Stabilizing the output voltage of flexible graphene supercapacitors by adding porous Ag/N-doped TiO2 nanocomposites on their anodes

Abstract

This work reports the electrochemical performance of flexible graphene-based supercapacitors (SCs) fabricated with Ag/TiO2 and Ag/N-doped TiO2 nanocomposites. According to the analysis by high resolution transmission electron microscopy (TEM), the Ag-TiO2 and Ag-N-TiO2 nanocomposites are formed by porous quasi-spherical TiO2 nanoparticles with sizes of 27 ± 1 nm and 24 ± 1 nm, respectively. In addition, the pore size distribution was obtained for both nanocomposites and found that the Ag-TiO2 and Ag-N-TiO2 nanocomposites have 73% and 83% of pores with sizes in the range of 1–20 nm, respectively. Thus, doping with N, increased the content of pores by 10% in the Ag-TiO2 nanocomposites. The electrochemical performance of the SCs was also evaluated, and we found that devices made with Ag/TiO2 (Ag-Ti/SC) and Ag/N-doped TiO2 (Ag-N-Ti/SC) presented 9-62 times more capacitance and energy density than the SCs made without TiO2 nanocomposites. The anode electrodes coated by the Ag/TiO2 and Ag/N-doped TiO2 nanocomposites were analyzed by X-ray photoelectron spectroscopy, optical absorbance and Fourier transform infrared techniques, and found carboxylic groups, oxygen vacancies, Ag0/Ag+/Ag3+ and Ti3+/Ti4+ species. All those species acted as redox centers to store charge by Faradaic reactions. Moreover, the use of the Ag/N-doped TiO2 nanocomposite in the SCs caused a stabilization of the output voltage (1.91 V). To the best of our knowledge, this last value of output voltage is the highest value reported for titania based SCs so far. Thus, the presence of Ag/TiO2 doped with N into the SCs prolonged the discharge times and produced a battery-like behavior. This last effect could be useful for the development of competitive and attractive graphene-based batteries.