WO2024116062A1

WO2024116062A1 - Membrane separator for electrolysis of alkaline water

Info

Publication number: WO2024116062A1
Application number: PCT/IB2023/061960
Authority: WO
Inventors: Diego FIORENTINI; Marta Ewa BONORA; Luca QUERZE'
Original assignee: Gvs S.P.A.
Priority date: 2022-11-30
Filing date: 2023-11-28
Publication date: 2024-06-06

Abstract

The present invention relates to a symmetrical separator membrane for electrolysis of alkaline water and with homogeneous distribution of the pores.

Description

MEMBRANE SEPARATOR FOR ELECTROLYSIS OF ALKALINE WATER

In order to prevent global warming, worldwide there have been created many proj ects for reducing emissions of carbon dioxide .

For a long time now there has been debate around a sustainable alternative to the conventional production o f hydrogen which exploits renewable and that is " carbon zero" energy . From mobility to energy storage for renewables , the research aims at ensuring that green hydrogen becomes a tangible option for reducing the emissions .

Green hydrogen has been at the centre of international focus with the aim of meeting decarbonisation and energy transition goals . Besides meeting low carbon emission threshold requirements , green hydrogen i s also generated using renewable energy sources such as photovoltaic, wind or hydroelectric energy sources .

Within the latter activity, fuel cells are being used for various applications such as in vehicles and power-supply systems . Today, the most common industrial hydrogen generation process is the steam reforming process . However, this process has the problem of emitting carbon dioxide and other pollutants given that it uses fossil gas as resource . Therefore , alkaline water electrolysis which uses renewable energy as the driving force is drawing attention worldwide as hydrogen supply means given that it does not have parallel emissions of carbon dioxide .

In an alkaline water electrolysis process , the alkaline electrolyte enters into the anode and cathode regions on both sides of the membrane and the water molecules can permeate through the membrane on the other side . Due to the electric current , the water molecules of the electrolyte in the cathode region combine with the electrons to form molecular hydrogen and hydroxide ions . Furthermore , in the anode region, the hydroxide ions lose electrons to generate oxygen and water . Due to the impediment of the permeable membrane , the gas generated by the electrolysis cannot pass through the separator on the other side and the generated gas and the electrolyte are discharged together from the chamber for the treatment .

In the alkaline water electrolysis system, one of the key materials that af fect the hydrogen production ef ficiency is the diaphragm or separator .

A separator for the alkaline water electrolysis requires : a high resistance to the hydrogen and oxygen crossed permeation; and

- a low ohmic resistance with resulting high ionic conductivity .

Furthermore , given that the alkaline water electrolysis operates at ultra-drastic conditions , such as high temperature and high alkaline concentration, very few separators could be used for alkaline water electrolysis . The separators used previously were often made with asbestos nets . This material is now forbidden in most countries worldwide , rendering the production of separators without this carcinogenic compound compulsory .

The choice moved towards to synthetic separators consisting of a supported polymeric membrane .

Various types of these separa tor membranes are already available on the market .

Despite having good properties in terms of ionic resistance and resistance in oxidative environment , these separator membranes reveal the disadvantage of having to be preserved in a moist environment ( see for example , patent EP3933069A1 ) . The disadvantage of preserving the membranes in a moist environment involves additional transportation and storage costs and greater operational di f ficulties for the user of the separator . Advantageously, and unexpectedly, the separator membrane obtained with the process of the present invention overcomes the disadvantage o f the prior art products and it can be stored dry, without water . Advantageously, the separator membrane of the present invention may be stored in these conditions due to the process through which it is obtained . Such process does not provide for the use of water as a whole .

The fact that it can be stored without water also reveals the advantage of minimising the ris k of bacterial/ fungal contamination of the separator membrane .

Furthermore , the separator membrane obtained with the process of the present invention has further advantages with respect to the products of the prior art and available on the market . As a matter of fact , the separator membrane obtained with the process of the present invention provides for a material with double lining, a symmetrical surface structure and a homogeneous porosity . This leads to improving, with respect to the prior art , the properties below

- electrolytic absorption,

- ohmic resistance ;

- ionic conductivity; and

- ef fective gas separation .

