Research On High-efficiency Self-driven Photoelectric Catalytic Water Splitting Based On Silicon Thin Film Materials

Photoelectrochemcial(PEC)water splitting is an effective method to develop clean energy especially for converting sunlight into hydrogen.The semiconductor material used in the system is considered a significant part for the PEC for the charge transfer across the semiconductor-catalyst interface plays an important role in the PEC process.Herein,the charge transfer process and mechanism at electrolyte/cocatalyst/semiconductor interfaces were studied using amorphous silicon thin film as a model photoelectrode.Firstly,based on p-n junction in silicon solar cells,we realized the charge transfer among the stack layers and improved the solar cell efficiency and device applications by controlling the deposition parameters.Then we listed some key factors influencing the activity of oxide film and metal electrocatalyst,such as the composition,crystallinity,morphology,specific surface area and active sites.Subsequently,we utilized them as cocatalyst coupled with silicon thin film photoelectrode in the PEC system,and studied their impacts to the charge transfer,PEC performance and stability.Detailed characterization has been conducted using many kinds of apparatuses and analytical instruments,including physical characterization,electrochemical testing,as well as photovoltaic and surface photovoltage testing equipment.We also developed an"electron extraction layer"to extract photogenerated electrons rapidly from thin film silicon and protected the photoelectrodes from corrosion.The ultimate goal is to develop an effective and stable self-driven PEC system and explore its mechanisms and kinetics in the process.The dissertation is summarized as follows:(1)A chemical method was introduced to prepare quasi-amorphous nickel catalyst by two steps.In Step 1,the laminated hydrotalcite skeleton was modified by ion exchange,and then in Step 2,it was further reduced to disordered quasi-amorphous metallic structure by NH3 600? last 90 min.The prepared quasi-amorphous catalysts show excellent activity for alkaline hydrogen evolution with onset potential close to 0 mV and the overpotential 160 mV lower than the crystalline nickel catalyst to achieve current density 10mAcm-2.By characterizing the crystallinity of the catalyst,bulk specific surface area,electrochemical specific surface area,TOF as well as Tafel,we confirmed the higher activity of the quasi-amorphous nickel is mainly influenced by internal disorder structure.In strong alkaline electrolyte,it could bear continuous or intermittent polarization for tens of hours.It is expected that the quasi-amorphous nickel HER catalyst not only shows a promising potential,it also provides a new strategy to prepare amorphous metal electrocatalyst.(2)Hydrogen evolution reaction catalyst(Ni and Pt metal nanoparticles)and oxygen evo:lution reaction catalyst(NiFeOx and CoOx metal oxide)were prepared by magnetron sputtering method.The film thickness,physical and chemical properties are controlled by deposition time and power accurately to investigate their activity for water splitting.Based on their high light transmittance,effective passivation abilities as well as the protection for the photoelectrodes to withstand in alkaline and acid solution,they have been commonly used as cocatalysts for photoelectrochemical water splitting system.(3)By loading Pt and CoO,as cocatalyst on the amorphous silicon thin film photoelectodes,the charge transfer across the photoelectrode-electrolyte interface was investigated.The apparent charge recombination is inhibited on the surface of the photoelectrodes after cocatalysts were introduced.In alkaline electrolyte,photogenerated carriers were quickly extracted to the surface for reaction,which prevents the charge accumulation on the surface and thin film silicon photocorrosion.At last,two thin film silicon photoelectrodes were assembled in a PEC device,and the ef:ficiency of solar to hydrogen(STH)reached a promising 0.92%without any extra bias.This design of combining high efficiency photovoltaic materials and cost-effective cocatalysts for PEC system provides hope for solar conversion to hydrogen.(4)Thin film silicon photocathode as the target material insulated the photoelectrodes with harsh environment by developing an"electron extraction layer"represented by ferrihydrite oxide(Fh).It helps to capture and store photogenerated electrons effectively from the surface of silicon photocathode.So,the photostability of silicon photocathode is increased to 3.5 hours comparing to the bare silicon that may be dissolved in a few minutes.This provides a new solution to the photocorrosion existed on the photoelectrodes in water decomposition.With help of the enhanced photocathode,the overall amorphous silicon photoelectrodes achieved the maximum efficiency of self-driven water splitting up to 3.35%.