Nano‐Ni‐Induced Electronic Modulation of MoS2 Nanosheets Enables Energy‐Saving H2 Production and Sulfide Degradation

Electrocatalytic hydrogen evolution and sulfion (S2−) recycling are promising strategies for boosting H2 production and removing environmental pollutants. Here, a nano‐Ni‐functionalized molybdenum disulfide (MoS2) nanosheet was assembled on steel mesh (Ni‐MoS2/SM) for use in sulfide oxidation reaction‐assisted, energy‐saving H2 production. Experimental and theoretical calculation results revealed that anchoring nano‐Ni on high‐surface‐area slack MoS2 nanosheets not only optimized catalyst adsorption of polysulfides but also played an important role in promoting hydrogen evolution reaction kinetics by absorbing OHad, thereby greatly enhancing the catalytic performance toward sulfide oxidation reaction and hydrogen evolution reaction. Meanwhile, the Ni/MoS2‐based hydrogen evolution reaction + sulfide oxidation reaction system achieved nearly 100% hydrogen production efficiency and only consumed 61% less power per kWh than the oxygen evolution reaction + hydrogen evolution reaction system, which suggested our proposed Ni‐MoS2 and novel hydrogen production system are promising for sustainable energy production.


Introduction
Currently, in the context of carbon neutrality, H 2 production based on electrochemical water splitting has been in the spotlight as an approach for sustainable, clean energy production, [1][2][3][4] with hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of critical importance to achieving efficient electrocatalytic water splitting. [5,6]However, anodic OER is rather sluggish and requires high anodic potential to produce only low-value O 2 , which has hampered its use in practical applications. [7,8][11][12] Specifically, the sulfide oxidation reaction (SOR, S 2À À 2e À = S, À0.48 V vs reversible hydrogen electrode, RHE) is an attractive candidate for use in meeting this goal since the SOR is much more thermodynamically favorable than the OER. [13,14][17] Nevertheless, the design and construction of bifunctional catalysts for achieving efficient SOR and HER remain extremely challenging, especially for catalysts used under alkaline conditions.
Generally, there are two reaction pathways in alkaline HER processes: Volmer-Heyrovsky or Volmer-Tafel step. [18,19]In terms of improving the overall kinetics of alkaline HER, many reported works have focused on optimizing the dissociation energy of H 2 O or H ad adsorption energy (ΔG H ). [20] However, little attention is paid to OH À and hydroxyl molecules (OH ad ), which are also important for HER performance. [21]Markovic [22] first found that generated OH ad staying on Ni (OH) 2 /Pt can affect the kinetics of the alkaline HER.Cao [23] indicated that accelerating the kinetics of the OH ad transfer (OH ad + e À ⇄ OH À ) could improve the sluggish kinetics of alkaline HER process.Koper [24] through experimental measurement and DFT modeling demonstrated that optimization of hydroxyl binding strength can accelerate the kinetics of alkaline HER process.The study of OH ad and OH À has contributed to improved understanding of HER kinetics, but consensus has yet to be reached.Therefore, studying the effect of OH ad on the kinetics of HER remains a huge challenge.27] To achieve this goal, one straightforward strategy would entail the development of metal-sulfide catalysts with good SOR activity and stability due to good thiophilic and sulfur corrosion resistance.One such catalyst, MoS 2 , has been extensively investigated and shown to hold promise for use in HER processes. [28,29]However, the electrocatalytic activity of MoS 2 catalyst is still limited by insufficient numbers of exposed active sites and slow water dissociation kinetics. [30,31]Thus, to enhance MoS 2 performance, new active sites must be created and adsorption energies of different reaction intermediates must be DOI: 10.1002/eem2.12644Electrocatalytic hydrogen evolution and sulfion (S 2À ) recycling are promising strategies for boosting H 2 production and removing environmental pollutants.Here, a nano-Ni-functionalized molybdenum disulfide (MoS 2 ) nanosheet was assembled on steel mesh (Ni-MoS 2 /SM) for use in sulfide oxidation reaction-assisted, energy-saving H 2 production.Experimental and theoretical calculation results revealed that anchoring nano-Ni on highsurface-area slack MoS 2 nanosheets not only optimized catalyst adsorption of polysulfides but also played an important role in promoting hydrogen evolution reaction kinetics by absorbing OH ad , thereby greatly enhancing the catalytic performance toward sulfide oxidation reaction and hydrogen evolution reaction.Meanwhile, the Ni/MoS 2 -based hydrogen evolution reaction + sulfide oxidation reaction system achieved nearly 100% hydrogen production efficiency and only consumed 61% less power per kWh than the oxygen evolution reaction + hydrogen evolution reaction system, which suggested our proposed Ni-MoS 2 and novel hydrogen production system are promising for sustainable energy production.
