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

Efficient and cost‐effective electrocatalysts that can operate across a wide range of pH conditions are essential for green hydrogen production. Inspired by biological systems, Fe7S8 nanoparticles incorporated on polydopamine matrix electrocatalyst were synthesized by co‐precipitation and annealing process. The resulting Fe7S8/C electrocatalyst possesses a three‐dimensional structure and exhibits enhanced electrocatalytic performance for hydrogen production across various pH conditions. Notably, the Fe7S8/C electrocatalyst demonstrates exceptional activity, achieving low overpotentials of 90.6, 45.9, and 107.4 mV in acidic, neutral, and alkaline environments, respectively. Electrochemical impedance spectroscopy reveals that Fe7S8/C exhibits the lowest charge transfer resistance under neutral conditions, indicating an improved proton‐coupled electron transfer process. Continuous‐wave electron paramagnetic resonance results confirm a change in the valence state of Fe from 3+ to 1+ during the hydrogen evolution reaction (HER). These findings closely resemble the behavior of natural [FeFe]‐hydrogenase, known for its superior hydrogen production in neutral conditions. The remarkable performance of our Fe7S8/C electrocatalyst opens up new possibilities for utilizing bioinspired materials as catalysts for the HER.


INTRODUCTION
Water electrolysis to produce hydrogen has been widely studied as an environmental and sustainable energy technology. [1]n the real case, electrochemical hydrogen evolution reaction (HER) required overpotential to overcome the activation energies. [2]For minimizing input energy, the development of electrocatalysts has been considered a crucial factor in its commercialization. [3]Likewise, the grid scale of hydrogen production requires the development of highly efficient and robust catalysts. [4]So far, platinum-based catalysts have been conventionally used as HER catalysts for their high activity and durability. [5]However, due to its intrinsic scarcity in nature and high cost, the development of efficient earth-abundant metal-based catalysts is necessary for practical electrochemical water splitting. [6]Recently, transition metal-based oxide, [7,8] phosphide, [9,10] and sulfide [11][12][13] have been extensively studied for HER catalysts.Metal sulfide catalysts have garnered significant attention as they aim to mimic the active sites of hydrogenase enzymes. [14]Hydrogenase enzymes, present in bacteria, archaea, and certain eukaryotes, exhibit an impressive turnover frequency of approximately 10,000 s −1 for proton reduction reactions. [15]This exceptional activity surpasses that of synthetic catalysts, approaching the thermodynamic efficiency limit.The active site of hydrogenase consists of an iron-sulfur cluster surrounded by functional proteins. [14,15]he structural analysis of hydrogenase active sites has inspired the development of organometallic chemistry methods for synthesizing artificial hydrogenase systems. [16,17][23] Previous studies have demonstrated that the pyrite structure exhibits the highest hydrogen evolution activity among the iron sulfide materials, primarily attributed to a high sulfur-to-iron ratio. [18]However, these Fe-S approaches alone have exhibited low stability and activity in HER. [24]][27] To address these challenges, recent advancements have focused on heteroatom doping or carbon coating techniques, which have shown remarkable improvements in the efficiency and stability of inorganic iron-sulfur electrocatalysts for the HER.
Herein, we designed a carbon matrix-incorporated ironsulfur cluster (Fe 7 S 8 /C) inspired by the structure of hydrogenase to enhance the stability and activity of iron sulfur-based catalysts.Similar to hydrogenase, protein assemblies surround the iron-sulfur metal site and serve as an effective matrix for facilitating electron charge transfer during electrocatalytic reactions.To stabilize the iron sulfide nanoparticles, we utilized polydopamine molecules as a matrix.Polydopamine, structurally resembling natural melanin (eumelanin), contains various functional groups such as catechol, hydroxyl, amine, and imine.Catechol compounds, including DOPA, can covalently or non-covalently attach to diverse surfaces, resembling the adhesive proteins found in mussels.The Fe 7 S 8 /C electrocatalyst prepared in this study exhibits low overpotentials of 90.6, 45.9, and 107.4 mV in acidic, neutral, and alkaline conditions, respectively.Notably, the iron species undergoes a change in oxidation state to monovalent during the HER, a catalytic mechanism similar to that of hydrogenase.In hydrogenase, a carbonyl or cyanide group adjacent to the active iron site induces low valency in a low spin state, promoting efficient hydrogen production.Similarly, the presence of monovalent iron species in the Fe 7 S 8 /C catalyst enhances its electrocatalytic activities toward HER.This significant improvement achieved in our work highlights the potential of biomimetic materials as efficient electrocatalysts for HER across all pH conditions.

