Patent Publication Number: US-2020291537-A1

Title: Arrangement for the electrolysis of carbon dioxide

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage of International Application No. PCT/EP2018/078031 filed 15 Oct. 2018, and claims the benefit thereof. The International Application claims the benefit of German Application No. DE 10 2017 219 766.8 filed 7 Nov. 2017. All of the applications are incorporated by reference herein in their entirety. 
    
    
     FIELD OF INVENTION 
     The invention relates to an arrangement for carbon dioxide electrolysis. 
     BACKGROUND OF INVENTION 
     The combustion of fossil fuels currently covers about 80% of global energy demand. These combustion processes in 2011 emitted around 34 000 million tonnes of carbon dioxide into the atmosphere globally. 
     Discussion about the adverse effects of the greenhouse gas CO2 on the climate has led to consideration of reutilization of CO2. CO2 is a strongly bonded molecule and can therefore be reduced back to usable products only with difficulty. 
     In nature, CO2 is converted to carbohydrates by photosynthesis. This complex process can be reproduced on the industrial scale only with great difficulty. One currently technically feasible route is the electrochemical reduction of CO2. The carbon dioxide is converted here with supply of electrical energy to a product of higher energy value, for example CO, CH4, C2H4 or C1-C4 alcohols. The electrical energy in turn advantageously comes from renewable energy sources such as wind power or photovoltaics. 
     For electrolysis of CO2, in general, metals are used as catalysts. The type of metal affects the products of the electrolysis. For example, CO2 is reduced virtually exclusively to CO over Ag, Au, Zn and, to a limited degree, over Pd and Ga, whereas a multitude of hydrocarbons are observed as reduction products over copper. As well as pure metals, metal alloys are also of interest, as are mixtures of metal and metal oxide having cocatalytic activity, since these can increase selectivity for a particular hydrocarbon. 
     In CO2 electrolysis, a gas diffusion electrode can be used as cathode in a similar manner to that in chlor-alkali electrolysis in order to establish a three-phase boundary between the liquid electrolyte, the gaseous CO2 and the solid silver particles. This is done using an electrolysis cell, as also known from fuel cell technology, having two electrolyte chambers, wherein the electrolyte chambers are separated by an ion exchange membrane. 
     The working electrode is a porous gas diffusion electrode (GDE). It typically comprises a metal mesh, to which a mixture of PTFE, activated carbon, a catalyst and further components has been applied. It has a pore system into which the reactants penetrate and react at the three-phase interfaces. 
     The counterelectrode is sheet metal coated, for example, with platinum or a mixed iridium oxide. The GDE is in contact with the electrolyte on one side. On the other side it is supplied with CO2. The mode of function of a GDE is known, for example, from EP 297377 A2, EP 2444526 A2 and EP 2410079 A2. 
     For a continuous process, the electrolysis cell can be operated in flow-by mode. In this mode, the reactant gas diffuses into the pores of the GDE. The reaction in the pores of the GDE forms OH −  ions that cause a locally high pH. If the reactant gas CO2 meets this alkaline liquid that additionally also contains alkali metal cations, for example potassium, sparingly soluble carbonates are formed, which precipitate out in salt form and block the pores. 
     There are various ways of avoiding this, including the utilization of the transpiration of the electrolyte through the GDE. This clears the pores in situ, runs off downward on the gas side of the GDE and is discharged from the cell at the base of the cell together with the unconverted reactant gas and the product gases. 
     In order to bring about good mixing of the gas in the gas space and hence promote diffusion of the carbon dioxide into the pores of the GDE, flow grids are used. These ensure vortexing and crossmixing and hence promote the mass transfer of reactant and product gases. Furthermore, the flow grids support the GDE, such that it cannot bend. 
     Although this enables stable long-term operation (&gt;1000 h) of the gas diffusion electrode in CO2 electrolysis, the high liquid content in the gas space reduces the efficiency of the process. Firstly, the liquid film on the GDE makes it difficult for the reactant gases to diffuse into the pores of the GDE. Furthermore, known flow grids in the form of grid structures or knits make it difficult for the transpiration liquid to flow away since it adheres to and accumulates in the flow grids. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to specify an improved arrangement for carbon dioxide electrolysis, by which stable long-term operation is enabled with avoidance of the disadvantages mentioned at the outset. 
     This object is achieved by an arrangement having the features of the independent claim. The dependent claims relate to advantageous configurations of the arrangement. 
