Studies On The Photo-electrocatalytic Water Splitting Performance Of BiVO₄-Cu₂O Photoelectrochemical Cell

A series of problems such as resource shortage,environmental pollution and local conflicts brought about by human society’s dependence on fossil energy have severely restricted the development of today’s society.Converting solar energy into hydrogen energy for storage and usage is considered to be one of the effective ways to ease the energy crisis.As we know,photoelectrochemical（PEC）water splitting is regarded as a promising technique for converting solar energy into green and sustainable hydrogen fuels.In which,photoelectrode can absorb solar energy to generate photogenerated carriers,and the photogenerated electrons can reduce H₂O molecules to release hydrogen on the surface of photocathode.However,the over low photo-electric conversion efficiency and poor photoelectrode stability severely restrict the practical application of PEC water splitting technology.Thus,the preparation of high-efficient and stable photoelectrodes for constructing zero-bias photo electrolytic cells and improving the efficiency of PEC water splitting are the key points to promote the large-scale application of PEC water splitting technology.The purpose of this thesis is solving the problems of low photoelectric conversion efficiency of the current semiconductor photoelectrode,poor long-term stability and difficulty in practical application.Here,BiVO₄ and Cu₂O with higher theoretical photoelectric conversion efficiency were selected as the research objects.By consideration of controlling the heterostructures and microstructures of semiconductors,cocatalyst modification,and metal organic polymer coating,I designed and constructed FeOOH（P-II）/BiVO₄ photoanode,OEC/BiVO₄/WO₃NB photoanode and PhC₂Cu/Cu₂O photocathode with high stability,which all effectively improved the separation efficiency of carriers in semiconductor materials.Related researches will pave the way to develop BiVO₄ and Cu₂O-based semiconductor composite materials with high efficiency and stability,which can be referenced for the production of hydrogen from PEC water splitting.The main researches are listed as follows:1.In order to improve the PEC performance of BiVO₄,I propose a simple and efficient chemical bath deposition（CBD）method to immobilize a thin layer ofα-Fe OOH with high crystallinity on the surface of BiVO₄.The experimental results show that the obtainedα-FeOOH（P-II）/BiVO₄ photoanode exhibits a nearly unity faradaic efficiency,remains steady over 20 hours,and presents more than 21-fold higher solar to fuel conversion efficiency compared with pure BiVO₄ photoanode.By exploring the precursor solution containing different Fe sources,I found that using Fe²⁺as the reaction precursor is crucial for preparation of this well-crystallizedα-FeOOH cocatalyst.Systematic studies reveal that the high efficiency and excellent stability of theα-FeOOH（P-II）/BiVO₄ photoanode are attributed to the well-crystallizedα-FeOOH that facilitated the charge carrier separation and transfer in BiVO₄.The above results provide the potential of crystallineα-FeOOH as a cocatalyst modified BiVO₄ for the preparation of highly stable and efficient photoanodes.2.WO₃ nanobowl（WO₃NB）array was synthesized by MCC process and used for constructing the highly-matched conformal BiVO₄/WO₃NB heterojunction photoanode,which was utilized as a new type of photoanode to enhance the charge separation of BiVO₄.In addition,this work used Ni OOH/FeOOH as an oxygen release promoter（OEC）layer to modify the BiVO₄/WO₃NB.The obtained OEC/BiVO₄/WO₃NB photoanode presents the maximum photocurrent density of ca.4 mA cm^-2 at 1.6 V vs RHE in 0.2 mol L^-1 Na₂SO₄electrolyte under AM 1.5 G illumination,as well as 5 times higher IPCE at 450 nm compared with BiVO₄ photoanode,leading to about 95%faradaic efficiency.Systematic studies attribute the significantly enhanced PEC performance to the reasons as follows:1)the highly ordered single-layer WO₃NB nanobowl array can minimize the defects of WO₃ at the grain boundary and decrease the interfacial resistance between WO₃NB and FTO;2)the small size of BiVO₄ nanoparticles（～90 nm）was perfectly deposited on the bottom layer of WO₃nanobowl with the large inner diameter of 920 nm to maximize the contact area between WO₃NB and BiVO₄ nanoparticles;3)the smaller size of BiVO₄ than its hole diffusion length（～100 nm）ensures the holes transferring to its surface effectively,and then participating in the oxidation of water.Overall,this work provides an efficient strategy to design and fabricate a“multiple”BiVO₄-based heterojunction with advantage architectures for efficient PEC water splitting.3.I obtained Cu₂O photocathodes on different substrates by thermally reducing Cu（OH）₂.Subsequently,the surface of Cu₂O was further modified with phenylacetylene copper（PhC2Cu）as a protective layer by light irradiation-assisted polymerization.The results of the PEC stability test indicate that the Ph C2Cu thin layer can effectively inhibit the photo-corrosion of the Cu₂O photocathode and significantly improve the PEC stability.Studies have also shown that the excellent stability of PhC2Cu/Cu₂O comes from the super hydrophobic properties of Ph C2Cu and its well stable structure in air.Moreover,deposition of Cu₂O on the base substrates,including FTO,copper foil and foamed copper,the protective layer of Ph C2Cu can be successfully prepared by the same method,indicating that the modification method of Cu₂O proposed in this work is a universal strategy.Finally,combining the optimal PhC2Cu/Cu₂O photocathode with the OEC/BiVO₄/WO₃NB photoanode,I successfully constructed a series-type two-electrode unbiased PEC cell,which presents 1.26 mA cm^-2 photocurrent density at zero bias and remains steady over 5 hours.The solar energy conversion efficiency（STH）of the device is 1.55%by converting the photocurrent density.