WO2002031901A2

WO2002031901A2 - Segmented electrode tubular solid oxide fuel cell, and method of manufacture

Info

Publication number: WO2002031901A2
Application number: PCT/US2001/042721
Authority: WO
Inventors: Caine Finnerty; Geoffrey Tompsett; Basil Fenton; Yanhai Du
Original assignee: Acumentrics Corporation
Priority date: 2000-10-12
Filing date: 2001-10-12
Publication date: 2002-04-18
Also published as: WO2002031901A3; AU2002211918A1

Abstract

A tubular solid oxide fuel cell system is disclosed. The fuel cell system comprises, in one embodiment according to the invention: a tubular electrolyte layer (250); a plurality of separate anode segments(250, 252) mounted on a first surface of the tubular electrolyte layer; and a plurality of separate cathode segments(253,254) mounted on a second surface of the tubular electrolyte layer, opposite the first surfface, in corresponding positions to positions occupied by the separate anode segments on the first surface; wherein corresponding anode and cathode segments form a plurality of fuel cell sections (231,232) along the length of the tubular electrolyte layer. Related methods of manufacturing and operating tubular solid oxide fuel cell systems, and of reforming hydrocarbon fuels, are also disclosed.

Description

Attorney Docket: 2354/134WO

SEGMENTED ELECTRODE TUBULAR SOLID OXIDE FUEL CELL, AND METHOD OF MANUFACTURE

DESCRIPTION Cross-Reference to Related Application This application claims the benefit of our United States provisional application

serial number 60/240,114, filed October 12, 2000, the disclosure of which is hereby

incorporated herein by reference.

Technical Field This invention relates to fuel cells, particularly tubular solid oxide fuel cells, and

methods of manufacture.

Background of the Invention

Worldwide forecasts show electricity consumption increasing dramatically in the

next decades, largely due to economic growth in developing countries that lack national

power grids. This increased consumption, together with the deregulation of electrical

utilities in industrialized nations, creates the need for small scale, distributed generation

of electricity.

Fuel cells are a promising technology for providing distributed generation of

electricity. A fuel cell places an oxidizing gas, such as air, and a hydrogen-containing

fuel, such as hydrogen or natural gas, on opposite sides of an electrolyte in such a way that they combine to form water and electricity. Such a reaction requires a cathode and

an anode composed of porous materials, and an ionically-conducting electrolyte. In

solid oxide fuel cells, the electrolyte conducts negatively-charged oxygen ions.

Solid oxide fuel cell systems can be made less expensively than other kinds of

fuel cells, and thus have particular potential for facilitating distributed power

generation.

Important parameters of such systems include cell performance and fuel

utilization, and tendency to undergo adverse effects such as carbon deposition.

Summary of the Invention

In one embodiment according to the invention, a tubular solid oxide fuel cell

system comprises: a tubular electrolyte layer; a plurality of separate anode segments

mounted on a first surface of the tubular electrolyte layer; and a plurality of separate

cathode segments mounted on a second surface of the tubular electrolyte layer,

opposite the first surface, in corresponding positions to positions occupied by the

separate anode segments on the first surface; wherein corresponding anode and

cathode segments form a plurality of fuel cell sections along the length of the tubular

electrolyte layer.

In further related embodiments, the plurality of fuel cell sections may include a

primary pre-reformer section and a primary active section, a secondary active section,

and a primary regenerating section. The system may also include at least one mixing buffer section between fuel cell sections. The first and second surfaces each may be the

inside or outside surfaces of the tubular electrolyte layer.

In other related embodiments, the fuel cell system comprises an anode or

cathode current collector, which each may have associated meshes or coils of current

collector wires wrapped around an anode or cathode segment (respectively), and

electrically contacting the current collector. In one embodiment, the tubular electrolyte

layer comprises yttria-stabilized zirconia; the anode segments comprise a mixture of

nickel and yttria-stabilized zirconia; and the cathode segments comprise LaSrMnO_s.

In another related embodiment, each of the fuel cell sections is a fuel reformer.

In a further related embodiment, the plurality of separate cathode segments are

formed to fit between both (i) axial cuts extending in a direction parallel to a lengthwise

axis of the tubular electrolyte layer, and (ii) circumferential cuts extending around the

circumference of the tubular electrolyte layer.

