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Perfluorinated composite membranes with organic antioxidants for chemically durable fuel cells
Electrochimica Acta 298 (2019) 901e909
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Perfluorinated composite membranes with organic antioxidants for chemically durable fuel cells Sung-Hee Shin a, Abdul Kodir a, b, Dongwon Shin a, Seok-Hee Park a, Byungchan Bae a, b, * a b
Fuel Cell Laboratory, Korea Institute of Energy Research (KIER), 152, Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea Renewable Energy Engineering, University of Science & Technology (UST), 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2018 Received in revised form 26 December 2018 Accepted 26 December 2018 Available online 27 December 2018
A perfluorinated composite membrane incorporating an organic antioxidant was prepared to improve the chemical durability of a polymer electrolyte fuel cell. Bipyridine and benzoquinone (BQ) were added as organic antioxidants, and the oxidative stability of the composite electrolyte membranes containing them was evaluated via accelerated aging tests, i.e., an ex situ Fenton's test and in situ open-circuit voltage (OCV)-holding test. The effects of the antioxidants in the Fenton's test were verified using scanning electron microscopy, fluorine ion detection, Fourier-transform infrared spectroscopy, and tensile strength measurements. Moreover, a commercial membrane, NRE211, and a composite membrane, Nafion-BQ, were incorporated into a membrane electrode assembly (MEA) that then underwent an OCV-holding test for 500 h. The change in the MEA state over time was analyzed using cyclic voltammetry and linear sweep voltammetry. While NRE 211 showed a significant reduction in OCV from 300 h, Nafion-BQ maintained a stable OCV for 500 h. Hence, the organic antioxidants added to the electrolyte membrane effectively improved the oxidative stability of the proton exchange membrane fuel cell without sacrificing the ionic conductivity of the perfluorinated electrolyte membrane. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Perfluorinated composite membrane Organic antioxidant Proton exchange membrane fuel cell Fenton's test Open-circuit voltage-holding test
1. Introduction Fuel cells that use hydrogen, which has been highlighted as a future energy source, as a fuel and discharge only water as a byproduct are an eco-friendly technology. Thus, various types have been actively developed. In particular, the proton exchange membrane fuel cell (PEMFC) has been most actively developed because it has a higher power density and fewer corrosion issues compared to other fuels cells, and it does not require electrolyte replenishment [1]. The cost competitiveness and life cycle are the most important factors for the successful entry of PEMFCs into the market. The durability of polymer electrolyte membranes is directly related to the life cycle of the fuel cells. Thus, many studies have been conducted on improving the durability of the polymer electrolyte membrane to improve the life cycle of fuel cells [2e9]. Perfluorosulfonic acid (PFSA) is the most widely used polymer electrolyte membrane. PFSA polymers have excellent chemical durability, mechanical strength, and proton conductivity [6].
* Corresponding author. Fuel Cell Laboratory, Korea Institute of Energy Research (KIER), 152, Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea. E-mail address: [email protected] (B. Bae). https://doi.org/10.1016/j.electacta.2018.12.150 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
Despite these advantages, PFSA electrolyte membranes are known to cause chemical deterioration due to hydrogen peroxide and hydroxyl radicals generated during fuel cell operation. This is because the hydroxyl radical attacks the end of the polymer electrolyte membrane chain or the sulfonic acid group to decompose the polymer chain [10]. Hence, studies focusing on the introduction of various kinds of additives into the polymer electrolyte membrane have been conducted to solve this problem. Previously used antioxidants were inorganic, including metal ions of transition metals such as Ce and Mn [11e14], oxides [15,16], and solid nanoparticles [17,18]. These inorganic antioxidants significantly improved the chemical durability of the electrolyte membrane by inhibiting attacks from radicals. However, the proton conductivity in the electrolyte membrane is lowered by the ionic crosslinking between the metal ion and the sulfonic acid functional group [11]. In addition, the performance of the cathode catalyst layer deteriorates due to the leakage of the antioxidant during the long-term operation of the fuel cells, which was identified to be closely related to the crystallite size of the ceria nanoparticles [19,20]. Efforts have been made to prevent the leakage of antioxidants from the polymer electrolyte membrane to solve this problem. For example, the Solvay Specialty Polymer group conducted
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studies to immobilize single metals (Ce [9], Cr, Co, and Mn [21]) or double metals (CeCr [10]) on a silica substrate containing sulfonic acid functional groups. In addition, researchers attempted to immobilize a polymeric antioxidant synthesized via radiation grafting on an electrolyte membrane [22]. Organic antioxidants, as well as metal-based antioxidants, have been actively developed in the food, bio, and pharmaceutical industries. This is because chemical reactants such as peroxyl radicals and hydroxyl radicals damage cells and accelerate the degradation of food [23e27]. Organic antioxidants can be classified into three types: phenolic, quinone, and N-heterocyclic, depending on the chemical structure. They prevent the auto-oxidation chain reaction by converting to stable radicals or radical resonance structures [28e31]. Thus, we proposed four kinds of organic antioxidants for use in fuel cellsdatocopherol (TOH), 2,6-dimethoxy-1,4-benzoquinone (BQ), hydroquinone (HQ), and 2,20 -bipyridine (BPY)dby considering the chemical structures of the classified organic antioxidants. In particular, the oxidation stability of the composite membrane was evaluated after the introduction of four antioxidants into the hydrocarbon-based polymer electrolyte membrane. The organic antioxidants present the possibility of improving the durability of electrolyte membranes for fuel cells as well as being used in the food, bio, and pharmaceutical industries [32]. In this study, two kinds of organic antioxidants, BQ and BPY, which exhibited excellent oxidative stability in hydrocarbon membranes in our previous study, were selected and applied to PFSA polymers. We aimed to verify whether the organic antioxidants can be used in perfluorinated polymer electrolyte membranes and hydrocarbon membranes. The oxidative stability of the fabricated PFSA composite membrane containing organic antioxidants was determined via an ex situ Fenton's test, and the deterioration of the electrolyte membrane was confirmed via scanning electron microscopy (SEM), fluoride emission rate (FER) measurements, Fourier-transform infrared spectroscopy (FT-IR), and tensile strength measurements. In addition, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed periodically during the in situ open-circuit voltage (OCV)-holding test for 500 h to determine if the organic antioxidants are effective in improving the long-term performance of the fuel cells of the PFSA electrolyte membrane. Therefore, we demonstrated in this work that organic antioxidants also effectively improve the chemical durability of the PFSA membranes without sacrificing ionic conductivity. 2. Experimental 2.1. Materials and chemicals Nafion® Dispersion DE 2021 (31175-20-9, Copolymer Resin (20%e22%)) was purchased from DuPont Fluoroproducts, USA. NMethyl-2-pyrrolidone (NMP, 872-50-4, anhydrous, 99%), 2,2Bipyridyl (BPY, 366-18-7, 99%), and 2,6-Dimethoxy-1,4benzoquinone (BQ, 530-55-2, 97%) were obtained from SigmaAldrich, USA. FeSO4∙7H2O (122020, DC Chemical Co., Ltd, South Korea) and H2O2 (E9MB61, Extra Pure Grade, Duksan Pure Chemicals, South Korea) were used as received. A glass plate and a hot plate were used to fabricate the cast membrane. As-received NRE211 (Nafion, equivalent weight (EW) ¼ 1100 g/mol, DuPont, USA) was used as a reference membrane for the OCV-holding test. 2.2. Preparation of Nafion composite membranes with antioxidant A 10 wt.% Nafion/NMP solution was prepared by replacing the original solvent with NMP. First, the required amount of NMP for the target concentration of Nafion solution was added to the
