AC IMPEDANCE AND CYCLIC VOLTAMMETRIC CHARACTERISTICS OF ELECTROOXIDIZED CARBON-FIBERS AND PALLADIUM CARBON-FIBER ELECTROCATALYTIC SYSTEMS IN ACIDIC AQUEOUS-SOLUTIONS

The modification extent of PAN-based carbon fibres through discontinuous anodic oxidation achieved by potentiostatic double-pulse application is controlled by means of a.c. impedance and cyclic voltammetric data. Through the above procedure, the intercalation compounds, which are initially formed, are converted into a thick layer of surface oxides exhibiting cation-exchanging properties. Palladium-carbon fibre electrocatalytic systems are prepared via exchange of Pd cations with the electrooxidized carbon fibres, followed by cathodic reduction to metallic palladium. The electrocatalytic activity of the above systems in the hydrogen evolution reaction is proved by the double-layer capacity and charge-transfer resistance values estimated from the impedance spectroscopic data. The ratio of concentration [H]/[Pd], calculated from the cyclic voltammograms of the Pd-modified carbon fibres in H2SO4 solutions, indicates that the whole quantity of Pd electrodeposited on the carbon fibres is catalytically active. Pd-modified carbon fibres have several advantages over conventional Pd materials, i.e. they show much more pronounced catalytic efficiency in electrochemical and chemical reactions, they retain their catalytic activity and mechanical stability even after long storage in aqueous solutions and they have the capability of forming multilayers as well as mixed-metal catalyst layers through a procedure of repetitive cation exchange and cathodic reduction.