Controlling the Oxygen Electrocatalysis on Perovskite and Layered Oxide Thin Films for Solid Oxide Fuel Cell Cathodes

Abstract

Achieving the fast oxygen reduction reaction (ORR) kinetics at the cathode of solid oxide fuel cells (SOFCs) is indispensable to enhance the efficiency of SOFCs at intermediate temperatures. Mixed ionic and electronic conducting (MIEC) oxides such as ABO₃ perovskites and Ruddlesden-Popper (RP) oxides (A₂BO₄) have been widely used as promising cathode materials owing to their attractive physicochemical properties. In particular, oxides in forms of thin films and heterostructures have enabled significant enhancement in the ORR activity. Therefore, we aim to give a comprehensive overview on the recent development of thin film cathodes of SOFCs. We discuss important advances in ABO₃ and RP oxide thin film cathodes for SOFCs. Our attention is also paid to the influence of oxide heterostructure interfaces on the ORR activity of SOFC cathodes.

1. Introduction

Solid oxide fuel cells (SOFCs) have shown great promise to contribute to the ultimate aim of environmentally friendly, efficient energy production and conversion [1,2,3,4]. However, overcoming the slow kinetics of the oxygen reduction reaction (ORR) at the cathode is essential to fulfill the increasing demand for developing intermediate temperature SOFCs [5]. At high temperatures such as 1000 °C, La_1−xSr_xMnO₃ (LSM₁₁₃) is commonly used as a cathode for SOFCs owing to their good stability and high electronic conductivity, but simultaneously has low ionic conductivity and a narrow triple-phase boundary (TPB) region for the cathode reaction [1]. Therefore, new cathode materials with faster ORR kinetics are required to facilitate the development of intermediate temperature SOFCs.

Cubic perovskite based mixed ionic and electronic conductors (MIECs) such as ABO₃ oxides [6,7,8,9] and layered perovskite based MIECs such as Ruddlesden-Popper (RP) oxides [10,11,12,13,14] have been extensively studied to promote oxygen electrocatalysis at intermediate temperatures such as 600 °C. Compared to LSM₁₁₃, these MIECs possess considerably higher oxygen surface exchange kinetics, which enables to extend the electrochemically active region to the entire oxide surface from the electrode/electrolyte interface. In addition, they also have higher ionic conductivity than LSM₁₁₃, and thus are promising cathode materials for intermediate temperature SOFCs. The oxygen diffusion and surface exchange coefficients for ABO₃ and RP oxides are summarized in Table 1.

In ABO₃ and RP oxides, the oxygen ion diffusion and oxygen surface exchange kinetics can be significantly influenced by cation substitution as substitutions on the A or B sites of MIECs are known to affect the oxygen stoichiometry. For example, substituting Sr for lanthanum on the A-site of La_1−xSr_xCoO_3−δ (LSC₁₁₃) results in an increase in oxygen vacancies, enhancing the oxygen transport and surface exchange kinetics [9,15,16,17,18,19]. In addition, doping with different-size cations in ABO₃ perovskites can also facilitate oxygen migration owing to a reduction in the ion migration energy [20]. The effect of Sr substitution on the activation energy for oxygen ion diffusion and oxygen surface exchange kinetics in LSC₁₁₃, reviewed by Berenov et al. [21], is that the activation energies for oxygen diffusion and surface exchange kinetics decrease with increasing Sr content owing to the reduced vacancy formation energy. Cater et al. [22] compared the oxygen diffusivity between Sr-doped cobaltites and Sr-doped manganites, concluding that B-site cations with a lower valence state increase in oxygen diffusivity. In the case of RP oxides, however, the effect of cation substitutions on the ORR kinetics is poorly understood due to difficulties in synthesizing RP oxides with a wide range of cation substitutions.

Table 1. Oxygen self-diffusion (D*) and surface exchange coefficients (k*) for solid oxide fuel cells (SOFC) cathode materials.

Material	D*/cm2∙s−1 @ 600 °C	k*/cm∙s−1 @ 600 °C	Reference
La0.8Sr0.2MnO3−δ	10−17	1.12 × 10−10	[8]
La0.5Sr0.5MnO3−δ	4.22 × 10−18	10−10	[8]
La0.8Sr0.2CoO3−δ	1.69 × 10−13	1.38 × 10−8	[8]
La0.6Sr0.4CoO3−δ	2.86 × 10−9	9.09 × 10−8	[21]
La0.5Sr0.5CoO3−δ	2.51 × 10−10	3.29 × 10−7	[8]
La0.6Sr0.4Co0.2Fe0.8O3−δ	5.83 × 10−10	2.35 × 10−7	[23]
Ba0.5Sr0.5Co0.8Fe0.2O3−δ	1.28 × 10−7	5.21 × 10−5	[24]
La2NiO4+δ	1.35 × 10−8	1.02 × 10−6	[13]
La1.9Sr0.1NiO4+δ	4.66 × 10−9	3.56 × 10−8	[13]
La1.8Sr0.2NiO4+δ	2.02 × 10−10	3.51 × 10−9	[13]
La2CuO4+δ	2.76 × 10−9	5.79 × 10−7	[11]
La2Ni0.9Co0.1O4+δ	1.07 × 10−8	6.52 × 10−7	[14]
La2Ni0.9Co0.1O4+δ	1.34 × 10−8	7.89 × 10−7	[14]
La2CoO4+δ	2.98 × 10−8	3.91 × 10−6	[25]

All materials are polycrystalline.

Table 1. Oxygen self-diffusion (D*) and surface exchange coefficients (k*) for solid oxide fuel cells (SOFC) cathode materials.

