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

Abstract

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.

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.

1. Introduction

Climate change is the harsh reality of our planet, mainly triggered by the accumulation of greenhouse gases generated through the hysterical use of fossil fuels to satisfy the requirement for energy. A boost in the production of clean energy through green processes could be a significant alternative to curb harmful carbon emissions into the atmosphere. In this context, H₂ is a promising source of energy owing to its high energy density, zero-emission nature, versatility, and facile integration with the existing gas infrastructure.¹ It is naturally available and called “geologic hydrogen”, produced through the serpentinization process under the Earth's crust.² It can also be generated through various methods, such as water reduction,³ methane pyrolysis,⁴ biomass reforming,⁵ and thermochemical⁶ and steam-methane reforming.⁷ The reduction of water to produce H₂ is known as the hydrogen evolution reaction (HER) and can be done either via photocatalytic^8,9 or electrocatalytic processes.^10,11 This method is considered renewable and one of the most appropriate, utilizing only water as a source and yielding no harmful byproducts. The electrochemical HER involves the combination of two protons and two electrons to generate H₂ gas at the cathode under an applied potential. This reaction is not as simple as it appears and necessitates a higher potential than the actual thermodynamic potential to surpass the kinetic barrier. The required additional potential is known as the overpotential, and its lower value is anticipated to produce H₂ at a high rate. Accordingly, continuous efforts have been made to develop robust and efficient electrocatalysts involving various metals,^12–14 nanomaterials,^15,16 and their heterostructures,^17,18 focusing on minimizing the overpotential. In addition to high activity, the cost, stability, and ease of synthesis are also deciding factors for the applicability of the developed electrochemical method at the industrial level. In this context, 2D graphene derivatives (graphene oxide, reduced graphene oxide, and fluorographene) are excellent candidates due to their high resistance to acidic and basic conditions, high specific surface area, and tunable electronic structure.^19–21 Moreover, the scope of their surface functionalization with various organic molecules,^22,23 such as amines,²⁴ polymeric porphyrins,^25,26 covalent organic frameworks,²⁷ and fullerenes,²⁸ is enormous to prepare competent electrocatalysts for the HER. Hence, they are also a promising choice for commercial operations because of their comparatively easy synthesis from the naturally abundant graphite material. Generally, physical and chemical functionalization approaches are applied, and both have their own significance, utilizing the collective virtues of the supported organic molecules and support material. However, chemical functionalization is more desirable because it offers key benefits such as improved stability and faster electron transfer reactions owing to covalent bonding.²⁹

Fluorographene (FlGr) is a comparatively underutilized graphene derivative owing to its high exposed surface area and great stability against temperature and chemicals.³⁰ It comprises highly electronegative F atoms attached to carbon atoms via covalent, ionic, and semi-ionic bonds, and their abundance decides its electronic properties.³¹ Importantly, the availability of F atoms allows facile substitution reactions with potential nucleophiles, establishing covalent bonding and paving the way for the attachment of diverse molecular functionalities.³² In the last few years, fluorographene has been used to fabricate high-performance metal-free electrocatalytic materials^31,33 by covalently attaching different organic molecules.^28,34 Primary amines are good nucleophiles, and their electron-donating nature can vary, depending on the alkyl chain length. Consequently, the covalent attachment of amines with varied alkyl chain lengths to form a π-conjugated system, such as graphene derivatives, would modify their electron density and amphiphilic nature, which in turn can influence the electrochemical HER accordingly.^35,36 With this in mind, we have synthesized N-alkylated fluorographene materials through the chemical covalent functionalization of fluorographene using three different long-chain n-amines [octylamine (OA), dodecyl amine (DDA), and octadecyl amine (ODA)] to study the effect of their carbon chain length and covalent linkage on the electrochemical activity for the HER in an acidic medium.

2. Experimental

2.1. Materials and reagents

Fluoride graphite and octadecyl amine were purchased from Sigma Aldrich. Octylamine and dodecyl amine were purchased from Thermo Scientific. Dimethylformamide and ethanol solvents were procured from VWR International. Nafion was supplied by Ion-Power Inc. All materials were used as received without further purification. Deionized water was used for material synthesis, washing and electrochemical studies.

