Structural Control Of Transition Metal Electrocatalysts And Their Application In Energy Conversion

As the level of productivity continues to increase,environmental pollution and energy consumption become more and more serious,so it is particularly important to seek a highly efficient and clean renewable resource.Hydrogen,as a new type of clean energy with no pollution and high energy density,has gradually entered people’s field of vision.At present,the most popular hydrogen production method is obtained by electrolyzing the water.The electrical energy can be converted into chemical energy in the form of hydrogen,and further be uesd for subsequent energy conversion or direct combustion.Under normal temperature and pressure,the theoretical voltage required for water electrolysis is 1.23V.However,due to the existence of the overpotential,the voltage tends to be high.Therefore,it is usually necessary to use a certain electrocatalyst to reduce the water decomposition voltage and improve the hydrogen production efficiency.Among the existing hydrogen-evolving catalysts,noble metals such as platinum are the most efficient,but their high price,the shortage of reserves and other problems severely limit the scope of use.Therefore,the development of cheap and efficient catalysts has become an important solution to the future energy crisis.In this paper,the authors selected the appropriate electrocatalyst to directly or indirectly improve the hydrogen production efficiency by analyzing the reaction mechanism involved in water electrolysis.In the preparation process of the catalysts,the active sites and electron transport behavior of the non-precious metal catalysts are optimized by the defect engineering,active surface exposure,amorphization and synergistic optimization strategies to improve the energy conversion efficiency.This paper mainly includes the following aspects:1.Based on the understanding of the edge active sites of molybdenum disulfide catalysts,the authors prepared the defect-rich molybdenum disulfide nanowall electrocatalyst vertically grown on conductive glass with a controlled thickness by constructing defects and preferentially exposing the active edge.Thanks to the highly exposed active edges,rough surfaces and the thickness-limited,defect-rich molybdenum disulfide nanowall structure,the catalyst exhibits an ultralow initial overpotential of 85mV and excellent electrochemical stability.This strategy of enriching active sites by more defects and anchoring thecatalysts on the substrate provides new insights into the design and development of future catalysts.2.The authors used a nickel-iron foam alloy to grow partially amorphous structure of nickel-iron layered double hydroxide（NiFe LDH）in situ.In this work,partially amorphous NiFe LDH nanosheet arrays with native Ni³⁺ions and optimal Ni:Fe ratio were developed,realizing robust performance on both electrocatalytic oxygen evolution and urea oxidation.The partially amorphous structure can not only bring in native Ni³⁺ions and lead to facile generation of high-valence active species for these electrooxidation reactions,but also offer 2D charge transfer pathway to facilitate the electrocatalytic process.Besides,the optimized Ni:Fe ratio as well as the large active surface area further enhance the catalytic activity,and the partially amorphous structure guarantees high stability towards long-term operation.The partially amorphous catalyst exhibites a higher OER current of 284.4 mA cm^-2 at an overpotential of 500 mV,which was2.2-10.0 times stronger than the counterpart of other Ni:Fe ratios.In addition,the UOR current density of the partially amorphous catalyst at 1.8V vs.RHE shows 1.6 and 2.4 times increment relative to fully amorphous and highly crystalline catalysts,and 2.7–9.4 fold larger than the catalysts with other Ni:Fe ratiosThis work enriches the design strategy of electrocatalysts for electrolysis of water electrolysis and energetic small molecules,as well as providing powerful dual-function electro-oxidation catalysts for simultaneous wastewater treatment and clean energy production for future electrocatalysts.