Electrospun manganese-based perovskites as efficient oxygen

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Electrospun manganese-based perovskites as efficient oxygen exchange redox materials for improved solar thermochemical CO splitting 2

Asim Riaz, Peter B. Kreider, Felipe Kremer, Hassina Tabassum, Joyce S Yeoh, Wojciech Lipi#ski, and Adrian Lowe ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01994 • Publication Date (Web): 07 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019

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ACS Applied Energy Materials

Electrospun Manganese-Based Perovskites as Efficient Oxygen Exchange Redox Materials for Improved Solar Thermochemical CO2 Splitting Asim Riaza,*, Peter Kreidera, Felipe Kremerb, Hassina Tabassumc, Joyce S Yeoha, Wojciech Lipińskia,*, Adrian Lowea,* a

Research School of Engineering, The Australian National University, Canberra, ACT 2601, Australia.

b

Centre for Advanced Microscopy, The Australian National University, ACT 2601, Australia.

c

Beijing Key Laboratory of Theory and Technology of Advanced Battery Materials, Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China.

*

Corresponding author. E-mail: [email protected]; Tel: +61 47 032 8104.

*


*


Abstract Developing durable redox materials with fast and stable redox kinetics under high-temperature operating conditions is a key challenge for an efficient industrial-scale production of synthesis gas via two step solar thermochemical redox cycles. Here, we investigate novel electrospun nano-structured La3+-doped strontium manganites, LSM (LaxSr1-xMnO3, x = 0, 0.25, 0.50, and 1) for an efficient CO production with high redox kinetics. The oxidation behaviour of these LSM powders was assessed in terms of oxygen recovery and CO yield via thermogravimetric analysis by using air and CO2 as oxidation medium. Strontium manganate (SrMnO3) shows the highest CO yield per cycle of 854.20 µmol g-1 at a rate of ~400 µmol g-1min-1 when reduced at 1400°C and re-oxidized at 1000°C, with high oxygen exchange capacity in terms of oxygen non-stoichiometry of up to 0.29, during CO2 splitting cycles. However, lanthanum manganite (LaMnO3) demonstrated high yield of CO of 329 µmol g-1 with a rate of 329 µmol g-1 when reduced at 1000°C and re-oxidized at 700°C, which is three times higher than the yield for SrMnO3 at the same conditions. The oxygen recovery in LSM samples was 4–15% higher during oxidation with air than with CO2. Moreover, the improved structural stability of these nano-powders indicates the potential of electrospinning technique for an up-scale synthesis of oxygen carriers. These findings show that a selective LSM system can be utilized for enhanced CO yield with high kinetics and structural stability at reduction temperatures 1000–1400°C. Keywords: Thermochemistry, reduction, electrospinning, perovskites, syngas, CO2 splitting

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Introduction Solar-driven two-step thermochemical cycles for production of synthesis gas (syngas) from water and carbon dioxide is a promising approach for production of carbon-neutral synthetic hydrocarbon fuels such as petrol, diesel and kerosene.1–3 Using optical concentrators solar radiation is focused at receiver aperture to obtain high-temperature process heat.4–6 A two-step thermochemical cycle (STC) comprises high temperature endothermic reduction of metal oxide (MO𝑥 ― 𝛿ox) to a reduced state (MO𝑥 ― 𝛿red), followed by its exothermic re-oxidation back to M O𝑥 ― 𝛿OX in a H2O or CO2 stream for water splitting (WS) or carbon dioxide splitting (CDS), respectively:7,8 Step 1: Endothermic reduction at Tred (~1000–1600 °C) 𝛿

MO𝑥 ― 𝛿𝑜𝑥 + ∆𝐻  M𝑥 ― 𝛿𝑟𝑒𝑑 (s) + 2 O2 (g)

(1)

Step 2: Exothermic oxidation at Tox (~800–1300 °C) WS:

M𝑥 ― 𝛿𝑟𝑒𝑑 + δH2O (g)  M𝑥 ― 𝛿𝑜𝑥 (s) + 𝛿H2(g) +∆𝐻

(2a)

CDS:

