Design and Performance of a Proton-Conducting Solid Oxide Reversible Cell

Abstract

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.