Durability of a novel sulfonated polyimide membrane in polymer electrolyte fuel cell operation

Electrochimica Acta 54 (2009) 1076–1082

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Durability of a novel sulfonated polyimide membrane in polymer electrolyte fuel cell operation Akihiro Kabasawa a,b , Jumpei Saito b , Hiroshi Yano c , Kenji Miyatake c,d,1 , Hiroyuki Uchida c,d,1 , Masahiro Watanabe c,d,∗,1 a

Fuji Electric Advanced Technology Co., Ltd., Hino, Tokyo 191-8502, Japan Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4 Takeda, Kofu 400-8511, Japan Fuel Cell Nanomaterials Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan d Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan b c

a r t i c l e

i n f o

Article history: Received 16 July 2008 Received in revised form 22 August 2008 Accepted 24 August 2008 Available online 30 August 2008 Keywords: PEFC Polymer electrolyte Sulfonated polyimide Durability

a b s t r a c t A novel sulfonated polyimide membrane containing triazole groups (SPI-8) was subjected to long-term fuel cell operation. Excellent durability of the SPI-8 membrane was confirmed by single cell operation for 5000 h at 80 ◦ C. Open circuit voltage and hydrogen crossover through the membrane showed only minor changes during cell operation, indicating a lack of catastrophic damage for the SPI-8 membrane. It was found by post-test analyses of the membrane that the ion exchange capacity (IEC) decreased only slightly, but the molecular weight decreased to 1/10, resulting in a loss of mechanical strength. It was concluded that the major degradation mode of the sulfonated polyimide membrane involves the ring-opening of the imide linkages via hydrolysis, while a certain degree of side chain degradation occurs as a result of oxidative attack by radical species. © 2008 Elsevier Ltd. All rights reserved.

1. Introduction The polymer electrolyte fuel cell (PEFC) is a promising energy source for residential co-generation systems and electric vehicles. Perfluorosulfonic acid (PFSA) membrane materials such as Nafion® (DuPont) are most commonly used as the electrolyte for PEFCs because of their high proton conductivity as well as chemical and thermal stability. However, the durability of the PFSA membrane is still insufficient for the commercialization of PEFCs [1–4]. Radical attack of the PFSA via H2 O2 formation was proposed to be a probable decomposition mechanism [2–5], but thermal decomposition via local combustion is also possible, especially when a pinhole is formed in the membrane. It has been recognized that the decomposition of the PFSA membrane can be induced by crossover of reactant gases, generating H2 O2 and/or radical species (HO• and HOO• ) that can attack the membrane [2,3,5]. Non-fluorinated hydrocarbon ionomers have been developed as alternative electrolyte membranes, particularly for high tem-

∗ Corresponding author at: Clean Energy Research Center, University of Yamanashi, 4 Takeda, Kofu 400-8510, Japan. Tel.: +81 55 220 8620; fax: +81 55 254 0371. E-mail address: [email protected] (M. Watanabe). 1 ISE member. 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.08.042

perature operating PEFCs [6–8]. Some of these exhibit proton conductivity and cell performance comparable to those for PFSA membranes [8]; however, there have only been a few reports on their durability and degradation in fuel cell operation [9–11]. Recently, we have reported successful fuel cell operation for 5000 h, using our sulfonated polyimide membrane containing aliphatic segments (SPI-5) [12–14] and sulfonated poly(arylene ether) membrane containing fluorenyl groups (SPE-1) [15–17]. These membranes were durable, particularly under high humidity conditions, whereas rather serious degradation was observed under low humidity conditions, with membrane thinning being observed. More recently, we have proposed novel sulfonated polyimide ionomers containing triazole groups (SPI-8, Fig. 1) [18]. The SPI8 membranes exhibit comparable proton conductivity, thermal, oxidative, and chemical stability and much better mechanical stability than those of the SPI-5 membranes. The aim of the present research is to evaluate the durability of the SPI-8 membrane in PEFC operation. We have confirmed that the SPI-8 membrane is also durable for 5000 h at 80 ◦ C in single fuel cell operation. The changes in the terminal voltage (Ecell ) at constant current density, open circuit voltage (OCV), mass activity (MA), and the ohmic cell resistance (Rcell ) were monitored during cell operation. Post-test analyses of the membrane characteristics were carried out, including thickness, ion exchange capacity (IEC) and molecular weight.

