CoOx film along the curved surface of the carbon fiber paper. The high uniformity of the film can be attributed to the self-limiting nature of our ALD cobalt oxide process, which has been proven in our previous paper.24 As shown in Fig. S3, the thin film on


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CP clearly contains Co species, according to the EDS line profile. In addition, this film has a polycrystalline phase, as shown in Fig. 3b. The fast Fourier transform (FFT) results show that the deposited CoOx film consists of CoO (200) and (111) regardless of the location of the film. The polycrystalline properties of ALD cobalt oxide are in good agreement with the XRD results of previous studies.25

Figure 3. (a,b) TEM images of ALD-CoOx grown along a fiber in carbon fiber paper. (c,d) fast Fourier transform (FFT) result of CoOx in selected area in (b). To investigate the valence state and oxygen stoichiometry of ALD-CoOx, this film (CPCo350) was analyzed by electron energy loss spectroscopy (EELS). Figure 4a shows the highangle annular dark field (HAADF) image of the ALD-CoOx on carbon fiber paper, with the top (near the surface) and bottom (adjacent to carbon fiber paper) locations exposed to the electron beam. The L3/L2 intensity ratio, from the measured L2,3 edges of transition metal oxides, is a powerful tool to quantify the valence state of the transition element. More specifically, the valence states of transition elements such as Co and Mn are known to be inversely proportional to the intensity ratio L3/L2.26 In our results, the intensity ratio L3/L2 of the lower position of the


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film is much higher than that in the upper position. This indicates that increasing the thickness of ALD-CoOx increases the valence state of Co on the surface.

Figure 4. (a) High-angle annular dark-field (HAADF) image of ALD-CoOx on carbon fiber paper. “Top” and “Bottom” indicate the positions of electron energy loss spectroscopy (EELS) analysis. (b) EELS result of the selected areas indicated in (a). Inset bar chart shows the intensity ratio of Co L3 to L2. X-ray photoelectron spectroscopy (XPS) was employed to study the surface chemistry of the electrocatalyst samples. All samples showed a strong satellite peak with a spin-orbit splitting (SOS) of 16 eV at the Co 2p core level, which means that Co is in the 2+ oxidation state (CoO or Co(OH)2) (Fig. 5a and Fig. S4).27-28 The O 1s region of all samples was analyzed to account for the activity of the electrocatalyst of the film. The O 1s spectrum can be decomposed into three components: oxygen in the oxidation lattice (O2-), oxygen in the hydroxyl group (-OH), and oxygen in adsorbed water. It is known that if the portion of oxygen sub-bands associated with the oxygen of the oxide lattice is large while the band related to oxygen in the hydroxyl group is relatively small, the oxidation state of the film is high, overall.29 In Figs. 5b-f, we can


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see that as we increase the ALD-CoOx thickness, the oxygen peak of the lattice oxygen increases while the sub-band peak area associated with the oxygen of the -OH group decreases. This indicates that the major content of the film should be CoO, considering the SOS value and strong satellite peak, but the fraction of Co3O4 may increase slightly as the film thickness increases. In addition, it should be noted that the CoOx film will be more oxidized after electrochemical tests, as reported previously.30 Based on the XPS data, we could conclude that the surface chemistry of the film is dependent on the thickness of ALD-CoOx, which is in agreement with the EELS result shown in Fig. 4. ALD is based on a self-limiting reaction through a cycle-by-cycle process that allows precise control of film thickness at the atomic layer scale.31 It is therefore expected that the ALD process can be used to accurately control the valence state of Co on the film surface. The reason for the change in oxidation state with film thickness variation is not yet clear; however, according to the literature, there is evidence that CoO is present in the initial growth stage and Co3O4 is gradually included as the film thickness increases.32 This can be related to the microstructural changes during film growth, since structures such as facets can be attributed to the presence of low-coordination oxygen sites.33


