Patent Publication Number: US-2011073491-A1

Title: Electrochemical cell

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. 119 from German Application No. 10333853.5 filed Jul. 24, 2003, the content of which is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an electrochemical cell, comprising at least an anode half-cell with an anode, a cathode half-cell with a cathode, and an ion exchange membrane arranged between the anode half-cell and the cathode half-cell, the anode and/or the cathode being a gas diffusion electrode. The present invention furthermore relates generally to a method for the electrolysis of an aqueous solution of alkali metal chloride. 
     2. Description of Related Art 
     WO-A 01/57290 discloses an electrolysis cell with a gas diffusion electrode in which a porous layer is provided in the gap between the gas diffusion electrode and the ion exchange membrane. The electrolyte flows from the top downwards through the gap, via the porous layer, under the effect of gravity. The porous layer according to WO-A 01/57290 may consist of foams, wire gauzes or the like. 
     U.S. Pat. No. 6,117,286 likewise describes an electrolysis cell with a gas diffusion electrode for the electrolysis of a sodium chloride solution, in which there is a layer of a hydrophilic material in a gap between the gas diffusion electrode and the ion exchange membrane. The layer of hydrophilic material preferably has a porous structure, which contains a corrosion-resistant metal or resin. Gauzes, fabrics or foams, for example, may be used as the porous structure. Sodium hydroxide, the electrolyte, flows downwards under gravity via the layer of hydrophilic material to the bottom of the electrolysis cell. 
     EP-A 1 033 419 furthermore discloses an electrolysis cell for the electrolysis of a sodium chloride solution, having a gas diffusion electrode as its cathode. A hydrophilic porous material, through which the electrolyte flows, is provided in the cathode half-cell where the electrolyte flows downward while being separated from the gas space by a gas diffusion electrode. In EP-A 1 033 419, metals, metal oxides or organic materials are suitable as the porous material, so long as they are corrosion-resistant. 
     A disadvantage of electrolysis cells with a gas diffusion electrode of the prior art is that the porous material prevents the gap between the gas diffusion electrode and the ion exchange membrane from being completely filled with electrolyte. The gap therefore contains regions in which gas is found and accumulates. Electric current cannot flow in these regions. Since current flows only through electrolyte-filled regions in the gap, there is locally a higher current density which causes a higher electrolysis voltage. If the gas accumulates at the ion exchange membrane, then the membrane can become damaged due to the lack of electrolyte. Porous layers also have the further disadvantage that it is difficult for gas to be released again once it has entered the porous structure. Thus, the gas can accumulate inside the porous layer, which leads to the aforementioned disadvantages, as well as others. Under operating conditions, gas in the gas space may also pass through the gas diffusion electrode from the gas space into the gap. 
     SUMMARY OF THE INVENTION 
     It was therefore an object of the present invention to provide an electrolysis cell which avoids disadvantages of the prior art. 
     The invention relates to an electrochemical cell, comprising an anode half-cell with an anode, a cathode half-cell with a cathode, and an ion exchange membrane arranged between the anode half-cell and the cathode half-cell. The anode and/or the cathode comprise a gas diffusion electrode and there is a gap arranged between the gas diffusion electrode and the ion exchange membrane. The half-cell with the gas diffusion electrode has an electrolyte feed and an electrolyte discharge as well as a gas inlet and a gas outlet, and the electrolyte feed is hermetically connected to the gap. 
     Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a schematic cross section of a first embodiment of the electrochemical cell according to the invention, without flow guide structures in the gap between the gas diffusion electrode and the ion exchange membrane; 
         FIG. 2  shows a schematic cross section of a second embodiment of the electrochemical cell according to the invention, with flow guide structures in the gap between the gas diffusion electrode and the ion exchange membrane. 
     
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     During operation of an electrochemical cell according to the instant invention, electrolyte flows from the top downward through the half-cell in the gap between a gas diffusion electrode and an ion exchange membrane. In this case the gap is preferably completely filled with electrolyte. The remaining space of the half-cell, i.e. the gas space, is filled with gas which is supplied through the gas inlet and discharged through the gas outlet. According to the present invention, the electrolyte feed is preferably hermetically connected to the gap. Hermetic sealing minimizes or prevents gas from the gas space from entering the gap via the electrolyte feed. Due to the hermetic connection between the electrolyte feed and the gap, electrolyte can be delivered through the gap with the aid of a pump, so that the electrolyte flow does not freely travel along the gas diffusion electrode in the gap. The flow rate of the electrolyte which is flowing through the gap can be adjusted in any manner such as with the aid of a pump. The flow rate is preferably adjusted so that the flow velocity of the electrolyte is less than in freefall, that is, the flow that would occur naturally due to gravity. 