2.2. Instruments

FTIR spectra were measured using a Jasco FT/IR-4700 spectrometer (Jasco, Easton, MD, USA) in the range of 400–4000 cm⁻¹ using the attenuated total reflection (ATR) technique on a diamond plate. X-ray powder diffraction patterns were measured using a MiniFlex600 (Rigaku Corporation, Tokyo, Japan) equipped with the Bragg–Brentano geometry and iron-filtered Cu K_α1,2 radiation (λ = 1.54056 Å). The angular range of measurements was set as 2θ = 10°–80°. N₂ adsorption–desorption isotherms were measured at 77 K using the Autosorb iQ adsorption analyzer from Quantachrome Instruments, Anton Paar, which used a static volumetric technique. Prior to measurements, each sample was outgassed for 12 hours at 150 °C and subsequently for 12 h at room temperature. The BET areas of the prepared samples were computed according to the multipoint Brunauer–Emmett–Teller (BET) model, with the range of BET adsorption points established following Rouquerol's criteria. The pore size distribution was derived from the N₂ adsorption–desorption isotherms using the nonlocal density functional theory (NLDFT) model in the Autosorb software. A Raman microscope (DXR, Thermo Fisher Scientific Inc., USA) was used under the following conditions: 532-nm laser, 900 lines/mm grating, spectrograph aperture: 50-µm pinhole, and 10×/0.25 objective. The measurement parameters were as follows: laser power = 9.0 mW (or 0.1 mW), exposure time = 2.00 s, number of exposures = 150, and number of background exposures = 150. A scanning electron microscope (SEM, VEGA3 LMU, TESCAN, Brno, Czech Republic) was used to examine the morphological features of the samples. This device was equipped with a LaB6 cathode and an Everhart–Thornley detector. The elemental composition was examined by energy-dispersive spectroscopy (EDS) with an XFlash silicon drift detector (410-M, Bruker Nano GmbH, Berlin, Germany). The EDS spectra were quantified using the PB-ZAF correction model implemented in Bruker ESPRIT 1.9 software. The software applied a semiempirical least-squares fitting procedure as well as matrix corrections for the atomic number, absorption, and fluorescence effects. The uncertainty of the quantitative EDS results was estimated using the counting statistics provided by the ESPRIT software, which were considered within the typical limits for this analytical method. The samples were dispersed in ethanol using a laboratory sonicator (Badelin Sonorex Digitec, Badelin Electronic, Berlin, Germany) for 1 h. Further, the dispersion was dropped onto a 1-cm² Si wafer and dried at 40 °C in air. The EDS measurements were performed at various acceleration voltages; 10 kV and 7 kV were used for EDS mapping. An X-band electron paramagnetic resonance (EPR) spectrometer (SPINSCAN X, LINEV Systems) was used to measure the samples in the solid state under a modulation frequency of 100 000 Hz and a modulation field of 100 µT. XPS spectra were measured using an AXIS Supra X-ray photoelectron spectrometer (Kratos Analytical, Manchester, UK). The emission current used to capture the resulting information was set at 5 mA for wide-scan spectra and 10 mA for detailed elemental spectra. During the process, a resolution of 80 was utilized for the measurement of the wide-scan spectra and 20 for the elemental spectra. The sample was dispersed in isopropanol (IPA) and sonicated for 15 minutes, and one drop was placed on a copper grid with a Lacey carbon film. After drying in air at room temperature, the samples were measured by a high-resolution transmission electron microscope (HRTEM, Titan G2 60–300, FEI) with an image corrector (CETCOR, CEOS) at an accelerating voltage of 300 kV. Images were taken using an UltraScan 1000XP CCD camera (Gatan).

2.3. Synthesis of N-alkylated fluorographene

200 mg of fluoride graphite and 40 mL of DMF were taken in a 100-mL flask. The corresponding mixture was sonicated for 80 min using an ultrasonic probe sonicator (70% power) and for 1 h using an ultrasonic bath sonicator. During sonication, the temperature was maintained near 30 °C using an ice bath. Subsequently, 2.5 mL of octyl amine (OA) was added to the same mixture and further sonicated for 1 h using an ultrasonic bath sonicator. A separate solution was prepared in a 20-mL beaker by dissolving 550 mg of K₂CO₃ in 10 mL of water, and it was subsequently added to the prepared dispersion of fluorographene and OA. The resulting mixture was heated under continuous stirring at 110 °C for 48 hours. Later, the reaction mixture was cooled to room temperature and centrifuged to collect the black solid material. The obtained black solid material was then thoroughly washed with DMF, water, and ethanol, vacuum-dried at 50 °C and labeled as OA-FGr. The same method was followed to synthesize other functionalized materials, namely, DDA-FGr and ODA-FGr, by taking equimolecular amounts of dodecyl amine (DDA) and octadecyl amine (ODA), respectively, instead of octyl amine (OA). Both sonication and the subsequent reaction under the mentioned reaction conditions might have resulted in the transformation of fluoride graphite to functionalized fluorographene in the presence of the respective amine molecules and K₂CO₃.

