N-alkylated fluorographene: a metal-free electrocatalyst for the hydrogen evolution reaction

Designing graphene-based metal-free electrocatalysts is crucial due to their reduced costs, tunable molecular structures, superior electronic conductivity and substantial tolerance under fluctuating pH conditions. In this report, we have covalently functionalized fluoride graphite with n-octyl amine (OA), n-dodecyl amine (DDA), and n-octadecyl amine (ODA) via a facile nucleophilic substitution reaction to prepare three different N-alkylated fluorographene materials, OA-FGr, DDA-FGr, and ODA-FGr, respectively. Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) analysis, and scanning electron microscopy (SEM) have collectively confirmed their successful formation by providing information about their functionality, stacking, chemical interactions, morphology and textural properties. Moreover, electron paramagnetic resonance (EPR) spectroscopy has been used to verify not only the functionalization but also the electrocatalytic properties. The synthesized materials have been utilized to prepare working electrodes to record linear sweep voltammetry (LSV) curves under acidic conditions (0.5 M H₂SO₄) and in an inert atmosphere at a scan rate of 5 mV s^-1. ODA-FGr has demonstrated superior performance to its counterparts (OA-FGr and DDA-FGr), with an overpotential (η) of 530 mV at 10 mA cm^-2 and a Tafel slope of 80 mV dec^-1. Moreover, ODA-FGr has shown excellent durability, stability under accelerated test conditions (a scan rate of 100 mV s^-1 and 2000 cycles) and reproducibility. Notably, the positive role of the alkyl chain length of amine molecules and their covalent functionalization is observed in improving HER performance and is found to be related to their amphiphilic nature and fluorine-to-nitrogen (F/N) ratio.