title: Synthesis and investigation of MX2 (M = Ti, V, Cr; X = S, Se) as electrocatalysts for the hydrogen evolution reaction
Creators: name: Masaityte, Liudvika
Publisher: name: University of Glasgow
Year: 2023

description: Rising attention to the climate crisis has boosted the search for alternative and clean energy. One of the most promising technologies to achieve it is water electrolysis. However, the process is thermodynamically unfavourable, therefore, catalysts are needed to lower the costs. The best catalysts known to this day are noble metal based. More cost-efficient and abundant materials have been investigated recently, such as transition metal phosphides, nitrides and transition metal dichalcogenides. The latter group of materials is at the centre of attention of this thesis, more specifically, early transition metal (Ti, V, Cr) sulphides and selenides. The investigation of TMDCs begins with the synthesis of VS2. Conflicting literature reports called for a very careful synthesis to obtain a genuine highly crystalline layered morphology of this compound, as most reports focused on nanostructured VS2. Two routes were investigated – through alkali metal intercalates and hydrothermal synthesis. The alkali metal intercalate route was key in synthesising highly crystalline and stoichiometric VS2, while the hydrothermal synthesis resulted in a nanostructured product. Contrary to literature reports, neither crystalline, bulk, nor nanostructure VS2 was found to be good electrocatalysts for the hydrogen evolution reaction due to poor electrochemical stability and a large overpotential value. VS2 became a steppingstone in the investigation of isostructural transition metal dichalcogenides. The attention was then turned to TiS2 and CrS2 – materials that adopt the same crystal structure as VS2. By preserving the crystal structure and only changing transition metal, the effect of transition metal on hydrogen evolution reaction was investigated. It was not possible to isolate CrS2 as a pure phase preventing further electrochemical testing. However, as a single-phase TiS2 sample was isolated, it was compared electrochemically with VS2. The results hinted at a trend toward improvement in catalytic activity as Ti was replaced with V. Therefore, the investigation was expanded to selenides. Facing the difficulties with chromium disulphide, the attention was turned to selenides. TiSe2, VSe2 and CrSe2 are all isostructural between themselves and their corresponding sulphides. A successful synthesis of all selenides provides us with an opportunity to get insights into the transition metal effect on the electrocatalytic activity of transition metal selenides. While the electrochemical performance of TiSe2 and VSe2 followed the same trend as for sulphides, CrSe2 broke the underlying trend by revealing extremely high overpotential. Lastly, V – S system was revisited, and the thermodynamically stable phases of VS4, V5S8 and VS were investigated. This allowed for understanding whether the unexpectedly poor catalytic performance of VS2 was a characteristic behaviour of V-based sulphides or originated from the metastability of the VS2 phase. Varying the S-content allowed for preliminary insight into the effect metal to sulphur ratio has on the electrocatalytic activity of this system. The stoichiometric VS showed the lowest overpotential, reasonable current densities and good stability. However, the observed overpotential of –640 mV vs NHE at –10 mA cm–2 still makes it unattractive for any applications.