The superior catalytic effect of N vs. Ni for improving hydrogen storage kinetics of LiBH₄@X-doped-C-MSU-H (X = N or Ni) nanoporous carbon composites

Abstract

Lithium borohydride (LiBH₄) is a promising hydrogen storage material releasing 13.8 wt% H₂ upon decomposition in lithium hydride and boron, significantly surpassing other complex hydrides. However, sluggish dehydrogenation kinetics still hinder the use for practical applications. The infiltration of LiBH₄ into carbon nanoscaffolds has proved to be effective in improving the hydrogen absorption/desorption (a/d) kinetics. Further improvement of storage kinetics can be achieved by modification of the nanocarbon with dopant elements. The present work compares nanoporous carbon (C-MSU-H) and C-MSU-H doped either with 1 at% N or 1 at% Ni as the matrix for infiltration of LiBH₄. The catalytic effect of nitrogen proved to be superior to that of nickel (keeping the same doping level) for improving the hydrogen a/d kinetics of LiBH₄ infiltrated in doped C-MSU-H. X-ray photoelectron spectroscopy was used to detect the amount and chemical proximity of nitrogen in nanoporous carbon following the thermal treatment in ammonia flow. The morphology and porosity of doped C-MSU-H were investigated by X-ray diffraction, FTIR, TEM, and BET. Hydrogen a/d kinetics of LiBH₄@C-MSU-H nanocomposites was investigated by a volumetric method. The desorption peak temperatures (measured at 2 °C min⁻¹ rate) are 339 °C for the undoped LiBH₄@C-MSU-H, 328 °C for the LiBH₄@C-MSU-H doped with 1 at% Ni and 318 °C for the LiBH₄@C-MSU-H doped with 1 at% N nanocomposites. The activation energies of hydrogen desorption for the investigated nanocomposites were obtained from Kissinger plots: 142.7 kJ mol⁻¹ for undoped LiBH₄@C-MSU-H, 123.8 kJ mol⁻¹ for LiBH₄@C-MSU-H 1 at% Ni and 119.5 kJ mol⁻¹ for LiBH₄@C-MSU-H 1 at% N nanocomposites. The catalytic effect on LiBH₄ dehydrogenation due to N-doping of nanocarbons is discussed.

---

1. Introduction

Due to the greenhouse effect caused by the burning of fossil fuels, a top priority for today's society is the development of a non-polluting and sustainable economy. Moreover, fossil fuels could meet society's needs for a limited period of time. Hydrogen, as an energy carrier, represents a non-polluting energy alternative. Storing hydrogen in hydrides is a safe and inexpensive method. Metal hydrides have a hydrogen mass content below 2 wt% H₂. Lithium borohydride, LiBH₄, upon decomposition into LiH and B provides an amount of hydrogen equal to 13.8 wt% H₂, well above the 7 wt% H₂ obtained for magnesium hydride or sodium alanate NaAlH₄.¹ Unfortunately, for the rehydrogenation of LiBH₄ after hydrogen desorption, very harsh conditions are required, namely 350 atm H₂ at 600 °C.²

The difficult absorption/desorption of hydrogen is caused by sluggish kinetics and thermodynamic barriers that negatively influence the dehydrogenation temperature. Significant efforts have been made to improve the hydrogenation/dehydrogenation kinetics of LiBH₄ through the use of catalysts such as those described in very recent reviews.^3,4 Another strategy for improving kinetics is nanoconfinement, by embedding borohydrides in various matrices that prevent the growth of hydride grains during hydrogen absorption/desorption cycles, adversely affecting hydrogen diffusion in the hydride bed. For this purpose, various matrices have been used: porous hollow carbon nanospheres,⁵ double-layered carbon nanobowls,⁶ silica and carbon scaffolds,⁷ nanoporous carbon,^8,9 CoNi doped hollow carbon networks,¹⁰ carbon wrapped Fe₃O₄ nanospheres,¹¹ TiO₂ decorated porous carbonaceous networks,¹² N-doped carbon nanosheets embedded with Co nanoparticles,¹³ and Ni nanoparticles coated with porous hollow carbon micropheres.¹⁴ Also mixing with carbon nanotubes¹⁵ or 2D structures like graphene doped with FeF₂/FeO_x¹⁶ or high entropy oxide nanoplates¹⁷ have proved to be useful in avoiding the growth of borohydride grains and improving hydrogenation/dehydrogenation kinetics. An interesting approach was to synthesize nanoparticles of LiBH₄ supported on Ni doped graphene.¹⁸ Thermodynamic barriers can be reduced through thermodynamic destabilization, which means lowering the decomposition enthalpy by creating new compounds that stabilize the dehydrogenated state as an alternative to the decomposition reaction of LiBH₄ into LiH and B.¹⁹ For LiBH₄ containing composites, thermodynamic destabilization has been achieved through the systems: LiBH₄/MgH₂,¹⁹ LiBH₄/LiAlH₄,²⁰ LiBH₄/YH₃,²¹ LiBH₄/Mg₂NiH₄.²²

The interaction between LiBH₄ and the carbon matrix or between multi-component systems based on lithium borohydride was investigated using modern methods such as in situ X-ray Raman spectroscopy²³ and QENS (quasielastic neutron scattering and neutron vibrational spectroscopy),²⁴ whereas the identification of poorly crystallized phases and reaction mechanisms can be explored by ¹¹B magic angle spinning (MAS) nuclear magnetic resonance.²⁵

Both Ni catalysis and LiBH₄ nanoconfinement have a synergistic effect on the reversible absorption/desorption of hydrogen.²⁶ Nitrogen-doped nanoporous carbon has proven to be a very effective support for catalytic reactions or for hydrogen storage^27,28 and has had a significant effect on improving the absorption/desorption kinetics of hydrogen for MgH₂ confined in such matrices.²⁹ Besides the physicochemical catalytic mechanism, carbon facilitates thermal conduction in the hydride mass (usually having low thermal conductivity), which enhances the hydrogen desorption kinetics (endothermic reaction).

Very recently, solar-driven reversible hydrogen storage was proposed as an efficient technique to generate H₂ reversibly, without using any external heat source, except for solar radiation. MgH₂ was catalyzed with Cu nanoparticles distributed on MXene nanosheets, having a dual effect: (i) photothermal effect under solar radiation and (ii) catalytic effect on hydrogenation/dehydrogenation of MgH₂. Such a system can reversibly store 5.9 wt% H₂ using only solar radiation as the heat source.³⁰ A similar system using MgH₂ catalyzed with flower-like microspheres made from N-doped TiO₂ nanosheets coated with TiN nanoparticles has a hydrogen storage capacity of 6.1 wt% H₂.³¹ Mg₂Ni(Cu) and its hydrogenated compound exhibit intra/inter-band transitions generating 85% light absorption across the entire spectrum range raising the surface temperature of MgH₂ to 261.8 °C under a light intensity of 2.6 W cm⁻². Mg₂Ni(Cu)/Mg₂Ni(Cu)H₄ acts as a light-enhanced “hydrogen pump” for MgH₂ having both photothermal and catalytic effects, leading to a reversible hydrogen storage amount of 6.1 wt% H₂ with 95% retention under 3.5 W cm⁻¹.³² MgH₂ catalyzed using a TiO₂/Fe₂O₃ heterojunction photocatalyst releases 3.1 wt% H₂ within 45 min under a light intensity of 1.78 W cm⁻², exhibiting a dehydrogenation activation energy of 77.3 kJ mol⁻¹.³³ Among other hydrides, LiBH₄ catalyzed using g-C₃N₄/Fe₂O₃ heterojunctions can desorb 2.70 wt% H₂ very quickly within 1 min and 3.87 wt% in 30 min under a light intensity of 1.78 W cm⁻².³⁴ A Ni/ZrO₂ catalyst derived from metal–organic frameworks has been proven useful both for solar-driven hydrogen storage and photothermal conversion of CO₂ to CH₄.³⁵ Reversible solar-driven hydrogen absorption/desorption of 4.9 wt% H₂ was achieved for the sodium cyclohexanolate/phenoxide pair with 99.9% conversion and selectivity. The initial dehydrogenation rate was 23.4 mmol H₂ g⁻¹ h⁻¹ (i.e. 2 orders of magnitude higher than that obtained via thermocatalysis).³⁶

The present work investigates the catalytic efficiency of nitrogen and respectively nickel as doping elements for ordered nanoporous carbon on the hydrogenation/dehydrogenation reaction of LiBH₄@doped nanoporous carbon nanocomposites. Via thermal treatment of the ordered nanoporous carbon matrix in gaseous ammonia, nitrogen doping of carbon exhibiting a strong catalytic effect in improving the kinetics of the hydrogen absorption/desorption reactions is achieved. The amount of nitrogen and its chemical proximity in the structure of the nanoporous carbon were carefully investigated through X-ray photoelectron spectroscopy. The Ni-doped ordered nanoporous carbon was obtained by reduction of Ni-salts deposited within the nanocarbon structure. The modification of the nanoporous carbon structure via thermal treatment in ammonia was analyzed through morphological and porosity investigations. The present work presents a comparison between the catalytic efficiency of C–MSU-H doped with the same amount of either N or Ni for improving the hydrogenation/dehydrogenation kinetics of LiBH₄ demonstrating the superiority of N over Ni. This will be very useful for replacing the critical or toxic metal in the hydrogenation/dehydrogenation catalyst. We have comparatively discussed the origin of the catalytic effect for Ni and N doping. To our knowledge, this is the first time such a comparison (keeping the same amount of N and Ni dopants) has been carried out.

