Concept of spatial surface heterogeneity in impedance modelling of electrochemically activated glass-like carbon electrode

Abstract

For the first time, the concept of spatial surface heterogeneity has been implemented to the impedance modelling of unmodified, i.e. only polished and progressively electrochemically oxidized/reduced, i.e. activated glass-like carbon, GC, electrodes. The GC electrode surface is generally treated as spatially heterogeneous in a sense of exhibiting at least two different domains with different electrochemical properties. Consequently, the overall electrode impedance is formally divided into the parallel sum of two global impedances, characteristic for two types of the surface and related by corresponding fractional surface coverage. Assuming the common electrolyte resistance term and indirect influence of surface coverage on impedance parameter values, the impedance/frequency equations are derived for a spatially heterogeneous electrode surface showing: 1) surface roughness due to formation of separate damage sites of surface and subsurface layers with high content of oxygen containing species and 2) partial surface porosity due to formation/reduction of oxide films at damage sites which by progressive oxidation have progressively been spread laterally and towards the bulk of the electrode.Regression of derived impedance/frequency equations to experimentally measured impedance spectra of GC electrodes in H2SO4 solution showed predominance of almost smooth, spatially homogeneous surface for unmodified, GC electrode, increased contributions of rough and/or porous parts for weakly to medium activated GC electrodes and domination of completely porous surface for strongly activated GC electrode.