Metal oxides are attractive candidates for low cost, earth-abundant electrocatalysts. However, owing to their insulating nature, theirwidespread application has been limited. Nanostructuring allows the use ofinsulating materials by enabling tunneling as a possible charge transportmechanism. We demonstrate this using TiO2 as a model system identifyinga critical thickness, based on theoretical analysis, of about ∼4 nm fortunneling at a current density of ∼1 mA/cm2. This is corroborated byelectrochemical measurements on conformal thin films synthesized usingatomic layer deposition (ALD) identifying a similar critical thickness. Wegeneralize the theoretical analysis deriving a relation between the criticalthickness and the location of valence band maximum relative to thelimiting potential of the electrochemical surface process. The critical thickness sets the optimum size of the nanoparticle oxide electrocatalyst and this provides an important nanostructuring requirement for metal oxide electrocatalyst design.