Method and reactor for electrochemically reducing carbon dioxide

0001METHOD AND REACTOR FOR ELECTRO CHEMICALLY REDUCING CARBON DIOXIDE The invention is directed to a method for electrochemically reducing carbon dioxide, and to an electrochemical reactor.

0002The combustion of fossil fuels in activities such as electricity generation, transportation, and manufacturing produces billions of tons of carbon dioxide annually. Increasing concentrations of carbon dioxide in the atmosphere may be responsible for climate change, changing the pH of the ocean and other potentially damaging effects. As a result, a lot of effort is put in seeking ways to mitigate emissions of carbon dioxide.

0003One possible way of mitigating carbon dioxide emissions is to convert carbon dioxide into economically valuable materials, such as fuels and industrial chemicals. If the carbon dioxide is converted using energy from renewable sources, both mitigation of carbon dioxide emissions and conversion of renewable energy into a chemical form that can be stored for later use will be possible.

0004At the moment, only a very minor fraction of carbon dioxide emissions is actually used. As an end product of combustion, carbon dioxide has a high thermodynamic stability, often demanding for an energy intensive activation. Consequently, many processes for converting carbon dioxide into useful end products are energy -intensive and/or dangerous.

0005An interesting alternative conversion process involves the electrochemical reduction of carbon dioxide. Toxic or hazardous reducing agents are then replaced by clean electrons. In this case, the high

0006thermodynamic stability of carbon dioxide is by-passed by a simple one-electron reduction at an electrode, leading to in situ generation of reactive intermediates. Often, room temperature conditions are sufficient, considering that the energy of the electrons is determined by the applied voltage. Since the electrochemical reduction takes place on a cathode surface, the need for complex homogeneous organometallic catalysts is minimised. Furthermore, electricity will be increasingly of renewable origin in the future, making organic electro-synthesis a promising technology for environmentally friendly chemical processes. The electrochemical reduction of carbon dioxide can be applied for the synthesis of fuels like formic acid, methanol or methane. This allows storage of electric energy from periodic sustainable origin, like solar or wind energy.

0007Electro-synthesis is typically carried out in an electrochemical reactor connected to an external voltage or current source. The

0008electrochemical reactors can be categorised into undivided and divided cells. A divided cell has a cathode compartment with a cathode, an anode compartment with an anode, and an interface (or divider or separator) between cathode compartment and anode compartment. The interface in a divided cell functions as a separator. It is a barrier for specific species while allowing the transport of other species. The separator, for example, controls the flow of ions between the two compartments. In particular, the separator may selectively allow the transport of cations from the anode compartment into the cathode compartment while simultaneously preventing the transport of anions in the opposite direction. Alternatively, the separator may selectively allow the transport of anions from the cathode compartment into the anode compartment while preventing the transport of cations in the opposite direction. Such control over ion flow provides a means to enhance or enforce the desired reaction chemistry to occur in the fluids contained in the individual compartments with respective electrodes, and increase the current efficiency by preventing the cycling of species by sequential oxidation and reduction.

0009As mentioned above, the electrochemical reduction of carbon dioxide is a promising application in the field of electrochemistry. A variety of materials can be produced including formic acid, formates, carbon monoxide, hydrogen, syngas, methanol, methane, and propane. Products and process efficiencies depend amongst others on cell configuration, cathode material, surface area of the electrode, porosity of the electrode, catalyst, electrolyte (anolyte as well as catholyte) participating in reaction chemistry or acting as pH buffer, and process conditions like temperature, pressure, applied fluid flow, (over)potential, etc.

0010A conventional configuration in the electrochemical reduction of carbon dioxide comprises a divided cell with a cation exchange membrane (proton exchange membrane or cation membrane) separating an aqueous anolyte and catholyte. Protons (from water splitting) and/or cations from the specific anolyte, transfer to the cathode compartment where they react with the reduced carbon dioxide. Products formed are retained in the cathode compartment, while typically oxygen evolves at the anode. For example, US-A-2008/0 223 727 describes a continuous co-current electrochemical reduction of carbon dioxide using a catholyte mixture with a specific volume ratio of carbon dioxide gas and liquid catholyte solvent.

0011In another reactor configuration a fuel like hydrogen is fed as a gas to the anode compartment. The protons, produced by direct splitting of the hydrogen fuel at the anode, transfer through the cation exchange membrane to react with the reduced carbon dioxide as in the previously mentioned configuration.

0012In yet a further reactor configuration, an anion exchange membrane is employed as separator. The reduction of carbon dioxide at the cathode under wet conditions leads to the production of hydroxide ions that transfer through the anion exchange membrane and react with a hydrogen fuel at the anode to produce water. The product of the carbon dioxide reduction is retained in the cathode compartment.

