Metal-Organic Framework Nanosheets for Electrocatalysis

Abstract

Metal-organic frameworks (MOFs) have aroused great interest in many fields due to their appearing properties such as tailorable structure and function, high specific surface area and porosity. Electrocatalysis is attractive and significant for the academia and industry because it underpins various clean and renewable technologies like water splitting, fuel cell, metal-air batteries, etc. In recent years, MOFs-derived materials prepared through post hightemperature calcination have been widely investigated for electrocatalysis. However, the pyrolysis process always destroys the structure of the MOFs, resulting in the agglomeration of metal nodes and loss of organic ligands, which are not favourable for electrocatalysis. Meanwhile, only very limited number of works directly used pristine MOFs as electrocatalysts. To this end, this thesis aims to design and synthesize 2D MOF nanosheets and 2D MOF-based hybrid nanosheets for electrocatalysis. The first aspect of this thesis is about Ni-MOF nanosheets with high oxidation state for urea oxidation reaction (UOR). High oxidation state of metal cations is critical in achieving outstanding performance of many transition metal-based materials towards electrochemical oxidation reactions such as UOR, which acts as a vital half reaction for several practical applications. However, it is still a great challenge to explore such a kind of materials for high-performance oxidation reactions. Herein, 2D MOF comprising nickel species and organic ligand of 1,4-benzenedicarboxylic acid (BDC) is fabricated and explored as an electrocatalyst for UOR, which exhibits high activity (120 mA cm-2 at 1.6 V vs. RHE) and strong catalyst durability after continuous operation for 10 hours. The excellent UOR performance is due to high active site density of the 2D MOF, and high oxidation state of the nickel species, which are proved by both X-ray photoelectron spectroscopy and Synchrotronbased X-ray absorption near edge spectra. Our findings provide a suitable material for practical application of UOR, and this 2D MOF strategy could be used to fabricate other electrocatalyst with high oxidation state for a wide range of oxidation reactions. The second aspect of this thesis is about Ni-BDC/Ni(OH)2 hybrid nanosheets for oxygen evolution reaction (OER). Just like graphene, 2D MOF has an unwanted tendency to aggregate, which reduces the specific surface area. Ni(OH)2 is a typical catalyst for OER, but the reaction activity is far from satisfactory probably due to its low oxidation state. The Ni- BDC/Ni(OH)2 hybrid nanosheets prepared through a facile sonication-assisted solution method can perfectly solve these two problems. After hybridization with Ni(OH)2, the large 1 surface area of Ni-BDC is well retained. Moreover, due to the strong electron interactions between BDC from Ni-BDC and Ni cations from Ni(OH)2, the electronic structure of Ni cations from Ni(OH)2 component can be well modified, leading to the generation of Ni cations with higher oxidation state, which surely contribute to enhanced OER activity. As a result, the Ni-BDC/Ni(OH)2 hybrid nanosheets exhibited remarkable OER performance in 1.0 M KOH, outperforming pure Ni-BDC, Ni(OH)2 and even commercial Ir/C. The third aspect of this thesis is about Co-BDC/MoS2 hybrid nanosheets for alkaline hydrogen evolution reaction (HER). Generally, the reaction activity of a catalyst for alkaline HER is about 2-3 orders of magnitude lower than that for acidic HER. This is because the hydrogen intermediate (H*) comes from the dissociation of water in alkaline solution, and this step introduces an additional energy barrier for alkaline HER. At present, the oxidation reaction (e.g. OER) performance of MOFs are comparable or even superior to benchmark noble metals, but the HER activities of MOFs are far from satisfactory. To this end, Co- BDC/MoS2 hybrid nanosheets are constructed for alkaline HER. The pristine 2H-MoS2 are transformed to 1T-MoS2 partially after the hybridization. This is beneficial for HER as 1TMoS2 is a much better HER catalyst. Moreover, the well-constructed Co-BDC/MoS2 interface is vital for alkaline HER, as both components play specific roles in different elementary steps of alkaline HER. In specific, Co-BDC facilitates the dissociation of water to provide enough protons to the nearby MoS2, while phase-modified MoS2 is favourable for the following H2 generation. As expected, the as-fabricated Co-BDC/MoS2 nanosheets exhibit remarkable HER performance in 1.0 M KOH, outperforming those of Co-BDC nanosheets, MoS2 nanosheets and almost all the previously reported MOFs-based electrocatalysts.