ACTS-P00577

THERMOCHEMICAL TWO-STEP WATER-SPLITTING USING PEROVSKITE

OXIDE FOR SOLAR HYDROGEN PRODUCTION

Yuta Sugiyama1, Nobuyuki Gokon2*, Hyun-Seok Cho2, Selvan Bellan2, Tsuyoshi Hatamachi3, Tatsuya

Kodama3

1 Graduate School of Science and Technology, Niigata University, Japan 2 Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi 2-nocho, Nishi-ku, Niigata 950-2181,

Japan 3 Dep. of Chem. & Chem. Eng., Faculty of Engineering, Niigata University, Japan

Presenting Author: F16B052A@mail.cc.niigata-u.ac.jp

*Corresponding Author: ngokon@eng.niigata-u.ac.jp

ABSTRACT

Perovskite oxide (LaSr(Al, Cr)MnO3-δ) powder was examined as a redox material on a thermochemical two-step

water-splitting for converting high-temperature heat into chemical fuels. The perovskite oxide was synthesized by

using a modified pechini method, and calcined in air atmosphere during 6 hours at temperature of 1350-1400 °C.

Firstly, a reactivity of LaSrAlMnO3-δ powder was studied for oxygen release and hydrogen production from water

via the thermochemical two-step water-splitting cycle of thermal reduction (TR) temperature of 1350 °C and water-

decomposition (WD) temperature of 1000 °C. The amount of oxygen and hydrogen, its stoichiometry, and

reproducibility of oxygen evolution and hydrogen production were evaluated for different chemical composition of

the perovskite oxide. Finally, LaSrCrMnO3-δ powder was prepared and tested in order to increase hydrogen

productivity. The reactivity and reproducibility of oxygen evolution and hydrogen production were evaluated for

different Cr doping amount of the perovskite oxide. The oxygen evolution increased with increasing Mn content of

the perovskite oxide. The thermally-reduced LaSrAlMnO3-δ powder suffered from high-temperature melting and

coagulating of the material during the T-R step. The hydrogen production for LaSrCrMnO3-δ powder was enhanced

in comparison to LaSrAlMnO3-δ powder.

KEYWORDS: Hydrogen production, Thermochemical water-splitting cycle, Perovskite oxide, High–temperature

solar heat, Concentrated solar radiation

1. INTRODUCTION

Development of technology for secure and efficient production of hydrogen is a topical issue pertaining to

establishment of the hydrogen economy. Abundant but intermittent solar radiation is the largest renewable

energy resource on earth, and tireless efforts have been devoted to investigating the conversion of energy from

solar radiation into chemical fuels such as hydrogen. Thermochemical water-splitting cycles can achieve

attractive and environment-friendly production of hydrogen and oxygen from the dissociation of water by the

provision of high temperature based on concentrated solar radiation [1]. Thus, solar-driven thermochemical

water-splitting cycles are regarded as prospective permanent solutions for the production of hydrogen from water.

This approach enables utilization of renewable technologies that are independent of fossil fuels via exploitation

of the incomparable magnitude and availability of solar resources [2].

The thermochemical two-step water-splitting cycle using a metal oxide as a redox pair is generally represented

as follows [3-5]:

1

Proceedings of the Asian Conference on Thermal Sciences 2017, 1st ACTS March 26-30, 2017, Jeju Island, Korea

MxOy → MxOy- + /2O2 (Thermal Reduction or TR step) (1)

MxOy- H2O → MxOy+ H2 (Water Decomposition or WD step) (2)

where M is a metal, MxOy is the corresponding metal oxide, and MxOy- is the reduced oxide. In the first step,

oxygen is released and evolved from the metal oxide when the oxide is thermally reduced at high temperature. In the

second step, the reduced oxide is oxidized by reacting with steam at relatively low temperature, thereby generating

hydrogen. The oxide is then recycled to the first step.

In general, prospective materials for use as redox materials for the thermochemical cycles can be divided into two

categories: metal oxide/metal (e.g., ZnO(s)/Zn(g)), and metal oxide/metal oxide [3-5]. The latter case can be further

categorized into subcategories, i.e., a single multivalent metal (e.g., Fe3O4(s)/FeO(s), Mn3O4(s)/MnO(s)), or the more

recently tested CeO2(s)/CeO2-(s), and multivalent mixed metals (ferrites, doped ceria, hercynite, Perovskites). Solid-

gas phase transitions occur during the TR step of metal oxide/metal redox cycles, whereas in the redox cycles of

metal oxides, the species remain in the solid phase [3-5].