M𝑥 ― 𝛿𝑟𝑒𝑑 + 𝛿CO2 (g)  M𝑥 ― 𝛿𝑜𝑥 (s) + 𝛿CO (g) +∆𝐻

(2b)

The reduction temperature Tred typically varies from 1000°C to 1600°C1,9,10. The oxidation temperature Tox is typically below 1300°C. 3,5,10 The maximum amount of fuel (H2/CO) is highly dependent on the oxygen storage capacity (δ) of metal oxides. Thus, an efficient STC process is directly related to high oxygen exchange capacity of the metal oxides with high phase stability and low thermal losses.3,11 A range of metal oxides have been investigated as promising intermediate materials for WS and CDS processes.1,12,13,14 Volatile stoichiometric redox pairs such as ZnO/Zn and SnO2/SnO offer high oxygen exchange capacities per unit mass, but formation of volatile metal products is a challenge for potential industrial applications.10,13,15–17 Non-volatile stoichiometric redox systems may involve formation of an intermediate liquid phase, e.g. in the Fe2O3/FeO system, adding to the complexity of an industrial application. In contrast, non-stoichiometric redox systems involve oxygen vacancy exchange as in the CeO2/CeO2-δ system, while the problem of phase change and separation of gaseous metal products is not of concern.3,13 Ceria (CeO2) is one of the most investigated non-stoichiometric oxygen carrier for syngas production due to its high oxygen vacancy concentration and ability to maintain its cubic fluorite structure cycling over a range of operating conditions and reduction extents.4,17–20 However, achieving high reduction extent of ceria requires high temperatures (T > 1500°C), which imposes constraints on selection of reactor materials.21–25 Doping ceria with Zr2+ 17, Co3+


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26,

Mn3+ 27 or Fe3+ 28 results in an increased oxygen exchange capacity at T < 1500°C, but at a

cost of slower reduction kinetics.18,29,30 Alternatively, perovskites provide high reduction limits at temperatures as low as 1200°C, promising high fuel yield and a less-constrained reactor design process. These oxides typically possess an ABO3 or ABO4 type structure with numerous dopant possibilities at the A-sites (e.g. Ba2+, Sr2+, Ca2+, Ce3+, La2+, Y3+) and at the B-sites (e.g. Zr2+, Mn2+, Fe2+, Al3+, Co2+, Cr2+).1,3– 5,7,31

The thermodynamic properties of LSM perovskites as a function of composition (i.e.

doping scheme) have been extensively studied theoretically and experimentally.10,12,13,32–34 The A-site doping with lanthanum improves the redox performance of perovskites as compared to other lanthanides, due to an efficient vacancy generation mechanism. Doping perovskites with ceria has been considered for its potential to increase oxygen ion mobility. However, ceria is not suitable for the A-site doping of perovskites due to complex reaction chemistry.35 McDaniel et al. reported a promising class of perovskites i.e. LaAlO3-δ with Mn2+ / Mn 3+ / Mn 4+

on B sites and Sr2+ on A sites, resulting in increased H2 and CO yields by factors of 9 and 6,

respectively, as compared to ceria when reduced at 1250°C and oxidized at 1000°C.36 Rao and Dey reported successful doping using La3+, Sr2+, Y3+ and Ca2+ on the A-site of a manganitebased perovskite through performance evaluation for CDS processes via thermochemical redox cycles.15 A 50:50 combination of Y3+ and Sr2+ on the A-site showed the highest CO production rate of 571 mol g-1, which is three times higher than ceria at 1500°C. Deml et al. reported theoretical and experimental performance of aluminium and manganese dopants in La3+/Sr2+based perovskites. SrxLa1-xMnyAl1-yO3 perovskite outperformed ceria in solar thermochemical CO production with improved structural stability and high rates or reduction and oxidation at lower temperatures.37 Similarly, doping A-sites with Ca2+ and Sr2+ and B-sites with Al3+ in lanthanum manganites showed a five to thirteen-fold improvement in the extent of reduction as compared to ceria, during thermochemical redox cycling.12 The high structural stability and re-oxidation capabilities of manganite based perovskites with La3+ and Sr2+ on the A sites makes it the preferable system among other perovskites for thermochemical splitting of CO2.38 These materials are capable of producing 6 to 9 times more H2 or CO than ceria if subsequently re-oxidized by 40 vol% of H2O or CO2 at 1000°C, respectively. 5,23 However, their re-oxidation is thermodynamically less favourable and results in incomplete oxidation, which might be due to impeded reactivity with CO2 and H2O. Thus, the oxidation potential of these perovskite oxides must be under consideration for evaluating its actual redox performance.



