Introduction

Reversible solid oxide cells (RSOCs) have attracted immense attention as one of the hydrogen energy technologies. They provide several benefits, including high efficiency, reliability, and environmental sustainability [1,2,3]. RSOCs seamlessly integrate fuel cell (FC) and electrolysis technologies, providing efficient, scalable, and intermittent energy storage and conversion capabilities. In solid oxide fuel cells (SOFCs) mode, electrochemical reactions efficiently convert hydrogen into electrical energy. Conversely, solid oxide electrolysis (SOEC) mode uses excess electrical energy from renewable sources to split water and generate hydrogen [4]. Specifically, the reversible protonic ceramic electrochemical cells (R-PCECs) developed on this foundation exhibit remarkable properties. Moreover, due to the low activation energy (Ea < 0.5 eV) of the proton-conducting electrolyte, the reversible protonic ceramic electrochemical cells (R-PCECs) exhibit remarkable properties (high efficiency, fast reaction rate, low operating temperature). This enables R-PCECs to maintain efficient energy conversion at the comparatively low operating temperature of 400–700 °C. Moreover, reducing the working temperature to 400–700 °C aids in preventing issues associated with sealing and material aging resulting from high temperature [5,6,7]. However, the kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) on the oxygen electrode are severely restricted in the low-temperature range, which affects the effectiveness and efficiency of R-PCECs [8]. Thus, there is an urgent need to develop new oxygen electrode materials with high catalytic activity, making it a prominent research focus in R-PCECs. Cobalt-based perovskites stand out among other materials due to their exceptional surface activity and conductivity, making them popular oxygen electrode materials in R-PCECs. These materials, Ba0.5Sr0.5Co0.8Fe0.2O3−δ, La0.6Sr0.4Co0.2Fe0.8O3−δ, and PrBa0.5Sr0.5Co1.5Fe0.5O5+δ, exhibit excellent O2−/e− mixing conductivity and demonstrate high catalytic performance within low-temperature range for R-PCECs [9,10,11]. However, the proton conductivity of these oxygen electrode materials is restricted, and their active sites exist only at the electrode–electrolyte–air triple-phase boundary (TPB) [12]. Introducing proton conductivity into O2−/e− mixing conducting perovskite oxides can expand active sites from TPB to the entire oxygen electrode surface, thereby improving the catalytic performance of the electrode. In addition to this drawback, there are other concerns, such as the high cost of cobalt-containing raw materials, the tendency for the cobalt element to evaporate, and the undesirable high thermal expansion coefficients. Hence, there is an urgent need to design perovskite cathode materials free from cobalt that exhibit desirable performance at intermediate temperatures.

Iron-based perovskite has emerged as a feasible replacement for cobalt-based perovskite due to its high potential performance, low cost and ease of hydration, and enhanced chemical and thermal stability [13,14,15]. These BaFeO3−δ-based perovskites also exhibit high electronic and ionic conductivities due to the variable valency of Fe ions and large free lattice volumes, indicating significant promise for R-PCEC applications. However, the ionic radii of Ba and Fe do not align perfectly, making BaFeO3−δ compounds heterogeneous. To achieve a cubic phase with enhanced conductivity, the doping technique is investigated to stabilize the cubic phase at low temperatures, as Ba0.95La0.05FeO3−δ, BaNb0.05Fe0.95O3−δ, and BaFe0.9Zr0.1O3−δ [16,17,18,19]. Doping BaFeO3−δ-based perovskite oxides with large amounts of Zr elements can improve ORR activity and phase stability. The La0.9Sr0.1Ga0.8Mg0.2O3−δ-supported symmetrical SOFC with a BaFe0.9Zr0.1O3−δ electrode obtains an impressive peak power density of 1097 mW/cm2 at 800 °C [16]. The anode-supported single cell of PCEC with Ba0.95La0.05Fe0.8Zr0.1Y0.1O3−δ cathode reaches a peak power density of 305 mW/cm2 at 600 °C, while the power density of 300 mW/cm2 at 600 °C for the cell with BaZr0.1Fe0.75Ni0.15O3−δ cathode [20, 21]. Moreover, researchers have demonstrated that doping Y3+ ions enable charge compensation for oxygen vacancies. Utilizing phase doping of elements is a common strategy to modulate the catalytic activity of perovskite oxides. In this method, B-site metal doping plays a crucial role. The effect of B-site element doping on the oxygen vacancy formation process and the stability of the perovskite lattice must be investigated systematically. Understanding this mechanism enables the development of high-performance catalysts [18]. Incorporating Y3+ ions can modify the charge equilibrium within a material, potentially creating oxygen vacancies to uphold charge neutrality [22,23,24]. Moreover, protons localize as a conjugated superstructure near the dopant Y that makes the landscape on which the protons move [25]. Considering the abovementioned factors, the oxygen electrode materials created by Zr and Y co-doped BaFeO3−δ-based perovskite oxides are anticipated to demonstrate outstanding electrochemical performance for PCECs.

