Enhanced activity and chlorine protection in prolonged seawater electrolysis using MoS₂/sulfonated reduced graphene oxide

Abstract

Electrolyzer technology necessitates the use of seawater instead of freshwater to achieve a comprehensive supply of clean and economical energy. However, the tendency of chloride ions (Cl⁻) to significantly erode the metal surface is a major challenge during seawater electrolysis. Therefore, designing an electrode that is resistant to chloride ions is of great importance to develop an efficient seawater electrolyser. In this work, we present a double layer anode consisting of a molybdenum sulfide electrocatalyst uniformly deposited over sulfonated graphene sheets coated over an Ni foam. The developed electrode (GNiMoOS) helps selectively convert H₂O into H₂ and O₂ rather than chloride (Cl⁻) ions into ClO⁻ in a seawater environment by resisting corrosion due to the Cl⁻ ions in seawater. The chromogenic substrate 3,3′,5,5′-tetramethylbenzidine (TMB) provides solid evidence that the GNiMoOS electrocatalyst blocks the chloride oxidation reaction owing to its distinct resistance to Cl⁻. In addition, density functional theory (DFT) calculations clearly validated the preference of sulfonic moieties towards OH⁻ compared with Cl⁻ ions, confirming the chlorine repelling properties of the GNiMoOS electrode. The successful in situ functionalisation of sulfonic moieties into the reduced graphene oxide (RGO) skeleton with simultaneous development of flower-like MoS₂ was well confirmed using XPS, Raman, SEM, TEM, and FT-IR techniques. GNiMoOS delivered an impressive current density of 100 mA cm⁻² for OER and HER at room temperature, requiring remarkably low overpotentials of just 180 mV and 201 mV, respectively. Industrial faradaic current densities (400–600 mA cm⁻²) were reported with the active electrode at combined overpotentials of ≤600 mV at room temperature. The unique morphology of MoS₂ provides more active sites for the HER/OER, while sulfonated functional groups over graphene impart much-needed anticorrosion properties to the system. Moreover, the electrical coupling between MoS₂ and RGO can make the electron transfer to RGO easier. Therefore, the synergistic interactions among MoS₂, SO₃H and RGO lead to improved catalytic activity and prolonged stability.

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1. Introduction

The advancement of electrolyzer technology increasingly necessitates the use of seawater over freshwater to enable a sustainable and economically viable clean energy supply.¹ Utilizing abundantly available seawater rather than limited freshwater resources for electrolysis presents a promising pathway for generating clean hydrogen fuel. However, the major obstacle in seawater electrolysis is the chlorine evolution reaction (CER), which occurs on the anode owing to the existence of chloride anions (∼0.5 M) in seawater and competes with the oxygen evolution reaction (OER).² Although the equilibrium potential of oxygen evolution is lower than that of chlorine evolution, chlorine evolution is kinetically favored. Hence, the formation of toxic chlorine on the anode is generally unavoidable in seawater electrolysis. The fundamental understanding of seawater splitting indicates that, while the CER predominates at low pH, the OER and hypochlorite production compete at high pH at the anode.³ These aggressive chloride/hypochlorite ions in seawater damage the electrodes, severely impeding the progression of seawater splitting. Moreover, the presence of various elements in seawater can interfere with electrochemistry, making the OER in seawater extremely challenging.⁴ Although Pt-based catalysts and iridium/ruthenium oxides (IrO₂/RuO₂) are most effective for the electrochemical HER and OER, respectively, their high cost and limited supply make them impossible to be used commercially.⁵ Therefore, finding robust electrocatalysts that can sustain seawater splitting without chlorine corrosion could address the issue of freshwater scarcity on earth.² In this direction, Vos and co-workers⁶ proved that putting the MnO_x layer on the surface of IrO₂ could effectively improve the efficiency of OER over CER. The MnO_x layer acts as a sieve that disfavours the transport of chloride ions. However, such materials are unable to generate high current densities at low cell voltage, leading to a decrease in electrolyzer efficiency. Kuang et al.⁷ developed a multilayer electrode that produced a polyanion sulfate/carbonate-passivated NiFe/NiS_x–Ni anode with high OER activity and resilience to corrosion for chlorides in saline electrolytes after activation. The enhanced corrosion resistance was accomplished by negatively charged polyanions that were integrated into the anode through anodization of the underlying nickel sulfide layer and carbonate ions in the alkaline solution. When the activated polyanion sulfate/carbonate-passivated NiFe/NiS_x–Ni anode was paired with the Ni–NiO–Cr₂O₃ cathode, a current density of 400 mA cm⁻² was achieved at a potential of 1.72 V in 6 M KOH + 1.5 M NaCl electrolyte at 80 °C. This is the first and only report to date that proved the implications of sulfonated and carbonated moieties in imparting corrosion resistance and acting as a chloride ion repellent. However, no specific explanation for the CER/hypooxychloride reaction's inhibition was given in the study. Yu et al.⁸ reported a S doped Ni/Fe hydroxide catalyst S-(Ni, Fe)OOH that exhibited extraordinary OER performance in both alkaline brine and seawater electrolyte. In the seawater electrolyte with 1 M KOH, the overpotentials of S-(Ni, Fe)OOH were 300 and 398 mV at current densities of 100 and 500 mA cm⁻², respectively. In another report, Feng et al.⁹ reported that the sulfonate functional groups on carbon related materials, particularly graphene, can capture H⁺ from *OH or *OOH during OER catalysis, thus significantly reducing OER overpotential due to the numerous advantages associated with doping inside the graphene matrix.^9–15 In this direction, Zhu et al.¹⁰ used electrical-field polarization to cause the enrichment of the sulfonated group on the surface nickel–iron layered double hydroxide (NiFe LDH)/carbon dots (CDs) composite catalysts. The polarized-SO₃-CDs/NiFe LDH could be used as a highly efficient OER electrocatalyst with a low overpotential of 200 mV at the current density of 10 mA cm⁻². From the above reports, it is concluded that sulfur and its associated anionic forms have good adjusting properties for working in saline water conditions. In the past, several methods have been utilized to generate sulfur moieties into the graphene matrix for developing an efficient electrocatalyst, including chemical functionalization of graphene oxide (GO),¹⁶ hydrothermal sulfonation of GO¹⁷ and edge sulfonic acid functionalized graphene via ball-milling of graphite powder.¹⁸ Despite their conductivity and stability, functionalized graphene systems alone lack the intrinsic catalytic activity and fail to reduce activation energy effectively for HER/OER. However, a stable, conductive, high-performing hybrid catalyst can be developed where the transition metals provide the actual catalytic active sites to enhance the activity and the functionalized sheets bring stability and conductivity when these sulfur rich graphene or sulfonated graphene systems are coupled with transition metals or their sulfides/oxides/phosphides. Molybdenum sulfides with different stoichiometries and polymorphs have been intensively investigated as efficient electrocatalyst material alternative to noble metals for water splitting applications.^19–32 However, it has been predominantly employed as a HER catalyst due to its inherent activity, and is rarely explored for OER applications. Therefore, to transform MoS₂ into an efficient and robust OER electrocatalyst for seawater splitting, we started the present work on MoS₂ by coupling it with functionalized (S doped) reduced graphene oxide (RGO). Previous reports^7–9 highlighting the effectiveness of sulfur or sulfonic moieties in developing anode materials for seawater splitting guided the incorporation of –SO₃H functional groups to combat the harsh chloride environment in seawater and enhance system efficiency. Therefore, the polyanion rich RGO layer was expected to improve the hydrophilicity and introduce a negative surface charge to repel Cl⁻ ions (especially relevant for seawater electrolysis) which would not only resist corrosion but also compensate for MoS₂'s inherent limitations in OER activity, thereby boosting the stability and overall electrocatalytic performance of the developed electrode. We chose Ni foam as our base substrate to grow our electrocatalyst due to its numerous advantages.^31,32 All previously reported papers utilizing MoS₂ and RGO over Ni foam as electrocatalyst material^25–30 show that the synergistic effects between MoS₂ and GO/RGO is accountable for the enhanced electrochemical activity obtained with the respective systems. While synergy between MoS₂ and RGO plays a central role in complex multi-component systems, potential combinations like Mo–S, Ni–S, or their interactions with GO introduces ambiguity in mechanistic understanding in our present system utilizing Ni foam, MoS₂ and RGO (which integrates Mo, Ni, S, and GO). This raises an important question regarding material attribution and mechanistic clarity in catalyst design for such multi-component systems. No previous studies have accurately elucidated the formation of the target electrocatalyst with full accuracy while systematically ruling out the possible formation of other undesired species that may arise in complex multicomponent systems. Keeping all this in mind, we moved forward towards the fabrication of a target specific catalyst utilizing the state-of-the-art MoS₂ along with polyanion functionalized graphene via a simple one-step hydrothermal approach, wherein oxidized sulfur moieties are in situ generated without the use of sophisticated techniques that usually requires multiple steps. Sulfur is simultaneously introduced on the surface of reduced graphene oxide and in the lattice of Mo catalyst during the reaction. Importantly, the developed sulfonic functionalized reduced graphene oxide supported over Ni introduces two essential features inside the catalyst system: mechanical integrity and specificity for the electrode process (typically OER over CER). Thus, we present a double layer electrode (GNiMoOS) consisting of a molybdenum sulfide electrocatalyst layer uniformly deposited on sulfonated graphene–Ni foam electrode which offers superior catalytic activity and corrosion resistance properties in alkaline seawater electrolysis. The GNiMoOS electrode exhibits outstanding OER and HER activities in simulated seawater requiring a very low overpotential of 180 and 201 mV, respectively, to reach current densities of 100 mA cm⁻² at room temperature. When GNiMoOS was employed as the cathode and anode in a two-electrode setup, it achieved current densities of 100 mA cm⁻² and 500 mA cm⁻² at low overall cell voltages of 1.58 V and 1.80 V, respectively at room temperature, demonstrating its exceptional bifunctional activity and long-term stability in simulated seawater. Raman, SEM, TEM, XPS and FT-IR spectroscopic techniques confirm the 100% formation of MoS₂ along with the abundance of oxidized sulfur moieties doped inside reduced graphene oxide in the GNiMoOS electrode which were responsible for the excellent performance of the electrode towards seawater splitting. In addition, DFT studies validated the role of sulfonated graphene towards imparting stability to the GNiMoOS system during seawater electrolysis. Therefore, a facile hydrothermal method was developed to steer commercial Ni foam into a robust novel electrode with superior catalytic activity for oxygen evolution and corrosion resistance properties for seawater electrolysis. This has important implications for the hydrogen economy and environmental remediation by allowing direct splitting of seawater into renewable fuels without desalination.

