Synthesis,Regulation And Electrolytic Applications Of Iron-Based Metal-Organic Frameworks Grown On Nickel Foam

Currently,the long-term overuse of fossil fuel has caused a series of environmental problems.Human are in desperate need of a clean,sustainable and high-energy-density energy carrier for energy conversion and storage.Hydrogen draws researchers’attentions for its efficient,carbon-free,and low-density properties.Electrolysis of water can produce hydrogen with little pollution.However,the high energy barriers of electrodes reactions limit its industrial application on large scale,which requires an ideal electrocatalyst to lower the energy barriers.Noble metal like platinum,palladium,ruthenium and their oxide（like Rb O₂,Ir O₂,etc.）are good commercial catalysts for hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.However,high cost,earth scarcity and partial dissolution limit their applications on large scale.Therefore,catalysts with low cost,high efficiency and good stability are expected to be developed as substitutes for noble metals.Metal-organic frameworks（MOFs）,porous crystalline,feature a variety of metal nodes,linked by various ligands.Meanwhile,as electrode,nickel foam substrate can offer conductive path and bubble resistance.Therefore,MOFs in-situ grown on nickel foam electrodes are well known for their convenient operation,high efficiency and low cost.Based on the above merits,Mn_xFe_yNi-MOF-74,Fe_xM_y-BTC and MIL-88B with different Fe（III）:Fe（II）ratios on nickel foam were synthesized in the thesis through solvothermal methods.By regulating the metal ratios,the electronic structures and morphologies were successfully regulated,and further promoted the catalysts’activity.The main contents of this thesis are as follows:（1）Fusiform Mn_xFe_yNi-MOF-74 bar were successfully in-situ grown on nickel foam and used as cathode for HER,anode for OER directly and for overall water splitting.Researches indicated that Mn_xFe_yNi-MOF-74 displayed good crystallinity as fusiform array,approximately 175 nm in diameter;Elements analysis demonstrated that Mn,Fe and Ni dispersed evenly in Mn_xFe_yNi-MOF-74 fusiform bar,and Mn:Fe ratios could be regulated;Introduction of Mn in bimetallic MOF-74 could promote the exposure of active sites,enlarge the electrochemical surface area and increase the Fe（III）-O-Fe（II）motifs.Overpotential of optimized Mn_0.52Fe₇₁Ni-MOF-74 only needed 99 m V at the current density of 10 m A·cm^-2 for HER and 268 m V at the current density of 100 m A·cm^-2 for OER;Assembled as two electrode system,overpotential only needed 245 m V and 462 m V at the current density of 10m A·cm^-2 and 100 m A·cm^-2.XPS indicated that Mn served as electrons regulator maintaining the OER catalysis performance for 100 h.（2）Fe_xM_y-BTC was in-situ grown on nickel foam through solvothermal method,taking1,3,5-Benzenetricarboxylic acid as ligand.Nickel foam with Fe_xM_y-BTC were used as electrodes directly.Overpotential of optimized Fe Co₂-BTC only needed 260 m V at the current density of 500 m A·cm^-2 for HER and Fe₂Ni-BTC 257 m V at the current density of 100m A·cm^-2 for OER.Fe_xM_y-BTC were verified as MIL-100;When metals were in proper ratios（Fe:Ni=2:1;Fe:Co=1:2）,morphologies turned up to be crowded and big films;metal oxidation states of Fe was+3,Ni was+2 and Co was+2,which could exert synergistic effect for catalysis of OER and HER in MIL-100.（3）Fixing the Ni:Fe ratio,this thesis synthesized MIL-88B material by controlling feeding ratios of Fe（II）:Fe（III）.When feeding ratios of Fe（III）:Fe（II）were 1:0 and 3:1,the morphologies of MIL-88B were bulk.When feeding ratios of Fe（III）:Fe（II）were 1:1,the morphologies of MIL-88B were flakes.When feeding ratios of Fe（III）:Fe（II）were 1:3,the morphologies of MIL-88B were flakes mixed with grains.When feeding ratios of Fe（III）:Fe（II）were 0:1,the morphologies of MIL-88B were pillar.MIL-88B performed best when feeding ration of Fe（III）:Fe（II）were 1:1,with only 263 m V overpotential for OER at current density of 200 m A·cm^-2.In summary,MOFs grown on nickel foam draw researchers’attention for its low price,simple preparation,direct use as electrode and excellent electrocatalysis performance,which have a promising future as electrocatalysts and play important roles in energy realm.