Proton conducting sulfonated polysulfone and polyphenylsulfone multiblock copolymers with improved performances for fuel cell applications

Proton conducting sulfonated polysulfone and polyphenylsulfone multiblock copolymers with improved performances for fuel cell applications Sydonne Swaby a , Nieves Ureña a , María Teresa Pérez-Prior a , Carmen del Río b , Alejandro Várez a , Jean-Yves Sanchez a,c,d , Cristina Iojoiu c,d , Belén Levenfeld a, ⇑ a Universidad Carlos III de Madrid, Departamento de Ciencia e Ingeniería de Materiales e Ingeniería Química, IAAB. Avda. Universidad, 30, 28911 Leganés, Madrid, Spain b Instituto de Ciencia y Tecnología de Polímeros (ICTP-CSIC), C/Juan de la Cierva, 3, 28006 Madrid, Spain c University Grenoble Alpes, LEPMI, 38000 Grenoble, France d CNRS, LEPMI, 38000 Grenoble, France article info Article history: Received 21 December 2022 Revised 14 February 2023 Accepted 24 February 2023 Available online 28 February 2023 Keywords: Block copolymer Polysulfone Polyphenylsulfone Proton exchange membranes Impedance spectroscopy Fuel cell abstract A series of proton exchange membranes based on sulfonated multiblock copolymers with three polysulfone (PSU) and polyphenylsulfone (PPSU) ratios (50/50, 60/40 and 75/25) are prepared following a synthesis strategy that aims to achieve a microphase separation. A selective sulfonation of PSU blocks was observed in copolymers with a high proportion of PSU. The water uptake is higher in these materials (31% and 57% for SPES 50/50 and SPES 75/25, respectively at 60 °C) while the tensile strength was lower (56.0 MPa and 40.6 MPa for SPES 50/50 and SPES 75/25 in the H + form, respectively). Ionic conductivity of SPES 75/25 membranes measured both ex situ and in situ at 80 °C is 25 and 31 mScm 1 , respectively. Fuel cell tests reveal that SPES 75/25 shows the highest value for the maximum power density (670 mWcm 2 at 70 °C and 100% of RH) which is higher than that achieved for SPES 50/50 (400 mWcm 2 ). In addition, the high current density obtained for SPES 75/25 (1000 mAcm 2 at 0.6 V and 70 °C) compared with SPES50/50 (600 mAcm 2 ) and Nafion 112 (450 mAcm 2 at 75 °C) shows its promising properties as solid electrolyte in polymeric fuel cells. Ó2023 The Authors. Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. This is an open access article under the CC BY-NC-ND license (http://creativecommons. org/licenses/by-nc-nd/4.0/). Introduction The consequences of the climate change are generating multiple issues in a worldwide range. Many countries are developing new regulations to solve the problems, including the reduction of CO 2 produced from fossil fuels [1,2]. The automobile industry is one of the most affected due to the high impact of the vehicles emissions in pollution [3]. ‘‘Zero emissions” is the new trend adopted by many governments, to reduce pollution, inducing the new solutions to clean energies like fuel cells (FCs). Thus, FC devices are proposed to be the future energy source in automotive industry. The main advantages of these devices are their high-power density and excellent energy conversion efficiencies, in addition to the fact that the only product obtained after the reaction is H 2 O[3,4]. FCs exceed the efficiency of combustion engines because they exhibit an electrical energy conversion exceeding 60% with low emissions. The FCs have found their major in electronics and milihttps://doi.org/10.1016/j.jiec.2023.02.037 1226-086X/Ó2023 The Authors. Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Abbreviations: FCs, Fuel cells; PEMFCs, Proton exchange membrane fuel cells; PEM, Proton exchange membrane; PES, Poly(ether sulfone); PEK, Poly(ether ketone); PEEK, Poly(ether ether ketone); WU%, Water uptake; PSU, Polysulfone; PPSU, Polyphenylsulfone; 1 H NMR, Proton nuclear magnetic resonance spectroscopy; T g , Glass transition temperature; TGA, Thermogravimetric analysis; DSC, Differential scanning calorimetry; FE-SEM, Field emission scanning electron microscopy; MEA, Membrane electrode assembly; THF, Tetrahydrofuran; DMSO, Dimethyl sulfoxide; DMAc, N,N-dimethylacetamide; TMSCS, Trimethylsilyl chlorosulfonate; DCE, 1,2-Dichloroethane; DMF, Dimethylformamide; DFDPS, 4,4 0 -Difluo ro-diphenylsulfone; BPA, 4,4 0 -Isopropylidenediphenol; BP, 4,4 0 -Dihydroxybiphenyl; SPES, Sulfonated PES copolymers of PSU and PPSU segments; DS, Degree of sulfonation; TMS, Tetramethylsilane; PTFE, Polytetrafluoroethylene; IEC, Ionexchange capacity; DMA, Dynamo mechanical analysis; EIS, Electrochemical impedance spectroscopy; RH, Relative humidity; r m , Membrane ionic conductivity; I p , Polydispersity index; TS, Tensile strength; T a , Temperature associated with a a relaxation. ⇑ Corresponding author. E-mail address: [email protected] (B. Levenfeld). Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 Contents lists available at ScienceDirect Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec tary devices, such as housing power, power plants and electric vehicles because of its high efficiency [5,6]. Several electric vehicles (EV) powered by proton exchange membranes fuel cells (PEMFCs) have demonstrated the high potential of this technology. Indeed, the main advantage for an automotive application with regard to battery lies in the recharge time close to the tank’s filling up of the tank of a combustion engine vehicle. Due to its modularity, PEMFC is adapted to any scale of electricity production; namely from portable electronic to EV and up to stationary devices [7,8]. Electrolyte and electrodes are the main components of PEMFCs, they had been overstudied [9]. The proton exchange membrane (PEM), used as solid electrolyte, is responsible of the correct [10,11]. The goal in new generations of PEMs is to increase temperature and humidity ranges tolerances, at the same time, they must have mechanical strength to support the higher power density and durability demands on the future [10]. Nafion Ò is the most used commercial membrane in low temperature fuel cell (under 90 °C). This material, which exhibits suitable mechanical properties and high ionic conductivity [11], face, nonetheless, limitations associated with their low conductivity at temperatures exceeding 80 °C that encouraging the development of alternative polymeric electrolytes different to Nafion [12]. In this context, hydrocarbonated structures [9] (polymers, polymer networks, copolymers) such as poly(ether sulfones) (PES) [13] and poly(ether ketones) (PEK, PEEK) have been widely studied due to their excellent properties among other thermoplastic polymer as thermal stability, chemical inertness, flame retardancy, electrical isolating and high heat distortion temperature performance [12]. The proton conductivity of these ionomeric membranes increases with the sulfonic group ´s content due both to the increase in dissociated ionpairs and in water uptake [14]. But at the same time, a worsening of mechanical stability –from excessive membrane swelling to membrane dissolutionis observed under high relative humidity. Multiblock copolymers are an emerging class of synthetic polymers that exhibit different macromolecular architectures and behaviors, based on phase separation hydrophobic and hydrophilic blocks, to those of homopolymers or di/triblock copolymers [15,16]. Recently, multiblock copolymers based on different functionalized polymeric structures have shown good combination of ionic transport and mechanical properties attributed to the phase separation between the hydrophobic and hydrophilic regions [9,17–19]. The water uptake (WU%) of the membrane and its mechanical stability is controlled by the balance between the hydrophilic blocks responsible of the membrane proton conduction and the hydrophobic blocks that prevent the excessive deformation of the membrane as a consequence of the water retention; thus ensuring the mechanical stability of the membrane [20]. So, the use of multi-blocks copolymers for PEM synthesis allows obtaining membranes with improved characteristics in terms of thermomechanical stability and of proton conductivity. In our previous work, Ureña et al. [21] synthetized for the first time PEMs from sulfonated copolymers made of a multiblock structure of polysulfone (PSU) and polyphenylsulfone (PPSU). These materials exhibit several advantages such as the use of commercially available and fairly cheap monomers, and easy synthesis prone to be upscaled. In addition, as compared with their PSU homopolymer, membranes based on PSU/PPSU multiblock copolymers: (i) are endowed with high mechanical strength due to the inert matrix of non-functionalized blocks [22], (ii) exert an efficient control of the water uptake and swelling degree of the ion conducting segments by surrounding low swelling non functionalized segments [22], and (iii) show higher proton conductivity since the number of sulfonic groups that can be covalently anchored to the polymer backbone is higher without suffering the risk of the membrane being partially or totally solubilized in water, and, in mechanistic terms, the potential formation of microdomains which promote the vehicular mechanism could also enhance the membrane’s proton conductivity [23]. It was found that one of the main limitations of the first membranes deals with a poorer single-cell behavior, as compared to Nafion 112, the single-cell behavior was poorer under the same experimental conditions. Also, a clear phase separation was not observed in these materials. With the aim to solve this problem, this article extends our preliminary work and reports, in detail, the preparation of ionomers in different PSU/PPSU ratios. Blocks of sulfonated PSU should favour the hydrophilic aggregation to induce phase separation, and higher ionic conductivity. The influence that the length of both blocks has on the ionic conductivity due to the formation of ionic domain on the material was also reported [20,24–26]. Thus, it has been observed, for example, that multiblock copolymers of poly(p-phenylene)-co-poly(arylene ether sulfone ketone) exhibit a clear phase separation and show high ionic conductivity that turned into good performance of the material in the fuel cell [27]. As well, the water uptake values reported are considered inappropriate to maintain the consistency of the membranes. This recent work presents membranes highly sulfonated, leading to a remarkable dimensional change by cause of the increase in water uptake (WU% = 130 at 25 °C). The singlecell behavior shows power density of 800 mWcm 2 as maximum and current density of 1400 mAcm 2 at 80 °C and 100% RH. However, lack of control in water absorption capacity strongly limits the use of these membranes at an industrial scale. In this work, sulfonated multiblock copolymers with different proportions of polysulfone and polyphenylsulfone are synthesized (PSU blocks are increased in the copolymer) to prepare proton exchange membranes. A complete characterization of the resulting membranes is also included, by using the following techniques: proton nuclear magnetic resonance spectroscopy ( 1 H NMR), field emission scanning electron microscopy (FE-SEM), Size exclusion chromatography (SEC), water contact angle measurements, thermogravimetric analysis (TGA), dynamo mechanical analysis (DMA), and impedance spectroscopy (EIS). In addition, MEA tests are also performed to evaluate the behavior of these electrolytes in a single fuel