Ion-conducting, sub-100 nm-thick film of amorphous hafnium silicate

Ceramic electrolytes operating in the temperature range of 200 to 500 °C under dry atmosphere are a key material for the next-generation fuel cell and related applications. We discovered that nanometer-thick films of amorphous hafnium silicate (Hf_nSi_{1 - n}O_x) exhibited efficient ionic conduction at 100-400 °C in dry air. When the fraction of hafnium doping was around 0.1, the nanofilm showed a low area-specific-resistance (< 0.15 Ω cm²) at around 350 °C that was small enough for the practical fuel cell application. The sub-100 nm-thick membranes of Hf_0.13Si_0.87O_x, could be fabricated on porous Pt/alumina substrate to provide gas concentration cells. The electromotive force observed with H₂ concentration cell indicated that the ceramic nanomembrane acted as predominant proton conductor without permeation of H₂ gas. In addition, the Hf_0.13Si_0.87O_x and Zr_0.11Si_0.89O_x membranes responded to the change of O₂ pressures in O₂ concentration cells, while the Al_0.16Si_0.84O_x and Ce_0.06Si_0.94O_x membranes did not produce the electrical voltage by gradient of O₂ pressure. We conclude that Hf_0.13Si_0.87O_x nanomembrane is promising as electrolyte material for fuel cells and related ionics devices.