Synthesis And Electrochemical Properties Of Phosphides And Their Composites Based On Prussian Blue Analogues

As a pollution-free and efficient method to obtain hydrogen,electrocatalytic hydrolysis technology for hydrogen production has become an international research frontier and hotspot.At present,commercial water electrolysis mainly uses Pt,Ru or Ir based precious metal catalysts.The scarcity and high cost of these catalysts greatly restrict the large-scale application of water electrolysis.It is of great significance to develop an efficient and low-cost overall water-splitting electrocatalyst based on transition metal compounds,which can be used for cathodic hydrogen evolution reaction（HER）and anodic oxygen evolution reaction（OER）.In terms of the questions,as a metal-organic framework,Prussian blue analogue（PBA）,which is used as a precursor of transition metal phosphates,has become the research object of this paper.Through the rearch on chemical composition design,crystal/micro/electronic structure control and conductive carbon materials selection,we have prepared the efficient and stable transition metal phosphide catalysts with unique nanostructure,and explored its electrocatalytic performance and reaction mechanism.The main research contents are as follows:1.By using cocobalt-iron Prussian blue analogue（CF PBA）as the precursor,Co-Fe-P nanocubes（CFP NCs）and their composites with N,P-doped graphene（NPG）and carbon nanotubes（NPCNT）are prepared by room temperature co-precipitation and subsequently phosphating method.The electrocatalytic performance of CFP NCs and their composites in 1M KOH are investigated.The results show that the electrocatalytic HER and OER performance of CFP NCs/NPCNT and CFP NCs@NPG are significantly improved compared with that of CFP NCs.In particular,CFP NCs@NPG showed more excellent electrochemical performance for OER（or HER）with low overpotential of 289（or 193）mV at the current density of 10 mA/cm² and small Tafel slope of only 47.8（or69.6）mV/dec.The main reason for the great improvement of electrocatalytic performance are as follows:The introduction of graphene can not only provide more active sites but also enhance the electrical conductivity of the composites.2.By using CF PBA/CNT as the precursor,then selectively etching and in-situ phosphorization,Co-Fe-P nanaframes immobilized on N,P-codoped CNTs（CFP NFs/NPCNT）is prepared.The HER and OER performance of CFP NFs/NPCNT in alkaline electrolyte are investigated.The results show that the hollow CFP NF_S/NPCNT has better electrocatalytic performance than the above solid cubic catalysts.The overpotentials at 10 mA/cm² of CFP NFs/NPCNT for OER and HER are only 278 mV and 132 mV,and the Tafel slopes decrease down to 39.5 mV/dec and 62.9 mV/dec.When it is employed as a bifunctional catalyst for overall water splitting,to afford 10mA/cm² only requires 1.56 V overpotential.The excellent electrocatalytic performance mainly comes from the following reasons:the highly conductive NPCNT network is conducive to electron transfer,the CoFe bimetallization improves the overall electron band structure of the material,and the hollow nanoframe structure can provide more active sites for catalytic reaction,and is conducive to electrolyte penetration,ion diffusion and O₂/H₂ escape.3.We use CF PBA as the precursor and hypophosphorous acid as the reductant and etchant to synthesize hollow Co-Fe-P nanoframes ecapsulated into N,P-codoped graphene aerogel（CFP NFs@NPGA）through one-step in situ encapsulation and selective etching,followed by phosphorization.The results show that the Tafel slopes of CFP NFs@NPGA for OER and HER in alkaline electrolyte are as low as 36.7 mV/dec and 59.8 mV/dec and the overpotential at 10 mA/cm² are only 267 mV and 59.8 mV,which is superior to CFP NFs/NPCNT.10 mA/cm² current density can be obtained with only 1.59 V overpotential by using CFP NFs@NPGA as both anode and cathode material in overall water-splitting cell.The reason why CFP NFs@NPGA has better electrocatalytic performance is that the protection of the hollow skeleton by graphene aerogel is better than carbon nanotubes,making the product more homogeneous and avoiding agglomeration between the hollow skeletons,which leads to a larger specific surface area of the material and more active sites exposed.