Oxygen reduction reaction over (Ba,Sr)₆RE₂Co₄O₁₅–Ba(Ce,Pr,Y)O₃ composite cathodes for proton-conducting ceramic fuel cells†

Toshiaki Matsui *^a, Naoki Kunimoto ^a, Kohei Manriki ^a, Kazunari Miyazaki ^a, Naoto Kamiuchi ^b, Hiroki Muroyama ^a and Koichi Eguchi ^a
aDepartment of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan. E-mail: matsui@elech.kuic.kyoto-u.ac.jp
^bThe Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

First published on 10th June 2021

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Introduction

Proton-conducting ceramic fuel cells (PCFCs) have been attracting much attention due to the potential for achieving high energy conversion efficiency at intermediate temperatures, especially below 600 °C. This can be realized by the lower activation energy for the proton migration in proton-conducting oxides, such as acceptor-doped BaCeO₃ and BaZrO₃.^1–10 The avoidance of fuel dilution also contributes to enhancing the power output; this PCFC system can be operated at higher fuel utilization as compared to solid oxide fuel cells with oxide ion conductors since the steam is generated in the cathode. Recently, some groups have demonstrated remarkably high performance by applying novel materials and fabrication methods.^11–14 At the current state, however, the performance of PCFCs is mainly limited by the cathode activity;^11,15–17 it is required to develop a high-performance cathode with an area-specific resistance less than 0.1 Ω cm² at 600 °C.

Great efforts have been devoted to developing high performance PCFC cathodes.^18–26 The fabrication of composite electrodes consisting of a mixed oxide ion-electron conductor and a proton-conducting oxide is one of the promising approaches. In this case, a proton conductive electrolyte or a mixed proton–electron conductor is applied to introduce the proton conduction path, as well as to increase active reaction sites. Some oxides have also been reported to be triple-conducting materials, which allow simultaneous transport of protons, oxide ions, and electronic defects, e.g., BaCo_0.4Fe_0.4Zr_0.1Y_0.1O_3−δ (ref. 11), BaGd_0.8La_0.2Co₂O_6−δ (ref. 27), and Ba_0.5Sr_0.5Fe_0.8Zn_0.2O_3−δ (ref. 28). Though some electrodes have been reported to have relatively high activity for the oxygen reduction reaction (ORR), sufficient durability and reliability have not been achieved yet.

Recently, we found that a new oxide of Ba₄Sr₂Sm₂Co₄O₁₅, which is one of the compounds of (Ba_6−xSr_x)RE₂Co₄O₁₅ (x = 1, 2, RE = some rare-earth elements) (ref. 29) serves as an electrocatalyst for the ORR.³⁰ Furthermore, a remarkable composite effect was confirmed for the electrode based on the triple-conducting material of Pr and Y co-doped BaCeO₃ (Ba(Ce,Pr,Y)O_3−δ) (ref. 31 and 22). In particular, the composite of Ba₄Sr₂Sm₂Co₄O₁₅–BaCe_0.5Pr_0.3Y_0.2O_3−δ (BSSC4224–BCPY) with a mixing ratio of 30:70 wt% exhibited higher activity below 600 °C in 3% humidified synthetic air, as compared with high-performance cathodes reported;³⁰ in this case, the volume ratio of BSSC4224 in this composite is almost the threshold value for the charge carrier conduction. Thus, it was suggested that the triple phase boundary of the BSSC4224/BCPY/gas phase is the electrochemically active reaction site and BSSC4224 serves as the active site for the dissociative adsorption of oxygen. However, the design guide for this composite system has not been established yet. It is required to verify the validity of the proposed electrode microstructure as well as the reaction mechanism. Moreover, this composite system has a potential to achieve higher ORR activity by modifying the chemical composition, optimizing the fabrication process, changing the mixing materials, etc. In this study, then, the effect of elemental substitution, especially the Ba/Sr ratio and rare-earth elements, in (Ba,Sr)₆RE₂Co₄O₁₅ and the composite effect with BCPY on the electrocatalytic activity were studied. We also focused on elemental interdiffusion in (Ba,Sr)₆RE₂Co₄O₁₅–Ba(Ce,Pr,Y)O₃ composites to clarify the reason for the high ORR activity.

