Thermochemical Energy Storage Characteristics And Mechanism Study Of Composite Metal Oxides

The development of green and low-carbon renewable energy power generation technology is an important force for implementing the dual-carbon strategy.The solar thermal power generation technology coupled with large-scale thermal storage has the advantages of outstanding stability,fast response rate,and good peak shaving flexibility,which can provide key support for the stable grid-connected power generation operation of fluctuating renewable energy.Raising the collector temperature is the most effective way to improve the operating efficiency of the solar thermal power generation system and reduce the system cost.However,limited by the thermal storage operating temperature（<560°C）,currently commercial solar salt thermal storage medium cannot meet the high-temperature thermal storage（>850°C）requirements of the solar high-temperature Brayton cycle power generation system.Aiming at this bottleneck,a series of experimental and theoretical studies have been carried out on the thermal storage characteristics and reaction mechanism of thermochemical energy storage media based on metal oxides suitable for CSP in this paper,laying a research foundation for the engineering application of high temperature thermochemical energy storage.The cation substitution doping can form composite crystal phases and improve the reaction kinetic parameters of the Mn₂O₃/Mn₃O₄ thermal storage system.In this work,the homogeneous solid solution of Mn-Fe is formed by introducing Fe³⁺to improve the reactivity,and the effects of doping ratios,preparation methods and reaction conditions on the reversibility of the reaction of Mn-Fe composite metal oxides are explored.When the Fe doping ratio reaches 20 mol%,the material of(Mn_0.8Fe_0.2)₂O₃ can achieve a completely reversible re-oxidation reaction process,and its comprehensive mass thermal storage density can reach 797.3 k J kg^-1.The material still maintains 88.16%and 81.28%reduction and oxidation reactivity after 600 cycles of testing.(Mn_0.8Fe_0.2)₂O₃ can still maintain good reversibility at higher oxygen partial pressure,while lower oxygen partial pressure（5 vol%）may inhibit its re-oxidation reaction process.The analysis of the reaction kinetics showed that the reduction and oxidation reactions can be described by the A4 and A3 mechanism functions of the Avrami-Erofeev nucleation and growth model respectively.In addition,the reaction characteristics of formed honeycomb thermal storage mediums via(Mn_0.8Fe_0.2)₂O₃powders are studied.Obvious temperature plateaus were observed during the thermal storage/release process,indicating that the material had a relatively constant endothermic and exothermic temperature in thermal storage/release process.The reactivity of the(Mn_0.8Fe_0.2)₂O₃ honeycombs can reach up to 99.4%,and maintain more than 85%reactivity after 100 cycles.To investigate the modification mechanism of Fe³⁺doping on the Mn₂O₃/Mn₃O₄system,the microscopic reaction mechanism of Mn-Fe composite metal oxides is investigated in detail through experimental study and first-principles calculations.Through XRD and in-situ XRD analysis,it is found that the introduction of Fe can make the reduction state of Mn₂O₃ change from single Mn₃O₄ phase to composite crystal phases composed of Mn Fe₂O₄ and Mn_2.7Fe_0.3O₄,and in the re-oxidation process Mn_2.7Fe_0.3O₄ and Mn Fe₂O₄ will be oxidized successively to form the uniform(Mn_0.8Fe_0.2)₂O₃ phase.The results of high-resolution TEM and EDS show that the material exhibits a core-shell layered structure of Mn Fe₂O₄@Mn_2.7Fe_0.3O₄ in the reduced state,and the transition between the homogeneous structure and the core-shell structure could be a significant improvement in the reaction performance.Through XPS analysis,the proportion of lattice oxygen in the surface oxygen of the material increases after doping with Fe,thereby increasing the reactive sites.Through the density functional theory calculations,it is revealed that Fe doping can promote the diffusion of O₂ on the surface and inside of the crystal,enhance its adsorption capacity,and reduce the migration barrier of O^2-in the crystal lattice,hence promoting the re-oxidation process.To solve the problems of slow re-oxidation rate of CuO/Cu₂O system and easy sintering and deactivation in multiple reaction cycles,doping Al₂O₃ to form CuAl₂O₄support to improve the cyclic reaction characteristics of CuO/Cu₂O is proposed in this work.We also further study the effects of different doping ratios,preparation methods and reaction conditions to endothermic/exothermic reaction properties of materials.When doped with 10wt%Al₂O₃,the re-oxidation process of copper oxide is almost completely reversible,and the mass energy storage density can reach 940.88 k J kg^-1.Similarly,it is found that higher oxygen concentration（>50 vol%）would inhibit the reduction reaction process of the material,while the material could still maintain fully reversible reactivity at lower oxygen concentration（5%vol%）.After doping the CuAl₂O₄ support,the cycling stability of the material is significantly improved,and its reduction and oxidation activity are 96.58%and 87.23%after 1000 cycles respectively.Al doping also affects the functional model of the reaction mechanism of the material.The reduction and oxidation processes of pure copper oxide are described by the three-dimensional diffusion model,and then the redox reaction processes are transformed into segmented descriptions by the second-order and first-order models after Al addition.The mechanism of CuAl₂O₄ support on the anti-sintering effect of CuO/Cu₂O is studied in this work.XRD analysis shows that CuAl₂O₄ would participate in the redox reaction and can be reduced to CuAl O₂.Combined with thermal analysis,the synergistic reaction mechanism of CuO/Cu₂O and CuAl₂O₄/CuAl O₂ is explored.Through SEM-EDS and TEM-EDS analysis,it is found that CuAl₂O₄/CuAl O₂ can be stably and uniformly supported on the surface of CuO/Cu₂O crystal in the form of the support to form a substrate-support composite structure.Through DFT calculation,it is revealed that oxygen diffusion is not the limiting factor of the re-oxidation rate of CuO/Cu₂O,and its higher Cucation vacancy formation energy can easily form cationic defects on the crystal surface,which is easy to cause grain sintering and agglomeration.The relatively lower surface of CuAl₂O₄/CuAl O₂ enables it to stably adhere to the surface of the CuO/Cu₂O substrate,and can resist the driving force of crystal sintering by applying a pinning force to the substrate,and the CuO/Cu₂O substrate grains are stably dispersed in the pore space by spatial limitation.Therefore,the grain size of CuO/Cu₂O remains relatively stable to maintain the high reactivity,thereby reducing the reactivity degradation caused by crystal sintering and growth.This work is expected to provide a reference for the formulation design and microscopic reaction mechanism research of solar high-temperature thermochemical energy storage materials.