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Rare earth elements based oxide ion conductors

a,b b a Cite this: Inorg. Chem. Front., 2021, Xiaohui Li, Xiaojun Kuang * and Junliang Sun * 8, 1374 Rare-earth elements (REs) based oxide ion conductors show promising performance and potential applications in solid oxide fuel cells (SOFCs). To meet the requirements of intermediate-temperature (IT) SOFCs operating in the range of 600–800 °C, oxide ion conductors with various structures were developed and summarized here, namely, pure oxide ion conductors and mixed oxide ionic and electronic conductors (MIECs). For the former, their structures range from traditional structure types (perovskite, fluorite, etc.) to new structures with isolated polyhedral units. For MIECs, perovskite and related structures have dominated this field due to their high oxide ion and electronic conductivities. Among these oxide ion conductors, REs play a crucial role in the modification of the structure and performances due to their unique characters, such as lanthanide contraction effect and stable valence (+3). This review emphasizes the structure–performance relationships among the REs. Furthermore, challenges particularly towards Received 14th July 2020, obtaining high oxide ion conductivity at low temperatures for lowering the operating temperature and Accepted 17th August 2020 thereby enhancing the device performance, and future prospects of RE-based oxide ion conductors are DOI: 10.1039/d0qi00848f also discussed, together with advanced research techniques and new research directions being the poss- rsc.li/frontiers-inorganic ible strategies to overcome these challenges.

1. Introduction

Rare-earth elements (REs)—such as lanthanides, yttrium, and scandium—are critical components in numerous modern technologies, including, permanent magnets in wind turbines, a Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. College of Chemistry and Molecular Engineering, Peking University, Beijing National electric car batteries, lasers, phosphors, and medical imaging Laboratory for Molecular Science (BNLMS), Beijing 100871, People’s Republic of agents. Consequently, REs have wide applications in modern – China. E-mail: [email protected]; Tel: +86 10 6275 5700 society.1 5 China—the world’s biggest producer of REs—has bCollege of Chemistry and Bioengineering, Guilin University of Technology, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, Guilin 541004, prospective RE reserve of 227 million tons. REs have become a People’s Republic of China. E-mail: [email protected] strategic resource for China. In 2016, China produced approxi-

Xiaohui Li received his PhD from Xiaojun Kuang received his BSc Sun Yat-Sen University in 2019. in Chemistry from Nanchang Currently, he is a Postdoctoral University in 1999 and PhD in Fellow in Junliang Sun’s group Inorganic Chemistry from Peking at Peking University. His University in 2004. After his research interests include the postdoctoral research in the development of new oxide ion University of Liverpool and conductors, inorganic lumines- University of Durham, he was cent materials, and nitride appointed as an Associate dielectric materials. Professor in the Chemistry College, Sun Yat-Sen University, in 2010 before he settled down Xiaohui Li Xiaojun Kuang in Guilin in 2013. His current research interests include the discovery of new oxide ion conductors, oxide and (oxy)nitride dielec- trics, and their structure–property relationships.

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sufficient active sites for the electrode reactions, both electrodes also need to provide appropriate pathways for oxygen ion transport during the electrode reactions. The electrolyte in a SOFC facilitates oxygen ion migration between the cathode and anode, while it blocks the transport of electrons and gas molecules to avoid short circuit in the cell. Therefore, solid Fig. 1 Schematic of the working principle of SOFCs. electrolytes need to have high ionic conductivity with negli- – gible electronic conductivity.17 19 Although SOFCs have so many advantages and broad appli- mately 125 000 tons, which accounted for 86% of the world’s cation prospects, they have not been widely commercialized total production.6 In recent decades, research projects invol- due to their high operating temperatures (>750 °C), which is ving the development and applications of REs have also been mainly determined by the oxide ion conductivity of the electro- the subject of the key national state support. lytes. Such high operating temperatures lead to a series of With the growth of energy consumption in the fields of issues, such as expensive fuel-cell components, higher plant industrial production, transportation, and electricity gene- cost, slow start up and shut down, poor cycling capability, and ration, energy crisis has become an important issue. In poor durability arising from thermal and chemical compatibil- addition, the use of fossil fuel releases a large amount of ity between the components.9,20 Over the past 20 years, to over- greenhouse and pollution gases, such as CO2, CO, and SO2, come the major issues of SOFCs, the dominant trend has been which has resulted in global warming and serious environ- to lower the temperature from 800 to 1000 °C down to an inter- mental pollution. Therefore, the search for new energy strat- mediate-temperature range (600–800 °C), and even lower temp- egies to meet energy demands and reduce exhaust gas emis- eratures, which has stimulated the discovery of new oxide ion 7–12 sions has attracted widespread attention. conductors that can sufficiently maintain higher oxide ion As an electrochemical device, solid oxide fuel cells (SOFCs) mobility at lower temperatures. Although the thermal and are all-solid-state systems that can efficiently generate electri- chemical compatibility issues can be alleviated when the oper- city via conversion from fuels and is also not limited by the ating temperature is reduced below 600 °C, the polarization ffi Carnot cycle e ciency. Therefore, SOFCs have become a sus- between the electrolytes and electrodes significantly increases, tainable energy technology for the future, owing to several which leads to an exponential decrease in the oxide ion trans- ffi critical advantages such as high overall energy e ciency (up to port between the different components. Therefore, new oxide 13–16 80%), fuel flexibility, and relatively low catalyst cost. As ion conductors for SOFCs need to enable not only high ionic shown in Fig. 1, a SOFC consists of three main components, conductivity but also high electrocatalytic activity, apart from namely, an oxygen electrode, a fuel electrode, and a dense elec- the good compatibility with other cell components under oper- – trolyte. Among these components, the oxygen electrode as the ating conditions.21 24 cathode of the SOFC provides active sites for the oxygen REs have also demonstrated important roles in oxide ion Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. reduction/evolution reactions (ORR/OER). The fuel electrode conductors owing to their unique chemical characteristics as as the anode of the SOFC provides active sites for the oxidation indicated by the trend that most leading oxide ion conductors of fuel, facilitating the use of various types of hydrocarbon contain REs. The contributions of REs to oxide ion conduction fuels such as natural gas, methane, and solid carbon fuels can be divided into the following four major categories. (coal, petroleum coke, and biomass). Apart from providing (1) REs act as the main constituents of the host structures of oxide ion conductors. REs have the electron configuration of [Xe]4f n5dm6s2 (0 ≤ n ≤ 14; 0 ≤ m ≤ 1), as shown in Table 1 Junliang Sun received his and Fig. 2. Most REs tend to lose the outermost d- and s-sub- BSc (2001) and PhD (2006) layer electrons to form a stable trivalent state.25,26 The unique in Chemistry from Peking characteristics that can be obtained by combining the trivalent University (Prof. Jianhua Lin). states with proper sizes of REs fitting various sites with coordi- After finishing his postdoctoral nation numbers ranging from 6 to 12 make them important as research at Cornell University the key constituents for the frameworks of several structures. and Stockholm University, he Compared with active alkaline and alkaline-earth elements, became an Assistant Professor at REs generally have lower electronegativities, which are much Stockholm University in 2009. In more stable toward moisture and carbon oxide atmospheres, 2012, he moved to Peking and their chemical bonds with oxygen have a less ionic charac- University under the Thousand ter; such a character coupled with higher charges but smaller Talents Program. His research sizes can make REs have more polarization power, and there- Junliang Sun interests include the develop- fore, the oxygen sublattice becomes more polarizable, which ment of structure determination facilitates oxide ion conduction. methods by X-ray and electron crystallography as well as the (2) REs act as a fine regulator of cationic size for the struc- syntheses and applications of porous materials and dense oxides. tures to optimize the oxide ion conductivity. Variations in the

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Table 1 Lanthanide atoms and the outermost electron conﬁguration of lanthanide ions

The outermost electron configuration

Elemental symbol Ionic radius (Å) Electronegativity Common valence Atom +2 +3 +4

La 1.06 1.1 +3 4f 05d16s2 4f 0 Ce 1.03, 0.92 1.1 +3, +4 4f 15d16s2 4f 1 4f 0 Pr 1.01, 0.9 1.1 +3, +4 4f 36s2 4f 2 4f 1 Nd 1.06 1.2 +3 4f 46s2 4f 3 Pm 0.98 1.2 +3 4f 56s2 4f 4 Sm 1.11, 0.96 1.2 +2, +3 4f 66s2 4f 6 4f 5 Eu 1.12, 0.95 1.1 +2, +3 4f 76s2 4f 7 4f 6 Gd 0.94 1.1 +3 4f 75d16s2 4f 7 Tb 0.92, 0.84 1.2 +3, +4 4f 96s2 4f 8 4f 7 Dy 0.91 1.1 +3, +4 4f 106s2 4f 9 4f 8 Ho 0.89 1.2 +3 4f 116s2 4f 10 Er 0.88 1.2 +3 4f 126s2 4f 11 Tm 0.94, 0.87 1.2 +2, +3 4f 136s2 4f 13 4f 12 Yb 1.13, 0.87 1.1 +2, +3 4f 146s2 4f 14 4f 13 Lu 0.85 1.2 +3 4f 145d16s2 4f 14

Now, REs based oxygen ion conductors have been constantly – attracting considerable attention,33 37 and knowledge regard- ing REs based oxide ion conductors has been significantly accumulating. This has stimulated our motivation to provide a timely review on the structure–property relationships of REs for REs based oxide ion conductors. In this review, we sum- marize various oxide ion conductors, such as mixed oxide ionic and electronic conductors (MIECs), in relation to their structural characters, oxide ion conductivity, and structure– performance relationships, and we provide perspectives on the design of new oxide ion conductors and improvements in the oxide ion conductivity from the viewpoint of REs. Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. Fig. 2 Ionic radius versus coordination number for some common RE 2. REs based pure oxide ion ions. (Adapted with permission from ref. 33. Copyright 1976 Wiley.) conductors as electrolytes for SOFCs

Electrodes with pure oxide ion conductivity and negligible electron configurations of REs has a minor eﬀect on the ionic electronic conductivity have played a role in transporting oxide − radius and chemical properties of REs, which facilitates the ions (O2 ) from the air electrode (cathode) side to the fuel elec- formation of a series of isomers and tailoring the structure trode (anode) side, where an electrical voltage is generated.38 and oxide ion conduction properties via substitution. The electrolyte material is one of the most critical factors for (3) REs can be used as dopants to tailor and improve the lowering the operating temperature of SOFCs; this has stimu- oxide ion conductivity. lated ideas for the discovery of new oxide ion conductors. The (4) REs with variable oxidation states can control the mixed mechanism of oxide ion conduction is generally based on oxide ionic and electronic conductivities via the redox process. oxygen vacancies and interstitial oxygen. Oxygen vacancies as Because of the complexation–extraction method and its mobile oxide species are widely spread in oxide ion conduc- – development toward countercurrent extraction technology,27 29 tors, such as conventional fluorite- and perovskite-structured – single RE oxides have been produced at the kilogram scale oxide ion conductors.39 41 Oxide ion conductors based since the 1960–1970s, which has created a firm foundation for on interstitial oxygen ions as charge carriers are rare. the research of new RE compounds and materials. As a conse- Nevertheless, interstitial oxide ion conduction has been found

quence, many new RE compounds have been discovered, to exist in apatite, scheelite, mellite, and β-SnWO4-structured paving the way for the discovery of new oxide ion conductors. systems with excess oxide ions, which exhibit high interstitial – The interest in REs as oxide ion conductors began from the oxide ion conductivities.31,42 46 The Arrhenius plots of the con- 1980s. New typical REs based oxide ion conductors such as ductivities of oxide ion conductors with various structures are

La10Si6O27 apatite, LaGaO3-based perovskite, and lanthanum shown in Fig. 3. Next, we will extensively study these oxide ion 30–32 molybdate (La2Mo2O9) were discovered in the late 1990s. conductors with various structures.

