Atomic layer deposited Ir nanostructures on titania nanotube layers for efficient alkaline hydrogen evolution reaction

Abstract

Herein, we present the deposition and use of Iridium (Ir) electrocatalysts by Atomic Layer Deposition (ALD) on TiO2 nanotube layers (TNTs) towards Hydrogen Evolution Reaction (HER) in alkaline media. We demonstrate a precise control over Ir ALD deposition by varying the number of ALD cycles from 25 to 300, obtaining singleatoms (SAs), clusters and nanoparticles (NPs). TNTs decorated with Ir using 200 ALD cycles exhibited superior HER performance with a low overpotential value of 34 mV, minimal charge transfer resistance of 0.2 Omega and a low Tafel slope of 37 mV dec-1. Long-term stability tests revealed gradual activity degradation due to a partial oxidation of Ir. Notably, while the 200 cycles coating showed the lowest overpotential, the 150 cycles coating demonstrated the highest turnover frequency (TOF) of 1.08 s-1, indicating that optimal use of the available active sites for improved HER performance. This results from the balance between the Ir content and the available active sites, rather than solely relying on electrochemically active surface area (ECSA) or overpotential values. This work provides insights into the complex relationship between Ir nanostructure, oxidation states and catalytic efficiency, contributing to the development of more effective HER catalysts.