Synergistically Enhanced Oxygen Evolution Catalysis with Surface Modified Halloysite Nanotube

Article information

J. Electrochem. Sci. Technol. 2023;14(1):96-104

Publication date (electronic) : 2023 February 10

doi : https://doi.org/10.33961/jecst.2022.00906

Hyeongwon Jeong, Bharat Sharma, Jae-ha Myung

Department of Materials Science and Engineering, Incheon National University, Incheon 22012, Korea

^*E-mail address: bharatsharma796@gmail.com (Bharat Sharma) mjaeha@inu.ac.kr (Jae-ha Myung)

Received 2022 November 2; Accepted 2023 January 12.

Abstract

Synergistically increased oxygen evolution reaction (OER) of manganese oxide (MnO₂) catalyst is introduced with surface-modified halloysite nanotube (Fe₃O₄-HNTs) structure. The flake shaped MnO₂ catalyst is attached on the nanotube template (Fe₃O₄-HNTs) by series of wet chemical and hydrothermal method. The strong interaction between MnO₂ and Fe₃O₄-HNTs maximized active surface area and inter-connectivity for festinate charge transfer reaction for OER. The synergistical effect between Fe₃O₄ layer and MnO₂ catalyst enhance the Mn³⁺/Mn⁴⁺ ratio by partial replacement of Mn ions with Fe. The relatively increased Mn^3+/Mn⁴⁺ ratio on MnO₂@FHNTs induced σ^* orbital (e_g) occupation close to single electron, improving the OER performances. The MnO₂@FHNTs catalyst exhibited the reduced overpotential of 0.42 V (E vs. RHE) at 10 mA/cm² and Tafel slope of (99 mV/dec), compared with that of MnO₂ with unmodified HNTs (0.65 V, 219 mV/dec) and pristine MnO₂ (0.53 V, 205 mV/dec). The present study provides simple and innovative method to fabricate nano fiberized OER catalyst for a broad application of energy conversion and storage systems.

Keywords: Oxygen Evolution Reaction; Catalyst; Nano Tube; Manganese Oxide; Halloysite Nano Tube

1. Introduction

Increasing demand for clean and sustainable energy has stimulated research on energy conversion and storage systems, including fuel cells, water splitting, alkaline electrolysis, and metal-air batteries. The catalytic materials for these systems play a key role in the development of high-performance and durable devices at an affordable price [1,2]. The electrode materials for such energy devices should deliver high catalytic activity as well as great conductivity for fast reaction kinetics. Oxygen electrode catalyst, in which oxygen evolution reaction (OER) takes place, has been regarded as the main challenge due to the sluggish kinetics of the 4-electron involved reaction pathway. Noble metal-based OER catalysts, such as IrO₂, RuO₂, and RhO₂ have been adopted for effective electrode materials for commercialized systems [3–5]. Their low overpotential and high electrical conductivity showed great OER performances, however, the high cost and scarcity of noble metal still hinder their practical application [6,7].

The non-precious metal or metal oxide-based OER catalyst is attracting attention as an alternative solution in OER electrode design [8–10]. MnO₂ has been studied and regarded as a feasible candidate in the development of transition metal-based OER catalysts [11,12]. The manganese oxide catalysts exhibited great redox stability both in acid and alkaline electrolytes compared with that of other transition or noble metal-based catalysts [13–15]. These electrode materials have the potential to be an effective OER electrode catalyst for various energy devices, however, further optimization is required to overcome the relatively higher overpotential and low electronic conductivity [16–18].

Recent studies on manganese oxide catalysts were mainly underway to increase their active surface area and inter-connectivity by tailoring their electrode scaffold design with various preparation methods, including porous or fiberized structure [19,20]. For example, the electrospinning method could also be a possible way to fabricate MnO₂ based nanofibers with increased active surface area and inter-connectivity [21–24]. The mesoporous structure is also studied for an electrode material in water electrolysis and supercapacitor, increasing electrode surface as well as structural stability [25,26]. The structure modified MnO₂ catalysts improved catalytic performances by successfully maximizing active surface areas, however, these pristine materials require high cost and complicated fabrication processes, which limit their practical commercialization.

