Journal ofMaterials Chemistry A

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Thermodynamic

aMaterials Science, California Institute of

E-mail: yamazaki@caltech.edu; smhaile@cabPRESTO, Japan Science and Technology Ag

Cite this: J. Mater. Chem. A, 2014, 2,13612

Received 29th May 2014Accepted 4th July 2014

DOI: 10.1039/c4ta02694b

www.rsc.org/MaterialsA

13612 | J. Mater. Chem. A, 2014, 2, 136

and kinetic assessments ofstrontium-doped lanthanum manganiteperovskites for two-step thermochemical watersplitting

Chih-Kai Yang,a Yoshihiro Yamazaki,*ab Aykut Aydina and Sossina M. Haile*a

Solar-driven thermochemical water splitting using non-stoichiometric oxides has emerged as an attractive

technology for solar fuel production. The most widely considered oxide for this purpose is ceria, but the

extreme temperatures required to achieve suitable levels of reduction introduce challenges in reactor

design and operation, leading to efficiency penalties. Here, we provide a quantitative assessment of the

thermodynamic and kinetic properties of La1�xSrxMnO3�d perovskites, targeted for a reduced

temperature operation of thermochemical water splitting. Sr-doping into lanthanum manganite

increases the thermodynamic fuel production capacity, which reaches 9 ml g�1 for 0.4 Sr for a

thermochemical cycle operated between 1400 and 800 �C. The hydrogen yields are moreover in good

agreement with expected values based on analysis and extrapolation of thermogravimetric data available

in the literature. High levels of Sr doping, however, result in low steam-to-hydrogen conversion rates,

implying high energy penalties in an operational reactor. Furthermore, the rate of fuel production

decreases with increasing Sr content, suggesting that intermediate compositions may yield the most

suitable combination of properties.

1. Introduction

‘Solar fuel’ has emerged as a concept for storing the earth's vastsolar resource in a high energy-density medium. Amongst manyapproaches currently pursued to generate solar fuels, thermo-chemical dissociation of water splitting is particularly attractive.It provides the benets of full utilization of the solar spectrumas well as inherent temporal separation of hydrogen and oxygengases.1–3 Given these advantages, numerous multi-step cycleshave been considered over the past several decades.1,2 Recently,two-step cycles making use of non-stoichiometric oxides havereceived attention because of the simplicity of their imple-mentation.4 The fuel production process operates on the basicthermodynamic principle that (partial) reduction of the oxideoccurs at high temperature and reoxidation by steam (or carbondioxide) occurs at low temperature. In such a process, solar-thermal heating releases oxygen from the bulk structure at hightemperature, and the subsequent reaction with steam at lowertemperature generates hydrogen. In the ideal case, both surfacereaction and bulk diffusion kinetics are rapid, and thermody-namic equilibrium is quickly achieved in each half of the cycle.Accordingly, the ideal oxide demonstrates not only high

Technology, Pasadena, CA 91125, USA.

ltech.edu

ency, Kawaguchi, 332-0012, Japan

12–13623

sensitivity in oxygen content to environmental conditions, butalso high oxygen diffusivity and high surface reaction constants.This set of characteristics is more commonly found in variablevalence oxides, which can undergo partial reduction, than inthose that undergo stoichiometric changes in oxygen content.5

Two classes of non-stoichiometric (variable valence) oxideshave been evaluated for solar-driven thermochemical fuelproduction: uorites based on ceria (CeO2�d)4–8 and perovskitesbased on lanthanum manganite (LaMnO3�d, in which d may be< 0)9,10 or on lanthanum aluminate (LaAlO3�d).11 The relativelyextensive studies of the ceria-based class of compounds hasrevealed that this group of materials generally requires ratherhigh temperatures, >1500 �C, to induce reduction of the oxide toan extent that the cycling yields non-trivial amounts of fuel.5,8

Such high temperatures, however, create signicant challengesfor reactor design and operation, although proof of principlehas been successfully demonstrated with CeO2.4,12 The perov-skite class of materials has been considered explicitly for ther-mochemical cycling in just two recent experimental studies.McDaniel et al.11 evaluated three compositions in theSrxLa1�xMnyAl1�yO3�d system using direct cycling and demon-strated high levels of fuel productivity for a high temperaturereduction step of just 1350 �C. Scheffe et al.10 carried out a moredetailed evaluation of the potential of La1�xSrxMnO3�d

compounds (x ¼ 0.2, 0.3, 0.4) for thermochemical fuelproduction on the basis of literature data for the bulk

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thermodynamic redox properties and supplemented the anal-ysis with an experimental study of the reduction behavior of thecomposition with x ¼ 0.35. Although the fuel production half-cycle was not directly probed, the authors were able to showgreater oxygen release from the perovskite than from ceria atmoderate temperatures, indicating the potential for greater fuelproductivity under reduced temperature cycling conditions.These authors further suggested, however, that obtaining highhydrogen yields would require large quantities of excess steamto drive the reaction to completion.10 Prior to these studies ofthermally driven CO2 and H2O dissociation, a handful ofresearchers have demonstrated CH4 to syngas conversion overLa1�xSrxMnO3�d.13–15 This process also relies on the oxygenuptake and release of the oxide, however, the redox propertieswere not explicitly considered in these studies. In sum, thesereports indicate that perovskite-structured materials havesignicant promise as reactive media for solar-driven thermo-chemical fuel production, but that the direct evaluation of thisclass of materials for this application is extremely limited. Thepresent work has been carried out with the aim of providing agreater understanding of perovskite-structured oxides for solarfuels generation. We examine the La1�xSrxMnO3�d system inparticular because of the availability of thermogravimetric data.We complement an analytical evaluation of these materials withextensive thermochemical cycling experiments.

