Highly Efficient And Stable Silicon-based Photoelectrodes For Photoelectrochemical Water Splitting

Photoelectrochemical(PEC)water splitting for hydrogen production employing semiconductors as photoelectrodes is widely accepted as one of the most promising routes to relieve the energy crisis and environmental issues.Silicon(Si)is emerging as a suitable candidate for a photoelectrode material due to its nearly ideal band structure,excellent charge carrier transport properties,and relatively low cost.However,the actual experimental PEC water splitting efficiency and stability for current Si-based photoelectrodes are far from satisfactory to meet the practical need for solar hydrogen production.This thesis introduces the definition and progresses of the efficiency and stability for Si photoelectrodes,then present several basic strategies for designing highly efficient and stable Si photoelectrodes,including the design of micro-nano surface textures,the preparation of protective layer,the decoration of catalyst and the integration of the system.Based on these design and experiment researches,an unassisted solar water splitting system with Si-based photoelectrodes was finally achieved.Our mainly works are listed below:(1)We introduce a SiO2 protective layer on a Pt decorated multicrystalline n+p-Si photocathode,using an atmospheric plasma oxidation method.Pt remains the direct contact with Si and the left part of Si is passivated by the SiO2 layer,leading to the increased carrier lifetime and subsequently enhanced change separation and transfer on the electrode/electrolyte interface.Compared with the un-treated one,the energy conversion efficiency(?)of the plasma-treated Pt/n+ p-Si photocathode increases from 6.2%to 8.9%,and its stability improves from less than 1 to 22 h.The corresponding results have been published on Applied Physics Letters 2016,109,233901.(2)For further improve the efficiency and stability of Si photocathode,crystalline Si(c-Si)is adopted as substrate.Uniform and small Pt nanoparticles(NPs)were successfully prepared on an n+p-Si pyramid surface by a cheap electroless deposition method,based on the hydrophilic character of SiO2 formed by above-mentioned plasma methode.Pt NPs are small enough to not only avoid the optical loss of Si substrate,but also provide a large number of reaction sites for hydrogen evolution reaction(HER).A high ? of 10.8%along with a superior stability over one week were obtained by the introduce of a TiO2 protective layer.The corresponding results have been published on Journal of Materials Chemistry A 2017,5,18744-18751.(3)Pt is reported as one of the most efficient and stable catalyst for HER.However,it is rare and expensive.There is a need to explore a cheap and efficient catalyst.Here we fabricate highly efficient and stable photocathodes by integrating crystalline molybdenum sulfide(MoS2)catalyst with?2 nm Al2O3 protected n+p-Si.A high-quality MoS2 layer with vertically standing grasslike nanowires was produced by magnetron sputtering,leading to rich active sites for PEC water splitting.Simultaneously,A12O3 acts as both protective and passivative layer for Si surface during MoS2 deposition.As a result,a high? of 3.6%and a stability of 120 h were achieved.The corresponding results have been published on ACS Applied Materials&Interfaces 2017,9,6123-6129.(4)Similar to MoS2,tungsten disulfide(WS2)has also been identified as a new class of catalyst for HER.In order to increase the edge sites and thereby improve the HER activity,various engineering approaches have been investigated.Herein,we integrated WS2 and Co-doped WS2(Co-W-S)onto n+p-Si photocathodes based on a simple wet chemical method to.Co-W-S/n+p-Si photocathode exhibited much better PEC performance,resulting from the CoW-S catalyst which has more electrochemically active sites and better electrical conductivity.A thin Ti interlayer between Co-W-S and Si was inserted to further optimize the PEC performance,especially the stability of the photocathode.The Co-W-S/Ti/n+p-Si photocathode exhibited a ? of 4%with a long-term stability of 6 days under operating conditions.The corresponding results have been published on Applied Catalysis B:Environmental 2018,237,158-165.(5)Based on above study on Si photocathodes,we try to make Si photocathode and photoanode as a pair in a photovoltaic(PV)-PEC tandem device to implement unassisted solar water splitting.The design of pn junction for both Si photocathode and photoanode allows back illumination from the Si substrate,spatially and functionally decoupling the optical absorption and the catalytic activity.By further employing a Ni protective layer together with a bifunctional Ni-Mo catalyst for both the Si photocathode and photoanode,the Si-based PV-PEC tandem cell can perform unassisted water splitting with a high solar to hydrogen conversion efficiency of 9.8%and a stability of over 100 h in alkaline solution The corresponding results have been published on Journal of Materials Chemistry A 2019,7,2200-2209.