First Principle simulations of electrochemical interfaces - a DFT study

In this thesis, I have looked beyond the computational hydrogen electrode (CHE) model, and focused on the first principle simulations which treats the electrode-electrolyte interfaces explicitly. Since obtaining a realistic electrode-electrolyte interface was difficult, I aimed to address various challenges regarding first principle electrochemical interface modeling in order to bridge the gap between the model interface used in simulations and real catalyst at operating conditions.

Atomic scale insight for the processes and reactions that occur at the electrochemical interface presents a challenge to both, the experimentalists, and the theorists. Energetics of charge transfer reactions over the electrochemical interface, determines, to a great extent, the efficiency of energy conversion.

Therefore, gaining an atomic-level understanding of the interface, have utmost importance. Experimentalists measure macroscopic quantities, e.g., current versus voltage and have no direct information about the corresponding interfacial atomic structure. However, scanning tunneling microscope (STM) might be useful in disclosing information about the atomic structure, but it can not be performed in situ in aqueous electrolytes in order to reveal metal-water interfacial structure.

First principle calculations are useful in disclosing interfacial atomic structure, however, theorists, have other challenges to deal. Atomic scale modeling of the electrochemical interface, is still far from realistic. The real electrochemical interface is challenging to model because processes that take place over the interface are complicated.

First principle methods have limitations due to the various approximations in implementations and may sometimes lead to incorrect electronic structure at the electrochemical interface, which can result in an improper/ill-defined electrochemical interface. Considering the electronic structure of the interface, I have mentioned some of the pitfalls in modeling electrochemical interfaces, and I have also shown how to avoid these pitfalls.

The electrode-electrolyte interface models constructed without care for electronic structure, could exhibit an unphysical charge transfer due to the DFT's notorious under-estimation of the HUMO-LUMO gap. For such systems, electrode potential cannot be tuned. I have shown that the HOMO-LUMO gap of the electrolyte have to straddle the Fermi level, in order for the whole system to qualify as a proper electrochemical interface.

I have also contributed to the model, which accounts for pH in the first principle electrode-electrolyte interface simulations. This is an important step forward, since electrochemical reaction rate and barrier for charge transfer can strongly dependent on pH. I have shown that pH can have influence over the interface structure, and hence can influence the adsorbate free energies with direct hydrogen bonding or chemical interactions with the electrolyte dipole.

Therefore, in order to study the reactions at constant electrochemical potential, pH has to be kept constant together with the chemical potential of protons and electrons. However, this was not the case for some of the calculations reported in the literature for constant electrochemical potential, where the calculations are not really done at constant electrochemical potential, as the chemical potential of proton (or pH) was not considered.

However, in most of the cases, the effect of pH was negligible. We have applied this developed model to Pt(111)-water interface as an example, and constructed the corresponding Pourbaix diagram, which shows the effect of pH and potential on adsorbate coverage and interface structure. I have also investigated the pH effect on the electrochemical adsorption of hydrogen for Pt(100) and Pt(111) surfaces by applying the above model that account for pH in the simulations.

As a consequence of negligible interaction between electrolyte and adsorbed hydrogen, I found that modeled electrochemical interface and pH have no influence over hydrogen adsorption energy. In fact, hydrogen adsorption is well defined by considering just CHE model. However, barrier for charge transfer, can depend on the pH, as pH can influence the water structure at the interface.

I have also discussed a scenario, where proton is more stable at the electrochemical interface, where the water layer is almost chemi-adsorbed at the surface. This is an interesting case as proton being stable in the Outer Helmholz Plane (OHP), significantly change the electrostatic potential in the double layer region.

This might also have an impact over barrier for charge transfer considering hydrogen evolution reaction (HER). I have also calculated the pseudo-capacitances for hydrogen adsorption region for both surfaces, which agrees with experiments.