Preparation Of WO₃ And BiVO₄ Photoanodes For Photoelectrocatalytic Water Oxidation

The heavy dependence on fossil fuels has caused people to worry about the sustainability of modern energy production and environmental impact.Therefore,China has also proposed the grand goal of"carbon peaking"and"carbon neutrality".To achieve this vision,there is an urgent need to find a green and sustainable way of energy utilization.The use of photoelectrochemical cells to decompose water for hydrogen production can effectively achieve this goal.The water oxidation reaction involves complex charge transfer processes,as well as the generation of oxygen,which is the bottleneck of the water decomposition reaction.Therefore,preparing an efficient and stable photoanode is crucial for achieving the utilization of solar energy to decompose water for hydrogen production.BiVO₄ and WO₃ have become popular materials for constructing water oxidation photoanodes due to their suitable band positions,ability to meet the oxidation of water,absorption of visible light utilization,and stable chemical properties.However,both have defects such as slow surface water oxidation kinetics,low internal carrier mobility,and insufficient light utilization.Therefore,further modification is needed to enhance the overall photocatalytic performance of the photoanode.Composite WO₃ with TiO₂ to construct a type II heterostructure photoanode.The photoanode provides an internal electric field,promoting the separation of photo generated charge pairs,greatly improving the internal charge separation efficiency of the photoanode.Compared to WO₃ photoanode,it increases by nearly 40%,and the photogenerated charge lifetime recombination time is extended from 0.1 ms to 10 ms.Three types of metal porphyrin molecular co catalysts with chlorophyll structures,Fe TCPP,Co TCPP,and Ni TCPP,have been synthesized.Theoretical calculations show that Ni TCPP has the lowest surface electrostatic potential,which is conducive to its reaction with water molecules,thereby reducing the potential barrier of water decomposition reaction at the semiconductor electrolyte interface.At the same time,the energy difference between the HOMO of Ni TCPP and the valence band of TiO₂ is the largest,More effectively extracting holes from the valence band of TiO₂ to promote photo generated charge separation,and experiments have also shown that Ni TCPP loaded WO₃/TiO₂ photoanode exhibits the most excellent catalytic water decomposition performance.The surface hole injection efficiency has increased from 43%of the original blank sample to 82%,and the photocatalytic water photocurrent response reaches 1.75 m A/cm²,which is nearly 70%higher than before loading.Since Ni TCPP and composite photoanode are mainly linked in the form of covalent bond,they will fall off due to the disconnection of covalent bond under long-term light,which will reduce the stability of photoanode.Afterwards,WO₃ and BiVO₄ were composite to solve the problem of insufficient photogenerated charge separation efficiency between the two materials.To this end,a dense WO₃ film is added as a passivation layer between porous BiVO₄ and FTO substrate.The type II heterostructure formed by WO₃ and BiVO₄ can also effectively promote the separation of photo generated charges.In addition,it was found that W⁶⁺ions in WO₃ can be doped into BiVO₄ during the sintering process,further improving the charge transfer efficiency inside the photoanode.Under the combined action of these factors,this new structure of WO₃/BiVO₄ composite photoanode achieved a photocurrent response of 2.85m A/cm² in a potassium borate buffer solution at p H=9 under an applied bias voltage of 1.23 V（vs RHE）,which improves the catalytic efficiency by nearly100%compared to pure BiVO₄ photoanode.Further loading a p-type inorganic co catalyst,Ni Co₂O_x,significantly improved the stability and catalytic performance of the photoanode.At an applied bias voltage of 1.23V,the maximum current reached 4.05 m A/cm² and remained stable after 3 hours.