Composite proton-conducting membranes based on perfluorinated short side chain polymers modified with nanodiamonds having a hydrophilic surface owing to grafted carboxyl groups have been produced. Membrane films are prepared by precipitating the polymer from a dimethylformamide solution with introduced nanodiamonds. The proposed method for synthesizing the composites provides a high mechanical strength and an increase in the proton conductivity of the material with an increase in temperature in a 20–50°С range at rather low diamond concentrations (0.25–1 wt %). These features are important for using the composites in fuel cells at high temperatures.
Ion-exchange membranes hold a significant place in the operation of modern power facilities and sensor systems and can be used in various electromembrane processes (water treatment, electrolysis, etc.). An important function is performed by perfluorinated proton-exchange membranes comprising a hydrophobic polymer matrix and a hydrophilic phase, forming a branched network of pores, which inner surface is covered with ionic –SO3H groups providing proton transport upon water saturation of the membrane . These membranes are used in hydrogen fuel cells (FCs) in power facilities with a broad power range. The development of the hydrogen power engineering industry involves studies of membranes based on copolymers with various side chain lengths. Nafion-type long side chain membranes, which have been the focus of attention for 50 years, exhibit fairly high conductive and strength characteristics. However, researches of the last decade have shown the advantages of Aquivion short side chain membranes .
The development of proton-conducting membrane technologies in recent years has been associated with the possibilities of targeted synthesizing of ordered composite structures by introducing organic and inorganic nanoparticles into polymer matrices to improve the functional properties of the membranes [3–7] and achieve higher conductive and strength characteristics than those of the original polymers [8–10]. The synthesis of hybrid membranes based on perfluorinated proton-conducting membranes exhibiting a unique combination of electrochemical and physicomechanical properties and chemical and thermal stability for use in FCs is of particular interest. The possibilities of modifying perfluorinated sulfonic acid membranes—mostly Nafion and its analogs—with inorganic oxides, heteropoly acids, and organic fillers with acid groups were discussed in detail in [11–14].
In particular, some fillers, such as SiO2, contribute to a high water retention, tensile strength, and proton conductivity of Nafion and can lead to a decrease in methanol crossover owing to the smaller pore size . Metal oxides (e.g., TiO2) exhibit mechanical and temperature stability and hygroscopicity and, therefore, provide enhanced water retention properties and improved stability and performance of membranes in FCs. The introduction of ZrO2 into Nafion leads to a significant increase in the proton conductivity of the material owing to the high surface acidity of ZrO2 .
It was found for carbon materials that, despite the fact that carbon nanotubes exhibit electron-conducting properties, small amounts of these materials provide an increase in the mechanical strength and temperature stability of the membranes . Graphene oxide is of interest, which does not conduct electrons and can generate proton conductivity itself at the Nafion level. Graphene oxide introduced into Nafion by solution casting leads to an increase in the proton conductivity and a decrease in the methanol permeability of the membrane .
Fundamentally new nanosized fillers for the modification of perfluorinated membranes are detonation nanodiamonds (DNDs) with acid functional groups. For DNDs, the problems of synthesizing crystals without defective graphite-like carbon layers in the sp2-hybridization state around the diamond core were solved in . Perfect DND crystals (size of 4–5 nm), depending on the groups grafted onto the surface (–H, –OH, –COOH, etc.), have an adjustable positive or negative ζ-potential in hydrosols. Therefore they form stable aqueous suspensions; and above the threshold concentration (about 4–5 wt %) they are transformed into stable hydrogels with a spatial network of diamond particles (cell scale of ~40 nm) [16–19].
Nanodiamonds are able to significantly improve the morphology, mechanical and electrical properties, and other characteristics of polymers even in the case of a small amount of an ND filler . It was shown that the modification of various polymer materials, particularly fluorinated elastomers, with NDs provides an improvement of the strength properties of the materials (increase in the elastic modulus and tensile strength) . Using the example of a polysiloxane block copolymer, it was shown that the introduction of NDs leads to an improvement of the physicomechanical parameters of the material due to a more efficient organization of supramolecular structural units of the polymer .
