Preparation Of Iron-doped Cobalt Phosphide/Carbon Composite From Porphyrinic Metal-Organic Framework For Electrocatalytic Oxygen Evolution

Oxygen evolution reaction（OER）is an important semi reaction in the process of water electrolysis.The research of OER catalysts is of great significance to reduce the energy consumption of electrolytic water reaction and realize large-scale hydrogen production.Transition metal phosphides show great potential as non-noble metal based OER catalysts.However,their catalytic activity of OER is still lower than that of noble metal based catalysts.Metal-organic frameworks（MOFs）are a kind of materials with high porosity,abundant metal active sites and various selectivity of organic linkers,which can be easily transformed to transition metal phosphides and carbon composites as precusors,achieving higher conductivity of phosphides as well as uniform dispersion of active sites.Meanwhile,MOFs have diverse morphologies,and the type and quantity of the metal centers can be flexibly adjusted,so as to improve the OER catalytic activity of their phosphated derivatives.Based on the above,this subject has chosen the nanoplate products of the thickness-controllable bimetal Co-TCPP（Fe）MOF of porphyrin paddle wheel-frameworks（PPFs）series,which was synthesized as a precursor for the preparation of transition metal phosphide and carbon composite material.The purpose is to expose abundant active sites with the nanoplate morphology of materials,and improve the OER catalytic activity by taking advantage of the synergistic effect between the two kinds of metal centers.The research contents in this article mainly include the following two aspects:1.The synthesis of bimetal Co-TCPP（Fe）MOF nanoplates was explored by adjusting the concentration of reactants in a surfactant assisted method,and the transformation of crystal structure of the product from PPF-3 to PPF-5 with the decrease of grain sizes was studied.In order to explain this phenomenon,a series of samples were synthesized by gradient control of reactant concentration and studied systematically by a series of characterizations.The results showed that,on the basis of the synthesis of PPF-3 bulk Co-TCPP（Fe）MOF reported in the literature,when the concentration of reactants was reduced to 1/2.25 or lower,the crystal type of the products transformed into PPF-5 type.The possible reason for this change was that the structure mobility was inhibited by the smaller crystal size.In addition,the PPF-5 type Co-TCPP（Fe）MOF nanoplates were successfully prepared by adding surfactant polyvinylpyrrolidone（PVP）into the reactants,and the inhibition effect of PVP on crystal growth was used to verify the relationship between the size change of Co-TCPP（Fe）MOF grains and crystal transformation.2.An iron-doped cobalt phosphide and nitrogen-doped carbon composite（represented as Fe/Co P@NC）was obtained through carbonization and phosphorization successively of the PPF-5 Co-TCPP（Fe）MOF nanoplates as precursor.According to structure and morphology characterizations,the material maintains the nanoplate morphology of the precursor,where Co P nanoparticles were dispersed uniformly in the nitrogen doped carbon substrate and Fe was uniformly doped.According to the OER tests（in 1.0 mol/L KOH）,the Fe/Co P@NC-600 nanocomposite prepared under the optimal conditions showed OER activity better than that of the noble metal based Ir O₂catalyst（overpotential of 290 m V at10 m A/cm²）.Through the comparison with the bulk and monometallic contrast samples in OER tests,the good OER performance of the Fe/Co P@NC-600 nanoplate composite can be attributed to the introduction of Fe element optimized the electronic structure of this material and increased the electrochemical active surface area,and the nanoplate morphology that exposed more active sites,furthermore,the organic ligand derived nitrogen-doped carbon substrate improved conductivity of the material.Meanwhile,after24 h potentiostatic test,the current density of the material did not decrease significantly,indicating its excellent stability.Characterization of the material after stability test demonstrated that metal hydroxide/hydroxyl-oxide generated during OER testing were the real active ingredients of OER.Further more,the overall water splitting system with Fe/Co P@NC-600 as the anode and the reported non-noble Ni/Ni Fe Mo O_x/NF catalyst as the cathode exhibited excellent electrolytic water performance（potential of 1.55 V at 10m A/cm²）.