Source molarity affected surface morphological and electrochemical transitions in binder-free FeO(OH) flexible electrodes and fabrication of symmetric supercapacitive device

Abstract

Binderless chemical synthesis of flexible electrodes (FEs) of FeO(OH) has been carried out by simple and cost-effective successive ionic layer adsorption and reaction (SILAR) technique. In aqueous route synthesis, FeCl₃ and NaOH solutions were used as cationic and anionic sources, respectively. Molar concentration of NaOH was kept constant while the molar concentration of FeCl₃ was varied from 0.2 to 0.8 M with step increments of 0.2 M to study its effect on physical and electrochemical characteristics of prepared FEs. X-ray diffraction (XRD) patterns of the FEs exhibit existence of tetragonal FeO(OH). SEM images show the rice grain-like morphologies. Cyclic voltammetric analyses indicate the decrease in specific capacitance value with the increase in molar concentration of cationic precursor. For the electrode prepared with 0.2 M FeCl₃ as precursor, the observed maximum specific capacitance (SC) was 444.44 F g⁻¹ at 5 mV s⁻¹ in 1 M NaOH, hence these electrodes were used to fabricate the symmetric solid state supercapacitors. Prepared symmetric supercapacitive devices (SSD) were electrochemically analyzed. The maximum SC for the symmetric supercapacitor was found to be 320.50 F g⁻¹ at 5 mV s⁻¹ which was nearly same as that given by GCD analysis which is 313.27 F g⁻¹ at 0.5 mA. Nyquist plot of the device shows minute semicircle in the high frequency region and the mid-low frequency region shows straight line with inclination of nearly $40^\circ$ with X-axis. The equivalent series resistance (ESR), charge transfer resistance (R_ct) and Warburg impedance (R_w) are found to be 2.58, 2.56 and 0.8 Ω, respectively. The prepared SSD shows high cycling stability with 88% of capacitive retention even after 2000 cycles.