     The arrangement of the invention for carbon dioxide electrolysis comprises an electrolysis cell having an anode and a cathode, where anode and cathode are connected by a power supply, where the cathode is configured as a gas diffusion electrode adjoined on a first side by a gas space and on a second side by a cathode space. The arrangement for carbon dioxide electrolysis further comprises an electrolyte circuit adjoining the electrolysis cell and a gas feed for supply of carbon dioxide-containing gas to the gas space. Finally, the arrangement for carbon dioxide electrolysis comprises one or more channels in the gas space, where the channels at least partly adjoin the gas diffusion electrode and are configured for transport of electrolyte liquid penetrating through the gas diffusion electrode to a side region of the gas space. 
     The channels advantageously bring about removal of electrolyte that penetrates through the cathode as transpiration liquid and wets the surface of the gas diffusion electrode. If the electrolyte layer on the surface of the gas diffusion electrode is thick enough, the electrolyte begins to run off. The channels guide the electrolyte away to the side and hence reduce the thickness of the electrolyte layer in the region beneath a respective channel. This creates better access for the carbon dioxide to the surface of the gas diffusion electrode and hence enables an improvement in electrolysis efficiency. 
     Furthermore, the channels, by virtue of their position and arrangement, also ensure flow resistance for the carbon dioxide flowing across the surface of the gas diffusion electrode. This interrupts the laminar flow of the gas and generates vortices. This likewise brings about better utilization of the carbon dioxide present in the gas. 
     Advantageous configurations of the arrangement of the invention for carbon dioxide electrolysis are apparent from the dependent claims. The embodiment according to the independent claim can be combined here with the features of one of the dependent claims or else with those from multiple dependent claims. Accordingly, the following features may additionally be provided for the arrangement:
         The channels may have an essentially linear configuration and an oblique arrangement with an angle to the horizontal between 1° and 30°, especially between 1° and 10°.   According to the basic length of the vertical extent of the gas diffusion electrode, a channel may be present, where the basic length is between 3 cm and 10 cm. In other words, the distance between the channels is between 3 cm and 10 cm.   The channels are advantageously connected by a common support structure, where the support structure is spaced apart from the gas diffusion electrode. This enables anchoring and mechanical integrity for the channels independently of the cathode since the channels are retained by the support structure. However, the support structure does not prevent the access of gas to the surface of the gas diffusion electrode.   The support structure has one or more support pins in mechanical contact with the surface of the gas diffusion electrode. This enables better mechanical strength for the support structure.   The support pins advantageously have a diameter of less than  1  mm. This achieves the effect that the surface coverage of the gas diffusion electrode by the support pins is small and hence the support pins have only a minor influence on the electrolysis.
           It is particularly advantageous when at least some of the support pins have vortexing elements arranged at a distance from the surface of the gas diffusion electrode of at least 1 mm, especially at least 2 mm. As a result, the gas stream that otherwise flows across the surface of the gas diffusion electrode in a largely laminar manner experiences vortexing, which distinctly improves the access of the carbon dioxide to the cathode. At the same time, the distance from the surface of the cathode ensures that electrolyte does not collect on the surface of the vortexing elements and hence is prevented from running off. The vortexing elements may have a corrugated configuration, for example.   
           The support structure, the support pins and/or the channels advantageously comprise a material having low hydrophobicity, for example PE. This material advantageously constitutes the surface of the individual elements, for example as a coating. Alternatively, it is possible for the support structure, the support pins and/or the channels to be largely or entirely made from the material. As a result, the liquid is distributed more easily over the material surface and the runoff of the liquid is facilitated.   The support structure, the support pins and/or the channels may also have an electrically conductive material for contacting the gas diffusion electrode. In the case of the connection of multiple cells to form what is called a stack, it is possible in this way, via the flow grid formed from support structure, support pins, vortexing elements and channels, to electrically connect a cell unit to the next cell unit.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An advantageous, but in no way limiting, working example of the invention will now be elucidated in detail with reference to the figures of the drawing. The features are shown here in schematic form. The figures show: 
         FIG. 1  an electrolysis system for CO2 electrolysis and 
         FIGS. 2 and 3  a side view and a top view of a flow grid. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The structure of an electrolysis cell  11  shown in schematic form in  FIG. 1  is typically suitable for undertaking carbon dioxide electrolysis. This embodiment of the electrolysis cell  11  comprises at least one anode  13  with an adjacent anode space  12 , and a cathode  15  and an adjacent cathode space  14 . Anode space  12  and cathode space  14  are separated from one another by a membrane  21 . The membrane  21  is typically manufactured from a PTFE-based material. According to the electrolyte solution used, another conceivable structure is one without a membrane  21 , in which case pH adjustment extends beyond that by the membrane  21 . 
