气体中吸附表征和储氢研究检测方案(蒸汽吸附仪)

There has been a recent restoration of interest in gasstorage media， which has stemmed from the drive touse hydrogen as a fuel. Hydrogen has one mainbenefit over other fuels. Water is the only by-productproduced when using hydrogen as a fuel. Hydrogen istherefore a zero emission fuel. Zero emission transportation is becoming more andmore of a necessity due to air pollution and the limitedresources of oil available. The drive to develop a”hydrogen economy” has been increased by evertightening legislation. The interest in hydrogen as a fuel was brought to focusin the early 1960’s. At this time rechargeable metalhydrides were studied for hydrogen storage. Hydrogenhas the potential to be a low pollution， multi-purposefuel. In particular the storage of hydrogen for Hfuelled internal combustion engines was commonlydiscussed. Today the emphasis is more on fuel cells but for any“hydrogen economy"to be viable a suitable means ofhydrogen storage is required. What is required for a good hydrogen storage material? Hydrogen gas can be stored as a compressed gas orsorbed onto a solid material (Hydrogen Storage Material). A good hydrogen storage material needs todemonstrate clear advantages over other methods. Safety -Low operating pressures are desirable.Compressed gas storage methods have high operatingpressures (100’s of bars) whereas it is possible to havea similar capacity in a metal-hydride alloy but anoperating pressure of only a few bars. Capacity-A good gas storage material has to hold alarge quantity of hydrogen. Compactness - The storage needs to be compact andlight for use on vehicles. Cycle life -The materials need to be reusable andtherefore require the ability to adsorb and desorb thesame amount of hydrogen repeatedly withoutdeterioration. Cost-The cost of the material needs to beeconomically viable. What type of materials are being studied for use asHydrogen Stores? The most studied materials are metal alloys the bestknown of which is LaNis. The search is still on to findless expensive materials with equivalent capacities. Atpresent there is a great deal of interest in a secondbranch of materials the carbon nanotubes. The application of the IGA system for theCharacterisation of Gas Storage Media The Intelligent Gravimetric Analyser (IGA)wasinitially developed for the accurate determination ofhydrogen anddeuterium aabsorption(desorption)characteristics in various storage alloys andintermetallics. Accurate， high-resolution measurementand reproducibility， afforded by the IGA system， areessential qualities when performing such studies. Theintelligent nature of the IGA method is a pre-requisiteto performing such experiments where equilibrationtime scales can increase by orders of magnitude duringahydride phase transition. Measurement of thedesorption branch enables the full hysteresis envelopeto be determined. The nature of the IGA method is tohold the pressure constant (after step change in systempressure) during the weight equilibration phase. Thisguarantees that the true absorption/desorption branch ismeasured as the boundary conditions， and hence thedriving chemical potential， are held constant. This isnot the case in a volumetric measurement where thepressure is allowed to relax： if the pressure relaxes thefinalequilibrationn value is not the trueabsorption/desorption branch but some pseudo-valuewithin the hysteresis loop. Hence the volumetricisotherm will be dependent on the pressure step size. Theel real-timee analysis of the individualmassrelaxation in response to a step change in systempressure gives valuable insight into the dynamics ofthe absorption/desorption process and an added extradimension to the P-C-T data. BET surface area measurements are easily performedin-situ by immersing the sample reactor in liquidnitrogen. Nitrogen or Argon can be used as theworking gas and automatic analysis in software allowsfor quick calculation of the surface area. Such surfacearea determinationncanibe carried Out withoutremoving the sample from the vacuum system and canthus be performed periodically during the course of aseries of hydrogen absorption/desorption isotherms. The extensive IGASwin software package can be pre-programmed to performa suiteofisothermameasurements at several temperatures and a built inevent sequencer can be used to perform cyclic pressureand temperature operations during sample activation.Semi-automatic features allow for pressure ramp andpressure cycle operations to be performed. In summary the IGA can measure capacity， kinetics， P-C-T maps， operating pressures and can determinelifetimes using cycling studies. Examples of Hydrogen Storage Studies with the IGA. Hydrogen absorption/desorption cycling studies can beperformed using the IGA to assess the useful lifetimeof storage materials. Cycles can be set-up in the userfriendly software and run automatically. Figure 1，shows deuterium adsorption and desorption isothermsduring two cycles measured on a PdY alloy， notice thechange in the second adsorption branch The capacity of hydrogen in materials as a function ofpressure and temperature (P-C-T maps) can bemeasured using the IGA by measuring the absorptionisotherms of materials. Figure 2 shows hydrogenabsorption isotherms measured on a PdY alloy at 5different temperatures. In addition to measuring capacity the IGA also provides kinetic information inthe form of chemical diffusion coefficients， figure 3.The kinetic parameters provide important informationwhen considering a material for hydrogen storageapplications， the speed of hydriding and dehydriding. Figure 4 is an example of an hydrogenabsorption/desorption isotherm measured using the IGA on carbon nanotubes. This measurement wasperformed at liquid nitrogen temperatures in order to enhance the hydrogen capacity. Reports have beenmade of capacities of 50% in the literature but thesewere determined using the volumetric method. Theyhave not yet been verified using the more accurategravimetric method. The debate at present is centredon proving that the adsorption measured is trulyhydrogen and not impurities in the hydrogen such aswater. The interesting feature in figure 4 is thereversibility of the isotherm. Any impurity sorption at77K would be irreversible..The high degree of reversibility indicates that the sorption measured isprimarily hydrogen. An example of a BET analysis on the storage materialLaNi5 after activation and cycling is shown in figure5. BET analysis can be performed before and afteractivation and cycling in-situ， avoiding the need toremove the sample from the apparatus andthusavoiding exposure to air. There are many referenced papers in the literaturediscussing hydrogen sorption measurement performedusing the IGA. Some references are provided below. References： “The Measurement of Concentration-DependentHydrogen Diffusion Coefficients in the Solid-SolutionAlloy Pd-Y”Stonadge P.R.， Benham M.J. and Ross D.K. Separation Technology (1994) 129 “Towards the Elucidation of Hydrogen Diffusion in anHomogeneous Medium”Ross D.K. and Stonadge P.R. Advanced Matrerials 1993 /B： Shape Memory Materials and Hydridses“Deuterium Absorption in Pdo.9Yo.1 Alloy”Poyser M.A.， Kemali M. and Ross D.K. TJ. Alloys andCompounds 253-254 (1997) 175-180“Experimental Determination of Absorption-Desorption Isotherms by Computer-ControlledGravimetric Analysis”Benham M.J. and Ross D.K. Z.Physik. Chemie NF 163(1989) www.HidenAnalytical.com IGA Application Note iden Analytical Ltdwww.HidenAnalytical.com