Material	D*/cm2∙s−1 @ 600 °C	k*/cm∙s−1 @ 600 °C	Reference
La0.8Sr0.2MnO3−δ	10−17	1.12 × 10−10	[8]
La0.5Sr0.5MnO3−δ	4.22 × 10−18	10−10	[8]
La0.8Sr0.2CoO3−δ	1.69 × 10−13	1.38 × 10−8	[8]
La0.6Sr0.4CoO3−δ	2.86 × 10−9	9.09 × 10−8	[21]
La0.5Sr0.5CoO3−δ	2.51 × 10−10	3.29 × 10−7	[8]
La0.6Sr0.4Co0.2Fe0.8O3−δ	5.83 × 10−10	2.35 × 10−7	[23]
Ba0.5Sr0.5Co0.8Fe0.2O3−δ	1.28 × 10−7	5.21 × 10−5	[24]
La2NiO4+δ	1.35 × 10−8	1.02 × 10−6	[13]
La1.9Sr0.1NiO4+δ	4.66 × 10−9	3.56 × 10−8	[13]
La1.8Sr0.2NiO4+δ	2.02 × 10−10	3.51 × 10−9	[13]
La2CuO4+δ	2.76 × 10−9	5.79 × 10−7	[11]
La2Ni0.9Co0.1O4+δ	1.07 × 10−8	6.52 × 10−7	[14]
La2Ni0.9Co0.1O4+δ	1.34 × 10−8	7.89 × 10−7	[14]
La2CoO4+δ	2.98 × 10−8	3.91 × 10−6	[25]

All materials are polycrystalline.

Recently, a remarkable enhancement of the ORR kinetics has been achieved by using epitaxial oxide thin films, which have yielded many intriguing physical and chemical properties that cannot be achieved in bulk counterparts. Epitaxial thin films can also allow the understanding of the fundamental physicochemical properties of oxide materials, and thus have been used as model systems to develop design principles for enhancing the ORR kinetics [26,27,28]. In particular, the use of oxide thin films has the following advantages for manipulating and evaluating electrocatalytic properties [29]. First, epitaxial thin films can be grown with different orientations, which can allow evaluating anisotropic behaviors (appropriate selection of the single crystal substrates and optimization of the deposition conditions). Second, the termination plane of each surface structure can be controlled using epitaxial thin films, and thus the different surface exchange kinetics can be elucidated. Third, dense thin films can allow studying oxygen transport kinetics with no influence of non-kinetic issues, such as particle morphology and connectivity, and porosity and tortuosity typical in porous electrodes. Lastly, the influence of the strain induced by the substrate on the electrocatalytic properties can be investigated. Furthermore, the use of oxide multilayer heterostructures has opened attractive opportunities in the design of new cathode material systems as oxide interfaces can drastically facilitate the ORR kinetics. So far, oxide heterostructure interfaces have been utilized to achieve unique physical and chemical properties such as phase transition [30], electronic reconstruction [31], magnetic domain rearrangement [32], and anomalous oxygen electrocatalysis [33], which are unattainable in the bulk oxides.

This review aims to provide a synopsis of the major developments and achievements in thin film MIEC cathodes of SOFCs, which can help facilitate the development of intermediate temperature SOFCs. We first discuss the ORR mechanisms on the cathode of SOFCs. We then cover in detail recent advances in ABO₃ and RP oxide thin film cathodes focusing on cobaltites, ferrites, nickelates, and cuprates. The effect of oxide heterostructure interfaces on the ORR kinetics and electrochemical stability is also overviewed. Finally, we present conclusions, perspectives and future developments. As this review deals with mostly epitaxial thin films, we do not focus on the kinetics of oxygen transport. Readers can be referred to relevant comprehensive reviews of oxygen transport in ABO₃ and RP oxide [12,34,35].

2. Oxygen Reduction in SOFCs

2.1. Oxygen Reduction Reaction Steps and Pathways in SOFC Cathodes

Among the entity of charge transport and transfer processes in an operating SOFC, the activation loss η associated with the ORR at the cathode is a main contribution to the overall voltage drop [36]. In MIEC cathodes, ABO₃ oxides are generally known as oxygen-deficient perovskites because oxygen vacancies are their dominant anion defect while RP oxides can be both oxygen-deficient and oxygen-excess, depending upon their majority oxygen defects. In the case of oxygen-deficient RP oxides, oxygen nonstoichiometry (δ) arises from oxygen vacancies, whereas oxygen interstitials result in oxygen hyperstochiometric RP oxides. Therefore, two ORR reactions may occur in MIEC cathodes depending on majority oxygen defects [13,37,38]. The overall ORR kinetics for oxygen-excess RP oxides can be expressed as:

O₂(g) + 4e⁻ → 2O_i″

(1)

where e⁻ is an electron and O_i″ is an oxygen interstitial. Unlike oxygen-excess RP oxides, the following ORR reaction can occur in ABO₃ and oxygen deficient RP oxides.

O₂(g) + 4e⁻ + 2V_o^∙∙ → 2O_o^x

(2)

where V_o^∙∙ is an oxygen vacancy and O_o^x represents an oxygen ion in the cathode or electrolyte lattice.

However, ORR reaction is composed of numerous elementary reaction steps. These steps can be surface or bulk related reactions such as adsorption, dissociation, electronation, and diffusion (Figure 1). As shown in Figure 1, pure electronic conductors such as Pt and LSM₁₁₃ have the surface pathway for the possible ORR reaction steps, which are adsorption and dissociation of molecular oxygen on the cathode surface, surface diffusion of adsorbed oxygen atoms to the TPB, formation of oxide ions by electron transfer and incorporation into the electrolyte. Previous studies [39,40,41,42,43,44] identified using polycrystalline LSM₁₁₃ samples that the ORR occurs only at the one-dimensional TPB, where cathode, electrolyte, and gas phase are in contact in pure electronic conductors.