     Anode  13  and cathode  15  are electrically connected by a power supply  22 , which is controlled by the control unit  23 . The control unit  23  may apply a protective voltage or an operating voltage to the electrodes  13 ,  15 , i.e. the anode  13  and the cathode  15 . The anode space  12  of the electrolysis cell  11  shown is equipped with an electrolyte inlet. The anode space  12  depicted likewise comprises an outlet for electrolyte and, for example, oxygen O 2  or another gaseous by-product which is formed at the anode  13  in carbon dioxide electrolysis. The cathode space  14  likewise in each case has at least one product and electrolyte outlet. It is possible here for the overall electrolysis product to be composed of a multitude of electrolysis products. 
     The electrolysis cell  11  is also executed in a three-chamber structure in which the carbon dioxide CO 2  is introduced into the cathode space  14  via the cathode  15  in the form of a gas diffusion electrode. Gas diffusion electrodes enable contacting of a solid catalyst, a liquid electrolyte and a gaseous electrolysis reactant with one another. For this purpose, for example, the catalyst may be porous and may assume the electrode function, or a porous electrode assumes the catalyst function. The pore system of the electrode is such that the liquid and gaseous phases can equally penetrate into the pore system and may be present simultaneously therein or at its electrically accessible surface. One example of a gas diffusion electrode is an oxygen-depolarized electrode which is used in chlor-alkali electrolysis. 
     For configuration as a gas diffusion electrode, the cathode  15  in this example comprises a metal mesh to which a mixture of PTFE, activated carbon and a catalyst has been applied. For introduction of the carbon dioxide CO2 into the catholyte circuit, the electrolysis cell  11  comprises a carbon dioxide inlet  24  into the gas space  16 . The carbon dioxide in the gas space  16  reaches the cathode  15 , where it can penetrate into the porous structure of the cathode  15  and hence react. 
     In addition, the arrangement  10  comprises an electrolyte circuit  20  by means of which the anode space  12  and the cathode space  14  are supplied with a liquid electrolyte, for example K2SO4, KHCO3, KOH, Cs2SO4, and the electrolyte is returned to a reservoir  19 . The electrolyte is circulated in the electrolyte circuit  20  by means of an electrolyte pump  18 . 
     The gas space  16  in the present example comprises an outlet  25  disposed in the base region. The outlet  25  is configured as an opening of sufficient cross section such that both electrolyte that passes through the cathode  15  and carbon dioxide and product gases can pass through the outlet into the connected tube. The outlet  25  leads to an overflow vessel  26 . The liquid electrolyte is collected and accumulates in the overflow vessel  26 . Carbon dioxide and product gases coming from the gas space  16  are separated from the electrolyte and accumulate above it. 
     From a point at the upper end of the overflow vessel  26 , a further pipe  28  leads to a pump  27 , a membrane pump in this working example, and further to the gas feed  17 . The pump  27  may also be a piston pump, reciprocating pump, extruder pump or gear pump. Part of the gas feed  17 , the gas space  16 , the pipe  28  and the overflow vessel  26  together with its connection to the outlet  25  thus collectively form a circuit. By means of the pump  27 , the carbon dioxide and product gases present are guided from the overflow vessel  26  back into the gas feed and hence the gas is partly circulated. The volume flow rate of the pump  27  here is distinctly higher than the volume flow rate of new carbon dioxide. As yet unconsumed reactant gas is thus advantageously guided once more past the cathode  15  and has one or more further opportunities to be reduced. Product gases are likewise partly circulated here. The repeated guiding of the carbon dioxide past the cathode  15  increases the efficiency of the conversion. 
     There is a further connection from the overflow vessel  26  that leads back to the electrolyte circuit  20 . This connection begins with an outlet  29  disposed on a side wall of the overflow vessel  26 , advantageously close to the base, but not in the base. The outlet  29  is connected to a throttle  30  in the form of a vertical pipe section having a length of 90 cm, for example. The diameter of this pipe section is much greater than that of the feeds to the throttle  30 . The feed has, for example, an internal diameter of 4 mm; the pipe section has an internal diameter of 20 mm. The throttle  30  is connected to the electrolyte circuit  20  on the outlet side, i.e. at the upper end of the pipe section. 
     In the course of operation, the throttle  30  establishes and maintains a pressure differential between the electrolyte circuit  20  connected at the upper end and hence also the cathode space  14  on the one hand, and the overflow vessel  26  and the gas space  16  on the other hand. This pressure differential is between 10 and 100 hPa, meaning that the gas space  16  remains at only a slightly elevated pressure relative to the cathode space  14 . 
