A Multipurpose Hydrogen Storage System Using AB₅– and AB₂–Type Metal Hydrides for Flexible Hydrogen Storage and Delivery

Abstract

Metal hydrides can safely store hydrogen in the solid state at high volumetric density under moderate temperature and pressure. Their hydrogen sorption characteristics are represented by pressure–composition–temperature (PCT) curves. AB₅–type metal hydrides, which have low plateau pressures, store and release hydrogen at low pressures. AB₂–type metal hydrides, which have high plateau pressures, store and release hydrogen at relatively high pressures. Compared with AB₅–type metal hydrides, AB₂–type metal hydrides generally have lower raw material costs and higher hydrogen storage capacity. This makes them more suitable for storing large quantities of hydrogen. Green and blue hydrogen are produced using commercial alkaline water electrolyzers and natural gas reformers, respectively. After downstream purification, this hydrogen is typically supplied at pressures below 1 MPa. However, the high plateau pressures of AB₂–type metal hydrides make it difficult to store this low-pressure hydrogen directly. AB₅–type metal hydrides can store it but release it only at low pressures. A single hydride type therefore operates within a narrow pressure range for both storage and delivery. In this study, a multipurpose hydrogen storage system (MHSS) using AB₅– and AB₂–type metal hydrides was proposed to broaden the applications of metal hydride-based systems. The feasibility of the MHSS was experimentally evaluated through lab-scale tests. The AB₅ and AB₂ modules were first tested as standalone units. The integrated MHSS was then tested assuming that waste heat was continuously available. The MHSS can store a large quantity of low-pressure hydrogen and deliver it across a wide pressure range. This range covers diverse end uses, from fuel cells at 0.5 MPa to hydrogen pipelines at 4.0 MPa.

1. Introduction

Hydrogen is widely recognized as a key energy carrier for achieving carbon neutrality. It has high gravimetric energy density and can be converted into electricity through fuel cells, with water as the only byproduct. For low-carbon hydrogen utilization, green hydrogen generated by water electrolysis using renewable energy and blue hydrogen generated by natural gas reforming with carbon capture are used. In practical hydrogen production systems, hydrogen generated by these processes is purified using downstream purification units and is typically supplied at pressures below 1 MPa. At these supply pressures, hydrogen exists as a low-density gas, making it difficult to store compared with liquid fuels.

To address this challenge, various hydrogen storage methods, including high-pressure gaseous storage, cryogenic liquid storage, and solid-state storage using metal hydrides, have been developed. Among various hydrogen storage methods, metal hydride-based solid-state storage can safely store a large quantity of hydrogen in the solid state under moderate temperature and pressure conditions [1,2,3,4,5]. This is because hydrogen atoms are located in the metal lattice at interatomic distances smaller than the van der Waals radius of the hydrogen molecule [6].

Among various metal hydrides, AB₅– and AB₂–type metal hydrides have been widely investigated for hydrogen storage and delivery because of their reversible hydrogen sorption, relatively fast reaction kinetics, and applicability under moderate operating conditions [7,8,9,10]. AB₅–type metal hydrides, represented by LaNi₅, show low plateau pressures and flat plateau regions. With these characteristics, AB₅–type metal hydrides are suitable for absorbing hydrogen supplied at low pressure, such as hydrogen produced by water electrolysis. AB₂–type metal hydrides, represented by TiMn₂, show high plateau pressures and higher gravimetric hydrogen storage capacity than AB₅–type metal hydrides [11]. With these characteristics, AB₂–type metal hydrides are suitable for storing large quantities of hydrogen and delivering hydrogen to diverse end uses, such as hydrogen pipelines requiring high-pressure hydrogen [12].

Hydrogen storage systems using only AB₅– or AB₂–type metal hydrides have a limited pressure range for hydrogen storage and delivery [13,14,15,16]. A system using only AB₅–type metal hydrides can store low-pressure hydrogen. However, because of their low plateau pressures, it is difficult to deliver hydrogen at higher pressures. A system using only AB₂–type metal hydrides can store a large amount of hydrogen and deliver hydrogen at relatively high pressures. However, because of their high plateau pressures, it is difficult to directly store low-pressure hydrogen. Therefore, a metal hydride-based hydrogen storage system is required to connect low-pressure hydrogen storage with higher-pressure hydrogen delivery.

