Study On The Characteristics Of Hydrogen Production By Microwave Two-step Thermochemical Cycle Water Splitting Based On High-Entropy Oxides

Hydrogen production via solar two-step thermochemical water splitting is a green fuel production method using concentrated solar energy to produce hydrogen and carbon monoxide,which has become a potential strategy for storing sparse and intermittent solar energy.However,in a typical two-step thermochemical cycle reaction,the thermal reduction temperature is as high as 1300℃ and the reduction time is at least 0.5 h.This high temperature process hinders the application of the two-step thermochemical hydrogen production technology.Efficient energy utilization and efficient rapid reaction process are the common pursuit of technological innovation in energy,environment and chemical industry.Well,microwave,as a representative of electromagnetic energy,can achieve non-contact,high penetration and selective energy intervention,thus greatly improving the rate of chemical reaction.In this dissertation,a series of research work has been carried out on how to reduce the energy imbalance of the two-step thermochemical cycle process to improve energy conversion efficiency,and to achieve low-cost,controllable and stable construction of microwave high-energy sites.A technical route for hydrogen production by microwave driven thermochemical water splitting based on high entropy oxide has been proposed.To this end,a high entropy oxide with equimolar ratio of Fe,Mg,Co and Ni cations,supported by SiC ceramic foam FeMgCoNiOx@SiC(FMCN@SiC),was prepared by sol-gel method.Firstly,the effects of microwave power(300-900 W,2.45 GHz),thermal reduction time and discharge plasma on the water splitting performance of FMCN@SiC were studied under microwave conditions,that is,the reduction and hydrolysis reactions were carried out at the same microwave power.Secondly,the water splitting performance of FMCN@SiC was studied under microwave synergistic conventional heating process conditions,that is,microwaves for thermal reduction reaction and conventional heating for water splitting reaction.Further,based on experiments and test results,the mechanism of highly efficient formation of oxygen vacancies and rapid water splitting for hydrogen production by microwave induced FMCN@SiC was deduced.Then,under the condition of microwave synergistic conventional heating,the water splitting ability of FMCN/Zry(y=0.0,0.1,0.2,0.4,0.6,0.8 and 1.0)was systematically studied.Finally,based on the literature review,the hydrogen production capacity of FMCN/Zry was compared with that of advanced materials,and the energy efficiency of microwave and conventional thermal systems was analyzed.The specific research contents are as follows:(1)Study on the hydrolysis characteristics of FMCN@SiC under microwave irradiation conditions.Under the power of 500-900 W,thermal reduction step could be completed after 4-6 minutes of microwave irradiation.The results of water splitting step(300 mL/min-N2/28 voL.%H2O)suggested that the introduction of steam under different microwave powers could immediately trigger the generation of hydrogen.Especially at 700 W,the hydrogen release rate increases rapidly from the initial stage to the peak rate of 468 μmol/min/g.The total amount of hydrogen generated within 30 minutes was 5.42 mmol/g,which was much higher than that of other microwave power levels(500 W-3.05 mmol/g,900 W-1.23 mmol/g).In addition,due to the uneven loading of the FMCN on the SiC foam,the microwave induced FMCN@SiC discharge to generate ·OH,·H and·O radicals,which strengthened the decomposition of water and showed an extraordinarily high hydrogen yield,and also led to sintering and loss of the sample.Meanwhile,the results of this study prove the feasibility of microwave driven two-step thermochemical cycle to split water.(2)Study on the hydrolysis characteristics of FMCN@SiC under microwave synergistic conventional heating conditions.Compared with microwave irradiation conditions,the WS reaction of FMCN@SiC was more stable at 700 W/800℃(300 mL/min-N2/29 voL.%H2O),4.75 mmol/g hydrogen could be produced after hydrolysis for 60 min.After 5 consecutive cycles,the hydrogen yield of FMCN@SiC hardly decreased,with an average hydrogen yield of 4.34 mmol/g.Furthermore,the water vapor content has a significant effect on the release rate of hydrogen.When H2O(g)content was above 8 vol.%,hydrogen yield increased linearly with the increase of H2O(g)content.However,in the conventional thermal hydrolysis process,FMCN@SiC showed a relatively slow hydrolysis kinetics,and the hydrogen release rate was still up to 50 μmol/min/g after hydrolysis for 60 min.(3)Study on the hydrolysis characteristics of Zr4+doped FMCN/Zry.With the increase of Zr4+content,the hydrogen yield of FMCN/Zry increased at first and then decreased gradually,and the doping of 60%Zr4+had a positive effect on the formation of oxygen vacancy and the hydrolysis reaction of FMCN.At 700 W/800℃(100 mL/min-N2,27 voL.%-H2O),FMCN/Zr-60 had the highest hydrogen yield of 4.84 mmol/g,which was twice that of FMCN.The effects of temperature and hydrogen background on the hydrolysis capacity of FMCN/Zr-60 were further studied.The results showed that increasing the temperature within 900℃ was beneficial to the forward progress of water splitting reaction.Moreover,FMCN/Zr-60 still had a hydrogen yield of 1.5 mmol/g under the extreme condition of H2O:H2 ratio of 287:1.(4)The comparison and analysis between microwave and conventional system.In this dissertation,a variety of reported novel materials with excellent hydrogen production potential have been compared.These catalytic materials are usually tested by traditional heating method,the reduction temperature of which could ranges from1300 to 1500℃ with relatively long time for reduction and oxidation.Therefore,the hydrogen yield per unit of time using the traditional catalytic material requires more energy input than that of this experiment.Efficiency analysis showed that the energy consumption of microwave system in thermal reduction reaction was only 3%of that using the conventional heating system.Unlike conventional processes,which require heating to above 1300℃,microwave systems operate at temperatures between 500 and 650℃ during thermal reduction reactions.Even domestic microwaves can be used to drive water splitting cycles,and this temperature has practical significance for current industrial equipment.