FIELD OF THE INVENTION

The present invention is in the field of processes of separation using semi-permeable membranes, e.g. dialysis, osmosis, ultrafiltration, and an apparatus specially adapted therefor. It may also be considered to relate to a climate change mitigation technology in that carbon dioxide is converted by electrolysis to carbon comprising molecules, as well as to a technology for transfer of charged chemical species.

BACKGROUND OF THE INVENTION

Electrolysis is a method using a direct electric current (DC) to drive an otherwise non-sponta- neous chemical reaction, converting first chemical species into further chemical species. Electrolysis may be used in the separation of elements, such as from naturally occurring sources using an electrolytic cell. The voltage providing the direct electric current, needed for electrolysis to occur, is referred to as the decomposition potential. The word “electrolysis” finds its origin in the Greek language.

The main components involved in electrolysis are an electrolyte, a positive and a negative electrode, and an external power source providing the voltage and direct electric current. Typically a separator is present, such as an ion-exchange membrane, to prevent diffusion of species to the vicinity of the opposite electrode. The electrolyte is a chemical substance which contains free ions, and carries the electric current. Ions typically are mobile, in order for electrolysis to occur. A liquid electrolyte may be produced by solvation, by reaction of an ionic compound with a solvent, and by melting of an ionic compound. When immersed, in an example the electrodes are separated by a distance, such that a current flows between them through the electrolyte. They are connected to the external power source, which therewith completes the electrical circuit. Materials of which electrodes are formed are typically a metal, graphite, and a semiconductor material. Suitable electrodes may be selected in view of chemical reactivity between the electrode and electrolyte, and manufacturing cost. Historically, graphite and platinum were often chosen.

A membrane is a selective barrier, allowing certain (chemical) species to pass through and preventing others from passing through. Membranes can be classified into synthetic membranes and biological membranes; the present invention relates to synthetic membranes. A first large scale use of membranes was in microfiltration and ultrafiltration technologies. A degree of selectivity of a membrane depends amongst others on the membrane pore size. Depending on the pore size, they can be classified as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) membranes. The present invention is in the field of NF and/or RO. Membranes can also be of various thickness, with homogeneous or heterogeneous structure. Membranes can be neutral or charged, and particle transport can be active or passive. The latter can be facilitated by pressure, concentration, chemical or electrical gradients of the membrane process. Important aspects of a membrane process operation relate to membrane permeability (k), operational driving force per unit membrane area and fouling and cleaning of the membrane surface. A thin film is a layer of material with a thickness ranging from a monolayer to several micrometers. Thin films are typically deposited, such as on a substrate, typically under well-controlled conditions. Upon deposition a controlled synthesis of materials forming the thin film occurs. A stack of thin films is called a multilayer. Deposition may take place using chemical [vapor] deposition, physical [vapor] deposition, epitaxial growth mechanisms, atomic layer deposition, and so on. Thin films find application in many fields of technology, ranging from batteries, to small apparatuses, such as acoustic wave resonators, to coatings, and so on.

A research trend relates to the conversion of CO2. The electrochemical reduction or electrocatalytic conversion of CO2 can produce value-added chemicals, such small alkanes as methane, small alkenes, such as ethylene, small alcohols as ethanol, etc. The electrolysis of carbon dioxide can result in formate (COOH ) or carbon monoxide, but sometimes more elaborate organic compounds such as ethylene. The technology is under research as a carbon-neutral route to organic compounds.

However the conversion of CO2 is often not high enough, as often a high percentage of the input quantity of CO2 is lost. Losses may be in the order of 40-60%, which makes processes uneconomical and difficult to maintain.

