Preparation Of Carbon Materials And Their Use In Clean Energy

It is now widely recognized that emissions from fossil fuels not only pollute the air,but also cause astonishing global warming consequences.Moreover,our country's dependence on foreign oil or natural gas creates potential social vulnerability and national instability.These problems have attracted nation's attention.Scientists are trying to find new energy sources to replace fossil fuels.Hydrogen generated by electrochemical water decomposition is a non-polluting,high energy fuel.Precious metal platinum(Pt),noble metal oxides(RuO2 and IrO2)are highly efficient catalysts for hydrogen evolution reaction(HER)and oxygen evolution reaction(OER),respectively.However,the high cost and rareness of precious metals limit their large-scale use.Therefore,the synthesis of highly efficient non-precious metal catalysts for water decomposition is well worth studying.On the other hand,lithium-ion rechargeable batteries are being considered to be the most promising energy storage systems because of their high energy storage densities.Graphite has been commercialized as a lithium ion battery anode material,but its low theoretical specific capacity has limited its extended use.The development of new high-capacity anode materials is currently one of the reaearch hot spots in materials chemistry.In the field of electrochemical energy storage and conversion,carbon materials are widely used in lithium ion batteries(LIBs),metal-air batteries,lithium-ion batteries,hydrogen fuel cells,electrochemical supercapacitors,etc.because of their excellent electron conductivity,large specific surface area,good chemical stability and low cost.In this thesis,we aim to prepare carbon materials with certain morphology and pore structures by using new synthetic methods and explore their use as electrochemical energy storage and conversion materials in clean energy fields.The following work has been carried out:1.A highly efficient water decomposition catalyst based on transition-metal phosphide-carbon nanosheet composites was synthesized in two steps by using a layered two-dimensional MOF(Metal-organic framework)material as a precursor.2.A new mesoporous carbon material was synthesized by using a relatively low temperature method and showed good performance as a lithium ion battery cathode material.The research in this thesis mainly includes the following two aspects:1.Transition metal phosphide-carbon nanosheet composites derived from 2-dimensional metal-organic frameworks for highly efficient electrocatalytic water-splitting.In this chapter,we report the fabrication of a bi-functional metal phosphide-carbon composite catalyst for HER and OER.The catalyst was derived through carbonization and subsequent phosphorization of 2-dimensional cobalt porphyrinic metal-organic framework nanosheets.It consists of cobalt phosphide nanoparticles embedded in mesoporous N-doped graphitic carbon materials.The catalyst shows good electrocatalytic activities for HER and OER with overpotentials of 98 and 370 mV at a current density of 10 mA/cm2 and the Tafel slopes of 74 and 79 mV/dec,respectively.In addition,the catalyst also shows good durability.The method used in this study could be applied to prepare new,highly efficient water-splitting catalysts by using diverse 2-dimensional metal-organic frameworks as templates.2.Mesoporous carbon for use as anode materials in lithium ion batteries(LIBs).We used CaC2 and CCl4 as carbon sources to synthesize mesoporous carbon materials under the acidic conditions of PCl3.After heat treatment,the specific surface area of the material reached 1322 m2/g,and the average pore size was further reduced to ca.4 nm,while the degree of graphitization increased.The yield of carbon was high,reaching 54%.The obtained carbon material has excellent properties in terms of capacity,cycle stability and rate performance as a LIB anode.At a current density of 0.1 A/g,the capacities of this material were 862 mA h/g,nearly three times more than that of the commercial graphite negative electrode material(372 mA h/g).After 230 charge-discharge cycles,the specific capacity of this material only attenuated from the initial 862 mA h/g to 800 mA h/g.