Insights in to the Electrochemical Activity of Fe-Based Perovskite Cathodes toward Oxygen Reduction Reaction for Solid Oxide Fuel Cells

Abstract

The development of novel oxygen reduction electrodes with superior electrocatalytic activity and CO₂ durability is a major challenge for solid oxide fuel cells (SOFCs). Here, novel cobalt-free perovskite oxides, BaFe_1−xY_xO_3−δ (x = 0.05, 0.10, and 0.15) denoted as BFY05, BFY10, and BFY15, are intensively evaluated as oxygen reduction electrode candidate for solid oxide fuel cells. These materials have been synthesized and the electrocatalytic activity for oxygen reduction reaction (ORR) has been investigated systematically. The BFY10 cathode exhibits the best electrocatalytic performance with a lowest polarization resistance of 0.057 Ω cm² at 700 °C. Meanwhile, the single cells with the BFY05, BFY10 and BFY15 cathodes deliver the peak power densities of 0.73, 1.1, and 0.89 W cm⁻² at 700 °C, respectively. Furthermore, electrochemical impedance spectra (EIS) are analyzed by means of distribution of relaxation time (DRT). The results indicate that the oxygen adsorption-dissociation process is determined to be the rate-limiting step at the electrode interface. In addition, the single cell with the BFY10 cathode exhibits a good long-term stability at 700 °C under an output voltage of 0.5 V for 120 h.

1. Introduction

Perovskite-type materials are the promising cathodes for solid oxide fuel cells (SOFCs) [1,2,3]. Among the perovskite oxides, the Co-containing perovskite materials with mixed ionic-electronic conducting feature have been widely investigated due to their remarkable electrochemical performance for ORR [4,5,6]. However, Co-containing oxides show some other drawbacks, such as poor chemical stability, higher thermal expansion coefficient, and strong volatility, which inhibit their wide applications in SOFCs [7,8]. To address these issues, developing new cathodes with improved electrochemical performance and good chemical stability is an important trend. Recently, Fe-based perovskite materials exhibit attractive chemical compatibility and excellent electrocatalytic activity, such as SrFe_1−xTi_xO_3−δ, SrFe_0.8Nb_0.2−xTa_xO_3−δ, La_1−xSr_xFeO_3−δ, and Ba_1−xLa_xFeO_3−δ [9,10,11,12].

Among the Fe-based oxides, BaFeO_3−δ presents attractive oxygen permeation flux and fast oxygen surface exchange kinetics [13,14]. This is mainly due to variable valence and excellent chemical stability of Fe ions, as well as lower valence and larger ionic radius of Ba²⁺, which facilitates the electrochemical performance and oxygen transport of the materials [15]. Cation-doping is commonly adopted to stabilize the cubic lattice of BaFeO_3−δ with disordered oxygen vacancies, such as La³⁺, Sm³⁺, and Ca²⁺ in the A site and Nb⁵⁺, Sn⁴⁺, In³⁺, and Ni²⁺ in the B site [12,16,17,18]. Lu et al. found that In³⁺ doping in the B site of BaFeO_3−δ can enhance oxygen permeation flux, in which BaFe_0.9In_0.1O_3−δ presented the higher oxygen permeation flux of 1.11 mL cm⁻² min⁻¹ at 950 °C [19]. Song et al. reported an attractive cathode candidate of BaFe_1−xBi_xO_3−δ for SOFCs. The BaFe_0.9Bi_0.1O_3−δ cathode exhibited a lower polarization resistance of 0.133 Ω cm² at 750 °C and a high oxygen vacancy concentration of 0.408. However, the thermal expansion coefficient of BaFe_0.9Bi_0.1O_3−δ is very large (26.697 × 10⁻⁶ K⁻¹). Additionally, the oxygen permeability and oxygen non-stoichiometry of BaFe_1−xY_xO_3−δ have been investigated by Liu et al. They found that Y-doping promotes the oxygen vacancy concentration and oxygen ion migration, which is believed to be favorable for transport of the oxygen ion in the bulk electrode, thereby resulting in the enhanced electrocatalytic performance [20].

