Fabrication Of Polymer Based TiO₂-Pt Composites And Photocatalytic Hydrogen Generation Performance

Energy is indispensable for the production and life in human society.In the 21 st century,the combined stress from energy crisis and environmental crisis has made it imperative to develop and apply new energy.Hydrogen is a kind of energy with the largest energy density so far.It is efficient and environmentally friendly in principle since its combustion will not produce any by-products or substances harmful to the environment.Therefore,hydrogen is the future development direction of energy.The existing technology of large-scale hydrogen production relies heavily on traditional non-renewable fossil energy,and the production process is accompanied by severe environmental pollution and high energy consumption.In contrast,the technology of photocatalytic water splitting hydrogen production by using semiconductor materials has made it possible to directly convert sunlight energy into chemical energy in hydrogen and store it in one step.The whole process is rapid without any by-products.The hydrogen produced by this technology is the real “renewable”energy and the “clean” energy which is pollution-free from production to use.TiO₂ is a typical semiconductor photocatalytic material with good chemical stability and easy preparation,and it is currently the most widely used semiconductor photocatalytic material.In order to allow semiconductor materials to participate in photocatalytic water splitting more efficiently,these materials often need to be modified before use.Common methods for modification include noble metal deposition,semiconductor compounding,element ion doping and organic dye sensitization.At present,most of the existing researches on photocatalytic water splitting hydrogen production using semiconductor materials focus on the modification of the semiconductor photocatalytic materials.However,the modification of the material itself is hard to solve the contradiction between the“hydrogen production efficiency of the photocatalyst” and the “energy conversion efficiency of the system” in the photocatalytic water splitting hydrogen production liquid reaction system,which greatly limits the performance of the material in the actual reaction system.This paper points out that the key to solving this contradiction is to realize the orderly arrangement of photocatalytic materials in limited space,and it is important and necessary to introduce matrix material to fix photocatalyst.By designing and preparing a series of polymer matrix-photocatalyst composite materials and comparing their performance in photocatalytic hydrogen production,this problem has been studied and discussed in depth.The main research contents and conclusions are as follows:（1）With the TiO₂-Pt photocatalytic water splitting hydrogen production liquid reaction system as an example,specific experiment was conducted to show that several influencing factors exist in the actual operation of the reaction system,and there is a contradiction between the hydrogen production efficiency of photocatalyst and the energy conversion efficiency of the reaction system.This contradiction is difficult to resolve through the design and control of the photocatalyst itself,which demonstrates the importance and necessity of introducing polymer as the matrix material.Subsequently,the polyurethane（PU）sponge with a micron-sized network structure was introduced as the matrix material,and the TiO₂ particles were loaded on the PU sponge matrix with the help of the huge surface energy of the TiO₂ particles in a highly dispersed state,so as to construct self-assembled PU/TiO₂-Pt composite network material.Compared with the photocatalytic hydrogen production reaction system using TiO₂-Pt powder material,the reaction system with the best performance of PU/TiO₂-Pt composite network material improved the hydrogen production efficiency of the photocatalyst by about 8 times,and the energy conversion efficiency of the system was also similar.However,due to the weak interaction between the PU matrix material and TiO₂,the mass fraction of the photocatalyst that can be loaded on the composite network material prepared by this method is limited,less than 5 wt%.This not only significantly restricts the performance of the composite network material in photocatalytic hydrogen production,but also leads to the poor stability of TiO₂ loaded on the composite network material.It will fall off during reuse,and only 65 % of the initial performance can be maintained after 10 cycles.（2）Graphene is a two-dimensional material with low density and large specific surface area.Owing to its good electron transport ability,it has been widely applied in composite photocatalytic materials.Graphene oxide（GO）was coated on the PU sponge to obtain the PU@GO network material,which was used as the matrix material to load TiO₂,so that the PU@GO/TiO₂-Pt composite network photocatalytic material was prepared.Compared with the PU/TiO₂-Pt composite material produced using the PU sponge without GO coated as the matrix,the PU@GO/TiO₂-Pt composite material greatly increased the loading capacity of TiO₂,exhibiting better performance in photocatalytic hydrogen production.Thus,the stability of loaded TiO₂ was also significantly improved.This is attributed to the introduction of GO,which increases the specific surface area of the matrix material and the interaction between GO and TiO₂.Compared to the system of photocatalytic hydrogen production using TiO₂-Pt powder material,the prepared reaction system with the best performance of the composite material enhanced the hydrogen production efficiency of the photocatalyst by about 10 times,and the energy conversion efficiency of the system by 6.5 times.In addition,a small part of the TiO₂ loaded on the composite material developed by this method still fell off during repeated use,but 82 % of the initial performance can be maintained after 10 cycles.This part of the work proved that increasing the specific surface area of the matrix material and strengthening the combination of TiO₂ and the matrix material are effective methods to improve the performance of composite material in photocatalytic hydrogen production.