Preparation Of Transition Metal Phosphorus/sulfide Nanostructures And Their Electrocatalytic Water Splitting Performance

Hydrogen,as an efficient,renewable and clean energy carrier,is considered to be the most potential fossil fuel alternative.In comparison with the conventional steam methane reforming and coal gasification hydrogen production technologies,electrochemical water splitting is a promising and eco-friendly hydrogen generation means with advantages including low energy consumption,high hydrogen purity and zero carbon emission.Thereinto,electrocatalyst plays a critical role for accelerating the hydrogen evolution kinetics.Up to now,platinum group(Pt and Pd)noble metals are still the benchmark electrocatalysts for hydrogen evolution.However,the scarcity and high cost limit their further practical application for commercial hydrogen generation.Therefore,developing the high-efficiency and budget non-noble electrocatalysts is the fundamental way to realize practical application.Currently,the transition metal sulfides and phosphides have been comprehensively demonstrated to be desirable candidates toward the hydrogen generation due to their earth-abundance,distinct electronic structures,high mechanical strength and excellent electrochemical stability.In this thesis,we aimed to synthesize effective electrocatalysts for hydrogen evolution,regulated the structure of each components as the main line and prepared several types of well-defined molybdenum sulfide(MoS2),nickel phosphide(Ni2P)and cobalt phosphide(CoP)nano-powder and self-supported electrocatalysts by attempting different and novel synthetic methods.In order to further enhance the catalytic performance,the strategies like loading of the nanocrystals,modifing of the porous structures,doping of the heteroatom and fabricating of the hierarchical nanostructures were employed,which can be provide valuable perspectives for novel electrocatalysts fabrication and development.Meanwhile,we also investigated the hydrogen evolution mechanism by means of the first-principles density functional theory(DFT).The details of this thesis are as follows:(1)The MoS2 nanotubes were prepared by facile solvothermal method.After the follow-up reactions with different Co/Ce mass ratios,the CoxCe1-x2-y-MoS2 hybrid nanotubes were obtained.The study found that the optimal doping contents of Ce/Co are 30%and 70%,respectively.The Ce0.3Co0.7Ox-MoS2 hybrid nanotubes exhibited superior HER performance with a low overpotential of 187 mV vs.RHE at 10 mA cm-2 and a small Tafel slope of 70 mV dec-1.The Ce0.3Co0.7Ox-MoS2 hybrid nanotubes also possessed excellent HER stability after 24 h of durability test,the current density well-maintained at 30 mA cm-2.The excellent HER activity can be attributed to the synergistic effects between CoxCe1-xO2-y nanocrystals and MoS2 nanotubes,the large length-diameter ratio nanotubes were conductive to loading the small-sized nanocrystals and promoting the infiltration of electrolyte.The CoxCe1-xO2-y nanocrystals brought in new active sites for hybrid system and increased the conductivity of the whole electrocatalyst.(2)The NiO nanosheets were synthesized via a facile hydrothermal method.Moreover,2,5-dihydroxyterephthalic acid was chosen as the organic ligand and coordinated with Ni metal ions for the synthesis of NiO-MOF-74 using a controllable oil bath strategy.The NiO-MOF-74 was treated by phosphorization to obtain porous Ni2P nanosheets.The porous Ni2O nanosheets exhibit excellent electrocatalytic performance towards the hydrogen evolution reaction(HER)with a low overpotential of 168 mV vs.RHE at a current density of 10 mA cm-2 in 1.0 M KOH and a small Tafel slope of 63 mV dec-1.The overpotential and Tafel slope of porous Ni2P nanosheets for the oxygen evolution reaction(OER)are 340 mV vs.RHE(20 mA cm-2)and 105 mV dec-1,respectively.In addition,both the HER and OER measurements demonstrate that porous Ni2P nanosheets have superior electrochemical stability in alkaline solution.The desirable electrocatalytic properties of the porous Ni2P nanosheets may be due to their larger surface area and favorable electrical conductivity.The porous structures of the Ni2P nanosheets provide pathways for electron conduction,which facilitates electron transfer and accelerates bubble(H2 and O2)diffusion on the surface of the electrode.Besides,the alternating connections between C=C and C-C might enhance the electrical conductivity of the porous Ni2P nanosheets and accelerate the charge transfer rate during the electrochemical process.(3)The Ni nanoprism precursors were synthesized through a facile oil bath method.After regulating the pH value of reaction solution and reacting with(NH4)2MoO4,the Ni-Mo hollow nanostructures(HNs)were acquired.Subsequently,the synthesized Ni-Mo hollow nanostructures were processed by phosphorization to obtain the Mo-doped Ni2P HNs.The Mo-doped Ni2P HNs demonstrate excellent HER activity in 1.0 M KOH solution with a low cathodic overpotential of 81 mV vs.RHE(10 mA cm-2,glassy carbon)and a small Tafel slope of 53.4 mV dec-1.DFT calculations reveal the mechanism of the alkaline HER.For pristine Ni2O,the P site serves as the active center.After Mo doping,the active center transforms to a bridging Mo-Ni site and the corresponding Gibbs free energy of H*absorption(ΔGH#)decreases from 0.33 eV(P site)to 0.21 eV(bridging Mo-Ni site).The optimal active site with a smaller ΔGH*can accelerate the charge transfer process for H*intermedium and H2 production.For the OER,the Mo-doped Ni2P HNs require 270 mV vs.RHE to attain 20 mA cm-2(glassy carbon),with a small Tafel slope of 68.5 mV dec-1.Impressively,both the HER and OER catalysis exhibit durable performance in alkaline electrolyte.When the Mo-doped Ni2P HNs are applied as bifunctional electrocatalysts for alkaline overall water splitting,the demanded cell voltage is only 1.54 V(10 mA cm-2,nickel foam).Our research results provide studying base for the rational design of electrocatalyst.(4)The self-supported Co nanorods(NRs)are grew on conductive carbon cloth and directly serve as the self-sacrificing template.After the solvothermal treatment,Co NRs are converted into well-ordered Co-Mo nanotubes(NTs).Subsequently,the small-sized Fe oxyhydroxide nanorods arrays are hydrothermally grew on the surface of Co-Mo NTs to form Fe-Co-Mo hierarchical nanostructures(HNSs),which are then converted into FeP-CoMoP HNSs through a facile phosphorization treatment.FeP-CoMoP HNSs reveal super activity for hydrogen evolution reaction(HER)with an ultralow cathodic overpotential of 33 mV vs.RHE at 10 mA cm-2 and a Tafel slope of 51 mV dec-1.Moreover,FeP-CoMoP HNSs also display excellent electrochemical durability in alkaline media.DFT calculations demonstrate that the remarkable HER activitiy of FeP-CoMoP HNSs originates from the synergistic effect between FeP and CoMoP.