Metal Hydrides and Related Materials. Energy Carriers for Novel Hydrogen and Electrochemical Storage

As important as the production of CO2-free energy is, so is the motivation to develop efficient storage of renewable energies for mobile and stationary applications. There is no doubt that metal hydrides continue to attract the overall materials science community, and it is not only restrained to a specialized hydrogen field. This is mainly the case when it is the matter of coupled technologies with other systems such as fuel cells, heat management, and batteries. Hydrogen is considered an energy carrier and its chemical energy can be converted into electricity through a chemical reaction with oxygen from a fuel cell. Therefore, coupling energy storage systems with renewable energy sources through an electrolyzer, which can transform electric energy into hydrogen chemical energy, is considered a high sustainable process of production and exploitation of renewable energies. Integrated systems are constituted by a metal hydride tank and a PEM fuel cell, in which the waste heat generated in the fuel cell is used to supply the necessary heat required for desorption of hydrogen from the tank. The field of application of the integrated power system is in combination with renewable sources: The hydrogen can be produced by electrolysis of water using the energy from a renewable source (e.g., photovoltaic); it is then stored and converted into electric energy by the proposed integrated power system, that allows energy storage in the form of hydrogen and its reuse when the renewable source is not available, for example, at night if solar power is exploited. The developed power system could replace batteries and could be applied in the case of a production plant not connected to the power grid, such as in remote areas. As an example, an integrated power system, showing a total energy production of 4.8 kW h, over more than 6 h of working activity, is reported in ref 4. In the SSH2S (Fuel Cell Coupled Solid-State Hydrogen Storage Tank) project, a solid-state hydrogen storage tank based on complex hydrides has been developed and it was fully integrated with a High-Temperature Proton Exchange Membrane (HT-PEM) fuel cell stack. The hydrogen storage tank was designed to feed a 1 kW HT-PEM stack for 2 h to be used for an Auxiliary Power Unit (APU).61 With respect to batteries, hydrides can be utilized as anodes with high capacity (e.g., 2 Ahg-1 for MgH2). A lot of effort has been expended to improve the cyclability, and significant results have been reached in a solid-state battery with LiBH4 solid electrolyte. Demonstration of high-energy density fuel cells with suitable cathodes will be the challenge of upcoming research studies.62,63 As mentioned for the beneficial contribution of electrodes, solid-state electrolytes based on borohydrides are a typical example that the battery community is now taking seriously along with the popular garnet-type solid electrolytes.64,65 It has been demonstrated that the ionic conductivity is a prerequisite for application in batteries, but unfortunately it is not win; in fact, other important issues need to be tackled, such as chemical compatibility, interfaces, heterogeneity, and mechanical properties, so important for the cell engineering and design, in addition to the structural and volumetric changes during cycling. At first, borohydrides meet some of these criteria regarding conductivity and ductility, (thermo)chemistry, and low-density materials. Future research might be directed to the understanding and assessment of interfaces and physical and mechanical properties of the selected solid-electrolyte and electrodes. The specificity of the application may become a determining aspect in the selection of the suitable configuration. Substantial research efforts are being conducted to study new approaches toward