Proton-conducting ceramic fuel cell architecture

A method of manufacturing a proton-conducting fuel cell includes assembling a green anode-electrolyte half-cell by forming an anode substrate layer having an upper surface and a lower surface, forming an anode functional layer on the upper surface of the anode substrate layer, forming an electrolyte layer on an upper surface of the anode functional layer, and forming a stress balancing layer on the lower surface of the anode substrate layer. The method further includes positioning the green anode-electrolyte half-cell on kiln furniture inside a sintering kiln and sintering the green anode-electrolyte half-cell using SSRS to an anode-electrolyte half-cell.

BACKGROUND

The present application relates generally to the field of proton-conducting ceramic fuel cell (PCFC) systems and, more particularly, the manufacture of PCFC systems at a commercially viable size and cost.

Generally, a fuel cell includes an anode, a cathode, and an electrolyte layer that together drive chemical reactions to produce electricity. Specifically, a PCFC is a solid electrochemical cell comprising a ceramic electrolyte sandwiched between a porous anode and porous cathode. Fuel, such as hydrogen gas or hydrocarbon gas, is supplied to the anode. The anode causes the hydrogen atom electrons to dissociate from the hydrogen protons. The protons travel across the proton-conducting electrolyte to the cathode, where they bond to oxidants, such as oxygen gas. The electrons travel through an external circuit from the anode to the cathode to generate electric power.

PCFC systems may be preferable to Solid Oxide Fuel Cell (SOFC) systems in certain circumstances, because they can provide enhanced performance at lower operating temperatures, resulting in lower operating costs and fewer material compatibility challenges. While SOFCs generally operate at temperatures in the range of 600-1000° C., PCFCs can provide good performance under 600′C.

PCFCs are traditionally manufactured using high-temperature calcination and sintering processes that require relatively long processing times, which in turn can contribute to a relatively high production cost. Additionally, prior PCFC production techniques that utilized solid state reaction sintering (SSRS) presented issues such as bonding of the materials to kiln furniture and/or warpage of the parts due to high shrinkage compared to conventional sintering processes. Accordingly, it would be advantageous to develop a PCFC manufacturing process that allows for cells of commercially viable size to be manufactured using a lower-temperature sintering process.

SUMMARY

In some embodiments of the present disclosure, a method of manufacturing a PCFC includes assembling a green anode-electrolyte half-cell by forming an anode substrate layer having an upper surface and a lower surface, forming an anode functional layer on the upper surface of the anode substrate layer, forming an electrolyte layer on an upper surface of the anode functional layer, and forming a stress balancing layer on the lower surface of the anode substrate layer. The method further includes positioning the green anode-electrolyte half-cell on kiln furniture inside a sintering kiln and sintering the green anode-electrolyte half-cell using SSRS to an anode-electrolyte half-cell.

In some aspects of the method, the method further comprises forming a cathode layer on an upper surface of the electrolyte and cathode sintering the anode-electrolyte half-cell and cathode layer.

In some aspects, the assembling of the green anode-electrolyte half-cell further comprises forming a coarse NiO layer on a lower surface of the stress balancing layer such that, when the green anode-electrolyte half-cell is positioned on the kiln furniture, the stress balancing layer does not directly contact the kiln furniture. In some aspects the coarse NiO layer may comprise NiO powder with average particle size above about 20 micrometers and below about 2.0 mm. The NiO powder may have an average particle size of about 60 micrometers.

In some aspects, the coarse NiO layer may be brushed off after sintering. In other aspects, the coarse NiO layer may be reduced to nickel metal by operating the PCFC.

In some aspects, the method may include forming a layer of coarse NiO paste on the kiln furniture such that the anode-electrolyte half-cell is not in contact with the kiln furniture. In some aspects, the method may include placing a sheet of yttria paper between the anode-electrolyte half-cell and the kiln furniture such that the anode-electrolyte half-cell is not in contact with the kiln furniture.

