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Journal of Power Sources 288 (2015) 121e127

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Development of membrane electrode assembly for high temperatureproton exchange membrane fuel cell by catalyst coating membranemethod

Huagen Liang, Huaneng Su*, Bruno G. Pollet, Sivakumar PasupathiHySA Systems Competence Centre, South African Institute for Advanced Materials Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535,South Africa

h i g h l i g h t s

� MEA with low Pt loading for HT-PEMFC was developed by CCM method.� The fabrication parameters were investigated for the performance optimization.� The CCM-based MEA has good stability during a short-term fuel cell operation.

a r t i c l e i n f o

Article history:Received 11 November 2014Received in revised form17 April 2015Accepted 19 April 2015Available online 23 April 2015

Keywords:High-temperature proton exchangemembrane fuel cellPolybenzimidazoleMembrane electrode assemblyCatalyst coating membraneCell performance

* Corresponding author.E-mail address: suhuaneng@gmail.com (H. Su).

http://dx.doi.org/10.1016/j.jpowsour.2015.04.1230378-7753/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

Membrane electrode assembly (MEA), which contains cathode and anode catalytic layer, gas diffusionlayers (GDL) and electrolyte membrane, is the key unit of a PEMFC. An attempt to develop MEA for ABPBImembrane based high temperature (HT) PEMFC is conducted in this work by catalyst coating membrane(CCM) method. The structure and performance of the MEA are examined by scanning electron micro-scopy (SEM), electrochemical impedance spectroscopy (EIS) and IeV curve. Effects of the CCM prepa-ration method, Pt loading and binder type are investigated for the optimization of the single cellperformance. Under 160 �C and atmospheric pressure, the peak power density of the MEA, with Ptloading of 0.5 mg cm�2 and 0.3 mg cm�2 for the cathode and the anode, can reach 277 mW cm�2, while acurrent density of 620 A cm�2 is delivered at the working voltage of 0.4 V. The MEA prepared by CCMmethod shows good stability operating in a short term durability test: the cell voltage maintained at~0.45 V without obvious drop when operated at a constant current density of 300 mA cm�2 and 160 �Cunder ambient pressure for 140 h.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Proton exchange membrane fuel cell (PEMFC) are considered asa promising next generation of clean energy conversion technologydue to its high power density, high efficiency, low emissions andfast start-up [1,2]. With increasing the operating temperature offuel cell (100e200 �C), the Pt catalyst poisoning by CO impurities atthe anode can be significantly mitigated and the cell performancealso can be further enhanced because of the improved kinetics ofcathode and anode reaction [3,4]. In addition, the humidificationsystem is not necessary and water management become easier at a

relatively higher temperature. Moreover, fuel cell operated atelevated temperature has high thermodynamic efficiency andsimplified thermal management, which is ideal for combined heatand power (CHP) systems. Hence, researchers have made efforts todevelop HT-PEMFCs based on phosphoric acid (PA) doped poly-benzimidazole (PBI) membrane in the last decades [5e10]. How-ever, to date, this promising technology has not yet been put on themarket, resulting from the low cell performance caused by the slowoxygen reduction reaction (ORR) kinetics and the transport limi-tation of the reactants and proton, due to the presence of phos-phoric acid [5,8]. Therefore, one of the most critical challenge indeveloping HT-PEMFCs is to enhance the cell performance [5,6,11].

The most important part of PEMFC is membrane electrodeassembly, which is consisted of catalyst layers, electrolytemembrane and two gas diffusion layers (GDLs). In the MEA, the

Table 1The differences in the MEAs preparation based on CCM method.

MEA-type Membrane status CL fabrication PA doping

Type-a Dry Directly on the membrane GDLType-b Wet, PA-doped Decal transfer MembraneType-c Dry Directly on the membrane CCM