     In one preferred embodiment, flow guide structures can be arranged in the gap. The flow guide structures likewise prevent freefall of the electrolyte in the gap, so that the flow velocity is reduced compared with freefall. At the same time, however, the flow guides should not cause electrolyte to stagnate in the gap. The flow guide structures can be selected for any desired function such as to compensate for head loss of hydrostatic liquid column in the gap. If flow guide structures are provided, then they may, if desired, fully undertake the function of the pump, i.e. to reduce the flow velocity in the gap, so that a pump or similar device is unnecessary. It is, however, also possible to use both, that is, a pump in combination with one or more flow guides. 
     The flow guide generally comprise thin plates, sheets or the like, which have openings for the electrolyte to flow through. They can preferably be arranged transversely, i.e. perpendicularly or obliquely, to the flow direction of the electrolyte in the gap. The plate-like flow guide structures can preferably be inclined in some embodiments with respect to horizontal, in which case they can be inclined with respect to either only one axis or both axes. If the flow guide structures are arranged obliquely to the flow direction, then they may be inclined in any desired fashion such as both in the direction of the ion exchange membrane and in the direction of the gas diffusion electrode. Inclination in the direction of the gas diffusion electrode or the ion exchange membrane corresponds to an inclination about an axis which extends parallel to the gas diffusion electrode, or the ion exchange membrane, and horizontally. The flow guide structures may furthermore be inclined in any other desired way such as across the width of the electrochemical cell. In one embodiment inclination is made about an axis which extends perpendicularly to the gas diffusion electrode, or the ion exchange membrane. This inclination may be any desired angle such as preferably from about 0 to about 45°, particularly preferably from about 3 to about 15°. 
     During operation of the electrochemical cell, small amounts of gas from the space behind the gas diffusion electrode, i.e. space of the half-cell remote from the ion exchange membrane, will still pass through the gas diffusion electrode into the gap through which electrolyte flows. Therefore, it is often desirable to ensure that the gas is discharged from the gap as much as possible, preferably completely. If the gas content in the electrolyte rises, then the resistance of the electrolyte will generally increase. If there are flow guide structures in the gap, then the gas may either escape upward through openings in the flow guides or else gas will be entrained downward by the electrolyte flow. Inclination of the flow guides particularly favors upward discharge of gas bubbles. 
     The flow guide structures can furthermore be arranged so that they are in contact with both the gas diffusion electrode, and the ion exchange membrane. The electrolyte therefore would pass only through openings of the guide structures. The flow guide structures may be connected in any desired fashion, to the gas diffusion electrode and the ion exchange membrane, e.g. permanently or removeably. The flow guides can preferably be clamped between the gas diffusion electrode and the ion exchange membrane. In a particularly preferred embodiment, the flow guide structures are fastened to a support structure arranged substantially vertically in the gap, i.e. substantially parallel to the gas diffusion electrode and the ion exchange membrane. For example, the support structure can extend through the middle of the gap, so that the flow guide structures project in the direction of the ion exchange membrane, on the one hand, and in the direction of the gas diffusion electrode on the other hand. The support structure can be, for example, a thin plastic rod, the diameter of which is preferably less than the gap width between the gas diffusion electrode and the ion exchange membrane. Any number of support structures, for example, in the form of plastic rods, over the length of the gas diffusion electrode, and therefore the number of flow guide structures, can be employed. In preferred embodiments, the number of support structures can depend on the material thickness of the flow guide structures since the plastic rods can be used to provide stability, for example, when the electrolyser is being assembled. 
     The flow guide structures may be any shape and in one embodiment the guides can be flat. In order to facilitate clamping of the flow guide structures between the gas diffusion electrode and the ion exchange membrane, the flow guide structures may, for example, have any desired profile, e.g. a Z-, L-, T-, double T-shaped or trapezoidal profile. The flow guide structures may also be angled or curved in any desired way. Preferably they can also be formed of a resilient plate which is preferably wider than the width of the gap. When the guides are clamped between the gas diffusion electrode and the ion exchange membrane, due to the effect of the electrolyte flow in the gap, the guides may bend downwards. The flow guide structures can therefore preferably be downwardly curved. It is, however, also possible to use upwardly curved flow guide structures. Curved flow guide structures are advantageous since they compensate for manufacturing tolerances of the electrochemical cell, for example, those manufacturing tolerances affecting the width of the gap. 