2. Experimental methods

2.1. Materials

Graphite powder [Qualikems Fine Chem Pvt. Ltd], sulfuric acid (H₂SO₄) [Finar Ltd], ortho-phosphoric acid (H₃PO₄) [Merck life science Pvt. Ltd], potassium permanganate (KMnO₄) [Sisco Research Laboratories Pvt. Ltd], hydrogen peroxide (H₂O₂) [Merck life science Private Limited], thiourea (CH₄N₂S) [Merck Pvt. Ltd], molybdic acid (H₂MoO₄) [Spectrochem Pvt. Ltd], (Na₂S·9H₂O) [Avantor Performance Materials India Limited], ethanol (C₂H₅OH) [ChangshuHongsheng Fine Chemical Co. Ltd], acetone (C₃H₆O) SDFCL [S D Fine-Chem Limited], methanol (CH₃OH) SDFCL [S D Fine-Chem Limited], potassium hydroxide (KOH) [Finar Ltd], sodium chloride (NaCl) [Finar Ltd], nickel foam (Sigma Aldrich-GF28024657-5EA), and Ni plate (Sigma Aldrich-GF86063654-1EA). All reactants were of analytical purity and used as received. Deionized water (DDW) was prepared by redistillation of the double distilled water in a glass distillation apparatus and used throughout the experiments.

2.2. Material characterization

Functional group identification was done using a Thermo Electron Scientific Instruments LLC Nicolet iS20 Fourier transform infrared spectrometer (FTIR) in the spectral range of 350 cm⁻¹ to 11 000 cm⁻¹. Raman spectra of the samples were recorded in a Jasco NRS-5500 laser confocal Raman spectrophotometer equipped with 1200 grooves per mm gratings, and a laser spot intensity of 0.0001% of the microscope with objective. The samples were excited with a 532 nm wavelength laser from an Ar⁺ laser with fluorescent correction. The XPS measurements were recorded using a Thermo Fisher Scientific K-Alpha X-ray photoelectron spectrometer that was outfitted with Al Kα X-ray (1486.6 eV) as a primary excitation source and an auto-firing, 3 filament TSP software. The curve fitting of the high-resolution spectra was performed with combined Gaussian–Lorentzian functions. The surface morphology was recorded using a field emission scanning electron microscope (FESEM) JSM-7900F (JEOL Ltd), and the elemental distribution was analyzed using the energy dispersive X-ray spectroscopic technique (EDS). Particle morphology was characterized by transmission electron microscope (TEM) imaging using a JEOL JEM-F200. The UV-vis measurements were carried out using a Cary 5000 instrument (Agilent Technology).

2.3. Electrochemical measurements

The electrochemical performance of the electrocatalysts were evaluated using MULTI AUTOLAB M204 from Metrohm Autolab involving three electrode configurations with platinum mesh as a counter electrode and Ag/AgCl as a reference electrode. The reference electrode was well saturated with 3 M KCl to ensure the reliability of our reference electrode. Before all our experimental tests, we used [Fe(CN)₆]^3−/4− as a non-reactive redox probe to monitor reference electrode drift; negligible drift was noted, making it feasible for use. All the electrochemical measurements were carried out in harsh simulated seawater (0.1 M KOH and 0.5 M NaCl) and alkaline seawater electrolyte (2 M KOH + 0.5 M NaCl). The catalyst was first stabilized by cyclic voltammetry (CV) at a 30 mV s⁻¹ scan rate in the potential window of −1.5 to 0.7 V vs. Ag/AgCl. Then, the linear sweep voltammetry (LSV) experiments were conducted at a scan rate of 5 mV s⁻¹. Chronoamperometry studies were carried out to check the stability and the durability of the working electrode with time. For prolonged stability tests, a two compartment cell with a Nafion membrane between the working and counter electrode compartments was used to prevent the migration of dissolved Pt (if any) to the working electrode area and obtain the desired current response solely from the electrocatalyst. The current density was calculated considering the planar geometry surface area of all the electrodes. All the potentials in this study were adjusted to the reversible hydrogen electrode (RHE) reference scale using the formula equation E (vs. RHE) = applied potential (vs. Ag/AgCl) + E⁰ [Ag/AgCl (3 M KCl)] + 0.059 V (pH). The electrochemical impedance experiment was carried out over the frequency range of 0.1 Hz to 1 × 10⁵ Hz at the OCP potential in 2 M KOH + 0.5 M NaCl. The present study is dedicated to understanding the phenomenon associated with the electrode during OER in harsh medium and also understanding the principle behind the remarkable stability of the working electrode in such a harsh medium.

2.4. Spin-polarized density functional theory (DFT)

Spin-polarized density functional theory (DFT) calculations were performed using the Vienna Ab initio simulation package (VASP). The exchange–correlation energy was evaluated using the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional, while the projector-augmented wave (PAW) method was employed to describe electron-ion interactions.^33,34 The energy cutoff for plane wave expansion was set to 500 eV, and the convergence threshold was set as 10⁻⁶ eV in energy and 0.01 eV Å⁻¹ in force. To describe van der Waals interactions, we used the DFT-D3 correction in the Grimme scheme.

For ribbon model calculations, a 20 Å vacuum layer was added to prevent interactions between periodic images. Graphene ribbons with zigzag edge morphology were modeled, saturated with hydrogen atoms to satisfy all in-plane carbon bonds (32 carbon and 4 hydrogen atoms). Separate calculations were performed for MoS₂ and SO₃H-decorated ribbons to reduce the computational costs. A Monkhorst–Pack mesh with a 1 × 4 × 1 k-point grid was used for Brillouin zone sampling during geometry optimizations and subsequent calculations.

Gibbs free energy variations for electron transfer reactions were computed using the computational hydrogen electrode (CHE) model developed by Nørskov et al.³⁵ The Gibbs free energy was defined as:

ΔG = ΔH + ΔZPE − TΔS

where ΔH denotes the change in enthalpy from the DFT calculation, T is the temperature, ΔS is the change in entropy, and ΔZPE represents the change in zero-point energy of the reaction. The entropy values for free gas molecules were obtained from the NIST database, while entropy was calculated based on the vibrational frequencies of the adsorbed intermediates for adsorbed species, considering only vibrational entropy in the calculation of SSS for these species. The theoretical onset potential (U_min) is defined as the minimum potential required to make the entire reaction pathway thermodynamically favorable (downhill) in the Gibbs free energy diagram, calculated using U_min = −maximum (ΔG)/e.

The adsorption energies (E_ad) of ions on the active site were calculated by

E_ad = E_ion@site − E_site − E_(ion)

where the E_ion@site represents the total energy of an ion adsorbed on the active site, the E_site is the total energy of either MoS₂@Gr or SO₃H@Gr and E_ion is the energy of an isolated OH or Cl molecule.

3. Catalyst synthesis

3.1. Synthesis of graphene oxide (GO)

GO was synthesized using modified Hummer's method. Typically, 3 g of natural graphite flakes were added into a 9 : 1 mixture of concentrated H₂SO₄ : H₃PO₄ (360 : 40 mL). After that, 18 g of KMnO₄ was added very slowly over about 1 h and the reaction was heated to 55 °C and stirred for 12 h. The resultant solution mixture was poured into ice-cold water contained 3 mL of 10% H₂O₂. Thereafter, the resultant product was repeatedly washed with excess DI water by centrifugation at 10 000 rpm for 30 minutes, and the process was repeated until the pH of the solution reached 7. Finally, the product was washed with absolute ethanol to dissolve any organic debris, dried, and stored at room temperature for use.

3.2. Preparation of RGO/NF electrocatalyst

Ni foam (NF) (20 × 20 mm, thickness 1.6 mm) was carefully cleaned with concentrated HCl solution (37 wt%) in an ultrasound bath for 5 min to remove the surface oxide layer. Then, the process was repeated with water and absolute ethanol for 5 min each to ensure the surface of Ni foam was well cleaned. The foams were fully dried in air at 60 °C. These electrodes were dip coated in graphene oxide multiple times to develop an even layer of graphene oxide all around the Ni foam electrode [GO coated nickel foam; GO thickness (0.67 mm)] and the electrodes were dried at room temperature before use. These electrodes were placed inside a 100 mL Teflon-lined stainless autoclave containing 30 mL of water and 10 mL of methanol and heated at 180 °C in an electric oven for 24 hours. The electrodes obtained after washing with water and ethanol followed by drying were named as RGO-NF.