Typical examples of prior art membranes may be found in the following documents. Petrov et al. (DOI: 10. 1039/d2se00858k) recite an electrochemical reduction of carbon dioxide (CO2R) poses substantial promise to convert abundant feedstocks (water and CO2) to value-added chemicals and fuels using solely renewable energy. Membrane-electrode assembly (MEA) devices that have been demonstrated to achieve high rates of CO2R are limited by water management within the cell, due to both consumption of water by the CO2R reaction and electro-osmotic fluxes that transport water from the cathode to the anode. Additionally, crossover of potassium (K+) ions poses concern at high current densities where saturation and precipitation of the salt ions can degrade cell performance. A device architecture is presented incorporating an anion-exchange membrane (AEM) with internal water channels to mitigate MEA dehydration and demonstrated. A macroscale, two-dimensional continuum model is used to assess water fluxes and local water content within the modified MEA, as well as to determine the optimal channel geometry and composition. The modified AEMs are then fabricated and tested experimentally, demonstrating that the internal channels can both reduce K+ cation crossover as well as improve AEM conductivity and therefore overall cell performance. This work demonstrates the promise of these materials, and operando water-management strategies in general, in handling some of the major hurdles in the development of MEA devices for CO2R. Yan et al. (DOI: 10.1038/S41557-020-00602-0) recite efficient conversion of electricity to chemicals needed to mitigate the intermittency of renewable energy sources. Driving these electrochemical conversions at useful rates requires not only fast electrode kinetics, but also rapid mass and ion transport. However, little is known about the effect of local environments on ionic flows in solid polymer electrolytes. Here, we show that it is possible to measure and manipulate the local pH in membrane electrolysers with a resolution of tens of nanometres. In bipolar-membrane-based gas-fed CO2 electrolysers, the acidic environment of the cation exchange layer results in low CO2 reduction efficiency. By using ratiometric indicators and layer-by-layer polyelectrolyte assembly, the local pH was measured and controlled within an ~50-nm -thick weak-acid layer. The weak-acid layer suppressed the competing hydrogen evolution reaction without affecting CO2 reduction. This method of probing and controlling the local membrane environment may be useful in devices such as electro- lysers, fuel cells and flow batteries, as well as in operando studies of ion distributions within polymer electrolytes., US 4 954 388 A recites an abrasion-resistant, tear-resistant, multilayer composite membrane, useful in electrolysis, is provided comprising a continuous perfluoro ion exchange polymer fdm attached to a reinforcing fabric by means of a porous, expanded polytetrafluoroethylene (EPTFE) interlayer. The fabric and EPTFE are rendered hydrophilic and non-gas-locking by coating the interior and exterior surfaces thereof with a perfluoro ion exchange resin of equivalent weight less than 1000. The composite preferably is treated with an ionic perfluoro surfactant. Also provided is a multilayer composite according to the above in which the continuous perfluoro ion exchange film is itself a multilayer construction of a perfluorosulfonate polymer and a thin layer of perfluorocarboxylate polymer in which the perfluorosulfonate polymer interfaces with the EPTFE and the interior and exterior surfaces of the EPTFE and fabric are coated with perfluorosulfonate polymer. Also provided are a method of making the composites and methods of use for these fabric reinforced thin membrane structures as separators in electrolytic cells, and as selective barriers in permeation separation and facilitated transport operations. AU 2020 393 869 Al recites membrane electrode assemblies (MEAs) for CO _X reduction. According to various embodiments, the MEAs are configured to address challenges particular to CO _X including managing water in the MEA. Bipolar and anion exchange membrane (AEM)-only MEAs are described along with components thereof and related methods of fabrication. Delacourt et al. (DOI: 10. 1149/1.2801871) recites an electrolysis-cell design for simultaneous electrochemical reduction of CO2 and H2O to make syngas (CO+H2) at room temperature (25 C) was developed, based on a technology very close to that of proton-exchange-membrane fuel cells (PEMFC), i.e., based on the use of gas-diffusion electrodes so as to achieve high current densities. While a configuration involving a proton-exchange membrane (Nafion) as electrolyte was shown to be unfavorable for CO2 reduction, a modified configuration based on the insertion of a pH-buffer layer (aqueous KHCO3) between the silver-based cathode catalyst layer and the Nafion membrane allows for a great enhancement of the cathode selectivity for CO2 reduction to CO [ca. 30 mA/cm ² at a potential of -1.7 to 1.75 V vs SCE (saturated-calomel reference electrode)]. A CO/H2 ratio of 1/2, suitable for methanol synthesis, is obtained at a potential of ca. -2 V vs SCE and a total current density of ca. 80mA/cm ² . An issue that has been identified is the change in product selectivity upon long-term electrolysis. Results obtained with two other cell designs are also presented and compared. The present invention relates to an improved CO2 conversion, which overcomes one or more of the above disadvantages, without jeopardizing functionality and advantages.

