Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell

0–18 nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nafion 117 membranes, and used as thin spacer electrolyte layers between the Nafion and a 3 nm Pt catalyst film. Electrochemical characterisation of the films in terms of oxygen reduction activity, high frequency impedance and cyclic voltammetry in nitrogen is performed in a fuel cell at 80 °C and full humidification. Titanium oxide films with thicknesses up to 18 nm are shown to conduct protons, whereas zirconium oxide and tantalum oxide block proton transport already at a thickness of 1.5 nm. The performance for oxygen reduction is higher for a bi-layered film of 3 nm platinum on 1.5 or 18 nm titanium oxide, than for a pure 3 nm platinum film with no spacer layer. The improvement in oxygen reduction performance is ascribed to a higher active surface area of platinum, i.e. no beneficial effect of combining platinum with zirconium, tantalum or titanium oxides on the intrinsic oxygen reduction activity is seen. The results suggest that TiO2 may be used as electrolyte in fuel cell electrodes, and that low-temperature proton exchange fuel cells could be possible using TiO2 as electrolyte.