Below is the detailed description of the present invention .

DEFINITIONS - METHODS

Hydrophilic

The expression "hydrophilic" membrane is used to indicate a membrane for which the contact angle between a liquid ( for example a water drop ) and the surface of the polymer forming the membrane is smaller than 90 ° .

Macroporous

According to the TUPAC ( International Union of Pure and Applied Chemistry) classification, membranes are referred to as macroporous if they have pores with a diameter larger than 50 nm (Mulder et al., "Basic Principles of Membrane Technology", Kluwer Academic Publishers, Dordrecht, 2nd Edition, p. 159, 1996;) . In the measurements using advanced porometry, the diameters of flow pores of the to-size membranes are in the range between 0.1 and 5.23 um I so tropy/Ani so tropy Isotropic/symmetric membranes have "a wholly uniform composition and structure; such membranes may be porous or dense .

Anisotropic (or asymmetric) membranes, instead consist of several layers, each with different structures and permeability. A typical anisotropic membrane has a relatively dense and thin surface layer supported on microporous substrate that is open and much thicker " (Baker et al., "Membrane Technology and Application", Wiley and Sons, 3rd edition, 2012) BP

The point where there can be seen a gas flow indicates that the pressure is sufficiently high to eject the liquid from the largest pore. The pressure at which there appears a constant flow of bubbles in this test is the bubble point pressure (ASTM F316 - 03; Standard test methods for pore size characteristics of membrane filters by bubble point and mean flow pore test) .

Distribution of the pore size/mean pore size

There can be obtained information on the various parameters for measuring the sizes of the pores through the advanced dehumidification of the pores through permanent porometry with gas/liquid.

The sizes of the pores may actually be defined "at the bubble point" and "at the mean" (Dapeng, et al., "Characterization of nanofibrous membranes with capillar flow porometry", Journal of Membrane Science, Elsevier, Volume 286, Issues 1- 2., pages. 104-114, 2006; M. Khayet and T, Matsuura "Membrane Distillation: Principles and Applications" Elsevier 2011, Chapter 8 — Membrane characterisation MD) .

To measure the parameters relating to pore size, in the membranes according to the invention there was used WET- UP/DRY-DOWN porometry measuring method, using a certified advanced capillary flow porometer (obtained from the company PMI) . In this method, a defined area membrane is first submerged in the appropriate wetting liquid and placed under constantly increasing gas pressure. The flow of a gas volume is measured for each imparted gas pressure. The point in which it is possible to measure a gas flow indicates that the pressure is sufficiently high to eject the liquid from the largest pore (bubble point pressure) ; this pressure is used to define the maximum pore size, known as "at the bubble point". With the further pressure increase, the liquid is ejected even from the smallest pores and the flow rate increases until all pores are emptied.

In the second part of the characterisation the gas flow is measured as a function of the pressure in the dry membrane. The point in which the flow rate - through the wet sample - is the same as that through the dry sample, it is the minimum pore size. The mean pore size "at the mean flow rate" is determined at 50% of the "wet flow" (during the wet measurement) with respect to the gas flow through the dry membrane at the same pressure (dry-down) .

The membranes prepared using the method according to the present invention have pore size, at mean flow, comprised between 0.1 pm and 5.2 pm. Comparing the dry flow with the wet flow for each pressure (corresponding to a particular specific porosity according to the Laplace's equation) there can be obtained the pore size distribution. Ohmic Resistance/Ionic Conductivity

With regard to membrane resistance measurements , there is commonly used a linear scanning voltage (potent iodynamic scanning) . The results of the voltage sweep, i f traced as Voltage (V) vs Current (A) , are a straight line , whose slope is the resistance (Q) (voltage sweep reported in Figure 3 ) .

