Changes in Microstructure of NiCo-Dispersed SDC Hydrogen Electrodes for the Improvement of Durability via Reversible Cycling Operation Between SOEC and SOFC-Modes
Authors: Uchida Hiroyuki , Nishino Hanako , Da'as Eman H. 
Published: July 2021

A reversible solid oxide cell (R-SOC, operating as solid oxide electrolysis cell, SOEC and solid oxide fuel cell, 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). Very recently, we found that the durability of a double-layer H2 (DL) electrode, consisting of a SDC scaffold [samaria-doped ceria (CeO2)0.8(SmO1.5)0.2] with highly dispersed Ni0.9Co0.1 nanoparticles as the catalyst layer (CL) and a thin current collecting layer (CCL) of Ni–YSZ cermet, was improved greatly by reversible cycling operation between the SOEC and SOFC modes (12). This presentation focuses on a stabilization of the microstructure of the CL in our hydrogen electrode via a reversible cycling operation.
We prepared a coin-size cell with an air reference electrode (ARE) (6, 9-12):
DL-H2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O2 electrode
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 Ni0.8Co0.2 (12, 13), and 60 vol.% Ni‒YSZ CCL was prepared on the CL. Test cells were operated in 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) at 800ºC. Changes in the microstructure of the CL were analyzed by scanning ion microscopy (SIM) in a focused-ion beam system (FIB) in the depth direction from the CL/YSZ interface.
In order to compare the actual degradation rates in the SOEC-mode for the two operation protocols, the values of IR-free potential E (vs. ARE) of each electrode and the ohmic resistance of the O2 and H2 electrode sides (R
O2-side and R
H2-side) during the operation at −0.50 A cm−2 are plotted in Fig. 1, while the inset shows changes in E as a function of the overall operation time. The degradation rate for R
H2-side in the initial 200 h decreased as low as ca. 1/10 as a result of the cycling operation. In contrast to the negative shift in E for the SOEC-mode only, the positive shift in E at the H2 electrode in the cycling operation is clearly seen, i.e., the performance was improved.
In the SIM image of pristine sample, fine Ni‒Co particles (ca. ⩽ 50 nm) are highly dispersed on the SDC scaffold. After the operation with SOEC-mode only, SDC particles are found to have agglomerated considerably, enclosing Ni‒Co particles, while fine Ni‒Co particles (ca. 50 nm) are dispersed on the surface. After the cycling operation, fine Ni‒Co particles still remain and agglomerated ones are also observed. Interestingly, the lower parts of many Ni‒Co particles are anchored tightly on the SDC support, and some portions are coated with SDC. Quantitative analyses of Ni‒Co are in progress, as a function of the distance from the interface.
This work was supported by funds for the “Collaborative Industry-Academia-Government R&D Project for Solving Common Challenges toward Dramatically Expanded Use of Fuel Cells” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

References

S. D. Ebbesen, S. H. Jensen, A. Hauch, and M. B. Mogensen, Chem. Rev., 114, 10697 (2014).

H. Uchida, N. Osada, and M. Watanabe, Electrochem. Solid-State Lett., 7, A500 (2004).

N. Osada, H. Uchida, and M. Watanabe, J. Electrochem. Soc., 153, A816 (2006).

Y. Tao, H. Nishino, S. Ashidate, H. Kokubo, M. Watanabe, and H. Uchida, Electrochim. Acta, 54, 3309 (2009).

R. Nishida, P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, M. Watanabe, and H. Uchida, RSC Adv., 4, 16260 (2014).

H. Uchida, P. Puengjinda, K. Miyano, K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and M. Watanabe, ECS Trans., 68 (1), 3307 (2015).

K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, Electrochim. Acta, 225, 114 (2017).

K. Shimura, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Ceram. Soc. Jpn.,
125, 218 (2017).

P. Puengjinda, H. Nishino, K. Kakinuma, M. E. Brito, and H. Uchida, J. Electrochem. Soc., 164, F889 (2017).

H. Uchida, P. Puengjinda, K. Shimura, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 78 (1), 3189 (2017).

H. Uchida, H. Nishino, K. Kakinuma, and M. E. Brito, ECS Trans., 91 (1), 2379 (2019).

H. Uchida, H. Nishino, P. Puengjinda, and K. Kakinuma, J. Electrochem. Soc., 167, 134516 (2020).

Figure 1