Energetics of Dysprosia-Stabilized Bismuth Oxide Electrolytes

Article pubs.acs.org/cm

Energetics of Dysprosia-Stabilized Bismuth Oxide Electrolytes Tien B. Tran† and Alexandra Navrotsky*,† †

Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, California 95616, United States S Supporting Information *

ABSTRACT: With ionic conductivities superior to conventional doped zirconia and ceria at intermediate temperatures (IT, 700−800 °C), bismuth oxide (BiO1.5) materials based on the defect fluorite structure are promising electrolyte candidates for solid oxide fuel cells (SOFCs) operating at reduced temperatures. In order to investigate the energetics of stabilized BiO1.5 in the fluorite structure, DyO1.5-stabilized BiO1.5 (DSB) over a range of compositions was synthesized by solid state reaction and quenched. Using high temperature oxide melt solution calorimetry in molten 3Na2O·4MoO3 at 702 °C, enthalpies of formation at 25 °C were determined. Relative to the phases of the oxide end-members stable at room temperature (monoclinic BiO1.5 and C-type DyO1.5), the formation of Bi1‑xDyxO1.5 is endothermic at x < 0.30 and becomes slightly exothermic toward the upper phase boundary (x = 0.50). These data suggest that this system is slightly stabilized, and there is only a moderate (1−2 orders of magnitude) decrease in the volatility and susceptibility to reduction of bismuth oxide at high temperatures. However, high conductivity still makes the system potentially useful at 700 °C and below. Similar to findings for rare earth-doped zirconia, hafnia, and ceria, a negative interaction parameter for mixing in the solid solution suggests a tendency for short-range ordering, and the increasingly exothermic ΔHmix with increasing x parallels the conductivity decrease with increasing dopant content. KEYWORDS: solid oxide fuel cells, bismuth oxide, fluorite, thermodynamics

■

Pure BiO1.5 has two stable polymorphs. The monoclinic α phase exists at temperatures below 730 °C. The more oxygen ion conducting defect fluorite (δ) phase persists from 730 °C to the melting point (825 °C).3 In addition to a conductivity drop-off, the δ→α transformation results in a large volume change that compromises mechanical integrity. Therefore, there exists a need to stabilize the δ phase to room temperature. Fluorite-phase stabilization has been achieved by doping with metal oxides, particularly rare earth (RE) oxides.2 Interestingly, when BiO1.5 is stabilized with the same amount of REO1.5, where RE = Dy, Ho, Er, Tm, Yb, and Y, the resulting conductivity as a function of temperature is roughly independent of the specific choice of RE.6 However, ionic conductivity decreases with increasing dopant concentration, and can be dramatically diminished by a phase transformation to the rhombohedral structure (ε), or decay during low temperature aging (T < 600 °C) due to an order−disorder transition of the anion sublattice.6 The rate of this decay is ameliorated with increasing dopant radius, increasing dopant polarizability, or at the cost of conductivity, increasing dopant concentration.6,7 Perhaps because Dy is the most polarizable of the lanthanide dopant candidates, the conductivity of DyO1.5stabilized BiO1.5 (DSB) decreases less rapidly than its YO1.5-

INTRODUCTION In the effort to deploy renewable energy technologies, solid oxide fuel cells, which have benefitted from decades of research, are finally beginning to see commercial use. SOFCs do not require a developed hydrogen infrastructure, since they can use any hydrocarbon fuel. However, lowering the operating temperature of SOFCs remains a primary goal, with current technologies requiring ∼700−1000 °C. By reducing operating temperatures, the selection of balance-of-plant components is simplified to common engineering materials, their costs are dramatically reduced, and the entire fuel cell system becomes more receptive to thermal cycling.1 The oxygen ion conductivity of bismuth oxide (BiO1.5)-based electrolytes has been shown greatly superior to that of gadolinia-doped ceria (GDC) and yttria-stabilized zirconia (YSZ), and diminishes less dramatically with decreasing temperature.2 As a result, BiO1.5 materials based on the fluorite structure are ideal candidates for intermediate temperature SOFCs. Unfortunately, without adequate thermochemical data, previous research on the system could only draw loose correlations between myriad dopant types, compositions, and their contributions to phase stability and ionic conductivity.2−4 Furthermore, the questions of bismuth oxide volatility and reduction during use cannot be addressed properly without thermodynamic data.5 The objective of this work is to determine the underlying energetics of doped BiO1.5 in order to systematically optimize phase stability and ionic conductivity for IT-SOFC applications. © 2012 American Chemical Society

