Progress in La-doped SrTiO₃ (LST)-based anode materials for solid oxide fuel cells

Abstract

Solid oxide fuel cells (SOFCs) have appeared as a promising technology for a wide variety of potential commercial applications to lessen the urgency of energy shortage and environmental pollution associated with using conventional fossil fuels. Among the worldwide SOFCs research activities, the progress of SOFCs fed with hydrocarbon fuels that contain trace amount of H₂S is one of the most important research directions. Thereby, it becomes crucial to design novel electrode materials with enhanced catalytic activity, stability and tolerances to carbon deposition and sulfur poisoning. La-substituted SrTiO₃ (LST) based perovskite anodes have been widely investigated because of their high electronic conductivity in reducing atmospheres, excellent dimensional and chemical stability upon redox cycling and outstanding sulfur and coking tolerances. In this review paper, we will describe the development of LST-based anode materials for SOFCs in recent years. The synthesis, structure and fuel cell performance of the doped LST and LST-based composite anode materials are summarized in detail. The mechanism of H₂S-induced enhancement effect for electrochemical reactions on the LST-based anode materials is explored. The challenges related to the future developments of LST-based anode materials for SOFCs are also discussed.

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1. Introduction

Conventional fossil fuels are the main energy sources at present and for the foreseeable future. However, the energy sources of this type are limited in nature and non-renewable, which can lead to serious energy crises in the future. Also, the environmental problems associated with using conventional fossil fuels are causing public concerns and frequently appear as the headlines in various social media. Solid oxide fuel cells (SOFCs), on the other hand, have been considered as one of the most efficient and environmental-friendly technologies to tackle some of challenges that energy industry is currently facing. The distinguishing advantages of SOFCs include high energy conversion efficiency which is projected to be up to 60%¹ and great fuel flexibility. In theory, SOFCs can be fueled by essentially any fuels ranging from gaseous hydrogen, hydrocarbons to solid carbon. A typical SOFC single cell is consisted of three major components: a cathode, an anode and a ceramic electrolyte which is usually a solid oxygen ionic conductor. On the cathode side, O₂ is reduced and incorporated into the electrolyte defects, namely oxygen vacancies, resulting in the formation of O²⁻ ions. These O²⁻ ions will be driven by the difference of chemical potential, which is established between the two opposite electrodes, to move through the electrolyte to the anode where they can oxidize the fuels. All the electrons generated during the electrochemical oxidation process will be released to the external circuit for power generation.

Presently, one of the major concerns associated with utilizing hydrocarbon fuels in SOFCs is the anode catalyst coking and sulfur poisoning since H₂S exists in most raw feedstock. The commonly employed Ni-based anode catalyst can be easily deactivated by carbon deposits and suffers irreversible sulfur poisoning. Although steam reforming and desulfurization enable the suppression of coking and H₂S removal, these processes inevitably add the extra cost and complexity into the overall system. Therefore, intensive efforts have been devoted to developing novel anode catalysts.

In recent years, many excellent review papers have been published with focusing on cathodes,^2–4 electrolytes,^5–8 interconnects,^9,10 sealants,^11,12 current collector materials,^13,14 SOFCs models^15,16 and other correlative studies^17,18 about SOFCs. Also, a number of excellent review papers are available in the literature summarizing the progress of SOFCs anode catalysts.^19–21 Thus, in this review, first, we will only very briefly introduce the commonly used anode materials for SOFCs. Then, we will focus on the recent progress of La-substituted SrTiO₃ (LST) based anode materials since the doped SrTiO₃ has attracted increasing attentions as a promising anode candidate. The development of a variety of LST-based anode materials will be reviewed in details. Finally, the phenomena and mechanisms of the H₂S-induced enhancement of the SOFC performance in SOFCs fed by H₂S containing fuels will be summarized via using the LST-based anode. Meanwhile, the LST-based anode fabrication process, the relationship between microstructure and the fuel cell performance, and miscellaneous issues regarding to LST-based materials will also be discussed. A complete and comprehensive coverage of all the research activities on the LST-based anode materials may not be an easy task, therefore, this short review paper is meant to be a summary of most of the correlative research in recent years.

2. A brief overview of common anode materials for SOFCs

As described in the earlier review articles,^2,20,21 the general requirements for SOFCs anode materials should include: high electronic conductivity, excellent catalytic activity towards electro-oxidation of fuels, high ionic conductivity, chemical and thermal stability, adequate durability and compatibility with other SOFCs components, easy fabrication, and low cost. The other desirable properties are tolerances to carbon deposition or coking when using hydrocarbon fuels,²² and to sulfur poisoning when using fuels containing H₂S.^23,24

In general, the anode materials in SOFCs can be categorized as metal-based cermet, simple oxides, complex oxides and other alternative anode materials.

Advantages of the metal-based anode materials (e.g. Ni–yttria-stabilized-zirconia (Ni–YSZ)) include most requirements for anode materials mentioned earlier. However, the metal-based anode materials are easily poisoned by the trace impurities (e.g. H₂S) in fuels, which could lead to serious fuel cell performance degradation. It has been revealed that Ni/YSZ anode supported SOFCs can only tolerate up to 1 ppm H₂S and 10 ppm HCl without significant performance degradation.²⁵ Carbon deposition, redox cycling instability and Ni agglomeration upon prolonged usage are the other major problems observed on Ni/YSZ anode. Recently, Ni and Cu-based cermet anode catalysts have been reviewed.^20,21,24,26 Some metal-based anode materials such as Ir/Ce_0.9Gd_0.1O_2−δ,²⁷ Ni–Gd_0.1Ce_0.9O_2−δ^28,29 have been reported. Zirconia-based (e.g. Y_0.2Ti_0.18Zr_0.62O_1.90³⁰) and ceria-based (e.g. CeO₂,³¹ Ce_0.8Gd_0.2O_1.9³²) fluorites are the common simple oxide anode materials and have been reviewed particularly.^20,21,33

Perovskite-based oxide (general stoichiometry ABO₃, where A and B are metal cations) is a typical complex oxide catalyst which is known to have excellent thermal and mechanical stability, physical compatibility with typical electrolyte materials, flexibility, and low production cost, which makes it attractive as electrode material in SOFCs. Recently, different perovskite-based materials including chromite (e.g. La_0.75Ba_xSr_0.25−xCr_0.5Mn_0.5O₃³⁴), vanadate (e.g. La_0.7Sr_0.3VO_3.85³⁵), titanate (e.g. Sr_0.895Y_0.07TiO_3−δ,³⁶ Nb-doped SrTiO₃³⁷) etc. have been reported to be used as electrode materials in SOFCs. Double perovskites A₂BB′O₆ containing alternating BO_6/2 and B′O_6/2 corner-shared octahedral³⁸ are the first on the list of interest due to their reasonable ionic conductivity, redox stability and stability in presence of sulphurous impurities.^39,40 Some new double perovskites such as LnBaCo_1.6Ni_0.4O_5+δ (Ln = Pr, Nd, and Sm).⁴¹ Sr₂MMoO₆ (M = Mg, Mn),⁴² Sr_2−xVMoO_6−δ,⁴³ La₂ZnMnO₆⁴⁴ have been explored as electrode materials in SOFCs. Different perovskite-based anode materials applied for SOFCs have been reviewed.^{20,21,45–47} In addition, other type of complex oxides such as rutile,⁴⁸ tungsten bronze,⁴⁹etc. are also explored as anode catalysts.^20,21

For perovskite-based materials ABO₃, varying A and/or B can generate a large number of different compounds. Among them, strontium titanate (SrTiO₃) whose A and B-sites are occupied by divalent Sr²⁺ and tetravalent Ti⁴⁺ (Fig. 1)⁵⁰ is an excellent example to explain the origin of the increased electronic conductivity upon doping. O-2p orbitals and the empty conduction band from Ti-3d orbitals make the electronic energy band of SrTiO₃ exhibit an n-type semiconducting behavior in reducing atmospheres due to the redox couple Ti⁴⁺/Ti³⁺.⁵¹ An A-site or a B-site doped SrTiO₃ can improve its total electrical conductivity. Lanthanum is an appropriate donor dopant because its ionic radius is similar to that of Sr²⁺. Its stability in the trivalent state ensures its incorporation into the perovskite lattice as La³⁺. Due to the difference in valence between La³⁺ and Sr²⁺, introduction of La into the lattice requires that the lattice defect structure be modified to maintain electroneutrality.⁵² At the same time, the introduction of La to Sr sites causes the formation of oxygen-rich planes, which will improve the ionic conductivity of LST.²⁰ La-substituted SrTiO₃ (LST) based anode materials have been widely investigated because of their high electronic conductivity in reducing atmospheres, outstanding dimensional and chemical stability upon redox cycling and excellent sulfur and coking tolerances.

