High performance reversible electrochemical cell for H2O electrolysis or conversion of CO2 and H2O to fuel

Abstract

The present invention relates to a reversible electrochemical cell, such as an electrolysis cell for water splitting or for conversion of carbon dioxide and water into fuel. The present invention relates also to an electrochemical cell that when operated in reverse performs as a fuel cell. The electrochemical cell comprises gas5 diffusion electrodes and a porous layer made of materials and having a structure adapted to allow for a temperature range of operation between 100-374 C. and in a pressure range between 3-200 bars.

Claims

1. An electrochemical cell comprising: at least two porous gas diffusion electrodes, wherein at least one of said at least two porous gas diffusion electrodes comprises doped strontium titanate; a ceramic porous layer, wherein said porous layer is located in between said at least two porous gas diffusion electrodes and wherein said ceramic porous layer comprises an aqueous electrolyte in said ceramic porous layer, wherein said ceramic porous layer has pore diameters between 2 and 50 nm; and wherein said ceramic porous layer has a perovskite structure having the formula of AB.sub.YB.sub.1yO.sub.3 wherein: A is Ca, Sr, or Ba, or a combination thereof; B is Ti, Zr, Hf, or Ce, or a combination thereof; and B is Y, Sc, or Ga, or a combination thereof; and 0y1; or wherein said ceramic porous layer is an alkaline earth metal titanate; or wherein said ceramic porous layer is a metal carbide; or wherein said porous layer is a metal nitride; whereby said electrochemical cell withstands a temperature range between 100-374 C. and a pressure range between 3-200 bars, allowing for said electrochemical cell to operate within that temperature and pressure range.

2. The electrochemical cell according to claim 1, wherein said porous layer has pore diameters that are greater than 50 nm.

3. The electrochemical cell according to claim 1, wherein said aqueous electrolyte is an aqueous solution of MOH, M.sub.2CO.sub.3, MHCO.sub.3, M.sub.2SO.sub.4, MX.sub.2PO.sub.4, M.sub.2XPO.sub.4, MXO.sub.4, MXO.sub.3, or MX, wherein M is H, Li, Na, K, Rb or Cs, and X is Cl, Br or I.

4. The electrochemical cell according to claim 1, wherein said at least one of said at least two porous gas diffusion electrodes comprises Nickel, a Nickel alloy or NiCo.

5. The electrochemical cell according to claim 1, wherein said at least one of said at least two porous gas diffusion electrodes comprises a ceramic material.

6. The electrochemical cell according to claim 1, wherein said at least one of said at least two porous gas diffusion electrodes comprises perovskites with the formula ABO.sub.3, wherein A is a combination of an alkali metal, an alkaline earth element, a lanthanide or Y, and B is a combination of a transition metal, lanthanide, Ga or Mg.

7. The electrochemical cell according to claim 1, wherein said at least one of said at least two porous gas diffusion electrodes comprises spinels with the formula A.sub.3O.sub.4, wherein A is a combination of transition metals, Mg, Ga or Al.

8. The electrochemical cell according to claim 1, wherein said at least one of said at least two porous gas diffusion electrodes comprises a transition metal or Zr, Hf, Ce, Y, or Sc or alkaline earth or lanthanide or Si, Ga, or Al.

9. The electrochemical cell according to claim 1, wherein at least one of said at least two porous gas diffusion electrodes is loaded with a catalyst.

10. A method of using an electrochemical cell according to claim 1 comprising: providing the electrochemical cell according to claim 1 and producing hydrogen and oxygen from water with said electrochemical cell.

11. A method of using an electrochemical cell according to claim 1 comprising: providing the electrochemical cell according to claim 1 and producing a synthetic fuel from carbon dioxide and water with said electrochemical cell.

12. The electrochemical cell according to claim 1, wherein at least one of said at least two porous gas diffusion electrodes comprises a metal foam.

