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Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells
international journal of hydrogen energy xxx (xxxx) xxx
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Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells Ji Eon Chae a,b, Sung Jong Yoo a, Jin Young Kim a, Jong Hyun Jang a, So Young Lee a,*, Kwang Ho Song b,**, Hyoung-Juhn Kim a,*** a
Center for Hydrogen and Fuel Cell Research, Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, 02792, Republic of Korea b Department of Chemical and Biological Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul, 02841, Republic of Korea
highlights
graphical abstract
Synthesis of polymer electrolytes which have different molecular weight (MW). Fabrication of membrane electrode assembly with different MW polymer electrolyte binder. Optimization of electrode configuration
for
high
fuel
cell
performance.
article info
abstract
Article history:
The electrode ionomer is a key factor that significantly affects the catalyst layer
Received 13 December 2019
morphology and fuel cell performance. Herein, sulfonated poly(arylene ether sulfone)-
Received in revised form
based electrode ionomers with polymers of various molecular weights and alcohol/water
9 February 2020
mixtures were prepared, and those comprising the alcohol/water mixture showed a higher
Accepted 1 March 2020
performance than the ones prepared using higher boiling solvents, such as dimethylace-
Available online xxx
tamide; this is owing to the formation of the uniformly dispersed ionomer catalyst layer. The relation between ionomer molecular weight for the same polymer structure and the
Keywords:
sulfonation degree was investigated. Because the chain length of polymer varies with
Hydrocarbon-based electrode
molecular weight and chain entanglement degree, its molecular weight affects the elec-
ionomer
trode morphology. As the ionomer covered the catalyst, the agglomerates formed were of
Membrane-electrode assembly
different morphologies according to their molecular weight, which could be deduced
Polymer electrolyte
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (S.Y. Lee), [email protected] (K.H. Song), [email protected] (H.-J. Kim). https://doi.org/10.1016/j.ijhydene.2020.03.003 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
2
international journal of hydrogen energy xxx (xxxx) xxx
Proton exchange membrane fuel cell
indirectly through dynamic light scattering and scanning electron microscopy. Addition-
Sulfonated poly(arylene ether
ally, the fuel cell performance was confirmed in the current-voltage curve.
sulfone)
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells produce electricity and thermal energy through electrochemical reactions and are one of the most promising energy generation devices in the wake of the climate change crisis; this is due to advantages such as zero-emission and high efficiency. Among various types of fuel cells, proton exchange membrane fuel cells (PEMFCs) offer higher power density and lower weight as well as volume. Among the constituent PEMFC materials, the polymer electrolyte membrane (PEM) and electrode ionomer binder have significant proton-conducting properties. Nafion® is a perfluorosulfonated polymer (PFSA), which exhibits good proton conductivity, and superior mechanical and chemical properties. Furthermore, Nafion® has been extensively studied and optimized to date [1e4]. However, its high price and high gas permeability are the main bottlenecks in the commercialization of fuel cells comprising Nafion®. In addition, as a fluorine-based polymer, Nafion® causes environmental problems during preparation and disposal. Studies to address these limitations by the development of hydrocarbon-based PEMs that complement the advantages and alleviate the disadvantages of Nafion® are also underway. The polymerization processes of hydrocarbon-based PEMs, such as poly(ether ether ketone)s, poly(ether sulfone)s, polyimides, poly(phenylene oxide)s, and polybenzimidazoles, are relatively straightforward and the materials exhibit high thermal and mechanical stabilities [5]. Various hydrocarbon-based PEMs have shown to attain exceedingly high levels as compared to PFSA. Recent studies have revealed the synthesis of hydrocarbon-based PEMs that display high conductivity, good mechanical and thermal stability, low gas permeability and well-ordered morphology for phase separation [5,6]. However, these high-performance hydrocarbon PEMs still employ PFSA ionomers as the electrode binder, during the fabrication of the membrane-electrode assembly (MEA). Although this approach delivers a high performance, it presents high costs and compatibility issues such as the delamination of the membranes and electrodes [7e9]. Although the development of hydrocarbon-based PEMs has been achieved, hydrocarbon-based electrode ionomers have not been studied extensively. Rahnavard et al. reported that the use of the sulfonated poly(ether ether ketone) (PEEK) ionomer solution (5 wt%) in dimethylacetamide (DMAc) as an electrode ionomer with a nanocomposite PEEK membrane, generated a current density of 250 mA cm2 at 0.6 V with H2 and O2 [10]. Moreover, Lee and co-workers were successful in improving the interfacial stability of the PEEK membrane and electrode, including the PEEK binder solution in DMAc at low relative humidity (RH 30%) [11]. However, in the hydrocarbon-based proton exchange ionomers, the catalyst layer was prepared mainly using a high boiling point organic solvent. In such
systems, the solvent might dissolve the PEM when the catalyst slurry is applied directly, resulting in low actual cell performance. Recently, alcohol-soluble hydrocarbon-based ionomers comprising poly(arylene ether)s were reported. Lee et al. reported the use of a sulfonated copolymer as the membrane and cathode catalyst layer ionomer in MEA, which showed a highperformance of 700 mW cm2 under the H2/O2 condition [12]. In the groups of various hydrocarbon polymer candidates, sulfonated poly(arylene ether sulfone) (s-PAES) has been regarded as a feasible PEM to address the shortcomings of Nafion® [13,14]. Until recently, researchers has studied that the modification and functionalization of s-PAES structure has improved the membrane morphologies and fuel cell properties [12,15e17]. Chemical modifications have been accomplished with a number of different processes involving modification of the polymer structure such as block copolymer [12,16], combshaped s-PAES [15], novel side chain [18e20] and cross linking system [21,22]. These suitable structures could be introduced for improved membrane properties. In this study, the basic s-PAES structure was introduced to examine the hydrocarbon polymeric binder. This basic structure is expected to be compatible with a variety of the enhanced s-PAES-based membranes. We investigated several conditions of the hydrocarbon-based electrode binder toward improving the cell performance. The choice of the solvent for the hydrocarbon electrode binder and catalyst ink is a vital aspect of the design. Furthermore, several studies have revealed the critical impact of the solvents used for ionomer and ink dispersion on the electrode morphology and cell performance of PFSA ionomers. Kim et al. recently reported that the desired microstructure in the catalyst layer was obtained with the use of the solvent that allowed high mainchain mobility of the Nafion® ionomer [3]. Welch et al. investigated Nafion® ionomer in dilute systems, which included both dispersion and solution [4]. This catalyst layer study leads to the study of polymer-solvent interaction, polymer-particle interaction and particle-solvent interaction. However, since it is practically inaccessible, the nano-scale interactions need research furthermore. In addition, the molecular weight (MW) of the polymer influences the behavior of the electrode ionomer in the catalyst layer, as shown in Fig. 1. Because the length of the polymer chain and the degree of entanglement vary with the MW, the ionomer MW affects the interactions in the catalyst layer. In particular, there are some studies on the influence of polymeric binder MW on the morphology of electrode and the relative performance in the field of lithium ion batteries and dye-sensitized solar cells. The correlation between the binder MW and electrode morphology for dye-sensitized solar cells et al. was established [23]. Lee et al. also investigated by Tasic examined a detailed investigation of the influence of MW as
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Fig. 1 e Schematic illustration of agglomerates controlled by polymer molecular weight of ionomer in catalyst layer. anode binder in lithium ion battery [24]. As a result, the MW of the polymer used as the binder has been found to affect the formation of the electrode microstructure or the electrochemical performance and to have an optimal MW region. Zhou et al. reported that lower MW are preferred probably due to the increased free volume and leads to higher mass transport and/or catalyst utilization [25]. Consequently, it has been found that the MW of the polymer used as the binder affects the formation of electrode microstructure or electrochemical performance and has an optimal region. Here, MEAs comprising hydrocarbon-based electrode ionomers were prepared. The anode and cathode of the MEAs were composed of a newly developed electrode binder (BPS series). For PEMFC operation, the Nafion® membrane, s-PAES membrane, and commercial Pt/C catalyst were employed. Different MWs and solvent of the same polymer structure as the electrode binder contribute to the formation of the catalyst layer on the commercial Nafion® membrane. With appropriate ionomer conditions that enable the uniform electrode morphology, the performance of polymeric material was enhanced. Finally, sPAES membranes were used instead of commercial PFSA membranes to investigate fuel cell performance with polymeric materials based on all-hydrocarbon-based polymer.
