Hydrogen Production From Water Splitting Reactions Promoted By Solar Radiative Non-thermal And Thermal Effects

As a secondary energy source,hydrogen energy can maximize its green and clean advantages only when coupled with renewable energy.Sun is the most common renewable energy source and always provides abundant,clean and pollution-free radiant energy to earth.Directly converting solar radiation to hydrogen can well ease its shortcomings of periodicity,low energy density and difficulty of transport.The solar hydrogen prodution involves simultaneous energy conversion and storage,thus matching the existing energy structure and mitigating climate change.When solar radiation irradiates on semiconductor,the thermal effect of temperature increment and the non-thermal effect which involves high-energy carriers such as electron-hole pairs will exhibit at the same time.Considering that infrared accounts for about 50%of the solar spectrum,only using the non-thermal effect from solar energy will cause a substantial quantity of energy lost.If all solar energy is converted into heat energy by thermal effect will prominently decrease the quality of short-wave radiation energy.Therefore,in an ideal solar cascade utilization system,the non-thermal effect can be used to drive the chemical reaction to overcome high energy barrier and the thermal effect can promote the thermal chemical reaction and output high quality thermal energy to take both‘quality’and‘quantity’of solar energy into account.In this thesis,the photothermal catalysis water splitting reaction is promoted by radiative non-thermal effect and thermal effect to obtain a high hydrogen production rate and a high solar conversion temperature at the same time.Based on the former study,a series of semiconductor oxides,which possess the potential for photothermal catalysis water splitting were synthesized and tested.It was found that the band structure and the characteristics of the EHPs are essential factors in water splitting reaction.Due to the wide radiation absorption range,good carrier migration and low EHPs reacombiantion,In₂O₃ and Ti O₂ show an excellent potential for photothermal catalysis water splitting reaction among all tested oxides.Then transition metal was used to enhance the non-thermal effect on Ti O₂ and promote water splitting reaction.Transition metal can extend the photoresponse range and reduce the carriers migration resistance.It was also found by DFT that the non-thermal effect was enhanced by the existance of impurity level in the bandgap,which can facilitate carriers separation and make it possible for electrons to be excited from the valence band through impurity level to the conduction band.The enhancement of non-thermal effects can provide more energy for chemical reaction and improve the H₂ production.After that,the influences of different two-phase interfaces and temperature rising caused by thermal effects were studied by combination of experiments and calculations.Water splitting reaction can be benefited from high temperature by the optimism of carriers migration resistance and the transient photocurrent response.Moreover,as temperature rises,the energy barrier of the rate-determining step is reduced,which may explain the positive correlation between the reaction yield and temperature.But the energy barrier of the water adsorption step also gradually rises,making the performance of liquid-solid interface better than that of the gas-solid interface.The difference of low valence and high valence Cu loading were also studied.As the copper content increased,the Cu on the surface of the Ti O₂ changed from an elemental state to an oxidized state.It was found that low valence Cu can form a suitable impurity level in bandgap which will improve the photo absorption of the catalyst,reduce the energy barrier of the rate-determining step,and promote the water splitting reaction.But Cu O will form electron-hole recombination centers and change the rate-determining step of the reaction.In the last chapter,the non-thermal effects were coupled with the thermal effects to increase the solar conversion temperature while promoting the conversion of short-wave photons to chemical energy,and explore the harvest of solar energy in terms of both‘quality’and‘quantity’.In the In₂O₃,transition metal doping showed the selectivity for the promotion of radiation absorption.Fe doping promoted the short-wave radiation absorption,which can improve the non-thermal effects,and showed better hydrogen yield at the same temperature.But Fe doping weakened the visible-infrared radiation absorption,making the lowest photothermal conversion temperature which limited water splitting reaction.Cu doping showed the opposite effect but the unsuitable band structure also caused a bad hydrogen yield.The oxygen vacancies in the catalyst may form a series of impurity energy levels in the bandgap extending the long-wave photo response.Theoretical calculations found that Cu doping significantly reduces the oxygen vacancy formation energy on the surface of In₂O₃,which will facilitate the long-wave radiation absorption.Fe doping increased the formation energy of the second oxygen vacancy making the surface oxygen vacancies harder to accumulate which may explain the long-wave radiation absorption deterioration.The reaction path bias after transition metal element doping was further calculated.As for In₂O₃ and Fe doped samples,the water splitting reaction tended to be completed through oxygen vacancies,while Cu doping greatly reduced the oxygen vacancy formation energy,worsened the H₂ generation and desorption on the oxygen vacancy and changed the water splitting reaction path.In order to couple the non-thermal effects and thermal effects by combining the advantages of Fe doping and Cu doping,the double-transition metal-doped In₂O₃ samples were designed and synthesized,which realized the co-promotion of non-thermal effects in short-wave radiation and thermal effects in long-wave radiative.After double doping,Fe&Cu-In₂O₃ showed the highest hydrogen production rate of28.63μmol·g^-1·h^-1 which was 1.8 times of In₂O₃ in constant temperature reaction and got 75 K higher than Fe-In₂O₃in radiation-driven reactions.