Next Article in Journal
Sustainability Assessment of Alternative Energy Fuels for Aircrafts—A Life Cycle Analysis Approach
Previous Article in Journal
The Updated Scope of Fuels
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Fuels
Volume 5
Issue 4
10.3390/fuels5040045
fuels-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Review

Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells

by
Jamila Nisar
1,2,
Gurpreet Kaur
1,
Sarbjit Giddey
1,*,
Suresh Bhargava
2 and
Lathe Jones
2
1
CSIRO Energy, Private Bag 10, Clayton South, VIC 3169, Australia
2
CAMIC (Centre for Advanced Materials and Industrial Chemistry), School of Science, RMIT University, Melbourne, VIC 3000, Australia
*
Author to whom correspondence should be addressed.
Fuels 2024, 5(4), 805-824; https://doi.org/10.3390/fuels5040045
Submission received: 6 May 2024 / Revised: 24 June 2024 / Accepted: 27 August 2024 / Published: 14 November 2024
Browse Figures
Versions Notes

Abstract

:
Intermediate temperature solid oxide fuel cell (SOFC) operation provides numerous advantages such as high combined heat and power (CHP) efficiency, potentially long-term material stability, and the use of low-cost materials. However, due to the sluggish kinetics of the oxygen reduction reaction at intermediate temperatures (500–700 °C), the cathode of SOFC requires an efficient and stable catalyst. Significant progress in the development of cathode materials has been made over recent years. In this article, multiple strategies for improving the performance of cathode materials have been extensively reviewed such as A- and B-site doping of perovskites, infiltration of catalytic active materials, the use of core-shell composites, etc. Emphasis has been given to intrinsic properties such as chemical and thermal stability and oxygen transport number. Furthermore, to avoid any insulating phase formation at the cathode/electrolyte interface, strategies for interfacial layer modifications have also been extensively reviewed and summarized. Based on major technical challenges, future research directions have been proposed for efficient and stable intermediate temperature solid oxide fuel cell (SOFC) operation.

1. Introduction

1.1. Background

Fossil fuels have been the prime source of electricity generation since the mid-19th century with nuclear energy as the next major source. Electricity production from coal is the largest among fossil fuels worldwide; for instance, in Australia alone, 54% out of the total 76% of the energy was produced from coal in 2020, as shown in Figure 1 [1].
Electricity production from coal emits harmful gases (CO2, CH4, SO2, etc.) and other pollutants that significantly contribute to global warming and cause consequential health-related issues. The transition to green renewable-based energy production is the solution to improving air quality and reducing greenhouse gas emissions. However, the renewable power generation capacity must meet the expectation of growing energy demand in comparison to the well-established fossil fuel energy sector. A solid oxide fuel cell (SOFC) is one of the technologies being considered for efficient power generation with a high electrical efficiency of 60–65% [2] and combined heat and power efficiency of up to 80% [3]. Power can be generated by a solid oxide fuel cell using a range of renewable fuels [4,5]. In a case study, Yilin et al. reported a 6.22% reduction in harmful emissions and an 18.52% reduction in the acidification potential of SOFC to conventional thermal power generation in a life cycle assessment of a solid oxide fuel cell system [6].

1.2. Fundamentals of SOFC

A fuel cell is an electrochemical device that generates electricity from the chemical energy of fuel in the presence of oxygen (from air). Fuel cells have three main cell components, namely a cathode, electrolyte, and anode. The schematic of the solid oxide fuel cell is represented in Figure 2.
The oxygen supplied to the cathode is reduced to an oxide anion by capturing an electron at the cathode/electrolyte interface (reaction 1). This oxygen anion is then transported through a ceramic electrolyte and reacts with hydrogen supplied at the anode, resulting in the release of electrons and the formation of water (reaction 2) and heat as the by-products. The electrons move through the external circuit and produce electricity. Each cell can generate an open circuit voltage (OCV) of about 1 V. Many cells can be stacked together to produce the required voltage/power for practical applications.
½ O2 (g) + 2e− → O2−         (Cathode)
O2− + H2(g) → H2O(g) + 2e− (Anode)
The overall reaction is
½ O2(g) + H2 (g) → H2O(g)

2. Requirements of Different Components of Solid Oxide Fuel Cells

2.1. Anode

An anode is the fuel electrode of the cell where oxidation of the fuel takes place. An anode should have high electronic conductivity (~100 S.cm−1) and catalytic activity towards the oxidation of the fuels. In addition, an anode should have thermal stability and chemical compatibility with the other cell components. The most important region where the reaction takes place is the triple phase boundary (TPB), where the fuel, electrolyte and anode come into contact.
A mixed-ionic and electron-conducting (MIEC) Ni–Yttria-stabilized Zirconia (Ni/YSZ) composite is a state-of-the-art anode material and provides extended TPB sites from the electrode/electrolyte interface to the bulk of the electrode for efficient oxidation of fuels. However, the major issue with this material is its degradation [7] with time (degradation rate of up to −0.094% per hour [8] or 0.014 mVh−1 [9]) due to Ni agglomeration, which can cause changes in Ni morphology in the matrix [10,11] and cross-reaction with interconnects [12].

2.2. Electrolyte

In SOFC, the electrolyte is a ceramic membrane sandwiched between the air electrode and fuel electrode that only conducts ionic species through its lattice. The electrolyte is required to be dense and should have negligible electronic conductivity. Materials with oxide ion conductivity exceeding 0.01 S.cm−1 and an ionic transport number > 0.99 are typically used as electrolytes in SOFC. Traditionally SOFCs are operated at high temperatures of 800–1000 °C to enable enough ionic transport through the electrolyte. For SOFC operation (above 800 °C) the state-of-the-art oxygen ion-conducting electrolyte material is 8–10 mol% Yttria-stabilized Zirconia (YSZ), which has good chemical stability and thermal expansion coefficient (TEC) compatibility with Ni/YSZ (anode) at operating temperatures. At the cathode/electrolyte interface, YSZ reacts with cathode materials (LaMnO3) at high temperatures, i.e., >900 °C [13], contributing to cell performance degradation.
The other prominent option for electrolytes includes ceria-based electrolytes with high ionic conductivity, such as Gd-doped ceria Gd-CeO2 (GDC), Sm-doped ceria Sm-CeO2 (SDC) and La1-xSrxGa1-yMgyO3−(LSGM) [14]. However, these electrolytes are thermally incompatible with cathodes due to a large difference between the TEC values of doped ceria electrolytes (GDC = ~12.0 × 10−6 K−1 [15] and SDC = 12.6 × 10−6 K−1 at 800 °C [16]) and LSCF cathode materials (15.6 × 10−6 K−1 at 700 °C [17] and 16.3 × 10− 6 K−1 at 800 °C [18]), which needs to be addressed.

2.3. Cathode

The cathode is the oxygen electrode of the cell where oxygen reduction takes place. Like the anode, the cathode material should have electrocatalytic activity, chemical compatibility with other components, and thermal stability in cell operating conditions. The high operating temperature favors a faster oxygen reduction reaction (ORR) at the cathode, which leads to the high power density of fuel cells. However, faster degradation rates, expensive cell components and more stringent requirements for sealings are the major drawbacks of high-temperature cell operation. Reducing the temperature to 500–700 °C can potentially resolve some of these issues in addition to SOFC lifetime challenges. Some features of the high- and low-temperature solid oxide fuel cells are highlighted in Figure 3.
At low-temperature cell operations, major voltage losses are mainly due to slow reaction kinetics, which leads to a decrease in the performance of the SOFC. In recent years, extensive R&D efforts have been devoted to developing new efficient materials [19] for intermediate-temperature cell operations. This article has extensively reviewed the cathode materials but also covered in detail the various strategies adopted to improve the kinetics of the existing materials in addition to future prospects.

3. Strategies for Improvement in the Performance of Cathode Materials

3.1. Doping

Mixed-ionic and electron-conducting (MIEC) perovskite materials have been doped by many researchers to improve the electrochemical properties of the cathode materials for IT-SOFCs. The main examples of MIEC cathode materials are LaSrCoFeO3-δ (LSCF), BaSrCoFeO3-δ (BSCF) and SmCoO3-δ (SSC), etc. For solid oxide fuel cells and electrolysis applications, these materials can be used as anodes and cathodes based on their electrochemical properties and redox stability [20,21,22].
Co-based perovskites [23] have been the material of choice in many fields because of their mixed ionic and electron conductivity, thermal stability, and chemical compatibility with cell components. However, the loss of Co from the lattice at high temperatures could trigger Co reduction [24]. To tackle the physical and chemical stability issues, these perovskites have been incorporated with other metals either at the A-site [25,26], B-site [27,28,29], and O-site (Table 1).

3.1.1. Effect on Structural Stability

The doping approach has been proven beneficial as it simplifies the procedures and can easily harvest higher oxygen vacancy concentrations to enhance the oxygen flux through the material (Figure 4). Resistance of some of the LSCF-based doped cathodes has been reported by Jia et al. [18]. The sample doped with Bi+3 showed the highest electrocatalytic performance, resulting in improved ORR due to the lone pair of electrons of Bi+3. The comparatively small particle size of LBSCF (Bi-doped LSCF) resulted in an extension of a three-phase boundary. The La+3 in lanthanum cobalt ferrites at the A-site is often substituted with cations that have similar ionic radii to improve the structural stability [30].
In addition to the thermal mismatch between the cathode materials and electrolytes [14], the phase instability (e.g., the phase transition from the cubic to the hexagonal phase of BSCF at high working temperature) can also impact the oxygen ion flux. Zhu et al. [27] reported an enhancement in the oxygen permeability as well as an enhanced electrocatalytic performance of the BSCF by optimizing the doping amount of the Ba at the A-site of the perovskite. The study suggested the transition to a hexagonal phase is more pronounced by increasing the Ba content at the A-site due to the difference in the ionic radii of the Ba and Sr [34]. Weber et al. [35] reported that Ti doping at the B-site of BSCF reduced the formation of secondary phases by up to 95%. They argued that the reason for the phase stability is the existence of Co+2, which is triggered by the high valency of dopant cations. The whole process eradicates the hexagonal phase transition by controlling the CoO and other Co species, thus stabilizing the system [35].

