Electrochemical energy storage devices, like supercapacitors and batteries, are needed to meet the constantly increasing demands of portable energy devices. Many types of supercapacitor electrodes were synthesized and reported which include carbon-based materials, polymers, oxides and perovskites. In this work, we report on the fabrication of Cu doped bismuth ferrite (Cu-BiFeO3, Cu-BFO) nanoparticles via sol-gel procedure. In most of the doping cases the costly rare earth elements like La, Nd, Sm, Ce and Yb are used to replace Bi. Electrodes of Bi1-xCuxFeO3 where x have the values 0.0, 0.025, 0.05, 0.075, 0.1 and 0.15 were deposited on graphitic sheets. Optimum Cu doping value was found to be at x = 0.05. Cu2+ ions doping in place of Bi3+ ions will reveal the roles of oxygen vacancies, as determined from XPS. The estimated values of specific capacitance was found to be 732 F/g at 0.5 A/g for the electrode at x = 0.05 in three electrode system. 5%Cu-BFO//Carbon black asymmetric supercapacitor (ASC) device can be charged and discharged reversibly at cell voltage of 0.0 up to 1.5 V when using a 0.5 M Na2SO4 aqueous electrolyte. This configuration allows the ASC device to deliver a great energy density of 37.25 Wh/kg at a power density of 752 W/kg. The ASC device demonstrates excellent cycling stability, retaining 88.64 % of its initial specific capacitance after 5000 cycles. Increasing the Cu content up to 15 % causes instability in the crystal structure of BFO resulting on phase segregation into Bi2Fe4O9 along with BFO and deterioration in the electrode overall performance. XPS showed that replacement of Cu2+ in place of Bi3+ disturbs the neutrality and the chemical environment of the compound. Hence to achieve neutrality and ionic balance, oxygen vacancies increase and the Fe2+/Fe3+ ratio increases as well. Both oxygen vacancies and the change in the Fe ions ratio reduce electrochemical impedance and give rise to charge density. The homogenous distribution of the pores at x = 0.05 contributed greatly to its fast electrolyte ion diffusion that improved the observed capacitive properties. One has to mention here, that despite doping that contains ions with different ionic size and/or different oxidation state is unfavorable, to some extent, due to the changes they produce in the electronic structure and the chemical environment, nevertheless it is sometimes beneficial in creating new parameters in the structure that promote specific application like in the case of the current work, oxygen vacancies, increase in surface area and the mixed state valency.
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