Theoretical Treatment of Electrocatalytic Reactions on 2D Transition Metal Dichalcogenides under Electrochemical Conditions

Two-dimensional semiconductors such as MoS2 and WSe2, etc. are known (photo)electrocatalysts for the electrochemical reduction of water to form H2, CO2 to form C1 and higher products, N2 to form NH3, and other relevant reactions for a future low-carbon economy. These reactions are complex and involve multiple steps. When performed on the 2-dimensional systems, they are often catalyzed by defects or step edges or to need a semiconductor to metal phase transition to be efficiently catalyzed on intact basal planes. The reactions also involve multiple electron and proton transfer events that are often thought to happen in an intricately coupled manner. At the same time, it is known from experiment that electrochemical conditions, such as electrical bias, strongly modify the electronic properties of the (photo)electrocatalyst by effects such as bandgap renormalization, non-uniform shifts of catalyst and molecular electronic states with applied potential, phase transitions, and other effects that can radically alter the energetics and kinetics of the steps involved. Also, these materials can have very strong light-matter interactions that modify their electrochemical behavior. This complexity makes it hard to make predictions about the electrocatalytic behavior of these materials.<br/>In this contribution, we investigate the above-mentioned reactions using density functional theory calculations performed in the Grand Canonical Ensemble, as implemented in the JDFTx software package that can accurately address realistic electrochemical conditions (electrified interfaces, dielectric response, ionic strength, etc.) while still fully treating spin- polarization and other electronic effects. We study the reaction pathways, intermediate energies and the electronic properties of the intermediate complexes to gain a better understanding of the electrochemical reduction reactions in their actual microenvironments. We find that the electrochemical conditions modify electrocatalytic pathways in a non-trivial manner and that the local structure of the 2-D semiconductor changes considerably during the reaction sequence. We investigate the bonding environment between catalytic site and substrate and find that effects such as pi-back bonding to the transition metal sites and local relaxation of the microenvironment are essential in understanding the reaction pathway and the dependence of the reaction thermodynamics on electrochemical bias. Most significantly we find that traditional methods of calculating intermediate state energies in vacuum environment, while informative to study intermediate conformation, are not able to treat the reaction particulars well and, in many cases, the wrong reaction step can be identified as rate limiting. <br/>Lastly, in this contribution, we connect our theoretical calculations to experiments performed in our group and in the literature and talk about the implications of the new theoretical insight on our understanding of fuel-forming reactions on 2-dimensional semiconductors and other related electrocatalytic systems. We demonstrate that it is essential to more fully treat the electrochemical microenvironment in order to model and understand these complex multi-step reactions and that local bias effects are much more important than previously understood and cannot be ignored or approximated by simple Coulomb approximations. This new insight has the potential to alter our understanding of electrocatalytic reactions in (photo)electrochemical systems considerably.