Solvent engineering of molybdenum disulfide electrocatalyst for hydrogen evolution.

Energy is at an exponentially growing demand, and to keep up with these demands new technologies for renewable energy have received increased attention. Hydrogen plays a vital role in water electrolysis and fuel cells, as the hydrogen evolution reaction (HER) is the main step water splitting process. Most of the current electrocatalysts for HER are dominated by platinum and other precious metals due to their low over-potential and small Tafel slope, however, they are extremely costly. For this reason, cost-effective non-precious metal catalysts must be developed. Transition metal dichalcogenides, such as molybdenum disulfide (MoS2), are abundant and have recently shown promising results for HER. The challenge facing the commercialization of MoS2 is the synthesis process. Hydrothermal synthesis using a precursor, ammonium tetrathiomolybdate, with hydrazine as a solvent is a common route for obtaining MoS2. Despite successful obtainment of MoS2, hydrazine is not favored due to high combustibility and toxicity. For this reason, we have investigated the electrochemical performance of MoS2 obtained by using different solvents. Electrochemical studies reveal best onset potentials of -116 mV for ethylene glycol followed by - 126 mV and -129 mV for water and diethyl glycol solvents, respectively. MoS2 synthesized in 1- methyl pyrilidone exhibit best Tafel slope of 51 mV/dec, followed by ethylene glycol and water at 54.9 mV/dec and 55.2 mV/dec, respectively. In comparison, hydrazine shows Tafel slope of 41 mV/dec and onset potential of -100 mV. Scanning electron microscopy reveals MoS2 catalysts synthesized in dimethylformamide and ethylene glycol produce more active edge sites allowing for better hydrogen proton adsorption. This study’s results found Ethylene Glycol to show most promising to replace hydrazine as a current solvent for synthesis of MoS2 catalysts with a possible link to higher solvent densities improving catalysts performance

Modified electrode surfaces with hydrogen evolution reaction catalysts derived from electropolymerized complexes with redox active ligands.

The demand for energy is growing exponentially, and to keep up with these demands new technologies for renewable energy have received increased attention. Hydrogen is one of the most promising energy sources for the future and plays a vital role in water electrolysis and fuel cells, as the hydrogen evolution reaction (HER) is the main step in the water splitting process. To increase the reaction rate and improve efficiency for the water electrolysis, catalysts are used to minimize the overpotential. Most of the current electrocatalysts for HER are heterogeneous in nature and are dominated by platinum and other precious metals due to their high current density and small Tafel slope; however, they are extremely costly and have rare-earth abundance. For this reason, cost-effective catalysts must be developed. Previously, many have seen the best success by employing the use of earth-abundant transition metal chalcogenides to use as homogeneous molecular electrocatalysts, the most promising of which is molybdenum sulfide. These electrocatalysts do display low overpotentials and high HER activity; however, they contain a low number of active sites. Many have worked to address these issues. The biggest challenge with heterogeneous catalysts as a whole is the inability to do detailed mechanism investigations. Homogeneous catalysts, alternatively, have attractive properties of activity and selectivity. The main issues with homogeneous catalysts are recycling and separation from product. To combine the benefits of both heterogeneous and homogeneous catalysts, immobilization of characterized catalysts onto the solid electrode surface to allow them to work under heterogeneous conditions is proposed. This heterogenization of a homogeneous catalyst onto the electrode surface is an ideal way to study a catalyst. The goal of this work was to develop and engineer new carbon materials, while heterogenizing new and existing homogeneous thiomesicarbazone (TSC) compounds, supplied by the Grapperhaus/Buchanan Research Group, as electrocatalysis of HER. Thermodynamics, kinetics, and transport were the driving forces in the study. A series of metal complexes based on inexpensive bis-thiomesicarbazone ligands including bis-thiophenepyrrolebutylamine(BTP4A), diacetyl-bis(N-4-methyl-3-thiosemicarbazide) (ATSM), and ATSM with pyrrole attached (ATSMpy) were synthesized and characterized by NMR, IR, cyclic voltammetry, and square wave voltammetry. Modified electrodes were prepared with films deposited on glassy carbon, standard pencils, and carbon paste electrodes, and evaluated as potential HER catalysts using cyclic voltammetry, linear sweep voltammetry, and electrochemical impedance spectroscopy. HER studies in 0.5 M aqueous H2SO4 (10 mA cm−2) revealed that modified electrode surfaces of glassy carbon, carbon paste, and standard pencils with TSCs gave promising electrochemical activity to be used for HER catalysis application. Pencil electrodes have shown to report improved activity due to increased surface interactions. Specifically, the blank pencil with C15 (Ni-ATSM) reported the lowest overpotential, Tafel slope, and charge transfer resistance of any sample, with overpotential values of 0.214-0.328 V. This sample combination will be further studied to prove its viability to be used as electrocatalysts with modified electrodes for HER

Mechanistic Electrochemistry: Investigations of Electrocatalytic Mechanisms for H2S Detection Applications

This thesis describes the development of electrochemical analytical approaches for the investigation of sulphide detection in stagnant and fluidic environments. The project reports the use of Fourier transform large amplitude alternating current voltammetry (FTACV) as a novel analytical technique for the investigation of sulphide sensing. Novel reactor technology and FTACV measurements carried out using macro and microelectrodes in stagnant and fluidic conditions are reported for the first time. The novel strategy adopts the use of an electrocatalytic (EC') mechanism by using a redox mediator to facilitate the reaction with sulphide in aqueous solutions. In order to support the analysis of FTACV, other electrochemical analytical techniques, cyclic voltammetry (CV) and linear sweep voltammetry (LSV), were also employed to support the observations from FTACV. Chapter 3 reports the application of the CV and FTACV for the detection of sulphide in stagnant conditions at a macroscale electrode. A split wave phenomenon, which is related to the reaction with sulphide, was observed both in the CV and FTACV. By measuring the current behaviour of the split wave, sulphide content in aqueous solution can be determined. Importantly, the split wave phenomenon of the FTACV is the first documented observation using macroscale electrodes. These observations highlight the potential of FTACV to support the detection of sulphide detection. Numerical models of the system are also presented from the calculation to support the experimental interpretation of the voltammetric responses of the CV and FTACV. In Chapter 4 measurements were focused on the voltammetric response of sulphide containing aqueous solutions using microelectrodes. In conventional CV measurements, the split wave behaviour observed at macroelectode disappears from the DC signal; however, for the FTACV measurements, the split wave can still be observed in the higher harmonics providing a clear and simple strategy for detecting sulphide. The results achieved in the FTACV are the first documented observation under the steady state at microelectrodes. Again numerical simulations are reported for this case to support the experimental results. Chapter 5 extends the FTACV measurements for sulphide detection to hydrodynamic environments. The design, development and application of a microfluidic electrochemical system are reported. Split wave characteristics were for the first time detected in both dc and FTACV measurements. The results support the possibility of using dc and ac voltammetry to detect sulphide, while also being used as a guide to assess the split-wave behaviour of the EC' mechanism under fluidic conditions. Numerical models were used to support the analysis of the experimental measurements