cell. Experimental Materials The monomers (4,4 0 -difluoro-diphenylsulfone (DFDPS), 4,4 0 - dihydroxybiphenyl (BP) and 4,4 0 -isopropylidenediphenol (BPA) were obtained from Alfa Aesar and recrystallized in isopropanol. Reagents such as potassium carbonate (K 2 CO 3 ,99.0%), and N,Ndimethylacetamide (DMAc, 99.0%), dimethyl sulfoxide-d 6 (DMSOd 6 , 99.9%), were supplied by Acros Organics and used as received. Chloroform-d (CDCl 3 -d, 99.9% D), toluene (99.8%), trimethylsilyl chlorosulfonate (TMSCS, 99.0%), dimethylformamide (DMF,  99.8%) and 1,2-dichloroethane (DCE, 98.9%) were purchased from Sigma-Aldrich. Synthesis of membranes Synthesis of sulfonated copolymers Multiblock copolymers based on PSU and PPSU blocks (PES) were synthesized via polycondensation in ‘‘one pot two-steps synthesis” varying the PSU/PPSU ratio. Subsequently, the functionalization reaction to obtain conductive polymers, i.e., sulfonated copolymers (SPES), was followed as described in our previous work [21]. PES copolymer with a 50/50 ratio (PES 50/50) was firstly prepared with a molecular weight of each block of 5000 gmol 1 .In this work two new PSU/PPSU copolymers with different ratio S. Swaby, N. Ureña, María Teresa Pérez-Prior et al. Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 367 (60/40 and 75/25) are prepared. First, PES 60/40 with molecular weights of 7500 gmol 1 and 5000 gmol 1 for PSU and PPSU, respectively, and PES 75/25 whose molecular weights are 7500 gmol 1 and 2500 gmol 1 for PSU and PPSU, respectively. The Scheme 1 shows the chemical structure of the nonsulfonated (PES) and sulfonated copolymers (SPES) prepared (A) and a schematic illustration showing the different SPSU/PPSU proportions (B). The synthesis pathway of PES is shown in (Fig. S1 A). In a 250 mL three-neck-round-bottom flask with a dean–stark trap, N 2 inlet, thermometer, and mechanical stirring, the monomers (DFDPS and BP), DMAc, and anhydrous K 2 CO 3 were added. After the components of the mixture were dissolved, toluene was added. The system was dehydrated following the procedure reported by Ureña et al. [21]. After this step, toluene and water had been distilled off, and the temperature was maintained at 120 °C for 18 h to acquire a viscous solution. Next, BPA, previously dissolved in DMAc, K 2 CO 3 , and toluene were added. Before adding the monomer DFDPS (previously solved in DMAc) it was carried out the azeotropic process, prior removing the toluene. The solution was stirred for 18 h to obtain a highly viscous solution. The resulting copolymer was precipitated in a 1 M HCl solution and dried under vacuum at 60C for 48 h. The commercial monomers besides the different proportions prepared are gathered on Fig. S1 B. Sulfonation of copolymers were performed with TMSCS in accordance with the procedure described by Ureña et al. [21]. Thus, TMSCS amounts ranged from 3.16 mL to 16 mL depending on the proportion of the blocks used (50/50, 60/40, and 75/25) are added into the flask containing solved copolymer in DCE. The resulting sulfonated copolymers (SPES) were precipitated in a 0.1 M NaOH solution. Then, the copolymers were filtered, rinsed with destilled water, and dried under vacuum at 60 °C for 24 h. Sulfonated copolymers are abbreviated as SPES 50/50, 60/40, and 75/25 as a function on the SPSU/PPSU ratio (50/50, 60/40 and 75/25, respectively) as can be seen in Scheme 1. Membrane preparation PEMs were obtained by casting procedure as follow: 0.9 g of SPES dissolved in 12 mL of DMF were cast on a petri dish and dried under vacuum at 80 °C over 48 h to get the membranes in the Na + form with thicknesses of approximately 50 l m. The membranes were then soaked in a 1 M HCl solution for 24 h over 60 °Cto exchange Na + with H + ions. Material characterization 1 H NMR The synthetized copolymers were analyzed by 1 H NMR spectroscopy in a Bruker Avance (Billerica, MA, USA) DPX-300 (300 MHz). The solvents used for the analysis of PES and SPES were CDCl 3 and DMSO-d 6 , respectively. The internal reference solvent was tetramethylsilane (TMS). Size exclusion chromatography (SEC) SEC analyses were performed at 40 °C using a Waters 2414 Refractive index detector coupled to a Waters 2489 UV/Visible detector with a 2xPLgel-Mixed-D column and a Waters 515 HPLC pump. The solvent (THF) was used as eluent with flow rate of 1mLmin 1 . The solutions of 1% of PES were filtered through a 0.45 mm Millipore filter based on polytetrafluoroethylene (PTFE). The calibration was performed using polystyrene standards. Each sample was analyzed twice. Scheme 1. (A) PES and SPES multiblock copolymers with different SPSU/PPSU ratios. (B) Schematic representation of SPES copolymers where PSU and PPSU are symbolized by green and brown color, respectively, and sulfonic groups are denoted with blue circles. S. Swaby, N. Ureña, María Teresa Pérez-Prior et al. Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 368 Ion-exchange capacity (IEC) The IEC of the SPES membranes was determined by acid-base titration following the procedure described by Gou et al.