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

Nanoporous boron-doped diamond films with highly ordered pore structures and various pore dimensions. fabricated by oxygen plasma etching of polished polycrystalline diamond films through porous alumina masks, were characterized using electrochemical impedance measurements. The porous structures exhibited wide electrochemical potential windows (2.46-2.70 V). although somewhat less than that for the ax-deposited films (3.04 V) because of surface damage due to the oxygen plasma treatment. A film of pore diam 400 nm and pore 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 frequencies due to the high pore impedance. Impedance measurements carried out at -0.5 V vs. Ag/AgCl indicated a faradaic reaction. most likely hydrogen evolution. 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 were also observed for a representative acetylene black and activated carbon. The as-deposited diamond and 60 x 500 nm honeycomb exhibited significantly higher effective potential limits for anodic charging due to the lack of surface oxidation current. This allows more charge to be stored per unit capacitance for diamond materials than for graphitic carbons. Due to the surface damage. the honeycomb sample did not store as much charge per unit capacitance as the as-deposited sample. (C) 2001 The Electrochemical Society.