A Bismuth Vanadate/silicon Nanoarrays Photoelectrochemical Tandem Cell For Overall Solar Water Splitting

Hydrogen（H₂）is one of the very promising candidates as a clean energy carrier in the future,if it can be produced using the world’s most abundant energy source,the sun.Photoelectrochemical（PEC）water splitting is considered as a promising strategy for solar hydrogen generation,and has been a hot topic for several decades.It is evident that an ideal system converting solar light into chemical fuel should be cost-effective,environmentally safe,and meet the efficiency required for applications on medium and large scales.To overcome the thermodynamic and kinetic barriers for water splitting,although only about 1.6-1.8V of solar-generated photo-potential is needed,a single semiconductor has not been found which can achieve high solar-to-hydrogen（STH）conversion efficiency as a large external bias is required for overall water splitting.Therefore,to develop novel approaches for efficient PEC water splitting without external bias is highly desired.To compete with"brute force" consisting of wired to a commercial potentiostat,connecting a solar cell to the photoelectrode（PV-PEC）/Photovoltaic-electrolysis（PV-EL）is a kind of feasible method.State-of-the-art PV-PEC and PV-EL tandem cells have been reported using a crystalline p-GaInP2 photocathode in contact with a GaAs solar cell,oxide photoanode/single-or double-p-i-n junction amorphous silicon and electrocatalysts/triple p-n junction amorphous silicon.However,this photovoltaic（PV）plus electrolysis approach usually requires complicated setups and solar cells with a high open-circuit voltage.And,expensive solar cells and complex electric connection in these systems limit their practical application in a large scale.An alternative photo-electrochemical approach for water splitting is to utilize a series-connected photocathode and photoanode in a tandem configuration,a p-n PEC tandem cell.In this configuration,solar photons not absorbed by the top electrode are transmitted and absorbed by the electrode underneath.Then two semiconductors with smaller band gaps can be chosen since each needs only to provide part of the water splitting potential.The smaller band gap means more absorption in the visible region of the solar spectrum where possesses the majority photon flux from the sun.The proper selection of both semiconductor electrode characteristics ensures the energy necessary for water photoelectrolysis is gathered entirely from the illumination,eliminating the necessity of applying energy from an external source.In this system,water splitting potential is generated directly at the semiconductor-liquid interfaces to perform water oxidation and reduction,in other words the number of junctions used is minimized,thus,the complexity and potentially cost of the device is reduced.In this dissertation,we evaluated the feasibility of a BiVO4/Si nanoarrays photoanode/photocathode（p/n PEC）tandem cell.The main conclusions are as follows:A bismuth vanadate-silicon nanoarrays tandem cell without external bias for overall solar water splitting.Through examination of the photoelectrochemical properties of BiVO4 and Si nanoarrays photoelectrodes,we evaluate the feasibility of a BiV04/Si nanoarrays photoanode/photocathode tandem cell for overall solar water splitting without external bias.By employing water oxidation and reduction electrocatalysts（Co-Pi and Pt,respectively）together with an operating point analysis,we show that an unassisted solar photocurrent density of 0.5 mA cm-2 is possible.Finally we fabricated the unassisted 2-electrode operation of the tandem cell with the solar-to-hydrogen（STH）efficiency of 0.57%,and had somewhat stalility for 3.5 hours.The electrolyte resistance and the distance between the two electrodes brought losses,which are not considered in the 3-electrode J-V curve intersection,though not to be important loss channel.After 100s of operation,a rapid descend was observed for the BiVO4/Si nanoarrays tandem cell,and reached a stabilized photocurrent for 3.5h.Upon rerunning the J-V analysis of the BiVO4 photoanode and Si nanoarrays photocathode after use in the tandem cell,the two electrodes showed the opposite the J-V behaviors.Characterized by some means and comparative experiments,we prove that the performance degradation of photoanode comes from the corrosion of Co-Pi off,which is why rapid decline in the tendem device,and the promoted property of silicon nanowires cathode promotioncome from the gradually clear of surface states.