Investigation of Perovskite Structures as Oxygen-Exchange Redox

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Investigation of Perovskite Structures as OxygenExchange Redox Materials for Hydrogen Production from Thermochemical Two-Step Water-Splitting Cycles Antoine Demont, Stéphane Abanades, and Eric Beche J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5034849 • Publication Date (Web): 28 May 2014 Downloaded from http://pubs.acs.org on May 30, 2014

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The Journal of Physical Chemistry

Investigation of Perovskite Structures as OxygenExchange Redox Materials for Hydrogen Production from Thermochemical Two-Step Water-Splitting Cycles Antoine DEMONT, Stéphane ABANADES*, Eric BECHE Processes, Materials, and Solar Energy Laboratory, PROMES-CNRS (UPR 8521), 7 Rue du Four Solaire, 66120 Font-Romeu, FRANCE

Abstract: This study addresses the synthesis, characterization, and thermochemical redox performance evaluation of perovskites and parent structures (Ruddlesden Popper phases) as a class of oxygenexchange materials for hydrogen generation via solar two-step water-splitting. The investigated materials are LaxSr1-xMO3 (M=Mn, Co, Fe), BaxSr1-x(Co,Fe)O3, LaSrCoO4 and LaSrFeO4, also used as mixed ionic-electronic conductors in fuel cells. Temperature-programmed reduction, powder X-ray diffraction and thermogravimetric analysis were used to obtain a preliminary assessment of these materials performances. Most of the perovskites studied here stand out by

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larger thermal reduction capabilities and oxygen vacancies formation at modest temperatures in the range 1000-1400°C when compared to reference non-stoichiometric compounds such as spinel ferrites or fluorite-structured ceria-based materials. In addition, these materials offer noticeable access to metallic valence transitions during reoxidation in steam atmosphere that are not available in stoichiometric oxides. The promising behaviors characterized here are discussed in regard of the crystal chemistry of the perovskite and parent phases.

Keywords: Solar energy – Solar fuel – Water dissociation – Mixed metal oxide – Oxygendeficient material – Non-stoichiometry

1. Introduction Reducing climate changes and dependency on fossil fuels are some of the challenging tasks faced by emerging technologies. In this context, the use of H2 as an energy carrier appears both as a credible and clean alternative, for example in fuel cells. H2-based technologies have gained considerable steps forward over the last 20 years, and could efficiently operate soon. However, one of the current drawbacks of these technologies is that, despite hydrogen element abundance on the earth, it is not naturally encountered in the H2 molecular form, and thus has to be produced artificially using cost effective, clean and efficient routes. In this context, solar thermochemical two-step water-splitting cycles (STWC) appear as an ideal sustainable process for molecular hydrogen generation. STWC involve the thermal reduction of a metal oxide (MO)


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at a given T1 temperature (Eq. 1) followed by the re-oxidation under H2O exposure (Eq. 2) at another T2 temperature, inducing H2 evolution. MO → MO1-δ + (δ/2) O2

(endothermic, T1)

(1)

MO1-δ + δ H2O → MO + δ H2

(exothermic, T2 < T1)

(2)

Such reactions have been performed using stoichiometric compounds such as the Fe3O4 spinel decomposing into FeO and O2 during thermal reduction.1 However, Fe3O4 thermal reduction requires high temperature (1600°C at least) and substituted magnetite have then been widely studied in order to improve the reduction yields at lower temperatures.2-9 Other optimizing efforts have focused on the microstructure improvement for enhancement of the solid-gas reaction involved during the water-splitting step. Considering both composition and microstructure optimization, the best H2 production yields are currently obtained on ZrO2supported NiFe2O4.6-9 More recently, the CeO2/Ce2O3 redox couple was also investigated, resulting in high reaction rates for both the reduction and hydrolysis steps.10 Here again, however, CeO2 reduction temperature into Ce2O3 (2000°C) is somewhat practically prohibitive in view of coupling with a solar concentrating facility for large scale H2 production. Nevertheless, further studies have demonstrated the possibility to promote significant H2 productions using non stoichiometric CeO2-based materials, with considerably decreased reduction temperatures (1400-1500°C),11-14 owing to the ability of the fluorite structure to sustain large oxygen deficiency. Such studies contributed in demonstrating that non-stoichiometric oxygen-deficient materials could compete with the performances practically achieved in classical stoichiometric compounds during STWC. Since then, various studies have made profit of the oxygen deficiency available in fluorite to synthesize some of the most efficient materials