3.1.2. Effect on Conductivity

The enhanced performance of metal cation-doped perovskites is attributed to mixed-valence couple formation (Co+3/Co+4 or Fe+2/Fe+3) and assists in the oxygen transport in the cathode microstructure. Substituting or doping smaller valence cations (larger ionic radius cations) than Co or Fe in the B-site leads to the increment in the lattice parameter creating higher amounts of oxygen vacancies in the lattice structure [36]. Introducing stress to the lattice dimensions of perovskites by doping the grains has been shown to effect the electrical conductivity due to altering of the porosity of the cathode material [37]. However, it has been observed that when considering perovskites with similar grain size yet entirely different dopants, the perovskite cathode material with more charge carrier concentration always showed higher conductivity, i.e., it had more mixed-valence cation concentration [29].
The larger number of vacancy formations is believed to reduce the electrical conductivity and hinder the oxygen transport channel along the crystal lattice through these vacant spaces. Hongfei et al. [38] reported a reduction in electrical conductivity of Nb-doped Pr0.5Sr0.5FeO3−δ (PSFN) at a high temperature (800 °C). This is due to the disrupted movement of electrons due to the increased number of vacancy formations. There are two theories related to the effect of the unit cell volume on oxygen transport; some authors reported that the increase in the unit cell volume has a positive effect on oxygen transport by increasing the size of the transport channel, which facilitates oxygen transport, while others suggest a smaller unit cell volume helps in the shortening of the distance between oxygen moieties, which favors oxygen transport [39].
Anbo et al. [20] attempted to find out the effect of the ionic radii of the rare earth metals in Nd1-xLnxBaCo2O6-δ (Ln represents La = 1.032 A°, Sm = 0.958 A°, Gd = 0.938 A° as compared to Nd = 0.938 A°) on the concentration of the oxygen vacancy and crystal structure. The Nd0.9La0.1BaCo2O6-δ was reported to have better electrochemical performance than other systems in terms of activation energy Ea, and polarization resistance Rp with larger ionic radius. The free oxygen volume facilitates oxygen movement through the lattice structure by lowering the activation energy needed for oxygen transport. The free oxygen volume Vf is calculated by the following expression.
Vf = a. b. c. − 0.75π (r3 (A1) + r3 (A2) + 2r3 (B) (6 − δ) r3 (O)
where a,b,c represent the three lattice parameters of the system, and r (A1), r (A2), r (B) and r (O) are the ionic radii of A1, A2, metal cations and the oxygen anion, respectively. Although the results supported the claims, the high TEC value (23 × 10−6 K−1) of NdLaBaCoO questions the thermal compatibility with the electrolytes, such as GDC and LSGM (11 − 13.2 × 10−6 K−1 at 800 °C [15,18]), and consequently the durability of the system [25].

3.1.3. Effect on Coefficient of Thermal Expansion (TEC)

Although metal cation substitution at the A-site is helpful for increasing the electronic conductivity and surface oxygen diffusivity, the TEC of the electrode material has been observed to be increased in many cases, for example, TEC increases with La content in Pr2-xLaxNi0.85Cu0.1Al0.05O4-δ (PNCA) cathode material, as reported by Zhou. Q et al. [40].
On the other hand, doping with high valence cations like Nb, Ti and Zr in the B-site of a perovskite is observed to improve the thermal compatibility issues. For example, TEC of Nb doped at the B-site of BSCF is 18 × 10−6K−1 in comparison to non-doped BSCF (21 × 10−6 K−1), as reported by Huang, Y et al. [41]. The lattice volume contraction caused by increased bond strength due to Nb +5 doping decreases the TEC value of the system.
Another emerging strategy to simultaneously address the conductivity-related issues and thermal mismatch is O-Site substitution (refer to Table 1). This approach showed a positive impact on the oxygen diffusion coefficient and on improving thermal expansion compatibility, as observed in PrBaCo2O5+δ [42].
Doping fluorine at the O-Site also maintained the tetragonal phase structure (0.1 < x < 0.2). The strengthening of the bond between Co-F due to more electronegative and smaller ‘F’ than ‘O’ is helpful in controlling the Co reduction at higher temperatures. The substitution of O by F reduced TEC from 24.0 × 10−6 K−1 to 20.86 × 10−6 K−1 and 16.78 × 10−6 K−1 for x = 0.1 and x = 0.2, but further improvements are still required in comparison to the TEC of SDC (12.6 × 10−6 K−1) and YSZ (10.3 × 10−6 K−1) at 800 °C. A similar trend is observed in BaCo0.4Fe0.4Zr0.1Y0.1O3-δF by Wang et al. [43]. The cubic structure of BaCo0.4Fe0.4Zr0.1Y0.1O3-δ F is stabilized by ‘O’ anion substitution. The more electronegative F− ion helped in changing the lattice structure in cubic form, as analyzed by Raman spectroscopy. The effect of the O substitution by fluorine doping seems promising for improving the cathode performance. Some of the prominent cathode materials doped with various metal cations at different lattice sites are given in Table 1.

3.2. Composites

To induce thermal compatibility between the cathode and the electrolyte, the use of composite cathodes has been observed to be a promising approach. This also helps in the formation of new oxygen ion transport channels throughout the electrodes and extends the TPB, which can significantly enhance the oxygen reduction reaction.
Table 1. The effect of dopant, doping site and optimum dopant value on the performance of perovskite/double perovskite.
Table 1. The effect of dopant, doping site and optimum dopant value on the performance of perovskite/double perovskite.
Perovskites/Double PerovskitesDopingOpt.
Value of Dopant (%)		Doping SiteCell Configuration
Electrolyte/
Anode
(Cell Type)		Testing Temp. (°C)Rp
(Ω. cm2)		Power Density
mW.cm−2 (of Cathode Material)		TEC (10−6 K−1)Cell Testing
Time (h)		Ref.
Nd1-xLnxBaCo2O6Ln = La, Sm & GdLa = 0.1 A-siteYSZ/(CGO Buffer layer)/NiO-Zr0.85Y0.15O1.95(Single cell)7000.0831.045 W.cm−2-100 h[25]
Ba0.5Sr0.5Co0.2Fe0.7Mo0.1O 3-dMo Mo = 0.1B-siteYSZ/Ba0.5Sr0.5Co0.2Fe0.7Mo0.1O3-d (BSCFM)
(Symmetrical cell)		800 0.035 418
(At 700 °C in H2 with 3% water)		-115 h[27]
PrBaCo2-xMoxO5+p Mo (x = 0–0.07)Mo = 0.03B-siteSDC/NiO-SDC
(Single Cell)		7000.067339 18.142 h[28]
PrBaCo2O5+Fx
(O-site doping)		x = 0–0.2%F− = 0.1 O-siteGDC/Ni-GDC
(Single cell)		6500.0620.679
W.cm−2		17.46100 h[42]
BaFeO3-δLa+3, Sm+3, Gd+3 (A-Site)
Zr+4 and Ce+4 (B-Site)		La+3 and Zr+4 = 0.005 A-site and B-siteSDC/Ba0.9La0.05 FeO3-δ (BFO)
(A-site Symmetrical cell)
SDC/BaFe0.95Zr0.05O3-δ (BFO)
(B-site Symmetrical cell)		7000.029 (La+3 at A-Site)
0.020 (Zr+4 B-Site)		---[44]
La0.6-xMxSr0.4Co0.2Fe0.8O3M = Ca, Ba & BiBi = 0.2%A-siteYSZ/La0.4Bi0.2Sr0.4Co0.2Fe0.8O3(LBSCF
(Symmetrical cell)		7500.1321.002 W.cm−218.1100 h at 600 °C[18]
LnxBa1-xCo0.5Fe0.3O3-δ (LnxBCF) Ln = La, Pr, NdPr = 0.1%A-siteSDC/Pr0.1Ba0.9Co0.5Fe0.3O3-δ
(Symmetrical cell)		7000.026
(ASR)		1236 16.2150 h[45]
La2-xAxNi1-yByO4+δ (LNO)x = Pr
y = Co		Pr = 0.5%
Co = 0.2%		A-site and B-siteYSZ/La1.5Pr0.5Ni0.8Co0.2O4+δ (LPNCO)
(Symmetrical cell)		6001.95400--[39]
Pr2-xLaxNi0.85Cu0.1Al0.05O4+δx = LaLa = 1%A-siteLSGM/PrLaNi0.85Cu0.1Al0.05O4+δ
(Symmetrical cell)		700 0.037
(ASR at 800 °C)		341 (at 700 °C)14.672 h[40]
Ba0.5Sr0.5(Co0.8Fe0.2)1-xZnxO3-δ (BSCFZ)x = Zn+2Zn+2 = 0.04%B-siteYSZ/Ba0.5Sr0.5 (Co0.8Fe0.2)0.96Zn0.04O3-δ (BSCFZ)
(Symmetrical cell)		6000.230.58 W.cm−2-140 h [36]
Ba0.5Sr0.5Co0.8-xFe0.2NbxO3-δ (BSCFNb) y = NbNb = 0.05%B-siteSDC/NiO-BaZr0.1 Ce0.7Y0.2O3-δ
(Single cell)		7001.14882 18.7470 h[41]
La1-xCexSr0.4Co0.2Fe0.8O3 (LCSCF)x = Ce+3Ce+3 = 0.6%A-siteGDC/Pt
(Single cell)		7500.09 ---[30]
The layered perovskites, like SmBaSrCaCoFeO5+δ (SBSCCF) with the general formula of AA’B2O5+δ, exhibit a stacked layered structure that provides high oxygen diffusivity and an O2 transport coefficient. However, these materials suffer from high TEC due to the spin state of the transition metal. Wang et al. [46] studied the effect of the addition of electrolyte material (GDC) up to 50% in a cathode material of SmBa0.5Sr0.25Ca0.25CoFeO5+δ. They observed a decrease in TEC with an increasing amount of GDC. Although the electrical conductivity of the composite material was less than the optimum value required for a good cathode material in SOFC operation (100 S.cm−1), this improved the power output by 75%. This is mainly expected to be due to the increase in TPB for the reaction. Along with the composite formation, doping also has an interesting effect on layered perovskites. Catarina et al. [47] reviewed the electrochemical properties of double-layer perovskites. They reported that these properties can be tailored in many ways as there are multiple sites available for substitution with a variety of metals, including alkaline earth metals, rare earth metals and transition metals. This can lead to an interesting trend in behavior regarding the electrical conductivity, TEC, oxygen diffusivity and the electrochemical performance of SOFC.
Similarly, ferrites have also been studied with various mixed-ion conducting phases such as composite cathodes. Gao et al. [48] used GDC in Bi0.5Sr0.5FeO 3-δ-Ce 0.9Gd0.1O1.95 and reported a power density of 709 mW.cm2 with 30 wt.% of GDC. It has been observed that the optimum amount of ion-conducting phases prevents the aggregation of the cathode particles and provides an improved microstructure. Most researchers reported the use of an ion-conducting phase of between 30 and 40 wt.% with different single/double perovskites. Doped ceria is commonly used in composites due to high ionic conductivities, as shown in Figure 5.
Although fluorites have been extensively used as a composite material for electrochemical reactions in solid oxide fuel cells and electrolysis applications, there are very limited studies on their use as single-withstanding electrodes due to low electronic conductivity [50].
Other composites have been widely explored, such as LSM/SDC by Eksioglu et al. [51] and PrBaCoO6-δ/PrBaCoTaO6 (PBC/PBCT) by Antipinskaya et al. [52]. The latter study used a composite cathode material, i.e., PBC/PBCT, which has a similar TEC and demonstrated thermal compatibility with the electrolyte (SDC) while maintaining low chemical interaction.

