Hydrophilic fillers for anione exchange membranes of alkaline water electrolyzers

. Alkaline water electrolysers are widespread in many industries, including systems with hydrogen cycle of energy storage. One of the problems of modern alkaline water electrolysers is insufficient purity of generated electrolysis gases relative to electrolysis systems with solid-polymer electrolyte. In this regard, work on modification of existing porous diaphragms is actively carried out. One new area of research is the impregnation of new hydrophilic fillers into the composition of existing diaphragms and the transition to ion-solvate membranes. In this work the synthesis of zirconium hydroxide hydrogel inside a porous diaphragm with the hydrophilic filler TiO2 was carried out. This synthesis makes it possible to obtain a membrane with anion-exchange properties. A possible mechanism of OH-hydroxyl ion transfer by immobilized K+ ion was also proposed. The obtained results demonstrated the resistance of the membrane to concentrated alkaline solutions.


Introduction
On a global scale, the share of electrolytic hydrogen accounts for about 4% of the total volume of its production by various methods. Unlike the most common methods of hydrogen production -oxygen vapor and water vapor conversion, the electrolysis method of hydrogen production is the most environmentally neutral. In 2021 The Government of the Russian Federation has approved the Concept for the Development of Hydrogen Energy (Order No. 2162-r of August 5, 2021), which sets the task of developing all types of electrolyzers, including alkaline water electrolysis (AWE). The high energy intensity of the AWE requires research work in the field of developing a modern and energy-efficient electrochemical element base.
Electrolytic hydrogen is used in the radioelectronic industry for the processes of hydrogen reduction of silicon tetrachloride using gas-phase epitaxy, in the food industry for the processes of hydrogenation of fats, in powder metallurgy for the production of hard alloys, in the power industry for cooling powerful turbo generators at thermal and nuclear power plants, as well as in the energy systems based on renewable sources that have been gaining momentum in the last decade energy with a hydrogen energy storage cycle.
In Russia, there are enough resources for the production of AWE for various industries, which requires a number of works to select promising materials and technologies for the electrolytic production of hydrogen. Research in this direction will help to develop technologies for obtaining modern domestic electrochemical element base for alkaline water electrolyzers.

Diaphragms and membranes for alkaline electrolysis of water
The design of alkaline electrolysis batteries of the filter-press type required the creation of a diaphragm material for separating the cathode and anode chambers. Chrysotile asbestos, which is a natural silicate mineral, has been used as a diaphragm for a long time [1,2]. To date, all deposits of long-fiber asbestos in the world have been almost completely exhausted. The domestic industry has been using AT-16 asbestos fabric for a long time with a number of disadvantages, the main of which is irreversible degradation of the fiber structure [1,3].
In order to reduce the effect of alkali on the diaphragm, various upgrades were carried out, namely, it was previously subjected to a multiple leaching process, in order to form a brucite (Mg-OH) structure, hydrophilic additives were impregnated, and also impregnated with alkali-resistant polymer materials, which significantly reduced the electrical conductivity of the diaphragm. As a result, all further work on the development of diaphragms continued on the basis of alkali-resistant polymers.
The following alkali-resistant polymers are used as the basis of diaphragms: polysulfone, polyphenylene sulfide, polyphenylene sulfone, polyestersulfone, polyaryl ether sulfone. Based on these materials, work was carried out to create an alkali-resistant diaphragm. Technologies for producing braided diaphragms based on polymer fibers were studied. But all the diaphragms manufactured using this technology did not provide the required gas density due to too large pore diameter. Attempts to impregnate hydrophilic filler into these diaphragms did not yield results [1]. Synthesis by the phase inversion method has become a breakthrough in the field of alkali-resistant diaphragms. This method has been used for a long time in the manufacture of membranes [4]. In 1993, the first industrial serial diaphragm of the Zirfon Perl brand was manufactured [5][6][7]. This diaphragm consists of alkali-resistant polysulfone (C27H22O4S), the chemical formula of which is shown in Fig. 1, of the Udel brand, and a hydrophilic filler ZrO2 or TiO2 is impregnated into its structure to impart hydrophilicity properties [8]. To impart porosity, poroforming agent, polyvinylpyrrolidone, can also be included in the diaphragm composition [9]. Currently, this type of diaphragm is the main one for industrial alkaline water electrolyzers [10]. The hydrophilic filler ZrO2 -TiO2 is also used in fuel cells with a solid polymer electrolyte [11].  Currently, studies are underway on the use of zirconium hydroxides, magnesium oxides, magnesium hydroxides, titanium oxides, titanium hydroxides and barium sulfate as a hydrophilic inorganic filler [12]. The structure of zirconium hydroxide is shown in Figure  2.1. This structure is based on a zirconium (IV) tetramer, i.e [Zr4(µ-OH)8(OH)8(H2O)8]•xH2O was expended, where the Zr atoms are arranged in a distorted square and are connected to each other by a pair of hydroxobonds. Zirconium hydroxide (ZHO) hydrogel has a similar structure (Figure 2a,b) [13].