Fig. 1 Idealized structure of cubic perovskite SrTiO₃. Reprinted from ref. 50, Copyright (2013), with permission from Royal Society of Chemistry.

3. LST-based anode materials

This section reviews the synthesis, structure, thermal and electrical properties, and fuel cell performance of LST-based candidates including LST, further doped LST and LST composite anode materials.

3.1 La_xSr_1−xTiO₃

It is well known that LST materials sintered in reducing atmospheres cause a change in the titanium valence from Ti⁴⁺ to Ti³⁺, thus enhancing the electronic charge carrier concentration.⁵¹ Moos etal.⁵³ investigated the solubility of lanthanum in La_xSr_1−xTiO₃ in oxygen-rich atmospheres. A pure strontium vacancy compensation mechanism was observed for lanthanum contents up to x = 0.3. Above x = 0.4, additional titanium vacancies form, but their concentration remains negligible compared to the predominating strontium vacancies. At x = 0.5 and 0.6 the lattice structure was found to be slightly distorted.

LST-based materials used as potential anode for SOFCs were firstly investigated by Irvine et al.⁵⁴ in 1997. They found that single phase A-site deficient perovskites La_xSr_1−3x/2TiO_3−δ could be achieved in air at 0 ≤ x ≤ 0.6, and the materials remained at single phase under both high and low oxygen partial pressures at 930 °C. As La content increased, the electrical conductivity also increased to as high as 7 S cm⁻¹ (P_O2 = 10⁻²⁰ atm) at 930 °C for x = 0.6. This kind of A-site deficient LST-based anode materials was further studied by the group.^55–57 New family of perovskite oxides such as La₂Sr_n−2Ti_nO_3n+1^58,59 and La₄Sr_n−4Ti_nO_3n+2⁶⁰ were also explored as a potential anodes for fuel cells. Due to the high electrical conductivity, LST-based materials were also used as the interconnect materials⁶¹ and support layer⁶² for resisting carbon deposition.

The thermal, electrical, and electrocatalytical properties of LST powders (La_xSr_1−xTiO₃, x = 0.1, 0.2, 0.3, 0.35, and 0.4) were studied by Marina et al.⁵² The samples sintered in air exhibited an electrical conductivity in the order of 1–16 S cm⁻¹. But La_xSr_1−xTiO₃ sintered in hydrogen at 1650 °C showed 80–360 S cm⁻¹ under the typical SOFCs experimental operation conditions. No significant chemical expansion or contraction of La_xSr_1−xTiO₃ with x < 0.4 was observed when exposed under a wide range of P_O2 because the thermal expansion of LST was close to that of YSZ. La_xSr_1−xTiO₃ was found to be dimensionally and chemically stable when subjected to oxidation–reduction cycling. The structural and electrical properties of La_0.3Sr_0.7TiO_3−α and other donor-doped SrTiO₃ based oxides were also investigated by Hashimoto et al.^63,64 They found that the conductivity showed a strong dependence on the P_O2. When the P_O2 was over 100 Pa, the conductivity drastically dropped with increasing P_O2. The La_1−xSr_xTiO₃ (x = 0.5, 0.6, 0.7, 0.8) samples have been tested for possible use as a matrix phase for producing composite anodes for SOFCs.⁶⁵ It was found that La_0.5Sr_0.5TiO₃ demonstrated some ionic conductivity and could be used as a basic phase for producing composite anodes. The thermal expansion mechanism of La_xSr_1−xTiO₃,⁶⁶ sintering characteristics of (La_xSr_1−x)_1−yTi_1−zO₃⁶⁷ and La_xSr_1−xTiO₃⁶⁸ were also studied.

A series of A-site deficient perovskite oxides (La_0.3Sr_0.7)_1−xTiO_3−δ (x = 0, 0.03, 0.05, 0.07, 0.10) were investigated by Li et al.⁶⁹ Standard four terminal DC methods and electron-blocking method were applied to measure the total electrical conductivity and ionic conductivity, respectively. As A-site deficiency level increased, the ionic conductivity increased but the electronic conductivity decreased (Fig. 2). The ionic conductivity of (La_0.3Sr_0.7)_0.93TiO_3−δ was as high as 0.2–1.6 × 10⁻² S cm⁻¹ at 500–950 °C and 1.0 × 10⁻² S cm⁻¹ at 800 °C, more than twice that of La_0.3Sr_0.7TiO_3−δ. Its electrical conductivity was in the range of 83–299 S cm⁻¹ at 50–950 °C and 145 S cm⁻¹ at 800 °C. It was also assumed that A-site deficiency could improve the thermal stability of (La_0.3Sr_0.7)_1−xTiO_3−δ. Burnat et al.⁷⁰ found that the phase stability of A-site deficient La_0.2Sr_0.7TiO₃ could be enhanced significantly by adding some Sr to form La_0.2Sr_0.706TiO_3. The secondary phase such as TiO₂ could invoke the loss of electrical performance.

Fig. 2 Electrical conductivities of (La_0.3Sr_0.7)_1−xTiO_3−δ (x = 0, 0.03, 0.05, 0.07, 0.10) as functions of A-site-deficient level and temperature in 50–950 °C (a) and ln σT versus 1000/T in 50–350 °C (b). Ionic conductivities of (La_0.3Sr_0.7)_1−xTiO_3−δ (x = 0, 0.03, 0.05, 0.07, 0.10) as functions of A-site-deficient level and temperature (c) and ln σT versus 1000/T (d) in 500–950 °C. Reprinted from ref. 69, Copyright (2010), with permission from International Association for Hydrogen Energy.

The performances of fuel cells were compared using LST anode with that using Y doped SrTiO₃ (YST).⁷¹ It was found that adding the extra electrolyte materials, such as YSZ, into the doped SrTiO₃ anode could improve cell performance, suggesting insufficient ionic conductivity of LST. Moreover, the electrical conductivity of YST was lower than that of LST at the same temperature. Recently, A-site deficient Y_0.07Sr_0.895TiO₃ (YST) and La_0.2Sr_0.7TiO₃ (LST) were synthesized through spray pyrolysis method and compared for the electrical conductivity and chemical expansion.⁷² Under same reducing conditions, YST had better chemical expansion, while LST had better conductivity. To accommodate both conductivity and redox stability, LST was sintered at 1400 °C in air and reduced at around 1000–1200 °C, they then became suitable candidates for full ceramic based SOFCs anodes. Computational prediction was also explored to study the properties of LST-based materials.⁷³

Under the operating conditions of SOFCs, the chemical compatibility between the electrode materials and the electrolyte was a major concern. Burnat et al.⁷⁴ investigated the chemical interactions between common electrolyte materials and 20 wt% La doped Sr titanates with various A-site occupancy by SEM/EDX microscopy and XRD. It was found that all A-site deficient LSTs promoted a reaction with Sc and YSZ, while stoichiometric LST was more stable.

From the above analysis, we can conclude the following. The maximal solubility of La in SrTiO₃ is 0.6. However, the common x in La_xSr_1−xTiO₃ is 0.3 or 0.4. The structure will be distorted when the x exceeds 0.5. La-doped LST materials with a normal stoichiometry and A-site deficiency are the two main candidates having been examined. Compared to the stoichiometric LST, A-site deficient LST has higher conductivity but lower phase stability and chemical stability.

3.2 Doped La_xSr_1−xTiO₃

The main disadvantages of LST anode materials are their low ionic conductivity and poor electrocatalytic activity. Presently, the two main methods of increasing the ionic conductivity and improving the fuel cell performance including doping LST with other metal ions and introducing of active second phase to LST to form composite anode.

