CPOS Seminar: Dissecting Turnover Number Heterogeneity in Electrocatalysts: Insights into Platinum Nanoparticle Hydrogen Evolution

Abstract: The longevity of chemical catalysts is a critical consideration in catalyst design alongside cost, activity, and selectivity. Turn-over number (TON) is a valuable metric to quantify catalyst durability, defined as the maximum number of attained catalytic cycles per active site before deactivation. TON for a given catalyst has conventionally been measured on a macroscopic scale, yielding a single number value with error bounds under given reaction conditions. Our approach studies electrocatalytic deactivation processes at the single catalyst level using a technique entitled single-entity electrochemistry for freely diffusing catalysts. We believe electrocatalyst degradation, therefore TON is inherently heterogeneous. The hydrogen evolution reaction (HER) is investigated with platinum nanoparticles as the model catalyst because it deactivates at the nanoscale when the catalytic activity decreases over time.

At specific applied potentials, the current measured when a single platinum nanoparticle impacts the electrode creates a transient spike and decays back to the baseline current. The turnover number is quantified by fitting the current decay response and integrating the fit above to find the charge transferred to catalyze the reaction. Our results show that the area above the deactivation curve corresponding to the turnover number is heterogeneous even with a monodispersed sample of PtNPs. Unlike the convention in literature, this shows a distribution of TON where some catalysts are more stable than others. We probed other parameters that may affect this degradation process and discovered that TON is also size-dependent and potential independent. To study how the surface facets of platinum affect the TON, scanning electrochemical cell microscopy (SECCM) was coupled with electron microscopy techniques to compare the deactivation of a polycrystalline platinum surface in the macro scale with immobilized platinum nanoparticles on a glassy carbon surface. Combining local structural, electrochemical activity, and durability will bridge our fundamental understanding of macro and nanoscale catalyst deactivation processes to assist in the bottom-up approach of material development and design of large-scale industrial electrocatalysts.