It is assumed that as a result of the coagulation process, ZHO forms a crystalline skeleton in which the ZrOOH + ion serves as the main structural unit [14]. The resulting crystalline skeleton of aqueous zirconium dioxide is shown in Figure 3. In turn, when exposed to ZrO2 with concentrated solutions of alkalis, a reaction of the formation of zircons can occur: It is also possible to form a hydrogel ZrOx(OH)2-x or K2ZrO3 by reactions (2) and (3): The study of the isobar dehydration of ZHO zirconium (ZrO2×xH2O) confirmed that water is not bound in stoichiometric ratios [15]. Other researchers believe that ZrO2×xH2O is obtained as a result of aging of the compound Zr(OH)4 by the following reaction: It is known that this agent is used in the processes of chemisorption of anions. This compound participates in the substitution reactions of anion exchange. The presence of anion-exchange properties in the experiments was indicated by the ions and 3 -. Due to its high chemical stability, high adsorption capacity, and fast response time, ZrO2×xH2O appears to be a promising new anion exchange material. However, in alkaline solutions (0.01 or 1 M aqueous NaOH solution) of the ZrO2•xH2O system, was expended loses of its anion-exchange properties, but at the same time begins to adsorb positive ions, that is, it exhibits cation-exchange properties [16].
The control of the diaphragms for resistance to gas penetration is provided by using the "first bubble" method (pressure at the bubble point). High resistance to gas permeability ensures a low rate of gas crossover between the electrode chambers. Changing the composition of the diaphragm (the ratio of polysulfone, hydrophilic filler and poroobrazovatel) allows you to vary its performance in the direction of improving the specific conductivity or lower gas permeability. In a number of studies, the particle size of the hydrophilic filler (ZrO2) was reduced to 40 nm, which made it possible to reduce the porosity of the diaphragm material after the phase inversion process. In turn, a reduction in the pore size available for electrolysis gases and electrolyte made it possible to increase the gas density of the diaphragm while reducing the crossover of hydrogen and oxygen between the electrode chambers [17]. A group of ion-solvated polymers is singled out separately, the basis of which was polyethylene oxide (PEO) with oxyethylene chains. Currently, this electrolyte is one of the most studied systems used in Ni-Zn, Ni-MH or Zn-air solid-state batteries [18]. Ionsolvating polymers consist of a matrix, which is a pair: a water-soluble polymer -a hydroxide salt (most often potassium hydroxide). This system combines the mechanical properties of a polymer (polymer base) and electrochemical (conductive properties of an alkaline salt). The polymer contains electronegative heteroatoms, such as oxygen, nitrogen or sulfur, which interact with salt cations by a donor-acceptor mechanism ( Figure 4) [19].
Studies have shown that in this type of electrolyte, the mechanism of ion transport is provided by the segmental movement of polymer matrices and the binding energy between cations and anions. Therefore, flexible polymer bases with low glass transition temperatures are used to improve the characteristics of polymer electrolytes. Recently, along with polyethylene oxide, the use of polyvinyl alcohol (PVA) began. For example, KOH/PVS hydrogel was used as an electrolyte in the electrochemical current source Zn/air. This type of polymer was previously kept in a 12 M aqueous solution of potassium hydroxide for 24 hours [20]. During this time, the K + ion shown in Figure 5 was immobilized into the polymer matrix.
