Design and Performance of Transition Metal-based Hydrogen Evolution Catalysts

Author:Wang Min Qiang

Supervisor:bao shu juan


Degree Year:2019





Traditional fossil energy(such as oil,coal,natural gas and other energy)still dominate in the existing world energy development pattern.However,the traditional fossil fuel storage ratio has been decreasing year by year,and the environmental pollution problem that comes with its consumption is increasing.Looking for a variety of green,renewable new energy to replace fossil fuels has been highly concerned by countries around the world.Hydrogen,solar energy,wind energy,tidal energy,etc.have been extensively studied and compared,and hydrogen energy is regarded as the most ideal energy carrier due to its high calorific value,high energy density and abundant reserves.And the preparation of hydrogen can be realized by various renewable clean energy sources,such as hydrogen production by electrolysis of water,hydropower,nuclear power,and the resulting high-purity hydrogen can be used to supply end energy users such as fuel cells,and the combustion products are only water,instead of greenhouse gas(carbon dioxide),which completes a closed hydrogen cycle,and also makes the implementation of the hydrogen economy possible.Therefore,hydrogen energy is considered to play an important role in the composition of the future energy landscape.Hydrogen economic importance other than environmental factors also includes higher energy efficiency:hydrogen fuel cells can easily achieve efficiencies of more than 60%compared to heat efficiency of less than 35%.However,the catalytic performance of the electrolyzed water cathode catalyst material severely restricts the hydrogen production efficiency of the electrolyzed water.Although the conventional platinum-based noble metal electrode catalyst has excellent electrocatalytic activity,the yield is insufficient and the price is expensive and cannot be promoted.Therefore,the development of electrolyzed water cathode hydrogen evolution catalytic materials with abundant reserves,low price,easy preparation and good catalytic activity has become the focus of research.Since the transition metal possesses an empty d orbital or excess d orbital electrons,in which empty orbit can be provided as an electrophile during the catalysis process,or lone pair of electrons can be provided as a nucleophile to form an intermediate product,which reduces the activation energy of the reaction and promotes the reaction.It is carried out,and its natural reserves are abundant,the types of elements are many,the price is low,and the environment is friendly.Based on the improvement of the catalytic efficiency of transition metal-based hydrogen evolution catalysts,a series of transition metal-based hydrogen evolution catalysts were prepared and catalyzed by hetero-doping,in-situ phosphating,in-situ phosphating,nitriding and metal displacement.The mechanism and performance have been thoroughly explored.The thesis is mainly summaried as the following section:1.Nanosized Metal Phosphides Embedded in Nitrogen-Doped Porous CarbonNanofibers for Enhanced Hydrogen Evolution at All pH ValuesDescribed in situ preparation of embedded ultrafine nickel phosphides in N-doped porous carbon nanofibers(NPCNFs)by electrospinning,followed by controllable pyrolyzed reduction in a H2 atmosphere.In our method,a precise ratio of nickel cation,phosphate anion,and polyvinyl pyrrolidone(PVP)was first mixed,and then electrospun into 1D NFs.In the subsequent pyrolysis reduction,primary phosphate was transformed into phosphide nanoparticles and then embedded in the N-doped porous carbon matrix.The accompanying ultrafine porosity residues from PVP were used as a molecular sieve to effectively prevent the aggregation of in-situ-generated phosphide nanocrystals,which is beneficial to confine Ni2P nanoparticles to a very small size(ca.10 nm in diameter).The unique N-doped porous carbon matrix introduced preeminent conductivity,specific surface area,and numerous pores,which support excellent electroactivity.Because of this unique property,Ni2P@NPCNFs as a HER catalyst revealed excellent catalytic performance in acidic media,and good durability in neutral and basic media.This method was used to prepare Fe2P@NPCNFs,Co2P@NPCNFs,and Cu3P@NPCNFs with the same nanostructure as Ni2P@NPCNFs;all of which are anticipated to be highly active electrocatalysts for HER.This work provides a general approach for fabrication of TMP structures with enhanced conductivity and catalytic activity.This work proposes