Experimental

Powder preparation and characterization

The powders of (Ba_6−xSr_x)RE₂Co₄O₁₅ (x = 1–3, RE = La, Pr, Nd, Sm, Gd), BaCe_0.5Pr_0.3Y_0.2O_3−δ (BCPY), BaCe_0.5−xCo_xPr_0.3Y_0.2O_3−δ (BCCPYx, x = 0.05–0.25), and BaCe_0.8Y_0.2O_3−δ (BCY) were synthesized by the Pechini method. Stoichiometric amounts of starting reagents were dissolved into distilled water: Ba(NO₃)₂ (Wako Pure Chemical Industries, Ltd.), Sr(NO₃)₂ (Wako Pure Chemical Industries, Ltd.), Co(NO₃)₂·6H₂O (Wako Pure Chemical Industries, Ltd.), Ce(NO₃)₃·6H₂O (Wako Pure Chemical Industries, Ltd.), Pr(NO₃)₃·6H₂O (Sigma-Aldrich, Co.), Y(NO₃)₃·6H₂O (Sigma-Aldrich, Co.), Sm(NO₃)₃·6H₂O (Wako Pure Chemical Industries, Ltd.), Gd(NO₃)₃·6H₂O (Wako Pure Chemical Industries, Ltd.), Nd(NO₃)₃·6H₂O (Wako Pure Chemical Industries, Ltd.), and La (NO₃)₃·6H₂O (Wako Pure Chemical Industries, Ltd.). Citric acid (C₆H₈O₇·H₂O, Wako Pure Chemical Industries, Ltd.) and ethylene glycol (Wako Pure Chemical Industries, Ltd.) were added to the resulting solution at the molar ratio of citric acid/ethylene glycol/total metal cations of 1.5/1.5/1. The pH value of the solution was controlled to be ca. 8.0 with adding an aqueous ammonia solution (Wako Pure Chemical Industries, Ltd.). After that the solution was stirred at 80 °C until the water was evaporated. The obtained gel was pre-heated at 350 °C in air, and subsequently calcined at 1100 °C for 5 h in air. The crystal structures of resultant powders were identified by X-ray diffraction (XRD, Rigaku Ultima IV X-ray diffractometer, Cu Kα). The working conditions were 40 kV and 40 mA with a scanning rate of 20° min⁻¹. The diffraction patterns of (Ba_6−xSr_x)RE₂Co₄O₁₅ (x = 1–3, RE = La, Pr, Nd, Sm, Gd) are summarized in Fig. S1 and S2 in the ESI.† The oxide powder of Ba₅SrGd₂Co₄O₁₅ (BSGC5124) was mixed with BCPY to form the composite, and subsequently annealed at 1000 °C for 5 h in air. After that, the local structure was analyzed using transmission electron microscopes (TEM), Tecnai G2 F20 (FEI Co.) and ARM-200F (JEOL Ltd.) equipped with an energy dispersive X-ray spectrometer (EDX, JED-2300).

Cell preparation

The symmetric cells consisting of various electrodes and a BCY disk electrolyte were fabricated as follows. The BCY powder was uniaxially pressed at 40 MPa into a pellet, followed by cold isostatic pressing at 300 MPa. The obtained pellet was fired at 1600 °C for 10 h in air, and subsequently polished to be 1 mm in thickness. The diameter of the pellet was ca. 19 mm. The relative density of the sintered BCY disk was higher than 98%. Some powders were mechanically mixed to form composites with varying the weight ratio. The slurries of electrodes were prepared by adding polyethylene glycol (molecular weight 400, Wako Pure Chemical Industries, Ltd.) to electrode powders. The slurry was screen-printed onto both sides of the BCY electrolyte, and then fired at 1000–1200 °C for 5 h in air. The diameter and thickness of the electrodes were 6 mm and ca. 20 μm, respectively. The electrode microstructure observation was performed by using a dual-beam focused ion beam-scanning electron microscope (FIB-SEM, NVision 40, Carl-Zeiss-SIINT) equipped with an EDX (Oxford).

Electrochemical measurements

The symmetrical cells were used to evaluate the ORR activity. The electrode performance was measured in 3 vol% humidified synthetic air (21% O₂–79% N₂) at 450–700 °C by impedance spectroscopy (CellTest system; potentiostat/galvanostat 1470E and frequency response analyzer 1455, Solartron Analytical, UK). The applied frequency was in the range of 0.1 Hz to 10⁵ Hz with a voltage amplitude of 10 mV. The humidification was conducted by bubbling the gas through the water. Note that the electrode performance included the influence of the electronic leakage current because of the usage of symmetrical cells with a BCY electrolyte in the oxidizing atmosphere. However, the difference in the electrode performance obtained can be compared fairly. This is because the experimental conditions, such as the electrolyte thickness and the atmosphere, were fixed throughout this study. The electrode design concept obtained in this study can also be applied to cells using other electrolytes, such as BaZrO₃-based oxides.