2.4. Electrochemical study

2.4.1. Instrument and electrode fabrication

The HER activity of all the N-alkylated fluorographene nanosheets was evaluated using an electrochemical workstation (CHI 600C, CH Instruments Inc). A standard three-electrode cell was constructed, consisting of a glassy carbon electrode (GCE) with a 3 mm diameter, Ag/AgCl (3 M KCl/saturated AgCl separated with a diaphragm), and a platinum coil as the working electrode, reference electrode, and counter electrode, respectively. Firstly, the GCE was polished using an alumina slurry (0.05 mm) and then sonicated in DI water for 10 min to remove any possible contamination. Subsequently, 5 mg of the respective N-alkylated fluorographene material was mixed with 100 µL of Nafion (5 wt%, alcohol-based) and 400 µL of isopropanol, and the resulting mixture was sonicated in a bath sonicator for 30 min to prepare the slurry. 10 µL of this slurry was drop-cast on the polished surface of the GCE to fabricate the working electrode. The catalyst loading was calculated to be ∼1.4 mg cm⁻². The working electrode was pre-activated by performing at least 20 cycles at a scan rate of 50 mV s⁻¹ in the potential window from 0 to −1.2 V.

2.4.2. Cyclic voltammetry measurements

Cyclic voltammetry and linear sweep voltammetry (LSV) measurements were conducted at room temperature in an acidic electrolyte (0.5 M H₂SO₄) under an inert (argon) atmosphere. LSV curves were recorded at a 5 mV s⁻¹ scan rate and calibrated with respect to a reversible hydrogen electrode (RHE) using the following equation:where E_(Ag/AgCl) and are the calculated and standard electrode potentials, respectively. and pH = 0.

No iR compensation was applied due to the negligible change in the overpotential. Such a behavior can be observed in the case of high-concentration electrolytes, e.g., 0.5 M H₂SO₄, due to their highly conductive nature. The electrochemical stability of the electrode materials for the HER was evaluated by recording an LSV curve after performing 2000 cycles at a scan rate of 100 mV s⁻¹ (accelerated stability test conditions). Tafel plots were derived from the corresponding linear sweep voltammograms by fitting the linear portion to the Tafel equation as follows:η = a + b log |j|,where η, a, b, and j are the overpotential, intercept, Tafel slope, and current density, respectively.

The electrochemically active surface area (ECSA) was calculated by determining the double-layer capacitance (C_dl) values from the cyclic voltammograms recorded in the non-faradaic region at different scan rates in the range of 25–200 mV s⁻¹ (Fig. S7). The C_dl was measured by plotting the average current ((i_a + i_c)/2) against the scan rate (Fig. S8). Subsequently, the C_dl value was divided by the specific capacitance (C_s) of the electrode to calculate the ECSA as follows:ECSA = C_dl/C_s,where C_s = 0.035 mF cm⁻².

The electrochemical impedance spectroscopy (EIS) plots of all the materials were measured in the frequency range from 0.1 Hz to 100 kHz with an AC amplitude of 0.01 V. Accordingly, an equivalent electrical circuit was drawn.

3. Results and discussion

3.1. Synthesis and characterization

We synthesized N-alkylated fluorographene using n-amines of varying chain lengths (C = 8, 12, and 18) via a substitution reaction with the intention to modulate the charge transfer and electron density, which in turn can improve HER activity. The synthesis process included the sonication of fluoride graphite in DMF, followed by the addition of the respective long-chain n-amine. Subsequently, potassium carbonate and water were added to the resulting solution and then heated at 110 °C under continuous stirring for 48 hours (Fig. 1).

Fig. 1. Schematic of the preparation of N-alkylated fluorographene.

