Optimized Electrochemical Performance Of Double Perovskite Oxides For Hydrogen Evolution Reaction

The overuse of fossil fuels and the increase of carbon emissions have caused energy crisis and serious environmental problems.In order to promote the sustainable development,the development and efficient utilization of clean and renewable energy has gained intensive attention.However,the intermittent and regional characteristics of renewable energy sources such as hydropower,wind power,and solar power have limited their applications.Employing advanced electrochemical energy conversion and storage devices can potentially solve the problems of the intermittent and regional supply of renewable energy.For example,the deployment of renewable energy to produce sustainable fuel such as hydrogen has attracted widespread attention.However,the hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?are commonly limited by the sluggish kinetics.So far,the practical applications of the electrochemical water splitting in alkaline solutions still inevitably require a large overpotential?Overpotential,??to obtain the required current density,leading to the considerable energy loss.,Hence,it is urgent to develop earth-abundant and low-cost electrocatalyst to improve the catalytic efficiency and stability.On one hand traditional perovskite oxides are considered as low-cost and multifunctional active catalysts,but their electrocatalytic hydrogen evolution activity and stability are still not satisfactory.On the other hand,recent studies have shown that double perovskites may exhibit better catalytic stability and catalytic performance in comparison to simple perovskites.In this thesis,via optimizing the annealing temperature and adjusting the A site elements in lanthanides,we have developed efficient and robust double perovskite oxides for HER in alkaline solutions.In the first work,we successfully demonstrate that a crystal-structure regulation by simply altering the annealing temperature might be an effective way to enhance the electrocatalytic activity of perovskite oxides.Via annealing praseodymium barium cobalt oxides at 900°C,1000°C,1100°C,and 1150°C in air atmosphere,a series of perovskite oxides were obtained,labeled PBC-900,PBC-1000,PBC-1100 and PBC-1150.We report a high-performance electrocatalyst with an orthorhombic double perovskite structure(i.e.,Pr Ba Co₂O_5+?,??=??0.52,PBC-1100),which shows superior activity and durability towards HER in comparison to a cubic perovskite oxide(i.e.,Pr_0.5Ba_0.5Co O_3-?,PBC-900and PBC-1000)and a tetragonal one(i.e.,Pr Ba Co₂O_5+?,??=??0.76,PBC-1150),despite the similar composition.The improvement of the electrocatalytic activity for hydrogen evolution reaction is mainly attributed to a high-degree structure distortion resulted from the high oxygen vacancy concentration,a large electrochemical active surface area,and a low charge transfer resistance.In addition,lattice oxygen species may provide an extra contribution to the enhancement of the hydrogen evolution reaction for Pr_0.5Ba_0.5Co O_3-?sintered at 1000?°C.Furthermore,a considerable correlation between O p-band centers and HER overpotentials is observed,and PBC-1100 with the highest O p-band center exhibits the best HER activity.These results indicate that the electrochemical performance of perovskite oxides could be effectively enhanced.Especially orthorhombic double perovskite oxides could be a new class of promising electrocatalysts.In the second work,lanthanide cations?Ln=La,Pr,Nd and Sm?with different ionic radii occupied A site in double perovskite Ln Ba Co₂O_5.5+?were prepared,and the effects of lanthanide cations on HER were evaluated.All double perovskites,i.e.La Ba Co₂O_5.5+??LBC?,Pr Ba Co₂O_5.5+??PBC?,Nd Ba Co₂O_5.5+??NBC?and Sm Ba Co₂O_5.5+??SBC?were prepared with sol-gel method.Our results demonstrate that LBC with a larger lanthanide cation at the A site shows the superior activity toward HER,and the relative activity follows the trend of LBC>PBC>NBC>SBC.Moreover,double perovskite oxides all show excellent stability for HER.Our results suggest that the type of the A site significantly affects the activity of electrocatalysts,i.e.,the larger the radius of the lanthanide cations at the A site,the higher the activity toward HER.These results may also provide a new way to develop the next-generation high-performance electrocatalysts.