α-Fe₂O₃ And WO₃ Photoelectric Water Oxidation Systems

In the 21st century,Energy and environmental issues are more concerned.Now people mainly rely on fossil energy such as coal,oil,natural gas,but these non-renewable energy sources reserves limited and emit gas that pollute the environment.Therefore,looking for clean energy can not only solve the energy problem,but also solve the environmental pollution caused by the energy problem.Among them,hydrogen is considered to be the most ideal clean energy which can be obtained from water splitting.At present,large-scale hydrogen production mainly uses coal gasification or electro-water splitting method,but it needs to consume a large amount of thermal or electrical energy.In contrast,using light driven water splitting to produce hydrogen can save fossil energy,but the efficiency is low though it has been successfully tested in the laboratory.Now,photoelectric water splitting system is a good choice,it can make good use of visible light to drive water splitting under electricity.However,water splitting is related to water oxidation and protons reduction.Water oxidation needs to transfer four electrons to produces a oxygen molecule and proton reduction only needs to transfer two electrons to produces a hydrogen molecule.Therefore,the kinetic bottleneck of water splitting mainly depends on water oxidation part.In this paper,we studies the following photoelectric water oxidation system:1.A molecular catalyst containing earth-abundant,low-cost cobalt was integrated with α-Fe2O3 film electrode for photoelectrochemical water oxidation.Under illumination of LED(λ=420 nm),the hybrid photoanode exhibits a 5-fold enhancement in photocurrent density relative to bare α-Fe2O3 in 0.1 M Na2SO4 at pH 7.Accompanied by the highly stable photocurrent,stoichiometric oxygen and hydrogen are generated with a faradic efficiency over 85%respectively for 4 h photolysis.With hydrogen peroxide(H2O2)serving as the hole scavenger,it demonstrated that integration with molecular catalyst can greatly prompt hole diffusion length of α-Fe2O3 and improve its charge transfer properties.Mechanistic study and stability test supports that highly efficient and stable molecular catalyst plays the crucial role in charge separation,which successfully inhibits electron-hole recombination,achieving great enhancement in photocurrent.2.CoOx-decorated WO3 electrode(C-M2P-CoOx/WO3)engineered via an organic linkage 3,3-Diphosphonopropanate(C-M2P)for photoelectrochemical water oxidation in a neutral media has been well developed,characterized and tested.Under this protocol,C-M2P functions as a stabilizer to control the colloidal COOx size,and serves to bridge the catalytic center and WO3 anode.As compared to the pristine WO3 with a carrier density Nd of 9.9×1019 cm-3,Na for the assembled electrode is determined as 1.9×1021 cm-3,confirming that the enhancement in photocurrent is boosted up from the strengthened charge separation,which is further evidenced by open circuit voltage decay(OCVD)measurements.Photovoltage decay reveals the improved photoexcited electrons transfer is attributable to the effective passivation of surface trap states.This allows for a generation of unexpected photocurrent as high as 2.42 mA/cm2 with 0.75 V vs.Ag/AgCl bias.Together with the dramatically shortened charge lifetime and large transient photovoltage on the assembled electrode,a successive hole transfer mechanism with M2P as a shuttle to speed up holes migration was established.The current study establishes an intelligent design of functional component for ameliorating charge separation in the specific hole or electron transfer process,which provides a new paradigm for leading to highly efficient photoelectrochemical device amenable to further application.3.EDTA·2Na was used to synthesize C QDs solution by hydrothermal method.C QDs solution can effectively improve the photoelectric performance of α-Fe2O3 and WO3 systems as electrolyte.the photocurrent density of α-Fe2O3 and WO3 electrodes increased by 48 and 3 times at 0.62V vs.Ag/AgCl,IPCE increased by 36 and 2.5 times respectively.Meanwhile,the C QDs was still stable using NMR or fluorescence spectrum analysis after electrolyzed for 2 hours.Testing the electrical properties of C QDs solution and OCVD show that the distance between C QDs and electrodes become closer,so the holes on the surface of electrode can quickly transport to the oxidized water through C QDs,which improve α-Fe2O3 and WO3 photoelectric properties.