3.2.1 A-site doped LST. Cerium-based compounds were particularly interesting since these materials were known to be highly active towards the direct oxidation of fuels. Périllat-Merceroz et al.⁷⁵ investigated the possibility of improving the catalytic and electrocatalytic properties of La_0.33Sr_0.67TiO_3+δ materials by introducing the Ce⁴⁺/Ce³⁺ redox couple through partial substitution of La by Ce in the structure of the x = 0.33 member of the La_xSr_1−xTiO_3+δ series. The pure phase of La_0.33Sr_0.67TiO_3+δ (LST) and La_0.23Ce_0.1Sr_0.67TiO_3+δ (LCST) were synthesized through Pechini route in reducing atmosphere. Sintering the LCST in oxidizing and reducing atmosphere at different temperatures could ensure the presence of nanoparticles of active Ce-rich phase (Fig. 3a) within LST network and then enhance their activity towards CH₄ oxidation. The catalytic tests in methane steam reforming at 900 °C (CH₄–H₂O = 10/1) showed that the properties of the Ce-rich phase within LST network (LCST-ox) are greatly improved compared with pure LST and LSCT materials (Fig. 3b). More importantly, this material maintained a high resistance to the carbon deposition. It was then considered that the LCST materials of this kind would be a promising anode material for SOFCs directly operating on slightly wet CH₄.

Fig. 3 (a) FEG-SEM image of LCST after oxidation at 1200 °C in air. The arrows point at Ce-enriched phases. (b) Hydrogen formation rates vs. time upon reaction of a 10 : 1 : 9 CH₄–H₂O–N₂ mixtures over LST, LCST and LCST-ox samples. T = 900 °C. Reprinted from ref. 75, Copyright (2011), with permission from American Chemical Society.

Vincent et al.⁷⁶ found that Ba partial substitution for Sr in LST (La_0.4Sr_0.6−xBa_xTiO₃, 0 < x ≤ 0.2 (LSBT)) could increase its ionic conductivity, catalytic activity to the oxidation of CH₄ and improve the stability of the LST structure. Fuel cells were fabricated using commercial YSZ disks (300 μm thick and 25 mm in diameter) as the electrolyte and the composited YSZ/LSM 50/50 wt% as the cathode. It was observed that all fuel cells using LST or LSBT had limited activity for the conversion of hydrogen and methane, and the activity increased with the level of substitution by Ba. Most importantly, the fuel cell performance was significantly enhanced when H₂S was present in either CH₄ or H₂ fuel. The different amount of La_0.4Sr_0.5Ba_0.1TiO₃ was impregnated into porous YSZ by Vincent et al.⁷⁷Fig. 4 compares the performance with the number of times LSBT was impregnated into porous YSZ. The temperature for each test was 850 °C and the sequences of the fuels used were: H₂, H₂ + 0.5% H₂S, CH₄ + 0.5% H₂S and CH₄. The experiments clearly suggested that when the feed was CH₄ + 0.5% H₂S, the power density was consistently higher than that in pure H₂ and pure CH₄. Six impregnations provided maximum power density in all the fuels. The maximum power density was 84 mW cm⁻² when the feed was CH₄ + 0.5% H₂S, which was about 3.5 times of that when using pure CH₄ as the fuel. Based on the Ba-doped LST materials, Li et al.⁷⁸ found that pure BaTiO₃ anode had higher tolerance to carbon deposition, better electrochemical performance and much higher stability during long term operation compared to SrTiO₃ and La₂Ti₂O₇ anodes, especially in H₂S-containing atmospheres.

Fig. 4 Dependence of compensated maximum power density on the number of times LSBT was impregnated into YSZ obtained at 850 °C. Reprinted from ref. 77, Copyright (2012), with permission from Elsevier.

In addition to the Ba-doping in Sr site, LST with Ca-doping in Sr site was also studied by Verbraeken et al.⁷⁹ The pure La_0.20Sr_0.25Ca_0.45TiO₃ (LSCT) anode initially showed a poor performance. Then the LSCT was impregnated with ceria and nickel oxide to form different anode materials including pure LSCT, LSCT + 10 wt% CeO₂, LSCT + 5 wt% Ni, LSCT + 10 wt% CeO₂ + 5 wt% Ni, LSCT + 8 wt% CeO₂ + 3 wt% Ni. When both nickel and ceria were added as impregnates, a drastic improvement of the anode performance was observed comparing to cases of impregnating with ceria only and pure LSCT. These new anodes also showed excellent stability upon redox cycling, and appeared as a promising alternative to Ni–cermet electrodes. Further studies were done on the structure and conductivity of a series of Ca-doping in A-site deficient perovskite La_0.2Sr_0.7−xCa_xTiO₃.⁸⁰ The results indicated that La_0.2Sr_0.7−xCa_xTiO₃ with 0.1 ≤ x ≤ 0.4 were tetragonal and samples with 0.45 ≤ x ≤ 0.7 were orthorhombic at room temperature. A 0.3% increase in lattice volume of the samples was observed after their reduction at 900 °C in 5% H₂. The conductivity was definitely improved when compared to La_0.2Sr_0.7TiO₃. By increasing Ca doping content, the conductivity of the reduced samples increased dramatically, reaching the peak of 27.53 S cm⁻¹ in La_0.2Sr_0.25Ca_0.45TiO₃. However, the conductivity decreased as x was further increased. Later, a solution method was introduced to synthesize La_0.25Sr_0.25Ca_0.45TiO₃ by Yaqub et al.⁸¹

3.2.2 B-site doped LST. In relation to A-site doping, the majority of the research activities were concentrated on the B-site substitution. LST-based materials doped with various transition metal elements (Cr, Mn, Fe, Co, Ni, Co, Ga, Al, Mg, Sc and Nb etc.) in B-site have been investigated in recent years.

The La_0.7Sr_0.3Cr_0.8Ti_0.2O_3−δ with chromium-rich composition was synthesized using the Pechini method⁸² and showed a n-type conductivity and high stability under reducing conditions. The perovskite electrodes were electrochemically active towards hydrogen oxidation, although the electronic conductivity (0.1 S cm⁻¹ at 850 °C and 10⁻²⁰ bar P_O2) was not sufficient. A series of perovskite oxides La_0.75Sr_0.25Cr_0.5X_0.5O_3−δ (X = Co, Fe, Ti, Mn) as SOFCs anode electrocatalysts have been studied by Danilovic et al.⁸³ They concluded that the performance of the perovskite oxides depended on the nature of the substituent element X. Temperature programmed reaction (TPR) of CH₄ under O₂-free conditions showed that the catalytic activity of the oxides for conversion of CH₄ was in the order of Co > Mn > Fe > Ti. Within this series of catalysts, the order of maximum fuel cell power density depended on feed: CH₄, X = Fe > Mn > Ti; H₂, X = Fe > Mn > Ti; and 0.5% H₂S/CH₄, X = Ti > Fe > Mn. It was also found that gadolinia doped ceria (GDC) or YSZ combined with La_0.75Sr_0.25Cr_0.5Ti_0.5O_3−δ could improve the performance. The La_0.3Sr_0.55Ti_1−xCr_xO_3−δ single-phase perovskite structure can be obtained in air when the dopant content of Cr did not exceed 20 mol%.⁸⁴ The particle diameter obtained at 800 °C was less than 60 nm and the particles showed an excellent chemical compatibility with YSZ at 1400 °C. At 800 °C, the conductivity of La_0.3Sr_0.55Ti_0.8Cr_0.2O_3−δ pellet was 1.96 × 10⁻³ S cm⁻¹ in static air, which needed to be further improved. The Cr-doping decreased the lattice parameter while it increased the sinterability of La_0.3Sr_0.7Ti_1−xCr_xO_3−δ (LSTC, x = 0, 0.1, 0.2) materials.⁸⁵ The total electrical conductivity decreased as Ti cations were partially substituted by Cr. However, the electrical conductivity of the LSTC sample with x = 0.2 was still 53 S cm⁻¹ at 800 °C in 5% H₂/Ar. The electrical conductivity of Cr-doped samples declined significantly when the testing atmosphere shifted from 5% H₂/Ar to air. When the atmosphere was switched back to 5% H₂/Ar, the values of the electrical conductivity could mostly be recovered except for the initial cycle. The highest power density obtained with La_0.3Sr_0.7Ti_0.8Cr_0.3O_3−δ as the anode reached 43.86 mW cm⁻² at 900 °C using H₂ as the fuel, which was higher than that of undoped La_0.3Sr_0.7TiO_3−δ anode material (∼30 mW cm⁻²). The better fuel cell performance of LSTC anode materials could be attributed to the better contact between LSTC and YSZ, and the increased electrical conductivity.