2. Experimental

2.1 Synthesis of materials

In order to obtain functionalized nanoporous carbon–hydride nanocomposites, the nanoporous carbon was prepared as a replica of nanoporous silica with an ordered structure. Then it was functionalized by treating in gaseous ammonia or by depositing metal ions, followed by chemical reduction. Nanoporous silica with ordered hexagonal pores was obtained starting from cheap precursors (sodium silicate). To obtain MSU-H,^37–39 sodium silicate solution (27% SiO₂, 14% NaOH) (Sigma Aldrich, extra pure) and pluronic surfactant P123 (Sigma Aldrich, average M_n ∼ 5800 block copolymers PEG–PPG–PEG) were used in the stoichiometric molar ratio 1 SiO₂ : 0.017 P123 : 0.83 CH₃COOH : 0.78 NaOH : 230 H₂O. Thus, 1.2 g of P123 were mixed with 10 mL of 1.0 M acetic acid solution and 10 mL of water. To this microemulsion were added 2.7 g of sodium silicate solution (27 wt% SiO₂ and 14 wt% NaOH) and 30 mL H₂O. The resulting product was stirred for 24 hours at ambient temperature. Subsequently, it was treated at 100 °C for 24 h. The product was filtered and dried and finally the remaining surfactant was removed by calcination in air at 550 °C for 5 h. The carbon replica of MSU-H, named in the following C-MSU-H,⁴⁰ was made by impregnating sucrose solution into the MSU-H matrix in two reaction steps. In the first step, 4 g of nanoporous silica MSU-H were dispersed in a solution consisting of 20 g H₂O and 0.64 g H₂SO₄ (the acid is the catalyst). Then, 5 g of sucrose (carbon source) were added to the formed mixture so that the solution filled the pores of MSU-H. The slurry was heated in an oven for 6 h at 100 °C followed by another 6 h at 160 °C so that the sucrose became polymerized. After that, in the second impregnation step 3.2 g of sucrose and 0.36 g of H₂SO₄ dissolved in 20 g of water were added. The heating in the oven was repeated for 6 h at 100 °C followed by another 6 h at 160 °C so that the sucrose became polymerized and completely filled the MSU-H pores. The product obtained was washed of impurities, filtered and subsequently calcined for 3 h at 400 °C and 4 h at 900 °C in a nitrogen flow (N₂, purity 99.999%) to transform sucrose into carbon. Thereby, a carbon–SiO₂ composite was obtained. The removal of SiO₂ was done via treatment with an aqueous solution of 10% hydrofluoric acid and washing with double distilled water until neutral pH. A treatment in gaseous ammonia flow (NH₃, purity 99.999%) with a rate of 100 ml min⁻¹ for 4 h was performed on the nanoporous carbon C-MSU-H in order to achieve the doping with N atoms. Special precautions were taken the ammonia being neutralized at the outlet of the heating system. The treatment temperatures were 400 °C, 450 °C and 500 °C.

Besides nitrogen doping, another functionalization procedure of C-MSU-H involved doping with nickel ions. A level of 1 at% Ni doping of nanoporous carbon was envisaged, which meant 4.65 wt% Ni in carbon in order to keep the same doping level previously obtained for N doped C-MSU-H. For this purpose, 0.233 g of nickel nitrate hexahydrate Ni(NO₃)₂·6H₂O (Sigma Aldrich > 99.9% trace metal basis) (corresponding to 0.047 g of Ni) dissolved in 10 ml of absolute ethanol was used to impregnate 1 g nanoporous carbon by the incipient wetness method (6 steps). Using this method we obtained a loading of 4.65 wt% Ni/nanoporous carbon (respectively 1 at% Ni/nanoporous carbon). After drying at 70 °C for 10 hours, the nanoporous carbon impregnated with an ethanol solution of Ni(NO₃)₂·6H₂O was treated in a 5% H₂/Ar (purity 99.999%) flow (100 ml min⁻¹) for 4 h at 430 °C. Subsequently, nanoporous carbon doped either with N or Ni was mixed with LiBH₄ in a proportion of 50/50 wt% and the nanocomposites were synthesized by the melt infiltration under hydrogen pressure of 100 atm at 300 °C, so that LiBH₄ (melting temperature 278 °C) infiltrates into the ordered pores of doped C-MSU-H. This method produces LiBH₄@(Ni or N) doped C-MSU-H nanocomposites, avoiding the growth of LiBH₄ grains during hydrogen absorption/desorption cycles, which would worsen the hydrogenation/dehydrogenation kinetics. Additionally, the heat transfer in the hydride mass was improved. The processing of hydrides was done in a Labstar Mbraun glove box (Garching, Germany) in a controlled atmosphere (<1 ppm of oxygen and moisture) in order to avoid sample contamination.

2.2 Characterization methods

X-ray diffraction measurements were performed using an Anton Parr diffractometer with Cu K-alpha radiation starting from very small angles for pristine ordered nanoporous carbon and respectively nitrogen doped samples with a step of 0.02° in the 2θ range of 0.1–60°. XRD measurements were performed in the 2θ range of 15–80° for LiBH₄@ordered nanoporous carbon nanocomposites.

FTIR measurements were performed on pressed pellets containing 120 mg KBr and the IR-active material. To avoid contamination, the powders were ground in a MBraun Labstar glove box. Measurements were performed in the wavenumber range of 300–4000 cm⁻¹ using a JASCO-6600 FTIR spectrometer with a measuring step of 4 cm⁻¹.

Nitrogen physisorption analysis was performed at −196 °C using a Micromeritics 3FLEX apparatus. Before determining the surface area, all powders were degassed at 200 °C for 4 h. The Brunauer–Emmett–Teller (BET) formalism was used to calculate the specific surface area from the data acquired at P/P₀ between 0.025 and 0.3. The pore size distribution of the samples was determined from the desorption branch of the N₂ isotherm. The pore size and volume analysis were calculated using the Barrett–Joyner–Halenda (BJH) formalism.

XPS (X-ray photoelectron spectroscopy) measurements were performed using a SPECS spectrometer based on a PHOIBOS 150 analyzer with a monochromatic X-ray source of 300 W with Al Kα radiation—1486.61 eV. Charge compensation was done using a Specs FG15/40 flood gun. The XPS core level lines were recorded using a pass energy of 20 eV while the survey spectra were recorded with a pass energy of 50 eV. The fittings were performed using Spectral Data Processor software using Voigt functions and usual relative sensitivity factors.

TEM images were obtained using a JEM-2100 analytical transmission electron microscope (Jeol, Tokyo, Japan) working at 200 kV endowed with a dispersive X-ray spectrometer. Prior to TEM investigation the samples were dispersed in hexane using an ultrasonic device with high power (VCX 750 Sonics, Newton, CT, USA), followed by drying onto Lacey carbon-coated Cu TEM grids.

Hydrogen storage kinetics measurements were performed using a commercially available Sievert (volumetric) apparatus built by Advanced Material Corporation, Pittsburgh, USA.

