Electrolysing cell stack with interchangeable flow inlets and outlets for carbon dioxide electrolysis

Abstract

The present invention relates to a carbon dioxide (CO2) electrolyser provided in the form of an electrolysing cell stack equipped with carbon dioxide and anolyte inlets, as well as product gas or anolyte outlets that are interchangeable from the point of view of respective substance flows. The essence of the electrolyser is in its symmetrical design. At stack level, the symmetrical design is represented in equal stack inlet / outlet connections provided by cathode side outlet / inlet connection ports (12, 12') and anolyte outlet / inlet connection ports (22, 22') and chimneys (13). At cathode or anode half-cell level, the symmetrical design is represented in equal inlet / outlet side cell-connector channels (16) and flow patterns within the membrane electrode assembly (18).

Description

[0001] ELECTROLYSING CELL STACK WITH INTERCHANGEABLE FLOW INLETS AND OUTLETS FOR CARBON DIOXIDE

[0002] ELECTROLYSIS

[0003] Field of the invention

[0004] The present invention relates to the field of carbon dioxide (CO2) electrolysis. In particular, the invention relates to a carbon dioxide electrolysing cell stack equipped with carbon dioxide and anolyte inlets, as well as product gas or anolyte outlets that are interchangeable from the point of view of respective substance flows. As a result of the geometrical construction of said cell stack, the direction of substance flow, either on the anode side or the cathode side or on both sides, is reversible during / for the operation of the carbon dioxide electrolysing cell stack which leads to a decrease in the asymmetricity of load on any sides of the cell stack, thereby increasing the lifetime of the CO2 electrolyser.

[0005] Background Art

[0006] Along the cell path, the elimination of variable stresses caused by changes in pressure, temperature, humidity, cross-membrane pressure difference or reactant concentration acting on the cell elements is critical to increase the cell lifetime. There are several solutions to this problem, for example, U.S. patent no. 6,589,678 Bl discloses a method of operating a fuel cell that includes flowing a reactant gas (e.g., an oxidant gas or a fuel gas) through the fuel cell in one direction so that the fuel cell produces a power output greater than zero. The flow of reactant gas through the fuel cell is then stopped so that the power output of the fuel cell is about zero. The reactant gas is then flowed through the fuel cell in the opposite direction so that the power output of the fuel cell is greater than zero. The realization that switching the flow direction of a reactant gas through a flow field plate during a period, when the reactant gas is not flowing through the flow field plate, can prolong the useful life of a protonexchange membrane (PEM) by reducing drying out of the leading edge of the PEM while also avoiding other potential situations that can reduce fuel cell performance. An advantage of this method is that the operation can be performed without hydrating the reactant gas so that flooding can be avoided. Another potential advantage is that without increasing the ratio of oxidant gas to fuel gas flowing through the fuel cell can avoid operating the fuel cell in electrolysis mode. As a result, this extends the useful life of a PEM.

[0007] European patent no. 1,036,422 Bl describes a method to optimize water distribution in an electrochemical fuel cell system and thereby increase lifetime. Namely, the solution is to periodically reverse the direction of the oxidant stream flowing through the cathode oxidant flow field to utilize water accumulating in the oxidant stream. In the present method, water accumulated in an oxidant stream in an oxidant flow field of a solid polymer electrochemical fuel cell flow field is utilized to provide moisture to the ion-exchange membrane. This electrochemical fuel cell assembly - beyond the conventional parts - further includes an oxidant stream flow switching device. This device periodically reverses the direction of the oxidant supply stream without affecting the fuel supply stream direction. For larger fuel cell arrays, similar principles apply, with staggered reversals of oxidant stream flow direction across different fuel cell stacks to ensure continuous operation and efficient water management. The configuration of the electrolysis cell according to the present invention we use, differs from the fuel cell in that only one of the half-reaction processes is fed by gas injection (humidified CO2 is fed to the cathode). On the anode side, an electrolyte solution is circulated. The technical solutions are very similar to the one we have in mind, essentially reversing the inlet and outlet with different technological solutions (valves with pumps). The technical solution requires a liquid transfer pump / pump on the anode side.

