Lanthanum–Strontium–Manganese Perovskites as Redox Materials

Ceria (Lehmann & Voss & Co., 99.99%) was uniaxially cold-pressed at 2 tons and sintered at 1773 K into ∼500 mg cylindrical pellets. ... Defect Theory ...
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Lanthanum−Strontium−Manganese Perovskites as Redox Materials for Solar Thermochemical Splitting of H2O and CO2 Jonathan R. Scheffe,*,† David Weibel,† and Aldo Steinfeld†,‡ †

Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zürich, Switzerland Solar Technology Laboratory, Paul Scherrer Institute (PSI), 5232 Villigen PSI, Switzerland

‡

ABSTRACT: A thermodynamic and experimental investigation of a new class of solar thermochemical redox intermediates, namely, lanthanum−strontium−manganese perovskites, is presented. A defect model based on low-temperature oxygen nonstoichiometry data is formulated and extrapolated to higher temperatures more relevant to thermochemical redox cycles. Strontium contents of x = 0.3 (LSM30) and x = 0.4 (LSM40) in La1−xSrxMnO3−δ result in favorable reduction extents compared to ceria in the temperature range of 1523−1923 K. Oxidation with CO2 and H2O is not as thermodynamically favorable and largely dependent upon the oxidant concentration. The model is experimentally validated by O2 non-stoichiometry measurements at high temperatures (>1623 K) and CO2 reduction cycles with commercially available LSM35. Theoretical solar−fuel energy conversion efficiencies for LSM40 and ceria redox cycles are 16 and 22% at 1800 K and 13 and 7% at 1600 K, respectively.

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CO2 or H2O.7 Redox kinetics may be estimated on the basis of the knowledge of oxygen conductivity or ambipolar diffusion coefficients.7 In this paper, we present results of a thermodynamic and experimental study on a new class of solar thermal redox materials: lanthanum-based perovskites. The attention is focused on La1−xSrxMnO3−δ (LSM, where δ is the degree of non-stoichiometry), primarily because of its well-characterized thermodynamic data available at lower temperatures.15−17 We discuss the applicability of extrapolating this data to higher temperatures using the defect model and validate the results with measurements conducted at elevated temperatures via thermogravimetric analysis. Thermodynamic equilibrium fuel yields are compared to those of ceria and those obtained experimentally. Theoretical solar−fuel energy conversion efficiencies are determined.

INTRODUCTION Several metal oxide redox systems have been considered for application in two-step solar thermochemical cycles for splitting of H2O and CO2, including volatile ZnO,1,2 non-volatile ferrites,3−5 and ceria-based oxides.6−8 Non-stoichiometric ceria is increasingly more attractive as an intermediate because of its relatively high structural and crystalline stability9,10 at elevated temperatures and its ability to rapidly conduct oxygen through its lattice. Additionally, the thermodynamic and kinetic properties of ceria can be altered with the addition of metal oxide dopants.11,12 To date, other than a few papers based on nonstoichiometric ferrites around 15 years ago,13,14 ceria and doped ceria are the only non-stoichiometric oxides that have been extensively investigated as intermediates for solar thermal fuel production. However, the numbers of non-stoichiometric metal oxides for which thermodynamic data exist are extremely extensive. In fact, a quick search for “perovskite oxygen nonstoichiometry” yields nearly 10 times the number of hits as “ceria oxygen non-stoichiometry”, indicating the sheer number of materials that may prove to be thermodynamically attractive as redox materials. The desirable properties of potential redox materials include large degrees of reduction at moderate temperatures, favorable oxidation thermodynamics, rapid oxidation and reduction kinetics, and morphological stability. Many of these parameters can be elucidated from data that already exists in the literature. For example, although non-stoichiometry data commonly exists at temperatures lower than those applicable to thermochemical redox cycles, appropriate defect models can be used to extrapolate the results to higher temperatures and estimate the deviation from stoichiometry at more relevant temperatures.11 Knowledge of oxygen non-stoichiometry as a function of the temperature and oxygen partial pressure can also be used to extract thermodynamic parameters, which, in turn, can be applied to predict the ability of a reduced oxide to react with © 2013 American Chemical Society