0013AH the above reactor configurations have in common that they use an ion selective membrane of either the cation or the anion exchange type, and the use of an aqueous phase where at the cathode compartment carbon dioxide is solubilised in the aqueous phase. Reduction and oxidation take place in the cathode compartment and the anode compartment of the electrochemical reactor, respectively.

0014Bipolar membrane technology is relatively unexplored and finds an increasing number of applications in electro-catalytic membrane reactors (Balster et al., Chemical Engineering and Processing 2004, 43, 1115-1127). A bipolar membrane is catalytically active and may contribute to

0015electrochemical conversions. Bipolar membranes are known to be part of electro-dialysis stack design, and are configured together with anion and cation exchange membranes. A bipolar membrane is a synthetic membrane comprising two oppositely charged ion-exchanging layers in contact with each other. The bipolar membrane may be considered the combination of a cation exchange membrane and an anion exchange membrane. By this arrangement of charged layers, the bipolar membrane is not effective in transporting either cations or anions across the full width of the membrane, and is to be distinguished from the ion selective membranes employed in the conventional electrochemical reduction of carbon dioxide.

0016The use of a bipolar membrane in electrochemical reduction of carbon dioxide is known, however, from CN-A-102 912 374. This document describes the electrochemical reduction of carbon dioxide in an electrolytic tank comprising a cathode electrolysis compartment, catholyte, an anode electrolysis compartment, anolyte, and a bipolar membrane for dividing the cathode electrolysis compartment and the anode electrolysis compartment. The electrochemical electrolytic reduction of carbon dioxide in this document, indicating the use of anolyte and catholyte, implies ambient temperature and pressure.

0017While a process for the electrochemical reduction of carbon dioxide at elevated pressure is known e.g. from WO-A-2014/043651 and US-A-2013/0 105 304, in these processes the carbon dioxide is dissolved in water, which does not allow for the carbon dioxide densities in the reactor as achieved in the present invention. Consequently, these processes at elevated pressure but based on an aqueous solution also require the use of an electrolyte. In the present invention, carbon dioxide is the main reaction medium (the solvent) and a small fraction of water is added to form ionic reaction species at the cathode side. These ionic reactive species assure electric conductivity and will (simultaneously) participate in the overall reduction reaction of carbon dioxide. These two aspects (conductivity and participation in the reaction) assure a minimal use, or no use at all, of an electrolyte/catholyte solution.

0018Problems associated with the conventional electrochemical processes include low availability of gaseous reactant at atmospheric pressure, substantial over potentials at the electrodes (stack), a high internal resistance to overcome (low power density), co-transport of anolyte with protons through cation exchange membrane and competitive reaction of these with the anion in the cathode compartment and a requirement for a secondary chemical conversion outside the main electrochemical reactor, low water dissociation rate, difficulties in achieving full conversion of a liquid and a gaseous reactant in a single pass through an electrochemical reactor operated in continuous mode, integrity of a thin membrane during pressurisation or depressurisation in particular with a gaseous phase on one side and a liquid phase on the other side.

0019Objective of the invention is to overcome one or more of these problems faced in the prior art.

0020The inventors surprisingly found that this objective can, at least in part, be met by employing a specific type of separator that participates in the reaction chemistry.

0021Accordingly, in a first aspect the invention is directed to a method for electrochemically reducing carbon dioxide, comprising

0022a) introducing a water feed to an anode compartment of an electrochemical reactor, said anode compartment comprising an anode; b) introducing a carbon dioxide feed to a cathode compartment of an electrochemical reactor, said cathode compartment comprising a cathode; c) applying an electrical potential between the anode and the cathode in the electrochemical reactor sufficient for the cathode to reduce carbon dioxide into a reduced carbon dioxide product or product mixture,

0023wherein said anode compartment is separated from said cathode

0024compartment by a separator comprising a bipolar membrane, a

0025charge-mosaic membrane, or a layered mixture of anion and cation exchange resins, and wherein the pressure in the electrochemical reactor is 20 bara or more.

0026In accordance with the method of the invention, the separator participates in the intended reaction chemistry instead of only transferring either cations or anions as being state of the art in carbon dioxide

0027electrochemical reduction employing a cation exchange membrane or an anion exchange membrane, respectively. The method of the invention advantageously allows for high electric power density, low voltage

0028over-potentials and a low internal resistance for electricity/electric current, a reactive layer with potential high services for direct gas/dense phase and liquid/supercritical phase conversion of carbon dioxide, a tremendously increased dissociation rate of water into H⁺ and OH^" in the separator as compared to the equilibrium dissociation rate of water, production of OH^" at the internal interface in the membrane as compared to production at the anode, versatility to operate the electrochemical reactor in continuous, as well as in semi-continuous or batch mode, and prevention of mechanical rupture or leakage of the membrane during start up or shut down of the reactor.

