| With the gradual increase of world population and the rapid development of science and technology,the urgent demand for energy use has been aggravated.At present,the energy of the earth is mainly from the burning of fossil fuels.However,these resources are not renewable,the abuse of them has led to serious environmental problems,such as the greenhouse effect.In view of the above problems,it is imminent to develop cheap and environmentally friendly energy instead of fossil fuels.Among them,hydrogen energy is a renewable energy with high heat and no pollution.The use of hydrogen energy can reduce the use of fossil fuels and the emissions of carbon dioxide effectively,and curb the trend of global warming.The most direct preparation of hydrogen energy is the decomposition of water.The water decomposition reaction usually includes the process of water oxidation and proton reduction.From a dynamic point of view,the process of water oxidation includes slow proton transfer and the formation of O–O bonds.It is the bottleneck of the whole water catalytic reaction.Therefore,it is very important to develop an efficient water oxidation catalyst.After more than 200 years of development,polyoxometalate has become a widely used cluster in inorganic chemistry.They have adjustable acidity,reversible redox properties,antioxidant properties and excellent thermal stability.These special properties make it widely studied in the field of catalysis.According to the catalytic effect on water splitting of Mn4CaO5 in the photosynthesis of green plants,people are also looking for compounds with similar structure in polyoxometalates as catalysts for water splitting.Water oxidation catalysts include photocatalysts and electrocatalysts.The electrocatalysis refers to imposing driving force of the reaction on the electrode,and the reaction of hydrogen evolution and oxygen evolution occurs under the action of catalyst.Therefore,searching for electrocatalysts for water splitting has always been a hot topic.Although kinds of electrocatalysts have been studied,it is the aim of people to find water decomposition electrocatalysts with high catalytic activity.Based on this research idea,we have selected several polyoxometalate containing manganese clusters that have similar structures with Mn4CaO5 as the research object,and used it as a catalyst in the process of electrocatalytic water oxidation.We studied their catalytic activity in solution and films respectively,in order to find catalyst materials with high catalytic activity.Firstly,we synthesized two compounds including sixteen manganese.They are Na19.5Cs8.5[Mn?10Mn6?O6?OH?6?PO4?4(A-?-SiW9O34)4]?Mn16–Cs?and Na18Rb10[Mn4Mn?12?OH?12?PO4?4(A-?-SiW9O34)4]?Mn16–Rb?,respectively.They have similar structures,but the ratios of MnIIIII and MnIII are different.In order to compare their catalytic activity for electrocatalytic water splitting.On the one hand,we measured their oxygen evolution currents in the neutral electrolyte solutions when they were homogeneous catalysts.On the other hand,we prepared film materials with PDDA and Rubpy on the surface of the electrode,respectively and measured their oxygen evolution currents in neutral electrolyte solutions when they were used as heterogeneous catalysts.The result shows that under the same catalytic conditions,the catalytic activity of Mn16-Cs is higher than that of Mn16-Rb.The catalytic activity of Rubpy film is higher than that of PDDA film.In addition,the stability test showed that the thin films had good stability in the process of electrocatalytic water splitting.Secondly,we studied the catalytic activity of the composites prepared with graphene and Na19.5Cs8.5[Mn?10Mn6?O6?OH?6?PO4?4(A-?-SiW9O34)4]?Mn16–Cs?and Na18Rb10[Mn4?Mn?12?OH?12?PO4?4(A-?-SiW9O34)4]?Mn16–Rb?.As a semiconductor material,graphene is the largest and thinnest material known.Its excellent conductivity,low cost and larger surface area make it an ideal film material.We have prepared two kinds of thin film materials by layer-by-layer method and the method of combining electrodeposition with layer-by-layer assembly.The results show that the film containing Mn16-Cs has good catalytic activity,that is,the activity of MnIIIII is better.For electrodeposited and layer-by-layer films,the concentration of graphene solution and the time of electrodeposition affect the activity of electrocatalytic water splitting.Thirdly,after that,we synthesized three other polyoxometalate compounds containingmanganeseclusters.Theyare Na2.5K4.5[Mn3??OH?3?H2O?3(A-?-SiW9O34)]·15H2O,K4[Mn3??OH?3?H2O?3(A-?-SiW9O34)]·10H2Oand Na3.5K2.5[Mn3?Mn?O3?CH3COO?3(A-?-SiW9O34)]·20H2O·NaCH3COO·0.5KCH3CO O and studied the electrocatalytic activity of water splitting in solution under the same experimental conditions.It was found that the compound containing four Mn had high catalytic activity.We then assembled the four manganese compound with Co3O4material on the electrode surface to prepare the thin film.Compared to four manganese and PDDA assembled films,the films assembled by four Mn and Co3O4have better catalytic activity. |
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