Confined Electrocatalysis in Carbon Nanotubes

Nano-confinement of electrocatalytic reactions is a promising strategy to influence the reaction kinetics and selectivity. The degree of confinement affects both electronic and mass transport parameters and breaks the scaling laws of surface activity in electrocatalysis.<br/><br/>Encapsulation of Pt nanoparticles in carbon nanotubes (CNTs) has previously shown excellent hydrogen oxidation reaction (HOR) performance and durability in corrosive electrolyte due to the high selective diffusion of the species H⁺ and H₂ vs. the corrosive anions Br^- and Br₃^-.¹ A slightly different approach has been followed to encapsulate Pt SAC in CNT which demonstrates a high selectivity towards HOR vs. oxygen reduction reaction (ORR) in acidic medium.²<br/><br/>Herein, a strongly confined system has been designed and synthesized to demonstrate the nanoconfinement effects on HOR in alkaline medium. CNTs with inner diameter as low as 14 Å have been filled with a Pt single atom catalyst (SAC). The kinetics of the HOR reaction in alkaline are slowed down by the confinement effect, with a high overpotential observed for Pt SAC in CNT compared with a non-confined Pt catalyst. This effect was observed to a lower extent in Pt SAC in CNT with a larger diameter. On the other hand, nanoconfinement does not slow down the kinetics in acidic medium for any of these three types of catalyst. This phenomenon can be explained by the mass transport limitations of OH^- in 14 Å CNT, affecting the Heyrovsky rate determining step in alkaline medium; density functional theory calculations confirm the energy barrier for OH^-to diffuse in the CNT.<br/><br/><b>References</b><br/>[1] Hardisty, S. S.; Saadi, K.; Nagaprasad Reddy, S.; Grinberg, I.; Zitoun, D. Ionically Selective Carbon Nanotubes for Hydrogen Electrocatalysis in the Hydrogen–Bromine Redox Flow Battery. <i>Mater Today Energy</i> <b>2022</b>, <i>24</i>. https://doi.org/10.1016/j.mtener.2021.100937.<br/>[2] Hardisty, S. S.; Lin, X.; Kucernak, A. R. J.; Zitoun, D. Single-Atom Pt on Carbon Nanotubes for Selective Electrocatalysis. <i>Carbon Energy</i> <b>2024</b>, <i>6</i> (1). https://doi.org/10.1002/cey2.409.