Cobalt and nickel doping influence on physical and electrochemical characteristics of spray-deposited SnO2 thin film supercapacitor electrodes

The present study aims to enhance the electrochemical performance of SnO2 thin film as a supercapacitor electrode by doping different amounts of Co and Ni (0, 2, 4, 6, and 10 at%) into spray-deposited SnO2 films. Both dopants reduced the thickness from 865 nm for pure SnO2 to 525 nm and 720 nm for 10 at% Co and Ni doping, respectively. Apart from the decrease in specular reflectance resulting from Co-doping, the transmittance and estimated band gap experienced the least change for the doped spray-deposited SnO2 films. Pure SnO2 thin film showed a fairly weak electrochemical response. However, introducing both dopants led to a considerable promotion in the ion storage of the spray-deposited SnO2 electrodes. Up to 6 at% of Co-doping led to a pseudocapacitive behavior, while thin film electrodes with 10 at% of Co-dopant exhibit a hybrid pseudocapacitive and battery-type electrochemical response. Although the Ni-doped SnO2 thin film electrodes also respond in a hybrid manner, the stronger faradaic redox couple in their cyclic voltammetry (CV) curve indicates the dominance of battery-type performance. The calculated capacitance from CV data exhibited an increase from 0.01 mF/cm(2) for pure SnO2 to 0.68 mF/cm(2) and 0.96 mF/cm(2) for 10 at% of the Co and Ni-doped SnO2 electrodes, respectively. Moreover, using the Nyquist plot, different forms of cell resistance were estimated for the Co and Ni-doped SnO2 thin films. Although the internal and equivalent series resistance was promoted with both dopant increases, the equivalent distribution resistance (which favors better ion diffusion) declined.