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Fig. 1. Chemical structure of the sulfonated polyimide SPI-8.

2. Experimental 2.1. Membrane SPI-8 membrane (45–55 ␮m thick) was prepared according to the method described previously [18]. Briefly, SPI-8 was synthesized by the polycondensation of triazole-containing dianiline (3,5-bis(4-aminophenyl)-1H-1,2,4-triazole), acid functionalized benzidine (3,3 -bis(sulfopropoxy)-4,4 -diaminobiphenyl), and naphthalene tetracarboxylic dianhydride. Membranes were prepared by solution casting. The IEC of the SPI-8 membrane was 2.33 meq g−1 , determined by titration. Commercial Nafion® membrane (NRE212, DuPont, 50 ␮m thick) was also used for comparison. 2.2. Preparation of membrane and electrode assemblies (MEA) Gas diffusion electrodes with a three-layer structure (carbon paper/gas diffusion layer/catalyst layer) were prepared as follows [12]. A slurry of Pt catalyst supported on carbon black (Pt/CB TEC10E50E, 46.5 mass%-Pt/CB, Tanaka Kikinzoku Kogyo K.K.), containing commercial Nafion® solution (DE-521, DuPont) and a solvent (2-propanol/water = 1/1 by weight) were mixed in a planetary ball mill for 30 min. The electrolyte (binder) in the catalyst layer was Nafion® for both cells, with SPI-8 and Nafion® NRE212 being used as membranes. The mass ratio of Nafion® (dry basis) to CB was adjusted to 0.7. The slurry was coated over the commercial microporous layer on carbon paper (24BC, SGL Carbon AG). The geometric electrode area was 25 cm2 (5 cm × 5 cm) and the Pt loading amount in the electrodes was 0.50–0.58 mg cm−2 . The membrane and electrode assembly (MEA) was prepared by hotpressing an SPI-8 or NRE212 membrane sandwiched between two identical gas diffusion electrodes at 140 ◦ C for 3 min at 1.0 MPa. The MEA was mounted in a single-cell apparatus (Japan Automobile Research Institute (JARI) standard cell) consisting of two carbon separator plates with single serpentine flow fields. Both of the prehumidified reactant gases (oxidant and fuel) were introduced into the top of the separator plate and exhausted from the bottom. 2.3. Cell operation Cells were operated at a constant current density of 0.20 A cm−2 at a cell temperature Tcell of 80 ◦ C and ambient pressure for 5000 h. The cell voltage (Ecell ) and ohmic resistance (Rcell ) were monitored during the operation. The Rcell values were measured by the current interruption method (890CL, Scribner Associates Inc.). Two cells were tested for SPI-8; one operated under high humidity conditions (100% relative humidity, RH, for both H2 and air) and the other under low humidity conditions (100% RH for H2 and 40% RH for air). For comparison, the cell using Nafion® NRE212 as electrolyte membrane was operated under high humidity conditions. Hereinafter, these cells are denoted as SPI-8-high cell, SPI-8-low cell and Nafion® cell, respectively. The utilization of reactant gases was 70% for H2 and 50% for air.

