Ultra-high capacitance hematite thin films with controlled nanoscopic morphologies

We synthesized α-Fe₂O₃ (hematite) thin films with two different nanoscopic morphologies through self-assembly between a Fe-precursor and a Pluronic tri-block copolymer (F127) followed by aging and calcination. Relative humidity (RH) during the aging step of the spin-coated films was found to be critical in determining the morphologies. A network structure of nanowires ∼6 nm in diameter formed when the RH was 75%. The resulting nanowire hematite thin film (NW) had 150-250 nm-sized macropores. When the RH was 0%, a mesoporous hematite thin film (MP) with a wormlike pore structure and a pore size of ∼9 nm formed. Investigation of the electrochemical properties of these films revealed that they had very high specific capacitances of 365.7 and 283.2 F g(-1) for NWs and MPs, respectively, at a current density of 3 A g(-1) in a 0.5 M Na₂SO₃ electrolyte. Both of these capacitance values are considerably higher than those previously reported for hematite-based electrodes. We attributed this to the high porosity of the thin films, which enables ready access of electrolyte ions to the electrode surfaces, and their ultra-thin size, comparable to that of the depletion layer, allowing the low conductivity of hematite to be overcome. The higher capacitance of NWs than MPs is likely due to the accelerated electron transport through the crystalline nanowires in NWs.