1. Introduction
Ion-exchange membranes are intensively used in fuel cells [
1], as well as in electrolysis, dialysis, electrodialysis, and other membrane methods for obtaining, separating, and purifying various mixtures in sensors and transducers during gas transportation [
2]. The most extensively applied materials for such membrane types are Nafion-117
®, produced by DuPont de Nemours (Wilmington, DE, USA), and its analogues, such as MF-4SC (LTD Plastpolymer, Saint Petersburg, Russia) and Dow (Dow, Midland, MI, USA) [
3,
4]. Incorporation in polymer frame-varying modifiers influences physicochemical properties, for example, ion transport, mechanical properties, diffusion, and electroosmotic permeability. One of approaches to improve efficiency of transport properties, as well as reducing fuel cell cost, is use of cheap natural modifiers capable of forming a nanoporous membrane structure, which is necessary for ion transport process. In this regard, aluminosilicate nanotubes (halloysite) and their variations, which are ecologically safe, obtainable, and cheap [
5], can be applied as a dopant. In addition, the halloysite structure has features that allow its modification [
6]. Halloysite has a positively charged inner lumen and a negatively charged outer surface [
6] that is used for intercalation and encapsulation tubes by various metals [
7,
8,
9]. Nanotubes, coated or intercalated with metal and metal oxides (TiO
2, Pd, Ni), are efficient mesoporous media for advanced catalysis [
8,
10,
11]. The ability of halloysite to be mixed with various kinds of polymers (e.g., polysaccharides, polyacrylates, polyamides, epoxy, poly(vinyl chloride), polyethylene) has been described in many research studies [
7,
12,
13]. Nanotubes form frameworks inside the polymer matrix, improving strength, and increasing elongation limits and porosity of membrane materials.
According to [
14], incorporation of halloysite nanotubes modified by platinum in ion-exchange membranes leads to advanced properties of membrane used as a solid electrolyte in fuel cells. Using halloysite as a dopant influences asymmetry of diffusion permeability and current–voltage characteristics (CVC), decreases the numbers of water transport, and increases the selectivity of hybrid membranes, and their power characteristics in the membrane–electrode assembly of the fuel cell [
15]. The theoretical base of the asymmetry effect for diffusion permeability of bi-layer membranes is established in our papers [
16,
17]. A search of effective solutions in the field of alternative energy sources is impossible without a detailed study of the fundamental principles of membrane processes.
The aim of the present paper is to investigate novel bi-layer membranes synthesized on the base of MF-4SC and halloysite nanotubes modified by Pt nanoparticles, to perform its characterization and to apply previously developed models, which will allow calculating asymmetry of membrane diffusion permeability and membrane CVC in order to reach the best quality for use in fuel cells and membrane switches (membrane relays or diodes).
2. Experiment
2.1. Materials and Instruments
Dehydrated halloysite nanotubes were obtained from Applied Minerals Inc., New York, NY, USA, solution of sulfopolymer of the MF-4SC in lithium form (7.2 wt % in dimethylformamide solution, with 0.98 mgeq/g exchange capacity) was purchased from Plastpolymer, Sankt-Peterburg, Russia.
For halloysite nanotubes modification following reagents were used: 3-aminopropyltriethoxysilane, APTES (99%, Sigma Aldrich, Saint Louis, MO, USA), hexachloroplatinic acid hexahydrate H2PtCl6·6H2O (99.9%, Sigma Aldrich), toluene (99.8%, Sigma Aldrich), sodium tetrahydridoborate NaBH4 (98%, Sigma Aldrich), and distilled water for rinsing.
Electron microscope JEM-2100 (JEOL, Tokyo, Japan) was applied for transmission electron microscopy (TEM) analyses at an accelerating voltage of 10 kV. Atomic force microscopy (AFM) studies were carried out by the scanning probe microscope SmartSPM®-1000 (AIST-NT, Novato, CA, USA) in semi-contact mode using fpN11 cantilever (beam length 130 µm, hardness 2.6–9.8 N/m, resonance frequency of 118–190 kHz, radius of curvature of the probe needles—10–25 nm).
2.2. Сlay Nanotubes Modification
Deposition of platinum nanoparticles onto the surface of the nanotubes was carried out by grafting NH2 groups on the outer surface of the halloysite nanotubes. For this purpose, we used APTES, halloysite nanotubes, and dry toluene. Nanotubes were diffused in concentrated APTES solution and left for 12 h with stirring. The obtained nanotubes were centrifuged, washed three times with toluene, and dried at 60 °C. The nanotubes modified by APTES were diffused in H2PtCl6∙6H2O solution in ultrasonic bath for 30 min followed by reduction by NaBH4. After the nanotubes were centrifuged, washed three times, and dried.
TEM was used to confirm the position of platinum onto synthesized nanomaterials.
