Controllable Synthesis And Electrocatalysis Performance Of Trifunctional Electrocatalyst Of N-doped Graphitic Carbon Encapsulated With Metal Nanoparticles

With the escalating demand for energy,the rampant consumption of fossil fuels has catalyzed the outburst of environmental issues.Among the myriad energy conversion and storage systems,metal-air batteries and water electrolyzers have been recognized as the most efficacious and facilitate ways of energy conversion and storage.However,the energy conversion efficiency and basic operation of these systems are limited by the slow kinetics and higher overpotentials of three key electrochemical reactions:oxygen reduction reaction（ORR）,oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）.Pt-based catalysts are one of the most prestigious ORR and HER catalysts,while the energetic OER catalysts are usually Ir-based and Ru-based catalysts.Therefore,designing the low-cost,multifunctional,efficient and stable ORR/OER/HER trifunctional electrocatalyst to replace the expensive,scarce and poorly stable noble metal-based electrocatalyst is crucial for the promotion of rechargeable Zn-air battery and water electrolyzer.The paper focused on the controllable synthesis and electrocatalysis performance of trifunctional electrocatalyst of N-doped graphitic carbon encapsulated with metal nanoparticles mainly discusses the following three aspects:1.N-doped graphitic carbon nanosheets encapsulated with Co Fe alloy nanocrystals（Co Fe@NC/NCHNSs）were prepared by simple reflux and annealing treatment,and the trifunctional electrocatalytic performances of ORR,OER and HER were tested.In this chapter,Co Fe-MOF particles can be easily deposited on the surface of the thin C₃N₄ nanosheets to obtain Co Fe-MOF/C₃N₄ nanosheets by a facile reflux method with H₂bpydc as a N-rich organic ligand.A brisk and stable trifunctional electrocatalyst for ORR,OER and HER based on N-doped graphitic carbon hybrid nanosheets encapsulated with Co Fe alloy nanocrystals（Co Fe@NC/NCHNSs）by a facile one-step pyrolysis of Co Fe-MOF/C₃N₄ nanosheets.In the pyrolysis process in N₂ atmosphere,C₃N₄ nanosheets could act as the sacrificial templates and nitrogen sources to further converted into N-doped graphitic carbon nanosheets.Moreover,direct pyrolysis of Co Fe-MOF only formed a vast amount of agglomerated Co Fe alloy particles on the surface of N-doped carbon（Co Fe@NC nanohybrids）,indicating that C₃N₄ as the sacrificial template is beneficial for controlling the particle sizes and hampering the agglomeration of alloy particles.The pyrolysis temperature was adjusted in order to explore the best-performing activity of the catalyst,and the obtained Co Fe@NC/NCHNSs-700 catalyst possesses the most eminent activity.The as-prepared catalyst evinces a positive half-wave potential(E_1/2)of 0.92 V for ORR,and low overpotentials of 285 m V and 120 m V for OER and HER at the current density of 10 m A cm^-2,respectively.The superior trifunctional electrocatalytic performance of the as-obtained Co Fe@NC/NCHNSs can be subject to the synergetic effect between the rich Co Fe alloy sites and the high-content graphitic N dopants.2.Utilizing Density functional theory（DFT）calculations to reveal the performance enhancement mechanism of N-doped graphitic carbon nanosheets encapsulated with Co Fe alloy nanocrystals（Co Fe@NC/NCHNSs）,and further disclose the effects of different metals on the electrochemical properties of the catalysts.The performance test of rechargeable Zn-air battery and water electrolyzer system assembled with the Co Fe@NC/NCHNSs-700 was carried out.In this chapter,based on the previous work,the Co Fe@NC/NCHNSs-700possesses impressively trifunctional electrocatalytic activity of ORR,OER,and HER.This work conducted an in-depth study on the trifunctional performance enhancement mechanism of the Co Fe@NC/NCHNSs-700 electrocatalyst,and revealed the key role of the Co Fe alloy and high content of graphitic N by combining experiments and theoretical calculations.Density functional theory（DFT）discloses that the existence of graphitic N and Co Fe alloy cooperatively enhances the adsorption of