3.3. Preparation of NiMoOS electrocatalyst

NiMoOS electrocatalyst was prepared hydrothermally by taking molybdic acid (source of Mo) and thiourea (source of S) in the molar ratio of 1 : 2. The HCl treated cleaned Ni foam (20 × 20 mm, thickness 1.6 mm) and powder mixture were transferred into a 100 mL Teflon-lined stainless autoclave that consisted of 30 mL of water and 10 mL of methanol. The autoclave was heated at 180 °C in an electric oven for 24 hours. The following electrode was washed with water and ethanol followed by drying and named as NiMoOS.

3.4. Preparation of GNiMoOS electrocatalyst

GNiMoOS electrocatalyst was prepared hydrothermally by taking GO coated nickel foam (GO-Ni foam, 20 × 20 mm, thickness 2.27 mm), molybdic acid (source of Mo) and thiourea (source of S) in the molar ratio of 1 : 2 (Fig. 1). The GO coated Ni foam and powder mixture were transferred into a 100 mL Teflon-lined stainless autoclave that consisted of 30 mL of water and 10 mL of methanol. The autoclave was heated at 180 °C in an electric oven for 24 hours. The resulting electrodes obtained after washing with water and ethanol followed by drying were labelled as GNiMoOS. The synthesis scheme is shown in Fig. 1.

Fig. 1 Synthesis scheme for GNiMoOS.

4. Results and discussion

Electrochemical studies were conducted to evaluate the electrocatalytic performance of five distinct electrode materials; namely, RGO/NF (hydrothermally treated GO coated Ni foam), NiS (S doped over Ni foam), NiMoOS (Mo and S doped in Ni foam), GNiS (S doped over GO coated Ni foam), and GNiMoOS (Mo and S codoped over GO coated Ni foam) which were prepared and taken along with bare Ni foam (NF) for comparison to check their performance as electrocatalysts in harsh simulated seawater (E1, ESI†). This medium is more favourable for CER than OER, as Cl⁻ ions are in abundance compared with OH⁻ ions. The electrochemical performance of NF, RGO/NF, NiMoOS, NiS, GNiS and GNiMoOS samples were first evaluated by LSV and the corresponding polarization curves are presented in Fig. 2A. It is noticeable to see that the GNiMoOS catalyst exhibits the lowest overpotential amongst all, requiring only 56 mV to achieve a current density of 10 mA cm⁻² in comparison with GNiS, NiMoOS, NiS, RGO/NF and NF catalysts, which showed overpotentials of 89, 101, 199, 237 and 246 mV, respectively, at a current density of 10 mA cm⁻² in the same media. GNiMoOS catalyst exhibited lower overpotential 40 mV and 52 mV in simulated seawater and real seawater media, respectively, at a current density of 10 mA cm⁻² which were lower than even the recently reported CA-1T-MoS₂/rGO (466 mV);³⁶ rGO/MnO₂/MoS₂ (205 mV);³⁷ MoS₂/rGOhybrids (242 mV);³⁸ MoS₂/NiSe₂/rGO (127 mV)³⁹ and 2H-1T MoS₂/rGO hybrid (71 mV)⁴⁰ electrocatalysts for simulated/seawater media. This remarkable HER activity is quite obvious considering literature reports^36–43 stating that MoS₂/RGO is an efficient HER catalyst in alkaline medium. However, there are rare reports citing MoS₂/RGO as an efficient OER electrocatalyst or a robust bifunctional electrode that could withstand seawater oxidation for a long period of time.

Fig. 2 (A) Linear sweep voltammetry (LSV) profiles of various electrodes recorded from −0.5 V to 1.7 V vs. RHE in 0.1 M KOH + 0.5 M NaCl electrolyte. (B) Chronoamperometry curves of the electrodes during the oxygen evolution reaction (OER) at a fixed potential of 0.7 V in the same electrolyte. (C) Long-term stability profiles of the catalysts, showing current density variations with time. (D) UV-visible spectra of the electrolyte post-electrolysis, indicating oxidation products. (E–J) Photographic images of the electrodes named bare Ni foam (NF), RGO/NF, NiMoOS, NiS, GNiS, and GNiMoOS before and after anodic polarization/oxidation in the same electrolyte, alongside images of their respective electrolytes post-OER using TMB dye for the qualitative detection of hypochlorite ions (OCl⁻). (K–L) Schematic of the hypochlorite (OCl⁻) detection mechanism using TMB dye. (M) Analysis of species leached into the electrolyte after electrolysis.

The LSV polarization curve (Fig. 2A) demonstrates promising OER activity for the prepared electrodes. However, when assessing the long-term sustainability of these electrodes in the harsh medium, several important conclusions were drawn. Initially, the stability of the electrodes was observed to be limited, with Ni foam lasting no more than a minute and NiMoOS less than 10 minutes in the harsh medium (0.1 M KOH + 0.5 M salt). This highlighted the significant challenge of ensuring the sustainability of the electrode material in the harsh environment without material degradation. To address this, a GO/NF composite was developed by dip-coating Ni foam multiple times in graphene oxide to enhance the mechanical strength of the substrate and form a uniform coating on the Ni surface, followed by air drying. Unlike the bare Ni foam, which degraded within 90 seconds, the GO-coated Ni foam demonstrated improved durability. However, exfoliation of the graphene sheets began after extended exposure, leading to material degradation over time. The same electrode was then hydrothermally treated to convert the GO coating into reduced graphene oxide (RGO) on the Ni foam. The increase in conductivity was immediately evident from the higher current values, as RGO is known for its significantly higher electrical conductivity compared with GO, which has relatively low conductivity.⁴⁴ This RGO-coated Ni foam electrode demonstrated improved stability, withstanding the anodic environment for approximately 400 seconds before the electrode began to leach out. A nearly fourfold increase in durability observed for the RGO/NF electrode compared with bare Ni foam prompted us to extend the same strategy of providing a solid carbon support to the NiMoOS electrode as well, anticipating a similar enhancement in durability. Initially, an inner layer of graphene oxide (GO) was coated onto a conductive nickel foam (NF) substrate, forming a GO/NF composite. Subsequently, a uniform outer layer of molybdenum sulfide (MoS₂) was hydrothermally deposited onto the GO/NF surface, resulting in the final robust electrocatalyst containing high-performance anodic electrode material: GNiMoOS. Surprisingly, the GNiMoOS electrode demonstrated a remarkable durability and activity during the OER in a harsh simulated seawater environment. The enhanced conductivity, coupled with newly imparted resistance to corrosive chloride ions, enabled the electrode to withstand chlorine-rich conditions for up to three hours of testing, significantly outperforming its counterparts, which degraded within minutes as seen in Fig. 2B. The current densities and corresponding survival times of all the electrodes under anodic polarization are shown in Fig. 2C. Notably, GNiS and GNiMoOS both exhibit the highest OER current densities and enhanced durability in harsh seawater media. Furthermore, UV-visible spectroscopic analysis showed the presence of a sharp peak at 650 nm which revealed the presence of hypochlorite ions OCl⁻ (a byproduct of chloride ion oxidation) post electrolysis in the electrolytes used with less stable electrodes like RGO/NF, NiS and NiMoOS (Fig. 2D). The degradation of all electrodes due to chlorine corrosion, except GNiS and GNiMoOS, is clearly evident in Fig. 2E–J. The images of the vials in each panel of Fig. 2E–J shows the presence of hypochlorite ions in the electrolyte solutions post electrolysis, as detected using a dye detection method. The whole mechanism of thorough identification of OCl⁻ ions is depicted in Fig. 2K and L. After the OER reaction, 10 μL of the electrolyte solution of the respective electrodes was taken out and mixed with a solution made with 3,3′,5,5′-tetramethylbenzidine (TMB) dye and a buffer solution to check for the presence of oxidized species: OCl⁻.^45,46 A change in the colour of the electrolyte from colourless (TMB) to blue (oxy-TMB; 3,3′,5,5′-tetramethylbenzidine diamine) indicated the presence of these harsh ions, which oxidize the TMB dye, resulting in the blue colour. As expected, the electrolyte bearing GNiS and GNiMoOS electrode remained colourless after dye addition whereas the electrolytes bearing the other electrocatalysts immediately turned blue. This indicates that chlorine oxidation reactions were negligible with the GNiS and GNiMoOS electrodes. These OCl⁻ species were also identified with the help of UV-visible analysis (Fig. 2L), where the presence of a sharp peak at 650 nm corresponds to oxy-TMB formed by the oxidation of TMB by oxidizing OCl⁻ ions. The green coloured leached product from the electrodes during anodic polarizations is supposed to be a corrosion product as a result of the undesired reaction of Cl⁻ ions with Ni metal resulting in the formation of NiCl₂, as confirmed by XPS analysis (Fig. 2M), where the Ni 2p spectrum contains two main peaks that are attributed to 2p_3/2 and 2p_1/2 centered at 853.8 eV and 871.9 eV, respectively which is in good agreement with the Ni²⁺ oxidation state.⁴⁷

Notably, the GNiS and GNiMoOS catalyst exhibited exceptional stability, showing no signs of leaching even under prolonged anodic polarization in harsh electrolytic conditions. Furthermore, this solid electrode effectively suppressed the formation of chlorine and hypochlorite ions as no such formation was detected post electrolysis in the electrolytes using dye detection and UV-visible analysis. These observations suggest that chloride ions were somehow unable to penetrate the Ni foam surface to initiate corrosion and oxidation processes.⁴⁸ This further highlighted the fact that sulfur and RGO imparted anticorrosive and resistant features for these electrodes which accounted for their remarkable stabilities. The functionalization of the reduced graphene oxides embedded with sulfonated moieties (which are validated in the subsequent sections) benefitted the system with superior specificity for OER over CER reactions.

In this article, we come-up with a novel Ni foam with sulfonated graphene, which eventually helps to selectively convert H₂O to O₂ rather than Cl⁻ to ClO⁻ in a seawater environment.