SUMMARY OF THE INVENTION

The present invention relates in a first aspect to a thin film composite membrane (TFCM) for CO2 electrolysis (100), comprising a substrate, in particular a semipermeable membrane substrate, more in particular an ion exchange membrane substrate, preferably a high strength mem- brane, wherein the substrate is selected form an anion-exchange membrane substrate, a cation-ex- change membrane, and a bipolar membrane substrate, and on at least one side of the substrate, at least one polymeric fdm, being a dense polymeric fdm, with a size exclusion of < 1 nm as determined with size exclusion chromatography (Shimadzu LC-2010AHT, ISO 16014-1:2019), in particular with a size exclusion of < 0.5 nm, for example with a size exclusion of < 0.35 nm, in particular at least one first polymeric film on a first side of the substrate and at least one second polymeric film on a second side of the substrate. A film with such a size exclusion characteristics is considered to relate to a dense film. In the electrolysis CO2inay be converted to CO, unsaturated or saturated C1-C4 compounds, such as C=C, C1-C4 alcohols, such as methanol, ethanol, propanol, butanol, and isopropanol, and C1-C4 carboxylic acids, such as formic acid, acetic acid, propionic acid, and combinations thereof. For instance, formic acid may be formed in an electrolytic cell, wherein the cell operates at a current density of about 140 mA/cm ² at a cell voltage of 3.5 V. Power consumption is in the order of 4.5 kWh/kg of product. For forming CO a cell has been operated at current densities of 200 to 600 mA/cm ² at about 3 V. The present composite membrane comprises a substrate, and at least one, typically one, thin film. The present thin film prevents carbonate (CO3 ² ) and bicarbonate (HCO3 ) from passing the membrane. The present membranes typically comprise a homogeneous structure, that is, with little or substantially no variation in composition and structure. The present membrane composite is typically charged, though a net surface charge may still be substantially 0, that is, it comprises substantially the same amount of positive charge and negative charge. Chemical species transport over the composite membrane is typically active, that is requiring a driving force, such as a pressure, a concentration difference, a voltage, or the like. Typically the present membrane is used in a cross-flow mode of operation. The present TFCM may be considered as a bi-fimc- tional membrane. The added polymeric (polyamide) layer is responsible for the size exclusion; by coating an ion-exchange membrane with this polymeric layer the bifimctional composite membrane is obtained, typically having two functions being the ion-exchange and the size exclusion. It can be applied to solve critical problems with CO2 electrolysis. It is noted that in prior art CO2 electrolysis more than 50% of the CO2 input is lost, as CO2 dissolves typically as bicarbonate. The present TFCM reduces losses of CO2 well below 50%, typically below 40%, such as to 1-30%, e.g. 5-20%, depending on the precise conditions. Therewith an alkaline anolyte medium, having a relatively high pH is now possible. In addition, the use of rather expensive catalysts, such as Ir, is also no longer required.

In a second aspect the present invention relates to a system for electrolysis comprising at least one first electrode of a first polarity, at least one second electrode of a second polarity, the second polarity being opposite of the first polarity, at least one first chamber comprising a first electrolyte, at least one second chamber comprising at least one second electrolyte, and at least one thin film composite membrane according to the invention, the membrane physically separating the first and second chamber, in particular wherein a volume of the respective at least one first chamber and the at least one second chamber each individually is from 1-2500 cm ³ , such as 10-1000 cm ³ .

In a third aspect the present invention relates to a method of converting CO2, comprising providing a system according to the invention, providing CO2 to the system, and converting CO2 into a chemical compound selected from CO, unsaturated or saturated C1-C4 compounds, such as C=C, C1-C4 alcohols, and C1-C4 carboxylic acids.

In a fourth aspect the present invention relates to a method of forming the thin fdm composite membrane according to the invention, comprising providing a substrate, in particular a membrane substrate, wherein the substrate is selected form an anion-exchange membrane substrate, and a bipolar membrane substrate, and providing at least one polymeric film on at least one side of the substrate by interfacial polymerization, in particular a dense polymeric film, more in particular with a size exclusion of < 10 nm.

The present invention also relates to a use of a thin film composite membrane according to the invention or a system according to the invention, for transfer of charged chemical species, in particular charged chemical species selected from cations and anions, in particular for electrochemical separation, for electrolysis, and for combinations thereof. Electrolysis may be performed in a fluid, such as a gas, in an aqueous environment, such as an aqueous electrolyte, in relatively pure conditions, such as an mainly aqueous electrolyte, or in more complex electrolytes, such as salty electrolytes, e.g. NaCl comprising electrolyte.

