Patent Application
20250062379
The present invention relates to a doped cubic bismuth oxide that is phase stable in the temperature range of from about 550° C. to about 700° C. In particular, the doped cubic bismuth oxide of the disclosure has a higher conductivity than DWSB.
Solid oxygen-ion conductors have a wide variety of applications in biomedical, semiconductor, chemical, transportation, and energy industries. One of the examples of applications of solid oxygen-ion conductors in transportation industries is the oxygen sensor used to control air/fuel ratios in automobile internal combustion engines for improved fuel economy as well as the oxygen-storage component of the three-way catalytic convertor that utilized that air/fuel ratio control to reduce the NOx, CO, and unburned hydrocarbons pollutants in automotive exhaust. Controlling oxygen atmospheres is also critical in fabrication of semiconductors and numerous industrial chemical reactions. Ability to separate oxygen from air provides concentrated O2 and N2 industrial gasses and the opportunity for greater combustion efficiency with the prevention of NOx formation. In addition, ceramic oxygen generators (COGs) have allowed people with limited lung capacity mobility by electrochemically concentrating the oxygen available in air. Recently this technology was extended in providing exo-planet life-support by electrolytically reducing CO2 in the Martian atmosphere to breathable O2.
In the energy sector, solid oxide fuel cells (SOFCs) are being developed and commercialized for conversion of hydrogen and hydrocarbon fuels to electric power with greater efficiency and thus lower greenhouse gas (GHG) emissions; however, to achieve the desired efficiency SOFC electrolytes not only need high ionic conductivity but stability and low electronic conductivity across a wide pO2 range (0.21 to 10−21 atm). Unfortunately, the highest conductivity oxide electrolytes tend to have the lowest stability to reduction in low pO2 atmospheres.
By using Ce0.8Gd0.2O2 (GDC) and Bi1.6Er0.4O3 (ESB) in a bilayer configuration the present inventor was previously able to achieve stability with two high conductivity oxygen-ion electrolytes resulting in record high SOFC power densities (2 W/cm2) for lower temperature (650° C.) operation. By substituting a more conductive Bi1.76Dy0.16W0.08O3 (DWSB) for the ESB electrolyte, an additional 75% improvement in performance, up to 3.5 W/cm2 at 650° C., was projected to be achieved by the present inventor. These SOFCs have the potential to also operate in reverse as a solid oxide electrolysis cell (SOEC) for hydrogen production, and by combining SOEC and SOFC operation for long duration energy storage. A more general term used herein for all the above technologies is a solid oxide cell (SOC).
Unfortunately, the primary barrier to greater use of SOC technologies and development of new applications is the high temperatures (typically ≥800° C.) required for sufficient oxygen-ion conductivity. For example, traditional SOFCs/SOECs are comprised of a zirconia-based electrolyte that does not possess sufficient oxygen ion conductivity below 700° C. to generate high performance (see, for example, Arachi et al., Solid State Ionics 1999, 121, 133-139 and Skinner et al., Mater. Today 2003, 6, 30-37), whereas if the operational temperature was reduced to ≤600° C. manufacturing and processing costs and the mechanisms that cause degradation would be decreased concomitant with an increase in system efficiency (see, for example, Steele, J. Mater. Sci., 2001, 36, 1053-1068 and Wachsman et al., Science 2011, 80, 334).
Lowering the operational temperature of SOC will enable reduction in fabrication complexity and cost as well as faster start-up. Moreover, reduction in operational temperature of SOC would dramatically increase life-support applications, such as COGs, where the oxygen produced needs to be at ambient temperature for human consumption. Ultimately developing an electrolyte material that is stable at a relatively lower temperature with higher ionic conductivity would facilitate further application and development of SOCs.
