Electroless Deposition of Conformal Nanoscale Iron Oxide on Carbon Nanoarchitectures for Electrochemical Charge Storage

Abstract

We describe a simple self-limiting electroless deposition process whereby conformal, nanoscale iron oxide (FeO_x) coatings are generated at the interior and exterior surfaces of macroscopically thick (∼90 μm) carbon nanofoam paper substrates via redox reaction with aqueous K₂FeO₄. The resulting FeO_x-carbon nanofoams are characterized as device-ready electrode structures for aqueous electrochemical capacitors and they demonstrate a 3-to-7 fold increase in charge-storage capacity relative to the native carbon nanofoam when cycled in a mild aqueous electrolyte (2.5 M Li₂SO₄), yielding mass-, volume-, and footprint-normalized capacitances of 84 F g⁻¹, 121 F cm⁻³, and 0.85 F cm⁻², respectively, even at modest FeO_x loadings (27 wt %). The additional charge-storage capacity arises from faradaic pseudocapacitance of the FeO_x coating, delivering specific capacitance >300 F g⁻¹ normalized to the content of FeO_x as FeOOH, as verified by electrochemical measurements and in situ X-ray absorption spectroscopy. The additional capacitance is electrochemically addressable within tens of seconds, a time scale of relevance for high-rate electrochemical charge storage. We also demonstrate that the addition of borate to buffer the Li₂SO₄ electrolyte effectively suppresses the electrochemical dissolution of the FeO_x coating, resulting in <20% capacitance fade over 1000 consecutive cycles.