Mechanistic Understanding of the Interactions and Pseudocapacitance of Multi-Electron Redox Organic Molecules Sandwiched between MXene Layers

Using a combined theoretical and experimental approach, a mechanistic
understanding of the interactions and pseudocapacitance of different quinone-
coupled viologen and pyridiniumium molecules sandwiched between
titanium carbide (Ti3C2Tx) MXene layers has been provided. Three different
derivatives of quinone-coupled viologen and pyridiniumium are synthesized
using nucleophilic substitution reaction and subsequently hybridized with
Ti3C2Tx MXene (organic@Ti3C2Tx) using self-assembly approach. The atomic
structure of pristine Ti3C2Tx and organic@Ti3C2Tx hybrid films is investigated
using grazing incidence X-ray diffraction and X-ray pair distribution function
analysis using synchrotron radiation. Spectroscopic results confirm the
coupling of quinones with viologen and pyridiniumium molecules and their
non-covalent functionalization to the MXene without their catalytic decomposition.
First-principles calculations confirm that the preferred orientation of
organic molecules upon intercalation/adsorption is horizontal to the Ti3C2Tx
surface. The authors reveal that these molecules attach to the Ti3C2Tx surface
with a significantly high binding energy (up to -2.77 eV) via a charge transfer
mechanism. The electronic structure calculations show that all organic@
Ti3C2Tx hybrids preserved their metallic behavior. Free-standing organic@
Ti3C2Tx hybrid films show a more than three times higher capacitance at ultrahigh
scan rates (up to 20 V s-1) compared to their pristine counterpart due
to molecular pillaring of organic molecules between Ti3C2Tx layers via strong
binding energies and charge transfer.