[0008] European patent no. 1,447,870 Bl presents a method of operating a fuel cell by feeding it gaseous fuel and oxidant streams and oscillating these via reversing their flow direction. These oscillations are phase-shifted by an amount equal to the average time it takes for anions to be transported from the cathode chamber to the anode-electrolyte interface of the fuel cell. This synchronization improves the efficiency of the fuel cell operation. U.S. patent no. 6,830,844 B2 relates to solid oxide fuel cells (SOFCs), wherein the temperature difference within the cell is maintained below 200°C by using directional flow reversal. A reverse flow air tempering and supply system for supplying air from a source through air flow passages of a fuel cell having first and second ports connected to the air flow passages. International publication pamphlet no. WO2011 / 032644 Al solves heat recovery problems and improves energy efficiency by using oscillating reactant gas supply. The study of Ping Yuan and Syu-Fang Liu (Effect of air flow rate distribution and flowing direction on the thermal stress of a solid oxide fuel cell stack with cross-flow configuration, Int. J. of Hydrogen Energy, Volume 47, Issue 10, p 6799-6810) also highlights that the effect of air flow rate distribution and flow direction do not affect the power generation, however, changing flow direction of the air obviously drops down the thermal stress without affecting the power generation of the SOFC stack.

[0009] In the U.S. patent application no. 2010167146AI, it was discovered that the effects arising from the differences in moisture content and temperature - along the flow direction in a fuel cell stack - could be mitigated by periodically inverting the flow direction of the cathode gas and / or the anode gas. This was found to be an effective method of preventing the deterioration of the performance and durability of the cell stack over time. Inverting the flow direction also entails interchanging the flow inlets and outlets.

[0010] Contrary to fuel cells, in (zero gap) carbon dioxide (CO2) electrolysers, an electrolyte (e.g. an aqueous solution of KHCO3 or CsHCOa) is used on the anode side during the operation. Depending on the location, different amounts of water pass through the membrane - along the cathode flow channel - carrying different amounts of K+or Cs+ions. Proper regulation of cation transport is particularly important in reducing the "flooding" phenomenon and is one of the keys to long-term operation. In addition, control of cation transport is critical to maintaining the selectivity of the cathode halfreaction. Too low cation concentration results in a local decrease in reaction rate, while too high a cation concentration promotes unwanted H2 evolution by increasing the hydrophilicity of the gas diffusion layer (GDL) applied. In the case of CO2 electrolysers, an appropriate flow management is also needed for handling the anolyte, too. This flow management should also be realized with a capability of reversing the flow direction of both the liquid anolyte and the gaseous catholyte, too. The latter is of especial importance, as in CO2 electrolysers, due to the various chemical reactions taking place in the electrolyser cells and the cation migration, various products might be produced in the electrolysis and said gradients might have significantly different effects on these various products during cell operation. Surprisingly, even very minor changes in the temperature cause a significant change in the product selectivity.

[0011] According to current knowledge, periodic reversal of the flow direction is not employed in zero gap CO2 electrolysis, partly because suitable flow direction control had not previously been achieved.

[0012] Technical problem to be solved

[0013] In the context of continuous-flow electrochemical reactors, it is important to note that parameters such as pressure, temperature, flow rate and reactant concentration are known to vary along the flow channel or path length, which can be defined as a characteristic distance between the CO2 inlet side line 100 or the anolyte inlet side line 110 and CO2 outlet side line 100' or the anolyte outlet side line 110' in Figure 1 and Figure 2, respectively The aforementioned gradients in various physical-chemical parameters have been demonstrated to induce considerable stress within the CO2 electrolyser stacks, thereby reducing their operational lifetime.

[0014] Its consequences on the cathode side of the carbon dioxide electroreduction cell are the followings: The local reaction rate varies as a function of the flow channel length along the cathode gas path due to the varying reactant (CO2) and product gas (CO, H2) concentration ratios. The local concentration ratios are also affected by the pressure drop or temperature change along the gas path and the progress of the reaction. The local relative humidity, and consequently the transport of water and ions across the membrane, is also a variable parameter as a function of path length, as the former is affected by temperature, pressure, and flow rate. As a result, in case of improper design or control of the above listed parameters, undesirable crystallization or viscous, highly concentrated electrolyte solution (precipitate formation) or flooding of the gas diffusion layer can occur to varying extent along the gas path. Its consequences on the anode side of the carbon dioxide electro-reduction cell are the followings: The local pressure and temperature of the anolyte varies as a function of the flow channel length along the anolyte path due to the varying flow resistance, varying formed gas amount and transferred heat. The local differential pressure between the two sides of the membrane varies as a function of the flow channel length along the anolyte path due to the varying local pressure. The local transport of water and ions across the membrane, is also a variable parameter as a function of path length, as the former is affected by temperature, pressure and formed gas amount. As a result, in case of improper design or control of the above listed parameters, undesirable crystallization or viscous, highly concentrated electrolyte solution (precipitate formation) or flooding of the gas diffusion layer can occur to varying extent along the gas path.

[0015] The provision of a structural design for CO2 electrolysers built up of several electrolyser stacks would be advantageous in eliminating the stress caused by the gradient of various physical-chemical parameters in the CO2 electrolyser stacks.

