Preparation of MoS2 Composites and Their Applications in Independent Hydrogen Generation System

Author:Li Yin Chang

Supervisor:li zhen liu xue qin


Degree Year:2019





Hydrogen energy will undoubtedly become the protagonist of future energy because of its environmental friendliness,cyclic utilization and so on.Hydrogen is mainly obtained by water splitting.At present,electrocatalytic decomposition of water to generate hydrogen is the most mature way to produce hydrogen.However,the electrocatalytic decomposition of water for hydrogen production has some problems,such as complicated electric power device,poor energy mobility and consumption of secondary energy.These problems can be solved by combining photovoltaic devices with water splitting system to build an independent hydrogen generation system.The voltage generated by the photovoltaic device is used to drive the hydrogen generation.It can not only get rid of the dependence on external power supply,but also solve the problem of power transmission generated by photovoltaic devices.At the same time,it is also convenient to build a simple airtight system to collect hydrogen.The combination between photovoltaic device and electrolyzer is an interesting research direction to solve the current energy problems.The independent hydrogen generation system consists of three key parts:the preparation of electrocatalysts with good stability and low overpotential;the assembly of photovoltaic devices with simple structure and excellent performance;the combination of photovoltaic devices and water splitting system.This paper developed series experiments around the construction of independent hydrogen generation system.To prepare the hydrogen evolution electrocatalysts with excellent performance,the size of molybdenum disulfide(MoS2)was decreased to increase its catalytic activity sites;and the conductivity of MoS2 was improved by combining with graphene.Dye-sensitized solar cell(DSSCs)with high performance was obtained by using three-dimensional network poly(3,4 ethylenedioxothiophene)and graphene composites(3D-PEDOT/rGO)as counter electrodes.The DSSCs based on 3D-PEDOT/rGO counter electrodes are combined with the water splitting system based on the MoS2 composite materials,to build the independent hydrogen production system and study its hydrogen production performance.The main results achieved are as follows:(1)Controlling the particle size of MoS2 to increases its active sites for hydrogen evolution.Globular flower 2H-MoS2 was prepared by hydrothermal method,the particle size of the globular flower MoS2 was 12μm.The effects of hydrothermal temperature,hydrothermal time and molybdenum concentration on the electrocatalytic performance of globular flower micro-MoS2 were discussed.The onset overpotential of globular flower micro-MoS2 is 265 mV,which makes globular flower micro-MoS2 a promising electrocatalyst material for hydrogen evolution.Polyvinylpyrrolidone(PVP)was used as surfactant to induce the nucleation and growth of MoS2,to improve the dispersibility of globular flower MoS2 and inhibit the growth of globular flower MoS2.The nanometer size globular flower MoS2(nano-MoS2)with uniform particle size,good dispersion was prepared.The influence of PVP addition amounts on the particle size and electrocatalytic performance of globular flower MoS2 was discussed.When the addition amounts of PVP was 150 mg mL-1,the globular flower MoS2 with particle size of about 100 nm was prepared.The globular flower nano-MoS2 with good dispersibility exposed more active sites for hydrogen evolution,and its onset overpotential is as low as 154 mV,which is lower than that of globular flower micro-MoS2.Using PVP to decrease of the particle size of globular flower MoS2,could increase its edge concentrations and the catalytic activity sites.Thus,leading to an improved electrocatalytic performance.(2)Embedding MoS2 into graphene layers to improve the electrocatalytic performance.MoS2 confined in graphene layers(MoS2@G)was prepared by a simple one-step hydrothermal synthesis method.The influence of graphene oxide(GO)addition amounts on the electrocatalytic performance of the composites was discussed.It was found that when the GO content was 50 wt%,the MoS2@G composites showed the best electrocatalytic performance.The unique structure of MoS2@G composite not only increased the number of active sites,but also improved the charge transfer performance of the composite,making the onset overpotential of 31 mV and the taffel slope of 45 mV dec-1.Besides,the MoS2@G composites have excellent electrochemical stability,which could be operated for a long time to split the water.Relying on the low overpotential and excellent electrochemical stability,MoS2@G composites are comparable to commercial Pt/C electrocatalysts.The high reserves and low cost of MoS2 make it unlimited potential in the large-scale application of water splitting.(3)Graphene combined 3D-PEDOT to accelerate electron transport and enhance photoelectrochemical properties.Three-dimensional network PEDOT was polymerized on fluorine-doped SnO2 transparent conductive glass(FTO)substrate by two-step voltage polymerization method.3D-PEDOT presents a film-like structure stacked by spherical particles which was formed by self-winding nanowire.The inherent pores in the microspheres and the voids between the microspheres endow 3D-PEDOT with a large number of electrocatalytic active sites,which can effectively reduce I3-.The effects of polymerization time,polymerization voltage and 3,4-ethylenedioxythiophene(EDOT)monomer concentration on the photoelectrochemical properties of 3D-PEDOT were studied.The power conversion efficiency(η)of DSSCs assembled with3D-PEDOT is 6.40%.Reduced graphene oxide(rGO)was coated on the surface of3D-PEDOT by spin coating to improve its conductivity,the addition of rGO reduces the charge transfer resistance between 3D-PEDOT and electrolyte interface,and improves the electron transmission rate.The 3D-PEDOT/rGO has better properties than that of Pt.Theηof 3D-PEDOT/rGO-based DSSCs is 7.12%,which is higher than that of Pt-based DSSCs(6.49%).This study provides a new idea for the preparation of PEDOT and the acceleration of electron transmission on PEDOT.(4)Constructing and optimizing the independent hydrogen production system for splitting water to produce hydrogen.Epoxy resin was used to encapsulate DSSCs to improve its stability.It was found that the DSSCs encapsulated with epoxy resin had better stability and could maintain the original open circuit voltage of 97.73%after 3 h irradiation with simulated sunlight.Through parallel mode series connection,the output voltage of the tandem DSSCs is improved.The results show that 4 tandem DSSCs can provide a voltage obout 2.83 V,which also has good stability.An independent hydrogen generation system was constructed by combining the tandem DSSCs with the MoS2electrocatalytic system,the voltage generated by the tandem DSSCs could drive the water splitting.The hydrogen evolution rate was measured by an automatic photocatalytic system.The results show that the independent hydrogen production system based on MoS2@G hydrogen-evolution electrode and Pt oxygen-evolution electrode achieved a solar to hydrogen conversion efficiency(ηSTH)of 6.28%in acidic environment,and the hydrogen evolution rate is 187.4μmol cm-2 h-1.The independent hydrogen production system achieved aηSTH of 7.30%in alkaline environment by using Pt and CuO@Co3O4 as hydrogen and oxygen-evolution electrode respectively,and the hydrogen evolution rate is 221.8μmol cm-2 h-1.Among them,the high activity and more catalytic active sites of CuO@Co3O4 make it show better oxygen evolution electrocatalytic performance than Pt in alkaline environment.Insteading Pt by MoS2@G as hydrogen-evolution electrode,the independent hydrogen production system presents aηSTH of 4.53%,and its hydrogen evolution rate is 137.6μmol cm-2 h-1.The independent hydrogen production system can directly split water into hydrogen under the irradiation of sunlight,which could provide a new idea for hydrogen production in the future.