Solvation Shells in Ions

 

An ion's path is dependent on several complex processes. Ions need to reorganize their solvation shell. This reorganization must occur before they can interrelate into battery cathodes, find ion channels in biochemical membranes or even absorb and change into chemicals like green hydrogen. 

An interesting study about transition state theory is coming from the Interface Science Department of the Fritz-Haber Institute. It was published in the journal Nature Communications. 

In the past, researchers found that the kinetics of interfacial ion solvation are ruled by compensation effects between entropy and enthalpy. An example is that if the elevation of the mountain in front of this ion is raised, the number of available hiking trails increases. This makes it more likely for an ion to go for a hike. 

Michael Polanyi was head of the Physical Chemistry Department until 1933. The current team interpreted the kinetics according to statistical physics and the Erying-Evans-Polanyi equations, an important part of the transition state theory. 

Almost 90 years later, the team at the Interface Science Department are observing the transition state theory. This research shows  the activation enthalpy and activation entropy with millisecond resolution!

Francisco Sarabia is the Marie Curie Postdoctoral Fellow and first author of the study. He states, "Our findings are really substantial on many fundamental levels. Using this technique, we can directly access the electrosorption kinetics of hydroxide ions that occur at specific structural surface motifs, e.g., step edges or defects, and show how they are linked to electrocatalyst kinetics. Further, we studied the dynamic poisoning behavior at the Pt surface during the ammonia oxidation reaction and how it impacts the solvation kinetics. This level of insight has remained completely hidden, so far."

The work shows that activation entropy changes at the catalyst surface. The interfacial solvents are important to understanding the electrocatalyst activity. An example of this is pH. The researchers learned that the pH can directly impact the activation entropy. It can also induce non-Nernstian activity changes with pH. It is widely assumed that the activation energy is the primary factor in the bias dependence of all electrocatalytic reactions. 

Dr. Sebastian Oner is a group leader at the Interface Science Department. He is also corresponding author of this study. He reports, "Abundant operando spectroscopy and microscopy evidence, including from my colleges here at the Inorganic Chemistry and Interface Science Departments, show that catalyst surfaces are highly dynamic. Beyond studying solvation kinetics, we now have a tool that we can apply to capture true kinetic information in real-time and overlay it with spectroscopic and microscopic information. 

This study shows the importance of bias dependent change in the environment of a catalyst. It also shows how the solid structure and liquid electrolyte are interconnected and can impact each other.