CO₂ Poisoning of Solid Oxide Fuel Cell Cathodes: Mechanisms, Solutions, and Progress

Abstract

Conventional energy resources have been constrained by their inefficient utilization and present a severe impact on the human living environment, and there is an urgent need to develop energy technologies with high efficiency, low carbon emissions, and environmental cleanliness. Solid oxide fuel cells (SOFCs) have been recognized as a highly efficient and clean energy conversion device that directly converts chemical energy in fuels into electricity, holding promising prospects for addressing the issues of low efficiency and environmental concerns associated with conventional energy resources. However, under practical operation conditions, the cathodes of SOFCs are often exposed to various contaminations including working environment-induced degradation, cathode poisoning, and corrosion. This review summarizes the severe performance degradation of SOFC cathodes caused by CO₂ poisoning, analyzes recent research findings on cathode durability under CO₂-containing atmospheres, and provides an overview of the reported strategies for enhancing CO₂ tolerance.

1. Introduction

In recent years, growing concerns over environmental pollution and energy crisis issues have intensified the demands for sustainable solutions, as illustrated in Figure 1a [1,2]. Solid oxide fuel cells (SOFCs) have emerged as a promising power generation system owing to their ability for direct, efficient, and low-pollution energy conversion [3]. SOFCs find potential applications in diverse areas such as distributed generation, transportation, combined heat and power (CHP), and trigeneration systems (Figure 1b) [4]. This broad applicability comes from the fact that SOFCs are all-solid-state electrochemical energy conversion devices capable of directly and efficiently converting the chemical energy from various hydrocarbon fuels, including hydrogen, methane, and ethanol, to electricity [5]. However, under real-life operating conditions, the high temperatures for SOFC operation pose challenges, (Figure 1c) [6], such as compatibility issues among cell components (Figure 1d) [7], shortened operational lifespan (Figure 1e) [8], mismatched thermal expansion coefficients (Figure 1f) [9], cathode delamination, and degradation induced by the surrounding environment [10,11,12]. Consequently, exploring the fundamental mechanisms underlying the degradation of intermediate-temperature SOFCs (IT-SOFCs) is of particular importance.

SOFCs consist of anodes, electrolyte, and cathodes, and among them, the cathode plays a particularly crucial role that significantly determines the overall high-performance output of the cell. The performance is intrinsically dependent on the kinetics of oxygen reduction and transport occurring at its surface [13]. Additionally, the stability of the cathode material constitutes a core factor governing the long-term operational performance, service span, and commercialization of SOFCs. As a key cell component, the cathode operates under harsh high-temperature conditions (700–900 °C), catalyzing the oxygen reduction reaction (ORR) [14,15], with its stability directly impacting the energy conversion efficiency and system reliability. During practical operation, cathodes must continuously endure high-temperature oxidizing atmospheres, fluctuations in gas composition, and thermal cycling stresses induced by frequent start–stop operations. Insufficient stability leads to significant performance degradation or even failure, with the primary failure mechanisms categorized as chemical degradation and mechanical failure. Chemical degradation manifests as the segregation of active elements forming insulating phases. For instance, the intermediate-temperature cathode La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF), known for its excellent mixed conductivity, experiences performance decay during prolonged operation due to the formation of segregating phases like SrCO₃ or SrCrO₄. Similarly, cathodes with perovskite-related structures, such as (Ba,Sr)(Co,Fe)O_3−δ (BSCF), (La,Sr)CoO_3−δ (LSC), and the layered perovskite PrBa_0.5Sr_0.5Co_0.5Fe_0.5O_5+δ (PBSCF) [16,17,18,19], display sharp breakdown due to similar segregation phenomena. Mechanical failures, on the other hand, originate from structural instability under thermal cycling stresses. Mismatched thermal expansion coefficients (TECs) readily induce interfacial delamination and crack formation. Zhang et al. developed the Sr_x(Y_y(Nb_0.1Co_0.9)_1−y)O_3−δ (SYNC) cathode by combining a negative thermal expansion component, achieving synergistic enhancement in thermomechanical compatibility and high catalytic activity [20,21]. Additionally, a chemically induced hydration strategy was introduced to reinforce the mechanical strength of protonic ceramic fuel cell cathodes. An as-prepared BaCo_0.7Ce_0.15Y_0.15O_3−δ (s-BCC-Y) cathode exhibited strengthened grain intergranular bonding and an 86% increase in fracture strength after hydration treatment, maintaining structural integrity even after 35 thermal cycles between 300 and 600 °C [22]. Similarly, a Nb-doped BaCe_0.1Fe_0.8Nb_0.1O_3−δ (BCFNb) cathode demonstrates excellent mechanical stiffness along with robust ORR/oxygen evolution reaction (OER) activity [23].

It is noteworthy that during prolonged SOFC operation, the cathode is inevitably and continuously exposed to CO₂ in ambient air. Such exposure leads to cathode material poisoning, triggering significant chemical degradation and mechanical failure, which severely impair the cell’s activity and durability [24,25]. This degradation mechanism is particularly pronounced in ABO₃-type perovskite cathode materials, especially in those containing alkaline earth metal elements, which are structurally susceptible to CO₂ poisoning [26]. For instance, the performance degradation of cathode materials such as Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) and Pr_0.6Sr_0.4Co_0.5Fe_0.5O_3−δ (PSCF) has been demonstrated to be closely associated with the presence of carbon dioxide [27,28]. Considering that practical SOFC systems demand stable operation for thousands of hours or even longer, it is essential to achieve the long-term stability of cathodes under the combined effects of thermal fields, chemical environments (particularly CO₂), and mechanical stresses in this field. To address this challenge, researchers are consistently driven to develop various cathode material modification strategies, including techniques such as multi-step solution infiltration, forming composite cathodes with materials like gadolinium-doped ceria (GDC), and introducing isovalent metal ions for doping. These strategies aim to significantly enhance the cathode’s tolerance and long-term stability under harsh operating conditions. Therefore, assessing the poisoning behavior of a cathode and its underlying mechanisms is crucial for developing commercially viable SOFCs. This paper systematically evaluates the poisoning impact of CO₂ on SOFC cathode performance.

Figure 1. (a). Schematic of an SOFC operating with hydrogen as the fuel. Reproduced with permission from Ref. [2]. (b). SOFC-absorption cooling building cogeneration system configuration. Reproduced with permission from Ref. [29]. (c). The possible poisoning and corrosions of cathode materials. (d) Delamination of SOFC cell components due to high operation temperature. Reproduced with permission from Refs. [7,9]. (e) Cracking behavior in SOFC electrolyte due to heating–cooling cycling, subfigure (a) displays an initial anode and its substrate as its co-fired state, subfigure (b) displays a reduced state, and (c) displays a re-oxidized state. Reproduced with permission from Refs. [8,9]. (f). Possible mechanism of cell cracking and delamination during cell operation at high temperature. Reproduced with permission from Ref. [9].

