METHOD AND SYSTEM FOR CO2 REDUCTION

Abstract

The invention relates to a method and system for gas-phase CO2 reduction using a composite electrolyte comprising an ionic liquid incorporated within a mesoporous host matrix. The composite electrolyte functions as a dual sorbent for CO.sub.2 and H.sub.2O while providing ionic conductivity, facilitating the electrochemical reduction of CO.sub.2 directly from a gas stream. The method involves sorption of CO.sub.2 into the composite electrolyte, followed by electrochemical reduction at an interface with a catalyst. The system integrates the composite electrolyte into a sorbent-catalyst assembly), enabling efficient CO.sub.2 reduction and simplifying the system architecture by combining sorption and conduction in one material.

Claims

1. A method for gas-phase CO.sub.2 reduction, comprising: providing a composite electrolyte with an ionic liquid incorporated within a mesoporous host matrix, wherein said composite electrolyte functions as a dual sorbent for CO.sub.2 and H.sub.2O, and provides ionic conductivity, exposing the composite electrolyte to a gas stream comprising CO.sub.2, wherein CO.sub.2 is sorbed into the composite electrolyte, electrochemically reducing the sorbed CO.sub.2 at an interface between the composite electrolyte and a catalyst, wherein the composite electrolyte acts as both a sorbent and ionic conductor to facilitate the electrochemical reaction.

2. The method according to claim 1, wherein the gas stream also comprises H.sub.2O which is sorbed into the composite electrolyte and wherein the provided composite electrolyte comprises a hydrophobic modification of the host matrix for maintaining decoupled transport of CO.sub.2 and H.sub.2O within the composite electrolyte.

3. The method according to claim 1, wherein the mesoporous host comprises silica.

4. The method according to claim 3, wherein the silica host matrix is modified with methyl sidechains to decrease hydrophilicity of the host matrix.

5. The method according to claim 4, wherein the modification of the silica host matrix is achieved using methyltrimethoxysilane.

6. The method according to claim 5, wherein the ratio of methyltrimethoxysilane to tetraethyl orthosilicate used to form the silica host matrix is varied to decrease the hydrophilicity of the host matrix.

7. The method according to claim 1, wherein the ionic liquid is selected from the group comprising bis(trifluorosulfonyl)imide-based ionic liquids.

8. A system for gas-phase CO.sub.2 reduction, comprising: a composite electrolyte comprising an ionic liquid incorporated in a mesoporous host matrix, wherein said composite electrolyte provides ionic conductivity and functions as a dual sorbent for CO.sub.2 and H.sub.2O, a sorbent-catalyst assembly comprising a catalyst interfacing with the composite electrolyte, where CO.sub.2 sorbed in the composite electrolyte is reduced at the interface between the composite electrolyte and the catalyst.

9. The system according to claim 8, wherein the host matrix is hydrophobically modified to decouple the transport of CO.sub.2 and H.sub.2O sorbed in the composite electrolyte.

10. The system according to claim 8, wherein the mesoporous host matrix comprises silica.

11. The system according to claim 10, wherein the silica host matrix is modified with methyl sidechains to decrease the hydrophilicity of the host matrix.

12. The system according to claim 11, wherein the modification of the silica host matrix is achieved using methyltrimethoxysilane.

13. The system according to claim 12, wherein the ratio of MTMS to tetraethyl orthosilicate (TEOS) used to form the silica matrix is selected to obtain a desired hydrophilicity of the host matrix.

14. The system according to claim 8, wherein the ionic liquid is selected from the group consisting of bis(trifluorosulfonyl)imide-based ionic liquids.

15. The system according to claim 9, further comprising a gas distribution system for supplying gas to the composite electrolyte and for retrieving gas from the composite electrolyte.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] FIG. 1 shows a schematic drawing of a prior art CO.sub.2 reduction cell.

[0045] FIG. 2 shows a zoomed in schematic drawing of the cathode of FIG. 1.

[0046] FIG. 3 shows a schematic drawing of a catalyst and thin electrolyte layer for illustrating that wetting limits CO.sub.2 availability at the catalyst.

[0047] FIG. 4 shows an exemplary flow chart of a method in accordance with embodiments of the present invention.

