Preparation Of New Silicon-based Photocathodes And Study On Hydrogen Evolution Behavior Of Them

"Carbon peak and carbon neutrality" has been included in the overall layout of ecological civilization construction in China,setting the national green development strategy.To achieve carbon neutrality,we need to adjust and optimize the energy structure and vigorously develop new energy sources.Hydrogen is considered to be the first choice to replace fossil fuels,and photoelectrochemical(PEC)hydrogen evolution reaction(HER)is a promising method of hydrogen production.Silicon is widely studied as photoelectric cathode because of its abundance in the earth’s crust,narrow band gap,and suitable conduction band position.However,silicon also has some defects,such as low open circuit and the inert kinetics of hydrogen production.The key to increasing the hydrogen production rate of silicon-based photocathodes is to change the internal energy band structure of the electrode and to support high-performance catalysts on the electrode surface.In this thesis,based on n+p/Si/Ti with low reflectivity and chemical stability,the heterojunction interface was designed to change the energy band structure,increase the open circuit voltage.At the same time,a facile method was used to support the catalyst on the surface of the silicon wafer for research and improvement.The main research contents are as follows:(1)In view of the high carrier recombination rate,low open circuit voltage and low photoelectric conversion efficiency of Si,a semiconductor layer was loaded on the surface of the silicon wafer,and the NiCoSx catalyst was supported by vulcanization of chloride precursor to construct a "silicon-semiconductor-catalyst" composite photoelectrode structure.The study found that the onset potential of n+p-Si/Ti/NiCoSx was 0.70 V vs.RHE(J=-0.1 mA cm-2)and the current density under 0 V vs.RHE was-11.3 mA cm-2.The PEC-HER performance of constructed n+p-Si/Ti/CdS/NiCoSx,n+p-Si/Ti/Cu2S/NiCoSx and n+p-Si/Ti/WS2/NiCoSx have all been improved.Studies have shown that WS2 has the most obvious promotion effect.Its onset potential was 0.79 V vs.RHE(J=-0.1 mA cm-2),and the current density under 0 V vs.RHE was-16.2 mA cm-2.The analysis showed that after compounding WS2 layer,the impedance was reduced,and the hydrogen production kinetics was accelerated.At the same time,the carrier concentration increased,resulting high degree of band bending,which was conducive to hydrogen evolution.The band position of WS2 was favorable for electron-hole separation,reducing the electron-hole recombination rate and enhancing the hydrogen production performance of the photocathode.(2)In view of the slow hydrogen production kinetics of silicon and the poor compatibility of the silicon substrate with the catalyst interface,the photo-assisted electrodeposition method was used to form efficient tungsten-doped cobalt hydroxide(W-Co(OH)2)nanosheets array on the surface of the silicon wafer to improve the hydrogen evolution kinetics on the silicon surface.The onset potential of n+p-Si/Ti/W-Co(OH)2 was 0.66 V vs.RHE(J=-0.1 mA cm-2),which was positively shifted by 0.52 V relative to n+p-Si/Ii,and the photocurrent density at 0 V vs.RHE was-22 mA cm-2,which was significantly higher than that of n+p-Si/Ti(-1.4 mA cm-2).The stability test of n+p-Si/Ti/W-Co(OH)2 electrode at-10 mA cm-2 for 7 hours showed no significant attenuation.On one hand,the excellent performance of n+p-Si/Ti/W-Co(OH)2 was attributed to the effective coupling of the surface catalyst and the semiconductor layer.On the other hand,the W-Co(OH)2 nanosheets array made it have a high density of active sites,and the doped W adjusted the electronic structure of Co.The doping of W reduced the impedance of the catalyst/electrolyte interface,accelerated the kinetics of hydrogen production,and at the same time increased the carrier concentration,which made the band bending degree greater.The DFT theoretical calculation results showed that Co(OH)2 doped with W can optimize the hydrogen adsorption energy,reduce the Gibbs free energy of the rate-determining step,which was more conducive to the HER reaction.