Hydrogen Evolution Reaction on Pt Nanoaggregates by <i>Operando</i> Scanning Transmission Electron Microscopy: The Electron Beam, Friend or Foe?

Understanding the behavior of electrocatalysts under operating conditions is essential to improving their performance. Electrochemical (scanning) transmission electron microscopy [EC-(S)TEM] enables real-time, high-resolution imaging of materials undergoing electrochemical processes; however, it provides limited information about the products of these processes in the solution phase, and the high-energy electron beam can perturb their distribution and reactivity through radiolysis. Previously, we demonstrated that Ni 2+ not only enhances the electrocatalytic performance of Pt for the hydrogen evolution reaction (HER) but also, through Ni(OH) 2 precipitation, that it serves as a quantitative in situ marker of HER activity at the single-nanoparticle level. Extending this quantitative footprinting methodology to EC-(S)TEM to report catalytic yield, we observe a different mechanism: while optical measurements indicate the expected Ni(OH) 2 precipitation on the EC-(S)TEM chip, in situ EC-(S)TEM experiments reveal beam-induced reduction of Ni 2+ to metallic Ni via radiolysis. Finite element modeling supports a mechanism involving H • intermediates and allows discrimination of the respective contributions of the HER and the electron beam. These results highlight the critical role of the beam in apparent electrocatalytic reactivity and provide a framework to quantify catalytic yields in EC-(S)TEM and to interpret operando data more cautiously.