Studies Of Preparation Of Electrocatalyst Using Ionic Liquids As Precursors And Their Catalytic Behaviors Toward Hydrogen Evolution Reaction

The growing energy crisis and environmental pollution have prompted us to develop clean and renewable energies that can replace traditional fossil fuels.Hydrogen is very advantageous to have wide source,cleanliness and high efficiency.Hydrogen evolution reaction?HER?underpins many clean-energy technologies since it could generate hydrogen from water electrolysis without CO₂ emission,and can greatly alleviate the energy crisis and environmental pollution.The water electrolysis contains two half-electrode reactions:hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.The high overpotential in electrolysis process increases the cost of hydrogen production.The catalyst can effectively reduce the electrode polarization and save the cost.Although platinum-based precious metals are the most effective catalysts for HER,they are expensive and relatively scarce,which limit the large-scale application of these catalysts.Therefore,it is of great significance to develop efficient and less expensive electrocatalysts.Under acidic conditions,non-precious metal electrocatalysts have been studied.It is found that transition metal based and heteroatom-doped carbon materials have good catalytic activities toward HER.However,the acvtivity and stability of these catalysts for HER still need to be improved,and the preparation of these materials is complicated.Therefore,it is essential to design highly efficient,stable,but easy synthesizable catalysts for HER,and the main contents of this dissertation are as follows:1.Iron phosphide as HER catalyst in previous studies was prepared from iron-containing ionic liquid?IL?,while its catalytic activity still has great room to be further improved.In Chapter 3,we applied cobalt-containing IL,namely N,N-bis?4?methoxycarbonyl?benzyl?-N-methyl-d-glucaminium dibromodichlorocobaltate?II???MBMG?₂-CoCl₂Br₂?,was mixed with carbon nanotubes?CNTs?as a precursor to prepare CNTs-supported CoP through 300 ^oC phosphidation.This CoP_?MGMB?/CNTs possesses a relatively high HER activity with a low onset overpotential of 55 mV,a Tafel slope of 58 mV dec^-11 and 95%Faradaic efficiency?FE?,while demonstrating the catalytic activity for at least 27 h.Additionally,it only needs overpotentials of 135 and 160 mV to deliver current densities of 10 and 20 mA cm^-2,respectively.The anion of?MBMG?₂-CoCl₂Br₂ formed CoP and glucaminium-based cation was carbonized to generate amorphous carbon after phosphidation.This amorphous carbon has dramatically improved the HER activity of CoP_?MGMB?/CNTs.2.It is reported that transition metal phosphates?TMPs?are promising catalysts for HER.In Chapter 3,TMPs using ionic liquids as metal sources are prepared and their catalytic activities for HER have been studied.The prepared TMPs show good HER activity,but it takes a long time and complex steps for preparation.In Chapter 4,two phosphine-based ionic liquids[P_4,4,4,4]₂[CoCl₄]and[P_6,6,6,14]₂[CoCl₄],which can be used as phosphorus and cobalt sources,have been synthesized.Firstly,[P_4,4,4,4]₂[CoCl₄]and[P_6,6,6,14]₂[CoCl₄]were physically mixed with CNTs,respectively.Then Co₂P/CNTs was prepared by a simple microwave radiation method or a one-step phosphating method in a tube furnace.In the microwave radiation method,tetrabutylphosphonium tetrachlorocobaltate?II?([P_4,4,4,4]₂[CoCl₄])with CNTs was utilized to obtain the Co₂P/CNTs-1 within 6 min.The electrocatalytic performance of microwave-prepared Co₂P/CNTs-1 toward HER was investigated in acidic solution,which shows a good catalytic activity with an onset overpotential of 80 mV,Tafel slope of 58 mV dec^-1,current densities of 10 and 20 mA cm^-2 at overpotentials of 135 and 170 mV,respectively.In addition,It is discovered that CNTs can promote the formation of Co₂P nanoparticles and improve the catalytic activity of HER.This microwave-assisted strategy for synthesis of Co₂P provides a remarkable advantage of preparation process in a quite short time,which also is a simple operation and energy efficient method.3.Heteroatom-doped carbon materials can be used as catalysts for HER.In Chapter 5,IL was easily and uniformly infiltrated into the graphene as nitrogen and phosphorus sources to prepare N,P-double doped graphene through a facile method,which could have a promising catalytic application.The material was investigated as a metal-free electrocatalyst for HER,exhibiting a good catalytic performance with high activity,favorable kinetics and strong catalyst stability due to its N,P-heteroatom doping and highly porous structure.Moreover,further comparison against other ILs indicates that the P-dopant atoms originated from IL anion has more contribution to the high HER activities.This work shows a method of using IL as a heteroatom source to prepare metal-free electrocatalyst for HER.In brief,three materials based on ionic liquids were prepared and utilized as efficient catalysts for HER.CoP_?MGMB?/CNTs was prepared by phosphating ionic liquid at low temperature,Co₂P/CNTs was prepared by simple microwave method;IL was easily and uniformly infiltrated into the graphene as nitrogen and phosphorus sources to prepare N,P-double doped graphene.These three kinds of IL-based electrocatalysts exhibited good HER activities,obtaining a great potential for practical application.Finally,perspectives of the HER catalysis research is provided.