2. Background: structural,thermodynamic and kinetic propertiesof La1�xSrxMnO3�d

Several structural distortions of the parent cubic perovskitestructure have been reported for La1�xSrxMnO3�d, dependingon the Sr concentration, oxygen partial pressure, and temper-ature, as determined most reliably by neutron powder diffrac-tion.16,17 Near-stoichiometric LaMnO3, in which the Mn has anaverage 3+ oxidation state, displays a Jahn–Teller distortion,and adopts an orthorhombic structure. Under typical process-ing conditions, the undoped composition contains oxygenexcess and the accompanying high concentration of Mn4+

eliminates the distortion, resulting in a rhombohedral structurewith space group R�3c. Introduction of Sr similarly generates ahigh concentration of Mn4+ under ambient conditions (ratherthan oxygen vacancies), resulting in the same rhombohedralstructure. Very high concentrations of Sr (x $ 0.47) lead to atetragonal structure (I4/mcm) and ultimately a cubic structure(Pm�3m) beyond 0.7 Sr.16,17 The present studies are limited to x ¼0.4, at which, under ambient conditions, the cubic phase is notexpected. In analogous cobaltite and ferrite perovskites,extreme reduction and the concomitant high concentration ofvacancies can lead to the vacancy-ordered Brownmilleritephase.18,19 In the case of strontium-doped lanthanum manga-nite, this phase has been observed only upon reduction withhydrides or upon exposure to 50% hydrogen gas.20,21 Suchextreme reducing conditions are not encountered under typicalthermochemical cycling conditions and accordingly the

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Brownmillerite phase is not anticipated to form as part of thefuel generation process.

The thermodynamic capacity of an oxide for fuel productionby thermochemical cycling is embedded in the reductionreaction

1

aABO3�d4

1

aABO3�d�a þ 1

2O2ðgÞ (1)

where the designation of the oxide as an ABO3 compoundreects the use of a perovskite, a represents an innitesimalchange in oxygen content, and the thermodynamic descriptors,the enthalpy and entropy of the reduction reaction, are recog-nized to be functions of nonstoichiometry, d. Knowledge of theextent of this reaction for a given temperature (T) and oxygenpartial pressure (pO2) is equivalent to knowledge of the equi-librium dependence of d on these environmental parameters.

Because of the technological importance of Sr-dopedlanthanum manganites as cathodes for solid oxide fuel cells,experimental studies of the oxygen content of this class ofmaterials at moderate temperatures and oxygen partial, typi-cally up to �1000 �C and in the range of 10�5 to 1 atm in oxygenpartial pressure, as relevant for fuel cell technology, are avail-able in the literature.22–25 Extrapolation of these results for theprediction of oxygen content at the high temperatures of rele-vance for thermochemical cycling requires extraction of ther-modynamic parameters from the raw experimental data. Mostcommonly, thermodynamic parameters are extracted with thegoal of understanding defect chemistry and hence are con-cerned with the specics of a defect model that can address, forexample, possible defect association or clustering.22,26 Whileuseful for meeting the objective of elaborating defect chemistry,such an analysis ultimately imposes some functional form forthe dependence of enthalpy and entropy on d, and discrep-ancies between the model and measured data can be antici-pated to lead to large errors upon extrapolation to temperaturesat which measurements were not performed.10

To address this challenge and provide robust predictions offuel production capacity, we reevaluate the literature thermog-ravimetric data for strontium-doped lanthanum manganiteusing an alternative approach that bypasses an interpretation ofdefect behavior and instead provides the enthalpy and entropyvalues for the reduction reaction as arbitrary functions of d. Inbrief, as described by Panlener et al.,27 the equilibrium constantfor reaction (1) in the limit a / 0 is given by

Kredox ¼�pO*

2

�1=2 ¼ exp

�� DH0

redox � TDS0redox

RT

�(2)

where pO*2 is the oxygen pressure relative to standard state, DH0

and DS0 are the standard enthalpy and entropy changes of thereduction reaction, respectively, and R is the universal gasconstant. Rearranging this expression yields

1=2 ln pO*2

��d¼d0

¼ DS0redoxðd0ÞR

� DH0redoxðd0ÞRT

(3)

Thus, if DH0redox and DS0redox are independent of temperature,

an Arrhenius plot of the set of T and pO2 that yield a given d will

J. Mater. Chem. A, 2014, 2, 13612–13623 | 13613

Fig. 2 Thermodynamic characteristics of the reduction ofLa1�xSrxMnO3�d: (a) enthalpy and (b) entropy of reduction as functionsof oxygen non-stoichiometry; data for undoped ceria27 shown for

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appear as a straight line, with a slope that yields the enthalpy atthat d and an intercept that yields the entropy. Presuming thetemperature independence of DH0

redox and DS0redox holds at alltemperatures (and no fundamental changes in material chem-istry occur), DH0

redox(d) and DS0redox(d) can be used to through aniterative treatment of eqn (3) to determine d under anycondition.

Fig. 1 and 2 show the results of this analysis using thethermogravimetric data reported by Mizusaki and coworkers forLa1–xSrxMnO3–d with x ¼ 0 to 0.5.22,28 These are selected foranalysis from the variety of datasets available in the literaturebecause they span the widest range of temperatures (600–1000 �C) and widest number of compositions. As already notedin those authors’ original report and reproduced here, Fig. 1,the oxygen stoichiometry in La1�xSrxMnO3�d is highly depen-dent on Sr content. In the absence of Sr, the material has a highconcentration of excess oxygen (d < 0), believed to be charge andsite balanced by cation vacancies (on both the A and B sites).With increasing Sr content, this oxygen excess region dimin-ishes and, simultaneously, the oxygen decient region enlarges.In the presence of Sr, charge balance is achieved either by anenhanced Mn4+ concentration or the presence of oxygen

Fig. 1 Oxygen content in La1�xSrxMnO3�d for several different valuesof x, as indicated. Solid blue lines are the experimental data for Miz-usaki and co-workers,22,23,28 collected for the temperature range from600 to 1000 �C. High temperature experimental data for the x ¼ 0.35composition10 are shown on the x ¼ 0.4 plot as solid black circles.Dotted lines are fit and/or extrapolated values, derived as described inthe text. Red dotted lines refer to extrapolated behavior at 1400 �C, atwhich reduction was carried out in the present thermochemicalcycling experiments. Green dotted lines refer to fit data at 800 �C, atwhich oxidation was performed. Open red and green circles indicate,respectively, expected values of oxygen content upon reduction under10 ppm O2 and oxidation under 20% H2O. The difference in oxygencontent between these open circles corresponds to Ddmax shown inFig. 10.

comparison.