In this work, Fe-based perovskite BaFe_1−xY_xO_3−δ oxides have been investigated as the prominent cathodes for SOFCs. The crystalline structure, CO₂ tolerance, and electrocatalytic activity for ORR of the BaFe_1−xY_xO_3−δ cathodes are systematically investigated. The results provide an effective strategy for designing novel cathode electrocatalysts for SOFCs.

2. Experimental

2.1. Material Preparation

The BaFe_1−xY_xO_3−δ (x = 0.05–0.15) samples were synthesized by the solid-state reaction. Stoichiometric amounts of BaCO₃ (99.99%, Tianjin Guangfu Co. Ltd., Tianjin, China), Fe₂O₃ (99.99%, Tianjin Guangfu Co. Ltd.), and Y₂O₃ (99.99%, Tianjin Guangfu Co. Ltd.) were mixed and ground via the ball milling using ethanol as the dispersant. Afterwards, the mixture was pre-fired at 1000 °C for 10 h with a heating rate of 5 °C min⁻¹ in air and then re-milled for 1 h, followed by calcining at 1300 °C for 12 h to obtain the final products.

2.2. Characterization

The crystal structure of BFYx cathodes were identified using X-ray diffraction (Bruker D8 advance) with filtered Cu-Kα radiation (λ = 1.5148 Å) source in a 2θ range of 10°–80°. The XRD data was analyzed to obtain the structural parameters by using Rietveld refinement method using the Rietica software (Version 1.7.7.8) program. The oxygen desorption property of the cathode catalysts was carried out by O₂ temperature-programmed desorption (O₂-TPD) with the TP-5076 instrument (Tianjin Xianquan, Co. Ltd., Tianjin, China). The electrical conductivity was measured between 100 and 800 °C in air by standard four-probe DC method with a Keithley 2400 SourceMeter Keithley Instruments Inc., Cleveland, OH, USA). The oxygen non-stoichiometry of the samples at room temperature was determined with the iodometric titration method, as described elsewhere [21,22]. The oxygen non-stoichiometry at high temperature was measured by thermogravimetric analysis (TGA, SETARAM, Caluire et Cuire, France).

2.3. Electrochemical Test

The dense Ce_0.9Gd_0.1O_1.95 (CGO) electrolyte was fabricated by pressing CGO powders (SOFCMAN Co. Ltd., Ningbo, China) uniaxially at 220 MPa, and sintered at 1450 °C for 24 h. For the fabrication of symmetrical cells (BFYx|CGO|BFYx), the BFYx electrode powders were mixed with terpineol and ethyl cellulose to prepare the cathode slurry. The slurry was symmetrically coated on the Ce_0.9Gd_0.1O_2−δ (CGO) electrolyte and sintered at 900 °C for 4 h. The electrochemical impedance spectroscopy (EIS) of symmetric cell was acquired by an Autolab PGSTAT302N workstation in the frequency range of 10⁻²–10⁶ Hz under open voltage conditions at 500–700 °C. To explore the electrochemical process for ORR of the electrode, the EIS spectra were collected under different oxygen partial pressure (PO₂).

The anode-supported half cell (NiO-YSZ|YSZ|CGO) was bought from Ningbo SuoFuRen Energy Co. Ltd. (Ningbo, China). The BFYx cathode slurry was printed onto the CGO barrier layer, and subsequently co-fired at 900 °C for 4 h. The electrochemical performance of anode-supported fuel cells was tested using electrochemical workstation (ZAHNER, IM6e, Kronach, Germany. The single cell was mounted on the alumina tube, while the humidified H₂ (3% H₂O) with a flow rate of 80 mL min⁻¹ and ambient air were used as the fuel and oxidant, respectively.

3. Results and Discussion

Figure 1a displays the XRD patterns of BFYx samples. The diffraction profiles reveal that the BFYx oxides crystallize in a cubic perovskite structure with Pm-3m space group. The magnified XRD patterns between 29 and 33° are presented in Figure 1a. The diffraction peaks gradually shift to a lower angle direction with increasing the doping fraction, indicating the expansion of the lattice constants. To further obtain the lattice constants of the materials, Rietveld refined XRD data of BFY10 samples are given in Figure 1b. The refined lattice constants of BFYx samples are summarized in

Table 1. Lattice parameters of BFYx samples.