13. A method of producing an electrochemical cell according to claim 1, said method comprising: assembling a porous layer for supporting a liquid electrolyte located in between at least two electrodes; pressing said assembly; sintering said assembly in a reducing atmosphere; and loading an electrocatalyst.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) The electrochemical cell according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

(2) FIG. 1 shows a schematic drawing of the production of fuels by means of co-electrolysis.

(3) FIG. 2a shows a schematic drawing of an embodiment of the invention, i.e. the co-electrolysis cell.

(4) FIG. 2b shows a schematic drawing of an embodiment of the invention, i.e. the electrochemical cell for water electrolysis.

(5) FIG. 3 shows the structure of an electrochemical cell 20 according to the invention.

(6) FIG. 4 shows a Cyclic sweep voltammogram for an electrochemical cell according to one embodiment of the invention.

(7) FIG. 4a shows a Cyclic sweep voltammogram for an electrochemical cell according to another embodiment of the invention.

(8) FIG. 5 shows an electrochemical cell stack structure according to one embodiment of the invention.

(9) FIG. 6 is a flow-chart of a method according to the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

(10) FIG. 1 shows the idea behind the use of co-electrolysis for production of fuels. From a source of CO.sub.2 such as a CO.sub.2 absorber 1, CO.sub.2 present in the atmosphere, e.g. released 2 by internal combustion engine in the atmosphere 3, may be collected so as to be fed 4 into a co-electrolysis cell 5. Water is also fed 7 into a co-electrolysis cell 5 and upon application of electrical current 6, e.g. from renewable sources of energy 8, fuel 9, such as methanol can be produced. Through the use of co-electrolysis, as shown in FIG. 1, CO.sub.2 and H.sub.2O can be converted into synthesis gas, such as CO and H.sub.2 and O.sub.2, or directly to CH.sub.3OH or other hydrocarbons.

(11) FIG. 2a shows the electrochemical cell when used for co-electrolysis.

(12) The overall reaction taking place in the co-electrolysis cell is:

(13) CO.sub.2(g)+4H.sub.2O(g)+electricity <-> hydrocarbons(g)+O.sub.2(g)

(14) e.g. 2CO.sub.2(g)+4H.sub.2O(g)+electricity <->2CH.sub.3OH(g)+3O.sub.2(g)

(15) This process produces hydrocarbons, such as methanol, CH.sub.3OH from CO.sub.2 and water. As shown in FIG. 2a, water, H.sub.2O and Carbon dioxide, CO.sub.2 enter the cell 14 at the cathode, i.e. GDE 15 and are reduced to form methanol Oxygen gas and the co-produced water leave the cell at the anode, i.e. GDE 17 while electrons move through the external circuit.

(16) Other examples of producible hydrocarbons include methane, ethane, propane, ethylene, propene, ethanol, propanol, dimethylether and formaldehyde.

(17) The electrochemical cell of the invention may also produce hydrogen and oxygen from water.

(18) FIG. 2b shows the electrochemical cell when used for water electrolysis. This process splits water into hydrogen and oxygen gas. As shown in FIG. 2b, water, H.sub.2O, enters the cell 10 at the cathode, GDE 13 and is split to form protons, H.sup.+, and hydroxide anions, OH.sup.. Protons are reduced to form hydrogen gas while the hydroxides move through the porous layer and are oxidized to oxygen gas. The oxygen gas and the co-produced water leave the cell at the anode, i.e. GDE 12 while electrons move through the external circuit.

(19) The key reaction occurs at the surface 11 of the porous layer.

(20) The overall reaction involved in the water electrolysis is:

(21) 2H.sub.2O(g)+electricity <->2H.sub.2(g)+O.sub.2(g)

(22) FIG. 3 shows the structure of an electrochemical cell 20 according to the invention.

(23) A porous layer 19, such as ST, is sandwiched in between two GDEs 18, such as STN. The structure of the GDEs 18 shows porosity so as to allow gas diffusion through the GDEs.

(24) FIG. 4 shows a cyclic sweep voltammogram for an electrochemical cell according to one embodiment of the invention having a Ni-foam/ST/Ni-foam structure. The voltammogram shows the performance of two electrochemical cells, A and B.