The morphology and microstructure of the prepared electrodes i.e., RGO-coated Ni foam, NiMoOS, GNiMoOS, GNiS, NiS along with Bare Ni foam were characterized with a scanning electron microscope (SEM). The SEM images of Ni foam before hydrothermal growth shows the smooth metallic surface with minimal micro- or nano-scale roughness Fig. 3A, which develops large, flat sheets with few wrinkles upon GO coating (Fig. 3B) and shows the typical crumpled and rippled sheet like morphology when GO gets converted to RGO (Fig. 3C) where twisted graphene sheets are clearly visible. SEM images of the thiourea-treated Ni foam surface (NiS) reveal the growth of inverted wedge-shaped flakes, pointed at the center and firmly adhered to the base material. These flakes appear uniformly across the entire surface and cross-sectional area, indicating homogeneous NiS wedge formation throughout (Fig. 3D–F).⁴⁹ Despite this well-distributed flaky structure, it contributes little to long-term stability, as the electrode shows poor durability under applied oxidation potentials. X-ray diffraction (XRD) profiles of NiS taken using longer scan times and narrow step sizes (Fig. S1, ESI†) exhibits peaks at 22.45°, 38.26°, 50.0° and 55.6°, which are indexed to the (101), (003), (211) and (122) planes of hexagonal Ni₃S₂ (JCPDS no: 44-1418), respectively.⁴⁹ The other two strong peaks at 45° and 52.0° and a peak at 76.0° are well assigned to cubic Ni of Ni foam (JCPDS no: 04-0850).⁵⁰ Upon solvothermal in situ deposition of Mo and S precursors over the Ni foam surface, characteristic flower like features of MoS₂ were visible, and these flowers cling to the surface of base foam as seen in Fig. 3G–I. However, the flower pattern was not uniform and only observed at a few locations. This resulted in the bare foam surface exposed to the harsh electrolyte during oxidation, causing structure collapse during harsh simulated seawater electrolysis with time. Therefore, enhanced OER activity could be observed with the inclusion of Mo and S compared with bare Ni foam material, but the stability of the developed electrode with time was still a challenge. The SEM images of GNiS shows that the NiS nanoparticles are not able to grow and come up to the surface because of the RGO coating which is acting as a barrier preventing the direct exposure of Ni with sulfur component. This could be a probable reason for the enhanced mechanical strength of GNiS compared with NiS, as shown in Fig. 3J–L. This observation is further supported by the XRD spectrum of GNiS (Fig. S1, ESI†), where the absence of characteristic NiS peaks indicates negligible formation of the NiS phase. Fig. 3M–O present the SEM images of our active catalyst, GNiMoOS, showing folded, flower-like nanostructures composed of petal-like MoS₂ lamellae densely clustered together. These structures closely resemble marigold-like formations, with an average diameter of approximately 200–300 nm, anchored onto the underlying graphene sheet.⁵⁰ The Raman spectrum of GNiMoOS confirmed the presence of MoS₂ (discussed later in detail). The elemental mappings of C, N, Mo and S elements taken by energy-dispersive X-ray spectroscopy (EDX) reveal that Mo, S, C and O elements were distributed uniformly throughout the whole GNiMoOS structure (Fig. 3P–T). The absence of inverted wedge-shaped NiS patterns confirms that only MoS₂ structures are present in GNiMoOS, with no indication of NiS formation.

Fig. 3 SEM images of (A) bare Ni foam, (B) GO/NF, (C) RGO/NF, (D–F) NiS, (G–I) NiMoOS, (J–L) GNiS, (M–O) GNiMoOS electrodes. (P–T) Corresponding EDX (EDAX) elemental mapping of the GNiMoOS electrode.

Fig. 4 highlights the cross-sectional SEM image of active electrocatalyst GNiMoOS electrode in the center, which validates the double layer characteristics of our active electrode. The porous foam surface is loaded with sulfonated graphene sheets above which the MoS₂ nanoflower-like structure are adhered following the hydrothermal deposition of active components (nanoparticle catalyst over GO coated foam).

Fig. 4 Cross-sectional SEM image of the GNiMoOS electrocatalyst.

The TEM image of GNiMoOS is shown in Fig. 5A and B, where many MoS₂ nanoflowers with diameters between 250–300 nm were homogeneously incorporated over the RGO sheets, suggesting that the MoS₂ nanospheres tended to nucleate and grow over the RGO sheets. The EDX and element mappings reveal that the nanocomposite contains C, O, Mo, and S as the main components and confirms the uniform distribution of each element (Fig. 5C). The atomic percentages of Mo and S atoms are 23.33 and 76.6%, respectively, confirming the formation of MoS₂. Selected area diffraction pattern (SAED) of GNiMoOS displayed diffraction rings of MoS₂ and RGO corresponding to 002 [MoS₂], 100 [rGO] and 110 [rGO] planes (Fig. 5D). The d-spacing value of MoS₂ nanosheets was calculated to be 0.26 nm, which was ascribed to the (100) plane of MoS₂. The d-spacing values for the other two 100 [RGO] and 110 [RGO] planes were 0.208 and 0.122 nm, respectively. Fig. 5E showed the particle size distribution of GNiMoOS, where the maximum number of MoS₂ flowers were within the diameter range 250–350 nm, homogeneously incorporated within the RGO sheets, previously verified by SEM. To further garner the structural information of the samples, X-ray diffraction (XRD) and Raman spectroscopy were carried out (Fig. 5F and G). MoS₂ powder synthesized hydrothermally by mimicking the conditions used in GNiMoOS was taken as a reference to observe the changes incorporated in the active catalyst GNiMoOS (E1, ESI†). As shown in Fig. 5F, no sharp peaks were observed for free MoS₂, indicative of the poorly crystallized MoS₂ powder synthesized by the solvothermal method. All the XRD diffraction peaks were matched to JCPDS Card No. 65-1951, which confirmed the formation of the pure hexagonal crystal phase of the MoS₂ nanosheets.⁵¹ Diffraction peaks detected at 2θ ∼ 13.8°, 32.8°, 39.5°, and 58.3° can be assigned to (002), (100), (103), and (110) atomic planes of MoS₂ nanosheets, respectively.^52,53 Other peaks at 2θ = 29°, 39.6° and 49.8° corresponding to planes (004), (103) and (105) of bulk MoS₂ faces were detected, respectively (JCPDF 37-1492).⁵⁴ The other peaks at 2θ = 23.3°, 25.7° and 27.3° can be attributed to the (110), (040) and (021) crystal planes of molybdenum trioxide (MoO₃), respectively due to the oxidation of Mo atoms.^55,56 The XRD spectra of RGO/NF, NiS, NiMoOS, GNiS and GNiMoOS electrodes are shown in Fig. S2, ESI.† It was very difficult to obtain information about the material phase in any of the electrodes from XRD as nickel foam is a highly crystalline material with strong, sharp diffraction peaks. Hence, the Ni peaks at 44.5°, 51.8°, and 76.4° corresponding to Ni (111), (200), and (220) planes, respectively, dominate the XRD patterns. Also, NiS or MoS₂ might have formed amorphous phases during hydrothermal synthesis and amorphous materials yield broad humps or weaker peaks, so they do not distinctly appear in XRD. Therefore, the GNiMoOS layer was scraped off the GNiMoOS electrode and ground into fine powder. The XRD results with this powder (Fig. 5F) showed the appearance of a broad peak centered at around 26.5° which was associated with a graphitic crystal structure, implying that GO was efficiently deoxidized (reduced) during the hydrothermal process.⁵⁷ The decreased intensity of the diffraction peaks of MoS₂ in the composite indicates that the incorporation of graphene considerably restrains the aggregation of layered MoS₂ during the solvothermal process, which leads to the growth of a few-layers of MoS₂ nanosheets of poor crystallinity.⁵⁸ Recently, it was demonstrated that amorphous MoS₂ exhibited higher HER activity than crystalline MoS₂ as the amorphous structure afforded a higher number of exposed edges.⁵⁹ The Raman spectra of RGO/NF, NiMoOS and GNiMoOS samples in the frequency range of 100–1800 cm⁻¹ are presented in Fig. 5G. Generally, the peak at about 1580 cm⁻¹ (G band) in carbon based samples like graphene is assigned to an E_2g mode of graphite, which is related to the vibration of sp² bonded carbon atoms in a two-dimensional hexagonal lattice, while the peak at about 1350 cm⁻¹ (D band) is easily influenced by the lattice distortion (defects) in the hexagonal graphitic layers or atomic substitution (doping).⁶⁰ More functional groups or defect sites can increase D band intensity and slightly shift the bands, depending on local bonding environments. As seen in Fig. 5G, the D and G bands in GNiMoOS have shifted significantly towards higher wavenumbers, indicating oxidation or functionalization in the graphene moieties. Also, doping with sulfur moieties have increased the D band intensity compared with the G band in GNiMoOS which is consistent with our proposed conclusion that sulfonic moieties are embedded into the graphene matrix. The characteristic peaks of MoS₂ in the E_1g and E_2g¹ (in-plane) and A_1g (out-of-plane) modes around 289 cm⁻¹, 385 cm⁻¹ and 405 cm⁻¹, respectively are clearly identified in both the NiMoOS and GNiMoOS electrode.⁶¹ Besides, two new peaks resulting from sulfur oxidation are observed between 900–950 cm⁻¹, which is due to the SO symmetric stretching vibration. This suggests a prominent presence of oxidized sulfur moieties in the catalyst which could be due to the sulfonation of graphene sheets.⁶²

Fig. 5 (A and B) TEM images of the GNiMoOS electrode; (C–E) EDAX spectrum, SAED pattern, and particle size distribution of GNiMoOS; (F) XRD spectra of MoS₂ and GNiMoOS; (G) Raman spectra of RGO/NF, NiMoOS and GNiMoOS electrodes; (H) FT-IR spectra of GNiMoOS, RGO/NF, and GNiS. Note: MoS₂ powder was synthesized in the lab; the detailed procedure is provided in the Experimental section (E1, ESI†).

