Two-Dimensional Catalysts for Hydrogen Evolution Reaction (HER)

Abstract

Green hydrogen is seen as the fuel of the future as various efforts are made to further implement it in different industries and sectors. It offers a clean and emissions free renewable source of high energy density that helps work towards the global net zero emissions goal. Green hydrogen, which is produced via the hydrogen evolution reaction (HER), requires a technological breakthrough in the electrolyzer; the latter necessitates for the presence of nonprecious, stable, and highly active electrocatalysts. Two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides (MXenes) are promising future electrocatalysts that have great reported exhibiting potential due to their excellent intrinsic properties and structural flexibility. In this thesis, density functional theory (DFT) was utilized to investigate the HER activity where the main HER descript used here is the hydrogen adsorption Gibbs free energy. Firstly, the effect of bromide termination on titanium carbide was explored which was found ineffective for the HER and thus, the introduction of three types of mono-vacancies was done for the manipulation of the electronic properties. The mono-vacancies initiated physisorption and sometimes chemisorption for bromide and titanium vacancies. Nonetheless, the free energy values were non-favorable in both cases, rendering mono-vacancies inefficient for this type of MXene. Secondly, a high entropy MXene composed of titanium, tantalum, molybdenum, and niobium showed extremely promising energetics superior to Pt (|∆GH∗| ≤ 0.09 eV) on one surface. The excellent energetics are attributed to the unique charge distribution gained by the surface oxygen atoms that were favorable for the HER unlike fluoride and hydroxyl terminations. In terms of electronic properties, both MXenes are found to be metallic with (TiNbMoTa)3C2O2 demonstrating ferromagnetism.

Date of Award	27 Dec 2023
Original language	American English
Supervisor	Kyriaki Polychronopoulou (Supervisor)