A New Efficient and Reliable Air Electrode Material for Proton

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A New Efficient and Reliable Air Electrode Material for Proton Conducting Reversible Solid Oxide Cells Daoming Huan, Nai Shi, Lu Zhang, Wenzhou Tan, Yun Xie, Wanhua Wang, Changrong Xia, Ranran Peng, and Yalin Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16703 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 29, 2017

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ACS Applied Materials & Interfaces

A New Efficient and Reliable Air Electrode Material for Proton Conducting Reversible Solid Oxide Cells Daoming Huana, Nai Shia, Lu Zhanga, Wenzhou Tana, Yun Xiea, Wanhua Wanga, Changrong Xiaa, Ranran Peng*a,b,c & Yalin Lu*a,b,c,d a

CAS Key Laboratory of Materials for Energy Conversion, Department of Materials

Science and Engineering, University of Science and Technology of China, Hefei, 230026 Anhui, China. b

Synergetic Innovation Center of Quantum Information & Quantum Physics,

University of Science and Technology of China, Hefei, Anhui 230026, China. c

Hefei National Laboratory of Physical Science at the Microscale, University of

Science and Technology of China, Hefei, 230026 Anhui, China. d

National Synchrotron Radiation Laboratory, University of Science and Technology

of China, Hefei 230026, P. R. China. * Corresponding author: E-mail: [email protected]

[email protected]

KEYWORDS: proton-conducting reversible solid oxide cells, air electrode, novel Ruddlesden-Popper oxide, electrochemical property, long-term stability

ABSTRACT: Driven by the demand to minimize fluctuation in common renewable energies, reversible solid oxide cells (RSOCs) have drawn increasing attention for that they can operate either as fuel cells to produce electricity or as electrolysis cells to store electricity. Unfortunately, development of proton-conducting RSOCs (P-RSOCs) faces a major challenge of poor reliability due to the high content of steam involved in air electrode reactions, which could seriously decay air electrode materials' lifetime. 1

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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. XRD analysis and HAADF-STEM images of SEFC reveal that Sr atoms occupy the center of perovskite slabs, whlie Eu atoms arrange orderly in the Rock-salt layer. Such special structure of SEFC largely depresses its Lewis bacisity, and therefore, its reactivity with steam. Applying the SEFC air electrode, our button switches smoothly between both FC and EC modes with no obvious degradation over a 135-hour long-term test under wet H2 (~3% H2O) and 10% H2O-air atmospheres. A record of over 230 hours is achieved in the long-term stability test in EC mode, doubling the longest test that had been previously reported. Besides good stability, SEFC demonstrates great catalytic activity towards air electrode reaction when compared with traditional LSCF-BZCY air electrodes. This research highlights the potential of stable and efficient P-RSOCs as important part in a sustainable new energy power system. 1. INTRODUCTION 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 in order 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 2


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electricity and as a electrolysis cell (EC mode) to store electricity power in chemical fuels, offering a great convenience for the 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, 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 (IT, 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 EC mode) or generated (in 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

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Figure 1. (a) The 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) air electrode chamber. (b) schematic of the reaction at the single phase air electrode for P-RSOCs in FC and EC modes.

It is well known that the polarization resistance of P-RSOCs 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: 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. Since 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 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(Zr0.1Ce0.7Y0.2)O3-δ (BZCY),17 La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF)- BZCY,18 and some new single phase perovskite oxides as potential triple conductors, such as BaCo0.4Fe0.4Zr0.1Y0.1O3-δ (BCFZY0.1),12 and BaZr0.6Co0.4O3 (BZC),21 have been proposed to achieve this goal, and fairly acceptable peak power densities were achieved in those cells using the above air electrodes.

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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 due to the localized formation of oxygen molecules in this reaction. Meanwhile, the injected steam reactant (≥10%), which can regarded as a Lewis acid, is prone to react with the above-mentioned composite or single-phase air electrodes (mainly Lewis bases), leading to deterioration of the used materials and the subsequent decay in cell performance.22-24 As summarized in Table S1, working in the EC mode the span life of majority of the past investigated P-RSOCs is usually a few hours, only the latest air electrode Sr2Fe1.5Mo0.5O6-δ- BaZr0.8Y0.2 (6:4) last 100 hours 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 n-layered 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 5



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 P-RSOCs, as illustrated in Figure 2a. In this work, crystal structure, chemical stablity, 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.

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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.