The microstructural and electrochemical properties of Mn-doped LST anode material La_0.4Sr_0.6Ti_0.8Mn_0.2O_3±δ (LSTM) fabricated via liquid-phase impregnation have also been investigated.⁸⁶ The thermal stability was improved with decreasing Mn content. The electrical conductivity of a 10 wt% CeO₂–50 wt% LSTM–YSZ composite anode was higher than that of a 50 wt% LSTM–YSZ anode and was stable under reducing conditions. The maximal power densities of 50 wt% LSTM–YSZ anode were less than 100 mW cm⁻² over the entire measured temperature range. The addition of 10 wt% of CeO₂ and 1 wt% of Pd as catalysts increased the power density in H₂ to 150 and 210 mW cm⁻² at 800 and 850 °C, respectively. The performance of A-site-deficient (La_1−xSr_x)_0.95Mn_0.5Ti_0.5O_3−δ and A-site-stoichiometric La_1−xSr_xMn_0.5Ti_0.5O_3−δ perovskites were studied by Kolotygin et al.⁸⁷ The total conductivity, thermal and chemical expansion, and steady-state oxygen permeation limited by oxygen surface exchange kinetics increased with increasing x. The presence of H₂S traces (5 ppm) in H₂-containing gas mixtures did not result in detectable decomposition of the perovskite phases. La_0.6Sr_0.4Ti_1−xMn_xO_3−δ (LSTM, x = 0.2, 0.4, 0.6, 0.8) were also studied.⁸⁸ The increase in electrical conductivity as well as oxygen vacancy concentration in a reducing atmosphere resulted in an increase in the higher H₂ and CH₄ oxidation rate in SOFCs. The single cell with a La_0.6Sr_0.4Ti_0.2Mn_0.8O_3−δ anode, Ba_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ–GDC cathode and GDC electrolyte reached maximum power densities of 0.29 W cm⁻² and 0.24 W cm⁻² in humidified H₂ and CH₄ at 800 °C, respectively. The recent study on the electrochemical and thermomechanical properties of La_0.5Sr_0.5Mn_0.5Ti_0.5O_3−δ and (La_0.55Sr_0.45)_0.95Mn_0.5Cr_0.3Ti_0.2O_3−δ⁸⁹ confirmed the suitability of LSTM as an alternative anode material.

The ionic transference numbers of La_0.4Sr_0.5Ti_0.6Fe_0.4O_3−δ were determined to be between 2 × 10⁻⁴ to 6 × 10⁻⁴ in air by faradaic efficiency measurements, indicating that the substitution of Sr for La would reduce the ionic and total conductivity.⁹⁰ Nonetheless the La containing compositions showed sufficient stability and a matched thermal expansion coefficient with YSZ. A series of candidate dopants X (X = Al³⁺, Ga³⁺, Feⁿ⁺, Mg²⁺, Mnⁿ⁺ and Sc³⁺) have been investigated in their search for La_0.33Sr_0.67Ti_0.92X_0.08O_3+δ anodes by Miller et al.⁹¹ The impedance results for the electrical half cell tests in H₂–3% H₂O at 900 °C are shown in Fig. 5a. The La_0.33Sr_0.67TiO_3+δ had the lowest polarisation resistances; doped Sc and Ga gave had the second lowest polarisation resistances followed by doped Al, Fe, Mn and Mg. Fig. 5b shows the over-potential versus current density curves corresponding to the La_0.33Sr_0.67Ti_0.92X_0.08O_3+δ–YSZ anodes. A reduction in polarisation resistance was observed when the anode was under the load compared to that at the open circuit potential. Ti site Fe doped LST was also explored as solid oxide electrolysis cells (SOECs) cathodes for hydrogen production.⁹²

Fig. 5 (a) The variation of polarization resistance with temperature for all LSTX–YSZ anodes. (b) Anodic overpotentials of La₄Sr₈Ti₁₁XO_36+ε–YSZ anodes operating in H₂–3% H₂O at 900 °C. Reprinted from ref. 91, Copyright (2011), with permission from Elsevier.

It is well known that catalytic activities of transition metals generally follow the order of Co > Mn > Ni > Fe > Cr on the oxidation–reduction process.⁹³ Thus, various amount of Co was doped into Ti-site in La_0.2Sr_0.8TiO₃ (LSTC) by Yoo et al.⁹⁴ The stability of LST anode was greatly improved with the Co doping. A single cell with Ni-impregnated LSTC–GDC composite as the anode, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) as the cathode and La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) as the electrolyte exhibited a maximum power density of ∼250 mW cm⁻² at 800 °C. The R_p value of anode (∼0.9 Ω cm²) was responsible for most of the total R_p value (∼0.95 Ω cm²). Li et al.⁹⁵ also considered LSTC as a promising anode candidate and found that at 1500 °C, the solid solution limits of La in La_xSr_1−xTiO_3−δ and Co in La_0.3Sr_0.7Co_yTi_1−yO_3−δ were about 40 mol% and 7 mol%, respectively. Co-doping into La_0.3Sr_0.7TiO_3−δ increased the oxygen vacancy concentration and decreased the migration energy for oxygen ions, leading to a significant increase in ionic conductivity but at the expense of some electrical conductivity loss. The electrical and ionic conductivities of La_0.3Sr_0.7Co_0.07Ti_0.93O_3−δ at 700 °C were 63 S cm⁻¹ and 6 × 10⁻³ S cm⁻¹ respectively. At the same time, both La_0.3Sr_0.7TiO_3−δ and La_0.3Sr_0.7Co_0.07Ti_0.93O_3−δ showed relatively stable electrical conductivities under oxygen partial pressure of 10⁻¹⁴ to 10⁻¹⁹ atm at 800 °C. Recently, Cui et al.⁹⁶ found that the doped Co in La_0.3Sr_0.7Co_0.07Ti_0.93O_3−δ could exsoluted to the LSCT surface after reduction in H₂ at 900 °C. The active Co nanoparticles could reduce the anode polarization resistance. A maximum power density of 300 mW cm⁻² was achieved when the cell was fueled with H₂ containing 5000 ppm H₂S at 900 °C. Also, the LSCT exhibited high sulfur resistance and redox stability. The performance of La_0.2Sr_0.8Ti_0.98Co_0.02O₃ anode material was reported by Yoo et al.⁹⁷ The maximum power density of the cell with Ni-impregnated LSTC–GDC anode increased from 638 to 924 mW cm⁻² at 800 °C for 30 h in H₂. This value was very high compared to other reported values using LSTC anode. In CH₄ fuel, while the cell performance with Ni–GDC anode decreased ∼44% for 30 h, the cell performance with Ni-impregnated LSTC–GDC anode was relatively stable for 30 h. The study further proved that LST anode exhibited high tolerance to carbon induced deactivation in the internal utilization of hydrocarbon fuels while sustained a reasonable performance in H₂ fuel compared to the conventional Ni-based anode. The reproducibility of crystallographic information of La_0.4Sr_0.6Ti_1−xCo_xO_3+δ (0.0 ≤ y ≤ 0.5) was also studied by XRD and TEM techniques.⁹⁸

A new La_0.5Sr_0.5Ti_0.75Ni_0.25O_3−δ (LSTN) compound was synthesized using a nitrate–citrate gel technique.⁹⁹ The LSTN presented a metallic behavior after reduction at 1200 °C in H₂. The high-temperature treatment led to the precipitation of Ni nanoparticles on the surface and an improved electrical conductivity that may be due to the formation of excess Ti³⁺. The obtained polarization resistance was 0.55 Ω cm² at 800 °C under H₂–H₂O (97/3) using symmetrical cells.