Figure 1. (a). Schematic of an SOFC operating with hydrogen as the fuel. Reproduced with permission from Ref. [2]. (b). SOFC-absorption cooling building cogeneration system configuration. Reproduced with permission from Ref. [29]. (c). The possible poisoning and corrosions of cathode materials. (d) Delamination of SOFC cell components due to high operation temperature. Reproduced with permission from Refs. [7,9]. (e) Cracking behavior in SOFC electrolyte due to heating–cooling cycling, subfigure (a) displays an initial anode and its substrate as its co-fired state, subfigure (b) displays a reduced state, and (c) displays a re-oxidized state. Reproduced with permission from Refs. [8,9]. (f). Possible mechanism of cell cracking and delamination during cell operation at high temperature. Reproduced with permission from Ref. [9].

2. CO₂ Poisoning Mechanism

2.1. Description of the Basic CO₂ Poisoning Process

The mechanism of CO₂ poisoning in SOFC cathode materials primarily involves both physicochemical adsorption and structural transformations. Among these, the affinity of alkaline earth metal oxide (AEMO) surfaces for CO₂ follows the trend: BaO > SrO > CaO > MgO [30]. Driven by physical adsorption, CO₂ initially captured on the AEMO surface forms monolayer or multilayer coverage. This not only impedes oxygen adsorption and diffusion but also obstructs the cathode’s porous structure and covers active sites, ultimately inhibiting oxygen transport and reaction kinetics.

During chemical adsorption, CO₂ readily reacts with alkaline elements (e.g., Ba²⁺ and Sr²⁺) present on the cathode surface to form stable carbonates (e.g., SrCO₃ and BaCO₃). The stability of these carbonates increases with the ionic radius of the alkaline earth metal (Figure 2a) [31,32]. Taking Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF), a classic high-performance mixed-conductor cathode material, as an example [33], it exhibits higher ionic conductivity and oxygen permeability compared to lanthanum-based perovskite materials due to the larger ionic radius of barium [34]. BSCF demonstrates exceptional oxygen reduction electrocatalytic activity, high electrochemical performance, superior oxygen permeation flux, and good phase stability at 600 °C. These properties enhance both the bulk oxygen diffusion rate and the surface exchange kinetics [35,36].

However, as illustrated in Figure 2b, research by Yan et al. [37] revealed that the BSCF cathode is extremely sensitive to CO₂ within a temperature range of 450–750 °C, particularly under low-temperature operating conditions and even at very low CO₂ concentrations (<1%). For instance, in an air atmosphere with a low content of 0.28% CO₂, the open-circuit voltage and peak power density of BSCF decrease dramatically. When the CO₂ concentration reaches 0.85%, the peak power density at 600 °C is nearly halved. Moreover, the performance degradation is even more severe and largely irreversible at 500 °C and 450 °C [38]. This clearly demonstrates that even relatively small amounts of CO₂ in air cause significant and irreversible poisoning effects on BSCF. Yan et al. [39] further reported that after exposure to air at 600 °C for 7 days, the degradation rate of oxygen exchange kinetics in BSCF approached approximately 80%.

Furthermore, CO₂ adsorption may induce structural transformations in cathode materials. For instance, Zhang et al.’s investigation of the BaFeO_3−δ (BF) cathode revealed that CO₂ exposure triggers in situ formation of barium carbonate (BaCO₃) [40]. This subsequently drives a comprehensive structural reconstruction of the cathode, resulting in a core-shell architecture comprising a BaCO₃ outer shell and a dual-phase core (BaFe₂O₄ and BaFeO₃ phases). Similarly, Tan et al. reported that the exposure to CO₂ induces surface strontium segregation and corrosion in La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes [41]. Other prominent cathode materials, including (Ba,Sr)(Co,Fe)O_3−δ (BSCF), (La,Sr)CoO_3−δ (LSC), and PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF), also undergo severe degradation through elemental segregation mechanisms, leading to detrimental alterations in material structure [18].

Figure 2. (a). Ellingham diagram for the decomposition of carbonates under different CO₂ partial pressures. Reproduced with permission from Ref. [42]. (b). Electrochemical impedance spectra measured at various times after CO₂ was supplied at 450 °C. Redrawn from Ref. [37].

Figure 2. (a). Ellingham diagram for the decomposition of carbonates under different CO₂ partial pressures. Reproduced with permission from Ref. [42]. (b). Electrochemical impedance spectra measured at various times after CO₂ was supplied at 450 °C. Redrawn from Ref. [37].

2.2. General Impact Patterns of A-Site Elements on Poisoning

The crystal structure is crucial for the performance of SOFC cathodes, as it directly determines their electrocatalytic activity and long-term stability. Various oxides with distinct crystal structures, such as perovskite-type and double perovskite-type oxides, are widely demonstrated to act as superior SOFC cathode materials. Among these, perovskite-structured oxides are considered the most promising cathode materials due to their high catalytic activity toward the ORR. For these materials, the physicochemical properties (e.g., electrical conductivity) can be tuned over a wide range by substituting the A-site ions (typically La, Sr, Ba, etc.), achieving conductivities from nearly pure electronic to almost pure ionic. Representative high-performance perovskite cathodes include La_1−xSr_xCo_1−yFe_yO_3−δ (LSCF), Ba_1−xSr_xCo_1−yFe_yO_3−δ (BSCF), Sm_0.5Sr_0.5CoO_3−δ (SSC), and PrBaCo₂O_6−δ (PBC) [43,44,45,46]. These materials typically contain abundant alkaline earth metal cations (e.g., Sr²⁺, Ba²⁺) occupying the A-site, which induce the formation of substantial oxygen vacancies, thereby significantly enhancing oxygen transport properties. Consequently, they are widely employed for their excellent power output and stability. However, these very A-site alkaline earth elements make the cathodes susceptible to significant performance degradation or even material failure when exposed to CO₂, severely compromising cell durability [47,48].

As reported by Shao et al. [35], the barium-based material BSCF undergoes surface segregation under ambient air due to CO₂ exposure, forming carbonate layers and secondary oxide phases that impair surface oxygen exchange kinetics through microstructural and conductivity changes [49,50]. Strontium-based cathode materials typically exhibit poor CO₂ tolerance, since CO₂ readily reacts with Sr to form SrCO₃ [51,52], while CO₂ absorption concurrently blocks active sites on the cathode surface [53]. Experimental and simulation studies by Khromushin et al. revealed significant interactions between CO₂ and strontium-doped La_1−xSr_xMnO_3−δ (LSM), as shown in Figure 3a. Their CO₂ temperature-programmed desorption (TPD) spectra demonstrated that the amount of CO₂ desorbed increased markedly with higher strontium doping levels and directly proved that the higher Sr content in LSM enhances its CO₂ adsorption capacity [54]. This poisoning phenomenon, initiated by high-basicity elements at the A-site, is prevalent across a wide range of cathode materials, such as BaCo_xFe_yZr₂O_3−δ (BCFZ), SrCo_0.9Nb_0.1O_3−δ (SCN), SrCo_0.9Sc_0.1O_3−δ (SCS), BaCe_0.9Y_0.1O_3−δ, Sr(CoFe)O_3−δ, and BaPr_0.7Gd_0.3O_3−δ [49,55,56,57,58,59,60]. Zhang et al. evaluated the CO₂ tolerance of the Sr-based cathode SrSc_0.175Nb_0.025Co_0.8O_3−δ (SSNC) using electrochemical impedance spectroscopy (EIS) [61]. After exposure to 10 vol% CO₂ for 15 min at 500 and 600 °C, followed by its removal, SSNC and BSCF exhibited an increase in area-specific resistance (ASR) by a magnitude of 20–45 times, whereas at 700 °C the relative increase was only 4–9 times. SSNC demonstrated superior CO₂ tolerance compared to BSCF, attributed to the higher basicity of BaO versus SrO, which facilitates more facile carbonate formation, and the surface faced a sharp change, as illustrated in Figure 3b. As shown in Figure 3c, Hu et al. documented that SrCO₃ formation on the surface of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes in 0–10% CO₂-air mixtures causes initial performance degradation [62]. The fundamental reason lies in the high basicity of the alkaline earth metals, which significantly promotes the chemisorption of CO₂; the resultant carbonate species or surface segregation layers cover ORR-active sites. Furthermore, the persistent accumulation of these carbonates can lead to irreversible structural damage.