These NDs are also promising as modifiers of perfluorinated proton-conducting membranes for improving their performance in FCs, which is particularly important for FCs consuming alcohol-based fuels. The introduction of fillers can decrease the crossover of the fuel and the oxidizer across the membrane, because hydrophilic NDs are able to form an additional network of conduction channels in the polymer matrix via associating in linear (branched) structures. It is reasonable to expect that protons in these membranes will intensively migrate along the surface of diamond particles by the hopping mechanism through proton adsorption sites and sulfonic acid groups of the side chains of the perfluorinated copolymer . The implementation of this mechanism is provided by the hydrophilic surface of the ND as a proton accumulator, because a water-sorbing free volume is formed at the ND–polymer interface. Under these conditions, proton transport occurs in two stages; initially, protons accumulate on the diamond surface; after that, due to the generated gradient, they overcome the diffusion activation barriers and migrate along the diamond particles bound in chains via the diamond–polymer interface. Thus, the composite is characterized by the formation of regular (planar) diffusion channels, unlike Nafion® and Aqiuvion® materials, which mostly have narrow (linear) channels due to the segregation and association of ionic groups, upon the formation of a network of free-volume elements between the polymer chains. An active diamond component is able not only to form regular stable channels, but also to structure the matrix, improving the mechanical strength, and increasing the service life of the material, while increasing its conductive properties.
The aim of this study is to search for methods of producing membrane composites using DND-modified perfluorinated polymers of the Aquivion type and analyze the physicochemical properties of the synthesized new materials for further development of effective proton-conducting membrane technologies.
Sample Synthesis Procedure
The matrix material was a perfluorinated membrane copolymer of the Aquivion type, which was synthesized using aqueous–emulsion technology [24, 25]. The copolymer synthesis process was implemented by the copolymerization of tetrafluoroethylene (TFE) with a perfluorinated sulfo-containing monomer (perfluoro(3-oxa-4-penten)sulfonyl fluoride) in a 0.45-L steel reactor, which was thermostatted at 40–60°C and equipped with an anchor stirrer, at an automatically controlled operating pressure of TFE supplied to the system in a range of 0.7–1.3 MPa. Tetrafluoroethylene was supplied directly into the emulsion of a presynthesized sulfonyl fluoride monomer stabilized with a perfluorinated surfactant (ammonium perfluorononaate).
The synthesis of the membrane material was based on the preparation of precursors of ion-exchange sulfonic acid groups in the form of sulfonyl fluoride groups of the copolymer, because this structure of the sulfo-containing monomer allows us to exclude the occurrence of side reactions during both copolymerization and storage of the monomer. In addition, it is a copolymer in the sulfonyl fluoride form that can be used for synthesizing extrusion film materials at high processing temperatures (up to 300°C) without degrading the sulfofluoride groups. For a copolymer in the sulfonic acid form, this processing mode is unacceptable because of the degradation of sulfonic acid groups and the high corrosive activity of the copolymer. For Aquivion membranes, the optimum equivalent weight (EW) of the copolymer varies in a range of 790–980 g/mol SO3H. The EW parameter is crucial for the complex of the electrochemical and physicomechanical properties of proton-conducting membranes.
Composites with NDs were obtained using an Aquivion-type Sh-1 membrane copolymer precursor with EW = 780 g/mol –SO2F, as determined by Fourier transform infrared spectroscopy, and a melt flow index of 0.8 g/10 min, as measured on an IIRT-5 instrument at 270°С (load of 21.2 N, a capillary diameter of 2.095 mm). Typically, the conversion of a copolymer precursor to a proton-conducting membrane involves the extrusion or molding of a copolymer melt in the –SO2F form and the subsequent conversion of it to the SO3H membrane form or the conversion of a copolymer from the –SO2F form to the –SO3Li(K,Na) form and its subsequent dissolution in an appropriate solvent, then the formation of a membrane from the copolymer solution, and its conversion to the –SO3H form . In this case, composites with DNDs were prepared by the solution casting of a membrane material and the conversion of the copolymer to the –SO3Li form using dimethylformamide (DMF) as a solvent.
