The extensive adoption of renewable energy is essential for the attainment of sustainable development [
1]. Protonic ceramic fuel cells (PCFCs), as a new generation of clean energy devices, have become a focal point in recent green energy research owing to their high fuel utilization, low cost, and excellent sustainability [
2–
5]. However, the widespread development of PCFCs is still constrained by the limited kinetics of the oxygen reduction reaction (ORR) and insufficient durability of cathodes [
6–
9]. Consequently, for the practical implementation of PCFCs, it is imperative to develop cathodes with both high electrocatalytic activity for ORR and exceptional durability under operating conditions [
10,
11]. The structural versatility of perovskite-type oxides, resulting from their tunable cationic substitution of A and B sites, creates dynamic redox active sites and controllable oxygen vacancy concentrations [
12]. These properties make them highly attractive as cathode materials for PCFCs. So far, La
0.6Sr
0.4Co
0.2Fe
0.8O
3−δ (LSCF), Ba
0.5Sr
0.5Co
0.8Fe
0.2O
3−δ (BSCF), BaCo
0.4Fe
0.4Zr
0.1Y
0.1O
3−δ (BCFZY),
etc., representing the perovskite-type oxides, have been extensively applied in PCFCs and achieved outstanding electrochemical performance [
13–
15]. PrBaCo
2O
5+δ (PBC), a conventional air electrode material with mixed ionic and electronic conductivity, enables remarkable electrocatalytic activity for ORR and superior oxygen diffusion kinetics [
16,
17]. The segregation of Ba at the A-site in the PBC cathode induces the stability of PCFCs during operation [
18,
19]. Therefore, it is necessary to employ targeted modification strategies to improve the structural integrity and electrochemical performance of PBC for effective integration.
Various practicable methodologies were applied to further improve the electro-catalytic activity and structural integrity of the PBC-based cathodes. For example, Gao
et al. infiltrated a multi-cationic oxide catalyst coating with a composition of Pr
0.2Ce
0.2Ni
0.2Co
0.2Fe
0.2O
x (PCNCF) to boost the ORR activity and longevity of the PBC cathode [
20]. The PCNCF-coated PBC air electrode achieved more satisfactory fuel cell performance than the bare PBC air electrode at 700–600 ℃. Meanwhile, as the cerium oxide in the catalyst coating reacted with separate Ba to form active ingredients, both the catalytic reaction activity and the operational stability of PBC have been improved [
20]. Entropy engineering can endow perovskite oxides with multifunctionality, including electrocatalytic activity, phase stability, and rapid ion transport [
21]. Zhu
et al. modulated the conformational entropy of the PBC perovskite by doping alkaline earth metals at the A-site to develop a medium-entropy Pr
0.5Ba
1/6Sr
1/6Ca
1/6CoO
5+δ (PBSCC) electrode with enhanced functionality [
22]. The PCFCs incorporating the PBSCC cathode demonstrated exceptional performance (0.80 W/cm
2 at 550 ℃) and stability (for over 250 h) [
23]. In addition, precisely engineered doping with cations has been considered as a widely adopted modification strategy to improve ORR activity and reinforce structural robustness for PBC [
24]. Du
et al. adopted an In-doping method to develop a PrBa
0.95Co
1.85In
0.09O
5+δ (PBCIn) air electrode with a BaCo
0.85In
0.15O
3−δ (BCIO) cubic second phase [
25]. Indium doping has been demonstrated to enhance ORR and OER kinetics, enabling the PBCIn air electrodes to achieve remarkable electrochemical performance (1.33 W/cm
2 at 600 ℃) in PCFCs [
25].
