| Resource shortages and environmental pollution are still two major problems that restrict the development of current society.Thus,people are actively looking for an ideal green energy to replace non-renewable fossil energy,such as coal,oil and natural gas.Hydrogen energy,with high energy value,green environmental protection and no secondary pollution during combustion,has become one of the best candidates as a new generation of clean energy.As we all know,photoelectrochemical(PEC)water splitting technology is the most ideal method to obtain hydrogen energy in the future.The PEC catalysts can absorb solar energy to generate photo-generated carriers,and the separated photo-generated electrons can reduce water to release hydrogen on the electrode surface.This technology also has the following advantages:solar and water are abundant renewable resources;no by-products and secondary pollution;hydrogen and oxygen released separately via photocathode and photoanode are easy collection;The technology has the potential for large-scale production;Theoretical solar conversion efficiency can reach about 30%.Therefore,PEC water splitting technology can achieve the perfect conversion of solar energy to hydrogen energy and meet the increasing energy demand of modern society.It is considered one of the most promising ways to solve the global energy shortage and environmental pollution problems.However,the PEC conversion efficiency of the photoelectrode is still far from the theoretical efficiency and the preparation cost is high at present,which cannot meet the application requirements.Therefore,further enhancing the PEC performance to improve the solar-to-hydrogen conversion efficiency(STH)is the key to promoting the large-scale application of photoelectrochemical water splitting technologyThe purpose of this doctoral thesis is to design and prepare efficient photoelectrodes and apply them to the field of PEC water splitting.Focus from expand the spectral response range(?abs)of semiconductor photoelectrodes,improve the efficiency of photo-generated carrier bulk separation(?sep),and interfacial injection efficiency(?inj),to solve the problems of low conversion efficiency of photoelectrodes and difficulties in practical application.PEC performance of semiconductor materials is effectively improved from four perspectives:i)Form solid solution via doping and thus change the band position and forbidden band of the semiconductor electrode and improve the efficiency of photo-generated carrier separation;ii)Prepare new efficient photoelectrode by developing and exploring visible light semiconductors with high crystallinity;iii)Reduce the internal recombination of photo-generated carriers and improve the efficiency of photo-generated carrier separation through morphological control;iv)Improve interfacial injection efficiency of photogenerated carriers through surface modification of cocatalyst.In this thesis,the details are as follows:In chapter one,the development process,basic principles and influencing factors of PEC water splitting technology are briefly introduced;Then introduced the research status of photoelectrodes in PEC cells,including meeting the conditions,preparation methods,basic properties,advantages and disadvantages;Finally,we describe in detail the research progress by researchers in recent years and problems in PEC water splitting,as well as effective strategies summarized to solve the existing problems,thus which leads to the significance of this topic and research content.In chapter two,we innovatively synthesized(Cui-xAgx)2ZnSnS4 solid solution with controllable different Ag doping concentrations by simple hydrothermal and post-calcination methods.Experimental characterization combined with theoretical analysis showed that the doped silver atoms can replace copper atoms and cause CAZTS lattice distortion,which optimized the band gap and energy band position of the CAZTS semiconductor,and thus significantly enhanced its PEC hydrogen evolution performance.Subsequently,the effects of different Ag doping concentrations on the photoelectrocatalytic performance and conductivity type of the CAZTS photoelectrode were explored via the photocurrent density-applied potential(J-V)tests and Mott-Schottky tests.The change of band structure and intrinsic defect in CAZTS semiconductor was the essential reason for the change of PEC performance and conduction type.The above results showed that CuAgZnSnS4(x=0.5)and Ag2ZnSnS4(x=1.0)had great application potential as high-efficiency new photocathode and new photoanode,respectively.In chapter three,we innovatively prepared a new pure-phase AZTS photoanode on a molybdenum mesh substrate by a simple electrodeposition method.In the PEC performance test,the AZTS photoanode exhibited excellent activity:the unmodified new AZTS photoanode with a lower starting potential and a higher photocurrent density;the highest incident photon-to-current efficiency IPCE=25%and the highest applied bias photon-to-current efficiency ABPE=2.95%in the water splitting reaction.In addition,it was confirmed that the use of atomic layer deposition(ALD)technology to deposit amorphous TiO2 on the surface of AZTS can effectively suppress photo-corrosion and thereby improve the stability of photoelectrocatalysis.The PEC cell showed a good hydrogen evolution rate and Faraday efficiency.The experimental characterization,theoretical calculations,and test results demonstrated that the excellent visible light absorption(-2.05 eV)and matched band position,the superiority of the molybdenum mesh substrate,the high crystallinity,and the presence of sacrificial agents and interfacial electric fields all played a key role in improving PEC performance on new AZTS photoelectrode.In chapter four,we proposed an electrochemical synthesis process similar to the printing method,that was immersing the FTO substrate in the electrolyte at a constant rate,then quickly depositing the precursor film,to creatively prepared a large-sized uniform BiVO4 photoanode.Firstly,the method preferentially generated a unique nanoporous morphology,which accelerated the transport of holes from the inside to the surface and greatly promoted the separation efficiency of internal carriers during the photoelectrocatalytic process,thereby enhancing the photoelectric response of pure BiVO4.Subsequently,the oxygen evolution co-catalyst NiOOH was supported on the surface of the film to improve the charge transfer ability and hole separation efficiency at the interface,thus further improving the PEC performance.It showed that the initial potential was significantly reduced;IPCE and ABPE were improved several times;coupled with the excellent stability and the Faraday efficiency of nearly 100%.In general,the method improved the practical feasibility of the electrode by optimizing the synthesis process and post-processing conditions,and it was confirmed that the large-sized nano-BiVO4 photoanode prepared by this method had a very broad application prospect.In chapter five,we first designed a tetragonal zircon phase BiVO4 nanocrystalline photocathode and a monoclinic scheelite phase BiVO4 nanoporous photoanode by simple hydrothermal method and optimized electrodeposition method based on the previous work.A series of explorations were subsequently carried out to improve the PEC performance of the electrode in multiple ways.For example,the high-exposure(200)crystal plane engineering of the photocathode and the surface modification of the hydrogen evolution co-catalyst Pt,the adjustment of the nano-network morphology of the photoanode and the surface modification of the Co-Pi oxygen evolution co-catalyst.Photocurrent density,IPCE,HC-STH,and ABPE and other important indicators increased several times.Experimental characterizations and calculation results confirmed that the morphology control and the co-catalyst modification synergistically promoted the internal carrier separation efficiency and interface carrier injection efficiency,accordingly enhanced PEC performance.Finally,we successfully built a tandem PEC cell of P-BVO/Pt photocathode and N-BVO/Co-Pi photoanode based on the performance test of two electrodes and the study of the mechanism of water splitting,and realized unbiased solar water splitting.The solar energy conversion efficiency of this device is STH=0.14%.In chapter six,it summarizes the main research contents and innovations of this paper,then analyzes and puts forward some problems and deficiencies in the current work,and finally makes a prospect for the next stage in response to these problems. |
PDF Full Text Request