Thereby the present invention provides a solution to one or more of the above mentioned problems.

Advantages of the present description are detailed throughout the description. References to the figures are not limiting, and are only intended to guide the person skilled in the art through details of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates in a first aspect to the thin film composite membrane according to claim 1 .

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the polymeric film has a thickness of 10-500 nm, in particular 50-300 nm, more in particular 100-200 nm..

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the substrate is resistant to alkaline substances, in particular to OH’, more in particular resistant up to a temperature of 70 °C at a molar concentration of 1 mole/1 during 24 hours.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the substrate is selective for bicarbonate, in particular wherein the substrate has a selectivity for OH’ of >70%, in particular > 85%, more in particular > 95% (at 0 °C and 100 kPa, versus H2).

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the polymeric film comprises surface charges, in particular with a surface charge density of > 11 * 1 O’ ¹⁵ C/mm ² | at a pH of 7, that is, when exposed to a neutral electrolyte or solution with a pH of about 7, in particular > 11 * 1 O’ ¹⁴ C/mm ² |, more in particular < 11 * 1 O’ ⁹ C/mm ² |.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis a surface charge is selected from an anion, a cation, and combinations thereof. In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis a surface charge is selected from an anion, a cation, a localized charge, a partial charge, and combinations thereof.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the substrate is at least partly formed of a chemical compound comprising at least one nitrogen atom, in particular at least two nitrogen atoms, wherein the chemical compound is selected from saturated and unsaturated organic molecules.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the chemical compound is selected from 5 -ring and 6-ring comprising molecules.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the 5 -ring and 6-ring comprising molecules comprise at least one nitrogen, in particular at least two nitrogens, such as imidazole.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the substrate has a thickness of 1-500 pm, in particular 4-240 pm, more in particular 12-120 pm, even more in particular 20-60 pm, such as 25-35 pm.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis the polymeric film is selected from a polyamide film, a polypropylene (PP) film, a Polyvinylidene fluoride (PVDF) film, a cellulose acetate film, in particular a Cellulose di(or tri)acetate film, a Piperazine film, a graphene film, a Graphene oxide film, and a PTFE film.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis a surface area of the thin film composite membrane is 1-10 ⁵ cm ² , in particular 2-10 ⁴ cm ² , more in particular 10-10 ³ cm ²

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis a ratio in permeance of OH’ versus the permeance of carbonate ions of the thin film composite membrane is larger than 5, in particular larger than 20.

In an exemplary embodiment of the present thin film composite membrane for CO2 electrolysis wherein a ratio in permeance of H ⁺ versus the permeance of Na ⁺ ions of the thin film composite membrane is larger than 5, in particular larger than 20 [under which conditions measured by applying a current for a period of time, measuring a concentration change, e.g. via pH/titration, and ion chromatography.

In an exemplary embodiment the present system comprises a catalyst, in particular an Ag catalyst, or a Cu catalyst, more in particular wherein the catalyst is provided on the thin film composite membrane and in electrical and physical contact with the thin film composite membrane, such as by pressing.

In an exemplary embodiment of the present system the system is selected from a system wherein the first and second electrode are physically attached to the thin film composite membrane, and a system wherein the first and second electrode are physically separated from the thin film composite membrane.

In an exemplary embodiment of the present system the system a ratio of the combined first chamber and second chamber volume: the surface area of the thin film composite membrane is 10" ² - 10 cm ³ :cm ² , in particular 10" ² -2 cm ³ :cm ² , more in particular 10" ¹ - 1 cm ³ :cm ² , even more in particular 2* 10 ^-1 -0.5 cm ³ : cm ² .

In an exemplary embodiment of the present method in operation the pH of the at least one first chamber comprising an anolyte is 7.5-12, in particular 9-11, and/or wherein in operation the pH of the at least one second chamber comprising a catholyte is 4-7, in particular 5-6.

In an exemplary embodiment of the present method, in case of an anion exchange membrane, in operation the pH of the at least one first chamber comprising an anolyte is 7.5-14, in particular 10-13.5, more in particular 11-12.

In an exemplary embodiment of the present method an Ag catalyst, or a Cu catalyst, is used.