（3）Polydimethylsiloxane（PDMS）can be rapidly molded by the heating and solidification of its precursor,and the precursor with appropriate concentration has good fluidity and ductility.Based on these characteristics,the microstructure of the nickel foam was replicated,and then the metal nickel foam functioning as a molding template was removed by etching to successfully obtain a matrix material with a denser network structure.At the same time,with the help of the solvent selectivity of the PDMS precursor and TiO₂ particles,and the difference in solvent volatilization rate,the TiO₂ particles to be loaded were added into the unformed precursor mixture in advance to realize one-step integral molding of the composite material.The tests and characterizations of the prepared PDMS/TiO₂-Pt composite material showed that the biggest advantage of integral molding is that it can ensure the dispersion of TiO₂ particles,thereby guaranteeing high hydrogen production efficiency of the photocatalyst in the prepared composite material.Compared with the reaction system of photocatalytic hydrogen production using TiO₂-Pt powder material,the reaction system with the best performance of the composite material increased the hydrogen production efficiency of the photocatalyst by about 14 times,and the energy conversion efficiency of the system by 8.4 times.Besides,since the composite material are integrally formed,the TiO₂ particles and the polymer matrix are integrated,which fundamentally solves the problem that the TiO₂ particles will fall off after repeated use of the composite material,and tremendously improves the stability of loaded TiO₂.（4）In order to further improve the specific surface area of composite,a PLA/TiO₂-Pt porous composite fiber membrane with a special structure was designed and fabricated.For the first time,the PLA/TiO₂ system was used to catalyze the hydrogen production by photocatalytic water splitting,which expands the application of the material combination system.When the composite fiber membrane was integrally formed by electrospinning,the difference in the volatilization rate of different solvents was used for assistance.Meanwhile,based on the principle of the “breath figure” and “gas phase induced phase separation” in electrospinning,a nanofiber membrane with a special internal pore structure was developed.On the one hand,this structure greatly increases the specific surface area of the fiber membrane,and the large specific surface area and integral molding method together ensure the high dispersion of the TiO₂ particles in the composite fiber membrane.On the other hand,the internal pores also facilitate the full utilization of the incident light by the composite material,which lays a good structural foundation for the photocatalytic performance.The porous composite fiber membrane showed excellent hydrogen production performance and contributed to the extremely high hydrogen production efficiency of the photocatalyst and satisfying energy conversion efficiency of the system in the test.Compared to the photocatalytic hydrogen production system using TiO₂-Pt powder material,the reaction system with the best performance of the composite fiber membrane increased the hydrogen production efficiency of the photocatalyst by nearly 30 times,and the energy conversion efficiency of the system by nearly 6 times.The fiber membrane also has good recycling stability.The photocatalytic hydrogen generator assembled using the composite fiber membrane with the best performance achieves a maximum output power of 42 m W and a maximum power density of 47 W/m2,as well as a UV energy conversion rate of up to 92.8 %.Therefore,it can be meet the demand for energy supply in general scenarios.The use of the PLA matrix and the full polymer composition also make the hydrogen generator ultra-thin and flexible with degradable core parts,which significantly enhances the practicality and flexibility in the application.（5）Polyacrylonitrile（PAN）is a material with strong chemical weather resistance.Firstly,the electrospinning method was adopted to prepare PAN fiber as the matrix material for loading TiO₂.The fiber membrane fabricated by electrospinning has a large specific surface area,so that it can provide more sites for loading TiO₂.Secondly,the method of fractional steps was employed to prepare the polymer matrix material,on which TiO₂ was load.In this way,the amount of TiO₂ added will not be affected by the molding conditions to ensure a large amount of TiO₂ loaded.At the same time,the tannic acid（TA）was introduced as the second component of the fiber.Through the complexation between TA and the TiO₂ particles,the TiO₂ particles were fixed on the surface of the fiber,which not only increased the actual loading amount of TiO₂,but also promoted the stability of loaded TiO₂.TA with strong reducibility also enables the composite fiber membrane to independently catalyze hydrogen production in an environment without additional sacrificial reagent.Therefore,a PAN/TA/TiO₂-Pt composite fiber membrane material with excellent performance in photocatalytic hydrogen production was developed,which not only possesses high hydrogen production ability of the overall material system,but also has the high hydrogen production efficiency of the photocatalyst and the excellent energy conversion efficiency of the reaction system.Compared with the photocatalytic hydrogen production reaction system using TiO₂-Pt powder material,the reaction system using the composite fiber membrane with the best performance enhanced the overall hydrogen production capability by 3.5 times,the hydrogen production efficiency of the photocatalyst by 12 times,and the energy conversion efficiency of the system by 10 times.Meanwhile,the composite fiber membrane also has good recycling stability.The sweat-photocatalytic hydrogen generator assembled by the composite fiber membrane with the best performance achieves a maximum output power of 56 m W and a maximum power density of 63 W/m2,as well as a hydrogen capacity of up to 2.69 k J.Moreover,it has strong water absorption,pollution-resistance and antibacterial properties,and can be safely and non-toxically attached to human skin.