Progress in Developing LnBaCo₂O_5+δ as an Oxygen Reduction Catalyst for Solid Oxide Fuel Cells

Abstract

Solid oxide fuel cells (SOFCs) represent a breed of eco-friendly, weather-independent, decentralized power generation technologies, distinguished for their broad fuel versatility and superior electricity generation efficiency. At present, SOFCs are impeded by a lack of highly efficient oxygen reduction catalysts, a factor that significantly constrains their performance. The double perovskites LnBaCo₂O_5+δ (Ln = Lanthanide), renowned for their accelerated oxygen exchange and conductivity features, are widely acclaimed as a promising category of cathode catalysts for SOFCs. This manuscript offers a novel perspective on the physicochemical attributes of LnBaCo₂O_5+δ accumulated over the past two decades and delineates the latest advancements in fine-tuning the composition and nanostructure for SOFC applications. It highlights surface chemistry under operational conditions and microstructure as emerging research focal points towards achieving high-performance LnBaCo₂O_5+δ catalysts. This review offers a comprehensive insight into the latest advancements in utilizing LnBaCo₂O_5+δ in the field of SOFCs, presenting a clear roadmap for future developmental trajectories. Furthermore, it provides valuable insights for the application of double perovskite materials in domains such as water electrolysis, CO₂ electrolysis, chemical sensors, and metal–air batteries.

1. Introduction

The escalating issues of climate change and energy shortages, predominantly driven by the pervasive and inefficient use of fossil fuels, have intensified the search for novel energy conversion methodologies. Among various power generation technologies, solid oxide fuel cells (SOFCs) hold a unique position. They are particularly noted for their exceptional efficiency rates: 45% to 65% for independent applications and exceeding 85% for combined heat and power applications. Moreover, their fuel compatibility is versatile, ranging from hydrogen to hydrocarbons and even to carbon, making SOFCs a pivotal component in the design of innovative energy solutions [1,2,3]. An SOFC is a solid-state device comprising two porous electrodes and a dense electrolyte. This electrolyte conducts solid oxygen ions and is the key functional component of each individual cell. Oxygen introduced at the cathode side is reduced to form O²⁻. Driven by the concentration gradient, these O²⁻ ions travel through the dense electrolyte layer to reach the anode. At the anode side, the fuel is directly oxidized to H₂O and/or CO₂ by O²⁻, releasing electrons to the external circuit [1,2]. Traditional SOFCs typically use the following materials: oxygen ion conductor yttria-stabilized zirconia (YSZ) for the electrolyte, the pure electronic conductors strontium-substituted manganites (LSM) for the cathode, and NiO–YSZ for the anode. Due to the inherent properties of these components, high operating temperatures, approximately 1000 °C, are required to achieve an economically viable power density [4]. However, such high operating temperatures result in significant fabrication and operational costs, severe material complications, and extended start–stop durations, all of which hinder the widespread commercialization of SOFCs [1,4,5].

In recent years, significant efforts have been made to lower the operational temperature of SOFCs to a range of 500–800 °C [1,4,6]. Identifying innovative oxygen catalysts with high catalytic activity at these reduced temperatures is a critical challenge for SOFCs, particularly due to the exceptionally high activation energy of LSM [1,2,5,7]. For instance, as the operational temperature decreases from 1000 °C to 500 °C, the polarization resistance of LSM increases dramatically from 1 Ω cm² to 2000 Ω cm². This decline in cathode catalytic performance is largely attributed to the limitation of the oxygen reduction reaction (ORR) to the narrow triple-phase boundary (TPB) at the interface between the cathode, electrolyte, and oxygen gas (air). Within this TPB, the transportation of electrons, oxygen vacancies or ions, and oxygen gas occurs, facilitating their movement to or from the reaction site [5,8].

