Solid-state hydrogen storage: materials, systems and the relevance of a gender perspective

This paper aims at addressing the exploitation of solid-state carriers for hydrogen storage, with attention paid both to the technical aspects, through a wide review of the available integrated systems, and to the social aspects, through a preliminary overview of the connected impacts from a gender perspective. As for the technical perspective, carriers to be used for solid-state hydrogen storage for various applications can be classified into two classes: metal and complex hydrides. Related crystal structures and corresponding hydrogen sorption properties are reviewed and discussed. Fundamentals of thermodynamics of hydrogen sorption evidences the key role of the enthalpy of reaction, which determines the operating conditions (i.e. temperatures and pressures). In addition, it rules the heat to be removed from the tank during hydrogen absorption and to be delivered to the tank during hydrogen desorption. Suitable values for the enthalpy of hydrogen sorption reaction for operating conditions close to ambient (i.e. room temperature and 1-10 bar of hydrogen) are close to 30 kJ·molH2 1. The kinetics of hydrogen sorption reaction is strongly related to the microstructure and to the morphology (i.e. loose powder or pellets) of the carriers. Usually, kinetics of hydrogen sorption reaction is rather fast, and the thermal management of the tank is the rate determining step of the processes. As for the social perspective, various scenarios for the applications in different socio-economic contexts of solid-state hydrogen storage technologies are described. As it occurs with the exploitation of other renewables innovative technologies, a wide consideration of the social factors connected to these processes is needed to assess the extent to which a specific innovation might produce positive or negative impacts in the recipient socio-economic system and to explore the potential role of the social components and dynamics in fostering the diffusion of the innovation itself. Attention has been addressed to the gender perspective, in view of the enhancement of hydrogen-related energy storage systems, intended both in terms of the role of women in triggering the exploitation of hydrogen-based storage as well as to the impact of this innovation in their current conditions, at work and in daily life.


Introduction
According to the analysis performed by the International Energy Association, [ On the other hands, thanks to the significant introduction of renewable energies, the CO2 production per capita increased much less at the global level (i.e. 14%), changing from 3.88 tCO2/capita in 1990 to 4.42 tCO2/capita in 2018. Even in this case, the values are very different comparing data for 2018 in Europe (5.72 tCO2/capita) and in Africa (0.98 tCO2/capita).
[1] It is worth noting that, considering the values of the same indicator in 1990, a significant decrease has been obtained in Europe (-29%), whereas it increased in Africa (+15%), as it can be observed in Figure 1. The decreased CO2 emission in Europe can be correlated to an increment of renewable energy production over the last 20 years that replaced the production of electricity by non-renewable sources. Though, increased emissions in Africa are mainly related to the increased production of electricity using non-renewable sources, even if a significative increment in renewables electricity production has been implemented moving from 326 GWh produced in 1990 to 28286 GWh in 2018.
[1] These trends underline the need of implementation of renewable energy production in areas that can benefit on increased green energy production while decreasing CO2 emissions. In fact, to get rid of pollution and global temperature increase, it is fundamental to look for alternative to fossil fuels.
The Paris Agreement aims at holding the increase in global average temperature below 2 °C above the pre-industrial levels. [2] The Clean Energy for all Europeans Package (CEP) represents the response of the European Union to the Paris Agreement and its purpose is to drive European Union towards a complete de-carbonization within 2050. The turning point for renewables exploitation (e.g wind, solar, water..) can be reached only through the efficeint storage of their energy. In fact, renewable energies are highly considered for the distributed production of energy, but their fluctuations in time and geography call for the use of energy storage systems that can deliver energy when needed. The development of good, clean and efficient materials for energy storage is the bottleneck for using only renewable energies, instead of fossil fuels. [4] It is therefore important to design integrated systems at the large scale that allow storing excess energy to meet future demand and utilisation at another place or time. The phasingout of the fossil fuels cannot be solved by a single technology but must involve the development of different approaches, which could offer economic and environmental benefits and cover any requirements concerning application, cost and footprint, so considering all its life cycle assessment.
