Insights to Industrial Electrocatalytic Sulfide Oxidation: Electrode Fabrication, System Engineering, and High-Value Product Acquisition

Insights to Industrial Electrocatalytic Sulfide Oxidation: Electrode Fabrication, System Engineering, and High-Value Product Acquisition | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL Energy & Environmental Materials This is a preprint and has not been peer reviewed. Data may be preliminary. 21 October 2025 V1 Latest version Share on Insights to Industrial Electrocatalytic Sulfide Oxidation: Electrode Fabrication, System Engineering, and High-Value Product Acquisition Authors : Zhongyuan Wang , Yinxi Han , Yufei Zhang , Jinhuan Chen , and Jiade Wang 0000-0002-9496-8551 [email protected] Authors Info & Affiliations https://doi.org/10.22541/au.176108062.20281769/v1 Published ENERGY & ENVIRONMENTAL MATERIALS Version of record Peer review timeline 339 views 200 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract The electrocatalytic sulfide oxidation reaction (SOR) has emerged as a promising strategy for removing reduced sulfur, owing to its low thermodynamic energy barrier and capacity to generate high-value products. However, current progress remains largely restricted to laboratory studies, and industrial-scale implementation is urgently needed to achieve effective sulfur pollution control and resource recovery. This review provides a comprehensive overview of recent advances in catalyst fabrication, reaction system engineering, and product acquisition, all of which offer opportunities to accelerate the industrial development of SOR. Particular attention is paid to the existence of diverse reduced sulfur species (S2-, HS-, H2S) under varying industrial conditions, as these strongly affect catalyst mechanisms and application feasibility. Building on this foundation, several catalyst design strategies are discussed to enhance catalyst stability and activity. Beyond catalyst design, emphasis is placed on coupling SOR with diverse cathodic reactions for integrated applications and on leveraging novel electrolytic devices to improve process efficiency. Pathways to high-value products are also highlighted, with a focus on diversifying product types and developing cost-effective recovery strategies. This review concludes by discussing current challenges and future opportunities, aiming to provide guidance for advancing SOR from laboratory research to sustainable industrial practice. 1. Introduction Reduced sulfur species (S(-II)), which are commonly present in aqueous systems as hydrogen sulfide (H 2 S), hydrosulfide (HS - ), and sulfide ions(S 2- ), are generated from natural gas processing, [1] petroleum refineries, [2] and municipal wastewater treatment. [3] Owing to its electron-rich characteristics and strong electron-donating capability, reduced sulfur often induces corrosion of steel pipelines during industrial production and treatment processes. [4] Moreover, their strong reductive properties enable them to readily bind with the metal centers of metalloenzymes in living organisms, thereby posing significant health risks to human. [5] Current methods for treating reduced sulfur species include thermal, chemical, biological, and photochemical approaches. However, these techniques often incur high operational costs and result in the generation of undesirable byproducts such as SO 2 and SO 4 2- . [6,7] The electrocatalytic SOR enables the direct conversion of reduced sulfur species into elemental sulfur (S 0 ) at the anode by precisely controlling the applied potential, thereby preventing further oxidation into sulfate ions. Importantly, the low thermodynamic energy barrier of SOR (0.14 ~ 0.48 V vs RHE) [8,9] enables energy-efficient treatment of reduced sulfur species, thus providing both environmental and economic benefits. Despite these advantages, most SOR research remains at laboratory scale. Achieving industrial application is therefore essential for effective large-scale sulfur remediation and resource utilization. The key barriers include the design of high-performance catalysts, the development of practical reaction systems, and the cost-efficient recovery of high-value products. This review systematically examines recent developments in these areas, with the aim of advancing SOR toward industrialization ( Figure 1 ). Three major challenges are highlighted. First, catalyst studies at the laboratory scale often prioritize either high catalytic activity or resistance to sulfur poisoning, but rarely address both simultaneously. For instance, Zhou et al. [10] demonstrated a tip-enhanced electric field strategy that enriched sulfur at catalyst tips, significantly accelerating reaction kinetics. By contrast, Deng et al. [11] proposed encapsulation protection strategy that prevented insulating S 8 deposits from covering active sites and lowered the reaction barrier. An effective industrial SOR catalyst, however, must integrate both strong adsorption for reaction initiation and efficient desorption to maintain long-term activity and stability. [12] To this end, we review diverse catalyst design strategies that could contribute to the development of multifunctional SOR catalysts for industrial applications. Second, anodic SOR studies conducted under idealized laboratory conditions rarely account for the complexity of real industrial environments, where sulfide species coexist with other pollutants. Designing appropriate reaction coupling systems can address these challenges while improving reaction kinetics. For example, coupling SOR with hydrogen evolution reaction (HER), [13] carbon dioxide reduction reaction (CO 2 RR), [14] or nitrate reduction reaction (NO 3 RR) [15] enables more flexible adaptation to industrial demands. Furthermore, the adoption of flow electrolyzers and other advanced electrochemical devices can enhance current densities, thereby meeting the performance requirements for industrial-scale operation. [16,17] Finally, product recovery remains a bottleneck for practical applications. In most laboratory studies, sulfur recovery involves adding hydrochloric acid to convert soluble polysulfides into solid S 8 , a process that increases cost and limits scalability. [18] Moreover, a single-product strategy provides only limited economic benefits for large-scale deployment. To address these issues, we review recent advances aimed at reducing the cost of S₈ recovery and diversifying product streams, with the goal of improving both economic feasibility and industrial value. Figure 1. Schematic illustration of the industrialization progress of the SOR process in terms of electrode design strategies, multi-scenario application systems, and high-value product recovery. In this work, we first propose that SOR research should distinguish the specific forms of reactants according to different application scenarios. We also provide a systematic overview of the theoretical calculation methods and product analysis techniques applied in SOR studies. Furthermore, we comprehensively summarize advanced catalyst design strategies for SOR based on the d-band center theory, [19] the soft acid-soft base theory, [20] the encapsulation protection strategy, [21] and the electric field regulation theory. [22] These well-established theoretical frameworks and strategies are expected to collectively address the dual requirements of high catalytic activity and sulfur-poisoning resistance for the industrial application of SOR catalysts. Moreover, we review and classify existing coupling systems and potential application scenarios to support the implementation of SOR in complex industrial environments. We also summarize newly emerging pathways for S 8 low cost recovery and the synthesis of other high-value products. Finally, we revisit and provide future perspectives on SOR industrialization from three key dimensions: electrode fabrication, application systems, and high-value product recovery, aiming to facilitate the transition of SOR toward industrial deployment. 2. Sulfur Electrooxidation 2.1 SOR Mechanisms Involving Different Reduced Sulfur Species Recently, few studies have systematically clarified the distinct branches of the SOR, and prior works have proposed differing views on its direct reactants. [23,24] In this section, based on the practical application scenarios of SOR in different industrial environments, we classify the SOR process into the oxidation of H 2 S, HS - , and S 2- , and provide a mechanistic analysis for each pathway. In the natural gas extraction and processing industry, [25] the wastewater treatment industry, [26] and the sulfate-based pulping industry, [27] H 2 S is generated as an acidic by-product. When electrolyzing weakly acidic wastewater containing gaseous H 2 S directly, the H 2 S molecules are first adsorbed onto the catalyst surface. After two successive deprotonation steps, H 2 S is ultimately electrooxidized to form solid sulfur (S 8 ). This process proceeds via the sequence shown in Equations (1)-(4) [28] , where * denotes the catalytic active site on the electrode surface. For H 2 S, alkaline absorption is commonly applied in industrial settings. Based on the dissociation constant (pK) of H 2 S and the Henderson-Hasselbalch equation, [29] it can be inferred that the predominant species of the weakly acidic H 2 S gas in alkaline solution is HS - . The anodic electrooxidation of H 2 S following alkaline absorption proceeds via the sequence shown in Equations (5)-(7), [30] where HS - is the principal electrooxidized species. HS * and S * represent adsorbed catalytic intermediates on the electrode surface. HS - undergoes deprotonation and oxidation on the electrode surface (HS - + * → H * + S * + e - ), and elemental sulfur accumulates stepwise through polysulfide intermediates (S * → S 2 * → S 3 * → S 4 * → S 8 * ). Wastewater containing sulfide ions (S 2- ) is commonly produced in metal smelting [31] and leather manufacturing processes. [32] The electrooxidation of S 2- involves a two-electron transfer and leads to the formation of polysulfides, which resembles