Experimental Materials The 4,40 -dichlorodiphenylsulfone (DCDPS), 4,40 -dihydroxybiphenyl (BP), potassium carbonate (K2CO3), anhydrous N, Ndimethylacetamide (DMAc), and toluene required for the synthesis of the poly(arylene ether sulfone) copolymer were obtained from Aldrich Chemical Co (USA). Sulfuric acid (98%) was obtained from Daejung Chemical Co. The Pt/C (46.7 wt%, Tanaka), 1-propanol, and 2-propanol required for the catalyst ink preparation were sourced from Aldrich Chemical Co (USA) and were used for the preparation of the catalyst slurry.
Synthesis of sulfonated poly(arylene ether sulfone) The sulfonated poly(arylene ether sulfone) (s-PAES) copolymer required for the electrolyte membrane was synthesized
according to reported literature [26]. The sulfonation degree was 40%, and the weight average molecular weight (Mw) of the polymer was 94,165 g mol1 (Mw/Mn ¼ 2.003). The s-PAES copolymers (BPS series) required for the electrode binder were synthesized via direct condensation polymerization according to the reported procedure [26], and the degree of sulfonation of the sulfonated poly(arylene ether sulfone) was fixed at 40%. The MW of the polymer required for the preparation of the electrode binder was adjusted to a 10,000 to 50,000 g mol1 range based on the weight average molecular weight (Mw). Also, the s-PAES with a 30% degree of sulfonation was synthesized for comparison. All polymers were soaked and stirred in 1 M aqueous sulfuric acid at 60 C for 2 h followed by washing with distilled water for 24 h. The acidified copolymers were dried overnight in a vacuum oven at 80 C.
Characterization of copolymers The structural characterization of the synthesized copolymers was carried out using 1H NMR on a 400 MHz Bruker AV400 spectrometer. The samples of the copolymers (0.01 g) were dissolved in DMSO-d6 (0.1 g) and were filtered via a 0.45 mm pore membrane before loading into the NMR tubes. The MW (Mn and Mw) of the copolymers was determined by gel permeation chromatography (GPC) on Styragel HR 3 and 4 columns in a system comprising a Waters 1515 isocratic HPLC pump, a Waters 2707 auto-sampler, and a Waters 2414 refractive index detector. The GPC sampling was carried out by mixing 0.006 g of each copolymer in the eluent, which consisted of 1.994 g of 0.05 M Lithium bromide in NMP (HPLC grade) as the eluent, and the sample solution was passed through a 0.45 mm pore syringe filter. The flow rate was 1.0 mL min1 at 40 C. The polymethylmethaycrylate Mid MW standard Kit (Waters Co.) was used for the GPC MW calibration for all tests.
Preparation of electrode ionomer solution Acidified BPS copolymers were dissolved in a mixture of 1propanol and distilled water in a 70/30 wt ratio at 40 C. All ionomers were fabricated at a 5 wt % concentration. For comparing with the high boiling point solvent-based binder, DMAc was also used at the same concentration.
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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MEA fabrications The Pt/C catalyst (46.2 wt% Pt content, Tanaka) was dispersed in a mixture of electrode ionomer solution, water, and 2propanol by sonication to obtain the homogeneous catalyst ink slurry. The catalyst slurry was directly sprayed on each Nafion® NRE-212 and s-PAES membranes. In all MEAs, the Pt loading content of the anode and cathode was the same at 0.4 mgpt cm2, and the ratio of the Pt/C-ionomer was fixed at 20 wt %. Both sides of the catalyst-coated membranes were then sandwiched with gas diffusion layers (GDLs, 39 BC, Sigracet) and Teflon gasket and were finally assembled into single unitcells. Dynamic light scattering (DLS) was used to analyze the size of the agglomerated particles in the catalyst ink depending on the electrode ionomers with an ELS-Z Zeta-potential & particle size analyzer (Otsuka Electronics Co.). Each ionomer solution (6.5 mg) was mixed with Pt/C (46.2 wt %, 1.3 mg) and 2propanol (7.8 g) to prepare 0.1 wt % solutions. The temperature was fixed as 25 C, and 2-propanol was employed as the diluent. The measurements for the tests were replicated at least nine times.
(Mw) of the s-PAES copolymers was determined to be 12k, 27k, and 51k, respectively (k: 103 g mol1). The polymers were designated as BPS-12k, 27k, and 51k, according to the Mw of the polymer. The Mw and polydispersity (PDI) were obtained by GPC (Table 1). The polymer structures and their 1H NMR peak assignments are shown in Fig. 2. The analysis revealed that the structure of S-PAES included the biphenyl sulfone (BPS) backbone, as shown in Fig. 2. The 1H NMR of the BPS-12k copolymers indicated a sharp peak of low intensity, which was observed in the spectra of other samples, indicating that the BPS-12k is in its oligomeric state and is not a complete polymer. The MW of polymers determines their physical properties. The higher the MW, the greater is the entanglement of the polymer chains. The variations in the degree of the polymer MW of BPS were reflected in their physical forms, and the12k was obtained as a powder, 27k as beads, and 51k as a fiber. In the case of casting membranes, while membranes of MWs less than 30,000 g mol1 do not free-stand or result in brittleness, membranes with Mw above 50,000 g mol1 displayed free-standing, as seen in Fig. S1. However, when using proton-conducting polymers as the electrode ionomers of the catalyst layer, the free-standing property may not be necessary.
Characterization of catalyst layer Analysis of electrode structure The cross-sectional morphology of the catalyst layer was observed by field emission-scanning electron microscopy (FESEM, Inspect F50, FEI Co). The cross-section of the MEAs were analyzed before assembling them as single-cells. The SEM was operated at 10 kV in the SE mode with a working distance of 10 mm.