At present, the storage facilities that do exist use pumped hydropower, [5] a system that pumps water uphill to a reservoir when excess electricity is available and then lets the water flow downhill through turbines to generate electricity when it is needed. However, it can be only located in very limited areas. Developing new energy storage technologies that are comparable in reliability and cost to pumped hydropower, and that are deployable at any location, could enable the storage of vast amounts of electricity anywhere on the grid worldwide and would enable the increased use of renewable electricity generation while maintaining high reliability in electric supply. A continuous flow of clean energy can be obtained with the development of smart grids connected with different energy storage systems, such as batteries or heat storage systems.
One of the best options for the storage is the production of green hydrogen from water. In fact, electricity excess can be exploited to produce hydrogen to be used in various applications and in different periods, for example in power production to satisfy domestic or multiple users.
For this reason, the paper aims at providing an overview of the opportunities and the challenges to be addressed for the available hydrogen-based solutions to become key technologies and boost energy transition improving energy storage efficiency and smart grids.
Although it is mostly focused on technical aspects, the ambition is to shade a light also on social aspects that the scientific literature (and the experience) suggests always to consider when dealing with innovation processes. It is widely recognized in fact that in order for innovation to be able to produce a systemic change and exploit all its potential, in order for it to 'get out from the niche' and become the normality, it has to be addressed as a socio-technical innovation, that is to consider the many social, economic, regulatory, cultural aspects that characterize the system within which the innovation is expected to spread [183]. Therefore, the ambition is to grasp the impacts of innovation on the wider communities and different social groups (passive perspective) and to take advantage of the social dynamics as a potential catalyser of innovation diffusion and acceptance (active perspective).
Therefore, the paper firstly gives an overview of hydrogen-based technologies for energy storage,

Hydrogen-based solutions for energy storage
The hydrogen cycle from renewables is completely CO2-free and water is the only by-product. [6] The energy storage can be obtained using hydrogen (H2) that is a secondary energy vector, which shows several advantages: it can be produced from other primary energy sources, resulting unlimited, it can be stored for a long period of time, and its storage is a key enabling technology for the advancement of hydrogen and fuel cell technologies in applications including stationary power, portable power, and transportation. [7,8] Moreover, the interest of hydrogen as an energy vector is due to its highly exothermic reaction with oxygen to produce only water. Compared to fossil fuels, 1 kg of H2 has the same energy as 2.4 kg of CH4 or 2.8 kg of gasoline. This means that hydrogen has a higher energy-to-weight ratio compared to other fuels. On the other hand, for the energy-tovolume ratio, the situation is inverted. Liquid hydrogen has 8.5 MJL -1 against 32.6 MJL -1 of gasoline. [9] This is a drawback for hydrogen storage, because a much higher volume of hydrogen is required to have the same amount of energy provided by most of fossil fuels. The aim of hydrogen storage research is to overcome this problem, increasing energy density to limit systems volume.
Conventionally, H2 is stored as compressed gas or liquid. It requires significant energy consumption to reach hyperbaric pressures and cryogenic temperatures, resulting non-economically advantageous. On the contrary, the use of hydrogen carriers is a promising solution, since they allow storing H2 at low pressure and close to room temperature, achieving large volumetric densities compared to compressed or liquid hydrogen. Considering a generic hydrogenation reaction of an intermetallic compound (M) to form the corresponding hydride (MH): M + x/2 H2 MHx, the thermodynamic of the equilibrium is characterized by the interrelation between H2 equilibrium pressure (Peq), its concentration (C) in the carrier and temperature (T), which is described in PCTdiagrams. The Peq is given as a function of temperature by the Van't Hoff equation, Peq=ΔH/RT-ΔS/R, where ΔH and ΔS are the standard enthalpy and entropy of the reaction, respectively, and R is the gas constant. The kinetic properties of H2 sorption reactions determine the rate at which a practical handling can be realized in the material. Moreover, handling hydrogen as hydride results to be safer than liquid and compressed gas. In addition, the environmental impact evaluation, through a Life Cycle Analysis (LCA), of the use of solid-state carriers in a tank has demonstrated that the environment impact could be limited with respect to other H2-storage technologies. [10,11] The energy produced by a renewable source (e.g. sun and wind) is used to produce H2 from water through an electrolyser. The gas is then stored in the designed tank using a carrier. A heat storage system allows to collect solar heat, which can be used during the desorption event to release hydrogen from the MH. Finally, the released green H2 supplies a fuel cell (FC) producing electricity to refill electric vehicles or feed stationary fuel cells for on demand electricity production, in order to meet the needs of end users (either prosumers or energy communities). The flows of hydrogen, heat and electricity during the hydrogen production and use are described in Figure 2, a and   In order to allow an even more intense exploitation of non-programmable renewable sources (sun and wind) in the electricity generation system, it is of fundamental importance to develop distributed generation and end-users must be encouraged to install energy production systems equipped with energy storage thus becoming prosumers.