The naturally abundant halloysite nanotubes (HNTs), therefore, has recently introduced as an innovative template to fabricate nano fiberized catalyst on many applications. The nano-sized tabular structure of the HNTs template has a high length to diameter ratio and an open end with hallow morphology, which maximizes the catalytically active surface area as well as gas diffusion channel [27,28]. The surface properties of HNTs could be manipulated by substituting their hydroxyl ions into organic or inorganic materials to overcome the inherently low electronic conductivity and poor adhesion of nano catalysts [29,30]. Compared with the pristine nanofiber fabrication processes the HNTs template could deliver an easy preparation method at a low cost.

In this work, we fabricated the surface decorated Fe₃O₄-HNTs (FHNTs) with a MnO₂ nano catalyst (MnO₂@FHNTs). The Fe₃O₄ coated HNTs template (FHNTs) is designed to increase the electronic conductivity as well as adhesion between the nano tube template and MnO₂ catalyst. The MnO₂@FHNTs nano catalyst was fabricated by the reduction-precipitation method and following the hydrothermal process. The morphologies and chemical composition of MnO₂@FHNTs nano catalyst were investigated before and after surface modification. The synergistically enhanced OER activity between Fe₃O₄ layer and surface MnO₂ catalyst was also investigated to understand their improved reaction kinetics. The electrochemical performances of oxygen evolution reaction were analyzed by rotating disk electrode (RDE) method with alkaline electrolyte condition.

2. Experimental

2.1. Materials synthesis

Halloysite nanotubes (HNTs), sodium sulfite (Na_2-SO₃), sodium hydroxide (NaOH), Ammonium persulfate ((NH₄)S₂O₈), Iron (III) chloride hexahydrate (FeCl₃.6H₂O), and potassium permanganate (KMnO₄) were attained from Sigma Aldrich, Germany.

In the synthesis method, 0.5 g of HNTs was added to 50 ml of DI water and the suspension was thoroughly ultrasonicated and stirred for 1 hr. Then FeCl₃.6H₂O solution (2%) was added to the mixture and then the resultant mixture was reflexed for 30 min. It is followed by the addition of Na₂SO₃ (0.3%) into the mixture. That results in the wine-red color of the mixture solution. After the addition of 30 mL of NaOH, the mixture solution turned to black colored precipitate of the Fe₃O₄ loaded HNTs (FHNTs) and an external magnet separated the FHNTs powder. The FHNTs were washed meticulously with DI water, methanol, and ethanol to eliminate residues.

The attained powder was dried in an oven at 50°C for 24 hr and followed by the grinding by mortar and pestle for further usage. The F-HNTs were decorated by MnO₂ nanoflakes by following the same procedure in our previous [30]. To summarize the procedure, 2 g of FHNTs was added to the 50 mL of DI water. The solution mixture was ultrasonicated and refluxed for 1 hr. The subsequent solution mixture was added by KMnO₄ (3.6 g) and ((NH₄)S₂O₈) (4.2 g) under constant refluxing. The attained solution mixture was dispensed in a 250 ml Teflon-lined stainless-steel autoclave and set aside at 105°C for 24 hr. An external magnet separated the gained bluish-black colored precipitate of F-HNTs-MnO₂. The as-prepared powder material was washed with DI water and dried at 50°C for 24 hr and then the powder was grinded by mortar and pestle and used for characterization and electrochemical analysis. The schematic illustration of MnO₂@FHNTs synthesis processes is presented in Fig. S1.

2.2. Materials characterization and electrochemical analysis

The crystalline purity of the HNTs and F-HNTs-MnO₂ was characterized by the XRD (Rigaku, smart LAB.) analysis in a range of 5°~90° with 2°/min scanning speed. The morphological properties of all the prepared samples were investigated by SEM (TES-CAN, Vega3) and HR-TEM (JEOL, JEM-2100F), respectively. The surface chemical properties of synthesized materials were analyzed by using TEM/EDS and X-ray photoelectron spectroscopy (Thermo Fisher scientific, K-Alpha+). The electrochemical properties were characterized by a rotating disk electrode system (RRDE-3A, ALS), with a glassy carbon disk electrode as a working electrode, Pt wire, and 1.0 M NaOH saturated Hg/HgO as a counter and reference electrode, respectively. Alkaline electrolyte of 0.1 M KOH (Sigma Aldrich, 99.99%) solution was N₂ purged for 1h before electrochemical measurement. The catalyst suspension ink was prepared by mixing catalyst materials (2 mg), conductive carbon (6 mg, Vulcan XC72R), 5 wt.% Nafion solution (70 uL, Sigma Aldrich) and isopropyl alcohol (1 mL), respectively. The suspension ink was ultrasonicated for 30 min before drop coating (6 uL) on the glass carbon electrode. Cyclic voltammetry (CV) with different scanning speeds (5~100 mV/s) was conducted to determine the double layer capacitance (C_DL) between electrode catalyst and electrolyte. The OER performance of prepared materials was characterized in O₂ saturated 0.1 M KOH electrolyte solution at a scanning rate of 10 mV/s with a rotating speed of 1600 rpm.

3. Results and Discussion

3.1. Characterization of catalyst

The surface-modified HNTs catalyst before and after the fabrication processes are illustrated in Fig. 1. The pure HNTs (HNTs ref.) templates showed the average diameter of ~160 nm and various lengths ranging from 100 nm ~ 2 um after series of wet chemical and hydrothermal synthesis processes. This result suggests that the HNTs template could be a feasible candidate in developing fiberized 3-D catalysts using conventional wet chemical and hydrothermal methods without any structural deformation. The Fe₃O₄ layer was attached to the HNTs surface after the wet chemical precipitation process, delivering ferromagnetic properties on FHNTs (Fig. 1(d,g)). The surface abundant hydroxyl ions on the HNTs template enabled the strong interaction between HNTs and Fe₃O₄. The magnetized HNTs (FHNTs) are covered by nano-flake MnO₂ catalyst after the following hydrothermal synthesis process (Fig. 1(e,h)). The nano-flake-shaped MnO₂ particles first attached to the surface magnetized FHNTs structure and crystalized with reducing temperature. This surface property of FHNTs improved the structural inter-connectivity between MnO₂ and FHNTs template. The strong interaction between MnO₂ and FHNTs constrained the agglomeration of MnO₂ during the hydrothermal process, compared with the pristine MnO₂ powder catalyst prepared without the FHNTs template.

Fig. 1

Structural and chemical analysis of MnO₂@FHNTs. (a) Schematic illustration of MnO₂@FHNTs fabrication process, (b) TEM/EDS mapping result of MnO₂@FHNTs, and (c–h) surface morphological differences characterized by FE-SEM and HR-TEM.

The surface chemical properties of MnO₂@FHNTs catalyst were exhibited in Fig. 1b and Fig. 2 after TEM/EDS and XPS analysis, respectively. The EDS mapping results before and after each fabrication steps are characterized in Fig. S2 and Fig. 1b, respectively. The results clearly showed that the partially covered Fe₃O₄ on HNTs template is attracting nanoflake MnO₂ catalyst, increasing catalytically active surface area.

Fig. 2

HR-XRD and X-ray photoelectron spectroscopy (XPS) results of prepared catalyst samples. (a) HR-XRD spectra of all prepared samples. (b) XPS spectra of MnO₂@FHNTs, FHNTs, and HNTs ref., (c) Mn2p spectra of MnO₂@FHNTs, MnO₂@HNTs, and MnO₂, (d) Fe2p spectra of MnO₂@FHNTs and FHNTs, (e) O1s spectra of MnO₂@FHNTs, MnO₂@HNTs, and MnO₂, respectively.

The phase characterization of all the prepared samples was also illustrated in Fig. 2a, after HR-XRD analysis. The HNTs ref. the sample showed typical crystalline planes as per the (JCPDS 29-1487) [31]. The FHNTs exhibited additional Fe₃O₄ peaks after the wet-chemical precipitation process. The MnO₂@FHNTs showed a typical crystalline plane (001) of HNTs ref. such as (2θ=12.50°). Subsequently, the surface modifications significantly decreased the intensity and sharpness of the HNTs ref. peaks at a higher diffraction angle. This observation probably originated from the thick coating of the Fe₃O₄ and 3-D MnO₂ nano-flake on MnO₂@FHNTs as exhibited by SEM and TEM images in Fig. 1. The slightly clear peak intensity of partially modified MnO₂@HNTs and FHNTs samples also supports this result. The typical tetragonal phase of α-type MnO₂ (JCPDS 44-0141) was detected on the MnO₂@FHNTs, MnO₂@HNTs, and powder MnO₂, which is synthesized under the same hydrothermal process. Compared with various MnO₂ polymorphs, α-MnO₂ tends to have the higher OER activity [32]. The result obtained here indicates that the hydrothermal condition for MnO₂ nano-flake formation is optimized in the development of MnO₂ based OER catalyst.