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currently reported,14-17 and ceria and substitution-based related phases are considered as state of the art materials in the field. Surely even more than the fluorite, the ABO3 perovskite is a prototypical example of crystal structure having the ability to sustain topotactical oxygen disincorporation, therefore inducing materials of the form ABO3-δ. The δ oxygen deficiency displayed by numerous perovskites far exceeds the maximum values reported in fluorite-based oxides. For instance, some of the best known perovskites are Ca2FeAlO5 (CaFe0.5Al0.5O2.5) and YBa2Cu3O7 (Y0.33Ba0.67CuO2.33), famous for entering in the composition of Portland cement clinker and being a high critical temperature superconductor respectively, which retain a perovskite related crystal structure with δ = 0.5 and 0.67 respectively. In contrast, the 0.5 value in the CeO2-δ fluorite would ideally correspond to CeO1.5 which has a totally distinct crystal structure in the form of Ce2O3. Furthermore, several ABO2 compounds (δ = 1) exist, illustrating well the adaptability of the perovskite crystal structure to oxide anion deficiency. When combined with the presence of transition metals able to adopt a variety of oxidation states, such intriguing crystal chemistry enhances rich redox properties that have recently attracted attention in the context of solar-driven thermochemical cycles using oxygen-deficient perovskites. Thus, elevated activity for watersplitting was demonstrated in La1-xSrxMO3-δ materials,18-21 although it must be underlined that the use of reducing agents such as CH4 is surely beneficial to the thermal reduction yield, which in turn results in a larger potential for re-oxidation, given the increase of δ prior to the watersplitting step. Even more recently, it was theoretically shown that some strontium-doped lanthanum manganites could display favorable thermodynamics for STWC,22 and performances comparing favorably to that of CeO2 were evidenced in Sr- and Mn-substituted LaAlO3,23 with H2 productions that are only slightly lower than those measured in Zr-substituted ceria.


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A class of perovskites that may retain attention of the community in the context of STWC is that of solid oxide fuel cell (SOFC) electrodes, notably SOFC cathodes. In order to be perfectly efficient, a SOFC cathode must display mixed ionic-electronic conduction and show good surface exchange kinetics for the oxygen reduction reaction (ORR). Enhancement of ionic conductivity in these materials often partly relies on large oxygen vacancy generation upon heating to the SOFC operating temperature, i.e. on large thermal reduction capabilities at relatively modest temperatures when compared to state of the art materials used for STWC. This feature can be seen as very promising to allow performing STWC at operating temperatures lower than those currently used for ceria-based materials, therefore potentially achieving efficient STWC for lower solar power input. Additionally, we may also underline here that significant ionic conductivity at high temperature, which SOFC cathodes display by definition, should be beneficial for bulk diffusion processes involved during both the thermal reduction and the water-splitting step. Thus, several features displayed by these materials appear promising for STWC, and their performances have yet to be studied in this context. We therefore investigated material compositions related to well known SOFC cathodes,24 such as BSCF, LSCF, or LSM (see experimental part for detailed compositions), using relevant techniques for a preliminary assessment of their capabilities for STWC. Reduction behaviors were firstly investigated from O2 release detection during temperature-programmed reductions under Ar atmosphere, followed by the systematic gravimetric measurement of O2 productions for all materials. These experiments confirmed the large reduction capabilities for most of the compounds investigated here, exceeding that of ceria in term of reduction yield, at lower temperatures. The water-splitting activity was subsequently studied by thermogravimetric (TG) analysis and performances comparable to that of ceria in term of H2 productions were found,


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with significantly lower operating temperatures for some of the compounds. In parallel, the existence of structural changes during thermal treatment was controlled by powder X-ray diffraction (PXRD). These results, achieved without much optimization in term of chemical composition and microstructure processing, show that this class of materials has strong potential for the generation of high performing water splitting catalysts at lower temperatures than those currently in use in order to obtain relevant H2 productions. Interestingly, these perovskite materials involve redox couples that are chemically inert in simple stoichiometric oxides. These results are essentially discussed in the context of the perovskite crystal chemistry.

2. Experimental section Table 1 summarizes the abbreviations used for the compositions described hereafter. Note that LaSrCoO4 and LaSrFeO4 are referred to as written throughout the article. Commercial powders of LSC, LSCF, LSF, LSM20 and LSM35 were purchased from Sigma Aldrich (all purities ≥ 99%). The series of BSCF compounds was prepared by solid state reaction of stoichiometric amounts of BaCO3, SrCO3, Co3O4 and Fe2O3. These oxide and carbonate precursors were weighted and intimately ground with an agate mortar and pestle. The resulting mixtures were fired at 1000°C in alumina crucibles for a total duration of 8h with one intermediate grinding after 4h. LSM50 was prepared by solid state reaction of stoichiometric amounts of La2O3, SrCO3 and MnO2. La2O3 was fired at 950°C for 6h prior to being weighted, in order to decompose contaminant La(OH)3. The weighted precursors were mixed and ground, then fired at 1000°C for 2h and 1450°C for 8h in alumina crucibles, with one intermediate grinding after 4h. LaSrCoO4 was synthesized from La2O3, SrCO3 and Co3O4. Mixed and ground powders were fired at


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1000°C for 2h and 1400°C for 4h. LaSrFeO4 was made following the same procedure with La2O3, SrCO3 and Fe2O3. A small quantity of a perovskite phase was then detected in the sample by PXRD analysis, and an additional firing was therefore performed at 1450°C for 4h, which allowed obtaining a single phase sample. Compound Ba0.5Sr0.5Co0.8Fe0.2O3-δ Ba0.5Sr0.5Co0.6Fe0.4O3-δ