Unit fuel cell tests and measurement of electrochemical properties The single-cell performances of the MEAs were evaluated using a fuel cell station system (CNL Energy, Korea) and an electric loader apparatus (E.L.P tek, ESL-300Z). Humidified hydrogen (0.4 L min1) was supplied at the anode, and air (1.2 L min1) was fed at the cathode. Upon complete activation at a constant voltage of 0.4 V at 80 C for 24 h, the currentevoltage i-V polarization curve was measured. Electrochemical impedance spectroscopy (EIS) was used for estimating the impedance of the devices (Biologics, HCP-803). EIS was employed for monitoring the ohmic resistance of the single-cells at 0.85 V. Also, the flow of air at the cathode was changed to a supply of nitrogen. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements were performed after purging nitrogen at the cathode. The LSV and CV analyses were conducted at the scan rates of 2 mV s1 and 50 mV s1, respectively.
Results and discussion Synthesis and characterization of sulfonated poly(arylene ether sulfone) (s-PAES; BPS series) polymers of various MWs
The catalyst-ionomer particle size was measured using DLS, and the results provided a relative comparison of agglomerate sizes in the alcohol-based proton exchange ionomers of the three different MWs. The data was obtained with a 0.1 wt% dilute catalyst slurry solution. In the 0.1 wt% dilute solution, the data was observed in the 200e250 nm range, which is considered to represent agglomerates [27]. Under the same dilution, the size of the agglomerated Pt/C particles was about 230 nm, as shown in Fig. 3. The particle sizes increased slightly with the addition of each ionomer, except for BPS-12k, for which, a significant increase in the size of the agglomerates was not observed even with the addition of ionomers, and it is speculated that the influence of the BPS-12k ionomer would be insufficient for polymeric network in the entire electrode system. In the case of the catalyst slurry sample with the PFSA ionomer Nafion®, the size was about 250 nm. In comparison, the BPS-27k sample showed a higher value (270 nm) than those of PFSA and BPS-51k, which indicated that they had smaller agglomerates than those of BPS-27k. The agglomerates are formed when groups of aggregates combine to form
Table 1 e Molecular weight and proton conductivity. Molecular weight by GPCa Mnb (g mol1) Mwc (g mol1) BPS-12k BPS-27k BPS-51k a
b c
s-PAES of three different MWs were synthesized for preparing the electrode binder. The weight average molecular weight
d
10,274 18,576 25,499
11,923 26,547 51,225
Proton conductivityd (S cm1) e e 94
GPC using an eluent of 0.05 M LiBr in NMP at 40 1.0 mL min1. Number averaged molecular weight. Weight averaged molecular weight. Measured at 80 C, full hydration.
C and
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Fig. 2 e Sulfonated poly(arylene ether sulfone) structure and 1H NMR of polymers with various molecular weights. Sample names indicate weight average molecular weight of ionomer; BPS-12k: 12,000 g mol¡1, BPS-27k: 27,000 g mol¡1, BPS-51k: 51,000 g mol¡1, and for BPS membrane.
larger structures, and pores formed between them are defined as “secondary pores”[28]. As the ionomer covers the catalyst particle (Pt nanoparticle and carbon support surface), the agglomerates could be affected by the ionomer. In principle, the polymer MW is reflected by the entanglement of the polymer chains in the polymer dispersion or solution. As the polymer chain length increases, the molecular force between the ionomers increases. Therefore, the 51k ionomer with the higher intermolecular force could form smaller agglomerates. These differences in the sizes of the agglomerated particles leads to the differences in catalyst layer thicknesses after MEA fabrication. Fig. 4 presents the cross-sectional SEM images of freshly fabricated MEAs for different ionomer MWs. Although each
MEA was fabricated with the same amount of ionomer, the thickness of the catalyst layer was slightly different, and this trend indicates the influence of the agglomerate size. The catalyst layer with Pt/C alone exhibited a thickness of 8.7 mm and has a tight cross-sectional structure, as shown in Fig. 4 (a). Further, the very low MW BPS-12k sample (Fig. 4 (b)) showed a very slight increase (9.9 mm) in thickness. The BPS-27k sample with the larger agglomerate size had the thickest cross-section (12.7 mm), as seen in Fig. 4 (c). For the BPS-51k sample with a size smaller than that of BPS-27k, the agglomerates showed a relatively dense and thin catalyst-sectional layer (10.7 mm), as seen in Fig. 4 (d). Due to the presence of a high amount of agglomerate in the catalyst layer, it was indirectly confirmed that the MW of the ionomer could affect the catalyst layer morphology through the associated DLS results and SEM cross-sectional images.
Electrochemical analysis Ionomer molecular weight effect
Fig. 3 e Agglomeration particle size analysis of dilute catalyst ink solution by DLS. Test condition: 0.1 wt% dilute solution in isopropyl alcohol at 25 C.
Three different MEAs with ionomers of different MWs were tested for electrochemical performance with Nafion® (NRE212, 50 mm) membrane. To investigate the MW and aggregate size effects, all ionomers were prepared in 5 wt % 1propanol/water mixture dispersion. As shown in Fig. 5, the MEAs with BPS-27k and 51k had a high open-circuit voltage (OCV) while the sample with the lowest MW (12k) showed an OCV of 0.89 V. Moreover, the overall performance of BPS12k on the polarization curve was measured to be of low current density (260 mA cm2 at 0.6 V). The extremely low MW polymers have very short polymer chains and disconnections between the aggregates. Therefore, the BPS-12k
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Fig. 4 e Cross-sectional SEM images of freshly prepared MEAs. (membrane: Nafion®, (a) without ionomer (b) BPS-12k (c) BPS27k (d) BPS-51k).
Fig. 5 e (a) Polarization curve, (b) EIS spectra measured at 0.85 V of MEAs with ionomers of three different molecular weights (BPS-12k, 27k, and 51k) on Nafion® membrane.
ionomer does not form suitable pathways for proton transport. On the other hand, the BPS-27k and 51k samples showed an overall increase in performance (750 ± 50 mA cm2 at 0.6 V) due to the possibility of proton transport and other differences, particularly in the mass transport region for 27k and 51k. The 51k, which belongs to the high MW group, had a peak power density of 560 mW cm2, and that for 27k was 670 mW cm2. Also, both ohmic resistance and charge transfer resistance were small in the 27k MEA. As observed from the differences in the morphology depending on the ionomer MW in the catalyst layer, the ohmic resistance of the agglomerates in the electrode was assumed to decrease slightly with higher MW because of the enhanced entanglements of the polymer chains as displayed in Fig. 5 (b). The recent trend for the preparation of electrodes involves the fabrication of the thin catalyst layer to improve the mass transfer efficiency at the interface, such as at the three-phase boundary (protons, electrons, and reactant gas) [29]. However, in our case, the MEA with the BPS-27k binder, which has a thicker electrode than that with BPS-51k, showed better cell performance than that with BPS-51k. Because the same amount of solid content (catalyst, ionomer) was used, the change in catalyst layer thickness indicates the formation of different microstructure of the catalyst layer. In this catalyst layer, the secondary pores between the agglomerates could serve as gas diffusion paths in the catalyst layer [28]. Therefore, the BPS-27k MEA had the catalyst layer morphology that enabled effective water and gas transport and showed superior performance.
