WO2024146821A1

WO2024146821A1 - Electrochemical carbon dioxide reduction to formic acid

Info

Publication number: WO2024146821A1
Application number: PCT/EP2023/087187
Authority: WO
Inventors: Bert Erik M DE MOT
Original assignee: Oxylum B.V.
Priority date: 2023-01-06
Filing date: 2023-12-21
Publication date: 2024-07-11

Abstract

The present invention relates to improved electrolytic devices comprising one or more electrolytic cells for the reduction of gaseous carbon dioxide to formic acid, the cell comprising a cathode configured to reduce carbon dioxide and produce formate; an anode configured to oxidise water or hydroxide ions and produce oxygen; a separator provided between the cathode and the anode, comprising an anion-exchange membrane contacting the cathode, a bipolar membrane contacting the anode, an electrolyte compartment provided between the anion-exchange membrane and the bipolar membrane comprising an electrolyte contacting the anion-exchange membrane and the bipolar membrane. The invention further relates to methods for the reduction of gaseous carbon dioxide into formic acid employing said devices.

Description

ELECTROCHEMICAL CARBON DIOXIDE REDUCTION TO FORMIC ACID

FIELD OF THE INVENTION

[0001] The present invention relates to improved electrolytic devices comprising one or more electrolytic cells forthe reduction of gaseous carbon dioxide to formic acid. The invention further relates to methods for the reduction of gaseous carbon dioxide into formic acid employing said devices.

BACKGROUND ART

[0002] Recently, electrochemical CO2 reduction has gained a lot of interest as a potential solution for the increasing atmospheric CO2 concentration, wherein, at the same time, CO2 is converted into valuable carbon-based compounds. Past research has focused on a better understanding of the effect of different parameters (e.g., temperature, pressure, pH, aqueous or non-aqueous solvents, type and concentration of electrolytes, type and morphology of catalysts, impurities, type of electrodes, type of membranes, cell configuration and flow, impurities, etc.) on the CO2 reduction reaction. In orderto move towards an industrially mature application, the focus of the research has now shifted towards reactor design. The current state of the art reactor is a gas diffusion electrode (GDE) based zero-gap reactor where both electrodes and a polymer electrolyte membrane are pressed together, the membrane separating the electrodes. In this arrangement, the redox reactions take place at the interface between each electrode and the membrane. In particular, the electrochemical reduction of CO2 typically takes place at the interface between the cathode and the membrane. This set-up shows a high CO2 mass transfer and energy efficiency.

[0003] A major problem in GDE-based zero-gap CO2 electrolysis is the formation of salts at the cathode, which is detrimental to the performance of the reactor. Precipitation of (bi)carbonate is observed as a consequence of the reaction between the supplied CO2 and the hydroxide ions generated at the cathode in alkaline media. In addition, solid or partially soluble salts like oxalate or formate salts can also cause problems. Other problems related to this set-up include dehydration of the membraneelectrode interface and poor removal of products. These problems are typically related to poor water management in the system. In the currently known processes, CO2 may be purged/bubbled through water at elevated temperatures in order to introduce water (in gaseous form) into the electrochemical cell in the form of humidified gas. For instance, WO2019051609 discloses a process and apparatus for electrocatalytically reducing carbon dioxide, wherein the carbon dioxide gas may be humidified with water vapour (i.e. in gaseous form), such as to a relative humidity of e.g. about 90%, before delivering the humidified gas to the cathode. The gas may be humidified by bubbling the carbon dioxide through water heated to a sub-boiling temperature. W02021110824 discloses further improvements to the water management in zero-gap electrolytic devices. On the other hand, when too much water is added, the pores of the cathode fill with water, increasing the contribution of H2 formation and decreasing faradaic efficiency for CO2 reduction. This complicated water management is a serious impediment to the commercial viability of zero-gap electrolytic devices.

[0004] Another disadvantage of zero-gap electrolytic devices as described above is that formic acid is produced in the form of formate, requiring a subsequent acidification step in order to obtain the commercially relevant formic acid. Additionally, the formate stream may be contaminated with other components like alkali metals from the anolyte, CO, H2, corroded cathode catalyst particles or ions therefrom and corroded anode catalyst particles or ions therefrom.

[0005] US2020080211 discloses an electrochemical cell comprising proton exchange membranes for the electrolysis of carbon dioxide to produce carbon monoxide. US202008021 1 only describes formate formation in the context of an acidic anode reaction used in combination with a cation exchange membrane at the anode.

[0006] US2019010620A1 discloses an electrochemical cell comprising both a cation exchange membrane and an anion exchange membrane, employing an acidic anolyte and thus requiring noble metal anode catalysts.

[0007] There is a need for improved electrochemical devices and methods for the conversion of CO2 into formic acid which are less dependent on external humidification, allow the production of high-purity formic acid, and reduce the need for a separate acidification step to convert formate to formic acid.

[0008] It is an object of the present invention to provide an electrolytic device and associated methods for the conversion of gaseous carbon dioxide into formic acid.

[0009] It is an object of the present invention to provide an electrolytic device and associated methods for the conversion of carbon dioxide into formic acid which minimizes or eliminates the need for an acidification step to convert formate to formic acid, which allows high-purity formic acid to be recovered, and/or which allows high-concentration formic acid solutions to be produced (e.g. in the range of 10-30 wt.%) without requiring an additional evaporation step.

[0010] It is an object of the present invention to provide an electrolytic device and associated methods for the conversion of carbon dioxide into formic acid which has low dependency on humidification of the CO2 feed.

SUMMARY OF THE INVENTION

[0011] The present inventor(s) have found that an electrolytic device for formic acid production from gaseous CO2 comprising an electrolyte compartment between cathode and anode delimited by an anion exchange membrane (AEM) on the cathode side and by a bipolar membrane (BPM) on the anode side, using an alkaline anolyte achieves one or more of the above-mentioned objects of the invention.

[0012] The working principle will first be explained in general terms, after which the various embodiments of the invention are detailed. The cathode is a porous or gas-diffusion electrode, allowing gaseous CO2 to be fed to the system, and comprises a catalyst which preferentially catalyzes the reduction of CO2 to formate. The CO2 will be forced in the porous cathode structure toward the interface between the cathode and the AEM. There, the CO2 will be reduced (generally according to: CO2 + H2O + 2e^_ —> HCOO- + OH ) when a sufficiently high cell and/or reduction potential is applied. The water in this equation is mainly provided by wetting of the AEM with water coming from the electrolyte compartment (humidification of the CO2 prior to entry in the cathode is optional but not necessary). The formed formate and hydroxide anions are able to pass through the AEM towards the electrolyte compartment. At the anode, an oxidation takes place, preferably the oxygen evolution reaction (generally according to 4OH- -^ 02 + 2H2O + 4e ) in an alkaline or neutral solution. The bipolar membrane facilitates water dissociation into protons and hydroxide ions, and comprises an anion exchange membrane facing the anode, and a cation exchange membrane facing the interior of the electrolyte compartment. The proton supply from the BPM to the electrolyte compartment acidifies the electrolyte compartment and converts the formate at least partly into formic acid. The hydroxide ion supply from the BPM to the anode compartment replenishes at least part of the hydroxides that are consumed by the anodic reaction. This increases the amount of time the anolyte can be used before requiring replenishment (or reduces the anolyte flow rate in case the anolyte is continuously supplied), resulting in less consumption of the anolyte, and also contributes to lowering or eliminating salt buildup and the associated the need to rinse the anode seen with alternative systems. By employing an alkaline environment in the anode compartment, cheap and abundant metal catalysts like nickel can be used as anode catalyst. The water dissociation by the BPM acts in synergy with the need to neutralize formate to formic acid.