Abbreviation BSCF5582 BSCF5564

Ba0.5Sr0.5Co0.2Fe0.8O3-δ

BSCF5528

Ba0.25Sr07.5Co0.8Fe0.2O3-δ

BSCF25/75/8/2

La0.8Sr0.2MnO3-δ

LSM20

La0.65Sr0.35MnO3-δ

LSM35

La0.50Sr0.50MnO3-δ

LSM50

La0.6Sr0.4Co0.2Fe0.8O3-δ

LSCF

La0.6Sr0.4FeO3-δ

LSF

La0.8Sr0.2CoO3-δ

LSC

Table 1. Summary of the abbreviations used for the different compositions Additionally, BSCF5582 and LSCF were also synthesized using a modified Pechini method. For BSCF5582, stoichiometric amounts of Ba(NO3)2, Sr(NO3)2, Co(NO3)2.6H2O and Fe(NO3)3.9H2O were dissolved into water with citric acid. The molar ratio between citric acid and the sum of metallic cations was 3/2. The solution was heated at 90°C and ethylene glycol was added with a 3:2 weight proportion for citric acid:ethylene glycol. The solution was stirred and maintained at 90°C until a gel was formed. The gel was then dried at 200°C for 3h and fired at 350°C for 3h. The resulting black powder was calcined at 1000°C for 1h. LSCF was made following the same procedure, except that it was prepared from stoichiometric amounts of La(NO3)3.6H2O, Sr(NO3)2, Co(NO3)2.6H2O and Fe(NO3)3.9H2O.


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Phase identification and subsequent crystallographic unit cell parameter determinations were performed by PXRD analysis of data collected at room temperature with an X’Pert Pro PANalytical diffractometer equipped with an X’celerator detector and working with Cu Kα1 and Kα2 radiations (λ = 0.15418 nm). The X-ray diffraction measurements of θ-θ symmetrical scans were made in the range 10 – 100°. The step size and the time per step were respectively fixed at 0.01° and 5 s. The X-ray diffraction spectra were recorded with a PANalytical HighScore Plus software. Crystalline phases were identified by comparison with standard reference patterns (Powder diffraction file PDF-2, International Centre for Diffraction Data, ICDD). LSM35 and BSCF5582 thermal reductions were studied with a high-temperature tubular furnace coupled with an online oxygen trace analyzer (SETNAG JC48V). Approximately 3 g of sample was introduced in an alumina crucible, and measurements were performed under Ar flow (200 ml.min-1), with a ramp rate of 10°C.min-1. TG experiments were performed with a SETARAM Sestys Evolution device coupled with a steam generator, and using about 120 mg of sample placed in a platinum crucible. The thermal reduction step was carried out under Ar flow (40 ml.min-1), and water (80% relative humidity in Ar at 40°C) was injected for the water splitting step. Ramp rates of 20°C.min-1 were applied during heating and cooling steps.

3. Results and discussion 3.1. Thermal reduction behavior Figure 1 shows the oxygen evolution for LSM35 (Fig. 1a) and BSCF5582 (Fig. 1b) upon heating under Ar atmosphere. In the case of LSM35, the furnace temperature was dwelled at 650°C in air prior to thermal reduction, in order to avoid pollution of the oxygen signal by


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possible furnace contamination. Data collection was therefore started at 650°C. Since reports show that BSCF5582 thermal reduction starts below 400°C,25 we applied the dwell and started data collection at 275°C in order to not miss important reduction features. For both compounds, the collected data are in good quantitative agreement with the TG data discussed later in this article (Fig. S1). A large O2 production is clearly observed, reaching 172 and 626 µmol.g-1, for LSM35 and BSCF5582 after 30 min dwells at 1500°C and 1100°C respectively. LSM35 shows several O2 production peaks at 1165°C, 1380°C and 1497°C. BSCF5582 shows a strong maximum at 409°C, with another weaker peak at 778°C, followed by a diffuse O2 production until 1100°C, with no well defined peak clearly emerging. Thus, complex profiles of the oxygen evolution curves were observed, especially in the case of BSCF5582, with large discontinuities in the O2 production rate as a function of temperature. This most probably relates to the peculiar and complex crystal chemistry of the compounds studied here. For instance, BSCF is a disordered perovskite displaying a large oxygen non stoichiometry26 resulting in the coexistence of Co3+, Fe3+ and Fe4+, within various oxide ion based environments such as octahedral or square based pyramidal due to the presence of disordered oxygen vacancies. As the reduction occurs, the chemical species present in the compound become Co2+, Co3+ and Fe3+, with an average coordination number gradually decreasing. In summary, several features may account for a distribution in M-O binding energies in these perovskites (where M is the transition metal): (i) the presence of several transition metals (Co and Fe) with various oxidation states. (ii) various polyhedral oxide ion environments, which imply different local repartitions of oxide anion vacancies.


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(iii) a continuous topotactic evolution of the compound during the reduction. In this case, oxide ion departure from the crystal lattice would tend not to be governed by a single energy threshold, therefore explaining the discontinuities observed here in the oxygen evolution profiles.

Figure 1. Oxygen evolution profiles collected for a) LSM35 and b) BSCF5582

Nevertheless, these data confirm that a strong thermal reduction capability at high temperature is expected for some of the perovskites related to SOFC cathode materials studied here. Moreover, as shown here for BSCF5582 (Fig. S2), the samples retained a perovskite crystal structure throughout the whole reduction temperature range, confirming the flexibility of the perovskite toward topotactic oxide anion disincorporation. Figure S2 indeed shows very similar


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powder X-ray diffractograms before and after the reduction, with all the peaks indexed on the basis of an ideal perovskite with a cubic symmetry and a ܲ݉3ത݉ space group. A noticeable change lies in a slight shift of the diffraction peaks toward lower angles, which reflects the evolution of the cell parameters after the reduction, with a volumetric expansion of the unit cell from 63.258(1) to 64.191(1) Å3, as analyzed by LeBail refinements of the PXRD data. This is in good agreement with the reduction of the average metal oxidation state, leading to greater ionic radii for the considered chemical species. This confirms via an O2 detection based method that large quantities can be produced over the course of topotactical reductions of such materials.