In this study, we investigate the reduction and oxidation behaviour of LaxSr1-xMnO3 (LSM, x = 0…1) nanoparticles during thermochemical CO2 splitting cycles. Samples are synthesized via the electrospinning technique with compositional uniformity and morphological control.39 The studied nano-powders are observed to possess controlled increase in particle size after cycles at Tred = 1400°C with less sintering effect. Oxygen recovery is found to be more efficient during air oxidation than with CO2. Increasing the temperature difference ∆T = Tred – Tox from 0 K to 400 K results in increasing the reduction extent. Incorporation of soluble amount of La3+ in LSM enables its reduction at Tred = 1400°C with an extraordinary theoretical oxygen exchange capacity of one mole of oxygen. Strontium-rich LSM is found to perform better at Tred >1250°C due to better high-temperature stability, while the larger lanthanum ions induce more deformity in LSM structure than strontium ions. The detailed surface study of LSM samples reveals that the oxidation state of manganese ions also decreases from +4 to +2.9 with higher content of La3+ present in the lattice. The specific surface area of the investigated nanopowders plays a key role in terms of structural stability in the redox reactions. The findings of this study can inform studies of other families of perovskite oxides to overcome one of the key challenges for the efficient oxygen recovery during thermochemical redox cycles. Experimental Sample preparation Lanthanum (III) chloride LaCl3·7H2O (Sigma Aldrich), strontium carbonate SrCO3 (Sigma Aldrich) and manganese (II) acetate (C6H9MnO6.4(H2O)) (Ajax Finechem) are the main precursor materials for the synthesis of LSM while polyvinyl pyrollidone (PVP) (Sigma Aldrich, Mw 1.3x106) is the fibre forming polymer. Commercial Mn3O4 (Sigma Aldrich) is used for the purpose of comparing CDS performance and material structure. De-ionized (DI) water, absolute ethanol (Ajax Finechem) and acetic acid (Ajax Finechem) are used as solvents. LSM perovskites are synthesized by a standard electrospinning technique as reported elsewhere.40 This technique is utilized to obtain structural and stoichiometric uniformity of the LSM structures, with low setup cost, batch production capability and morphology control. The other liquid phase preparation techniques of nanoparticles e.g. sol-gel and hydrothermal methods involve expensive and toxic precursors and solvents. Volume shrinkage and difficulty in morphology control is another disadvantage in sol-gel method. In addition, formation of gel is a slow process and extra steps for by-products/solvent removal and cleaning of gel make the sol-gel method complicated for perovskite production.41 The hydrothermal process utilizes an expensive equipment and the crystal growth cannot be observed during the reaction.42


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Table 1. Elemental composition of LaxSr1-xMnO3 for sample synthesis determined by stoichiometric analysis vs measured composition of as-prepared1 samples using energy-dispersive X-ray spectroscopy. The measurement errors represent 95%-confidence intervals about the mean. Molar fraction Sample