Currently, among all solid oxygen-ion conductors the face centered cubic (FCC) phase of bismuth oxide (δ-81203) has exhibited the highest known oxygen ion conductivity of any material. See, for example, Jiang et al., Solid State Ionics, 2002, 150, 347-353 and Sammes et al., J. Eur. Ceram. Soc., 1999, 19, 1801-1826. Unfortunately, pure δ-Bi2O3 is not phase stable below 700° C. As a result, major research efforts in the 1980's were focused on stabilizing the cubic phase down to room temperature by doping rare earth cations into the Bi+3 site resulting in the development of ESB as the highest conductivity phase-stable oxygen-ion electrolyte. ESB held the record for decades until the present inventor and co-workers developed a co-doping strategy to stabilize δ-Bi2O3 with a reduced total dopant concentration in 2010 resulting in DWSB, which is the highest oxygen-ion conductivity material known to date. See, for example, Jung et al., Acta Mater., 2010, 58, 355-363 and Jung et al., J. Electrochem. Soc., 2016, 163, F411-F415.
Unfortunately, while the cubic phase of Bi2O3 can be stabilized down to room temperature, long range ordering of the anion sublattice is responsible for a considerable decay in conductivity when the electrolyte is annealed below 600° C. Jiang, N. et al., Mater. Lett., 1995, 22, 215-219. At temperatures above approximately 600° C., the oxygen vacancies in the cubic Bi2O3 lattice are disordered, while below 600° C. the oxygen vacancies order along the <111> direction and reduce the number of possible oxygen anion jump directions. Boyapati et al., Solid State Ionics, 2001, 140, 149-160. The decrease in anion equivalent jump directions causes a several orders of magnitude drop in conductivity that causes ESB and DWSB to be unsuitable as an SOC electrolyte below 600° C.
To avoid the instability that plagues the cubic phase of Bi2O3, other phases of Bi2O3 have been considered as SOC electrolytes. Unfortunately, the low conductivity of these materials usually renders them inadequate. Naturally occurring, low temperature polymorphs of bismuth oxide like β-Bi2O3 and γ-Bi2O3 display a conductivity that is orders of magnitude lower than the cubic phase. Harwig et al., J. Solid State Chem., 1978, 26, 265-274. The rhombohedral phase of Bi2O3 likewise exhibits much lower conductivity than the cubic phase. Takahashi et al., J. Appl. Electrochem., 1972, 2, 97-104 and Drache et al., J. Solid State Chem., 1999, 142, 349-359. The benefit of a lower symmetry phase of Bi2O3 resides in its stability at lower temperatures. Below 700° C., the rhombohedral phase of Bi2O3 has even been demonstrated to be thermodynamically favorable over the cubic phase, as the cubic lattice undergoes a transformation to the rhombohedral lattice when annealed for hundreds of hours. Fung et al., J. Am. Ceram. Soc., 1993, 76, 2403-2418 and Joshi, J. Mater. Sci., 1990, 25, 1237-1245.
Therefore, there is a continuing need for developing a stable conductivity solid electrolyte that can operate efficiently at a relatively lower operating temperature.
Some aspects of the disclosure are based on the discovery by the present inventors of a stable doped cubic Bi2O3 electrolyte having a high conductivity. One particular aspect of the disclosure provides a doped cubic bismuth oxide composition that is phase stable in a temperature range of from about 550° C. to about 700° C. The doped cubic bismuth oxide of the disclosure comprises a mixture of a first dopant and a second dopant. In some embodiments, a total amount of said first and said second dopant is about 15 mole % or less of a total metal content. Still in other embodiments, a total amount of said first and said second dopant is about 10 mole % or less of a total metal content.
Yet in other embodiments, said doped cubic bismuth oxide has a higher conductivity than Bi1.76Dy0.16W0.08O3 (DWSB) at 650° C. In some instances, said doped cubic bismuth oxide has at least about 20% higher conductivity than Bi1.76Dy0.16 W0.08O3 (DWSB) at 650° C. Still in other instances, said doped cubic bismuth oxide has at least about 30% higher conductivity than Bi1.76Dy0.16W0.08O3 (DWSB) at 650° C.