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zirconia in SOFCs, yttrium or scandium as the dopant, which stabilizes the cubic-fluorite-structured zirconia, has been 49,50 widely used. Further, 11% Sc2O3-stabilized zirconia shows the highest ionic conductivity, but the higher cost of scandium extremely limits its application in SOFCs.51 Therefore, 8% YSZ with the cubic fluorite structure is considered to be the most reliable candidate as an electrolyte for SOFCs. With respect to applications, YSZ has been used as an oxygen ion sensor since 1976. However, it requires higher operating temperatures (1273 K), which induces several interfacial reactions between the electrode/solid electrolyte, electrode/interconnector, and interconnector/solid electrolyte, resulting in bad compatibility and stability of the SOFCs.52 Therefore, the trend of lowering the operating temperature to the intermediate-temperature range of 500–800 °C (called IT-SOFCs) is necessary to partially Fig. 3 Temperature-dependent Arrhenius plots of the conductivities overcome the problems associated with higher working of pure oxide ion conductors with diﬀerent structures such as temperatures. yttria-stabilized zirconia (YSZ), Gd-doped CeO2 (GDC), Ge-apatite (La9.33+x(GeO4)6O2+3x/2), La2Mo2O9,La1−xSrxGa3−yMgyO3−δ (LSGM), and 2.1.2 CeO2-based fluorite structure. To develop an electro- La1.54Sr0.46Ga3O7.27 (LSGO). (Adapted with permission from ref. 46. lyte with high oxygen ion conductivity for IT-SOFCs, ceria- Copyright 2004 The Chemical Society of Japan.) based solid electrolytes have been developed since their con- − ductivity can be up to 0.01 S cm 1 at ∼500 °C.35 Cerium oxides have a fluorite structure and exhibit low oxygen ion conduc- 2.1 REs based fluorite structure tivity. Owing to the lack of oxide-anion vacancies, the substitution of Ce by lower-valent cations can be used to enhance RE-based oxide ion conductors with the fluorite structure have the oxide ionic conductivity. RE cations were selected as high ionic conductivity because their open structures can approximate dopants due to their stable trivalent states and provide large tolerance toward higher levels of atomic dis- lanthanide contraction eﬀects. Experimentally, it has been order.47 Traditional fcc-fluorite-type oxide ion conductors reported that the substitution of ceria with Sm, Nd, La, and Gd based on ZrO ,CeO, and Bi O are the most widely investi- 2 2 2 3 can remarkably improve the oxygen ion conductivity.53 Fig. 5 gated materials. As shown in Fig. 4, in the fluorite structure, shows that the ionic radius of REs depends on the oxygen ion the host cations get replaced by lower-valent cations, and the conductivity of doped ceria at 800 °C. Among these dopants, formation of oxygen vacancies in the oxide ion sublattice is 3+ Sm -doped CeO2 with the highest ionic conductivity (0.01 S used to balance the missing charge. Because of the partial − cm 1 at ∼500 °C) could be obtained by the microscopic modu-

Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. occupancy of an energetically equivalent set of oxygen-ion lation of the ionic radius of REs.54 Even though Ca2+-doped lattice sites, the oxide ions can diﬀusively move by overcoming ceria shows superior conductivity in alkaline-earth doping a smaller energy barrier. systems, its conductivity is much lower than that of REs-doped 2.1.1 ZrO -based fluorite structure. Stabilized zirconia 2 ceria, proving that the lanthanide contraction eﬀect plays a adopts the cubic fluorite structure, which was used for the first uniquely important role in the modulation of the oxygen ion time in a fuel cell in 1937.48 Among the trivalent REs-stabilized

Fig. 5 Arrhenius plots of the conductivities of CeO2 at 800 °C against the radius of the dopant cation. (Adapted with permission from ref. 47.

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conductivity. The crucial disadvantage of cerium oxide is the 2.2 REs based pyrochlore structure presence of electronic conduction from the partial reduction REs based pyrochlore-type oxides with the general formula of Ce4+ to Ce3+ under high-temperature reducing conditions A2B2O7 (A: REs; B: 3d, 4d, or 5d transition-metal elements) (>600 °C). have attracted considerable attention owing to their unique δ 2.1.3 Bi2O3-based fluorite structure. Fluorite-type -Bi2O3 structural characteristics, such as low thermal conductivity, ∼ −1 with ionic conductivity of 1Scm at 750 °C is one of the high melting point, high chemical stability, and excellent 55,56 highest among the known oxygen ion conductors. The oxide ion conductivity.65 These unique properties render RE 3+ 2 high polarizability of the Bi cation with 6s lone-pair elec- pyrochlore type oxides as promising candidates for use as trons as well as the occupational and positional ordering on solid electrolytes in SOFCs. The fully ordered pyrochlore struc- the oxygen ion sublattice are responsible for the high oxygen ture adopts the cubic symmetry with the Fdˉ3m (no. 227) space 57 58 ion conductivity. Harwig reported the structural model of group, which can be considered as a superstructure of the δ -Bi2O3 via the Rietveld refinement of the neutron powder ideal defect (A,B)O -type fluorite structure with one vacant ﬀ λ 2 di raction (NPD) data ( = 2.57 Å) at 744 °C, indicating that oxygen site per formula unit, where the cation and anion the oxide ions were displaced from the ideal tetrahedral 8c vacancies are ordered (Fig. 7).66 This structure can tolerate 〈 〉 sites to 32f sites along the 111 direction (the so-called pos- vacancies at the A and O sites to a certain extent, which can 59 itional disorder), as shown in Fig. 6. Yashima and Ishimura generate pathways for oxygen transport. Generally, pyrochlores verified the complicated disorder spreading over a wide area can be divided into two major classes depending on the com- 〈 〉 and shift to the 111 direction in cubic bismuth oxide bination of oxidation states, namely, (2+, 5+) pyrochlores and δ ( -Bi2O3) by the maximum entropy method (MEM)-based (3+, 4+) pyrochlores. pattern fitting combined with the Rietveld method using the Many of the reported pyrochlore-based oxide ion conduc- δ NPD data. However, -Bi2O3 is only stable between 730 °C and tors are of the (3+, 4+) type, which are governed by the relative 3+ its melting point of 825 °C. The partial substitution of Bi by ionic radii or the ionic radius ratio (RR: r /r ) and the oxygen δ A B various cations in Bi2O3 can yield the -Bi2O3 phase, particu- parameter.67 3+ larly for trivalent RE cations such as Y and lanthanides (Sm, The partial disordering of the pyrochlore phases (P phases) 60 Eu, Gd, Tb, or Dy). For example, among lanthanide-stabilized have a smaller migration energy than that of the same compo- δ -Bi2O3, 20 mol% erbia-stabilized bismuth oxide (ESB) exhibits sition of phases with a fully disordered fluorite structure −2 −1 the highest oxygen ion conductivity of 1.5 × 10 Scm at (F structure); further, the optimal conductivity can be 61,62 500 °C. Because of the superior oxygen ion conductivities obtained in partially disordered pyrochlore materials.68,69 For δ of REs-stabilized -Bi2O3, they prove to be promising candi- example, P-Gd2Zr2O7 is a better oxygen ion conductor than dates as electrolytes of SOFCs. However, there are some draw- 70 F-Gd2Zr2O7. Structural disorder in the (3+, 4+)-pyrochlore- backs that limit their applications as electrolytes for SOFCs, type compounds is closely dependent on the size mismatch such as partial reduction at low oxygen partial pressures, high between the A and B cations.68 Therefore, the modulation of thermal expansion, and poor mechanical properties.63,64

Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. disordering in the pyrochlores can be realized by A- and/or B-site substitutions. A-site Gd3+ doped by smaller lanthanides (Tb3+–Lu3+ and 3+ Y ) in P-Gd2Zr2O7 induces a pyrochlore-to-fluorite transition, while doping by larger lanthanides (La to Gd) yields a partially disordered pyrochlore. The corresponding ion conductivities of the fluorite-type materials are normally lower than those of pyrochlore materials. Moreover, the oxygen migration energies

Fig. 7 (a) Unit cell of pyrochlore-structured Gd2Zr2O7. (b) One-eighth

Fig. 6 Structural model of δ-Bi2O3. (a) Schematic of the average occu- of the unit cell of Gd2Zr2O7. The blue sphere represents the A cation at pancy of the oxide ion sublattice (occupancy disorder) in δ-Bi2O3. (b and the 16d site and the green one represents the B cation at the 16c site. c) Displacement of the oxide ions away from the ideal tetrahedral 8c The red sphere represents the oxygen anions at the 48f and 8b sites. sites along the 〈111〉 direction to the 32f sites. (d) Positional disorder The oxygen at the 48f site has a variable position coordinate, which structure along the [001] direction. (Adapted with permission from ref. varies from 0.309 to 0.375. At higher temperatures, oxygen at the 48f 58. Copyright 1978 Wiley.) site can occupy the vacant 8a site, leaving the original site.

