Patent Application
20220093935
The present disclosure relates to long-term stable ceramic anodes for solid oxide fuel cells (SOFCs) and methods for producing and using the same. In particular, the anodes for solid oxide fuel cells disclosed herein are capable of both operating at significantly lower temperatures than conventional SOFCs and enabling thermal and anode gas cycling under transient conditions, such as start-up and shut-down. More significantly, such anodes are stable over a long period of operation.
Solid oxide fuel cells (SOFCs) are efficient electrochemical energy conversion device that can convert chemical energy stored in a variety of fuels into electrical energy. Conventional SOFCs operate at high temperatures (e.g., ≥800° C.), leading to degradation and other issues that increase cost. Efforts are underway to lower the operating temperature of SOFCs to demonstrate reliable SOFCs at a lower cost.
One of the critical factors that hampers the deployment of low temperature solid oxide fuel cells (LT-SOFC) is anode instability. In particular, reduction/oxidation (redox) cycles during fuel-rich and fuel-starved conditions induce anodes of SOFC to fail. To overcome such limitations various ceramic anodes have been proposed for SOFC, including titanates (e.g., Sr0.99Ti0.9Nb0.1O3-δ), molybdates (e.g., Sr2MgMoO6-δ), chromates (e.g., La0.2Sr0.7Cr0.5Mn0.5O3-δ), ferrites (e.g., La0.6Sr0.4Fe0.9Mn0.1O3-δ) and vanadates (La0.8Sr0.2VO3-δ). Of which, Sr2MgMoO6-δ (SMMO) based ceramic anodes, Sr2FeNb0.2Mo0.8O6-δ, Sr2Fe1.5-xCoxMo0.5O6-δ (X=0, 0.5, 0.75, and 1) have shown to have excellent redox stability, and operate steadily even in H2S-containing fuels at 800° C. Also, variants of SMMO such as Sr2CoMoO6 and Sr2NiMoO6 operate in hydrocarbon fuels (utilizing Co3+/Co2+ and Mo6+/Mo5+ as redox couples) at high temperatures (e.g., >800° C.). However, the insufficient catalytic activity of these ceramic anodes for fuel oxidation at low temperatures (e.g., ≤600° C.) greatly limits their application in a low-temperature solid oxide fuel cell (LT-SOFC).
Therefore, there is a need for SOFC anodes that can operate efficiently at a low-temperature. In addition, there is a continuing need for SOFC anodes that can withstand or undergo a large number of reduction/oxidation cycles and/or anodes that have a long-term stability during operation.
Some aspects of the present disclosure are based on the discovery by the present inventors that ceramic anodes, such as strontium-iron-cobalt-molybdenum (SFCM), can withstand a large number of redox cycles without cracking that is observed in conventional SOFC anodes. More significantly, the present inventors have discovered that limiting the amount of electrocatalyst present in the ceramic anode increases the long-term stability of ceramic anodes. For example, when the electrocatalyst comprises nickel and gadolinium-cerium oxide, by limiting the total amount of electrocatalyst in the ceramic anode to less than 10 wt %, typically less than 8 wt %, often less than 5 wt % or less, combined with reducing the ratio of Ni to GDC in the electrocatalyst, one can increase the operability of the anode by at least 10%, typically by at least 25%, and often by at least 50% compared to a similar anode having the total amount of electrocatalyst of about 10 wt % with a 1:1 atomic/molecular ratio of Ni to GDC. Unless context requires otherwise, the weight % of the electrocatalyst refers to its weight relative to the total weight of the ceramic anode and the amount of nickel discussed herein refers to the amount of nickel in the electrocatalyst relative to the amount of ion conducting oxide in the electrocatalyst, e.g., Ni50-GDC corresponds to 50% Ni and 50% GDC on an atom basis (i.e., 50% Ni atom and 50% GDC compound) within the infiltrated electrocatalyst.
Other aspects of the disclosure are based on the discovery by the present inventors that addition of ion-conductor composition to SFCM-based anodes allow SOFCs to operate at a significantly lower temperature (e.g., less than about 800° C., typically less than about 750° C., often less than about 700° C., more often less than about 650° C., and most often about 600° C. or less) compared to conventional SOFCs.
Still other aspects of the disclosure provide a solid oxide fuel cell comprising: a cathode layer, an anode layer, and an electrolyte layer located between the cathode layer and the anode layer. Typically, the anode layer is any anode layer disclosed herein.
One particular aspect of the disclosure provides a stable ceramic anode composition for a solid oxide fuel cell (SOFC) having a porous surface. The stable ceramic anode composition comprises:
Yet in other embodiments, a ratio of SFCM to said ion-conductor composition is from about 5:1 to about 1:1 by weight. Still in other embodiments, said rare-earth metal is a lanthanide metal. Yet in other embodiments, said lanthanide metal comprises Gd, Pr, Nd, Sm, Y, La, Eu, Tb, Dy, Ho, Er, Tm, Yb, or a combination thereof. In one particular embodiment, said lanthanide metal comprises Gd, Pr, Nd, Sm, or a combination thereof.
In another embodiment, the total amount of said electrocatalyst composition is less than about 10 wt % relative to a total amount of said ceramic anode composition, typically about 8 wt % or less, often about 5 wt % or less, and most often less than about 5 wt %. In one particular embodiment, said electrocatalyst composition comprises nickel or an alloy of nickel in combination with a second ion-conductor comprising an oxide of cerium or cerium that is doped with a rare-earth metal. Still in other embodiments, said nickel alloy comprises cobalt, iron, tin, other transition metals, or a combination thereof. In one particular embodiment, said nickel alloy comprises cobalt, iron, tin, or a combination thereof.
