A reversible solid oxide cell (R-SOC) is a reciprocal direct energy converter between hydrogen and electricity (1). We have engaged in the research and development of high-performance electrodes with novel architecture for the R-SOC (2-9). A mixed conducting samaria-doped ceria (CeO₂)_0.8(SmO_1.5)_0.2 (denoted as SDC) has been used in both hydrogen and oxygen electrodes. A double-layer (DL) hydrogen electrodes consisting of SDC with highly dispersed Ni_0.9Co_0.1 catalysts as a catalyst layer and a Ni-SDC cermet attached as a current collecting layer (4, 7) was proposed and tested as a hydrogen electrode. We used an oxygen electrode consisting of a composite of La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF) and SDC with SDC interlayer (5), in which SDC acted as a high oxide ionic conductor in oxygen atmosphere. In the present work, we demonstrate that the SDC is a key material in increasing the durability of both electrodes under reversible operations.

We examined the durability of the LSCF-SDC electrode with SDC interlayer by using a symmetrical cell: LSCF‒SDC|SDC interlayer|YSZ electrolyte|SDC interlayer|LSCF‒SDC (8, 9). Pure oxygen gas at ambient pressure was supplied to both electrodes. The anodic overpotential (η_A) and cathodic overpotential (η_C), together with the ohmic resistances of the anode side R_A and the cathode side R_C, were measured by the current-interruption method with the use of a Pt/air reference electrode.

The symmetrical cell was operated at 900 °C and a constant current density of 0.5 A cm⁻². It was found that the values of η_A and η_C were virtually constant over whole operation of 5500 h. In contrast, the value of R_C increased somewhat markedly, although the change in the R_A was very small. Figure 1 shows the elemental distribution of Ce, Sr, and Zr for a cross-section of the LSCF‒SDC/SDC interlayer/YSZ region observed by SEM equipped with EDX. For the pristine electrode (Fig. 1(A)), we confirmed negligible inter-diffusion of Sr and Zr components at the LSCF‒SDC/SDC interlayer/YSZ. This is ascribed, with certainty, to a low fabrication temperature (1050 °C for 1 h). On the anode side after 5500 h of operation (Fig. 1(B)), the Sr component penetrated into the SDC interlayer with a layer-like distribution, which could be ascribed to rapid diffusion along the sub-layer of the SDC. It is noteworthy that the presence of Sr was limited within the SDC interlayer on the anode side, whereas the Sr component from the cathode reached the YSZ surface just below the SDC interlayer. It was suggested that a rapid diffusion of Sr over the YSZ surface could form SrZrO₃, leading to the increase in R_C. Because the concentration of Sr was found to be high in the vicinity of defects (dips or voids) of the SDC interlayer, the formation of a dense, uniform SDC interlayer is very important to obtain high durability with high performance in R-SOCs.

We are performing the durability test of a full cell with the configuration: DL H₂ electrode│YSZ or ScSZ│SDC interlayer│O₂ electrode. The initial IR-free applied voltage at 0.5 A cm⁻² was 1.16 V at 800 °C and 1.24 V at 750 °C. Effects of microstructure of DL hydrogen electrodes on the durability will be discussed.

This work was supported by the funds for “Advanced Low Carbon Technology Research and Development Program” (ALCA) from the Japan Science and Technology Agency (JST).

 

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