Preparation Of Platinum-loaded MOFs Derived Materials And Their Electrocatalytic Oxygen Reduction / Hydrogen Evolution Performance

Environmental issues have become one of the major challenges facing human society.Fuel cell has the advantages of high efficiency,long service life and environmental friendliness,which is a new energy supply scheme.Hydrogen energy has been widely concerned as a clean and renewable energy source.Oxygen reduction reaction（ORR）and hydrogen evolution reaction（HER）are the key cathodic reactions in fuel cells and water electrolysis,respectively.At present,platinum-based catalysts show excellent ORR,HER catalytic performance and commercial application.However,platinum is expensive and scarce,which hinders its large-scale application.Metal-organic frameworks（MOFs）materials have high specific surface area,adjustable pore structure,flexible and diverse metal centers and ligand composition,which are commonly used in the development of materials in the field of catalysis and energy conversion.In this paper,a variety of MOFs derived materials were developed and platinum was loaded for ORR and HER catalysis.The development of high-performance catalysts with low platinum loading was achieved by optimizing the MOFs treatment scheme and precisely controlling the platinum loading form.In addition,the catalytic mechanism was studied in depth by means of advanced research methods such as synchrotron radiation and theoretical calculation.The specific content includes the following aspects:（1）Using ZIF-8 as the precursor,the nitrogen-doped porous carbon with higher specific surface area and pore volume than ZIF-8 was obtained by introducing argon-hydrogen mixed gas and improving the heat treatment process.Its BET specific surface area and pore volume are as high as 2476 m² g^-1 and 1.325 cm³ g^-1,respectively.Studies have shown that hydrogen evaporates the zinc in ZIF-8 at high temperature,avoiding the carbothermal reduction reaction between the zinc component and the carbon skeleton of the MOFs,protecting the skeleton of the MOFs and leaving a large number of microporous structures.The obtained porous carbon is used as a carrier for platinum nanoparticles for ORR catalysis,showing excellent performance.Its half-wave potential is 0.883 V（vs.RHE）,which is superior to commercial Pt/C（Pt 20 wt%）and has excellent stability,while the platinum content is only 43.3%of Pt/C.Studies have shown that the high specific surface area and nitrogen-doped structure of the support are beneficial to the dispersion of platinum and improve the performance of the catalyst.（2）Copper atom-dispersed nitrogen-doped porous carbon was prepared by using copper-doped MOFs as precursors.Copper-platinum alloy nanoparticles dispersed nitrogen-doped porous carbon（C-ZIF-Cu Pt）was obtained by copper atom-targeting platinum atom deposition,and the deposition process was verified by theoretical calculation.C-ZIF-Cu Pt exhibits excellent ORR and HER bifunctional catalytic performance.Its platinum mass activity is 4.4 times（ORR）and 6.7 times（HER）of Pt/C.X-ray photoelectron emission electron microscopy（X-PEEM）tests show that there is a strong interaction between the carbon carrier and the copper-platinum alloy,which helps to improve the stability of the material.Theoretical calculations show that copper doping increases the d-band center of platinum,reduces the ORR overpotential,and the activation energy barrier for water molecules,which is beneficial to ORR and HER catalysis.（3）Double-layer nitrogen-doped porous carbon nanocages were obtained by etching-calcination of ZIF-8.It has a wide pore size distribution and large pore volume and a unique hollow double-layer structure,providing a large number of accessible adsorption sites for platinum loading.Platinum single atom and platinum cluster double active site structures were constructed on the nanocage.The obtained material NPCN-Pt has better alkaline HER catalytic activity and stability than commercial Pt/C.The turnover frequency（TOF）and mass activity of NPCN-Pt are 5.0 and 7.2 times that of Pt/C,respectively.Theoretical calculations show that the activation energy barrier of platinum clusters to water molecules is lower than that of platinum single atoms,which is beneficial to the Volmer step.The Gibbs free energy of hydrogen adsorption(ΔG_H*)at platinum single atom sites is smaller than that of platinum clusters,which is beneficial to the release of H₂.Studies have shown that platinum single atoms and platinum clusters can synergistically catalyze alkaline HER reactions,providing design ideas for the development of highly efficient alkaline HER catalysts.（4）Controlled loading of platinum single atoms,amorphous platinum clusters and crystalline platinum nanoparticles of MOFs-derived nitrogen-doped porous carbon materials was achieved by freeze-drying technology and heat treatment process control.The effects of different platinum loading forms on the acidic HER catalytic performance of the materials were studied.Studies have shown that the platinum single atom+platinum cluster combination loading form exhibits excellent activity.The prepared catalyst Pt_1+Cs-NPC has an overpotential of only 24 m V at 10 m A cm^-2,a Tafel slope of only 12.5 m V dec^-1,and a platinum loading of only 3.8 wt%.The mass activity and TOF of Pt_1+Cs-NPC are 10.2 times and 5.4 times that of commercial Pt/C,respectively.Theoretical calculations show that the platinum cluster adjusts the electronic state structure of the adjacent platinum single atom,so that theΔG_H*of the platinum single atom site approaches 0.The study of HER reaction pathway shows that platinum clusters and adjacent platinum atoms can synergistically catalyze the Tafel step and reduce the energy barrier of H-H coupling.At the same time,platinum clusters reduce the energy barrier of the adjacent platinum single atom site to the Heyrovsky step and accelerate the reaction with hydrated hydrogen ions.Studies have shown that platinum clusters and platinum single-atom composite loading structures exhibit excellent activity for the Volmer-Tafel or Volmer-Heyrovsky reaction paths of HER reactions.This study provides guidance for optimizing the form of metal loading and developing efficient supported catalysts.