2. RESULTS AND DISCUSSION 2.1 Crystal structure and stability of SEFC 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 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 500-1000 °C, during which SrFeO3-x and Eu2O3 form first and then react to form SrEu2Fe2O7 gradually and totally. A Rietveld refinement of the XRD data for 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 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

microscope (STEM) images of the SEFC powders with both (0 1 0) and (1 -1 0) 7



crystal faces are shown in Figure 3a and 3b, respectively. All the atomic spots in the images are regularly arranged and can be divided into three types according to their colour and size: bright white spots, big grey spots and small grey spots. Since 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 grey and the small grey 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 centre of the perovskite slabs, while 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 analysed using electron energy-loss spectroscopy (EELS) as shown in Figure 3c and Figure S2. EELS clearly reveals that the Eu and Sr ions occupy different sites, the former in the rock salt layers, the latter in the centre of perovskite slabs. Meanwhile, the individual distribution of Fe and Co ions in the slabs can not be distinguished due to 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 and 3b also suggests that the Fe/Co atoms are slightly off centred 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, 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. 8


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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).

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 hours. 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 high steam content. Figure 4a shows the XRD patterns of pure SEFC powders before and after heating at 600 °C for 100 hours in a 10% H2O-air atmosphere. No peaks 9



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 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 high steam content, where steam reacts as a Lewis acid such as CO2. The calculation details are shown in the Electronic Supplementary Information. Figure 4b shows the ABE values of BaZr0.8Y0.2O3-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 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.

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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 hours, and of SEFC-BZCY mixtures (blue line, 1:1 in weight ratio) calcined at 1000 °C for 2 h. (b) Average metal–oxygen bond energies (ABEs) for the BZY, SEFC, LSCF, SSC and BSCF air electrodes. XPS spectra (c) and proportion (with error bars) (d) of O 1s species on the surface of the SEFC and SEF samples.

Figure S4a shows the XRD patterns of SEFC-BZCY composite powders of 95:5 wt.% (the same ratio of the air electrode used in our cell characterization) before and after treated at the same condition. And no peaks corresponding to BaCO3 impurity are observed thanks to the very low content of the BZCY (5 wt.%) content in our air electrode, which is infavor of the stability of air electrode. XRD patterns of BZCY electrolyte pellets sintered at 1350 oC for 5 h before and after treated at 600 °C for 11



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 illustrated in Figures 4c and 4d. O 1s spectra of SEFC and SEF show four XPS binding energy (BE) peaks centred at 528.65, 530.25, 531.2 and 532.3 eV. In general, the peak with the lowest binding energy can be attributed to the lattice oxygen species O2-, the ones with the two highest binding energies are due to chemisorbed H2O or CO32- and -OHor O2,42-45 and the peak with a binding energy 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, while Co shows a +2/+3 state with a low signal-to-noise ratio.46-49 The relatively low oxidation state of the Co compared with that of iron may account for the enhanced amount of reactive oxygen

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species in SEFC, which may accelerate the kinetics of the electrode reaction in RSOCs. 2.3 Electrochemical performance and long-term stability test 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 voltages (OCVs) of the cells measured at 600 °C are 0.99 V, very close to the theoretical electromotive force (EMF) of 1.05 V calculated from the Nernst equation, indicating that the electrolyte film is very dense.

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 13



curves of the RSOC button cell using the SEFC cathode measured in FC mode in a wet H2 and ambient air atmosphere. (d) Temperature dependence of Rp extracted from the impedance data in Figure S9a.

Operating in FC mode (the right part of Figure 5a), the charging current densities are 169 and 432 mAcm-2 at 0.7 V at 600 and 700 °C, respectively. In EC mode (the left part of Figure 5a), the electrolysis current densities are -1049 and -2058 mAcm-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 that the one that uses 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 conductution of 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 mAcm-2 at a voltage of 0.7 V, and a peak power density of 562 mWcm-2 can be achieved at a voltage of 0.53 V. Although 14


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it is somewhat less than that using Sr3Fe2O7 (683 mWcm-2 at 700 °C),30 this peak power density is still higher than has been observed for many composite cathodes, such as NdBaFe1.9Mn0.1O5+δ- BZCY (453 mWcm-2, 700 °C),51 Nd1.95NiO4 + δBZCYYb (154 mWcm-2, 750 °C),52 and Pr0.6Sr0.4Cu0.2Fe0.8O3−δ- SDC (546 mWcm-2, 700 °C).53 This result can further certify the effectiveness of SEFC air electrode in the P-RSOC working conditions.. 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 polarization resistance (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 condition, the impedance spectra were measured in both EC and FC modes, as shown in Figure 6 and Figure S10. And the fitted polarization resistances (Rp) and bulk resistances (Rs) are plotted in Figure 6c and 6d, respectively, as functions of testing temperatures and operating conditions. It can be seen those cells have the lowest Rs in EC mode, which mainly comes from the electronic conduction of BZCY electrolyte.50 Interestingly, values of Rs at FC mode are higher than at OCV condition, especially at enhanced temperatures. This seems to suggest that BZCY has lower electronic conduction in FC mode, or electrical conduction of SEFC (a p-type conductor) is depressed in FC mode because of relatively low O2 partial pressure resulting from O2 consumption. It is 15



worth noting that polarization resistances (Rp) has similar dependence on the operating modes with Rs. This similarity seems to suggest that electronic conduction in 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 EC and FC modes, which can be attributed to their different rate-limiting steps, as illustrated in Figure 1. Calculations based on 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 the opposite when of that observed in O-RSOCs, where the Rp in FC mode is smaller than that in EC mode due to a different ionic conduction mechanism6. 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.