In other embodiments of the present disclosure, a PCFC is provided which comprises an anode substrate layer comprising an upper surface and a lower surface, an anode functional layer coupled to the upper surface of the anode substrate layer, an electrolyte layer coupled to an upper surface of the anode functional layer, and a stress balancing layer coupled to the lower surface of the anode substrate layer.

In some aspects, the PCFC further comprises a cathode layer coupled to an upper surface of the electrolyte layer.

In some aspects, the PCFC further comprises a coarse NiO layer coupled to a lower surface of the stress balancing layer. In some aspects, the PCFC includes a layer of nickel metal coupled to a lower surface of the stress balancing layer, the layer of nickel metal formed by heating a layer of coarse NiO

In some aspects, the stress balancing layer may be more than about Sum thick and less than about 100 micrometers thick.

In other embodiments of the present disclosure, a PCFC is provided which comprises an anode substrate layer comprising an upper surface and a lower surface, an anode functional layer coupled to the upper surface of the anode substrate layer, an electrolyte layer coupled to an upper surface of the anode functional layer, and a coarse NiO layer forming a lower surface of the proton-conducting fuel cell.

In some aspects, the coarse NiO layer may comprise NiO powder with an average particle size above about 20 micrometers and below about 2.0 mm, and preferably about 60 micrometers.

It will be appreciated that these and other features and/or aspects maybe used in any combination.

DETAILED DESCRIPTION

PCFCs may be manufactured using a lower temperature sintering process through the use of solid-state reactive sintering (SSRS). The use of SSRS allows for the sintering of anode-electrolyte half-cells at temperatures of about 1450° C. or less, compared to temperatures as high as 1700° C. for traditional PCFC sintering.

Production of PCFCs using SSRS includes a first solid-state reactive sintering of the anode-electrolyte half-cell containing a thin layer of anode substrate, an anode functional layer, and an electrolyte layer. During the SSRS step, the anode substrate layer, generally the bottom layer, may be placed directly on the kiln furniture inside the sintering kiln. The layers bond during SSRS process to form the half-cell. PCFC fabrication may be completed by screen printing a layer of cathode onto the upper surface of the electrolyte layer and conventionally sintering the cell a second time at a lower temperature, about 800-1000° C., a process called cathode sintering.

While the SSRS method has been successful in producing small test cells, or button cells, fabrication of PCFCs at a commercially viable size—about 81 cm2active electrode area or larger—poses additional problems. First, fuel cells sintered using SSRS shrink up to 1.5 times as much as conventionally sintered cells, causing the cell to warp during processing due to the inconsistent shrink rates in the layers. Second, the carbonates and oxides used to produce the barium zirconates in the anode substrate layer, which is in contact with the furniture inside the sintering kiln, react strongly with the kiln furniture material, generally zirconia or silicon carbide. The reaction with the kiln furniture causes the half-cells to deform and break due to the large surface area in contact with the kiln furniture. For smaller button cells, an additional layer of electrolyte can be bonded to the lower surface of the anode substrate, i.e. the surface opposite the anode functional layer. This layer can be ground off after sintering to expose the anode substrate for use in a test cell. Because there is less shrinkage due to the size of the button cells, the reaction between the half-cell and the kiln furniture is less likely to be destructive. However, this solution is not viable for larger cells because of the larger contact area between the half-cell and the kiln furniture.

The present disclosure discusses the production of PCFCs that contain a stress balancing layer coupled to the bottom surface of the anode substrate layer as well as methods of fabricating such a PCFC. Various fabrication methods include steps to prevent the reaction of the anode substrate with the sintering kiln furniture using a coarse layer of NiO paste or a sheet of yttria paper between the half-cell and kiln furniture. These methods overcome previous limitations which prevented the production of PCFCs of commercially viable size due to the PCFC warping during sintering and bonding to the kiln furniture, causing deformation and breakage.

