Hydrogen energy is considered to be a desired energy storage carrier because of its high-energy density, extensive sources, and is environmentally friendly. The development of hydrogen storage material, especially liquid organic hydrogen carrier (LOHC), has drawn intensive attention to address the problem of hydrogen utilization. Hydrogen carrier is a material that can reversibly absorb and release hydrogen using catalysts at elevated temperature, in which LOHC mainly relies on the covalent bonding of hydrogen during storage to facilitate long-distance transportation and treatment. In this review, the chemical properties and state-of-the-art of LOHCs were investigated and discussed. It reviews the latest research progress with regard to liquid organic hydrogen storage materials, namely N-ethylcarbazole, and the recent progress in the preparation of efficient catalysts for N-ethylcarbazole dehydrogenation by using metal multiphase catalysts supported by carbon–nitrogen materials is expounded. Several approaches have been considered to obtain efficient catalysts such as increasing the surface area of the support, optimizing particle size, and enhancing the porous structure of the support. This review provides a new direction for the research of hydrogen storage materials and considerations for follow-up research.

Hydrogen storage in the form of liquid organic hydrides, especially N-ethylcarbazole, has been regarded as a promising technology for substituting traditional fossil fuels owing to its unique merits such as high volumetric, gravimetric hydrogen capacity and safe transportation. However, unsatisfactory dehydrogenation has impeded the widespread application of N-ethylcarbazole as ideal hydrogen storage materials in hydrogen energy. Therefore, designing catalysts with outstanding performance is of importance to address this problem. In the present work, for the first time, we have synthesized Pd nanoparticles immobilized on carbon nanotubes (Pd/CNTs) with different palladium loading through an alcohol reduction technique. A series of characterization technologies, such as X-ray diffraction (XRD), inductively coupled plasma-atomic emission spectrometer (ICP-AES), X-ray photoelectron spectroscopy (XPS) and transmission electron spectroscopy (TEM) were adopted to systematically explore the structure, composition, surface properties and morphology of the catalysts. The results reveal that the Pd NPs with a mean diameter of 2.6 ± 0.6 nm could be dispersed uniformly on the surface of CNTs. Furthermore, Pd/CNTs with different Pd contents were applied in the hydrogen release of dodecahydro-N-ethylcarbazole. Among all of the catalysts tested, 3.0 wt% Pd/CNTs exhibited excellent catalytic performance with the conversion of 99.6% producing 5.8 wt% hydrogen at 533 K, low activation energy of 43.8 ± 0.2 kJ/mol and a high recycling stability (>96.4% conversion at 5th reuse).

Graphite carbon nitride (g-C₃N₄) supported PtNi alloy nanoparticles (NPs) were fabricated via a facile and simple impregnation and chemical reduction method and explored their catalytic performance towards hydrogen evolution from ammonia borane (AB) hydrolysis dehydrogenation. Interestingly, the resultant Pt_0.5Ni_0.5/g-C₃N₄ catalyst affords superior performance, including 100% conversion, 100% H₂ selectivity, yielding the extraordinary initial total turnover frequency (TOF) of 250.8 mol_H2 min⁻¹ (mol_Pt)⁻¹ for hydrogen evolution from AB at 10 °C, a relatively low activation energy of 38.09 kJ mol⁻¹, and a remarkable reusability (at least 10 times), which surpass most of the noble metal heterogeneous catalysts. This notably improved activity is attributed to the charge interaction between PtNi NPs and g-C₃N₄ support. Especially, the nitrogen-containing functional groups on g-C₃N₄, serving as the anchoring sites for PtNi NPs, may be beneficial for becoming a uniform distribution and decreasing the particle size for the NPs. Our work not only provides a cost-effective route for constructing high-performance catalysts towards the hydrogen evolution of AB but also prompts the utilization of g-C₃N₄ in energy fields.