[0013] It was found that the electrolytic device and methods described herein allow surprisingly high faradaic efficiencies to be achieved, even upon sustained long-term operation of the electrolytic device, with very low anolyte consumption. The formic acid produced is of high purity and present in protonated form to a large extent, minimizing or even eliminating the need for subsequent acidification. High concentrations of formic acid, up to e.g. 30 wt.% are obtainable directly from the electrolytic device without requiring a subsequent concentration step.

[0014] In a first aspect of the present invention, there is thus provided a carbon dioxide electrolytic device for the production of formic acid, comprising at least one electrolytic cell, the electrolytic cell comprising:

(i) a cathode configured to reduce carbon dioxide and produce formate;

(ii) a gas flow path facing the cathode, configured to supply gas comprising carbon dioxide to the cathode;

(iii) an anode configured to oxidise water or hydroxide ions and produce oxygen;

(iv) a liquid flow path facing the anode configured to supply an anolyte to the anode;

(v) a separator provided between the cathode and the anode; characterised in that the separator comprises:

• an anion-exchange membrane contacting the cathode;

• a bipolar membrane contacting the anode;

• an electrolyte compartment provided between the anion-exchange membrane and the bipolar membrane, wherein the electrolyte compartment comprises an electrolyte contacting the anion-exchange membrane and the bipolar membrane; wherein the bipolar membrane comprises an anion-exchange layer contacting the anode and a cation-exchange layer contacting the electrolyte.

[0015] In a preferred embodiment of the invention, the liquid flow path facing the anode is configured to supply an anolyte to the anode having a pH of 6 or more, preferably 7 or more. In some embodiments the liquid flow path facing the anode is configured to supply an anolyte to the anode having a pH of 8 or more, such as 10 or more or 12 or more. Accordingly, in a highly preferred embodiment of the invention, the liquid flow path facing the anode comprises an anolyte, having a pH of 6 or more, preferably 7 or more. In some embodiments the liquid flow path facing the anode comprises an anolyte having a pH of 8 or more, such as 10 or more or 12 or more. The anolyte is an aqueous composition.

[0016] In a preferred embodiment of the invention, the electrolyte compartment further comprises a first inlet for providing a liquid to the electrolyte compartment, and a first outlet for withdrawing a liquid comprising the formic acid from the electrolyte compartment, wherein it is preferred that the first outlet is fluidly connected, optionally via one or more conduits, to cooling means configured for cooling the liquid comprising the formic acid withdrawn via the first outlet.

[0017] In a preferred embodiment of the invention, the electrolytic device comprises an anode wherein the anode comprises a metal or metal oxide catalyst, preferably a metal or metal oxide catalyst comprises a d-block or f-block element, more preferably a metal or metal oxide catalyst comprising Ni, Zn, Ti, Co or Fe, most preferably a metal or metal oxide catalyst comprising Ni.

[0018] In a preferred embodiment of the invention, the anolyte comprises a base selected from alkali metal hydroxides and alkaline earth metal hydroxides.

[0019] In a preferred embodiment of the invention, the electrolytic device further comprises a plurality of electrolysis cells, an anode current collector connected to the anode of one of the plurality of electrolysis cells, a cathode current collector connected to the cathode of one of the plurality of electrolysis cells, and a power supply configured to supply an electric current between the anode and the cathode.

[0020] In another aspect of the invention, there is provided a method for the electrochemical conversion of gaseous carbon dioxide into formic acid using the electrolytic device of the present invention, comprising the steps of:

(a) providing a gas comprising carbon dioxide to the cathode of the electrolytic device via the gas flow path;

(b) providing an anolyte to the anode of the electrolytic device via the liquid flow path;

(c) converting carbon dioxide into formic acid by applying an electric potential to the electrolytic device.

BRIEF DESCRIPTION OF THE FIGURES

[0021] Fig 1. shows an embodiment of the electrolytic device (100) of the invention comprising an energy source (110), a cathode compartment (111), an anion exchange membrane (107) contacting the cathode, an anode compartment (112), a bipolar membrane (108) contacting the anode, and an electrolyte compartment (109).

[0022] Fig.2 shows another embodiment of the electrolytic device (200) of the invention wherein the gas stream comprising carbon dioxide is humidified in a humidification means (202) before being provided to the system via a gas inlet (201).

[0023] Fig.3 shows another embodiment of the electrolytic device (300) wherein the first outlet (305) is fluidly connected to cooling means for cooling the liquid comprising formic acid withdrawn via the first outlet.

[0024] List of references: 100 - system; 101 - gas inlet; 102 - gas outlet; 103 - first inlet; 104 - first outlet; 105 - anolyte inlet; 106 - anolyte outlet; 107 - anion exchange membrane; 108 - bipolar membrane; 109 - electrolyte compartment; 110 - energy source; 111 - cathode compartment; 112 — anode compartment; 201 - gas inlet; 202 - humidification unit; 203 - gas outlet; 204 - first inlet; 205 - first outlet; 206 - anolyte inlet; 207 - anolyte outlet; 208 - cathode compartment; 209 - anode compartment; 210 - anion exchange membrane; 211 - bipolar membrane; 212 - electrolyte compartment; 213 - energy source; 301 - gas inlet; 302 - gas outlet; 303 - first inlet; 304 - cooling means; 305 - first outlet; 306 - anolyte inlet; 307 - anolyte outlet; 308 - energy source; 309 - cathode compartment; 310 - anode compartment; 311 - anion exchange membrane; 312 - bipolar membrane; 313 - electrolyte compartment.

[0025] Fig. 4 shows another embodiment of the electrolytic device comprising two cells according to the invention.

DESCRIPTION OF THE EMBODIMENTS

[0026] In the following detailed description, preferred embodiments are described in detail to enable the practise of the invention. Although the invention is described with reference to these specific preferred embodiments, it will be understood that the invention is not limited to these preferred embodiments. But to the contrary, the invention includes numerous alternatives, modifications and equivalents as will become apparent from consideration of the following detailed description. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

[0027] The expression “comprise” and variations thereof, such as, “comprises” and “comprising” as used herein should be construed in an open, inclusive sense, meaning that the embodiment described includes the recited features, but that it does not exclude the presence of other features, as long as they do not render the embodiment unworkable. The word 'distinct' highlights that the distinct steps of the process steps are different.

[0028] The expressions “one embodiment”, “a particular embodiment”, “an embodiment” etc. as used herein should be construed to mean that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of such expressions in various places throughout this specification do not necessarily all refer to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. For example, certain features of the disclosure which are described herein in the context of separate embodiments are also explicitly envisaged in combination in a single embodiment.

[0029] In the present description of the invention, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration only of specific embodiments in which the invention may be practised. Parenthesized or emboldened reference numerals affixed to respective elements merely exemplify the elements by way of example, with which it is not intended to limit the respective elements. It is to be understood that other embodiments may be utilised and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

[0030] The singular forms “a,” “an,” and “the” as used herein should be construed to include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its broadest sense, that is as meaning “and/or” unless the content clearly dictates otherwise.