In an exemplary embodiment of the present method conversion of CO2 is provided at an operation energy of <3 kWh/kg of product, in particular <1 kWh/kg product, [current of <300 mA and voltage of 3 V]

In an exemplary embodiment of the present method of forming the thin film composite membrane the substrate is an anion exchange membrane, and wherein the at least one polymeric film is a polyamide, and wherein the polymerization is by reacting m-phenylenediamine with 1,3,4- benzenetricarbonyl trichloride.

In an exemplary embodiment of the present method of forming the thin film composite membrane the reaction is carried out during 1-60 minutes, at a temperature of 20-80 °C, at a pressure of 90-110 kPa, at a concentration of 0.01-1 mol m-phenylenediamine, at a concentration of 0.01-1 mol 1,3,4-benzenetricarbonyl trichloride, and at a ratio of m-phenylenediamine : 1,3,4-ben- zenetricarbonyl trichloride of 0.5-2.

The present invention further relates to a use of a thin film composite membrane according to the invention, for transfer of charged chemical species, in particular selected from cations and anions, in particular for electrochemical separation, such as wherein the use is in acid-base production, in a flow battery, or in electrolysis.

The invention is further detailed by the accompanying figures and examples, which are exemplary and explanatory of nature and are not limiting the scope of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be conceivable falling within the scope of protection, defined by the present claims.

SUMMARY OF FIGURES

Figure la shows principles of a prior art redox flow battery.

Fig. lb, 2 and 3a, b, and 4 show schematics of a present flow cell; figs. 5 and 6 show experimental results.

DETAIEED DESCRIPTION OF FIGURES

100 redox flow battery

10 membrane

11 catholyte tank 12 anolyte tank

13 contact (current collector)

14 pump

15 current flow

16 first chamber

17 second chamber

18 third chamber

31 first electrolyte flow

32 second electrolyte flow

40 thin film

Figure la shows principles of a prior art redox flow battery. Therein a single cell is shown. The cell comprises a membrane 10, and contacts 13 (current collector). Also a catholyte tank 11 and an anolyte tank 12 is shown. Two pumps 14 are provided for driving a flow; a first electrolyte flow 31 and a second electrolyte flow 32 is shown. As a result an electrical current 15 flows. Also first and second chambers 16,17 are shown.

Figure lb shows a similar layout as fig. la, only the current flows from membrane 10 to a contact 13.

In a similar manner fig. 2 shows schematically the functioning of the present flow cell, comprising two catholyte tanks, and an extra chamber 18, parallel to chamber 16. Such may be in particular relevant if a first tank 11 comprises a liquid, and a second tank 11 comprises a gas. The separator 13 may be a gas diffusion electrode. The figures are further detailed in the description of the experiments below.

In fig. 3a it is shown that a contact 13 and a membrane 10 are physically separated by a respective first chamber 16 and second chamber 17, whereas in fig. 3b the contacts 13 are in physical contact with membrane 10, and first chamber 16 and second chamber 17 are on opposite sides of the con- tact/membrane/contact stack.

Fig. 4 shows a membrane with a thin film, forming the present thin film composite membrane.

Figure 5 - Transport numbers of OH- and CO32- under a stable current. M Base is the bare AEM and M1-M4 are different tested TFCMs.

Figure 6 - Products at the anode side of a CO2 electrolyzer employing a TFCM.

Figs. 7 and 8 show the diffusion coefficient (m ² /s) and reduction of mobility (relative) as a function of hydrated radius (A), respectively.

Figs. 9a, b show comparison of CO2 crossover.

EXAMPLES/EXPERIMENTS

Selectivity to OH" vs CO3 ² ’

Figure 5 shows the result of the cross-over experiments of hydroxide vs carbonate, before optimization of the coating process. However, it can already be observed that the transport number of OH" has a clear increase for the modified membranes. Since CO3 ² ’ carries twice the charge of OH’, it is concluded that at least 85 % of the ions crossing over the modified membranes were OH’ in this experiment.