In contrast, the surface pathway is complemented by a bulk pathway in MIEC cathodes, where adsorbed oxygen species are reduced at the cathode surface and transported to the electrolyte by solid-state diffusion through the cathode interior regardless of the dominant oxygen defects. Consequently, the active area for the ORR on MIEC cathodes spans over the entire cathode surface. The overall rate of the multitude of reaction steps in MIEC cathodes is determined by a single rate determining step. Then, all remaining steps can be considered to be close to equilibrium so that the voltage loss associated with the rate-determining step constitutes the cathodic activation loss η.

2.2. Oxygen Diffusion and Surface Exchange Kinetics in MIEC Cathodes

The total electrical conductivity σ_tot of perovskite oxides can be expressed as:

σ_tot = σ_el + σ_ion

(3)

where σ_el and σ_ion are electronic conductivity and ionic conductivity, respectively. Depending on the relative amount of σ_el and σ_ion, perovskite oxides are classified into pure electronic conductors or MIECs.

Oxide cathode materials are typically extrinsic semiconductors, where σ_el results from the movement of holes or excited electrons through the surrounding lattice. In addition, MIEC cathodes conduct O^2- ions via oxygen vacancies or interstitials at a significant rate. The mobility of O²⁻ ions in MIEC cathodes is correlated with the diffusion coefficient (D*) for O²⁻. In addition, the oxygen surface exchange coefficient (k*) is also a key parameter for determining the rate of oxygen transport through a MIEC cathode material. Therefore, D* and k* play a determining role in the ORR kinetics on MIEC cathodes.

In the case of thin film MIEC cathodes, the ORR kinetics can be limited by either oxygen diffusion or oxygen surface exchange kinetics depending on the critical film thickness, t_crit, which can be defined as D*/k* [45]. The ORR kinetics of thick film cathodes are mainly governed by the bulk transport oxygen whereas in thin film cathodes the ORR kinetics can be governed by the surface exchange of oxygen. In general, t_crit is in the range of 100 μm for many perovskite oxides [46]. Therefore, the ORR kinetics of thin film MIEC cathodes can be enhanced by an increase in the oxygen surface exchange kinetics.

Electrochemical impedance spectroscopy (EIS) measurements are commonly used to analyze the kinetics of oxygen surface exchange on thin film cathodes [6,26,28,47]. Typical EIS tests are conducted in the frequency range from ~10 MHz to ~100 μHz using an AC voltage amplitude with zero DC bias in a wide range of temperatures and oxygen partial pressures. Without dc bias, the Nyquist plot represents three well-separated features analyzed by an equivalent circuit [48], which are denoted as high frequency (HF), medium frequency (MF) and low frequency (LF) feature. The HF feature (10⁴–10⁵ Hz) is associated with the transport of O²⁻ ions through an electrolyte, while the MF features (10³–10⁴ Hz) is associated with the ionic transfer resistance at the electrode/electrolyte interface. In addition, the magnitude of the capacitance at the MF feature is relatively small (~10⁻⁶ F) compared to the LF feature (~10⁻³ F), which is the main resistive process representing the surface exchange reaction [6,49,50,51,52]. In the case of thin film cathodes, the magnitude of capacitance is due to the oxygen content change in the films, and therefore the oxygen surface reaction on thin film electrodes generally corresponds with the LF feature. The electrical oxygen surface exchange coefficient (k^q) can be determined using the expression [53,54],

k^q = R∙T/4F²∙R_ORR∙A_electrode∙c_o

(4)

where R is the universal gas constant (8.314 J·mol⁻¹·K⁻¹), T is the absolute temperature, F is the Faraday’s constant (96,500 C·mol⁻¹), R_ORR is the resistance obtained from the LF feature, A_electrode is the area of the electrode, and c_o is the lattice oxygen concentration in thin film cathodes.

3. Thin Film Cathodes

As discussed earlier, the use of epitaxial thin films offers many advantages for controlling the ORR kinetics in SOFC cathodes. Strain engineering is one of the most effective ways to tailor the ORR kinetics of perovskite and layered oxide thin films. Many studies have reported that epitaxial strain strongly influences bulk and surface properties of oxide materials including the electronic structure [55], oxygen transport [56,57,58,59], oxygen defect formation [60], and oxygen surface exchange kinetics [61,62], ultimately enhancing the high temperature oxygen electrocatalysis in ABO₃ and RP oxides [63,64]. In addition, the viability of controlling the crystallographic orientation of epitaxial thin films has enabled investigation of the anisotropic nature of oxygen transport and kinetic properties in RP oxides [61,65]. Furthermore, oxide interface engineering has brought a new concept to design highly active and stable cathode materials for intermediate SOFCs [66,67,68]. The growth of high quality epitaxial thin films is made possible using several deposition methods such as pulsed laser deposition (PLD), molecular beam epitaxy (MBE), atomic layer deposition (ALD), and sputtering. However, it is not the aim of this review to describe in detail those techniques; an extended review on the different techniques can be found elsewhere [69,70,71,72], In the following subsections, we focus on how these factors have brought advances in the oxygen surface exchange kinetics in ABO₃ and RP oxide thin films.

