Metal Hydride Hydrogen Storage (Compression) Units Operating at Near-Atmospheric Pressure of the Feed H 2

: Metal hydride (MH) hydrogen storage and compression systems with near-atmospheric H 2 suction pressure are necessary for the utilization of the low-pressure H 2 produced by solid oxide electrolyzers or released as a byproduct of chemical industries. Such systems should provide reasonably high productivity in the modes of both charge (H 2 absorption at P L ≤ 1 atm) and discharge (H 2 desorption at P H = 2–5 atm), which implies the provision of H 2 equilibrium pressures Peq < P L at the available cooling temperature (T L = 15–20 ◦ C) and, at the same time, Peq > P H when heated to T H = 90–150 ◦ C. This work presents results of the development of such systems based on AB 5 -type intermetallics characterized by Peq of 0.1–0.3 atm and 3–8 atm for H 2 absorption at T L = 15 ◦ C and H 2 desorption at T H = 100 ◦ C, respectively. The MH powders mixed with 1 wt.% of Ni-doped graphene-like material or expanded natural graphite for the improvement of H 2 charge dynamics were loaded in a cylindrical container equipped with internal and external heat exchangers. The developed units with a capacity of about 1 Nm 3 H 2 were shown to exhibit H 2 ﬂow rates above 10 NL/min during H 2 charge at ≤ 1 atm when cooled to ≤ 20 ◦ C with cold water and H 2 release at a pressure above 2 and 5 atm when heated to 90 and 120 ◦ C with hot water and steam, respectively.


Introduction
High-temperature steam electrolysis, primarily based on solid oxide cells (SOCs), is a promising method of hydrogen production [1].However, due to technical difficulties related to cell sealing, almost all commercial SOCs operate at near-atmospheric pressures.In doing so, the generated hydrogen releases at P~1 atm (absolute), thus requiring further compression.Similar near-atmospheric pressures are typical for hydrogen byproducts released in vast amounts by several chemical industries.For example, the hydrogen production capacity of chlor-alkali plants in the U.S.A. ranges from 20 to 100 thousand tons annually [2], and the utilization of this hydrogen could bring significant benefits.At the same time, the minimum input/suction pressure of conventional mechanical hydrogen compressors does not typically exceed 4-5 atm [3].Thus, it will be promising to collect low-pressure H 2 in a metal hydride (MH) and to further deliver it at a higher pressure using thermally driven compression [4].
An obvious material solution to realize this approach is the use of hydride-forming material characterized by hydrogen equilibrium pressure (Peq) below 1 atm when absorbing the feed H 2 .However, the challenge will be in providing reasonably high productivities in both charge (H 2 absorption) and discharge (H 2 desorption) modes.Indeed, maximizing the pressure driving force for H 2 absorption requires the values of Peq to be close to zero.Such MH materials (hydrogen getters) are well known [5].However, hydrogen release from the Inorganics 2023, 11, 290 2 of 10 H-saturated getter, even at low pressures requires too high temperatures, up to 650 • C [6].At the same time, for the target application, the MH should provide sufficiently high H 2 discharge pressure (at least the 2-5 atm necessary for the suction in the next compression stage) when heated up to a moderate temperature, from 70 to 90 • C (provided by solar collectors) to 150 • C (provided by low-grade industrial steam).
The most suitable hydride-forming materials which satisfy these requirements are AB 5 -type intermetallics based on LaNi 5 where Ni is substituted with elements (Al, Mn, Co, etc.), increasing the thermal stability of the intermetallic hydride as compared to LaNi 5 H x .Such intermetallics can be easily activated, and their H sorption characteristics are less sensitive to the impurities in H 2 (first of all, water vapors) than the Ti-based AB-and AB 2 -type hydrogen storage alloys [7].The influence of various substituting elements in LaNi 5−x M x intermetallics on their structural, thermodynamic, magnetic, and hydrogen sorption properties has been analyzed by Joubert et al. in their review [8].The substitution of Ni with Al [8][9][10] and Mn [11] has the strongest effect on the increase in the stability of the intermetallic hydrides.Multicomponent AB 5 -type alloys (A − La or La + Ce; B − Ni, Co, Al, and Mn) were shown to exhibit H 2 plateau pressures at ambient temperature far below 1 atm and to compress H 2 to 5-10 atm when heated to 150-170 • C [12,13].
Apart from thermodynamic constraints specifying the values of Peq to be below H 2 charge pressure, P L ≈ 1 atm, at the cooling temperature, T L ≈ 15-20 • C, and above H 2 discharge pressure, P H ≈ 2-5 atm, at the heating temperature, T H ≈ 90-150 • C, high rates of H 2 uptake and release should also be provided on both the material and system level.Considering the limited pressure driving force when absorbing near-atmospheric pressure H 2 , the acceleration of this process is the most critical problem.On the material level, it relates to the improvement in hydrogenation kinetics, while the improvement in H 2 uptake dynamics in the system mainly depends on the augmentation of heat exchange between the MH material and the cooling means of the MH reactor.
The improvement in hydrogenation kinetics, along with facilitating activation and adding poisoning tolerance, can be achieved by introducing catalysts of the dissociative H 2 chemisorption, including Pd or Pd-Ni nanoparticles deposited onto the AB 5 substrate [14].The less expensive but still efficient approach is the creation of MH-based composites containing minor additives of graphene-like material with deposited Ni nanoparticles (Ni/GLM).Apart from catalyzing hydrogenation reactions, such composites are characterized by improved effective thermal conductivity (ETC), thus assisting in the acceleration of the H 2 charge dynamics of the MH reactor [15].Alternatively, ETC can be increased by adding expanded natural graphite (ENG) to the MH material [16].
Finally, the thermal augmentation methods include optimising the size and geometry of the MH reactors.The most frequently used methods combine the increase in (1) ETC (e.g., by using ENG), (2) heat transfer surface area (finned or coiled heat exchangers), and (3) the overall heat transfer coefficient (internal heat exchanger + external cooling jacket) [17].
This work presents results of the activities of the authors representing FRC PCP&MC RAS (Russia) and HySA Systems (South Africa) on the development of systems which can absorb low-pressure H 2 and to further desorb it at higher pressures.The studies of the Russian team aimed toward the development of MH H 2 storage and supply unit fed by H 2 from a solid oxide electrolyzer were focused on the use of LaNi 5−x Al x intermetallics with Ni/GLM additives.The activities of the HySA Systems team in South Africa were focused on the development of an MH hydrogen compression unit with suction pressure below 1 atm comprising a powder mixture of a multi-component AB 5 -type battery alloy with ENG additives loaded into a standard HySA Systems MH container for medium-pressure hydrogen compression [4].

