Biomimetic Fe₇S₈/Carbon electrocatalyst from [FeFe]-Hydrogenase for improving pH-Universal electrocatalytic hydrogen production

2.1 Structural characterization

The Fe₇S₈/C samples were prepared by a facile wet-chemical method followed by calcination (Figure 1A). The X-ray diffraction (XRD) pattern of the prepared Fe₇S₈/C nanoparticles annealed at each temperature shows well-defined and high crystallinity peaks, which correspond to the standard JCPDS No. 98-015-1766 pattern of pyrrhotite 4C Fe₇S₈ (Figure 1B). The XRD pattern of the sample annealed at a low temperature of 400°C showed a broad peak due to the amorphous structure of the sample conjugated by the accretion of polydopamine. The crystallinity of the sample increases corresponding to the annealing temperature due to the contraction of the carbon layer. The XRD result confirms the formation of stable pyrrhotite 4C Fe₇S₈/C nanoparticles throughout the annealing process. The minor XRD peaks arising at 800°C annealing temperature can be due to the recrystallization of Fe and S samples as insignificant impure phases of monoclinic FeS (JCPDS: 98-008-7500), as shown in Figure 1A. The samples prepared at 600°C do not exhibit such insignificant impurity peaks which indicate the recrystallization of impure FeS phases at temperatures higher than 700°C. X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical valence states for the prepared Fe₇S₈/C electrocatalyst. The deconvoluted Fe 2p spectra in Figure S1A reveal two peaks for Fe²⁺ and Fe³⁺ at 711 and 713 eV for Fe 2p_3/2, and at 724 and 726 eV for Fe 2p_1/2, respectively. Figure S1B for S 2p spectra exhibits two peaks around 164 eV and 165.2 eV for S 2p_3/2 and S 2p_1/2, respectively. An additional peak at 168.8 eV corresponding to S-O bonds arises due to the passivation of the electrocatalysts with air exposure. Besides, the C 1s spectra, shown in Figure S1C, reveal four major peaks at 284 eV, 285.2 eV, 286 eV, and 288 eV corresponding to sp² bonded C=C bonds, C–C bonds, passivated C–O bonds, and the carboxyl groups (O–C=O) in the prepared electrocatalysts. O 1s spectra in Figure S1D depict three signals at 530 eV, 531.5 eV, and 532.4 eV corresponding to adsorbed OH, C–O–C bonds, and carboxyl groups, respectively.

The morphological characterization of the prepared Fe₇S₈/C@600 was identified using scanning electron microscopy (SEM) analysis. The obtained SEM image reveals a uniform distribution of Fe₇S₈/C spherical nanoparticles embedded within a three-dimensional porous carbonized polydopamine matrix (Figure 2A). This arrangement enhances mass transport and augments the catalytic surface area. The interconnected porous network facilitates increased electrolyte ion access to electroactive sites, improving the catalyst's active surface area and consequently enhancing its electrocatalytic activity. The inset of Figure 1A shows the high-magnification SEM image of Fe₇S₈/C. Substantially, the transmission electron microscopy (TEM) image in Figure 2B reveals network-like clusters of Fe₇S₈/C spherical nanoparticles similar to the SEM image. The HRTEM images in Figure 2C reveal the interface of carbon structures and crystalline Fe₇S₈ structures. The inset of Figure 2C shows well-defined lattice fringes indicating the interplanar spacing of about 0.2 nm corresponding to (322) lattice planes of Fe₇S₈ supported by the FFT image also reveals the bright spot for (322) lattice planes in the reciprocal lattice as observed from XRD analysis. In Figure 2D, the selected area electron diffraction (SAED) image shows dot diffraction patterns implying the high crystallinity nature of the prepared Fe₇S₈/C@600 sample.

Energy-dispersive X-ray spectroscopy (EDS) analysis was conducted to assess the chemical composition of the prepared Fe₇S₈/C@600 sample, as shown in Figure 2E–J. Elemental mapping images show the co-existence of the Fe, S, N, and C in the Fe₇S₈/C@600 sample. Therefore, the structural and morphological analysis explicates the phase pure formation of interconnected network-like Fe₇S₈/C spherical nanoparticles, which can enhance electrocatalytic activity.

2.2 Electrocatalytic activity

The electrocatalytic activity of Fe₇S₈/C electrocatalysts was characterized by the linear sweep voltammetry (LSV) technique using a rotating disk electrode in a three-electrode system. To optimize the annealing temperature, which enhances HER activity, LSV was conducted for the samples prepared at different annealing temperatures (400–800°C), as shown in Figure 3A. Comprehensively, the LSV curve of the sample annealed at 600°C showed the largest positive shift toward the equilibrium potential of 0 V (vs. reversible hydrogen electrode [RHE]) compared to the samples annealed at 400, 500, 700, and 800°C.