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Figure 5. X-ray photoelectron spectroscopy (XPS) of ALD-CoOx on carbon paper electrodes. (a) High-resolution Co 2p spectra of CP-Co350. High-resolution O 1s spectra of (b) CP-Co50, (c) CP-Co100, (d) CP-Co350, and (e) CP-Co700. (f) Summary of the relative O 1s sub-band intensities in (b-e). We investigated the effect of surface chemistry and film thickness on the OER activity of ALD-CoOx on carbon fiber paper electrodes in 1 M KOH electrolytes. First, the electrochemical performance of the samples was analyzed by linear sweep voltammetry (LSV) with 0 to 0.8 V vs. Ag/AgCl. The resulting IR-corrected polarization curve is shown in Fig. 6a. It can be observed that the overpotential for the OER decreases with increasing number of ALD cycles (film thickness). The best electrochemical performance was recorded in samples with 350 cycles of ALD. For example, η10, the overpotential required for a current density of 10 mA cm-2 which corresponds to 10% efficiency in solar-to-fuel synthesis,34 is recorded as 406, 380, 343, and 354 mV for CP-Co50, CP-Co100, CP-Co350, and CP-Co700, respectively. The optimal one among these (343 mV, CP-Co350) is competitive with the catalytic activity of


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previously reported state-of-the-art cobalt oxide catalysts such as CoOx nanoparticles on porous N-carbon (349 mV),35 Co3O4 nanowire on Ni foam (340 mV),36 and CoO nanosheet on carbon fiber cloth (350 mV).37 It should be noted that the OER performance of CP-Co350 is still inferior to that of a reported noble metal oxide catalyst (270 mV, RuO2/Ti).38 When the thickness increased beyond a certain value (28 nm for CP-Co700), it was observed that the overpotential increased. This indicates that there is a critical thickness for OER activity in CoOx. The OER performance of bare carbon paper was not as good as expected. The overvoltage required for a current density of 1 mA cm-1 was recorded at 550 mV (Fig. S5). Electrochemical impedance spectroscopy (EIS) was performed to evaluate the charge transfer resistance (Rct). The fit results for the Nyquist plot in Fig. 6b show that the series resistance, which is the sum of the contact and electrolyte resistance and can be approximated by the x-axis intersection of the Nyquist plot in the high-frequency region, are similar to one another regardless of coating thickness, as expected. Rct, which can be approximated by the semicircular diameter of the Nyquist plot, decreases as the number of ALD cycles increases. From the fit results, Rct was determined to be 5.3, 3.6, 1.7, and 2.2 Ω cm2 for CP-Co50, CPCo100, CP-Co350, and CP-Co700, respectively. The Rct of CP-Co350 is the lowest, indicating the fastest charge transfer among the samples. To account for the differences in electrochemical performance, the electrochemically active surface area was estimated by analyzing the electrochemical double layer capacitance (Cdl). The detailed procedure (C-V curve and current difference analysis) is shown in the Supporting Information (Figs. S6 and S7). As a result, the Cdl of CP-Co350 was 17.45 mF cm-2, which was the highest value among the electrodes (for example, 12.4 times of CP-Co50) (Fig. 6c). This indicates that the number of active sites on the electrocatalyst surface increases significantly as the film thickness increases.39 Since charge transfer kinetics are closely related to the number


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of active sites in an electrocatalyst, an increase in Cd1 can explain a decrease in Rct when increasing the film thickness.39 Tafel plot analysis can provide insights into the intrinsic kinetics (exchange current density) and reaction mechanism of the electrode catalyst, or the rate-determination step (Tafel slope). For the intrinsic reaction kinetics, the exchange current density i0 can be extracted from the Tafel plot, which is the x-axis intersection, log(i0), of the extrapolated linear line in Fig. 6d. The resulting i0 of CP-Co350 was the highest among the samples of different cycle number, which indicates that the intrinsic reaction kinetics at the surface of CP-Co350 are the fastest. This is in good agreement with other catalytic activity parameter analysis results, including η10, Rct, and Cdl. The Tafel slope, which can be calculated from the slope of the Tafel plot, increases as the film thickness increases. In general, electrocatalysts with small Tafel slopes are preferred because of the small overpotential required at high current densities. However, the electrochemical performance of the CP-Co350 sample in this study is highest in all current ranges despite its large Tafel slope. This is because the difference in Tafel slope between samples is not very large, but the exchange current density of CP-Co350 is significantly larger than that of all the other electrodes. The durability of CP-Co350 was evaluated using chronoamperometry with various potential steps (600, 700, and 800 mV vs. Ag/AgCl), as shown in the Supporting Information (Fig. S8). The current density remained almost constant during the test.