     The opening in the flow guide structures may have any desired shape, for example, round or angular. Openings in flow guide structures can be arranged above one another or below one another, and/or may lie either above one another or below one another, e.g. the openings can overlap. The electrolyte flow can then travel substantially perpendicularly through the gap. The openings may, however, also be offset with respect to one another so that the electrolyte can flow through the gap in a zigzag or meandering fashion, for example, rather than in a straight line. Such an arrangement may, reduce the formation of dead zones. 
     The flow guide structures may be made of any desired material, e.g. an alkali-resistant material, especially an alkali-resistant metal or plastic. Nickel or PTFE, for example, may suitably be used as an advantageous material. 
     The number of flow guide structures, as well as the number and cross-sectional area of the openings, are preferably selected so that the flow velocity of the electrolyte is less than in freefall. With an overall electrolyser height of for example 1.3 m, and an electrolyte quantity of, for example, 180 l/h, 26 flow guide structures with 64 openings may, for example, be employed. The openings preferably have a diameter of 1 mm, for example. As an alternative, it is also possible to use 6 flow guide structures with 127 openings measuring 0.5 mm in diameter. Depending on the throughput, corresponding pressure compensation can be achieved by controlling the diameter and the number of the openings, as well as the number of flow guide structures. 
     The electrolyte flowing downward in the gap should preferably not stagnate at the flow guide structures. It is therefore often desirable to provide that the sum of the cross-sectional areas of all the openings of a flow guide structure is the same for all the flow guide structures. This can be done by any desired method such as by varying the number of openings or the cross-sectional area. 
     Regardless of whether the electrolyte is flowing through the gap with the aid of a pump or whether flow guide structures are provided, or whether both apply, a preferred flow rate of the electrolyte in the gap (with a gap width of 3 mm, for example), is from about 100 to about 300 l/h. The flow rate is particularly preferably at most about 500 l/h. The flow velocity is preferably at most about 1 cm/s. 
     An advantage of including flow guide structures verses using porous layers known from the prior art, is better discharge of the gas bubbles which enter the gap through the gas diffusion electrode. The electrolyte is furthermore pumped through the gap between the gas diffusion electrode and the ion exchange membrane, so that the gap can be completely filled with electrolyte. Porous structures, through which the electrolyte flows in freefall according to the prior art, are usually not completely filled with electrolyte, thus resulting in a higher electrolysis voltage. 
     An electrochemical cell according to the instant invention can be used for a variety of electrolysis processes involving a gas diffusion electrode. The gas diffusion electrode preferably functions as a cathode, particularly preferably an oxygen-consuming cathode. The gas fed to the electrochemical cell is preferably a gas containing oxygen, for example, air, oxygen-enriched air and/or pure oxygen. A cell according to the present invention is preferably used for the electrolysis of an aqueous solution of an alkali metal halide, particularly sodium chloride. 
     In the case of the electrolysis of an aqueous sodium chloride solution, the gas diffusion electrode can advantageously be constructed in the following way. For example, the gas diffusion electrode preferably includes at least an electrically conductive substrate and an electrochemically active coating. The electrically conductive substrate is preferably a gauze, fabric, lattice, mesh, nonwoven or foam made of metal, especially nickel, silver or silver-coated nickel. The electrochemically active coating includes a catalyst, for example, silver(I) oxide, and a binder, for example, polytetrafluoroethylene (PTFE). The electrochemically active coating may be made up of one or more layers. It is also possible to provide a gas diffusion layer, for example, a layer comprising a mixture of carbon and polytetrafluoroethylene, which can be applied to the substrate. 
     Titanium electrodes which are coated with ruthenium-iridium oxides or ruthenium oxide, for example, may suitably be used as the anode. 
     A commercially available membrane, for example Nafion NX2010 from DuPont, may be used as the ion exchange membrane. 
     An electrolysis cell according to the invention, which is suitable for the electrolysis of an aqueous sodium chloride solution, preferably has a gap with a width of the order of about 3 mm between the gas diffusion electrode and the ion exchange membrane. The flow guide structures are preferably made of thin PTFE or PVDF plates and have a thickness of from 0.1 to 0.5 mm. 
     The electrolyte feed is a channel, for example, a tube, which preferably extends over an entire length of the gas diffusion electrode. In this case, the electrolyte may be fed uniformly from above into the gap between the gas diffusion electrode and the ion exchange membrane, over the entire length thereof, via a channel-like electrolyte feed. Instead of an electrolyte feed which extends over the entire length of the gas diffusion electrode, the feed may take place in only one region, for example, in an upper region of one of the two ends of the gas diffusion electrode. A uniform distribution of the electrolyte, over the entire length of the gap, may be brought about in this case by employing flow guide structures, which can be inclined with respect to an axis perpendicular to the gas diffusion electrode or the ion exchange membrane. 