[0031] All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

The Electrolytic Device

[0032] In a first aspect of the present invention, there is provided a carbon dioxide electrolytic device for the production of formic acid, comprising at least one electrolytic cell, the electrolytic cell comprising:

(i) a cathode configured to reduce carbon dioxide and produce formate;

• an anion-exchange membrane contacting the cathode;

• a bipolar membrane contacting the anode;

• an electrolyte compartment provided between the anion-exchange membrane and the bipolar membrane, wherein the electrolyte compartment comprises an electrolyte contacting the anion-exchange membrane and the bipolar membrane; wherein the bipolar membrane comprises an anion-exchange layer contacting the anode and a cation-exchange layer contacting the electrolyte. [0033] A particular embodiment of the device of the present invention is shown in Fig. 1 . Fig 1 . shows an embodiment of the electrolytic device (100) comprising an energy source (110), a cathode compartment (111), an anion exchange membrane (107) contacting the cathode, an anode (112) compartment, a bipolar membrane (108) contacting the anode, and an electrolyte compartment (109). A gas stream comprising carbon dioxide is provided to the system via a gas inlet (101). The depleted carbon dioxide stream, potentially enriched with gaseous by-products such as carbon monoxide or hydrogen exits the system via a gas outlet (102). Similarly, a liquid anolyte, typically an alkaline electrolyte is fed to the anode via an anolyte inlet (105) and removed through the anolyte outlet (106). The first electrolyte compartment inlet (103) provides liquid electrolyte or an aqueous composition like demineralized water (in case a solid electrolyte is used) to the electrolyte compartment so that the formed formic acid is removed from the electrolyte compartment via the first outlet (104).

[0034] In accordance with the invention, the electrolytic device comprises an anion-exchange membrane (AEM) contacting the cathode. As the skilled person will understand, the anion exchange membrane is a semipermeable membrane which allows anion migration from the cathodic compartment to the electrolyte compartment, but is essentially impermeable to other components of the cathodic compartment. The anion exchange membrane comprises an anion-conducting ionomer which comprises a polymer backbone functionalized with ionizable groups which allow the exchange of anions (the cation being bound to the polymer backbone). In preferred embodiments of the invention, the polymer backbone is selected from the group consisting of perfluorinated polymers, poly(styrene) polymers, poly(arylene ether sulfone) polymers, poly(arylene ether ketone) polymers, poly(benzimidazole) polymers, poly(vinylchloride) polymers, poly(aryl ether) polymers, poly(sulfone) polymers, poly(ethersulfone) polymers, poly(ether sulfone ketone) polymers, poly(ether ethyl ketone) polymers, poly(phthalazinone ether sulfone ketone) polymers, poly(acrylonitrile) polymers, poly(olefin) polymers, cellulose acetate polymers, poly(vinyl alcohol) polymers, poly(benzimidazole) polymers, poly (aery late) polymers and poly(vinyl acetate) polymers, copolymers thereof, and combinations thereof, preferably selected from perfluorinated polymers, poly(styrene) polymers, poly(acrylate)polymers, poly(alkyleneimine) polymers, copolymers thereof and combinations thereof. The ionizable groups are preferably selected from the group consisting of ammonium, imidazolium, pyridinium, guanidinium, pyrrolidinium, morpholinium, phosphonium or combinations thereof, preferably ammonium. The ammonium group may be provided in the form of a quaternary ammonium ion, a primary amine, a secondary amine or a tertiary amine. As the skilled person understands, primary, secondary or tertiary amines can be ionized to a quaternary ammonium ion depending on the pH conditions. In preferred embodiments of the invention, the AEMs is stable in alkaline media. As will be understood by the skilled person, some CO2 will be converted to bicarbonate at the cathode-AEM interface and subsequently permeate via the AEM into the electrolyte compartment. Once in the electrolyte compartment, most of the bicarbonate is converted back into CO2 under the influence of the proton supply from the BPM, thereby advantageously allowing very pure formic acid to be recovered despite the occurrence of some CO2-pumping. The CO2 can be vented or recovered from the electrolyte compartment.

[0035] In accordance with the invention, the electrolytic device comprises a bipolar membrane (BPM) comprising an anion-exchange layer contacting the anode and a cation-exchange layer contacting the electrolyte. As the skilled person will understand, BPMs are essentially impermeable both to anions and cations and prevent amongst others the diffusion of gaseous carbon dioxide to the anode. The BPM may comprise further layers between the cation-exchange layer contacting the electrolyte and the anion-exchange layer contacting the anode. Typically the BPM consists of the cation-exchange layer contacting the electrolyte in the electrolyte compartment, the anion-exchange layer contacting the anode, and an interfacial layer in between. The cation exchange membrane comprises a cationconducting ionomer which comprises a polymer backbone functionalized with ionizable groups which allow the exchange of cations (the anion being bound to the polymer backbone). In preferred embodiments of the invention, the polymer backbone is selected from the group consisting of perfluorinated polymers, poly(styrene) polymers, poly(arylene ether sulfone) polymers, poly(arylene ether ketone) polymers, poly(benzimidazole) polymers, polyvinylchloride) polymers, poly(aryl ether) polymers, poly(sulfone) polymers, poly(ethersulfone) polymers, poly(ether sulfone ketone) polymers, poly(ether ethyl ketone) polymers, poly(phthalazinone ether sulfone ketone) polymers, poly(acrylonitrile) polymers, poly(olefin) polymers, cellulose acetate polymers, poly(vinyl alcohol) polymers, poly(benzimidazole) polymers, poly(acrylate) polymers and poly(vinyl acetate) polymers, copolymers thereof, and combinations thereof, preferably selected from perfluorinated polymers, poly(styrene) polymers, poly(acrylate)polymers, poly(alkyleneimine) polymers, copolymers thereof and combinations thereof. The ionizable groups are preferably selected from the group consisting of sulfonic acid, carboxylic acid, phosphoric acid, phosphonic acid, and combinations thereof. Suitable examples of cation exchange membranes which can be comprised in the BPM comprise a polymer backbone selected from perfluorinated polymers functionalized with sulfonic acid ionizable groups (e.g. Nation® brand unreinforced types N117 and N120 series) or a polymer backbone selected from perfluorinated polymers functionalized with carboxylic acid ionizable groups (e.g. Flemion® brand).

[0036] The anion exchange layer comprised in the BPM is typically a polymer membrane as described herein earlier in the context of the AEM. The interface layer of the BPM can be smooth, corrugated or heterogenous. In highly preferred embodiments of the invention, the BPM is stable in both alkaline and acidic media.

[0037] In some embodiments of the invention, the electrolyte compartment has a thickness between 25-2000 pm, preferably between 30-1500 pm, preferably 100-1250 pm, more preferably 200-1000 pm, even more preferably 300-900 pm and most preferably 300-600 pm.

[0038] In accordance with preferred embodiments of the invention, the electrolyte compartment further comprises a first inlet for providing a liquid to the electrolyte compartment, and a first outlet for withdrawing a liquid comprising the formic acid from the electrolyte compartment. In some embodiments of the invention, the first inlet is configured to provide an aqueous liquid to the electrolyte compartment, preferably demineralized water. The first inlet may be fluidly connected to an aqueous liquid reservoir, preferably a demineralized water reservoir.