Received: August 1, 2012 Revised: October 1, 2012 Published: October 2, 2012 4185

dx.doi.org/10.1021/cm302446e | Chem. Mater. 2012, 24, 4185−4191

Chemistry of Materials

Article

Figure 1. (a) X-ray diffraction patterns of select quenched samples show evidence of a single fluorite phase. (b) Lattice constants of the DSB samples follow Vegard’s law. measured, ground, and milled in an agate jar with agate balls for two hours. The powders were then pressed into pellets and placed into a narrow alumina crucible with an overlapping alumina cap. The alumina crucible was then sealed into a silica glass vial to avoid excessive BiO1.5 evaporation. The samples were heat treated at 850 °C (x = 0.10 to 0.20) to 950 °C (x = 0.20 to 0.45) for twelve hours, quenched by removal of the sample vials from the furnace into a room temperature atmosphere, and then ground, pressed, sealed, and heat treated again. This cycle was repeated until the reaction was complete, and a single fluorite phase was detected by X-ray diffraction (XRD). Structural analysis was conducted by XRD using a Bruker-AXS D8 Advance diffractometer (Bruker-AXS, Inc., Fitchburg, WI) operating at 40 kV and 40 mA using Cu−Kα radiation. Data were acquired from 20°−80° 2θ using a step size of 0.016° and a collection time of 2 s·step−1. The sample was rotated at 15 rpm during data collection. Lattice parameters were calculated by whole pattern fitting as implemented in Jade MDI 6.1 (Materials Data, Inc., Livermore, CA). Elemental analysis was conducted by wavelength dispersive spectroscopy (WDS) using a Cameca SX-100 electron microprobe operating at 15 kV and 20 nA, with a beam size of 1 μm. Bi12GeO20 (C.M. Taylor Co., San Jose, CA) and DyPO4 (Smithsonian Microbeam Standards, Washington, DC) were used as standards, and at least ten measurements were made for each sample. Sample homogeneity was evaluated by backscattered electron (BSE) imaging. Differential scanning calorimetry (DSC) was conducted using a Setaram LabSys Evo thermoanalyzer (Setaram, Cailure, France) with a heat capacity sensor using lidded platinum crucibles. The instrument was operated in DSC mode with an arrested balance. To measure the enthalpy of the monoclinic to fluorite transformation in pure Bi2O3, an empty crucible, α-Al2O3, and Bi2O3 were each heated from 50 to 780 °C at 10 °C·min−1 in flowing oxygen (40 mL·min−1) for two cycles. The heat flow sensitivity was calibrated using high purity α-Al2O3, which was heat treated at 1500 °C overnight prior to the experiment. Temperature calibration was performed by melting a Au standard in an alumina crucible. High temperature oxide melt drop solution calorimetry was performed in a house-built isoperibol Tian-Calvet microcalorimeter as described by Navrotsky.15 For each measurement, two pellets weighing a total of ∼15 mg were dropped, at the same time, into molten sodium molybdate (3Na2O-4MoO3) solvent at 702 °C. The calorimeter was calibrated using the heat content of high purity αAl2O3. For the DSB samples, oxygen gas was flushed through the calorimeter assembly at 55 mL·min−1 to maintain a constant atmosphere, and bubbled through the solvent at 7 mL·min−1 to aid dissolution and to prevent local solvent saturation. For the pure BiO1.5 samples, no gas flushing or bubbling was used, since these factors may accelerate the evaporation of BiO1.5 from the solvent. To address issues that may stem from the high vapor pressure of BiO1.5, furnace tests were conducted wherein 30 mg of pelletized BiO1.5 was dropped into 10 g of molten 3Na2O-4MoO3 at 700 °C. No significant mass loss was discernible after one hour, the length of a typical drop solution experiment, without the use of gas flushing and bubbling.

and other REO1.5-stabilized counterparts during aging at a given low temperature.6 The thermodynamics of cation substitution in fluorite structured oxides has been studied extensively using high temperature oxide melt solution calorimetry.8−12 The energetic effect of doping ZrO2, HfO2, CeO2, ThO2, and UO2 with yttrium- and lanthanide oxides was investigated. Relevant to solid oxide electrolyte applications, in Gd- and La-doped CeO2,13 as well as La- and Y-doped ThO2,14 compositions yielding the highest ionic conductivity were associated with the most endothermic heats of formation. That is, with additional aliovalent cation substitution, the probability of vacancy− vacancy interactions increases, and additional thermodynamic stabilization can be offered by defect cluster formation. Defect clustering, which causes an exothermic deviation from regular solution behavior, lowers ionic conductivity by trapping mobile vacancies. The primary goal of doping BiO1.5 is to stabilize the δ-phase, and not to introduce additional oxygen vacancies. Since Dy3+ and Bi3+ are isovalent, increasing the concentration of DyO1.5 in DSB does not affect the total oxygen vacancy concentration, but can affect the positions of the oxygen vacancies. Moreover, RE3+-O2‑ bonds are much stronger than Bi3+-O2‑ bonds, and the formation of cation−anion associates can reduce conductivity by decreasing the mobility of vacancies, or reducing the total concentration of mobile oxygen ions.6 In addition to stabilization of the δ-phase, the use of BiO1.5 at elevated temperatures or in reducing atmospheres also requires stabilization against volatilization and reduction.5 Phase stability, associate formation, volatility, and susceptibility to reduction can all be assessed using calorimetric techniques. In this work, the quenched δ-phase of BiO1.5 was stabilized by doping with DyO1.5 over the mol fraction range of x = 0.11 to 0.45. The DSB materials were characterized by high temperature oxide melt drop solution calorimetry, and the resulting thermochemical data are discussed in light of published structural investigations and conductivity studies. Special attention is paid to the energetic stabilization offered by DyO1.5 doping against BiO1.5 volatilization and reduction, respectively, under common SOFC operating conditions.