Here, the Fe₇S₈/C catalyst annealed at 600°C, with its optimized amorphous-carbon interface, exhibits superior electrocatalytic hydrogen production compared to other annealing temperatures for Fe₇S₈/C. Consequently, the Fe₇S₈ sample annealed at 600°C strikes an optimal balance in terms of crystallinity, indicating the presence of an amorphous carbon interface.^[28] Thus, the Fe₇S₈/C annealed at 600°C demonstrates heightened electrocatalytic performance for the HER due to its mixed crystalline-amorphous phases, surpassing samples annealed at lower and higher temperatures. Further, to estimate the optimal synthetic condition, the LSV curves of the Fe₇S₈/C catalyst prepared by varying the molar ratio of Fe precursor from 2 to 16 mM were obtained (Figure 3B). The sample prepared from 8 mM Fe precursor showed improved electrocatalytic hydrogen production. Additionally, the Fe₇S₈/C@600 electrocatalysts were distinguished from exhibiting superior HER performance across all pH conditions, and commercial Pt/C samples as shown in Figure 3C. In addition, the electrochemical HER performances were performed by mixing the Fe₇S₈ samples prepared in the absence of dopamine with carbon black (Fe₇S₈/CB). The Fe₇S₈/CB samples reveal higher electrocatalytic activity than Fe₇S₈ without C indicating significant conductivity effects of carbon coating (Figure 3C). However, the Fe₇S₈/C samples reveal higher HER activity than Fe₇S₈/CB due to the significant electrocatalytic effects of N doped C matrix. The results reveal higher HER electrocatalytic activity under various pH conditions for Fe₇S₈/C compared to Fe₇S₈ without C and Fe₇S₈/CB electrocatalysts, and commercial Pt/C under neutral pH conditions.

Under all pH conditions, the current density associated with the HER exponentially increased when the potential was swept from 0.15 to −0.3 V vs. RHE. In addition, iron sulfide nanoparticles without dopamine additives were also tested to investigate the dopamine-derived polymerization effect. The overpotential necessitated by the Fe₇S₈/C was significantly lower than that of the pristine Fe₇S₈ sample prepared without carbon (η = 263 mV), substantiating the role of carbon in promoting improved charge transfer and catalytic activity. Additionally, the Fe₇S₈/C catalyst was observed to be highly active under the neutral (η = 45.9 mV vs. RHE), base (η = 107.4 mV vs. RHE), and acid condition (η = 90.6 mV vs. RHE). To our knowledge, the value of overpotential (η) is the lowest among the reported iron sulfur-based electrocatalysts (Tables S1–S3).

The results reveal higher electrocatalytic activity for Fe₇S₈/C prepared in the presence of dopamine with C and N elements. Due to different ionic components, acidic and alkaline electrolytes exhibit different non-Faradaic reaction mechanisms. The latest literature reported that the Volmer reaction (H₃O⁺ + e⁻ → H_ads + H₂O in acidic / H₂O + e⁻ → H_ads + OH⁻ in alkaline) barrier was higher in acidic conditions rather than alkaline conditions.^[29] Therefore, the reactant adsorption on the catalytic surface easily occurs in alkaline electrolytes, resulting in lower overpotential in the non-Faradaic region. However, the Faradaic reaction, including the Heyrovsky reaction and Tafel reaction (which occurred only in acidic conditions), occurred in acidic conditions rather than in alkaline conditions. For these reasons, lower overpotential in pH = 14 at low current density while at high current density, Fe₇S₈ shows lower overpotential in pH = 0. The turnover frequency (TOF) values in Figure S2 indicate the rate of charge transfer reactions at the electrode surface for hydrogen evolution. Notably, the Fe₇S₈/C electrocatalysts exhibit significantly higher TOF values in neutral electrolytes across a range of HER overpotentials compared to their performance in acidic or alkaline electrolytes.

To further examine the electrocatalytic reaction of the Fe₇S₈/C catalyst, electrochemical impedance spectroscopy (EIS) analysis was conducted at HER condition (−0.1 V vs. RHE). The Nyquist plot from the EIS measurement is displayed in Figure 4A.

