Re-Evaluating the Stability of Al₂O₃ Barriers Prepared by Atomic Layer Deposition under Electrochemical Conditions

Atomic layer-deposited (ALD) films are widely used as insulating barriers in (photo)electrochemical systems, yet their stability and charge-transfer behavior under operational conditions remain poorly characterized. Here, we systematically investigate how film thickness and electrolyte composition influence the performance of ALD-grown amorphous Al₂O₃ films on indium tin oxide. Using cyclic voltammetry and electrochemical impedance spectroscopy, we find that a thickness of ∼4-5 nm is required to achieve stable insulation and tunneling-limited electron transfer, which is significantly more than the minimum needed to form a continuous film. Moreover, the extracted tunneling decay constant, 0.30 Å^-1, is lower than values reported for crystalline Al₂O₃, indicating noticeable charge transport through amorphous thin films. On the other hand, a reduction in the effective diffusion of redox active molecules at the electrode surface is suggested for films thicker than 3 nm. We further demonstrate that specific ions strongly influence film lifetime. Unexpectedly, we found that acetate buffers are significantly less detrimental to film stability compared to commonly used phosphate buffers. Moreover, the addition of low concentrations of Al³⁺ ions dramatically delays film degradation. In contrast, pH effects between 4 and 8 are minimal. Notably, film failure shows stochastic behavior while also being broadly consistent with gradual homogeneous dissolution rather than discrete pinhole formation previously reported for TiO₂ and Al₂O₃ insulating films. These results reveal the critical and previously underappreciated role of electrolyte composition in determining the lifetime of insulating oxide films. Our findings offer practical design guidelines and highlight the need for controlled conditions when implementing ALD barriers in electrochemical devices.