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Article

One-Step Construction of Co2P Nanoparticles Encapsulated into N-Doped Porous Carbon Sheets for Efficient Oxygen Evolution Reaction

by
Ke Wang
1,
Ruimin Zhang
1,
Yun Guo
1,
Yunjie Liu
1,
Yu Tian
1,
Xiaojun Wang
1,
Peng Wang
1,* and
Zhiming Liu
1,2,*
1
Shandong Engineering Laboratory for Preparation and Application of High-Performance Carbon-Materials, College of Electromechanical Engineering, Qingdao University of Science & Technology, Qingdao 266061, China
2
Qingdao Industrial Energy Storage Research Institute, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, China
*
Authors to whom correspondence should be addressed.
Energies 2023, 16(1), 478; https://doi.org/10.3390/en16010478
Submission received: 28 November 2022 / Revised: 23 December 2022 / Accepted: 29 December 2022 / Published: 1 January 2023
(This article belongs to the Special Issue Nanomaterials for Advanced Energy Storage and Conversion)
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Versions Notes

Abstract

:
It is critical and challenging to develop high performance transition metal phosphides (TMPs) electrocatalysts for oxygen evolution reaction (OER) to address fossil energy shortages. Herein, we report the synthesis of Co2P embedded in N-doped porous carbon (Co2P@N-C) via a facile one-step strategy. The obtained catalyst exhibits a lower overpotential of 352 mV for OER at a current density of 10 mA cm−2 and a small Tafel slope of 84.6 mV dec−1, with long-time reliable stability. The excellent electrocatalytic performance of Co2P@N-C can be mainly owed to the synergistic effect between the Co2P and highly conductive N-C substrate, which not only affords rich exposed active sites but also promotes faster charge transfer, thus significantly promoting OER process. This work presents a promising and industrially applicable synthetic strategy for the rational design of high performance nonnoble metal electrocatalysts with enhanced OER performance.

1. Introduction

As societies become further industrialized, fossil energy shortages have become a serious problem; the energy crisis, environmental pollution and the greenhouse effect are the main challenges facing the world in this century [1]. The high energy density, cleanliness and sustainability of hydrogen produced by the electrolysis of water make it one of the most promising substitutes for traditional fossil fuels [2]. In the overall water-splitting systems, oxygen evolution reaction (OER) acts as an anodic half-reaction, limiting the production of hydrogen in water splitting units due to its multi-step proton-coupled transfer process, which has slower kinetics than hydrogen evolution reaction (HER) [3,4]. To overcome this obstacle in OER, efficient catalysts are urgently required to facilitate OER kinetics and improve energy conversion efficiency [5]. Noble metal-based electrocatalysts such as RuO2 and IrO2 are the basic electrocatalysts used for OER, but the low material fractionation, high cost and lack of durability of noble metal oxides greatly limit their further industrial development [6,7]. For this reason, cheap and efficient non-precious metal electrocatalysts have been developed for overall water splitting, mainly transition metals and their oxides [8,9], transition metal sulfides [10,11], transition metal phosphides (TMPs) [12,13]. Among them, transition metal phosphides have received increasing attention due to their abundant reserves and low cost [14].
Depending on the type of phosphorus source and the reaction method, there are three general methods for the synthesis of TMPs. The liquid-phase reaction method uses organic phosphorus sources (TOP and TPP) as the phosphorus source [15,16]. The organic solvent and the reaction temperature make the entire reaction system flammable and corrosive, and a continuous inert gas supply is required to ensure safety. The gas–solid reaction method uses hypophosphates (NaH2PO2 and NH4H2PO2) as the phosphorus source, and the high-temperature heating of the phosphorus source will produce PH3 highly toxic gas, which needs to be treated for toxic exhaust gas [17,18]. Pyrolytic reduction uses phosphates [19], polymetallic oxides [20], phytic acid [21], and other reagents with PO4 groups as phosphorus sources. The PO4 groups are easily complexed with metal ions, and under high temperatures and a reducing atmosphere, the P-O bonds break and combine with metal ions to form TMPs; however, the high temperature reaction makes it more energy intensive and limits large-scale preparation [22,23]. The complexity of the process and the high-risk factors hinder the practical production of TMPs.
Although TMPs have a good performance on OER, their inherent characteristics such as poor electrical conductivity and easy agglomeration limit the electrochemical kinetics and lead to reduced catalytic activity [24]. Forming a composite structure of TMPs with heteroatom-doped porous carbon materials is a practical solution to address the aforementioned issues [25]. For example, Sun and co-workers reported a series of transition metal phosphides encapsulated in nitrogen-phosphorus co-doped carbon, a carbon substrate that not only effectively enhances the catalytic performance of electrocatalysts, but also limits the growth and aggregation of nanoparticles [26]. It has been shown that porous carbon materials as a substrate can effectively disperse the TMPs to facilitate the exposure of active sites; in addition, the doping with heteroatoms such as nitrogen and phosphorus significantly improves the catalyst activity due to the synergistic effect [27]. To date, designing a low-cost, feasible and convenient method of coupling nanomaterials to carbon substrates for electrochemical catalysts remains a daunting task.
Herein, we report on the large-scale preparation of Co2P nanoparticles encapsulated into N-doped porous carbon (Co2P@N-C) by a facilitative one-step approach using glucose, phosphoric acid, urea and cobalt acetate as precursors. The synergistic effect between the cobalt phosphide nanoparticles and the N-doped carbon substrate results in remarkable catalytic properties. Additionally, it requires a low overpotential of 352 mV for OER to deliver a current density of 10 mA cm−2 under alkaline conditions. As well, the catalyst shows a negligible overpotential increase after 24 h stability test. Our work presents a promising and industrially accessible synthetic strategy for the rational design of high-performance catalysts.