Mixed ionic and electronic conductors (MIECs) that exhibit elevated oxygen ion conductivity within the temperature range of 500–800 °C have the potential to expand the oxygen reduction region from the TPB to several micrometers within the cathode. Consequently, these materials are anticipated to display exceptional catalytic activity for the ORR [9,10,11,12,13]. For instance, at a temperature of 700 °C, the area-specific resistance (ASR) of a La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ cathode on a gadolinium-doped ceria (GDC) electrolyte has been reported to be approximately 0.1 Ω cm² [12]. Furthermore, Pang et al. reported a notably lower resistance of less than 0.035 Ω cm² for La_0.5Ba_0.5CoO_3−δ on a GDC electrolyte under identical operating conditions [13]. In the search for advanced cathode materials for SOFCs, significant advancements have been realized over the past decade. However, polarization resistances at lower temperature ranges are often considered suboptimal. Notably, Hwang et al. observed a significant rise in the ASR of a La_0.6Sr_0.4Coe _0.2Fe_0.8O_3−δ cathode on a GDC electrolyte, increasing from ~0.1 Ω cm² at 700 °C to 12 Ω cm² at 500 °C [12]. The primary reason for this observation originates in the pronounced decrease in O²⁻ conductivity as the temperature drops, due to the relatively high activation energies associated with oxygen transport and exchange processes [12,14]. As such, the ongoing quest for high-performance cathodes for SOFCs underscores the need for the development of materials that facilitate faster oxygen transport and surface exchange.

Over the past few years, owing to its unique crystal structure, considerable efforts have been directed towards investigating the MIEC double perovskite oxides LnBaCo₂O_5+δ (Ln = Lanthanide). These materials find potential applications across a multitude of domains, such as magnetism [15,16,17], SOFCs, proton-conductive ceramic fuel cells [18,19,20,21,22,23,24,25], water electrolysis [26,27,28], CO₂ electrolysis [29], chemical sensors [30,31,32], ceramic semi-permeable membranes [33,34,35], metal–air batteries [36,37], soot combustion [38], supercapacitors [39], photocatalysis [40], and solar-driven thermal storage [41,42]. Given the diverse requirements in terms of physicochemical properties for each application, this article will exclusively focus on novel strategies employed in advancing double perovskites for use as cathode catalysts in SOFCs. It is worth noting that, due to the considerable interest in double perovskites, comprehensive reviews of these materials have previously been published [43,44]. However, the ongoing advancements in understanding the properties and applications of these materials underscore the need for updated reviews, such as the one presented in this article. Our discussions will span the exploration of physicochemical properties, optimization of composition, and enhancement of application methodologies. Additionally, we will deliberate on potential research breakthroughs concerning high-performance double perovskite-based cathode materials.

2. Physicochemical Properties of LnBaCo₂O_5+δ

As depicted in Figure 1, the LnBaCo₂O_5+δ compound exhibits a perovskite structure of the 112 type. Relative to their disordered analogs, these orderly structures have been widely reported to considerably enhance the rate of oxygen transport [45,46]. Notably, Taskin et al. [45] were pioneers in observing a notably high oxygen diffusion coefficient (D_chem) of approximately 3.0 × 10⁻⁹ cm² s⁻¹ at 350 °C and 10⁻⁵ cm² s⁻¹ at 600 °C for the GdBaCo₂O_5+δ double perovskite. The oxygen transport characteristics of the PrBaCo₂O_5+δ double perovskite were subsequently evaluated by Kim et al. [47,48]. Their results demonstrated appreciably higher rates of oxygen transport (D_chem) for PrBaCo₂O_5+δ in comparison to GdBaCo₂O_5+δ, suggesting an enhancement in oxygen transport properties corresponding to the increased size of the Ln cation. Tarancón et al. carried out a detailed comparative study between the double perovskite LnBaCo₂O_5+δ (Ln = Pr, Gd) and other classes of oxygen catalysts [44]. As illustrated in Figure 2, the double perovskite outperformed in terms of oxygen transport properties, emphasizing its considerable potential as a cutting-edge cathode material for SOFCs. It is important to recognize that significant variations exist in the LnBaCo₂O_5+δ oxygen tracer diffusion and the oxygen surface exchange coefficient as reported by different research groups [49]. Such disparities mainly stem from differences in the precise composition and/or microstructure of the samples used by distinct researchers. Thus, readers are encouraged to assess the data in Figure 2 judiciously and objectively.