[28]. Phenolphthalein was used as an indicator. The titrations were repeated at least three times for each sample. The IEC values were calculated as follow: IEC ¼V NaOH NaOH½ W dry ð1Þ where, V NaOH and [NaOH] are the volume and concentration of NaOH used in the titration, respectively, and W dry is the weight of the dry membranes. Thermal properties Thermogravimetric analysis (TGA) was performed in a Perkin Elmer Pyris STA 6000 equipment. The thermal stability of the copolymers was studied in the range of temperature 30–600 °C under nitrogen atmosphere at a heating rate of 10 °Cmin 1 . Field emission scanning electron microscopy (FE-SEM) The morphology of membranes was characterized by FE-SEM using a FEI TENEOlow vacuum equipped with an energydispersive detector (EDS-EDAX). The voltage employed to record the images were 15 kV. To improve the quality of the images, the mobile H + coming from sulfonic groups were replaced by Pb 2+ ions [21]. Briefly, the membranes in the Na + form were immersed in a 1 M HCl solution several times at 60 °C to replace Na + with H + . The resulting membranes were repeatedly rinsed with deionized water. Finally, the membrane samples were immersed in a 1 M Pb(NO 3 ) 2 solution stirred for 48 h and then they were dried in an oven at 60 °C. Water contact angle Wettability of membranes was examined through water contact angle measurements. A Dataphysics OCA15 plus goniometer and a SCA20 software (DataPhysics Instruments GmbH, Germany) were employed. First, the membranes in the Na + form were immersed in a 1 M HCl solution for 24 h at 60 °C to replace Na + with H + . Finally, the membranes were dried under vacuum for 6 h at 60 °C before the test. The water contact angle was obtained using a sessile drop technique. In the procedure, a 3 l L of deionized water was dropped onto the dried membrane surface. At least five measurements were performed on each membrane. Water uptake Water uptake (WU%) of the membranes was evaluated in the range of temperature from room temperature to 60 °C. Membranes in acidic form were vacuum-dried at 60 °C for 48 h and weighted (W dry ). Next, they were immersed in deionized water for 72 h and weighted (W wet ). Thus, WU% was calculated according to the Eq. (2). WU%¼W wet W dry W dry 100 ð2Þ Mechanical properties The mechanical properties of the membranes were evaluated by dynamo mechanical analysis (DMA) with a DMA Q800 equipment (TA Instruments, USA). The tests were performed in tensile test mode. The dimensions of the specimens were 2.5 10 mm 2 . The membranes were tested in both the dry (Na + ) and wet (H + ) form. To obtain the last one, the membranes in Na + form were immersed in a 1 M HCl solution for 24 h at 60 °C and rinsed several times. An initial static force was fixed at 0.15 N. Stress-Strain tests were performed at 30 °C using controlled force mode and a ramp force of 0.3 Nmin 1 until 18.0 N. For dynamo mechanical test, the frequency was set in 1 Hz and the temperature was increased from 30 to 250 °C with a rate of 2 °C min 1 . Each test was repeated at least three times and the average and standard deviation values were considered. Ex situ proton conductivity Ex situ proton conductivity measurements were determined by Electrochemical Impedance Spectroscopy (EIS) using a Solartron 1260 equipment in a frequency range between 10 -1 and 10 6 Hz, from 30 to 80 °C at 90% of RH. A homemade ionic conductivity cell was used to carry out the measurements. Samples were sputtered with gold before doing the assays. The ionic conductivity of the membranes ( r m ) was determined from the resistance value (intersecting point in the abscissa axis at Nyquist diagram) through the Eq. (3). r m ¼L R m Að3Þ where L,R m , and Aare the thickness (cm), resistance ( X ), and active area (cm 2 ) of the membrane. MEA test To carry out the fuel cell test, MEA was placed in a Scribner 850e multi range fuel cell test system. Cathode and anode catalyst layers consisted of Pt/C catalyst (70 wt % Pt, Paxitech, 0.5 mg Pt cm 2 ). The SPES membranes thickness used was 50 l m. Pure oxygen and pure hydrogen were used as oxidant and fuel, respectively. The flow rate was set of 200 mLmin 1 . The measurements were performed at atmospheric pressure, 100% of RH, and at a range of temperature from 50 to 80 °C. Once the MEA reached to stable values, i.e.potential remained constant over time at a fixed current, the polarization curves were obtained. MEA cyclability tests were performed by means of the evolution of voltage over time through cycles of current variation and keeping always the cell operating. In situ through-plane proton conductivity of the membranes was determined by means of EIS on the MEA at 100% RH and different cell temperatures using a potentiostat Autolab PGStat30 provided with a FRA module. The test was performed with humidified hydrogen (SHE, anode) and nitrogen (cathode) at a flow rate of 200 mLmin 1 to supply the system. Data were collected every 10 °C. The amplitude voltage was 10 mV, and the frequency was in the range between 10 Hz and 10 5 Hz. A DC bias potential of 0.45 V was used to record the spectra. Each sample was measured at least five times after it had obtained a constant value to ensure good data reproducibility. The experimental values resulted from the EIS measurements were analyzed using the software Z-View analysis impedance (Scribner Associates, Inc., Southern Pines, NC, USA). The proton conductivity of the membrane was determined from the resistance value (intersecting point in the abscissa axis at Nyquist diagram) through the Eq. (3). Results and discussion Strategy Requirements of polymer electrolytes in hydrogen fuel cells are summarized as follows: high ionic conductivity, excellent thermal properties, good chemical and mechanical stabilities under high relative humidity (about 100% RH) and temperature conditions of a PEMFC. On the other hand, it has been observed that multiblock copolymers that exhibit a clear separation of the hydrophilic and hydrophobic domains show a good combination of ionic conductivity and mechanical stability, which turns into good performance of the material in the fuel cell [29]. S. Swaby, N. Ureña, María Teresa Pérez-Prior et al. Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 369 The present work is focused on the design and synthesis of a multiblock ionomer with limited deformation upon water swelling, due to the alternation of hydrophilic and hydrophobic blocks in charge of proton conductivity and mechanical performances, respectively. Indeed, to get a multiblock copolymer with a neat phase separation, the first option, widely documented, implies a sequential polycondensation between ionic and non-ionic monomers. However, this synthesis faces some issues regarding its upscaling, as the ionic monomer purification at large scale is often not so easy. On the other hand, the clever approach developed by Ureña et al. [21] aims at functionalizing multiblocks copolymers prepared from cheap and very pure commercially available monomers, i.e., 4,4 0 -isopropylidenediphenol (BPA), 4,4 0 - dihydroxybiphenyl (BP) and 4,4 0 -difluoro-diphenylsulfone (DFDPS). Polycondensation of BPA with DFDPS leads to the commercial PSU while that of BP with DFDPS leads to the commercial PPSU. Due to its electron-donating isopropylidene moiety, BPA is more reactive in electrophilic substitution than BP. The sulfonation degree (DS) required for PEM is high and supposes to sulfonate twice BPA. The mono substitution of BPA by the electronwithdrawing group SO 3 (indeed SO 3 -SiMe 3 ) counterbalances the electron-donating effect of the isopropylidene moiety, thus allowing PPSU blocks to be sulfonated and, therefore, losing partly the phase separation benefits. To overcome this drawback, a first approach was to optimize the synthesis protocol (temperature, TMCS excess, etc.) to disfavor the BP sulfonation. The second approach consists of increasing the PSU block length in such a way to reach the required DS while avoiding a di-substitution of BPA that favors a further sulfonation of BP. In the present work, the second option has been chosen to prepare conductive membranes. To address this aim, copolymers with higher proportion of SPSU blocks (PSU/PPSU ratios of 60/40 and 75/25) have been synthesized and characterized. The characterization comprises a microstructural, mechanical (influenced by the water level of the membranes), and electrochemical study, including the electrochemical performances in the MEA. Synthesis and structural characterization of SPES copolymers The synthesis of PES is performed by polycondensation as described by Ureña et al. [21]. Multiblock copolymers with different block lengths are obtained by controlling the molar ratio between the BPA:DFDPS and BP:DFDPS monomers. In a first step, PPSU blocks are formed from DFDPS and BP monomers. From these blocks and by using BPA and DFDPS monomers, the PSU block is obtained. In the second step, the 7500 gmol 1 PSU blocks are synthesized by adding BPA and DFDPS to the PPSU mixed. The successful formation of the PES with different proportions is confirmed by 1 H NMR analysis (Table 1). The characteristic peaks in the aromatic proton range of aromatic protons are shown in Fig. S2 [30]. In this figure, peaks corresponding to the PSU blocks are more intense in PES 60/40 and 75/25, respectively, due to the higher percentage of PSU block in these copolymers. Fig. S3 shows full 1 H NMR of the PES. The comparison of the integration of the peaks related to PPSU and PSU results in molar ratios close, within the measurements uncertainties, to those expected. Size exclusion chromatography is used to determine the molecular weight of the copolymers. The ones reported here are equivalent to standard polystyrene hydrodynamic volume. Table S1 shows the number (M n ) and weight (M w ) average molecular weight and polydispersity index (I p ) of multiblock copolymers. The presence of well-defined peaks indicates a quasi-homogeneous structure for the ionomers. Therefore, strongly supporting the synthesis of multiblock copolymers with a polydispersity of segment (PSU, PPSU) lower than 2.6 excludes the formation of oligomers and sustains the formation of multiblock copolymers. From the values of M n shown in Table S1 it has been calculated the polymerization degree (X n ), used to determine the conversion degree (p) through the following equation: p¼11 Xn ð4Þ The values obtained (Table 1) shows conversion degree of the copolymers over 98% to SPES 60/40 membrane. For SPES 50/50 and SPES 75/25 a conversion over 99% is reached, that allows a scalable and reproducible synthesis with high conversion degree for the series of copolymers prepared. The sulfonation of PES is carried out with a mild reagent (TMSCS) that avoids chain breakage when performed under a sufficient flux of inert gas [21,30–32]. Sulfonation is an electrophilic aromatic substitution reaction that occurs preferentially on aryl rings bearing electron-donating groups, thus on blocks incorporating BPA, i.e., preferentially in the ortho position of BPA ether group. Then the reaction follows in ortho position of BP-ether group. The activation of both positions for the sulfonation was confirmed in SPES 50/50. In the case of the new copolymers containing higher SPSU/PPSU ratio, i.e., SPES 60/40 and SPES 75/25, the BP position can be deactivated [33]. Each PSU unit has two electron-donating groups, the ether, and the isopropylidene groups, whereas the PPSU block contains only the ether functionality. So sulfonation reaction occurs mainly in PSU blocks as can be observed by means of 1 H NMR (Fig. 1). The protons adjacent to the sulfonic group attached to the aromatic ring show a peak which is shifted. In the case of PSU blocks, it shifts from 7.25 to 7.69 ppm (4 00 ) while in the case of the PPSU unit, it shifts from 7.60 to 8.06 ppm (5 00 ). The degree of sulfonation (DS) of the PPSU repeat units is calculated according to Eq. (5) using the integral area of the peaks at 8.06 ppm (A(5 00 )), and 1.70 ppm (A(9)) [21] associated