Introduction

Proton exchange membrane fuel cells are considered as environmentally friendly because decreasing of long-term dependence on oil and other fossil fuels are in the alternative source of energy [1]. It also has attracted extensive attention because of potential application in portable electronic devices such as fuel cells, batteries, electrolysis, desalinations, and separations [2,3,4,5,6]. The proton exchange membrane fuel cells (PEMFCs) are one of the most important applications of PEM, which have promising power applications such as vehicular applications [7]. PEMFCs have lot of benefits such as high-temperature operation, environmentally friendly, high energy density, and appropriate fuel cell storage [8]. For the above applications, lots of PEM have been developed. Nowadays, Nafion is a benchmark material used in PEMFCs. Some major draw backs are available in the Nafion such as it cannot operate at high temperature, water management problem, high cost, and low mechanical stability. Since recent years, there has been an effort to develop a low-cost inorganic and organic polymer composite membrane. In recent years, several kinds of proton exchange membranes such as sulfonated poly(ether sulfone) (PES) [9], sulfonated poly(arylene ether sulfone) [10], and sulfonated poly(benzimidazole) [11] have been studied. In hybrid organic-inorganic composite membranes, the inorganic part usually offers good thermal and mechanical stability, while the organic functional groups such as SO3H gives specific chemical reactivity [12]. The aromatic hydrocarbon polymer poly(ether ether ketone) is a semi-crystalline polymer that has a high chemical and thermal stability. It is a high-performance thermostable engineering polymer with non-fluorinated backbone. The proton conductivity of sulfonated poly(ether ether ketone) (SPEEK) can be easily controlled by the degree of sulfonation (DS) and DS can be balanced by sulfonation criteria such as reaction time, temperature, and concentration of sulfuric acid. In order to give hydrophilicity in the pure PEEK polymer, it is sulfonated with the help of sulfuric acid. The hydrophilic/hydrophobic phase separation in SPEEK is narrower and more branched, which results in low electroosmotic drag and low methanol permeation [13, 14]. Sulfonated poly (ether ether ketone) displays numerous advantages such as excellent film-forming property, excellent thermal stability, high mechanical strength, and sufficient proton conductivity that all depend upon the degree of sulfonation. To this end, the membranes require to be suitably composed with hygroscopic fillers such as TiO2, SiO2, Fe2O3, and alumina nanostructures which have been largely studied over the past decade for use in PEM of high-temperature fuel cells [15,16,17,18]. Nowadays, perovskite structured materials are required in the fuel cell. Proton conductors with ABO3 structure have been particularly studied due their high proton conductivity, moderate chemical stability, and excellent mechanical and thermal stability. The perovskite currently expressed by ABO3 is structurally stable because of its well-balanced geometrical array of constituent atoms and their valences [19]. Perovskites can be synthesized with a widespread variation of array of chemical elements because cations of large site (A) and small (site B) ionic radius are well suitable in crystalline structures. Perovskite structures from group II elements such as Sr, Ba, and Ra can be used as hygroscopic materials to improve the humidity sensing. So perovskite materials are predominantly proton conductors at high temperatures. Recently, barium zirconate is widely used for proton exchange membrane fuel cells. BaZrO3 is a well-known proton conductor with ABO3 perovskite structure which has lattice parameter of a = 4.2 Å, high melting point, a small thermal coefficient, and excellent mechanical and thermal stability. BaZrO3 is a well-known refractory material with very high melting point (2600 °C) and low chemical reactivity toward corrosive compounds [20]. It has excellent thermal stability and resistance due to a low coefficient of thermal expansion (α = 87 × 10−7/°C). In the present work, SPEEK-based nanocomposite membranes are prepared by solvent casting technique and characterized by XRD, FTIR, NMR, IEC, etc. The best composite membrane has been identified in terms of current density and power density applications.

Experimental

Materials and method

Poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4 phenylene) (PEEK; Mw = 20,800, Victrex (USA)), N-methyl-2-pyrrolidone) (NMP; 99%), concentrated sulfuric acid (H2SO4) (98%), and methanol (99%) were purchased from SRL (India), and barium nitrate (99.8%), zirconium nitrate (99%), and cetyltrimethylammonium bromide (95%) were procured from Alfa Aesar (India).