original 20 wt.% Nafion dispersion. Then, the original solvent mixture was evaporated at 50 C for 3 h using a centrifuge evaporator (kdScientific, USA). One of the organic antioxidants (BPY and BQ) was mixed with 10 wt.% NMP-based Nafion solution at 25 C for 8 h under a nitrogen atmosphere. The content of organic antioxidants was 2.5 mol% relative to the sulfonic acid group content of Nafion. The final Nafion solution with organic antioxidants was filtrated at a speed of 2 mL/min using a syringe filter (10746053, GE Healthcare UK Limited, UK) and a syringe pump (Norm-Ject 20 mL, 4G14048, Henke Sass Wolf, Germany). Perfluorinated composite membranes (PCMs) with organic antioxidants were prepared via a solution casting method. The filtrated polymer solution was cast on a glass plate and dried overnight at 70 C to form a thin polymer film. Then, the residual solvent was removed by drying at 80 C for 6 h under vacuum and washing with deionized (DI) water. The final PCMs were obtained by drying at room temperature for 1e2 days. 2.3. Proton conductivity of membranes The proton conductivity of the membranes was measured by using a four-electrode conductivity cell (MCC, WonAtech, South Korea). The cell was equipped with a Solartron 1260 impedance/ gain-phase analyzer and Solartron 1287 electrochemical interface. Then, electrochemical impedance spectroscopy was conducted in the range of 105 Hze101 Hz with an amplitude of 100 mV. The conductivity was tested at different relative humidities (RHs) of 30%, 50%, 70%, and 90%. The proton conductivity was calculated using the following equation:
Proton conductivity ðS=cmÞ ¼
D LBR
where D is the distance between electrodes, L is the membrane width, B is the membrane thickness, and R is the resistance obtained by extrapolating the impedance plot to the axis on the highfrequency side of the plot. 2.4. Water uptake based on relative humidity of membranes The water uptake of the PCMs was measured based on the RH using a dynamic water absorption analysis (TGA Q5000SA, TA Instruments, New Castle, USA). The PCMs were dried overnight at 100 C before measurement. The water absorption trend of the dried samples was investigated at 80 C under 30% RH, 50% RH, and 70% RH. Each RH condition was maintained for 1 h to reach equilibrium. Finally, the water uptake of PCMs at each RH was calculated as follows:
Water uptakeð%Þ ¼
Wwet Wdry 100 Wdry
where Wwet represents the wet weight of the membrane at a specific RH and Wdry represents the dry weight of the membrane. 2.5. Ex situ Fenton's test An ex situ Fenton's test was used to examine the chemical stability of the membranes. Fenton solutions of 3, 5, 7, and 10 ppm were prepared depending on the Fe2þ concentration, and the amount of H2O2 was fixed as 10 wt.% in DI water. The degradation of the membrane was investigated by increasing the concentration of the Fenton solution. 5 5 cm2 Nafion composite membranes were assembled with a sample holder, as shown in Fig. 1(a). Subsequently, the sample holder with the membrane was soaked in a
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Fig. 1. Ex situ Fenton's test. (a) Assembly of sample holder with membrane and (b) setup for Fenton's test.
Fenton solution and placed in a water bath (Eyela NTS 4000, Tokyo Rakikai Co. Ltd., Japan) at 60 C for 24 h at a shaking speed of 50 rpm (Fig. 1(b)). After this, the membrane was washed with DI water for 1 day. 2.6. Analysis of deteriorated membranes The chemical changes in the deteriorated membranes were observed using FT-IR spectroscopy before and after the Fenton's test. In particular, FT-IR spectroscopy was performed to examine the chemical degradation of the side chain of the Nafion structure. After the test, the membrane samples were rinsed with DI water and dried at 30 C for 24 h prior to analysis. The measurement was conducted using Nicolet 5700 FT-IR spectrometer (Thermo Electron Corporation, Madison, WI, USA) equipped with an attenuated total reflection (ATR) sampling accessory (Smart MIRacle™, Diamond, PIKE technologies, USA). The dried samples were placed on the diamond crystal of the ATR device, which is called an internal reflection element, and fixed by an upper probe. The spectrum was recorded with a wave number of 4 cm1 in the range of 700e4000 cm1. The obtained spectra were normalized with the absorbance peak of CF2 (1145 cm1), which is a major bond in the PTFE backbone, to investigate the variation in the peak intensity for two ethers in the side chain. In addition, the amount of fluoride in the reacted aqueous solution after membrane degradation was detected using a fluoride-selective electrode (6.0502.150, Metrohm AG, Switzerland). A calibration curve was drawn to estimate the fluoride concentration by using standard fluoride solutions of 1, 2, 10, 20, and 100 ppm. The physical changes in the deteriorated membranes due to accelerated degradation were investigated based on an analysis of
the weight change ratio, membrane turbidity, tensile strength, and SEM images. The degradation rate of the membranes based on the Fenton concentration was calculated based on the weight change of the membrane before and after the Fenton's test. All the samples were dried at 120 C for 24 h prior to measurement. The variation in the membrane turbidity was investigated using a haze meter (NDH 7000, Nippon Denshoku, Japan) before and after the Fenton's test. The pinholes formed on the surface of the deteriorated membrane were observed using SEM analysis (Hitachi S-4800 N SEM, Hitachi High-technologies Corporation, Japan). In addition, the variation in the mechanical strength of the deteriorated membranes was observed via an analysis of the tensile strength (TXA™ Texture Analyzer, Yeonjin Corporation, South Korea). The samples were cut in a dumbbell shape and both ends of the sample were clamped using the claws of the Texture Analyzer. The stress and strain responses were obtained for samples at a crosshead speed of 0.1 mm/ s at room temperature. 2.7. In situ OCV-holding test The long-term stability of the PCMs was evaluated by assembling a unit cell for an in situ OCV-holding test and incorporating it in membrane electrode assemblies (MEAs). Firstly, MEAs with Nafion and Nafion-BQ membranes were fabricated using a decal transfer method using a catalyst-coated substrate [33]. An electrode decal was formed on a decal substrate of a common polyethylene terephthalate film by coating it with a catalyst ink composed of a carbon-supported Pt catalyst (TEC10F50E, Tanaka Kikinzoku Kogyo K. K., Japan) and a perfluorinated ionomer (Aquivion D83-24B, Solvay Specialty Polymers Italy SpA, Italia) with an ionomer to carbon (I/C) ratio of 1.0. The transfer of the electrode
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decal onto the membrane was carried out by hot pressing at 100 C and 30 bar for 20 min. The prepared MEAs had platinum loadings of 0.36 and 0.38 mg/cm2 on the anode and cathode, respectively. Then, the MEAs were assembled into a unit cell (VeN Type, InWooTech, South Korea) with an active area of 9 cm2 using graphite bipolar plates and 10BC gas diffusion layers (GDLs, SGL Group). The unit cell was compressed at a pressure of 30 kgf/cm2. Finally, the OCV-holding test of the unit cells was performed following a New Energy and Industrial Technology Development (NEDO, Japan) protocol at 90 C and 30% RH under a hydrogen flow rate of 0.20 L/min for an anode and an air flow rate of 0.42 L/min for a cathode [34]. The time-dependent status of the MEAs was investigated with CV and LSV using a potentiostat (HCP-803, BioLogic Science Instruments, France) every 100 h. CV measurements were conducted with hydrogen supplied to the anode, flowing at 0.2 L/min, and nitrogen to the cathode at 0.0 L/min. The CV curves of MEAs with Nafion and Nafion-BQ were recorded in the range of 0.05e0.90 V at a scan rate of 50 mV/s. Then, the electrochemically active surface area (ECSA) of Pt in the MEAs was estimated from the hydrogen desorption charge of the 5th scan as follows [35]:
. ECSA m2 gPt ¼
QH 102 210mC=cm2Pt LPt
where QH is a hydrogen desorption charge (mC/cm2), and Lpt is the Pt loading (mgPt/cm2). The residual ECSA was obtained from the relative ratio to the initial value. LSV measurements were performed between 0.1 and 0.6 V at a scan rate of 1 mV/s, with hydrogen (nitrogen) supplied to the anode (cathode) at 0.2 L/min (0.5 L/min). A hydrogen crossover current of MEAs was obtained from the current at 0.5 V in the LSV curves. In addition, the short-circuit resistance (i.e., the electronic resistance) was calculated from the inverse of the slope of the straight-line portion of the LSV plot between 0.4 and 0.5 V.
