Visiting the roles of Sr‐ or Ca‐ doping on the oxygen reduction reaction activity and stability of a perovskite cathode for proton conducting solid oxide fuel cells

While double perovskites of PrBaCo2O6 (PBC) have been extensively developed as the cathodes for proton‐conducting solid oxide fuel cells (H‐SOFCs), the effects of Sr‐ or Ca‐doping at the A site on the activity and stability of the oxygen reduction reaction are yet to be fully studied. Here, the effect of A‐site doping on the oxygen reduction reaction activity and stability has been studied by evaluating the performance of both symmetrical and single cells. It is shown that Ca‐doped PBC (PrBa0.8Ca0.2Co2O6, PBCC) shows a slightly smaller polarization resistance (0.076 Ω cm2) than that (0.085 Ω cm2) of Sr‐doped PBC (PrBa0.8Sr0.2Co2O6, PBSC) at 700°C in wet air. Moreover, the degradation rate of PBCC is 0.0003 Ω cm2 h−1 (0.3% h−1) in 100 h, about 1/10 of that of PBSC at 700°C in wet air. In addition, it is also confirmed that single cells with PBCC cathode show higher peak power density (1.22 W cm−2 vs. 1.08 W cm−2 at 650°C) and better durability (degradation rate of 0.1% h−1 vs. 0.13% h−1) than those with PBSC cathode. The distribution of relaxation time analyses suggests that the better stability of the PBCC electrode may come from the fast and stable surface oxygen exchange process in the medium frequency range of the electrochemical impedance spectrum.

are severely constrained to the triple-phase boundaries of the cathode. 13 As a result, many cathode materials that work well at high temperatures in oxygen ion-conducting SOFCS may perform limited activity or durability on H-SOFCs. 12,14 Through extensive research, mixed ionic electronic conductors with enlarged electrochemical reaction regions have been successfully developed and achieved higher electrochemical activity. [15][16][17][18][19] With higher chemical diffusion and surface exchange coefficients, layered LnBaCo 2 O 5+δ -based perovskites exhibit better electrochemical performance, compared to ABO 3 -type perovskites. 20,21 Among them, PrBaCo 2 O 6 (PBC) system materials appear to have the best electrochemical properties and are widely investigated. [22][23][24][25][26] For example, Jiang et al. studied the impact of the Pr to Ba on the oxygen vacancy content of the material. 27 A single cell with a Pr 1.1 Ba 0.9 Co 2 O 5+δ cathode obtained a peak power density of 0.732 W cm −2 at 800 • C. 27 A group of PrBaCo 2-x Fe x O 5+δ materials with a low thermal expansion coefficient were evaluated by Xia et al. as promising cathode materials for intermediate-temperature (IT)-SOFCs. 28 Kim et al. reported an A-site cation-ordered double perovskite cathode material PrBa 0.5 Sr 0.5 Co 2-x Fe x O 5+δ , which reached a peak power density of 1.61 Wcm −2 (750 • C) on an H-SOFC, 29 which is substantially larger than the same sort of cell using La 0.6 Sr 0.4 Co 0.8 Fe 0.2 (LSCF) as the cathode.
Optimization of the A-site cation in the perovskite structure to improve both performance and durability has been widely explored. 30 For example, Irvine et al. demonstrated that the non-stoichiometry of A-site elements facilitated the segregation of B-site cations, resulting in a surface with more catalytically active sites. 31 This strategy has been widely adopted to obtain highly active materials with in situ exsolved active metals. 32 While in the double perovskite LnBaCo 2 O 5+δ , the LnO-and BaO-at the A-site are arranged alternately along the c-axis. Due to the large radius of Ba ion, it tends to form a complete 12 co-ordination with oxygen, directly leading to the tendency of oxygen vacancies to be concentrated in the LnOlayer. Therefore, the regulation of the content of oxygen vacancies can be achieved by adjusting the radius difference of the A-site cations (r(Ba 2+ ) -r(Ln 3+ )). 20 The crystal structure, symmetry, and consequently the physical and chemical properties of the double perovskite are also affected by the radius difference of the A-site cations. For example, the use of a smaller radius Sr instead of Ba can lower the oxygen vacancy of the material, thus improving the conductivity of the material itself and reducing the O-Co-O perturbation, which is reflected in the higher conductivity and surface exchange coefficients and bulk phase diffusion coefficients than those of monochalcogenide materials. [33][34][35][36] However, the presence of Sr tends to reduce the structural stability as well as the electrochemical stability of perovskites. For instance, Simner et al. found that a 35-75 nm thick SrO layer formed on the surface of LSCF after a 500 h test at 750 • C, resulting in substantial performance deterioration. 37 Jiang et al. found that the dense LSCF would have numbers of SrO particles of different sizes segregated on its surface after an 800 • C treatment. 38 The precipitated strontium reacts with impurities like H 2 O, CO 2 , or Cr to generate Sr(OH) 2 , SrCO 3 , or SrCrO 4 , which exacerbates deterioration when exposed to air containing these contaminants. 11 The strong electrostatic attraction between the surface oxygen vacancies and the hetero-valent elements in the bulk phase is likely the main reason for the phase separation of the perovskite cathode. 39 Therefore, the material components can be tuned to improve the activity and stability of the cathode through doping. [39][40][41] Similarly, Lee et al. reported that a smaller size mismatch between the main body and the dopant cation would reduce the level of dopant segregation, leading to a more stable cathode surface. 40 Recently, several Ca-containing layered perovskites have also been reported and are claimed to be extremely stable. 42 For example, materials with Ca at the A-site exhibit excellent stability in an atmosphere containing CO 2 . [42][43][44] With density functional theory calculation, Kim et al. demonstrated that the improved stability is caused by the higher electron affinity of mobile oxygen species for Ca. There are numerous perovskite materials involving Ca-and Sr-containing elements. 43,45,46 For example, Bi et al. demonstrated that the lower V O formation energy in La 0.5 Ca 0.5 MnO 3-δ resulted in higher oxygen reduction activity, compared to Sr-doped La 0.5 Sr 0.5 MnO 3-δ . 47 However, it is still not clear which doping is better, in terms of reaction activity and durability when applied to the cathode of H-SOFCs, which generate water vapor during operation. Herein, we compare the activity and stability of Ca-or Sr-doped PBC (PrBa 0.8 Sr 0.2 Co 2 O 5+δ [PBSC]) as potential cathode materials through electrochemical tests of symmetrical cells and single cells. These fundamental comparisons could inspire more in-depth mechanistic studies to advance the development of H-SOFCs cathodes.

