Improving Reproducibility in Hydrogen Storage Material Research

Reproducibility in chemistry research

Chemistry is a reproducible science whose pillars - synthesis and analysis - actually comprise a huge collection of highly reproducible experimental methods to synthesize and analyze substances. The historical development of chemistry, furthermore, shows that reproducibility of methods has been the companion of novelty and creative innovation. The "publish or perish" principle dominating global academia since over two decades, however, intrinsically contributes to the publication of non-reproducible research outcomes also in chemistry. A study on reproducibility of chemistry research seems therefore timely, especially now that chemists are slowly but inevitably adopting open science and its tools such as the preprint, open access, and data sharing. We conclude presenting three simple guidelines for enhanced publication of research findings in chemistry.

The Effectiveness of Silver and Gold in Catalytic Homogenous and Heterogenous Borohydride Hydrolysis - a DFT Study

Energy demands, and environmental aspects raised the need to study hydrogen-carrying material such as borohydride for the practical usage of hydrogen as a cleaner and more efficient fuel. A proper understanding of the hydrogen generation mechanism is a key requirement for the designing of efficient catalysts, as the non-catalytic hydrolysis of borohydride in non-acidic media is a slow process. The hydrolysis mechanism of borohydride varies considerably using homogeneous and heterogeneous catalysts. A comparison of the hydrolysis mechanism of borohydride using gold and silver as homogenous and heterogeneous catalysts is given in this review. Unexpectedly, with gold catalyst, Au+ or Au(111), only two steps of hydrolysis occur and BH(OH)2 is produced, while with silver catalyst, Ag+ or Ag(111), the hydrolysis can proceed to completion.

Establishing ZIF-8 as a reference material for hydrogen cryoadsorption: An interlaboratory study

Hydrogen storage by cryoadsorption on porous materials has the advantages of low material cost, safety, fast kinetics, and high cyclic stability. The further development of this technology requires reliable data on the H2 uptake of the adsorbents, however, even for activated carbons the values between different laboratories show sometimes large discrepancies. So far no reference material for hydrogen cryoadsorption is available. The metal-organic framework ZIF-8 is an ideal material possessing high thermal, chemical, and mechanical stability that reduces degradation during handling and activation. Here, we distributed ZIF-8 pellets synthesized by extrusion to 9 laboratories equipped with 15 different experimental setups including gravimetric and volumetric analyzers. The gravimetric H2 uptake of the pellets was measured at 77 K and up to 100 bar showing a high reproducibility between the different laboratories, with a small relative standard deviation of 3-4 % between pressures of 10-100 bar. The effect of operating variables like the amount of sample or analysis temperature was evaluated, remarking the calibration of devices and other correction procedures as the most significant deviation sources. Overall, the reproducible hydrogen cryoadsorption measurements indicate the robustness of the ZIF-8 pellets, which we want to propose as a reference material.

ZIF-8 Pellets as a Robust Material for Hydrogen Cryo-Adsorption Tanks

Cryoadsorption on the inner surface of porous materials is a promising solution for safe, fast, and reversible hydrogen storage. Within the class of highly porous metal-organic frameworks, zeolitic imidazolate frameworks (ZIFs) show high thermal, chemical, and mechanical stability. In this study, we selected ZIF-8 synthesized mechanochemically by twin-screw extrusion as powder and pellets. The hydrogen storage capacity at 77 K and up to 100 bar has been analyzed in two laboratories applying three different measurement setups showing a high reproducibility. Pelletizing ZIF-8 increases the packing density close to the corresponding value for a single crystal without loss of porosity, resulting in an improved volumetric hydrogen storage capacity close to the upper limit for a single crystal. The high volumetric uptake combined with a low and constant heat of adsorption provides ca. 31 g of usable hydrogen per liter of pellet assuming a temperature-pressure swing adsorption process between 77 K - 100 bar and 117 K - 5 bar. Cycling experiments do not indicate any degradation in storage capacity. The excellent stability during preparation, handling, and operation of ZIF-8 pellets demonstrates its potential as a robust adsorbent material for technical application in pilot- and full-scale adsorption vessel prototypes.