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well known that materials crystallizing in highly symmetric structures are fairly interesting since they may exhibit superior oxygen ion conductivity in SOFCs. In perovskites, the degree of distortion from fully symmetric and stress-free lattice plays a crucial role in controlling the oxide ion conductivity; there- 3+ 3+ fore, the oxygen vacancy in the A B O3 perovskite has the highest mobility and lowest activation energy for vacancy hopping.75 Perovskite oxides have garnered considerable interest as materials for SOFCs, but most perovskite oxides are p-type semiconductors at high oxygen partial pressures and n-type semiconductors at low oxygen partial pressures.

Alkaline-earth-doped perovskite oxides (LaYO3 and LaAlO3) are promising electrolytes, affording pure oxide ionic conductivity at low oxygen pressures, which can be developed for use in very low oxygen pressures.76 In the Ga-based perovskite Fig. 8 Arrhenius plots of conductivities for Gd Ln Zr O (Ln: Er, Dy, 1.2 0.8 2 7 oxide of LaGaO (Fig. 9), LaGaO doped with Sr and Mg for Sm, Nd, or La) samples. (Adapted with permission from ref. 71. Copyright 3 3 ff 2009 Elsevier Science B.V.) the La and Ga site a ords high oxide ionic conductivity, which is comparable to the conductivities of Gd3+-doped 30 CeO2. Acceptor-doped LaGaO3 with the ability to accommo- 3+ 3+ 3+ 3+ 3+ 3+ in the Gd2−xLnxZr2O7 (Ln: Er ,Y ,Dy ,Sm ,Nd ,orLa ) date a large amount of vacancies has a superior characteristic, series monotonically decrease with an increase in the average exhibiting stable oxygen ion conductivity in a wide range of size of the A cation (Fig. 8).71 In addition, Kutty et al.72 oxygen partial pressures. Generally, transition-metal-cations reported that the thermal expansion coefficient (TEC) doping (e.g., Fe/Co) enhances electron or hole conduction. decreases with an increase in the A-site cationic radius in However, first-principles calculations for Sr- and Mg-doped

lanthanide pyrochlore zirconate and hafnate. Furthermore, LaGaO3 have revealed that the conductivity of LaGaO3-based structural disorder and consequent enhancement in the ionic materials can be further improved by Cu2+/Hg2+ doping on the – 2+ conductivity are also obtained in Gd2GaSbO7 Gd2Zr2O7 solid Ga site due to their lower solution energy than Mg doping solutions by the B-site doping strategy.73 on the Ga site.77 Following this idea, doping Co for the Ga site

There are relatively fewer (2+, 5+) pyrochlores because of of La0.8Sr0.2Ga0.8Mg0.2O3 on the oxide ion conductivity was which fewer numbers of A2+ and B5+ cations have suitable investigated, and it was also found that Co was an eﬀective ionic radii for the formation of the pyrochlore structure.67 dopant for increasing the oxide ion conductivity without sig- However, the (2+, 5+) type in pyrochlores are unique since the nificantly increasing the p-type electronic conduction.78 These covalency, polarizability, and electronegativity of the constitu- perovskite materials show relatively considerable application Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. ent ions result in a wide variety of interesting physical pro- prospects as electrolytes for SOFCs. However, there are certain perties. A combination of the two types of pyrochlore struc- drawbacks in these perovskite materials, mainly Ga volatility tures forming quaternary pyrochlore systems with the general at high temperatures under reducing conditions and their 2+ 3+ 4+ 5+ formula A1 A2 B1 B2 O7 were proposed toward the design reactivity with Ni (prevalent in most of the currently used of new oxygen ion conductors. When both A- and B-site none- anode materials). quivalence doping was simultaneously performed, the influ- ences of disorder on the structure, lattice thermal expansion, and oxide ion conducting properties were studied in

(CaxGd1−x)2(Zr1−xMx)2O7 (M: Nb, Ta) pyrochlore solid solutions.74 It was found that with an increase in doping, progress- ive ordering from a defect pyrochlore to an ordered pyrochlore occurs. Studies on quaternary-type pyrochlores by structural order–disorder modulation provide the basis for the further development of oxide ion conductors as solid electrolytes for SOFCs.

2.3 REs based perovskite structure It is well known that fluorite-structured oxides consisting of tetravalent cations exhibit good oxide ion conductivity, and Zr-based oxides are generally used as an oxide ion conductor, but their performances are not satisfactorily high.30 Therefore, oxide ion conductors with superior conductivity in

the wide oxygen partial pressures are strongly needed. It is Fig. 9 Structure of the perovskite oxide of LaGaO3.

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2.4 REs based La2Mo2O9 structure suppress mechanical instability upon heating. Among these As mentioned above, many eﬀorts have been devoted toward doping ions, W doping for Mo in La2Mo2O9 can stabilize the 81 the fabrication of fluorite-, pyrochlore-, and perovskite-type structure against reduction under low oxygen pressures. oxide ion conductors due to their suitable structural features However, W doping often raises the low-temperature ionic 82 facilitating high mobility of oxygen ions; however, there are conductivity but not the high-temperature ionic conductivity. some bottlenecks in these systems. Therefore, the search for In addition, the Nb-doped La2Mo2O9 compound of new oxide ion conductors with superior properties is still La2Mo1.94Nb0.06O9−δ with the lowest Nb doping shows the −1 needed. Consequently, Lacorre et al. were the first to report a highest ionic conductivity in La2Mo2O9 systems (0.113 S cm 83 at 800 °C). Anion substitution was also realized in La2Mo2O9; family based on the parent compound La2Mo2O9 that shows 31 for example, fluorine substitution at the oxygen site can be fast oxygen ion conductivities. La2Mo2O9 exhibits a reversible phase transition at 580 °C from the low-temperature used to increase the conductivity by decreasing the transition 84 α β temperature of the structural order–disorder. -La2Mo2O9 phase to the high-temperature -La2Mo2O9 phase. β β The -La2Mo2O9 phase adopted by the -SnWO4 structure exhi- − − 2.5 REs based apatite structure bits ionic conductivity as high as 6 × 10 2 Scm1 at 800 °C 79 α The apatite structure has the general formula A10(MO4)6O2+y, (Fig. 10). The structural evidence of -La2Mo2O9 reveals that the variable coordination environment of the Mo cation is the where A is generally a large divalent cation (alkaline earths or 3− 3− key element to provide a low-energy migration path for oxygen REs) and MO4 is a trivalent anionic group (SiO4 , GeO4 , 3− 3− 3− ions.80 PO4 ,VO4 , AsO4 , etc.). Large cations were situated at the 2− β two sites (seven-coordinate and nine-coordinate sites) and O With regard to the practical applications of the -La2Mo2O9 phase, it is necessary to stabilize the high-temperature cubic anions (2a site) occupied the channels along the c-axis direc- β-La Mo O phase at room temperature. The substitution of tion, while M coordinated with the four-oxygen-formed iso- 2 2 9 3− 85 lanthanum by bismuth can increase the cell volume and stabil- lated TO4 tetrahedra. The oxygen at the 2a site coordinated only with the La ions at the 6 h positions, indicating that the ize the high-temperature cubic β-La2Mo2O9 phase, but it does not improve the oxide ion conductivity due to the reintroduc- oxygen at the 2a site was mobile. tion of a lone pair because of Bi3+ substitution. The reintro- AO6 octahedra and MO4 tetrahedra were connected, duced 6s2 lone pair prefers to block the oxygen-ion migration forming the structural framework of A10(MO4)6X2, as shown in path, suggesting that substituting the lone-pair element with Fig. 11. the oxidation state doped by a non-lone-pair element of the RE apatite materials have been attracting considerable same size is a way to design new oxide ion conductors. interest as a new family of oxide ion conductors due to their Consequently, there are numerous studies on stabilizing the high oxygen ion conductivities and low activation energies, 86 β which can be an alternative solid electrode for SOFCs. -La2Mo2O9 phase at lower temperatures by means of substitution at the cationic sites, including A-site substitutions (e.g., Various stoichiometric compositions and their corresponding ionic conductivities have been investigated in apatite systems

Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. other REs, alkaline earths, or alkali metals) or B-site substitutions (e.g., W, V, S, Cr, Nb, or Ta). Because of the various sub- (Fig. 12); for example, stoichiometric compositions “ ” stitutions at the La and Mo sites, cationic substitutions can ((RE0.8AE0.2)10(MO4)6O2), cation-deficient compositions “ ” lead to the suppression of distortion, aﬀording the cubic (RE9.33(MO4)6O2), and oxygen-excess compositions β (RE10(MO4)6O3 and (RE10−xAEx)(MO4)6O3−x/2); here, RE denotes -La2Mo2O9 phase. Most cationic substitutions are likely to La, Nd, Sm, Gd, or Dy; AE denotes Ca, Sr, or Ba; and M denotes Si or Ge.87,88 In contrast to traditional perovskite- and fluorite-based oxide ion conductors that aﬀord ionic conductivity via oxygen vacancies, the ionic conductivity of RE-based

Fig. 11 Apatite structure of La9.33(SiO4)6O2 along the b axis. The pink

Fig. 10 La2Mo2O9-type structure of β-SnWO4. spheres denote oxygen at the 2a site.