In situ XRD of Sr-or Ca-doped PBC in steam
The schematics for the standard H-SOFCs operating principle are shown in Figure 1A. On the cathode side, a significant amount of steam is produced under practical operating conditions. The performance of the oxgen reduction reation (ORR) could be negatively impacted by the steam. As a result, we performed in situ XRD measurements to track the potential changes in the phase structure of the materials after treatment with wet air (3% H 2 O) at 600 • C for 6 h to examine the stability of two materials doped with various elements in steam. In particular, during the first 0.5 h, the primary peaks of both materials shift to the left (as seen in Figure 1B,C). This phenomenon is most likely caused by the lattice expansion and saturation due to the water absorption. In addition, no new phase formation was observed during the testing, indicating that both powders may have good phase stability in vapor. 4,49

Performance of symmetrical cell
The electrochemical impedance spectra (EIS) of the symmetrical cells with PBCC or PBSC cathodes were measured in dry air or wet air with 3% H 2 O at 550∼750 • C using symmetrical cells configured with a cathode| BZCYYb | cathode. The thickness of the cathode is approximately 15 μm ( Table S1. The R p of both materials under wet air is smaller than the R p under dry air, which is supposed to be attributed to the increased ORR activity of the electrodes due to the provision of proton transport by wet air. 50 As shown in the Arrhenius plots ( Figure 2D), the polarization resistance of PBCC decreases more quickly with temperature increase. Although the impedance value of PBCC was bigger at lower temperature intervals (e.g., at 550 • C, the value of PBSC was 0.05 Ω cm 2 smaller than that of PBCC), as temperature increased, the polarization resistance of PBCC became smaller than that of PBSC (e.g., at 750 • C, PBCC: 0.045 Ω cm 2 , smaller than PBSC: 0.053 Ω cm 2 ). Accordingly, the symmetrical cell with a PBCC cathode demonstrated an activation energy (Ea) of 1.07 eV in wet air, which was slightly higher than that of PBSC (0.98 eV), as seen from the slope of the Arrhenius plot. The situation was similar when dry air is used as the testing atmosphere. PBCC exhibited an Ea of 1.05 eV, slightly higher than that of PBSC (0.99 eV). The above results imply that PBSC has a marginally lower energy barrier and more ORR catalytic activity (at low temperatures).
Although the polarization resistance of the two materials, PBCC and PBSC, do not differ significantly at different temperatures, to obtain more detailed information on the reaction kinetic processes occurring on the two different A-site doped PBC cathodes, we collected the EIS of symmetrical cells for electrodes of PBCC or PBSC at 700 • C with different oxygen partial pressures (p O2 ) (varied from 0.1 to 0.8) and applied a careful distribution of relaxation time (DRT) analysis on the EIS. 51 Regulation of the p O2 is achieved by adjusting the ratio of pure oxygen to nitrogen and controlling the total flow rate to 100 ml min −1 . Besides, we applied MATLAB R2018b to perform the DRT analysis, and the regularization parameter was chosen to be 10 −3 . 52-55 When the p O2 gradually decreases, the R p rises noticeably. For example, as shown in Figure 3A, when the p O2 increased from 0.1 to 0.8, the R p of PBCC decreased from 0.139 to 0.061 Ω cm 2 , which is consistent with the fact that increased oxygen concentration can promote the ORR reaction. Shown in Figure 3B,E is the DRT plots of PBCC and PBSC at different p O2 . Each impedance spectrum is resolved into a series of sub-peaks that can be noted as low frequency (LF), intermediate frequency (IF), and high frequency (HF) peaks, depending on the frequency range. 56 Moreover, the IF area occupies most of the area. The inte-gral area of each frequency range is corresponding to the resistance of different types of electrochemical processes. 