part of the HS - oxidation pathway. However, many existing studies do not clearly define the application context or the primary reactant involved. During experimental design, researchers should consider whether to use an NaOH solution containing H 2 S gas or an Na 2 S solution as the test electrolyte. The former primarily involves HS - as the reactive species, while the latter features S 2- as the main reactant. This distinction is essential for mechanistic interpretation of catalytic behavior and for accurately constructing adsorption models in DFT calculations. The electrocatalytic oxidation pathway of S 2- is illustrated in Equations (4). [33,34] 2.2 Catalyst Performance and Theoretical Evaluation 2.2.1 Electrocatalytic Performance Testing At the fundamental research stage, the electrochemical reaction system is typically constructed using an H-type electrolytic cell combined with a three-electrode configuration to evaluate the performance and durability of electrode materials. The reactor can be separated using either a proton exchange membrane or a cation exchange membrane. For investigating SOR reactions under acidic and alkaline conditions, Ag/AgCl and Hg/HgO electrodes could be used as reference electrodes, respectively. In studies focusing on HS - formed by absorbing H 2 S with alkaline solution, the electrolyte should consist of NaOH saturated with H 2 S gas. [35] For simulating sulfide-rich wastewater containing S 2- , a mixture of 1 M Na 2 S and 1 M NaOH or KOH is generally used. The use of a 1 M S 2- concentration allows for convenient calculation of the actual electrode potential relative to the standard redox potential, as shown in Equation (8). A higher pH environment can accelerate the dissolution of polysulfides in solution, thereby mitigating sulfur poisoning of the electrode surface to some extent. Reaction kinetics can also be improved by increasing the concentration of Na 2 S, with S 2- concentrations typically raised up to 3 M. One research group has reported that by employing a flow-type electrolyzer and optimizing kinetic conditions, the current density of the SOR process was increased to the ampere level. [36] From an engineering perspective, the study of SOR kinetics is of great significance, as the reaction rate directly determines the efficiency and economic viability of product generation. Therefore, a well-designed electrochemical system should incorporate appropriate parameters such as electrolyte concentration and temperature. These factors can enhance the diffusion and exchange of reactants and products. [37-40] For linear sweep voltammetry (LSV), the onset potential at a current density of 10 mA cm -2 is often used to compare the catalytic performance of materials reported in different studies. It is important to note that the standard redox potentials vary depending on the type of electrolyte. Therefore, the accurate theoretical redox potential should be calculated using the Nernst equation according to the specific concentration of the electrolyte. [41] The measured onset potential should not be lower than the theoretically calculated redox potential. Based on this, the overpotential corresponding to a given current density can be determined with precision. [42] By transforming the LSV curve, the relationship between the logarithm of current density and overpotential can be obtained, enabling the construction of a Tafel plot (η = a + b log j). The Tafel slope (b) directly reflects the reaction kinetics of SOR, [28,43,44] and an outstanding catalyst is characterized by a smaller Tafel slope in the SOR process. Long-term stable electrolysis is a direct indicator of a catalyst’s resistance to sulfur poisoning. Therefore, either chronopotentiometry or chronoamperometry can be used for extended electrolysis testing. If electrolysis is not conducted in a flow-type electrolyzer, the electrolyte should be refreshed periodically during the process to ensure stable mass transport and diffusion of HS - or S 2- . [11,14] 2.2.2 Theoretical Analysis of SOR via DFT Methods Density functional theory (DFT) enables the description of charge distribution within a reaction system and is widely employed to investigate its thermodynamics and kinetics, thereby assessing both the feasibility of the reaction and its rate-determining step (RDS). Specifically, DFT enables the calculation of adsorption energies of intermediates, the structure of active sites, activation energy barriers, as well as preferred reaction pathways and mechanisms. [45-47] In electrocatalytic reactions, the adsorption strength of reactants and products on the catalyst surface plays a critical role in determining both catalytic activity and long-term stability. Interestingly, the definition of a suitable adsorption energy varies among different electrocatalytic reactions. For example, in the HER, a promising catalyst typically exhibits a hydrogen adsorption free energy close to zero, which facilitates both the adsorption of protons and the desorption of hydrogen. [48-50] Figure 2. a) The left figure depicts a schematic illustration of the interfacial interaction between S₈ and solid surfaces, and the right figure shows contact angle measurements of molten sulfur droplets at 120 ℃ on different substrates. [12] Copyright 2021, Wiley. b) Free energy diagram for the accumulation of sulfur atoms into polysulfides on various crystal facets of NiMoN during the SOR process. [51] Copyright 2024, Wiley. c) UV-Vis absorption spectra of the SOR electrolyte after different numbers of reaction cycles. [23] Copyright 2025, Elsevier. d) ATR-FTIR spectra of the NiSe/NF electrode during chronoamperometric testing in a mixed solution of 1.0 M Na 2 S and NaOH. e) In situ Raman spectra of the NiSe/NF electrode in 1.0 M Na 2 S + NaOH solution within the potential range of 0.277 ~ 0.602 V vs RHE. [52] Copyright 2022, Elsevier. In DFT studies of SOR, strong interactions between the catalyst and reactants (HS - , S 2- , or H 2 S) are required in the initial stages, as indicated by more negative adsorption energies. In contrast, the desorption of products such as S 8 calls for weaker interactions and thus more positive adsorption energies. For example, Zhang et al. [12] proposed that the theoretical desorption behavior of S 8 on catalysts can be intuitively visualized by the contact angle of molten sulfur on the catalyst surface ( Figure 2a ), a correlation that has been confirmed by other researchers. [36] Free energy diagrams are commonly used to determine energy barriers between reaction intermediates, assess the spontaneity of reactions, and identify the RDS, and this approach has been widely adopted by researchers studying SOR. For instance, Wang et al. [51] calculated the energy barriers for the stepwise oxidation of S 2- to S 8 on NiMoN/MNF (Figure 2b), and the results indicated that strong chemical interactions between sulfur intermediates and the catalyst facilitate the reaction by lowering the energy barriers. The overall energy barriers on all crystal facets of NiMoN were found to be negative, suggesting thermodynamically favorable conditions for SOR. In addition, electronic structure analyses such as Bader charge analysis, differential charge density, and density of states (DOS) calculations can further reveal charge transfer and orbital interactions between the catalyst and reactants. 2.3 Advanced Techniques for Product Analysis Nowdays, electrochemical techniques such as cyclic voltammetry (CV), LSV, electrochemical impedance spectroscopy (EIS), and chronoamperometry (IT) remain the primary methods for evaluating the electrochemical performance of SOR catalysts. However, these methods are incapable of capturing the real-time chemical states on the surface of electrode materials during the reaction. In other words, electrochemical measurements alone cannot provide direct insight into the reaction pathway from reactants to products. To address this limitation, conventional spectroscopic methods and in situ spectroscopic techniques have been increasingly employed in SOR studies. These approaches offer valuable information about the actual chemical transformation pathways, thereby guiding the rational design and modification of catalysts to better match the reaction mechanism. [53-55] 2.3.1 Ex Situ Characterization Techniques Ultraviolet-visible (UV-vis) spectroscopy is a technique based on absorption spectra, which enables rapid detection of polysulfides. It is widely regarded as a convenient and efficient analytical method. However, UV-vis spectroscopy is not capable of precisely identifying the number of sulfur atoms in polysulfide chains. [11,56] In practice, it is typically used to detect characteristic peaks of short-chain polysulfides, [9,23] such as S 2 2- to S 4 2- , which appear in the 350-450 nm range (Table 1). Although UV-vis has limitations in identifying long-chain polysulfides (S 5 2- to S 8 2- ), it remains useful for verifying the progress of the SOR reaction, particularly in determining whether S 2- has been successfully oxidized (Figure 2c). [32] X-ray diffraction (XRD) utilizes short-wavelength X-rays to accurately determine the crystalline phases of solid products. After the SOR reaction, researchers often add hydrochloric acid to the polysulfide-containing electrolyte, yielding a suspension. This suspension is then centrifuged and dried to obtain a pale yellow solid. XRD analysis confirms that the obtained product under these conditions is S 8 . [9,57] The typical diffraction peaks of S 8 appear at 23.1°, 25.8° 26.7°, and 27.7° (Table 1). When combined with UV-vis results, XRD can help clarify the oxidation pathway from S 2- to S 8 2- during the SOR process. Table 1 Summary of the characteristic peak position in Raman, FT-IR, UV-vis, XRD of S 8 polysulfide dianions. Techniques S 8 S 8 2- S 7 2- S 6 2- S 5 2- S 4 2- S 3 2- S 2 2- FT-IR/cm -1 504 495 490 485 479 475 Raman/cm -1 150, 220 480 380 448 453 UV-vis/nm 350-450 XRD/2θ（°） 23.1, 25.8, 26.7, 27.7 2.3.2 In Situ Characterization Techniques In situ Fourier-transform infrared spectroscopy is an infrared absorption spectroscopy technique that detects molecular bond vibrations and rotations resulting from infrared absorption. This technique enables non-destructive detection of polysulfides and byproduct sulfates generated during the sulfur oxidation process (Equations 9-10). Table 1 summarizes the characteristic peak positions of divalent polysulfide ions (S x 2- , x = 2-8) detected by FT-IR, [58,59] including: S 2 2- (475 cm -1 ), S 3 2- (479 cm -1 ), S 4 2- (485 cm -1 ), S 5 2- (490 cm -1 ), S 6 2- (495 cm -1 ), and S 8 2- (504 cm -1 ). Zhou et al. [52] also identified byproduct sulfates in addition to polysulfides using FT-IR. These include SO 3 2- , S 2 O 3 2- , and SO 4 2- , which exhibit characteristic peaks at 935 cm -1 , 997 cm -1 , and 1115 cm -1 or 1099 cm -1 , respectively (Figure 2d). Raman spectroscopy is one of the most widely used techniques for polysulfide detection. It operates by irradiating the sample with visible, near-infrared, or ultraviolet light. The scattered photons exhibit energy shifts that reflect the vibrational modes of the molecules. This technique allows for accurate identification of polysulfides under both ex situ [60] and in situ [61,62] conditions (Figure 2e). Table 1 lists the typical Raman shifts corresponding to various soluble polysulfides and elemental sulfur: S 2 2- (453 cm -1 ), S 4 2- (448 cm -1 ), S 6 2- (380 cm -1 ), S 8 2- (480 cm -1 ), and S 8 (150 cm -1 and 220 cm -1 ). 