Preparation Of Cobalt Based Compounds Derived From ZIF-67 And Their Performance Of Electrocatalytic Oxygen Evolution

Electrocatalytic water splitting is an efficient energy conversion process for replacing traditional fossil fuels,which needs to be completed by overcoming the reaction kinetic barrier.Therefore,the research on low-cost and efficient non-noble metal-based catalysts is crucial for the large-scale generation of green energy.Cobalt-based materials are regarded as one of the promising candidates for electrocatalytic water splitting owing to their low cost,abundant reserves and environmental friendliness.As a MOF material coordinated with 2-methylimidazole（2-MIM）and Co ions,ZIF-67 has the characteristics of high porosity,large specific surface area and ordered pore structure,and the synthesis process is simple.Therefore,in this paper,a variety of cobalt based catalysts were prepared from the aspects of morphology regulation,oxygen defect construction and surface reconstruction with ZIF-67 as template and precursor.The relationship between the structure and electrochemical properties of the catalysts and the formation mechanism were discussed.The specific research contents are as follows:According to the ligand and multi metal coordination competition in MOFs,Co N/Mo N complexes with hollow 3D/2D structure were prepared.By changing the content of H₂O and 2-MIM in the synthesis process,the morphology of the material was regulated and the active area was improved.The experimental results also show that the high temperature treatment in NH₃ does not significantly change the morphology and specific surface area of the samples.X-ray photoelectron spectroscopy（XPS）shows that the electronic structure of Co and Mo can be adjusted by complex structure,and the pyridine nitrogen can enhance the electrocatalytic oxygen evolution（OER）performance of the material.Therefore,at a current density of 10 m A cm^-2,the OER overpotential of Co-Mo-N-PHP material in 1 M KOH is 294 m V.The Co O@Co₃O₄/C heterogeneous composites with oxygen defects was successfully synthesized with ZIF-67 as template,and the oxygen defect in the material can be controlled by changing the vacuum degree during heat treatment.Combined with a variety of test methods,it indicated that the charge imbalance distribution caused by oxygen defects can produce built-in electric field to accelerate charge transfer and enhance OER activity.Density functional theory（DFT）and in-situ Raman spectroscopy show that oxygen defects can effectively reduce the reaction energy barrier and promote the adsorption and transformation of OER reaction intermediates（*OH and*O）.At the same time,oxygen defects and heterogeneous interfaces increase the active area of the material.In addition,the pore structure arising from ZIF-67 provides an effective path for the transmission of charge and gas.The experimental results show that the Co O@Co₃O₄/C composites in 1 M KOH offered an OER overpotential of 287 m V at a current density of 10 m A cm^-2.A two-dimensional（2D）MOF was synthesized at room temperature with ZIF-67 as precursor.The specific surface area and mesoporous content of MOF were increased by doping Ni and Fe ions,respectively.Rely on surface activation,2D MOF is transformed into highly catalytically active Fe-Ni-Co OOH.The experimental results show that the material contains amorphous phase and crystalline phase structure.The activation process can promote the d-band center of the sample to move to the Fermi level and provide active sites for the adsorption/desorption of OER intermediates.DFT results show that 1,4-terephthalic acid（TPA）in the material is beneficial to reduce the reaction barrier,improve the electron density around the active site,and further enhance the OER catalytic activity of Fe Ni Co OOH.In addition,the nano network structure avoids agglomeration and the use of adhesives in the OER process.According,the Fe-Ni-Co OOH-TPA composites offered an OER overpotential of 236 m V at a current density of 10 m A cm^-2 in the 1 M KOH.Novel Co-Ni bimetallic phosphides with a 2D/3D structure were developed by in-situ phosphorization and rapid surface reconstruction strategies for use as an electrocatalyst toward overall water splitting.In-situ phosphorus diffusion from the substrate was confirmed by high-resolution transmission electron microscopy（HRTEM）and X-ray photoelectron spectroscopy（XPS）depth profiles.The in-situ diffusion of phosphorus in the synthesis process was confirmed by high-resolution transmission electron microscopy and XPS depth analysis.Co OOH was rapidly generated by electrochemical treatment to realize the surface reconstruction of the material.Combined with the results of in-situ Raman spectroscopy and DFT,it is further proved that Co OOH can provide more catalytic active sites,which is conducive to the adsorption and desorption of intermediates,and effectively reduce the reaction barrier of OER.The designed materials showed efficient and stable electrocatalytic performance,with HER and OER overpotentials of 117 m V and 272 m V at a current density of 10 m A cm^-2,respectively.A low cell voltage of 1.59 V was achieved for overall water splitting,and there is no obvious attenuation of current after 400 h stability test.