Potassium doping and phase engineering of TiO2 nanotube arrays: high-performance mixed-phase architectures for supercapacitor applications
摘要
Potassium doped TiO2 nanotube electrodes of different phases were synthesized by electrochemical anodization followed by doping, adopting two different strategies. The K-doped mixed-phase electrodes, prepared by an innovative ‘water-bath temperature-controlled electrochemical anodization and doping method, exhibited specific capacitance values four to six times higher than those of the K-doped anatase electrodes prepared by the conventional electrochemical anodization, doping, and annealing. A maximum specific capacitance of 429 mF cm−2 (714 F g−1) at a current density of 0.75 mA cm−2 in a 1.23 V potential was obtained for the K-doped mixed-phase electrode, while the K-doped anatase yielded a specific capacitance of only 96 mF cm−2 (172 F g−1) at 3.8 mA cm−2. The former showed consistent performance in a wider potential window of 1.5 V, manifesting enhanced values of specific capacitance ~ 639 mF cm−2 (1065 F g−1) at 3.3 mA cm−2. The mixed-phase electrode manifested excellent cyclic stability retaining 84% of its initial capacitance value over 10,000 charge discharge cycles. An asymmetric supercapacitor, developed using the K-doped mixed-phase electrode and an activated carbon electrode as the negative and positive electrodes, respectively, operated in 2.1 V potential window, achieving a specific capacitance of 167 mF cm−2 (173 F g−1) at a scan rate of 5 mV s−1. The supercapacitor retained 83% of its initial specific capacitance value after 4800 cycles of operation. Three supercapacitors in series successfully powered a red LED of forward voltage of 1.8 V for 15 min.