Solvent effect The polarization curve of the MEAs incorporating BPS-27k ionomer in DMAc and alcohol mixture with the Nafion® (NRE-212, 50 mm) membrane at 80 C is shown in Fig. 6. The RH for the condition was 100%, and H2/air operation conditions were employed. Because the BPS-27k-DMAc based on the organic solvent with a high boiling point was used, the activation, ohmic, and mass transport regions exhibited low performances overall. The current density at 0.6 V was 245 mA cm2. The corresponding EIS results at 0.85 V are represented by the Nyquist plot shown in Fig. 6 (b). The ohmic resistance of the two samples was different, with that for BPS27k-DMAc being 0.22 Ohm cm2 and the one for BPS-27k-Alc being 0.15 Ohm cm2. In addition, the more significant difference was observed in the charge transfer resistances which are relevant in the well-defined electrode structure of the catalyst layer [31]. Therefore, these major increases in the resistances of BPS-27k-DMAc are related to the poor agglomeration in the catalyst layer. The BPS-27k polymer was completely dissolved in the DMAc solvent. Because the catalyst ink was prepared using a solution type binder, the isopropyl alcohol was observed to be a poor solvent; this should theoretically lead to the precipitation of the dissolved BPS-27k polymer in the isopropyl alcohol as the dispersion solvent. Consequently, in this work, a larger agglomeration was formed when the catalyst particles were encountered in the mixed solution, as shown in Fig. S2. Furthermore, the distribution of the agglomerate sizes was rough and caused uneven dispersion of the catalyst ink, including BPS-27k-DMAc, which resulted in reduced cell performance. On the other hand, the
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Single-cell performance of hydrocarbon material based MEA
Fig. 6 e (a) Polarization curve, (b) EIS measured at 0.85 V and (c) CV curve for comparison of high boiling point organic solvent (DMAc) and alcohol-water mixture solvent constituting the BPS-27k ionomer.
charge transfer resistance of BPS-27k-alc was greatly reduced because of the fine structure with the dispersion state and the alcohol solvent-based ionomer with BPS-27k polymer showed improved performance. The electrochemical surface area (ECSA) was calculated using the hydrogen region (hydrogen adsorption/desorption) CV graph (Fig. 6 (c)). The ECSA indicated the active surface area of the platinum catalyst [30]. As the solvent used for the BPS27k ionomer was DMAc, it exhibited a slightly low ECSA value (BPS-27k-DMAc showed the ECSA value of 30.81 m2 g1and BPS-27k-Alc represented 33.79 m2 g1). However, it was not significantly different from its alcohol mixture due to the same amount of the ionomer content.
In present study, MEAs based on hydrocarbon polymers were fabricated for PEMFCs. Hydrocarbon-type MEAs were composed of the electrode binder of both anode and cathode, the s-PAES PEM, and commercial Pt/C catalyst. It is noteworthy that it was possible to fabricate the hydrocarbon-based MEA, which was difficult to apply when using the conventional organic solvent-based electrode ionomer, by using the alcohol-based ionomer, without pinhole generation or shorting the electrode. The BPS-27k alcohol-based ionomer was used as the electrode binder, and the s-PAES membrane (40 mm) was used as the proton exchange membrane. The hydrocarbon-based MEAs were operated under hydrogen and oxygen conditions at an RH of 100%. Fig. 7 shows these single-cell performances with sPAESs, which have different sulfonation degrees (s-PAES 30 and 40). Because the same catalyst layer was applied, the performance in the activation region was similar; further, the performance in the ohmic region was different according to the difference in the degree of sulfonation. The s-PAES membrane that had the higher sulfonation degree, showed the higher current density at low voltage. Furthermore, the ohmic resistance of s-PAES 40 is 0.1142 Ohm cm2, which is lower than that of s-PAES 30 (0.1588 Ohm cm2) due to the greater proton-substitution of the s-PAES 40 membrane. The s-PAES 40 had a maximum power density of 426 mW cm2, and the s-PAES 30 showed a maximum power density of 287 mW cm2. However, the performance of the s-PAES membrane with the Nafion® ionomer as the electrode binder was better, as seen in Fig. S3, which is attributed to the hydrocarbon polymer-characteristics, such as the lower proton activity and oxygen permeability [31]. Therefore, the investigation of the gas permeability as well as the ionomer conditions is essential. Nafion® is known to be a high gas-permeable and well phase-separated material [1,2], whereas the s-PAES polymer is known to have a lower gas permeability than Nafion® and high hydrophilicity and random phaseseparation morphology [32]. Therefore, even if the same hydrocarbon electrode ionomer and electrolyte membrane are used, it is presumed that it would be difficult to obtain a more positive effect at the interface between the membrane and catalyst layer due to the lower gas permeability and higher water uptake in the s-PAES compared to Nafion®. Fig. S4 provides a comparison of the cell performance by lowering the degree of sulfonation of the electrode binder. The effect of reduced hydrophilicity of the electrode was also investigated. When the degree of sulfonation was lowered, not only the hydrophilicity but also the ionic conductivity decreased. However, even with low proton conductivity, the BPS30 binder showed better performance due to the increased hydrophobic group ratio. The performance of the BPS40 binder was reduced due to water flooding or dead zone formation due to high hydrophilicity, which was confirmed by the performance improvement in the activation region, and the greatly reduced charge transfer resistance seen in Fig. S4. Therefore, to realize high-performance with the hydrocarbon-based MEA, the various ionomer conditions, the suitable polymer structure for high gas
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Fig. 7 e (a) Polarization curve, (b) EIS spectra measured at 0.6 V of the MEAs with BPS-27k ionomer on hydrocarbon membranes and Nafion® membrane. permeability [33], and appropriate ion exchange capacity and water uptake are essential [11,31].
Conclusions We investigated s-PAES-based electrode ionomers under various conditions. BPS-12k, 27k, and 51k were compared according to their MWs and exhibited the same structure and degree of sulfonation. The lowest MW ionomer, BPS-12k, showed an inferior performance due to the lack of a role of binder. With the increased MW and chain entanglement, BPS-27k and 51k functioned as the binder and improved the overall fuel cell performance. However, the structure of each electrode was formed differently. In the mass transport region, the BPS-27k ionomer, which had a catalyst layer morphology that allowed an effective water and gas transport, showed superior performance. In addition, the solvent constituting the ionomer was changed from the high boiling point DMAc to the 1-propanol/water solvent mixture. The agglomeration, including in the BPS-27k-DMAc ionomer, had a rough distribution and dispersion that consequentially degraded the fuel cell performance. Additionally, the hydrocarbon-based MEAs were fabricated directly by the catalyst coated membrane method using s-PAES as PEM and were evaluated. The performance of the hydrocarbon-based MEA could be further improved by employing the developed polymer structure.
