Site-Specific Stochastic Rates and Energetics of Ag Nucleation on Highly Ordered Pyrolytic Graphite.

Nucleation is a fundamentally important step in electrochemical phase transition reactions, e.g., in electrodeposition, which is pertinent for emerging battery technology, nanoparticle synthesis, and many industrial processes. Surface defects have been suggested to enhance nucleation rates. However, directly quantifying the nucleation rates at specific surface sites is challenging due to the ensemble averaging effect in bulk measurements. Herein, we report the measurement of rates and energetics of electronucleation across the model surface of highly oriented pyrolytic graphite (HOPG). Specifically, scanning electrochemical cell microscopy (SECCM) is used to confine the nucleation spatially in the nanoscale cell, allowing one nucleation event to be measured at one time. The scanning capability further allows the mapping of Ag nucleation at the step edge vs basal plane. A stochastic model is developed to extract the nucleation rate and energetics from voltammetric experiments. We observed a ∼57 mV difference in the median nucleation overpotential between the step edge and basal plane, corresponding to a ∼12 kJ mol^-1 difference in the nucleation energy barrier. The voltammetric method to measure the nucleation rate explored here can be extended to understand the heterogeneity of nucleation rates in other electrochemical nucleation systems, e.g., metal anode batteries.