3. Results and discussion

3.1 X-ray diffraction

For the pristine ordered nanoporous carbon C-MSU-H and nanoporous carbon samples thermally treated in ammonia flow at 400 °C and 500 °C a hexagonal structure is observed with peaks at very low angles in the X-ray diffraction pattern provided in Fig. 1. The reflections at (1 0 0), (1 1 0) and (2 0 0) indicate a hexagonal lattice constant a = 10.9 nm. The peaks (0 0 2) and (1 0 0) from 2θ of about 23° and 43°, respectively, correspond to a graphitic phase in a very small amount with a lattice constant close to that of pure graphite. This phase appears in a significantly higher amount for the sample treated at 500 °C. The initially ordered nanoporous carbon C-MSU-H was functionalized either by treatment in a flow of gaseous ammonia or by doping with Ni ions. The XRD pattern of C-MSU-H doped with 1 at% Ni (Fig. 1D) exhibits differences from the undoped one due to the presence of very broad peaks corresponding to Ni and NiO which show very small crystallite size of a few nanometers.

Fig. 1 X-ray diffraction spectra of ordered nanoporous carbon C-MSU-H: (A) as obtained, (B) treated in ammonia gas at 400 °C, (C) treated in ammonia gas at 500 °C and (D) C-MSU-H doped with 1 at% Ni.

The X-ray diffraction spectra for the re-hydrogenated (after cycles of hydrogen absorption/desorption) nanocomposites – 50 wt% LiBH₄ – 50 wt% C-MSU-H (named LiBH₄@C-MSU-H) (Fig. 2A) and 50 wt% LiBH₄ – 50 wt% C-MSU-H doped with 1 at% N (named LiBH₄@C-MSU-H 1 at% N) (nitrogen doping level was measured from XPS) (Fig. 2B) are very similar, and show mainly LiBH₄ (ICDD file 01-084-8599) and a small amount of LiH (ICDD file 04-013-9487) according to the hydrogen generation reaction:

LiBH₄ ↔ LiH + B + 3/2H₂ (1)

Fig. 2 X-ray diffraction spectra of rehydrogenated nanocomposites: (A) 50 wt%LiBH₄ – 50 wt% C-MSU-H (LiBH₄@C-MSU-H), (B) 50 wt%LiBH₄ – 50 wt% 1 at% N-doped C-MSU-H (LiBH₄@C-MSU-H 1 at% N), (C) 50 wt%LiBH₄ – 50 wt% 1 at% Ni-doped C-MSU-H (LiBH₄@C-MSU-H 1 at% Ni).

Boron cannot be detected in the XRD pattern due to amorphous state.

A relatively small amount of Li₃BO₃ (ICDD file 00-018-0718) can be seen in the XRD pattern, probably appeared during the diffraction measurements, although we mounted a protective foil over the sample. The most important amorphous contribution is due to LiBH₄ at 2θ ≈ 25°, but some amount of LiBH₄ crystallizes outside of the ordered nanoporous carbon matrix during the hydrogen absorption/desorption cycles. The X-ray diffraction spectrum for the rehydrogenated composites 50 wt% LiBH₄ – 50 wt% C-MSU-H doped with 1 at% Ni (named LiBH₄@C-MSU-H 1 at% Ni) (Fig. 2C) presents novel elements compared to the previous ones. Thus, the presence of nickel borides with different percentages of B is observed, namely Ni₃B (ICDD file 00-048-1223) and Ni₂B (ICDD file 00-048-1222) in addition to the LiBH₄, LiH and Li₃BO₃ phases. There is no evidence of metallic Ni or NiO in the XRD pattern, probably due to the reduction effect of LiBH₄ and the affinity of Ni to the reaction with boron provided from LiBH₄.

3.2 FTIR spectroscopy

The measurements were performed in the wavenumber range of 300–4000 cm⁻¹ using a JASCO-6600 FTIR spectrometer with a measuring step of 4 cm⁻¹ for both the initial C-MSU-H and LiBH₄ and for the LiBH₄@C-MSU-H and LiBH₄@C-MSU-H 1 at% N rehydrogenated nanocomposites as shown in Fig. 3.

Fig. 3 FTIR spectra of C-MSU-H and the original LiBH₄ and LiBH₄@C-MSU-H and LiBH₄@C-MSU-H 1 at% N rehydrogenated nanocomposites.

The band at 1568 cm⁻¹ corresponds to the stretching mode of the carbon atoms in the aromatic ring (C=C) that exists in C-MSU-H. The band at 1036 cm⁻¹ in C-MSU-H corresponds to the stretching mode for the C–O interaction, and the band at 3445 cm⁻¹ can be assigned to the stretching mode for the O–H bond that appears due to hygroscopicity of KBr.

The vibrational bands at 2390, 2294 and 2226 cm⁻¹ present in LiBH₄@C-MSU-H correspond to the stretching mode for the B–H bond in LiBH₄. The band at 1126 cm⁻¹ can be assigned to the bending B–H vibration in the original LiBH₄ and LiBH₄@C-MSU-H re-hydrogenated nanocomposites. In LiBH₄@C-MSU-H nanocomposites, bands corresponding to the O–H (3440 cm⁻¹), C=C (1572 cm⁻¹) and C–O (1004 cm⁻¹) stretching vibrations also appear, originating from the initial C-MSU-H. Similar to X ray-diffraction, the FTIR spectra proved that after re-hydrogenation the LiBH₄ phase is recovered. In the FTIR spectra of the rehydrogenated nanocomposites (Fig. 3) there is no evidence of Li₂B₁₂H₁₂ whose distinctive feature would be the band at 2485 cm⁻¹. Borane is not released below 450 °C for LiBH₄ infiltrated in nanoporous carbon.⁴¹ We consider that the incomplete hydrogenation is caused by a kinetic barrier. The FTIR spectrum of LiBH₄@C-MSU-H is very similar to that of LiBH₄@C-MSU-H 1 at% N reydrogenated nanocomposites. There is no obvious change in the FTIR spectra due to the presence of N. This can be understood as being due to the very low nitrogen content in the sample. As expected, the XRD pattern of LiBH₄@C-MSU-H 1 at% N is very similar to that of undoped LiBH₄@C-MSU-H (Fig. 2). The reason why the presence of nickel borides can be detected in the XRD pattern of LiBH₄@C-MSU-H 1 at% Ni rehydrogenated nanocomposites is the much higher atomic scattering factor of Ni compared to that of N, C, B and Li. However, we performed N 1s HRXPS (with a noisy spectrum due to very low N content) on LiBH₄@C-MSU-H 1 at% N rehydrogenated sample showing the presence of Li₃N/Li₃BN₂.

3.3 X-ray photoelectron spectroscopy

We performed X-ray photoelectron spectroscopy on C-MSU-H undoped and doped with nitrogen by thermal treatment under ammonia flow in order to find the amount and chemical state of N embedded in the carbon nanostructure (Table 1). We also performed XPS on Ni doped samples.

Table 1 The composition of nanoporous carbon as derived from X-ray photoelectron spectroscopy

Sample	C (at%)	O (at%)	N (at%)
C-MSU-H pristine	96.5	3.5	—
C-MSU-H treated 4 h NH3 at 400 °C	96.8	2.6	0.6
C-MSU-H treated 4 h NH3 at 450 °C	96.9	2.4	0.7
C-MSU-H treated 4 h NH3 at 500 °C	96.5	2.6	0.9

---

The reference energy was taken as the C 1s line C=C bonds, which we set at 284 eV. The spectra from each sample were calibrated according to this reference. The samples have very few contaminants. Other elements, apart from C, O and N, were not observed in the samples. Monotonic increase in the amount of nitrogen is observed with increasing treatment temperature in NH₃ (about 1 at% N doping for 4 h NH₃ at 500 °C). A decrease in the O content is observed following treatment in NH₃. In Fig. 4A–D are depicted the extended XPS spectrum and HRXPS C 1s, O 1s and N 1s spectra for nanoporous carbon sample without N doping. In C 1s HRXPS spectrum (Fig. 4B and Table 2) the binding energy of 284.00 eV corresponds to C-sp² from the hexagonal carbon ring from nanoporous ordered structure, binding energy of 285.07 eV can be assigned to C–C, C–H, single bonds, binding energy of 286.43 eV indicates very likely C–O–C bonds, binding energy of 288.29 eV indicates C=O double bonds, whereas binding energy of 290.20 eV can be assigned to specific shake-up C sp²-π–π* transition.^42–44 In HRXPS O 1s spectrum (Fig. 4C and Table 3) the binding energy of 531.17 eV indicates molecular O₂ adsorbed or C=O double bonds, whereas the binding energy of 532.86 eV can be assigned to C–O single bonds.⁴⁴ In HRXPS N 1s spectrum for C-MSU-H – NH₃ (4 h, 500 °C) (sample doped with ≈ 1 at% N) (Fig. 4E and Table 4) the binding energy of 397.94 eV indicates N in the pyridinic position whereas the binding energy of 399.35 can be assigned to N in the pyrrolic position.^42,45

Fig. 4 X-ray photoelectron spectra (XPS) for whole binding energy range (A) and high resolution X-ray photoelectron spectra (HRXPS) for C 1s (B), O 1s (C) and N 1s (D) for pristine C-MSU-H ordered nanoporous carbon sample and N 1s HRXPS for 1 at% N-doped C-MSU-H (E) and N 1s HRXPS for LiBH₄@C-MSU-H 1 at% N rehydrogenated nanocomposites (F) and Ni 2p_3/2 HRXPS for 1 at% Ni-doped C-MSU-H (G) and Ni 2p_3/2 HRXPS for LiBH₄@C-MSU-H 1 at% Ni rehydrogenated nanocomposites (H).