As for the end users, it is worth to higlight that prosumers (both as individual housholds and collective initiatives, such as energy coop and communities) have been gaining more and more relevance for the past 2 decades as a catalyzer for renewables technologies diffusion mainly triggered by the feed-in-tariffs incentives schemes in many EU countries. [12] Again, a major challenge that prosumers and energy communities have to face for being actually able to satisfy their own energy demand is the discontinuity of power production from RES. Therefore, hydrogenrelated storage solution can play a crucial role in fostering their diffusion and development, thus making the national energy system able to exploit the funding and policy opportunities coming from the extraordinary convergence of the emergency funds provided by Next Gen EU and the policy framework resulting from the national transposition of th EU directives on Energy Market and Renewables. [13,14] Therefore, electricity prosumers and energy communities indeed could be able in the near future to play a central role as an integral part of the electric system, for the spreading of hydrogen technologies and for the overall energy transition by reducing network losses, by encouraging the implementation of innovative technologies and by reducing pollutant emissions and energy import, thanks to energy self-production.
Basic and applied research and knowledge on new class of multifunctional materials will lead the way for the development of such flexible and efficient energy storage, the key to a reliable clean electricity supply as it will be resumed in the present paper, focusing on hydrogen storage

Social aspects of energy transition: a gender perspective
Climate change is making our world overall less safe for the entire biosphere. Human beings (and human culture and societies) are at risk of being overwhelmed by this global process that they consistently contribute to trigger. Both the increase in the frequency and magnitude of catastrophic events and the negative trends in environmental assets, such as temperature, droughts and pollution might produce economic and social negative impacts, able to affect individuals and social groups, and therefore hamper and challenge the current social order. A changing climate therefore affects any living and human beings, but as for the most delicate natural ecosystems, it is the world's poorest and most vulnerable groups, such as the elderly, migrants, indigenous groups (and women and girls within these groups) who withstand the worst of environmental, economic, and social shocks. The reason why women and girls are more vulnerable to climate change, especially in the developing countries, is often socially constructed with women and girls predominantly responsible for food production, household management, water supply, and energy supply for heating and cooking. As climate change impacts increase, these tasks become more difficult and more time consuming [15][16][17]. But due to their role in organizing social life, women are at the same time at the forefront of the solutions of many of the societal concerns connected with energy transition. In developing as well as in developed countries, they are early adopters of climate-friendly agriculture and clean energy, and offer solutions and valuable insights into better managing climate risks. [18] The centrality of women in the transition processes has entered the international policy agenda for sustainability. At the last United Nations Climate Change Conference (UNCCC), governments approved the Gender Action Plan (GAP), recognising the importance of involving men and women equally in climate action for an efficient and equal energy transition that will ensure human rightsbased and gender-just implementation in the energy transition. [19] With the GAP, we have entered in a new era in which we must enable women leadership in decision-making at all level of society.
By empowering women and girls, we can both address a right issue, and give ourselves a better chance to meet the sustainable development goals and the Paris agreement. In fact, in the 2015 Paris agreement, calls for gender equality and women's empowerment were clearly established.
