Preparation of Iron-based Oxygen Carrier for Hydrogen Storage via Chemical Looping Oxygen Diffusion
Author:Zeng De Wang
The difficulties of using renewable energy and the wast of wind and water energy have become a prominent problem in renewable energy power generation systems.Hydrogen production from electrolyzed water using the excess electric energy and the subseqeunt hydrogen storage as well as fuel cell power generation is considered to be an effective solution.However,the large-scale application of this technology is still severely constrained by the low safety and volumetric hydrogen storage density of traditional hydrogen storage methods(e.g,high pressure hydrogen storage,hydraulic hydrogen storage).To address this issue,this paper proposes a new hydrogen storage method based on the principle of circulating oxygen vacancy,i.e.chemical looping hydrogen storage.Focusing on how to improve the hydrogen storage density and lifetime of materials,the effects of various reaction conditions and material structures on hydrogen absorption and desorption and hydrogen storage stability of oxygen carrier in chemical looping hydrogen storage process were investigated.the multi-scale structure and property relationship was established.thus,a two-dimensional Cu-Co-Fe-Al hydrogen storage composites material was designed and synthesized,which has high volumetric hydrogen storage density 80 kg/m~3 and 60 continuous cycle stability.Finally,a kilowatt-scale electrolyzed water hydrogen-hydrogen storage-fuel cell power generation demonstration system was built to study the energy optimization between each module,and the feasibility of the new chemical looping hydrogen storage-fuel cell power generation technology was demonstrated.In order to implement the rational design of high-performance hydrogen storage materials,this paper first studied the influence of the oxidation and reduction atmosphere,the reaction temperature,the choice of hydrogen storage medium,the pore structure of the material and the interface structure of the material etc on hydrogen absorption and desorption characteristics and stability of the material.The results show that with the increase of hydrogen concentration,the rate of hydrogen absorption is faster and faster.When the concentration of hydrogen reaches above 45%,the rate of hydrogen absorption is basically unchanged.The reaction temperature is from 500 ~oC to 900 ~oC,as the reaction temperature increases,the hydrogen absorption rate increases rapidly.When the reaction temperature reaches 800 ~oC the hydrogen absorption rate no longer increases significantly.In terms of material structure characteristics,the CuFe material with single super-large pore distribution,dimension structure and interface-bound structure exhibits better cyclic reaction performance.And the CoFe spinel material with small pore large specific surface area shows better hydrogen absorption and desorption kinetics.In order to obtain the deeper lattice dynamics of the hydrogen absorption and desorption process,the insitu photoelectron spectroscopy was used to trace the oxygen species changes during hydrogen absorption and desorption,and the evolution of oxygen species during hydrogen absorption and desorption was obtained.On this basis,the DFT+U method was used to simulate the change of oxygen ions in the process of hydrogen absorption and desorption.The results show that the oxygen atoms on the surface react with the reaction gas to form the corresponding oxygen vacancies during the reduction reaction and then the subsurface oxygen atoms migrate to the surface to fill surface oxygen vacancies,the third layer of surface oxygen atoms migrate to the subsurface to fill the oxygen vacancies on the subsurface,and so on.At the late stage of the reduction reaction,the energy barrier of oxygen atom migration increases with the progress of the reaction,while the surface characteristics are unchanged,so the reaction rate is limited by the oxygen atom migration rate.In the oxidation reaction stage,the energy barrier of the water vapor directly forming the Fe-O bond on each surface of the catalyst body phase is greater than the energy barrier of the oxygen atom moving from the surface layer to the bottom layer.So,the process of catalyst oxidation occurs at the surface,and oxygen atoms are transferred layer by layer until the oxygen vacancies in the bulk phase of the catalyst are filled.In the early stage of the oxidation reaction,the reaction rate is controlled by the process of formation of surface Fe-O bonds.At the later stage of the reaction,the rate is limited by rate of lattice oxygen transport in the bulk phase.Based on the experimental and simulation results,the LDH material was used as the precursor to construct the CoCuFeAl oxygen carrier hydrogen storage material with composite nanostructures from the three dimensions of phase composition,pore distribution and reaction interface.The pure LDH precursor and the spinel oxide obtained after calcination were obtained by method of urea ultra-uniform coprecipitation.The characterization and experimental results show that:(1)LDH material as a layered metal oxide material,the material itself has no void structure but the secondary pores formed between the layers have large pore characteristics.In the high temperature chemical looping cycle it alleviates the collapse of the pores due to lattice tension and increases the pore structure stability of the material.(2)The addition of Co and Cu elements to the iron-based LDH material,the incorporation of Co element improves the hydrogen storage capacity of the material.Cu and Fe element form vickers which improves the phase cycle stability of the material.(3)The molecular structure of the LDH precursor determines the uniform distribution of the active component on the inert component.Since the longitudinal dimension of the two-dimensional layer is significantly smaller than that of the particulate oxygen carrier,the active component is largely encapsulated inside the inert component,which effectively alleviates the agglomeration and sintering of the active component inside the material.(4)The longitudinal dimension of the layered structure is small,the migration path of O ions is short,which enhances the transmission of lattice oxygen and the kinetics of hydrogen absorption and desorption of the material.In the 60 cycles,volumetric hydrogen storage density of LDH material is about 80 kg/m~3.After the cycle,the hydrogen storage density of the material is still as high as 70 kg/m~3,which is four times that of pure iron.Based on the hydrogen storage technology of chemical looping,a kilowatt-scale demonstration device of electrolyzed water to produce hydrogen-chemical looping hydrogen storage-fuel cell power generation was designed and built,and the feasibility of applying chemical looping hydrogen storage technology to hydrogen storage system was demonstrated.The developed CoFeCuAl LDH oxygen carrier was used as the hydrogen storage material.The operating characteristics and energy matching relationship of each module of the device were studied.The experimental results show that the device can achieve stable operation under normal pressure conditions,and the operating indexes of the modules in the system have not changed significantly in 20 cycles.The average hydrogen storage density of the hydrogen storage system can reach89.7kgH2/m3,and the average hydrogen release rate is 5.09 L/min,which is greater than the theoretical hydrogen consumption rate of the fuel cell(4.07 L/min),which can achieve self-balancing of the hydrogen flow.The operating temperature of the system is medium and high temperature(600~oC~900~oC),which forms a good temperature match with the solid oxide fuel cell with the highest power generation efficiency.In actual operation,the hydrogen production efficiency of electrolyzed water is 52.6%,and the average efficiency of hydrogen storage and release can reach 90%,the fuel cell power generation efficiency is46.1%,and the total power conversion efficiency of the device is 21.8%,reaching the advanced level of the existing hydrogen storage device.The technical and economic analysis of the device was carried out on the results of this experiment.It was found that the total cost of preparing one ton of chemical looping hydrogen storage material was 56.22 thousand yuan/ton,and the per cost of hydrogen storage material was 4.92 yuan/kg,to compare to current commercial high pressure hydrogen storage methods there is significant cost advantage.For the 1MW-scale hydrogen storage power plant,the total cost of constructing the device is about 2.36 million yuan,about 2300 yuan/kW.