Numerous experimental and theoretical studies have been undertaken to delve deeper into the oxygen diffusion behaviors in double perovskites [53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71]. Seymour et al. utilized static atomistic simulations based on the Born model to methodically examine the intrinsic defect processes of the double perovskite LnBaCo₂O_5.5 (Ln = Y, La, Pr, Nd, Sm, Gd, Dy, Ho, Er, Yb) [53]. Their research indicated that the defect reaction with the lowest energy stemmed from the Ln/Ba antisite disorder energy, which diminishes with decreasing Ln size. This suggests that the ordered structure’s primary foundation is the size difference between the Ln and Ba cations [53]. Parfitt et al. combined molecular dynamics with Born model potentials to study the oxygen transport behavior of GdBaCo₂O_5+δ at 900 K [54,55]. They posited that A-site cation ordering, in contrast to its disordered equivalent, can amplify oxygen bulk diffusivity while decreasing transport in the c-axis direction [54,55]. Importantly, the distinctively anisotropic oxygen diffusion in the double perovskite GdBaCo₂O_5+δ takes place exclusively within the [GdO_δ] and adjacent [CoO₂] layers, as illustrated in Figure 3 [54,55,56,57]. Shiiba et al. probed the distribution of oxygen vacancies in GdBaCo₂O_5+δ under various oxygen vacancy concentrations (0 ≤ δ ≤ 1) and temperatures using a fusion of density functional theory and Monte Carlo simulation [57]. Their analysis showed that oxygen vacancies, which function as oxygen ion carriers, are restricted to the [GdO_δ] and neighboring [CoO₂] layers, reinforcing the anisotropic oxygen diffusion mechanism. Seymour et al. performed theoretical investigations on the oxygen transport properties of layered PrBaCo₂O_5+δ at 650 and 1000 °C, employing the MD method [59,60,61,62]. These proposed mechanisms for oxygen conducting were later confirmed experimentally via in situ high-temperature neutron powder diffraction and isotope exchange depth profile methods [59,60,61,62]. Additionally, it has been shown that PrBaCo₂O_5+δ has a lower energy barrier for oxygen diffusion perpendicular to the c-axis compared to Nd, suggesting enhanced oxygen ion diffusivity with larger Ln sizes [53,59]. Wang et al. detected rapid cobalt redox reactions in epitaxial LaBaCo₂O_5+δ within a temperature bracket of 260–700 °C, intimately tied to the processes of oxygen release and uptake processes [72]. This finding hints at the potential application of these films in SOFC cathodes. Notably, Wang et al. found the cobalt oxidation in the epitaxial thin films to be substantially swifter than the reduction process, denoting a more rapid oxygen uptake compared to the oxygen release (Figure 4) [72]. Bao et al.’s research further revealed a layer-by-layer oxygen transport mechanism in epitaxial double perovskites, specifically LnBaCo₂O_5+δ (Ln = Pr, Er), which likely originates in their intrinsic anisotropic oxygen diffusion properties [73].

LnBaCo₂O_5+δ, owing to its exceptionally promising properties, has been extensively studied as a cathode material for SOFCs [34,74,75,76,77,78,79,80,81,82,83]. Researchers have undertaken thorough studies into the structural performance, thermal expansion behavior, electrical conductivity, and electrochemical performance of these double perovskites. Studies on ions such as La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, and Gd³⁺ have shown that these oxides exhibit good chemical compatibility with commonly used electrolytes, including GDC, La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8 (LSGM), and samarium oxide-doped ceria (SDC), at temperatures below 1000 °C [74,75,76,84]. After firing LnBaCo₂O_5+δ double perovskites at 850 °C in air for durations ranging from 60 to 100 h, no impurity phases or phase transitions were detected. This finding highlights the remarkable structural stability of these oxides under the standard operating conditions of SOFCs [34,77]. Additionally, the electrical conductivities of LnBaCo₂O_5+δ compounds tend to increase with growth in the size of the Ln ion, leading to a rise in the number of electronic holes created by interstitial oxygen [75,76]. The electrical conductivity values of these materials surpass 100 S cm⁻¹ between 100 and 800 °C in air, meeting the electrical conductivity requirements for SOFC cathodes [34,75,76,77]. What is more, oxides with larger Ln ions exhibit superior electrochemical performance, stemming from enhanced oxygen transport and exchange rates [34,75]. For instance, as the Ln ion shifts from Gd³⁺ to La³⁺, the maximum power density (PPD) values of SOFCs utilizing these double perovskite cathodes increase from 443 to 516 mW cm² [75].

