Timothy T. Yang 

PhD Dissertation Defense

Materials Science and Engineering Graduate Program
 

Title:

First Principles Investigations of Hydrogen Evolution Reaction and Electrochemical Modeling

ABSTRACT:

      Green Hydrogen, a clean and renewable energy carrier, is a sought solution for decarbonization and meeting net-zero greenhouse gas emissions by 2050. The central challenge in producing green hydrogen is to improve electrolysis performance via designing low-cost, high-efficient catalysts for hydrogen evolution reaction (HER) – the cathodic reaction in water splitting.

First-principles simulations provide a simple, yet effective, strategy to screen and identify HER catalysts. For the past two decades, the computational hydrogen adsorption free energy, proposed by Nørskov, has been the norm to gauge the exchange current – the electrochemical current that is generated at zero overpotential and is analogous to the reaction rate. However, this model misses a linkage with the fundamentals of electrochemistry and notably leads to inaccurate predictions of experimental measurements.

My research aims to develop an easy-to-compute model for HER exchange currents building on the Butler-Volmer relation for a one-step, one-charge transfer process. I show that the exchange current is solely a function of the hydrogen adsorption free energy that can be obtained from first-principles methods. Benefiting from an absolute rate constant and the universality of the transfer coefficient, the new model successfully predicts the experimental exchange currents within two-orders of magnitude for various catalysts. This model is further validated by employing a data-driven approach based on experimental cyclic voltammograms – current-potential characteristics obtained from analytical measurements. Beyond Nørskov’s approach, the new model, with its rigorous linkage to theoretical electrochemical methods, not only boosts the prediction accuracy of the exchange current, but also addresses several decades-long controversies in the field.

Using this model in conjunction with first-principles, ab initio thermodynamics, and cluster expansion, I determine the catalytic activity towards hydrogen evolution of several low-cost novel catalysts, focusing on molybdenum carbides (Mo_xC). The computational screening show that doping with titanium or iridium, and the coupling of Mo_xC with graphene increase the HER activity of Mo_xC. These findings are validated experimentally and further confirm the high fidelity of the model.  In addition, my overarching theoretical approach for developing the model is transferable to other electrochemical reactions.