3.1. ABO₃ Oxide Thin Films

ABO₃ perovskite oxides are conventional materials for electrochemical applications, in particular, SOFCs owing to their good reactivity, flexibility in terms of oxygen stoichiometry, and viability of A- and B-site cation substitutions. In most cases, the A-site cation is a combination of rare and alkaline earths for SOFC cathodes. However, the B-site cation is generally a transition metal, such as Mn, Fe, and Co. The atomic structure of perovskites consists in a 3D network of BO₆ octahedra connected by apex, which are maintained by a cubic lattice of A atoms of coordination 12, as shown in Figure 2. The symmetries of ABO₃ perovskites can theoretically be directly determined from the ionic radii of A and B cations, which govern strains inside the materials, since the size mismatch is responsible for the distinct stacking.

In the early stage, LaCoO₃ (LCO₁₁₃) [36,73] and LaMnO₃ (LMO₁₁₃) [36,74] based ABO₃ perovskite oxides were utilized as a cathode material for SOFCs. These stoichiometric compositions, however, were found unsuitable in terms of electrochemical performance for a cathode. Usually, the increase of ionic conductivities is more influenced by Sr concentration at the A-site while the increase of the electronic conductivities is more influenced by Fe and Co concentration at the B-site. The substitution of Sr²⁺ for La³⁺ can enhance the oxygen ion transport kinetics due to an increase in the oxygen vacancy concentration in the perovskite structure [17]. Therefore, high electrochemical performance at the cathode could be achieved relative to undoped ABO₃ perovskite oxides. In this sense, LSC₁₁₃ [17,21,27,28,46,64,73,75,76] and La_1−xSr_xCo_1−yF_yO_3−δ (LSCF₁₁₃) [22,45,47,66,77,78,79,80,81,82,83,84,85,86] perovskite oxides are commonly used as these oxides are known to show oxygen deficiency and the oxygen nonstoichiometry (δ) affects the electrochemical properties, conductivity and lattice expansion of the materials.

3.1.1. Cobaltites

Epitaxial strain induced by lattice mismatch between a thin film and a substrate can effectively control the oxygen migration and oxygen defect formation in ABO₃ perovskite oxides [64,87,88,89,90,91,92,93]. Using density functional theory (DFT) calculations, Han et al. [94] demonstrated that tensile strain increased the one-dimensional mobility of oxygen vacancy and adsorbed atomic oxygens migrations on the LCO₁₁₃ surface, which can enhance the ORR kinetics. According to the authors, the maximum tensile strain for the enhanced ORR kinetics is 3%, which has a lower oxygen vacancy formation energy by 0.32 eV compared to the strain-free surface. Later, Mayeshiba and Morgan [90] predicted the oxygen migration barriers in LaMO₃ (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Ga) perovskite oxides under tensile strain via DFT calculations. They showed that tensile strain can significantly reduce the oxygen migration barriers, resulting in enhanced oxygen diffusion (one order of magnitude) at 500 °C compared with the diffusion under unstrained conditions. laO’ et al. [28] experimentally showed that tensile strained epitaxial La_0.8Sr_0.2CoO_3−δ (LSC82) films exhibit significantly increased ORR activity compared to LSC82 bulk. According to the authors, the oxygen surface exchange coefficients (k^q) of epitaxial LSC82 films are about two orders of magnitude higher than those of LSC82 bulk, which is attributed to the increased oxygen vacancy concentration in LSC82 thin film relative to LSC bulk. Using secondary ion mass spectroscopy (SIMS) measurements, Kubicek et al. [64] also investigated the effect of strain on the oxygen surface exchange kinetics of epitaxial LSC82 films. In this work, the epitaxial LSC82 films were synthesized on SrTiO₃ (STO) and LaAlO₃ (LAO) substrates to introduce tensile and compressive strain, respectively, into the LSC82 films, where the oxygen surface exchange kinetics was found to be faster in tensile strained films. The authors postulated that the lower formation energy of oxygen vacancy by tensile strain leads to the enhanced ORR kinetics. However, Crumlin et al. [27] did not observe the strong dependency of the oxygen surface exchange kinetics of epitaxial La_0.6Sr_0.4CoO_3−δ (LSC64) films on strain. The authors attempted to control the degree of tensile strain in LSC64 films with various film thicknesses but the k^q values of LSC64 films were thickness-independent and comparable with those of bulk LSC64 (k^q can be approximated as k*). Similarly, epitaxial LCO thin films also showed little dependence of their surface exchange kinetics as a function of film thickness although the films exhibited higher ORR kinetics than LCO bulk [95]. In contrast, a clear trend of the thickness-dependent ORR activity in epitaxial LSC64 thin films was recently demonstrated by Lee et al. [96] (Figure 3a). According to this work, the oxygen surface exchange kinetics of LSC64 films can be enhanced as decreasing the film thickness until 50 nm, where strain relaxation occurs. The authors proposed that the increased ORR activity results from the increased oxygen vacancies in epitaxial LSC64 films due to the large tensile strain of high-quality films. Employing in situ impedance spectroscopy during thin film growth, Rupp et al. [97] also showed an enhancement in the oxygen surface exchange kinetics of tensile-strained epitaxial LSC64 films on (001)-oriented La_0.95Sr_0.05Ga_0.95Mg_0.05O_3−δ substrates compared to unstrained films on yttria-stabilized zirconia (YSZ) substrates. Similar to the results by Lee et al. [96], the authors also found the same trend of enhanced oxygen surface exchange kinetics as film thickness decreases in epitaxial LSC64 films but they interpreted that this trend may be attributed to the capability of in-situ measurement as the films never exposed to any thermal cycling such as cooling from PLD preparation temperature or heating for impedance measurements. However, regardless of strain, epitaxial LSC₁₁₃ thin films were found to have higher k^q values compared to their bulk counterpart (Figure 3b).

