Theoretical study of ether group substitution effects on the electrochemical properties of Ti-modified vanadium-oxide clusters

Abstract

In this study, the effects of ether group substitution on the electrochemical properties of [Ti₂V₄O₅(OCH₃)₁₄] (Ti₂V₄) and [Ti₃V₃O₄(OCH₃)₁₅]⁺ (Ti₃V₃⁺) were investigated by combining density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The lowest unoccupied molecular orbital energy decreases from Ti₂V₄ (−1.98 eV) to the ether-substituted derivatives of Ti₂V₄ (−2.04 eV), and from Ti₃V₃⁺ (−2.55 eV) to the ether-substituted derivatives of Ti₃V₃⁺ (−2.64 eV). The ether substitution affects the electrochemical window of Ti₃V₃⁺ derivatives that increases from 2.69 V for Ti₃V₃⁺ to 2.97 V for [Ti₃V₃O₄(OCH₃)₁₂(OCH₂)₃CCH₂OC₂H₄OCH₃]⁺ (Ti₃V₃TRIOL^C+). The ether substitution also affects the interaction between vanadium-oxide clusters and both the solvent CH₃CN and the supporting electrolyte tetrabutylammonium hexafluorophosphate ([NBu₄][PF₆]), leading to a substantial increase in the diffusion coefficient of the Ti₃V₃⁺ series: from 5.8 × 10⁻⁶ cm² s⁻¹ for Ti₃V₃⁺ up to 7.8 × 10⁻⁶ cm² s⁻¹ for [Ti₃V₃O₄(OCH₃)₁₂(OCH₂)₃CCH₂OCH₃]⁺ (Ti₃V₃TRIOL^B+) and 9.2 × 10⁻⁶ cm² s⁻¹ for Ti₃V₃TRIOL^C+. Radial distribution function (RDF) and electrostatic potential (ESP) analyses further indicate that ether substitution modulates the surface charge density and enhances the hydrogen bonding interactions between CH₃CN and vanadium-oxide clusters. These findings suggest that Ti₃V₃TRIOL^C+ possesses superior electrochemical performance, highlighting its potential as a promising electroactive material for RFBs.