optimized. [32,33]Importantly, theoretical calculations in this work show that nickel possesses strong adsorption capacity for OH ad and sulfur intermediates, suggesting that creation of Ni-MoS 2 electrocatalysts with molecularly heterogeneous interfaces may accelerate the reaction kinetics.Besides, the creation of heterogeneous interfaces could greatly increase the number of available active sites for electrocatalytic reactions.Indeed, Ren [34] reported MoS 2 /Ni 3 S 2 heterostructure nanowires not only promoted a synergistic electrocatalytic effect stemming from the MoS 2 -Ni 3 S 2 interaction but also provided enormous number of active sites, good conductivity, and excellent wettability that enabled this electrocatalyst to provide excellent HER performance.For these reasons, use of this electrocatalyst provides a two-pronged approach for improving catalytic performance of HER and SOR systems based on the creation of a heterogeneous interface between MoS 2 and Ni. [20,23]ith these points in mind, herein a Ni nanoparticle-modified flower-like MoS 2 electrocatalyst was assembled on steel mesh (Ni-MoS 2 /SM) via a hydrothermal process followed by electrodeposition.As predicted, the Ni-MoS 2 /SM electrocatalyst exhibited attractive catalytic performance that supported both anodic SOR and cathodic HER processes when used with a simple electrochemical cell constructed from the Ni-MoS 2 /SM bifunctional electrocatalyst.Such a hybrid electrolyzer may achieve more cost-effective and sustainable hydrogen production than is achieved compared to current systems, which is illustrated in Figure 1a.In fact, this system requires only a low voltage of 0.49 V at 10 mA cm À2 , a much lower voltage than that needed for electrochemical water splitting (theoretically 1.23 V).Furthermore, relative to electrochemical water-splitting systems, this system remained stable for over 100 h of operation and required 61% lower electricity consumption to produce the same amount of H 2 , which achieved efficient energy-saving H 2 production and sulfion (S 2À ) recovery.

Structural and Morphological Analyses
To promote a strong interaction between Ni and MoS 2 and optimize the catalytic activity of MoS 2 , MoS 2 nanosheets were first grown on steel mesh (SM) using a hydrothermal method.As shown in Figures S3-S5, Supporting Information, flower-like MoS 2 nanospheres uniformly grew to cover the entire surface of the SM.Meanwhile, due to the attraction between Ni 2+ and negative charges of MoS 2 (Figure S6, Supporting Information), Ni 2+ was electrodeposited onto MoS 2 to form Ni-MoS 2 , which resulted in generation of Ni-MoS 2 /SM that retained the original MoS 2 flower-like morphology that was incorporated within the overall thin interconnected nanosheet structure (Figure 1b-f and Figure S7, Supporting Information).Furthermore, high-resolution transmission electron microscopy (TEM) images of Ni-MoS 2 (Figure 1e,f) revealed lattice fringe structures of lengths 0.618 and 0.203 nm corresponding to the (002) crystal plane of MoS 2 and the (111) plane of Ni, respectively.Importantly, energy-dispersive Xray (EDX) elemental mapping (Figure 1g) revealed that Ni, Mo, and S elements were uniformly distributed throughout the nanosheets, thus indicating that the Ni nanoparticles were well dispersed on MoS 2 nanosheet surfaces.