3. Results and discussion 3.1. Characterization of Nafion composite membranes The average thicknesses of the Nafion, Nafion-BQ, and NafionBPY membranes prepared by solution casting method were similar (~37 mm). After the incorporation of organic antioxidants, there is no significant change in the ion exchange capacity (IEC), as determined by an acidebase back-titration [32]. Fig. 2(a) shows a comparison of the change in the proton conductivity of the composite membrane based on the RH after the introduction of organic antioxidants. The electrolyte membrane containing BQ had a
conductivity similar to that of the pristine Nafion membrane at both 70% and 90% RH, and the conductivity was confirmed to be slightly higher under conditions of low humidity. Since BQ is highly hydrophilic, the conductivity of the Nafion-BQ membrane may have been slightly higher than that of the pristine Nafion membrane at a low RH. In contrast, the composite membrane with BPY exhibited a slightly lower conductivity than the pristine Nafion membrane at all RH conditions, but the decrease in conductivity was negligible compared to that of electrolyte membranes containing inorganic antioxidants [14]. As BPY has a weak alkalinity, it is likely to have weak ionic interactions with the sulfonic acid group of PFSA, but it does not seem to have affected the conductivity of the PFSA electrolyte membrane significantly. Fig. 2(b) shows that the pristine Nafion and Nafion-BQ membranes had similar water uptake values under all RH conditions. However, Nafion-BPY had slightly lower water uptake values than the other membranes, presumably due to the weak ionic interaction between BPY and the sulfonic acid group. Nevertheless, this result is consistent with the findings of our previous study that organic antioxidants do not significantly inhibit the conductivity and moisture content of a hydrocarbon electrolyte membrane [32].
3.2. Effect of Fe2þ concentration on pristine Nafion membrane durability The deterioration of the pristine PFSA membrane based on the Fe2þ concentration was monitored to identify the Fenton's accelerated deterioration test conditions suitable for evaluating the oxidation stability. Fe2þ solutions of 3, 5, 7, and 10 ppm were prepared and used in the test. The deterioration of the membrane in the Fenton's oxidation experiment was observed based on changes in the dry weights and FT-IR spectra. As shown in Fig. S1(b), the dry weight of the pristine Nafion membrane tended to decrease as the Fe2þ concentration increased from 3 ppm to 7 ppm and the weight loss was similar at 10 ppm and 7 ppm. The FT-IR spectra showed a peak due to stretching vibrations of SeO at 1051 cm1 and two peaks (A and B) corresponding to ether stretching vibrations at 981 cm1 and 967 cm1, respectively. To more accurately compare the absorbance peaks corresponding to the side chains in the FT-IR spectra, the peaks were normalized to an absorbance peak corresponding to the main chain (CF2, 1145 cm1). As a result, as shown in Fig. S1(c), the absorbance peaks corresponding to the side chains of PFSA (SeO, CeOeC) exhibited a tendency to decrease with the Fe2þ concentration. Therefore, 10 ppm, which is considered to cause sufficient chemical deterioration, was selected as the optimum concentration for the experiment.
Fig. 2. (a) Proton conductivity and (b) water uptake of pristine Nafion, Nafion-BQ, and Nafion-BPY as function of the relative humidity at 80 C.
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3.3. Effect of antioxidant type on durability of PFSA composite membranes Based on the results of section 3.2, the oxidative stability based on the presence or absence of organic antioxidants was evaluated under optimized conditions for the Fenton's oxidation experiment. The degree of deterioration of the composite membrane in the experiment was observed via turbidity measurements, FE-SEM, fluorine ion detection, FT-IR, and tensile strength measurements. The turbidity of the electrolyte membrane was measured before and after the experiment to predict the degree of pinhole formation. The pristine Nafion membrane exhibited a turbidity of 0.81% before deterioration and 3.95% after deterioration, which was an ~4.9-fold increase. However, the initial turbidity of the Nafion-BQ and Nafion-BPY membranes was 0.87% and 1.04%, respectively, which increased to 1.54% and 1.21% after membrane deterioration, which correspond to 1.7- and 1.2-fold increases. To analyze the damage to the membranes in further detail, the surface characteristics of the pristine and composite Nafion membranes were observed using FE-SEM. Fig. 3 shows the FE-SEM images of the surface morphology of the electrolyte membrane damaged by hydrogen peroxide. After the experiment, several pinholes were observed to appear on the surface of the PFSA pristine membrane (the pinholes are marked with yellow circles). However, the composite membrane containing organic additives had fewer pinholes than the pristine membrane. The number of pinholes identified in the SEM image exhibited the same tendency as the turbidity increase rate. In addition to SEM analysis, measuring the amount of F ions generated by the decomposition of the PFSA membrane in the Fenton's experiment can be used to predict degree of decomposition more quantitatively. Therefore, the amount of F ions generated from PFSA in the Fenton's test solution were analyzed for each composite membrane. The results of FER owing to the decomposition of the electrolyte membrane during the experiment are shown in Fig. 4, which also shows bars from two measurements to demonstrate the reliability. A quantitative FER was found for BQ and BPY at 0.40 ppm/d and 0.61 ppm/d, respectively. These results confirm that the organic antioxidants present in the electrolyte membrane prevented the deterioration of the membrane. Fig. 5(a) shows a graph obtained by recording the FT-IR spectra
Fig. 4. Fluoride concentration of remaining solution for pristine Nafion, Nafion-BQ, and Nafion-BPY after Fenton reaction.
of a PCM and normalizing the peaks based on the peak of the main chain. Peaks of the main chain due to the vibration of CeC and CeF were observed at 1300 cm1 and 1201 cm1, respectively. Peaks of the side chain due to SeO and two types of CeOeC were described above. Unfortunately, peaks in the fingerprint region for BQ and BPY were overlapped by peaks of Nafion and thus cannot be defined [36]. In the case of the composite membrane without antioxidants, the intensity of the SeO peak decreased after the Fenton's oxidation experiment, while that of the composite membrane with antioxidants was almost unchanged. The changes in the two types of ether peaks, A and B, in the side chains before and after the experiment are shown in Fig. 5(b). The rate of decrease of both peaks in the composite membrane containing organic antioxidants was lower than that of the pristine Nafion membrane. In particular, BQ was considered a better antioxidant because it presented the lowest decreasing rate of the two ether peaks.
Fig. 3. SEM images and photographs of (a) pristine Nafion, (b) Nafion-BQ, and (c) Nafion-BPY before membrane degradation and (d) Nafion pristine_Fenton, (e) Nafion-BQ_Fenton, and (f) Nafion-BPY_Fenton after degradation of membranes at a Fe2þ concentration of 10 ppm.
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Fig. 5. (a) FT-IR spectra and (b) peak change ratio of doublet ether peaks at 981 and 967 cm1 for pristine Nafion, Nafion-BQ, and Nafion-BPY membranes before and after the Fenton's test in a solution with 10 ppm Fe2þ at 60 C for 24 h.
However, the peak due to SeO shifted to the left after the experiment. This peak was observed at 1055 cm1 and 1057 cm1 for the pristine Nafion membrane and PCMs, respectively. This was due to the shift in the vibration peak when the Hþ counter ions of the sulfonic acid was exchanged with Fe3þ after the experiment [37]. In addition, after the experiment, the degree of shift of the SeO peak was the same for the three membranes, which indicates that there was no ionic interaction between the sulfonic acid group and the organic antioxidant. Fig. 6 shows the stressestrain response characteristics before and after the Fenton's oxidation experiment. The experiments were repeated thrice for each sample, and the average values of the Young's modulus, elongation break, and tensile strength were calculated, as shown in Table 1. After the experiment, the membranes with antioxidants were more stressed at the breaking point, and their modulus was maintained to be almost the same as that of the pristine Nafion. In particular, there was no or little decrease in the elongation of the composite membranes containing antioxidants, while the decrease was significant for the pristine Nafion membrane. This may be due to a decrease in the molecular weight of the electrolyte membrane without antioxidants. The elongation is significantly affected by the degree of entanglement of the polymer, which indirectly reveals that the molecular weight of the composite membrane containing antioxidants did not decrease much. In the Fenton's test, the antioxidants in the composite membrane inhibited the chemical deterioration and maintained
the molecular weight, showing a tendency similar to the that of the SEM, FER, and FT-IR results. Thus, the organic antioxidants were effective in preventing the chemical deterioration of the perfluorinated electrolyte membrane and maintaining its mechanical strength. 3.4. OCV-holding test of PFSA composite membrane Among the two organic antioxidants, the PCM containing BQ exhibited a lower FER, higher side-chain absorbance FT-IR peak, and higher tensile strength than that containing BPY. Hence, the MEA was created with the Nafion-BQ PCM and a commercial membrane, NRE211. Then, the long-term durability effect of the organic antioxidant was verified via an OCV-holding test (under NEDO conditions). As shown in Fig. 7, the PCM containing BQ maintained a stable OCV for 500 h, whereas the performance of NRE211 began to decrease rapidly after 300 h. In addition, the voltage decay rates of the cells with NRE211 and Nafion-BQ PCMs for 500 h were 426 and 200 mV/h, respectively. During the OCV-holding test, the electrochemical characteristics of the MEA were analyzed by performing CV and LSV at 100-h intervals. The charge density and ECSA calculated from the CV and LSV obtained in the initial state of the MEA were compared with those from before the OCV-holding test. The oxidative stability of the BQ composite membrane containing antioxidants was demonstrated by the CV and ECSA results.
