# Molecularly modified ultrathin Al2O3 layers as proton-conductive, oxygen-impermeable nanomembranes for catalytic surfaces

## TL;DR

This paper studies ultrathin alumina layers that block oxygen but allow protons to pass through, which could improve catalytic reactions in energy technologies.

## Contribution

The study introduces molecularly modified alumina layers that balance proton conductivity and oxygen impermeability for catalytic applications.

## Key findings

- Al2O3 layers of 3–5 nm show proton diffusivity of ~10−13 m²/s and block oxygen effectively.
- Adding oligo(ethylene glycol) chains lowers proton transport resistance but increases charge-transfer resistance.
- Design priorities differ for high-current electrocatalysis versus low-current photocatalysis based on resistance trade-offs.

## Abstract

Ultrathin inorganic oxide coatings can improve selectivity in photo- and electrocatalysis, but they also bury active sites and impede transport of the desired reactants. Here we quantify proton and O2 permeability of 3–5 nm amorphous alumina (Al2O3) overlayers on poly-crystalline Pt using electrochemical impedance spectroscopy (EIS) and fourier-transform infrared reflection–absorption spectroscopy (FT-IRRAS). The apparent proton diffusivity amounts to ∼10−13 m2 s−1 in the atomic-layer-deposited (ALD) films. FT-IRRAS reveals hydrated AlOOH motifs whose presence correlates with the measured diffusion coefficients, highlighting their role as the dominant proton-transport pathways. The through-(Al2O3) film resistance is growing non-linear with thickness (17 → 37 Ω cm2 for 3 → 4 nm) and becomes close to infinity at 5 nm. Embedding oligo(ethylene glycol) chains within the alumina reduces the through-film resistance to 2.6 Ω cm2 at 3 nm. This is associated with enhancing proton access, albeit with a higher charge-transfer resistance (∼38 → 250 Ω cm2), consistent with diminished activity of the underlying Pt active sites. In O2-saturated electrolyte the total impedance increases and the diffusion contribution moves below the measurement threshold (1 Hz), indicating preserved oxygen-blocking character. Practically, this sets different design priorities. For high-current electrocatalysis, performance is governed by the overlayer's area-specific resistance, which can be improved by molecular functionalization. In low-current photocatalysis, the ohmic resistance penalty is small, so maintaining (or boosting) the intrinsic activity of buried active sites is more important to justify selectivity gains from O2 blocking.

Ultrathin inorganic oxide coatings can improve selectivity in photo- and electrocatalysis, but they also bury active sites and impede transport of the desired reactants.