Study On Nickel、Cobalt、Iron Based Semiconductor Heterojunction For High-Efficiency Photo-Assisted Electrocatalytic Oxygen Evolution

The development of abundant,efficient,and stable non-precious metal materials for electrocatalytic oxygen evolution reaction（OER）is of considerable significance to alleviate the energy crisis.In recent years,photo-assisted electrocatalytic OER,as an emerging method,can drive photo-responsive materials to generate photo-generated holes under sunlight irradiation conditions,and the photo-generated holes can promote the increase of material active sites or directly participate in the OER process to promote OER performance.However,photo-assisted electrocatalysts need to have both the electrocatalytic oxygen evolution capability and the absorption of visible light,that is,materials selection is extremely rigorous.Among transition metal materials,Ni,Co,and Fe-based semiconductor heterojunctions have electrocatalytic and photocatalytic activities,and are therefore applied in the fields of electro/photocatalytic water splitting.Heterojunction construction is a strategy to improve the performance of photoassisted OER catalyst.On the one hand,constructing a heterojunction can improve the electrocatalytic performance by improving the charge distribution between the interfaces and exposing more active sites;on the other hand,constructing semiconductor heterojunction can improve the bandgap structure,enhance light absorption and charge carriers separation ability to promote photocatalytic performance.In summary,in this paper,three transition metal-semiconductor heterojunction catalysts are synthesized and their performance and mechanism in the photo-assisted electrocatalytic oxygen evolution process are investigated.The details are as follows:Firstly,the assembly of"optical+electric"combination is designed.The p-n type Sn S₂/NiO heterojunction is formed by combining photosensitive Sn S₂ semiconductor with NiO semiconductor with good electrocatalytic activity.The experimental results shows that the Sn S₂/NiO heterojunction exhibits good electrocatalytic performance under visible light irradiation.The overpotential of Sn S₂/NiO is only 310 m V to attain a current density of 10m A cm^-2,which is 78 m V lower than that under no irradiation.XPS spectroscopic analysis shows that the strong light absorption of Sn S₂ promotes the accumulation of holes on NiO after illumination,thus promoting the conversion of low Ni²⁺to high Ni³⁺on NiO,and Ni³⁺as the active site can promote OER reaction.In addition,we propose a non-parallel photo-assisted catalysis mechanism based on the change of photocurrent density at different potentials:at low potentials,light is the dominant factor in reducing the overpotential of the OER reaction,while at high potentials,the applied potential is the main driving force to further promote the OER process.This work provides an effective strategy for manipulating the electronic and band structures of semiconductors to enhance the electrocatalytic performance of OER.To further improve the photo-assisted electrocatalytic OER performance,the assembly of"dual optical/electric"is designed,which combines Co₃O₄ and Co S₂ semiconductor with both photoresponse and electrocatalytic activity to form a double cobalt Co₃O₄/Co S₂ p-n heterojunction.After photo-assisted electrocatalysis tests,the Co₃O₄/Co S₂ heterojunction exhibits excellent OER performance,and only 255 m V of overpotential is required to attain a current density of 10 m A cm^-2,which is 40 m V lower than that under no illumination.Meanwhile,under illumination,the water splitting device assembled with Co₃O₄/Co S₂ as anode only requires a potential of 1.57 V to attain 10 m A cm^-2.XPS shows that due to the strong electronic interaction between Co₃O₄ and Co S₂,the strongly electronegative S is more inclined to combine with Co,generating more Co-S species.The Co-S will be further converted into Co³⁺which can act as an active center to promote the OER reaction.In addition,under light irradiation,the semiconductor generates and separates photogenerated electrons and holes after absorbing light energy,and the holes with oxidative ability can promote the conversion of Co²⁺to Co³⁺,thereby further increasing the OER activity.This work provides an effective perspective for the development of high efficiency OER catalysts based on non-noble metals combined with solar energy as a renewable energy source.Following the"dual optical/electric"combination,Co O/Ni Fe LDH photo-responsive electrocatalysts are prepared by a simple ultrasonic self-assembly method.Consequently,the Co O/Ni Fe LDH heterojunction exhibits significantly enhanced OER performance,requiring only an overpotential of 257 m V to achieve 10 m A cm^-2,which is 50 m V lower than that under without irradiation.XPS shows that there is a strong electron transfer between Co O and Ni Fe LDH,which is beneficial to change the electronic structure of catalyst and promote OER reaction.Meanwhile,the Co O/Ni Fe LDH heterojunction absorbs photons under illumination to generate photogenerated electron and hole,and the existence of the built-in electric field hastens the transfer of electrons from Co O to Ni Fe LDH,and the holes from Ni Fe LDH to Co O.The fast transfer of photogenerated charges promotes the accumulation of holes in the valence band（VB）of Co O.Since the VB potential of Co O is more positive than the oxygen evolution potential,the photogenerated holes with oxidative ability can effectively promote the OER reaction,thereby further enhancing the OER performance.The LED lamp is used instead of simulated sunlight to assist the OER water splitting process of the catalyst.It is found that the potential of water splitting decreased from 1.66 V to 1.62 V under the driving of LED lamp.This work provides direction and application value for developing light-responsive non-precious metal materials and utilizing solar energy resources.