Every 500 or 1000 h during the operation, the current–voltage (I–V) curves were measured under high humidity conditions (100% RH for both H2 and air) to evaluate the cell performance parameters, i.e., Ecell (IR-free, at 0.20 A cm−2 ), Rcell (at 0.20 A cm−2 ), open circuit voltage (OCV), and the mass activity (MA). MA is defined as the current at 0.85 V per unit Pt weight (A g−1 ) and is a measure of catalyst utilization in the catalyst layer. At the start (0 h) and end (5000 h) of the operation, hydrogen crossover across the membrane was measured by linear sweep voltammetry under 100% RH conditions at Tcell = 40 ◦ C. The anode acted as both the counter and the reference electrode (reversible hydrogen electrode, RHE). The potential of the cathode (in N2 ) was swept at 0.5 mV s−1 from the rest potential (ca. 110 mV) to 500 mV vs. RHE. Hydrogen crossover was evaluated as the diffusion-limited hydrogen oxidation current obtained in the range of 400–500 mV vs. RHE. 2.4. Post-test analysis of the membrane After 5000 h fuel cell operation, the membranes were removed carefully from the cells and examined with respect to changes in thickness, IEC and molecular weight. As for the Nafion® NRE212 membrane, only the thickness change was evaluated. The thickness of the membranes was measured by scanning electron microscopy (SEM, Hitachi S-5200) of the cross section of small pieces (5 mm wide). The membrane samples were cut from two different locations (gas inlet and outlet) of the electrodes to investigate the effect of location on the membrane degradation. IEC changes in the membranes, i.e., before vs. after cell operation, were determined by titration. Sections of the membranes were cut from the MEAs, from which the catalyst layers were carefully removed by immersing in ethanol. These were then equilibrated with a large excess of 0.01 M NaCl aq. for 15 h. The amount of protons released from the membrane sample was measured by titration with 0.01 N NaOH aq. using phenolphthalein as an indicator. Molecular weights were measured with gel permeation chromatography (GPC) in an instrument equipped with two Shodex KF-805 columns and a Jasco 805 UV detector (270 nm) with DMF containing 0.01 M LiBr as eluent. Standard polystyrene samples were used for calibration. 1 H (400 MHz) NMR spectra were recorded on a Bruker AVANCE 400S spectrometer with deuterated dimethyl sulfoxide (DMSO-d6 ) as a solvent and tetramethylsilane (TMS) as an internal reference to check the chemical structure of the SPI-8 ionomer. 3. Results and discussion 3.1. Cell operation The initial I–V curves for the Nafion® cell and the two SPI-8 cells operated under high humidity conditions (100% RH for both H2 and air) are shown in Fig. 2. The two SPI-8 cells were constructed

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Fig. 2. Initial I–V curves for PEFCs operated under high humidity conditions (H2 /air, 100% RH) at Tcell = 80 ◦ C. (䊉) Nafion® cell; () SPI-8 cell (1); () SPI-8 cell (2). Two SPI-8 cells were constructed with the same batch of MEA for the long-term tests under different humidity conditions; SPI-8 cell (1) for high humidity conditions (100% RH), SPI-8 cell (2) for low humidity conditions (cathode 40% RH).

with the same batch of MEA for the long-term tests under different humidity conditions. The performances of the two SPI-8 cells were comparable to that of the Nafion® cell, which was ascribed to the similar proton conductivities of the SPI-8 and NRE212 membranes under high humidity conditions [18]. Thus, the proton conduction pathway between the SPI-8 membrane and the Nafion® binder in the catalyst layer was properly formed, similar to that between the NRE212 membrane and Nafion® binder. Fig. 3 shows the time courses of the cell voltage (Ecell ) and cell resistance (Rcell ) during the operation at a constant current den-

sity of 0.20 A cm−2 . All three cells were successfully operated for 5000 h. This is one of the longest successful operations reported for non-fluorinated hydrocarbon membranes [12–15]. The Ecell value for the Nafion® cell under high humidity conditions was nearly constant (ca. 0.75 V) during the 5000 h operation, as was the Rcell value (0.07  cm2 ). In contrast, the initial Ecell for the SPI-8-low cell (0.60 V) was lower than those for the other two cells. This was mostly due to the lower proton conductivity of the SPI-8 membrane at low humidity [18], resulting in the highest Rcell value (0.25  cm2 ) among the three cells. After the continuous operation started, the Ecell values for the SPI-8 cells decreased gradually. In particular, the SPI-8-low cell showed the largest drop in Ecell (from 0.60 V at 0 h to 0.43 V at 300 h) during operation. However, Ecell for the SPI-8-low cell recovered to nearly the initial value (ca. 0.6 V), when the continuous operation was interrupted to measure the cell performance under high humidity conditions at 300 h. Then, Ecell gradually decreased again and reached ca. 0.43 V at 1000 h. Similar behavior was observed repeatedly every 500 or 1000 h throughout the continuous operation. The Rcell value for SPI-8-low cell showed a rapid increase during the initial 1000 h and increased gradually thereafter. The Rcell value for SPI-8-low cell did not recover completely to the initial value by interrupting the continuous operation for the measurement of cell performances under high humidity conditions. These results suggest that the losses in Ecell for the SPI8 cells were due to a temporary decline in electrode performance, as discussed below, and thus were recoverable, while the increase in Rcell for SPI-8-low was associated with the membrane and was non-recoverable. In a previous communication, we have reported that decomposition products from the PFSA membrane degraded the cell performance due to their adsorption on the Pt cathode catalyst, blocking the oxygen reduction reaction [19]. However, the cathode performance was recovered during high humidity (100% RH) operation, which removed the decomposition products. Hence, the degradation and recovery in the Ecell values for the SPI-8 cells seen in Fig. 3 may also be ascribed to the adsorption/desorption of the decomposition products from the SPI-8 membrane. Further investigations on the decomposition products are in progress and will be reported elsewhere. 3.2. Cell performance during operation