In one specific embodiment, said electrocatalyst comprises nickel-gadolinium cerium oxide (Ni-GDC). In some instances, nickel in the electrocatalyst has a particle size of about 200 nm or less, typically 100 nm or less, often 75 nm or less, and most often 50 nm or less.
Still in other embodiments, said ion-conductor composition comprises gadolinium-doped cerium oxide (GDC). In some instances, said GDC is doped with cobalt. While the amount of cobalt can vary, in one particular embodiment, the amount of cobalt in said GDC is about 10 wt % or less, typically about 5 wt % or less, and often about 2 wt % or less.
Other aspects of the disclosure provide a solid oxide fuel cell (SOFC) comprising:
In some embodiments, the solid oxide fuel cell further comprises a cathode functional layer (CFL) located between said cathode layer and said electrolyte layer. In some instances, said CFL comprises cobalt-doped GDC (Co-GDC). Still, in other embodiments, the CFL comprises (a) Fe, Cu, Bi, or a combination thereof and (b) GDC. Yet in another embodiment, the CFL comprises CeO2.
Still in other embodiments, said stable ceramic anode layer known to one skilled in the art including SFCM anodes disclosed in a commonly assigned U.S. Pat. No. 10,938,052, which is incorporated herein by reference in its entirety, in combination with nanoparticles of electrocatalyst comprising (a) a second ion-conductor and (b) nickel, an alloy of nickel, or a combination thereof. In one particular embodiment, the stable ceramic anode layer comprises:
Yet in other embodiments, the total amount of said electrocatalyst in said stable ceramic anode of said infiltrate is about 5% by weight or less. In one specific embodiment, said electrocatalyst comprises nickel and gadolinium cerium oxide (Ni-GDC). In some instances, the amount of nickel in the Ni-GDC electrocatalyst is about 25% or less on an atom basis.
Some aspects of the disclosure are based on the discovery by the present inventors of new ceramic anode compositions that were found to provide a significantly higher operational stability by limiting the amount of electrocatalyst infiltration. In some embodiments, the term “stable” refers to a ceramic anode that does not lead to SOFC failure after 20 or more, typically after 30 or more, often after 40 or more, still more often after 50 or more, and most often after 100 or more operational cycles or redox-cycles as determined by any of the methods disclosed herein. Alternatively, the term “stable” refers to a ceramic anode having a less than about 15%, typically about 10% or less, and often about 5% or less reduction of cell voltage in galvanostatic mode testing over a period of 200 h. Moreover, the term “stable” is used to denote increased time of anode operability, e.g., ceramic anode that can operate at least 95% of the initial cell voltage capacity (in galvanostatic mode testing) of at least about 10% or more, typically by at least about 25% or more, and often by at least about 50% or more operation time compared to a similar ceramic anode having the total amount of electrocatalyst of about 5 wt % or more. When referring to a numerical value, the terms “about” and “approximately” are used interchangeably herein and refer to being within an acceptable error range for the particular value as determined by one skilled in the art. Such a value determination depends at least in part on how the value is measured or determined, e.g., the limitations of the measurement system, i.e., the degree of precision required for a particular purpose. For example, the term “about” can mean within 1 or more than 1 standard deviation, per the practice in the art. Alternatively, the term “about” when referring to a numerical value can mean±20%, typically ±10%, often ±5% and more often ±1% of the numerical value. In general, however, where particular values are described in the application and claims, unless otherwise stated, the term “about” means within an acceptable error range for the particular value, typically within one standard deviation.
In one aspect of the disclosure, a stable ceramic anode composition for a solid oxide fuel cell (SOFC) is provided. In one particular embodiment, the stable ceramic anode composition comprises (i) strontium-iron-cobalt-molybdenum oxide material (SFCM); (ii) a first ion-conductor composition comprising an oxide of cerium or cerium that is doped with a rare-earth metal; and (iii) nanoparticles of an electrocatalyst comprising (a) a second ion-conductor and (b) nickel, a nickel alloy, or a combination thereof, wherein said nanoparticles are infiltrated within said porous surface of ceramic anode. Yet in other embodiments, the total amount of said electrocatalyst in said ceramic anode is less than about 10 wt %, typically 8 wt % or less, often about 7 wt % or less, and more often about 5 wt % or less.
In some embodiments, the amount of nickel, nickel alloy, or a combination thereof in the electrocatalyst is less than 50% on an atom basis, typically about 25% or less on an atom basis, often about 20% or less on an atom basis, more often about 15% or less on an atom basis, and most often about 10% or less on an atom basis.
Yet in other embodiments, said second ion-conductor of electrocatalyst comprises an oxide of cerium or cerium that is doped with a rare-earth metal. Still in other embodiments, said nickel alloy in said electrocatalyst comprises cobalt, iron, tin, or a combination thereof.
In one particular embodiment, said electrocatalyst comprises nickel and gadolinium cerium oxide (Ni-GDC). Within this embodiment, in some instances the ratio of nickel to gadolinium cerium oxide in said electrocatalyst is less than about 1:2, typically about 1:4 or less, often 1:6 or less, more often about 1:8 or less, and most often about 1:9 or less.
Still in further embodiments, said SFCM oxide material is of the formula: SrFexCo((1-x)/2)Mo((1-x)/2)O3±d, wherein x is 0.1-0.5 and d is 0-1.5. In general, the SFCM oxide material can be any SFCM oxide material that is known to one skilled in the art including, but not limited to, those disclosed in a commonly assigned U.S. Pat. No. 10,938,052, which has previously been incorporated by reference.