3.3. Infiltration/Impregnation

An infiltration technique has been reported to be effective for performance improvement of cathode materials. Conventionally, it consists of depositing thin films or nanoparticles of electrocatalytically active cathode materials into ion-conducting frameworks, such as YSZ, ScSZ, GDC, etc. The most attractive feature is that a wide range of catalytic active materials can be used in combination with MIEC [53,54,55], as discussed below.

Effect of Infiltration on TPB

The oxygen ion conduction in pure ionic conductors is only along a two-phase boundary (cathode and electrolyte surface), while in mixed ionic and electronic conductors, the ORR is extended to the whole cathode–electrolyte interface called TPB. A schematic representation of the oxide ion conduction in MIEC and pure ionic conductors is depicted in Figure 6. The extension of the triple phase boundary (TPB) is achieved by infiltrating the ion-conducting material [56]. The most frequently used MIEC are LSCF (Sr-doped Lanthanum cobalt ferrite) [57], BSCF (Sr-doped Barium Cobalt Ferrite), LSM (Sr-doped lanthanum manganite), LSC (Sr-doped lanthanum cobaltite) [58] and LCN (La0.95Co0.4Ni0.6O3) [59], etc. A schematic representation of infiltration is depicted in Figure 7. Doped ceria is a material of interest because of the high ionic conductivity and good chemical and thermal compatibility with cobaltite [60]. Doped CeO2 nanoparticles on the LSCF surface can improve the surface exchange coefficient due to the introduction of additional oxygen vacancies. Significant improvements of 59% and 50% in Rp at 650 °C and at 750 °C were reported when infiltrating SDC into LSCF, as shown by Nie et al. [61].
In addition, uniform distribution of the infiltrate can be achieved by controlling several extrinsic factors like the wetting property of the composite backbone by using organic solvents [63]. The epitaxy deposition of SSC nanoparticles on the LSCF-GDC composite cathode surface can also result in good contact between the electrolyte and the LSCF-GDC composite cathode interface, as reported by Song et al. [64].

3.4. Core–Shell Composites

Core–shell assembly is emerging as a new strategy to explore new combinations of materials. Several groups around the world are adopting new approaches to achieve a stable and efficient core–shell cathode material. A variety of combinations of the ion-conducting phase as the core and MIEC as the shell and vice versa have been investigated, as shown in Table 2. Lee et al. [65] reported controlling the particle size of the SDC (50 nm)/LSCF core/shell material resulting in a well-distributed MIEC phase shell (LSCF) to evenly cover the surface of the core underneath. This helps to increase the electrocatalytic activity and durability of the system over multiple thermal cycles. Ai et al. [66] used the rapid sintering treatment with thermal cycles of 20 secs to deposit the LSCF shell onto the LSM core to take advantage of the electrocatalytic activity of the LSCF and durability of the LSM core with the electrolyte while controlling the coarsening of particles. Jiajia et al. [67] reviewed several studies reporting impetuous and swift sintering techniques called ultrafast high-temperature sintering and flash sintering. Both these techniques are reported to have been recently adopted by several researchers for co-sintering of perovskites, specifically LSCF and composite of LSCF with ceria like GDC. It is believed that this type of fast sintering helps in co-sintering different components at a comparatively low temperature, thus helping in controlling impurity phase formation. A detailed review can be found in reference [67]. However, in the mentioned review, only a small section is dedicated to the use of these techniques in the fabrication of perovskites as cathode materials, reflecting the fact that there are not much data available on these robust techniques adopting perovskites as cathodes for SOFC.
Using nanofibrous forms of the cathode materials with a casual arrangement of MIEC and ionic phases in a core–shell assembly also helps with the modification of the microstructure and prevents aggregation upon sintering [70]. Nanofibrous microstructures are proven durable during operation due to uninterrupted oxygen diffusion and an increase in TPB [79]. An efficient cathode with hollow nanofiber morphology resulting in increased ORR, and thus high power density of 1.11 W.cm−2 at 550 °C has been reported to be due to virtuous oxygen diffusion [80]. Similarly, Zhang et al. [81] reported that the cathode polarization resistance of the LSCF/CeO2 nanofiber composite improved fivefold compared to a bare LSCF cathode.

3.5. Interface/Interfacial Layer Modifications

There are additional intrinsic and extrinsic factors that can lead to degradation of the cathode performance, like interfacial diffusion of the metal cation into the electrolyte, resulting in the formation of an insulating-phase layer at the reaction interfaces. Multiple reports on the interfacial cross-reaction between cathodes and electrolytes (GDC, SDC and even YSZ) at high temperatures suggest the need for interfacial layer modifications.
Sr segregation on the LSCF/YSZ phase is reported by Horita et al. [82]. The formation of the Sr segregation at LSCF/GDC/YSZ was evident from the SIMS (secondary ion mass spectrometry) analysis by the high peak intensity of the 18O at the GDC/YSZ interface. The Sr segregation was also evident from the SEM images, which showed the SrZrO3 layer near the GDC/YSZ interface, and the results were supported by EDS mapping. They explained two possible reasons for SrZrO3 formation at the high 18O peak intensity: (i) the voltage assisted in a faster incorporation of 18O near GDC/YSZ at the increased polarization, (ii) another possibility is the hindrance of 18O diffusion across the Sr segregation layer due to lower ion diffusivity, which caused the delay in transport of the 18O from GDC/YSZ area.
Two types of interdiffusion of elements in the LSCF/GDC cathode electrolyte interface are reported by Li et al. [83]. First is the mutual diffusion of elements between the cathode and electrolyte. An STEM-EDX analysis was performed, which showed that the interdiffusion of the cations Sr, La, Co, or Fe from the LSCF cathode into the electrolyte can take place across the interface. From the comparison of the ionic radii of the cations and their diffusion length or distance from the interface, mutual interdiffusion was suggested as a preferable diffusion mechanism. This mutual diffusion can extend up to 200 nm in diameter from the interface.
The second type of diffusion is around the grain boundaries as it is believed that grain boundaries create the pathways for the segregation of the cations. The presence of the La and other A-site cations near the grain boundary that had similar ionic radii between the elements on the two sides made them diffuse around grain boundaries.
The other factor that dictates the resistance at the interface between the cathode and electrolyte is the effect of the sintering temperature on the microstructure. The higher sintering temperature results in grain agglomeration, leading to coarsening of the particles, while lower sintering temperature causes an incomplete or poor connection—both scenarios contributing to the increased resistance of the cathode/interlayer.
Jang et al. [84] thoroughly investigated the performance efficiency of the bilayer system LSCF/GDC/YSZ interface as a function of sintering temperature. They reported an increased performance of the nanoweb-structured LSCF cathode deposited on the electrolyte with the GDC barrier layer. Impurity segregation takes place towards the cathode/electrolyte interface above the optimum sintering temperature range (1250 °C), while splitting/ruptures in the deposited GDC layers after sintering were reported when at lower temperatures. In this comprehensive study of the effect of sintering temperature on the morphology of the cathode material, the main area of concern was the GDC/YSZ interfacial layer where the Ce–Zr layer deposition caused an increased polarization resistance. However, further work is required regarding the changes to TPB and an explanation of the oxygen ion transport.
The interlayer can prevent the formation of the resistive phases with high power density. Park et al. [85] applied the interlayer of GDC at the LSCF/GDC interface. By optimizing the heat treatment during calcination, they were able to suppress the formation of the secondary insulating phases with Sr or Zr. Similarly, the importance of interfacial layer modification by use of a GDC buffer layer is well-demonstrated by Sheraz et al. [19] by controlling Sr-containing secondary phase formation during long-term operation of a single-cell based on an Sm0.5Sr0.5CoO3 (SSC) cathode. However, the thickness of the interlayer can increase the ionic transport resistance and therefore increase the overall resistance of the cell. Importantly, a substantial modification to the interface can be performed by depositing a catalyst interfacial layer between the cathode and electrolyte. The infiltrated catalyst interfacial layer must meet certain performance criteria, for example, it should be chemically stable and have a thermal expansion compatibility between the cathode and electrolyte.