Furthermore, chemical compositions and value states of samples were investigated using X-ray photoelectron spectroscopy (XPS), with results shown in Figure S9, Supporting Information and Figure 1h-j.Peaks observed at 229.1 and 233.1 eV in the Mo 3d XPS spectrum of MoS 2 correspond to the binding energies of Mo 4+ , [35] while peaks at 236.2 and 232.3 eV correspond to binding energies of Mo with a higher oxidation state (Mo 6+ ) due to MoS 2 exposure to air, [36] and peaks at 226.2 eV corresponded to S 2s. [31,37]As compared with the binding energy of MoS 2 , Ni-MoS 2 /SM-binding energy values of Mo 3d 5/2 (228.6 eV) and Mo 3d 3/2 (235.7 eV) shifted in the negative direction by 0.4 eV, suggesting the strong electronic coupling of MoS 2 and Ni occurred during heterostructure construction.Besides, the decrease in peak intensity of Mo 4+ and S 2s in Ni-MoS 2 may be due to the deposition of Ni on the surface of MoS 2 and the formation of strong interactions between Ni and MoS 2 .In contrast, the binding energy of the S 2p 3/2 (161.0 eV) in the Ni-MoS 2 was shifted in a positive direction by 0.4 eV relative to that of MoS 2 (Figure 1i), thus confirming a strong interaction between Ni and MoS 2 .These results were consistent with XPS results showing similar shifts in XPS spectral peak positions corresponding to the Ni 2p 3/2 and Ni 2p 1/2 signal, with Ni-MoS 2 /SMnegative shifts observed of about 0.3 eV relative to corresponding Ni/ SM spectrum peak positions (Figure 1j).Taken together, the abovementioned results suggest that Ni can act as an electron donor site to support generation of good coupling interfaces within Ni-MoS 2 that can further enhance Ni-MoS 2 electrocatalytic activity.

Electrocatalytic Performance of Ni-MoS 2 /SM
Hydrogen evolution reaction performance of the Ni-MoS 2 /SM was investigated in 1 M KOH aqueous solution using a typical threeelectrode system, where the self-supported electrode directly served as the working electrode.The Ni-MoS 2 /SM catalyst was optimized by adjusting the electrodeposition time (Figure S10, Supporting Information).Comparisons of electrocatalytic performances between Ni-MoS 2 /SM and reference electrocatalysts (pure SM, Ni/SM, MoS 2 /SM, and 20% Pt/C) using linear sweep voltammetry (LSV) curves (Figure 2a) revealed active Ni-MoS 2 /SM HER participation requiring a lower overpotential (109 mV) to generate the required current (10 mA cm À2 ) as compared to overpotentials required by SM (509 mV), Ni/ SM (324 mV), and MoS 2 /SM (214 mV).Moreover, the Tafel curve slope value obtained for Ni-MoS 2 /SM (128 mV dec À1 ) was lower than corresponding values obtained for SM (189 mV dec À1 ), Ni/SM (136 mV dec À1 ), and MoS 2 /SM (141 mV dec À1 ) (Figure 2b).This result implies that the Volmer step of HER can be fasted by nano-Ni that is integrated into MoS 2 nanosheets.Taken together, the abovementioned results revealed a synergistic effect between Ni and MoS 2 that enhanced electrocatalyst intrinsic activity and HER dissociation kinetics.