H. Liang et al. / Journal of Power Sources 288 (2015) 121e127122

electrochemical reaction for both anode and cathode only takeplace at ‘triple-phase boundaries’, where reactant, catalyst particlesand electrolyte contact together [12]. The fuel cell performance candiffer greatly depending on the method of the MEA fabrication andother key parameters such as catalyst loading, binder and ionomercontent [5,6]. Many methods have been developed to prepareMEAs, including gas diffusion electrode (GDE) method and CCMmethod [13,14]. As an alternative to the GDE method, in the CCMprocess, the catalyst inks are directly applied onto both sides of theproton exchange membrane. Hence, it is believed that the CCMmethod can avoid the loss of catalyst particles immersed into thepore network of gas diffusion layer (GDL) and establish a betterinterfacial contact between the catalyst layer (CL) and the electro-lyte membrane, which can enhance the catalyst utilization andimprove the cell performance [15,16]. However, one technicalchallenge is that the surface of the PBI-based membrane with pre-doped PA will remain moist state due to the strong moisture ab-sorption and the exudation of PA, resulting in a poor adhesion ofthe catalyst particles on the wet surface of the ABPBI membrane.Wannek et al. [17e20] reported that PA redistribution is a quickprocess within the HT-MEAs consisted of dry ABPBI and PA pre-doped GDEs. A stable cell performance can be reached in severalminutes after commissioning. Inspired by this line of thought,MEAs with enhanced Pt utilization prepared by CCM method andby acid impregnated GDLs have been reported by our group [21]. Itwas found that the serious distortion of the membrane can beavoided, then a good contact between the CL and the membranecan be kept. At low platinum loadings, the CCM method exhibitedmuch higher performance and Pt utilization compared with theMEA fabricated by GDE method.

In this work, we prepared MEAs by the CCM method and theeffects of different parameters, such as preparation method, bindertype as well as the Pt loadings of the cathode and the anode, on thefuel cell performance of the so-prepared MEA were investigated.The cell performances were evaluate at 160 �C with pure hydrogenand air as the reactants under ambient pressure. Polarizationcurves (IeV) and electrochemical impedance spectroscopies (EIS)were used to characterize various potential losses and variation ofelectrochemical properties. The results provide a more completeunderstanding for MEAs prepared by using CCM method for ABPBImembrane-based HT-PEMFC.

2. Experimental

2.1. Preparation of catalyst inks and fabrication of MEAs

Before the CCM based-MEAs fabrication, homogeneous sus-pension of the catalyst inks were prepared by dispersing Pt/Ccatalyst (JM 40 wt.% Pt), binder (PTFE, PBI or PVDF) in extra solvent(DMAc for PVDF and PBI binder, IPA for PTFE binder) and thenultrasonicated for 1 h at room temperature. In the CL, the dry PTFE/PVDF content is ~15 wt.%, while the PBI content in the CL is~10 wt.%.

In the work, all the MEAs with an active area of 2.3 � 2.3 cm2

were prepared by using an automated ultrasonic spraying tech-nique [9]. Three types of CCM-based MEAs were investigated usingthe above-prepared catalyst inks. For clarity, the differences in thepreparation of the three types of MEA are presented in Table 1.

For the type-a MEAs, the catalyst inks were directly depositedonto the both sides of the dry ABPBI membranes (fumapem®AM,~30 mm of thickness, FuMA-Tech). After the formation of CLs, theresulting electrodes were left in a vacuum oven for overnightdrying. Finally, the MEAs were assembled by contacting CCM andtwo commercially GDLs (H2315-CX196, Freudenberg, Germany)impregnated with PA without a preceding hot-pressing step. The

details of introducing PA can be found in our previous work [21].The amount of PA pre-impregnated in the GDLs was calculated bythe weight of the dry membrane (before CL coating) with theactual electrode area considering that PA redistribution mainlyhappened around the actual electrode area. The H3PO4 dopinglevel is 3.8 molecules of H3PO4 per polymer repeating unit (PRU)[6].

Type-b MEAs were constructed by the decal transfer method[22e24], which is considered as a suitable way for CCM massproduction. In this work, we are then motivated to examine itsapplicability on the preparation of HT-PEMFC MEAs. To preparetype-b MEAs (CCM-decal transfer), the CL was formed byspraying the catalyst inks onto the surface of a PTFE piece andthen transferred onto the surface of the PA pre-doped ABPBImembrane by hot-pressing at 130 �C under the pressure of200 kgf cm�2 for 5 min. The H3PO4-doping process was carriedout by soaking the ABPBI membranes in 85 wt.% PA solution forseveral hours at 100 �C. The acid doping level in the membranewas about 3.8(±0.4) molecules of H3PO4 per PRU, which is similarwith that for type-a MEA. The MEAs were assembled by con-tacting CCM and two commercially GDLs together without hot-pressing.