2. Materials and Methods

2.1. Materials

Cobalt(II) acetate tetrahydrate (Co(CH3COO)2·4H2O, 99.5%), D-(+)-Glucose (C6H12O6, 99.5%), Urea (CO(NH2)2, 99.5%), and phosphoric acid (H3PO4, 85 wt%) were purchased from Aladdin Reagents Ltd., Shanghai, China. All reagents were of analytical grade and required no further purification. All experiments used deionized water (DIW). The commercial RuO2 (99.9%) catalyst was bought from Sigma-Aldrich, St. Louis, MI, USA.

2.2. Synthesis of Co2P@N-C

Typically, 3 g of urea, 1 g of glucose and 0.1 g of Co(CH3COO)2·4H2O were dissolved in 35 mL of DIW at room temperature, then 10 µL of phosphoric acid was added with continuous stirring for 30 min. Next, the precursor was heated at 85 °C for 4 h to remove moisture, then placed in a quartz tube and annealed at 900 °C for 1 h in an NH3 atmosphere at a heating rate of 5 °C min−1. After cooling down to room temperature, the obtained black powder was Co2P@N-C-900. For comparison, Co2P@N-C-800 and Co2P@N-C-1000 were synthesized by the same procedure, except for the pyrolysis temperatures of 800 °C and 1000 °C, respectively.

2.3. Electrochemical Measurement

All the electrochemical measurements were performed on a CHI760E electrochemical workstation in a standard system at room temperature. A glass carbon electrode (GCE), a graphite electrode and a saturated calomel electrode (SCE) served as the working electrode, counter electrode and reference electrode, respectively. Typically, 5 mg of the sample was dispersed in a mixture of 400 µL water and 80 µL ethanol with 20 µL Nafion solution, and then the mixture was under continuous ultrasonication for 30 min to obtain a homogenous ink. Then, 8 µL of the catalyst ink was loaded on the glass carbon electrode, resulting in a catalyst loading of 0.64 mg cm−2 (GCE: 4 mm in diameter). Polarization data were obtained at a scan rate of 5 mV s−1 in 1.0 M KOH. In all electrochemical measurements, all potentials were referenced to the reversible hydrogen electrode (RHE) by the equation ERHE = ESCE + 0.0591 pH + 0.241. The electrochemical impedance spectroscopy (EIS) was carried out in the range of 105 to 0.01 Hz under a current signal amplitude of 5 mV. The stability tests were measured by cyclic voltammetry with 2000 scans (CV, sweep frequency, 200 mV s−1) and long-term chronoamperometry.