To prepare composite membranes with DNDs, both the possibility of adding DNDs into a perfluorinated membrane copolymer solution immediately before the membrane formation (physical method) and the version of a deeper modification of the polymer with the introduction of DNDs at the phase of our copolymerization of fluoromonomers (chemical method) were considered. At the current phase of our research, the physical method was used. A Sh-1 membrane, which is the analogue of the Aquivion E-87 brand produced by Solvay Solexis, was used. The membrane was characterized by the following parameters: EW = 890 g/mol –SO3H, which was determined from ion-exchange capacity; an equilibrium water content of 37.0 wt %; and a proton conductivity of 0.145 ± 0.002 S/cm at 20°С and a relative humidity (RH) of 100%.
The following procedure was developed to synthesize composites with DNDs. The copolymer solution was prepared as follows: initially, a powder of the Sh-1 copolymer in the –SO2F form was subjected to alkaline hydrolysis with a 5% LiOH aqueous solution at a twofold excess of alkali and a temperature of 90°С; the reaction mixture was stirred for 2 hours. The resulting polymer was filtered off and washed with deionized water from LiOH and LiF residues; the polymer was then dried to 50% humidity (by weight). The hydrolyzed polymer in the –SO3Li form was dissolved in freshly distilled DMF at 90°C under stirring for 2 hours to obtain a transparent colorless solution of the Sh-1 copolymer in DMF with a concentration of 2 wt %.
The copolymer was modified using DNDs in the carboxylated form dissolved in DMF (concentration of 1 wt %), which was derived from a hydrosol of deagglomerated DNDs by drying and ultrasonic dispersion in DMF. The solutions of the copolymer and DNDs in DMF were mixed at room temperature for 30 minutes. The resulting homogeneous mixtures with a DND : copolymer weight ratio of 1/400–1/20 (0.25–5 wt % DNDs of the weight of the polymer) were filtered off in a vacuum through a 5-μm Schott filter.
Films of the Sh-1–DND composites were prepared by the solution casting method . After the formation of the films, the functional groups of the membrane composite materials were converted from the –SO3Li form to the –SO3H form by acid washing in 15% nitric acid. The resulting films had a thickness of 30–50 μm and a DND content of 0.25–5.0 wt %.
Particle sizes in DMF solutions were analyzed by dynamic light scattering (DLS) on a Zetasizer Nano ZS ZEN3600 analyzer (Malvern Instruments, United Kingdom).
The surface structure of the synthesized membranes was studied by atomic force microscopy (AFM) on a P47 instrument (NT-MDT, Russia) in the tapping mode at a scanning frequency of 1 Hz using an RTESPA MPP-11120-10 probe.
Proton conductivity was measured by the impedance spectroscopy method in the state of equilibrium water saturation at the maximum water content of the membrane achieved by boiling at 100°C for 1 h. A Z-3000X impedance meter (Elins, Russia) with a four-electrode circuit for connecting the measuring cell was used; the frequency range was 10–150 000 Hz.
The water content in the membranes (CW) was determined from the weight of the samples dried in a vacuum at 80°С to constant weight (Мdry) and subjected to equilibrium saturation in water at 20°С for 24 h (МW) according to the formula: CW = (MW – Mdry)/Mdry × 100%.
The mechanical properties of the composites were tested on an AG-100X Plus universal testing machine (Shimadzu, Japan) in the mode of the uniaxial tension of samples with a working length of 25 mm at a fixed rate of extension of the sample of 100 mm/min. The measurements were conducted at a fixed RH value of 50%, which was maintained by a Polaris PUH 8505 TFD air humidifier and analyzed using a GAL WS-1403 weather station at a temperature of 23 ± 2°С. The following characteristics of the material were determined in the tests: elastic modulus E, plastic limit σpl, ultimate tensile strength σbr, and ultimate strain εbr. These parameters were determined by averaging the test results of seven samples of each film.