The energy transition is an essential component of the climate action and there is a high need for the consideration of social aspects and dynamics as a crucial component of the transformation of energy systems. Not only from the passive perspective of the social risks connected to the transition process' impact and the ambition to reduce social problems such as energy security, energy poverty, gender gap but also, and even more, in terms of the relevance that the social factors (culture, values, education...) and dynamics (interaction, solidarity, imitation…) might play in fostering the transition process. Within this social turn in the transition research and policy design, the adoption of a gender perspective to investigate the processes at stake and to promote effective policies although highly underestimated has shown to have a great potential.
First of all, it has been evidenced that effectiveness of energy policies improves when gender is acknowledged, through the optimization of outcomes for all the actors involved in the energy system, as highlighted by some policy design approaches such as women in development, gender in development, transformative approach, intersectional approach and social justice. [20] Women roles in society equips them with an understanding of the cultural and community context, which is useful for introducing behavioural change with regard to energy consumption, starting from the household level. [21,22] Therefore, women empowerment in the energy transition through their enhanced inclusion in energy access and use and through the strengthening of their role in the energy chain and decision making processes, can play a decisive role for addressing the impacts of the sustainable transformation in energy systems on the social, cultural, economic and political contexts they are embedded in. [20] Nonetheless, for this involvement to be effective and sustainable, energy policies have to be properly implemented, allocations of time and resources between genders should be levered to further include women as participants of the entire energy supply chain, in order to support energy planners and consumers in making informed energy choices and to acknowledge and support women's livelihood priorities. [18,20] Taking into account gender roles and relations in energy policies design might trigger a shift towards energy justice in implementing availability, affordability, sustainability, intra-and inter-generational equity and responsibility in the development of energy system infrastructures. [20] Secondly, when it comes to the already introduced prosumers exploitation, with energy consumers becoming energy producers for their self-consumption and the market, women role can bridge and play as agent of change in the whole energy system. As a matter of fact, women have a high potential in developing and implementing energy solutions locally. This can be mainly related to women different energy dynamics outlooks, decision-making areas, energy needs, responses to crises or coping mechanisms with respect to men, that lead to more inclusive prospective to energy technologies and services that match those dynamics. [20].
Third, the design and implementation of more sustainable ways to produce, distribute and consume energy affect gender equality if women are involved in the process, by hindering stereotypes, levering opportunity and access to education and culture, stimulating energy research to better understand and account for energy transitions, particularly around envisioning a more just transition along political, socio-ecological, economic, technical dimensions, and creating more democratic systems and processes. [23] Therefore, the transition towards renewable energies, starting from their production till use, seems to be a unique opportunity to foster women empowerment and, the way around, involving women might play a role in fostering the transition process itself. Greater gender equality and equity in energy would result in social impacts that will lead to greater participation to the energy transition, beneficial and efficient management of energy resources at a lower climate impact level, more savings, more free time and lowering workloads. Furthermore, given the competences required to project design and implementation. [26] As for the hydrogen-related storage solutions, the reciprocal influence between technical and social systems is evident if we consider how important is the general social acceptance of new technologies that should replace energy systems based on fossil fuel, a process that might be supported by women involvement. The social acceptance connected to a fair transition can be performed only including and empowering minorities in the transformative pathways and should include all gender perspectives towards the new energy regime. [27] As a provisional conclusion, it seems clear that the involvement of women in the energy transition is crucial for the transition itself to be properly and fairly implemented. Because, given their role in organizing social life, women are likely to be more affected than men by the negative and positive impacts of the transition (especially in developing countries) and at the same time they can play as agent of energy transition. A woman's prospective offers a more comprehensive perception of inequalities and power imbalances in the current energy transition. [28] As women take on more leadership roles can lead to addressing climate change by promoting a transition to renewable energy, redistributing power, with more balanced community and public control in the energy sector benefits and risks, and prioritize equity and justice with community ownership and distributed governance. [29] Hydrides for energy storage As introduced, the bottleneck towards an affordable and efficient storage of RES through hydrogen is related to the development of high-density hydrogen storage and integrated systems.