57 The dependence of each R p on the oxygen partial pressure can be deduced by fitting the data to an empirical formula, R p = k (p O2 ) −n . 10,52 As shown in Figure 3C, for PBCC, the n values for the LF, IF, and HF range are 0.18, 0.51, and 0.30, respectively, likely corresponding to the process of mass transfer, adsorption/desorption of O 2 and charge transfer. 58 As Figure 3D,F shows, the situation is similar for PBSC. This indicates that the main steps limiting the oxygen reduction reactions of both materials are situated in the mid-frequency region. In combination with previous reports, 59-61 the mid-frequency region is recognized as the impedance associated with the surface exchange process. The impedance arising from the charge transfer at the electrode/electrolyte interface is classified as HF. The low-frequency resistance is generated by the mass transfer process associated with gas diffusion. 51 In addition, we analyzed the change in R p values of the two different cathodes in dry and wet air at 700 • C to compare their stability. Figure 4A,D shows the decay of the two polarization impedances in dry and wet air over 100 h and the corresponding DRT analysis plots. It is shown that PBCC demonstrated much-improved durability over time in dry and wet air. As Figure 4A,B shows, under a dry air atmosphere, the R p of PBSC increased from 0.052 to 0.095 Ω cm 2 (increased by 83%), while R p of PBCC slightly increased from 0.083 to 0.115 Ω cm 2 (increased by 38.5%) over 95 h. The comparison is more pronounced in wet air ( Figure 4C,D), where the R p of PBCC remains almost stable after the initial decay (from 0.097 to 0.129 Ω cm 2 , with only a 28% increase), while the R p of PBSC continues to increase, growing by 250% in 100 h (from 0.112 to 0.389 Ω cm 2 ). Summarized in Figure 4E is the stability of PBSC and PBCC electrodes in dry air and wet air. Furthermore, as shown by the corresponding DRT results ( Figure S3), PBSC exhibited significantly increased resistance at LF and IF, indicating that the oxygen adsorption/desorption and the surface exchange processes suffer from severe degradation. In addition, the microscopic morphology of the electrodes after the stability test (about 100 h) can also explain the difference in the ORR durability of the two cathodes. From the SEM results, a visible agglomeration was observed on the surface of PBSC after a long time of testing in wet air. However, under the same condition, PBCC showed finer and more uniform particles as well as more adequate porosity. Due to the mixed ionic-electronic conduction property of the doped PBCO cathode, its oxygen reduction reaction can occur on the whole cathode surface. The stable microstructure facilitates the diffusion of oxygen (and even the transport of oxygen ions), and therefore the stable microstructure of PBCC is beneficial to the efficient ORR for a long time. Together with the rate-limiting steps analyzed in the previous section from DRT and the electrode morphology after long-term testing in wet air (shown in Figures 4F,G and S4), it may suggest that PBSC is more prone to coarsening in the long-term tests, making the resistance to processes related to surface exchange increase, which ultimately leads to performance deterioration. In addition, we measured the conductivity change as a function of testing time via a standard fourprobe direct current (DC) method at 700 • C in dry/wet (3% H 2 O) air. The powders were mixed with 1% polyvinyl butyral and then uniaxially pressed into rectangular bars. Afterward, the green bars were sintered at 1200 • C for 10 h to obtain dense bars (the PBCC bar is with a dimension of about 5.45 ×4.33 ×1.33 mm, and the PBSC bar is with a dimension of about 7.79 ×4.41 ×1.56 mm). As shown in Figure S5, the conductivity of both materials showed favorable stability under dry and wet air. Besides, the electronic conductivity of PBSC is higher than that of PBCC, indicating that higher conductivity is more easily obtained by doping Sr. 62,63