MH Materials
The analysis of data in the literature [8][9][10] and our own preliminary results on hydrogen absorption in aluminium-substituted LaNi 5 at P(H 2 ) = 1 atm and T = 20-40 • C (Figure 1) shows that the maximum hydrogen absorption capacity can be achieved when the content of aluminium in LaNi 5−x Al x corresponds to x = 0.55.At the lower x values, when Peq in the plateau region of the pressure-composition isotherm approaches 1 atm, the capacity drop, especially pronounced when increasing the absorption temperature, is observed.A further increase in x results in reducing the H absorption capacity that is typical for the Al-substituted alloys, despite significant decreases in the plateau pressures in H-LaNi 5−x Al x systems when increasing x [8].In addition, hydrogen desorption from the hydrogenated alloys LaNi 5−x Al x (x > 0.55) at moderate (<100 • C) temperatures is characterized by lower plateau pressures resulting in high residual H concentrations and, in turn, low reversible H sorption capacities at the operating conditions.pressure hydrogen compression [4].

MH Materials
The analysis of data in the literature [8][9][10] and our own preliminary results on hydrogen absorption in aluminium-substituted LaNi5 at P(H2) = 1 atm and T = 20-40 °C (Figure 1) shows that the maximum hydrogen absorption capacity can be achieved when the content of aluminium in LaNi5-xAlx corresponds to x = 0.55.At the lower x values, when Peq in the plateau region of the pressure-composition isotherm approaches 1 atm, the capacity drop, especially pronounced when increasing the absorption temperature, is observed.A further increase in x results in reducing the H absorption capacity that is typical for the Al-substituted alloys, despite significant decreases in the plateau pressures in H-LaNi5-xAlx systems when increasing x [8].In addition, hydrogen desorption from the hydrogenated alloys LaNi5-xAlx (x > 0.55) at moderate (<100 °C) temperatures is characterized by lower plateau pressures resulting in high residual H concentrations and, in turn, low reversible H sorption capacities at the operating conditions.The LaNi4.45Al0.55alloy was prepared in an upscaled (×10 kg) amount and used in further studies by the Russian co-authors of this work.
In the studies carried out by HySA Systems in South Africa, a multi-component AB5type alloy was used.According to the results of EDX analysis (see Supplementary Information; Figure S1 and Table S1), the composition of the used alloy corresponded to the formula La0.41Ce0.59Ni3.71Co0.52Mn0.36Al0.29 (B/A = 4.88).
XRD analysis of the as-delivered and hydrogenated alloys confirmed that each sample contains a single phase of CaCu5-type intermetallic (intermetallic hydride), space group P6/mmm/#191 (Figure S2 and Table S2).Rietveld refinement of the XRD patterns yielded acceptable residuals when Rp values varied between 3.1 and 5.4%, except for the starting LaNi4.45Al0.55alloy (Rp = 13.6%), which exhibited a bigger crystallite size and noticeable preferred orientation in the (1 0 0) plane (Table S2).
The data on lattice periods and unit cell volumes calculated from the results of Rietveld refinement of the XRD patterns are summarized in Table 1.The lattice periods and unit cell volume of LaNi4.45Al0.55 are in good correspondence with the reference data The LaNi 4.45 Al 0.55 alloy was prepared in an upscaled (×10 kg) amount and used in further studies by the Russian co-authors of this work.
In the studies carried out by HySA Systems in South Africa, a multi-component AB 5 -type alloy was used.According to the results of EDX analysis (see Supplementary Information; Figure S1 and Table S1), the composition of the used alloy corresponded to the formula La 0.41 Ce 0.59 Ni 3.71 Co 0.52 Mn 0.36 Al 0.29 (B/A = 4.88).
XRD analysis of the as-delivered and hydrogenated alloys confirmed that each sample contains a single phase of CaCu 5 -type intermetallic (intermetallic hydride), space group P6/mmm/#191 (Figure S2 and Table S2).Rietveld refinement of the XRD patterns yielded acceptable residuals when Rp values varied between 3.1 and 5.4%, except for the starting LaNi 4.45 Al 0.55 alloy (Rp = 13.6%), which exhibited a bigger crystallite size and noticeable preferred orientation in the (1 0 0) plane (Table S2).