To determine the calculated membrane and conductivity resistance one must accurately measure the resistance R_ceii ) • This resistance for the test environment of interest can be determined by assembling the cell without the sample to be tested . The R_ceii is a function o f the conductivity o f the solution in question

The conductivity of the sample is :

(Nourani et al . , "Elucidating Ef fects of Faradaic Imbalance on Vanadium Redox Flow Battery Performance : Experimental Characteri zation" ; Journal of The Electrochemical Society Volume 166 , Number 15 , 2019 )

Resistance to oxidation

The chemical stability in the alkaline and oxidative means is a vital property in the practical application of the membranes of the present invention . The oxidative stability of the samples of the present invention and of the concurrent samples was estimated by observing the weight loss of the membrane using the Fenton' s reagent . Fenton' s reagent could generate free radicals and lead to clear degradation of the membrane . The stability at oxidation of the AWE membrane was carried out in 50 ppm of Fe ²⁺; 5% H2O2 in pH=3 at RT . The weight reduction of the membranes was gradually observed as the time progresses after 72 hours . There can also be analysed the presence of cracks or dissolution of the membranes . (Method based on the article by Zhanga et al . "Development of a high-performance anion exchange membrane using poly ( isatin biphenylene ) with flexible heterocyclic quaternary ammonium cations for alkaline fuel cells" , Journal of Materials Chemistry A, I ssue 12 , 2019 and validated internally) Scanning Electron Microscope (SEM)

A direct view of the morphological structure of the membranes can be obtained by means of a scanning electron microscope ( SEM) . The membranes according to the present invention were examined in the upper and lower surface .

DOUBLE SIDE CASTING

This is a process that is known to the person skilled in the art and commonly used, which can be carried out vertically or hori zontally using a doctor blade , a die coat or spreading by submersion in a double blade kni fe . The spread solution is then submerged in a coagulation bath where the inversion step is carried out .

OPEN AREA

The open area is the total area of the openings divided by the total area of the material is expressed in percentage .

DESCRIPTION OF THE FIGURES

Figure 1 : Figure 1 ( 1 . 1 , 1 . 2 , 1 . 3 and 1 . 4 ) reports images o f both sides ( indicated as "top" and "bottom" ) obtained using the SEM of the two samples of separator membranes obtained with the process of the present invention ( symmetrical GVS 1 and GVS 4 ) . Figure 2 : Figure 2 (2.1 and 2.2) reports images of both sides (indicated as "top" and "bottom") obtained using the SEM of a sample of a separator membrane available on the market.

Figure 3: Figure 3 reports the voltage sweep chart.

Figure 4 : Figure 4 reports the results of the simulation test for the ageing and deterioration of the membrane subject of the present invention.

For this test there was taken into account the sample of the present invention (GVS) and compared with a sample currently available on the market (Competitor 3) .

Below is the detailed description of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In an embodiment, the present invention relates to a symmetrical separator membrane for alkaline water electrolysis with homogeneous pore distribution, obtained with the following process: a) Dissolving a thermoplastic polymer in a dispersion comprising inorganic filler and organic solvent, where :

- said thermoplastic polymer is selected from the group consisting of: polysulfone (PSU) , polyethersulfone, polyphenylsulfide, polyether ether ketone ( PEEK) , polyurethane ( PU) , polyvinylidene fluoride ( PVDF) , polyacrylonitrile ( PAN) , polyvinyl alcohol (PVA) , polyvinyl acetate (PVAc) ;

- said inorganic filler is selected from the group consisting of: zirconium oxide, zirconium hydroxide, yttrium-doped zirconium oxide, magnesium oxide, magnesium hydroxide, titanium oxide, titanium hydroxide and barium sulphate;

- said organic solvent is selected from the group consisting of: Dimethylacetamide (DMAc) , N- methylpyrrolidone (NMP) , N-ethylpyrrolidone (NEP) , N-butylpyrrolidone (NBP) Dimethylformamide (DMF) , Dimethylsulfoxide (DMSO) ; and wherein said components are present in the following ranges :

- Thermoplastic polymer 7-18% (weight/weight ) ; Inorganic filler: 20-35% (weight/weight) ;

- Organic solvent: 48-72% (weight/weight) wherein the sum of said components is equal to 100% (weight/weight) b) Degassing the solution obtained in point a) ; c) Creating a membrane by applying the solution obtained in step b) to a permeable medium positioned at the centre, with the "double side casting" technique in a coagulation bath, where :