Perovskite oxide is known to remain in the condensed state during the two-step reaction, and the perovskite

phase maintains its structure across a wide range of content of reduced species, . This leads to formation of

oxygen vacancies that promote the transport of oxygen atoms through the material. In contrast to ceria, where

vacancies are intrinsically formed at low temperatures, perovskite oxides reported in literatures induced extrinsic

vacancies by ionic doping [6-9]. Recently, perovskite oxides-based on (La, Sr, Ca)MnO3- and their alumina

solid solutions were reported as a potential material of solar thermochemical water-splitting cycle [6, 10].

Generally, it was well-known that the redox performances for perovskite oxides provide high extents of thermal

reduction and low levels of water-oxidation of thermochemical cycle.

In this study, we experimentally screened a reactivity of the perovskite oxides-based on (La, Sr)(Al,Mn)O3- for

the thermochemical two-step water-splitting cycle. The perovskite oxides for promising chemical composition

were improved by substituting Al ions into Cr ions in the perovskite oxide to enhance a rate of hydrogen

production during WD step. The redox performances were experimentally evaluated in this study.

2. EXPERIMENTAL

2.1 PREPARATION OF REDOX MATERIALS

Perovskite oxide (LaSr(Al, Cr)MnO3-δ) powders were synthesized by a wet

process using a modified pechini method involving the polymerization of the

complex precursor from an aqueous mixed solution of La(NO3)3·9H2O

(purity 99.9 %) with Al(NO3)3·6H2O (purity 99.9%), Sr(NO3)2 (purity 98 %),

Mn(NO3)3·9H2O (purity 98 %), and Cr(NO3)3·9H2O (purity 99.9 %). These

reagents were purchased from Wako Pure Chemical Industries

(http://www.wako-chem.co.jp/english/), Ltd, and used without further

purification. Deionized water free of oxygen and CO2 was prepared by

passing N2 through the deionized water for a few hours. The reagents above

were dissolved in the deionized water at appropriate concentrations. Ethylene

glycol and citric acid were added in the aqueous solution, thus resulting in

the formation of the complex precursor. The solution was stirred in a beaker,

and heated at 80 ºC for 1 h and subsequent 170 ºC for 1.5 h while the stirring

process was maintained during the heating. A gel formed from the solution

was collected, and dried in air stream in an oven at 180 ºC for 1 h, thus

resulting in the formation of the oxide powder. The dried powders were then

calcined at 1350-1400ºC for 6 h in air, before performing the high-

temperature cyclic reactions. The synthesized oxide powders were

characterized by X-Ray Powder Diffraction (XRD) (MX–Labo, MAC

Science) using CuK radiation for identification of the phases formed.

Table 1 summarizes the abbreviations used for the chemical compositions

Chemical composition abbreviation

La0.3Sr0.7Al0.3Mn0.7O3-δ LSAM3737

La0.4Sr0.6Al0.4Mn0.6O3-δ LSAM4646

La0.5Sr0.5Al0.5Mn0.5O3-δ LSAM5555

La0.6Sr0.4Al0.6Mn0.4O3-δ LSAM6464

La0.7Sr0.3Al0.7Mn0.3O3-δ LSAM7373

La0.4Sr0.6Al0.6Mn0.4O3-δ LSAM4664

La0.6Sr0.4Al0.4Mn0.6O3-δ LSAM6446

La0.7Sr0.3Al0.3Mn0.7O3-δ LSAM7337

La0.8Sr0.2Al0.2Mn0.8O3-δ LSAM8228

La0.9Sr0.1Al0.1Mn0.9O3-δ LSAM9119

LaMnO3-δ LM

La0.7Sr0.3Cr0.2Mn0.8O3-δ LSCM7328

La0.7Sr0.3Cr0.3Mn0.7O3-δ LSCM7337

La0.7Sr0.3Cr0.4Mn0.6O3-δ LSCM7346

La0.7Sr0.3Cr0.7Mn0.3O3-δ LSCM7373

Table 1 Summary of abbreviations used

for the different chemical compositions.

2

described hereafter. Note that LaSr(Al,Mn)O3-δ are referred to as written throughout the paper.