In this study, BaFe0.9−xZr0.1Y0.xO3−δ (BFZYx, x = 0, 1, 2, 3, 4) perovskite oxides are investigated co-doping with Zr and Y to serve as oxygen electrodes for PCECs. The BFZYx substances sustain their stable cubic phase structure at room temperature following B-site co-doping with Zr and Y. Furthermore, incorporating the Y element notably enhances the catalytic performance of the electrode and reduces the polarization resistance. Thus, the cell with the BFZY3 oxygen electrode delivers a low polarization impedance of 0.13 Ω cm2 and high peak power density (PPD) of 546.59 mW/cm2 at 650 °C, with excellent visibility. This study emphasizes a prospective oxygen electrode for high-performance R-PCECs, which could expedite the implementation of this technology in the real world.

Experimental Method

Material Synthesis and Cell Fabrication

A sol–gel method [26] was used to prepare the BaFe0.9-xZr0.1Y0.xO3−δ (BFZYx, x = 0, 1, 2, 3, 4) and BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb) powders. Chemical precursors included Ba(NO3)2 (Aladdin 99%), Fe(NO3)3·9H2O (Aladdin 99%) ZrO(NO3)2·xH2O, Y(NO3)3·6H2O (Aladdin 99%), Ce(NO3)3·6H2O (Aladdin 99%), and Yb(NO3)3·5H2O. First, the stoichiometric chemical precursors, citric acid (CA, Sinopharm Chemical Reagent, purity > 99.5%), and ethylenediaminetetraacetic acid (EDTA, Sinopharm Chemical Reagent, purity > 99.5%) were dissolved in water according to the molar ratio of metal ions (Me): EDTA: CA = 1:1:1.5. Subsequently, ammonia hydroxide was added to the solution to adjust the pH to 7. Heating and continuous stirring at 90 °C thickened the solution to produce a stable gel. The gel was then dried in an oven at 300 °C for 5 h. Finally, the precursor was calcined in air at 800–1000 °C for 5 h to obtain powders. The tape casting method was used to prepare the cell’s Ni-BZCYYb support with a weight ratio of 65:35 (NiO:BZCYYb). BZCYYb electrolyte paste was coated on the support using the screen-printing method, which was subsequently cosintered at 1450 °C for 5 h. Finally, the screen-printed BFZYx was calcined at 1000 °C for 3 h to form a final cell. The active cell area is 0.2 cm2, and Pt was used as the current collector. In symmetric cell measurements, dense BZCYYb pellets, exhibiting a relative density of approximately 98%, were subjected to a preparation process involving 1% NiO powders. This mixture was then processed via the dry-pressing method and calcined at 1450 °C for 5 h. Subsequently, slurries containing the oxygen electrode were used on both sides of the electrolytes using a screen-printing technique. These assemblies were calcined at 1000 °C for 3 h in ambient air to finalize the formation of symmetric cells.

