Identification Of The Active Structures In P-doped Nano Carbon Materials Towards Oxygen And Hydrogen Evolution Reactions

As a clean and renewable energy,hydrogen has been regarded as one of the most potential substitutes to oils and coal.Among many methods of producing hydrogen,hydrogen production from water electrolysis without carbon emissions is considered to be the most promising technology for producing high-purity hydrogen.At present,Ir/Ru-based and Pt-based catalysts are the best electrocatalysts for anodic oxygen evolution reaction(OER)and cathode hydrogen evolution reaction(HER)in electrolyzed water,respectively.Nevertheless,the massive application of precious metals in industry has been hindered because of the exorbitant cost and poverty.Consequently,it is imperative to reduce the loading of noble metal catalysts and design economical and green OER/HER electrocatalysts.Some works have demonstrated that incorporating heteroatoms into the carbon network can give the materials better OER/HER electrocatalytic activity of late year,and can effectively improve the activity of the loaded noble metal and reduce the amount of it.However,researchers have poorly understanding about the specific active structures in the materials,which seriously hinder high-performance water directional preparation of electrolysis catalysts.Therefore,in this work,phosphorus doped carbon nanomaterials were prepared and loading with a small amount of noble-metal(Pt).Meanwhile,the active structures of OER/HER were explored and identified,the specific contents are as follows:(1)Influences of P species in P-doped graphite layers on the catalytic activities of oxygen and hydrogen evolution reactionsThe distribution of different phosphorus species in the P-doped graphite layers materials were adjusted by changing the heating temperature.SEM,TEM,Raman,XPS and other instruments were used to characterize the physical and chemical properties of catalysts,and the electrochemical workstation was used to investigate the HER/OER activity and stability.The results showed that the OER activity of the P-doped graphite layers materials with the highest content of C-O-P species was even better than that of the commercial Ir O₂ catalyst.Meanwhile,the material with the highest degree of defects showed the best HER activity.Density functional theory(DFT)calculations indicated that:C-O-P species can activate the three adjacent carbon atoms to make it easier to adsorb OH^-,thereby effectively promoting the OER performance of graphite layer materials.The five-membered ring defect produced by the C₃-P=O species located at the edge of the carbon matrix is the main source of activity of the catalyst HER.(2)Influences of P species in P-doped carbon nanotubes on the catalytic activities of oxygen and hydrogen evolution reactionsIn order to introduce C-P species into carbon materials,in this paper,we prepared P-doped carbon nanotubes by chemical vapor deposition(CVD),and controlled the contents of C-P species in P-doped carbon nanotubes by changing the temperature of vacuum heating.SEM,TEM,Raman,XPS and other instruments were used to characterize the physical and chemical properties of materials.Electrochemical testing results showed that there was no significant positive correlation between the content of C-P species and the HER and OER activities of the catalysts.P-doped carbon nanotubes with the least contents of C-P species exhibited the best HER performance and excellent stability,and the material with the highest C-O-P species still showed the best OER catalytic activity.The DFT calculation results revealed that the C-P species will form a stable five-membered ring and nine-membered ring defects after decomposition,and there were seven active carbon atoms in the defect structure that can be used as the active sites of HER.(3)Influences of P species in P-Doped carbon nanotubes on the HER catalytic activities of Pt catalystPt was loaded on the carbon nanotubes with different phosphorus species contents by using the ethylene glycol microwave heating reduction method.TEM,Raman,LA-ICP-MS,XPS and other instruments were used to characterize the physical and chemical properties of the material,and electrochemical workstation was used to study the HER performance.The test results indicated that the HER activity and durability of Pt-based catalyst with the least C-P species contents and 8.66 wt.%Pt loading exceeded those of 40 wt.%Pt/C,whether in acidic or alkaline solutions.The DFT calculation results showed that after the decomposition of the C-P species containing pyridine phosphorus and graphite phosphorus structures,the defects formed may be beneficial to the reduction of the positive charge of the Pt atoms loaded on the surface,which may promote the adsorption of Pt on H⁺in the acid solution.Meanwhile,the increased electronegativity of the two carbon atoms that bond with the Pt atom facilitates the diffusion of the OH^-generated by the Volmer step in the alkaline solution,which may be beneficial to the Volmer reaction and promote the HER activity of Pt atoms in the alkaline medium.