Construction Of Active Sites For Electrocatalysts In Alkaline Water Splitting

Water electrolysis is a clean and sustainable technology for the production of hydrogen energy.The efficiency of the water electrolysis mainly depends on the level of the two half-reaction energy barriers: Hydrogen evolution reaction（HER）and Oxygen evolution reaction（OER）.Fortunately,the introduction of a catalyst materials into the water electrolysis system can effectively promote the occurrence of the reaction,reduce the reaction energy barrier and improve the energy conversion efficiency.Therefore,it is necessary to develop efficient,stable and cheap catalytic materials to reduce cost consumption for water electrolysis.The ideal catalytic materials need to have the following characteristics:（1）high activity,which need to be close to or even more than the noble metal;（2）high stability,which need to be stable for a long time at both large current density and small current density;（3）The synthesis method is simple and easy to expand for large-scale industrial demand.To achieve the above requirements,a lot of experiments are needed.It is necessary to exploitation catalytic materials with different composition,structures and discover the laws to further guide the design and synthesis of catalytic materials.In this paper,we designed and synthesized a series of highly active and stable catalytic materials by optimizing the active sites of corresponding material（adjusting the coordination of the active sites,increasing the number of active sites,and optimizing the conductivity of the materials）,such as crystals phase regulation and introduction of heterogeneous interfaces.The main contents of this paper are as follows:1.At present,Pt,as a catalyst,with the best hydrogen evolution performance.However,the reaction rate of HER in alkaline solutions is lower than that in acidic solutions.This is mainly due to the need to dissociate water under alkaline conditions to generate adsorbed hydrogen,and Pt has a larger dissociative adsorption energy.Therefore,we need to find active Pt-like and non-Pt catalytic materials.Intermetallic compounds are a large family of comprising two or more metals/metalloids and they usually possess specific electronic structures.And the compositional richness and structural diversity of intermetallic compounds,which provide new opportunities for the development of advanced catalytic materials.Based on the above ideas,we selective synthesis of four phase-pure ruthenium-boron intermetallic,including Ru₇B₃,Ru₂B₃,RuB and RuB₂.Through the study of the crystal structures of four ruthenium-boron intermetallic compounds,we found that the Ru 4d-B 2sp orbital hybridization was weakened,which was attributed to the enhanced B-B interactions with high boron content.The HER performances tests found that the catalytic activity and stability are the same under acidic and alkaline conditions.Ru₇B₃ is the worst catalytic activity among the four ruthenium-boron intermetallic compounds,and it is similar to that of metal Ru.RuB,Ru₂B₃ and RuB₂ exhibit Pt-like HER activity.The catalytic activity of RuB₂ is close to the commercial 20 wt% Pt/C.The stability test found that Ru₇B₃ shows a rapid attenuation of activity,while the activities of RuB and Ru₂B₃ decrease slowly.RuB₂ displays the optimal activity and stability（at least 50 h）.We calculated the hydrogen adsorption free energy(ΔG_H*)of RuB₂.It was found that ΔG_H* was close to zero at many sites in RuB₂.It shows that the material contains a large number of catalytically active sites.The better catalytic activity of RuB₂ than Pt in alkaline media which is attributed to the lower barrier of water dissociation adsorption on Ru sites in comparison with Pt sites.Moreover,for RuB₂,there are another two types of predicted structures（RuB₂-II and RuB₂-III）via density functional theory（DFT）calculations.We found that ΔG_H* was close to zero of RuB₂（RuB₂-I）.Therefore,boat-like boron sheet in RuB₂ is an important structural unit for achieving high-performance electrocatalysis.2.Ni₃S₂ is a conductive metal sulfide due to the Ni-Ni metal bonds throughout its structure.The nanosheet array structure of the material can expose a large number of catalytically active sites.Most of the research on Ni₃S₂/NF under a small current density（10 mA/cm²）at alkaline conditions,while the related research on high-efficiency and stable catalytic materials under large current density（1000 mA/cm²）is scarce.But commercial alkaline electrolytic cells often require current densities of 1000 mA/cm².In order to achieve the large-current-density oxygen evolution electrocatalysis,catalytic materials need to have high intrinsic activity,high-density catalytic active sites,good electrical conductivity,corrosion resistance under strong oxidation conditions and good adhesion between the catalytic material and the electrode.However,it is still difficult to find a suitable OER electrocatalyst that can meet all the above standards at the same time.Based on the classic heterogeneous interface nucleation-growth theory,we have further developed an ultrafast（5 s）conductive sulfide surface modification synthesis strategy,and synthesized five different bimetal hydroxide films on 3D conductive sulfide materials（Ni-M-OH@Ni₃S₂/NF,M = Fe,Co,Ni,Mn,Cr）.We have studied the oxygen evolution catalytic activity of five materials under alkaline conditions.And we found that the introduction of manganese and chromium decreases the activity,the introduction of nickel has little effect on the activity,the introduction of cobalt slightly improves the catalytic activity.The introduction of iron has significantly improved the activity.We further studied the structural characteristics and catalytic activity of Ni-FeOH@Ni₃S₂/NF.It was found that Ni-Fe-OH@Ni₃S₂/NF integrated the structural characteristics and catalytic advantages of Ni-Fe-OH and Ni₃S₂/NF.The mass and load transfer rates of the materials are optimized to achieve excellent catalytic activity under high current density.It has high catalytic activity and stability in high current density and high concentration KOH solution.At a current density of 1000 mA/cm²,the catalytic activity of Ni-Fe-OH@Ni₃S₂/NF is 3.5 times that of Ni₃S₂/NF.It is 2.5 times that of IrO2.This research work provides new ideas for the design synthesis of water electrolysis in industrial applications.