Hydrogen Evolution Reaction on Pt Nanoaggregates by Operando 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 Ni2+ 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 Ni2+ 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.