Table 2 HRXPS C 1s fitting results (binding energies and concentrations corresponding to subspectra) for nanoporous carbon samples treated at various temperatures in ammonia flow

Sample	C-sp^2		C–C, C–H, C–N		C–O–C		C=O		C sp^2-π–π* shake up
	BE	Conc.	BE	Conc.	BE	Conc.	BE	Conc.	BE	Conc.
	(eV)	(at%)	(eV)	(at%)	(eV)	(at%)	(eV)	(at%)	(eV)	(at%)
C-MSU-H	284.0	69.70	285.07	13.70	286.43	7.40	288.29	4.20	290.2	5.00
C-MSU-H	283.9	69.60	285.15	13.00	286.48	7.40	288.42	5.00	290.4	5.10
4 h/400 °C
C-MSU-H	284.0	73.80	285.00	13.90	286.16	5.00	287.48	3.00	289.6	4.30
4 h/450 °C
C-MSU-H	284.0	76.30	285.18	12.80	286.60	5.50	288.48	2.90	290.1	2.40
4 h/500 °C

---

Table 3 HRXPS O 1s fitting results (binding energies and concentrations corresponding to subspectra) for nanoporous carbon samples treated at various temperatures in ammonia flow

Sample	O ads, C=O		C–O
	BE	Conc.	BE	Conc.
	(eV)	(at%)	(eV)	(at%)
C-MSU-H	531.17	49.60	532.86	50.40
C-MSU-H 4 h/400 °C	530.88	41.00	532.82	59.00
C-MSU-H 4 h/450 °C	531.11	25.40	532.61	74.60
C-MSU-H 4 h/500 °C	531.06	36.40	532.65	63.60

---

Table 4 HRXPS N 1s fitting results (binding energies and concentrations corresponding to subspectra) for nanoporous carbon samples treated at various temperatures in ammonia flow

Sample	N-pyridinic		N-pyrrolic
	BE	Conc.	BE	Conc.
	(eV)	(at%)	(eV)	(at%)
C-MSU-H	—	—	—	—
C-MSU-H 4 h/400 °C	398.00	17.10	399.15	82.90
C-MSU-H 4 h/450 °C	397.92	34.90	399.30	65.10
C-MSU-H 4 h/500 °C	397.94	39.60	399.35	60.40

---

An increase in the C-sp² content is observed with increasing treatment temperature in ammonia flow for C-MSU-H (Table 2). A decrease in the content of adsorbed oxygen and/or C=O bonds and an increase in the content of C–O bonds is observed following the thermal treatment in NH₃ (Table 3). An increase in the N-pyridinic content and a decrease in the N-pyrrolic content are observed as a result of increasing the treatment temperature in gaseous ammonia (Table 4). However, C-MSU-H thermally treated in NH₃ samples have a majority of nitrogen atoms into pyrrolic configurations. The nitrogen atoms incorporated into graphitic, pyridinic, or pyrrolic configurations modify the local electronic density and create electron-rich regions that can interact strongly with the metal cations of the hydride (e.g., Li⁺, Na⁺, Mg²⁺). These interactions lead to polarization of the Li–H bonds within the hydride, weakening them and lowering the activation energy for hydrogen desorption.⁴⁶ Consequently, N-doped carbons not only provide physical confinement but also exert a catalytic effect on hydrogen release. The degree of doping and the type of nitrogen species play a decisive role in tuning these effects: graphitic N enhances conductivity and stabilizes the framework, while pyridinic and pyrrolic N sites provide strong chemical anchoring and catalytic reactivity. The theoretical simulations further demonstrate that N-doped sp²/sp³ hybrid carbon can migrate the Fermi level to the conduction band, leading to an n-type conductivity due to the additional electrons attributed to the N dopant.⁴⁷ Particularly, pyridinic nitrogen from C-MSU-H thermally treated in ammonia has a strong catalytic effect. In this state, the N atom is sp² hybridized with carbon atoms from the 2D hexagonal aromatic ring, participating in one π and two σ bonds with the nearest neighboring carbon atoms, providing a lone pair of electrons available for additional bonding. Pyridinic nitrogen embedded in the nanoporous carbon scaffold could act as a Lewis base and donate significant electron density to the confined hydride layered onto the scaffold's surface with an effect on hydride thermodynamic destabilization. XPS data confirm the pyridinic and pyrrolic nitrogen in our prepared N-doped C-MSU-H nanocarbons and the catalytic activity of such supports is expected to be higher than that of neat C-MSU-H as we will further notice from hydrogen storage behavior. The HRXPS N 1s spectrum of 50 wt% LiBH₄–50 wt% 1 at% N-doped C-MSU-H rehydrogenated nanocomposites (noted as LiBH₄@C-MSU-H 1 at% N) is depicted in Fig. 4F. The majority peak corresponding to binding energy of 397.2 eV can be attributed to the formation of Li₃N or Li₃BN₂, both having binding energies close to 397 eV.^48–50 The binding energy given in the literature for Li₂NH is 398.2 eV,⁵¹ which is significantly higher than that of our experimental value. In the Ni 2p_3/2 HRXPS spectrum for 1 at% Ni-doped C-MSU-H (Fig. 4G) the binding energy of 852.63 eV corresponds to Ni metal, the binding energy of 854.31 eV can be assigned to the Ni²⁺ oxidation state which belongs to NiO, and the binding energy of 855.91 eV corresponds to a higher oxidation state of Ni⁴⁺ which can be assigned to NiO₂. In Ni 2p_3/2 HRXPS for LiBH₄@C-MSU-H 1 at% Ni re-hydrogenated nanocomposites (Fig. 4H) the binding energy of 852.96 eV can be assigned to Ni–B bonds.^52,53

3.4 TEM investigation

The transmission electron microscopy image for as prepared C-MSU-H sample is presented in Fig. 5A. The size of the hexagonal nanopores of the ordered structure is approximately 4 nm. It can also be observed that in the case of the C-MSU-H thermally treated in NH₃ flow at 500 °C (Fig. 5B), the ordered structure remains similar to the untreated C-MSU-H sample, with hexagonal nanopores of almost the same size. In Fig. 5C are shown TEM images at various magnifications for C-MSU-H decorated with Ni nanoparticles. The samples indicate an ordered nanoporous carbon structure with uniformly distributed Ni/NiO clusters, without agglomeration. In the EDAX spectrum (Fig. 5D) taken over a large area where nanoparticles are present, a maximum corresponding to nickel is observed and the composition of the nanoparticles is confirmed. The nanoparticles are uniformly distributed in the structure, as can be seen from Fig. 5C. The histogram corresponding to their size distribution and the lognormal fit is presented in Fig. 5E, indicating a mean diameter of 5.3 nm.

Fig. 5 TEM images for (A) as prepared C-MSU-H, (B) C-MSU-H thermally treated in NH₃ at 500 °C, (C) C-MSU-H decorated with Ni nanoparticles and corresponding EDAX spectrum (D) and histogram of the measured values of the diameter of the Ni nanoparticle with a fitted lognormal distribution, showing the mean and width of the underlying size distribution (E). (F) TEM/EDS mapping of LiBH₄@C-MSU-H 1 at% Ni, (G) TEM/EDS mapping of LiBH₄@C-MSU-H 1 at% N.