PCFCs made according to the embodiments described herein have demonstrated peak power densities of about 521 mW/cm2when tested at an operating temperature of 550′C, which is more than double the power density of typical solid oxide fuel cells at that relatively low temperature.

FIG.1Ais a representation of a typical PCFC anode-electrolyte half-cell100athat does not include a stress balancing layer. The half-cell100ashown has warped during sintering due to the lack of a stress balancing layer. The anode substrate layer110is formed from the anode substrate base material, generally a mixture of barium and zirconium oxides and/or carbonates and nickel oxide, which is combined with binders and solvents to form a paste. The paste is then laid out in a thin (between approximately 0.2 and 2.0 millimeters (mm)) layer, typically using tape casting, and dried. Note that the various ceramic materials that make up the completed cell are in the form of oxide and carbonate precursors of the ceramic components prior to sintering, and form barium zirconate ceramics during the sintering process. Next, the anode functional layer120is formed from the anode functional layer120base material, which is mixed with binders and solvents to form a paste and screen printed, doctor bladed, or painted onto the dried anode substrate layer110. The anode functional layer120is generally a mixture of a barium and zirconium oxides and/or carbonates and NiO with a thickness of between approximately 5 and 50 micrometers. Finally, the electrolyte layer130is formed from the electrolyte base material, which is combined with binders and solvents to form a paste and screen printed, doctor bladed, or painted onto the dried anode functional layer120. The electrolyte layer130is generally a mixture of barium and zirconium oxides and/or carbonates having a thickness of between approximately 5 and 50 micrometers. These layers are sintered together using SSRS to form the PCFC anode-electrolyte half-cell100acontaining an anode substrate layer110, an anode functional layer120, and an electrolyte layer130. A cathode layer may then be sintered to the half-cell100aon top of the electrolyte layer130at lower temperatures in a separate step to complete the cell.

During sintering, anode-electrolyte half-cells shrink as the base materials densify into ceramic. To fabricate an ideal anode-electrolyte half-cell, the electrolyte layer130is sintered to a fully dense state, while the anode functional layer120is slightly more porous with a fine microstructure, and the anode substrate layer110is even more porous with a coarser microstructure. Thus, the electrolyte layer130experiences the most shrinkage, the anode functional layer120slightly less shrinkage, and the anode substrate layer110the least shrinkage.

Due to the differing rates of shrinkage, as well as temperature differentials in the half-cell and differences in the stress state in the middle of the half-cell (fully constrained) compared to the edges of the cell (partially constrained), half-cells can warp during sintering, as shown inFIG.1A. Compared to a traditional fuel cell sintering process, there is additional chemical shrinkage during SSRS as the carbonates and oxides are converted into the final perovskite phase. While excessive shrinkage does not have a strong effect on small button cells made using SSRS, large-area, thin half-cells that are required for commercial PCFC operation may be so warped as to require multiple time-consuming and expensive ironing steps, or may be entirely unusable.

FIG.1Bis a representation of an exemplary embodiment of a PCFC anode-electrolyte half-cell110bthat includes a stress balancing layer140. The stress balancing layer140is made from a material similar to that of the anode functional layer120and may be between approximately 5 and 100 micrometers thick. In some embodiments, the stress balancing layer140may be approximately the same thickness as the anode functional layer120. The stress balancing layer140is formed from the stress balancing layer base material, a material similar to or the same as the anode functional layer base material, which is combined with binders and solvents to form a paste and screen printed, doctor bladed, or painted on to the lower surface of the anode substrate layer110, i.e. the surface opposite the anode functional layer120. The stress balancing layer140roughly matches the rate of shrinkage of the anode functional layer120and electrolyte layer130, preventing or reducing the amount of warping in the PCFC during SSRS by balancing the mismatched shrinkage rates between the anode substrate layer110, the anode functional layer120, and electrolyte layer130. After the anode-electrolyte half-cell100bis fabricated according to the various embodiments described herein, a complete PCFC may be fabricated by applying a cathode layer to the upper surface of the electrolyte layer130(i.e., the surface opposite the anode functional layer120), and sintering again at a lower temperature in the range of between approximately 800 and 1000 degrees Celsius. This step is called cathode sintering.FIG.9illustrates the microstructure of a cathode layer after it has been sintered to an anode-electrolyte half-cell.