Space Group	x = 0.05	x = 0.1	x = 0.15
	Pm-3m	Pm-3m	Pm-3m
a (Å)	4.066	4.091	4.104
b (Å)	4.066	4.091	4.104
c (Å)	4.066	4.091	4.104
V (Å3)	67.220	68.468	69.148
χ2	2.171	1.906	4.190
Rwp (%)	7.762	7.664	9.811
RP (%)	5.975	6.023	7.878

. The increase in cell volume from 67.220 ų (x = 0.05) to 69.148 ų (x = 0.15) is identified, which is attributed to the larger ionic radius of Y³⁺ (0.90 Å) relative to that of Fe³⁺ (0.55 Å). This phenomenon indicates that the Fe sites in BaFeO₃ are partially replaced by the Y ions. Furthermore, to examine the chemical compatibility between electrode and electrolyte, the mixtures of BFYx and CGO were co-fired at 1000 °C for 12 h. Figure 1c presents the XRD patterns of the calcined BFYx-CGO mixtures. No obvious impurities can be detected, revealing the favorable chemical compatibility of the BFYx cathodes with CGO electrolyte at 1000 °C.

The oxygen mobility and reducibility of Fe ions for the BFYx samples were studied by O₂-TPD. Figure 2a presents the representative O₂-TPD profiles of the BFYx samples. The curves show two desorption peaks at around 200 and 650 °C in all samples. The first broad peak located at ~200–300 °C is associated with the desorption process of the chemisorbed oxygen on the material surface. The second oxygen desorption peak may be ascribed to the reduction of Fe⁴⁺ to Fe³⁺ at 300–650 °C [23]. It is noteworthy that the BFY10 material shows the largest area of second desorption peak among the BFYx samples, indicating the highest oxygen vacancy concentration and favorable oxygen mobility of the BFY10. The oxygen non-stoichiometry (δ) of the BFYx samples at elevated temperatures was explored by TGA in air, as presented in Figure 2b. The δ values were determined by TGA results and the initial oxygen non-stoichiometry (δ₀) values at room temperature were obtained by the iodometric titration. The initial weight loss from room temperature to 300 °C is associated with the evaporation of adsorbed water. When the temperature is above 300 °C, the obvious weight loss is due to the reduction of Fe⁴⁺ to Fe³⁺. It can be seen that the δ values of the samples decreases with the doping content from room temperature to 600 °C. However, when increasing the temperature to 600 °C, the BFY10 possesses the largest oxygen vacancy concentration, meaning its excellent oxygen ions mobility and promoted catalytic activity for ORR.

The temperature dependence of electrical conductivity for the BFYx samples within the temperature range of 100–800 °C in air is presented in Figure 3a. The electrical conductivity of all samples shows a similar trend as a function of temperature. When increasing the temperature, the electrical conductivity of BFYx initially increases and reaches a maximum value at about 400 °C, and subsequently decreases between 400 and 800 °C. This indicates a transition from semi-conducting behavior to metal-like conduction. In addition, it is observed that the electrical conductivity gradually decreases with increasing Y-doping fraction. This phenomenon may be due to the higher Y content leads to the reduction of Fe⁴⁺ to Fe³⁺ or Fe²⁺ for the charge compensation, resulting in a decrease in the electrical conductivity. The maximum values of electrical conductivity are 9.81, 5.03, and 1.67 S cm⁻¹ for BFY05, BFY10 and BFY15, respectively. These values are comparable to those of other reported BaFeO_3−δ-based cathodes, such as BaFe_0.95Nb_0.05O_3−δ (9.5 S cm⁻¹) [16], BaFe_0.95Zr_0.05O_3−δ (7.5 S cm⁻¹) [14] and BaFe_0.8In_0.2O_3−δ (2.3 S cm⁻¹) [19]. Furthermore, the Arrhenius plots of electrical conductivity are presented in Figure 3b. The calculated activation energies (E_a) of BFYx are 0.29, 0.38, and 0.43 eV for BFY05, BFY10 and BFY15, respectively.