In further embodiments, the first ion-conductor composition comprises gadolinium-doped cerium oxide (GDC). In some embodiments, GDC is doped with cobalt. In some instances, the amount of cobalt in said GDC is about 10 wt % or less, typically about 5 wt % or less, and often about 2 wt % or less compared to the amount of GDC.
In general, the stable ceramic anode is not limited to SFCM oxide material. In fact, the scope of the disclosure includes all ceramic anodes known to one skilled in the art, provided that the total amount of nickel or electrocatalyst is within those prescribed herein. Furthermore, the nickel or the electrocatalyst present in the stable ceramic anode should be such that they are present in the surface pores of ceramic anodes. More specifically, nickel or the electrocatalyst should be present as infiltrates of surface pores and should be nanoparticles. One of the key requirements of having a stable ceramic anode is to provide electrocatalyst or nickel infiltrates at a sufficiently low enough concentration to allow a long-term operation of the anode without cracking or diminishing its performance, e.g., cell voltage.
Another aspect of the disclosure provides stable SFCM anode-supported SOFC. In some embodiments, stable SFCM anodes of the invention operate efficiently in low-temperature range, e.g., less than about 800° C., typically about 750° C. or less, often about 700° C. or less, more often about 650° C. or less, and most often 600° C. or less.
In some embodiments, the first ion-conductor composition comprises an oxide of cerium or cerium that is doped with a rare-earth metal. Exemplary rare-earth metals include yttrium, scandium, and the lanthanide metals. In some embodiments, ion-conductor composition comprises a lanthanide metal comprising Gd, Pr, Nd, Sm, Y, La, Eu, Tb, Dy, Ho, Er, Tm, Yb, or a combination thereof. In one particular embodiment, said lanthanide metal comprises Gd, Pr, Nd, Sm, or a combination thereof.
Still, in other embodiments, the ratio of SFCM to said first ion-conductor composition ranges from about 5:1 to about 1:1 by weight, typically from about 4:1 to about 3:1 by weight, and often from about 2:1 to 1:1 by weight. In one specific embodiment, the ratio of SFCM to said first ion-conductor composition is about 2:1 by weight.
In further embodiments, a surface of said ceramic anode composition is porous. This porosity allows infiltration of said electrocatalyst on the surface of the stable ceramic anode.
The average pore size of the ceramic anode surface can range from about 5 μm to about 10 μm. However, it should be appreciated that the scope of the present disclosure is not limited to these particular average pore size. In fact, any pore size that is sufficient to accommodate the electrocatalyst can be used. In general, however, the pore size on the surface of the ceramic anode composition should be sufficiently large enough to accommodate the electrocatalyst but sufficiently small enough to prevent dislodging of the electrocatalyst.
As discussed herein, in some embodiments, the ceramic anode composition of the disclosure also includes nanoparticles of the electrocatalyst. The electrocatalyst is infiltrated within the pores of the ceramic anode surface. As used herein, the term “infiltrated” refers to putting the electrocatalyst inside the pores, typically as a liquid precursor, and calcinating or heating the resulting composition. Typically, the electrocatalyst occupies the pore spaces on the surface of the ceramic anode. One can place the electrocatalyst within the pores of the ceramic anode using any of the methods known to one skilled in the art. One particular example is to dissolve the electrocatalyst in a solvent and allow the solution to penetrate the pores and remove the solvent, e.g., by drying using heat and/or vacuum, or simply by letting the solvent evaporate. In some embodiments, the electrocatalyst is a nanoparticle such that it infiltrates the surface pores of the ceramic anode. Generally, the average particle size of nickel in the electrocatalyst is about 200 nm or less, typically about 100 nm or less, often about 75 nm or less, and most often about 50 nm or less. In some instances, the total amount of the electrocatalyst is less than about 10 wt % or less, typically less than about 8 wt %, often about 7.5 wt % or less, more often about 5 wt % or less, and most often less than about 5% relative to a total amount of said ceramic anode composition. Yet in other embodiments, the electrocatalyst comprises a nickel alloy. In some instances, nickel alloy comprises cobalt, iron, tin, or any other transition metal, or a combination thereof.
In further embodiments, the second ion-conductor comprises an oxide of cerium or cerium that is doped with a rare-earth metal. In one particular embodiment, the second ion-conductor is gadolinium-doped cerium oxide (GDC).
In one specific embodiment, the electrocatalyst comprises nickel-gadolinium cerium oxide (Ni-GDC). Yet in other embodiments, the electrocatalyst or nickel in said electrocatalyst has a particle size of about 200 nm or less, typically 100 nm or less, often 75 nm or less, and most often 50 nm or less. Without being bound by any theory, based on extensive research by the present inventors, one of the reasons for the failure of anodes in SOFCs is due to the change in the size of nickel that is present in the electrocatalyst during its operation. In particular, it is believed the particle size of nickel when combined with oxygen to form nickel oxide increases and when it is reduced to nickel the particle size is reduced. Still in other instances, the present inventors have observed nickel agglomeration leading to large nickel particles during operation can lead to SOFC failures. This expansion and reduction and/or agglomeration of nickel during SOFC operation is believed to be one of the main reasons that creates cracks, and hence failure, of anodes as well as decreased efficiency. As used herein, the terms “decreased efficiency” and “decreased performance” are used interchangeably herein and refer to the reduction of cell voltage in galvanostatic mode testing over a period of 200 h of about 10% or more, typically about 15% or more, and often 20% or more.
