Synergistic Coupling of Proton-Conductors BaZr0.1Ce0.7Y0.1Yb0

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Energy, Environmental, and Catalysis Applications

Synergistic Coupling of Proton-Conductors BaZr0.1Ce0.7Y0.1Yb0.1O3# and La2Ce2O7 to Create Chemical Stable, Interface Active Electrolyte for Steam Electrolysis Cells Wenyuan Li, Bo Guan, Liang Ma, Hanchen Tian, and Xingbo Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00303 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 4, 2019

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ACS Applied Materials & Interfaces

Synergistic Coupling of Proton-Conductors BaZr0.1Ce0.7Y0.1Yb0.1O3-δ and La2Ce2O7 to Create Chemical Stable, Interface Active Electrolyte for Steam Electrolysis Cells Wenyuan Lia, Bo Guana, Liang Mab, Hanchen Tiana, Xingbo Liua, aMechanical

& Aerospace Engineering Department, Benjamin M. Statler College of Engineering &

Mineral Resources, West Virginia University, Morgantown, WV 26506, USA bSchool

of Materials Science and Engineering, Hebei University of Engineering, Handan, 056038, China Abstract

For the first time, proton conductors BaZr0.1Ce0.7Y0.1Yb0.1O3-δ (BZCYYb) and La2Ce2O7 (LCO) are combined to create an interface active and steam-tolerant electrolyte for high-performance protonconducting solid oxide electrolysis cells. LCO shows good chemical compatibility with BZCYYb. The readily fabricated LCO/BZCYYb bilayer electrolyte can be densified at a temperature as low as 1300oC vs. ~1600oC for the benchmark steam-stable BaZr0.8Y0.2O3-δ electrolyte. With Pr2NiO4+δ as the anode and Ni as the cathode catalyst, this bilayer electrolyte cell yields a current density of 975 mA/cm2 and 300 mA/cm2 under a 1.3 V applied potential at 700oC and 600oC, respectively. This performance is among the best of all H-SOECs equipped with a chemically stable electrolyte so far. BZCYYb layer in the bilayer electrolyte promotes the hydrogen evolution reaction at the cathode side, resulting in a 108% improvement over the cell without this layer. The LCO layer, on the other hand, effectively protects this functional BZCYYb layer from high concentration of steam in a practical SOEC operation condition. The cell without LCO layer shows degradation in terms of an increased electrolyzing potential from 1.07 V to 1.29 V during a constant 400 mA/cm2 operation at 700oC. In contrast, the bilayer electrolyte cell maintains the same electrolyzing potential of 1.13 V under the same conduction for a 102 h operation. These findings demonstrate that this synergic bilayer electrolyte design is a vital strategy to overcome the dilemma between performance and stability faced by the current benchmark Zr- or Cerich Ba(CeZr)O3-δ electrolysis cells, achieving excellent performance and stability at the same time. Keywords: steam electrolysis, proton-conductors, triple-conducting, BZCYYb stability, La2Ce2O7, barrier layer 1. Introduction Proton-conducting solid oxide electrolysis cells (H-SOECs) have been attracting increasing attention recently in high-temperature electrolysis investigations.1-4 Due to the low activation energy for ion conduction, H-SOECs can be operated at a temperature 100-150oC lower than that of the state-of-theart oxygen-conducting yttrium-stabilized zirconia (YSZ)-based SOECs without compromising the 

Corresponding author, E-mail address: [email protected]