[0016] Underlying technical finding

[0017] We found that by periodically reversing the gas or anolyte inlet and outlet on the cathode or on the anode side, the effects of path length variable concentration, pressure, temperature, crossmembrane pressure difference and humidity can be reduced or minimized to ensure long-term operation of scaled-up CO2 electrolysing stacks (if reversing the inlet and the outlet means reversing the path length distance).

[0018] In terms of technical featrures, the interchangeability of the CO2 or anolyte inlet and product gas or anolyte outlet, is a pre-requisite for this process. That is, in order to implement the process, it is necessary to incorporate the following features into the system: the CO2 electrolyser according to the invention need to be capable of changing CO2 or anolyte inlet and product gas or anolyte outlet flow direction during electrolysis. While the directional change of gas flow was described to maintain membrane hydration throughout the fuel cell; in this instance, different gases are also present (CO2, CO), and the cell architecture differs significantly since these fuel cells operate with gas-phase feeding only. Our zero-gap electrolyser operates by a continuous circulation of aqueous anolyte. Moreover, the alkali ion content of the anolyte plays a crucial role in the electrolysis process both in terms of activity, selectivity and lifetime. In conclusion, this is straightforward due to the structural design of fuel cells, the implementation of this interchangeable solution for electrolyser cells presents difficulties to be solved and not obvious to a person skilled in the art.

[0019] We found that designing a symmetrical design and using a directional control valve 9 and actuator 10, e.g. a 4-way ball valve or any equivalent valve or valves is used to reverse the flow both at the cathode and the anode sides. Tubes, pipes or hoses with equal length, internal diameter and heat insulation or active heating can be used to connect the stack fluidical ly. The carefully designed electrolyser stack allows for flow directional change, since flow path cross-sections, lengths and shapes are equal from both directions, resulting in equal overall pressure drop, heat and mass transfer. From fluid paths point of view of the electrolyser stack is symmetrical.

[0020] We found that intermittent reversal of the flow direction on the cathode side (and optionally on the anode side) help to maintain a more uniform cation transport along the cathode flow channel, thereby prolonging the useful life with the desired selectivity.

[0021] Definitions and abbreviations

[0022] The term "zero-gap electrolyser" is well known for the skilled person, and refers to an electrolyser, in which the separator (e.g., an ion exchange membrane) is in direct contact with the catalyst layers of the anode and the cathode. In the context of the invention, direct contact means that there is no intentional gap, and no substantial liquid layer between the ion exchange membrane and the catalyst layers, and between the catalyst layers and the electrode substrates. However, as liquid (anolyte or water originating from the humidified carbon dioxide) is present in the system, it is evident that e.g. a thin liquid film may be present. This design reduces the distance that ions must travel through the layers, compared to non-zero gap cell designs.

[0023] The term "cathode" as used herein refers to the electrode at which the cathodic reduction of carbon dioxide occurs. The feed gas referred to as CO2 contains CO2 at least in 50% molar ratio. Additional feed gas components can be, but not limited to, one of or combinations of the followings: nitrogen, methane, ethane, ethylene, acetylene, hydrogen, carbon monoxide, oxygen, nitrogen oxides, hydrogen sulfide, sulfur dioxide and water vapor. At a minimum, the cathode comprises a catalyst; however, it typically includes a supporting electrode positioned beneath the catalyst layer. The electrode may be composed of any electrically conductive material, although the current standard involves the use of porous electrodes, commonly referred to as gas diffusion layers.

[0024] The catalyst layer may be immobilized on the gas diffusion layer, in which case the assembly is commonly referred to as a gas diffusion electrode. Alternatively, the catalyst layer may be formed on the membrane, resulting in a catalyst-coated membrane configuration. A further technologically equivalent configuration involves the use of self-supporting catalyst layers, which may be sandwiched between the electrode and the membrane, or may themselves serve both as the catalyst and the electrode.

[0025] The term "membrane electrode assembly" refers to the combination of cathode, membrane, and anode in an electrolyser cell, regardless of the specific method or location of catalyst layer deposition. An "electrolyser cell", or shortly "cell", is the basic functional unit of the electrolyser in which the electrochemical reactions occur. It typically comprises a cathode, an anode, an ion-conductive membrane (or separator), and compartments or chambers to manage gas and liquid flows.

[0026] An "electrolyser cell-stack", often referred to as electrolyser stack or simply stack, is comprised of multiple electrolyser cells, wherein the individual cells are connected in series in terms of the electrical connections of the cells and connected in series / parallel in terms of the flow management of the electrolyser, i.e., the liquid flows and the gaseous flows directed through the electrolyser. The overall performance of the electrolyser cell-stack comprises the cumulative performance of the contained electrolyser cells. Therefore, any process to optimise the cell performance implies a similar effect on the electrolyser stack.