[0048] FIG. 5 shows an example of the gas/sorbent composite electrolyte/catalyst interface at the cathode comprising a composite electrolyte interfacing with a catalyst for a gas-phase CO.sub.2 reduction system in accordance with embodiments of the present invention.

[0049] FIG. 6 shows a graph illustrating the CO.sub.2 uptake of a composite material, in accordance with embodiments of the present invention.

[0050] FIG. 7 shows graphs illustrating the CO.sub.2 uptake at different CO.sub.2 partial pressures for different composite electrolytes, with different ionic liquid-to-silica molar ratios, denoted x, and with different MTMS-to-TEOS molar ratios, denoted y.

[0051] FIG. 8 illustrates water uptake for composite electrolytes, with different ionic liquid-to-silica molar ratio x, and with different MTMS-to-TEOS molar ratio y, in accordance with embodiments of the present invention.

[0052] FIG. 9 shows graphs illustrating CO.sub.2 uptake in a composite electrolyte, in accordance with embodiments of the present invention, as a function of water content for different MTMS-to-TEOS molar ratios, denoted y.

[0053] FIG. 10 shows a schematic drawing of an ATR-FTIR (Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy) set-up under a gas flow.

[0054] FIG. 11 shows the differential absorbance spectrum measured under dry CO.sub.2 in an ATR-FTIR flow cell set-up for CO.sub.2 gas as such (bottom curve) and for a CO.sub.2 sorbent nanocomposite coating (top curve).

[0055] FIG. 12 shows the differential absorbance spectrum measured under humid CO.sub.2 in an ATR-FTIR flow cell set-up for a CO.sub.2 sorbent nanocomposite coating with ionic liquid-to-silica molar ratio of x=2, and MTMS-to-TEOS molar ratios of y=0 or y=1.

[0056] FIG. 13 shows a schematic drawing of a sorbent-catalyst assembly for gas phase CO.sub.2 reduction in accordance with embodiments of the present invention.

[0057] FIG. 14 shows a graph illustrating the production rate for Hydrogen evolution reaction and for CO.sub.2 reduction from composite electrolytes in accordance with embodiments of the present invention.

[0058] FIG. 15 shows a conceptual drawing of a sorbent-catalyst assembly in a membrane electrode configuration comprising an electrode on each side of the composite electrolyte of a system for gas-phase reduction, in accordance with embodiments of the present invention.

[0059] FIG. 16 shows a practical implementation of an electrolyzer comprising a sorbent-catalyst assembly in a membrane electrode configuration, comprising an electrode on each side of the composite electrolyte of a system for gas-phase reduction, in accordance with embodiments of the present invention.

[0060] FIG. 17 shows the Faraday efficiencies for Hydrogen evolution reaction and for CO.sub.2 reduction for different current densities obtained using the practical implementation illustrated in FIG. 16.

[0061] FIG. 18 illustrates the CO.sub.2 uptake for sol-gel synthesized TFSI-based ionic liquid-silica composites.

[0062] Any reference signs in the claims shall not be construed as limiting the scope.

[0063] In the different drawings, the same reference signs refer to the same or analogous elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0064] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0065] The terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

[0066] Moreover, the terms top, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

[0067] It is to be noticed that the term comprising, used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression a device comprising means A and B should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

[0068] Reference throughout this specification to one embodiment or an embodiment means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases in one embodiment or in an embodiment in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0069] Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

[0070] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0071] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0072] Where in embodiments of the present invention reference is made to a mesoporous host matrix, reference is made to a material that contains pores with diameters ranging between 2 and 200 nm. In embodiments of the present invention the host matrix may comprise micro- and microporous domains for high ionic liquid contents. The mesoporous nature allows it to effectively host other substances, like ionic liquids. In the context of the present invention, a mesoporous host matrix, such as silica, is used to immobilize an ionic liquid. In embodiments of the present invention the ionic liquid volume fraction of the ionic liquid immobilized in the pores of the mesoporous host matrix may be between 0% and 96% of the total volume of the host matrix or even between 40% and 96%, or even between 60% and 96% or even between 84% and 96% of the total volume of the host matrix. This fraction may be measured after extraction of the ionic liquid. The ionic liquid volume fraction may for example be checked via thermal gravimetric analysis wherein the mass of the mesoporous host matrix is measured after removing the ionic liquid by heating.