To estimate the electrochemical active surface area (ECSA) of MoS 2 / SM and Ni-MoS 2 /SM electrocatalysts, electrochemical double-layer capacitance (C dl ) was calculated by cyclic voltammetry (CV) curves plotted for different scan rates within the non-Faraday region (Figure S11, Supporting Information).The results obtained for Ni-MoS 2 /SM were associated with a greater C dl value (61.8 mF cm À2 ) than that obtained for MoS 2 /SM (52.2 mF cm À2 ) and Ni/SM (4.79 mF cm À2 ), indicating that the generated Ni-MoS 2 interface can provide more reaction sites for HER and SOR.Meanwhile, Nyquist plots revealed that the charge transfer resistance value (R ct ) of Ni-MoS 2 /SM was lower than those obtained for Ni/SM and MoS 2 /SM (Figure S12, Supporting Information).This improved charge transfer is attributed to nano-Ni altering the electronic structure of the MoS 2 surface to eventually accelerate HER reaction kinetics.In addition, microscopic-level Energy Environ.Mater.2024, 7, e12644 2 of 8 examination revealed that bubbles generated on the Ni-MoS 2 /SM surface during the HER process were very small and were quickly released (Video S1, Supporting Information), characteristics that promoted hydrogen evolution at high current density. [38]Next, chronoamperometry test results showed that Ni-MoS 2 /SM still maintained excellent catalytic activity after more than 32 h of operation (Figure S13, Supporting Information).Furthermore, the durability of the Ni-MoS 2 /SM electrode was evaluated at consecutive time points for different current densities, with no obvious degradation of electrode activity detected at both low and high current densities (Figure S14, Supporting Information).Additionally, SEM, TEM images (Figure S15, Supporting Information), and XPS results (Figure S16, Supporting Information) indicated no obvious electrode structural changes after long-term operation over multiple reaction cycles.It is worth noting that the stability of an electrocatalyst is also affected by bubbles since bubbles generated by the HER process gathering on the catalyst surface can block active sites to thereby increase internal resistance and reduce water decomposition efficiency, while bubble detachment from electrode surfaces can trigger mechanical shock that damages the  electrocatalyst. [39]In our experiment, HER-generated tiny bubbles quickly left the Ni-MoS 2 /SM surface (Video S1, Supporting Information), resulting in improved electrode mechanical stability.Collectively, these results indicate good durability and outstanding stability of Ni-MoS 2 /SM.
In order to evaluate SOR performance of the Ni-MoS 2 /SM electrode, the electrode was operated in 1 M NaOH containing 1 M Na 2 S. The LSV curves (Figure 2c) exhibit that in the absence of Na 2 S, Ni-MoS 2 /SM electrode operation required 1.51 V to generate the required OER current density of 10 mA cm À2 .By contrast, after adding Na 2 S, only 0.35 V was needed to generate the required 10 mA cm À2 for the SOR process, thus indicating that the SOR is thermodynamically more favorable than the OER process.Moreover, CV curves (Figure S17, Supporting Information) further indicated that the potential of SOR was significantly negative than that of the OER.Meanwhile, the Ni-MoS 2 /SM electrode exhibited high SOR catalytic activity that was comparable to that reported for other SOR electrodes (Table S1, Supporting Information), with Ni-MoS 2 /SM activity observed at 0.35 V@10 mA cm À2 and a minimum Tafel slope value observed of 80 mV dec À1 (Figure 2d,e).By contrast, Ni/SM and MoS 2 /SM electrodes delivered relatively lower currents and slower SOR kinetics upon polarization as compared to the Ni-MoS 2 /SM electrode.Electrochemical impedance spectroscopy (EIS) (Figure 2f) showed the R ct of Ni-MoS 2 /SM was lower than that of Ni/ SM and MoS 2 /SM.In addition, Ni-MoS 2 /SM electrode maintained relatively stable current density with almost negligible decay after i-t testing (as revealed through analysis of LSV curves) (Figure S18, Supporting Information).Furthermore, SEM and XPS analyses conducted after SOR i-t testing (Figures S19 and S20, Supporting Information) revealed good sulfur tolerance of the Ni-MoS 2 /SM electrode during the SOR process.