The process of diffusion, migration and convection of ions is considered as alternative mechanisms for the transfer of hydroxyl ion OHwithin this class of membranes. At the same time, it is known that the closed structure of the polymer (including structural defects of the polymer), as well as the strong interaction between the hydroxyl ion and polymer chains, can hinder the movement of OH -. In many studies, there is a direct correlation between the concentration of potassium hydroxide immobilized in the membrane and the specific electrical conductivity of the membrane.
It is suggested that the amount of water inside the polymer can affect the electrical conductivity in several ways. More intensive water absorption increases the pore size between polymer chains and significantly reduces the strength of the OHinteraction with polymer chains. An optimal amount of water is needed inside the polymer matrix for the formation of hydrogen bonds along the entire length of the polymer chain for the further formation of hydroxyl anions with stable solvate shells. Currently, work is underway to study the possibility of using ion-solvated polymers for promising alkaline water electrolyzers [22]. To ensure ionic conductivity, the polymer matrix of the electrolyte is impregnated with an aqueous KOH solution in order to obtain an immobilized potassium ion in the polymer structure ( Figure 6).

Production of polymer diaphragm with ceramic hydrophilic filler
The presence of iron compounds in titanium dioxide is a particular danger in the synthesis of polymer diaphragms for alkaline water electrolysis. As a result of prolonged operation of the diaphragm as part of the electrolysis cell, metal inclusions are restored from the cathode side of the diaphragm. This can lead to the germination of metal dendrites through the diaphragm cloth and cause a short circuit when in contact with the anode and cathode. That is why, at the very beginning of the work, iron compounds in titanium dioxide were removed.
Qualitative reactions with potassium hexacyanoferrate (II) (K4[Fe(CN)6]) and potassium hexacyanoferrate (III) (K3[Fe(CN)6]) were used to detect residual traces of iron. The initial titanium dioxide powder was boiled in deoinized water, after which the water with the powder deposited in it was analyzed. In the presence of iron ions, staining of solutions and slight precipitation of turbulent blue was observed, indicating the presence of two-and three-valet iron in the initial reagent in accordance with reactions: (6) Trivalent iron is particularly dangerous, for its removal, TiO2 powder was poured with nitric and hydrochloric acid in a ratio of 1:3. The resulting solution was boiled and stirred for 30 minutes. After the powder settled at the bottom of the chemical beaker, the resulting precipitate was decanted by draining the acid solution. The remaining titanium dioxide was again poured with concentrated nitric and hydrochloric acid, and the process was repeated several times. In case of a negative reaction to hexacyanoferrate, the washing of titanium dioxide was stopped.
After removal of iron compounds, titanium dioxide powder was dried in a laboratory oven for 3 hours at a temperature of 100 ° C. The dried powder was a mixture of large fractions, for the crushing of which it was placed in a planetary ball mill Fritisch 7. Balls of zirconium dioxide with a diameter of 2 mm were used for grinding. Then the crushed powder fraction was sieved on a Sieve Shaker M100 vibrating screen with a sieve cell diameter of 20 microns.
The solution of polysulfone was prepared separately, acting as a porous polymer matrix for fixing a hydrophilic filler in it. To do this, PSF-150 polysulfone granules were dissolved in an organic polar solventdimethylacetamide. The polymer was dissolved in a polar aprotic solvent (for example, dimethylacetamide) in a sealed box at a temperature of 50 ℃ for 36 hours. Then TiO2 powder was added to the polymer solution.
For subsequent experiments, two types of diaphragms were prepared, the composition of which is shown in Table 1. The resulting forming composition is applied to a polypropylene mesh with the help of rollers. Rollers quickly dissipate, forming a composition over the entire area of the grid with the same thickness. Then the reinforcing mesh with cosmetic polymer was placed for 25 minutes in a container of deionized water at ambient temperature. The process of inversion transformation occurs in the washing out of the organic solvent of water with the formation of a polymer sponge base. Then the resulting diaphragm was boiled in deionized water in order to finally remove the remaining solvent from the polymer matrix.