Figure 1 demonstrates the presence of metallic nanoparticles and their clusters outside the halloysite nanotubes (over their outer surface). Two opposite surfaces of the bi-layer membrane having one layer modified with halloysite nanotubes and platinum nanoparticles obtained by AFM method is presented in
Figure 2. Regular white circular dots (elevations) and small dark areas (valleys) in
Figure 2a indicate minor defects appeared at pure membrane MF-4SC layer (without modifiers) contacted with glass during the drying procedure.
Figure 2b,c shows the modified layer surface presented in semi-contact and phase-contrast mode of AFM, respectively. In both figures, we see woven polymer chains. White dots and slightly curved white “worms” in
Figure 2c are halloysite nanotubes randomly and uniformly distributed in a polymer matrix.
2.3. Hybrid Membrane Synthesis
Bi-layer membranes were prepared by a novel approach that included two steps. First, to create thicker membrane layer (to be referred as 2-nd layer), the polymer solution was placed into a glass former and kept at least 2 h to ensure uniform distribution over the surface and removal of air bubbles. Then, the polymer solution was dried at a temperature 65 °C until completely solvent evaporation (about 1.5–2 h). The second step was coating of the 2-nd warmed up layer by the polymer solution with non- and modified nanotubes (to be referred as 1-st layer) by means of airbrush. Spraying was carrying out gradually to prevent dissolution of the 2-nd layer by new portions of the suspension solvent at a pressure 3 atm. Further, the 1-st membrane layer was dried at a temperature of 80 °C to remove residual solvent. After that, the film was neatly removed from the glass surface. The membrane was visually homogeneous over the entire area of both the sample surfaces (
Figure 3). The light-grey membrane color resulted from the color of halloysite nanotubes modified by Pt. The ratio of the width of layers 2 to 1 was 4:1, and the total thickness of the obtained membranes was approximately 160–220 μm. The content of the halloysite nanotubes was 4 wt % of the 2-nd membrane layer. The content of the modifying metal was 2 wt % of the nanotubes’ mass. Such percentage of platinum was chosen based on previous research [
18]. We conducted additional studies of the mechanical properties of synthesized hybrid membranes depending on the percentage of halloysite that confirmed the optimal value of halloysite nanotubes is equal to 4 weight percent (see
Section 2.4). This value corresponds to the largest Young’s modulus and the tensile strain among the composite films doped with halloysite [
19]. It means that such membranes have better mechanical characteristics. Three bi-layer membranes were prepared for investigations, and their composition is given in
Table 1.
Platinum nanoparticles (20–40 nm in diameter) were encapsulated on the outer surface of halloysite nanotubes. For simplicity, we will denote membranes as follows: No. 1: P/P + H + Pt; No. 2: P/P + H; No. 3: P + H/P + H + Pt.
To confirm bi-layer structure of synthesized composites, we used micrography of the normal cut-off of No. 1 membrane (
Figure 4a).
It is seen that there is a modified layer having a different structure (at the bottom of the film) which is approximately four times thinner than non-modified layer. The contrast of two membranes layers can be seen clearly in
Figure 4b, which is a photo made using optical microscope.
2.4. Mechanical Testing
To find the optimal content in view of the best mechanical properties we cast and tested several one-layer membranes with different percentages of halloysite nanotubes. Investigations of the mechanical characteristics of hybrid membranes were carried out using a TT-1100 tearing machine (Cheminstruments, Fairfield, OH, USA) at room temperature (25 °C). The traverse speed was 3.8 cm/min. Samples were pieces of films of rectangular shape about 90 mm long and about 10 mm wide. The initial distance between the clamps was 60–75 mm in dependence on the length of the film stripes. The modulus of elasticity was determined from the slope of the stress–strain curve close to the rectilinear section, with strain values not exceeding 5%. The stresses were calculated based on the initial cross-section of the sample. Typical stress-strain curve is shown in
Figure 5.
The curve has pronounced elastic and viscous plastic regions. The destruction of the film occurs on a fragile scenario with an elongation of about 150% at the time of destruction.
Figure 6 presents dependences of elastic modulus (
E) and strength limit (σ) on content of halloysite in the membrane. For comparison, low density polyethylene has Young’s modulus equal to
E = 150–250 MPa. It can be seen from
Figure 6 that the membrane with halloysite content of 4% by weight has the largest modulus of elasticity and strength limit. Therefore, in our studies, we used just such a percentage of the mineral.
