SOEC System with Heating Ability

Abstract

A Solid Oxide Electrolysis System has electrolytes with increased Area Specific Resistance, ASR yet is thin as compared to known electrolytes in the field, to obtain heating of the endothermic reducing process performed in the electrolysis cells directly where it is needed without any extra heating appliances or integrated heating elements, a simple efficient solution which does not increase the volume of the stack.

Claims

1. A solid oxide electrolysis system comprising a planar solid oxide electrolysis cell stack comprising a plurality of solid oxide electrolysis cells, each cell comprising layers of an oxidizing electrode, a reducing electrode and an electrolyte, comprising of a first electrolyte layer, a second electrolyte layer, and a layer formed by interdiffusion of the first electrolyte layer and the second electrolyte layer, wherein the area-specific resistance of the electrolyte, measured at 700 C., is higher than 0.2 cm.sup.2 and the total thickness of the electrolyte is less than 25 m.

2. A solid oxide electrolysis system according to claim 1, wherein the total thickness of the electrolyte is between 5 m and 25 m.

3. A solid oxide electrolysis system according to claim 1, wherein the first electrolyte layer is composed primarily of stabilized zirconia, the second electrolyte layer is composed primarily of doped ceria, and a third layer between the above layers is formed by interdiffusion (interdiffusion layer).

4. A solid oxide electrolysis system according to claim 3, wherein the first electrolyte material is primarily (Y.sub.2O.sub.3).sub.x(ZrO.sub.2).sub.1-x, where 0.02x0.10 or (Y.sub.2O.sub.3).sub.y(L.sub.2O.sub.3).sub.z(ZrO.sub.2).sub.1-y-z or (Sc.sub.2O.sub.3).sub.y(L.sub.2O.sub.3).sub.z(ZrO.sub.2).sub.1-y-z, where 0.0y0.12, 0z0.06, and L is Ce, Gd, Ga, Y, Al, Yb, Bi, or Mn.

5. A solid oxide electrolysis systems according to claim 3, wherein the second electrolyte materials is primarily (Ln.sub.2O.sub.3).sub.x(CeO.sub.2).sub.1-x, where 0.02x0.30, and Ln is a lanthanide or mixture of two lanthanides.

6. A solid oxide electrolysis system according to claim 1, wherein the thickness of the interdiffusion layer is at least 300 nm.

7. A solid oxide electrolysis system according to claim 1, wherein at least 65% of the area-specific resistance of the electrolyte originates from the interdiffusion layer.

8. A solid oxide electrolysis system according to claim 4, wherein the interdiffusion layer is obtained by sintering the electrolyte layers at temperatures above 1250 C.

9. A solid oxide electrolysis system according to claim 1, wherein the in-plane electrical conductivity of the oxidizing electrode, measured at 700 C. in air, at is higher than 30 S/cm.

10. A solid oxide electrolysis system according to claim 1, wherein the oxidizing electrode comprises two or more layers.

11. A solid oxide electrolysis system according to claim 10, wherein the oxidizing electrode layer closest to the electrolyte is a composite of doped ceria and Ln.sub.1-x-aSr.sub.xMO.sub.3, where Ln is a lanthanide or mixture thereof, M is Mn, Co, Fe, Cr, Ni, Ti, Cu or mixture thereof, 0x0.95, 0a0.05, and 00.25, and the oxidizing electrode layer farthest from the electrolyte is primarily Ln.sub.1-x-aSr.sub.xMO.sub.3, Ln.sub.1-aNi.sub.1-yCo.sub.yO.sub.3, or Ln.sub.1-aNi.sub.1-yFe.sub.yO.sub.3, where 0y1, or mixtures thereof.

12. A solid oxide electrolysis system according to claim 1 wherein the operating temperature is in the range of 650 C.-900 C.

13. A solid oxide electrolysis system according to claim 1 where the reaction occurring in the reducing electrode comprises the electrochemical reduction of CO.sub.2 to CO.

Description

EXAMPLE 1 (COMPARATIVE EXAMPLE)

[0039] The example shows the performance of a planar solid oxide electrolysis cell stack, comprising 75 cells and 76 metallic interconnect plates. The cells comprised an LSCF/CGO based first oxidizing electrode, an LSM-based second oxidizing electrode, a Ni/YSZ reducing electrode, a Ni/YSZ support and an electrolyte, comprising of 8YSZ first electrolyte layer, a CGO second electrolyte layer, and a layer formed by interdiffusion of the first electrolyte layer and the second electrolyte layer. The thickness of the 8YSZ electrolyte layer was approximately 10 microns, and the thickness of the CGO electrolyte layer was approximately 4 microns. The sintering temperature of the bi-layer electrolyte was 1250 C., which, based on scanning electron microscopy investigations, results in an interdiffusion layer that is approximately 300 nm in thickness. The cells were 12 cm by 12 cm in size. The interconnect plates were made of Crofer22 stainless steel.

[0040] The cells used in the stack were tested in a single-cell test setup in fuel cell mode in a furnace with air fed to the cathode and humidified H.sub.2 to the anode. The total ASR of such cells at a constant current density of 0.3125 A/cm.sup.2 was estimated to be 0.372 cm.sup.2 at 750 C. and 0.438 cm.sup.2 at 720 C.

[0041] The stack described above was tested in CO.sub.2 electrolysis mode with air fed to the air-side of the cells and a 5% H.sub.2 in CO.sub.2 mixture fed to the fuel-side of the cells. The stack was operated in a furnace held at a constant temperature of 750 C. in co-flow mode. The electrolysis current was varied from 0 to 85 A. The resulting temperature profiles were recorded using internal thermocouples placed along the flow direction from the inlet of the stack (0 cm) to the outlet of the stack (12 cm). Stack internal temperature profiles corresponding to electrolysis current values of 50 A and 85 A are shown in FIG. 1. Inlet, outlet, maximum, and minimum temperatures, as well as relevant temperature differences, are summarized in FIG. 2.

EXAMPLE 2

[0042] The example shows the performance of another planar solid oxide electrolysis cell stack, similarly comprising 75 cells and 76 metallic interconnect plates. The cells were otherwise identical to cells in Example 1, except that the sintering temperature of the bi-layer electrolyte was 1300 C., which, based on scanning electron microscopy investigations, results in an interdiffusion layer that is approximately 360 nm in thickness. The interconnect plates were identical to these in Example 1.

[0043] The cells used in the stack were tested in a single-cell test setup in fuel cell mode in a furnace with air fed to the cathode and humidified H.sub.2 to the anode. The total ASR of such cells at a constant current density of 0.3125 A/cm.sup.2 was estimated to be 0.446 cm.sup.2 at 750 C. and 0.515 cm.sup.2 at 720 C.

[0044] The stack was tested under identical conditions to Example 1. The resulting temperature profiles were recorded using internal thermocouples placed along the flow direction from the inlet of the stack (0 cm) to the outlet of the stack (12 cm). Stack internal temperature profiles corresponding to electrolysis current values of 50 A and 85 A are shown in FIG. 1. Inlet, outlet, maximum, and minimum temperatures, as well as relevant temperature differences, are summarized in FIG. 2.

[0045] The inlet-to-outlet temperature difference, as well as the maximum-to-minimum temperature difference is lower in Example 2 than in Example 1 at both 50 A as well as at 85 A. This improvement is due to the higher electrolyte ASR, and thus higher heating ability of the cells used in Example 2 compared to Example 1.