PSI - Issue 60

3

Ram Niwas Singh et al. / Procedia Structural Integrity 60 (2024) 411–417 RNSingh/ Structural Integrity Procedia 00 (2023) 000 – 000

413

( ) + ( ) + → + → + + → +

(2) (3) (4)

Hydrogen can be produced by splitting the water molecule by passing electric current using a device called electrolyzer that converts electrical energy into chemical energy, produces hydrogen and oxygen, and consists of two electrodes, suitable electrolytes and appropriate separating membrane. The electrolysers could be alkaline, proton exchange membrane or solid oxide cells based (Filippov and Yaroslavtsev 2021, Ajanovic 2022, Holladay 2009). Alkaline electrolysers composed of electrodes, a microporous separator and an aqueous alkaline electrolyte are most developed, cheapest but has lowest efficiency. Proton ion exchange membrane electrolyser use Pt black, iridium, ruthenium, and rhodium for electrode catalysts and a Nafion membrane. Both the electrolysers operate at temperatures less than 100°C and pressure less than 100 bar (Filippov and Yaroslavtsev 2021, Ajanovic 2022, Holladay 2009). Solid oxide electrolysis cells are solid oxide fuel cells operating in reverse, most efficient, operate at temperature as high as 650-1000°C, but most capital intensive (Filippov and Yaroslavtsev 2021, Ajanovic 2022, Holladay 2009). Though there is no standard in literature, different colors are commonly used to represent the source from which it is derived (Table 1) and these colors provide measure of the environmental cost for producing hydrogen (Filippov and Yaroslavtsev 2021, Ajanovic 2022). The global hydrogen production from natural gas and coal is 76 and 23%, respectively (Filippov and Yaroslavtsev 2021, Ajanovic 2022). Approximately 1% of hydrogen comes as a by-product from alkaline electrolysis used to produce chlorine and sodium hydroxide (caustic soda), and only 0.1% of hydrogen are produced by water electrolysis (Filippov and Yaroslavtsev 2021, Ajanovic 2022). Method of hydrogen storage decides the mode of transportation. Gaseous hydrogen has been traditionally stored in steel cylinders under pressure usually as 200 bars and are designated as Type I cylinders (Barthelemy 2012, Langmi, 2022). However, to achieve high volumetric and gravimetric density stronger partially hoop reinforced steels, and alloys like Al-alloys and Ti-alloys having high specific strength are used for construction for cylinders and are called Type II cylinders and can be used up to pressure of 400 bars. Type III cylinders are made of fully hoop reinforced alloys possessing high specific strength or carbon-based composites with metallic liner as permeation barrier and can be used for pressures up to 700 bars, whereas in Type IV cylinders polymer liner is used as permeation barrier and can be used up to pressures of 700 bars albeit with slightly higher volumetric and gravimetric density (Barthelemy 2012, Langmi 2022). Both Type III and Type IV cylinders are costly and are considered for specific applications in aerospace industries and transportation. Hydrogen stored in liquid form has higher volumetric density but requires storage at cryogenic temperature and is more suitable for static applications (Züttel 2003) and is also used as fuel for cryogenic applications in space programme. Hydrogen adsorbed in metal lattice is safest, has high volumetric density but has low gravimetric density. Highest gravimetric and volumetric density has been achieved for low atomic number complex compounds like [AlH4] - or [BH4] - but desorption takes place at elevated temperatures and absorption requires high pressure (Ali and Ismail 2021, Ouyang 2020). Hydrogen can also be transformed to ammonia or methane, stored in tanks or container, transported to the site of consumption and decomposed before use, thereby avoiding the need of very low temperature and/or high pressure. Gaseous hydrogen can be transported through pipelines by mixing it with natural gas, liquid hydrogen through cryo-cooled containers, and hydrogen stored in metal lattice can be transported in cartridges. The relative comparison between the various storage methods operating temperature and pressure is shown in Table 2 (Ajanovic 2022, Holladay 2009). The requirements to be satisfied for Hydrogen storage systems that can be used for transport applications are Multicycle reversibility of hydrogen uptake/release (not less than 500 cycles), Fast kinetics of hydrogen uptake/release, Low-operating pressure (less than 4 bar), Operating temperature in the range from - 50 to 150°C, High gravimetric