New, Efficient, and Reliable Air Electrode Material for Proton

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 1761−1770

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New, Efficient, and Reliable Air Electrode Material for ProtonConducting Reversible Solid Oxide Cells Daoming Huan,† Nai Shi,† Lu Zhang,† Wenzhou Tan,† Yun Xie,† Wanhua Wang,† Changrong Xia,† Ranran Peng,*,†,‡,§ and Yalin Lu*,†,‡,§,∥ †

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, ‡Synergetic Innovation Center of Quantum Information & Quantum Physics, and §Hefei National Laboratory of Physical Science at the Microscale, University of Science and Technology of China, Hefei, 230026 Anhui, China ∥ National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, P. R. China S Supporting Information *

ABSTRACT: Driven by the demand to minimize fluctuation in common renewable energies, reversible solid oxide cells (RSOCs) have drawn increasing attention for they can operate either as fuel cells to produce electricity or as electrolysis cells to store electricity. Unfortunately, development of protonconducting RSOCs (P-RSOCs) faces a major challenge of poor reliability because of the high content of steam involved in air electrode reactions, which could seriously decay the lifetime of air electrode materials. In this work, a very stable and efficient air electrode, SrEu2Fe1.8Co0.2O7−δ (SEFC) with layer structure, is designed and deployed in P-RSOCs. X-ray diffraction analysis and High-angle annular dark-filed scanning transmission electron microscopy images of SEFC reveal that Sr atoms occupy the center of perovskite slabs, whereas Eu atoms arrange orderly in the rock-salt layer. Such a special structure of SEFC largely depresses its Lewis basicity and therefore its reactivity with steam. Applying the SEFC air electrode, our button switches smoothly between both fuel cell and electrolysis cell (EC) modes with no obvious degradation over a 135 h long-term test under wet H2 (∼3% H2O) and 10% H2O−air atmospheres. A record of over 230 h is achieved in the long-term stability test in the EC mode, doubling the longest test that had been previously reported. Besides good stability, SEFC demonstrates great catalytic activity toward air electrode reactions when compared with traditional La0.6Sr0.4Co0.2Fe0.8O3−δ air electrodes. This research highlights the potential of stable and efficient PRSOCs as an important part in a sustainable new energy power system. KEYWORDS: proton-conducting reversible solid oxide cells, air electrode, novel Ruddlesden−Popper oxide, electrochemical property, long-term stability

1. INTRODUCTION

including: (1) a low migration energy barrier for protons, which can facilitate good ionic conduction and accelerate the electrode reaction kinetics at the desired intermediate temperatures (500−700 °C). This could lead to further optimization of the stack materials and extension of the stack lifespan;7,8 (2) absence of the need for off-gas treatment (H2) for the steam either injected (in the EC mode) or generated (in the FC mode) in the air electrode chamber (as shown in Figure 1a);9 and (3) avoidance of the oxidation of Ni particles because of the low steam injected into the fuel electrode.10,11 It is well-known that the polarization resistance (Rp) of PRSOCs mainly comes from the air electrode in both FC and EC modes. As illustrated in Figure 1a, operating in the FC mode, the air electrode reaction includes the following steps:

Driven by the rapidly increasing economic growth and a crisis awareness of the need for environmental protection, common renewable energies, including solar, wind, and tide, have been greatly promoted in recent decades to build a sustainable and decarbonizing society.1−3 Unfortunately, their discontinuities or fluctuations still present severe challenges to gain smooth access to the power grid. Reversible solid oxide cells (RSOCs) have been drawing extensive attention over the recent decades as an efficient clean energy conversion device. The major benefit of a RSOC lies in that it can work both as a fuel cell (FC mode) to oxidize chemical fuels to produce electricity and as an electrolysis cell (EC mode) to store electricity power in chemical fuels, offering great convenience for storage and utilization of the renewable energies.4−6 Compared to conventional RSOCs based on oxygen-ion-conducting electrolytes (e.g., using stabilized zirconia), RSOCs using proton-conducting electrolytes (P-RSOCs) have several additional advantages, © 2017 American Chemical Society

Received: November 2, 2017 Accepted: December 28, 2017 Published: December 28, 2017 1761

