Stabilizing layered Sr-intercalated CoO2 nanotubes for bifunctional energy applications: enhanced OER catalysis and energy storage

Abstract

Layered cobalt oxides with tunable intercalation chemistry are promising for sustainable energy applications. However, the synthesis and stabilization of layered CoO₂ with larger cations such as Sr²⁺ in low-dimensional morphologies remain a challenge due to structural instability and competing phase formation. Here, we report the first successful synthesis of phase-pure Sr-intercalated CoO₂ nanotubes (Sr_xCoO₂, denoted as SCONTs) via a top-down hydrothermal approach from bulk Sr₆Co₅O₁₅. This process involves a dimensional transformation, converting face-sharing CoO₆ chains into edge-sharing CoO₆ octahedra arranged in a 2D crystal structure of CoO₂ layers, which roll into nanotubes stabilized by Sr intercalation. This work highlights the critical role of controlled reaction parameters for nanostructuring and precise intercalation of Sr to optimize the electronic structure for bifunctional activity. Comprehensive structural characterization confirms the formation of metastable, phase-pure nanotubes with tunable Sr content. The resulting SCONTs demonstrate excellent OER activity in alkaline media surpassing that of the bulk Sr₆Co₅O₁₅ catalyst, due to an enhanced surface area, optimized electronic structure, and improved Co–O covalency. Additionally, SCONTs (SCONT-5h) demonstrate superior supercapacitor performance in 1 M LiOH, significantly outperforming the bulk material. A maximum energy density of 21.5 Wh kg⁻¹ and a power density of 1.2 kW kg⁻¹ are achieved in a symmetrically assembled supercapacitor device. This dual functionality as both an OER catalyst and high-capacity supercapacitor electrode positions SCONTs as a potential material for both energy conversion and storage applications.