■

MATERIALS AND METHODS

Bismuth(III) oxide powders (Sigma Chem. Co., St. Louis, MO) and dysprosium(III) oxide powders (99.99%, Aldrich Chem. Co., Milwaukee, WI) were dried at 500 and 700 °C, respectively, prior to solid state synthesis of dysprosia-stabilized bismuth oxide (DSB, DyxBi1‑xO1.5). Appropriate amounts of BiO1.5 and DyO1.5 were 4186

dx.doi.org/10.1021/cm302446e | Chem. Mater. 2012, 24, 4185−4191

Chemistry of Materials

■

Article

RESULTS

Stabilized δ-Bi2O3 samples with compositions DyxBi1‑xO1.5 at x = 0.11 to 0.45 were synthesized by solid state reaction. All samples used for calorimetry were homogeneous by backscattered electron imaging and did not contain any impurity elements greater than 1% by wavelength dispersive spectroscopy. The actual compositions determined by wavelength dispersive spectroscopy, with uncertainties less than x = 0.01, are used in all calculations. Actual compositions can differ from nominal compositions, mainly due to the high vapor pressure of BiO1.5 at synthesis temperatures, but are often not satisfactorily characterized in the literature. This oversight could explain the inconsistencies found between studies regarding phase stability ranges. By X-ray diffraction, all quenched samples exhibit a single cubic fluorite phase (Figure 1a). The lattice constants of the stabilized samples, determined by whole pattern fitting, are shown in Figure 1b. They decrease linearly with increasing DyO1.5 content, consistent with the ionic radii of Bi3+ (1.11 Å) and Dy3+ (1.03 Å).16 The enthalpies of drop solution (ΔHds) of DSB decrease with increasing DyO1.5 content, as shown in Figure 2. The ΔHds values have been used to calculate the DSB heats of formation from the room temperature phases of the oxide endmembers (ΔHf, ox, Figure 3) and the heats of formation from the fluorite end-members (ΔHf, c, Figure 4) using the thermodynamic cycle in Table 1.

Figure 3. Enthalpies of formation of DSB relative to monoclinic BiO1.5 and C-type DyO1.5.

Table 1. Thermochemical Cycle for Determining DSB Heats of Formation from the Room Temperature End-Member Phasesa reaction Bi1‑xDyxO1.5 (c, 25 °C) → xDyO1.5 (soln., 700 °C) + (1-x) BiO1.5 (soln., 700 °C) DyO1.5 (C, 25 °C) → DyO1.5 (soln., 700 °C) BiO1.5 (m, 25 °C)→ BiO1.5 (soln., 700 °C) xDyO1.5 (C, 25 °C) + (1-x)BiO1.5 (m, 25 °C)→ Bi1‑xDyxO1.5 (c, 25 °C) ΔHf, ox(4) = −ΔHds(1) + xΔHds(2) + (1-x)ΔHds(3)

Figure 4. Enthalpies of formation of DSB relative to the fluorite phase of the end-member oxides. A quadratic fit representing regular solution treatment is shown.

ΔH ΔHds(1) ΔHds(2) ΔHds(3) ΔHf, ox(4)37

In Figure 2, the ΔHds of the fluorite end-members were calculated from ΔHds of their respective room temperature phases, monoclinic BiO1.5 and C-type DyO1.5, using their heats of transformation (ΔHt) to the cubic fluorite phase (Table 2).

Here, c = cubic fluorite phase, C = C-type phase, and m = monoclinic phase. a

Table 2. Experimental Heats of Drop Solution of the Room Temperature End-Member Phases, Heats of Transformation from Monoclinic BiO1.5 and C-type DyO1.5 to the Cubic Fluorite Phase, and Calculated Heats of Drop Solution of the Fluorite Phasesa

BiO1.5 DyO1.5

ΔHds (RT phase) (kJ·mol‑1)

ΔHt (kJ·mol‑1)

ΔHds (fluorite phase) (kJ·mol‑1)

5.21 ± 0.53 (14) −57.44 ± 1.11 (9)37

16.39 ± 0.47 20.6 ± 2.59

−11.18 ± 0.71 −78.04 ± 2.74

a

Number of drop solution calorimetry measurements are presented in parentheses.

The positive deviation from a linear trend in ΔHds, indicating negative heats of mixing between fluorite end-members, increased with increasing DyO1.5 content. Thermochemical data are summarized in Table 3. The monoclinic to fluorite (α→δ) transformation of bismuth oxide was investigated by DSC. The measured transformation enthalpy (ΔHt) of BiO1.5 is compared to published values in Table 4. The results of this study agree relatively well with published values previously obtained by DSC and DTA. Since

Figure 2. Drop solution enthalpies of DSB, shown with square data markers, deviate from ideal mixing behavior with increasing DyO1.5 content. Error bars equal to two standard deviations of the mean are presented, but all are within the open data markers. Drop solution enthalpies of the room temperature end-member phases are shown with diamond data markers.

