Design And Synthesis Of Pyrite-Type Nano Catalysts For Hydrogen/Oxygen Electrode Reactions

The hydrogen economy describes a clean,safe and sustainable economic structure system with hydrogen as the energy medium:Hydrogen is obtained through water electrolyzers,where the electrical energy generated by renewable energy such as solar,wind and tidal energy is stored to high-energy-density hydrogen.Then the hydrogen fuel cell releases the chemical energy of hydrogen in an efficient and orderly manner without pollutants.However,the high cost and scarcity of noble metal catalysts(Ru,Ir,Pt,etc.),that drive the operation of water electrolyzers and hydrogen fuel cells,makes it impossible to address the huge global market demand.In contrast,non-noble metals are abundant and inexpensive,becoming the new choices for catalysts in water electrolyzers and hydrogen fuel cells.However,compared with noble metal catalysts,non-noble metal catalysts exhibit poor activity,low selectivity and instability,which cannot meet the requirement of water electrolyzers and hydrogen fuel cells.Among non-noble metal materials,inorganic minerals such as pyrite have been widely used in the catalysis field due to their diversity properties about physicochemical properties,crystal phase and electronic structure.As an important member of pyrite structural materials,cobalt diselenide(CoSe2)exhibits many advantages such as simple synthesis,structural stability,environmental friendliness,and excellent electrical conductivity.However,CoSe2 electrocatalyst shows the poor activity,low selectivity and instability.Based on this,we used CoSe2 as a model catalyst to construct a highperformance catalyst structure model by designing its crystal phase,interface structure and interlayer spacing.The structure-activity relationship between the catalyst structure and the performance of hydrogen/oxygen electrode reaction was established via a combination of theoretical exploration and experiment,thereby creating a series of CoSe2-based nano-electrocatalysts with high activity,selectivity and stability,which provided a new research dimension for the design and preparation of novel non-noble metal-based nanocatalysts with high activity and stability.The main achievements of this dissertation can be summarized as follows:1.Crystal phase mixing strategy to design extremely stable and highperformance electrocatalysts for hydrogen evolution reaction in acidic media.A novel mixed-phase CoSe2 was successfully prepared by treating the cubic CoSe2 via alkali-thermal method.During this process,the harsh condition causes the leaching of Co and Se at defective sites of cubic CoSe2,leaving atomic vacancies,which induced a 90° rotation of Se-Se,resulting in formation of orthorhombic CoSe2.The homogeneous phase mixing between cubic and orthorhombic phases in CoSe2 offer a favorable electronic structure that enhance Co 3d-Se 4p orbital hybridization,resulting in a boosted covalency between Co and Se atoms and an enhanced Co-Se bond energy,which substantially enhances the lattice robustness and thereby the material stability.The catalyst shows the hydrogen evolution reaction(HER)polarization curve is well retained after 50,000 potential cycles and no sign of deactivation after more than 400 h of continuous operation in acid media.We expect that such phase-mixed engineering methodology could be intensively extended for designing cost-effective and betterperforming catalysts in acid.2.An efficient Turing-type multi-interfacial oxygen evolving electrocatalyst.We used the "reaction-diffusion" mechanism to construct the Ag2Se Turing pattern on CoSe2.The resultant Ag2Se-CoSe2 catalyst possesses abundant interfacial structures.The optimized electronic structure of Co atoms at the interface make the d-band center of Co atom deviates more from the Fermi level,leading to a weak binding of*OH intermediate on the interfaces,making the desorption of*OH to generate*O is easier to ocuur.Thus,the oxygen evolution reaction(OER)energy barrier of the Ag2Se-CoSe2 catalyst is greatly reduced.The Ag2Se-CoSe2 needs an overpotential of mere 221 mV to reach the 10 mA/cm2 with an 84.5%anodic energy efficiency,outperforming previously reported non-noble metal OER catalysts.Moreover,the intrinsic OER activity correlates linearly with the length of Ag2Se-CoSe2 interfaces,proving that the interface sites are the optimized active sites.The work provides a new research direction for designing higher-performance OER catalysts via interfacial structures.3.Strongly coupled CoSe2 for selective electrocatalytic oxygen reduction to H2O2 efficiently under acidic conditions.We narrow interlayer spacing of CoSe2 by ion exchange method,creating a novel strongly coupled CoSe2 catalyst.The enhancement of the interlayer coupling between CoSe2 atomic layers offers a favorable surface electronic structure that make the d-band center of Co atom deviates more from the Fermi level.Thus weakens the two-electron oxygen reduction reaction intermediate*OOH adsorption,which enables binding energy of*OOH on CoSe2 to be 4.16 eV,approaching to the optimum value of 4.2 eV.Therefore,the HOO*hydrogenation to H2O2 instead of the O-O bond cleavage to generate H2O.Consequently,on the strongly coupled CoSe2 catalyst,we achieved Faradaic efficiency of 96.7%,current density of 50.04 milliamperes per square centimeter,and product rate of 30.60 mg/(cm2 h).Moreover,this catalyst shows no sign of degradation when operating at-63 milliamperes per square centimeter over 100 h.This work proposed a strategy to optimize the surface electronic structure of non-noble metal catalysts by adjusting the interlayer distance,improving the activity,selectivity and stability performance of the catalyst,which provides a new strategy for the design of other high-performance layered catalysts.