Hydrogen may be stored in pure metals or in inorganic compounds, forming metal and complex hydrides. In some cases, hydrogen is uptaken and released in solid solutions (e.g. ZrV2), without changing the crystal structure of the compound, but only increasing the lattice parameters. On the contrary, some pure elements (e.g. Mg) and intermetallic compounds (e.g. LaNi5) form a hydride phase upon hydrogen absorption. [30,31] In the following a brief introduction on selected materials for solid-state hydrogen storage will be reported (i.e. pure metals, intemetallic compounds, complex hydrides) to introduce and discuss realised tanks or integrated system applications, that will be describe in the last section.

Pure metals
Considering pure metals, MgH2 is an excellent material due to its high hydrogen storage capacity (i.e. volumetric H2 density about 130 kg H2 m -3 and gravimetric H2 density around 7.6 wt.%). [32] According to available thermodynamic databases (ΔH = -75 kJ/molH2 and ΔS = -130 J/molH2/K), [33] the equilibrium dehydrogenation temperature is high, around 297 °C under 1 bar of H2 and the ab/desorption kinetics is undesirably slow. The presence of nanostructured materials, prepared by ball milling (BM), rapid solidification, sputtering and inert gas condensation, may have significant effects on the thermodynamics and kinetics of the hydrogenation/dehydrogenation reaction.
A number of techniques have been applied to improve the ab/desorption kinetics of MgH2, for example by ball milling with different additives, e.g. transition metals (TM) and transition metal oxides (TMO). [34] Typically, 3d transition metals can play a catalytic role in hydrogen chemisorption, due to their d-electrons. [35] Moreover, transition metals may reduce the nucleation barrier, speeding up the formation of hydrides. [36] In addition, they were shown to affect the desorption kinetics, decreasing the activation energy for hydrogen sorption in MgH2. [36] However, they do not change significantly the thermodynamic properties of MgH2. A summary of Mg additives used to improve hydrogen sorption properties has been discussed by Baran et al. [37].

Borohydrides
Borohydrides are inorganic ionic compounds suitable for hydrogen storage in the solid state and they are a multifunctional class of materials that may also be used as fast ion conductors for new types of batteries, for gas adsorption or CO2 capture and recycling. [69] They may also have optical, electronic and magnetic properties and can be used as reducing agents in organic synthetic chemistry. [69] Since they contain a large amount of hydrogen, they are promising materials as neutron shield from ionizing radiation such as high energy electrons and protons in the space. [32] The most promising ones contain light alkali or alkali-earth metal cation ionically bonded to the complex borohydride anion (BH4 -). [69] For their rich chemistry and tuneable properties in relation to their structures and interactions in mixtures, the attention of the latest research focused on the synthesis and characterization of new borohydrides. [69,73] A large variety of crystal structure characterize borohydrides, which shows also a tendency to form polymorphs as a function of temperature or pressure.

Amides and Imides
Amides and imides of alkali and alkaline-earth metals in combination with metal hydrides have the potential to meet the needs of on-board hydrogen storage. Lithium amide-imide can form a solid solution that plays a functional role in the hydrogen storage reactions. A recent review by Garroni et al. has been published recently and will be resumed hereafter. [115] Li3N was reported to reversibly store more than 9 wt.% of hydrogen in a two-step process that includes the formation of lithium imide firstly and then lithium amide. Even if pure lithium amide (LiNH2) decomposes above 300 °C releasing hydrogen and ammonia gas during the decomposition, mixtures of lithium amide and lithium hydride improve the thermodynamic of hydrogen release, achieving a reversible capacity of 6.5 wt.% and a releasing temperature lower than 300 °C. [115] The tailoring of particles size by ball milling or the addition of catalyst such as TiCl3 are effective strategies to improve kinetics of hydrogen release and to suppress ammonia release during decomposition.

Reactive Hydrides Composites (RHC)
As introduced for N-containing systems, another way to tailor and improve hydrogen storage in complex hydrides is the formulation of mixtures which enables reversible and kinetically/thermodynamically favourable hydrogen desorption/absorption. [

Applications of hydrides for hydrogen storage
Looking forward applications of the above-mentioned materials for solid state hydrogen storage in integrated energy systems, the thermodynamic modelling of materials and systems is essential to define and tailor hydrogenation properties and their stability as a function of temperature and pressure in the direction of defining their thermo-fluido-dynamic behaviour. In the following, an overview of modelling approaches will be resumed and hydride-based integrated systems available at both the lab-scale (storing a hydrogen amount lower than 1 kg) and at the industrial scale (storing a hydrogen amount higher than 1 kg) will be presented.