13614 | J. Mater. Chem. A, 2014, 2, 13612–13623

vacancies, the balance of which depends on T and pO2.22 Mostsignicant for this discussion is the observation that thecomputed values of oxygen content (dotted lines) are in goodagreement with the experimental values (solid lines). Thisagreement provides condence in the extracted DH0

redox(d) andDS0redox(d) values, Fig. 2, as well as the extrapolation of theoxygen nonstoichiometry values to high temperature.

Overlain on the data plots in Fig. 1 are vertical lines corre-sponding to the values of oxygen partial pressure for thereduction and oxidation half-cycles employed in this work, 10�5

and 1.6 � 10�7 atm, respectively. Oxidation was performed at800 �C using a gas stream of 20% H2O in Ar. The equivalentoxygen chemical potential is computed assuming equilibriumfor the thermolysis reaction, H2O(g) 4 H2(g) + 1/2O2(g), andmass balance in the product generation (moles of H2 ¼ twicethe moles of O2). The equilibrium change in oxygen content,Dd ¼ df � di, is indicated on these plots as the differencebetween oxygen content at the intersection of these respectivevertical lines with the non-stoichiometry curves at 800 and1400 �C. Excluding the undoped composition, it is evident thatthe greater the Sr content, the greater the expected change inoxygen content and hence fuel production. In the case ofLaMnO3�d, a contribution to Dd appears due to the possibility ofentering the oxygen excess region. Because accessing this non-stoichiometry would require signicant atomic level structuralrearrangements (cyclic generation and annihilation of cationvacancies), the undoped composition is not evaluated inthis study.

The extracted thermodynamic functions, shown in Fig. 2,also reveal important trends. Ignoring for the moment theoxygen excess region, a monotonic decrease in the enthalpy ofreduction is evident with increasing Sr content, falling from

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�350 kJ per mol-O for the undoped composition to �230 kJ permol-O for x ¼ 0.5. In contrast, the entropy is relatively constant.This behavior directly reects the fact that the material attains ahigher oxygen deciency with increasing Sr content at any givenexperimental condition of temperature and oxygen partialpressure and further reveals that the greater reducibility is dueto a decrease in the enthalpy penalty of reduction rather thandue to an increase in the entropy gain. A rough metric ofreducibility is given by Teq ¼ DH0

redox(d0)/DS0redox(d0), which

species the temperature at which a nonstoichiometry value ofd0 would occur under a standard oxygen pressure of 1 atm.Again, the monotonic decrease in DH0

redox with minimal changein DS0redox implies the reduction temperature monotonicallydecreases with Sr content. A second important observation fromthe results in Fig. 2 is the relative insensitivity, within theoxygen decient region, of DH0

redox to oxygen content. This is afeature of ideal solution behavior, in which the enthalpy of thereaction is independent of the number of defects. In such case,there is no apparent tendency towards either clustering orordering of defects. Mizusaki et al. drew the same conclusion,that strontium-doped lanthanum manganite displays idealsolution behavior, from a more detailed analysis of the defectchemistry.22 Within the oxygen decient region one also seesthat the enthalpy and entropy values of the manganite aresubstantially smaller than those of ceria,27 considered abenchmark material for thermochemical fuel production.Because the enthalpy is particularly high for ceria, the temper-atures required for reduction are higher than they are forstrontium-doped lanthanum manganite. Indeed, the highenthalpy is the fundamental reason that thermochemicalcycling with ceria must be carried out at extremely hightemperatures.5

Both the enthalpy and entropy of reduction ofLa1�xSrxMnO3�d undergo abrupt changes at the boundarybetween the oxygen decient and oxygen excess regions, wherethe latter region is observed only in low Sr content compositions(undoped or 10 at% Sr). Given the substantially different meansby which the structure accommodates excess oxygen from thatin which it accommodates oxygen vacancies, such a result is notsurprising. The enthalpy of reduction undergoes a particularlystrong decrease in value, indicating that reduction occursrelatively easily for a material that has an excess of oxygen.Conversely, it implies that oxidizing the material with steam toutilize the oxygen excess capacity will be difficult. Anticipationof such behavior is another reason that the undoped materialhas not been experimentally evaluated in this study.

Turning from thermodynamic expectations to kinetic char-acteristics, the ideal reaction substrate for thermochemicalcycling will display both rapid bulk diffusion and high surfacereaction rates. Diffusion here refers to chemical (or ambipolar)diffusion of neutral oxygen species, a combination of ionic andelectronic transport contributions. While several studies havebeen carried out using isotope exchange methods to probeoxygen self diffusion coefficient, D*, and isotopic surfaceexchange rate, k*,29–33 surprisingly few have targeted directmeasurements of chemical diffusivity, Dchem (or Dd) andchemical surface exchange constant, kS (or k

d) under the driving

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force of a chemical potential gradient,34–36 the kinetic parame-ters of relevance to thermochemical cycling. While it is possible,in principle, to transform between D* and Dchem and analo-gously between k* and kS using knowledge of the thermody-namic behavior of the material,37 the discussion here focuses onthe direct measurements of Dchem and kS.

Yasuda et al.35 determined Dchem for compositions in therange of x ¼ 0.05 to 0.2 using the conductivity relaxationmethod, whereas Belzner et al.34 determined Dchem for compo-sitions with x ¼ 0.2 and 0.5 using the potentiostatic stepmethod. Yasuda's studies showed that at the low pO2 values thatcould be probed by conductivity relaxation (10�10 to 10�17 atm,depending on temperature, over the range 850 to 1000 �C), thechemical diffusivity falls between 10�5 and 4 � 10�4 cm2 s�1,decreases with increasing pO2 (a linear dependence wasobserved on a double-logarithmic plot) and is insensitive to Srcontent. More oxidizing conditions could not be examinedbecause of the insensitivity of conductivity to changes in oxygenpartial pressure above�10�9 atm. Belzner et al.measured Dchem

under somewhat more oxidizing conditions and at slightlylower temperatures. The two sets of results are in general,though not complete, agreement and indicate that the oxygenchemical diffusion at 800 �C during the oxidation half-cycle ofthermochemical fuel productionwill likely be at least 10�8 cm2 s�1.For a typical diffusion length, l, of �3 mm, this diffusioncoefficient implies a characteristic time (sdiff ¼ l2/4Dchem) ofapproximately 2 s, suggesting that diffusion is unlikely to berate-limiting for the overall fuel production process.

