Proton-conducting oxides are extremely promising for playing a central role in future energy supplies based on renewables – for example as electrolyte and electrode materials in solid oxide fuel or electrolysis cells, or as mixed proton- and electron-conducting ceramic membranes for hydrogen purification. Materials with perovskite-type structures have emerged as possible candidates for both applications. However, their composition, and in particular, their basicity influence the achievable concentration of protons and thus the ionic conductivity of the material. What makes the situation even more difficult is that the proton concentration in an oxide cannot be easily measured, especially not in-situ, which would be important if one wants to obtain this information on a running device.

This study is dedicated to examining the achievable proton concentration in the perovskite-type model material BaFe_0.8Y_0.2O_3-δ (BFY) in various oxidation and protonation states, as the protonation of BFY is intricately linked to the oxidation state of iron. Since this material is an auspicious candidate as a potential electrode material in proton-conducting fuel/electrolysis cells, its proton uptake has already been extensively studied, revealing favorable behavior[1]. The determination of the proton concentration in BFY, however, remains a challenge and primarily relies on techniques that involve indirect analysis of protons, such as thermogravimetry and conductivity measurements, which are considered state-of-the-art methods.

Here we present two alternative analytical methods that can be used for direct detection of protons in perovskites and are also very well suited for in-situ quantification: Infrared (IR) spectroscopy and laser induced breakdown spectroscopy (LIBS). IR spectroscopy provides a potential avenue for directly analyzing protons, since they can be expected to exhibit a characteristic OH vibration band. Indeed such an absorption band is observable on BFY, and its position in the IR spectrum is in good accordance with simulations in literature[2]. LIBS allows direct measurement of the proton concentration via analyzing the characteristic emission from the plasma generated by irradiating the material with an UV-laser. Here we present a customized sample chamber that enables LIBS measurements in different and even changing atmospheres, allowing in-situ analysis of protons in oxides.

The results obtained were validated by gravimetric measurements and supplemented by X-ray diffraction analyses, and both techniques demonstrated their ability to directly detect protons in perovskite-type oxides. This is particularly noteworthy as it makes them immune to artifacts arising from competing reactions, such as oxygen uptake, a challenge often encountered in thermogravimetry. The findings underscore the reliability of our analytical approach, and validate it as a valuable alternative to conventional methods, thus contributing to advancements in renewable energy applications.