2.2 ACTIVITY TEST OF REDOX MATERIALS FOR THERMOCHMICAL TWO-STEP

WATER-SPLITTING CYCLE

The powdered materials of LaSr(Al, Cr)MnO3-δ were tested for activity in the two-step water-splitting

thermochemical cycle under the same reaction conditions. Approximately 1 g of the powder material was packed

into a platinum crucible (10 mm in diameter and 7 mm in depth) and mounted on the ceramic bar in a quartz reaction

chamber (SSA–E45, Ulvac–Rico) with an inner diameter of 45 mm (Fig. 2(a). In the TR step, the powder materials

were heated to 1350ºC within 1 min using an infrared furnace (RHL–VHT–E44, Ulvac–Rico) while passing N2 gas

(purity 99.999%) through the reactor at a flow rate of 1.0 dm3·min–1 at normal state. The temperature of the powder

material was controlled using an R-type thermocouple in contact with the platinum crucible. After heating at a

constant temperature of 1350ºC for 30 min, the powder material was cooled to room temperature. An aliquot of the

effluent gas from the reaction chamber was fed through a capillary tube and introduced into a residual gas analyzer

mass spectrometer (RG–102P, Ulvac) to determine the chemical composition of product gases, and to measure the

production rate of oxygen. Variations in oxygen partial pressure in the product gases were measured with respect to

reaction time during the TR step, and the amounts of oxygen evolved were determined from these variations. The

mass spectrometer was equipped with a standard oxygen gas tank that can supply oxygen gas at a constant flow rate

of 1.3×10-10 mol/s. This oxygen releasing equipment was used to calibrate the relationship between partial pressure

and the amount of oxygen evolved.

The thermally reduced powder material was pulverized using a mortar and pestle and then packed into a quartz tube

reactor (Fig. 1(b)) with an inner diameter of 7 mm in order to perform the WD step. A gas mixture containing H2O

and N2 was introduced into the reactor. This mixture was produced by bubbling N2 gas at a flow rate of 4–12

Ncm3·min–1 through a glass tube into distilled water at 80 or 95ºC. The partial pressures of water vapor in the

resultant gas stream were estimated to be 50 and 84%, respectively, by considering the water vapor pressure at 1 bar

at these temperatures. The powder material was heated to the given WD temperatures in the 1000ºC range within 10

min in an infrared furnace (RHL–E45P, Ulvac-Rico), and the WD step was performed for 60 min. The temperature

of the powder material was controlled using a K-type thermocouple in contact with the materials bed located inside

the reactor. To determine the amount of hydrogen produced during the WD step, the effluent gas was collected in a

bottle by the water displacement method, and the gas compositions were determined using a gas chromatograph

(GC–8A, Shimadzu) with a thermal conductivity detector. The powder material was then characterized using XRD.

After the WD step, the powder material was again pulverized using a mortar and pestle, mounted in the platinum

crucible, and placed in the quartz reaction chamber (Fig. 1(a)) to repeat the TR step. The TR and WD steps were

alternately repeated up to a maximum of three cycles.

The powdered material was pulverized after each step in order to examine oxygen and hydrogen productivities at

each cycle and to evaluate the reactivity of the powdered material throughout the cyclic reaction. The LaSr(Al,

Cr)MnO3-δ powders appeared to be sintered at high temperature, and became a porous pellet of fine sintered particles

after the TR step at 1500ºC. The materials could be pulverized using a mortar and pestle, so that the subsequent WD

step to generate hydrogen could be performed. The materials did not appear to be sintered at the temperatures of

1000ºC during the WD step, but remained as a powder.

Fig. 1 Experimental setup for (a) TR and (b) WD steps of the thermochemical two-step water-splitting cycle using

LaSr(Al, Cr)MnO3-δpowders.

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3. RESULTS AND DISCUSSION

Fig. 2 shows XRD patterns of LSAM7337, LSCM7328, LSCM7337, LSCM7346 and LACM7373 synthesized as

an original material. As seen for LSCM7328 (Fig. 2), a series of peaks due to the perovskite structure were evident

in the XRD patterns at LSAM7337 and all LSCM powders synthesized. No new peaks for the LSCM powders were

also observed as well as the LSAM powder synthesized. This result means that LSCM powder retains the perovskite

structure of the LM (LaMnO3)-based oxide as a single phase, indicating that the Sr, Al and Cr additions were

incorporated into the crystal structure as a solid solution.