An important difference between the conclusions of Belznerrelative to those of Yasuda concerns the role of Sr content on thechemical diffusivity. As discussed by Belzner, Dchem can, underselect conditions, have a signicant dependence on partialpressure. The dependence is embodied in the expression

Dchem ¼ DVotel

�� 1

2

v ln pO*2

v ln½VccO��

¼ DVotelL (4)

where DVois the diffusivity of oxygen vacancies, tel is the trans-

ference number of electronic species, and ½VccO� is the fractional

oxygen vacancy concentration (indicated according to Kroger–Vink notation38). At the relatively low oxygen vacancy concen-trations present in strontium-doped lanthanum manganiteunder moderately oxidizing conditions, both DVo

and tel arelargely independent of ½Vcc

O�, the latter being close to 1throughout the experimental conditions, and both are effec-tively independent of pO2. The term in parenthesis in eqn (4),named here L, is clearly dependent on pO2. This quantity isrelated, but not identical, to the thermodynamic factor, G,dened formally as d ln aO/d ln cO, where aO and cO are,respectively, the activity and concentration of oxygen in thesolid. Because the specic manner in which oxygen stoichi-ometry in La1�xSrxMnO3�d depends on pO2 is sensitive to Srcontent (Fig. 1), one can anticipate that Dchem will, in turn, alsobe sensitive to Sr content. Indeed, Belzner directly measured thethermodynamic factor and found that, at close to 1 atm pO2, Gwas about an order of magnitude higher for La0.8Sr0.2MnO3�d

than for La0.5Sr0.5MnO3�d. The authors concluded that this

J. Mater. Chem. A, 2014, 2, 13612–13623 | 13615

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enhancement was responsible for the observed order ofmagnitude difference in the diffusion coefficients between thetwo compositions. In the more limited compositional rangeexamined by Yasuda, Sr ¼ 5 to 20 at%, no signicant depen-dence on doping level was observed. Although the Belzner resultsuggests the possibility of a systematic variation of Dchem with Srcontent, particularly beyond Sr ¼ 20 at%, even an order ofmagnitude reduction of the diffusivity, resulting in sdiff � 20 s,is unlikely to render diffusion the rate-limiting step in ther-mochemical fuel production.

Determination of the surface reaction constant in theLa1�xSrxMnO3�d system by the conductivity relaxation methodare limited to Yasuda's evaluation of La0.8Sr0.2MnO3�d underCO–CO2 gas mixtures over the temperature range 850 to 1000 �C(ref. 35) and to Yan's study of La0.7Sr0.3MnO3�d under reducedoxygen pressure over the temperature range 610 to 920 �C.36

While some intriguing results have been revealed by thesestudies (a strong dependence on pO2 in the case ofLa0.8Sr0.2MnO3�d and a strong dependence on crystallographicorientation in the case of La0.7Sr0.3MnO3�d), the differingexperimental conditions preclude drawing conclusions aboutthe role of Sr concentration on the surface reaction step. Incontrast, many different compositions have been examined byisotopic exchange methods.29–33 While no single study providesa comprehensive comparison over a wide range of composi-tions, in general, the data suggest there is no systematic trendfor the dependence of the surface exchange rate constant on Srcontent. Furthermore, because the surface reaction constantcan be expected to be extremely sensitive to the exact nature ofthe gaseous species,39 and measurements have not been carriedout under H2O-containing atmospheres, it is not possible toprovide a meaningful estimate of the characteristic time for asurface-reaction-limited fuel production half-cycle. Similarly, inthe absence of very high temperature data, estimates of thecharacteristic time for the reduction half-cycle are premature.

3. Experimental methods

La1�xSrxMnO3�d (x ¼ 0.1, 0.2, 0.3, 0.4) powders were preparedby solid-state reaction. Stoichiometric quantities of La2O3 (AlfaAesar, REacton®, 99.99%), SrCO3 (Sigma-Aldrich $99.9%), andMnCO3 (Sigma-Aldrich $99.9%) were combined and attritor-milled in isopropanol for 6 hours at 500 rpm. Aer drying, thepowder was calcined at 1000 �C for 3 hours under air. A porousmonolith was fabricated from this powder by mixing the latterwith isopropanol (3 ml liquid per gram of powder) to obtain athick paste. This paste was then placed (without application ofpressure) into an alumina cylindrical mold with an innerdiameter of 10 mm and red at 1500 �C for 6 hours under air.The result was a loosely sintered, but mechanically rigid, porousbody. The porosity was measured using mercury porosimetry(AutoPore IV, micromeritics) and the morphology examined byscanning electron microscopy (ZEISS 1550VP Field EmissionSEM). Phase characterization was performed by X-ray powderdiffraction (Panalytical, PW3040-PRO, Cu Ka radiation), forwhich the material was lightly hand-milled.

13616 | J. Mater. Chem. A, 2014, 2, 13612–13623

Thermochemical hydrogen production was carried out over athermal cycle with reduction at 1400 �C and fuel generation at800 �C. The sample was placed inside an alumina tube reactorin turn placed in an infrared furnace with rapid heating andcooling capability. Temperature was measured with an S-typethermocouple placed in the axial center of the reactor and incontact with the porous pellet. The sample temperature wasramped to 1400 �C at 500 �C min�1 under owing 10 ppmoxygen premixed with Ar (certied by Air Liquid), and held therefor a given period of time (12–70 min, depending on composi-tion) to observe oxygen release. At the end of the reduction step,the material was ramped down to 800 �C (also at 500 �C min�1),and UHP Ar gas saturated with puried water at 60 �C (pH2O ¼0.20 atm, corresponding to pO2 ¼ 1.6 � 10�7 atm) was intro-duced. For the next reduction half-cycle, the gas was switchedfrom H2O-saturated argon to Ar (10 ppm O2), and aer a 3 minpurge under this gas, the rapid heating from TL (800 �C) to TH(1400 �C) was initiated. Evolved oxygen and hydrogen gaseswere detected using mass spectroscopy (OMNI Star, Pfeiffer-vacuum). For quantitative determination of gas content, the ioncurrent signal of the mass spectrometer was calibrated dailyusing 6 different hydrogen partial pressures ranging from 0 to5.05 � 10�3 atm and 7 different oxygen partial pressuresranging from 1 � 10�5 to 3.83 � 10�3 atm.