[0027] In the context of the invention, "cell performance", or shortly "performance" of the CO2electrolyser refers to how effectively the electrochemical cell converts carbon dioxide (CO2) into desired or target products (like carbon monoxide (CO), methane, formate, ethylene, etc.) under certain operating conditions. Thus, the cell performance is an indicator of the energy efficiency, selectivity, single pass conversion efficiency (shortly conversion), and durability of the electrolyser system.

[0028] Brief description of the drawings

[0029] In what follows, the invention is explained in more detail with reference to the appended drawings wherein

[0030] Figure 1 illustrates the path length within an electrolyser cathode current collector, here line 100 and line 100' represents the CO2inlet side and the CO2outlet side, respectively.

[0031] Figure 2 shows the path length within an electrolyser anode current collector: here line 110 and line 110' represents the anolyte inlet side and the anolyte outlet side, respectively.

[0032] Figure 3 shows the pressure dependence along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 100 cm2.

[0033] Figure 4 shows the change of CO2concentration along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 100 cm2.

[0034] Figure 5 illustrates the temperature change along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 100 cm2.

[0035] Figure 6 illustrates the change of CO2molar fraction along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 100 cm2.

[0036] Figure 7 illustrates the change of transferred water vapor along the flow channel on the cathode side of an electrolyser cell prepared by machining and having an active geometric area of 100 cm2. Figure 8 illustrates the pressure drop along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 2500 cm2.

[0037] Figure 9 illustrates the change of CO2 concentration along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 2500 cm2.

[0038] Figure 10 shows the temperature change along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 2500 cm2.

[0039] Figure 11 shows the change of CO2 molar fraction along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 2500 cm2.

[0040] Figure 12 shows the change of transferred water vapor along the flow channel of an electrolyser cell prepared by machining and having an active geometric area of 2500 cm2.

[0041] Figure 13 illustrates the pressure drop along the flow channel of an electrolyser cell prepared by chemical etching or stamping and having an active geometric area of 2500 cm2.

[0042] Figure 14 illustrates the change of CO2 concentration along the flow channel of an electrolyser cell prepared by chemical etching or stamping and having an active geometric area of 2500 cm2.

[0043] Figure 15 illustrates the temperature change along the flow channel of an electrolyser cell prepared by chemical etching or stamping and having an active geometric area of 2500 cm2.

[0044] Figure 16 shows the change of CO2 molar fraction along the flow channel of an electrolyser cell prepared by chemical etching or stamping and having an active geometric area of 2500 cm2.

[0045] Figure 17 shows the change of transferred water vapor along the flow channel on the cathode sire of an electrolyser cell prepared by chemical etching or stamping and having an active geometric area of 2500 cm2.

[0046] Figure 18 presents the effect of temperature on CO and H2 partial current density.

[0047] Figure 19 is a schematic representation of a possible embodiment of the CO2 electrolyser according to the present invention with symmetrical cell stack design that enables interchangeability of the flow inlet and outlet on any of the anode / cathode sides.

[0048] Figure 19A presents a possible embodiment of a cathode or anode side flow directional change scheme realized with a CO2 electrolyser that has the symmetrical cell stack design shown in Figure 19.

[0049] Figure 19B presents a possible further embodiment of the cathode or anode side flow directional change scheme realized with a CO2 electrolyser that has the symmetrical cell stack design shown in Figure 19.

[0050] Figure 20 shows possible combinations of the cathode and anode side flow direction patterns relative to each other.

[0051] Figure 21A illustrates an exemplary 3-cell stack design in top view.

[0052] Figure 21B shows the exemplary 3-cell stack design of Figure 21A in sectional view (A-A).

[0053] Figure 21C shows the exemplary 3-cell stack design of Figure 21A in sectional view (B-B). Figure 22: Schematic illustration of the MEA components.

[0054] Figure 23 presents the cell voltage and the ratio of product gases (CO / H2) as a function of time during electrolysis at I = 40 A; the data is represented as a function of the flow rate of carbon dioxide (normalized with the surface area) in the cathode compartment of the electrolyser cell.

[0055] Detailed description of the invention

[0056] Materials and General Methods

[0057] Materials

[0058] The CsOH-F O and Ag nanopowder (davg< 100 nm, 99.5%, 5.0 m2g1) was purchased from Sigma- Aldrich. The KOH was purchased from VWR. The PiperlON (40 pm thick PiperlON-A40-HCO3) membrane and PiperlON ionomer dispersion (PiperlON-A5-HCO3-EtOH, 5 wt% in EtOH) was purchased from Versogen. The I rOxcatalyst and the Freudenberg H23C6 gas-diffusion layer were purchased from FuelCellStore. MilliQgrade (p = 18.2 MO cm) ultrapure deionised water was produced using a Millipore Direct-Q 3 UV instrument and was used to prepare all the solutions.