Still in further embodiments, said first dopant comprises La, Ce, Pr, Pa, U, or a mixture thereof. In other embodiments, said second dopant comprises Zr, Y, Nb, Sn, Hf, Ta, W, or a combination thereof.
In further embodiments, said doped cubic bismuth oxide is stable at 650° C. for at least 100 hours. As used herein, the term “stable” means the doped cubic bismuth oxide of the disclosure does not exhibit conductivity decay or decrease of 5% or more after 100 hours at 650° C.
Yet still in other embodiments, the total amount of said first and said second dopant in said doped bismuth oxide comprises about 10 mole % or less of the total metal content. Still in other embodiments, the amount of said first dopant ranges from about 2 mole % to about 8 mole % of the total metal content. In other embodiments, the amount of said second dopant is about 3 mole % or less of the total metal content.
Other aspects of the disclosure provide a solid-oxide cell (SOC) electrolyte comprising a doped cubic bismuth oxide composition disclosed herein. In some embodiments, the SOC electrolyte of the disclosure has an electrical conductivity of at least about 0.58 S/cm at a temperature of about 650° C. for at least about 100 hours.
Still other aspects of the disclosure provide a solid oxide fuel cell (SOFC) comprising a solid oxide cell electrolyte disclosed herein.
Further aspects of the disclosure provide a doped bismuth oxide composition comprising δ-phase bismuth oxide doped with a mixture of a first dopant and a second dopant. In some embodiments, the total amount of said first and said second dopant in said doped bismuth oxide comprises about 15 mole % or less of a total metal content. Still in other embodiments, said δ-phase bismuth oxide is stable at a temperature range of from about 550° C. to about 700° C. In other embodiments, said doped bismuth oxide is phase stable for at least 100 hours at a temperature of about 650° C. In one particular embodiment, said δ-phase bismuth oxide is phase stable for at least 100 hours at 650° C.
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 of ordinary skill in the art, which will depend 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” typically means within 1 standard deviation, per the practice in the art. Alternatively, the term “about” 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.
The term “stable” when referring to a phase of bismuth oxide means that at least about 75%, typically at least about 80%, often at least about 90%, more often at least about 95%, and most often at least about 98% of the initial crystal lattice structure of bismuth oxide is maintained under the given conditions. For example, at 550° C. for at least 100 h, typically at least 150 h, often at least about 200 h, and most often at least 300 h. Alternatively, at 650° C. for at least 100 h, typically at least 150 h, often at least about 200 h, and most often at least 300 h. Still in other embodiments, the term “stable” refers to having about 15% or less, typically 7% or less, often 5% or less, and most. often 3% or less decay in conductivity at 650° C. In another embodiment, the term “stable” refers to ability of bismuth oxides of the disclosure to maintain conductivity with no or minimal decay. For example, in some embodiments the term “stable” means bismuth oxides of the disclosure maintain stable conductivity within given conditions. with no more than about 25%, typically no more than about 20%, often no more than about 15%, more often no more than about 10%, and most often no more than about 5% loss or decay in conductivity (e.g., at 550° C. for at least 100 h, typically at least 150 h, often at least about 200 h, and most often at least 300 h).
The term “phase stable” refers to the crystal structure that does not change or changes no more than about 25%, typically no more than about 20%, often no more than about 15%, more often no more than about 10%, and most often no more than about 5% of at 650° C. after at least about 100 h, typically after at least about 150 h, often after at least about 200 h, and most often after at least 300 h.