Thermochemical two-step water splitting with LSAM powders was conducted to examine the oxygen and hydrogen

productivities and reproducibility of the redox cycle. The TR step was performed at a temperature of 1350ºC for all

of the materials, while the subsequent WD step was conducted at temperatures of 1000ºC. The amounts of oxygen

evolution and hydrogen production per gram of material are listed in Table 2. For the LSAM3737, 15.4 and 2.8

Ncm3/g-material of oxygen was evolved on each cycle, while 1.6 Ncm3/g-material of hydrogen was produced. For

the LSAM4646, LSAM5555, LSAM6464, LSAM7373, the amounts of oxygen releases for TR step of 1st cycle

were 12.1, 9.5, 7.6, 3.5 Ncm3/g-material, respectively. The amounts of hydrogen production for WD step of 1st cycle

were 0.5-2.9 Ncm3/g-material. The results mean that the series of LSAM powders provides higher oxygen release

than stoichiometric H2/O2 as the Mn contents in the LSAM powders increases, indicating that Mn ion is closely

related to redox reaction to produce oxygen and hydrogen. However, hydrogen productivity for the LSAM powders

is too low. In addition, it was observed for the high Mn contents of the LSAM powders to enhance the melting and

sintering of the reduced material. The melting and sintering was alleviated as La and Al contents in the LSAM

powders increases. Then, the following series of the LSAM powders (LSAM4664, LSAM6446, LSAM7337,

LSAM8228, LSAM9119 and LaMnO3(LM) ) which La and Mn contents both increased were synthesized and tested

for the thermochemical two-step water-splitting. For the LSAM6446, LSAM7337, LSAM8228, LSAM9119 and

LM with high Mn contents, the melting of the reduced material was not observed during TR step at 1500 ºC, but was

a mild sintering. Thus, the cyclability was improved for the second series of the LSAM powders. As seen in Table 2,

the amounts of oxygen release for the second series of the LSAM powders decreased, while those of hydrogen

production increased in comparison to the first series of the LSAM powders. In addition, for the LSAM7337,

LSAM8228, LSAM9119, the hydrogen production was enhanced, and the H2/O2 ratio approached to the

stoichiometry. The results for the second series of LSAM powders indicate the need to improve hydrogen

productivity.

0

100

200

300

400

500

600

700

800

900

20 30 40 50 60 70 80

LSAM7337

LSCM7328

LSCM7337

LSCM7346

LSCM7373

La2O3

Perovskite

structureSample

Evolved amount of H₂ and O₂/Ncm³g⁻¹-material1st 2nd 3rd Ave. H₂/O₂

LSAM3737O₂ 15.4 2.78 - 9.1

0.18H₂ 1.6 1.6 - 1.6

LSAM4646O₂ 12.1 3.9 - 8

0.18H₂ 1.7 1 - 1.4

LSAM5555O₂ 9.5 1.3 - 5.4

0.22H₂ 1.2 - - 1.2

LSAM6464O₂ 7.6 1.8 - 4.7

0.28H₂ 1.5 1 - 1.3

LSAM7373O₂ 3.5 1.4 - 2.5

0.28H₂ 0.5 0.8 - 0.7

LSAM4664 O₂ 4.4 - - -

-H₂ - - - -

LSAM6446O₂ 2.3 1.6 1.6 1.8

1.02H₂ 1.3 1.9 2.4 1.9

LSAM7337O₂ 2.4 1.9 1.9 2.1

1.05H₂ 2.8 1.8 1.9 2.2

LSAM8228O₂ 1.9 1.8 1.5 1.7

1.41H₂ 1.7 3.8 1.8 2.4

LSAM9119O₂ 1.2 0.9 0.8 1

2H₂ 1.6 2.2 2.1 2

LMO₂ 2.5 1.4 0.9 1.6

1.31H₂ 1.5 2.2 2.7 2.1

Table 2 Oxygen and hydrogen productions for LSAM powders

Fig. 2 XRD patterns of LSCM powders synthesized before

reaction. The LSAM7337 was a control for the LSCM powder.

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In order to improve the hydrogen productivity of LSAM powder, we newly synthesized a series of LSCM powders

(LSCM7328, LSCM7337, LSCM7364, LSCM7373) which all Al ions were completely substituted into Cr ions in

the perovskite structure. Thermochemical two-step water splitting with LSCM powders was conducted to examine

the oxygen and hydrogen productivities and reproducibility of the redox cycle. The amounts of oxygen release and

hydrogen production were plotted against cycle number (Fig. 3). In comparison to the LSAM7337, the LSCM7337

shows that the amounts of oxygen release were almost the same through the 3 cycles, but those for hydrogen

production evidently increase. Thus, the Cr substitution for the LSAM powders impacts on hydrogen productivity

with thermochemical two-step water-splitting cycle. Among the LSCM powders synthesized, the LSCM7328 and

LSCM7337 were superior in the viewpoint of oxygen and hydrogen productivities and cyclicity.