It has been experimentally established that the maximum TiO2 content is 70%; at a higher content, this hydrophilic filler is washed out from the diaphragm body.

Manufacturing of a polymer diaphragm with hydrogel as a hydrophilic filler
As an alternative to the hydrophilic filler (TiO2) proposed in the previous section, a zirconium hydroxide hydrogel (ZrO2×xH2O) can be used. To obtain this polymer diaphragm, PSF-150 polysulfone granules were also dissolved in dimethylacetamide. Zirconium oxychloride was dissolved separately in an aqueous alcohol solution (30% C-2H5OH and 70% H2O). A polymer solution was applied to the reinforcing mesh of polyamide and immediately placed in an aqueous alcohol solution of zirconium oxychloride to start the coagulation process. In the process of phase inversion, zirconium oxychloride particles were bound by a polymer and held by it inside the diaphragm matrix.
Separately, a diaphragm was prepared, in which, in addition to the hydrogel, a hydrophilic filler was introduced. To do this, TiO2 powder was added to the polymer solution.
At the second stage, the resulting diaphragm was boiled in a 6M KOH solution to remove ethyl alcohol from the polymer and hydrolyze zirconium chloride particles, forming a ZrOx(OH)2-x hydrogel structure. The resulting diaphragm was immediately placed in deionized water. In order to prevent the destruction of the hydrogel structure, this diaphragm must be stored in deoinized water. In general, the hydrogel structure resembles hydrated zirconium dioxide ZrO2×xH2O.

Research methods
The porosity of the synthesized diaphragms was determined using the method of contact reference porometry. All measurements were carried out on an automated Porotech 3.1 reference porometer.
The specific electrical conductivity of the diaphragms was measured using a method based on measuring the voltage difference arising on a package of diaphragms clamped in a research cell. A package of diaphragms may consist of five consecutive diaphragms assembled together. This method is based on direct current measurements. The voltage on the measuring cell has never approached the decomposition voltage of water. The electrodes are platinized platinum. The specific electrical conductivity of the diaphragm was determined by the formula (7): U = 2 *ΔUc + n*l*i*S/(Ϭ*S) The study of diaphragms for resistance to pressure drops was carried out in a laboratory cell (Figure 7). For research, a sample of a diaphragm with a diameter of 30 mm was placed and fixed in the cell. On both sides, the diaphragm was compressed by electrodes. Then one of the chambers was filled with water, and compressed air was supplied to the second from a pressurized cylinder. The air pressure was regulated by a reducer and increased until the first bubble appeared, which was recorded visually. In all measurements, the GPR-3520 HD served as a DC source, the voltage was measured using a voltammeter M2044.

Investigation of gas permeability of diaphragms
In case of pressure differences between the anode and cathode chamber, the diaphragm must have a low gas permeability value ( Table 2) while maintaining an acceptable value of electrical conductivity (Table 3).
Synthesized samples of diaphragms can be conditionally divided into two groups: with hydrophilic filler (TiO2) in the composition and without (porous polymer matrix).