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Fig. 12 Arrhenius plots of conductivities for “oxygen-stoichiometric” Fig. 13 Conductivities (700 °C) of Ln10(SiO4)6O3 (Ln: La, Nd, Sm, Gd, or (La8Sr2(SiO4)6O2 and La8Sr2(GeO4)6O2); “oxygen-excess” Dy) versus the ionic radius of Ln3+. (Adapted with permission from ref. (La8.8Sr1.2(SiO4)6O2.4,La8.65Sr1.35(SiO4)6O2.32, and La8.65Sr1.35(SiO4)6O2.32); 97. Copyright 1995 The Chemical Society of Japan.) and “cation-deﬁcient” (La9.33(SiO4)6O2,La9.33(Si0.33Ge0.67O4)6O2, La9.33(Si0.5Ge0.5O4)6O2, and La9.50(Ge0.916Al0.083O4)6O2) apatite samples. (Adapted with permission from ref. 88. Copyright 2005 Royal Society Chemistry.) decreases, a general decrease in the conductivity and an increase in the activation energy is observed. In order to optimize the oxide ion conductivity of RE-based apatite materials, apatite systems is mediated by interstitial oxide. In recent a wide range of doping investigations have been performed for years, several earth-based apatite systems have been reported, apatites with larger RE systems, because a larger-REs-contain- especially lanthanum silicate and germanate systems comprising system aﬀords higher conductivities in comparison to ing larger REs (La, Pr, or Nd), exhibiting the highest oxide ion other systems with smaller REs. In addition, the flexibility of

conductivities and higher oxygen transference numbers (to > the Ge/SiO4 tetrahedral framework plays a crucial role in facili- 0.9) in a wide range of oxygen partial pressures, e.g., tating oxide ion migration. Evidently, RE-based apatite systems

La10(GeO4)6O3,La9Sr(GeO4)6O2.5,La10(SiO4)6O3, and are tolerant to an unusually broad range of dopants and the 89–92 La9.75Sr0.25(SiO4)6O2.895. In the following section, we will observed conductivities are sensitive to the substitution 85,87,98 introduce in detail lanthanum-silicate- and germanate-based regime and cation/anion nonstoichiometry. In general, Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. oxide ion conductors. fully stoichiometric systems, such as La8Sr2Si6O26 (Fig. 12), 2.5.1 REs based Si-containing apatite systems. For “oxygen- exhibit poor conductivity as the alkaline content increases

excess” Si-based systems, such as (RE10−xAEx)(SiO4)6O3−x/2, and the number of cation vacancies decreases. In contrast, “ ” oxygen-excess compositions exhibited enhanced conduc- for cation-deficient compositions, such as RE9.33(SiO4)6O2, 3+ 3+ tivities. For example, the conductivities of La9.67(SiO4)6O2.5 and the aliovalent doping of Ga and Al for the Si site −4 −3 −1 La9.33(SiO4)6O2 at 500 °C were 1.1 × 10 and 1.3 × 10 Scm , can significantly enhance the ionic conductivity, e.g., 93–95 87,94 respectively. Studies on the oxygen ion conductivity RE9.33(Si1−xMxO4)6O2 (RE: La; M: trivalent cation). Here, Si (Fig. 13) were also performed for oxygen-excess lanthanum sili- can be partially replaced by Al with charge compensation by

cates, such as Ln10(SiO4)6O3 (Ln: La, Nd, Sm, Gd, Dy, Y, Ho, Er, the incorporation of La according to the following mechanism: or Yb), revealing that the important factors for ion diﬀusion þ þ þ Si4 ! Al3 þ 1=3La3 : ð1Þ are the electrostatic interactions between the positively charged framework and the mobile interstitial oxide ions or These results indicate that lower-valent doping on the Si Ln3+ ions, as well as the steric eﬀects from the size of the site tends to enhance the conductivity in nonstoichiometric cations and the flowing oxygen in the channel system.96 systems, while the same doping strategy on the La site tends to Interestingly, it was also found that the hexagonal phase was result in a decrease in conductivity in fully stoichiometric

adopted for the Ln10(SiO4)6O3 system (Ln: La, Nd, Sm, Gd, or systems. Dy), while when Ln was Y, Ho, Er, or Yb, a monoclinic struc- In addition, the magnitude of ionic conductivity is strongly ture could be obtained due to the lanthanide contraction related to the size of the dopants. The substitution of La3+ by eﬀect. larger alkaline-earth cations for creating cation vacancies can For smaller RE analogs, such as Gd, lower nonstoichiome- enhance the ionic conductivity, suggesting that the A-site try ranges could be obtained, indicating that less space is avail- cation vacancies may induce local distortions and provide the able to accommodate the additional interstitial oxide ions in driving force for the displacement of channel oxygen ions into 97 the reduced size of the unit cell. As the size of the RE ions the interstitial sites, e.g.,RE9.33(Si1−xMxO4)6O2 (RE: La; M:

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larger divalent cation).91 Furthermore, NPD structural studies and atomistic simulations have revealed that cation vacancies increase the interstitial oxygen concentration by promoting the formation of Frankel defects.91,99,100 However, smaller dopants (Mg) on the La site can be detrimental to the ionic conductivity. A decrease in conductivity can be inherently ascribed to a change in the coordination environment, where the coordinate numbers change from 9 (for La) to 6 (for smaller dopants such as Mg), aﬀecting the oxygen-ion transport channels.101 For equivalent doping in RE-based apatite systems, the substitution of Si with Ge can significantly enhance the conductivity,102,103 while the substitution of Si with Ti or substitution of Bi with La can result in a decrease in – conductivity.89,104 107 In the abovementioned studies, it has Fig. 14 (a) Morphologies of the as-grown single crystals of been reported that the conductivities of La9Ba Pr9.33(SiO4)6O2,Nd9.33(SiO4)6O2, and Sm9.33(SiO4)6O2. (b) Arrhenius plots (SiO4)4(TiO4)2O2.5 and La7BaBi2(SiO4)6O2.5 at 500 °C are 4 and of the conductivities of the Nd9.33(SiO4)6O2 single crystal parallel and 3 orders of magnitude lower than that of La9Ba(SiO4)6O2.5, perpendicular to the c axis. (Adapted with permission from ref. 112. respectively. In the case of Ti substitution, it has been indi- Copyright 2004 Elsevier Science B.V.) cated that interstitial oxygen gets trapped by Ti because of which its coordination number increases. For Bi-doped samples with lower conductivity, it is highly probable that the origin of the reduction in conductivity can be ascribed to the presence of the Bi2+ lone pair. At this point, for lead-alkali-

based apatite Pb8K2(PO4)6, it has been reported that the lone pair of Pb2+ occupies the vacant channel space, resulting in no 108 channel anions in the crystal structure of Pb8K2(PO4)6. Therefore, the presence of Bi2+ lone pairs may be the intrinsic “ ” cause that encroaches on the oxide ion channels and therefore Fig. 15 Proposed SN2 -type mechanism involving the rotation of the partially blocks the migration pathway. tetrahedra. (Adapted with permission from ref. 114. Copyright 2007 Royal Society Chemistry.) To investigate the mechanism of interstitial oxide ion migration, atomic simulation studies were performed for

La9.33(SiO4)6O2 with high conductivity and La8Sr2(SiO4)6O2 with poor conductivity.99,100 A principle result from the atomic provided new insights toward investigating the local structure Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. simulation was obtained, predicting the location of a new ener- and disorder of apatite silicate systems due to doping getically favorable oxygen site, where the interstitial oxygen eﬀects.86,113 Local structural investigations have demonstrated was accommodated at the channel periphery and stabilized by that this dopant behavior can be ascribed to the flexibility of the displacement of the nearby silicate unit toward the La the Si substructures, which allows large local distortions and channels. Subsequent studies confirmed the predicted alternation of the site volumes. A careful inspection of the

location of the interstitial oxygen site in the La9.33+x(Si/ local structural distortion in the vicinity of the oxygen-ion 85 29 GeO4)6O2+3x/2 system. Si nuclear magnetic resonance interstitial site based on computer modeling provides one (NMR) studies for a range of alkaline-earth-doped apatite sili- possibility of inter-channel conduction.114 This investigation cates further support this interstitial oxide ion site, confirming suggests that the local relaxation around the interstitial site

that samples with poor conductivity demonstrate single reso- leads to a decrease in the distance between the adjacent SiO4 nance and highly conducting samples exhibit more than one units, which eﬀectively creates a pathway for an interstitial Si environment signal.109 This occurs due to the local distor- oxide ion between the adjacent channels via a series of two

tions induced by the presence of interstitial oxygens and cooperative “SN2”-type processes (SN2: bimolecular nucleophi- cation vacancies. The presence of these oxygen interstitial sites lic substitution) accompanied by the rotation of the tetrahedra

is also supported by Mössbauer studies in La10Si5FeO26.5 (Fig. 15). systems.89 In addition, the oxide ion conductivity for single- 2.5.2 REs based Ge-containing apatite systems. With

crystal Ln9.33(SiO4)6O2 (Ln: Pr, Nd, or Sm) shows that the con- regard to silicates, the superior oxide ion conductivities are ductivity is highly anisotropic (Fig. 14), exhibiting significantly favorable for “cation-deficient” and/or “oxygen-excess” compo-

higher conductivity parallel to the c axis than that perpendicu- sitions. For germanate systems, such as La9.33+x(GeO4)6O2+3x/2, σ lar to the c axis (e.g., for Nd9.33(SiO4)6O2, c (500 °C) = 6.4 × a higher magnitude of oxygen excess can be achieved (up to −2 −1 −3 −1 110–112 10 Scm and σab (500 °C) = 1.3 × 10 Scm ). 1.0 per formula unit) compared with silicates. An additional Combining extended X-ray absorption fine-structure complexity of germanate apatite is that its structural symmetry (EXAFS) spectroscopy and atomic modeling techniques have changes with an increase in the oxygen content. The single-

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phase region of La9.33+x(GeO4)6O2+3x/2 series was reported to be could be directly imaged successfully by a combination of 0.19 ≤ x ≤ 0.42 with a structural change from hexagonal to tri- annular bright-field scanning transmission electron clinic. The solid solutions in the region of 0.19 ≤ x ≤ 0.27 are microscopy (ABF-STEM) experiments and frozen phonon mul-

hexagonal with the space group P63/m, and it becomes triclinic tislice simulations. The accurately imaged ABF-STEM experi- in the region of 0.33 ≤ x ≤ 0.42.98 Subsequently, Pramana et al. ments combined with the frozen phonon multislice simu- reported the synthesis of the end member for x = 0.67, which lations could reach ∼0.8% of the total electron content; for is also triclinic.115 Triclinic symmetry distortions are caused by example, levels of 8 out of 1030 electrons in a unit cell of the

the excess oxide ions in the tunnels and deformation of new apatite-type oxide ion conductor of Bi2La8[(GeO4)6]O3 116 40 the GeO4 tetrahedron to accommodate these oxide ions. could be determined.