Fig. 3 Cyclic reactivity of LSCM and LSAM powders (a) for oxygen evolution of the 1st–3rd cycles during TR step

at 1500 ºC; (b) for hydrogen production of the 1st–3rd cycles during WD step at 1000 ºC.

Fig. 4 XRD patterns of the original LSCM7328, the solid materials obtained after the TR step and that formed after

the subsequent WD step.

Fig. 4 shows the XRD patterns of the original material, the material obtained after the 1st –3rd TR step, and the

material obtained after the 1st –3rd WD step using LSCM7328. A series of peaks due to the perovskite structure were

initially apparent in the spectra of all LSCM powders. In addition, new series of peaks due to the doping Cr element

were not observed in the XRD patterns of the original samples. These results indicate that all LSCM powders

comprises a Cr-substituted perovskite structure without second phase. After the TR step of the first run, the peaks

due to the LSCM powder with the perovskite structure were slightly shifted to lower diffraction angles compared to

the peaks of original LSCM7328, while no new peak was observed in the XRD pattern. These variations of the

series of peaks indicate that non-stoichiometric phase was formed from the LSCM7328 powder in the TR step.

46 47 48

0

100

200

300

400

500

600

700

20 30 40 50 60 70 80

original

TR1st

WD1st

TR2nd

WD2nd

TR3rd

WD3rd

Inte

nsity

(cps

)

(a) Oxygen release (b) Hydrogen production

0

0.5

1

1.5

2

2.5

3

1st 2nd 3rd

LSCM7328

LSCM7337

LSCM7364

LSCM7373

LSAM7337

Cycle number

Am

ou

nt

of

O2

pro

du

ctio

n(N

cm3

/g-m

ater

ial)

0

0.5

1

1.5

2

2.5

3

3.5

4

1st 2nd 3rd

LSCM7328LSCM7337LSCM7364LSCM7373LSAM7337

Cycle number

Am

ou

nt

of

H2

pro

du

ctio

n(N

cm3

/g-m

ater

ial)

5

After the WD step of the first run, the peak due to the LSCM with the perovskite structure was slightly shifted

toward a higher diffraction angle. For the second and third runs of the cycle, similar behavior of peak shift involving

redox reaction in the oxygen release–uptake mechanism was observed.

Finally, there is a clear benefit of using LSCM powders in order to favor the oxidation (hydrogen production)

kinetics which are the limiting factor for the round-trip efficiency of the materials evaluated in this work. In fact, it is

shown in the results how the oxidation kinetics are in overall slower than reduction kinetics. Therefore, LSCM

powders can favor the overall efficiency of the perovskite oxide-based thermochemical cycles. Challenges for the

future are to further improve a hydrogen productivity and oxygen kinetics by optimization of reaction conditions and

chemical composition of the LSCM powders.

4. SUMMARY

Perovskite oxide (LaSr(Al, Cr)MnO3-δ) powder was studied for a thermochemical two-step water-splitting cycle to

produce hydrogen from water. The perovskite oxide was synthesized by using a modified pechini method, and

calcined in air atmosphere during 6 hours at temperature of 1350-1400 °C. The perovskite oxides of some series of

LSAM and LSCM powders were successfully synthesized as a single phase. The reactivity and reproducibility of

oxygen release and hydrogen production was evaluated through the cyclic reaction. The results for a series of LSAM

powders indicate the amount of oxygen release was enhanced, but those of hydrogen production was low than the

stoichiometory. In addition, in the case of high Mn contents in the LSAM powder, it was tend to become melting

and sintering of the material at high-temperature during the TR step of the thermochemical cycle. By increasing La

contents in the LSAM powders, the melting could be alleviated during the TR step. In order to improve the

hydrogen productivity for the LSAM powders, new perovskite oxides, LSCM powders, which Al was substituted

into Cr in the perovskite structure was synthesized and tested to evaluate oxygen and hydrogen productivities and

cyclicity for the thermochemical two-step water splitting cycle. The hydrogen production for LSCM powders was

enhanced in comparison to LSAM powder with the same chemical composition.

ACKNOWLEDGMENT

This research was partially supported by the Ministry of Education, Science, Sports, and Culture, Grant-in-Aid

Challenging Exploratory Research, JSPS KAKENHI Grant Number 26630495.

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