Higher conductivity electrolytes are important for the reduction of operating temperature and increased viability of numerous solid-state oxygen-ion conducting technologies. Unfortunately, the highest oxygen ion conducting electrolyte, doped cubic bismuth oxide, is currently only useable above 600° C. Disclosed herein are tailored structures of doped bismuth oxide electrolytes that allow functionality at both high and low temperatures. At higher temperatures (e.g., ≥600° C.), a doped cubic Bi2O3 disclosed herein resulted in the highest oxygen-ion conductivity ever recorded to date for a phase stable electrolyte. Meanwhile, a doped rhombohedral Bi2O3 electrolyte disclosed herein showed exceptional stability, exhibiting no observable conductivity decay for 100 hours of aging at 500° C., thus making it the highest oxygen ion conducting electrolyte known with stable performance for lower temperature solid oxide cells (SOCs).
Solid oxide fuel cells (SOFCs) are a promising technology for efficient and high power energy conversion of hydrocarbon fuels. One of the main barriers to commercialization and widespread use is the high temperatures (≥800° C.) required for operation. Currently, the highest oxygen ion conducting electrolyte, cubic bismuth oxide, is only useable above 650° C. due to structural instability below this temperature.
Above approximately 730° C., pure Bi2O3 exists in the face centered cubic phase (FCC or δ-cubic) and exhibits a greater oxygen ion conductivity than any known oxygen ion conductor. As the operating temperature reaches below 730° C., Bi2O3 transitions through a set of lower symmetry phases including the monoclinic phase, which ultimately lowers the ionic conductivity by orders of magnitude. Traditionally, cations with a smaller ionic radius than Bi+3 are used to stabilize the high symmetry δ-cubic phase to room temperature to protect the highly conductive cubic phase. Although this substituted δ-cubic phase exhibits superior conductivity initially, below approximately 600° C. the oxygen ion sub-lattice begins to order and the ionic conductivity drops significantly over time. For example, 20% Er-doped Bi2O3 (ESB) is a standard doped Bi2O3 electrolyte in the δ-cubic phase that undergoes oxygen ion ordering, making it a poor candidate for a high performing solid oxide fuel cells (SOFCs) below 600° C. due to the inherent degradation of conductivity.
By decreasing the operational temperature below 600° C., it is possible to achieve reduced manufacturing and processing costs, an increase in theoretical efficiency, and a decrease in the mechanisms that cause degradation.
Some aspects of the disclosure are based on the discovery by the present inventors of stable conductivity electrolytes that can operate efficiently at a relatively lower operating temperature, e.g., about 650° C. or lower, typically about 600° C. or lower, and often about 550° C. or lower. Other aspects of the disclosure are based on the discovery by the present inventors that the phase of Bi2O3 is dependent on the size and amount of dopant present in the lattice. In particular, the present inventors have noticed that utilizing a particular average radius (e.g., <1.07 Å) of double dopants, as shown in Table 1, favors the δ-cubic phase, while an average radius of >1.07 Å of double dopants appears to favor the rhombohedral phase.
One particular aspect of the disclosure provides a stable doped δ-Bi2O3 having a high conductivity and stable performance at intermediate temperatures (e.g., 650° C.). In some embodiments, the dopant comprises a first dopant and a second dopant. Unlike ESB and DWSB, the total amount of metal dopants in this stable doped δ-Bi2O3 is less than about 20%, typically about 15% or less, and often about 10% or less relative to the total metal content. In other embodiments, the relative dopant ionic radius ranges from about 1.00 Å to about 1.10 Å, typically from about 1.00 Å to about 1.09 Å, often from about 1.01 Å to about 1.08 Å, more often from about 1.02 Å to about 1.07 Å, and most often from about 1.03 Å to about 1.07 Å. Typically, the first dopant has a relatively large ionic radius, whereas the second dopant has a relatively smaller ionic radius. The ionic radius of dopants can be readily determined by one skilled in the art having read the present disclosure.