3. Design Strategies for SOR Catalysts The proposed design strategy for the SOR catalyst aims to enhance the adsorption of sulfur reactants and mitigate sulfur passivation. To provide comprehensive guidance for catalyst design, we summarize the contributions of d-band center theory, hard and soft acids and bases (HSAB) theory, encapsulation protection strategy, and tip-enhanced electric field in improving catalytic performance. 3.1 d-Band Center Theory For the industrial deployment of SOR, electrocatalysts with high intrinsic activity, long-term stability, and low cost are essential prerequisites for a viable electrolytic system. Due to their tunable d-orbital states, transition metals have frequently been used in the form of sulfides or oxides as catalysts for SOR applications. [63] To rationalize experimental observations, descriptors such as the d-band center, coordination number, bond length, and electron occupancy of orbitals are often employed to characterize the interactions between electrocatalysts and reactants. [64,65] Among these, the d-band center, introduced by Nørskov, [66] has emerged as a widely used and promising descriptor that correlates the binding strength of surface adsorbates with the electronic structure of transition metal catalysts. This concept has been extensively applied in various electrocatalytic reactions, including oxygen evolution reaction (OER), HER, and CO 2 RR. However, its application and development in the context of SOR have not yet been systematically summarized. Figure 3. a) Schematic illustration of the dynamic adsorption and desorption of sulfur species on the electrode surface. b) Diagram showing the regulation of the catalyst d-band center through atomic doping. Shifts in the d-band center can significantly influence the adsorption and desorption behaviors of reactants, intermediates, and products, thereby modulating the energy barrier of the RDS in SOR and facilitating the reaction thermodynamically. Whether adjusted upward or downward, the regulation of the d-band center generally aims to achieve three key objectives: (1) to enhance the adsorption of sulfur-containing reactants, (2) to accelerate the conversion of polysulfide intermediates, and (3) to promote the desorption of sulfur products ( Figure 3a ). Strategies for tuning the d-band center include heteroatom doping, high-entropy engineering, vacancy introduction, and heterointerface construction. Whether main group metals, transition metals, or non-metallic atoms are selected for doping to enhance catalytic performance, the fundamental objective is to regulate the electronic structure of the pristine catalyst. According to frontier molecular orbital theory, doping primarily modifies the electronic interaction between the d orbitals of transition metals in the catalyst and the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of the sulfur species (Figure 3b), thereby influencing the ease of populating anti-bonding orbitals and strengthening or weakening the overall electronic interactions. Wang et al. [70] systematically investigated the effects of substitutional doping with Mg, V, Cr, Fe, Ga, and Mo on the SmMn 2 O 5 . Their study demonstrated the relationship between sulfur species adsorption and doping with either transition metals or main group elements. Both experimental data and DFT simulations indicated that the d-band centers of main group metals such as Mg and Ga are higher than those of transition metals, allowing Mg and Ga doped SmMn 2 O 5 to more effectively anchor and convert polysulfide species. In general, the d-band center of transition metals tends to decrease along the periodic table from top to bottom and from left to right. [66,71] For instance, Co has a higher d-band center than Ni, which reduces the probability of populating anti-bonding orbitals ( Figure 4a ), [72] thereby enabling stronger adsorption between Co and sulfur species. Yu et al. [73] reported that cobalt-doped Ni 3 S 2 exhibited enhanced adsorption capacity for S 2- and superior catalytic performance. The Co-Ni 3 S 2 catalyst achieved a current density of 209 mA cm -2 at 0.7 V vs RHE, representing a 30% improvement over undoped Ni 3 S 2 . Figure 4. a) Schematic illustration of the adsorption and catalytic conversion processes for Ni, CoNi, and Co in SOR. [67] Copyright 2025, Wiley. b) Work functions of Fe, Cu, Co, Ni, and Pt. c) Projected density of states (PDOS) and d-band centers of Pt in PC-LEAs, PCN-MEAs, PFCN-MEAs, and PCFCN, along with the PDOS and d-band center of pure Pt. d) d-band center positions of Pt, PC-LEAs, PCN-MEAs, PFCN-MEAs, and PCFCN-HEAs. e) Schematic diagram of the sulfur redox reaction pathway on the surface of PCFCN-HEA. [68] Copyright 2025, ACS. f) Polarization curves of SOR-SRR on HEA-Mo₂C/HPC and corresponding control samples. g) PDOS analysis of adsorbed sulfur species (S) on Mo 2 C (102), Ni (111), and the high-entropy alloy (HEA). h) Differential charge density distributions between the adsorbed S and Mo site on Mo 2 C (102), the Ni site on Ni (111), and the Ni site on HEA. [69] Copyright 2023, ACS. Compared with single-atom doping, alloying and high-entropy strategies not only optimize the overall d-band center of the catalyst [74,75] but also provide a nearly continuous adsorption energy band, [76] which enables outstanding synergistic catalytic effects. For instance, Li et al. [68] offered a compelling explanation for the d-band center modulation in high-entropy catalysts based on the work functions of different metals. The metal’s work function determines its ability to donate or accept electrons, thus allowing high-entropy alloys to regulate the local electronic structure of catalysts. As shown in Figure 4b, the work functions of Fe, Cu, Co, Ni, and Pt were compared, with Pt exhibiting the highest value. The authors first presented the d-band centers of Pt within the Cu 3 Pt ( - 1.76 eV), PtCoNi ( - 2.38 eV), PtFeCoNi ( - 2.46 eV), and PtCuFeCoNi ( - 2.50 eV) alloys (Figure 4c), relative to that of pure Pt, which was - 2.28 eV. Furthermore, the overall d-band centers of PtCuFeCoNi, PtFeCoNi, PtCoNi, and Cu 3 Pt were calculated as - 1.70, - 1.77, - 1.83, and - 1.87 eV, respectively (Figure 4d). These results indicate that the incorporation of low work function elements such as Fe, Co, and Ni into the alloy caused partial electron transfer to Pt, which has a higher work function. This electron redistribution led to a downward shift in the d-band center of Pt in the alloy, altering the surface electronic structure of the catalyst and increasing the activation energy for SOR (Figure 4e). Yuan and co-worker [69] developed HEA-Mo 2 C/HPC as a high-entropy SOR catalyst, which achieved a current density of 300 mA cm -2 at 0.5 V vs RHE (Figure 4f). At the same potential, the high-entropy Mo 2 C/HPC catalyst doped with multiple metals (Fe, Ni, Mn, Co, Mo) exhibited approximately a 50% improvement in performance compared to Mo 2 C/HPC, which delivered 200 mA cm -2 . This remarkable improvement was attributed to two key factors. First, the Mo sites in Mo 2 C exhibited significantly stronger d-p orbital hybridization with the p orbitals of adsorbed reduced sulfur species. Specifically, the overlap between the p orbitals of S * and the d orbitals of surface Mo atoms was substantially larger than that in other control groups (Figure 4g). Second, the variation in d-band centers across different metal sites in the HEA, caused by differences in their work functions, modified the surface charge distribution. This promoted faster capture of polysulfide intermediates and facilitated their subsequent electrochemical oxidation on the Mo 2 C surface (Figure 4h). Figure 5. a) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of AD-LCO b) Annular bright-field scanning transmission electron microscopy (ABF-STEM) image with atomic resolution; the inset shows the corresponding fast Fourier transform (FFT) pattern. c) Lattice strain distribution map of AD-LCO. [77] Copyright 2022, Wiley. d) HAADF-STEM image of strained MoS₂ nanosheets (s-MoS₂). e) HAADF-STEM image of unstrained MoS 2 nanosheets (u-MoS 2 ). f) PDOS calculations for s-MoS 2 and u-MoS 2 . [78] Copyright 2022, Elsevier. The introduction of vacancies in transition metal catalysts can create additional active sites [79] and enhance electron transport properties. [80] And the presence of either anion or cation vacancies can alter the electronic structure of the coordinating atoms, [81] thereby lowering the reaction energy barrier for intermediate species. Furthermore, the vacancies can also cause lattice strain, thereby regulating the