Revealing the Volcano-Shaped Activity Trend of Triiodide Reduction Reaction: A DFT Study Coupled with Microkinetic Analysis

Triiodide/iodide (I₃^-/I^-) represents a widely used redox couple and plays an important role in some photovoltaic devices. However, the understanding of the triiodide reduction kinetics occurring at the liquid/electrode interface is very limiting, which largely hinders the identification of highly efficient electrode material. In this work, by virtue of DFT calculations, we systematically investigated the I₃^- electroreduction at some acetonitrile/electrode interfaces and uncovered two new BEP relations for the key elementary steps, I₂ dissociation and I∗ desorption through one-electron reduction. Furthermore, by utilizing a steady-state microkinetic model, we successfully identified a general volcano-shaped activity trend of triiodide electroreduction as a function of a single descriptor, the adsorption energy of I atom (E_ad^I) at the interface. Our results show that a good catalyst should possess an E_ad^I within the range of 0.3-0.6 eV, while the optimal E_ad^I is 0.43 eV, where the surface coverages of free sites and iodine atoms are equal. In particular, the dependences of the volcano shape on the electrochemical conditions (external voltage, temperature, concentration, and transfer coefficient) are quantitatively discussed. Some suggestions for the optimization of experimental conditions and design of better catalysts are also provided.