Pt Single-Atom Catalysis in Carbon Nanotubes as a Platform for Confinement Limited Electrocatalysis

Nanoconfinement of electrocatalytic reactions is a promising strategy to influence the reaction kinetics. The degree of confinement affects the electronic and mass transport parameters and breaks the scaling laws of surface activity in electrocatalysis. Herein, a strongly confined system has been designed to demonstrate the nanoconfinement effects on the hydrogen oxidation reaction (HOR) in an alkaline medium. Carbon nanotubes (CNTs) with an inner diameter of 14 Å have been filled with a Pt single-atom catalyst (SAC). The kinetics of the HOR reaction in alkaline solution are slowed down by the confinement effect, with a high overpotential observed for Pt SAC in CNT compared with a nonconfined Pt catalyst. This effect was observed to a lower extent in Pt SAC in a CNT with a larger diameter. On the other hand, nanoconfinement does not slow down the kinetics in an acidic medium for any of these three types of catalysts. This phenomenon can be explained by the mass transport limitations of OH^- in 14 Å CNT, affecting the Heyrovsky rate-determining step in an alkaline medium; density functional theory calculations confirm the energy barrier for OH^- to diffuse in the CNT.