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EXPERIMENTAL SECTION

Commercially available La0.65Sr0.35MnO3 (LSM35, Aldrich, catalog number 704261) particles were sintered at 1773 K into ∼500 mg cylindrical pellets. Ceria (Lehmann & Voss & Co., 99.99%) was uniaxially cold-pressed at 2 tons and sintered at 1773 K into ∼500 mg cylindrical pellets. Sintering of both materials was performed in an argon atmosphere. Oxygen non-stoichiometry was measured using a thermogravimetric analyzer (TGA, Netzsch STA 409 SD). Samples were placed on a flat 17 mm diameter Al2O3 crucible. The oxygen partial pressure was controlled by mixing high-purity Ar (Messer, Argon 5.0) with three different O2/Ar mixtures (0.5, 1.0, and 1.5% O2 in Ar, Messer Ar/O2 5.0). Gases were delivered with a mechanical mass flow controller, and gas compositions were measured with a micro gas chromatograph (GC, Varian cp4900 equipped with Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: November 26, 2012 Revised: February 4, 2013 Published: February 5, 2013 4250

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oxides are capable of being reduced at moderate temperatures and oxygen partial pressures, only a handful are capable of reoxidizing in the presence of H2O or CO2. Oxidation is thermodynamically favorable when Δgo̅ xd − ΔgH̅ 2O < 0 or Δgo̅ xd − ΔgC̅ O2 < 0, where ΔgH̅ 2O and ΔgC̅ O2 are the Gibbs energy of oxidation of H2 to H2O and CO to CO2, respectively, and Δgo̅ xd is the Gibbs free energy of oxidation of the reduced oxide in the presence of O2, defined as

Molsieve-5A/Poraplot-U columns). Temperatures were varied between 1623 and 1773 K. CO2 splitting cycles were conducted in the aforementioned TGA at reduction temperatures of 1773 K and oxidation temperatures between 1073 and 1273 K. During reduction, 300 standard cubic centimeters per minute (sccm) of high-purity Ar (Messer, Argon 5.0) was delivered as a sweep gas and 300 sccm of 40% CO2 (Messer, ≥99.995%) was delivered during oxidation. During all experiments, a heating rate of 20 K min−1 was used. To account for buoyancy effects, duplicate runs were conducted without the presence of a sample. The minimum O2 concentration achieved during reduction was on the order of 1 × 10−4 atm. Defect Theory. Several defect models have been developed to describe the oxygen non-stoichiometry in LSM.15,16,18,19 Models that describe the oxygen excess region differ substantially from those describing the oxygen-deficient region,16 and only the latter are further discussed because the focus of this study is on the oxygen deficiency of LSM. In LSM perovskites, the divalent dopant strontium replaces the trivalent lanthanum on the A site of the crystal, creating a hole that is located at the Mn site. Its chemical formula in the Kröger−Vink20 notation reads as La1−xSr′xMnx1−xMn•x O3. The reduction of LSM in the oxygen-deficient region in Kröger−Vink notation is x OOx + 2Mn•Mn ↔ V •• O + 2Mn Mn +

1 O2 (g) 2

δ

Δgoxd ̅ =

∫δ i ΔgO̅ dδ f δi − δf

(9)

where δi is the degree of non-stoichiometry following thermal reduction, δf is the final degree of non-stoichiometry after oxidation, and ΔgO̅ is the partial oxygen molar free energy. ΔgO̅ can be directly obtained from oxygen non-stoichiometry data as a function of the temperature and oxygen partial pressure.10,11 According to the aforementioned criteria, lanthanum−strontium− manganese-based perovskites are a promising class of redox materials. In Figure 1, Δgo̅ xd is plotted as a function of the temperature for

(1)

(V•• O)

and reduction of and involves the formation of oxygen vacancies tetravalent manganese (Mn•Mn) to trivalent manganese (MnxMn) and the evolution of O2(g). The equilibrium constant associated with this reaction is expressed by K red =

x 2 1/2 [V •• O ][Mn Mn] pO 2

[OOx][Mn•Mn]2

(2)

Disproportionation of trivalent manganese to tetravalent and divalent ′ ) oxidation states can occur according to the following reaction: (MnMn x 2Mn Mn ↔ Mn•Mn + Mn′Mn

(3)

It has been shown that, when defect concentrations exceed 0.1%, a random defect model and a fixed A/B cation ratio15 cannot describe deviations from stoichiometry adequately. Van Roosmalen et al.18 included the addition of a cluster to the random defect model and observed behavior that was more consistent to experimental data at higher concentrations. The so-called defect cluster model is based on ′ ) generated by the assumption that all divalent manganese ions (MnMn charge disproportionation form defect clusters with neighboring oxygen vacancies, as shown by x 2Mn Mn + OOx ↔ ⟨Mn′Mn − V •• O − Mn′Mn⟩ +