3. Results and Discussion

3.1. Design and Characterizations of Catalysts

A one-step gelation method and subsequent thermal treatment were used to prepare Co2P nanoparticles encapsulated in N-doped carbon (Co2P@N-C) from urea and glucose (C, N source), phosphoric acid (P source) and cobalt (Co source), respectively (Figure 1). In brief, the light pink gel precursor was synthesized by heating and stirring glucose, phosphoric acid, urea and cobalt acetate. Then the precursor was annealed under an NH3 atmosphere. During the annealing process, the evaporation of water molecules creates a porous form for the carbon substrate and the ammonia produced by the decomposition of urea causes the carbon substrate to be doped with nitrogen. In addition, the partial P source in phosphoric acid forms cobalt phosphide particles with the Co source, resulting in particles embedded in N-doped porous carbon due to the limitations of the carbon substrate. To investigate the effect of different temperatures on the morphology and properties of the catalysts, Co2P@N-C samples were obtained at 800 °C and 1000 °C, respectively.
The morphology was examined by scanning electron microscopy (SEM). Figure 2a shows a low magnification SEM image of Co2P@N-C-900, showing a fluffy porous structure with a large specific surface area exposing multiple active sites for OER process. More importantly, Co2P@N-C-900 can be easily scaled up (inset of Figure 2a). Figure 2b further shows that these nanoparticles are immobilized on porous carbon and surrounded by pore space. To explore the effect of different temperatures on morphology, Co2P@N-C-800 and Co2P@N-C-1000 samples consist of amorphous particles with uneven distribution. (Figure S1). The transmission electron microscopy (TEM) image shown in Figure 2c demonstrates the Co2P nanoparticles with an average diameter of 65 nm are homogeneously confined within the carbon layers. The detailed structural information of these particles is demonstrated by high-resolution TEM (HRTEM) images shown in Figure 2d,e. The surface of the Co2P nanoparticle with a crystalline surface spacing of 0.209 nm is covered with an amorphous carbon layer, corresponding to the (211) crystalline surface of Co2P; this core–shell structure facilitates the acceleration of electron transfer in the catalytic process. Due to the unique core–shell encapsulation of Co2P@N-C-900, it efficiently prevents the agglomeration and corrosion of Co2P nanoparticles during OER and consequently endows its excellent durability and stability. Figure 2f depicts the selected area electron diffraction (SAED) pattern, which further confirms the successful synthesis of Co2P. In addition, element mapping images confirm the presence of Co2P nanoparticles and the homogeneous distribution of N elements in the porous carbon substrate (Figure 2g and Figure S2). All these results convincingly suggest that the Co2P nanoparticles are successfully synthesized and uniformly anchored in N-doped porous carbon.