O*,OH*,OOH*,thus greatly improving the intrinsic activity.Furthermore,to disclose the effects of different metals on the electrochemical properties of the catalysts,another two samples of N-doped carbon hybrid nanosheets with Co or Fe nanoparticles encapsulated in N-doped graphitic carbon（Co@NC/NCHNSs-700 and Fe@NC/NCHNSs-700 catalysts）were prepared for comparison.Interestingly,it has been found that the introduction of Co species can help anchor N in the carbon matrix and Fe species contribute to the generation of graphitic N species in the synthesis of Co Fe@NC/NCHNSs,which together promote the formation and retention of high content of graphitic-N in the final product and lead to an enhancement of electrocatalytic activity.As expected,the assembled liquid Zn-air battery exhibits a high-power density of 184 m W cm^-2 and a long-term cyclability for 50 h,and the water electrolyzers just need 1.665V to deliver a current density of 10 m A cm^-2.Furthermore,an open-circuit voltage of 1.46 V and a maximum power density of 125m W cm^-2 are achieved with the Co Fe@NC/NCHNSs-700-based solid-state Zn-air battery,demonstrating the practical applications of the as-prepared catalyst in advanced energy storage and conversion devices.3.Through the hard template method and the nanopore confinement effect,the controllable synthesis of honeycomb-like N-doped carbon matrix with Co nanoparticles encapsulated in N-doped graphitic carbon layers（Co@NC/HNC）was achieved,the trifunctional electrocatalytic performances of ORR,OER and HER were tested,and the performance test of rechargeable Zn-air battery and water electrolyzer system assembled with the Co@NC/HNC-2-700 was carried out.In this chapter,the silica nanospheres are first modified by the cobalt source followed by the addition of imidazole ligand.The abundant deposition sites at the silica spheres would be beneficial for the uniform deposition of ZIF-67 in the interspace of silicon nanospheres.The subsequent annealing process endows the production of interconnected nanocarbons embedded between the silica nanosphere assemblies.After removing silica templates by the KOH treating,the Co-based honeycomb-like N-doped carbon materials（Co/HNC）are achieved.The Co-MOF/HNC is formed by situ growth of ligands（H₂bpydc）and cobalt-based complex onto the inner wall of the nanopores.After heat treatment under N₂atmosphere,honeycomb-like N-doped carbon matrix with Co nanoparticles encapsulated in N-doped graphitic carbon layers（Co@NC/HNC）can be prepared.In order to enhance the catalytic activity,adjust the input of organic ligand（H₂bpydc）and the pyrolysis temperature to obtain Co@NC/HNC-2-700 with the best electrochemical activity and stability.To better highlight the role of the nanopore confinement effect,single Co-based polyhedron nanocarbons derive from ZIF-67nanoparticles were synthesized in place of the 3D macroporous interconnected carbon matrix（Co@NC nanohybrids）.The as-prepared catalyst evinces a positive half-wave potential(E_1/2)of 0.90 V for ORR,and low overpotentials of 370 m V and 135 m V for OER and HER at a current density of 10 m A cm^-2,respectively.The assembled liquid Zn-air battery with Co@NC/HNC-2-700 as the air cathode exhibits a high power density of 198 m W cm^-2 at 326 m A cm^-2.The stable voltage plateau in the galvanostatic discharging-charging curve can be maintained well for 375 h at a current density of 10 m A cm^-2.However,the rechargeable Zn-air battery assembled by Co@NC nanohybrids has a poor stability,merely for 50 h.The solid-state Zn-air battery displays an open-circuit voltage of 1.415 V and a high power density of 107m W cm^-2.The water splitting only needs 1.75V to deliver a current density of 10 m A cm^-2.Benefiting from the continuously interconnected skeletonthe and the synergetic interactions between the rich metallic Co sites and the high-content N dopants,the rechargeable Zn-air battery and overall water splitting system present excellent catalytic performances with outstanding stabilities,demonstrating the Co@NC/HNC-2-700 catalyst holds bright prospect to be practically applied in energy conversion and storage fields.