In another embodiment according to the invention, a method of manufacturing a

tubular solid oxide fuel cell system comprises: layering a plurality of separate anode

segments onto a first surface of a tubular electrolyte layer; and layering a plurality of

separate cathode segments onto a second surface of the tubular electrolyte layer,

separate anode segments on the first surface, such that corresponding anode and

electrolyte layer. In further related embodiments, layering the anode segments comprises coating

separate anode segment layers onto a hollow center of the tubular electrolyte layer, and

layering the cathode segments comprises coating cathode material onto the outside of

the tubular electrolyte layer. In one related embodiment, the cathode segments are

coated onto the tubular electrolyte layer by a roller having higher diameter portions

alternating with lower diameter portions, the higher diameter portions applying a coat

of cathode material. In another related embodiment, more than one of the plurality of

fuel cell sections are electrically connected in series.

In another embodiment according to the invention, a method of operating a

tubular solid oxide fuel cell system comprises: applying a load to a first fuel cell

segment mounted on a common tubular electrolyte layer with other segments of a

plurality of fuel cell segments, the applied load producing reformation of a

hydrocarbon fuel flowing through the fuel cell system; and operating at least one

remaining fuel cell segment, of the commonly-mounted plurality of segments, on the

reformate produced by the first fuel cell segment.

Further related embodiments comprise operating the at least one remaining fuel

cell segment to reform the hydrocarbon fuel, when the first fuel cell segment degrades,

and regenerating the first fuel cell segment when it degrades. Regenerating the first

fuel cell segment may comprise altering the applied load on the first fuel cell segment

while it is under operating conditions, or flowing an inert gas through the fuel cell

system while the first fuel cell segment is drawing a load. In another embodiment according to the invention, a method of reforming a

hydrocarbon fuel comprises: flowing the hydrocarbon fuel through a fuel cell system

comprising a plurality of fuel cell segments mounted on a common tubular electrolyte

layer, each of the fuel cell segments reforming the hydrocarbon fuel; and adjusting the

applied load on each of the plurality of fuel cell segments to produce a desired product

composition at an exhaust of the fuel cell system.

Brief Description of the Drawings Fig. 1 A shows a side view of a prior art tubular solid oxide fuel cell in which

there is a single active section;

Fig. IB shows a side view of a segmented tubular solid oxide fuel cell according

to an embodiment of the invention;

Fig. 1C shows a side view of a segmented tubular solid oxide fuel cell, in which

one segment is a regenerating segment, according to an embodiment of the invention;

Fig. 2 is a cross-sectional view of a segmented tubular solid oxide fuel cell

according to an embodiment of the invention;

Fig. 3 is a cross-sectional view of a segmented tubular solid oxide fuel cell that is

used as an electrochemical "smart" reformer for hydrocarbon fuels, in accordance with

an embodiment of the invention;

Fig. 4 illustrates a method for coating a segmented cathode onto an electrolyte

layer, in accordance with an embodiment of the invention; and Fig. 5 shows multiple cathode segments within each cell section along the

lengthwise axis of a segmented tube, in accordance with an embodiment of the

invention.

Description of Specific Embodiments

Fig. 1A shows a side view of a prior art tubular solid oxide fuel cell 100 in which

there is a single active section 110, between a combustion buffer section 120 and a

manifold section 121.

By contrast with the fuel cell of Fig. 1A, Fig. IB shows a side view of a segmented

tubular solid oxide fuel cell 130 according to an embodiment of the invention. In this

embodiment, segmented electrochemically active areas 131-133 form multiple cells

along the length of tube 130.

The embodiment of Fig. IB offers several advantages over the prior art fuel cell

of Fig. 1A. One advantage of the embodiment of Fig. IB is that it allows one to control

the electrochemical load on segmented regions 131-133, and thereby to tailor reforming

catalysis and electrochemical activity to the particular type of fuel that the fuel cell tube

is using. Research has shown that there is a direct correlation between applied load and

reforming activity. Higher hydrocarbons require higher load conditions to provide

sufficient ionic transfer to prevent long term carbon deposition. Accordingly, in the

embodiment of Fig. IB, one cell segment 133 is set at the appropriate specific load level

to aid in situ reforming of the fuel cell's hydrocarbon fuel. The two remaining cell segments 131 and 132 then operate on the reformate produced by the first segment 133.

Segments 131 and 132 are thus essentially operating on syngas (H₂, CO) and the

remaining unreformed hydrocarbons from the first segment 133. Carbon deposition in

segments 131 and 132 is thus reduced.