x

Sr

Stoichiometric La Mn

O

Sr

La

Measured Mn

O

SMO

0

0.52

–

0.52

1.57

0.54±0.16

–

0.57±0.14

1.36±0.21

LSM25

0.25

0.37

0.121

0.49

1.48

0.38±0.11

0.12±0.02

0.42±0.17

1.65±0.27

LSM50

0.5

0.23

0.23

0.46

1.39

0.26±0.09

0.20±0.06

0.43±0.13

1.12±0.15

LMO

1

–

0.41

0.41

1.24

–

0.45±0.11

0.37±0.10

1.08±0.11

First, the precursor materials with the required cation stoichiometric ratios (Table 1) were dissolved in a combination of solvents and stirred for 18 hours. The combinations of solvents for C6H9MnO6.4(H2O) was a 3:1 mix of DI water and ethanol, while a 3:2 mix of DI water and acetic acid was used to dissolve LaCl3 and SrCO3 in a separate container. The two solutions were mixed together and PVP was added with a molar ratio of 1:10 to the precursor material and stirred until a homogenized sol was achieved. The molar ratio of PVP was carefully adjusted to achieve a suitably viscous sol, which was then transferred into a syringe. Electrospinning was performed at an accelerating voltage of 25 kV, with a needle-tocollector distance of 15 cm at a feed rate of 0.5 mL hr-1. As-spun fibres were collected on a flat copper plate wrapped with aluminium foil. The samples were subsequently calcined in an air atmosphere at 1000°C, ramping with a rate of 3°C min-1 and held for 5 hr, followed by furnace cool. Water and organic constituent removal process during calcination was elaborated by thermogravimetric analysis (TGA). Removal of surface water and water of crystallization molecules was observed at ~110°C and ~229°C, respectively.43 The removal temperature for water of crystallization increases from 229°C to 286°C La3+ fraction increasing from 0 to 1. A much faster rate of mass loss was observed above 229°C, which is attributed to removal of organic molecules, predominantly PVP. No significant mass change was observed above 530°C, validating the selection of the calcination temperature of 1000C for the materials and their fabrication process considered in this study.44 The sample fabrication process is schematically depicted in Figure 1.

1

Samples obtained after the electrospinning step of the fabrication process are referred to as as-spun. Samples obtained after

the calcination step of the fabrication process are referred to as as-prepared or before thermochemical cycling. Samples obtained after the thermochemical cycling are referred to as cycled.



Figure 1. Fabrication of LSM samples via electrospinning method

Morphological and structural characterization LSM samples were characterized for structural, chemical and morphological investigation before and after CDS cycles. A Bruker D2 Phaser Diffractometer was utilized to perform Xray diffraction (XRD). Each powder sample was scanned using a Cu Kα (1.54Å) radiation source with an operating voltage of 30kV and a current of 10 mA. The XRD patterns were recorded with a scan rate of 0.75° min-1 in the 2 range of 10–80° at an increment of 0.02°. The Scherrer formula was applied for the most intense peaks to calculate the crystalline domain size. Nitrogen adsorption–desorption isotherms at 7 K were acquired for the surface and porosity analysis by using TriStar II, Micromeritics. These isotherms were utilized in the determination of Brunauer–Emmett–Teller (BET) surface area values. Powder samples were degassed at 180°C under vacuum (0.1 mBar) for 4 h prior to the measurements. Information about the morphology, particle size, lattice plane spacing was acquired using a high-resolution transmission electron microscope (JEOL 2100F HRTEM) operated at 200 kV. A small amount of powder sample was added to ethanol and sonicated for 20 min to achieve a stable suspension, followed by casting a drop onto Lacey carbon 200 mesh copper grids. Field emission electron microscopy (Zeiss Ultraplus FESEM) was utilized to observe the morphology of powder samples before and after CDS cycles. Chemical composition data were acquired from energy dispersive X-ray spectroscopy (EDS). Surface characterization of elemental electronic states was measured by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra imaging photoelectron spectrometer). The instrument was equipped with a monochromic Al-Kα X-ray source (h = 1468.7 eV) at a power of 15 kV 12 mA =180 W with a hemispherical analyser. The analysis was carried out in a fixed analyser transmission mode with the analysis area of 0.3 mm0.7mm and a pressure of 10-8 mbar maintained in the analysis chamber. Powder samples were loaded into custom-built sample