The rehydrogenated sample LiBH₄@C-MSU-H 1 at% Ni consists of large amorphous particles containing well-dispersed nanoparticles (Fig. 5F). EDX chemical mapping of the amorphous particles reveals a uniform distribution of B and C, while the nanoparticles are rich in Ni. The nickel distribution appears to be concentrated in the nanoparticles, and the speckles appearing on maps outside of them can be attributed to noise. The rehydrogenated LiBH₄@C-MSU-H 1 at% N sample consists of large amorphous particles with irregular shapes that do not show any special features (Fig. 5G). EDX chemical mapping reveals a uniform distribution of B, N, and C throughout the surface of the investigated particles, although the concentration of N is low, close to the detection quantification limit.

3.5 Porosity measurements

The porosity parameters of the samples are summarized in Table 5. The surface area calculated using the BET formalism for the samples C-MSU-H and C–MSU-NH₃ (4 h, 500 °C) is 1264 m² g⁻¹ and, respectively, 1198 m² g⁻¹ indicating that the ordered nanoporous structure is maintained after the treatment at 500 °C in ammonia gas.

Table 5 Surface area, pore volume, and average pore diameter for as prepared C-MSU-H and C-MSU-H thermally treated in ammonia flow at 500 °C

Sample	Surf. area	Surf. area	Surf. area	Pore vol	Pore vol	Pore diam.	Pore diam.
	BET	BJH ads	BJH des	BJH ads	BJH des	BJH ads	BJH des
	(m^2 g^−1)	(m^2 g^−1)	(m^2 g^−1)	(cm^3 g^−1)	(cm^3 g^−1)	(nm)	(nm)
C-MSU-H	1264	1126	1197	1.158	1.288	4.113	4.1057
C-MSU-H	1198	1073	1131	1.051	1.105	3.915	3.9078
NH3, 4 h, 500 °C

---

The isothermal N₂ adsorption–desorption plots, shown in Fig. 6(A and C), have isotherms of type IV shape. The pore volume and average pore diameter for the pristine C-MSU-H and C–MSU-NH₃ (4 h, 500 °C) are presented in Fig. 6(B and D) and Table 5 and the values obtained indicate once again the preservation of the nanostructure of the sample after the treatment at 500 °C in NH₃. The combination of nanoporosity and N-doping offers synergistic benefits. The nanoporous network ensures that hydrides are spatially confined, maximizing surface contact with the doped carbon walls, while the doped sites locally destabilize the hydride, effectively lowering both the desorption enthalpy and activation energy. This results in improved hydrogen release at significantly lower temperatures compared to bulk hydrides. Additionally, the carbon host enhances thermal conductivity with positive effect on hydrogen desorption which requires external heating because it is an endothermic reaction and mitigates phase segregation or irreversible decomposition during cycling.

Fig. 6 N₂ adsorption/desorption isotherms and pore size distributions for as-prepared C-MSU-H (A) and (B) and C-MSU-H thermally treated in ammonia flow at 500 °C (C) and (D).

3.6 Hydrogen storage behavior

Hydrogen absorption–desorption measurements were taken using an automated volumetric apparatus (Sievert). We performed isothermal kinetic hydrogen absorption and desorption and thermal programmed desorption measurements. In Fig. 7A, it can be observed that a large amount of hydrogen is released after the first desorption. The theoretical amount of hydrogen reversibly stored in the reaction LiBH₄ ↔ LiH + B + 3/2H₂ is 13.8 wt% H₂. The weight ratio between LiBH₄ and C-MSU-H was 1 : 1. That means that the theoretical amount of hydrogen reversibly stored by LiBH₄@C-MSU-H should be 6.9 wt% H₂. However, the theoretical reversible amount should be approximately 6.5–6.7 wt% H₂ (less than 6.9 wt% H₂) because nickel borides or Li₃N/Li₃BN₂ are also formed in the rehydrogenated nanocomposites containing either Ni or N. By making nanocomposites of LiBH₄ infiltrate under H₂ pressure in ordered nanoporous carbon, the aim is to avoid the growth of LiBH₄ grains following the hydrogen absorption–desorption cycles, which would lead to a drastic deterioration of the hydrogen absorption–desorption kinetics. LiBH₄@C-MSU-H desorbs 6.2 wt% H₂ up to 450 °C for the first desorption with a temperature ramp of 2 °C min⁻¹ (Fig. 7A). For complete desorption a much longer desorption time at 450 °C is required. Due to the presence of 1 at% Ni or N as doping elements, the LiBH₄@doped C-MSU-H nanocomposites should desorb less than the theoretical amount of 6.9 wt% H₂. LiBH₄@C-MSU-H 1 at% Ni desorbs up to 450 °C, the least among all the samples, approximately 5.3 wt% H₂ for the first desorption. After four hydrogen absorption–desorption cycles LiBH₄@C-MSU-H releases only 3.1 wt% H₂ up to 450 °C (Fig. 7B). LiBH₄–C-MSU-H 1 at% N and LiBH₄–C-MSU-H 1 at% Ni samples desorb 4.1 and 3.8 wt% H₂ up to 450 °C, respectively, using the same temperature ramp of 2 °C min⁻¹ (Fig. 7B). For complete desorption, a much longer desorption time at 450 °C is required. Faster desorption kinetics is observed for the N-doped sample compared to Ni-doped sample, but both doped samples have much faster desorption kinetics than the sample containing undoped C-MSU-H.

Fig. 7 Thermal programmed desorption with temperature ramp of 2 °C min⁻¹ (A – first desorption, B – after 4 cycles of absorption/desorption). (C) Desorption peak temperature (measured for 2 °C min⁻¹ heating rate) and (D) absorption kinetics after 4 H₂ absorption/desorption cycles for LiBH₄@C-MSU-H, LiBH₄@C-MSU-H 1 at% N and LiBH₄@C-MSU-H 1 at% Ni. The amount of hydrogen is reported relative to the total weight of the composite (including the weight of the nanoporous carbon).

The experimental maximum reversible capacity was determined at 450 °C by allowing hydrogen uptake for at least 20 h under 100 atm and prolonged hydrogen desorption at the same temperature (at least 8 h). Fig. S1 (SI) describes thermal programmed desorption up to 450 °C and prolonged desorption at this final temperature. We obtained a reversible capacity of 6.14 wt% H₂ for LiBH₄@C-MSU-H 1 at% N and 6.05 wt% H₂ for LiBH₄@C-MSU-H 1 at % Ni rehydrogenated nanocomposites. The amount of hydrogen released after several absorption–desorption cycles is lower than that in the first desorption due to the impossibility of complete rehydrogenation of the dehydrogenated nanocomposites due to a kinetic barrier. Fig. 7C shows the desorption peak temperatures after four absorption/desorption cycles for undoped LiBH₄@C-MSU-H nanocomposites and the ones doped with N or Ni. A desorption peak temperature of 339 °C is observed for LiBH₄@C-MSU-H, 328 °C for LiBH₄@C-MSU-H 1 at% Ni and 318 °C for LiBH₄@C-MSU-H 1 at% N, thus a clear advantage for the N-doped nanocomposites compared with the other two samples. Fig. 7D shows the absorption kinetics after 4 a/d cycles for LiBH₄ infiltrated in nanoporous carbon containing undoped, N doped and Ni doped C-MSU-H performed at 450 °C and 100 atm H₂. In 40 000 s (approximately 11 h) the LiBH₄@C-MSU-H sample absorbs 3.6 wt% H₂, LiBH₄@C–MSU-H 1 at% Ni absorbs 4.3 wt% H₂ and LiBH₄@C–MSU-H 1 at% N almost 4.6 wt% H₂. Once again, also regarding the absorption kinetics, N-doped nanocomposites show the best behavior, while undoped nanocomposites behave the worst.

The Kissinger plot⁵⁴ allows obtaining the activation energy of hydrogen desorption from the slope of the ln(β/T_p²) vs. 1/T_p graph according to the relation

where β is the heating rate in (°C min⁻¹) and T_p is the desorption peak temperature at different heating rates. The desorption peak temperatures for different heating rates and the related Kissinger linear plots are presented in Fig. 8A–C, from which the activation energies of desorption are extracted. For LiBH₄@C-MSU-H we obtained 142.7 kJ mol⁻¹, whereas for LiBH₄@C-MSU-H 1 at% Ni we got 123.8 kJ mol⁻¹ and the lowest activation energy of 119.5 kJ mol⁻¹ was found for LiBH₄@C-MSU-H 1 at% N. According to the activation energy of desorption values, LiBH₄ infiltrated in C-MSU-H doped with 1 at% N has the fastest hydrogen desorption kinetics among all samples.