     When the electrolysis is started, in spite of the slightly elevated pressure on the gas side, i.e. in the gas space  16 , electrolyte is “pumped” out of the catholyte space  14  through the gas diffusion electrode, i.e. the cathode  15 , in the direction of gas space  16  on account of the electrical potential applied at the cathode  15 . Droplets arise at the surface of the cathode  15  on the side of the gas space  16 , which coalesce and collect in shape in the lower region of the cathode  15 . 
     The OH −  ions passing through the cathode  15  do cause salt formation together with the carbon dioxide and the alkali metal cations from the electrolyte, but the pressure differential at the cathode  15  is so small that sufficient liquid is purged through the cathode  15  and brings the salt formed into solution, washes it away permanently and transports it out of the gas space  16  into the overflow vessel  26 . A further pressure rise that would lead to crystallization of the salt formed is prevented by the throttle  30 . 
     A flow grid  40  is disposed on the gas diffusion electrode. This flow grid  40  is arranged such that the gas flow between the carbon dioxide inlet  24  and the outlet  25  is between the surface of the gas diffusion electrode and a support structure  41  of the flow grid  40 . The specific construction of the flow grid  40  is shown in  FIG. 2  and  FIG. 3 . 
       FIG. 2  shows an enlarged side view of the flow grid  40 . The flow grid  40  adjoins the cathode  15  by the right-hand side in  FIG. 2 .  FIG. 3  shows a top view of the flow grid  40  from the side of the cathode  15 . 
     The flow grid  40  comprises a support structure composed of struts or plates that mechanically connects the further elements. The flow grid  40  is concluded on the outside by an essentially rectangular frame  46  that permits gas access and gas exit only at orifices  47  and  48 . In the region of the orifices  47 ,  48 , the flow grid  40  has parallel ridges  50  aligned in gas flow direction and one or more baffles  49  to shape the gas flow. 
     In a middle region of the flow grid  40  is disposed a multitude of support pins  42 . The support pins  42  serve to increase the mechanical strength of the flow grid and bring about a fixed minimum distance of the support structure  41  from the surface of the gas diffusion electrode. In the present example, 8 horizontal rows of 9 and 10 support pins  42  in alternation are present. The support pins  42  have a distance from one another of about 6 mm. In other executions of the flow grid  40 , therefore, it is also possible for more support pins  42  or fewer support pins  42  to be present, according to the size of the flow grid  40 . The distance between the support pins  42  is advantageously between 3 mm and 12 mm. The support pins should cover not more than 10% of the area of the gas diffusion electrode, the coverage advantageously being less than 5%. 
     At a distance from the surface of the cathode  15  of 1.5 mm, or in another example of 2.5 mm, a vortexing element  43  is disposed on each of the support pins  42 . The vortexing elements  43  in the present example are in the form of a flat, essentially rectangular piece of material, but one that has been bent to form a corrugation. The vortexing elements  43  are arranged essentially transverse to the main flow direction of the gas. By virtue of their shape and the remaining flow regions between the vortexing elements  43 , the gas flow is made turbulent to a considerable degree, i.e. laminar flow past the gas diffusion electrode is eliminated. 
     Likewise in the middle region of the flow grid  40 , the flow grid  40  also has two channels  44 . The channels  44  are secured to multiple support pins  42  in each case and arranged such that they adjoin the surface of the gas diffusion electrode. They are arranged at a small angle from the horizontal of 10°, for example, i.e. are not entirely horizontal. By virtue of their arrangement on the surface of the cathode  15 , they take up transpiration liquid, i.e. electrolyte passing through the cathode  15 , that runs off downward at the surface of the cathode  15 , and transport the liquid to the side by virtue of their inclination. At the side of the flow grid  40 , the channels  44  in the frame  41  conclude in a runoff channel  45  that allows the liquid to run off downward to the orifice  48 . This achieves the effect that the transpiration liquid wets the surface of the cathode  15  to a lesser degree and hence the entry of gas into the pores of the gas diffusion electrode is hindered to a lesser degree. 
     Just as in the case of the support pins  42 , the number of channels  44  depends on the total size of the flow grid  40  and hence on the size of the cathode  15 . The channels are advantageously arranged at a distance from one another of between 3 cm and 10 cm. 
     In the present example, the flow grid  40  has been manufactured from polyethylene to a significant degree. In other execution variants, it is possible to choose other materials, advantageously having low hydrophobicity. The flow grid  40  may be manufactured wholly or essentially from the material, or the material is applied as a surface coating. By virtue of the low hydrophobicity, the contact angle between the flow grid  40  and the material is minimized, such that the liquid is distributed over the material surface and the best possible runoff is assured.