with the 5 00 peak of the PPSU block and isopropylidene moiety (9) of the PSU block, respectively (Fig. S3). DS NMRPPSU ¼6A5ðÞ 00 A9ðÞ m n  ð5Þ The DS of the PSU blocks is estimated from both IEC and DS NMRPPSU . The IEC is measured by heterogeneous standard titration (Table 2). The highest value obtained in SPES 60/40. These results lead to a high DS for these membranes [34]. The DS for the PSU unit (DS PSU ) is obtained using Eq. (6). Table 1 Theorical and experimental PPSU/PSU molar ratio of PES copolymers. Membrane m a n a m/n theoretical m/n experimentalb p PES 50/50 12.5 11.3 1.1 1.1 0.994 PES 60/40 12.5 17 0.73 0.75 0.986 PES 75/25 6.2 17 0.36 0.36 0.996 (a) m and n correspond to the number structural units of PPSU and PSU blocks, respectively. (b) m/n exp was determined by 1 H NMR. S. Swaby, N. Ureña, María Teresa Pérez-Prior et al. Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 370 DSPSU ¼10000 þDSNMRPPSU 80ðÞIEC DSNMRPPSU mðÞ1000ðÞ 1000 80 IECðÞnðÞ ð6Þ where 10,000 is the backbone average molecular weight of the PSU and PPSU blocks, 80 gmol 1 is the molecular weight of the sulfonic group, m and n are the number of the structural units of PPSU and PSU blocks, respectively, for each copolymer. The low values of DS PPSU for SPES 60/40 and 75/25 reveal that sulfonation reaction occurs preferentially in the PSU blocks. These two copolymers have, in turn, DS PSU values higher than 1, associated with the attach of two sulfonic groups per repeat unit. So, by modifying the PSU/PPSU ratio, high selectivity in the sulfonation of copolymers can be easily achieved. This fact promotes phase separation in the 60/40 and 75/25 copolymers. The morphology of the membranes, analyzed by SEM (Fig. S4), is homogeneous with non-porous surfaces of the SPES membranes studied (50/50, 60/40, 75/25). Thermal properties The thermal stability of the SPES membranes is also evaluated by means of thermogravimetric analysis and differential scanning Fig. 1. 1 H NMR spectra of SPES at different SPSU/PPSU proportions (Solvent: DMSO-d 6 ). Table 2 IEC, DS PSU and DS PPSU values of SPES membranes. Membrane IEC (meq. H + g 1 ) DS PSU DS PPSU SPES 50/50 a 1.58 0.76 0.82 SPES 60/40 1.95 1.99 0.1 SPES 75/25 1.51 1.40 0.1 a [14]. Fig. 2. TGA and DTG curves of membranes under N 2 atmosphere of the SPES 75/25. S. Swaby, N. Ureña, María Teresa Pérez-Prior et al. Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 371 calorimetry in a STA equipment. Fig. 2 shows the TGA and DTG curves for SPES 75/25 membrane under N 2 atmosphere. All membranes here studied show a similar behavior. A first weight loss occurs around 150 °C is associated with the water removal [21,35]. From 200 to 400 °C a second weight loss takes place, which is attributed to the sulfonic group’s losses. From 450 °C the decomposition of the polymer chains takes place. No significant differences are observed when the SPSU/PPSU ratio changes. Below 150 °C, these membranes show high thermal stability. So, they are stable in the temperature range at which PEMFCs generally operate and could even be used at higher temperature to make easier the thermal management. Water contact angle and water uptake of SPES membranes The hydrophilicity of the SPES membranes at different SPSU/ PPSU ratios is evaluated by using water contact angle measurements. A similar angle value is observed for SPES 50/50 (80°± 1), SPES 60/40 (74.5°± 0.5), and SPES 75/25 (81°± 2). The water drops in the surface of all the membranes show a contact angle under 90° with the baseline. These results reveal that all the membranes studied have a high hydrophilic nature which is associated with the presence of sulfonic groups in all membranes [36]. So, the nature of hydrophobic segments of the copolymers seem not to influence significantly on this parameter. The contact angle of SPES membrane is between the value for SPSU (lower than 70°) and SPPSU (130.1°) membranes [37]. These results are in accordance with the water uptake of the membranes. Table 2 shows WU% values for SPES membranes at 30 °C and 60 °C. The WU% obtained for SPES 60/40 and for SPES 75/25 are substantially higher than the WU% reported for SPES 50/50 [21]. As the SPSU blocks content increases, which are preferably sulfonated, the ability of the membranes to absorb water is also promoted [38]. In these two membranes, the domains can be differentiated since the water molecules retained in the structures solvate the ionic groups bonded to the backbones and dissociate the ion pairs. This fact causes the repulsion of the fixed charges in the main chain originating changes in the morphology of the membranes favoring thus the separation between hydrophilic and hydrophobic domains [39]. Mechanical properties The study of the thermo-mechanical properties of the membranes is crucial to predict their performances and durability in the PEMFC. The membranes must have high mechanical strength to overcome gas pressure changes during operation and they should also have some elasticity to compensate the volume changes associated with changes in relative humidity and temperature. Stress-Strain tests Stress–strain curves for dry and wet SPES 50/50, 60/40 and 75/25 membranes are shown in Fig. 3. From this figure, three different effects can be discussed. As a general result, dry membranes exhibit higher tensile strength than wet membranes (Table 3). Water molecules interact with the sulfonic groups of the copolymer, mainly located in the hydrophilic blocks of SPSU. From this interaction, an improved ductility of the material in wet form is observed, and we could conclude that water molecules act as plasticizers [42,43]. SPES 50/50 membrane shows the highest tensile strength