As stated above, at a certain nickel concentration, when infiltrated nickel particles are close together, operation of SOFC leads to formation of nickel agglomerates, thereby increasing the likelihood of anode failure and decreased anode performance or efficiency. Accordingly, to reduce the likelihood of agglomerate formation, the amount of nickel present in Ni-GDC, or other electrocatalyst with nickel, is less than about less than 50% on an atom basis, typically about 25% or less on an atom basis, often about 20% or less on an atom basis, more often about 15% or less on an atom basis, and most often about 10% or less on an atom basis. Still in other embodiments, the ratio of Ni to GDC in the electrocatalyst is about 1:2 or less on an atom basis, typically about 1:3 or less on an atom basis, often about 1:4 or less on an atom basis, more often about 1:5 or less on an atom basis, still more often 1:7 or less on an atom basis, and most often about 1:9 or less on an atom basis. Still in some embodiments the amount of nickel present in Ni-GDC is such that even after 200 h of operation under galvanostatic mode the change in the particle size of nickel using SEM (i.e., initial vs. 200 h) is less than about 25%, typically less than about 20%, often less than about 15%, more often less than about 10%, still more often less than about 5%, and most often less than about 2%.
Yet in other embodiments, said first ion-conductor composition in the ceramic anode comprises gadolinium-doped cerium oxide (GDC), i.e., the ceramic anode composition comprises SFMC-GDC. In some instances, said GDC of ion-conductor composition is doped with cobalt. While the amount of cobalt can vary, in one particular embodiment, the amount of cobalt in said GDC is about 10 wt % or less, typically about 5 wt % or less, and often about 2 wt % or less.
Other aspects of the disclosure provide a solid oxide fuel cell (SOFC) comprising: (a) a cathode layer; (b) a stable ceramic anode layer having a less than 15% cell voltage reduction in galvanostatic mode over a period of 200 h; and (c) an electrolyte layer located between said cathode layer and said ceramic anode layer. In one particular embodiment, the stable ceramic anode layer comprises strontium-iron-cobalt-molybdenum (SFCM) and (ii) the first ion-conductor composition comprising an oxide of cerium or cerium that is doped with a rare-earth metal. In one particular embodiment, the first ion-conductor composition of said ceramic anode layer comprises gadolinium-doped cerium oxide.
Still, in other embodiments, the ceramic anode layer of the SOFC comprises a porous surface containing nanoparticles of an electrocatalyst. In some instances, the electrocatalyst comprises Ni-GDC. The amount of nickel relative to GDC in Ni-GDC is less than about 50% on an atom basis, typically about 40% or less on an atom basis, often about 30% or less on an atom basis, more often about 20% or less on an atom basis, and most often about 10% or less on an atom basis.
In further embodiments, the amount of electrocatalyst in the stable ceramic anode is less than 10 wt %, typically about 8 wt % or less, typically about 7 wt % or less, and often about 5 wt % or less.
Yet in other embodiments, the solid oxide fuel cell further comprises a cathode functional layer (CFL) located between said cathode layer and said electrolyte layer. In some instances, said CFL comprises cobalt-doped GDC (Co-GDC). Without being bound by any theory, it is believed that the CFL increases the active area for oxygen reduction and charge transfer through the cathode/electrolyte interface, thereby improving performance of SOFC. In addition, it is believed that the presence of CFL also increases the stability of the cathode from thermal stresses during SOFC operation.
Still, in other embodiments, the surface of said ceramic anode layer is infiltrated with nanoparticles of nickel gadolinium-cerium oxide. Surface modification of conductive ceramic anodes using nanostructured electrocatalysts, e.g., infiltration of nanoparticles of electrocatalyst to the ceramic anode, is a facile approach to introduce catalytic activity which in turn can make them appropriate for LT-SOFCs. Nanoscale surface modifications using an electrocatalyst offers a variety of advantages including, but not limited to, suppressing carbon formation or coking when a hydrocarbon-based fuel, such as CH4, propane, an alcohol, etc. is used as a fuel source, enhancing fuel tolerance to sulfur, and facilitating oxidation of hydrocarbon fuels at lower temperatures, as well as providing other advantages. In addition, the porous ceramic support structures allow expansion and contraction of nanosized Ni, thereby preventing failure of Ni-GDC supported cells during a long-term operation or due to redox cycling instability.
In some aspects of the disclosure, a stable SFCM (e.g., SrFe0.2Co0.4Mo0.4O3) ceramic anode that is infiltrated with nanoparticles of Ni-GDC is used instead of conventional Ni anodes to provide LT-SOFCs. SFCM-anode supported SOFC has shown to be stable over a number of redox cycles with a high open circuit voltage (OCV) of 0.89 V and a peak power density (PPD) of 500 mW/cm2 at 600° C. in H2/3% H2O. Detailed study by the present inventors also revealed that oxygen non-stoichiometry of SFCM compensates for the dimensional changes during redox cycles. Without being bound by any theory, it is believed that the dimensional changes during redox cycles can result in the formation of cracks in conventional nickel or Ni-cermet anodes, thereby leading to SOFC failure. In examples of the present disclosure, ceramic anodes comprising SFCM infiltrated with a limited amount of nanoparticles of nickel or electrocatalyst have shown to be superior anodes for LT-SOFC technology.