Sulfonation reaction process

Ten grams of PEEK was added into 200 mL of sulfuric acid and the mixture was subjected to vigorous stirring under the N2 atmosphere at room temperature for 7 h. The obtained solution was poured into ice-cold deionized water to precipitate the sulfonated polymer (SPEEK), followed by filtration and thorough washing of the filtrate using deionized water until neutral pH was obtained. It is important to remove even the last trace of acid as it may affect the product. The obtained SPEEK was dried in a vacuum oven at 90 °C for 24 h [21]. The synthesis process can be explained by Scheme 1.

Scheme 1
scheme 1

Sulfonation process for SPEEK

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Synthesis of BaZrO3

Barium nitrate (2 g), zirconium nitrate (1 g), and cetyltrimethylammonium bromide were dissolved in a solution of ethanol water solution at room temperature and the resultant solution was vigorously stirred for 30 min. Under strong stirring, sodium hydroxide was added and the solution was stirred again for 1 h at room temperature. The hydrothermal reaction was carried out in a Teflon autoclave under the autogenous pressure. For different reaction times, the precipitate was then washed with distilled water and dried in oven at 100 °C followed by calcination at 1000 °C [22].

Polymer nanocomposite membranes

SPEEK was dissolved in 20 mL of NMP under continuous stirring to form a homogeneous viscous solution. Interaction of typical SPEEK-BaZrO3 composite took place when BaZrO3 is added into the solution. The resulting solution was cast in a petri dish and the solvent was evaporated at 70 °C under vacuum for 14 h. The thicknesses of the membranes were found to be ~ 160 μm.

Pre-treatment of membrane

The membranes were immersed in 1 M H2SO4 solution for 4 h at 30 °C to activate the acidic functional groups. Finally, those activated in the membranes were immersed in DI water for 24 h to remove the free acid [23].

Instrument and apparatus details

The crystallinity of the proton-conducting polymer electrolytes was studied by XRD pattern with the help of X’Pert PRO PANalytical X-ray diffractometer. The complex formation between SPEEK and BaZrO3 was identified by FTIR spectrophotometer in the range of 400–4000 cm−1. NMR spectra were recorded on a Bruker 500 MHZ NMR spectrometer. Morphology of the composite membrane was examined by a high-resolution scanning electron microcopy (HRSEM) (FEI Quanta 250 microscope, Netherlands). Thermal stability of the polymer electrolytes was carried out using TGA model (SDT Q600) by heating under N2 atm. The surface topography and roughness factor of the sample were observed by atomic force microscope with the help of AFM (A100SGS). The elemental analysis was carried out using CHNSO elemental analyzer using elemental Vario EL III Germany. A catalyst ink was prepared by a conventional method. A combination of 0.2 g platinum nominally 40% on carbon black (Alfa Aesar), 6 g of isopropanol, and 5 wt% Nafion solution was mixed in catalyst ink slurry that was mixed in an ultrasonic bath overnight to reduce the catalyst powder and got in a homogeneous ink. Catalyst membranes were prepared by hand painting this catalyst ink that was applied on both sides of the membrane with an air brush. The Pt:C loading was 0.5 mg cm−2 for both sides of the membrane. The composite membrane was then sandwiched between the electrodes by hot pressing [24]. MEAs with an active area of 5 cm−2 were prepared by incorporating two gas diffusion layers (GDL sigracet GDL 10BB, 420 μm, SGL carbon group) on both sides of the catalyzed membrane and flow field is serpentine type. The evaluation of fuel cell performance at 90 °C by using fuel cell model 850e (Scribner Associates, Inc.). The polarization curves were recorded at atmospheric pressure using humidified oxygen (0.2 L min−1) and hydrogen (0.2 L min−1) with dry conditions. Polarization curves were obtained by using current step potentiometry.