The systematic study of the thermal reduction behavior of the synthesized and commercial materials was carried out by TG analysis for various compositions related to SOFC cathode materials. The TG data shown here were collected at various temperatures depending on the thermal stability and melting point of the compounds. Though no melting point was precisely measured, it was fairly obvious from pre-reduction annealing experiments performed at various temperatures that (for example) LSM compounds had a higher melting point than BSCF compounds. This correlates well with the fact that solid state reactions in order to obtain LSM structure are commonly performed at 1400°C or above, while very well crystallized powders of BSCF5582 can be obtained at 1000°C using the same synthesis technique. This discrepancy in melting points influenced the choice for reduction and water-splitting temperatures. Reduction temperatures were selected to obtain a significant amount of reduction for each compound while remaining below the melting point for each composition. This trade-off is necessary for obtaining maximal reduction potential while avoiding extended material sintering that would spoil the reactivity with water. Water-splitting temperatures were chosen to ensure a significant


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gap (at least 200°C) between the reduction and the oxidation temperatures for thermodynamic limitation reasons. However, the oxidation temperature must be high enough because of kinetic limitations and an elevated temperature also promotes oxide ion bulk diffusion, which is a key property during the re-oxidation of the non-stoichiometric compounds. These temperatures thus resulted from a trade-off involving different criteria and were not definitely optimized. Reduction data collected for identical thermal treatments are presented on single figures for three LSM compounds (Fig. 2), for BSCF with various Fe/Co and Ba/Sr ratios (Fig. 3), and for LSC, LSCF, and LSF (Fig. 4). Results from the different temperature-programmed reductions performed by TG analysis were gathered together to summarize the quantitative data in term of O2 productions (Fig. 5).

Figure 2. TG analysis performed during the thermal reduction of LSM20, LSM35 and LSM50 under Ar atmosphere, for an identical thermal treatment

Figure 2 shows for LSM that the reduction extent increases with Sr2+ substitution for La3+. We reason this by simple charge balance considerations, as the increase of Sr2+ content on the A site results in a higher nominal oxidation state on the B site of the perovskite. As LaxSr1-xMnO3


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perovskites are reported to be fully stoichiometric in term of oxygen content under air atmosphere at room temperature,27 LSM20, LSM35 and LSM50, initially contain Mn+3.20, Mn+3.35 and Mn+3.50 respectively. In this mixed Mn3+/Mn4+ valence state, the higher the Mn4+ content is, the more the reduction of the compound is favored, which results in the tendency observed here within the LSM series.

Figure 3. TG analysis performed during the thermal reduction of BSCF5582, BSCF5564, BSCF5528 and BSCF25/75/8/2 under Ar atmosphere, for an identical thermal treatment

Figure 3 shows the weight losses observed for the BSCF series during thermal reduction experiments performed up to 1000°C in Ar flow. Various reports range the oxygen stoichiometry of BSCF5582 from 2.48 to 2.69,25-26, 28-33 which ranges the average M oxidation state (M = Co, Fe) from +2.96 to +3.38. The presence of M4+ species was nevertheless shown by measurements of a positive Seebeck coefficient accounting for p-type polaron hoping involving M4+ and M3+ cations.31 Such a high initial oxidation state for Co and Fe results in a large reduction extent upon heating, beginning below 300°C, with an oxygen production reaching 600 µmol.g-1 after a dwell of 45 min at 1000°C for BSCF5582. At a fixed Ba/Sr ratio of 1, the oxygen production slightly increases with the Fe content. We relate this to the oxygen content increase in BSCF as Fe


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substitutes for Co,31 leading to a greater amount of M4+ species in the compounds. In parallel, at a fixed Co/Fe ratio, the increasing weight loss from BSCF5582 to BSCF25/75/8/2 can be related to the initial oxygen content of the compounds as the end member, SrCo0.8Fe0.2O3-δ, is reported to have a larger oxygen content by various authors making direct comparisons with BSCF5582.33 Sr2+ substitution for Ba2+ in the BSCF series may therefore be expected to promote the generation of M4+ cations, thereby favoring higher reduction yields.

Figure 4. TG analysis performed during the thermal reduction of LSC, LSCF and LSF under Ar atmosphere, for an identical thermal treatment

Figure 5. Summary of the oxygen productions determined from TG analysis for compounds studied in this work.