The type-c MEAs was fabricated by soaking the prepared CCMsin 85 wt % PA solution for several hours at 100 �C, then contact withtwo GDLs without hot-pressing. The PA doping level was alsocontrolled at ~3.8(±0.4) molecules of H3PO4 per PRU of themembrane.

2.2. Single-cell tests

The prepared MEA was assembled with two gaskets made offluorinated polymer into an HT-PEMFC cell fixture (BalticFuelCellsGmbH, Germany) and then installed in a Cell Compression Unit(CCU, Pragma Industries, France). The cell fixture consists of twographite plates with single serpentine channels(1.0mm� 1.0mm� 23mm) and ribs (1.0mm� 23mm). The activearea is about 5 cm2 (23 mm � 23 mm). Electrical heaters and athermocouple were embedded into the plates and connected to theCCU which controlled the cell temperature at 160 �C and the pistonpressure at 1 N mm�2 in this study to minimize the electrical andthermal resistances of the GDLs [25]. A procedure and set-up de-tails for fuel cell performance evaluation is referred in the previouswork [21]. The flow rate of hydrogen and air is 0.2 and 0.5 L min�1,respectively. The cell was activated at 160 �C and 0.5 V until thevariation of current density was less than 5 mA in 5 min.

2.3. Physical and electrochemical characterization of the MEAs

The surface morphology and the cross-section images of theMEAs were obtained by scanning electron microscopy (SEM)(Oberkochen, Germany). The cross-section of the samples wereprepared by freeze-fracturing the MEAs in liquid N2.

To determine the resistances of the MEAs, the in-suit electro-chemical impedance spectroscopy (EIS) was performed at 0.6 Vwith a 5 mV amplitude and the frequency range of0.1 Hze20,000 Hz.

Fig. 2. Polarization curves of three typed MEAs with PVDF as binder, operated at160 �C under atmosphere pressure and 1 N mm�2 assembling torque of the test cell.

H. Liang et al. / Journal of Power Sources 288 (2015) 121e127 123

3. Results and discussion

3.1. Effect of preparation method on the cell performance

Fig. 1 shows the digital photographs of the PA-doped CCMs forthe three typed MEAs with PVDF as binder. As seen as in Fig. 1(a),the size of the CCM after PA doping is almost same as its original,and the PA redistribution was restrained only around electrodeactive area (i.e., catalyst coated area). Therefore, the CCM surfacestill kept flat and no serious distortion occurred. However, using thedecal transfer method, as showed in Fig. 1(b), the catalyst layercannot be completely transferred to the PA-doped membrane,because the surface of the PA pre-doped membrane was wet due tothe presence of PA. Therefore, the contact between the catalystlayer and the membrane may be not secure. As shown in Fig. 1(c),the type-c MEA seriously distorted after PA doping, which mayresult in CL detachment, then a poor contact between the CCM andthe GDLs when being assembled.

The polarization curves of the three prepared MEAs are pre-sented in Fig. 2. It is clear that the type-a MEA shows better per-formance than the MEAs obtained from the other two methods. At

Fig. 1. Digital photographs of the PA-doped CCMs prepared by three methods. (a) CCMfor type-a MEA before (left) and after (right) PA doping, (b) PTFE substrate after CLtransfer (left) and the CCM for type-b MEA (right), (c) CCM for type-c MEA before (left)and after (right) PA doping.