Additionally, based on the Lewis acid-base theory, BaO exhibits the strongest basicity, surpassing that of SrO and La₂O₃. Correspondingly, oxides with stronger basicity react more readily with CO₂. Furthermore, the Ellingham data presented in Figure 2a corroborate this trend, demonstrating that Ba²⁺ possesses a greater thermodynamic driving force for carbonate formation compared to Sr²⁺ and La³⁺ [63]. Therefore, collectively, the basicity of the A-site element is the key factor governing a cathode’s susceptibility to CO₂ poisoning. The stronger the basicity, the higher the tendency for chemisorption reactions with CO₂, leading to more severe poisoning effects such as carbonate formation or surface segregation.

To make the discussion quantitative, we consider the surface-carbonation equilibria of A-site oxides with CO₂. The overall reaction can be described as follows (Equations (1) and (2)):

BaO (SrO, La₂O₃) + CO₂(g) ⇌ BaCO₃ (or SrCO₃, La₂O₂CO₃),

(1)

$$\mathsf{\Delta }\mathrm{G}°\left(\mathrm{T}\right)+\mathrm{RT}\ln {\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}=0.$$

(2)

Here, R is the gas constant, and T is the temperature. The magnitude of ln ${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$ reflects the extent of the carbonates forming reaction, and a value above 105 indicates complete reaction completion. Therefore, the CO₂ resistance of different metal elements can be compared by evaluating their corresponding ln ${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$ values.

At a given temperature, carbonate stability follows Equation (2). Carbonates are stable when the ambient ${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$ lies above the boundary, and they decompose when it lies below. The stability boundaries follow the order BaCO₃ < SrCO₃ < La₂O₂CO₃, indicating that Ba and Sr carbonates stabilize more readily than La oxy-carbonate. In LSC/LSCF, Sr surface segregation effectively lowers the threshold for SrCO₃ formation, so even sub-percent CO₂ at approximately 600–700 °C can sustain persistent surface coverage, whereas La-rich surfaces are less susceptible under the same conditions. Increasing the temperature and decreasing ${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$ drive decarbonation, which helps maintain accessible oxygen-exchange sites and limits interfacial secondary-phase formation.

2.3. General Impact Patterns of B-Site Elements on Poisoning

The B-sites of mixed ionic–electronic conductor (MIEC) oxide cathodes are typically occupied by transition metal ions. Based on the dominant cation type, they are classified as Mn-based cathodes (e.g., La_1−xSr_xMnO_3−δ, LSM) [50,64], Fe-based cathodes (e.g., SrFeO_3−δ) [65,66], Ni-based cathodes (e.g., LaNiO_3−δ), Co-based cathodes (e.g., (La,Sr)CoO_3−δ) [36,39], and mixed transition metal-based cathodes (e.g., La(Ni,Fe)O_3−δ and Sr(Co,Fe)O_3−δ) [67,68]. Among them, cubic perovskite SrCoO_3−δ-based cathodes inherited excellent combined ionic and electronic conductivity, yet they still suffer from insufficient structural stability at high temperature. To address this, various cation dopants (e.g., Ni, Cu, Zn, Cr, Fe, Al, Ga, In, Ce, Ti, Zr, Sn, V, Nb, etc.) are introduced to sustain cubic phase stability [52]. Recently developed high-performance Co-based and Fe-based cathode materials, such as SrCo_0.9Nb_0.1O_3−δ (SCN), SrFe_0.9Ti_0.1O_3−δ (SFT), and SrNb_0.1Co_0.9−yFe_yO_3−δ (SNCF) [69,70,71,72], perform particularly well in the low-temperature region (400–600 °C). Taking Co-based SrSc_0.2Co_0.8O_3−δ (SSC) and SrNb_0.1Co_0.9O_3−δ (SNC) as examples, they combine high conductivity with high oxygen vacancy concentration [73]. However, an enriched cobalt component leads to a larger thermal expansion coefficient (TEC), such as ~19.1 × 10⁻⁶ K⁻¹ for SNC [74] and ~24 × 10⁻⁶ K⁻¹ for SSC [75], posing an increased risk of cathode/electrolyte delamination during thermal cycling [76]. Additionally, CO₂ sensitivity further exacerbates performance degradation.

To address this, Shao et al. systematically investigated the influence of the Co/Fe ratio at the B-site in SrNb_0.1Co_0.9−yFe_yO_3−δ (y = 0–0.9) in relation to the CO₂ tolerance mechanism [64]. Their results indicate that the increase in the iron content significantly reduces the material’s sensitivity to CO₂, which can be certified by the smaller increase in the area-specific resistance (ASR) after CO₂ exposure. This phenomenon is attributed to the higher bond strength of Fe-O compared to Co-O, where the resulting increase in average metal–oxygen bond energy (ABE) effectively suppresses the carbonation reaction. Therefore, ABE can serve as a key descriptor for evaluating CO₂ tolerance. Further studies confirmed that the substitution of the B-site of SrCoO_3−δ with high-valence cations (e.g., Nb⁵⁺, Mo⁶⁺, Ti⁴⁺) can significantly enhance the perovskite’s CO₂ tolerance [77,78,79]. This is verified by Zhu’s work, in which more negative ABE values denoted better CO₂ tolerance, shown by the BSNM, LSCF, and BSCF (of which the ABEs were −414, −316, and 274 k mol⁻¹, respectively), suggesting that BSNM has the best CO₂ resistance (Figure 4a,b) [80]. In summary, the high ABE value induced by high-valence B-site cations is the fundamental factor in the improvement in CO₂ tolerance.

Furthermore, Tejuca et al. investigated the extent of CO₂ adsorption and the heat of adsorption on LaBO₃ (B = Cr, Fe, Co) oxides over a wide temperature range [81]. They found that the chemical nature of the B-site cation significantly influences the CO₂ coverage and its variation with the adsorption temperature. The results demonstrate that CO₂ coverage follows the order LaCrO₃ > LaFeO₃ > LaCoO₃. Consequently, the CO₂ adsorption capacity is strongly dependent on the identity of the B-site cation.