Despite the numerous advantages of LnBaCo₂O_5+δ as a cathode catalyst for SOFCs, there are certain technical challenges that require further improvements. Firstly, enhancing the catalytic activity of these oxides for ORR is paramount. Chen et al. [74] observed that the ASR of PrBaCo₂O_5+δ on SDC electrolytes increases from 0.18 to 5.68 Ω cm² as the temperature drops from 650 to 500 °C. Moreover, the PPD of SOFCs utilizing PrBaCo₂O_5+δ as the cathode material decreases from 866 mW cm² (at 650 °C) to 115 mW cm² (at 500 °C). Secondly, it is essential to minimize the thermal mismatch between these cobalt-based cathode materials and other SOFC components. Kim et al. [75] reported that the thermal expansion coefficients (TECs) of LnBaCo₂O_5+δ double perovskites increase from 16.6 × 10⁻⁶ K⁻¹ (Ln = Gd³⁺) to 24.3 × 10⁻⁶ K⁻¹ (Ln = Pr³⁺) with larger Ln sizes at 80–900 °C. Given that the TECs of standard electrolytes for SOFCs, such as GDC, SDC, and LSGM, are around 11 × 10⁻⁶ K⁻¹, this notable thermal mismatch between LnBaCo₂O_5+δ and the electrolyte could adversely affect fuel cell stability. Thirdly, tuning the physicochemical properties of the surface is essential. The surface physicochemical properties serving as catalysts for the ORR significantly influence cathode performance. Findings by Téllez et al. [79] suggest that the surface composition and morphology of LnBaCo₂O_5+δ (Ln = Pr, Gd) double perovskites are profoundly influenced by exposure time, temperature, and ambient atmosphere. A quick covering of the electrocatalytic transition metal by inactive Ln³⁺ or Ba²⁺ cations, observed under certain conditions, can be detrimental to the ORR. Therefore, the subsequent sections will provide a comprehensive overview of advancements in studying the physicochemical property attributes of double perovskites and in adjusting the composition and nanostructure of LnBaCo₂O_5+δ.

3. Compositional Optimization of LnBaCo₂O_5+δ

To enhance the performance of double perovskite-based cathodes, extensive efforts have been made to optimize the composition of LnBaCo₂O_5+δ. These modifications involve A-site and B-site doping, or a combination of both, aiming to improve structural stability, enhance chemical compatibility with the electrolyte, increase electrocatalytic activity, and finely tune the TECs (

Table 1. Typical electrical conductivity, TEC, and ASR values of double perovskites.