Fuel flexibility is a unique feature of SOFCs. The instability of conventional Ni-based cermet anodes in hydrocarbon fuels has thus far impeded the advancement of low-temperature solid oxide fuel cells (LT-SOFCs). The present inventors have demonstrated that in some embodiments, highly stable LT-SOFCs prepared by catalytically modifying the surface of a conductive ceramic oxide, e.g., SFCM, using nanoparticles of electrocatalyst, can readily overcome many limitations of conventional Ni-based cermet anodes. In some embodiments, the nano-sized Ni-GDC electrocatalysts, resulting from Ni-to-GDC ratio disclosed herein, and subsequent low-temperature calcination process, enhance the fuel oxidation kinetics and stability of SFCM anode significantly. For example, an electrocatalyst with Ni-to-GDC ratio of 1:9 on SFCM-supported SOFC can deliver peak power density of about 0.60 W/cm2 or higher, typically about 0.65 W/cm2 or higher, often about 0.70 W/cm2 or higher, and most often about 0.75 W/cm2 or higher at 650° C. in humidified H2. In other embodiments, a Ni-to-GDC ratio of 1:9 on SFCM-supported SOFC can deliver peak power density of about 0.50 W/cm2 or higher, typically about 0.55 or W/cm2 higher, often about 0.60 W/cm2 or higher, and most often about 0.65 W/cm2 or higher at 600° C. in humidified Hz. Yet in other embodiments, a Ni-to-GDC ratio of 1:9 on SFCM-supported SOFC can deliver peak power density of about 0.20 W/cm2 or higher, typically about 0.25 or W/cm2 higher, often about 0.30 W/cm2 or higher, and most often about 0.35 W/cm2 or higher at 550° C. in humidified H2.
In further embodiments, in CH4/H2 gas mixtures as an example of hydrocarbon gas performance, a Ni-to-GDC ratio of 1:9 on SFCM-supported SOFC can deliver peak power density of about 0.45 W/cm2 or higher, typically about 0.50 W/cm2 or higher, often 0.55 W/cm2 or higher and most often about 0.60 W/cm2 or higher at 650° C. Yet in other embodiments, under this gas mixture, a Ni-to-GDC ratio of 1:9 on SFCM-supported SOFC can deliver peak power density of about 0.25 W/cm2 or higher, typically about 0.30 W/cm2 or higher, often 0.35 W/cm2 or higher, and most often about 0.40 W/cm2 or higher at 600° C. Still in other embodiments, under this gas mixture a Ni-to-GDC ratio of 1:9 on SFCM-supported SOFC can deliver peak power density of about 0.10 W/cm2 or higher, typically about 0.15 W/cm2 or higher, often about 0.20 W/cm2 or higher, and most often about 0.25 W/cm2 or higher at 550° C.
Surprisingly and unexpectedly, stable ceramic anodes comprising nanoparticles of infiltrated electrocatalyst (e.g., Ni-GDC) can maintain a stable cell voltage of 0.82V over at least about 100 h, typically over at least 150 h, often over at least 200 h, still more often over at least 250 h, and most often over at least 300 h of operations (under current) at 600° C. in CH4/H2 gas mixtures. As used herein, the term “stable” refers to maintaining cell voltage within about 15%, typically within about 10%, often within about 5%, and most often within about 3% of the initial cell voltage.
Additional objects, advantages, and novel features of this invention will become apparent to those skilled in the art upon examination of the following examples thereof, which are not intended to be limiting. In the examples, procedures that are constructively reduced to practice are described in the present tense, and procedures that have been carried out in the laboratory are set forth in the past tense.
Example 1: SFCM-GDC anode vs. Ni-GDC Anode: This example shows comparative examples of SFCM-GDC anode vs. Ni-GDC anode.
SFCM Synthesis: SFCM was prepared by a conventional solid-state synthesis route. Stoichiometric amounts of strontium carbonate (SrCO3, Sigma-Aldrich), iron oxide (Fe2O3, Sigma-Aldrich), cobalt oxide (Co2O3, Inframat-Advanced Materials), and molybdenum oxide (MoO3, Alfa Aesar) were ball-milled in ethanol medium for 24 h. The resulting mixed oxides were then dried in an oven to evaporate ethanol. The dried powder was heat-treated at 1100° C. for 4 h. The phase purity was determined using a Bruker D8 powder X-ray diffractometer (XRD) with Cu Kα radiation. The XRD data show a pure perovskite phase.
Fabrication of Ni-GDC Anode-Supported SOFCs: Ni-GDC anodes for comparison with the SFCM-GDC anode were prepared using a tape-casting method. The tape-casting recipe for the Ni-GDC anode-supported SOFC constitute ethanol (solvent), Menhaden fish oil (dispersant), polyvinyl butyral (PVB; binder), benzyl butyl phthalate, and BBP (plasticizer). The SOFC configuration comprises 30 μm-thick GDC tape laminated with NiO-GDC anode functional layer (AFL) tape and NiO-GDC anode-support layers (ASLs). The lamination was performed using a hydraulic hot press. The laminated tapes were stepwise heat-treated to burn out organic binders and sintered at 1450° C. for 4 h. The resulting half-cell consisted of a 550 μm-thick porous NiO-GDC ASL scaffold and an ˜20 μm dense GDC electrolyte. The LSCF-GDC cathode was then deposited and sintered at 1100° C. for 2 h to form the full SOFC button cells.
Fabrication of Ceramic Anode-Supported SOFCs: An anode-supported SOFC configuration was used with an anode support consisting of a SFCM-GDC composite in a weight ratio of 2:1. GDC used in making the SFCM-GDC composite anode was separately doped with 2 mol % Co2O3 to adjust the shrinkage mismatch with the electrolyte. A cathode functional layer (CFL) consisting of 2 wt % Co-modified GDC (Co-GDC) was applied between the Sm0.5Sr0.5CoO3-GDC (SSC-GDC) cathode and GDC electrolyte.