[0073] In a first aspect, embodiments of the present invention relate to a method for gas-phase CO.sub.2 reduction. FIG. 4 illustrates the method steps involved in the gas-phase CO.sub.2 reduction process, as described in the invention. The method begins with step 110, which involves providing a composite electrolyte (210) that comprises an ionic liquid (212) incorporated within a mesoporous host matrix (211). The composite electrolyte functions as a dual sorbent for CO.sub.2 and H.sub.2O, providing ionic conductivity. In step 120, the composite electrolyte (210) is exposed to a gas stream comprising CO.sub.2, which is subsequently sorbed into the composite electrolyte. Finally, step 130 involves the electrochemical reduction of the sorbed CO.sub.2 at an interface between the composite electrolyte (210) and a catalyst (220), with the composite electrolyte functioning as both a sorbent and ionic conductor to facilitate the electrochemical reaction.

[0074] In a second aspect, embodiments of the present invention relate to a system (200) for gas-phase CO.sub.2 reduction (see for example FIG. 5 and FIG. 13). The system comprises a composite electrolyte (210) that includes an ionic liquid (212) incorporated in a mesoporous host matrix (211), where the composite electrolyte (210) provides ionic conductivity and functions as a dual sorbent for CO.sub.2 and H.sub.2O. The system also includes an sorbent-catalyst assembly (230) comprising a catalyst (220) interfacing with the composite electrolyte (210) wherein the sorbent-catalyst assembly (230) is configured such that, in operation, CO.sub.2 sorbed in the composite electrolyte (210) is reduced at the interface between the composite electrolyte (210) and the catalyst (220).

[0075] In embodiments of the present invention an electrochemical cell comprises a working electrode (250a) in contact with the composite electrolyte and a counter electrode (250b) to balance the charge in the cell by completing the circuit during the electrochemical process. A system in accordance with embodiments of the present invention may comprise such an electrochemical cell.

[0076] In embodiments of the present invention the electrochemical cell comprises a pseudo-reference electrode (250c) to provide a stable reference potential.

[0077] In embodiments of the present invention the mesoporous host may comprise silica. In embodiments of the present invention the ionic liquid (212) is selected from the group comprising bis(trifluorosulfonyl)imide-based ionic liquids.

[0078] In embodiments of the present invention the gas stream also comprises H.sub.2O which is sorbed into the composite electrolyte (210). In embodiments of the present invention the provided (110) composite electrolyte (210) comprises a hydrophobic modification of the host matrix (211) for maintaining decoupled transport of CO.sub.2 and H.sub.2O within the composite electrolyte.

[0079] In embodiments of the present invention the silica host matrix (211) is modified with methyl sidechains to decrease the hydrophilicity of the host matrix (211). The modification of the silica host matrix (211) may for example be achieved using methyltrimethoxysilane. The ratio of methyltrimethoxysilane to tetraethyl orthosilicate used to form the silica host matrix is varied may for example be selected to obtain a specific hydrophobicity of the host matrix (211).

[0080] FIG. 5 shows an sorbent-catalyst assembly (230) comprising a composite electrolyte (210) interfacing with a catalyst (220) for a gas-phase CO.sub.2 reduction system in accordance with embodiments of the present invention. Please note that for simplicity of the figure no electrodes are shown. Examples of an sorbent-catalyst assembly with electrodes are shown in FIG. 13 and FIG. 15 (in these graphs the catalyst and electrode are one same metal).

[0081] At the catalyst interface, the following reactions may occur:

[0082] In an alkaline environment: aCO.sub.2+bH.sub.2O+ce.sup..fwdarw.Product+dOH.sup.

[0083] In an acidic environment: aCO.sub.2+bH.sup.++ce.sup..fwdarw.Product+dH.sub.2O