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performance.5-7 This lowered operating temperature brings many benefits to the SOEC system, including less demanding sealing and interconnecting requirement, slower thermo-activated degradation, less operation and maintenance costs.8 Moreover, since the proton-conducting mechanism, dry and pure H2 is produced at the cathode side of the H-SOECs. This unique feature allows for the use of Ni as cathode catalyst without concerns of Ni being oxidized by the high concentration of steam as in the O-SOECs. In recent years, exciting progress has been made in the materials development of HSOECs. For instance, Wu et al invented a 3D self-Architectured PrBa0.5Sr0.5Co2-xFexO5+δ steam electrode, achieving a 2.02 A/cm2 current density at 1.6 V 600oC.9 Huan et al. reported a new steam electrode material, SrEu2Fe1.8Co0.2O7-δ, achieving a ~1 A/cm2 current density at 1.3 V 700oC on the Ba(Zr0.1Ce0.7Y0.2)O3-δ electrolyte.10 Li et al. exploited layer-structured Pr2NiO4+δ (PNO) as steam electrode on the BaZr0.2Ce0.6Y0.2O3-δ electrolyte, yielding a 0.977 A/cm2 current density at 1.3 V 700oC.3 Marrony et al. obtained a 0.5 A/cm2 current density at 1.3 V 700oC on a Ni//BaCe0.8Zr0.1Y0.1O34 δ//Ba0.5Sr0.5Co0.8Fe0.2O3-δ electrolysis cell. Of those advanced H-SOECs, perovskite structure BaCeO3based proton-conductors are the most popular electrolyte. This family exhibit excellent conductivity and good compatibility with Ni cathode. Unfortunately, it suffers from the susceptibility towards steam, which is predicted by the thermodynamic calculation and also confirmed in experimental results.11, 12 Substitution of Zr for Ce is a commonly used strategy. However, Zr-rich recipes usually show conductivities 5-10 times lower than those of the Ce-rich ones.13, 14 In addition, Zr-rich compounds require an extremely high temperature to densify, e.g. 1600oC.15 Alternative proton-conducting electrolyte materials have been developed by co-workers in hopes of achieving a good trade-off between the chemical stability, ionic conductivity and processability,16 such as scheelite-type LaNbO4,17 rareearth tungstate Ln6WO12,18 fluorite La2Ce2O7 and perovskite LaYbO3-based materials. Among them, La2Ce2O7 is found a promising candidate, showing fairly high proton conductivity and excellent stability towards steam.19 Applications of LCO in relevant proton exchange ceramic membranes and solid oxide fuel cells (SOFCs) technologies have been reported.20, 21 But not surprisingly, the electrochemical cells using LCO as electrolyte are still not a match to the doped BaCeO3 electrolytebased cells, due to the superior proton-conductivity of BaCeO3 family.22 New strategies are needed to largely retain the high ionic conductivity and overcome the instability of the electrolyte for a better performance. Our previous study has revealed that in solid oxide cells, the electrolyte not only determines the resistance of the ion transport process through this dense layer, i.e. the ohmic resistance, but also heavily affects the kinetics of the electrode reaction taking place primarily at the electrolyte/electrode interface.23 This phenomenon results from the fact that a highly conducting electrolyte, such as BZCYYb, would lead to a higher exchange current density at the electrolyte/electrode interface under the equilibrium state, which will give rise to a faster charge transfer process and also a broadened surface reaction area when overpotential is applied during operation. Inspired by these closely related materials and reaction kinetics findings, in this study, we synergistically couple the state-of-the-art proton conductor BZCYYb with the newly-emerged robust LCO to create a stable and active bilayer electrolyte for high-performance stable H-SOECs. LCO functions as a protective layer of BZCYYb from decomposing in the high concentration of steam at the anode side. BZCYYb boosts the relatively sluggish hydrogen evolution reaction at the LCO electrolyte/Ni cathode