[0059] The CsHCOa or KHCO3 electrolyte solution was obtained from CsOH or KOH solution by bubbling CO2 gas through these until saturation (for at least 30 minutes).

[0060] General Methods

[0061] Electrode preparation

[0062] The cathode catalyst dispersion consisted of Ag nanopowder with 5 wt% PiperlON ionomer (m(ionomer) / (m(ionomer)+m(Ag)) dispersed in a 1:1 isopropanol / water solvent mixture with a concentration of 24 mg cm'3(Ag). The anode catalyst dispersion consisted of I rOxnanoparticles with 15 wt% PiperlON ionomer (m(ionomer) / (m(ionomer)+m(lrOx) dispersed in an identical solvent with a concentration of 17 mg crrr3(lrOx). The lrOxdispersion was homogenised with a magnetic stirring bar at 600 rpm. The silver nanoparticles were dispersed with a high-power immersion sonotrode (3 min) and a regular ultrasonic bath for 20 min, and the dispersion was kept sonicated in the ultrasonic bath for the duration of spray coating (while keeping the bath temperature below 35 °C by additions of ice cubes).

[0063] The GDEs were fabricated using a hand-held airbrush. The Ag dispersion was spray coated onto preheated Freudenberg H23C6 GDLs on a hotplate at 100gC until reaching a loading of 1.0 ± 0.1 mg cm-2(Ag). The anode catalyst dispersion was spray coated similarly, onto a porous Ti frit to reach a loading of 1.0 ± 0.1 mg cm-2(lrOx). The loading was calculated from the substrate weight difference before and after the spray coating. The GDE was soaked in CsOH equivalent concentration to the anolyte 10 minutes before use, and washed with ultrapure water immediately before use. For example, for 0.1 M CsHCOa anolyte the GDE was soaked in 0.1 M CsOH.

[0064] Membrane pretreatment

[0065] The membranes were ion-exchanged before use for at least 24 hours in a 1 M CsOH solution, which was exchanged for a fresh solution after the first 5 hours. Immediately before cell assembly, the membrane was cut to size and placed into ultrapure water for 10 minutes and was thoroughly rinsed with ultrapure water.

[0066] Electrochemical measurements and cell assembly

[0067] All electrochemical measurements were performed in a custom-designed direct gas feed zerogap electrolyser cell with an active area of 8 cm2. Commercially available AEM (40 pm thick PiperlON- A40-HCO3) was used to separate the anode and the cathode chambers. The cell was assembled starting from the anode flowplate, continuing with the catalyst-coated Ti frit (anode), the anion exchange membrane, the PTFE gasket, the catalyst coated gas-diffusion layer (cathode) and finished with the cathode flowplate. The PTFE gasket was 200 pm thick to achieve the necessary compression for the approximately 250 pm thick catalyst-coated gas-diffusion layer. The cell assembly is secured with 6 bolts and nuts, tightened in three steps to 3 Nm. Either a CsHCOa or a KHCO3 solution was used as the anolyte, which was prepared from the respective hydroxide solution by saturating it with CO2.

[0068] A Biologic VMP-300 potentiostat was used for chronoamperometry, chronopotentiometry and EIS measurements. The cell voltage refers to the voltage between the anode and cathode flowplates without iR compensation. The CO2 inlet gas flow rate was 12.0 cm3cm'2, controlled with a Bronkhorst MASS-STREAM D-6321 type mass flow controller. The CO2 was humidified by bubbling it through ultrapure water heated to 60 °C. The gas line between the humidifier and the cell was heated to avoid any condensation. An external heating mantle heated the anolyte to achieve a cell temperature of 60°C.

[0069] Gas product detection (cathode)

[0070] The gas products of the CO2 electrolysis were detected using a Shimadzu Nexis-GC-2010 equipped with a BID detector. A Restek Shincarbon ST column was used to separate the zero-gap cell outlet gas, using grade 6.0 He carrier gas. The outlet flow rate was measured with an Agilent ADM flow meter. For long-term measurements, an online infrared-thermal conductivity gas analyser (Gasboard- 3100, customised for CO2-CO-H2 mixtures, Hubei Cubic-Ruiyi) was used, coupled with a McMillan S- 110-4 Flo-Meter for continuous outlet flow rate measurement. The partial current densities were calculated from the outlet gas flow rate and the composition of the outlet gas.