Acknowledgment This work was partially supported by the Korea Institute of Science and Technology (KIST) Institutional Program (2E30380) and partially supported by a National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT & Future Planning (Grant No. 2015M 1A2A2058015 and 2016M 1A2A2937136).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.03.003.
references
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Available online at www.sciencedirect.com
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Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells Ji Eon Chae a,b, Sung Jong Yoo a, Jin Young Kim a, Jong Hyun Jang a, So Young Lee a,*, Kwang Ho Song b,**, Hyoung-Juhn Kim a,*** a
Center for Hydrogen and Fuel Cell Research, Korea Institute of Science and Technology (KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul, 02792, Republic of Korea b Department of Chemical and Biological Engineering, Korea University, Anam-ro 145, Seongbuk-gu, Seoul, 02841, Republic of Korea
highlights
graphical abstract
Synthesis of polymer electrolytes which have different molecular weight (MW). Fabrication of membrane electrode assembly with different MW polymer electrolyte binder. Optimization of electrode configuration
for
high
fuel
cell
performance.
article info
abstract
Article history:
The electrode ionomer is a key factor that significantly affects the catalyst layer
Received 13 December 2019
morphology and fuel cell performance. Herein, sulfonated poly(arylene ether sulfone)-
Received in revised form
based electrode ionomers with polymers of various molecular weights and alcohol/water
9 February 2020
mixtures were prepared, and those comprising the alcohol/water mixture showed a higher
Accepted 1 March 2020
performance than the ones prepared using higher boiling solvents, such as dimethylace-
Available online xxx
tamide; this is owing to the formation of the uniformly dispersed ionomer catalyst layer. The relation between ionomer molecular weight for the same polymer structure and the
Keywords:
sulfonation degree was investigated. Because the chain length of polymer varies with
Hydrocarbon-based electrode
molecular weight and chain entanglement degree, its molecular weight affects the elec-
ionomer
trode morphology. As the ionomer covered the catalyst, the agglomerates formed were of
Membrane-electrode assembly
different morphologies according to their molecular weight, which could be deduced
Polymer electrolyte
* Corresponding author. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (S.Y. Lee), [email protected] (K.H. Song), [email protected] (H.-J. Kim). https://doi.org/10.1016/j.ijhydene.2020.03.003 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
2
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Proton exchange membrane fuel cell
indirectly through dynamic light scattering and scanning electron microscopy. Addition-
Sulfonated poly(arylene ether
ally, the fuel cell performance was confirmed in the current-voltage curve.
sulfone)
© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Fuel cells produce electricity and thermal energy through electrochemical reactions and are one of the most promising energy generation devices in the wake of the climate change crisis; this is due to advantages such as zero-emission and high efficiency. Among various types of fuel cells, proton exchange membrane fuel cells (PEMFCs) offer higher power density and lower weight as well as volume. Among the constituent PEMFC materials, the polymer electrolyte membrane (PEM) and electrode ionomer binder have significant proton-conducting properties. Nafion® is a perfluorosulfonated polymer (PFSA), which exhibits good proton conductivity, and superior mechanical and chemical properties. Furthermore, Nafion® has been extensively studied and optimized to date [1e4]. However, its high price and high gas permeability are the main bottlenecks in the commercialization of fuel cells comprising Nafion®. In addition, as a fluorine-based polymer, Nafion® causes environmental problems during preparation and disposal. Studies to address these limitations by the development of hydrocarbon-based PEMs that complement the advantages and alleviate the disadvantages of Nafion® are also underway. The polymerization processes of hydrocarbon-based PEMs, such as poly(ether ether ketone)s, poly(ether sulfone)s, polyimides, poly(phenylene oxide)s, and polybenzimidazoles, are relatively straightforward and the materials exhibit high thermal and mechanical stabilities [5]. Various hydrocarbon-based PEMs have shown to attain exceedingly high levels as compared to PFSA. Recent studies have revealed the synthesis of hydrocarbon-based PEMs that display high conductivity, good mechanical and thermal stability, low gas permeability and well-ordered morphology for phase separation [5,6]. However, these high-performance hydrocarbon PEMs still employ PFSA ionomers as the electrode binder, during the fabrication of the membrane-electrode assembly (MEA). Although this approach delivers a high performance, it presents high costs and compatibility issues such as the delamination of the membranes and electrodes [7e9]. Although the development of hydrocarbon-based PEMs has been achieved, hydrocarbon-based electrode ionomers have not been studied extensively. Rahnavard et al. reported that the use of the sulfonated poly(ether ether ketone) (PEEK) ionomer solution (5 wt%) in dimethylacetamide (DMAc) as an electrode ionomer with a nanocomposite PEEK membrane, generated a current density of 250 mA cm2 at 0.6 V with H2 and O2 [10]. Moreover, Lee and co-workers were successful in improving the interfacial stability of the PEEK membrane and electrode, including the PEEK binder solution in DMAc at low relative humidity (RH 30%) [11]. However, in the hydrocarbon-based proton exchange ionomers, the catalyst layer was prepared mainly using a high boiling point organic solvent. In such
systems, the solvent might dissolve the PEM when the catalyst slurry is applied directly, resulting in low actual cell performance. Recently, alcohol-soluble hydrocarbon-based ionomers comprising poly(arylene ether)s were reported. Lee et al. reported the use of a sulfonated copolymer as the membrane and cathode catalyst layer ionomer in MEA, which showed a highperformance of 700 mW cm2 under the H2/O2 condition [12]. In the groups of various hydrocarbon polymer candidates, sulfonated poly(arylene ether sulfone) (s-PAES) has been regarded as a feasible PEM to address the shortcomings of Nafion® [13,14]. Until recently, researchers has studied that the modification and functionalization of s-PAES structure has improved the membrane morphologies and fuel cell properties [12,15e17]. Chemical modifications have been accomplished with a number of different processes involving modification of the polymer structure such as block copolymer [12,16], combshaped s-PAES [15], novel side chain [18e20] and cross linking system [21,22]. These suitable structures could be introduced for improved membrane properties. In this study, the basic s-PAES structure was introduced to examine the hydrocarbon polymeric binder. This basic structure is expected to be compatible with a variety of the enhanced s-PAES-based membranes. We investigated several conditions of the hydrocarbon-based electrode binder toward improving the cell performance. The choice of the solvent for the hydrocarbon electrode binder and catalyst ink is a vital aspect of the design. Furthermore, several studies have revealed the critical impact of the solvents used for ionomer and ink dispersion on the electrode morphology and cell performance of PFSA ionomers. Kim et al. recently reported that the desired microstructure in the catalyst layer was obtained with the use of the solvent that allowed high mainchain mobility of the Nafion® ionomer [3]. Welch et al. investigated Nafion® ionomer in dilute systems, which included both dispersion and solution [4]. This catalyst layer study leads to the study of polymer-solvent interaction, polymer-particle interaction and particle-solvent interaction. However, since it is practically inaccessible, the nano-scale interactions need research furthermore. In addition, the molecular weight (MW) of the polymer influences the behavior of the electrode ionomer in the catalyst layer, as shown in Fig. 1. Because the length of the polymer chain and the degree of entanglement vary with the MW, the ionomer MW affects the interactions in the catalyst layer. In particular, there are some studies on the influence of polymeric binder MW on the morphology of electrode and the relative performance in the field of lithium ion batteries and dye-sensitized solar cells. The correlation between the binder MW and electrode morphology for dye-sensitized solar cells et al. was established [23]. Lee et al. also investigated by Tasic examined a detailed investigation of the influence of MW as
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Fig. 1 e Schematic illustration of agglomerates controlled by polymer molecular weight of ionomer in catalyst layer. anode binder in lithium ion battery [24]. As a result, the MW of the polymer used as the binder has been found to affect the formation of the electrode microstructure or the electrochemical performance and to have an optimal MW region. Zhou et al. reported that lower MW are preferred probably due to the increased free volume and leads to higher mass transport and/or catalyst utilization [25]. Consequently, it has been found that the MW of the polymer used as the binder affects the formation of electrode microstructure or electrochemical performance and has an optimal region. Here, MEAs comprising hydrocarbon-based electrode ionomers were prepared. The anode and cathode of the MEAs were composed of a newly developed electrode binder (BPS series). For PEMFC operation, the Nafion® membrane, s-PAES membrane, and commercial Pt/C catalyst were employed. Different MWs and solvent of the same polymer structure as the electrode binder contribute to the formation of the catalyst layer on the commercial Nafion® membrane. With appropriate ionomer conditions that enable the uniform electrode morphology, the performance of polymeric material was enhanced. Finally, sPAES membranes were used instead of commercial PFSA membranes to investigate fuel cell performance with polymeric materials based on all-hydrocarbon-based polymer.