In this study, a multipurpose hydrogen storage system (MHSS) was proposed to broaden the range of applications of metal hydride-based hydrogen storage systems. The MHSS consists of an AB₅ module and an AB₂ module connected in series, which contain AB₅– and AB₂–type metal hydrides, respectively. The AB₅ module is used to store low-pressure hydrogen. When waste heat is available, the AB₅ module increases the pressure potential of the hydrogen stored in the AB₅ module, and the hydrogen is stored in the AB₂ module. The AB₂ module is used to deliver hydrogen at higher pressures. The feasibility of the MHSS was experimentally evaluated through lab-scale tests. The performances of the standalone AB₅ and AB₂ modules were first evaluated, and the integrated MHSS was then tested under the assumption that waste heat was continuously available.

2. Multipurpose Hydrogen Storage System

Figure 1 shows a schematic diagram of the MHSS. The MHSS is composed of an AB₅ module and an AB₂ module. The AB₅ module and the AB₂ module are connected in series. The AB₅ module is used to upgrade the hydrogen pressure to store hydrogen in the AB₂ module. The AB₅ module consists of two sub-modules, AB₅ module–A and –B, which are connected in parallel. Each AB₅ sub-module has an outer diameter of 60.5 mm and contains 375 g of La_0.9Ce_0.1Ni₅ with 5 wt% expanded natural graphite [17]. The AB₂ module serves as the main hydrogen storage module of the MHSS, storing a large amount of hydrogen and delivering hydrogen over a wider pressure range. The AB₂ module has an outer diameter of 89.1 mm and contains 1607 g of Ti_0.95Zr_0.05Mn_1.33V_0.51Fe_0.06 with 5 wt% expanded natural graphite. The AB₅ sub-modules and the AB₂ module were each installed in water jackets. Water was used as the heat transfer medium (HTM) for temperature control of the AB₅ sub-modules and the AB₂ module. The HTM was supplied to each water jacket at a flow rate of 3 L/min. The MHSS operates just by shifting the HTM. Hydrogen is stored in the AB₅ module when low-temperature HTM flows through it, and is released at an increased pressure and transferred to the AB₂ module when high-temperature HTM flows through it. Low-temperature and high-temperature HTMs are alternately supplied to AB₅ module–A and –B, allowing one sub-module to store hydrogen while the other releases it. Hydrogen transferred from the AB₅ sub-modules is stored in the AB₂ module under low-temperature HTM supply. Under high-temperature HTM supply, the hydrogen pressure in the AB₂ module further increases, allowing for hydrogen delivery at a higher pressure. Check valves are installed in the system to prevent backflow and ensure unidirectional hydrogen flow from the hydrogen gas cylinders through the AB₅ module to the AB₂ module.

3. Results and Discussion

3.1. Characteristics of Hydrogen Storage Materials

Figure 2 shows the activation of AB₅– and AB₂–type metal hydrides. Each hydrogen absorption process was conducted at 6 MPa for 30 min, followed by vacuuming for 30 min before the next absorption process. The hydrogen absorption process was repeated five times, and both metal hydrides were fully activated through two hydrogen absorption processes. The amount of hydrogen stored in the activated AB₅–type metal hydride was 1.35 wt% (approximately 99% of theoretical storage capacity). The amount of hydrogen stored in the activated AB₂–type metal hydride was 1.73 wt% (approximately 97% of theoretical storage capacity). The elapsed time to store these amounts of hydrogen was 2 min for the AB₅–type metal hydride and 3 min for the AB₂–type metal hydride.

Figure 3 shows pressure–composition–temperature (PCT) curves of AB₅– and AB₂–type metal hydrides at 283 K, 313 K, and 353 K. The PCT curves present the thermodynamic characteristics of metal hydrides, which consist of pressure, composition, and temperature in hydrogen absorption and desorption. Based on the PCT curves, the AB₅–type metal hydride can absorb hydrogen at 0.6 MPa at 283 K and desorb hydrogen at 1.5 MPa at 353 K. The AB₂–type metal hydride can absorb hydrogen at 2.0 MPa at 283 K and desorb hydrogen at 4.0 MPa at 353 K. These results indicate that the AB₅–type metal hydride is suitable for low-pressure hydrogen storage, whereas the AB₂–type metal hydride, with a higher hydrogen storage capacity, is suitable for storing a large quantity of hydrogen and delivering hydrogen at higher pressures. Hydrogen absorption enthalpies of AB₅– and AB₂–type metal hydrides obtained from the PCT results were 31.34 kJ/mol and 26.47 kJ/mol, respectively.