[0039] In a preferred embodiment of the invention, the first outlet is fluidly connected, optionally via one or more conduits, to cooling means configured for cooling the liquid comprising the formic acid withdrawn via the first outlet. The liquid stream recovered from the electrolyte compartment via the first outlet will typically be above room temperature during normal operation of the electrolytic device. Hence, cooling prevents significant evaporation and thus product loss of the formic acid. Cooling can be performed by any cooling means known to the skilled person, such as heat exchange with a liquid coolant (e.g. an aqueous coolant like water) or with a gas (e.g. air). In preferred embodiments, the cooling means comprises an air-cooled heat exchanger.

[0040] A particular embodiment of the system of the present invention is shown in Fig. 3. The system (300) comprises an energy source (308), a cathode (309), an anion exchange membrane (311) contacting the cathode, an anode (310), a bipolar membrane (312) contacting the anode, and an electrolyte compartment (313). A gas stream comprising carbon dioxide is provided to the system via a gas inlet (301). The depleted carbon dioxide stream, potentially enriched with gaseous by-products such as carbon monoxide or hydrogen exits the system via a gas outlet (302). Similarly, a liquid anolyte, typically an alkaline electrolyte is fed to the anode via an anolyte inlet (306) and removed through the anolyte outlet (307). The first electrolyte compartment inlet (303) provides liquid electrolyte or an aqueous composition like demineralized water to the electrolyte compartment so that the formed formic acid is removed from the electrolyte compartment via the first outlet (305) which is fluidly connected to cooling means for cooling the liquid comprising formic acid withdrawn via the first outlet.

[0041] In a preferred embodiment of the invention, the liquid flow path facing the anode is configured to supply an anolyte to the anode having a pH of 6 or more, preferably 7 or more. In some embodiments the liquid flow path facing the anode is configured to supply an anolyte to the anode having a pH of 8 or more, such as 10 or more or 12 or more. Accordingly, in a highly preferred embodiment of the invention, the liquid flow path facing the anode comprises an anolyte, having a pH of 6 or more, preferably 7 or more. In some embodiments the liquid flow path facing the anode comprises an anolyte having a pH of 8 or more, such as 10 or more or 12 or more. By using an alkaline anodic reaction, there is no need to use noble metal anode catalysts, while the combination with the BPM allows the lifetime of the anolyte to be prolonged before replenishment is needed (or allows the anolyte feed rate to be low in case a continuous anolyte feed is employed).

[0042] In preferred embodiments of the invention, the anolyte comprises a base selected from alkali metal hydroxides, alkaline earth metal hydroxides, preferably selected from the group consisting of NaOH, CsOH, Ca(OH)₂ and KOH, most preferably KOH.

[0043] In preferred embodiments of the invention, the anode comprises a metal or metal oxide catalyst, preferably a metal or metal oxide catalyst comprising a d-block or f-block element, more preferably a metal or metal oxide catalyst comprising Ni, Zn, Ti, Co or Fe, most preferably selected from the group consisting of Ni, Zn or Ti. A preferred anode catalyst material is Nickel, for example in the form of nickel foam.

[0044] The anode catalyst should be suitable for catalysing the production of oxygen, according to the reaction: 4OH- —> O₂ + 2H₂O + 4e^_ .

[0045] In a preferred embodiment of the invention, the metal or metal oxide catalyst comprised in the anode does not comprise a metal selected from the group consisting of Au, Pt, or Pd, more preferably not selected from the group consisting of Au, Pt, Ru, Rh, Pd, Os or Ir .

[0046] The anode may take the form of a gas diffusion electrode, porous electrode or solid electrode and is the site at which the oxidation half-reaction takes place. In one embodiment, the anode may be bonded to the bipolar membrane by means of a suitable ionomer, for example an anionic ionomer. In another embodiment, the anode may be embedded partially in the bipolar membrane. In another embodiment of the invention, the anode is a non-continuous three-dimensional structure, for example a mesh or an expanded metal or metal oxide that adjoins the bipolar membrane. In another embodiment, the anode is an ionomer-supported catalyst layer which is placed in contact with the bipolar membrane. The anode may also contain materials that are customary in electrodes such as binders, ionomers, fillers, hydrophilic additives, conductive aids, etc. which are not particularly restricted. In some embodiments of the invention, the anion comprises an ionomeric binder. Examples of suitable ionomeric binders are the cation-conducting ionomers and/or the anion-conducting ionomers described herein in the context of the anion-exchange membrane and the cation-exchange layer of the bipolar membrane.

[0047] In a preferred embodiment of the invention, the cathode comprises a metal or metal oxide catalyst, preferably a metal or metal oxide catalyst comprising a d-block of f-block element, more preferably a metal or metal oxide catalyst comprising Sn, Bi, In, Mn, Al, Cu, Au, Ag, or Pt, most preferably a metal or metal oxide catalyst comprising Sn, Bi, or In. In highly preferred embodiments of the invention, the cathode comprises Bi2O3. In some embodiments of the invention, the cathode comprises the catalyst particles or polymers on a supporting carbon fibre structure.

[0048] The cathode catalyst should be suitable for catalysing the reduction of carbon dioxide to formic acid, according to the following overall reaction scheme: CO2 + 2e + H2O —> HCOO- + OFT. Without wishing to be bound by any theory, it is believed the reactive species may be bicarbonate formed by dissolution of CO2 in water drawn from the electrolyte compartment (or present due to humidification of the C02-containing gas feed) at or near the cathode-membrane interface. Similarly, without wishing to be bound by any theory, the present inventors cannot exclude that formic acid is formed, which is subsequently deprotonated to produce formate which migrates through the AEM. For the purposes of the present invention, this is also considered to be encompassed by the production of formate at the cathode.

[0049] The cathode takes the form of a gas diffusion electrode or porous electrode such that gaseous CO2 can be fed to the cathode, and is the site at which the reduction half-reaction takes place. In one embodiment, the cathode may be bonded to the AEM by means of a suitable ionomer, for example an anionic ionomer. In another embodiment, the cathode may be embedded partially in the AEM. In another embodiment of the invention, the cathode is a non-continuous three-dimensional structure, for example a mesh or an expanded metal or metal oxide that adjoins the AEM. In another embodiment, the cathode is an ionomer-supported catalyst layer which is placed in contact with the AEM. The cathode may also contain materials that are customary in electrodes such as binders, ionomers, fillers, hydrophilic additives, conductive aids, support materials, etc. which are not particularly restricted. In some embodiments the cathode contains a support material selected from as C, Si, boron nitride (BN), and/or boron-doped diamond. In some embodiments the cathode contains a conductive fillers (e.g. carbon), a nonconductive fillers (e.g. glass) and/or a hydrophilic mineral additive (e.g. AI2O3, MgC>2). In some embodiments, the cathode comprises an ionomeric binder. Examples of suitable ionomeric binders are the cation-conducting ionomers and/or the anion-conducting ionomers described herein in the context of the anion-exchange membrane and the cation-exchange layer of the bipolar membrane.

[0050] In some preferred embodiments, the cathode is an ionomer-supported catalyst layer obtainable by spraying a solution or dispersion of metal or metal oxide catalyst powder, preferably tin oxide or bismuth oxide, in an appropriate solvent together with an ionomer resin, such as a cation-exchange resin or an anion-exchange resin, preferably an anion-exchange resin. Examples of suitable ionomeric resins are the cation-conducting ionomers and/or the anion-conducting ionomers described herein in the context of the anion-exchange membrane and the cation-exchange layer of the bipolar membrane. Other suitable methods of depositing the cathode catalyst include sputter deposition, electroplating, immersion coating, physical vapour deposition or chemical vapour deposition.

