Achieving Wide-Temperature-Range Physical and Chemical Hydrogen Sorption in a Structural Optimized Mg/N-Doped Porous Carbon Nanocomposite

Abstract

The as-synthesized rN-pC exhibited H₂ uptake of ~0.9 wt% at 77 K and ultralow pressure of ~0.1 bar, with an isosteric adsorption enthalpy (Q_st) of ~14 kJ mol^-1 H₂ at zero coverage.

The 60MgH₂@rN-pC started to decompose at 175 °C and released H₂ of 3.38 wt% at 300 °C within 30 min, which showed outstanding desorption kinetics of MgH₂ among Mg-carbon material nanocomposites.

The drawback of nanoconfinement scaffolds that cannot store hydrogen was firstly overcome.

Nanoconfinement is a promising approach to simultaneously enhance the thermodynamics, kinetics, and cycling stability of hydrogen storage materials. The introduction of supporting scaffolds usually causes a reduction in the total hydrogen storage capacity due to “dead weight.” Here, we synthesize an optimized N-doped porous carbon (rN-pC) without heavy metal as supporting scaffold to confine Mg/MgH₂ nanoparticles (Mg/MgH₂@rN-pC). rN-pC with 60 wt% loading capacity of Mg (denoted as 60 Mg@rN-pC) can adsorb and desorb 0.62 wt% H₂ on the rN-pC scaffold. The nanoconfined MgH₂ can be chemically dehydrided at 175 °C, providing ~ 3.59 wt% H₂ with fast kinetics (fully dehydrogenated at 300 °C within 15 min). This study presents the first realization of nanoconfined Mg-based system with adsorption-active scaffolds. Besides, the nanoconfined MgH₂ formation enthalpy is reduced to ~ 68 kJ mol⁻¹ H₂ from ~ 75 kJ mol⁻¹ H₂ for pure MgH₂. The composite can be also compressed to nanostructured pellets, with volumetric H₂ density reaching 33.4 g L⁻¹ after 500 MPa compression pressure, which surpasses the 24 g L⁻¹ volumetric capacity of 350 bar compressed H₂. Our approach can be implemented to the design of hybrid H₂ storage materials with enhanced capacity and desorption rate.