Preparation Of Metal-Organic Frameworks Derivatives And Their Properties For Urea-assisted Electrolysis For Hydrogen Production

At present,the ideal method of hydrogen production is electrocatalytic water splitting.Water splitting consists of anodic oxygen evolution reaction（OER）and cathode hydrogen evolution reaction（HER）.The slow kinetics of anodic reaction greatly limits the practical application of electrolytic water.To reduce the energy consumption required for water splitting,catalysts with high efficiency electrocatalytic performance can be used to reduce the reaction barrier,or a small molecule oxidation reaction with low theoretical overpotential can be selected to replaced OER.In terms of small molecule oxidation,the theoretical potential of urea oxidation reaction（UOR）at the anode is only-0.46 V vs.SHE,which is much smaller than that of OER,and its products are only CO₂ and N₂.CO₂ is absorbed by electrolyte and does not affect the collection of hydrogen at the cathode.Therefore,replacing OER with UOR is an effective method to reduce the energy consumption of electrolytic water.As for the selection of catalysts,catalysts with open three-dimensional（3D）structures can not only avoid the disadvantages of traditional powders that are easy to agglomerate and require Nafion adhesive,but also tend to expose more active sites,making them perform well in water splitting.Combined with the above two points,this paper used a reasonable construction of metal organic framworks（MOF）derivatives to make them have a unique 3D structure and have good catalytic performance in urea electrolysis.The specific research contents are as follows:Ni-Fe based MOF composite（MOF-Ni@MOF-Fe）grown on Ni foam was prepared by combining two different MOF materials,and then MOF-Ni@MOF-Fe was transformed into bimetallic sulfide（MOF-Ni@MOF-Fe-S）.The results of SEM and TEM show MOF-Ni@MOF-Fe-S presents the structure of interwoven nano-network.Electrochemical test results show that MOF-Ni@MOF-Fe-S shows a low potential of 145.0 mV（vs.RHE）for the HER at 10 mA cm^-2.MOF-Ni@MOF-Fe-S as the anode catalyst,needs 1.346 V（vs.RHE）to get 50 mA cm^-2 in 1.0 M KOH containing 0.5 M urea,the electrode potential of the UOR is 110.0 mV less than that of the OER.By assembling MOF-Ni@MOF-Fe-S into symmetrical electrolytic cell,MOF-Ni@MOF-Fe-S||MOF-Ni@MOF-Fe-S electrolyzer only needs 1.539V to reach 10 mA cm^-2 in 1.0 M KOH containing 0.5 M urea while it needs 1.602 V to reach10 mA cm^-2 in 1.0 M KOH.Using the strategy of combining the original MOF material with the active species,MOF-Ni with 3D structure was grown on nickel foam by hydrothermal method.In situ nickel-iron Prussian blue was grown using MOF-Ni as template.After further phosphating,the Ni-Fe Prussian blue was transformed into Ni₂P,Fe₂P and nitrogen-doped carbon（MNPBA-P）.The results of SEM and TEM show that MNPBA has the struture of nanocube-like Ni-Fe Prussian blue uniformly growth on the nanocsheets at 30℃for 24 h,and the size of nanocube is about 50 nm.Electrochemical test results show that MNPBA-P as HER catalyst only need 134.0 mV（vs.RHE）to get 10 mA cm^-2.MNPBA-P as an anode catalyst,the potential for UOR is 94.00 mV lower than that for OER at 20 mA cm^-2.MNPBA-P was used as a multifunctional catalyst and assembled into a symmetric electrolyzer to test its catalytic performance of urea electrolysis and water splitting.MNPBA-P||MNPBA-P requires 1.500 V to drive a current density of 10 mA cm^-2 in urea electrolyte,and has good stability.The possibility of using MOF-Ni as a template was further studied after the successful preparation of Mo F-Ni as a template.Ni-Co Prussian blue（PBA@MOF-Ni）grown in situ with MOF-Ni as a template was synthesized by hydrothermal method.PBA@MOF-Ni was converted to Co-Ni/Se（PBA@MOF-Ni/Se）in the reactor.The results of SEM and TEM show that PBA@MOF-Ni/Se has the morphology of hollow nanocube.Electrochemical test results show that PBA@MOF-Ni/Se obtained at 60℃for 24 h has the best catalytic performance of HER and UOR.PBA@MOF-Ni/Se need the ponteial of 138.0 mV（vs.RHE）for HER and 1.318 V（vs.RHE）for UOR to reach 10 mA cm^-2.PBA@MOF-Ni/Se was used as both cathode and anode to be assembled into a symmetrical electrolytic cell.In 1.0 M KOH containing 0.5 M urea electrolyte,PBA@MOF-Ni/Se only needs 1.490 V to reach 10mA cm^-2,which is 0.2450 V lower than the potential required for water electrolysis.After the study of nickel-based catalyst,cobalt-based catalyst was studied.Co（OH）₂ was converted into Co-MOF with 3D urchin struture in tubular furnace by gas phase method.After low temperature water bath vulcanization treatment,the surface vulcanized CoS_x/Co-MOF was obtained.The results of SEM and TEM show that CoS_x/Co-MOF have3D urchin-like morphology.Electrochemical test results show that CoS_x/Co-MOF as HER catalytic only need 73.00 mV（vs.RHE）to reach 10 mA cm^-2.CoS_x/Co-MOF as UOR catalytic only need 1.315 V（vs.RHE）to reach 10 mA cm^-2 in 1.0 M KOH containing 0.5 M urea.CoS_x/Co-MOF was used as cathode and anode at the same time,and assembled into symmetrical electrolytic cell.The CoS_x/Co-MOF electrolyzer only needs a cell voltage of1.480 V to achieve a current density of 10 mA cm^-2 in 1.0 M KOH containing 0.5 M urea,while in 1.0 M KOH electrolyte,CoS_x/Co-MOF||CoS_x/Co-MOF requires 1.66 V.In order to further optimize the experimental scheme,a reaction route with simple method,short time consumption and controllable morphology was designed.Co（OH）₂ grown on nickel foam was synthesized by electrodeposition.Then,using Co（OH）₂ as template,ZIF-67 with rhombic dodecahedron is in situ on Co（OH）₂ nanosheet.After further high-temperature selenization reaction,CoSe₂ with a yolk-shell structure（Co-Z/Se-2）was obtained.The results of SEM and TEM show that when the ratio of water and ethanol is 4:6,ZIF-67 was grown at the edge of Co（OH）₂ nanosheet without destroying the morphology of Co（OH）₂.Electrochemical test results show that CoS_x/Co-MOF as HER catalytic only need106.0 mV（vs.RHE）to reach 10 mA cm^-2.CoS_x/Co-MOF as UOR catalytic only need 1.320V（vs.RHE）to reach 10 mA cm^-2 in 1.0 M KOH containing 0.5 M urea.In 1.0 M KOH containing 0.5 M urea,a symmetric cell assembled by Co-Z/Se-2 can drive 10 mA cm^-2 with only 1.490 V and can operate continuously for 20 hours.