To identify the structure of the materials, X-ray powder diffraction (XRD) measurements were performed. XRD patterns of various Co2P@N-C obtained by synthesis at different temperatures are presented in Figure 3a. The Co2P@N-C composite material has four obvious diffraction peaks at 40.7°, 40.9°, 43.3° and 48.7° corresponding to the (121), (201), (211) and (031) planes of the orthorhombic Co2P, respectively, while the broad peak at 25.8° is attributed to the (002) graphite plane. The crystalline planes (121), (201), (211) and (031) correspond to lattice spacings of 0.221 nm, 0.220 nm, 0.209 nm and 0.187 nm, where the (211) plane is consistent with the HRTEM in Figure 2e. The specific surface area and pore size distribution of Co2P@N-C were obtained by N2 adsorption–desorption isotherm (Figure 3b). The curves clearly show a type IV hysteresis loop demonstrating the presence of a large number of mesopores in Co2P@N-C-900. The BET surface areas of Co2P@N-C-800, Co2P@N-C-900 and Co2P@N-C-1000 are 148, 338 and 40 m2 g−1. Moreover, the average pore size of Co2P@N-C-900 is 4 nm (Figure S3). This mesoporous morphology of Co2P@N-C-900 may offer an efficient diffusion pathway and more electroactive sites, thus facilitating the enhancement of the catalytic process. To further analyze the degree of graphitization and the defect level for Co2P@N-C, the Raman spectrum of the samples prepared at different heating temperatures is recorded (Figure 3c). The defect-induced (D) band located at about 1340 cm−1 and the graphitic-induced (G) band at about 1583 cm−1 yield an ID/IG intensity ratio. The intensity ratio ID/IG of Co2P@N-C-900 is about 1.06, is higher than Co2P@N-C-800 (1.02) and Co2P@N-C-1000 (0.97), reflecting that there are more defects and disordered structures in Co2P@N-C-900.
X-ray photoelectron spectroscopy (XPS) analysis was performed to examine the valence states and surface chemical composition of Co2P@N-C-900. The survey XPS spectra reveal that Co2P@NPC-900 consists of C, N, Co and P elements (Figure S4). To further explore the bonding structure, high-resolution XPS spectra were also gained. As illustrated in Figure 4a, the C 1s spectrum has three peaks at 284.7, 285.6 and 287.9 eV, which can be corresponded to C-C/C=C, C-N/C-O and C=O, respectively [28]. The existence of the N-doped carbon skeleton improves the porousness and electronegativity of the whole composite. Rapid contact between the electrolyte and active sites provides a shorter pathway for charge transfer. The high-resolution Co 2p spectrum shows four main peaks at 778.4, 781.6, 797.2 and 801.6 eV, which are characteristic of Co 2p3/2 and Co 2p1/2. Among them, the peaks located at 778.4 and 797.2 eV correspond to metallic Co-P in Co2P. The peaks centered at 778.4 and 797.2 eV correspond to Co 2p3/2 and Co 2p1/2 of Co3+, while the peaks at 781.6 and 801.6 eV can be assigned to Co2+. Meanwhile, the peaks observed at 781.6 and 801.6 eV, both with obvious satellite peaks (denoted as Sat.), are assigned to oxidized Co species (Figure 4b) [29]. The high-resolution N 1s spectrum is provided in Figure 4c. It can be deconvoluted into four peaks at 398.0, 398.8, 400.9 and 402.3 eV, corresponding to pyridinic N, pyrrolic N, graphitic N and oxygenated N, respectively [30]. It was previously reported that N-doping can modulate the electronic structure of its neighboring C atoms, improving the electrical conductivity of the carbon obtained and synergistically inducing the creation of a large number of deficiencies in the carbon, thus further enhancing its electrocatalytic performances [31]. The binding energies of P 2p peaks located at 129.7 and 130.5 eV correspond to P 2p3/2 and P 2p1/2 in Co2P while the broad peak at 133.4 eV belongs to P-O [32]. These spectroscopic analyses agree well with the aforementioned results, further confirming the formation of Co2P nanoparticles on the porous carbon substrate.