3.2. Performance of AB₅ Module

Figure 4 and Figure 5 show the performance of the AB₅ module during hydrogen storage and release. The inlet and outlet pressures of the AB₅ module were controlled at 0.6 MPa and 1.5 MPa, respectively. The HTM with different temperatures, 358 K and 288 K, was shifted between AB₅ sub–modules (AB₅ module–A and –B). When the HTM was shifted, the temperature of the HTM flowing through AB₅ module–A decreased from 353 K to 300 K for 1 min and to 283 K for 10 min. The hydrogen pressure of AB₅ module–A immediately decreased from 1.5 MPa to 0.6 MPa. After the pressure decreased to 0.6 MPa, AB₅ module–A started to store hydrogen. The hydrogen storage flow rate reached 6000 Scm³/min for 2 min. Thereafter, the hydrogen storage flow rate gradually decreased, and hydrogen storage was completed after approximately 15 min. The total amount of hydrogen stored in AB₅ module–A was 26,000 Scm³. At the same time, the temperature of the HTM flowing through AB₅ module–B increased from 283 K to 343 K for 1 min and to 353 K for 7 min. The hydrogen pressure of AB₅ module–B immediately increased from 0.6 MPa to 1.5 MPa. After the pressure increased to 1.5 MPa, AB₅ module–B started to release hydrogen. The hydrogen release flow rate reached 3000 Scm³/min for 1 min. Thereafter, the hydrogen release flow rate gradually decreased to 500 Scm³/min at 30 min. The total amount of hydrogen released from AB₅ module–B was 26,000 Scm³ after 30 min.

3.3. Performance of AB₂ Module

Figure 6 shows the performance of the AB₂ module during hydrogen storage. During hydrogen storage, the temperature of the HTM flowing through the AB₂ module was maintained at 283 K. The inlet pressure of the AB₂ module was controlled at 2.0 MPa, and the hydrogen storage flow rate was controlled not to exceed 30,000 Scm³/min. When the inlet valve was opened, the inlet pressure immediately dropped from 2.0 MPa to 1.8 MPa, and the hydrogen storage flow rate reached 30,000 Scm³/min. This state was kept for approximately 2 min. Thereafter, the inlet pressure gradually increased to its initial value of 2.0 MPa after 60 min, while the hydrogen storage flow rate gradually decreased. Hydrogen storage in the AB₂ module was completed after approximately 80 min. The total amount of hydrogen stored in the AB₂ module was 325,800 Scm³.

Figure 7 shows the performance of the AB₂ module during hydrogen delivery. Before hydrogen delivery, the AB₂ module stored hydrogen at 2.0 MPa and 283 K. The hydrogen delivery flow rate was controlled not to exceed 30,000 Scm³/min. The HTM was shifted from low-temperature HTM at 283 K to high-temperature HTM at 353 K. The outlet pressure of the AB₂ module was controlled at 0.5, 1.0, 2.0, and 4.0 MPa. At all outlet pressure conditions, the temperature of the HTM flowing through the AB₂ module showed a similar increase when the HTM was shifted, increasing from 283 K to 347 K within 1 min and reaching 353 K within 40 min. At outlet pressures of 2.0 and 4.0 MPa, the pressure of the AB₂ module increased and reached the target outlet pressure within 1 min. The hydrogen delivery flow rate reached 29,000 Scm³/min and 17,000 Scm³/min for 1 min at 2.0 and 4.0 MPa, respectively. Thereafter, the hydrogen delivery flow rate gradually decreased, and hydrogen delivery was completed after 45 min and 80 min at 2.0 and 4.0 MPa, respectively. At outlet pressures of 0.5 and 1.0 MPa, the pressure of the AB₂ module remained relatively high because the hydrogen delivery flow rate was controlled to 30,000 Scm³/min, and then reached the target outlet pressure after approximately 4 min. Thereafter, the hydrogen delivery flow rate gradually decreased, and hydrogen delivery was completed after 32 min and 35 min at 0.5 and 1.0 MPa, respectively. The total amount of hydrogen delivered from the AB₂ module was 325,200 Scm³, 308,600 Scm³, 286,600 Scm³, and 261,100 Scm³ at outlet pressures of 0.5, 1.0, 2.0, and 4.0 MPa, respectively.

