Synchrotron Radiation Study On The Transition Metal Bsed Electrocatalyst For Water Decomposition

The development of clean hydrogen energy acts as a positive performer in pushing forward the social progress around the world.This hydrogen production technology from renewable energy is economical and eco-friendly,but it involves many complex reaction steps,so more external force consumption is needed to promote this reaction.Among them,catalyst is an important part to realize the rapid development of hydrogen energy,which can reduce the reaction energy and increase the output rate of the products.The commercial precious metals have advantage in the activity,the high cost is the inherent defect.The transition metals can make up for it that are abundant reserves and low cost.Due to their rich electronic structures,they play an important role in a large number of energy conversion reactions.Through the crystal plane control,heteroatom doping,coordination with the supporter,nano construction and other strategies,the transition metal catalysts showed comparable commercial performance can be obtained.In this paper,Ni,Fe and Co based transition metal electrocatalysts were designed and synthesized,and the catalytic activity was improved by using hybridization and crystal phase control strategies.Combined with synchrotron radiation XAFS,the local structure of transition metal catalysts was studied.With the help of operando XAFS,the valence state changes,structure evolution,the adsorption and desorption of the intermediate state on the transition metal active sites under working condition were sensitively and accurately revealed.The relationship between real active structure and active efficiency was also obtained,which provided valuable guidance for the design of low-cost and high-efficiency transition metal catalysts.The three main parts of this work were listed in the following points:1.Studying the effect of strong Ni-S hybrid on the activity of crystalline NiS electrocatalyst under the acid oxygen evolution by synchrotron radiation XAFSHigh crystallinity NiS electrocatalyst was prepared by a simple one-step reaction,where the H2S produced by pyrolysis of thiourea at high temperature acted with Ni precursor.Among them,there is a strong orbital hybridization between Ni 3d and S 3p,which provides a favorable electronic structure for the adsorption and separation of water in acid medium.Benefiting from the stable high crystalline state of NiS and strong Ni-S hybridization,NiS catalyst showed good acid OER activity that shows a low overpotential of 337 mV to reach the current density of 10 mA cm-2.After 5000 CV cycles,it still maintained high activity and high stability,and its crystal structure and micro morphology remained unchanged.And it shows strong acid corrosion resistance under long-term strong acid conditions,only 4wt%Ni dissolved after reaction.This work gives an important thinking for the design of transition metal catalysts.2.Revealing the structural evolution of Fe single atom active site in hydrogen evolution reaction with the help of Operando XAFSThe single-atom electrocatalyst(Fei/NC)was successfully synthesized by high temperature pyrolysis method,which showed high hydrogen production(HER)with only 111 mV overpotential at current density of 10 mA cm-2.The synchrotron radiation XAFS and spherical aberration electron microscopy showed that Fe atoms linked with N atom on N-C support as structure "Fe-N4".Operando XAFS reveal that the coordination number of Fe-N decreased gradually under the working condition,then the oxidation state of Fe decreases,which was beneficial for the Fe sites adsorbed the oxygen-containing intermediates,forming the key site "Fe-O".In the whole reaction,the monoatomic Fe site with more d electrons is beneficial to the first step of hydrolysis,which improves her activity.3.Exploring the self optimization of Co/CoO-GO electrocatalyst in alkaline oxygen evolution reaction based on Operando XAFSThe Co/CoO-GO catalyst was successfully synthesized by hydrothermal kettle boiling and pyrolysis,and showed excellent alkaline OER activity.It was found that metallic state Co as well as CoO characteristic peaks as well as oscillation shape existed in the Co/CoO-GO catalyst,meaning that the initial sample has Co and CoO coexisting phases.Operando XAFS results showed that the two crystalline phases in the Co-based catalyst were oxidized under electric drive,the valence state of Co increased and the structure was optimized at the same time.The electronic structure changed,then the phase transformed to CoOOH.In alkaline OER,the sample needed 408 and 492 mV overpotential to reach current of 20 and 40 mA cm-2,and could run for 18 hours over 20 mA cm-2 current density.