To the test the permeability of the thin film composite membranes (TFCM), they were placed between two compartments: one with 0.25 M of KOH and 0.25M of K2CO3; and the other with only KC1, to allow the determination of the anions which have crossed over. A stable current was applied in order to observe an, at least, 50 % concentration change, and then a sample was taken out of each compartment. The samples were subsequently titrated to determine the concentrations of OH’ and CO3 ² ’. Figure 1 shows the result of the cross-over experiments, before optimization of the coating process. It can already be observed that the transport number of OH’ has a clear increase for the modified membranes, in comparison to the non-modified membrane (M_base).

After optimizing reagent concentrations, drying time and ensuring a uniform film, the ionic resistance of a non-coated AEM and that of a TFCM were measured in 0.1 M KOH and K2CO3. The table shows that the coating gives a very low increase in terms of resistance towards OH’ but the resistance in carbonate has increased at least 22-fold.

Table 1 - Ionic resistance measured in a 6 compartment setup with 4 electrode configuration.

Ionic Resistance (Q.cm2)

0. IM KOH O. IM K2CO3

AEM 4.52 ± 0.02 5.59 ± 0.07

TFCM 4.89 ± 0.01 >110* (* very low limiting current)

Furthermore, the TFCM was tested in a CO2 electrolyzer. Carbonate cross-over toward the anolyte is a major issue in prior art CO2 electrolysis, since there it oxidizes back to CO2. This leads to a loss of around 50% of the reagent, making the prior art process inefficient and less economically viable. It has been shown in literature that during stable operation, the molar ratio of CO2 to O2 gas produced at the anode is 2: 1. Figure 6 shows the gasses produced at the anode side in our experiment using a TFCM (O2 left lower points, right higher points; arrow). It can be observed, that there is a low amount of CO2 produced that is independent of current density. If this CO2 was due to carbonate cross-over it would increase with current density, therefore it can be concluded that is not its origin. It can be due to the pre-column of the gas chromatograph for example. Meaning, our TFCM allows little to no cross-over of carbonate during operation.

In terms of cell potential, there is an increase of ~1 V in comparison to a non-modified AEM. The resistance of the membrane can be further optimized by changing the polyamide film density.

Stability of the polyamide films

Long term operation tests and analysis after exposure to different solutions are still required to confirm the stability of these films.

In terms of delamination, we believe that the film will be extremely stable since the PA film is entangled in the polymeric structure of the AEM. Firstly, because in order to create this film using interfacial polymerization, we let the water phase soak the membrane, and then completely remove the excess from the top, until the membrane appears almost dry. Only then is the organic phase with the second monomer added, meaning the interface where the polymerization happens is the surface of the AEM. Secondly, the XPS analysis (Table 2) in the first 10 nm of the sample, already shows a low amount of a different NH2 structure (NH2*) from the one of the PA films. These amines correspond to the immobilized amine groups of the AEM. And it is known from literature (refs) that the PA films created with this concentration of reagents have a thickness of 100 to 200 nm. Meaning, the majority of the film is within the polymeric structure of the AEM. The XPS analysis also confirms the film’s atomic structure is consistent with literature (Table 3).

Table 2 - XPS results for 10 nm depth of a polyamide film. Analysis of states of NH2 present. NH2* denominates a different state of NH2.

Sample Name Position %At Cone

TFCMl_001 NH ₂ 399.98 97.21

NH ₂ * 402.43 2.79

TFCMl_002 NH ₂ 399.97 90.64

NH ₂ * 402.49 9.36

TFCM1 003 NH ₂ 399.91 87.47

NH ₂ * 402.48 12.53

Table 3 - XPS results for 10 nm depth of a polyamide film. Percentages of C, N and O atoms.

CNO, atomic % C, aver % N, aver % O, aver %

TFCM1 75.1 7 15.0 deviation 0.7 1 0.1

In terms of chemical stability, RO membranes have been optimized to work for quite a wide range of feeding solutions. There is a vast choice of materials which can be used for different applications, so it is a matter of finding the correct material, among the already available ones.

Charge density characterization

Typical prior art ion exchange membranes have an immobilized [volumetric] charge density between 0.1 and 7 M (mol fixed charge/L sorbed in membrane), with the majority being between 0.5 and 3 M. So the substrate, an anion-exchange or bipolar-exchange membrane will have an immobilized charge density between 0.1 and 7 M.

