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

The surface of catalysts is extremely heterogeneous and comprises different structural units; therefore, the resulting macroscopic chemical activity is an average behavior due to a complex combination of many different sites. Studies of these sites during the catalytic work, i.e., operando, can provide key information for the rational design of new catalysts and the comprehension of key phenomena such as activation, poisoning, and degradation. Electrochemical STM (EC-STM) allows the direct observation of surface changes at the atomic scale in the presence of an 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 conditions. 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 a case study, we demonstrate that the quantitative analysis of the noise in the tunneling current allows quantifying the local onset potential and provides information about the microscopic mechanism of electrochemical reactions on sub nanometric electrocatalytic sites, such as chemically heterogeneous flat interfaces, nanoparticles, and even single atom defects. Furthermore, this novel method allowed us to identify and quantitatively compare the chemical activity of several chemical and morphological defects, such as single iron atoms trapped in several configurations within the graphene basal plane, step edges, and graphene-covered metal interfaces. In this way, we determined that single Fe atoms are even more active than platinum, the benchmark catalyst for the hydrogen evolution reaction.