FIG.1Cshows a PCFC anode-electrolyte half-cell100b, including a stress balancing layer140as described above, on the kiln furniture190inside the sintering kiln180. The stress balancing layer140is in direct contact with the kiln furniture190, with the anode substrate layer110on top of the stress balancing layer140, the anode functional layer120on top of the anode substrate layer110and the electrolyte layer130on top of the anode functional layer120. The direct contact of the half-cell100bwith the kiln furniture190may cause the stress balancing layer140to react with and bond to the kiln furniture190during sintering. This may cause the half-cell100bto warp and/or crack, which may require the half-cell100bto be ironed flat prior to use or may render the half-cell100bunusable altogether. In a half-cell with no stress balancing layer (e.g., half-cell100a), the anode substrate layer110may be in direct contact with the kiln furniture190during firing and may similarly react with the kiln furniture190. Thus, separation between the stress balancing layer140or anode substrate layer110and the kiln furniture190may be required to prevent or reduce the amount of warping and cracking of the half-cell.

A layer of appropriate material between the kiln furniture and the half-cell may be used to prevent the cell materials from reacting with the kiln furniture. Kiln furniture is generally made of zirconia or silicon carbide which can withstand the extreme temperatures inside the sintering kiln. The material used to prevent the cell materials from reacting with and bonding to the kiln furniture must also be able to withstand these temperatures. In order to overcome the cell materials' reacting with and bonding to the kiln furniture, extensive experimentation was done with various materials placed between the kiln furniture and the cell. Some of the materials tested include zirconia foam, zirconia plate, zirconia fiber paper, dense alumina plate, porous alumina plate, alumina fiber paper, silicon carbide plate, and NiO powder finer than 20 micrometers. In each case, the cell either still reacted with the kiln furniture, reacted with the test material, cracked, warped, or was completely destroyed. Two solutions were eventually discovered: (i) a layer of coarse NiO powder paste applied between the half-cell and the kiln furniture, either on the kiln furniture, on the cell itself, or both, and (ii) yttria paper placed between the half-cell and the kiln furniture.

FIG.1Dshows a PCFC anode-electrolyte half-cell100cwith a coarse NiO layer150according to an embodiment of the present disclosure. The coarse NiO layer150is formed from the coarse NiO layer base material, coarse NiO powder with an average particle size above about 20 micrometers and below about 2.0 mm, which is combined with binders and solvents to form a paste. In one successful test cell according to an exemplary embodiment, the average particle size of the NiO powder was about 60 micrometers. The coarse NiO layer paste loosely sinters during SSRS and forms a coarse microstructured layer, and prevents or reduces the reaction between the kiln furniture and the oxides and carbonates used to make barium zirconates of the half-cell. The coarse NiO particles of the coarse NiO layer150do not bond or react strongly with the kiln furniture. Thus, the coarse NiO layer150prevents or reduces warpage and cracking of the half-cell100cdue to reactions with the kiln furniture and the stress balancing layer140prevents or reduces warpage due to mismatched shrink rates.