The embodiment of Fig. IB also offers a redundancy mechanism, in which a

single cell segment may degrade (for example, through carbon deposition), but the

remaining cells continue to operate normally. In Fig. IB, cell segment 133, which is

closest to tube 130's fuel inlet, acts as an in situ electrochemical reformer for the other

two cell segments 131 and 132. That is, cell segment 133 is the active primary pre-

reformer cell; cell segment 132 is the primary active section of the fuel cell; and cell

segment 131 is the secondary active section of the fuel cell. However, as cell segment

133 degrades (for example, through carbon deposition), cell segment 132 begins to act as

a reformer for cell 131. Tube 130 then functions as shown in the embodiment of Fig. 1C:

cell segment 132 becomes the active primary pre-reformer cell, while cell segment 131

becomes the primary active section of the fuel cell.

Cell segment 133 or any other cell segment may be regenerated after it has been

degraded; segment 133 is shown in Fig. 1C as the primary regenerating cell. Such

regeneration may be produced by altering the applied load on the degraded cell while

the cell is under operating conditions; or by passing an inert gas through the cell while

it is drawing a load, to electrochemically oxidize carbon species and remove them as

CO or CO,. In the embodiments of Figs. IB and 1C, regions between cell segments 131-133

form the primary mixing buffer section 141 and the secondary mixing buffer section

142. These embodiments also have a combustion buffer section 120 and a manifold

section 121, as in Fig. 1A.

In addition to the advantages noted above, segmented cells offer the potential for

increased cell performance and fuel utilization, by comparison with cells with the same

overall active area but a single cell design. Also, use of segmented cells decreases the

effective thermal gradient along the active length of the cell. Thus, there is less

performance drop along a tube compared to a tube with a single active length cell of the

same area.

Fig. 2 is a cross-sectional view of a segmented tubular solid oxide fuel cell 230

according to an embodiment of the invention. A continuous electrolyte tube 250 forms

the middle layer of tube 230. Separate cell segments 231 and 232 are formed on

electrolyte layer 250, by forming segmented inner anode layers 251 and 252, and

segmented outer cathode layers 253 and 254. In an alternative embodiment according

to the invention, the cathode layers may be on the inside of tube 230, while the anode

layers are on the outside of tube 230. In Fig. 2, cell segment 231 is the primary active

section, having a primary fuel cell anode 251 and a primary fuel cell cathode 253.

Similarly, cell segment 232 is the primary reformer section, having a primary reformer

anode 252 and a primary reformer cathode 254.

Current collection from the anodes and cathodes may be performed by a variety of methods, including by wrapping current-collecting wires or meshes around the

inside and outside of each segment. In the embodiment of Fig. 2, separate wire coils 261

and 262 (shown in cross-section) wrap around each anode segment 251 and 252. Larger

metal wires 271 and 272 electrically contact these coils, and thus act as the primary fuel

cell anode current collector 271 and reformer anode current collector 272, respectively.

Similarly, separate wire coils 263 and 264 (shown in cross-section) wrap around each

cathode segment 253 and 254. Larger metal wires 273 and 274 electrically contact the

cathode coils, acting as the primary fuel cell cathode current collector 273 and reformer

cathode current collector 274, respectively. Wire coils 261-264 may also be replaced

with current-collecting meshes.

In accordance with one embodiment according to the invention, electrolyte tube

250 is an extrusion of 8 mol% yttria-stabilized zirconia (YSZ). Anodes 251 and 252 are

formed of a Ni/YSZ mixture or a Ni/YSZ/CGO mixture. Cathodes 253 and 254 are

formed of a LaSrMn0₃/YSZ mixture, or of LaSrMn0₃. Anode current collectors and

wires 261, 262, 271, 272 are silver, while cathode current collectors and wires 263, 264,

273, 274 are nickel. The reformer cathode current collector 274 is set at 0.6V. However,

in accordance with other embodiments according to the invention, different

components and values may be used, as will be appreciated by those of ordinary skill in

the art, including: different materials for the electrolyte, anodes, and cathodes; different

metals, or mixtures of metals, for the anode and cathode current collectors and wires or

meshes; and different voltage settings. Fig. 3 is a cross-sectional view of a segmented tubular solid oxide fuel cell 330

that is used as an electrochemical "smart" reformer for a range of hydrocarbon fuels, in

accordance with an embodiment of the invention. In this embodiment, the cell

segments function as reformers, such as primary reformer 332 and secondary reformer

331; more reformer segments may be used as necessary. In a similar fashion to that of

Fig. 2, the cell segments 331, 332 are formed using an electrolyte tube 350; a secondary

reformer anode 351 and a primary reformer anode 352; a secondary reformer cathode

353 and a primary reformer cathode 354; anode current collection wires 361, 362 and

cathode current collection wires 363, 364; secondary and primary reformer anode

current collectors 371 and 372; and secondary and primary reformer cathode current

collectors 373 and 374. Primary reformer cathode current collector 374 may be set, for

example, at 0.6N. In the embodiment of Fig. 3, the applied load on each cell segment is

adjusted, as a hydrocarbon fuel is passed through tube 330, to produce the desired

product composition at the cell exhaust. Such an embodiment may be useful, for

example, in the synthetics industry, which uses pre-reformers to produce H₂ and CO.