Measurements

Proton conductivity

The prepared polymer composite was subjected to AC impedance analysis in order to calculate the proton conductivity with the help of computer-controlled microautolab type III potentiostat/galvanostat in the frequency range of 100 Hz–300 KHz over the temperature range of 303–353 K. The following equation was used for the calculation of proton conductivity:

$$ \sigma =\frac{l}{R_bA} $$

where A = 3.14 cm2 is the cross-sectional area of the membrane, l is the thickness of the membrane, and RB is the bulk resistance. The measurement was taken at different temperatures respectively.

Water uptake and swelling ratio

Water uptake and swelling ratio of the composite membranes were performed by dipping the composites in water for 2 days. The prepared polymer composite membranes were dried in vacuum oven for 24 h at 80 °C. The prepared composite membranes were dipped in distilled water to determine the water uptake of the membrane. Weight of the wet membranes was measured after drying the sample overnight in a vacuum at 80 °C. The water uptake of the membrane was calculated from the following equation

$$ \mathrm{Water}\ \mathrm{uptake}\ \left(\%\right)=\left(\frac{W_{\mathrm{wet}}-{W}_{\mathrm{dry}}}{W_{\mathrm{dry}}}\right)\times 100 $$

Here Wwet and Wdry are the weight of the wet and dry membranes respectively.

The swelling ratios of the membranes were calculated as follows:

$$ \mathrm{Swelling}\ \mathrm{ratio}\ \left(\%\right)=\left(\frac{L_{\mathrm{wet}}-{L}_{\mathrm{dry}}}{L_{\mathrm{dry}}}\right)\times 100 $$

where LWet and LDry are the lengths of the wet and dry membranes respectively.

Ion exchange capacity

Classical back titration technique was used to determine the IEC of the membranes. First, the membranes in the acid form (H+) were converted to sodium form by immersing the membranes in 1 M NaCl solution for 24 h at 40 °C [25]. Then the exchanged H+ ions within the solutions were titrated with 0.0 1 M NaOH solution using phenolphthalein as an indicator [26]. The IEC values (meq g−1) of the membranes can be calculated from the following equation:

$$ IEC=\frac{\left({V}_{\mathrm{NaOH}}\right)\left({N}_{\mathrm{NaOH}}\right)}{W_{\mathrm{dry}}} $$

where VNaOH is the volume of NaOH solution, NNaOH is the normality of NaOH solution, and Wdry is the mass of the dry sample.

Oxidative stability

The chemical stability was measured by measuring the weight loss of the proton-conducting composite membrane (2 × 2 cm) in Fenton’s solution (3 wt% H2O2 containing 2 ppm FeSO4) at 60 °C for 1 day.

Results and discussion

Degree of sulfonation

  1. (A)

    IEC

The degree of sulfonation (DS) is defined as the percentage of repeating units of PEEK that has been sulfonated and calculated by using the following equation.

$$ DS\left(\%\right)=\frac{M_{\mathrm{PEEK}}\times IEC\times 100}{\left(1000-\left({M}_{\mathrm{SPEEK}-}{M}_{\mathrm{PEEK}}\right)\times IEC\right)} $$

MPEEK and MSPEEK are the molecular weights of PEEK and SPEEK repeating units and the values are 288 and 368 respectively [27] [28] by substituting these values into the above equation. The number (80) resulted from the difference between these two unit molecular weights.

$$ DS\left(\%\right)=\frac{288\times IEC\times 100}{\left(1000-80\times IEC\right)} $$

The degree of sulfonation was found to be 65%.

  1. (B)

    Nuclear magnetic resonance spectroscopy

Figure 1 represents hydrogen-nuclear magnetic resonance (1H) spectroscopy that was used to calculate the DS and it was calculated using comparative integration of distinct aromatic signals according to the following equation:

Fig. 1
figure 1

1Proton NMR Spectrum of SPEEK

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$$ {\displaystyle \begin{array}{l}\frac{n}{12-2n}=\frac{SH_{13}}{\sum {SH}_{\left(1,2,3,4,5,6,7,8,9,10,11,12,13,14,15\right)}}\\ {}\left(0\le n\le 1\right)\end{array}} $$

where n is the number of H13 repeat unit. SH13 is the area under the graph for H13 region and integrated peak area ∑SH(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15) of the signals corresponding to all aromatic hydrogen [29]. The degree of sulfonation is obtained by the equation

$$ DS=n\times 100\% $$

The presence of sulfonic acid group causes a characteristic singlet (H13) at 7.5 ppm.