Mechanism Investigation into the Electrochemical SOR and HER of Ni-MoS 2 / SM
The mechanism by which Ni greatly improves MoS 2 -based catalyst sulfur oxidation and hydrogen evolution performance is unknown, prompting this study to clarify key forces driving improved performance based on experimental results and theoretical calculations.First, performances of electrochemical S 2À conversion processes conducted using different electrocatalysts were evaluated through the CV test and ultraviolet and visible (UV-Vis) spectrophotometric detection of reaction products (Figure 3).Results obtained for Ni-MoS 2 /SM revealed the highest CV curve oxide peaks, which indicated a greater polysulfide formation rate relative to rates obtained for other electrocatalysts.Moreover, electrolyte UV-vis spectra (Figure 3b) contained an obvious absorption peak at 300 nm, demonstrating generation of short-chain polysulfides (S 2À 2 ). [13,40]To further explore products that form during long-term SOR, galvanostatic electrolysis was conducted for over 20 h using the Ni-MoS 2 /NF electrode, during which the color of the anodic electrolyte gradually changed from transparent to light yellow then gradually changed to dark yellow as electrolysis duration increased (Figure S21, Supporting Information).Meanwhile, a new peak at 370 nm appears in the UV-vis spectra of electrolytes, corresponding to the S 2À 4 polysulfides (Figure 3c).As compared to results presented in Figure 3b, a new peak was visible after long-term SOR, indicating that our electrode continuously catalyzed the conversion of polysulfides from short-chain to long-chain forms, as consistent with the observed formation of a yellow sulfur-containing substance around the electrode.Notably, collection of this yellow substance from the S 2À 2 -containing anolyte followed by addition of H 2 SO 4 , centrifugation, washing, and drying produced a sulfur-containing product that was confirmed via XRD to contain S 8 molecules formed via disproportionation of polysulfides (Figure 3d).Thus, these results collectively verified that polysulfides were produced during the SOR, as has been widely reported for Na-S battery systems, [41,42] and nanonickel enhances the performance of the catalyst for SOR.
To further investigate the possible mechanism underlying enhanced SOR and HER electrocatalytic activity of Ni-MoS 2 , density functional theory (DFT) calculations were performed using Ni (111) and MoS 2 (002) as comparative models that were selected based on TEM and XRD results.As for SOR process, based on our experimental results, only considering the adsorption of intermediates (S Ã and S Ã 2 ) (Figure 4a and Figure S22, Supporting Information).In general, the binding strength between the catalyst and the reaction intermediate should reflect the electrocatalytic activity of the catalyst.Based on this concept, when adsorption energy of S species on a catalyst is approximately zero (ΔG = 0), this catalyst is considered as a good candidate for SOR, with computed corresponding free energy profiles presented in Figure 4b.From these results, it can be seen that overly weak adsorption of S Ã 2 onto MoS 2 (002) and overly strong adsorption of S Ã and S Ã 2 onto Ni (111) limit the catalytic activity of single MoS 2 and Ni.However, the

introduction of Ni onto MoS 2 optimizes adsorption of intermediates (S Ã and S Ã
2 ) while reducing the energy barrier to thereby enhance catalytic activity.Specifically, S Ã 2 on Ni-MoS 2 exhibited a positive energy of 0.13 eV, suggesting that short-chain S 2 species are main products that form in the electrolyte.Therefore, the high alkaline SOR activity can be attributed to the synergistic effect from the Ni/MoS 2 interface: 1) the strong binding energy of Ni with S of MoS 2 reduces the energy barrier of conversion of S 2À to S Ã and S Ã to S Ã 2 ; and 2) the interaction of nano-Ni with MoS 2 facilitates the near-optimal adsorption of intermediates (S Ã and S Ã 2 ) and its subsequent desorption.Furthermore, the results of DFT analysis in Figures S23 and S24, Supporting Information revealed that Ni (111) interacts more strongly with OH ad than with MoS 2 (002).Notably, the