Ruiz-Morales et al.¹⁰⁰ has demonstrated the concept of inducing functionality through disorder of extended defects in LST. Mn was known to accept lower coordination numbers in perovskites and thus it may facilitate oxide-ion migration. Similarly Ga was well known to adopt lower coordination than octahedral in perovskite-related oxides. They further studied the Ga and Mn doped in Ti site for La₄Sr₈Ti₁₂O_38−z. The maximum power density in wet H₂ was close to 0.5 W cm⁻² and the power density in wet CH₄ was two times higher than that in 5% H₂, reaching the value of about 0.35 W cm⁻². After running for two days, cycling from 950 °C to 850 °C, in wet 5% H₂, wet H₂ and wet methane, no trace of carbon could be detected visually or by thermogravimetric analysis. It was thought that the materials design concept could lead to the devices that enable more-efficient energy extraction from fossil fuels and carbon-neutral fuels. Based on the research of the La_0.33Sr_0.67Ti_0.92X_0.08O_3+δ,⁹¹ La_0.4Sr_0.4Ga_xTi_1−xO_3−x/2−δ (0 ≤ x ≤ 0.15) were chosen to study the improvement of n-type conductivity in perovskite oxides by enhancing bulk oxide ion mobility.¹⁰¹ The study suggested that both oxygen deficiency (δ) and conductivity (σ) of the reduced La_0.4Sr_0.4Ga_xTi_1−xO_3−x/2−δ were increased significantly with Ga doping. The oxygen deficiency was increased from 0.035 to 0.060 when Ga stoichiometry was enhanced from x = 0 to x = 0.15 after reduction. The corresponding conductivity values were also increased with doping and showed a maximum conductivity of 50 S cm⁻¹ for x = 0.05. They also found that the Ga doping could promote a fast reduction of the samples and significantly increase the stability of the reduced phase in oxidizing conditions at high temperatures.

La_1−xSr_xSc_1−yTi_yO_3−d (LSST) was synthesized through a glycine nitrate process by Hatchwell et al.¹⁰² A-site deficiency led to an increased ionic conductivity over LSST. The highest total conductivity achieved for (La_0.8Sr_0.2)_0.99Sc_0.9Ti_0.1O_3−d in air with ∼1% H₂O at 800 °C was 23 mS cm⁻¹, in which 6 mS cm⁻¹ was ionic. Li et al.¹⁰³ found that the Sc-doping in LST increased the oxygen vacancy concentration and decreased the oxygen migration energy. The ionic conductivity of La_0.3Sr_0.7Sc_0.1Ti_0.9O_3−δ was 1 × 10⁻² S cm⁻¹ at 800 °C at the oxygen partial pressure of 10⁻¹⁹ atm. This value increased about 230% compared with pure La_0.3Sr_0.7TiO_3−δ.

Gorte et al.¹⁰⁴ thought that the morphologies design and control synthesis of anode were particularly crucial because a larger triple phase boundary (TPB) and the thermal compatibility were required to obtain high fuel cell performance. Based on this viewpoint, Chang et al.¹⁰⁵ reported a novel core–shell anode material, which was prepared with Nb-doped LST perovskite core of (La_0.3Sr_0.7)(Ti_1−xNb_x)O₃ (LSTNx) and the shell of multiple elements doped ceria (La_0.75Sr_0.2Ba_0.05)_0.175Ce_0.825O_1.891 (LSBC) by a citric acid-based combustion coating process. The obtained LSTNs powders morphologies were shown in Fig. 6a and c. The elemental analysis on the core–shell SLTN0.1–12 mol% LSBC powders was shown in Fig. 6b. The shell LSBC nanoparticles adhered on the larger calcined SLTN0.1 core particles were distinguished, as shown in Fig. 6c. It indicated the element of Ce existed on the nanoparticles of the shell (Fig. 6e), which further proved the core–shell formation. The Sr and Ti distributed over core–shell main body (Fig. 6d and f). The Sr and Ti mapping images indicated the core SLTN also existed below the nanoparticles except the large core as seen at the center of Fig. 6c. The power density increased three times by using the core–shell structural anode than that without using the core–shell anode in a half-cell test. This can be attributed to the increased the effective TPB sites and the reduced the interface thermal expansion and lattice matching, as well as extended the ionic conduction path from the LSBC electrolyte to the core–shell anode. The authors thought that the benefits of core–shell structure include simple anode material and structure preparation for the anode without the tedious mixing or impregnation of secondary functional particles. It was also suggested that the core–shell structured anode could be applied in the design of other anode materials.

Fig. 6 FESEM images and EDS elemental analysis of the prepared core–shell anode powders, (a) FESEM image of LSTN0.1–1.5 mol% LSBC, (b) EDS pattern of LSTN0.1–12 mol% LSBC, (c) FESEM image of LSTN0.1–12 mol% LSBC, and the EDS elemental mappings on the LSTN0.1–12 mol% LSBC, (d) Ti, (e) Ce as well as (f) Sr. Reprinted from ref. 105, Copyright (2012), with permission from International Association for Hydrogen Energy.

3.3 LST-based composite anode materials

Combining LST with other conductor or active catalyst to form a composite anode is an effective way to improve the conductivity and electrocatalytic performance. In this part of the review, the developments of the LST-based composite anode materials are summarized.

3.3.1 Direct addition to LST. It is well known that the LST materials showed poor electro-catalytic performance. As a good catalyst, CeO₂ may enhance the overall electrochemical performance. Composite anode materials including LST and CeO₂-based additives such as La_0.20Sr_0.25Ca_0.45TiO₃ + CeO₂,⁷⁹ La_0.20Sr_0.25Ca_0.45TiO₃ + CeO₂ + Ni,⁷⁹ La_0.4Sr_0.6Ti_0.8Mn_0.2O_3±δ + CeO₂ + Pd,⁸⁶ La_0.3Sr_0.7TiO₃ + CeO₂ + Pd^107,108,110 have been reported in the above work. La_0.4Sr_0.6TiO₃ (LST) was ball milled with different weight ratios of CeO₂.¹⁰⁶ Increasing CeO₂ content in LST matrix decreased the cell polarization resistance and increased the maximum power density. The cell with LST–50 wt% CeO₂ anode created maximum power densities of about 172.3, 139.6 mW cm⁻² in humidified H₂ and CH₄ at 900 °C, respectively. The higher fuel cell performances were attributed to the additional amount of CeO₂, which extended the anode TPBs.

Ahn et al.⁶² have studied YSZ and La_0.3Sr_0.7TiO₃ (LST) composite support layer for SOFCs. Excellent performance of the SOFCs was obtained using an anode having a functional layer made from Pd-doped CeO₂ in YSZ and a current collector from LST.¹⁰⁷ The maximum power densities were 208 mW cm⁻² at 973 K and 539 mW cm⁻² at 1073 K using humidified CH₄ as fuel. At the same time, the anode showed no evidence for carbon formation during an overnight exposure to CH₄. They also found that the functional layer could be prepared by impregnation of 1 wt% Pd and 10 wt% CeO₂ into porous LST and YSZ.¹⁰⁸ This strategy allowed much greater flexibility in electrode design to include different layers. Furthermore, the performance of the LST + Pd-doped CeO₂ in YSZ was considered to be only modest. The conductivities of composites with 35 vol% LST were only 0.1 S cm⁻¹ at 1173 K in humidified H₂, more than 200 lower than the value reported for bulk LST.¹⁰⁹ LST was added to porous YSZ by infiltration method to form LST–YSZ composite anode materials.¹¹⁰ The conductivity of this composite depended strongly on the pretreatment conditions but was greater than 0.4 S cm⁻¹ after being heated to 1173 K in humidified H₂. With the addition of 0.5 wt% Pd and 5 wt% CeO₂, the maximum power density of LST–YSZ composite anode materials increased from less than 20 to 780 mW cm⁻² for operating in humidified H₂ at 1073 K. Hill et al.¹¹¹ investigated the electrochemical conversion pathway of CH₄ on Ni/YSZ and La_0.3Sr_0.7TiO₃ bi-layer anode materials in details. They found that the pathway include the CH₄ decomposition and subsequent oxidation C and H₂, which strongly depended on the anode polarization.