1 O2 2

Figure 1. Gibbs free energy of oxidation of LSM30, LSM40, ceria, and H2 as a function of the temperature. Oxidation of LSM30, LSM40, and ceria occur according to the generic oxidation reactions MOδi + ((δf − δi)/2)O2 → MOδf, where MOδ refers to an oxygen-deficient oxide, δi = 0.1, and δf = 0.03. La0.7Sr0.3MnO3 (LSM30) and La0.6Sr0.4MnO3 (LSM40), calculated from non-stoichiometry data from Tagawa et al.,21 and compared to ΔgH̅ 2O derived from NIST-JANAF tables. Additionally, Δgo̅ xd for the oxidation of ceria is included for reference. All calculations based on ceria are discussed in detail by Scheffe et al.11 As seen, for oxidation between δi = 0.1 and δf = 0.03, Δgo̅ xd of LSM30 is lower than ΔgH̅ 2O below ∼600 K and LSM40 below ∼400 K. Although no data are shown for strontium concentrations below x = 0.3, the oxidation thermodynamics become more favorable as the strontium concentration decreases. However, the reduction potential at a given temperature and oxygen partial pressure also continues to decrease as the strontium concentration decreases. In fact, only LSM-based materials with greater than x = 0.3 strontium content are expected to reduce to a greater extent than ceria at 1273 K. It is also apparent that the oxidation potential of reduced ceria is greater than both LSM30 and LSM40. It’s oxidation in the presence of H2O is thermodynamically favorable up to temperatures as high as 1200 K. Nevertheless, the temperatures required to obtain the same degree of reduction as LSM30 and LSM40 are substantially greater and will be discussed in the forthcoming sections. Knowledge of Kred and Kcf as a function of the temperature are necessary to predict equilibrium non-stoichiometry at temperatures above those where experimental data are available. Calculated values of

(4)

and the associated equilibrium constant is given by Kcf =

1/2 [⟨Mn′Mn−V •• O −Mn′Mn⟩]pO 2

x 2 [Mn Mn ] [OOx]

(5)

Electroneutrality, crystal site conservation, and oxygen site balances yield • 2[V •• O ] + [Mn Mn] = [Sr′La]

(6)

x [Mn Mn ] + [Mn•Mn] + 2[⟨Mn′Mn−V •• O −Mn′Mn⟩] = 1

(7)

x •• [V •• O ] + [OO] + [⟨Mn′Mn−V O −Mn′Mn⟩] = 3

(8)

where Sr′La is a strontium ion localized on a lanthanum lattice site. From eqs 2 and 5−8, the equilibrium constants Kred and Kcf may be determined as a function of the temperature by minimizing the sum of square differences between experimental data and calculated δ, where δ •• = [V•• O ] + [⟨Mn′Mn−VO −Mn′Mn⟩]. Thermodynamic Modeling. One of the main criteria for a H2O or CO2 splitting candidate is favorable oxidation thermodynamics following reduction to a non-stoichiometric state. While numerous 4251

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Figure 2. Least square fits of (a) Kred and (b) Kcf based on data from Tagawa et al.21

Figure 3. Equilibrium non-stoichiometry of LSM30 (left) and LSM40 (right) as a function of pO2 between 873 and 1973 K. Data are compiled from Tagawa et al.21 well with experimental data from Tagawa et al.21 Figure 3 shows model and experimental data points for LSM30 (left) and LSM40 (right) for temperatures between 873 and 1273 K. For both LSM30 and LSM40, the agreement between the model and data is better for larger deviations from non-stoichiometry. For small deviations from stoichiometry (δ < ∼0.04), however, the model overpredicts δ, where contributions because of cluster formation are small. The experimental data of LSM30 showed a variation at δ = 0.2 that can probably be attributed to a change in defect behavior; the model was not compared below this point. Extrapolations to temperatures as high as 1973 K are included, but a comparison with experimental data is not possible at such elevated temperatures. Although only model predictions for LSM30 and LSM40 are included, δ is expected to increase as the strontium concentration is increased.21 For example, at 1773 K and pO2 of 10−5 (×105 Pa), LSM40 is predicted to have a