4. Modifications to Cathode to Mitigate Carbon Dioxide (CO2) and Humidity Effect

The presence of CO2 and humidity in reactant gas (air, oxygen) can significantly affect the performance of the cells. For example, CO2 adsorption competes with O2 adsorption on the active sites of the cathode, thus hindering ORR at the cathode. Notably, the mechanism of the CO2 adsorption depends upon the nature of the cathode material and configuration, for example, the effect of the CO2 exposure on the ORR in pure electronic-conducting materials like LSM (where ORR is confined to the TPB) is different from the MIEC cathode material (where ORR takes place over the entire cathode rather than only at TPB).
Another important factor affecting the response of the cathode material towards CO2 exposure is the working temperature of the cell. The temperature-dependent response towards the CO2 exposure in a pure electronic conducting cathode material and in MIEC cathode materials is different due to the difference in the chemical composition, microstructure, as well as the catalytic nature [86]. Enhanced CO2 tolerance of different cathode materials is studied by exploring various strategies. Almar et al. [87] reported that using acidic 10% Y+3 doping showed an improvement not only in ORR but also the resistivity of BSCF in a CO2 atmosphere, which, according to the Lewis acid–base theory, is less likely to be affected by the acidic nature of CO2.
The improved bond energy of metal-–oxygen by substitution of cobalt with certain metals, stabilizes the chemical structure, thus decreasing the chemical adsorption of CO2. Alkaline earth metal (Sr, Ba)-containing cathode materials have been reported to form carbonates in CO2-containing environments, which rapidly degrades the cathode material for ORR. Hu et al. [88] reported the improvement and stability of antimony (Sb)-doped strontium cobalt oxide (SrCoO3-δ) in 10% CO2/air atmosphere due to an increased metal-to-oxygen bond energy. Doping 18.75% Sb shows lower affinity to carbonates as compared to the lower content of Sb doping in SrCoO3-δ.
Similarly, cells operating in humid air can affect the cathode performance. Like CO2, it is believed that water vapors undergo competitive adsorption for active sites and thus hinder ORR. Huang et al. [89] reported a severe oxygen partial pressure dependent effect of humidity on HT-SOFC LSM cathode material as compared to an IT-SOFC cathode material—LSC [89], with a more hostile effect at lower temperatures than high temperatures [90]. The cell electrocatalytic processes are not very affected by humidity at high temperatures compared to low operating temperatures. This is because the thermal energy requirement for water molecule dissociation is not met at low temperatures and the competitive adsorption of water molecules on oxygen sites affects the oxygen reduction reaction. Details on the humidity adsorption can be found in reference [84]. This is also evident from Figure 8, showing a more pronounced effect of humidity and even CO2 on the stability of an LSCF cathode material at a lower temperature of 600 °C compared to 750 °C under the same operating conditions [90].
An LSCF:GDC composite showing more tolerance to humidity has been reported in the literature even at high vapor content compared to LSM:YSZ [91]. The LSM: YSZ composite showed reversible performance in dry air. Interestingly, OCV was considerably stable suggesting that the humidity response under voltage is totally reflected by the polarization resistance. The possible explanation of the LSM:YSZ composite ORR degradation is the Sr segregation and surface morphology change [92]. The U.S. Department of Energy reported in a project some of the fundamental concepts for differences in the mechanism of Sr segregation between LSM:YSZ and LSCF:GDC. They stated that Sr segregation is directly dependent on Sr-content, along with Mn and La segregation, which is also reported at the interface toward YSZ. According to Chen et al. [93], ORR at TPB is also affected by humidity due to MnOx-insulating phase formation, which engorged towards the interface over time (Figure 9). At first, the adsorption of H2O is triggered on the YSZ surface forming hydroxyl groups (OH) upon reaction. Initiating at the TPB, the OH moves along the LSM–YSZ interface. The oxygen vacancy concentration increase is caused by the reduction of Mn3+/4+ to Mn2+ at the LSM/YSZ interface during operation under polarization conditions. This results eventually in the formation of Mn-containing precipitates at the TPB due to its impenetrability in the zirconia lattice. During prolonged operation under cathodic conditions, this marks the delamination at the interface [93].
In the case of LSCF, H2O molecules are chemically bound to Co, thus filling the lattice vacancies, and hindering oxygen transport. Co (cobalt) and Fe (Iron) percentage content seem to be interfering with the degree of lattice expansion/contraction, LSCF lattices containing a higher percentage of iron expanding more with humidity [94].
Similarly, a comparative study of electrochemical performance and Cr poisoning in humid conditions between LSM:YSZ and LSF:GDC composites was carried out by Wang et al. [95]. The results suggested that the LSF:GDC composite is less affected in humid air comparatively and even showed improved electrochemical performance because of the presence of H2O molecules in the ambient environment. Nanocrystals of Sr accumulation on the surface in the presence of humid air were reported in LSC (La6.0Sr4.0CoO3-δ) by Egger et al. [96], who studied the dominant effect of the microstructure and synthesis method in offering the protection of LSC material against humidity. The LSC infiltrated on GDC scaffolds with larger surface area and optimum pore content in microstructures was reported as helpful in providing stability in humid air. Thus, approaches like doping with metals, which are believed to increase the stability of the cathode material, are tested for increasing stability in humid conditions [97]. Good adhesion between the cathode and electrolyte is also reported to decrease the adverse effects of humidity on the cathode compared to the unimproved interface [98]. The composite formation in core–shell assembly by introducing a protective layer is reported to have improved humidity tolerance of different cathode materials for IT-SOFC.

5. Conclusions

Perovskites are the material of choice for cathodes at intermediate temperatures in SOFC operation. The unique electrical characteristics offered by the two metals in the crystal lattice provide a substantial scope for R&D towards the optimization of cathode properties. Figure 10 is the schematic representation of overall discussion. The possibility of a vast variety of dopants at the A-site and/or B-site can change the material’s intrinsic properties. The change in the subatomic levels of the material can offer improvements in the electrical properties, such as the charge carrier concentration and their movement across the lattice due to the introduction of new energy levels and changes in the band gaps. Altering the charge transport mechanism and redox potential offers improved ionic/electrical conductivity due to mixed-valence cation formation (M+3/M+4 or M+2/M+3) and this can lead to improvement in the electrocatalytic activity as well as thermal properties, such as TEC.
Dopants such as rare earth metals, alkaline earth metals and transition metals of stable oxidation states and similar ionic radii have been explored as a substitution in perovskites. Iron doping in perovskites is believed to reduce the TEC. In addition to improvements in ionic/electronic conductivity, the stability can also be tailored by such substitutions. Interestingly, non-metallic substitution at the O-site has also been demonstrated and has shown improved electrical conductivity and TEC simultaneously. However, this strategy has not been widely explored.
Notably, the cathode characteristics improved by doping do not meet all the requirements for a good cathode material in terms of conductivities and thermal compatibility with state-of-the-art electrolytes. In many cases, there is a trade-off on the TEC while improving electrical properties or vice versa. Extensive studies and innovative research still need to be adopted to fully explore the potential of this strategy.
On the other hand, composite formation between the cathode materials, such as cobaltite, manganite or ceria-based cathode materials and the electrolyte material (doped ceria or doped zirconia) results in an improvement in TEC compatibility as well as in the extension of TPB. However, co-sintering of the constituent phases often results in the insulating phase formation or agglomeration along the grain boundary in the extended active area (TPB), resulting in the degradation of the performance. The optimization of factors like chemical and thermal compatibility, synthesis technique, phase stability, particle distribution, sintering temperature, concentration of the constituents and external parameters, like temperature and pressure, could help in the improved performance while controlling the growth of secondary phases.
Infiltration is a widely used technique in producing efficient catalyst materials for IT SOFC cathodes. A wide range of selection materials broadens the scale of tuning the electrochemical properties. Still, the use of complex methods adds to the cost and effort and restricts the scalability and reproducibility of some of the infiltration techniques. Non-uniform infiltration can also disrupt the oxygen conduction path, which can significantly affect the performance of SOFC. The interfacial modification by smearing barrier layers in the micro/nm range through different advanced techniques can control the cross diffusion across interfaces. This review suggests that the role of ionic barrier layers on ORR mechanisms and kinetics is still not clear, suggesting a direction of future research in this field.
In conclusion, perovskites have been studied broadly for energy conversion and storage applications. The cathode of the IT-SOFC presents a significant contribution to performance degradation over time. The development of an efficient cathode material in the intermediate temperature range is still in the premature stage. The research up to the present time lacks comprehensive fundamental insight into the relationship between the electrochemical performance of the perovskites to different parameters like material chemistry, synthesis route, electrochemical properties and durability over a long period of operation. New approaches offering significant headway in understanding surface and bulk chemistry should be adopted for cathode material development. One of the possible approaches is to accustom the in-situ characterization of the materials to identify real-time processes taking place during operation. A significant amount of dedication is required to realize the full potential of perovskites as an efficient cathode material for IT-SOFC. Apprehending all these challenges can lead to the guidelines to explore perovskite in other fields like gas sensors, permeable membranes, solar cells and batteries, etc.

Author Contributions

Conceptualization of the article, S.G., G.K. and J.N.; writing—original draft preparation, J.N.; writing—review and editing, S.G., G.K., L.J. and S.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the CSIRO Hydrogen Future Science Platform (FSP).

Acknowledgments

This review was supported by the CSIRO Hydrogen Future Science Platform (FSP). he authors gratefully acknowledge the funding support from the CSIRO Hydrogen Energy Systems Future Science Platform HES FSP. The authors would also like to acknowledge the Royal Melbourne Institute of Technology (RMIT), Melbourne, Australia, for collaborating in this review. We are thankful to Dattatray Dhawale for reviewing the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

SOFCSolid Oxide Fuel Cell
HT-SOFCHigh-Temperature solid oxide fuel cell
IT-SOFCIntermediate Temperature solid oxide fuel cell
SOECSolid oxide electrolysis cell
PEMFCPolymer electrolyte membrane fuel cell
ORROxygen reduction reaction
TPBTriple phase boundary
NOxOxides of nitrogen
SOxOxides of Sulphur
DESDistributed Energy system
BOPBalance of plant
NGNatural gas
OCVOpen circuit voltage
ABO3A general formula for Perovskites
MIECMixed ionic and electronic conductor
Ni-YSZNickle–Yttrium stabilized zirconia
YSZYttrium stabilized zirconia
ScSZScandium stabilized zirconia
GDCGadolinium doped ceria
SDCSamarium doped ceria
LSGMStrontium magnesium doped Lanthanum gallate
LSMLanthanum strontium manganite
LSCFLanthanum (La) Strontium (Sr) Cobalt (Co) ferrite
BSCFBarium (Ba) Strontium (Sr) Cobalt (Co) ferrite
SSCSamarium (Sm) doped Strontium (Sr) cobalt oxide
R&DResearch and development
ASRArea-specific resistance
TGAThermal Gravitational Analysis
SEMScanning Electron Microscope
SIMSSecondary Ion Mass Spectroscopy
EDSEnergy Dispersive Spectroscopy
Symbols
δOxygen non-stoichiometry
K−1Per Kelvin
S.cm−1Siemens per centimeter
VfFree oxygen volume
D *Electronic conductivity
RpCathode Polarization/resistance
RohmElectrolyte polarization/resistance
EaActivation energy
RIonic radius
a,b,cLattice parameters of the cubic system along with three dimensions
Ω.cm2Ohm centimeter square
W.cm−2Watt per centimeter square
mW.cm−2Milliwatt per centimeter square
LnRare earth metals

