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

: The development of novel oxygen reduction electrodes with superior electrocatalytic activity and CO 2 durability is a major challenge for solid oxide fuel cells (SOFCs). Here, novel cobalt-free perovskite oxides, BaFe 1 − x Y x O 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 cm 2 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.


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−x Ti x O 3−δ , SrFe 0.8 Nb 0.2−x Ta x O 3−δ , La 1−x Sr x FeO 3−δ , and Ba 1−x La x FeO 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 Ba 2+ , 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 La 3+ , Sm 3+ , and Ca 2+ in the A site and Nb 5+ , Sn 4+ , In 3+ , and Ni 2+ in the B site [12,[16][17][18]. Lu et al. found that In 3+ doping 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.

Results and Discussion
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. The increase in cell volume from 67.220 Å 3 (x = 0.05) to 69.148 Å 3 (x = 0.15) is identified, which is attributed to the larger ionic radius of Y 3+ (0.90 Å) relative to that of Fe 3+ (0.55 Å). This phenomenon indicates that the Fe sites in BaFeO 3 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.
Coatings 2020, 10, x FOR PEER REVIEW 3 of 10 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.

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. The increase in cell volume from 67.220 Å 3 (x = 0.05) to 69.148 Å 3 (x = 0.15) is identified, which is attributed to the larger ionic radius of Y 3+ (0.90 Å) relative to that of Fe 3+ (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 cofired 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  The oxygen mobility and reducibility of Fe ions for the BFYx samples were studied by O 2 -TPD. Figure 2a presents the representative O 2 -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 Fe 4+ to Fe 3+ 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 (δ 0 ) 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 Fe 4+ to Fe 3+ . 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.
Coatings 2020, 10, x FOR PEER REVIEW 4 of 10 second oxygen desorption peak may be ascribed to the reduction of Fe 4+ to Fe 3+ 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 Fe 4+ to Fe 3+ . 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 Fe 4+ to Fe 3+ or Fe 2+ 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 BaFeO3−δ-based cathodes, such as BaFe0.95Nb0.05O3−δ (9.5 S cm −1 ) [16], BaFe0.95Zr0.05O3−δ (7.5 S cm −1 ) [14] and BaFe0.8In0.2O3−δ (2.3 S cm −1 ) [19]. Furthermore, the Arrhenius plots of electrical conductivity are presented in Figure 3b. The calculated activation energies (Ea) of BFYx are 0.29, 0.38, and 0.43 eV for BFY05, BFY10 and BFY15, respectively.  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 Fe 4+ to Fe 3+ or Fe 2+ 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 0.95 Nb 0.05 O 3−δ (9.5 S cm −1 ) [16], BaFe 0.95 Zr 0.05 O 3−δ (7.5 S cm −1 ) [14] and [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 cm 2 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. 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 BaFeO3−δ-based cathodes, such as BaFe0.95Nb0.05O3−δ (9.5 S cm −1 ) [16], BaFe0.95Zr0.05O3−δ (7.5 S cm −1 ) [14] and BaFe0.8In0.2O3−δ (2.3 S cm −1 ) [19]. Furthermore, the Arrhenius plots of electrical conductivity are presented in Figure 3b. The calculated activation energies (Ea) 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 (Rp) value of the electrode is a crucial descriptor for the cathode performance, and the lower Rp value means a superior electrochemical activity for ORR. The Rp of BFY05, BFY10 and BFY15 cathodes are 0.136, 0.057, and 0.107 Ω cm 2 at 700 °C, respectively, suggesting highly electrocatalytic performance of the BFYx cathodes. The Rp 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 Ea values are 1.40, 1.33 and 1.45 eV for BFY05, BFY10, and BFY15, respectively. Moreover, the BFY10 cathode presents the lowest Ea 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 [Rohm-(RH-CPEH)-(RM-CPEM)-(RL-CPEL)] (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 Rp and PO2 can be expressed by the following formula: Rp = k(PO2) −m (1) [30,31]. The dependence of the RHF, RMF and RLF for the BFY10 cathode on the PO2 at 700 °C is presented in Figure 5c  To further clarify the electrochemical processes of the cathode, EIS spectra of the BFY10 electrode are systematically studied under varied Po 2 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 Po 2 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 PO 2 can be expressed by the following formula: R p = k(PO 2 ) −m (1) [30,31]. The dependence of the R HF , R MF and R LF for the BFY10 cathode on the PO 2 at 700 • C is presented in Figure 5c [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.
Coatings 2020, 10, x FOR PEER REVIEW 6 of 10 can be found that the RMF is higher than RHF and RLF, 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 Rp value significantly increases with increasing the concentration of CO2. However, the Rp 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]: where xA and xB are the molar fraction of A and B metals; 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 Ba0.5Sr0.5Co0.8Fe0.2O3−δ (−274 kJ mol −1 ) [37], Bi0.5Sr0.5FeO3−δ (−276.67 kJ mol −1 ) and Bi0.5Sr0.5Fe0.9Ta0.1O3−δ (−296.97 kJ mol −1 ) [26]. These results confirm that the BFYx cathodes have satisfactory CO2 tolerance. Some Ba-based cathodes for SOFCs show chemical instability under CO 2 -containing atmospheres because of the formation of BaCO 3 on the cathode surface, diminishing oxygen reduction kinetics [33,34]. To evaluate the CO 2 tolerance of the electrode, EIS spectra of the BFY10 cathode were acquired in CO 2 -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 CO 2 . However, the R p recovers to the initial value after removing CO 2 atmosphere, which demonstrates the outstanding CO 2 tolerance of the BFY10 cathode. Generally, the average metal oxide binding energy (ABE) is normally used to assess the CO 2 durability of the cathode materials [35]. More negative ABE value indicates that oxide has excellent CO 2 tolerance. The ABE is calculated based on the following equation [36]: where 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. 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.

Conclusions
In summary, Fe-based perovskite BaFe1−xYxO3−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 RP value of 0.057 Ω cm 2 at 700 °C. In addition, the single  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.

Conclusions
In summary, Fe-based perovskite BaFe1−xYxO3−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 RP value of 0.057 Ω cm 2 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 adsorptiondissociation of the oxygen molecule process is the rate-limiting step on the cathode.

Conclusions
In summary, Fe-based perovskite BaFe 1−x Y x O 3−δ oxides with excellent ORR performance and CO 2 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 cm 2 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.

Conflicts of Interest:
The authors declare no conflict of interest.