The doped cubic bismuth oxide composition of the disclosure is phase stable in a temperature range of from about 550° C. to about 700° C., typically from about 550° C. to about 675° C., often from about 575° C. to about 675° C., more often from about 575° C. to about 650° C., and most often from about 600° C. to about 650° C. The doped cubic bismuth oxide of the disclosure comprises a mixture of a first dopant and a second dopant. In some embodiments, a total amount of said first and said second dopant is about 15 mole % or less of a total metal content. Still in other embodiments, a total amount of said first and said second dopant is about 10 mole % or less of a total metal content. In some embodiments, said first dopant ranges from about 1 mole % to about 9 mole %, typically from about 2 mole % to about 8 mole %, often from about 2 mole % to about 7 mole % of the total metal content. Still in other embodiments, the amount of said second dopant is about 8 mole % or less, typically about 6 mole % or less, often about 5 mole % or less, and most often about 3 mole % or less of the total metal content. In one particular embodiment, the first dopant comprises about 6-8 mole % and the second dopant comprises about 4-2 mole %, respectively, of the total metal content. Yet in other embodiments, the amount of said second dopant is about 3 mole % or less of the total metal content.
Yet in other embodiments, the first dopant comprises a cation of a lanthanide or an actinide series having a relatively large ionic radius, e.g., at least about 0.95 Å, typically at least about 1.00 Å, often at least about 1.01 Å, more often at least about 1.02 Å, still more often at least about 1.03 Å, and most often at least about 1.04 Å. In some embodiments, the first dopant comprises La, Ce, Pr, Pa, U, or a mixture thereof.
In some embodiments, the second dopant typically has a smaller ionic radius relative to the first dopant. In one particular embodiment, the ionic radius of the second dopant is about 1.00 Å or less, typically 0.98 Å or less, often 0.95 Å or less, more often 0.93 Å or less, still more often about 0.91 Å or less, and most often about 0.89 Å or less. In one particular embodiment, the second dopant comprises Zr, Y, Nb, Sn, Hf, Ta, W, or a combination thereof.
Surprisingly and unexpectedly, the doped cubic bismuth oxide of the disclosure has a significantly higher conductivity than Bi1.76Dy0.16W0.08O3 (DWSB) at 650° C. In some instances, the doped cubic bismuth oxide of the disclosure has at least about 10%, typically at least about 15%, often at least about 20%, more often at least about 25%, and most often at least about 30% higher conductivity than DWSB at 650° C. In further embodiments, said doped cubic bismuth oxide is stable at 650° C. for at least 100 hours. In one particular embodiment, the doped cubic bismuth oxide does not exhibit conductivity decay or decrease of 5% or more after 100 hours at 650° C.
Yet in other embodiments, the doped cubic bismuth oxide composition disclosed herein has conductivity of at least about 0.56 S/cm, typically at least about 0.57 S/cm, often at least about 0.58 S/cm, and most often at least about 0.59 S/cm at a temperature of about 650° C. for at least about 100 hours.
The polarizability of bismuth is higher than the dopant cations. Thus, in contrast to conventional methods where a relatively large amount (e.g., 20 mole % or higher) of dopants are used, by decreasing the total cation substitution resulted in increased lattice polarizability, thereby allowing for greater oxygen ion mobility and facilitated anion diffusion. Without being bound by any theory, it is believed that higher polarizability results in larger cation charge separation and improved anion diffusion due to the reduction in coulombic repulsion that oxygen ions experience when moving through the saddle point of the conductivity pathway. Accordingly, some aspects of the disclosure relate to increasing conductivity of doped bismuth oxide by using less than about 20 mole %, typically about 15 mole % or less, often about 10 mole % or less, and more often less than 10 mole % of the total dopant.
The doped bismuth oxides of the disclosure can be used in any of the application disclosed herein including in a wide variety of electronic devices such as, but not limited to, solid state gas sensors (e.g., to control air/fuel ratio and control emissions), ion-transport membranes for oxygen separation and purification (e.g., for biomedical and aerospace application), batteries, components of batteries (e.g., as a cathode). Other uses for bismuth oxides of the invention are readily recognized by one skilled in the art having read the present disclosure.
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.