Based on the LST–CeO₂ composite anode materials, different doped CeO₂ including GDC (Gd-doped CeO₂), YDC (Y-doped CeO₂), SDC (Sm-doped CeO₂), LDC (La-doped CeO₂) etc. were further explored as the additive to improve the performance of LST-based anode materials.

The addition of GDC to LST has been proved to be an efficient way to improve the performance of the cell in the above mentioned work.^83,94,97 Dense and porous A-site deficient La_0.2Sr_0.7TiO₃ were prepared by Savaniu et al.¹¹² Two types of fuel cells, electrolyte supported and anode supported, were produced using YSZ as the electrolyte, porous LST impregnated with Ce_0.8Gd_0.2O₂ and Cu as the anode, and La_0.6Sr_0.4CoO₃ as the cathode. Maximum power densities of 0.22 and 0.5 W cm⁻² at 750 °C were achieved using dense and porous La_0.2Sr_0.7TiO₃ in humidified H₂, respectively. In another work, La_0.2Sr_0.8TiO₃ powers were mixed with Ce_0.8Gd_0.2O_2−δ (GDC) powders in 50 : 50 weight ratio to form LST–GDC composite in which Ni was impregnated afterwards.¹¹³ GDC could provide the ionic conduction path whereas the small amount of Ni would enhance the catalytic activity. The polarization resistance (R_p) of LST–GDC anode was greatly reduced relative to that of pure LST anode. When Ni was impregnated into LST–GDC composite, the R_p value was further reduced to 1 Ω cm² at 800 °C in humidified H₂ (Fig. 7), which was ∼10% of that for pure LST anode. The maximum power density of the single cell with LST–GDC + Ni anode was about 275 mW cm⁻² at 800 °C, much larger than 60 mW cm⁻² for the cell with LST–GDC anode. A similar study was conducted in ref. 114 in which La_0.2Sr_0.8TiO₃–GDC–Ni anodes of various composition ratios were prepared as the anode. With LSGM as the electrolyte and La_0.6Sc_0.4Co_0.2Fe_0.8O₃ (LSCF) as the cathode material at 800 °C in humidified H₂, the performance of single cell was about 300 mW cm⁻², increasing drastically from 67 mW cm⁻² for the cell without Ni impregnation. It was suggested that in order to further reduce the polarization resistance, the optimization of anode microstructure and the content and distribution of Ni particles might be helpful. A-site deficient LST backbone impregnated with GDC and Ni¹¹⁵ or Cu¹¹⁶ was also investigated by Savaniu et al. A-site deficient La_0.2Sr_0.7TiO₃ was impregnated using solutions with 20 mol% GDC and a metal Cu. YSZ and La_0.6Sr_0.4CoO₃ (LSC) thin layer were used as the electrolyte and the cathode material, respectively. Fuel cells tests demonstrated that remarkable power densities about 0.5 W cm⁻² at 750 °C could be achieved using pure, humidified H₂ as fuel.

Fig. 7 The polarization resistance values of symmetrical LSGM electrolyte-supported cell with LST, LST–GDC and Ni-impregnated LST–GDC anodes as a function of time. All data were extracted from the impedance spectra obtained under open circuit conditions at 800 °C. All values increased with time. Reprinted from ref. 113, Copyright (2009), with permission from Elsevier.

Y_0.2Ce_0.8O_2−δ (YDC) was also explored as the additive to LST-based composite because of its high ionic conductivity and catalytic performance. It was found that La_0.4Sr_0.6TiO_3±δ (LST) and YDC composite was a stable, high performance material for use as anode in SOFCs fed by 0.5% H₂S-containing syngas, whereas pure CeO₂ was not chemically stable using the same fuel.¹¹⁷ Moreover, both LST and YDC were synthesized using citrate–nitrate gel combustion method.¹¹⁸ The obtained LST and YDC powders were ball-milled and calcined to obtain the LST–YDC composite anode. The maximum power densities at 850 °C were 169, 102 and 39 mW cm⁻² when the cell was fueled with syngas having 0.5% H₂S, pure syngas and 0.5% H₂S balanced with Ar, respectively. Gas chromatographic and mass spectrometric analyses revealed that the presence of H₂S improved the rate of fuel electrochemical oxidation while H₂S itself was not converted and its oxidation was not the source of the enhanced performance. The presence of H₂S in different feeds was evidenced to have negligible influence over the fuel cell stability.

Pillai et al.¹¹⁹ studied the SOFCs with La_0.2Sr_0.8TiO₃ anode-side supports, along with NiO–Ce_0.8Sm_0.2O₂ (SDC) (1 : 1 by weight) adhesion layer, NiO–YSZ (1 : 1 by weight) anode active layer, YSZ electrolyte, and LSM–YSZ cathode. The colloidal solutions of NiO–SDC, NiO–YSZ, and YSZ were sequentially dropped and coated onto the LST pellets. The obtained LST-supported SOFCs showed improved stability against coking in natural gas, compared with conventional Ni–YSZ anode-supported SOFCs. There was no long-term degradation in H₂ fuel with 50–100 ppm H₂S, and no degradation during redox cycling. However, the maximum power density was decreased from 850 mW cm⁻² to about 700 W cm⁻² when the fuel was changed from dry H₂ to dry H₂ containing 100 ppm H₂S, which further indicated the poor sulfur tolerance of Ni-based catalyst.

Besides the CeO₂-based oxide, the addition of Bi₂O₃ into LST was also reported to be able to significantly reduce the fuel cell's polarization, refine the grains, increase the triple phase boundary, and then to improve the electrochemical performance.¹²⁰

3.3.2 Exsoluted nanoparticles from doped LST. Certain doping elements in LST were not stable in reducing environment at elevated temperature. They tended to exsolute from the LST matrix and form nano-scale electrocatalytic active particles which promoted the anode performance considerably. As mentioned in the above doped LST-based materials, different nanoparticles such as electrocatalytic active nano Ce-rich phase⁷⁵ (see Fig. 3), Ni,^92,99 Co⁹⁶ and Fe⁹⁹ nanoparticles were exsoluted to the LST surfaces. Ru nanoparticles exsoluted from La_0.8Sr_0.2Cr_0.82Ru_0.18O_3−δ,¹²¹ Ni nanoparticles from La_0.8Sr_0.2Cr_1−yNi_yO_3−δ¹²² and Ni, Cu, Co nanoparticles from Ce_1−xM_xVO_4−0.5x (M = Ni, Co, Cu)¹²³ were also observed. Thereby, the resulting nanoparticles extended the length of TPB and enhanced the fuel cell performance. Recently, Neagu et al. investigated the metallic, oxides or mixtures nanoparticles in situ growth on the surface of stoichiometry or non-stoichiometry LST.¹²⁴

Although the nanostructured electrocatalytic active composition exsoluted from the doped LST matrix is a potential way to form high performance LST composite anode. The effective control of the exsoluted particle quantity and the prevention of their aggregation at high temperature are crucial and remain to be solved in future research. In addition, the LST-based composite anode materials including Ni nanoparticles could not be used directly for the hydrocarbon fuel because of their poor carbon resistance character.

3.4 LST synthesis methods

Overview of all the LST materials covered in this review suggests that conventional solid state method^{53–60,62–67,71,76,78–80,85,90–96,100,101,108,115,116,119,120} was still the main route to prepare LST materials due to its simplicity and relatively low processing cost. However, the solid state method needs a long period of time to facilitate the diffusion at high temperature (usually >1200 °C). The obtained products were often coarse and agglomerated, exhibiting limited purity and homogeneity. Therefore, the new approaches for the development of nano-structured electrodes are needed and some soft ways have drawn increasing attentions in recent years.¹²⁵ The other different routes to synthesize LST-based materials include Pechini route,^61,75,82 impregnation or infiltration method,^{77,86,107,110} combustion synthesis,^83,88 citric acid–nitrate process,^{68,84,98,99,105,118} glycine–nitrate and citrate synthesis routes,^52,87,102 spray pyrolysis method,⁷² and nanoparticle-based spray pyrolysis^70,74 to synthesize LST-based materials. The LST powders obtained through the above soft methods have small particle size and narrow size distribution or porous structure which would enlarge the TPBs and then improve the performance. Recently, Birol et al.¹²⁶ reviewed the cellulose-assisted glycine nitrate process of preparing ceramic nanoparticles. Some other soft methods such as co-precipitation method,¹²⁷ sol–gel route¹²⁸ and rheological phase reaction^129,130 were the potential techniques to synthesize LST-based anode materials.

For the synthesis of LST composite anode materials, the common way was to mix pure LST with other materials automatically.^{62,74,106,108,113,114,120,137} Impregnation or infiltration technology^{107,110,112,116} was another commonly used and effective way to add the active components to the LST materials. Combination of the combustion method and automatical mix,¹¹⁸ colloidal deposition technique¹¹⁹ were also applied to blend LST and other materials. Exsolution method was a compellent way to obtain LST-based anode materials^{76,92,96,99,121–123} and would be further explored in the future work.

3.5 Conductivity and performance of LST-based anode materials

Table 1 summaries the conductivities of selected LST-based anode materials. We can see that the pure SrTiO₃ has a low ionic conductivity. The co-doped La and other metal ions in A-site and B-site doping can obviously improve the conductivity. The type of doping element and the doping quantity should be experimentally optimized in the future work.

Table 1 Conductivity of selected LST-based anode materials

Anode composition	Electronic conductivity (S cm^−1)	Ionic conductivity (S cm^−1)	Tem. (°C)	Fuel	Ref.
La0.6Sr0.1TiO3−δ	7	—	930	5% H2/Ar	54
La2Sr4Ti6O19−δ	40	—	950	5% H2/Ar	58
La0.4Sr0.4TiO3	∼100	—	900	5% H2/Ar	60
La0.3Sr0.7TiO3−α	∼100	—	1000	9% H2/N2	63
La0.1Sr0.9TiO3	12	—	1000	30% H2/N2	68
(La0.3Sr0.7)0.93TiO3−δ	145	1.0 × 10^−2	800	5% H2/Ar	69
La0.2Sr0.7TiO3	0.3	—	800	5% H2/Ar	72
La0.2Sr0.7TiO3	30–40	—	1000	5% H2/Ar
La0.2Sr0.7TiO3	∼100	—	1200	5% H2/Ar
La0.2Sr0.7TiO3	600	—	600	5% H2/N2	70
La0.2Sr0.25Ca0.45TiO3	27.53	—	900	5% H2/Ar	80
La0.3Sr0.55Ti0.8Cr0.2O3−δ	1.96 × 10^−3	—	800	Static air	84
La0.3Sr0.7Ti0.8Cr0.2O3−δ	53	—	800	5% H2/Ar	85
La0.33Sr0.67Ti0.92Al0.08O3+δ	1.5	—	900	5% H2/Ar	91
La0.33Sr0.67Ti0.92Sc0.08O3+δ	5.5	—	900	5% H2/Ar
SrTiO3	0.03	—	700	5% H2/Ar	95
La0.3Sr0.7TiO3−δ	247	1 × 10^−3	700	5% H2/Ar
La0.3Sr0.7Co0.07Ti0.93O3−δ	63	6 × 10^−3	700	5% H2/Ar
La0.5Sr0.5Ti0.75Ni0.25O3−δ	2.2 × 10^−3	—	800	Air	99
La0.5Sr0.5Ti0.75Ni0.25O3−δ	4.9 × 10^−4	—	800	2% H2/Ar
La0.5Sr0.5Ti0.75Ni0.25O3−δ	2.3	—	800	2% H2/Ar
La0.4Sr0.4Ga0.05Ti0.95O2.95/2−δ	∼50	—	880	5% H2/Ar	101
(La0.8Sr0.2)0.99Sc0.9Ti0.1O3−d	6 × 10^−3	—	800	1% H2O/air	102
La0.3Sr0.7Sc0.1Ti0.9O3−δ	49	1 × 10^−2	800	5% H2/Ar	103
La0.3Sr0.7TiO3	>0.4	—	900	3% H2O/H2	110

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Table 2 lists the fuel cell performances of selected LST-based anode materials. It can be seen that the pure LST materials have a poor electrocatalytic performance for H₂ oxidation. The common methods to improve the performance include: (1) exploring new methods to synthesize LST-based anode materials with a nanostructure; (2) mixing, impregnation or infiltration of high ionic conductor (CeO₂-based materials, Bi₂O₃, etc.) and high catalytic materials (Pd, Cu, Ni, etc.) into porous LST; (3) exsolution method is a potential technology to form LST-based composite anode materials.

Table 2 Fuel cell performance of selected LST-based anode materials

Anode composition	Electrolyte (thickness)/cathode	Tem. (°C)	Fuel	MPD^a (mW cm^−2)	Ref.
a MPD = maximal power density.
La0.4Sr0.6TiO3	YSZ (160 μm)/LSM	1000	Humidified H2	260	52
La2Sr4Ti6O19−δ	YSZ (2 mm)/LSM	900	Humidified H2	76	59
La0.3Sr0.7TiO3 + CeO2 + Cu	YSZ (60 μm)/LSM	700	H2	200	62
La0.2Sr0.8TiO3	YSZ (280 μm)/Pt	900	5% H2 + 95% Ar	12	71
La0.4Sr0.45Ba0.15TiO3	YSZ (300 μm)/LSM	850	H2	∼39	76
La0.4Sr0.5Ba0.1TiO3	YSZ (300 μm)/LSM	850	H2	20	77
La0.20Sr0.25Ca0.45TiO + CeO2 + Ni	ScSZ (310 μm)/LSM	900	Humidified H2	∼300	79
La0.3Sr0.7TiO3−δ	YSZ (400 μm)/LSM	900	Humidified H2	∼31	85
La0.3Sr0.7Ti0.8Cr0.2O3−δ				44
La0.4Sr0.6Ti0.8Mn0.2O3±δ	YSZ (50 μm)/LSM	800	Humidified H2	30	86
La0.4Sr0.6Ti0.8Mn0.2O3±δ + Pd				∼70
La0.4Sr0.6Ti0.8Mn0.2O3±δ + CeO2				100
La0.4Sr0.6Ti0.8Mn0.2O3±δ + Pd + CeO2				150
La0.6Sr0.4Ti0.2Mn0.8O3−δ	GDC (500 μm)/Ba0.6Sr0.4Co0.5Fe0.5O3−δ	800	Humidified H2	290	88
La0.2Sr0.8Ti0.98Co0.02O3 + GDC + Ni	LSGM (500 μm)/Ba0.5Sr0.5Co0.8Fe0.2O3−δ	800	Humidified H2	250	94
La0.3Sr0.7TiO3−δ	YSZ (300 μm)/LSM	900	Humidified H2	90	96
La0.3Sr0.7Ti0.93Co0.07O3−δ				230
La0.2Sr0.8Ti0.98Co0.02O3 + GDC + Ni	LSGM (250 μm)/La0.6Sr0.4Co0.2Fe0.8O3−δ	800	Humidified H2	638	97
La4Sr8Ti11Mn0.5Ga0.5O37.5	YSZ (2 mm)/LSM	950	Humidified H2	500	100
La0.4Sr0.6TiO3 + CeO2	YSZ (300 μm)/ScSz + LSM	900	Humidified H2	172	106
La0.3Sr0.7TiO3	YSZ (60 μm)/LSF	800	Humidified H2	20	110
La0.3Sr0.7TiO3 + CeO2 + Pd				780
La0.3Sr0.7TiO3 + Ni/YSZ	YSZ (300 μm)/Pt	750	Humidified H2	102	111
Dense La0.2Sr0.7TiO3 + GDC + Cu	YSZ (150 μm)/La0.6Sr0.4CoO3 (LSC)	750	Humidified H2	220	112
Porous La0.2Sr0.7TiO3GDC + Cu				500
La0.2Sr0.8TiO3 + GDC	LSGM (500 μm)/Ba0.5Sr0.5Co0.8Fe0.2O3−δ	800	Humidified H2	60	113
La0.2Sr0.8TiO3 + GDC + Ni	(BSCF)			275
La0.2Sr0.8TiO3 + GDC	LSGM (150 μm)/La0.6Sc0.4Co0.2Fe0.8O3	800	Humidified H2	67	114
La0.2Sr0.8TiO3 + GDC + Ni	(LSCF)			300
La0.44Sr0.56TiO3 + GDC + Ni	YSZ (32 μm)/LSC64	850	Humidified H2	185	115
La0.2Sr0.7TiO3 + GDC + Cu	YSZ (50–75 μm)/La0.6Sr0.4CoO3	750	Humidified H2	500	116
La0.4Sr0.6TiO3±δ + YDC	YSZ (300 μm)/LSM	850	40% H2 + 60% CO	102	118
La0.2Sr0.8TiO3 + NiO–SDC + NiO–YSZ	YSZ/LSM	800	Dry H2	850	119
La0.4Sr0.6TiO3	YSZ (500 μm)/LSM	1000	Humidified H2	175	136