[0051] The metal or metal oxide catalyst comprised in the cathode preferably comprises a major amount (by weight) of particles having a size in the range of 0.5 to 500 nm, preferably within the range of 10 to 200 nm, more preferably within the range of 50 to 150 nm.

[0052] In preferred embodiments of the invention, the cathode is obtainable by spraying a solution or dispersion of metal or metal oxide catalyst powder, preferably tin oxide or bismuth oxide, in an appropriate solvent together with an ionomer resin, such as a cation-exchange resin or an anion- exchange resin, preferably an anion-exchange resin, and the catalyst loading is less than 100 mg/cm², preferably less than 50 mg/cm², more preferably less than 10 mg/cm². The catalyst loading is preferably more than 0.1 mg/cm², preferably more than 1 mg/cm².

[0053] The electrolyte comprised in the electrolyte compartment may be a solid or liquid electrolyte. When a liquid electrolyte is used, it will be mixed with the formic acid which is recovered from the electrolyte compartment and depending on the desired use of the formic acid an additional separation step may be needed. The liquid electrolyte is not particularly limited and can be any aqueous solution of a salt, preferably an aqueous solution comprising a carbonate or bicarbonate salt. Preferred liquid electrolytes are aqueous compositions comprising a salt selected from alkali metal salts of (bi)carbonate, alkaline earth metal salts of (bi)carbonate, quaternary amine salts of (bi)carbonate (in particular tetraethyl ammonium, tetramethyl ammonium, or NHT). The term “(bi)carbonate” should be construed to mean carbonate, bicarbonate, or a combination thereof. When a solid electrolyte is used, the electrolyte compartment can simply be flushed with water, thereby recovering high-purity formic acid which was previously not obtainable by electrolytic methods. A preferred solid electrolyte is a cation-conducting ionomer also called a cation-exchange resin. The solid electrolyte comprised in the electrolyte compartment is a cation-conducting ionomer also called a cation-exchange resin. This resin is typically employed in combination with a solvent such as water (sometimes referred to as swollen or hydrated), such that a so-called gel-polymer electrolyte is comprised in the electrolyte compartment. The cation-conducting ionomer comprises a polymer backbone functionalized with ionizable groups which allow the exchange of cations (the anion being bound to the polymer backbone). In preferred embodiments of the invention, the polymer backbone is selected from the group consisting of perfluorinated polymers, poly(styrene) polymers, poly(arylene ether sulfone) polymers, poly(arylene ether ketone) polymers, poly(benzimidazole) polymers, poly(vinylchloride) polymers, poly(aryl ether) polymers, poly(sulfone) polymers, poly(ethersulfone) polymers, poly(ether sulfone ketone) polymers, poly(ether ethyl ketone) polymers, poly(phthalazinone ether sulfone ketone) polymers, poly(acrylonitrile) polymers, poly(olefin) polymers, cellulose acetate polymers, poly(vinyl alcohol) polymers, poly(benzimidazole) polymers, poly(acrylate) polymers and poly(vinyl acetate) polymers, copolymers thereof, and combinations thereof, preferably selected from perfluorinated polymers, poly(styrene) polymers, poly(acrylate)polymers, poly(alkyleneimine) polymers, copolymers thereof and combinations thereof. The ionizable groups are preferably selected from the group consisting of sulfonic acid, carboxylic acid, phosphoric acid, phosphonic acid, and combinations thereof. Hence, in some preferred embodiments of the invention, the electrolyte comprised in the electrolyte compartment comprises a gel polymer electrolyte which comprises a cation-conduction ionomer and water, the cation-conducting ionomer comprising a polymer backbone selected from perfluorinated polymers, poly(styrene) polymers, poly(acrylate)polymers, copolymers thereof and combinations thereof functionalized with ionizable groups selected from the group consisting of sulfonic acid, carboxylic acid, phosphoric acid, phosphonic acid, and combinations thereof, preferably sulfonic acid.

[0054] In the context of the ionomers described herein for the ion-exchange membranes and the solid electrolyte:

• Examples of suitable perfluorinated polymers include polytetrafluoroethylene (PTFE), perfluorosulfonic acid polymers (PFSA), perfluoroalkyl carboxylic acid polymers (PFCA) and perfluorosulfonic acid ionomer (PFSI), as well as partially per-fluorinated polymers such as copolymers of polyvinylidene fluoride and hexafluoropropylene (PVDF/HFP), copolymers of polytetrafluoroethylene and polyferrocenylsilanes (PFS), copolymers of ethylene and tetrafluoroethylene, and copolymers in which at least one constituent monomer is an a,p,p- trifluorostyrene monomer.

• Examples of suitable polystyrene polymers include poly(p-methyl styrene), poly(m-methyl styrene), poly(p-tertiary-butyl styrene), poly(p-chlorostyrene), poly(m-chlorostyrene), poly(p- fluorostyrene), hydrogenated polystyrene.

• Examples of suitable poly(arylene ether ketone) polymers include polyether ketone (PEK), polyether ether ketone (PEEK), polyetherketone ketone (PEKK) and polyetheretherketonketone (PEEKK).

• Examples of suitable poly(olefin) polymers include polyethylene and polypropylene.

• Examples of suitable cellulose acetate polymers include polycellulose diacetate and polycellulose triacetate.

[0055] In some embodiments of the invention, the electrolyte comprised in the electrolyte compartment comprises a gel polymer electrolyte which comprises a cation-conduction ionomer and water, the cation-conducting ionomer comprising an optionally cross-linked styrene-divinylbenzene copolymer backbone functionalized with sulfonic acid groups (e.g. AmberChrom® 50WX2 50-100 mesh, PORO® XS). In some embodiments of the invention, the electrolyte comprised in the electrolyte compartment comprises a gel polymer electrolyte which comprises a cation-conduction ionomer and water, the cation-conducting ionomer comprising a crosslinked polystyrene backbone functionalized with sulfonic acid groups. In some embodiments of the invention, the electrolyte comprised in the electrolyte compartment comprises a gel polymer electrolyte which comprises a cation-conduction ionomer and water, the cation-conducting ionomer comprising a poly(acrylate) backbone functionalized with sulfonic acid a groups (e.g. polyAMPS®).

[0056] It is preferred that at least one anode compartment in the electrolytic device and at least one cathode compartment in the electrolytic device comprises a conductive plate, preferably a metal or graphite conductive plate. Typically the anode conductive plate is on the other side of the anode than the BPM and the cathode conductive plate is on the other side of the cathode than the AEM. The conductive plates are also called current collectors, and are provided with suitable connectors for connecting the electrolytic device to an energy source. The liquid flow path and gas flow path respectively may be comprised in said conductive plate, or placed between the conductive plate and the anode/cathode. In case the flow path(s) are not provided in the conductive plate, they may be provided e.g. in a graphite plate placed between and in contact with the electrode and the current collector. Thus, in preferred embodiments, the electrolytic device of the present invention comprises an anode current collector connected to the anode, a cathode current collector connected to the cathode, and a power supply configured to supply an electric current between the anode and the cathode. In case the device comprises a plurality of electrolysis cells as described herein (also known as a “stack” of cells), the skilled person will understand that it is sufficient if one of the anodes and one of the cathodes (typically the ones at the ends of the stack) are provided with a current collector and power supply.

[0057] The components of the electrolytic device of the present invention can be interposed by seals. Suitable sealings are made from an electric isolating material and ensure a liquid and gas-tight operation.