[0084] The ionic liquid (212) is incorporated within a mesoporous host matrix (211), for example comprising silica. For illustrative purposes, only a limited number of channels are shown; however, in reality, the composite comprises a significantly higher volumetric ionic liquid fraction, contained in a largely interconnected network. H.sub.2O is expected at the interface of the silica, indicated by 212b, while CO.sub.2 is expected ionic liquid region of the channel, indicated by 212a. The silica, by its nature, remains predominantly hydrophilic; for a ratio of 1 mole of TEOS to 1 mole of MTMS, only one in eight Si bonds will be CH.sub.3 instead of O. Therefore, it is expected that a water layer will still be present at the silica interface with the channel, with its thickness depending on the relative humidity. An ionic conductivity ranging from 1.8 to 2.8 mS/cm was obtained for relative humidities ranging from 7 to 90%, for an ionic liquid-to-TEOS molar ratio of 2, and methyl functionalization with MTMS-to-TEOS molar ratio of 1. For non-methyl modified ionic liquid-silica composites, a maximum uptake was reached around 0.13 mmol/g for x=0.5 and 2 mol [BMP][TFSI]:mol SiO.sub.2 at 303 K and 1 bar, 0.12 mmol/g for x=1 and 0.10 mmol/g for x=1.5. A 5:1 molar selectivity of CO.sub.2 sorption compared to N.sub.2 was assessed under dynamic conditions from a 50-50 vol % feed. Additionally, the water uptake of composites at 85% RH was assessed for different ionic liquid-to-SiO.sub.2 contents and for various MTMS-to-TEOS contents, spanning from 0.3 to 2.3 mmol/g. Thus, by modifying the SiO.sub.2 hydrophobicity, the H.sub.2O uptake can be tuned.

[0085] Different problems may be solved by a method and system in accordance with embodiments of the present invention amongst which: limited CO.sub.2 solubility in aqueous CO.sub.2 electroreduction; loss of CO.sub.2 to the bicarbonate equilibrium in alkaline aqueous electrolyte, complicating the electrode surface pH control and reducing selectivity for CO.sub.2 reduction products.

[0086] Another problem that may be solved by a method and system in accordance with embodiments of the present invention is the competition between H.sub.2O and CO.sub.2 for sorption which is needed for lower energy of regeneration.

[0087] The first problem is the low solubility of CO.sub.2 in aqueous electroreduction processes. This issue is solved by using a composite electrolyte (210) with an ionic liquid (212) incorporated within a mesoporous host matrix (211), which enhances CO.sub.2 uptake and provides ionic conductivity.

[0088] The CO.sub.2 sorption from flue gas-like streams was studied using CO.sub.2 adsorption isotherms at 303 K. The confinement of the ionic liquid within the silica matrix was found to enhance CO.sub.2 uptake, particularly at lower partial pressures and with higher ratios of ionic liquid to host matrix. The uptake for a nanocomposite with a composition ratio of 2 (mol ionic liquid:mol SiO.sub.2) at different partial pressures is shown as the top curve in FIG. 6. The CO.sub.2 uptake capacity was approximately 0.03 mmol/g of the composite at 0.1 bar and about 0.125 mmol/g at 1 bar. A breakdown of the expected uptake was included to evaluate the performance of the composite relative to its individual componentsnamely silica and [BMP][TFSI]. The expected uptake for pure SiO.sub.2 (310) represents the uptake per unit surface area normalized to the SiO.sub.2 weight fraction in the composite. The expected uptake for [BMP][TFSI], indicated by reference number (320), is based on its reported uptake at various partial pressures. The region indicated by reference number (330), indicates the uptake enhancement, which is the difference between the actual uptake of the nanocomposite and the sum of the expected uptake for the individual components.

[0089] The modification of the silica network with methyl sidechains was explored to decrease the hydrophilicity of the silica network. In an exemplary embodiment of the present invention the functionalization was achieved by varying the silica precursors and their ratio, namely TEOS and MTMS. The silica-to-ionic liquid molar ratio, denoted x, was varied between 0.5 and 2. The MTMS-to-TEOS molar ratio, denoted y, was varied between 0 and 1. These variations in nanocomposite composition did not appear to impact the CO.sub.2 uptake in dry environments, as exemplified in FIG. 7.

[0090] In this exemplary embodiment of the present invention the materials synthesized were shown to have a conductivity around 2 mS/cm (between 1.14-2.75 mS/cm) for different compositions and relative humidity ranging from 5-90%, as measured by impedance spectroscopy for coated films on interdigitated electrode assemblies. The conductivity was seen to increase with relative humidity for all measured samples. The obtained results are in the order of magnitude of typical ionic liquid conductivities (2 mS/cm) and electrolytes used in electrochemical cells (10 mS/cm).

