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

We describe a simple self-limiting electroless deposition process whereby conformal, nanoscale iron oxide (FeOx) coatings are generated at the interior and exterior surfaces of macroscopically thick (similar to 90 mu m) carbon nanofoam paper substrates via redox reaction with aqueous K2FeO4. The resulting FeOx-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 FeOx loadings (27 wt %). The additional charge-storage capacity arises from faradaic pseudocapacitance of the FeOx coating, delivering specific capacitance >300 F g(-1) normalized to the content of FeOx 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 FeOx coating, resulting in <20% capacitance fade over 1000 consecutive cycles.