Kred and Kcf, determined from a least-squares analysis based on data from Tagawa et al.,21 are shown in panels a and b of Figure 2, respectively, for La0.8Sr0.2MnO3 (LSM20), LSM30, and LSM40. Both Kred and Kcf have an exponential dependence with respect to inverse temperature. This trend is expected for Kred because the number of oxygen vacancies created are known to increase with an increasing temperature at a given pO2. These trends are consistent with the results by Nowotny et al., who studied doped and undoped LaMnO3 systems,17 but their absolute values are about 3 orders of magnitude greater than the values presented here. This can be explained by the differences in the assumptions between the defect model used by Nowotny et al.17 and the model applied in this study based on Van Roosmalen et al.18 The former assumes that all vacancies created form a cluster with divalent manganese and excess divalent manganese exists independent of oxygen vacancies.17 On the other hand, the latter assumes that all divalent manganese forms a cluster with oxygen vacancies and oxygen vacancies still exist as isolated species within the lattice. The positive correlation of cluster formation and temperature seen here is contrary to the behavior observed for ceria-based systems. Cluster formations between localized electrons and oxygen vacancies in ceria have been shown to decrease with the temperature.22,23 Predicted equilibrium non-stoichiometry as a function of the temperature and oxygen partial pressure based on linear least-squares fits of log Kred and log Kcf with respect to inverse temperature agrees

deviation from stoichiometry equal to δ = 0.178, whereas LSM30 is expected to achieve δ = 0.124. As a reference, equilibrium δ of ceria under the same conditions is 0.077. Equilibrium hydrogen yields can be obtained from knowledge of ΔgO̅ and the equilibrium constant of H2O dissociation (H2O ⇄ H2 + 1/2O2), Kw, according to7,11 4252

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Figure 4. Predicted equilibrium H2 yields of LSM30, LSM40, and ceria as a function of the oxidation temperature for (a) nH2O = δi and (b) nH2O = 100δi. δi was computed for a reduction temperature of 1773 K and pO2 of 10−5 (×105 Pa) and was 0.178, 0.124, and 0.077 for LSM40, LSM30, and ceria, respectively. ⎛ nH OK w ⎞ ⎟⎟ ΔgO̅ (δ = δi − nH2 , T ) = RT ln⎜⎜ 2 ⎝ nH2 ⎠

(10)

where δi is the non-stoichiometry achieved during thermal reduction, R is the ideal gas constant, T is the temperature, and nH2 and nH2O are the equilibrium yields of H2 and H2O, respectively. Figure 4 shows nH2 as a function of the oxidation temperature for LSM30, LSM40, and ceria. For an initial water concentration equal to δi (see the caption of Figure 4), both LSM30 and LSM40 are expected to produce more H2 than ceria at low temperatures (< ∼600 K), but their yields decrease quickly with an increasing temperature compared to ceria (Figure 4a). Ceria, on the other hand, is predicted to produce stable amounts of H2, roughly equal to its initial degree of reduction until temperatures of 900 K. The differences in the behavior can be explained by the more favorable Δgo̅ xd for ceria compared to both LSM materials, as discussed in Figure 1. For example, even at the lowest oxidation temperature, where oxidation is most thermodynamically favorable for LSM40, the H2 yield is only 0.115 mol compared to 0.178 mol if it were to be stoichiometrically oxidized. The advantage of increasing the initial water concentration to 100 times δi can be seen in Figure 4b. At 400 K, LSM40 is expected to be stoichiometrically oxidized and produce more H2 than ceria until temperatures of 1100 K (130% greater at 400 K). At equilibrium, LSM30 produces less H2 than LSM40 at low temperatures but still produces roughly 60% more H2 at 400 K compared to ceria. Its decrease with the temperature is less severe than LSM40, and it produces more H2 than LSM40 and ceria in the range of 750−1300 K. Ceria is expected to oxidize nearly stoichiometrically over all temperatures. Experimental Investigation. Although thermodynamic data exist for LSM30 and LSM40, experimentation was carried out with LSM35 because of its commercial availability. The oxygen release and uptake of LSM35, as measured by the mass change with the thermogravimeter, was stable to temperatures as high as 1773 K in the range of oxygen partial pressures of interest. Figure 5 shows the relative mass change as a function of the temperature while cycling between a relatively high pO2 (0.2 × 105 Pa), where the non-stoichiometry approaches zero, to a lower pO2 of 1.9 × 10−3 (×105 Pa). As expected, the mass change increases incrementally with the temperature, and the sample is able to be completely reoxidized when exposed to a high oxygen partial pressure. The reduction is slower than oxidation for all temperatures investigated, but anything beyond a qualitative inference is beyond the scope of this work. The stepwise pO2 shown on the upper left axis is not the temporally measured pO2 but rather the pO2