FIG.8is an image from a scanning electron microscope showing the microstructure of a cross-section of the upper layers of a PCFC anode-electrolyte half-cell manufactured with a stress-balancing layer (e.g., stress balancing layer140) and a sacrificial layer (e.g., coarse NiO layer150that may be removed or converted to nickel metal) according to an exemplary embodiment. The stress-balancing layer and sacrificial layer have prevented the half-cell from warping or cracking.FIG.9is an image from a scanning electron microscope showing the microstructure of the upper layers of a PCFC after a cathode layer has been sintered to an anode-electrolyte half-cell according to an exemplary embodiment.FIGS.10and11are images from a scanning electron microscope showing the microstructure of a PCFC with a stress-balancing layer according to an exemplary embodiment.FIG.12is an image from a scanning electron microscope showing the microstructure of the lower layers of a PCFC with a stress-balancing layer and a sacrificial layer according to an exemplary embodiment.

Various embodiments of the present disclosure include methods of producing PCFCs as described above.FIG.2illustrates an embodiment of a method of producing a PCFC of commercially viable size. At step210, anode substrate paste is tape cast or otherwise deposited to form the anode substrate layer110. At step220, anode functional layer paste is screen printed or otherwise applied onto the top of the anode substrate layer110to form the anode functional layer120. At step230, electrolyte paste is screen printed or otherwise applied onto the top of the anode functional layer120to form the electrolyte layer130. At step240, stress balancing layer paste is screen printed or otherwise applied onto the bottom of the anode substrate layer110to form the stress balancing layer140. As described above, the stress balancing layer140is coupled to the anode substrate to prevent or reduce bowing, warping, and cracking of the half-cell100cduring sintering. At step250, coarse NiO layer paste is screen printed or otherwise applied onto the bottom surface of the stress balancing layer140(i.e., the surface opposite the anode substrate layer110) to form the coarse NiO layer150. These layers form a green half-cell that may not warp due to mismatched shrinkage rates or due to reaction with the kiln furniture190during sintering. In other exemplary embodiments, steps210-250may be completed in different orders. For example, the stress balancing layer140may be applied to the bottom of the anode substrate layer110before the anode functional layer120and electrolyte layer130are applied.

At step260inFIG.2, the green half-cell is sintered in a sintering kiln180using SSRS at approximately 1450° C. to form the half-cell100ccontaining a stress balancing layer140and a coarse NiO layer150. The arrangement of the half-cell100cduring the sintering step260is shown inFIG.1D. The coarse NiO layer150is a coarse layer of NiO loosely coupled to the stress balancing layer140. At step270, the coarse NiO layer is manually brushed off of or otherwise removed from the half-cell100c. At step280, the cathode paste is screen printed or otherwise applied onto the top surface of the electrolyte layer130, i.e. the surface opposite the anode functional layer120, to form the cathode layer. At step290, the cell is sintered again with the cathode layer at a lower temperature such as 900° C. (cathode sintering) to form the completed PCFC.

In other exemplary embodiments, the coarse NiO layer150may be designed such that it remains attached to the cell. During PCFC operation, the coarse NiO layer150that remains attached may be reduced to nickel metal, which may be conductive and compatible with the other cell materials and may not interfere with the operation of the cell.

In some embodiments, as shown inFIG.3A, a layer of coarse NiO paste155may be applied directly to the kiln furniture190rather than to the half-cell. After sintering, the NiO layer on the kiln furniture190is generally reduced to a loose powder bed. The NiO can be swept up and reprocessed into a paste for subsequent firings. Some of the NiO that was applied to the kiln furniture190may bond to the PCFC after firing, and can be manually brushed off.

In some embodiments, a coarse layer of nickel oxide may be applied both to the anode-electrolyte half-cell100cand to the kiln furniture190.FIG.3Bshows the arrangement of a half-cell100cwith a coarse NiO layer150in a sintering kiln180with a layer of coarse NiO paste155applied to the kiln furniture190. As in the embodiments described above, the NiO can be brushed off the kiln furniture190and the half-cell100c, or may be brushed off the kiln furniture190and remain on the half-cell100c, where it will be reduced to nickel metal during PCFC operation.

