Abstract
A sound understanding of any sorption system requires an accurate determination of the enthalpy of adsorption. This is a fundamental thermodynamic quantity that can be determined from experimental sorption data and its correct calculation is extremely important for heat management in adsorptive gas storage applications. It is especially relevant for hydrogen storage, where porous adsorptive storage is regarded as a competing alternative to more mature storage methods such as liquid hydrogen and compressed gas. Among the most common methods to calculate isosteric enthalpies in the literature are the virial equation and the Clausius–Clapeyron equation. Both methods have drawbacks, for example, the arbitrary number of terms in the virial equation and the assumption of ideal gas behaviour in the Clausius–Clapeyron equation. Although some researchers have calculated isosteric enthalpies of adsorption using excess amounts adsorbed, it is arguably more relevant to applications and may also be more thermodynamically consistent to use absolute amounts adsorbed, since the Gibbs excess is a partition, not a thermodynamic phase. In this paper the isosteric enthalpies of adsorption are calculated using the virial, Clausius–Clapeyron and Clapeyron equations from hydrogen sorption data for two materials—activated carbon AX21 and metalorganic framework MIL101.
It is shown for these two example materials that the Clausius–Clapeyron equation can only be used at low coverage, since hydrogen’s behaviour deviates from ideal at high pressures. The use of the virial equation for isosteric enthalpies is shown to require care, since it is highly dependent on selecting an appropriate number of parameters. A systematic study on the use of different parameters for the virial was performed and it was shown that, for the AX21 case, the Clausius–Clapeyron seems to give better approximations to the exact isosteric enthalpies calculated using the Clapeyron equation than the virial equation with 10 variable parameters.
It is shown for these two example materials that the Clausius–Clapeyron equation can only be used at low coverage, since hydrogen’s behaviour deviates from ideal at high pressures. The use of the virial equation for isosteric enthalpies is shown to require care, since it is highly dependent on selecting an appropriate number of parameters. A systematic study on the use of different parameters for the virial was performed and it was shown that, for the AX21 case, the Clausius–Clapeyron seems to give better approximations to the exact isosteric enthalpies calculated using the Clapeyron equation than the virial equation with 10 variable parameters.
Original language  English 

Pages (fromto)  373384 
Number of pages  12 
Journal  Adsorption 
Volume  20 
Issue number  23 
DOIs  
Publication status  Published  Feb 2014 
Keywords
 isosteric enthalpies of adsorption
 hydrogen storage
 thermal management
 storage systems
 porous materials
 physisorption
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Profiles

Tim Mays
 Department of Chemical Engineering  Professor
 Institute for Sustainable Energy and the Environment  Director
 Centre for Sustainable and Circular Technologies (CSCT)  CoDirector
 Water Innovation and Research Centre (WIRC)
 Institute for Policy Research (IPR)
 Centre for Advanced Separations Engineering (CASE)
 EPSRC Centre for Doctoral Training in Statistical Applied Mathematics (SAMBa)
Person: Research & Teaching
Datasets

Data for cryocharging and cryokinetics analysis of hydrogen storage in MIL101 (Cr) and AX21
Bimbo, N. (Creator), Ting, V. (Creator), Mays, T. (Data Curator), Xu, W. (Researcher) & Sharpe, J. (Researcher), University of Bath, 8 Jun 2015
DOI: 10.15125/BATH00099
Dataset