References

  1. Australia’s Department of Industry, Innovation, and Science, Office of the Chief Economist. Australian Energy Update 2021; Australia’s Department of Industry, Innovation, and Science, Office of the Chief Economist: Canberra, Australia, 2021; pp. 27–30. [Google Scholar]
  2. Chen, J.; Ni, M. Economic analysis of a solid oxide fuel cell cogeneration/trigeneration system for hotels in Hong Kong. Energy Build. 2014, 75, 160–169. [Google Scholar] [CrossRef]
  3. Baldi, F.; Wang, L.; Pérez-Fortes, M.; Maréchal, F. A Cogeneration System Based on Solid Oxide and Proton Exchange Membrane Fuel Cells With Hybrid Storage for Off-Grid Applications. Front. Energy Res. 2019, 6, 139. [Google Scholar] [CrossRef]
  4. Nishimura, A.; Hayashi, Y.; Ito, S.; Kolhe, M.L. Performance Analysis of Hydrogen Production for a Solid Oxide Fuel Cell System Using a Biogas Dry Reforming Membrane Reactor with Ni and Ni/Cr Catalysts. Fuels 2023, 4, 295–313. [Google Scholar] [CrossRef]
  5. Amhamed, A.I.; Shuibul, Q.S.; Hewlett, S.; Sodiq, A.; Abdellatif, Y.; Isaifan, R.J.; Alrebei, O.F. Ammonia Production Plants—A Review. Fuels 2022, 3, 408–435. [Google Scholar] [CrossRef]
  6. Shen, Y.; Yang, Y.; Song, L.; Lei, T. Environmental Impact Assessment of Solid Oxide Fuel Cell Power Generation System Based on Life Cycle Assessment—A Case Study in China. Sustainability 2024, 16, 3863. [Google Scholar] [CrossRef]
  7. Song, B.; Ruiz, -T.E.; Bertei, A.; Brandon, N.P. Quantification of the degradation of Ni-YSZ anodes upon redox cycling. J. Power Sources 2018, 374, 61–68. [Google Scholar] [CrossRef]
  8. Jiao, Z.; Lee, G.; Shikazono, N.; Kasagi, N. Quantitative Study on the Correlation Between Solid Oxide Fuel Cell Ni-YSZ Composite Anode Performance and Sintering Temperature Based on Three-dimensional Reconstruction. J. Electrochem. Soc. 2012, 159, F278–F286. [Google Scholar] [CrossRef]
  9. Iwata, T. Characterization of Ni-YSZ anode degradation for substrate-type solid oxide fuel cells. J. Electrochem. Soc. 1996, 143, 1521–1525. [Google Scholar] [CrossRef]
  10. Liu, Y.; Shao, Z.; Mori, T.; Jiang, S.P. Development of nickel based cermet anode materials in solid oxide fuel cells—Now and future. Mater. Rep. Energy 2021, 1, 100003. [Google Scholar] [CrossRef]
  11. Vafaeenezhad, S.; Hanifi, A.R.; Laguna-Bercero, M.A.; Etsell, T.H.; Sarkar, P. Microstructure and long-term stability of Ni–YSZ anode supported fuel cells: A review. Mater. Futures 2022, 1, 042101. [Google Scholar] [CrossRef]
  12. Heidarpour, A.; Saidi, A.; Abbasi, M.H.; Choi, G.M. In situ fabrication mechanism of a dense Sr and Ca doped lanthanum chromite interconnect on Ni-YSZ anode of a solid oxide fuel cell during co-sintering. Ceram. Int. 2013, 39, 1821–1826. [Google Scholar] [CrossRef]
  13. Horita, T. Microstructures and oxygen diffusion at the LaMnO3 film/yttria-stabilized zirconia interface. Solid State Ion. 2002, 152–153, 439–446. [Google Scholar] [CrossRef]
  14. Hussain, S.; Yangping, L.J.E.T. Review of solid oxide fuel cell materials: Cathode, anode, and electrolyte. Energy Transit. 2020, 4, 113–126. [Google Scholar] [CrossRef]
  15. Zhang, L.; Liu, M.; Huang, J.; Song, Z.; Fu, Y.; Chang, Y.; Li, C.; He, T. Improved thermal expansion and electrochemical performances of Ba0.6Sr0.4Co0.9Nb0.1O3−δ–Gd0.1Ce0.9O1.95 composite cathodes for IT-SOFCs. Int. J. Hydrogen Energy 2014, 39, 7972–7979. [Google Scholar] [CrossRef]
  16. Yin, S.; Li, M.; Zeng, Y.; Li, C.; Chen, X.; Ye, Z. Study of Sm0.2Ce0.8O1.9 (SDC) electrolyte prepared by a simple modified solid-state method. J. Rare Earths 2014, 32, 767–771. [Google Scholar] [CrossRef]
  17. Fan, B.; Yan, J.; Yan, X. The ionic conductivity, thermal expansion behavior, and chemical compatibility of La0.54Sr0.44Co0.2Fe0.8O3-δ as SOFC cathode material. Solid State Sci. 2011, 13, 1835–1839. [Google Scholar] [CrossRef]
  18. Jia, W.; Huang, Z.; Sun, W.; Wu, L.; Zheng, L.; Wang, Y.; Huang, J.; Yang, X.; Lv, M.; Ge, L. Flexible A-site doping La0.6-xMxSr0.4Co0.2Fe0.8O3 (M = Ca, Ba, Bi; x = 0, 0.1, 0.2) as novel cathode material for intermediate-temperature solid oxide fuel cells: A first-principles study and experimental exploration. J. Power Sources 2021, 490, 229564. [Google Scholar] [CrossRef]
  19. Ahmed, S.; Kazmi, W.W.; Hussain, A.; Khan, M.Z.; Bibi, S.; Saleem, M.; Song, R.H.; Sajid, Z.; Ullah, A.; Khan, M.K. Facile and low-temperature synthesis approach to fabricate Sm0.5Sr0.5CoO3−δ cathode material for solid oxide fuel cell. J. Korean Ceram. Soc. 2023, 60, 272–282. [Google Scholar] [CrossRef]
  20. Jung, W.; Tuller, H.L. Investigation of surface Sr segregation in model thin film solid oxide fuel cell perovskite electrodes. Energy Environ. Sci. 2012, 5, 5370–5378. [Google Scholar] [CrossRef]
  21. Liu, Q.; Yang, C.; Dong, X.; Chen, F. Perovskite Sr2Fe1.5Mo0.5O6−δ as electrode materials for symmetrical solid oxide electrolysis cells. Int. J. Hydrog. Energy 2010, 35, 10039–10044. [Google Scholar] [CrossRef]
  22. Dey, S.; Mukhopadhyay, J.; Lenka, R.K.; Patro, P.K.; Sharma, A.D.; Mahata, T.; Basu, R.N. Synthesis and characterization of Nanocrystalline Ba0·6Sr0·4Co0·8Fe0·2O3 for application as an efficient anode in solid oxide electrolyser cell. Int. J. Hydrogen Energy 2020, 45, 3995–4007. [Google Scholar] [CrossRef]
  23. Chen, F.; Zhou, D.; Xiong, X.; Pan, J.; Cai, D.; Wei, Z.; Chen, X.; Liu, Y.; Luo, N.; Yan, J.; et al. Doping strategy on improving the overall cathodic performance of double perovskite LnBaCo2O5+δ (Ln = Pr, Gd) as potential SOFC cathode materials. J. Mater. 2023, 9, 825–837. [Google Scholar] [CrossRef]
  24. Stevenson, J.W.; Armstrong, T.R.; Carneim, R.D.; Pederson, L.R.; Weber, W.J. Electrochemical Properties of Mixed Conducting Perovskites La1−x MxCo1−yFey O3−δ (M = Sr, Ba, Ca). J. Electrochem. Soc. 1996, 143, 2722–2729. [Google Scholar] [CrossRef]
  25. Anbo, Y.; Tian, X.; Liping, S.; Qiang, L.; Lihua, H.; Hui, Z. Effects of rare earth doping on electrochemical properties of NdBaCo2O6- cathode materials. J. Alloys Compd. 2020, 837, 155563. [Google Scholar] [CrossRef]
  26. Barros Julião, P.S. A-site cation influences on performance, structure and conductivity of a lanthanide-based perovskite electrode for symmetrical solid oxide fuel cells. J. Power Sources 2020, 450, 227723. [Google Scholar] [CrossRef]
  27. Yang, X.; Li, R.; Yang, Y.; Wen, G.; Tian, D.; Lu, X.; Ding, Y.; Chen, Y.; Lin, B. Improving stability and electrochemical performance of Ba0.5Sr0.5Co0.2Fe0.8O3-δ electrode for symmetrical solid oxide fuel cells by Mo doping. J. Alloys Compd. 2020, 831, 154711. [Google Scholar] [CrossRef]
  28. Xu, J.; Cai, H.; Hao, G.; Zhang, L.; Song, Z.; Long, W.; Zhang, L.; Wang, L. Characterization of high–valence Mo–doped PrBaCo2O5+ cathodes for IT–SOFCs. J. Alloys Compd. 2020, 842, 155600. [Google Scholar] [CrossRef]
  29. Sowjanya, C.; Mandal, R.; Abhinay, S.; Mohanta, A.; Das, S.; Pratihar, S.K. Effect of B-site substitution on the crystal structure, electrical conductivity and oxygen transport properties of La0.5Sr0.5M0.2Fe0.8O3-δ (M = Co, Al, and Zn) perovskite. J. Solid State Chem. 2020, 285, 121237. [Google Scholar] [CrossRef]
  30. Zhou, F.; Liu, Y.; Zhao, X.; Tang, W.; Yang, S.; Zhong, S.; Wei, M. Effects of cerium doping on the performance of LSCF cathodes for intermediate temperature solid oxide fuel cells. Int. J. Hydrogen Energy 2018, 43, 18946–18954. [Google Scholar] [CrossRef]
  31. Chen, K.; He, S.; Li, N.; Cheng, Y.; Ai, N.; Chen, M.; Rickard, W.D.A.; Zhang, T.; Jiang, S.P. Nb and Pd co-doped La0.57Sr0.38Co0.19Fe0.665Nb0.095Pd0.05O3-δ as a stable, high performance electrode for barrier-layer-free Y2O3-ZrO2 electrolyte of solid oxide fuel cells. J. Power Sources 2018, 378, 433–442. [Google Scholar] [CrossRef]