The EIS spectra were used to demonstrate electrocatalytic performance of the symmetric cells of BFYx|CGO|BFYx. Figure 4a displays the Nyquist plots of the BFYx cathodes at 700 °C in air. In general, the polarization resistance (R_p) value of the electrode is a crucial descriptor for the cathode performance, and the lower R_p value means a superior electrochemical activity for ORR. The R_p of BFY05, BFY10 and BFY15 cathodes are 0.136, 0.057, and 0.107 Ω cm² at 700 °C, respectively, suggesting highly electrocatalytic performance of the BFYx cathodes. The R_p value (700 °C) of BFY10 cathode is smaller than that of the Fe-based perovskite electrodes (Figure 4b) [24,25,26,27]. Furthermore, the Arrhenius plots of the polarization resistance are presented in Figure 4c. The calculated E_a values are 1.40, 1.33 and 1.45 eV for BFY05, BFY10, and BFY15, respectively. Moreover, the BFY10 cathode presents the lowest E_a value, implying the highest electrocatalytic performance.

To further clarify the electrochemical processes of the cathode, EIS spectra of the BFY10 electrode are systematically studied under varied Po₂ at 700 °C, as shown in Figure 5a. Clearly, the impedance spectra are consisted of three separable high-frequency, intermediate-frequency and low-frequency arcs, respectively, suggesting that the three different electrode processes occur on the cathode. The EIS data are further fitted with an equivalent circuit using the model of [R_ohm-(R_H-CPE_H)-(R_M-CPE_M)-(R_L-CPE_L)] (inset in Figure 5a) and analyzed by the distribution of relaxation time (DRT) method. Figure 5b displays the DRT results of BFY10 cathode under different Po₂ at 700 °C. It can be seen that the typical DRT plots presents three distinct peaks, the high-frequency peak P1 (HF), and intermediate-frequency peak P2 (MF) and low-frequency peak P3 (LF), corresponding to charge transport process, adsorption-dissociation process of oxygen molecule, and gaseous diffusion, respectively [28,29]. Additionally, the relationship between R_p and PO₂ can be expressed by the following formula: R_p = k(PO₂)^−m (1) [30,31]. The dependence of the R_HF, R_MF and R_LF for the BFY10 cathode on the PO₂ at 700 °C is presented in Figure 5c. One should note that the m values are 0.26, 0.48, and 1.03 in high-frequency, intermediate-frequency and low-frequency region, respectively, which are ascribed to the charge transfer process (${\mathrm{O}}_{\mathrm{ads}.}{+2\mathrm{e}}^{/}{+\mathrm{V}}_{\mathrm{O}}^{..}\iff {\mathrm{O}}_{\mathrm{O}}^{\mathrm{x}}$, m = 1/4), the adsorption-dissociation of the oxygen molecule process (${{\mathrm{O}}_2}_{,\mathrm{ads}.}\iff 2{\mathrm{O}}_{\mathrm{ads}.}$, m = 1/2) and the adsorption and diffusion of gaseous oxygen on the electrode surface (${\mathrm{O}}_2(g)\iff {\mathrm{O}}_{2\mathrm{ads}.}$, m = 1) [32]. Furthermore, it can be found that the R_MF is higher than R_HF and R_LF, implying that the rate-determining step for ORR is dominated by intermediate-frequency arc assigned to the oxygen adsorption-dissociation.

Some Ba-based cathodes for SOFCs show chemical instability under CO₂-containing atmospheres because of the formation of BaCO₃ on the cathode surface, diminishing oxygen reduction kinetics [33,34]. To evaluate the CO₂ tolerance of the electrode, EIS spectra of the BFY10 cathode were acquired in CO₂-containing air (3%, 5%, 10%) at 700 °C, as presented Figure 6a,b. It can be seen that the R_p value significantly increases with increasing the concentration of CO₂. However, the R_p recovers to the initial value after removing CO₂ atmosphere, which demonstrates the outstanding CO₂ tolerance of the BFY10 cathode. Generally, the average metal oxide binding energy (ABE) is normally used to assess the CO₂ durability of the cathode materials [35]. More negative ABE value indicates that oxide has excellent CO₂ tolerance. The ABE is calculated based on the following equation [36]:

ABE = ABE(A-O) + ABE(B-O)