It is known that lattice strain can tune the electronic structure and charge transfer in ABO₃ oxides [98,99]. Using scanning tunneling spectroscopy and density functional theory calculations, Cai et al. [55] showed that the surface electronic state depends on the strain state of the epitaxial LSC82 thin films. In this work, STO and LAO substrates were utilized to induce tensile and compressive strains, respectively, into the LSC82 films, where tensile-strained LSC films exhibited a more increased electronic density of states (DOS) near the Fermi level compared to compressive strained LSC films. It was hypothesized that the increased DOS and the transition to the metal-like state are attributed to the easier formation of oxygen vacancies on tensile strained LSC films, which was supported by DFT calculations.

A recent study for a wide range of bulk perovskites demonstrated that the position of the oxygen 2p-band center with respect to the Fermi level is linearly correlated with the activation barrier of oxygen surface exchange, acting as a descriptor for the oxygen surface exchange kinetics [100]. According to this work, the formation energy of bulk oxygen vacancies decreases with increasing the oxygen 2p band center relative to the Fermi level, indicating the enhanced oxygen reduction reaction (Figure 4a). Later, Lee et al. [96] demonstrated that the oxygen 2p-band center relative to the Fermi level can be linearly correlated with strain state, where tensile strain results in the upshift of the oxygen 2p-band center (Figure 4b). According to the authors, As the oxygen 2p-band center also has a linear relationship with the vacancy formation energy in perovskite oxides [100], and thus the tensile strain is expected to lower the energy of vacancy formation due to an increase in the oxygen 2p-band center in the LSC₁₁₃ films. Hong et al. [95] showed that in epitaxial LCO₁₁₃ films, strain can be used to reduce the spin transition temperature and promote the occupation of higher spin states that weaken the Co-O bond, which results in an increase in the oxygen surface exchange kinetics. The authors also pointed out that the decreased Co-O bond strength due to the increased fractional occupancy of high-spin states can lead to the oxygen 2p-band center upshift relative to the Fermi energy. Recently, Zhu et al. [101] measured the ORR activities of polycrystalline LSC₁₁₃ thin films with a wide range of Sr concentration (0 ≤ Sr ≤0.8). The authors found that with increasing the Sr content, the O 2p-band center can be close to the Fermi level, which favors the oxygen vacancy formation. However, they claimed that at Sr > 0.4, Co²⁺ becomes optimal for the surface instead of Co³⁺/Co⁴⁺ and may act as electron traps, which may decrease the electronic conductivity, leading to an overall poor ORR activity of LSC₁₁₃ films. These results showed a different trend with the previously reported data [95,96] which showed enhanced ORR kinetics due to an upshift of the oxygen 2p-band center to the Fermi level.

3.1.2. Ferrites

Iron-based perovskites (LaFeO₃, LFO₁₁₃) are typically oxygen deficient materials [102,103]. Therefore, LFO₁₁₃ has high oxygen mobility within the material [104] and is suitable for catalytic applications such as oxygen reduction and ion transport in SOFCs [105,106]. Substituting Sr for La on the A-site of LFO₁₁₃ is widely used to promote oxygen defects, enhancing oxygen diffusion and surface exchange processes due to the charge neutrality [107]. However, the ORR activity of La_1−xSr_xFeO₃ (LSF₁₁₃) thin films is still insufficient for the use of oxygen electrocatalysis. For example, using electrochemical impedance spectroscopy (EIS) measurements, la O et al. [108] found that the oxygen surface exchange coefficients (k^q) of La_0.8Sr_0.2FeO₃ (LSF82) thin films are comparable to those of La_0.8Sr_0.2MnO₃ (LSM82) thin films.

Compared to LSF₁₁₃, LSCF₁₁₃ exhibits better ionic conductivity, reasonable high-temperature stability, and catalytic activity [109,110,111,112,113]. However, there was no systematic study on the strain dependent ORR kinetics (Figure 5a). Baumann et al. [47] and Ingram et al. [85] reported the ORR kinetics of La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF6482) thin films (Figure 5b). The authors concluded that LSCF6428 films can enhance the ORR kinetics compared to bulk LSCF6428 due to the increased oxygen vacancies. So far, substantial efforts have been focused on tailoring the surface microstructure of LSCF₁₁₃ thin films to enhance the ORR kinetics, based on the hypothesis that many grain boundaries on the surface of LSCF₁₁₃ films may modify the oxygen bulk diffusion and catalytic properties to enhance the ORR kinetics. For instance, Prestat et al. [114] investigated the ORR kinetics of La_0.52Sr_0.48Co_0.18Fe_0.82O_3−δ thin films with various thicknesses (Figure 5b). In this study, the polarization resistance of LSCF films was reduced by decreasing the film thickness, indicating that the number of grain boundaries on the surface increases. Consequently, the oxygen surface exchange and diffusion coefficients were significantly enhanced in LSCF thin films compared to bulk LSCF [84]. Angoua et al. [115] investigated the ORR kinetics of LSCF6428 thin films with a Ce_0.8Gd_0.2O_1.9 (CGO) layer. Adding CGO to LSCF6428 films reduced the polarization resistance for LSCF-CGO films, which leads to a higher density of grain boundaries. Bagarinao et al. [116] discussed the oxygen surface exchange and diffusion kinetics of LSCF6428 thin films by comparing a nanograined microstructure film. The authors found that introducing grain boundaries on LSCF6428 films can enhance the oxygen surface exchange reactivity, which is in good agreement with previously reported data [115].