Compared to pristine Fe₇S₈ without carbon (733 Ω cm²), the Fe₇S₈/C sample showed a much smaller semicircle, corresponding to lower charge transfer resistance (R_ct) values of 33.2 Ω cm² at pH 0, 24.8 Ω cm² at pH 7, and 37.5 Ω cm² at pH 14, respectively (Figure 4B). The charge transfer resistance obtained from the fitted equivalent circuit is closely related to kinetic barrier energy for faradaic reactions (HER) at the interface between electrocatalysts and the electrolyte. The charge transfer resistance is also inversely proportional to the exchange current for the faradaic reaction. Therefore, the lower charge transfer resistance implies the enhanced HER performance with a lower overpotential. The electrochemical surface area analysis (ECSA) infers the higher active surface available for charge transfer reactions over prepared Fe₇S₈/C electrocatalysts. The scan rate versus current plots were derived from the cyclic voltammetry (CV) curves obtained for various scan rates in the non-Faradic region, as shown in Figure S3A–C. The double layer capacitance (C_dl) values for the prepared Fe₇S₈/C samples annealed at 500, 600, and 700°C were estimated to be 0.45, 1.31, and 0.62 mF, respectively (Figure S3D). The corresponding ECSA values of Fe₇S₈/C samples annealed at 500, 600, and 700°C are 11.25, 32.75, and 15.5 cm², respectively. The higher ECSA values for Fe₇S₈/C annealed at 600°C indicate highly exposed active sites for electrocatalytic reactions compared to samples annealed at 500 and 700°C.

The g value of the catalyst was calculated by 71.4484 v/B (mT). In the resting state, the Fe³⁺ and carbon radical signals were at g values of 4.24 and 2.00, respectively. After CA testing at the potential of −0.05 V vs. RHE, the Fe¹⁺ signal was detected at a g value of 2.082 (B = 3308 G) in the vicinity of the carbon radical signal (g = 2.00 and B = 3444 G), while the intensity of the Fe³⁺ signal had slightly decreased (g = 4.24 and B = 1624 G) and the carbon radical signal had increased. When the potential was increased to −0.1 V vs. RHE, the Fe¹⁺ signal further increased and the Fe³⁺ signal further decreased, as shown in Figure 5A. These results suggest that iron species are involved during the HER and that the iron valency changes from 3+ to 1+. This behavior can be found in the iron-sulfur cluster in natural hydrogenase, called [FeFe]-hydrogenase, which is a well-known hydrogen-evolving catalytic reaction mechanism.^{[35, 36]} Such enzymes have a unique organometallic cofactor, called the H-cluster, composed of a canonical [4Fe-4S] cluster, as shown in Figure 5B. This structural arrangement makes the active site a blocked Lewis pair system, prompting efficient and reversible heterolytic H⁺/H⁻ coupling. In the [FeFe]-hydrogenase, [Fe-Fe] center with Fe-S clusters as active sites can catalyze the reduction of protons with extremely high efficiency in some anaerobic microorganisms.^[37]

During redox reactions under the influence of electrolytic ions and applied potentials, Fe atoms can undergo significant changes in their valence states. These reduction performances are pivotal in inducing a shift from Fe²⁺ to Fe¹⁺ valence states, ultimately enhancing electrocatalytic activity.^[38] It is worth noting that Leonard et al. have observed this Fe²⁺/Fe¹⁺ valence state change through operando XAS studies during CO₂ reduction conducted on porous Fe-nitrogen-carbon (Fe-N-C) materials.^[39] Thus, the presence of lower valence states (2+/1+) in Fe is suggested to occur during electrochemical reduction processes.

Figure S4A–C shows the long-term stability tests for the Fe₇S₈/C samples under neutral, acidic, and alkaline pH conditions which reveal negligible differences after 12-hour tests. The XPS analysis in Figure S5A shows that under acidic and neutral conditions negligible changes were noted to the characteristic peaks of Fe 2p relating to the as-prepared sample. However, under alkaline conditions, Fe 2p spectra revealed a partially oxidized state compared to as prepared samples. The small positive shift can be attributed to the partially adsorbed intermediates during HER activity. Besides, the S 2p spectra reveal almost similar peak intensity for S-O, S 2p_3/2, and S 2p_1/2 for the samples after the stability test under acidic and neutral pH conditions, as shown in Figure S5B. The S 2p spectra for alkaline pH conditions show decreased peak intensity for S 2p_3/2 and S 2p_1/2. Hence, the alkaline pH condition for Fe₇S₈/C annealed at 600°C reveals relatively higher chemical oxidation state changes than acidic or neutral electrolytes. However, the XRD results in Figure S5 after the stability test under acidic, alkaline, and neutral pH conditions reveal insignificant changes to the crystal phase of Fe₇S₈. The XRD analysis confirms that the bulk Fe₇S₈ did not show any structural variations and the strong amorphous peak around 24° corresponds to the carbon cloth substrate. Besides, morphological analysis by FESEM after stability tests under different pH conditions has revealed insignificant changes due to the partially adsorbed reaction intermediates (Figure S6A–D).

During the HER process, the oxidation state of Fe is partially changed with H⁺ adsorption.^[63] From the effective [FeFe]-hydrogenase system, we designed the biomimetic Fe₇S₈/C electrocatalyst and revealed superior HER performance compared to state-of-the-art transition metal-based electrocatalysts under all pH conditions. As shown in Figure 6, our Fe₇S₈/C electrocatalyst showed a lower overpotential than other reported transition metal-based HER electrocatalysts.