DOI: 10.1021/acsami.7b16703 ACS Appl. Mater. Interfaces 2018, 10, 1761−1770

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Reaction mechanism of P-RSOCs when working in FC and EC modes. The steam generated from (in FC mode) or injected into (in EC mode) the air electrode chamber. (b) Schematic of the reaction at the single-phase air electrode for P-RSOCs in FC and EC modes.

latest air electrode Sr2Fe1.5Mo0.5O6−δ−BaZr0.8Y0.2 (6:4) lasts 100 h but in a very low steam-involved (∼3%) atmosphere.25 Therefore, exploring single-phase air electrodes with good triple conductivities and high stability in steam is the key to promote the practical application of P-RSOCs in a sustainable FC and EC mode switching energy power system. Ruddlesden−Popper (R−P) oxides (An+1BnO3n+1), with an nlayered perovskite slab sandwiched between two rock-salt layers, have gained special attentions as promising solid oxide fuel cell electrode materials because of their high oxygen ionic and electronic conductivities.26−29 Very recently, we have found that Sr3Fe2O7−δ (SFO) possesses a low proton formation energy (∼−0.23 eV) and a low proton migration energy (∼0.62 eV). 30 Unfortunately, SFO is unstable in high steam atmospheres because of the high basicity of Sr, and therefore, a dopant with higher acidity is proposed to effectively suppress its basicity based on the acidity of metal oxides. Among the common lanthanide elements, we take Eu element into consideration as a donor dopant to form SrEu2Fe2O7−δ (SEF), whose basicity is supposed to be suppressed at the expense of oxygen vacancy concentration lowered simultaneously.31 To make up this loss, Co is partially substituted for Fe; and thus, SrEu2Fe1.8Co0.2O7−δ (SEFC) is obtained and proposed as a novel, stable, and efficient air electrode for PRSOCs, as illustrated in Figure 2a. In this work, crystal structure, chemical stability, and catalytic activity of SEFC were investigated. Long-term effectiveness of the SEFC air electrode in a P-RSOC was fully characterized while operating and conveniently switching between EC and FC modes.

(1) O2 molecules are dissociatively adsorbed over the electrocatalysts; (2) the dissociated oxygen species transfer to the triple-phase boundaries to react with protons, which migrate from the fuel electrode, to generate water; and (3) water molecules are desorbed into the gas phase. Because the migration of oxygen ions and protons are thought to be the limiting steps in the reaction, a highly demanded air electrode for the FC mode should be a proton, oxygen ion, and electron triply conducting material.12−16 Many composite FC mode air electrodes composed of oxygen-ion−electron conductors and proton conductors,17−20 including Sm0.5Sr0.5CoO3−δ (SSC)− Ba(Zr0.1 Ce 0.7 Y 0.2 )O 3−δ (BZCY), 17 La 0.6Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF)−BZCY,18 and some new single-phase perovskite oxides as potential triple conductors, such as BaCo0.4Fe0.4Zr0.1Y0.1O3−δ12 and BaZr0.6Co0.4O3,21 have been proposed to achieve this goal, and fairly acceptable peak power densities were achieved in those cells using the above air electrodes. Unfortunately, the above composite air electrodes are not so effective for the EC mode. In the EC mode, water molecules are adsorbed on the air electrode and then dissociated to form protons and oxygen atoms by losing electrons. Then, the oxygen atoms produced combine to yield dioxygen molecules to be released into the gas phase, and the protons transfer to the fuel electrode to produce hydrogen. As shown in Figure 1b, less oxygen-ion conduction is required because of the localized formation of oxygen molecules in this reaction. Meanwhile, the injected steam reactant (≥10%), which can be regarded as a Lewis acid, is prone to react with the above-mentioned composite or single-phase air electrodes (mainly Lewis bases), leading to the deterioration of the used materials and subsequent decay in cell performance.22−24 As summarized in Table S1, working in the EC mode, the lifespan of majority of the past investigated P-RSOCs is usually a few hours; only the

2. RESULTS AND DISCUSSION 2.1. Crystal Structures and Stability of SEFCs. In this work, we propose a novel air electrode material, SEFC, with a two-layer R−P structure for P-RSOCs. Figure 2b reveals the 1762