It was also shown that the ORR kinetics can be enhanced by using the LSCF₁₁₃ thin film as an interfacial layer at the electrode-electrolyte interface. Dumaisnil et al. [120] investigated the ORR kinetics of adding LSCF6482 thin films between porous LSCF113 cathode and Gd-doped ceria (GDC) substrate. In this study, the addition of LSCF6482 thin films was found to result in lower polarization resistance, which may be attributed to the increased active area due to the enhanced adhesion between cathode/substrate. Jang et al. [121] also investigated the ORR kinetics by addition of porous nanoparticle network structured LSCF6428 thin film between the cathode and electrolyte. The cell with LSCF6428 films showed higher peak power density and smaller polarization resistance compared to the cell without LSCF6428 film. Later, the role of nanoparticle network structured LSCF6428 thin film, which is called nano-web structured LSCF6428 (NW-LSCF) was further discussed [122].

3.2. Ruddlesden-Popper (RP) Oxides

The general formula of Ruddlesden-Popper (RP) phases can be written as A_n+1B_nO_3n+1 [123]. The RP phases are comprised of n consecutive perovskite layers (ABO₃) alternating with rock-salt layers (AO) along the crystallographic c-axis direction. Their formula can be represented by (AO)(ABO₃)_n, where n represents the number of connected layers of vertex sharing BO₆ octahedra [124]. In the case of SOFCs, the n = 1 members of some RP series, notably those based on La₂NiO_4+δ (LNO₂₁₄) are of particular interest. These RP oxides were shown to accommodate oxygen interstitials in the AO layers [10], leading to fast ion conduction and hence potential application as cathodes at intermediate temperatures. Recently, lanthanum strontium cobaltites have attracted much attention as a cathode material for SOFCs, which exhibited interestingly fast oxygen exchange kinetics when interfaced with its perovskite counterpart LSC₁₁₃ [67,125]. In addition, (La,Sr)₂CuO₄ (LSCu₂₁₄) has also demonstrated its availability as a promising cathode for SOFCs showing comparable oxygen surface exchange kinetics to that of LSCF₁₁₃ thin films at intermediate temperatures [126]. Figure 6 presents the ideal tetragonal unit-cells for n = 1, which correspond to the stoichiometric compounds all of same space group, I4/mmm.

It is well-known that RP oxides exhibit strong anisotropic features as a result of the adjustment of two different structural units, i.e., ABO₃ and AO within the lattice. While the B-site cations are coordinated by six oxygen anions, the B-O bond lengths are different due to the Jahn-Teller effect caused by the valence state of the B-site cations [127]. This results in two types of oxygen species in the BO₆ octahedra, which are referred to as “apical” and “equatorial” oxygen. In the RP system, interstitial sites are located in the AO layer, where RP oxides can accommodate an excess oxygen as an interstitial oxygen defect. Oxygen vacancies can also be formed by appropriate doping in the system. Consequently, the oxygen transport properties of RP oxides can be strongly influenced by either oxygen interstitials or oxygen vacancies as discussed earlier.

Nickelates are interesting alternative materials to LSC₁₁₃ for intermediate temperature SOFCs owing to its high oxygen surface exchange kinetics and oxygen transport properties [10,11,13,14,61,128,129,130,131,132,133]. However, due to difficulties in synthesizing epitaxial thin films of nickelates, few studies have examined the ORR kinetics of nickelates in a form of epitaxial films. Yamada et al. [134] first reported the oxygen electrocatalysis of epitaxial RP oxide thin films using (011)-oriented epitaxial Nd₂NiO_4+δ (NNO₂₁₄) films on (001) YSZ substrates. According to this study, compression along the c -axis of NNO₂₁₄ films reduces the oxygen surface exchange kinetics. Later, Tsvetkov et al. [135] compared the δ values of tensile- and compressive-strained NNO₂₁₄ thin films, demonstrating that the larger δ in tensile-strained NNO₂₁₄ can enhance the oxygen surface exchange kinetics relative to those of compressive-strained NNO₂₁₄ thin films (Figure 7a). These results are in good agreement with the previous observation by Yamada et al. [134].

Among various RP oxides, LNO₂₁₄ is the most widely studied member of the series. Kim et al. [136] proposed that epitaxial LNO₂₁₄ thin films with a thickness of 300 nm may possess two different surface exchange rates as a result from the coexistence of two different film microstructures. Later, they demonstrated that the surface exchange reaction of polycrystalline LNO₂₁₄ samples limits the electrode reaction, showing comparable k^q values with those obtained previously for LNO₂₁₄ bulk [132]. However, to evaluate the effect of strain on the ORR kinetics in RP oxides, high quality epitaxial thin films are required. Burriel and co-workers [61] synthesized (001)-oriented epitaxial LNO₂₁₄ thin films on STO and NbGaO₃ (NGO) substrates, which introduce tensile and compressive strain states, respectively, into the LNO₂₁₄ films. They evaluated the oxygen diffusion and surface exchange kinetics for tensile- and compressive-strained LNO₂₁₄ films, concluding that both compressive and tensile strain led to a reduction in the oxygen diffusivity in the first 175–200 nm of the film, whereas neither strain affected the oxygen surface exchange. However, Lee et al. [63] later demonstrated that tensile strain along the c-direction can lead to an increase in the driving force to form interstitial oxygen atoms in LNO₂₁₄, resulting in enhanced oxygen surface exchange kinetics of (100)-oriented epitaxial LNO₂₁₄ thin films (Figure 7b). In this study, the authors successfully controlled strain in LNO₂₁₄ films by modulating the film thickness. The discrepancy in the effects of strain on oxygen surface exchange kinetics in LNO₂₁₄ films may be attributed to the fact that the critical thickness required to maintain an acceptable strain state is fairly smaller than what Burriel et al. [61] used. However, the effect of strain on the ORR kinetics in RP oxides needs further investigation.