0029Without wishing to be bound by any theory, the inventors believe that in accordance with the invention the reduction of carbon dioxide occurs in a reaction volume wherein the concentration of reactive species is exceptionally high. The result is a highly efficient conversion, wherein a very pure product can be obtained in one single production step.

0030The separator can comprise (or consist of) a bipolar membrane, a charge-mosaic membrane, or a layered mixture of anion and cation

0031exchange resins. A bipolar membrane is defined as a synthetic membrane comprising two oppositely charged ion-exchanging layers in contact with each other. A charge-mosaic membrane is defined as a membrane having a charge structure comprised of cation-exchange domains and anion-exchange domains which are alternately aligned and each of which penetrates the membrane from one side to the other side. The separator may also be a permeable layer confining a mixture of a cation exchange resin and an anion exchange resin. Preferably, the separator comprises or consists of a bipolar membrane.

0032The separator may be in direct electrical contact with the cathode and/or anode of the electrochemical reactor. Alternatively, the separator may be in indirect electrical contact with the cathode and/or anode of the electrochemical reactor via an electrically conductive liquid, such as an electrolyte, or more specifically a catholyte or anolyte.

0033The method of the invention may be performed in various embodiments that allow reduction of carbon dioxide using different configurations of the electrochemical reactor.

0034For example, water can be split at the separator, thereby producing OH- ions and H⁺ ions. The feed with carbon dioxide (which can be present in different forms such as carbon dioxide, carbonic acid, bicarbonate, carbonate, etc.) can be reduced in the cathode compartment while

0035consuming the H⁺ ions, and the OH^" ions can be oxidised in the anode compartment between to produce water. In this example, the separator preferably comprises a cation exchange layer and an anion exchange layer in contact with each other at a junction, wherein the cation exchange layer faces the cathode and the anion exchange layer faces the anode. This configuration may be represented by the following overall reactions.

0036In the cathode compartment: C0₂ + 2 H⁺ + 2 e^"→ HCOOH (1)

0037At the separator (junction): 2 H₂0→ 2 H⁺ + 2 OH^" (2) In the anode compartment: 2 OH^"→ ½ 0₂ + H₂0 + 2 e^" (3) The net result of these three reactions is:

0038C0₂ + H₂0→ HCOOH + ½ O₂ (4)

0039This exemplary configuration is shown in figure 1, wherein an

0040electrochemical reactor 100 comprises a cathode compartment 110, a separator 120, and an anode compartment 130.

0041In another example, carbon dioxide can be reduced in the cathode compartment (where the carbon dioxide can be present in different forms such as carbon dioxide, carbonic acid, bicarbonate, carbonate, etc.), thereby producing OH- ions. In the anode compartment, water can be oxidised to produce H⁺ ions. The OH- ions and H⁺ ions can be recombined at the separator, thereby producing water. In this example, the separator preferably comprises a cation exchange layer and an anion exchange layer in contact with each other at a junction, wherein the anode exchange layer faces the cathode and the cation exchange layer faces the anode.

0042This configuration may be represented by the following overall reactions.

0043In the cathode compartment: CO₂ + H₂O + 2 e^"→ HCOOH + 2 OH^" (5)

0044At the separator (junction): 2 H⁺ + 2 OH^"→ H₂O (6)

0045In the anode compartment: H₂O→ 2 H⁺ + 2 e^" + ½ O2 (7) The net result of these three reactions is:

0046CO₂ + H₂O→ HCOOH + ½ O₂ (4)

0047This exemplary configuration is shown in figure 2, wherein an

0048electrochemical reactor 100 comprises a cathode compartment 210, a separator 220, and an anode compartment 230. Hence, depending on the configuration of the electrochemical reactor, different electrochemical conversions can be performed with the method of the invention. In one embodiment, the separator comprises a cation exchange layer and an anion exchange layer in contact with each other at a junction, wherein the cation exchange layer faces the cathode and the anion exchange layer faces the anode. In a further embodiment, the separator comprises a cation exchange layer and an anion exchange layer in contact with each other at a junction, wherein the anion exchange layer faces the cathode and the cation exchange layer faces the anode.

