Flow-type electrochemical cell

Abstract

Flow type electrochemical cells are disclosed. The electrochemical cell has an anode half-cell, a cathode half-cell, and permeable separating layer. The half-cells are bounded by side elements. Respective porous electrodes are housed in the half-cells. The permeable separating layer is disposed between the anode half-cell and the cathode half-cell. An electrolyte region connected to an electrolyte feed and an electrolyte outflow region connected to an electrolyte drain are further provided. An electrolyte inflow region and an electrolyte outflow region are disposed on opposite sides of the porous electrodes such that inflowing electrolyte flows through the porous electrode perpendicularly to the permeable separating layer.

Claims

1. A flow type electrochemical cell, comprising: an anode half-cell and a cathode half-cell bounded by side elements, each half-cell further comprising porous electrodes; a permeable separating layer disposed between the anode half-cell and the cathode half-cell; an electrolyte inflow region connected to an electrolyte feed; and an electrolyte outflow region connected to an electrolyte drain; wherein: the electrolyte inflow region and the electrolyte outflow region are disposed on opposite sides of the porous electrode; inflowing electrolyte flows through the porous electrode perpendicularly to the permeable separating layer; and at least one of the electrolyte inflow region and the electrolyte outflow region further comprises a wide-mesh support structure, wherein the wide-mesh support structure presents a lower flow resistance than the porous electrode.

2. The electrochemical cell of claim 1, wherein: the electrolyte inflow region is disposed between the permeable separating layer and the porous electrode; and the electrolyte outflow region is disposed between the porous electrode and the side elements, or vice versa.

3. The electrochemical cell of claim 1, wherein: the electrolyte inflow region is disposed between the porous electrode and the side elements; and the electrolyte outflow region is disposed between the permeable separating layer and the porous electrode.

4. The electrochemical cell of claim 1, wherein at least one of the electrolyte inflow region and the electrolyte outflow region is integrated into at least one of the porous electrodes and the side elements by means of one or more flow channels.

5. The electrochemical cell of claim 1, wherein the wide-mesh support structure is a woven fabric or a knitted fabric.

6. The electrochemical cell of claim 1, wherein the wide-mesh support structure is made of an electrically conducting material or of a material with electrically conductive coating.

7. The electrochemical cell of claim 1, wherein the wide-mesh support structure represents a carbon support structure.

8. The electrochemical cell of claim 1, wherein the porous electrode comprises a nonwoven carbon web, foams, or metal foams.

9. A cell stack of a flow type electrolytic cell as claimed in claim 1.

10. A method for operating a flow type electrochemical cell, wherein flow of an electrolyte is caused to pass through the porous electrode perpendicularly to the permeable separating layer; wherein: the electrolyte is supplied via an electrolyte inflow region connected to an electrolyte feed; the electrolyte is guided from the cell via an electrolyte outflow region which is disposed on the opposite side of the porous electrode from the electrolyte inflow region; and at least one of the electrolyte inflow region and the electrolyte outflow region further comprises a wide-mesh support structure, wherein the wide-mesh support structure presents a lower flow resistance than the porous electrode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The prior art and the present invention are represented in more detail with reference to a variety of figures:

(2) FIG. 1: Schematic representation of a redox flow storage medium from the prior art.

(3) FIG. 2: Schematic construction of a redox flow cell from the prior art.

(4) FIG. 3: Schematic construction of an electrochemical cell of the invention, in which flow is caused to pass through the porous electrodes perpendicularly to the permeable separating layer.

(5) FIG. 4: Schematic construction of a further electrochemical cell of the invention, in which flow is caused to pass through the porous electrodes perpendicularly to the permeable separating layer.

(6) FIGS. 5a, 5b, 5c: Different arrangements of the constituents of an electrochemical cell of the invention.

(7) FIGS. 6a, 6b, 6c: Three-dimensional representation of an electrochemical cell of the invention, comprising the different arrangements of the constituents as shown in FIGS. 5a, b, and c.

DETAILED DESCRIPTION OF THE INVENTION

(8) The invention provides an electrochemical cell of the flow type, comprising

(9) (a) an anode half-cell and a cathode half-cell, which are bounded by side elements, and which the respective porous electrodes are comprised in the half-cells, and also

(10) (b) a permeable separating layer which is disposed between the anode half-cell and the cathode half-cell,

(11) wherein

(12) (i) an electrolyte inflow region connected to an electrolyte feed, and an electrolyte outflow region connected to an electrolyte drain, are provided, where

(13) (ii) electrolyte inflow region and electrolyte outflow region are disposed on opposite sides of the porous electrode, and so

(14) (iii) inflowing electrolyte flows through the porous electrode perpendicularly to the permeable separating layer.

(15) Surprisingly it has been found that flow through the electrode in the horizontal direction, i.e., in the z-direction relative to FIG. 3, makes the pressure drop smaller by a multiple, allowing the cell to be designed with greater dimensions.