4187

dx.doi.org/10.1021/cm302446e | Chem. Mater. 2012, 24, 4185−4191

Chemistry of Materials

Article

ΔHmix = Ωx(1 − x)

Table 3. Heats of Drop Solution (ΔHds), Heats of Formation Relative to Room Temperature Oxide End-Members (ΔHf, ox), and Heats of Formation Relative to the Fluorite End-Members (ΔHf, c) Determined for DSB Using 3Na2O4MoO3 at 702 °Ca x 0.11 0.16 0.23 0.30 0.41 0.45

ΔHds (kJ·mol‑1) −8.09 −13.20 −16.35 −18.47 −18.06 −22.23

± ± ± ± ± ±

0.61 0.52 0.71 0.25 0.51 0.72

ΔHf,ox (kJ·mol‑1)

(8) (8) (12) (9) (9) (10)

6.41 8.21 7.20 4.89 −2.42 −0.75

± ± ± ± ± ±

0.78 0.71 0.86 0.56 0.75 0.92

The regular solution parameter, Ω = −72.99 ± 1.43 kJ·mol−1, can be calculated directly from the heats of drop solution using a quadratic fit constrained by the ΔHds of the fluorite endmembers, as shown in Figure 2. ΔHmix can be considered in terms of

ΔHf,c (kJ·mol‑1) −10.45 −8.87 −10.16 −12.77 −20.53 −19.04

± ± ± ± ± ±

0.93 0.91 1.09 0.99 1.30 1.48

ΔHf,ox(Bi1 − xDyxO1.5) = (1 − x)ΔHt(BiO1.5) + xΔHt(DyO1.5) + ΔHmix (3)

Thus, ΔHmix is equal to the enthalpy of DSB formation from the fluorite end-members (ΔHf, c, Figure 4). Both ΔHmix and Ω are negative, suggesting some degree of short-range ordering. ΔHmix becomes more exothermic with increasing dopant concentration, in parallel to the decrease in ionic conductivity of DSB with DyO1.5 content, further supporting the possibility of cation−anion associate formation. In comparing the efficacy of REO1.5 dopant species, the magnitude of Ω reflects the degree of ordering in the structure and may offer some insights into the tendency toward cation−anion association, susceptibility to aging, and ionic conductivity. The enthalpy of formation of DSB in the range of x = 0.11 to 0.45 from α-BiO1.5 and C-type DyO1.5 (ΔHf, ox) is endothermic at x ≤ 0.30, and becomes very slightly exothermic near the phase boundary at x = 0.50. The endothermic ΔHf, ox at low dopant contents suggests that the fluorite phases are entropystabilized at synthesis temperatures. DSB at these compositions may therefore be metastable at low temperature.

a

Compositions were measured by WDS, errors are reported as two standard deviations of the mean, and the number of drop solution calorimetry measurements is presented in parentheses for each sample.

Table 4. Temperature and Enthalpy of the α-to-δ Transformation in BiO1.5 temperature (°C)

ΔHt (BiO1.5) (kJ·mol‑1)

732.2 ± 0.3 730

16.393 ± 0.473 14.85

DSC DSC

730 730 ± 5 727

14.75 20.711 ± 1.046 18.4

DSC DTA DTA

730

15.3

705 718

28.5 21.7

drop calorimetry emf emf

707

19.9

emf

724 729

22 15

emf

704.85

28.451

717 ± 7

---

ΔGt = ΔHt − T ΔSt = 0

method

(2)

reference this work Korobeinikova, Kholmov, Rezmitskii (1976)38 Harwig, Gerards (1979)25 Levin, McDaniel (1965)39 Rao, Rao, Ramdas (1969)40 Cubicciotti, Eding (1967)41 Rao, Tare (1971)42 Mehrotra, Frohberg, Kapoor (1976)43 Isecke, Osterwald (1979)44 Fitzner (1980)45 Risold, Hallstedt, Gauckler, Lukas, Fries (1995)46 Barin, Knacke, Kubaschewski (1977)47 Gattow, Schroder (1962)28

■

DISCUSSION In solid oxide fuel cell and other elevated-temperature applications, an important consideration is the volatility of bismuth oxide. BiO1.5 volatilization has been studied in several oxide systems, including Bi−Fe−O,17 Bi−Sb−O,18 and Bi− Sn−O,19 since BiO1.5 loss under such operating conditions can lead to the formation of deleterious concentration gradients in the bismuth oxide-based component. Coupled with low-oxygen or fuel-rich atmospheres, such as on the anode side of SOFCs, BiO1.5 can also be reduced to Bi metal. In fact, Azad et al. directly pointed out the need to investigate the thermodynamics of doped BiO1.5 in reducing atmospheres in their 1994 review paper, but few, if any, efforts have been made in this area since then.2 The new thermodynamic data can be used to assess the degree of stabilization offered by DyO1.5 doping against BiO1.5 volatilization and reduction under operating conditions, respectively. In this analysis, intermediate to conventional operating temperatures (500−1000 °C), low oxygen partial pressure atmospheres, and fuel-rich environments are considered. The partial molar free energy (i.e., chemical potential) of BiO1.5 in DSB (ΔμBiO1.5) containing x mole fraction of DyO1.5 can be expressed by