0049The electrochemical reactor can advantageously be free of liquid electrolyte, such as catholyte and/or anolyte. Preferably, the electrochemical reactor is free of catholyte. The cathode and bipolar membrane can be in direct contact in order to minimise electrical resistance. Carbon dioxide can, for example be supplied as dense gas to the cathode compartment, and water may be supplied either to the cathode compartment or to the anode compartment in the appropriate amount to wet the cathode and anode structure thereby forming a reactive layer. The absence of liquid electrolyte is highly preferred, as the presence of liquid electrolyte can lead to the production of undesirable by-products.

0050The method may be operated continuously, semi-continuously, or in batch. It is preferred to perform the method of the invention in a continuous mode.

0051Preferably, the method of the invention further comprises releasing oxygen from the anode compartment, and collecting reduced carbon dioxide product or product mixture from the cathode compartment.

0052The reduced carbon dioxide product or product mixture can comprise one or more components from a group of molecules referred to as platform chemicals, such as alkanes, alkenes, syngas, carboxylic acids, alcohols, including diols, aldehydes, and ketones. More specifically, the carbon dioxide product or product mixture can comprise one or more components selected from the group consisting of carbon monoxide, hydrogen, syngas, formic acid, a formate, methanol, acetaldehyde, methane, and ethane. Preferably, the reduced carbon dioxide or product mixture comprises one or more selected from the group consisting of methanol, formic acid, a formate, and acetaldehyde. Accordingly, the method can involve the conversion of carbon dioxide into one or more of these

0053compounds. In a preferred embodiment, the method comprises the

0054conversion of carbon dioxide to formic acid. The product that is obtained may be controlled by varying the applied voltage.

0055A water feed is introduced into the anode compartment. This water feed may be supplied in liquid form, in vapour form or in gaseous form. The water feed comprises at least water, but may further comprise other components such as one or more selected from the group consisting of pH control agents, conductivity control agents, and reactivity inhibitors. It is also possible that the water feed is water. Suitably, the water feed is a feed that has water as main component.

0056A carbon dioxide feed is introduced into the cathode compartment. This carbon dioxide feed may be supplied in one of the following phase, as a supercritical phase, in a liquid phase, in a dense phase, in a vapour phase or in a gaseous phase. Preferably, the carbon dioxide feed is in a dense phase. As is commonly known in the art, dense phase carbon dioxide refers to carbon dioxide in either liquid form or supercritical form. In the case of a liquid, the carbon dioxide itself may be liquid, or it may be a solute in a liquid solvent, such as water. The carbon dioxide feed comprises at least carbon dioxide, but may further comprise other components, such as one or more selected from the group consisting of water droplets, aerosols, pH control agents, conductivity control agents, and compounds for mediated electrochemical conversion. Suitably, the carbon dioxide feed is a feed that has carbon dioxide as main component. Preferably, the carbon dioxide feed comprises a majority of carbon dioxide and a minority of water. It is preferred that the carbon dioxide feed is pre-conditioned in a dedicated vessel to obtain high concentrations of reactive species (CO2, carbonic acid, bicarbonate, and carbonate) for carbon dioxide reduction. The carbon dioxide feed is suitably pre-conditioned in terms of pressure, temperature, and pH, and preferably consists of a mixture of carbon dioxide and water with an excess of carbon dioxide. With respect to the pressure, the carbon dioxide is preferably fed to the reactor in a dense phase and water is preferably partly solubilised in the dense carbon dioxide phase, whereby the dense phase carbon dioxide will be highly concentrated with ionic reactive species. These reactive species are formed by a reaction between water and the carbon dioxide during the pre-conditioning step, and (additional) ionic reactive species can be formed during the contacting in the electrochemical reactor of the dense phase carbon dioxide with the membrane where the membrane is saturated with water.

0057The pressure of the carbon dioxide feed is preferably 20 bara or more, such as 30 bara or more, even more preferably 50 bara or more, and most preferably 60 bara or more. Usually, the pressure of the carbon dioxide feed will not exceed 200 bara. More preferably, the pressure of the carbon dioxide feed will be 150 bara or less, even more preferably 100 bara or less, and most preferably 80 bara or less.

0058The carbon dioxide feed may have a density of 10-1000 kg/m³, preferably a density of 100-980 kg/m³, and more preferably a density of

0059600-960 kg/m³.

0060Both water feed and carbon dioxide feed may be introduced into the electrochemical reactor continuously or intermittently. It is also possible to introduce the water feed and carbon dioxide feed into the electrochemical reactor in a batch wise fashion.

0061While reduced carbon dioxide product or product mixture is produced in the cathode compartment, typically oxygen is produced in the anode compartment. This oxygen can be released from the anode compartment via an anode compartment outlet. It is preferred that the rate of introducing the carbon dioxide feed and the rate of releasing the oxygen is monitored and controlled. This may be realised, for instance, by releasing oxygen when the absolute pressure thereof is 0.2 bar or more higher than the pressure of the carbon dioxide feed, preferably 0.3-1.0 bar higher than the pressure of the carbon dioxide feed, and more preferably 0.4-0.8 bar higher than the pressure of the carbon dioxide feed. This control can advantageously prevent mechanical damages by avoiding large pressure differences in the system.