- Said permeable medium is selected from the group consisting of: paraphenylene sulphide (PPS) , polypropylene, polyethylene, polyether ether ketone (PEEK) ,

- Said coagulation bath consists of solvent and alcohol wherein : o Said solvent is selected from the group consisting of: Dimethylacetamide (DMAc) , N- methylpyrrolidone (NMP) , N-ethylpyrrolidone (NEP) , N-butylpyrrolidone (NBP)

Dimethylformamide (DMF) , Dimethylsulfoxide (DMSO) ; o Said alcohol is selected from the group consisting of: Ethyl alcohol, isopropyl alcohol, methyl alcohol; d) Washing the membrane obtained in point c) with alcohol, e) Drying the membrane obtained in point d) ; wherein said separator membrane the resistance per specific area is comprised in the range between 0.03 and 0.3 Q*cm2 measured at RT and 30% KOH concentration. Preferably, said separator membrane is preserved in a dry environment without reducing the mechanical characteristics and the electrochemical performance.

Preferably, said inorganic filler of point a) is selected from the group consisting of: zirconium oxide, yttrium-doped zirconium oxide.

Preferably, said permeable medium of point c) is paraphenylene sulphide (PPS) with thickness comprised between 60 and 450 um and open area between 40 and 60%.

Preferably, said permeable medium is a permeable medium of the "woven" type.

Preferably, in said separator membrane the pore size is comprised in the range between 0.1- 5.2 pm (according to the WET-UP/DRY-DOWN method) .

Preferably, in said separator membrane the weight loss in the oxidative stability test in the first 96 hours through Fenton's reaction (pH =3; 5% H2O2 50 ppm Fe²⁺) is in the range between 3 and 5%.

Preferably, before being used, the thermoplastic polymer is placed in an oven overnight (8h) to remove any water present which can be absorbed during storage (standard laboratory practice) .

Preferably, in the dispersion of zirconium oxide (ZrO2) in the solvent of point a) there is used the whole of ZrO2 and part of DMAc for concentrating % by weight of the filler up to 42% during the dispersion step. In particular, there is used a by weight ratio ZrO2/DMA of 42/58. In order to reach the final % in the solution of Zr there is added the reminder of DMA after the dispersion step.

Preferably, said dispersion of point a) is carried out according to the techniques known to the person skilled in the art, such as for example using a ball mill, rotor - stator; high shear mixers etc.

Preferably, the dissolution of point a) is carried out by stirring said solution for a period of time comprised between 2 and 8 hours , preferably for 6 hours . The solution i s considered " completed" once the whole PSU has been dissolved and a uni form white dispersion has been created .

Preferably, said degassing of point b ) is carried out for a period of time comprised between 0 . 5 and 3 hours , preferably 1 hour .

In such degassing step the solution is allowed to stand so as to allow the exit of the air bubbles from said solution so as to avoid defects during deposition on the medium .

Preferably, said permeable medium of point c ) has thicknesses comprised in the range between 60-450 um .

Preferably, the temperature of the coagulation bath is at room temperature (RT , 25 ° C ) .

The "doubl e side casting" technique allows to obtain a symmetric membrane on both sides : there is obtained a product which has the polymer on both sides and the medium at the centre . The technique provides for the use of NIPS (nonsolvent induced phase separation) . In this method, a liquid solution is trans formed into a membrane in solid state by de-mixing, a process in which the solvent in a solution moves in the coagulation bath, while the non-solvent moves from the coagulation bath to the solution .

Preferably, the dwell time in the coagulation bath of point c ) is comprised in the range between 5 and 15 minutes .

Preferably, said washing of the separator membrane of point d) is carried out for a period of time comprised between 5 and 15 minutes .

Preferably, said washing is carried out under static or dynamic conditions .

Preferably, said drying of point e ) is carried out at a temperature comprised between 50 and 100 ° C preferably 80 ° C, for a period of time comprised between 5 and 10 minutes , preferably between 5 and 7 minutes . Advantageously, the separator membrane of the present invention may be stored without water, speci fically due to its implementation process , which, advantageously, does not provide for the use of water .