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Similarly, with an average M oxidation state in the vicinity of +3.40, the reduction of the initially nearly oxygen stoichiometric LSCF 34 and LSF 35 draws large reduction yields at 1200°C (Fig. 4). The presence of Co favors a larger weight loss when comparing LSCF to LSF, in agreement with cobalt oxides generally reducing more easily than iron oxides for an equal oxidation state. An even larger O2 production is observed during the reduction of LSC. However, post experiment X-ray diffraction patterns showed that the LSC compound undergoes decomposition during the reduction process (Fig. S3). The identified impurities suggest that the following reaction occurs: 2(La,Sr)CoO3 → (La,Sr)2CoO4 + CoO + ½ O2

Therefore the LSC perovskite partially decomposes into a Ruddlesden Popper (RP) phase. Interestingly, RP phases are also often subject to significant topotactic oxygen disincorporation3638

due to a crystal chemistry that is reminiscent of that of the perovskite. They are even

frequently referred to as layered perovskites, owing to a parent crystal structure. Thus, the thermal reduction behavior of the RP phases (LaSrCoO4 and LaSrFeO4) was also explored in this study. O2 productions of 268 and 68 µmol.g-1 were measured at 1300°C for LaSrCoO4 and LaSrFeO4 respectively (Fig. 6). Lower O2 productions were observed when compared to LSF and LSCF reduced at 1200°C, which is partly related to an initial oxidation state of +3 for M in the LaSrMO4 phases, while the oxidation state was greater in LSF, LSCF, and LSC. Similarly to the comparison of LSCF versus LSF, the cobalt containing LaSrCoO4 shows greater reducibility at high temperature than the iron containing LaSrFeO4 does, most probably for the same aforementioned reasons.


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Figure 6. TG analysis performed during the thermal reduction of LaSrCoO4 and LaSrFeO4 under Ar atmosphere, for the same thermal treatment

3.2. Water-splitting capabilities In each series of samples, the water splitting capabilities of the materials producing the largest O2 quantities were studied by TG analysis and powder X-ray diffraction (Fig. 7a-i; Fig. 8a-i). As shown by powder X-ray diffractograms presented on figure 7 and 8, none of the materials studied here underwent decomposition during the thermal reduction under Ar followed by the water-splitting step. LeBail fits of these data were carried out according to previously reported crystallographic unit cell parameters and symmetries for these compounds. For each compound, a volumetric expansion of the unit cell was systematically observed when comparing the unit cell parameters before and after the thermochemical treatments, which agrees with the re-oxidations observed by TG analysis, that are only partial during the water-splitting step. Although LSCF was refined with a distorted perovskite rhombohedral symmetry, one should note that the unit cell parameters after the thermal treatment can also be transferred into a perovskite that is metrically cubic within an interval of three standard deviations, as a result of a distortion that is


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significantly reduced. Refinements performed with a cubic symmetry (ܲ݉3ത݉) led to a goodness of fit indicator of 1.38, to be compared with 1.36 for the rhombohedral symmetry. Therefore an unambiguous determination of the space group of the reduced form present after the reduction and water-splitting steps would require further analysis, such as analysis of high resolution powder X-ray diffraction data with a monochromatic beam or convergent beam electron diffraction. In any case, the decrease of the extent of distortion correlates well with the behaviors reported in the parent LSC and LSF systems, for which comparative studies made under N2 and O2 atmospheres show that the extent of distortion decreases when anion deficiency increases, also showing that the presence of oxygen vacancies decreases the temperature of transition from rhombohedral to cubic symmetry.39-40


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Figure 7. a), b), c) LeBail fits of PXRD data collected before the TG experiments for BSCF25/75/8/2, BSCF5582 and LSCF respectively. d), e), f) TG curves collected during hightemperature reduction under Ar followed by exposure to H2O at lower temperature for BSCF25/75/8/2, BSCF5582 and LSCF respectively. g), h), i) LeBail fits of PXRD data collected after the TG experiments for BSCF25/75/8/2, BSCF5582 and LSCF respectively. Space groups: ܲ݉3ത݉ (BSCF25/75/8/2 and BSCF5582); ܴ3തܿ (LSCF, hexagonal setting)

From analysis of the TG experiments, H2 productions of 74, 83 and 90 µmol.g-1 were determined for BSCF25/75/8/2, BSCF5582, and LSCF, respectively. When compared to the initial O2 production, the yields of the water-splitting step are then 5.3, 6.9, and 8.9% respectively. These values are to be correlated with reported enthalpies (∆Hox) and entropies of oxidation (∆Sox) for BSCF5582 and LSCF,41 which results in elevated free enthalpies of oxidation (∆Gox) for these compounds at 700°C. This suggests that their oxidation with H2O


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should be thermodynamically unfavorable for high oxygen contents (i.e. close to the nominal content of the as-made samples) in the water-splitting conditions used here (800°C). Therefore, a total re-oxidation could not be expected and is not observed here. Nevertheless, a partial reoxidation with water occurs, certainly as a result of a more favorable ∆Gox for the lower oxygen stoichiometries, because the employed thermal reduction conditions allow surpassing the amount of oxygen deficiency for which thermodynamics were determined in reference 40. This is reminiscent of several studies of non-stoichiometric perovskite oxides showing that ∆Hox clearly lowers for smaller oxygen contents (i.e. transition metal lesser oxidation state), 42-43 sometimes with an abrupt decrease as a function of δ, as is the case in the double perovskite GdBaCo2O6-δ,44 wherein both ∆Hox and ∆Sox undergo a dramatic decrease as δ reaches 1.


