Fabrication Of Electrospun Nanofiber-based Composites And Their Electrocatalytic Properties Toward Water Splitting

Increasing global demand for energy has led to a growing consumption of fossil fuels,accompanied by environmental degradation and the energy crisis,which has prompted the search for new renewable alternatives.Hydrogen is a carbon-free and clean energy carrier with the highest energy density,which is expected to replace traditional fossil fuels.Compared with traditional hydrogen production methods,electrochemical water splitting is an environmentally friendly and effective approach to produce high purity hydrogen.Generally,water electrocatalysis contains two half reactions,namely the cathodic hydrogen evolution reaction?HER?and the anodic oxygen evolution reaction?OER?.However,the kinetics of these two half reactions is slow,thus requiring efficient electrocatalysts to reduce the overpotential of water dissociation.Although noble metal Pt and Ru/Ir-based catalysts possess the optimum HER and OER activity,respectively,the disadvantages of their high price and scarce reserves make them unsuitable for large-scale practical production.Therefore,the development of efficient and inexpensive electrocatalysts for water decomposition is the focus of current research.In recent years,electrospun nanomaterials have been widely used in the field of energy conversion and storage due to their large specific surface area,unique chemical structure,easily adjustable components,and fast electron and mass transport properties.Based on the above discussion,in this thesis,a series of efficient and cheap electrocatalysts have been designed and synthesized with electrospinning nanofiber composites as the main research object,and the catalytic activity was significantly improved by regulating the composition,morphology and electron transport characteristics of the materials.The main contents of this thesis are as follows:1.It is a promising strategy to couple highly active noble metals with transition metals as efficient electrocatalysts,which can not only effectively reduce the cost of the preparation procedure,but also greatly improve the performance of catalysts through a synergistic effect.Herein,two series of work has been developed.a)Fabrication of Ni/nitrogen-doped carbon nanofibers?NCNFs?/Pt composites for universal pH HER application.First,Ni nanoparticles embedded in NCNFs were prepared via an electrospinning and annealing treatment.The content of Ni nanoparticles in NCNFs can be controlled by tuning the content of Ni?Ac?₂·4H₂O in the precursors.Then,a chemical reduction route has been used to deposit ultrafine Pt nanoparticles on the surface of the Ni-NCNFs.The content of metal Ni has a significant effect on the electrocatalytic performance.The as-prepared Ni-NCNFs-Pt not only reduce the usage of noble metal Pt but also reveal an excellent electrochemical activity over a wide pH range.The HER catalytic activity of Ni-NCNFs-Pt is comparable to commercial Pt/C.The remarkable HER performance could be ascribed to the synergistic interactions between Ni and Pt nanoparticles and the introduction of the conductive nitrogen-doped carbon nanofibers?NCNFs?substrate which facilitated the fast electron transport.Although an efficient HER electrocatalyst has been prepared in this experiment,the single performance could not achieve the requirements of catalyzing the overall water splitting.In order to simultaneously catalyze HER and OER in the same electrolyte,we further explored the synthesis of highly efficient bifunctional electrocatalysts.b)The composite material of Ru and Ni nanoparticles supported on nitrogen-doped carbon nanofibers was prepared via simple electrospinning and carbonization processes,and the dual-function electrocatalytic water splitting performance was achieved.We have investigated the effects of the molar ratio between Ru and Ni as well as the carbonization temperature on the electrocatalytic performance.The synthesized Ru₁Ni₁-NCNFs catalyst possesses high conductivity,large electrochemical active surface area,fast charge transport and abundant active sites,thus resulting in superior activity and stability.The optimized Ru₁Ni₁-NCNFs represent excellent Pt-like electrocatalytic activity for the hydrogen evolution reaction?HER?in both alkaline and acidic conditions.Furthermore,the RuNi-NCNFs also exhibit an outstanding oxygen evolution reaction?OER?activity with an overpotential of 290 mV to achieve a current density of 10 mA cm^-22 in an alkaline electrolyte.Strikingly,owing to both the HER and OER performance,an electrolyzer with Ru₁Ni₁-NCNFs as both the anode and cathode catalysts requires only a cell voltage of1.564 V to drive a current density of 10 mA cm^-22 and exhibits good stability in an alkaline medium.2.To further reduce the cost,non-noble metal-based electrocatalysts for water splitting have become the focus of research.Among them,transition metal carbide materials not only exhibit high catalytic activity,but also possess excellent stability.Herein,we have also carried out two series of research work:a)We have designed and synthesized Mo/Mo₂C/N-CNFs electrocatalysts for universal pH HER application.A typical metal-semiconductor heterostructure with metallic Mo and Mo₂C nanoparticles encapsulated in N-CNFs has been fabricated via the pyrolysis of electrospun polyacrylonitrile?PAN?/cellulose acetate?CA?/bis?acetylacetonato?dioxomolybdenum?MoO₂?acac?₂?nanofibers.During carbonization process at a high temperature,CA containing a large number of acyl and hydroxyl groups was decomposed,resulting in pores and channels in the carbonized fibers,thereby increasing the specific surface area and facilitating the exposure of active components.Notably,both Mo₂C and Mo nanoparticles?NPs?come from the single-source MoO₂?acac?₂ precursor,so the immediate contact between Mo₂C and Mo can be realized,which is necessary to construct heterojunction electrocatalysts.The unique porous and channel rich structure of Mo/Mo₂C/N-CNFs can be manipulated by varying the mass ratio of PAN and CA,leading to the exposure of abundant active centers and the acceleration of rapid mass transport.In addition,the synergistic effect among metal and semiconductor as well as the excellent conductivity of the N-CNFs result in excellent HER activity and stability over a wide pH range.b)A bifunctional electrocatalyst based on transition metal carbide was constructed to catalyze the overall water-splitting reaction.Through electrospinning and carbonization process,metal Ni and Mo₂C nanoparticles were successfully dispersed on NCNFs.The ratio between Ni and Mo₂C metal precursors,the carbonization temperature,and the proportion of the metal precursors to polyacrylonitrile?PAN?on the electrocatalytic performance were investigated.The optimized hybrid?Ni/Mo₂C?1:2?-NCNFs?delivers low overpotentials of 143 mV for HER and 288 mV for OER in an alkaline medium at a current density of 10 mA cm^-2.An alkaline electrolyzer with Ni/Mo₂C?1:2?-NCNFs as catalysts for both anode and cathode exhibits a current density of 10 mA cm^-22 at a voltage of 1.64 V,which is comparable with the benchmark of Pt/C-RuO₂ electrodes.In addition,an outstanding long-term durability was achieved,which is superior to most of the noble-metal-free electrocatalysts reported to date.The excellent HER and OER electrocatalytic performance of Ni/Mo₂C?1:2?-NCNFs is attributed to the synergistic effect between Ni and Mo₂C nanoparticles and the large electrochemically active surface area.This work provides a facile and feasible strategy to fabricate a low-cost and high-performance bifunctional electrocatalysts for efficient electrocatalytic overall water splitting.