Proton Accumulation Modulated Surface Potential in Proton Conducting Ceramics Revealed by Near-Ambient Pressure X-ray Photoelectron Spectroscopy

Proton conducting electrochemical cells (PCECs) are efficient and clean intermediate-temperature energy conversion devices. The proton concentration across the PCECs is often nonuniform, and characterizing the distribution of proton concentration can help to locate the position of rate-limiting reactions. However, the determination of the local proton concentration under operating conditions remains challenging. Here, we employed in situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) to investigate an Au/BaZr_0.9Y_0.1O_3-δ/Au symmetric cell with DC bias of 1 V applied between the working and counter electrodes (CE). The relative intensity of hydroxyl groups, deconvoluted from the O 1s XPS spectra, reveals the distribution of proton concentration across the electrolyte. The applied electric field induces proton accumulation at the counter electrode, imposing binding energy shifts of the surface components for metal elements relative to their lattice components. Combined XPS and impedance analysis suggests that the accumulation layer of protons is much thicker at 500 K compared to that at 670 K, as a result of a larger amount of hydroxyl groups at the lower temperature. This nonuniform distribution of proton concentration affects the chemical environment of metal elements, and the local electrical potential, as revealed by the in situ XPS. This work demonstrates in situ NAP-XPS as a tool to probe the distribution of proton concentration and its impact on the defect chemistry and local electrical potential of PCECs, thereby advancing the understanding of the impact of proton defect chemistry and the performance improvement of PCECs.