Study On Highly Efficient Silicon-based Photoelectrodes For Photoelectrochemical Water Splitting

Producing hydrogen by photoelectrochemical(PEC)cell is a way of realizing the sustainable use of hydrogen as well as solving the global energy crisis.The core of the PEC cell is the semiconductor photoelectrode.Silicon is a highly promising semiconductor electrode material for photoelectrochemical solar water-splitting due to its low cost,suitable band gap,excellent photoelectric conversion and charge transport characteristics.However,the application of Si is also restricted by its slow surface reaction kinetics and instability.This thesis summarized the main ways to improve the efficiency and stability of Si photoelectrode and illustrated the current research progress in Si photoelectrode.Then,we studied on the efficient and stable Si photocathode and photoanode respectively.Eventually,the unassisted Si-based solar water splitting system was built.The specific works are as follows:(1)Many earth-abundant transition metal dichalcogenides(TMDs)have been employed as catalysts for H2 evolution reaction(HER)to replace the noble metal catalysts,however,their impactful integration onto photocathodes for PEC-HER is less developed.In this study,we directly sputtered a newly MoSe2 catalyst onto n+p-Si photocathode for efficient and stable PEC-HER.An onset potential of 0.4 V vs.RHE(VRHE),saturated photocurrent of 29.3 mA/cm2,fill factor of 0.32 and an energy conversion efficiency of 3.8%were obtained under 100 mA/cm2 Xe lamp illumination.Such superior PEC properties were ascribed to the nearly vertically standing two dimensional MoSe2 rough surface layer,and the sharp interface between Si and MoSe2 with small charge transfer resistance.The balance between the reflectivity of the electrode surface and the absorptivity of MoSe2 was also discussed.In addition,the MoSe2 layer can protect the n+p-Si photocathode with a 120 h stability,due to its initial growth on Si with high flatness and compactness.This study provides a path to the effective and scalable growth of TMDs onto the Si photocathode aiming for high efficiency and stability.The corresponding results have been published on Applied Physics Letters,2018,112,13902.(2)Ni oxides(Ni-O)have been studied as an effective catalyst for O2 evolved reaction in basic solution during PEC water splitting based on Si photoanodes,however,it is difficult to obtain a porous surface which is critical for promoting its catalytic activity.In this study,we firstly successfully integrated porous Ni-O on Ni protected pn+-Si photoanode by electrodepositing a porous Ni-S layer,and subsequently converting it to Ni-O through an activation process while inheriting the porous character.Special attention was then paid to clarify the contribution of non-faradaic process,etc,the oxidation of Ni2+to Ni3+,on PEC current-potential curves,and to provide the methods how to get the true PEC properties.Finally,excellent PEC activity was demonstrated for the porous Ni-O/Ni/Si photoanode,which has an onset potential of 0.93 VRHE,a photocurrent of 39.7 mA/cm2 at 1.23 VRHE,and an energy conversion efficiency of 3.2%under 1 sun back-illumination,which is much superior to that of the dense Ni/Si photoanode.The corresponding results have been published on Chemical Communications,2019,55,377.(3)Producing hydrogen by unassisted solar water splitting is an important way to realize the sustainable use of hydrogen energy.In order to be viable in industry and commerce,the energy conversion efficiency from solar energy to hydrogen should be more than 10%in basic electrolyte.Here we deposited Ni-Mo-S catalyst,which is of high catalytic activity,on the surface of Ni/Si by electrodeposition method.In this way,a Ni-Mo-S/Ni/Si photocathode with an energy conversion efficiency of up to 8.48%was obtained.Being connected in series with our porous Ni-O/Ni/Si photoanode and two solar cells,an unassisted PV-PEC solar water splitting system with solar to hydrogen conversion efficiency over 11.48%1nder 110 mW/cm2 simulated sunlight was constructed.This conversion efficiency is the highest one among the reported Si water splitting systems using cheap catalyst.This research also provides a viable solution to building a highly efficient,stable,low-cost and sustainable solar water splitting system.