The DS obtained was 65% for the prepared SPEEK

  1. (C)

    C H N S O analyzer

The elemental content of the membrane is reported as weight percentage. Carbon is 65.5%, hydrogen 5.23%, sulfur 4.95%, and empirical formulae C35H34O10S1. From the sulfur content, the DS was calculated by the following equation:

$$ DS=\frac{S_E\times 100}{S_T} $$

where SE and ST represents the experimental and theoretical value of (–SO3 H) per repeat unit of PEEK and sulfur content of PEEK [30]. The DS is calculated as 65% for the prepared SPEEK.

XRD analysis

A phase change helps to take obtain from XRD analysis due to the polymer sulfonation and addition of BaZrO3 nanoparticles in the polymer composites. Figure 2(A–G) shows the XRD pattern of PEEK, SPEEK, BaZrO3, and polymer composite membrane. From Fig. 2(F), four major peaks of pure PEEK are found at 2θ = 18.87°, 20.7°, 23°, and 28.9° which correspond to the diffractions of (110), (111), (200), and 211 crystalline planes [31]. Sulfonation of PEEK changes the chain conformation and thus brings about loss of crystallinity which reveals a mixture of both amorphous and crystalline nature with a broad peak at 20°. Figure 2(A) shows the XRD pattern of BaZrO3 nanoparticle. Characteristic peaks at 2θ = 30.11°, 37.10°, 43.10°, 48.51°, 53.49°, 62.61°, 71.03°, 75.08°, and 79.08° confirmed the presence of BaZrO3 [21]. Figure 2(B–E) reveals that the amorphous nature of SPEEK increased with the incorporation of the BaZrO3 nanoparticles; the degree of crystallinity is calculated using the relation

Fig. 2
figure 2

XRD spectra for (A) BaZrO3, (B) 98 SPEEK-02 BaZrO3, (C) 96 SPEEK-04 BaZrO3, (D) 94 SPEEK-06 BaZrO3, (E) 92 SPEEK-08 BaZrO3, (F) PEEK, G-SPEEK

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$$ K=\frac{S}{S0}\times 100\% $$

where s denotes sum of areas of all the crystalline peaks and s0 represents sum of areas of crystalline peaks and amorphous hump. The SPEEK corresponds to the crystallinity value of 3.368%. It is seen that the percentage of crystallinity of the nanocomposite membranes decreased up to 6 wt% of BaZrO3. For the prepared samples A, B, C, and D, the degree of crystallinity is found to be 2.102, 1.132, 0.956, and 1.323% respectively. Hence, the absence of peaks pertaining to BaZrO3 in the complexes indicates the thorough mixing of nanoparticle in the polymer matrix is completely dispersed. In 8 wt% BaZrO3 in the prepared polymer composite, the peak intensity increases due to aggregation of BaZrO3 nanoparticles that has occurred within the polymer matrix.

FTIR analysis

Figure 3(A–C) shows the FTIR spectra of PEEK, SPEEK, and BaZrO3. The vibrational peaks observed at 1655 cm−1 (C=O), 1509 cm−1 (–CH), 1244 cm−1 (C–O–C), and 1484 cm−1 (C–C) are the characteristic peaks of PEEK. The peak identified at 1484 cm−1 indicates the presence of C–C aromatic ring [32]. The presence of broadband observed at 3427 cm−1 is assigned to the O–H stretching vibration of sulfonic acid groups. The vibrational bands related to the sulfonic acid groups appearing at 1220 cm−1 (asymmetric O=S=O), 1082 cm−1 (symmetric O=S=O), 1024 cm−1 (S=O) stretching, and 710 cm−1 (S–O) stretching confirm the successful synthesis of SPEEK [33]. The presences of vibrational bands at 400–750 cm−1 correspond to the metal–oxygen stretching vibrations of ABO3 perovskite [34]. The small peaks around 2350 cm−1 observed in the sample may be to atmospheric CO2. Figure 4(A, B) represents the FTIR spectra of 98 wt% SPEEK-02 wt% BaZrO3 and 94 wt% SPEEK-06 wt% BaZrO3 respectively. The vibrational peaks observed at 1640–1650 and 3450 cm−1 are due to bending water molecule present in the prepared polymer electrolytes. The absorption peak at 3450 cm−1 is present in all the prepared composites. The vibrational peaks observed at 1020 and 1078 cm−1 are attributed to the S=O=S vibrations. The vibrational peak observed at 1470 cm−1 may be attributed to C=C stretching of the aromatic ring [35]. The absorption peak at 1645 cm−1 corresponds to the carbonyl group of SPEEK. In the prepared polymer composites, the peaks at 400–750 cm−1 are observed at metal–oxygen stretching vibrations at the B site of ABO3 perovskite. The present polymer electrolyte is due to the formation of hydrogen bonds between sulfonic acid group of SPEEK and functional group of BaZrO3.

