Exploration On Novel Strategies To Enhance Photoelectrochemical Water Splitting Activity

Energy is the driving force to promote the progress of human society and the development of science and technology.For a long time,human society has formed an energy structure dominated by fossil energy such as coal,oil and natural gas.In recent years,with the continuous consumption of fossil energy,on the one hand,the fossil energy reserves are limited,and some fossil energy is about to be exhausted.On the other hand,the large consumption of fossil energy causes a large number of emissions of carbon dioxide and other "greenhouse gases",which seriously damages the earth's ecological environment.Therefore,in order to alleviate the current global energy shortage and ecological environment deterioration,and continue to maintain and promote the continuous progress of human society and the innovation and development of science and technology,exploring and finding new clean renewable energy has become one of the research focuses and hot spots of global scientists.As one of the new energy sources with great potential in the future,hydrogen energy has the characteristics of high energy density,clean and pollution-free,easy to connect with traditional energy and technology,and is expected to replace traditional fossil energy in the near future.At present,various technologies based on hydrogen energy utilization have been widely concerned and continuously improved.However,the hydrogen energy generation is still mainly dependent on the cracking of fossil energy or the production of electrolytic water.These methods still can not completely avoid the consumption of fossil energy and the emission of "greenhouse gases".Therefore,it is very vital to explore and develop new technologies for efficient hydrogen production based on new energy utilization.As another important form of new energy,solar energy has the characteristics of abundant reserves,wide sources,green and clean.In recent years,with the development of photocatalysis,photoelectrochemistry(PEC)and other technologies,it is possible to use solar energy to split water to produce hydrogen.This makes photocatalysis,PEC water splitting and other technologies based on solar energy expected to become one of the potential solutions for hydrogen production in the future.Compared with photo catalytic water splitting to produce hydrogen,PEC water splitting to produce hydrogen has higher solar energy conversion efficiency,and the hydrogen and oxygen produced are easier to separate,which is more suitable for large-scale industrial production.It is considered to be one of the most promising new hydrogen production technologies.Although after years of research,PEC water splitting technology has been rapid development,but the solar-to-hydrogen(STH)efficiency is still far from the requirements of large-scale application.Therefore,how to further improve the PEC water splitting efficiency,exploring and preparing efficient PEC electrode is a research hotspot in this field.In order to further improve the PEC water splitting efficiency,we choose the photoanode as the main research object,which is the bottleneck of improving the PEC water splitting efficiency.Based on the traditional strategies of enhancing the photoanode activity,and drawing lessons from the basic concepts of supercapacitor and Schottky junction solar cell,two new strategies were proposed to improve the PEC photoanode water splitting activity from the perspective of thermodynamics and kinetics of PEC water splitting,i.e."efficient composite photoanode based on photogenerated electrons modulation" and "Schottky-PEC series photoanode ".The universality and effectiveness of these two strategies to improve PEC photoanode water splitting activity are verified by experiments,which provides a new route to the explorate and design new high efficiency PEC water splitting photoanode.The specific experimental design and research contents are as follows:In the first chapter,firstly,the importance of energy for the human society development and the problems of energy shortage and environmental pollution are summarized.Then,during renewable new energy development,hydrogen energy has become the focus because of its many advantages.Therefore,to seek efficient and cheap hydrogen production methods has become the research focus.Among many hydrogen production methods,PEC water splitting to produce hydrogen is outstanding because of its unique advantages.Then the research basis of PEC water splitting technology is introduced,including the basic principle,basic device,performance evaluation parameters,influencing factors and the development status of PEC water splitting technology.Then,the key photoelectrode materials of PEC water splitting technology are introduced.Firstly,the conditions that need to be satisfied as photoelectrode materials need to be clear.Secondly,the photocathode and photoanode material systems widely studied at present are introduced.It is pointed out that the research on photo anode materials of PEC water splitting is the key to improve the PEC efficiency.Then,the current modification strategies of photoanode are introduced,and the problems and challenges of the photoanode materials are pointed out.Finally,on the basis of these research background,we put forward our own unique views and research ideas,which leads to the significance and research content