The fabrication of all materials followed a standard solid state synthesis route. The raw materials include Bi2O3 (Alfa Aesar 99.99%), ZrO2 (Sigma Aldrich 99.9%), La2O3 (Alfa Aesar 99.99%), and Y2O3 (Alfa Aesar 99.9%). To begin, a stoichiometric ratio of precursor powders were ball milled in ethanol for 24 hours. The resulting slurry was dried prior to calcination at 800° C. for 16 hours with a heating and cooling rate of 5° C./minute. After cooling, the calcined powder was initially ground in a mortar in pestle and ball milled for another 24 hours in ethanol. Once complete, the slurry was dried and the emerging powder was compacted into disks using a uniaxial press and sintered at 800° C. for 16 hours with a heating and cooling rate of 5° C./minute.
X-ray diffraction (XRD) patterns were collected using a Bruker D8 X-ray Diffractometer with Cu Kα radiation. High temperature XRD patterns were collected Rietveld refinement was performed on the XRD data to extract the lattice parameters of all materials. In-situ high temperature XRD studies were done on a Bruker C2 Discover X-ray Diffractometer with Cu Kα radiation and 2D area detector. An Anton Paar DHS 1100 hot stage was used for sample heating on top of platinum foil.
Conductivity measurements were performed using electrochemical impedance spectroscopy (EIS). Gold paste was used as the current collector and painted on the faces of sintered disks. Four silver wires were affixed to the sample faces and used as the working and counter electrodes, as well as the two reference electrodes. The AC impedance was measured using a Solartron 1260A frequency response analyzer with a Solartron 1470E acting as a multiplexer. A frequency range of 1 MHz to 10 Hz and an AC voltage of 50 mV was used for EIS measurements. All impedance data was analyzed and fit with Scribner Zview software using equivalent circuit modeling approach.
The transference number was calculated from the OCV measured across the electrolyte sample in a two-gas environment. Silver paste was applied to both faces of the sintered disk and acted as electrodes for the measurements. The sample was placed between two gas lines and secured with a compressive seal made from Thermicullite 866 O-rings. One gas line supplied air, while the other gas line supplied gas mixtures with a range of PO2 values. An oxygen sensor was used downstream to measure the exact PO2 being supplied to the sample.
Ab initio molecular dynamics (AIMD) simulations were performed using the Vienna Ab initio Simulation package (VASP) within the projector augmented-wave approach with Perdew-Burke-Ernzerhof (PBE) generalized-gradient approximation (GGA). A supercell model with Γ-centered k-point in the non-spin-polarized DFT calculations. The time step was set to 2 fs, and NVT ensemble using Nosé-Hoover thermostat was adopted. The total time of AIMD simulations were 100 to 300 ps until a significant number of O-ion jumps were observed. The ionic conductivity was calculated using the established method. See, for example, //dx.doi.org/10.1039/c5cp02181b and //doi.org/10.1038/s41524-018-0074-y.