(1)

at the transformation temperature, the entropy change associated with the monoclinic-to-fluorite transformation, ΔSt(α→δ, BiO1.5), is 16.31 ± 0.47 J·mol−1·K−1. The C-type to cubic transformation of DyO1.5 has not been observed experimentally, and direct enthalpy measurements remain difficult due to the high transformation temperature (>2000 °C).9 ΔHt(DyO1.5) has been reported previously by Simoncic et al.9,10 In the first study, ΔHt(DyO1.5) was found to be 39 ± 5 kJ·mol−1 by extrapolating a quadratic fit of ΔHf,ox for DyO1.5-doped HfO2 relative to the room temperature phases of the oxide end-member.10 In the second study, ΔHt(DyO1.5) was found to be 20.6 ± 2.5 kJ·mol−1 by simultaneously applying quadratic fits to both DyO1.5-doped ZrO2 and HfO2 and constraining the fits so that Ht(DyO1.5, ZrO2−DyO1.5) = Ht(DyO1.5, HfO2−DyO1.5). The latter transformation enthalpy for DyO1.5 is used in this study, since its determination is wellconstrained. Applying a regular solution model for mathematical simplicity

Δμ BiO = μ BiO

1.5(ss)

1.5

2

− μ BiO

1.5(xl)

= Ωx + RT ln(1 − x) + Δμt

(4)

Here, ss refers to BiO1.5 the DSB solid solution, and xl refers to pure BiO1.5 in its most stable phase at a given temperature. Therefore, at temperatures below 732.2 °C, the free energy of the α→δ transformation (Δμt = ΔHt − TΔSt, Table 4) is considered. It is assumed that Bi3+ and Dy3+ mix randomly, and 4188

dx.doi.org/10.1021/cm302446e | Chem. Mater. 2012, 24, 4185−4191

Chemistry of Materials

Article

vacancy clustering does not change upon mixing the fluorite end-members. The activity of BiO1.5 (aBiO1.5) can then be calculated, using Ω = −72.99 kJ·mol−1, from ss ⎛ Δμ BiO ⎞ pBiO 1.5 ⎟⎟ = 0 1.5 aBiO1.5 = exp⎜⎜ pBiO ⎝ RT ⎠ 1.5

some improvement in vaporization-resistance is predicted, but the effect is not very large since the thermodynamic stabilization of the solid solution is moderate. In a SOFC, low oxygen partial pressures on the anode side of the cell are necessary to create the concentration gradient that drives oxygen anion conduction through the electrolyte. At low oxygen partial pressures (p(O2)), the reduction of BiO1.5 in DSB to Bi(l) and O2(g), according to the reaction below, is of concern.

(5)

p0BiO1.5

While refers to the saturated vapor pressure of pure BiO1.5 at some temperature, pssBiO1.5 refers to the vapor pressure of BiO1.5 in the DSB solution. (Note that for this order-ofmagnitude exercise, propagating the error from Ω (±1.43 kJ·mol−1) does not significantly affect the final results.) The vapor pressure of pure BiO1.5 as a function of temperature was determined by ⎛ −ΔG 0(xl → g) ⎞ 0 ⎟ pBiO = exp⎜ 1.5 RT ⎝ ⎠

BiO1.5(ss) → Bi(l) +

(6)

where ΔG (xl→g) is the standard free energy change associated with the solid to vapor transformation of pure BiO1.5, and the activity of all gas species is determined by defining 1 atm as the standard pressure. ΔG0(xl→g) is equal to ΔG0(α→δ) + ΔG0(δ→l) + ΔG0(l→g) at T < 732.2 °C and to ΔG0(δ→l) + ΔG0(l→g) at T > 732.2 °C (Table 5). ΔG0(α→δ) = ΔHt − TΔSt is calculated using the values in Table 4. Table 5. Standard Enthalpy and Entropy Changes Associated with Pure BiO1.5 Phase Transformations δ→l l→g

ΔS0t (J·mol-1·K-1)

reference

8.519 136.831

7.427 97.696

Levin, McDaniel (1965)38 Freiser (1930)48

(7)