	Electrical Conductivity (S cm−1)			TEC (10−6 K−1)	ASR (Ω cm2)				Refs
	600	700	800		600	700	800	Electrolyte
LaBaCo2O5+δ	558	447	355	-	0.195	0.039	0.010	GDC	[93]
LaBa0.9Co2O5+δ	483	386	306	-	0.118	0.023	0.007	GDC	[93]
PrBaCo2O5+δ	208	164	128	23.4 (30–900 °C)	0.181	0.038	0.009	GDC	[94]
	212	170	138	24.6 (30–800 °C)	-	0.078		SDC	[100]
PrBa0.92Co2O5+δ	233	187	146	22.8 (30–900 °C)	0.093	0.024	0.007	GDC	[94]
PrBa0.5Sr0.5Co2O5+δ	-	-	-	-	0.688	0.154	-	GDC	[87]
PrBaCoFeO5+δ	91	68	62	24.9 (30–800 °C)	-	0.105	-	SDC	[100]
NdBa0.5Sr0.5Co2O5+δ	695	556	442	-	-	0.139	0.039	LSGM	[90]
	263	204	191	25.2 (100–800 °C)	2.800	0.676	0.086	SDC	[73]
SmBaCo2O5+δ	560	440	-	-	0.192	-	-	GDC	[89]
SmBa0.5Sr0.5Co2O5+δ	1000	810	-	-	0.141	-	-	GDC	[89]
	-	-	-	-	0.631	0.092	-	GDC	[87]
SmBaCoCuO5+δ	27	28	31	15.5 (30–850 °C)	-	0.382	0.086	GDC	[107]
GdBaCo2O5+δ	311	249	196	-	-	-	-	GDC	[85]
	-	-	-	20.0 (30–900 °C)	0.4	-	-	GDC	[104]
	472	374	305	16.6 (80–900 °C)	-	-	-	GDC	[86]
GdBa0.5Sr0.5Co2O5+δ	591	492	409	-	-	-	-	-	[85]
	-	-	-	-	1.260	0.561	-	GDC	[87]
GdBa0.4Sr0.6Co2O5+δ	1099	930	591	19.5 (80–900 °C)	-	-	-	GDC	[86]
GdBaCo1.7Ni0.3O5+δ	-	-	-	15.5 (30–900 °C)	0.54	0.297	-	GDC	[104]

) [73,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118]. Marrero-Jerez et al. [85] found that substituting Sr for Ba in GdBaCo₂O_5+δ completely stabilizes the high-temperature tetragonal symmetry even at room temperature. Kim et al. [86] observed improved chemical stability when GDC and LSGM electrolytes are used. Numerous investigations have systematically examined the electrical and electrochemical properties of the LnBa_1−xSr_xCo₂O_5+δ system, with Ln representing La³⁺ [73], Pr³⁺ [87,92], Nd³⁺ [90,91], Sm³⁺ [87,88,89,112,114], and Gd³⁺ [85,86,87,111]. Kim et al. [86] demonstrated that the electrical conductivity of GdBa_1−xSr_xCo₂O_5+δ increases with rising Sr content, attributed to increased oxygen content, which is believed to be linked to the difference in A-site cation radii between (Ba_1−xSr_x)²⁺ and Gd³⁺. Additionally, Subardi et al. [88] found that the double perovskite SmBa_0.6Sr_0.4Co₂O_5+δ has a relatively high D_chem (1.63 × 10⁻⁶ cm² s⁻¹ at 500 °C and 1.41 × 10⁻⁵ cm² s⁻¹ at 700 °C) and a notably low activation energy (E_a = 68.03 kJ mol⁻¹) for oxygen bulk diffusion at 500–700 °C. Jun et al. [89] demonstrated that Sr substitution in SmBa_1−xSr_xCo₂O_5+δ can boost the catalytic activity of double perovskites. For instance, on a GDC electrolyte, the ASR decreases from 0.192 Ω cm² (x = 0.00) to 0.138 Ω cm² (x = 0.75), and the maximum power density grows from 0.848 to 1.039 W cm⁻² at 600 °C (Table 1).

The presence of A-site cation deficiency has been found to significantly influence the physical and chemical properties of perovskite oxides, as reported in previous studies [119]. Extensive investigations have been undertaken to understand the effects of Ba²⁺ [93,94,95,96,97,120,121] and Ln³⁺ [122,123,124,125] deficiencies on the crystal structure, oxygen content, electrical conductivity, and electrochemical performance of double perovskite LnBaCo₂O_5+δ. Pang et al. [94,95] observed that with an increase in Ba deficiency from x = 0.00 (0.181 Ω cm²) to x = 0.08 (0.093 Ω cm²) at 600 °C, the ASR value of PrBa_1-xCo₂O_5+δ drops by approximately 50%. This indicates a substantial improvement in oxygen catalytic activity associated with A-site deficiency. Dong et al. [98] further revealed that a higher Ba deficiency in PrBa_1−xCo₂O_5+δ oxides results in an increased concentration of oxygen vacancies, thus boosting oxygen transport and exchange kinetics. These findings strongly suggest that A-site cation deficiency can enhance the electrochemical performance of double perovskite LnBaCo₂O_5+δ.

