Future towards renewable hydrogen storage and powered applications

G.VijayalakshmiP.SankaraGomathy

Electrical and Electronics EngineeringGovernment college of Engineering

Tirunelveli

Our ecosystem will no longer be able to nurture the human species if the exploitation and consumption of fossil fuel sources continues at its current rate. Hydrogen holds great promise to meet many of our future energy needs, and it addresses national security and our environmental concerns. Hydrogen is the simplest, most abundant element in the universe. The environmental benefits of hydrogen are also outstanding.

  Metal hydride, formed when metal atoms bond with hydrogen to form stable compounds, are ideal mediums for portable hydrogen fuel storage. Metal hydrides are extremely well suited for use in consumer products requiring safety and ease of operation. Refueling can be simply performed by replacing a small hydrogen metal hydride canister. 

Schematic of a Metal Hydride - hydrogen is inserted in

between the atoms in a solid

As an example, the volumetric hydrogen density of LaNi5H6  is much higher than that of compressed hydrogen or liquid hydrogen

• Long term stability

• Long life (over 1000 charge cycles)

• Ambient temperature operation (25C nominal)

• Fast re-filing time

• Light weight aluminum metal hydrides

• Good resistance to impurities

• Safe storage due to low pressure system

• Compact in size (compared to compressed hydrogen cylinder)

• Portability & mobility

Nanocrystalline Mg and Mg2Ni obtained by ball milling exhibit much faster hydrogen sorption rates at relatively low temperatures eg: Nanocrystalline Mg and Mg2Ni

Recent work shows that reducing the size of Mg-based hydrides to the nanometer scale can also alter their stability, which can be characterized by the desorption energy. The desorption energy of MgH2, which is 74 kJ/mol-H2 for bulk, decreases to 65.3 kJ/mol-H2 when it is in nanowire form (30–50 nm in diameter). This experimental result shows that hydrogen desorption energy decreases significantly when the grain size is smaller than 1.3 nm.

The nanocomposite materials for hydrogen storage encompass a catalyst and composite chemical hydrides at the nanometer scale. The catalyst increases reaction rate. The thermodynamic stability of the nano-composite materials can be controlled by the composite chemical hydride shaving protide (hydride) (Hδ-) and proton (Hδ+). In addition, the hydrogen absorption kinetics is accelerated by the nanosize materials and they may change the thermodynamic stability of the materials.

Catalysts help the dissociation of H2 molecules or the recombination of H atoms. Ball milling is a simple and effective way to introduce and disperse catalysts into hydrides, and thus to generate nano sized catalysts. Various transition metals (Pd, V, Ni), oxides (Cr2O3, Nb2O5), non-metals (graphite) and compounds play catalytic roles in hydriding/dehydriding reactions of Mg. TiF3-catalyzed MgH2 nanoparticles desorb 4.5 wt.% hydrogen in 6 min at 573K, and hydrogen uptake can take place even at room temperature. The activation energy for MgH2 decomposition was found to decrease significantly owing to the addition of nanosized catalyst. Mechanisms such as the ‘hydrogen pump’ and the ‘spillover effect’ have been proposed to explain the catalytic effect. The underlying catalytic mechanism, however, needs to be further clarified.

The introduction of complex hydrides to hydrogen storage research has undoubtedly broadened the materials scope and enhanced researchers’ capability to design and optimize new materials. Typical complex hydrides include amide–hydride systems,borohydrides, and alanates. Alanates, e.g. NaAlH4, have been the subject of intensive studies

• Sodium Borohydrate is the most popular of many hydrate solutions

• Solution passed through a catalyst to release H2

• Commonly a one-way process (sodium metaborate must be returned if recycling is desired.)

• Some alternative hydrates are too expensive or toxic

• The “Millennium Cell” company uses Sodium Borohydrate technology

Sodium Borohydrate