Characterisation and modelling of conducting composite electrodes

Abstract

Conducting composite electrodes are versatile devices for biomedical applications showing good biocompatibility, low cost, ease of construction. They can be bulk modified with stabilised enzymes to make biosensors and electrodes for biofuel cells. Low sensitivity to flow conditions arises from the microelectrode array like behaviour but this is accompanied by high capacitance and bulk resistance which can blur the voltammetric detail. We have undertaken a systematic investigation of the effects of composition and formulation on the voltammetric behaviour and non-faradaic properties. Monodisperse glassy carbon spherical powders have been used to simplify the modelling and the relative importance of the patterns of surface conductivity and bulk 3-D connectivity has been investigated. Three dimensional numerical models based on percolation theory have been constructed which allow calculation of the distributed resistances in the composite and which enable qualitative prediction of the voltammetric properties. Voltammetric results showed a bias of E1/2 (half-wave potential) and an unstable il (diffusion limiting current) for conducting composite electrodes. A.C. Impedance showed major changes of Rct (charge transfer resistance) and Cd (double layer capacitance) as the electrodes’ ratio and thickness are varied. Comparison with carbon fibre arrays separates the effects of a distributed interface from three dimensional disparities in the conductance. Key findings are: the distributed resistance in electrodes results in bias for E1/2 and the overlap of diffusion layers on electrode surface leads to an uneven il. Numerical results have shown that the bias of E1/2 and il are in good agreement with experimental works. Different geometric configuration allows the investigation of diffusion layer difference caused by electrode array location and different internal resistance.