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Hydrogen is a promising sustainable energy carrier for the future due to its high energetic content and no emissions, other than water vapor. However, its full deployment still requires technological advances in the renewable and cost-effective production of hydrogen, cost reduction of fuel cells and especially in the storage of hydrogen in a lightweight, compact and safe manner. One way to achieve this is by using materials in which hydrogen bonds chemically, or by adsorption. Different kinds of Hydrogen Storage Materials have been investigated, such as Metal-Organic Frameworks (MOFs), Simple Hydrides (including Magnesium Hydride, MgH2), AB5 Alloys, AB2 Alloys, Carbon Nanotubes, Graphene, Borohydrides, Alanates and Ammonia Borane. Billions have been invested in Storage Materials research, resulting in tens of thousands of papers. Thus, it is challenging to track how much effort has been devoted to each materials class, by which countries, and how the field has evolved over the years. Quantitative Science and Technology Indicators, produced by applying Bibliometrics and Text Mining to scientific papers, can aid in achieving this task. In this work, we evaluated the evolution and distribution of Hydrogen Storage Materials research using this methodology. Papers in the 2000–2015 period were collected from Web of Science and processed in VantagePoint® bibliometric software. A thesaurus was elaborated relating keywords and short phrases to specific Hydrogen Storage Materials classes. The number of publications in Hydrogen Storage Materials grew markedly from 2003 to 2010, reducing the pace of growth afterwards until a plateau was reached in 2015. The most researched materials were MOFs, Simple Hydrides and Carbon-based materials. There were three typical trends in materials classes: emerging materials, developed after 2003, such as MOFs and Borohydrides; classical materials with continuous growth during the entire period, such as Simple Hydrides; and stagnant or declining materials, such as Carbon Nanotubes and AB5 Alloys. The main publishing countries were China, countries from the European Union (EU) and the USA, followed by Japan. There is a division between countries with continued growth in recent years, such as China, and those with stagnant production after 2010, such as the EU, the USA and Japan. The results of this work, compared to a previous study in storage materials patenting by our group, and the recent launch of commercial hydrogen cars and trains and stationary hydrogen production and fuel cell solutions, indicates that although the Hydrogen Energy field as a whole is transitioning from lab and prototype stages to commercial deployment, materials-based hydrogen storage still has base technological challenges to be overcome, and therefore still needs more scientific research before large scale commercialization can be realized. The developed thesaurus is made available for refinement and future works.

Magnesium hydride (MgH2) is a promising candidate as a hydrogen storage material. However, its hydrogenation kinetics and thermodynamic stability still have room for improvement. Alloying Mg with Al has been shown to reduce the heat of hydrogenation and improve air resistance, whereas graphite helps accelerating hydrogenation kinetics in pure Mg. In this study, the effects of simultaneous Al alloying and graphite addition on the kinetics and air-exposure resistance were investigated on the Mg60Al40 system. The alloys were pulverized through high-energy ball milling (hereinafter HEBM). We tested different conditions of milling energy, added graphite contents, and air exposure times. Structural characterization was conducted via X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). H2 absorption and desorption properties were obtained through volumetry in a Sieverts-type apparatus and Differential Scanning Calorimetry (DSC). The desorption activation energies were calculated using DSC curves through Kissinger analysis. Mg60Al40 with 10 wt% graphite addition showed fast activation kinetics, even after 2 years of air exposure. Graphite addition provided a catalytic effect on ball-milled Mg–Al alloys by improving both absorption and desorption kinetics and lowering the activation energy for desorption from 189 kJ/mol to 134 kJ/mol. The fast kinetics, reduced heat of reaction, and improved air resistance of these materials make them interesting candidates for potential application in hydride-based hydrogen storage tanks.

Hydrogen is a promising future energy carrier due to its high energetic content and sustainable appeal when produced via clean manufacturing processes. One of the technological challenges concerns its storage in a safe, compact, low mass and high gravimetric capacity manner. In this sense, many Hydrogen Storage Materials (HSM) have been investigated to house this source of energy, such as Simple Hydrides, Borohydrides, Metal-Organic Frameworks (MOFs), Alanates, AB5 Alloys, Ammonia Borane, Carbon Nanotubes and Graphene. Scientific advances aside, less attention has been paid in establishing a panorama of the technological developments in HSM. To assess the technological advances in HSM, patent analysis can be carried out using bibliometrics and text mining approaches in order to forecast the future trend of development and the main players involved in this process. In this work, we evaluated the technological life cycle stage, HSM class prominence and the role of different countries in HSM patenting. The results show that overall HSM patenting decreased after 2007, except in the case of China. On the other hand, the USA, Japan, China and the European Union (EU) were the main patenting territories. Simple Hydrides and Borohydrides were the main classes of HSM that received more attention from the USA and the EU, while Japan had a high share in Solid Solution Alloys. The life cycle stage of HSM seems to be between the first prototype market experiences and full market deployment, even though future assessment is needed to fine-tune the analysis. The developed indicators may also support the funding of new projects and decision making.

Severe mechanical processing routes based on high-energy ball milling (HEBM) or severe plastic deformation (SPD) can be used to produce Mg nanomaterials for hydrogen storage applications. In the last few years, we have been exploring in our research group different SPD processing routes in Mg systems to achieve good activation (first hydrogenation) and fast H-absorption/desorption kinetics, combined with enhanced air resistance. In this paper, we compare SPD techniques applied to Mg with HEBM applied to MgH2. Both advanced – melt spinning (MS), high-pressure torsion (HPT) – and more conventional – cold rolling (CR), cold forging (CF)- techniques are evaluated as means of production of bulk samples with very refined microstructures and controlled textures. In the best SPD processing conditions, attractive H-absorption/desorption kinetic properties are obtained, which are comparable to the ones of MgH2 milled powders, even if the needed temperatures are higher – 350°C compared to 300°C.CR and CF stand out as the processes with higher potential for industrial application, considering the level of the attained hydrogen storage properties, its simplicity and low cost.