  32. Serra, J.M.; Buchkremer, H.-P. On the nanostructuring and catalytic promotion of intermediate temperature solid oxide fuel cell (IT-SOFC) cathodes. J. Power Sources 2007, 172, 768–774. [Google Scholar] [CrossRef]
  33. Liu, Y.; Zhou, F.; Chen, X.; Wang, C.; Zhong, S. Enhanced electrochemical activity and stability of LSCF cathodes by Mo doping for intermediate temperature solid oxide fuel cells. J. Appl. Electrochem. 2021, 51, 425–433. [Google Scholar] [CrossRef]
  34. Zhu, Y.; Cai, L.; Li, W.; Cao, Z.; Li, H.; Jiang, H.; Zhu, X.; Yang, W. A permeation model study of oxygen transport kinetics of BaxSr1-xCo0.8Fe0.2O3-δ. AIChE J. 2020, 66, e16291. [Google Scholar] [CrossRef]
  35. Weber, V.; Meffert, M.; Wagner, S.; Störmer, H.; Unger, L.S.; Ivers, T.E.; Gerthsen, D. Influence of B-site doping with Ti and Nb on microstructure and phase constitution of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ. J. Mater. Sci. 2019, 55, 947–966. [Google Scholar] [CrossRef]
  36. Zeng, Q.; Zhang, X.; Wang, W.; Zhang, D.; Jiang, Y.; Zhou, X.; Lin, B. A Zn-Doped Ba0.5Sr0.5Co0.8Fe0.2O3-δ Perovskite Cathode with Enhanced ORR Catalytic Activity for SOFCs. Catalysts 2020, 10, 235. [Google Scholar] [CrossRef]
  37. Mushtaq, N.; Xia, C.; Dong, W.; Abbas, G.; Raza, R.; Ali, A.; Rauf, S.; Wang, B.; Kim, J.S.; Zhu, B. Perovskite SrFe1-xTixO3-δ (x <= 0.1) cathode for low temperature solid oxide fuel cell. Ceram. Int. 2018, 44, 10266–10272. [Google Scholar]
  38. Chen, H.; Lü, Z.; Wu, Y. Enhanced Cathode Performance in Pr0.5Sr0.5FeO3−δ of Perovskite Catalytic Materials via Doping with VB Subgroup Elements (V, Nb, and Ta). Materials 2024, 17, 1635. [Google Scholar] [CrossRef]
  39. Navarrete, L.; Fabuel, M.; Yoo, C.Y.; Serra, J.M. Tailoring La2-XAXNi1-YBYO4+δcathode performance by simultaneous A and B doping for IT-SOFC. Int. J. Hydrogen Energy 2020, 45, 15589–15599. [Google Scholar] [CrossRef]
  40. Zhou, Q.; Qu, L.; Zhang, T.; He, Y.; Zhao, C.; Wang, M.; Wei, T.; Zhang, Y. Preparation and electrochemical properties of an La-doped Pr2Ni0.85Cu0.1Al0.05O4+δcathode material for an IT-SOFC. J. Alloys Compd. 2020, 824, 153967. [Google Scholar] [CrossRef]
  41. Huang, Y.; Ding, J.; Xia, Y.; Miao, L.; Li, K.; Zhang, Q.; Liu, W. Ba0.5Sr0.5Co0.8-xFe0.2NbxO3-δ (x ≤ 0.1) as cathode materials for intermediate temperature solid oxide fuel cells with an electron-blocking interlayer. Ceram. Int. 2020, 46, 10215–10223. [Google Scholar] [CrossRef]
  42. Wan, Y.; Xing, Y.; Li, Y.; Huan, D.; Xia, C. Thermal cycling durability improved by doping fluorine to PrBaCo2O5+δ as oxygen reduction reaction electrocatalyst in intermediate-temperature solid oxide fuel cells. J. Power Sources 2018, 402, 363–372. [Google Scholar] [CrossRef]
  43. Wang, W.; Zhang, X.; Zhang, D.; Zeng, Q.; Jiang, Y.; Lin, B. Highly promoted performance of triple-conducting cathode for YSZ-based SOFC via fluorine anion doping. Ceram. Int. 2020, 46, 23964–23971. [Google Scholar] [CrossRef]
  44. Wang, J.; Saccoccio, M.; Chen, D.; Gao, Y.; Chen, C.; Ciucci, F. The effect of A-site and B-site substitution on BaFeO3−δ: An investigation as a cathode material for intermediate-temperature solid oxide fuel cells. J. Power Sources 2015, 297, 511–518. [Google Scholar] [CrossRef]
  45. Zhang, W.; Zhang, L.; Guan, K.; Zhang, X.; Meng, J.; Wang, H.; Liu, X.; Meng, J. Effective promotion of oxygen reduction activity by rare earth doping in simple perovskite cathodes for intermediate-temperature solid oxide fuel cells. J. Power Sources 2020, 446, 227360. [Google Scholar] [CrossRef]
  46. Wang, S.; Yang, H.; Zheng, Y.; Ge, L.; Chen, H.; Guo, L. Effect of electrolyte composite on the performance of SmBa0.5Sr0.25Ca0.25CoFeO5+δ cathode for IT-SOFCs. Ionics 2019, 26, 281–291. [Google Scholar] [CrossRef]
  47. Mendonça, C.; Ferreira, A.; Santos, D.M.F. Towards the Commercialization of Solid Oxide Fuel Cells: Recent Advances in Materials and Integration Strategies. Fuels 2021, 2, 393–419. [Google Scholar] [CrossRef]
  48. Gao, J.; Li, Q.; Guo, M.; Sun, L.; Huo, L.; Zhao, H. Improved electrochemical activity of Bi0.5Sr0.5FeO3-δ–Ce0.9Gd0.1O1.95composite cathode electrocatalyst for solid oxide fuel cells. Ceram. Int. 2021, 47, 748–754. [Google Scholar] [CrossRef]
  49. Eguchi, K.; Setoguchi, T.; Inoue, T.; Arai, H. Electrical properties of ceria-based oxides and their application to solid oxide fuel cells. Solid State Ion. 1992, 52, 165–172. [Google Scholar] [CrossRef]
  50. Ishihara, T.; Xie, J.; Shin, T.H.; Ju, Y.W.; Ida, S.; Kilner, J.A. Bi doped Pr6O11 as fluorite oxide cathode for all-fluorite solid oxide fuel cells. J. Power Sources 2015, 275, 167–174. [Google Scholar] [CrossRef]
  51. Eksioglu, A.; Colakerol, A.L.; Sezen, M.; Ow, Y.C.; Buyukaksoy, A. Formation of Nanocomposite Solid Oxide Fuel Cell Cathodes by Preferential Clustering of Cations from a Single Polymeric Precursor. ACS Appl. Mater. Interfaces 2019, 11, 47904–47916. [Google Scholar] [CrossRef]
  52. Antipinskaya, E.A.; Politov, B.; Osinkin, D.; Suntsov, A.; Kozhevnikov, V. Electrochemical performance and superior CO2 endurance of PrBaCo2O6–δ–PrBaCoTaO6 composite cathode for IT-SOFCs. Electrochim. Acta 2021, 365, 137372. [Google Scholar] [CrossRef]
  53. Ding, X.F.; Zhu, W.L.; Hua, G.X.; Li, J.F.; Wu, Z.C. Enhanced oxygen reduction activity on surface-decorated perovskite La0.6Ni0.4FeO3 cathode for solid oxide fuel cells. Electrochim. Acta 2015, 163, 204–212. [Google Scholar] [CrossRef]
  54. Shen, J.; Chen, Y.B.; Yang, G.M.; Zhou, W.; Tade, M.O.; Shao, Z.P. Impregnated LaCo0.3Fe0.67Pd0.03O3-delta as a promising electrocatalyst for “symmetrical” intermediate-temperature solid oxide fuel cells. J. Power Sources 2016, 306, 92–99. [Google Scholar] [CrossRef]
  55. Giuliano, A.; Carpanese, M.P.; Clematis, D.; Boaro, M.; Pappacena, A.; Deganello, F.; Liotta, L.F.; Barbucci, A. Infiltration, Overpotential and Ageing Effects on Cathodes for Solid Oxide Fuel Cells: La0.6Sr0.4Co0.2Fe0.8O3-delta versus Ba0.5Sr0.5Co0.8Fe0.2O3-delta. J. Electrochem. Soc. 2017, 164, F3114–F3122. [Google Scholar] [CrossRef]
  56. Endler-Schuck, C.; Joos, J.; Niedrig, C.; Weber, A.; Ivers-Tiffée, E. The chemical oxygen surface exchange and bulk diffusion coefficient determined by impedance spectroscopy of porous La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF) cathodes. Solid State Ion. 2015, 269, 67–79. [Google Scholar] [CrossRef]
  57. Shah, M.; Voorhees, P.W.; Barnett, S.A. Time-dependent performance changes in LSCF-infiltrated SOFC cathodes: The role of nano-particle coarsening. Solid State Ion. 2011, 187, 64–67. [Google Scholar] [CrossRef]
  58. Song, X.; Lee, S.; Chen, Y.; Gerdes, K. Electrochemically influenced cation inter-diffusion and Co3O4 formation on La0.6Sr0.4CoO3 infiltrated into SOFC cathodes. Solid State Ion. 2015, 278, 91–97. [Google Scholar] [CrossRef]
  59. Ovtar, S.; Chen, M.; Samson, A.J.; Kiebach, R. In-situ formed Ce0.8Gd0.2O1.9 barrier layers on yttria stabilized zirconia backbones by infiltration—A promising path to high performing oxygen electrodes of solid oxide cells. Solid State Ion. 2017, 304, 51–59. [Google Scholar] [CrossRef]
  60. Hu, B.; Wang, Y.; Xia, C. Effects of Ceria Conductivity on the Oxygen Incorporation at the LSCF-SDC-Gas Three-Phase Boundary. J. Electrochem. Soc. 2015, 162, F33–F39. [Google Scholar] [CrossRef]
  61. Nie, L.; Liu, M.; Zhang, Y.; Liu, M. La0.6Sr0.4Co0.2Fe0.8O3−δ cathodes infiltrated with samarium-doped cerium oxide for solid oxide fuel cells. J. Power Sources 2010, 195, 4704–4708. [Google Scholar] [CrossRef]
  62. Irshad, M.; Siraj, K.; Raza, R.; Ali, A.; Tiwari, P.; Zhu, B.; Rafique, A.; Ali, A.; Kaleem Ullah, M.; Usman, A. A Brief Description of High Temperature Solid Oxide Fuel Cell’s Operation, Materials, Design, Fabrication Technologies and Performance. Appl. Sci. 2016, 6, 75. [Google Scholar] [CrossRef]
