Effect of Proton Concentration on the Proton Conductivity in Calcium Niobium Oxide Nanosheets

Proton-intercalated layered perovskite metal oxides have garnered significant interest as proton-conducting materials because of their easy fabrication and extraordinary physical and chemical properties. However, there are few reports on fuel cells using proton-intercalated layered oxides as proton-exchange membranes. Herein, we have controlled the interlayer proton concentration of Dion–Jacobson (DJ)-type layered perovskites calcium niobium oxide (HCNO; HCa 2 Nb 3 O 10 ) through doping with La and Ti on the Ca-sites and Nb-sites, and have applied the materials for proton-exchange membranes in fuel cells. The layered HCNO was exfoliated into calcium niobium oxide (CNO) nanosheets, and then, thin-film membranes of doped and undoped H/CNO nanosheets were fabricated through the vacuum filtration method. Proton conductivity was found to increase with an increase in proton concentration. The in-plane proton conductivity for Ti-doped HCNO (H 1.5 Ca 2 Nb 2.5 Ti 0.5 O 10 ) (σ = 5.30 × 10 –4 S cm –1 ) was ∼60 times higher than that of La -doped HCNO (H 0.5 Ca 1.5 La 0.5 Nb 3 O 10 ) under 80 °C and 100% RH conditions. The out-of-plane proton conductivity for Ti-doped HCNO was 20 times higher than that of La-doped HCNO. A fuel cell using the Ti-doped H/CNO nanosheet membrane showed an optimum power density of 11.20 mW cm –2 at 80 °C, which was 15 times higher than that of the undoped H/CNO membrane. This high proton conductivity is due to the high proton concentration of the Ti-doped HCNO (H:1.5) compared with HCNO (H:1.0) and La-doped HCNO (H:0.5). The present findings hold great promise for the design and development of metal oxide-based proton conductors.