Effect of chromium dopant on the capacitance of CoFe2O4 nanoparticles for high performance flexible supercapacitors

CoFe 2 O 4 (CFO) and Cr 3 + doped CoFe 2 O 4 (CFO:Cr (2 mol%)) nanoparticles were used to fabricate flexible supercapacitors (SCs). Firstly, SC electrodes were fabricated by printing graphene ink onto recycled polyethylene substrates. Next, CFO or CFO:Cr nanoparticles were deposited on these electrodes using a Dr. Blade technique. Symmetric SCs made with CFO and CFO:Cr redox powders (deposited on the electrodes) were named CFO-SC and CFO:Cr-SC, respectively. After the electrochemical evaluation of both devices, the CFO-SC reached a capacitance of 545 F g −1 and energy density of 75.9 Wh kg −1 , whereas the CFO:Cr-SC device exhibited superior capacitance of 970 F g −1 and energy density of 152.4 Wh kg −1 . Thus, adding CFO:Cr nanoparticles on SC electrodes boosted the capacitance by 78%. The devices above were made using only one separator/gel-electrolyte layer (cellulose-membrane), but a third device was made with two separator/gel-electrolyte layers. This last device had the highest capacitance-retention of 95% and 88% after 1000 cycles of charge-discharge and 1000 bending cycles, respectively. This higher performance was associated with better mechanical resistance. Furthermore, Raman and XPS spectroscopies revealed the presence of active redox species (Fe, Co, Cr and oxygen vacancies) on CFO and CFO:Cr nanoparticles. Those ones participated for the storage of charge, but we noted that the content of Fe 3+ and Co 2+ species was 90–111% higher in the electrodes of the CFO:Cr-SC device. Hence, the results of this investigation demonstrated that doping with Cr is a feasible strategy to increase the energy density to competitive values, which are similar to these for commercial-LIBs.