In this reaction, the terminal amino groups of OA, DDA, and ODA interacted and replaced the F atoms on the surface of FlGr, forming a covalent –NH– linkage to yield the N-alkylated fluorographene materials OA-FGr, DDA-FGr, and ODA-FGr, respectively. All synthesized materials were separated, washed through centrifugation, dried under a vacuum, and characterized using XPS, FTIR spectroscopy, XRD, Raman spectroscopy, BET analysis, and SEM. The FTIR spectra of FlGr and the N-alkylated fluorographene materials (OA-FGr, DDA-FGr, and ODA-FGr) are depicted in Fig. 2a. FlGr exhibits a strong band at around 1206 cm⁻¹ and a weaker band at 1326 cm⁻¹, corresponding to the stretching vibrations of covalent C–F bonds and peripheral C–F₂ bonds, respectively. By contrast, OA-FGr, DDA-FGr, and ODA-FGr materials display some new peaks related to the respective covalently functionalized amine molecules. The strong bands centered around 750 cm⁻¹ can be assigned to the characteristic rocking vibration of CH₂ groups in long-chain alkylated amines. The two promising peaks in the range of 2910–2852 cm⁻¹ are due to the C–H stretching vibration of alkyl groups in the amines. The N–H and C–H bending vibrations of amines can be confirmed by the peaks at around 1543 and 1454 cm⁻¹, respectively. The formation of the C–N bond, due to the covalent attachment of long-chain amine molecules onto the surface of fluorographene, can be attributed to the existence of new bands between 900 and 1250 cm⁻¹.³⁷ Moreover, the presence of residual F atoms can be confirmed by the existence of the C–F stretching at 1206 cm⁻¹ in all the materials. Notably, the relative intensity of the C–F stretching is found to increase when using longer-chain amines, indicating a lower degree of F-substitution. This could be due to the higher steric hindrance in the case of bulky amine molecules. The XRD patterns of FlGr and N-alkylated fluorographene are presented in Fig. 2b. In the XRD diffractogram of FlGr, one highly intense peak and two broad peaks appear at 2θ values of 12.7°, 25.81°, and 41.04°, respectively. The intense peak at 12.56° corresponds to the (001) reflection, signifying the fluorine intercalation between the graphite layers. However, the less-intense peak at 25.81° corresponds to defluorinated graphite. The reflection at 41.04° indicates the (100) in-plane hexagonal lattice, and the broadening can be due to some defects. However, these peaks are not observed in OA-FGr, DDA-FGr, and ODA-FGr materials, but multiple new sharp peaks appear, implying a significant enhancement in the crystallinity of all the materials. This could be due to the following reasons: (1) restoration of the planarity of graphitic nanosheets or (2) an increase in the interlayer D-spacing due to the orderly functionalization of long-chain amines.³⁸ Importantly, the crystalline nature of the functionalized materials can also participate in improving their electrocatalytic performance by enabling high electrical conductivity and durability.³⁹ Raman spectroscopy can effectively investigate the degree of graphitic and non-graphitic regions in the N-alkylated fluorographene by examining the nature of D and G bands. These bands appear between 1320–1370 and 1550–1600 cm⁻¹ due to the availability of sp³ and sp² carbon, respectively. FlGr does not show any well-resolved peak but does show two very broad humps in the region from 1100 to 3300 cm⁻¹, as it is a Raman-silent material due to a high content of C–F bonding (Fig. 2c).⁴⁰ However, two distinct peaks, i.e., D and G bands, appear after amine functionalization for all three materials, indicating a severe structural change. The D bands in the first region at around 1345, 1351, and 1345 cm⁻¹ and the G bands in the second region at around 1592 cm⁻¹, 1592 cm⁻¹, and 1587 cm⁻¹ for OA-FGr, DDA-FGr, and ODA-FGr, respectively, arise due to the defluorination and attachment of amine molecules, generating graphitic and non-graphitic regions, respectively. The intensity of D bands appears to be higher than that of G bands for all the materials, confirming a comparatively lower graphitic nature. Moreover, their intensity ratios (I_D/I_G) are calculated to be 1.05, 1.03, and 1.04 for OA-FGr, DDA-FGr, and ODA-FGr, respectively, implying a higher degree of functionalization.

Fig. 2. (a) FTIR spectra, (b) XRD patterns, (c) Raman spectra and (d) N₂ adsorption–desorption isotherms of N-alkylated fluorographene materials.