[0058] The flow channel layout for the gas flow path and the liquid flow path is not particularly limited, and may be parallel, serpentine, spiral, interdigitated, pin, etc. As will be understood by the skilled person, in some flow channels, like an interdigitated flow channel, the inlet and outlet of the flow channel are physically separated by a permeable material which allows fluid connection between a fluid delivery part of the flow channel and a fluid removal part of the flow channel. Depending on the exact implementation the permeable material is typically part of the flow plate or of the electrode. The present invention does not require a liquid flow path for the cathode, hence it is preferred that the cathode compartment does not comprise a liquid flow path. The gas flow path comprises an inlet, which is typically fluidly connected to a means to supply gas comprising carbon dioxide (e.g. a gas reservoir like a gas cylinder), and an outlet. The gas flow path outlet is provided for removing the at least partially depleted gas from the electrolytic device. The gas flow path outlet may simply open into the atmosphere, such that during normal operation the at least partially depleted gas is removed from the electrolytic device by venting to the atmosphere. The gas flow path outlet may also be provided with means for at least partially recycling the partially depleted gas, for example by recirculating it to the gas flow path inlet, or to another useful purpose. Similarly, the liquid flow path comprises an inlet, which is typically fluidly connected to a means to supply anolyte (e.g. an anolyte reservoir like a storage container), and an outlet. The anolyte outlet is provided for removing the at least partially depleted anolyte from the electrolytic device. The anolyte outlet may be fluidly coupled to a gas-liquid separation means in order to allow the separation and recovery of the produced oxygen from the spent liquid anolyte.

[0059] In some embodiments the gas flow path is fluidly connected to humidifying means configured for humidifying the gas comprising carbon dioxide before entry into the gas flow path. In other words, the humidifying means is placed upstream from the gas flow path inlet. Humidification may take place by bubbling the gas through an aqueous composition such as water, by spraying water droplets into the gas comprising carbon dioxide, or by any other means available to the skilled person.

[0060] A particular embodiment of the system of the present invention is shown in Fig. 2. The system (200) comprises an energy source (213) a cathode compartment (208), an anion exchange membrane (210) contacting the cathode, an anode compartment (209), a bipolar membrane (21 1) contacting the anode, and an electrolyte compartment (212). A gas stream comprising carbon dioxide is humidified in humidification means (202) before being provided to the system via a gas inlet (201). The depleted carbon dioxide stream, potentially enriched with gaseous by-products such as carbon monoxide or hydrogen exits the system via a gas outlet (203). Similarly, a liquid anolyte, typically an alkaline electrolyte is fed to the anode via an anolyte inlet (206) and removed through the anolyte outlet (207). The first electrolyte compartment inlet (204) provides liquid electrolyte or an aqueous composition like demineralized water to the electrolyte compartment so that the formed formic acid is removed from the electrolyte compartment via the first outlet (205).

[0061] The first outlet, gas flow path outlet and/or the liquid flow path outlet can be connected to suitable means of monitoring the reaction such as gas or liquid chromatography coupled to a UV-Vis or IR spectrometer, and optionally coupled to a mass spectrometer, titration unit or ion chromatograph.

[0062] Advantageously, the system of the present invention is easily scalable by connecting multiple electrolytic cells. Accordingly, in particular embodiments, the system comprises a plurality of electrolysis cells as described herein. Preferably, the electrolytic device of the invention comprises a plurality of electrolysis cells, an anode current collector connected to the anode of one of plurality of electrolysis cells, a cathode current collector connected to the cathode of one of the plurality of electrolysis cells, and a power supply configured to supply an electric current between the anode and the cathode.

Method for the electrochemical conversion of CO2 to formic acid

[0063] In a different aspect of the invention, there is provided a method for the electrochemical conversion of gaseous carbon dioxide into formic acid using the electrolytic device of the present invention, comprising the steps of:

[0064] The electrolytic device employed in the method for the electrochemical conversion of gaseous CO2 is the device described herein earlier. Any embodiments of said device described herein are thus equally applicable to the method described herein. The method of the invention is termed a method for the electrochemical conversion of gaseous carbon dioxide since the carbon dioxide supply takes place directly to the cathode in gaseous form, without a preceding step taking place outside the cathode to absorb the CO2 in an aqueous solution (typically by conversion to (bi)carbonate), followed by supplying the aqueous solution (typically comprising (bi)carbonate) to the cathode, as is sometimes known in the art. Without wishing to be bound by any theory, the inventors believe that within the cathode, in particular at the cathode-membrane interface, a localised dissolution of CO2 into bicarbonate may in fact take place, such that the bicarbonate is the actual reactive species. Alternatively, dissolved CO2 without conversion to bicarbonate may be the reactive species. The water may originate from the electrolyte compartment and/or from humidity present in the CO2 containing gas stream which is fed to the system.

[0065] The CO2 containing gas stream provided in step (a) typically comprises between 0.4 % (v/v) and 100 % (v/v) of CO2, such as at least 1 % (v/v), at least 5 % (v/v) or at least 10 % (v/v). The CO2 containing gas stream provided in step (a) highly preferably comprises at least 15 % (v/v) of CO2, preferably at least 20 % (v/v) of CO2. Preferably the CO2 containing gas stream is a concentrated CO2 stream comprising at least 80% (v/v) of CO2, such as at least 90 % (v/v), at least 95 % (v/v) or at least 98 % (v/v). The method described herein has an acceptable efficiency also at lower CO2 concentrations. Thus, in some embodiments the CO2 containing gas stream comprises between 10 and 50% (v/v) of CO2, such as between 10 and 40% (v/v) of CO2 or between 20 % (v/v) and 50 %(v/v) of CO2. In some embodiments, the C02-containing gas stream is a combustion flue gas, in particular a flue gas from fossil fuel combustion, wood pellet combustion, biomass combustion or municipal waste combustion. Fossil fuel combustion may be coal, petroleum coke, petroleum, natural gas, shale oil, bitumens, tar sand oil, or heavy oils combustion, or any combination thereof. The combustion flue gas may optionally have been treated to reduce the water content, the O2 content, the SO2 content, and/or the NOx content.

[0066] The gas comprising carbon dioxide is preferably provided to the electrolytic device of the present invention at a rate between 0.5 to 20 mL/cm² per minute, preferably at a rate between 2 to 10 mL/cm² per minute, more preferably at a rate of 4 to 6 mL/cm² per minute. The gas supply is expressed based on the geometrical (i.e. macroscopic) surface area of the cathode.

[0067] The present inventors have found that during normal operation the water provided by wetting of the AEM is sufficient to facilitate the electrochemical reduction of CO2 at the cathode. Hence, in some embodiments of the invention, the gas comprising carbon dioxide comprises less than 1000 ppm of water, preferably less than 800 ppm of water, more preferably less than 500 ppm of water, even more preferably less than 100 ppm of water. In some embodiments the gas comprising carbon dioxide is substantially free of water.

[0068] In preferred embodiments of the invention, step (b) comprises providing an anolyte having a pH of 6 or more, preferably 7 or more to the anode. In some embodiments step (b) comprises providing an anolyte having a pH of 8 or more, such as 10 or more or 12 or more. As described herein earlier, the anolyte preferably comprises a base selected from alkali metal hydroxides, alkaline earth metal hydroxides, preferably those selected from the group consisting of NaOH, CsOH, Ca(OH)2 and KOH, most preferably KOH. It is preferred that the base is comprised in the anolyte at a concentration of at least 0.1 mol/L, preferably at least 0.5 mol/L, more preferably at least 0.75 mol/L.

