INVESTIGATION OF ELECTROCATALYTIC ENERGY CONVERSION REACTIONS ON 2D LAYERED MATERIALS: HYDROGEN EVOLUTION ON MoS2 AND CARBON DIOXIDE REDUCTION ON Ti3C2 AND Mo2C

Abstract

Anthropogenic release of the greenhouse gas carbon dioxide is believed to be a leading cause in the global rise in temperature. The main source of the carbon dioxide released is from combustion of fossil fuels. Thus, its necessary to mitigate the release of CO2, look for alternatives for fossil fuels and capture and sequester or capture and convert CO2 to other useful fuels and chemicals hence creating carbon neutral or carbon negative energy cycles. This thesis work was primarily focused on design, adapt and understand the chemistry of two-dimensional (2D) layered materials, particularly transition metal dichalcogenide (TMD) molybdenum disulfide and transition metal carbides (MXenes) as catalytic materials for the conversion of renewable energy into fuels and chemicals as an alternative for fossil fuels. This investigation was accomplished by combining electrochemistry, state of the art characterization and density functional theory (DFT) calculations. We hypothesized that it would be possible to improve the electrocatalytic hydrogen evolution reaction (HER) on MoS2 by engineering catalytically active sites on the plane, their edges and their interlayer regions. We also hypothesized 2D MXene sheets would serve as good carbon dioxide reduction reaction (CO2RR) catalysts under aprotic conditions. Conceivably the broad impact of this thesis work utilizing experimental and theoretical studies is the realization of transition metal doped metallic MoS2 as a potential candidate towards HER in alkaline conditions. Initially the interlayer region of MoS2 were investigated for the HER by introducing Na+, Ca2+, Ni2+ and Co2+ cations in the interlayers of metallic phase MoS2. Experimental results show that intercalation of cations (Na+, Ca2+, Ni2+, and Co2+) into the interlayer region of 1T-MoS2 to lower the overpotential for the HER. In acidic media the overpotential to reach 10 mAcm-2 for 1T-MoS2 with intercalated ions is lowered by ~60 mV relative to pristine 1T-MoS2 (~230 mV). DFT calculations suggest that the introduction of states from the intercalated metals whether sp or d, to lower the Gibbs free energy for H-adsorption (ΔGH) relative to intercalant-free 1T-MoS2. The DFT calculations suggest that Na+ intercalation results in ΔGH closest to zero, which is consistent with our experiments where the lowest overpotential for the HER is observed with Na+ intercalation. In order to explore the activity of the edge sites of MoS2 and the effect of a conductive support we used a microwave-assisted growth technique to synthesize interlayer expanded MoS2 with a vertically orientation on conductive two-dimensional Ti3C2 MXene nanosheets (MoS2⊥Ti3C2). Judicious choice of reaction temperature allows a control over the density of the edges obtained. Compared to pure MoS2 this unique inorganic hybrid structure allows an increased exposure of catalytically active edge sites of MoS2. The produced materials were investigated as electrocatalysts for the hydrogen evolution reaction (HER) in acidic conditions. The MoS2⊥Ti3C2 catalyst synthesized at 240 0C exhibited a low onset potential (-95 mV vs RHE) for the HER and a low Tafel slope (~40 mV dec-1). The decrease in the overpotential is linked to decrease in the charge transfer resistance of the materials with the electrode and the increased edge site density. In a third study the basal plane of metallic MoS2 was engineered by doping with transition metals Co and Ni to be evaluated as a catalyst for the alkaline HER. Due to a lack of oxygen evolution catalysts that can oxidize water at the anode under acidic conditions, there is an urgency to realize HER catalysts that can efficiently reduce water to hydrogen gas under alkaline conditions. Though metallic MoS2 has an optimum H binding free energy for the HER, the sluggish water dissociation step under alkaline conditions has made the implementation of MoS2 as a catalyst at higher pHs harder. We hypothesized that doping transition metals in the basal plane of metallic MoS2 that can efficiently catalyze the water dissociation step in alkaline conditions would help to reduce the overpotential required for the HER under alkaline conditions. Ni and Co were doped in orthorhombic MoO3 which was then converted metallic MoS2 under hydrothermal conditions. The polarization plots obtained in 1.0 M KOH solution shows a low onset overpotential of -75 mV vs RHE for the 10% Ni doped metallic MoS2 with an overpotential of -145 mV to reach a current density of 10 mA/cm2. Pure metallic MoS2 reaches the same current density at an overpotential of -238 mV vs RHE while samples doped with 10% Co atoms reached 10 mA/cm2 at -165 mV. This improvement in the doped samples is attributed to the improved kinetics of the water dissociation step under the alkaline reaction conditions. DFT calculations suggests that an optimal binding of water for the water dissociation step, H binding free and low free energy of binding for OH intermediates. Rigorous cycling of the catalysts shows extremely high stability with the doped samples while the pure metallic MoS2 loses its activity with continuous cycling. DFT calculations show that the doped samples provide extra stability to the metastable metallic MoS2 thus improving their long-term stability. Photo/electrochemical conversion of CO2 is an important step in the path to renewable production of carbon-based fuels and chemicals. Activity and selectivity have been major concerns on the CO2RR catalysts. The activity of known materials are hindered by the scaling relationship in the binding energies of the many intermediates involved in the CO2RR. Thus, the simplest of CO2RR products CO and HCOOH are of great value. Nano structured precious metals like silver and gold have shown promise as cathode materials for the conversion of CO2 to CO. In this thesis work we evaluate the electrocatalytic properties of Mo2C and Ti3C2 MXenes towards the electrochemical CO2 reduction reaction (CO2RR) as cheaper alternatives for precious metals. Though there have been theoretical predictions of the ability of MXenes with certain composition to have the ability to reduce CO2 to hydrocarbons, there are no experimental findings to support these calculations. In this study we observe very high faradaic efficiencies, ~90% for the CO2 reduction to CO at low overpotentials ~250 mV in acetonitrile/ionic liquid electrolytes on Mo2C MXene while Ti3C2 shows ~65% FE at an overpotential of ~600 mV for the cathodic half reaction. Density functional theory calculations suggests that the enhanced activity of Mo2C relative to Ti3C2 is due to relative lowering of the energy barrier for the initial proton couple electron transfer step of CO2 and the spontaneous dissociation of the absorbed *COOH species to *CO and H2O on the Mo2C surface. The calculations also predict the most probable active sites for the CO2 conversion to be vacant oxygen sites. High selectivity and high FE of CO2 reduction to CO makes these earth abundant materials an attractive electrocatalyst for the CO2RR.