The present thin film composite membrane layer comprising the at least one polymeric (e.g. polyamide) layer, has a much denser polymer and lower pore size. The charge density typically depends on the polymer used in the at least one polymeric layer, on the pH, and on electrolyte concentration, but is typically much, three orders of magnitude, lower (5-30 mM):for example, between pH 5 and 9, the charge density is between 9 and 20 mM. At pH 5, the present charge density is between 28 and 30 mM, depending on the salt. Between pH 3 and 10, the present charge density is between 0 and 4.5 mM, sometimes referred to as surface charge. Most modeling works take between 10 and 30 mM.

Size exclusion

Based on our cross-over experiments, in the non-modified membrane, the diffusion coefficients for OH’ and CO3 ² ’ are 1.18* IO’ ¹⁰ and 4.10* 10 ^-11 m ² /s. In the modified membranes, depending on how dense the polyamide layer is, the diffusion coefficient for OH’ is between 8.41* 10’ ¹² and 2.10* 10’ ¹¹ m ² /s; and the diffusion coefficient for CO3 ² ’ is between 4.10* 10’ ¹¹ and 7.78* 10’ ¹³ m ² /s. In other words, the diffusion coefficient for OH’ is reduced in the modified membrane by 6 to 14 times, and the CO3 ² ’ coefficient is reduced by 21 to 53 times.

The figure 7 shows the diffusion coefficients plotted against the hydrated radius of the ions (OH’ on the left and CO3 ² ’ ions displayed on the right of the graph). Since the polyamide layer has a low charge density, this larger reduction of CO3 ² ’ mobility compared to OH’ mobility can only be explained by the (partial) size-exclusion. The cut-off size is likely between 3.1 and 3.8A (in line with literature, htps, .nature .com/aiticles/s41467-020-15771 -2 ), but naturally there will be a pore size distribution in the polyamide layer, causing a less sharp cut-off. The thin film composite membrane has an increased selectivity for ions below this size (higher selectivity than the substrate).

An alternative way to plot this could be the figure 8. Here we show how many times the mobility of the ions has decreased in comparison to the base membrane. This is the ratio of the diffusion coefficients in the non-modified membrane and TFCMs.

From these figures one may conclude that the present TFCM reduces the diffusion of larger species, i.e. CO3 ² ’ ions, to almost zero, and therewith excludes larger ions from passing trhogh the present TFCMs

Figs. 7 and 8 show the diffusion coefficient (m ² /s) and reduction of mobility (relative) as a function of hydrated radius (A), respectively, for a non-modified membrane, and two modified membranes (TFCM1 and TFCM 2 respectively). TFCM 1 is made with a 2% w/v MPD (m-phe- nylene diamine) aqueous solution and 0.05% TMC (trimesoyl chloride) cyclohexane solution. TFCM 2 is made with a 3% MPD aqueous solution and 0.15% TMC cyclohexane solution. They are typical reagents for the present polyamide layer for reverse osmosis membranes; therewith the present dense layer is made, by changing the concentrations of the monomers. Clearly the diffusion coefficient reduces by about an order of magnitude, and wherein the reduction of mobility is largest for larger radii.

CO2 crossover

It has been found that with the present TFCMs the CO2 crossover is reduces significantly. In general it is well known that AEM-membranes are poor in rejecting bicarbonates, and therefore not particularly suited for the present CO2 electrolysis. As a consequence, also the prior art AEM is not showing a stable pH due to bicarbonate crossover. CO2 utilization in electrolysis with such prior art membranes is therefore very limited, typically less than 50%.

Fig. 9a, b show (a) pH of the anolyte in AEM- and BPM-based cells at 100 mA cm ^-2 , with the simulation-based pH for AEM if the consumed OH- was sourced only from the CO2ER and (b) the pH drop of the KOH anolyte of the AEM-based cell in function of the applied charge for 100 and 300 mA cm ^-2 in comparison to the simulation-based pH (calculated based on x = 1, where pH is determined by the loss of OH- molecules due to applied charge) if the consumed OH- was sourced only from the CO2ER. This pH shift is caused by the consumption of OH“ at the anode (for the OER) as well as at the cathode (reacting with CO2). Given the high alkaline environment at the cathode, carbonate is formed, which requires two hydroxide ions.

So, the standard AEM is not showing a stable pH due to bicarbonate crossover.

The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying figures.

It should be appreciated that for commercial application it may be preferable to use one or more variations of the present system, which would similar be to the ones disclosed in the present application and are within the spirit of the invention.