---

4. H₂S-induced enhancement for electrochemical reaction on LST-based anode materials

H₂S is a common impurity in many industrial feedstocks such as coal derived syngas and raw natural gas. Although H₂S may serve as an increasingly significant source for sulfur production, it is a major industrial pollutant that is toxic and corrosive. Direct electrochemical conversion of H₂S in SOFCs would offer both economical and environmental benefits.^131–135 It is known that even trace amount of H₂S could lead to severe performance degradation of the fuel cells when using Ni-based anode materials.^24–26 Therefore, a series of LST-based anode materials have been developed in our lab, which showed not only a high H₂S poisoning resistance and high chemical stability but also a H₂S-induced enhancement effect for electrochemical reaction when using a fuel containing H₂S.

The sulfur tolerance of La_1−xSr_xBO₃/YSZ anodes (B = Mn, Cr, and Ti) was examined at 1273 K in a H₂/H₂O fuel using YSZ electrolyte, LSM cathode.¹³⁶ The study indicated that the La_0.4Sr_0.6TiO₃ anode showed no degradation in the presence of up to 5000 ppm of H₂S in H₂. It was found that the performance of the fuel cell improved significantly in the presence of 5000 ppm of H₂S especially at the higher current densities. But the reason for this improvement was not clear in this work. The sulfur tolerance of the perovskite-based anodes decreased in the order La_0.4Sr_0.6TiO₃ (LST) > La_1−xSr_xCrO₃ (LSC) > La_1−xSr_xMn_yCr_1−yO₃ (LSCM).

Table 3 compares the fuel cell performances of LST-based anode materials when the fuel contained trace amount of H₂S. It is clearly shown that the performance was dramatically improved when H₂S was present in the fuels such as CH₄,^76,77,83,137 H₂,^76,77,136 syngas (H₂ + CO)^117,118. This can also be observed clearly in Fig. 8. In ref. 119, the performance was decreased when using the dry H₂ containing 100 ppm H₂S, which could be attributed to the poisoning of Ni-based active catalyst in LST-based composite anode.

Table 3 Fuel cell performance of LST-based anode materials using fuel containing H₂S

Anode composition	Electrolyte/cathode	Tem. (°C)	Fuel	MPD^a (mW cm^−2)	Ref.
a MPD = maximal power density.
La0.4Sr0.45Ba0.15TiO3	YSZ (300 μm)/LSM	850	H2	∼39	76
			H2 + 0.5% H2S	∼50
			CH4	∼1.5
			CH4 + 0.5% H2S	∼7.5
La0.4Sr0.5Ba0.1TiO3	YSZ (300 μm)/LSM	850	H2	20	77
			H2 + 0.5% H2S	137
			CH4	24
			CH4 + 0.5% H2S	75
La0.3Sr0.55Ti1−xCrxO3−δ	YSZ (300 μm)/Pt	900	CH4	∼2	83
			CH4 + 0.5% H2S	∼140
La0.3Sr0.7Ti0.93Co0.07O3−δ	YSZ (300 μm)/LSM	900	H2	230	96
			H2 + 0.5% H2S	300
La0.4Sr0.6TiO3±δ + YDC	YSZ (300 μm)/LSM	850	40% H2 + 60% CO	102	118
			40% H2 + 60% CO + 0.5% H2S	169
La0.2Sr0.8TiO3 + NiO–SDC + NiO–YSZ	YSZ/LSM	800	Dry H2	850	119
			Dry H2 + 100 ppm H2S	700
La0.4Sr0.6TiO3	YSZ (500 μm)/LSM	1000	Humidified H2	175	136
			Humidified H2 + 0.5% H2S	∼210
La0.4Sr0.6TiO3−δ	YSZ/Pt	850	CH4	∼2	137
			CH4 + 5% H2S	320
			CH4 + 20% H2S	450

---

Fig. 8 Maximal power density (after compensation) as a function of H₂S concentration in CH₄ (after flow corrections) at 850 °C. Fuels were fed at 50 mL min⁻¹. Reprinted from ref. 137, Copyright (2011), with permission from Elsevier.

A series of experiments were designed to systematically investigate the electrochemical performance and compositions of anode gases to explore the mechanism of the significant performance enhancement with H₂S presence in either CH₄ or H₂ fuel. La_0.4Sr_0.6TiO_3−δ was mixed with commercial YSZ power with a 50/50 weight ratio to form the anode materials.¹³⁷ Platinum paste was used as the cathode and the current collector. Electrolyte was the commercial YSZ dicks. The power density improved dramatically from 2 mW cm⁻² with pure CH₄ as fuel to more than 450 mW cm⁻² for CH₄ containing 20% H₂S (Fig. 8). From thermodynamic calculations and mass spectroscopic data obtained at different potential stages during a regular potentiodynamic run, it was concluded that H₂S was not the only fuel converted, and its interaction with CH₄ was the key factor for the enhancement of performance. The anode effluent gas mixture included H₂S, SO₂, H₂ and CS₂, which was consistent with thermodynamic predictions. The conclusion was that H₂S had a synergistic effect on CH₄ oxidation, i.e., CH₄ and H₂S must not be considered as two separate fuels. H₂S actually ran as a powerful promoter for fuel electro-oxidation. At low current density, there was a rapid conversion of surface species, whereas at high current density there was a mass transfer blocking effect from build-up of the surface intermediates. The predominant form of sulfur in the gas phase was H₂S previously presented. There was at least 3% H₂ in the mixture. CH₄ conversion proceeded via rapid formation of syngas, and the reaction path probably included CS₂ and other sulfur species. The selectivity of the obtained products could be controlled by adjusting the applied voltage. In order to further explore the mechanism of the H₂S enhancement effect in details, in situ technologies such as in situ Raman,¹³⁸in situ XPS¹³⁹etc. were recommended to be applied in the field in the future work.

5. Conclusions

The recent progress in the field of LST-based anode materials in SOFCs are summarized. Although pure LST had very limited activity for oxidation of H₂S-free fuels, it exhibited superior balance between high chemical stability in sulfur/carbon environment and adequate electrical conductivity compared to other anode candidates. Thus it could be used as an anode matrix material and its properties could be further tailored depending on the fuel selected. Doping and composting of LST are the most effective ways to increase the ionic conductivity and the electrocatalytical activity of LST-based anode.

Future work on the LST-based anode materials in SOFCs can focus on the following aspects: (1) Exploring new methods to synthesize LST materials, doping LST with appropriate elements in different site, impregnating, mixing or coating nano-structured ionic conductor or exsoluted electrocatalytical active nanoparticles to LST anode. (2) Combining LST-based materials with other catalytic active material to form LST-based composite anode materials using impregnation, infiltration technique or some other new methods. The added secondary phases should sustain sufficient thermal stability at elevated temperature. (3) Exploring the fuel conversion mechanism of LST-based anode especially the H₂S-induced enhancement effect for electrochemical reaction using in situ experimental technologies.

Acknowledgements

This research was supported through funding to the Solid Oxide Fuel Cell Canada Strategic Research Network and Strategic Grant from the Natural Science and Engineering Research Council (NSERC).

Notes and references

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