Results and discussion

First of all, the effect of elemental substitution, especially the Ba/Sr ratio and rare-earth elements, in (Ba,Sr)₆RE₂Co₄O₁₅ on the ORR activity was studied. Fig. 1 shows impedance spectra at 550 °C and the temperature dependence of area-specific resistance (ASR) for (Ba_6−xSr_x)Sm₂Co₄O₁₅ (x = 1–3) electrodes in 3 vol% humidified synthetic air. All oxides were fired on the BCY disk with a symmetrical configuration at 1200 °C. The polarization resistance decreased with an increase in the Ba/Sr ratio of (Ba_6−xSr_x)Sm₂Co₄O₁₅, which was prominent below 650 °C. The same tendency was confirmed for (Ba,Sr)₆Gd₂Co₄O₁₅, as shown in Fig. S3.† Furthermore, the difference in the total electrical conductivity was confirmed; the conductivity for Ba₅SrSm₂Co₄O₁₅ was one order of magnitude higher or more than that for Ba₃Sr₃Sm₂Co₄O₁₅, as shown in Fig. S4.† These results indicate that the basicity, as well as the conductivity, strongly affects the ORR activity; in other words, the material with higher basicity and electrical conductivity accelerates the ORR.

In Fig. 2, the influence of elemental substitution in the rare-earth site of Ba₅SrRE₂Co₄O₁₅ (RE = La, Pr, Nd, Sm, Gd) electrodes on the ORR activity was also studied. The activity did not change systematically along the periodic table, and the remarkable elemental dependence on the ASR was not observed. At the current state, factors other than the basicity, which contributed to the activity, are unclear. Furthermore, it is apparent that the electrocatalytic activity of (Ba_6−xSr_x)RE₂Co₄O₁₅ was insufficient for the practical use. Since Ba₅SrGd₂Co₄O₁₅ (BSGC5124) exhibited the lowest ASR with the lowest activation energy among electrodes studied, this material was applied for the composite electrodes with BCPY in the following part; note that BCPY was reported as a triple-conducting material.^22,31

The composite electrodes of BSGC5124–BCPY were prepared by mixing two oxides with various weight ratios. Fig. 3(a) displays the impedance spectra of BSGC5124–BCPY (30:70 wt%) and BCPY electrodes. In Fig. 3(b), ASR as a function of the weight percent of BSGC5124 in the composites at 550 °C in 3 vol% humidified synthetic air is summarized. The composite effect was clearly observed in the mixing ratio of 20–40 wt% BSGC5124, and the BSGC5124–BCPY composite with a weight percent of 30:70 exhibited the lowest ASR. In the impedance spectra of these composites, a significant reduction in the low-frequency arc was confirmed as compared with that of BCPY. Since the weight ratio can be replaced almost equally to the volume ratio in this composite system, the volume ratio of BSGC5124, ca. 30%, is near the threshold for the charge carrier conduction, based on the percolation theory. This means that BSGC5124 did not serve as the main charge carrier conduction path. The ASR of BSGC5124–BCPY (30:70 wt%) as a function of reciprocal temperature in 3 vol% humidified synthetic air is presented in Fig. 4. For comparison, the results of BSGC5124, BCPY, and BSSC4224–BCPY (30:70 wt%) (ref. 30) are also plotted. Although the ASR of BSGC5124 was about one order of magnitude lower than that of BSSC4224 at 500 °C (see Fig. S3†), the performance of the BSGC5124–BCPY composite was almost comparable to that of BSSC4224–BCPY. Furthermore, both composites exhibited similar activation energy: BSGC5124–BCPY – 61.8 kJ mol⁻¹, BSGC4224–BCPY – 68.5 kJ mol⁻¹. Judging from the frequency response in impedance spectra and the activation energy, therefore, we can guess that BSGC5124 mainly serves as an active site for the dissociative adsorption of oxygen in the BSGC5124–BCPY composite, as in the case of the BSSC4224–BCPY composite.³⁰ In this case, the triple phase boundary of the BSGC5124/BCPY/gas phase is the main electrochemical reaction site.

In the following parts, the reason for the performance enhancement in the composite system was studied in detail. Fig. 5 shows the XRD patterns of BSGC5124–BCPY composites before and after calcination at 1000 °C for 5 h in air; this calcination condition is the same as that of the firing process of electrodes on the BCY disk. Although no impurity phase was formed after heat-treatment, a slight shift of diffraction patterns was confirmed for both oxides. This means that the elemental interdiffusion occurred during the heat-treatment. Then, TEM observation was performed for the composite powder to elucidate the local structure (see Fig. 6). The composite consisted of particles with a size from 500 nm to several μm as shown in Fig. 6(a) and (d). The EDX mapping revealed that Y and Co elements, derived from BCPY and BSGC5124, respectively, were present throughout the composite powder (Fig. 6(b) and (c)). Such elemental interdiffusion led to the peak shift in the XRD patterns in Fig. 5. The bulk of BCPY was highly crystalline in Fig. 6(f). In contrast, the amorphous layer was formed with a good wettability on the BCPY surface while including the crystallites with a size of 5–10 nm as indicated by the yellow circles in Fig. 6(e). This nanostructure of the amorphous layer including the crystallites will be derived from BSGC5124, since the same structure was confirmed for the powder of BSGC5124. In addition, the vulnerability of the surface oxide on BCPY to the electron beam irradiation during STEM-EDX analysis was also observed for the original BSGC5124 powder. Consequently, the local structure analysis has revealed that the BCPY phase is partially covered by the BSGC5124 phase, concomitant with the considerable elemental interdiffusion between two components. Such a surface modification will be one of the effective design strategies for PCFC cathodes.

In the next part, to clarify the impact of elemental interdiffusion on the ORR activity, the average composition of the BCPY phase after firing the composite electrode on the BCY disk was estimated by an SEM-EDX analysis (see Fig. S6 and Table S1†); it was impossible to distinguish two components from the cross-sectional SEM image of the BSGC5124–BCPY (30:70 wt%) composite. We found that the Co element diffusing from the BSGC5124 phase dissolved in the BCPY phase in the range of 1–3 mol%. Then, cobalt-containing BCPY of BaCe_0.5−xCo_xPr_0.3Y_0.2O_3−δ (BCCPYx, x = 0.05–0.25) was synthesized and their structural and electrochemical properties were investigated; note that only the Ce/Co ratio was varied with fixing the Pr and Y molar ratios to simplify the discussion. Fig. 7 shows the XRD patterns of BCPY and BCCPYx (x = 0.05–0.25). Only the diffraction patterns ascribable to the perovskite-structure were detected up to x = 0.15, while BaCoO₃-based oxide and CoO were formed as impurities at x = 0.2, 0.25. Thus, the powder was the mixture of BCCPY and impurities more than x = 0.2. According to the EDX analysis in Table S1,† the x value was in the range of 0.05–0.15 at each point in Fig. S6† with assuming the fixed Pr and Y molar ratios. Thus, the cobalt-containing BCPY phase in the composite electrode was within the single phase range. Fig. 8 displays the temperature dependence of ASR for BCPY and BCCPYx (x = 0.05–0.15) in 3 vol% humidified synthetic air and corresponding impedance spectra collected at 550 °C. Cobalt-containing BCPY exhibited almost the same or higher ORR activity as compared with BCPY. The ORR activity enhanced with an increase in the amount of cobalt up to x = 0.1, and then reduced at x = 0.15. It is clear from impedance spectra in Fig. 8(b) and (c) that the low frequency arc shrank significantly for BCCPY0.1, as compared with BCPY. The elementary processes related to the surface reaction, such as the dissociative adsorption of oxygen and the subsequent migration of adsorbed oxygen, will be mainly accelerated by the substitutional dissolution of cobalt. Interestingly, this spectral change also affected the activation energy; the activation energy of cobalt-containing BCPY was lower than that of the cobalt-free one. Thus, the rate limiting step is different depending on the presence or absence of the Co element. Consequently, the ORR at the two phase boundary of the BCCPYx/gas phase will be facilitated since pristine BCPY was reported as a triple-conducting material. In the case of BCPY0.15, the frequency response of the impedance spectrum was analogous to that of BCCPY0.1. The formation of small amounts of impurities may affect the ORR activity, considering the results in Fig. 7.