(25) The dashed line shows the performance of cell A, composed of an Inconel foam cathode, a nickel foam anode and, mesoporous SrTiO.sub.3 with 45 wt % KOH (aq) immobilized electrolyte at 39 bars and 237 C. The full line shows the performance of cell B, composed of an Inconel foam cathode, a nickel foam anode with Ag deposited electro-catalyst and, mesoporous SrTiO.sub.3 with 45 wt % KOH (aq) immobilized electrolyte at 241 C. and 37.7 bars. In both cells A and B, the porous layer is ST and the electrolyte used is aqueous KOH; despite the high temperature, the liquid KOH is immobilized in the porous layer.

(26) Both cyclic voltammograms have been recorded with a sweep rate 5 mV/s. The only difference between the two cells is the presence of silver as electrocatalyst.

(27) It can be clearly seen from the cyclic voltammetry that the presence of an electrocatalyst such as silver, increases the performance of the cell.

(28) FIG. 4a shows cyclic voltametry analysis of an electrochemical cell produced according to the invention. Cu foam was used as cathode and nickel foam was used as anode. FIG. 4A shows the cell performing electrolysis of H.sub.2O(g) at 250 C. and 40 bar (full line), and co-electrolysis of H.sub.2O(g) and CO.sub.2 at 150 C. and 20 bar (dashed line).

(29) An electrochemical cell was produced according to the procedure previously described herein. The anode was Ni-foam and the cathode was Cu-foam, SrTiO.sub.3 or YSZ (yttria stabilized zirconia) was used to obtain the porous structure to immobilize the aqueous electrolyte. The cell has been tested by cyclic voltametry at 250 C. and 40 bar with 45 wt % KOH as electrolyte for steam electrolysis and at 150 C. and 20 bar for co-electrolysis of steam and CO.sub.2. It was demonstrated that the proposed electrochemical cell operates with a copper foam based gas diffusion electrode (cathode) and a nickel foam based gas diffusion electrode (anode) for both steam and co-electrolysis. Current densities of ca. 500 mA/cm.sup.2 and ca. 200 mA/cm.sup.2 were measured for steam electrolysis at 250 C./40 bar and co-electrolysis of steam and CO.sub.2 at 150 C./20 bar, respectively, as shown in FIG. 4a.

(30) This test shows that this type of cell can perform both electrolysis of steam and co-electrolysis of steam and CO.sub.2.

(31) FIG. 5 shows an electrochemical cell stack structure according to one embodiment of the invention.

(32) The stack is formed by a repetitive structure where electrochemical cells 32, comprising a layer of ST 25 sandwiched in between a layer of metal foam, such as Ni-foam 24 and a layer of Inconel foam 26, are stacked on top of each other and separated by a layer structure 33 comprising a Ni-foil 22 sandwiched in between two layers of coarse Ni-foam 23.

(33) Thus, in general the electrochemical cells are stacked having a thin foil of dense metal in between at least two sheets of coarse metal-foam.

(34) FIG. 6 is a flow-chart of a method of producing an electrochemical cell according to one aspect of the invention.

(35) In step (S1) 28, a porous layer for supporting liquid electrolyte is assembled in between at least two electrodes by, e.g. multilayer tape casting.

(36) In step (S2) 29, the assembly is pressed together.

(37) In step (S3) 30, the cell is sintered in reducing atmosphere, e.g. at T>1000 C. in hydrogen gas such as 9% H.sub.2.

(38) In step (S4) 31, the electrocatalyst is loaded into the cell.

(39) In some embodiments according to a third aspect of the invention, step (S4) and step (S3) are inverted so that the step of loading an electrocatalyst is performed before sintering in reducing atmosphere the assembly.

(40) The electrochemical cell of the invention may be reversely operated as a fuel cell. In this case hydrogen gas is fed to the anode and oxygen gas is fed to the cathode.

(41) Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms comprising or comprises do not exclude other possible elements or steps. Also, the mentioning of references such as a or an etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.