Figure 5. Relative mass change of LSM35 between 1623 and 1773 K and oxygen partial pressures of 1.9 × 10−3 (×105 Pa) and 0.2 (×105 Pa). measured at steady state once equilibrium was reached following reduction. Oxygen non-stoichiometry measurements between 1623 and 1773 K at oxygen partial pressures of 1.1 × 10−3, 1.9 × 10−3, 3.5 × 10−3, and 5.4 × 10−3 (×105 Pa) agree well with the extrapolations from lower temperature data discussed in Figure 3. Figure 6 shows the relative mass changes of LSM35 and ceria. Yields increase with the temperature and decreasing oxygen partial pressure. The reduction extent for ceria was substantially lower than that for LSM35, even when reduced at 1773 K, as compared to reduction at 1673 K for LSM35. There was also a notable change in the slope of relative mass change with respect to the temperature somewhere between Δm% = 0.1 and 0.2. This can be seen for the highest pO2, where the first two data points at 1623 and 1673 K fall at 0.045 and 0.085%, yet the relative mass change at 1723 K is substantially greater at 0.23%. The same sharp increase in mass loss can be seen between 1623 and 1673 K for oxygen partial pressures of 1.9 × 10−3 (×105 Pa) and 3.3 × 10−3 (×105 Pa). Experimentally determined non-stoichiometry of LSM35 at elevated temperatures is compared to model predictions of LSM30 and LSM40 in Figure 7. Although thermodynamic data for LSM35 at lower temperatures was not available for model calculations to directly compare our results, it is reasonable to expect that the results fall between LSM30 and LSM40. Apart from very small nonstoichiometries (δ < ∼0.02), experimentally determined values for LSM35 fall nearly in the middle of model predictions of LSM30 and 4253

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Figure 8. CO2 [pCO2 = 0.40 (×105 Pa)] splitting cycles for LSM35 (dashed lines) and ceria (black lines) reduced at 1773 K [pO2 = 1.5 × 10−3 (×105 Pa)] and oxidized at 1073, 1173, and 1273 K. The total Ar flow rate during reduction and CO2/Ar flow rate during oxidation were 300 sccm.

Figure 6. Relative mass change of LSM35 (black) and ceria (gray) as a function of the oxygen partial pressure and temperature.

yields after oxidation were still substantially greater than ceria at 1173 and 1273 K. The initial reduction extent of LSM35 was roughly 2 times greater than ceria, yet neither material reacted long enough to reach its thermodynamic equilibrium. The oxidation rates of ceria were faster than LSM35, and it was capable of being reoxidized completely, consistent with thermodynamic predictions. Although comparisons between the experimental results from an open system, such as the TGA, to thermodynamic modeling, which assumes a closed system, should be made judiciously, the experimental trends observed here qualitatively agree with the thermodynamic predictions. Fuel yields and reaction rates may improve if a higher surface area structure or powder is used instead of the dense pellet used here. Preliminary observations have indicated the sintering of LSM35 particles and material decomposition above 1823 K and pO2 = 1 × 10−4 (×105 Pa). Efficiency. Although fuel yields per mole of oxide are high for LSM30 and LSM40 based on thermodynamic calculations and for LSM35 based on experimental evidence, a consideration of all energy inputs required to produce the fuel must be considered when comparing potential materials. The solar−fuel energy conversion efficiency, ηsolar−fuel, is such a metric. For the case of H2 production, it is defined as

Figure 7. Experimentally determined non-stoichiometry of LSM35 (dashed lines) compared to LSM30 (solid lines) and LSM40 (gray dashed lines) model predictions at 1673 and 1773 K. LSM40. The trends with pO2 are also consistent. The deviation at low non-stoichiometries occurs at the same point as the change in behavior of the mass loss observed in Figure 6. This is presumably due to a change in the reduction mechanism, which the model does not take into account. Contrary to Figure 3, where the model predicts smaller deviations from stoichiometry compared to the data, the results of Figure 7 indicate that the model overpredicts the non-stoichiometry compared to LSM35. In general, the model predictions agree well with the experimentally determined values, where non-stoichiometry is of interest to thermochemical fuel production (δ > 0.02), validating the theoretical methodology applied to study the high-temperature behavior of LSM compounds. Figure 8 shows the relative mass change of LSM35 and ceria as a function of the temperature for 3 consecutive cycles, with reduction at 1773 K and oxidation with CO2 at 1073, 1173, and 1273 K. The total Ar flow rate during reduction and CO2/Ar flow rate during oxidation were 300 sccm. The oxidation with CO2 rather than H2O was performed because of practical limitations of the TGA with condensable gases, but the oxidation with H2O is expected to be qualitatively similar.24 The experimental results show that, even though oxidation is more thermodynamically favorable at 1073 K compared to 1273 K, the rates are much slower. LSM35 was not capable of being completely reoxidized under any of the conditions investigated for the duration of the experiments, which is consistent with the trends predicted by thermodynamic modeling. Nevertheless, the total fuel