3. Correlation Analysis Between SOFC Operating Parameters and Cathode CO₂ Tolerance

Although the CO₂ tolerance of perovskite cathodes is primarily governed by the elemental composition at the A/B-sites, the operating temperature also serves as a critical variable significantly impacting the performance. As shown in Figure 5a, Zhao et al. [47] investigated the CO₂ tolerance of the La_0.6Sr_0.4CoO_3−δ (LSC) cathode at different temperatures. The adsorption behavior in the low-temperature range (550–650 °C) accords with the Temkin model, while at a higher temperature range (700–800 °C), the model conforms to the Freundlich model. Notably, under constant CO₂ partial pressure (e.g., 1.85 kPa), the increase in polarization resistance rises exponentially with decreasing temperature: only a 0.01 Ω cm² increase at 700 °C, rising to 0.21 Ω cm² at 600 °C, and surging to 1.20 Ω cm² at 550 °C. This temperature effect is further reflected in the significantly higher rate of change in the area-specific resistance (ASR) with increasing CO₂ partial pressure observed in the low-temperature range compared to the high-temperature range. Zhang et al. [61] studied the CO₂ tolerance of SSNC and BSCF cathodes in atmospheres containing 10 vol% CO₂ at 500–700 °C, as shown in Figure 5b. After exposure to 10 vol% CO₂ at 500 °C and 600 °C, the relative increase rate of ASR reached 20–45 times, whereas at 700 °C, the relative increase rate was only 4–9 times. Furthermore, after CO₂ removal from the atmosphere, the electrochemical performance of cathodes tested at 700 °C fully recovered within 15 min, while no complete recovery was observed for cathodes tested at 500 °C and 600 °C.

On perovskite cathodes, O₂ preferentially adsorbs at B-site/oxygen-vacancy ensembles, forming O₂(ad) to O²⁻/O⁻ via charge transfer before incorporation as O²⁻. In contrast, CO₂ chemisorbs more strongly at A-site-terminated BaO/SrO domains, producing surface carbonate-like ACO₃ species. These pathways compete for surface vacancies and electrons: carbonate formation blocks O₂ adsorption/incorporation sites and reduces the population of mobile oxygen intermediates, thereby depressing the surface-exchange kinetics. The balance is dynamic: at higher temperature (or lower ${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$), transient carbonates desorb, and O₂ readsorbs rapidly (recovery), whereas at lower temperature and/or higher ${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$, persistent Ba/Sr-carbonates accumulate (poisoning). The schematic in Figure 5c visualizes these steps and the competition between CO₂ and O₂ on Ba/Sr-containing perovskites.

Nomura et al. discovered that the peritectic oxide formed in Ba_0.95Ca_0.05Co_1−xFe_xO_3−δ that filled with plenty of lattice oxygen at high temperatures readily adsorbs CO₂ [82], suggesting that lattice oxygen in perovskites may catalyze carbonate formation. Significantly, the relationship between co-adsorption and competitive adsorption of CO₂ and O₂ undergoes a fundamental transformation with temperature. Yan et al.’s research on the BSCF system revealed that above 400 °C, CO₂ and O₂ exhibit significant competitive adsorption. When CO₂ adheres on the cathode surface and occupies active sites, it significantly suppresses the adsorption, dissociation of oxygen molecules, and a reduction in adsorbed oxygen atoms, thereby degrading cell performance. However, in the high-temperature range of 600–700 °C, oxygen takes over the active sites on the surface and thereby leads the key reactions [83]. Collectively, these findings demonstrate that operating temperature not only modulates CO₂ adsorption intensity but also governs the competitive relationship between CO₂ and O₂. Furthermore, thermodynamic data at lower temperatures indicate that materials containing Ba or Co elements are more susceptible to CO₂ adsorption within perovskite structures [84]. In summary, increasing the operating temperature effectively mitigates CO₂ poisoning effects.

When evaluating the CO₂ tolerance of cathodes, there are significant differences in the test conditions adopted (e.g., CO₂ partial pressure, exposure time, operating temperature), which poses challenges for direct performance comparisons among different material systems. Zhao et al. suggested that the CO₂-poisoning effect is strongly dependent on parameters during tests. In their research on the La_0.6Sr_0.4CoO_3−δ (LSC) cathode, a sharp increase in polarization resistance occurred when the CO₂ partial pressure increased from 1.85 kPa to 4.68 kPa at 550 °C. However, at 700 °C, the effect of the same partial pressure change was much less significant, and the performance was quickly restored after CO₂ removal [67]. The study was conducted under multiple temperature gradients (550–800 °C) and a series of precisely controlled CO₂ partial pressures (0.53, 1.85, 2.99, 4.68 kPa). The coupling effect of temperature and CO₂ partial pressure and its profound influence on the toxicity mechanism were clearly revealed. Therefore, to achieve fair and effective comparison of research results, future work urgently needs to clarify and unify the following core test parameters as much as possible when reporting CO₂ tolerance.

4. Strategies for Enhancing Cathode Resistance to CO₂ Poisoning

4.1. High-Acidity Cation Doping Strategy (Strategy via High-Acidity Cation Doping)

As discussed in the previous sections, the CO₂ resistance of perovskite-type cathodes links to both A-site and B-site cations. The key influencing factors include ABE and the nature of alkaline earth metal elements. Crucially, the acidity level of the dopant cations is a decisive factor for CO₂ tolerance (cation acidity order: Nb⁵⁺ > Co⁴⁺ > Co³⁺ > Fe³⁺ > Sc³⁺ > Co²⁺ > Fe²⁺ > Sr²⁺ > Ba²⁺). Introducing more acidic cations clearly enhances theCO₂ resistance of perovskites.

Chen et al. [85] developed a calcium-doped double perovskite material, PrBa_0.8Ca_0.2Co₂O_5+δ (PBCC), which exhibits exceptional ORR activity and CO₂ tolerance as a cathode. Almar et al. [86] demonstrated improved CO₂ tolerance in BSCF through minor Y doping at the B-site. Focused ion beam (FIB) and scanning electron microscopy (SEM) analysis of the cell’s internal microstructure revealed that (Ba_0.5Sr_0.5)(Co_0.8Fe_0.2)_0.9Y_0.1O_3−δ (BSCFY) exhibits significantly enhanced CO₂ adsorption resistance in air at 700 °C. This improvement is primarily attributed to the higher acidity of Y³⁺ compared to A-site cations and the high concentration of Y dopants. Similarly, Huang et al. [87]. reported that Nb doping at the B-site enhances CO₂ tolerance in BSCF. Stability measurements and XRD analysis showed that increasing the Nb doping content in Ba_0.5Sr_0.5Co_0.8−xFe_0.2Nb_xO_3−δ (BSCFNb_x) reduces BaCO₃ peak intensity, indicating superior resistance to CO₂ corrosion, likely due to the high acidity of Nb dopants. Kim et al. [88]. found that La³⁺ doping stabilizes the cubic perovskite structure in BSCF. Calculations based on the Goldschmidt tolerance factor indicate that substituting La³⁺ into the A-site of BSCF (forming La_0.5Ba_0.25S_r0.25Co_0.8Fe_0.2O_3−δ, LBSCF) results in smaller cubic symmetry distortions, enhancing the structural stability and CO₂ tolerance, while maintaining excellent phase stability under low p(O₂) and CO₂-containing atmospheres. Bi et al. [89] investigated Ti substitution for partial Co/Fe in BSCF as a cathode for proton-conducting SOFCs. The strong Ti-O bond reduces CO₂ reactivity in Ti-doped BSCF, improving its chemical stability in pure CO₂ at 600 °C. Li et al. [90] reported that partial substitution of Ba with Ca enhances CO₂ tolerance in the proton-conducting Ba_0.95Ca_0.05Co_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (BCaCFZY) cathode. Zhang et al. [91] documented that co-doping enhances ORR activity and CO₂ resistance. The presence of 7.5 mol% Sc³⁺ promotes high oxygen valence states in Fe and oxygen vacancy formation in SrSc_0.075Ta_0.025Fe_0.9O_3−δ (SSTF75), enhancing ORR activity, while 2.5 mol% Ta⁵⁺ strengthens CO₂ resistance due to its high valence state and acidity. Su et al. [92] reported that Ca-doped LaFeO₃ cathodes exhibit good CO₂ tolerance and low thermal expansion coefficients. DFT calculations revealed the CO₂ adsorption mechanism on the surface, providing a strategy for designing Ca-doped lanthanum-based cathodes. In summary, doping strategies effectively enhance CO₂ tolerance. Selected dopants should possess high acidity, high valence state, and high ABE characteristics; elements such as Ti⁴⁺, Zr⁴⁺, Nb⁵⁺, Sb⁵⁺, Ta⁵⁺, W⁶⁺, and Mo⁶⁺ show promise in mitigating CO₂ poisoning in perovskites [93].

