Solid oxide fuel cells (SOFCs) have been developed over half century as highly efficient and clear power generation devices, and are expected to play an important role in the electrical grids for absorbing output fluctuations of renewable energy power sources by providing new function as solid oxide electrolytic cells and working as SOFC / SOECs. For two decades, replacing of oxygen ionic conductors in SOFC to proton conductors, which show higher ionic conductivity at intermediate temperatures (400 - 600℃), have been trying to increase fuel utilization, and reduce the cost of cells and stacks. Such proton conducting SOFCs are also expected to be work as proton conducting SOECs. Among many proton conductors discovered for almost four decades, proton conductors based on perovskite type oxides such as strontium zirconate, barium zirconate and barium cerate show sufficient proton conductivity for practical application. Recent reports show that related materials Y and Yb doped Ba(Ce, Zr)O₃ has excellent conductivity and chemical stability, and further improvement of proton conducting SOFC’s performance is achieved. However, such acceptor doped perovskite oxides exhibit not only proton conduction but also oxygen ionic, and electronic conduction. Among them, hole conduction under high oxygen partial pressure has non negligible impact on the performance of SOFC/SOECs because it exposes to air atmospheres when working as a generator and an electrolyser. The influences of leakage current caused by hole conduction are observed as low electro motive forces, and appeared as the decrease of faraday efficiency as the electrolyser. As developing progressed, such influence on proton conducting SOFC/SOECs recently have come to the attention. Onishi et al. reports that faraday efficiency is influenced by hole conductivity as a function of oxygen partial pressure, applied bias voltage, and over potential at air electrode, and their calculation suggests that if air and fuel electrodes over potential are sufficiently small for the SOEC operation, its faraday efficiency become quite low[1]. Therefore, suppressing of electronic conduction is essential problem for feasible proton conducting SOECs. However, since both holes and protons are introduced via oxygen vacancies, it is unavoidable to introduce holes, and no material that can achieve both high proton conductivity and low hole conductivity has been found at present. Introducing an interlayer to prevent leakage current is an effective method and many examples have been reported for oxygen ionic conductors. However, the adoption of this concept for proton conducting SOECs considering whole electrodes are remained as new challenges especially to material selecting, and reduction of electrode overpotential at air side. We propose nanoscale multilayered electrolyte includes both surface modification layer, which is thin and inserted to only improve air overpotential, and hole blocking layer to solve this challenge.This research focuses on the material selection for the hole blocking layer. The influence of interlayer with various hole and proton conductivities and thickness range of 100 nm to several micron meter for the hole current suppressing was numerically studied. In the research, the ionic and electronic current densities and oxygen partial pressure distribution inside the multilayered electrolyte were calculated to investigate the faraday efficiency in SOECs operation. In the calculation, according to the calculation method of Onishi et al., the faraday efficiency was evaluated for various interlayers with different proton and hole conductivities. Figure 1 shows typical example of the calculation assuming sufficiently small electrode overpotential (0.1 Ωcm²), and an interlayer with low proton and hole conductivities. As an interlayer, a proton conductor, which had lower proton and hole conductivities than 20 mol% yttrium doped barium zirconate (BZY20) was employed. In addition, the ratio of hole to proton conductivities was one tenth of BZY20. This ratio is referred from the most promising reported value [2]. Fig.1 clearly indicates that the faraday efficiency could be improved significantly only by the thin interlayer. For example, the 100 nm interlayer improved faraday efficiency to be over 80% from 51% of BZY single layer at applied voltage of 1.3 V, although the protonic current only reduced by 4.7%, and the efficiency reached over 90 % for 500 nm interlayer. However, protonic current decreased by 17%. Hence, this result suggest that optimal thickness exist for specific operation conditions.
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Dec 3, 2020
Paolo Lazzeretti 
Open Access