Highly efficient textile supercapacitors fabricated with graphene/NiO:Yb electrodes printed on cotton fabric

We report in this work the electrochemical performance of textile supercapacitors (SCs) fabricated with conductive electrodes of cotton-fabric+graphene. NiO:Yb microparticles (YbNi MPs) with cubic phase and sizes of 1.5–45 µm were introduced into the SCs in order to enhance their capacitance and energy density. Two different asymmetric devices were fabricated with electrodes of cotton+graphene (cathode) and cotton+graphene+YbNi MPs (anode). The first device (named as YbNi-SC) employed as separator a biodegradable rice paper and the second one (named as YbNi-S-SC) used as separator a recycled dishwasher sponge. According to the electrochemical characterization, the YbNi-SC and YbNi-S-SC devices presented a maximum capacitance of 640.6 and 1153.1 F/g, respectively. These devices also presented a maximum energy density of 128.1 and 230.6 Wh/kg, respectively, which are among the highest reported so far for textile SCs. A reference G-SC device made without YbNi MPs was also fabricated, but it presented a poor capacitance and energy density of 11.2 F/g and 22.1 Wh/kg, respectively. Moreover, The GC-SC device was completely discharged after only 1683 s (28.05 min), while the devices made with YbNi MPs (YbNi-SC and YbNi-S-SC) had discharge times of 4171 s (69.6 min) and 16,604 s (276.7 min), respectively. Moreover, the YbNi-S-SC device had superior capacitance retention (≈95%) over the YbNi-S-SC device (≈83%) after 500 cycles of charge/discharge. The optical absorbance, Raman and XPS spectra confirmed the presence of oxygen vacancy defects, Yb3+/Yb2+ and Ni3+/Ni2+ species in the SC electrodes, which acted as redox centers for the charge storage. Impedance measurements were also carried out and a reduction of the electrical resistance (≈36%) at the electrode/electrolyte interface was produced in the SCs containing the YbNi MPs, which facilitated the ion diffusion/transport in the SC electrodes. Hence, the results obtained here demonstrate that the textile SCs made with YbNi MPs are efficient for wearable applications.