a simple method for constructing highly efficient transition metal phosphides using conventional electrospinning,and points out the direction for the construction of other transition metal-based one-dimensional catalysts.2.Engineering the nanostructure of molybdenum nitride nanodot embedded N-doped porous hollow carbon nanochains for rapid all pH hydrogen evolutionThe composite catalyst obtained in the previous work has a small pore structure leading to the active site is not sufficiently exposed.In this work,we further introduced a hard template and successfully Hierarchical cavity structured MoN nanodots embedded in N-doped porous carbon nanochains(MoN@NPCNCs)were successfully achieved.In our proposed unique structure,PAN was decomposed to form N-doped porous carbon,carbon,and further serve as an efficient conductive network and molecular sieve to effectively prevent the aggregation of MoN.Meanwhile,it provides a three dimension(3D)pathway for efficient electron transfer from MoN particles to the electrode.In addition,the ultrathin MoN nanodots combined with the ultrathin wall cooperatively allow for excellent electrolyte diffusion and maximized exposure of catalytically active sites.Furthermore,density function theory(DFT)calculations reveal that the close coupling between N-doped carbon and MoN can synergistically optimize the adsorption free energy of H*at the active sites.As HER catalysts,MoN@NPCNCs exhibit a low overpotential of 72.35 mV at a current density of 10 mA cm-2 in acidic media,a small Tafel slope of 53.21 mV dec-1,and excellent stability.Furthermore,this catalyst offers excellent electrocatalytic ability and durability in both neutral and alkaline media.3.MoP nanoparticles with a P-rich outermost atomic layer embedded in N-doped porous carbon nanofibers:Self-supported electrodes for efficient hydrogen generationFirstly,MoP was selected as a model cathode material to comprehensively investigate the effect of a terminated surface for HER by complementary theoretical and experimental approaches,indicating that the P-terminated surface plays a crucial role in determining its high-efficiency HER catalyst properties.And then,we designed an efficient strategy to construct a P-rich outermost atomic layer in MoP nanoparticles for the first time.Annular dark-field scanning transmission electron microscopy(ADF-STEM)shows that MoP nanoparticles with a P-rich outermost atomic layer were successfully constructed.Electrochemical tests show that this MoP with a P-rich outermost atomic layer displays prominent HER activity.Carbon coating protects the phosphide nanoparticles from oxidation,giving them long-term durability under HER operation conditions.In addition,the unique MoP nanoparticles embedded N-doped porous carbon nanofiber(MoPNPCNF)catalyst system can be directly used as an electrode for HER,and it displays excellent stability.4.Ternary NixCo3-xS4 with Fine Hollow Nanostructure as Robust Electrocatalyst for Hydrogen EvolutionScalable synthesis of highly active Ni-doped Co3S4 nanotube arrays on conductive carbon cloth(NixCo3-xS4/CC)as a monolithic catalyst for HER in 1 M KOH.We discover that the microstructure,morphology,and electrocatalytic activity of NixCo3-xS4/CC are strongly related to the Ni doping ratio.The tubular array structure is not only significantly influenced by appropriate Ni-doping,but the outer wall of the nanotube consists of ultra-tiny nanoparticles,and has a greater number of exposed surface atoms,more defects,richer pores,and bigger surface areas.Notably,the Ni0.5Co2.5S4/CC catalyst sustains the microtopography of nanotube arrays and displays the lowest overpotential of72 mV at10 mA·cm-2,Tafel slope of49.44 mV·dec-1,and excellent stability over 45 h.On further enhancing the Ni doping ratio,the fine robust array of composites destroyed gradually and its electrochemical activity decreased significantly.This work provides an idea for the transition metal doping to enhance the intrinsic activity of the catalyst.5.NiSe surface single-layer Rh metal atom replacement:platinum-like hydrogen evolution catalystFirstly,a NiSe nanowire array uniformly grown on the Ni foam was prepared,and then the obtained NiSe nanowire array was soaked in an ultra-low concentration cerium salt solution(cerium chloride),the surface single-layer atom on NiSe was replaced by the noble metal Rh(NiSe-Rh/NF)due to the metal mobility differences.It was found that the NiSe-Rh/NF material obtained after the replacement of the Rh element on the surface was much improved compared with the original NiSe.