Atom-by-atom identification of catalytic active sites in operando conditions by quantitative noise detection

Electrochemical scanning tunneling microscopy (EC-STM) allows direct observation of surfacechanges at theatomic scale in thepresenceofan electrolyte at different electrochemical potentials. Recently, it has been demonstrated that the noise in the tunneling current of EC-STM allows identifying electrocatalytically active sites under reaction condi- tions. However, this method has never been applied to atom-by-atom investigations and could not provide a quantitative evaluation of the catalytic activity. Using the hydrogen evolution reaction as case study, we demonstrate that the quantitative analysis of the noise in the tunneling current allows quantifying the local onset potential and pro- vides informationaboutthemicroscopicmechanismofelectrochemical reactions on subnanometric electrocatalytic sites, such as chemically heterogeneous flat interfaces, nanoparticles, andeven single-atom de- fects. This unique method allows surpassing the current limits of not only the state-of-the-art EC-STM but also other operando and micro- scopy techniques.