Electrochemical investigations on functionalized 2D graphene, molybdenum disulfide, and nickel sulfide

Widespread implementation of electrochemical-based energy generation and storage techniques is the indispensable strategy for addressing the issues resulting from the constant use of fossil fuels. After decades of searching for the best candidates to be used as electrodes in the electrochemical-based devices, electrochemists have started to investigate the details of electrochemical reactions, such as mechanisms, reaction site, and electrode alterations under operando conditions. Despite invaluable findings about these issues, there are still unanswered or unclearly answered significant questions. Two dimensional (2D) materials provide unique features to design highly active electrodes and have comprehensive investigations about the details of electrochemical reactions. The distinctive structure of 2D materials delivers a high number of available sites for functionalization. This paves the way to alter the electrochemically essential properties such as surface morphologies, electronic structure, carrier concentration, charge transfer efficiency, etc. As a result, it is possible to design new electrodes with enhanced electrochemical activities. In addition, the combination of the availability of a high number of active sites and accessibility of these sites with the surface-sensitive characterization methods provide exceptional conditions to study the electrochemical reactions in detail. This issue has gained prime importance with recent findings about the structural and compositional alterations of electrodes during electrochemical measurements. In this thesis, the unique features of 2D graphene, MoS2, and NiSx derived from Ni(OH)2 combined with various physical and electrochemical characterization techniques have been used to study the alterations in electrode materials during electrochemical reactions. The initial part of the study has been focused on graphene, which is the simplest and the best-known 2D material. It has been shown that N-doping significantly enhances the electrochemical activity of graphene toward oxygen reduction reaction (ORR). After studying the effect of different N configurations on the activity of N-doped graphene, it has been shown that pyridinic N improves the activity of bilayer N-graphene by modulating the interaction between the layers. The increased π- π interaction in pyridinic N-doped graphene, promotes the electron transfer kinetics between layers and results in an improved activity for the N-doped bilayer graphene. In order to have a better insight into the role of different N configurations, active sites, and ORR mechanisms, bilayer pristine and N-doped graphene were investigated by the spectroelectrochemical Raman technique. It has been shown that based on the configuration of the N dopants, it is possible to have a competition between adsorption of H/OH and O2 groups. After showing Raman fingerprints for the possible reactions between O2 (or O2-) with H/OH, it has been proposed that the ORR could have multiple mechanisms with the same transferred electron numbers. Despite enormous efforts in advancing the hydrogen evolution reaction (HER) activity of MoS2, the presence of different phases and relatively complex structure of MoS2 has resulted in a significant number of controversial results in the published reports so far. Unfortunately, most of the operando techniques which have been used to answer some fundamental questions, such as the active site, the mechanism on different MoS2 structures, and the structural changes of MoS2 during HER, have been focused on amorphous MoS2. Here a systematic investigation has been performed to study the effect of modifying a synthesis parameter on the phase, composition, flake size, and defect density of MoS2. It has been shown that N doping improves the HER activity of MoS2. However, this improvement is not stable, and the strain and phase conversion induced by N dopants fade away when they abandon the MoS2 structure. Finally, the HER activity of different nickel sulfides phases in the alkaline electrolyte was investigated. Despite the 3D structure for NiSx, it has been shown that it is possible to synthesize pure 2D NiS2 using a Ni precursor, which has a layer-by-layer structure (Ni(OH)2). It has been shown that the electrochemical measurements performed on these samples have better results compared to the samples with 3D structures. The non-similar dynamic behavior in linear sweep voltammetry results of the samples was investigated by performing Raman spectroscopy and XPS investigations during the course of structural transition. It has been shown that the co-presence of hydroxylated Ni and NiSx promotes the HER activity of the electrodes.