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Figure 8. a), b), c) LeBail fits of PXRD data collected before the TG experiments for LaSrCoO4, LSM35 and LSM50 respectively. d), e), f) TG curves collected during high-temperature reduction under Ar followed by exposure to H2O at lower temperature for LaSrCoO4, LSM35 and LSM50 respectively. g), h), i) LeBail fits of PXRD data collected after the TG experiments for LaSrCoO4, LSM35 and LSM50 respectively. Space groups: ‫ܫ‬4/݉݉݉ (LaSrCoO4); ܴ3തܿ (LSM35 and LSM50, hexagonal setting)

Existing reports indicate that the thermodynamics for water-splitting should be rather more favorable in the case of strontium-doped lanthanum manganites,22 when compared to the BSCF or LSCF compounds. Thus, larger H2 productions were observed for LSM35 and LSM50 with 124 and 195 µmol.g-1 respectively. In parallel, with an H2 production of 161 µmol.g-1, LaSrCoO4 also shows more favorable thermodynamics when compared to the BSCF and LSCF compounds. Moreover, LaSrCoO4, LSM35 and LSM50 display substantially improved water-splitting yields with respective values of 30.0%, 32.3% and 32.7%. As such, the H2 productions observed here appear to be restrained by kinetic limitations, since a plateau is never reached despite the 45 min dwell time applied for the water-splitting step. The kinetics for these perovskites appear slower when compared to that of CeO2, which is almost fully re-oxidized during water-splitting steps of a few minutes.45 This is consistent with the fact that oxide ion bulk diffusion coefficients and derived ionic conductivities are reported to be much lower in strontium-doped lanthanum manganites than in oxygen-deficient fluorites.46-49 In addition, LSM compounds may display lower catalytic activity for H2O molecule dissociation: for instance, such reports exist for O2 molecule dissociation with lower surface exchange coefficients in LSM compounds.48-49 Therefore, water-splitting enhancement for these perovskites shall require further optimization of the synthesis conditions, notably in order to decrease the particle size for promotion of a larger specific surface area. A decrease in the H2 production was observed upon cycling the reduction and water-splitting steps (Fig S4). This phenomenon was also observed for thermochemical


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cycles using substituted ceria and was assigned to sintering of the sample,14 both lowering specific surface area and restraining gas diffusion throughout the packed powder sample. Finally, Figure 9 is presented to summarize and illustrate the H2 productions observed for the different investigated compounds.

Figure 9. Summary of the hydrogen productions determined from TG analysis for compounds studied in this work. 1st and 2nd after the compound label refer to the productions determined after the first and second water-splitting step respectively. Legend of the histograms indicates the temperatures of reduction/re-oxidation.

3.3. Potential for further optimization Accounting for the discontinuities shown in oxygen evolution profiles (Fig. 1), a major source of performance improvement should also be the optimization of the temperatures applied to the two-step thermochemical cycle, especially since the aforementioned discontinuities are hardly predictable. Figure 1a exemplifies well this type of behavior for LSM35 as it shows that if a significant amount of oxide anion vacancies are already generated at 1000°C, an enhancement of the O2 production is only observed at 1390°C. This is consistent with a large increase of the


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oxygen production at 1400°C, when compared to the modest improvement from 1300 to 1350°C (Fig. 10). While optimization of the thermal treatment in the context of STWC is crucial for any material,50 this type of behavior contrasts with that of stoichiometric compounds for which, once a significant oxygen evolution has started, an increase in temperature only promotes more favorable kinetics but should not imply better yields in term of the maximum reduction potentially achieved. Therefore, in addition to performance improvements related to materials synthesis and composition, optimization of the thermal cycle in the case of perovskites should prove to be crucial not only in term of kinetics but also in term of the reduction yields potentially achievable thermodynamically.

Figure 10. Thermal reductions of LSM35 performed at 1300, 1350 and 1400°C

As stated above, we expect that variations of several parameters such as synthesis route, composition and temperatures applied to the thermochemical cycles may induce notable improvements in the performance of these materials. In order to approach this, BSCF5582 and LSCF6428 were synthesized via the Pechini method. This led to a modest improvement of the water-splitting yield for BSCF5582 made by the Pechini method (Fig. 11a), with 102 µmol.g-1


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comparing to 83 µmol.g-1 for the same compound made by solid state reaction. For LSCF, an enhancement of the reactivity is observed (Fig. 11b), with the H2 production reaching 162 µmol.g-1 as compared to 90 µmol.g-1. As such, these primary experiments tend to demonstrate that there is a large potential for optimizing the performances of these compounds, although a lower water-splitting yield was also observed here after the first thermochemical cycle (Fig. S5).

Figure 11. TG curves collected during high-temperature reduction under Ar of samples made by the Pechini method followed by exposure to H2O at lower temperature for a) BSCF5582 and b) LSCF6428

3.4. Discussion Purely in term of H2 production, the water-splitting capabilities of the perovskites studied here are comparable to that of pure ceria reduced at 1400 °C. This is achieved while reduction