Performance verification from single cells
To verify the above conclusion that PBSC is less stable than PBCC (especially in wet air) derived from the electrochemical tests of the symmetrical cell, we further performed the electrochemical tests on single cells, using a Ni-BZCYYb anode-supported cell with an approximately 8-μm thick BZCYYb electrolyte. As shown in Figure 5A, the porous cathode, the dense and smooth electrolyte layer, and the anode support are in close contact. As exhibited in Figure 5B,C, the cells with PBCC cathode demonstrated remarkable peak power densities of 1.69, 1.22, and 0.88 W cm −2 at 700, 650, and 600 • C, respectively. Cells with PBSC cathode are also good but slightly lower than those of cells with PBCC cathode: 1.60, 1.08, 0.72 W cm −2 at 700, 650, and 600 • C, respectively. The demonstrated peak power densities are higher than most of the cells with similar cathodes that have been reported in the literature, 29,64-68 demonstrating that doping with either Ca or Sr is a promising strategy to develop high-performance H-SOFCs. Shown in Figure 5D,E are the EIS of single cells with PBCC and PBSC cathode, measured at 600, 650, and 700 • C, respectively. The difference between the two intersection points between the EIS curve and the real axis denotes the R p of the cell. The single cells with Ca-or Sr-doped cathodes show similar R p under OCV conditions. However, the R p of the PBCC cell is slightly smaller than that of the PBSC cell, as shown in the histograms ( Figure 5F,G), indicating that PBCC may show higher activity at the operating conditions since the anode and electrolyte of cells were controlled the same.
We further tested the short-term stability at 650 • C of the single cell with PBSC or PBCC cathode at a constant current of 0.5 Acm −2 . The initial voltage of the cell with the PBCC cathode is 0.934 V, which is higher than that of the PBSC cathode at 0.907 V under the same conditions. As shown in Figure 6A, both cells showed degradation during the test of 60 h. However, the cell with PBCC showed a slightly lower degradation. 69 The response voltage of PBCC single cell declined only 57 mV over 60 h of testing (a degradation rate of 0.10% h −1 ), while that of PBSC declined 73 mV over 60 h (a degradation rate of 0.13% h −1 ). The decay rate of PBCC is less than that of PBSC, which indicates that the PBCC cathode exhibits better stability than the PBSC cathode and is consistent with the results shown in the symmetrical cells. The comparison of the EIS before and after the test in Figure 6B,C shows that the polarization resistance of the PBCC cell decreased from 0.12 to 0.07 Ω cm 2 after the stability test, which may be attributed to the relatively better stability of PBCC against steam. The degradation of the PBCC single cell may be related to degradation caused by other components of the cell. In contrast, the polarization resistance of PBSC increased from 0.12 to 0.14 Ω cm 2 . This may be a consequence of the coarsening of PBSC caused by the water generated during the operation, which may lead to increased resistance of gas diffusion and/or surface exchange process.