     The invention is also directed to a method for the electrolysis of an aqueous alkali metal halide solution in an electrochemical cell. The cell preferably includes at least an anode half-cell with an anode, a cathode half-cell with a cathode, and an ion exchange membrane arranged between the anode half-cell and the cathode half-cell. The anode and/or the cathode is preferably a gas diffusion electrode and a gap is arranged between the gas diffusion electrode and the ion exchange membrane. The half-cell with a gas diffusion electrode preferably has an electrolyte feed and an electrolyte discharge as well as a gas inlet and a gas outlet. The electrolyte preferably flows from the top downward in the gap by a pump, such that the gap is capable of being completely filled with electrolyte. 
     The invention will be explained in more detail below with reference to the appended drawings, in which: 
       FIG. 1  represents an illustrative electrochemical cell  1  according to the invention which is constructed from an anode half-cell  2 , with an anode  21 , and a cathode half-cell  3  having a gas diffusion electrode  31  as the cathode. The two half-cells  2 ,  3  are typically separated from each other by an ion exchange membrane  4 . The gas diffusion electrode  31  is preferably separated from the ion exchange membrane  4  by a gap  32 . Seals  39  close off the half-cell  3  from the outside. The cathode half-cell  3  has an electrolyte feed  33  and an electrolyte discharge  34 , as well as a gas inlet  35  and a gas outlet  36 . The electrolyte feed  33  is preferably hermetically connected to the gap  32 . The electrolyte is fed into the half-cell  3  through the electrolyte feed  33 , and flows downwards in the gap  32  before it is discharged from the half-cell  3  through the electrolyte discharge  34 . The gap  32  is preferably completely filled with electrolyte during operation of the electrolysis cell  1 . Gas is fed to the gas space  37  of the half-cell  3  through the gas inlet  35 , then flows upwards in the gas space  37  and is discharged from the half-cell  3  through the gas outlet  36 . The hermetic connection of the electrolyte feed  33  to the gap  32  makes it possible to deliver the electrolyte through the gap  32  with the aid of a pump, and to set up a desired flow rate (and/or a desired flow velocity) of the electrolyte in the gap  32 . The hermetic connection should generally minimize or even prevent the gas from flowing out of the gas space  37  into the gap  32 . The electrolyte feed  33  can generally be completely filled in order to achieve this. In such a case, the compensation opening  38  should typically be dimensioned so that a very small flow rate of the electrolyte runs off into the gas space  37  through the opening  38 . The flow rate through the opening  38  into the back space is preferably less than about 5% of the total flow rate. At the same time, the compensation opening  38  lets out the gas when, during operation of the electrolysis cell  1 , small quantities of gas from the gas space  37  enter the gap  32  through the gas diffusion electrode  31  and rise upwards in the form of gas bubbles. In this way, the gas can travel out of the gap  32 , through the compensation opening  38  in the electrolyte feed  33  and into the gas space  37 . 
     In comparison with the embodiment represented in  FIG. 1 , and in addition to the hermetic connection of the electrolyte feed  33  to the gap  32 , the electrolysis cell  1  in  FIG. 2  has flow guide structures  51 ,  52 ,  53 ,  54  in the gap  32 . The flow guide structures  51 ,  52 ,  53 ,  54  reduce the flow velocity of the electrolyte in the gap  32  compared with the flow velocity which the electrolyte would have in freefall. The flow guide structures  51 ,  52 ,  53 ,  54  include thin plates with openings  56 , which allow the electrolyte to pass through. In the embodiment of  FIG. 2  the flow guides  51 ,  52 ,  53 ,  54  are clamped between the ion exchange membrane  4  and the gas diffusion electrode  31 . The flow guide structures  51  can be arranged substantially horizontally in the gap  32 , i.e. transversely to the flow direction of the electrolyte. The flow guide structures  53  may also be arranged obliquely, i.e. at an angle to the flow direction, for example, inclined in the direction of the ion exchange membrane  4 . In another embodiment, the flow guide structures  53  can be designed with a V-shape. The flow guide structures  54  can also be downwardly curved. 
     Additional advantages, features and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices, shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents. 
     All documents referred to herein are specifically incorporated herein by reference in their entireties. 
     As used herein and in the following claims, articles such as “the”, “a” and “an” can connote the singular or plural.