Research On The Controllable Preparation And Electrochemical Performance Of Molybdenum-based Electrode Materials

The exploitation and utilization of the state-of-the-art energy storage and conversion technologies depends on the development of the earth-abundant electrode materials with low cost,improved activity and durability.Currently,the best commercial electrode materials for both hydrogen reduction reaction(HER)in water electrolysis and supercapacitors are still strongly relying on Pt group noble metals.The resource scarcity and high cost of precious metals seriously impede the practical promotion and applications of these two technologies.Due to their similar electronic properties with Pt-group,earth-abundant and low-cost molybdenum(Mo)-based materials show electrocatalytic hydrogen evolution.Besides,possessing various chemical value states and high theoretic specific capacitance,allow for the wide use of the Mo-based electrode materials widely used in supercapacitors.Though large number of Mo-based electrode materials have been reported,their catalytic activity and specialfic capacitance still need to be further improved.Therefore,the development of facile and easily scalable ways for producing low-cost,high-performance of non-noble Mo-based electrode materials for HER and supercapacitors,and the exploration of the relationships between the performance and the materials' structure,composition and morphologies are becoming an important topic but still challenging in the scopes.Motivated by this challenge,this dissertation mainly focused on the cost-effective noble metal-free Mo-based electrode materials with high activity and durability for HER and supercapacitors,and carried out the following four projects.Firstly,a facile strategy was developed to produce Mo2C nanocrystals embedded in carbon network(Mo2C@C)by the direct pyrolysis of the mixture of ammonium molybdate and polyvinylpyrrolidone(PVP).The strong interaction of PVP with molybdate led to the uniform distribution of Mo source in the mixture,and thus the good dispersion of the formed Mo2C nanocrystals in the porous carbon network after the pyrolysis.The interconnected porous carbon network derived from the decomposition of PVP not only effectively prevented the aggregation of the in-situ formed Mo2C nanocrystals during the pyrolysis,but also facilitated the mass and electron transport in HER process.Benefiting from these features,the as-prepared Mo2C@C exhibited superior electrocatalytic activity and a good long-term durability for HER in alkaline electrolyte.Secondly,an effective method was developed for the synthesis well-defined of pomegranate-like Mo2C@C nanospheres by using phosphomolybdic acid(PMo12)to initiate the polymerization of polypyrrole(PPy)and as a single source for Mo and P to produce N,P-doped Mo2C nanocrystals.The existence of PMO12 at molecular scale in the polymer network allows the formation of pomegranate-like MO2C@C nanospheres with porous carbon shell as peel and Mo2C nanocrystals well-dispersed in the N-doped carbon matrix as seeds.This nanostructure provides several favorable features for hydrogen evolution application:1)conductive carbon shell and matrix effectively prevents the aggregation of Mo2C nanocrystals and facilitates the electron transportation,2)the uniform N,P-doping in carbon shell/matrix and plenty of Mo2C nanocrystals provide abundant catalytically highly active sites.After the systematical investigation on the influences of annealing temperature,mass loading,pomegranate-like structure,carbon shell and doping effect of the as-prepared materials for HER performance,the best results in terms of an ultralow overpotential of 47 mV at 10 mA cm-2,exhibitsed excellent catalytic activity for HER and superior stability.Thirdly,a self-templated strategy was developed to fabricated binder-free efficient MoNi4/MoO3_x nanorod arrays with 3D open structure using Ni foam as both scaffold and Ni source to form NiMoO4 precursor,followed by the reduction annealing.It was discovered that the in-situ conversion NiMoO4 into MoNi4 nanocrystals and MoO3-x as dual active components dramatically boosted its hydrogen evolution activity,comparable to the benchmark Pt/C catalyst and outperformed the reported non-precious-metal Ni-based or Mo-based HER electrocatalysts.Together with the results from the control experiment and the annealing atmosphere,it could be reasonably ascribed the high activity for MoNi4/MoO3-x is due to the presence of low-valence states of Mo and Ni.Last,a PMo12 mediated one-pot redox relay strategy was developed for the in-situ synthesis of novel PPy-PMo12/rGO ternary hybrid materials(TNHs).The PPy-PMo12 nanoparticles were uniformly anchored on the rGO surfaces,which can effectively alleviate the re-stacking of rGO and thus preserved sufficient charge transport channel.Benefiting from these unique features,the developed TNHs exhibited significantly enhanced performance for supercapacitor in 0.5 M H2SO4 and superior to pure rGO and PPy/rGO.The TNHs were further used to fabricate flexible,all-solid-state supercapacitors(PPG-SSCs),which exhibiting excellent flexibility and mechanical stability,good rate capability and cycling performance.