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temperatures are significantly decreased in the case of BSCF, LSCF and LaSrCoO4, to 1000, 1200 and 1300°C respectively. During the first cycle, LaSrCoO4 and LSM50 even surpass the performances reported for ceria, with respective H2 productions of 161 and 195 µmol.g-1, comparing favorably to 125 µmol.g-1 measured for ceria under similar conditions.45 Mechanistically however, there are strong distinctions between the TG experiments performed on LaSrCoO4, LSM50, and CeO2. From purely qualitative comparison, it can be safely stated that the kinetics are favorable to CeO2. In addition, existing reports suggest that the oxidation thermodynamics (∆Gox) may also be more favorable for CeO2, especially when the Sr content is increased in LSM compounds.22 Therefore, the reason for the productions observed here mainly lies in the strong capability of the perovskite structure to incorporate oxygen vacancies upon reduction, allowing significant H2 evolution even for relatively poor re-oxidation yields arising from oxygen capture by the non-stoichiometric crystal lattice (for the first cycle: 30.0% for LaSrCoO4 and 32.7% for LSM50 versus complete re-oxidation yield for CeO2). When acknowledging the vast chemical tunability that perovskite materials offer, which should allow for optimization of the thermodynamics and kinetics, this feature is undoubtedly a promising one. Relating to previous results for substituted ceria, where for example, isovalent substitution of Ce for Zr (25%) combined with optimization of the synthesis enhances the H2 production from 125 to 432 µmol.g-1,45 one can clearly envision that the multiple composition variations available within the perovskites studied here may induce similar enhancement. This is also emphasized by the recent report of productions reaching 300 µmol.g-1 in Mn- and Sr-doped LaAlO3, even though the experimental set up was not exactly the same as the one used herein.23


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Another interesting feature of the perovskites studied here is the nature of the redox couples involved in the water-splitting reaction, notably for LSM50 and LaSrCoO4, which are the materials displaying the highest productions (Fig. 12). In the form of an Ellingham plot, figure 12a shows that water-splitting relying on the MnO2/Mn2O3 and Co3O4/CoO systems (Fig. 12a), which involve Mn4+/Mn3+ and Co3+/Co2+ redox couples, is highly unfavorable in term of thermodynamics. From analysis of the TG data, figure 12.b shows the evolution of Mn oxidation state in LSM50 during the first thermochemical cycle, based on the reported initial stoichiometry La0.5Sr0.5MnO3 for the as made material.27,51 At the start of the cycle, Mn oxidation state is at +3.50, before being decreased to +3.24 during the thermal reduction, and being re-oxidized during the water-splitting step into +3.32. The variation from +3.24 to +3.32 implies a partial oxidation of Mn3+ into Mn4+, showing that the Mn4+/Mn3+ redox couple is active during the water-splitting step. In parallel, starting at +3 for the reported LaSrCoO4 stoichiometry,52 cobalt oxidation state in the RP phase varies from +2.59 to +2.71 during the water-splitting step, involving the Co3+/Co2+ redox couple, as shown in figure 12c. Therefore, perovskite and RP materials enable activation for water-splitting of redox couples that are not available in stoichiometric oxides.


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Figure 12. a) Ellingham plots for H2O/H2, MnO2/Mn2O3 and Co3O4/CoO. b) variation of Mn oxidation state in LSM50 during the first thermochemical cycle involving water-splitting. c) variation of Co oxidation state in LaSrCoO4 during the first thermochemical cycle involving water-splitting


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Like in any material, thermal reduction and water splitting ability of the ABO3 perovskites strongly depend on the redox chemistry, and therefore on the B-O bond properties, such as B-O bond energy, B atom ionization potential, or the extent of orbital overlap between O 2p and B 3d states. In some of the perovskites studied here, these properties are favorable to water-splitting for redox couples that cannot be used with simple oxides. This can be put in relationship with the rich crystal chemistry of the perovskite, which allows the tuning of some of these properties. For example, transition metals in perovskite oxides and derivates (such as RP phases) commonly adopt oxidation states that are not observed in well-known stoichiometric oxides. As an illustration of this statement, one of the best known examples is found in superconducting cuprates. While commonly encountered copper oxides are Cu2O (Cu+) and CuO (Cu2+), the stabilization of Cu3+/Cu2+ mixed valence within perovskites and derivate layered phases was crucial in the discovery of high TC superconductivity.53-54 More closely to the compounds studied here, SrFeO3 55 and SrCoO3 56 are archetypal examples of pure Fe4+ and Co4+ containing perovskites, with an oxidation state significantly higher than that encountered into well known Fe and Co stoichiometric oxides. It is arguable that the former compounds are made under oxidizing conditions. However, under ambient air conditions, Sr and Fe form the oxygendeficient SrFeO2.87 perovskite,55 which still contains a significant amount of Fe4+, while Co4+ containing perovskites can be easily made under air atmosphere.57 The ability of the perovskite to stabilize different valences when compared to stoichiometric oxides arises from a large contribution of the interplay between the transition metal oxidation state in the B site and the A site preferential coordination. For example, Ca and Fe stabilize the brownmillerite Ca2Fe2O5,58 which can be seen as a pure Fe3+ oxygen-deficient perovskite (CaFeO2.5). This compound is formed because the A site Ca2+ cation is highly stable in the 9-fold coordinated environment thus