The characterisation of the separator membrane of the present invention was carried out with the following techniques : SEM, Feeler gauge , porometry, BP, electrolytic uptake , H- cell ( in which ohmic resistance is measured, thanks to which there is derived the ionic conductivity and the area resistance ) , solution density, oxidative stability .

Advantageously, the separator membrane obtained with the process of the present invention is capable of guaranteeing the production of pure gases thanks to the homogeneous pore distribution .

Advantageously, the separator membrane obtained with the process of the present invention shows high electrolyte retention, due to the symmetrical morphology on the surface of the product and spongy inside .

Advantageously, the separator membrane of the present invention has high ionic conductivity and low ohmic resistance , due to the homogeneous distribution of the nanoparticles measuring < di 40 nm .

Advantageously, the separator membrane of the present invention allows to be stored dry, limiting all problems relating to a product stored wet .

Advantageously, the separator membrane of the present invention shows high resistance to oxidative degradation, with a weight loss < 5% in a highly oxidative environment .

Advantageously, the process indicated in the present invention allows to avoid the use of water in all the steps for preparing the separator membrane , and the amount o f inorganic filler used reduces with respect to the state-of- the-art , thanks to the nanometric si ze thereof . Below are some non-limiting exemplifying experiments aimed at better describing the technical aspects and advantages of the present invention.

EXAMPLES

The ZrO2 used in the experiments is produced by Inframat Advanced Materials, with a diameter 20~30nm, and a purity equal to 99.9+%.

The polysulfone used in the experiments of the present invention is Solvay.

EXAMPLE 1

The symmetric membrane according to the invention is prepared starting from a 1:1.5 solids : liquids by weight ratio whose inorganic : polymer ratio is 3:1.

The solid part consists of a chemically resistant polymer, part of the family of polysulfones (PSU or PES) , and of an inorganic filler consisting of a zirconium oxide.

The obtained membrane has a thickness that may range between 190 and 450 um and it is reinforced with a porous medium made of PPS .

The obtained membrane allows to have at least the same properties in terms of ionic resistance of the membranes currently available on the market, with the advantage lying in the fact that it can be stored and used in dry and not wet form.

The difference between the sample of the present invention 1 and 3 lies in the type of polymer (PSU and PES) , while between 1 and 2 the difference lies in the casting thickness. There are also analysed 3 samples of products currently available on the market, hereinafter indicated with Competitor 1, Competitor 2 and Competitor 3, and wherein the main characteristics are reported below:

EXAMPLE 2

The solid part consists of a chemically resistant polymer, part of the family of polysulfones, and of an inorganic filler consisting of a zirconium oxide. The obtained membrane has a thickness that may range between 190 and 450 um and it is reinforced with a porous medium made of PPS .

The membranes of the present invention also show greater resistance in oxidative environment .

The test is used to simulate the ageing and the deterioration of the membrane .

The test shows the decrease in weight as time increases .

For this test the sample of the present invention ( GVS ) was taken into account and compared with a sample currently available on the market (Competitor 3 , defined above ) . The results are reported in Figure 4 .

EXAMPLE 3

The membrane according to the invention is prepared starting from a 1 : 1 . 5 solids : liquids by weight ratio whose inorganic : polymer ratio is 3 : 1 .

The solid part consists of a chemically resistant polymer, part of the family of polysul fones , and of an inorganic filler consisting of a zirconium oxide .

In the table below there is shown the di f ference for the formulation of the present invention ( as reported in example 1 ) in the properties upon variation of the phase separation technique .

Speci fically, the new membranes are obtained using NIPS or

VIPS+NIPS .

EXAMPLE 4

The symmetric membrane according to the invention is prepared starting from a 1:1.5 solids : liquids by weight ratio whose inorganic : polymer ratio is 3:1. The solid part consists of a chemically resistant polymer, part of the family of polysulfones, and of an inorganic filler consisting of a zirconium oxide. The obtained membrane has a thickness that may range between 190 and 450 um and it is reinforced with a porous medium made of PPS .

The inorganic filler used in the new membranes of this invention may be doped with a % Yttrium oxide.