Ca doping and optimization of the Ln component have been identified as other effective strategies to boost the electrochemical performance of double perovskite cathodes [86,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153]. Yoo et al. proposed novel cathode materials, such as Ca-doped NdBaCo₂O_5+δ, that have ionic radii similar to Nd and demonstrated their impressive structural stability and outstanding electrochemical performance [99]. The ASR of NdBa_1-xCa_xCo₂O_5+δ double perovskites was observed to decrease from 0.091 Ω cm² (x = 0.00) to 0.066 Ω cm² (x = 0.25) at 600 °C. As illustrated in Figure 5, compared to the Ca-free sample, single cells using NdBa_0.75Ca_0.25Co₂O_5+δ as the cathode exhibited a significantly higher PPD of 2.114 W cm⁻² at 600 °C. Moreover, while the power density of the single cell with an NdBaCo₂O_5+δ cathode experienced a decrease of approximately 50%, nearly no degradation in power density was seen for NdBa_0.75Ca_0.25Co₂O_5+δ, highlighting its remarkable stability. This result is believed to correlate with the increased electron affinity of mobile oxygen species in the presence of Ca.

Similar to other cobalt-based cathode materials, LnBaCo₂O_5+δ often displays relatively high TECs, typically ranging from 19 to 25 × 10⁻⁶ K⁻¹ at 80–900 °C. These values are substantially higher than those of conventional electrolytes (10–13 × 10⁻⁶ K⁻¹) [154] and sealing materials (11–14 × 10⁻⁶ K⁻¹) [155]. Such differences can lead to significant compatibility issues between the double perovskites and other components of SOFCs during cell fabrication and thermal cycling, potentially causing performance degradation. Besides lattice anharmonic vibrations, the elevated TECs of cobalt-based oxides are mainly attributed to the conversion of smaller Co⁴⁺ ions to larger Co³⁺ ions at higher temperatures. This is due to the liberation of lattice oxygen upon heating and the spin-state changes of Co³⁺ ions [156,157]. To address these drawbacks, researchers have examined the substitution of cobalt with various elements, including Fe [100,101,102,158,159,160,161,162,163], Ni [103,104,164,165,166], Cu [105,106,107], Mn [167,168], Zn [169], Zr [170,171], W [172], Sc [173], Mo [174], Ga [175], and Bi [176]. Jo et al. [104] reported that partial substitution of Fe and Cu for Co in GdBaCo₂O_5+δ (GdBaCo_2/3Fe_2/3Cu_2/3O_5+δ) can reduce the TECs from 19.9 × 10⁻⁶ K⁻¹ to 14.6 × 10⁻⁶ K⁻¹ at 80–900 °C. Zhao et al. [100] conducted a comprehensive investigation into the impact of Fe content on the physicochemical properties of double perovskite PrBaCo₂O_5+δ, discovering a continuous decrease in TECs with higher Fe content. However, this substitution also led to reduced electrical conductivity, oxygen vacancy concentration, and electrochemical performance compared to the Fe-free compound. For an in-depth exploration of this subject, readers are encouraged to refer to relevant review articles [177].

