Study On Hydrogen Production With Prepared M-SnO_x?Sb₂O₃?/Ti?M=Co,Pt? Electrocatalysts

Hydrogen is widely used in chemical,pharmacy,energy and food industry.Almost96%of hydrogen comes from fossil fuels?petroleum,coal and natural gas?,and 4%from water dissociation.However,with the exhaustion of fossil energy sources,and with the stringent requirements of the ecological environment for pollutants such as CO_x,NO_x,C_xH_y and for greenhouse gas emissions,it is necessary to adopt a cleaner way to obtain hydrogen.Moreover,the storage and transportation of electric energy,coming from the conversion of solar,wind and water energy,is still a problem which should be considered and even be solved in the current society.Hydrogen,as an energy carrier,can make the conversion of electric energy,chemical energy and thermal energy into reality.Therefore,it is of great significance to study how to obtain hydrogen energy efficiently for the development of society as a whole.Fortunately,hydrogen comes from a wide range of sources,besides conventional fossil fuel,it can be obtained by electrolysis of water and ammonia at room temperature and atmospheric pressure.Nevertheless,during water electrolysis,high price of noble metals,poor stability and easy separation from the matrix in OER?Oxygen Evolution Reation?are serious for hydrogen production.During ammonia electrolysis,large amount of noble metals,easy poisoning of catalyst,poor conductivity and high price of matrix materials,and easy dissolution of some metals are prominent either.Based on aforementioned issues,this thesis introduced innovations into Co-SnO_x/Ti and Pt-SnO_x/Ti electrocatalyst preparation for hydrogen production.Firstly,SnO_x/Ti can be obtained by coating SnO_x on Ti surface polymeric membrane layer by layer method,and then loading Co and Pt metals on it.The OER activity of Co-SnO_x/Ti and the performance of Pt-SnO_x/Ti in ammonia decomposition are further studied.The main six conclusions are as follows.?1?The morphology,species,valence and structure of Co-SnO_x/Ti and Pt-SnO_x/Ti electrocatalysts were analyzed by SEM,XRD,FT-IR,XPS and EDS.It was found that the surface of the Co-SnO_x/Ti contained TiO₂,SnTiO₃,SnO₂,Co₂SnO₄ and Co₃O₄,and all the oxides existed in both crystalline and amorphous forms.The existence of SnTiO₃and Co₂SnO₄ indicated that the possible conductive chain of Ti-O-Sn-O-Co was formed.Simultaneously,Pt,SnO₂,SnO and Sn-OH were found on the surface of the Pt-SnO_x/Ti,and both Pt and SnO_x were polycrystalline.?2?The OER activity of Co-SnO_x/Ti electrocatalyst and the hydrogen production activity by ammonia decomposition via Pt-SnO_x/Ti electrocatalyst were measured by Cyclic Voltammetry.It was found that the OER activity of the Co-SnO_x/Ti was higher than that of Pt electrode,and its tolerance was higher than that of IrO₂/Ti electrocatalyst.Especially,the OER activity and tolerance of the Co-SnO_x/Ti under alkaline conditions depended strongly on the calcination temperature,the morphology,and the Co valence state.Meanwhile,it was firstly found that the Pt-SnO_x/Ti can efficiently electrolyse NH₃to H₂ under native pH value?pH=11.3-12.2?,without producing by-products such as NO_x in the reaction system.?3?Co valence states of Co-SnO_x/Ti electrocatalysts,which were obtained at different calcination temperatures,were studied in the process of water electrolysis.It was found that there was a redox cycle of Co²⁺?Co³⁺?Co⁴⁺on the surface,which was the intrinsic essence of OER reaction.Once this cycle was destroyed,the OER activity of the Co-SnOx/Ti was decreased or even completely losed.?4?The activity and stability of the Co-SnO_x/Ti electrocatalysts were tested in alkaline aqueous solution.The Co-SnO_x/Ti had good activity and tolerance,and no deactivation occurred after 840 min?14 h?continuous operation,whatever the calcination temperature is lower or higher.When calcined at higher temperature?600??,its current density was larger and can be maintained at 80-100 mA.cm^-2 for a long time,which was about twice as high as that of calcined at lower temperature?480??.?5?In the electrolysis of NH₃ to H₂,the influence of applied voltage on the formation of NO_x was analyzed.Under alkaline conditions,the low applied voltage was beneficial to selective electrolysis of NH₃ to H₂ and N₂.However,the high applied voltage easily resulted in peroxidation of NH₃ to NO_x.Under native pH condition,voltage had little effect for selective electrolysis of NH₃ to H₂ except for the concentration of reactant?NH₃ concentration?.The favorable ammonia concentrations were 1.9-7.5 mol.L^-1.?6?According to the species on the surface of Pt-SnO_x/Ti electrocatalyst before and after the reaction and reported references,the reaction mechanism of the Pt-SnO_x/Ti as anode and cathode for electrolysis NH₃ to H₂ was proposed.In proposed mechanism,the Pt-SnO_x/Ti electrode acts as a tri-functionalised catalyst.That is,SnO has the function of adsorbing NH₃,-Sn-O-H has the function of transferring H and OH^-,and Pt has the function of adsorbing H to form H₂.