[0091] Another problem which may be solved by a system or method in accordance with embodiments of the present invention is the competition between H.sub.2O and CO.sub.2 sorption in nanocomposites. As discussed before this is solved by a composite electrolyte which comprises a hydrophobic modification of the host matrix (211) for maintaining decoupled transport of CO.sub.2 and H.sub.2O within the composite electrolyte. In embodiments of the present invention the H.sub.2O and CO.sub.2 transport are decoupled by decreasing the silica network hydrophilicity, thereby providing control over the respective CO.sub.2 and water uptake.

[0092] In embodiments of the present invention the CO.sub.2 and H.sub.2O uptake were successfully decoupled by the addition of a methyl group in the host matrix (e.g. silica structure). To achieve this, in an exemplary embodiment of the present invention, a MTMS precursor was used in combination with TEOS, with their molar ratio defined as y (mol MTMS:mol TEOS). As shown in FIG. 8, higher silica contents (lower x, mol IL:mol SiO.sub.2) resulted in higher water uptake, while addition of MTMS decreased the water uptake.

[0093] In embodiments of the present invention a non-competitive CO.sub.2/H.sub.2O uptake is achieved by addition of MTMS. This modifies the silica host matrix with methyl sidechains and decreases hydrophilicity of the host matrix. Many sorbent CO.sub.2 materials showcase CO.sub.2 displacement in presence of H.sub.2O, resulting in lower CO.sub.2 uptake in humid environment. In an exemplary embodiment of the present invention MTMS was added to a host matrix, which has an ionic liquid-to-silica molar ratio of x=2, with a MTMS:TEOS molar ratio of 1, resulting in a constant CO.sub.2 uptake for the synthesized nanocomposite, regardless of the relative humidity in the feed stream. FIG. 9 shows the CO.sub.2 uptake as a function of the water content in nanocomposites with silica:ionic liquid molar ratio x varied between 0.5 and 2, and MTMS:TEOS molar ratio y, varied between 0 and 1. The samples were exposed to high relative humidity, and the CO.sub.2 uptake was measured by thermogravimetric analysis with He as carrier gas, at a total gas flow rate of 25 mL/min. Results were corrected with an empty pan measurement to account for buoyancy effects of the pan volume, and corrected for the additional sample volume based on the sample density. Subsequent drying steps were implemented via Thermal Gravimetric Analysis (TGA), and between each drying step, the uptake was measured again. As visible in FIG. 9, MTMS contents equal to 1 for x=0.5 and x=2 yield the flattest slopes, that is, the least dependence of CO.sub.2 uptake with regards to H.sub.2O content. Note that the water content range over which the measurement is performed is not directly equal to the equilibrium water uptake measured for the composites. Equilibration time prior to the dynamic measurement, performed in order to have a stable baseline, resulted in water evaporation. Even though the amount of sample loaded was kept around 50 mg, some variations in amount and particle size of the sample loaded can have an influence on the water desorption rate. Some slightly negative water contents are reported, which can be attributed to measurement errors from the water uptake.

[0094] The previous results, obtained using exemplary embodiments of the present invention, showed no dependency of CO.sub.2 uptake on water content for higher MTMS contents (1 mol MTMS:mol TEOS). Complementarily to these results, ATR-FTIR was used to evidence that under high relative humidity flows, CO.sub.2 is not being displaced by H.sub.2O. As visible on FIG. 10 (left hand side), an ATR-FTIR crystal was coated with a composite electrolyte, as used in embodiments of the present invention, and mounted in a gas-tight cell. The chamber of the cell was flushed under a gas flow, either dry N.sub.2, dry CO.sub.2, or humidified CO.sub.2 (85% relative humidity). The incident beam passes via the bottom of the crystal, as depicted on the right hand side of the figure, before being reflected over its length. The resulting signal detected showcases the chemistry present in the layer, given that the thickness of the coating is higher than the penetration depth of the IR beam.

[0095] For each sample, a background transmission spectrum was measured under dry N.sub.2 until a stable signal was obtained. CO.sub.2 in the gas form has four vibrational modes: symmetric and asymmetric stretching, and two degenerate bending modes. Its asymmetric stretching mode produces two peaks at 2337 cm.sup.1 and 2362 cm.sup.1 accounting for P- and R-branches corresponding to the coupling of rotational-vibrational excitations. These can be seen in the bottom curve which shows ATR-FTIR differential spectra for CO.sub.2 gas. Subsequently, dry CO.sub.2 was passed and a differential absorption spectrum could be obtained. Upon sorption in the composite electrolyte, the asymmetric stretch signature changes shape, with a marked peak at the position of the previously defined P-branch, and a shoulder at lower frequencies. This can be seen in the top curve in FIG. 11.