BET measurements were conducted to observe the porous nature and surface area of all functionalized materials (Fig. 2d). The shape of the N₂ adsorption–desorption isotherms of N-alkylated fluorographene materials is observed to be similar to that of typical graphene materials, with no indication of microporosity. The BET surface areas are calculated to be 25, 28, and 35 m² g⁻¹ for OA-FGr, DDA-FGr, and ODA-FGr, respectively, with a hierarchical pore size distribution in the range of 2–30 nm (Fig. S1). This could be due to the attachment of the respective amine molecules onto the surface. Electron paramagnetic resonance (EPR) spectroscopy is a powerful tool that requires unpaired electrons to construct an EPR signal, and it can characterize defects and electronic structures in graphene derivatives. Generally, the active sites in metal-free electrocatalysts originate from vacancies, functional groups, edges, and heteroatom dopants, which directly influence their performance. EPR parameters can help to reveal these features by detecting the spin density, radical type, and dynamics. Therefore, the synthesized N-alkylated fluorographene materials were also characterized by the EPR technique (Fig. 3a). FlGr shows a very weak EPR signal, as it could be an EPR-silent material like other fluoropolymers (Fig. S2a).⁴¹ However, strong EPR signals are acquired for the functionalized materials, indicating a substantial structural change, which could be because of the covalent attachment of longer alkyl-chain amines. Moreover, the g-factors of alkyl chain amine-functionalized materials are in the range of g ≈ 2.004–2.005, signifying a delocalized state with influence from neighboring heteroatoms such as O and N, thus indicating successful covalent functionalization.⁴² The surface morphology and microstructure of FlGr and N-alkylated fluorographene materials were scrutinized using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). Based on the SEM images, FlGr, OA-FGr, DDA-FGr, and ODA-FGr materials seem to have similar morphologies with crumpled, irregularly shaped, and stacked nanosheets (Fig. S3). Unfortunately, no distinct morphological features are observed due to the limitations of the SEM technique for these materials. However, the charging profile of the FlGr sample is observed to be different from that of the N-alkylated fluorographene materials. The bright section in the SEM images of FlGr indicates the charged region due to the low-conductive surface, resulting in blemished and distorted images. By contrast, the SEM images of OA-FGr, DDA-FGr, and ODA-FGr look comparatively sharp and with good resolution. Based on these results, we can presume that the conductivity of the FlGr material might have been enhanced significantly after the n-amine functionalization.

Fig. 3. (a) EPR spectra. High-resolution C 1s (b), N 1s (c) and F 1s (d) XPS spectra of ODA-FGr. (e) and (f) HRTEM images of ODA-FGr at different resolutions.

Moreover, the SEM-EDS technique was used to determine the elemental composition and exclude the possibility of transition metals in all the samples (Fig. S4, S5, and S6). EDS data clearly identify the presence of C, F, and N elements, but there is no sign of transition metals, indicating the successful attachment of amine molecules and the synthesis of metal-free N-alkylated fluorographene materials. The presence of other unwelcomed elements, such as K, Cl, Si, and O, could be due to synthesis and analysis constraints. XPS analysis was conducted to examine the chemical interactions and elemental compositions of the functionalized materials (Fig. 3 and S7–9). The XPS survey scan of FlGr mainly shows peaks related to the C, O and F elements at 284.8, 527.8 and 683.7 eV, respectively. However, after functionalization, the emergence of an N peak at 399.1 eV in the survey scans of OA-FGr, DDA-FGr and ODA-FGr indicates the successful functionalization of alkyl amine molecules onto the fluorographene nanosheets. Moreover, the XPS elemental quantification of all the required elements (C, N, O, and F) in the respective materials was performed to quantify the substitution level in the form of the F/N ratio (Table S1). Based on the results, the F/N ratio is found to decrease with increasing alkyl chain length of the attached amine molecules. The F/N ratios are calculated to be 5.1, 1.4 and 1.1 for OA-FGr, DDA-FGr and ODA-FGr, respectively, thus suggesting a direct relation between the F/N ratio and HER activity. Subsequently, the high-resolution XPS spectra of C 1s, N 1s, and F 1s were deconvoluted to observe the types of chemical bonding of each element (Fig. 3, S8 and S9). The deconvoluted C 1s spectrum of FlGr shows five peaks positioned at 284.5, 286.3, 288.2, 289.6 and 291.2 eV, corresponding to C–C, C–O, C–F, C–F2, and C–F3, respectively.⁴³ After functionalization, the C–F3 peak disappears completely from all the materials, indicating that most of the alkyl amine molecules attach to the edges of FlGr, irrespective of their chain length. However, the peak corresponding to C–F2 is only observed in OA-FGr and not in DDA-FGr and ODA-FGr. Concurrently, the intensity of the peaks corresponding to C–C increases in all the functionalized materials. These results indicate that alkyl amine molecules are preferably attached to the edges of FlGr, replacing the peripheral F molecules. Moreover, the C–F peaks are observed in all the materials, indicating irreplaceable F, most probably on the surface. A similar phenomenon is observed in the high-resolution F 1s XPS spectra of all the functionalized materials, except for the appearance of peaks related to ionic F. The intensity of the ionic F peak is higher than that of the covalent F–C peak in DDA-FGr but lower than that in OA-FGr and ODA-FGr, suggesting that an optimum ratio of ionic and covalently bonded F is necessary for the best activity. Importantly, the high-resolution N1s XPS spectra of all the materials clearly show a single peak at 398.9 eV, confirming the successful covalent attachment of the alkyl amine molecule to the surface of FlGr, replacing the F atom and forming the –NH– bond.⁴⁴ This covalent attachment can facilitate the electron transfer between nanosheets and functionalized amine molecules and thus can improve the HER activity. The HRTEM images of ODA-FGr were also acquired to observe its morphological features (Fig. 3e, f and S10). The material appears to be stacked 2D nanosheets with a mostly smooth surface. The edges of the nanosheets stacked onto one another are clearly visible (pointed by the yellow arrow) and can also be observed by contrast variation. Hence, all characterization techniques have collectively confirmed the successful formation of the respective N-alkylated fluorographene.