The XPS analysis results of MnO₂@FHNTs catalysts were also shown in Fig. 2 for further investigation. All the elements of HNTs, Fe₃O_4, and MnO₂ appeared in the survey spectra of MnO₂@FHNTs, which correspond to 74.47, 100.40, 527.50, 642.59, and 711.06 eV for Al2p, Si2p, O1s, Mn2p, and Fe2p, respectively (Fig. 2b). This observation also indicates that the surface-modified MnO₂@FHNTs catalyst was successfully obtained under series of fabrication processes without impurities or secondary phases. Fig. 2c shows the high-resolution spectra of Mn2p for MnO₂@FHNTs, MnO₂@HNTs, and MnO₂, which include the specific binding energy of 654.57 and 642.64 eV. The binding energy difference (~11.45 eV) between two separated peaks indicates the Mn⁴⁺ in the MnO₂ phase [33]. The high-resolut ion Fe2p spectra of MnO₂@FHNTs and MnO₂@HNTs were also analyzed and illustrated in Fig. 2d. The binding energy difference of ~12.93 eV is attributed to the presence of Fe₃O₄, which confirms that the surface Fe₃O₄ layer on HTNs template [34]. The O1s peaks could be deconvoluted into oxygen ions in an oxide lattice structure (Mn-O) and adsorbed oxygen ions (Mn-OH and Mn-O-OH) on the surface of catalytic materials, respectively [33]. Fig. 2e exhibits the area fraction of deconvoluted O1s peaks of MnO₂@FHNTs, MnO₂@HNTs, and MnO₂ catalysts, respectively. The deconvolution result of MnO₂@FHNTs sample indicates 27.3% of lattice oxygen (Mn-O) and 72.7% of absorbed oxygen ions (Mn-OH and Mn-O-OH). The high concentration of surface adsorbed oxygen ions (Mn-OH and Mn-OOH) is widely regarded as a prerequisite for OER, providing enough starting or intermediate ions in the catalysis process [35,36]. The relative ratio of adsorbed oxygen ions for MnO₂@HNTs (O_Ads.: 80.2%) and MnO₂ (O_Ads.: 80.6%) were higher than that of MnO₂@FHNTs (O_Ads.: 72.2%). This observation is probably originated from the surface covering Fe₃O₄ layer in MnO₂@FHNTs sample. The Fe₃O₄ layer on HNTs template is believed to increase the relative amount of surface lattice oxygen, while reducing the pristine hydroxyl ions on pure HNTs surface. The overall results obtained here demonstrate that the potential application of MnO₂@FHNTs catalyst as an OER catalytic material with superior activity.