Recently, lanthanide elements have been extensively utilized as dopants in PCFCs. Ytterbium, an element in the lanthanide series, has attracted considerable attention [
26,
27]. Yb doping has been widely proven as an effective strategy for developing advanced proton-conducting electrolytes [
28]. Yang
et al. reported that BaZr
0.1Ce
0.7Y
0.2-xYb
xO
3-δ (BZCYYb), a Y and Yb co-doped mixed ion conductor, enabled rapid transport of protons and oxide ion vacancies, achieving high ionic conductivity at 700–500 ℃ [
29]. Further, Guo
et al. discovered that higher Yb doping levels led to a simultaneous rise in proton concentration and ionic conductivity, suggesting a critical role of Yb in BZCYYb [
30]. Luo
et al. synthesized a Yb-doped BaHf
0.1Ce
0.7Yb
0.2O
3-δ (BHCYb172) proton-conducting electrolyte, which has high ionic conductivity and excellent chemical stability [
31]. When applied in PCFCs, the cell showed outstanding performance of 1.73 W/cm
2 at 600 ℃, caused by the fact that Yb doping can increase oxygen migration rates and reduce both activation energy and thermal expansion coefficient in diverse crystalline materials [
32–
34]. To date, studies on the substitution of Yb are primarily focused on electrolytes, with limited investigation of their application in the cathodes of PCFCs. Due to the lower valence state, Yb
3+ doping at the B-site may increase the oxygen vacancy concentration [
31]. These intrinsic vacancies could promote water incorporation to provide proton sources that contribute to the charge transfer process on the electrode surfaces [
35]. Furthermore, in perovskites, the Gibbs free energy of hydration exhibits a linear dependence on the average cationic electronegativity, with lower electronegativity favoring stronger water adsorption [
36,
37]. The substitution of Co with lower-electronegativity Yb
3+ at the B-site could enhance hydration capability, consequently improving electrocatalytic activity. Studies are showing that doping with low-electronegativity elements effectively improved the performance of PBC air electrodes, such as PrBa
0.9Cs
0.1Co
2O
5+δ (PBCsC) and PrBaCo
1.8Nb
0.1Y
0.1O
5+δ (PBCNY) [
38,
39].
Inspired by the above findings, we designed a ytterbium-doped PBC-based perovskite material with the designed composition of PrBaCo1.95Yb0.05O5+δ (PBCYb0.05). The perovskite consists of a major perovskite phase of deficient-PrBa1-xCo1.95–0.99x Yb0.05–0.01xO5+δ (D-PBCYb0.05) and a minor secondary perovskite phase BaCo0.99Yb0.01O3-δ (BCYO). The nanoscale synergy between these two phases significantly enhances both the electrocatalysis and durability of the cathode. From the results of the experiments, compared to the PBC, PBCYb0.05 displays more oxygen vacancies and faster oxygen surface exchange and bulk conduction kinetics. Therefore, the PBCYb0.05 cathode achieved a relatively low area-specific resistance of 0.249 Ω cm2 (vs. 0.410 Ω cm2 from the PBC). The PCFCs employing the developed PBCYb0.05 cathode achieved an outstanding power density of 2.25 W/cm2 at 700 ℃ and a desirable short-term stability for 103 h at 600 ℃.
The details of chemicals and materials, material preparation, cell fabrication, electrochemical measurements, and characterization of materials are provided in Supporting information.
To characterize the phase composition of PBCYb
0.05, we performed the X-ray diffraction (XRD) and related Rietveld refinement analysis. As shown in
Fig. 1a, the PBCYb
0.05 powder is composed of a major perovskite phase of deficient-PrBa
1-xCo
1.95–0.99xYb
0.05–0.01xO
5+δ (D-PBCYb
0.05,
a =
b = 3.888 Å and
c = 7.705 Å, 95.08 wt%) and a minor secondary perovskite phase of BaCo
0.99Yb
0.01O
3-δ (BCYO,
a =
b =
c = 4.143 Å, 4.92 wt%). The corresponding fitting parameters are
Rwp = 4.02%,
Rp = 3.06%, and χ
2 = 2.008, indicating that the Rietveld refinement analysis is reasonable. XRD patterns of PBC and other materials with Yb doping ratios are given in
Fig. 1b and Fig. S1 (Supporting information). The BCYO secondary phase emerges when the Yb doping ratio is 1.25%, and its volume fraction increases significantly at 2.50%. The heterogeneous phase increases when the Yb content reaches 3.75%. It is worth noting that the main characteristic peak of PBCYb
0.05 exhibits a distinct shift to a lower angle at 32°−34°, compared to PBC, suggesting a lattice expansion (Fig. S2 in Supporting information). This phenomenon indicates that ytterbium has successfully been doped into the B-site of the PBC perovskite since the ionic radius of Yb
3+ is between the Pr
3+/Pr
4+ and Co
3+/Co
4+ [
38–
40]. It has been confirmed that Yb preferentially occupies the B-sites of perovskites containing large A-site elements such as Pr, Ba, and Sr [
41]. The chemical compatibility between the PBCYb
0.05 powder and BaZr
0.1Ce
