| Nowadays,the increasingly serious energy crisis and environmental problems force people to explore clean and renewable energy to replace traditional fossil energy.Hydrogen has high energy density and its combustion product is water implying the nearly zero discharge.Therefore,hydrogen is a new clean and renewable energy with great potential.Photoelectrochemical water splitting cells can convert the largest and renewable solar energy in the world into hydrogen energy that can be stored.The key of a photoelectrochemical water splitting cell with a high energy conversion efficiency is developing high performance semiconductor photoelectrode materials(including photoanode and photocathode materials).An ideal photoelectrode material should not only have an appropriate band gap,but also have strong resistance to chemical corrosion and photocorrosion.In present reports,most metal-oxide semiconductor photoelectrodes are not only inexpensive,but also have strong visible light response.They have relatively good chemical stabilities and photo-stabilities,and their performances can hardly reduce when exposure to air.The above advantages make metal-oxide semiconductor photoelectrodes expected to take the lead in achieving large-scale applications.However,the solar conversion efficiency of the metal-oxide photoelectrodes is still far from satisfactory,which is also a common problem for most other types of photoelectrodes.The recombination of photo-generated carriers in semiconductor and the water splitting reaction barrier at the semiconductor-electrolyte interface are two main factors limiting their performance.Therefore,the main way for improving the performance of photoelectrodes is to inhibit carriers recombination,promote the separation of carriers and enhance the surface water reaction of the photoelectrodes(oxidation reaction for photoanodes and reduction reaction for photocathodes).In this paper,BiVO4 and CuBi2O4 were selected as the research objects.Different strategies were developed to deal with the problems of serious photogenerated electron hole recombination and slow surface water oxidation in these photoelectrodes.The charge transport of BiVO4 photoelectrodes have been promoted by combining with conductive carbon materials,ion doping and electrocatalysts.With the highly efficient composite electrocatalysts,the bulk charge separation of BiVO4 is promoted,and the rate of water oxidation is improved simultaneously.The photoelectrochemical performances of CuBi2O4 photoelectrodes are enhanced by surface passivation.Nanoporous CuBi2O4 photoelectrodes are synthesized and their photoelectrochemical performances are enhanced by surface passivation.The specific contents and conclusions are as follows:Reduced graphene oxide promoting charge separation and transport in BiVO4.BiVO4 is a potential photoanode material for Photoelectrochemical water splitting.The bandgap of BiVO4 is 2.4-2.6 eV,which is a kind of visible light response material.BiV04 is a direct bandgap semiconductor.Its conduction band electrons can directly transfer to the valence band after the excitation of light,corresponding to the high efficient utilization of light.In addition,BiVO4 has the lowest open potential in all photoanode materials.However,the combination of electrons and holes in BiVO4 is serious,which restricts its photoelectrochemical properties.Increasing the transport efficiency of photo-generated electrons or holes can avoid or reduce the combination.The reduced graphene oxide has excellent electrical conductivity.It is not only stable in structure but also has good chemical and electrochemical stability.In this paper,the charge transport of BiVO4 thin films was improved by RGO with excellent conductivity.Benifiting from the improved charge transport,the photocurrent of BiVO4 was enhanced in two different electrolytes.The charge separation efficiency of BiVO4 was increased by 32.2%,and its photoelectrochemical stability was also enhanced.Mo6+ ion doping enhancing the bulk transport of BiVO4,and Ag-Bi electrocatalyst accelerating the surface water oxidation of BiVO4.The process of photoelectrochemical water splitting by semiconductor photoanode is divided into three parts:Electrons-holes pairs are produced by semiconductor absorption photons;the photo-generated electrons-holes are separated in the electric field in the space layer;oxidation of water by photo-generated holes on semiconductor surfaces.The performance of a photoanode is mainly determined by the above three parts.If the bandgap of a semiconductor is constant,the factors affecting the performance are the latter two aspects.The charge transport efficiency of photo-generated electrons and holes in BiVO4 thin films is low,so the bulk phase recombination is rather serious.Besides,the surface water oxidation rate of BiVO4 is very slow in dynamics,resulting in low surface charge injection efficiency.Therefore,for the BiVO4 photoanode,we should avoid or reduce the recombination of its photo-generated carriers and accelerate its surface water oxidation reaction.In order to reduce the bulk carrier recombination of BiVO4,we have improved the electrical conductivity of BiVO4 by Mo6+ ion doping,thus improving the bulk charge separation efficiency.Then,we enhanced the surface water oxidation efficiency through the new Ag-Bi electrocatalyst,and further enhancing the photoelectrochemical performances of BiVO4.Composite AgOx/NiOx electrocatalyst simultaneously promoting charge separation efficiency and surface charge injection efficiency.There are two views on the role of oxygen evolution electrocatalysts in photoelectrochemical water splitting.One view is that the surface water oxidation reaction can be accelerated by electrocatalysts.Another view is that the band bending of the semiconductor increases,and its bulk charge separation strengthens,due to the combination of the semiconductor and electrocatalyst.The experimental results show that the composite AgOx/NiOx electrocatalyst can not only promote the surface water oxidation of BiVO4,improving the surface charge injection efficiency,but also increase the band bending of BiVO4,improving the charge separation efficiency.In other words,composite AgOx/NiOx electrocatalyst have both kinetic and thermodynamic functions in photoelectrochemical water splitting.The results of the photo-assisted electrochemical impedance spectroscopy confirmed the dynamic function of the composite AgOx/NiOx electrocatalyst,corresponding to the improved charge injection efficiency of BiVO4.The photo-assisted Kelvin probe microscope technique and the open circuit voltage method demonstrated the thermodynamic function of composite AgOx/NiOx electrocatalyst,corresponding to the enhanced bulk charge separation efficiency of BiV04.Nanoporous structure promoting bulk charge transport in CuBi2O4,and surface states of CuBi2O4 are passivated by MgO.Nanoporous structures can shorten the distance of photogenerated carriers transported from the bulk to the surface,and facilitate the adequated contact between semiconductors and electrolytes.Therefore,if the film thickness is the same,the nanoporous photoelectrode has better bulk charge transport compared with the dense flat photoelectrode.In this paper,a nanoporous CuBi2O4 photoelectrode was fabricated by metal-organic droplet coating.Besides bulk charge transport,semiconductors are also affected by surface charge transfer.Before semiconductors participate in the water spitting reaction,the surface of the semiconductor will accumulate a certain amount of photogenerated carrier.If a semiconductor has too much surface states,the photo-generated carriers will be trapped before the water splitting reaction and a serious recombination will occur.As a type of ternary compound,CuBi2O4 is very easy to form surface states during the synthesis process,which will restrict its photoelectrochemical performances.When we prepared CuBi2O4,we added passivation material MgO to prepare the composite photoelectrode MgO-CuBi2O4.Experimental results demonstrated that a proper amount of MgO reduced the recombination of photocarriers in the surface states and increased the surface charge injection efficiency of CuBi2O4.MgO was able to suppress the CuBi2O4's Fermi level pinning and increase the range of its space charge layer.Therefore,the passivation effect of MgO on the surface states of CuBi2O4 greatly improved its photoelectrochemical performances. |
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