Research On The Application Of MOFs As Hydrogen Storage Materials In Fuel Cell Electricpropulsion System For Ships

Hydrogen fuel cell is an ideal energy storage mode for renewable energy powered ships,but it lacks efficient hydrogen storage technology that matches the characteristics of energy density and ship basic load variation.Metal-organic frameworks(MOFs)have been extensively studied in the research field of solid-state hydrogen storage,especially in those of hydrogen storages at low temperatures due to their easily adjustable and modifiedable pore structures and surfaces.However,there is no research report on the application of MOFs hydrogen storage on ships.The purpose of this paper is to carry out research on the applicability of MOFs hydrogen storage in fuel cell powered electric propulsion system for ships.The Grand Canonical Monte Carlo(GCMC)simulation was conducted to determine the density of adsorbed phase of hydrogen molecules confined within the spaces of the MOFs.The specific volume of adsorbed phase was much accurately determined through a modified approach of nonlocal density functional theory(NDFT).The main factors affecting the capacity of hydrogen storage on MOFs at low temperature were determined.Accordingly typical MOFs were selected out and then synthetized,and their dehydrogenation performance was tested.The matching experiment between MOFs low temperature adsorption hydrogen storage system and fuel cell was carried out under typical navigation conditions.Because of the variety of MOFs,it is necessary to analyze the main factors affecting the adsorption behavior of hydrogen on MOFs,and select the suitable MOFs for hydrogen storage.For this reason,using GCMC simulation as a theoretical analysis tool,21 typical MOFs in MOFs crystal database were selected.After determing the characterization parameters of MOFs such as crystal density,specific surface area,specific pore volume and helium void fractions of MOFs were determined the analysis software.GCMC simulation of hydrogen adsorption on MOFs was carried out at-196 C and 10 MPa.The simulation results show that lightweight structure with super high specific surface area is crucial factor to ensure the high hydreogen adsorption capacityMOFs.Considering the adsorption specific volume,the adsorption capacity of IRMOF-1 is similar to that calculated by equivalent liquid hydrogen density,while the hydrogen storage capacity of MIL-101 is larger than that of IRMOF-1 because of its mesoporous structure.IRMOF-1 and MIL-101 were selected for synthesis after comparing the effects of different pore structures on hydrogen storage performance.In order to determine the adsorption specific volume of MOFs,IRMOF-1 and MIL-101 with complete crystals were respectively synthesized by mechanical chemical method and hydrothermal reaction method.For evaluating the integrity of the MOFs crystals,the prepared samples were analyzed using XRD,SEM and thermal TG method.In order to accurately calculate the pore size distribution(PSD)of the MOFs,the adsorption isotherm of argon at 87 K on two MOFs was firstly measured,and the surface roughness correction function of modified adsorbents was introduced to modify the classcial NLDFT theory.The prediction accuracy of adsorption isotherms of argon at 87 K on two MOFs was improved,and the calculation accuracy of PSD and the adsorption specific volume of two MOFs was increased.In order to further screen suitable MOFs hydrogen storage materials from IRMOF-1 and MIL-101,the adsorption equilibrium of hydrogen on the prepared samples was analyzed at-196 C,and the experiment for testing the stability of crystal structure was also conducted.Isotherms of excess amount of hydrogen adsorption on IRMOF-1 and MIL-101 were measured.The absolute adsorption amount and the isosteric heat of adsorption were then determined by Toth equation.The results of stability experiments show that the mass densities of IRMOF-1 and MIL-101 are 7.93 wt% and 7.79 wt% respectively at-196 C and 10 MPa when they are intact crystals..After 168 h storage,the structure of IRMOF-1 collapsed and the hydrogen storage capacity was only 37% of that of the complete crystal,while the structure of MIL-101 remained unchanged.Therefore,MIL-101 was selected as the hydrogen storage adsorbent.In order to verify the feasibility of MOFs cryo-adsorption hydrogen storage system in ships,a test platform,which consists of a fuel cell electric propulsion test rig and a hydrogen charging and discharging test rig,was built for evaluating the performance of the electric propulsion system powered by fuel cell.Tests for the hydrogen desorption kinetics of MIL-101 adsorbent bed were carried out on the experimental platform,which was further applied to perform the matching experiment between the flow rate of the released hydrogen from hydrogen storage system via cryo-adsorption on MOFs and the output power from the fuel cell stack.A cruise ship navigating around the lake in Xiamen was selected as the prototype ship whose composition of the power was taken as a reference for constructing ship's hybrid electric propulsion system powered by the fuel cell and lithium battery.The technical parameters of the fuel cell and the simulated electronic load equipped on the test rig were determined as per the requirements in power supply on the electrical propulsion systemin selected ship under typical navigation modes.Meanwhile,the batch synthesis of MIL-101 and the design of a conformable tank for hydrogen storage were carried out,and the test unit of the electrical propulsion system powered by the fuel cell was built.Secondly,the requirement for the flow rate of the released hydrogen form the hydrogen storage system using MOFs cryo-adsorption was determined in terms of the characteristics of the output power from the fuel cell,and the kinetics of hydrogen discharging from the adsorbent bed packing with MIL-101 were tested at different current densities of the fuel cell,and the output power points of fuel cell which can meet the requirement for typical navigation conditions of navigation were finally determined.Finally,the low-temperature adsorption hydrogen storage system of MOFs was used as the hydrogen supply unit of the fuel cell powered electric propulsion system.The matching between the flow rate of hydrogen release and the output power of the fuel cell under typical navigation conditions was investigated and analyzed to verify the applicability of the MOFs hydrogen storage system on ships.