The data on lattice periods and unit cell volumes calculated from the results of Rietveld refinement of the XRD patterns are summarized in Table 1.The lattice periods and unit cell volume of LaNi 4.45 Al 0.55 are in good correspondence with the reference data for LaNi 4.5 Al 0.5 : a = 5.03 Å, c = 4.03 Å, c/a = 0.80, V = 88.3Å 3 [18].At the same time, the multi-component AB 5 intermetallic studied in this work exhibits a unit cell volume 1.6-2.4% lower than the values reported in [19,20] for the battery alloys of similar compositions but containing La-rich mischmetal (~70 at.%La) as an A-component.The reason for the deviation appears to be the higher content of cerium (atomic radius 185 pm) in our case, while the intermetallics studied in [19,20] contained more lanthanum, characterized by a bigger atomic radius (195 pm).Both alloys were found to exhibit reproducible hydrogen absorption/desorption performances after 1 h long evacuation at T = 90 • C followed by exposure in hydrogen at P = 5 atm, cooling to ambient temperature and being kept under H 2 pressure for 5-6 h.
Figure 2 presents isotherms of hydrogen absorption and desorption for the studied intermetallics.For the correct comparison, the experimental PCT data taken for LaNi   • C, 0.02-30 atm) were processed using the model of phase equilibria in hydrogen-metal systems [21], and the absorption (T L = 15 • C) and desorption (T H = 100 • C) isotherms shown in Figure 2 were calculated using the refined fitting parameters.
component AB5 intermetallic studied in this work exhibits a unit cell volume 1.6-2.4% lower than the values reported in [19,20] for the battery alloys of similar compositions but containing La-rich mischmetal (~70 at.%La) as an A-component.The reason for the deviation appears to be the higher content of cerium (atomic radius 185 pm) in our case, while the intermetallics studied in [19,20] contained more lanthanum, characterized by a bigger atomic radius (195 pm).Both alloys were found to exhibit reproducible hydrogen absorption/desorption performances after 1 h long evacuation at T = 90 °C followed by exposure in hydrogen at P = 5 atm, cooling to ambient temperature and being kept under H2 pressure for 5-6 h.
Figure 2 presents isotherms of hydrogen absorption and desorption for the studied intermetallics.For the correct comparison, the experimental PCT data taken for LaNi4.45Al0.55(20-90 °C; 0.04-15 atm) and La0.41Ce0.59Ni3.71Co0.52Mn0.36Al0.29 (20-120 °C, 0.02-30 atm) were processed using the model of phase equilibria in hydrogen-metal systems [21], and the absorption (TL = 15 °C) and desorption (TH = 100 °C) isotherms shown in Figure 2 were calculated using the refined fitting parameters.As it can be seen, both intermetallics exhibit similar reversible hydrogen sorption capacities: 116-119 NL/kg (1.03-1.05wt.% H) when operating in the range of T L = 15 • C/P L = 1 atm − T H = 100 • C/P H = 2 atm.However, due to the slightly lower thermal stability of La 0.41 Ce 0.59 Ni 3.71 Co 0.52 Mn 0.36 Al 0.29 hydride (the fitted values of hydrogenation entropy and enthalpy correspond to ∆S o = −112.20 ± 0.01 J/(mol H 2 K) and ∆H o = −35.615± 0.004 kJ/mol H 2 ) as compared to LaNi 4.45 Al 0.55 H x (∆S o = −108.22± 0.02 J/(mol H 2 K); ∆H o = −36.90± 0.02 kJ/mol H 2 ), the latter exhibits a significant drop (by 3.3 times) in the reversible capacity when the discharge pressure increases to P H = 5 bar, while for the multi-component AB 5 -type alloy, the capacity remains close to 100 NL/kg (0.85 wt.% H).Apparently, low errors of the refined values of ∆S o and ∆H o are related to the specifics of the modeling procedure [21], which assumes the error of the refined fitting parameter to be equal to its biggest increment (decrement), which results in the 1% increase in the sum of the squared shortest distances of the experimental points from the calculated PCI curves built in coordinates C/Cmax − ln P. Thus, we recommend using LaNi 4.45 Al 0.55 for buffer hydrogen storage in systems characterized by near-atmospheric pressures of the produced H 2 and relatively low (up to 1 atm gage) pressures of H 2 consumption typical for the renewable energy systems based on reversible solid oxide fuel cells [1].The La 0.41 Ce 0.59 Ni 3.71 Co 0.52 Mn 0.36 Al 0.29 alloy can be used as a material for the first stage of a thermally driven MH H 2 compressor with a near-atmospheric suction pressure for the utilization of low-pressure H 2 .

System Development 2.2.1. MH Hydrogen Storage Unit Charged from Solid Oxide Electrolyzer
The low-pressure hydrogen storage unit developed by FRC PCP&MC RAS (Figure 3) comprises a stainless-steel cylindrical container (1) equipped with an inner heat exchanger (2) made as a double-circuit coiled copper tubing, and an outer heating/cooling jacket (3).The containment ends are closed by two flanges (4,5).The top flange (4) carries a H 2 inlet/outlet port (4.1) connected to tubular filter (4.2), as well as ports (4.3) for the input and output of cooling/heating fluid (cold and hot water), a connector for the pressure sensor (4.4), and a thermocouple probe (4.5) for measuring the temperature of the MH bed.The inner space of the containment (1), 20 mm in the diameter and 290 mm in the height, was filled with 8.5 kg of LaNi 4.45 Al 0.55 powder mixed with 100 g (1.16 wt.%) of Ni/GLM powder for the improvement in hydrogen absorption kinetics and the increase in the effective thermal conductivity of the MH bed [15].
when the discharge pressure increases to PH = 5 bar, while for the multi-component AB5type alloy, the capacity remains close to 100 NL/kg (0.85 wt.% H).Apparently, low errors of the refined values of S o and H o are related to the specifics of the modeling procedure [21], which assumes the error of the refined fitting parameter to be equal to its biggest increment (decrement), which results in the 1% increase in the sum of the squared shortest distances of the experimental points from the calculated PCI curves built in coordinates C/Cmax − ln P.
Thus, we recommend using LaNi4.45Al0.55 for buffer hydrogen storage in systems characterized by near-atmospheric pressures of the produced H2 and relatively low (up to 1 atm gage) pressures of H2 consumption typical for the renewable energy systems based on reversible solid oxide fuel cells [1].The La0.41Ce0.59Ni3.71Co0.52Mn0.36Al0.29 alloy can be used as a material for the first stage of a thermally driven MH H2 compressor with a nearatmospheric suction pressure for the utilization of low-pressure H2.

MH Hydrogen Storage Unit Charged from Solid Oxide Electrolyzer
The low-pressure hydrogen storage unit developed by FRC PCP&MC RAS (Figure 3) comprises a stainless-steel cylindrical container (1) equipped with an inner heat exchanger (2) made as a double-circuit coiled copper tubing, and an outer heating/cooling jacket (3).The containment ends are closed by two flanges (4,5).The top flange (4) carries a H2 inlet/outlet port (4.1) connected to tubular filter (4.2), as well as ports (4.3) for the input and output of cooling/heating fluid (cold and hot water), a connector for the pressure sensor (4.4), and a thermocouple probe (4.5) for measuring the temperature of the MH bed.The inner space of the containment (1), 20 mm in the diameter and 290 mm in the height, was filled with 8.5 kg of LaNi4.45Al0.55powder mixed with 100 g (1.16 wt.%) of Ni/GLM powder for the improvement in hydrogen absorption kinetics and the increase in the effective thermal conductivity of the MH bed [15].The developed unit exhibited the maximum hydrogen absorption capacity of 1200 NL, of which, about 1000 NL was absorbed at P(H 2 ) ≤ 1 atm when cooled to 15-20 • C with running water.The amount of hydrogen further desorbed at P(H 2 ) ≥ 2 atm during water heating to 70-90 • C exceeded 900 NL, which corresponds to 100 NL/kg, or ~84% of the theoretical (equilibrium) reversible H storage capacity of LaNi 4.45 Al 0.55 in the range of operating temperatures and pressures 15-100 • C and 1-2 atm, respectively (see Figure 2a).The unit was successfully tested in integration with a solid oxide electrolyzer.

MH Unit for Low-Pressure Hydrogen Compression
As mentioned above, the low-pressure H 2 compression unit developed by HySA Systems used a standard MH container for medium-pressure hydrogen compression, described in ref. [4].The cylindrical stainless-steel container (625 mm long and 2.68 dm 3 in the inner volume) with an inner heat exchanger (finned tube) and an outer heating/cooling jacket was filled with 8.3 kg of the powdered mixture of La 0.41 Ce 0.59 Ni 3.71 Co 0.52 Mn 0.36 Al 0.29 with 1wt.% of ENG.
Figure 4 schematically shows the connection of the MH container to the testrig.During the tests, hydrogen pressure was measured both at the front (H 2 supply/removal pipeline; P(front)) and back (P(back)) sides of the container using a two-channel absolute/differential PD39 pressure sensor (0-10 atm absolute).For the connection of the rear side of the container to the pressure sensor, as well as for the input of a thermocouple measuring the temperature in the MH bed, the MH loading port of the container was used.Additionally, the mass flow rate (FR) of hydrogen supplied to or released from the MH container was measured.The temperatures of the supplied (T(in)) and drained (T(out)) cooling/heating fluid (water/steam), along with the MH bed temperature (T(MH)) in the container, close to its back side, were measured as well.
theoretical (equilibrium) reversible H storage capacity of LaNi4.45Al0.55 in the range of o erating temperatures and pressures 15-100 °C and atm, respectively (see Figure 2 The unit was successfully tested in integration with a solid oxide electrolyzer.

MH Unit for Low-Pressure Hydrogen Compression
As mentioned above, the low-pressure H2 compression unit developed by HySA Sy tems used a standard MH container for medium-pressure hydrogen compression, d scribed in ref. [4].The cylindrical stainless-steel container (625 mm long and 2.68 dm 3 the inner volume) with an inner heat exchanger (finned tube) and an outer heating/cooli jacket was filled with 8.3 kg of the powdered mixture of La0.41Ce0.59Ni3.71Co0.52Mn0.36Alwith 1wt.% of ENG.
Figure 4 schematically shows the connection of the MH container to the testrig.Du ing the tests, hydrogen pressure was measured both at the front (H2 supply/removal pip line; P(front)) and back (P(back)) sides of the container using a two-channel absolute/d ferential PD39 pressure sensor (0-10 atm absolute).For the connection of the rear side the container to the pressure sensor, as well as for the input of a thermocouple measuri the temperature in the MH bed, the MH loading port of the container was used.Additio ally, the mass flow rate (FR) of hydrogen supplied to or released from the MH contain was measured.The temperatures of the supplied (T(in)) and drained (T(out)) co ing/heating fluid (water/steam), along with the MH bed temperature (T(MH)) in the co tainer, close to its back side, were measured as well.The results of the tests (see Figure 5) showed that the total hydrogen capacity of t MH container exceeded 1000 NL, of which, about 850 NL was absorbed at the pressu below 1 atm during the cooling of the container with running water at T = 16-18 °C.Abo 80% of the low-pressure hydrogen (670 NL) was absorbed in 30 min with a maximum flo rate of 30 NL/min.The MH temperature increased to ~75 °C almost immediately after t start of the low-pressure H2 supply, and the pressure drop between the entrance to t container, P(front), and its rear side, P(back), did not exceed 0.05 atm.Further heating the container by steam (T~120 °C) resulted in the desorption of hydrogen at higher pr sures; in doing so, about 650 NL H2 was desorbed from the container at a pressure abo 5 atm and a flow rate of 10-30 NL/min.The results of the tests (see Figure 5) showed that the total hydrogen capacity of the MH container exceeded 1000 NL, of which, about 850 NL was absorbed at the pressure below 1 atm during the cooling of the container with running water at T = 16-18 • C.About 80% of the low-pressure hydrogen (670 NL) was absorbed in 30 min with a maximum flow rate of 30 NL/min.The MH temperature increased to ~75 • C almost immediately after the start of the low-pressure H 2 supply, and the pressure drop between the entrance to the container, P(front), and its rear side, P(back), did not exceed 0.05 atm.Further heating of the container by steam (T~120 • C) resulted in the desorption of hydrogen at higher pressures; in doing so, about 650 NL H 2 was desorbed from the container at a pressure above 5 atm and a flow rate of 10-30 NL/min.

Discussion
Figure 6 compares the H2 charge dynamic performance of the low-pressure hydrogen storage unit developed at FRC PCP&MC RAS (1) and the unit for the low-pressure H2 compression developed by HySA Systems (2).Both units can absorb hydrogen supplied below atmospheric pressure; the hydrogen absorption capacity achieved after 2 h long operation was about 90% of the maximum one observed after the charge at P(H2) = 5 atm and ambient temperature.In doing so, the average H absorption rate exceeded 10 NL/min for the MH load between 8.3 and 8.6 kg, which corresponded to the specific rate of more than 1.15-1.20NL/(min kg).This shows the technical feasibility of charging MH units with near-atmospheric-pressure hydrogen with reasonable productivity when the absorption plateau pressure of the used AB5-type MH material is about 0.1-0.3atm (Figure 2).Slower H2 charge dynamics of the FRC PCP&MC RAS unit (1), first of all, are caused by the lower efficiency of the used inner heat exchanger (coiled tube) as compared to the HySA Systems unit (2), in which the heat exchanger was made as a finned tube with fin

Discussion
Figure 6 compares the H 2 charge dynamic performance of the low-pressure hydrogen storage unit developed at FRC PCP&MC RAS (1) and the unit for the low-pressure H 2 compression developed by HySA Systems (2).Both units can absorb hydrogen supplied below atmospheric pressure; the hydrogen absorption capacity achieved after 2 h long operation was about 90% of the maximum one observed after the charge at P(H 2 ) = 5 atm and ambient temperature.In doing so, the average H absorption rate exceeded 10 NL/min for the MH load between 8.3 and 8.6 kg, which corresponded to the specific rate of more than 1.15-1.20NL/(min kg).This shows the technical feasibility of charging MH units with near-atmospheric-pressure hydrogen with reasonable productivity when the absorption plateau pressure of the used AB 5 -type MH material is about 0.1-0.3atm (Figure 2).

Discussion
Figure 6 compares the H2 charge dynamic performance of the low-pressure hydrogen storage unit developed at FRC PCP&MC RAS (1) and the unit for the low-pressure H2 compression developed by HySA Systems (2).Both units can absorb hydrogen supplied below atmospheric pressure; the hydrogen absorption capacity achieved after 2 h long operation was about 90% of the maximum one observed after the charge at P(H2) = 5 atm and ambient temperature.In doing so, the average H absorption rate exceeded 10 NL/min for the MH load between 8.3 and 8.6 kg, which corresponded to the specific rate of more than 1.15-1.20NL/(min kg).This shows the technical feasibility of charging MH units with near-atmospheric-pressure hydrogen with reasonable productivity when the absorption plateau pressure of the used AB5-type MH material is about 0.1-0.3atm (Figure 2).Slower H2 charge dynamics of the FRC PCP&MC RAS unit (1), first of all, are caused by the lower efficiency of the used inner heat exchanger (coiled tube) as compared to the HySA Systems unit (2), in which the heat exchanger was made as a finned tube with fin Slower H 2 charge dynamics of the FRC PCP&MC RAS unit (1), first of all, are caused by the lower efficiency of the used inner heat exchanger (coiled tube) as compared to the HySA Systems unit (2), in which the heat exchanger was made as a finned tube with fin pitch of 5 mm.However, the advantage of the former unit is the almost constant hydrogen absorption flow rate throughout the operation period, which improves the overall system on the instrument used, being from 12 to 35 cm 3 .The as-collected PCT data were processed using the model of phase equilibria in hydrogen-metal systems [21]; the average fitting error corresponded to ∆C/Cmax below 0.06%.
Testing of the developed units was carried out using in-house built testrigs, allowing us to monitor the H 2 charge/discharge flow rates, hydrogen pressures, and temperatures.

Conclusions
This work has shown the technical feasibility of the storage and thermally driven compression of low-pressure (1 atm absolute) hydrogen with a compression ratio from 2 to 5 in the temperature range from 15 to 20 to 70 to 120 • C by using AB 5 -type hydride-forming intermetallics characterized by plateau pressures of 0.1-0.3atm at the ambient temperature and 3-8 atm at T = 100 • C. The use of powdered mixtures of these alloys with minor additives of the reduced graphite oxide (expanded natural graphite) or graphene-like material with deposited nickel nanoparticles allowed us to achieve the rate of hydrogen uptake and release above 10 NL/min for the 1 Nm 3 H 2 capacity unit, or above 1.2 NL/min per 1 kg of the used material.

Patents
The following patented solutions contributed to the results reported in this work:

•
Metal hydride bed, metal hydride container, and method for the making thereof, by M.

Figure 4 .
Figure 4. Schematics of the tests of MH container for the low-pressure H2 compression.T(in)-ste (water) inlet temperature, T(out)-steam (water) outlet temperature, T(MH)-temperature in MH bed, P/ΔP-absolute/differential pressure sensor, P(front)-H2 pressure at the input/output the MH container, P(back)-H2 pressure from the opposite side, FR-mass flow rate of the sorbed/desorbed H2.

Figure 4 .
Figure 4. Schematics of the tests of MH container for the low-pressure H 2 compression.T(in)-steam (water) inlet temperature, T(out)-steam (water) outlet temperature, T(MH)-temperature in the MH bed, P/∆P-absolute/differential pressure sensor, P(front)-H 2 pressure at the input/output of the MH container, P(back)-H 2 pressure from the opposite side, FR-mass flow rate of the absorbed/desorbed H 2 .

Figure 5 .
Figure 5. Results of tests of MH container for low-pressure H2 compression: H2 absorption (a) and H2 desorption (b).

Figure 6 .
Figure 6.Dynamics of H2 charge of the MH hydrogen storage (1: FRC PCP&MC RAS) and compression (2: HySA Systems) units at the pressure of the feed H2 below 1 atm (absolute).

2 Figure 5 .
Figure 5. Results of tests of MH container for low-pressure H 2 compression: H 2 absorption (a) and H 2 desorption (b).

Figure 5 .
Figure 5. Results of tests of MH container for low-pressure H2 compression: H2 absorption (a) and H2 desorption (b).

Figure 6 .
Figure 6.Dynamics of H2 charge of the MH hydrogen storage (1: FRC PCP&MC RAS) and compression (2: HySA Systems) units at the pressure of the feed H2 below 1 atm (absolute).

2 Figure 6 .
Figure 6.Dynamics of H 2 charge of the MH hydrogen storage (1: FRC PCP&MC RAS) and compression (2: HySA Systems) units at the pressure of the feed H 2 below 1 atm (absolute).

Table 1 .
Lattice periods and unit cell volumes of the studied samples calculated from Rietveld refinement of their XRD patterns.

Table 1 .
Lattice periods and unit cell volumes of the studied samples calculated from Rietveld refinement of their XRD patterns.