Measured Hydrogen Uptake Research Articles

Performance analysis of a Sieverts apparatus for measuring hydrogen uptake

The most important measurement made on a new hydrogen storage material is the pressure–composition isotherm that captures the storage conditions and the amount of hydrogen stored. The most popular experimental approach for constructing such isotherms is the Sieverts technique, in which the amount of hydrogen absorbed or desorbed by the sample is inferred from measurements of the hydrogen pressure in a stepwise procedure. An in-depth discussion of the factors contributing to good performance is presented, based on a model of the sensitivity of the Sieverts manifold to changes in hydrogen uptake. Quantification of satisfactory performance may be achieved through a Figure of Merit for the apparatus in combination with the sample to be measured. The Figure of Merit is derived for the general case of a Sieverts apparatus in which the constituent volumes are not at the same temperature, and related to the supposed maximum amount of hydrogen taken up by the sample. The analysis confirms that (i) low sample temperature and high sample molar volume pose challenges for authoritative measurements and (ii) tests with a dense sample like LaNi 5 do not validate a Sieverts manifold for accurate measurements with high-surface-area adsorbers such as carbons and metal-organic frameworks. The calculation of a Figure of Merit for a proposed experiment provides an indication of the expected accuracy of the results, as well as scope to vary quantities such as the amount of sample in order to improve the Figure of Merit. In addition, a procedure for devising an optimised experiment is proposed, based on the reference and sample volumes of the Sieverts manifold being configurable. • Fully quantitative theory presented of the performance of a Sieverts hydrogenator. • Effects of low sample temperature and high hydrogen pressure explained. • Single Figure of Merit applicable to any Sieverts experiment developed. • Method presented to optimise reference and sample volumes to available sample mass. • Examples given of the application of the Figure of Merit to published results.

Accurate Coulometric Quantification of Hydrogen Absorption in Palladium Nanoparticles and Thin Films

We report here an electrochemical method for precise and accurate quantification of hydrogen absorption in palladium materials. We demonstrate that conventional chronocoulometry over-reports adsorbed hydrogen due to charge from the accompanying hydrogen oxidation reaction (HOR). We designed and built a bespoke electrochemical flow cell that mitigates the concurrent HOR reaction and consequently provides improved accuracy and reproducibility relative to other existing electrochemical techniques. The efficacy of this technique is demonstrated experimentally for a series of palladium sample types: a 100 nm electron-beam deposited thin film, a 20 μm electrodeposited palladium film, a casting of 21 nm edge-length cubic nanoparticles, and a casting of 27 nm edge-length octahedral nanoparticles. We contend that this method is the most effective for measuring hydrogen uptake in different palladium samples.

Milling time effect of Reactive Hydride Composites of NaF[sbnd]NaH[sbnd]MgB2 investigated by in situ powder diffraction

Light metal complex borohydrides have high hydrogen storage capacities but suffer from drawbacks of slow hydrogen sorption kinetics, poor reversibility and high thermodynamic stability. The NaF+9NaH+5MgB2 composite has a theoretical hydrogen capacity of 7.7 wt% H assuming the formation of 10NaBH3.9F0.1+5MgH2. Hydrogenation and dehydrogenation properties as well as the effect of different ball milling times have been investigated. The in situ hydrogenation is faster in the composite ball milled for 87 h than the 5 h milled composite. A boron-rich phase with space group Pa-3, a = 7.4124(5) Å was formed during hydrogenation at 325 °C and 50 bar hydrogen for both short and long milling times. In the long milled composite the boron-rich phase disappeared after 3 h of hydrogenation, whereas it became a major phase in the short milled composite after 1.5 h of hydrogenation. NaBH4 was formed at 206 °C. NaMgH1-xFx was formed at 290 °C instead of the assumed MgH2. The same phases formed at 268 °C and 325 °C, respectively, and only in minor amounts in the short milled composite. Ex situ hydrogenation in a Sieverts' type apparatus at the same temperature and hydrogen pressure conditions followed a different reaction pathway with formation of MgH2 in addition to NaBH4 and NaMgH3-xFx (0 ≤ x ≤ 1). The measured hydrogen uptake was 6.0 and 6.3 wt% for the long and short milled composites, respectively.

Uptake of atmospheric hydrogen by soils: a case study from southern Poland

SummaryUptake of atmospheric hydrogen by soils constitutes the most important sink of this gas at the global scale. However, little is known about the strength and spatio‐temporal variability of this sink. We present the results of a systematic study aimed at characterizing hydrogen uptake by urban soils. The study was carried out at two field sites in Krakow, southern Poland. The measured fluxes of hydrogen to soils revealed distinct seasonality, with large values (up to 40 µmol hour−1 m−2) recorded during summer months and small values (between 5 and 10 µmol hour−1 m−2) during winter. Temperatures in the top 6 cm of the soil profile varied during the study from −9 to +24°C, while volumetric soil water content fluctuated between 15 and 50%. Measurable hydrogen fluxes were also observed when soil temperatures were less than zero. The results were interpreted in the context of seasonal changes of local climate and air and soil temperature, soil water content and amount of rainfall. Mixing conditions at the air–soil interface were also considered. Modelling of H2 uptake was performed with a two dimensional diffusion transport model, with contrasting assumptions about distribution and intensity of the hydrogen sink in the soil profile. The model results suggest that conditions in the uppermost soil layer exert a decisive control on measured hydrogen uptake rates. The measured hydrogen flux can be reproduced by the transport model only if significant reduction of the soil water content in the uppermost layer of the soil is postulated. This finding has important implications for the parameterization of hydrogen uptake rates in models of the global H2 budget.