Fig. 3. Time courses of (a) cell voltage (Ecell ) and (b) cell resistance (Rcell ) at constant current density (0.20 A cm−2 ) at Tcell = 80 ◦ C. (䊉) Nafion® cell (100% RH); () SPI-8high cell (100% RH); () SPI-8-low cell (cathode 40% RH).

Fig. 4 shows changes in the IR-free cell voltage [Ecell (IR-free)], cell resistance (Rcell ) at 0.20 A cm−2 , open circuit voltage (OCV), and mass activity (MA), obtained from I–V measurements under high humidity conditions every 1000 h during cell operation. The Ecell (IR-free) values for all cells were constant during the continuous operation, indicating that the performance under high humidity conditions was very stable for all three cells. It is also noteworthy that the OCV values remained high (>0.95 V) for all of the cells during the operation. Thus, no catastrophic or significant membrane degradation occurred up to the 5000 h point, indicating that the membranes were quite resistant to crossover of reactant gases. We have measured the hydrogen crossover by the linear sweep voltammetry method. Comparisons of the hydrogen crossover current densities [j(H2 )cross ] for the three cells measured at the initial point and 5000 h operation are shown in Fig. 5. The values of j(H2 )cross of pristine SPI-8 cells were ca. 0.2 mA cm−2 , which is less than half that for the Nafion® cell (0.45 mA cm−2 ), due to the low hydrogen permeation coefficient of the SPI-8 membrane compared with that of the Nafion® membrane [18]. The j(H2 )cross value for the SPI-8-low cell increased by 50% after 5000 h, while that for the SPI-8-high cell did not change. The operation under low humidity conditions appeared to cause minor degradation of SPI-8 membrane, leading to increase in the

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Fig. 4. Changes in performance parameters: (a) IR-free cell voltage [Ecell (IR-free)] at 0.20 A cm−2 , (b) open circuit voltage (OCV), (c) mass activity (MA) at 0.85 V, and (d) cell resistance (Rcell ) at 0.20 A cm−2 . Symbols are the same as those in Fig. 3. All parameters were measured under H2 /air under high humidity conditions (100% RH) at Tcell = 80 ◦ C. Fig. 6. I–V curves for (a) Nafion® cell, (b) SPI-8-high cell, and (c) SPI-8-low cell before (䊉) and after 5000 h operation test (♦). I–V measurements were carried out under high humidity conditions (100% RH for both H2 and air) at Tcell = 80 ◦ C. Inset (d) shows IR-corrected I–V curves for SPI-8-low cell. The data for the initial I–V curves are cited from Fig. 2.

Fig. 5. H2 crossover current density [j(H2 )cross ] for the SPI-8-high, SPI-8-low and Nafion® cells measured initially and at 5000 h. H2 and N2 (100% RH) were fed to the anode and cathode, respectively, at Tcell = 40 ◦ C.

Fig. 7. Membrane thicknesses of the SPI-8-high, SPI-8-low and Nafion® cells after 5000 h operation. The membrane samples were cut from regions of the electrodes near the gas inlet and the gas outlet.