in both Na + and H + forms (dry and wet membranes, respectively). For SPES 60/40 and 75/25 membranes, as the percentage of the SPSU block increases, the tensile strength decreases markedly with similar values (in the dry form). This decrease is less pronounced in wet membranes. However, from this fact, we could conclude that lowering the content in hydrophobic blocks results in poorer mechanical properties of the copolymers. Comparing the values of TS in Na + and H + forms, the differences between SPES 60/40 and 75/25 membranes are negligible. In both cases, the presence of sulfonic group-rich regions (higher DS in PSU blocks) guarantees a better organization of the nanostructure. In all cases, even when the content of absorbed water in the membrane is high, the membranes show good mechanical properties. Thus, the TS values obtained for the SPES copolymers exceed Nafion 112 ones, of similar thickness (Table 3), and those reported for sulfonated polyphenyl sulfone membranes (13 MPa) [44], SPSU (11 MPa) [32] or sulfonated PEEK-b-PES copolymers (23.8 and 35.4 MPa) [45]. Regarding the elongation at break parameter, membranes in the dry form show low values (lower than 4% in all cases) whereas wet membranes exhibit, generally, higher values due to the plasticizing effect of the water embedded in the membranes vide supra. Dynamic mechanical analysis To evaluate the thermo-mechanical properties and the relaxation processes of the membranes in the Na + and H + forms (SPES-Na and SPES-H, respectively), the variation of the storage modulus (E 0 ) and the modulus of loss (E 00 ) as a function of temperature is analyzed. The loss in the storage modulus and the peak of Fig. 3. Stress–strain curves for SPES membranes in dry (A) and wet (B) form at 30°C. S. Swaby, N. Ureña, María Teresa Pérez-Prior et al. Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 372 tangent dare directly related to the a relaxation process associated, in turn, with the T g of the sample studied. For each membrane, the value of T a is determined with the peak of the loss modulus. These values, as well as the storage modulus of the membranes are shown in Table 3. The relaxation processes of SPES-Na and SPES-H membranes are also studied. Dry membranes show higher values for both E 0 50 °C and T a than the wet samples. The E 0 50 °C values obtained by DMA are >2000 MPa for all membranes in the H + form, being higher than those reported for similar membranes [46,47]. The lower values for T a obtained for wet membranes reveal the loss in rigidity associated with the presence of water molecules in the membrane. The effect that the SPSU/PPSU ratio has on both E 0 50 °C and T a parameters is also studied. Thus, E 0 50 °C decreases in both types of membranes (dry and wet) when the percentage of SPSU blocks increases. However, in T a this effect is less pronounced due to, the possible formation of chemical species (e.g., anhydrides), which may act as crosslinkers between the polymer chains. These species delay the destabilization of the electrostatic interactions, and therefore increasing the transition temperature [47,48].In general, the presence of SPSU blocks, highly sulfonated, seems to significantly reduce the rigidity of these materials. Ex situ proton conductivity The ionic conductivity of the membranes is measured ex situ in a homemade ionic conductivity cell by electrochemical impedance spectroscopy. The Nyquist plot of SPES 75/25 at 70 °C and 90% RH is shown, as an example, in Fig. S5A. Al well as the admittance versus frequency plots of this membranes in the temperature range from 30 to 90 °C (S5B). All membranes here studied follow a similar behavior. Ionic conductivity of the material is obtained from the inverse of the intersection of the Z real component axis in the Nyquist plot. Fig. 4 shows the evolution of proton conductivity of SPES copolymers with temperature at 90% RH. (Proton conductivities are displayed in Table S2). We can emphasize the significantly lower conductivities obtained, from R.T. to roughly 50 °C, with the copolymers having lower hydrophobic blocks content. It is observed in the studied range that the SPES 75/25 copolymer exhibits higher ionic conductivity values than SPES 60/40. At 80 °C, the membranes reached 19.5 mScm 1 for SPES 60/40; 25.4 and 24.6 mScm 1 for SPES 50/50 and 75/25, respectively. At high temperatures, i.e., around the operating temperature of MEAs, there is not a clear tendency on the ionic conductivity when the PSU/PPSU ratio increases. Thus, SPES 60/40 shows lower proton conductivity than SPES 50/50, but SPES 75/25 exhibits similar values at high temperatures. Activation energy (E a ) for the proton transport across the membrane is determined though Arrhenius equation as follows: ln r ¼ln r 0 E a RT ð7Þ where r 0 is the pre-exponential factor, R is the ideal gas constant. The obtaioned Ea values are 40 and 20 kJmol 1 for SPES 60/40 and 75/25, respectively. These values are higher than found for SPES 50/50 (9 kJmol 1 [21]). These results suggest that a vehicular mechanism prevails in membranes containing higher amount of PSU blocks where proton interacts with water molecules, which transfers in the form of hydrated hydrogen ions, such as H 3 O + , H 5 O 2 + and H 9 O 4 + species, similar to molecular diffusion [49]. However, in SPES 50/50 Grotthuss mechanism seems to be predominant (Ea <15kJmol 1 ). In this mechanism water can assist proton hopping from one proton carrier to neighboring one down a chain of hydrogen-bonded network. The same behavior obtained on styrene-ethylene-butylene-styrene (SEBS) triblock copolymers exhibit a predominant vehicular mechanism of proton transport [50]. The combination of both