4. Results and discussions

X-ray powder diffraction patterns of the as-synthesizedLa1�xSrxMnO3�d and that of the material aer thermochemicalcycling (1400 �C/800 �C) are presented in Fig. 3. Consistent withliterature,16,17 all of the as-synthesized materials adopt therhombohedral structure (R�3c). No structural changes wereobserved on cycling of materials with Sr content of 20 at% orhigher. The 0.1 Sr sample, on the other hand, underwent atransformation from rhombohedral to orthorhombic, asrevealed by the splitting of the peak at around 47� 2q. Thisbehavior reects the fact that the fuel production step under20% steam at 800 �C presents a less oxidizing atmosphere thanthe original synthesis condition of laboratory air (pO2 z 0.2atm). Slight reduction of La0.9Sr0.1MnO3�d has been shown byMitchell et al. to induce transformation to the orthorhombicphase, as a result of the previously described Jahn–Tellerdistortion.12

Secondary electron microscopy images, Fig. 4, reveal a hier-archical structure in the porous monoliths, in which large(�60 mm) porous granules are formed from smaller sized crys-tallites or grains. The grain size decreases with increasing Srcontent, from �5 mm for 0.1 and 0.2 Sr to �1 mm for 0.3 and 0.4Sr. In contrast, the porosity (in as-synthesized samples) remainsessentially constant with Sr content, Fig. 5, with sample-to-sample variations more likely due to variations in fabricationsteps rather than a systematic trend with composition. Thelarge pores of these monoliths can be anticipated to be bene-cial for ensuring gas transport through the porous body,whereas the small pores can be anticipated to ensure access to alarge number of active sites at the surface. In addition, the nerstructure of the high Sr content materials suggests enhanced

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Fig. 4 Microstructure of La1�xSrxMnO3�d before thermochemicalcycle between 800–1400 �C: (a) typical porous structure for x ¼ 0.2,(b) impact of strontium content on grain size.

Fig. 5 Porosity of La1�xSrxMnO3�d as a function of strontium contentbefore thermochemical cycle.

Fig. 3 X-ray powder diffraction patterns of La1�xSrxMnO3�d beforeand after thermochemical cycling between 800–1400 �C: (a) widerrange and (b) narrower range showing phase change from rhombo-hedral (R�3c) to orthorhombic (Pbnm) in La0.9Sr0.1MnO3�d after cycling.Dashed lines in (a) indicate the region shown in (b).

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kinetics may be observed for these materials due to thepresumably higher specic surface area and the shorter solid-state diffusion length.

Typical oxygen release and hydrogen production proles arepresented in Fig. 6a for La0.8Sr0.2MnO3�d (TH ¼ 1400 �C,atmosphere¼ 10 ppm O2 in Ar; TL ¼ 800 �C, atmosphere¼ 20%H2O in Ar). In this example, reduction and oxidation werecarried out for 40 and 30 min, respectively. Oxygen is immedi-ately released at the initiation of the heating step, whereashydrogen is immediately produced upon introduction of steamat the low temperature step. The observation that strontium-doped lanthanum manganite can dissociate H2O through athermochemical process is fully consistent with the thermody-namic expectations. The data further reveal that the reductionreaches 90% of completion aer 46 min, as determined from anevaluation of the peak decay behavior, whereas the oxidationreaches the same extent of completion aer just 6.2 min. Basedon these observations, cycling experiments were performedusing reduction and oxidation periods of 45 and 16 min,respectively. The results, Fig. 6b, show relatively stable yields ofboth oxygen and hydrogen over 21 cycles.

Figures 7–9 show the inuence of the Sr content on thethermochemical cycling behavior. The raw oxygen andhydrogen evolution proles, Fig. 7, reveal that the oxygenrelease and hydrogen production per cycle, on a per gram basis,increase with increasing Sr content, in general agreement with

This journal is © The Royal Society of Chemistry 2014

the thermodynamic expectations. Furthermore, the oxygenrelease kinetics, as roughly characterized by how quickly the gasevolution decays from its peak value, are largely unaffected by Srcontent. The fuel production kinetics, in contrast, are stronglyaffected by Sr doping level, and even with a 65 min oxidationperiod, fuel production is clearly incomplete for the x ¼ 0.4composition. The difference in reduction and oxidation kinetics

J. Mater. Chem. A, 2014, 2, 13612–13623 | 13617

Fig. 6 Thermochemical cycle of 1400–800 �C usingLa0.8Sr0.2MnO3�d: (a) typical cycle between high temperature dry stateand low temperature wet state for 40 min in each step, (b) 21continuous cycles for thermochemical water splitting. Red and bluecolors correspond to oxygen and hydrogen evolution, respectively.The high-temperature reduction and low-temperature oxidation stepswere held for 47 and 16 min, respectively.

Fig. 7 Impact of strontium substitution in La1�xSrxMnO3�d on ther-mochemical water splitting during 1400–800 �C cycle: (a) oxygenevolution at 1400 �C, (b) hydrogen evolution at 800 �C. The reductiontimes for 0.1, 0.2, 0.3 and 0.4 Sr at 1400 �C were 12, 47, 60 and 70 min,respectively, while the oxidation times at 800 �C were 8, 16, 36 and65 min, respectively.

Fig. 8 Characteristic hydrogen production and oxygen release timesas functions of Sr content. Characteristic time is defined as thatrequired for the off-gas detection to fall to 10% of the peak value.

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is further evident from an examination of the time required forthe gas concentration to decrease to a value of 10% of the peak,Fig. 8. In the case of oxygen release, the required time is just 1–2minutes irrespective of oxide composition, whereas forhydrogen production it increases from about 1 min at x¼ 0.1 toalmost an hour at x ¼ 0.4. Possible origins of this kineticbehavior are considered below. The cycling proles presented inFig. 7(b) reect oxidation half-cycle times that correspond toapproximately 90% of reaction completion for the ‘slow’compositions, for 0.3 and 0.4 Sr, as determined from evaluationof the prole decay behavior. While the reduction behaviorappears, at rst glance, to be rapid, particularly in comparisonto these oxidation reactions, it is noteworthy that the oxygenrelease proles are characterized by extremely long tails, noteasily visible in Fig. 7(a).

