Fabrication of spinel ferrite based alkaline anion exchange membrane water electrolysers for hydrogen production

Abstract

Alkaline anion exchange membrane water electrolysis (AEMWE) is considered to be an alternative to proton exchange membrane water electrolysis (PEMWE), owing to the use of non-noble meta/metal oxides in AEMWE. Here, we report a highly durable and low-cost AEM-based electrolysis cell with active spinel ferrite catalysts for hydrogen production. Ce-substituted MnFe₂O₄ was synthesized by a combustion method and investigated as the electro catalyst for oxygen evolution reaction (OER). Substitution of Ce in the cubic lattice of MnFe₂O₄ increases the conductivity of Ce_xMnFe_(2−x)O₄, which results in a negative shift in the OER onset potential. At 25 °C, the single cell with Ce_0.2MnFe_1.8O₄ exhibited a current density of 300 mA cm⁻² at 1.8 V. Notably, Ce_0.2MnFe_1.8O₄ demonstrates a durability of >100 hours in continuous electrolysis.

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1. Introduction

The gradual progression of the world toward a serious energy crisis has motivated intense studies on alternative energy conversion and storage systems.^1–5 To determine the ultimate renewable energy source for the future, a number of energy conversion and storage techniques have been developed, such as water splitting, fuel cells, and batteries, etc.^6–8 One of the most promising energy conversion methods, electrochemical water splitting, has received much attention due to its commercial efficiency and minimal impact on the environment.^3,9,10 Alkaline water electrolysis (AWE) is a simple and cheap method for producing hydrogen, which is recognized as an efficient fuel for energy storage systems. Moreover, AWE has dominated large-scale hydrogen production technology for several decades, because of cost-effective electrolyser fabrication and the simple control of corrosion.^11–15 However, conventional liquid alkaline electrolysers face a set of challenges, such as limited current density, low operating pressure, and low partial load range.^11,16 These challenges are due to the use of an electrolyte of 30% KOH solution in liquid alkaline water electrolysers for the electro catalytic water splitting.

The problems associated with the use of a concentrated alkali are largely related to the formation of carbonate ions, which can decrease the electrolyser lifetime and purity of hydrogen.^17–19 As a result, water electrolysis via solid polymer electrolyte (SPE), viz. proton exchange membranes (PEMs), and anion exchange membranes (AEMs) have recently received greater attention. Moreover, an alkaline polymer electrolyte possesses the advantages of both a solid polymer electrolyte and an alkaline pH.

While PEM water electrolysis (PEMWE) offers high current densities, compact electrolyser designs, and high hydrogen production rates, the non-platinum group metal lacks durability in an acidic environment.^9,16 Furthermore, the fabrication of the membrane electrode assembly (MEA) for PEMWE is expensive, resulting in resources being diverted to the development of alkaline anion exchange membrane water electrolysis (AEMWE).^20,21

The efficiency of electrolysers depends on the MEA, a key component of water electrolysis that hinges on the sluggish kinetic reactions of oxygen evolution (OER) and oxygen reduction reaction (ORR). Although the OER activity of noble metal oxides (Ir, Ru, and Pt) was outstanding in acidic medium, stability problems under alkaline conditions prevent the utilization of noble metal oxides as a practical electro catalyst for OER.^22–24

The exhaustive search for non-noble catalysts, conducted over many years, has identified the mixed metal oxides,^25,26 specifically the AB₂O₄ spinel,^27–31 spinel ferrites,^32–34 organic compounds,³⁵ and perovskites,^6,36–38 as potential OER catalysts capable of replacing expensive noble metal-based compounds. A detailed discussion of the electrocatalytic activities of non-noble metal oxides can be found in the cited ref. 37. Moreover, several studies have reported Ni and Co-based materials as excellent catalysts for water oxidation,³⁷ but only in alkaline solution; changing the electrolyte to deionized water significantly decreases the efficiency of electrolysers. To date, only a few studies have addressed in AAEMWE in deionized water. For example, Xu Wu et al. have reported a current density of 100 mA cm⁻² at 1.8 V (25 °C) for MEA with Cu_0.7Co_2.3O₄.³⁹ In addition, a current density of 300 mA cm⁻² at 2.2–2.5 V with a Li_0.21Co_2.79O₄ anode has been reported.⁴⁰ More recently, Lin Zhuang and co-workers⁴¹ demonstrated a current density of 400 mA cm⁻² at 1.85 V with a Ni–Mo-based AEM electrolyser using only deionized water. Similarly, Chao-yang Wang et al.¹⁸ report a life-time of >500 h with AEM water electrolysis. However, the Ni–Mo-based AEM electrolyser exhibits a durability of only 8 h, and the latter requires noble metal oxides for long-term stability. Moreover, each of these electrolysers required an operating temperature of 50 to 75 °C.