3.2. Electrochemical study

The application of metal-free electrocatalysts for the HER is not as widely reported as that of metal-based electrocatalysts. Subsequently, the electrochemical performance of the synthesized OA-FGr, DDA-FGr, and ODA-FGr materials was examined for the HER in an acidic electrolyte (0.5 M H₂SO₄) at room temperature under an inert atmosphere using a standard three-electrode system. Moreover, a comparison was made based on the results obtained with respect to FlGr deposited on a glassy carbon electrode (GCE) to assess the effectiveness of all the functionalized materials. The polarization curves of the as-prepared materials were recorded by linear sweep voltammetry (LSV) at a scan rate of 5 mV s⁻¹. Based on the LSV curve, a significant enhancement in the current density was recorded for all the synthesized materials by applying a negative bias in contrast to the FlGr-deposited GCE electrode (Fig. 4a). These results signify the active role of long-chain alkyl amines covalently attached to the surface of fluorographene nanosheets in improving the current at the applied voltage. This could be due to the generation of a higher number of desirable, easily accessible active sites after functionalization. The overpotential (η) and Tafel plots are important parameters to assess the effectiveness of the electrocatalysts for a highly efficient HER. A lower overpotential is desirable at a given current density to improve the energy conversion efficiency. FlGr, OA-FGr, DDA-FGr, and ODA-FGr recorded overpotentials of −702, −649, −559, and −530 mV vs. RHE at 10 mA cm⁻², respectively. Electrochemical impedance spectroscopy (EIS) was utilized to determine the charge transfer resistance (R_ct). Notably, the semicircular radius of all the functionalized materials was found to decrease after functionalization and with increasing alkyl chain length of the functionalized amine molecules (Fig. 4e). Accordingly, ODA-FGr had the smallest semicircular radius, thus owning better electrode conductivity and lower charge transfer resistance. Moreover, Tafel slopes are important for studying the charge transfer, reaction kinetics, and type of catalyst deposited, which are evaluated by their nature and values. A smaller Tafel slope value suggests that a high current density can be attained at a lower overpotential. Subsequently, Tafel slopes were measured for FlGr, OA-FGr, DDA-FGr, and ODA-FGr by linear fitting within the kinetic region, and the values were calculated to be 204, 140, 104, and 80 mV dec⁻¹, respectively (Fig. 4b). Notably, ODA-FGr recorded comparatively lower overpotential and Tafel values than the other functionalized materials (FlGr, OA-FGr, and DDA-FGr), implying faster HER kinetics.⁴⁵ Moreover, the Tafel slope can provide information about the possible reaction mechanism: Volmer (H⁺ + e⁻ + active site → H_ads), Heyrovsky reaction (H_ads + H⁺ + e⁻ → H₂) and Tafel (H_ads + H_ads → H₂). The Tafel slope values between 80 and 140 mV dec⁻¹ for ODA-FGr, DDA-FGr, and OA-FGr suggest a Volmer–Heyrovský reaction mechanism. In the acidic medium, the hydrophilic amino groups can act as trapping sites, accumulating protons and forming –NH₂⁺– to facilitate the initial Volmer step.⁴⁶ By contrast, the hydrophobic alkyl chain can accelerate the desorption of the hydrogen bubble in the Heyrovsky step through the recombination of adsorbed hydrogen with a proton.^36,47 The electrochemical surface areas (ECSAs) of all the materials were also calculated to further realize the intrinsic electrocatalytic activity. A high value of the ECSA corresponds to a high number of active sites accessible to the electrolyte, which can lead to enhanced performance. Firstly, the double-layer capacitance (C_dl) values were measured with the help of cyclic voltammograms (CVs) recorded for the respective N-alkylated fluorographene in the non-faradaic region at different scan rates in the range of 25–200 mV s⁻¹ (Fig. S11a–d). The current measured in this region is attributed to the charging and discharging of the electrochemical double-layer, avoiding faradaic processes (such as H-adsorption/desorption). It ensures that the measured current is predominantly capacitive, thus allowing the “accurate” calculation of C_dl. The C_dl was measured by plotting the average current ((i_a + i_c)/2) against the scan rate (Fig. S12).