Since BCCPY0.1 exhibited the highest activity among the samples studied, this material was applied as the main electrical conduction component in the composite electrode. The temperature dependences of the area-specific resistance of BSGC5124–BCCPY0.1 (30:70 wt%) and BSGC5124–BCPY (30:70 wt%) composite electrodes in 3 vol% humidified synthetic air are shown in Fig. 9. Corresponding impedance spectra at 550 °C are also displayed. As described above, the mixing weight ratio can be almost equivalently converted to the volume ratio in the case of BSGC5124–BCPY. The same thing can be applied to the BSGC5124–BCCPY0.1 since the difference in the molecular weights between BCCPY0.1 and BCPY is less than 3%. Thus, the volume ratio of BSGC5124 in both composites is close to the threshold value for the charge carrier conduction through this phase, considering the percolation theory. Surprisingly, the drastic performance deterioration was confirmed for BSGC5124–BCCPY0.1 though BCCPY0.1 with the highest ORR activity was used. Furthermore, its activation energy also increased as compared with that of each constituent component, BSGC5124 and BCCPY0.1. This means that the distribution of cobalt in the composite decisively affected the ORR activity. The Co element comes out from BSGC5124 in this case, whereas a certain amount of cobalt already dissolved in BCCPY0.1. Thus, an excess amount of cobalt in BCCPY0.1 may lead to the formation of impurities such as BaCoO₃-based oxide, resulting in the reduction in the ORR activity of cobalt-containing BCPY as observed in Fig. 7 and 8. At the current state, however, the diffraction lines ascribable to impurities were not detected directly in an XRD analysis for BSGC5124–BCCPY0.1. The suppression of cobalt diffusion from BSGC5124 also inhibits the dissociative adsorption of oxygen over BSGC5124, judging from the impedance spectrum and activation energy. In other words, the formation of BSGC5124-derived oxide with a barium-rich surface induced by the cobalt diffusion will be an important factor to accelerate the dissociative adsorption of oxygen.

Summarizing the results and discussion so far, the plausible ORR mechanism on the BSGC5124–BCPY (30:70 wt%) composite can be drawn as shown in Fig. 10. BSGC5124-derived oxide partially covered the cobalt-containing BCPY surface with a good wettability; both phases were formed by elemental interdiffusion through the contacting interface. In this composite, two factors affect the electrocatalytic activity in a complex manner. Cobalt-containing BCPY is a mixed conductor and serves as ionic and electronic conduction paths. The enhancement in the ORR activity was confirmed concomitantly with the dissolution of cobalt cations into BCPY, leading to the promotion of the two phase boundary reaction. Furthermore, BSGC5124-derived oxide also played an important role in the ORR. The barium-rich surface with high activity for the dissociative adsorption of oxygen is formed accompanied by cobalt diffusion. In conclusion, we propose that in situ surface modification of electrocatalysts through elemental interdiffusion is one of the effective design strategies for PCFC cathodes. In this composite, however, the CO₂-resistivity of materials still remains as a drawback to be settled since the basicity is a trade-off relation with the ORR activity in CO₂-containing atmospheres.

Conclusions

The effect of elemental substitution in (Ba,Sr)₆RE₂Co₄O₁₅ on the ORR activity was studied, and we found that the basicity and the electrical conductivity were key factors. However, the order of the electrocatalytic activity of (Ba,Sr)₆RE₂Co₄O₁₅ was found to be not the same as that of composites with BCPY; the performance of the BSGC5124–BCPY (30:70 wt%) composite was almost comparable to that of BSSC4224–BCPY (30:70 wt%). The local structure analysis of BSGC5124–BCPY revealed that BSGC5124-derived oxide partially covered the surface of cobalt-containing BCPY with a good wettability because of the considerable elemental interdiffusion during the firing process. As a result, the promotion of two reaction routes resulted in high ORR activity in this composite: (1) the two phase boundary reaction over cobalt-containing BCPY, and (2) the dissociative adsorption of oxygen over BSGC5124-derived oxide and the subsequent electrochemical reaction at the triple phase boundary of the BSGC5124-derived oxide/cobalt-containing BCPY/gas phase. Therefore, in situ surface modification of electrocatalysts through elemental interdiffusion will be a good design guide to develop high-performance PCFC cathodes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This paper is partially based on results obtained from projects, Development of Ultra-High Efficiency Protonic Ceramic Fuel Cell Devices, WP1 Development of Innovative High-Performance Electrodes, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO) and the research grant of the Toyota Mobility Foundation (TMF). TEM analysis in the present work was supported by “Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials” from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan.

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Footnote

† Electronic supplementary information (ESI) available: XRD patterns (Fig. S1 and S2), temperature dependence of area-specific resistance (Fig. S3), temperature dependence of total electrical conductivity (Fig. S4), temperature dependence of ohmic resistance (Fig. S5), SEM image (Fig. S6), and the result of EDS analysis (Table S1). See DOI: 10.1039/d1ta02485j

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