Proton ceramic electrochemical cells (PCECs) are emerging as a promising energy conversion technology for clean production of power (fuel cells), hydrogen (electrolyser) and chemicals (i.e., NH₃ synthesis) – particularly due to their potential for intermediate temperature of operation (400-500°C). One of the main obstacles towards realizing intermediate-temperature operation is the kinetics of the O₂/H₂O-electrode (positrode). An emergence of novel positrodes based on mixed proton electron conducting oxides have resulted in excellent performance with electrode resistances below 0.5 Ωcm² at temperatures above 500°C. However, the fundamental electrochemistry of positrode reaction mechanisms and the rate limiting surface reactions are not well understood, particularly the role of bulk and surface protonic species in the elementary reaction steps. Most electrode studies are performed under conditions in which the electrolyte material (typically BZCY-based) is a mixed conductor with comparable transport numbers for electron holes and protons (and to a certain extent oxide ions). Accordingly, the apparent electrode resistances due to accurately reflect the proton-mediated electrochemical processes which will occur in pure proton ceramic electrochemical cell.

In this work, we present a detailed study on interdigitated model electrodes (IDEs) prepared by PLD and lithography to create geometries with controlled electrochemically active area and triple-phase boundary length, with BGLC37 as the model electrode material. The electrode processes area measured at intermediate temperatures (200-500°C), low pO₂ (< 0.05 bar) and high pH₂O (>0.1 bar) to ensure the reactions are proton-dominated. A combination of standard EIS with voltammetry and Butler-Volmer analysis is employed to discriminate the true DC-polarization resistance and the separate time-resolved contributions occurring at or along the electrode surface. The surface species present and active on the electrode surface during operation is investigated using in-situ and operando XPS and XAS analysis and corroborated by ab-initio DFT simulations. The composition and structure of the electrode surface and the electrode-electrolyte interface is established by (S)TEM-EELS characterization of cross-sectional pieces of the samples. Finally, the impact of surface protonic species and their diffusion (surface protonics) on positrodes for PCECs is discussed based on combined input from DFT modelling and experimental evidence.