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interface. This H-SOEC with LCO/BZCYYb bilayer electrolyte is proven easy to fabricate, one of the best electrolyzing performances, 975 mA/cm2 at 1.3 V at 700oC, and excellently stable in a ~100 h operation. These findings represent a significant technical leap from the current low-performance unary electrolyte configuration to the future’s highly performing and stable binary electrolyte design. 2. Experimental Both La2Ce2O7 and BaZr0.1Ce0.7Y0.1Yb0.1O3-δ electrolyte powders were synthesized by the ethylenediaminetetraacetic (EDTA)-citric sol-gel method.24, 25 Stoichiometric nitrates together with citric acid were dissolved into distilled water. EDTA was dissolved into diluted ammonia water. The mole ratio for metal cation:citric acid:EDTA was set to 1:1.5:1. The nitrate and EDTA solutions were mixed together in a glass beaker, followed by adjusting pH value to 8~10 using ammonia water. Afterward, the solution was held at ~80oC on a heating plate and magnetically stirred until gelation. The gel was heated at 500oC for 12 h to decompose the nitrates and organic residual. The resultant LCO powders were calcined in air at 850oC for 4 h, BZCYYb powders in air at 1400oC for 4 h. Calcined powders were ball-milled in a planetary miller for 12 h to decrease the particle size. The phase purity was examined by X-ray diffraction (XRD, PANalytical X’pert PRO, Cu Kα radiation). To confirm the compatibility between each component, LCO and BZCYYb, Pr2NiO4+δ and LCO were thoroughly mixed at a 1:1 wt. ratio in a mortar and calcined at different temperatures. XRD was conducted to examine phase stability after these treatments. All full cells adopted a NiO-BZCYYb cathode-supported structure with electrolyte varied. Cathode powders of NiO:BZCYYb:flour (6:4:2 wt. ratio) were mixed thoroughly. BZCYYb electrolyte slurry was made by blending 4 mol.% Zn(NO3)2 sintering aid and an ink vehicle (Fuel Cell Materials Co.) with electrolyte powders (powder: vehicle = 4:5 wt.), then grinding in a mortar until homogeneous electrolyte slurry was formed. LCO electrolyte slurry was made in the same way except that no sintering aid was added. 0.7 g cathode powders were pressed into ~1 mm thick pellets at 100 MPa in a 16 mm die. Cathode functional layer consisting of NiO:BZCYYb=6:4 wt. in slurry was spin-coated first to the pellets. Afterward, these pellets were spin-coated with two cycles of BZCYYb electrolyte slurry only, or one cycle BZCYYb slurry then one cycle of LCO slurry, or two cycles of LCO slurry only, yielding three kinds of cells different in electrolyte configuration. To increase the packing density of the electrolyte layer for an improved sinterability, after dried at 120oC for 30 min, the cathode/electrolyte assemblies were pressed to 300 MPa.26 The green pellets were heated to 250oC at a 2oC/min ramp and held for 2 h to burn out the flour slowly in a box furnace in stagnant air. A 2 g BZCYYb dense pellet as weight was placed over each sample to prevent warping during sintering. The sintering temperature was set to increase to 1300oC at a 3oC/min ramp and dwell for 4 h, decrease to 1100oC at a 2oC/min ramp and dwell for 0.5 h, cool down to 800oC at a 3.5oC/min ramp and finally cool down naturally to room temperature. Single phase PNO anode slurry was made via blending PNO powders and ink vehicle at powders:vehicle=5:4 wt., then grinding into homogeneous slurry in a mortar. PNO was screen-printed to the electrolyte surface with an area of 0.5 cm2, dried in an oven at 200oC for 1h, followed by sintering in a box furnace. The temperature was programmed to increase to 1150oC at a 3oC/min ramp, dwell for 3 h, cool down to 800oC at the same rate, then cool down to room temperature naturally. Ag paste was



applied to the anode and cathode as current collector. Ag wire was used as lead wire. Single cell was sealed to one end of a 15mm OD alumina tube by a high-temperature ceramic bond (Ceramabond 552, Aremco Products Inc.) for performance characterization. During testing, dry H2 was fed to the cathode side, and 60 vol.% water steam-containing air to the anode side. Electrochemical measurements including I-V and electrochemical impedance spectroscopy (EIS) were carried out using Metrohm Autolab b.v. Impedance was collected over a frequency range from 0.1 Hz to 1 MHz with an AC perturbation of 10 mV. The resulting impedance spectra were deconvoluted using Z-view software.27, 28 Constant current electrolyzing was carried out by using B&K PRECISION 8500 as the current source and Keithley 2400 SourceMeter as voltage measurement. The hydrogen production rate was measured by gas-water displacement method. During measurement, a small dry H2 stream was flown through the mass flow controller to compensate for tiny gaseous leakage throughout the system and e, H+ bipolartransport leakage through the electrolyte.29 The cathode chamber outlet was led to a water-filled graduated cylinder through a 1/8 inch OD Teflon tube. The cylinder was inversely hung over the water tank. A single bubble from the Teflon tube displaced ~0.05 mL water. A specific electrolyzing current was not applied until the gas sealed in the Teflon tube was steadily still. Once the electrolyzing potential was applied, the time elapsed to reach every integer reading was recorded. The microstructure of samples was examined by scanning electron microscopy (SEM, Hitachi S-4700). 3. Results and Discussion