The equilibrium oxygen partial pressure (peq(O2)), below which this reaction proceeds, can be determined as a function of temperature using the thermochemical cycle in the Supporting Information (Table S1). Thus, peq(O2) is found when the free energy change of reduction (ΔGR) is equal to zero, as shown in Figure S1a as a function of temperature. At the highest dopant level investigated (x = 0.45), BiO1.5 reduction is stabilized by approximately one and a half orders of magnitude in p(O2) at 700 °C in oxygen-poor environments. Bismuth oxide reduction is also problematic in fuel-rich environments. The simplest fuel, H2(g), is considered as an example. For this condition, the maximum partial pressure ratio, p(H2)/p(H2O), above which reduction occurs can be calculated from the thermochemical cycle in the Supporting Information (Table S2), and is shown in Figure S1b as a function of temperature. As the amount of DyO1.5 increases in DSB, the electrolyte is able to withstand higher p(H2)/p(H2O) levels. At the highest dopant level investigated (x = 0.45), in terms of p(H2)/p(H2O), BiO1.5 reduction is stabilized by approximately half an order of magnitude in the fuel-rich environment at 700 °C. Of course, this analysis is for bulk, nonfunctioning DSB, so that the calculated p(H2)/p(H2O) limits are not direct prescriptions for inlet fuel composition. In an operating SOFC, only excess H2(g) not oxidized by the oxygen anions conducted through the electrolyte would contribute to BiO1.5 reduction. Thus, the p(H2)/p(H2O) encountered by the electrolyte is lower than that of the inlet fuel mixture. Nevertheless, one needs to realize that DyO1.5 doping does not substantially stabilize BiO1.5 against reduction at IT-SOFC operating temperatures. While some stabilization is achieved against BiO1.5 loss and reduction, DyO1.5 doping is probably not a sufficiently stabilizing mechanism on its own. Other methods, such as the deposition of more-stable barrier coatings or layers, must be

0

ΔH0t (kJ·mol-1)

3 O2(g,p(O2)) 4

BiO1.5 volatilization can become an issue at IT to conventional SOFC operating temperatures, especially in the presence of flowing gas. From the measured heats of formation (Figures 3 and 4), it is clear that the fluorite-structured solid solutions of BiO1.5 and DyO1.5 are stabilized with respect to their end-members. In determining how this stabilization translates to volatility, Figure 5 shows that, for x = 0.41, the activity of BiO1.5 is approximately 0.1, and that vapor pressure is diminished by an order of magnitude relative to pure BiO1.5. For acceptable vaporization losses, we assume that the vapor pressure should be less than 0.01 atm. This would occur at 740 °C for pure BiO1.5 and at 860 °C for Dy0.41Bi0.59O1.5. Thus,

Figure 5. (a) The activity of BiO1.5 in DSB relative to δ-BiO1.5 above 732.2 °C. (b) The vapor pressure of BiO1.5 in DSB as a function of inverse temperature. 4189

dx.doi.org/10.1021/cm302446e | Chem. Mater. 2012, 24, 4185−4191

Chemistry of Materials

Article

Positional ordering has also been investigated in the context of aging by neutron diffraction,36 since the displacement of anions to non-8c sites results in vacancy ordering on the 8c sites, and leads to inhibited oxygen ion diffusion. Intensive structural investigations are therefore necessary in order to determine the presence and characteristics of Dy 3+ -O2‑ associates and the nature of vacancy occupation. These affect both the enthalpy and the entropy of the material. Furthermore, the enthalpy of doping in the rhombohedral phase is not yet known. Understanding the details of energetics and structure as a function of composition in both the cubic and rhombohedral phases, and as functions of temperature and preparation/operation conditions, is required for accurate thermochemical modeling to assist in tailoring the resistance of stabilized bismuth oxides against aging, and will be the focus of future studies.

