One‐Step Electrodeposition of Iron Oxyhydroxide Onto 3D Porous Graphene Substrates for on Chip Asymmetric Micro‐Supercapacitors

Abstract Electrochemical capacitors based on redox active materials can achieve greater capacitance values than traditional electric double layer composites. Herein, electrodeposition of iron oxyhydroxide from a mildly acidic acetate precursor is reported. The one‐step deposition resulted in a submicron film composed of FeOOH phase, which was confirmed via Raman and x‐ray photoelectron spectroscopy. The capacitance increased linearly with loading amount and achieved a maximum at 1600 mC deposition with 120 mF cm −2 at 25 mV s −1 after which the film became more resistive, limiting electrolyte access to the porous graphene substrate. The deposited FeOOH demonstrated promising rate capability and good cycling stability, without phase changes, retaining 82 % of the initial capacitance after 5000 consecutive charge/discharge cycles. The charge storage mechanism of FeOOH was determined via in situ Raman spectroscopy, which followed reversible iron oxygen vibration changes upon cycling which become more intense upon reduction as a result of sodium ion intercalation. Furthermore, an asymmetric configuration full cell combining FeOOH/MnO 2 allowed the working voltage to be extended to 2 V, maintaining an ideal capacitor behaviour, and achieving a maximum energy and power density of 21 μWh cm −2 and 2.5 mW cm −2 respectively.