Hidden defects unveiled: Solvent-driven kinetic accessibility in anatase TiO2 pseudocapacitance

Abstract

Organic solvent selection in anatase TiO₂ nanoparticle synthesis profoundly influences microstructure and electrochemical performance despite comparable intrinsic defect chemistry. Comprehensive multi-scale characterization of ethanol-, ethylene glycol-, and glycerol-assisted TiO₂ reveals that ethanol-derived monodispersed ∼10-15 nm low-strain nanoparticles with clean surfaces achieve superior galvanostatic capacitance (44.4 mF g⁻¹ at 0.2 mA g⁻¹), linear charge-discharge profiles, and minimal charge-transfer resistance (∼12 Ω), reflecting 9.4% defect utilization. In stark contrast, glycerol-assisted TiO₂—featuring ∼15-20 nm primary crystallites, 9 times higher lattice strain, and residual organic coatings (5.32 at% C)—exhibits excellent equilibrium capacitance (563 F g⁻¹ at 2 mV s⁻¹) but catastrophic practical failure (7.8 mF g⁻¹ at 0.2 mA g⁻¹), trapping 98.6% of Ti³⁺/oxygen vacancy sites due to 12 times elevated impedance (R_ct∼150 Ω). Ethylene glycol occupies the performance intermediate. Cyclic voltammetry, galvanostatic charge-discharge, Randles-Sevcik, and electrochemical impedance spectroscopy establish a kinetic accessibility framework: solvent microstructure governs not defect quantity but practical electroactivity. This work provides rational design principles—ethanol TiO₂ for high-energy pseudocapacitors, glycerol TiO₂ for ultra-long cycle life (93% retention after 1000 cycles)—with broad implications for defect-engineered electrochemical energy storage.