3.4. Performance of MHSS

3.4.1. Preparation of MHSS

Figure 8 shows the pressure variation in the AB₅ and AB₂ modules during MHSS preparation. Figure 9 shows the volumetric flow rates of hydrogen entering the MHSS and the AB₂ module and the amounts of hydrogen stored in the MHSS and the AB₂ module during MHSS preparation. The amount of hydrogen stored in the MHSS represents the total amount stored in AB₅ module–A, AB₅ module–B, and the AB₂ module. The inlet pressure of the MHSS was controlled at 0.6 MPa, and the volumetric flow rate entering the MHSS was controlled not to exceed 30,000 Scm³/min. The temperatures of the HTM flowing through AB₅ module–A, AB₅ module–B, and the AB₂ module were maintained at 353 K, 283 K, and 283 K, respectively. When the inlet valve of the MHSS was opened, the pressures of the AB₅ and AB₂ modules gradually increased and reached the target outlet pressure after 20 min. The hydrogen storage flow rate entering the MHSS immediately reached 30,000 Scm³/min and then gradually decreased. The hydrogen storage flow rate entering the AB₂ module reached 24,000 Scm³/min within 1 min and then gradually decreased. Hydrogen storage in the MHSS and the AB₂ module during preparation was completed within 30 min, and the total amount of hydrogen stored in the MHSS and the AB₂ module was 84,000 Scm³ and 41,000 Scm³, respectively. These results indicate that only a limited amount of hydrogen can be stored in the AB₂ module at an inlet pressure of 0.6 MPa, and that additional hydrogen storage in the AB₂ module requires pressure-upgraded hydrogen transferred from the AB₅ module.

3.4.2. Operation of MHSS

Figure 10 and Figure 11 show the performance of the MHSS during operation. Before the operation, the AB₅ and AB₂ modules were filled with hydrogen at 0.6 MPa. The inlet pressure of the MHSS was controlled at 0.6 MPa. The HTM at different temperatures, 353 K and 283 K, was set to be shifted between the AB₅ sub-modules every 15 min. The temperature of the HTM flowing through the AB₂ module was maintained at 283 K. The AB₅ sub-modules, AB₅ module–A and –B, repeatedly stored hydrogen at pressures below 0.6 MPa and released hydrogen into the AB₂ module. The pressure of the AB₂ module varied with the increases in the pressures of the AB₅ sub-modules, AB₅ module–A and –B, which were heated to 353 K by the HTM. The hydrogen storage flow rate into the AB₂ module was 5600 Scm³/min at the beginning of the MHSS operation. As hydrogen storage in the AB₂ module progressed, the driving force for hydrogen transfer decreased because the pressure difference between the plateau pressures of the AB₅– and AB₂–type metal hydrides became smaller, resulting in a gradual decrease in the volumetric hydrogen storage flow rate. The total amount of hydrogen stored in the AB₂ module through the MHSS operation was 286,400 Scm³ for 210 min. The total amount of hydrogen stored in the AB₂ module during MHSS preparation and operation was 327,400 Scm³, which is almost the same amount of hydrogen stored in the standalone AB₂ module, 325,800 Scm³.

4. Conclusions

A multipurpose hydrogen storage system (MHSS) using AB₅– and AB₂–type metal hydrides was proposed and experimentally evaluated. The pressure–composition–temperature (PCT) curves of the fully activated AB₅– and AB₂–type metal hydrides were obtained. Each standalone AB₅ sub-module stored 26,000 Scm³ of hydrogen at 0.6 MPa and 283 K for 15 min and released the same amount of hydrogen at 1.5 MPa and 353 K for 30 min. The standalone AB₂ module stored 325,800 Scm³ of hydrogen at 2.0 MPa and 283 K for 80 min and delivered 325,200 Scm³, 308,600 Scm³, 286,600 Scm³, and 261,100 Scm³ of hydrogen at 353 K and outlet pressures of 0.5, 1.0, 2.0, and 4.0 MPa, respectively. The AB₅–type metal hydride was suitable for increasing low-pressure hydrogen storage and pressure, whereas the AB₂–type metal hydride was suitable for storing a large quantity of hydrogen and delivering hydrogen at higher pressures. Assuming that waste heat was continuously available, the integrated MHSS was tested to evaluate hydrogen transfer from the AB₅ module to the AB₂ module. During the preparation process, the MHSS stored 84,000 Scm³ of hydrogen, of which 41,000 Scm³ was stored in the AB₂ module, at an inlet pressure of 0.6 MPa for 30 min. During operation, the MHSS stored an additional 286,400 Scm³ of hydrogen in the AB₂ module over 210 min at an inlet pressure of 0.6 MPa by shifting the HTM every 15 min. The total amount of hydrogen stored in the AB₂ module was 327,400 Scm³, which was comparable to that stored in the standalone AB₂ module (325,800 Scm³). The proposed MHSS can be used as a flexible hydrogen storage and delivery system for storing low-pressure hydrogen and delivering it over a wider pressure range.