d-band center. For instance, Zheng et al. [77] demonstrated, using HAADF-STEM imaging ( Figure 5a,b ) and geometric phase analysis (GPA), that vacancy defects induce local lattice mismatches (Figure 5c), which in turn lead to changes in lattice strain. Such lattice strain has been shown to modulate the width of the d-band of transition metals, resulting in upward shifts in the d-band center (Figure 5d,e,f), and is recognized as an effective method for tuning electronic structure. [78,82] In the SOR reaction, this strategy has excellent practical cases. Wang and co-worker [83] synthesized ultrathin Pd 4 S metal nanoribbons (MNRs) with abundant cationic Pd vacancies (V pd -Pd 4 S MNRs) via a hydrothermal sulfidation process. Bader charge analysis and PDOS calculations revealed that the S atoms coordinated with Pd gained a net charge of 0.15 e, and the overall d-band center of V pd -Pd 4 S MNRs was shifted upward by 0.86 eV compared with pristine Pd 4 S. These modifications led to significantly enhanced SOR performance, with the V pd -Pd 4 S MNR achieving a current density of 100 mA cm -2 at 0.776 V. Similarly, Ai et al. [84] developed a FeSe 2 -V se catalyst with selenium anion vacancies. Compared to pristine FeSe 2 , FeSe 2 -V se exhibited a higher density of electronic states near the Fermi level, indicating improved electrical conductivity. The introduction of Se vacancies raised the d-band center of Fe, increasing the overlap between Fe d orbitals and the p orbitals of adsorbed sulfur. This enhanced the interaction between the catalyst and reduced sulfur species. At 0.95 V vs RHE, the FeSe 2 -V se catalyst achieved a SOR current density of 200 mA cm -2 , which was higher than those of the Se and FeSe 2 electrodes by 88 and 37 mA cm -2 , respectively. not-yet-known not-yet-known not-yet-known unknown Figure 6. a) Energy band diagram illustrating the Mott-Schottky contact formed between MXene (work function Wₘ = 4.37 eV) and SnO2 (work function Wₛ = 3.84 eV) before contact. b) Energy band diagram after contact. c) Electron localization function (ELF) and sulfur species adsorption energy for - O terminated MXene, SnO₂-loaded - O terminated MXene, -OH terminated MXene, and SnO2-loaded -OH terminated MXene, respectively. The color scale represents the degree of electron localization, ranging from blue (localized) to red (delocalized). d) PDOS analysis of the metal d-band and nonmetal p-band in MXO and MXOH, with and without SnO2.[85] Copyright 2024, Spring. e) Schematic illustration of the reaction process on the CoS/MoS2 heterostructure. f) Scanning electron microscopy (SEM) image of CoS/MoS2. g) Selected area electron diffraction (SAED) pattern of CoS/MoS2.[86] Copyright 2024, Elsevier. Heterostructures are considered an effective strategy for modulating the adsorption strength of reaction intermediates on heterogeneous interfaces.[87] In particular, this approach relies on the Schottky barrier to regulate the d-band center of transition metals.[88-90] Additionally, heterointerface construction can reconstruct the charge density near the Fermi level, reduce the band gap of individual catalysts,[91,92] and thereby enhance their electron transport properties.[93] For instance, Zhang et al.[85] prepared a SnO2@MXene heterostructure, which formed a Schottky barrier that induced a strong built-in electric field within the catalyst, thereby modulating its electronic structure. Figure 6a and 6b illustrate the energy band diagrams of SnO2@MXene before and after charge redistribution. Due to the difference in Fermi levels between SnO2 and MXene, charge carriers migrate to establish equilibrium. Differential charge density mapping confirmed that the heterojunction significantly affected the surface charge distribution of the catalyst, and this localization effect showed a linear correlation with the adsorption energy of sulfur species (Figure 6c). PDOS calculations (Figure 6d) indicated that, for - OH terminated MXene (MXOH), coupling with SnO2 resulted in a 0.33 eV upward shift in the d-band center and a 0.70 eV shift in the p-band center. For - O terminated MXene (MXO), the d-band and p-band centers were raised by 1.43 eV and 0.17 eV, respectively, upon coupling with SnO2. The reduced energy difference between the d- and p-band centers suggests enhanced bonding strength between the catalyst and sulfur species. Zhao et al.[23] synthesized a RuO2-Co3O4-x/foam-Co heterostructure through electrodeposition, cation exchange, and subsequent calcination. The d-band center of RuO2-Co3O4-x was located at 0.405 eV, which was closer to the Fermi level than those of RuO2 (0.436 eV) and Co3O4 (0.472 eV), thereby enhancing adsorption of reactants and reducing the reaction barrier of the RDS. As a result, the catalyst achieved a current density of 375 mA cm-2 at 0.6 V vs RHE in the SOR process. Wang et al.[86] in situ synthesized “yarn-ball”-like core/shell CoS/MoS2 microflowers on a flexible carbon fiber cloth substrate (Figure 6e,f,g). The work function difference between CoS and MoS2 generated dual built-in electric fields at their layered interface. The authors used DOS calculations to demonstrate that this heterostructure avoided the slow electron transport problem caused by the band gap of pristine MoS2. not-yet-known not-yet-known not-yet-known unknown 3.2 Hard and Soft Acid-Base Theory The Lewis acid-base theory proposed in 1923[94] defines acids and bases as electron acceptors and donors, respectively, which can interact to form Lewis adducts. In 1963, R. G. Pearson further developed this concept and proposed the HSAB theory,[95] which describes the interaction strength between Lewis acids and bases. In general, soft (or hard) acids exhibit stronger interactions and affinity toward soft (or hard) bases.[96-98] With the theoretical advancements made in 1983[99], the descriptor of absolute hardness (η) has been introduced as a semi-quantitative parameter to predict the interaction strength between transition metals and their ligands, such as reduced sulfur species.[100-102] This value is closely related to polarizability, electronegativity, and ionization energy. For example, among commonly used transition metal catalysts, Cu+ (η = 6.3 eV) is more likely to coordinate with the soft base S2- than Zn2+ (η = 10.9 eV), Co2+ (η = 8.2 eV), or Fe2+ (η = 7.2 eV).[98] Remarkably, these predictions are consistent with the experimentally determined binding energies of TM-S bonds.[103] Therefore, HSAB theory can serve as a predictive guideline for selecting catalysts that exhibit strong adsorption toward reduced sulfur species in SOR processes.[104-106] Based on this concept, Zhong et al.[107] and Wu et al.[108] suggested that high-performance SOR catalysts tend to include soft acid metal ions, which typically have low charge density, large ionic radius, and a tendency to donate electrons, enabling strong coordination or adsorption with soft base anions such as S2-. From the perspective of molecular orbital theory, this interaction depends on the manner in which electrons in the HOMO of the Lewis base metal ions enter the LUMO of sulfur species acting as Lewis acids.[109] It is also worth noting that empirical predictions based on this principle should be combined with DFT calculations for verification, because the relationship between absolute hardness and operational chemical hardness may be nonlinear.[110] Figure 7. Electrocatalytic performance of metal sulfide electrodes for the SOR in an electrolyte containing 1 mol L -1 NaOH and 1 mol L -1 Na 2 S: a) LSV curves; b) Tafel slopes. c) EIS spectra. [107] Copyright 2021, RSC. d) Schematic illustration of Cu-S bonds in undoped and hard-acid-cation-doped (labeled as M) electrodes. e) DFT free-energy profiles for the conversion of sulfur to S 8 on CuCoNiMnCrS x and on the Cu 2 S (102) surface. f) Charge transfer numbers between the Cu active sites and S 8 in CuS and CuCoNiMnCrS x . g) Sulfophilic and sulfophobic tests for CuCoNiMnCrS x /NF, CuS x /NF, and NF electrodes. [36] Copyright 2024, Wiley. Previous studies, guided by the HSAB theory, have predicted and synthesized soft acid-type catalysts such as Cu₂S and CoS for SOR. For instance, Jin et al. [107] first proposed a simple hydrothermal method to synthesize Cu 2 S nanosheets grown on nickel foam. Benefiting from the porous morphology of the electrode, its high electron transport properties, and the soft acid sites provided by Cu + , a synergistic enhancement of sulfur adsorption was achieved. This electrode reached a current density of 100 mA cm -2 at a potential as low as 0.44 V vs RHE, with a Faradaic efficiency exceeding 97%. Control electrodes including Co-S/NF, Ni-S/NF, and Fe-S/NF were also prepared. The SOR electrochemical performance of these transition metal sulfides ( Figure 7a,b,c ) was found to be consistent with the order of the absolute hardness of the corresponding transition metal ions (Cu + < Co 2+ < Ni 2+ < Fe 3+ ). On this basis, the authors further combined DFT calculations to demonstrate that different crystal facets of Cu 2 S, namely (034), (630), and (106), exert distinct influences on the accumulation of short-chain polysulfides into long-chain polysulfides. Therefore, as mentioned above, absolute hardness predictions based on the HSAB theory should be complemented with DFT calculations to identify other theoretical factors affecting the experimental catalytic performance. Composite soft acid sites combines the strategies of high-entropy alloying and HSAB-based predictions. The multimetallic interactions can strengthen the adsorption capability of individual soft acid sites toward reduced sulfur species. As a typical example, Zhang et al. [36] introduced a series of relatively hard acid metal cations into Cu 2 S, creating a “hard-soft” combination. The repulsion between cations and anions induced an asymmetric electronic environment, which shortened the Cu-S bond length and strengthened the adsorption of S 2- by Cu (Figure 7d). Theoretical calculations verified the feasibility of this strategy, showing that the adsorption energy of S on CuCoNiMnCrS x was - 0.084 eV, which is 14 times greater than that of Cu 2 S ( - 0.006 eV). Furthermore, a DFT free energy diagram (Figure 7e) revealed that the adsorption energy of S 8 on CuCoNiMnCrS x (0.002 eV) is lower than that on Cu 2 S (0.007 eV), indicating superior sulfur-repelling behavior of the alloy. Bader charge analysis (Figure 7f) demonstrated that in the Cu 2 S * S 8 model, 0.12 e - was transferred from Cu atoms to S 8 , whereas only 0.02 e - was transferred in the CuCoNiMnCrS x system. This result indicates that doping with other elements weakens the interaction between Cu sites and S 8 . The results shown in Figure 7g confirm that such composite sites provide an electronic buffering effect, mitigating sulfur poisoning on Cu active sites and thereby improving the SOR performance. 