Mechanism Investigation Of The Coupling Rule Of Photocathode And Photoanode For Unbiased Solar Water Splitting

The massive burning of fossil fuels has caused a dramatic increase in greenhouse gas emissions,resulting in global warming,air pollution and other hazards.In this context,the goal of"carbon neutrality"is proposed,i.e.,the pursuit of net zero emissions of greenhouse gases such as carbon dioxide.One promising approach to achieve"carbon neutrality"is to develop renewable energy that can replaces traditional fossil energy.The inexhaustible solar energy is an important renewable energy,but its decentralized and intermittent nature poses a significant challenge to energy supply.Unbiased solar water splitting powered only by solar energy is a promising approach to convert sunlight and water into hydrogen（a carbon-free fuel）and oxygen,which has significant immediate interest for the efficient utilization of solar energy.The tandem cell consists of crystalline silicon（Si）and bismuth vanadate（Bi VO₄）is one of the most promising candidates for unbiased solar water splitting because it balances the open circuit voltage and saturation current of the system.Si is an excellent choice of photocathode due to its proper bandgap（1.12 e V）,earth abundance,and mature production technolog.However,Si photoelectrodes suffer from parasitic light absorption of cocatalyst that severely limits their photocurrent density.Bi VO₄ has attracted much attention for pairing with Si,motivated by its low cost,and large photovoltage generated from the semiconductor-liquid junction.But its bulk recombination limits the photocurrent density.In addition,the interface defects caused by lattice mismatch need to be considered in the integrated structure of Si and Bi VO₄.To eliminate the parasitic light absorption of catalyst in conventional p-Si based photocathodes,we designed and fabricated photocathode using n-Si,a semiconductor traditionally used for photoanode,to realize the spatial decoupling of light absorption region and reaction sites.This illumination-reaction decoupling structure utilizes the majority carriers of n-Si,rather than the commonly used minority carriers,to drive the reaction,thus the reaction sites can be placed away from the light absorption region.The illumination-reaction decoupled n-Si MIS photocathode exhibited a remarkable applied bias photon-to-current efficiency of 10.26%for hydrogen evolution,which surpassed previous p-Si MIS junction photocathodes.To address the paradox between high crystallinity and small grain size in Bi VO₄photoanode through convential thermal treatment,we introduced a rapid thermal process（RTP）,revealing the nucleation growth mechanism of Bi VO₄ during heat treatment.By increasing the heating rate,the fast diffusion and decomposition of the precursor can be promoted,which increases the nucleation rate.In addition,the emission spectrum of the RTP lamp exhibits blue-shifts with increasing heating power,which achieves efficient heating of the Bi VO₄ with large bandgap.Thus,Bi VO₄ with high crystallinity and small grain size can be obtained,which facilitates the carrier transport to achieve high performance.To improve the sluggish carrier transport at the Si/Bi VO₄ interface,we introduced the design and fabrication of an Al₂O₃/ITO interfacial bi-layer for Si/Bi VO₄ integrated photoelectrode,which promoted the interfacial carrier transport by enlarging the band offset while healing interfacial defects.Specifically,the introduction of the ITO layer enables the fully utilization of the interfacial band offset by simultaneously enlarging the band bending in n-Si and Bi VO₄ at the n-Si/Bi VO₄ interface.Thus,sufficient driving force can be provided for carrier transport.Further,the Al₂O₃ layer is inserted to passivate the defects at the newly formed n-Si/ITO interface,which ensures unimpeded carrier transport.Thanks to the interfacial bi-layer,n-Si and Bi VO₄ could be integrated into an integrated n-Si/Al₂O₃/ITO/Bi VO₄/Ni Fe（OH）_x photoanode,which realized unbiased solar water splitting when coupled to a Pt foil cathode.In order to reduce the resistance loss caused by the enlarged area of the conducting substrate in the photoelectrode scale-up,a metal wire-assisted photoelectrode preparation strategy is proposed.Combined with the industrialized RTP method,the metal grid wire is used to reduce the ohmic loss,so that the photoelectrode exhibited good performance even after the area is expanded by 10 times,which provides experimental support for solar water splitting towards large-scale applications.