Fig. 6. Stressestrain responses of pristine Nafion, Nafion-BQ, and Nafion-BPY (a) before and (b) after reaction with 10 ppm Fenton solution.
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Table 1 Mechanical properties of pristine Nafion, Nafion BQ, and Nafion BPY before and after Fenton's test. Membranes
Pristine Nafion Nafion-BQ Nafion-BPY
Before test
After test
Young's modulus (MPa/%)
Elongation Break (%)
Tensile Strength (MPa)
Young's modulus (MPa/%)
Elongation Break (%)
Tensile Strength (MPa)
5.37 6.02 5.14
54.92 57.94 72.31
24.85 31.31 28.61
3.23 4.87 3.98
13.91 60.69 30.86
24.13 32.13 25.69
Fig. 7. Open circuit voltage (OCV) degradation of single cell assembled with NRE211 and Nafion-BQ membranes.
As shown in Fig. 8(a), NRE211 exhibited a similar curve shape in the CV graph up to 300 h, but after 400 h, the hydrogen adsorption peak disappeared, and the graph gradually tilted. However, Nafion-BQ maintained the curve shape of the initial CV graph for 500 h. Moreover, as shown in Fig. S2, the ECSA of NRE211 decreased from 63.8 to 32.4 m2/gPt, i.e., the residual ECSA after 400 h was 51% of the initial value. By contrast, the ECSA of Nafion-BQ decreased from 58.6 to 50.6 m2/gPt. This residual ECSA is only 81% of the initial value, even after 500 h, thus revealing that the oxidative stability was maintained. LSV was used to predict the hydrogen gas permeability based on the OCV measurement time. As shown in Fig. 8(b), the crossover current densities at 0.5 V of NRE211 and Nafion-BQ were similar to the initial values of 1.99 mA/cm2 and 2.30 mA/cm2, respectively. However, after 500 h, they were 7.47 mA/cm2, and 5.95 mA/cm2, respectively. These results reveal that the chemical durability of the electrolyte membrane was improved because the organic antioxidant suppressed the radicals, thus maintaining a low gas permeability. As shown in Table S1, the short-circuit resistance of NRE211 from 100 h to 500 h decreased 9.5-fold from 0.19 mU cm2 to 0.02 mU cm2, respectively, whereas that of Nafion-BQ decreased 1.8-fold from 0.0.28 mU cm2 to 0.16 mU cm2. The short-circuit resistance means the electrical resistance inside the polymer electrolyte membrane, which generally plays an essential role as an insulator in a fuel cell system. The reduction in the short-circuit
Fig. 8. Time-dependence (a) CVs and (b) LSVs of NRE211 and Nafion-BQ at 90 C and 30% RH.
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resistance implied that the membrane had suffered local damage such as pinholes or membrane thinning due to its deterioration [38]. These results suggest that Nafion-BQ experienced less membrane thinning than NRE211. Eventually, the perfluorinated electrolyte membrane containing organic antioxidants improved the long-term durability of the unit cell of the fuel cell. 4. Conclusion We developed a perfluorinated electrolyte membrane that incorporates organic antioxidants for use in high-durability polymer fuel cells. The composite membrane was fabricated by incorporating two antioxidants, BQ and BPY, which were already verified to exhibit superior oxidative stability in hydrocarbon membranes in a previous study, with perfluorinated polymers. In particular, the fabricated PCMs exhibited a proton conductivity and water uptake similar to those of the pristine membrane even after organic antioxidants were introduced. An ex situ Fenton's test and in situ OCVholding test were conducted to confirm the improvement in the durability as a result of the organic antioxidants. The composite membrane containing antioxidants had fewer pinholes, a lower increase in turbidity, and a lower emission rate of F ions than the pristine one after the Fenton's test. The reduction rate of the absorbance peak corresponding to the side chain for the composite membranes was found to be lower than that of the pristine membrane. The OCV-holding test for the NRE211 and Nafion-BQ membranes in a unit cell revealed a sharp decline in the OCV after 300 h for NRE211 and a stable OCV for 500 h for the Nafion-BQ membrane. In addition, the CV and LSV measurements showed that Nafion-BQ experienced a smaller ECSA reduction and less crossover current density due to hydrogen permeation than NRE211. The perfluorinated electrolyte membrane containing organic antioxidants exhibited improved chemical durability, as confirmed by OCV-holding test, implying that its antioxidant capability is always effective, regardless of the kind of polymer matrix, i.e., PFSA or hydrocarbon polymers. Meanwhile, the organic antioxidants introduced into the polymer electrolyte membrane were limited in that determining the content of organic antioxidants present in the membrane is difficult due to the membrane-like structure. Thus, it is also difficult to determine whether the organic antioxidants leaked out during fuel cell operation. Therefore, immobilizing an organic antioxidant directly on a polymer electrolyte membrane or finding a method to track organic antioxidants are potential areas of future study. Acknowledgment This work was supported by the framework of the Research and Development Program of the Korea Institute of Energy Research (B9-2412) and the Korea Evaluation Institute of Industrial Technology Grant (10067135). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.12.150. References [1] J.M. Andújar, F. Segura, Fuel cells: history and updating. A walk along two centuries, Renew. Sustain. Energy Rev. 13 (2009) 2309e2322. [2] K. Hongsirikarn, X. Mo, J.G. Goodwin Jr., S. Creager, Effect of H2O2 on Nafion® properties and conductivity at fuel cell conditions, J. Power Sources 196 (2011) 3060e3072. [3] T. Kinumoto, M. Inaba, Y. Nakayama, K. Ogata, R. Umebayashi, A. Tasaka, Y. Iriyama, T. Abe, Z. Ogumi, Durability of perfluorinated ionomer membrane
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Liu, J.E. Owejan, Mitigation of perfluorosulfonic acid membrane chemical degradation using cerium and manganese ions, ECS Trans. (2008) 1735e1747. [14] H. Lee, M. Han, Y.W. Choi, B. Bae, Hydrocarbon-based polymer electrolyte cerium composite membranes for improved proton exchange membrane fuel cell durability, J. Power Sources 295 (2015) 221e227. [15] P. Trogadas, J. Parrondo, V. Ramani, Degradation mitigation in polymer electrolyte membranes using cerium oxide as a regenerative free-radical scavenger, Electrochem. Solid State Lett. 11 (2008) B113eB116. [16] M. Breitwieser, C. Klose, A. Hartmann, A. Büchler, M. Klingele, S. Vierrath, R. Zengerle, S. Thiele, Cerium oxide decorated polymer nanofibers as effective membrane reinforcement for durable, high-performance fuel cells, Adv.Energy Mater. 7 (2017), 1602100. [17] D. Zhao, B.L. Yi, H.M. Zhang, H.M. Yu, MnO2/SiO2-SO3H nanocomposite as hydrogen peroxide scavenger for durability improvement in proton exchange membranes, J. Membr. 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Gubler, Polymer-bound antioxidants in grafted membranes for fuel cells, J. Mater. Chem. 2 (2014) 5870e5882. [23] G.W. Burton, K.U. Ingold, Autoxidation of biological molecules. 1. The antioxidant activity of vitamin E and related chain-breaking phenolic antioxidants in vitro, J. Am. Chem. Soc. 103 (1981) 6472e6477. [24] D. Bagchi, A. Garg, R.L. Krohn, M. Bagchi, M.X. Tran, S.J. Stohs, Oxygen free radical scavenging abilities of vitamins C and E, and a grape seed proanthocyanidin extract in vitro, Res. Commun. Mol. Pathol. Pharmacol. 95 (1997) 179e189. [25] F. Regoli, G.W. Winston, Quantification of total oxidant scavenging capacity of antioxidants for peroxynitrite, peroxyl radicals, and hydroxyl radicals, Toxicol. Appl. Pharmacol. 156 (1999) 96e105. [26] O.I. Aruoma, B. Halliwell, B.M. Hoey, J. Butler, The antioxidant action of Nacetylcysteine: its reaction with hydrogen peroxide, hydroxyl radical, superoxide, and hypochlorous acid, Free Radic. Biol. Med. 6 (1989) 593e597. [27] A. 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Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Perfluorinated composite membranes with organic antioxidants for chemically durable fuel cells Sung-Hee Shin a, Abdul Kodir a, b, Dongwon Shin a, Seok-Hee Park a, Byungchan Bae a, b, * a b
Fuel Cell Laboratory, Korea Institute of Energy Research (KIER), 152, Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea Renewable Energy Engineering, University of Science & Technology (UST), 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2018 Received in revised form 26 December 2018 Accepted 26 December 2018 Available online 27 December 2018