Fig. 8 Desorption peak temperature measured at various heating rates and corresponding Kissinger plots for (A) LiBH₄@C-MSU-H, (B) LiBH₄@C-MSU-H 1 at% Ni and (C) LiBH₄@C-MSU-H 1 at% N.

The incorporation of Li₃N, LiNH₂, or Li₂NH into LiBH₄@N-doped mesoporous carbons significantly enhances both hydrogen desorption kinetics and reversibility, primarily via the formation of stable Li–B–N–H intermediates and the reduction of activation energy barriers. Within N-doped nanoporous carbons, the dispersion of Li₃N or LiNH₂ nanoparticles ensures intimate contact with LiBH₄ and maximizes interface area. Nitrogen functionalities on the carbon surface (pyridinic or pyrrolic N) act as Lewis base sites that coordinate to Li⁺ ions from LiBH₄ or Li₃N, inducing polarization of B–H bonds and facilitating H⁻ transfer. In confined geometries, this reaction is spatially controlled, producing defect-rich interphases that sustain reversibility by minimizing B segregation and LiH aggregation. During cycling, the LiNH₂ ↔ Li₂NH ↔ Li₃N coupled equilibrium acts as a hydrogen reservoir, allowing dynamic hydrogen exchange.⁵⁵ Additionally, these equilibria maintain reactive species within the composite, stabilizing the system during multiple hydrogen absorption–desorption cycles.

Li₃N + 2H₂ ↔ Li₂NH + LiH + H₂ ↔ LiNH₂ + 2LiH (3)

The underlying chemistry encompasses formation of Li₄BN₃H₁₀⁵⁶ and its dehydrogenation to another catalytic species, Li₃BN₂.⁵⁰ When LiBH₄ is mixed or confined with Li₃N, or equivalently with LiNH₂ (derived in situ from Li₃N + H₂), a solid–solid reaction occurs upon heating, yielding the complex hydride Li₄BN₃H₁₀:

3LiNH₂ + LiBH₄ → Li₄BN₃H₁₀ (4)

This reaction represents a chemical destabilization of LiBH₄ through amide/nitride incorporation, resulting in a hydrogen-rich compound (∼11.4 wt% H) that can dehydrogenate at lower temperatures (∼250 °C). Li₄BN₃H₁₀ is the most thermodynamically favorable intermediate in the Li–B–N–H system.⁵⁷ Upon heating, it may undergo several reactions leading to Li₃BN₂ as the more probable reaction product according to the reaction⁵⁸

Li₄BN₃H₁₀ → Li₃BN₂ + LiNH₂ + 4H₂ (5)

Our HRXPS N 1s spectrum is consistent with the formation of Li₃BN₂ as an active catalytic species in LiBH₄ dehydrogenation/rehydrogenation. The catalytic enhancement of LiBH₄ hydrogen sorption by Li₃BN₂, can be attributed to the formation and decomposition of the Li₄BN₃H₁₀ intermediate, and the interfacial electronic effects induced by N-doped carbon supports. The synergy between LiBH₄ and N-doped carbon nanostructures—which provides conductive, confining, and catalytically active environments—yields a composite system with enhanced kinetics, reduced activation energy, and improved reversibility for hydrogen storage applications. Ni as a dopant for C-MSU-H forms catalysts such as Ni₂B and Ni₃B after interaction with LiBH₄ which are less efficient than Li₃BN₂. Moreover, Li₃BN₂ catalytic centers formed around nitrogen inside the C-MSU-H matrix are much more dispersed compared to Ni₂B and Ni₃B clusters (provided from Ni/NiO initial clusters of 5.3 nm dispersed onto the C-MSU-H surface), leading to higher catalytic efficiency. Nickel borides actively participate in H₂ dissociation/recombination. Using first principles calculations Liu et al.¹⁴ demonstrated that the dissociation energy for removing one H from LiBH₄ decreased down to 1.00 eV for LiBH₄ on Ni₂B, much lower than the value of 4.22 eV for bulk LiBH₄ and even lower than that of LiBH₄ on Ni (1.27 eV) showing a superior catalytic effect of nickel boride compared to pure nickel. Additionally, using EXAFS spectroscopy at the Ni K-edge for 5 wt% Ni-catalyzed LiBH₄, Ngene et al.⁵⁹ identified Ni_xB phases with different Ni–B and Ni–Ni coordinations for the dehydrogenated and rehydrogenated samples, which behave reversibly. Specifically, for the dehydrogenated samples Ni–Ni coordination decreased and Ni–B coordination increased whereas for the rehydrogenated samples Ni–Ni coordination increased and Ni–B coordination decreased.

The purpose of the work was to experimentally demonstrate the superiority of N over Ni as a doping element for the C-MSU-H-based catalyst for improving the dehydrogenation kinetics of LiBH₄ and to explain this experimental finding. Table S1 (SI) shows the comparison of the hydrogen storage performances of the LiBH₄@C-MSU-H 1 at% N studied in the present work with those presented in state-of-the-art literature for LiBH₄-based systems. Our material was obtained through a simple procedure (melt infiltration), which does not consume much time and resources such as expensive precursors or toxic and critical elements. Further optimization of the catalyst is required to achieve better performances.

4. Conclusions

Ordered nanoporous carbon with nitrogen substitution was prepared using the nanotemplating method starting from nanoporous silica by sucrose impregnation and pyrolysis followed by etching in HF. Finally, the treatment in a gaseous ammonia flow at temperatures between 400 °C and 500 °C was used for nitrogen doping. For the doping of the ordered nanoporous carbon with 1 at% Ni, nickel nitrate hexahydrate dissolved in absolute ethanol was used. The final thermal treatment was performed for 4 h at 430 °C in a 5% H₂/Ar flow. The ordered carbon nanoporous structure is maintained even after nitrogen doping according to diffraction spectra and TEM images. Doping levels of up to 1 at% are obtained by treatments in a gaseous ammonia flow at 500 °C. For the ordered nanoporous carbon C–MSU-H doped with N, an increase in the N-pyridinic content and a decrease in the N-pyrrolic content are observed as a result of increasing the treatment temperature in gaseous ammonia flow. However, the samples have mostly N-pyrrolic. After nickel doping, homogeneously distributed clusters with average size of 5.3 nm containing Ni and NiO are observed in TEM decorating the ordered nanoporous carbon structure C–MSU-H. Nanocomposites 50 wt% LiBH₄–50 wt% undoped or doped with N or Ni ordered nanoporous carbon (LiBH₄@C-MSU-H, LiBH₄@C-MSU-H 1 at% N, LiBH₄@C-MSU-H 1 at% Ni) were prepared using the method of infiltration under hydrogen pressure above the melting temperature of LiBH₄. According to XRD, for the re-hydrogenated LiBH₄@C-MSU-H 1 at% Ni nickel borides are also observed. The fastest hydrogen absorption and desorption kinetics were obtained for the LiBH₄@C-MSU-H 1 at% N, followed by LiBH₄@C-MSU-H 1 at% Ni and much slower desorption kinetics for the undoped nanocomposites. The desorption peak temperatures (2 °C min⁻¹ ramp) are 339 °C for LiBH₄@C-MSU-H, 328 °C for LiBH₄@C-MSU-H 1 at% Ni and 318 °C for the LiBH₄@C-MSU-H 1 at% N. From the Kissinger plots the activation energies of hydrogen desorption for the nanocomposites were obtained: 142.7 kJ mol⁻¹ for LiBH₄@C-MSU-H, 123.8 kJ mol⁻¹ for LiBH₄@C-MSU-H doped with 1 at% Ni and 119.5 kJ mol⁻¹ for LiBH₄ infiltrated in 1 at% N-doped C-MSU-H.

Author contributions

Conceptualization: P. P. and C. C.; investigation: P. P., C. N., A. G. M., and C. R., formal analysis: P. P., C. C., C. N. A. G. M., and C. R.; software: C. C., P. P., C. R., and C. N.; data curation: P. P. and C. C.; writing original draft: P. P., C. C., C. N., and A. G. M.; writing – revised editing: all authors. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

The data that support the findings of this study are available from the authors upon reasonable request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5cp03997e.

Acknowledgements

This work was funded by the Core Program of the National Institute of Materials Physics, granted by the Romanian Ministry of Research, Innovation and Digitization through the Project PC3-PN23080303.

