New Pathways and Metrics for Enhanced, Reversible Hydrogen Storage in Boron-Doped Carbon Nanospaces

Progress Report and Future Directions Carbon-based materials have recently shown promise for hydrogen storage at moderate pressures. Our group has focused on the development of materials derived from synthetic precursors in order to optimize, measure and control pore geometries at the sub-nm scale; and to enhance the adsorption of H2 (particularly at low pressure, high temperature) by increasing the depth of the H2-carbon potential by chemically functionalizing the adsorbent’s surface. Our group has spearheaded the effort to improve these materials and in previous grant periods have reported: (i) record-breaking H2 storage in very high-surface area activated carbons; (ii) ab initio theoretical predictions that boron doping of carbon at 5-10% B:C concentration raises the H2 binding energy from ~5 kJ/mol to 10-14 kJ/mol; (iii) Grand Canonical Monte Carlo (GCMC) simulations that successfully reproduce the experimental adsorption of H2 in heterogeneous pore structures, and that demonstrated enhanced H2 storage in B-doped carbon; (iv) demonstrated experimentally the existence of B-C bonds in B-doped carbon; (v) demonstrated experimentally that in B-doped carbons the isosteric heat of adsorption nearly doubles from 5-7 kJ/mol to 9-12 kJ/mol, this is accompanied by an enhancement of the H2 sorption at cryogenic and room temperature; (vi) demonstrated that activated carbon from synthetic precursors have a nearly monodisperse network of narrow pores; (vii) developed pore characterization methods based on small-angle X-ray scattering (SAXS); and (vii) developed the theoretical background to utilize incoherent inelastic neutron scattering (IINS) off adsorbed H2 to characterize the interaction potentials as seen by molecular H2 in sub-nm pores. In what follows we present the most relevant results attained in the current reporting period. a) Observation of anomalous adsorption of H2 in synthetic carbon: significantly higher excess adsorptions normalized per Brunauer-Emmett-Teller (BET) surface area at both cryogenic and room temperature (e.g., at toom temperature synthetic sample HS;0B has more than doubles the performance of our best lignocellulose carbon, sample 3K). This indicates higher binding energies (consistent with a narrower pores). In addition, synthetic carbons show anomalous excess adsorption isotherms, with the maximum of the excess adsorption occurring at higher than normal pressures. See Figure 1. b) Determination of adsorbed film characteristics: we succeeded in measuring important adsorbed film characteristics (excess and absolute adsorption, isosteric heat of adsorption, film thickness and volume, saturation density) by using an integrated combination of experimental methods (N2 characterization, He “picnometry”, H2 adsorption isotherms at cryogenic and room temperature up to 200 bar, SAXS), data processing (extrapolation to calculate saturated IV.H.7 New Pathways and Metrics for Enhanced, Reversible Hydrogen Storage in Boron-Doped Carbon Nanospaces Pfeifer – University of Missouri IV.H Hydrogen Storage / Basic Energy Sciences IV–234 DOE Hydrogen and Fuel Cells Program FY 2012 Annual Progress Report film density), theoretical modeling (isosteric heats from Clausius-Clapeyron eq., absolute adsorption, film thickness from monotonicity of isosteric heat), and computational efforts (GCMC). Film densities are significantly in excess of the density of liquid H2. See Figure 2. c) Design and construction of suband super-critical H2 Sievert instrument: we have built a Sievert sorption instrument to be used for sub-critical and super-critical H2 adsorption (temperature range: 4-300+ K). This will permit determination of BET surface areas, and pore and skeletal volumes using H2 rather than N2; and permit a more precise determination of adsorbed film densities, especially in synthetic precursors that have maximum of the excess adsorption at anomalously high pressures. d) High resolution transmission electron microscopy: Aberration corrected scanning transmission electron microscopy performed at the Oak Ridge National Laboratory (ORNL) Center for Nanophase Materials Sciences show detailed atomic structure of synthetic carbon: these have regions of graphitic and amorphous carbon consistent with 700 m/g BET surface areas. See Figure 2. e) IINS: we conducted experiments at ORNL over an unprecedented broad range of energy and momentum transfer. We developed a novel theoretical methodology that permitted the classification of the H2 excitations into localized and mobile states. This provides a measure of the planarity of the adsorption surface on the >1 nm scale, and gives insight on the quantum states of adsorbed H2. f) Pore conformability: we have performed a mechanical analysis of the stability of pores in carbon, this indicates that pores with lateral (in plane) dimensions larger than 2-4 nm would naturally collapse and close, consistent with our SAXS experiments. More interestingly, we observe that pores that are above this “critical length” may be partially opened by H2 at P >20-30 bar. Interestingly, this model results in excess adsorption isotherms that do not show a clear maximum at low and moderate pressures. Figure 1. Pore size distribution (left), and H2 excess adsorption per unit area at 80 K (center) and 303 K (right) for synthetic carbon HS;0B (ΣBET = 900 m /g) and lignocellulose carbon 3K (ΣBET = 2,600 m /g). Figure 2. Determination of saturated film densities from extrapolation of the excess adsorption (left). Determination of film thicknesses (and volume) from the thermodynamic requirement that the isosteric heat of adsorption is a monotonically decreasing function of coverage (center). At the lower bound for film thickness, tfilm = 4.1 Å, these results are consistent with the adsorption values (Figure 1). High resolution transmission electron microscopy of synthetic carbon HS;0B (right).