Characterization and Electrochemical Test

The phase structure of BFZYx was characterized using an X-ray diffractometer (XRD, Cu Kα, 40 kV, 40 mA, Shimadzu XRD-7000S). The BFZY1 and BZCCYYb powders (50:50 wt%) were mixed evenly and calcined at 1000 °C for 2 h, and the chemical compatibility was assessed using XRD. A field emission scanning electron microscope (FE-SEM, Carl Zeiss, GeminiSEM300) was used to characterize the microstructure of powders and cells. Moreover, a thermogravimetric analyzer (TGA, STA 449 F5, NETZSCH) was used to assess the mass change of the powders in the air. The temperature was increased to 700 °C to measure the weight loss in dry air. An electrochemical workstation (Zennium XC, Zahner) was used to measure electrochemical performance, including the electrochemical impedance spectroscopy (EIS) measurement and I − V curves of cells based on a four-probe configuration. The fuel electrode of the cell was supplied with wet H2 (3% H2O) at a flow rate of 60 mL/min, while the air electrode was supplied with wet air (3% H2O) at a flow rate of 100 mL/min. The impedance spectroscopy was recorded in the frequency range of 10−1 − 106 Hz, with the amplitude set to 20 mV.

Results and Discussion

Phase Structure and Physical Property Characterization

Figure 1a depicts the XRD patterns of BaFe0.8Zr0.1Y0.1O3−δ (BFZY1) samples calcined at different temperatures. Compared with the standard XRD card PDF#01-075-0426, the synthesized BFZY powders exhibit a favorable perovskite structure [21]. Moreover, when the sintering temperature is less than 1000 °C, the patterns assigned to ZrO can be observed, indicating the presence of unincorporated ZrO in the powder at this stage. Figure 1b compares the XRD patterns of samples calcined at 800 °C and 1000 °C. The comprehensive analysis depicted that 100 °C is the optimum sintering temperature of BFZY powders.

Fig. 1
figure 1

a X-ray diffractometer (XRD) patterns of BFZY1 powders at different calcination temperatures; b XRD patterns of BFZY1 calcined at 800 °C and 1000 °C

Full size image

Figure 2a depicts the XRD patterns of BFZY samples with various Y-doping ratios after calcination at 1000 °C. Compared with PDF#01-075-0426, the BFZYx samples exhibit no redundant peaks, indicating that the obtained BFZYx samples are in a pure phase under these conditions. Furthermore, as the Y element doping ratio increases gradually, the diffraction peak position shifts toward a small angle. Doping elements with larger atomic radii cause the peak position to shift toward a small angle, indicating that the Y element enters the B-site of the perovskite unit cell [18]. The chemical compatibility between BFZY1 powders and BZCYYb electrolyte was examined by calcining the mixture at 1000 °C for 2 h. Figure 2b depicts the XRD patterns of BZCYYb, BFZY1, and the calcined mixture. The results reveal a superposition peak of the two samples without a second phase, indicating that BFZY1 and BZCYYb have outstanding chemical compatibility.

Fig. 2
figure 2

a X-ray diffractometer (XRD) patterns of BFZY powders with different doping ratios; b XRD patterns of BFZY1, BZCYYb, and the mixture with 1:1 after calcination in air

Full size image

Figure 3 depicts the morphology and element distribution results of the BFZY1 sample. The particles are relatively uniform, and the distribution of the elements Ba, Fe, O, Y, and Zr is extremely homogeneous, with no obvious elemental segregation. This indicates the incorporation of Zr and Y into the BaFeO3−δ perovskite, confirming the successful synthesis of BFZY1 perovskite material.

