Performances of Ni−SDC Hydrogen Electrodes in Reversible Operation Between SOEC and SOFC-Modes

A reversible solid oxide cell (R-SOC), which can be operated as solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC), is a highly efficient direct energy converter between hydrogen and electricity (1). We have engaged in the R&D of high-performance, durable electrodes for the R-SOC (2-13). Recently, it was found that the durability of a double-layer H 2 (DL) electrode, consisting of a SDC scaffold [samaria-doped ceria (CeO 2 ) 0.8 (SmO 1.5 ) 0.2 ] with highly dispersed Ni 0.9 Co 0.1 nanoparticles as the catalyst layer and a thin current collecting layer of Ni–YSZ cermet, was improved greatly by a reversible cycling operation between the SOEC and SOFC modes (12). The microstructure was observed to be stabilized by the cycling operation, i.e., lower parts of many Ni‒Co particles were anchored tightly on the SDC support and some portions were coated with SDC film, with certainty due to a strong interaction between Ni‒Co and SDC (13). However, the procedure for dispersing Ni‒Co nanoparticles on SDC (via an impregnation method) is not always suitable for large-scale fabrication. Aiming to develop a durable, practical hydrogen electrode with such a stabilized microstructure, we prepared new Ni–SDC hydrogen electrodes by a convenient protocol and examined the performances in the reversible operation. We prepared a coin-size cell with an air reference electrode (ARE) (6, 9-12): Ni–SDC H 2 electrode│YSZ (0.5 mm)│SDC interlayer│LSCF–SDC O 2 electrode For the H 2 electrode, a paste was prepared by mixing Ni nanoparticles with uniform size (prepared by Noritake Co., Limited), SDC powder, and pore former. The paste was painted on YSZ electrolyte, followed by sintering. The LSCF–SDC (40 vol.% SDC) oxygen electrode was prepared on the SDC interlayer in the same manner as that described in our work (7). Test cells were operated at 800ºC by supplying humidified H 2 ( p [H 2 O] = 0.4 atm) to the H 2 electrode compartment and dry O 2 to the O 2 electrode, irrespective of operation modes (SOEC or SOFC). The steady-state IR-free polarization curves ( I–E curves) of the electrodes with area-specific ohmic resistances were measured by the current-interruption method in a three-electrode configuration. Figure 1 shows a SEM image acquired with a back-scattered electron (BSE) detector and Ni mapping by an energy-dispersive X-ray spectrometer (EDX) for the cross section of Ni–SDC electrode in pristine condition. In the BSE image, SDC is observed as light gray, Ni as dark gray, and pores as black. The distribution of Ni particles are fairly uniform in the electrode. The Ni–SDC electrode thus prepared exhibited a comparable initial performance to that of our DL-H 2 electrode with Ni‒Co dispersed SDC prepared by the impregnation method (12). The durability tests of the electrodes are under progress in reversible cycling between −0.50 A cm −2 (SOEC-mode for 11 h) and 0.50 A cm −2 (SOFC-mode for 11 h). 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. Jon., 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). H. Uchida, H. Nishino, and E. Da’as, ECS Trans ., 103 (1), 6119 (2021). Figure 1