The ability to incorporate dopants at both the A-site and B-site offers a broader spectrum for customizing double perovskite cathode materials [102,108,109,110,178,179,180]. As earlier elaborated, appropriate A-site doping in LnBaCo₂O_5+δ, like substituting Ba with Sr or inducing a Ba deficiency, has the potential to enhance electrical conductivity, oxygen bulk diffusivity, surface exchange kinetics, and the oxygen catalytic activity pertinent to ORR [86,89,94,181]. On the other hand, replacing Co with different metal ions, including Fe, Ni, and Cu, has proven to successfully enhance the structural resilience and thermal expansion coefficients, thus improving compatibility with the electrolyte [100,101,103,104,105]. For instance, Kim et al. [110] deftly adjusted the manganese content in NdBa_0.5Sr_0.5Co_2-xMn_xO_5+δ to refine its physicochemical attributes as a cathode catalyst for SOFCs. They found that an increase in manganese content led to a decrease in TEC from 20.27 × 10⁻⁶ K⁻¹ (x = 0.0) to 14.33 × 10⁻⁶ K⁻¹ (x = 0.5), while maintaining acceptable electrochemical performance. Similarly, Choi et al. [108] documented a robust cathode material, PrBa_0.5Sr_0.5Co_2-xFe_xO_5+δ, which exhibited increased oxygen ion mobility and surface oxygen exchange reactions, superior electrochemical performance (~0.056 Ω cm⁻² at 600 °C), and strong compatibility and stability with a GDC electrolyte. Persistent optimization of double perovskite composition is essential to uncover innovative cathode materials boasting excellent structural stability, advantageous chemical and thermal compatibility with the electrolyte, adequate electrical conductivities, swift oxygen transport and exchange kinetics, high catalytic ORR activity, and outstanding durability.

4. Nanostructure and Nanoscience of LnBaCo₂O_5+δ

Nanostructures offer significantly enhanced surface area-to-volume ratios and expanded interphase and interfacial areas. As such, they have the potential to augment electrochemical reaction sites. Perovskite oxides with nanostructured morphologies have been rigorously studied and employed in solid oxide fuel cells [182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198] as well as other energy-related applications [199,200,201,202]. Reducing the operating temperature creates an opportunity to use nanostructured materials, which can sidestep the slow ORR and, in turn, boost the catalytic performance of the cathode [182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198]. Infiltration is a common and straightforward method for developing nanostructured cathode materials tailored for SOFCs [182,183,184]. A nanostructured cathode material, represented by the formula SmBa_0.5Sr_0.5Co₂O_5+δ, was created by infusing its precursor solution into the porous LSGM framework, followed by calcining at 850 °C. This material showcased commendable electrochemical performance [185]. For instance, it showed an ASR as low as 0.12 Ω cm² and a PPD of up to 0.70 W cm⁻² at 500 °C. Electrospinning, praised for its scalability and precision, was utilized to fabricate a GdBaCo₂O_5+δ cathode material possessing a nanofiber configuration, achieving a comparatively low ASR, approximately 0.10 Ω cm² at 700 °C [194].

Ding et al. [185] managed to produce unique needle-like nanospikes of the cathode material PrBaCo₂O_5+δ by applying a discharge voltage of 0.1 V to the anode-supported single cell, arranged as NiO-Sm_0.2Ce_0.8O_1.9/Sm_0.2Ce_0.8O_1.9/PrBaCo₂O_5+δ, and then firing the PrBaCo₂O_5+δ cathode slurry at 450 °C. As illustrated in Figure 6, these nanospikes, with an average diameter of 20 nm and lengths spanning from tens to hundreds of nanometers, are uniformly distributed along the pore boundaries of the porous cathode. For the single cell that used the nanospikes PrBaCo₂O_5+δ as the cathode, exceptionally high maximum power densities of 1.453 W cm⁻² at 550 °C and 1.044 W cm⁻² at 500 °C, coupled with excellent endurance, were recorded.