RESULTS AND DISCUSSION
Dynamic Light Scattering
It was found that, in a concentrated polymer solution (sample 1, 2 wt %), the particle size distribution is monomodal in the range of 400–2000 nm; this distribution indicates the association of Sh-1 macromolecules (Fig. 1a). The peak maximum at ~1000 nm corresponds to the average size of these associations (Table 1). In the case of a copolymer–DND mixture (sample 2), the peak of the copolymer is preserved; the peak position remains unchanged. Hence, the components do not undergo significant binding. The smaller objects represented in the spectrum are DND clusters with characteristic sizes of 17 and 67 nm (Fig. 1a, Table 1). The bimodal distribution, where the larger size is almost three times higher than the smaller size, may be interpreted as a result of secondary aggregation, in which a small cluster forms a single-layer shell of similar particles around itself.
Structuring is specific for ND dispersions producing chain (branched) aggregates of similar scales composed of a few tens of particles in a polar (aqueous) medium . In this case, carboxylated DNDs, which in general are negatively charged in solution, are repelled from similarly charged polymer chains. This factor stimulates the mutual binding of diamonds along oppositely charged facets  on scales ~70 nm, higher than those in aqueous solutions of pure DNDs (~40 nm).
Similar effects were observed in diluted systems 3 and 4 (Fig. 1b). Upon tenfold dilution, the sizes of the Sh-1 copolymer associates decreased to 400–450 nm. The scale of DND aggregation is limited to a value of ~45 nm for a monomodal distribution. Individual particles and moieties of clusters are not detected in this case; this effect is characteristic of dilute DND solutions in water  and repeated in DMF. Thus, the DLS data suggest that, under the DMF solution conditions, the DND functional groups do not bind to the sulfonic acid groups of the copolymer or form weak bonds. However, diamonds are able to interact with the hydrophobic part of a polymer matrix, which appears during the drying of the solutions and the formation of membranes.
Atomic Force Microscopy
Atomic force microscopy data for the original diamond-free Sh-1 membrane show the presence of globules with a size of 100–150 nm on the membrane surface (Fig. 2a); this finding characterizes the hydrophobic part of the polymer matrix in accordance with [28, 29]. The hydrophilic part with sulfonic acid groups in the side chains of the copolymer occupies the gaps between the hydrophobic moieties to form a connected network of conduction channels penetrating the membrane volume.
In the presence of DNDs (1 wt %), the packing of polymer globules is preserved; however, it contains inclusions with the size two to three times larger than that of the globules; these inclusions can be interpreted as polymer–diamond particles, which, according to their size, can combine up to ten globules (Fig. 2b). An increase in the diamond fraction leads to the enlargement of the inclusions to a size of ~500 nm and their linear ordering (Fig. 2c). It should be concluded that the incorporation of DNDs into the membranes by the selected method of simple physical mixing of the components and further precipitation and drying of the resulting films does not provide a uniform distribution of DND particles on scales comparable to the size of the polymer globules. Diamonds are not uniformly distributed in either of the both phases; instead, they are concentrated around only some of the hydrophobic globules to form larger globules. Diamonds apparently have no affinity for either the hydrophilic or hydrophobic phases of the polymer; this assumption is confirmed by the results of DLS in DMF solutions. In solution, diamond–diamond interactions are more advantageous than diamond–polymer interactions. As a result, after the removal of the solvent and the formation of a hybrid membrane, the diamonds are condensed into a separate phase at the interface between the hydrophobic and hydrophilic parts of the membrane. In this case, they can occupy a part of the volume intended for the hydrophilic regions of the membrane and thereby block the conductive channels. At low temperatures, this factor leads to a decrease in conductivity. With an increase in temperature, the protons contained in the diamond phase obtain additional mobility and become involved in the conduction mechanism.
It should be noted that, in the future, diamonds should be introduced into a polymer directly during aqueous–emulsion copolymerization to achieve a finer structuring of the membrane matrices with diamonds.
