Highly active catalysts for the oxygen reduction reaction are essential for the widespread and economically viable use of polymer electrolyte fuel cells. Here we report the oxygen reduction reaction activities of single‒crystal platinum electrodes in acidic solutions containing tetraalkylammonium cations with different alkyl chain lengths. The high hydrophobicity of a tetraalkylammonium cation with a longer alkyl chain enhances the oxygen reduction reaction activity. The activity on Pt(111) in the presence of tetra‒n‒hexylammonium cation is eight times as high as that without this cation, which is comparable to the activities on Pt3Co(111) and Pt3Ni(111) electrodes. Hydrophobic cations and their hydration shells destabilize the adsorbed hydroxide and adsorbed water. The hydrophobic characteristics of non‒specifically adsorbed cations can prevent the adsorption of poisoning species on the platinum electrode and form a highly efficient interface for the oxygen reduction reaction.

Understanding microscopic mechanism of multi-electron multi-proton transfer reactions at complexed systems is important for advancing electrochemistry-oriented science in the 21st century.

The activity of the oxygen reduction reaction (ORR) is significantly affected by the nonspecifically adsorbed ions at the electrode/electrolyte interface as well as the surface structures of the catalytic substrate. In this study, we found that an organic cation activates the ORR on Pt single-crystal electrodes in alkaline media, and we evaluated ORR activity using shape-controlled Pt nanoparticles. The ORR activity on Pt(111) was significantly enhanced by a tetramethyl­ammonium cation (TMA+), which correlates with its hydrophobicity as is the case for alkali metal cations. However, an alkyl­ammonium cation with a long chain deactivated electrochemical reactions because these cations assemble at high concentrations on the surface through their hydrophobic interionic interactions with alkyl chains. The ORR activity on Pt nanoparticles was also enhanced by TMA+; cuboctahedral Pt nanoparticles with a (111) plane exhibit a markedly high rate of increase in ORR activity.

Weakening of the poisoning by the specifically adsorbed anions assists in developing next-generation electrocatalysts for use in low-temperature fuel cells. In this study, we evaluated how hydrophobic cations with different alkyl chain lengths affect the oxygen reduction reaction (ORR) activities on the single-crystal Pt surfaces in contact with sulfuric acid solution and Nafion ionomers. Interfacial tetraalkylammonium cations with longer alkyl chains activated the ORR on the Pt(111) surface. In a solution containing tetrahexylammonium cations (THA + ), the ORR activities on Pt(111) in sulfuric acid solution and on Nafionmodified Pt(111) in perchloric acid solution were four and eight times higher than those in the solutions without THA + , respectively. Infrared spectroscopy revealed the reduction of the amount of (bi)sulfate anions and the sulfonate group of Nafion adsorbed on Pt(111) due to the presence of THA + . The hydrophobic cations weaken the noncovalent interactions between specifically adsorbed species and promote the ORR.