3.2. Electrochemical properties of catalyst

The OER activity of MnO₂@FHNTs, MnO₂@HNTs, MnO₂, FHNTs, and HNTs ref. catalysts are summarized in Fig. 3. The OER polarization curve shows further enhanced OER activity after doping MnO₂ on the surface of the FHNTs structure. The MnO₂@FHNTs showed the lowest onset potential and highest current density, compared with that of other samples (Fig. 3a). The overpotential at the current density of 10 mA is widely adopted for evaluating the performance of OER catalyst materials, such as alkaline electrolysis and water splitting. The MnO₂@FHNTs catalyst exhibited an overpotential of 0.46 V (V vs. RHE) at 10 mA current density, which is higher than pristine bulk MnO₂ catalyst materials [32]. Fig. 3a inset shows the magnified OER characteristics near onset potential region. The FHNTs exhibited a slightly increased current density than the raw HNTs template (HNTs ref.), while the onset potential for OER remained similar. This result implies that Fe₃O₄ coating on HNTs only improved the electronic conductivity of the HNTs template without tailoring its catalytic activity. The MnO₂@HNTs sample without Fe₃O₄ layer was also prepared and characterized to understand the effect of Fe₃O₄ layer on the HNTs template. The MnO₂@HNTs exhibited higher overpotential and lower current density then MnO₂@FHNTs. The pristine powder MnO₂ catalyst showed even lower overpotential and higher current density then that of MnO₂@HNTs. This result indicates that the surface Fe₃O₄ coated HNTs (FHNTs) could improve the electrochemical properties of MnO₂ catalyst. The catalytic properties of MnO₂ based OER catalysts were further investigated by calculating their overpotential and mass-specific current density. Fig. 3c exhibits the overpotential and mass-specific current density of the prepared MnO₂ based catalysts deposited on the FHNTs and HNTs template. The MnO₂@FHNTs showed the lowest overpotential of 0.42 V (V vs. RHE) at 5 mA/cm₂ current density, which is much lower than 0.53 V and 0.65 V for pristine MnO₂ and MnO₂@HNTs samples, respectively. The mass-specific current density of MnO₂@FHNTs catalyst at 1.6 V (V vs. RHE) exhibited 13.30 A/cm², while pristine MnO₂ and MnO₂@HNTs only recorded similar results of 4.52 A/cm² and 4.36 A/cm². The sluggish overpotential and the lower specific current density of MnO₂@HNTs (0.65 V, 4.36 A/cm²) than pristine MnO₂ (0.53 V, 4.52 A) are probably originated from the additional raw HNTs template, which delivers inherently no catalytic activity as well as low electronic conductivity.

Fig. 3

Electrochemical properties of HNTs based catalysts. (a) OER polarization curve under 0.1 M KOH electrolyte, (b) Tafel plots of OER catalysis, and (c) double-layer capacitance of prepared sample.

Fig. 3b shows the Tafel plot of the prepared catalyst samples obtained from the LSV polarization curve. The Tafel slope of MnO₂@FHNTs, MnO₂@HNTs, and HNTs ref. was 99 mV/dec, 219 mV/dec, and 247 mV/dec, respectively. The pristine MnO₂ and FHNTs recorded 205 mV/dec and 200 mV/dec, which is almost overlapping that of MnO₂@HNTs. The lowest Tafel slope of MnO₂@FHNTs confirms that the reaction kinetics of OER near onset potential region is drastically improved by nano-flake MnO₂ with FHNTs, indicating that the energy required for activation of OER is probably reduced after MnO₂ deposition on the FHNTs template. This observation also implies that a synergistic effect could exist between MnO₂ and Fe₃O₄ coating and FHNTs could be a favorable template for nano-sized MnO₂ catalysts. The catalytic activity of various MnO₂ based OER catalysts were listed in Table S1 for comparison.

Fig. 3d exhibits the double-layer capacitance of the prepared sample, which is consistent with an electrochemically active surface area of each catalytic material. The MnO₂@FHNTs sample showed a double-layer capacitance of 3.00 uF, which is 3 times higher value than FHNTs (1.00 uF) and HNTs ref. (0.77 uF). The pristine MnO₂ and MnO₂@HNTs sample exhibited the similar double-layer capacitance of 1.00 and 0.79 uF, respectively. This tendency could be explained by the morphological properties of HNTs catalysts in Fig. 1, which show the increasing active surface area of MnO₂@FHNTs as serial doping of Fe₃O₄ and MnO₂. The addition of FHNTs template restrained the agglomeration of MnO₂ catalyst under synthesis process, maximizing the catalytically active surface area. Also, the sluggish electronic conductivity of raw HNTs is believed to reduce the electrochemically active surface area of the reference MnO₂@HNTs sample. The full CV curves at different scanning speeds for calculating double-layer capacitance and the related calculating processes were summarized in Fig. S3.

3.3. Effect of Fe₃O₄ layer on OER activity

The above electrochemical analysis results suggest that the MnO₂@FHNTs catalyst could be a feasible candidate for enhancing OER activity. Understanding the fundamental catalytic activity of MnO₂@FHNTs is vital in the development of further improved OER catalysts. The oxygen evolution reaction steps on the surface of MnO₂ are illustrated in Fig. 4a [4,37–39]. The series of OER steps on the MnO₂ oxide catalyst involves a 4-electron transfer pathway with adsorption and desorption of hydroxyl (OH⁻) ions. The increased electronic conductivity of MnO₂@FHNTs catalyst by additional Fe₃O₄ coating on HNTs template, therefore, is believed to improve the OER activity as discussed in the previous studies of MnO₂ based electrochemical catalyst materials with transition metal oxide [26,40,41]. Furthermore, the nano fiberized FHNTs structure not only increased the active surface area by attracting nano-flack MnO₂ catalysts but also improved their inter-connectivity like other fiberized catalytic materials.

