Construction Of Heteroatom-doped Carbon Encapsulating Metal Phosphides And Their Application In Hydrogen Evolution Reaction

With the development of the society,traditional energy resources,such as coal,oil and natural gas can not meet the social demands,and energy consumption will lead to climate changes and environmental issues.Therefore,the research and development of new energy has been widely concerned.Hydrogen is regarded as the real energy source for the future due to its high energy density and zero carbon dioxide emission.A promising approach to producing hydrogen on a large scale is via water electrolysis,due to its high product purity,simple technology,reliable in operation and so on.An advanced hydrogen evolution reaction（HER）catalyst is required to decrease the overpotential for high efficiency.Up to now,the most efficient electrocatalyst for HER is still commercial Pt-based catalysts（Pt/C）.Unfortunately,its large-scale application is seriously limited by its high cost and scarcity.Therefore,highly active HER alternative catalysts based on low-cost and high efficiency have aroused intense research interests.In the thesis,we focused on the preparation of heteroatom-doped carbon encapsulating metal phosphide and their application in hydrogen evolution reaction.The specific contents are as follows:1.We reported a general strategy for synthesis of a series of transition metal phosphides（TMPs）(WP,MoP,Fe₂P,FeP,Co₂P,CoP,Ni₂P,and Ni₁₂P₅)nanoparticles（NPs）with different metal phases embedded in a N-doped carbon（NC）matrix by changing metal salt,ammonium dihydrogen phosphate,melamine,molar ratios and thermolysis temperatures.The resultant TMPs can serve as highly active and durable electrocatalyst toward HER and oxygen evolution reaction（OER）.In particular,the WP@NC phase only requires an overpotential of～102 mV to derive HER in 0.5 M H₂SO₄ at the current density（j）of 10 mA cm^-2.The Ni₂P@NC needs～320 mV for OER in 1.0 M KOH at j=10 mA cm^-2.In addition,WP@NC and Ni₂P@NC exhibit almost unfading catalytic performance.2.We demonstrated a green method for fabrication of a number of TMPs by pyrolyzing melamine and self assembled phytic acid（PA）cross-linked metal complexes.The obtained materials consisting of TMP NPs are encapsulated in N,P-codoped carbon（NPC）.Among TMPs,the resultant FeP NPs encapsulated in the NPC matrix（FeP NPs@NPC）shows the highest HER activity at all pH values.At a current density of 10 mA cm^-2,FeP NPs@NPC displays overpotentials of 130,386and 214 mV in 0.5 M H2SO4,1.0 M phosphate buffer solution（PBS）and 1.0 M KOH,respectively.Additionally,the encapsulation by NPC effectively prevents FeP NPs from corrosion,exhibiting almost unfading catalytic activity after 10 h testing in acidic,neutral and basic electrolytes.More importantly,other TMPs wrapped in NPC（CoP NPs@NPC and Ni₂P NPs@NPC）can be easily obtained by this method,which also exhibit relatively high activity toward HER.3.We reported a general strategy for synthesis of a series of di-metal phosphides NPs embedded in a NC matrix using PA as phosphrous source,including Rh₂P@NC,Ru₂P@NC and Fe₂P@NC etc.In a wide pH range,Rh₂P@NC catalyst not only possesses small overpotentials at 10 mA cm^-2（⁹ mV in 0.5 M H₂SO₄,⁴6mV in 1.0 M PBS and¹0 mV in 1.0 M KOH）,but also demonstrates high stability.Importantly,all these performances are superior to commercial Pt/C for HER.Density function theory（DFT）calculations reveal that the introduction of phosphorus can significantly lower the proton adsorption energy of Rh/NC,thereby benefiting the surface hydrogen generation.4.For the first time,we developed a Pt-like catalyst through P-rich strategy.Based on DFT predictions,we designed and synthesized a novel N,P dual-doped carbon-encapsulatedrutheniumdiphosphide（RuP₂@NPC）nanoparticle electrocatalyst for HER.Electrochemical tests reveal that,compared with the Pt/C catalyst,RuP₂@NPC not only has Pt-like HER activity with small overpotentials at10 mA cm^-2（38 mV in 0.5 M H₂SO₄,57 mV in 1.0 M PBS and 52 mV in 1.0 M KOH）,but demonstrates superior stability at all pH values,as well as 100%Faradaic yields.More importantly,the price of Ru is only 7%of Pt which is very attractive for commercialization.