The tape-casting method was used to prepare the porous SFCM-GDC scaffold. The tape-casting recipe constitutes 80% ethanol as the solvent, 1.6% Menhaden fish oil as the dispersant, 17.6% polyvinyl butyral (PVB) as the binder, 10% benzyl butyl phthalate (BBP) as the plasticizer, and poly(methyl methacrylate) (PMMA) as the pore-former (16 wt %). All percentages are relative to the SFCM-GDC solid content. For making dense GDC tape, the 30 μm thick as prepared GDC tape was laminated with prelaminated SFCM-GDC sheets using a hot press. The laminated tapes were stepwise heat treated to burn out the PMMA pore formers and an organic binder followed by sintering at 1200° C. for 4 h. The resulting half-cell consists of a porous SFCM-GDC scaffold and an ˜20 μm dense GDC electrolyte. The prepared half-cell was then deposited (doctor-blade method) with the CFL and dried in an oven (100° C.) for 2 h followed by deposition of the SSC-GDC cathode. The assembly was then sintered at 950° C. for 2 h. Compared to conventional high-temperature sintering for Ni-GDC-based cells (˜1450° C.), the entire processing of SFCM-based cells requires a relatively low sintering temperature of 1200° C. in air, reducing cost and process complexities.
Infiltration of Ni-GDC Nanoparticles: The prepared anode-supported full cells were then infiltrated with the NiO-GDC precursor on the anode side. The required quantity of NiO-GDC (Ce0.9Gd0.1O2-δ) precursor was prepared by dissolving nitrates of nickel, cerium, and gadolinium (Alfa Aesar) in H2O. A few drops of the infiltrate solution were added on the porous scaffold of samples. The samples were then kept under vacuum for 10 min. Between each successive infiltration step, the sample was heat-treated at 400° C. for an hour to decompose the nitrates. The infiltration cycles were repeated to get an approximate loading of ˜10 wt % in all the samples.
Electrochemical Measurements: I-V characteristics and power densities of SOFCs were determined using Solartron 1470E. Electrochemical impedance spectroscopy (EIS) was performed using a Solartron 1425 frequency response analyzer. The electrochemical performance including I-V characteristics and impedance spectra was determined using a fuel cell testing fixture loaded with the SOFC. A commercial two-part sealant (ceramabond-517, Aremco) was used to separate the anode and cathode compartments. The anode and cathode side of the cells were exposed to 100 mL/min of humidified H2 and synthetic air, respectively. The active area of the fuel cell was 0.31 cm2. Gold wires and silver paste were used as leads and the current collector, respectively, for all the SOFCs characterized in this study.
Scanning electron microscopy (SEM) of the post-tested SOFCs was performed using a Hitachi SU-70 with a field-emission gun equipped with a Bruker XFlash silicon drift EDS detector.
Results and Discussion
The performance of Ni-GDC-infiltrated SFCM-GDC anodes was investigated using the anode-supported SOFC configuration (porous anode support|electrolyte|cathode functional layer (CFL)|cathode).
The SOFC under investigation was treated under two different conditions, as shown in
Although the OCV of the SFCM-GDC cell decreased slightly after the third redox cycle, it remained stable up to 30 cycles in both conditions. In contrast, the OCV of the Ni-GDC cell gradually decreased throughout seven cycles and at the eighth cycle the OCV decreased to 0.75 V, indicating initiation of cell failure.
SFCM-GDC has been successfully demonstrated herein to survive high-temperature sintering processes in air and is compatible with the low-temperature GDC electrolyte. SFCM provides the necessary high electronic conductivity otherwise provided by Ni and is the first to demonstrate redox stability at the full cell level. SFCM does not, however, have the catalytic activity toward hydrogen oxidation that Ni does, particularly for low-temperature SOFC operation (<600° C.). However, SFCM can provide a stable scaffold with the required electronic conductivity and the infiltrated nano-Ni-GDC particles can then provide the required catalytic activity, resulting in stable high-performance SFCM-GDC-based anode-supported cells. EIS was performed on both Ni-GDC and SFCM-GDC cells.
Conclusion: Experimental results suggest that SFCM based ceramic oxide is a unique, redox stable anode-material for LT-SOFCs. Further, SFCM-GDC cells show robust SOFC operation after 30 redox cycles under load at 600° C. in H2/3% H2O. Unlike the commonly used Ni-GDC, ceramic anodes retained the dimensional stability and microstructure as that of pristine SFCM-GDC even under harsh cycling conditions. Oxygen stoichiometry of ceramic SFCM plays an important role in maintaining the dimensional stability after multiple redox cycles. The excellent redox stability and performance of SFCM based ceramic anode-supported cell makes it an excellent replacement for Ni based LT-SOFC anodes. The surprising and unexpected dimensional stability and performance of SFCM ceramic anode is viewed as significant progress towards replacement of Ni anode, which provides access to LT-SOFC technology.
