Structure-dependent electrochemical response characteristics of antimony tin oxide nanoparticle-based porous electrodes

Antimony tin oxide (ATO) nanoparticle-based porous electrodes have been investigated for use in fast-response electrochromic devices. However, despite their low resistivity, the electrochemical response characteristics of these electrodes are inferior to those of TiO2, which was attributed to the effect of small particle and pore size based on structural simulation. Therefore, we investigated the electrochemical response characteristics of ATO porous electrodes with different nanoparticle sizes, to clarify the effect of the porous electrode structure on response characteristics. The time required for charging an electric double layer (EDL) on the surface of a porous electrode increased as the particle size decreased. The ratios of the time constants of the EDL charging current between each porous electrode were larger than the ratios of the effective surface areas although the porous electrodes had almost the same resistivity. When the particle diameter was small (around 20 nm), the electrochromic reaction of dye modification on the porous electrode started 10 s after the application of a potential, because of the extremely low EDL formation rate. It was confirmed that the delay in EDL formation was induced by a lack of electrolyte ions inside the porous electrode. Therefore, to achieve ideal fast-response electrochemical reactions in low-resistivity nanoparticle-based porous electrodes, it is important to optimize the relationship between the electrode structure and the electrolyte ion concentration.