Ba-Zr-based proton conductor is expected as an electrolyte material for protonic ceramic fuel cells (PCFCs). Electron-hole conduction in the proton conductor emerging in oxidizing atmospheres should be addressed to improve power generation efficiency. For that purpose, the nature of protons and electron holes, which are believed to be very similar in electrolyte materials, should be clarified in detail by experimental observation in addition to computational methods. One of the challenges for the experimental observation of charged carriers in the proton conductor is to prepare a fully oxidized sample with only electron holes, unlike a fully hydrated sample with protons, which can be easily obtained by heat treatment in a humidified atmosphere.^[1] Therefore, this study focuses on preparing fully oxidizing Ba-Zr-based proton conductors using high pressure with KClO₄ as an oxygen source. The candidate material suitable for incorporating electron holes is Sc-doped BaZrO₃, which has been suggested to have moderate stability of oxygen vacancies.
BaZr_1-xSc_xO_3-δ (BZS; x=0.1-0.5) is prepared by the solid-state reaction method. Protons in samples are removed by drying under a vacuum at 800 °C for 10 h. Electron-hole incorporation was conducted by high-pressure oxidation under 6 GPa for 12 h. ESR and NMR were used to confirm sample oxidation and estimate hole concentration.
¹H NMR confirmed the existence of no proton in the oxidized BZS samples; meanwhile, ESR indicated the presence of non-pair electrons only in the oxidized BZS samples. This suggests that hole incorporation into the BZS samples was successful by high-pressure oxidation. ⁴⁵Sc NMR spectra also supported the full oxidation of the samples and hole incorporation; that is to say, a peak attributed to ScO₅ decreased, and a new peak attributed to an electron-hole-coordinated Sc environment emerged in the oxidized BZS samples. This implies that oxygen vacancies are filled, and the high-pressure oxidation incorporates electron holes. The hole concentration in the oxidized BZS samples was estimated to be 9.6, 15.5, and 44.0% for 10%Sc, 20%Sc, and 50%Sc-doped samples, respectively, which reached almost stoichiometric maximum values, meaning full oxidation. The differences and similarities of local structures of fully hydrated and oxidized BZS will also be discussed.