Nano-TiO ₂ /Graphene Using Pulsed Laser Ablation (ULPING) for Aqueous Sodium-Ion Pseudocapacitors

The growing demand for sustainable energy storage solutions has accelerated research into sodium-ion systems, such as sodium-ion batteries (SIBs) and sodium-ion capacitors (NICs), as promising alternatives to lithium-ion energy storage. However, SIBs face challenges such as low energy density, sluggish kinetics, and limited cycle life, necessitating the development of advanced pseudocapacitor electrode materials. This study reports the fabrication of nanostructured TiO 2 and TiO 2 -graphene multilayered electrodes using ultrashort laser pulses for in situ nanostructure generation (ULPING) for aqueous sodium-ion pseudocapacitors. The TiO 2 layer was directly grown on titanium substrates via pulsed laser ablation using a ytterbium picosecond fiber laser, achieving a highly porous floret-like morphology conducive to ion diffusion. An additional graphene layer, applied with a carboxymethyl cellulose (CMC) binder, improved conductivity and surface area. Structural characterization revealed a mixed anatase/rutile TiO 2 phase with predominant rutile, which is advantageous for high-power performance. Electrochemical characterization in a 1 M Na 2 SO 4 electrolyte using a three-electrode testing system demonstrated that the TiO 2 -graphene electrodes exhibited a significantly higher areal capacitance of 46.9 mF/cm 2 and an energy density of 22.3 μWh/cm 2, approximately 3.5× higher than pristine TiO 2 . Cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) confirmed a pseudocapacitive behavior with a stable capacitance retention of over 85% after 200 cycles. Moreover, the TiO 2 -graphene electrode showed a reduced charge transfer resistance and a higher sodium-ion diffusion coefficient (2.124 × 10 – 14 cm 2 /s) compared to pristine TiO 2, indicating enhanced electrochemical kinetics. This study highlights the potential of ULPING as a scalable and cost-effective technique for engineering high-performance electrode materials, positioning TiO 2 -graphene hybrids as viable candidates for next-generation aqueous sodium-ion pseudocapacitors.