3.3 Encapsulation Protection Strategy Recently, under harsh conditions such as strong acidity or alkalinity, sulfur poisoning, and high temperatures, a two-dimensional encapsulation protection strategy for non-noble metal catalysts has demonstrated outstanding advantages. [111,112] In the SOR process, such harsh conditions can be described as sulfur poisoning, where S 8 covers the transition metal active sites, resulting in inefficient and unstable operation. This phenomenon has been confirmed by our experiments ( Figure 8a ) and is consistent with previous studies. [113] Encapsulating transition metals with two-dimensional shells such as graphene, carbon nitride, or MoS₂ not only protects the active sites but also prevents the catalytic activity of the transition metals from being masked by the shell. The working principle is that the encapsulated transition metal transfers electrons to the two-dimensional crystal shell, which activates the inert outer surface for efficient catalytic reactions. Research on this strategy has mainly focused on regulating the properties of the 2D shells and the encapsulated transition metals for different harsh environments (HER, OER, lithium-sulfur batteries, etc.). However, a systematic summary of encapsulation-protected catalysts for SOR is still lacking, which motivates the following discussion. Figure 8 a) Photographs of the platinum electrode surface before and after SOR testing in alkaline Na 2 S solution. b) Schematic illustration of the encapsulation protection strategy for preventing sulfur poisoning. [114] Copyright 2024, wiley. c) Graph of C240, Co@C240 and Co@N-C240 molecular model. d) SOR polarization curves of Co@N-CNTs/CC and the corresponding control samples. e) Comparison of PDOS between S atoms (3s3p) and the bonding atoms C (2s2p) and N (2s2p) when sulfur is adsorbed on C 240 and on Co@N-CNTs. f) Free energy diagrams of the SOR reaction on different models. [115] Copyright 2024, Elsevier. g) SOR polarization curves of CoNi@NGs and control samples. h) Adsorption free energy (ΔG) of sulfur atoms on different catalysts. i) Differential charge density distribution of sulfur adsorbed on the CoNi@NGs surface, where the red regions represent charge accumulation and the blue regions indicate charge depletion. [23] Copyright 2020, RSC. Adjusting the properties of the 2D crystal shell in encapsulated catalysts can induce optimization of the electronic structure. By controlling the number of shell layers, [116] heteroatom doping within the shell, [117,118] or varying the shell composition, [119] researchers can effectively protect the inner transition metals and simultaneously tune the surface electric field for sulfur adsorption. For instance, Sun et al. [114] employed a sequential process involving precipitation, chemical vapor deposition, and template etching to synthesize CoNi composites encapsulated by few-layer, multilayer, and thick-layer graphene (denoted as CoNi@fG, CoNi@mG, and CoNi@tG). Such an encapsulated structure enhanced sulfur redox kinetics and protected the CoNi active sites (Figure 8b). Experimental results revealed that a thinner shell (CoNi@fG) provides insufficient protection, while an excessively thick shell (CoNi@tG) hinders the full release of the catalytic activity from the inner active sites. A key challenge for the application of graphene shells in SOR lies in the weak interaction between the nonpolar carbon surface and sulfide species. [119] Heteroatom doping can overcome this limitation. Introducing N, S, O, P, or B atoms increases the adsorption capability of carbon shells toward reduced sulfur species. Based on this strategy, Zhou et al. [115] fabricated a nitrogen-doped carbon nanotube-encapsulated Co catalyst on carbon cloth (Co@N-CNTs/CC) (Figure 8c). This electrode achieved a current density of 99.36 mA cm -2 at an anode potential of 0.6 V vs RHE, which was approximately 75% higher than those of bare Co nanoparticles and N-CNTs (Figure 8d). Compared with S-C bonding states on undoped Co@-CNTs, the S-C bonding states on Co@N-C 240 showed a downward shift in the band center (Figure 8e). Analysis of the free-energy diagram (Figure 8f) revealed that the stronger N-S interaction enhanced sulfur adsorption on Co@N-C 240 , thereby improving SOR electrochemical performance. Two-dimensional crystalline materials such as graphene, BN, and MoS 2 typically exhibit chemically inert surfaces. [120-122] However, when these materials cover transition metals such as Co, Ni, or Fe, the half-filled d orbitals of the metals interact with the bonds of the outer shell. This interaction facilitates electron transfer from the transition metals to the otherwise inert outer surface. Studies [123-125] have shown that this activation of the inert surface enables strong coupling with reaction intermediates. Therefore, understanding how the properties of the encapsulated transition metals affect their interaction with reduced sulfur species is critically important. For example, Bao and co-worker [11] proposed a template-assisted synthesis of a CoNi alloy catalyst encapsulated in nitrogen-doped graphene (CoNi@NGs). This alloy-regulated design significantly enhanced the current density of CoNi@NGs in LSV tests, achieving 155 mA cm -2 at 0.6 V vs RHE, which was approximately 400% and 10% higher than those of Ni@NGs and Co@NGs, respectively (Figure 8g). DFT calculations showed that the CoNi alloy greatly strengthened the adsorption of sulfur compared with single-metal encapsulated catalysts (Figure 8h). Charge transfer analysis further indicated that strong adsorption was likely due to the enriched charge density around the carbon atoms and nitrogen dopants near the CoNi clusters (Figure 8i). Moreover, the encapsulated catalyst exhibited excellent stability, maintaining electrolysis for over 1200 h at 20 mA cm -2 and 0.5 V vs RHE in an alkaline electrolyte containing syngas (49% CO, 49% H 2 , and 2% H 2 S). These results demonstrate the remarkable stability of such catalysts for the SOR. 3.4 Tip-Enhanced Electric Field Theory Electronic structure and morphology are two critical factors influencing the performance of electrocatalytic reactions. [126-128] In recent years, nanomaterials with tip-rich morphologies ( Figure 9a,b ) have been shown to generate strong localized electric fields, which significantly promote electrocatalytic reactions from both thermodynamic and kinetic perspectives. [129] This phenomenon is commonly referred to as the “tip-enhanced electric field effect.” Localized electrons induced by the sharp tips can form charged excited states, which effectively modulate the electronic structure of catalysts and reduce the thermodynamic barriers of reaction intermediates. In addition, such tip-rich structures can enrich electrolytes such as K + and S 2- on their tips, [130-132] regulating the microenvironment of the catalyst surface and thereby improving the kinetics of the electrocatalytic process (Figure 9c). Under working conditions with low-concentration H 2 S or S 2- containing solutions, achieving high current densities remains a critical challenge for the industrial application of SOR. We propose that the tip-enhanced electric field strategy offers an important solution to this challenge. The intensity of the local electric field of nanoneedle array catalysts is primarily determined by the curvature of the tips and the spacing between adjacent tips. [133] Figure 9. a, b) SEM images of the needle-shaped catalyst. [134] Copyright 2022, Elsevier. c) Schematic illustration showing that the tip-rich nanostructures accelerate the migration of S 2- ions toward the catalyst surface in alkaline solution. d) Finite element simulations of Co 3 S 4 @NF with a rough morphology (r-Co 3 S 4 @NF) and nanoneedle-shaped Co 3 S 4 @NF (n-Co 3 S 4 @NF). The color scale indicates the distribution of current density near the needle tips, and the white lines represent the electric field distribution of the core-shell structure. f) Sulfide ion adsorption test. [10] Copyright 2024, Springer. g) Distribution of K + concentration in the gaps between nanoneedles, showing that narrower gaps lead to a higher local K + concentration. From left to right, the needle-tip spacing is 10 nm, 20 nm, and 40 nm, respectively. [135] Copyright 2019, Wiley. Nanoneedle-array catalysts differ significantly from conventional columnar or spherical catalysts in terms of morphology, and this difference can be described by variations in curvature. Regions of the catalyst with higher curvature induce stronger charge localization, forming more intense electric fields that attract more S 2- and increase the local current density. [136] For instance, Lum et al. [137] systematically studied the Stark tuning rates of single-atom Ni catalysts with different surface curvatures. The results demonstrated that an increase in curvature leads to an increase in the Stark tuning rate, which enhances the interfacial electric field. These findings highlight the importance of curvature in controlling the strength of the electric field at active sites. Moreover, Zhou et al. [10] designed a study that confirmed the mechanism and principle of tip-rich catalysts in SOR. Using finite element simulations, they showed that a nanoneedle array Co 