3.2. Electrocatalytic Performances

The electrocatalytic OER performance of Co2P@N-C and commercial RuO2 catalysts was investigated in 1.0 M KOH solution using a typical three-electrode system. Obviously, the linear sweep voltammetry (LSV) polarization curves in Figure 5a show that Co2P@N-C-900 performs with a small overpotential of 353 mV for reaching a current density of 10 mV cm−2 as compared with Co2P@N-C-800 (417 mV) and Co2P@N-C-1000 (409 mV), and is even better than commercial RuO2 (375 mV). The overpotential of Co2P@N-C-900 also outperforms many recently reported noble metal-free OER electrocatalysts (Table S1). Figure 5b shows the Tafel plots based on LSV polarization curves. The Tafel slope of Co2P@N-C-900 is 84.6 mV dec−1, which is close to that for RuO2 (79 mV dec−1), and lower than those of Co2P@N-C-800 (87.3 mV dec−1), indicating the higher catalytic kinetics of Co2P@N-C-900 for the OER process. Interestingly, Co2P@N-C-1000 has the smallest Tafel slope (65.5 mV dec−1), which can be attributed to the increased conductivity of the carbon substrate due to the pyrolysis temperature. In addition, electrochemical impedance spectroscopy (EIS) was performed to explore the reaction kinetics (Figure 5c and Table S2) [33]. From the Nyquist plots, the charge transfer resistance (Rct) of Co2P@N-C-900 is found to be the smallest among the control samples under the same conditions, demonstrating that the kinetics for OER is much faster. Furthermore, cyclic voltammetry (CV) measurements were carried out to determine the electrochemically active surface area (ECSA) at different scan rates (Figure S5). As exhibited in Figure 5d, the calculated double-layer capacitance (Cdl) values for Co2P@N-C-900 is 16 mF cm−2, which is much larger than those of 1.9 and 2.4 mF cm−2 for Co2P@N-C-800 and Co2P@N-C-1000, respectively. Such a high value of Cdl value indicates that the Co2P@N-C-900 catalyst with a large ECSA favors the exposure of the active site to the OER. Stability is another important indicator to evaluate whether catalysts have the potential for industrial application. The polarization curves of Co2P@N-C-900 presented in Figure 5e display a negligible loss of activity after 2000 CV cycles. Furthermore, the current–time curve further demonstrates that Co2P@N-C-900 exhibits a stable current density of 10 mA cm−2 for 24 h, verifying good long-term stability. Based on the above results, Co2P@N-C-900 could be used as a powerful and high-efficiency electrocatalyst for highly active OER reactions.

4. Conclusions

In summary, we have successfully fabricated Co2P nanoparticles embedded in N-doped porous carbon via a one-step pyrolysis strategy. The heteroatom-doped carbon layers provide physical and chemical protection for the Co2P nanoparticles in alkaline electrolytes. Thanks to the large surface area and mesoporous morphology of Co2P@N-C-900, which can facilitate the penetration of the electrolyte and reduce electron/ion diffusion pathways and maximize the exposure of active sites, the as-prepared Co2P@N-C-900 exhibits outstanding OER performance. It only requires an overpotential of 352 mV to afford 10 mA cm−2, which is better than those of recently reported noble metal-free catalysts. Meanwhile, it also shows superior long-term stability for 24 h. Furthermore, we have demonstrated that our synthesis strategy can be easily scaled up for large-scale industrial production. This work provides a facile, promising and cost-effective strategy to prepare highly active catalysts for various energy storage and conversion applications.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en16010478/s1, Figure S1: SEM images of (a) Co2P@N-C-800 and (b) Co2P@N-C-1000; Figure S2: Low magnification HAADF-STEM image of Co2P@N-C-900; Figure S3: Pore size distribution plot of Co2P@N-C-900; Figure S4: XPS survey spectra of Co2P@N-C-900; Figure S5: CV curves of Co2P@N-C catalysts in the non-Faradaic region (1.05–1.35 V) were obtained at different scanning rates. (a) Co2P@N-C-800, (b) Co2P@N-C-900 and (c) Co2P@N-C-1000; Table S1: Comparison of OER performance of Co2P@N-C-900 with other recently reported electrocatalysts in alkaline electrolyte [34,35,36,37,38,39,40,41,42]; Table S2: EIS calculation parameters of Co2P@N-C-900 and other samples for OER.