The proton conductivity of the composites was studied in comparison with the conductivity of the original Sh-1 membrane by impedance spectroscopy in the state of equilibrium swelling of samples in water (RH = 100%). The measured electrochemical characteristics are listed in Table 2. In a 20–50°C temperature range, the effect of DNDs on the conductivity of the material is different. At room temperature (20°C), with an increase in the fraction of DNDs, the conductivity decreases, whereas at high temperatures (35, 50°C), the addition of the diamonds leads to an increase and then a decrease in conductivity (Table 2). The decrease in conductivity at 20°C is attributed to two factors: (1) a decrease in the water content in the composites and (2) the use of DNDs with functional acidic carboxylic and amphoteric hydroxyl groups . In fact, the fraction of water in the composite decreases by ~20 wt % even at a low DND concentration (0.25 wt %); however, with the further introduction of DNDs (up to 5 wt %), the water content remains at a level not less than 75% of the level for the original polymer (Table 2). The ability of the anions of the polymer matrix and DNDs to be hydrated and to retain bound water is determined by the heat of hydration of the anions. For sulfate anions, it is 265 kcal/mol; for carboxylate anions, it is three times less, 90 kcal/mol . The decrease in the proton conductivity of the composites is directly caused by the difference in the pK values of the functional groups of the matrix copolymer and the DNDs. Acids with a fluorine-containing substituent are close to strong inorganic acids (pK = 0.6) , which is the case of the sulfonic acid matrix copolymer, whereas hydrocarbon-containing acids in DNDs are classified as weak acids (pK = 4.8); this factor weakens the proton conductivity of the composites with the addition of DNDs. However, for these composites based on perfluorinated sulfonic acid copolymers (Nafion, Aquivion) and NDs, a significant increase in proton conductivity was observed at a low humidity . To interpret the effect, the authors of  used the hypothesis of limited elasticity of water-conducting pores of membranes, which was first proposed in ; the effect of diamond additives on conductivity was attributed to a change in the size of the pores and channels of the membrane material.
To a first approximation, the discussed factors (heat of hydration, pK) explain the decrease in conductivity at 20°C with an increase in the amount of diamonds added to the composite, taking into account a slight decrease in water content (Table 2). In this case, attention should be paid to the fact that the water content remains within a limited range (37–28 wt %), while the fraction of DNDs increases by more than an order of magnitude and the diamond–polymer interface with an area of up to ~20 m2/cm3 at a diamond content of 5 wt % is formed in proportion to this increase. In addition, upon the aggregation of diamonds inside the matrix, the formation of an additional free volume for the accumulation and retention of water is expected, which is important for the operation of membranes which conductivity should be maintained during heating to ~130°C. In this case, in the composites, the normalized conductivity SN(T) = S(T)/S20°C increases with temperature by more than a factor of 1.5 with the addition of a small (0.5 wt %) amount of DNDs (Table 2, Fig. 3).
The original copolymer shows an increase in proton conductivity (~40%) during heating; this feature is generally specific for perfluorinated sulfonic acid membranes. In addition, the introduction of small amounts of DNDs—not higher than 1 wt %—is sufficient to provide this effect (Fig. 3a). A further enrichment in diamonds leads to a decrease in the effect (Fig. 3b). The diamonds are apparently in the associated state in the membrane films, because in solutions, being mixed with the copolymer, they form aggregates with sizes of ~10–100 nm (Fig. 1); these namely aggregates will precipitate together with the polymer on substrates during samples preparation. The presence of these inclusions is confirmed by AFM data (Fig. 2). To improve the conductivity of the original matrices at high temperatures, it is sufficient to introduce small amounts of DNDs (up to 1%) to provide sizes of the precipitated diamond aggregates of a few tens of nanometers (Fig. 1b); thus, a composite material with small diamond inclusions was formed (Fig. 2b), unlike the version of highly enriched composites with significant inhomogeneities in the distribution of the DNDs (Fig. 2c).