Taken together (

Table 1. Representative doped SOFC cathodes under CO₂ suggests that a higher valence improve apparent CO₂ tolerance via stability.

Material	Temperature (°C)	CO2 (vol%)	Time (h)	Impedance: Start to End
BSCF	700	1 3	50	Relative value: 1.0 to 2.8 Relative value: 1.0 to 12
BSCF-10%Y (BSCF10Y)	700	1 3	50	Relative value: 1.0 to 1.7 Relative value: 1.0 to 8
BCFZY	700	1 10	120	Rp: 0.368 to 0.574 Ω cm2 Rp: 0.503 to 0.682 Ω cm2
BCaCFZY	700	1 10	120	Rp: 0.357 to 0.423 Ω cm2 Rp: 0.431 to 0.495 Ω cm2
PBCC	750	1	1001	Rp: ≈0.024 Ω cm2
SSTF75	600	10	0.5	Relative value: 1 to ~8.2
LCFe	700	10	72	Rp: ~0.24 to 0.29 Ωcm2

), the available evidence suggests that higher valence, more Lewis-acidic B-site dopants, and/or stronger M-O bonds (e.g., Y³⁺/Nb⁵⁺/Ti⁴⁺/Ta⁵⁺) can improve apparent CO₂ tolerance. That said, comparisons are complicated by the varying test conditions (1–10% CO₂, 600–700 °C, 10 h) and by co-evolving factors such as tolerance-factor geometry, A-site basicity/Ba segregation, humidity, and conductivity. For example, Y-doped BSCF often shows smaller surface-exchange decay and less carbonate, but part of this advantage likely reflects phase stabilization; La on the A-site tends to stabilize the cubic phase (a geometric rather than acidity effect). Nb/Ti may weaken BaCO₃ formation or improve chemical robustness yet can depress conductivity/ORR. Partial Ba to Ca substitution appears to suppress Ba segregation, especially under humid/protonic conditions. In Sc/Ta ferrites, CO₂-induced ASR rises can be partly reversible, consistent with reversible surface coverage rather than persistent carbonate. Accordingly, “acidity-driven” improvements should be interpreted under matched conditions, with co-reporting of activity and stability to avoid over-attribution.

4.2. Composite SOFC Cathodes with Ceria-Based Electrolytes (SDC/GDC)

Incorporating electrolyte materials such as Sm_0.2Ce_0.8O_1.9 (SDC) or Gd_0.1Ce_0.9O_1.95 (GDC) into SOFC cathodes to form composite cathodes has been demonstrated as an effective approach to significantly enhance CO₂ tolerance. The core mechanism involves the reduced relative content and surface exposure of alkaline earth metal elements (e.g., Sr and Ba) within the cathode phase after compositing with SDC/GDC. Simultaneously, SDC/GDC provides more stable and highly active ORR sites while forming physical barriers that restrict direct contact pathways between CO₂ molecules and susceptible components in the cathode matrix at the microstructural level. This synergy improves the stability of the composite cathode in CO₂-containing atmospheres. As shown in Figure 6a, Xue et al. proposed a CO₂-tolerant stable oxygen-permeable membrane (60Ce_0.9Gd_0.1O_2−δ-40Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ) that exhibits higher oxygen permeability alongside superior stability in CO₂-containing atmospheres compared to pure BSCF [93]. Li et al. found that mechanically incorporating samarium-doped ceria (SDC) into SrCo_0.85Ta_0.15O_3−δ (SCT15) cathodes yielded composites with significantly lower sensitivity to 10% CO₂ than pure SCT15 cathodes (Figure 6b) [94]. Thermogravimetric analysis, energy-dispersive spectroscopy, and conductivity data indicate that SDC nanoparticles improve triple-phase boundaries while simultaneously serving as protective layers. As shown in Figure 6c, Gu et al. studied SrCo_0.8Nb_0.1Ta_0.1O_3−δ (SCNT)-Ce_0.9Gd_0.1O_2−δ (GDC) mechanically mixed composite cathodes, which exhibit enhanced CO₂ resistance [95,96]. This system demonstrates superior ORR activity and lower thermal expansion coefficients compared to BSCF, with GDC blending reducing CO₂-induced resistance sensitivity. Yang et al. [97] developed a cobalt-free Ba_0.95La_0.05FeO_3−δ (BLF) cathode where the surface coating with GDC nanoparticles via mechanical mixing enhances O₂ selectivity due to GDC’s high oxygen conductivity. This suppresses CO₂ surface exchange, providing an effective approach to improve cathode performance and mitigate CO₂ poisoning in air.

Adding SDC/GDC appears to improve CO₂ tolerance (

Table 2. Stability of representative doped SOFC cathodes under CO₂ before and after adding SDC/GDC.

Material	Temperature (°C)	CO2 (vol%)	Time (h)	Impedance: Start to End (Ω cm2)
BLF (Ba0.95La0.05FeO3−δ)	700	3 (+5% H2O)	24	Rp: 0.161 to 0.773
BLF-GDC (mechanical mixture)	700	3 (+5% H2O)	24	Rp: 0.148 to 0.274
BLF-GDC (nano-GDC surface coating)	700	3 (+5% H2O)	24	Rp: 0.126 to 0.160
BLF	700	10	2	Rp: 0.190 to 0.264
BLF-30% SDC (biphasic)	700	10	2	Rp: 0.121 to 0.150
SCNT (SrCo0.8Nb0.1Ta0.1O3−δ)	550	1	100	Rp: ~0.13 to ~1.3
SCNT-GDC composite	550	1	100	Rp: ~0.17 to ~0.8

), plausibly through the chemical shielding of Sr/Ba sites and by maintaining wider and more TPB that help sustain ORR in CO₂. The magnitude of the benefit seems sensitive to the architecture and loading: conformal coatings or percolating networks tend to perform better than simple mixes, whereas excessive ceria can dilute electronic pathways and increase ASR in clean air. Across both Co-containing and Co-free hosts, well-designed ceria phases often suppress ASR growth under CO₂, albeit with a potential trade-off in peak ORR activity. Therefore, these observations point toward moderate ceria fractions implemented in optimized architectures as a pragmatic target.

