Synthesis Of MOF And MOF Derivatives Co₃O₄ And Their Performance In Photoreduction Of CO₂ And Oxygen Evolution In Water Electrolysis

The average surface temperature of the planet has risen about 0.9? since the late19th century,a change driven largely by the sharp increase of greenhouse gases such as CO₂ in the atmosphere.The global warming,sea level rise,forest fires,ocean acidification and other increasingly serious problems caused by the greenhouse effect have become an urgent problem to be solved.And the increase of energy consumption led directly to the CO₂ emissions of greenhouse gases.Thus,there is an urgent need to use clean energy,reduce CO₂emission,capture CO₂,store CO₂ and consume CO₂.Among the strategies,converting carbon dioxide into fuels(e.g.CO,HCOOH,CH₄,CH₃OH and C₂H₅OH etc.)using solar energy is one promising method to achieve zero net emission of CO₂.Because the light is relatively clean and safe,photocatalytic CO₂reductions using majority sun-light and visible light are particularly favored by the scientific community.MOFs have already received considerable attention from scientists because of their extraordinary porosity,exceptional chemical and structural diversity,tailorable structures and properties.Among MOFs,porphyrinic MOFs(PMOFs)have attracted much more attention due to that porphyrins plays important role in chemical and biological processes.Driven by solar energy,wind energy,tidal power,hydrogen production by water-electrolysis technology can provide clean,sustainable hydrogen instead of the fossil fuels,thereby reducing environmental pollution.It is an effective way of energy storage.Water electrolysis consists of two half reactions,hydrogen evolution(OER)and oxygen evolution reaction(OER).And the oxygen evolution reaction(OER)is also an important half reaction in energy conversion of metal-air cells and fuel cells.In this paper,three 3D,anionic Cd(II)porphyrinic metal-organic frameworks(PMOFs),1(3.5:1)-2d-120([Cd_3.2(H₂TCPP)₂][(CH₃)₂NH₂]_1.6),its Li⁺-exchanged product(e.g.Li⁺-1-120,formula:[Cd_3.2(H₂TCPP)₂]Li_1.6)and 1(10:1)-4d-120([Cd_3.7H₂TCPP)₂][(CH₃)₂NH₂]_0.6)were synthesized.The molecular formula of PMOFS was calculated according to ICP-AES combined with TG test results.Compound1(10:1)-4d-120 represents a framework with much less charges compared with those of the other two PMOFs.The band positions of them in nonaqueous electrolyte are almost identical,showing they can photocatalytically reduce CO₂ to CO.Consistently,the reductions were observed experimentally.Combined with the investigation of mechanism,we found that the roles of these PMOFs in photocatalytic system were mainly co-catalysts,while Ru(bpy)₃Cl₂ was the actual photocatalyst.Li⁺-1-120 was found to be able to react with Ru(bpy)₃Cl₂ in the presence of sufficient amounts of water,preventing the photocatalytic property of Ru(bpy)₃Cl₂ to be released.This paper put forward clearly the role of PMOFs materials in CH₃CN/TEA(or TEOA)/H₂O 400?L)/Ru(bpy)₃Cl₂·6H₂O/PMOF system by photocatalytic mechanism studies of the CO₂reductions,identifying that in some cases the actual catalyst is Ru(bpy)₃Cl₂ and PMOFs are cocatalysts.These discoveries are of great significance for the synthesis and application of PMOFs.Secondly,using the carbon cloth as substrate,we synthesized with nano lamellar structure Co(OH)₂/CC,the structure of the cube ZIF-67/CC and concave cube structure Co₃O₄/CC by electrochemical deposition,simple hydrothermal and annealing treatment methods.The catalyst Co₃O₄/CC has the highest catalytic activity,having an overpotential of only 260 m V at a current density of 10 m A·cm^-2,a Tafel slope of only50 m V·dec^-1.It also has smaller interfacial transfer resistance,larger electrochemical active area and good stability.The carbon cloth substrate promotes electrolyte diffusion,which also leads to the excellent OER activity of Co₃O₄/CC.In the future,we can further explore the electrocatalytic performance of Co₃O₄ nanocrystals by further regulating the synthesis of nanocrystals on the substrate surface.