A perfluorinated composite membrane incorporating an organic antioxidant was prepared to improve the chemical durability of a polymer electrolyte fuel cell. Bipyridine and benzoquinone (BQ) were added as organic antioxidants, and the oxidative stability of the composite electrolyte membranes containing them was evaluated via accelerated aging tests, i.e., an ex situ Fenton's test and in situ open-circuit voltage (OCV)-holding test. The effects of the antioxidants in the Fenton's test were verified using scanning electron microscopy, fluorine ion detection, Fourier-transform infrared spectroscopy, and tensile strength measurements. Moreover, a commercial membrane, NRE211, and a composite membrane, Nafion-BQ, were incorporated into a membrane electrode assembly (MEA) that then underwent an OCV-holding test for 500 h. The change in the MEA state over time was analyzed using cyclic voltammetry and linear sweep voltammetry. While NRE 211 showed a significant reduction in OCV from 300 h, Nafion-BQ maintained a stable OCV for 500 h. Hence, the organic antioxidants added to the electrolyte membrane effectively improved the oxidative stability of the proton exchange membrane fuel cell without sacrificing the ionic conductivity of the perfluorinated electrolyte membrane. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Perfluorinated composite membrane Organic antioxidant Proton exchange membrane fuel cell Fenton's test Open-circuit voltage-holding test
1. Introduction Fuel cells that use hydrogen, which has been highlighted as a future energy source, as a fuel and discharge only water as a byproduct are an eco-friendly technology. Thus, various types have been actively developed. In particular, the proton exchange membrane fuel cell (PEMFC) has been most actively developed because it has a higher power density and fewer corrosion issues compared to other fuels cells, and it does not require electrolyte replenishment [1]. The cost competitiveness and life cycle are the most important factors for the successful entry of PEMFCs into the market. The durability of polymer electrolyte membranes is directly related to the life cycle of the fuel cells. Thus, many studies have been conducted on improving the durability of the polymer electrolyte membrane to improve the life cycle of fuel cells [2e9]. Perfluorosulfonic acid (PFSA) is the most widely used polymer electrolyte membrane. PFSA polymers have excellent chemical durability, mechanical strength, and proton conductivity [6].
* Corresponding author. Fuel Cell Laboratory, Korea Institute of Energy Research (KIER), 152, Gajeong-ro, Yuseong-gu, Daejeon 34129, Republic of Korea. E-mail address: [email protected] (B. Bae). https://doi.org/10.1016/j.electacta.2018.12.150 0013-4686/© 2019 Elsevier Ltd. All rights reserved.
Despite these advantages, PFSA electrolyte membranes are known to cause chemical deterioration due to hydrogen peroxide and hydroxyl radicals generated during fuel cell operation. This is because the hydroxyl radical attacks the end of the polymer electrolyte membrane chain or the sulfonic acid group to decompose the polymer chain [10]. Hence, studies focusing on the introduction of various kinds of additives into the polymer electrolyte membrane have been conducted to solve this problem. Previously used antioxidants were inorganic, including metal ions of transition metals such as Ce and Mn [11e14], oxides [15,16], and solid nanoparticles [17,18]. These inorganic antioxidants significantly improved the chemical durability of the electrolyte membrane by inhibiting attacks from radicals. However, the proton conductivity in the electrolyte membrane is lowered by the ionic crosslinking between the metal ion and the sulfonic acid functional group [11]. In addition, the performance of the cathode catalyst layer deteriorates due to the leakage of the antioxidant during the long-term operation of the fuel cells, which was identified to be closely related to the crystallite size of the ceria nanoparticles [19,20]. Efforts have been made to prevent the leakage of antioxidants from the polymer electrolyte membrane to solve this problem. For example, the Solvay Specialty Polymer group conducted
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studies to immobilize single metals (Ce [9], Cr, Co, and Mn [21]) or double metals (CeCr [10]) on a silica substrate containing sulfonic acid functional groups. In addition, researchers attempted to immobilize a polymeric antioxidant synthesized via radiation grafting on an electrolyte membrane [22]. Organic antioxidants, as well as metal-based antioxidants, have been actively developed in the food, bio, and pharmaceutical industries. This is because chemical reactants such as peroxyl radicals and hydroxyl radicals damage cells and accelerate the degradation of food [23e27]. Organic antioxidants can be classified into three types: phenolic, quinone, and N-heterocyclic, depending on the chemical structure. They prevent the auto-oxidation chain reaction by converting to stable radicals or radical resonance structures [28e31]. Thus, we proposed four kinds of organic antioxidants for use in fuel cellsdatocopherol (TOH), 2,6-dimethoxy-1,4-benzoquinone (BQ), hydroquinone (HQ), and 2,20 -bipyridine (BPY)dby considering the chemical structures of the classified organic antioxidants. In particular, the oxidation stability of the composite membrane was evaluated after the introduction of four antioxidants into the hydrocarbon-based polymer electrolyte membrane. The organic antioxidants present the possibility of improving the durability of electrolyte membranes for fuel cells as well as being used in the food, bio, and pharmaceutical industries [32]. In this study, two kinds of organic antioxidants, BQ and BPY, which exhibited excellent oxidative stability in hydrocarbon membranes in our previous study, were selected and applied to PFSA polymers. We aimed to verify whether the organic antioxidants can be used in perfluorinated polymer electrolyte membranes and hydrocarbon membranes. The oxidative stability of the fabricated PFSA composite membrane containing organic antioxidants was determined via an ex situ Fenton's test, and the deterioration of the electrolyte membrane was confirmed via scanning electron microscopy (SEM), fluoride emission rate (FER) measurements, Fourier-transform infrared spectroscopy (FT-IR), and tensile strength measurements. In addition, cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were performed periodically during the in situ open-circuit voltage (OCV)-holding test for 500 h to determine if the organic antioxidants are effective in improving the long-term performance of the fuel cells of the PFSA electrolyte membrane. Therefore, we demonstrated in this work that organic antioxidants also effectively improve the chemical durability of the PFSA membranes without sacrificing ionic conductivity. 2. Experimental 2.1. Materials and chemicals Nafion® Dispersion DE 2021 (31175-20-9, Copolymer Resin (20%e22%)) was purchased from DuPont Fluoroproducts, USA. NMethyl-2-pyrrolidone (NMP, 872-50-4, anhydrous, 99%), 2,2Bipyridyl (BPY, 366-18-7, 99%), and 2,6-Dimethoxy-1,4benzoquinone (BQ, 530-55-2, 97%) were obtained from SigmaAldrich, USA. FeSO4∙7H2O (122020, DC Chemical Co., Ltd, South Korea) and H2O2 (E9MB61, Extra Pure Grade, Duksan Pure Chemicals, South Korea) were used as received. A glass plate and a hot plate were used to fabricate the cast membrane. As-received NRE211 (Nafion, equivalent weight (EW) ¼ 1100 g/mol, DuPont, USA) was used as a reference membrane for the OCV-holding test. 2.2. Preparation of Nafion composite membranes with antioxidant A 10 wt.% Nafion/NMP solution was prepared by replacing the original solvent with NMP. First, the required amount of NMP for the target concentration of Nafion solution was added to the