Structural characterization
The Fe 7 S 8 /C samples were prepared by a facile wet-chemical method followed by calcination (Figure 1A).The X-ray diffraction (XRD) pattern of the prepared Fe 7 S 8 /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 Fe 7 S 8 (Figure 1B).The XRD pattern of the sample annealed at a low temperature of 400 The morphological characterization of the prepared Fe 7 S 8 /C@600 was identified using scanning electron microscopy (SEM) analysis.The obtained SEM image reveals a uniform distribution of Fe 7 S 8 /C spherical nanoparticles embedded within a three-dimensional porous carbonized polydopamine matrix (Figure 2A).This arrangement enhances mass transport and augments the catalytic surface shows well-defined lattice fringes indicating the interplanar spacing of about 0.2 nm corresponding to (322) lattice planes of Fe 7 S 8 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 Fe 7 S 8 /C@600 sample.
Energy-dispersive X-ray spectroscopy (EDS) analysis was conducted to assess the chemical composition of the prepared Fe 7 S 8 /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 Fe 7 S 8 /C@600 sample.Therefore, the structural and morphological analysis explicates the phase pure formation of interconnected network-like Fe 7 S 8 /C spherical nanoparticles, which can enhance electrocatalytic activity.

Electrocatalytic activity
The electrocatalytic activity of Fe 7 S 8 /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 Fe 7 S 8 /C catalyst annealed at 600 • C, with its optimized amorphous-carbon interface, exhibits superior electrocatalytic hydrogen production compared to other annealing temperatures for Fe 7 S 8 /C.Consequently, the Fe 7 S 8 sample annealed at 600 • C strikes an optimal balance in terms of crystallinity, indicating the presence of an amorphous carbon interface. [28]Thus, the Fe 7 S 8 /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 Fe 7 S 8 /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 Fe 7 S 8 /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 Fe 7 S 8 samples prepared in the absence of dopamine with carbon black (Fe 7 S 8 /CB).The Fe 7 S 8 /CB samples reveal higher electrocatalytic activity than Fe 7 S 8 without C indicating significant conductivity effects of carbon coating (Figure 3C).However, the Fe 7 S 8 /C samples reveal higher HER activity than Fe 7 S 8 /CB due to the significant electrocatalytic effects of N doped C matrix.The results reveal higher HER electrocatalytic activity under various pH conditions for Fe 7 S 8 /C compared to Fe 7 S 8 without C and Fe 7 S 8 /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 Fe 7 S 8 /C was significantly lower than that of the pristine Fe 7 S 8 sample prepared without carbon (η = 263 mV), substantiating the role of carbon in promoting improved charge transfer and catalytic activity.Additionally, the Fe 7 S 8 /C catalyst was observed to be highly active under the neutral (η = 45.9 mV vs. RHE), base (η = 107.4mV 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 Fe 7 S 8 /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 (H 3 O + + e − → H ads + H 2 O in acidic / H 2 O + 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, Fe 7 S 8 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 Fe 7 S 8 /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 evaluate the HER mechanism in more detail, pH dependency measurements were also performed at wide pH ranges from 0 to 14 in 0.5 increments, as shown in Figure 3D.From the pH dependency test, an average overpotential is calculated by 87.3 ± 20 mV vs. RHE and a partial derivative of potential with respect to pH, ( E pH ) j was calculated to be 60.3 mV/pH (Figure 3F).The LSV curves were converted into a plot of potentials as a function of the logarithm of J; this plot is called a Tafel plot and is also expressed as a partial derivative of potential with respect to logarithm current density, ( E logj ) pH .The Tafel plot shows an average Tafel slope of 69.2 ± 18 mV/decade (Figure 3E).Proton activity dependence on current density is derived as: Substituting the value of ( a nearly first-order dependence on proton activity.As a result, the electrochemical law of the Fe 7 S 8 /C catalyst is followed as: where, k 0 , a H + , m, α, E, F, R, and T are the potentialindependent constant, proton-activity, ( , transfer coefficient, potential, Faraday constant, gas constant, and temperature, respectively.Although the electrocatalytic hydrogen production process has not yet been fully verified, one electron and one proton-involved reaction appear to exist as a quasi-equilibrium step, followed by the rate-determining step (RDS). [30]he inherent catalytic activity, such as the Tafel slope and pH dependency, provide information on the rate-limiting step in the HER.The mechanism for HER is generally depicted in two ways.In an acidic solution, hydronium ion reduction occurs with the following reaction steps: where H ads is the adsorbed H atom.In the experimental condition, the value of the Tafel slope relates to the adsorbed hydrogen coverage (θ H ) on the surface of the electrode.If the electrochemical desorption step were the RDS, a Tafel slope of 118-40 mV/dec should measure as a variant of θ H between 0 and 1.The observed Tafel slope of 69.2 mV/dec in the current work indicates that the Heyrovsky reaction instead determines the kinetics of the HER on Fe 7 S 8 /C and that the extent of coverage adsorbed hydrogen (θ H ) on the surface of the catalytic structure is favorable for the HER. [31,32]o further examine the electrocatalytic reaction of the Fe 7 S 8 /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 Fe 7 S 8 without carbon (733 Ω cm 2 ), the Fe 7 S 8 /C sample showed a much smaller semicircle, corresponding to lower charge transfer resistance (R ct ) values of 33.2 Ω cm 2 at pH 0, 24.8 Ω cm 2 at pH 7, and 37.5 Ω cm 2 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 Fe 7 S 8 /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 Fe 7 S 8 /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 Fe 7 S 8 /C samples annealed at 500, 600, and 700 • C are 11.25, 32.75, and 15.5 cm 2 , respectively.The higher ECSA values for Fe 7 S 8 /C annealed at 600 • C indicate highly exposed active sites for electrocatalytic reactions compared to samples annealed at 500 and 700 • C.
To monitor the change of Fe oxidation state during the hydrogen evolution catalysis, continuous-wave electron paramagnetic resonance (CW-EPR) analysis was conducted by varying the applied potentials (Figure 5A).Chronoamperometry (CA) at −0.05 V and −0.1 V versus RHE potentials was performed for about 5mins.After the electrolysis, we collected the samples and then quenched them immediately.Using the conversion equation [33,34] : The g value of the catalyst was calculated by 71.4484 v/B (mT).In the resting state, the Fe 3+ 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 Fe 1+ 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 Fe 3+ 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 Fe 1+ signal further increased and the Fe 3+ 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 hydrogenevolving 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]uring 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 Fe 2+ to Fe 1+ valence states, ultimately enhancing electrocatalytic activity. [38]It is worth noting that Leonard et al. have observed this Fe 2+ /Fe 1+ valence state change through operando XAS studies during CO 2 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 Fe 7 S 8 /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 Fe 7 S 8 /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 Fe 7 S 8 .The XRD analysis confirms that the bulk Fe 7 S 8 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 Fe 7 S 8 /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 Fe 7 S 8 /C electrocatalyst showed a lower overpotential than other reported transition metal-based HER electrocatalysts.