Fig. 4

(a) Oxygen reduction reaction steps on the MnO₂ based catalyst and related electronic configuration of Mn ions. (b) XPS spectra of Mn2p on MnO₂@FHNTs and MnO₂@HNTs.

Fig. 4b shows the XPS spectra of Mn2p on MnO₂@FHNTs and MnO₂@HNTs catalyst after deconvolution. The Mn2p_1/2 and Mn2p_3/2 peaks of MnO₂@FHNTs catalyst were slightly shifted to the lower binding energy, compared with that of MnO₂@HNTs catalyst. This observation suggests that the average oxidation state of Mn ions was partially decreased with the surface coating of Fe₃O₄ on the HNTs template. The Mn2p_3/2 peak can be deconvoluted into two peaks of 641.1 and 642.2 eV, which represent Mn³⁺ and Mn⁴⁺ species [42]. The ratio of Mn³⁺/Mn⁴⁺ ions was revealed to 2.58 and 2.20 for MnO₂@FHNTs and MnO₂@HNTs catalyst, respectively (Table 1). The higher Mn³⁺/Mn⁴⁺ ratio of MnO₂@FHNTs catalyst was probably originated from ionic interdiffusion between MnO₂ and Fe₃O₄ interface, in which Mn⁴⁺ ions were partially replaced by Fe³⁺ ions under synthesis process, resulting in the formation of additional oxygen vacancy in the hydrothermal synthesis process [43,44]. This observation is also supported by MnO₂-Fe₃O₄ composite catalyst materials prepared by various methods [45,46].

Table 1

Mn ions ratio of MnO₂@FHNTs and MnO₂@HNTs catalyst from XPS results

The rate-determining step (RDS) of OER for MnO₂ catalyst was generally proposed as the O-O double bond formation and the proton extraction from OOH⁻ group, which attribute to the step 2 and step 3 reaction in Fig. 4a [47–49]. The electronic configuration of Mn³⁺ and Mn⁴⁺ species is illustrated in Fig. 4a inset [38,50]. The higher Mn³⁺/Mn⁴⁺ ratio of MnO₂@FHNTs is probably facilitated the OER process by manipulating the σ* orbital (e_g) occupation close to single electron, therefore reducing the required free energy difference for the formation of O-O bond on Mn ions [47–49,51]. The higher Mn³⁺/Mn⁴⁺ ratio of MnO₂@FHNTs catalyst driven from synergistic effect between Fe₃O₄ layer, therefore, is believed to improve its OER performance than pristine MnO₂ and MnO₂@HNTs catalyst.

4. Conclusions

In summary, the hollow nano fiberized MnO₂@FHNTs catalyst was synthesized by series of wet precipitation processes and hydrothermal method using earth-abundant HNTs template. The Fe₃O₄ layer on HNTs template were attracted the surface nano flack MnO₂ catalyst increasing electronic conductivity as well as electrochemical active surface area for OER. The Fe₃O₄ layer increased Mn³⁺/Mn⁴⁺ ratio of MnO₂ catalyst, causing synergistic effect by single electron occupation σ* orbital. The LSV curve exhibited that overpotential of MnO₂@FHNTs at 5 mA current density shifted negatively by 110 mV and 230 mV compared with that of pristine MnO₂ and MnO₂@HNTs catalyst. The reduced Tafel slope of MnO₂@FHNTs also showed the improved reaction kinetics with the surface Fe₃O₄ layer on HNTs structure. The results suggest that the MnO₂@FHNTs could be a promising nano template in the development of OER catalytic materials. The MnO₂@FHNTs catalyst is believed to be utilized for various energy conversion and storage systems, such as alkaline electrolysis and metal-air batteries.

Acknowledgments

This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government (MOTIE) (G032542411), and by Incheon National University (International Cooperative) Research Grant in 2018.

Supporting Information

Schematic illustration of materials synthesis process, TEM/EDS images, Cyclic voltammetry curves, and comparison table for OER performances

jecst-2022-00906-Supplementary.pdf

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