Synthesis and characterization of boron substituted carbon deposits on PFA-derived carbon substrates for hydrogen adsorption

The effect of boron substituted carbon (BC_x, x ∼ 3-5) coatings on the hydrogen adsorption properties of porous carbons was investigated via spectroscopic characterization and hydrogen adsorption measurements. Thin films of BC_x were synthesized by reacting BCl₃ and benzene in a chemical vapor and then depositing the product as a solid (CVD). The BC_x deposits were collected on microporous carbon substrates and mesoporous silica aerogel. High-resolution solid-state boron NMR, ¹¹B HR-NMR, detected boron atoms in a symmetric chemical environment with trigonal coplanar coordination. Results from analyses based on NMR, X-ray photoelectron spectroscopy (XPS), and d-spacings, calculated from electron diffraction and X-ray diffraction (XRD) patterns, indicated that 17 at.% boron was substitutionally incorporated into the carbon framework. Thus the material has an empirical formula of BC_4.9. Hydrogen adsorption data were collected at 100 bar and 25 °C. The BC_x coatings reduced the surface area of the porous substrates by 30-50%. However, at the same time, they increased the heat of adsorption and the adsorption capacities per unit area by as much as a factor of five. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) revealed wagging energies at 1190 cm^-1 attributable to hydrogen interactions with C-B-C bonds.