3 S 4 @NF (n-Co 3 S 4 @NF) with high curvature exhibits a denser distribution of electric field lines in the terminal region than that of the rough columnar catalyst (r-Co 3 S 4 @NF), resulting in a significantly higher current density at the sharpest tips (Figure 9d). In addition, the group calculated the S 2- concentration near the electrode surface using the Gouy-Chapman-Stern model. Notably, the simulation results were consistent with UV-Vis spectroscopy data, indicating that the amount of S 2- adsorbed on n-Co 3 S 4 @NF is more than twice that on r-Co 3 S 4 @NF (Figure 9e,f). These results demonstrate that tip-rich catalysts can indeed enhance mass transfer of ions at the catalyst surface. LSV measurements showed that n-Co 3 S 4 @NF requires only 0.233 V vs RHE and 0.279 V vs RHE to achieve SOR current densities of 100 and 300 mA cm -2 , respectively. The proximity effect describes the synergistic influence between neighboring active sites in enhancing electrocatalytic performance. Similarly, the spacing between tips can influence the overlap of localized electric fields, a phenomenon referred to as the tip-proximity effect, which can effectively regulate the local concentration of reactants. [133,138] For example, Yu et al. [135] demonstrated (Figure 9g) that as the spacing between nanoneedles decreases, the local K + concentration increases due to the enhancement of the electric field in the inter-tip region. Reducing the tip spacing from 40 nm to 10 nm led to an approximately 28-fold increase in K + concentration. K + ions are known to interact noncovalently with CO 2 molecules, enabling the rapid enrichment and stabilization of CO 2 on the tips and lowering the activation barrier for the reaction. Although studies on the influence of tip spacing on SOR remain scarce, evidence from other electrocatalytic systems suggests that controlling the spacing between nanoneedles is also a key factor that can enhance the catalytic performance of tip-rich catalysts in SOR. 4. Coupling Applications of SOR and the economic benefits The industrial application of anodic SOR electrocatalysis must satisfy the requirements of adaptability to diverse application scenarios, low energy consumption, and high economic efficiency. Although the SOR was originally developed for the treatment of sulfur-containing wastewater and H 2 S gas to convert these pollutants into valuable resources, [139-141] its low thermodynamic energy barrier and resulting low reaction potential make it a promising replacement for the HSAB in conventional water electrolysis (HER + OER). In water electrolysis, the OER step is limited by the sluggish kinetics of the four-electron transfer process, which leads to high energy consumption exceeding 4.5 kWh per m 3 of H 2 . [24,142,143] In addition to coupling with HER, SOR can also be paired with CO₂ reduction [144] and NO 3 - reduction, [145] which greatly broadens its potential for use in complex industrial environments. Furthermore, researchers are advancing the application of SOR catalysts in flow electrolyzers and solar-driven systems. [146] These efforts not only enable the resource recovery of pollutants but also increase the SOR current density to industrially relevant levels and reduce dependence on fossil fuel power generation. Such progress can be viewed as “killing multiple birds with one stone.” In the following sections, we summarize the roles of SOR catalysts in various coupling systems and evaluate their economic benefits. 4.1 Coupling of HER and SOR Although fossil fuels have driven the development of human society, they have also accelerated the continuous deterioration of the global environment. Furthermore, the rising cost and depletion of fossil fuel resources have become pressing challenges. As a result, the pursuit of renewable energy sources with high energy density and zero pollution has become a global priority. Hydrogen energy has an extremely high specific energy density (282 kJ mol -1 ) and produces no pollutants upon combustion. Water electrolysis, as a zero-emission method for producing hydrogen, has therefore attracted extensive attention. After years of research, seawater, which accounts for more than 96% of the Earth’s water resources and is abundantly available, has been adopted as a sustainable aqueous electrolyte for hydrogen production on an industrial scale. However, seawater electrolysis still faces two major obstacles: the anodic side reaction of chlorine evolution (CER) and the high thermodynamic barrier of the OER. CER produces corrosive chlorine-containing species that can damage and poison the electrodes, reducing the service life of electrolyzers. Meanwhile, OER significantly increases the energy consumption of hydrogen production. Figure 10. a) Comparison of LSV curves of the SOR + HER system and the OER + HER system. b) LSV curves of SOR compared with the corresponding OER. [147] Copyright 2024, ACS. c) Schematic diagram of the solar-driven reaction setup. d, e) Two-dimensional in situ Raman spectra collected on HEA and HEA-SP catalysts. f) Comparison of the calculated energy consumption of HEA-SP in conventional seawater electrolysis (SWE), SOR-assisted SWE, and SOR-assisted SWE with a bipolar membrane (BPM). [148] Copyright 2022, Elsevier. In contrast, the SOR, with its lower reaction potential, avoids the occurrence of CER and replaces the low-value OER. Lu et al. [107] first synthesized CoMoO 4 nanorods through a hydrothermal method and used them as the anodic catalyst for SOR. Subsequently, CoMoO 4 was subjected to CVD treatment in an NH 3 atmosphere to obtain N-doped CoMoN, which was used as the cathodic catalyst for HER. In the coupled HER + SOR system, the same pair of electrodes required only 0.43 V to achieve 50 mA cm -2 , which is 1.25 V lower than that of the conventional HER + OER system ( Figure 10a ). This means that for the same amount of hydrogen production, the HER + SOR system reduces energy consumption by approximately 74% while producing elemental sulfur, a product with higher economic value than oxygen. The low energy consumption can be attributed to two factors: the lower thermodynamic energy barrier of SOR compared to OER (Figure 10b), and the unique S 2- conversion capability of CoMoO 4 . More importantly, the authors demonstrated that the reaction can be directly driven by a commercial solar photovoltaic panel (1.5 V) (Figure 10c). To utilize the abundant seawater resources, Zhao et al. [148] proposed a strategy that incorporates FeCoNiCrMn high-entropy alloy catalysts co-doped with sulfur and phosphorus (HEA-SP) into an asymmetric electrolyzer. In this system, HER takes place in acidic seawater (SWE) at the cathode, while SOR occurs in an alkaline electrolyte at the anode, with a bipolar membrane (BPM) in between to lower the requirement for external energy input. TEM characterizations revealed abundant lattice defects and lattice strain in HEA-SP. In situ Raman spectroscopy detected a signal at 355 cm -1 corresponding to the interaction between S 2- and the HEA catalyst at open-circuit potential (Figure 10d). Furthermore, a Raman signal corresponding to S 8 was observed at 0.60 V vs RHE. Compared with HEA, HEA-SP exhibited a more prominent Raman signal due to sulfur adsorption and was able to detect the S 8 signal at an even lower potential of 0.50 vs RHE (Figure 10e). These results demonstrate the spontaneous adsorption of S 2- on the surface of the HEA catalyst and confirm that co-doping with sulfur and phosphorus enhances SOR activity. Interestingly, when a bipolar membrane was introduced into the asymmetric SWE system, the energy consumption of electrolysis was further reduced. At 100 mA cm -2 , only 0.253 kWh was required for SOR-assisted SWE to produce one cubic meter of H₂, which is approximately 7.7 times lower than that required for conventional SWE (Figure 10f). 4.2 Coupling of CO 2 RR and SOR The CO 2 RR technology can effectively reduce the emission of greenhouse gases into the atmosphere while generating specific hydrocarbons. [149] Over the past five to ten years, this field has attracted widespread attention. [150] The growing interest is driven by two major factors. First, CO 2 reduction, as a green and sustainable concept, not only supports the achievement of carbon neutrality but also reduces dependence on fossil carbon-based energy sources. Second, due to the intermittent availability of renewable electricity (such as wind, solar, and nuclear power) and the general patterns of electricity demand in human society, the storage of high-value energy makes CO 2 electroreduction particularly attractive. With the use of flow electrolyzers and gas-diffusion electrode designs, CO 2 can be supplied to the cathode in the gas phase, enabling the reaction rate to reach or even exceed the industrial ampere level and producing value-added products such as methane, ethylene, and formate. [151-154] However, CO 2 RR is also constrained by the high thermodynamic barrier of the OER occurring at the anode. For this reason, much previous research has focused on designing advanced OER catalysts to reduce the overall cell potential. Moreover, compared to oxygen (0.085 $ kg -1 ), the sulfur produced from SOR (0.22 $ kg -1 ) has a significantly higher market value, which enhances the economic benefit of the anodic process. Therefore, coupling CO 2 RR with SOR enables the production of value-added chemicals from industrial waste at a lower energy cost, providing a promising pathway toward achieving carbon neutrality. Another practical factor is that as global energy demand continues to rise, natural gas is increasingly replacing traditional fossil fuels. Natural gas purification generates CO 2 as more than 30% of the total gas stream, [155,156] and as much as 28% of H 2 S is also present in natural gas. [157,158] These facts indicate that a CO 2 RR-SOR system has broad application prospects. Figure 11. a) LSV curves of Co-S nanosheet (Co-S NSs) electrodes compared with control electrodes for SOR and OER in an H-type electrolyzer. b) Schematic diagram of a flow-type