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gas crossover, as has often been observed for PFSA membranes [11,20,21]. Nevertheless, the hydrogen crossover for the SPI-8-low cell after 5000 h was still much lower than the initial value for the Nafion® cell, demonstrating the great advantage of the SPI-8 membrane. Hence, the SPI-8 membrane was thus considered sufficiently durable in terms of crossover of reactant gases, at least up to 5000 h of operation. The MA values for all three of the cells did not change significantly during the operation, as shown in Fig. 4c. The catalytic activities and/or the utilization of Pt did not deteriorate significantly for any of the cells, at least under high humidity conditions, during the operation. In spite of the losses and recovery of Ecell shown in Fig. 3a, the MA of the SPI-8-low cell was nearly constant. This discrepancy can be explained by the timing of the cell performance measurements. MA was measured after the recovery of Ecell that resulted from increasing the cathode humidity to 100% RH every 1000 h. In contrast, Rcell for the SPI-8-low cell increased gradually up to 2000 h and approached a constant value, while Rcell for the SPI-

8-high and Nafion® cells remained constant during operation, as shown in Fig. 4d. The increase in Rcell for the SPI-8-low cell is probably due to the lowered proton conductivity of the SPI-8 membrane even under 100% RH condition. Since one of the possible reasons for the decrease in the proton conductivity of the SPI-8 membrane is chemical degradation during operation, we carried out post-test analyses of the membrane after the cell operation. Fig. 6 shows I–V curves under high humidity conditions (100% RH for both H2 and air) for the Nafion® cell, SPI-8-high cell, and SPI-8-low cell before and after the long-term operation. I–V curves were not changed after 5000 h operation for both Nafion® cell and SPI-8-high cell, consistent with the results stated above. In contrast, as shown in Fig. 6c for SPI-8-low cell, Ecell values at high current density region (>0.3 A cm−2 ) decreased slightly after 5000 h operation. One of the reasons for such a voltage drop is increase in Rcell for SPI-8-low cell described above (see Fig. 4d). However, an additional voltage drop was still seen in the IR-corrected I–V curve (Fig. 6d). We consider that interfacial resistance between SPI-8 membrane and catalyst layer (which cannot be measured by a current inter-

Fig. 8. SEM images (left row: low magnification, right row: high magnification) of cross-sections of the electrolyte membranes (gas inlet) after 5000 h operation. (a) SPI-8-high cell, (b) SPI-8-low cell, (c) Nafion® cell. Membrane thicknesses are indicated with arrows.

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ruption method for Rcell ) might increase during the operation due to the degradation of SPI-8 membrane as discussed below. 3.3. Characterization of membranes after cell operation The membrane thicknesses for the SPI-8-high, SPI-8-low and Nafion® cells after 5000 h operation are shown in Fig. 7. The thickness was averaged over 2 to 3 different samples, which were cut from two different locations of the electrodes (gas inlet and outlet). The SPI-8 membranes were found to be fragile after operation, but the membrane thicknesses of all cells did not change noticeably from those of the pristine membranes (SPI-8, 45–55 ␮m; Nafion® NRE212, 50 ␮m). No significant differences in thickness were observed between the samples taken from inlet and outlet for either SPI-8 or NRE212 membranes. The slight decreases in the membrane thickness were probably due to either minor decomposition of the membrane or creep due to compression, or a combination of the two effects. Fig. 8 shows typical SEM images of cross sections of the membranes (gas inlet) for each cell after 5000 h cell operation. No locally thin areas or pinholes could be found in the membranes after the operation, even in the lower or higher magnification images, respectively. These analytical results are consistent with the results shown in the previous section, i.e., OCV and hydrogen crossover were nearly constant during the cell operation. Changes in the IEC values at two different locations (gas inlet and outlet) of the SPI-8 membranes are shown in Fig. 9. The average IEC of the membrane in the SPI-8-high cell did not change, within experimental error, during operation. In contrast, the IEC of the membrane at the gas outlet in the SPI-8-low cell was found to have decreased from 2.33 to 2.07 meq g−1 after 5000 h operation. Since sulfate ions and aliphatic sulfonate ions were detected in the drain water from the SPI-8 cells, we conclude that side chain scission occurred in the decomposition of SPI-8. The decrease in IEC and increase in Rcell for the SPI-8-low cell suggests that the membrane degradation was more pronounced under lower humidity conditions, as mentioned above, which is similar to the results obtained in the cells using PFSA membranes [11,20,21]. The molecular weights of the SPI-8 membranes were measured after cell operation to evaluate the chemical decomposition. Gel