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which SrFeO3−x and Eu2O3 form first and then react to form SrEu2Fe2O7 gradually and totally. A Rietveld refinement of the XRD data for the SEFC powders calcined at 1200 °C is shown in Figure S1 using the P42/mnm space group. The good consistency of these patterns further confirms the formation of an R−P-structured SEFC solid solution with no impurities. The refined lattice parameters (a = 5.4827(1) and c = 19.8345(8) Å) and the deduced atomic positions and occupancy of the SEFC are all shown in Table S2. The reliability of the Rietveld refinement is confirmed by the Rp value of 5.79% and the Rwp value of 3.80%, indicating the effectiveness of the lattice fitting. High-angle annular dark-filed (HAADF) scanning transmission electron microscopy (STEM) images of the SEFC powders with both (0 1 0) and (1 −1 0) crystal faces are shown in Figure 3a,b, respectively. All the atomic spots in the images are regularly arranged and can be divided into three types according to their color and size: bright white spots, big gray spots, and small gray spots. Because the intensity of the spots in a HAADF-STEM image is approximately proportional to the square of the atomic number, Z, the bright white, the big gray, and the small gray spots should correspond to Eu, Sr, and Fe/ Co atoms, respectively. Oxygen atomic spots are not visible because of their low intensity. It should be noted that all the Sr atoms are positioned in the center of the perovskite slabs, whereas the Eu atoms twice as many as the Sr atoms sit in the rock-salt layers with close packing. The detailed distribution of the cations in the lattice structure was analyzed using electron energy-loss spectroscopy (EELS), as shown in Figures 3c and S2. EELS clearly reveals that the Eu and Sr ions occupy different sites; the former in the rock-salt layers and the latter in the center of perovskite slabs. Meanwhile, the individual

Figure 2. (a) Design sketch of the new material: from SFO to SEFC and (b) room temperature X-ray diffraction (XRD) patterns of SEFC powders calcined at increasing temperatures.

structural evolution of SEFC powders prepared via a combustion process and calcined at different temperatures. It can be clearly seen that when calcined at low temperatures (≤500 °C), the powders are amorphous. When the calcination temperature is ≥1000 °C, the crystal structures of the powders are almost unchanged with temperature. However, a major phase transformation occurs between 500 and 1000 °C, during

Figure 3. (a) (0 1 0) and (b) (1 −1 0) crystal faces. The insets show the enlarged and SAED patterns and the corresponding models of the cationic positions in SEFC. (c) EELS analysis of the elemental distribution Eu, Sr, Fe, and Co along the yellow line marked in (a). 1763


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Figure 4. (a) XRD patterns of the SEFC powders before (orange line) and after (purple line) heating at 600 °C in a 10% H2O−air atmosphere for 100 h and of SEFC−BZCY mixtures (blue line, 1:1 in weight ratio) calcined at 1000 °C for 2 h. (b) ABEs for the BZY, SEFC, LSCF, SSC, and BSCF air electrodes. X-ray photon spectroscopy (XPS) spectra (c) and proportion (with error bars) (d) of O 1s species on the surface of the SEFC and SEF samples.

and ionic conductivity.31,34−37 Shao’s group has suggested that more negative ABE values for an oxide denote lower basicity and better CO2 tolerance.38−40 Here, we applied ABE values to evaluate the stability of air electrode materials in an atmosphere with a high steam content, where steam reacts as a Lewis acid such as CO2. The calculation details are shown in the Supporting Information. Figure 4b shows the ABE values of B a Zr 0 . 8 Y 0 . 2 O 3 − δ (BZY), SEFC, LSCF, SSC, and Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF), which were calculated to be −369, −329, −317, −292, and −273 kJ mol−1, respectively. The largest negative ABE value was observed for SEFC among the electrode materials, suggesting that SEFC has the lowest basicity, and thus the best H2O resistance among the four materials above. Figure S4a shows the XRD patterns of the SEFC−BZCY composite powders of 95:5 wt % (the same ratio of the air electrode used in our cell characterization) before and after being treated at the same condition. In addition, no peaks corresponding to BaCO3 impurity are observed thanks to the very low content of BZCY (5 wt %) in our air electrode, which is in favor of the stability of the air electrode. XRD patterns of BZCY electrolyte pellets sintered at 1350 °C for 5 h before and after treatment at 600 °C for 100 h in 10% H2O−air atmosphere. As shown in Figure S4b, the peaks corresponding to any impurities are not detected in the treated samples, suggesting that the dense electrolyte is kinetically stable in our testing conditions.8,41 2.2. XPS Analysis. Figure S5 shows the XPS full survey spectra of the SEFC powders. Peaks corresponding to europium (Eu 3d at ∼1130 eV), strontium (Sr 3d at ∼134 eV), iron (Fe 2p at ∼710 eV), oxygen (O 1s at ∼530 eV), and Co (Co 2p at ∼780 eV) can be clearly detected and provide useful information about the chemical states of the elements in the near-surface region. The O 1s XPS measurements are