In the table below there are reported the resistance values obtained with the new filler. The difference between the samples of the present invention GVS 1 (as reported in the example 1) and GVS 4 is only of the inorganic filler type, and specifically if this filler is pure Zirconia or if it is Yttrium-doped zirconia.

EXAMPLE 5

The solid part consists of a chemically resistant polymer, part of the family of polysulfones, and of an inorganic filler consisting of a zirconium oxide.

The membrane of this invention is obtained by spreading the same solution on both sides, therefore obtaining a symmetric membrane .

Obtaining a symmetric and porous membrane on both sides improves many properties, such as electrolytic absorption, ohmic resistance and ionic conductivity.

Figures 1 and 2 report images obtained with the scanning electron microscope (SEM) , of two symmetric membranes of the present invention (GVS1 and GVS4) .

EXAMPLE 6

The membrane according to the invention is prepared starting from a 1:1.5 solids : liquids by weight ratio whose inorganic : polymer ratio is 3:1.

The solid part consists of a chemically resistant polymer, part of the family of polysulfones, and of an inorganic filler consisting of a stabilised or non-stabilised zirconium oxide. The obtained membrane has a thickness that may range between 190 and 450 um and it is reinforced with a porous medium made of PPS .

The table below reports the electrolytic absorption values taking into account the prototypes with thickness in the range between 190 250 um and comparing it with the competitor in the same thickness range .

Claims

1 ) Symmetrical separator membrane for electrolysis of alkaline water with homogeneous distribution of pores, obtained with the following process: a) Dissolving a thermoplastic polymer in a dispersion comprising inorganic filler and organic solvent, where :

- said thermoplastic polymer is selected from the group consisting of: polysulfone, polyethersulfone, polyphenylsulfide, polyether ether ketone, polyurethane, polyvinylidene fluoride, polyacrylonitrile, polyvinyl alcohol, polyvinyl acetate ;

- said organic solvent is selected from the group consisting of: Dimethylacetamide, N- methylpyrrolidone, N-ethylpyrrolidone, N- butylpyrrolidone, Dimethyl formamide,

Dimethyl sulfoxide ; and wherein said components are present in the following ranges :

- Organic solvent: 48-72% (weight/weight) wherein the sum of said components is equal to 100% (weight/weight) ; b) Degassing the solution obtained in point a) ; c) Creating a membrane by applying the solution obtained in step b) to a permeable medium positioned at the centre, with the "double side casting" technique in a coagulation bath, where :

- Said permeable medium is selected from the group consisting of: paraphenylene sulphide, polypropylene, polyethylene, polyether ether ketone,

- Said coagulation bath consists of solvent and alcohol wherein : o Said solvent is selected from the group consisting of: Dimethylacetamide, N- methylpyrrolidone, N-ethylpyrrolidone, N- butylpyrrolidone, Dimethyl formamide,

Dimethyl sulfoxide ; o Said alcohol is selected from the group consisting of: Ethyl alcohol, isopropyl alcohol, methyl alcohol; d) Washing the membrane obtained in point c) with alcohol, e) Drying the membrane obtained in point d) ; wherein in said separator membrane the resistance per specific area is comprised in the range between 0.03 and 0.3 Q*cm2 measured at RT and 30% KOH concentration.

2) Separator membrane according to claim 1, wherein said separator membrane is preserved in a dry environment without reducing the mechanical characteristics and the electrochemical performance.

3) Separator membrane according to one or more of the preceding claims, wherein said inorganic filler of point a) is selected from the group consisting of: zirconium oxide, yttrium-doped zirconium oxide.

4) Separator membrane according to one or more of the preceding claims, wherein said permeable medium of point c) is paraphenylene sulphide with thickness comprised between 60 and 450 pm and open area between 40 and 60%. 5) Separator membrane according to one or more of the preceding claims, wherein in said separator membrane the pore size is comprised in the range between 0.1 and 5.2 pm (according to the WET-UP/DRY-DOWN method) .

6) Separator membrane according to one or more of the preceding claims, wherein in said separator membrane the weight loss in the oxidative stability test in the first 96 hours through Fenton's reaction (pH =3; 5% H2O2 50 ppm Fe2+) is in the range between 3 and 5%.