The lack of other ionizable groups on the surfaces of graphene nanosheets was confirmed by IR analysis (Fig. 5H), wherein all the labile groups i.e., the OH and the carbonyl groups were absent at 1620 cm⁻¹ and 1730 cm⁻¹ in GNiMoOS. Meanwhile, there are abundant sulfonic groups and oxidized sulfur moieties as S=O stretching vibrations at 1188 cm⁻¹, 1107 cm⁻¹ and 1051 cm⁻¹.⁶³ These results give solid evidence of the formation of sulfonate graphene as abundant sulfonic groups were detected.

For further information regarding the elements and chemical composition, XPS measurements were performed. Fig. 6A–C displays the XPS spectra of the GNiMoOS electrode. In Fig. S3 (ESI),† the full spectrum shows a C 1s peak at 284 eV, a strong O 1s peak at 532 eV, an N 1s peak at 402 eV, S 2p peak at 168.7 eV and the peak for Mo at 233.8 and 235.6 eV, confirming the presence of MoS₂ and SO₃H groups over graphene coated Ni foam. In Fig. 6A, the deconvolution of the Mo 3d spectrum of GNiMoOS and the bare MoS₂ compounds in the powdered state (excluding graphene oxide) are compared. In the Mo 3d spectrum of MoS₂ powder (similar to the Mo 3d spectrum of NiMoOS), two peaks located at the binding energy (BE) values of 229.5 and 232.3 eV can be assigned to the Mo 3d_5/2 and Mo 3d_3/2 core levels, respectively; meanwhile, the peak at 226.5 eV is associated with the S 2s core level, which confirms the Mo⁴⁺ and S²⁻ charge states due to Mo–S charge transfer.⁶⁴ Another peak located at 235.4 eV is ascribed to Mo 3d_3/2, indicative of an oxidation state of 6⁺. These peaks were attributed to molybdenum trioxide (MoO₃) due to the oxidation of low-oxidation-state Mo atoms.⁵⁶ GNiMoOS displayed two broad peaks representing two set of doublets: one being ascribed to MoVI (235.6 and 232.2 eV corresponding to 3d_3/2 and 3d_5/2 of Mo VI), the other to Mo V (233.8 and 230.6 eV corresponding to 3d_3/2 and 3d_5/2 of Mo V).⁶⁵ It is worth noting that the deconvolution of the Mo 3d spectrum (Fig. 6A) for NiMoOS and GNiMoOS correlates well with the SEM observations. The shift in the oxidation state of molybdenum from Mo⁴⁺ to higher states (Mo⁵⁺ and Mo⁶⁺) in the MoS₂-based structures likely contributes to the morphological transformation from lotus-like flower patterns composed of assembled MoS₂ petals in NiMoOS, to marigold-like MoS₂ nanostructures anchored over graphene sheets in GNiMoOS. Besides, the coexisting oxidation states of Mo V and Mo VI indicates that the oxidation state of Mo is affected by the presence of sulfonic acid moieties, which is the key component in enhancing the absorption of OH⁻ anions over chloride ions leading to faster OER kinetics and this has been proven by DFT studies (shown in the later sections). However, the peak of Mo V might also result from the oxidation of the catalyst in air to form MoO₃ or MoO₄²⁻, which inevitably happens with these kind of species.⁶⁶Fig. 6B shows the deconvoluted S 2p XPS spectrum of GNiMoOS where the peak at 169.63 eV corresponds to S⁶⁺ and S⁴⁺ species in their oxide form (SO₃²⁻ and SO₄²⁻) and these groups could be located at the edges of graphene layers.^67,68 The peaks at 169.84 eV (S 2p_1/2) and 168.85 eV (S 2p_3/2) typically correspond to sulfonic acid groups.⁶⁹ They might be attributed to the consumption of sulfur for the formation of sulfonated graphene over the graphene carbonaceous substances. Therefore, the abundant species that could be responsible for imparting commendable strength to the GNiMoOS electrode are sulfonated graphene (which is the major species present in the electrode of interest) alongside MoS₂ flowers, which are embedded uniformly over its surface. The successful doping of sulfonic groups into carbon skeleton of the GNiMoOS electrode compared with the other electrode materials is clearly depicted in Fig. 6C, where the peak responsible for the apical S²⁻ and bridging S₂²⁻ sulfide groups is negligible compared with the prominent peak representing the oxidized sulfur moieties around 168 eV in the S 2p XPS spectrum of GNiMoOS. However, in the base precursor MoS₂ and the NiMoOS electrode, the peaks at 162–164 eV denoting S²⁻ groups were intense and abundant compared with sulfonic peaks. The ratio of the peaks corresponding to the oxidized sulfonic acid species were estimated from the respective integrated peak area of XPS spectra (Fig. 6D). Ninety-three percent of the total area corresponds to the abundant sulfonic acid groups present on the catalyst surfaces (Table S1, ESI†). Deconvolution of the C 1s and O 1s XPS spectrum of GNiMoOS gives additional support to the proposed abundance of sulfonic groups (Fig. 6E and F). The main peak of C 1s at 284.7 eV is related to the graphitic carbon (C=C linkage). The peaks at 285.6 and 288.6 eV were assigned to the C–S linkage and O–C=O moiety, respectively.^70,71 Besides, the O 1s core level of GNiMoOS could be deconvoluted into two curves at 532.5 (O–C) and 533.9 eV (O–S), confirming the sulfur–oxygen bonds.⁷¹ The in situ hydrothermal deposition results in the incorporation of sulfonated functional groups over graphene imparted the much-needed anti repelling properties. The MoS₂ nanoparticles are dispersed on the RGO due to the large surface of graphene oxide and this structure provides sufficient void space between the neighboring MoS₂ nanosheets and more catalytic edge sites on the RGO. In the meantime, the electrical coupling between MoS₂ and RGO can help facilitate electron transfer to RGO. Overall, the novel aspects are the formation of a layer of Mo–S catalyst formed over graphene supported nickel foam, with abundant sulfonic functionalized graphene, which are both responsible for the high HER/OER activity and repulsiveness to chloride anions in seawater.

Fig. 6 (A) Deconvoluted Mo 3d XPS spectra of GNiMoOS, NiMoOS, and MoOS electrodes; (B) deconvoluted S 2p XPS spectrum of the GNiMoOS electrode; (C) overlay of S 2p XPS spectra for GNiMoOS, NiMoOS, and MoS₂ precursor powder for comparison; (D) bar graph showing the percentage area of peaks A and B corresponding to different sulfur species in MoS₂ (1), NiMoOS (2), and GNiMoOS (3); (E) deconvoluted C 1s XPS spectrum of GNiMoOS; (F) deconvoluted O 1s XPS spectrum of GNiMoOS.

In Fig. 7A and B, the polarization curves before and after iR correction for HER in 0.1 M KOH + 0.5 M NaCl media are compared. From the LSV data in Fig. 7A, the overpotentials required to reach a current density of 100 mA cm⁻² follow the following trend: GNiMoOS (256 mV) < GNiS (340 mV) < NiMoOS (436 mV) < RGO/NF (472 mV) < NF (550 mV). After iR correction (see Fig. 7B and E2, ESI†), GNiMoOS demonstrates the best HER performance, requiring only 201 mV to achieve 100 mA cm⁻², significantly outperforming GNiS (210 mV), NiMoOS (236 mV), RGO/NF (276 mV), and bare NF (371 mV). The LSV results demonstrated that the GNiMoOS hybrid exhibited the best electrocatalytic activity for HER among all the other electrodes. A comparison of the overpotential before and after iR correction at 100 mA cm⁻² current density is shown in Fig. S4 (ESI).† Notably, the overpotentials after iR corrections required to deliver current densities of 200, 300, 400 and 500 mA cm⁻² are 238, 241, 245 and 252 mV, respectively for GNiMoOS in harsh alkaline media (Fig. S5, ESI†). To have a better understanding of the HER mechanism of these samples, Tafel plot were analysed. It is well known that the HER process could be divided into three possible reactions; namely, Tafel reaction (30 mV dec⁻¹), Heyrovsky reaction (40 mV dec⁻¹), and Volmer reaction (120 mV dec⁻¹). According to the kinetic model, an inherent index Tafel slope of 120, 40 or 30 mV dec⁻¹ signifies the Volmer, Heyrovsky or Tafel reaction as the rate-determining step, respectively.⁷² To investigate the rate-determining step of the GNiMoOS electrode, the Tafel plots were derived from the LSV curve by fitting the linear regions according to the Tafel equation mentioned below:

γ = a + b log j

where γ, b, and j denote overpotential, Tafel slope, and current density, respectively. As shown in Fig. 7C, the Tafel slope value for GNiMoOS was 64 mV dec⁻¹, whereas the Tafel slope of the NF, RGO/NF and NiMoOS catalysts were determined to be 124, 105 and 80 mV dec⁻¹, respectively. The GNiMoOS catalyst exhibits a lower Tafel slope compared with the other prepared catalysts, due to the strong electronic and chemical interaction between the anionic doped RGO and MoS₂ which provides more catalytic edge sites, hence speeding up the rate. The Tafel slope of the GNiMoOS catalyst falls within the range of 40–120 mV dec⁻¹, which indicate that the electrochemical HER reaction proceeds through the Volmer–Heyrovsky mechanism. Cyclic voltammetry (CV) with different scan rates (20 mV s⁻¹ to 100 mV s⁻¹) at the non-faradaic current region was conducted to estimate the electrochemically active surface areas (ECSA) for GNiMoOS (E3, ESI†). The ECSA of GNiMoOS was calculated to be 55.55 cm² (Fig. S6, ESI).†Fig. 7D compares the overpotential values of some recently reported electrocatalysts at 10 mA cm⁻² current density with GNiMoOS during HER. It can be seen that GNiMoOS requires much lower overpotential to reach the current density (10 mA cm⁻²) than any of the recently reported HER electrocatalysts for simulated seawater/real seawater systems and is among the most efficient electrocatalysts reported for overall seawater splitting.

