New Alkali Doped Pillared Carbon Materials Designed to Achieve Practical Reversible Hydrogen Storage for Transportation

We propose a new generation of materials to maximize reversible ${\mathrm{H}}_2$ storage at room temperature and modest pressures ($<20\mathrm{bars}$). We test these materials using grand canonical Monte Carlo simulations with a first-principles-derived force field and find that the Li pillared graphene sheet system can take up 6.5 mass% of ${\mathrm{H}}_2$ (a density of $62.9\mathrm{kg}/{\mathrm{m}}^3$ at 20 bars and room temperature. This satisfies the DOE (Department of Energy) target of hydrogen-storage materials for transportation. We also suggest ways to synthesize these systems. In addition we show that Li-doped pillared single-wall nanotubes can lead to a hydrogen-storage capacity of 6.0 mass% and $61.7\mathrm{kg}/{\mathrm{m}}^3$ at 50 bars and room temperature storage, which is close to the DOE target.

Figure 1

Dependence of the mass hydrogen storage capacity on the interlayer (intertube) distances under various pressures for (a) PGS (white) and Li-GIC or Li-PGS (black); and (b) (10,10) SWNTs (white) and Li-P-SWNT (black) systems. The doping concentration is $\mathrm{Li}:\mathrm{C}=1:6$. The pressures are square $=$ 50 bars; circle $=$ 10 bars, and triangle $=$ 1 bar.

Figure 2

(a) Effects of Li-doping concentration on the mass hydrogen storage capacity under various pressures. $\mathrm{ILD}=10\AA$. (b) Binding energy per Li atom under various Li-doping concentrations. Keys in (b) square $=$ equilibrium interlayer distance; circle $=$ interlayer distance $8\AA$; triangle $=$ interlayer distance $10\AA$. The zero energy reference responds to pure graphite crystal and pure Li metal.

Figure 3

Optimization of nanostructures of (a) Li-PGS and (b) Li-P-SWNT systems for mass (white) and volumetric (black) hydrogen storage capacities. The Li-doping concentrations are $\mathrm{Li}:\mathrm{C}=1:3$. The DOE target is shown by a line. The optimum interlayer or intertube distance is indicated by an arrow. Key: square $=$ 50 bars; circle $=$ 20 bars, and triangle $=$ 10 bars.

Figure 4

Temperature and pressure (unit: bar) effects on the mass hydrogen storage capacity. (a) Li-PGS: $\mathrm{Li}:\mathrm{C}=1:3$ and $\mathrm{ILD}=10\AA$; (b) Li-P-SWNT: $\mathrm{Li}:\mathrm{C}=1:3$ and $\mathrm{ITD}=9\AA$.

Figure 5

A scheme to synthesize the Li-doped pillared graphene or nanotubes of high Li-doping concentrations and large interlayer distances.