Oxygen reduction electrocatalysts sophisticated by using Pt nanoparticle-dispersed ionic liquids with electropolymerizable additivesElectronic supplementary information (ESI) available: Additional data (Fig. S1-S6 and Movie S1) and captions for Movie S1. See DOI: 10.1039/c8ta03465f

The electropolymerization reaction of protic organic salt (POS) diphenylammonium hydrogen sulfate ([DPA][HSO 4 ]) in the solid state proceeds in a N 2 -saturated 0.1 M HClO 4 aqueous solution, and conductive poly(diphenylamine) is formed without difficulty. This reaction has also been observed at the thin ionic liquid (IL) layer between Pt nanoparticles and carbon support on Pt nanoparticle-modified carbon electrocatalysts prepared using a N , N -diethyl- N -methylammonium hydrogen sulfate ([DEMA][HSO 4 ]) protic IL with a [DPA][HSO 4 ] POS. Similar Pt nanoparticle electrocatalysts are fabricated using different electropolymerizable additives, including phenylammonium hydrogen sulfate ([PhNH 3 ][HSO 4 ]). A local electropolymerization reaction at the IL layer can confer a better electrochemical surface area and mass activity retention rates on oxygen reduction electrocatalysts. The performance of the resulting electrocatalysts is dependent on the electropolymerizable species. We demonstrated that conductive polymer formation between Pt nanoparticles and carbon supports contributes to the catalytic performance for oxygen reduction.