Development Of Novel Non-Precious Metal Photoelectrocatalytic Materials For Near-Neutral Water Oxidation

Due to population growth and ascending level of industrialization,the global energy demand will continue to grow rapidly,and traditional energy sources such as oil,coal,and natural gas will eventually run out.Therefore,the development of energy innovation technologies to ensure access to renewable and clean energy is essential to sustainable social development.Photoelectrocatalytic（PEC）water splitting can take advantage of the abundant solar energy and the vast water resources.The oxygen and hydrogen can generate at the anode and cathode respectively to achieve renewable clean energy.In order to reduce the complexity,material corrosion,and cost of constructing a PEC cell and thereby improve the possibility of its practical application,a near-neutral electrolyte condition is necessary.For water splitting,oxygen evolution reaction（OER）involves four electron-proton coupled and requires more energy than that of hydrogen evolution reaction with two electron-transfer.Therefore,it is urgent to develop and construct a photoanode for near-neutral water oxidation.However,n-type semiconductor and oxygen evolution catalyst（OECs）materials required for constructing photoanode currently suffer poor solar-to-hydrogen efficiency and the large kinetic barrier of OER respectively under near-neutral conditions.In addition,precious metal oxides（Ir O₂and Ru O₂）still remain the most effective OECs under near-neutral conditions,but their scarcity hinders further large-scale use.In conclusion,we need to develop n-type semiconductor thin-film materials with better sunlight absorption,as well as more efficient and stable non-precious metal OECs under near-neutral conditions.In this dissertation,we developed an n-typeα-SnWO₄semiconductor thin film,and designed three near-neutral OECs（tungsten doped manganese silicate,niobium-cobalt mixed oxide,cobalt-iron spinel oxide）,and verified the OER properties of a V₂(Co_xSn_1-x)C MAX phase under alkaline and neutral conditions.The purposes are to reveal the great potential of narrow-gap semiconductor thin films in the application of PEC water splitting,and to elaborate the new understanding of the structure and composition of OECs under near-neutral conditions and thereby break the limited bottleneck of screening new non-precious metal OECs,which is conducive to promote the construction of narrow-gap semiconductor photoanode under near-neutral and then use for practical PEC water splitting application in the future.（1）We develop an uncomplicated and easily popularized chemical vapor deposition（CVD）method to prepare high-qualityα-SnWO₄thin films with excellent visible light absorption（band-gap is～1.99 e V）,suitable valence band and conduction band edge position and pure crystal phase.PEC tests under near-neutral conditions reveal that theα-SnWO₄films have low turn-on potential and good intrinsic structural stability.Meanwhile,theα-SnWO₄film can output stable PEC photocurrent after the modification of Co O_xcocatalyst,indicating that it is ideal to extract carriers effectively.These results indicate that theα-SnWO₄film prepared by the CVD method is excellent basic research material for photoanode and has great potential to be applied in practical photocatalytic water splitting through further semiconductor optimization and surface interface engineering.（2）By introducing silicate and W,we design and synthesize non-precious metal manganese-based film materials for the first time as stable and efficient OECs under near-neutral conditions.Manganese silicate exhibits excellent intrinsic catalytic activity.By doping,the tungsten doped manganese silicates show better apparent catalytic activity and stability than that of silicate manganese,which is mainly attributed to the fact that W doping not only promotes the exposure of more active sites on the surface of manganese silicate and does not reduce the overall conductivity of the film,but also significantly regulates the valence state of Mn and promotes the enrichment of surface hydroxyl groups.Density Functional Theory（DFT）calculation also shows that the W doping can make the structure of manganese silicate more stable.These results indicate that introducing high-valency metal elements will provide a new pathway to further develop more stable and more efficient non-precious metal-based OECs under near-neutral and even acidic conditions.（3）We successfully anchor Co₃O₄domains in Co Nb₂O₆to form a unique mixed oxide film with three-dimensional heterojunction structure through a modified sol-gel drop-casting protocol followed by a thermal annealing process in air.The tests show that the composite catalyst exhibited better stability and activity than pure Co₃O₄in neutral and weak acidic electrolytes.These enhancements are mainly attributed to the Coulombic interaction between the active site of Co₃O₄and the cobalt niobate phase,which inhibits the dissolution of the active sites of Co,improves the lower charge oxidation cycling path of active sites of Co,and provides more active sites and better internal charge transport capacity.These results indicate that constructing heterojunction metal oxides by introducing stable high-valency metal oxides presents a new strategy to improve the stability and performance of conventional transition-metal oxides for near-neutral OER.（4）Cobalt-Iron oxide thin films with spinel structure are prepared on fluorine-doped tin oxide（FTO）conductive glass by ultrasonic spray pyrolysis.The experiments show that the OER stability of spinel Co₃O₄can be significantly improved with appropriate Fe content in a neutral electrolyte,and 30%Fe content is the best.This improvement is mainly attributed to depend on the suitable site distribution of Fe³⁺and Fe²⁺between octahedral and tetrahedral in the spinel structure,producing the suitable super-exchange interactions between sublattices and the oxygen at the octahedral site more negatively charged.Thereby,Metal-Oxygen bonds have appropriate length and lower covalency,which is conducive to enhancing the energy barrier for oxidation of lattice oxygen,and inhibiting the formation of oxygen defects and further dissolution of catalyst in the OER process.These results suggest that reasonable construction of the distribution of doped elements in octahedral and tetrahedral sites for cobalt spinel structure provides a new strategy to develop efficient and stable neutral OECs.（5）MAX phases are a kind of material with a unique three-dimensional layered structure,which are made up of two-dimensional transition metal carbides or nitrides（MXenes）with good electrical conductivity and interlayer inserted A elements.Here we report for the first time that the V₂(Co_xSn_1-x)C MAX phase can be transformed into 2D"Co C"nanostructured particle in an alkaline solution through the OER polarization process.Compared with typical electrodeposited cobalt hydroxide（Co-H）,the activated V₂(Co_xSn_1-x)C exhibits exposed more active sites,better conductivity,and lower valence charge transition,resulting in better OER activity.In addition,the pristine V₂(Co_xSn_1-x)C MAX phase shows a certain OER activation under neutral electrolyte,and the activated performance is better than that of pure nickel foam.These results indicate that the widely adjustable M-site and A-site elements open up new concepts for further potential design and explore efficient and stable MAX phase OECs under alkaline and neutral conditions.