Transition Metal Dichalcogenide (TMDCs) Catalysts for Energy Storage and Conversion Applications

2016-02-17T00:00:00Z (GMT) by Mohammad Asadi
Energy conversion and storage systems could provide a viable route to store energy in the form of the chemical bond. In general, energy can be stored or converted into chemical bonds through electrochemical and photo-electrochemical processes such as Li-oxygen (Li-O2) batteries, fuel cells, hydrogen (H2) generation, carbon dioxide (CO2) reduction reactions and utilized as an energy source in the form of electricity or fuel. However, these systems have advanced far more slowly over the last two decades due to the lack of a suitable catalyst. Fundamentally, the electronic structures of the existing catalysts are not well suited for electrochemical reactions mainly due to their high work function and low density of d-orbital electrons near the fermi level of energy. Here for the first time we report that the inexpensive non-noble-metal transition metal dichalcogenide (TMDCs) family of catalysts performed remarkably well for any electro and photo chemical reactions owing to their advantageous electronic structure. The performance of these materials was also promoted by using an appropriate electrolyte, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4), as a reaction medium. In particular, the synergetic effect between the ionic liquid (IL) and the catalyst enhances the electrochemical reaction at the TMDC edge atoms where the IL plays multiple roles to facilitate the reaction pathway. We studied the performance of TMDC family for H2 generation, CO2 reduction reactions for energy storage systems. Moreover, we introduce a new generation of energy storage systems, a lithium oxygen (Li-O2) battery with five-fold higher energy density using a molybdenum disulfide/EMIM-BF4 co-catalyst system. Details are discussed into three separate chapters entitled “Hydrogen evolution reactions (HER)”, “CO2 reduction reactions” and “Li-O2 battery” due to different technical characteristics of each system. In brief, the state-of-the-art of systems for H2 generation, CO2 reduction and Li-O2 battery were discussed in the first chapter. In the second chapter, the chemical vapor deposition (CVD) method has been employed to synthesize crystalline 3D structured molybdenum disulfide (MoS2) with bare Mo edge atoms on graphene film, and demonstrated its outstanding activity and stability for the hydrogen evolution reaction (HER). In the third chapter, a novel 2D form of transition metal dichalcogenide (TMDC) has been used for catalyzing CO2 reduction in the ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) environment. Finally, in chapter 4, the concept of co-catalyst system is introduced in the Li-O2 batteries for the first time. In more details, it has been shown that MoS2 nanoflakes (NFs) in the ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) exhibits unique bi-functional catalytic activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). This MoS2/IL system is also found to perform remarkably well in a Li-O2 battery system with a high round trip efficiency, small discharge/charge polarization gap and good reversibility.