4.3. Cathode Fabrication via Infiltration Strategy

The infiltration strategy has been validated as an effective approach for enhancing CO₂ tolerance in SOFC cathodes. Qiu et al. [38] employed a solution infiltration technique to fabricate a novel core–shell structure for protecting BSCF. This design utilized LSM, known for its high catalytic activity and exceptional CO₂ resistance, as a dense shell to encapsulate porous BSCF cathodes with high oxygen conductivity. This configuration effectively prevented a direct reaction between BSCF and CO₂ while preserving optimal ORR activity and minimal polarization resistance (R_p) variation. Li et al. [79] demonstrated that infiltrating La₂NiO_4+δ (LNO) into a PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5+δ (PBSCF) cathode significantly enhanced CO₂ tolerance and ORR activity. The composite provided additional active sites for O₂ adsorption and achieved near-complete performance recovery after CO₂ removal. This improvement was attributed to LNO’s high oxygen exchange coefficient (k*), which facilitated dynamic oxygen surface exchange and localized CO₂ adsorption on the LNO coating. Similarly, La_0.94Ni_0.6Fe_0.4O_3−δ (LNF94) [98] infiltration into La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathodes suppressed Sr segregation and improved tolerance to CO₂ and chromium poisoning. Pei et al. [99] reported that barium cobaltate (BCO)-coated LSCF cathodes exhibited enhanced ORR kinetics and CO₂ resistance. Raman spectroscopy and DFT calculations confirmed that the BCO coating weakened CO₂ adsorption and inhibited Sr surface segregation, thereby promoting ORR activity and mitigating CO₂-induced degradation.

Taken together (

Table 3. Stability of representative doped SOFC cathodes under CO₂ before and after adding infiltration and coating.

Material	Temperature (°C)	CO2 (vol%)	Time (h)	Impedance: Start to End (Ω cm2)
LSCF (blank)	700	8	100	Rp: 0.26 to 1.92
LSCF-BaCoO3−δ coating (BCO-LSCF)	700	8	100	Rp: 0.11 to 0.32
PBSCF (PrBa0.5Sr0.5Co1.5Fe0.5O5+δ)	700	10	0.167 (10 min)	Rp: 2.50 to 3.63
PBSCF-LN (La2NiO4+δ-coated PBSCF)	700	10	20	Rp: 1.19 to ~1.56

), infiltration and coating appear to improve CO₂ tolerance by forming thin conformal shells that may mask Sr/Ba-rich surfaces and potentially shift ORR toward faster surface-exchange sites. The benefit seems to depend on the loading and architecture. On LSCF, Ni-Fe perovskite (LNF94) at moderate loading has been reported to form a near-continuous film that lowers ASR and apparent activation energy, whereas heavier loading can lead to agglomeration, porosity loss, and an ASR rebound. In addition, on LSCF, BCO coatings have been observed to weaken CO₂ adsorption, suppress the ~1066 cm⁻¹ carbonate signal, and limit Sr segregation, thereby helping to stabilize ORR kinetics in CO₂. For layered PBSCF, a La₂NiO_4+δ shell has been shown to reduce ASR and enable near-complete performance recovery after CO₂ removal, in contrast to uncoated cathodes under similar conditions. Therefore, thin continuous infiltrates or percolating networks at moderate loading tend to be more effective than simply adding more material.

4.4. CO₂-Induced In Situ Self-Reconstruction of the Cathode Surface

Enhancing cathode CO₂ tolerance can also be achieved through surface in situ reconstruction, beyond conventional methods. As shown in Figure 7a, Zhou et al. [100] developed a novel cathode material, SrY_0.05W_0.05Co_0.9O_3−δ, and established an optimization strategy utilizing CO₂-induced surface in situ reconstruction. Upon switching the treatment atmosphere from CO₂ to air, nano-sized SrCO₃ particles and oxide nanoparticles were generated. Both the SrCO₃ and oxide nanoparticles were found to enhance ORR kinetics at the surface. Consequently, the ASR of a symmetric cell employing the SrY_0.05W_0.05Co_0.05Co_0.9O_3−δ electrode decreased from 0.085 to 0.054 Ω cm² at 600 °C.

Similarly, Shao et al. [101] proposed a SrSc_0.025Nb_0.075Fe_0.9O_3−δ (SSNF) perovskite, where CO₂ exposure induced a beneficial transformation on the cathode surface (Figure 7b). The typically detrimental effect of carbonate formation was converted into a positive influence on the ORR activity for this perovskite. After treatment with high-concentration CO₂ (10 vol%), a SrCO₃ film formed on the perovskite surface, effectively shielding the underlying SSNF from further CO₂ corrosion. This resulted in favorable and stable ORR activity even under high CO₂ conditions (10 vol% in air). Furthermore, upon CO₂ removal from the air, the surface carbonate film readily decomposed, leading to a significant improvement in ORR activity at intermediate temperatures.

Zhang et al. [40] discovered that CO₂ treatment of a BaFeO_3−δ (BF) cathode induced the in situ generation of barium carbonate, which reconstructed the cathode’s internal architecture, ultimately forming a high-performance core-shell structured cathode. This structure comprises a BaCO₃ shell and a dual-phase core (containing BaFe₂O₄ and BaFeO₃ phases), synergistically facilitating charge transfer, surface oxygen binding, and durability. Crucially, carbonate species can induce either surface activation or passivation of ORR on MIEC perovskites [72], depending on the treatment method, carbonate type, and loading ratio. The Ba(Co_0.7Fe_0.3)_0.8Zr_0.1Sc_0.1O_3−δ (BCFZS) cathode shows Zr/Sc co-doping tailors CO₂ adsorption at Ba sites, causing beneficial nano-BaCO₃ surface decoration instead of poisoning [102]. Simultaneously, in situ La/Co/Mg interdiffusion enhances the interface and oxygen-ion conduction.

Controlled CO₂ exposure may transiently activate perovskite cathodes when it forms thin decomposable nano-carbonates that appear to protect or refresh the surface (e.g., SYWCo, SSNF, BF, BCFZS), whereas persistent coarser Ba/Sr carbonates tend to block transport and thus act as poisons. The balance seems to reflect a competition between A-site basicity (carbonate thermodynamics) and B-site acidity/oxygen-exchange kinetics (k*). Under 600–650 °C and 5–10 vol% CO₂, brief treatments can favor the “activating” route, while lower temperatures, higher CO₂, or dry feeds are more likely to promote passivation. Accordingly, strategies that seek to lower effective surface basicity while maintaining high k* may help bias reconstruction toward reversible nano-carbonates rather than bulk phases.

Beyond being recent, this review highlights CO₂-induced in situ self-reconstruction as a distinct designable line of work by synthesizing scattered case reports into a tentative process window and material-selection notes. We also provide a side-by-side comparison of mainstream strategies (high-acidity/high-valence doping, composite cathodes, surface infiltration/coatings, and CO₂-driven self-reconstruction) with operation complexity, advantages, and limitations to support practical choice (

Table 4. Comparative strategies for CO₂-tolerance enhancement.