Fig. 7 (A and B) LSV spectra of electrodes showing HER activity before and after IR correction, respectively, in harsh KOH (0.1 M) + NaCl (0.5 M) media from 0 to −0.5 V vs. RHE. (C) corresponding Tafel plots for the HER. (D) Electrocatalyst comparison data for the HER at 10 mA cm⁻² current density.^73–80 (E and F) LSV spectra of electrodes showing OER activity before and after IR correction in harsh media from 0 to 1.7 V vs. RHE. (G) corresponding Tafel plots for the OER. (H) electrocatalyst comparison data for the OER at 100 mA cm⁻² current density.^81–88 (I) Gas collection setup for calculating faradaic efficiency. (J) Volume of H₂ and O₂ produced by the GNiMoOS electrode during water splitting. (K) Images showing the OER selectivity of the electrodes over the CER at frequent time intervals. (L) Images showing OER selectivity of the electrodes over the CER during prolonged electrolysis at a fixed potential of 0.7 V in the medium of KOH (0.1 M) + NaCl (0.5 M) at room temperature.

OER is known to be bottleneck of overall water splitting owing to the large overpotential for driving the half reaction. As shown in Fig. 7E, GNiMoOS exhibited the lowest overpotentials of 90 and 180 mV at current densities of 10 and 100 mA cm⁻², respectively in simulated seawater media. For instance, the overpotentials of NF, RGO/NF, NiMoOS and GNiS at 10 mA cm⁻² were 450, 330, 200 and 150 mV respectively and the overpotentials of NiS, NiMoOS and GNiS at 100 mA cm⁻² were 550, 290 and 250 mV, respectively, which are very high compared with that of GNiMoOS, indicating its high electrocatalytic activity toward OER. In Fig. 7E and F, the polarization curves before and after iR correction for OER were compared. The overpotentials required to deliver a current density of 100 mA cm⁻² for OER decreases by 110 mV upon iR correction in the active GNiMoOS electrocatalyst. The Tafel slope was also used to evaluate the OER activity of these as-prepared samples. The sample with higher OER activity usually presents a lower Tafel slope. As shown in Fig. 7G, the Tafel slope of GNiMoOS for OER is 120 mV dec⁻¹, which is lower than that of NF (198 mV dec⁻¹), RGO/NF (165 mV dec⁻¹) and GNiS (134 mV dec⁻¹). Compared with the other recently reported OER electrocatalysts for seawater splitting, GNiMoOS stands out, requiring the lowest overpotential to reach the benchmark current density (100 mA cm⁻²) in simulated/real seawater systems (Fig. 7H). For the calculation of faradaic efficiency, gas collection and measurement of the evolved gases was performed through a manual setup shown in Fig. 7I, considering that the water displacement method and the measured volume ratio of H₂ to O₂ was close to the theoretical value of 2 : 1 (Fig. 7J). Faradaic efficiencies of 94% and 87.6% for HER and OER, respectively, indicated good selectivity of GNiMoOS for these reactions in simulated seawater medium. The faradaic efficiency of HER and OER with time is shown in Fig. S7 (ESI).† The selectivity of GNiMoOS towards OER was further monitored hourly by checking the presence of hypochlorite ions formed gradually in the medium during applied anodic potentials. The images after testing for OCl⁻ ions every hour are shown in Fig. 7K. GNiMoOS proved to be highly selective for OER compared with CER, which can be seen from the images taken after the electrolysis. Meanwhile, all the other electrodes produced hypochlorite up to a testing time of 6 hours. The solution from the GNiMoOS electrode remained colorless from the start and throughout the testing hours, indicating the sole occurrence of hydroxide ion to oxygen gas transformation with our active catalyst. To further confirm this selectivity, experiments were conducted in a two-compartment electrochemical cell, separating the anodic and cathodic electrolytes. After prolonged electrolysis lasting several days, the TMB dye test showed that the anodic electrolyte containing the GNiMoOS electrode remained colorless, with no detectable OCl⁻ (Fig. 7L). This strongly confirms the superior selectivity of GNiMoOS toward OER, even under harsh conditions in 0.1 M KOH + 0.5 M NaCl.

The long-term stability of the GNiMoOS catalyst for the hydrogen evolution reaction was assessed using chronoamperometry (i–t) at a constant potential of 0.354 V for 400 hours, as shown in Fig. 8A. The catalyst demonstrated exceptional durability, maintaining a nearly constant current density of 330 mA cm⁻² in pure alkaline media with negligible decay in HER activity over the entire testing period. Comparable stability was also observed in simulated seawater and real seawater, where respective current densities of 350 mA cm⁻² and 360 mA cm⁻² were recorded (Fig. 8A). Even after 400 hours of continuous operation, negligible decay was observed in current density, highlighting the excellent long-term durability of the GNiMoOS catalyst. Post-stability analysis revealed no significant morphological changes: the MoS₂ flower-like structures remained firmly anchored onto the RGO sheets, and the elemental distribution remained uniform, as evidenced by the SEM images in Fig. S8 (ESI).† These findings confirm the exceptional structural and electrochemical stability of the synthesized GNiMoOS hybrid catalyst under prolonged HER conditions. The HER alkaline stability of the GNiMoOS electrocatalyst was also investigated at current densities of 10, 20, 40, 60, 80, and 100 mA cm⁻² using a multistep chronopotentiometry method (Fig. 8B).The potential remains steady at each applied current density, but it instantly changes by altering the current density from one value to the another. The symmetry of potential changes during both the increase and decrease of current density further confirms the robust electrochemical stability and sustained HER performance of the GNiMoOS catalyst under dynamic operating conditions.

Fig. 8 (A) Chronoamperometry spectra of the electrodes during the OER at a fixed potential of 0.7 V in three marked mediums. (B) Multistep chronopotentiometric analysis for the hydrogen evolution reaction (HER) with the GNiMoOS catalyst. (C) Chronopotentiometry analysis of the electrodes during the OER at a fixed current density of 100 and 500 mA cm⁻² in simulated seawater media. (D) Overpotential comparison of GNiMoOS in all the three mediums during the HER and OER. (E and F) Cathodic and anodic LSV voltammograms of Ni–G–MoOS after 400 hours and 160 hours of HER and OER operation, respectively. (G) DC Source study of current dependency on the concentration and temperature of the electrolyte [conditions: 2 M KOH and 0.5 M NaCl, 24 °C, potential window: −0.5 to +1.7 V vs. RHE]; (H) Nyquist plots of the marked electrodes in the simulated seawater system. (I) Tafel plots of the electrocatalysts developed over Ni plates in 0.1 M KOH and 0.5 M NaCl for corrosion analysis. (J) Bifunctional activities of GNiMoOS based on the two electrode setup with a DC source in 2 M KOH and 0.5 M NaCl medium at the marked potentials. (K) Stability data of the GNiMoOS electrode for about 6 months. (L) Hydrogen and oxygen evolution activity of GNiMoOS.

