Advanced 2D Transition Metal Chalcogenide Materials for Energy Storage and Conversion

In the first and second project, Lithium metal gas batteries in the form of oxygen and carbon dioxide electrodes have been developed. In these projects by using MoS2 catalyst and a unique electrolyte blend we could develop batteries that could sustain a high number of consecutive charge and discharge cycles at a high current density of 500 mA g-1. Specifically, the decomposition of Li2O2 during the charge process requires charge transfer which is challenging because of the large band gap of solid Li2O2 likely covering the catalytic sites. Redox mediators (RM) are added to the electrolyte which have a low oxidation potential and whose solution phase cationic state can chemically decompose the Li2O2 product resulting in a low charge potential. The electrolyte used in our system is a blend based on a solvent composed of tetraethylene glycol dimethyl ether (TEGDME) and EMIM-BF4 ionic liquid with LiNO3 added as a salt along with LiI; and the cathode is based on MoS2 nanoflakes (NFs). This electrolyte/cathode combination enables a Li-O2 battery to achieve a high-energy efficiency (reduced polarization gap) as well as high cyclability (270 cycles) at a fixed capacity of 1000 mAh/g with a minimal (5%) Li2O2 capacity loss. Additionalt for Li-CO2 battery project, we developed a chemistry that would enable carbon and Li2CO3 reversibility in a Li-CO2 battery. Using a MoS2 electrocatalyst with an ionic liquid/DMSO electrolyte, we have been able to achieve a long cycle life Li-CO2 battery with evidence from various in-situ and ex-situ techniques for reversibility with carbon neutrality. In the third project, we report the prediction, synthesis, and multiscale characterization of two-dimensional (2D) high-entropy transition metal dichalcogenide (TMDC) alloys with four/five transition metals is reported. The synthesized alloys can be exfoliated into 2D structures. We studied the electrochemical performance of five metal compounds i.e., (MoWVNbTa)0.20S2 with the highest configurational entropy is investigated for CO2 conversion to CO, revealing an excellent current density of 0.51 A/cm2 and a turnover frequency of 58.3 s-1 at ~-0.8 V vs. RHE. In the fourth project, we address the extending the use of TMDCs at high temperature challenge by exploiting the high entropy stabilization of TMDCs and demonstrate that the large configurational entropy of these materials can be leveraged to overcome these thermal and electrical bottlenecks in 2D-TMDCs.The thermal stability and electrical properties of (MoWVNbTa)0.20S2 have been studied through experiments and computations. Multi-scale in-situ and ex-situ measurements confirm an exceptionally high thermal stability of this material at elevated temperatures tested up to ~1300 K. Moreover, this material exhibits a very low electrical sheet resistance (~0.7 mΩ.cm) at both thin-film and 2D forms comparable to other state-of-the-art materials but with much higher stability at ambient condition for an extended time tested up to 90 days. It also exhibits excellent electrical stability under cyclic mechanical strains. In the last two projects, the use of a copper based conductive metal organic framework (c-MOF) has been in investigated in energy storage and conversion systems i.e., Li-air battery and electrochemical CO2 conversion. Specifically, we report an excellent catalytic activity of a two-dimensional copper tetrahydroxyquinone (Cu-THQ), for aqueous CO2 reduction reaction at low overpotentials. It is revealed that Cu-THQ nanoflakes (NFs) with an average lateral size of 140 nm exhibit a negligible overpotential of 16 mV for the activation of this reaction, a high current density of ~173 mA cm-2 at -0.45 V vs RHE, an average Faradaic efficiency of ~91% towards CO production and a remarkable turnover frequency as high as ~20.82 s-1. Additionally, this material promotes the growth of nanocrystalline Li2O2 products with amorphous regions. This provides a novel platform for the continuous growth of Li2O2 units away from framework enabling a fast discharge at high current rates. Moreover, the Li2O2 structure works in an excellent synergy with the RM in the electrolyte. The conductivity of the amorphous Li2O2 structure allows the RM to act directly on the Li2O2 surface instead of catalyst edges and then transport through the electrolyte to the Li2O2 surface.