further investigated. A synergistic relationship has already been achieved in ESB-GDC bilayer electrolytes, wherein GDC prevents BiO1.5 loss on the anode side of the component, and ESB blocks electronic conduction to GDC.20,21 In such an electrolyte couple, the total conductivity of the composite is still greater than that of an alternative electrolyte (e.g., GDC) alone. Moreover, the remarkably high conductivity of BiO1.5 at intermediate temperatures may still outweigh its drawbacks, and warrants the development of additional chemical and physical stabilization routes. In terms of DSB conductivity and mechanical integrity, it is of paramount importance to maintain the δ-phase, which can transform to the rhombohedral phase (ε) upon cooling, throughout the operating temperature range. Previous studies have shown that, while the fluorite phase may be maintained by quenching samples containing x = 0.10 to 0.50, DyO1.5 contents of x = 0.285 to 0.50 are necessary to maintain the fluorite phase by slowly cooling from synthesis temperatures (>800 °C) at 0.5 °C·min−1.22 Other studies have shown that DyO1.5 contents as high as x = 0.325 are necessary to stabilize the δ phase to room temperature.23 By drop solution calorimetry, the room temperature formation enthalpy of DSB was measured without influence of the sluggish δ→ε transformation. Verkerk and Burggraaf22 showed that the rhombohedral to cubic transformation temperature increases from 575 to 745 °C for x = 0.10 to 0.25 in DSB. This is consistent with greater short-range order (i.e., clustering in the cubic fluorite phase) with increasing doping, which would diminish the entropy of transformation. To better understand clustering in DSB, it is worthwhile to revisit the results of previous experimental and computational investigations. It is well-known that vacancies in highly disordered, pure δBiO1.5 cannot be described by an average 8c occupancy model.24,25 (This explains the large discrepancy that exists between the measured ΔSt(α→δ), and a calculated ΔSconfig(α→ δ), which assumes random 8c occupancy.) Several models for the complex anion sublattice of δ-BiO1.5 have been proposed, and thorough descriptions of the structure can be found elsewhere.24,26−31 In stabilized δ-BiO1.5, Battle et al. showed by neutron scattering and extended X-ray adsorption fine structure (EXAFS) techniques that structural disorder in the immediate environments of host Bi3+ cations and dopant M3+ cations (M3+ = Y3+, Er3+, Yb3+) are, in fact, very different.32,33 The authors considered the role of the dopant cations as one to “absorb the anion disorder” about the Bi3+ cations. Therefore, order increases with increased doping, in agreement with the increasing δ→ε transformation temperatures measured by Verkerk and Burggraaf.22 Recently, Abrahams et al.,34 following the works of Norberg et al.35 on pure BiO1.5, probed anion distributions in δ-Bi3YO6 by combining total neutron scattering with the reverse Monte Carlo method, finding oxygen anions on 8c, 32f, and 48i sites. In contrast, only 8c and 32f sites are occupied in pure δ-BiO1.5; so anions occupying the 48i sites are likely associated with the Y3+ dopant cations. In this way, oxygen anions form a distorted octahedral coordination about the Y3+ cations. According to the results of Battle et al.,32,33 which found little difference between the effects of Y3+-, Er3+-, and Yb3+-doping, this type of “positional” ordering, which can be considered a form clustering, may reasonably be expected in RE-doped BiO1.5 as well.

■

SUMMARY The energetics of dysprosia-stabilized bismuth oxide, DyxBi1‑xO1.5, was investigated by drop solution calorimetry using 3Na2O·4MoO3 at 702 °C. Since thermodynamic stabilization of the solid solution is slight, DyO1.5 doping only moderately stabilizes BiO1.5 against volatilization and reduction under typical SOFC operating conditions. The negative heats of mixing and interaction parameter suggest the formation of cation−anion associates. Furthermore, heats of mixing become more negative with increasing dopant content, parallel to the decrease in ionic conductivity, and in support of greater defect clustering with increasing doping.

■

ASSOCIATED CONTENT

S Supporting Information *

Thermochemical cycles for calculating peq(O2) and p(H2)/ p(H2O) as well as figures showing peq(O2) and p(H2)/p(H2O) as functions of temperature at select dopant levels. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was performed with funding from the U.S. Department of Energy (Grant No. DE-FG02-03ER46053).

■

REFERENCES

(1) Wachsman, E. D.; Marlowe, C. A.; Lee, K. T. Energy Environ. Sci. 2012, 5, 5498−5509. (2) Azad, A. M.; Larose, S.; Akbar, S. A. J. Mater. Sci. 1994, 29, 4135− 4151. (3) Sammes, N. M.; Tompsett, G. A.; Näfe, H.; Aldinger, F. J. Eur. Ceram. Soc. 1999, 19, 1801−1826. (4) Malavasi, L.; Fisher, C. A. J.; Islam, M. S. Chem. Soc. Rev. 2010, 39, 4370−4387. (5) Shuk, P.; Wiemhöfer, H.-D.; Guth, U.; Göpel, W.; Greenblatt, M. Solid State Ionics 1996, 89, 179−196. (6) Jiang, N.; Wachsman, E. D. J. Am. Ceram. Soc. 1999, 82, 3057− 3064. (7) Wachsman, E. D.; Boyapati, S.; Jiang, N. Ionics 2001, 7, 1−6. 4190

dx.doi.org/10.1021/cm302446e | Chem. Mater. 2012, 24, 4185−4191

Chemistry of Materials

Article

(44) Isecke, B.; Osterwald, J. Z. Phys. Chem. Neue Folge 1979, 115, 17−24. (45) Fitzner, K. Thermochim. Acta 1980, 35, 277−286. (46) Risold, D.; Hallstedt, B.; Gauckler, L. J.; Lukas, H. L.; Fries, S. G. JPE 1995, 16, 223−234. (47) Barin, I.; Knacke, O.; Kubaschewski, O. Thermochemical properties of inorganic substances; Springer: 1977. (48) Freiser, J. Met. Erz 1930, 27, 585.