  63. Tomov, R.I.; Mitchell-Williams, T.; Gao, C.; Kumar, R.V.; Glowacki, B.A. Performance optimization of LSCF/Gd:CeO2 composite cathodes via single-step inkjet printing infiltration. J. Appl. Electrochem. 2017, 47, 641–651. [Google Scholar] [CrossRef] [PubMed]
  64. Song, Y.-H.; Rehman, S.-U.; Kim, H.-S.; Song, H.-S.; Song, R.-H.; Lim, T.-H.; Hong, J.-E.; Park, S.-J.; Huh, J.-Y.; Lee, S.-B. Facile surface modification of LSCF/GDC cathodes by epitaxial deposition of Sm0.5Sr0.5CoO3via ultrasonic spray infiltration. J. Mater. Chem. A Mater. Energy Sustain. 2020, 8, 3967–3977. [Google Scholar] [CrossRef]
  65. Lee, S.; Lee, K.; Kang, S.; Kang, J.; Song, S.; Bae, J. Investigation of electrospun Ba0.5Sr0.5Co0.8Fe0.2O3−δ-Gd0.1Ce0.9O1.95 cathodes for enhanced interfacial adhesion. Int. J. Hydrogen Energy 2018, 43, 21535–21546. [Google Scholar] [CrossRef]
  66. Ai, N.; Chen, K.; Jiang, S.P. A La0.8Sr0.2MnO3/La0.6Sr0.4Co0.2Fe0.8O3−δ core–shell structured cathode by a rapid sintering process for solid oxide fuel cells. Int. J. Hydrogen Energy 2017, 42, 7246–7251. [Google Scholar] [CrossRef]
  67. Wu, J.; Wu, X.; Gao, Y.; Yan, Z. Innovations in Electric Current-Assisted Sintering for SOFC: A Review of Advances in Flash Sintering and Ultrafast High-Temperature Sintering. Appl. Sci. 2024, 14, 3953. [Google Scholar] [CrossRef]
  68. Kamlungsua, K.; Lee, T.-H.; Lee, S.; Su, P.-C.; Yoon, Y.-J. Inkjet-printed Ag@SDC core-shell nanoparticles as a high-performance cathode for low-temperature solid oxide fuel cells. Int. J. Hydrogen Energy 2021, 46, 30853–30860. [Google Scholar] [CrossRef]
  69. Yang, T.; Ramasamy, D.; Queirós, R.P.; Loureiro, F.J.A.; de Almeida, C.M.R.; Julião, P.S. The annealing influence on the microstructure and performance of Au@Ni core-shell bimetal as the cathode of low-temperature solid oxide fuel cells. Int. J. Hydrogen Energy 2015, 40, 4980–4988. [Google Scholar] [CrossRef]
  70. Yang, J.; Wang, J.; Fu, L.; Wu, K.; Liu, Z.; Wu, K.; Zhou, J. Electrospun Core–Shell Fibers for High-Efficient Composite Cathode-Based Solid Oxide Fuel Cells. Energy Fuels 2021, 35, 1768–1778. [Google Scholar] [CrossRef]
  71. Ding, X.; Zhao, X.; Yan, Y.; Li, M. Enhancing oxygen reduction activity of perovskite cathode decorated with core@shell nano catalysts. Int. J. Hydrogen Energy 2019, 44, 22122–22128. [Google Scholar] [CrossRef]
  72. Qiu, P.; Wang, A.; Li, J.; Li, Z.; Jia, L.; Chi, B.; Pu, J.; Li, J. Promoted CO2-poisoning resistance of La0.8Sr0.2MnO3−δ-coated Ba0.5Sr0.5Co0.8Fe0.2O3−δ cathode for intermediate temperature solid oxide fuel cells. J. Power Sources 2016, 327, 408–413. [Google Scholar] [CrossRef]
  73. Lee, D.; Jung, I.; Lee, S.O.; Hyun, S.H.; Jang, J.H.; Moon, J. Durable high-performance Sm0.5Sr0.5CoO3–Sm0.2Ce0.8O1.9 core-shell type composite cathodes for low temperature solid oxide fuel cells. Int. J. Hydrogen Energy 2011, 36, 6875–6881. [Google Scholar] [CrossRef]
  74. Wang, L.; Wang, P.; Geng, C.; Cao, H.; Xu, C.; Cheng, J.; Hong, T. A novel core-shell LSCF perovskite structured electrocatalyst with local hetero-interface for solid oxide fuel cells. Int. J. Hydrogen Energy 2020, 45, 11824–11833. [Google Scholar] [CrossRef]
  75. Li, J.; Qiu, P.; Xia, M.; Jia, L.; Chi, B.; Pu, J.; Li, J. Microstructure optimization for high-performance PrBa0.5Sr0.5Co1.5Fe0.5O5+δ-La2NiO4+δ core-shell cathode of solid oxide fuel cells. J. Power Sources 2018, 379, 206–211. [Google Scholar] [CrossRef]
  76. Qiu, P.; Li, J.; Jia, L.; Chi, B.; Pu, J.; Li, J.; Chen, F. LaCoO3-δ coated Ba0.5Sr0.5Co0.8Fe0.2O3-δ cathode for intermediate temperature solid oxide fuel cells. Electrochim. Acta 2019, 319, 981–989. [Google Scholar] [CrossRef]
  77. Li, D.; Zhang, X.; Liang, C.; Jin, Y.; Fu, M.; Yuan, J.; Xiong, Y. Study on durability of novel core-shell-structured La0.8Sr0.2Co0.2Fe0.8O3-δ@Gd0.2Ce0.8O1.9 composite materials for solid oxide fuel cell cathodes. Int. J. Hydrogen Energy 2021, 46, 28221–28231. [Google Scholar] [CrossRef]
  78. Li, F.; Xu, Y.; Xia, S.; Liu, J.; Yan, Y.; Cheng, F.; Jiang, L.; Huang, Y. LaSrCoO4±@La0.5Sr0.5CoO3- core-shell hybrid as the cathode materials for solid oxide fuel cells. J. Alloys Compd. 2020, 819, 152996. [Google Scholar] [CrossRef]
  79. Kim, C.; Jang, I.; Kim, S.; Yoon, H.; Paik, U. Ba0.5Sr0.5Co0.8Fe0.2O3-δ/Gd0.1Ce0.9O2-δ Core/shell Nanofiber Via One-Step Electrospinning for Cathode of LT-SOFCs. ECS Trans. 2017, 78, 637–641. [Google Scholar] [CrossRef]
  80. Chen, Y.; Bu, Y.; Zhao, B.; Zhang, Y.; Ding, D.; Hu, R.; Wei, T.; Rainwater, B.; Ding, Y.; Chen, F.; et al. A durable, high-performance hollow-nanofiber cathode for intermediate-temperature fuel cells. Nano Energy 2016, 26, 90–99. [Google Scholar] [CrossRef]
  81. Zhang, W.; Wang, H.; Guan, K.; Wei, Z.; Zhang, X.; Meng, J.; Liu, X.; Meng, J. La0.6Sr0.4Co0.2Fe0.8O3−δ/CeO2 Heterostructured Composite Nanofibers as a Highly Active and Robust Cathode Catalyst for Solid Oxide Fuel Cells. ACS Appl. Mater. Interfaces 2019, 11, 26830–26841. [Google Scholar] [CrossRef]
  82. Horita, T.; Nishi, M.; Shimonosono, T.; Kishimoto, H.; Yamaji, K.; Brito, M.E.; Yokokawa, H. Visualization of oxide ionic diffusion at SOFC cathode/electrolyte interfaces by isotope labeling techniques. Solid State Ion. 2014, 262, 398–402. [Google Scholar] [CrossRef]
  83. Li, Z.-P.; Mori, T.; Auchterlonie, G.J.; Zou, J.; Drennan, J. Two types of diffusions at the cathode/electrolyte interface in IT-SOFCs. J. Solid State Chem. 2011, 184, 2458–2461. [Google Scholar] [CrossRef]
  84. Jang, I.; Kim, S.; Kim, C.; Lee, H.; Yoon, H.; Song, T.; Paik, U. Interface engineering of yttrium stabilized zirconia/gadolinium doped ceria bi-layer electrolyte solid oxide fuel cell for boosting electrochemical performance. J. Power Sources 2019, 435, 226776. [Google Scholar] [CrossRef]
  85. Park, S.-Y.; Ahn, J.H.; Jeong, C.-W.; Na, C.W.; Song, R.-H.; Lee, J.-H. Ni–YSZ-supported tubular solid oxide fuel cells with GDC interlayer between YSZ electrolyte and LSCF cathode. Int. J. Hydrogen Energy 2014, 39, 12894–12903. [Google Scholar] [CrossRef]
  86. Park, S.-Y.; Ahn, J.H.; Jeong, C.-W.; Na, C.W.; Song, R.-H.; Lee, J.-H. A comparison on effects of CO2 on La0.8Sr0.2MnO3+δ and La0.6Sr0.4CoO3−δ cathodes. J. Power Sources 2013, 222, 542–553. [Google Scholar]
  87. Almar, L.; Störmer, H.; Meffert, M.; Szász, J.; Wankmüller, F.; Gerthsen, D.; Ivers-Tiffée, E. Improved Phase Stability and CO2 Poisoning Robustness of Y-Doped Ba0.5Sr0.5Co0.8Fe0.2O3−δ SOFC Cathodes at Intermediate Temperatures. ACS Appl. Energy Mater. 2018, 1, 1316–1327. [Google Scholar] [CrossRef]
  88. Hu, X.; Xie, Y.; Wan, Y.; Yang, Y.; Wu, X.; Xia, C. Antimony-doped strontium cobalt oxide as promising cathode for low-temperature solid oxide fuel cell with excellent carbon dioxide tolerance. Appl. Catal. B Environ. 2021, 286, 119901. [Google Scholar] [CrossRef]
  89. Huang, Y.L.; Pellegrinelli, C.; Wachsman, E.D. Fundamental Impact of Humidity on SOFC Cathode ORR. J. Electrochem. Soc. 2015, 163, F171–F182. [Google Scholar] [CrossRef]
  90. Zhao, Z.; Liu, L.; Zhang, X.; Wu, W.; Tu, B.; Cui, D.; Ou, D.; Cheng, M. High- and low- temperature behaviors of La0.6Sr0.4Co0.2Fe0.8O3−δ cathode operating under CO2/H2O-containing atmosphere. Int. J. Hydrogen Energy 2013, 38, 15361–15370. [Google Scholar] [CrossRef]
  91. Nielsen, J.; Hagen, A.; Liu, Y.L. Effect of cathode gas humidification on performance and durability of Solid Oxide Fuel Cells. Solid State Ion. 2010, 181, 517–524. [Google Scholar] [CrossRef]
  92. Liu, R.R.; Kim, S.H.; Taniguchi, S.; Oshima, T.; Shiratori, Y.; Ito, K.; Sasaki, K. Influence of water vapor on long-term performance and accelerated degradation of solid oxide fuel cell cathodes. J. Power Sources 2011, 196, 7090–7096. [Google Scholar] [CrossRef]
  93. Chen, Y.; Fan, Y.; Lee, S.; Hackett, G.; Abernathy, H.; Gerdes, K.; Song, X. Interface and grain boundary degradation in LSM-YSZ composite Solid Oxide Fuel Cell cathodes operated in humidified air. J. Power Sources 2019, 438, 227043. [Google Scholar] [CrossRef]
