Carbon–TiO2 composites as high-performance supercapacitor electrodes: synergistic effect between carbon and metal oxide phases

Abstract

A series of carbon xerogels doped with different percentages of TiO₂ has been studied as a tentative means of preparing electrodes for supercapacitors. Carbon composites were obtained by an inverse emulsion method in n-heptane, and after carbonization at 900 °C the metal oxide phase was well dispersed in the carbon phase with a crystal size of less than 4 nm, and only an anatase phase was detected. An increase in the percentage of TiO₂ produced a decrease in the hydrophobicity of the composite, which improved the wettability of the electrodes. XPS results showed that Ti³⁺ and Ti⁴⁺ were present on the surface of samples, and the presence of both oxidation states can improve the electron mobility in the inorganic phase. The obtained composite materials possessed specific surface areas that ranged from 423 to 539 m² g⁻¹ and very well developed micro- and mesoporosity with a total pore volume that ranged from 0.361 to 0.480 cm³ g⁻¹. The mean size of supermicropores increased as the percentage of TiO₂ increased, whereas practically no variation was found in the size of ultramicropores. Two-electrode symmetric supercapacitors based on the carbon xerogel–TiO₂ composites exhibited high electrochemical performance, which was better than that of other similar materials in the literature, and displayed high capacitance (up to 137 F g⁻¹ at 0.250 A g⁻¹ for the composite containing 20% TiO₂), high capacitance retention (66–80%) at 20 A g⁻¹ and a high energy density of 20.15 W h kg⁻¹ at a power density of 138.11 W kg⁻¹ in the voltage range of 0 V to 1.1 V. A sample with a combination of low hydrophobicity and an adequate micro/mesopore network with an intermediate content of TiO₂ exhibited the best performance for energy storage. A floating test showed the very good cyclability of the synthesized materials.