Bi2O3 electrolytes for intermediate temperature SOC operation: Double doped δ-Bi2O3 was explored as an SOC electrolyte due to its high conductivity and stable performance at intermediate temperatures (650° C.). Two materials discussed herein illustrates the scope of the disclosure. These two illustrative materials include: the La and Y double doped Bi2O3 and La and Zr double doped Bi2O3. The La+3 cation was selected as a dopant due to its large radius, and Y+3 and Zr+4 were selected as co-dopants due to their relatively smaller ionic radius. Without being bound by any theory, it is believed that these smaller secondary dopants were needed to stabilize δ-Bi2O3, as the ionic radius of La+3 is too large to stabilize the cubic phase on its own. The ionic radius of all dopants was determined from Shannon radii for a coordination eight cation. Since two cations were substituted into the Bi2O3 lattice, a weighted average of the two dopant's radius was used to calculate the relative dopant ionic radius. Table 1 lists the Bi2O3 compound, relative ionic radius, and phase for each sample synthesized. The room temperature X-ray diffraction (XRD) patterns of the double doped bismuth oxide materials are shown in
The ionic conductivity of the materials were measured using electrochemical impedance spectroscopy (EIS) on sintered pellets. The EIS data, the modeling and fits of the data, and the conductivity as a function of temperature of the cubic Bi2O3 materials are shown in
Like DWSB, the high conductivity in La7Zr3 was an outcome of lowering the total dopant concentration in Bi2O3, while also retaining the highly conductive cubic phase. The conductivity of La7Zr3 was improved beyond DWSB by reducing the total dopant concentration to 10%, providing a larger concentration of highly polarizable Bi+3 in the lattice. Because the polarizability of bismuth is higher than the dopant cations, decreasing the total cation substitution increased the lattice polarizability, consequently allowing for greater oxygen ion mobility and facilitated anion diffusion. Higher polarizability equates to larger cation charge separation and improved anion diffusion due to the reduction in coulombic repulsion that oxygen ions experience when moving through the saddle point of the conductivity pathway. The path of anion transport through the highly polarizable cation network is illustrated in
The cubic La7Zr3 sample exhibited better conductivity than all other 10% substituted Bi2O3 compounds investigated. The lanthanum cation has the largest ionic radius of all lanthanide series cations, and since the ionic radius is linearly related to polarizability, the large La+3 cation is more polarizable than the other rare earth elements. The La7Zr3 compound contained over four times more lanthanum than the La1.5Y8.5 sample for a similar dopant ionic radius (Table 1). The higher lanthanum content imparted greater polarizability and thus conductivity for the La—Zr doping vs. La—Y doping. Unfortunately, at higher zirconium concentrations, there was a marked increase in La2Zr2O7 impurity phase formation. The substantial secondary phase within the La5Zr5 compound was most likely responsible for the decreased ionic conductivity compared to the La7Zr3 compound.
Bi2O3 Electrolytes for Low Temperature SOC Operation
Tailoring the electrolyte for SOC operation at low temperatures (<600° C.) was also a priority of this investigation. While cubic Bi2O3's generally displays stable performance at intermediate temperature, degradation at lower temperatures is a problem for SOCs operating below 650° C. In particular, anion ordering is most rapid in cubic Bi2O3 at 500° C. To ensure the Bi2O3 electrolytes were capable of operation at low temperatures, the samples were aged for one hundred hours at 500° C. Both ESB and DWSB exhibit a sharp decay in conductivity. The La7Zr3 compound meanwhile had a much more gradual decay in conductivity as it was aged at 500° C. (
Although the cubic La7Zr3 compound exhibited a reduced decay in conductivity at 500° C. compared to ESB and DWSB, the conductivity decay would still degrade long term performance of an SOFC. Thus, the rhombohedral phase of double doped Bi2O3 was investigated for SOCs operating at low (<600° C.) temperature. The conductivity stability of La1.5Y8.5, La2.2Y7.8, La2.9Y7.1, and La3.6Y6.4 compounds were tested at 500° C. These four materials were selected as they ranged from pure cubic phase to entirely rhombohedral. The aging behavior of these compounds at 500° C. and the phase stability before and after aging for cubic La1.5Y8.5 and rhombohedral La3.6Y6.4 are shown in
Ultimately, La5.1Y1.4 provided the highest conductivity of all rhombohedral Bi2O3 materials developed to date. The Arrhenius behavior of rhombohedral La5.1Y1.4 is compared to other SOC electrolyte materials in
AIMD simulations were performed on this new rhombohedral La5.1Y1.4 to study O-ion transport. As directly observed in AIMD simulations, the rhombohedral structure exhibits a fast three-dimensional O-ion conduction network, where O ions jump between O2 and O3 sites in the ab plane and also jump across planes through O1 sites (
Moreover, the stability of rhombohedral La5.1Y1.4 provides a clear advantage over other electrolytes.