original 20 wt.% Nafion dispersion. Then, the original solvent mixture was evaporated at 50 C for 3 h using a centrifuge evaporator (kdScientific, USA). One of the organic antioxidants (BPY and BQ) was mixed with 10 wt.% NMP-based Nafion solution at 25 C for 8 h under a nitrogen atmosphere. The content of organic antioxidants was 2.5 mol% relative to the sulfonic acid group content of Nafion. The final Nafion solution with organic antioxidants was filtrated at a speed of 2 mL/min using a syringe filter (10746053, GE Healthcare UK Limited, UK) and a syringe pump (Norm-Ject 20 mL, 4G14048, Henke Sass Wolf, Germany). Perfluorinated composite membranes (PCMs) with organic antioxidants were prepared via a solution casting method. The filtrated polymer solution was cast on a glass plate and dried overnight at 70 C to form a thin polymer film. Then, the residual solvent was removed by drying at 80 C for 6 h under vacuum and washing with deionized (DI) water. The final PCMs were obtained by drying at room temperature for 1e2 days. 2.3. Proton conductivity of membranes The proton conductivity of the membranes was measured by using a four-electrode conductivity cell (MCC, WonAtech, South Korea). The cell was equipped with a Solartron 1260 impedance/ gain-phase analyzer and Solartron 1287 electrochemical interface. Then, electrochemical impedance spectroscopy was conducted in the range of 105 Hze101 Hz with an amplitude of 100 mV. The conductivity was tested at different relative humidities (RHs) of 30%, 50%, 70%, and 90%. The proton conductivity was calculated using the following equation:
Proton conductivity ðS=cmÞ ¼
D LBR
where D is the distance between electrodes, L is the membrane width, B is the membrane thickness, and R is the resistance obtained by extrapolating the impedance plot to the axis on the highfrequency side of the plot. 2.4. Water uptake based on relative humidity of membranes The water uptake of the PCMs was measured based on the RH using a dynamic water absorption analysis (TGA Q5000SA, TA Instruments, New Castle, USA). The PCMs were dried overnight at 100 C before measurement. The water absorption trend of the dried samples was investigated at 80 C under 30% RH, 50% RH, and 70% RH. Each RH condition was maintained for 1 h to reach equilibrium. Finally, the water uptake of PCMs at each RH was calculated as follows:
Water uptakeð%Þ ¼
Wwet Wdry 100 Wdry
where Wwet represents the wet weight of the membrane at a specific RH and Wdry represents the dry weight of the membrane. 2.5. Ex situ Fenton's test An ex situ Fenton's test was used to examine the chemical stability of the membranes. Fenton solutions of 3, 5, 7, and 10 ppm were prepared depending on the Fe2þ concentration, and the amount of H2O2 was fixed as 10 wt.% in DI water. The degradation of the membrane was investigated by increasing the concentration of the Fenton solution. 5 5 cm2 Nafion composite membranes were assembled with a sample holder, as shown in Fig. 1(a). Subsequently, the sample holder with the membrane was soaked in a
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Fig. 1. Ex situ Fenton's test. (a) Assembly of sample holder with membrane and (b) setup for Fenton's test.
Fenton solution and placed in a water bath (Eyela NTS 4000, Tokyo Rakikai Co. Ltd., Japan) at 60 C for 24 h at a shaking speed of 50 rpm (Fig. 1(b)). After this, the membrane was washed with DI water for 1 day. 2.6. Analysis of deteriorated membranes The chemical changes in the deteriorated membranes were observed using FT-IR spectroscopy before and after the Fenton's test. In particular, FT-IR spectroscopy was performed to examine the chemical degradation of the side chain of the Nafion structure. After the test, the membrane samples were rinsed with DI water and dried at 30 C for 24 h prior to analysis. The measurement was conducted using Nicolet 5700 FT-IR spectrometer (Thermo Electron Corporation, Madison, WI, USA) equipped with an attenuated total reflection (ATR) sampling accessory (Smart MIRacle™, Diamond, PIKE technologies, USA). The dried samples were placed on the diamond crystal of the ATR device, which is called an internal reflection element, and fixed by an upper probe. The spectrum was recorded with a wave number of 4 cm1 in the range of 700e4000 cm1. The obtained spectra were normalized with the absorbance peak of CF2 (1145 cm1), which is a major bond in the PTFE backbone, to investigate the variation in the peak intensity for two ethers in the side chain. In addition, the amount of fluoride in the reacted aqueous solution after membrane degradation was detected using a fluoride-selective electrode (6.0502.150, Metrohm AG, Switzerland). A calibration curve was drawn to estimate the fluoride concentration by using standard fluoride solutions of 1, 2, 10, 20, and 100 ppm. The physical changes in the deteriorated membranes due to accelerated degradation were investigated based on an analysis of
the weight change ratio, membrane turbidity, tensile strength, and SEM images. The degradation rate of the membranes based on the Fenton concentration was calculated based on the weight change of the membrane before and after the Fenton's test. All the samples were dried at 120 C for 24 h prior to measurement. The variation in the membrane turbidity was investigated using a haze meter (NDH 7000, Nippon Denshoku, Japan) before and after the Fenton's test. The pinholes formed on the surface of the deteriorated membrane were observed using SEM analysis (Hitachi S-4800 N SEM, Hitachi High-technologies Corporation, Japan). In addition, the variation in the mechanical strength of the deteriorated membranes was observed via an analysis of the tensile strength (TXA™ Texture Analyzer, Yeonjin Corporation, South Korea). The samples were cut in a dumbbell shape and both ends of the sample were clamped using the claws of the Texture Analyzer. The stress and strain responses were obtained for samples at a crosshead speed of 0.1 mm/ s at room temperature. 2.7. In situ OCV-holding test The long-term stability of the PCMs was evaluated by assembling a unit cell for an in situ OCV-holding test and incorporating it in membrane electrode assemblies (MEAs). Firstly, MEAs with Nafion and Nafion-BQ membranes were fabricated using a decal transfer method using a catalyst-coated substrate [33]. An electrode decal was formed on a decal substrate of a common polyethylene terephthalate film by coating it with a catalyst ink composed of a carbon-supported Pt catalyst (TEC10F50E, Tanaka Kikinzoku Kogyo K. K., Japan) and a perfluorinated ionomer (Aquivion D83-24B, Solvay Specialty Polymers Italy SpA, Italia) with an ionomer to carbon (I/C) ratio of 1.0. The transfer of the electrode
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decal onto the membrane was carried out by hot pressing at 100 C and 30 bar for 20 min. The prepared MEAs had platinum loadings of 0.36 and 0.38 mg/cm2 on the anode and cathode, respectively. Then, the MEAs were assembled into a unit cell (VeN Type, InWooTech, South Korea) with an active area of 9 cm2 using graphite bipolar plates and 10BC gas diffusion layers (GDLs, SGL Group). The unit cell was compressed at a pressure of 30 kgf/cm2. Finally, the OCV-holding test of the unit cells was performed following a New Energy and Industrial Technology Development (NEDO, Japan) protocol at 90 C and 30% RH under a hydrogen flow rate of 0.20 L/min for an anode and an air flow rate of 0.42 L/min for a cathode [34]. The time-dependent status of the MEAs was investigated with CV and LSV using a potentiostat (HCP-803, BioLogic Science Instruments, France) every 100 h. CV measurements were conducted with hydrogen supplied to the anode, flowing at 0.2 L/min, and nitrogen to the cathode at 0.0 L/min. The CV curves of MEAs with Nafion and Nafion-BQ were recorded in the range of 0.05e0.90 V at a scan rate of 50 mV/s. Then, the electrochemically active surface area (ECSA) of Pt in the MEAs was estimated from the hydrogen desorption charge of the 5th scan as follows [35]:
. ECSA m2 gPt ¼
QH 102 210mC=cm2Pt LPt
where QH is a hydrogen desorption charge (mC/cm2), and Lpt is the Pt loading (mgPt/cm2). The residual ECSA was obtained from the relative ratio to the initial value. LSV measurements were performed between 0.1 and 0.6 V at a scan rate of 1 mV/s, with hydrogen (nitrogen) supplied to the anode (cathode) at 0.2 L/min (0.5 L/min). A hydrogen crossover current of MEAs was obtained from the current at 0.5 V in the LSV curves. In addition, the short-circuit resistance (i.e., the electronic resistance) was calculated from the inverse of the slope of the straight-line portion of the LSV plot between 0.4 and 0.5 V.