0.4 V, the current density of the type-a MEA reaches 620 mA cm�2,2.5 times and 3.1 times to that of the type-b MEA (248 mA cm�2)and type-c MEA (200 mA cm�2), respectively. The peak powerdensity of the type-a MEA can reach 278 mW cm�2 at 0.31 V andthe limiting current density reaches up to 1200 mA cm�2. The goodperformance of the type-a MEA can be attributed to the combina-tion of the better utilization of the catalyst, as well as the superiorinterface contact among the GDLs, membrane and CLs, resultingfrom directly depositing the CL on the membrane and the properway of PA doping. On the contrary, the type-b MEAmay suffer fromthe catalyst loss and the unsecured CL/membrane interfacial con-tact during the decal transfer process, resulting in an inferiorcatalyst utilization and high cell resistance, which should be thereason for the fast voltage drop in the ohmic polarization region.For type-c MEA, the similar trend of voltage drop in this regionshould originate from the seriously distorted CCM after PA doping(Fig.1c), whichmakes difficulties when being assembled, leading toa high contact resistance. Moreover, the distorted CCM could causedifferences in the stress state of the stiff catalyst coating area andsoft bare membrane area, which may result in small fissures orpinholes in the CCM when it is assembled in the cell fixture. Thiscould be the reason for the type-cMEA showing amuch lower opencircuit voltage (0.55 V) compared to the other two. Based on theseresults, the MEAs for the following studies were prepared in type-a.

3.2. Effect of binder on the cell performance

Compared to low temperature (LT) PEMFC, high-boiling PA issuggested to act as the proton conductor in the CL for PBI mem-brane based high temperature PEMFC, instead of Nafion resins. Thephosphoric acid is in the solution state, which can penetrate intothe porous structure of the CL, whereas the colloidal Nafion cannot.The difference in the structures of the pore networks formed byparticles of these binders within the CL direct affect the cell per-formance. Therefore, the binder properties have greatly influenceon the CL mechanical properties, the gas permeability, PAimpregnation, platinum utilization and ORR in the electrodes of theHT-PEMFC. Therefore, three common used binders for HT-PEMFC(PTFE, PVDF and PBI) were evaluated to investigate their effectson the MEA performance based on the CCM method, as shown inFig. 3. It should bementioned here that these binder contents in theCLs are different from each other, which were pre-optimized in ourpreliminary optimization experiments (not detailed here).

Fig. 3. Polarization curves and power density curves of the type-a MEAs with differentpolymer binders, operated at 160 �C under atmosphere pressure and 1 N mm�2

assembling torque of the test cell.

Fig. 4. In-situ impedance curves of the MEA with different binders at 0.4 V and at160 �C.

H. Liang et al. / Journal of Power Sources 288 (2015) 121e127124

It is clear from Fig. 3, the performance of MEA with PVDF asbinder in the CL is much better than that of theMEAs preparedwithPBI or PTFE in whole current density region, which is mainlyattributable to the nature physicochemical properties of the threebinders since this is the only difference in these MEAs. At 0.4 V, thecurrent density o reaches 620 mA cm�2 for the MEA with PVDFbinder, while the value for the MEA with PTFE and PBI is only500 mA cm�2 and 350 mA cm�2, respectively.

For the type-a MEA, the PA was contained in the GDLs. There-fore, during fuel cell operation, the PA in the GDLwill be transferredto the CL. PBI can easily absorb the aqueous PA to form a highconductive acidebase complex, which is beneficial to increase theproton conductivity in the CL [26]. Although PBI is considered agood candidate for membrane materials because the low gaspermeability, the strong hydrophilicity of the PBI can lead to seriousmass transport limitation due to the risk of the CL flooding byexcess PA impregnation and the polymer film covered on thecatalyst particles blocking gases transport [5,6]. This should be thereasonwhy the MEAwith PBI binder shows the worst performanceand the limiting current density is only 450 mA cm�2.

When PTFE is used as a binder, it exists as colloidal solid parti-cles in the catalyst ink and easily forms large size agglomerates,which resulting the distribution of PTFE in CLmight be less uniformand poor utilization of Pt catalyst [7]. Although in the case of MEAsprepared by GDE method, the danger of PA flooding and masstransport limitation by using PTFE as binder can be efficientlyreduced with a sufficiently thick CL (>30mm) [27]. However, in ourcase, it is difficult for the liquid PA doped in the GDL to penetratethrough the thick hydrophobic CL, which results in an inferior PAdoping level in the membrane, consequently leading to anincreased ohmic resistance and the decreased cell performance, aspresented in Fig. 3.

In contrast, the PVDF binder exists in the CLs as a fiber phase,which makes catalyst particles less likely to be encapsulated in thebinder, and thereby making more Pt surface available in the CLs [7].The CL prepared with PVDF binder shows a more uniform anddenser structure compared to that with PTFE binder, which wasobserved from previous study [7]. Moreover, the moderate hydro-phobicity of PVDF favors both PA distribution and lowering the riskof PA flooding in the CL. Therefore, the MEA with PVDF binderyielded the best cell performance, which suggests that PVDF is thepreferred CL binder for the MEAs created in this work.

