Studies On The Preparation And Properties Of Photoanode Materials Toward Photoelectrochemical Water Splitting

Solar hydrogen production by photoelectrochemical?PEC?water splitting technology is one of the most promising methods to solve the current worldwide environmental crisis and energy shortage.The current biggest challenge for this technology is to accelerate the sluggish water oxidation kinetics involving proton-coupled four-electron-transfer multi-step reactions.To solve this problem,people improve the photo-generated charge charge separation and transfer efficiency of semiconductor photoanodes by modification of photoanode with oxygen evolution catalysts,which effectively accelerates the water oxidation kinetics.However,there are still some problems to be solved in this strategy.Integration of oxygen evolution catalysts?OECs?with light-absorbing semiconductor photoanodes can enhance the separation and injection efficiency of photogenerated charge,thus effectively accelerating water oxidation kinetics,but there are still some problems that need to be solved.In addition,the development of new semiconductor photoelectrode materials is another important way to achieve efficient photoelectrocatalytic water splitting.This article aims at the above problems,through the composite electrode construction,defect engineering,new electrode material development and other ways to improve the photoanode material photoelectric conversion efficiency,the main research contents and results are as follows:?1?A ternary photoanode system was designed to enhance PEC efficiency of photoelectrodes through introducing carbon dots?CDs?,NiFe-layered double hydroxide?NiFe-LDH?nanosheets on BiVO₄ particles.Systematic research shows that NiFe-LDH serves as an OEC which accelerates oxygen evolution kinetics,while the introduction of CDs can further reduce charge transfer resistance and overpotential for oxygen evolution.Under the synergistic effect of NiFe-LDH and CDs,the photocurrent and incident photon to current conversion efficiency?IPCE?of the resulting CDs/NiFe-LDH/BiVO₄photoanode is improved significantly than those of the NiFe-LDH/BiVO₄ or CDs/BiVO₄electrode.?2?DFT calculations show that the introduction of oxygen vacancies(O_vac)can significantly reduce the H₂O adsorption energy and oxygen evolution energy barriers of nitrogen-doped TiO₂?N-TiO₂?thin films,which contributes to the reduction of oxygen evolution overpotentials,leading to a dramatically increase in oxygen evolution reaction?OER?efficiency.Based on theoretical calculations,we successfully prepared a N-TiO₂containing oxygen vacancies?TiO₂-N₂?thin-film photoanode,which showed a high charge injection efficiency of 94.6%at 1.23 V_RHE in the absence of any oxygen evolution catalyst.More interestingly,the TiO₂-N₂ electrode exhibits excellent stability with negligible photocurrent decay?2%?after 4 hours of PEC reaction at 1.23 V_RHE under AM 1.5G illumination.Systematic studies reveal that the O vacancies in the N-TiO₂ play an important role in increasing the electrochemically active surface area and the reactivity of active sites along with reduction of anodic overpotential for oxygen evolution.Consequently,the method provides an alternative route for efficient and stable PEC water splitting.?3?We designed and developed a two-step vapor deposition?TVD?process of depositing graphitic carbon nitride?g-CN?films for photoelectrochemical?PEC?application.The method is versatile for various monomers including cyanamide,dicyanamide and melamine as well as different substrates.The role of deposition temperature andamountof monomers has been investigated in detail.Structural and surface morphological studies suggest uniform and pinhole-free g-CN films could be achieved with this feasible method.Photon-induced oxygen evolution upon anodic polarization in aqueous electrolytes brings up a photocurrent density of 63?A cm^-22 for the g-CN films prepared with dicyanamide.This is the highest value to date for a pristine g-CN photoanode atthe bias of 1.23 V?versus reversible hydrogen electrode?without sacrificial reagents.The good performance could be attributed to an enhanced light harvesting and decreased charge transport resistance at the film/electrolyte interface with an increase of amount of monomers.Accordingly,the strategy of TVD brings g-CN another step to applying as photoactive material in various photoelectronic fields.