The cycling conditions of Fig. 7 were repeated over multiplecycles as a means of evaluating stability and of gaining statis-tically averaged fuel production data. The per cycle oxygen andhydrogen yields for the x ¼ 0.2 and 0.4 compositions (obtainedfrom an integration of the area of the peak proles) are pre-sented in Fig. 9. The error bars reect the uncertainty in the

13618 | J. Mater. Chem. A, 2014, 2, 13612–13623

integrated values due to background dri. Both materials showrather stable fuel productivity, indicating that, at least forthermal exposure periods of 22 and 15 h, respectively,morphological changes or mass losses from the material arenegligible. Direct measurement of the sample mass aercompletion of the cycling experiments showed no more than0.5% loss for all compositions. Accordingly, fuel productivitynumbers quoted below are the averaged values over the

This journal is © The Royal Society of Chemistry 2014

Fig. 9 Hydrogen and oxygen yield in La1�xSrxMnO3�d: (a) x ¼ 0.2 and(b) x ¼ 0.4. Reproducible hydrogen (blue) and oxygen (red) evolutionover 21 and 8 cycles was observed in 0.2 and 0.4 Sr pellets, respec-tively. The reduction and oxidation times for 0.2 Sr were 47 and 16min,respectively, whereas those for 0.4 Sr were 70 and 65 min, respec-tively. Uncertainty is estimated from the signal drift as observed duringcalibration. Where omitted, error bars lie within the data symbol.

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multiple cycles. The cycling data further reveal that the H2 : O2

molar ratio is within experimental error of the theoretical valueof 2, for 0.2 Sr, indicating that all oxygen vacancies createdduring reduction are consumed for fuel production duringoxidation. In contrast, for 0.4 Sr, the ratio is consistently slightlyless than 2, a result also obtained for the remaining twocompositions. Such behavior may reect incomplete oxidationby steam at 800 �C which is followed by oxidation by Ar/O2 at800 �C prior to high temperature reduction. In this way, theoxygen released on heating can exceed the correspondingproduction of hydrogen from reaction with steam. In addition,loss of hydrogen due to leaks is a ubiquitous challenge and canalso account for some of the deviation. Loss of hydrogen relativeto oxygen due to inherent material behavior, for example,inaccessibility of vacancies due to ordering, would be expectedto be cumulative (with a declining per cycle hydrogen andoxygen productivity), and the stable behavior argues against anysuch interpretation.

A summary of the fuel production capacity ofLa1�xSrxMnO3�d is provided in Table 1. Reported are the cycle-averaged measured values of oxygen and hydrogen production,as well as the projected quantity of oxygen release had longreduction times, sufficient to reach equilibrium, beenemployed. These projections are computed on the basis of theprole decay behavior and are provided along with estimates of

This journal is © The Royal Society of Chemistry 2014

the time required to reach equilibrium, again, based on theprole decay behavior. The projected values are useful forcomparison against the values estimated from an analysis of thethermogravimetric data, as both are representative of equilib-rium behavior. Beyond the monotonic increase in fuel produc-tivity with increasing Sr content already revealed in the rawproles, it is evident from Table 1 that the projected oxygenrelease (and hence fuel productivity) is in excellent agreementwith the thermodynamic predictions. The nite timescale of themeasurements implies only a small penalty of about 10% on thefuel productivity. However, accessing that nal 10% wouldrequire substantial increases in the total cycle time, as much asa factor of three in the case of La0.6Sr0.4MnO3�d. The experi-mental hydrogen yields are 0.91, 2.89, 5.68 and 8.91 ml g�1,respectively, for the four compositions. The latter two values arefavorable in comparison to ceria, which has a hydrogenproduction capacity of 3.6 ml g�1 under comparable cyclingconditions.27

While equilibrium fuel productivity provides one measure ofthe suitability of a material for thermochemical cycling, it hasthe potential to be misleading because it does not account forsteam to hydrogen conversion efficacy. That is, an oxide thatrequires a small amount of steam to reach full reoxidation willbe preferable to that which requires a large amount as theformer will inherently result in higher conversion rates,behavior that will ultimately benet system efficiency. An esti-mate of the number of moles of water required to induce a givenchange in oxygen content in the oxides studied here is pre-sented in Fig. 10. The reduction is taken to reach equilibriumunder an oxygen partial pressure of 10�5 atm and temperatureof 1400 �C. This condition xes the di for each composition. Thesubsequent oxidation is taken to occur in a closed volume ofvariable size that allows equilibration between the oxide and thegas phase under an initial condition of 0.2 atm of H2O partialpressure (pO2 ¼ 1.6 � 10�7 atm) and a temperature of 800 �C.The amount of fuel produced corresponds to Dd and must, onthermodynamic grounds, increase with increasing steam input.The maximum Dd possible occurs when df reaches the equilib-rium non-stoichiometry under pO2 ¼ 1.6 � 10�7 atm. The latteris close to 0 (Fig. 1), and thus Ddmax is just less than di. Theprecise value of Ddmax is unknown, however, as indicated by thelarge uncertainties given in the nal column entry in Table 1,due to the challenges of measuring small weight changes withpO2 as occurs in the vicinity of d ¼ 0.

The results show that the steam requirement for fuelproduction from strontium-doped lanthanum manganite israther high. For example, to achieve a Dd of 0.02, possible onlywhen x in La1�xSrxMnO3�d is 0.3 or higher, requires a steaminput ranging from about 1 to 2 moles H2O per mole of oxide.This reactant input requirement in turn implies a steam-to-hydrogen conversion rate in the range of 2 to 4%. While thiscalculation corresponds to a worst case scenario and in a real,owing system the inux of fresh reactant with high oxidizingpower will decrease the steam requirement, the results show aclear composition trend. Specically, the steam requirementincreases with Sr content for a given Dd, yet, conversely, asalready discussed, the fuel production capacity (Ddmax) also

