Return of the solid-state coulometric titration: A new hope to expand the p(O2) range

The catalytic and electrochemical properties of many non-stoichiometric oxides are governed by their defect chemistry. Therefore, detailed knowledge of their oxygen non-stoichiometry under operating conditions is desired. For this, coulometric titration can offer a valuable tool that can have advantages in terms of required sample mass, accuracy and reachable p(O₂) range over other established techniques, such as thermogravimetric analysis (TGA). Here, we present a new design for an easy to fabricate miniature coulometric titration setup using materials selected for optimal electrode kinetics. The small chamber volume (0.03–0.05 ml), small sample mass (about 30 mg) and kinetically fast electrodes allow for a precise variation of the p(O₂) from 1 bar down to 10⁻³² bar at 625 °C. This is a much wider range than typically achievable under gas flow in TGA or with other titration setups described in the literature. A characterisation of the titration setup showed that residual errors in the defect chemistry of the investigated materials are in the range of 10⁻⁴ to 10⁻³ p.f.u. Exemplary measurements on CeO_2-δ and Sr_1-xTi_0.6Fe_0.4O_3-δ (STF) showcase how this wide p(O₂) range can not only be used to study oxygen non-stoichiometry at very reducing conditions and the p(O₂) at which vacancy ordering phenomena occur (for CeO_2-δ), but also detect and quantify small amounts of redox-active secondary phases (for STF).