Author Contributions

Conceptualization, R.Z.; Formal analysis, Y.L.; Investigation, Y.G.; Data curation, K.W. and Y.T.; Writing—original draft, K.W.; Writing—review & editing, P.W.; Supervision, X.W.; Project administration, P.W. and Z.L.; Funding acquisition, Z.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (Grant Nos. 21905152, 52176076, and 22005167), the Youth Innovation Team Project for Talent Introduction and Cultivation in Universities of Shandong Province, the Taishan Scholar Project of Shandong Province of China, the China Postdoctoral Science Foundation (Grant Nos. 2022M713249, 2021M693256 and 2021T140687), and the Shandong Provincial Natural Science Foundation of China (Grant Nos. ZR2020QB125 and ZR2020MB045).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic of the synthesis of Co2P@N-C.
Figure 1. Schematic of the synthesis of Co2P@N-C.
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Figure 2. The morphology and structure of the Co2P@N-C-900. (a,b) SEM images (inset digital photo shows the production of Co2P@N-C-900 in the gram scale), (c) TEM (inset shows particle size distribution of Co2P@N-C-900) and (d,e) HR-TEM images, (f) SAED pattern, and (g) elemental mapping images of Co2P@N-C-900.
Figure 2. The morphology and structure of the Co2P@N-C-900. (a,b) SEM images (inset digital photo shows the production of Co2P@N-C-900 in the gram scale), (c) TEM (inset shows particle size distribution of Co2P@N-C-900) and (d,e) HR-TEM images, (f) SAED pattern, and (g) elemental mapping images of Co2P@N-C-900.
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Figure 3. (a) XRD spectra of samples calcined at different temperatures. (b) Nitrogen (N2) adsorption/desorption isotherm of Co2P@N-C-800, Co2P@N-C-900 and Co2P@N-C-1000. (c) Raman spectroscopy of Co2P@N-C-800, Co2P@N-C-900 and Co2P@N-C-1000.
Figure 3. (a) XRD spectra of samples calcined at different temperatures. (b) Nitrogen (N2) adsorption/desorption isotherm of Co2P@N-C-800, Co2P@N-C-900 and Co2P@N-C-1000. (c) Raman spectroscopy of Co2P@N-C-800, Co2P@N-C-900 and Co2P@N-C-1000.
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Figure 4. High-resolution XPS spectrum (a) C 1s, (b) Co 2p, (c) N 1s and (d) P 2p of Co2P@N-C-900.
Figure 4. High-resolution XPS spectrum (a) C 1s, (b) Co 2p, (c) N 1s and (d) P 2p of Co2P@N-C-900.
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Figure 5. (a) OER polarization curves and (b) corresponding Tafel plots of Co2P@N-C-800, Co2P@N-C-900, Co2P@N-C-1000 and commercial RuO2 in 1 M KOH. (c) Nyquist plots and (d) variation of double layer charging currents as a function of scan rate of Co2P@N-C-800, Co2P@N-C-900 and Co2P@N-C-1000. (e) Stability of Co2P@N-C-900 electrocatalysts before and after 2000 cycles in 1.0 M KOH. (f) Time-dependent current density curves for Co2P@N-C-900 at the constant potential for 24 h.
Figure 5. (a) OER polarization curves and (b) corresponding Tafel plots of Co2P@N-C-800, Co2P@N-C-900, Co2P@N-C-1000 and commercial RuO2 in 1 M KOH. (c) Nyquist plots and (d) variation of double layer charging currents as a function of scan rate of Co2P@N-C-800, Co2P@N-C-900 and Co2P@N-C-1000. (e) Stability of Co2P@N-C-900 electrocatalysts before and after 2000 cycles in 1.0 M KOH. (f) Time-dependent current density curves for Co2P@N-C-900 at the constant potential for 24 h.
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Wang, K.; Zhang, R.; Guo, Y.; Liu, Y.; Tian, Y.; Wang, X.; Wang, P.; Liu, Z. One-Step Construction of Co2P Nanoparticles Encapsulated into N-Doped Porous Carbon Sheets for Efficient Oxygen Evolution Reaction. Energies 2023, 16, 478. https://doi.org/10.3390/en16010478

AMA Style

Wang K, Zhang R, Guo Y, Liu Y, Tian Y, Wang X, Wang P, Liu Z. One-Step Construction of Co2P Nanoparticles Encapsulated into N-Doped Porous Carbon Sheets for Efficient Oxygen Evolution Reaction. Energies. 2023; 16(1):478. https://doi.org/10.3390/en16010478

Chicago/Turabian Style

Wang, Ke, Ruimin Zhang, Yun Guo, Yunjie Liu, Yu Tian, Xiaojun Wang, Peng Wang, and Zhiming Liu. 2023. "One-Step Construction of Co2P Nanoparticles Encapsulated into N-Doped Porous Carbon Sheets for Efficient Oxygen Evolution Reaction" Energies 16, no. 1: 478. https://doi.org/10.3390/en16010478

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