mechanisms had been described in proton exchange membranes as Nafion Ò [51,52]. Fuel cell test In situ ionic conductivity of SPES membranes is measured by impedance spectroscopy directly in the MEA. Table 4 shows the ionic conductivity ( r ) of SPES 50/50, 60/40 and 75/25 membranes at different temperatures from 50 °Cto80°C, atmospheric presTable 3 WU%, Tensile Strength (TS), Elongation at Break ( e ), Storage Modulus at 50C(E0 50C ) and Temperature associated with a a Relaxation (T a ) (from DMTA Experiments) Membrane WU % TS b (MPa) e a % E 0 50 °C (MPa) T a DMTA (°C) 30 °C60°C Dry Na + Wet H + Dry Na + Wet H + Dry Na + Wet H + Dry Na + Wet H + SPES 50/50 a 24 ± 1 31 ± 2 87 ± 8 56 ± 5 3.7 ± 0.4 - 5004 3424 230 223 SPES 60/40 49 ± 2 56 ± 1 51 ± 3 50 ± 1 1.6 ± 0.1 5 ± 2 4000 2100 250 225 SPES 75/25 42 ± 2 57 ± 3 53 ± 5 41 ± 7 2.8 ± 0.5 3 ± 2 2300 2000 240 230 Nafion 112 c - 28.5 d 19 - - - - - 230 e - a Ref. [19]. b T=30°C. c TS determined in H+ form. d Ref. [40]. e Ref. [40]. Fig. 4. Ex situ proton conductivity of the SPES membranes as a function of temperature at 90% RH. S. Swaby, N. Ureña, María Teresa Pérez-Prior et al. Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 373 sure, and 100% RH by using humidified gases. Additionally, in situ proton conductivity of Nafion 112 used as a reference material and measured under the same experimental conditions is also shown. The ionic conductivity of all membranes increases with temperature. Although the highest ionic conductivity is observed for 50/50 at 90 °C, their conductivity decays and membranes with high SPSU/PPSU ratio can maintain the conductivity. Fig. 5 shows the polarization (A), and power density (B) curves of SPES 50/50, 60/40, and 75/25 membranes at 80 °C and 100% RH. The highest performance in the fuel cell is achieved for SPES 75/25 membrane. Their maximum power and current density values are 600 mWcm 2 and 1950 mAcm 2 , respectively. If we compare these values with those obtained for the 50/50 membrane, we can observe that the maximum power density is almost doubled. MEA cyclability tests were performed for the MEA with SPES 75/25, the membrane with the best performance and highest maximum power density, to evaluate the efficiency, reliability, and level of degradation of the membrane in the single cell determining the degradation of voltage over time through cycles of current variation and keeping always the cell operating. These cycles consist of maintaining the MEA for 10 minutes at open circuit (OCV); then 15 minutes at 2.5 A (500 mAcm -2 ) and, finally, 15 minutes at 5 A (1000 mAcm -2 ), repeating these conditions during 9 cycles to evaluate the stability of the MEA (see Fig. 6). These load values correspond to the current producing around 50% and 80% of the maximum power density obtained in the performance tests, respectively at 70 °C (temperature cell), atmospheric pressure, and relative humidity of 100%. By applying two steps of external load (2.5 and 5 A), it can be observed in Fig. 6 that the voltage responds quickly to increases in current and then reach a stable state. The second load step (5 A) which is approximately the current leading to 80% of the maximum power density of performance tests is slightly higher cycle by cycle indicating a good dynamic cyclability behavior of this membrane. Another test was also carried out for this membrane under the same experimental conditions, setting a constant voltage of 0.55 V for 13400 s and measuring the stability of power density values that remain almost constant over time as it is shown in Fig. 7. The influence that temperature has on the fuel cell performance is also analyzed for SPES 75/25 membrane (see Fig. 8). The highest value for maximum power density is observed at 70 °C (670 mW cm 2 and 2200 mAcm 2 ). These results are quite interesting because although they are lower to those reported for Nafion 112 (power density of 729 mWcm 2 and current density of 2400 mAcm 2 at 70 °C and 100% of RH) they are clearly higher than those published for Nafion 117 (power density of 310 mWcm 2 and current density of 999 mAcm 2 at 70 °C and 100% of RH) [21]. These results are also in the same range than those reported on multiblock copolymer based on poly(p-phenylene)-co-poly(ary lene ether sulfone ketone) with sulfonated multiphenyl pendant group which showed a maximum power density of 800 mWcm 2 and a maximum current density of 1400 mAcm 2 at 80 °C and 100% RH [29]. The fuel cell performance of SPES 75/25 at 70 °C and 0.6 V provides a current density of 1000 mAcm 2 which is higher compared to Nafion Ò at 75 °C and 0.6 V 450 mAcm 2 [53]. From these comparative MEA tests, it can be concluded that the SPES 75/25 membrane exhibits promising performance for fuel cell applications. Fig. 9 shows a comparative of the three membranes synthetized on ionic conductivity, power density and water uptake. It can be seen that the ex situ proton conductivity of the membranes follows the tendency SPES 50/50 > SPES 75/25 > SPES 60/40. On the other hand, the power density increases as the water uptake due to the incorporation of PSU blocks which provide higher hydrophilicity on the SPES 60/40 and 75/25. Table 4 In situ proton conductivity ( r ) of SPES and Nafion 112 membranes. T (°C) r SPES 50/50 (mS cm 1 ) a r SPES 60/40 (mS cm 1 ) r SPES 75/25 (mS cm 1 ) r N112 (mS cm 1 ) 50 19.7 14.0 28.2 - 60 25.8 18.7 30.1 33.0 70 34.1 21.7 31.8 34.0 80 29.8 24.3 31.3 34.3 a [14]. Fig. 5. (A) Polarization and (B) power density curves of SPES 50/50, 60/40 and 75/25 membranes at 80 °C and 100% RH. (thickness 50 l m). S. Swaby, N. Ureña, María Teresa Pérez-Prior et al. Journal of Industrial and Engineering Chemistry 122 (2023) 366–377 374