In other exemplary embodiments, as shown inFIG.3C, the PCFC anode-electrolyte half-cell100bis sintered without the use of a coarse NiO layer on either the half-cell or the kiln furniture. Instead, yttria paper160is placed between the half-cell100band the kiln furniture190to prevent them from reacting with and bonding to each other. The yttria paper160may be reused for sintering multiple cells before needing to be replaced due to degradation.

Various embodiments include using yttria paper160or a coarse NiO layer150,155between the PCFC materials and the kiln furniture190without the use of a stress balancing layer140. Button cells may be produced with this method, and future development in PCFC fabrication may obviate the need for a stress balancing layer140.

EXAMPLE

A first test cell with an active electrode area of 81 cm2was produced in which a layer of coarse NiO paste was applied to the kiln furniture prior to sintering. Voltage and power density were measured during a first test at various operating temperatures and current densities, and the measurements are shown inFIG.4A.FIG.4Bshows the results from a repeat test performed with the same cell. Operating at 550 degrees Celsius (a relatively low temperature for most fuel cells of this type) the PCFC had a peak power density of approximately 521 mW/cm2.FIG.6Ashows the results of a steady-state hold test of the first test cell. The cell was held at a temperature of 700 degrees Celsius and a current density of 0.34 A/cm2while voltage was measured over an extended duration. The cell voltage measured between about 0.80V and 0.93V over the course of about 400 hours.

A second test cell with an active electrode area of 81 cm2was produced according to an embodiment of the invention in which yttria paper is placed between the half-cell and kiln furniture during sintering. Voltage and power density of the second test cell were measured at various operating temperatures and current densities, and the measurements are shown inFIG.4C. At 550 degrees Celsius, the second test cell performed as well as the first, also reaching a peak power density of about 521 mW/cm2.FIG.6Bshows the results of a steady-state hold test performed on the second test cell at a temperature of 550 degrees Celsius and a current density of 0.2 A/cm2. The cell voltage measured between about 0.89V and 0.84V over the course of about 250 hours.

A solid oxide fuel cell, not according to an embodiment of this disclosure, was tested under similar conditions for comparison to the PCFC test cells. Voltage and power density were measured during a first test at various operating temperatures and current densities, and the measurements are shown inFIG.5. The solid oxide fuel cell had a peak power density at 550 degrees Celsius of only about 200 mW/cm2, less than half that of the PCFC test cells produced according to the embodiments of the invention.FIG.7shows the results of a steady-state hold test performed on the solid oxide fuel cell. The solid oxide fuel cell voltage measured between about 0.70V and 0.67V over the course of about 1100 hours, more than 0.1V lower than the second PCFC test cell tested at the same temperature and current density (i.e., 550 degrees Celsius and 0.2 A/cm2). The test data shows that the PCFC cells produced according to embodiments of the present application outperform solid oxide fuel cells at the relatively low temperature of 550 degrees Celsius.

Notwithstanding the embodiments described above inFIGS.1-12, various modifications and inclusions to those embodiments are contemplated and considered within the scope of the present disclosure.

It is also to be understood that the construction and arrangement of the elements of the systems and methods as shown in the representative embodiments are illustrative only. Although only a few embodiments of the present disclosure have been described in detail, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter disclosed.

Accordingly, all such modifications are intended to be included within the scope of the present disclosure. Any means-plus-function clause is intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangement of the preferred and other illustrative embodiments without departing from scope of the present disclosure or from the scope of the appended claims.

Furthermore, functions and procedures described above may be performed by specialized equipment designed to perform the particular functions and procedures. The functions may also be performed by general-use equipment that executes commands related to the functions and procedures, or each function and procedure may be performed by a different piece of equipment with one piece of equipment serving as control or with a separate control device.

References herein to the positions of elements (e.g., “top,” “bottom,” “in front,” “behind,” “above,” “below”) are merely used to describe the orientation of various elements in the FIGURES. It should be noted that the orientation of various elements may differ according to other exemplary embodiments, and that such variations are intended to be encompassed by the present disclosure.