(16) As a result of the new, advantageous construction of the cell, moreover, the electrolyte does not have a sharply different state of charge over the height and width of the cell. As a consequence of this, the likelihood of secondary reactions is the same over the entire area of the cell, and it is therefore possible to achieve the maximum change in the SOC of the electrolyte per unit residence time in the cell, and also to operate at significantly higher current densities; as a result, a lower volume flow rate is needed, and hence less pumping power, and accordingly a higher system efficiency can be achieved.

(17) Either liquids or gases, or else both, may constitute the flow through the flow-type electrochemical cell of the invention. Solvents used here are typically organic or inorganic acids, with preference being given to the use of aqueous sulfuric acid. Possible redox couples used are titanium, iron, chromium, vanadium, cerium, zinc, bromine, and sulfur. It is also possible, however, to use the cell of the invention as a zinc-air energy storage medium, meaning that the flow through the cell is a flow of a zinc slurry and of air or oxygen. Other such applications are conceivable as well where a salt in solution in a liquid is electrochemically reacted in an electrochemical cell, where the formation of a gas does not constitute the primary reaction.

(18) The electrochemical cell of the invention may constitute an electrolysis cell in single-cell construction, of the type referred to as single cell elements, as disclosed in DE 196 41125 A1 (Uhdenora), for example, or else a construction of the filter press type, as described by way of example in EP 0095039 A1 (Uhde). The side elements are therefore monopolar elements in the case of the single-cell construction, and bipolar elements in the case of the electrochemical cells of the filter press type. The respective side elements used here are configured preferably as plates, and more preferably as bipolar plates.

(19) The permeable separating layer is selected from the group encompassing permeable membranes, selectively permeable membranes, semi-permeable membranes, diaphragms, ultrafiltration membranes, and ceramic separators.

(20) In an advantageous embodiment, the electrolyte inflow region is disposed between the permeable separating layer and the porous electrode, and the electrolyte outflow region is disposed between the permeable separating layer and the side elements, or vice versa.

(21) In a further advantageous refinement of the invention, the electrolyte inflow region and/or the electrolyte outflow region are integrated into the porous electrodes and/or into the side elements by means of one or more flow channels. These flow channels may be arranged parallel to one another in the porous electrode or the side elements, or may intersect. Any arrangement of flow channels is conceivable.

(22) In a further version of the invention, in the electrolyte inflow region and/or in the electrolyte outflow region there is a wide-mesh support structure provided. This wide-mesh support structure is preferably a woven fabric or a knitted fabric or another component which ensures a defined distance between permeable separating layer and electrode and which presents a low flow resistance. In this case, in the electrolyte inflow region and in the electrolyte outflow region, the same type of design of wide-mesh support structure or a different wide-mesh support structure is used. This wide-mesh support structure is also referred to as a percolator.

(23) The wide-mesh support structure here is made of an electrically conducting material or of a material with conductive coating, and is preferably a carbon support structure. Other materials may also be used, however. The wide-mesh support structure here has a lower flow resistance than the porous electrode and is stable with respect to the electrolyte.

(24) It is important here that the material is sufficiently connected electrically to the porous electrode and also has effective electrical connection to the side elements. This woven fabric can be omitted if the side elements and/or the porous electrode are/is provided with corresponding flow channels which ensure unhindered flow-off of the electrolyte and which produce a sufficient electrical connection to the electrode.

(25) At this support structure there may likewise be redox reactions, preferably but not necessarily.

(26) The porous electrode is advantageously a nonwoven carbon web, a foam, or a metal foam. Other materials may also be used.

(27) The construction may be expanded with further layers, these layers leading either to a more uniform electrolyte distribution or to an improved cell power, i.e., to a higher current density, a higher efficiency, or a better or more uniform current distribution or the like, or the construction may display other advantages. It is also possible for the cathode and anode half-cells of an individual cell to differ in construction, or for the construction of the two half-cells to be symmetrical.

(28) The present invention further relates to cell stacks of an electrochemical cell of the flow type as described at the outset.

(29) Lastly, the present invention also embraces a method for operating an electrochemical cell of the flow type, wherein electrolyte is caused to flow through a porous electrode perpendicularly to the permeable separating layer.

(30) The method is advantageously realized such that (i) electrolyte is supplied via an electrolyte inflow region connected to an electrolyte feed, (ii) flow is caused to pass through the porous electrode perpendicularly to the permeable separating layer, and (iii) the electrolyte is guided from the cell via an electrolyte outflow region which is disposed on the opposite side of the porous electrode from the electrolyte inflow region.