The compositions with triclinic distortions exhibit lower The volatilization of GeO2 is a serious problem and poses a conductivity at lower temperatures as the additional challenge for the application prospects of this system. It is well

defects are constrained in the lower-symmetry structure, e.g., known that GeO2 is volatile at higher temperatures 117 98,119,126 La10(GeO4)6O3. According to the conductivities of the (>1250 °C); for example, the prolonged heating of

La9.33+x(GeO4)6O2+3x/2 series, the conductivities at lower temp- La9.33(GeO4)6O2 produces the impurity phase of La2GeO5 due 126 eratures tend to be lower than those of the silicate systems, to the higher degree of GeO2 loss. Moreover, upon pro- while the conductivities at higher temperatures are enhanced. longed heating at higher temperatures, the hexagonal apatite- – Moreover, variable-temperature XRD studies for high-oxygen- based La2O3 GeO2 compositions change to lower symmetry. content compositions indicated a gradual structural transition This occurs due to the volatilization of GeO2 and the conse- from triclinic to hexagonal over the range of 600–800 °C, quent increase in the La/Ge ratio.98 According to the above causing a decrease in the responding activation energy with an studies, the oxygen ion conductivity of La-germanate-based increase in temperature.116,118 apatites is mainly aﬀected by the following factors: crystal sym-

In contrast to the large number of doping studies on sili- metry, composition, volatilization of GeO2, ceramic density, cate systems, it is possible to introduce a wide range of and microstructure. dopants into germanate systems, too. Doping the La site with divalent cations (e.g., Ca, Sr, Ba, Mg, Cu, Ni, or Co) favors the 2.6 REs based mellite structure stability of the hexagonal lattice, while the substitution of Ge Mellite is a common layered tetrahedral network structure. 42,98,102,103,116,119,120 by Al results in triclinic symmetry. The Gallate melilites have the general formula (RE, A)2M3O7, where impetus for the formation of the triclinic structure rather than RE is a lanthanide, A is an alkaline-earth element (Ca, Sr, or

the hexagonal one is provided by the strain induced by the Ba), and M is Ga or Al. LaSrGa3O7-based oxide ion conductors presence of large cations (e.g., Ge, As, or V).121 Since these have been constantly attracting increasing interest owing to

dopants prefer to increase the strain, the GeO4 tetrahedra tilt their ability to accommodate and transport oxygen interstitials, toward the triclinic symmetry to relieve stress at the metal and showing promising interstitial oxide ion conductivity for 127 metalloid sites for yielding reasonable bond valence sum- SOFCs. LaSrGa3O7 consists of alternating cationic (La/Sr)2 Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. mations. Compared with silicate systems, the diﬀerence in the and 2D extended corner-sharing tetrahedral network Ga3O7, features of germanate systems is that the conductivities appear which can accommodate excess oxide anions in the pentago- to be significantly less influenced by the nature of the dopant nal rings formed in the tetrahedral layers as well as sustain or the dopant site. The selective partial substitution of a their mobility when the Sr atoms are replaced by La to form a 128 smaller RE (e.g., Y) at the La site facilitates the stabilization of La1+xSr1−xGa3O7+x/2 solid solution. Kuang et al. reported the hexagonal lattice even for high-level doping by altering the that the x = 0.54 composition showed high oxide ion conduc- − size of the framework, which is an alternative way to relieve tivity (σ = 0.02–0.1 S cm 1 in the temperature range of these structural stresses from the extension stress at the large 600–900 °C).44 The tetrahedral central cations of Ga exhibit A site and the compression stress at the M site.114,122,123 variable coordination numbers, thereby allowing the accom-

Detailed NPD studies for the La9.33+x(GeO4)6O2+3x/2 series modation of excess oxygen ions; however, the presence of non- have revealed the interstitial sites at the periphery of the bridging oxygen in the layered tetrahedral network facilitates channels.115,122,124 It has been shown that these framework the cooperative deformation and rotation of the tetrahedron interstitial oxygens provide a reservoir that can migrate into for transporting the interstitial oxide ions (Fig. 16a and b). The the apatite conductive channels, instantaneously transforming melilite structure can cater to excess oxide anions and the sus-

the GeO4 tetrahedra into distorted trigonal bipyramids via an tainable mobility of interstitial oxygen ions in the layered tetra- inter-tunnel oxygen diﬀusion mechanism. In addition, the tra- hedral network, which opens up many new structural families jectory plots obtained from the simulation studies reveal that for fabricating new interstitial oxide ion conductors.43

there is a clear migration path perpendicular to the c axis, indi- Subsequently, the substitution of La by Ce in LaSrGa3O7

cating that the conduction mechanism is more isotropic rather results in the CeSrGa3O7 mellite phase at the CO-reducing con- than anisotropic.125 The experimentally performed conduc- dition. However, it is diﬃcult to extend the Ce-to-Sr ratio to

tivity studies of Mg and Bi dopants at the La site are consistent form a Ce1+xSr1+xGa3O7+0.5x solid solution. Alternatively, inter- with the simulated mechanism of isotropic transport.120 In stitial oxygen ions can be successfully introduced by the

addition, very low levels of disordered interstitial oxygen atoms annealing treatment for CeSrGa3O7 under an oxygen atmo-

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able to match the tetrahedral layer of Ga3O7 and they can tolerate the local relaxation for accommodating the excess oxygen atoms in the pentagonal ring. On the other hand, trivalent RE cations contribute toward the stabilization of the oxygen interstitials by forming ionic bonding with the excess oxide ions,132 which requires large RE cations. More information on the development of melilite-type oxide ion conductors can be found in a review recently published by Zhou et al.133

2.7 Other REs based isolated tetrahedral structures In the preceding section, local interstitial oxygen was accommodated in the isolated tetrahedral structure, which was confirmed using lattice dynamics calculations and NMR studies on the apatite system. It was revealed that interstitial oxygen

transforms from (Si/Ge)O4 to (Si/Ge)O5 due to the rotational Fig. 16 (a) Structure of mellite La1.54Sr0.46Ga3O7.27. (b) Pentagonal flexibility of the isolated tetrahedra. Herein, the scheelite struc- tunnel of the La1.54Sr0.46Ga3O7.27 structure with interstitial oxygen. (c) ture has two ABO4 (A: Ca or Pb; B: Nb or W) units in the primi- Structure of mellite CeSrGa O with partially occupied interstitial 3 7.39 tive cell in which the A cations are coordinated by eight oxygen oxygen sites and (d) the corresponding local defect structure of the pentagonal tunnel containing the interstitial oxygen. (Adapted with per- anions and the B cations are coordinated with four oxygen 3− missions from ref. 44 and 129. Copyright 2008 Nature Publishing Group anions, forming isolated tetrahedral BO4 anions (Fig. 17). and 2014 American Chemical Society.) Typically, scheelite-structured interstitial oxide ion conductors,

e.g.,Pb1−xLaxWO4,aﬀord pure oxide ion conductivity of 4.8 × − − 10 2 Scm 1 at 800 °C.45 Interstitial oxide ions were introduced 3+ 2+ sphere to form CeSrGa3O7.39 via the oxidation of Ce to into the tetrahedral structure by the substitution of Pb with 4+ 129 Ce . However, these oxygen interstitial oxide ions are not La3+. As mentioned above, the apatite- and scheelite-structured mobile, as evidenced by the conductivity of CeSrGa3O7.39 con- interstitial oxide ion conductors have isolated anions. taining a high level of interstitial oxygen ions but does not However, there are a few structures in which both isolated show remarkable conducting behavior of the interstitial polyhedral anions and variable coordination numbers can be oxygen ions. It was found that both location and stabilization obtained; therefore, it is worthwhile to introduce mobile inter- mechanisms of the interstitial oxygen in CeSrGa3O7.39 are not stitial oxygens in other common structures based on the the same as those in La1.54Sr0.46Ga3O7.27.Inlinked polyhedra. La1.54Sr0.46Ga3O7.27, the interstitial oxygen is almost at the Subsequently, extensive eﬀorts have been devoted toward same level as the Ga cation and is stabilized at the framework identifying oxide ion conductors in other RE-based structural Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. oxygen level by entering the (Ga2)O4 tetrahedra with terminal systems. The cuspidine structure can be described by the

O2. However, in CeSrGa3O7.39, as shown in Fig. 16c and d, the general formula A4(Si2O7)(O, OH, F)2, where A represents large interstitial oxygen is stabilized by entering Ga1O4 for which all divalent cations (RE) in octahedral (or roughly octahedral) the oxygens as bridging atoms form the (Ga1)O5 trigonal coordination134 comprising chains formed by edge-sharing

bipyramid. The Ga1O4 tetrahedra with the terminal oxygen AO7/AO8 polyhedra along the a axis interconnected via the have constrained rotation and deformation, reducing the mobility of interstitial oxygen ions. On the other hand, the trapping eﬀect from the smaller Ce4+ due to the oxidation of Ce3+ contributes toward the localization of the oxygen intersti-

tials. CeSrGa3O7+δ mellite highlights the diversity of the stabilization and migration mechanisms of interstitial oxygen in the mellite structure arising from the oxidation nature of the RE. When the RE cations are smaller than La and Ce, melilite gallate becomes much less favorable for stabilizing the interstitial oxide ions. Among these REs, the Pr composition of

Ln1+xSr1+xGa3O7+0.5x achieved the highest interstitial oxide ion content (Pr1.4Sr0.6Ga3O7.2), out of which the conductivity − reached ∼0.1 S cm 1 at 800 °C.130 The compositions containing a smaller RE than Pr could hardly maintain the melilite structure at high RE concentrations, which indicates that a cationic size eﬀect occurs on the stability of the interstitial oxide 131 ions in gallate melilites. Smaller RE cations are not favor- Fig. 17 Scheelite structure of PbWO4 along the b axis.

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sites. The generation of a proton from the vacancies and water incorporation are also consistent with the experiment, which further confirm that the carriers of the cooperative rotational mechanism are the oxygen ion vacancies rather than interstitial oxygen ions. Despite the interesting features of cuspidine systems, their conductivities are too low to meet the application requirements of SOFC electrolytes.