CONCLUSION
In this study, we assessed how Ca-or Sr-doping in PBC double perovskite affected the ORR activity and durability in H-SOFCs. The symmetrical cell results demonstrated that PBCC showed higher activity than PBSC at higher temperatures, while PBSC exhibited higher activity than PBCC at low temperatures. Careful DRT analyses suggested that the main rate-limiting steps of both materials are in the IF region, which is associated with the surface exchange process. PBCC showed better stability than PBSC, especially in wet air, as confirmed by the stability tests of symmetrical cells and single cells. At 650 • C, the anode-supported single cell with PBCC cathodes shows a good reaction activity (a P max of 1.22 Wcm −2 ) and durability (a degradation rate of 0.1% h −1 ), better than those of PBSC cells (a P max of 1.08 Wcm −2 and a degradation rate of 0.13% h −1 ). It is suggested by the DRT analysis and SEM observations that PBCC showed a relatively stable surface exchange process and microstructure in wet air. The rough surface of PBSC caused by the interaction with steam may block the surface exchange process and result in a significant increase in polarization resistance.  (BZCYYb) were produced using the conventional solid-state reaction process. 48 BaCO 3 , ZrO 2 , CeO 2 , Y 2 O 3 , and Yb 2 O 3 were mixed in a stoichiometric ratio and ball-milled in ethanol for 24 h. After being dried and crushed, the combined powders were pressed into a pellet under 10 MPa, which was then calcined at 1100 • C for 12 h. To eventually obtain the pure phase of BZCYYb, the procedures were repeated.

Cell fabrication
To produce electrolyte pellets supported symmetrical cells, 0.25g BZCYYb powder was uniaxially pressed into pellets (10 mm in diameter and 500 um in thickness) under 2 MPa and sintering at 1450 • C for 5 h. The anode-supported half-cells were created using a three-layer co-tape casting technique. Every 2 h, the slurries of the BZCYYb electrolyte, functional layer, and NiO-BZCYYb anode were successively cast onto a polymer film. Several 15 mm pellets were punched from the large green tape and allowed to cure in room air overnight. To eliminate all organic components and maintain the structure, the cells were then heated at 600 • C for 2 h at a slow firing rate. Afterward, the electrolyte layer was densified by heating the pre-fired pellets at 1450 • C for 5 h.
PBCC/PBSC powder was ball-milled to prepare the cathode for 2 h. Subsequently, a 160-mesh screen was used to ensure that the cathode powder was sufficiently uniform in particle size. Fine cathode powders were then mixed thoroughly with adequate 5 wt% ethylcellulose-terpineol binders to obtain a cathode slurry. The cathode slurry was screen-painted on the electrode surface with an area of 0.2826 cm 2 . After being dried thoroughly in a constant temperature oven at 70 • C, the cells were sent to a chamber furnace for calcination at 950 • C for 2 h. Besides, the silver paste (DAD-87, purchased from Shanghai Synthetic Resin Research Institute) was used as a current collector for cell testing.

Electrochemical measurements
The alternating current (AC) impedance method was used to examine the electrode performance of symmetrical cells and single cells. The applied frequency ranged from 10 −2 to 10 5 Hz. The signal amplitude under open-circuit voltage was 10 mV for the single-cell test and 30 mV for the symmetrical cells. The fuel cell test device was set up in a vertical chamber furnace where the temperature was controlled and adjusted by thermocouples. Hydrogen (3% H 2 O) was fed into the anode chamber at a flow rate of 30 ml min −1 . Ambient air was used as an oxidant to the cathode. A multi-channel electrochemical testing system, PARSTAT MC200, was used to collect current-voltage (I-V) and polarization curves.

Other characterizations
The crystal structures of PBCC and PBSC powder were measured by X-ray diffraction (XRD) with Cu Ka radiation. The morphology and structural characteristics of the electrodes and cells were examined by scanning electron microscopy (SEM, Hitachi SU8010).

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.