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Figure 1. (a) Phase structure of PNO, LCO and BZCYYb, and chemical compatibility between each component, (b) Peak-shift and formation of new phases due to the diffusion of La from LCO to PNO, (c) cross-section of stand-alone LCO pellet sintered at 1300oC, (d) LCO/BZCYYb bilayer electrolyte on NiO-BZCYYb cathode substrate, (e) fully-densified LCO/BZCYYb bilayer. Figure 1a shows the XRD results of the as-prepared powders and chemical compatibility between each component. Single phase is confirmed for LCO.21, 30 Despite the fact that 50% Ce cation is replaced by La, this compound still exhibits the disordered defect fluorite structure as CeO2-δ. The pyrochlore phase has not been observed in this compound, as the fingerprint (311), (331) and (511) peaks that distinguish the pyrochlore structure from fluorite structure have not been found.31, 32 No foreign peaks are observed when BZCYYb and LCO powders were sintered together at 1300oC for 4 h, confirming good chemical compatibility between these electrolyte materials. When LCO and PNO were heated together at 1150oC for 3 h, peak shift in LCO and tiny foreign peak for new phases are found. Figure 1b shows the enlarged 2θ range 25~34o. Due to the loss of La from LCO (rLa3+=1.16 Å, rCe4+=0.97 Å) to PNO, the lattice parameter for this cubic structure decreases from 5.578 Å to 5.539 Å, accompanied by an upward angle shift, e.g. (111) peak from 27.645o for pure LCO to 27.845o for LCO in the sintered LCO/PNO mixture. By comparing this pattern with those of CeO2-δ doped with La2O3 of different levels,33 the real stoichiometry of LCO in this LCO/PNO mixture is found close to La1.6Ce2.4O7+δ (~27.8o for La1.6Ce2.4O7+δ). La from LCO substitutes for Pr in PNO, forming R-P phase La2NiO4+δ. Meanwhile, Pr was squeezed out to form praseodymium oxide, Pr6O11. Their major peaks have been marked in Figure 1b. The interaction between LCO and PNO can be described as in Eq. 1. 2 6La 2 Ce 2 O7 +2Pr2 NiO 4+δ  xO 2   5La1.6 Ce 2.4 O7+δ +2La 2 NiO 4+δ + Pr6 O11 3

(1)

As neither the major peak for La2NiO4+δ nor that for Pr6O11 is significant compared to that of PNO or LCO, the portion of those materials in the sintered PNO cathode would be small. It has been previously proven that PNO is an excellent anode material for H-SOEC on Ce-rich doped BaCeO3-δ electrolyte.3 But violent reaction was found when Zr-rich recipe, BaZr0.8Y0.2O3-δ was used, which rules out PNO as an anode material for the BZY-based steam-tolerant H-SOEC (see Figure S1, chemical compatibility between PNO and BZY by XRD). Herein, the observed mild reaction between LCO and