ηsolar−fuel =

HHVH2nH2 Q solar + Epenalties

(11)

where HHVH2 is the higher heating value of H2, nH2 is the amount of H2 produced (in moles), Qsolar is the solar radiative energy input through the aperture of the solar reactor, and Epenalties is the systemspecific energy penalties, such as those derived from the consumption of inert gas, electricity, or pumping work for promoting the chemical reactions. Note that ηsolar−fuel does not include the optical efficiency of the solar concentrating system. Additionally, this definition assumes that only the high-temperature endothermic reduction step requires solar energy as an input. This definition has a direct impact on the economics of the process. Higher ηsolar−fuel implies a smaller solar concentrating system for the same fuel output, which directly translates to a lower fuel cost, because analogous to solar thermal electricity (CSP) plants, the major cost component derives from the investment of the solar collecting and concentrating infrastructure. Qsolar is calculated as ⎛ Q solar = ⎜Δh H ̅ 2O|298K → TL nH2O + nox ⎝ /ηabs 4254

∫T

TH

L

⎞ c p,ox dT + Δhred ̅ δnox ⎟ ⎠ (12)

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Figure 9. Solar−fuel energy conversion efficiency versus the oxidation temperature for LSM40, LSM30, and ceria reduced at (a) 1600 K and (b) 1800 K. pO2 is 10−5 (×105 Pa), and the water concentration is nH2O = δi.

Figure 10. Solar−fuel energy conversion efficiency versus the oxidation temperature for LSM40, LSM30, and ceria reduced at (a) 1600 K and (b) 1800 K. pO2 is 10−5 (×105 Pa), and the water concentration is nH2O = 100δi. Energy required to heat excess steam is not taken into account. where Δh̅H2O|298 K→TL is the enthalpy required to heat water from ambient temperature to the oxidation temperature (TL), nH2O is the number of moles of water, nox is the number of moles of metal oxide used, cp,ox is the specific heat capacity of the material, TH is the thermal reduction temperature, Δh̅red is the reduction enthalpy, δ is the degree of non-stoichiometry achieved during thermal reduction, and ηabs is the solar energy absorption efficiency. Thus, this calculation of Qsolar assumes that the sensible heat of solid and gases between the redox steps is not recovered. The reduction enthalpy is given by

similar to that of heavily doped LSM materials at lower temperatures.27 The heat capacity of ceria was extrapolated from Riess et al.28 To obtain the highest theoretical ηsolar−fuel, Epenalties = 0 is assumed. In practice, additional energy would be required for inert gas separations (e.g., 20 kJ/mol)29 or pumping work to achieve the reduced oxygen partial pressures. For the purpose of formulating a metric for comparison between the new materials cycle and the reference ceria cycle, Epenalties was omitted from consideration because it is strongly dependent upon the process design and it will ultimately be comparable for the same reactor technology. Figure 9 shows the theoretical ηsolar−fuel as a function of the oxidation temperature for LSM40, LSM30, and ceria reduced at pO2 of 10−5 (×105 Pa) and at 1600 K (Figure 9a) and 1800 K (Figure 9b). Only a stoichiometric amount of water was considered (nH2O = δi), and pO2 was taken as 10−5 (×105 Pa). All materials are predicted to have single-digit efficiencies when reduced at 1600 K, partly because of the low reduction extents at these conditions being 0.11, 0.045, and 0.017 for LSM40, LSM30, and ceria, respectively. Both LSM30 and LSM40 have higher predicted ηsolar−fuel than ceria below ∼450 K, but it decreases with the temperature because of decreasing H2 yields. Even at 400 K, where their oxidation thermodynamics are most favorable, their H2 yields are only 0.049 and 0.037 mol, much less than what is possible if they were able to be completely reoxidized. In contrast, ηsolar−fuel of ceria increases with the oxidation temperature because its H2 yields remain stable between 400 and 900 K, but less energy is required for heating to the reduction temperature. The effect of

δi

Δhred ̅ =

∫δ ΔhO̅ (δ)dδ f

δi − δf

(13)

where Δh̅O is the partial oxygen molar enthalpy and can be directly obtained from oxygen non-stoichiometry data as a function of the temperature and oxygen partial pressure.10,11 For a perfectly insulated solar cavity receiver, the solar energy absorption efficiency is given by25

