Structural design and electronic modulation of molybdenum disulfide for electrochemical energy storage and conversion

Abstract

Electrochemical energy storage and conversion are promising techniques to make use of the renewable energy sources (wind, solar, and tidal, etc.). MoS2 has received great attention in energy-related applications owing to its favorable physicochemical characteristics. Nonetheless, MoS2 suffers from capacity decay as an anode in energy storage systems and lacks efficient in-plane active sites as an electrocatalyst in energy conversion systems. In this thesis, various MoS2-based nanomaterials have been fabricated with layered ammonium zinc molybdate and its analogs as the starting materials. The structural and electronic optimization of the MoS2-based materials has been realized via different synthetic routes. The electrochemical performance of the obtained materials has been evaluated in lithium ion battery (LIB) and hydrogen evolution reaction (HER), and the structure-performance relationship has also been investigated via different techniques. To simplify the synthetic procedure in creating mesoporous and hierarchical structure of MoS2, a phase-segregation route was employed to afford MoS2/ZnS composite by direct sulfidation of (NH4)HZn2(OH)2(MoO4)2. The in-situ formed ZnS acts as a hard template and is etched to yield the MoS2 framework of porous architecture (HM-MoS2), which is beneficial for the subsequent conformal carbon coating. As an anode in LIB, the carbon-coated HM-MoS2 with an optimal carbon content shows larger capacity and better rate performance than that of the bare HM-MoS2 and the carbon-coated MoS2 without using a template, which confirms that both conformal carbon coating and porous structure play an important role for enhancing the battery performance. To further optimize the performance of MoS2 in LIBs, several MoS2-based hybrids were synthesized from (NH4)HZn2(OH)2(MoO4)2 via sulfidation, carbon coating, and cation exchange processes. The influence of the secondary component on the MoS2-based hybrid was also studied by replacing ZnS with CuS, partially or completely, during the Cu2+ cation exchange process. The MoS2-based hybrids show better capacity retention compared with the single-phase MoS2, which can be ascribed to the mutual alleviation of the structural variation. Moreover, the hybridization of MoS2 with the ZnS/CuS heterostructure enhances the specific capacity and the rate performance, which may be attributed to its enhanced kinetics of the S/Li2S redox couple and elevated Li+ ion migration rate. A selective acid etching method was demonstrated to achieve Co and/or Fe doping in MoS2 frameworks by using Co2+ and/or Fe3+ cations substituted (NH4)HZn2(OH)2(MoO4)2 as the precursors. Alien phases or clusters are not found in the final products, and Co and/or Fe atoms are homogeneously distributed in MoS2 matrix. The transition metal doped MoS2 electrocatalysts need lower overpotential to drive HER compared with the bare MoS2. In addition, the Co and Fe codoped MoS2 exhibits enhanced HER activity compared with the Co-doped and the Fe-doped MoS2, due to the introduction of alien atoms (Co and Fe) and the activation of in-plane sulfur sites by the nearby Co and Fe atoms.