Experimental Materials The 4,40 -dichlorodiphenylsulfone (DCDPS), 4,40 -dihydroxybiphenyl (BP), potassium carbonate (K2CO3), anhydrous N, Ndimethylacetamide (DMAc), and toluene required for the synthesis of the poly(arylene ether sulfone) copolymer were obtained from Aldrich Chemical Co (USA). Sulfuric acid (98%) was obtained from Daejung Chemical Co. The Pt/C (46.7 wt%, Tanaka), 1-propanol, and 2-propanol required for the catalyst ink preparation were sourced from Aldrich Chemical Co (USA) and were used for the preparation of the catalyst slurry.
Synthesis of sulfonated poly(arylene ether sulfone) The sulfonated poly(arylene ether sulfone) (s-PAES) copolymer required for the electrolyte membrane was synthesized
according to reported literature [26]. The sulfonation degree was 40%, and the weight average molecular weight (Mw) of the polymer was 94,165 g mol1 (Mw/Mn ¼ 2.003). The s-PAES copolymers (BPS series) required for the electrode binder were synthesized via direct condensation polymerization according to the reported procedure [26], and the degree of sulfonation of the sulfonated poly(arylene ether sulfone) was fixed at 40%. The MW of the polymer required for the preparation of the electrode binder was adjusted to a 10,000 to 50,000 g mol1 range based on the weight average molecular weight (Mw). Also, the s-PAES with a 30% degree of sulfonation was synthesized for comparison. All polymers were soaked and stirred in 1 M aqueous sulfuric acid at 60 C for 2 h followed by washing with distilled water for 24 h. The acidified copolymers were dried overnight in a vacuum oven at 80 C.
Characterization of copolymers The structural characterization of the synthesized copolymers was carried out using 1H NMR on a 400 MHz Bruker AV400 spectrometer. The samples of the copolymers (0.01 g) were dissolved in DMSO-d6 (0.1 g) and were filtered via a 0.45 mm pore membrane before loading into the NMR tubes. The MW (Mn and Mw) of the copolymers was determined by gel permeation chromatography (GPC) on Styragel HR 3 and 4 columns in a system comprising a Waters 1515 isocratic HPLC pump, a Waters 2707 auto-sampler, and a Waters 2414 refractive index detector. The GPC sampling was carried out by mixing 0.006 g of each copolymer in the eluent, which consisted of 1.994 g of 0.05 M Lithium bromide in NMP (HPLC grade) as the eluent, and the sample solution was passed through a 0.45 mm pore syringe filter. The flow rate was 1.0 mL min1 at 40 C. The polymethylmethaycrylate Mid MW standard Kit (Waters Co.) was used for the GPC MW calibration for all tests.
Preparation of electrode ionomer solution Acidified BPS copolymers were dissolved in a mixture of 1propanol and distilled water in a 70/30 wt ratio at 40 C. All ionomers were fabricated at a 5 wt % concentration. For comparing with the high boiling point solvent-based binder, DMAc was also used at the same concentration.
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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MEA fabrications The Pt/C catalyst (46.2 wt% Pt content, Tanaka) was dispersed in a mixture of electrode ionomer solution, water, and 2propanol by sonication to obtain the homogeneous catalyst ink slurry. The catalyst slurry was directly sprayed on each Nafion® NRE-212 and s-PAES membranes. In all MEAs, the Pt loading content of the anode and cathode was the same at 0.4 mgpt cm2, and the ratio of the Pt/C-ionomer was fixed at 20 wt %. Both sides of the catalyst-coated membranes were then sandwiched with gas diffusion layers (GDLs, 39 BC, Sigracet) and Teflon gasket and were finally assembled into single unitcells. Dynamic light scattering (DLS) was used to analyze the size of the agglomerated particles in the catalyst ink depending on the electrode ionomers with an ELS-Z Zeta-potential & particle size analyzer (Otsuka Electronics Co.). Each ionomer solution (6.5 mg) was mixed with Pt/C (46.2 wt %, 1.3 mg) and 2propanol (7.8 g) to prepare 0.1 wt % solutions. The temperature was fixed as 25 C, and 2-propanol was employed as the diluent. The measurements for the tests were replicated at least nine times.
(Mw) of the s-PAES copolymers was determined to be 12k, 27k, and 51k, respectively (k: 103 g mol1). The polymers were designated as BPS-12k, 27k, and 51k, according to the Mw of the polymer. The Mw and polydispersity (PDI) were obtained by GPC (Table 1). The polymer structures and their 1H NMR peak assignments are shown in Fig. 2. The analysis revealed that the structure of S-PAES included the biphenyl sulfone (BPS) backbone, as shown in Fig. 2. The 1H NMR of the BPS-12k copolymers indicated a sharp peak of low intensity, which was observed in the spectra of other samples, indicating that the BPS-12k is in its oligomeric state and is not a complete polymer. The MW of polymers determines their physical properties. The higher the MW, the greater is the entanglement of the polymer chains. The variations in the degree of the polymer MW of BPS were reflected in their physical forms, and the12k was obtained as a powder, 27k as beads, and 51k as a fiber. In the case of casting membranes, while membranes of MWs less than 30,000 g mol1 do not free-stand or result in brittleness, membranes with Mw above 50,000 g mol1 displayed free-standing, as seen in Fig. S1. However, when using proton-conducting polymers as the electrode ionomers of the catalyst layer, the free-standing property may not be necessary.
Characterization of catalyst layer Analysis of electrode structure The cross-sectional morphology of the catalyst layer was observed by field emission-scanning electron microscopy (FESEM, Inspect F50, FEI Co). The cross-section of the MEAs were analyzed before assembling them as single-cells. The SEM was operated at 10 kV in the SE mode with a working distance of 10 mm.