⎛ σT 4 ⎞ ηabs = 1 − ⎜ ⎟ ⎝ IC ⎠

(14)

where σ is the Stefan−Boltzmann constant, I is the direct normal irradiation (DNI), and C is the solar flux concentration ratio of the incident concentrated solar radiation. I was taken to be 1 kW m−2, and C was 5000. The specific heat capacity of LSM at high temperatures was based on undoped lanthanum manganite (LaMnO3),26 which is 4255

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increasing the reduction temperature to 1800 K can be seen in Figure 9b. Here, for all temperatures, ηsolar−fuel is higher for ceria and increases with the oxidation temperature approaching nearly 20%. ηsolar−fuel for LSM40 is somewhat lower than that for LSM30, even though it is reduced substantially more, because of low oxidation yields. The reduction extents for LSM40, LSM30, and ceria are 0.189, 0.136, and 0.096, respectively. At 400 K, ηsolar−fuel for ceria is ∼15% compared to 11% for LSM40, even though the H2 yield is lower for ceria (0.096 mol) than that for LSM40 (0.111 mol). This is primarily due to the larger specific heat of LSM40 compared to that of ceria. Thus, the energy required to heat LSM40 from 400 to 1800 K is ∼208 kJ mol−1, whereas it is only ∼112 kJ mol−1 for ceria. ηsolar−fuel for ceria reaches a maximum theoretical value at 1000 K of ∼20%, assuming no sensible heat recovery of solid and gases. Increasing the water concentration has a positive effect on ηsolar−fuel for both LSM-based materials. Figure 10 shows ηsolar−fuel as a function of the oxidation temperature for LSM40, LSM30, and ceria reduced at pO2 of 10−5 (×105 Pa) and at 1600 K (Figure 10a) and 1800 K (Figure 10b). Here, instead of considering a stoichiometric amount of water, the concentration was increased by 100 times (nH2O = 100δi). The energy required to heat the excess steam was not included, because the purpose of this calculation is to show the effect of increasing H2 yields by way of increased steam concentrations, assuming that the additional energy required to heat the steam could be recovered in an efficient way. If no heat recovery is considered, there is a detrimental effect of using steam beyond nH2O ∼ 2δi. For all oxidation temperatures, LSM40 has a larger ηsolar−fuel than either LSM30 or ceria. At 400 K, its H2 yield is 0.107 mol with nH2O = 100δi compared to 0.049 mol with nH2O = δi, yet even with 100 times excess steam, the H2 yields decrease as the oxidation temperature increases, resulting in decreasing ηsolar−fuel. The effect of increasing the water concentration is negligible for ceria, except at 1000 K, where the H2 yield is expected to increase in the presence of excess steam. The effect of increasing the reduction temperature to 1800 K while still conducting the oxidation with excess steam is shown in Figure 10b. Here, LSM40 only has higher predicted efficiencies than ceria at temperatures below ∼550 K. ηsolar−fuel for ceria is higher than LSM30 for all oxidation temperatures and reaches a maximum theoretical value at 1000 K of ∼22%.

temperatures of 1073, 1173, and 1273 K in the presence of CO2 after thermal reduction at 1773 K, and qualitatively, the results followed the trends predicted by the thermodynamic model. The investigation of reaction kinetics was not within the scope of this study. Because of the low specific surface area of the pressed pills, the observed slower rates of oxidation of LSM35 compared to ceria are consistent with lower oxygen chemical diffusion coefficients (Dchem). Dchem for ceria at 1073 K is reported to be roughly 2 × 10−5 cm2/s,7 which is higher than that reported for LSM20 at 1173 K (Dchem = 8 × 10−6 cm2/s) and 1273 K (Dchem = 2.8 × 10−5 cm2/s).30 For higher strontium contents, Dchem is expected to be even lower.31 Although larger oxidation yields were experimentally observed with LSM35 and predicted for LSM30 and LSM40, ceria is still expected to have a larger solar−fuel energy conversion efficiency under most conditions. This is primarily due to the fact that the LSM-based materials do not oxidize stoichiometrically and have nearly twice the heat capacity compared to ceria. The energy required to heat LSM40 from 400 to 1800 K is ∼208 kJ mol−1, whereas it is only ∼112 kJ mol−1 for ceria. However, if one considers a system with heat recuperation of excess steam, then LSM-based materials begin to look more attractive because oxidation proceeds nearly stoichiometrically. For instance, at reduction temperatures of 1600 K, LSM40 reaches ηsolar−fuel of ∼13% compared to ∼4.5% for ceria under the same conditions. At higher reduction temperatures, however, ceria is capable of achieving ηsolar−fuel of 22% compared to 16% for LSM40. These calculations assume that the sensible heat of solids and gases is not recovered between the redox steps. The results presented here provide a simple framework to investigate other potential redox materials. The class of LSM oxides was chosen for this study because of the exhaustive amount of lower temperature data available in the literature with which to build a robust thermodynamic model and compare our experimental data. Other perovskites, including chromium- and calcium-doped lanthanum manganites, may have more favorable oxidation thermodynamics than LSM perovskites.32 However, their thermodynamics and oxygen non-stoichiometry have not been characterized to the same degree. The work presented also highlights the importance of considering the effect of oxidation thermodynamics on the overall solar−fuel energy conversion efficiency.