0.7Y
0.1Yb
0.1O
3-δ (BZCYYb) powder is illustrated in Fig. S3 (Supporting information). The result indicates there are no chemical side reactions between the PBCYb
0.05 air electrode and the BZCYYb electrolyte. To determine the phase composition of PBCYb
0.05, HR-TEM was employed in combination with energy-dispersive spectroscopy (EDS) mapping analysis. Distinct regions of PBCYb
0.05 and BCYO can be identified in the HR-TEM image (
Fig. 1c). As shown in
Fig. 1d, the lattice spacing of BCYO, determined by fast Fourier transformation (FFT), is approximately 0.207 nm, likely corresponding to the (002) crystallographic plane. Analogously, the measured lattice spacing of 0.276 nm matches mostly with the (110) crystallographic plane of PBCYb
0.05 through FFT (
Fig. 1e). From the STEM and EDS mapping images, the absence of the Pr element is visible (
Fig. 1f). The SEM images of the PBCYb
0.05 and PBC powders are shown in Figs. S4 and S5 (Supporting information). In contrast, the BCYO nanoparticles can be found in PBCYb
0.05. Combined with the other results (XRD and TEM), it is demonstrated that PBCYb
0.05 is a two-phase composite with D-PBCYb
0.05 and BCYO.
Electrochemical impedance spectroscopy (EIS) curves of BZCYYb electrolyte-supported symmetrical cells with PrBaCo
2-xYb
xO
5+δ (
x = 0, 0.05, 0.10, and 0.15) (denoted as PBC, PBCYb
0.05, PBCYb
0.10, PBCYb
0.15) electrodes are performed to evaluate the Yb doping effect on the electrochemical performance of PBC-based electrodes (
Fig. 2a and Fig. S6-S8 in Supporting information). All the EIS curves were measured at an open-circuit voltage (OCV) condition in humid air (3 vol% H
2O). The area-specific resistance (ASR) values of the PBCYb
0.05 symmetrical electrode are 0.077, 0.150, 0.249, 0.441, and 1.013 Ω cm
2 at 700, 650, 600, 550, and 500 ℃, respectively. As exhibited in
Fig. 2b, the PBCYb
0.05 electrode achieves the minimum ASR among all Yb doping ratios, suggesting that the formation of unitary and pure BCYO secondary phase is crucial for electrochemical performance. The enhanced electrochemical performance after ytterbium doping compared to the PBC electrode may be attributed to the relatively lower electronegativity of Yb
3+, which can reduce the average electronegativity of cations in the system. As evidenced by the ASR comparison in
Fig. 2c, the PBCYb
0.05 air electrode exhibits remarkable electrocatalytic performance relative to reported advanced cathodes, demonstrating its superior catalytic activity [
13,
21,
23,
39,
42–
45]. Further details of these cells are in Table S2 (Supporting information). Meanwhile, the electrochemical stability of symmetrical cells is also an important index for evaluating the cathode. As shown in
Fig. 2d, the electrochemical stabilities of symmetrical cells with PBCYb
0.05 and PBC cathode are measured in humid air (3 vol% H
2O) at 600 ℃. The corresponding EIS spectra are presented in Figs. S9 and S10 (Supporting information), respectively. The polarization resistance (
Rp) values of the PBCYb
0.05 air electrode increased from 0.249 Ω cm
2 to 0.308 Ω cm
2 for 104 h, indicating that it maintained better stability than the PBC air electrode (from 0.410 Ω cm
2 to 0.506 Ω cm
2 for 104 h). The electrochemical performance stability of the PBCYb
0.10 air electrode with the corresponding EIS curves is also tested (Figs. S11 and S12 in Supporting information). After 104 h of operation, the
Rp values of the PBCYb
0.10 air electrode finally ended at 0.355 Ω cm
2, exceeding the final
Rp values of PBCYb
0.05. The result highlights the better electrochemical stability of the PBCYb
0.05 air electrode. To elucidate the degradation mechanisms of symmetrical cells, the distribution of relaxation times (DRT), a deconvolution method to resolve EIS curves into discrete electrochemical processes, was applied. Three distinct electrochemical processes can be resolved in a DRT spectrum through different frequency distributions: Low frequency (LF, 10
−3−10 Hz), intermediate frequency (IF, 10−10
4 Hz), and high frequency (HF, 10
4−10
5 Hz). Three typical EIS curves from the stability measurements of the PBCYb
0.05 and PBC cathodes are selected for the DRT analysis, covering the beginning, the middle, and the end. As shown in
Fig. 2e, the change in DRT curves is concentrated in the IF range, suggesting that the processes of oxygen surface exchange primarily govern the
Rp evolution in the PBCYb
0.05 air electrode [
46]. Similarly, the increased
Rp values of the PBC air electrode originate from modifications in both LF and HF ranges (
Fig. 2f). The LF and HF ranges are associated with gas diffusion and charge transfer processes, respectively [
46–
48]. The results confirm the efficacy of Yb
3+ doping in improving gas diffusion and charge transfer efficiency.