Unit fuel cell tests and measurement of electrochemical properties The single-cell performances of the MEAs were evaluated using a fuel cell station system (CNL Energy, Korea) and an electric loader apparatus (E.L.P tek, ESL-300Z). Humidified hydrogen (0.4 L min1) was supplied at the anode, and air (1.2 L min1) was fed at the cathode. Upon complete activation at a constant voltage of 0.4 V at 80 C for 24 h, the currentevoltage i-V polarization curve was measured. Electrochemical impedance spectroscopy (EIS) was used for estimating the impedance of the devices (Biologics, HCP-803). EIS was employed for monitoring the ohmic resistance of the single-cells at 0.85 V. Also, the flow of air at the cathode was changed to a supply of nitrogen. Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) measurements were performed after purging nitrogen at the cathode. The LSV and CV analyses were conducted at the scan rates of 2 mV s1 and 50 mV s1, respectively.
Results and discussion Synthesis and characterization of sulfonated poly(arylene ether sulfone) (s-PAES; BPS series) polymers of various MWs
The catalyst-ionomer particle size was measured using DLS, and the results provided a relative comparison of agglomerate sizes in the alcohol-based proton exchange ionomers of the three different MWs. The data was obtained with a 0.1 wt% dilute catalyst slurry solution. In the 0.1 wt% dilute solution, the data was observed in the 200e250 nm range, which is considered to represent agglomerates [27]. Under the same dilution, the size of the agglomerated Pt/C particles was about 230 nm, as shown in Fig. 3. The particle sizes increased slightly with the addition of each ionomer, except for BPS-12k, for which, a significant increase in the size of the agglomerates was not observed even with the addition of ionomers, and it is speculated that the influence of the BPS-12k ionomer would be insufficient for polymeric network in the entire electrode system. In the case of the catalyst slurry sample with the PFSA ionomer Nafion®, the size was about 250 nm. In comparison, the BPS-27k sample showed a higher value (270 nm) than those of PFSA and BPS-51k, which indicated that they had smaller agglomerates than those of BPS-27k. The agglomerates are formed when groups of aggregates combine to form
Table 1 e Molecular weight and proton conductivity. Molecular weight by GPCa Mnb (g mol1) Mwc (g mol1) BPS-12k BPS-27k BPS-51k a
b c
s-PAES of three different MWs were synthesized for preparing the electrode binder. The weight average molecular weight
d
10,274 18,576 25,499
11,923 26,547 51,225
Proton conductivityd (S cm1) e e 94
GPC using an eluent of 0.05 M LiBr in NMP at 40 1.0 mL min1. Number averaged molecular weight. Weight averaged molecular weight. Measured at 80 C, full hydration.
C and
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Fig. 2 e Sulfonated poly(arylene ether sulfone) structure and 1H NMR of polymers with various molecular weights. Sample names indicate weight average molecular weight of ionomer; BPS-12k: 12,000 g mol¡1, BPS-27k: 27,000 g mol¡1, BPS-51k: 51,000 g mol¡1, and for BPS membrane.
larger structures, and pores formed between them are defined as “secondary pores”[28]. As the ionomer covers the catalyst particle (Pt nanoparticle and carbon support surface), the agglomerates could be affected by the ionomer. In principle, the polymer MW is reflected by the entanglement of the polymer chains in the polymer dispersion or solution. As the polymer chain length increases, the molecular force between the ionomers increases. Therefore, the 51k ionomer with the higher intermolecular force could form smaller agglomerates. These differences in the sizes of the agglomerated particles leads to the differences in catalyst layer thicknesses after MEA fabrication. Fig. 4 presents the cross-sectional SEM images of freshly fabricated MEAs for different ionomer MWs. Although each
MEA was fabricated with the same amount of ionomer, the thickness of the catalyst layer was slightly different, and this trend indicates the influence of the agglomerate size. The catalyst layer with Pt/C alone exhibited a thickness of 8.7 mm and has a tight cross-sectional structure, as shown in Fig. 4 (a). Further, the very low MW BPS-12k sample (Fig. 4 (b)) showed a very slight increase (9.9 mm) in thickness. The BPS-27k sample with the larger agglomerate size had the thickest cross-section (12.7 mm), as seen in Fig. 4 (c). For the BPS-51k sample with a size smaller than that of BPS-27k, the agglomerates showed a relatively dense and thin catalyst-sectional layer (10.7 mm), as seen in Fig. 4 (d). Due to the presence of a high amount of agglomerate in the catalyst layer, it was indirectly confirmed that the MW of the ionomer could affect the catalyst layer morphology through the associated DLS results and SEM cross-sectional images.
Electrochemical analysis Ionomer molecular weight effect
Fig. 3 e Agglomeration particle size analysis of dilute catalyst ink solution by DLS. Test condition: 0.1 wt% dilute solution in isopropyl alcohol at 25 C.
Three different MEAs with ionomers of different MWs were tested for electrochemical performance with Nafion® (NRE212, 50 mm) membrane. To investigate the MW and aggregate size effects, all ionomers were prepared in 5 wt % 1propanol/water mixture dispersion. As shown in Fig. 5, the MEAs with BPS-27k and 51k had a high open-circuit voltage (OCV) while the sample with the lowest MW (12k) showed an OCV of 0.89 V. Moreover, the overall performance of BPS12k on the polarization curve was measured to be of low current density (260 mA cm2 at 0.6 V). The extremely low MW polymers have very short polymer chains and disconnections between the aggregates. Therefore, the BPS-12k
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Fig. 4 e Cross-sectional SEM images of freshly prepared MEAs. (membrane: Nafion®, (a) without ionomer (b) BPS-12k (c) BPS27k (d) BPS-51k).
Fig. 5 e (a) Polarization curve, (b) EIS spectra measured at 0.85 V of MEAs with ionomers of three different molecular weights (BPS-12k, 27k, and 51k) on Nafion® membrane.
ionomer does not form suitable pathways for proton transport. On the other hand, the BPS-27k and 51k samples showed an overall increase in performance (750 ± 50 mA cm2 at 0.6 V) due to the possibility of proton transport and other differences, particularly in the mass transport region for 27k and 51k. The 51k, which belongs to the high MW group, had a peak power density of 560 mW cm2, and that for 27k was 670 mW cm2. Also, both ohmic resistance and charge transfer resistance were small in the 27k MEA. As observed from the differences in the morphology depending on the ionomer MW in the catalyst layer, the ohmic resistance of the agglomerates in the electrode was assumed to decrease slightly with higher MW because of the enhanced entanglements of the polymer chains as displayed in Fig. 5 (b). The recent trend for the preparation of electrodes involves the fabrication of the thin catalyst layer to improve the mass transfer efficiency at the interface, such as at the three-phase boundary (protons, electrons, and reactant gas) [29]. However, in our case, the MEA with the BPS-27k binder, which has a thicker electrode than that with BPS-51k, showed better cell performance than that with BPS-51k. Because the same amount of solid content (catalyst, ionomer) was used, the change in catalyst layer thickness indicates the formation of different microstructure of the catalyst layer. In this catalyst layer, the secondary pores between the agglomerates could serve as gas diffusion paths in the catalyst layer [28]. Therefore, the BPS-27k MEA had the catalyst layer morphology that enabled effective water and gas transport and showed superior performance.