The Dilemma of Reproducibility of Gating Isotherms for Flexible MOFs

Porous materials receive a high level of scientific and technological interest due to their applications in various fields such as adsorption, separation and storage, catalysis, ion exchange, nanotechnology, etc. Gas adsorption is a well-established tool for the characterization of the texture of porous solids. Physisorption isotherms are generally expected to be well reproducible for rigid adsorbents, but this is not always the case for nonrigid (flexible) materials. The presence of a metastability region and sensitivity of the activation barriers to the material's texture often influence the isotherms' run. Here, we address the complexity that arises in terms of reproducibility and sample handling for flexible metal-organic frameworks, with the example of DUT-8(Ni). It belongs to the group of "gate opening" metal-organic frameworks and is a typical representative of the pillared layer compounds. We propose characteristic parameters for the analysis and comparison of adsorption isotherms, showing the "gate opening" step, associated with the adsorption-induced solid-state phase transition. A set of 50 nitrogen physisorption isotherms measured at 77 K were analyzed and correlated with the synthetic and outgassing conditions. The study highlights the importance of accurate descriptions and record-keeping of experimental details and their role in the replication of scientific results.

Recent Development in Nanoconfined Hydrides for Energy Storage

Hydrogen is the ultimate vector for a carbon-free, sustainable green-energy. While being the most promising candidate to serve this purpose, hydrogen inherits a series of characteristics making it particularly difficult to handle, store, transport and use in a safe manner. The researchers' attention has thus shifted to storing hydrogen in its more manageable forms: the light metal hydrides and related derivatives (ammonia-borane, tetrahydridoborates/borohydrides, tetrahydridoaluminates/alanates or reactive hydride composites). Even then, the thermodynamic and kinetic behavior faces either too high energy barriers or sluggish kinetics (or both), and an efficient tool to overcome these issues is through nanoconfinement. Nanoconfined energy storage materials are the current state-of-the-art approach regarding hydrogen storage field, and the current review aims to summarize the most recent progress in this intriguing field. The latest reviews concerning H2 production and storage are discussed, and the shift from bulk to nanomaterials is described in the context of physical and chemical aspects of nanoconfinement effects in the obtained nanocomposites. The types of hosts used for hydrogen materials are divided in classes of substances, the mean of hydride inclusion in said hosts and the classes of hydrogen storage materials are presented with their most recent trends and future prospects.

Experimental Volumetric Hydrogen Uptake Determination at 77 K of Commercially Available Metal-Organic Framework Materials

Storage is still limiting the implementation of hydrogen as an energy carrier to integrate the intermittent operation of renewable energy sources. Among different solutions to the currently used compressed or liquified hydrogen systems, physical adsorption at cryogenic temperature in porous materials is an attractive alternative due to its fast and reversible operation and the resulting reduction in storage pressure. The feasibility of cryoadsorption for hydrogen storage depends mainly on the performance of the used materials for the specific application, where metal-organic frameworks or MOFs are remarkable candidates. In this work, gravimetric and volumetric hydrogen uptakes at 77 K and up to 100 bar of commercially available MOFs were measured since these materials are made from relatively cheap and accessible building blocks. These materials also show relatively high porous properties and are currently near to large-scale production. The measuring device was calibrated at different room temperatures to calculate an average correction factor and standard deviation so that the correction deviation is included in the measurement error for better comparability with different measurements. The influence of measurement conditions was also studied, concluding that the available adsorbing area of material and the occupied volume of the sample are the most critical factors for a reproducible measurement, apart from the samples’ preparation before measurement. Finally, the actual volumetric storage density of the used powders was calculated by directly measuring their volume in the analysis cell, comparing that value with the maximum volumetric uptake considering the measured density of crystals. From this selection of commercial MOFs, the materials HKUST-1, PCN-250(Fe), MOF-177, and MOF-5 show true potential to fulfill a volumetric requirement of 40 g·L−1 on a material basis for hydrogen storage systems without further packing of the powders.