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Proton-conducting oxides have attracted much attention in recent years due to their good ion conductivity at intermediate temperatures (500-700°C), as opposed to conventional oxide ion-conducting oxides that require high temperatures (usually, >900°C). Lowering the operating temperature of reversible protonic ceramic electrochemical cells (RePCECs) is desirable as it reduces material, heating, and insulation costs, while also increasing the operating life of the cell. Nevertheless, along with the desired proton conductivity, typical proton-conducting solid electrolytes also allow the transport of other charged defects like oxygen-vacancies, electrons, and electron holes. The presence of these other defects, especially the electron holes, causes leakage current that significantly reduces the cell’s electrical efficiency. Therefore, an electrolyte capable of reducing leakage current is necessary. Unfortunately, there is usually a trade-off between high ionic conductivity and low hole conduction. For example, while lanthanum tungstate (LWO) has a low electron conductivity and is capable of suppressing hole conduction below 800°C, its proton conductivity is lower than that of commonly used perovskite materials like yttria-doped barium cerate (BCY) or yttria-doped barium zirconate (BZY). Nonetheless, previous studies showed that LWO67 can act as a leakage current-blocking layer when deposited on a BZY82 base electrolyte and the estimated leakage current was almost 200 times lower than that of a BZY82 single layer electrolyte under open-circuit conditions. This suggests that under certain operating conditions, bilayer electrolytes can achieve better performance when compared to their corresponding single-layer electrolytes. Therefore, in this study, we design proton-conducting bilayer electrolytes by investigating the incorporation and transport of charged defects using a steady-state Nernst-Planck-Poisson (NPP) model. The main goals of this work are twofold. Firstly, to find the combination of electrolytes that better suppresses leakage current. For this purpose, we calculate the defect concentration, flux, and electrostatic profiles inside common materials like BCY, BZY, BZCYYb, BZCY, LWO, and different combinations of bilayer electrolytes to better understand how the addition of a leakage current-blocking layer affects proton conduction and overall efficiency. We consider three mobile defects when solving the NPP equation, protons (OHO •), oxygen vacancies (vO ••), and electron holes that are treated as small polarons localized at oxygen sites (OO •). Afterward, the I-V curve and relevant theoretical efficiency are predicted within and A cm-2 (fuel cell and electrolyzer operation) at 500, 600 and 700°C, in order to identify the most promising one. Secondly, to design a cell by finding the optimal thickness of the better performing bilayer electrolyte that maximizes efficiency.

Introduction Renewable and clean energies have attracted much interest in recent years due to environmental and economic concerns related to the use of fossil fuels. For this reason, solar and wind energies have become popular alternatives and are widely used in many countries as reliable energy sources. However, these technologies are intermittent by nature which makes it challenging to successfully integrate them into power grids. Reversible electrochemical cells provide an attractive way to store solar and wind energy surplus by operating as an electrolysis cell to produce hydrogen that can be later used to generate electricity by operating as a fuel cell. Reversible cells using proton conductive solid oxide electrolytes have some advantages over other technologies, for instance, they show higher ionic conductivity in the intermediate temperature range which lowers the need for heating. Nonetheless, previous studies show low faradaic efficiencies (FEs) that are usually between 20 and 70%, which means that less than 70% of the applied current is used to produce hydrogen. A state-of-the-art reversible cell by Choi et al [1]. in 2019 yielded a remarkable current density of -1.8 A cm-2 at 600°C and 1.3 V, but the reported faradaic efficiencies ranged from 40 to 75%. In a recent study BCZYYb+1.0wt% NiO electrolyte was used to obtain one of the highest reported faradaic efficiencies to date, between 90-98%. According to the authors, “low faradaic efficiencies can be almost entirely attributed to higher current leakage” [2]. Therefore, an electrolyte capable of considerably reducing leakage current is necessary. Lanthanum tungstate (LWO) is a promising electrolyte material since it has a good ionic conductivity, low electron conductivity and it is capable of suppressing hole conduction below 800°C [3]. In this study, we assess the performance of a solid oxide reversible cell using LWO and other common electrolyte materials like BCY10, BZY20 and BZCYYb1711. Methods The performances for LWO67 and other commonly used electrolyte materials were predicted based on their transport properties using Choudhury’s model [4] with the required conductivity parameters obtained by fitting to reported experimental data (Table 1). Power efficiencies (PEs) were computed for fuel cell operation and faradaic efficiencies for electrolysis cell operation. Additionally, the performances of LWO67/BCY10 electrolyte-supported-platinum (Pt)-symmetric cells were also evaluated using a fuel cell testing unit with an in-line gas chromatograph (GC) to measure the produced hydrogen. A potentiostat/galvanostat was used to measure the characteristic I-V curves and electrochemical impedance. Both the predictions and experiments were performed with a gas composition of 20% O2 in Ar (3% H2O) for the air electrode side and 1% H2 in Ar (3% H2O) for the hydrogen side. Pt was chosen as both the fuel and air electrode to be able to easily compare the efficiencies of LWO67 and BCY10 under the exact same conditions. This is because LWO can react with other electrodes and a thin protective layer would be necessary to prevent this from happening. Results and discussion According to the predictions, LWO67 exhibits the highest PE in