Furthermore, the stability of the GNiMoOS catalyst as a bifunctional electrocatalyst was investigated by chronopotentiometry (η–t) measurements at a constant current density of 100 mA cm⁻² and 500 mA cm⁻² in simulated seawater media for 12 hours by keeping the same active catalyst as cathode and anode in a two-electrode setup. Fig. 8C showed that the catalyst maintained a constant potential of 1.58 V and 1.8 V at current densities of 100 mA cm⁻² and 500 mA cm⁻² for 12 hours, respectively, signifying superior stability and excellent bifunctional activity with a slight potential increment of 10 mV. The overpotentials exhibited by GNiMoOS at a current density of 10, 100 and 500 mA cm⁻² for HER, and 100 and 300 mA cm⁻² for OER in alkaline solution (1 M KOH), simulated seawater (1 M KOH + 0.5 M NaCl) and real seawater (1 M KOH + seawater media) respectively are collectively summarized in Fig. 8D. After the chronopotentiometry measurement, the polarization curve before and after a time period of 400 hours and 160 hours for both HER and OER in simulated seawater showed a small ignorable shift from the initial one (Fig. 8E and F). These results confirm the long-term electrochemical stability of the GNiMoOS catalyst during the electrochemical process in harsh alkaline electrolyte. The effect of the amount of sulfur source as a precursor (thiourea) with molybdic acid in the synthesis of GNiMoOS on the electrocatalytic performance was also investigated. Their corresponding HER electrocatalytic results are presented in Fig. S9 (ESI).† When the molar ratio of Mo to S is 1 : 2, the GNiMoOS sample has the best HER performance. This can be seen in inset image in Fig. S9,† where no residue precursors remained in the medium after the hydrothermal reaction. This is unlike the results obtained with a 1 : 1 ratio, where excess precursors remained in the solvent medium (inset image S9†) or the 1 : 3/1 : 4 ratios, where the precursor quantity is not optimal to give a current density better than Ni foam itself. Thus, the ratio of Mo to S (1 : 2) exhibits the highest HER and OER activity. The electrode's activity was measured after varying the concentration of the electrolyte and the reaction temperature to observe the rise in the current value besides the durability of the electrode material when exposed to high concentrations of OH⁻ ions and high temperature, as shown in Fig. 8G. The conductivity of the electrolyte becomes greater with increased concentration of electrolyte, leading to an enhanced current efficiency and a higher electrolysis rate. The sharp rise in current value with a 1 × 1 cm² area of the catalyst at room temperature without undergoing any leaching confirms the highly anti corrosive nature of the prepared electrode. The conductivity of the electrode increases upon further increasing the temperature due to the increasing ionic mobility of the ions and drop in resistance of the electrolytes.⁸⁹ The evolution of hydrogen gas is very rapid with increasing temperatures of the electrolyte above 40°, as high temperature decreases the potential required to rupture the water molecule, increasing the mass flow of hydrogen.⁹⁰ Besides, some of the energy is supplied as heat, which is cheaper than electricity. The current value increases from 600 mA cm⁻² to 1.6 A cm⁻² upon increasing the temperature of the electrolyte from 30 to 70 °C, respectively in 5 M KOH, as seen in Fig. 8G. Electrochemical impedance spectroscopy (EIS) measurements were performed for four electrodes: NF, NiMoOS, RGO/NF, and GNiMoOS in simulated seawater media at an open circuit potential (OCP). The representative Nyquist plots obtained from EIS measurements are shown in Fig. 8H. Theoretically, a smaller semicircle diameter at high frequencies corresponds to lower charge transfer resistance R_ct, indicating faster charge transfer and enhanced electrocatalytic kinetics. As shown in Fig. 8H, the Nyquist plot for GNiMoOS displays the smallest semicircle, signifying the lowest R_ct amongst all tested electrodes and confirming its superior hydrogen desorption efficiency and rapid electron transport at the active sites. The fitted EIS spectrum for GNiMoOS is provided in Fig. S10 (ESI).† As already mentioned, seawater contains a large amount of Cl⁻, which causes CER at the anode, causing it to corrode, and seriously affect the service life and electrolytic activity of the electrode. Therefore, a seawater-based electrode needs to be highly stable and corrosion resistant. So far, many investigations of anticorrosion designs have focused on the preparation of a protective layer. Although the physical protective layer can prevent the corrosion and improve the stability of the electrode, active sites may be corroded or blocked at the electrochemical reaction due to complex ions in seawater. To avoid this problem, the insertion of a polyanion layer for electrostatic repulsion into the interior of the active material can effectively prevent the adsorption of Cl⁻.⁹¹ In our system, we have proposed the presence of abundant sulfonic moieties over the graphene sheets and this anion insertion has imparted anticorrosion properties to our system towards chloride ions. To validate our findings, we carried out corrosion experiments with a bare Ni plate (Nip) modified with GO treated hydrothermally (RGO/Nip) and RGO/Nip treated with thiourea hydrothermally (RGO/S/Nip) (E1, ESI†) to confirm the importance of sulfonic functionalities in minimizing the corrosion of the surface in simulated seawater. The Tafel plots of pure Nip, RGO-Nip and RGO/S/Nip in 2 M KOH + 0.5 M NaCl are presented in Fig. 8I, from which the corrosion potential (E_corr) and corrosion current density (I_corr) have been calculated. Generally, higher E_corr and lower I_corr values indicate better corrosion resistance performance.⁹² The E_corr values of Nip, RGO-Nip and RGO/S/Nip were −0.486 V, −0.381 V and −0.236 V, respectively. The I_corr values of Nip, RGO-Nip and RGO/S/Nip were 1.52 × 10⁻⁶ A cm⁻², 2.2 × 10⁻⁷ A cm⁻² and 1.19 × 10⁻⁸ A cm⁻², respectively. Although severe corrosion existed in high salinity electrolytes, difference in E_corr and I_corr values were still observed. E_corr became more positive and I_corr became smaller in the sulfonic groups-modified with reduced graphene oxide nickel plates compared with bare graphene oxide coated plates, indicating a much weaker corrosion tendency of the surface. The above results demonstrated that the corrosion of Cl⁻ was inhibited by the formation of SO₃–C/SO₃H/SO₄²⁻ moieties into the graphene matrix, which re-confirmed our suggested findings.

Encouraged by the high catalytic activity of GNiMoOS for OER and HER in simulated seawater media, an electrolyzer was assembled by integrating GNiMoOS electrodes as the cathode and anode for overall water splitting in 2.0 M KOH + 0.5 M NaCl media with a DC source operating at 2 to 2.5 V. The assembled electrolyzer with the symmetrical electrodes could drive current densities of 2.2 A cm⁻² and 3.1 A cm⁻² at a combined overpotential of 2.2 V and 2.4 V, respectively (Fig. 8J), which are greater than the state-of-the-art catalyst reported. Further, the electrode was tested for its activity at regular intervals of 10 days (inset of Fig. 8K) for a period of nearly 6 months (Fig. 8K) as the same current density was obtained for HER and OER in each trial with negligible current decay. The vigorous hydrogen and oxygen evolution activity of the GNiMoOS electrode is shown in Fig. 8L.

5. Decoding functionality and synergy in the GNiMoOS electrocatalyst

Based on all the above discussions, GNiMoOS emerges as the most active and durable electrode for both simulated and real seawater, delivering industrial-scale current densities with remarkable long-term stability. This outstanding performance is largely attributed to the in situ formation of abundant sulfonic moieties and MoS₂ nanostructures over the graphene matrix during synthesis. However, it becomes inherently difficult to attribute the observed functionality to a single moiety with complete certainty in complex multi-component systems involving various metals and nonmetals. Multiple moieties can either contribute synergistically or serve overlapping roles. In the case of GNiMoOS, which integrates Mo, Ni, S, and GO, potential combinations like Mo–S, Ni–S, or their interactions with GO introduce ambiguity in mechanistic understanding. This raises an important question regarding material attribution and mechanistic clarity in catalyst design for such multi-component systems. To provide a clearer understanding, we have summarized all key findings below to support and validate our proposed rationale.

To clearly isolate and quantify the role of SO₃H-GO (sulfonated GO), all the prepared electrodes were assayed; namely, bare Ni-foam, RGO/NF (to observe the role of RGO), NiS (to observe the role of S), NiMoOS (to observe the individual role of MoS₂ developed in situ over Ni foam), GNiS (to observe the individual role of sulfonated graphene developed in situ), and GNiMoOS (to observe the combined role of MoS₂ and sulfonated graphene developed in situ over Ni foam). The following activity and stability results were obtained:

Activity order [OER]: NF < RGO/NF < NiS < GNiS < NiMoOS < GNiMoOS [0.5 M KOH].

Activity order [OER]: NF < NiS < NiMoOS < RGO/NF < GNiS < GNiMoOS [0.5 M NaCl].

Activity order [OER]: NF < RGO/NF ∼ NiS < NiMoOS < GNiS < GNiMoOS [0.1 M KOH + 0.5 M NaCl].

In pure KOH media (free of harsh Cl⁻ ions), the activity of NiMoOS > GNiS, which means that MoS₂ [MoS₂ flowers developed over Ni foam in NiMoOS] has a role in the enhanced activity but this trend is inversed in 0.5 M NaCl, and the activity follows GNiS > NiMoOS. Also, both of the sulfur doped RGO coated electrodes were the most stable in 0.5 M NaCl compared with the bare electrodes devoid of any RGO coatings. Our hypothesis that sulfur and RGO (i.e., in situ developed sulfonated graphene) play a significant role in imparting stability to the electrode in harsh environments explains why GNiS and GNiMoOS were stable in 0.5 M NaCl, while NiMoOS (which doesn't appear to contain any sulfonated graphene) was unstable in the same medium. However, it is still more active than GNiS in KOH media (but less active than GNiMoOS). To validate the trend observed and the conclusions proposed, we put forward some important findings supporting our proposed findings which are mentioned below:

The Raman spectra of GNiMoOS clearly indicated the characteristic peaks of MoS₂ and the SO₃H related vibrations in the 1000–1200 cm⁻¹ region. The functionalization of graphene sheets with sulfonic moieties was well justified due to the shift in the D and G band positions towards higher wavenumbers as well as the increase in the D band intensity compared with the G band. The peaks at 182, 199, 221 and 301 cm⁻¹ that coincide with the characteristic Raman peaks of Ni₃S₂ were missing in the Raman spectrum of GNiMoOS, although they were present in the Raman spectrum of NiS (Fig. S11, ESI†). The Raman spectra of GNiS (Fig. S11, ESI†) was similar to the Raman spectra of GNiMoOS in terms of the peaks corresponding to SO₃H vibrations. This might be because there is in situ doping of sulfonic moieties into the graphene matrix similar to the case of GNiMoOS, as the reaction conditions for the synthesis of both GNiS and GNiMoOS are similar, except for the use of molybdic acid in GNiMoOS. However, the peaks at 182, 199, 221 and 301 cm⁻¹ that coincide with the characteristic Raman peaks of Ni₃S₂ (which were found in NiS) were absent in GNiS in the lower frequency range, which confirms the formation of Ni₃S₂-like species over Ni foam in GNiS [inset of Fig. S11†]. Since GNiS and GNiMoOS were equally stable in harsh media, this led us to conclude that sulfonic doped graphene is definitely linked to the remarkable stability of the electrode. Further in the XPS spectra of GNiMoOS, the S²⁻ peak around 161.1 to 162.2 eV (attributed to S 2p_3/2) and 162.4 to 163.5 eV (attributed to S 2p_1/2) in the S 2p spectrum are absent in Ni₃S₂ and the peaks in the region 168–170 eV were abundant, suggesting the large presence of oxidized sulfonic moieties, which again confirmed the absence of Ni₃S₂. Also, the growth of inverted wedge shaped flakes of Ni₃S₂ were not observed in the SEM images of GNiMoOS when S was solely doped over Ni foam in the same operating conditions as GNiMoOS. Instead, MoS₂ flowers homogeneously adhered to graphene sheets in GNiMoOS were observed, suggesting the sole formations of MoS₂. Hence, the possibility of Ni₃S₂ formation during the in situ development of GNiMoOS was ruled out. Since our active catalyst GNiMoOS exhibits higher activity than GNiS and comparable stability in harsh media, we conclude that both MoS₂ and sulfonated graphene (SO₃H-GO) contribute synergistically to its high efficiency and chlorine-repelling capability, making it the most effective among the GNiS and NiMoOS electrodes for seawater electrolysis.