Fig. 3
figure 3

a The micrographs in BFZY1; the elemental mapping images of BFZY1, b Ba, c Fe, d Zr, e Y, f O

Full size image

Figure 4 depicts the thermogravimetry analysis (TG) curves of the BFZYx samples in air. As the temperature rises, all the samples exhibit varied weight loss. In the low-temperature range of 100–300 °C, the weight loss related to the desorption of the adsorbed water or CO2 loss is 0.28%, 0.41%, 0.40%, 0.48%, and 0.65% for BFZ, BFZY1, BFZY2, BFZY3, and BFZY4, respectively, whereas in the high-temperature range of 300–700 °C, the corresponding value is 1.03%, 1.13%, 1.21%, 1.24%, and 1.31%, respectively, primarily caused by the loss of lattice oxygen [27,28,29]. The Y-doped samples display more pronounced weight loss at 500–700 °C than the undoped sample due to the elevated concentration of oxygen vacancies at higher temperatures. BFZY4 experiences more weight loss than other doping ratios, indicating that Y-doped materials tend to form more oxygen vacancies. The removal of lattice oxygen facilitates the ORR and OER due to the creation of sufficient oxygen vacancies. These oxygen vacancies serve as active sites for the formation and desorption of proton defects at the oxygen electrode. The presence of oxygen vacancies primarily influences the activity of generated proton defects of perovskite oxides, and the addition of the Y element via doping can facilitate the generation of oxygen vacancies. While oxygen vacancies are necessary for the migration of oxygen ions, protons diffuse primarily by hopping between adjacent lattice oxygen sites in the lattice (Grotthuss mechanism) [30, 31]. Within the Grotthuss mechanism, protons conduct by forming a chemical bond with a neighboring oxygen ion (HO−), reorienting to OH−, and then transferring to another adjacent oxygen ion to initiate the formation of a new chemical bond [22, 24, 32]. Therefore, maintaining an appropriate concentration of oxygen vacancies promotes effective proton transport [32,33,34].

Fig. 4
figure 4

Thermogravimetry analysis curves of BFZ and BFZYx

Full size image

Electrochemical Performance Characterization

To further elucidate the effect of Y3+ doping on the catalytic activity of BFZYx oxygen electrode materials, the fuel electrode-supported cells with Ni-BZCYYb/BZCYYb/BFZYx configuration were fabricated and subjected to electrochemical performance tests. Figure 5 depicts the EIS measured at 600 − 700 °C with an open circuit voltage, employing BFZYx as the oxygen electrodes. At 600–700 °C, the ohmic and polarization resistances gradually decrease as the temperature increases, attributing to the increased rate of OER and ORR with rising temperature [35].

Fig. 5
figure 5

a–e EIS curves of Ni-BZCYYb/BZCYYb/BFZYx cells at 600–700 °C, a BFZ, b BFZY1, c BFZY2, d BFZY3, e BFZY4, f EIS curves of Ni-BZCYYb/BZCYYb/BFZYx cells at 650 °C

Full size image

As illustrated in Fig. S1, the electrochemical reactivity of the BFZY3 air electrode was systematically explored by measuring Rp in a symmetric cell configuration containing the BZCYYb electrolyte. The EIS was conducted under controlled conditions of both dry and humid atmospheres at 550–700 °C. A decreased Rp value generally signifies an elevated gas (vapor/O2) interfacial exchange capacity and an elevated triple conductivity within the air electrode, pivotal for facilitating the ORR and OER. The observed reduction in Rp during 550–700 °C implies that BFZY3 possesses rapid oxygen interfacial exchange, superior oxygen ion mobility, and sufficient electronic conductivity. Furthermore, the introduction of vapor reduces Rp, signifying the induction of proton conductivity upon hydration. Figure 5f depicts the EIS of the cells with different BFZYx oxygen electrodes measured at 650 °C. The polarization resistance (Rp) of BFZYx (x = 0, 1, 2, 3, 4) at 650 °C is 0.83, 0.57, 0.35, 0.13, and 0.24 Ω cm2, respectively. Moreover, the BFZY3 electrode exhibits the lowest polarization resistance, signifying its excellent surface reactivity and proton conductivity compared to other electrodes (Fig. S2). This observation suggests a competitive relationship between oxygen vacancies and proton conductivity, where an optimal balance improves electrode performance. The excessive concentration of oxygen vacancies can enhance oxygen ion conductivity but may simultaneously reduce proton conductivity and electron conductivity. Accordingly, the material can exhibit improved proton conductivity at low-temperature ranges by striking the right balance between different types of conductivity with an appropriate concentration of oxygen vacancies [31,32,33].