[0071] Detailed description of the drawings As illustrated in Figure 18, temperature inhomogeneities within the electrochemical cell and along the paths influence the partial current densities of carbon monoxide (CO) and hydrogen (Hz). Experimental measurements were conducted using an electrolyser with an active geometric area of 100 cm2, operated at a current density of 400 mA-crrr2. During these measurements, the cell voltage decreased from 3.4 V to 3.2 V as the operating temperature of the electrolyser was increased from 48°C to 60°C. The anolyte flow rate was maintained at 2 L-min-1. A direct correlation was identified between elevated temperatures and increased hydrogen partial current density. Additional influential parameters, quantified and detailed in Examples 1 to 3, yielded similarly significant effects. The challenge arising from temperature inhomogeneity is addressed through the implementation of a unique cell geometry and reversible structural modifications, as disclosed in Examples 4 to 7.

[0072] In what follows the present invention and its further advantages are discussed with reference to some examples.

[0073] Example 1 - Calculations

[0074] According to an embodiment, a cell with an active geometric area of 100 cm2with 10 cm characteristic cathode path length was prepared. The COZfeed flow rate was set to 1.2 standard dm3min1at a current density of 400 mA cm'2, while the anolyte (aqueous CsHCOa, c = 0.05 M) flow rate was set to 2 L / min. Under electrolysis conditions the measured pressure values at the in- and outlet can be seen on Figure 2. Pressure drop along the channels naturally apply, which is affected mostly by the set flow rate, flow cross section area, temperature and pressure.

[0075] Calculated CO2 concentration values at the inlet and outlet shows significant decrease, due to the effect of pressure drop and formation of product gases at the cathode (Figure 3).

[0076] In case of a co-flow configuration (see Figure 20) a temperature rise is measured between the in - and outlets (Figure 5).

[0077] Reaction rate influencing the calculated molar fraction decrease is shown in Figure 6. Water vapor amount transferred by the cathode gas was measured by relative humidity sensors at both the inlets and outlets (Figure 7). Humidification was set to result in low overall water loss at the cathode.

[0078] In what follows, several further Examples are discussed, wherein although the electrolysing cells of the CO2 electrolyser are of the same size, their channel patterns and geometries differ which result in different effects.

[0079] 2 - Calculations

[0080] Figures 8-12 refer to an embodiment of the electrolyser cell according to the invention, prepared by machining and having an active geometric area of 2500 cm2, the relevant characteristic cathode path length is 38 cm. The CO2 feed flow rate was set to 30 standard dm3min1at a current density of 400 mA / cm2, while the anolyte (aqueous CsHCOa, c=0.05 M) flow rate was set to 13.6 L / min. Generally, a longer path length resulted in higher pressure drop (Figure 8) and the elevated temperature difference between the in-and outlets can be observed (Figure 9). This leads to a more pronounced difference in the transferred water vapor amount along the flow path (Figure 11).

[0081] Example 3 - Calculations

[0082] Figures 13-17 refer to an embodiment of the electrolyser cell according to the invention, prepared by chemical etching or stamping and having an active geometric area of 2500 cm2, the relevant characteristic cathode path length is 38 cm. The CO2 feed flow rate was set to 30 standard dm3min1at a current density of 400 mA cm'2, while the anolyte (aqueous CsHCOa, c = 0.05 M) flow rate was set to 9.1 L / min. Generally higher pressures and higher pressure drop could be observed due to the smaller flow cross section area. Leading to even higher differences between the parameters measured or calculated at the inlets and outlets.

[0083] Example 4 - Cell stack design

[0084] Embodiments of CO2 electrolysers with symmetrical cell stack design are shown below. In Figures 19A-B, schematic representations of a CO2 electrolyser with symmetrical cell stack design are presented that enable interchangeability of flow inlet and outlet on any of the anode / cathode sides. As seen in Figure 19A or 19B, the cathode or anode side flow directional change scheme that can be realized, for example, with a CO2 electrolyser having symmetrical cell stack design shown in Figure 19. The pump or compressor 8 can supply CO2 or anolyte to the electrolyser cell 200 throughout the 4-way directional valve 9 and the stack inlet / outlet 4 or 4'. Within the cell both CO2 or the anolyte is distributed by stack outlet / inlet side chimney(s) sections 1 and 1', cell outlet / inlet side connector channels 2 and 2', and the flow pattern 3.

[0085] The various substance flow direction patterns that are equally obtainable by the schematic CO2 electrolyser shown in Figs. 19, 19A and 19B are presented in Fig 20.