To further understand the effect of total dopant concentration and dopant ionic radius on the conductivity of rhombohedral Bi2O3, the amount of La and Y substitution was varied. Optimization began with varying the ratio of La to Y substitution at a fixed total dopant concentration of 10%. For the 10% La and Y doped Bi2O3, the pure rhombohedral phase was present from a relative dopant ionic radius of 1.07 Å up to 1.14 Å. The conductivity of 10% rhombohedral Bi2O3 at 500° C. vs. average dopant ionic radius is shown in
The total dopant concentration was also varied to investigate the effect on conductivity of the rhombohedral bismuth oxide. For La and Y doped Bi2O3, the average dopant ionic radius was fixed at 1.12 Å and the total dopant concentration was varied between 10% and 6%. The conductivity of the rhombohedral sample vs. total dopant concentration is plotted in
Rhombohedral Bi2O3 Phase Transition and Ionic Transference Number
The stability of the rhombohedral La5.1Y1.4 compound was investigated over a range of temperatures. The phase stability was determined using high temperature X-ray diffraction, and conductivity stability was determined with EIS. XRD patterns of La5.1Y1.4 were recorded from room temperature up to 700° C. The XRD patterns from 550° C. to 600° C. are given in
To ensure the rhombohedral La5.1Y1.4 material would be suitable as an ionic conductor, the transference number was measured. The sintered disk was sealed between two separate gas environments, and the oxygen differential generated an electrical potential. The symmetric cell was held for an hour at each condition before the VOC was measured and EIS was performed. The impedance of the cell at different PO2 values is shown in
In this equation, Rb=bulk resistance, RT=total resistance, VOC=open circuit voltage, and VN=Nernst potential. Overall, La5.1Y1.4 demonstrated an average ionic transference number greater than 0.997 across the entire measured PO2 range. With a transference number approaching unity, it is evident that conductivity of the rhombohedral phase resulted from ionic rather than electronic transport.
Significant strides have been made in the development of electrolytes for SOCs operating at intermediate to low temperatures. For intermediate temperature operation (650° C.), double doped cubic Bi2O3 electrolytes show the greatest potential. In particular, the newly developed La7Zr3 not only demonstrated stable ionic conductivity, but it also exhibited the highest oxygen ion conductivity ever measured in any material. For low temperature operation (<600° C.), double doped rhombohedral Bi2O3 displayed the most promise of any electrolyte. La5.1Y1.4 is first bismuth oxide material with conductivity higher than ESB at 500° C., but with stable conductivity. In fact, La5.1Y1.4 has the highest stable oxygen ion conductivity ever recorded at low temperature. Overall, the exceptional conductivity shown here was a result of doping the larger La+3 cation into the lattice while simultaneously minimizing the total dopant without altering the phase of bismuth oxide. As disclosed herein, optimizing SOC electrode chemistry and overall design around these Bi2O3 electrolytes provided superior performance over a range of temperatures for a wide number of oxygen-functional applications.
The foregoing discussion of the invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. Although the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope of the invention, 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 is continuation-in-part of U.S. patent application Ser. No. 18/623,682, filed Apr. 1, 2024, which is a continuation of U.S. patent application Ser. No. 17/990,664, filed Nov. 19, 2022, now U.S. Pat. No. 11,978,938, issued May 7, 2024, which is a divisional of U.S. patent application Ser. No. 16/521,462, filed Jul. 24, 2019, now U.S. Pat. No. 11,539,065, issued Dec. 27, 2022, which claims the priority benefit of U.S. Provisional Application No. 62/702,730, filed Jul. 24, 2018, all of which are incorporated herein by reference in their entirety.
This invention was made with government support under DE-AR0000494 awarded by the U.S. Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 62702730 | Jul 2018 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 16521462 | Jul 2019 | US |
| Child | 17990664 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 17990664 | Nov 2022 | US |
| Child | 18623682 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 18623682 | Apr 2024 | US |
| Child | 18936110 | US |