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DISCUSSION AND CONCLUSION LSM-based materials are one class of non-stoichiometric oxides that provide an alternative to cerium-oxide-based systems for application in two-step solar redox cycles for splitting H2O and CO2. The potential of these materials was assessed on the basis of oxygen non-stoichiometry data directly available from the literature and a cluster defect model extrapolated to higher temperatures. In comparison to ceria, LSM30 and LSM40 are both expected to be reduced to a greater extent than ceria under the same conditions. The deviations from stoichiometry of LSM40 are expected to be nearly 6.5 times larger than those of ceria at 1600 K [pO2 = 10−5 (×105 Pa)] and 2 times larger at 1800 K [pO2 = 10−5 (×105 Pa)]. Thermodynamic predictions indicated that oxidation of LSM-based materials is less favorable than ceria and becomes less favorable as the amount of strontium is increased. Nevertheless, at low oxidation temperatures, the H2 yields of LSM40 are expected to be roughly 3 and 1.15 times greater at reduction conditions of 1600 and 1800 K [pO2 = 10−5 (×105 Pa)]. These yields become even greater and approach their stoichiometric limits when higher water concentrations (nH2O = 100δi) are considered. The oxygen non-stoichiometry predicted by the thermodynamic model were experimentally validated with LSM35 using thermogravimetric measurements in the range of 1623−1773 K. Cyclic experimental runs were also performed at oxidation

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +41-44-6339380. E-mail: jscheff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding by the Swiss Competence Center for Energy and Mobility is gratefully acknowledged.

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NOMENCLATURE C = solar flux concentration ratio of the incident concentrated solar radiation (suns) cp,ox = heat capacity (kJ mol−1 K−1) Dchem = oxygen chemical diffusion coefficient (cm2 s−1) Epenalties = system-specific energy penalties (kJ) HHVH2 = higher heating value of H2 (kJ mol−1) I = direct normal irradiation (DNI) (kW m−2) dx.doi.org/10.1021/ef301923h | Energy Fuels 2013, 27, 4250−4257

Energy & Fuels

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Kcf = equilibrium constant for defect cluster formation Kred = equilibrium constant for reduction of LSM Kw = equilibrium constant of H2O dissociation ⟨MnMn ′ −V•• ′ ⟩ = defect cluster O −MnMn Mn′Mn = divalent manganese Mn•Mn = tetravalent manganese MnxMn = trivalent manganese OxO = oxygen atom on the oxygen lattice site nH2 = moles of H2 (mol) nH2O = moles of H2O (mol) nox = moles of metal oxide (mol) pO2 = oxygen partial pressure (×105, Pa) Qsolar = solar radiative energy input through the aperture of the solar reactor (kJ) R = ideal gas constant (kJ mol−1 K−1) ′ = strontium ion localized on a lanthanum lattice site SrLa T = temperature (K) TH = reduction temperature (K) TL = oxidation temperature (K) V•• O = oxygen vacancy δ = degree of non-stoichiometry δf = the final degree of non-stoichiometry after oxidation δi = degree of non-stoichiometry following thermal reduction ΔgC̅ O2 = Gibbs energy of oxidation of CO to CO2 (kJ mol−1) ΔgH̅ 2O = Gibbs energy of oxidation of H2 to H2O (kJ mol−1) ΔgO̅ = partial oxygen molar free energy (kJ mol−1) Δgo̅ xd = Gibbs free energy of oxidation of the reduced oxide in the presence of O2 (kJ mol−1) Δh̅H2O = energy required to heat H2O (kJ mol−1) Δh̅O = partial oxygen molar free enthalpy (kJ mol−1) Δh̅red = reduction enthalpy (kJ mol−1) ηabs = solar energy absorption efficiency ηsolar−fuel = solar−fuel energy conversion efficiency σ = Stefan−Boltzmann constant (5.6705 × 10−8 W m−2 K−4)

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dx.doi.org/10.1021/ef301923h | Energy Fuels 2013, 27, 4250−4257