Fabrication Of Two-dimensional Transition Metal/Carbon Composite Catalysts And Their Electrocatalytic Properties

Oxygen reduction（ORR）,oxygen evolution（OER）and hydrogen evolution（HER）have highly dependence on precious metal catalysts,which limited the development of green electrochemical energy storage technologies such as metal air cells,fuel cells and electrochemical water cracking.Notwithstanding the good electrocatalytic performance of noble metals and corresponding oxides（Pt,Ru O₂,Ir O₂）,their natural characteristics such as high cost,scarcity,and poor methanol resistance hinder further commercial application.To address these problems,reasonable preparation of non-noble metal electrocatalysts with excellent performance is important and necessary.Two-dimensional（2D）materials have excellent physical properties such as high specific surface area（SSA）and high active sites,and show good development prospects in the research and development of novel and efficient electrocatalysts.In addition,transition metal/carbon composites can be promising catalytic materials owing to good acid and alkali resistance,high conductivity,and stable electrochemical performance.Based on these backgrounds,we have prepared a variety of two-dimensional transition metal/carbon composite through reasonable design of structure and composition.In this paper,we have studied the reaction mechanisms of different synthesis processes and the electrocatalytic performances of the samples,which can be conducive to the development of non-noble metal electrocatalysts with excellent electrochemical performances.The details are as follows:（1）Cysteine,FeCl₃ and Zn-mediated Mg O template were co-pyrolyzed to prepare highly dispersed S-modified Fe–N–C nanosheet catalysts,which delivered hierarchical porous architecture and ultra-high specific surface area（SSA）.In particular,the addition of Zn Cl₂ and FeCl₃ is evidenced to create mesopores and boost the preparation of thiophenic-S sites（up to 12.29 at.%）in resultant carbons,respectively.The electrochemical performance of the catalyst Fe NSC-ZM was tested and found that it exhibited outstanding ORR catalytic activity in p H-universal electrolytes(in 0.1 M KOH solution,voltage corresponding to the current density at-3 m A cm^-2:E_j3=0.87 V,Limiting current density:j _L=9.80 m A cm^-2;in 0.5 M H₂SO₄ solution,E_j3=0.64 V,j _L=10.3 m A cm^-2;in 0.1 M PBS solution,E_j3=0.66 V,j _L=6.1 m A cm^-2),excellent stability and methanol resistance.When the electrode as the cathode for liquid zinc-air battery,the open-circuit voltage of the battery is about 1.60 V,the power density and energy density are as high as 205 m W cm^-2 and 990.6 Wh kg _Zn^-1 respectively,which are much higher than that of the Pt/C-based battery(1.42 V,115 m W cm^-2,676.7 Wh kg Zn^-1).（2）In order to study the universality of the method for preparing S-modified Fe-N-C,FeCl₃ was adjusted to Ni Cl₂,Co Cl₂,and Cu Cl₂ respectively.The preparation processes for S-modified Ni-N-C,Co-N-C,and Cu-N-C composite catalysts have been presented,which all exhibited excellent ORR catalytic activity in p H-universal solution.Electrochemical performance testing indicated that the E_j3 and j _L of Cu NSC-ZM were0.84 V and 9.77 m A cm^-2 in 0.1 M KOH medium respectively,which were superior to the performance of commercial Pt/C.It also exhibits excellent electrochemical performance in acidic and neutral media(0.5 M H₂SO₄ solution:E_j3 and j _L of Cu NSC-ZM are 0.42 V,8.48 m A cm^-2 respectively;0.1 M PBS solution:E_j3 and j _L are 0.53 V and 10.22 m A cm^-2 respectively),which are comparable to the performance of commercial Pt/C.In p H-universal media,Cu NSC-ZM exhibited excellent stability and methanol resistance.Cu NSC-ZM possesses abundant active sites of Cu-N-C/N-C/S-C,which can be favorable for ORR reaction charge transfer,and exhibits excellent ORR performance in p H-universal media.（3）In order to improve the density and dispersion of active sites,the Co₄Ni₁S/CC nanosheet composite catalyst was obtained by arranging Co₄Ni₁S amorphous nanosheet arrays on carbon cloth by hydrothermal and room temperature sulfidation.The catalyst possesses ultra-highly atomic dispersion of binary metal active sites and excellent catalytic activity for OER and HER.It requires overpotentials of 296 and 192 m V to deliver the current density of 10 m A cm^-2 for OER and HER,respectively,and exhibited good stability and durability.As directly applied as anode and cathode for Co₄Ni₁S/CC catalyst in alkaline electrolyzer,a low cell-voltage of 1.60 V could endow the water splitting current of 10 m A cm^-2,outperforming the benchmark Ru O₂ and Pt/C-based electrolyzer at 1.72 V@10 m A cm^-2.（4）In order to further improve the electrocatalytic stability of the composites,hydrothermal and high temperature annealing methods were applied to composite layered bimetal hydroxide（Ni Fe-LDH）nanosheets with carbon nanofibers to obtain Ni Fe-LDH/CNFs nanocomposites with strong electronic coupling.Ni Fe-LDH/CNFs composite exhibited excellent OER catalytic activity（η=230 m V）with a Tafel slope of 34 m V dec^-1,outperforming pure Ni Fe-LDH nanosheets,commercial Ru O₂,and most reported non-precious metal catalysts.After 40 h of OER polarization at a high current of 30 m A cm^-2,the excellent structural and electrocatalytic stability can be still maintained.In addition,the electrolytic water electrolyzer assembled with Ni Fe-LDH/CNFs and commercial Pt/C as anode and cathode required only a cell voltage of1.50 V at a current density of 10 m A cm^-2,which is superior to（+）Ru O₂|Pt/C（-）double electrode system.