In such a setting, the embodiment of Fig. 3 could provide electrical energy return, and

improved fuel and product flexibility.

Segmented electrode cells according to embodiments of the invention may be

manufactured in several ways. A segmented anode, as in the embodiment of Figs. 1-3,

may be made by first forming an electrolyte tube, and then injecting segmented anode

layers into the hollow center of the electrolyte. The anode layers may also be coated on by other techniques. In one embodiment, anode material is drawn into the interior of

the electrolyte tube using a syringe. A segmented cathode, as in the embodiments of

Figs. 1-3, may be made by coating cathode material onto the outside of a pre-formed

electrolyte layer.

The cathode coating process may be automated and made continuous, in

accordance with an embodiment of the invention, by the "roller" method shown in Fig.

4. A first, larger roller has cylindrical portions 402 of higher diameter, alternating with

lower diameter portions 403. The higher diameter portions 402 are coated with cathode

material by rotating the first roller, on shaft 401, with its lower surfaces in contact with a

reservoir of cathode material 404. A smaller set of rollers 405 rotates in the opposite

direction on a shaft 406. By being in surface contact with the higher diameter portions

402 of the larger roller, the smaller rollers 405 are coated with cathode material.

Electrolyte tubes 407, to be coated with segmented cathodes, are conveyed along

a belt or set of rails 408 towards the rollers. Each electrolyte tube is then coated with

cathode material by being rolled against the smaller rollers 405. Since there are gaps

between the smaller rollers 405, the cathode layers will be segmented.

Optionally, a lexan tab 410, or other suitable mechanical device, may be used to

push the tube that is being coated against the rollers 405. This tab may be movable,

retracting when each electrolyte tube is rolling over it, and then pushing each tube 409

onto the rollers. The tab's shape may be altered so that it does not contact lengths of the

electrolyte tubes that are in the process of being coated. In order to remove each cell after coating, one or both of the rollers may contain

grooves in their cylindrical portions, parallel to the direction of the fuel cells' axes.

After being coated with one revolution of the roller, a cell will then be caught by the

grooves, taken up by the roller, and revolved around until either falling off or being

automatically removed from the groove.

Other automated manufacturing techniques for segmented cells may be used in

accordance with embodiments of the invention.

Different numbers of cell segments per tube than those illustrated above may be

used in accordance with embodiments of the invention. In one embodiment, two active

areas are used, one on each end of an open-ended fuel cell tube.

Another advantage is provided in an embodiment according to the invention by

placing cell segments electrically in series. This has the advantage of increasing voltage

per tube, and of reducing losses (losses being proportional to the square of current

through a cell). In one embodiment according to the invention, two active areas are

produced using syringe-coated anode layers, with one active area on each end of the

tube; the two active areas are then placed in series.

Fig. 5 shows multiple cathode segments within each cell section along the

lengthwise axis of a segmented tube, in accordance with an embodiment of the

invention. In this embodiment, axial cuts 501 are formed in each cell segment, in a

direction parallel to the lengthwise axis of the tube. Thus, the fuel cell tube benefits

from a further increase in segmentation, by comparison with a tube in which segment cuts are only made in circumferential direction 502.

Although this description has set forth the invention with reference to several

preferred embodiments, one of ordinary skill in the art will understand that one may

make various modifications without departing from the spirit and the scope of the

invention, as set forth in the claims.

Claims

What is claimed is:

1. A tubular solid oxide fuel cell system, the fuel cell system comprising:

a tubular electrolyte layer;

a plurality of separate anode segments mounted on a first surface of the tubular

electrolyte layer; and

a plurality of separate cathode segments mounted on a second surface of the

tubular electrolyte layer, opposite the first surface, in corresponding positions to

positions occupied by the separate anode segments on the first surface;

wherein corresponding anode and cathode segments form a plurality of fuel cell

sections along the length of the tubular electrolyte layer.

2. A fuel cell system according to claim 1, wherein the plurality of fuel cell sections

include a primary pre-reformer section and a primary active section.