[0086] As is shown in Figure 20 schematically, a directional control valve 9 and an actuator 10, e.g. a 4-way ball valve or any equivalent valve or valves is used to reverse the flow both at the cathode and the anode sides. Tubes, pipes or hoses with equal length, internal diameter and heat insulation or active heating can be used to connect the stack fluidically. The carefully designed electrolyser stack allows for flow directional change, since flow path cross-sections, lengths and shapes are equal from both directions, resulting in equal overall pressure drop, heat and mass transfer. From fluid paths point of view of the electrolyser stack is symmetrical. 5 - Cell stack

[0087] In one embodiment, the invention is constructed as a stack of three electrolyser cells. The number of the cells in the stack can vary in wide range, as it is clear to a skilled person in the art. Figures 21A shows the 3-cell stack design in a top view, while Figure 21B is a sectional view (A-A section) taken along a plane intersecting the cathode side inlet / outlet connection ports perpendicularly to the top of the CO2 electrolyser. Similarly, the B-B line in Figure 21A refers to a sectional view (not shown separately) of the exemplary CO2 electrolyser taken along a plane intersecting the anode side, i.e. the anolyte inlet / outlet connection ports perpendicularly to the top of said CO2 electrolyser.

[0088] Figure 21A shows the 3-cell stack design in top view, while Figure 21B illustrates said exemplary 3- cell stack design in its A-A sectional view.

[0089] Here, at stack level, the symmetrical design is represented in equal stack inlet / outlet connections (see Figure 21A; cathode side outlet / inlet connection ports 13, 13' and anolyte outlet / inlet connection ports 22, 22') and cathode side chimneys 37 and anode side chimneys 38. (see Figure 21A and B).

[0090] Said chimneys 37 and 38 at both the inlet and the outlet side of each of the anode side and the cathode side of the cells are responsible for equal distribution of fluid substance flows amongst the cells arranged in the stack.

[0091] At cathode (Figure 21B) or anode half-cell level, the symmetrical design is represented in equal inlet / outlet side cell connector channels 16 and flow patterns within the membrane electrode assembly 18.

[0092] Said cell-connector channels 16 are responsible for the fluid substance transfer between the chimney or chimneys 37 and the flow pattern in the membrane electrode assembly (MEA) 18. The flow pattern area then distributes the respective fluid substances across the whole active electrolyser area. Said electrolyser cell is comprised of the following structural elements to provide mechanical support and appropriate electrical connection to the MEAs: bolt 10, nut 20, end plate 11, insulator 26, current collectors 14, sealing / spacer 17, cathode end plate 15, anode and plate 25 and bipolar plate 19 represented in Figure 21A and 21B.

[0093] Figure 21A-C and Figure 23 illustrates schematically an embodiment of a zero-gap electrolyser cell 200 to convert gaseous CO2 to other products. Said electrolyser cell 200 comprises (here, from bottom to top) at least an anode end plate 25 with fluid inlet(s) and fluid outlet(s) 23, 23' on one side thereof and a fluid-flow pattern 44 formed on the other side, an anode electrode 43 with a catalyst layer (not shown) on one side, spacer element at the anode side 46, a membrane 43, in direct contact with said catalyst layer, a cathode catalyst layer (not shown) in direct contact with the membrane 42 on one side, and the cathode electrode 41 on the other side, and a bipolar plate 19, on which a gas-flow pattern 40 is formed on one side thereof (in direct contact with the cathode electrode 41). Where the spacer element at the cathode side 45 influences the compression of the cathode electrode 41. As is clear for a skilled person, each of the anode electrode 43 and the cathode electrode 41 can be provided in the form of a gas-diffusion electrode (with a respective catalyst layer), or may alternatively be constructed using a different electrode architecture. Said electrolyser cell 200 may also be constructed as an electrolyser cell-stack, consisting of multiple electrolyser layers (cells). In this case, multiple electrolyser layers are stacked on each other, repeating the anode, anode catalyst, membrane, cathode catalyst and cathode elements. Between the adjacent layers, preferably bipolar plates are used, which on one side act as anode, while serve as cathode on the other side. Such bipolar plates are known in literature.

[0094] The anode end plate 25, the cathode end plate 15, the anode electrode 43, the cathode electrode 41, the catalysts and the flow channels and fluid distribution elements 13, 16, 36, 37, 38 and the flow patterns 44, 40 applied in the electrolyser cell 200, as well as their functions and possible design are equally known in literature.

[0095] Furthermore, the membrane 42 is an anion exchange membrane, available under the trade names of e.g. Fumasep, Selemion, PiperlON and Sustainion, just to mention a couple of examples only, which theoretically allows, in operation, the migration of anions (e.g., OH“, HCOa- and COa2-ions; charges) between the cathodic and anodic sides of the electrolyser cell 200 through its bulk, while water (H2O) diffusing through said cell 200 from the anodic to the cathodic side takes part in the electrolytic reduction of CO2 at the cathodic side. As in this case no electrons are transported through the membrane 42, said membrane 42 actually acts as an ionic conductor between the cathodic and anodic sides of the cell 200.