5. Results and Discussion
In
Figure 9, as an illustration, the experimental and theoretical dependences of the integral coefficients of the diffusion permeability
Ps and
Pw in the case of their greatest discrepancy (membrane No. 2), on average equal to 1.16 μm
2/s per measured point, are given. In the second place, according to this discrepancy, there is membrane No. 1 (0.97 μm
2/s), in the third place, membrane No. 3 (0.59 μm
2/s). Note that membranes No. 1 and No. 2 have the same thicker layer 2 of pure polymer, synthesized by casting, so we used the same parameters
and
of this layer obtained in calculating the dependence of the diffusion permeability on the concentration for membrane No. 1 (see the penultimate column of
Table 4). For all membranes, the theoretical dependence of
Pw on the concentration (the upper curve in
Figure 9) is somewhat higher than the dependence of
Ps (the lower curve in
Figure 9), which is a consequence [
16] of the inequality
(the effective exchange capacity of the thicker layer 2 is higher than that of the modified layer 1).
Exchange capacities of three hybrid monolayer membranes were found in independent experiments to be equal to 1.08, 1.15, and 1.22 mole/l for pristine MF-4SC, MF-4SC doped with 4 wt % of halloysite nanotubes, and MF-4SC doped with 4 wt % of halloysite nanotubes encapsulated by platinum nanoparticles, respectively. After this, using the data in
Table 4, it became possible to determine the intrinsic coefficients of the equilibrium distribution and diffusion of the electrolyte molecules in the layers of the membrane (
Table 5). Despite the fact that membranes No. 1 and 3 have a modified layer of the same composition, this layer of membrane No. 3 is denser (has a lower diffusion coefficient
of the electrolyte) and has stronger positive adsorption of the electrolyte molecules, i.e., has lower coefficient
. This, in particular, might be due to the fact that the modified layer 1 in these membranes is applied by airbrush to different substrates (thick layers 2), and the thickness of membrane No. 3 is one third smaller.
In order to calculate the densities of the limiting currents from Equations (3) and (4) for different orientations of the membrane, one can use
Table 4 and
Table 5. In our case,
D =
DNaCl = 1622 µm
2/s, so
and
, consequently for membranes No. 1–3. Both systems of Equations (3) and (4) should be solved numerically. To obtain the dimensional values of voltage
and current densities
, their dimensionless analogs must be multiplied correspondingly by
and by
, 43.23 and 47.14 A/m
2 consequentially for 1-st, 2-nd, and 3-rd membranes. The system of Equations (3) and (4) can be regarded as a tool to find the thickness,
δ, of diffusion layers because we know the limiting current densities from experiments (
Table 3). We calculated the thicknesses of the diffusion layers for all hybrid membranes using specially created program for Mathematica 11, and the results, depending on the orientation of the membranes with respect to the applied electric field, were placed in
Table 6.
Figure 10 presents experimental CVCs of all three hybrid membranes for their two orientations inside measuring cell. From
Table 6 and
Figure 10, it follows that dimensionless values of
fluctuate around unity. It is interesting to compare the results obtained with the case of a perm-selective (ideal) cation-exchange membrane, when its current–voltage characteristic is given by the simple equation
and the limiting current density can be found from (5) applying the classical formula derived by Isaac Rubinstein:
In the case under consideration, we have
and
. Using (6), we calculated values of
, and put them after slash into third and fifth column of
Table 5. Analyzing the data from
Table 5, we see that there are practically precise relations between the thicknesses of diffusion layer for perm-selective and real membranes, namely
and
. The corresponding coefficients in the given ratios depend on the properties of the surface facing the anode, and the degree of imperfection of the membrane. In all cases, the thickness of the diffusion layer is less when the modified layer is oriented into the desalting cell (s-orientation). This can be explained by the fact that the modified surface is rougher (
Figure 2b,c), and the surface of the thicker layer (
Figure 2a) is smooth, as it was turned to the bottom glass of the Petri dish during the membrane casting. The rough surface forms vortices that partially destroy the diffusion layer [
23]. In addition, due to the presence of halloysite nanotubes on the modified surface, it has a mosaic charge structure, i.e., has alternating charged and uncharged areas, which also contributes to the formation of electroconvective vortices [
24].
Figure 10 shows that less asymmetry of the CVC is observed for the first membrane (
Figure 10a), and the largest for the second membrane (
Figure 10b), which also has more pronounced diffusion permeability asymmetry (
Figure 9).
Figure 10b,c illustrate, in comparison, that introduction of 4 wt % halloysite nanotubes in one layer of perfluorinated matrix MF-4SC increases the limiting current density and leads to asymmetry of CVC, while the addition of halloysite nanotubes functionalized with platinum partly compensates for the effect of halloysite adding. The addition of platinum nanoparticles to the external surface of halloysite nanotubes leads to a reduction in the plateau of the limiting current by 25%–50%, which is more significant in the case of the w-orientation of the membrane (a modified layer is turned to the cathode). The shortening of the plateau of the limiting current can be connected with the catalytic action of platinum on the process of water splitting, which leads to the appearance of additional charge carriers—protons and hydroxyl ions. In order to illustrate the high reproducibility of the results,
Figure 10c shows 2–3 CVCs measured for each orientation of the membrane. We see a practical coincidence of these curves, even in the overlimiting regime.