(31) FIGS. 3 and 4 show electrochemical cells 9 of the invention. In these cells, electrolyte 8a, 8b flows via an electrolyte feed 13a, 13b into an electrolyte inflow region 10 which is disposed between permeable separating layer 4 and porous electrode 3a. Located in the electrolyte inflow region 10 is a wide-mesh support structure 11, also referred to as a percolator. The electrolyte inflow region 10 has a closed end 12 at the end of the electrochemical cell opposite the electrolyte feed. As a result, inflowing electrolyte 8a, 8b is forced perpendicularly to the permeable separating layer 4, i.e., in the z-direction, through the porous electrode 3a, 3b. With inflowing electrolyte 8a, 8b into the electrolyte inflow region 10 filled with a support structure 11, this region 10 initially fills uniformly with electrolyte 8a, 8b. Thereafter the electrolyte 8a, 8b flows uniformly through the porous electrode 3a, 3b, which presents a greater flow resistance than the support structure 11. From there, the electrolyte 8a, 8b flows into an electrolyte outflow region 14, in which there is provided a further wide-mesh support structure 15, which consists of the same material as or of a different material from the support structure 11 in the electrolyte inflow region 10. The electrolyte 8a, 8b subsequently departs the electrochemical cell 9 through an electrolyte drain 16.

(32) FIG. 4 differs from FIG. 3 only in that the electrolyte outflow region 14 is integrated into the side element 7 via flow channels 17. There is then no need for the support structure.

(33) FIG. 5 shows different arrangements of the constituents in an electrochemical cell through which flow passes perpendicularly to the permeable separating layer. FIG. 5a shows the porous electrode 6a, 6b with integrated flow channels 17 for the inflow region 10 and for the outflow region 14. In FIG. 5b, the electrolyte inflow region 10 is realized via a wide-mesh support structure 11, and the flow channels 17 are incorporated in the porous electrode 6a, 6b. In FIG. 5c, the electrolyte inflow region 10 and the electrolyte outflow region 14 are represented by way of flow channels 17. In this case the flow channels 17 of the electrolyte inflow region 10 are located in the porous electrode 6a, 6b, and the flow channels of the electrolyte outflow region 14 are located in the side elements 7. The form and arrangement of the flow channels may be selected arbitrarily here.

(34) In FIGS. 6a, 6b, and 6c, the arrangements of constituents of an electrochemical cell as shown in FIGS. 5a, 5b, and 5c are represented in a three-dimensional view. The upper part 18 of the diagram shows the view of that side of the electrochemical cell from which electrolyte is taken out via an electrolyte drain 16, and the lower region of the diagram 19 shows the view of that side of the electrochemical cell from which electrolyte flows into the cell via an electrolyte feed 13.

(35) The present invention is described in more detail below by means of a working example.

EXAMPLE

(36) The pressure drop in a cell according to the prior art with an active area of 1 m.sup.2 and with dimensions of 1 m1 m can be calculated as follows:

(37) For discharge of the electrolyte by 20% per unit residence time with an assumed power density of 500 W/m.sup.2, it can be assumed that an electrolyte volume flow rate of just under 39 L/h is required. If the electrode used is a nonwoven web having a thickness of 6 mm and a permeability of 1.6 E.sup.10 m.sup.2, and if this web, as is usual, is compressed by 25% (permeability of the compressed web 4.0 E.sup.11), it is possible from these figures to calculate the resultant pressure drop within the cell by the following formula:
Pressure drop=Volume flow rate*Viscosity*Length/(Permeability*Cross-sectional area)

(38) With an average electrolyte viscosity of 1.0 E.sup.2 Pas, therefore, a pressure drop of around 0.6 MPa can be ascertained.

(39) By the technique proposed in accordance with the invention, with flow through the nonwoven web in the Z-direction, under otherwise identical conditions and with the pressure drops in the inflow and outflow regions 10 and 14 disregarded, the pressure drop would reduce significantly to around 1.2*10.sup.5 MPa. This corresponds to a ratio of approximately 50 000:1.

(40) Advantages resulting from the present invention:

(41) As a result of this construction, it is possible not only to reduce the pressure drop within the cell by a multiple but also to prevent the electrolyte being present with a sharply different state of charge over the height and width of the cell. A consequence of this is that the likelihood of secondary reactions is the same over the entire area of the cell, and therefore it is possible to achieve the maximum change in the SOC of the electrolyte per unit residence time in the cell and it is also possible to operate at significantly higher current densities; as a result, a lower volume flow rate is necessary, hence less pumping power, and therefore a higher system efficiency can be achieved. Furthermore, the individual components such as permeable separating layer, electrode, and side element see the same state of charge over the height and width, and this has positive consequences for cell performance and component durability.

(42) This provides the possibility for electrochemical cells with electrodes consisting of a nonwoven web or the like to be produced and to be operated economically with a greater geometrical dimension than in the state of the art to date.

LIST OF REFERENCE SYMBOLS

(43) 1 Tanks 2 Electrochemical cell 3a, 3b Porous electrodes/nonwoven web 4 Permeable separating layer 5 Network connection 6a, 6b Nonwoven web/porous electrode 7 Side element 8a Anolyte 8b Catholyte 9 Electrochemical cell of the invention 10 Electrolyte inflow region 11 Wide-mesh support structure/percolator 12 Closed end of the electrolyte inflow region 13a, 13b Electrolyte feed 14 Electrolyte outflow region 15 Further support structure (percolator) 16 Electrolyte drain 17 Flow channels 18 Upper region of FIG. 6 19 Lower region of FIG. 6