3. REs based MIECs as electrodes for

Fig. 18 Structures of the cuspidine oxides of (a) Nd4(Ga2O7)O2 and (b) SOFCs Nd2TiO5. The air electrode cathode operates in an oxidizing condition of air or oxygen at ∼1000 °C and participates in the following oxygen reduction reaction: tetrahedral disilicate Si2O7 group (Fig. 18a). Interestingly, the 2 A2TiO5 (A: La, Y, Nd, or Eu) structure also contains the A4O8 1=2O2ðgÞþ2e ¼ O ðsÞ: ð2Þ “octahedral” framework of the cuspidine structure with the 135–137 Oxygen is reduced to oxide ions, consuming two electrons formula A4(T2O8)O2. The columnar arrangement of Si2O7 in the process. Therefore, the cathode has to meet the rigorous is replaced by a single chain of corner-sharing TiO5 trigonal requirements for applications in SOFCs. Apart from the stabi- bipyramids in this structure, forming (Ti–O–Ti–O) infinite lity and compatibility requirements of any fuel cell, the chains (Fig. 18b). cathode materials need to have higher electronic conductivity Cuspidine-like titanites demonstrate that the cuspidine under oxidation conditions and excellent catalytic activity for structure with flexibility can accommodate up to 20% oxygen ORR/OER. With growing interest in decreasing the operating vacancies per formula unit, showing that compounds with temperature of SOFCs, materials research in the field of SOFC these structural features have potential as oxide ion conduc- cathodes is moving toward MIECs with perovskite or related tors. The obtained Nd4[Ga1.8Ge0.2O7.1]O2 with the cuspidine − − structures, as their performances are promising particularly at structure aﬀords respectable conductivity of 0.5 × 10 3 Scm 1 lower temperatures (<800 °C). Therefore, perovskite and at 800 °C due to its remarkably structural flexibility.138 The related structures have been consistently dominating this enhancement in the oxide ion conductivity can also be field.141 achieved in cuspidine-type RE4(Ga2O7)O2 (RE: La, Nd, or Sm) by the substitution of Ga3+ with Ge4+, and the introduction of this oxygen transforms the isolated digallate groups to infinite 3.1 REs based perovskite-type MIECs Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. distorted trigonal bipyramid chains with some interruptions Perovskite-based electrolytes are well-known MIECs commonly due to the partial occupancy of oxygens. The conductivity used as the cathode materials in SOFCs. REs-based perovskite σ of the obtained La4(Ga2−xGexO7+x/2)O2 (x = 0.6; (627 °C) = materials with the general formula LnMO3−δ (where Ln is a RE − − 6.6 × 10 5 Scm 1) is enhanced by 2 orders of magnitude as and M is a transition metal (Cr, Mn, Fe, Co, or Ni)) possess 139 compared to intrinsic La4(Ga2O7)O2. The substitution of mixed electronic and ionic conductivities along with intrinsic 3+ 4+ Ga by Ti in La4(Ga2O7)O2 aﬀords proton conductivity below catalytic properties, making them promising candidates for

∼700 °C. The variable conductivities of La4(Ga2−xTixO7+x/2)O2 applications as SOFC cathodes. Therefore, considerable eﬀorts slightly increase with the Ti content, with the maximum con- have been directed toward the discovery of perovskite-type

ductivity achieved for the Ti1.3 composition, followed by a cathodes and investigations of their structure–property −5 −1 gradual decrease in conductivity (σ (Ti1.3) = 6.5 × 10 Scm relationships.

at 850 °C). These studies show that the partial substitution of In the RE-based perovskite chromite of LnCrO3, a decrease Ga3+ by Ge4+ or Ti4+ can be accompanied by the introduction in the RE cationic radii leads to a decrease in the degree of of interstitial oxygen for charge compensation, resulting in the thermal dissociation; for example, the diﬀerent degrees of ﬀ enhancement of oxide ion conductivity. LaCrO3 and YbCrO3 have di erent degrees of decomposition Computer modeling studies were also performed for the after sintering at 1827 °C for 0.5 h.142 In addition, the crystal

La4(Ga2−xTixO7+x/2)O2 (x = 2) end number; therefore, the oxide- structure of LnCrO3 (Ln: Pr, Nd, Sm, or Y) show a general ion conduction mechanism and process were elucidated by increase in orthorhombic distortions with a decrease in the RE 143 computer modeling for the La4(Ga2−xTixO7+x/2)O2 (x = 2) end cationic radii at temperatures below 900 °C. Furthermore, number, which highlighted the importance of a cooperative the activation energy of conductivity of these systems is also vacancy process in this material, with the most favorable aﬀected by the 4f shells of the RE cationic radii; for example,

rotation occurring via the cooperative vacancy hopping mecha- the conductivity of LnCrO3 (Ln: Pr, Nd, Sm, or Y) decreases in nism.140 As per the computer modeling data, the most favor- the order of La > Nd > Sm > Y. This can be ascribed to the fact able position for water incorporation was at the oxygen defect that the overlapping integration of the d orbital of the chro-

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mium cations and the p orbital of the oxygen anions increases with an increase in the lanthanide cations, leading to an increase in the electron and hole mobilities. A-site and/or B-site substitutions result in remarkable improvements in certain aspects of RE-based perovskite

systems. Lanthanum manganate (LaMnO3) has high p-type electronic conductivity due to the presence of mixed Mn3+ and Mn4+, which can be enhanced by the A-site substitution of La3+ with divalent alkaline earths, particularly Sr2+.144

Although La1−xSrxMnO3 has high electronic conductivity at higher temperatures, its drawbacks are not negligible, such as its low oxygen ion conductivity, as well as the fact that the Fig. 19 Crystal structure of the R–P series for n = 1, 2, and 3, which material reacts with the YSZ electrolytes at higher operating consists of a block of n perovskite units separated by a layer of rock-salt temperatures forming an insulating layer. These problems layer. seriously damage the performance of the cell. In fact, lantha-

num-strontium manganite (La1−xSrxMnO3) and composites are still considered to be state-of-the-art cathode materials for extensively studied as new MIECs, which comprise alternating “ ” ﬀ SOFCs at 800–1000 °C. ABO3 perovskite units and AO rock-salt layers in an o set Among the various MIECs, cobalt-containing perovskite ABA′ B′ arrangement along the c axis. Among these materials, oxides aﬀord superior electrocatalytic activity than that obtain- the Ln2NiO4 series (Ln: lanthanides) has drawn the most atten- δ 153 able from LaMnO3. The substitution of La by Sr in the lantha- tion. In particular, La2NiO4+δ ( can be up to 0.25) possesses

num cobalt phase of La1−xSrxCoO3 can significantly improve oxygen excesses, thereby providing higher interstitial oxygen its catalytic activity and oxide ion conductivity.145 However, diffusion over a wide temperature range.154 The highly hyper- this material also reacts with the YSZ electrodes, and it yields stoichiometric phases (δ > 0.15) have promising applications bad compatibility with YSZ and other typical electrolytes. Each as fast oxide ion conductors for SOFC cathodes. Generally, the of these materials have some deficiency that prevents their structural stability of the ABO3 perovskite is associated with applications in SOFC technology. Subsequently, further the ionic radii of the A and B sites, which is usually evaluated studies in these cobaltite systems have shown considerable by the Goldschmidt tolerance factor “t”.155 3+ 2+ promise; for example, the A-site substitution of Sm by Sr in þ ¼ pffiffiffirA rO ð Þ Sm − Sr CoO revealed excellent electrical conductivity, catho- t 3 1 x x 3 2ðr þ r Þ dic polarization, and reactivity with the YSZ electrolyte.146 A O 156 Moreover, Sm0.5Sr0.5CoO3 was fabricated as the cathode of a For La2NiO4, t = 0.89, revealing that the NiO2 plane is SOFC cell with a single-chamber design, which enabled device under pressure as a result of the stretched La2O2 layer. The Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. 20 operation at lower temperatures. Perovskite-related cobaltite intercalation of interstitial oxygen ions in La2NiO4 actually

oxides possess considerable cathodic and transport properties, causes tilting of the NiO6 octahedra, which partially reduces but they also have higher thermal and chemical expansion. the steric strain induced by the lattice mismatch between the 2+ 2− The B-site substitution of Co with Fe in La1−xSrxCo1−yFeyO3 La2O2 and NiO2 layers. Furthermore, a combination of (LSCF) exhibits a better TEC match and improves its stability solid-state NMR spectroscopy and computational (density at higher temperatures.147,148 functional theory (DFT) calculations) methodologies clarified the atomically local distortions and the dynamics of La NiO δ – – 2 4+ 3.2 REs based Ruddlesden Popper (R P) structure MIEC, providing an insight into the design and improvement 157 The R–P structure with the general formula An+1BnO3n+1 or of next-generation MIECs. Single-crystal La2NiO4+δ exhibits

(AO)(ABO3)n, where A is an alkaline-earth element or lantha- anisotropic ionic transport properties, which is in agreement – ﬀ nide, B is a transition metal, and n =13, consists of n ABO3 with the di usion pathway predicated by local distortions and perovskite layers sandwiched between two AO rock-salt layers DFT calculations.158

(Fig. 19). Electronic conduction can occur in the perovskite 3.2.2 An+1BnO3n+1-based R–P type (n = 2 and 3). Owing to – – layers via the corner-sharing BO6 octahedra, because the B the special features of the R P structure, the properties of R P cations occupying the center of the polyhedra can exhibit a oxides, such as ionic and electronic conductivities, TECs, and variety of nonlocalized oxidation states. Moreover, the AO rock- surface oxygen catalytic activity, can be regulated by their salt layers may present oxygen vacancies or interstitial oxygen number of layers, types and concentrations of dopants, and – ions, resulting in oxygen ion conduction. As stated above, operating atmosphere.159 161 The partial substitution of the A these structural features of the R–P phases show unique trans- or/and B site with other cations oﬀers a facile way to develop port properties that possess high electronic and ionic conduc- their electrocatalytic performance and the introduction of – tivities as cathode materials for SOFCs.149 152 desired oxygen vacancies can result in the stabilization of

3.2.1 A2BO4-based K2NiF4 type (n = 1). A2BO4-based oxides unusual valence state of the B-site cations or the formation of – 162 as the R P family with the K2NiF4-type structure have been hybrid oxygen catalysts.

