The hydrogen economy: challenges and prospectives

The fossil fuels-based economy led to considerable growth in CO2 footprint and air pollution. The shift toward renewable and green energy sources is necessary and a hydrogen-based economy may be a solution for both climate change and the need for a secure energy supply. To make the transition from the present carbon-based economy to the hydrogen economy several problems have to be solved, regarding the production, safety issues, and the storage of hydrogen. Hydrogen has high gravimetric chemical energy (142 MJ/kg) and its electrochemical reaction with oxygen in the fuel cell leads to zero carbon emission. It is abundant, but in the bonded form in water, hydrocarbons, and other organic compounds. The most of hydrogen is produced by the steam reforming of hydrocarbons. However, this is so-called “grey hydrogen”, as a considerable amount of CO2 and CO is released during the process. To meet the sustainability criteria, it is necessary to make the methods of “green hydrogen” generation, like water electrolysis, more affordable and energy efficient. Thus, the development of low-cost, stable electrocatalysts with high activity for the hydrogen evolution reaction (HER) without noble metals, is a considerable challenge. Among all possible solutions for hydrogen storage (gas cylinders, liquid tank, solid-state storage), the one which relies upon storage in solid media such as hydrides, is the most attractive. Solid-state storage implies hydrides such as metal/intermetallic and complex- chemical hydrides. Since the release of hydrogen from hydrides takes place via an endothermic process, this method of storage is the safest of all mentioned, but the problem of the slow sorption process and high temperature of desorption has to be solved if hydrogen is to be used as an energy carrier. In the last three decades, many studies have been performed, both experimental and theoretical, on the same subject – how to overcome these drawbacks. Sluggish thermodynamics and slow sorption/desorption kinetics can be altered by MgH2 destabilization through surface modification and structural deformation. To improve the diffusivity of hydrogen in the metal hydride, various techniques such as ball milling and ion bombardment are applied, which in turn reduces the particle size, increases the defect concentration and shortens the diffusion path for hydrogen. Also, sufficiently fast hydrogen sorption kinetics has been achieved by using metals, metal oxides, transition metals, and transition metal oxides as additives in the process of milling. In this short review, we have summarized the possibilities for the storage and production of hydrogen by green synthesis methods.