PNO constrains the possible detrimental effect of the foreign phases, and allows PNO still to be used as an excellent anode material. Figure 1c shows the cross-section of a 0.5 mm thick LCO pellet pressed at 300 MPa and sintered at 1300oC for 4 h. Holes manifest in the whole pellet. LCO stand-alone cannot be densified under this condition due to its poor sinterability.20 Figure 1d shows the cross section of LCO/BZCYYb bilayer electrolyte deposited on a porous NiO-BZCYYb cathode substrate. The linear shrinkage of the LCO stand-alone pellet is only 7.1%. This value increases to 17.0% for the Ni-BZCYYb cathode. The compressive stress from the thick NiO-BZCYYb substrate underneath aids the in-plane shrinkage of the LCO layer during sintering. A dense bilayer LCO/BZCYYb electrolyte was successfully obtained. A similar phenomenon has been well-documented by Hyegsoon et al.34 In Figure 1e, it can be clearly seen that the bilayer electrolyte has been fully densified. No mismatch, crack or delamination has been observed. Figure 2 shows the performance of electrolysis cells with BZCYYb, LCO or LCO/BZCYYb bilayer electrolyte from 550 to 700oC. The performance of the cells can be ranked in the order of BZCYYb>LCO/BZCYYb>LCO according to the electrolyzing current under the applied potential of 1.3 V. At 700oC 1.3V, the electrolyzing current is 975 mA/cm2 for LCO/BZCYYb cell, and 748 mA/cm2 for LCO cell, which represents a 30% improvement upon using this bilayer electrolyte structure. Those cells possess exactly the same composition except for the electrolyte. To reveal the effect of the electrolyte, EIS results have been analyzed. It is confirmed that the electrolyte not only affect the ohmic resistance, e.g. 0.42 Ωcm2, 0.80 Ωcm2 and 0.67 Ωcm2 for BZCYYb, LCO, LCO/BZCYYb electrolytebased cells (~15 µm thick electrolyte for all sample), but also the polarization resistance (Rp), e.g. 1.58 Ωcm2, 2.90 Ωcm2 and 2.05 Ωcm2 for BZCYYb, LCO, LCO/BZCYYb cells at 550oC, respectively. The increased ohmic resistance after replacing BZCYYb with LCO is readily understandable considering the lower ionic conductivity of the latter relative to the former (see Figure S2, the electrical conductivities of LCO and BZCYYb).20, 22 The varied Rp, however, could stem from the reaction kinetics change at anodic and/or cathodic interface. To disclose the real cause behind this improvement, the EIS results were fitted into an R(R/CPE)(R/CPE) equivalent circuit,35 with one high-frequency (HF) and one low-frequency (LF) arc. It was found that the HF process primarily corresponded to the hydrogen evolution reaction (HER) at the Ni cathode, and LF process to the oxygen evolution reaction (OER) at the PNO anode (see Figure S3, devolution of HF, LF arc dependency on varying steam partial pressure and hydrogen partial pressure).


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Theoretical value PNO on BZCYYb PNO on LCO PNO on LCO/BZCYYb bilayer

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Figure 3. (a) Fitting results of EIS arcs for three cells at OCV condition, upper-left inset shows the equivalent circuit, upper-right inset shows the fitted area-specific resistances for HF and LF arcs, (b) OCV of these cathode-supported cells and the corresponding transference number. Figure 3a shows the fitting results of these three cells. The equivalent circuit used is also presented. This circuit yields good consistency between the measured and fitting results. The inset displays the resultant deconvoluted resistances for OER at the anode side and HER at the cathode side. The OER resistances for the LCO (1.23 Ωcm2) and LCO/BZCYYb cell (1.25 Ωcm2) are almost the same, and slightly higher than that of the BZCYYb cell (1.03 Ωcm2) at 550oC. But the HER resistance at the cathode side is significantly decreased in this bilayer electrolyte cell compared to the LCO cell, 0.80 Ωcm2 for the former and 1.66 Ωcm2 for the latter, which represents a more than 100% improvement. This remarked enhancement of the HER process clearly demonstrates that the functional BZCYYb layer effectively decreased the electrode resistance, and justifies the rationality of this synergic bilayer electrolyte structure design. Figure 3b shows the OCV of these three cells and the calculated ion transference number. The theoretical OCV at each condition is also provided for reference. The OCV of BZCYYb cell is slightly higher than the other two. The difference in OCVs could possibly be affected by random pinholes which is difficult to entirely eliminate in the thin layer electrolytes. But it should be largely attributed to a relatively lower ion transference number of LCO compared to BZCYYb, which is evidenced by the faradaic efficiency measurement as will be shown later. Table 1. Comparison of performance between most recent H-SOECs featured with chemically stable electrolytes.