The underlying mechanism for the superior electrochemical performance of the PBCYb
0.05 air electrode requires thorough exploration. The electrical conductivities of PBCYb
0.05 and PBC samples are shown in
Fig. 3a. Both samples exhibit the same temperature-dependent conductivity trend as reported in the literature, showing a decrease in conductivity with increasing temperature. Compared to the PBC sample, the relatively low conductivity of the PBCYb
0.05 sample originates from the substitution of Yb at Co sites, breaking the strongly covalent Co-O bonds [
49]. The surface chemistry of PBC and PBCYb
0.05 powders was characterized by XPS to determine Co valence states and oxygen vacancy concentration. As displayed in
Fig. 3b, according to the binding energies, the Co 2p
3/2 and Ba 3d
5/2 XPS spectra can be classified into three subsidiary peaks: Co
3+ (central peak is at ≈777.15 eV), Ba
2+ (central peak is at ≈779.29 eV), and Co
4+ (central peak is at ≈780.89 eV) [
22]. After Yb
3+ doping, the rise in Co
4+ content implies greater catalytic activity availability [
50]. Likewise, the O 1s XPS spectra can be resolved into four distinct peaks, which are the lattice oxygen (O
lat, central peak is at ≈528.75 eV), the highly oxidative oxygen (O
−/O
22−, central peak is at ≈531.01 eV), the adsorbed oxygen (O
ads, central peak is at ≈531.94 eV) and the hydroxyl oxygen (O
H, central peak is at ≈533.37 eV) (
Fig. 3c) [
25]. The relative distribution percentages of four distinct peaks for PBCYb
0.05 and PBC materials are quantified in
Fig. 3d. The oxygen vacancy concentration correlates with the value of O
−/O
22−. A higher value of highly oxidative oxygen of PBCYb
0.05 (O
−/O
22- = 37.38%) than that of PBC (O
−/O
22- = 25.36%) may suggest that Yb doping can lead to more oxygen vacancies in the cathode [
25]. To provide additional evidence of the improvement in oxygen transport kinetics and ORR activity, we measured the electrical conductivity relaxation (ECR) response curves of both PBCYb
0.05 and PBC samples from 700 ℃ to 600 ℃ (
Fig. 3e and Fig. S13 in Supporting information). The determination of surface exchange coefficients (
k*
chem) and bulk diffusion coefficients (
D*
chem) is based on Fick's second law. As exhibited in
Fig. 3f, the
k*
chem and
D*
chem values of PBCYb
0.05 are larger than those of PBC, revealing that the oxygen surface exchange and diffusion kinetics of the PBCYb
0.05 cathode are enhanced [
8]. The detailed
k*
chem and
D*
chem are listed in Table S3 (Supporting information). These experimental data confirm that the ORR activity of the PBC-based air electrode can be improved by effectively accelerating the oxygen transport kinetics through Yb doping. Thus, incorporating an appropriate amount of Yb into PBC perovskite generates multifunctional improvements, leading to superior electrocatalytic activity. In addition, thermal expansion coefficient (TEC) curves of the PBCYb
0.05 and PBC samples are exhibited in Fig. S14 (Supporting information). It is shown that PBCYb
0.05 exhibits a similar thermal expansion to other cobalt-based electrode materials [
21,
51,
52].