0062In another configuration, the pressure difference between the anode and the cathode compartment, i.e. the pressure difference over the membrane, will be used to control the rate of introducing the carbon dioxide feed and releasing the oxygen. The oxygen maybe released, for instance, when the pressure difference between the anode and the cathode

0063compartment is 0.2 bar or more, preferably in the range of 0.2-1.0 bar pressure difference, and more preferably in the range of 0.3-0.8 bar pressure difference.

0064This control of the feeding and the release of the various streams can be achieved, by an advanced pressure control system, employing pressure transducers, pressure transmitters, pressure controllers, and pressure differential transducers.

0065For example, the carbon dioxide feed can be fed to the

0066electrochemical reactor at a fixed pressure (e.g. in the range of 20-100 bara), thereby guaranteeing an automatic supplementation of this reactant.

0067Fluctuations in the water feed can be damped by using, for example, an expansion chamber, pusher bellows, etc. In the preferred configuration, for both the water feed stream and the carbon dioxide feed stream a

0068(pre)conditioning step is included. Preferably, for each stream a pressurised, thermostatic vessel or hydraulic accumulator is employed to assure stable pressure and temperature levels for the feed streams. A basic process flow diagram of the main process steps including the most relevant

0069instrumentation for pressure control is depicted in figure 3.

0070The water feed supplied at the anode can be in direct contact with the produced gaseous oxygen. The pressure thereof can be at an absolute pressure with a set-point for a control valve to release oxygen slightly higher than the carbon dioxide feed pressure, for example 0.5 bar higher than the carbon dioxide feed pressure. For the different modes of operation, batch semi-batch and continuous operation, the pressure difference over the membrane and the pressure difference between the various streams (water, carbon dioxide and oxygen) will not exceed the pressure of the operational window. Preferably, the pressure difference over the membrane should be in the range of 0.3-0.8 bar.

0071The command signal for the valve can be derived from a sensor that monitors an absolute pressure. Preferably the liquid reactants contact the pressure hull of the vessel.

0072During start-up, shutting down, or in emergency situations, distinct or even abrupt pressure differences may occur between the anode compartment and the cathode compartment. In order to avoid any problems related with such pressure differences, a sensor may be employed that monitors a pressure differential between the carbon dioxide feed and released oxygen and controls and overrules the absolute pressure signal command. The pressure differential signal can affect the operation of a valve connected to the cathode compartment inlet to adjust the rate of carbon dioxide feed introduction and a valve at the anode compartment outlet to adjust the rate of oxygen release. The discharge rate of the reduced carbon dioxide product or product mixture may be controlled using a control valve connected to the cathode compartment outlet.

0073The pressure in the electrochemical reactor is 20 bara or more, preferably 30 bara or more, even more preferably 50 bara or more, and most preferably 60 bara or more. Usually, the pressure in the electrochemical reactor will not exceed 200 bara. More preferably, the pressure in the electrochemical reactor will be 150 bara or less, even more preferably 100 bara or less, and most preferably 80 bara or less.

0074In a particular embodiment, an overpressure is maintained in the anode compartment with respect to the cathode compartment. The separator divides the reactor in two compartments, wherein the anode compartment suitably contains a liquid phase (such as an aqueous liquid), while the cathode compartment suitably contains a gaseous phase (such as a carbon dioxide gas). Typically, the liquid phase is difficult to compress, while the gaseous phase can be more easily compressed. Possible pressure fluctuations that may occur in the system could lead to transport of the liquid through the membrane in the direction of the gaseous phase. In order to prevent this from happening, it is preferred to employ a small

0075overpressure, with the bipolar membrane preferably supported at least at the interface with the cathode compartment. Moreover, this overpressure can facilitate the diffusion of H⁺ ions in the bipolar membrane. The over-pressure can, for example, be in the range of 0.2-0.5 bar, such as 0.3-0.4 bar.

0076The reduced carbon dioxide product or product mixture can be further purified after being collected using one or more of the state of the art separation methods, such as distillation, absorption or membrane

0077separation.

0078Such purification may comprise one or more from the group of membrane separations consisting of ultrafiltration, nanofiltration, pervaporation, vapour permeation, and membrane distillation. Such purification methods can be based on the difference in molecular size and/or affinity for a membrane material.

0079In an embodiment, the method comprises a step wherein the reduced carbon dioxide product or product mixture is subjected to dehydration. Such dehydration may, for instance, involve dehydration of methanol and/or acetaldehyde.