(8) Navrotsky, A.; Simoncic, P.; Yokokawa, H.; Chen, W.; Lee, T. Faraday Discuss. 2007, 134, 171. (9) Simoncic, P.; Navrotsky, A. J. Am. Ceram. Soc. 2007, 90, 2143− 2150. (10) Simoncic, P.; Navrotsky, A. J. Mater. Res. 2007, 22, 876−885. (11) Lee, T. A.; Stanek, C. R.; McClellan, K. J.; Mitchell, J. N.; Navrotsky, A. J. Mater. Res. 2008, 23, 1105−1112. (12) Shvareva, T. Y.; Alexandrov, V.; Asta, M.; Navrotsky, A. J. Nucl. Mater. 2011, 419, 72−75. (13) Avila-Paredes, H. J.; Shvareva, T.; Chen, W.; Navrotsky, A.; Kim, S. Phys. Chem. Chem. Phys. 2009, 11, 8580−8585. (14) Aizenshtein, M.; Shvareva, T. Y.; Navrotsky, A. J. Am. Ceram. Soc. 2010, 93, 4142−4147. (15) Navrotsky, A. Phys. Chem. Miner. 1997, 24, 222−241. (16) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1969, 25, 925−946. (17) Mikhailov, A. V.; Gribchenkova, N. A.; Kolosov, E. N.; Kaul’, A. R.; Alikhanyan, A. S. Russ. J. Phys. Chem. A 2010, 85, 26−30. (18) Gribchenkova, N.; Steblevskii, A.; Kolosov, E.; Alikhanian, A.; Nipan, G. Inorg. Mater. 2007, 43, 78−84. (19) Asryan, N. A.; Kol’tsova, T. N.; Alikhanyan, A. S.; Nipan, G. D. Inorg. Mater. 2002, 38, 1141−1147. (20) Ahn, J. S.; Camaratta, M.; Lee, K. T.; Yoon, H. S.; Lee, B. W.; Wachsman, E. D. ECS Trans. 2009, 135−144. (21) Ahn, J. S.; Pergolesi, D.; Camaratta, M. A.; Yoon, H.; Lee, B. W.; Lee, K. T.; Jung, D. W.; Traversa, E.; Wachsman, E. D. Electrochem. Commun. 2009, 11, 1504−1507. (22) Verkerk, M. J.; Burggraaf, A. J. J. Electrochem. Soc. 1981, 128, 75−82. (23) Watanabe, A. Solid State Ionics 1995, 79, 84−88. (24) Hull, S.; Norberg, S. T.; Tucker, M. G.; Eriksson, S. G.; Mohn, C. E.; Stølen, S. Dalton Trans. 2009, 8737−8745. (25) Harwig, H. A.; Gerards, A. G. Thermochim. Acta 1979, 28, 121− 131. (26) Battle, P. D.; Catlow, C. R. A.; Drennan, J.; Murray, A. D. J. Phys. C: Solid State Phys. 1983, 16, L561−L566. (27) Yashima, M.; Ishimura, D. Chem. Phys. Lett. 2003, 378, 395− 399. (28) Gattow, V. G.; Schroder, H. Z. Anorg. Allg. Chem. 1962, 318, 176−189. (29) Harwig, H. A. Z. Anorg. Allg. Chem. 1978, 444, 151−166. (30) Hund, F. Z. Anorg. Allg. Chem. 1964, 333, 248−255. (31) Jacobs, P. W. M.; Mac Dónaill, D. A. Solid State Ionics 1987, 23, 279−293. (32) Battle, P. D.; Catlow, C. R. A.; Moroney, L. M. J. Solid State Chem. 1987, 67, 42−50. (33) Battle, P. D.; Catlow, C. R. A.; Chadwick, A. V.; Cox, P.; Greaves, G. N.; Moroney, L. M. J. Solid State Chem. 1987, 69, 230− 239. (34) Abrahams, I.; Liu, X.; Hull, S.; Norberg, S. T.; Krok, F.; Kozanecka-Szmigiel, A.; Islam, M. S.; Stokes, S. J. Chem. Mater. 2010, 22, 4435−4445. (35) Norberg, S. T.; Eriksson, S. G.; Hull, S. Solid State Ionics 2011, 192, 409−412. (36) Wachsman, E. D.; Boyapati, S.; Kaufman, M. J.; Jiang, N. J. Am. Ceram. Soc. 2000, 83, 1964−1968. (37) Ushakov, S. V.; Helean, K. B.; Navrotsky, A.; Boatner, L. A. J. Mater. Res. 2001, 16, 2623−2633. (38) Korobeinikova, A. V.; Kholmov, V.; Rezmitskii, L. A. Vestn. Mosk. Univ., Ser. 2: Khim. 1976, 17, 381. (39) Levin, E. M.; McDaniel, C. L. J. Res. Nat. Bur. Stand 1965, 69A, 237−243. (40) Rao, C. N. R.; Rao, G. V. S.; Ramdas, S. J. Phys. Chem. 1969, 73, 672−675. (41) Cubicciotti, D.; Eding, H. J. Chem. Eng. Data 1967, 12, 548− 551. (42) Rao, A. V. R.; Tare, V. B. Scr. Mater. 1971, 5, 807−811. (43) Mehrotra, G. M.; Frohberg, M. G.; Kapoor, M. L. Z. Phys. Chem. Neue Folge 1976, 99, 304−307. 4191

dx.doi.org/10.1021/cm302446e | Chem. Mater. 2012, 24, 4185−4191