Photoelectrocatalytic Performances Of BiVO₄ Photoanodes With Controllable Construction Of Oxygen Evolution Cocatalysts

Under the two serious problems of energy shortage and environmental pollution today,the search and development of a renewable energy is extremely urgent.Solar energy is an inexhaustible and ubiquitous natural energy source.In recent years,photoelectrocatalytic（PEC）water splitting by solar energy to produce green and clean hydrogen energy has attracted extensive attention from scholars at home and abroad.PEC water splitting has become a research hotspot for researchers due to the environmentally friendly reaction process and the variety of feedstocks available,while the solar-to-hydrogen（STH）conversion efficiency of PEC water splitting is influenced by the performance of semiconductor electrode materials.Therefore,good semiconductor electrode materials are the core of PEC water splitting system to improve STH efficiency.Among many PEC water oxidation photoanodes materials,an n-type semiconductor material with a narrow band gap（～2.4 eV）and a long hole diffusion length（～70 nm）,bismuth vanadate（BiVO₄）,has come to the attention of scholars.However,the slow oxygen evolution reaction（OER）kinetics on the surface of BiVO₄ limits its actual conversion efficiency of water splitting.Based on this,this paper effectively enhances the PEC water splitting performances of BiVO₄ photoanodes through strategies such as reasonable introduction of oxygen vacancy,phosphorylation treatment and in situ growth of cocatalysts,and the reaction mechanism of which has been thoroughly investigated as follows:1.Enhanced photoelectrocatalytic activity of BiVO₄ photoanodes by FeCoOx（Ar）catalysts containing oxygen vacanciesBiVO₄/FeCoO_x（Ar）photoanodes were obtained by a combination of calcination and plasma treatment.A series of experimental results showed that the modification of BiVO₄photoanodes by FeCoO_x（Ar）containing oxygen vacancies could effectively improve the activities of PEC water splitting on the photoanodes.The photocurrent density of the composite photoanodes could reach up to 6.3 mA cm^-2 at 1.23 V_RHE under AM 1.5illumination(100 mW cm^-2),which is the higher photoelectrocatalytic activity and stability simultaneously among the cobalt-based catalysts modified BiVO₄ photoanodes reported so far.Subsequently,The oxygen vacancies in the BiVO₄/FeCoO_x（Ar）photoanodes were demonstrated to effectively promote photogenerated charge separation and improve the performance of PEC water splitting by EPR and other tests.2.Preparation of BiVO₄/FeCoPO_x photoanodes and its performances of photoelectrocatalytic water splittingBiVO₄/FeCoPO_x photoanodes were obtained by phosphorylation treatment.The experimental results showed that the introduction of P element led to the formation of FeCoPO_x cocatalysts layer on the surface of BiVO₄photoanodes.At 1.23 V_RHE under AM1.5 illumination(100 mW cm^-2),the composite photoanodes showed a high photocurrent density of 6.5 mA cm^-2,which was 3.6 times higher than the initial BiVO₄ photoanodes,and the photoelectrocatalytic stability of BiVO₄/FeCoPO_x photoanodes was also well improved.In addition,the XPS and other test results visually demonstrated that the FeCoPO_x structure can effectively promote the photogenerated charge separation and thus accelerate the PEC water oxidation process.3.Co-catalytic of FeF_x and FeCoOOH for the stability of water oxidation in BiVO₄photoanodesBiVO₄/FeF_x/FeCoOOH photoanodes were constructed in situ using a simple solution impregnation method.Surprisingly,this cocatalysts system greatly enhanced the PEC stability of the pristine BiVO₄ photoanodes.The photoelectrocatalytic activity of the composite photoanodes could reach up to 6.5 mA cm^-2 at 1.23 V_RHE under AM 1.5illumination(100 mW cm^-2),and remained around 6.0 mA cm^-2after 6 h of continuous light exposure.And the PEC stability of the composite photoanodes was experimentally demonstrated to be repeatedly measurable.Meanwhile,the IPCE value of BiVO₄/FeF_x/FeCoOOH photoanodes was up to 96.5%.Subsequently,the results of scanning Photoelectrochemical Microscopy（SPECM）,Kelvin Probe Force Microscopy（KPFM）and XPS confirmed that the improved PEC performance of the composite photoanodes was due to the fact that FeF_x/FeCoOOH could effectively promote the photogenerated charge separation and increase the active sites of OER reaction on the surface of the photoanodes.