The Ni-BZCYYb anode-supported PCFCs with PBCYb
0.05 or PBC cathode were assembled and systematically tested for electrochemical performance and durability to verify the improvement in ORR activity and oxygen transport. As depicted in
Fig. 4a, the current density-voltage-power density (
I-
V-
P) curves of the PCFC loaded with the PBCYb
0.05 air electrode are measured during fuel cell operation from 700 ℃ to 600 ℃. Remarkable peak power densities (PPDs) of 2.25, 1.52, and 1.09 W/cm
2 are achieved at 700, 650, and 600 ℃, respectively. Simultaneously, as shown in
Fig. 4b, under OCV conditions, the
Rp values of this PCFC remain relatively low, which are 0.022, 0.054, and 0.135 Ω cm
2 at 700, 650, and 600 ℃, respectively. After electrochemical testing under hydrogen reduction conditions, the cross-sectional scanning electron microscopy (SEM) image of this PCFC is presented in
Fig. 4c. The specific morphologies of the PBCYb
0.05 air electrode and Ni-BZCYYb fuel electrode are exhibited in Figs. S15 and S16 (Supporting information). The PCFC possesses a classic triple-layer structure, consisting of a porous PBCYb
0.05 air electrode layer (≈6 μm), a dense BZCYYb electrolyte layer (≈7 μm), and a porous Ni-BZCYYb fuel electrode layer. To enable direct comparison, the
I-
V-
P curve and area-specific polarization resistances of the PCFC loaded with PBC air electrode are recorded in Figs. S17 and S18 (Supporting information). The PCFC loaded with PBC air electrode exhibits peak power densities of 1.68, 1.22, and 0.96 W/cm
2 at 700, 650, and 600 ℃, respectively. The enhanced electrochemical performance can be attributed to the Yb doping in PBC. Meanwhile, at 700, 650, and 600 ℃, the
Rp values of the PCFC loaded with PBC air electrode are 0.040, 0.085, and 0.183 Ω cm
2, respectively. The electrochemical resistances, including both ohmic resistances (
Ro) and
Rp of PCFCs loaded with PBCYb
0.05 and PBC cathode, are quantitatively compared in
Fig. 4d. Although the
Ro of the PCFC with PBCYb
0.05 air electrode is marginally larger, the significantly lower
Rp values compared to those with the PBC air electrode are responsible for the enhanced power output, suggesting the polarization resistance of PCFCs contributes mostly to the performance improvement. To explore the influence of different Yb-doped cathodes on the performance of PCFCs, the
I-
V-
P curves and the corresponding impedance spectra of PCFCs loaded with PBCYb
0.10 and PBCYb
0.15 cathodes were measured (Figs. S19-S22 in Supporting information). Among all the Yb-doped cathodes, the PCFC loaded with PBCYb
0.05 exhibited the best electrochemical performance, as demonstrated by the test results of symmetrical cells. In recent literature, the performance of the PBCYb
0.05 cell has surpassed most of its counterparts with comparable electrolyte architectures (
Table 1 and Fig. S23 in Supporting information) [
10,
25,
38,
43,
44,
53–
57]. As shown in
Fig. 4e, at a current density of 0.5 A/cm
2 and 600 ℃, the PCFC loaded with PBCYb
0.05 air electrode maintained stable operation for 103 h with a slight degradation rate of 1.08 mV/h, demonstrating the Yb-doped PBC enables reliable application as cathode in PCFCs.
To summarize, we have successfully developed a Yb-doped PBC-based air electrode with a nominal composition of PBCYb0.05. As evidenced by XRD patterns, HR-TEM images, and EDS mapping, the coexistence of D-PBCY and BCYO phases in the PBCYb0.05 material is confirmed. According to the testing of symmetrical cells and DRT analysis, the PBCYb0.05 air electrode demonstrates excellent electrocatalytic activity and operational stability. From the combined analyses of XPS and ECR, compared to PBC, PBCYb0.05 possesses more oxygen vacancies and faster oxygen transport kinetics, accounting for enhanced oxygen reduction reaction (ORR) activity. In the single-cell tests, the PCFCs loaded with PBCYb0.05 cathode achieved an ideal peak power density of 2.25 W/cm2 at 700 ℃. In addition, it also showed promising durability for 103 h during operation. The testing demonstrates that the Yb-doped PBC serves as a highly reliable air electrode for the practical applications of PCFCs. The Yb-doping strategy efficiently broadens the modification methodologies for the cathodes in PCFCs.
PDF
Full
66
38
关闭全屏
No. 86 Xueyuan South Road, Haidian District, Beijing
010-62662699(Operation and Technical Support)
qkjq@cast.org.cn