3. Results and discussion 3.1. Characterization of Nafion composite membranes The average thicknesses of the Nafion, Nafion-BQ, and NafionBPY membranes prepared by solution casting method were similar (~37 mm). After the incorporation of organic antioxidants, there is no significant change in the ion exchange capacity (IEC), as determined by an acidebase back-titration [32]. Fig. 2(a) shows a comparison of the change in the proton conductivity of the composite membrane based on the RH after the introduction of organic antioxidants. The electrolyte membrane containing BQ had a
conductivity similar to that of the pristine Nafion membrane at both 70% and 90% RH, and the conductivity was confirmed to be slightly higher under conditions of low humidity. Since BQ is highly hydrophilic, the conductivity of the Nafion-BQ membrane may have been slightly higher than that of the pristine Nafion membrane at a low RH. In contrast, the composite membrane with BPY exhibited a slightly lower conductivity than the pristine Nafion membrane at all RH conditions, but the decrease in conductivity was negligible compared to that of electrolyte membranes containing inorganic antioxidants [14]. As BPY has a weak alkalinity, it is likely to have weak ionic interactions with the sulfonic acid group of PFSA, but it does not seem to have affected the conductivity of the PFSA electrolyte membrane significantly. Fig. 2(b) shows that the pristine Nafion and Nafion-BQ membranes had similar water uptake values under all RH conditions. However, Nafion-BPY had slightly lower water uptake values than the other membranes, presumably due to the weak ionic interaction between BPY and the sulfonic acid group. Nevertheless, this result is consistent with the findings of our previous study that organic antioxidants do not significantly inhibit the conductivity and moisture content of a hydrocarbon electrolyte membrane [32].
3.2. Effect of Fe2þ concentration on pristine Nafion membrane durability The deterioration of the pristine PFSA membrane based on the Fe2þ concentration was monitored to identify the Fenton's accelerated deterioration test conditions suitable for evaluating the oxidation stability. Fe2þ solutions of 3, 5, 7, and 10 ppm were prepared and used in the test. The deterioration of the membrane in the Fenton's oxidation experiment was observed based on changes in the dry weights and FT-IR spectra. As shown in Fig. S1(b), the dry weight of the pristine Nafion membrane tended to decrease as the Fe2þ concentration increased from 3 ppm to 7 ppm and the weight loss was similar at 10 ppm and 7 ppm. The FT-IR spectra showed a peak due to stretching vibrations of SeO at 1051 cm1 and two peaks (A and B) corresponding to ether stretching vibrations at 981 cm1 and 967 cm1, respectively. To more accurately compare the absorbance peaks corresponding to the side chains in the FT-IR spectra, the peaks were normalized to an absorbance peak corresponding to the main chain (CF2, 1145 cm1). As a result, as shown in Fig. S1(c), the absorbance peaks corresponding to the side chains of PFSA (SeO, CeOeC) exhibited a tendency to decrease with the Fe2þ concentration. Therefore, 10 ppm, which is considered to cause sufficient chemical deterioration, was selected as the optimum concentration for the experiment.
Fig. 2. (a) Proton conductivity and (b) water uptake of pristine Nafion, Nafion-BQ, and Nafion-BPY as function of the relative humidity at 80 C.
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3.3. Effect of antioxidant type on durability of PFSA composite membranes Based on the results of section 3.2, the oxidative stability based on the presence or absence of organic antioxidants was evaluated under optimized conditions for the Fenton's oxidation experiment. The degree of deterioration of the composite membrane in the experiment was observed via turbidity measurements, FE-SEM, fluorine ion detection, FT-IR, and tensile strength measurements. The turbidity of the electrolyte membrane was measured before and after the experiment to predict the degree of pinhole formation. The pristine Nafion membrane exhibited a turbidity of 0.81% before deterioration and 3.95% after deterioration, which was an ~4.9-fold increase. However, the initial turbidity of the Nafion-BQ and Nafion-BPY membranes was 0.87% and 1.04%, respectively, which increased to 1.54% and 1.21% after membrane deterioration, which correspond to 1.7- and 1.2-fold increases. To analyze the damage to the membranes in further detail, the surface characteristics of the pristine and composite Nafion membranes were observed using FE-SEM. Fig. 3 shows the FE-SEM images of the surface morphology of the electrolyte membrane damaged by hydrogen peroxide. After the experiment, several pinholes were observed to appear on the surface of the PFSA pristine membrane (the pinholes are marked with yellow circles). However, the composite membrane containing organic additives had fewer pinholes than the pristine membrane. The number of pinholes identified in the SEM image exhibited the same tendency as the turbidity increase rate. In addition to SEM analysis, measuring the amount of F ions generated by the decomposition of the PFSA membrane in the Fenton's experiment can be used to predict degree of decomposition more quantitatively. Therefore, the amount of F ions generated from PFSA in the Fenton's test solution were analyzed for each composite membrane. The results of FER owing to the decomposition of the electrolyte membrane during the experiment are shown in Fig. 4, which also shows bars from two measurements to demonstrate the reliability. A quantitative FER was found for BQ and BPY at 0.40 ppm/d and 0.61 ppm/d, respectively. These results confirm that the organic antioxidants present in the electrolyte membrane prevented the deterioration of the membrane. Fig. 5(a) shows a graph obtained by recording the FT-IR spectra
Fig. 4. Fluoride concentration of remaining solution for pristine Nafion, Nafion-BQ, and Nafion-BPY after Fenton reaction.
of a PCM and normalizing the peaks based on the peak of the main chain. Peaks of the main chain due to the vibration of CeC and CeF were observed at 1300 cm1 and 1201 cm1, respectively. Peaks of the side chain due to SeO and two types of CeOeC were described above. Unfortunately, peaks in the fingerprint region for BQ and BPY were overlapped by peaks of Nafion and thus cannot be defined [36]. In the case of the composite membrane without antioxidants, the intensity of the SeO peak decreased after the Fenton's oxidation experiment, while that of the composite membrane with antioxidants was almost unchanged. The changes in the two types of ether peaks, A and B, in the side chains before and after the experiment are shown in Fig. 5(b). The rate of decrease of both peaks in the composite membrane containing organic antioxidants was lower than that of the pristine Nafion membrane. In particular, BQ was considered a better antioxidant because it presented the lowest decreasing rate of the two ether peaks.
Fig. 3. SEM images and photographs of (a) pristine Nafion, (b) Nafion-BQ, and (c) Nafion-BPY before membrane degradation and (d) Nafion pristine_Fenton, (e) Nafion-BQ_Fenton, and (f) Nafion-BPY_Fenton after degradation of membranes at a Fe2þ concentration of 10 ppm.
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Fig. 5. (a) FT-IR spectra and (b) peak change ratio of doublet ether peaks at 981 and 967 cm1 for pristine Nafion, Nafion-BQ, and Nafion-BPY membranes before and after the Fenton's test in a solution with 10 ppm Fe2þ at 60 C for 24 h.