[0096] FIG. 12 shows the differential absorbance spectrum measured under humidified CO.sub.2 gas (curves a) and dry CO.sub.2 gas (curves b) with respect to a dry N.sub.2 background in an ATR-FTIR flow cell set-up for a CO.sub.2 sorbent nanocomposite coating with ionic liquid-to-silica molar ratio of x=2, and MTMS-to-TEOS molar ratios of y=0 (i.e. without MTMS; left graph) or 1 (i.e. with MTMS; right graph). The subtracted background was measured under dry N.sub.2 flow. Upon humidification of the CO.sub.2 stream, water peaks at 1650 cm.sup.1 and in the 3000-3500 cm-1 range became visible. The MTMS-free samples with x=2 mol IL:mol SiO.sub.2 showed a dry-to-humid P-branch peak height ratio of 0.71, which implies a competition between H.sub.2O and CO.sub.2 uptake. On the other hand, the MTMS:TEOS containing sample with a molar ratio of y=1 showcased a P-branch peak ratio of 0.94, thereby showing little-to-no competition between H.sub.2O and CO.sub.2 uptake, as previously shown from the TGA analysis. The peak height under humid environment was monitored periodically for 85 min, and no further decrease was observed for the sample with and without MTMS.

[0097] Another problem which may be solved by a system or method in accordance with embodiments of the present invention is that loss of CO.sub.2 due to the formation of bicarbonate in aqueous electrolytes complicates pH control at the electrode surface, reducing the selectivity for CO.sub.2 reduction products. In embodiments of the present invention this is solved by performing CO.sub.2 reduction in the gas phase, in absence of liquid electrolyte. Instead, a nanocomposite electrolyte enables decoupled H.sub.2O and CO.sub.2 transport.

[0098] FIG. 13 shows a schematic drawing of a system (200) for gas phase CO.sub.2 reduction in accordance with embodiments of the present invention. The figure shows a sorbent-catalyst assembly (230) comprising a catalyst (220) interfacing with the composite electrolyte (210) a working electrode (250a) in contact with the composite electrolyte, a counter electrode (250b), and a pseudo-reference electrode (250c). During operation of such a cell, CO.sub.2 sorbed in the composite electrolyte (210) is reduced at the interface between the composite electrolyte (210) and the catalyst (220). The sorbent-catalyst assembly (230) was obtained by depositing the composite electrolyte (210) onto a non-conductive wafer substrate (260) with three electrodes: the working electrode (250a) which may for example be a silver working electrode, the counter electrode (250b) which may for example be a platinum counter electrode, and the pseudo-reference electrode (250c) which may for example be a silver pseudo-reference electrode. In this exemplary embodiment of the present invention the electrodes are made of catalyst material, i.e. Ag (cathode 250a) and Pt (anode, 250b).

[0099] The invention is not limited to the design shown in FIG. 13. Also other designs are possible. It is for example possible to have an electrochemical cell comprising a sorbent-catalyst assembly with interdigitated electrodes.

[0100] For obtaining the production rate (expressed in mol/min/cm.sup.2) of this sorbent-catalyst assembly, the assembly was placed in a chamber under a humidified CO.sub.2 stream (241 is the gas feed, 242 is the gas outlet). CO.sub.2 reduction was successfully performed directly from the sorbent layer at a current density of 0.13 mA/cm.sup.2. CO.sub.2 reduction products were measured at the outlet of the chamber by gas chromatography. The production rate towards CO, a CO.sub.2 reduction product, was much higher for the MTMS-containing sample compared to the MTMS-free sample (FIG. 14). On the other hand, the Hydrogen Evolution Reaction (HER, the conversion of protons to H.sub.2) was inhibited for the MTMS-containing sample.