fuel cell operation at typical operating conditions of current p-SOFC systems (Fig. 1). In the same way, LWO67 shows the highest faradaic efficiency in electrolysis operation (Fig. 2) exceeding that of BZCYYb by more than 10%. These results are due to LWO67’s low electron and hole conductivities and they suggest that it has great potential as an electrolyte for reversible cells. It was also found that the optimum electrolyte thickness is around 10 µm and lower values produce a decrease in the FE. This occurs because as ionic conductivity increases with decreasing thickness, so does the leakage current. Therefore, there is a trade-off relationship between the electrolyte thickness, the current density, and the efficiency. Moreover, the FEs measured experimentally were higher for LWO67. For example, at 650°C the FE was 26% for BCY10 while it was 37% for LWO67. Nevertheless, this difference is expected to be even higher in anode-supported cells with thinner electrolytes, indicating that LWO67 has potential as an electrolyte material. Additionally, it has been shown that poor electrode performance can affect the FE. Thus, a more active electrode should improve the performance. References [1] S. Choi, T.C. Davenport, S.M. Haile, Energy Environ. Sci. 12 (2019) 206–215. [2] C. Duan et al., Nat. Energy 4 (2019) 230–240. [3] G. Kojo, R. Tsukimura, J. Otomo, Solid State Ion. 306 (2017) 89–96. [4] N.S. Choudhury, J.W. Patterson, J. Electrochem. Soc. 118 (1971) 1398–1403. Figure 1

For efficient operation of proton ceramic fuel cell (PCFC), transport properties of hole and electron are important factors to be controlled as well as that of protons. Ba-based perovskite-type proton conductors have been used as typical electrolytes for the PCFCs because of their high proton conductivity. However, they also exhibit apparent hole conduction under oxidative atmospheres which causes leakage current through the electrolyte. The leakage current leads to not only a decrease in cell voltages but also a reduction of the energy efficiency since the leakage current cannot be utilized as electric power while hydrogen is consumed to generate protons and electrons. Therefore, a novel approach to suppress the leakage current has been required. Lanthanum tungstate (La28−x W4+x O54+3/2x ; LWO) with a high La/W ratio of 6.7 is one of the attractive proton conductors with a quite low hole conductivity. Though its proton conductivity is lower than the typical perovskite-type electrolytes, a high protonic transport number is attractive as a hole blocking layer for the PCFCs. An introduction of a thin LWO hole blocking layer on the Ba-based proton conductors with the high proton conductivity will suppress the leakage current by changing a profile of oxygen chemical potential in the electrolytes as we have proposed previously (Y. Matsuzaki et al., ECS Trans. 91, 1009-1018 (2019)). In this study, we explored the influence of the LWO hole blocking layer on the cell performances of PCFCs by theoretical calculations and experiments. In the theoretical calculations, a BZY|LWO bilayer cell model in which the LWO layer was located at the cathode side was employed. The calculations suggested that the bilayer structure not only improves efficiency but also gives flexibility to control the cell performance as required. Moreover, we experimentally investigated the performance of anode-supported cells with a BZY monolayer electrolyte and with a BZY|LWO bilayer electrolyte to elucidate the effect of the LWO layer. An LWO layer thickness (L LWO) dependence of the OCV in the BZY|LWO bilayer model was calculated based on the transport properties of BZY and LWO. Dependences of the energy efficiency and the power density with respect to a ratio of to a total electrolyte thickness (L total) of 10 μm were calculated for the model at a fixed external current density (I ext) of 250 mA/cm2 and at a fixed terminal voltage (V T) of 1.05 V, respectively. ε was defined by ε = V T I ext/V th I ion, where V th and I ion are a Nernst voltage and an ionic current density. We assumed the following operation conditions; operation temperature of 600 °C, 3% H2O + 97% H2 for anode gas, and 3% H2O + 19.4% O2 + 77.6 % Ar for cathode gas. In the experiments, a BZY monolayer electrolyte and a BZY|LWO bilayer electrolyte were prepared on NiO-BZY cermet substrates by the pulsed-laser deposition (PLD). La0.6Sr0.4Co0.2Fe0.8O3− d (LSCF) cathode was applied for both the cells. To suppress chemical reactions of LWO with BZY and LSCF, thin La-doped ceria (LDC) layers were introduced. The resulted structure of the monolayer cell and the bilayer cell were Ni-BZY|BZY(3.5 μm)|LSCF and Ni-BZY|BZY(3.5 μm)|LDC(40 nm)|LWO(1 μm)|LDC(40 nm)|LSCF, respectively. Cell performance tests were performed at 600 °C supplying 3% humidified H2 + Ar gas for the anode and 3% humidified 20% O2 + Ar gas for the cathode. The calculated OCV of the BZY monolayer model was 1.08 V and steeply increased up to 1.13 V by employing the thin LWO layer with L LWO/L total of 0.017 owing to suppression of the leakage current. Calculated dependences of ε and the power density for the model are shown in Fig.1. The efficiency and the power density showed a steep increase with increasing L LWO/L total. They showed the maximum values at certain L LWO/L total values, and then gradually decreased as L LWO/L total approached 1. This calculation result indicates that the cells exhibiting both the high ε and the high power density can be developed by employing the bilayer structure with the thin LWO hole blocking layer. Moreover, It is notable that, as the performance of the bilayer cell can be tuned by changing as presented in Fig. 1, the bilayer structure increases a degree of freedom for the design of the cells to satisfy various performance requirements. In the experiments, an enhancement of the OCV from 0.95 V for the BZY monolayer cell to 1.01 V for the bilayer cell was actually observed as suggested by the calculation. We consider that these theoretical calculations and experimental results present the effectiveness of the LWO hole blocking layer for the development of the high-performance PCFCs with high-design flexibility. Figure 1