NaOH-Aided Platinum Nanoparticle Size Regulation on Polybenzimidazole-Wrapped Carbon Nanotubes for Use as Non-Humidified Polymer Electrolyte Fuel Cell Catalyst

We describe a method to control the size of platinum nanoparticles (Pt-NPs) on poly[2,2′-(2,6-pyridine)-5,5′-bibenzimidazole] (PyPBI)-wrapped pristine multi-walled carbon nanotubes (MWNTs) without changing the loading amount by adding various concentrations of sodium hydroxide (NaOH) to the ethylene glycol aqueous solutions. The Pt-NP sizes decreased from 3.8±0.4 to 1.7±0.1 nm with increasing NaOH concentrations. The mass activity of the oxygen reduction reaction was 224 mA mg_Pt^-1 for the electrocatalyst with 3 nm Pt-NPs, in which the power density reached 350 mW cm^-2, which was 1.4 times higher than that of the electrocatalyst synthesized without NaOH (250 mW cm^-2) at 120 °C and without any extra humidification. The electrocatalyst synthesized from a small amount of additional NaOH (0.5 mm) in the starting solution shows an enhanced performance and similar Pt stability. More importantly, the Pt-NP size showed no effect on the Pt stability owing to the use of the PyPBI.