Effect of thermal annealing on electrochemical and morphological properties of printed NiO electrodes for flexible asymmetric micro-supercapacitor applications

We report the effects of thermal annealing on screen-printed NiO electrodes for asymmetric battery-type supercapacitors on flexible substrates. Ni(OH)₂ was synthesized and annealed at 300 °C, 400 °C, 500 °C, 600 °C, 700 °C and 800 °C to obtain six morphologically diverse NiO samples. FE-SEM, XRD, BET, and four-probe measurements were used to investigate their morphology, crystallinity, surface area, and conductivity, respectively. Six symmetric supercapacitors were screen-printed on a PET substrate with 1 M H₂SO₄ electrolyte to probe their electrochemical properties. NiO annealed at 400 °C (NiO-400) demonstrated the highest areal capacitance of 19.54 mF/cm² coupled with reduced charge-transfer resistance, consistent with the highly porous yet continuous structure observed via FE-SEM and BET surface analysis. With this, an inter-digital asymmetric supercapacitor with NiO-400 cathode and graphene anode was fabricated to demonstrate high areal capacitance of 26.42 mF/cm², a wide potential window of 1.5 V (at 97.3% coulombic efficiency) and excellent flexibility with 96.4% retention (180°) after 1000 cycles.