Preparation And Electrocatalytic Properties Of Ruthenium Pyrochlore Oxides And Transistion Metal Compounds

Proton exchange membrane water electrolyzer(PEMWE)is a technology that convert electric energy into hydrogen energy as means to store the excessive electricity generated during the peak hours.There are several unique advantages of PEMWE,such as large working current density,fast response speed,and high electrolytic efficiency.Coupling PEMWE with renewable energy power generation technology can effectively perform peak clipping and valley filling to improve stability and safety of renewable energy generation.Nowadays,the electrode materials of commercial proton membrane electrolytic cells heavily rely on the high-cost and low-abundant precious metals Ir and Pt.Hence,there is an urgent need to develop efficient,stable and low-cost electrolytic water anode and cathode catalysts to reduce the use of precious metals.As such,this thesis focuses on Ru-based pyrochlore oxides and carbon-encapsulated transition metal compounds.To increase the surface active sites of the catalyst and optimize its surface adsorption ability,this thesis regulates the surface of catalyst composition,microstructure and defects,thus improving the intrinsic catalytic activity of electrode materials and studying the mechanism of promoting its catalytic activity,with an objective to replace or partially replace IrO2 and Pt/C catalysts,to accelerate the development of water electrolysis.The main research contents and results mainly include following aspects.Y2Ru2O7,with a much lower mass fraction of Ru than RuO2,has been found to have good oxygen evolution stability in acidic media.In terms of actual application,its catalytic activity needs to be further improved.The divalent Zn+is used to partially replace Y3+of pyrochlore oxide Y2Ru2O7.This thesis investigates the effect of A site doping of Y1.85Zn0.15Ru2O7 to the electronic and surface structure of Y2Ru2O7,and analyzes the promotion mechanism of electrocatalytic performance.After Zn2+partially replaced Y3+,the electron,crystal and surface structure of the catalyst is changed.An obvious hole effect is generated in catalyst,and the conductivity and surface oxygen vacancy concentration of Y1.85Zn0.15Ru2O7 catalyst are significantly enhanced.In 0.5 M H2SO4,the overpotential of Y1.85Zn0.15Ru2O7 at 10 mA cm-2 is 291 mV,lower than 308 mV of Y2Ru2O7 and 329 mV of IrO2.In addition,the electrochemical stability results show that the partial substitution of Zn2+for Y3+does not change the intrinsic stability of Y2Ru2O7 catalyst.To further improve the OER performance of study Y2Ru2O7 and study the mechanism of oxygen vacancy engineering on the catalyst,Y2-xCaxRu2O7 doped with different Ca2+concentrations is further synthesized.The correlation between surface oxygen vacancy concentration and electrochemical oxygen evolution performance is analyzed by electrochemical tests.Our experiments find that the electrochemical oxygen evolution reaction(OER)performance strongly relies on surface oxygen vacancy concentration of catalysts.The higher surface oxygen vacancy concentration provides more adsorption active sites for the intermediate process of OER.The OER overpotential of Y1.75Ca0.25Ru2O7 at 10 mA cm-2 is 275 mV,and the corresponding Tafel slope is 40.3 mV dec-1.The influence of the valence state of Ru in Y2-xCaxRu2O7 on the catalytic performance is discussed in depth.The mixed valence state of Ru4+/Ru5+improves redox ability of the catalyst and proton dissociation ability of water molecules.Y1.75Ca0.25Ru2O7 is then prepared as the anode of a proton exchange membrane electrolytic cell.At 70? and 1.7 V,the operating current density of the electrolytic cell reaches 1.25 A cm-2,and it operates stably for more than 1000 minutes In addition,the electrochemical performance of Y2-xCaxRu2O7 in the alkaline medium for the oxygen evolution and oxygen reduction reaction is further studied.The catalyst is applied to the air electrode of zinc-air battery(ZAB),demonstrating a high electrochemical performance.For the first time,this thesis shows that an oxygen catalyst can be used as an efficient and stable electrode for both PEMWE and ZAB,providing a good paradigm for new type of oxygen catalysts.Hydrogen evolution reaction(HER)is another important half reaction of water electrolysis.Tungsten carbide(WC)has platinum-like electron structure,but the H*adsorption energy of WC catalyst is too strong,resulting in poor HER activity.Pt is widely used in HER due to its high electrocatalytic activity.However,the low abundance and high cost of Pt restricts its mass usage.This thesis synthesizes carbon-coated ultrafine WC nanocrystal catalyst and studies its HER property.This thesis discovers that self-polymerization of dopamine hydrochloride and ammonium metatungstate can occur at the water-alcohol interface.This synthetic method can obtain uniform porous nitrogen doped carbon coated WC nanocrystal(WC@NC).The precursor ratio,pyrolysis temperature,water-to-alcohol ratio,and pH value have a great influence on the product phase,structure,and morphology.For a specific 10 mA cm-2,the overpotentials of WC@NC in acidic and alkaline hydrogen evolution reactions are 127 and 132 mV,respectively.The outer carbon shell provides excellent stability for electrochemical hydrogen evolution.The results of density functional theory calculations show that there is a strong electronic synergy effect between WC nanocrystals and the carbon shell,which effectively reduces the hydrogen adsorption binding energy.Transition metal phosphates have more unsaturated atoms and narrower band gap than the corresponding carbides.In order to further improve the hydrogen evolution activity of non-noble metal catalysts,this thesis studies the preparation method of carbon coated molybdenum phosphide(MoP)nanocrystalline and its HER performance.The mechanism of NaCl crystal during high temperature melting and low temperature recrystallization is studied.At an appropriate temperature and NaCl template ratio,the unique porous carbon nanosheet capsulated MoP nanocrystalline structure can be produced.The ultrathin nanosheet structure is beneficial to expose more active sites and facilitate the gas/liquid mass transfer process,and protect the active ingredient from leaching in corrosive environment.The MoP/C electrode prepared by NaCl template method has an overpotential of 118 mV at 10 mA cm-2 and a Tafel slope of 50.7 mV dec-1,which exceeds the most of non-precious metal catalyst.MoP/C is used as a cathode of proton exchange membrane electrolytic cell.At a voltage of 2 V and temperature of 80?,the single electrolyzer achieves a current density of 0.71 A cm-2.Meanwhile,the single electrolyzer has been running stably for more than 3300 minutes under a constant current density of 150 mA cm-2,which demonstrates an outstanding application prospective.