Example 2: SFCM-GDC anode with infiltrated second ion-conductor composition: This example shows stability of SFCM ceramic anode having Ni-GDC nanoparticle infiltrates on its surface. This embodiment is schematically illustrated in
Synthesis and characterization of SFCM: SFCM was prepared by conventional solid-state synthesis method. Stoichiometric amounts of strontium carbonate (SrCO3, Sigma-Aldrich), iron oxide (Fe2O3, Sigma-Aldrich), cobalt oxide (Co2O3, Inframat-Advanced Materials) and molybdenum oxide (MoO3, Alfa-Aesar) were ball-milled in an ethanol medium for 24 h. The resulting mixed oxides were then dried in an oven to evaporate the ethanol. The dried powder was heat-treated at 1100° C. for 4 h. The phase purity was determined using a Bruker D8 powder X-ray diffractometer (XRD) with Cu Kα radiation.
DC electrical conductivity of SFCM was measured on a bar sample (length=7.5 mm, width=2.8 mm and height=1.6 mm). The bar sample was prepared by compacting SFCM powder using a rectangular die and sintered at 1350° C. for 4 h in air. The density of the sintered sample is 96%, determined by Archimedes' principle using Mettler Toledo's Density measurement apparatus. For DC conductivity measurements using Keithley 2400 source meter, silver wires and paste were used as leads and current collector, respectively. The sample was placed in a reactor that operated under a controlled gas environment. The samples were first heated up to 650° C. in humidified 10% H2/N2 (pure H2 was not used for safety considerations) and kept overnight under the reducing gas conditions. Before taking measurements, 30 min stabilization time was provided at each interval and electrical measurements were made at 50° C. intervals down to 400° C.
Anode-supported SOFCs electrochemical characterization: An anode-supported electrolyte configuration was used to determine the SOFC characteristics. The anode support consisted of SFCM-GDC composite in the weight ratio of 2:1. SFCM-GDC composites are used for the anode to increase the triple phase boundary (TPB) length and help match the thermal expansion coefficient between the anode and GDC electrolyte. Further, GDC of the SFCM-GDC composite was doped with Co (2 mol. % cobalt oxide) to adjust the rate of shrinkage. A cathode functional layer (CFL) consisting of Co-modified GDC (Co-GDC) was used at the interface of Sm0.5Sr0.5CoO3-GDC (SSC-GDC) cathode and GDC electrolyte.
The tape-casting method was used to prepare the porous SFCM-GDC scaffold. Tape-casting recipe constituted 80% ethanol as solvent, 1.6% Menhaden fish oil as dispersant, 17.6% polyvinyl butyral (PVB) as binder, 10% benzyl butyl phthalate (BBP) as a plasticizer and poly (methyl methacrylate), PMMA as pore-former (16 wt. %). All percentages are relative to the SFCM-GDC solid content. The recipe was cast as a film of thickness 110 μm on a Mylar sheet and laminated using a hot press to achieve the desired thickness. Dense GDC tape was then laminated with already pre-laminated SFCM-GDC sheets using the hot press. The laminated tapes were stepwise heat-treated to burn out the PMMA pore-formers and an organic binder, followed by sintering at 1200° C. for 4 h. The half-cell that resulted consisted of the porous SFCM-GDC scaffold and ˜20 μm-thick dense GDC electrolyte. The prepared half-cell was then deposited (doctor-blade method) with Co-GDC cathode functional layer and dried in an oven (100° C.) for 2 h, followed by deposition of SSC-GDC cathode. The assembly was then sintered at 950° C. for 2 h. Co-GDC and SSC-GDC were formulated as screen printing ink using Thinky Mixer (ARE-310) and a commercial ink making vehicle (ESL ElectroScience, type 441).
The prepared anode-supported full cells were then infiltrated with NiO-GDC precursor on the anode side. NiO-GDC (Ce0.9Gd0.1O2-δ) precursor was prepared by dissolving nitrates of nickel, cerium, and gadolinium in H2O. Five compositions were prepared GDC (Ce0.9Gd0.1O2-δ), Ni10-GDC (10% Ni—Ce0.9Gd0.1O2-δ), Ni50-GDC (50% Ni—Ce0.9Gd0.1O2-δ), Ni75-GDC (75% Ni—Ce0.9Gd0.1O2-δ), and Ni. A few drops of infiltrate solution were added on the porous scaffold and kept under vacuum for 10 minutes. Between each successive infiltration step, the sample was heat-treated at 400° C. for an hour to decompose the nitrate salts. The infiltration cycles were repeated to get approximate loading of ˜5 wt. % in all samples.
I-V characteristics and power density of the SOFCs (with an area 0.31 cm2) were determined using Solartron 1470E. The electrochemical impedance spectroscopy (EIS) response of the cells was obtained using a Solartron 1425 frequency response analyzer. The electrochemical performance including I-V and impedance spectra were determined using a fuel cell testing fixture loaded with the SOFC. Commercial two-part sealant (Ceramabond-517, Aremco) was used to seal anode and cathode compartments. The anode and cathode gas flow rates were 100 ml/min of humidified CH4/H2 gas mixture (CH4:H2 ratio of 1:5) and air, respectively. Impedance spectra were measured by 4-point probe method over a frequency range from 100 mHz to 100 kHz under open-circuit voltage (OCV) conditions. The impedance spectra were fitted with an equivalent circuit model using the code ZsimpWin with a complex non-linear square fitting routine (CNLS). Gold wires and silver paste were used as leads and the current collector, respectively for all SOFCs characterized in this study. Scanning electron microscopy (SEM) of the post-tested SOFCs was performed using a Hitachi SU-70 with a field emission gun equipped with a Bruker XFlash silicon drift EDS detector.