Fig. 4. (a) LSV curves in 0.5 M H₂SO₄ at a scan rate of 5 mV s⁻¹ and (b) corresponding Tafel plots. (c) LSV curves and (d) long-term stability of the ODA-FGr electrocatalyst. (e) Electrochemical impedance spectroscopy (EIS) plots with circuit diagram.

Subsequently, the C_dl value was divided by the specific capacitance (C_s) of the electrode to calculate the ECSA (Table S2). Because C_dl is directly proportional to the ECSA, a higher C_dl value indicates a higher surface area accessible to electrolyte ions. The C_dl and ECSA values for FlGr, OA-FGr, DDA-FGr, and ODA-FGr were calculated to be 7.51, 8.61, 9.01, and 11.89 mF and 214, 246, 257, and 339 cm², respectively (Table S2). Moreover, the ECSA values were used to calculate the turnover frequencies (TOFs) (see the SI). The TOF values for FlGr, OA-FGr, DDA-FGr, and ODA-FGr were calculated to be 0.014, 0.021, 0.045, and 0.061 s⁻¹, respectively (Table S2). Based on these results, an activity trend is observed following the carbon chain length of attached amine molecules. Fluorographene functionalized with amine molecules with longer alkyl chains demonstrated higher HER activity in terms of the overpotential, EIS, Tafel slope, ECSA, and TOF values than that functionalized with amines with shorter alkyl chains, following the activity order of ODA-FGr > DDA-FGr > OA-FGr. These results clearly validate the influence of the carbon chain length in improving the HER activity of the respective fluorographene materials. These results are further supported by the EPR data, which demonstrate a lower g-factor value (g ≈ 2.004) for DDA-FGr and ODA-FGr, indicating a higher number of defects (Fig. 3a).⁴⁸ However, the comparatively high asymmetric nature of the EPR signal of ODA-FGr implied higher conductivity due to the lower resistance to the mobile electrons.⁴⁹ It might be due to the modification of the electron density of the graphene surface due to the attachment of longer alkyl-chain amines with a higher electron-donating nature. Notably, EIS data confirmed the decrease in the internal charge resistance in the case of the functionalized materials. Such adjustments can not only improve the electron transfer but also facilitate proton adsorption at neighboring active sites.⁵⁰ Consequently, such changes might have improved the proton adsorption at the active sites, electron transfer, and H₂ desorption, resulting in better HER performance. Moreover, another apparent reason behind the improved HER activity could be the amphiphilic nature of the alkyl amine-functionalized fluorographene nanosheets. The regulation of the hydrophobicity/hydrophilicity of an electrocatalyst can improve the mass transport of protons, stability and active site utilization by reducing the bubble-induced overpotential, preventing the blockage of active sites and maintaining a stable solid–liquid–gas interface.^36,47 Alkyl amines are amphiphilic molecules containing a hydrophilic head and a hydrophobic tail. Their covalent attachment might have created a microenvironment of hydrophilic amino groups and hydrophobic hydrocarbon chains. Hydrophilic sites facilitate water contact, ion transportation, and reaction kinetics. By contrast, hydrophobic sites attract gas and repel water, thus preventing flooding and promoting gas bubble release. Therefore, an intrinsically balanced amphiphilic electrocatalyst is ideal for ensuring continuous activity and reduced overpotentials. In order to investigate the effect of wettability on HER activity, we measured the contact angles of all the materials by the Sessile drop method using ImageJ software installed with the low-bond axisymmetric drop shape analysis (LB-ADSA) plugin (see the SI, Fig. S13). The contact angles were measured to be 147.63°, 152.64° and 156.73° for OA-FGr, DDA-FGr, and ODA-FGr, respectively, showing an evident relation with the alkyl chain length of functionalized amine molecules. Hence, ODA-FGr was deemed to be an optimum metal-free electrocatalyst for further assessments. Subsequently, the significance of the covalent functionalization approach was investigated by measuring the overpotential for covalently bonded ODA-FGr and a physical mixture of FlGr and ODA (FlGr + ODA) at the same current density (Fig. 4c). Based on the LSV data, the physical mixture of FlGr + ODA showed a significantly higher overpotential of −733 mV than ODA-FGr at a current density of 10 mA cm⁻², confirming the superiority of the covalent attachment of organic molecules in achieving high HER performance. This enhancement could be due to the faster electron transfer between fluorographene nanosheets and functionalized amine molecules through the covalent linkage, improving the efficacy of active sites. Based on experimental results and the existing literature, the plausible active sites could be the neighboring carbon atoms on the graphitic surface of the functionalized amine molecules.⁵⁰ Accordingly, we can presume that the –NH– linkage on the catalyst surface protonates to form the secondary ammonium ion (*FGr–NH₂⁺–) in the acidic medium (eqn (1)). *FGr–NH₂⁺– can facilitate the Volmer step by effortlessly providing the proton to form the adsorbed hydrogen intermediate (H_ads –*FGr–NH₂⁺–) by receiving an electron (eqn (2)).⁴⁶ In the Heyrovsky step, H_ads–*FGr–NH₂⁺– instantly combines with another proton provided by the local environment to produce the H₂ gas, thus leaving the active site available for further adsorption (eqn (3)). Importantly, the desorption of H₂ can also be influenced by the amphiphilic nature of the electrocatalyst, as discussed previously.^36,47