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In the research on Lan+1NinO3n+1 (n = 1, 2, or 3) for SOFC cathodes, in contrast to the R–P phases for n = 1 and 2,

La4Ni3O10−δ (n = 3) is more suitable as a cathode material for SOFCs because it has better electrochemical activities and compatibility with the electrolytes.163,164 These eﬀects can be primarily attributed to the increased concentration of the Ni– O–Ni bonds that are responsible for the enhancement in the electronic conduction and oxygen ion migration due to the gradual delocalization of the p-type electronic charge – carriers.165 167 Generally, Co-containing R–P oxides exhibit high catalytic activity for ORR/OER and high electronic conductivity, but these materials have relatively high TECs mainly due to the spin transitions of Co.168,169 Therefore, the substitution of Co by other 3d metals (Fe/Cu) is an eﬀective way to decrease the TEC. Moreover, the substitution of Co by Fe in

the new member of the n =2R–P phase of Eu2SrCo1.5Fe0.5O7 can provide relevant insights into the doping eﬀect on material performances.170,171 Fig. 20 (a) Crystal structure of NdBaInO4 in the [001] and [010] directions. (b) Apical-oxygen-facing motif of the R-P structure. (c) Edge- In other AO(ABO3)n intergrowth oxides where A is Sr or Ln, facing motif of the NdBaInO4 structure. B is Fe or Co, and n = 1, 2, or 3, the total electrical conductivity

and oxygen permeability of the AO(ABO3)n oxides increase with an increase in n as well as the Co content, but at the expense − 172–175 σ 4 of an increase in the TEC. The substitution of Co with Fe oxide ion migration. For NdBaInO4, total (850 °C) = 5.3 × 10 − − − Scm1 and σ (850 °C) = 3.1 × 10 5 Scm1. The diffusion in LaSr3Fe3−xCoxO10−δ (Ln: La) affords enhancement in con- ion ductivity because of the increase in the covalency of the (Fe, pathways of oxide ions in NdBaInO4 are also highly anisotropic – due to the intergrowth of the perovskite and rock-salt A–O Co) O bonds. Compared with the well-known La0.6Sr0.4CoO3−δ units, as evidenced by the bond valence method obtained perovskite cathode, the LaSr3Fe3−xCoxO10−δ cathode exhibits 180 better electrochemical performances with an important using the 3DBVSMAPPER program. The oxide-ion migration – ff advantage of significantly lower TEC. Moreover, the spreads in the A O layer, showing two-dimensional di usion along the b and c axes in NdBaInO . LaSr3Fe3−xCoxO10−δ cathode (n = 3) shows better performance 4 172 Further modifications of NdBaInO were carried out to than the (Sr, La)3(Fe, Co)2O7 (n =2R–P phase) cathodes. In 4 3+ particular, the Ln = Nd R–P oxides exhibit superior cathode improve the oxide ion conductivity. The substitutions of Nd / 3+ performances in SOFCs compared with the Ln = La or Gd In by divalent alkaline-earth metal or transition-metal ions

Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. 173 samples. Moreover, the substitution of smaller and less elec- in NdBaInO4 show enhanced oxide ion conductivity by creat- – ing mobile oxygen defects.181 183 In the case of alkaline-earth- tropositive Ca with Sr in NdSr3−xCaxFe1.5Co1.5O10 demonstrates an improvement in the cathode performance and a decrease in metal-doped NdBaInO4, molecular dynamics (MD) simulations 3+ the TEC value, which can be ascribed to the decreased lattice reveal that Ca is the most favorable dopant for doping Nd in 2+ distortion in the perovskite layer, thereby enhancing the oxide NdBaInO4; further, experimentally obtained Ca -doped ∼ ion conductivity. NdBaInO4 showed the enhanced oxide ion conductivity of 3× − − − 10 4 to 1.3 × 10 3 Scm1 within 600–800 °C.183,184 3+ 3.2 REs based new layered perovskite NdBaInO4-type MIECs Substituting the B-site In by other metal ions (Ce, Ga, Cr, Si, Mg, Zr, Nb, Ta, Ti, or Sn) in NdBaInO was also studied by The new perovskite-related structure of MIECs with the AA′BO4 4 182 3+ 4+ composition was first reported by Fujii et al.,176 where A and A′ Ishihara. Evidently, the substitution of In by Ti is 3+ 2+ ﬀ are relatively larger cations (Nd and Ba ), while B is a e ective for increasing the oxide ion conductivity. 3+ smaller cation (In ). As shown in Fig. 20a, the structure of Subsequently, the NdBaInO4-type-derived materials of BaLnInO (Ln: Sm, Y, Ho, Er, or Yb) were discovered and inves- NdBaInO4 consists of a rock-salt layer (Nd–O) and perovskite 4 tigated, including their lattice parameters and anisotropic layer (Nd2/8Ba6/8InO3), where the perovskite layer is formed by chemical expansion.185 the InO6 octahedron and it is surrounded by eight cations (two

Nd and six Ba). The structural feature of NdBaInO4 is that the – 3.3 Layered double perovskite structure edge of the InO6 octahedron faces the A O layer (Fig. 20c); however, in all the other known AA′BO4-perovskite-related Cation-ordered perovskite oxides provide channels for oxygen structures, the apical oxygen of the B-site octahedron faces the vacancies that can enhance the mobility of oxygen ions. A-site- A–O layers, such as the R–P type,177 Dion–Jacobson type,178 ordered double perovskites have the general formula AA′ 179 and Aurivillius type (Fig. 20b). The structural feature of the B2O6−δ formed by repeatedly layering A/A′ along the c axis,

A–O layer in NdBaInO4 is that it has the ability to accommo- where A is a RE ion or Y, A′ is an alkaline-earth ion, and B is a date interstitial oxygen ions and oxygen vacancies, facilitating transition-metal ion (e.g., Co, Fe, or Mn). The hierarchically

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Fig. 21 Crystal structures of A-site-ordered double perovskite with

the formula AA’B2O6 and B-site-ordered perovskite with the formula

A2BB’O6.

ordered structure leads to the deformation of the cubic struc- Fig. 22 Structure of LnBaCo2O5+δ (Ln: Nd) for various oxygen contents. ture toward other symmetries, and fast oxygen diﬀusion chan- (a) δ > 0.75 (δ = 1); (b) 0.5 < δ < 0.75 (δ = 0.69); (c) δ ≈ 0.5 (δ = 0.5); (d) δ = nels exist only in the RE plane. These fast oxygen channels can 0. (Adapted with permission from ref. 198. Copyright 2005 American − Chemical Society). be explained by the fact that the larger alkaline (Ba ) ions prefer 12-fold coordination, while the smaller lanthanide (Y3+) ions prefer lower coordination numbers.186 Therefore, the this phase shows a tendency to perturb this ordering vacancy localization of the oxygen vacancies within the RE layer leads and therefore leading to higher tetragonal symmetry with the to the highly anisotropic transport of oxygen ions.32,187 On the space group P4/mmm.197,198 other hand, B-site-ordered double perovskites have the general The particular Ln3+ RE dopant has a significant eﬀect on

formula A2BB′O6 with an alternate arrangement of corner- the phase structure/stability, oxygen content, electrical conduc- 200 sharing BO6 and B′O6 octahedra (Fig. 21). tivity, oxygen permeability, and cathode performance. In the

In recent years, A-site-ordered double perovskite oxides case of LnBaCo2O5+δ systems, a stable layered structure can be with alternate A- and A′-layered structures were extensively obtained when Ln is Pr, Nd, Sm, or Gd, while when Ln is Y investigated as promising materials for SOFC cathodes due to and La oxides with layered structures, they were in the meta- – 191 their high electronic conductivity above the metal insulator stable state. The crystal structure of LnBaCo2O5+δ adopt the transition temperature (∼27–127 °C) and excellent oxide ion tetragonal or orthorhombic symmetry and is strongly depen- – conductivity at intermediate temperatures.187 193 In particular, dent on the size of the Ln3+ ions and the oxygen content (5 + δ layered double perovskite oxides of LnBaCo2O5+δ (Ln: La, Pr, ). In GdBaCo2O5+δ systems, the oxygen content linearly Nd, Sm, Gd, or Y) have attracted tremendous attention due to increases with the size diﬀerence between the A-site cations

Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. 2+ 3+ the high conducting plane formed by the oxygen vacancies (r(Ba )–r(Ln )), indicating that the LnBaCo2O5+δ oxides show located near the REs, which facilitates oxygen ion migration a general decrease in the oxygen vacancies with a decrease in compared to that with the A-site-disordered perovskites.194,195 Ln3+ ions from Ln = La to Gd and finally Y.200 As a result, in

A wide range of oxygen vacancies can be accommodated LnBaCo2O5+δ systems, Ln = La (δ ≈ 0) adopts the cubic sym- – δ δ δ within the Ln O layers of LnBaCo2O5+δ (0 < < 1), and the metry, Ln = Pr ( = 0.85) and Nd ( = 0.78) adopt the tetragonal ordering of the oxygen vacancies occurs at about δ = symmetry, Ln = Sm (δ = 0.65) and Gd (δ = 0.61) adopt the 187,196–198 0.5. Recent studies on LnBaMn2O5+δ systems have orthorhombic symmetry, and Ln = Y (δ = 0.41) adopt the tetra- discussed the direct investigation of the oxygen vacancies pre- gonal symmetry. In addition, the double ordered perovskites 199 ferentially localized on the Ln–O planes. In fact, the oxygen of NdBaCo2O5 and NdBaCo2O6 were obtained under reduced content δ dominates the normal valence of the Co ions, which and oxidized conditions, respectively.201 It was also found that 3+ 5+ δ δ ≈ varies from 3.5+ (50% Co and 50% Co ) for = 1 to 2.5+ the oxygen content in PrBaCo2O5+δ could be tailored from (50% Co3+ and 50% Co2+) for δ =0.196 These structural features 0.2 to δ ≈ 0.0 under diﬀerent heat treatments.198 These results

result in the coexistence of Co ions in the octahedral (CoO6) show that the LnBaCo2O5+δ systems can reversibly catch and ff ff and pyramidal (CoO5) coordination environments, as shown in release oxygen under di erent temperatures and di erent Fig. 22 (δ = 0.5). NPD data can provide extraordinary details of partial oxygen pressures. the different arrangements associated with the anion sublat- Apart from the oxygen content, the crystal structure of

tice of the oxides. In this sense, the oxygen vacancy ordering LnBaCo2O5+δ at higher temperatures is also dominated by the and other ordering eﬀects leading to oxygen displacements spin transitions of cobalt ions as well as the thermally induced can be determined by the Rietveld refinements of the NPD disordering of the oxygen vacancies. The spin-state transition δ 3+ data. As a result, the oxygen-vacancy-ordered LnBaCo2O5+δ ( = of Co has been reported in GdBaCo2O5.5 in which the low- III 6 0 0.5) adopts lower orthorhombic symmetry with the space spin-state Co (t2ge2h) ions in the CoO6 octahedra become δ 4 2 group Pmmm. In contrast to LnBaCo2O5+δ ( = 0.5), further excited to the high-spin-state (t2ge2g) ions at specific transition decreasing or increasing the amount of oxygen vacancies in temperatures (∼77 °C).196 Since this transition from the low- to

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high-spin state is accompanied by a considerable improvement in the electronic conductivity, it is termed as metal–insulator (MI) transition (from insulator to metallic behavior).196