The results obtained in Table 2 allow us to conclude that the porous diaphragm (70% TiO2, 30% PSF-150) has the highest gas permeability of all the samples studied. The impregnation of ZrOx(OH)2-x hydrogel into the porous structure of this diaphragm made it possible to raise the minimum pressure of the appearance of the first bubble to a value of 0.26 MPa. This decrease in gas permeability can be explained by the internal volume of the diaphragm, not filled with a hydrophilic filler or polymer, inside the diaphragm matrix ( Figure 8). The complete absence of a hydrophilic filler in the porous matrix of the diaphragm under study makes it possible to increase the minimum pressure that the gas flow needs to pass through the diaphragm cloth to a value of 0.32 MPa. A further decrease in the gas permeability of this diaphragm was confirmed when filling its base with zirconium hydroxide (ZrOx(OH)2-x). This can be explained by the fact that the hydrogel has evenly penetrated and distributed throughout the available porous structure of the diaphragm, making it more resistant to pressure drops. The porous matrix of the diaphragm with hydrogel ZrOx(OH)2-x allows to withstand the pressure drop between the electrode chambers up to 0.39 MPa. When testing samples of diaphragms obtained by irrigation, the design of the test cell was changed for resistance to the pressure drop of gases. Irrigation diaphragms without reinforcing mesh in their structure were able to demonstrate exactly the same values of resistance to pressure drop only when they were tightly mechanically fixed on both sides with an electrode grid, completely repeating the design of the electrode-diaphragm unit. In the absence of an internal reinforcing mesh, the polymer base of the diaphragm mechanically rests on the mesh surface of the electrode, and the breakdown of gases occurred all the time in the internodes of the electrode base. Therefore, a nickel mesh with the smallest possible cell size was chosen as the electrode. The obtained integral and differential pore distribution curves (Figure 9) show that a diaphragm based on a polymer matrix has a pore system with a radius of about 100 nm, which causes high resistance to pressure drops. The addition of TiO2 to the diaphragm makes it possible to obtain a developed pore system in a wide range. In this type of diaphragm, three sections can be distinguished: pores of the order of 64 nm, 280 nm and 3 microns. It can be assumed that the presence of large pores is the reason for the low resistance of the diaphragms to pressure drops. Also, the filling of a developed system of large pores formed during phase inversion in a diaphragm with a ceramic hydrophilic filler with a hydrogel makes it possible to raise the resistance of the diaphragm to almost values comparable to diaphragms without a hydrophilic filler.
The obtained data on the specific electrical conductivity of the studied diaphragms are summarized in a single Table 3 at two temperatures ( Figure 10 and 11). The preservation of low values of the decomposition voltage of water is possible only if a high value of the specific electrical conductivity of the synthesized diaphragms is provided. Filling the porous structure of the diaphragm with a hydrophilic filler (70% TiO2, 30% PSF-150) with ZrOx(OH)2-x hydrogel led to a slight drop in the specific electrical conductivity of the diaphragm from 0.211 to 0.188 ohm -1 × cm -1 . These values can be explained by a decrease in the immobilized electrolyte available for hydroxyl ion transfer inside the pores of the diaphragm.
Samples of diaphragms without hydrophilic filler showed a sharp decrease in the specific electrical conductivity to 0.092 ohm -1 ×cm -1 . The absence of titanium dioxide in the diaphragm significantly blocks the development of pores during phase inversion and strongly affects the decrease in electrical conductivity. Subsequent filling of the pores of this diaphragm sample with ZrOx(OH)2-x hydrogel also revealed a drop in the electrical conductivity to 0.074 ohm -1 ×cm -1 . The comparison of the values of electrical conductivity and voltage characteristics, the worst characteristics are the diaphragms 1 and 2that is, a porous matrix without a hydrophilic filler, and a porous matrix with a hydrogel ZrOx(OH)2-x. Porous diaphragms with a hydrophilic TiO2 filler are characterized by a significant decrease in voltage on the volt-ampere characteristics obtained at atmospheric pressure ( Figure 12). Moreover, a comparison of curves 3.1 and 3.2 shows an improvement in performance when replacing electrodes with NiPx (cathode) and NiCo2O4 (anode) with Ni-Fe electrodes (anode and cathode). Filling of the porous structure of the polymer matrix and the hydrophilic filler leads to a decrease in the electrical conductivity and an increase in the voltage on the cell (curve 3.1). On the one hand, the resistance to charge transfer increases by a mechanism similar to microfiltration, on the other hand, there is no significant deterioration in the characteristics, which may indicate the implementation of charge transfer by the mechanism of ion exchange.  