Figure 6 depicts the I–V curves of Ni-BZCYYb/BZCYYb/BFZYx cells at different temperatures. Wet hydrogen (3% H2O) was introduced into the fuel electrode side, and humid air (3% H2O) was supplied to the oxygen electrode side. Under an applied electrolysis voltage of 1.3 V at 650 °C, the current density of the cell with BFZ, BFZY1, BFZY2, BFZY3, and BFZY4 was − 0.66, − 0.81, − 0.88, − 1.03, and − 0.93 A/cm2, respectively. In EC mode, the water electrolysis performance of the cell with BFZY3 was significantly higher than that of BFZYx electrodes with other Y-doping concentrations.

Fig. 6
figure 6

I − V curves of Ni-BZCYYb/BZCYYb/BFZYx cells at 650 °C

Full size image

Figure 7 depicts the I − V − P curves of Ni-BZCYYb/BZCYYb/BFZYx cells operating in FC mode at 600 °C, 650 °C, and 700 °C with wet hydrogen (3% H2O) at the fuel electrode and humid air (3% H2O) at the oxygen electrode. As illustrated in Fig. 7f, the PPD of the cell with BFZYx (x = 0, 1, 2, 3, or 4) at 650 °C is 279.82, 338.21, 406.44, 546.59, and 496.59 mW/cm2, respectively. Notably, the cell with BFZY3 oxygen electrode exhibits the highest electrochemical performance in FC mode. Optimizing Y3+ doping can improve the catalytic activity of BFZ and electrochemical performance. Following the Grotthuss mechanism, oxygen vacancies promote the formation of proton defects, whereas excessive oxygen vacancies inhibit proton transport within the perovskite lattice [22, 24, 31]. These results suggest that the BFZY3 is a highly promising oxygen electrode for R-PCECs.

Fig. 7
figure 7

I − V − P curves of Ni-BZCYYb/BZCYYb/BFZYx cells at 600–700 °C: a BFZ, b BFZY1, c BFZY2, d BFZY3, e BFZY4, f I − V − P curves of Ni-BZCYYb/BZCYYb/BFZYx cells at 650 °C

Full size image

Figure 8 depicts the reversibility of the cell with the BFZY3 electrode. The cell was subjected to a reversible operation by switching between EC and FC modes for up to eight cycles with 80 h operation time. During the eight cycles of alternative operation, the cell shows stable performance without apparent degradation, confirming the excellent stability and reversibility of the BFZY3 electrode. Figure 8b presents the cell microstructure after the test. The oxygen electrode is found to maintain a tight connection with the electrolyte, exhibiting no discernible rupture or separation. After long-term alternative operation, all three cell components remain intact, demonstrating excellent structural stability.

Fig. 8
figure 8

a Reversible operation of the cell with BFZY3 electrode between electrolysis cell (EC) and fuel cell (FC) modes at 600 °C, b The cross-sectional microstructure of the post-test cell

Full size image

Conclusion

This study proposed that Y3+ ions could partially replace Fe in BaFe0.9Zr0.1O3−δ to improve its phase stability and electrochemical performance as an oxygen electrode for reversible protonic ceramic electrochemical cells (R-PCECs). The results indicated that Y-doping could introduce more oxygen vacancies, and this excess oxygen vacancy can impact the electrochemical performance in R-PCEC applications. The cell with BFZY3 oxygen electrode exhibited the highest catalytic activity, with an electrolysis current density of − 1.03 A/cm2 at 1.3 V in SOEC mode and a PPD of 546.59 mW/cm2 in SOFC mode at 650 °C. This research emphasized the significance of Y in augmenting the proton uptake capacity and provided an effective strategy to enhance the proton conductivity of perovskite oxides.