[0101] FIG. 15 shows a conceptual drawing of a sorbent-catalyst assembly (230), comprising a composite electrolyte (210) between two electrodes (250), of a system in accordance with embodiments of the present invention. In embodiments of the present invention the sorbent-catalyst assembly (230) is configured such that, in operation, CO.sub.2 sorbed in the composite electrolyte (210) is reduced at the interface between the composite electrolyte (210) and the catalyst (220). In this exemplary embodiment of the present invention the sorbent-catalyst assembly is configured as a vapor-fed Membrane Electrode Assembly set-up with CO.sub.2/H.sub.2O sorbent separator. The composite electrolyte (210) has three functions: it acts as a CO.sub.2/H.sub.2O sorbent and transport medium, as a separator and as an ionic conductor. The sorbent-catalyst assembly comprises high surface area porous electrodes at both sides of the composite electrolyte (210). These serve both as electrodes (250) and as catalyst (220).

[0102] In embodiments of the present invention the system may be an electrolyser comprising a sorbent-catalyst assembly in a membrane electrode configuration for gas phase operation.

[0103] FIG. 16 provides a practical implementation of an electrolyser comprising a Membrane Electrode Assembly in accordance with embodiments of the present invention. The composite electrolyte (210) serves as the medium where CO.sub.2 and H.sub.2O are sorbed and subsequently transported to the catalyst interface for electrochemical reactions. In this implementation, the composite electrolyte could be a sorbent-impregnated porous matrix, such as Celgard, which is shown in the SEM image at the bottom of the figure. A MTMS-modified sorbent solid composite electrolyte may be used. The invention is, however, not limited thereto. Any non-chemically active porous material, with mechanical stability, high porosity and preferably a minimal thickness is adequate. In this example a polypropylene membrane with 55% porosity and 25 m thickness was used (Celgard 2500).

[0104] The catalysts (220) which also serves as electrodes (250) to facilitate the electrochemical reduction of CO.sub.2, directly interface with the composite electrolyte to enable efficient conversion of the sorbed CO.sub.2 into valuable products. The high surface area porous electrodes (250) provide both catalytic activity and electrical contact. The working electrode (250a), which may be a silver electrode, is shown in the SEM images at two different scalesa 25 m scale depicting a porous silver structure and a 500 nm scale illustrating a silver nanomesh. These structures provide a high surface area for enhanced reaction kinetics. The counter electrode (250b) may be a platinum (Pt) felt or nanomesh, serving as the complementary electrode in the cell to complete the electrochemical circuit.

[0105] Electrolysers in accordance with embodiments of the present invention may comprise flow fields. In alternative systems the flow fields may be integrated into a device.

[0106] The gas distribution systems (240) are used to provide flow fields for efficient gas distribution and also serve as electrical contacts for the cell. These systems ensure that CO.sub.2 and H.sub.2O are effectively delivered to the active sites within the composite electrolyte and maintain proper operation of the sorbent-catalyst assembly. Overall, the combination of a high surface area porous electrode, a well-distributed gas supply, and a sorbent-impregnated composite electrolyte ensures a good performance for the electrochemical reduction of CO.sub.2. The SEM images illustrate the specific structural properties of the electrodes and electrolyte, highlighting the porosity and nanostructures that enhance the efficiency of the cell.

[0107] At the working electrode (250a) the following reaction occurs:

##STR00006##

[0108] This represents the reduction of CO.sub.2 to carbon monoxide (CO) with the simultaneous production of water.

[0109] At the counter electrode (250b) the following reaction occurs:

##STR00007##

[0110] This is the oxidation of water, which produces oxygen gas (O.sub.2), releases electrons, and generates protons (H+). These complementary reactions at the electrodes complete the electrochemical process of CO.sub.2 reduction in the cell.

[0111] In an exemplary embodiment of the present invention, the cathode, which serves as working electrode 250a, consists of a titanium (Ti) felt used for electrical contact, combined with a commercially available porous silver (Ag) electrode. The anode, which serves as counter electrode, is composed of platinum (Pt) coated titanium (Ti) felt, also commercially available. The membrane used between the anode and cathode is a Celgard membrane filled with a silica-[BMP][TFSI] nanocomposite, which has been modified with methyltrimethoxysilane (MTMS) to enhance its properties.

[0112] The anode flow is supplied with 100 mL/min of humidified CO.sub.2, while the cathode flow is maintained with 100 mL/min of dry CO.sub.2. This arrangement facilitates different environmental conditions at each electrode, optimizing the reactions for efficient CO.sub.2 reduction and oxygen evolution in the sorbent-catalyst assembly.

