Design Of Cobalt/Nickel Based Electrocatalysts For Water Splitting In Alkaline Electrolyte

In recent years,the accelerated consumption of traditional fossil energy and serious environmental pollution have forced people to explore green and efficient sustainable energy.As a carbon-free carrier,hydrogen is considered as an ideal energy source to replace traditional fossil fuel systems because of its clean,small molecular mass and high energy density.Electrochemical water splitting is one of the effective ways to generate hydrogen.However,the sluggish four-electron reaction of the anode greatly limits its practical application.To date,noble metals?such as Pt,Pd?and noble metal oxides?such as IrO₂,RuO₂?are the most active HER and OER catalysts,respectively.However,the high cost,scarcity and instability of these noble-metal-based catalysts are the major obstacles in their large-scale applications.Therefore,it is necessary to develop electrocatalysts with abundant resources,low cost and high activity as substitutes for noble metal-based catalysts.Transition metal compounds have attracted great interest due to their low cost and wide application prospects.Transition metal alloys and transition metal halides are proved to be stable and active for water splitting.Based on the advantages of transition metal compounds,Ni-Mo alloy nanotubes,Prussian blue analogue borides and selenides were synthesized by controlling the morphology and electronic structure,aiming to improve the electrocatalytic activity of these materials.The main contents and achievements of this dissertation are as follows:?1?Ni-Mo alloy nanotube arrays were synthesized by hydrothermal method and followed reduction method for efficient cathode hydrogen production in a water electrolysis system.Specifically,ultralow overpotential of 44 mV?vs.RHE?is required to deliver 10 mA cm^-2current density with the Tafel slope is 55 mV dec^-11 for cathodic reaction.In the reduced Ni-Mo nanotubes,the presence of nickel indicates that the metallicity and conductivity of the alloyed Ni-Mo nanotubes are enhanced.The results of SEM,XRD and electrochemical measurements show that Ni-Mo nanotubes have strong stability,as the morphology and properties have hardly changed before and after HER stability test.Density functional theory calculation results show the Mo center is main reaction site for the chemisorption and O-H bond cleavage of H₂O while Ni center is identified as the hydrogen-evolving site.This chapter highlights the high efficiency of nickel-based nanocomposites as hydrogen evolution catalysts,and holds promising potential in future energy conversion technologies.?2?In this work,amorphous Co-Ni-B-O nanosheets?CNBO-NS?were synthesized by the chemical reduction of bimetallic Prussian blue analogues?Co-Ni PBA?using sodium borohydride for the first time.The as-prepared CNBO-NS exhibit excellent electrochemical catalytic activity and stability.They can steadily afford 10 mA cm^-2 current density at overpotentials of 140 mV for hydrogen evolution and 300 mV for oxygen evolution during 24h operation.Based on this CNBO-NS bifunctional electrocatalyst,a water electrolysis cell is proposed and the overall cell voltage of 1.69 V is achieved for outputting 10 mA cm^-2 current density,which is much better than that of Co-Ni PBA couple?1.85 V?and Pt/C||IrO₂ couple?1.70 V?in the same hybrid water electrolysis.The boosted catalytic activities are attributed to the formation of a metal–boron bond and the improved surface area along with the enhanced conductivity in the resultant CNBO-NS.This work could offer new clues for the utilization of metal-organic frameworks for designing functional nanomaterials,and more importantly valuable insights into crystallinity engineering in efficient electrolysis for energy conversion and beyond.?3?Hollow Fe-CoSe₂ nanocubes were synthesized by a one-step selenization with CVD.Benefiting from its hollow structure and high spin state electron configuration as well as rapid electron transfer rate,Fe-CoSe₂ shows excellent OER performance in 1.0 M KOH solution.It can steadily afford 10 mA cm^-2 current density at overpotential of 270 mV for oxygen evolution,the Tafel slope is only 36 mV dec^-1,and it can maintain high activity for 24 h.It is worth noting that,XPS and EPR are used to verify the change of Co valence state.Specifically,Co changes from the low-spin state of tetrahedron to the high-spin state of local octahedral field,so two more solitary electrons appear,the valence state of the active center Co is biased to+3 or even higher,which enhances the adsorption and fracture of O-H then improves the activity of OER.This work not only provides a highly efficient OER catalyst for future energy system,but more innovatively,also pioneers a promising pathway to clarifying heteroatom doping mechanism.