Results and Discussion
Surface modification of conductive ceramic anodes using nanostructured electrocatalysts is a facile approach to introduce catalytic activity which in turn can make them appropriate for LT-SOFCs. Nanoscale surface modifications using an electrocatalyst can suppress carbon formation, enhance fuel tolerance to sulfur, and facilitate oxidation of hydrocarbon fuels at lower temperatures. In addition, the porous ceramic support structures allow expansion and contraction of nanosized Ni, preventing the failure of Ni-GDC supported cells due to redox cycling instability.
This example demonstrates the performance of SFCM based anode-supported SOFCs at the temperature ranges between 550° C. to 650° C. The anode side of the SOFC was infiltrated with Ni-GDC to incorporate catalytic activity in ceramic oxides for fuel oxidation. The SOFC performance and characteristics were determined by varying the Ni concentration (modifying the Ni-to-GDC ratios) that was infiltrated in the SFCM ceramic scaffold. The long-term stability was determined over a period of 200 h in CH4/H2 gas mixtures for Ni10-GDC (low Ni-content, Ni-to-GDC ratio of 1:10), Ni50-GDC (high Ni-content, Ni-to-GDC ratio of 5:10) and Ni only infiltrated cells. The total Ni content in the anode of these infiltrates is <5 wt. % as compared to ˜50 wt. % of the typical Ni-cermet anode used to internally reform CH4. SOFCs using this infiltrate SFCM anode are expected to utilize externally reformed CH4 and as such a CH4/H2 gas mixture was used to evaluate SOFCs with SFMC having Ni-GDC infiltrates. The long-term microstructural stability of the SOFCs was also investigated in terms of the particle size distribution and resulting agglomeration based on various Ni-to-GDC ratios.
As discussed in detail below, the composition and the particle size of the infiltrated nanoparticles greatly affected the performance and long-term stability of the SOFCs. Some Ni-GDC infiltrated SOFCs showed peak power density of 0.75, 0.65 and 0.36 W/cm2 at 650° C., 600° C., and 550° C., respectively, in humidified H2 and 0.62, 0.39 and 0.22 W/cm2 at 650° C., 600° C., and 550° C., respectively, in CH4-containing gases. A stable cell voltage of 0.82V is obtained over 200 h operations at a constant current of 0.2 A/cm2 at 600° C. in CH4-containing gases. The results show that highly stable SOFCs can be produced by carefully adjusting the quantity of Ni in the Ni-GDC infiltrate composition and by suppressing the grain growth of Ni nanoparticles.
The crystal structure of SFCM is similar to that of a double perovskite oxide Sr2CoMoO6 (SCMO) or Sr2FeMoO6 (SFMO). The XRD data show the pure cubic double-perovskite phase without any impurities or secondary phases, e.g., SrMoO4 or SrFeO3-δ. The hopping of electrons in SCMO crystal lattice is sluggish; however, with the introduction of Fe-cation, the hopping of free electrons is significantly faster resulting in high electrical conductivity. The temperature-dependent electrical conductivity of SFCM in reducing gas conditions is shown in
The anode-supported SOFC under investigation is shown in
At low-temperatures, SFCM-based anodes have a negligible catalytic activity for fuel oxidation. Thus, a Ni-GDC electrocatalyst was infiltrated into the porous SFCM scaffold to introduce catalytic activity for fuel oxidation. Current-voltage (“I-V”) characteristics and power density curves of Ni10-GDC infiltrated SFCM measured in H2/H2O and CH4/H2 gas mixtures are shown in
The impedance spectra under OCV measured in CH4/H2 gas atmosphere are shown in
The performance of the SFCM based anode-supported SOFCs depend on the Ni-to-GDC ratio that was infiltrated in the porous support.
As shown in
The ohmic ASR shown in
I-V characteristics and EIS measurements at initial, 100th and 200th hour are shown in
For the Ni50-GDC infiltrated SFCM cell, the I-V characteristics and impedance spectra are shown in
The long-term performance of Ni10-GDC, Ni50-GDC, and Ni infiltrated SFCM cells are compared in
SEM images with the schematic sketches depicting the grain growth/particle sizes of Ni-GDC on SFCM-GDC porous scaffold are shown in
The particle size distributions of Ni, Ni50-GDC and Ni10-GDC electrocatalysts on SFCM-GDC ceramic anodes, determined by using SEM images are shown in
Conclusion: Anode-supported SOFCs were made with a SFCM-GDC ceramic anode infiltrated with an electrocatalyst having various Ni-to-GDC ratios. A composition with low Ni content (Ni-to-GDC ratio of 1:10) showed the high SOFC performance of 0.75, 0.65 and 0.36 W/cm2 at 650° C., 600° C., and 550° C., respectively, in humidified Hz. Further, the performance and long-term stability of the SOFCs were determined in CH4/H2 gas mixtures. The same composition (Ni-to-GDC ratio of 1:10) showed stable performance without degradation over a period of 200 h. It is believed that the reason for this improvement is due to suppressed particle growth of Ni nanoparticles coated or infiltrated on the surface of SFCM.
The foregoing discussion of the examples has been presented for purposes of illustration and description. The foregoing is not intended to limit the disclosure or claimed subject matter to the form or forms disclosed herein. Although the description included discussion of one or more example embodiments and certain variations and modifications, other variations and modifications are within the scope of the present disclosure, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which, include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter. All references cited herein are incorporated by reference in their entirety.
This application claims the priority to U.S. Provisional Application No. 63/080,452, filed Sep. 18, 2020, which is hereby incorporated by reference in its entirety.
This invention was made with government support under ARPA-E DE-AR0000494 and NETL DE-FE0027897 awarded by the Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63080452 | Sep 2020 | US |
