Nanostructured viologen-phosphate bridged titanium dioxide frameworks: Tuning thermal growth for improving supercapacitance

Titanium dioxide (TiO2) has much potential for usage in supercapacitors because of its unique structural characteristics, outstanding chemical stability, low cost and availability, and lack of environmental impact. However, TiO2 presents a low specific capacitance due to its limited electrical conductivity. Incorporating a conductive matrix (e.g. carbon) is a logical way to exploit TiO2's electrochemical potential, but the efficiency of electron transport from the carbon matrix to TiO2 nanoparticles has been challenged by the weak interfacial stability between the two phases. To further address this issue, we leveraged on the highest stability of phosphate conjugated titanium dioxide (P-O-Ti) and have crafted an ionically-tunable viologen phosphate from which open porous viologen-bridged titanium (oxide) phosphate frameworks could be obtained. Thermal annealing was performed between 180 and 900 degrees C to investigate the effect of the conductive carbon matrix on supercapacitor performance. During carbonization, viologen serves as a nitrogen-containing carbon precursor while phosphates behave as a stabilizer by end-capping the surface of crystalline anatase titanium dioxide, limiting its thermally-induced sintering to <7 nm at 700 degrees C and delaying its transition to rutile phase until 900 degrees C. Based on three-electrode measurement, the material pyrolyzed at 700 degrees C displayed the best electrochemical performance among samples, with an areal specific capacitance of 283 mF/cm(2) at 0.25 mA/mg (3.43 mA/cm(2)) in the potential range of -1 to +1 V vs. Ag/AgCl. Due to homogeneous dispersion of phosphate stabilized TiO2 nanoparticles within an N-doped conductive carbon matrix, VioP@TiO2-700 displays excellent rate capability. When the discharge current density increased by 16 times, the specific capacitance decrease was only of 14.3 %. The charge storage mechanism of TiO2-based electrodes showed that the surface-controlled process was dominant at the negative potential, while the diffusion-controlled process was dominant at the positive potential. Moreover, the symmetric cell exhibited a high energy density of 21.46 mu Wh/cm(2) at a power density of 272.02 mu W/cm(2) and good capacitance retention of 95.3 % over 4000 cycles. The outlined supercapacitor performance suggested that the resulting nitrogen-containing carbon/phosphate-stabilized TiO2 nanocomposite is promising for energy storage applications.