Electrochemical characterization of the nanoporous honeycomb diamond electrode as an electrical double-layer capacitor

Electrochemical properties of nanoporous honeycomb diamond electrodes in an aqueous electrolyte were investigated. Highly ordered honeycomb diamond st ructures were fabricated by etching the microwave plasma-deposited diamond films using an oxygen plasma. The diamond honeycomb exhibited a wide electr ochemical potential window (ca. 2.5 V), similar to the unetched diamond fil m. From impedance measurements, the capacitance of the diamond honeycomb fi lm was found to be 1.97 X 10(-3) F cm(-2) (geometric area), which is cn. 20 0 times greater than that for the unetched, as-deposited surface. The resul ts obtained with galvanostatic measurements were consistent with this value . The formation of the highly ordered porous structure, together with surfa ce oxidation, was found to be responsible for the observed enhancement in t he capacitance. The transmission line model for cylindrically porous electr odes was successfully applied to the present honeycomb structure. Based on an estimation of the capacitance of a hypothetical through-hole diamond mem brane, the specific capacitance is ca. 16 F g(-1), which is within an order of magnitude of the range for activated carbon capacitors (100 to 400 F g( -1)). (C) 2000 The Electrochemical Society. S0013-4651(99)05-044-2. All rig hts reserved.