To provide required conductive properties of the composites, it is important to optimize their morphology by introducing small diamond aggregates mostly of a linear (branched) structure into the polymer matrix by using mixtures diluted with respect to the diamond component to prepare membrane films. In the case of highly enriched mixtures, a highly heterogeneous filling of membranes with large diamond aggregates is inevitable. The blocking of the channel system of the matrix responsible for the conventional proton conduction mechanism of perfluorinated membranes can eventually occur. The requirements for the membrane material structure are also important to provide the mechanical and strength properties of the material and the membrane stability at high temperatures. These characteristics were analyzed in mechanical testing of the samples.
The synthesized composites are low-modulus polymer materials (elastic modulus of E = 200–250 MPa) with large marginal deformations (ultimate strain εbr = 230–340%). The stress–strain curves of all the tested materials show a clearly identifiable transition through the plastic limit at stresses σpl ~ 11–12 MPa and strains of ~5–6%. The further deformation processes of the tested materials exhibit an almost identical behavior. The curves do not have a pronounced region of neck propagation through the sample (either a local maximum, i.e., plastic limit): immediately after strains of 7–8%, the stress begins to increase sequentially with an increase in the strain of the samples; that is, the so-called strain hardening region of the material is observed. An increase in the filler concentration in the polymer matrix does not cause any regular changes in the mechanical characteristics of the material up to a DND concentration in the composite of 1%. A further increase in the fraction of the filler (2–5%) leads to an increase in the elastic modulus of the composite film to values of E ~ 250 MPa and a decrease in ultimate strain εbr and ultimate tensile strength σbr. For the composite containing 5% DNDs, the εbr value decreases to ~0.7 of the εbr value of the original Sh-1 material. The strength of the composite (σbr) decreases approximately to the same extent at an excessively high DND concentration.
It was shown in  that, for Aquivion membranes, a decrease in the amount of ion-exchange groups in the copolymer chain (increase in EW) leads to the weakening of the interactions between ionogenic groups and an increase in the crystallinity of the material and, eventually, to an increase in the elastic modulus and an improvement of the mechanical properties in general; therefore, the material becomes more promising for use as an FC electrolyte.
The data (Table 3) show that membrane filling with diamonds up to 2–5 wt % also makes it possible to solve the problem of increasing the Young’s modulus of the composite, which is equivalent to the effect of increasing the EW of the copolymer.
(1) Methods have been developed for the preparation of compatible solutions of a perfluorinated membrane material of the Aquivion type and DNDs with functional carboxyl, hydroxyl, and lactone groups in DMF for use in the synthesis of composite membranes by solution casting.
(2) Using the DLS and AFM methods, it has been shown that the functional groups of the membrane material and the DNDs do not interact either in their mixtures in DMF solutions or in the resulting composite membranes. Nanodiamond particles in the composite form conductive clusters at the interface between the hydrophobic and hydrophilic parts of the membrane copolymer.
(3) The introduction of DNDs into the structure of a sulfonic acid membrane composite leads to a decrease in the proton conductivity of the composite at a temperature of 20°C due to weakly acidic and uncharged carboxyl, hydroxyl, and lactone functional groups of the NDs. Conversely, with an increase in temperature to 35–50°С, the proton conductivity of the composites regularly increases by 40–70% depending on the degree of filling of the composite. At low degrees of filling (0.25–1.0 wt %) at 35–50°С, the proton conductivity of the composites becomes higher than that of the original membrane.
(4) The synthesized composites are low-modulus polymer materials characterized by large ultimate deformations and a high mechanical strength. An increase in the degree of filling of the material with a diamond component to 2–5 wt % leads to an increase in the elastic modulus of the composite film.
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This work was supported by the Russian Foundation for Basic Research (project no. 19-03-00249).
Translated by M. Timoshinina
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Primachenko, O.N., Kulvelis, Y.V., Lebedev, V.T. et al. Perfluorinated Proton-Conducting Membrane Composites with Functionalized Nanodiamonds. Membr. Membr. Technol. 2, 1–9 (2020). https://doi.org/10.1134/S2517751620010060
- proton conductivity