CONCLUSION
In summary, we developed the highly effective HER electrocatalyst of Fe 7 S 8 /C from the co-precipitation technique.This work highlights the bio-mimetic electrocatalyst of Fe 7 S 8 /C, The comparison of hydrogen evolution reaction (HER) performance of Fe 7 S 8 /C electrocatalyst with state-of-the-art transition metal-based electrocatalysts under all pH conditions.  which d enhance the HER activities under all pH conditions.Such an iron-sulfur cluster surrounded by a carbon matrix provided iron species in a low spin state, and this effect promotes hydrogen production by inducing iron active sites like natural hydrogenase.Fe 7 S 8 /C was optimized by controlling the annealing temperature of the sulfur-polydopamine complex, the molecular concentration of iron precursors, and the pH condition of the electrolyte.Among these various samples, the 8 mM Fe 7 S 8 /C annealed at 600 • C sample exhibited the lowest overpotential of 45.9 mV in neutral pH conditions.In addition, the Fe 7 S 8 /C also showed a low overpotential of 90.6 and 107.4 mV in acidic and alkaline conditions, respectively.The EIS results showed the charge transfer resistance of the Fe 7 S 8 /C electrocatalyst in various pH conditions.The charge transfer resistance was shown to be small in the order of neutral, acidic, and alkaline conditions, which revealed electrocatalytic activities depending on pH conditions.Furthermore, CW-EPR analysis confirmed the change in iron valence from 3+ to 1+ during the HER process, mirroring the behavior of natural hydrogenase enzymes.Our research demonstrates the potential of bio-inspired Fe 7 S 8 /C electrocatalysts to enhance HER performance across all pH conditions and opens avenues for their application in diverse electrochemical energy conversion systems.(Multilab 2000).The morphology and the SAED patterns were obtained using a high-resolution transmission electron microscope (JEM-3000F; JEOL) with an acceleration voltage of 300 kV.EPR measurement was performed using Bruker EMX/Plus spectrometer equipment with a dual-mode cavity (ER 4116DM).The temperature was controlled by liquid He quartz cryostat (Oxford Instruments ESR900) with a controller of temperature and gas flow (Oxford Instruments ITC503).The measurement was performed under a microwave frequency of 9.64 GHz (perpendicular mode), modulation amplitude at 10 G, modulation frequency of 100 kHz, and 0.94 mW microwave power.