Fig. 3
figure 3

FTIR spectra for (A) PEEK, (B) SPEEK, and (C) BaZrO3

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Fig. 4
figure 4

FTIR spectra for (A) 98 SPEEK-02 BaZrO3 and (B) 94 SPEEK-06 BaZrO3

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Physico-chemical properties (water uptake, swelling, and IEC properties)

Water uptake is one of the important properties for PEMs, which plays an important role in proton transport. The water uptake for the sulfonated polymer also increases with increasing amount of degree of sulfonation. Water uptake, swelling, and IEC values are shown in Table 1. Water uptake of the SPEEK membrane has been obtained as 35.2% at room temperature with the incorporation of BaZrO3. Water uptake of the nanocomposite membrane is increased due to water retention character and hydrophilic nature of the nanoparticle which is due to the formation of hydrogen bonding between the surfaces of the nanoparticle. However, 08 wt% filler content water uptake started decreasing due to the crystallization in the polymer matrix [36]. The membrane swelling is another important property of composites to specify the dimensional stability of PEMs. The composite with a high membrane swelling lead to low mechanical stability and low durability and it will decrease the cell performance. The swelling of the composite membrane decreased with the addition of BaZrO3 nanoparticle. Similar results have been observed in SPEEK-based composites in the literature [37]. Electrochemical properties of the composite mainly depend on the IEC and water uptake. IEC value depends on the concentration of sulfonic acid group present in the polymer electrolytes. At higher level (08 wt%) BaZrO3, the IEC value is decreased in the system due to the particle agglomeration and blocking effect of BaZrO3 nanoparticles.

Table 1 The water uptake, membrane swelling, and IEC values for prepared polymer composites
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AC impedance analysis

Proton conductivity is one of the most important properties which enable membranes to be applicable in PEMFC. Proton conductivities of SPEEK and nanocomposite membranes are shown in Fig. 5. It is found that 94 wt% SPEEK-06 wt% BaZrO3 membrane exhibits high proton conductivity value as 3.12 × 10−1 S/cm at 90 °C. As temperature increases, the proton conductivity values are found to increase. BaZrO3 nanoparticles have proton conduction, which results from their ability to dissolve protons from water in wet atmospheres [38]. Protons are incorporated into BaZrO3 nanoparticles by dissociative adsorption of water molecules at the surface, followed by diffusion toward the inner nanoparticles. There are three proton conduction pathways in cubic BaZrO3 such as three-dimensional O–H rotations across Zr4O4 square and two-dimensional O–H rotations within Zr4O4 square and O–O diagonal path. The schematic design of the proton transfer (T) and reorientation (R) pathways in cubic structure of BaZrO3 are displayed in Fig. 6. In the Grotthuss mechanism for proton transport in BaZrO3 nanoparticles, proton diffuses by an arrangement between a molecular reorientation (R) round the oxygen and jump of the proton from oxygen and it jumps from oxygen to the nearest neighboring ions [39]. The orientation of the O–H group directs the proton near the neighboring oxygen ion. The activation barrier for rotational diffusion is commonly low and hence these rotations are facile. The rotation period to a complete revolution is about 10–12 s and jumping time is about 9–10 s in which it is considered as the limiting step of conduction mechanism [40, 41]. In this case, an oxygen vacancy would be filled by hydroxide group from water dissociation, whereas the proton forms chemical bonding with lattice oxygen. The prepared polymer electrolyte using 94 wt% SPEEK-6 wt% BaZrO3 exhibits the highest proton conductivity. At 8 wt% BaZrO3, the proton conductivity is formed to decrease in the system due to the self-aggregation of BaZrO3 nanoparticles leading to decrease of the active surface of the nanoparticles, and hence, the proton conductivity of the composite membrane also decreased.