(2)

$$\mathrm{ABE}(\mathrm{A}-\mathrm{O})=\frac{x_{\mathrm{A}}}{12m}({\mathrm{\Delta }}_f{H}_{{\mathrm{A}}_{\mathrm{m}}{\mathrm{O}}_{\mathrm{n}}}^{\mathrm{o}}-m\mathrm{\Delta }{H}_{\mathrm{A}}^{\mathrm{o}}-\frac{n}{2}{D}_{{\mathrm{O}}_2})$$

(3)

$$\mathrm{ABE}(\mathrm{B}-\mathrm{O})=\frac{x_{\mathrm{B}}}{6m}({\mathrm{\Delta }}_f{H}_{{\mathrm{B}}_{\mathrm{m}}{\mathrm{O}}_{\mathrm{n}}}^{\mathrm{o}}-m\mathrm{\Delta }{H}_{\mathrm{B}}^{\mathrm{o}}-\frac{n}{2}{D}_{{\mathrm{O}}_2})$$

(4)

where x_A and x_B are the molar fraction of A and B metals; $\mathrm{\Delta }{H}_{\mathrm{A}}^{\mathrm{o}}$ and $\mathrm{\Delta }{H}_{\mathrm{B}}^{\mathrm{o}}$ are the sublimation heat of A and B metals; ${\mathrm{\Delta }}_f{H}_{{\mathrm{A}}_{\mathrm{m}}{\mathrm{O}}_{\mathrm{n}}}^{\mathrm{o}}$ and ${\mathrm{\Delta }}_f{H}_{{\mathrm{B}}_{\mathrm{m}}{\mathrm{O}}_{\mathrm{n}}}^{\mathrm{o}}$ are the formation heat of A_mO_n and B_mO_n oxides, and D_O2 is the dissociation energy of O₂. The ABE values of BFYx oxides are −287.18 kJ mol⁻¹, −291.35 kJ mol⁻¹, and −295.51 kJ mol⁻¹, respectively, which are higher than cobalt-free perovskite cathodes, such as Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (−274 kJ mol⁻¹) [37], Bi_0.5Sr_0.5FeO_3−δ (−276.67 kJ mol⁻¹) and Bi_0.5Sr_0.5Fe_0.9Ta_0.1O_3−δ (−296.97 kJ mol⁻¹) [26]. These results confirm that the BFYx cathodes have satisfactory CO₂ tolerance.

To further demonstrate the practical application of the BFYx cathodes, the single cells were fabricated and tested. Figure 7a–c displays the I–V and I–P curves of the single cells with the BFYx cathodes 600–700 °C using humidified hydrogen and air as the fuel and oxidant, respectively. At 700 °C, the peak powder densities of 0.73, 1.1, and 0.89 W cm⁻² are achieved in the BFY05, BFY10 and BFY15 cathodes, respectively. It should be noted that the single cell with the BFY10 cathode shows the highest peak powder density among the BFYx electrodes, which is associated with superior electrochemical performance for ORR. In addition, the peak powder density of single cell with the BFY10 cathode is higher than those of other cobalt-free perovskite cathodes [38,39,40,41,42,43,44]. Furthermore, Figure 7d displays the operating stability of the fuel cell with the BFY10 cathode during a period of 120 h. The single cell presents a stable current density and peak power density under an output voltage of 0.5 V without noticeable attenuation, suggesting that the BFY10 electrode has outstanding durability during the operating process. The remarkable electrocatalytic properties indicate that the BFY10 oxide is a highly attractive cathode candidate for SOFCs.

4. Conclusions

In summary, Fe-based perovskite BaFe_1−xY_xO_3−δ oxides with excellent ORR performance and CO₂ durability are evaluated as the cathode materials for SOFCs. Benefiting from cubic perovskite structure and large oxygen vacancy concentration, the BFY10 cathode presents outstanding electrochemical activity for ORR with a lower R_P value of 0.057 Ω cm² at 700 °C. In addition, the single fuel with the BFY10 cathode delivers a peak powder density of 1.1 W cm⁻² at 700 °C, along with negligible attenuation over a period of 120 h. Furthermore, DRT study verifies that the adsorption-dissociation of the oxygen molecule process is the rate-limiting step on the cathode.