Mechanistic Insights Into Proton and Oxygen Transport Through Ultrathin Amorphous Al₂O₃ and Al₂O₃-SiO₂ Electrocatalyst Overlayers

Ultrathin amorphous alumina layers are excellent barriers making them ideal (electro)catalyst overlayers to prevent undesirable side-reactions, e.g. in O2-containing environments. Here, 2.5, 5, and 10 nm ultrathin Al₂O₃ and 2Al₂O₃-3SiO₂ (mullite) films are deposited onto Pt electrodes using pulsed laser deposition to evaluate their permeability to protons and O₂. Cyclic voltammetry revealed that aluminosilicate layers are proton-permeable but fail to effectively block O₂, while amorphous alumina quantitatively suppresses oxygen reduction, enabling selective electrochemical conversions in oxygen-rich environments. Electrochemical impedance spectroscopy and FT-IR reflection-absorption spectroscopy revealed structural transformations in alumina upon applying cathodic potentials, leading to new proton diffusion pathways. The effective proton diffusion coefficient (D_eff,H+) remained in the range of 10⁻¹⁸ to 10⁻¹⁷ m²/s, as determined from Pt-H vibrational mode growth and Warburg analysis. The observed decrease in diffusion and charge transfer resistance results from structural relaxation or increased hydration at the Pt/alumina interface, enhancing proton transport without altering the fundamental diffusion properties of the material. This highlights the ability of Al₂O₃ overlayers to enable additional transport pathways without fundamentally altering proton diffusivity. Furthermore, it highlights the importance of active site accessibility at buried catalyst interfaces in governing proton reduction kinetics under electrochemical conditions.