Impedance characteristics of the nanoporous honeycomb diamond electrodes for electrical double-layer capacitor applications

Nanoporous boron-doped diamond films with highly ordered pore structures an d various pore dimensions. fabricated by oxygen plasma etching of polished polycrystalline diamond films through porous alumina masks, were characteri zed using electrochemical impedance measurements. The porous structures exh ibited wide electrochemical potential windows (2.46-2.70 V). although somew hat less than that for the ax-deposited films (3.04 V) because of surface d amage due to the oxygen plasma treatment. A film of pore diam 400 nm and po re depth 3 mum exhibited a 400-fold increase in the capacitance (3.91 x 10( 3) F cm(-2)) in comparison to an as-deposited surface film. A film with 30 nm diam pores showed little enhancement in the capacitance at useful freque ncies due to the high pore impedance. Impedance measurements carried out at -0.5 V vs. Ag/AgCl indicated a faradaic reaction. most likely hydrogen evo lution. Although the dc current was small (ca. 3.0 muA cm(-2) real area for the 60 nm honeycomb), it represents a pathway for charge leakage, limiting the negative potential limit for charge storage. However. similar limits w ere also observed for a representative acetylene black and activated carbon . The as-deposited diamond and 60 x 500 nm honeycomb exhibited significantl y higher effective potential limits for anodic charging due to the lack of surface oxidation current. This allows more charge to be stored per unit ca pacitance for diamond materials than for graphitic carbons. Due to the surf ace damage. the honeycomb sample did not store as much charge per unit capa citance as the as-deposited sample. (C) 2001 The Electrochemical Society.