proposed simplified Ni-MoS 2 structure-based model explained the observed structure-activity relationship by revealing that H ad adsorbed to Mo sites while OH ad adsorbed more stably to Ni sites, such that as additional H ad adsorbed to Mo sites, the ΔG decreased to 0.33 eV.This result indicated that H 2 readily underwent desorption (Figure 4c), as consistent with results obtained by exchanging Ni-MoS 2 model H ad and OH ad adsorption positions (Figure S25, Supporting Information), which increased the H 2 desorption barrier rate.Figure 4d shows the catalytic roles of Ni and MoS 2 in the alkaline HER process, with results indicating that Ni both improved hydroxyl adsorption and prevented OH ad from recombining with H ad to synergistically increase the HER rate, as consistent with the abovementioned experimental results (Figure 2a,b).After water dissociation, Ni sites strongly interacted with OH ad , which prevented OH ad from recombining with H ad , allowing H ad to adsorb to nearby vacant Mo sites in Ni-MoS 2 .Finally, two H ad atoms on the MoS 2 surface recombined to form H 2 .As a result, the HER occurred on the Ni-MoS 2 was greatly accelerated by integration of nano-Ni.Thus, these DFT results indicate that catalyst composition may affect adsorption of different intermediates to thereby optimize OH ad and H ad adsorption energy and increase H 2 desorption.Furthermore, the enhanced alkaline HER activity can be attributed to the synergistic effect from the Ni/ MoS 2 interface: 1) nano-Ni provides the active sites for favorable hydroxyl adsorption; and 2) thanks to the help of nano-Ni, MoS 2 facilitates adsorption of H Ã intermediates and subsequent H 2 desorption.In a word, the abovementioned calculated results indicate that nickel plays an important role in improving MoS 2 catalytic performance in both SOR and HER processes.

Ni-MoS 2 /SM Dual-Electrode System for Coupling SOR + HER Catalysis
To evaluate the practical applications of our proposed electrochemical SOR-assisted hydrogen production system, a dual-electrode cell was established for use in performing SOR + HER, with Ni-MoS 2 /SM acting as both anode and cathode (Figure 5a).During the operation of this dual-electrode cell, H 2 was continuously released from the cathode as anolyte color gradually changed from colorless to yellow, indicating polysulfides formation.In Figure 5b, the LSV curve of the HER + SOR system showed an apparently decreased operating voltage (below 0.49 V) as compared to that of the water-splitting HER + OER system.As shown in Figure 5c and Table S2, Supporting Information, the SOR-assisted hydrogen production system was more energy efficient than most previously reported room-temperature-operated aqueous electrochemical hydrogen production systems, while the performance of our dual-electrode was comparable to that of current state-of-the-art bifunctional catalysts used in SOR + HER systems and other systems.Moreover, long-term durability testing of the Ni-MoS 2 /SM electrode in our SOR + HER system revealed a high degree of durability, as reflected by a chronoamperometry value of 0.6 V (Figure 5d).Notably, during prolonged electrolysis, S 2À 2 in the solution was continuously consumed, as reflected by decreasing S 2À concentration and decreasing electrolytic current density with prolonged system operation.However, continual refreshing of the anode electrolyte during system operation restored current density to about 30 mA cm À2 and maintained this current density for over 100 h of operation, thus indicating the good durability and catalytic stability of the Ni-MoS 2 /SM for both HER and SOR.In addition, the hydrogen production efficiency of the SOR-assisted hydrogen production system approached 100% (Figure S26, Supporting Information), thus indicating this system provided >2.53-fold greater H 2 yield with 61% less electricity consumption per kWh than OER + HER system.Taken together, the abovementioned results demonstrate that development of low-cost systems that achieve hydrogen production coupled with S 2À removal is feasible through incorporation in these systems of a simple, nonprecious metal-containing Ni-MoS 2 / SM electrode.