To further understand the performance differences of the MEAswith different binders, the in-situ EIS measurements were con-ducted at 0.4 V and 160 �C. Fig. 4 shows the equivalent circuit(insert) and the impedance curves of the MEAs. It should be notedthat RU represents the cell ohmic resistance, measured from theintercept at the real axis of the high frequency. Ra and Rc representthe distributed ionic resistance and the charge transfer resistance[28,29], respectively. The CPEa and CPEc represent a constant phaseelement. From Table 2, it is clear that RU of the MEA with PTFE asbinder is significantly increased compared to that of with PBI andPVDF binders, which may arise from the increased proton transferresistance in the membrane due to the insufficient PA doping fromthe GDLs because the highly hydrophobic PTFE CLs. The chargetransfer resistance of the MEAwith PVDF as binder is lowest amongthese three MEAs, which suggests that the catalyst layer with PVDFhas more efficient electrochemical active layer. Therefore, PVDF ispreferred as the CL binder for this study.

3.3. Effect of Pt loading of cathode and anode on the cellperformance

The Pt loadings in the cathode and the anode were varied from0.1 to 0.8 mg cm�2, respectively, to find the optimum value for thecell performance. Fig. 5 compares the cell performance of MEAswith various anode and cathode Pt loadings. Generally, the catalyticactivity of CL improve with the increasing of Pt loading, whichcertainly helps the improvement of theMEA performance. It is clearthat the cell performance was greatly improved with the Pt loadingin the anode increasing from 0.1mg cm�2 to 0.3mg cm�2 (Fig. 5(a)).The current density at 0.4 V is increased by ~50%, from410 mA cm�2 to 620 mA cm�2. In our previous report [21], it wasobserved that the ohmic resistance of MEA prepared by CCMmethod was affected the PA content in the membrane. Therefore,the MEAs with lower Pt loading, which possessed a thinner CLthickness, can contain more PA in the membrane than the MEAwith high Pt loading since the PA doping level is same in each MEA.This could be the reason for the cell performance gradually reducedwith further increasing the anode Pt loading from 0.3 mg cm�2 to0.8 mg cm�2. Fig. 5(b) shows the performance changes with thechanges of cathode Pt loading. It is easy to understand that the cellperformance was greatly increased when the cathode Pt loadingincreased from 0.1 mg cm�2 to 0.5 mg cm�2 because the majorvoltage losses occur in cathode due to poor ORR kinetics. However,

Table 2Fitted impedance parameters of the MEAs with different binders.

Binder type RU (U cm2) Ra (U cm2) Rc (U cm2)

PBI 0.367 0.012 0.384PTFE 0.482 0.008 0.182PVDF 0.354 0.006 0.142

Fig. 6. Short-term discharge curve of the optimized CCM-type MEA operated at300 mA cm�2 and at 160 �C under ambient pressure.

H. Liang et al. / Journal of Power Sources 288 (2015) 121e127 125

for similar reason mentioned above, further increasing the Ptloading to 0.8 mg cm�2 caused a fast voltage drop in both ohmicand mass transfer regions, implying that an excessively thick CL isnot favorable for the distribution and transfer of the protonconductor PA, which will result in increased cell resistance andmass transport limitations. It should be stated that the optimized Ptloadings (0.3/0.5 mg cm�2 for anode/cathode) for the MEA pre-paredwith the CCMmethod are substantially lower comparedwiththe traditional GDE method (normally above 0.7 mg cm�2 in asingle GDE) [18,30e32], which is considered as a major advantageof the CCM method over GDE method in MEA fabrication.

