Regulating The Electronic Structure Of Electrocatalysts To Boost The Hydrogen Evolution Reaction

With the rapid development of modern society,energy and environmental problems have been widely concerned.Therefore,the development of new environmentally-friendly energy is of great significance.With the advantages of high energy density and zero carbon dioxide emissions,hydrogen is one of the hotspots in the field of renewable energy.The water splitting could use electricity from renewable energy?e.g.,wind and solar?to obtain high-purity hydrogen.While,the hydrogen evolution reaction?HER?requires electrocatalysts to reduce the energy barrier of reaction and improve the catalytic efficiency.It shows that Pt is relatively easy to absorb or desorb hydrogen due to the moderate binding energy of Pt-H,thus it exhibits good catalytic activity.The researchers adjust the electronic structure of electrocatalysts through various strategies?alloying,hybridization with non-metals,carbon-encapsulated structure,defect sites,etc.?to change the adsorption strength of hydrogen on the surface of electrocatalysts and optimize the catalytic activity for hydrogen evolution.This thesis adopts three strategies?alloying,carbon-encapsulated structure and defect sites?to design and synthesize high-performance electrocatalysts for HER.The main contents and conclusions are as follows:?1?Controlling the electronic structure of electrocatalysts by alloying method is explored.In the second chapter,a platinum-rhodium alloy supported on tungsten carbide/carbon electrocatalyst?Pt₉Rh-WC/C?is synthesized by intermittent microwave heating method and impregnation reduction method,and it is used for HER.The X-ray powder diffraction characterization shows that the WC is successfully prepared and the characteristic diffraction peaks move to higher 2?values than Pt/C,which confirms that the lattice of Pt is compressed.The compressive strain changes the electronic structure of Pt,which reduces the binding energy of Pt-H and thus is conducive to the desorption of hydrogen during the HER.Electrochemical characterization indicates that the Pt₉Rh alloy exhibits better catalytic activity than Pt/C.In addition,the presence of WC facilitates the desorption of hydrogen,which further boosts the desorption of hydrogen on Pt in alkaline solutions.In the third chapter,a worm-like S-doped Rh Ni alloy electrocatalyst?S-Rh Ni/C?is prepared by a solvothermal method.It is found that the introduction of Ni with a smaller atomic radius than Rh compresses the lattice of Rh and reduces the binding energy of Rh-H,which is conducive to the desorption of hydrogen on the surface of Rh.In addition,the introduced Ni has lower Gibbs free energy of hydrogen adsorption than Rh,so it can increase the coverage of hydrogen on the surface of electrocatalysts and improve the catalytic activity.Specifically,the polarization curves show that the S-Rh Ni/C exhibits higher catalytic activity for HER than that of S-Rh/C,and displays similar activity to that of Pt/C.Moreover,the 3D network structure of S-Rh Ni/C facilitates the transfer of protons and charges,and also avoids the accumulation of 1D or 2D electrocatalysts,improving the catalytic stability.These results show that the alloying is an effective way to regulate the electronic structure of electrocatalysts and provides the possibility for preparing high-performance electrocatalysts for HER.?2?The effect of the carbon-encapsulated structure on HER is studied.Metal compounds with non-metals?e.g.,S,P,C,N,O and Se?are good electrocatalysts for HER.However,non-metals with strong electronegativity will decrease the conductivity of electrocatalysts,turning the conductor into a semiconductor or even an insulator,and affect the electron transfer during the catalysis.The ultra-thin carbon layer has attracted wide attention due to its high conductivity and its effect on changing the Gibbs free energy of hydrogen adsorption.The fourth chapter uses phosphotungstic acid to oxidize and polymerize pyridine,and then carbonizes to synthesize carbon-encapsulated tungsten oxide?WO_x@C/C?at a high temperature.Electrochemical results show that WO_x@C/C exhibits Pt-like catalytic activity and has good stability.Theoretical calculations display that these C atoms over the W atom,O atom,and W-O bond of the WO₃?001?surface exhibit near-zero Gibbs free energy of hydrogen adsorption,which is much smaller than that of the W and O atom of WO₃.It indicates that the carbon shell plays a key role in regulating the Gibbs free energy of hydrogen adsorption for WO₃.Besides,the charge density difference of electrocatalysts indicates that,as an electron acceptor or donor,the carbon shell promotes the electron transfer and improves the conductivity of electrocatalysts.These results show that the carbon-encapsulated structure modifies the electronic structure and catalytic activity of electrocatalysts,and provides a new option for designing and preparing efficient materials for HER.?3?The effect of oxygen defect sites on the catalytic activity of hydrogen evolution for the tungsten oxide is explored.Oxygen defects,similar to the carbon shell at the fourth chapter,can increase conductivity.The reason is that the absence of oxygen atom weakens its restriction on the delocalized electrons of metal and facilitates the transfer of electrons in the catalytic process.In addition,the absence of oxygen atoms alters the inherent electronic structure of tungsten oxides,producing more active sites.In the fifth chapter,WO_x?x<3?electrocatalysts with different content of oxygen vacancies are synthesized by controlling the hydrogen treatment,and their catalytic activity for HER is estimated.Combining the physical and chemical characterizations,we find that these current densities of electrocatalysts at the same overpotential are both positively correlated with the content of oxygen vacancies,indicating that the content of oxygen vacancies is proportional to catalytic activity for HER.This work provides a strategy for the study of other metal oxides as electrocatalysts for HER or other catalytic reactions.