High Pressure Hydrogen Storage on Carbon Materials for Mobile Applications

Abstract

Recognising the difficulties encountered in measuring the adsorption of hydrogen at high pressure, a reliable volumetric differential pressure method of high accuracy and good repeatability has been developed for measurement up to ca 100 bar. The apparatus used has two identical limbs, a sample and a blank limb, between which a high accuracy differential pressure cell measures changes in pressure. By simultaneously expanding the two limbs and closely controlling the temperature of the entire system, many of the errors due to expansion of the gas can be avoided. In addition, helium blank measurements are used as a base line correction, which substantially reduces the effects caused by the rapid expansion of gas through a small port. Using this method, the hydrogen storage capacities of relatively small samples (1.0-2.5 g) of a selection of carbon materials have been accurately measured to a conservative limit of detection of 0.05 wt% and an accuracy of ±0.02 wt%. The accuracy of the apparatus has been proven using lanthanide nickel (LaNi5), which has a known hydrogen storage capacity of 1.5 wt%, as a standard. The method has also been developed in order to analyse samples at elevated temperatures of up to 270 °C. This has been demonstrated using lithium nitride (Li3N) compounds. The carbon materials studied include a series of activated carbons, carbon nanofibres (CNF) and carbon nanotubes (CNT). The activated carbons have displayed almost instantaneous hydrogen uptake independent of the degas method used, which indicates that sorption occurs via a physisorption mechanism. The series of powdered activated carbons have displayed direct correlation between the BET surface area and the hydrogen sorption capacity. The largest hydrogen sorption capacity observed for activated carbons was for a chemically activated carbon with a surface area of 3100 m2 g-1, achieving an uptake of 0.6 wt%. The preparation of CNF, grown from ethylene over mixed copper, iron and nickel alloy catalysts, has been extensively investigated. Control of the parameters of preparation has allowed the formation of CNF with surface areas of 10 - 500 m2 g-1, diameters of 100 - 1000 nm, lengths of 1-10s μm, gas conversions of 0-90 % and the formation of herringbone and platelet CNF structures. The CNF studied have been observed to be capable of adsorbing a maximum of 0.5 wt% hydrogen at 100 bar and ambient temperature. Abstract ____________________________________________________________________ iii Only one of the materials studied was observed to break by a significant amount the trend of surface area vs hydrogen sorption capacity, observed for the activated carbons. This was a single-walled nanotube (SWNT) sample which achieved ca 1.6 wt% after slow carbon dioxide activation at low temperature. This larger sorption is hypothesised to result from the hydrogen slowly diffusing into the SWNT through defects in the structure and between the graphite planes in the CNF.