0080The residual or permeate of a purification step of the reduced carbon dioxide product or product mixture can optionally be recycled to the carbon dioxide feed, optionally after recompression.

0081The temperature in the electrochemical reactor during the method of the invention is preferably 20 °C or less, and may be even cooler, such as 10 °C or less, or even 5 °C or less. The temperature will normally not be below 0 °C. Such relatively low temperatures can be achieved by active cooling of the electrochemical reactor and/or by introducing low temperature water feed and/or low temperature carbon dioxide feed (e.g. by active cooling of the feed).

0082The reduced carbon dioxide product or product mixture can be collected. This may be accomplished by removing reduced carbon dioxide product or product mixture from the cathode compartment via a cathode compartment outlet. Such outlet is typically located at the bottom of the cathode compartment. The product or product mixture can then suitably be collected in a separate volume, before optionally being processed further.

0083The electrical potential between the anode and the cathode can be applied using an external power supply that is connected to the electrodes (anode and cathode). The electrical potential should be sufficient to drive the reaction, and more in particular for the cathode to reduce carbon dioxide into a reduced carbon dioxide product or product mixture.

0084In a further aspect, the invention is directed to an electrochemical reactor, preferably for performing a method as described herein, said reactor comprising an anode compartment separated from a cathode compartment by a separator, wherein said cathode compartment is in contact with a cathode and said anode compartment is in contact with an anode,

0085said anode compartment comprising an anode compartment inlet for feeding water and an anode compartment outlet for releasing oxygen, said cathode compartment comprising a cathode compartment inlet for feeding carbon dioxide and a cathode compartment outlet for collecting reduced carbon dioxide product or product mixture,

0086wherein said separator is selected from the group consisting of a bipolar membrane, a charge-mosaic membrane, and a layered mixture of anion and cation exchange resins, said electrochemical reactor being configured to operate at a pressure of 20 bara or more.

0087The cathode and/or the anode may be embedded in the separator.

0088For example, in case of a bipolar membrane, the cathode and/or anode may be embedded in one of the ion exchange layers of the bipolar membrane. In such a case, the cathode is preferably embedded in one of the ion exchange layers, and the anode is embedded in the other ion exchange layer.

0089The separator preferably comprises a cation exchange layer and an anion exchange layer. The separator can then be oriented in the electrochemical reactor such that the cation exchange layer faces the cathode and the anion exchange layer faces the anode. Alternatively, the separator can be oriented in the electrochemical reactor such that the anion exchange layer faces the cathode and the cation exchange layer faces the anode.

0090Preferably, the electrochemical reactor is configured to operate at a pressure of 20 bara or more, more preferably 30 bara or more, even more preferably 50 bara or more, such as 60 bara or more. Usually, the pressure in the electrochemical reactor will not exceed 200 bara. The pressure in the electrochemical reactor is preferably 150 bara or less, more preferably 100 bara or less, such as 80 bara or less.

0091In order to improve the mechanically properties of the reactor, the separator may be mechanically supported on at least one side. It is also possible to mechanically confine the separator between two mechanical supports. The reactor can further be mechanically supported by a holder, which may hold the mechanical support. The reactor can suitably further comprise a water feed line, a carbon dioxide feed line, an oxygen release line, and a reduced carbon dioxide product collection line. The water feed line is then connected to the anode compartment inlet, the carbon dioxide feed line is then connected to the cathode compartment inlet, the oxygen release line is then connected to the anode compartment outlet, and the reduced carbon dioxide product collection line is then connected to the cathode compartment outlet.

0092Suitably, the oxygen release line and/or the anode compartment inlet comprises a pressure valve which is operably linked to a pressure sensor for measuring the pressure difference between the carbon dioxide feed line and the oxygen release line. Separately or additionally, the carbon dioxide feed line and/or the cathode compartment inlet may comprise a pressure valve which is operably linked to a pressure sensor for measuring the pressure difference between the carbon dioxide feed line and the oxygen release line. The pressure valve at the oxygen release side may further be operably linked to a pressure sensor for measuring the absolute pressure in the oxygen release line. Hence, the system pressure can be controlled by venting the released oxygen, while the differential pressure can be maintained at the appropriate level by controlling the carbon dioxide feed. As such, the system will be balanced.

0093In a further aspect the invention is directed to a electrochemical reaction module, comprising two or more electrochemical reactors as described herein in parallel.