J. Mater. Chem. A, 2014, 2, 13612–13623 | 13619

Tab

le1

Cyc

le-ave

ragedfuelp

roductivityofLa

1�xSr

xMnO

3�d.A

lsoreportedarethe(exp

erimental)measurementtime,themeasuredH2/O

2ratio,theprojectedgas

productionbasedonthe

profile

charac

teristicsan

dapresu

medsu

fficientlylongequilibrationtime,an

dtheestim

atedtimeto

reac

htheprojectedvalue.In

addition,thepredictedO

2releaseonthebasis

ofthe

therm

odyn

amic

analysisisprovidedforco

mparison

xof

La1�xSr x

MnO3�d

Measu

redga

sprod

uction(m

lg�

1)

Measu

remen

ttime(m

in)

H2/O2

(unitless)

Projectedga

sprod

uction

(mlg�

1)

Projection

time(m

in)

Pred

ictedO2

release

(mlg�

1)

Gas

species

O2

H2

O2

H2

O2

H2

O2

H2

0.1

0.55

�0.09

0.91

�0.04

128

1.64

�0.02

0.65

�0.12

0.91

�0.04

468

0.5�

0.4

0.2

1.45

�0.05

2.89

�0.07

4516

2.00

�0.04

1.53

�0.04

2.89

�0.07

9122

0.9�

0.3

0.3

3.4�

0.3

5.68

�0.07

6031

1.68

�0.05

3.8�

0.3

6.26

�0.09

177

923.3�

0.2

0.4

4.9�

0.4

8.9�

0.2

7060

1.84

�0.03

5.3�

0.5

10.3

�0.4

192

210

6.1�

0.8

13620 | J. Mater. Chem. A, 2014, 2, 13612–13623

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increases. The steam requirement for lanthanum strontiummanganite, irrespective of specic composition, is alsosubstantially greater than that when ceria is employed,computed here for comparison.27 For example, di for ceria is0.031, and attaining a Dd of 0.02 from this material (at therelatively unfavorable reduction temperature of 1400 �C) wouldrequire introduction of only slightly more than 0.02 moles ofH2O per mole of oxide, corresponding to almost a 100% steam-to-hydrogen conversion rate.

In general, the amount of steam required to achieve a targetamount of hydrogen production approaches innity as Dd

approaches Ddmax (limitations of the raw thermogravimetricdata in the region of d ¼ 0 slightly obscure this behavior ofLa1�xSrxMnO3�d), indicating that efficient fuel productionstrategies may not be those that attain maximum fuel outputper cycle. Instead, cycling that forgoes the nal stages ofoxidation and hence limits the steam input may be preferable.For example, doubling the fuel output from La0.6Sr0.4MnO3�d

from a Dd of 0.05 to 0.1 would require about an order ofmagnitude increase in steam input. While it is impossible toachieve such high levels of fuel productivity from ceria (underthe specied cycling atmospheres the maximum Dd is 0.031),one must carefully weigh the tradeoffs between increased fuelproduction per formula unit of oxide versus the anticipatedefficiency penalties of decreased conversion values beforeconcluding which is the superior choice. Scheffe et al. recog-nized the need to operate strontium-doped lanthanummanganites under high steam input conditions and computedfavorable solar-to-fuel conversion efficiencies from theseperovskites under the assumption of 100% heat recovery fromthis large excess of steam.10

The kinetic characteristics of the La1�xSrxMnO3�d materialsfor thermochemical fuel production merit further discussion.As already indicated in Fig. 8, the time-scales associated withreduction are much shorter than those associated with oxida-tion. Moreover, the time to reach the peak in the oxygen releaseprole is approximately 1.1 min from the rst detection ofoxygen above the background level, a value that is unchangedwith x. This time period is comparable to the 1.2 min requiredto heat the oxide from 800 to 1400 �C at the heating rate of500 �C min�1. From these observations, we conclude that,during the thermal reduction, each of the strontium-dopedlanthanum manganites largely maintains an oxygen contentthat is equilibrated with its T and the gas phase pO2. The longtail in the oxygen release proles (present for each of thecompositions) may reect the decrease in the thermodynamicdriving force as the oxygen chemical potential of the solid phaseapproaches that of the gas phase.

In contrast to oxygen release, fuel production is not onlysluggish, but also sensitive to Sr content, Fig. 8. Possibleexplanations for this include a systematic variation inmorphological features, a decrease in chemical diffusivity, or adecrease in the surface reaction constant with increasing Srdoping level. From the SEM images, Fig. 4, the decreasing grainsize with increasing Sr content suggests a morphology thatfavors rapid reaction at large x. Given the observation of acharacteristic time that increases with increasing Sr doping

This journal is © The Royal Society of Chemistry 2014

Fig. 10 Moles of water vapor required to induce the indicated stoi-chiometry change per mole of oxide at 800 �C using an input gasstreamwith pH2O¼ 0.2 atm (balance inert), after reduction at 1400 �Cunder 10 ppm oxygen (balance inert). Results for La1�xSrxMnO3�d arecomputed based on the thermogravimetric data reported by Mizusakiet al.19,20,25 Behavior is compared to that of CeO2�d, computed basedon the thermodynamic data reported by Panlener et al.24 Vertical linescorrespond to Ddmax for each composition. Where the curves areshown using dotted lines, the computation has large uncertainty due agap in the thermogravimetric data in the vicinity of d ¼ 0.

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level, the morphological variations can be ruled out as thesource of the differing kinetic responses. The chemical diffu-sivity, as noted, above can be expected to be dependent on Srcontent as a result of the differing d(pO2,T) functional forms forthe different compositions. As given in eqn (4), Dchem is directlyproportional to the quantity L, dened as

L ¼ � 1

2

v ln pO*2

v ln½VccO�

(5)

with the other terms, DVoand tel being largely independent of

oxygen partial pressure and composition over the experimentalconditions considered. The computed values at 800 �C, Fig. 11,show that L indeed varies with doping level. The value of this

Fig. 11 L plotted as function of oxygen partial pressure and strontiumcontent in La1�xSrxMnO3�d. Open circles indicate the value of L afterquenching from 1400 �C (under 10 ppm oxygen) to 800 �C.

This journal is © The Royal Society of Chemistry 2014

parameter at an internal oxygen chemical potential corre-sponding to a quench from 1400 �C is shown. Although L varieswith Sr content, as with the morphological variations, the trendis the opposite of that required to explain the trend in hydrogenproduction kinetics. Furthermore, as already discussed, theabsolute value of Dchem reported by both Yasuda et al. and byBelzner et al. implies a characteristic time that is far shorterthan the times represented in Fig. 8. Thus, diffusion can beruled out as either the rate-limiting step or the source of thediffering fuel production kinetics.

