(Invited) Improvement of Durability of NiCo-Dispersed SDC Hydrogen Electrodes via Reversible Cycling Operation Between SOEC and SOFC-Modes
Authors: Uchida Hiroyuki , Nishino Hanako , Kakinuma Katsuyoshi 
Published: December 2020

A reversible solid oxide cell (R-SOC, operating as SOEC and SOFC) is a promising direct energy converter between hydrogen and electricity (1). We have engaged in the research and development of high-performance, durable electrodes with novel architecture for the R-SOC (2-12). We used an oxygen electrode consisting of a composite of La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) and samaria-doped ceria (CeO2)0.8(SmO1.5)0.2 (SDC) with SDC interlayer (5, 7-11) and a double-layer (DL) hydrogen electrode consisting of porous SDC with highly dispersed Ni0.9Co0.1 catalyst as a catalyst layer (CL) and a Ni-SDC or Ni-YSZ cermet attached as a current-collecting layer (CCL) (4, 7, 10-12). This presentation focuses on a significant improvement of the durability of our hydrogen electrodes via a reversible cycling operation.
We prepared a coin-size cell with an air reference electrode (ARE):
DL-H2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O2 electrode
The preparation method of the cell was similar to that described previously (7, 10, 11). For the DL-H2 electrode, 7.2 vol.% Ni0.9Co0.1 nanocatalyst was dispersed on SDC scaffold in the CL with addition of 10 vol.% three-dimensionally-connected Ni or Ni0.8Co0.2 (11, 12), and 60 vol.% Ni‒YSZ CCL was prepared on the CL.
Figure 1 shows the time course of the IR-free potential E (vs. ARE) of each electrode and the ohmic resistance R
ohm of the oxygen and hydrogen electrode sides (R
O2-side and R
H2-side) during two operation protocols: a constant current density operation at −0.50 A cm−2 (SOEC mode only) and a reversible cycling operation between −0.50 A cm−2 (SOEC mode for 11 h) and 0.50 A cm−2 (SOFC mode for 11 h). As reported previously (11, 12), the DL-hydrogen electrode showed appreciable degradation in the SOEC mode only (j = −0.50 A cm−2), specifically, an increase in the R
H2-side with time. During the initial 100 h, the degradation rate of R
H2-side was rapid, and the cathode potential shifted in the negative direction (increase in overpotential). On the other hand, it is striking that the durability of the DL-hydrogen electrode was improved greatly via the reversible cycling operation. Even though the actual operation time in the SOEC was nearly half that with the constant current operation, the degradation rate of R
H2-side was very low. The cathode potential in the SOEC mode (red circles) shifted in the positive direction, and the anode potential (blue circles) shifted in the negative direction during the initial 400 h, i.e., the performances were improved via the cycling operation. Graves et al. have reported a remarkable improvement of the durability of an Sr-doped LaMnO3 oxygen electrode via a cycling operation (13). In contrast, our LSCF‒SDC oxygen electrode exhibited very stable operation, irrespective of the operation protocol. Thus, an improved durability of the H2 electrode via the cycling operation has been demonstrated for the first time. Research on the mechanism of the suppression of degradation of the DL-H2 electrodes is in progress in our laboratory.

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Figure 1