Investigation Of Electrocatalysts Based On Ionic Liquid For Water Splitting

The depletion of traditional fossil fuels and its negative impact on the environment have attracted extensive attention from all over the world.In addition,the non renewable and difficult exploitation of fossil fuels make their prices rise continuously.Therefore,it is vita to develop green and renewable energy sources that can replace fossil fuels.Hydrogen is one of the most promising green energies owing to its eco-friendly,high abundant and energy density.Hydrogen production by water splitting obtains mature process and high product purity,which an ideal hydrogen production technology.The critical part of this technology is development of electrocatalytic materials with high performance.Through possessing excellent activity and stability noble metal-based catalysts are expensive and scarce,which limits their large-scale application.Therefore,the preparation of electrocatalysts with excellent performance,low cost and simple procedure are highly desired.Recently,many efforts have been made to prepare transition metal-based and carbon-based electrocatalysts for water splitting.However,the preparation of transition metal-based electrocatalysts is tedious and has a risk of heavy metal pollution.Also,the stability and corrosion resistance of these materials can be further improved.Meanwhile,carbon-based metal-free materials including carbon nanotubes,graphene,carbon quantum dots,and graphdiyne have low activities for water splitting,so modification methods,such as heteroatom doping and structural defects are applied to ameliorate the catalytic activity.Compared to the Pt/C and RuO2 catalysts,the activity and stability of these metal-free electrocatalysts are still relativity low.Therefore,the development of electrocatalyst with low cost,simple preparation process,high activity and stability is still a challenge.In this dissertation,transition metal and modified carbon nanotubes based electrocatalysts were prepared using ionic liquids for water splitting.The research contents are as follows:(1)Ionic liquid(IL)was used as an iron source and phosphorus source for the preparation of iron phosphating.Trihexyl(tetradecyl)phosphonium tetrachloroferrate([P(C6H13)3C14H29][FeCl4])and carbon nanotubes were used as precursors to prepare CNTs-supported Fe2P through 400℃ phosphidation.The material(Fe2P(IL6)/CNTs)showed high HER activity and stability with a low onset overpotential of 75 mV,a Tafel slope of 68 mV dec-1.In addition,it only needs overpotentials of 115 and 150 mV to deliver current densities of 10 and 20 mA cm-2,respectively.The phosphatizing process with[P(C6H13)3C14H29][FeCl4]was also studied.All the experimental results showed that the IL could form Fe2P in situ on CNTs,and the introduction of CNTs contributed to the formation of Fe2P nanoparticles and improved electrical conductivity The IL-based in situ preparation is facile and eco-friendly and does not require the addition of other reagents.This method is expected to be applied in other electrochemical studies.(2)Ionic liquids were used as the dual-source of Fe and P to prepare iron phosphate through microwave radiation.These prepared iron phosphate/CNTs composites were used as electrocatalysts for water splitting,and IL can influence the morphology of iron phosphate and electrocatalytic performance.Compared to our previous study,this preparation process is simple and repaid.The sample can be directly used as an electrocatalyst without pyrolysis.Experimental results show that IL can change the morphology and electrocatalytic performance of the catalysts.In addition,carbon nanotubes are conducive to the formation of iron phosphate nanoparticles,thus improving the catalytic activity.As a hydrogen evolution catalyst,the phosphate/CNTs composite shows high HER activity with a low onset overpotential of 120 mV,a Tafel slope of 32.9 mV dec-1,and it only needs overpotentials of 185 mV to deliver current densities of 10 mA cm-2.Meanwhile,as an oxygen evolution catalyst,the onset potential is 1.48 V,the Tafel slope is 73.3 mV dec-1,and the overpotential is 300 mV at a current density of 10 mA cm-2.This bifunctional catalyst also shows good durability for HER and OER.In this study,ionic liquids were used as iron and phosphorus dual sources to prepare transition metal phosphates by microwave irradiation.The method is simple,rapid,and eco-friendly,and has good application prospects in other electrochemical studies.(3)In previous studies,we used IL functionalized CNTs as catalysts for hydrogen evolution,but ionic liquid and carbon nanotubes were connected by non-conductive carbon chains,which hindered the charge transfer process and reduced HER activity.In this chapter,ionic liquids in situ functionalized carbon nanotubes was prepared,where IL was directly bonded to CNTs surface.Theoretical calculation and experimental results show that the ionic liquids on carbon nanotubes promoted the electron transfer process,and exhibited excellent hydrogen adsorption performance,which improves the HER performance.In particular,this catalyst showed excellent electrocatalytic activity with an onset overpotential of 80 mV and a Tafel slope of 38 mV dec-1.This study highlights the great potential of IL in-situ functionalized carbon nanotubes in electrocatalytic HER and provides insights into the structural design of efficient electrocatalysts.