Modelling of hydride-based integrated systems
A full picture of the thermodynamic properties of a system can be obtained by the CALPHAD approach. [117] The goal of CALPHAD assessments is to obtain a description of the dependence of the free energy of all phases on temperature, pressure and composition. The interaction parameters can be obtained with a least square procedure, starting from experimental values of existing phase diagrams and thermodynamic data (Figure 3). For example, the thermodynamic properties of LiBH4-NaBH4, LiBH4-KBH4, and NaBH4-KBH4 binary systems have been investigated and the assessed binary systems have shown good agreement with experimental data (Figure 3). [123] Ab-initio results provided the enthalpy of mixing for the solid solutions on both the lithium-and sodium-rich side. Moreover, a full investigation of the ternary system LiBH4-NaBH4-KBH4 have been carried out. [124] As for divalent complex borohydrides, calcium [105] and magnesium [102] borohydrides have been also investigated with the powder on the basis of experimental data. [127] The geometry used in modelling is shown in Figure   4-a and represents a longitudinal section with two subdomains (one for the porous media and one for the free space), which is sorrounded by an isothermal bath. Once all the parameters have been set, [127] it was possible to obtain the time-space evolution of temperature, as shown in Figure 4-b and Figure 4-c for the hydrogen absorption and desorption process, respectively. In Figure 4-b, the temperature into the vessel increases in the first 60 s until to reach 25 °C due to the exothermic hydrogen absorption reaction. Later, the temperature of the vessel reaches the equilibrium temperature of the bath. In Figure 4-c, the temperature into the vessel decreases in the first 30 s due to endothermic hydrogen desorption reaction. As the external bath is fixed at 40 °C, after 30 s the temperature increases until it reaches the equilibrium. As a result, a full description of thermo-fluido-dynamic processes during hydrogen absorption and desorption became available.

Hydride-based systems available at lab-scale
In alanates. [143] The demonstration system module design and the system control strategies were enabled by experiment-based, computational simulations that included heat and mass transfer coupled with chemical kinetics. [143] From a perspective of industrial applications, a lab-scale tank has been tested using 98 g of Mg(NH2)2-2LiH-0.07KOH reactive composite as hydrogen storage media. [144,145] The tank ran under 60 bar in absorption and at 180-250 °C, the heat management of the system is of fundamental importance for efficient hydrogen delivery and storage. [146] Beside their use as solid-state hydrogen storage materials, borohydrides can be exploited in Direct Borohydride Fuel Cells (DBFC), very promising system for portable applications, [147] where hydrogen is generated by hydrolysis of complex hydrides. Usually, a DBFC employs an alkaline solution of sodium borohydride (NaBH4) as fuel and oxygen or hydrogen peroxide as oxidant.
Recent developments in DBFC research have been reviewed in ref. [148]. involving sodium alanates and reactive hydride composites mixtures. [129] Hydrolysis of borohydride has been also considered for one-shot hydrogen production on demand, it is however still restricted to niche and limited applications. [150,151]

Hydride-based systems available at industrial scale
Metal hydrides are known since late 1970s as low risk option to store hydrogen. [128] Intermetallic Applications for mobility are limited by the low gravimetric density of metal hydrides.
Nevertheless, when an increased weight is a benefit for the application, hydrogen storage by metal hydrides become an added value. On the contrary, for mobile applications on roads (i.e. automotive, heavy-duty, trains), compressed gas is preferred for the hydrogen storage.
An important heavy mobile application is forklift. exploited. [152] A special focus on hydrogen storage media at the solid state for submarine applications has been review by Fiori et al. [153]. Many other reviews describe the development of fuel cell vessels and potential of hydrogen for maritime and submarine applications. [154][155][156][157][158] Hydrogen fuelled submarines have about 300 kW FC and the storage system is based on an AB2 alloy, which works between 20 and 50 °C. [128] Another example of application in the maritime sector is for the AB2 alloy (Ti0.93Zr0.05)(Mn0.73V0.22Fe0.04)2 used for the canal boat Ross Barlow, in UK, [159] where eight cylinders containing about 30 kg of metal hydride powder each were used and 4 kg of H2 were stored and used to supply a 1 kW PEM FC, which allows 10 h of operation.