References

1. L. Schlapbach and A. Züttel, Hydrogen-storage materials for mobile applications, Nature, 2001, 414, 353–358.
2. S. Orimo, Y. Nakamori, G. Kitahara, K. Miwa, N. Ohba, S. Towata and A. Züttel, Dehydriding and rehydriding reactions of LiBH₄, J. Alloys Compd., 2005, 404–406, 427–430.
3. Y. Xu, Y. Zhou, Y. Li, M. Ashuri and Z. Ding, Engineering LiBH₄-based materials for advanced hydrogen storage: A critical review of catalysis, nanoconfinement, and composite design, Molecules, 2024, 29, 5774.
4. D. Yin, X. Wang, F. Wu, D. Shang, H. Li, P. Huang and L. Zhang, A mini review on the state-of-the-art progress of LiBH₄ for hydrogen storage: Challenges and perspectives, J. Alloys Compd., 2025, 1041, 183883.
5. J. Zheng, Z. Yao, X. Xiao, X. Wang, J. He, M. Chen, H. Cheng, L. Zhang and L. Chen, Enhanced hydrogen storage properties of high-loading nanoconfined LiBH₄-Mg(BH₄)₂ composites with porous hollow carbon nanospheres, Int. J. Hydrogen Energy, 2021, 46, 852–864.
6. R. Wu, X. Zhang, Y. Liu, L. Zhang, J. Lu, M. Gao and H. Pan, A unique double-layered carbon nanobowl-confined lithium borohydride for highly reversible hydrogen storage, Small, 2020, 16, 2001963.
7. Suwarno, P. Ngene, A. Nale, T. M. Eggenhuisen, M. Oschatz, J. P. Embs, A. Remhof and P. E. de Jongh, Confinement effects for lithium borohydride: comparing silica and carbon scaffolds, J. Phys. Chem. C, 2017, 121, 4197–4205.
8. S. D. House, X. Liu, A. A. Rockett, E. H. Majzoub and I. M. Robertson, Characterization of the dehydrogenation process of LiBH₄ confined in nanoporous carbon, J. Phys. Chem. C, 2014, 118, 8843–8851.
9. A. Gasnier and F. C. Gennari, Graphene entanglement in a mesoporous resorcinol-formaldehyde matrix applied to the nanoconfinement of LiBH₄ for hydrogen storage, RSC Adv., 2017, 7, 27905–27912.
10. Y. Ding, C. Li, X. Zhang, W. Chen, X. Yu and G. Xia, Spatial confinement of lithium borohydride in bimetallic CoNi-doped hollow carbon frameworks for stable hydrogen storage, ACS Appl. Mater. Interfaces, 2024, 16, 50717–50725.
11. S. Wang, M. Gao, Z. Yao, Y. Liu, M. Wu, Z. Li, Y. Liu, W. Sun and H. Pan, A nanoconfined-LiBH₄ system using a unique multifunctional porous scaffold of carbon wrapped ultrafine Fe₃O₄ skeleton for reversible hydrogen storage with high capacity, Chem. Eng. J., 2022, 428, 131056.
12. K. Xian, B. Nie, Z. Li, M. Gao, Z. Li, C. Shang, Y. Liu, Z. Guo and H. Pan, TiO2 decorated porous carbonaceous network structures offer confinement, catalysis and thermal conductivity for effective hydrogen storage of LiBH4, Chem. Eng. J., 2021, 407, 127156.
13. W. Chen, Y. Liu, C. Li, Y. Pang, X. Yu and G. Xia, Enhancing reversible hydrogen storage performance of LiBH₄ catalyzed by N-doped carbon nanosheet networks embedded with Co nanoparticles, ACS Appl. Mater. Interfaces, 2025, 17, 50603–50611.
14. Y. Liu, W. Chen, S. Ju, X. Yu and G. Xia, Stable hydrogen storage of lithium borohydrides via the catalytic effect of Ni₂B induced by thermodynamic destabilization reaction, J. Mater. Sci. Technol., 2024, 202, 192–200.
15. Z. Z. Fang, X. D. Kang, P. Wang and H.-M. Cheng, Improved reversible dehydrogenation of LiBH₄ by milling with as-prepared single-walled nanotubes, J. Phys. Chem. C, 2008, 112, 17023–17029.
16. G. Na, W. G. Cui, H. Shi, Z. Li, F. Gao, X. Wang, K. Wang, Y. Gao, Y. Yang, Z. Shen, Y. Liu, J. Miao and H. Pan, In situ generation and stabilization of multiple catalysts by introducing a graphene-supported FeF₂/FeO_x additive for enhancing the hydrogen storage of LiBH₄, ACS Appl. Energy Mater., 2025, 8, 3802–3811.
17. Y. Pang, X. Hu, X. Zhang, X. Yu and G. Xia, Two-dimensional transition metal-based high-entropy oxide nanoplates for enhanced hydrogen storage of LiBH₄, J. Alloys Compd., 2025, 1040, 183613.
18. X. Zhang, L. Zhang, W. Zhang, Z. Ren, Z. Huang, J. Hu, M. Gao, H. Pan and Y. Liu, Nano-synergy enables highly reversible storage of 9.2 wt% at mild conditions with lithium borohydride, Nano Energy, 2021, 83, 105839.
19. J. J. Vajo, S. L. Skeith and F. Mertens, Reversible storage of hydrogen in destabilized LiBH₄, J. Phys. Chem. B, 2005, 109, 3719–3722.
20. Q. He, D. Zhu, X. Wu, D. Dong, M. Xu and Z. Tong, Hydrogen desorption properties of LiBH₄/xLiAlH₄ (x = 0.5, 1, 2) composites, Molecules, 2019, 24, 1861.
21. J. H. Shim, Y. S. Lee, J. Y. Suh, W. Cho, S. S. Han and Y. W. Cho, Thermodynamics of the dehydrogenation of the LiBH₄-YH₃ composite: Experimental and theoretical studies, J. Alloys Compd., 2012, 510, L9–L12.
22. N. Bergemann, C. Pistidda, M. Uptmoor, C. Milanese, A. Santoru, T. Emmler, J. Puszkiel, M. Dornheim and T. Klassen, A new mutually destabilized reactive hydride system: LiBH₄-Mg₂NiH₄, J. Energy Chem., 2019, 34, 240–254.
23. P. S. Miedema, P. Ngene, A. M. J. van der Eerden, D. Sokaras, T. C. Weng, D. Nordlund, Y. S. Au and F. M. F. de Groot, In-situ X-ray Raman spectroscopy study of hydrogen sorption properties of lithium borohydride composites, Phys. Chem. Chem. Phys., 2014, 16, 22651–22658.
24. X. Liu, E. H. Majzoub, V. Stavila, R. K. Bhakta, M. D. Allendorf, D. T. Shane, M. S. Conradi, N. Verdal, T. J. Udovic and S. J. Hwang, Probing the unusual anion mobility of LiBH₄ confined in highly ordered nanoporous carbon frameworks via solid state NMR and quasielastic neutron scattering, J. Mater. Chem. A, 2013, 1, 9935–9941.
25. Y. Yan, H. Wang, M. Zhu, W. Cai, D. Rentsch and A. Remhof, Direct rehydrogenation of LiBH₄ from H-deficient Li₂B₂H_12−x, Crystals, 2018, 8, 131.
26. P. Ngene, M. R. van Zwienen and P. E. de Jongh, Reversibility of the hydrogen desorption from LiBH₄: A synergetic effect of nanoconfinement and Ni addition, Chem. Commun., 2010, 46, 8201–8203.
27. F. Wang, J. Xu, X. Shao, X. Su, Y. Huang and T. Zhang, Palladium on nitrogen-doped mesoporous carbon: A bifunctional catalyst for formate-based, carbon-neutral hydrogen storage, ChemSusChem, 2016, 9, 246–251.
28. K. Koh, M. Jeon, D. M. Chevrier, P. Zhang, C. W. Yoon and T. Asefa, Novel nanoporous N-doped carbon-supported ultrasmall Pd nanoparticles: Efficient catalyst for hydrogen storage and release, Appl. Catal., B, 2017, 203, 820–828.
29. I. Muhammad, J. Saddique, C. Wu, M. ur Rahman, Z. U. Khan, W. Ali and R. Zhang, Nitrogen-doped -graphene-supported nickel nanoparticles reveal low dehydrogenation temperature and long cyclic life of magnesium hydrides, ACS Omega, 2024, 9, 19261–19271.
