Polymer Electrolytic Membranes

In 1964, the American firm "Dupont" patented a method for producing fluorocarbon vinyl esters containing sulfo groups. After their polymerization, polymeric membrane materials, known under the name "Nafion" (English "Nafion"), were obtained. Later, similar TEM began to be produced in Russia under the name MF-4SK. The world's first industrial plants using Nafion membranes were launched in Japan in 1975-1976. In the 1970s, extensive scientific research began on the properties of these polymer electrolytes, mainly the mechanism of their conductivity.

Externally, the Nafion membrane is optically transparent in the visible region of the spectrum sheets with a thickness of 0.1 to 1.0 mm. Since the material "Nafion" is extremely inert, the membrane is resistant to chemical attack (it withstands boiling in concentrated nitric acid), is thermally resistant to 100 ° and mechanically strong.

The membrane "Nafion", manufactured by "Dupont", is the most common and well studied. It is a branched fluorocarbon chain ending in a sulfone group. The fluorocarbon chain has hydrophobic properties, whereas the sulfonic groups are hydrophilic. The chemical composition of "Nafion" can be different, since the modern technology of its production can provide different degrees of polymerization of fluorocarbon fragments and concentration of sulfonic groups.

To describe the behavior of "Nafion" proposed several structural models. The most common is the Gierke model ^[1] , which was historically the first. According to this model, sulfone groups aggregate inside the polymer matrix and form almost spherical clusters 2-4 nm in diameter with an inner surface filled with SO ₃ ^- H ⁺ groups. According to the data on electrical conductivity, it was found that such clusters are connected by channels with a diameter of about 1 nm. Upon contact of "Nafion" with water, water molecules gather around hydrophilic sulfone groups within clusters. Since the clusters are connected by channels, this provides a continuous stream of protons across the polymer membrane.

With an increase in the amount of sorbed water, the geometric size of an individual cluster grows. The cluster size is linearly dependent on the water content in the TEM. In a dry membrane, the density of clusters and their sizes have a finite value, that is, these clusters are already formed during the polymerization process.

A three-phase structure “Nafion” was also proposed ^[2] . The authors identified a region of a polymer chain with low porosity, a region formed by side chains, where the porosity is higher, and clusters filled with sulfone groups. An alternative structural model is the “core” model, in which it is assumed that side chains with sulfone groups at the end form something like a crystal of rods, the surface of which can adsorb water molecules ^[3] . Experimental data have been collected that are in good agreement with the assumption about the transformation of the “Nafion” structure from cluster to “core” type with an increase in the water content in the membrane.

Water Sorption

The description of the behavior of TEM during water sorption is an important part of the physical chemistry of TEM. TEM is saturated with water from liquid or steam. The water content in TEM is characterized by the parameter λ, which is equal to the ratio of water molecules to the number of sulfonic groups. The water content in TEM is nonlinear depending on the vapor activity, which was confirmed by numerous studies ^{[4] [5] [6] [7]} . When interpreting the sorption curves of water in TEM, the model of an athermal solution of water in a polymer matrix is ​​the most successful.

An important feature of water-TEM systems is the different water content in the membrane when it is saturated from the liquid phase and from water vapor. Such a distinction is typical mainly for gels, and the effect itself is called the Schroeder paradox. So far, this effect has not received a clear explanation.

One attempt to explain it is to assume that the change in the water content in the TEM is associated with a first-order phase transition, like van der Waals condensation, in which the density of water at the transition point changes dramatically. However, this assumption was not experimentally confirmed, since the transport properties of "Nafion" in the range of compositions λ = 14-22 changed insignificantly ^[8] . Another attempt to explain is to take into account the capillary forces when considering the equilibrium between the water in the membrane and the saturating phase in the narrow channels of the FEM. It was assumed that the surface tension at the vapor – water interface in the TEM channel is much higher than that at the liquid – water interface in the TEM channel ^[9] . This approach allowed us to satisfactorily describe the discrepancies observed in the framework of the Schroder paradox. However, from the point of view of a rigorous theory, this approach is also uneven, since there are alternative theories describing the behavior of polymer membranes in aqueous solutions.

TEM swelling upon contact with water

The phenomenon of swelling TEM under the action of water is important not only in terms of the operational properties of the membrane, but also in terms of thermodynamic properties. Since the water molecules in the TEM occupy a small volume, it is considered that the size of the membrane grows in proportion to the water content. However, direct measurements of this dependence were not carried out ^[10] . An important characteristic required to describe the properties of TEM in the presence of water is the internal pressure. Two main models of stretching of the TEM polymer core under the action of internal pressure — linear and biaxial — have been proposed. The latter best describes the behavior of TEM in real conditions. However, it should be noted that the internal pressure increases non-monotonously with increasing water content. This suggests that a cluster structure filled with water may also have a non-monotonic nature and break up into phases with different concentrations of water. This, in turn, can explain the Schröder paradox.

Transfer of protons and water molecules

Already in early studies it was found that the proton in the membrane is transferred in the form of the hydroxonium ion H ₃ O ⁺ . The diffusion coefficient of the hydroxonium ion through the membrane is several times higher than the diffusion coefficient of water at λ> 10 ^[8] . This observation is explained using two diffusion mechanisms: direct transfer of the hydroxonium ion and "structural diffusion". The second mechanism suggests the existence of intermediate structural complexes - the so-called Zundel-H ₅ O ₂ ⁺ and Eigen-H ₉ O ₄ ⁺ ions. These complexes characterize separate stages of proton transfer in TEM, limiting the speed of the process. These mechanisms make it possible to explain the anomalously high proton mobility in TEM in comparison with other ions.

The transfer of charge and water molecules in TEM is interrelated. This relationship is usually described using the drag coefficient, that is, the number of water molecules carried along with the transfer of one proton. The drag coefficient is equal to unity at low TEM water contents and reaches ~ 50% of its maximum possible value at high water concentrations ^{[8] [11]} . This maximum value corresponds to the simultaneous movement of all water molecules contained in the membrane.

At low water contents, diffusion is the driving force of the electric transfer, and at higher, the pressure drop. In this case, the proton conductivity strongly depends on the concentration of water and increases with its increase. With a low water content, protons are captured by dissociated sulfonic groups and lose mobility, which is reflected in a sharp drop in electrical conductivity. A different explanation takes into account the structural features of TEM. So, it is considered that at low concentrations of water, clusters in the membrane are not connected with each other, whereas an increase in the water content leads to their unification into a single channel.