Stationary applications are mainly referring to the volumetric density rather than to the gravimetric density of the hydrogen carrier. In particular, the final footprint per weight of stored hydrogen becomes the key performance to be considered. Most of the integrated system are based on AB5 compounds, such as LaNi5 and LaNi5-based alloys. As an example, an automated hydrogen-based auxiliary power unit was developed by Doucet et al. [160], with a storage capacity of about 1 Nm 3 , to be used by a PEM FC of 1 kW. A stationary system commercially available based on an AB5 intermetallic compound is the H2ONE developed in Japan and commercialized by Toshiba. [161] Systems are scaled to be easily installed anywhere, with a H2 storage capacity of ≈ 150 Nm 3 per module. An example of stationary application, based on a commercial LaNi4.8Al0.2 alloy, was developed by Rizzi et al. [162]. The hydrogen tank worked at 60 °C and it was integrated with a 1 kW PEM FC. A scheme of the developed tank is reported in Figure 5-a, while a general view of the developed system is reported in Figure 5 al. [162].
A hydrogen storage system based on a AB intermetallic (i.e. Ti(FeMn)) alloy was developed in South Tyrolean Alps by GKN,[163] with a system providing energy for a house, which has no access to the grid. At the Brookhaven National Laboratory, engineering-scale tests were performed in the '70s and a large TiFe-based system prototype was developed. [164][165][166][167] A final optimised tank stored 6 kg hydrogen using 405 kg of TiFe for the storage of off-peak electricity in an integrated system that combined an electrolyser, hydrogen tank and a fuel cell. [168] In the framework of the European project HyCARE, supported by FCH-JU, a hydrogen storage system based on AB alloys is planned. [169,170] The aim of the project is to store ≈50 kg of H2 integrating the storage tank with an electrolyser upstream and a fuel cell downstream to produce energy for the grid (stationary application). The tank is based on an innovative concept that links hydrogen and heat storage for stationary storage of excess renewable energy.
Mg-based systems are working at high temperatures (e.g. 360 °C) and 0.1 MPa. As an example, the (volumetric storage capacity of 10 g l -1 ), releasing hydrogen at 170 °C. [177,178] An overview of the developed tank is reported in Figure 6. The storage tank consisted of three concentric tubes and the hydrogen carrier was based on a mixed lithium amide/magnesium hydride system, coupled with an intermetallic compound. [177] The system was planned to supply a 1 kW HT-PEM FC stack for 2 h. The developed hydrogen tank is reported in Figure 6-a. The complete storage system combined twelve vessels in parallel (Figure 6-b) and a general view of the integrated APU system is reported in Figure 6- Baricco et al. [177].
The development of the tank has been followed by a study involving a Life Cycle Assessment (LCA), [10] which demonstrated that, when the electricity consumption for hydrogen gas compression is included into the analysis, a solid-state hydrogen storage tank has similar greenhouse gas emissions and primary energy demand than those of type III and IV gas tanks. [10]

Discussion
Metal hydrides allow to store H2 at low temperature (e.g. even close to room temperature) and at low pressures (e.g. < 50 bar), being easily integrated with low pressure electrolyser upstream and/or fuel cell downstream. A drawback is their low gravimetric capacity, (< 2 wt.%), which implies higher amount of alloys compared to high gravimetric capacity hydrides, such as complex ones, to reach the same amount of H2 stored. Nevertheless, easier design thanks to low temperature and pressure conditions are allowed, compared to complex hydrides, which usually require high temperatures. Intermetallic compounds are particularly interesting in stationary applications, [128] in which there are not limitation on system weight, and in heavy mobile applications, like forklift [179]  In order to estimate possible applications of solid-state hydrogen storage for energy communities, different scenarios have been considered. As a starting point, the green hydrogen production by water electrolysis is considered. Taking as a reference the production of 1 kg of hydrogen and an energy consumption for hydrogen production of 50 kWh/kg H2, the time for production can be  From the gender perspective, the dark side of the H2 development as reported by the study, [180] is the role of males in the decision and adoption of renewable energy system and in their maintenance. [180] This process, nowadays, is still men-skewed due to the requirement of more technical skills, and, as a result of an historical but still persistent trend, is characterized by a highly unbalanced distribution of STEM competencies. [180] Given this evidence, the goal must be to increase women involvement in the process as bearers of expert knowledge and therefore able to steer the decision making process. A goal and an ambition are that the H2-based technologies development might help in fulfilling through two main dynamics/strategies. First, and at present, by collecting women present STEM skills that are relevant for H2-based solutions. More than other RES technologies H2-based need the mere technical-engineering skills to be complemented by other knowledge (e.g. chemistry and physics) where the female component is relatively more represented.
