Synthesis And Electrocatalytic Water Splitting Performances Of Transition Metal Phosphides

Electrochemical water splitting driven by intermittent solar,wind,and tidal energy sources is a desirable technology for the generation of clean and sustainable hydrogen.This technology enables the conversion of unevenly distributed renewable energy to stable chemicals,which could replace traditional fossil fuels and alleviate energy crisis.The actual voltage of water splitting is generally higher than 1.23 V because of the two sluggish half-reactions,i.e.,the anodic oxygen evolution reaction（OER）and cathodic hydrogen evolution reaction（HER）.Therefore,effective electrocatalysts are crucial for decreasing the overpotential and improving the energy conversion efficiency.To date,noble-metal-based compounds are the benchmark of HER and OER catalysts,but the high price and limited reserves cast a shadow over their large-scale application.It is thus highly urgent to develop low-cost and efficient non-noble-metal-based catalysts,which would promote the fast development of electrochemical water splitting technique.According to the analysis of the research progress of non-noble metal-based catalysts for electrocatalytic water splitting,transition metal phosphides were selected as research object in the present dissertation.We have prepared four kinds of metal phosphides with rational designed compositions,morphologies,and structures.The electrocatalytic performances of the as-prepared metal phosphides were investigated by using various characterization techniques.The main research works and corresponding results are summarized as follows:1.The Fe-doped Ni_xFe_1-x（OH）_y nanosheets are in-situ grown on the surface of Ni submicrowires and then coverted to(Ni_x Fe_1-x)₂P nanosheets by a low-temperature phosphidation treatment.The as-obtained(Ni_xFe_1-x)₂P-Ni with 2D/1D hybrid structures are used as electrocatalysts for OER.Specifically,(Ni_0.87Fe_0.13)₂P-Ni exhibits comparable catalytic performances to noble metal-based RuO₂ in 1 M KOH,delivering a current density of 10 mA cm^-22 at an overpotential(η₁₀)of 257 mV along with Tafel slope of 96 mV dec^-1.The(Ni_0.87Fe_0.13)₂P-Ni catalyst is quite stable during the 10 h electrolysis,showing an output current density retention of 96.7%.These excellent OER catalytic performances of(Ni_0.87Fe_0.13)₂P-Ni are attributed to the unique structure combining metallic Ni submicrowires as good electronically conductive substrate and the Fe-doped phosphide nanosheets.On account of these two components,the(Ni_0.87Fe_0.13)₂P-Ni shows small charge transport resistance,increased active sites and intrinsic activity,fast reaction kinetics,and enhanced stability,finally realizing excellent OER catalytic performances.2.We report a metal-organic frameworks（MOFs）templating approach to synthesize Co-doped Co-Fe-P nanotubes,aiming at improving the HER catalytic performance of FeP.The Co-Fe-P nanotubes are prepared by facile calcination and phosphidation treatments using CoFe MIL-88B MOFs as a template.The Co-Fe-P nanotubes are demonstrated to be efficient electrocatalysts for HER at wide pH range,featuring lowη₁₀ values of 86 mV,138 mV,and 66 mV in 1 M KOH,1 M PBS,and0.5 M H₂SO₄,respectively.The corresponding Tafel slopes are 66 mV dec^-1,138 mV dec^-1,and 72 mV dec^-1.In addition,the Co-Fe-P nanotubes shows stable output current over 20 h of use,and their morphologies and phase structures remain ucchanged.Experimental and theoretical results indicate that the high surface area of tubular structure and the synergetic effect induced by Co substitution endow Co-Fe-P catalyst with abundant catalytic sites,short mass diffusion pathways,low charge transport resistance,desirable adsorption free energies of the intermediates,and increased intrinsic activity for hydrogen evolution.3.N-doped carbon encapsulated Co_xFe_1-xP nanoparticles(Co_xFe_1-x-x P/NC)electrocatalysts are prepared using Co_xFe_y NH₂-MIL-88B MOFs precursors and following appropriate thermal treatments.Simultaneous Co doping and carbon encapsulation are achieved in Co_xFe_1-xP/NC by taking the advantages of unique composition and structure of MOFs.The carbonized temperature and Co/Fe ratio effects of MOFs on final catalytic performances are investigated.The Co_0.17Fe_0.79P/NC obtained at carbonized temperature of 600?C shows the best catalytic activities toward HER and OER in 1 M KOH,delivering a current density of 10 mA cm^-22 at overpotentials of 139 mV and 299 mV with Tafel slopes of 57 mV dec^-1 and 44 mV dec^-1,respectively.The water electrolyzer assembled by two Co_0.17Fe_0.79P/NC electrodes could reach the current density of 10 mA cm^-2 at a voltage of 1.66 V,and exhibit excellent long-term stability for 35 h.These excellent catalytic performances are mainly ascribed to the synergistic effects between Co doping and NC scaffold,providing large surface area,abundant active sites,improved intrinsic activity,fast reaction kinetics,and enhanced stability.4.Cu₃[Co（CN）₆]₂ coated Cu（OH）₂ submicrowires array is grown on the surface of3D copper foam（CF）by simple wet-chemical methods.The core-shell CH@PBA-P/CF electrocatalysts with the composition of Cu₃P@Co-Cu₃P are further prepared by phosphidation treatment.The Co-Cu₃P shell derived from Cu₃[Co（CN）₆]₂ serves as the source of active sites.Co doping and construction of core-shell structure endow the CH@PBA-P/CF catalyst with abundant catalytic sites,enhanced intrinsic activity,and low charge transport resistance.The catalytic electrode integrated with 3D copper foam and 1D submicrowires array is highly conductive and stable,which promotes the charge transport and improves the structural stability.As consequences,the CH@PBA-P/CF shows impressive catalytic performances toward HER and OER in terms of low overpotentials of 231 mV and 312 mV at a current density of 50 mA cm^-2 in 1 M KOH,respectively.Notably,the water electrolyzer using CH@PBA-P/CF electrode exhibits better water splitting performance than the one using noble metal-based couple.