Fig. 5
figure 5

Nyquist plot for SPEEK and prepared polymer composite at RT

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Fig. 6
figure 6

Schematic illustration of the proton pathway in BaZrO3

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Temperature-dependent conductivity

For a hopping-like conduction mechanism, the conductivity follows the Arrhenius behavior.

$$ \sigma ={\sigma}_0\exp \left(\frac{-{E}_a}{k_BT}\right) $$

where σ is the proton conductivity of the membrane (S cm−1), Ea is the proton-conducting activation energy (KJ mol−1), kB is a Boltzmann constant, T is temperature, and σ0 is pre-exponential factor. Arrhenius plots of temperature-dependent proton conductivity are shown in Fig. 6, and the conductivity values are shown in Table 2. The activation energies of proton conductivity are calculated from the slope of the Arrhenius plots. The activation energy often indicates the prevailing mechanism for proton transport. The proton conduction mechanism of SPEEK-BaZrO3 is explained by a Grotthuss mechanism. From Fig. 7, the activation energy is found to decrease with increasing BaZrO3 nanoparticles from 12.01 to 10.61 KJ/mol. This indicates easier proton transfer with increase of BaZrO3. Improvement in the proton conductivity of nanocomposite membranes is belonging to interconnection of hygroscopic BaZrO3 with OH moieties in the polymer matrix and water channeling at the polymer/perovskite interface which can create more pathways for proton transfer. Hence, it is concluded that the proton transport in the SPEEK and SPEEK-BaZrO3 composite membranes occurs predominantly by the Grotthuss mechanism.

Table 2 Proton conductivity of prepared composites
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Fig. 7
figure 7

Arrhenius plot for pure SPEEK and prepared polymer electrolytes

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Oxidative stability

The oxidative stability is essential for the composite in PEM usage. The chemical stability test was performed in hydrogen peroxide and the observations are shown in Table 3. The oxidative stability of SPEEK and SPEEK-BaZrO3 composite membranes were examined by observing their dissolving characters in Fenton’s reagent at 80 °C. From Table 3, the addition of BaZrO3 nanoparticles to the pristine SPEEK membrane decreases its weight residue from 93.50 to 91.81% after Fenton’s test. The pristine SPEEK membrane began to break after 72 min. There was no weight loss observed in 92 wt% SPEEK-08 wt% BaZrO3.

Table 3 Oxidative stability for SPEEK and prepared polymer composites
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Thermogravimetric analysis

Thermal stability is one of the key parameters for the PEM to ensure a long-life guarantee of the PEMFCs. Thermal stability of the prepared polymer composites was analyzed by TGA from 50 to 1000 °C as shown in Fig. 8. The first weight loss occurred at around 100 °C which indicates the adsorbed water at the time of loading sample. The second weight loss region starting from 200 to 250 °C is attributed to the decomposition of sulfonic acid groups of SPEEK [42]. The third weight loss occurred in the region 380–550 °C that corresponds to the melting of the non-polar crystallite back bone of PEEK polymer. However, the presence of the BaZrO3 leads to increase of the decomposition temperature of nanocomposite membranes compared with SPEEK membranes. The perovskite with cubic BaZrO3 is a promising one in HT-PEM fuel cells due to its high melting point (2920 °C). In addition, BaZrO3 has excellent thermal stability and resistance due to a low coefficient of thermal expansion (α = 87 × 10−7/°C between RT and 1080 °C) [43]. The results obtained from the TGA show that 94 wt% SPEEK-06 wt% BaZrO3 polymer composite membrane exhibits excellent thermal stability and is a potential candidate for HT-PEM fuel cell applications with appropriate high conductivity as well as good thermal stability.