0094For example, a stack of repeat units (300) as shown in figure 4a may be placed in a high pressure reaction vessel as shown in figure 4b. The repeat unit (300) shown in figure 4a contains a bipolar membrane (100) confined between two constructive parts of similar design that form the cathode compartment and the anode compartment, respectively. Each part consists of a holder (320) for conductive mechanical support (310). The cathode compartment has a port (330) opening to a common cathode feed volume. The anode compartment has a port (340) opening to a common anode feed volume. Figure 4b shows a stack of the repeat units (300) in a high pressure reaction vessel. In this example, the high pressure vessel consists of two parts. The upper part (420) is only a cover and can be removed for access to the electrodes and membranes. The lower part (410) supports the stack of cathode-bipolar membrane-anode repeat units, which physically separates the cathode compartment of the reactor vessel at the bottom of the reactor vessel from the anode compartment of the reactor vessel at the top of the reactor vessel. The lower part has two entry ports; one for the cathode feed (430), for example carbon dioxide at high pressure, and for the anode feed (440), for example water. The lower part has two exit ports; one for the reaction product(s) at the cathode side (450), and one for the anode side (460). The tubing connected to entry and exit ports can be brought to the desired level inside the high-pressure electrochemical reactor vessel. The lower part of the reactor vessel also contains the connection ports to a differential pressure sensing device (470). Albeit not show, it is clear that pressure sensing devices for sensing cathode compartment pressure and/or anode compartment pressure can be mounted on the lower part of the reactor vessel.

0095In accordance with the invention, a number of such high pressure reaction vessels may, for instance, be operated in parallel. An example thereof is shown in figure 3. In this example, water (1) and carbon dioxide (2) feeds are fed to respectively the anode and the cathode chambers of the high pressure reaction vessel. The product mixture (3) is collected at the bottom of the reaction and is purified in a separation unit. The product (4) (here formic acid), leaves the system. Oxygen (5) evolves at the anode and leaves the system as a gas from the anode chamber. The permeate of the separation unit (10), is recompressed and recycled into the dense carbon dioxide feed stream (2). A further example of an electrochemical reactor is shown in figure 5. In this example, the reactor (500) contains a bipolar membrane (100) in a cylindrical configuration. The bipolar membrane has a mechanical support (510). This mechanical support allows a dense gas phase reactant (e.g.

0096carbon dioxide, or carbon dioxide plus water vapour at high pressure) to pass from the cathode compartment. The product is collected in to the cathode compartment volume (530). At the other side of the bipolar membrane is a conductive screen (520) that allows a liquid phase reactant (such as water or water plus electrolyte) to pass from the anode

0097compartment. The anode chamber is in open contact with the common anode chamber volume (540). A stack of these electrochemical reactors (500) may be placed in a high pressure reaction vessel, for example in a similar fashion as shown in figure 4b, or as a single unit in a high pressure tube, the aggregate being a tube-in-tube electrochemical reactor.

0098All references cited herein are hereby completely incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

0099The use of the terms "a" and "an" and "the" and similar referents in the context of describing the invention (especially in the context of the claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising", "having", "including" and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to") unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. For the purpose of the description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Also, all ranges include any

0100combination of the maximum and minimum points disclosed and include and intermediate ranges therein, which may or may not be specifically enumerated herein.

0101Preferred embodiments of this invention are described herein. Variation of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject-matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. The claims are to be construed to include alternative embodiments to the extent permitted by the prior art.

0102For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include

0103embodiments having combinations of all or some of the features described.

0104The invention will now be further illustrated by the following non- limiting example. Examples

0105A high pressure electrochemical cell was designed in order to have optimal dimensions for the reactor. This is, in particular, relevant for the volume of the electrolyte compartments, the surface area of the membrane, and the surface area of the electrodes. The operating conditions are, typically, in the range of 1- 100 bar and temperatures are in the range of 5-50 °C.

0106The use of a high pressure, dense phase mixture of carbon dioxide and water has as main advantage that high concentrations of ionic, reactive species especially, bicarbonate (HCO3^") and carbonate (CO3^2") will be formed. The presence of the ionic carbonate species (HCO3^" and CO3^2") avoids the need of using an additional electrolyte solution. More specifically, in the proposed reactor configuration there is no need to use a catholyte and/or anolyte solution.

0107Table 1 shows the amount of CO2 (concentration of CO2 per litre) when CO2 is solubihsed in water up to a pressure of 10 bar. In this case the CO2 concentration is also referred to as the CO2 solubility. Table 2 shows the maximum amount (concentration of CO2 per litre) that can be achieved at the given conditions in the case where a small amount of water is added to a CO2 dense phase. For the two situations a temperature of 20 °C is taken, which might require some cooling of the reactor.

0108Table 1: Concentration of CO2 for different conditions, starting with an aqueous solution.