3. A fuel cell system according to claim 2, wherein the plurality of fuel cell sections

further include a secondary active section.

4. A fuel cell system according to claim 1, wherein the plurality of fuel cell sections

include a primary regenerating section.

5. A fuel cell system according to claim 1, further comprising at least one mixing buffer section between fuel cell sections.

6. A fuel cell system according to claim 1, wherein the first surface is the inside surface

of the tubular electrolyte layer, and the second surface is the outside surface of the

tubular electrolyte layer.

7. A fuel cell system according to claim 1, wherein the first surface is the outside

surface of the tubular electrolyte layer, and the second surface is the inside surface of

the tubular electrolyte layer.

8. A fuel cell system according to claim 1, further comprising an anode current

collector.

9. A fuel cell system according to claim 8, further comprising an anode mesh wrapped

around an anode segment, and electrically contacting the anode current collector.

10. A fuel cell system according to claim 8, further comprising a coil of anode current

collector wires wrapped around an anode segment, and electrically contacting the

anode current collector.

11. A fuel cell system according to claim 1, further comprising a cathode current collector.

12. A fuel cell system according to claim 11, further comprising a cathode mesh

wrapped around a cathode segment, and electrically contacting the cathode current

collector.

13. A fuel cell system according to claim 11, further comprising a coil of cathode current

collector wires wrapped around a cathode segment, and electrically contacting the

cathode current collector.

14. A fuel cell system according to claim 1, wherein the tubular electrolyte layer

comprises yttria-stabilized zirconia.

15. A fuel cell system according to claim 1, wherein the anode segments comprise a

mixture of nickel and yttria-stabilized zirconia.

16. A fuel cell system according to claim 1, wherein the cathode segments comprise

LaSrMnO_s.

17. A fuel cell system according to claim 1, wherein each of the fuel cell sections is a fuel

reformer.

18. A fuel cell system according to claim 1, wherein the plurality of separate cathode

segments are formed to fit between both (i) axial cuts extending in a direction

parallel to a lengthwise axis of the tubular electrolyte layer, and (ii) circumferential

cuts extending around the circumference of the tubular electrolyte layer.

19. A method of manufacturing a tubular solid oxide fuel cell system, the method

comprising:

layering a plurality of separate anode segments onto a first surface of a tubular

electrolyte layer; and

layering a plurality of separate cathode segments onto a second surface of the

positions occupied by the separate anode segments on the first surface, such that

corresponding anode and cathode segments form a plurality of fuel cell sections along

the length of the tubular electrolyte layer.

20. A method according to claim 19, wherein layering the anode segments comprises

coating separate anode segment layers onto a hollow center of the tubular

electrolyte layer.

21. A method according to claim 19, wherein layering the cathode segments comprises coating cathode material onto the outside of the tubular electrolyte layer.

22. A method according to claim 21, wherein the cathode segments are coated onto the

tubular electrolyte layer by a roller having higher diameter portions alternating with

lower diameter portions, the higher diameter portions applying a coat of cathode

material.

23. A method according to claim 19, further comprising electrically connecting more

than one of the plurality of fuel cell sections in series.

24. A method of operating a tubular solid oxide fuel cell system, the method

comprising:

applying a load to a first fuel cell segment mounted on a common tubular electrolyte

layer with other segments of a plurality of fuel cell segments, the applied load

producing reformation of a hydrocarbon fuel flowing through the fuel cell

system; and

operating at least one remaining fuel cell segment, of the commonly-mounted

plurality of segments, on the reformate produced by the first fuel cell segment.

25. A method according to claim 24, further comprising:

operating the at least one remaining fuel cell segment to reform the hydrocarbon fuel, when the first fuel cell segment degrades.

26. A method according to claim 25, further comprising:

regenerating the first fuel cell segment when it degrades.

27. A method according to claim 26, wherein regenerating the first fuel cell segment

comprises altering the applied load on the first fuel cell segment while it is under

operating conditions.

28. A method according to claim 26, wherein regenerating the first fuel cell segment

comprises flowing an inert gas through the fuel cell system while the first fuel cell

segment is drawing a load.

29. A method of reforming a hydrocarbon fuel, the method comprising:

flowing the hydrocarbon fuel through a fuel cell system comprising a plurality of *

fuel cell segments mounted on a common tubular electrolyte layer, each of the

fuel cell segments reforming the hydrocarbon fuel; and

adjusting the applied load on each of the plurality of fuel cell segments to produce a

desired product composition at an exhaust of the fuel cell system.