Therefore, the challenge of developing a non-noble metal catalysts for AEMWE that exhibits a current density of j > 0.5 A cm⁻² at an over potential of η_O2 < 0.3 V with long-term stability, at room temperature remains to be achieved.

Among non-noble metal catalysts, spinel ferrites³² are of particular interest because they possess low-cost, high catalytic activity, and durability at high pH; such attributes make spinel ferrites relevant for use in catalysis, sensors, and magnetic devices. Moreover, the spinel ferrite based catalysts demonstrate a great potential for OER^{33,34,42–44} and ORR,⁴⁵ which is comparable to commercial Pt. However, in practice, the low electrical conductivity restricts the application of spinel ferrite as a catalyst. Here, we report the performance of a laboratory scale AAEMWE with spinel ferrite catalysts for pure hydrogen production.

2. Experimental section

2.1. Membrane pre-treatment

The Alkaline Anion Exchange Membrane (Fumasep® FAA-3-PK-130) was purchased from the Fumatech company (Germany), delivered in bromide form and dry state. The membrane thickness was 130 μm and the specific conductivity was 7.6 mS cm⁻¹ at 25 °C. For alkaline applications, the membrane must be converted into a hydroxyl form. To achieve this, the membrane was kept in an aqueous solution of 1 M KOH for 24 h at 20–30 °C, and then transferred to deionized water for 1 h. To avoid CO₂ contamination, the pre-treated membrane was stored under humidified and CO₂ free conditions.

2.2. Ionomer preparation

Instead of using commercial ionomer solution, we used the AAEM dissolved in dimethyl sulfoxide (DMSO) solvent. To prepare AAEM ionomer solution 0.1 g of AAEM was added to 3 ml of DMSO solvent. However, the substrate used as a support for membrane not dissolved completely in the DMSO and settle down as precipitate. So we discard the precipitate and used the solution as ionomer for catalyst ink preparation.

2.3. Catalyst ink preparation

To prepare the catalyst ink, 3.5 mg of the catalyst powder dispersed, in the mixture of 60μ ml AAEM solution, 2 ml of 2-propanol and 30μ ml deionized water. After 30 minutes of sonication, the solution became homogeneous and was used for the MEA preparation.

2.4. MEA preparation

The MEA was fabricated by brush coating the catalyst mixture onto each side of the membrane, which was allowed to dry and hot pressed at 70 °C. The catalyst loadings on the anode and cathode were 3.5 mg cm⁻² of Ce_0.2MnFe_1.8O₄ and Ni powder (Sigma-Aldrich) respectively. This was followed by the cell assembly, utilizing a pair of platinum-coated titanium mesh as current collectors. The current collectors were placed on each side of the MEA to ensure proper contact between electrodes and the current carrier plate. Finally, a stainless steel end plate was placed for mechanical support. Fig. 7 shows the schematic diagram of the single cell adopted for this work. The working area of the single cell was 2 cm × 2 cm. The single-cell experiments were carried out using deionized water. Polarisation curves of the single cell with Ce_xMnFe_2−xO₄ were measured with a scan rate of 1 mV s⁻¹.

3. Results & discussions

3.1. Half-cell studies

In our previous work,⁴⁶ we performed the initial screening of catalysts in a standard three-electrode system using various concentrations of KOH electrolytes and showed that 0.2 M cerium substitution enhances the electrocatalytic activity of MnFe₂O₄ for OER in alkaline solution.

The Working electrode for electrochemical studies was prepared by sonicating 3 mg of the catalyst, in 60 μl isopropyl alcohol for 15 minutes. 20 μl of the suspension was pipette out and dried on a Pt metal substrate (1 cm × 1 cm) by heating, in a pre-heated oven at 80 °C. Subsequently the substrate was sintered, at 1200 °C in presence of air for 2 h.

The X-ray diffractograms of the as prepared MnFe₂O₄ and Ce_0.2 Fe_1.8O₄ are shown in Fig. 1. The diffraction patterns are distinct and sharp, clearly delineating the crystalline nature of the Ce_x Fe_2−xO₄. Furthermore, the observed diffraction pattern of MnFe₂O₄ are good agreement with the spinel-type MnFe₂O₄ (JCPDS: 073-1964), with a cubic lattice spinel structure. Due to the large ionic radius the incorporation of Ce³⁺ in the spinel lattice increases the lattice constant value of MnFe₂O₄ ⁴⁷ from a = 8.515 Å to 8.614 Å. The substitution of Ce³⁺ on MnFe₂O₄ was confirmed by X-ray photoelectron spectroscopy (XPS).

Fig. 1 XRD patterns of Ce_xMnFe_2−xO₄ with x = 0.0, 0.2, 0.4, 0.6, 0.8.

Fig. 2 illustrates the XPS spectrum of the Ce_0.2 Fe_1.8O₄ after sintering at 1200 °C, in ambient air for 2 hours. The survey spectrum presented in Fig. 2A, shows the compositional elements of the Ce_0.2 Fe_1.8O₄. Moreover, the elemental composition of Ce³⁺, Mn²⁺, and Fe³⁺ were further analyzed with the EDAX spectrum (Fig. 2B).

Fig. 2 (a). XPS spectra of the Ce_0.2MnFe_1.8O₄ after sintering (b). EDAX spectra of Ce_0.2MnFe_1.8O₄ after sintering. Reproduced from ref. 46.

Fig. 3A shows the XPS spectra for Mn 2p which provides clear evidence for the presence of Mn²⁺ in the Ce_xMnFe_2−xO₄. It can be seen from the spectrum that the peaks at 641.5 and 652.3 eV are caused by Mn 2p_3/2 and Mn 2p_1/2 respectively, with a satellite peak at 41.3 eV.^43,48 In Fig. 3C, the peaks at 711.0 and 724.6 eV were assigned to Fe 2p_3/2 and Fe 2p_1/2.⁴³ Moreover, two main peaks at 718.5 and 732.5 eV ^49–51 specifies the presence of Fe³⁺. Similarly, the characterized peaks with binding energies 845 and 905 eV ^52–54 in the XPS spectra (Fig. 3B) occurs due to the presence of Ce³⁺ on MnFe₂O₄. Thus, the above XPS data clearly suggest that Ce³⁺ incorporated on MnFe₂O₄ and sintering the catalysts at 1200 °C, resulted in the reduction of Fe³⁺ on the ferrite system.

Fig. 3 XPS spectra of the catalysts. Insert XPS spectra of (a) Mn²⁺ (b) Ce³⁺ (c) Fe²⁺ & Fe³⁺ on as prepared Ce_0.2MnFe_1.8O₄ and (d) Fe²⁺ & Fe³ on Ce_0.2MnFe_1.8O₄ after sintering. Reproduced from ref. 46.

Fig. 4 shows the morphology of the prepared Ce_x Fe_2−xO₄ (x = 0.0, 0.2) were analyzed using a scanning electron microscope (SEM). In all cases, we observed the collapsed microstructures of ferrites due to the high-temperature treatment. Further, sintering the samples at 1200 °C increased the agglomeration of ferrites; this resulted in the formation of a thin-film layer.³⁴ Subsequently, it insulates the Pt surface from the electrolyte solution. Therefore, no redox peaks appeared for Pt on cyclic voltammetry studies of samples in KOH solution, which ruled out the synergetic effect or diffusion of Pt on the catalysts layers.⁴² Fig. 4C & D shows SEM recorded on samples coated on Pt surface and sintered in air for 2 hours at 1200 °C.

Fig. 4 SEM images of Ce_xMnFe_2−xO₄ (x = 0.0, 0.2). Insert (A & B) before sintering and (C & D) after sintering. Reproduced from ref. 46

The cyclic voltammograms (CV) of catalysts were recorded in 1 M KOH solution with a scan rate of 10 mV s⁻¹ at 25 °C. Fig. 5 shows the CV curves recorded between 1.2–1.8 V vs. RHE). No redox peaks were seen in the CV,⁴² indicating that the ferrite does not undergo any oxidation process in the reaction conditions. However, the OER onset of Ce-substituted manganese ferrite shifts towards more negative than the manganese ferrite.

Fig. 5 Cyclic voltammograms of Pt/Ce_xMnFe_2−xO₄ (x = 0.0 ≤ 0.8) at 25 °C, with scan rate 10 mV s⁻¹ in 1 M KOH solution. Adapted from ref. 46.

The electrochemical impedance spectrum (EIS) of the catalysts recorded during the OER is shown in Fig. 6. The Ce-substituted MnFe₂O₄ shows smaller charge transfer resistance, compared to MnFe₂O₄. Such a drastic change in electrochemical behavior is due to the increase in the electronic conductivity of Ce_0.2MnFe_1.8O₄.

Fig. 6 Electrochemical impedance spectra of catalysts as a function of Ce substitution, measured at 1.6 V during oxygen evolution. Reproduced from ref. 46.

In general, the electrical conductivity of ferrite is explained by Verwey–de Boer mechanism and the polaron effect. However, the electrical conduction in ferrites originates between Fe²⁺ and Fe³⁺ ions at site B, conduction is mostly influenced by the Fe²⁺ concentration in the ferrite system. Due to the high temperature sintering, some lattice oxygen escapes from the spinel lattice, causing an oxygen deficiency in the spinel ferrites. Therefore, to balance the electrical charge created in the lattice, Fe³⁺ is reduced to Fe²⁺,^55,56 which facilitates the formation of excess Fe²⁺ ions in the system (as shown in Fig. 3D). Hence, the hopping rate of electrons in ferrites increases which is the cause of their higher electronic conductivity.^55,57

The X-ray photoelectron spectroscopy (XPS) spectrum, shown in Fig. 3C & D, confirms the co-existence of Fe²⁺ ions with Fe³⁺ in the spinel lattice. In addition, the replacement of metal ions with rare earth metal ions forms grain boundaries in ferrites but using high concentration (≥0.4) of such metal ions lead to the formation of secondary phases (i.e. ABO₃) on the ferrite system. These secondary phases hinder the mobility of charge carriers and increase the electrical resistivity. However, in the case of Ce_0.2MnFe_1.8O₄, the electronic conductivity was further enhanced by the cooperative effect⁵⁸ between the Ce³⁺ and Mn²⁺. The interaction between the electron-rich Mn²⁺ and electron-deficient Ce³⁺ ion enhances the Lewis acid properties of the mixed valence centers, as a result, the electrocatalytic activity of Ce_0.2MnFe_1.8O₄ increases.

3.2. Single cell studies

Based on results of half-cell studies, we selected Ce_0.2MnFe_1.8O₄ as a catalyst for single-cell experiments, since Ce_0.2MnFe_1.8O₄ demonstrates an excellent electro-catalytic activity and stability for OER, in alkaline solutions.

In general, the addition of the ionomer (during catalyst ink preparation) to the catalyst layer shows a direct effect on the efficiency of the electrolysers.^59,60 This is because the ionomer addition promotes ion (H⁺ or ⁻OH) transport from the bulk of the catalyst layer to the membrane, which consequently enhances the electrolyser efficiency by reducing the interfacial resistance between the membrane and electrode, or the ionomer and catalyst. Conversely, the electrical conductivity of catalysts is decreased by the addition of electron resistant ionomer solutions. Hence, it is important to optimize the ionomer loading for the catalyst ink and MEA preparation.

Furthermore, we modified the current collectors by platinum-coating on Ti mesh for single cell studies. As commercial AEMs possess a lower conductivity than Nafion membranes, single cells based on AEMs are essentially sensitive to the requirement of a good current collector; which should be corrosion resistant, have strong electrical conductivity, and provide mechanical support to the membrane.^16,61 Such a current collector should effectively expel the gases and allow the water to reach the catalytic sites in its counter flow. Hence, Pt-coated Ti meshes are better choice, as they display good electrical properties and do not passivate over time. As a result, the performance and durability of the MEA may increase in electrochemical studies.

The electro catalytic performances of single cell with Ce_0.2MnFe_1.8O₄ and MnFe₂O₄ anodes were first evaluated by recording a Linear Sweep Voltammograms (LSV) at room temperature. Fig. 8 shows the LSV curves of the MEA with Ce_0.2MnFe_1.8O₄ and MnFe₂O₄ anodes. At 25 °C, the Ce_0.2MnFe_1.8O₄ exhibits an over potential of 320 mV with a current density of 10 mA cm⁻², which is 80 mV lower than the MnFe₂O₄ by passing deionised water. When the Ce_0.2MnFe_1.8O₄ initiates the water splitting at 1.48 V, the electrolyser starts with a current density of 3 mA. Moreover, gradual increase the applied potential increases the current density to a maximum of 300 mA cm⁻² at 1.8 V. As shown in Fig. 9, the potential above 1.55 V the charge transfer resistance values of MEA using Ce_0.2MnFe_1.8O₄ shifts significantly to smaller values and it accelerates the water splitting to a greater extent. To validate the synergistic effect or catalytic effect of Pt coated Ti meshes, we measured the polarization curve without catalysts on AAEM (Fig. 8). However, the single cell did not show any significant catalytic activity on water electrolysis, which ruled out the synergistic effect of Pt.

Fig. 7 Schematic diagram of the single cell.

Fig. 8 Polarisation curves of the MEA with Ce_0.2MnFe_1.8O₄, MnFe₂O₄ and without catalysts in deionised water at 25 °C.

Fig. 9 Nyquist plots for MEA with Ce_0.2MnFe_1.8O₄ as a function of potential from 1.50 to 1.75 V during oxygen evolution reaction.

In contrast, the PEMWES using noble metal oxides will probably yield a maximum current density of >1 A cm⁻² under similar voltage, due to the high conductivity of per fluorinated membranes (Nafion). However, our results are superior to the electrolysers with Co-based catalysts such as Cu_0.7Co_2.3O₄ ³⁹ and Li_0.21Co_2.79O₄ ⁴⁰ anodes. Though, the MEA with the above catalysts exhibit promising activity in KOH solution, they show only a small current density performance with AAEM. Most significantly, the electro catalytic activity of Ce_0.2MnFe_1.8O₄ was comparable to the AAE water electrolysers using Ni–Fe as the anode.⁴¹ Previously, Hickner and Wang ¹⁸ reported a maximum current density of 399 mA cm⁻² at 1.8 V (50 °C) with IrO₂ anode and Pt cathode. However, recently, Zhuang et al. demonstrated the same current density with a slightly higher over potential and operating temperature of 75 °C in AAEMWE using Ni–Fe anode working only with pure water. Although our experiments were carried out at room temperature, we achieved a performance comparable to that of the Ni–Fe anode performance at higher temperature. The electrocatalytic activities of various electrodes using AAEM are given in Table 1.

Table 1 OER activity of various anode materials recorded in deionized water

S. no	Anode material	Operating potential (V)	Temperature (°C)	Current density (mA cm^−2)	Reference
(1)	Ce0.2MnFe1.8O4	1.8	25	300	This work
(2)	Li0.21Co2.79O4	2.2–2.05	20–45	300	39
(3)	IrO2	1.8	50	399	17
(4)	Cu0.7Co2.3O4	2.0	25	200	38
(5)	Ni–Fe	1.85	70	400	40

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The efficiency of MEA depends on the effective utilization of catalysts for electrochemical reactions viz., OER and HER, which are ultimately governed by the quantity of intermediates that reach the catalyst surface. Here, we used an AEM ionomer solution prepared by dissolving pre-treated membrane in DMSO. As a result, which enhances the hydroxyl ions transportation from the bulk catalyst layer to the membrane, and vice versa. Hence, the ionic conduction in the catalyst layer is significantly enhanced due to AAEM ionomer, and improves the electrolyser efficiency. Similarly, the Pt coated Ti meshes improve the durability of the electrolysers by reducing the corrosion of anode components.^40,61 Together with the ionomer effect and current collectors, the reduction of Fe³⁺ at high temperature also contributes for the overall efficiency of the single cell.

To evaluate the kinetic parameters (Table 2), we fit the polarization curves achieved with Ce_0.2MnFe_1.8O₄ and MnFe₂O₄ to the following equation.

η = a + b log j (1)

where η is the over potential and j is the current density.

Table 2 Electrode Kinetic parameters for OER in deionized water

S. no	Catalysts	Lattice parameter (Å)	ηO2 (mV) at current density (j)			Tafel slope value (b) mV per decade
			10 mA cm^−2	100 mA cm^−2	300 mA cm^−2
1	Ce0.2MnFe1.8O4	8.614	320	460	570	64
2	MnFe2O4	8.515	390	690	910	95

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Fig. 10 depicts the Tafel slope plots of MEA with Ce_xMnFe_2−xO₄ for OER measured in deionized water. The Tafel slope of Ce_0.2MnFe_1.8O₄ is 64 mV per decade, lower than 45 mV per decade obtained on the MEA using MnFe₂O₄. The observation of lower Tafel slope value for Ce_0.2MnFe_1.8O₄ compared to that of MnFe₂O₄ indicates the increased electron transfer for OER. A common mechanism for OER listed in eqn (2)–(4) has been used to account for the observed the kinetic parameters.

S + OH⁻ ↔ S–OH + e⁻ (2)

S–OH + OH⁻ ↔ S–O + H₂O + e⁻ (3)

2S–O ↔ 2S + O₂ (4)

where “S” is an active species.

Fig. 10 Tafel plots of OER current in Fig. 8.

The Tafel slope (b) value of Ce_0.2MnFe_1.8O₄ indicates that the eqn (3) is the rate determining step for OER. The literature studies^39,42 show that if eqn (3) is the rate determining step then it gives a second order reaction in OH⁻ and a typical b value of 60 mV per decade. Again, it can be noticed from the discussions that, Ce_0.2MnFe_1.8O₄ was the most active catalyst for the OER among all cerium substituted manganese ferrite.

3.3. Stability studies

The short term stability of the MEA estimated under an electrolysis current of 200 mA cm⁻² at room temperature. During this study the cell voltage steadily increased with time (Fig. 11). However, the commercial utilization of any catalyst requires long-term stability toward OER. As shown in Fig. 12, the stability of the Ce_0.2MnFe_1.8O₄ and MnFe₂O₄ was investigated by running chronoamperometric responses at 2 V (vs. RHE) by circulating deionized water. Current density of >390 mA cm⁻² remained constant. There was no apparent drop in the performance of Ce_0.2MnFe_1.8O₄ electrodes after 100 h of operation. Moreover, we analyzed the electrolyte every 10 h to ensure the stability of the catalysts during continuous electrolysis. Atomic absorption spectroscopy (AAS) analysis shows the absence of metal ions, further supporting that these catalysts are stable during AEM water electrolysis.

Fig. 11 Chronopotentiometry curve of single with Ce_0.2MnFe_1.8O₄ under current density of 200 mA cm⁻².

Fig. 12 Current density profile for constant electrolysis of MEA using MnFe₂O₄ and Ce_0.2MnFe_1.8O₄ with applied potential of 2 V at 25 °C.

4. Conclusion

In summary, we concluded that the enhanced electrocatalytic activity of Ce_0.2MnFe_1.8O₄ results from the incorporation of Ce³⁺ into the MnFe₂O₄ spinel lattice. The single cell fabricated with Ce_0.2MnFe_1.8O₄ catalysts exhibit a current density of ∼300 mA cm⁻² at 1.8 V with a lifetime of >100 h on constant electrolysis. Moreover, our studies reveal that using spinel ferrite as a catalyst, we can fabricate cost-effective, highly active AEM-based electrolysers for water oxidation. Moreover, studies are on-going to investigate the effect of temperature on water electrolysis and to improve the efficiency of the cell.

Acknowledgements

T. Pandiarajan thanks UGC, India for providing Junior Research Fellowship. We also thank Mr R. Ram Kumar and M. Baskar, CSIR-CECRI, Karaikudi – for the fabrication of single cell. We are greatly indebted to Dr Vijayamohanan K. Pillai, Director, CSIR-CECRI for his keen interest and encouragement.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01123j

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