[0113] FIG. 17 shows the measured Faraday efficiency for different current densities. The Faraday Efficiency (FE) is calculated using the following formula:

[00002]$\mathrm{FE}\left[\%\right]=\frac{\mathrm{charge}\mathrm{used}\mathrm{for}\mathrm{product}\mathrm{formation}}{\mathrm{total}\mathrm{charge}\mathrm{passed}}=\frac{\mathrm{zNF}}{Q}$ [0114] Where: [0115] Q is the total charge passed, [0116] N is the number of moles electrolyzed, [0117] z is the number of electrons involved in the reaction, [0118] F is the Faraday constant

[0119] The Faraday efficiency represents the percentage of the total charge that is used for the formation of the desired product. The error bars in FIG. 17 show the upper and lower Faraday efficiency measured for 2 subsequent measurements. The Membrane Electrode Assembly achieved a higher current density compared to the planar configuration (comprising a planar substrate onto which 3 electrodes are deposited as illustrated in FIG. 13), while also operating at a lower cell potential. CO production was still observed. Additionally, the total Faradaic efficiency FE.sub.CO+FE.sub.H2 was close to 100%, as parasitic reactions involving anodic products were effectively preventedan improvement over the planar setup where such side reactions were present.

[0120] In embodiments of the present invention, the method comprises producing the composite electrolyte using a one-pot sol-gel method. The sol-gel method enables in-situ confinement of the ionic liquid in the mesoporous host matrix. The use of a one-pot synthesis method allows for conformal coatings of CO.sub.2 sorption nanocomposites, which enhances the supply of CO.sub.2 to the electrode. In an exemplary embodiment of the present invention the sol-gel synthesis of ionic liquid-silica nanocomposites involves a one-pot synthesis process using formic acid, [BMI][TFSI] (1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), and TEOS (tetraethyl orthosilicate). Initially, the components are mixed in solution, and as the reaction proceeds, the solution undergoes a transition to a gel state. The synthesis process includes a series of hydrolysis and condensation reactions:

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[0121] The condensation reaction (4) results in the formation of a silica network, which encapsulates the ionic liquid, creating a nanocomposite structure. This encapsulation effectively integrates the ionic liquid within the silica matrix, providing a dual-functional material for enhanced CO.sub.2 uptake and electrochemical performance. In embodiments of the present invention compounds may be added in the initial solution for attaching methyl sidechains to the silica matrix (for example MTMS may be added).

[0122] Alternative methods, such as impregnation and grafting, may be used for producing the composite electrolyte. In impregnation methods, pre-synthesized or commercial silica particles are mechanically mixed with an ionic liquid, until a gel is obtained.

[0123] Various ionic liquids can be used in the composite electrolyte to enhance CO.sub.2 sorption and provide ionic conductivity. Examples include imidazolium-based ionic liquids, such as 1-ethyl-3-methylimidazolium (EMI) and 1-octyl-3-methylimidazolium (OMI), as well as pyrrolidinium-based ionic liquids like 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][TFSI]). These ionic liquids were chosen for their CO.sub.2 sorption properties, which ensure that CO.sub.2 can be efficiently adsorbed without forming covalent bonds, allowing for easier and energy-efficient desorption during regeneration. The selection of ionic liquids can be tailored based on the specific requirements of the application, including factors like CO.sub.2 partial pressure, operational temperature, and desired sorption capacity.

[0124] FIG. 18 illustrates the CO.sub.2 uptake for sol-gel synthesized TFSI-based ionic liquid-silica composites at a pressure of 0.4 bar. The figure shows the performance of different ionic liquid cations, specifically EMI (1-ethyl-3-methylimidazolium), BMI (1-butyl-3-methylimidazolium), HMI (1-hexyl-3-methylimidazolium), and BMP (1-butyl-1-methylpyrrolidinium), all with TFSI (bis(trifluoromethylsulfonyl)imide) as the anion.

[0125] The confinement of the ionic liquid within the silica matrix results in a CO.sub.2 uptake that is six times higher compared to the uptake of the non-confined ionic liquid at a partial pressure of CO2 (P.sub.CO2) of 0.4 bar. This significant enhancement in uptake highlights the positive impact of the confinement effect on improving CO.sub.2 sorption capacity in the nanocomposite material.