With morphology and bulk diffusivity eliminated, we turn tothe surface reaction step. It is common to express the ux,specically in this case the oxygen ux, JO, across a surface atwhich a reaction occurs as

JO ¼ �ksurfDCO (6)

where ksurf is the surface reaction rate constant, DCO [¼ CO,solid-(interface) � CO,gas(interface)] is the difference in oxygenconcentration across the solid-gas interface, and the reaction istaken to be rst order with a rate constant that is independentof concentration. The expression above takes the driving forcefor the reaction to be the difference in concentration, butthermodynamic principles dictate that the driving force is, infact, the difference in oxygen chemical potentialDmO ¼ mO,solid(interface)�mO,gas(interface). At the initiation ofthe hydrogen production step, these differences, DCO and DmO,can be obtained directly from the oxygen non-stoichiometrycurves, Fig. 1. The DCO values are essentially given by the Dd

values indicated between oxidizing and reducing conditions(also corresponding to Ddmax described above). Recognizingthat mO ¼ m0O + RT ln pO2, the DmO values correspond to thedifference in effective oxygen partial pressures between thequenched state at 800 �C and the oxidized state at the sametemperature.

The correlation between the peak in the hydrogen evolutionand two parameters, Dd (directly proportional to DCO) and DmO,is shown in Fig. 12. In the case of Dd, an anticorrelation isobserved, with the peak generally decreasing in intensity withincreasing concentration difference. In contrast, the peakheight appears to be positively correlated with the magnitude ofDmO. In both cases, the behavior of La0.8Sr0.2MnO3�d is some-what anomalous. Ignoring this outlier, these results (incombination with the conclusion that diffusion cannot be rate-limiting) suggest that the hydrogen production half-cycle issurface reaction limited, and that the rate limitation arises fromdifferences in the thermodynamic driving force for the reaction.It is of some value to note that, because the magnitude ofmO,gas(interface) is xed by the reactor conditions (irrespectiveof the nature of the reactive oxide), DmO directly scales withmO,solid(interface). The latter is a measure of the oxygen chemicalpotential in the material upon quenching and, in turn, directlyscales with DHredox, suggesting that materials with smallenthalpies of oxidation will be at a kinetic disadvantage relativeto those with large enthalpies. On the other hand, in theabsence of direct measurements of the surface reactionconstant, it is premature to rule out a possible dependence of

J. Mater. Chem. A, 2014, 2, 13612–13623 | 13621

Fig. 12 Peak flux of hydrogen plotted against (a) change in oxygencontent, and (b) change in chemical potential over the course of theoxidation by 0.2 atm steam (balance inert) at 800 �C after quenchingfrom reduction under 10 ppm O2 (balance inert) at 1400 �C.

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ksurf on Sr doping level as the cause of the depression in kineticswith increasing Sr content. Such behavior could arise indirectlyfrom a dependence of ksurf on the (initial) bulk oxygen vacancyconcentration, which increases monotonically with Sr contentfor the cycling conditions explored (Fig. 1). Regardless of thesepossibilities (reduction of the driving force or inherent variationin ksurf), the elimination of diffusion as the rate-limiting steppoints towards a surface reaction limited process.

5. Conclusions

Thermodynamic and kinetic properties of Sr-doped lanthanummanganites, La1�xSrxMnO3�d up to x ¼ 0.4, were assessed fortwo-step thermochemical water dissociation. Existing ther-mogravimetric data were analyzed without reference to aspecic defect chemical model to obtain generic values of theenthalpy and entropy of the reduction reaction as functions ofnonstoichiometry. The equilibrium fuel productivity expectedfrom thermochemical cycling between 800 and 1400 �C wascomputed using these extracted thermodynamic functions. Sr-doping was shown to increase the expected fuel productivitymonotonically, reaching 8.9 ml H2 per ml per cycle for x ¼ 0.4.Oxygen evolution and hydrogen generation from steam werethen experimentally conrmed. The materials exhibited stablefuel production over multiple cycles, with hydrogen

13622 | J. Mater. Chem. A, 2014, 2, 13612–13623

productivities approaching the thermodynamically predictedvalues and hydrogen to oxygen production ratios close to theexpected value of 2. Although Sr-doping lowers the amount ofoxide required to produce 1 mole of hydrogen, it increases theamount of steam required. The kinetics of oxygen release onheating were found to be largely limited by the rate of temper-ature change of the material for heating rates of�500 �Cmin�1.In contrast, the fuel production kinetics were slower than couldbe explained by bulk (solid-state) mass diffusion limitations,indicating a surface reaction limited process. The rate wasfurther found to decrease with increasing Sr content, possiblydue to a decrease in the thermodynamic driving force foroxidation of the oxide with increasing dopant amount.

In sum, it is apparent there is a subtle trade-off betweenmaterial characteristics in selecting the optimal material forsolar-driven thermochemical fuel production. Whilelanthanum strontium manganite generates more fuel per cyclefor reduced temperature cycling than ceria, this benet comesat the price of increased steam requirements. In addition,within the La1�xSrxMnO3�d system, the increase in fuelproductivity with x comes at the price of a reduction in fuelgeneration kinetics. Thus, one can expect that the optimalmaterial will be dependent on the cycling conditions, the detailsof the reactor and success in designing systems that providehigh levels of gas phase heat recovery. Although these tradeoffsdo not permit clear designation of La1�xSrxMnO3�d as a supe-rior material relative to ceria, it is to be recognized that, incontrast to the uorite structure, the perovskite structure is hostto an immense combination of elements. Thus, the potential foruncovering a perovskite composition with superb thermody-namic and kinetic characteristics for solar-driven thermo-chemical can be considered high.

Acknowledgements

This work was supported by the Precursory Research forEmbryonic Science and Technology (PRESTO), Japan ScienceTechnology Agency. We thank Prof. William Chueh for fruitfuldiscussions and Mr Stephen Wilke for performing mercuryporosimetry measurements. We are further grateful to Prof.Yong Hao for assistance with the preparation of poroussamples.

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