distribution of Fe and Co ions in the slabs cannot be distinguished because of the low content of Co in samples or their random dispersion. These observations agree well with the results of the XRD Rietveld refinements listed in Table S2 and are consistent with the orderly Eu and Sr assignments in Eu2SrCo1.5Mn0.5O7.32,33 The enlarged HADDF images (inset images in Figure 3a,b) also suggest that the Fe/Co atoms are slightly off-centered between the Eu and Sr atoms, with a slightly shorter distance to the Eu layers, causing a slight lattice distortion. The lattice parameters a and c are measured to be approximately 5.48 and 19.83 Å, respectively, and d is 7.74 Å ≈ 2 × a, coinciding well with the XRD refinements. The steric crystal structure of SEFC is shown in Figure S3. Before evaluating SEFC as a potential air electrode material, the chemical compatibility between SEFC and BZCY, the electrolyte, was first considered. Figure 4a shows the XRD pattern of a mixture of SEFC and BZCY (1:1 in weight ratio) heated at 1000 °C for 2 h. It can be seen that both SEFC and BZCY retain their own phase structures, and no additional diffraction peaks were detected, indicating good chemical compatibility between the two materials. Considering the high steam concentration in the air electrode of P-RSOCs, it is important to evaluate the structural stability of SEFC in an atmosphere with a high steam content. Figure 4a shows the XRD patterns of pure SEFC powders before and after heating at 600 °C for 100 h in a 10% H2O−air atmosphere. No peaks corresponding to any impurities are observed, suggesting good structural stability for SEFC. The high stability of SEFC may be related to its low basicity, but the assessment of the basicity of an oxide, especially an R−P oxide, remains a challenge. The average metal−oxygen bond energies (ABE) of metal oxides, estimated from thermodynamic calculations, can serve as a quantitative value to estimate the properties of perovskite-related oxides, such as their basicity 1764


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Figure 5. (a) I−V curves for the RSOC button cell using the SEFC air electrode measured at different temperatures in wet H2 and a 10% H2O−air atmosphere. (b) I−V performance of the cells using the SEF and SEFC air electrodes. (c) I−V and I−P curves of the RSOC button cell using the SEFC cathode measured in the FC mode in a wet H2 and ambient air atmosphere. (d) Temperature dependence of Rp extracted from the impedance data in Figure S9a.

Figure 5a), the electrolysis current densities are −1049 and −2058 mA cm−2 at an applied voltage of 1.5 V measured at 600 and 700 °C, respectively. As shown in Table S1, the cells using the SEFC air electrode demonstrate the best electrolysis current densities, much larger than those using the BaZr0.6Co0.4O3−δ air electrode and those using composite air electrodes. It should be mentioned that Faradaic efficiencies of electrolysis currents of P-RSOCs are not so high, about 20− 40%, as shown in Figure S7, indicating a severe electronic conduction of the electrolyte film.50 Nevertheless, the performance comparison between SEFC and SEF (Figure 5b) and traditional LSCF−BZCY (Figure S8) electrodes can obviously demonstrate that SEFC exhibits much better performance in both the EC and FC modes, considering the uniform anode/ electrolyte fabrication process for cell fabrications. Considering the fact that high steam concentrations may affect the FC performance, wet H2 containing ∼3% H2O and ambient air atmospheres were also used to characterize the electrochemical performances of the RSOFCs, as shown in Figure 5c. Operating at 700 °C, the current density is 700 mA cm−2 at a voltage of 0.7 V, and a peak power density of 562 mW cm−2 can be achieved at a voltage of 0.53 V. Although it is somewhat less than that using Sr3Fe2O7 (683 mW cm−2 at 700 °C),30 this peak power density is still higher than has been observed for many composite cathodes, such as NdBaFe 1.9 Mn 0.1 O 5+δ −BZCY (453 mW cm −2 , 700 °C), 5 1 Nd1.95NiO4+δ−BZCYYb (154 mW cm−2, 750 °C),52 and Pr0.6Sr0.4Cu0.2Fe0.8O3−δ−SDC (546 mW cm−2, 700 °C).53 This result can further certify the effectiveness of SEFC air electrode in the P-RSOC working conditions.