Fig. 8
figure 8

TGA curve for SPEEK, (A) 98 SPEEK-02 BaZrO3, (B) 96 SPEEK-04 BaZrO3, (C) 94 SPEEK-06 BaZrO3, and (D) 92 SPEEK-08 BaZrO3

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Scanning electron microscopy analysis

The surface morphology and dispersion of BaZrO3 nanoparticles in the SPEEK composite membranes were investigated by scanning electron microscopy (SEM) analysis. Figure 9a shows smooth and uniform surface morphology of SPEEK and without any obvious agglomeration that occurred in the polymer matrix. Figure 9b shows the surface image of BaZrO3, and it depicts the crystalline nature and also agrees with XRD [20]. BaZrO3 nanoparticles are uniformly distributed in the system; Fig. 9c–f uniform distributions of BaZrO3 nanoparticles in 94 wt% SPEEK-06 wt% BaZrO3 polymer composite membranes indicate the interaction between SPEEK and BaZrO3 nanoparticle which plays an important role in the proton conductivity. When the amount of BaZrO3 was increased to 8 wt%, there is agglomeration that occurred in the prepared polymer electrolyte system.

Fig. 9
figure 9

SEM images for a SPEEK, b BaZrO3, c 98 SPEEK-02 BaZrO3, d 96 SPEEK-04 BaZrO3, e 94 SPEEK-06 BaZrO3, and f 92 SPEEK-08 BaZrO3

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Atomic force microscopy surface topography

Surface topography was studied by employing atomic force microscopy (AFM) to explore the topographical characteristics of SPEEK and 94 wt% SPEEK-06 wt% BaZrO3 polymer electrolyte systems. Figure 10(A) shows the three-dimensional smooth surface of SPEEK. Figure 10(B) shows the increase of the number of pores in the proton-conducting polymer electrolyte after incorporating BaZrO3 nanoparticles. The average roughness (Rrms) parameter was calculated using the expression

Fig. 10
figure 10

AFM images for (A) SPEEK and (B) 94 SPEEK-06 BaZrO3

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$$ {R}_{\mathrm{rms}}=\left(\frac{1}{N}\sum \limits_{i=1}^N\left|{Z}_m-{Z}_i\right|\right) $$

where N is the number of deviations in height (Zi) from the profile means value (Zm) [14]. In addition, the rms roughness of the sample has been calculated and it is of the order of 55 nm. Hence, this type of proton-conducting polymer electrolyte can be used in fuel cell application to achieve better proton conductivity which is confirmed by AFM.

Fuel cell performance

94 wt% SPEEK-06 wt% BaZrO3 polymer composite membranes were also used for their fuel cell test in PEMFC in H2/O2 by making membrane electrode assemblies (MEAs). The catalyst loading on both anode and the cathode (active area = 5 cm2) was kept at 0.5 mg/cm2. Figure 11 depicts the cell voltage, current density, and power density curve for 94 wt% SPEEK-06 wt% BaZrO3 at 90 °C. The maximum current density of 180 m A cm−2 and power density of 153 m W cm−2 at a cell voltage of 0.5 V were obtained at 90 °C. The result suggests that 94 wt% SPEEK-06 wt% BaZrO3 composite membrane is a potential candidate for use in PEMFC.

Fig. 11
figure 11

Polarization curve for 94 SPEEK-06 BaZrO3 at 90 °C

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Conclusion

SPEEK-BaZrO3 polymer nanocomposite membranes were prepared by solvent casting technique. The degree of sulfonation of 65% was obtained for the SPEEK through NMR, IEC, and CHNSO studies. The structural functionalities and polymer chain interaction were identified by XRD and FTIR techniques. The composite membranes 94 wt% SPEEK-06 wt% BaZrO3 showed the maximum proton conductivity of 3.12 × 10−1 S/cm at 90 °C. The temperature-dependent proton conductivity of the above system seems to obey Arrhenius behavior and Grotthuss mechanism. The above composite membrane exhibited current density and power density of 280 m A cm−2 and 183 m W cm−2 respectively. It is concluded that the 94 wt% SPEEK-06 wt% BaZrO3 polymer composite membrane is a potential candidate for PEM fuel cell applications.