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Figure 6. (a) IR-corrected linear sweep voltammetry curves of ALD-CoOx on carbon fiber paper. (b) Electrochemical impedance spectra of the electrodes at 600 mV vs. Ag/AgCl; the inset shows the equivalent circuit for data fitting. (c) Double layer capacitance (Cdl) for the electrodes, and (d) corresponding Tafel analyses of the electrodes. The electrochemical activity factors (η10, Rct, Cdl, i0, and Tafel slope) can be correlated with the surface chemistry information about the films obtained from EELS and XPS with varying thicknesses. It is noteworthy that many well-known high-performance water oxidation catalysts, including RuO2, IrO2, and Mn2O3, have metal cations in a highly oxidized state at the surface.40-41 Similarly, for CoOx, it is well-known that the overpotential decreases linearly with increasing Co3+ fraction.42 This is because the surface must be oxidized to Co4+, which is the active catalytic species, from the lower valence states, such as Co2+ or Co3+ during oxygen


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evolution.43 In the case of a metal oxide electrocatalyst, a lack of surface hydroxyl groups on the metal oxide may represent a film having a higher oxidation state.29 Similarly, in our study, a thick film with a low concentration of oxygen related to -OH functional groups in the XPS analysis was found to have a higher surface oxidation state according to EELS measurements. Thus, an increase in oxidation state at the surface, which is indicated by the decrease in concentration of surface functional groups (-OH), can be a major cause of improved surface catalytic activity (η10, Rct, Cdl, and i0) in thick films. Interestingly, despite the lowest concentration of the -OH functional group, the electrochemical performance of the CP-Co700 sample (the thickest) was not further improved over CP-Co350. To explain the electrochemical behavior of the thickest film, the Tafel slope was analyzed by film thickness (Fig. 6d). One of the lessons learned from the Tafel slope is about the reaction mechanism. More specifically, a change in the Tafel slope indicates that the reaction mechanism has changed.44 Interestingly, the correlation between the Tafel slope and the amount of surface hydroxyl groups (or oxidation state) of this study is different from previously reported studies in the literature. Smith et al. have shown that lowering the relative concentration of hydroxyl groups on the metal oxide surface can lead to lower Tafel slopes.29 In this study, thicker samples such as CP-Co350 and CP-Co700 recorded a larger Tafel slope than thinner samples (61, 66, 74, and 75 mV dec-1 for CP-Co50, CP-Co100, CP-Co350, and CP-Co700, respectively), even though the oxygen concentration of the hydroxyl species was lower than that of the thinner CoOx thin films. This discrepancy can be accounted for by the nanoscale limitation which indicates the presence of a critical thickness for charge transport in the thin metal oxide electrocatalyst. For example, Bent and Norskov et al. have shown that OER catalytic activity decreases gradually as the thickness of the thin TiO2 layer increases beyond a threshold limit (4 nm).16 The critical thickness of the thin-film metal oxide is due to the charge transport limitations present in wide-bandgap materials such as MnOx and TiO2. For


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CoOx, the critical thickness is known to be about 6 nm.16 In our study, the majority of the CoOx film is thicker than 6 nm; thus, increasing the CoOx film thickness enhances the charge transport limit. As a result, when increasing the film thickness, the increased length for charge transport can affect the rate-determining step for the OER, and consequently, the Tafel slope can be increased (Fig. 7).

Figure 7. Schematic diagram of the change in oxidation state and charge transport length at various film thicknesses and the consequent OER activity.

CONCLUSION The catalytic activity of nanoscale thin-film CoOx in the oxygen evolution reaction has been evaluated. The oxidation state of the surface of the film can be controlled by precisely controlling the film thickness (2-28 nm) with ALD. The surface concentration of -OH functional groups associated with the surface oxidation state of the film can be finely tuned by controlling the number of ALD cycles. More specifically, higher oxidation states were obtained in thicker CoOx films (14 and 28 nm) than in the thinner ones (2 and 4 nm), as observed via


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STEM-EELS and XPS. As a result of these findings, we can conclude that the electrochemical performance of the film can be improved by increasing its thickness. However, we observed performance saturation when the film thickness reached 28 nm. We identified a larger Tafel slope for the thicker film, which could be a result of the inevitable charge transport limitation in the bulk region. In this work, we successfully demonstrated the usefulness of ALD-CoOx on carbon fiber paper for the OER. Our results also provide an important prerequisite for designing nanoscale thin-film catalysts, namely, that they must satisfy simultaneously the need to stabilize the surface for higher oxidation states and the film thickness must be minimized.

ASSOCIATED CONTENT Supporting Information. Additional figures including those depicting microstructure, chemical composition, surface chemistry, and electrochemical performance of the OER catalysts (Figs. S1–S8). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (No.


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Highly Active Oxygen Evolution on Carbon Fiber Paper Coated with

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