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holders with shallow wells and analysed on two different locations at a photoelectron emission angle of 0° with respect to the surface normal. For more detailed information about oxidation states and chemical structure of powder samples, high resolution survey spectra were acquired at a pass energy of 40 eV. CasaXPS processing software version 2.3.15 (Casa Software Ltd., Teignmouth, UK) was utilized for the XPS data processing of samples. Binding energies (BE) of samples were referenced to the C 1s peak at 284.8 eV. C 1s, La 3d, Sr 3d, Mn 2p, Mn 3s and O 1s were analysed from survey spectra. Thermochemical cycling Thermochemical cycling performance of LSM samples was studied by TGA (Netzch STA 449 F3 Jupiter). 20 ± 2.5 mg samples were placed in 70 µl alumina crucibles without a crucible lid. Gas analysis was performed using a quadrupole mass spectrometer (QMS200, Stanford Research Systems), allowing for continuous monitoring with 700°C is thermodynamically favourable for high reduction extents, efficient oxygen recovery and CO production. This typical behaviour suggests a strong relation between reduction and oxidation temperatures with the CDS performance. However, hindrance in the solid-gas transfer due to the possible sintering of nanoparticles may result in a non-linear variation in CDS performance. The highest calculated CO of 921.37 µmol g-1 was recorded during the air oxidation of SMO nano powders at Tred = 1400°C and Tox = 1000°C. However, LMO demonstrated the highest yields of around 364.25 µmol g-1 at Tred = 1000°C and Tox = 700°C. Depending on the capability of the material to react with CO2, stoichiometric moles of CO are also determined from TGA data. The average CO yield by LSM and Mn3O4 is presented in table 5. An increase in CO yield is observed for LSM samples with x = 0, 0.25 and 0.50, at higher Tred and Tox. At Tred = 1400°C and Tox = 1000°C, SMO showed an average of 854.20 µmol g-1, followed by LSM25 and LSM50 with 909 µmol g-1 and 661 µmol g-1 respectively. LMO showed the highest fuel production of 329 µmol g-1 at Tred = 1000°C and Tox = 700°C. Table 5 represents the average CO and O2 evolved during CDS cycles on various perovskites variant in this study and reported elsewhere. Clearly, the manganese oxide based perovskites demonstrate a high end redox capability in range of reduction and oxidation temperatures during CDS cycles, as compared to ferrite and cobaltate based perovskites. However, the nanosized LSM powders synthesized by electrospinning method exhibited a superior redox capability as compared to that of samples prepared by solid state reaction method.



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Table 5. Performance comparison of different perovskites variants used in this study and reported elsewhere, in terms of average CO and O2 evolved determined from the TGA data obtained during TC–CDS cycling.

SMO

1400

O2 evolved (µmol g-1) 532.52+36.5

SMO

1250

301.25+24.8

1000

511.52+41.2

This work

LSM25

1400

516.24+19.55

1000

786.32+41.6

This work

LSM50

1000

55.25+3.55

1000

76.37+15.2

This work

LSM50

1000

105.35+6.9

700

134.65+17.2

This work

LSM50

1400

512.55+43.1

1000

701.91+48.2

This work

LMO

1000

242.16+36.2

700

329.32+31.2

This work

LMO

1000

149.35+11.2

1000

223.94+19.5

This work

Y0.5Sr0.5MnO3

1400

483

900

757

61

La0.5Ca0.5MnO3

1400

315

1100

525

62

La0.5Sr0.5MnO3

1400

201

1100

325

62

La0.6Sr0.4Al0.6Mn0.4O3

1350

120

1000

247

63

LaSrCoO4

1300

268

800

161

64

La0.6Sr0.4MnO3

1350

348.8

1000

469.1

65

La0.6Sr0.4Mn0.4Fe0.6O3

1350

333.2

1000

277.2

65

La0.6Sr0.4FeO3

1350

427.3

1000

250.6

65

La0.6Sr0.4FeO3

1200

337

1050

53

38

Y0.5Sr0.5MnO3

1400

551

1050

112

38

La0.5Sr0.5Mn0.5Co0.5O3

1300

538

1050

152

38

Material

Tred (°C)

1000

CO evolved (µmol g-1) 854.2+66.4

This work

Tox (°C)