EXPERIMENTAL SECTION
Electrochemical cell preparation and measurements: The glassy carbon (GC) was employed as the electrode for all the electrochemical reactions.The samples of Fe 7 S 8 /C were prepared by the drop-casting method.The catalyst ink was prepared by coalescing the prepared active material, carbon black (Super P), and polyvinylidene fluoride using N-methyl-2-pyrrolidone.The electrochemical measurements were performed in a three-electrode cell using an electrochemical analyzer (CHI 760E; CH Instruments, Inc.).The catalyst-coated GC electrode and Pt ring act as a working and counter electrode, respectively, while the Ag/AgCl and Hg/HgO were used as the reference electrode for respective pH conditions. 1 M PBS solution (pH = 7) is used as the electrolyte and the ionic strength of all the electrolytes was maintained at 1 M with various pH.The pH of the electrolyte was adjusted by H 2 SO 4 (pH < 7) and KOH (pH > 7), which was monitored in real-time through a pH meter.The reference electrode was carefully calibrated with respect to RHE at 25

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I G U R E 1 (A) Schematic illustration of Fe 7 S 8 /C electrocatalyst synthesis procedure.(B) X-ray diffraction (XRD) patterns of Fe 7 S 8 /C annealed at 400, 500, 600, 700, and 800 • C. F I G U R E 2 The morphologies and surface component analysis of Fe 7 S 8 /C@600.(A) Scanning electron microscopy (SEM) image.(B,C) Low-and high-magnification transmission electron microscopy (TEM) images.(D) Selected area electron diffraction (SAED) patterns.(E) TEM-Energy-dispersive X-ray spectroscopy (TEM-EDS) image and element mapping are represented as (F) Fe, (G) S, (H) C, (I) N, and (J) overlapping images.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 Fe 7 S 8 /C.Substantially, the transmission electron microscopy (TEM) image in Figure 2B reveals network-like clusters of Fe 7 S 8 /C spherical nanoparticles similar to the SEM image.The HRTEM images in Figure 2C reveal the interface of carbon structures and crystalline Fe 7 S 8 structures.The inset of Figure 2C

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I G U R E 3 (A) Polarization curves of Fe 7 S 8 /C depending on the annealing temperature (400-800 • C). (B) Polarization curves of Fe 7 S 8 /C depending on the molar ratio of Fe precursor from 2 to 16 mM.(C) Polarization curve of Fe 7 S 8 /C in the acid (pH 0), neutral (pH 7), base (pH 14) conditions, pristine Fe 7 S 8 without carbon, Fe 7 S 8 /CB, and commercial Pt/C in the pH 7. (D) Polarization curves of Fe 7 S 8 /C@600 depending on the pH.(E) Tafel slope transition of Fe 7 S 8 /C@600 with changing pH.(F) Potential vs. NHE (blue line) and potential versus reversible hydrogen electrode (RHE) (red line) at −10 mA cm −2 of the Fe 7 S 8 /C@600 electrocatalyst as a function of pH value.
87, which indicates that the reaction rate follows

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I G U R E 4 (A) Nyquist plots for Fe 7 S 8 and Fe 7 S 8 /C electrocatalysts at various pH conditions.(B) Comparison of the charge transfer resistance of Fe 7 S 8 and Fe 7 S 8 /C electrocatalysts.F I G U R E 5 (A) Continuous-wave electron paramagnetic resonance (CW-EPR) spectra at the resting state and after bulk electrolysis at the potential of −0.05 V vs. reversible hydrogen electrode (RHE) and −0.1 V vs. RHE for 5 min.(B) Design of biomimetic Fe 7 S 8 /C electrocatalyst for effective hydrogen evolution reaction (HER).
• C. The reference potential was converted to RHE following equation:E (RHE) = E (Reference) + E o (Reference) + 0.0591 pH A C K N O W L E D G M E NT S This research was supported by the Outsourced R&D Project of Korea Electric Power Corporation (KEPCO) (Grant number: R23XO04), the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (NRF-2021M3H4A6A01045764, 2020M3H4A3106313, 2021R1C1C1004264, and 2021R1A4A1032114), the Korea Institute for Advancement of Technology (KIAT) grant Powder XRD was carried out on a D-8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056Å) to confirm the structure of Fe 7 S 8 /C.The chemical states of the prepared samples were examined by high-performance XPS • C to 800 • C. Physicochemical characterizations: