(201g) Modeling Interfacial Dynamics on Single Atom Electrocatalysts: Explicit Solvation and Potential Dependence

Single atom electrocatalysts, with noble metal-free composition, maximal atomic efficiency, and exceptional reactivity towards various energy and environmental applications, have become a research hot spot in the recent decade. Their simplicity and isolated nature of the atomic structure of their active site has also made them an ideal model catalyst system for studying reaction mechanisms and activity trends. However, the state of the single atom active sites during electrochemical reactions may not be as simple as usually assumed. To the very contrary, the single atom electrocatalysts have been reported to be under a greater influence of the interfacial dynamics, with solvent and electrolyte ions perpetually interacting with the electrified active center under an applied electrode potential. These complexities render the activity trends and reaction mechanisms derived from simplistic models dubious.

In this collection of works, we demonstrate, with a few popular single atom electrocatalysis systems, how the change in electrochemical potential induces non-trivial variation in the free energy profile of elemental electrochemical reaction steps, how the active centers with different electronic structure features can induce different solvation structure at the interface even for the same reaction intermediate of the simplest electrochemical reaction, and the implication of the complexities on the kinetics and thermodynamics of the reaction system to better address the activity and selectivity trends. We also venture into more intriguing interfacial phenomena, such as alternative reaction pathways and intermediates that are favored and stabilized by solvation and polarization effects, the long-range interfacial dynamics across the region far beyond the contact layer, and the dynamic activation or deactivation of single atom sites under operation conditions. We exemplify the necessity in including the realistic aspects (explicit solvent, electrolyte, electrode potential) into the model to correctly capture the physics and chemistry at the electrochemical interface and to correctly understand the reaction mechanisms and reactivity trends. We also demonstrate how the popular simplistic design principles fails, and how they can be revised by including the kinetics and interfacial factors in the model. All these rich dynamics and chemistry would remain hidden or overlooked otherwise.

We believe that the complexity at an electrochemical interface is not a curse but a blessing, in that it enables deeper understanding and finer control of the potential-dependent free energy landscape of electrochemical reactions, which opens up new dimensions for further design and optimization of single atom electrocatalysts and beyond. Limitations of current methods and challenges faced by the theoretical and experimental community are discussed, along with the possible solutions awaiting development in the future.