Influence Of Element Doping On Water Splitting Performance Of CuWO₄ And BiVO₄ Photoanodes

The drawbacks of traditional fossil fuels,such as limited reserves and environmental pollution problems,have been amplified with the increasing energy requirement of modern society.To find the alternatives of fossil fuels have been put on the agenda of many countries.Through photoelectrochemical cell to decompose water molecule into hydrogen show promising research and application potential.The U.S.Department of Energy(DOE)has presented 25%solar-to-hydrogen(AM 1.5G 100 mW cm-2)efficiency target for photoelectrochemical water splitting cell.Unfortunately,due to limited photoanode performances(low photocurrent density,poor photoelectrochemical stability),the pratical conversion efficiency is far below the 25%target.Metal oxides photoanode materials receive much concerns due to the advantages of low manufacturing cost and desirable photoelectrochemical stability,but the photocurrent density are restricted by their poor conductivity.Element doping is a comment stragety has been utilized to increase the majority concentration or carriers mobility of these materials,which aims to improve charge transport property.Meanwhile,we should also evaluate the possible impacts of dopants on the onset potential or photoelectrochemical stability on these materials.Therefore,it is crtical to choose proper dopants and modified process to get an optimized performance.In this dissertation,we will take the CuWO4 and BiVO4 photoanode materials as examples,to study the influence of element doping on photocurrent density and onset potential.The main contents are as follows:(1)Improving the photocurrent density of CuWO4 photoanode with Sn4+doping.CuWO4 photoanode materials received much concerns recently,due to the incorporation of Cu2+,the band gap was reduced to 2.25-2.4 eV,which has broaden the utilization wavelength range of solar spectrum.However,the poor conductivity restricts its photocurrent density.We introduce 3%proportion Sn4+ into CuWO4 thin film by substituting Cu2+ lattice to improve the charge transport property.The majority concentration is demonstrated to be increased by a factor of 40.Under simulated AM 1.5G sunlight irradiation,the plateau photocurrent density is improved from 0.54 mA cm-2 to 1.05 mA cm-2.(2)Identifying the influence of Mo6+ doping proportion on onset potential of BiVO4 photoanode.Through Mo6+ doping to improve the BiVO4 photoanode plateau photocurrent density has been well studied.But the most reported onset potential for Mo6+ doped BiVO4 photoanode are about 0.4 V vs.RHE,which is positive than the undoped BiVO4 photoanode(0.2 V vs.RHE).We study the onset potential of BiVO4 photoanode with 0.5%and 3%Mo6+ doping proportion.With the existence of hole scavanger Na2SO3,we find the onset potential for 3%Mo6+ doped BiVO4 photoanode is 100 mV positive than 0.5%Mo6+doped BiVO4 photoanode.Then we also find the flat potential for the former photoanode was 90 mV positive than the later photoanode which suggests femi level pinning on the Mo6+ doped BiV04 photoanode surface.We believe that the Mo6+ on photoanode surface served as the source of surface states which cause femi level pinning and flat potential positive shift.The charge seperation ellicicency is decreased with the Mvio doping proporation raised.By dipping the photoanodes in 1 M NaOH,partial Mo6+ on the surface can be etched through the surface corrosion process which can slightly negative shift the onset potential by 20-30 mV.