  94. Hardy, J.S.; Coyle, C.A.; Bonnett, J.F.; Templeton, J.W.; Canfield, N.L.; Edwards, D.J.; Mahserejian, S.M.; Ge, L.; Ingram, B.J.; Stevenson, J.W. Evaluation of cation migration in lanthanum strontium cobalt ferrite solid oxide fuel cell cathodes via in-operando X-ray diffraction. J. Mater. Chem. A 2018, 6, 1787–1801. [Google Scholar] [CrossRef]
  95. Wang, R.; Sun, Z.; Lu, Y.; Gopalan, S.; Basu, S.N.; Pal, U.B. Comparison of chromium poisoning between lanthanum strontium manganite and lanthanum strontium ferrite composite cathodes in solid oxide fuel cells. J. Power Sources 2020, 476, 228743. [Google Scholar] [CrossRef]
  96. Egger, A.; Perz, M.; Bucher, E.; Gspan, C.; Sitte, W. Effect of Microstructure on the Degradation of La0.6Sr0.4CoO3–δ Electrodes in Dry and Humid Atmospheres. Fuel Cells 2019, 19, 458–471. [Google Scholar] [CrossRef]
  97. Wang, J.; Yang, Z.; He, X.; Chen, Y.; Lei, Z.; Peng, S. Effect of humidity on La0.4Sr0.6Co0.2Fe0.7Nb0.1O3−δ cathode of solid oxide fuel cells. Int. J. Hydrogen Energy 2018, 44, 3055–3062. [Google Scholar] [CrossRef]
  98. Hagen, A.; Neufeld, K.; Liu, Y.L. Effect of Humidity in Air on Performance and Long-Term Durability of SOFCs. J. Electrochem. Soc. 2010, 157, B1343. [Google Scholar] [CrossRef]
Fuels 05 00045 g001
Figure 1. Net Energy generation in 2020 by fuel type in Australia. Figure recreated from the U.S. Energy Information Administration with data from the Australian Department of Industry, Innovation, and Science [1].
Figure 1. Net Energy generation in 2020 by fuel type in Australia. Figure recreated from the U.S. Energy Information Administration with data from the Australian Department of Industry, Innovation, and Science [1].
Fuels 05 00045 g001
Fuels 05 00045 g002
Figure 2. Schematic representation of SOFC based on an oxide-ion conducting (O-SOFC) electrolyte.
Figure 2. Schematic representation of SOFC based on an oxide-ion conducting (O-SOFC) electrolyte.
Fuels 05 00045 g002
Fuels 05 00045 g003
Figure 3. Comparison between intermediate- and high-temperature SOFCs.
Figure 3. Comparison between intermediate- and high-temperature SOFCs.
Fuels 05 00045 g003
Fuels 05 00045 g004
Figure 4. Resistance of LSCF-based cathodes at 750 °C. (a) Nb- and Pd-doped LSCF (LSCFNPd) [31], (b) Rh-doped LSCF (Rh-LSCF) [32], (c) Pd-doped LSCF (Pd-LSCF) [32], (d) Mo-doped LSCF (LSCFM) [33], (e) Bi-doped LSCF (LBSCF) [18] and (f) Ce-doped LSCF (LCSCF) [30].
Figure 4. Resistance of LSCF-based cathodes at 750 °C. (a) Nb- and Pd-doped LSCF (LSCFNPd) [31], (b) Rh-doped LSCF (Rh-LSCF) [32], (c) Pd-doped LSCF (Pd-LSCF) [32], (d) Mo-doped LSCF (LSCFM) [33], (e) Bi-doped LSCF (LBSCF) [18] and (f) Ce-doped LSCF (LCSCF) [30].
Fuels 05 00045 g004
Fuels 05 00045 g005
Figure 5. Arrhenius plots for ionic conductivities of ceria-based oxides: (a- (CeO2)0.8(SmO1.5)0.2; (b) (CeO2)0.8(GdO1.5)0.2; (c) (CeO2)0.8(YO1.5)0.2; (d) (CeO2)0.8(CaO)0.2; (e) CeO2; (f) (ZrO2)0.85(YO1.5)0.15. Figure redrawn from [49].
Figure 5. Arrhenius plots for ionic conductivities of ceria-based oxides: (a- (CeO2)0.8(SmO1.5)0.2; (b) (CeO2)0.8(GdO1.5)0.2; (c) (CeO2)0.8(YO1.5)0.2; (d) (CeO2)0.8(CaO)0.2; (e) CeO2; (f) (ZrO2)0.85(YO1.5)0.15. Figure redrawn from [49].
Fuels 05 00045 g005
Fuels 05 00045 g006
Figure 6. Oxygen conduction path: (a) triple-phase boundary (TPB) in MIEC cathodes; (b) two-phase boundaries in non-MIEC cathodes. Figure redrawn from [62].
Figure 6. Oxygen conduction path: (a) triple-phase boundary (TPB) in MIEC cathodes; (b) two-phase boundaries in non-MIEC cathodes. Figure redrawn from [62].
Fuels 05 00045 g006
Fuels 05 00045 g007
Figure 7. Two types of infiltration: (a) solution infiltration on the backbone; (b) thin film coating; (c) discrete particle coating.
Figure 7. Two types of infiltration: (a) solution infiltration on the backbone; (b) thin film coating; (c) discrete particle coating.
Fuels 05 00045 g007
Fuels 05 00045 g008
Figure 8. LSCF stability behavior at low (600 °C) and high (700 °C) temperatures in CO2 and humidity exposure test. Figure redrawn from [90].
Figure 8. LSCF stability behavior at low (600 °C) and high (700 °C) temperatures in CO2 and humidity exposure test. Figure redrawn from [90].
Fuels 05 00045 g008
Fuels 05 00045 g009
Figure 9. (a) TEM image of the original TPB region and the LSM/YSZ interface in the fresh cell. (b) TEM image of the active layer of the cell operated for 120 h. The interfacial defects initiate from the original TPBs, while the LSM/YSZ interface and YSZ/YSZ grain boundaries initially remain intact. (c) The enlarged TPB region in b shows the cracking at the LSM/YSZ interface is associated with the nano precipitates. (d) The enlarged TPB region in b showing the precipitates have an epitaxial orientation with the YSZ grains and an amorphous layer at the LSM/YSZ interface. Figure used from [93] with permission.
Figure 9. (a) TEM image of the original TPB region and the LSM/YSZ interface in the fresh cell. (b) TEM image of the active layer of the cell operated for 120 h. The interfacial defects initiate from the original TPBs, while the LSM/YSZ interface and YSZ/YSZ grain boundaries initially remain intact. (c) The enlarged TPB region in b shows the cracking at the LSM/YSZ interface is associated with the nano precipitates. (d) The enlarged TPB region in b showing the precipitates have an epitaxial orientation with the YSZ grains and an amorphous layer at the LSM/YSZ interface. Figure used from [93] with permission.
Fuels 05 00045 g009
Fuels 05 00045 g010
Figure 10. Schematic representation of the different cathode fabrication strategies, their advantages and future R&D prospects.
Figure 10. Schematic representation of the different cathode fabrication strategies, their advantages and future R&D prospects.
Fuels 05 00045 g010
Table 2. Polarization resistance (Rp) and power density of some of the core–shell cathodes.
Table 2. Polarization resistance (Rp) and power density of some of the core–shell cathodes.
Core–Shell CathodeElectrolyteTemperature (°C)Rp (Ω.cm2)Power Density (W.cm−2)Reference
Ag@SDCSDC4500.70.0562 [68]
Au@NiGDC5000.250.464[69]
YCo0.5Fe0.5O3(YCF)@Gd0.1Ce0.9O1.95(GDC)GDC5500.66 0.4265 [70]
Ag @ Pr0.2Ce0.8O2-δ -LSCFSDC5500.65-[71]
La0.8Sr0.2MnO3-δ (LSM)@Ba0.5Sr0.5Co0.8Fe0.2O3-δ(BSCF)GDC6001.141-[72]
Sm0.5Sr0.5CoO3 (SSC)@Sm0.2Ce0.8O1.9 SDC6500.098 1.07 (600 °C)[73]
La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)@La0.6Sr1.4Co0.2Fe0.8O4-δ (LSCF)SDC6500.17 0.57[74]
La0.8Sr0.2MnO3 (LSM)@La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)YSZ7002.1 -[66]
PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF)-La2NiO4+δ (LN)YSZ7000.51 0.71[75]
LaCoO3-d (LC)@Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF)GDC7500.020 0.929[76]
La0.8Sr0.2Co0.2Fe0.8O3-δ (LSCF)@Gd0.2Ce0.8O1.9 (GDC)GDC7500.16 -[77]
LaSrCoO4±δ(LSC) @La0.5Sr0.5CoO 3-δ (LSC)LSGM8000.030.87[78]
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Nisar, J.; Kaur, G.; Giddey, S.; Bhargava, S.; Jones, L. Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells. Fuels 2024, 5, 805-824. https://doi.org/10.3390/fuels5040045

AMA Style

Nisar J, Kaur G, Giddey S, Bhargava S, Jones L. Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells. Fuels. 2024; 5(4):805-824. https://doi.org/10.3390/fuels5040045

Chicago/Turabian Style

Nisar, Jamila, Gurpreet Kaur, Sarbjit Giddey, Suresh Bhargava, and Lathe Jones. 2024. "Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells" Fuels 5, no. 4: 805-824. https://doi.org/10.3390/fuels5040045

APA Style

Nisar, J., Kaur, G., Giddey, S., Bhargava, S., & Jones, L. (2024). Cathode Materials for Intermediate Temperature Solid Oxide Fuel Cells. Fuels, 5(4), 805-824. https://doi.org/10.3390/fuels5040045

Article Metrics

Back to TopTop