Hydrogen absorption in epitaxial bcc V (001) thin films analysed by statistical thermodynamics

Andersson, Aits and Hjörvarsson of Uppsala University measured hydrogen uptake in epitaxial bcc (body centred cubic) vanadium (V) (001) thin films of thickness, 50 nm and 100 nm, over temperature range between 443 K and 513 K. The reported equilibrium pressure–temperature–composition ( P– T– C) relationships for the epitaxial bcc V (001) thin films showed appreciable extent of enhancement of H solubility compared with that for bulk bcc V. In this work, the reported equilibrium P– T– C relationships for the epitaxial bcc V(001) thin films by Andersson et al. were analysed in terms of statistical thermodynamics for H 2 gas partial pressure p(H 2) up to 100 Pa and H/V mole atom ratio x in VH x up to 1. The present analysis results showed that, up to x = 0.75, the state of H in the V lattice was comparable to that in bulk VH x specimen but that, in the range of x higher than 0.75, state of H in the thin film with the constrained basal plane condition was evidently distinguishable from that in non-constrained bulk VH x . This was concluded to be the consequence of the tetragonal distortion of the bcc lattice with biaxially constrained condition at the bottom surface of the VH x (001) thin film in the range of x exceeding 0.75.

Hydrogen storage of nanostructured TiO 2-impregnated carbon nanotubes

Hydrogen uptake study of carbon nanotubes (CNTs) impregnated with TiO 2-nanorods and nanotubes has been performed at room temperature and moderate hydrogen pressures of 8–18 atm. Under hydrothermal synthesis conditions, nanorods (NRs) and nanoparticles (NPs) are found to form either of the two polymorphic phases, i.e., nanorods are formed of predominantly anatase phase while nanoparticles are formed of rutile phase. NRs and NPs are introduced into the CNT matrix via the wetness-impregnation method. These composites store up to 0.40 wt.% of hydrogen at 298 K and 18 atm, which is nearly five times higher the hydrogen uptake of pristine CNTs. The excess amount of hydrogen stored in TiO 2-impregnated CNTs is determined from the amount of TiO 2 in the sample and the measured hydrogen uptake of TiO 2 nanoparticles. Higher hydrogen uptake of NP-impregnated CNTs when compared pristine CNTs is accounted for by considering initial binding of hydrogen on TiO 2 and subsequent spillover in CNT–TiO 2-NPs.

Hydrogen-adsorption Properties of a Novel Lantern-type Dinuclear Co(BDC)(DABCO)1⁄2

Abstract A three-dimensional (3-D) cobalt coordination polymer Co(BDC)(DABCO)1⁄2 (BDC = 1,4-benzenedicarboxylate; DABCO = 1,4-diazabicyclo[2.2.2]octane), which has a novel lantern-type structure, was synthesized and characterized by X-ray powder diffraction patterns and elemental analysis. In addition, hydrogen-adsorption characteristics were evaluated at 77 K. The highest measured hydrogen uptake was 2.28 wt % at 77 K and 0.1 MPa for Co(BDC)(DABCO)1⁄2.

Sieverts apparatus and methodology for accurate determination of hydrogen uptake by light-atom hosts

Accurate determination of the quantity of hydrogen absorbed by a potential hydrogen storage material is crucial to progress in the field. The most common techniques for measuring hydrogen uptake from the gas phase by a solid host, the Sieverts technique and gravimetry, both become susceptible to systematic errors as the density of the host material decreases. We focus here on the Sieverts technique, which in a poorly designed apparatus may produce errors ∼100% in the quantity of absorbed hydrogen owing to a realistic 25% error in the density of a light-atom sample. Using hydrogen absorption isotherms measured for low-density materials, including carbon nanotubes, potassium-intercalated graphite and lithium nitride, we show that designing the Sieverts apparatus with carefully chosen volumes greatly lessens the impact of uncertainty in the sample density. Rules-of-thumb for the volumes in the apparatus and the volume occupied by the sample itself, and a figure of merit for the sensitivity of the system to changes in the hydrogen content of the sample, are introduced.

Techniques for Investigating Hydrogen-Induced Cracking of Steels With High Yield Strength

Abstract As part of a Song range study to develop grades of steel with improved resistance to hydrogen-induced cracking, techniques for measuring hydrogen uptake and hydrogen permeability were investigated and these parameters were correlated with resistance to hydrogen-induced cracking at different strength levels. The steel used for these studies was a Ni-Cr-Mo steel heat treated to four yield strength values ranging from 95 ksi to 150 ksi. The environment studied was a hydrogen sulfide saturated aqueous solution of 3% sodium chloride and 0.5% acetic acid. The technique used to measure hydrogen permeability consisted of exposing one side of a steel specimen to the sulfide environment and electrochemically measuring the hydrogen permeating to the opposite side. Hydrogen uptake was measured by the vacuum extraction method with specimens exposed to the environment for various time periods. The results of this comparison Stowed that both the resistance to cracking and the apparent diffusivity of hydrogen decreased as the yield strength was increased. Hydrogen uptake increased with increasing yield strength but the steady state permeation rate was not affected by strength level. A concept of hydrogen trapping is proposed to explain the observed relationships among the parameters studied and the mechanical strength of the steel.