[0069] The method of the invention preferably comprises a further step

(d) recovering formic acid from the electrolyte compartment.

[0070] The supply of carbon dioxide and anolyte, and the recovery of formic acid, may each independently take place in a continuous or discontinuous manner. Typically, each of the supply of carbon dioxide and anolyte, and the recovery of formic acid will take place continuously, such that the electrolytic device operates in a so-called steady-state mode for a prolonged period of time.

[0071] In preferred embodiments of the invention, step (d) comprises recovering the formic acid in the form of an aqueous solution comprising at least 1 wt.% (by total weight of the aqueous solution) formic acid, preferably comprising at least 5 wt.% formic acid, more preferably comprising at least 10 wt.% formic acid and most preferably at least 15 wt.% formic acid. The present inventors have found that the device and methods described herein allow the recovery of formic acid from the electrolyte compartment in highly concentrated aqueous solutions of about 30 wt.%. Hence, in some embodiments of the invention step (d) comprises recovering the formic acid in the form of an aqueous solution comprising at least 20 wt.% (by total weight of the aqueous solution) formic acid, preferably comprising at least 25 wt.% formic acid, more preferably comprising at least 27 wt.% formic acid. Step (d) preferably comprises recovering the formic acid in the form of an aqueous solution wherein the molar ratio of formic acid:formate is more than 3:1 , preferably more than 10:1 , more preferably more than 15:1. In some embodiments step (d) comprises recovering the formic acid in the form of an aqueous solution wherein the molar ratio of formic acid:formate is within the range of 3:1 to 20:1 .

[0072] Step (d) may comprise flushing the electrolyte compartment with an inert gas stream, such as a gas stream comprising at least 90 vol% of N2 or a noble gas, preferably at least 95 vol% of N2 or a noble gas, more preferably at least 99 vol% of N2 or a noble gas, such as at least 99.9 vol% of N2 or a noble gas. In a highly preferred embodiment of the invention, step (d) comprises providing a first flow of a first aqueous composition, preferably demineralized water, into the electrolyte compartment via the first inlet, and recovering a second flow of a second aqueous composition comprising the formic acid from the electrolyte compartment via the first outlet. Such embodiments are preferably combined with the embodiments described herein earlier wherein a solid electrolyte is used in the electrolyte compartment. This allows highly concentrated and pure formic acid to be recovered. The first aqueous composition is preferably demineralized water. In some embodiments the first aqueous composition comprises formic acid. By recirculating the formic acid flow recovered from the electrolyte compartment back to the electrolyte compartment, an enrichment in formic acid concentration may be achieved.

[0073] In a further highly preferred embodiment of the invention, step (d) comprising cooling the second aqueous composition recovered from the electrolyte compartment. Cooling to a temperature of less than 25 °C, preferably less than 20 °C is preferred. In some embodiments of the invention the step of cooling the second aqueous composition comprises decreasing the temperature of the second aqueous composition by at least 5 °C, preferably by at least 10 °C. Cooling can be performed by any cooling means known to the skilled person. In preferred embodiments, the cooling step comprises cooling employing an air-cooled heat exchanger. In some embodiments, the second aqueous composition recovered from the electrolyte compartment has a temperature before cooling in the range of 40-60 °C.

[0074] In some embodiments of the invention, the method described herein further comprises cooling the anolyte provided to the anode. Cooling the anolyte is preferably performed by cooling an anolyte storage tank which is fluidly connected to the liquid flow path facing the anode.

[0075] In preferred embodiments of the invention, the electrolytic cell is operated at a pressure within the range of 0.1 to 1 MPa, preferably within the range of 0.1 to 0.7 MPa, more preferably within the range of 0.1 to 0.5 MPa, and most preferably within the range of 0.1 to 0.3 MPa. As will be understood by the person skilled in the art, a backpressure regulator can be used to maintain the defined pressure range between 0.1 and 1 MPa, preferably between 0.1 and 0.5 MPa and more preferably between 0.1 and 0.3 MPa. In a particular embodiment, the backpressure regulator may be provided at the cathode compartment, the anode compartment, or the electrolyte compartment. In some embodiments of the invention a backpressure regulator is provided at two or all of the cathode compartment, the anode compartment, and the electrolyte compartment.. The back-pressure regulator allows to operate the system as envisaged herein in a high-pressure set-up, such as between 0.3 and 0.8 MPa, preferably between 0.5 and 0.7 MPa. Advantageously, since in the present system a gaseous compound, in particular carbon dioxide, is provided to the electrolytic device, increasing the pressure, in particular while maintaining a constant temperature, will increase the carbon dioxide concentration provided to the electrolytic device, according to the principles of the ideal gas law. This allows higher partial current densities for the carbon dioxide electroreduction as envisaged herein at lower cell voltages. Advantageously, by providing a back-pressure regulator at the different compartments, high pressure differences between the different compartments of the electrolytic device as envisaged herein can be prevented, reducing the risk of membrane failure.

[0076] A sufficient electrical potential between the anode and the cathode in the electrolysis device is applied for the cathode to reduce the CO2 into formic acid with the catalyst being used. In preferred embodiments of the invention, step (c) comprises operating the electrolytic device at a cell potential within the range of 2-10 V, preferably within the range of 3-8 V, more preferably within the range of 3-7 V. Higher electric potentials result in higher energy consumption and potentially in degradation of reactor components, such as electrodes.

[0077] In preferred embodiments of the invention, step (c) comprises operating the electrolytic device at a current density between 25 mA/cm² and 1000 mA/cm² , such as between 50 and 750 mA/cm², preferably between 100 and 500 mA/cm², more preferably between 200 and 300 mA/cm². The current density is expressed based on the geometrical (i.e. macroscopic) surface area of the cathode.

[0078] In some embodiments of the invention, the method is performed such that production of formic acid at a rate in the range of 0.08 to 2.60 pmol/cm²/s, preferably in the range of 1 .0 to 2.4 pmol/cm²/s, more preferably in the range of 1.5 to 2.2 pmol/cm²/s is achieved. The production rate is expressed based on the geometrical (i.e. macroscopic) surface area of the cathode.

[0079] In some embodiments of the invention, the method is performed such that production of formic acid at a yield in the range of 2.0 to 4.2 mol/kWh, preferably in the range of 2.2 to 4.0 mol/kWh, more preferably in the range of 2.5 to 3.5 mol/kWh is achieved.

EXAMPLES

1. Electrode preparation

Using SnOz

[0080] Commercial nickel foam was used as the anode.

[0081] To form the cathode, an ink spraying solution was prepared by mixing 1.2 g of tin (IV) oxide nanoparticles mixed with a Nation perfluorinated resin solution in an 85:15 w/w ratio respectively. The resulting mixture was diluted with a solution of isopropyl alcohol/water (1 :1 v/v, 36 mL) and thoroughly sonicated (NextGen, Lab120) for 30 minutes to ensure a homogenous mixture. The homogenised ink was airbrushed (Fengda, FE-180K) on a 192 cm² GDE (Sigracet 39 BB GDL). A hotplate (IKA, RH digital) was set to 60°C to accelerate the drying. Argon was used as carrier gas. The electrodes were weighed before and after the spray coating procedure to determine the final loading of the catalyst and ensure it was > 2.5 mg/cm².