Conclusion
In summary, we successfully fabricated a Ni-MoS 2 /SM catalyst via a two-step process involving hydrothermal step followed by electrodeposition for efficient desulfurization and H 2 production.Experimental and theoretical calculation results revealed that anchoring nano-Ni on highsurface-area slack MoS 2 nanosheets not only reduces the energy barrier of S 2À conversion but also played an important role in promoting HER kinetics by absorbing OH ad , thereby greatly enhancing the catalytic performance toward SOR and HER.Meanwhile, HER + SOR system based on Ni/MoS 2 significantly reduces energy consumption by 61% (per kWh) compared to OER + HER system.This work proposed an effective strategy for low-cost hydrogen production and electrochemical desulfurization.
Synthesis of MoS 2 /SM: First, SM was ultrasonically cleaned in 1 M HCl, ethanol, and deionized water, respectively.Second, 1 mmol Na 2 MoO 4 (0.243 g) and 1 mmol L(+)-cysteine (0.121 g) were added to 35 mL deionized water to form a clear solution under stirring.Finally, the obtained solution and cleaned SM (4 × 3 cm 2 ) were transferred into a 50 mL Teflon-lined stainless-steel autoclave, and kept at 200 °C for 24 h to grow MoS 2 .Then, the obtained black sample was taken out, wash it thoroughly with deionized water and ethanol, and then dried under vacuum at 60 °C for 6 h.
Synthesis of Ni-MoS 2 /SM: Ni-MoS 2 /SM was prepared by a simple electrodeposition process in a three-electrode electrochemical system with MoS 2 /SM and Pt flake as the working and counter electrodes.The electrolyte was made of a bimetallic solution consisting of 4 M HBO 3 and 0.8 M NiCl 2 Á6H 2 O.After operating at À1.0 V (vs SCE) for 1 min, the obtained sample was washed with deionized water and dried at 60 °C in vacuum oven.For comparison, Ni/SM was also prepared under the same condition, and the corresponding SEM (Figure S1, Supporting Information) and XRD pattern (Figure S2, Supporting Information) indicate that nano-Ni is uniformly deposited on the surface of SM.
Electrochemical measurements: An electrochemical workstation (CHI 760E; Shanghai CH Instruments, Co., China) was used for all electrochemical tests.HER catalysts were tested in 1 M KOH solution.All linear sweep voltammetry (LSV) curves were measured at 5 mV s À1 .The long-term durability performance was measured by using a chronopotentiometry test under 50 mA cm À2 for 20 h.Electrochemical impedance spectroscopy (EIS) data were collected at À200 mV (vs RHE) with frequencies ranging from 100 kHz to 0.01 Hz.Cyclic voltammetry (CV) tests were measured in a potential window from À0.9 to À0.8 V versus Hg/HgO without Faradic current at different rates from 20 to 100 mV s À1 .All the potentials are referred to as the reversible hydrogen electrode (RHE).
The electrocatalytic performance of catalysts for SOR was evaluated in a three-electrode system with carbon and Hg/HgO (1 M KOH) as the counter and reference electrodes, respectively.The electrolyte consisted of 1 M NaOH and 1.0 M Na 2 S. Before the electrochemical test, the N 2 was bubbled into the electrolyte for more than 30 min.The CV measurements were conducted at a scan rate of 5 mV s À1 .In all the experiments, the surface area of the working electrode was controlled around 0.5 cm 2 .
Alkali-alkali SOR/HER cell were evaluated in H-type cell with a Nafion membrane (N-117, Dupont) to separate anode chambers and cathode chambers.Ni-MoS 2 /SM were used as cathode and anode.The catholyte and anolyte consisted of 1 M NaOH solution and consisted of 1 M Na 2 S and 1 M NaOH, respectively.
The volume of H 2 evolved from the working electrode at 20 mA cm À2 for HER was measured using displacement method.The theoretical amount of H 2 evolved during HER process was calculated by the following formula: n (H 2 ) = It/ 2F, where n is the amount of H 2 (mol), t is HER time (s), I is the measured current (A), and F is the Faraday constant (96 485 C mol À1 ).
