Laser-Processed 2D Germanane on Graphene for Organohydrogel-Based Zinc-Ion Hybrid Capacitors

Group 14 monoelemental two-dimensional (2D) materials beyond graphene, such as silicene and germanene, have gained significant attention in the scientific community. Covalent functionalization of germanene with hydrogen and methyl leads to germanane (hydrogen/methyl-terminated germanene; HGe/MGe). While the optical and electronic properties of HGe and MGe were explored previously, there is no report on their zinc ion storage electrochemistry. Though the layered HGe/MGe sheets have tunable interlayer spacing, which cushions the volume expansion during ion storage, their inferior electrical conductivity limits the charge transfer kinetics. Herein, we demonstrate a single-step, facile approach for in situ decoration of 2D HGe/MGe sheets over laser-induced graphene (LIG) using a pulsed laser and examine their morphological, chemical, and electrochemical (EC) characteristics. The HGe/MGe-decorated LIG is tested as a cathode for a zinc ion hybrid capacitor (ZHC) in an aqueous electrolyte and polyacrylamide organohydrogel to unveil the selective sites for zinc ion electrochemistry by experimental and theoretical aspects. This ZHC design enables a notable Zn²⁺ storage capacity (104 F g^-1 @ 0.25 A g^-1 in aqueous electrolyte) for HGe-decorated LIG, whereas MGe-decorated LIG records impressive cyclic stability (capacity retention 83% after 12000 cycles). Density functional theory calculations elucidate favorable adsorption of Zn at MGe and HGe networks. These findings summarize the applicability of 2D functionalized germanane, which has the potential to expand by numerous alkyl chains and terminal groups for targeted energy storage applications.