Reference

Figure 9 shows the TEM images of SMO, LSM25, LSM50 and LMO powders after CDS cycling at Tred = 1400°C. SMO underwent distinct restructuring during thermochemical carbon dioxide splitting (TC–CDS), resulting an increase in the particle size from 18.12  6.1 nm to 256.12  25 nm with an 89% decrease in specific surface area. However, SMO still had the highest pore volume of 0.16 cm3g-1 (Table 3) among all LSM powders. This high pore volume increases the surface available to the redox reactions, which contributes to enhanced oxygen transport during CDS cycles. The most intense restructuring was observed in the cycled LMO morphology, as shown in Figure 9, where the nano-sized LMO particles underwent substantial grain growth, resulting in highly sintered sub-micron particles of ca. 320.2  81.24 nm. In fact, LMO particles had a specific surface area of 2.256 m2 g-1 (Table 3) after 5 TC-CDS cycles, which is significantly lower than the original value of 76.29 m2 g-1. Crystal growth from 24 nm to 61 nm is also observed in LMO powders after CDS cycles. This sintering effect resulted in a 92% decrease in the specific surface area and 98% loss of pore volume reduced the total available surface for redox reactions. Consequently, reflecting the drop in reduction extents of LMO at Tred > 1000°C. In addition, increase in particle size led to slower reaction kinetics of LMO.


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Figure 9. Transmission electron microscope images depicting the morphological changes in the samples of (a) SMO, (b) LSM25, (c) LSM50, and (d) LMO after cycling at Tred = 1400°C.

LSM25 and LSM50 underwent moderate structural changes during CDS cycling as observed in the data of Table 3. For LSM25, the specific surface area and the pore volume decreased by 92% and 81.3%, respectively. For LSM50, the specific surface area the pore volume decreased by 93.6% and 83.2%, respectively. Conclusions We investigated the synthesis of nano-sized manganese-based perovskite oxides by electrospinning technique and their potential application for high-temperature thermochemical CO2 splitting. The effects of specific surface area, oxidizing medium (air/CO2), reduction– oxidation temperatures and concentration of La3+ on the redox performance of the materials with different La3+ and Sr2+ concentrations were elucidated. The electrospinning technique produced faceted and uniform nano-sized particles, with diameter ranging from 18.12 ± 6.1 nm to 35.22 ± 4.2 nm, high specific surface area up to 145.55 m2g-1 and pore volume of 0.561 cm3 g-1. These structural properties seen in LSM allowed for high CO yields, up to  mol g1,

with fast kinetics of ~400 mol g-1min-1 at Tred = 1400°C in case of SMO. The highest redox

capacity was observed in SMO (δ = 0.29) at Tred = 1400°C during thermochemical cycling with



CO2 splitting, whereas high La3+ fractions increased the redox capacity to δ = 0.08 at relatively low reduction temperatures (1000°C) with CO production rate up to ~200 mol g-1min-1. In addition, Sr2+-rich LSM compounds retained the grain morphology with definite grain boundaries, which demonstrates their high suitability for thermochemical redox cycling at reduction temperatures up to 1400°C. The combination of La3+ and Sr2+ in manganite systems can be useful for performance improvement at moderately-high temperatures (1250°C). The effect of manganese oxidation state in LSM systems on the CO2 splitting performance was critically examined. SMO demonstrated the highest CO yield with fast rates, showing a high oxygen exchange capacity at high reduction temperatures. In sum, this study resulted in nanostructured LSM materials with a set of robust structural properties such as specific surface area and high pore volume, which can in turn serve as a roadmap towards engineering of premium quality redox materials by electrospinning technique for enhanced solar fuel production via high-temperature thermochemical redox cycling. Acknowledgments Financial support from the Australian Research Council (ARC Future Fellowship FT140101213 by W. Lipiński) is gratefully acknowledged. This study used the facilities and the scientific and technical assistance at the Centre of Advanced Microscopy at The Australian National University. We thank Dr Michael Gao for his assistance in preparation of the thermochemical cycling experiments. Supporting Information SEM images of as-spun fibres. SEM images of Mn3O4 and SMO after cycling at Tred = 1400°C. XRD patterns of powder samples before and after cycles. XP spectra of C 1s, Sr 3d, La 3d of as-prepared powder samples. XP spectra of Mn 2p, O 1s and Mn 3s. TGA graph of Mn3O4 at Tred = 1000°C.

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For Table of Contents Only


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Graphic depicting the process of Solar thermochemical CO2 splitting and the yield of CO produced at reduction temperatures (1400C) 312x104mm (150 x 150 DPI)


Electrospun manganese-based perovskites as efficient oxygen

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