30. X. Zhang, Y. Sun, S. Ju, J. Ye, X. Hu, W. Chen, L. Yao, G. Xia, F. Fang, D. Sun and X. Yu, Solar-driven reversible hydrogen storage, Adv. Mater., 2023, 35, 2206946.
31. X. Hu, X. Chen, X. Zhang, Y. Meng, G. Xia, X. Yu, D. Sun and F. Fang, In situ construction of interface with photothermal and mutual catalytic effect for efficient solar-driven reversible hydrogen storage of MgH₂, Adv. Sci., 2024, 11, 2400274.
32. X. Zhang, S. Ju, C. Li, J. Hao, Y. Sun, X. Hu, W. Chen, J. Chen, L. He, G. Xia, F. Fang, D. Sun and X. Yu, Atomic reconstruction for realizing stable solar-driven reversible hydrogen storage of magnesium hydride, Nat. Commun., 2024, 15, 2815.
33. Y. Jiang, Y. Sun, Y. Liu, M. Yue, Y. Cao, Q. Yuan and Y. Wang, Solar-driven reversible hydrogen storage in metal oxides-catalyzed MgH₂, Int. J. Hydrogen Energy, 2025, 149, 150101.
34. Y. Sun, Y. Jiang, Y. Liu, L. Feng, M. Yue, Y. Cao and Y. Wang, Solar-driven rapid dehydrogenation of LiBH₄ catalyzed by g-C3N4/α-Fe₂O₃ heterojunction, Int. J. Hydrogen Energy, 2026, 200, 152958.
35. H. Wang, Q. Li, J. Chen, J. Chen and H. Jia, Efficient solar-driven CO₂ methanation and hydrogen storage over nickel catalyst derived from metal-organic frameworks with rich oxygen vacancies, Adv. Sci., 2023, 10, 2304406.
36. K. Tan, Q. Pei, Y. Yu, L. Liu, J. Li, L. Han, Y. Yu, Z. Li, A. Munyentwali, J. Guo, Y. Wang, L. Rao, T. He and P. Chen, Solar-driven reversible hydrogen storage of sodium cyclohexanolate/phenoxide pair, Angew. Chem., Int. Ed., 2025, 64, e202506275.
37. S. S. Kim, A. Karkamkar, T. J. Pinnavaia, M. Kruk and M. Jaroniek, Synthesis and characterization of ordered, very large pore MSU-H silicas assembled from water-soluble silicates, J. Phys. Chem. B, 2001, 105, 7663–7670.
38. S. S. Kim, T. R. Pauly and T. J. Pinnavaia, Non-ionic surfactant assembly of ordered, very large pore molecular sieve silicas from water soluble silicates, ChemComm, 2000, 17, 1661–1662.
39. S. H. Joo, R. Ryoo, M. Kruk and M. Jaroniec, Evidence for general nature of pore interconnectivity in 2-dimensional hexagonal mesoporous silicas prepared using block copolymer templates, J. Phys. Chem. B, 2002, 106, 4640–4646.
40. S. S. Kim and T. J. Pinnavaia, A low cost route to hexagonal mesostructured carbon molecular sieves, ChemComm, 2001, 23, 2418–2419.
41. X. Liu, D. Peaslee, C. Z. Jost and E. H. Majzoub, Controlling the decomposition pathway of LiBH₄ via confinement in highly ordered nanoporous carbon, J. Phys. Chem. C, 2010, 114, 14036–14041.
42. M. Oschatz, J. P. Hofmann, T. W. van Deelen, W. S. Lamme, N. A. Krans, E. J. M. Hensen and K. P. de Jong, Effects of the functionalization of the ordered mesoporous carbon support surface on iron catalysts for the Fischer-Tropsch synthesis of lower olefins, ChemCatChem, 2017, 9, 620–628.
43. B. Lesiak, L. Kövér, J. Tóth, J. Zemek, P. Jiricek, A. Kromka and N. Rangam, C sp²/sp³ hybridisations in carbon nanomaterials – XPS and (X)AES study, Appl. Surf. Sci., 2018, 452, 223–231.
44. H. Darmstadt, C. Roy, S. Kaliaguine, S. J. Choi and R. Ryoo, Surface chemistry of ordered mesoporous carbons, Carbon, 2002, 40, 2673–2683.
45. L. M. Chew, W. Xia, H. Düdder, P. Weide, H. Ruland and M. Muhler, On the role of the stability of functional groups in multi-walled carbon nanotubes applied as support in iron-based high-temperature Fischer–Tropsch synthesis, Catal. Today, 2016, 270, 85–92.
46. Y. J. Cho, S. Y. Kang, B. C. Wood and E. S. Cho, Heteroatom-doped graphenes as actively interacting 2D encapsulation media for Mg based hydrogen storage, ACS Appl. Mater. Interfaces, 2022, 14, 20823–20834.
47. Q. Wang, J. Su, H. Chen, D. Wang, X. Tian, Y. Zhang, X. Feng, S. Wang, J. Li and H. Jin, Highly conductive nitrogen-doped sp²/sp³ hybrid carbon as a conductor-free charge storage host, Adv. Funct. Mater., 2022, 32, 2209201.
48. S. D. S. Fitch, G. E. Moehl, N. Meddings, S. Fop, S. Solue, T. L. Lee, M. Kazemian, N. G. Araez and A. L. Hector, Combined electrochemical, XPS, and STXM study of lithium nitride as a protective coating for lithium metal and lithium-sulfur batteries, ACS Appl. Mater. Interfaces, 2023, 15, 39198–39210.
49. W. Hou, Y. Li, S. Li, Z. Liu, P. G. Ryan, M. Xu, J. K. Kim, B. Yuan, R. Hu and Z. Luo, Lithium dendrite suppression with Li₃N-rich protection layer formation on 3D anode via ultra-low temperature nitriding, Chem. Eng. J., 2022, 441, 136067.
50. S. Emani, C. Liu, M. Ashuri, K. Sahni, J. Wu, W. Yang, K. Nemeth and L. L. Shaw, Li₃BN₂ as a transition metal free, high capacity cathode for Li-ion batteries, ChemElectroChem, 2019, 6, 320–325.
51. Y. Pang, C. Wei, X. Ye, X. Li, H. Sun, S. Luo, T. Chen, S. Xia, T. Yuan and S. Zheng, H^δ−–H^δ+ comproportionation enables stable Li-N-H-F solid electrolyte, Angew. Chem., Int. Ed., 2025, 64, e202511344.
52. J. Legrand, A. Taleb, S. Gota, M. J. Guittet and C. Petit, Synthesis and XPS characterization of nickel boride nanoparticles, Langmuir, 2002, 18, 4131–4137.
53. J. Hong, M. Miola, D. Gerlach, M. C. A. Stuart, P. Rudolf, D. M. Morales, L. Protesescu and P. P. Pescarmona, An exploration of the electrocatalytic activity of nickel boride nanocrystals in the oxidation of 5-HMF, Catal. Sci. Technol., 2025, 15, 457–475.
54. H. E. Kissinger, Variation of peak temperature with heating rate in differential thermal analysis, J. Res. Natl. Bur. Stand., 1956, 57, 217–221.
55. J. L. White, A. A. Baker, M. A. Marcus, J. L. Snider, T. C. Wang, J. R. I. Lee, D. A. L. Kilcoyne, M. D. Allendorf, V. Stavila and F. El Gabaly, The inside-outs of metal hydride dehydrogenation: imaging the phase evolution of the Li-N-H hydrogen storage system, Adv. Mater. Interfaces, 2020, 7, 1901905.
56. Y. E. Filinchuk, K. Yvon, G. P. Meisner, F. E. Pinkerton and M. P. Balogh, On the composition and crystal structure of the new quaternary hydride phase Li₄BN₃H₁₀, Inorg. Chem., 2006, 45, 1433–1435.
57. D. J. Siegel, C. Wolverton and V. Ozoliņs, Reaction energetics and crystal structure of Li₄BN₃H₁₀ from first principles, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 75, 014101.
58. J. F. Herbst and L. G. Hector, Jr, Electronic structure and energetics of the quaternary hydride Li₄BN₃H₁₀, Appl. Phys. Lett., 2006, 88, 231904.
59. P. Ngene, M. H. W. Verkuijlen, Q. Zheng, J. Kragten, P. J. M. van Bentum, J. H. Bitter and P. E. de Jongh, The role of Ni in increasing the reversibility of the hydrogen release from nanoconfined LiBH₄, Faraday Discuss., 2011, 151, 47–58.

---