However, the peak due to SeO shifted to the left after the experiment. This peak was observed at 1055 cm1 and 1057 cm1 for the pristine Nafion membrane and PCMs, respectively. This was due to the shift in the vibration peak when the Hþ counter ions of the sulfonic acid was exchanged with Fe3þ after the experiment [37]. In addition, after the experiment, the degree of shift of the SeO peak was the same for the three membranes, which indicates that there was no ionic interaction between the sulfonic acid group and the organic antioxidant. Fig. 6 shows the stressestrain response characteristics before and after the Fenton's oxidation experiment. The experiments were repeated thrice for each sample, and the average values of the Young's modulus, elongation break, and tensile strength were calculated, as shown in Table 1. After the experiment, the membranes with antioxidants were more stressed at the breaking point, and their modulus was maintained to be almost the same as that of the pristine Nafion. In particular, there was no or little decrease in the elongation of the composite membranes containing antioxidants, while the decrease was significant for the pristine Nafion membrane. This may be due to a decrease in the molecular weight of the electrolyte membrane without antioxidants. The elongation is significantly affected by the degree of entanglement of the polymer, which indirectly reveals that the molecular weight of the composite membrane containing antioxidants did not decrease much. In the Fenton's test, the antioxidants in the composite membrane inhibited the chemical deterioration and maintained
the molecular weight, showing a tendency similar to the that of the SEM, FER, and FT-IR results. Thus, the organic antioxidants were effective in preventing the chemical deterioration of the perfluorinated electrolyte membrane and maintaining its mechanical strength. 3.4. OCV-holding test of PFSA composite membrane Among the two organic antioxidants, the PCM containing BQ exhibited a lower FER, higher side-chain absorbance FT-IR peak, and higher tensile strength than that containing BPY. Hence, the MEA was created with the Nafion-BQ PCM and a commercial membrane, NRE211. Then, the long-term durability effect of the organic antioxidant was verified via an OCV-holding test (under NEDO conditions). As shown in Fig. 7, the PCM containing BQ maintained a stable OCV for 500 h, whereas the performance of NRE211 began to decrease rapidly after 300 h. In addition, the voltage decay rates of the cells with NRE211 and Nafion-BQ PCMs for 500 h were 426 and 200 mV/h, respectively. During the OCV-holding test, the electrochemical characteristics of the MEA were analyzed by performing CV and LSV at 100-h intervals. The charge density and ECSA calculated from the CV and LSV obtained in the initial state of the MEA were compared with those from before the OCV-holding test. The oxidative stability of the BQ composite membrane containing antioxidants was demonstrated by the CV and ECSA results.
Fig. 6. Stressestrain responses of pristine Nafion, Nafion-BQ, and Nafion-BPY (a) before and (b) after reaction with 10 ppm Fenton solution.
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Table 1 Mechanical properties of pristine Nafion, Nafion BQ, and Nafion BPY before and after Fenton's test. Membranes
Pristine Nafion Nafion-BQ Nafion-BPY
Before test
After test
Young's modulus (MPa/%)
Elongation Break (%)
Tensile Strength (MPa)
Young's modulus (MPa/%)
Elongation Break (%)
Tensile Strength (MPa)
5.37 6.02 5.14
54.92 57.94 72.31
24.85 31.31 28.61
3.23 4.87 3.98
13.91 60.69 30.86
24.13 32.13 25.69
Fig. 7. Open circuit voltage (OCV) degradation of single cell assembled with NRE211 and Nafion-BQ membranes.
As shown in Fig. 8(a), NRE211 exhibited a similar curve shape in the CV graph up to 300 h, but after 400 h, the hydrogen adsorption peak disappeared, and the graph gradually tilted. However, Nafion-BQ maintained the curve shape of the initial CV graph for 500 h. Moreover, as shown in Fig. S2, the ECSA of NRE211 decreased from 63.8 to 32.4 m2/gPt, i.e., the residual ECSA after 400 h was 51% of the initial value. By contrast, the ECSA of Nafion-BQ decreased from 58.6 to 50.6 m2/gPt. This residual ECSA is only 81% of the initial value, even after 500 h, thus revealing that the oxidative stability was maintained. LSV was used to predict the hydrogen gas permeability based on the OCV measurement time. As shown in Fig. 8(b), the crossover current densities at 0.5 V of NRE211 and Nafion-BQ were similar to the initial values of 1.99 mA/cm2 and 2.30 mA/cm2, respectively. However, after 500 h, they were 7.47 mA/cm2, and 5.95 mA/cm2, respectively. These results reveal that the chemical durability of the electrolyte membrane was improved because the organic antioxidant suppressed the radicals, thus maintaining a low gas permeability. As shown in Table S1, the short-circuit resistance of NRE211 from 100 h to 500 h decreased 9.5-fold from 0.19 mU cm2 to 0.02 mU cm2, respectively, whereas that of Nafion-BQ decreased 1.8-fold from 0.0.28 mU cm2 to 0.16 mU cm2. The short-circuit resistance means the electrical resistance inside the polymer electrolyte membrane, which generally plays an essential role as an insulator in a fuel cell system. The reduction in the short-circuit
Fig. 8. Time-dependence (a) CVs and (b) LSVs of NRE211 and Nafion-BQ at 90 C and 30% RH.
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resistance implied that the membrane had suffered local damage such as pinholes or membrane thinning due to its deterioration [38]. These results suggest that Nafion-BQ experienced less membrane thinning than NRE211. Eventually, the perfluorinated electrolyte membrane containing organic antioxidants improved the long-term durability of the unit cell of the fuel cell. 4. Conclusion We developed a perfluorinated electrolyte membrane that incorporates organic antioxidants for use in high-durability polymer fuel cells. The composite membrane was fabricated by incorporating two antioxidants, BQ and BPY, which were already verified to exhibit superior oxidative stability in hydrocarbon membranes in a previous study, with perfluorinated polymers. In particular, the fabricated PCMs exhibited a proton conductivity and water uptake similar to those of the pristine membrane even after organic antioxidants were introduced. An ex situ Fenton's test and in situ OCVholding test were conducted to confirm the improvement in the durability as a result of the organic antioxidants. The composite membrane containing antioxidants had fewer pinholes, a lower increase in turbidity, and a lower emission rate of F ions than the pristine one after the Fenton's test. The reduction rate of the absorbance peak corresponding to the side chain for the composite membranes was found to be lower than that of the pristine membrane. The OCV-holding test for the NRE211 and Nafion-BQ membranes in a unit cell revealed a sharp decline in the OCV after 300 h for NRE211 and a stable OCV for 500 h for the Nafion-BQ membrane. In addition, the CV and LSV measurements showed that Nafion-BQ experienced a smaller ECSA reduction and less crossover current density due to hydrogen permeation than NRE211. The perfluorinated electrolyte membrane containing organic antioxidants exhibited improved chemical durability, as confirmed by OCV-holding test, implying that its antioxidant capability is always effective, regardless of the kind of polymer matrix, i.e., PFSA or hydrocarbon polymers. Meanwhile, the organic antioxidants introduced into the polymer electrolyte membrane were limited in that determining the content of organic antioxidants present in the membrane is difficult due to the membrane-like structure. Thus, it is also difficult to determine whether the organic antioxidants leaked out during fuel cell operation. Therefore, immobilizing an organic antioxidant directly on a polymer electrolyte membrane or finding a method to track organic antioxidants are potential areas of future study. Acknowledgment This work was supported by the framework of the Research and Development Program of the Korea Institute of Energy Research (B9-2412) and the Korea Evaluation Institute of Industrial Technology Grant (10067135). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.electacta.2018.12.150. References [1] J.M. Andújar, F. Segura, Fuel cells: history and updating. A walk along two centuries, Renew. Sustain. Energy Rev. 13 (2009) 2309e2322. [2] K. Hongsirikarn, X. Mo, J.G. Goodwin Jr., S. Creager, Effect of H2O2 on Nafion® properties and conductivity at fuel cell conditions, J. Power Sources 196 (2011) 3060e3072. [3] T. Kinumoto, M. Inaba, Y. Nakayama, K. Ogata, R. Umebayashi, A. Tasaka, Y. Iriyama, T. Abe, Z. Ogumi, Durability of perfluorinated ionomer membrane
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