illustrated in Figure 4c,d. O 1s spectra of SEFC and SEF show four XPS binding energy (BE) peaks centered at 528.65, 530.25, 531.2, and 532.3 eV. In general, the peak with the lowest BE can be attributed to the lattice oxygen species O2−, and the ones with the two highest BEs are due to chemisorbed H2O or CO32− and −OH− or O2,42−45 and the peak with a BE of 530.25 eV likely corresponds to reactive oxygen species (for O22−/O−). Compared to those for SrEu2Fe2O7, SEFC clearly shows an increased proportion of O22−/O−, as shown in Figure 4d. The magnified peaks corresponding to the Eu, Sr, Fe, and Co cations are shown in Figure S6 for SEFC and SEF (without Co). From these spectra, the oxidation states of Eu, Sr, and Fe ions are likely to be +3, +2, and +3, respectively, whereas Co shows a +2/+3 state with a low signal-to-noise ratio.46−49 The relatively low oxidation state of Co compared with that of iron may account for the enhanced amount of reactive oxygen species in SEFC, which may accelerate the kinetics of the electrode reaction in RSOCs. 2.3. Electrochemical Performance and Long-Term Stability Tests. P-RSOC button cells with a structure of Ni−BZCY|BZCY|SECF-5 wt % BZCY were fabricated, and their I−V curves, ranging from 0 to 1.6 V, are shown in Figure 5a, with humid hydrogen (∼3% H2O) and 10% H2O−air fed into the hydrogen and the air electrode, respectively. The open circuit voltage (OCV) of the cells measured at 600 °C is 0.99 V, very close to the theoretical electromotive force of 1.05 V calculated from the Nernst equation, indicating that the electrolyte film is very dense. Operating in the FC mode (the right part of Figure 5a), the charging current densities are 169 and 432 mA cm−2 at 0.7 V at 600 and 700 °C, respectively. In the EC mode (the left part of 1765


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Figure 6. (a,b) Nyquist impedance plots for a P-RSOC single cell both in EC (OCV +0.3 V) and FC (OCV −0.3 V) modes under wet H2/10% H2O−air from 700 to 600 °C. For clarity of comparison, the Ohmic resistances have been extracted. (c) Comparisons between the two opposite operating modes at 600 and 700 °C. (d) Temperature dependence of Rp for the RSOC button cell using the SEFC air electrode measured in OCV, EC (+0.3 V), and FC (−0.3 V) modes.

density functional theory could be performed to further investigate the details of the reactions in both FC and EC modes. It should also be noted that this Rp change between the EC and FC modes is opposite when observed in O-RSOCs, where the Rp in the FC mode is smaller than that in the EC mode because of a different ionic conduction mechanism.6 Impedance spectra of symmetric cells using the SEFC air electrode were also provided as shown in the Figure S11 as well. The reproducibility of symmetrical cells and single cells was demonstrated by repeated measurements in the same conditions, as shown in Figure S12. To the best of our knowledge, we obtain the longest reported long-term test in the P-SOEC mode, which demonstrates the high stability of this SEFC air electrode. Operating at 1.3 V in the EC mode, no performance degradation is observed within the 230 h test (in Figure 7a). The microstructures of the button cells using the SEFC air electrode before and after the 230 h long-term test are shown in Figure 8a−f. No obvious particle agglomeration can be observed, indicating good stability of SEFC in the high-steam-content atmosphere and in the EC mode. For a comparison, the electrochemical performance of a button cell using the typical LSCF−BZCY air electrode (7:3 in weight ratio), where BZCY is needed to get high performance,18 was also characterized. As shown in Figure S8, the cell using the LSCF−BZCY air electrode has a smaller electrolysis CD and a larger Rp than the cell using the SEFC−BZCY air electrode measured at 700 °C. Importantly, the button cell using LSCF−BZCY (7:3) breaks down after a 20 h EC operation, suggesting an unsatisfactory long-term stability. This