Solvent effect The polarization curve of the MEAs incorporating BPS-27k ionomer in DMAc and alcohol mixture with the Nafion® (NRE-212, 50 mm) membrane at 80 C is shown in Fig. 6. The RH for the condition was 100%, and H2/air operation conditions were employed. Because the BPS-27k-DMAc based on the organic solvent with a high boiling point was used, the activation, ohmic, and mass transport regions exhibited low performances overall. The current density at 0.6 V was 245 mA cm2. The corresponding EIS results at 0.85 V are represented by the Nyquist plot shown in Fig. 6 (b). The ohmic resistance of the two samples was different, with that for BPS27k-DMAc being 0.22 Ohm cm2 and the one for BPS-27k-Alc being 0.15 Ohm cm2. In addition, the more significant difference was observed in the charge transfer resistances which are relevant in the well-defined electrode structure of the catalyst layer [31]. Therefore, these major increases in the resistances of BPS-27k-DMAc are related to the poor agglomeration in the catalyst layer. The BPS-27k polymer was completely dissolved in the DMAc solvent. Because the catalyst ink was prepared using a solution type binder, the isopropyl alcohol was observed to be a poor solvent; this should theoretically lead to the precipitation of the dissolved BPS-27k polymer in the isopropyl alcohol as the dispersion solvent. Consequently, in this work, a larger agglomeration was formed when the catalyst particles were encountered in the mixed solution, as shown in Fig. S2. Furthermore, the distribution of the agglomerate sizes was rough and caused uneven dispersion of the catalyst ink, including BPS-27k-DMAc, which resulted in reduced cell performance. On the other hand, the
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Single-cell performance of hydrocarbon material based MEA
Fig. 6 e (a) Polarization curve, (b) EIS measured at 0.85 V and (c) CV curve for comparison of high boiling point organic solvent (DMAc) and alcohol-water mixture solvent constituting the BPS-27k ionomer.
charge transfer resistance of BPS-27k-alc was greatly reduced because of the fine structure with the dispersion state and the alcohol solvent-based ionomer with BPS-27k polymer showed improved performance. The electrochemical surface area (ECSA) was calculated using the hydrogen region (hydrogen adsorption/desorption) CV graph (Fig. 6 (c)). The ECSA indicated the active surface area of the platinum catalyst [30]. As the solvent used for the BPS27k ionomer was DMAc, it exhibited a slightly low ECSA value (BPS-27k-DMAc showed the ECSA value of 30.81 m2 g1and BPS-27k-Alc represented 33.79 m2 g1). However, it was not significantly different from its alcohol mixture due to the same amount of the ionomer content.
In present study, MEAs based on hydrocarbon polymers were fabricated for PEMFCs. Hydrocarbon-type MEAs were composed of the electrode binder of both anode and cathode, the s-PAES PEM, and commercial Pt/C catalyst. It is noteworthy that it was possible to fabricate the hydrocarbon-based MEA, which was difficult to apply when using the conventional organic solvent-based electrode ionomer, by using the alcohol-based ionomer, without pinhole generation or shorting the electrode. The BPS-27k alcohol-based ionomer was used as the electrode binder, and the s-PAES membrane (40 mm) was used as the proton exchange membrane. The hydrocarbon-based MEAs were operated under hydrogen and oxygen conditions at an RH of 100%. Fig. 7 shows these single-cell performances with sPAESs, which have different sulfonation degrees (s-PAES 30 and 40). Because the same catalyst layer was applied, the performance in the activation region was similar; further, the performance in the ohmic region was different according to the difference in the degree of sulfonation. The s-PAES membrane that had the higher sulfonation degree, showed the higher current density at low voltage. Furthermore, the ohmic resistance of s-PAES 40 is 0.1142 Ohm cm2, which is lower than that of s-PAES 30 (0.1588 Ohm cm2) due to the greater proton-substitution of the s-PAES 40 membrane. The s-PAES 40 had a maximum power density of 426 mW cm2, and the s-PAES 30 showed a maximum power density of 287 mW cm2. However, the performance of the s-PAES membrane with the Nafion® ionomer as the electrode binder was better, as seen in Fig. S3, which is attributed to the hydrocarbon polymer-characteristics, such as the lower proton activity and oxygen permeability [31]. Therefore, the investigation of the gas permeability as well as the ionomer conditions is essential. Nafion® is known to be a high gas-permeable and well phase-separated material [1,2], whereas the s-PAES polymer is known to have a lower gas permeability than Nafion® and high hydrophilicity and random phaseseparation morphology [32]. Therefore, even if the same hydrocarbon electrode ionomer and electrolyte membrane are used, it is presumed that it would be difficult to obtain a more positive effect at the interface between the membrane and catalyst layer due to the lower gas permeability and higher water uptake in the s-PAES compared to Nafion®. Fig. S4 provides a comparison of the cell performance by lowering the degree of sulfonation of the electrode binder. The effect of reduced hydrophilicity of the electrode was also investigated. When the degree of sulfonation was lowered, not only the hydrophilicity but also the ionic conductivity decreased. However, even with low proton conductivity, the BPS30 binder showed better performance due to the increased hydrophobic group ratio. The performance of the BPS40 binder was reduced due to water flooding or dead zone formation due to high hydrophilicity, which was confirmed by the performance improvement in the activation region, and the greatly reduced charge transfer resistance seen in Fig. S4. Therefore, to realize high-performance with the hydrocarbon-based MEA, the various ionomer conditions, the suitable polymer structure for high gas
Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003
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Fig. 7 e (a) Polarization curve, (b) EIS spectra measured at 0.6 V of the MEAs with BPS-27k ionomer on hydrocarbon membranes and Nafion® membrane. permeability [33], and appropriate ion exchange capacity and water uptake are essential [11,31].
Conclusions We investigated s-PAES-based electrode ionomers under various conditions. BPS-12k, 27k, and 51k were compared according to their MWs and exhibited the same structure and degree of sulfonation. The lowest MW ionomer, BPS-12k, showed an inferior performance due to the lack of a role of binder. With the increased MW and chain entanglement, BPS-27k and 51k functioned as the binder and improved the overall fuel cell performance. However, the structure of each electrode was formed differently. In the mass transport region, the BPS-27k ionomer, which had a catalyst layer morphology that allowed an effective water and gas transport, showed superior performance. In addition, the solvent constituting the ionomer was changed from the high boiling point DMAc to the 1-propanol/water solvent mixture. The agglomeration, including in the BPS-27k-DMAc ionomer, had a rough distribution and dispersion that consequentially degraded the fuel cell performance. Additionally, the hydrocarbon-based MEAs were fabricated directly by the catalyst coated membrane method using s-PAES as PEM and were evaluated. The performance of the hydrocarbon-based MEA could be further improved by employing the developed polymer structure.
Acknowledgment This work was partially supported by the Korea Institute of Science and Technology (KIST) Institutional Program (2E30380) and partially supported by a National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science, ICT & Future Planning (Grant No. 2015M 1A2A2058015 and 2016M 1A2A2937136).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.03.003.
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Please cite this article as: Chae JE et al., Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.003