electrolyzer. c) Comparison of the LSV curves of Co-S NSs measured in an H-type electrolyzer and in a flow-type electrolyzer. [56] Copyright 2023, Wiley d) Faradaic efficiencies of all products (CH 3 OH, CH 3 CH 2 OH, CH 4 , CO, and H 2 ) at different applied potentials, as determined by NMR and GC, for NiCo (1:1) Sn electrodes. e) FTIR spectra of the cathode surface, measured in a CO 2 -saturated 0.1 M KHCO 3 solution containing 0.1 M NaOH saturated with H 2 S. f) LSV comparison of NiCo (1:1) Sn electrodes with and without H 2 S. g) Chronopotentiometric graphs of NiCo (1:1) Sn electrodes at 5 mA cm -2 in CO 2 RR-SOR and CO 2 RR-OER systems. The inset shows bar plots of the methanol and ethanol production rates after chronopotentiometric tests. [144] Copyright 2025, Wiley. To enable the efficient coupling of two distinct reactions, Shi and co-work [56] synthesized porous Bi nanosheets (p-Bi NSs) via a solvent-assisted electrore construction method for use as the cathode catalyst in CO 2 RR to produce formate, while CoS prepared by electrochemical deposition was employed as the anode catalyst for SOR. As shown in Figure 11a , in an H-type cell, the coupled CO 2 RR + SOR system required only 1.0 V to achieve 10 mA cm -2 , much lower than the 1.83 V needed for CO 2 RR + OER. This coupling significantly reduces energy consumption while simultaneously producing high-value chemicals at both electrodes. The same coupled system was further implemented in a flow-type electrolyzer (Figure 11b,c), where a potential of only 0.8 V was required at 40 mA cm -2 , compared with 2.0 V in an H-type cell, corresponding to 40% of the voltage. The protons generated from the anodic oxidation of HS - can migrate through the proton exchange membrane to the cathodic reaction surface, which may facilitate the CO₂RR process. For instance, Nagaiah et al. [144] proposed a bifunctional catalyst, NiCo (x:y) Sn, which can simultaneously convert H 2 S and CO 2 in alkaline media into high-value sulfur and ethanol (Figure 11d). As shown in Figure 11e, when SOR occurs at the anode, the intensity of the M-OH peak increases and continues to grow as the reaction proceeds. Figure 11f shows that replacing OER with SOR at the anode promotes the production of ethanol and methanol. These FTIR and 1 H NMR analyses of intermediates and products clearly indicate that protons from HS - at the anode diffuse toward the cathode, facilitating methanol production. Compared with the CO 2 RR-OER system, the CO 2 RR-SOR system reduces the energy required for ethanol production by 82.5% (Figure 11g). 4.3 Coupling of NO 3 RR with SOR In recent years, human activities such as pharmaceutical manufacturing and the extensive use of nitrogen fertilizers have resulted in the excessive generation of nitrate-containing wastewater, with an annual amount exceeding billions of tons [159-161] This has caused a severe imbalance in the nitrogen cycle, which is essential to human health and natural ecosystems. [162,163] In heavily polluted regions, excessive nitrate contamination in soil and groundwater has led to methemoglobinemia in infants [164] and eutrophication of water bodies. [165,166] Various technologies, including reverse osmosis [167] , microbial denitrification, [168] and electrodialysis, [169] have been developed for the removal of nitrates and the production of freshwater. However, these strategies are hindered by low efficiency, high sludge generation, and high operating costs. Consequently, the electrocatalytic nitrate reduction reaction (NO 3 RR) has emerged as a promising alternative. NO 3 RR can follow two distinct pathways that ultimately produce either ammonia (NH 3 ) as a valuable resource [170-172] or harmless nitrogen gas (N 2 ). [173,174] The choice of pathway should depend on the application scenario. For wastewater with high nitrate concentrations, the resource-oriented route to produce NH 3 is preferred, [175] whereas for the purification of low-nitrate groundwater, the generation of N 2 is more suitable. [176] It is worth noting that industrial wastewater, particularly from the petroleum industry and thermal power plants, often contains not only high concentrations of nitrate but also large amounts of toxic sulfide species. [177] Therefore, the development of a coupled NO 3 RR (ammonia production) and SOR system provides a promising approach to simultaneously achieve the valorization of these two hazardous pollutants. This section focuses on recent progress in designing high-performance bifunctional catalysts for such coupled NO 3 RR-SOR systems. Figure 12. a) Schematic diagram of the coupled electrolysis system combining NO 3 RR and SOR. b) UV-Vis absorption spectra of cathodic NO 3 RR at different applied potentials, with the inset showing the corresponding optical images. c) FE NH3 and calculated ammonia production rate for MFe 2 O 4 catalysts. [178] Copyright 2025, Wiley. d) XRD patterns of Co 3 O 4 @CC and CuCo 2 O 4 @CC. e, f) Low-magnification and high-magnification SEM images of CuCo 2 O 4 @CC. g) In situ FTIR spectra of CuCo 2 O 4 @CC in 0.1 M KOH containing 2000 ppm NO 3 - at different applied potentials. h) LSV curves of CuCo 2 O 4 @CC measured in NO 3 RR-SOR and NO 3 RR-OER systems. [177] Copyright 2025, Elsevier. To simultaneously and effectively treat S 2- and NO 3 - containing wastewater, Liu et al. [178] synthesized a dual-network porous spinel MFe 2 O 4 (M = Ni, Co, Fe, and Mn) catalyst by combining melt-spinning with a chemical dealloying process. This bifunctional catalyst consists of a Mn-doped porous NiFe 2 O 4 /CoFe 2 O 4 heterostructure network and a Ni/Co/Mn co-doped Fe 3 O 4 nanosheet network, enabling efficient ammonia production through NO 3 RR as well as high-performance SOR ( Figure 12a ). UV-Vis spectroscopy was used for qualitative and semi-quantitative product analysis at different applied potentials, confirming NH 3 as the primary product and showing a linear relationship between more negative potential and higher production rate (Figure 12b). The catalyst exhibited a high NH 3 /N 2 selectivity in NO 3 RR, with a Faradaic efficiency for ammonia (FE NH3 ) of 95.2% at - 0.80 V vs RHE and a maximum ammonia production rate of 608.9 μmol h -1 cm -2 at - 1.60 V vs RHE (Figure 12c). These values were significantly higher than those of the control samples (CoFe 2 O 4 : 94.7%, 490.5 μmol h -1 cm -2 ; NiFe 2 O 4 : 91.7%, 548.9 μmol h -1 cm -2 ). For SOR, the MFe 2 O 4 catalyst achieved a current density of 100 mA cm -2 at 0.98 V vs RHE. Spinel oxides possess a versatile structure that readily accommodates a wide range of divalent and trivalent transition metal elements. As a result, multi-active-site spinel oxides combine flexible catalytic properties with high activity and long-term stability for both NO 3 RR and SOR. Similarly, Fan and co-worker [177] used a spinel CuCo 2 O 4 electrode grown on carbon cloth (CuCo 2 O 4 @CC) (Figure 12d,e,f) for coupled NO 3 RR towards NH 3 and SOR. The incorporation of Cu improved the conductivity of the inherently less conductive Co 3 O 4 , thereby enhancing electron transfer in both NO 3 RR and SOR. Meanwhile, the synergistic effect of Cu-Co dual active sites promoted NO 3 RR at the cathode and SOR at the anode. CuCo 2 O 4 @CC achieved a maximum FE NH3 of 98.5 ± 0.8% at - 0.4 V vs RHE, with a corresponding ammonia production rate of 445.6 ± 21.2 μmol h -1 cm -2 , demonstrating a high NH 3 /N 2 selectivity. The mechanism of NO 3 RR on CuCo 2 O 4 @CC was investigated using in situ FTIR spectroscopy, which revealed that applying more negative potentials facilitates the generation of intermediate species such as *NH 2 OH, NH 3 , *NO, NO 2 , and *NH 2 during the catalytic process (Figure 12g). In the coupled NO 3 RR-SOR electrolyzer, CuCo 2 O 4 @CC required a cell voltage of only 0.45 V to reach 100 mA cm -2 (Figure 12h), highlighting its excellent bifunctional performance toward both SOR and NO 3 RR. 4.4 Product Recovery and Benefit Evaluation Currently, the anodic sulfur electrooxidation in alkaline media generally produces soluble polysulfides. After electrolysis, the electrolyte must be acidified to precipitate solid sulfur from solution. This post-treatment not only incurs additional acid costs but also releases harmful H 2 S gas, resulting in partial loss of sulfur resources. To accelerate the industrial application of sulfur electrooxidation, it is important to develop low-cost treatment methods that avoid the need for additional acid consumption depending on the application environment. In scenarios where acidic or hazardous gases such as CO 2 and H 2 S are present, these acidic gases can be bubbled into the alkaline polysulfide solution. This process lowers the pH of the solution, recovers solid sulfur, and simultaneously reduces the cost of treating the acidic off-gases. Recently, Shi et al. [56] demonstrated that CO 2 can be effectively used to recover solid sulfur from alkaline electrolytes that would otherwise require acidification with hydrochloric acid. Furthermore, the byproduct NaHCO 3 can be directly reused as the electrolyte for cathodic CO 2 RR ( Figure 13a,b ). In addition, most SOR studies appear to overlook another important aspect: the target pollutant itself, H 2 S, can also be utilized to extract sulfur. For example, Zhou et al. [179] reported as early as 2000 that the acidic gas H 2 S can convert polysulfides into solid sulfur. This approach not only solves the product recovery issue but also helps address the residual tail gas generated when NaOH is initially used to absorb H 2 S. not-yet-known not-yet-known not-yet-known unknown Figure 13. a) Schematic illustration of the CO2RR-SOR coupled system and the process of separating sulfur from the anodic electrolyte using CO2 after SOR. b) Optical images of the anodic electrolyte: A shows the electrolyte after long-term SOR electrolysis, B shows the electrolyte after bubbling CO2, and C shows the electrolyte after sulfur extraction.[56] Copyright 2023, Wiley. c) Reaction process of thiosulfate formation. d) Conversion process of sulfide into thiosulfate with and without the catalyst. e) XRD pattern of the products.