DFT calculation: All calculations are based on density function theory (DFT), [43] using the Cambridge Serial Total Energy Package (CASTEP) codes [44] of the Materials Studio software package. [45]The Perdew-Burke-Ernzerhof (PBE) [46] exchange-correlation functional of the generalized gradient approximation (GGA) [47] is adopted.Based on our experimental analysis, we choose Ni (110) crystal plane, pristine MoS 2 , and Ni-doped MoS 2 as the computational models in the calculations.The geometric optimization uses a 5 × 5 × 1 k-point sample of the Monkhorst-Pack grid in the Brillouin zone.To ensure accuracy, the cutoff energy is chosen to be 300 eV, and the convergence tolerances for energy and force are set to 1 × 10 À4 eV atom À1 and 0.03 eV ÅÀ1 , respectively.The adsorption energy of the intermediates is calculated by the following equation: E abs = E(AB) À E(A) À E(B), where E(AB) is the energy of system AB after the adsorption of substance A (intermediates) on substrate B (pristine Ni, MoS 2 , and Ni-doped MoS 2 ), E(A) is the energy of the isolated substance A, and E(B) is the energy of the isolated substrate B. Negative adsorption energy indicates that the adsorption process is exothermic and the adsorption system is thermodynamically stable. [48]

Figure 1 .
Figure 1.a) Schematic illustration of simultaneous desulfurization and hydrogen evolution using Ni-MoS 2 /SM; b) SEM; c-f) HR-TEM images of Ni-MoS 2 (where f is an enlargement of the yellow circle in e); g) dark-field TEM image and relevant elemental mapping images of Ni, Mo, and S elements in the samples; h) Mo 3d; and i) S 2p XPS spectra of MoS 2 /SM and Ni-MoS 2 /SM; and j) Ni 2p XPS spectra of Ni/SM and Ni-MoS 2 /SM.

Figure 2 .
Figure 2. a) LSV curves of MoS 2 /SM, Ni/SM, Ni-MoS 2 /SM, and Pt/C without IR correction; b) corresponding Tafel slopes; c) comparison of the LSV between the SOR and OER on the Ni-MoS 2 /SM electrode; d) LSV curves; e) Tafel slopes; and f) EIS plots of the MoS 2 /SM, Ni/SM, and Ni-MoS 2 /SM for SOR.

Figure 3 .
Figure 3. a) CV curves of Ni/SM, MoS 2 /SM, and Ni-MoS 2 /SM electrode in 1.0 M NaOH + 1.0 M Na 2 S solutions.b) The corresponding UV-vis spectra of electrolyte after CV test.c) UV-vis spectra before and after stability test.d) XRD characterization of product S.

Figure 4 .
Figure 4. a) Binding energy of S 1 and S 2 on the Ni 1 -MoS 2 surfaces for DFT calculation; b) the free energy profiles of the formation of polysulfides (S Ã x ) on Ni (111), MoS 2 (002), and Ni-MoS 2 surface; c) Gibbs free energy of OH ad , H ad , and H 2 adsorb on Ni-MoS 2 ; and d) The schematic illustration of the catalytic role of Ni and MoS 2 during the alkaline HER process.

Figure 5 .
Figure 5. a) Optical photograph of the two-electrode system for HER + SOR within 1 M NaOH and 1 M Na 2 S + 1 M NaOH solutions, respectively; b) a comparison of the LSV curves for SOR + HER cell and OER + HER cell; c) comparison of several room-temperature electrolytic hydrogen production systems in alkaline medium; d) durability measurement of Ni-MoS 2 /SM catalysts for SOR + HER cell at 0.6 V.The vertical lines mark the timespan to replace fresh electrolyte; and e) comparison of H 2 production consuming 1 kWh electricity between SOR + HER cell and OER + HER cell at 10 mA cm À2 on the Ni-MoS 2 /SM electrode.