Therefore, Raman, SEM, TEM and XPS analysis confirm the 100% possibility of MoS₂ formation along with the abundance of oxidized sulfur moieties doped inside graphene in the GNiMoOS electrode, which were responsible for the excellent performance of the electrode towards seawater splitting. In addition, DFT studies validated the role of sulfonated graphene towards imparting stability to the GNiMoOS system during seawater electrolysis. The details are discussed in the next section.

6. Validation of the sulfonic group's repulsive potential by computational analysis

To gain a deeper understanding of the catalytic performance of GNiMoOS, density functional theory (DFT) calculations were performed to examine the mechanism of the oxygen evolution reaction (OER) for this catalyst. To optimize computational efficiency, separate calculations were conducted for MoS₂ and SO₃H-decorated graphene ribbons (SO₃H@Gr). These ribbons were used as theoretical models to represent the edges of the graphene sheet, where MoS₂ typically forms (MoS₂@Gr). A Mo₃S₁₂ cluster consisting of three Mo and twelve S atoms was used to simulate the flower-like MoS₂ morphology, with the optimized structure shown in Fig. S12 (ESI).†

First, the study focused on the catalyst's stability in an alkaline medium by examining the interaction between Cl⁻ ions and the catalyst (Fig. S13, ESI†). The adsorption energy of Cl⁻ on MoS₂ was calculated to be slightly positive at 0.045 eV. Meanwhile, Cl⁻ ions were found to interact with the hydrogen atom of SO₃H@Gr, leading to deprotonation and the formation of HCl, with a formation energy of −0.34 eV. Further evaluation of Cl⁻ adsorption on SO₃@Gr, specifically on the SO₃⁻ ions, yielded an adsorption energy of 0.68 eV, confirming Cl⁻ repulsion. This suggests that SO₃⁻ ions remain stable and could potentially participate in the OER process. The interaction between Cl⁻ and SO₃H creates a localized acidic environment within the alkaline medium, thereby enhancing the catalyst's stability.

The OER catalytic process in alkaline media can follow two mechanisms: the conventional adsorption evolution mechanism (AEM) and the lattice oxygen participation mechanism (LOM).⁹³ We investigated both mechanisms on SO₃@Gr. For the AEM, the process begins with OH adsorption at an oxygen site, which then reacts with another OH radical to form *O and a water molecule, as depicted in Fig. 9A. This is followed by the reaction of *O with OH⁻ to form *OOH, eventually leading to O₂ evolution and H₂O production through interactions with OH⁻. Gibbs free energy calculations revealed a theoretical onset potential (U_min) of 2.31 V, as shown in Fig. 9B, with the rate-determining step being the initial adsorption of OH.

Fig. 9 (A) OER reaction mechanism with the optimized structure for intermediates adsorbed on SO₃H@Gr; (B) calculated free energy diagram for the OER on MoS₂@Gr and SO₃H@Gr. Color code: C, brown; O, red; S, yellow; and H, white.

Next, we studied the LOM mechanism (Fig. S14a, ESI†), where OH adsorbs onto SO₃ and reacts with an oxygen atom from SO₃ to form OOH (SO₂–OOH). This is followed by an interaction between OH and OOH, resulting in O₂ evolution and H₂O formation. Subsequently, OH adsorbs onto SO₂, and further interaction with OH releases H₂O, regenerating SO₃. The free energy diagram in Fig. S14(b) (ESI)† indicates a U_min of 2.66 V, with the rate-determining step identified as the final step in the process.

Additionally, the OER activity of MoS₂@Gr was investigated using AEM. As depicted in Fig. S15 (ESI),† various intermediate structures were identified throughout the reaction process. Initially, the Mo atom in MoS₂@Gr adsorbs an OH radical, which then reacts with another OH radical to form *O and a water molecule. The *O species subsequently desorbs from the Mo atom and adsorbs onto a neighboring S atom. This process continues as *O reacts with OH⁻ to form *OOH, eventually leading to O₂ evolution while producing H₂O through interactions with OH⁻. Gibbs free energy calculations determined a U_min of 2.16 V, as shown in Fig. 9B. The rate-determining step was identified as the formation of the OOH intermediate. Both MoS₂ and SO₃ follow the AEM mechanism, with theoretical overpotentials of 0.93 V and 1.08 V, respectively. Although MoS₂ has a lower overpotential than SO₃, SO₃ is expected to be more available and stable, as Cl⁻ could adsorb at the MoS₂ site.

7. Conclusion

In summary, an efficient robust electrode GNiMoOS has been developed which eventually helps to selectively convert H₂O to O₂ rather than Cl⁻ to ClO⁻ in a seawater environment. Prolonged sea water electrolysis was possible due to the formation of protective layer of abundant sulfonated graphene over Ni-foam. The successful in situ functionalisation of the sulfonic moieties into the reduced graphene oxide skeleton with simultaneous embedding of flower-like MoS₂ (GNiMoOS) was well confirmed using XPS, Raman, SEM, TEM, and FT-IR methods. The DFT results showed that the Cl⁻ repelling properties and endurance of the GNiMoOS electrode were due to the sulfonic moieties that were in situ produced over reduced graphene oxide that was coated over Ni foam. The GNiMoOS electrode exhibited the highest OER activity and durability compared with the bare NiMoOS or RGO/NF owing to the synergistic cooperation of all the components: i.e., Ni foam, sulfonated graphene, and MoS₂ compounds. The chromogenic dyes (TMB) demonstrated that the GNiS and GNiMoOS electrodes stop the formation of ClO⁻ during seawater electrolysis. This is because the sulfonic group above GNiS and GNiMoOS keeps Cl⁻ ions away. DFT studies strongly validated the repulsive properties of sulfonated functionalities developed over graphene. The positive adsorption energy of 0.68 eV for Cl⁻ over SO₃⁻ ions confirmed its repulsive nature towards Cl⁻ ions. Overall, the fabricated electrodes offered large surface area, high activity, and durability which make it an ideal material for sea water splitting application. Hence, careful design of anodes and electrolytes can fully solve the chloride corrosion problem and allow direct splitting of seawater into renewable fuels without desalination.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its ESI.† Raw data that support the findings of this study are available from the corresponding author, upon reasonable request.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the Department of Science and Technology, the Inspire Faculty Program of India (DST/INSPIRE/04/2018/002018), the Department of Science and Technology—Ministry of Science and Technology, India (DST/TMD-EWO/AHFC-2021/2021/111), and NTPC-NETRA (9100000216-151-1001). PT acknowledges the Rajiv Gandhi Institute of Petroleum Technology for providing the institute's assistantship. AKV acknowledges the University Grant Commission, India, for providing a research assistantship (191620017619). The authors express their gratitude to the Institute of Science, Bangalore (IISc), and the Rajiv Gandhi Institute of Petroleum Technology for their invaluable assistance with characterisations.

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Footnote

† Electronic supplementary information (ESI) available: Preparation of NiS, GNiS, RGO/Nip and RGO/S/Nip electrocatalysts; XRD spectra of NiS powder and the GNiS electrode; combined XRD spectra of RGO/NF, NiS, NiMoOS, GNiS and GNiMoOS electrodes; full survey XPS spectrum of GNiMoOS; percentage area of peak A (162–164 eV) and B (168.8 eV) corresponding to different sulfur species in MoS₂, NiMoOS and GNiMoOS; iR correction procedure, iR drop corrected data of GNiMoOS, HER overpotential values of the electrodes before and after iR correction at 100 mA cm⁻² current density; iR corrected potentials of GNiMoOS at 100–500 mA cm⁻² current density in harsh alkaline media; determining the electrochemical active surface area (ECSA); CV curves of GNiMoOS at different scan rates from 20 to 100 mV s⁻¹ in the potential window of 0.5–0.7 V vs. RHE; ECSA linear plot of GNiMoOS, faradaic efficiency % of GNiMoOS for the HER and OER with time in harsh alkaline media; SEM images of GNiMoOS at 1 μm (a) and 100 nm (b) after a 400-h durability test; HER electrocatalytic results of GNiMoOS with varying molar ratios of Mo:S; Fitted EIS spectra of GNiMoOS, Raman spectra of the NiS and GNiS electrodes; DFT optimized structure of (a) MoS₂@Gr and (b) SO₃H@Gr; DFT optimized structure for adsorption Cl⁻ ion on (a) MoS₂@Gr, (b) SO₃H@Gr and (c) SO₃@Gr; OER-LOM reaction mechanism with the optimized structure for intermediates adsorbed on SO₃H@Gr; calculated free energy diagram for the AEM and LOM mechanism on SO₃H@Gr; the OER reaction mechanism with the optimized structure for intermediates adsorbed on MoS₂@Gr; images of electrodes demonstrating GNiS and GNiMoOS endurance in the harsh NaCl medium. See DOI: https://doi.org/10.1039/d5se00541h

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