The Nyquist impedance plots for the P-RSOC button cell using the SEFC air electrode are shown in Figure S9a, measured from 600 to 700 °C under OCV conditions. Three depressed arcs can be observed in these plots, indicating three rate-limiting steps. Figure S9b shows the way to distinguish the overall Ohmic resistance (Rs) and the interfacial Rp. The activation energy for Rp can be calculated as 1.41 eV, as shown in Figure 5d. To intensively investigate the reaction mechanism in different working conditions, the impedance spectra were measured in both EC and FC modes, as shown in Figures 6 and S10. Also, the fitted Rp and Ohmic resistances (Rs) are plotted in Figure 6c,d, respectively, as functions of testing temperatures and operating conditions. It can be seen that those cells have the lowest Rs in the EC mode, which mainly comes from the electronic conduction of the BZCY electrolyte.50 Interestingly, the values of Rs at the FC mode are higher than those at OCV condition, especially at enhanced temperatures. This seems to suggest that BZCY has lower electronic conduction in the FC mode or the electrical conduction of SEFC (a p-type conductor) is depressed in the FC mode because of relatively low O2 partial pressure resulting from O2 consumption. It is worth noting that Rp has similar dependence on the operating modes with Rs. This similarity seems to suggest that electronic conduction in the BZCY electrolyte is one of the main reasons for the different electrode reaction resistances in different operating conditions. Furthermore, the impedance arcs have different shapes and characteristic frequencies in the EC and FC modes, which can be attributed to their different ratelimiting steps, as illustrated in Figure 1. Calculations based on 1766


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result further indicates that a good cathode material for protonconducting SOFCs (P-SOFCs) may not be suitable in PRSOCs because of their different involved atmospheres. The stability of the button cells using the SEFC air electrode was also characterized by switching its operating mode between the FC and EC modes. Applying a voltage of 1.3 and 0.7 V in turn, the CDs of the cell change accordingly, remaining constant at −420 mA cm−2 for 1.3 V and at 120 mA cm−2 for 0.7 V. The Rs and Rp values for the cell measured under OCV conditions after each cycling test are shown in Figure 7b. Within the 135 h test, Rs remains stable, whereas regular fluctuations are observed in Rp. When measured immediately after operating in the EC mode, Rp is somewhat greater than that measured after operating in the FC mode. This can be attributed to the impact of different species adsorbed on the electrode surface in the EC and FC operating modes. In summary, the button cell using the SEFC air electrode demonstrates a stable performance over the 135 h FC-/ECswitching tests. Additionally, the thermal cycling stability of the button cell using the SEFC air electrode was characterized, as shown in Figure 7a. Cycling the operating temperature between 600 and 550 °C, the cell demonstrates stable operation over 13 thermal cycles in a 45 h short-term test at an electrolysis voltage of 1.3 V, indicating good thermal cycling stability. It is worth noting that this is the first time a P-RSOC button cell has been reported to operate well in both FC and EC modes and additionally the first report to record long-term stability.

3. CONCLUSIONS In summary, we demonstrate a novel stable and highperformance air electrode SEFC with the R−P layered structure for P-RSOCs. The special crystal structure of SEFC, where Eu and Sr atoms arrange orderly and respectively in the rock-salt layer, and the perovskite slab greatly promotes its stability in high-steam-involved atmosphere. Operating in the EC mode, the cell using the SEFC air electrode lasts 230 h without degradation, demonstrating an excellent stability. Moreover, applying this SEFC air electrode, our P-RSOC button cell can switch easily between the FC mode and the EC mode with no degradation over the 135 h long-term test. To the best of our

Figure 7. (a) Long-term stability and thermal cycling stability of the PRSOC button cell using the SEFC air electrode, operating at the EC mode under a voltage of 1.3 V. (b) Stability of the current densities (CDs), Ohmic resistance (Rs) and Rp of the button cell over a 135 h reversible cycling test, switching between 0.7 V (FC mode) and 1.3 V (EC mode) measured at 600 °C. The values of Rs and Rp were measured under open circuit conditions right after the EC or FC tests.

Figure 8. Section microstructure of the button cells before (a−c) and after (d−f) the 230 h long-term test with humidified H2 and 10% H2O−air injected into hydrogen and air electrodes, respectively. 1767


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ACS Applied Materials & Interfaces knowledge, this is the first study to report a P-RSOC with good long-term operation switching between the FC and EC modes, highly indicating the importance of a stable air electrode. These results also indicate that the P-RSOC can operate as a highperformance and a stable energy conversion device, which may be useful to build a sustainable and a renewable energy system.