Using BizOs

[0082] Commercial nickel foam was used as the anode.

[0083] To form the cathode, an ink spraying solution was prepared by mixing 1.2 g of bismuth (III) oxide nanoparticles mixed with a Sustainion XA-9 Alkaline ionomer in an 85:15 w/w ratio respectively. The resulting mixture was diluted with a solution of isopropyl alcohol/water (4:1 v/v, 35.2 mL) and thoroughly sonicated (NextGen, Lab120) for 30 minutes to ensure a homogenous mixture. The homogenised ink was airbrushed (Fengda, FE-180K) on a 192 cm² GDE (Sigracet 39 BB GDL). A hotplate (IKA, RH digital) was set to 60°C to accelerate the drying. Argon was used as carrier gas. The electrodes were weighed before and after the spray coating procedure to determine the final loading of the catalyst and ensure it was > 2.5 mg/cm².

2. Electrolytic device setup

[0084] A schematic presentation of the electrolytic device used is shown in Fig.4. The device comprises a Front plate (401), Front plate insulator (402), Current collector (403), Cathode gas flow path (first cell) (404), Cathode (first cell) (405), Anion exchange membrane (first cell) (406), Sealing and electrolyte compartment (first cell) (407), Bipolar membrane (first cell) (408), Anode (first cell) (409), Bipolar plate with anode liquid flow path (first cell) cathode gas flow path (second cell) (410), Cathode (second cell) (411), Anion exchange membrane (second cell) (412), Sealing and electrolyte compartment (second cell) (413), Bipolar membrane (second cell) (414), Anode (second cell) (415), Anode liquid flow channel (second cell) (416), Current collector (417), Back plate Insulator (418), Back plate (419).

3. Operation of the electrolytic device

[0085] The flow of CO2 is controlled by a gas flow controller (35639L, MTC-Analytic, Germany) and fed to the electrolytic device at a rate of 1 L/min where the CO2 reacts to form formate at the catalyst surface. The gaseous stream exiting the electrolytic device is sampled periodically to detect the amount of CO and H2 by-products. The sample is injected into a GC (Shimadzu, Japan) equipped with a ShinCarbon St 100/120 2mx1 mm column (Restek, USA). 10 ml/min helium was used as carrier gas. The column temperature was held constant at 40°C for 3 minutes, afterwards the temperature increased 40°C/min to 250°C.

[0086] Water is fed to the middle compartment by a peristaltic pump (Minipuls 2, Gilson, USA) at flows between 1 and 5 mL/min. The exiting flow is sampled periodically and subsequently analysed by HPLC. Samples were taken over several minutes (>5 min) to determine the actual flow rate. The HPLC (Alliance 2695, Waters, USA) is combined with a packed column (IC-Pak, Waters, USA) and a PDA detector (2996, Waters, at 210 nm). A sulphuric acid solution (10 mM) was used as the eluent for the HPLC. The samples are treated with perchloric acid to remove bicarbonate and filtered to remove potentially formed potassium perchlorate before injection.

[0087] On the anode side 100 mL/min of 1 M KOH was provided to the electrolytic device by a pump (2035 Manual, Verder, The Netherlands) where it reacts towards O2 on the surface of the Ni-Foam.

[0088] The energy for the reaction was provided by a power source (EA Elektro-Automatik EA-PS 9080-120 2U, country) operated under 6.5 V cell potential and a current under 19.2 A, whichever one was limiting.

[0089] The reactor reaches FE’s at the cathode which are over 70% when using the SnO2 catalyst, and over 85% when using the Bi2Os.

[0090] The aqueous solution recovered from the electrolyte compartment has a composition which depends on the flow in the electrolyte compartment. Higher flows give rise to formic acid concentrations in the range of 0.5-2.5 m% whereas lower flows give rise to concentrations in the range of 5-25 m%. The recovered solution also comprises formate in a ratio of formate/formic acid of 0.05-0.33. The pH of the solution is between 2.1 and 3.4.

[0091] The temperature of the aqueous solution recovered from the electrolyte compartment was not measured but the solution was cooled to 20°C using a heat exchanger, with the outer shell being kept at 20°C by a Huber Unichiller Ole, in order to avoid evaporation of the product.

Claims

1. A carbon dioxide electrolytic device for the production of formic acid, comprising at least one electrolytic cell, the electrolytic cell comprising:

(i) a cathode configured to reduce carbon dioxide and produce formate;

• an anion-exchange membrane contacting the cathode;

• a bipolar membrane contacting the anode;

• an electrolyte compartment provided between the anion-exchange membrane and the bipolar membrane comprising an electrolyte contacting the anion-exchange membrane and the bipolar membrane; wherein the bipolar membrane comprises an anion-exchange layer contacting the anode and a cationexchange layer contacting the electrolyte.

2. The electrolytic device of claim 1 wherein the electrolyte compartment further comprises a first inlet for providing a liquid to the electrolyte compartment, and a first outlet for withdrawing a liquid comprising the formic acid from the electrolyte compartment.

3. The electrolytic device of claim 2 wherein the first outlet is fluidly connected, optionally via one or more conduits, to cooling means configured for cooling the liquid comprising the formic acid withdrawn via the first outlet.

4. The electrolytic device of any one of the previous claims wherein the liquid flow path facing the anode comprises an anolyte, the anolyte having a pH of 7 or more, preferably a pH of 8 or more, more preferably a pH of 10 or more, more preferably a pH of 12 or more.

5. The electrolytic device of claim 4 wherein the anolyte comprises a base selected from alkali metal hydroxides, alkaline earth metal hydroxides, preferably selected from the group consisting of NaOH, CsOH, Ca(OH)2 and KOH, most preferably KOH.

6. The electrolytic device of any one of the previous claims, wherein the electrolyte comprised in the electrolyte compartment comprises a gel polymer electrolyte.

7. The electrolytic device of claim 6 wherein the gel polymer electrolyte comprises a cationexchange resin wherein the exchangeable cations are protons.

8. The electrolytic device according to any one of the previous claims further comprising an anode current collector connected to the anode, a cathode current collector connected to the cathode, and a power supply configured to supply an electric current between the anode and the cathode.

9. The electrolytic device according to any one of the previous claims further comprising a plurality of the electrolysis cells, an anode current collector connected to the anode of one of the plurality of electrolysis cells, a cathode current collector connected to the cathode of one of the plurality of electrolysis cells, and a power supply configured to supply an electric current between the anode and the cathode.

10. A method for the electrochemical conversion of gaseous carbon dioxide into formic acid using the electrolytic device of any one of claims 1-9, comprising the steps of:

(a) providing a gas comprising carbon dioxide to cathode of the electrolytic device via the gas flow path;

11 . The method of claim further comprising a further step:

(d) recovering formic acid from the electrolyte compartment.

12. The method of claim 11 comprising providing a first flow of a first aqueous composition, preferably demineralized water, into the electrolyte compartment via the first inlet, and recovering a second flow of a second aqueous composition comprising the formic acid from the electrolyte compartment via the first outlet.

13. The method of claim 12 comprising cooling the second aqueous composition recovered from the electrolyte compartment.

14. The method of any one of claims 10-13 wherein the anolyte has a pH of 7 or more, preferably a pH of 8 or more, more preferably a pH of 10 or more, more preferably a pH of 12 or more.

15. The method of any one of claims 10-14 wherein the cell is operated at a pressure within the range of 0.1 to 1 MPa.