Secondly, and in the mid-term, the inclusion of STEM women in H2 development might play as a driving force for steering women educational and professional careers towards the STEM fields.
In conclusion, H2 might positively impact women both as a trigger for a more balanced distribution of daily activities connected to the family and house management, and as a trigger for enhancing the role of women in the decision process towards the adoption of H2 solutions through the enhancement of their STEM skills.
The implementation of a storage system based on hydrogen within an integrated energy system would enable new organisational in-house tasks leaving more freedom in the use of smart energy and empowering women implementing their free time and developing an ungendered house space towards a balanced division of household tasks and maintenances. Furthermore, climate change impact will affect differently the north and the south of the world, the vulnerability of women during the energy transition should be considered to protect these population and supporting the unique values and virtues of women, to foster the energy transition locally and to lead towards an increased women's responsibility and communication role in the energy market. [181] Conclusions Intermetallic compounds are good candidates for safe H2-carrier systems in the storage of renewable energies, thanks to their low working temperature and pressure. Thermodynamic and kinetic properties of metal hydrides allow simple and suitable design for solid-state hydrogen tanks.
AB2 compounds have found a high interest for mobility applications, but also AB5-based systems are under developments. Although the use of novel hydrogen storage media such as complex hydrides could be advantageous, their integration into hydrogen storage tanks need further assessment in scaled up systems under real operating conditions. Thermal exchange remains the main issue in the system management.
Hydrogen tanks based on solid-state materials are already available in the marked, but limited applications have been demonstrated. Because of the main properties of the hydrogen carriers (i.e. gravimetric and volumetric H2 density), large weights and volumes must be considered for the tank system, if the request for stored hydrogen is high. The limited market strongly limits the availability and increases the cost of metal powders for hydrogen storage.
In conclusion, hydrides can provide a technical suitable solution for the storage of renewable energies and therefore can play a relevant role in fostering the transition towards an energy system characterized by a wide adoption of renewables technologies. But it is now recognized, both in the research and the policy fields, that technical aspects are only part of the solutions and the social aspects have been gaining relevance in the past decades as crucial components for investigating the transition pathways (e.g. the long lasting socio-technical transition approach) [182][183][184] and to design effective policies for their implementation. For a technical innovation to be properly developed and diffused and to be adequately accepted and adopted by the users (ultimately, the citizens) attention has to be paid to the social, economic and cultural context within which the innovation should be embedded, to better design the innovation itself, to minimize the negative impacts and to maximise the potential benefits. Among the many social aspects and dynamics that would deserve to be considered, this paper focused on the promotion of a gender perspective as a driver for more innovative and inclusive solutions in the clean energy transition and in the hydrogen-based solution exploitation. Given their role in the organization of social life and in fostering innovative solutions, both in the developing and developed countries, even though different mechanisms, women inclusion in the development of hydrogen-related technologies will provide a more comprehensive view on the exploitation of its storage potential in the overall reorganization of the daily and working life and, the way around, it might trigger to an actual empowerment of women through an increase of their talent and skills (i.e. share in STEM) and their involvement in the energy chain decision making. A process that has the potential to result in a safer, more affordable, sustainable and inclusive clean energy future.