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Figure 6. (a) and (b) show 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) exhibits the comparisons between the two opposite operating modes at 600 and 700 °C, respectively. (d) Temperature dependence of Rp for RSOC button cell using the SEFC air electrode measured in OCV, EC (+0.3 V) and FC (-0.3 V) modes.

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Figure 7. (a) Long-term stability and thermal cycling stability of the P-RSOC button cell using the SEFC air electrode, operating at EC mode under a voltage of 1.3 V. (b) 18


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Stability of the current densities (CD), ohmic resistance (Rs) and polarization resistance (Rp) of the button cell over a 135-hour 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 test.

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 EC mode, no performance degradation is observed within the 230 hours' test (in Figure 7a). The microstructures of the button cells using the SEFC air electrode before and after the 230-hour long-term test are shown in Figure 8a-8f. No obvious particle agglomeration can be observed, indicating good stability of SEFC in the high-steam-content atmosphere and in electrolysis mode.

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

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For a comparison, the electrochemical performance of a button cell using the typical LSCF-BZCY air electrode (7:3 in weight ratio), where BZCY are 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 current density and a larger polarization resistance than that of the cell using the SEFC-BZCY air electrode measured at 700 °C. Importantly, the button cell using the LSCF-BZCY (7:3) breaks down after a 20-hour EC operation suggesting a unsatisfactory long-term stability. This result further indicates that a good cathode material for proton conducting SOFCs (P-SOFCs) may not be suitable in P-RSOCs due to 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 V and 0.7 V in turn, the current densities of the cell change accordingly, remaining constant at -420 mA cm-2 for 1.3 V and at 120 mAcm-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-hour test, Rs remains stable, while regular fluctuations are observed in Rp. When measured immediately after operating in EC mode, Rp is somewhat greater than that measured after operating in 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-hour FC/EC switching test.

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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 thirteen thermal cycles in a 45-hour 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 of a record long-term stability. 3. CONCLUSIONS In summary, we demonstrate a novel stable and high-performance 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 EC mode, the cell using SEFC air electrode lasts 230 hours without degradation, demonstrating an excellent stability. Moreover, applying this SEFC air electrode, our P-RSOC button cell can switch easily between FC mode and EC mode with no degradation over the 135-hour long-term test. To the best of our knowledge, this is the first time to report a P-RSOC with good long-term operation switching between FC and EC modes, highly indicating the importance of a stable air electrode. These results also indicate that P-RSOC can operate as a high performance and stable energy conversion device, which may be useful to build a sustainable and renewable energy system. 21



4. EXPERIMENTAL SECTION 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. SrEu2Fe2O7 (SEF), BaZr0.3Ce0.5Y0.2O3-δ (BZCY), La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and NiO powders were all prepared using a similar combustion synthesis procedure.54,55 All chemicals were purchased from the Sinopharm Chemical Reagent Co. Ltd. P-RSOC button cells with the SEFC air electrode were fabricated using a modified co-pressing 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, respectively. 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 2h. 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. 22


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4.2 Characterization of the powders X-ray diffraction (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 X-ray photon spectroscopy (XPS, Thermo ESCALAB 250). Atomic structures were observed using an aberration corrected scanning transmission electron microscope (STEM) (JEM-ARM200F, JEOL) equipped with electron energy-loss spectroscopy (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 A.C. impedance spectra and the electrochemical performances of the cells were investigated using an electrochemistry workstation (IM6e, Zahner). To investigate the Faradaic efficiency of electrolysis, 23



hydrogen evolution rate was measured using the a mass flowmeter and an online gas chromatograph (FULI, GC9790II). The fracture microstructures of the button cells using SEFC air electrode after testing were characterized using scanning electron microscopy (SEM, JSM-6700F). The hydrogen evolution rate was measured using a mass flowmeter and online gas chromatograph (FULI, GC9790II).

ASSOCIATED CONTENT Supporting Information Additional experimental results of XRD Rietveld refinement, HAADF-STEM image, 3D crystal structure, XPS spectra, Stability analysis, I-V, Faradaic efficiency and EIS analysis available.

AUTHOR INFORMATION Corresponding Authors Email: [email protected] ,Tel&Fax: ++86-551 63600594 Email: [email protected] ,Tel&Fax: ++86-551 63603004 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS

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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, 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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A New Efficient and Reliable Air Electrode Material for Proton

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