Sm0.5Sr0.5CoO3-δ -BCZY//BaCe0.5Zr0.3Y0.2O3−δ (20 μm)//Ni-BCZY (LaSr)CoO3-δ-BZCYbCo//BaCe0.48Zr0.4Yb0.1Co0.02O3−δ (45 μm)//Ni-BCZYbCo

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La2NiO4+δ-BaCe0.2Zr0.7Y0.1O3−δ//BCZY27(25μm)//Ni-BCZY27 La2NiO4+δ-BaCe0.5Zr0.3Dy0.2O3−δ//BCZD(30μm)//Ni-BCZD

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Table 1 summarizes the performance of the most recently investigated H-SOECs featured with a chemically stable electrolyte. To improve the stability, instead of using a buffer layer to protect the highly active doped-BaCeO3 electrolyte, traditional approach like the high-level substitution of Zr for Ce was generally adopted by co-workers.38 This strategy suffers from higher loss from big ohmic resistance and also largely lowered interface activity.42 Compared to these cells, the performance of the present bilayer LCO/BZCYYb cell is among the best under similar electrolyzing conditions. o

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Figure 4. (a) Faradaic efficiency under different current densities at 700oC, (b) the stability of electrolysis cells with different electrolytes. Figure 4a show the faraday efficiency (FE) of these three cells measured under different current densities at 700oC by gas-water displacement method (see Figure S4, H2 production rates for these samples). FE for all samples decreases with increasing current density. Among them, BZCYYb electrolyte exhibits the highest efficiency, LCO the lowest. Since there is no oxygen source in the cathode compartment, the current through the electrolyte can only be carried by protons and electrons/holes. The departure of FE from unit value is due to the electronic current leaking in the electrolytes. Like other doped ceria,43 LCO could experience a significant increase in the electronic conductivity upon reduced by high equivalent H2 pressure inside the electrolyte.44, 45 From 0.2A/cm2 to 1A/cm2, FE for LCO cell drops from 54% to 12%, meaning the electronic current leaking starts to dominate this electrolysis process. This leakage is effectively blocked by the BZCYYb layer in the bilayer electrolyte cell. The most reducing environment in the H-SOECs is the cathode/electrolyte interface.44 The equivalent H2 partial pressure inside the electrolyte drops abruptly along the depth into the electrolyte, and reverses to oxidizing environment near the anode/electrolyte interface. BZCYYb layer placed at the cathode side buffers LCO from the most reducing region, preventing the rise of electronic conductivity. FE increases from 24% for LCO cell to 51% for the bilayer cell at 0.4 A/cm2, and from 12% for LCO cell to 29% for the bilayer cell under 1 A/cm2. Though these FEs still depart from unit, they are typical values as reported in the state-of-the-art H-SOECs studies. Literature found



that FE for H-SOECs ranged from 70~80% at 0.1 A/cm2, dropped to ~64% at 0.2 A/cm2 and to 20~40% for higher current density at 600~700oC.10, 29, 37, 39 The tolerance of this LCO layer towards high concentration of steam has been examined in a ~100 h operation. Figure 4b shows the applied voltage as a function of time during a 400 mA/cm2 constant current electrolysis process. Though the BZCYYb cell shows better initial performance than the bilayer electrolyte cell, which is consistent with the results in Figure 2, this high performance has not lasted long in high concentration of steam. The electrolyzing potential increases from 1.07 V at the beginning to 1.29 V after just 10 h of operation. In contrast, with the protection of the LCO layer, the bilayer electrolyte cell experienced almost no degradation in a 102 h operation. The applied potential maintains ~1.13 V under this 400 mA/cm2 electrolysis process, showing promising stability under practical operation condition.


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Figure 5. Cross-sectional and surface microstructure of BZCYYb cell (a, e) before, and (c, g) after measurement; cross-sectional and surface microstructure of LCO/BZCYYb bilayer electrolyte cell (b, f) before, and (d, h) after measurement. Surface images were taken from free electrolyte surface that was not covered by PNO anode. Figure 5 shows the microstructural of BZCYYb electrolyte and LCO/BZCYYb bilayer electrolyte cells before and after measurement. Firm cathode/electrolyte, anode/electrolyte interface bond can be verified in both cells in Figure 5a and b. A ~10 μm thick cathode functional layer (CFL) was used to promote the HER activity. Individual LCO and BZCYYb layer cannot be distinguished from the bilayer electrolyte as shown in Figure 5b, proving good sintering compatibility between these two components. After measurement, the reduced CFL layer becomes a porous layer with fine gas channels for hydrogen transport. Good interface bond remains in both tested cells. The cross-sectional view in Figure 5c shows that BZCYYb electrolyte tends to break along the grain-boundary after measurement, which might be associated with the grain-boundary ZnO enrichment.46 ZnO accumulated at the grain-boundary could be reduced by the strong reducing environment in the electrolyte during operation, leaving atomic defects behind and weakening the grain-boundary. Similar phenomenon has not been observed for LCO. It still shows a dense and flat cut after measurement as in Figure 5d. More remarkable is the surface change of BZCYYb electrolyte after operation. A dense clean surface (Figure 5e) deteriorates to a poreand flake-full rough surface (Figure 5g), confirming the poor stability of this material in high