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4. EXPERIMENTAL SECTION

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone/Fax: ++86-551 63600594 (R.P.). *E-mail: [email protected]. Phone/Fax: ++86-551 63603004 (Y.L.).

4.1. Preparation of the SEFC Powders and Single Cells. The SEFC powders were prepared via a combustion process. Stoichiometric amounts of Eu2O3 (99.9%), Sr(NO3)2 (99.5%), Fe(NO3)3· 9H2O (98.5%), and Co(NO3)2·6H2O (99.5%) were dissolved in a dilute nitric acid solution. Citric acid (99.5%) and EDTA (99.5%) were used as a reducing agent and a chelating agent, respectively. The molar ratio of metal cations to citric acid to EDTA was set at 1:1.5:1. After adjusting the pH to 7.0 using ammonia, the solution was heated under stirring until it self-ignited to form brown ashes. The ashes were calcined at 1200 °C for 10 h to obtain good crystallization and to remove carbon residues. SEF, BZCY, LSCF, and NiO powders were all prepared using a similar combustion synthesis procedure.54,55 All chemicals were purchased from Sinopharm Chemical Reagent Co. Ltd. P-RSOC button cells with the SEFC air electrode were fabricated using a modified copressing method, as has been reported previously, with NiO−BZCY as the hydrogen electrode,54 BZCY as the electrolyte, and SEFC-5 wt % BZCY as the air electrode. The air electrode slurry consisting of SEF/SEFC-5% BZCY powders and terpilenol was brush-painted onto the BZCY surface, followed by firing at 1000 °C for 2 h. The effective area of the air electrode was approximately 0.2376 cm2. Here, 5 wt % BZCY was added to increase the binding between the air electrode and the electrolyte. The thickness of the electrolyte film was approximately 15 μm after sintering at 1350 °C for 5 h. 4.2. Characterization of the Powders. XRD patterns were recorded on a Rigaku TTR-III powder X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å), employing a 0.02° step size and a scan rate of 2° min−1 in the range of 10−80°. Rietveld refinements of the XRD patterns were performed with the GSAS program and the EXPGUI interface to determine the atom sites and cell parameters.56 The XRD patterns were applied to investigate the structure evolution of the single phase for SEFC at increasing firing temperatures and the stability of SEFC and BZCY before and after heating at 600 °C for 100 h in a 10% H2O−air atmosphere. Oxygen species in the SEFC and SEF powders were investigated using XPS (Thermo ESCALAB 250). Atomic structures were observed using an aberration corrected scanning transmission electron microscope (JEM-ARM200F, JEOL) equipped with EELS (GIF Quantum 965, Gatan). To evaluate the chemical compatibility between SEFC and BZCY, SEFC−BZCY powders with a 1:1 weight ratio were heated at 1000 °C for 2 h. 4.3. Electrochemical Measurements. Button cells using the SEFC and SEF air electrodes were tested in a homemade cell-testing system with humidified (∼3% H2O) hydrogen and 10% H2O−air fed into the hydrogen electrode and the air electrode, respectively. The flow rates of the gases were both controlled at ∼30 mL min−1. The ac impedance spectra and the electrochemical performances of the cells were investigated using an electrochemistry workstation (IM6e, Zahner). To investigate the Faradaic efficiency of electrolysis, a hydrogen evolution rate was measured using a mass flowmeter and an online gas chromatograph (FULI, GC9790II). The fracture microstructures of the button cells using the SEFC air electrode after testing were characterized using scanning electron microscopy (SEM, JSM6700F). The hydrogen evolution rate was measured using a mass flowmeter and online gas chromatograph (FULI, GC9790II).

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Additional experimental results of XRD Rietveld refinement, HAADF-STEM image, 3D crystal structure, XPS spectra, stability analysis, I−V, Faradaic efficiency, and EIS analysis (PDF)

ORCID

Changrong Xia: 0000-0002-4254-1425 Ranran Peng: 0000-0003-4558-6757 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the Natural Science Foundation of China (51472228), the National Key Research and Development Program of China (2016YFA0401004), the External Cooperation Program of BIC, the Chinese Academy of Sciences (211134KYSB20130017), Hefei Science Center CAS (2016HSC-IU004), and the Fundamental Research Funds for the Central Universities (WK3430000004).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b16703. 1768


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New, Efficient, and Reliable Air Electrode Material for Proton

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