Generally, the MI transition in LnBaCo2O5+δ occurs at δ = 0.5, III 6 0 where the low-spin-state Co (t2ge2g) ions in the CoO6 octahe- III 5 1 dra and intermediate-spin-state Co (t2ge2g) ions in CoO5 square pyramids are in a well-ordered arrangement along the b III 6 0 axis. After the MI transition, the spin state of Co (t2ge2g)/ III 5 1 III 4 2 III 5 1 196,202 Co (t2ge2g) changes to Co (t2ge2g)/Co (t2ge2g). Above the MI transition temperature at around 500 °C, it was found that

in the PrBaCo2O5.48 phase, the ordered oxygen vacancies along the b axis get rearranged and randomly distributed in the Pr-O plane. Correspondingly, the structural symmetry gets increased from orthorhombic (Pmmm) to tetragonal (P4/mmm) due to the

vanishing of the ordered arrangement of the CoO6 octahedra Fig. 23 Total conductivity of the LnBaCo2O5+δ (Ln: La, Pr, Nd, Sm, Gd, 202 – and CoO5 square pyramids. The order disorder transition or Y) samples. (Adapted with permission from ref. 186. Copyright 2015 has also been found in other LnBaCo2O5+δ oxides (Ln: Nd, Sm, Royal Society Chemistry.) or Gd).203,204 With respect to the transition temperature, the

order–disorder transition of the LnBaCo2O5+δ oxide (Ln: Pr) −3 occurs from 500 °C at pO2 =10 atm down to 350 °C at pO2 = layered perovskite materials can be used as the potential − 10 4 atm, revealing that the transition temperature depends cathode materials for SOFCs. Kim et al. reported that layered 205 on the oxygen partial pressure. Further studies on the SmBa0.5Sr0.5Co2O5+δ perovskite materials comprising order–disorder structural transitions and the oxygen transport Ce0.9Gd0.1O2−δ electrolyte exhibit advanced electrical pro- properties show that ordered oxygen vacancies suppress perties, suggesting that they are highly promising cathode oxygen diﬀusion within the lattice, while the disordered materials for SOFCs.210 Owing to the smaller ionic radius of oxygen vacancies facilitate the oxygen bulk diﬀusion prop- Sr2+ (r = 1.44 Å; coordination number: 12) compared with Ba2+ 204 211 erty. Additionally, this structural transition in LnBaCo2O5+δ (r = 1.60 Å; coordination number: 12), the average ionic 2+ oxides induces negligible lattice expansion since the a axis radius of (Ba1−xSrx) decreases with an increase in the Sr con-

expands while the c axis simultaneously contracts, showing centration in Ln(Ba1−xSrx)Co2O5+δ; consequently, the size ﬀ 3+ 2+ 2+ – that LnBaCo2O5+δ oxides can be applied as cathodes for di erence between Ln and (Ba1−xSrx) (r((Ba1−xSrx) ) SOFCs. r(Ln3+)) decreases. On the basis of the relationship between 200 As stated above, the crystal structures of LnBaCo2O5+δ the A-site cation and oxygen content stated in the literature, oxides and their structure–performance relationships have it is evident that the oxygen content decreases with the Sr Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. been comprehensively studied. Therefore, the optimization of content. Finally, since the ionic radius of Sr2+ is smaller than 2+ 3+ the cathode performance of LnBaCo2O5+δ can be achieved by that of Ba and oxidation of Co ions with the oxygen modifying their compositions via A- and/or B-site content decreases, the substitution of Ba by Sr results in a

substitutions. decrease in the unit cell of Ln(Ba1−xSrx)Co2O5+δ. 3+ Generally, the size of the A-site cation strongly aﬀects the LnBaCo2O5+δ oxides in which Ln are smaller REs (Ln < 3+ crystal structure and oxygen content of LnBaCo2O5+δ. In the Gd ) show phase instability at elevated temperatures. The

study on LnBaCo2O5+δ oxides as the cathode materials for phase decomposition of HoBaCoO5+δ and YBaCo2O5+δ at SOFCs, both conductivity and catalytic activity for ORR/OER 800 °C was observed by an in situ XRD study. In this regard, 3+ decrease with a decrease in the Ln size from La to Gd, as the phase instability of LnBaCo2O5+δ (Ln: Ho or Y) could be

shown in the total conductivities of the LnBaCo2O5+δ (Ln: La, overcome by an appropriate amount of Sr substitution for Ba. Pr, Nd, Sm, Gd, or Y) samples (Fig. 23).206 Therefore, the physi- Their structural rigidity could be enhanced because the cal and/or chemical properties can be tuned by modulating decrease in the Ln–O bonds resulted from a decrease in the the size of the A-site cation. Some researchers have reported oxygen vacancies in the Ln–O layer.212

that the substitution of Ba with Sr in LnBaCo2O5+δ can Co-Based perovskites have higher TEC values as a result of improve the conductivity and catalytic activity for the ORR of oxygen loss and spin-state transition of Co at increased temp- layered perovskite oxides.207,208 The chemical stability between eratures, which hampers their use in practical SOFCs com- the electrode and electrolytes as well as oxygen transport can pared with other standard electrode materials.186,213,214 It has

be enhanced by the substitution of Ba with Sr in GdBaCo2O5+δ been reported that LnBaCo2O5+δ oxides show a trend of cathode materials.200 It was also found that transitions from lowered TEC with a smaller size of Ln, but their TEC values are the orthorhombic to tetragonal structures induced by the sub- still high compared with traditional electrolytes, such as YSZ,

stitution of Sr for Ba in YBa1−xSrxCo2O5+δ (x = 0.5) systems GDC, and LSGM. This is the key issue limiting their appli- resulted in a considerable 32-fold enhancement in the electri- cations as cathodes for SOFCs.215 Therefore, many eﬀorts have 209 cal conductivity. These studies indicate that Sr-doped been devoted toward obtaining LnBaCo2O5+δ cathodes with

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superior thermal and electrochemical properties obtained via certain challenges for their practical applications, which are the partial substitution of Co3+ with other transition-metal required to be further investigated, such as chemical instabil-

ions. First, the oxygen content in LnBaCo2−xMxO5+δ depends ity under operating conditions, low oxide ion conductivity on the chemical nature of the transition-metal dopants. The under intermediate temperatures (600–800 °C), long-term resulting oxygen contents dominate the ordering or disorder- instability due to unwanted reaction and corrosion, bad com- ing of the oxygen vacancies in the corresponding patibility between the diﬀerent components of the cell, high

LnBaCo2−xMxO5+δ phase. Second, it has been reported that the cost of materials, and so on. spin transition of cobalt ions is responsible for the higher TEC Despite these challenges, current research into new oxide values in cobalt-based perovskite oxides.213,214 The B-site sub- ion conductors is fairly narrowly focused on the types of struc-

stitution of Co by Fe, Ni, or Cu in LnBaCo2−xMxO5+δ afford tures covered in this review, which also have the abovemen- effectively decreased TEC values, which can be attributed to a tioned problems. Therefore, research on alternative structures – decrease in the Co content.216 219 Generally, the substitution of oxide ion conductors hold promise for overcoming these of Co with other transition-metal ions (M: Ni, Fe, Mn, or Cu) problems. Furthermore, a combination of advanced character- offers promising MIEC properties, but it results in decreased ization techniques and dynamic simulations is needed to – electronic conductivity.216,217,220 222 Among these various tran- further study the relationships among the structure, oxide ion sition-metal-ion-based doping strategies, although the substi- conductivity, and electrocatalytic activity. In particular, in situ

tution of Co by Ni in NdBaCo2−xMxO5+δ decreases the elec- measurement techniques, such as in situ NMR, EXAFS, NPD, tronic conductivity, Co-doped NdBaCo2−xMxO5+δ can still show synchrotron powder diﬀraction, and TEM can be used to inves- − higher electronic conductivity of >300 S cm 1 for up to 900 °C, tigate the evolution of local defects and average structures, as which is suﬃcient for a cathode for SOFCs.216 well as the experimental performance, thereby providing newer insights and guidance for the development of new oxide ion conductors. 4. Conclusions and future prospects

In summary, this review has shown that RE-based oxide ion Conﬂicts of interest conductors with various structures ranging from traditional fluorites, perovskites, and pyrochlores to the popular isolated There are no conflicts to declare. polyhedral structures and layered perovskite structures have promising applications in the electrolyte and cathode fields. In addition, REs and the relationships between their structures Acknowledgements and oxide ion and/or electronic conducting properties have This work was financially supported by the National Basic been systematically and comprehensively consolidated in this Research Program of China (No. 2016YFA0301004) and the review. REs play many crucial roles in the design and develop-

Published on 20 August 2020. Downloaded 10/1/2021 3:21:50 AM. National Natural Science Foundation of China (No. 21527803, ment of oxide ion conductors, including the use of REs in the 21871009, and 21621061). Prof. X. J. Kuang acknowledges main constituents of the host structure, use of specific features funding from the Guangxi Natural Science Foundation (No. of the ionic radius of REs to fine-tune the local defect struc- 2019GXNSFGA245006). Great sincere thanks to Prof. Xiping tural environment to improve the oxide ion conductivity, and Jing (Peking University) and Prof. Jungu Xu (Guilin University using variable valence of REs to tune the mixed oxide ionic of Technology) for his highly thoughtful and constructive sug- and electronic conductivities. gestions on this review. Many successful efforts have been made toward the development of new oxide ion conductors and the improvement of known oxide ion conductors with superior performances at References higher operating temperatures. For example, the development of some ceramic techniques could efficiently enhance the per- 1 T. I. Kostelnik and C. Orvig, Radioactive Main Group and formance of SOFCs. Among these ceramic techniques, the Rare Earth Metals for Imaging and Therapy, Chem. Rev., addition of sintering acids could reduce the limited barriers 2019, 119, 902–956. because of grain boundaries by improving the ceramic density, 2 V. Zepf, Rare Earth Elements: What and Where They Are, as well as effectively avoiding material stability and corrosion in Rare Earth Elements, 2013, pp. 11–39. problems that occur in SOFCs. Moreover, a variety of depo- 3 H. A. Hoppe, Recent developments in the field of in- sition techniques have also been employed to fabricate thin- organic phosphors, Angew. Chem., Int. Ed., 2009, 48, 3572– film or nanometer-sized ceramic electrolytes, including pulsed 3582. laser deposition (PLD), chemical vapor deposition (CVD), 4 X. H. Li, L. Zhou, J. Y. Hong, S. M. He, X. P. Jing, electrochemical vapor deposition (EVD), sol–gel deposition, M. D. Dramićanin, J. X. Shi and M. M. Wu, Structural and sputtering among others, which can be used to efficiently modulation induced intensity enhancement of full color 3+ reduce the area specific resistance (ASR), resulting in higher spectra: a case of Ba3ZnTa2−xNbxO9:Eu phosphors, – power densities for SOFCs.223 225 However, there are still J. Mater. Chem. C, 2020, 8, 6715–6723.

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