[36] Copyright 2024, Wiley. The concept of “alkaline polysulfide valorization” aims to convert alkaline polysulfides into products with a higher economic value than sulfur, such as sodium thiosulfate (37.2-55.8 $ kmol-1sulfur), which is significantly more valuable than elemental sulfur (1.6-4.8 $ kmol-1sulfur). Zhang et al.[36] enhanced the air oxidation kinetics of polysulfides by adding amorphous NiSx as a catalyst, rapidly converting polysulfides into sodium thiosulfate. Without the NiSx catalyst, the conversion yield of sodium thiosulfate was only 36.5%, whereas the addition of NiSx increased the conversion yield to 89.6% (Figure 13c,d). Moreover, it has been proposed that the active development of industrial applications that directly utilize alkaline polysulfide products is an effective way to increase the market demand for SOR products. For example, alkaline polysulfide solutions can be applied to the treatment of wastewater containing heavy metal ions.[180-182] Polysulfides react with heavy metal ions to form stable metal sulfide precipitates, thereby effectively removing heavy metals from wastewater. Figure 14. a) Comparison of HSE with previously reported electrolyzers coupling HER with various anodic reactions. b) Economic comparison of sodium thiosulfate production with conventional SOR and OER processes. c) Relationship between cell voltage and current density for HER coupled with OER and HER coupled with SOR systems. [10] Copyright 2024, Springer. The economic feasibility of SOR technology is the key factor determining whether this research can be successfully scaled up for industrial applications. Regardless of which cathodic reaction is coupled with the SOR system, an efficient SOR catalyst offers clear advantages in terms of overall benefits compared with other anodic reactions. Given that the application scenarios and the value of the products vary depending on the cathodic reaction, this section focuses on the anodic contribution rather than discussing individual cathodic processes in detail. Zhou et al. [10] reported that a hybrid seawater electrolysis (HSE) system coupled with SOR exhibited a 67.9% reduction in energy consumption compared with an alkaline seawater electrolyzer (ASE), and more importantly, the HER + SOR system could be stably powered by solar devices. As shown in Figure 14a , the hydrogen production system based on HER + SOR consumes only 1.21 kWh m -3 H₂ , which is significantly lower than that of other reported electrolysis systems for hydrogen generation. Furthermore, the diversity of product recovery routes significantly improves the economic viability of SOR systems. For example, Zhang et al. [36] proposed a low-cost method to convert polysulfide-containing alkaline solutions into sodium thiosulfate, a higher-value product. As shown in Figure 14b, economic analysis indicates that per kilowatt-hour of electricity consumed, the profits from sodium thiosulfate produced in an HER + SOR system are 166 times higher than those of O 2 produced in a conventional HER + OER system and 6.64 times higher than those of sulfur produced in a standard HER + SOR system. As shown in Figure 14c, LSV data for the electrolyzer demonstrate that a current density of 200 mA cm -2 requires a cell voltage of 1.82 V for HER + OER, whereas only 0.69 V is needed for HER + SOR. These advantages make sodium thiosulfate-targeted HER + SOR systems capable of achieving a total revenue of 1.652 $ kWh -1 , which represents an increase of 400% compared to conventional HER + SOR (0.377 $ kWh -1 ) and 2500% compared to traditional HER + OER (0.066 $ kWh -1 ). 5. Conclusions and Perspectives In recent years, SOR studies have attracted widespread attention and made substantial progress. However, the critical research factors driving the successful commercialization of SOR have not been systematically summarized. In this review, we focus on examining the current gaps between laboratory research and industrial implementation of SOR from multiple perspectives, and propose potential development directions including electrode fabrication, application system engineering, and high-value product acquisition. Based on a comprehensive summary of current SOR catalyst fabrication strategies, we propose the key factors influencing efficient reactant adsorption and sulfur-passivation resistance, which may facilitate a better understanding of the essential characteristics required for industrial SOR catalysts. Considering the varied application scenarios of reduced sulfur, we highlight the potential for developing diverse coupled systems and electrolyzer configurations. Furthermore, we also summarize various product recovery techniques, which, in contrast to other anodic reactions that typically yield a single product, significantly enhance the economic benefits of SOR. The following contents discuss the future challenges and opportunities for industrializing SOR from different perspectives. Despite significant advances in catalyst fabrication strategies, the majority of reported SOR electrodes are still constructed by loading powder catalysts with polymeric binders (Nafion) onto conductive substrates such as metal foam or carbon paper. Coated electrodes suffer from limited catalyst adhesion and are prone to catalyst detachment during prolonged electrolysis. We propose that integrating high-performance catalysts into monolithic electrodes via in situ growth is essential to effectively overcome the issue of weak adhesion, while significantly enhancing electron transport efficiency and long-term stability. While laboratory-scale electrodes are typically around 1-2 cm 2 , practical applications impose stringent requirements for electrode size, often necessitating large-area electrodes ranging from several hundred to several thousand square centimeters. In future research, the development of SOR electrodes should extend beyond performance testing and focus on the scalable fabrication of large-area electrodes with uniform active layers, which is essential for practical implementation. In addition to the fabrication of electrodes for SOR, researchers should also place greater emphasis on the reaction kinetics of the entire system. Previous studies have demonstrated that sulfur passivation is a key factor hindering the continuous and efficient progress of the electrocatalytic SOR process. Consequently, it is crucial to investigate how key operational parameters such as pH, temperature, and pollutant concentration influence accumulation behavior of sulfur on the electrode surface and the reaction rate. Although this review highlights several advances in SOR-coupled systems, it remains imperative to actively explore additional reductive reaction pathways compatible with SOR. And a deeper understanding of their prospective application scenarios could unlock new solutions for sulfur transformation technologies. Furthermore, future SOR research should focus on implementing to test electrodes under simulated industrial conditions. For example, when treating reduced sulfur species in natural gas, the electrolytes should be prepared with mixed gases containing H 2 S together with CH 4 or CO 2 . The economic feasibility of industrial SOR depends largely on the consumption and market value of sulfur products. By expanding the applications of elemental sulfur in fields such as textiles, personal care products, the rubber industry, and heavy-metal wastewater treatment, the industrial promotion and application of SOR technology can be further advanced. It is believed that the ultimate objective of SOR application research is to achieve substantial economic returns. Therefore, the way for recovery of products should be further optimized. In facilities where large volumes of acidic off-gases require treatment, these gases can serve as substitutes for the acid typically used in polysulfide recovery, thereby reducing both acid consumption and gas treatment costs. Collectively, these strategies will serve as a direction for our collective efforts to advance the development and commercialization of SOR technology. Acknowledgements The authors are grateful to the financial support from the National Natural Science Foundation of China (No. 52270110) and the Special Support Plan for High Level Talents in Zhejiang Province (No. 2017R52018). Conflict of Interest The authors declare no conflict of interest. Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff)) References [1] J. W. Zhang, D. Lei, W. X. Feng, J. Hazard. Mater. 2014 , 264, 350. [2] M. Q. Salih, R. R. Hamadamin, J. R. Hama, Int. J. Environ. Sci. Technol. 2023 , 20, 4727. [3] J. Sun, I. Pikaar, K. R. Sharma, J. Keller, Z. G. Yuan, Water Res. 2015 , 71, 150. [4] Q. Gong, H. Wu, F. Yang, Z. H. Tang, Gas Sci. Eng. 2023 , 109, 104846. [5] Y. H. Chan, S. S. M. Lock, M. K. Wong, C. L. Yiin, A. C. M. Loy, K. W. Cheah, S. Y. W. Chai, C. Li, B. S. How, B. L. F. Chin, Z. P. Chan, S. S. Lam, Pollut. Res. 2022 , 314, 120219. [6] S. 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Collection Energy & Environmental Materials Keywords electrode fabrication strategy industrialization reduced sulfur species sulfide oxidation reaction Authors Affiliations Zhongyuan Wang Zhejiang University of Technology View all articles by this author Yinxi Han Zhejiang University of Technology View all articles by this author Yufei Zhang Zhejiang University of Technology View all articles by this author Jinhuan Chen Zhejiang University of Technology View all articles by this author Jiade Wang 0000-0002-9496-8551 [email protected] Zhejiang University of Technology View all articles by this author Metrics & Citations Metrics Article Usage 339 views 200 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Zhongyuan Wang, Yinxi Han, Yufei Zhang, et al. Insights to Industrial Electrocatalytic Sulfide Oxidation: Electrode Fabrication, System Engineering, and High-Value Product Acquisition. Authorea . 21 October 2025. 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