concentration of steam.11 In contrast, LCO surface remains the same before and after measurement as shown in Figure 5f and h, proving again its trait of good stability towards high concentration of steam. 4. Conclusion In this study, a synergic effect for the first time was demonstrated on the LCO/BZCYYb bilayer as electrolyte for H-SOECs. The cell with this bilayer electrolyte exhibited excellent performance and stability. Electrolyzing currents of 975 mA/cm2 and 330 mA/cm2 under an applied potential of 1.3 V were achieved for 700oC and 600oC respectively, which was among the best performance of H-SOECs with chemically stable electrolyte measured at similar conditions to date. BZCYYb layer in this bilayer structure promoted both the cathode HER kinetics and the faradaic efficiency. LCO layer, on the other hand, protected the BZCYYb from the attack of high concentration of steam, resulting in an operation in 60 vol.% steam-containing air for 102 h without degradation. The bilayer electrolyte full cell with a cathode-support structure also can be readily fabricated with good electrolyte quality at as low as 1300oC. The synergic coupling of these two electrolyte materials overcame the longstanding difficulty that good performance and stability cannot be achieved at the same time in the benchmark Zr&Ce tuned Ba(ZrCe)O3-δ electrolyte family. These findings proved that this new electrolyte design was a vital strategy to crucially advance the development of H-SOECs for efficiency H2 production. Acknowledgments This investigation is funded by U.S. Department of Energy, office of Energy Efficiency and Renewable Energy (EERE) under the contract Number DE-EE0008378. We would like to thank the program managers Drs. Eric Miller and David Peterson for the technical guidance and financial support. Supporting Information Available: chemical compatibility between PNO and steam-stable proton electrolyte; electrical conductivities of LCO and BZCYYb; deconvolution of HF and LF arcs in EIS; faradaic efficiencies at different conditions. References (1) Bi, L.; Boulfrad, S.; Traversa, E., Steam Electrolysis by Solid Oxide Electrolysis Cells (SOECs) with Proton-Conducting Oxides. Chem. Soc. Rev. 2014, 43, 8255-8270. (2) Bi, L.; Boulfrad, S.; Traversa, E., Reversible Solid Oxide Fuel Cells (R-SOFCs) with Chemically Stable Proton-Conducting oxides. Solid State Ionics 2015, 275, 101-105. (3) Li, W.; Guan, B.; Ma, L.; Hu, S.; Zhang, N.; Liu, X., High Performing Triple-Conductive Pr2NiO4+δ Anode for Proton-Conducting Steam Solid Oxide Electrolysis Cell. J. Mater. Chem. A 2018, 6, 18057-18066. (4) Marrony, M.; Dailly, J., Advanced Proton Conducting Ceramic Cell as Energy Storage Device. J. Electrochem. Soc. 2017, 164, F988-F994. (5) Laguna-Bercero, M., Recent Advances in High Temperature Electrolysis Using Solid Oxide Fuel Cells: A Review. J. Power Sources 2012, 203, 4-16. (6) Kreuer, K., Proton-Conducting Oxides. Annu. Rev. Mater. Res. 2003, 33, 333-359.


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Table of Contents Graphic

LCO

Stable, inactive

BZCYYb

Unstable, active

LCO/BZCYYb Stable, active


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Synergistic Coupling of Proton-Conductors BaZr0.1Ce0.7Y0.1Yb0

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