Hydrogen adsorption of O/N-rich hierarchical carbon scaffold decorated with Ni nanoparticles: Experimental and computational studies

Hierarchical carbon scaffold (HCS) with multi-porous structures, favoring hydrogen diffusion and physisorption is doped with 2–10 wt % Ni for storing hydrogen at ambient temperature. Due to N- and O-rich structure of melamine-formaldehyde resin used as carbon precursor, homogeneous distribution of heteroatoms (N and O) in HCS is achieved. 2 wt % Ni-doped HCS shows the highest hydrogen capacity up to 2.40 wt % H₂ (T = 298 K and p (H₂) = 100 bar) as well as excellent reversibility of 18.25 g H₂/L and 1.25 wt % H₂ (T = 298 K and p (H₂) = 50 bar) and electrical production from PEMFC stack up to 0.7 Wh upon eight cycles. Computations and experiments confirm strong interactions between Ni and heteroatoms, leading to uniform distribution small particles of Ni. This results in enhancing reactive surface area for hydrogen adsorption and preventing agglomeration of Ni nanoparticles upon cycling. Ni K-edge XANES spectra simulated from the optimized structure of Ni-doped N/O-rich carbon using DFT calculations are consistent with the experimental spectra and suggest electron transfer from Ni to hydrogen to form Ni–H bond upon adsorption. Considering low desorption temperature (323 K), not only chemisorbed hydrogen is involved in adsorption mechanisms but also physisorption and spillover of hydrogen.     

Reversible hydrogen sorption and kinetics of hydrogen storage tank based on MgH2 modified by TiF4 and activated carbon

By doping with 5 wt % TiF₄ and activated carbon (AC), onset and main dehydrogenation temperatures of MgH₂ significantly reduce (ΔT = 138 and 109 °C, respectively) with hydrogen capacity of 4.4 wt % H₂. Up-scaling to storage tank begins with packing volume and sample weight of 28.8 mL and ～14.5 g, respectively, and continues to 92.6 mL and ～60.5–67 g, respectively. Detailed hydrogen sorption mechanisms and kinetics of the tank tightly packed with four beds of MgH₂TiF₄-AC (～60.5 g) are investigated. De/rehydrogenation mechanisms are detected by three temperature sensors located at different positions along the tank radius, while hydrogen permeability is benefited by stainless steel mesh sheets and tube inserted in the hydride beds. Fast desorption kinetics of MgH₂TiF₄-AC tank at ～275–283 °C, approaching to onset dehydrogenation temperature of the powder sample (272 °C) suggests comparable performances of laboratory and tank scales. Hydrogen desorption (T = 300 °C and P(H₂) = 1 bar) and absorption (T = 250 °C and P(H₂) = 10–15 bar) of MgH₂TiF₄-AC tank provide gravimetric and volumetric capacities during the 1^st-2nd cycles of 4.46 wt % H₂ and 28 gH₂/L, respectively, while those during the 3^rd-15th cycles are up to 3.62 wt % H₂ and 23 gH₂/L, respectively. Due to homogeneous heat transfer along the tank radius, de/rehydrogenation kinetics superior at the tank center and degrading forward the tank wall can be due to poor hydrogen permeability. Particle sintering and/or agglomeration upon cycling yield deficient hydrogen content reproduced.     

Compaction of LiBH4-MgH2 doped with MWCNTs-TiO2 for reversible hydrogen storage

According to catalytic effects of TiO₂ on kinetic properties of hydrides and thermal conductivity of multiwall carbon nanotubes (MWCNTs) favoring heat transfer during de/rehydrogenation, improvement of dehydrogenation kinetics of compacted 2LiBH₄-MgH₂ by doping with MWCNTs decorated with TiO₂ (MWCNTs-TiO₂) is proposed. Via solution impregnation of Ti-isopropoxide on MWCNTs and hydrothermal reaction to produce TiO₂, high surface area and good dispersion of TiO₂ on MWCNTs surface are obtained. Composite of 2LiBH₄-MgH₂ is doped with 5–15 wt. % MWCNTs-TiO₂ and compacted into the pellet shape (diameter and thickness of 8 and 1.00–1.22 mm, respectively). By doping with 15 wt. % MWCNTs-TiO₂, not only fast dehydrogenation kinetics is obtained, but also reduction of onset dehydrogenation temperature (ΔT = 25 °C). Besides, gravimetric and volumetric hydrogen storage capacities of compacted 2LiBH₄-MgH₂ increase to 6.8 wt. % and 68 gH₂/L, respectively, by doping with 15 wt. % MWCNTs-TiO₂ (～twice as high as undoped sample). The more the MWCNTs-TiO₂ contents, the higher the apparent density (up to ～1.0 g/cm³ by doping with 15 wt. % MWCNTs-TiO₂). The latter implies good compaction, resulting in the development of volumetric hydrogen capacity. In the case of mechanical stability during cycling, compacted 2LiBH₄-MgH₂ doped with at least 10 wt. % MWCNTs-TiO₂ maintains the pellet shape after rehydrogenation. Although increase of porosity (up to 30%), leading to the reduction of thermal conductivity, is detected after rehydrogenation of compacted 2LiBH₄-MgH₂ doped with 15 wt. % MWCNTs-TiO₂, comparable kinetics during cycling is obtained. This benefit can be achieved from thermal conductivity of MWCNTs.     

Compaction of LiBH4-LiAlH4 nanoconfined in activated carbon nanofibers: Dehydrogenation kinetics,reversibility, and mechanical stability during cycling

To enhance volumetric hydrogen capacity for on-board fuel cells, compaction of LiAlH₄-LiBH₄ nanoconfined in activated carbon nanofibers (ACNF) is for the first time proposed. Loose powders of milled and nanoconfined LiAlH₄-LiBH₄ samples are compacted under 976 MPa to obtain the pellet samples with thickness and diameter of ～1.20–1.30 and 8.0 mm, respectively. Dehydrogenation temperature of milled LiAlH₄-LiBH₄ increases from 415 to 434 °C due to compaction, while those of both compacted and loose powder samples of nanoconfined LiAlH₄-LiBH₄ are lower at comparable temperature of 330–335 °C. Hydrogen content liberated from milled LiAlH₄-LiBH₄ pellet is 65% of theoretical capacity in the temperature range of 80–475 °C, while that of nanoconfined LiAlH₄-LiBH₄ pellet is up to 80% at lower temperature of 100–400 °C. Besides, nanoconfined LiAlH₄-LiBH₄ pellet shows significant reduction of activation energy (ΔE_A up to 69 kJ/mol H₂) as compared with milled sample. Significant enhancement of volumetric hydrogen storage capacity up to 64% (from 32.5 to 53.3 gH₂/L) is obtained from nanoconfined LiAlH₄-LiBH₄ pellet. Hydrogen content released and reproduced of nanoconfined LiAlH₄-LiBH₄ pellet are 67 and 50% of theoretical capacity, respectively, while those of milled LiAlH₄-LiBH₄ pellet are only 30 and 10%, respectively. Moreover, upon four hydrogen release and uptake cycles, nanoconfined LiAlH₄-LiBH₄ pellet can preserve its shape with slight cracks, suggesting good mechanical stability during cycling. Curvatures and fibrous structure woven on one another of ACNF in nanoconfined LiAlH₄-LiBH₄ pellet not only favor hydrogen permeability through pellet sample during de/rehydrogenation, resulting fast kinetics, but also reinforce the pellet shape during cycling under high temperature and pressure condition.