[0096] Example 7 - measurement

[0097] In case of an electrolyser cell prepared by machining and having an active geometric area of 100 cm2, the relevant characteristic cathode path length is 10 cm. The CO2 feed flow rate was set to 1.2 standard dm3min1at a current density of 400 mA cm'2, while the anolyte (aqueous CsHCOa, c = 0.05 M) flow rate was set to 2 L / min. Cathode outflow gas composition was analyzed inline.

[0098] Figure 22 illustrates the cell voltage and ratio of product gases (CO / H2) over time during electrolysis at I = 40 A as a function of carbon-dioxide flow rate (normalized with the surface area) in the cathode compartment of the electrolyser cell. Again, as for the cathode, 2 mg cm2Ag cathode catalyst layer was immobilized on Sigracet39BC carbon paper by spray coating. As for the anode, 2 mg cm2Ir black was immobilized on a porous titanium frit. Both catalyst layers contained 15 wt% Sustanion ionomer. The measurements were performed feeding 0.05 M CsHCOa anolyte continuously to the anode compartment (at a feed rate of ~20 cm3cm'2min1, T = 50 °C).

[0099] Measurement data marked with A represents a baseline measurements at equilibrium conditions of a single cell electrolyse^ where the gas feed and the cathode gas direction was kept constant (A). Then the electrolysis process was stopped, and the cell was kept idle under standard room temperature conditions for more than 1 week.

[0100] Measurement data marked with O exhibits slow performance increase (cell voltage drop and selectivity increase after the idle period) until approximately 2.25 h, where the gas feed and the cathode gas direction was kept constant (A). After 2.25 h the gas feed and the cathode gas direction were periodically changed between "A" and "B" directions in every 30 min.

[0101] By applying gas flow reversal in every 30 minutes, higher product selectivity values at the same cell voltage were achieved which is an unpredictable characteristic. That is, this method affected the selectivity of the conversion process - no similar effect was observed earlier in the field of fuel cell technology.

[0102] List of reference numbers

[0103] 100 CO2 inlet side

[0104] 100' CO2 outlet side

[0105] 110 anolyte inlet side

[0106] 110' anolyte outlet side

[0107] 200 Electrolyser cell

[0108] 1 stack outlet / inlet side chimney(s) section

[0109] 1' stack inlet / outlet side chimney(s) section

[0110] 2 cell outlet / inlet side connector channels

[0111] 2' cell inlet / outlet side connector channels

[0112] 3 cell flow pattern

[0113] 4 stack inlet / outlet

[0114] 4' stack outlet / inlet

[0115] 8 pump or compressor

[0116] 9 4-way directional valve

[0117] 10 Valve actuator

[0118] 11 Bolt

[0119] 12 End plate

[0120] 13 Cathode side inlet / outlet connection port

[0121] 13' Cathode side outlet / inlet connection port

[0122] 14 Current collector

[0123] 15 Cathode end plate

[0124] 16 Cell-connector channel

[0125] 17 Sealing / spacer

[0126] 18 Membrane electrode assembly (MEA)

[0127] 19 Bipolar plate

[0128] 20 Nut

[0129] 22 Anolyte inlet / outlet connection port

[0130] 22' Anolyte outlet / inlet connection port Anode end plate

[0131] Insulator

[0132] Cell-connector channel

[0133] Cathode side chimney

[0134] Anode side chimney

[0135] Gas-flow pattern

[0136] Cathode electrode

[0137] Membrane

[0138] Anode electrode

[0139] Fluid-flow pattern

[0140] Spacer element at the cathode side

[0141] Spacer element at the anode side

Claims

CLAIM1. An electrolysing cell stack to perform electrolysis of gaseous carbon dioxide, comprising a plurality of individual electrolysing cells, each cell having a membrane electrode assembly with a system of cathode side cell flow channels, each cathode side cell flow channel being connected through a respective cell cathode inlet side connector channel to a continuous stack inlet side chimney extending on a cathode side of the cell and to a continuous stack outlet side chimney extending on the cathode side of the cell; a system of anode side cell flow channels, each anode side cell flow channel being connected through a respective cell anode inlet side connector channel to a continuous stack inlet side chimney extending on an anode side of the cell and to a continuous stack outlet side chimney extending on the anode side of the cell; said stack inlet side chimney on the anode side is connected to an anolyte inlet port, said stack outlet side chimney on the anode side is connected to an anolyte outlet port, said stack inlet side chimney on the cathode side is connected to an inlet port for gaseous carbon dioxide, and said stack outlet side chimney on the cathode side is connected to a product outlet port; wherein the anolyte inlet port and the anolyte outlet port are configured to be interchangeable with each other from the point of view of a flow of anolyte supplied to the anode side, and / or the inlet port for gaseous carbon dioxide and the product outlet port are configured to be interchangeable with each other from the point of view of a flow of gaseous carbon dioxide supplied to the cathode side / a flow of product discharged from the cathode side.

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