of this paperIn the second chapter,the design idea of using supercapacitor to modulate photogenerated electrons to promote the reaction kinetics of photoanode water oxidation is proposed for the first time.During the experimental verification,four kinds of carbon materials were selected as electron storage materials to composite with TiO2 photoanode.Through comparative study,it is found that carbon spheres with rich OH groups on the surface could form good contact with photoanode materials.Then,the PEC related properties of carbon sphere composite TiO2(C-TiO2)photoanode were studied.The experimental results show that C-TiO2 photoanode has higher PEC water splitting activity and higher specific capacitance than pure TiO2 photoanode.At the bias of 1 VRHE,the injection efficiency of C-TiO2 photoanode is increased from 35.3%to 71.4%,achieving a 2.5-fold increase,which effectively improve the water oxidation reaction rate of TiO2 photoanode.Therefore,the feasibility of using the modulation effect of carbon spheres on photogenerated electrons to improve the PEC water oxidation kinetics is verified.In the third chapter,in order to verify the universality of carbon sphere modulated photo generated electron to enhance PEC photoanode water oxidation reaction kinetics strategy and further clarify its reaction mechanism,based on the research in Chapter 2,C-BiVO4(C-BVO)composite photoanode composed of BiVO4 and carbon spheres was designed and prepared.PEC performance tests show that at the bias of 1 VRHE the photocurrent density of C-BVO composite photo anode increases by 6.5 times compared with pure BVO photoanode.At 0.8 VRHE bias,the injection efficiency increases from 18%to 81%,which verifies the effectiveness and universality of the strategy.The photocurrent density of C-BVO composite photoanode increases with the increase of carbon sphere content in the composite photoanode within a certain range of carbon sphere concentration,which indicates that the modulation ability of carbon sphere to photogenerated electrons plays an important role in the PEC water spliting performance.The mechanism of PEC water oxidation reaction kinetics of carbon sphere enhanced BVO photoanode was studied.It is revealed that the storage and release of photogenerated electrons in C-BVO composite photoanode promoted the effective separation of photogenerated electron-hole pairs and greatly improved its water oxidation reaction kinetics,which provided a new universal strategy for the design and preparation of efficient PEC photoanode.In the fourth chapter,Schottky-PEC series photoanode is innovatively proposed and constructed to improve the STH efficiency of PEC water spltting.Firstly,the single crystal photoanode with less surface defects was used to construct an effective Schottky junction,so the dark-red ZnSiP2 single crystal was grown as the photoanode.Then schottky-ZnSiP2 series photoanode is successfully constructed by adjusting the semiconductor-metal contact interface.I-V measurements show that the Schottky junction at the interface of ZnSiP2 single crystal has a 0.64 V photovoltage.Therefore,when it is connected in series with ZnSiP2 photoanode,which can directly provide thermodynamic drive for PEC water splitting reaction,thus reducing the thermodynamic onset potential of water oxidation reaction,and even realizing the single electrode water splitting without bias and the the maximum applied bias photon-current efficiency(ABPE)of 2.6%,which is in the forefront of current research results This indicates that the simple and integrated Schottky-PEC series photoanode designed and constructed can eliminate the contradiction between the photoanode light absorption and the reaction overpotential of water splitting,and greatly improve the STH efficiency of PEC water splitting.So far,this work provides a new method to prepare efficient and unbiased PEC photoanode water splitting.In the fifth chapter,based on the research results in Chapter 4 that Schottky-PEC series photoanode can significantly improve the PEC water splitting efficiency,the universality of this method is verified,and the influence factors of semiconductor-metal interface contact characteristics are preliminarily studied.First of all,GaN single crystal which is less difficult to process is selected as the photoanode,and Schottky-Gan series photoanode is successfully constructed by adjusting the contact interface.I-V characteristic curve shows that Schottky-GaN photoanode can produce 0.26 V photovoltage.Compared with GaN photoanode with ohmic contact,Schottky-GaN photoanode has higher PEC water splitting activity and lower onset potential of water oxidation reaction.Therefore,it is verified that Schottky-PEC series photoanode can enhance the PEC waterspltting efficiency by reducing the reaction thermodynamics onset potential.At the same time,the influence factors of semiconductor-metal interface contact properties are also explored.Different contact metals,annealing treatment and doping will have important influence on the interface contact propertiesIn sixth chapter,the paper summarizes the research ideas,research contents and innovation points,then analyzes and summarizes some problems,deficiencies and inspiration in the current research work,and puts forward the next research plan for these problems and deficiencies,and looks forward to the next stage of research direction from the development.