Strategy	Mechanism	Operation Complexity	Advantages	Limitations
High-acidity/high-valence cation doping	Increases ABE and lowers carbonate-formation ΔG; intrinsically stabilizes lattice/surface	Moderate	Intrinsic and durable stabilization; scalable with standard ceramic processing; improved long-term CO2 tolerance	Possible conductivity/ORR trade-off; requires optimization of sintering, phase compatibility and TEC; some dopants add cost
Composite cathodes (e.g., SDC/GDC)	Dilutes Ba/Sr exposure and supplies stable ORR sites; microstructural/interface buffering against CO2	Simple	Drop-in friendly (mixing/light infiltration); fast, tunable gains via phase ratio/porosity control; maintains ORR while improving CO2 robustness	Must ensure phase/TEC compatibility; potential interdiffusion/interface reactions over long operation if co-fired
Surface infiltration/coating	LSM/LNO/BCO nano-layers tune adsorption/exchange and transport; shield susceptible sites	Moderate	Retrofittable to existing electrodes; fine surface control; targeted suppression of CO2 adsorption/carbonates	Performance hinges on thickness/uniformity/durability; excessive layers add resistance
CO2-induced in situ self-reconstruction	Controlled CO2 forms reversible carbonate/oxide nano-films or core-shell; adaptive passivation with potential mid-T ORR boost	Complex	Adaptive protection without foreign phases; reversible performance rebound; possibility of performance enhancement	Narrow process window (time/temperature/${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$); over-accumulation to poisoning; higher demands on reproducibility and in-process control; strict material selectivity/compatibility required

). While not exhaustive, we hope this framing offers a concise actionable overview for readers considering CO₂ tolerance design.

5. Characterization

5.1. Operando Characterizations

Recent operando investigations directly connect CO₂-induced surface chemistry with performance degradation in SOFC-relevant cathodes. Pagliari et al. [103] used operando NAP-XPS and synchrotron XRD to examine state-of-the-art cathodes at 700 °C (21% O₂/79% CO₂, up to 20 bar) and found SrCO₃ on LSCF/LSC but not on LSM, with the performance largely reversible over 350 h under load (0.85 V). Opitz et al. [104] performed operando NAP-XPS on perovskite model electrodes during CO₂ electrolysis (under cathodic polarization) and observed a high-binding-energy carbon species (~290 eV) consistent with a carbonate-type intermediate, requiring oxygen vacancies and electrons; graphitic carbon formed at strong bias could be removed upon depolarization. Yu et al. [105] conducted operando AP-XPS on ceria cells (~600 °C, 0.5 Torr CO/CO₂, +2.0 V) and directly identified surface CO₃²⁻ intermediates whose coverage increased under bias with concurrent Ce³⁺/Ce⁴⁺ changes, supporting a mechanism of pre-coordination to carbonate followed by electron-transfer reduction. Matras et al. [106] used (in situ/operando-style) XRD-CT on working BSCF membranes with CO₂-containing feeds and reported a negative impact on product yields but no detectable carbonate phases; instead, degradation involved BSCF phase collapse and interphases including Ba₆Co₄O₁₂ and BaWO₄ (reaction-zone interfaces). Therefore, these operando results indicate that CO₂ poisoning is governed by A-site-derived carbonate formation on Sr/Ba-containing perovskites and load-dependent surface redox; conversely, LSM-type or ceria-based surfaces show distinct responses. These insights suggest practical levers for material selection (limiting A-site basicity), bias/temperature windows, and surface-stabilization strategies.

5.2. DFT Calculations

Strengthening the role of theoretical guidance in the assessment and prediction of the chemical stability of cathode materials is of great importance. In addition to empirical stability assessment through exposure testing, computational methods based on density functional theory (DFT) provide profound insights into the intrinsic chemical stability of cathode materials from a fundamental perspective, offering a new paradigm beyond the traditional trial-and-error experimental approach. Dai et al. [107] demonstrated this by calculating the Gibbs free energy of the interaction between CO₂ and Ca-doped LaMnO₃ (LCaM) surfaces. Their DFT calculations incorporated vibrational entropy, and the results indicated that the adsorption of CO₂ on the LCaM was thermodynamically unfavorable throughout the entire operating temperature range. This theoretical prediction of high stability was subsequently confirmed experimentally: after long-term exposure to a 600 °C CO₂ atmosphere, the LCaM phase remained unchanged, and the fuel cell with an LCaM cathode exhibited highly stable performance. The strong agreement between computational predictions and experimental results shows that DFT-based energetics can serve as a powerful predictive descriptor for screening and designing CO₂-tolerant cathode materials, thereby establishing a more rational foundation for material selection. DFT calculations provide key theoretical support for understanding the adsorption behavior of CO₂ on the cathode surface. Studies have shown that the adsorption energy of CO₂ on the surface of alkaline earth metal oxides (AEMO) follows the trend of BaO > SrO > CaO > MgO, which aligns with DFT-computed adsorption energies. DFT simulations further reveal that the adsorption energy of CO₂ on AEMO surfaces is closely related to their basicity. Sites with stronger basicity (e.g., Ba²⁺) have a lower CO₂ adsorption energy barrier and are thus more likely to form carbonates. During the chemical adsorption process, CO₂ reacts with basic elements on the cathode surface (such as Ba²⁺, Sr²⁺) to form carbonates (such as SrCO₃, BaCO₃). DFT calculations indicate that the driving force of this process is closely related to the electronegativity of alkaline earth metal ions and the formation energy of oxygen vacancies. For instance, BaO has a relatively low oxygen vacancy formation energy, suggesting that its lattice oxygen is more readily involved in reactions, thereby promoting carbonate formation. In addition, DFT simulations have elucidated the dependence of CO₂ adsorption on surface oxygen vacancy concentration: materials with higher oxygen vacancy concentrations are more susceptible to CO₂ chemical adsorption, which in turn hinders oxygen adsorption and diffusion. Combined with thermodynamic analyses such as Ellingham diagrams, DFT has also demonstrated that Ba²⁺ possesses a higher driving force for carbonate formation than Sr²⁺ and La³⁺. It further quantifies alkalinity descriptors of A-site elements, such as local charge density and the oxygen p-band center and has revealed that the oxygen p-band center at Ba sites is closer to the Fermi level, indicating higher surface reactivity and a stronger tendency to interact with CO₂. For example, Tsvetkov et al.’s research on La_1−xSr_xMnO₃ DFT (LSM) materials shows that with the increase in the Sr doping amount, the CO₂ in surface adsorption can significantly reduce, leading to more serious carbonate formation and attenuation of the performance [18]. This result is consistent with the experimental data of programmed temperature desorption (TPD), verifying the effectiveness of DFT in predicting the CO₂ tolerance of materials. DFT can also serve to evaluate “acidity descriptors” for B-site cations, such as the average metal–oxygen bond energy (ABE) and B-site electronegativity. Studies indicate that high-valence and highly electronegative B-site cations (e.g., Nb⁵⁺, Mo⁶⁺, Ti⁴⁺) can significantly increase the ABE, thereby inhibiting CO₂ adsorption and carbonate formation. For example, the DFT calculation of the SrNb_0.1Co_0.9−yFe_yO_3−δ system shows that as the Fe content increases, the ABE of the material rises, and the CO₂ adsorption energy increases, resulting in the CO₂ tolerance being enhanced [108]. Furthermore, DFT simulations have revealed the relationship between CO₂ coverage and the type of B-site ion: in LaBO₃ (B = Cr, Fe, Co), CO₂ coverage follows the order LaCrO₃ > LaFeO₃ > LaCoO₃, which is closely related to the electronic structure of the B-site ion and the activity of surface oxygen [109].