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generated. Upon Ca2+ replacement by the larger Sr2+, the coordination number is increased on the A site of the perovskite yielding the SrFeO2.87 perovskite. This mechanism is also well exemplified in the LnBaCo2O5+δ (Ln = Y3+, Gd3+, Sm3+, Nd3+ and La3+) layered double perovskites. In this series of compounds, the coordination number of Ln is gradually extended with the Ln ionic radius, leading to an increase of the oxygen stoichiometry from 5.41 to 6.00, combined with an increase of the Co oxidation state from +2.91 to +3.50 respectively for the smallest Y3+ and largest La3+, inducing the presence of Co4+ cations in the latter case.59 This interplay between A site preferential coordination and transition metal valence should also be crucial in the ability of perovskites to provide a range of favorable B-O bond properties and overlap between O 2p and B 3d orbitals, which can promote Co2+ and Mn3+ oxidation during water-splitting. These feature results in significant H2 productions when working with Mn4+/Mn3+ and Co3+/Co2+ redox couples whereas this is not feasible within stoichiometric oxides. The fact that the perovskite crystal chemistry opens access to different redox couples in order to enable water dissociation is both intriguing and promising for the generation of materials displaying high performance in the context of STWC. Therefore future work should focus on the investigation of perovskite structures featuring enhanced reducibility and subsequent watersplitting activity. 4. Conclusions Material compositions related to well-known mixed ionic-electronic conductors were studied in the context of STWC, owing to their reactivity toward oxygen vacancy formation upon heating, combined with their ability to transport oxide ions, which is crucial during the re-oxidation water-splitting step. Complex reduction behaviors were observed during temperatureprogrammed studies of the oxygen evolution that can be correlated to the rich crystal chemistry


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of oxygen-deficient perovskites. Powder X-ray diffraction analysis was conducted in parallel, in order to characterize potential decomposition and/or validate structural changes resulting from the applied thermal treatments. Although small re-oxidation yields were observed during watersplitting experiments performed via TG experiments, the gravimetric H2 productions of the perovskites were nonetheless comparable to that of pure ceria, with for some of the materials, an operating temperature significantly reduced, potentially demanding less solar power input. These performances are due to larger reduction capabilities at lower temperature when compared to ceria, while kinetic limitations suggest that the observed performances could be improved by optimizing the synthesis route in order to enhance specific surface area and thermal stability of the microstructure. Of note is the fact that some of the redox couples involved here are Co3+/Co2+ and Mn4+/Mn3+ whereas such couples are thermodynamically inert upon water-splitting within stoichiometric oxides. This fact is discussed in regard of the perovskite crystal chemistry. In conclusion, although the performances of the best substituted ceria are not reached here, the data we report contribute in showing that perovskites and parent compounds such as Ruddlesden Popper phases are a promising class of redox materials in the context of solar thermochemical water-splitting, provided that further improvement can be achieved by optimizing both composition and processing of the oxygen-exchange materials. ASSOCIATED CONTENT Supporting Information. Additional TG curves and PXRD analysis. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author


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* Corresponding author: Tel.: +33 4 68 30 77 30; Fax: +33 4 68 30 77 99 E-mail address: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This study was funded by Airbus Group Corporate Foundation (CNRS CT 084210). The authors thank D. Perarnau for XRD data collection. REFERENCES [1] Nakamura, T. Hydrogen Production from Water Utilizing Solar Heat at High Temperatures. Sol. Energy 1977, 19, 467-475. [2] Tamaura, Y.; Steinfeld, A.; Kuhn, P.; Ehrensberger, K. Production of Solar Hydrogen by a Novel, 2-Step, Water-Splitting Thermochemical Cycle. Energy 1995, 20, 325-330. [3] Tamaura, Y.; Kojima, M.; Sano, T.; Ueda, Y.; Hasegawa, N.; Tsuji, M. Thermodynamic Evaluation of Water Splitting by a Cation-Excessive (Ni, Mn) Ferrite. Int. J. Hydrogen Energ. 1998, 23, 1185-1191. [4] Han, S. B.; Kang, T. B.; Joo, O. S.; Jung, K. D. Water Splitting for Hydrogen Production with Ferrites. Sol. Energy 2007, 81, 623-628. [5] Kaneko, H.; Yokoyama, T.; Fuse, A.; Ishihara, N.; Tamaura, Y. Synthesis of New Ferrite, AlCu Ferrite, and Its Oxygen Deficiency for Solar H2 Generation from H2O. Int. J. Hydrogen Energ. 2006, 31, 2256-2265. [6] Kodama, T.; Gokon, N.; Yamamoto, R. Thermochemical Two-Step Water Splitting by ZrO2Supported NixFe3−xO4 for Solar Hydrogen Production. Sol. Energy 2008, 82, 73-79. [7] Gokon, N.; Muramyama, H.; Nagasaki, A.; Kodama, T. Thermochemical Two-Step Water Splitting Cycles by Monoclinic ZrO2-Supported NiFe2O4 and Fe3O4 Powders and Ceramic Foam Devices. Sol. Energy 2009, 83, 527-537. [8] Fresno, F.; Fernandez-Saavedra, R.; Gomez-Mancebo, M. B.; Vidal, A.; Sanchez, M.; Rucandio, M. I.; Quejido, A. J.; Romero, M. Solar Hydrogen Production by Two-Step Thermochemical Cycles: Evaluation of the Activity of Commercial Ferrites. Int. J. Hydrogen Energ. 2009, 34, 2918-2924. [9] Fresno, F.; Yoshida, T.; Gokon, N.; Fernandez-Saavedra, R.; Kodama, T. Comparative Study of the Activity of Nickel Ferrites for Solar Hydrogen Production by Two-Step Thermochemical Cycles. Int. J. Hydrogen Energ. 2010, 35, 8503-8510.


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Investigation of Perovskite Structures as Oxygen-Exchange Redox

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