*FGr–NH– + H₃O⁺ ⇌ *FGr–NH₂⁺– + H₂O 1

2

3

Furthermore, the stability of the electrode materials is paramount for their potential commercial applications. An electrocatalyst's stability can be defined in terms of short-term cycling stability and true long-term durability. The former indicates the electrocatalyst's robustness against structural degradation under fluctuating potentials and is generally measured via CV. By contrast, true long-term durability indicates the capability of the electrocatalyst to maintain a steady current or potential over an extended period under harsh acidic conditions and is calculated using chronoamperometry (CA) or chronopotentiometry (CP). The short-term cycling stability of ODA-FGr was evaluated by recording the LSV curve to measure the rise in the value of the overpotential before and after performing a higher number of CV scans. The tested material demonstrated a higher current density and only a minor rise of 17 mV at 10 mA cm⁻² in overpotential value under accelerated stability test conditions (100 mV s⁻¹ for 2000 cycles), indicating its robust and active nature (Fig. S14). Moreover, long-term durability was examined by measuring the current under a constant potential of 0.56 V for a period of 105 000 seconds (∼29 h) (Fig. 4d). The electrocatalyst retained 81.25% of its current density, indicating good durability. Reproducibility is also an important factor in determining the efficacy of a catalyst. Accordingly, a reproducibility study was conducted by fabricating four working electrodes and measuring their LSV curves (Fig. S15). All four electrodes, named electrode-1, electrode-2, electrode-3, and electrode-4 fabricated using ODA-FGr materials, showed consistent activity, showing overpotentials of 533, 537, 538 and 555 mV at 10 mA cm⁻², respectively. Finally, the intactness of the active sites of the used catalysts was also verified by measuring FTIR spectra. The used catalyst showed similar features to the fresh catalyst in terms of peak positions, indicating its robust nature (Fig. S16). However, a true comparison of the synthesized N-alkylated fluorographene materials with the reported electrocatalysts is not reasonable due to their different catalytic materials, electrochemical reaction conditions, ease of electrocatalyst synthesis, and cost. However, a table listing recently reported electrocatalysts for the HER, including metal-based electrocatalysts, has been incorporated for general comparison (Table S3). Importantly, the overpotential and Tafel values and stability of ODA-FGr are found to be superior and even comparable to those of some metal-based electrocatalysts.

4. Conclusion

The successful covalent attachment of three different long-chain alkyl amines (OA, DDA, and ODA) on the surface of fluorographene via a comparatively safe and simple reaction was achieved. Based on the chemical and structural characterization data, F atoms on the surface were replaced with the respective alkyl amines, forming the –NH– linkage and restoring the π-conjugated network. The amine functionalization process enhanced the intrinsic electrocatalytic activity of the functionalized materials by introducing additional active centers and modifying their charge density and amphiphilic nature. ODA-FGr demonstrated better HER activity than FlGr, OA-FGr, DDA-FGr and the physical mixture of FlGr and ODA in an acidic medium. Moreover, it showed high stability and good reproducibility. These results support our hypothesis about the role of longer alkyl chains, covalent bonding, and the amphiphilic nature of the materials in enhancing HER activity, and thus, they can be crucial in designing future electrocatalysts.

Author contributions

S: conceptualization, methodology, investigation, validation and data curation and writing – original draft; SL: formal analysis (BET measurement); MK: formal analysis (SEM-EDX); BS: formal analysis (Raman measurement); KK: formal analysis (EPR analysis); DS: formal analysis (XPS analysis); PK: writing – review and editing and supervision; and SK: writing – review and editing, supervision and project administration.

Conflicts of interest

There are no conflicts of interest to declare.

Data availability

Data will be made available upon request.

Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d6ra03157a.