This work reports the electrochemical performance of sustainable solid-state supercapacitors (SCs) made with cellulose (extracted from diaper waste) and graphene. The electrodes of these SCs were decorated with nanoparticles of PbFe₁₁CrO₁₉ (PbFeCr) ferrite to increase the capacitance of the devices by redox reactions. SCs were fabricated by coating the surface of a diaper with graphene ink, producing a continuous and conductive network and diaper + graphene was employed as the SC electrode. Subsequently, two electrodes were assembled to form a solid-state device (the dielectric separator was a soft textile layer taken from the diaper). The flexible devices were electrochemically characterized and capacitances of 1894.8 and 411.4 F g⁻¹ were obtained for the SCs fabricated with and without PbFeCr, respectively. Thus, the addition of ferrite increased the capacitance by 360%. Furthermore, there was an increase in the energy density of the SCs after the incorporation of ferrite from 36.6 to 164.4 W h kg⁻¹. Also, the SC made using ferrite provided a steady operating voltage of 0.48 V after 10 minutes of uninterrupted discharge, while the device fabricated without ferrite only maintained 0.12 V for the same discharging time. Surface characterization techniques such as absorbance spectroscopy and XPS confirmed the formation of oxygen vacancies, Fe²⁺/Fe³⁺ and Cr³⁺/Cr⁶⁺ species on the surface of the electrodes, which worked as redox-centers for the storage of charge in the SCs. In general, the results of this investigation demonstrated that using diaper waste is a feasible option for the fabrication of efficient and sustainable flexible SCs. This benefits the environment because recycled materials were employed.