0109CO2 in water

0110conditions maximum CO 2 concentration p = 1 bar, T = 20 °C 1.4 g C0₂/l

0111p = 10 bar, T = 20 °C 12.5 g CO2/I Table 2: Concentration (based on density) of CO2 for different conditions, starting with a dense CO2 phase.

0112<img file="WO2017014635A1_D0001.tif" />

0113Based on the use of a high pressure, dense CO2 phase and the absence of an electrolyte solution, a relatively small reaction volume can be used, as compared to the conventional electrochemical cells that employ an electrolyte solution. In a conventional electrochemical cell, the typical conditions that are applied are atmospheric pressures, and as a result the mass-based ratio of the reactant CO2, to be consumed in the reduction reaction, to the water (in the electrolyte solution) is well below unity. For a typical electrochemical cell the mass-based CO2 to water ratio is in the order of 0.001-0.005 (1 to 5 g CO2 per litre water). While in the case of starting with a dense phase CO2, the envisaged CO2 to water (mass-based) ratio will be close to unity, based on densities and operation above 50 bar and temperatures below 20 °C.

0114A small reaction volume assures a small(er) distance between the electrodes and the bipolar membrane, with as main result a minimal resistance for the electrical current, and the optimal (minimum) ratio of reaction volume to electrode surface area also assures a high current density. Furthermore, because of the difference in density between the dense CO2 phase and the liquid formic acid phase, with a density of formic acid of 1.22 kg/1 there will be a separation of the product from the reactants.

0115In the absence of an (extra) electrolyte solution no unwanted side reactions, with species typically present in the electrolyte solution, will occur, and therefore in the proposed reactor configuration the selectivity towards the desired product will be increased. Furthermore, no significant change in pH value occurs as a result of the high concentration of

0116bicarbonate and carbonate species.

0117In addition, the use of the bipolar membrane will assure a high flux of (only) protons (hydrogen ions, H⁺ ions) across the membrane into the dense CO2 phase. The use of the bipolar membrane for the water splitting, into OH^" and H⁺, is an essential step for high selective reduction of carbon dioxide, with a bipolar membrane only hydrogen ions will flow into the dense CO2 phase with the bicarbonate and carbonate ions. Again, the absence of additional ions that are normally exchanged across ion exchange membranes used in electro-catalytic reduction of CO2 will assure no unwanted side reactions.

0118The reactor configuration is ideally suited for the production of formic acid, by providing flexible operation to achieve the required and selected over-potential for carbon dioxide reduction, according to the following reactions/steps:

0119- high pressure mixing of water in dense CO2 phase

0120<img file="WO2017014635A1_D0002.tif" />

0121- equilibrium and formation of (bi)carbonate species by solvation

0122H2CO3 = H⁺ + HCO3- = 2 H⁺ + CO3²- (9)

0123- water splitting

0124H₂O = H⁺ + OH- (10)

0125- main reaction

0126HCO3- + H⁺ = HCOOH + ½ O₂ (11) The overall reaction for the direct electrochemical conversion of

0127CO2 to formic acid is then as follows.

0128CO₂ + H₂O→ HCOOH + ½ O₂ (4) Based on the (starting) amount of CO2 in a reactor volume of 1 1 (1000 ml) as given in table 1 and table 2, the following concentrations for formic acid can be obtained, see case 1 and case 2 below. From the molar mass it follows that about 1 gram of CO2 (MW = 44.00 g/mol) gives about 1.05 gram of formic acid (MW = 46.03 g/mol). For the formation of formic acid from CO2 a current efficiency of around 80 % has been reported, the density of formic acid is 1.22 g/ml, for water a density of 1 g/ml is taken.

0129Case 1:

0130Starting with a dense phase mixture of 500 ml CO2 and 500 ml water, using a reactor volume of 1 1 (1000 ml), at 100 bar and 20 °C gives: (500 / 1000) * 850 = 425 g C0₂, which yields: (0.8 * 1.05 * 425 =) 357 g of formic acid.

0131For the concentration in vol.%, and after the pressure has been reduced this gives: (357 * 1.22 =) 436 ml or 42.6 vol.% formic acid.

0132Case 2:

0133Starting with an aqueous solution at 10 bar and 20 °C, the amount of CO2 is 12.5 g/1, which is a factor of (425 / 12.5 =) 34 lower than for case 1 which uses a dense phase. This corresponds to a formic acid concentration of (43.6 / 34 =) 1.3 vol.%.

0134For an aqueous system at 1 bar and 20 °C, the amount of CO2 is a factor (425 / 1.4 =) 304 lower, resulting in a (maximum) formic acid concentration of (42.6 / 304 =) 0.14 vol.%.