According to the literature data obtained from various sources, another mechanism of hydroxyl ion transfer is also possible. In alkaline media, ZHO begins to exhibit cationexchange properties, that is, the polymer matrix can reliably hold the potassium cation (K + ). Moreover, this cation can react with a negative atom (oxygen) by a donor-acceptor mechanism. The potassium ion can be associated with the structure of ZrO2×xH2O during its synthesis or further impregnation in a concentrated aqueous solution of alkali. It is also known that a possible hydroxyl ion transfer takes place in the interfacial region between the ceramic filler and the polymer matrix ( Figure 14). Therefore, for further consideration, we will single out the side chain ZrO2×xH2O bordering the ceramic filler (TiO2) and denote on this chain the double bond between Zr and the oxygen atom ( Figure 15). The resulting structure is very similar to the crystal structure of ZHO shown in Figure 16. Also, an oxygen atom can form bonds between the ceramic filler of the diaphragm and the structure of ZrO2×xH2O. This fact can explain the high degree of adhesion between the hydrophilic filler and the hydrogel. According to the presented literature data, the potassium cation can participate in the transfer of the hydroxyl ion through the mechanism of segmental movement in an amorphous structure or "hopping". The high degree of content of ceramic hydrophilic filler in the diaphragm makes it possible to ensure the movement of the hydroxyl ion along the interphase boundary. This fact can explain the slight increase in voltage between curve 3.1 and 3.2 in Figure 12. The deterioration of electrical conductivity and the increase in voltage on the cell is compensated by greater gas density, and allows the use of this type of diaphragm in electrolyzers operating under high pressures and /or at high current densities. According to the data from the volt-ampere characteristics obtained at atmospheric pressure, a cell with a diaphragm sample filled with hydrogel without hydrophilic filler showed significantly higher voltage values in all ranges of current densities (curve 2). High voltage values on the volt-ampere characteristics taken for a diaphragm based on a polymer matrix without a hydrophilic filler are also confirmed by high values of specific electrical conductivity.

Conclusion
The obtained volt-ampere characteristics of the diaphragm with the hydrogel impregnated in it fully confirmed the high values of the specific electrical conductivity and data on the resistance to gas permeability of the synthesized diaphragms. The highest values of the decomposition voltage of water were shown by a cell consisting of diaphragms based on a matrix of polysulfone without hydrophilic filler. The introduction of a hydrogel into the composition of this membrane causes an increase in voltage to 2.0 V at a current density of 300 mA / cm 2 . The use of this technology does not allow to obtain low specific energy consumption for hydrogen production.
Cells with diaphragms, which include a ceramic filler TiO2, on the contrary, showed low values of the decomposition voltage of water. At a current density of 300 mA/cm 2 , the voltage was about 1.825 V. The introduction of aqueous zirconium dioxide into the diaphragm caused an increase in voltage over the entire range of current densities. At the same time, this diaphragm may have an ionic mechanism of hydroxyl ion transfer.
The presented types of diaphragm with hydrophilic filler make it possible to obtain hydrogen with the same energy efficiency as existing foreign diaphragms, but already at high current densities relative to domestic analogues.
The most promising diaphragm for the operation of alkaline electrolysis plants under pressure is a sample whose porous structure is filled with hydrogel.
The data on the drop in the decomposition voltage of water with an increase in pressure inside the electrode chambers from 1 to 30 atm were confirmed. The pressure range in which the water decomposition voltage decreases coincides with the calculated data of the thermoneutral voltage.
Further increase in pressure from 30 to 100 atm. leads to an increase in the decomposition voltage of water, which fully corresponds to modern thermodynamic concepts. To date, from the point of view of energy efficiency and ease of operation of alkaline electrolyzers of water under pressure, the pressure level of 15 atm is the most appropriate. At this pressure value, it is possible not to use a bulky discharge housing, but at the same time there is an effect of reducing the voltage on the electrolysis cell.
Studies of synthesized diaphragms have shown that the inclusion of a hydrophilic ceramic filler (TiO2) in the polymer matrix makes it possible to obtain a biporous structure. However, this structure is not able to provide the required resistance to the pressure drop between the electrode chambers during alkaline electrolysis of water under pressure. The introduction of ZrOx(OH)2-x hydrogel into the biporous structure made it possible to increase the resistance to pressure drop by 0.8 atm. The greatest gas density was shown by a diaphragm based on a polymer matrix with a hydrogel without the inclusion of a hydrophilic filler. But this type of diaphragm has a very low value of electrical conductivity.
At the same time, the use of hydrogel in a biporous structure allows you to maintain the specific electrical conductivity at a sufficient level. And the use of a hydrophilic filler allows you to reliably impregnate ZrOx(OH)2-x into your structure.