3.4. Stability

The stability of MEAs is of significance for the commercializationof HT-PEMFC [5]. To check the MEA stability prepared by the CCM

Fig. 5. Polarization curves and power density curves of MEAs with different anode andcathode Pt loadings and PVDF as binder, operated at 160 �C under atmosphere pressureand 1 N mm�2 assembling torque of the test cell.

method, a short-term operation at 160 �C and 300 mA cm�2 wasperformed (Fig. 6). After the activations, the CCM-MEA exhibitedgood stability at the working current density of 300 mA cm�2: thevoltage remained at ~0.45 V without obvious drop after ~140 hoperation. Generally, it is believed that the loss of PA from the MEAis a major mechanism for the degradation of the PBI/ABPBI-basedHT-PEMFCs [33], which caused by the high load conditions andhigh operating temperature, leading to PA removal from the MEAbecause the steam distillation mechanism [33]. Therefore, the goodperformance stability of the CCM-basedMEA suggests that the CCMmethod was effective to keep required PA in the membrane and theCLs, resulting in low PA loss rates under the operating conditions.The average degradation is about 0.35 mV h�1, which is close theinitial performance degradation rates estimated from other re-searchers' long-term durability results [34e36], implying thefeasibility of the CMM-based MEA for long-term operation.

A SEM analysis on the cross-section of the MEA prepared byCCMmethod before and after stability test shows in Fig. 7. The MEAafter the durability test were pretreated by the following proce-dure: peel off the GDLs and then frozen fracturing the CCM in liquidN2. It can be seen from Fig. 7(a) that the good contact between theCLs and the ABPBI membrane was established from the CCMmethod. The dry ABPBI membrane is about 33.5 mm thick. Nor-mally, the original thickness of PA pre-doped ABPBI membrane isnormally about 80 mm [5]. After the durability test, as shown inFig. 7(c), the ABPBI membrane still kept a thickness of about 72 mm,suggesting a satisfactory PA content remained in the membraneafter the test. A reduction of CL thickness also can be observed fromFig. 7(c) after the durability test, which should result from the longcell compression process and the part catalyst loss during GDLremoving. However, the good contact is still kept between the PAdoped ABPBI membrane and the CLs. Fig. 7(b) and (d) show thepartial enlarged views of the CL before and after the short-termoperation, respectively. It is clear that the CL still keep a similaruniform porous structure after the short term operation. Fromthese results, the CCM-based MEA showing high stability isattributable to the excellent interface contact between the CLs andthe dry ABPBI membrane, as well as the good CL porous structureresulting from CCM method.

4. Concluding remarks

ABPBI-based MEAs were fabricated by catalyst coating mem-brane (CCM) method for HT-PEMFC application. The effect of key

Fig. 7. SEM images of the surface and the cross-section of the MEA prepared by CCM method before (a) and after (c) a short-term operation. (b) and (d) is the partial enlarged viewof the CL in (a) and (c), respectively.

H. Liang et al. / Journal of Power Sources 288 (2015) 121e127126

parameters related on CCM fabrication, i.e. CCM type, binderproperties and anode/cathode Pt loading, were investigated for theoptimal performance of the resultant MEA. Following findings canbe drawn from the data obtained:

(1) For CCM-based MEA, pristine (dry) ABPBI membrane shouldbe used for catalyst deposition, and the required PA shouldbe pre-impregnated into the GDLs to avoid the CCM distor-tion, which can greatly reduce the ohmic resistance of theMEA.

(2) PVDF was found to be a suitable CL binder for the CCMmethod owing to its polymer form and moderate hydro-phobicity, which favors both PA distribution and loweringthe risk of PA flooding in the CL.

(3) The CCM method can be considered a promising way toreduce the Pt loading of HT-PEMFCs. The optimized Ptloading is only 0.3/0.5 mg cm�2 for anode/cathode, which aresubstantially lower compared to the traditional GDE methodwith an average Pt loading of ~0.8 mg cm�2.

Although good stability of the MEA was observed in a short-term (140 h) operation, long-term durability should be addressedin the future to validate the practicability of the CMM-based MEA.Also, further lowering the Pt loading to a comparable level withNafion-based LT-PEMFCs (normally less than 0.2 mgPt cm�2) will bea target, also a challenge, in the future endeavors.

Acknowledgments

This work is supported by Hydrogen and Fuel Cell TechnologiesRDI Programme (HySA), funded by the Department of Science andTechnology in South Africa (project KP1-S01). Financial supportfrom the NRF Free-standing Postdoctoral Fellowship (Grant No:88347) is also acknowledged.

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