Design And Preparation Of Tungsten/Iron Oxide-cocatalyst Composite Photoanode For Water Oxidation

Photoelectrochemical?PEC?water splitting process contains the steps of light absorption,the separation of photogenerated charge and the water splitting reaction using photogenerated charge.The second and third steps usually have low efficiency,which lead to low efficiency of water splitting.In this thesis,WO₃ and Fe₂O₃ are used as photoanodes for PEC water splitting.Water oxidation cocatalysts are loaded on these photoanodes to improve the efficiency of water splitting reaction.The details are summarized briefly as follows:?1?Loading FeOOH on WO₃ nano bars using photodeposition method and investigating its photoelectrochemical water oxidation performance.WO₃ nano bars were fabricated on FTO substrate using hydrothermal method.The amount of FeOOH could be controlled by varying deposition time.WO₃/FeOOH electrode obtained with the deposition time of 20 min has the highest photocurrent of 1.3 mA/cm2 at 1.23 V vs.RHE under AM 1.5G light irradiation.The stability of WO₃ is improved by loading FeOOH because FeOOH can act as a protective layer and oxidize H2 O to be O2 easily rather than H₂O₂,thus suppressing the formation and accumulation of H₂O₂ on WO₃.A p–n junction between WO₃ and FeOOH formed due to the different solvation effects on WO₃ and FeOOH.The FeOOH acts as not only the cocatalyst but also a hole extractor to facilitate the separation of photoinduced electrons and holes.So WO₃/FeOOH electrode has high photoelectrochemical water oxidation activity.?2?Loading CoO_x nanoparticles on WO₃ nanoflakes and investigating its PEC water oxidation performance.WO₃ nanoflakes were fabricated on FTO substrate and loaded with CoO_x nanoparticles to form CoO_x/WO₃ electrode using hydrothermal method.The charge separation efficiency and charge injection efficiency,as well as the oxidation selectivity of WO₃ for photoelectrochemical water oxidation are improved by loading CoO_x nanoparticles.This can be ascribed to the formation of p-n heterojunctions between CoO_x and WO₃,in which an internal built-in electric field improved the charge separation efficiency of WO₃ electrode in PEC water oxidation reaction.Meanwhile,as excellent water oxidation cocatalyst,CoO_x nanoparticles improve the charge injection efficiency of WO₃ electrode.The produced oxygen is monitored by a Clark electrode,which shows a high Faradaic efficiency on CoO_x/WO₃.In an effort to gain insight into the role of CoO_x on the high Faradaic efficiency,the rotating ring-disk electrode system was used to detect the products of the CoO_x/WO₃ and WO₃ electrodes in the photoelectrochemical water oxidation process.The results show that the loading of CoO_x nanoparticles can improve oxidation selectivity of WO₃ and therefore improve the Faradaic efficiency of WO₃.?3?Doping Mn element into Fe₂O₃ and coating it with NiFe layered double hydroxide?NiFe-LDH?,then investigating its PEC water oxidation performance.The Mn doping improves the charge separation efficiency from 20% to 35% at high potential.The coating of NiFe-LDH not only improves the charge injection efficiency but also reduces the onset potential of Fe₂O₃ electrode.The coating amount of NiFe-LDH has a significant effect on the performance of Fe₂O₃.At low precursor concentrations,the formed thin NiFe-LDH has little effect on the light absorption of Fe₂O₃,while NiFe-LDH fabricated at high precursor concentrations forms a thick layer that reduces the light absorption of Fe₂O₃ and leads to lower photocurrent.The enhancement of charge separation efficiency and charge injection efficiency lead to the increase of photocurrent.