Fig. 10. Gel permeation chromatograms of the SPI-8 membranes before and after cell operation. The membrane samples were cut from the same locations as those in Fig. 7. Table 1 Molecular weights of SPI-8 membranes before and after cell operation Cell

Sampling location

Mn (kDa)

Mw (kDa)

Mw /Mn

SPI-8

Pristine

–

241

773

3.2

SPI-8-high

5000 h

Inlet Outlet

22 33

68 97

3.1 2.9

SPI-8-low

5000 h

Inlet Outlet

28 23

76 79

2.7 3.4

The membrane samples were cut from the same locations as those in Fig. 7.

permeation chromatograms of the SPI-8 membrane are shown in Fig. 10, and the number-averaged molecular weights (Mn ) and weight-averaged molecular weights (Mw ) are summarized in Table 1. These values are not absolute molecular weights but are relative to the polystyrene standards; the absolute error is less than a factor of 2–3. The molecular weights of the SPI-8 membrane for all samples decreased significantly during the 5000 h cell operation. In the NMR spectra of the SPI-8 membrane samples measured after the cell test, three minor peaks appeared at 6.88, 7.01 and 7.14 ppm, which were assigned to naphthaleic moieties with ringopened imide groups. Similar decreases in molecular weight and appearance of new NMR peaks were also observed for SPI-8 samples after ex situ tests of hydrolytic and oxidative degradation using hot water and Fenton’s reagent, respectively [18]. Therefore, we conclude that the major degradation modes are the hydrolytic and oxidative decomposition of the main chains involving the imide linkages, while a certain degree of side chain degradation occurs, reducing the IEC of the membrane as a result of oxidative attack by radical species (HO• and HOO• ). In spite of the significant decrease in Mw , the membrane thickness slightly decreased (see Fig. 7). This suggests that the SPI-8 ionomer retains sufficient structural integrity as long as its molecular weight remains above a certain level. Also, the values of Mn = 22 kDa and Mw = 68 kDa were sufficiently high for the SPI-8 membranes to maintain gas impermeability at a practically acceptable level. It is recognized, however, that there is still a need to improve the stability of the main chains as well as the side chains (including sulfonic groups) in these polyimide ionomers by appropriate molecular design. 4. Conclusions

Fig. 9. IEC values for the membranes in the SPI-8-high and SPI-8-low cells after 5000 h operation. The membrane samples were cut from the same locations as those in Fig. 7.

We have confirmed that the novel sulfonated polyimide membranes containing triazole groups (SPI-8) were reasonably durable

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for 5000 h in the single fuel cell operation at 80 ◦ C, under both high and low humidity conditions. This is one of the longest successful operations reported for non-fluorinated hydrocarbon membranes. Changes in open circuit voltage and hydrogen crossover through the membrane were very small; no catastrophic damage occurred for the SPI-8 membrane during the operation. It was found that the cell operation under low humidity conditions (cathode 40% RH) caused minor degradation of the SPI-8 membrane, resulting in a slight increase of the gas crossover and the ohmic resistance. The SPI-8 membrane became fragile during the operation due to hydrolytic and oxidative decomposition, which must be minimized by appropriate molecular design. Detailed investigation of the decomposition products of the SPI-8 membranes is in progress. Acknowledgements This work was partly supported by a grant from the fund “Leading Project: Next Generation Fuel Cells” and a Grant-in-Aid for Scientific Research (20350086) of the Ministry of Education, Science, Culture, Sports and Technology of Japan. Authors thank Prof. Donald A. Tryk (Fuel Cell Nanomaterials Center, University of Yamanashi) for his kind advises. References

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