Hydrogen Absorption on Single Palladium Nanoparticles

Abstract

This study presents a comprehensive investigation into the electrocatalytic behavior of single palladium nanopar-ticles (Pd NPs) during the hydrogen evolution reaction (HER), with a particular focus on their unique ability to absorb hydrogen into the bulk lattice. Using stochastic single-particle collision experiments at carbon fiber ultramicroelec- trodes, we captured and analyzed transient current signals arising from individual NP–electrode interactions under acidic conditions. These current traces revealed distinct kinetic phases associated with hydrogen adsorption (Volmer step), hydrogen evolution (Heyrovsky and Tafel steps), and subsurface hydrogen absorption. Quantitative analysis shows that Pd NPs absorb a significant fraction of the adsorbed hydrogen, leading to reduced HER efficiency compared to bulk Pd electrodes. The observed steady-state and transient currents suggest a strong competition between HER and hydrogen absorption, which alters the effective reaction pathway and suppresses molecular hydrogen formation. Systematic evaluation of proton concentration and applied overpotential further revealed that the HER kinetics on Pd NPs deviate from classical mechanisms, exhibiting unusually low transfer coefficients and small apparent rate constants. These findings indicate that hydrogen absorption—not hydrogen desorption—is the rate-limiting step under many experimental conditions. By isolating single-particle responses, this work provides critical mechanistic insight into nanoscale hydrogen electrosorption and establishes single-NP collision electrochemistry as a powerful tool for resolving complex catalytic processes involving coupled interfacial and bulk transformations.