Ti–Cu alloy-based converted suboxides: robust electrode scaffolds for enhanced electrocatalytic glycerol oxidation

Abstract

Titanium dioxide (TiO₂) nanotubes exhibit high surface area, chemical stability and geometrically favorable electronic properties, posing as a suitable support material for embedded nano-catalysts. However, being semiconductive in nature, TiO₂ nanotubes suffer from limited conductivity. This hampers their use as electrodes especially in anodic applications where they experience significant current blockage. In this study, we investigate the intrinsic decoration of TiO₂ nanotubes with Cu sites and the subsequent defect engineering of the electrodes for enhanced glycerol oxidation. Despite its good electrocatalytic glycerol oxidation performance, Cu suffers from low stability, requiring a sturdy scaffold as a support material. Ti–Cu alloy foils containing trace concentrations of copper were anodized, simultaneously decorating titania nanotubes with Cu species as they grow on the metal substrate, followed by treating the metal–metal oxide electrodes in an optimized reducing environment. Cu nanoparticles as small as 2–3 nm in diameter are formed as a result as well as point defects in the titania lattice such as Ti³⁺ and oxygen vacancies. The presented methodology results in significantly uplifting the conductivity of TiO₂, robustly securing Cu-based catalytic sites on the titania tube walls and allowing for the synergistic interactions between Cu sites and point defects in the titania lattice. These physicochemical changes were confirmed by electron microscopy techniques (FESEM, TEM, HAADF) and surface characterization (XPS, EDX), revealing homogeneously distributed Cu nanoparticles, lattice distortion, and increased Ti³⁺/Ti⁴⁺ ratios. The substoichiometric TiO₂ nanotubes also demonstrated an increased capacity to scaffold co-catalysts; this was exemplified using Co deposition, which showed uniform nucleation and stable anchoring, indicative of improved chemical compatibility and interface engineering. Electrochemical analysis demonstrated a significant improvement in activity, where binder-free Cu-decorated electrodes treated in a hydrogen environment at 500 °C showed up to 239 times higher glycerol oxidation activity than air-annealed CuTNA, and up to 325 times higher activity than pristine TNAs. The enhancement is attributed to the combination of improved charge transport, increased active surface area, and catalytic synergy between Cu and TiO₂ defect sites. Furthermore, the modified electrodes exhibited Faradaic efficiency as high as 33.69% toward the partial oxidation of glycerol, suggesting favorable selectivity toward value-added oxidation products, mainly formate and glycolate. These findings demonstrate the effectiveness of intrinsic nanostructure and interface engineering in achieving highly conductive, active, and potentially selective electrodes for anodic electrochemical reactions.