Engineering electronic structure of MOF-derived materials and their electrocatalytic Applications

Abstract

Efficient electrodes with suitable electronic structures are highly demanded for practical applications. Thus, developing effective strategies to modulate the electronic structure of electrode materials in with a controllable manner is highly significant. Metal-organic frameworks (MOFs) with precise composition and flexible structures provide great opportunities to pre-design or post-modify the structure from a molecular level. The resulted structure design/change of MOFs can be well inherited in their derivatives upon pyrolysis. Therefore, the final electrode materials with target functionalities can be easily obtained by careful design of the MOFs precursors, realizing a controllable manner. In view of the great importance of electronic structures in electrocatalysts, this thesis sought to develop effective strategies for electronic structure modulation of MOFs-derivatives for efficient electrocatalysis. The main contents of this thesis are listed below: (1) Previous work usually used oxygen-free ZIF to prepare carbon materials, which led to small pore size. Different from that, in this work, N-doped porous carbon (NPC) with a hierarchically porous structure and a high concentration of pyridinic-N and graphitic-N reaching 68.31% was successfully prepared by calcinating the mixture of oxygen-rich Zn-MOF-74 and melamine. The obtained NPC shows excellent activity and selectivity for CO2 to CO conversion with a high FECO of 98.4%. (2) A new coordination mode of Zn-N6 was achieved by a pre-coordination method. The atomically distributed Zn with a high coordination number of Zn-N6 is anchored on N-doped porous carbon by grafted -NH2 on the Zn-MOF-74 to immobilize unsaturated metal sites. With the synergetic effect of doped N and enlarged exposure of Zn-N6 active sites, the Zn single atoms immobilized in nitrogen-doped porous carbon (ZnSA/NPC) exhibit superior CO2 reduction reaction (CO2RR) with 94.7 % FECO. (3) Ultrafine Mo2C is quite difficult to prepare. This work shows an effective method to prepare ultra-fine Mo2C embedded in NPC for efficient ORR. Controlled encapsulation of molybdenum-based polyoxometalates (PMo12) inside of ZIF-8 lead to the successful fabrication of ultrafine Mo2C nanoparticles embedded in porous N and P co-doped carbon matrix. Significantly, from experimental and theoretical investigations, the highly porous structure, highly dispersed ultrafine Mo2C as well as the N and P co-doping in Mo2C@NPC result in a decreased ΔG of the rate determining step for efficient ORR activity and zinc-air batteries. (4) A Co-Mo-S hybrid consisting of CoS2/MoS2 embedded in N-doped carbon was prepared from Keggin-type polyoxometalate (PMo12) encapsulated ZIF-67 via a one-step sulfurization process. The presence of Mo induced reconfiguration of the electronic structure around Co in NC@CoS2/MoS2. It is revealed that the excellent catalytic performance (activity and stability) of this material is attributed to the cooperative effect of interfacial charge redistribution between CoS2 and MoS2 as well as the nitrogen doped carbon with a suitable amount of pyridinic N for promoting electron transfer. In summary, different strategies including heteroatom doping, coordination environment regulation and heterojunction construction have been developed to convert MOFs to carbon-based materials with modified electronic structure for efficient electrocatalysis in CO2RR, oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). This kind of molecular-level strategy allows the structural design in a predictable manner and provides an effective way for both the electrocatalyst synthesis and electrocatalytic mechanism study, which would be highly significant in the fields of electrocatalysis as well as energy device development.