6. Conclusions and Perspectives

SOFCs face the significant challenge of cathode poisoning by atmospheric CO₂ in real-life applications. CO₂ adsorbs onto perovskite cathode surfaces, particularly reacting with alkaline earth metals such as barium, which exhibits far higher reactivity than strontium or lanthanum to form carbonates. This process not only inhibits surface oxygen exchange and the oxygen reduction reaction but also creates persistent carbonates that resist decomposition even after CO₂ removal, critically impacting the long-term material stability. The interaction between CO₂ and O₂ at the cathode surface involves dynamic co-adsorption and competitive adsorption, which varies with temperature. Under competitive adsorption conditions, CO₂ occupies valuable active sites, hindering oxygen molecule adsorption.

Despite notable progress in the study of CO₂ poisoning, developing effective solutions remains a major challenge, due in part to insufficient systematic characterization and theoretical guidance. To address this, we propose the following perspectives and potential strategies to guide more in-depth investigations.

(1)

Developing new low-alkali/alkali-free perovskite cathode materials. The thermodynamic driving force of carbonate formation can be fundamentally avoided by designing and synthesizing high-performance perovskite cathode materials with no alkaline earth metals or low alkaline earth metal content. From this aspect, future research can focus on perovskite structures with rare earth elements (La, Pr, Sm, etc.) as the dominant A-sites.

(2)

Systematic screening and optimization of high-acidity/high-valence B-site dopants. High-acidity dopants are the key factor determining CO₂ tolerance, and systematic evaluation of the mechanism will promote an in-depth understanding. Establishing the structural–activity relationship between dopants, structural stability, and CO₂ tolerance by combining theoretical calculations and high-throughput experiments can open up novel strategies for synthesizing CO₂-resistent perovskite.

(3)

Construct material systems with high ABE values. The value of ABE can be well used to evaluate the priority of carbonation reactions and thus can reflect the tolerance of the perovskite system to CO₂. Priority should be given to developing material systems with high ABE values. For instance, by introducing elements such as Fe, Ti, and Zr to strengthen the B-O bond, the chemical adsorption of CO₂ and the formation of carbonates can be inhibited. It is also expected to synthesize materials with high CO₂ resistance.

(4)

Optimization of composite cathode structures and interface engineering. At present, research on composite cathodes mainly focuses on verifying the effectiveness, while in-depth study is required for the understanding of the microstructure of the composite interface, the uniformity of phase distribution, and long-term stability. In the future, efforts should be made to optimize the composite ratio and preparation process and to study at the microscopic scale how the SDC/GDC electrolyte phase acts as a physical barrier and highly active sites to precisely regulate gas transport and surface reaction pathways, so as to maximize the synergistic effect.

(5)

In-depth development of surface modification and in situ reconstruction strategies. Future research should focus on systematically revealing the thermodynamic boundaries and dynamic mechanisms of the reconstruction process. By precisely controlling the atmosphere, temperature, and time, the negative impact of CO₂ is explored to be transformed into a controllable and beneficial surface self-assembly and activation process, thereby developing smart cathode materials with adaptive and self-repairing functions.

(6)

Innovation of operational strategies and system integration. By regularly introducing CO₂-free air or controllable reducing atmosphere into the cathode can potentially restore the cathode activity by promoting carbonate decomposition; therefore, periodic atmosphere purging, temperature modulation, controlled reducing gas treatment, and other methods can be adopted to promote the decomposition and activity recovery of surface carbonates, and the impact of these dynamic strategies on the overall battery life, energy consumption, and system complexity should be systematically evaluated.

(7)

Strengthen the guidance of theoretical calculation in the assessment and prediction of the chemical stability of cathode materials. In addition to empirical stability assessment through exposure testing, the computational method based on density functional theory (DFT) provides profound insights into the intrinsic chemical stability of cathode materials from a fundamental perspective and offers a new angle for the traditional experimental trial-and-error approach. Dai et al. [107] demonstrated this by calculating the Gibbs free energy of the interaction between CO₂ and Ca-doped LaMnO₃ (LCaM) surfaces. Their DFT calculations incorporated vibrational entropy, and the results indicated that the adsorption of CO₂ on the LCaM was thermodynamically unfavorable throughout the entire operating temperature range. This theoretical prediction of high stability was eventually confirmed by experimental observations: after long-term exposure to 600 °C CO₂ atmosphere, the LCaM phase remained unchanged, and the fuel cell using the LCaM cathode exhibited highly stable output. The good consistency between computational predictions and experimental results indicates that the DFT-based energetics provides a powerful predictive descriptor for screening and designing CO₂-resistant cathode materials, thereby laying a more reasonable foundation for material selection beyond the traditional trial-and-error method.

(8)

Application of practical operando tools coupled with electrochemical readouts. For CO₂ poisoning, near-ambient pressure X-ray photoelectron spectroscopy (NAP/AP-XPS) (C 1s for carbonate/bicarbonate, O 1s for lattice/OH/CO₃²⁻, Sr/Ba core levels for A-site segregation) under realistic ${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$-${\mathrm{P}}_{{\mathrm{H}}_2\mathrm{O}}$-T-bias can be paired with synchrotron or lab in-situ XRD to follow phase changes, while operando Raman/DRIFTS tracks ν₃(CO₃²⁻) and OH features. Complementary online MS (CO₂/CO/H₂O) together with simultaneous EIS/ASR helps relate surface chemistry to kinetics; occasional ¹³CO₂/¹⁸O switching can clarify intermediate lifetimes and exchange routes. In related multi-pollutant contexts, in situ/operando SERS with mapping has already shown promise: for example, Chen et al. [110] detected and located SrCrO₄ forming on LSCF surfaces in real time under operating conditions, providing direct evidence of the poisoning pathway. Building on such demonstrations, carefully designed broad-spectrum anti-poisoning strategies (targeting CO₂, Cr, H₂O) can be evaluated with the same operando toolset. To keep results comparable, we suggest reporting basic test histories (${\mathrm{P}}_{{\mathrm{C}\mathrm{O}}_2}$, ${\mathrm{P}}_{{\mathrm{H}}_2\mathrm{O}}$, T, flow, bias/time, and recovery steps) and presenting simple operating “maps” indicating where carbonate or other surface species persist or decompose. This measured approach should gradually clarify load- and humidity-dependent behavior on Sr/Ba-containing perovskites versus LSM/ceria-type surfaces and provide practical guidance for material choices and operating windows.

Therefore, enhancing long-term cathode CO₂ tolerance demands both continuous material optimization (e.g., developing high-ABE high-acidity/high-valence dopants to simultaneously boost ORR activity and poisoning resistance) and the comprehensive evaluation and development of effective operational strategies. This integrated approach represents a crucial research direction for the practical implementation of SOFC technology.