Functionality of the Cathode–Electrolyte Interlayer in Protonic Solid Oxide Fuel Cells
Katsuya Akimoto, Ning Wang, Chunmei Tang, Kai Shuto, Seongwoo Jeong, Sho Kitano, H. Habazaki, Yoshitaka Aoki
Abstract
Efficient power generation with protonic solid oxide fuel cells (H-SOFCs) remains challenging because the mismatch between the primary ion carriers of the electrolyte and the cathode limits the effective cathode reaction area to the gas–electrolyte–cathode triple-phase boundary (TPB), resulting in large cathodic overpotentials at low operating temperatures. Herein, we report the role of functional layers between an electrolyte and a cathode in reducing the cathodic reaction resistance at the TPB. Thin-film fuel cells with BaZr0.1Ce0.7Y0.1Yb0.1O3−δ (BZCYYb1711) electrolytes were fabricated using a dense La0.5Sr0.5CoO3−δ (LSC) nanofilm (approximately 100 nm) as a cathode functional layer (CFL) with the typical oxide ion/electron mixed conductor La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF), LSC, and the less-active Sb0.1Sn0.9O2 (ATO). The peak power densities of the cells increased by >100% when using a CFL, and cells having an LSC cathode and CFL achieved power densities of 500 mW cm–2 at 500 °C. The distribution of relaxation times in the impedance spectra revealed the CFL’s effect on the ohmic and polarization resistances. Crucially, cells without a CFL had large ohmic resistances because the proton-accessible electrode areas were confined to the gas–cathode–electrolyte TPB. However, the resistance decreased with the CFL because coupled partial proton conductivity and electrocatalytic activity of the LSC nanofilms increased the proton-accessible electrode area. The cells without a CFL showed a large polarization resistance because of the sluggish diffusion of O adatoms over the cathode resulting from the increased TPB length. This resistance decreased by >70% with an LSC CFL because the cells did not require long-range O diffusion because of the significantly extended proton-accessible reaction area near the gas–CFL–cathode TPB. Thus, using interfacial layers is an alternative way to design new cathode materials having low cathodic polarization for H-SOFCs.