The fabrication of double perovskites in a thin-film architecture not only facilitates fundamental studies to evaluate inherent properties of materials [73,183,203,204] but also illuminates a new avenue for the development of high-performing cathode materials [72,186,188]. The influence of orientations on the electrochemical performance of double perovskites was appraised by Gao et al. [186]. They produced PrBaCo₂O_5+δ thin films with different orientations, including (110), (001), and (111), using pulsed laser deposition. The thin film with the (111) orientation showed superior performance, achieving an ASR of 0.302 Ω cm² at 600 °C. Liu et al. [187,188] fabricated symmetric half-cells by coupling single-crystal, highly epitaxial LnBaCo₂O_5+δ (Ln = Pr, La) thin-film cathodes with Gd_0.8Ce_0.2O₂:Y_0.08Zr_0.92O₂ electrolytes and subsequently characterized their oxygen surface exchange and catalytic activity. For instance, the symmetric half-cell featuring the epitaxial LaBaCo₂O_5+δ thin film displayed remarkable properties, such as an impressively fast surface exchange rate of 0.017 cm s⁻¹ at 600 °C and an exceptionally low activation energy value of 0.49 eV. These outcomes might be ascribed to the structural entropy arising from the nano-ordered oxygen vacancy framework.

5. Conclusions and Outlook

This manuscript addresses the development of novel strategies concerning the double perovskites LnBaCo₂O_5+δ, which possess rapid oxygen bulk diffusivity and a high surface exchange rate. These parameters serve as the cornerstone for achieving advanced catalytic activity for the ORR; hence, they are essential for cathode materials operating within intermediate-to-low temperature SOFCs. The discourse encompasses physicochemical characteristics, compositional fine-tuning, and the implementation of nanostructure and nanoscience within double perovskites. Potential research focuses for advancing high-performance double perovskite-based cathode materials include:

(1)

Surface Chemistry of LnBaCo₂O_5+δ Under Operating Conditions

The surface physicochemical properties of double perovskite LnBaCo₂O_5+δ, as catalysts for the ORR, are essential for their practical application. Téllez et al. [79] characterized the surface chemistry evolution in double perovskite LnBaCo₂O_5+δ (Ln = Pr, Gd) using low-energy ion scattering, spectrometry, and atomic force microscopy. They found that the surface composition and morphology of LnBaCo₂O_5+δ (Ln = Pr, Gd) are sensitive to their ambient environment. Inactive Ba-rich layers emerged on the double perovskite surface following annealing, adversely affecting oxygen surface exchange processes and, subsequently, the electrochemical performance. Druce et al. [205] observed a similar surface termination and subsurface restructuring for GdBaCo₂O_5+δ. According to Lee et al. [206], the segregation originates from the dopant’s elastic and electrostatic interactions with the host lattice. A slight size mismatch between the dopant and host cations could reduce this segregation, promoting a more stable cathode surface [205,206,207,208,209,210,211,212,213,214,215]. Nonetheless, the majority of research on the surface microstructure of double perovskite cathodes relies on basic sintering processes in the air, which differs significantly from actual battery operating conditions. Hence, guidance for the practical use of double perovskites remains limited. Investigating surface microstructure evolution under the SOFCs’ actual operational conditions and creating targeted enhancement strategies offer a promising path for practical deployment of these cathode materials.

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Microstructure of LnBaCo₂O_5+δ Cathode Materials

The structural design of double perovskite metal oxides offers advantages for their use in the cathodic parts of SOFCs. Nevertheless, in practical applications, these double perovskites often present as polycrystalline particles. It is believed that features such as grain orientation, microstructure, lattice strain, and chemical imperfections strongly influence their catalytic activity [72,73,183,186,188,189,193,203,216,217,218]. Consequently, a comprehensive study on the microstructure of these double perovskite particles is crucial for high-performance SOFC cathodes. For instance, Fu et al. found that a dual-phase cathode containing both double perovskite PrBa(Co_1-xFe_x)₂O_5+δ and simple perovskite Pr_0.5Ba_0.5Co_1-xFe_xO_3-δ significantly enhanced the cathode’s oxygen catalysis [193]. Likewise, Pang et al. engineered a biomimetic ceramic catalyst resembling tree leaves, incorporating Ce_0.9Gd_0.1O_1.95 “epidermis” and “veins” externally and inside the bulk of the PrBaCo₂O_5+δ [189]. This unique design substantially improved cell performance, inducing a 79% rise in the cell’s output power density, reversing the rapid decline trend, yielding a 23% power density gain in the initial 20 h, and stabilizing at 0.91 W cm⁻² (at 750 °C and 0.7 V) [189].