Ferrocene-functionalized covalent organic framework exceeding the ultimate hydrogen storage targets: a first-principles multiscale computational study

Abstract

The development of efficient hydrogen storage materials is crucial for advancing the hydrogen economy and meeting the U.S. Department of Energy's targets of 6.5 wt% and 50 g H₂/L for automotive applications. We present a computational study of ferrocene-functionalized covalent organic frameworks (COFs) for hydrogen storage. Following the <b>M</b>ulti-binding <b>S</b>ites <b>U</b>nited in <b>C</b>ovalent-<b>O</b>rganic <b>F</b>ramework (MSUCOF) approach, we introduce MSUCOF-4-FeCp, designed by incorporating ferrocene (FeCp₂) moieties into IRCOF-102. Notably, it achieves exceptional performance with gravimetric and volumetric uptakes of 18.0 wt% and 72.6 g H₂/L at 298 K and 700 bar. The material exhibits optimal binding energies (15-20 kJ/mol) ensuring both high storage capacity and deliverable hydrogen under practical conditions. This work establishes ferrocene functionalization as a cost-effective alternative to precious metal incorporation in COFs.

Figures (12)

• Figure 1: Leading MOF/COF hydrogen storage performanceKlontzasGuomendoza-cortes_covalent_2012pramudya_design_2016Xiao-Dong_2016Xiao-Dong_2019GhoshDjokic2024 compared to U.S. Department of Energy (DOE) light-duty vehicle (LDV) hydrogen storage targets.doe_hydrogen_targets_2020 Total hydrogen uptakes reported at 298 K and up to 100 bar are shown as: (a) volumetric uptake (g L$^{-1}$) versus publication year, (b) gravimetric uptake (wt%) versus publication year, and (c) volumetric versus gravimetric uptake, illustrating the trade-off between storage density metrics. In (c), the DOE Ultimate LDV target corresponds to the upper-right quadrant (dark blue). Circles ($\circ$) represent literature-reported materials, $\times$ denotes our group’s prior MSUCOF study, and $+$ indicates results from the present work; identical colors indicate data points originating from the same publication.
• Figure 2: Schematic illustration of framework synthesis and post-synthetic metallation. (a, top left) Representative metallocene formation reaction, FeCl + Cp + NaCp -> NaCl + FeCp + HCl. (b, top right) Boroxine condensation of tetrakis(4$'$-borono-[1,1$'$-biphenyl]-4-yl)methane (Linker 1) to form IRCOF-102. (c, bottom) Boroxine ring formation of tetrakis(4-(4-boronoinden-7-yl)phenyl)methane (Linker 2) to yield MSUCOF-4, a modified variant of IRCOF-102 in which biphenyl units are replaced by indene moieties; the cyclopentadienyl (Cp) portion of the indene linker is highlighted in light blue. Subsequent post-synthetic metallocene formation affords the metalated framework MSUCOF-4-FeCp. Atom colours: Fe (orange), B (pink), O (red), and C (black).
• Figure 3: Validation of the QM-fitted force field for ferrocene--H2 interactions. Hc denotes hydrogen configuration number. FeCp labels correspond to eclipsed ferrocene (as observed in MSUCOF-4-FeCp), while stagFeCp denotes staggered conformers.
• Figure 4: Electronic structure of (a) MSUCOF-4-FeCp and (b) IRCOF-102, showing band structure (left) and density of states (right). Both materials exhibit flat bands typical of porous materials. Functionalization in MSUCOF-4-FeCp narrows the band gap, enhancing the potential for electronic or catalytic applications.
• Figure 5: Comparison of total gravimetric (a,b) and volumetric (c,d) uptake at 298 K for the MSUCOF-4 family against top-performing MSUCOF variants. Log-log plots (a,c) provide detail in the low-pressure region, while linear plots (b,d) show high-pressure behavior. MSUCOF-4-FeCp demonstrates superior performance across all pressure ranges.