As PFV and CPE primarily show the effect of CA on net catalytic electron flow, EIS provides new insights on the potential-and frequency-dependent charge carrier behaviours for IO-TiO2|enzyme electrodes. This complements two previous reports investigating [FeFe]-H2ase 40 and [W]-FDH immobilised on carbon electrodes, as detailed EIS fittings have not been conducted for [NiFeSe]-H2ase and [W]-FDH. Quantitative analysis was undertaken by fitting the Nyquist plots with equivalent circuits , comprising of a series resistor (Rs) to describe cell resistance; a double layer capacitor (Cdl) connected in parallel with a charge transport resistor (Rct) and a Warburg element (Zw) in series to represent the electrical double layer; and an RC circuit (Re, Ce) to depict electron transfer in the faradaic process (Fig. and detailed plots in Fig. ). Specifically, the top, middle and bottom circuits were employed to fit the bare IO-TiO2, IO-TiO2|enzyme (pre-onset) and IO-TiO2|enzymes (post-onset) electrodes, respectively.