Defect-assisted vertical proton channels in highly oriented bismuth strontium tantalum oxide nanosheet laminar films

Abstract

Two-dimensional oxide nanosheets are regarded as a new class of inorganic proton conductors, particularly when assembled into highly oriented laminar films. However, c-axis oriented architectures typically exhibit strong proton transport anisotropy, with in-plane conductivity exceeding out-of-plane values by several orders of magnitude. Enhancing out-of-plane conductivities is therefore essential for practical electrochemical device applications. Here, we investigate anisotropic proton conduction in highly oriented laminar films assembled from A-site-deficient [Sr_0.70Bi_0.21□_0.10Ta₂O₇]²⁻ (□: A-site-vacancy) nanosheets and vacancy-free [SrTa₂O₇]²⁻ nanosheets, enabling a direct comparison of vacancy-regulated proton transport. At 100 °C and 100% relative humidity, the A-site-deficient films exhibit high in-plane and out-of-plane proton conductivities of 6.95 × 10⁻² and 2.24 × 10⁻⁵ S cm⁻¹, respectively, giving an anisotropy ratio (σ in-plane/σ out-of-plane) of ∼3.1 × 10³. In contrast, the vacancy-free films show in-plane and out-of-plane proton conductivities of 4.30 × 10⁻² and 5.76 × 10⁻⁶ S cm⁻¹, respectively, yielding a larger anisotropy ratio of ∼7.5 × 10³. Despite having essentially identical geometrical proton migration distances, the A-site-deficient films exhibit ∼5-fold higher out-of-plane proton conductivity than their vacancy-free counterparts, leading to a marked reduction in transport anisotropy. This enhancement is attributed to defect-assisted interlayer proton hopping mediated by A-site vacancies, which act as proton-conduction channels that shorten migration distances and enable continuous out-of-plane hydrogen-bond networks. The observed H/D isotope effect and low activation energies further support a hydration-assisted Grotthuss-type proton conduction mechanism. These findings establish that atomic-scale A-site vacancy engineering combined with nanosheet self-assembly is a powerful strategy for constructing highly oriented inorganic proton-conducting membranes with suppressed transport anisotropy, providing new design principles for advanced electrochemical energy-conversion devices.