Similar to the correlation between the oxygen 2p-band center and ORR kinetics in ABO₃ oxides, Lee et al. [126] also proposed that the oxygen 2p-band center relative to the Fermi energy can be an effective descriptor to predict the oxygen surface exchange as well as the activation energy for oxygen transport in RP oxides. In contrast to ABO₃ oxides, the oxygen 2p-band center relative to the Fermi level was found to be inversely proportional to the oxygen surface exchange kinetics in RP oxides (Figure 8). According to the authors, the opposite slopes of the oxygen 2p-band center correlations between the RP oxides and the perovskite materials are due to the intrinsic mechanistic differences of their oxygen surface exchange kinetics and bulk anionic transport. The authors also found that the calculated interstitial formation energies of La₂BO_4+δ (B = Co, Ni, and Cu) strongly depend on the B cation in the sequence LCuO₂₁₄ > LNO₂₁₄ > LCO₂₁₄, which corresponds to the trend of B-cation–dependent oxygen diffusivity. Recently, Xie et al. [137] also calculated the formation energy of oxygen interstitials in the same RP oxide systems, demonstrating that, regardless of δ, the formation energy of oxygen interstitials decreases with decreasing the atomic number of the B cation, which is in good agreement with the data reported by Lee et al. [126].

As discussed earlier, the anisotropy of oxygen migration is one of the intrinsic properties of RP oxides owing to its structural feature. Therefore, different oxygen diffusion and surface exchange kinetics along different crystallographic directions were reported as shown in Figure 9. Using NNO₂₁₄ and Pr₂NiO_4+δ (PNO₂₁₄) single crystalline oxides, Bassat et al. [128] observed a large anisotropy for the oxygen surface exchange kinetics for both oxides, where the k* values in the a-b plane are 1.5 orders of magnitude larger than those along the c-axis. Burriel et al. [61] reported that the oxygen diffusion and oxygen surface exchange kinetics in the a-b plane are faster than those along the c-direction in epitaxial LNO₂₁₄ thin films. However, later the authors also evaluated the anisotropy for the oxygen surface exchange kinetics of LNO₂₁₄ single crystalline, reporting the lack of anisotropy in the oxygen surface exchange values, which is attributed to the surface segregation process [133]. The anisotropy of the oxygen surface exchange kinetics was also observed in Sr-doped LNO₂₁₄ films. Lee et al. [65] demonstrated that substituting Sr for lanthanum in LSNO₂₁₄ thin films can result in the structural reorientation of the films because of the reduction in the surface energy of the (001) surface, reporting the anisotropic oxygen surface exchange kinetics.

La_2-xSr_xCuO_4±δ (LSCu₂₁₄) oxides are famous for their high T_c superconductivity [138,139,140]. Interestingly, the superconducting properties of LSCu₂₁₄ depend on the Sr concentration and oxygen stoichiometry, which also strongly influence the kinetics of oxygen exchange and oxygen diffusion. Opila et al. reported the anisotropic oxygen transport properties and proposed the defect model for Sr-doped curpates [38,141]. Recently, using epitaxial LSCu₂₁₄ thin films, the oxygen surface exchange kinetics were evaluated at intermediate temperatures [126]. In this study, the k^q values of undoped La₂CuO₄ (LCuO₂₁₄) thin films were found to be higher than those of Sr-doped La₂CuO₄ thin films (Figure 9a), which is attributed to that adding Sr content resulted in changes of the oxygen 2p-band center relative to the Fermi energy. More recently, Meyer and co-workers [142] revealed that the oxygen nonstoichiometry commonly reported for strained LSCu₂₁₄ is mediated by the strain-modified surface exchange kinetics, rather than reduced thermodynamic oxygen formation energies. The authors also demonstrated that tensile-strained LSCu₂₁₄ shows nearly an order of magnitude faster oxygen exchange rate in tensile-strained LSCu₂₁₄ films compared to a compressively strained film.

La_2-xSr_xCoO_4±δ (LSC₂₁₄) oxides are widely used as a perovskite counterpart to promote the oxygen electrocatalysis of ABO₃ perovskite such as LSC₁₁₃ [33,68,125,143]. However, LSC₂₁₄ showed very attractive transport and surface exchange properties at intermediate temperatures. For instance, La₂CoO_4+δ (LCO₂₁₄) showed higher oxygen diffusivity and lower activation energy in the temperature range of 450–700 °C as compared to nickelates and cuprates [11,13,25]. Later, Lee et al. [126] calculated the interstitial formation energies of LCO₂₁₄, LNO₂₁₄, and LCuO₂₁₄ and found the minimum energy for interstitial formation in LCO₂₁₄. In addition, the authors also reported that the k^q values of LSC₂₁₄ thin films are comparable to LSC₁₁₃ thin films, indicating a promising alternative cathode material to LSC₁₁₃. Recently, Chen et al. [144] successfully fabricated LSC₂₁₄ (Sr = 0.25) thin films grown in two different crystallographic orientations, i.e., (100) and (001), using (100) LaSrAlO₄ (LSAO) and (001) SrTiO₃ (STO) substrates. They demonstrated that the oxygen diffusivity along the a-b plane in the (100)-oriented LSC₂₁₄ film was three orders of magnitude lower than that in the (001)-oriented LSC₂₁₄ film (Figure 9b). Moreover, the D* value along the c-direction in the (001)-oriented LSC₂₁₄ film was found to be one order of magnitude higher than that along the a-b plane in the (100)-oriented LSC₂₁₄ film. However, no dependence of oxygen surface exchange kinetics was observed on the crystal orientation, which may be attributed to the strong Sr segregation at the surface overriding the effect of the structural anisotropy.

