Preparation Of Fe-doped Mesoporous GC₃N₄ And Its Hydrogen Evolution Performance In Water Electrolysi

As a high-quality clean energy,hydrogen energy has the characteristics of high combustion calorific value and clean zero pollution.Hydrogen production from water electrolysis is one of the most promising hydrogen production methods,but its slow hydrogen evolution kinetics requires a high overpotential to drive it,which means high energy loss.Platinum-based catalysts can effectively improve this problem,but the low reserves and high cost of precious metals hinder their large-scale applications.Therefore,the development of inexpensive,stable,and non-noble metal electrocatalysts with excellent catalytic performance is necessary.Graphitic carbon nitride（g-C₃N₄）has been widely used in electrocatalysis due to its tunable structure,abundant hydrogen absorption sites,and low cost.However,bulk g-C₃N₄ is prone to inter-layer accumulation,resulting in a decrease in its active sites and an increase in sheet resistance,which is not conducive to the progress of HER（Electrocatalytic hydrogen evolution reaction）.In this thesis,iron-doped mesoporous g-C₃N₄（Fe-C₃N₄-TU）and CQDs（carbon quantum dots）composite iron-doped mesoporous g-C₃N₄（CQDs/Fe-C₃N₄）,and its hydrogen evolution performance in alkaline environment was investigated.（1）Fe-doped mesoporous g-C₃N₄ was prepared by one-step pyrolysis using melamine and ferric nitrate as precursors and thiourea as self-templated pore-forming agents.The morphology and structure of the catalysts were analyzed by various characterizations such as SEM,FTIR,and XPS.The results show that Fe-N（III）active sites are formed by the coordination of Fe with the pyridine nitrogen.Meanwhile,thiourea induces the catalyst to generate a large mesoporous structure and specific surface area,which provides favorable conditions for the exposure of active sites.Therefore,Fe-C₃N₄-TU exhibits a low overpotential of 206 m V to obtain the current density of 10 m A·cm^-2.The Tafel slope and the charge transfer resistance of Fe-C₃N₄-TU are 82 m V·dec^-1 and 44 Ω,respectively.In addition,the effects of Fe-N（III）active sites and the pore-forming effect of thiourea on the catalytic performance were further illustrated by the comparative study of different Fe and thiourea addition amounts.（2）Fe-doped mesoporous g-C₃N₄ composites of CQDs were prepared by pyrolysis and thermal exfoliation using melamine and ferric nitrate as precursors and P123 as soft templating agent and CQDs carbon source.The morphology and structure of each control sample were studied by TEM,XRD,XPS and other characterizations.The study showed that the CQDs formed by the pyrolysis of P123 combined with g-C₃N₄ in the form of the C=N-C bond,which effectively enhanced the conductivity of the catalyst.During thermal exfoliation,the excess CQDs were removed,allowing the release of the P123-induced pore structure and large specific surface area,which facilitated the exposure of active sites.Meanwhile,excessive CQDs form a protective effect on Fe-N（III）to avoid damage to its structure by thermal exfoliation.Therefore,the overpotential required for CQDs/Fe-C₃N₄ to drive the current density of 10 m A·cm^-2 is only 112 m V.The Tafel slope and the charge transfer resistance of CQDs/Fe-C₃N₄ are 86 m V·dec^-1 and 4.1 Ω,respectively.The excellent activity of CQDs/Fe-C₃N₄ is mainly due to the high intrinsic activity of Fe-N（III）active sites and the enhanced conductivity of CQDs.In addition,the effects of Fe-N（III）active sites and CQDs on the catalytic performance were further illustrated by the comparative study of different Fe and P123 additions.