To further probe the influence of scan rate on PEDOT thin film electrosynthesis, CV cycling experiments were carried out at 1 and 100 mV•s -1 , as described in Supplementary Section S9. Using aqueous Ce 4+ concentrations of either 2 or 4 mM (Supplementary Figs. ), the PEDOT thin films formed at 100 mV•s -1 were notably thinner and had a more metallic or "golden" colour (Supplementary Fig. ) than a PEDOT thin film formed at 25 mV•s -1 under otherwise identical conditions. A high concentration of EDOT oligomers is confined to a thinner organic EDL at 100 mV•s -1 than at 25 mV•s -1 , potentially leading to differences in the morphology and thickness of the film formed. At the lower scan rate of 1 mV•s -1 , no PEDOT interfacial film was formed after cycling using aqueous Ce 4+ concentrations of either 2 or 6 mM, respectively (Supplementary Fig. ). This was despite the characteristic CVs of PEDOT interfacial electrosynthesis being observed during the initial cycles with 6 mM Ce 4+ (Supplementary Figs. and). At 1 mV•s -1 , the relatively long contact time with the large excess of interfacial Ce 4+ may deplete the interfacial concentration of EDOT monomers and overoxidise oligomers present in the organic diffuse layer. These overoxidised oligomers may then be incapable of forming longer chains during subsequent CV cycles to reach the critical chain length. Direct visual evidence of such overoxidation was the coating of the liquid|liquid interface with a white precipitate after cycling at 1 mV•s -1 (Supplementary Fig. ). The latter immediately redissolved upon agitating the L|L electrochemical cell.