3.3. Oxide Heterostrcuture Interface

To achieve enhanced surface exchange kinetics and stability, recent efforts have been focused on developing advanced cathode materials based on MIECs with surface modification [27,33,66,68,76,82,86,125,143,145,146,147,148]. Oxide heterostructure interfaces, the combination of a Ruddlesden-Popper (RP, LSC₂₁₄) layer on top of the perovskite LSC₁₁₃, have shown remarkably high oxygen surface exchange kinetics [27,33,68,86,125,143,148,149]. Yashiro et al. reported ~1 order of magnitude enhancement in activity for the composite cathode screen-printed with the mixture of LSC64 and LSC₂₁₄ [143]. Sase et al. [33] have also reported ~3 orders of magnitude higher oxygen surface exchange coefficient (k*) at the interfacial region between polycrystalline LSC64 and LSC₂₁₄ compared to their bulk value [21]. Later, Crumlin et al. [27,148] utilized epitaxial LSC₁₁₃ and LSC₂₁₄ thin films, achieving higher oxygen surface exchange coefficients of LSC₁₁₃. Recently, Zhao et al. [150] also observed ~10 times higher k^q values in heterostructure LSC₁₁₃/LSC₂₁₄ thin films than those in LSC₁₁₃, which is attributed to increased number of active oxygen vacancies that manipulate the oxygen 2p-band center.

A recent study using Coherent Brag Rod Analysis (COBRA) revealed the atomic structure and concentrations of the (001)-oriented LSC₁₁₃ thin film on a STO substrate, which shows strontium (Sr) segregation toward the LSC₁₁₃ surface and Sr depletion near the interface between LSC₁₁₃ and STO [151]. It was also reported that heating the (001)-oriented LSC₁₁₃ surface leads to the formation of surface Sr-enriched particles upon annealing while the LSC₂₁₄-decorated LSC₁₁₃ surface chemistry is stable upon heating [152]. More recently, Lee et al. [86] compared the time-dependent surface exchange kinetics and stability of epitaxial LSCF₁₁₃ and LSC₁₁₃ thin films with LSC₂₁₄ surface decoration. Using density functional theory (DFT) computation, the authors revealed greater surface Sr segregation for LSCF₁₁₃, which is predicted to have an SrO termination, than LSC₁₁₃, which is predicted to have a less Sr enriched (La_0.25Sr_0.75)O termination. Furthermore, the authors pointed out a lower energy gain to move Sr from LSCF₁₁₃ into LSC₂₁₄ relative to the LSC₂₁₄-LSC₁₁₃ surface (Figure 10). These observations have suggested that the surface decoration can modulate the surface Sr segregation and the surface phase stability, which can greatly influence the oxygen surface exchange kinetics and the surface stability in LSC₁₁₃ and LSCF₁₁₃. Therefore, understanding the surface decoration effect on the surface chemistry of perovskites is critical and other potential surface modification materials need to be investigated to design highly active and stable cathodes for SOFCs.

Similar to the LSC₁₁₃ surface modification with LSC₂₁₄, several studies have also reported the enhanced surface electrocatalytic activity of porous LSCF cathodes with surface decoration [82,147,153,154]. For example, depositing thin La_0.85Sr_0.15MnO_3−δ coatings on porous LSCF6428 electrodes using an infiltration process, Lynch et al. [82] showed the enhanced surface electrocatalytic activity of decorated LSCF₁₁₃ cathodes upon polarization. Vast majority of research has been performed on porous LSCF₁₁₃ electrodes, which lead to ambiguous structure and geometry, and therefore the physical origin responsible for enhanced cathodic performance associated with surface decoration of perovskites is not yet completely understood. However, Lee et al. [66] recently demonstrated using new forms of heterostructure oxide interfaces—(1) LSCF6428 thin films with a single layer decoration of mixed LSC₂₁₄ and LSC₁₁₃ and (2) LSCF6428 thin films with a double layer decoration of stacked LSC₂₁₄ and LSC₁₁₃ – significantly enhanced ORR activity of LSCF6428. More recently, Chen et al. [155] showed the improved electrocatalytic activity and durability of LSCF6428 thin films by coating PrO₂. The authors claimed that PrO₂ coating facilitates the rate of oxygen dissociation due to increased oxygen vacancies. Figure 11 shows the influence of heterointerfaces on k* for LSC₁₁₃ and LSCF₁₁₃.

4. Conclusions

Developing intermediate temperature SOFCs requires discovering new cathode materials with faster ORR kinetics. MIEC oxides such as ABO₃ oxides and RP oxides are promising alternative cathode materials to LSM113, pure electronic conductor. Recently, introducing oxide thin films and oxide heterostructure interfaces has been recognized as an extremely effective means to control the ORR kinetics of MIEC oxides, opening a plethora of new possibilities useful for intermediate temperature SOFCs. We have briefly overviewed how epitaxial strain can influence the oxygen surface exchange kinetics in ABO₃ and RP oxide thin films. It has been pointed out that strain can tune the oxygen 2p-band center relative to the Fermi energy as a result of the modulated concentration of oxygen defects. In the case of RP oxides, epitaxial thin films offer a direct way to evaluate the intrinsic anisotropy of oxygen diffusion and surface exchange kinetics. Lastly, oxide heterointerfaces can be used to improve the ORR kinetics and stability of ABO₃ oxides. This review has given a comprehensive overview of the current state of research. However, a full understanding of the effect of strain and crystallographic orientation on the ORR kinetics in ABO₃ and RP oxides is still being developed for intermediate SOFCs. In addition, understanding the surface decoration effect on the surface chemistry of perovskites and other potential surface modification materials still need to be investigated to design highly active and stable cathodes for SOFCs.