Semiconductor TiO₂ thin film as an electrolyte for fuel cells

Electrolyte layer, made up of an ionic conductor with ignorable electronic conductivity, plays vital roles in transporting ions as well as blocking electron passage in electrochemical devices like a solid oxide fuel cell (SOFC). The electronic conductivity of the electrolyte has been always blamed for bringing in the short-circuiting problem. In this study, however, we demonstrate that the dominant issue is not the electronic conductivity of electrolytes but the energy band diagram of the device. Using a semiconductor TiO₂ thin film as an electrolyte, we present a novel design of fuel cell devices from the perspective of the energy band structure and alignment. A TiO₂ thin film was fabricated by a mass-productive spin coating method. An OCV of 1.1 V and maximum power output of 364 mW cm^-2 at 550 °C were achieved, which proves that TiO₂ plays the role of an electrolyte with sufficient ionic transportation while no electronic short-circuiting problem occurs. The online intercalation of Li into TiO₂ enables the creation of more oxygen vacancies. Additionally, proton incorporation and conducting mechanisms in TiO₂ have been verified and discussed. This work provides a new method for suppressing the electronic conductivity of electrolytes as well as developing functional electrolytes from a well-known semiconductor for advanced low-temperature SOFCs.